defining a 3 dimensional trishear parameter space to understand the temporal evolution of fault propagation folds abstract the application of trishear in which deformation occurs in a triangular zone in front of a propagating fault tip is often used to understand fault related folding a key element of trishear in comparison to kink band methods is that non uniform deformation within the triangle zone allows the layer thickness and length to change during deformation by varying three controlling parameters independently trishear propagation slip ratio trishear apical angle and fault dip we construct a three dimensional parameter space to demonstrate the variability of resultant geometry feasible with trishear we plot published natural examples in this parameter space and identify two clusters and show that the most applicable typical trishear propagation slip ratio is 2 to 3 while the trishear apical angle varies from 30 to 100 we propose that these findings can help estimate the best fit parameters for natural structures we then consider the temporal evolution of specific geometric examples and factors that increase the complexity of trishear including 1 fault dip changes and 2 pre existing faults to illustrate the applicability of the parameter space and complex trishear models to natural examples we apply our results to a sub surface example from the qaidam basin in northern tibetan plateau

true three dimensional trishear a kinematic model for strike slip and oblique slip deformation most structural kinematic models are inherently two dimensional even several recent three dimensional models are pseudo three dimensional in that they consist of a series of parallel two dimensional cross sections lack of a true three dimensional formulation hampers our abilities to simulate three dimensional structures such as oblique and strike slip faulting and displacement gradients perpendicular to the slip vector the mathematical formulation of trishear deformation using incompressibility of flow is well suited to a solution in three dimensions we derive one plausible velocity field for true three dimensional flow in a triangular shear zone this formulation allows us to simulate the deformation in oblique slip deformation zones as well as flower structures associated with strike slip fault zones the strain distribution in flower structures combined with some simple mechanical assumptions suggests that faults in these zones would have a helicoidal geometry the results of the kinematic model compare well to well described structures in the colorado plateau andaman sea and death valley as well as to data from analogue experiments

geometric analysis of fold development in overthrust terranes abstract fault bend folding fault propagation folding and detachment or decollement folding are three distinct scenarios for fold thrust interaction in overthrust terranes simple kink hinge models are used to determine the geometric associations implicit in each scenario bedding maintains constant thickness in the models except in the forelimb of the fold the forelimb is allowed to thicken or thin without limit the models address individual folds and the calculated fold geometries are balanced structures each mode of fold thrust interaction has a distinct set of geometric relationships final fold geometry is adequate in itself to discern many fault bend folds this is not the case for fault propagation and detachment folds these two fold forms have very similar geometric relationships some knowledge of the nature of the underlying thrust or decollement zone is usually needed to distinguish between them the geometry of a fold is altered in a predictable fashion by transport through an upper ramp hinge and by fault parallel shearing of the structure the shearing results in a tighter fold whereas transport through the ramp hinge produces a broader fold the viability of the geometric analysis technique is demonstrated through its application to a pair of detachment folds from the canadian cordillera the geometric analysis is also used to evaluate cross sections through subsurface structures in an example from the turner valley oil field the analysis indicates how the interpretation should be altered so as to balance the cross section the analysis reveals hidden assumptions and specific inconsistencies in structural interpretations

piggyback basin in the sevier orogenic belt utah implications for development of the thrust wedge timing data on a frontal thrust system indicate that the sevier thrust wedge of central utah advanced rapidly to its ultimate structural front in 30 m y between early albian and campanian time it then persisted by out of sequence thrusting for an additional 25 m y until the early eocene the key to the history of the frontal thrust is a piggyback basin that formed on the hanging wall unconformity bounded sedimentary units within the basin thin both westward and eastward onto structures generated by thrust faulting up to 860 m of upper campanian to lower eocene strata accumulated between these structures which consist of a ramp anticline in the west and a thrust duplex in the east the newly recognized interaction of coeval deposition and thrust deformation permits revised limits on the inception and longevity of the frontal thrust system

bed by bed fold growth by kink band migration sant llorenc de morunys eastern pyrenees ahstract rowth strata deposited over and against the flank of the sant llorenc de morunys fold during its final stages of deformation have been mapped at high resolution as the basis for unraveling the kinematics of fold growth we use restoration techniques based on normal balancing assumptions to decipher the detailed kinematic history of folding the progressive restorations as well as balanced forward modeling show that the last few hundred meters of fold growth were dominated by kink band migration of a sort that is typical of much faultrelated folding the kink band migration has produced complex anticlinal hinge zone geometry including segmented fold hinges linked by disconformities and unconformities which has direct and detailed explanation in terms of fluctuations in deposition rate relative to curved hinge kink band migration rate large fluctuations in the convolution of non steady sedimentation and deformation are demonstrated although the absolute fluctuations in deformation and sedimentation are unknown at a length scale of 100 m kink band migration with little or no deposition is interspersed with sedimentation with little or no deformation at the length scale of 500 m deposition ranges from 200 to 50 of uplift 0 1997 elsevier science ltd all rights reserved

subsurface structural interpretation by applying trishear algorithm an example from the lenghu5 fold and thrust belt qaidam basin northern tibetan plateau abstract the application of trishear algorithm in which deformation occurs in a triangle zone in front of a propagating fault tip is often used to understand fault related folding in comparison to kink band methods a key characteristic of trishear algorithm is that non uniform deformation within the triangle zone allows the layer thickness and horizon length to change during deformation which is commonly observed in natural structures an example from the lenghu5 fold and thrust belt qaidam basin northern tibetan plateau is interpreted to help understand how to employ trishear forward modelling to improve the accuracy of seismic interpretation high resolution fieldwork data including high angle dips dragging structures thinning hanging wall and thickening footwall are used to determined best fit trishear model to explain the deformation happened to the lenghu5 fold and thrust belt we also consider the factors that increase the complexity of trishear models including a fault dip changes and b pre existing faults we integrate fault dip change and pre existing faults to predict subsurface structures that are apparently under seismic resolution the analogue analysis by trishear models indicates that the lenghu5 fold and thrust belt is controlled by an upward steepening reverse fault above a pre existing opposite thrusting fault in deeper subsurface the validity of the trishear model is confirmed by the high accordance between the model and the high resolution fieldwork the validated trishear forward model provides geometric constraints to the faults and horizons in the seismic section e g fault cutoffs and fault tip position faults intersecting relationship and horizon fault cross cutting relationship the subsurface prediction using trishear algorithm can significantly increase the accuracy of seismic interpretation particularly in seismic sections with low signal noise ratio

is the north altyn fault part of a strike slip duplex along the altyn tagh fault system although the altyn tagh fault system has played an important role in the indo asian collision its geometry and tectonic evolution remain poorly known between 86 and 92 e this system is at least 100 km wide and is bounded to the north and south by the north altyn and altyn tagh faults respectively mapping along the jianglisai reach of the north altyn fault indicates that miocene to pliocene motion was predominantly left to left reverse slip with transport vectors trending n45 60 e map relationships suggest that total offset on the fault is 120 km these results are inconsistent with previous models of the altyn tagh fault system in which oblique convergence along the northern margin of the tibetan plateau is partitioned into thrusting on the north altyn fault and left slip on the altyn tagh fault an alternative hypothesis is that the north altyn fault is the northern boundary of a transpressional strike slip duplex within which the structurally elevated altyn mountains were created our model suggests that transpressional deformation may be restricted to this strike slip duplex and need not characterize the entire margin

intraplate deformation and basin formation during the tertiary within the northern iberian plate origin and evolution of the almazan basin the tertiary almazan basin was a topographic high during the mesozoic rifting at the northern part of the iberian plate it was bordered by two mesozoic basins 1 the castilian basin with a maximum thickness of more than 2000 m to the south and 2 the cameros basin with a maximum thickness of 9000 m to the north to the east the almazan basin is bordered by paleozoic rocks of the nw se trending aragonian range which underwent little subsidence during the mesozoic the two mesozoic basins show an e w direction during the tertiary nne ssw compression mainly late eocene early miocene both mesozoic basins were inverted and uplifted and the almazan basin with an overall synclinal geometry was filled with syn tectonic terrestrial sediments reaching a maximum thickness of 3500 m deformation during the tertiary was strongly controlled by preexisting structures both mesozoic extensional faults and reactivated variscan folds the depocenter of the tertiary basin migrated southward during the paleogene this migration is consistent with a piggyback transport of the whole basin northward due to the movement on the underlying crustal scale north verging cameros thrust the geometry and sedimentary evolution of the almazan thick skinned piggyback basin was controlled by the uplift associated with the main crustal scale thrust which was the main source area and was separated from the foreland basin since the early stages of filling late eocene this makes the main difference with thin skinned piggyback basins controlled by the foreland thrust belt and separated from the foreland basin only at their late stages

neogene extension and volcanism in the kunlun fault zone northern tibet new constraints on the age of the kunlun fault 1 the geology of the jingyu basin north tibet provides constraints on the relationship between localized e w extension in the qiangtang block and strike slip motion along the kunlun fault eocene to oligocene sw ne compression formed topographic relief in the present western kunlun shan pull apart basins started forming in the western kunlun ranges around 15 ma possibly coincident with the initiation of subduction of the tarim qaidam lithosphere under the kunlun shan extensional tectonism is still active and the jingyu basin is extending as a normal fault connected with the kunlun fault was recently active localized extension allows tertiary and quaternary shoshonitic magmas to reach the surface from crustal depths of 50 to 60 km miocene strike slip faulting indicates that the initiation of the kunlun strike slip fault is early neogene in age and that the total amount of horizontal movement that occurred along the fault has to be reconsidered

a wide angle seismic reconnaissance survey of the crust and upper mantle in the celtiberian chain of eastern spain abstract data from seismic refraction profiles to investigate the crust and upper mantle in the celtiberian chain are presented and discussed a rather heterogeneous deep crustal structure with lateral variations can be inferred from the available data in the central part of the chain crustal depths of about 32 km are coincident with those in the iberian meseta whereas close to the mediterranean shore the moho is located at about 20 km depth the varying thickness of the crust can be interpreted in terms of different tectonic regimes the shallowing towards the mediterranean is associated with the neogene rifting process which has its main expression in the gulf of valencia but whose effects seem to reach well towards the interior of iberia the upper mantle has a structure typical for the hercynian domain in europe a seismic phase reflected from about 80 km is the most important feature this may be interpreted as a mantle discontinuity with a positive velocity jump or alternatively as an indication for the bottom of the lithosphere

deformation patterns and structural vergence in brittle ductile thrust wedges an additional analogue modelling perspective abstract scaled physical models of brittle ductile thrust wedges investigated the causes leading to the development of the various structural styles observed in fold and thrust belts and accretionary prisms this study focused on some potential controlling factors such as 1 relative strength between the brittle overburden and the viscous decollement layer brittle ductile coupling 2 the effect of very low basal friction and 3 pre shortening rheological stratigraphy including decollement strength type 1 models had a single basal decollement whereas in type 2 models the decollement was embedded in brittle material simulating an upper roof sequence and a lower floor sequence other models investigated the effects produced by an horizon of mechanical weakness at the base of the viscous decollement type 1a and the role of decollement offset type 3 the experiments suggest the occurrence of a genetic relationship between the absolute values of shear stresses and the development of dominating hinterland or foreland verging thrust faults or dually verging thrusts by controlling the orientation of the 1 axis basal friction and decollement offset is found to effectively influence the deformation pattern and structural vergence and could explain the development of dominating backthrusts in cascadian type margins the decoupling of a frontal monocline via the upper decollement type 2 models produced passive roof duplex structures termed composite roof duplex when the frontal monocline coexists with outward fold propagation

a unified kinematic model for the evolution of detachment folds abstract detachment folds represent a major structural element in a number of fold belts they are common in the jura mountains the zagros fold belt the central appalachian fold belt the wyoming fold belt the brooks range the parry islands fold belt and parts of the subandean belt these structures form in stratigraphic packages with high competency contrasts among units the competent upper units exhibit parallel fold geometries whereas the weak lower unit displays disharmonic folding and significant penetrative deformation two distinct geometric types disharmonic detachment folds and lift off folds have been recognized however these structures commonly represent different stages in the progressive evolution of detachment folds the structures first form by symmetric or asymmetric folding with the fold wavelength controlled by the thickness of the dominant units volumetric constraints require sinking of units in the synclines and movement of the ductile unit from the synclines to the anticlines continuing deformation results in increasing fold amplitudes and tighter geometries resulting from both limb segment rotation and hinge migration initially limb rotation occurs primarily by flexural slip folding but in the late stages of deformation the rotation may involve significant internal deformation of units between locked hinges the folds eventually assume tight isoclinal geometries resembling lift off folds variations in the geometry of detachment fold geometry such as fold asymmetry significant faulting and fold associated with multiple detachments are related to variations in the mechanical stratigraphy and pre existing structure

mechanical stratigraphy and the transition from trishear to kink band fault propagation fold forms above blind basement thrust faults a discrete element study abstract we employ a discrete element technique to investigate the influence of mechanical stratigraphy on the transition from trishear to kink band fault propagation fold forms above a blind basement thrust fault we find that a classic trishear fold consisting of a broad upwards widening monocline with limb dips increasing towards the fault tip occurs where the cover stratigraphy is weak and has no strong mechanical layering in contrast we find that marked bedding parallel mechanical layering in the cover promotes much narrower kink like fold geometries with more constant limb widths and dips we discuss the modelling results in terms of their implications for structural interpretation and the application of kinematic models of fault related folding in contractional settings

pseudo 3 d modeling of trishear fault propagation folding abstract basement structures to which trishear fault propagation models have most successfully been applied are commonly three dimensional folds formed at the tip line of a fault we present here a pseudo 3d trishear model in which various parameters are permitted to vary along strike and oblique slip can be modeled these variations may be combined in an infinite number of ways facilitating the simulation of many real structures a thrust changing from blind to emergent can be produced by a change in the slip or propagation to slip p s along strike folds with forelimbs changing from overturned to upright along strike can be modeled either by changing the slip p s or trishear angle also some minor folds perpendicular or oblique to the main structure can result from changes in the trishear angle or fault angle along the strike models including growth strata show that it is practically impossible to distinguish between growth and pre growth strata using the map patterns as a field test we have modeled the oblique slip east kaibab monocline demonstrating a good fit between the field observations and model predictions

numerical modeling of trishear fault propagation folding in contrast to kink band migration modeling methods trishear numerical models produce fault propagation folds with smooth profiles and rounded hinges modeled fold hinges tighten and converge downward within a triangular zone of distributed deformation which is focused on the fault tip such features have been reported from field studies and are also seen in analogue models of compressional deformation however apart from its initial application to laramide folds little quantitative work has been undertaken on trishear fault propagation folding in other settings in addition no study has been undertaken into the growth strata which might be associated with such structures this paper uses an equivalent velocity description of the geometric model of trishear together with models of erosion and sedimentation to investigate trishear fault propagation folding of both pregrowth and growth strata the trishear model is generalized to include a variety of fault propagation to slip ratios and fault propagation from a flat decollement the models show continuous rotation of the forelimb with the characteristic development of cumulative wedges within growth strata when total slip on a structure is high the model predicts overturned pregrowth and growth strata during the initial stages of deformation beds in the forelimb thicken but later thin when they become steep or overturned the effect of variations in fault propagation to slip ratios on two dimensional finite strain in the models is assessed by the use of initially circular strain markers high fault propagation to slip p s ratios lead to narrow zones of high finite strain while lower p s ratios lead to more ductile deformation and broader zones of high strain in all cases hanging wall anticlines and footwall synclines originate as early ductile folds which are later cut by the propagating fault modeled structures are compared with natural examples

application of trishear fault propagation folding to active reverse faults examples from the dalong fault gansu province nw china abstract determining accurate fault slip rates at 1 ka to 1 ma timescales requires well constrained palinspastic reconstructions of dateable geomorphic and or geologic markers although general kinematic models have been developed to simultaneously reconstruct both bedrock e g bedding and fault attitudes and neotectonic markers e g strath terraces along active strike slip and thrust faults it is not clear if these models can also account for deformation along steeply dipping 45 reverse faults to address this problem we have investigated the active 50 dipping dalong reverse fault system this 40 km long fault system forms part of the aksai restraining stepover along the active left slip altyn tagh fault in northwestern china our geometric and kinematic analyses show that conventional fault bend fold models cannot satisfy the steeply dipping fault geometry we observe in the bedrock record likewise standard fault propagation fold models fail to match our measurements of a set of fluvial terraces however by expanding the trishear model of fault propagation folding to track both bedrock and neotectonic markers we are able to match both sets of records in particular we have developed trishear kinematic models for two sites liuchenzi and qingyazi using the numerical modeling program fault fold v 5 0 this work indicates that an important implication of active trishear fault propagation folding is that terrace deformation extends for over 1 km on either side of the fault trace thus to accurately measure the total magnitude of vertical separation between matching terraces in the hanging wall and footwall terrace profiles across active reverse faults must extend 1 2 km on either side of this zone of deformation

a quadshear kinematic model for folding and clay smear development in fault zones abstract a new kinematic model based on the trishear model has been developed to simulate shearing and folding of a ductile interval sandwiched between two brittle layers as faults propagate from the brittle layers into the ductile interval this quadshear model can be used to study fault zone evolution and the development of clay smears which often form when a stratigraphic sequence including ductile clay rich beds is sheared into a developing fault zone the model also gives the critical clay smear factor ccsf the ratio of fault displacement to bed thickness at which a clay smear becomes discontinuous and hence a potential leak point the main control on clay smear development is the position of the clay rich beds relative to brittle fracture prone layers clay rich beds near the centre of a ductile interval can shear to form much longer continuous clay smears than clay rich beds adjacent to brittle layers fault propagation rates are also a strong control on clay smear development slow propagating faults form longer continuous clay smears than fast propagating faults however the width of the shear zone the dip of the fault and the presence of releasing or restraining bends in the fault are minor controls on clay smear development

numerical investigation of dip slip fault propagation effects on offshore seabed sediments abstract an enhanced understanding of the phenomenon of dip slip fault propagation through young unconsolidated offshore sediments is essential for the evaluation of the response of shallow offshore structures and subsea oil gas pipelines substantial and permanent seafloor deformation constitutes a severe geohazard for these marine structures and associated offshore elements components this paper addresses the response of seabed sand deposits subjected to underlying dip slip fault movement the resulting propagation of the faulting offset in seabed sediments is explored using 2d finite element modeling in abaqus while accounting for nonlinear soil behavior with strain softening a 2d plain strain fem seabed model is adopted to investigate the effect of different seabed soil properties and layer thicknesses on the extent and magnitude of seabed surface deformation induced by normal and reverse faults with different dip angles the results indicate that 1 the adoption of sand properties that depend on depth is necessary to realistically model the fault propagation mechanism particularly for fault propagation through thick and dense sand deposits 2 the presentation of fault propagation results in a normalized manner with respect to sand cover thickness does not eliminate the dependency on the actual layer thickness and 3 the deformation of the beds in the cover as a result of basement faulting can be modeled realistically using trishear kinematic approaches

discrete element modelling of contractional fault propagation folding above rigid basement fault blocks many studies have shown that discrete blind faults at depth are commonly linked to more distributed deformation in particular folding at higher levels one category of fault related folds forced folds is common where there is a distinct mechanical contrast between faulted basement and sedimentary cover outcrop numerical and analogue modelling studies indicate that such folds form as upward widening zones of distributed deformation monoclines above discrete faults at depth with increasing displacement the folds are often cut by faults as they propagate upwards into the cover while the trishear kinematic model of fault propagation folding appears to approximately represent the geometric development of such structures comparatively little is known of the mechanical controls on their development here we present a 2d discrete element model of sedimentary cover deformation above a contractional fault in rigid basement the elements consist of a series of soft spheres that obey newton s equations of motion and initially interact with elastic forces under the influence of gravity particles are bonded until the separation between them exceeds a defined breaking strain at which time the bond breaks simulated by the transition from repulsive attractive forces to solely repulsive forces the model is used to investigate the influence of basement fault dip and sedimentary cover strength on the geometry of the folds developed and the rate of fault propagation in all cases an upward widening monocline occurs above the basement fault we find that shallow basement fault dips produce homogenous thickening of the monocline limb while steeper dips produce contemporaneous thinning and thickening within the monocline thinning and thickening within the monocline are accommodated by a combination of small scale faulting and folding with decreasing cover strength the zone of deformation becomes wider localization does not occur on a single fault and fold geometries resemble trishear fold profiles with low propagation to slip ratios p s 1 in contrast a stronger cover produces a narrower zone of deformation localization on a single fault and more rapid fault propagation similar to trishear fold profiles where p s 2 3 the fault propagates into the cover at approximately the same angle as the basement fault the model reproduces well many of the features observed in analogue modelling and reported from outcrop and seismic studies

structural deformation pattern within the nw qaidam basin in the cenozoic era and its tectonic implications abstract the qaidam basin is located in the northeastern tibetan plateau and provides an excellent field laboratory in understanding the history and mechanism of the plateau growth it deformed widely over the northwest during the cenozoic but with little thrust loading along the margins where no foreland depression is observed based on satellite images seismic and borehole data we investigated the structural deformation pattern including the structural style and timing of deformation and its formation mechanism within the northwestern qaidam basin during the cenozoic era mapping of surface geology shows that the modern qaidam basin is characterized by five se trending anticlinal belts each belt consists of several right step en echelon anticlines with plenty of normal and strike slip faults crossing the crests those anticlines are generally dominated by double fault systems at different depths an upper thrust fault system controlling the anticlines identified on the surface and a lower dextral transpressional fault system characterized by typical flower structures they are separated by weak layers in the upper xiaganchaigou or the shangganchaigou formations the upper system yields shortening strain 2 5 times larger than that of the lower system and the additional strain is interpreted to be accommodated by hinge parallel elongation in the upper system growth strata indicate that deformation within the qaidam basin initiated in the middle miocene 15 ma and accelerated in the late miocene 8 ma a simple riedel p shear model is used to explain the deformation mechanism within the northwestern qaidam basin

cenozoic deformation history of the qaidam basin nw china results from cross section restoration and implications for qinghai tibet plateau tectonics abstract the deformation and uplifting history of the qinghai tibet plateau is a controversial issue and so far there has been insufficient evidence for interpretation investigations of the cenozoic deformation in the northeastern edge of this plateau may provide some relevant information in this paper the cenozoic deformation history of the qaidam basin has been studied based on results from the balanced cross section restoration of six geological sections they indicate that 1 the qaidam basin has been undergoing continuous shortening since the beginning of cenozoic with two relatively fast shortening phases the first during 42 8 40 5 ma and the second during 2 8 ma present 2 the shortening rate of the qaidam basin reached its maximum value since 2 8 ma and approximately 32 of the average total cenozoic shortening took place in this period and 3 the shortening of all sections reached approximately 50 of their respective total shortening by miocene the cenozoic deformation history of the qaidam basin shows good synchroneity with uplifting history of the qinghai tibet plateau it is therefore proposed that the deformation and uplifting in the south and north edges of the qinghai tibet plateau was almost synchronous with the altyn tagh fault formed at the beginning of india asia collision accordingly it is postulated that since the india asia collision at the beginning of cenozoic the indian plate moved northward continuously with two relatively fast phases the first during 42 8 40 5 ma and the second during 2 8 ma present approximately 50 of the total cenozoic northward displacement of the indian plate of the total amount of crustal shortening and uplifting of the qinghai tibet plateau and of the total cenozoic strike slip displacement of the altyn tagh fault were probably reached by the early miocene moreover another 30 40 might have taken place during 2 8 ma present the periods during 42 8 40 5 ma and during 2 8 ma present may also be the two fast phases for crustal shortening and uplifting of the qinghai tibet plateau and strike slip displacement of the altyn tagh fault

tectonic and sedimentary evolution of basins in the northeast of qinghai tibet plateau and their implication for the northward growth of the plateau the northeast of the qinghai tibet plateau from the hoh xil basin to the hexi corridor basin demonstrates a basin ridge geomorphy that is the result of the long term uplift of the qinghai tibet plateau the geological evolution of the qinghai tibet plateau is recorded in its associated basins from the cenozoic sedimentary filling pattern in the hoh xil qaidam and jiuquan basins we find that the evolution of these basins is similar they began as strike slip basins evolved into foreland basins and then to intermontane basins foreland basins are the direct result of orogenic activity in the northeast of qinghai tibet plateau and can be documented to have formed over the following time periods 49 23 ma for the hoh xil foreland basin 46 2 45 ma for the qaidam foreland basin and 29 5 0 ma for the jiuquan foreland basin this northward migration in the timing of foreland basin development suggests that the northeastern part of tibet grew at least in part by accretion of cenozoic sedimentary basins to its northern margin

reconstruction of the altyn tagh fault based on u pb geochronology role of back thrusts mantle sutures and heterogeneous crustal strength in forming the tibetan plateau 1 reconstructing deformation along the northwestern margin of the tibetan plateau is critical for evaluating the relative importance of microplate versus continuum models of the indo asian collision questions regarding this margin s evolution are as follows 1 what is the total offset along the sinistral altyn tagh strike slip system 2 how has that offset been absorbed in the western kunlun shan 3 why does the n s width of the plateau vary along strike ion microprobe u pb zircon apparent ages of 17 plutons from nw tibet together with regional geologic observations define a discrete e w trending boundary between two tectonic belts that has been offset along the altyn tagh system by 475 70 km kinematic arguments indicate that this offset cannot be the result of north directed thrusting in the western kunlun shan therefore we propose that south directed faulting in the tianshuihai thrust belt both offset the tectonic boundary and produced the asymmetry in the plateau shortening appears to have been absorbed in the upper crust by thin skinned thrusting in the middle lower crust by east directed ductile flow and or subduction and in the mantle by north dipping subduction factors controlling the formation of the south directed thrust system appear to be the contrast between the rigid tarim and the weaker songpan ganzi flysch belt and a mantle suture inherited from late paleozoic subduction the evolution of western tibet leads to a view of continental deformation that integrates elements of the microplate model e g plate like mantle and crust mantle decoupling with aspects of the continuum model weak crustal flow beneath the plateau

cenozoic tectonic evolution of the qaidam basin and its surrounding regions part 3 structural geology sedimentation and regional tectonic reconstruction the qaidam basin is the largest topographic depression inside the tibetan plateau because of its central position understanding the tectonic origin of the qaidam basin has important implications for unraveling the formation mechanism and growth history of the tibetan plateau in order to achieve this goal we analyzed regional seismic reflection profiles across the basin and a series of thickness distribution patterns of cenozoic strata at different time slices the first order structure of the basin is a broad cenozoic synclinorium which has an amplitude ranging from 16 km in the west to 48 in the west to 15 s 1 to 1 3 10 17 s 1 the eastward decrease in upper crustal shortening requires a progressive shift in crustal thickening mechanisms across qaidam basin from dominantly upper crustal shortening in the west to dominantly lower crustal shortening in the east although sedimentation began synchronously at 65 50 ma across the entire basin the initiation ages of the southern and northern basin bounding structures are significantly different deformation started at 65 50 ma in the north and at 29 24 ma in the south this information and the existing inference that the uplift of the eastern kunlun range south of qaidam basin began after 30 20 ma imply that the paleogene 65 24 ma qaidam and hoh xil basins on both sides of the eastern kunlun range may have been parts of a single topographic depression 500 km wide in the north south direction between the qilian shan and fenghuo shan thrust belts in the north and south the development of this large paleogene basin in central tibet and its subsequent destruction and partitioning by the neogene uplift of the eastern kunlun range requires a highly irregular sequence of deformation possibly controlled by preexist ing weakness in the tibetan lithosphere

temporal and spatial patterns of cenozoic deformation across the qaidam basin northern tibetan plateau we conducted a systematic analysis of the temporal and spatial patterns of cenozoic deformation of the northeastern ne and southwestern sw parts of the qaidam basin the largest and thickest cenozoic sedimentary basin within the tibetan plateau by constructing balanced cross sections traversing the whole basin using high resolution seismic reflection data we found that that these two parts deformed synchronously with similar temporal patterns involving nearly constant ne sw contraction before c 2 5 ma and much more intense shortening possibly enhanced by fierce wind erosion since then however the spatial deformation patterns of these two areas differ markedly an eastward weakening trend in the sw part is not observed in the ne part we believe that this difference is due to their different boundary conditions this result places new constraints on the formation mechanisms of the northern tibetan plateau

origin of the permian triassic iberian basin central eastern spain abstract the iberian basin was an intracratonic rift basin in central eastern spain developed since early permian times the basin boundary faults were normal listric faults controlling an asymmetric extension propagating northeast with time hercynian or older lineaments controlled the orientation of the iberian basin and extension was accommodated basically in the hanging wall block by the formation of secondary grabens and a central high the basin was related with the coeval ebro catalan and cuenca mancha basins and their connections are discussed subsidence curves show that the early permian early jurassic period of extension can be subdivided into three rifting episodes and a flexural one extension factor increases from 1 17 in the northwest to 1 29 near the mediterranean coast the increasing extension rate was accommodated by transfer faults trending nne ssw more important in the levante area the rift evolution is intermittent and seems to reflect distinct stress fields the collapse of the late hercynian orogen and related increased heat flux extension and rifting is the most probable origin of the iberian basin and related basins the origin of the catalan and the valencia prebetic basins is related to the southwards migration of the hesse burgundy rift along the eastern margin of the iberian microplate

experimental insights on the geometry and kinematics of fold and thrust belts above weak viscous evaporitic decollement abstract this paper discusses the differences in mechanical properties and kinematics between fold and thrust belts detaching on evaporitic decollements and those detaching on stronger detachments physical experiments are described that model shortening of a thick brittle cover overlying a weak viscous decollement to gain a better understanding of these differences we tested the influence of 1 the decollement layer thickness and 2 the presence of a deformable backstop on the hinterland side and of a decollement pinch out on the foreland side because of the very low shear strength of the viscous decollement folds and thrusts did not propagate according to a piggy back sequence but by centripetal back and forth propagation additional shortening was accommodated by growth of all existing structures fold symmetry and thrust vergence varied between experiments models confined between two rigid vertical endwalls always deformed by symmetric folding and thrusting in models having a deformable backstop and a foreland decollement pinch out forethrusting initially dominated folds were asymmetric and fault blocks rotated in models having a thick decollement layer folds kept growing asymmetrically diapirism also depended on the initial decollement geometry diapirs formed only in models having a deformable backstop and were restricted to the hanging wall of a fault related fold finally the implications of such kinematics on the thermal history are discussed as well as the likelihood of hydrocarbon maturation and preservation

deformation and secondary faulting associated with basement involved compressional and extensional structures experimental clay modeling is used to study the geometry and evolution of deformation zones formed above basement faults the nature of deformation is compared for reverse faults dipping 30 45 and 60 vertical faults and normal faults dipping 60 and 75 basement faulting results in the formation of a sheared triangular deformation zone that widens upward in the cover units most of the deformation is focused within a central zone of secondary faults which propagate upward and eventually break through the entire section basal units exhibit steeper dips and higher fault densities whereas stratigraphically higher units exhibit gentler dips and lower fault densities the width of the deformation zone is generally greater for reverse faults with low dips the anticlinal axial surface typically dips in the same direction as the synclinal axial surface for low to moderate angle reverse faults however the axial surfaces dip in opposite directions for vertical faults and normal faults particle paths suggest a convex upward rotation within the deformation zone suggesting transfer of material across the extension of the master fault reverse faults are associated with secondary faults with reverse and thrust separations vertical faults are associated with mostly vertical and reverse secondary faults whereas normal faults are generally associated with normal separation of most secondary faults therefore the nature of secondary faults varies with structural position possibly within the same layer these results provide some key insights that are useful in the interpretation of macroscopic surface and subsurface basement involved structures

evolving alluvial stratigraphy and thrust front development in the west jaca piggyback basin spanish pyrenees in the southwestern pyrenees distinctive alluvial mega sequences correlate with the development of a shallow mountain front thrust structure the pena flexure the flexure evolved during the oligocene to early miocene and is interpreted as a duplex marking the limit of thrust front advance three main depositional facies preserved completely across the pena flexure are described a pre thrust emergence high sinuosity fluvial sequence the c 2300 m thick sheet sandstone facies of the campodarbe formation a syn thrust emergence conglomeratic alluvial fan sequence the c 1100 m thick conglomeratic facies of the bernues formation and a post thrust emergence low sinuosity fluvial sequence the 700 m thick ribbon sandstone facies of the uncastillo formation collectively their continuous exposure across the pena flexure provides a more or less complete record of the sedimentary evolution of a mountain front syn deformational features in the alluvial fan sequence of the bernues formation and marked contrasts in palaeochannel morphology sandstone body geometry and palaeocurrent patterns between the two fluvial sequences are best understood by invoking a strong tectonic control over their deposition an increased rate of vertical sediment accretion was a key process in determining fluvial style the onset of major thrusting led to more rapid basin floor subsidence thereby promoting a change from lateral to vertical accretion as channels responded to the rising base level

cenozoic fault systems in southwest qaidam basin northeastern tibetan plateau geometry temporal development and significance for hydrocarbon accumulation we present a detailed structural analysis of geometry and temporal development of the cenozoic faults in the southwestern part of the qaidam basin the largest petroliferous sedimentary basin and the only one producing oil and gas within the tibetan plateau northwest china based on three dimensional 3d seismic and well log data the cenozoic faults are mostly steep 50 70 basement involved reverse faults trending primarily west northwest and secondarily north south the two fault sets are commonly linked to each other by west northwest oriented faults bending either southward at their eastern tips or northward at the western tips and originated possibly from inversion of pre existing extensional faults in response to the far field effect of the cenozoic india eurasia collision faults or fault segments active during different time periods were identified from isopach maps and verified via seismic reflection profiles showing that the faults became active as early as the paleocene in the southern part of the southwest qaidam basin and propagated dominantly northward and subordinately eastward over time measurement of throws on major faults indicates that fault activity intensified over time and culminated since the mid miocene these faults have been an important factor in forming the oil fields in the southwest qaidam basin by improving permeability forming anticlinal traps and acting as conduits for oil migrating from source rocks to the reservoirs

geometric constraints derived from the law of conservation of volume and applied to evolutionary models for detachment folding this paper presents geometric arguments for modifying the conventional concept for the evolution of concentric fold trains above nonductile detachment zones the common geometric model for concentric folding above a detachment invokes a wave train with fixed fold axes regularly spaced along the fold panel limb lengths from crest to trough remain constant as the limb dips increase to uplift the anticlinal crests while the synclinal troughs slide parallel to the detachment plane the basic depth to detachment calculation which derives from the law of conservation of volume requires that the area of uplift above regional for a concentric anticline be equal to the arithmetic product of the shortening and the depth to detachment this imposes a geometric constraint with which a constant limb length anticline growing by increasing limb dip above a fixed detachment plane cannot comply unless in the early stages mobile material in the detachment zone flows from the synclinal troughs to the anticlinal crests and then in the later stages the flow direction reverses relatively few detachment zones contain a significant amount of mobile salt or shale therefore in most concentric detachment fold trains a basic geometric incompatibility exists between the conventional geometric model and themore law of conservation of volume the conflict can be resolved by an evolutionary fold model wherein the anticlinal fold limbs are short at the inception of folding and grow longer as dips increase and the fold grows in this model the anticlinal axes generally are fixed and the synclinal axes active which has structural and economic implications for the distribution of permeability and syntectonic sediments as well as for the migration and entrapment of hydrocarbons 18 figs less

influence of mechanical stratigraphy and initial stress state on the formation of two fault propagation folds abstract kinematic and mechanical modeling of the rip van winkle se new york usa and la zeta sw mendoza argentina anticlines illustrate the influence of mechanical stratigraphy and initial stress state on the kinematics of fault propagation folding in both anticlines faults nucleating at distinct stratigraphic levels open upward into triangular zones of folding folding intensity and finite strain attenuates with distance from the fault tip trishear reproduces the bulk geometry and finite strain of the relatively homogeneous limestone sequence of the rip van winkle anticline and predicts an initial location of the fault tip consistent with the field observations folding of the heterogeneous sedimentary section of la zeta anticline however cannot be simulated by the trishear model alone an additional mode of deformation involving transport of material from the backlimb into the hinge area and extension parallel to the direction of fault propagation is necessary to reproduce the geometry and finite strain of the anticline mechanical distinct element modeling dem of the anticlines indicates that their contrasting kinematics could have resulted from differences in mechanical stratigraphy and initial stress state folding of a homogenous normally consolidated assemblage initial horizontal to vertical stress ratio k o 1 is trishear like and resembles the rip van winkle anticline folding of a heterogenous layered such as la zeta over consolidated assemblage k o 1 departs from the trishear model and resembles la zeta anticline based on the dem simulations we postulate that the rip van winkle anticline formed at high depths high overburden loads and lithostatic stress conditions and that la zeta anticline formed at shallow depths after substantial uplift and erosion of the andean mountain front which induced over consolidation and high k o

active thrusting and folding in the qilian shan and decoupling between upper crust and mantle in northeastern tibet fieldwork south of the city of gaotai gansu province china shows that active shortening of surface sediments in the foothills of the yumu shan a large fore mountain of the qilian shan at the northeastern edge of tibet involves both overthrusting and flexural slip folding north of this mountain we found and mapped a prominent north facing thrust scarp that offsets a holocene fan sloping gently 3 4 to the north part of this scarp appears to be related to the m 7 5 180 a d earthquake that may have led to the demise of the han dynasty city of luo tuo chen in the hexi corridor a set of 10 100 150 m long profiles measured across this scarp 3 2 m high on the average can be made to fit the diffusion degraded morphology of a surface break related to the 180 a d event using a value of about 3 3 m 2 10 3 yr for the mass diffusivity of fanglomerates in this part of gansu province smaller mountain facing scarps on a terrace capped foothill result from bedding slip concurrent with active folding of underlying steeply northdipping plioquaternary sandstone and conglomerate beds holocene uplift rates along the yumu shan which is only one of the qilian shan ranges are estimated to be between 0 4 and 1 9 mm yr which implies that much of the mountain formed in the quaternary the periclinal structure of the plioquaternary envelope under which the paleozoic core of the yumu shan plunges towards the west suggests that the whole 3200 m high mountain is a basement ramp anticline mountains striking parallel to the yumu shan with similar structure and comparable or greater sizes north and south of the hexi corridor probably also correspond to recent crustal ramp anticlines this implies that the wide mountainous upper crustal wedge making the northeastern edge of the tibet qinghai plateau is detached from the underlying lower crust and upper mantle

examining fault architecture and strain distribution using geospatial and geomechanical modelling an example from the qaidam basin ne tibet the investigation of complex geological setting is still dominated by traditional geo data collection and analytical techniques e g stratigraphic logging dip data measurements structural ground mapping seismic interpretation balance section restoration forward modelling etc despite the advantages of improving our understanding in structural geometry and fault architecture the geospatial modelling applying computer aided three dimensional geometric design visualization and interpretation has rarely been applied to such complex geological setting this study used the lenghu fold and thrust belt in qaidam basin ne tibetan plateau to demonstrate that the application of geospatial and geomechanical modelling could improve our understanding and provide an effective technique for investigating the fault architecture and strain distribution the three dimensional configuration of the lenghu fold and thrust belt was initially derived from traditional analysis techniques such as regional stratigraphic logging cross section construction meso scale ground mapping and landsat image interpretation the high resolution field data and landsat image were integrated to construct the geospatial model which was subsequently used to quantitatively investigate the fault throw changes along the lenghu thrust fault zone and to understand its control on the lateral structural variation the geospatial model was then restored in three dimensions to reveal the kinematic evolution of the lenghu fold and thrust belt geomechanical modelling using a mass spring algorithm provided an effective three dimensional tool for structural strain analysis which was used to predict the strain distribution throughout the overall structure e g normal faults with throws ranging from meters to tens of meters in the hanging wall the strain distribution predicted by geomechanical modelling was then validated by the natural normal faults in the hanging wall the high accordance between the strain prediction and statistics of natural normal faults demonstrates good applicability of geospatial and geomechanical modelling in the complex geological setting of the lenghu fold and thrust belt the geospatial models and geomechanical models therefore can provide a robust technique for analyzing and interpreting multi source data within a three dimensional environment we anticipate that the application of three dimensional geospatial modelling and geomechanical modelling integrating both multi source geologic data and three dimensional analytical techniques can provide an effective workflow for investigating the fault architecture and strain distribution at different scales e g ranging from regional to meso scale

rounding hinges to fault bend folding geometric and kinematic implications fault bend folding is a common folding mechanism in thrust and fold belts worldwide the widely used kink band geometric model of fault bend folding necessitates complex ramp segmentations to reproduce the rounded shape of many natural thrust related anticlines curvilinear hinge sectors provide a geometric and kinematic alternative solution to kink bands for modelling curved hinge folds we developed an analytical solution for modelling fault bend folding using circular hinge sectors the velocity field of this kinematic solution is different from that associated with the classical kink style model our solution predicts the development of curvilinear anticlines above staircase fault geometries the occurrence of limb rotation and consequently the development of rotational syngrowth wedges on both the forelimb and the crest conversely to the kink style kinematics curvilinear hinge sectors imply a dependence of deformation intensity from the fold shape and stratigraphic position of the folded layer application to natural thrust related anticlines validates the effectiveness of curvilinear fault bend folding

geometry and kinematics of single layer detachment folds detachment folds form above below or above and below bed parallel thrusts where thrust displacement is transferred into folding geometric and kinematic models are developed for individual single layer detachment folds formed above the tip line of a thrust we analyze three different detachment fold shapes kink bands chevron and box detachment folds and consider three principal detachment fold models 1 constant limb dip where the detachment fold grows by limb lengthening 2 constant limb length where the detachment fold grows by limb rotation and 3 variable limb length and variable limb dip where the detachment fold grows by both limb rotation and limb lengthening the allowance for variable forelimb thickness and excess layer parallel shear enables us to de elop a wide range of possible detachment fold geometry the geometric validity fields for folds with excess parallel shear are reduced when forelimb thinning takes place folds formed with constant limb length are likely to lock up in the initial amplification stages whereas larger amounts of shortening can be taken up by folds formed with variable limb length the manner in which a fold evolves affects the fracture and porosity features of the folded strata therefore determining which mechanism operated to form a specific detachment fold is of particular importance in hydrocarbon exploration the models we present are aimed at permitting more accurate geometric and kinematic interpretations of individual detachment folds examples of detachment folds are analyzed to illustrate the po ential benefits and limitations of the geometric model presented

cenozoic qaidam basin china a stronger tectonic inversed extensional rifted basin qaidam basin is a composite sedimentary basin developed on typical continental crust it comprises a jurassic foreland basin and a cenozoic extensional basin the geologic history of the cenozoic qaidam basin can be divided into two megastages on the basis of stratigraphy and structure the first megastage from latest cretaceous to oligocene consisted of two periods of rifting due to upwelling of the hot upper mantle this is demonstrated by two tectono sequences formed by the rifting the synrifting magmatic thermal activity and the control of protobasin development by syndepositional faults the second megastage comprises three tectono sequences that developed in the miocene and pliocene and was a period of structural inversion that consisted of compressive downwarping and reverse faulting this tectonic inversion may be ascribed to the long distance propagation of compressive stress caused by the collisions of the indian and eurasian plates the thick sequence of primary oil source rocks was deposited in the major rifted depression and the restricted drainage graben of the rifted protobasin during the tectonic inversion megastage they were subject to deep burial and prolonged heating a major and a minor oil generating basin have developed the tectonic inversion processes produced several structural features that may contain potential hydrocarbon reservoirs and traps

mid crustal detachment in the betic system of southeast spain abstract the tertiary orogenic belts of eastern spain can be interpreted using the principles of thin skinned thrusting with a fundamental detachment at mid crustal level in the eastern betics the compression involves an oblique restacking of a stretched continental margin much of the shortening is transmitted on permian and triassic detachments which emerge as forward and back thrust duplexes an interpretation of the guadarrama range of central spain is presented in which significant shortening is shown on a mid crustal detachment propagating from the southeast it is suggested that this represents the external limit of a compressive system linked to the betics the pyrenees and eastern cantabrians are a separate tertiary compressive system again resulting from restacking of an extensional basin during the collision of iberia and france previous authors have proposed mid crustal detachment in the pyrenees observation of the geological map of the iberian ranges suggests a broadly similar structural origin may apply in which case the iberian ranges may represent the most external zone of a major pyrenean system again linked by a mid crustal detachment thus it is suggested that the whole of eastern spain is made up of two major compressive systems linked at mid crustal level

evolution of coseismic fault related folds induced by the chi chi earthquake a case study of the wufeng site central taiwan by using 2d distinct element modeling abstract coseismic fault propagation folds are generally associated with blind faults which are of considerable academic interest and have recently been recognized as critical for assessing seismic hazards the wufeng excavation site was characterized by a major east dipping basal thrust exhibiting a dip angle of 34 and 2 opposing vergent thrusts generated a pop up anticlinal fold in soil cover induced by the 1999 chi chi earthquake mw 7 6 in order to represent the soft soil behavior we set up a direct shear simulation test to obtain valid parameters for soil a series of 2d distinct element models possessing different bonding types and strengths was conducted to determine the deformation pattern near the surface and the evolution of the fault tip propagation the coseismic deformation features of soft and plastic soil cover like the mutative limb thickness complex ruptures and the overturned forelimbs in small scale caused by the propagation of the blind fault tip were accurately predicted by contact bond model in which grains were allowed to rotate and slip without cements breaking our results show that the alternative thrusts and pop up structure developed before the main basal thrust fault ruptured through the ground surface at the wufeng excavation site we evaluated the slipping distance of the main fault at approximately 6 m in wufeng during the 1999 chi chi earthquake

geometry numerical models and revised slip rate for the reelfoot fault and trishear fault propagation fold new madrid seismic zone abstract the geometry and structural relief of late holocene sediments folded across the reelfoot scarp are characterized with trench excavations shallow borings a digital elevation model dem of topography and reelfoot lake rl bathymetry results suggest the scarp forms by heterogeneous shear in the forelimb of a fault propagation fold seismic data and the accompanying structural analysis support the argument that the reelfoot fault does not offset the surface and that the reelfoot scarp is a fold limb structural relief measured across the reelfoot scarp is slightly higher than previous determinations and varies from 7 to 11 m along the western shoreline of rl numerical models based on trishear kinematics constrain growth of the reelfoot monocline and indicate that the underlying reelfoot thrust fault is steeply dipping the shallowest portion of the central reelfoot thrust segment dips 75 where the fault tip has propagated upward from 911 m to its current location 465 m beneath the surface the models indicate that the shallowest section of the southern thrust segment in the reelfoot thrust system dips even more steeply at 80 but has propagated upward only a few tens of meters to its present location 1016 m beneath the surface the thrust is flatter at deeper levels based on the location of earthquake hypocenters strain across the reelfoot scarp is partitioned into two or three separate fold scarps that collectively record a late holocene fault slip rate of 3 9 0 1 mm yr the slip rate is based on 9 m of structural relief the 2290 60 ybp age of folded sediment and a 75 dip for the fault this implies a rate of horizontal contraction in the seismic zone of 1 0 mm yr this rate is strongly dependent on the dip of the thrust calculated by the structural analysis the numerical models argue for reactivation of faults that previously offset paleozoic strata

multiple geometries and modes of fault propagation folding in the canadian thrust belt abstract a multitude of fold models have been proposed to explain the variety of fold geometries which develop in front of thrust faults detailed field fabric and photogrammetric studies of 4 fault cored asymmetrical folds in the thin skinned canadian thrust belt were used to test models of fault propagation folding fold geometries include combinations of angular and rounded fold surfaces highly contorted anticlinal hinge areas and minimal penetrative deformation or changes in bedding thickness interlimb angles generally decrease with increasing shortening indicating progressive fold tightening about fixed anticlinal hinges extensive flexural slip thrusting toward the anticlinal axes of angular folds suggests that kink folding in thin skinned thrust belts is aided by material transfer from both fold limbs into hinge areas fold geometries change dramatically along the strike of individual structures demonstrating the non uniqueness of fault propagation fold geometries no single mode of fault propagation folding can explain the diverse fold geometries seen in the canadian thrust belt this geometric variability can be ascribed to the complex interplay of multiple modes of folding in strata near the causal thrusts oblique shear and flexural slip in triangular shear zones distribute thrust displacements into both rounded and angular folds simultaneous angular folding of overlying strata commonly occurs by progressive kink folding where folds tighten by flexural slip on all fold limbs until the thrust breaks through the fold regional and local differences in the amount of pervasive top to the craton shear needed for progressive kink folding may be partially responsible for the variability of fault propagation fold geometries in thin and thick skinned orogens world wide

trishear modeling of fold bedding data along a topographic profile this paper describes an algorithm that extends the use of the trishear kinematic model to the commonly encountered case of a set of fold bedding observations along a profile in a universe of trishear models that can possibly fit the structure the algorithm searches for the model that minimizes the difference between observed and modeled beds i e best fit model the robustness and versatility of the algorithm are shown by applying it to a synthetic fold geometry and to a transect of the waterpocket monocline in southern utah in the first case the best fit parameters are exactly those used in the synthetic model in the second case the best fit model reproduces fairly well the observable features of the monocline

variscan syncollisional extension in the iberian massif structural metamorphic and geochronological evidence from the somosierra sector of the sierra de guadarrama central iberian zone spain abstract the rocks affected by the variscan orogeny in the sierra de guadarrama central iberian zone can be separated into two domains a high grade western domain and a medium to low grade eastern domain these domains are separated by the berzosa riaza shear zone a new integrated analysis of structural metamorphic and u pb monazite age data indicates that the berzosa riaza shear zone is a major d2 extensional structure associated with oblique crustal scale extensional shearing and important decompression low p high t metamorphism associated with the d2 event overprints earlier barrovian metamorphic assemblages formed during the variscan d1 compressional event the kilometre scale berzosa riaza shear zone exhibits a s c mylonitic front at its base and a crenulation front at its top with successive fabric development under different p t conditions indicating crustal thinning textural relationships and u pb dating of monazite indicate that the main d2 extensional event took place between 337 2 and 326 3 ma early mid carboniferous a late d2 extensional episode overprinted the major d2 ductile shear zone along narrow low grade oblique normal detachment zones kinematic indicators in both d2 and late d2 fabrics indicate a displacement of the upper structural levels to the southeast field relationships microstructures and qualitatively derived p t t d paths suggest that the early mid carboniferous syncollisional extension in the sierra de guadarrama was associated with the major extensional collapse of the variscan hinterland central iberian zone and was contemporaneous with the earliest thrusting in the foreland

an unstable kinematic state of the himalayan tectonic wedge evidence from experimental thrust spacing patterns abstract the frontal tectonic wedge in the nw himalayas shows a sequence of imbricate thrusts with increasing spacing in the foreland direction we used analogue model experiments to analyze the thrust pattern in relation to the kinematics of wedge evolution experimental findings reveal two kinematic states of the foreland ward propagation of a wedge unstable state and stable state the unstable state is characterized by continuous vertical growth of the wedge with progressive horizontal contraction on the other hand in the stable state the wedge ceases to grow vertically but propagates laterally in the foreland direction in the experimental runs thrust wedges remained in the unstable state even after large horizontal shortening 50 when the basal friction was high 0 46 our analysis suggests that successive thrusting occurs always with increasing spacing in unstable wedges and the rate of increase is larger for larger basal friction it can maintain uniform spacing only when the wedges turn into the stable state using finite element models we determined the stress distribution in the deforming wedge to find the potential locations of new thrusts in front of a wedge which also show larger spatial distances with increasing vertical thickness of the wedge both physical and numerical models suggest that the tectonic wedge in the nw himalayan frontal belt has evolved in an unstable state

inference of trishear faulting processes from deformed pregrowth and growth strata near surface strata deformed by propagation of a fault situated underneath these strata hold explicit records of fault activities the delicate deformed strata exposed in investigation trenches for paleoseismologic study conform to fault propagation folds growing above a blind thrust growth strata deposited subsequently across the coseismic fold scarps are preserved by onlap and unconformities information from these sedimentary and folding processes was used in conjunction with inverse modeling using the trishear model technique proposed by allmendinger allmendinger r w 1998 inverse and forward numerical modeling of trishear fault propagation folds tectonics 17 640e656 in estimating the times and amounts of slips this methodology becomes necessary especially when the extent of a deformed profile is insufficiently exposed to observation or the location of the fault tip is invisible the proposed method can deal with growth strata that can be selectively removed during fold restoration the interpretations on the deformation history obtained by inverse analysis and trench investigation were then compared the combination of these two approaches seemed to enhance and complement each other and allows a better and more complete understanding of the trishear faulting processes and is useful in seismic hazard assessment 2007 published by elsevier ltd

role of hydraulic sorting in the origin of fluvial placers a necessary condition for placer development is selective sorting at the grain scale by size and density the effects of differential entrainment suspension and transport on an initial distribution composed of medium size quartz and 10 fine size magnetite were modeled by solving the einstein bedload function for specific grain friction velocities u super in the range 3 63 cm sec super 1 and bed roughnesses of 0 55 2 5 and 10 mm for any value of u super and for both mineral densities the transport rate for all sizes in the initial distribution decreases with increasing roughness the decrease being greatest in the finer sizes the concentration of magnetite transported in the flow increases with increasing u super for each roughness and decreases with increasing roughness for each u super the settling velocity ratio of magnetite to quartz in transport decreases with increasing u super for any value of roughness for any value of u super the ratio first decreases then increases with increasing roughness ranging from 1 36 at low u super and roughness to 0 76 at high u super and intermediate roughness these results are due to variations in the reactive angles of grains and the extent to which grains hide in the lower inner zone of the boundary layer concentrations of heavy minerals at the bed bar and system scales are explained using these results

geometric and numerical model of progressive limb rotation in detachment folds a simple area balanced geometric model of progressive limb rotation in detachment folds allows the quantification of displacement uplift and rotation and the assessment of their effect upon growth strata geometries translation of this geometric model into a velocity model of deformation allows the modeling of associated growth strata resulting from background sedimentation and local erosion transport and deposition we show that with constant displacement and sedimentation rate this model predicts a progressive unconformity indicating the onlap and eventual overlap of the fold limb caused by a decreasing uplift rate this decrease is a consequence of the kinematics of progressive limb rotation increases in displacement rate with sedimentation rate kept constant can lead to offlap and angular unconformities although subsequent onlap does not necessarily reflect a return to a lower displacement rate modeled growth strata geometries exhibit many of the features reported from the literature

seismic stratigraphy of a neogene piggyback basin argentina depositional sequences composed of neogene nonmarine strata are imaged on seismic reflection data from the iglesia piggyback basin argentina the strata accumulated in an internally drained basin located between the frontal cordillera the high andes and the precordillera thrust belt two reflection geometries onlap and truncation are observed along four unconformities within the neogene section onlap of reflections onto unconformities defines five distinct intervals of basin growth local truncation of reflections beneath unconformities defines four intervening intervals of nondeposition or erosion along strike variation in the amount of truncation suggests that erosion was limited to zones through which major streams entered the basin postdepositional thrusting o the east side of the basin produced field exposures of the five sequences and allowed examination of the nature of the sequence boundaries the iglesia depositional sequences are interpreted to result from neogene crustal shortening within the central andes the iglesia basin has been passively transported above a horizontal decollement that links mid crustal deformation beneath the frontal cordillera with surface thrusting in the precordillera deposition resulting generally from simultaneous uplift of the frontal cordillera and the precordillera can be explained by two scenarios thrust controlled accumulation where space for deposition was created only during faulting or drainage controlled accumulation where deposition occurred only when thrusting cut off through going drainages linking the iglesia basin with the foreland

strike slip fault network of the huangshi structure sw qaidam basin insights from surface fractures and seismic data abstract the huangshi structure as one of the nww trending s shaped structures in the southwestern qaidam basin holds important implications for unraveling the regional structural pattern there are four dominant sets of surface strike slip fractures at the core of the huangshi structure the fractures with orientations of n28 e n47 e and n65 e correlate well with conjugate riedel shears r tension fractures t and riedel shears r in the riedel shear model respectively two conjugate strike slip fracture sets occur at the surface of the hongpan structure secondary to the huangshi structure and the southwestern part of the huangshi structure in seismic sections the huangshi structure is present as a positive flower or y shaped structure governed by steeply dipping faults whereas hongpan and xiaoshaping structures located symmetrically to the huangshi structure are thrust controlled anticlines the riedel shear pattern of surface strike slip fractures the positive flower or y shaped structure in seismic sections and the nw trending secondary compressional anticlines consistently demonstrate that the huangshi structure is dominated by left lateral strike slip faults which comprise a strike slip fault network considering the similar s shaped configuration and nww trend of structures across the southwestern qaidam basin it can be further speculated that these structures are also predominantly of left lateral strike slip types

multibend fault bend folding this paper shows how slightly complex but angular fault shapes with only a few bends lead to broadly curved and or highly complex folds a common perception exists that simple angular fold geometries are a necessary consequence of the normal assumptions of fault bend fold theory namely angular fault bends straight faults and flexural slip to the contrary it is shown here that exact application of these low level assumptions produces great complexity from simple fault shapes this complexity is caused by the combination of 1 generation of new axial surfaces by displacement of hangingwall cut offs past successive fault bends and 2 fragmentation of axial surfaces by mutual interference the result is entwined arrays of fold axial surfaces that produce quasi curved and or complex folds from discrete angular fault bends fold complexity grows in a highly non linear way in the most extreme case the number of fold axial surface segments grows as the fourth power of the number of fault bends this paper presents examples of the immense variety of complex fold geometries that are thus generated 0 1997 elsevier science ltd all rights reserved

intracrustal detachment within zones of continental deformation geologic mapping of active to recently active geologic structures in panamint valley western united states and in the haiyuan region and northern tibetan plateau china suggests detachment in the middle and lower crust on a scale of tens of kilometres to at least several hundred kilometres detachment occurs similarly in predominantly extensional panamint valley and in predominantly compressional china environments it involves structures with displacements of more than 10 km and displacement rates of more than 3 mm yr perhaps more than 10 mm yr the steeply dipping strike slip faults present in all three areas hunter mountain fault haiyuan fault and altyn tagh fault zone terminate in zones of extension or compression and geometric relations indicate that all structures including strike slip faults are thin skinned and restricted to the upper crust in panamint valley and in the haiyuan region deformation within these systems can be reconstructed in three dimensions displacement on the strike slip faults is absorbed by extension or compression occurring at the termination of the faults so strike slip displacement is roughly equal to and in the same direction as shortening or extension we propose that left slip on the altyn tagh fault zone in northern tibet is similarly absorbed by shortening southeast of the fault zone within the qaidam basin and the nan shan

paleomagnetism of eocene and miocene sediments from the qaidam basin implication for no integral rotation since the eocene and a rigid qaidam block the qaidam basin is the largest topographic depression inside the tibetan plateau and it is a key factor to understanding the cenozoic evolution of the northern tibetan plateau paleomagnetic data was obtained from the middle to late eocene xiaganchaigou formation and the early to middle miocene xiayoushashan formation from seven localities the paleomagnetic results indicate that the qaidam basin has not undergone obvious basin scale vertical axis rotation with respect to the eurasia plate since the eocene local clockwise rotation took place only at a few special locations along the northern margin of the qaidam basin the uniform paleomagnetic results at different localities support that the qaidam basin is a relatively rigid block regional paleomagnetic and geodetic observations also suggest that crust south of the kunlun fault moves eastward faster than crust north of the kunlun fault

cenozoic tectonic evolution of qaidam basin and its surrounding regions part 1 the southern qilian shan nan shan thrust belt and northern qaidam basin cenozoic qaidam basin the largest active intermountain basin inside tibet figures importantly in the debates on the history and mechanism of tibetan plateau formation during the cenozoic indo asian collision to determine when and how the basin was developed we conducted detailed geologic mapping and analyses of a dense network of seismic reflection profiles from the southern qilian shan nan shan thrust belt and northern qaidam basin our geologic observations provide new constraints on the timing and magnitude of cenozoic crustal thickening in northern tibet specifically our work shows that the southernmost part of the qilian shan nan shan thrust belt and contractional structures along the northern margin of qaidam basin were initiated in the paleocene early eocene 65 50 ma during or immediately after the onset of the indo asian collision this finding implies that stress was transferred rapidly through tibetan lithosphere to northern tibet from the indo asian convergent front located 1000 km to the south the development of the thrust system in northern qaidam basin was driven by motion on the altyn tagh fault as indicated by its eastward propagation away from the altyn tagh fault the eastward lengthening of the thrust system was spatially and temporally associated with eastward expansion of qaidam basin suggesting thrust loading was the main control on the basin formation and evolution the dominant structure in northern qaidam basin is a southwest tapering triangle zone which started to develop since the paleocene and early eocene 65 50 ma and was associated with deposition of an overlying southwest thickening growth strata sequence recognition of the triangle zone and its longevity in northern qaidam basin explains a long puzzling observation that cenozoic depocenters have been located consistently along the central axis of the basin this basin configuration is opposite to the prediction of classic foreland basin models that require the thickest part of foreland sediments deposited along basin edges against basin bounding thrusts restoration of balanced cross sections across the southern qilian shan nan shan thrust belt and northern qaidam basin suggests that cenozoic shortening strain is highly inhomogeneous varying from 20 to 60 both vertically in a single section and from section to section across the thrust belt the spatially variable strain helps explain the conflicting paleomagnetic results indicating different amounts of cenozoic rotations in different parts of qaidam basin the observed crustal shortening strain also implies that no lower crustal injection or thermal events in the mantle are needed to explain the current elevation 3000 3500 m and crustal thickness 45 50 km of northern qaidam basin and the southern qilian shan nan shan thrust belt instead thrusting involving continental crystalline basement has been the main mechanism of plateau construction across northern qaidam basin and the southern qilian shan nan shan region

geometry and kinematics of the arlar strike slip fault sw qaidam basin china new insights from 3 d seismic data abstract the right stepping en echelon kunbei fault system consisting of the hongliuquan arlar xiii and kunbei faults features importantly in the tectonic evolution of the southwest qaidam basin the 3d seismic cross section interpretation and key horizon similarity attribute analysis show that the nww striking arlar fault is composed of two steeply dipping faults f1 and f2 in cross sectional view and branches eastward into two west dipping faults f5 and f6 in map view the fact that the strata above t 2 the base of the early miocene xiayoushashan formation become dramatically thinner toward the high points of the arlar fault suggests an early miocene initiation ca 22 0 ma for this fault as indicated by the windowed amplitude related attribute maps the arlar fault and f5 one branch of the arlar fault have cumulative sinistral offsets of 8 6 8 7 km and 5 2 5 4 km respectively these considerably greater strike slip components in conjunction with the respective maximum true dip slip displacements of