Wrench Fault Research Articles

The geology and tectonic evolution of Waigeo Island, NE Indonesia

Waigeo occupies a critical position between the Halmahera-Philippine arcs to the northwest and Australia-New Guinea to the southeast. The island consists of a deformed ophiolitic basement of supra-subduction zone type overlain by probable Paleogene forearc sedimentary rocks. The forerarc basement is cut by mylonite shear zones and the basement and sedimentary cover rocks were deformed by southward-directed thrusts and associated folds during the Oligocene. The deformed sequences are intruded by basic dykes, thought to be coeval with basalts and andesites of Late Oligocene age. The entire Paleogene sequence is overlain unconformably by a thick (up to 2000 m) sequence of Miocene limestones which accumulated during a tectonically quiet period. A final phase of deformation occurred during the Pliocene which caused the development of two very large anticlines and an intervening syncline, associated with left-lateral wrench faults. Waigeo is interpreted to have been situated in a forearc position in an intra-oceanic island arc during the early Paleogene, forming part of the East Halmahera-Waigeo forearc terrane. The Waigeo arc terrane collided with a continental block in about the Middle Oligocene, contemporaneous with similar arc-continent collision in northern New Guinea. A period of tectonic quiescence during the Miocene was followed by Pliocene deformation in Waigeo related to a left-lateral wrench faulting on splays of the Sorong Fault in northern New Guinea. The Pliocene deformation is interpreted as resulting from compression on a right-stepping restraining bend in this wrench fault system.

The post-Triassic sedimentary cover of Tunisia: Seismic sequences and structure

A series of geological cross sections based on data from the new geological map of Tunisia coupled with stratigraphic and structural studies indicate that a folded, post-Triassic sedimentary cover was separated from the basement by a decollement zone located in Triassic evaporitic deposits. Four depositional basins were formed during the Triassic and Liassic in an extensional regime, on a presumably thinned continental crust. Geological fieldwork, studies of metamorphic grade and gravimetric and magnetic surveys indicate that their depocentres shifted with time from basin to basin, finally resulting in today's Tellian and Tunisian troughs to the north, Gafsa Trough to the west of Gabes and the Tuniso-Libyan Trough to the east. Subsidence in the latter trough commenced at the end of the Cretaceous and is still continuing today. There are four well-defined seismic stratigraphic sequences comprising Mesozoic and Cenozoic sedimentary rocks, each with distinctive lithoseismic characteristics. We display the distribution and structure of these seismic sequences in a series of cross sections. In addition, we note that the regional tectonic style of Tunisia has been greatly influenced by a conjugate strike-slip fault system. The role of wrench faulting has been well-documented in the Kasserine and other areas by detailed structural studies.

Clay diapiric structures within the strike-slip margin of the southern leg of the Barbados prism

The study area is situated 200 km to the east of Trinidad; in the southern sector of the Barbados accretionary wedge. Analysis of seismic profiles and Seabeam bathymetric data, permit the identification of an important dextral wrench fault system oriented N95°E which induced morphostructural elements, characteristic of this type of kinematics in a thick sedimentary cover, separated from its basement by a décollement level. This décollement level is due to the presence of Miocene clay beds, supersaturated with water which under the effect of stresses, trigger clay diapiric processes in this transtensional sector. We shall demonstrate that the boundary between the Caribbean Plate and the South American plate continues as far as 250 km to the east of Trinidad.

Kinematics of compressional fold development in convergent wrench terranes

Kinematic models are presented for compressional fold development in wrench and convergent wrench terranes that relate fold shortening, axial rotation, and axial extension. Fold shortening may be derived from final fold geometry. Existing fold geometry and axial orientation, two readily measurable quantities, provide the data needed to determine the relative components of shearing and convergence within the fold system. Analyses utilizing these kinematic models indicate that folds developed in sedimentary rocks in the wrench borderlands of both the Rineonada and San Andreas wrench faults in central California are the product of strongly convergent wrenching. The axes of these folds have been rotated no more than a few degrees during the course of their development. In contrast, folds developed in the Alpine Schists along the Alpine fault in New Zealand and in Pleistocene sediments along the southern limit of the San Andreas fault suggest an almost pure wrench setting and large (>25 °) axial rotations. Significant axial extension is inherent in wrench-related compressional folds. This axial extension is commonly manifest in the form of normal and strike-slip faults that are internal to the folds and trend at high angles to the fold axes. The relative amount of axial extension diminishes as the degree of convergence increases. This axial extension, and the associated extensional features, can be a diagnostic indication of the influence of wrenching.

Geophysical evidence for Early Permian igneous activity in a transtensional environment, Denmark

Integrated interpretation of gravity, magnetic and seismic data in the area of the strong Silkeborg Gravity High in Central Denmark has led to a structural geological model involving a transtensional rift feature which probably formed in connection with Permo-Carboniferous wrench faulting and the development of the Oslo-Horn Graben system. Reinterpretation of the deep seismic data from profile 2 of the EUGENO-S project in the area of the Silkeborg Gravity High detected a new thin high-velocity layer (6.25 km/s) at about 7 km deep within the Palaeozoic sedimentary sequence. In the crystalline basement a separate body of high velocity (6.9–7.0 km/s) was found at 11 km deep. Beneath this body the Moho reaches its shallowest position in the area. The seismic results compare well with an interpretation based on magnetism of a shallow volcanic or subvolcanic layer at a depth of 6–8 km. This layer shows strong remanent and induced magnetization with a southward inclination of about 10°. Gravity reinterpretation of the area allows the separation of an upper body of density contrast 0.1 g/cm 3 from a lower body of density contrast 0.2 g/cm 3. The upper body roughly coincides with the magnetic body. The lower body, the major one, coincides with the deep seismic body and it is responsible for the main gravity anomaly in the area.

Devonian basin initiation in East Greenland: a result of sinistral wrench faulting and Caledonian extensional collapse

Following the Caledonian orogeny, large scale ESE–WNW extensional faulting and N–S directed sinistral wrenching affected East Greenland leading to the initiation of the Devonian basin. The NNE–SSW trending extensional faults have very large downthrows to the east exceeding 10 km and separate up to 90 km wide fault blocks, which during deformation were tilted approximately 12°. The tilting of one of the fault blocks led to uplift of deep-seated hot crystalline rocks in the eastern part causing contact metamorphism in the adjacent downfaulted block to the east. Based on K-Ar cooling ages, the faulting seems to have taken place during the Middle Devonian around 385 Ma, coinciding with the initiation of deposition within the Devonian basin. A N–S trending sinistral wrench fault zone developed in addition to the extensional faults leading to formation of synthetic and antithetic Riedel shears along the present-day western border of the Devonian basin. The timing of the wrench displacement cannot be younger than Middle Devonian. The extension faults and the wrench fault can be followed as regional structures in East Greenland and can be compared to similar Devonian extensional structures in northern Great Britain and to the major wrench faults in Scotland. No clear separation in time is possible between the extension and the wrenching, and a transtensional kinematic history is the most plausible. The basin-forming tectonic evolution in East Greenland during Middle Devonian can be related to an extensional collapse of an overthickened Caledonian crustal welt associated with wrench deformations due to late Caledonian shear displacements along plate boundaries. This evolution is analogous to the Devonian basin formation in northern Great Britain.

Experiments on simultaneous faulting and folding above a basement wrench fault

Wrench zones commonly show en-echelon patterns of folds in the stratified cover above a basement fault (examples are the Darien Basin in Panama and the Inglewood fault trend in California). Folds are often associated with synthetic and antithetic wrench faults, but the time relationships between folds and faults may not be obvious. We have done experiments where simultaneous faulting and folding occur above a basement fault. Four experiments were performed using a simple apparatus capable of imposing localised strike-slip motions. This apparatus consists of two plastic plates. One can be moved laterally past the other by a piston, thus modelling lateral motion on a single basement fault. Models were properly scaled to account for gravitational forces, brittle behaviour of sandstone and ductile behaviour of clay or shales in the upper crust. Each model is a vertical succession of silicone layers and sand layers. Each layer is 2 mm thick except for the basal silicone layer which is 2, 4, 6 or 8 mm thick in experiments 1, 2, 3 or 4, respectively. All models were deformed at the same slip rate on the basal fault (1 cm h −1). Experiments 1, 2 and 3, with 8 cm total horizontal sinistral offset, were designed to investigate the influence of basal silicone layer thickness on the deformation of a multilayered model. Experiment 4, with 20 cm of total offset, was designed to investigate the influence of an increasing left-lateral offset on folding and faulting. With a 2 mm thick basal silicone layer (experiment 1), synthetic strike-slip faults appeared, en-echelon above the basement fault. No folds were visible at the surface of the experiment, but the interfaces between sand and silicone layers showed millimetric folds en-echelon above the basement fault. With a 4 mm thick basal silicone layer (experiment 2), en-echelon synthetic strike-slip faults and folds appeared above the basement fault. Faults and folds rotated progressively anticlockwise, and synclines were activated as dextral antithetic shear zones. With a 6 mm thick basal silicone layer (experiment 3), synthetic faults and en-echelon folds appeared as before, but the fold amplitude was larger than in experiment 2. An increasing left-lateral offset produced anticlockwise rotation of faults and folds, as before, and synclines were activated as dextral antithetic shear zones. In experiment 4, after 16 cm of left-lateral offset, the anticlockwise rotation of the folds was such that antithetic dextral shear zones became almost parallel to the basement fault and were then reactivated as synthetic sinistral shear zones.

Possible Oil and Gas Traps Related to a Wrench-Fault Zone in Hungary

The Szolnok-Maramures flysch zone, penetrating deep into the Great Hungarian Plain, is an appendix of the inner Carpathian flysch. The biggest gas field of Hungary (found in 1960) is in this zone. Since 1960, the flysch zone was explored thoroughly but nothing comparable was found; however, most of the boreholes had some hydrocarbon indications. The Szolnok area represents the western termination of the flysch zone with some minor gas fields. From recent seismic data, the authors determined the dominant tectonic style of the extremely complex tectonic features: wrench faulting. In the presented seismic time sections, all possible hydrocarbon traps are in connection with strike-slip movement: (1) domes in upper Pannonian Pliocene clastic sediments (probability is low because of listric faults in the top zone); (2) channels in the lower Pannonian delta series (probability is higher but the danger of open listric fault still exists); (3) closed highs above the dome of positive flower structure of the fly, in the lower Pannonian marine sequence; (4) similar situation in Badenian-Sarmatian neritic to terrestrial sediments and volcanic-sedimentary formations (these last two represent most finds); and (5) the overthrust plane (terminating the flysch with crushed crystalline schists) and the Miocene sediments trapped beneath itmore » while the two blocks of Mesozoic and Precambrian basement moved by, forming ideal trapping conditions for the hydrocarbons moved by, forming ideal trapping conditions for the hydrocarbons generated in the flysch and in older Mesozoic source rocks.« less

Clark Fork Lineament, West-Central Montana: A Model for Wrench Anticlines

The Clark Fork lineament is a 150-km-long left-lateral wrench fault zone developed between Missoula and Helena, Montana. It is part of the extensive Montana Laramide wrench system that includes classic wrench faults like Cat Creek, Lake Basin, and Nye-Bowler. The Clark Fork is remarkable in that it consists of a dazzling array of en echelon wrench anticlines. To better define it as a model for wrench anticlines, field mapping, gravity, and compilation were undertaken. Fourteen wrench anticlines occur between Missoula and Helena. Generally, they are symmetrical, south plunging, about 5 km wide, and 13-35 km long. They are arranged en echelon along the Clark Fork lineament forming a series of wrench anticline umbrellas over a basement fault. The anticlines are age-constrained by cross-cutting plutons of the Garnet stocks, the Elkhorn Mountain volcanics, and the Boulder batholith, all dating about 80 to 78 Ma. Following folding, a south-facing monocline formed along the entire length of the Clark Fork lineament, increasing from zero throw to about 5 km westward. The approximately 70 Ma Sapphire block, a 3,000 km{sup 3} thrust sheet from the southwest, was emplaced against the lineament and tectonically loaded the south side of the Clark Fork wrench zone, addingmore » to (or causing) down-to-the-south, tilt-to-the-west movement. Wrench anticlines adjacent to the Sapphire block margin were squeezed toward the northeast by the block and structurally modified.« less

Tectonic Evolution of the Northern Caribbean/Bahamas/Florida Region: Implications for Exploration

ABSTRACT The northern Caribbean/Florida/Bahamas region consists of (1) a major thrust belt striking from western Cuba to Puerto Rico and (2) a broad carbonate foreland to the north in Florida and the Bahamas. A major exploration problem in Florida and the Bahamas has been the interpretation of many anomalous vertical seismic features in the carbonate platform. Are they vertical faults or the effect of statics and paleokarst problems? If these features are true faults, uncertainties remain concerning fault geometry, timing, and trap potential. In order to clarify these uncertainties the following tectonic scenario was developed by (1) analyzing regional structure and stratigraphy and (2) testing palinspastic interpretations on an interactive 3D computer graphics system. Triassic/Jurassic. The study area serves as a transform zone in continental crust between the opening Central Atlantic and Gulf of Mexico basins. Continental crust is variably stretched between NW striking wrench faults and subsidiary NNE striking normal faults. By the latest Jurassic, the Gulf of Mexico stops opening and transform motion in the study area ceases. Early Cretaceous. Seafloor spreading begins east of Yucatan in the proto-Caribbean ocean and the proto-Cuba, Bahamas, and Florida area develops into a young continental margin sequence characterized by rapid subsidence and deposition of pelagic limestone (good source rocks). Mid Cretaceous. Changes in regional stresses surrounding the enlarging Atlantic Ocean disrupt the continental margin. Older rift and wrench-related structures in the basement reactivate and propagate into the overlying Neocomian section creating highs and lows at the surface. Late Cretaceous-Tertiary. The Cuban arc collides with the continental margin and forms a south-dipping thrust stack. Traps form in thrust-related anticlines that are charged as continental margin source rocks are overthrust and matured. Convergent stresses reactivate rift- and wrench-related structures in the foreland causing a jostling of crustal blocks. New faults propagate upward into overlying carbonates creating low-amplitude anticlinal closures. For explorationists, the tectonic scenario developed by this study suggests that in Florida/Bahamas, the anomalous seismic features in the carbonate platform are true vertical faults making this area more structurally complicated (but also more prospective) than previously recognized. End_of_Record - Last_Page 321-------

Possible Strand of the North Anatolian Fault in the Thrace Basin, Turkey--An Interpretation (1)

The North Anatolian fault is a 1200-km-long major transform fault bounding the Anatolian plate on its north side. The fault formed during the late middle Miocene as a broad shear zone with a number of strands splaying westward in a horsetail pattern. Later, movement became localized along a central strand, and the southerly and northerly splays became inactive. The west-northwest-striking Thrace strike-slip fault system is one of these right-lateral, presently inactive, splays. It consists of three subparallel strike-slip faults. From north to south these are the Kirklareli, Luleburgaz, and Babaeski fault zones, which extend for about 130 km through the Thrace basin. The Thrace fault zone probably connected to the southeast with the presently active northern strand of the North Anatolian fault in the Marmara Sea. To the northwest, the zone may have extended to the Plovdiv graben zone in Bulgaria. The Cenozoic Thrace basin contains middle Eocene to Pliocene sedimentary rocks. The Thrace right-lateral wrench fault system formed prior to the Pliocene and had become inactive by the Pliocene. Strike-slip fault zones with normal and reverse separation are detected by seismic reflection profiles and subsurface data. As the system became inactive, the motion on the North Anatolian fault zone began to be accommodated in the Marmara Sea region. Thus, the Thrace fault system represents the oldest strand of the North Anatolian fault to the west of Istanbul. Releasing-bend extensional structures and restraining-bend compressional structures are abundant along the fault zones. Several hydrocarbon-producing anticlines lie en echelon to the Luleburgaz fault zone. Regionally, the Thrace fault ystem has a horsetail shape, with the various strands becoming younger southward.

Ground Magnetic Anomalies in the Tanna Fault and Their Implications.

The Tanna fault is a left lateral fault extending in the middle of the Izu Peninsula. We carried out a detailed ground magnetic survey at a grid spacing of 50 to 150m along the Tanna fault and constructed a magnetic model based on the ground survey with the help of drilling data in the Tanna basin and measurement of rock magnetism.Total intensity distribution obtained by the ground magnetic survey represents a distinguished contrast forming relative magnetic highs in the west to the fault and lows in the east. The Tanna fault is a remarkable border terminating magnetic highs.Drillings revealed that formations filling the Tanna basin are mainly the middle Pleistocene Taga volcanics which possess high susceptibility as a whole. A recent work suggests that the Taga volcanics were produced in normal polarity and had strong remanent magnetization. Therefore, remanent magnetization is inferred to be dominant in the total magnetization.The magnetic contrast forms almost a north-south trend throughout the study area. The magnetic modeling using E-W profiles delineated strong normally magnetized sources of 6A/m confined to the west to the fault. Three drillings indicated that the east formations to the fault were wholly disintegrated and the west formations, by contrast, included fresh lavas. These evidences suggest that the major cause of magnetic contrast is not the difference of geologic unit but instead the weakened remanent magnetization in the east adjacent area to the fault.Seismic reflection and shallow drillings indicate the possibility of existence of shallow wrench fault. Bending usuallly appears along a strike slip and actually several basins such as Tanna basin and the Tashiro basin exist along the fault, and some magnetic lows are supposed to be caused by sinking of volcanic layer or pull-apart movements. The striking magnetic discontinuity, on the other hand, implies that the fault surface been displaced ever since Pleistocene has always been the known Tanna fault.In conclusion, the cause of relative magnetic lows in the east to the fault is responsible for effects of, either remanent magnetization reduced, and sinking and truncating of magnetic sources deformed, by the fault activities.

Study of Multilayered Divergent Wrench-Fault Systems by Using Sandbox Models

Sandbox models are used to evaluate the effects of mechanical layering (structural stratigraphy) on the deformation response (structural styles) of sediments during basement-controlled divergent wrench faulting. Three divergent boundary conditions are generated by moving two adjacent plates along a 45{degree}-dipping precut (basement fault). Dry sand and silicone putty (overlying the basement fault) simulate the brittle and ductile behavior of lithified rocks, respectively. Four combinations of sand and silicone putty are used to investigate the effects of structural stratigraphy on the deformation response. Similar to the behavior of simple and convergent models, these divergent models show that with increasing putty thickness (1) the effect of dip-slip motion decreases, (2) there is a better development of Riedel shears and splay faults, (3) the angle between Riedel shear and basement fault increases, and (4) the Riedel shears do not propagate to the basement/putty interface so that the corresponding negative flower structures or grabens become indistinct.

Identification of Wrench Faults Using Subsurface Structural Data: Criteria and Pitfalls: Reply (1)

Identification of Wrench Faults Using Subsurface Structural Data: Criteria and Pitfalls: Reply (1)

Tectonic Influences on Carbonate Deposition and Diagenesis, Buckhorn Asphalt, Deese Group (desmoinesian), Arbuckle Mountains, Oklahoma

ABSTRACT The Dry Branch Section is a mixed siliciclastic/carbonate sedimentary sequence containing carbonate rocks that experienced incomplete diagenesis due to early introduction of oil from the Oil Creek Formation (Ordovician). The section is part of the Deese Group (Desmoinesian) exposed in the western portion of the Mill Creek Graben, Arbuckle Mountains, Oklahoma. The sequence was deposited in shallow marine shelf setting within a tectonically active basin; deposition was strongly influenced by adjacent terrigenous coastal depositional environments. The section was synclinally deformed by wrench faults to the north which were active soon before and after deposition. Two of these faults cut both the Dry Branch Section and the underlying Oil Creek Formation, a known petroleum reservoir and t e source of fault-related oil seeps throughout the central Arbuckle region. These faults acted as avenues for reservoir fluids to enter section as a result of remigration driven by orogenic stress. Carbonate rocks which show evidence of diagenesis influenced by reservoir fluids are present in the upper part of the section. Limited cementation, overcompaction, and arrested carbonate diagenesis resulted from the effects of interstitial asphalt which formed as a result of intraformational biodegradation of Oil Creek oil. Stratigraphic variation in petrologic characteristics suggests that oil introduction into Dry Branch sediments occurred as a single event within 10,000 years of deposition of uppermost part of section, coinciding regionally with the Desmoinesian Ouachita orogenic pulse. Dia enesis has resulted in moderate isotopic depletion of altered fossils (-418O; -3.5 13C) relative to unaltered skeletal aragonite (18O = 0 to -1; 13C = 3.7 to 5.0). Petrographic and stable isotopic study shows that two phases of early carbonate cementation affected Dry Branch carbonates: 1) early clear spar that is more depleted in 18O (-5) than the altered fossils; 2) later spar which contains asphalt inclusions. Microspar and poikilotopic sparry cement present in the upper Dry Branch are anomalously depleted in 13C (< -20). This is close to the isotopic compositional range of asphalt in the section, indicating that organic carbon derived from oil biodegradation was present in the porewater during diagenesis. Carbonate geochemical and petrographic evidence, and the early post-depositional tectonic introduction of oil, indicate that subsurface water was a principal diagenetic agent.

Identification of Wrench Faults Using Subsurface Structural Data: Criteria and Pitfalls: Reply (1)

Identification of Wrench Faults Using Subsurface Structural Data: Criteria and Pitfalls: Reply (1)

Structure, Stratigraphy, and Climate of the Mesozoic Chignecto Subbasin, Bay of Fundy, Canada

The Chignecto subbasin is one of three Triassic/Jurassic subbasins in the Bay of Fundy rift system of southeastern Canada. Although no wells have been drilled in the subbasin, offshore seismic data and onshore outcrop data show that the basin contains typical rift structures and stratigraphy. The Chignecto subbasin is an asymmetric rift basin bounded on the west by a series of gently to moderately dipping normal faults. The seismic data show that the total heave on these boundary faults locally exceeds 10 km. Most strata within the basin dip gently to the west toward the boundary faults. Field data show that a divergent wrench fault zone composed of numerous strike-slip, oblique-slip, and normal faults bounds the Chignecto subbasin on the south. The normal faults bounding the basin on the west and the divergent wrench fault zone on the south are reactivated Paleozoic compressional structures. Major stratigraphic features in the Chignecto subbasin include an unconformity that subdivides the basin strata into pre- and syn-rift sequences and an asymmetric arrangement of sedimentary and seismic facies and corresponding lithologies. The Triassic/Jurassic syn-rift strata within the basin thicken toward the western boundary faults. Coarser-grained strata of alluvial-fan and fluvial origin interfinger with finer-grained stratamore » of fluvial and lacustrine origin near the boundary faults. Eolian deposits and evaporites in adjacent Mesozoic rift basins indicate an arid climate for the Late Triassic/Early Jurassic. Large-amplitude reflections observed within the western half of the basin probably represent high-velocity evaporites or carbonates deposited within the deepest part of the Chignecto subbasin.« less

Petroleum Geology of Rift Basins in Niger, Chad, and Central African Republic

Ten Cretaceous-Tertiary rift basins in Niger, Chad, and the Central African Republic (C.A.R.) are defined and the petroleum geology is overviewed. This paper is based on proprietary exploration results derived from more than 1 million km{sup 2} of aeromagnetics, 10,520 line km of gravity profiles, 49,721 km of reflection seismic, and 50 exploration wells. The data were acquired by Exxon with partners Shell, Chevron, Elf, Conoco, Texaco, and Amax Oil Gas, Inc., during the years 1969-1989. In Niger and Chad, the West African rift subsystem includes the extensional basins of Termit, Tefidet, Tenere, Grein/Kafra, N'Djel Edji, and Bongor. These rift basins contain up to 15,000 m of Cretaceous to Cenozoic continental and marine clastics. Key exploration elements are Tertiary and Cretaceous fluvial to tidal sandstone reservoirs, Tertiary and Cretaceous marine to lacustrine shale source rocks, and seals, with traps in normal fault blocks and anticlinal closures. There are six oil discoveries in the Termit basin. In Chad and the C.A.R., the Central African rift subsystem incorporates the extensional Doba and transtensional Doseo and Salamat basins flanking the Borogop dextral wrench fault. These basins contain up to 7,500 m of chiefly Cretaceous continental clastics. Key exploration elements are Lower and Uppermore » Cretaceous fluvial to lacustrine sandstone reservoirs, Lower Cretaceous lacustrine shale source rocks, lacustrine to flood plain shale and mudstone seals, with traps in mainly faulted anticlinal closures. There are six oil discoveries in the Doba basin and three in the Doseo basin. The studied petroleum geology in the rifts of Niger, Chad, and the C.A.R. indicates that potentially commercial volumes of oil remain to be discovered.« less

Right‐lateral shear along the Northwest Pacific Margin and the India‐Eurasia Collision

Right‐lateral shear along the eastern margin of Asia, from the Eocene to the Present has led to the opening of pull‐apart basins, intracontinental such as the Bohai basin, or oceanic such as the Japan Sea. We suggest in this paper that this right‐lateral shear is a consequence of indentation of Asia by India. As in small‐scale analog experiments, we conclude that antithetic wrench faults accommodate the counterclockwise rotation of large domino blocks between two major left‐lateral shear zones (Tien Shan‐Baikal‐Stanovoy for the northern one, and Qin Ling for the southern one). We discuss the compatibility of this mechanism, which involves a rather small amount of extrusion, with the fast eastward expulsion described for southeast Asia. We re‐emphasize the role played in the opening of marginal basins by the Pacific subduction as a free boundary to the east.

Tectonic evolution of the NW Sabah continental margin since the Late Eocene

The NW Sabah continental margin in the northern part ofNW Borneo, consists of 2 main elements: a Tertiary trench-associated sedimentary basin with up to 12 km of siliciclastic sediments and the Southern South China Sea Platform, separated by the deep and relatively narrow, NE-trending NW Sabah Trough. The Tertiary sedimentary sequences were deposited during two main phases of basin development: 1. a pre-early Middle Miocene phase of generally deep-marine clastic sedimentation (Stages I, II and III). The sediments were subjected to strong compression to form an imbricate wedge related to the subduction of the South China plate. This was concomitant with the counter-clockwise rotation of the Borneo plate, and the relative motion of the two plates may have produced N-S wrench faults. 2. a post-early Middle Miocene phase of clastic shelf/slope deposition (Stage IV) which prograded northwestward over the underlying imbricate wedge and separated from the latter by a major regional unconformity. On the basis of differences in structural styles and sedimentation histories, the NW Sabah continental margin can be subdivided into 7 tectonostratigraphic provinces: (1) the CrockerAccretionary Prism, (2) the Inboard Belt, (3) the Outboard Belt, (4)the East Baram Delta, (5) the NW Sabah Margin, (6) the NW Sabah Trough and (7) the Southern South China Sea Platform. Some 90 exPloration and exploratory appraisal wells have been drilled in the NW Sabah offshore. Apart from minor oil and gas shows in the pre-early Middle Miocene deep­ marine sediments, all commercial accumulation~ discovered to date have been found in the Middle Miocene or younger clastic reservoirs in the Inboard Belt, Outboard Belt and East Baram Delta. The other tectonostratigraphic province have not been drilled to date. The tectonic evolution of the NW Sabah continental margin occurred in four stages: 1. The Late Eocene to early Middle Miocene subduction of the South China Sea oceanic crust beneath Borneo with deposition and subsequent of deep-marine sediments into an accretionary prism. 2. The collision and subduction of the South China Sea attenuated continental crust with Borneo in early Middle Miocene which led to the regional uplift and erosion of the accretionary prism resulting in the Deep Regional Unconformity. This was followed by NW progradation over the Inboard Belt from Middle Miocene to early Late Miocene. 3. Cessation of active subduction in middle late Miocene was accompanied by major tectonic activities. The Inboard Belt was subjected to strong compressional deforma­ tion, probably associated with deep-seated major N-S shear zones. The area was strongly folded, uplifted and eroded resulting in the Shallow Regional Unconformity.