The Central Southern Alps (N. Italy) paleoseismic zone: a comparison between field observations and predictions of fault mechanics

The internal sectors of the Orobic Alps (Northern Italy) are characterised by Alpine age regional shortening showing a transition, through time, from plastic to brittle deformation. Thrust faults cut Alpine ductile folds and are marked by cataclasites and, locally, by pseudotachylytes, suggesting that motion was accommodated by seismic frictional slip. In the Eastern Orobic Alps the thrusting initiated at depths deeper than 10 km (the emplacement depth of the Adamello pluton) and possibly continued at shallower depths. This demonstrates that thrust motion occurred between 10 km depth and the brittle-ductile transition, i.e., at mid-crustal depths. The Orobic Alps exhumed paleoseismic zone shows different geometries along strike. In the central sectors of the Orobic Alps, thrust faults, associated with pseudotachylytes, have average dips around 40° and show no pervasive veining. Much steeper thrusts (dips up to about 85°) occur in the eastern Orobic Alps. In this area, faults are not associated with pervasive veining, i.e., fluid circulation was relatively scarce. This suggests that faulting did not occur with supralithostatic fluid pressure conditions. These reverse faults are severely misoriented (far too steep) for fault reactivation in a sublithostatic fluid pressure regime. We suggest that thrust motion likely started when the faults were less steep and that the faults were progressively rotated up to the present day dips. Domino tilting is probably responsible for this subsequent fault steepening, as suggested by a decrease of the steepness of thrust faults from north to south and by systematic rotations of previous structures consistently with tilting of thrust blocks. When the faults became inclined beyond the fault lock-up angle, no further thrusting was accommodated along them. At later stages regional shortening was accommodated by newly formed lower angle shear planes (dipping around 30–40°), consistently with predictions from fault mechanics.     

Thrust structures in the eastern Dauphinois Zone (French Alps), north of the Pelvoux Massif

An examination of thrust structures in the eastern part of the Dauphinois Zone of the external French Alps (referred to in the literature as the Ultradauphinois Zone) shows that major basement thrusts climb up section to produce cover-basement synclines. These thrusts also climb laterally and are continuous with thrust in the cover rocks. The external basement massifs are recognized as thrust sheets with variably deformed and thrust cover sequences. The distinction made in the previous literature between the Dauphinois and Ultradauphinois Zones is no longer tenable. Cover thrusting proceeded by both smooth slip and rough slip, the latter producing a duplex of cover thrust slices. Restoration of this duplex indicates that a shortening of 70 km in the cover occured during its formation. Possible errors in this estimate include uncertainties in the original stratigraphic thickness and in the overall shape of the duplex. Another duplex is thought to have formed at a basement ramp created by the presence of an early basement normal fault. Partial footwall collapse of this basement ramp gave rise to a basement horse at the bottom of the duplex. The overall relation between cover and basement thrusting is indicated using a hanging wall sequence diagram. Recent geophysical studies suggest that the basement thrusts developed from a mid-crustal décollement which passes down dip to offset the Moho. Model studies of thin-skinned tectonics may not be appropriate to such thrust geometries.     

The origin of the Basse Normandie duplex,Boulonnais, France

Two Hercynian duplexes are developed in Viséan limestones in the Basse Normandie quarry. The lower duplex is completely exposed in a subvertical quarry wall; the partially exposed upper duplex lies immediately above the lower duplex. The duplexes are both located in the footwall of the Hydrequent thrust which emplaced Devonian clastic sediments above the Viséan limestones. The lower duplex exposes all the internal thrusts, a reference bed of chalky limestone, the roof and floor thrusts, and the duplex tip. The duplex has been graphically restored to its pre-deformation geometry by line-length and area balancing and its resultant geometry is close to the model of Boyer & Elliott. The lower duplex shortened by two different mechanisms, an initial phase of layer-parallel shortening which produced no cleavage, followed by thrust imbrication. The average contraction of the front portion of the duplex was ?49% (natural strain) of which ?27% is layer-parallel shortening and ?22% is thrust imbrication. However, locally the bulk shortening increases from zero at the duplex tip to over ?120% in a down-dip direction. The area balancing provides the most accurate estimates of bulk shortening; line-length balance calculations give minimum estimates only. An area balance on the whole of the lower duplex gives a bulk shortening of ?84%. An area balance of the upper duplex yields an average contraction of ?75% and the total contraction produced by both duplexes is ?92%.     

Kinematic analysis of mélange fabrics: examples and applications from the McHugh Complex, Kenai Peninsula, Alaska

Permian to Cretaceous mélange of the McHugh Complex on the Kenai Peninsula, south-central Alaska includes blocks and belts of graywacke, argillite, limestone, chert, basalt, gabbro, and ultramafic rocks, intruded by a variety of igneous rocks. An oceanic plate stratigraphy is repeated hundreds of times across the map area, but most structures at the outcrop scale extend lithological layering. Strong rheological units occur as blocks within a matrix that flowed around the competent blocks during deformation, forming broken formation and mélange. Deformation was noncoaxial, and disruption of primary layering was a consequence of general strain driven by plate convergence in a relatively narrow zone between the overriding accretionary wedge and the downgoing, generally thinly sedimented oceanic plate. Soft-sediment deformation processes do not appear to have played a major role in the formation of the mélange. A model for deformation at the toe of the wedge is proposed in which layers oriented at low angles to σ₁ are contracted in both the brittle and ductile regimes, layers at 30–45° to σ₁ are extended in the brittle regime and contracted in the ductile regime, and layers at angles greater than 45° to σ₁ are extended in both the brittle and ductile regimes. Imbrication in thrust duplexes occurs at deeper levels within the wedge. Many structures within mélange of the McHugh Complex are asymmetric and record kinematic information consistent with the inferred structural setting in an accretionary wedge. A displacement field for the McHugh Complex on the lower Kenai Peninsula includes three belts: an inboard belt of Late Triassic rocks records west-to-east-directed slip of hanging walls, a central belt of predominantly Early Jurassic rocks records north–south directed displacements, and Early Cretaceous rocks in an outboard belt preserve southwest–northeast directed slip vectors. Although precise ages of accretion are unknown, slip directions are compatible with inferred plate motions during the general time frame of accretion of the McHugh Complex. The slip vectors are interpreted to preserve the convergence directions between the overriding and underriding plates, which became more oblique with time. They are not considered indicative of strain partitioning into belts of orogen-parallel and orogen-perpendicular displacements, because the kinematic data are derived from the earliest preserved structures, whereas fabrics related to strain partitioning would be expected to be superimposed on earlier accretion-related fabrics.     

对逆冲断层一些普遍观念的质疑──以美国西部科迪勒拉及中国华北的野外研究为例(英文)

在过去的25年里,由于许多原因,作为最常见、分布也最广泛的地质构造形迹之一,逆冲断层成为倍受关注的科学研究主题。文中指出,关于逆冲断层及其几何学特征的许多普遍认识（或观念）,并不像以往文献中所阐述的那样简单。其中之一的"薄皮"冲断构造是受地层控制的,极少有或者没有结晶基底物的卷入。文中主张,"薄皮"一词只有逆冲板片的几何学形态含义,而不应包含地层意义,并列举了一些完全由结晶岩石所构成的薄皮逆冲构造的例子来说明这一主张。近来,逆冲双重构造成为构造文献中的热点。关于逆冲双重构造的成因,引用得最多的是1982年Boyer和Elliot在其重要论文"逆冲断层系统"中所作的解释。他们认为,双重道冲构造是通过在冲断坡底部发生下盘破裂。新生断裂不断向前扩展并进入先存断层下盘的一系列变形过程中逐渐形成的。根据Boyer和Elliot提出的这种变形过程,将形成一个具有平面状顶板断层的边冲双重构造,这个顶板断层只在活动断坡的顶部是主动向前扩展的。依笔者之见,在实际的构造变形当中,是不可能具备形成平顶过冲双重构造的地质条件的。而能对平顶过冲双重构造形成作出最好解释的是反序（out－of－sequence,OOS）边冲断层的发育,即断层向着主冲断层的后方发展,在先存道冲构造的上部?     

Sequences of thrusts and displacement transfer in the superposed duplexes of the Esla Nappe Region (cantabrian zone, nw spain)

In a cross-section through the southern arm of the Cantabrian Zone, several duplexes have been identified below the Esla Nappe, which is the uppermost and main thrust sheet of the area. The folds deforming the Esla Nappe are culmination walls linked to frontal and lateral ramps belonging to the lower thrust sheets. The thrust sequence can be established on the basis of quantitative analysis of displacement transfer and out of sequence thrusting. The primitive footwall ramps of the Esla Nappe Region were often subsequently broken by décollements developed in successively lower stratigraphic levels of these footwalls. The kinematics of the lowest duplex are more complicated than those of typical duplexes described elsewhere: some thrusts transfer only part of their displacement to the roof thrust, while the remaining part is accommodated along the higher thrusts of previously emplaced duplexes, cutting out of sequence one or more floor or roof thrusts. Cumulative displacement of the thrusts in this region is about 90 km, giving a present thickness 3 times that of the original pre-orogenic sequence, together with a translation of at least 60 km, for the synorogenic basin.     

A new kinematic evolutionary model for the growth of a duplex — an example from the Rangit duplex, Sikkim Himalaya, India

The Lesser Himalayan duplex (LHD) is a prominent structure through much of the Lesser Himalayan fold–thrust belt. In the Darjeeling - Sikkim Himalaya a component of the LHD is exposed in the Rangit window as the Rangit duplex (RD). The RD consists of ten horses of the upper Lesser Himalayan Sequence (Gondwana, Buxa, Upper Daling). The duplex varies from hinterland-dipping in the north, through an antiformal stack in the middle to foreland-dipping in the south. The Ramgarh thrust (RT) is the roof thrust and, based on a balanced cross-section, the Main Himalayan Sole thrust is the floor thrust at a depth of ~ 10 km and with a dip of ~ 3.5° N.Retrodeformation suggests that the RD initiated as a foreland-dipping duplex with the Early Ramgarh thrust as the roof thrust and the RT as the floor thrust. The RT became the roof thrust during continued duplexing by a combination of footwall imbrication and concurrent RT reactivation. This kinematic history best explains the large translation of the overlying MCT sheets. The restoration suggests that RD shortening is ~ 125 km, and the original Gondwana basin extended ~ 142 km northward of its present northernmost exposures within the window.     

Structural context of meso-scale and cartographical duplex structure genesis in thrusting tectonics: role of tear faults. Zaghouan thrust belt. Case of Bir M'Cherga-Ain Asker area

Bir M'Cherga-Ain Asker area, situated in the hinterland of Zaghouan thrust (Tunisian dorsal), was the land example treated with metric and cartographic scale in order to identify duplex genesis criteria and to include thrusting tectonics associated with tear faults, which are in fact the directory response generating duplex structures identified in outcrop for the first time through Tunisia in this case. Given its geological location between the "dôme" and the "dorsal" zone of Tunisia, this area was the most exposed to a highly paleostress history expressed by a huge fault system remobilization and reactivation through several tectonic events from NE–SW middle Cretaceous distension to a NW–SE and NNW–SSE Paleogene compression. Regarding fault planes generated analysis, they show numerous streak generation of normal, strike-slip, and reverse faults that go with geodynamic and paleostress evolution of the studied area; we note that each streak generation is perfectly matching with one of the tectonic event (mentioned before) affecting the area. In this paper, we analyze duplex structure elaboration scenarios to assess the involved kinematics and their geometrical recognition criterious. We propose to discuss the causes of duplex structures installation in a thrust belt system and the predictable geometrical styles after its installation on foreland or backland. Using the geometrical criteria acquired through this analysis, we will show that such, however, exceptionable structures exist on the land, and that they record the mechanisms of their genesis linked to the tear faults acting in this case. We describe "tear faults" as the sliding breaks which disunite two compartments during deformation, allowing them to undergo different independent deformations in their drawing and their width (for example more or less stretched folds). These types of faults differ from that of the true stick-slip faults, which slice and shift preformed structures (it's even this shift which makes it possible to highlight them); here, there are no shift but dissimilarity of the structures on both sides of the fault; therefore, deformations are the direct results of displacements; they are expressed in thrust belts by ramp folds, intense internal deformations, and even by complex duplex structures. A duplex feature that is not mainly studied is made up by tilted imbricate sedimentary sequences (or horses), separated by link thrusts and underlined respectively at their tops and bottoms by roof and floor thrusts. Imbrications cause a shortening, a thickening, or a thinning of stratigraphic columns and even its crushing and inverting. In thrust belts, duplexes are usually set up following two or even more deformational events; those structures start typically with decollement and imbricate sedimentary unit ones which are made cover by a roof thrust sometimes visible at outcrop. Through this paper, we suggest to discuss geometrical duplex criteria, and we will try a zooming through different scales, from regional to local one in order to show how the shape (expression) of the deformation differs.     

Intracontinental thrusts and inclined transpression along eastern margin of the East Dharwar craton, India

Recent works suggest Proterozoic plate convergence along the southeastern margin of India which led to amalgamation of the high grade Eastern Ghats belt (EGB) and adjoining fold-and-thrust belts to the East Dhrawar craton. Two major thrusts namely the Vellikonda thrust at the western margin of the Nellore Schist belt (NSB) and the Maidukuru thrust at the western margin of the Nallamalai fold belt (NFB) accommodate significant upper crustal shortening, which is indicated by juxtaposition of geological terranes with distinct tectonostratigraphy, varying deformation intensity, structural styles and metamorphic grade. Kinematic analysis of structures and fabric of the fault zone rocks in these intracontinental thrust zones and the hanging wall and footwall rocks suggest spatially heterogeneous partitioning of strain into various combinations of E-W shortening, top-to-west shear on stratum parallel subhorizontal detachments or on easterly dipping thrusts, and a strike slip component. Although relatively less prominent than the other two components of the strain triangle, non-orthogonal slickenfibres associated with flexural slip folds and mylonitic foliation-stretching lineation orientation geometry within the arcuate NSB and NFB indicate left lateral strike slip subparallel to the overall N-S trend. On the whole an inclined transpression is inferred to have controlled the spatially heterogeneous development of thrust related fabric in the terrane between the Eastern Ghats belt south of the Godavari graben and the East Dharwar craton.     

SHRIMP U–Pb zircon ages of pyroclastic rocks in the Bansong Group, Taebaeksan Basin, South Korea and their implication for the Mesozoic tectonics

The Bansong Group (Daedong Supergroup) in the Korean peninsula has long been considered to be an important time marker for two well-known orogenies, in that it was deposited after the Songnim orogeny (Permian–Triassic collision of the North and South China blocks) but was deformed during the Early to Middle Jurassic Daebo tectonic event. Here we present a new interpretation on the origin of the Bansong Group and associated faults on the basis of structural and geochronological data. SHRIMP (Sensitive High-Resolution Ion MicroProbe) U–Pb zircon age determination of two felsic pyroclastic rocks from the Bansong Group formed in the foreland basin of the Gongsuweon thrust in the Taebaeksan Basin yielded ages of 186.3 ± 1.5 and 187.2 ± 1.5 Ma, respectively, indicating the deposition of the Bansong Group during the late Early Jurassic. Inherited zircon component indicates ca. 1.9 Ga source material for the volcanic rocks, agreeing with known basement ages.The Bansong Group represents syntectonic sedimentation during the late Early Jurassic in a compressional regime. During the Daebo tectonic event, the northeast-trending regional folds and thrusts including the Deokpori (Gakdong) and Gongsuweon thrusts with a southeast vergence developed in the Taebaeksan Basin. This is ascribed to deformation in a continental-arc setting due to the northwesterly orthogonal convergence of the Izanagi plate on the Asiatic margin, which occurred immediately after the juxtaposition of the Taebaeksan Basin against the Okcheon Basin in the late stage of the Songnim orogeny. Thus, the Deokpori thrust is not a continental transform fault between the North and South China blocks, but an “intracontinental” thrust that developed after their juxtaposition.     

A cross-section through the Scandinavian Caledonides constructed with the aid of branch-line maps

Fault surfaces have a finite area enclosed by branch- and tip-lines. A tip-line separates the slipped from the unslipped region. A branch-line forms where one fault splays off another and occurs at the trailing or leading ends of thrust sheets and along frontal, oblique and lateral ramps. Hence potentially complicated patterns of branch- and tip-lines outline or surround the fault surface. The branch-lines determine which parts of the fault geometry, off a line of section, can be projected on to the section; help to define the fault movement direction; and identify horses or fragments left behind by the faulting. The technique of analysing branch- and tip-lines is demonstrated on the thrusts of the Trondheim area to derive a more rigorous section which is also constrained by gravimetric, aeromagnetic and metamorphic data. Lateral branch-lines, parallel to the thrust slip-direction, suggest slip vectors between 155 and 165° (SE) for three of the thrusts. Horses, left behind by the thrusts, suggest minimum displacements of 50 and 100 km for two of these thrusts.     

Rupture mechanism of the 1993 Killari earthquake, India: constraints from aftershocks and static stress change

The Killari earthquake of September 29, 1993 (Mw=6.2) in peninsular India triggered several aftershocks that were recorded by a network of 21 stations. We computed the change in regional static stress caused by coseismic slip on the earthquake rupture and correlated it with the aftershocks with a view to constrain some of the rupture parameters of this earthquake. We evaluated the six available estimates of fault plane solutions for this earthquake and concluded that reverse slip on a 42° dipping, N112° trending fault, which extends up to the surface from a depth of 7 km, produces maximum correlation between the increased static stress and aftershock distribution. Our analysis suggests that the majority of coseismic slip occurred on the part of the rupture that lies in the depth range of 3–6.5 km.     

Thick- and thin-skinned tectonics in the Pyrenees

A comparison is made between the Gavarnie thrust and the Mérens Fault in the Axial zone of the Pyrenees. The former has a gentle dip and quite a large displacement (at least 12 km) but does not cut through either Hercynian or Alpine isograds. The latter has a smaller displacement (~ 5 km) but dips steeply and cuts through both Hercynian and Alpine isograds at a high angle. On this basis and on the basis of shear zone geometries immediately north of it, it is proposed that the Mérens Fault nucleated as a steeply (65°–80°) dipping structure, while the Gavarnie thrust nucleated with a shallow attitude. The Mérens Fault is not a backward-rotated thrust fault, nor is it the root zone for any major nappe structure. Similar steep ductile structures occur within the Gavarnie nappe and may reflect considerable internal strain in basement lithologies.The relationship between steep and shallow structures is not yet clear; the shear zones may pre-date the thrusting in which case they may be thick-skinned structures affecting the whole lithosphere, or they may be contemporary with thrusting reflecting only local thickening above a décollement.Rheological models can be used to test proposed geometrical and kinematic models for the lithosphere-scale evolution of the Pyrenees. Suggested models are dominated by a cool, rigid, high-level mantle wedge beneath the North Pyrenean zone which probably controlled the location of north-dipping thrust faults. Thick-skinned shortening is possible in thick crust in the Axial zone but is very unlikely in the North Pyrenean zone where steeply rooted structures would have to cut through the strongest part of the lithosphere.     

Structural inversion and its controls : examples from the Alpine foreland and the French Alps

Abstract

Positive structural inversion involves the uplift of rocks on the hanging-walls of faults, by dip slip or oblique slip movements. Controlling factors include the strike and dip of the earlier normal faults, the type of normal faults — whether they were listric or rotated blocks, the time lapsed since extension and the amount of contraction relative to extension. Steeply dipping faults are difficult to invert by dip slip movements; they form buttresses to displacement on both cover detachments and on deeper level but gently inclined basement faults. The decrease in displacement on the hanging-walls of such steep buttresses leads to the generation of layer parallel shortening, gentle to tight folds — depending on the amount of contractional displacement, back-folds and back-thrust systems, and short-cut thrust geometries — where the contractional fault slices across the footwall of the earlier normal fault to enclose a “floating horse”. However, early steeply dipping normal faults readily form oblique to strike slip inversion structures and often tramline the subsequent shortening into particular directions.

Examples are given from the strongly inverted structures of the western Alps and the weakly inverted structures of the Alpine foreland. Extensional faulting developed during the Triassic to Jurassic, during the initial opening of the central Atlantic, while the main phases of inversion date from the end Cretaceous when spreading began in the north Atlantic and there was a change of relative motion between Europe and Africa. During the mid-Tertiary well over 100 km of Alpine shortening took place; Alpine thrusts, often detached along, or close to, the basement-cover interface, stacking the late Jurassic to Cretaceous sediments of the post-extensional subsidence phase. These high level detachments were joined and breached by lower level faults in the basement which, in the external zones of the western Alps, generally reactivated and rotated the earlier east dipping half-graben bounding faults. The external massifs are essentially uplifted half-graben blocks. There was more reactivation and stacking of basement sheets in the eastern part of this external zone, where the faults had been rotated into more gentle dips above a shallower extensional detachment than on the steeper faults to the west.

There is no direct relationship between the weaker inversion of the Alpine foreland and the major orogenic contraction of the western Alps; the inversion structures of southern Britain and the Channel were separated from the Alps by a zone of rifting from late Eocene to Miocene which affected the Rhone, Bresse and Rhine regions. Though they relate to the same plate movements which formed the Alps, the weaker inversion structures must have been generated by within plate stresses, or from those emanating from the Atlantic rather than the Tethyan margin.     

Cenozoic post-collisional brittle tectonic history and stress reorientation in the High Zagros Belt (Iran, Fars Province)

The Zagros fold-and-thrust belt of SW-Iran is among the youngest continental collision zones on Earth. Collision is thought to have occurred in the late Oligocene–early Miocene, followed by continental shortening. The High Zagros Belt (HZB) presents a Neogene imbricate structure that has affected the thick sedimentary cover of the former Arabian continental passive margin. The HZB of interior Fars marks the innermost part of SE-Zagros, trending NW–SE, that is characterised by higher elevation, lack of seismicity, and no evident active crustal shortening with respect to the outer (SW) parts. This study examines the brittle structures that developed during the mountain building process to decipher the history of polyphase deformation and variations in compressive tectonic fields since the onset of collision. Analytic inversion techniques enabled us to determine and separate different brittle tectonic regimes in terms of stress tensors. Various strike–slip, compressional, and tensional stress regimes are thus identified with different stress fields. Brittle tectonic analyses were carried out to reconstruct possible geometrical relationships between different structures and to establish relative chronologies of corresponding stress fields, considering the folding process. Results indicate that in the studied area, the main fold and thrust structure developed in a general compressional stress regime with an average N032° direction of σ₁ stress axis during the Miocene. Strike–slip structures were generated under three successive strike–slip stress regimes with different σ₁ directions in the early Miocene (N053°), late Miocene–early Pliocene (N026°), and post-Pliocene (N002°), evolving from pre-fold to post-fold faulting. Tensional structures also developed as a function of the evolving stress regimes. Our reconstruction of stress fields suggests an anticlockwise reorientation of the horizontal σ₁ axis since the onset of collision and a significant change in vertical stress from σ₃ to σ₂ since the late stage of folding and thrusting. A late right-lateral reactivation was also observed on some pre-existing belt-parallel brittle structures, especially along the reverse fault systems, consistent with the recent N–S plate convergence. However, this feature was not reflected by large structures in the HZB of interior Fars. The results should not be extrapolated to the entire Zagros belt, where the deformation front has propagated from inner to outer zones during the younger events.     

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.     

The terminology of structures in thrust belts

A review of structures and geometric relationships recognized in thrust belts is presented. A thrust is defined as any contractional fault, a corollary being that thrusts must cut up-section in their transport direction. ‘Flats’ are those portions of a thrust surface which were parallel to an arbitrary datum surface at the time of displacement and ‘ramps’ are those portions of thrusts which cut across datum surfaces. Ramps are classified on the basis of their orientation relative to the thrust transport direction and whether they are cut offs in the hangingwall or footwall of the thrust. Lateral variations in the form of staircase trajectories are joined by oblique or lateral ramps which have a component of strike-slip movement.An array of thrusts which diverge in their transport direction may form by either of two propagation models. These are termed ‘piggy-back’ propagation, which is foreland-directed, and ‘overstep’ propagation which is opposed to the thrust transport direction. An array of thrust surfaces is termed an ‘imbricate stack’ and should these surfaces anastamose upwards a ‘duplex’ will result; the fault-bounded blocks are termed ‘horses’. A duplex is bounded by a higher, ‘roof’ thrust and a lower, ‘floor’ thrust. The intersection of any two thrust planes is termed a ‘branch line’.Thrusts can be classified on the basis of their relationship to asymmetric fold limbs which they cut. A further classification arises from whether a particular thrust lies in the hangingwall or footwall of another one.The movement of thrust sheets over corrugated surfaces, or the local development of thrust structures beneath, will fold higher thrust sheets. These folds are termed ‘culminations’ and their limbs are termed ‘culmination walls’. Accommodation of this folding may require movement on surfaces within the hangingwall of the active thrust. These accommodation surfaces are termed ‘hangingwall detachments’ and they need not root down into the active thrust. This category of detachment includes dip-slip ‘hangingwall drop faults’ which are developed by differential uplift of duplex roofs, and ‘out-of-the-syncline’ thrusts which develop from overtightened fold hinges. Back thrusts, as well as forming as hangingwall detachments, may also form due to layer-parallel shortening above a sticking thrust or by rotation of the hangingwall above a ramp.     

Multiphase structural evolution of the western margin of the Girardot subbasin, Upper Magdalena Valley, Colombia

More than 1400 km of two-dimensional seismic data were used to understand the geometries and structural evolution along the western margin of the Girardot Basin in the Upper Magdalena Valley. Horizons are calibrated against 50 wells and surface geological data (450 km of traverses). At the surface, low-angle dipping Miocene strata cover the central and eastern margins. The western margin is dominated by a series of en echelon synclines that expose Cretaceous–Oligocene strata. Most synclines are NNE–NE trending, whereas bounding thrusts are mainly NS oriented. Syncline margins are associated mostly with west-verging fold belts. These thrusts started deformation as early as the Eocene but were moderately to strongly reactivated during the Andean phase. The Girardot Basin fill records at least four stratigraphic sequences limited by unconformities. Several periods of structural deformation and uplifting and subsidence have affected the area. An early Tertiary deformation event is truncated by an Eocene unconformity along the western margin of the Girardot Basin. An Early Oligocene–Early Miocene folding and faulting event underlies the Miocene unconformity along the northern and eastern margin of the Girardot Basin. Finally, the Late Miocene–Pliocene Andean deformation folds and erodes the strata along the margins of the basin against the Central and Eastern Cordilleras.     

Heating during thrust faulting in the rocky mountains: friction or fiction?

Measurement of the levels of organic maturation below and within shear zones of thrust faults in the Rocky Mountains has revealed no general thermal metamorphism that can be attributed to faulting, with the exception of very localized areas. With few exceptions the vitrinite reflectance values obtained are in the range expected if the maximum level of organic maturation was produced as a result of increasing temperature during progressive burial accompanying sedimentation. Only at Marias Pass evidence has been found to suggest additional maturation as a result of post-orogenic burial below the thrust sheets. Anomalously high vitrinite reflectances obtained from the Lewis thrust, McConnell thrust, Coleman thrust and two unnamed thrusts, are restricted to very narrow films immediatly adjacent to, or within the shear zone which, considering any reasonable thermal conductivity, indicates elevated temperatures were very short lived. The anomalously high vitrinite reflectances within these films, when compared to laboratory heated coals, suggest temperatures in the order of 350°-650°C were locally generated during faulting. Such high temperatures are considered to have been generated during stick-slip faulting at macro-asperities or at ramps on the fault plane where local, and possibly transient, high frictional stresses existed. The absence of evidence for extensive frictional heating supports previous arguments that stable-sliding and/or low frictional stress must exist during thrusting.     

Rupture progression along discontinuous oblique fault sets: implications for the Karadere rupture segment of the 1999 Izmit earthquake, and future rupture in the Sea of Marmara

Large earthquakes in strike-slip regimes commonly rupture fault segments that are oblique to each other in both strike and dip. This was the case during the 1999 Izmit earthquake, which mainly ruptured E–W-striking right-lateral faults but also ruptured the N60°E-striking Karadere fault at the eastern end of the main rupture. It will also likely be so for any future large fault rupture in the adjacent Sea of Marmara. Our aim here is to characterize the effects of regional stress direction, stress triggering due to rupture, and mechanical slip interaction on the composite rupture process. We examine the failure tendency and slip mechanism on secondary faults that are oblique in strike and dip to a vertical strike-slip fault or “master” fault. For a regional stress field well-oriented for slip on a vertical right-lateral strike-slip fault, we determine that oblique normal faulting is most favored on dipping faults with two different strikes, both of which are oriented clockwise from the strike-slip fault. The orientation closer in strike to the master fault is predicted to slip with right-lateral oblique normal slip, the other one with left-lateral oblique normal slip. The most favored secondary fault orientations depend on the effective coefficient of friction on the faults and the ratio of the vertical stress to the maximum horizontal stress. If the regional stress instead causes left-lateral slip on the vertical master fault, the most favored secondary faults would be oriented counterclockwise from the master fault. For secondary faults striking ±30° oblique to the master fault, right-lateral slip on the master fault brings both these secondary fault orientations closer to the Coulomb condition for shear failure with oblique right-lateral slip. For a secondary fault striking 30° counterclockwise, the predicted stress change and the component of reverse slip both increase for shallower-angle dips of the secondary fault. For a secondary fault striking 30° clockwise, the predicted stress change decreases but the predicted component of normal slip increases for shallower-angle dips of the secondary fault. When both the vertical master fault and the dipping secondary fault are allowed to slip, mechanical interaction produces sharp gradients or discontinuities in slip across their intersection lines. This can effectively constrain rupture to limited portions of larger faults, depending on the locations of fault intersections. Across the fault intersection line, predicted rakes can vary by >40° and the sense of lateral slip can reverse. Application of these results provides a potential explanation for why only a limited portion of the Karadere fault ruptured during the Izmit earthquake. Our results also suggest that the geometries of fault intersection within the Sea of Marmara favor composite rupture of multiple oblique fault segments.