Comparison of the history of the Southwest Alpine Foreland Basin (SWAFB) with that of the frontal North Alpine Foreland Basin (NAFB) provides a new insight into the evolution of the western Alpine arc. These basin stratigraphies show that the Alpine foreland basin first developed as a simple flexural basin from the Mid Eocene until the Early Oligocene. The stratigraphy of this phase is consistent around the arc and represents an underfilled foreland basin. Mapping and restoration of the diachronous pinchouts of the marine transgression suggest that the basin became gradually more arcuate with time. In the Early to Late-Oligocene shortening and flexural subsidence of the Central Alps were partitioned from those of the Southern Subalpine Chains (SSC) which became confined to the western sidewall of the advancing and thickening orogenic wedge. The NAFB continued to subside flexurally in front of the orogenic wedge accumulating a total of over 5 km of sediment. In contrast, the flexural basin in SE France was abandoned as the orogenic load slipped past to the NW generating very little deep rooted SW shortening or loading, although the Embrunais-Ubaye nappes were emplaced onto the basin largely by gravity gliding. The SSC (Southern Subalpine chains) underwent mainly uplift from this time onward due to continuous shortening. The Burdigalian seaway penetrated northward to the NAFB along the European rift system to provide conditions for deposition of the Upper Marine Molasse. The internal and external Alps continued to shorten and rise, providing detritus to the NAFB and leading to the return of continental conditions (Upper Freshwater Molasse). Finally uplift of the external massifs, coupled with thrusting at the outer margins of the Jura and Digne fold-thrust belts caused uplift of the whole external zones including the NAFB. Coevally the internal zones of the Alps were experiencing extension.
Dating of detrital minerals from synorogenic sediments allows to reconstruct the cooling history of the source area of these sediments. The major potential of this approach is the possibility to constrain paleo-cooling-paths which can be converted to exhumation rates of rocks that were exposed to the surface at the time when the sediments were deposited. Paleo-cooling-paths of the hinterland together with data on sediment composition constrain the linkage between the orogenic belt and the related foredeep basin.
We have dated detrital white micas from four sandstone samples representing four stratigraphic levels (31, 25, 21, and 14 Ma) of the Honegg-Napf dispersal system of the Central Swiss Molasse Basin. Petrographic data of sandstones and associated conglomerates indicate the erosion of carbonates and sandstones of Austroalpine and Penninic sedimentary cover nappes at 31 Ma. At 25 Ma and 21 Ma, there is a signifikant contribution of crystalline, mostly granitic rocks from Austroalpine and/or Penninic basement nappes. The youngest deposits at 14 Ma have a more complex provenance including the above mentioned source rocks and a high amount of micas and chlorites in the sand-sized fraction.
The Ar/Ar white mica ages display three distinct clusters: Early Carboniferous ages (350 - 330 Ma), late Carboniferous to Permian ages (310 - 265 Ma), and Tertiary ages (44 - 24 Ma). In the oldest sediments we find only micas of the oldest age cluster which were recycled from older most probably Penninic flysch-like sediments. The 25 and 21 Ma sediments display mica ages of both Paleozoic age clusters. The younger one (Carboniferous to Permian) corresponds to Late Variscan granite cooling ages. The first Tertiary cooling age ocurred in the sample with a 21 Ma stratigraphic age. The ages in the youngest sample (stratigraphic age 14 Ma) are mostly Tertiary with the youngest grain yielding 24 Ma. These young mica ages correspond to even younger zircon-FT-ages (see Spiegel et al.,1999).
The data reflect a normal unroofing sequence of the Central Alpine hinterland. The sudden increase of Tertiary ages from the 21 Ma to the 14 Ma sample is interpreted to reflect a significant contribution of material from the Lepontine core to the post 20 Ma Central Swiss Molasse basin. Assuming a geothermal gradient of 30°C/km results in an exhumation rate of the Lepontine of >1.2 mm/a for the Lower/Middle Miocene. For the same time interval average erosion rates based on sediment volume calculations are less than 0.2 mm/a. The data suggest that the exhumation of the Lepontine core complex was largely controlled by tectonic processes.
Spiegal et al., J. Conf. Abs. 4, (1999)
The clastic wedge of the Gonfolite Lombarda Group (GLG) accumulated during Oligocene-Miocene times in the Southern Alps foredeep. It was generated on the inner side of the Alpine belt by the back-thrusting of the southalpine units on the Po Plain crust, contemporaneously with the exhumation of Central Alps metamorphic and magmatic units. The Oligocene to Lower Miocene Gonfolite Group (Como Conglomerates and Val Grande Sandstones) represents the depositional counterpart of the exhumation and erosion of the Central Alps units. Within the GLG the occurrence of clasts from the Tertiary Bergell Intrusion (TBI) is particularly interesting because their radiometric ages with the biostratigraphic age of the GLG, provide a strong, and still troublesome, chronological constraint to the reconstruction of the Central Alps uplift history. The aim of this study is to investigate how the fast exhumation of the TBI is recorded by the composition of the related clastic products. The study of the detrital mode of both sandstones and conglomerates of the GLG was undertaken in different stratigraphic sections within the stratigraphic interval studied.The main results of this study are as follows: i) the sandstone detrital mode within the Como Conglomerate clearly record a compositional evolution from feldspatolithic to feldspatic sandstones; ii) the related Q/F-F/L provenance indexes change from 0.7-1.1 to 0.6-3.4. clearly reflecting the evolution from a mixed plutonic-metamorphic source to a mainly plutonic source; iii) the conglomerate detrital mode records a progressive increase of plutonic pebbles coupled with a decrease of the metamorphic one; iv) attempts to estimate quantitatively the meaning of these results in terms of the geology of the source area, suggest an increase of the plutonic contribution to the detritus from 30 to 56%, and a contemporaneous decrease of the metamorphic contribution from 63 to 43% and of the cover rocks from 7 to nearly 0%. This general trend is abruptly interrupted during the Aquitanian when a return to a more mixed source is recorded both by sandstones and conglomerates at the top of the Como Conglomerate and in the Val Grande Sandstone. This reorganization of the Gonfolite source area probably reflects an infra-Aquitanian tectonic acme recorded also in several other neighboring areas.
Bernoulli D, Giger M, Muller DW, Ziegler UR, Eclogae geol. Helv, 86/3, 751-767, (1993).
In the study of collisional orogenic systems, the available set of information is generally bimodal: the structural and kinematic evolution is best recorded in the deformed pre-orogenic sequences outcropping in the fold-and-thrust belt, whereas the timing of deformation is generally inferred from the synorogenic deposits which occupy the adjacent foredeep basins. Although independent investigations from either setting are widely reported, studies where structural information from fold-and-thrust belts is related to straigraphic record from foredeep basins are relatively less abundant. Well exposed fold belt-foredeep contacts are, therefore, particularily favourable settings for the study of orogenic processes in that they provide valuable information for both kinematic history and timing of mountain building.
The Mt Sibillini Range and the adjacent Laga Basin, in the Umbria-Marche Apennines of Italy, provide an excellent opportunity to investigate the interplay between deformation and sedimentation processes. Hinterland-dipping normal faults affect the Messinian Laga Fm., and are truncated by the Late Messinian-Early Pliocene Mt Sibillini thrust. These relationships poses several questions on the origin of pre-thrusting normal faults: i) What is their tectonic and geodynamic significance? ii) Should we expect extension in a foredeep basin adjacent to an advancing thrust front? iii) How fast were both pre-thrusting extension and subsequent thrusting? Stratigraphic-structural evidence provided a key to successfully answer these questions.
The results from cross-section restoration along the Mt Sibillini thrust permit to define the areal extent and volume of the Laga Basin, and to evaluate the amount of contraction produced by both folding and thrusting across the belt. These results provide relevant quantitative constraints to the Neogene evolution of the Central Apennines, and show that an integrated structural-stratigraphic approach is essential for a better understanding of foredeep basin evolution at active plate margins.
The evolution of chain-foredeep-foreland systems is characterised by contractional and extensional structures: the former operate at the chain-foredeep boundary, whereas the latter develop at the foredeep-foreland transition zone, often isolating uplifted peripheral bulges. As a consequence, the architecture of most foredeep depressions is generally envisioned as controlled by coeval thrusts and normal faults active in their inner and outer edges, respectively.
New surface data, integrated with available commercial seismic profiles across the outer zone of the Central Apennines of Italy, indicate that normal faults were, in fact, active in the inner edges of Tortonian, Messinian and Early Pliocene foredeeps. These faults are abundant and particularly clear in the youngest (i.e. Early Pliocene) basin, where the overprint of later contraction was mild; however, pre-thrusting normal faults are also recognised in the oldest (i.e. Tortonian and Messinian) basins, which experienced significant folding and were eventually thrust towards the foreland. A common feature of all foredeep basins is a central trough, triangular in map view, which is bounded by synsedimentary extensional faults on all sides. This geometry is reflected by the arcuate shape of the thrust fronts, and by the present distribution of fold axial culminations. When seen in cross-section, instead, the location of the main thrust ramps approximately coincides with that of structural highs bounded by normal faults, further outlining the controls of pre-thrusting extension in the final shape of the fold-thrust system. Foreland-dipping normal faults were generally buckled, rotated in the forelimbs of thrust-related anticlines and eventually reactivated as high-angle thrusts, whereas hinterland-dipping normal faults were more frequently truncated by younger thrusts propagating upwards with shortcut trajectories.
The architecture of sequentially younger foredeeps, as reconstructed using surface and subsurface data, yields important implications for the tectonic evolution of the Central Apennines. The differences in structural elevation, characteristic of the belt, appear to result mainly from pre-orogenic extensional deformations. Detailed stratigraphic controls on the age of synorogenic deposits indicate that the transition from extension to contraction was extremely rapid. These integrated lines of evidence constrain the estimate of the shortening experienced by the
Our study demonstrates that monazite ages from Himalayan foredeep sediments can constrain metamorphic events, erosion and exhumation history of the mountain belt that is consistent with thermal calculations from the hinterland. The sedimentary record preserved in the Himalayan foreland basin provides a record of the orogens evolution through time, a record often obscured in the mountain belt itself by later metamorphism, tectonism or erosive events. Previous studies of single grain ages from the Himalayan foredeep have used a number of different minerals e.g. zircons (DeCelles et al. 1998) and muscovite (Najman et al. 1997) to constrain Himalayan source terrains, tectonic evolution, and the timing and extent of erosion and exhumation.
In this study we focus upon the use of detrital monazite as a sensitive provenance indicator. We have measured the U-Th-Pb ages of single grains using laser ablation sampling coupled to a plasma ionisation multicollector VG P54 mass spectrometer. Samples were obtained from a magnetostratigraphically dated 2 km section of the Dharamsala Formation. This detritus, fluvial continental molasse, was shed from the uplifting Himalaya during the period 20-11 Ma (Maithani & Burbank, unpublished data). Unlike conventional U-Pb dating, ICPMS laser ablation techniques can detect inheritance, thereby providing more accurate single-grain age determination. Comparison with the depositional age of the host sediment permits constraints on exhumation rates.
206Pb/238U and 208Pb/232Th ages obtained using the laser ablation technique have errors of approximately 4-10% and are interpreted to represent the age of metamorphism or protolith in the Himalayan hinterland. Our results show populations between 1000-1300 Ma, 800-900 Ma and 500 Ma. These are similar to ages of detrital zircons within the Greater Himalayan sequence (GHS) of Nepal, which was subsequently strongly metamorphosed during the Tertiary (Parrish and Hodges, 1996). However, monazites also record significantly younger ages. Inheritance was detected in one monazite with a mixed age, interpreted as being an older grain of approx. ~ 250 Ma with a rim of 30-40 Ma. Three further grains gave young ages of between 30-35 Ma, two of which came from the base of the sedimentary succession, with a depositional age of 20 Ma. These data provide strong evidence for an Eo-Himalayan metamorphic event. Our results imply that this early metamorphic event, which occurred within 20 Ma of collision, was being exhumed and eroded within 10-20 Ma of metamorphism. This is consistent with a period of rapid exhumation of the Indian plate in the interval between 20-25 Ma (Chamberlain, 1991).
Decelles PG et al, GSA Bulletin, 110, 2-21, (1998).
Najman YMR et al, Geology, 25, 535-538, (1997).
Parrish RR & Hodges KV, GSA Bulletin, 108, 904-911, (1996).
Chamberlain C et al, Journal of Geology, 99, 829-849, (1991).
The older (Palaeogene) sediments of the Himalayan foredeep preserve a record of early Himalayan evolution, largely obscured in the mountain belt itself by later metamorphic, tectonic and erosive processes. For the foredeep deposits to be used as an effective record of events in the mountain belt, the sediments must be accurately dated. This has been possible in the oldest sediments (Late Palaeocene - early Mid Eocene) using marine fauna (Mathur 1978) and the younger sediments (Early Miocene to recent) using magnetostratigraphy (Meigs et al. 1995) but sediments of the intervening period are fossil free and too deformed for magnetostratigraphic dating. Constraining the ages of these sediments has been possible by dating individual detrital muscovites using Ar-Ar techniques (Najman et al. 1997) and fission-track dating of detrital zircons.
Where an independent dating method exists for the depositional age of the sediments, periods of rapid exhumation and erosion of the mountain belt can be deduced from times when the detrital mineral age is equivalent to the host sediments depositional age. Detrital mineral ages have also helped constrain sediment provenance and therefore the timing of thrusting and uplift of various Himalayan tectonic source domains. Constraint of provenance and consequently early Himalayan tectonic evolution has been further constrained by the use of Sr and Nd isotopes, geochemical analyses (XRF) and petrographic study, which have shown a switch from suture-zone type rocks to Higher Himalayan thrust belt source, and erosion to deeper metamorphic levels with time.
This multi-disciplinary approach has been successful in using the dated sedimentary record to document the many facets of the early history of Himalayan evolution.
Mathur NS, Recent Researches in Geology, 5, 96-112, (1978).
Meigs AJ, Burbank DW & Beck, RA, Geology, 23, 423-426, (1995).
Najman YMR, Pringle MS, Johnson MRW, Robertson, AHF & Wijbrans, JR, Geology, 25, 535-538, (1997).
The Alpine evolution of basins developing to the North of the Great Caucasus has been studied in detail using geological and geophysical data including a database of more than 130 wells located in North Caucasus region. The evolution of these basins is related to three main stages [Mikhailov et al, 1999]: 1. Early Jurassic/Late Cretaceous. This stage correspond to the initial rifting ,phase. The formation of the basins was driven by left-lateral transform movements [Stamply and Pillevuit, 1993] that controlled the development of pull-apart basins in the central part of the Great Caucasus trough. Shear stresses within the lithosphere of the Great Caucasus can be related to oblique subduction and even transform movements at the plate boundary to the south of the Caucasus [Dercourt et al, 1993].
2. Late Cretaceous/Middle Eocene. It is characterised in the North Caucasus area by alternating subsidence and uplift events of considerably low amplitude (at least up to the Maastrichtian). Since the Late Paleocene almost all the subsidence curves reflect the same short term events, on the background of rather fast subsidence at nearly constant rate. Nevertheless there are strong differences in the rate of movements, that are considerably higher in the west and to the north of the central part. The strong differences in the evolution of the eastern, western and central parts are related to the closure of the Lesser Caucasusoceanic basin and the collision of the Nakhichevan block. This led to changes in the configuration of the plate boundary, in reorganisation of stress and displacements within the Caucasus region. The rapid subsidence of the western and central parts of the area in the end of this stage is related to these events and the opening of the Eastern Black Sea.
3. Middle Eocene/Present. The development of the foreland basins is coeval with shortening and uplift in the adjacent Great Caucasus range. Four main stages of compression are revealed by the subsidence curves in Early Maykopian (between 39.5 and 36.0 Ma),Tarkhanian (16.6-15.8 Ma), Konkian-Early Sarmatian (14.3-12.3 Ma) and Pontian (7.0-5.2 Ma) time. Analysis of geological and geophysical data has shown that the formation and evolution of the North Caucasus foredeep can not be dealt exclusively with elastic flexure of .the lithosphere. A model of a small-scale convection within the asthenosphere [Mikhailov et al., 1996] has been used to explain deep subsidence at both sides of collisional belt. Different width of the Great Caucasus trough by the beginning of the compression, as well as variations in thickness of the lithosphere and different thermal state that can cause interruption of foredeep formation at the Stavropol high.
Mikhailov, V.O., Panina, L.V., Polino, R., Koronovsky, N.V., Klavdieva, N.A., Kiseleva, E.A., Tectonophysics, (subm.).
Stamply B, Pillevuit A., Atlas Tethys Paleoenvironmental Maps, 55-62, (1993).
Dercourt, J, Ricou, L.E., Vrielynck, B., Atlas Tethys Paleoenvironmental Maps, 397 pp, (1993).
Mikhailov, VO, Myasnikov, VP, Timoshkina, EP, Physics of the Solid Earth, 32(6), 496-502, (1996).
Collisional history of the Caucasus segment of the Alpine-Himalayan fold belt started at the end of the Eocene and came into the main phase at the midle Miocene. In-plane configuration of the Northern Caucasus molasse basin contradicts the hypothesis, which considers topographical loading as the main control on foreland subsidence. The deepest areas of the basin are on the tips of the orogen, where mountain heights are negligible; the area adjacent to the highest mountains (Central Caucasus) is uplifted. A clear anticorrelation between basin depths and orogen heights exists. This fact has forced us to investigate influence of other (than topographical) types of loading. A good candidate for this role is the change of the crustal/lithosphere thickness due to shortening and stacking of material or removal of lithospheric roots. It can produce either additional buoyancy forces due to the thickening of the crust or a loading due to the thickening of the subcrustal lithosphere, which is colder and heavier than asthenosphere, as it was pointed by Brunet (1986) and Royden (1993) among others. The last factor is especially important during the initial stages of collision when orogen is not high, later the removal of the lithospheric root could take off this loading and produce uplift of the area. The importance of these two factors during orogenesis was recognised for many orogens (e.g. Burg and Ford, 1997). The results of flexural and gravity modelling carried out for different parts of the Caucasus and its foreland demonstrate, that crustal thickening and removal of lithospheric roots are responsible for the support of high Central Caucasus mountains and uplift of the adjacent area. The subsidence of the basins on the orogen tips is explained by the loading of lithospheric roots. Both crustal thickening and loading of lithospheric roots are important in the geographically intermediate areas. In general, we conclude that topography should not be considered as the main control of foreland subsidence, but only as one of the counterbalancing mechanisms (side by side with foreland subsidence) to the other, more significant types of loading. So, the existing flexural model needs improvements to relate directly "subsurface loading" to the real structures and processes during collision.
Brunet M-F, Tectonophysics, 12, 343-354, (1986).
Burg J-P & Ford M, Burg J-P & Ford M(eds), Orogeny through time, Geol Soc Spec Publ, 121, 1-17, (1997).
Royden L.H., Tectonophysics, 303-325, (1993).
The Baltic Basin was defined as the foreland basin that evolved along the North German-Polish Caledonides (NGPC) during Silurian times.
The thickness of the Silurian carbonaceous-shaly succession increases towards the southwest reaching 4 km, while the thickness in the east is about 80 m. Deep-marine graptolite shales predominate in the southwest grading to the shallow-water marls, limestones and dolomites in the east. It shows deepening of the basin towards NGPC. Though it was associated with no remarkable increase in sediment thickness during Early Silurian, that implies the starvation stage of the basin development. The succeeding significant increase of the subsidence was accopmanied by increase of sediment influx, gradual narrowing and shallowing of lithofacies during Late Silurian that implies the compensated regime of the sedimentation. During Early Silurian the inflow of terrigenous sediments derived from the eastern sources prevailed. Since Late Wenlock sediments were derived mostly from the active orogens in the west (Lapinskas,1996).
The quantitative modelling of the Silurian sedimentary in-fill of the Baltic Basin was compiled by using the programme SEDPAK 2.0 developed by Ch. Kendall (1991), allowing to incorporate the influx of sediments from both sides, the combination of clastic sediments and in situ carbonate growth and component of crustal rigidity (Kendall et al., 1989).
The model results show Fennoscandia-Sarmatia inland and adjacent Caledonian Orogen being main provenances for the two-side sediments supply of the Silurian Baltic Basin. Some models of clastic and carbonate sediments evolved through time were created to test the sensitivity and interaction of tectonic movements, eustasy, sediment accumulation rate and their effect to the architecture of the basin. The modelled lithofacies distribution show close connection of the sedimentation regime to the tectonic processes in the Caledonian Orogen. Influence of rheological properties of the lithosphere was incorporated to the model as well.
Kendall Ch, Mitch H, Strobel J, Cannon R, Moor P, Bezdek J & Biswas G, Abstracts of the 28th International Geological Congress, Waschington, 2-174, (1989).
Lapinskas P, The oil-bearing complexes in Lithuania, Geological Institute, 27-34, (1996).
The early orogenic evolution of the Eastern Alps was characterized by southward oblique subduction of Penninic oceanic lithosphere below the Austroalpine microplate at the northern margin of the Adriatic plate. During the Cretaceous an orogenic wedge was initiated, which shows a complex history of compression vs. extension. Sedimentary basins formed on top of the orogenic wedge, e.g. within the Northern Calcareous Alps (NCA). The northern foreland of the wedge comprises deep-water basins of the Rhenodanubian Flysch zone and shelf basins along the margin of the Bohemian massif. Synorogenic basins of the NCA show in-sequence-arrangement starting in southern (internal) units and propagating to the northern (external) units. Subsidence rates of forethrust and piggyback basins of the Early and Mid Cretaceous are relatively low, reaching 100 m/Ma. During the Mid Cretacoeus an extremely long (> 500 km) and narrow (30 km) deep-water basin developed in front of thrust sheets, which can be followed from the western NCA to the western Carpathians. Provenance of sandstones is mainly from accretionary slices to the north. After Mid-Cretaceous compression pull-apart and extensional Gosau basins in the Late Cretaceous are characterized by subsidence rates as high as 500 m/Ma. A significant younging of subsidence from the western NCA to the western Carpathians is proven. Peak sedimentation rates (580 m/Ma) are reached during the Late Turonian-Santonian. Flysch basins to the north of the NCA suggest a first weak coupling to orogenic wedge evolution during the Late Cretaceous, when sedimentation rates significantly increased from below 20 m/Ma to more than 150 m/Ma, coeval to a major subsidence pulse in the Austroalpine basins. Nevertheless, the provenance of deep-water sandstones is still mainly from sources outside the orogenic wedge. Strata at the shelf of the Bohemian massiv show no correlation to Cretaceous tectonic events within the Austroalpine plate. Sedimentation rates stayed below 100 m/Ma. Indications for forebulge migration are recognized only from Eocene onwards.
Current paleogeographic models of the Alpine foreland basin in early Oligocene times assume a tens of km wide, marine connection between the western Mediterranean, the Paratethys, and the North Sea. The foreland basin is supposed to form an axial orogen-parallel, eastward deepening submarine depression, directing sandy and pebbly material to the east, forming pebbly turbidite layers. Marls, characteristic for the eastern Swiss and German molasse sedimentation of this period, indicate a quiet environment. The finding of reworked Cretaceous foraminifera from Alpine sources in the Rhine Graben and further to the N (FISCHER 1965) is in conflict with this paleogeographic scenario. Our zircon-FT data from sand samples in the Rhine graben support the hypothesis that large amounts of material from the Alps have been deposited. Medium-grained, well sorted sandstones intercalated in early Oligocene marine clay (ca. 30-28.5 Ma), S of Mulhouse were transported from the S by currents, as indicated by frequent current ripple marks and occasional turbidites. A garnet-staurolite-apatite dominated heavy mineral spectrum indicates an Alpine source. The zircon-FT age spectrum shows peaks at around 60 and 140 Ma, and a broad range of mixed Alpine-Variscan ages. This spectrum clearly reflects the dominant Alpine provenance of the zircons and cannot be derived from the surrounding Variscan basement. The currently assumed foreland basin geometry would prevent the crossing of sandy material from the Alps to the Rhine Graben. Transport to N may have been performed by storms in a shallow marine environment in the western Swiss Molasse basin. This implies that the deeper, orogen-parallel depression would have a western ending in the eastern Swiss Molasse basin. Between the Molasse basin and the Rhine graben a marine N-S connection ("Rauracic depression") with fairly strong currents to the N must have existed.
Fischer H, Beitr. geol. Karte der Schweiz, 122, 106 pp, (1965).
In the upper continental crust, compressive tectonics are express by thrust faults, with erosion at their hanging wall and sedimentation at their footwall. The study of thrust systems around the edge of compressive intermountain Tertiary basins of the Iberian Chain suggests that synthrusting sedimentation should influence the structure of the thrusts. Indeed, the degree of complexity of fault systems seems to increase with increasing amonts of syntectonic deposits of alluvial fans at fault front. The settling rate of deposits during the deformation and their spatial localization are two parameters which appear to influence the fault geometry. The effect of these parameters has been investigated on 2-layer brittle-ductile analog models submitted to compression. In these experiments, the sedimentation rate controls the number and the dip of faults. For low sedimentation rates, a single thrust dippings at low angle is observed; whereas for high sedimentation rates, a series of steeper dipping thrusts is observed. In experiments whith changing sedimentation rates along strike, these two fault patterns alternate consistently. Comparison of experimental results and field data shows that the syntectonic sedimentation in a compressive context can substantially influence fault patterns at basin boundaries.
This study comprises part of a multidisciplinary approach aimed towards correlating the uplift and exhumation history of the Eastern Andean Cordillera with the development of the Oriente foreland basin in Ecuador.
The Ecuadorian continental margin has been active for ~210 My, starting within the Tethyan spreading system and currently overiding the Nazca plate. During the latter part of this history, the double-vergent Andean Cordillera and associated Oriente foreland basin developed. In the east, this geodynamic system can be resolved into an orogenic basement zone, an orogenic wedge defined by a series of frontal thrusts which have formed foothill highs (Sub-Andean Zone, SAZ), and the Oriente Basin.
The Maastrichtian-Paleocene Tena formation is commonly considered to represent the first sedimentary deposit in the SAZ and Oriente Basin. However, conspicuous unconformities between Upper-Jurassic to Lower-Cretaceous volcanics and oil-bearing Aptian-Campanian Hollin and Napo Formations suggests a pre-Maastrichtian orogenic wedge depositional environment.
To improve the understanding of the Oriente Basin we are using the combined approaches: (1) Sedimentological and stratigraphic analysis of facies distribution, the geometries of sediment sequences, uncomformities and provenance analysis of Lower Cretaceous to Tertiary formations. These results will be used to test different foreland basin models, e.g DeCelles & Giles, (1996) and Catenuanu et al., (1997) in order to understand the chronological development of the Oriente foreland Basin and the Cordillera so as to ultimately investigate the kinematics of the Nazca plate (2) Apatite and zircon fission-track data are being acquired from the SAZ and the Cordillera to quantify the thermal and tectonic histories of these units. The technique permits a determination of the thermal history over the temperature range of ~390-170°C (Yamada et al., 1996) for zircon and ~110-60°C for apatite. Fission-track data from the SAZ and the Eastern Cordillera, combined with a sedimentological study, simultaneously reveal the provenance age of the SAZ sediments as well as quantification of the timing and amount of crustal denudation. Lateral variations in the low temperature thermal history (<110°C) across the entire region will provide estimates of the rate of foreland propagation of the SAZ thrust system which may currently bury older sequences of a pre-Maastrichtian foreland basin.
DeCelles, PG, & Giles, KA, Basin Research, 8, 105-123, (1996).
Catuneanu, O, Beaumont, C, Waschbusch, P, Geology, v. 25, no. 12, 1087-1090, (1997).
Yamada, R, Tagami, T, Nishimura, S, and Ito, H, Chemical geology, 122, 249-258, (1996).
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