In the last 20 years, the progress of remote sensing techniques and renewed access to many regions of Tibet have yielded new insights into the processes of continental collision. But there is still no consensus on the mechanisms that have led to the growth and rise of the 3.106 km2 wide and 5000 m high plateau.
In one class of models, the whole lithosphere is inferred to deform as a thin viscous sheet. Concurrent thickening of the crust and lithospheric mantle, spread over a vast area, absorbs most of the plate convergence. In the thickest region (Tibet), topographic rebound is subsequently triggered by convective removal of the thickened mantle. This is followed by relief collapse and extension. The rebound has a profound effect on climate. Mostly the final collapse is associated to strike-slip faulting, which is thus seen as a shallow and minor side-effect.
In another class of models, strain localization along shear zones, and decoupling between mantle and crust, are thought to govern the deformation of Asia. Strike-slip faulting and thrusting combine to cause diachronic uplift of Tibet soon after the onset of collision. The crust thickens but the mantle beneath subducts. At any given time, a small number of boundaries extending to the base of the lithosphere absorb much of the convergence.
Testing such competing models rests on mapping and dating Tertiary strain, metamorphism and intrusion, measuring slip- and uplift-rates, and imaging the deep crust and mantle. We review ongoing studies of Tertiary deformation and magmatism in and around Tibet, incoming evidence from seismic tomography experiments, and the kinematic picture emerging from Holocene slip-rate measurements.
New results tend to support the second class of models. Tibet appears to have grown towards the north and east in three distinct stages, as large sinistral strike-slip faults successively sliced farther into the Asian lithosphere, causing mountain growth on oblique thrust splays. The propagation of such faults "pushed" one edge of the plateau inside the continent. Decoupled wedges of crust thickened in conjunction with south-dipping subduction of the Asian mantle along two reactivated suture zones. Overall, the large-scale deformation of Asia thus coinvolved subduction and extrusion of coherent blocks of lithospheric mantle. Major block boundaries, such as the Himalayas, Altyn Tagh and Kunlun, resemble oblique convergent margins, with crustal slip-partionning. Plate Tectonic-like mechanisms may therefore operate beneath the thickened crust. High resolution seismic imaging, and more accurate strain and gravity measurements should help resolve the issue.
A 2D numerical model of mantle convection, which incorporates rigidly moving surface plates, isused to study the effects of the accretion of a 2000 km wide block of continental crust to the margin of a large stationary continent. This continental block is a model representation of the Indian sub-continent. It is carried along with an oceanic plate towards a subduction zone at the leading edge of a much larger stationary continent, meant to represent Asia. Prior to continental collision, subducted oceanic plate material under-rides the margin of the larger continent for several hundred kilometers before peeling away from the surface boundary layer and descending into the mantle as a cold narrow slab. Consequently, once the moving continental block arrives at the continental margin and closes over the original subduction zone the surface location of the continental suture lies several hundred kilometers south of the location of the cold remnant of the sinking slab. Convective flow in the sub continental mantle adjusts to accommodate the change in mechanical boundary condition at the upper surface. We find that once the upper surface is immobilized, the continued descent of the cold slab is inhibited, and the cold remnant remains below the collision zone longer than expected based on 'normal' rates of subduction. Evolution of temperature and flow fields in the former subduction zone result in a relaxation of normal stresses at the upper surface and a consequent slow regional topographic uplift. Although the model is limited to two-dimensions, the observed time scales and orders of magnitude the observed variations are expected to be reasonably accurate.
The mantle under Tibet, India and the adjacent Indian Ocean reveals several zones of relatively high P-wave velocities at various depths. Under the Pamir-Hindu Kush region in northeastern Afghanistan and southern Tajikistan a localized northward-dipping slab is seen in the entire upper 600 km of the mantle and is apparently still attached to the lithosphere of the Indian plate, whereas under northern Pakistan it shows a roll-over structure with the deeper portion overturned and dipping southward. Farther east-southeast (e.g., in the vicinity of Nepal), the deeper (overturned?) portion seems separated from the Indian lithosphere. These upper mantle anomalies are interpreted as the remnants of sub-continental mantle lithosphere that went down when Greater India continued to converge northward with Asia after about 45 Ma.
The deeper high-velocity anomalies under the Indian sub-continent appear clearly separated from the shallower ones as well as from each other, and are inferred to be Tethyan slab remnants overridden by the Indian plate. They occur at depths between 1000 and 2300 km and form three parallel WNW-ESE striking zones. We interpret these zones as remnants of oceanic lithospheric material, which was subducted when the Paleo-and Neo-Tethys oceans closed between India and northern Tibet between Late Jurassic and earliest Tertiary times. The present-day latitudes of the deep slabs under the Indian sub-continent (10 - 30 degrees N) correspond to the paleolatitudes of the Cretaceous to earliest Tertiary southern margin of Asia before its collision with India and the subsequently continued northward movement of India. The slab remnants in the middle mantle occur therefore approximately under the ancient locations where they started their downward journey.
Project INDEPTH (International Deep Profiling of Tibet and the Himalaya) is a collaborative Sino, US, German, Canadian geoscience project aimed at elucidating the structure of the lithosphere beneath Tibet. The project began as a Sino-US collaboration in 1992 with the acquisition of 100 km of CMP and complimentary wide-angle deep reflection data in southern Tibet (INDEPTH 1). The principal result of this work was the imaging of the active Himalayan decollement to 270 km north of the Himalayan thrust front and to a depth of about 50 km. A greatly expanded phase of fieldwork was undertaken during the summers of 1995-95 (INDEPTH 2/GEDEPTH). This phase saw the advent of German and Canadian participation in the project, and included the acquisition of 200 km of additional CMP and wide-angle data, along with complimentary broadband passive seismic, magnetotelluric, and surface geologic data along and adjacent to the INDEPTH 1-2 route. This work produced a suite of observations, which together suggest that the middle crust beneath southern Tibet is partially molten. These include the MT observation that the crust below the INDEPTH 2 transect is electrically conductive below about 15 km, the passive seismic observation of a broadly coincident midcrustal low velocity zone, and the CMP/wide-angle observation of seismic bright spots coincident with the top of the midcrustal conductor. The existence of a midcrustal partial melt layer beneath southern Tibet is counter-intuitive given that the uppermost mantle beneath the region is cool (exhibits relatively high velocity and earthquakes). It is significant in that the existence of such a layer has substantial implications for the way crust deforms in orogenic belts, the way granites form, and on a broader scale, the way continental crust becomes chemically stratified. Other noteworthy results of the INDEPTH 2 work include the discovery of a gently dipping structure in the upper crust that cuts off the Yarlung-Zangbo suture at depth (wide-angle and MT observations), and the tentative observation of a north-dipping structure in the upper mantle beneath the Lhasa terrane that might mark the India/Asia "mantle suture" (receiver function profile).
INDEPTH 3 is currently underway in central Tibet. The program includes a year-long deployment of about 70 broadband, intermediate, and short-period seismic stations for passive seismic studies, a crustal wide-angle reflection/refraction experiment, broadband and long period MT recording, and geological studies. Key goals of the program are to test whether the inferred crustal partial melt layer is a plateau-wide phenomenon, characterize the uppermost mantle beneath the central plateau, and constrain the distribution and magnitude of Cenozoic uppercrustal deformation within the plateau.
In 1994 and 1998, scientists from German institutions joined colleagues from Chinese, American and Canadian institutions in the INDEPTH project in Tibet. In 1994, off-line wide-angle seismic recordings of the shots along the Sino-US reflection line indicated the presence of a reflector at about 20 km depth cutting the Indus-Yarlung suture. A similar structure was also recognized beneath the in-line seismic stations about 40 km to the east. From broadband seismic recordings along the 400 km long profile across the Indus-Yarlung suture in 1994, receiver functions analysis indicates Moho depths of 70-80 km and the presence of a second discontinuity at 50-60 km depth beneath the entire profile. North of the Indus-Yarlung suture at about 10-20 km depth a pronounced low-velocity zone was detected, which together with evidence from other seismic and magnetotelluric data was taken to infer the existence of a partially molten layer in the middle crust beneath much of southern Tibet. On the other hand, the locations of the upper mantle discontinuities at 410 and 660 km depth are in agreement with the IASP91 global reference model. A combined analysis of receiver functions from the 1994 broadband data and data from the 1991-2 Sino-US PASSCAL broadband experiment across Tibet suggests a boundary diverging from the Moho about 100 km north of the Indus-Yarlung suture and dipping northwards in the upper mantle under central Tibet and a south dipping segmented structure in the upper mantle under northern Tibet. A clearer picture of the upper mantle under central Tibet should be provided by broadband seismic data presently being recorded along a 400 km long N-S profile across the Banggong-Nujiang suture in central Tibet. First results from a few broadband seismic recordings between 1997-8 and wide-angle seismic data recorded from large shots in 1998 along this 400 km long profile will also be presented.
Evolutionary models proposed for the Indo-Asian collision usually transcend our ability to test them against geological knowledge of these remote regions. When theory outpaces data, an effort is often later required to modify or reject popular models that have become inconsistent with observations. The Indo-Asian orogen appears to have responded to collision by discrete changes in accommodation mechanisms over time, even though convergence was continuous and the boundary conditions relatively constant. Our traditional inclination to interpret episodic phenomena in the geological record in terms of discontinuous processes (e.g., pulse of the earth) needs revision in light of the growing appreciation that complex physical systems driven by structureless inputs can exhibit highly intermittent, non-linear responses (e.g., turbulence). The Tibetan crust began to thicken upon collision of the Lhasa Block with S Asia producing a fold and thrust belt between 144-110 Ma that remained elevated until the onset of the Indo-Asian collision. The first post-collisional evidence of crustal thickening is thin-skinned thrusting in the Tethyan Himalaya, Eo-himalayan metamorphism, and deformation in the Fenghuo Shan and Nan Shan regions of N Tibet. The latter, initiated between 45-32 Ma, may have led to development of the left-lateral Red River fault system along which ~500 km of left-slip motion occurred from ~35-17 Ma. At 31±1 Ma, crustal thickening began in S Tibet along the Gangdese Thrust, moving southward to the Himalaya shortly thereafter in a series of S-directed thrusts (MCT, MBT, MFT) that sole into a common decollement. Slip along the decollement produced the Himalayan granites between 24-9 Ma. The STDS apears to have ben active between 17-11 Ma. The pattern of foreland thrust propagation was interrupted by the N-directed Renbu Zedong Thrust, active in S Tibet between 19-11 Ma. Tien Shan thrusting and thickening in the W Kun Lun also initiated during the early Miocene. Thickening in NW Tibet, apparently related to transtension along the Atyn Tagh fault, began during the middle Miocene. Following initiation of the MBT, the MCT reactivated between 8-4 Ma producing the classic Himalayan inverted metamorphism. By 9±1 Ma, Tibet had begun to differentially extend E-W via a series of N-S graben that extend across the plateau. Models for the evolving Indo-Asian orogen include those that predict wholesale uplift (mantle delamination, delayed under-plating), progressive growth (Indian under-thrusting, Asian under-thrusting, continental injection,), lateral responses (orogenic collapse, horizontal extrusion), and inheritance of an elevated terrane (multiple collision, intra-arc thickening). The pattern of crustal displacements we infer is inconsistent with most permutations of these single mechanisms but instead requires a specific, time-dependent transfer among several of these processes, often with multiple mechanisms operating simultaneously. The parameters that appear most important in dictating which mechanisms are dominant at any one time are: the location and geometry of pre-existing lithospheric weakness, the distribution of topography before and during the collision, the geometry of the indenter and extruded blocks, the magnitude of boundary stresses, and the age of the lithosphere. The clearest lesson emerging from our study of the Indo-Asian collision is that the continental lithosphere's complex history and geometry exerts a powerful control on continuous plate convergence being manifested in the geological record as episodic phenomena. A corollary of this 'intermittent dynamic' interpretation is that the chain of geologic events we infer to have occurred during the Indo-Asian collision is unlikely to have direct relevance to the understanding other, and perhaps more ancient, orogenic belts.
KEYNOTE
A02 : 3A/11 : G0
The frame of the India-Asia convergence is set by the apparent polar wander paths of both plates. Microplates in between and the northern margin of the Indian Plate have to be considered for reconstruction of crustal shortening and block rotation processes in the collision zone. Primary remanences from former Gondwana blocks north of the IYS suture (e.g. Lin & Watts 1988) provide a rough information on the timing of their relative movement, their stepwise accretion to Asia, and the latitude of the former southern Asian margin before the India-Asia collision. Present studies within the GEODEPTH program should improve the data base in central Tibet. Here we focus on the crustal deformation of the Indian Plate. Klootwijk et al. (1985) introduced an oroclinal bending and rotational underthrusting model to estimate a minimum magnitude of shortening. Only few primary results are available from the key zone for such studies, the Tethyan Himalaya. They controversely indicate shortening between several hundreds of km (Besse et al. 1984) to about 1500 km (Patzelt et al. 1996). The lack of primary data forces concentration on secondary remanences. Stable secondary pyrrhotite remanences are frequently found in the Tethyan Himalaya and were probably acquired during last cooling below 300°C. Quantitative reconstruction of rotational shortening is affected by local and meso-scale block rotations superposed on regional movements. Such effects are clearly seen in the NW, where the Indian indenter creates a complicate rotation pattern on both sides of the IYS, which however, can be still interpreted by tectonic processes. In order to enhance the data base for quantitative modelling of rotational shortening, we presently conduct sampling of low grade metamorphic limestones from the central Tethyan Himalaya between about 77°E and 86°E. Sampling localities are spread in equal distances as much as possible to recognise the regional rotation pattern, with partly closer spacing to detect local variations. Already processed samples indicate that local disturbances exist only within the limit of significance in most areas. Systematic block rotations support a rotational shortening and a more significant quantitative approach seems to be possible when all data are available.
Lin J & Watts DR, Phil Trans R Soc, 327, 239-262, (1988).
Klootwijk CT, Conaghan PJ & Powell CMcA, Earth Planet Sci Lett, 75, 167-183, (1985).
Besse J, Courtillot V, Pozzi JP, Westphal M & Zhou YK, Nature, 311, 621-626, (1984).
Patzelt A, Li H, Wang J & Appel E, Tectonophysics, 259, 259-284, (1996).
The western boundary of the Philippine Sea Plate (PH) with Sundaland (SU) in the Philippines corresponds to a wide deformation zone that includes the stretched continental margin of Sundaland, and the Philippine Mobile Belt (PMB) extending from Luzon to the Molucca Sea. The GPS GEODYSSEA data are used to decipher the present kinematics of this complex area. One of the main results is the quantification of overlapping subductions on both sides of the PMB. To the west, convergence decreases from about 90-100 mm/yr along the Manila trench to a few mm/yr across Mindoro in the zone of collision of Palawan with the PMB. Transfer of 55% of this Manila Trench convergence motion to the Philippine Trench to the east is accommodated principally by rapid counterclockwise rotation of eastern Luzon. Southward, convergence decreases regularly along the Philippine Trench from 54 mm/yr near 13°N to 32 mm/yr east of Mindanao. Inside the PMB, the slip rate of the left-lateral Philippine Fault, which enables complete partitioning to occur at the trench, increases northward from 22 mm/yr at 7°N to 35 mm/yr in the Visayas. As a result, the East Philippine sliver, east of this fault, moves north and transfers this motion to the East Luzon block counterclockwise rotation. West of the Philippine Fault, the Visayas block in central Philippines, is rotating rapidly clockwise, transfering most of the remaining convergence to the Negros/Cotabato trenches on its western border, up to 32 mm/yr near 12°N and 50 mm/yr near 8.5°N. These estimates ignore the amount of convergence absorbed to the west, at the limit of undeformed SU. Two opposite rotations on both sides of the left-lateral Philippine Fault, clockwise to the southwest and counterclockwise to the northeast, illustrate the PMB internal deformation. As a result, the northern part of the Visayas block acts as a cog wheel with respect to the Luzon block. It is remarkable that this system is such that subduction is everywhere perpendicular to the trenches. South of Mindanao, within the Molucca sea, the overlapping divergent subduction system of the PMB is relayed by the convergent Sangihe-Halmahera subduction system. This one accommodates 80 mm/yr of the 105 mm/yr PH/SU convergence and about 15 to as much as 20 mm/yr of the remaining convergence are absorbed far to the west, within the northern Borneo margins.
We present an estimate of the modern erosion of the Himalaya based on riverine fluxes. The dissolved flux related to chemical erosion is estimated on the basis of yearly averages of the rivers. It is 32 and 29*106 ton/yr for Brahmaputra and Ganges respectively. The fluxes of suspended matter measured in Bangladesh are 402 to 608*106 ton/yr for Brahmaputra and 328 to 548 *106 ton/yr for Ganges. Isotopic tracers show that the large majority of these particles is derived from the Himalaya. However the total erosional flux must take into account bedload transport and sedimentation in the floodplain which cannot be simply measured. Their importance can be estimated using the contrast in chemical composition between the suspended load and the bedload or the accumulated sediments due to mineral sorting during transport.
Assuming steady state erosion in Himalaya, the chemical composition of the total erosional flux equals that of the average source rock. This later is relatively well established for silicate formations based on ca. 200 analyses of the Himalayan formations. Compositions of the riverine end members are based on suspended- and bedload sampled in Bangladesh. The composition of sediments accumulated in the floodplain is derived from an average composition of Siwaliks. The principal process driving chemical differenciation is the sorting of phyllosilicates with respect to quartz and feldspars in favor of the suspended load. Si/Al ratios are: 4.2 for the source rock, 2.7 for the suspended load, 6.5 for bedload and 7.8 for the Siwaliks. Mass balance calculation is performed for two extreme case: no bedload or no sediment sequestered in the flood plain. On this basis, the suspended load represents between 40 and 65% of the total erosional flux. Using the fluxes of suspended load mentioned above, total erosional fluxes can be calculated and expressed in term of erosion rate. They are 2.8 ± 1.1 mm/yr for the Brahmaputra Himalaya and 1.8 ± 0.6 for the Ganges Himalaya. The higher erosion rate in the Brahmaputra basin is likely related to its higher runoff.
Using an Atmospheric General Circulation Model, we simulate most of the spatial evolutions of the Asian monsoon only accounting for the changes of paleogeography including continental drift, orogeny and sea level change. The paleogeographic changes modify drastically the climate over the central and southern Asia between the Oligocene and the present day. The retreat of an epicontinental sea (the Paratethys) warms central Eurasia in summer. The heating of this area and the uplifts of the Tibetan plateau and of the Himalayas deepen the Asian low pressure cell and displaces it northwest subsequently shifting precipitation from Indochina towards the southern flank of the Himalayas. The agreement with proxy data is good. Therefore, our modeling studies support a shift and a strengthening of the Asian monsoon during the late Tertiary rather than a real onset. We also investigate the respective impact of the Paratethys shrinkage and of the Tibetan plateau uplift through sensitivity experiments and prove that the Paratethys retreat plays an important role in the monsoon evolution.
Numerous previous studies of Sm/Nd systematics in crustal rocks and sediments have shown the importance of sedimentary recycling for the evolution of the continental crust. The proportion of old sediments vs. granites in the continental crust is a key parameter to constrain if we want to address important questions such as the secular evolution of the continental crust composition or the long term consumption of atmospheric CO2 by the weathering of continental rocks. Recent weathering studies suggest that first cycle weathering is more efficient, in terms of CO2 consumption rates, than the weathering of rocks having passed through several weathering cycles. Thus, the role of sediment recycling in the evolution of climate needs to be considered.
We propose in this study to use the geochemistry of large river erosion products to estimate sedimentary recycling rates. Our study case is the major orogenic zone of East Asia, which is drained by the following large rivers: Huanghe, Changjiang, Xijiang, Hongha and Mekong rivers. The basic idea is to establish mass budgets for the dissolved and solid products of silicate erosion in rivers. In particular, insoluble over soluble element ratios, such as Sm/Na or Th/K ratios are especially sensitive to sedimentary recycling and are used to calculate the (minimum) proportions of recycled sediments involved in present day erosion. These chemical mass budgets are highly sensitive to the amount of sediments transported per year by the rivers (physical denudation rates). For the rivers under consideration, a good agreement exist between the multiyear average of river suspended sediment concentrations and the mass accumulated in the sea by these rivers over the last 2 million years (Métivier and Gaudemer, 1998). We thus consider these rates as reliable estimates of physical denudation.The recycling rates calculated using the different chemical ratios are close to 50% for the Huanghe river (mostly influenced by loess) and range from 70% to 95% for the other river systems. To our knowledge, these values are the first estimates of present day recycling rates and highlight the importance of sedimentary cannibalism in one of the major orogenic zone of the Earth.
On the Northern part of the Tibetan plateau, in a 1300 km long - 300 km wide area roughly parallel to the margin of the plateau, lies Neogene to Quaternary post-collisional potassic magmatism. On Satellite images we have mapped different features of this young volcanism: small cones, large pyroclastic deposits and lava plateaus, and have estimated the volume of volcanic products. In 1997, during a Sino-French mission, we sampled volcanic rocks in the Kokoshili Shan (latitude : N35°30', N36°30', longitude : E88°, E90°), near the western extremity of the Kunlun fault. Field observations of the structure of the edifices and the study of the samples show that this volcanism ranges from mafic to silicic. The more mafic deposits, usually cinder cones, contain deep crustal xenoliths. Thin sections show two types of texture and mineralogy. The mafic rocks contain abundant olivine, some of which may be xenocrystic, plagiocase, pyroxene and oxydes, and the groundmass consists sometimes of glassy matrix with laths of plagioclase and pyroxenes. The more silicic material contains plagioclase, quartz, some alkali feldspar and some biotite in a groundmass of granular texture.
The age, location and composition of these volcanic rocks provide constraints on their sources and on the degree of interaction with the warm crust of mafic magma that ascends from the mantle. We interpret these results in the framework of a fluid dynamic model of crustal melting induced by the passage of basaltic dykes, and show how the depth and amount of melting depends on the deep thermal structure of this thick continental crust.
Understanding the role of the subcontinental lithospheric mantle in accommodating deformation in collisional regions is fundamental to our understanding of continental tectonics as a whole. The formation of the Tibetan plateau has been linked with the Indo-Asia collision, and it provides an ideal setting in which to investigate these tectonic processes.
Changes in tectonic regime may be reflected by spatial and temporal variations in the associated magmatism. Previous studies have identified small-volume, potassic volcanics across the Tibetan plateau, and these are considered to represent partial melts of the mantle lithosphere (Turner et al, 1996). This broadly supports geodynamical models of convective removal and thinning of the mantle lithosphere that have been proposed to explain the elevation history and active extension of the plateau.
Here we present initial elemental and isotopic data for volcanics collected from southern Tibet. These lavas are predominantly trachyandesites with subordinate dacites and rhyolites. Samples are characterised by extreme enrichment in incompatible elements, and form distinct arrays in La/Yb - Tb/Yb space, implying variations in both source and melting regime. Isotope data shows that samples fall into several broad fields defined by radiogenic 87Sr/86Sr and unradiogenic 143Nd/144Nd. Preliminary laser Ar-Ar ages for these samples show that lavas from southwest Tibet are at least 24 Ma - considerably older than samples from southeast or northern Tibet, which range from 8-14 Ma.
These findings indicate that the geochemistry of these volcanics varies both temporally and spatially across the Tibetan plateau. The implication of these results is that models of Tibetan plateau evolution are considerably oversimplified and that the true picture is far more complex. New geochemical data from the 1998 field season, including a north-south trending lamprophyre swarm, will be included in this discussion.
Turner S P, Arnaud N, Lie J, Rogers N, Hawkesworth C, Harris N, Kelley S, Van Calsteren P & Deng W, J. Petrology, 37, 45-71, (1996).
Recent geophysical data constrain fairly well the deep structure of the Himalayas and Tibet. In particular, they reveal the Indian crust subducted to about 80 km depth. Eclogites discovered in the western Himalayas indicate that the Indian crust has already reached at least this depth at the beginning of the collision (ca. 50 Ma). Since that time, hundreds of kilometres of the Indian crust have been subducted under the Himalayas. The major challenge now is to organise all this information into a physically and geologically consistent evolutionary model, considering that the history of the Himalayas cannot be separated from that of Tibet because they both belong to the same geomechanical system. We started to develop such a model based on the new results of 2-D thermo-mechanical experimental modelling. The principal successive stages of this model are as follows: (1) subduction of the Indian crust to 200-250 km depth followed subduction of the oceanic lithosphere; (2) failure of the subducted crust at ~80 km depth and rapid buoyancy driven uplift of the crustal material from this depth to 20-30 km; (3) break-off of the Indian subducted lithospheric mantle attached to the previously subducted oceanic lithosphere; (4) shallow-angle (horizontal) subduction of the Indian lithosphere under Asia over a distance of a few to several hundreds of kilometres; (5) delamination and roll-back of the mantle layer of the Indian lithosphere until its break-off beneath the Himalayas; (6) failure of the Indian crust in front of the mountain belt (formation of the MCT) and underthrusting of a new portion of the Indian lithosphere under Tibet for a few hundred of kilometres. At the beginning of this stage (6), the crustal slice corresponding to the Crystalline Himalayas undergoes "erosion-activated" uplift and exhumation. The overriding Asian plate is horizontally undeformable in this model and plays a passive role. Its deformation in reality is essentially three-dimensional and thus a 3-D approach will be necessary.
We employ P-to-S converted teleseismic waves recorded by temporary broadband networks for imaging the seismic discontinuities in both the crust and the upper mantle across the whole Tibet. The data used consist of 353 teleseismic receiver functions (RF's) from INDEPTH II/GEDEPTH and 406 RF's from PASSCAL These observations are jointly interpreted together with published teleseismic tomography and gravity models.
A north-dipping interface, called here Zangbo Conversion Boundary (ZCB), is imaged starting 50 km north of the Zangbo suture at the Moho and reaching 200 km depth underneath the Bangong suture. The average amplitude of the converted wave at this boundary (4% of the direct P-wave) is modeled by an S-wave velocity increase downward by 6-8%. Such a velocity change may be explained by a temperature contrast of 500-700°C (considering the effect of anelasticity and chemical reactions) together with chemical differences (a higher Mg content in cratonic mantle). The ZCB probably separates the relatively cold and depleted cratonic lithospheric mantle of India with a temperature of 600-800°C at its top from the lower lithosphere of Asia with a temperature of 1100-1300°C. Teleseismic travel time tomography in this region reveals a North-South P-wave velocity contrast of about 4-5% at a depth of 200-250 km (Witlinger et al., 1996). Most of this contrast can be again explained by a lower temperature and higher Mg content of the Indian lithospheric mantle and the remainder (0-2%), by an upwelling of hot mantle (if any) with a temperature excess of 100-200°C or less. Our model of the gently subducting Indian lithospheric mantle is also consistent with observed gravity and surface topography in southern Tibet (Jin et al.,1996).
Under northern Tibet a less clear south dipping segmented structure is also identified by converted waves. This zone of strong and complicated conversions may indicate an ongoing process of destruction/southward-subduction of the Asian lithospheric mantle which appears to be more complicated than simple convective lithospheric thinning in a viscous media. Together these observations are interpreted as different styles of detachment of the Indian and Asian lithospheric mantles caused by a difference in their composition and buoyancy, where the Indian lithosphere is gently subducting but remains stable and the Asian lithosphere is in the process of destruction.
Jin et al., JGR, 101,11275-11290, (1996).
Witlinger et al., EPSL, 139, 263-279, (1996).
We present new paleomagnetic results obtained at 39 sampling sites from 5 sections of Tertiary redbed formations: 2 Eocene formations from the Qiangtang block of Tibet (Xialaxiu locality; 32.8°N, 96.6°E) and the Xining basin of Qaidam (Xining locality; 36.5°N, 102.0°E), and 3 Neogene formations from the Xining basin (Jungong locality; 34.7°N, 100.7°E) and the Kunlun block (Tuoluo lake and West Yushu localities; 35.3°N, 98.6°E and 33.2°N, 96.7°E respectively). Thermal demagnetization of the samples allowed to obtain a High Temperature Component which we interpret as the primary magnetization in all but one (Tuoluo lake) localities. For the 4 other localities, the paleopoles lie at 52.6°N/ 352°E (dp/dm=6.0°/10.7°) for Xialaxiu, 61.6°N/211.3°E (dp/dm=9.7°/16.1°) for Xining, 66.0°N/228.6°E (dp/dm=3.6°/6.9°) for Jungong and 53.9°N,/205.4°E (dp/dm=5.6°/10.0°) for W. Yushu. As in previous studies of Tertiary formations from Asia, the paleomagnetic inclinations we obtained are systematically shallower (by 22°-26°) than the magnetic field computed from the Eurasian APWP at 10 and 20 Ma for Neogene formations and at 50 and 60 Ma for the Eocene ones. This inclination shallowing, known as the «inclination anomaly», has previously been interpreted as either due to a magnetic field anomaly over Eurasia in the Tertiary, and/or due to compaction processes in sediments. Based on a detailed discussion of these possible causes, and on a compilation of Eocene data from the South China Block, Tibet, Central Asia and Kyrgyzstan, we conclude that these are probably not the main causes of the discrepancy. We propose instead that the «anomaly» arises from an inadequate description of Siberia craton movement based on the Eurasia APWP, the causes of which could be a non rigid behaviour of the Eurasia plate in the Tertiary. Combination of this with intracontinental shortening of Asia under the penetration of India provides a full explanation for the anomaly. Finally, a comparison of our Neogene results with the India APWP demonstrates that the formations we sampled can certainly not be assigned an age as young as Neogene. This addresses the critical problem of dating of the Tertiary redbed formations in Asia.
Thermal models based on simple sliding block kinematics have been successful in reproducing the peak P,T conditions of metamorphism in the Higher Himalayan Crystalline series. However, these models generally assume a constant rate of erosion in time and space and thus may not give realistic exhumation histories. In particular, the presence of aseismically active crustal thrust ramp beneath the high Himalayas north of Kathmandu is suspected and we investigate the possible consequences on the thermal structure and PTt paths. This ramp connects the main seismic decollement to the south with the main ductile decollement to the north and corresponds to a temperature range of 350 to 500°C. We computed temperature fields and PTt paths for steady-state kinematic models with and without ramp and with and without underplating. Uplift above the ramp only causes a small and local perturbation of the geotherms which would probably be undetectable with available geothermometers and geobarometers. However, geochronological data could be radically altered. We examined the example of the ~100 muscovite 40Ar/39Ar ages (corresponding to 350-400°C) and 10apatite fission-track ages (about 100°C) in a section extending across the Himalayan thrust system from ~50 km south to ~ 30 km north of Kathmandu. The muscovite data show a continuous trend of progressively younger ages from south to north, from a maximum of 22 Ma to a minimum of 5 Ma without change in crossing the Main Central Thrust (MCT). Models with a flat decollement fail to reproduce this feature as they give constant (±2 Myr) closure ages over the HHC section but models with a ramp all give significantly younger ages towards the north with an age minimum located above the ramp. We conclude that the crustal ramp is probably a steady state feature that influences the distribution of erosion and exhumation rates since at least 15 Ma. The oldest measured 40Ar/39Ar ages cannot, however, be reproduced with steady-state models and may be the consequence of exhumation along the South Tibetan Detachment or an older geometry of the thrust system not accounted for in our model. In the models, the Lesser Himalayan Series correspond to material continuously underplated at the ramp and possible reactivation of the MCT after underplating is ignored. Although simplified, this mechanism may explain some of the observations, such as why muscovite closure ages appear more or less contiuous across the MCT zone.
The Himalaya undergoes at present, about one-third of the today convergence between India and Eurasia (58 +/- 4 mm/a). The present-day deformation of the Himalaya is characterised by big earthquakes (nearly half of the chain has ruptured over the last century). The historical sismicity of Nepal indicates the occurrence of big earthquakes in eastern and central Nepal, western Nepal being characterised by a lack of recent big earthquake. To study the present-day deformation of western Nepal a GPS network consisting of 29 sites, was installed in central and western Nepal, measured in 1995, partially remeasured in 1997 and totally in November 1998. Fist data indicate 15 mm/yr of N180° convergence between Higher Himalaya and the Indian border south of Nepal and suggest a 4 mm/year E-W extension. The velocity field is consistent with creep on a dislocation striking N120° and dipping 9° to the north. This dislocation is locked at 17 km deep beneath the Lesser Himalaya and affected by a 19 mm/yr thrust component and a 7.5 mm/yr right lateral component. These results suggest an oblique underthrusting of the Indian crust below the High Himalaya of western Nepal consistent with N-S shortening across the arcuate shape of the Himalaya of Nepal. A N170 contraction and a N80 extension is observed for sites located East of 83°30, where quaternary faults (Darma-Bari Gad fault system and Thakkhola graben) delineate a crustal wedge. This wedge is located in the continuity of the Karakorum fault and may segment the Himalayan thrust belt. During this meeting, the new results obtained from the comparison 1995-1997-1998 will allow to precise the localisation and geometry of the dislocation surface and the amounts of displacement along the presumed active faults.
Processes of exhumation of HP are discussed in the light of the study of the Tso Morari dome (E-Ladakh, NW Himalaya). Petrological, structural and geochronological investigations show that the exhumation of the HP Tso Morari continental unit is a discontinuous process from a minimum depth of 70 km up to the upper crustal level. It is controlled by the major plate tectonics changes that occurred during the India-Asia convergence.
The beginning of the exhumation of the HP Tso morari unit is first characterized by a rapid isothermal and vertical extrusion (> 4 mm.yr-1) from 70 to 40 km depth. This active exhumation occurs during the upper Paleocene (55 ± 7 Ma) while the subduction of the Indian margin is still active. The association of the HP rocks with soft serpentinized mantle rocks and the tectono-metamorphic evolution of the Tso Morari suggest that the beginning of the exhumation is controlled by the return flow developed along the down-going subducting Indian plate. The coaxial D1-D2 deformations related to the vertical extrusion of the dome throughout the mantle are independent of the deformations occurring at the same time close to the surface. Since 48-45 Ma to 30 ± 1 Ma, from the base of the Indian crust (30-40 km) up to upper crustal levels, the slower vertical exhumation (~ 2 mm.yr-1) of the HP Tso Morari dome has been ruled by ductile normal shearing (D3). This second stage of the exhumation is associated with a slight temperature increase, and is contemporaneous with the underthrusting of the High Himalayan Crystallines below the Tso Morari dome. The India-Asia collision governed the second exhumation stage of the HP rocks. It implies a strong horizontal squeezing of the HP unit and the surroundings between the two landmasses, that lead to the pinching of the Tso Morari eclogitic dome and to its upward extrusion close to the surface.
In November 1996 we have carried out a gravity survey in the framework of the Nepalese-French IDYLHIM cooperation program. Using two Scintrex CG3 meters, we collected more than 150 new gravity measurements along two profiles perpendicular to the range, in the vicinity of Kathmandu. Data were tied to the IGSN71 network in Kathmandu. Accurate geographic coordinates were obtained from the Nepalese Geodetic Survey benchmarks and GPS positioning. We estimated inner zones terrain corrections in the field and computed outer zones ones, up to 167 km, using various DTM. The overall accuracy of the resulting complete Bouguer anomaly ranges between 0.5 and 5 mGal depending on the terrain roughness. Our data complemented with those collected during the 1982 French survey in Tibet, with data from the Bureau Gravimetrique International and with previously published ones, allow to get continuous Bouguer anomaly profiles from India to the Tarim Basin. At short wavelengths, these data are used to assess the geometry of the system and specifically the main thrusting faults such as the Main Central Thrust (MCT), the Main Boundary Thrust (MBT) and the Main Frontal Thrust (MFT). Especially, the use of 1D wavelets transforms on these profiles allows us to put quantitative constraints on several geological discontinuities. At long wavelengths, the observed gravity anomalies are consistent with a flexural model of the lithosphere. The modeling is performed using a plane strain finite element model (ADELI) that accounts for thermal and strain dependant rheology and for P,T conditions. We also assess the possibility for petrological changes during the underthrusting; in particular, eclogitization of the lower crust. We find no gravimetric signature for such effects.
Data from 12 broad-band magnetotelluric (MT) soundings, acquired across Central Nepal in November and December 1996, are analysed to determine the electrical structure of the crust of the active Himalayan region. We have estimated the MT impedance tensors by a robust method for frequencies between 0.001 to 500 Hz. A tensor distortion analysis, using the Groom-Bailey decomposition technique, shows that the electrical distortions are weak in the south and strong in the north. This indicates that Central Nepal's present day geoelectrical structure is of growing complexity from south to north, close to one-dimensional behaviour under the Gangetic plain and three-dimensional in the north. Two dimensional joint inversion of TE- and TM- mode data after decomposition reveals from south to north : 1) a depth to the Indian basement of about 5 km under the Siwaliks, in good agreement with seismic data 2) an upper Indian crust more conductive than the lower Indian crust 3) a shallow very resistive body under the Kathmandu klipp extending to depths of about 7 km, likely related to Ordovician granites and underlain by a thin conductor 4) a 20 km deep conductor near the boundary between the Lesser and Higher Himalayas coinciding with the Himalayan crustal ramp and located near a seismically active zone 5) an increasing of the deep conductivity from south to north. Comparison of conductivity models with other geophysical data and thermal modelling will also be presented.
The contact between the Kohistan-Ladakh Arc and the Asian paleomargin (Karakorum) is defined in Indian Ladakh as the Shyok suture zone. Westwards, in the Skardu area (Pakistan) the Arc-Karakorum contact is an imbricate South vergent thrust system, the MKT (Main Karakorum thrust), with thrust contacts underlined by serpentinite strips. Still farther West, North of the Nanga-Parbat /Haramosh spur (consisting of Himalayan high grade gneisses), the MKT is no more a thrust, but a recent vertical and rectilinear fault. In this Western segment, the actual Arc-Asian margin boundary is unclear: geochemical studies suggest that some amphibolitic rocks of the South Karakorum metamorphic pile are fragments of the Ladakh Arc, obducted onto the Asian margin, then metamorphosed and deformed during the early South vergent tectonics (from 67 ± 2 Ma to 35 ± 5 Ma: period of the apparently syn-tectonic Mango Gusar granite emplacement). Late metamorphic evolution along the MKT/Shyok suture is marked by local high temperature anomalies, known in the Shyok suture zone and best studied in Skardu area: here, late metamorphic isogrades define an E-W elongated pattern, oblique to the MKT and the earlier metamorphic pattern. They follow a line of kilometric scale domes, from the Baltoro glacier in the East to the Nanga Parbat Himalayan spur in the West. Various thermochronological data from gneisses in the domes or from syn-doming plutonic cores (Hemasil) indicate young ages: most of them are less than 10 Ma, to the North as well as to the South of the MKT. Thus they can be interpreted in the light of the very recent to present stress and kinematic data. The recent tectonic pattern was investigated using strain trajectories, as marked by the late metamorphic foliation, and the stress directions obtained by inversion of post metamorphic striated planes:- The isogrades pattern, the E-W alignement of domes and the lack of small scale extension shear criteria suggest that the thermal anomalies are linked to a transpressive deformation regime, with along strike and down dip movements on the MKT/Shyok zone,- The MKT, as the dextral Karakorum fault, offset the young metamorphic patterns confirming that the MKT is not the suture, but a very young reactivation of a former contact,- The stress pattern is quite heterogeneous: the principal stress can be vertical in the core of the domes (f.i. the Haramosh dome), or horizontal and either parallel or perpendicular to the MKT (shortening perpendicular and parallel to the belt, very irregular exhumation). As a whole, stress and thermal patterns can be linked to a transpressive deformation regime, with partitioning of the deformation during the oblique Indian-Asian convergence, and control of inherited major crustal discontinuities.
In addition to the magnetotelluric data acquired during the 1996 IDYLHIM survey, we have computed the magnetic field transfer functions, also known as induction vectors, using broad-band data at 12 sites as well as long-period records from four stations. The induction vectors are very small south of Kathmandu, indicating a primarily one- or two-dimensional conductivity structure there. North of Kathmandu however, the vectors are much longer and the opposite of their real parts (Parkinson vectors) generally point towards the Main Central Thrust (MCT): this is particularly spectacular for the Kakani and Pipaltar sites, on either side of the MCT, for which the Parkinson vectors are in near-perfect opposition. Qualitatively, the data show that the MCT does not extend further north than Syabru Bensi, since the Parkinson vectors there point towards the south at all frequencies. This high conductivity associated to the MCT is likely to be related to fluid circulation. To gain more insight into the large-scale conductivity structures around central Nepal, we have computed the response of 3D models for two end-members and present here the first results of our modelling of the magnetic transfer functions. The first end-member is an attempt to quantify the effect of the rough topography, especially along the Trisuli river valley where most of the data were acquired, on the magnetic induction vectors. To do so, we use a 30-second interval digital elevation model and consider an homogeneous earth from which we calculate the response using an integral equation scheme. For the second end-member, we neglect the effects of topography and express lateral variations of conductivity in terms cells of varying integrated conductance. The response is then computed using a thin-sheet algorithm. We show that a satisfying degree of fit can be obtained assuming that the MCT plays an important role in channelling electrical currents in central Nepal. 3D modelling from more realistic intermediate models is currently in progress.
Neodymium model ages are useful tools for deciphering ancient tectonic associations in complex polymetamorphic terrains. Here we present the results of a whole-rock Nd isotopic study of two contrasting regions of the western Himalaya which demonstrate the utility of the approach on the scale of a single orogen. New data for high-grade metasediments from Zanskar yield model ages that are similar to the High Himalayan Crystalline Series (HHCS) (TDM = 1.2-2.0 Ga, (sum)Nd = -6 to -16), and quite distinct from values from the Lesser Himalaya (TDM = 2.3-3.4 Ga, (sum)Nd = -18 to -27). Hence these two lithological sequences can be recognised for 2000 km along strike of the orogen. New Nd data for basement of the Nanga Parbat Haramosh Massif (NPHM) at the western extremity of the Himalaya (TDM = 2.3-2.8 Ga, (sum)Nd = -18 to -30) suggest that these rocks are not equivalent to the HHCS, as previously supposed, but have affinities with the Lesser Himalaya. A thin metasedimentary cover sequence on the margins of the NPHM is isotopically indistinguishable from the HHCS (TDM = 1.6-1.8 Ga, (sum)Nd = -10 to -14). The prior misidentification of the provenance of the NPHM stems from its high metamorphic grade, characteristic of the HHCS, but in this case related to the unique Neogene history of the NPHM, which has exhumed a deeper structural level of the LHF than is seen elsewhere. These findings have implications for the identification of terrains in older orogenic belts, where contrasting P-T-t histories may mask their pre-orogenic correlations.
The Phenomenon of extension in the India-Asia collisional belt was first noted nearly twenty years ago (Burg et al. 1981). The Southern Tibet Detachment System (STDS), subsequently has been identified all along the Himalaya, with an ever increasing indication that the STDS is a complex system of detachment horizons, and not a simple horizon with a restricted period of activity. This may be expected in a large mountain belt, and the STDS (the cat) has not run away. However, the orogen-wide relationship of the STDS with large bodies of leucogranite (that are ubiquitous along the Himalaya) has not been so easy to catch. Although this relationship is a spatial association, and from the Everest and Manaslu areas (and others) it seemed initially that there was a "specific period" when most of the anatexis and STDS displacement occurred, further mapping and geochronology has allowed the mouse to get away. Khula Kangri suggested that the "specific period" was later in the Bhutan Himalaya, or that there was a second occurrence. A northward younging between the High Himalayan and the "north Himalayan" granites was subsequently proposed and even a slip rate on the STDS was attempted, but the mouse escaped. Structural and metamorphic data have caught the STDS in many parts of the Himalaya, whose associations with subsequently dated melt products show the relationship to have continued since Eocene and Oligocene, with no clear constraint of an end. The INDEPTH results show that the "anatectic front" may have simply pushed on northward. The "anatectic front" (accompanied by large exhumation rates but no "detachment") is also seen at the Himalayan Syntaxis. The Nanga Parbat group results have uncovered a pattern of protracted melting (that includes the Upper Miocene age so common in the main Himalaya). A procession of plutonism interacting with shear zone displacement has a long history within the two main (conjugate) shear zones at Nanga Parbat, and the Indus River, that has maybe focused the deformation, has re-located the sediment to tell the tale. The last twenty years have seen the mouse (the timing of anatexis) run away frequently, however the education is to be able to recognise the signs (and then role) of anatexis interacting with deformation to unravel a long and complex interplay. This melting deformation interaction is a rising issue in studies of granites worldwide, and indeed the Himalaya are frequently turned to for example. But the story goes on and if the mouse is truly caught in the next twenty years, it will be a bad thing indeed.
Burg, J-P, Brunel, M, Chen, GM and Liu, GH, Deformation of the leucogranites of the Crystalline Main Central Thrust sheet in southern Tibet (China) Mitt. Geol. Eidgen. Techn. Hochs. Univ. Zürich, Neue Folge, 239a: 49-51, (1981)
The Higher Himalayan Crystalline (HHC) mainly comprises amphibolite to anchizone grade late Precambrian to early Cambrian metasediments (Haimantas Formation), which have been intruded by Cambro-Ordovician granites. Evidences for a crystalline basement of this huge sedimentary deposits are still missing. This sequence continues through the Paleozoic-Mesozoic up to the Early Tertiary (i.e. Tethyan Zone - TZ) and is locally detached from the HHC along a normal fault system (i.e. South Tibetan Detachment Zone - STDZ). This normal fault system acted synchronously with the Main Central Thrust Zone (MCTZ), which thrust the HHC over the Indian continent in the Early Miocene.
The low-grade metamorphic rocks of the TZ in the Pin Valley (Spiti, N-India) form a SW-vergent fold-thrust belt, with steep, NE dipping axial planes. 40Ar/39Ar ages of newly formed illite scatter between 42 and 45 Ma, indicating that this deformation did not correlate with the MCTZ/STDZ phase but probably formed after the India-Asia collision as a response of crustal thickening. Early Miocene and later brittle deformation is only recorded by a spaced cleavage with shallow dipping axial planes, local sets of extension gashes and minor slickensides, which did not significantly contribute to the overall deformation of this area.
In order to quantify the amount of shortening of the early fold-thrust deformation phase, a 25 km long line-length balanced cross-section through the Pin Valley, essentially perpendicular to the fold-axes and parallel to the thrusting direction, has been drawn and restored. The calculated amount of shortening is about 25% and has been mainly accommodated by break-thrust folding. Depth-to-detachment calculations have been made on the base of several justified assumptions such as constant bed-length, plain strain, no material to leave or enter pin-lines and parallel reference and detachment horizons in order to validate the area balancing by the Chamberlin Method. These calculations suggest a detachment at a depth of 10-11 km, with a calculated décollement dip of 3°.
These results bear important implications for the deformation phase between the India-Asia collision and the well known Early Miocene events: initial crustal thickening was probably partitioned in a more homogeneous thickening at depth, whereas near the brittle-ductile transition zone shortening at more shallow crustal levels was accommodated by the formation of a SW- vergent fold-thrust belt. A hypothetical detachment of the Precambrian to Early Tertiary sedimentary sequence provides an attractive solution to the unknown crystalline basement of the HHC and TZ.
The Structure of the High Himalayan Crystalline zone is dominated by the SW verging folds and thrusts of the Main Central Thrust zone which has been formed by underthrusting of the North Indian margin below Asia (Heim & Gansser, 1939, Frank, 1977, Le Fort, 1986). New investigations in the Lahul and Spiti regions show that these SW verging structures of the MCT are overprinting a stack of older NE directed intracontinental thrusts (Steck et al. 1993, Epard et al. 1995). Such NE-directed thrust structures may be followed over a distance of more than 200 km along the strike of the High Himalayan Crystalline Zone. The NE verging Shikar Beh Nappe stack represents an example of the transition from brittle to ductile conditions and has been formed by a SW-directed intracontinental underthrusting (reactivating an old listric fault) and accretion of Precambrian to Mesozoic sediments of the North Indian margin. The heating of the subducted sediments is responsible for the regional metamorphism of Barrowian type. P-T estimates indicate a thermal field gradient of 25°C/km and according to published geochronological data, thermal peak conditions have been reached between about 40 and 20 Ma. The transition from NE-verging folds in a high tectonic level to a mylonitic NE-directed thrust zone in a deep tectonic level may be observed in the NE-directed Miyar Valley Thrust Zone, located on the southern limb of the High Himalayan Crystalline dome structure of the Zanskar region.
Epard, J-L, Steck, A, Vannay, J-C & Hunziker J, Schweiz. mineral. petrogr. Mitt, 75, 59-84, (1995).
Frank, W, Thöni, M & Purtscheller, F, Colloq. int. CNRS (Paris), 268, 147-160, (1977).
Heim, A & Gansser, A, Mém. Soc. Helv. Sci. nat, 73, 1-245, (1939).
Le Fort, P, Collision tectonics. Geological Society Special Publication (london), 19, 152-172, (1986).
Steck, A, Spring, L, Vannay, JC, Masson, H, Bucher, H, Stutz, E, Marchant, R & Tièche, J-C, Eclogae gel. Helv, 86, 219-263, (1993).
Steck, A, Epard, J-L, Vannay, J-C, Hunziker, J, Girard, M, Morard, A & Robyr, M, Eclogae gel. Helv, 91, 103-121, (1998).
02 : 3P/09 : PO
Higher Himalayan Crystalline (HHC) rocks often show metamorphic zonation from lower green schist facies to migmatites associated with leucogranite intrusions. In the investigated area, the Kinnar Kailas (KK) granite was already described as a concordant low grade intrusion related to the inverted metamorphism developed in the surrounding rocks (Vannay and Grasemann, 1998). The present study based on structural and chronological relationships between the KK granite and surrounding HHC is evidences for a discordant intrusive contact of the KK granite.
The first deformation phase (D1) in basement rocks is reflected by an old poorly preserved schistosity and associated folding. The geometry of the main progressive ductile deformation (D2-D3) results from SW vergent doming and migmatisation. This progressive deformation produces an unique NE-SW stretching lineation. Axial planes of D2-D3 folds are parallel to the main schistosity (S2-S3) which dips slightly towards the NE. The fold axes are oriented NE-SW (D2) and NW-SE for D3 SW vergent folds. The late D4 deformation is specially localized (East of Wangtu Gneisses and West of KK Granite in Sangla Valley). This low grade deformation corresponds to wide angle ductile normal faulting. NW-SE oriented stretching lineations and C/S microstructure testify a lowering of southeastern blocks.
The KK granite crosscuts the high grade deformation structures (D2-D3) but is locally affected by the local late D4 extensional deformation. Moreover, the KK granite is mainly undeformed and contains xenoliths with D2-D3 upper amphibolite facies paragneisses. The geometry of the xenoliths reflects magmatic stoping mechanism. In several localities, the KK granite bears dark microdioritic and hybrid enclaves that show textural evidence for mixing and mingling during the magmatic process. The main body of KK pluton consist of coarse grained biotite rich granite. KF porphyritic granite and fine grained leucogranite are less abundant. KK granite is intruded by fine grained aplitic and leucocratic granitic dykes.
Geochemically, samples of the KK granite show a wide range of SiO2 content (64 to 75%). The KK granite shows characteristics of peraluminous, S-type granite and calc-alkaline affinities in geochemical diagrams. Rb/Sr whole rock ages of =480 Ma with initial 87Sr/86Sr = 0.7201 +.0023 (Kumar, 1986) and Rb/Sr muscovite ages (this study) indicate a pre-Tertiary age for the KK granite and therefore for the high-grade metamorphic deformations D2/D3 in this part of Himachal Himalaya. These petrological and geochemical results on the KK granite reflect a mixing between mantle and deep crustal melts which emphasize the difference with the typical Tertiary Himalayan leucogranites.
Vanney JC & Grasemann B, Schwiz. Mineral. Petrogr. Mitt., 78, 107-132, (1998).
Kumar S, Unpub. Ph.D. Thesis, Phys. Deptt. Panjab University, Chandigarh, India, (1986).
The Shergol-Baltikar blueschist unit (Honegger et al., 1989) of the Indus Suture Zone in Ladakh was investigated for structural, petrological and geochronological aspects. This HP/LT unit, one of only few that occur along the whole Himalayan orogenic belt, is situated between the Ladakh block, a Cretaceous island arc sequence in the north and the Lamayuru Group, Mid-Jurassic to Santonian (Danelian and Robertson, 1997) sediments and volcanics of the Indian passive margin in the south. This whole nappe stack, initially assembled by southward thrusting, is backthrusted to the north, including the post-Eocene Shergol Conglomerate Fm., which rests unconformally on top of the blueschist and the corresponding Indus Suture Melange units.
The blueschist unit consists mainly of basic volcanic and pyroclastic rocks, comprising ash tuffs to agglomerates, and some metasedimentary rocks, quartzites, some cherts and impure marbles. In general, the rocks are only weakly deformed, preserving volcano-sedimentary textures, often. Foliations dip steeply to the SE to S, stretching and mineral lineations trend N-S, rarely E-W. Other deformation features are monomineralic (Na-Ca-amphiboles) shear zones showing similar orientations and lineations. Foliations are sometimes folded around steep axes in a disharmonic manner. Both foliations, defined by Na-Ca-amphiboles (winchite), chlorite and phengite, and the shear zones are overgrown by late lawsonite, indicating that they formed on the burial path within the subduction zone.
Mineral parageneses allow to constrain metamorphic conditions at about 400-440°C and 10.5-12.5 kbar based on the petrogenetic grid of Will et al. (1998). The stability of albite + lawsonite places maximum P-T conditions of 12.9 kbar and 440°C. Their coexistence with Na-amphibole indicates maximum temperatures of 405°C (at 10.5 kbar), in disagreement with minimum temperatures of 430°C and a minimum pressure of 12.2 kbar for the paragenesis garnet + Na-amphibole + lawsonite. However, garnet could be stabilised to lower temperatures by the addition of Mn, not considered in the calculations of Will et al. (1998). These P-T conditions are in agreement with preliminary geothermobarometric investigations. Part of the prograde, compressional path is recorded in mineral zonations (garnet, phengite, Na-amphibole, Na-pyroxene) and their order of crystallisation, but there are no indications for any retrograde overprint related to exhumation of the rocks (except the transformation of aragonite to calcite). The latter points to rapid cooling and exhumation after the blueschist metamorphic event.
Ar/Ar and Rb/Sr dating of phengite (single and multiple grains), which are currently in progress (and will be presented) should allow to constrain the timing of the blueschist metamorphic event and thus of subduction and exhumation of the leading edge of the colliding Indian plate.
Danelian T & Robertson AHF, Mar. Micropaleont, 30, 171-195, (1997).
Honegger K, Le Fort P, Mascle G & Zimmermann JL, J. Met. Geol, 7, 57-72, (1989).
Will T, Okrusch M, Schmädicke E & Chen G, Contr. Min. Petrol, 132, 85-102, (1998).
Occurrences of medium-temperature eclogites are now recognized as a distinctive feature of many continental collision zones. In the Himalayas, eclogites, dated as late Eocene (Tonarini et al., 1993), were first described from the Higher Himalayan Crystallines of the upper Kaghan valley in Pakistan (Pognante & Spencer, 1991). Glaucophane eclogites, first reported from the Kaghan HHC (Pognante, 1992), were recently described in the North Himalayan Tso Morari Dome of Ladakh (De Sigoyer et al., 1997).
Equilibration conditions in the Kaghan glaucophane eclogites have been estimated at T = 600 ± 30°C and P > 1.3 GPa (Lombardo et al., 1998b). This estimate is close to that of De Sigoyer et al. (1997) for the eclogites of the Tso-Morari Dome (T = 580 ± 60°C, P > 1.6 GPa).
In contrast with the Tso Morari and Kaghan eclogites, which both record isothermal decompression (or cooling) after the eclogite peak, the eclogites recently found in the Kharta region of southern Tibet at the top of the Main Central Thrust zone (Lombardo et al., 1998a) record decompression under increasing temperature.
Reaction textures and geothermobarometry suggest that in the Kharta eclogites an early mineral assemblage, formed at metamorphic T of 600-650°C and minimum P between 1.2 and 1.4 GPa (garnet-omphacite-rutile-phengite), was overprinted by granulite facies compatibilities at medium P (0.55-0.65 GPa) and high T (750-770°C), followed by recrystallization in the amphibolite facies at low P (0.4 GPa) and high T (700°C ).
A simple tectonic model is proposed to explain the metamorphic evolution recorded in the Kharta eclogites: a) formation of an orogenic prism during the first phases of continental collision, with eclogitization of basic rocks in the deepest parts of the prism (Early Himalayan event); b) self-warming of the prism through radioactive heating, and extrusion of the Higher Himalayan Crystallines along zones mechanically weakened by partial melting and formation of leucogranite melts (Late Himalayan event).
At the regional scale, the main difference between the E Himalaya eclogites and those of the NW Himalaya lies probably less in equilibration conditions than in the different P-T paths they followed during their exhumation, the P-T path followed by the Kharta eclogites implying rapid extrusion of lower crustal rocks in a relatively narrow orogenic belt.
De Sigoyer J, Guillot S, Lardeaux JM & Mascle G, Eur. J. Mineral., 9, 1073-1083, (1997).
Lombardo B, Pertusati P, Rolfo F & Visonà D, Mem. Sci. Geol. Padova, 50, 67-68, (1998a).
Lombardo B, Rolfo F & Compagnoni R, Geol. Bull. Univ. Peshawar, 31, 116-118, (1998b).
Pognante U, Geodinam. Acta, 6, 5-17, (1992).
Pognante U & Spencer DA, Eur. J. Mineral, 3, 613-618, (1991).
Tonarini S, Villa IM, Oberli F, Meier M, Spencer DA, Pognante U & Ramsay JG, Terra Nova, 5, 13-20, (1993).
The initial India-Asia contact that occurred during the Upper Paleocene and the Eocene is related to collisional processes. However, constraints from stratigraphy, paleo-magnetism, geochronology and tectonophysics in NW Himalaya allow to dis- criminate during the India-Asia convergence three successive events : (1) the Indian continental subduction (2) the progressive thickenning of the Indian Crust and (3) the steady-state collision.
The Indian continental subduction : a major decrease in plate velocity (from 18 cm.yr-1 to 10 cm.yr-1) occurring from 55+ Ma to 52 Ma is interpreted as the first effects of the India-Asia collision. At the same time, onset of the India-Asia contact at the Paleocene/Eocene boundary (55 ± 0.5 Ma) is recorded by shoaling of the marine sediments in Zanskar, and related to the obduction of the Asian accretionary prism onto the Indian margin. It is followed by the end of the marine sedimentation and the beginning of the erosion of the first Indian reliefs in the Indus suture during the Late Ypresian (< 52 Ma). During this period, part of the subducted Indian continental margin was exhumed within the mantle wedge at the base of the Indian crust. These results suggest that during the Paleocene/Lower Eocene period, the Indian margin records the progressive ending of the cold subduction of the Indian plate and the beginning of the thickening of the Himalayan wedge by under-plating of continental units and superficial nappe emplacement. This period leads to the progressive apparition of reliefs in the suture zone.
The progressive thickenning: from ~50 Ma to 40 Ma the intracontinental subduction of the High Himalayan Crystallines allow the progressive thickening and warming of the Himalayan orogenic wedge, that leads to the establishment of the Eo-Himalayan metamorphism. Similarly, it corresponds to the progressive decrease of the plate velocity down to 5 cm.yr-1. A the surface, this period corresponds to the formation of perisutural basins.
The steady-state collision: since 40 Ma, the India-Asia convergence is stabilized at 5 cm.yr-1. It corresponds to the progressive exhumation and erosion of the metamorphic units and the global deformation of the whole units from the suture zone to the Himalayan front.
Study of the structural and geochronological evolution of the Ladakh Batholith in NW India tightly constrains the timing of continental collision in that part of the Himalayan orogen. The batholith is a multipluton calc-alkaline body, which developed above a subduction zone on the southern margin of Eurasia between 100 and 50 Ma ago. The sudden end of calc-alkaline magmatic activity is related to the collision of India, and disturbance of the subduction process. Dyke orientation, penetrative foliation, shear zones and microstructural features suggest that the 61 Ma Gyamsa Pluton records syn-magmatic deformation unrelated to collisional deformation. The Gyamsa Pluton solidified and hardened and did not record the younger deformation recorded by the Leh Pluton. The 49 Ma Leh pluton records syn-magmatic deformation analogous to young, collision-related, regional structures developed in softer country rocks. Thus, collision-related deformation must have started between 61 and ca. 50 Ma. An independent and tighter constrain on the time of collision is given by the magmatic activity and cooling history of the batholith. The Leh pluton is the youngest major magma pulse in eastern Ladakh. Its intrusion was followed by rapid cooling and the intrusion of a few small and narrow subvolcanic dykes at 46±1 Ma. A number of published Ar-Ar and K-Ar cooling ages indicate that when the Leh pluton crystallized at ca. 50 Ma, the Ladakh Batholith in general was undergoing cooling, reaching temperatures around 350°C in most places before 45 Ma. Thus, I conclude that 50 Ma marks disturbance of the magmatic cycle at the continental margin caused by collision. However, this does not date the exact timing of collision. Generalized cooling and syn-magmatic deformation of the Leh Pluton indicate that collision was already under way at 50 Ma and that it may have started somewhat before that, possibly in the Upper Ypresian (ca. 52 Ma), as concluded from the study of sedimentary sequences in NW Himalayas.
The conventional dislocation model of strain accumulation is a convenient approach to explain most of the observed displacements during interseimic phase. However, this approach have many shortcomings including the assumption of homogeneous elastic lithosphere and failure to approximate deformation associated with aseismic displacement. Furthermore this model cannot describe the plastic deformation that accumulates over many seismic cycles to form the geologically observed deformation.
We have developped another approach in order to model the wide range of informations available on this long-term deformation as well as on interseismic deformation in the Himalaya of Nepal. We use a plane strain finite element model (ADELI). We consider thermal and strain dependant rheologies for the crust and mantle derived from laboratory experiments.In addition surface processes are modeled from a 1-D diffusion equation, assuming flat deposition in the foreland. In all our experiments a section of lithosphere, initially loaded by the topography is submitted to 20 mm/yr horizontal shortening. We study the effect of varying erosion rate, rheological properties, thermal regime and boundary conditions to derive some constraints on these parameters.
Interseismic vertical velocity can be matched only if erosion is accounted for. Moreover the fact that most of the shortenig is accomodated by slip along the MFT (Main Frontal Thrust) requires a low effective friction coefficient on the seismic decollement. The observed long-term deformation depends mostly on geometry of the crustal thrust and on rheology. The interseismic deformation is also sensitive to rheology and is independant on the assumed goemetry of the decollement and ramp system. The best results are obtained assuming a quartz dominated crustal composition with high radioactive production in the upper crust.
The striking increase of the 187Os/188Os ratio of seawater during the Cenozoic Era (Pegram et al., 1992) has been attributed, by some authors, to enhanced weathering provoked by the Himalayan collision. In order to test this hypothesis, we analyzed river bedloads and bedrocks from Central Nepal and Bangladesh. The Himalayan Range consists of three main units. Moving progressively downstream (ie, from N to S), these are: (1) the Tethyan Sedimentary Series (TSS) composed of sediments of Cambrian to Eocene age. The TSS samples are unradiogenic with 187Os/188Os from 0.6 to 1.4 for the bedrocks and from 0.9 to 1.2 for bedloads. The Os concentration varies from 20 to 200 ppt for the bedrocks and from 30 to 100 ppt for bedloads. (2) The High Himalayan Cristallines (HHC), highly metamorphosed paragneisses and leucogranites, are the principal formation of the Range. The bedrocks have 187Os/188Os between 0.8 and 2.0 and bedloads with an 187Os/188Os ratio of 1.7 represent an average composition of the formation. Os concentrations of both bedloads and bedrocks are lower (3 to 50 ppt) than the upper continental crust concentration (50 ppt) (Esser & Turekian, 1993). (3) The Lesser Himalaya (LH) composed of variably metamorphosed bedloads with very old Nd model ages, displays very large variations in 187Os/188Os ratios (0.7 to 3.0 for bedloads, 1.4 to 8.3 among the bedrocks). The Os concentration also varies widely, ranging from 7 to 500 ppt. The highest Os concentrations are associated with the highest Os isotopes ratios and are found in black shales.
The 187Os/188Os of river bedloads collected at the outflow of the Range reaches 3.25. This radiogenic value can be explained by addition of a small fraction (<5%) of organic-rich rocks with high Os concentration from the LH to less radiogenic sediments from the HHC and the TSS. The mixing relationship is also apparent in a Sr vs Os isotopic diagram. The 187Os/188Os ratios of river bedloads in the Indo-Gangetic Plain (Ganges) are controlled by a single lithology (Black Shale), which is of minor occurence.
Bedloads from the Indo-Gangetic Plain were also analyzed. Samples from the Ganges and Tista (Central Nepal) are radiogenic with a 187Os/188Os ratios of 2.3-2.6. These ratios are consistent with those of bedloads collected at the outflow of the Range. In contrast, two samples from the eastern part of the Bangladesh (Brahmaputra) are less radiogenic than the Ganges (187Os/188Os = 1.5 and 0.8) and appear to reflect seasonal variations.
Pegram WJ, Krishnaswami S, Ravizza GE & Turekian KK, Earth Planet. Sci. Lett, 113, 569-576, (1992).
Esser BK & Turekian KK, Geochim. Cosmochim. Acta, 57, 3093-3104, (1993).
We present an estimate of the modern erosion of the Himalaya based on riverine fluxes. The dissolved flux related to chemical erosion is estimated on the basis of yearly averages of the rivers. It is 32 and 29*106 ton/yr for Brahmaputra and Ganges respectively. The fluxes of suspended matter measured in Bangladesh are 402 to 608*106 ton/yr for Brahmaputra and 328 to 548 *106 ton/yr for Ganges. Isotopic tracers show that the large majority of these particles is derived from the Himalaya. However the total erosional flux must take into account bedload transport and sedimentation in the floodplain which cannot be simply measured. Their importance can be estimated using the contrast in chemical composition between the suspended load and the bedload or the accumulated sediments due to mineral sorting during transport.
Assuming steady state erosion in Himalaya, the chemical composition of the total erosional flux equals that of the average source rock. This later is relatively well established for silicate formations based on ca. 200 analyses of the Himalayan formations. Compositions of the riverine end members are based on suspended- and bedload sampled in Bangladesh. The composition of sediments accumulated in the floodplain is derived from an average composition of Siwaliks. The principal process driving chemical differenciation is the sorting of phyllosilicates with respect to quartz and feldspars in favor of the suspended load. Si/Al ratios are: 4.2 for the source rock, 2.7 for the suspended load, 6.5 for bedload and 7.8 for the Siwaliks. Mass balance calculation is performed for two extreme case: no bedload or no sediment sequestered in the flood plain. On this basis, the suspended load represents between 40 and 65% of the total erosional flux. Using the fluxes of suspended load mentioned above, total erosional fluxes can be calculated and expressed in term of erosion rate. They are 2.8 +/- 1.1 mm/yr for the Brahmaputra Himalaya and 1.8 +/- 0.6 for the Ganges Himalaya. The higher erosion rate in the Brahmaputra basin is likely related to its higher runoff.
The growing set of data on fault rates and finite offsets permits to envisage a step-by-step backwards restoration of the intracontinental deformation between India and Asia. We here assume that the major part of the deformation is localized along a few majors faults separating less deformed lithospheric blocks. The motion of such blocks can thus be modelled by moving microplates on the sphere. To account for crustal thickening or extension at the limits of the blocks, we allow changes in the surface of these blocks. Finally, minor deformation of the blocks interiors (bending, local rotation...) permits to minimize gaps or overlaps due to motion along faults. Tectonic and geophysic data used to constrain these reconstructions are: present-day rates and finite offsets along fault (strike-slip, thrusts); amount of crustal thickening or extension; paleomagnetic rotations; kinematics of large plates. We use Euler poles and rotation parameters determined for India, Tarim, Tibet, China, and Indochina. The present reconstruction is done by steps corresponding to the timing of major changes in the active fault pattern during the collision evolution. Such changes correspond to the dextral reactivation of the Red River fault at 5 Ma, the activation of the Kunlun fault at 10 Ma, the end of the sinistral motion on the Red River fault at 15 Ma, and the end and beginning of motion along the Wang Chao and 3 Pagodas faults at 30 and 40 Ma. The recent contours of the blocks are drawn along major faults of Central Asia, whose segmented traces and junctions have been simplified. As we step back in time, the size of the area of deformation shrinks and the block contour pattern gets simpler. At each time step we propose a overall solution for the position of all the blocks of the deformation zone, in agreement with the data set available for this period. We finally obtain a synthetic evolution of the collision, back to its onset.This backward reconstruction allows us to test the validity of the assumption of coherent lithosperic blocks and permits to make kinematic and mass balances. This model confirms the importance of the extrusion mechanism that plays a role comparable to crustal thickening to absorb plate convergence.
Recent total convergence between Indian and Eurasian plates is at very high rate of about 45 mm/a (DeMets et al., 1990). Crustal shortening across the Tien Shan is found at a rate of about 20 mm/a (Abdrakhmatov et al., 1996), implying that convergence is transferred across the entire Pamir range. Questions arise, where and how the crustal deformation is distributed across the Tien Shan and Pamir region from the time of India-Eurasia collision until today. To constrain the propagation of deformation and to estimate how crustal shortening might be translated into uplift, we collected structural data and geochronological samples along a N-S traverse from S-Tien Shan to N- and Central Pamirs. Sampling was concentrated on E-W striking magmatic belts, which are associated to Late Paleocoic, Triassic-Jurassic and Early Creataceous suture zones. Ar-Ar biotite ages from granitoids in the Rushan Pschart zone (Early Creataceous suture zone) demonstrate regional cooling below 300° C in Late Cretaceous time. The apatite fission-track (FT) ages cluster around 11 Ma. The track-length distribution shows shortened tracks, indicating a longer time in the PAZ (partial annealing zone) or thermal overprint. Granites in the Muzkol metamorphic core complex of the Central Pamirs show very fast cooling in Miocene time (Ar-Ar hornblende: 15-22 Ma; Ar-Ar biotite: 15-17 Ma; zircone FT: 16 and 17 Ma; apatite FT: 14-17 Ma) reflecting fast exhumation of the dome. The emplacement of the granites was determined by a U/Pb zircon age of 31 Ma. In the northern Pamirs (Lake Karakul area) granitoids show the following apatite FT age distribution: in the south of the lake the oldest apatite FT age is around 52 Ma with track lengths partly shortened. To the north the ages decrease to 36 respectively 24 Ma. The track-length distributions indicate undisturbed cooling of the rocks. The batholith north of the lake yielded apatite fission track ages of 17 and 18 Ma with a narrow track-lengths distribution (mean 15 µm), expressing a much faster cooling in Miocene time. The age pattern and track-length distributions in the northern Pamirs may be interpreted in terms of block rotation above a listric fault, thrusting over a ramp or local back-thrusting.
DeMets C, Gordon RG, Argus DF & Stein S, Geophys. J. Int., 101, 425-478, (1990).
Abdrakhmatov KY, et al, Nature, 384, 450-453, (1996).
The Chuya basin in Gorno-Altai (South Siberia) and the Zaisan depression in South-Altai (East Kazakhstan) contain the most complete sedimentary sections of the Altai mountain range in the Former Soviet Union. Stratigraphic, morphotectonic, fault kinematic, remote sensing and paleomagnetic studies allowed to highlight the dynamic evolution of these basins during the Cenozoic, and also provide some time constraints on the mountain building process in Altai.
The Zaisan depression was initiated during the Permian and was active during most of the Mezosoic. It was reactivated during the Cenozoic. The Chuya basin is superimposed on a Devonian depocenter, and was initiated only in the Paleogene. In the Late Cretaceous-Paleocene, a vast peneplain developed in Central Asia in a warm and humid climate. The area of the present Chuya depression, now in the middle of the Altai massif, had a short marine incursion during the Santonian-Campanian, while sedimentation in the Zaisan depression remained continental.
The basins developed first in a transtensional setting during Oligocene-Miocene, with fluvio-lacustrine and lacustrine sedimentation. In the Pliocene, at about 3 Ma, strong transpression caused disappearance of the lacustrine environment, disruption of the lake beds, coarse clastic sedimentation, occasional block rotation and the formation of oblique thrusts at the basin margins. This strong transpressional deformation occurred approximately during a relatively short time span (2 Ma), until the early-middle Pleistocene. Similar intensification of tectonic movement (both compressional and extensional) has also been observed in other areas of the Tian-Shan mountains and in the Baikal Rift Zone. It can be expected that a significant part of the Late Cenozoic structures of Altai was formed during the Late Pliocene-Early Pleistocene. The cause for this tectonic event seems not straighforwardly related to the dynamics of the Asian plate boundary process.
From the Middle Pleistocene to Holocene, transpressional deformation continued, but in a less vigorous way, and was accompanied by rapid vertical movements. Asymmetric uplift of the mountain ranges bordering both basins on the southern side may have been stimulated by a modification of the weathering, erosion and transport conditions due to climate change.
Mainly based on paleomagnetic results from South and North China blocks (SCB & NCB), this eastern part of the Asian mosaic is generally considered to have been accreted to Siberia prior to the Cretaceous. In order to check this hypothesis, we performed in the summers 1995 and 1997 two field trips during which we collected Jurassic and Cretaceous effusives north and south of the Mongol-Okhotsk suture which separates the Mongol block to the south from Siberia to the north. South of the Mongol-Okhotsk suture, i.e. in the Mongol block, we have collected one Jurassic locality in the Transbaikal region along the Unda river at (51.7°N, 117.4°E, basaltes, 12 sites), and three lower Cretaceous effusive formations: the first one in Amur region around the town of Taldan (53.8°N, 124.5°E, andesites), and the two others in the Transbaikal at the Kriemlovka peak locality (51.8°N, 117.5°E, basalts, 12 sites) and the Torey lakes locality (50.2°N, 115.8°E, basalts, 14 sites). North of the Mongol-Okhotsk suture, i.e. in the Siberian continent, two Cretaceous localities have been sampled. The first one along the Ingoda river at (51.2°N, 112.3°E, trachy-basalts, 12 sites) and the second in the arounds of Bitchura town (50.6°N, 107.6°E, trachy-basalts, 10 sites). The Jurassic locality and three out the five Cretaceous localities yielded consistent results deserving attention, which can be summarized as follows:
1) We have determined an upper Jurassic and two lower Cretaceous poles for the Mongolian block, and a new Cretaceous pole for the Siberian block. 2) When compared on an equal area projection, the Cretaceous poles from the north and the south of the Mongol-Okhotsk suture lie on small circle containing the Cretaceous poles of the reference apparent polar wander path of Eurasia through the error bars demonstrating that Mongolia and Siberia were accreted by the lower Cretaceous. 3) The North China block-South China block-Mongolia-Siberia assemblage was already formed in the Cretaceous and did not suffer any relative N-S motion since that time. 4) Important relative rotations between the north (clockwise) and the south (anticlockwise) of the suture could indicate that it has acted as left lateral shear zone allowing a post-Cretaceous eastward extrusion of Mongolia. 5) Based on the preliminary Jurassic pole of Mongolia, we can not do better than estimate the width of the the Mongol-Okhotsk ocean to between 2500 and 4000 km. 6) The Mongol-Okhotsk ocean seems to be entirely closed between the upper Jurassic and the lower Cretaceous.
The Pamir mountain building is bounded on the east and on the west by narrow depressions filled with undeformed Quaternary sediments. They cut out from main orogen the marginal moun-tain chains that composed of the Tertiary molasse (Darvaz) or of the Paleozoic metamorphics (Kun-Lun). Together with basins located on the front of the Pamir Mountains, these boundary depressions reveal kinematics of recent structuring in the Central Asian region. A basement of the Darvaz depressions includes the faulted Mesozoic and Cenozoic formations. Pre-orogenic clastics and carbonates present shelf and lagoon facies those thickness strongly decreases eastward. Orogenic sediments (more 7000 m in thickness) comprise the lower red-colored Oligocene - middle Pliocene molasse that follows continuously the quasi-platform sequences and upper gray-colored post-orogenic molasse. The upper Pliocene - lower Pleistocene conglomerates are enveloped by two regional unconformities; uppermost middle Pleistocene to Holocene sediments are thin and located only within river valleys. Structure of the Darvaz arc de-velops in transpressive setting. The echeloned faults of the region consist left-lateral strike-slip zone, in which the faults rotate gradually toward the north from NS to NE, and become in the same time more flat transforming consequently from subvertical strike-slips to low-angle thrusts. Depressions are integrated in this structure and expressed by almost isolate linear valleys embedded between overstepping strike-slips, so they compose echeloned chain of oblique or Freund's pull-aparts.
Configuration of the East Pamir depressions is almost mirrorlike in respect to the Darvaz valleys. The local basins constitute right-lateral row and each of the basins is clearly segmented. Basin closures coincide with the fractures of the Karakorum fault zone that are unfolded southward fanlikely from WE to SE. The depressions is separated from Kun-Lun structures with large-scale normal faults (Brunel et al.,1994), and the Quaternary clactics within them lack remarkable deformations. Thus, we consider the East Pamir depressions as structures of strike-slip related extension (normal or Burchfield's pull-aparts). Thermochronological data establish that concordant uplift of bordering mountains and opening of basins started ~ 2 m.y. ago (Arnaud et al. 1993), i.e. synchronously with development of the Darvaz depressions. Transtensional evolution of the Kun-Lun border of the Pamir microplate versus transpression on the Darvaz border follows non-parallelism in the Pamir displacement to orientation of the bordering strike-slip zones reflected complex many-block interaction controlling dynamics of this region.
The work is supported by RFFI, grant 96-05-65521
The western boundary of the Philippine Sea Plate (PH) with Sundaland (SU) in the Philippines corresponds to a wide deformation zone that includes the stretched continental margin of Sundaland, and the Philippine Mobile Belt (PMB) extending from Luzon to the Molucca Sea. The GPS GEODYSSEA data are used to decipher the present kinematics of this complex area. One of the main results is the quantification of overlapping subductions on both sides of the PMB. To the west, convergence decreases from about 90-100 mm/yr along the Manila trench to a few mm/yr across Mindoro in the zone of collision of Palawan with the PMB. Transfer of 55% of this Manila Trench convergence motion to the Philippine Trench to the east is accommodated principally by rapid counterclockwise rotation of eastern Luzon. Southward, convergence decreases regularly along the Philippine Trench from 54 mm/yr near 13°N to 32 mm/yr east of Mindanao. Inside the PMB, the slip rate of the left-lateral Philippine Fault, which enables complete partitioning to occur at the trench, increases northward from 22 mm/yr at 7°N to 35 mm/yr in the Visayas. As a result, the East Philippine sliver, east of this fault, moves north and transfers this motion to the East Luzon block counterclockwise rotation. West of the Philippine Fault, the Visayas block in central Philippines, is rotating rapidly clockwise, transfering most of the remaining convergence to the Negros/Cotabato trenches on its western border, up to 32 mm/yr near 12°N and 50 mm/yr near 8.5°N. These estimates ignore the amount of convergence absorbed to the west, at the limit of undeformed SU.Two opposite rotations on both sides of the left-lateral Philippine Fault, clockwise to the southwest and counterclockwise to the northeast, illustrate the PMB internal deformation. As a result, the northern part of the Visayas block acts as a cog wheel with respect to the Luzon block. It is remarkable that this system is such that subduction is everywhere perpendicular to the trenches. South of Mindanao, within the Molucca sea, the overlapping divergent subduction system of the PMB is relayed by the convergent Sangihe-Halmahera subduction system. This one accommodates 80 mm/yr of the 105 mm/yr PH/SU convergence and about 15 to as much as 20 mm/yr of the remaining convergence are absorbed far to the west, within the northern Borneo margins.
GPS measurements acquired over Southeast Asia in 1994 and 1996 in the framework of the GEODYSSEA program revealed that stations located in Vietnam, Malaysia, Thailand, Borneo, Eastern Sumatra and Java have no resolvable relative motions. A large piece of continental lithosphere comprising Indochina, Sunda shelf and part of Indonesia thus behaves as a rigid "Sundaland" platelet. A direct adjustment of velocity vectors obtained in an Eurasian frame of reference shows that Sundaland block is rotating clockwise with respect to Eurasia around a pole of rotation located south of Australia. Although the GEODYSSEA network was carefully attached to the ITRF-94 reference frame using IGS stations, an internal systematic misadjustment cannot be ruled out from GPS data alone, raising the possibility that the derived Sundaland/Eurasia motion is contaminated by some whole network rotation artifact. We present here an original and totally independent check by using earthquakes slip vectors at Sunda and Philippines trenches. Seven stations of the GEODYSSEA network are close to the trenches and not separated from them by large active faults (two at Sumatra trench, three at Java trench and two at the Philippines trench). The difference between the vector at the station and the adjacent subducting plate vector defines the relative subduction motion, and should thus be aligned with the subduction earthquakes slip vectors. We thus first derive a frame-free solution that minimizes the upper plate (or Sundaland) motion. When corrected for Australia-Eurasia and Philippines-Eurasia NUVEL1A motion, the misfit between GPS and slip vectors azimuths is high and scattered (-10° to 15° for Sunda trenches stations, -45° and 22° for the two Philippines trench stations), indicating that the upper plate has significant motion with respect to Eurasia. We then determine by least squares inversion the best rigid upper plate rotation that minimizes this azimuth misfit. The misfit between GPS and slip vectors drops to about 5° for Sunda trenches, and about 10° for the Philippines trench. This best fit solution is close to the initial fit obtained through direct adjustment of the Eurasian frame GEODYSSEA solution. A series of theoretical modelling shows that elastic coupling at trenches introduces only a minor correction. We conclude that Sundaland motion is essentially correct and discuss some of the geodynamic implications.
Recent data obtained by the 600 permanent GPS sites of the Geographical Survey Institute of Japan in 1995 indicate that full mechanical coupling prevails in the Japan trench and the Nankai trough along the Japanese Island Arc (Le Pichon et al., 1998). We use the 1996 and 1997 GPS data to compute the strain rate tensors that we invert to estimate the average subduction velocity loading the central Nankai and Japan Trench seismogenic zones, independently of the data reference frame. The results show that both subduction planes are fully locked on a regional scale.We then use the GPS velocity vectors in order to look in greater details at the geometry and local coupling of the seismogenic zones. Defining partial coupling along a fault segment as the association of smaller scale fully locked and free-slip zones, we define small portions of the Pacific and Philippine slab surfaces that we assume to be either fully locked or fully unlocked. We then invert the GPS data for the distribution of free-slip and locked areas along both seismogenic zones. This method confirms that the Central Nankai trough and the Japan trench slabs are fully locked, with small consistent areas of free-slip. These area coincides, in the Japan trench, with a series of recent large earthquakes (M 7.4 1989, M 7.0 1992, M 7.7 1994), and in the Nankai trough with a zone of divergence between the isodepth and isotherm contours. We study the spatial and temporal variation of these free-slip zones from 1995 to 1997. We conclude that, at least around Japan, trenches are either fully locked or slipping and that "partial coupling" does not exist there.
Le Pichon, X, Mazzotti, S, Henry, P, Hashimoto, M, Geophysical Journal International, 134, 501-514, (1998).
Fieldwork along several segments of the Altyn Tagh Fault, between 85 and 95 °E, confirms that it ranks as one of the most active faults of Asia. In the East, near Aksay, the active fault trace offsets numerous stream channels, terrace risers and fans tens to hundreds of meters. 14C dating of organic remains and charcoal within terrace gravels, and 10Be and 26Al cosmogenic dating of surface pebbles, still in progress, indicate that the principal terraces were emplaced 2.5 and 5.5 Ka ago, implying a left slip rate between 2 and 3 cm/yr. Large mole tracks attest to the occurrence of great earthquakes. Even larger mole tracks are found north of Lenghu, within the Altun Shan push-up, a 6000 m-high range in a restraining bend of the fault, now sliced by its most active strand. North of Huatougou, at the transition between another push-up mountain and a recent pull-apart basin, a spectacular sequence of five flat-floored, hanging channels, beheaded by the fault from a unique source in the mountain, have been horizontally displaced by up to 1250 m. Cosmogenic dating of the abandonment of these channels is in progress. Several km to the west, pressure ridges exceeding 10 m in height across a large young fan, imply the repeat of several great earthquakes in a relatively short time span.Large mole tracks and hectometric cumulative offsets of post-glacial fans and terrace risers, sampled for cosmogenic dating in several localities, are also visible for tens of kilometers along the fault east and west of Tura. The cumulative pressure ridges of great earthquakes are preserved at five thousand meters, above the permafrost line, implying that they are due to fairly recent events. 10Be dating of quartz cobbles from a lateral moraine offset several kilometers by the fault yields exposure ages on order of 40 Ka. This suggests a slip rate in excess of 3 cm/yr, to be further tested by the dating, in progress, of other moraines. The consistency of our observations over a length of nearly 900 km confirms that the Altyn Tagh fault ruptures mostly with M*8 earthquakes and slips at a rate of several cm/yr.
The eastern segment of the sinistral Altyn-Tagh fault, and the NW-SE trending ranges that splay from it display remarkable examples of coupling between N70°E directed strike-slip and N125°E oriented thrust faulting. In the Subei area (95°E, 39.5°N), fieldwork and SPOT image or air-photo mapping of left-laterally offset alluvial terraces and fans along the Altyn-Tagh fault, and of uplifted and folded terrace surfaces along the Tanghenan Shan thrust, help constrain such coupling quantitatively. Topographic leveling, with a total station, of offsets and folds, and radiogenic dating of the main surfaces constrain both the strike slip- and uplift-rates. At a first site, a high alluvial terrace offset 140 m with a maximum age of 14693 ± 200 yr BP yields a mean left-slip-rate of 13 (+10/-6) mm/yr, with a lower bound of 9.5 ± 0.2 mm/yr. At a second site, two terraces offset 160 and 310 m with corresponding ages of 9235 ± 130 yr BP and 12400 ± 5400 yr BP imply left-slip rates of 25 and 17 mm/yr on the Altyn Tagh fault west and east of the junction with the Tanghenan Shan thrust-splay, respectively. The northeastward decrease in strike-slip rate at the thrust junction is compatible with a shortening rate of 5 mm/yr along the Tanghenan Shan thrust, otherwise consistent with uplift rates of 4 to 7 mm/yr along this thrust. The recent growth of the western Qilian mountain ranges is thus intimately coupled with sinistral motion on the Altyn Tagh fault, and hence a consequence of the extrusion of Tibet.
Gravity anomalies over the Tien Shan ranges, Ferghana Valley, Tadjik basin and other deep sedimentary basins in the western Tien Shan, differ from those expected for local Airy isostatic equilibrium by 50-120 mGal and imply a deficit of mass at Moho level. The overcompensated depressed Moho is deeper by 5-10 km than prevails where isostasy holds. Although some of the basins are filled by thick sediment fills, measurements of density show that this material cannot account for the deficit of mass. Gravity anomalies over surround mountain belts and the Kazakh Platform farther north imply that local Airy isostatic equilibrium is approached; flexure of a relatively thin plate (effective mechanical thickness ~ 10 -15 km) loaded by the present topography can account for the range of Bouguer anomalies of -50 to -450 mGal. Using gravity modelling and finite element brittle-elasto-ductile mechanical models taking into account surface processes (diffusional erosion and sedimentation) we infer that approximately north-south unstable shortening (folding) of the relatively thin lithosphere has created mountains north and south of the basin, has warped the basement of the immediate surroundings of the basin up by folding the mantle lithosphere, and has forced the basin floor down. In the center of the Tien Shan range, such downwarping of the lithosphere could localize in some place, resulting in growth of a single inverted mantle "megafold", accelerated by RT instability, and lead to large-scale faulting and initialization of subduction and mountain building. It appears that even though the pre-existing thermal structure and variations in crustal thickness probably have dictated the styles and distribution of deformation in this region, the processes of development of the compressional instability and of localization of the mountain growth could also be significantly eased by the surface processes which reduce gravity forces and localize vertical movements. We also suggest that the compressional instabilities, coupled with surface processes, could play an important role in the formation of the Tibetan plateau as well.
In the Kathmandu region of central Nepal, the Himalayan Thrust System is manifested by three major faults. The southernmost, the Main Boundary Thrust (MBT, and the two northernmost, the Main Central Thrust (MCT) and the Mahabharat Thrust (MT) which carry the Gosainkund and Kathmandu metamorphic nappes, respectively, forming a large composite crystalline nappe. The boundary between the two nappes is found just north of Kathmandu; we interpret this contact as the trace of the MCT as it has been traditionally mapped in Nepal. No major faults have been recognized other than the faults which bound the two nappes. Intruding the contact between the two nappes is a 0- to 300-m thick zone of pegmatite which is syn- to late-kinematic. One xenotime and two monazite fractions from two samples of this pegmatite yield U-Pb ages which suggest crystallization at approximately 25 Ma. Approximately 120 muscovite 40Ar/39Ar ages are now available from the region. These data range from a maximum ages of 21-22 Ma in the southern most portion of the Kathmandu nappe, becoming progressively younger to the north over a distance of ~70 km until reaching a minimum of ~5 Ma, in the northern part of the Gosaikund nappe, at the western part of the Langtang Valley. After this minimum, ages increase to 12 Ma going up the Langtang valley, approximately 20 km to the NE. Ten apatite fission-track ages are also available from this region. One datum from the southern part of the Kathmandu nappe is ~5 Ma; fission-track ages the Gosainkund nappe range from 6 to 4 Ma in the south and 3 to 1 Ma in the north. The field and isotopic data are consistent with an evolution in which the Gosainkund and Kathmandu nappes have been juxtaposed in roughly their current relative position since about 25 Ma and have undergone an apparent rotation during the entire Miocene with samples in the north experiencing more and later exhumation during this period. The muscovite age minimum of ~5 Ma in the northern part of the study area is similar in age to a series of 40Ar/39Ar and U-Pb ages from the Bhuri Gandaki south of Manaslu (~60 km to the W) which have been interpreted by Harrison et al. (1996) as the result of progressive footwall imbrication during slip on the MCT in the mid to late Miocene (after the development of the MBT but not necessarily linked to it). We speculate that the area south of the Langtang valley has been affected by a blind thrust and associated anticline which can be an along-strike prolongation of the zone of inferred footwall imbrication in Bhuri Gandaki.
ERS-2 radar data acquired before and after the Mw7.6, Manyi (Tibet) earthquake of November 8, 1997, provide geodetic information about the surface displacement produced by the earthquake in two ways. (1) The sub-pixel geometric adjustment of the before and after images provides a two dimensional offset field with a resolution of ~1 m in both the range (radar line of sight) and azimuth (satellite track) directions. Comparison of offsets in azimuth and range indicates that the displacement along the fault is essentially strike-slip and in a left-lateral sense. The offset map reveals a relatively smooth and straight, N78E surface rupture that exceeds 150 km in length, consistent with the EW plane of the Harvard CMT solution. The rupture follows the trace of a quaternary fault visible on satellite imagery (Tapponnier and Molnar, 1977; Wan Der Woerd, pers. comm.). (2) Interferometric processing of the SAR data provides a range displacement map with a precision of a few millimeters. The slip distribution along the rupture reconstructed from the range change map is a bell-shaped curve in the 100-km long central section of the fault with smaller, local maxima near both ends. The curve shows that the fault slip exceeds 2.2 m in range, or 6.2 m strike-slip, along a 30-km long section of the fault and remains above 1 m in range, ~3 m strike-slip, along most of its length. Preliminary forward modeling of the central section of the rupture, assuming a uniform slip distribution with depth, indicates that the slip occurred essentially between 0 and the depth of 10 km, consistent with a relatively shallow event (Velasco et al., 1998).
Tapponnier P & Molnar P, JGR, 82, 2905-2930, (1977).
Velasco Aet al, AGU Fall Meeting, (1998).
The 1000 km-long, sinistral, Haiyuan fault accomodates most of the eastward component of movement between NE Tibet and the Gobi Ala Shan platform. From the southwestern Qilian Shan (near 98°E) where it strikes N110E on average, the fault reaches the western tip of the Wei He graben near 107°E, after veering to N140E on either side of the Liupan Shan. The Holocene slip-rate on the western stretch of the fault, between 101°E and 104°E, is now well constrained to be 12 ± 4 mm/yr.
Inferred values of the total cumulative offset, on the other hand, range between ~ 10 km (Burchfiel et al., 1991) and ~ 120 km (Gaudemer et al., 1995), about a factor of 10! To better constrain this total offset, we combined fieldwork with the analysis of SPOT satellite images, of the topography (from maps at a scale of 1/50000 to the 30-arc second Defense Mapping Agency DEM), and of geological maps (1/200000 to 1/1000000 scale) along ~ 400 km of the fault (between 102°E and 106°E. Comparison of the different rock assemblages cut by the fault is in progress, based on structural mapping and samples collected in the field. Our synthesis of the geology and Phanerozoic history of the fault helps identify new geological markers of the offset (e.g.: Devonian red beds of Hasi Shan and Nanhua Shan, Quaternary fluvial conglomerates south-east of Hasi Shan and south-west of Jingtai). In order to match such markers north and south of the fault, a cumulative offset on order of 120 km is needed, west of the junction between the Haiyuan Fault and the Gulang Fault. Such an offset is about 20% greater than the 95 km offset of the Yellow River valley (Gaudemer et al., 1995). Due to the restraining bend of the fault along the Leng Long Ling, 120 km of sinistral slip require about 30 km of NE-SW shortening across the Shiyang thrusts that bound the eastern Qilian Shan, consistent with the minimum amount of Late-Tertiary thrusting (25 km) estimated from retrodeformed sections by Gaudemer et al., 1995. Counterclockwise rotation of the Ordos block helps reduce the amount of E-W shortening in the Liupan Shan to a realistic value of ~ 20 km, on order of that found by Zhang et al., 1991, but is not enough. Clockwise rotation of the region SW of the fault would alleviate this problem. The remaining offset on the fault has probably been accomodated by oblique extension along the Wei He graben.
Burchfiel BC, Zhang P, Wang Y, Zhang W, Song F, Deng Q, Molnar P & Royden L, Tectonics, 10, 1091-1110, (1991).
Gaudemer Y, Tapponnier P, Meyer B, Peltzer G, Guo S, Chen Z, Dai H & Cifuentes I, Geophys J Int, 120, 599-645, (1995).
Zhang P, Burchfiel BC, Molnar P, Zhang W, Jiao D, Deng Q, Wang Y, Royden L & Song F, Tectonics, 10, 1111-1129, (1991).
We present the spatial variation of the current state of stress of western New Guinea Island, to describe the escape of the western Irian Jaya (Bird's Head) relative to the Australia/Sundaland motion. Results are calculated from inversion of the whole shallow focal mechanism data (CMTS with DC ~ 60%) determined since 1976. Inversion is a statistical method for calculating the mean state of stress from a cluster of focal mechanisms, by selection one of the two nodal planes as active fault plane (Carey-Gailhardis & Mercier, 1987).Ten areas were determined, each of them corresponding to a specific structural province and to an epicentral distribution of focal mechanism location. Results are given from East to West. Result from Mamberamo area gives evidence for a N42°E trending compression. By contrast, strike-slip and E-W trending normal faulting occur farther South in the western part of the front of the Thrust Belt (<sigma>1: 236/1, <sigma>3: 327/11) and farther West in the Paniai area respectively. Due West, results around the Cendarawasih bay give evidence for a N-S trending compression on its eastern border (Waipona trough), strike-slip faulting on its southern part (<sigma>1: 181/15, <sigma>3: 273/6), and an E-W trending extension along its western border (Lengguru Belt). Strike-slip faulting occurred both in the northwestern Sorong area (<sigma>1: 224/5, <sigma>3: 134/5) and in the southwestern Tarera area (<sigma>1: 29/7, <sigma>3: 296/23). By contrast, south of this latter, result gives a N287°E trending extension in the Aru Trough. Results showing WNW-ESE extension in the Aru Trough and left-lateral wrenching along the SW end of the Thrust Belt front are in agreement with recent implications of the GPS surveys. The escape is guided by wrench motion along the Waipona Trough and the Paniai Fault Zone, meaning it does not only include the Bird's Head, but also part of central Irian Jaya. The wrenching is accommodated by transtension (E-W extension in Paniai and Lengguru) and local transpression (Waipona Trough).
Carey-Gailhardis E & Mercier JL, Earth Planet. Sci. Lett., 82, 165-177, (1987).
A clear picture of the tectonic evolution of the Himalaya can be gained only through application of various techniques and approaches. This paper presents an overview of fission-track (FT) geochronology to understand the orogenic and morphogenic history of the Himalaya. The FT database for the Himalaya are unevenly distributed both for geologic divisions and geographic regions. Most of the data come from the Trans-Himalayan Batholith (carried out by Zeitler in Kohistan, this author in Ladakh, and Pan in Gandese) and from the Higher Himalayan metamorphic and leucogranites (by Zeitler in Pakistan, Kumar/Lal in India, Ganzawa/Arita in Nepal, and this author in India and Nepal). A comparison of these two crystalline belts show quite different cooling histories, which in conjunction with other geologic and geochronologic evidence, demonstrate that a single-phase Miocene uplift (as suggested by some authors) for the whole Himalaya and Tibet is a "myth." Systematic (zircon-apatite pair) and numerous FT data exist for the NW Himalaya of Pakistan and India. These data indicate variations in cooling histories of rocks, which mimic tectonic features such as faults and domes. A similar picture is emerging from the FT study of NE Himalayan syntaxis (data by Seward/Burg). These data indicate that tectonism (not climate) has been the dominant control on the pattern and timing of denudation of rocks in the Himalaya. This has important implications for the ongoing "chicken-or-egg" debate over the causes of late Cenozoic uplift and erosion in orogenic belts. The FT data demonstrate that Late Pliocene-Quaternary was a wide-spread and distinct phase of uplift and erosion in the Himalaya. Owing to its low-temperature sensitivity, the FT dating documents the more recent events in the tectonic evolution of the Himalaya. Tectonic models merely relying on other radiometric techniques and ignoring the FT database are prone to be misleading.
The Cenozoic deformation on the northern edge of the Tibetan Plateau is poorly known but of great interest in understanding the mechanisms by which the Plateau has been formed. Systematic fission track dating of each mountain belt between the Kunlun fault to the South, the Altyn Tagh fault to the North and the Qilian Shan belt to the East, will give us the ages of the last denudation episodes and allow us to constrain tectono-thermal history for this region during the Cenozoic. Samples were collected during the 1996 and 1997 French-Chinese expeditions. Fission track dating was performed in E.T.H. Zurich. We would like to present here some preliminary results of a wide field area forecast several research leads:- Sample AT1 and AT60 from the Tarim and the Qaidam basements, evidence Triassic to Jurassic denudation. A following long period of stability, until 45 Ma, is observed, and the denudation starts again.- Sample AT147 from the easternmost thrust of the Qilian Shan belt again shows a Jurassic denudation but the stability period last until 12 Ma where denudation starts again.- Samples AT8 and WQ122 taken in the internal and western parts of the Qilian Shan belt show a similar increase in denudation around 10 Ma.- Sample AT68 taken from a thrust fault immediately North of the Altyn Tagh fault also reccords a denudation increase around 15 Ma, after a very long period of stability.- Sample AT118 from the western part of the Kunlun fault, shows a denudation process from 27 Ma to present, with an increase in the denudation rate since 7 Ma.- For all these samples we can observe that the denudation rates are very slow on the whole area even on very high peaks (up to 6000 m).
Several upshots can be pointed out from these data :
1- Fission Track data display a widespread Miocene denudation event in the Qilian Shan, which is probably related to tectonic uplift of the mountain ranges (Tapponier et al., 1990). More detailled analyses are necessary in view of a best understanding of the propagation of mountain growing in this region. 2- The old basements reccord a denudation episode around 45 Ma which may be due to the firts tectonic movements in relation with the India / Asia collision (Molnar and Tapponnier, 1975). 3- The activity of the Kunlun fault began during Oligocene, and denudation in relation with volcanism continues during late Miocene. 4- General very slow denudation rates, less than 0.5 mm per year, are in agreement with slow erosion and sedimentation rates measured in the basins (Metivier, 1996).
Tapponier P, Meyer B, Avouac JP, Peltzer G, Gaudemer Y, Guo Shunmin, Xiang Hongfa, Yin Kelun, Chen Zhitai, Cai Shuahua and Dai Huanang, Earth and Planetary Science Letters, 97, 382-403, (1990).
Molnar P & Tapponnier P, Science, 189, 419-426, (1975).
Metivier F, Phd Thesis Paris VII "Volumes sédimentaires et bilans de masses en Asie pendant le Cénozoique", 1, 1-255, (1996).
The tectonic setting of the Bhutan Himalaya (Gansser, 1983; Swapp and Hollister, 1991; Grujic et al., 1996 and Davidson et al., 1997) shares many similarities with the central Himalaya of Nepal and India, namely the continuation of major units and faults such as the Siwaliks, Main Boundary Thrust (MBT), Lesser Himalayan Sequence (LHS), Main Central Thrust (MCT) and Greater Himalayan Sequence (GHS). We present new U-Pb TIMS data for several rock units of the Greater and Lesser Himalayan Sequences in Bhutan to better understand the age of these major tectonic units.The Lesser Himalayan Sequence in Bhutan consists mainly of the Daling-Shumar- group, a Precambrian clastic sedimentary succession several kilometers thick. The Daling-Shumar-group hosts several large magmatic bodies which are structurally conformable with the sediments (Gansser, 1983). Detrital zircons from the Shumar quartzite display a complete range from rounded to euhedral morphologies. Three grains yielded ages in the range from 2.3 Ga to 1.0 Ga. These ages demonstrate that the Shumar quartzite was deposited not earlier than the Mesoproterozoic. Multiple and single grain analysis on homogeneous, inclusion-free magmatic zircons from a strongly foliated quartz porphyry yielded ages in the range 1.96 - 1.9 Ga, similar to a strongly foliated two-mica orthogneiss nearby. These are the oldest rocks so far discovered in the central eastern Himalaya. They could be of similar age to the Ulleri gneisses of Nepal. Because contacts are conformable, it is not clear whether these Palaeoproterozoic magmatic rocks are intrusions or tectonic slices of crystalline basement. Geochemical data are being acquired to evaluate their origin. The Greater Himalayan Sequence in Bhutan is very thick in comparison to Nepal and India. It comprises a large amount of augen gneiss and a major thrust structure above the MCT (Gansser, 1983; the Kaktang Thrust of Swapp and Hollister, 1991 and Hollister et al., 1995), which may have thickened or duplicated the GHS (Swapp and Hollister, 1991; Davidson et al., 1997; Parrish et al., 1997). An amphibolite-facies augen gneiss, which overlies the Kaktang Thrust near Tashi Yangtse, yielded an upper concordia intercept zircon age of 825 Ma. This age differs from a previously dated augengneiss in the GHS of north-central Bhutan at 514±3 Ma (Parrish et al., 1997) and is interpreted to date the time of protolith crystallisation. The lower concordia intercept reflects the time of amphibolite facies metamorphism in the GHS and is defined by near-concordant monazite analyses at 15 Ma. Three detrital zircon fractions from a quartzite in the GHS beneath the Kaktang thrust yielded ages from 1.9 Ga to 1.0 Ga. These imply Neoproterozoic or Palaeozoic deposition, younger than the quartzite in the GHS of Nepal (Parrish and Hodges, 1996).
Gansser A, Denkschr. Schweiz. Naturforsch. Ges., Birkhäuser Verlag, Basel, 96, 181 pp., (1983).
Swapp SM & Hollister LS, Canadian Mineralogist, 29, 1019-1041, (1991).
Grujic D, Casey M, Davidson C, Hollister L, Kuendig R, Pavlis T & Schmid S, Tectonophysics, 260, 21-43, (1996).
Parrish RR & Hodges KV, GSA Bulletin, 108, 7, 904-911, (1996).
Davidson C, Grujic D, Hollister L & Schmid SM, Journal of Metamorphic Geology, 15, 593-612, (1997).
Parrish R, Grujic D, Hollister L, Klepeis K, Kündig R & Dorji T, 12th Himalaya-Karakoram-Tibet International Workshop, Rome, Abs Vol, (1997).
The Indochinese block represents a major structural unit situated on the eastern side of the Himalayan collisional orogen. The Vietnam territory forms a complex structural assemblage: a) to the N, the conspicuous NW-SE Red River Fault Zone (RRF), b) the Truong Song Belt which occupies Central Vietnam and extends to the S of Danang, in contact with c) the composite Kontum Block in Southern Vietnam. Metamorphism within the Truong Song Belt has been recently dated at 240-245 Ma, as a result of the indosinian orogeny. In the Northern realm, the RRF separates two main crystalline units, the Fan Si Pan from the Dai Nui Con Voi and the Song Chay Massifs. We investigated this latter one, in order to decipher the influence of Cenozoic thermotectonic events, which affect the RRF, Dai Nui Con Voi and the Eastern edge of the Fan Si Pan. The Song Chay Massif represents a NE-SW dome-like structure involving orthogneisses and migmatites, overlied on its northern flank by muscovite bearing marbles. E-W striking faults bound the dome to the S along a 300 m deep valley, as well as the cover at the Northern edge. Kinematic indicators across the dome consistently indicate a top-to-the-North sense of shear wich drove the exhumation. The structural arrangement of these series and the topography allow an ideal sampling from the core to cover, along a near orthogonal cross section. Sampling was conducted in order to study the evolution of the Ar-Ar ages vs. the structural position of the specimens. From South to North radiometric ages are as follows: A muscovite from a mylonitic cordierite bearing micaschists yields ages from 60 Ma to 234 Ma. A muscovite from orthogneisses located in the uppermost structural levels of the dome gives maximum age plateau at 236 Ma. Then, biotites and muscovites from orthogneiss and migmatites give ages of 201, 166 and 164 Ma, while the northernmost muscovite from the cover marbles gives an age of 198 Ma. This evolution across the structure can be likely related to the updoming process, resulting in the older ages recorded by the upper levels (234-236 Ma), presently in an external position, whereas the inner parts display younger ages, related to a latter-reached blocking temperature. It clearly indicates that the up-doming of indosinian series ended during Late Jurassic (around 160 Ma). and that the northeasternmost indosinian landmark in the Vietnamese area, has not been directly influenced by the cenozoic tectonothermal activity along the RRF. The E-W trending strike slip fault which bounds the massif to the S was active as early as Paleocene.
Several generations of granitic plutons intrude Paleozoic rocks along the western edge of the Shan plateau, east of the active Sagaing fault, in Myanmar (Burma). Zircon U/Pb dating of zircons yields precise emplacement ages for four such intrusives, exposed over a distance of ~400 km between Moulmein and Mandalay. Three of the rocks were sampled in quarries at the foot of the Shan plateau.These comprise an undeformed biotite granite (MY23) from Bilin near Kyiakto, a sheared C-S biotite orthogneiss (MY25) from Moulmein, and a slightly foliated biotite granite dike (MY37) intruding foliated granite gneisses at Donshaw, east of Toungoo. A fourth hornblende granodiorite (MY50) was sampled within the plateau, along the road to Kalaw at Yepaung Zon.Six zircon fractions weighing from 0.45 to 0.098 mg from sample MY23 define a reverse discordance position on the concordia diagram. Five data points are close to the lower intercept, with 206Pb/ 238U ages ranging between 47 and 53.3 Ma. Linear regression of all data yield a lower intercept age of 48.5 ± 2.5 Ma and an upper intercept age of ~ 900 Ma. Eight zircon fractions from sample MY25, with weights ranging from 0.013 to 0.065 mg, plot in concordant to subconcordant positions, with 206Pb/ 238U ages ranging between 70.7 and 71.7 Ma (mean age = 71.3 ± 0.5 Ma). Eight zircon fractions from MY37 (0.041 to 0.0905 mg each) plot in a concordant to subconcordant position with a mean 206Pb/ 238U age of 33.9 ± 0.2 Ma. Four zircons fractions from sample MY50 define a reverse discordia, with a lower intercept of ~120 Ma and a Precambrian upper intercept age. A hornblende K-Ar age of 117 ± 4 Ma is consistent with this Early Cretaceous age.During ductile shear of these granitic rocks, peak metamorphic temperatures (< 500-600°C) did not exceed the closure temperature of the U-Pb system in zircon (> 800°C). Hence, the concordant or lower intercept ages correspond to the emplacement ages of the different granite generations. The upper intercepts of samples MY23 and 50 imply inheritance from Proterozoic basement rocks. The Cretaceous ages of 71 and 120 Ma could reflect either subduction-related magmatism along the active margin of Asia associated with that observed in Tenasserim and Gangdese, respectively, or alternatively, continental collision magmatism similar to that found along the Early Cretaceous Bangong suture between the Lhasa and Qiantang blocks collision. The ages of 49 and 34 Ma, on the other hand, probably relate to collission of India, and/or to subsequent dextral motion along the Shan shear zone.
A regional apatite fission-track thermochronology (AFTT) project was conducted throughout mainland Thailand. AFTT thermal histories of the shallow crustal provide new constraints on the Tertiary tectonics of Thailand, denudation and basin fill. AFTT has been able to clearly distinguish contrasting cooling patterns related to Late Cretaceous and Tertiary tectonic events. The AFTT data has been coupled with heat flow and thermal conductivity values to provide quantitative estimates of section loss, and the identification of erosive and tectonically driven denudation.
AFTT analysis indicates that inversion of the Mesozoic Khorat monocline was initiated during the Late Cretaceous/Early Tertiary (50-70 Ma). Since the Late Cretaceous/Early Tertiary, Eastern Thailand has cooled steadily (1.5°C Ma-1) by erosive denudation in response to isostatic rebound. The tectonic component is estimated to be ~600±200 m and the amount of section loss placed at ~3.6 km at an average rate of 50-75 m Ma-1, assuming a geothermal gradient of 25°C/km.
In Western Thailand, the early phase of inversion was superseded by rapid cooling in a discrete north-south belt of gneissic and plutonic rocks which took place during the Late Oligocene/Early Miocene (25-18 Ma). Minimum cooling rate through the partial annealing zone (PAZ) during the unroofing of the north-south belt is estimated have been between 6°C Ma-1 to 60°C Ma-1. Assuming a geothermal gradient of 30-35°C/km, this would equate to between 2.75 km and 3.5 km of section denuded in the Late Oligocene/Early Miocene over a ~3±2 m.y. time span at a rate of 550-3500 m Ma-1. It is most likely that this later cooling represented a phase of tectonic denudation and unroofing of a metamorphic core complex. This is attributed to transtension/transpression along major strike-slip faults. The relief generated by the tectonic movement caused accelerated erosion which resulted in increased sediment supply to adjacent Tertiary basins. Basin fill switched from lacustrine environments, representing underfilled (subsidence>sediment supply) basins to fluvial clastic dominated deposits (sediment supply>subsidence).
AFTT analysis of the shallowest detrital apatite grains from three wells located in the Gulf of Thailand, show central ages which are similar to the timing of accelerated hinterland denudation. AFTT analysis of the samples located deeper in the well, at present-day geothermal temperatures higher than the partial annealing zone, show that the anomalously high heat flow documented for the region is a recent condition developed over the last 100,000 years. These conditions may be related to basaltic eruptions found in the region or localised hydrothermal circulation within the basins.
We investigate the manner in which Tibet may have acquired its flat topography and high elevation by studying current deformation processes over =1/4 of its surface, east of the Altyn Tagh fault, between the Kunlun fault and the Gobi desert. Contemporaneous north-vergent thrusting there governs the growth of = NW-trending mountain ranges and intervening basins. Thrusts that plunge =15-20 km down beneath the ranges usually break the ground many kilometers north of the range-fronts, along the northeast limbs of growing, asymmetric ramp-anticlines. Typical rates of throw or shortening on the thrusts appear to be on order of a few millimeters per year, and typical amounts of cumulative displacement on the largest thrusts reach 10-20 km. Most of the relief growth north of the Kunlun fault appears to postdate 11 million years. Farther north, the large ranges south of the Hexi corridor probably rose in less than 6 million years. Smaller ranges along the eastern part of that corridor may be of Quaternary age only.From hill to range size, the elongated reliefs appear to follow a simple scaling law, with roughly constant length/width ratio, suggesting that they have grown self-similarly. The greatest mountain ranges, which are over 5.5 km high, tens of kilometers wide and hundreds of kilometers long may thus be interpreted to have formed as NW-trending ramp-anticlines, at the scale of the middle-upper crust. The fairly regular, large-scale arrangement of those ranges, with parallel crests separated by piggy-back basins, the coevality of many parallel, south-dipping thrusts, and a change in the scaling ratio (from =5 to 8) for range-widths greater than =30 km, further suggests that they developed as a result of the northeastward migration of large thrust-ramps above a broad décollement dipping SW at a shallow angle in the middle-lower crust. The northeastwards younging of the thickening process appears to be coupled with propagation of the Altyn Tagh fault because most thrust-traces merge northwestwards with active branches of this fault, after veering clockwise. We thus infer that the propagation of this fault triggered tectonic uplift of distant, narrow ranges. The growth of such ranges dammed fluvial catchment and huge accumulations of sediments filled the resulting intervening basins. This provides a mechanism for smoothing the topography and flattening the surface of orogenic plateaus over broad areas.
Cosmogenic dating, using in-situ 26Al and 10Be on quartz pebbles from terrace surfaces, and 14C dating of charcoal within terrace alluvium constrain the late Pleistocene-Holocene slip rate on the Kunlun fault in northeastern Tibet. In the west, at three sites along the Xidatan-Dongdatan segment of the fault, terrace riser offsets ranging from 24 to 110 m, with cosmogenic ages ranging from 1800 yr to 8000 yr, yield a mean left-lateral slip-rate of 11.7 ± 1.5 mm/yr. The full range of ages obtained (~ 25 k.y., most of them between 13 and 1.4 k.y.) confirms that terrace deposition and incision, hence landform evolution, are modulated by post-glacial climate change. Given observed minimum offsets of 9-12 m, the slip-rate found implies that great earthquakes (M ~8) with a recurrence time of 800-1000 yr, rupture the Kunlun fault near 94°E. At two sites along the central part of the fault, near 99°E, terrace riser offsets ranging from 57 to 400 m with corresponding 14C ages ranging from 5400 yr BP to 37000 yr yield a minimum slip-rate of ~10 mm/yr, consistent with that in the west. The measured cosismic displacement of the January 7, 1937, M=7.5 earthquake involves a horizontal left-lateral component of about 4 m and a small reverse component of about 0.4 m. The penultimate earthquake had similar amounts of slip. The recurrence time of such earthquakes appears to be 380 yrs. Farther east, near 100.5°E, along the Maqen segment, a lateral moraine offset by about 180 m dammed a high terrace with a 14C age of 11157 ± 157 yr BP. This yields a mean slip-rate of about 12 ± 3 mm/yr. The slip-rates thus appear to be fairly constant all along the central 600 km of the Kunlun fault. The average rate is 12 ± 2 mm/yr. Our ages show that most of the alluvial terraces studied were emplaced during the wetter period of the early Holocene optimum (9-5 ka), while younger terraces can be related to climate change in the late Holocene.
The stresses generated by the India-Eurasia convergence are usually assumed to drive the present-day deformation in Central Asia. However, relatively few quantitative data are available so far in its northernmost part (Baikalrift zone and Mongolia) to constrain the current deformation models.
Three years and four campaigns of Global Positioning System (GPS) measurements (1994-1997) in the Baikal rift zone, largest active continental rift system in Eurasia, show crustal extension at a rate of 4.5±1.2 mm/yr in a WNW-ESE direction. A comparison with moment release of large historical earthquakes suggests that elastic strain is currently accumulating in the Baikal rift zone along active faults that currently have the potential for a M=7.5 earthquake. The GPS-derived extension rate in the Baikal rift zone is at least two times greater than the prediction of most deformation models of Asia. This result could reflect the dynamic contribution of the Pacific-Eurasia subduction to intracontinental deformation in Asia, in addition to the effect of the India-Eurasia collision.
This hypothesis needs to be (1) confirmed by additional GPS measurements, in particular in Mongolia, (2) further tested using dynamic deformation models that we present here.
The Myanmar Central Basin extends from the Andaman sea in the south, to the Himalaya Syntaxis in the north. It is bounded by the Sagaing fault to the east, and the ArakanYoma/Naga hills (Indo-Burma ranges) to the west Tectonic studies developed on this basin and its margins, within the frame of the G.I.A.C. Project*, revealed that the N-S trending active Sagaing fault crosscuts an en echelon basin system comparable to the present Gulf of California. N50°E trending depocenters connected with N160°E dextral strike slip fault zones are bounded by thinned continental crust characterized by Cenozoic gneisses marked by pervasive sub-meridian stretching lineations. SE dipping ductile detachments affects the Mt Victoria metamorphics in the central part of the Indo-Burmese belt, while dominant NW dipping detachments characterizes the western margin of the Shan plateau and its northern edge in the Mogok area. Extensive Ar-Ar radiometric dating of the gneisses reveal an Early-middle Miocene age (around 15 Ma) for the final step of stretching of the continental crust now exposed along the Shan plateau. Seismic data within the central basin reveal that the main depocenters were active from the late Eocene to the upper Miocene with minor internal unconformities, testifying for a rather continuous sedimentation during the major part of the Cenozoic. This pull apart basin system was inverted during the late Miocene. En echelon NW-SE trending folds and thrusts were then developed in the whole basin. The NE-SW main compression axis is compatible with dextral motion along the Sagaing fault that connects southward with the spreading center of the Andaman Sea. Offset of crustal thinning along the edge of the Shan plateau coincides with incipient tectonic inversion within the Myanmar central basin. Both are interpreted as the response to a major kinematic change along the western margin of Sundaland.Following the path of India to the north, the central basin of Myanmar was developed between Sundaland (Shan Plateau) and the Indo Burmese belt, a detached continental fragment of India. The Late Miocene inversion is coeval with incipient opening of the Andaman Sea and dextral motion along the Sagaing fault, that can be considered as a relatively recent fault system.
* the G.I.A.C. Project (Geodynamics of the India-Asia Collision) is a cooperative project developed by the Ecole Normale Superieure of Paris (France), with the Universities of Yangon, Dagon, Mandalay (Myanmar) and Chulalongkorn (Thailand), and supported by TOTAL MYANMAR E&P, MOGE, UNOCAL and PTTEP.
The rectilinear N-S trending dextral Sagaing fault zone is a major active tectonic structure in Myanmar, between India and Sundaland. Using satellite images interpretations controlled by tectonic field studies, we have identified a N160 trending dextral transpressive fault system, close to the Sagaing fault, that forms the western edge of the Shan plateau (Shan scarp), and that may accommodates a part of the active motion between India and Sundaland plates. These active faults cross-cut ductile structures we have observed along the Mogok metamorphic belt and that resulted from an extension characterized by NNW-SSE trending stretching lineations on E-W trending low dipping foliation planes. The ages we have obtained, from 39Ar-40Ar analysis of high grade metamorphic and foliated intrusive rocks exposed along this belt, range from 25.9 to 26.9 My in the Thaton area (southern Shan scarp) and 18.4 to 25.4 My, with a significant cluster of ages around 22 My, in the Mandalay area (northern Shan scarp). The geometry of this ductile extension extends along the whole length of the Shan scarp, but can not be traced in northern Myanmar; instead, in the Mogok area, it bends sharply eastward, and forms a major ENE-WSW trending segment where we obtained the youngest ages, bracketed between 15.8 and 19.5 My. The undeformed Kabaing granites, northwest of Mogok, give an age of 15.8 ± 1.1 My (biotite), that marks the end of the ductile extension. This N60°E trending metamorphic belt, connects northeastward with a sub-meridian dextral ductile shear zone exposed along the Myanmar-China border (24°N). Here sub-vertical foliation planes and horizontal pencil-like lineations were dated at 15.8 ± 0.7 My (biotite). We then interpret the metamorphic belt exposed along the western and northwestern margin of the Shan plateau, as a major S-shaped transtensive ductile shear zone marked by a clear NNW-SSE direction of extension. The N60°E trending metamorphic belt near Mogok is here interpreted as a large metamorphic core complex connecting to sub-meridian branches of this shear zone. This "Paleo Sagaing" ductile shear zone became inactive around 15 My and was then crosscut by the presently active Sagaing Fault zone.
* the G.I.A.C. Project (Geodynamics of the India-Asia Collision) is a cooperative project developed by the Ecole Normale Superieure of Paris (France), with the Universities of Yangon, Dagon, Mandalay (Myanmar) and Chulalongkorn (Thailand), and supported by TOTAL MYANMAR E&P, MOGE, UNOCAL and PTTEP.
The Sagaing Fault is the strike-slip plate boundary between India and Eurasia south of the E-Himalayan syntaxis. In Myanmar (Burma) geomorphic evidence for active right-lateral slip is ubiquitous for at least 800 km along this N-S fault. Isolated outcrops of deformed rocks east of the fault and along western edge of Shan plateau (Shan escarpment) are interpreted as parts of a former Early Cenozoic right-lateral shear zone (Sagaing shear-zone) nearly parallel to the active Sagaing fault. Most of the shear zone is buried under the Sittang-Irrawaddy plains. Another major right-lateral fault zone (the Shan Fault), and NW-SE striking left-lateral faults within the Shan plateau are presently less active. K/Ar, 39Ar/40Ar , and apatite fission track (AFT) thermochronological data constrain the cooling history of Sagaing shear-zone rocks near Mandalay. The shear-zone cooled through 500°C at 27 Ma and experienced rapid cooling between 23 Ma and 15 Ma (ca. 30°C/Ma) to temperatures below 60°C. Rocks presently exposed along this part of the Sagaing shear zone cooled below brittle-ductile transition (350°C) at 20-17 Ma. Regionally widespread cooling ages constrain the timing and rates of Cenozoic motion along the Sagaing strike-slip system. We found evidence for diachronous cooling up to 80 km perpendicular to the fault and for 800 km along strike. Argon and AFT ages show overall younging towards the active Sagaing fault. AFT ages on samples less than 30 km from the fault are all younger than 22 Ma, whereas samples within 5-10 km of the fault have ages between 14.6 to 18.7 Ma. Such younging could result from higher heat flow along the fault due to localized dextral strain followed by regional denudation or by an early Miocene pulse of uplift and erosion localized along the fault. Alternatively, it could be due to progressive increase of finite uplift and erosion towards the fault. We prefer a combination of the two first hypotheses. Biotite K/Ar and 40Ar/39Ar ages close to the fault are also diachronous along strike, indicating a northward migration of the 300°C isotherm at a rate of 3-5 cm/yr from Oligocene to early Miocene times (35-25 to 17 Ma). Taking the Ailao Shan - Red River shear zone as a model, this rate compares with the strike-slip rate along the Sagaing fault sensu lato, during the lower Miocene, as India's NE tip bypassed Indochina. This range of rates is broadly compatible with the rates of relative motion between India and Eurasia (ca. 5.6 cm/yr) or Indochina (3.7 cm/yr).
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