Thin-skinned Thrust Belt Research Articles

Thrust tectonics in the Wetterstein and Mieming mountains, and a new tectonic subdivision of the Northern Calcareous Alps of Western Austria and Southern Germany

We investigate the tectonic evolution of the Wetterstein and Mieming mountains in the western Northern Calcareous Alps (NCA) of the European Eastern Alps. In-sequence NW-directed stacking of thrust sheets in this thin-skinned foreland thrust belt lasted from the Hauterivian to the Cenomanian. In the more internal NCA major E-striking intracontinental transform faults dissected the thrust belt at the Albian–Cenomanian boundary that facilitated ascent of mantle melts feeding basanitic dykes and sills. Afterwards, the NCA basement was subducted, and the NCA were transported piggy-back across the tectonically deeper Penninic units. This process was accompanied by renewed Late Cretaceous NW-directed thrusting, and folding of thrusts. During Paleogene collision, N(NE)-directed out-of-sequence thrusts developed that offset the in-sequence thrust. We use this latter observation to revise the existing tectonic subdivision of the western NCA, in which these out-of-sequence thrusts had been used to delimit nappes, locally with young-on-old contacts at the base. We define new units that represent thrust sheets having exclusively old-on-young contacts at their base. Two large thrust sheets build the western NCA: (1) the tectonically deeper Tannheim thrust sheet and (2) the tectonically higher Karwendel thrust sheet. West of the Wetterstein and Mieming mountains, the Imst part of the Karwendel thrust sheet is stacked by an out-of-sequence thrust onto the main body of the Karwendel thrust sheet, which is, in its southeastern part, in lateral contact with the latter across a tear fault.

Where is There Chance to Find Deep Prospects Below the Outer Western Carpathian Thrust Belt?

In the period of 80 and 90th, the intensive deep drilling and seismic profiling program in the Outer Western Carpathian belt has been realized for confirming the idea about the existence and opening of new promising plays beneath some of these thin-skinned belts. Many trans-Carpathian 2D seismic transects opened discussion on the understanding of the complex structure of thrust belts and its basement. The recent development of exploration, based on the new modern play concepts, has brought new light on this problem. The new 3D and the reprocessed 2D seismic sections from Czech and Slovak part of the Western Carpathian Flysch Belt disclosed several potentially new prospects, combined with the cover of the foreland of European plate. New significant reserves of hydrocarbons may occur in subthrust autochthonous and parautochthonous series buried below the frontal zones of thin-skinned thrust belts. The study of the deepest parts of the Carpathian Flysch belt has been based on the complex geochemical, structural analyses and geophysical Data reprocessing, supplied by verification along the chosen balanced sections. Four examples of 2D seismic transects, with registration up to 9-12 seconds, that present different tectonic style of structures of Flysch Belt and its platform basement, influenced by older large-scale faults, is presented from the western, northern and eastern parts of the Outer Western Carpathians. From point of view of hydrocarbon prospection, the most important features of the seismic profiles are the anticline structures of the North European Platform (NEP) below the thrust stack of the Flysch Belt. From the west to east the Týnec-Cunín, Drietoma, Orava and Zbudza elevations can be distinguished, created by a passive margin of the European plate, in the second and third structures probably by parautochthonous blocks of the same plate.

A sub-salt structural model of the Kelasu structure in the Kuqa foreland basin, northwest China

The Kuqa foreland basin, adjacent to the South Tianshan Mountains, is a major hydrocarbon accumulation basin in Western China. The Kelasu structural belt is the focus for hydrocarbon exploration in the basin due to the presence of ramp-related anticline traps and a thick salt seal. The model of the Kelasu sub-salt structure is still contentious because of the structural complexity and poor seismic imaging below the salt layer. The area–depth–strain (ADS) method is applied to the southern part of the Kelasu Fault, a regional fault that cuts basement rocks. The ADS results are consistent with the seismic data, which indicate that both thin-skinned thrusting and basement-involved deformation occur within the Kelasu structure, with the Kelasu Fault acting as the boundary between the two regions of contrasting deformation. The ADS results also suggest that the depth of the lower detachment of the thin-skinned thrust belt is 9.5–10 km, which may correspond to the base of the Triassic. The Kelasu structure has undergone approximately 8.15–10.76 km of horizontal shortening in the east and 16.34 km in the west of the structure.

Mesozoic geology of southwestern China: Indosinian foreland overthrusting and subsequent deformation

The southwestern part of the South China Block (SCB) records Triassic and subsequent deformations and is a key region that provides evidence of the post-amalgamation tectonic evolution of Southeast Asia. Here, we outline the tectonothermal evolution of early Mesozoic orogenesis in this region using structural analysis and fission track thermochronology. This region is divided into three tectonic units (south to north): the Youjiang Fold-and-Thrust Belt (YFTB), the Qianzhong Massif (QZM), and the Thin-skinned Thrust Belt (TTB) of the southeastern Sichuan Basin, all three of which record three deformation events, here termed D1, D2, and D3. D1 deformation is represented by Triassic top-to-the-north thrusts (F1) and fault-related folds (f1) that are indicative of N–S shortening. The intensity of deformation decreases toward the north. The subsequent D2 and D3 deformations are marked by Cretaceous top-to-the-NW thrusting and fault-propagation folding, and Cenozoic NE–SW trending normal faults (F3), respectively. The temperature–time (t–T) path obtained by apatite fission-track modeling of samples from the YFTB, QZM, and TTB areas provides evidence of uplift and denudation. The D1 deformation at ∼230–210Ma is characterized by buried thrusts within the relatively stable YFTB and QZM, and an increase in depth of the TTB. This was coeval with the Late Triassic northward migration of uplift and denudation that is evidenced by a Late Triassic stratigraphic gap within the YFTB, a thin layer of coarse-grained continental clastics within the QZM, and a thick layer of clastic rocks within the TTB. The study area records a Triassic thrust system (D1) that progressively migrated northwards and represents a foreland fold-and-thrust belt that formed during the Indochina–SCB collision. This initial deformation was subsequently overprinted by Late Jurassic to Cretaceous NW–SE thrusting (D2) and the development of Cenozoic NW–SE extensional structures (D3).

Deformación andina en el cordón del Hielo Azul al oeste de El Bolsón. Implicancias en la evolución tectónica de la Cordillera Norpatagónica en Río Negro, Argentina

This geological study, carried out in the Cordon del Hielo Azul, west of the town of El Bolson (Rio Negro province, Argentine), shows the different relationships between the Early Jurassic cordilleran volcano-sedimentary sequence, the Middle-Late Jurassic Cordilleran Patagonian Batholith and the Cenozoic volcanic rocks of the Ventana Fornation exposed in the Patagonian Cordillera of Rio Negro. Through structural analysis, a series of previously unknown complex deformational structures have been identified. It has been observed that the sequence of volcanic rocks that outcrop on the Cerro Lindo, just west of El Bolson, is bounded by a backthrust in the eastern side and by an out of sequence thrust to the west; two other thrusts affect the sequence, with associated folded structures. To the north, on the west side of Cerro Hielo Azul, these Lower Jurassic volcanic rocks are in tectonic contact with the Cenozoic rocks of the Ventana Formation. Based on these observations, we inferred that an extensional fault controlled deposition of the Cenozoic volcanic rocks, being subsequently inverted, superimposing these rocks on the Jurassic volcanics. The different recognized faults exerted a first order control in the building of the mountain front and are robust evidence to establish the tectonic framework and uplift history of the North Patagonian Andes. This deformation took place from the Eocene to the Late Miocene. A pre-Oligocene contractional pulse led to the development of a thin-skinned fold and thrust belt affecting the Jurassic volcanics. During the Oligocene a regional extensional event resulted in the formation of basins such as the one recognized on Cerro Silvia, where the Ventana Formation crops out. A second contractional pulse took place during the Miocene and led to the development of a thick-skinned fold and thrust belt with deep faults that built the present mountain front.

Late orogenic faulting of the foreland plate: An important component of petroleum systems in orogenic belts and their forelands

One of the many factors affecting the petroleum potential of thin-skinned thrust belts and their foreland plates is late orogenic normal, reverse, and strike-slip faulting, which occurs in the foreland plates and occasionally extends into the overlying thin-skinned thrust belts. This tectonism is typically active during the late stages of convergent orogeny, when subduction-driven thin-skinned thrusting ceases and the remaining convergence and geometric adjustment occur mainly within the foreland plate. The distribution, character, and orientation of particular structures generated during this collisional event, however, is to a great extent determined by the geometries of crustal blocks and orientations of preexisting faults and other weak zones within the foreland plates. As a result, despite the similar orogenic stress field, the dominant geometric expressions in various parts of the orogenic system may be different. Because massive petroleum generation and migration in both the thrust belt and in the foreland plate commonly occur later in the orogenic phase, late orogenic faulting can critically affect the whole petroleum system, including the generation, migration, and preservation of hydrocarbons. The potential existence of late orogenic tectonics should then be examined thoroughly, especially when exploring for petroleum at deeper structural levels of thrust belts and the underlying foreland plates. This is demonstrated by three examples from the Vienna Basin, Eastern Carpathians, and Dinarides-Hellenides, all parts of the European Alpine system. Several other examples, including the Rocky Mountain Laramide uplifts, the Eastern Cordillera in Colombia, and Timan-Pechora inverted structures in Russia, are mentioned to document the common occurrence of late orogenic faulting in orogenic systems.

Estimating basal friction and fluid pressure from the geometry and spacing of frontal faults in Naga fold thrust belt, India

Naga fold thrust belt is an emerging destination for petroleum exploration in eastern India. It is a thin skinned FTB where new initiatives are being taken after a long gap of initial discoveries. In absence of sufficient drilling or quality seismic data in the southern part of the fold thrust belt, fluid pressure regimes remain largely speculative. Using the concept and the formulation for efficient coefficient of basal friction, we have revisited the method and concept of pressure-dependent Coulomb wedge theory for thin skinned thrust belts for deriving the fluid pressure ratio in Naga fold thrust belt. The efficient coefficient of basal friction on the decollement and the fluid ratio of Naga fold thrust belt are estimated to be 0.22 and 0.85 respectively. This indicates an overpressure situation in the wedge. This method of estimating basal friction and fluid pressure is more case-specific and can be obtained from the data of thrust initiation angle and thrust spacing.

Anomalous clastic wedge development during the Sevier-Laramide transition, North American Cordilleran foreland basin, USA

New, high-resolution, regional correlation and isopach maps provide evidence that (1) Laramide-style deformation began as early as ca. 77 Ma in central Utah, and (2) rapid (208 km m.y. –1 ) and extensive (400 km) progradation of a clastic wedge was facilitated by reduced subsidence during the transition from Sevier- to Laramide-style deformation. This study defi nes three Campanian, alluvialto-marine clastic wedges that traversed 200– 400 km eastward across the Utah-Colorado segment of the Cordilleran foreland basin. Wedges A and C are thicker successions with rising-trajectory shoreline stacking patterns (Blackhawk Formation and Lower Castlegate Sandstone, Bluecastle Tongue and Rollins Sandstone) that refl ect relatively slow overall progradation (50–81 km m.y. –1 ) of narrow (10–20 km wide), wave-dominated shorelines. In contrast, wedge B consists of lower-volume successions with a fl at to falling shoreline stacking pattern (Middle Castlegate Sandstone, Sego Sandstone, Neslen Formation, Corcoran and Cozzette Members of the Iles Formation) that suggests rapid progradation (~208 km m.y. –1 ) of embayed (60–80 km wide), mixed-energy (wave- and tide-infl uenced ) shore lines. Wedges A and C prograded 200–250 km in more than ~3 m.y., whereas wedge B prograded 340–400 km in ~2 m.y. (~170 km m.y. –1 ). The anomalously extensive wedge B is unique in the Utah-Colorado segment because of its long extent, rapid progradation rate, dominance of tidally infl uenced facies, long-transit transgressions/regressions, and low-accommodation, nested sequence architecture. Stratigraphic relationships indicate development of wedge B coeval with both Sevier- and Laramide-style deformation in Utah. Assuming a constant sediment supply, the extensive (>300–400 km) “sheet-like” amalgamated wedge (wedge B) may have been caused by (1) reduced subsidence driven by a fl exural interference pattern, whereby two short-wavelength (±200 km) fl exural profi les are superimposed during the uplift of both basement-cored and thin-skinned thrust belt, (2) an increase in, or eastward migration of, dynamic subsidence during tectonic transitions associated with slab fl attening or rollback (i.e., long-wavelength fl exure), or (3) reduced subsidence due to short-wavelength fl exural interference augmented by the reduction, or migration, of long-wavelength fl

Longmen Shan fold-thrust belt and its relation to the western Sichuan Basin in central China: New insights from hydrocarbon exploration

The Longmen Shan fold-thrust belt is one of the key regions of demonstrable Mesozoic–Cenozoic tectonic evolution in China, and the Sichuan Basin was the first natural-gas-producing area in China. In this article, the structural features of the Longmen Shan belt are presented, using both seismic profiles and field data. The complex structures of the northeast-trending Longmen Shan fold-thrust belt and its foreland in the western Sichuan Basin are formed by southeast-directed thrusting. Several eastward-verging, rootless thrust sheets and imbricates of Cambrian–Triassic rocks have been recognized in the northern Longmen Shan belt. Evidence suggests that the northern Longmen Shan belt experienced at least two major periods of deformation in the Late Triassic and Cenozoic. However, the southern Longmen Shan belt is represented by the basement-involved thrust structures and klippen, and its major periods of deformation were in the latest Cretaceous–early Cenozoic. Sedimentary features in the western Sichuan Basin reflect a two-phase flexural-loading history and illustrate that the Late Triassic foreland basin extends along the foredeep of the entire length of the Longmen Shan belt, but the uppermost Cretaceous–Paleogene rejuvenated foreland basin is restricted in the southern part of the western Sichuan Basin. Structural geometries suggest that prospective traps are mainly developed in the frontal zone of the Longmen Shan fold-thrust belt and in the southern part of the western Sichuan Basin. One of the major contributions of this article is finding preexisting Paleozoic rift basins under the Cenozoic thin-skinned thrust belts, which represent a new potential hydrocarbon play.

Petroleum geology and resource assessment: 1002 area, Arctic National Wildlife Refuge

In 1998, a new assessment of the geology and hydrocarbon resources in part of the Arctic National Wildlife Refuge (ANWR) in Alaska was prepared by the U.S. Geological Survey and is here summarized. The ANWR is located between the petroleum-rich provinces of Prudhoe Bay to the west and the Mackenzie delta to the east. The new assessment is focused on the coastal-plain part of the ANWR (the so-called 1002 area), where hydrocarbon trends found in these adjacent areas are believed to exist. Significant new information from recent wells, field studies, laboratory analysis, and newly reprocessed and reinterpreted geophysical data support the identification of 10 major petroleum plays in the 1002 area. Four of these plays, identified by seismic facies mapping as topset, turbidite, wedge, and thin-skinned thrust belt plays, hold more than 85% of the interpreted total resources. These plays exist in Upper Cretaceous–Tertiary foreland basin strata (Brookian sequence) and can be related to three petroleum systems, which suggest that oil was generated, expelled, and entrapped mainly between the Eocene and Miocene. Total estimated volumes for the 1002 area are in the range of 11.6–31.5 billion bbl oil (95th to 5th percentile probability) for in-place resources and 4.8–11.8 billion bbl for technically recoverable resources. Because no drilling has occurred in the ANWR itself, these estimates must be understood as hypothetical. The volumes noted represent a significant increase over those derived by an earlier assessment of the same area jointly conducted by the Bureau of Land Management and the U.S. Geological Survey.

The Caledonian thin-skinned thrust belt of Kronprins Christian Land, eastern North Greenland

Kronprins Christian Land in the extreme north of the East Greenland Caledonides, exposes a thin-skinned thrust belt up to 50 km wide developed in Ordovician–Silurian platform limestones and dolostones of the Iapetus passive margin. This thrust belt is characterised by a series of SSW–NNE-trending and east-dipping Caledonian thrusts with westward displacements of generally a few kilometres each. It passes westwards into undisturbed autochthonous foreland. Based on a line and area restoration, total displacement along a well-exposed WNW–ESE section through the thrust belt amounts to 17.6 km, which represents a shortening of 45% in the line of section. Biostratigraphic control in the limestone and dolostone succession is based on conodonts and macrofossils. The alteration colours of the conodonts provide estimates of maximum burial temperatures, which show that the thickness of the overlying thrust sheets ranged from about 6 to 12.5 km from west to east across the thrust belt. Since the estimated former thickness of the Vandredalen thrust sheet above the thin-skinned parautochthonous thrust belt is insufficient to yield the temperatures attained, higher thrust sheets must once have extended across the region.

The control of pre-existing extensional structures on the evolution of the southern sector of the Aconcagua fold and thrust belt, southern Andes

The Aconcagua fold and thrust belt, located in the Andean mountains at 32°30′ to 34°S, has been described as a classic model of a thin-skinned thrust belt. However, new structural data from its southern sector have shown that it has a complex structural framework reflected in multiple Mesozoic extensional phases, overprinted by structural inversion, as well as thin- and thick-skinned tectonics. Two major superimposed extensional structural styles have been identified for the Mesozoic characterized by distinctly oriented stress fields. A key role in the evolution of this part of the fold and thrust belt was played by a Late Triassic to Early Jurassic depocentre and by Late Jurassic block faulting. Shortening was accommodated by a combination of inversion of pre-existing normal faults, development of footwall short cuts and both thin and thick-skinned thrusting. Synrift and postrift sedimentary rocks were uplifted by reactivation of normal faults, with further shortening along newly formed thin-skinned thrust faults. The geometry of thin-skinned fault systems is controlled by the architecture of the rift basin, competent footwalls forming barriers to the lateral propagation of detachments.

Initiation of the Himalayan Orogen as an Early Paleozoic Thin-skinned Thrust Belt

Research by many workers in various regions of the Himalaya, combined with our recent geologic and geochronologic studies in Nepal, indicate that fundamental aspects of the Himalayan orogen originated in an early Paleozoic thrust belt and are unrelated to Tertiary IndiaAsia collision. Manifestations of early Paleozoic tectonism include ductile deformation, regional moderate- to highgrade metamorphism, large-scale southvergent thrusting, crustal thickening and the generation of granitic crustal melts, uplift and erosion of garnet-grade rocks, and accumulation of thick sequences of synorogenic strata. Determining the relative contributions of early Paleozoic versus Tertiary tectonism constitutes a significant challenge in understanding the Himalayan orogen.

Late orogenic strike-slip faulting and escape tectonics in frontal Dinarides-Hellenides, Croatia, Yugoslavia, Albania, and Greece

The frontal zone of the Dinarides and its foreland are characterized by the presence of major late orogenic northwest-southeast faults trending parallel to and locally dissecting the leading edge of the thin-skinned thrust belt. On the basis of their geometry and regional distribution, these structures are interpreted as right-lateral strike-slip faults and are related to the formation of en echelon compres sional structures in the Split-Dubrovnik area of the central Adriatic region and to the opening of the post-Messinian pull-apart Albanian foredeep (South Adriatic Basin) in the Pliocene-Holocene. These strike-slip faults represent the last stage of the mountain-building process in the Pliocene and Holocene, when the forelandward prop agation of the Dinaric thrust belt was inhibited by stacking of thrust sheets and by overall thickening of the colliding continental crust. In that respect, the Dinaric side of the Apulia (Adria) differs from the Apenninic side, where the thin-skinned thrusting progressed normally toward the foreland until the Pliocene. On a regional scale, these northwest-southeast-trending strike-slip faults in the frontal Dinarides and their apparent continuation to the Hellenides of western Greece, as well as the dextral Vardar fault system, indicate the existence of escape tectonics in the Di naric-Hellenic region. During the last stage of orogeny, the strike-slip faults provided a means of tectonic transport from the collision zone of Apulia with Europe toward the subduction zone of the Hellenic Trench. In southern Greece, the southeastward-moving Dinaric-Hellenic blocks apparently interfered with the westward-moving Anatolian plate and deflected it southwestward. The Albanian foredeep, in terms of tectonic setting, resembles the pull-apart Vienna basin, which formed within the attenuated southwest-northeast-trending transfer zone of the West Carpathi ans during the last stages of the Alpine orogeny. The late orogenic strike-slip faulting may have breached seals and thus negatively affected the hydrocarbon potential of the Dinaric-Hellenic side of the peri-Adriatic region, as contrasted with that of the more prolific Apenninic side, which was not significantly disrupted by the late orogenic strike-slip faulting.

Neoproterozoic–Lower Palaeozoic stratigraphical relationships in the marginal thin-skinned thrust belt of the East Greenland Caledonides: comparisons with the foreland in Scotland

Throughout the 1300 km long East Greenland Caledonides, the western exposed marginal thrusts overlie foreland rocks of latest Neoproterozoic–Early Palaeozoic age, mainly exposed in tectonic windows. In the western, 100–130 km wide, marginal thrust belt, the thrust planes outlining the windows appear to follow long flats developed in Lower Palaeozoic carbonates. East of the marginal thrust belt, thrust inclinations steepen, and by implication the remaining part of the Caledonian orogen extending eastwards to the present Atlantic Ocean coast is allochthonous and thick-skinned. The contrast between the restricted Neoproterozoic–Lower Palaeozoic foreland succession and the very thick and almost continuous sedimentation of the allochthonous Neoproterozoic Eleonore Bay Supergroup–Tillite Group–Cambro-Ordovician sequence of the fjord zone of East Greenland confirms the presence of distinct N–S trending facies belts on the northwestern passive margin of Iapetus. Comparisons with the Caledonides of Northwest Scotland, which may originally have lain as little as 500 km south of the East Greenland Caledonides, provide further clues to the understanding of Neoproterozoic–Early Palaeozoic basin geometry on this sector of the developing Iapetus margin. The areas of the Laurentian margin represented in the foreland windows of East Greenland were inboard of Neoproterozoic rifting but, with respect to the Torridonian basins of Northwest Scotland, the Eleonore Bay Supergroup succession must have been laid down further outboard. Similarly the Lower Palaeozoic developments of the foreland of Northwest Scotland are thicker than the equivalent foreland sequences of East Greenland, but much thinner than the allochthonous East Greenland Cambro-Ordovician succession.

Mushwad: Ductile duplex in the Appalachian thrust belt in Alabama

Structural style in thin-skinned thrust belts is controlled substan tially by the stratigraphic succession and relative thicknesses of lith otectonic units (stiff layers and weak layers). In contrast to the rela tively predictable geometry of fault-related folds (fault-bend folds, fault-propagation folds, and detachment folds), ductile deformation of a stratigraphically thick weak layer generates a ductile duplex that elevates and distorts the overlying stiff layer. A ductile duplex is herein termed a (Malleable, Unctuous SHale, Weak-layer Accretion in a Ductile duplex). In the Appalachian thrust belt in Alabama, mushwads nucleated in a thick, shale-dominated suc cession of weak rocks in association with basement faults beneath the regional decollement. The stiff-layer roof is deformed by folds and faults and is elevated by tectonic accretion of weak rocks in a mushwad. Shortening within the mushwad may drive translation of the stiff layer from the roof over the foreland. Lateral termina tions range from plunge of the stiff-layer roof over a laterally thin ning mushwad to a transverse fault. The examples in the Appala chian thrust belt suggest a style of structure that may be more common than presently recognized. The structural geometry of a mushwad, as well as the potential for concentration of fractures in the distorted stiff layer, suggests important applications in petro leum exploration and reservoir development.

Piggyback basin development above a thin-skinned thrust belt with two detachment levels as a function of interactions between tectonic and superficial mass transfer: the case of the Subandean Zone (Bolivia)

The Subandean fold and thrust belt of Bolivia is characterised by two major detachment levels and large piggyback basins. ‘Sand-box’ and numerical models have been used to study sedimentation and erosion control on thrust belt evolution and to study the retroactive effects of tectonics on piggyback development in thin-skinned thrust belts with two detachment levels. Analogue models show that surface processes play a dominant role in controlling wedge evolutions: erosion promotes fault reactivation and tectonic delamination (passive roof duplex) while sedimentation promotes forward shifting of the frontal thrust and consequently piggyback basin development. Numerical models were used to understand the development of the Subandean fold and thrust belt of Bolivia. Numerical experiments show that the simultaneity of basement tilting and high sedimentation rates promotes the formation of a stable tectonic wedge. Outer and inner faults are alternately active during the beginning of deformation, a kinematic evolution that favours the development of piggyback basins between. The step-by-step history of the thrust belt predicts that each change in tectonic location is recorded with large unconformities in basins, but these unconformities are not well preserved from progressive erosion in the final geometry.

The Siwaliks of western Nepal: I. Geometry and kinematics

The Siwalik Group which forms the southern zone of the Himalayan orogen, constitutes the deformed part of the Neogene foreland basin situated above the downflexed Indian lithosphere. It forms the outer part of the thin-skinned thrust belt of the Himalaya, a belt where the faults branch off a major décollement (MD) that is the external part of the basal detachment of Himalayan thrust belt. This décollement is located beneath 13 Ma sediments in far-western Nepal, and beneath 14.6 Ma sediments in mid-western Nepal, i.e., above the base of the Siwalik Group. Unconformities have been observed in the upper Siwalik member of western Nepal both on satellite images and in the field, and suggest that tectonics has affected the frontal part of the outer belt since more than 1.8 Ma. Several north dipping thrusts delineate tectonic boundaries in the Siwalik Group of western Nepal. The Main Dun Thrust (MDT) is formed by a succession of 4 laterally relayed thrusts, and the Main Frontal Thrust (MFT) is formed by three segments that die out laterally in propagating folds or branch and relay faults along lateral transfer zones. One of the major transfer zones is the West Dang Transfer Zone (WDTZ), which has a north-northeast strike and is formed by strike-slip faults, sigmoid folds and sigmoid reverse faults. The width of the outer belt of the Himalaya varies from 25 km west of the WDTZ to 40 km east of the WDTZ. The WDTZ is probably related to an underlying fault that induces: (a) a change of the stratigraphic thickness of the Siwalik members involved in the thin-skinned thrust belt, and particularly of the middle Siwalik member; (b) an increase, from west to east, of the depth of the décollement level; and (c) a lateral ramp that transfers displacement from one thrust to another. Large wedge-top basins (Duns) of western Nepal have developed east of the WDTZ. The superposition of two décollement levels in the lower Siwalik member is clear in a large portion of the Siwalik group of western Nepal where it induces duplexes development. The duplexes are formed either by far-travelled horses that crop out at the hangingwall of the Internal Décollement Thrust (ID) to the south of the Main Boundary Thrust, or by horses that remain hidden below the middle Siwaliks or Lesser Himalayan rocks. Most of the thrusts sheets of the outer belt of western Nepal have moved toward the S–SW and balanced cross-sections show at least 40 km shortening through the outer belt. This value probably under-estimates the shortening because erosion has removed the hangingwall cut-off of the Siwalik series. The mean shortening rate has been 17 mm/yr in the outer belt for the last 2.3 Ma.

Along-strike segmentation of the Andean foreland: causes and consequences

Thrust belts of Tertiary age characterize the east flank of the Andes and part of the adjacent foreland from Venezuela to the southern tip of South America. This foreland deformation comprises three basically different structural styles: (1) thin-skinned thrust belts detached within the sedimentary cover, (2) thick-skinned thrust belts with an inferred basal detachment in the basement, at 10–20 km depth and (3) foreland basement thrusts which possibly affect the entire crust. Alternating foreland structural styles along the orogen, often with sharp boundaries, produce a segmentation of the Andean foreland. The changes in structural style are interpreted to be primarily controlled by inherited stratigraphic and structural features of the South American plate, for instance: thin-skinned thrust belts appear to require a more or less conformable sedimentary cover at least some 3 km thick over a metamorphic/crystalline basement unaffected by Mesozoic extension. In areas of Mesozoic rifting, normal faults have been reactivated during Tertiary contraction, resulting in the formation of thick-skinned thrust belts. The basement becomes involved even if potential décollement levels are present. Deep-seated foreland basement thrusts tend to form over basement arches with a thin sedimentary cover. The three basic structural styles are unequally efficient in terms of shortening: Thin-skinned belts are typically shortened by 40–70%, thick-skinned belts by 20–35%, and areas of foreland thrusts by less than 10%. The structural segmentation of the foreland therefore implies strong along strike variations in the amount of shortening. Available estimates of shortening along the Central Andes actually suggest that shortening does not vary steadily, but has several maxima and minima. Strongly and weakly shortened foreland segments correlate poorly with the segmentation of the subducted plate in steeper and subhorizontal segments (`flat slabs'), but closely with lithospheric structure: The most strongly shortened segment coincides with thick mantle lithosphere, suggesting that shortening of the edge of the South American plate has resulted in substantial underthrusting of lithospheric mantle beneath part of the Andes. This underthrusting of thick lithosphere may have caused steepening of a formerly `flat' slab in the Arica bend region to the `normal' 25–30° dip presently observed.

Abstract: Thick-skinned Versus Thin-skinned Fold and Thrust belts, Alternative Models for the Interpretation of Structures in the Sub-Andean Basins and their Significance in Terms of Basin Modelling

Abstract: Thick-skinned Versus Thin-skinned Fold and Thrust belts, Alternative Models for the Interpretation of Structures in the Sub-Andean Basins and their Significance in Terms of Basin Modelling