Recent geological and geophysical investigations clearly show that the continental crust can subduct at least to 100-150 km depth. The arc-continent collision should logically occur before that, during early subduction of the continental margin. What is the difference between this process and the deep continental subduction and overall, what an arc-continent collision means in terms of lithospheric mechanics. We address these problems by physical modelling of subduction. The essential element of the model is a weak zone within the overriding plate which corresponds to the volcanic arc. During oceanic subduction this zone can fail in different ways depending on the subduction regime: (1) under extension (formation of the back-arc basins); (2) under shear force (strain partitioning in oblique subduction zones), or (3) under compression along a continent-ward-dipping thrust (Kurils and Japan) which may result in the subduction reversal. Subduction of the continental margin causes considerable increase in the compression of the overriding plate and its failure in the arc zone becomes inevitable. The failure occurs in this case along the continent vergent fault dipping under the arc. This process is followed by the subduction under the arc of the fore arc block together with the underlined, previously subducted under this block margin. There are two possibility for the continuation of this process: the fore-arc block can be completely subducted into the mantle or a partial subduction of the block can be followed by the subduction reversal. There is one more mode of arc-continent collision corresponding to the situation when the continental margin arrives to the subduction zone with active back arc basin. Subduction of the margin results in a change of the overriding plate extension by the increasing compression. Finally, the overriding plate fails not in the arc area but in the axial zone of the back-arc ridge which is now the weakest zone. Failure occurs along the continent-vergent fault and is followed by the subduction of the arc plate including arc itself which can be either subducted completely into the mantle or scraped-off and accreted in front of and under the another half of the back-arc lithosphere. According to the modelling results, the arc-continent collision thus means a compression and failure of the overriding plate followed by subduction of the fore arc block or of the whole arc plate. It seems that the all presented above modes of arc-continent collision have natural analogues: in Taiwan Himalayas, Oman, and Urals which are analysed.
The Philippine Sea Plate (PSP) started colliding with the Eurasia continental margin in Pliocene time. Because of the obliquity of plate convergence, the collision propagated through time from the north to the south. The more advanced stages are presently observed to the north, whereas the subduction of the South China Sea beneath the PSP occurs to the south. Swath bathymetry and backscattering data, together with seismic reflection and geopotential data obtained during an R/V L'Atalante cruise, showed major N-S changes in the tectonic style in both the indenting arc and the host margin. Offshore, the collision zone is characterized by strong deformation of the arc including the forearc region to the south. This very active tectonic domain absorbs significant shortening between the metamorphic Central Range (exhumed part of the Eurasia margin) and the PSP moving towards N 310° E at about 8 cm/yr. The Longitudinal Valley (LV) is characterized by west vergent active thrusting. This major structure disappears offshore to the south, below the Southern Longitudinal Basin (SLB), a collisional basin filled with recent orogenic sediments laying unconformably on deformed and folded series from the former forearc domain. Seismic lines suggest that left-lateral transpressional movements occurred before SLB formation. Today, the deformation front has jumped to the east and is characterized by growing up of the Huatung Ridge (backpart of the former Manila oceanic accretionary wedge including forearc and intra-arc sequences) which overthrust the volcanic basement of the arc. North of N 22°30', active westward thrusting of the Costal Range (remnants of the volcanic arc and forearc basins) over the Lichi mélange develops along the LV. At the base of the slope, prominent fault scarps suggest an active eastward thrusting of parts of the arc (volcanic edifices and intra-arc or forearc sediments) onto the oceanic crust of the Philippine sea. The northernmost part of the Coastal Range (Hualien area), is presumably accreting to the rest of Taiwan as indicated by geodetic data. Today most of the convergence is accounted for by east vergent thrusts at the base of the arc slope. An east-west transfer zone of deformation accomodates the differential motion and rotation of the northern and southern tectonic units. Structural observations and geophysical data show that the Luzon forearc basement no longer exists north of the latitude of Taitung, suggesting that it has been subducted beneath the Luzon Arc. South of Taitung, only a small part of the forearc domain would remain under the thrust wedge. A tectonic evolutionary model involving the progressive subduction of large slices of the forearc domain lithosphere accounts for the contrasting styles of deformation encountered from south to north of the collisional orogen. The progressive subduction of the continental margin of China induces: 1) to the south, major eastward backthrusting and shortening of the forearc domain in between the former oceanic accretionary wedge and the Luzon Arc volcanic edifice, 2) to the north, accretion of parts of the arc domain to the collision belt associated to westward thrusting and blocks rotation.
The Kenting melange in the Hengchun Peninsula and the Lichi melange in the Coastal Range are the two ophiolite-bearing sheared chaotic rock units without discernible stratification in the Taiwan arc-continent collision belts. Detailed field surveys on land and marine investigations in active arc-continent collision region off southern Taiwan, age determinations on sheared matrix by microfossils, and fission track analyses on exotic blocks can provide independent constrains to share light in understanding origins and tectonic settings of the melanges. The Hengchun Peninsula in southernmost Taiwan represents accretionary prism of thrust sheets of Miocene deep-marine turbidites and Plio-Pleistocene accretionary slope sequences (Huang et al., 1997). In comparison, the Coastal Range in eastern Taiwan is northward extension of the Luzon arc and Plio-Pleistocene forearc sequences (Chai, 1972). Micropaleontological studies on intensively sheared muddy matrix of the Kenting melange show an existence of three modes of fossil occurrences: (I) barren of fossils due to rapid sedimenation, (II) late Miocene age, and (III) mixed Miocene and Pliocene-Pleistocene ages. Occurrence of (I) and (II) modes of fossils suggests that most of sheared sedimentary rocks of the Kenting melange are the autochthonous Miocene turbidites in the accretionary prism. Occurrence of (III) mode of fossils further indicates that the Kenting melange represents a subduction complex along which Miocene trench-slope turbidites and South China Sea oceanic crust were offscraped and then accreted into the accretionary wedge during Plio-Pleistocene time. The Lichi melange in the Coastal Range occurs in three regions (Hsu, 1956): (I) SW along the Longitudinal Valley, (II) along syncline axis of Loho forearc basin, and (III) along Chimei fault. In (I) region, the Lichi melange is westward over-thrust by thick coherent Plio-Pleistocene forearc turbidites. Along the Lichi melange, intensively sheared intervals with ophiolite and qtz-rich sandstone blocks intervene between non-sheared coherent Pliocene forearc turbidites. Microfossil studies indicate both sheared and non-sheared intervals are same in age of early Pliocene (NN14-15), younger than the overlying thrust sheet of the coherent forearc sequences (NN16-19). Fission track analyses on those quartz-rich sandstone blocks show a late Miocene age similar to the sandy turbidites in the accretionary prism of the Hengchun Peninsula. In (II) region, the Lichi melange (4.1-3.5 Ma) along the Loho syncline axis is even older than the underlying coherent forearc turbidites (3-2 Ma). The facts indicate the Lichi melange represents sheared lower forearc sequences with ophiolite and quartz-rich sandstone blocks eastward (arcward) back-thrust from the accretionary prism during the arc-continent collision in Pliocene, similar to what observed in the offshore Huatung Ridge in active arc-continent collision zone (Lundberg et al., 1997). However, after the arc-continent collison, the Lichi melange had further westward thrust and sheard during the later accretion of the Luzon arc onto the Eurasian continent to form the Coastal Range in eastern Taiwan.
Huang CY, Wu WY, Chang CP, Tsao S, Yuan PB, Lin CW & Xia KY, Tectonophysics, 281, 31-51, (1997).
Chai BHT, Amer. J. Science, 272, 389-422, (1972).
Hsu TL, Bull. Geol. Survey Taiwan, 8, 39-63, (1956).
Lundberg N, Reed DL, Liu CS & Lieske JRJ, Tectonophysics, 274, 5-23, (1997).
As an active orogenic belt in the western Pacific, the island of Taiwan allows better understanding of the evolution of a convergent plate boundary resulting in a major active suture zone. Juxtaposed against the remnant forearc basin sequences along thrust faults, the Lichi Melange of the Coastal Range is composed of exotic ophiolite and sedimentary blocks, metric to kilometric in size, and coherent turbidite beds, all embedded in a sheared scaly argillaceous matrix. The Lichi Melange is controversial in origin, being interpreted either as a "subduction complex", or as an "olistostrome". Our field studies suggest that the Lichi Melange most likely arose from the shearing of forearc sequences, instead from a subduction complex or olistostrome. The general evolution of the convergent boundary is thus reconstructed as follows. Since the Early Miocene, the oceanic lithosphere of the South China Sea began to subduct eastward beneath the Philippine Sea plate along a proto-Manila trench, thus forming a subduction wedge in front of the Luzon Arc. Continued subduction resulted in an oblique collision of the underthrusting Eurasian continental crust with the Luzon arc in the Late Miocene time. Because subduction turned into arc-continent collision, the site of proto-Manila trench became a plate suture zone with several thrusts. Starting in the Middle Miocene time, the Luzon arc and the forearc basin were westward accreted onto the Eurasian continent to generate the Coastal Range in eastern Taiwan. A new plate boundary between the Eurasian plate (eastern Central Range) and the Philippine Sea plate (Coastal Range) thus formed along the Longitudinal Valley. It has originated as a submarine arc-prism boundary, an innate weak zone of the overriding plate. The shortening caused by the arc-continent collision was accompanied by the formation of major thrusts and backthrusts. This inference is supported by observations in and around the recent suture zone, on the Lichi Melange in the Coastal Range and the Huatung Ridge off southeastern Taiwan.
167 earthquakes have been accurately relocated in the Taiwan area. Magnitudes Mb range from 5.5 to 6.6 for 139 of them, and from 3.9 to 5.4 for the 28 remaining ones. Ten major seismogenic structures could be delineated. Two are related to the Taiwan collision: the north collision zone (NCZ, 38 mechanisms) and south collision zone (SCZ, 26 mechanisms). Two belong to deep slabs with extensional earthquakes: the Ryukyu slab to the north (RSE, 8 mechanisms) and the Luzon slab to the south (LSE, 4 mechanisms). Two correspond to subduction fronts with thrusts, for Ryukyu (RSF, 38 mechanisms) and Luzon (LAC, 11 mechanisms). Two reveal zones of shallow extension near subduction zones, back-arc in type in the upper plate in the Ilan-West Okinawa trough area (IBE, 9 mechanisms) or due to flexuring of subducting plate near the Manila trench (MTF, 11 earthquakes). The last two ones correspond to particular behaviours in the Luzon slab (not discussed) and to the Ryukyu-Taiwan junction zone, at the northwestern tip of the Philippine Sea Plate (RTJ, 17 mechanisms). With these data subsets, inversion was carried out based on a new direct method established by one of us (J.A.). The inversion revealed homogeneous stress patterns. In the collision zones, trends of s1 axes indicate compression N113°E on average, with a counterclockwise change between the northernmost segment of the NCZ (N133°E compression) and the other four segments of the NCZ and SCZ giving identical results (N110°-114°E compression). All s3 axes plunge 73° or steeper, indicating that the reverse faulting mode prevails. Further south, the compression trends N94°E (LAC). All this brings confirmation of the fan-shaped pattern of compressional stress trajectories throughout the Taiwan collision zone. The extensional mechanisms in the Ryukyu back-arc basin (IBE) and near the Manila Trench (MTF) reveal nearly trench-perpendicular extensions, with s3 axes trending N168°E and N86°E, respectively; s1 axes plunge 76° or steeper (normal faulting mode). Near the Ryukyu subduction front, the compression trends N174°E (RSE). To the west, at the junction with the collision zone of Taiwan, the compression abruptly changes to N78°E (RTJ), the regime being principally of reverse-type mode in both cases. This major change in the direction of compression at the northern corner of the Philippine Sea Plate is interpreted as the effect of lateral shortening within the frame of the regional NW-SE convergence.
We perform 2-D finite element numerical modelling of subduction to study the deformation and failure pattern of the overriding plate. This plate has elasto-plastic rheology with strain weakening and contains a «volcanic arc» presented in the model as a thinned and rheologically weakened zone. The subducting plate is not modeled directly. Its effect on the overriding plate is simulated by the normal (pressure) and tangential (friction) stresses along the interplate surface which both cause the tectonic compression of this plate. The first approximation for the interplate pressure is derived from previous experimental modelling of the complete process of oceanic subduction. The numerical modelling shows that the overriding plate fails under the arc along the trench-ward dipping fault. We then vary both the interplate pressure and the interplate friction stress and show that this failure direction does not change. Subduction of the continental margin modifies (increases) the interplate pressure due to the buoyancy of thick continental crust. The compressive stress in the overriding plate increases and its failure direction changes. The failure occurs now along the continent-vergent fault, dipping under the arc. The failure is followed by the underthrusting of the fore-arc block under the arc which corresponds to the arc-continent collision. The modelling results are applied to analyze the seismicity pattern, gravity field and geological structure of southern Taiwan plate boundary segment which corresponds to the ongoing arc-continent collision. The 3D wavelet method (Ouillon et al., 1995) applied to the analysis of spatial distribution of seismicity reveals a well pronounced seismicity concentration zone along the western foot of the Luzon arc. This zone dips to the east to around 30 km depth and corresponds well to the failure of the overriding plate (the Philippine Sea Plate) predicted by the modelling. The underthrusting of the fore arc block along this zone is suported by the gravity modelling as well. This process results in the closure of the fore arc basin and the formation of the Huatung Ridge.
In the Miura and Boso Peninsulas, south of Tokyo, Japan, a unique set of deep-sea forearc sediments (the Misaki Formation) of middle Miocene to Pliocene, composed of coarse to fine arc-derived volcaniclastic materials is distributed, showing complicated but shallow-level deformation. This set preserves very detail styles of sedimentological and structural features. They exhibit Izu forearc sedimentation and deformation before as well as subduction zone deformation during the accretionary prism formation. Then they received Honshu forearc sedimentation. These processes are the necessary conlcusion of the world only known Boso TTT-type triple junction in the NW Pacific.
The Izu forearc sediments represent fall-type coarse basaltic to rhyolitic tuffaceous materials within deep-sea, fine muddy background of hemipelagic to contour current deposits. They are deformed firstly by vertical compression and earthquake shake to form vein structure, then secondly by layer-parallel sliding and duplex structures (Hanamura and Ogawa,1993; Yamamoto et al., 1998). The former is concentrated in diatomaceous claystone, while the latter along smectite-rich tufaceous claystone to shorten the sediments horizontally of SE vergence. Thirdly, many conjugate sets of thrust faults occur in all the formations, suggesting multiple layer-parallel shortening just after the second stage of deformation. This layer-parallel shortenings contemporaneously occurs during south-verging large-scale folding into accretionary prism, now situated to the Honshu forearc side.
Then finally the prism is overlain by shallow, cross bedded tuffaceous materials of gentle undulation of Pliocene age. Thus such accretionary prism formation of the Izu forearc sediments produces very quick deformation across the oblique plate convergent boundary along the Sagami trough between the Boso triple juction and the Izu arc collisional zone. Such unique tectonics is the necessary process on the arc-collision region at a TTT-type triple junction area.
Hanamura Y & Ogawa Y, The Island Arc, 3, 126-141, (1993).
Yamamoto et al, Journal Geol. Soc. Japan, 104, XVII-XVIII, (1998).
Collision and accretion along the entire Kamchatka margin is evident since Jurassic times. After the Aleutian island arc is established in Oligocene times convergence is restricted to the Aleutian - Kamchatka junction area and along the Kuril-Kamchatka trench to the south. The Aleutian - Kamchatka triple junction is an area of active arc-arc collision. The western segment of the Aleutian arc consists of a complex system of active fracture zones which spreads over some 150 km parallel to the Aleutian trench. This dextral strike-slip fault zone system marks the present boundary between the involved Pacific and North American plates. Between main fracture zones either pull-apart basins with high heat flow values or horst like slivers of island arc affinity are devoloped.
The fracture zones appear in single-channel seismic profiles and bathymetric maps as canyon like features cutting the Kamchatka slope up to the shelf. The northwest striking Pikezh Shear Zone in the central part of the Kamchatka Mys Peninsula is the onshore continuation of the Steller Fracture Zone, the southernmost one of the Aleutian Island Arc. The width of the Pikezh Shear Zone is about 6 km. It appears as a mélange of rocks exposed on the southern and northern Kamchatka Mys Peninsula. Young Pliocene to Quaternary marine sediments are involved in mélange formation. Neotectonic activity is indicated by a dextral strike-slip fault terminating as a thrust fault which forces the former drainage system to cut new valleys. This new results on the Kamchatka Mys Peninsula give evidence that these fracture zones are still active and continue onshore the Kamchatka Peninsula.
The mechanism of island arc collision seems to be controlled by the broad plate boundary at the western portion of the Aleutian Arc. Different kinematic conditions at each fracture zone support the fragmentation into basins and horst blocks which become attached to the Kamchatka Peninsula.
The principal structures of Kamchatka orogenic belt (Central and Eastern Ranges) extended now northeastwards over 1000 km have been formed during collision of the Upper Cretaceous-Lower Paleocene island arc with the Asian continental margin at the Early Eocene time. It is recognized two major syncollisional tectonic events: a westward overthrusting of the island arc structures onto the continental margin and an eastward (Pacific vergence) reverse faulting superimposed on back side of the deformed arc and accompanied by tectonic accretion to it from east the slices of oceanic crust.
The both syncollisional tectonic processes occurred discretly at an upper crustal level as they developed without notable interruption at a relatively deeper one, in zone of plastic deformations. In this zone syntectonic HT-MP graded metamorphism (up to amphibolite and granulite facies) affected the units of arc, its oceanic basement and continental margin and synkinematic gabbro-plagiogranite injections occurred along major decollements of the both vergences. According to geodynamic model, consequent occurrence during the arc-continent collision (ACC) firstly westward overthrusting in front of the arc and secondly eastward reverse faulting at its back side is resulted from early blocking of continental subduction followed by subduction reversal with beginning of westward oceanic subduction that takes in sum about 7 Ma. The similar geodynamic models of ACC are proposed for present active collision zones in the western Pacific (Taiwan and Timor areas), where subduction reversal is believed to be occurred now as beginning of collision is estimated as old as 5 Ma.
The geodynamic models proposed for mentionned above past and present arc - continent collision zones represent one of the possible scenarios in evolution of this process and seem to be quite different from those elaborated for some Paleozoic orogenic belts where ACC is accompanied by deep crustal subduction and consequent exhumation of HP-LT metamorphic rocks. One of the important factors which seems to control the geodynamic evolution way of ACC is rheology and tectonic structure of subducted continental margin that is supported by physical modeling experiments.
Different mineralogical types of amhibolites are widespread in accretionary structure of the Taigonos Peninsula, Northeastern Russia. These rocks have different tectonic setting at the area of sampling. Amphibolites occur as blocks in serpentinite melange, within the system of tectonic nappes composed of volcanic-sedimentary rocks (Kingiveem Formation), and disseminated fragments into mixtites. Amphibolites examined have been subdivided into following groups by their textural features: 1) massive (isotropic) amphibolites and hornblende schists; 2) amphibolites with vague relics of gabbro-ophitic texture; and 3) amhibole bearing metavolcanic rocks with clear relics of porphyritic texture. First two groups includes following mineralocical types of rocks: hornblende-plagioclase-ilmenite-sphene-epidote amphibolites; garnet-clinopyroxene-hornblende-plagioclase amphibolites; and garnet-hornblende-plagioclase-sphene-actinolite amphibolites. Amphibolites belonging to third group consist of hornblende and plagioclase predominantly. The metamorphic P-T conditions of formation of garnet free amphibolites and garnet amhibolites were determined by amphibole-plagioclase (Plyusnina,1983) and garnet-amhibole pairs (Perchuk, 1970) and correspond to 450 - 500°C and 8 kb. The formation of low alumina actinolite in amphibolites took a place by T=350-380°C and P ¾2 kb. Clinopyroxene-garnet pair in amphibolites from Taigonos Peninsula originated by T=760-820°C and P=8 kb judging by different geothermobarometer (Ellis, Green, 1979; Dahl, 1980; Krogh, 1988; Sengupta et al., 1989). Geochemical data (major elements as well REE) indicate that amphibolites were formed mainly after different members of MORB magmatic suite at temperature 450-820°C and 8 kb. Such P-T conditions of metamorphism may have occured during subduction of relatively young, warm oceanic crust. This study was supported by INTAS grant 96-1880.
The Kohistan-Ladakh formations are remnants of a large palaeovolcanic arc, obducted to the South on the Indian margin along the MMT, and separated to the North from the Karakorum (southern Asian margin) by the Shyok suture zone, or MKT. Strips of basic and ultrabasic rocks are also known north of the MKT, incorporated within the Karakorum metamorphic complex. We investigated the lithological and geochemical characteristics of mafic units both in Ladakh arc and in Karakorum metamorphic complex.
South of the MKT, the northern part of the Ladakh arc comprises volcano-sedimentary formations intruded by large dominantly granodioritic plutons. Lithologies in its western part (Skardu area, Pakistan) suggest a N-S evolution from a basin to a volcanic arc environment. Lavas are predominantly basaltic to andesitic. Farther to the East (Indian Ladakh), volcanics are less abundant, the amount of platform type sediments increases and lava compositions becomes andesitic to rhyolitic. The basalts and andesites are subdivided into three groups: immature arc or MORB compositions (1.5 > LaN/YbN), mature to evolved arc (6 > LaN/YbN > 1.5) and alcaline (LaN/YbN > 6) compositions. This provides further evidence that the northern part of the Ladakh belongs to a back-arc basin, situated between the Asian margin and the real Kohistan-Ladakh arc.
North of the MKT, metamorphism decreases from W to E. In the Masherbrum area, low grade basic and ultrabasic rocks overly high-grade gneisses in a folded nappe system. Here, volcanic and plutonic rocks have geochemical signatures like those of the Ladakh volcanics, and can be subdivided into three similar groups similar. We suggest that the Masherbrum nappe could actually be a klippe composed of Ladakh arc and back-arc formations. Early incorporation of arc terranes to the South Asian margin involved obduction of the Arc on both the Indian and Asian margins.
Some of the Karakorum amphibolites are interpreted as the metamorphic equivalents of the Masherbrum rocks: they could be parts of the Ladakh Arc that were incorporated within the Asian margin during several periods of intense folding and metamorphism.
The Kohistan complex is an island arc obducted during the Mesozoic onto India along the Indus Suture. We have recognised two units containing ultramafic rocks in this complex: (1) the Jijal unit, which is the lowermost part of the exposed arc and (2) the Chilas unit, which configures the core of the arc. Our observations indicate that the ultramafic rocks are exhumed mantle rocks that record distinct evolutions and different depths of equilibration. The Jijal unit includes the arc crust-mantle boundary and contains mantle rocks equilibrated in the Ariegite subfacies. The Chilas ultramafic associations are apexes of intra-arc mantle diapirs with ultramafic rocks equilibrated in the plagioclase peridotite facies.The lower part of the Jijal section ('the mantle section') is dominated by dunites and wehrlites interlayered with subordinate pyroxenites. The presence of harzburgites, with a high-temperature tectonite microstructure, indicates that peridotites were part of a residual 'arc mantle'. Flames of dunite in websterites indicate that these rocks have replaced peridotites through magma-consuming melt-rock reactions, at P-T conditions close to the peridotite solidus. The proportion of mafic rocks (websterites and minor hornblendites and garnetites) increases up-section, up to an intrusive sharp contact between ultramafic rocks and overlying garnet granulites. The latter facies is derived from garnet gabbro. The contact dips gently to the north, parallel to the pyroxenite-peridotite layering. We interpret this boundary as the exhumed petrological arc-'Moho'. The Chilas section includes a series of ultramafic bodies and gabbro norites. Gabbro-norites display magmatic/cumulitic textures. Ultramafic rocks, however, show microstructure typical of mantle rocks, notably, porphyroclastic textures in wehrlites and harzburgites and coarse-grain textures in plagioclase-bearing dunites. The contact between ultramafic rocks and gabbro-norites is steep and underlined by strongly metasomatised rocks, such as hornblende and/or plagioclase pyroxenites. This metasomatic aureole is indicative of extensive reaction between peridotites and infiltrated melt, implying that the mantle rocks were intruded at high, near-solidus conditions within partially consolidated norites. Interstitial plagioclase and diffuse impregnations of gabbro-norite are further indication of magma percolation in peridotites. Thereafter - upon cooling - the peridotites were fractured, intruded by magma dikes and eventually brecciated and finely dispersed in the gabbro-norite. These final evolutionary stages are exemplified by the occurrence of steeply dipping, intrusive gabbro-norite layers in peridotites, blocks of peridotites in gabbro-norites and olivine xenocrysts in graded cumulate layers. These cumulate layers describe a large antiform. To integrate small (reactional and intrusive contacts) and large (antiformal) scale observations, we interpret the Chilas ultramafic bodies as apices of intra-arc mantle diapirs. These mantle diapirs indicate rifting of the Kohistan arc during a major tectonic event that should be considered in any collisional model of the area.
I and my associates have worked on orogenies in Japan, China, Kazakhstan, Indonesia and California since 1970, and extendted recently the research to the Archean orogenic belts including 3.8-3.7 Ga Isua, Greenland, 3.5-1.9 Ga Pilbara, western Australia, and 3.5 Ga Barberton, S. Africa from 1991. Based on these experiences and reference compilation, I will discuss orogeny through geologic time.
Pacific-type orogeny has prevailed formig continental crust by plate tectonics on the Earth since 4.0 Ga, and contributed to accumulate the continental felsic crust of 7.76 x 109 km3. The Phanerozoic Pacific-type orogeny is accompanied with exhumation of high-P/T regional metamorphic belt. This may be related to the arrival of RTT junction, i.e., the gradually shallowing subduction angle by approaching mid-oceanic ridge caused the exhumation of deep-seated accretionary complex. The episodic increase of granitic crust was due to the slab-melting which is avalibale only by young slab subduction < 25 Ma. More common slab-melting increased the crustal volume efficiently in the Archean. The arc-arc collision orogeny has dominated in the Archean to form continent on the Earth's surface until 2.7 Ga. Since then, the continent-continent collision orogeny has appeared to form the first supercontinent on the Earth until 1.9-1.8 Ga. Thereafter, the Wilson cycle and relevant orogenies have repeated several times. The long-term continuation of the Earth's orogenic activities may be the result of 660 km-depth endothermic phase relation that made possible to release the internal heats gradually. The reactivated Phanerozoic orogeny was due to the initiation of returnflow of seawater into mantle at 750 Ma.
The eastern Pontides comprise the components of a collisional mountain range consisting of fragments of an active continental margin and a continent. The collision occurred during the late Cretaceous - early Eocene period.
In the region from the north to the south the following east - west trending tectonic zones may be distinguished; a magmatic belt, an ophiolite belt, and a metamorphic belt. Between these tectonic zones are the remnants of the basin fills of the late Cretaceous to the Eocene in age.
In the eastern Pontides the geological sequence reveal that a south - facing passive continental margin was formed during the late Jurassic, and survived till the late Cretaceous period. A magmatic chain of ensialic island arc type began to develop on the platform, during the Turonian. This was accompanied by a co-eval, mélange association with a high P metamorphism, at the leading edge of the continent. This suggests that the Neo - Tethyan ocean floor began to be consumed by the northward subduction under the eastern Pontides. Around the volcanic belt a volcanogenic flysch was developed extensively. During the same period, a narrow fore-arc basin was formed in front of the volcanic arc, which was filled with the arc-derived debris and volcanic fragments.
The subduction - accretion complex was uplifted as the mélange wedge continued to grow. While the mélange wedge kept growing in the north, the subduction retreated oceanward. Consequently the southward-migrating magmatic front swept through the Pontides range. The new volcanic products blanketed the previous fore-arc region and the subduction-accretion mélange wedge during the late Campanian-Maastrichtian. In this period, the Neo-Tethyan ocean floor was possibly completely eliminated. However a remnant basin, underlain by a mélange foundation, survived as evidenced by the continual deep-sea sediment deposition, situated between the mélange forebulge and the southerly-situated continental fragment; the Kirsehir massif. The sediment deposition in the remnant basin continued uninterruptedly till the end of early Eocene, in a progressively shallowing marine environment.
The early Eocene-Middle Eocene transition corresponds to the final stage of the collision between the Pontide arc and the Kirsehir block. This period witnessed the compressional regime everywhere, characterized by thrusting and ophiolite nappe emplacement.
The Taigonos Peninsula (TP) is a segment of the large Mesozoic convergent margin between NE Eurasia and Pacific plates. TP consists of two main tectonic units. Northern TP includes two suprasubduction volcanic belts of Late Paleozoic-Early Mesozoic and Late Jurassic-Neocomian age. Southern TP consists of an accretionary wedge of Late Jurassic - Early Cretaceous age. The TP accretionary structure includes numerous thrust sheets of Pz and Mz ophiolites and a serpentinite melange, high P-T Mz metamorphic rocks (Ar/Ar dating), a terrigenous accretionary melange of Late Jurassic - Early Cretaceous age, polymictic terrigenous turbidites of Dogger-Malm age, and ensimatic island-arc complexes of Late Jurassic-Neocomian age. The oceanic basalt-chert complexes of Late Triassic, Jurassic, and Tithonian-Berriasian ages are identified as well. Based on structural data, we can reconstruct three main deformation stages. Stage 1 is observed only in the accretionary wedge. All the stages display strike-slip structural assemblages. Based on a set of data, we propose a scenario for TP's Late Mesozoic evolution. This segment of the Eurasia-Pacific convergent margin was the site of a trench-trench-trench triple junction. This paleogeodynamic situation arose through an oblique collision of an ensimatic island arc and the NE Eurasia craton. The ensimatic island arc moved southward along the Eurasian convergent margin, while rotating counterclockwise. The tectonic and metamorphic events, as well as the terrigenous melange, result from this oblique collision. Paleomagnetic and paleobiogeographic data (radiolarian determinations by Valentina Vishnevskaya and Irina Pralnikova) show that the Late Jurassic-Neocomian intraoceanic island arc had a Late Triassic basement and marked the boundary of two Pacific oceanic plates. We believe that these were the Farallon and Izanagi plates. This study is supported by RFBR (grant 96-05-64359) and INTAS (grant 96-1880)
The metamorphic sole of the Khawr Fakkan block of the Semail ophiolite, exposed in the Bani Hamid area of the United Arab Emirates, appears to represent the oldest preserved phase of metamorphism in the ophiolite belt, and is characterised by UHT assemblages in metasedimentary and metabasaltic material. Petrographic, geochronological and structural evidence suggests that the Bani Hamid metamorphic sole is derived from Tethyan pelagic (Hawasina) and trench fill complex (Haybi) sediments, with intercalated basaltic extrusives, accreted to the base of the ophiolite during intra-oceanic subduction. A series of distinct granitiod dyke suites, ranging from peralkaline to peraluminous cordierite-andalusite-biotite monzogranites to peraluminous garnet-tourmaline leucogranites, is reported intruding the harzburgitic mantle and cumulate gabbro lower crustal sequences of the Khawr Fakkan block. Initial 87Sr/86Sr ratios of the dyke suite vary between 0.710 and 0.706 at <epsilon>Nd(90 Ma) values of between -6.3 and -0.5, and vary systematically with structural height above the Bani Hamid terrane. 87Sr/86Sr ratios and <epsilon>Nd(90 Ma) values of the metasedimentary and metabasaltic components of the Bani Hamid terrane appear to define end-members to the granitoid dyke array, and Nd-Sr isotope covariation implies a genetic relationship between the metamorphic sole components and the granitoid dyke suites. Nd-Sr isotope systematics suggest that source contamination of melts derived from the metamorphic sole by LILE enriched mantle wedge material produced a suite of hybrid magmas. Estimated crystallisation pressures of the dyke suites vary between 5 and 2 kbar. LILE (Sr, Rb and Ba) modelling of individual dyke suites suggests that the observed intra-suite covariation trends can be explained by the in situ fractional crystallisation of alkali feldspar and biotite. It is possible that the granitoid dyke suite represents the highest structural levels of a more extensive zone of granitoid intrusion at the base of the Khawr Fakkan block. The unique status of the Semail ophiolite as an exposed example of sub-arc mantle, and the study of the petrogenetic and metamorphic processes related to the generation and emplacement of the ophiolite, may have parallels with processes in similar intra-oceanic arcs observable today.
The Jijal complex, at the base of the Mesozoic Kohistan arc (Pakistani Himalayas), has been interpreted either as old oceanic lithosphere on which the arc was built or, most often, as ultramafic and mafic cumulates formed in a lower-crustal magma chamber beneath an island arc. From bottom to top, the Jijal complex comprises four main lithological units: (1) a peridotite unit composed of dunites and wehrlites interlayered with pyroxenites, and minor harzburgites and lherzolites; (2) a pyroxenite unit made of websterites, Cr-rich clinopyroxenites and subordinate dunite layers; (3) a garnet-hornblende pyroxenite unit mainly composed of garnet pyroxenites and hornblendites; and (4) a garnet granulite unit composed of mafic garnet- and subordinate two-pyroxene granulites. The transition from unit (1) to units (2) and (3) is gradual and marked by an up-section increase of the ratio of mafic to ultramafic rock-types. In contrast, the contact between units (3) and (4) is sharp and likely intrusive (Burg et al., 1999).
All Jijal rock-types have a geochemical 'island-arc' signature marked by a depletion of Nb, Ta, Zr and Hf relative to REE, and elevated Nb/Ta, U/Th and Ba/La ratios. Most samples from units (1), (2) and (3) have trace-element compositions that are inconsistent with a crustal cumulate origin. For instance, unit (2) pyroxenites display extremely low REE contents (MREE < chondrites) that cannot be explained by equilibrium with any known volcanism. Unit (4), garnet granulites display flat to LREE-enriched REE patterns displaying positive Eu anomalies. Their high FeO(total) and HREE contents indicate that garnet was a primary magmatic phase. These observations preclude that mafic granulites derive from low-pressure oceanic gabbros. They represent high-pressure crustal cumulates (with trapped melts) after high-Al arc-basalts. The garnet-hornblende pyroxenites are depleted in LREE relative to MREE, a feature that precludes a fractional crystallization genetic relationship with the overlying garnet granulites.
Our geochemical data exclude that the Jijal complex represents a remnant of oceanic crust or that all the rock-types are arc lower-crust magmatic cumulates. Geochemical observations are better explained with the Jijal complex representing a piece of subarc mantle-crust transition. Unit (1) represents the uppermost part of a residual arc mantle magmatically modified at lower pressures. Unit (4) rocks conform the arc lower crust accreted by large-volume underplating of high-Al basalts. Units (2) and (3) represent mafic segregations formed by replacement of mantle rocks in the arc crust-mantle transition and record small-volume accretion of the arc crust by mantle metasomatic melts.
Burg et al, J. Conf. Abs. 4, (1991)
The Antarctic Ross Orogen, well exposed in the Transantarctic Mountains over a distance of about 3000 km, was formed at the active plate margin of Gondwana during Early Paleozoic time. Evidence comprises subduction-related plutonism, inboard/outboard polarity, high pressure metamorphism, ophiolites, oceanic segments and accreted terranes. The main common feature of the orogen is its magmatic arc, consisting of large calc-alkaline granitoid bodies. The subduction-related intrusive activity had its peak at around 500 Ma.
In the Ross Sea sector of the Transantarctic Mountains, a major suture has been identified in North Victoria Land. The suture separates the inboard continental part with the granitic arc, hosted in medium to high-grade (low-pressure) metamorphic rocks, from two low grade outboard terranes of Cambro-Ordovician age. The inner terrane consists of volcanic rocks of primitive island arc signature, overlain by a clastic to conglomeratic sequence containing exotic limestone blocks and this in turn covered by a very thick, deltaic to fluviatile, quartzitic sandstone sequence. The outer terrane consists of thick, regularly folded turbidites.
The suture between the continent and the outboard terranes is characterized by the occurrence of:
* a small belt of medium to high pressure metamorphic rocks,
* lenses of ultramafic rocks in the form of cumulates, layered gabbros and eclogites,
* an overprint of greenschist metamorphism, retrograde on the arc side and prograde on the terrane side,
* a system of thrusts carrying the three major tectonic units outwards, the inboard units on top of the outboard ones.
A major problem poses the occurrence of thick deltaic to fluvial quartzitic sediments of continental character on top of the primitive volcanic rocks of the island arc. These two features are very difficult to reconcile.
After the accretion of the arc, the active margin setting between Gondwana and Proto-Pacific persisted through the whole of the Phanerozoic. Younger orogens in a more outboard position appear to be similar products of subduction accretion.
The accreted terranes along the eastern edge of the Main Uralian Fault suture zone of Southern Urals include remnants of island arcs of the Magnitogorsk Zone. These remnants consist of typical volcanic suites generated in oceanic island arcs. They range in age from Early to Middle Devonian according to conodont stratigraphy, and show evidence in their structural, petrographic and chemical characteristics of various stages of development of oceanic arcs through time, and in space. Early volcanic products of Emsian age include boninitic lavas and dykes which are overlain by tholeiitic to calc-alkaline, basaltic to andesitic and dacitic, lavas and volcaniclastic rocks (Baimak-Buribai complex and Tanalyk Formation). Younger volcanic rocks (Emsian to Eifelian) are lavas, breccias, tuffs and tefra, ranging between basalt to dacite compositions, and showing distinct affinities: tholeiitic and calc-alkaline (Irendyk and Karamalytash formations). These young suites include highly fractionated rocks, among which quartz dacites from the Karamalystash Formation are remarkable. We have investigated on the petrogenesis of the magmas from the Magnitogorsk volcanic suites with high-quality geochemical data obtained by ICP-MS plasma spectrometry on a set of representative, poorly fractionated, effusive rocks. The relatively immobile elements during seafloor hydrothermal metamorphism, Th, Ta, Nb, Y and rare earths have provided useful indications and discriminations. All suites show signatures of fluid-, and possibly melt-related, subduction components typical of oceanic island arcs lavas, that are: low Nb, high Th contents, relatively low Ta/Yb ratios, and mostly depleted REE patterns. All suites also display large variations of enriched and fluid-related components. The early boninitic rocks are clearly distinct; the tholeiitic and calc-alkaline rocks of the older sequences show many similarities with those of the younger sequences, except some calc-alkaline rocks of the Irendyk unit which show significant enrichment of a crustal component, similarly to lavas from active continental margins. We suggest that the Magnitogorsk arc lavas record the initiation of subduction (Emsian boninites) and subsequent intraoceanic convergence thorough Early and Middle Devonian. The inception of arc-continent collision, which could be possibly recorded by the youngest Karamalytash volcanic rocks, is suspected, but not clearly evidenced with the geochemical data so far obtained. A contribute to the history of the Palaeozoic Uralian ocean is therefore provided by Magnitogorsk arc rocks, relatively to its initial and progressive closure.
The Urals orogen consists of two major sectors. West Urals (paleocontinental sector) is composed of Russian platform basement and cover and complexes of its passive continental margin - shelf, continental slope and rise. Paleozoic ophiolites, and cherty-shales formations compose here overthrusts being formed during arc-continent collision. East Urals (paleoisland-arc sector) consists of two main island-arc terrains, successively collided with Russian platform: Tagil terrain (O3-D2), located in the Middle and Nothern Urals, and Magnitogorsk terrain (D-C) located in the South Urals and also microcontinents and Late-Paleozoic granite belt. Main Uralian Fault is the suture zone with high pressure eclogite-glaucophane belt dividing these two sectors. Almost all Early Paleozoic Uralian ocean's crust was subducted by the Middle of Late Devonian. Convergence and subsequent oblique collision of island-arc terraines of the Urals with Russian platform during Carboniferous-Permian was the reason for the compression, folding and bivergent deep region's structure formation. Collision direction of the Urals island-arc terraines with Russian platform was oblique (North-Western), that is proved by the following independent groups of data: 1. Direct structural observations (studying of mineral stretching lineation, S-C structures, directions of dykes of different generations and other data of Ivanov, Bankwitz, Echtler, 1993-1997); 2. Age rejuvenation of greywacke flysch of Zilair series (the complex-index of regional overthrusting) from South (Lower Famennian) to North. A similar difference of age is characteristic for high pressure complexes, collisional granites and others; 3. Paleomagnetic data (Svyazhina et al., 1992); 4. Paleobiocenotic and paleogeographic data; 5. The formation All-Uralian sinistral strike-sleeps system including the plastic ones in the East of the region. Tectonics and stratigraphy of the complexes of continental slope and rise and shelf terrigenous-carbonate deposits have been studied in the West of the Middle Urals. First phase of folding connected with the beginning of collision is determined here according to structural unconformity in Frasnian layered limestones. Higher on the section these deposits contain olistoliths of shallow reef limestones, and at the level of Late Famennian there are horizons of sedimentary carbonate breccias, caused by intensive tectonic movements. Second stage was here the post-Visean one (probably Middle-Late Carboniferous; intensive folding connected with sublatitudinal compression and the following overthrusts to the West) and it is the main collision stage. Third stage is the formation of normal faults dipping to west and south-west. To all appearance this last stage is a result of post-collisional limited extension of the Urals during Triassic.
Svyazhina IA, Puchkov VN, Ivanov KS, J. Russian Geology and Geophysics, 4, 17-22, (1992).
The southern Urals contains a well-preserved accretionary complex and forearc basin related to the Late Devonian collision between the Magnitogorsk arc and the East European Craton. Excellent exposure, aided by three parallel reflections seismic profiles, allow the structure and collisional processes to be determined with confidence. The accretionary complex is composed of offscraped continental shelf and rise material, syncollisional sediments derived from the arc, high-pressure gneisses with intercalated eclogites and blueschist, and, at the highest structural level, ultramafic complexes. It is bound at the base by a thrust and at the rear by the arc-continent suture. The forearc basin is composed of weakly deformed volcaniclastic turbidites and intercalated cherts that were deposited on a volcanic basement that includes Emsian-age boninitic lavas near the base of the sequence. The presence of boninites marks the onset of subduction, possibly in an intra-oceanic setting, followed by mature arc volcanism that continued throughout the Middle Devonian and into the Late Devonian. Deposition of the Late Devonian syncollisional volcaniclastic sediments in the forearc and on the continental margin appears to be related to collision, uplift, and erosion of the arc, possibly following the arrival of the full thickness of the East European Craton continental crust at the subduction zone. Widespread olistostromal development indicates intense seismic activity during this time. With the arrival of the continental crust at the subduction zone, Paleozoic slope and platform material were offscraped and underplated at the base of the accretionary complex. Uplift of the arc was followed by its collapse and the unconformable deposition of Lower Carboniferous shallow water carbonates on top of it.
The Zilair Nappe consists of 5 - 6 km of syncollisional, Upper Devonian (Late Frasnian and Famennian) to Lower Carboniferous (Tournaisian) volcanic arc-derived polymictic and graywacke turbidites with thin interbeds of chert and black shales at the base. The nappe was emplaced as part of an accretionary complex associated with the arc-continent collision that occurred between the East European Craton and the Magnitogorsk arc during the Late Devonian. In the west, kilometer-scale folds verge westward and have a moderately east dipping axial planar cleavage. The front limb of these folds is generally steep to overturned, and often a zone of intense cleavage development indactive of thrusting. Along its eastern contact, however, the cleavage fans until it dips moderately westward and the folds are east vergent. Internal imbrication in the nappe thrusts low-grade metamorphic rocks (illite crystallinity index indicates upper chlorite zone) westward over unmetamorphosed rocks along the Sosnovka thrust (or Zuratkul fault). In the northwest, the basal thrust of the Zilair Nappe coinsides with the roof thrust of the Timirovo thrust system which outcrops for several tens of kilometers along the northwestern margin of the Zilair Nappe, forming a west vergent thrust stack composed of a highly deformed and sheared Lower Devonian (?) reef complex. Southward, subsequent thrusting resulted in minor reworking of the basal contact. Detailed mapping in the volcaniclastic sediments have revealed structural styles similar to those in currently active accretionary complexes, suggesting that similar processes were active during the Devonian evolution of the Zilair Nappe.
Deep-sea sediments with abundant arc-derived volcanic materials are quickly accreted from the Izu forearc side to the Honshu forearc on the Boso triple junction in NW Pacific during Miocene to Pliocene (Ogawa et al., 1989). Several sets of such forearc accretion occur intermittently after the triple junction came to the present position off Boso Peninsula, south of Tokyo. This forearc material accretion is the necessary results by means of the Izu arc collision to the other Honshu arc. Before the time, a juvenile island arc (Proto-Izu and Kyushu-Palau ridge) volcanic materials were supplied to the Shimanto accretionary prism in Shikoku during Paleogene due to a littel effect of the island arc collision to the Honshu.
However, during middle Miocene, a specific and unique setting occurs in the present Boso region. Since then the normal to oblique Izu island arc collision has become most conspicuous, and produced W-shaped bending of the previous Honshu structure around the Izu arc collision zone. One of the remarkable phenomena is the formation of the "trinity clastics" which is composed of ophiolitic fragments, island arc clastics, and continetal clastics. This occurs particularly within the ophiolite melange belt in the Mineoka Mountains, Boso Peninsula, indicating very speicific sedimentary setting of ophiolite emplacements to the Sagami trough area which is in between the Pacific, Philippine Sea, and Eurasia plates, from each ophiolitic, volcanic arc and continental clastic materials were supplied, respectively. This is the unique sedimentary tectonics only expected in the world only known TTT-type triple junction, the Boso triple junction scenario.
Ogawa et al, 1989, Tectonophysics, 160, 135-150, (1989).
During four Polish Geodynamical Expedition to West Antarctica in 1980-1991, seismic measurements were made along 21 deep refraction profiles in the Bransfield Strait and the coastal area of Antarctic Peninsula. The shots of 50-100 kg of TNT were recorded by 16 5-channel land stations and 8 ocean bottom seismometers. The good quality recordings obtained up to about 250 km distance allowed a detailed study of the seismic wave field and crustal structure. After construction of 2-D models of the crust along the profiles, a 3-D modelling has been carried out, using the complete data set including off-line recordings. As a result we obtained P-wave velocity distribution and 3-D models of main crustal discontinuities in the study area. In the area adjacent to the Antarctic Peninsula coast, sedimentary cover of 0.2 to 3 km thickness was found, while in the shelf area and in the Bransfield Strait sedimentary basins with thickness from 5 to 8 km are observed. In the Bransfield Strait a high velocity body with Vp > 7.0 km/s was found at 10 km depth. The thickness of the crust varies from 35 to 42 km in the coastal area to 30-35 km in Bransfield Strait and South Shetland Islands and about 10 km in the Pacific Ocean NW ofSouth Shetland Islands.
Island arc complexes and terranes are widespread in northeastern Asia. They differ by tectonic position (allochthonous and autohthonous), internal structure (disrupted and coherented), composition (ensimatic and ensialic), and fauna (Boreal and Tethyan). So the reconstruction of convergent margin is a hard problem and there are different standpoints on this question. Uda-Murgal island arc system was marked the convergent margin of Eurasia continent and northwestern Pacific in Late Jurassic to Early Cretaceous. The complexes of this arc are tracked on long distance (3500 km) from Mongolo-Okhotsk folded belt to Chukotka. In southern part it was a marginal volcanic belt, in middle part - ensialic island arc, and in northern part (Pekulney segment) - ensimatic (?) arc. In this regard Uda-Murgal island arc system is similar with Kamchatka-Kuril segment of modern convergent margin. A lateral sequence of paleostructures (volcanic zone of island arc - forearc - accretionary prism) is reconstructed for the Taigonos and Penzhina segments. The accretionary prism consists of boninite-like rocks, serpentinite-matrix melange, turbidites and basalt-chert assemblages. Boninite-like rocks and related Late Jurassic clastics are interpreted as intraoceanic arcs. The paleotectonic position of these arcs is unknown. There are not any evidences that they belonged to Asian convergent margin. Melange includes blocks of peridotite, plagiogranite, sheeted dikes, boninite, basalt, chert and metamorphic rocks. Triassic-Jurassic basalt- chert assemblages (paleolatitude 35) are considered as off-scrapped fragments of the oceanic crust of the Izanagi plate. Basalts are of the MORB-like type; bedded cherts have the negative Ce-anomally that is typical for oceanic setting. Apart from the back arc complexes were recognized in Pekulney segment. In last instance Uda-Murgal island arc separated northwestern Pacific from South-Anuy oceanic basin. To north another zone of suprasubduction volcanism was located along southern margin of the Chukotka microcontinent in that time.
An important extensive regime connected with the opening of the Central Atlantic Ocean gave rise in the Mesozoic to the Ionian crustal opening and the consequent separation of the Adria Plate from the African megaplate. Two more or lesssymmetric fault systems related to this rifting process occur along the present Hyblean and, on the opposite side, Apulian Escarpments. Both of them represent old passive margins: the crustal thickness passes from about 30 km in the Apulian and Hyblean Platforms (continental crust) to about 15 km in the Ionian (oceanic crust). The following geological events were predominantly caused by the African-European megaplates convergence: they hardly changed the paleogeography of the Central Mediterranean but at the same time they masked the main parts of the former fracture zones, in particular the Apulian Escarpment. The compressive movements associated with the formation of the Calabrian and Hellenic Arcs affected the whole area at least from Neogene and produced numerous compressive features. They also caused the activation of new normal faults, the reactivation of some old ones and the creation of relevant transcurrent systems. The Mesozoic paleogeography of the two margins played a major role for the final tectonic structure of the area: it determined an oblique collision between the South Apennine/Calabrian Arc and the continental margin of the foreland represented by the western margin of the Apulian Platform. The reconstruction of the structural domain and of the different crustal thickness are fundamental in understanding the deformations of the upper crust: in addition to the compressive features of the orogenic system, a set of strike-slip faults cuts the mountain belt and adjusts the migrating orogenic front to the different conditions of the foredeep. A new important dataset of the Italian CROP Project allows to define the tectonic features of the Ionian-Apulian transition. Advanced techniques of seismic processing were applied to the CROP seismic profiles in order to obtain a better definition of both shallow and deeper reflections. The complexity of the deformations required an accurate analysis of all the available seismic lines. A large amount of seismic profiles has been interpreted for a regional but detailed reconstruction of the old margin between the Apulia continental plate and the Ionian oceanic crust. As it was to be expected, this margin has determined the different displacements of the external South Apennine/Calabrian Arc with his important left trascurrent fault systems and, on the opposite site, of the external W-Hellenic Arc with the right trascurrent fault system of Kefallinia.
The West Pilbara is a super-terrane that was accreted onto the East Pilbara super-terrane at circa 2.9 Ga to form the Pilbara Craton (Barley, 1997, Smith et al., 1998). Both are typical Archean granite greenstone terrains. The West Pilbara consists of a number of internal tectonostratigraphic terranes separated by major crustal scale shear zones. The WSW-ENE trending Sholl Shear Zone represents the boundary between two such terranes. On the basis of geochronological and geochemical studies, it was concluded that the Sholl Shear Zone separates the Roebourne Complex, a ~3260 Ma old terrane with island arc affinity, from the Sholl Belt, a ~3112 Ma terrane with back arc affinity (Smith et al., 1998). The Sholl Shear Zone has a width varying from 1 to 2 kms and is traceable for over 150 kms. Strain and kinematic partitioning are prominent within the shear and allow the kinematic history of the zone to be determined.
Initial deformation in the steeply north dipping Sholl Shear Zone and in the Roebourne Complex, in the hanging wall, took place at amphibolite grade conditions. All amphibolite grade rocks have mineral lineations plunging steeply to moderately to the NNW. Kinematic analyses indicate thrusting with a transport direction towards the SSE during this event and placed the above amphibolite grade rocks over the greenschist facies rocks of the Sholl belt. Following this compressional event the Sholl Shear Zone went through an intermediate phase of transpression before becoming active as a dextral transcurrent structure. This is indicated by mineral and stretching lineations showing a gradual shift from having an intermediate plunge to being subhorizontal. This deformation phase took place at upper-greenschist conditions. The change from thrust to transcurrent movement appears to be related to a single tectonic event and is similar to that recorded in modern convergent margins. This further supports the argument of Smith et al that the West Pilbara consists of a number of accreted terranes that resulted from the occurrence of plate tectonic-type processes at 2.9 Ga.
Barley ME, In: De Wit MJ and Ashwal LD (Eds) Greenstone Belts. Oxford University press, Oxford, U. K, 650-657, (1997).
Smith JB, Barley ME, Groves DI, Krapez B & McNaughton NJ, In: Barley ME and Loader SE (Eds), The tectonic and metallogenic evolution of the Pilbara granitoid-greenstone terrane, Precambrian Research, 88, 143-171, (1998).
The Lomonosov submarine massif (LSM) is situated at the Black Sea floor 26 miles distant south-westwards from Sevastopol. It stretches north-westwards for 44 km along the foot of the continental slope. Up to discovery of the LSM all the geodynamical ideas on the structure of this region where suboceanic crust of the Black Sea depression directly contacts with the Crimean continental crust have been based on the geophysical data. New petrological data obtained by the authors permit to form a material foundation for the remote sensing methods, in particular to draw on more confidently an analogy between the LSM and the classic oceanic island arcs. Within a real submarine exposure predominantly Cretaceous vulcanites of three typical island arc series have been established, namely boninite, calc-alkali and shoshonite ones. In their spatial distribution a clear petrochemical zoning has been detected. The LSM opens different parts of the island arc approximately across its strike, which is north-eastern that is subparallel to the Mountainous Crimea. Then the eastern part of the LSM (boninites and oceanic plagiogranites) represents the earliest rocks of the arc front and the western part (shoshonites) - its back. In such a case the dip of a hypothetical subduction plane must be directed towards the East-European platform.
The Jurassic folded constructions of the Crimea and the LSM Cretaceous arc are thought to be formed as a result of the collision between the European and the African continental plates proceeded by obduction mechanism. The LSM is suggested to be a small fragment of a large formerly existed island arc that had been dissected, separated and moved apart westwards and eastwards due to the thrusting of the African plate from the south.
Both Crimean and LSM constructions arose on the edge of the Scythian plate. The latter in its turn is disposed on the southern margin of the East-European platform, which southern boundaries are not exactly determined. Probably the East-European platform is smoothly overlapped by the Scythian plate together with the Mountainous Crimea and goes on far away to the south. The hard roof of the East-European platform limits the depth of the Crimean seismic centres. The Crimean folded structures in the form of melanges might slide over a surface of the East-European platform. The sutures might vary the dip direction like listric faults.
Thus, the Black Sea branch of the Mediterranean foldbelt may be considered as a combination of subconcordant island arcs and collision terranes that had been forming on the southern margin of the East-European platform beginning in Jurassic or perhaps even earlier and ending in Cenozoic. The continental crust had been growing from the north to the south on account of adjoining more and more young arcs.
The Ordenes Complex is the largest of the allochthonous ensembles containing the Variscan suture in the NW of the Iberian Massif. Its uppermost tectonostratigraphic unit overlies the ophiolitic units, and consists of a thick metasedimentary sequence, the Ordenes Series, intruded by orthogneisses and gabbros. In the lower part of the Ordenes Series, the large Monte Castelo gabbro (approx. 150 km2) is surrounded by high-grade migmatitic paragneisses. Several shear zones cutting-across the gabbro massif, depict intermediate-P granulite-facies, indicating a common metamorphic evolution with the surrounding paragneisses. Recent U-Pb geochronological data (Abati et al., in press) prove that the main tectonothermal evolution of the Ordenes Series took place in Early Ordovician times. These data suggest that the intrusion of the Monte Castelo gabbro (499±2 Ma; U-Pb in zircons) was immediately followed by a Barrovian metamorphic episode that reached the granulite-facies (493-498 Ma; U-Pb in monazites). Later on, a Variscan overprint is indicated by U-Pb rutile ages of 380-390 Ma.
Three major compositional types have been distinguished in the Monte Castelo gabbro: 1) olivine gabbronorites (Pl+Cpx+Opx+Ol+Hbl); 2) amphibole gabbronorites (Pl+Cpx+Opx+Hbl±Bt); 3) biotite gabbronorites (Pl+Cpx+Opx+Bt). The samples with olivine may contain up to 12.4% of this mineral, whereas the biotite does not exceed 4.1%; the proportion of amphibole may be occasionally high, but generally is lower than 10-12%. A very limited presence of quartz (<1%) was detected in two of the studied samples. The major and trace element composition of the Monte Castelo gabbro is indicative of a tholeiitic character (TAS, SiO2-K2O, AFM, REE abundance and variation diagrams). Using multi-element representations and some other diagrams for tectonic setting discrimination, the geochemical characteristics of the Monte Castelo gabbro can be compared to that of island arc tholeiites. In any case, the overall composition of the gabbros is clearly distinct from that of within-plate tholeiitic and alkaline magmas.
Considering the geochronological evidence for almost coeval magmatism and metamorphism during the Early Ordovician, together with the geochemical characteristics of the Monte Castelo gabbro, an accretionary complex related to an Early Ordovician island arc appears as the more probable setting for the uppermost allochthonous terrane in the NW of the Iberian Massif. This implies the presence of a convergent plate boundary in the oceanic realm between Laurentia and Gondwana, close to it, during the Lower Paleozoic. Later on, the island arc became involved in the Variscan convergence.
Abati A, Dunning GR, Arenas R, Díaz García F, González Cuadra P, Martínez Catalán JR & Andonaegui, P, Earth and Planetary Science Letters, (in press).
Major-and trace-element contents, Sr-Nd isotope ratios and K-Ar radiometric ages of rocks from the ophiolitic suite at Sirwa, Anti-Atlas, Morocco, have been determined. Petrographically, the rocks are mainly represented by ultramafic and mafic lithotypes with some intermediate differentiate. All the rocks were affected by greenschist metamorphism, that occurred during docking of the ophiolitic suite to the Western African Craton in the late stage of the Panafrican orogenesis, as indicated by the K-Ar radiometric ages (670-700 Ma). The spiderdiagrams of the rocks show N- to E-MORB affinity; immobile elements and Sr-Nd isotope ratios are similar to those of magmatic rocks from island arc environment; i.e. low TiO2, HFSE and REE concentrations; low Ti/V and Ti/Zr ratios and moderate enrichment in LILE, Ce and Pb. No geochronological indications have been obtained from the Sr- and Nd- isotopes. Moreover, the Sr- isotope ratios suggest that the rocks underwent some alteration operated by seawater.
The Cyclops ophiolites (Irian Jaya - Western Indonesia) displays all components of an ophiolitic sequence including residual mantle peridotites (harzburgites and dunites), cumulate gabbros, dolerites, normal mid-oceanic ridge basalts (N-MORB) composition and minor amounts of boninitic lavas. This ophiolitic series tectonically overlies high temperature (HT) High pressure (HP) mafic rocks metamorphosed during the Miocene. Mineral chemistry and bulk rock rare earth element (REE) abundances of the peridotites are characteristic of highly residual mantle rocks. The high Cr# [Cr#=100*Cr/(Cr+Al)] of spinel (up to 60) and very low heavy rare-earth element (HREE) concentrations of peridotites (<0.1 time the chondritic values) are in agreement with residues of 25 to 35% melting as expected for peridotites from supra-subduction zone environments. The existence of Ti-enrichments in spinels and secondary clinopyroxenes (up to 1% and 0.5%, respectively), is likely a consequence of reaction between mantle-derived melts and the host peridotites. From the high light rare-earth element (LREE) concentrations reaching up to the chondritic values and the high field strength element (HFSE) anomalies, it is suggested that the initial composition of the residual peridotites has been previously modified by the passage of boninitic melt(s). The associated basalts and related cumulate rocks display major and trace element contents with Nb-negative anomalies typical of back-arc magmas. New 40K/39Ar isotopic ages obtained from the back-arc basin basalts (BABB) and boninites combined with the geochemical signatures of the rocks studied here, indicates that the Cyclops Mountains may have formed in a single supra-subduction environment. This implies southward plunging subduction of the Pacific plate beneath the northern Australian margin. The ultramafic rocks and related lavas (boninites) likely formed during the Eocene (43 Ma) in a fore-arc environment, before their southward obduction onto the island arc crustal welt during the early Miocene. All these series were obducted by the back-arc basin representing the main ophiolitic series that includes gabbros, dolerites and lavas during early Oligocene.
An unusual type of Au-PGE mineralization has been found in the Ospa-Kitoy ophiolite complex, associated with unique carbonaceous ultrabasic rocks. Average carbon contains range from 0,5 wt.% up to 9,7 wt.% in the forms graphite, graphitite, kerogene ( 12C = -12,2;-12,4 per mil). Carbonaceous ultrabasic rocks presents of serpentinites, harsburgites, dunites. Besides process of carbonatization influenced on magmatic rocks (olistolites of granite) and metasomatic rocks (albitites, tremolitites). Carbonaceous ultrabasic rocks place in the forms of veins (from first meters up to 30 m x 250 m) and diffuse carbon and rarely in the form isometric body's. The body's of the magnesite and dolomite connect with zones graphitic (carbonaceous). The fluid inclusions contains of CH4 (CO and CO2 are absents) according to data of fluid inclusion microthermometry and laser Raman microprobe analysis. Water-salt inclusions contains CaCl2 (18-21 wt.%), NaCl (8-10 wt.%) and LiCl. Minimum temperature of process was 380-400oC and pressure 770 bar. The minerals of Au and PGE were found in the carbonaceous serpentinites. Microprobe analyses and SEM indicate that chemical composition of Au is very variable: 1) high fineness Au with admixture of Cu (average value - xav=0,22 wt.%.); 2) Au with high content of Cu (xav= 14,27 wt.%.); 3) composition of Au (xav= 26,26 wt.%), and Ag (xav= 46,0 wt.%), and Cu (xav= 26,02 wt.%); 4) electrum with admixture of Cu up to 2 wt.%; 5) Hg bearing Au (up to 27,22 wt.% Hg); 6) compositions of Au and Ag with contents Ag from xav= 8,36 wt.% up to xav= 43,48 wt.%. Gold copper alloy minerals characterize right dependence for Ag-Cu and inversely proportional dependence for Au-Ag and Au-Cu. Minerals of PGE presents of compositions Pt and Pd with different contents elements from Pt0,1Pd up to PtPd1,1 - PtPd1,5. All minerals of Pt and Pd contains of admixture Sn, Pb, Bi (up to 1,0-1,33 wt.%), Ba (up to 1,83-2,94 wt.%) but Ir, Rh, Ru, Os is absent. Besides hav been found of minerals (PtPd)3Sn, Pd3Sn, Pt3Sn. Pt (0,58 wt.%) and Pd (4,43 wt.%) fixing in the composition Sn2,5Pb. In narrow grow together with minerals Pt and Pd presents composition of PbSn, PbSb, BaPbSn, BaPbCdSn, BaBi. Minerals of Ag presents of AgS, AgPbSn, AgPbCu. Attendant minerals presents of NiAs, NiS, arsenopyrite, awaruite, zircon, galena, and Cr-spinels.
The formation of Au-, PGE-mineralization occur in reduced fluid flow in the condition of zone subduction according to Geological, petrological, and geochemical data. Origin of carbon bearing reduced fluid might be from mantle or at the expense of sediments carbonaceous rocks (black schist, limestone) in the result assimilation by the melt. Transport of Au and PGE might be realize in the form carbon-bearing composition, for example of carbonyl-complexes, carbonyl-halogen-complexes, and probably fullerenes (metallofullerenes). They composition is very stability near temperatures more 500o-600oC and reduced condition.
This work was supported by RFBR grant No 97-05-64845 and UIGGM SB RAS VMTK grant 1 1736.
The stability of rutile and ilmenite have been investigated experimentally in synthetic MORB (Fe(3+)/Fe(tot)=0.2), at TiO2 saturated, water undersaturated (3.5-5 wt% H2O) conditions, fO2 inferiour or equal to CCO (graphite capsules), P=9.5-17 kbar, and T=950-1200°C. Starting materials have been enriched by a cocktail of trace element yielding concentrations near 300 ppm in the MORB.Ilmenite "facies" is caracterised by cpx+opx(T<1050°C)+ilm±plag assemblages whereas rutile "facies" is characterized by gar+cpx+rut±plag assemblages. The upper pressure limit of ilmenite stability is between 11 and 15 kbar, increasing with temperature. Garnet formation is accompanied by ilmenite destabilisation according to the reaction (T< 1050°C)
ilmenite+orthopyroxene+anorthite = rutile+garnet.
Rutile/melt, ilmenite/melt and silicate/melt partition coefficients have been mesured with Laser Ablation ICP-MS. Nb-Ta-Hf-Lu-Yb-U-Zr are compatible in rutile but, compared to garnet, rutile has only major effects on melt compositions for Nb, Ta, U, (Hf). Melts equilibrated with residues in rutile facies have adakitic compositions. In rutile facies, only rutile can fractionate Th/U significantly. Fractionation becomes extremely effective for melting rates inferior to 10%, resulting Th/U ratios are irrealistic for rutile contents of more than 1%. For higher melting rates (15-30%), the measured Th/U ratios of adakites (e.g. AVZ, Sigmarsson et al. 1998, Th/U=3.4-5) can be modeled by adjusting TiO2 contents of the MORB source between 1.2 and 3.0 wt%. On the contrary, ilmenite does not have much of an effect on the measured trace element concentrations except for Nb and Ta. Even for low degrees of melting, resulting Th/U ratios of magmas in equilibrium with an ilmenite facies residue are similar to those of the MORB source. Nb and Ta contents of the melt can be similar to those of adakites but HREE and Y contents are much higher due to lack of garnet.
The experimental determination of partition coefficents and stabilities of rutile and ilmenite in MORB yields (i) that residual rutile can explain part of the trace element signature of adakites and (ii) a minimum depth of 40 km for the source region. Unfortunately, this depth can be reached by thickened continental crust and thus, rutile+garnet residue is not an unequivocal indication for melting of the subducted crust.
Sigmarsson et al, Nature, 394, 566-569, (1998).
The occurrence of a large ultramafic sheet covering one third of the island is the most prominent geologic feature of New Caledonia. Obduction occurred in the late Eocene-Early Oligocene as a consequence of subduction blocking. During the post-obduction period, the convergence of Pacific and Australian plates continued and before the onset of the New Hebrides arc, the lithosphere in excess was subducted in an unknown location. A possible location for post-obduction lithosphere resorbtion is along the west coast of New Caledonia where a prominent negative gravity anomaly can be related to a 5-7 kilometres thick Tertiary sedimentary pile, accumulated in a 500 km long trench, affected by duplexing and normal faulting. Below this accretionary complex, an abandoned slab has been detected at a depth of ca. 60 km. Therefore, a short-lived NE dipping subduction is suspected to have existed below the west coast; however, no related magmatic activity has been reported yet. Upper Tertiary granodioritic intrusives have been known for a long time to exist in New-Caledonia and have been classically interpreted as a post-obduction crustal melting; however, this interpretation does not account for their geochemical features.
Granodioritic plutons are typically 0.5 to 2 km wide and crop out on both coasts of the island: St Louis massif, on the west coast and Koum massif on the east coast. Both are composed of several stocks and dykes emplaced across the lower boundary of the Ophiolitic (ultramafic) Nappe, and typically display contact metamorphism. Cooling ages of c.a. 20 - 25 Ma (K-Ar) have been obtained on both granodiorites. Plutons are located along steeply dipping faults, with normal and transcurrent motions, that crosscut the basal thrust. Therefore, magma ascent has most likely been controlled by fault location, while the final emplacement of plutons probably resulted from the rheologic contrast between peridotite and sedimentary basement rocks.
St Louis intrusive is composed of relatively homogenous, hornblende-biotite granodiorite. The Koum intrusives appear more heterogenous, and display contrasting hornblende-biotite granodiorite and biotite tonalite. Both contain numerous microgranular dykes and comagmatic enclaves. The mesocratic hornblende-rich enclaves likely represent remobilized early hornblende cumulates. Preliminary geochemical data infer slightly potassic calc-alkaline affinities and trace element ratios are similar to continental related magmatism. Intrusive rocks from the Koum massif appear more potassic and differentiated than those from St Louis and display more complex features possibly due to magmatic melange and/or higher crustal contamination.
We propose that magmatic production may be related to the abandonned slab. The weak magma production associated with this slab may be explained by the short lived subduction or/and by the vicinity to the trench which do not allow enough water production to induce large melting of the overlying mantle wedge.
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