backthrusting in accretionary wedges backthrusting of the rear of the accretionary wedge arcward over the rocks of the forearc basin is a common aspect of accretionary tectonics an older assumption that backstops of accretionary wedges dip back toward the arc and that accreted material is emplaced below such backstops is contradicted by observations from many active forearcs that indicate 1 backthrusting is common 2 forearc basins are nearly ubiquitous associates of accretionary wedges and 3 forearc basement where imaged appears to diverge from the sedimentary package dipping under the wedge while the overlying sediments are often lifted up against it backthrusting may be favored where relief is high between the crest of the wedge and the surface of the forearc basin because the relief must be supported by shear stress along the backthrust we find ratios of lt lb averaging 2 64 0 8 where lt is the horizontal distance from the crest of the wedge to the front of the toe and lb is the distance from the crest of the wedge to the surface projection of the backthrust this ratio is predicted to increase with total convergence and with sedimentation because the toe undergoes higher deformation rates than the backthrust and sedimentation in the trench favors outbuilding of the toe whereas sedimentation in the forearc basin tends to cancel the gravitational component of the driving stress for the backthrust

quaternary blind thrusting in the southwestern sacramento valley california patterns of microearthquakes and quaternary surface deformation suggest that the tectonic setting of the sw sacramento valley is similar to areas of the western san joaquin valley known to be underlain by seismogenic blind thrust faults on the basis of previous work and analysis of geologic and seismic reflection data the following late cenozoic tectonic features and processes are identified 1 uplift of the northern coast ranges beginning approximately 3 4 ma and eastward propagation of uplift into the southwestern sacramento valley by 1 0 ma 2 uplift and homoclinal flexure of plio pleistocene strata at the eastern coast ranges mountain front 3 uplift and folding above blind thrusts approximately 15 km east of the mountain front in the southwestern sacramento valley similar associations of structures and processes have been observed in thrust belts in pakistan the peruvian andes and the canadian cordillera and are commonly attributed to thrusting within an intercutaneous wedge or triangle zone by using other thrust belts as analogs the propagation of an eastward tapering triangle zone is interpreted to be the principal mechanism for uplift and homoclinal flexure at the eastern coast ranges mountain front seismic reflection profiles reveal that 1 the triangle zone consists primarily of east vergent blind thrusts and 2 west vergent backthrusts exposed in the the eastern coast ranges and southwestern sacramento valley are rooted in the east vergent thrusts transfer of slip from the east vergent blind thrusts to the west vergent backthrusts occurs locally beneath the southwestern sacramento valley fault bend folding in the hanging walls of the backthrusts has created a north northwest striking chain of low hills approximately 15 km east of the mountain front the folds deform 3 4 1 0 ma fluvial sediments and thus are middle pleistocene in age or younger local variations in strike suggest that the fold chain is segmented like the new idria coalinga kettleman hills segmented fold chain in the southwestern san joaquin valley stein and ekstrom 1989 these data have implications for seismic hazard assessment anecdotal accounts indicate that two m 6 0 events of the 1892 winters vacaville earthquake sequence probably occurred beneath the eastern coast ranges dale 1977 toppozada et al 1981 ground cracking was observed following the main shocks along the mountain front in the southwestern sacramento valley we propose that the earthquakes were generated by slip on a blind thrust beneath the coast ranges and that the ground cracking in the valley represents propagation of the eastward tapering triangle zone the 1892 earthquake sequence suggests that blind thrusts beneath the southwestern sacramento valley are active and capable of generating moderate to large magnitude earthquakes

balanced section in thrust belts part 1 construction balanced geological cross sections are an important aid to understanding thrust belt structures and to estimating their hydrocarbon bearing potential two approaches to section balancing have been taken in the past construction of retrodeformable sections from raw data and modification of previous interpretations after an evaluation procedure these approaches may be unified by use of a langrangian grid projection of datum locations dips and stratigraphic thicknesses onto the section plane is performed directly by calculation or graphically using plunge lines spline interpolation and parallel fold modeling are the simplest ways of filling gaps in the sectional data but isogon interpolation yields rheologically more realistic results faults are interpolated using cutoff geometries as constraints fault tips are located from distance displacement plots whereas the depth to detachment is obtained by a modified chamberlin construction interpolations made on the section are directed back to the geologic map via plunge lines although these techniques of section construction do not guarantee a balance especially when complexities such as growth structures diapirs or strike slip faults are present they eliminate many potential errors

the tectonic evolution of the zoumi sandstone western moroccan rif the zoumi sandstone is an oligo miocene syn tectonic flysch sequence deposited on the back of the mesorif thrust sheet whose sediment source was from the foreland to the south variations within the zoumi sandstone from thick to thin bedded turbidite lobes are explained as the result of deposition in small confined basins or larger more extensive basins respectively formation of small basins is related to contractional deformation in the underlying mesorif thrust sheet propagation of the zoumi thrust through the developed basin with areas of relative uplift and subsidence produced complex hanging wall cutoff geometry a complicated footwall geometry exists because deformation in the more external mesorif thrust sheet preceded the final displacement of the more internal intrarif thrust sheet the latter incorporated the zoumi sandstone into its leading edge and thrust it over the mesorif during the miocene this deformation sequence of two or more simultaneously active thrust sheets is described as synchronous thrusting

relationship between uplift of the metamorphic core of the southernmost andes and shortening in the magallanes foreland fold and thrust belt tierra del fuego chile rocks containing upper amphibolite facies assemblages reflecting mesozoic cenozoic metamorphism form the 5000 km2 core of the southernmost andes in cordillera darwin approximately 55 s latitude results from mapping along a 50 km continuous transect across the southernmost andes on the main island of tierra del fuego show structural relationships between the late cretaceous to early tertiary uplift of the metamorphic core in northern cordillera darwin and mid cretaceous to paleogene shortening in the adjacent magallanes foreland fold and thrust belt new data indicate that pre late jurassic basement schists exposed along the northeastern edge of cordillera darwin were uplifted along a discrete brittle continent vergent thrust fault this thrust overlies a series of thin skinned thrusts and back thrusts that affect mesozoic tertiary rocks of the magallanes foreland basin the thrust that uplifts basement schists is interpreted to have formed out of sequence with respect to the thin skinned structures because it placed the schists over previously shortened lower cretaceous sedimentary strata of the magallanes basin the geometry of thrusts observed across the transect and their inferred sequence of deformation indicate a thick skinned contractional mechanism for uplift of the pre late jurassic schists of northern cordillera darwin relative to the structural level of metamorphic basement in the magallanes foreland the relative time of out of sequence thrusting and basement uplift inferred from thrust geometries in this paper is consistent with the latest cretaceous to early tertiary episode of rapid exhumation and cooling of the metamorphic core of the southern andean orogen determined in previous structural metamorphic and geochronological studies

kinematic evolution of andean fold thrust structures along the boundary between the eastern cordillera and middle magdalena valley basin colombia 1 surface and subsurface data support a kinematic reconstruction of cenozoic fold thrust deformation along the eastern cordillera magdalena valley transition in colombia the la salina fault lsf marks the boundary between west vergent eastern cordillera structures and hinterland deposits of the middle magdalena valley basin apatite fission track and u th he thermochronological results for the west directed lsf reveal initial hanging wall exhumation during middle eocene early oligocene 45 30 ma shortening renewed exhumation in the early middle miocene 18 12 ma and accelerated late miocene pliocene 12 3 ma exhumation vitrinite reflectance data suggest maximum burial of 4 6 km helping constrain cenozoic basin architecture mapping of the lsf reveals hanging wall cretaceous eocene rocks in a broad anticline syncline pair with limited faulting and footwall eocene quaternary basin fill in a complex series of tight thrust related folds limited displacement along the westernmost frontal thrust suggests that shortening is largely accommodated by east directed thrusting within a broader triangle zone of a passive roof duplex and probable minor strike slip deformation in the preferred kinematic restoration the most recent phase of shortening to transpressional deformation represents out of sequence reactivation of the lsf consistent with irregular crosscutting relationships among footwall structures earliest exhumation by 45 30 ma in the eastern cordillera fold thrust belt is correlated with increased sedimentary lithic fragments and high compositional maturity in sandstones of the adjacent magdalena valley basin exhumation since 15 ma coincided with decreased compositional maturity and elevated accumulation rates for the real group the compositional provenance shifts are attributed to westward advance of fold thrust deformation into the proximal eastern segments of the magdalena valley basin

detachment folding fold amplification and diapirism in thrust wedge experiments 1 the relations between detachment folding fold amplification and salt diapirism in contractional settings have been investigated by means of scaled analogue models the viscosity of the silicone layer simulating salt in nature and the shortening rates were combined in order to reproduce weak type 1 models and strong type 2 models decollements deformation patterns in the roof sequence exhibited two contrasting styles 1 outward propagation of detachment folding along the decollement ofp mode and 2 passive roof duplex prd mode in type 2 models detachment folding propagated away from the most external thrust in the floor sequence while in type 1 models long lived detachment folds almost invariably localized amplified above a floor thrust tip as a result of strain localization a silicone wall intruded occasionally into the crestal graben of detachment folds in type 1 and ofp models best fitting of transition models data points indicates nonlinear relations with regression curves close to the equilateral hyperbola equation for both ofp prd and amplified detachment folds box folds transitions a quantitative comparison of model results with nature has been attempted by plotting salt based fold and thrust belts data points on the scaled transition curves obtained from the modeling such a comparison relates shear stress products and ratios to the conditions favoring the amplification of detachment folds and the potential emplacement of ductile diapirs in their core by reducing the roof sequence strength pore fluid pressure b is inferred to shift the equilibrium of fold and thrust belts toward the field of ofp and diapirism

influence of syn sedimentary faults on orogenic structures in a collisional belt insights from the inner zone of the northern apennines italy abstract this paper discusses the possible influence of syn sedimentary structures on the development of orogenic structures during positive tectonic inversion in the inner northern apennines italy examples from key areas located in southern tuscany provided original cartographic structural and kinematics data for late oligocene early miocene thrusts organized in duplex systems verging in the opposite direction of the foreland propagation back thrusts which affected the late triassic oligocene sedimentary succession of the tuscan domain previously affected by pre orogenic structures these latter consist of mesoscopic to cartographic scale jurassic syn sedimentary normal faults and extensional structures which gave rise to effective stratigraphic lateral variation and mechanical heterogeneities structural analysis of both syn sedimentary faults and back thrusts were therefore compared in order to discuss the possible role of the pre existing anisotropies in influencing the evolution of the back thrusts as a result it can be reasonably proposed that back thrusts trajectories and stacking pattern were controlled by relevant syn sedimentary normal faults these latter were reactivated in some cases if properly oriented such an issue adds new inputs for discussing the potential role of structural inheritance during tectonic inversions and helps to better understand the processes suitable for the development of back thrusts in the inner zones of orogenic belts as it is the case of the inner northern apennines

cocorp seismic profiling of the appalachian orogen beneath the coastal plain of georgia a southeastward extension onto the coastal plain of an earlier cocorp traverse which confirmed large scale thin skinned thrusting of crystalline rocks of the southern appalachians has provided some of the most spectacular reflections yet seen in crustal seismic data most of the reflectors can be interpreted as either fault surfaces or as metamorphosed strata of late precambrian early paleozoic age they are consistent with the hypothesis that a major detachment extends eastward beneath this part of the orogen although other interpretations with a more complex pattern of detachments or sutures are also possible large scale overthrusting provides a mechanism for incorporating sedimentary rocks into the lower crust and may help to explain many of the layered features on crustal seismic data reflections from deep beneath the coastal plain indicate that the structural configuration of the rocks is complex and that the remains of a collision zone are being observed several east dipping horizons which bear strong similarities to thrust faults in valley and ridge sedimentary rocks are seen in the basement at shallow and mid crustal levels beneath the coastal plain the augusta fault for example displays a reflection which extends at a low angle some 80 km or more southeast of its surface position in conjunction with surface geologic information these new data demonstrate that late paleozoic compressive deformation was pervasive and resulted in lateral movements in the upper crust extending from the valley and ridge to the crystalline rocks beneath the coastal plain a distance of 400 km or more a large antiform cresting at about 2 3 sec or about 6 km below the surface and other structures beneath the coastal plain of georgia deserve further consideration for petroleum exploration although metamorphism may have eliminated petroleum from these rocks refracted arrivals and fault geometries indicate two triassic rift basins beneath coastal plain sedimentary rocks one of which has apparently not been recognized previously

revised deformation history of the central andes inferences from cenozoic foredeep and intermontane basins of the eastern cordillera bolivia 1 investigation of cenozoic deformation sediment accumulation and provenance in the eastern cordillera of bolivia at 17 21 s indicates major shortening 60 140 km and foredeep development followed by limited internal shortening and intermontane basin development contrasting histories of shortening deposition sediment dispersal and detrital composition distinguish a formerly extensive paleogene foredeep exposed along an eastern belt of synclines from a zone of principally neogene intermontane basins in the central eastern cordillera new 40ar 39ar ages of 25 17 ma for interbedded tuffs reveal latest oligocene early miocene accumulation in intermontane basins at rates 80 m myr several times lower than andean foredeeps although poorly dated foredeep evolution probably coincided with middle eocene oligocene deformation and denudation in adjoining regions to the west a mid cenozoic transition from foredeep to intermontane conditions may be attributable to emplacement of a basement involved tectonic wedge beneath the eastern cordillera in this interpretation wedge emplacement drove flexural foredeep subsidence from roughly 40 to 25 ma whereas subsequent accumulation occurred in localized internally drained basins in elevated intermontane areas regardless of the regional subsurface structural geometry 40ar 39ar ages define a 25 21 ma onset of intermontane sedimentation that signifies the termination of major upper crustal shortening over large parts of the eastern cordillera and possibly the age of initial shortening in the interandean and subandean zones to the east in contrast to several popular models the revised tectonic history presented here suggests significant pre neogene shortening in the eastern cordillera and underscores the large uncertainties in estimates of long term shortening rates in the andes

lateral and vertical changes of deformation style in the osen roa thrust sheet oslo region abstract changes in deformation style and amounts of shortening in the osen roa thrust sheet of the oslo region occur vertically and laterally approaching the thrust front in the south deformation in the cambromiddle ordovician sequence passes laterally from closely spaced imbricates in the north 50 60 shortening through triangle pop up and imbricate zones toward the south 20 37 shortening to widely spaced zones of deformation up to 20 shortening approaching the thrust front changes in deformation style are attributed to changing boundary conditions across the klekken thrust declining end of orogenic forces and an increase in thickness of competent units in the ordovician rocks to the south vertical changes in deformation style are attributed to the increasing percentage of competent units upward in the cambro silurian sedimentary rocks in the north the accompanying decrease in shortening upwards requires a structurally necessary upper detachment horizon to separate folded late middle ordovician silurian sediments from imbricated early cambro middle ordovician sediments below while southward in the oslo area the upper detachment needs to be placed between silurian and cambro ordovician units finally in eiker with less than 20 shortening the whole cambrosilurian sequence appears to have deformed as a single unit in the northern oslo region the upper detachment probably has a backthrust sense of motion above an imbricate stack passive roof duplex further south the upper detachment is probably directed toward the foreland

evolution of the triangle zone in the rocky mountain foothills near coalspur central alberta a triangle zone which commonly occurs along the external margin of a foreland thrust and fold belt with a buried thrust front is underlain by a subhorizontal blind foreland verging thrust that ends against a foreland dipping hinterland verging thrust these contemporaneous thrusts active towards the end of orogenesis enclose an intercutaneous wedge that moved towards the foreland during orogenesis a triangle zone evolves through periodic replacement of faults bounding the active wedge replacements occur in cycles during each of which a lower fault tends to be replaced by one in a lower stratigraphic horizon an upper fault by one farther away from the foreland each cycle ends with the lower fault moving to a younger horizon where it joins a new more external upper fault near coalspur the triangle zone exposes the remnants of several wedges involving upper cretaceous and paleocene molasse most of these wedges developed during the last cycle but one and have a combined displacement of about 5

seismic data geometry evolution and shortening in the active sulaiman fold and thrust belt of pakistan southwest of the himalayas surface and subsurface data from the sulaiman fold and thrust belt are integrated to analyze the deep structure tectonic shortening and kinematics of the sulaiman fold and thrust belt at the western margin of the indian subcontinent seismic reflection data show that nearly all the 10 km thick sequence of dominantly platform 7 km and molasse strata is detached at the deformation front these strata thicken tectonically to about 20 km in the hinterland without significant thrust faults in the foreland a balanced structural cross section suggests that structural uplift in the sulaiman fold and thrust belt is a result of a thin skinned passive roof duplex style of deformation the duplex sequence consists of jurassic and older strata it is separated from the roof equence by a passive roof thrust in thick cretaceous shales that has a backthrust sense of displacement over foreland propagating duplexes the roof sequence with incipient out of sequence thrusting is intact for about 150 km northwards where it emerges along a passive backthrust sequential restoration of the balanced section reveals a series of structural and geometrical features including 1 development of low amplitude broad concentric detachment folds sui and loti at the tip of the decollement 2 increase in amplitude of a detachment fold to a critical level for development of ramp and duplex structures and 3 out of sequence thrusting to create required critical taper for an outward translation of the foreland fold and thrust belt a balanced structural cross section 349 km long from the sulaiman fold and thrust belt restores to an original length of 727 km suggesting a maximum of 378 km of shortening since 21 ma in the cover strata of the indian subcontinent calculation of displacement rates over the sulaiman fold and thrust belt 18 mm yr added to the resolved rate of the chaman fault vector for the component parallel to the plate convergence direction 15 mm yr are close to the current india asia plate convergence rate 37 mm yr

the irish variscides a fold belt developed within a major surge zone an arcuate fold belt characterizes the irish variscides south of the dingle dungarvan line the folds are cross cut by numerous faults which tend to be sinistral in the west and dextral in the east clockwise transection of major fold axes by minor folds and by cleavage has been recorded in the east while anticlockwise transections are common in the west these and other features suggest that north south dextral and sinistral shearing occurred in the east and west respectively this is considered to be a consequence of the morphology of the munster basin which allowed the development of a surge zone in the centre of the fold belt the variscan sole thrust is believed to lie at the base of or within the old red sandstone however to the east thinning of the upper palaeozoic cover sequence forces the sole thrust to cut down laterally stratigraphic section into lower palaeozoic basement in the depocentre the sole thrust surged ahead by propagating along bedding surfaces or possibly a devonian carboniferous extensional detachment beneath the basin while to the east and offshore to the west its propagation was inhibited by the absence of easy decollement horizons within the basement consequently the tip line became strongly arcuate and the thrust developed into a surge zone the sole thrust propagated northwards by stick slip processes much of the deformation within the cover sequence occurred during sticking events particularly fold tightening and accommodation faulting

an acadian mountain front in the english lake district the westmorland monocline the structure of the southern and central english lake district is that of a southeast facing monocline named here the westmorland monocline this 10 km wide zone of highly cleaved southeast dipping rocks separates gently dipping poorly cleaved borrowdale volcanic group to the north from extensively folded but regionally subhorizontal windermere group foreland basin rocks to the south the monocline formed early in the local acadian deformation sequence and accommodates at least 8 km of uplift it coincides with the steep concealed margin of the lake district batholith a major northwest dipping shear zone is revealed in the deepest levels now exposed within the monocline in the skiddaw group rocks of the black combe inlier the monocline has the characteristics of a mountain front providing significant tectonic elevation across a foreland dipping panel of rocks with no hinterland dipping thrust visible at the surface we interpret the uplift as the consequence of a southeast vergent thrust with a gently northwest dipping ramp beneath the central lake district continuing southeastwards as a flat detachment beneath the windermere group a displacement up the ramp of about 20 km is accommodated by backthrusting within the monocline and by shortening within the windermere group of the hangingwall southeast of the monocline the tip lies beyond the limit of the lower palaeozoic inlier beneath carboniferous cover

reappraisal of hypothetical franciscan thrust wedging at coalinga implications for tectonic relations along the great valley flank of the california coast ranges 1 correlation of the character and orientation of seismic reflectors imaged on profiles across the coalinga anticline with known surface geology indicates that local franciscan thrust wedging favored for past geodynamic interpretations of the 1983 coalinga earthquake is an untenable hypothesis the inference that a subhorizontal detachment surface of simple shear is present beneath the anticline is consequently unnecessary and unlikely in view of the coherent areal pattern of wrench folding within the southern diablo range elsewhere along the structural front of the california coast ranges at the western flank of the great valley interpretations of similar thrust wedging based in part on analogy with supposed structural relations at coalinga are equally questionable in a wider context the seismogenic behavior of blind thrust and reverse faults in california cannot be interpreted with the invalid concept of thrust wedging at coalinga as a guiding template for geotectonic interpretations

structural and stratigraphic perspectives on the uplift and erosional history of djebel cherichira and oued grigema a segment of the tunisian atlas thrust front

transition from convergence to escape field evidence from the west carpathians abstract the tertiary deformation of the west carpathians occurred during northeast directed convergence of the european and apulian plates and was complicated by space constraints imposed by orogen advance palaeostress fields computed from fault slip data and structural interpretation of known gravimetric magnetic seismic palaeomagnetic lithostratigraphic sedimentological fission track and borehole data indicate that during deformation the orogen consisted of both frontal thrust stacking zones and internal extensional trans tensional zones synthesis of available data indicates that the episodes of tectonism in litho tectonic units were both time and plate position related and were caused by conditions which varied in different areas ranging from the foreland to back arc region in each time period discrete deformation phases record the transition from convergent to tectonic escape tectonisrn principal horizontal stress trajectories were approximately co linear with the plate convergence vectors prior to escape the onset of escape conditions is indicated by 1 clockwise rotation of palaeostress orientations during the time of known escape in the eastern alps and 2 the cessation of thrusting of the flysch belt nappes onto the neogene foredeep in the westernmost part of the west carpathians tectonic escape along strike slip faults subparallel to the collision suture zone began in early badenian time in the west carpathians various regions along the orogenic belt were affected by escape that became progressively younger towards the east this agrees with the observations of various authors that the final phase of thrusting towards the european platform became younger to the east along the west carpathian arc in addition palaeostress trajectory patterns for various periods are similar to those modelled by a rigid block indenting a viscous half space an attempt is made to complete the mosaic of information on the cenozoic evolution of the alpine carpathian pannonian area which in the models given by ratschbacher royden horvath bergerat and others is based on extrapolated data the object is to detail the timing and complexity of the local time and plate position related mechanisms in the context of the entire orogen in order to update previous rather simplified models

the neogene structural evolution of the western margin of the pelagian platform central tunisia abstract the deformation front of the atlas mountains in central tunisia is a structure termed the north south axis to the west of kairouan the north south axis consists of ne sw to nne ssw trending folds and thrusts a main decollement thrust in triassic evaporites has been mapped with secondary detachment of out of sequence thrusts in tertiary rocks and backthrusts at the base of the tertiary and in upper cretaceous sediments the upper cretaceous and tertiary sediments are backfolded in the hangingwalls of the backthrusts which are in turn folded in the se verging anticlines the backthrusts are roof thrusts to passive roof duplexes and imply the extensive development of thin skinned blind se verging thrusts a similar structural style is seen further west in the tunisian atlas but major nw verging backthrusts are not exposed adjacent to the pelagian platform the thrust structures are affected by later strike slip faults the thin skinned thrusting at the deformation front is middle miocene in age and was synchronous with africa europe collision the north south axis is therefore interpreted as the frontal structure of a collision related thrust belt the thrust belt is underlain by a nw se to e w trending middle jurassic to early cretaceous rift basin with probable mid cretaceous to oligocene post rift fill the ne sw trending thrust belt is detached from nw se to e w trending syn rift basement faults but changes in thrust belt geometry across the syn rift faults indicate that basement structures have compartmentalized the thrust belt the later strike slip faulting resulted from the reactivation of the syn rift basement faults in the post collisional framework of the thrust belt

deep structure of the merida andes and sierra de perija mountain fronts maracaibo basin venezuela the maracaibo basin is a 50 000 km2 19 305 mi2 topographic depression bounded to the east and south by the mrida andes of venezuela and to the west by the sierra de perij of venezuela and colombia both uplifted mountain blocks expose paleozoic basement rocks and mesozoic cenozoic carbonate and clastic rocks that were mainly folded and thrusted by regional shortening in the paleogene and late neogene using geologic maps seismic reflection data and wells from the steep mountain front areas we test different structural models of how late paleogene neogene convergence is accommodated along subsurface faults and folds at the mountain fronts seismic imaging of the deep 5 km 3 1 mi structure of both maracaibo mountain fronts shows basinward dipping monoclines with stratal dips ranging from 20 to overturned and an almost complete lack of faulting in the basin edge monocline or in adjacent horizontally bedded strata of the maracaibo basin seismic data reveal the presence of one or more triangle zones at depth along both the mrida andes and sierra de perij that exhibit characteristic thrust wedge geometries the lower part of the wedge is defined by imbricate thrust faults dipping beneath the mountain block and involving the paleozoic basement the upper part of the wedge is defined by a basinward dipping thrust associated with fault propagation folds at the surface and an overlain basin edge monocline the creation of the steep to overturned dips of the monocline is attributed to the effects of 6 10 km 3 7 6 2 mi of shortening along the lower zone of imbricated thrust faults this 6 10 km 3 7 6 2 mi of shortening calculated from triangle zones of both the sierra de prija and mrida andes is significantly less than regional estimates from 12 to 60 km 7 4 to 37 mi of shortening inferred by previous workers from regional balanced cross sections that assume low angle thrust type geometries we propose that a pop up style of deformation related to the inversion of jurassic rift features may be a more realistic interpretation of the convergent fault systems that have controlled uplift of the mrida andes and sierra de perij inversion of relatively steep basement normal faults at the edges of and within both ranges may explain lesser amounts of observed shortening

geometries of frontal fold and thrust belts insights from discrete element simulations abstract this study investigated the evolution of frontal structures of fold and thrust belts with 2d numerical simulations using the discrete element method dem of specific interest is the occurrence of triangle zones and how mechanical stratigraphy affects the formation of the triangle zones in our simulations we varied the strength of a two unit stratigraphy as well as the decollement horizons to determine the controlling parameters of different types of frontal structures for models with homogenous stratigraphy deformation was concentrated along forethrusts which involved both the upper and lower units with the addition of a mechanically weak upper unit deformation along forethrusts and popup structures largely occurred in the upper unit with relatively little deformation of the lower unit with thicker decollement surfaces and strong upper and lower units decoupling was enhanced and allowed for the formation of triangle zones in front of the deforming wedge the triangle zones were uplifted along lower forethrusts until a new frontal triangle zone formed results compared favorably with triangle zones found in the canadian cordillera in southern alberta

exploration in a classic thrust belt and its foreland bavarian alps germany the foreland of the bavarian alps consists of the tertiary molasse and its mesozoic substratum so far 38 oil and gas fields have been found with reserves of 14 million bbl 2 million mt of oil and 140 bcf 4 billion m3 of natural gas the fields are associated predominantly with antithetic normal faults and to a lesser extent with low relief anticlines or a combination of such anticlines and shaling out sands the lower part of the molasse and its mesozoic substratum dip to the south under the alpine thrust units despite its depth of 7 000 m 23 000 ft and more the subthrust is still considered to be within the oil window and is the prime target for exploration in the bavarian alps other possible targets are the kalkalpin and helveticum thrust units the chances in the folded molasse and flysch thrust units are considered to be extremely low seismic exploration in the alps is difficult because of the rough topography thick quaternary sediments a deep weathering zone and heterogeneous seismic velocities in the thrust units and their quaternary cover

the vredefort structure a perspective with regard to new tectonic data from adjoining terranes abstract the deformational history of the area surrounding the vredefort structure in south africa is long and complex involving several periods of contraction and extension a general chronology of events covering a time span of 1000 ma is established the vredefort deformation is identified as a separate structural event which is distinct from the earlier deformations it is essential therefore that deformational structures related to the pre vredefort event should be identified and subtracted from the total spectrum of tectonism so that only those structures which can be related with certainty to the formation of the vredefort structure are defined this approach could influence interpretations of the origin of the vredefort structure which attribute most structural and geological associations to the deposition of sediments onto a granitoid basement followed by a vredefort deformational event

a scenario for late neogene andean shortening transfer in the camisea subandean zone peru 12 s implications for growth of the northern andean plateau precise knowledge of the timing of deformation in the subandean zone of the andean plateau is a prerequisite for deciphering the late neogene growth of the andean plateau in this paper we report new apatite fission track aft and vitrinite reflectance ro data for a regional balanced cross section of the camisea basin in the central peruvian subandean zone adjacent to the northern andean plateau the balanced cross section shows that the structure of this basin is characterized by a broad internal passive roof duplex and external thrust related anticlines the balanced cross section restoration shows 53 km 39 of total horizontal shortening we sampled paleozoic to cenozoic sedimentary strata for aft and ro analyses along the 4 km thick vertical profile of the mainique back thrust passive roof thrust the innermost preserved subandean structure young components of aft ages are spread between ca 6 ma and ca 24 ma a break in the slope in the aft ages determines the geometry of the miocene partial annealing zone and the exhumation of the mainique back thrust at ca 6 ma sequential restoration calibrated by aft and ro data indicates that the last 23 km horizontal shortening were accommodated by the camisea thrust system over the past 6 m y giving a mean shortening rate of 3 8 mm yr using this shortening rate for the first 30 km horizontal shortening we calculate that the andean shortening transfer into the peruvian subandean zone initially started at ca 14 ma this result suggests that the transfer of shortening from the northern andean plateau to the subandean zone occurred prior to the removal of dense lithosphere previously reported to have occurred between ca 10 ma and ca 7 ma we rather propose that the late neogene growth of the northern andean plateau mostly resulted from a continuous crustal shortening combined with lower crustal flow

displacement distance methods in the analysis of fold thrust structures and linked fault systems a new method for analysing the internal strain in thrust sheets is described the displacement distance diagram can be used to quantify the relative shortening or relative stretch between the hangingwall and footwall beds of a thrust and is a useful way of analysing commonly occurring fold thrust structures the stretch is a function of the relative rates of thrust propagation and thrust slip the cumulative displacement distance diagram described here can be used to calculate the absolute shortening or extension when this is accommodated above a single decollement in a linked fault system this method offers a useful way of analysing duplex structures and large scale foreland thrust zones as well as extensional fault systems and can be used as an alternative to or in conjunction with balanced cross sections both methods can be used to quantify the variation in shortening above a single detachment zone this variation may be mechanically significant reflecting lithological control on thrust or normal fault growth

a new active tectonic model for the construction of the northern apennines mountain front near bologna italy 1 we integrate existing and new geologic data retreating trench extension and accretion tectonics retreat project particularly on the origin growth and activity of the mountain front at bologna italy into a new model that explains apennine orogenesis in the context of a slab rollback upper plate retreat process the bologna mountain front is an actively growing structure driving rock uplift 1 mm year cored by a midcrustal flat ramp structure that accommodates ongoing shortening driven by adria subduction at a rate of 2 5 mm year the data we use are assembled from river terraces and associated pleistocene growth strata geodesy including releveling surveys reinterpretation of published reflection lines and a new high resolution reflection line these data constrain a simple trishear model that inverts for blind thrust ramp depth dip and slip apennine extension is recognized both in the foreland as high angle normal faults and modest stretching in the carapace of the growing mountain front and in the hinterland with larger normal faults that accomplish some crustal thinning as the upper plate retreats this coevolution of extension and shortening shares some notable characteristics with other basement involved collisional orogens including the early tertiary laramide orogeny in the american west and the oligocene to miocene evolution of the alps we propose a possible relationship between underplating and the development of the po as a sag basin as a quaternary phenomenon that may also apply to past periods of apennine deformation tortonian continued shortening on the structure beneath the bologna mountain front represents by far the most important and underappreciated seismogenic source in the front of the northern apennines

structural and stratigraphic evolution of the west jaca thrust top basin spanish pyrenees thrust top basins form and fill during their detachment from the foreland margin by active thrust sheets strike normal n s sections from the west jaca thrust top basin southwest pyrenees to the ebro basin show that the pena flexure the local expression of the thrust front can be interpreted as a passive roof duplex the duplex accommodated up to 7 km bulk shortening at the tip of the gavarnie thrust sheet unravelling the timing of uplift and sedimentation in the west jaca basin has revealed that development of the pena flexure represented the climactic phase of deformation lateral e w diachroneity of gross stratigraphy and thrust emergence throughout the southwest pyrenees suggests that the thrust front propagated both toward the foreland and westward parallel to the orogen axis

continental margin tectonics submarine accretionary prisms submarine accretionary prisms are sites of incipient mountain building where sediments and rocks of the deep ocean are uplifted deformed and ultimately transformed to continental crust accretionary prisms develop at sites of lithospheric convergence that is subduction zones commonly accretionary prisms form along continental margins although accretionary prisms may also occur adjacent to island arcs which are ultimately coalesced into continental landmasses here we review the studies of submarine accretionary prisms during the past four years with primary focus on the contributions of geoscientists from the united states

geometry and evolution of the frontal part of the magallanes foreland thrust and fold belt vicuna area tierra del fuego southern chile the magallanes foreland thrust and fold belt is a thin skinned foreland thrust and fold belt of paleocene to oligocene age that deforms upper jurassic through tertiary volcanic volcaniclastic and siliciclastic strata of the magallanes basin southern andean cordillera chile this paper is a detailed description and analysis of the geology and structural evolution of the thrust front vicuna area of southern tierra del fuego reflection seismic and well data together with 1 50 000 scale geological mapping have been used in the analysis in the southern part of the vicuna area two different thrust systems have been found an upper imbricate fan that deforms upper jurassic and cretaceous strata and a younger lower duplex composed of cretaceous and probably upper jurassic rocks the imbricate fan is characterized by fault propagation folding in which listric thrust faults merge downward into a sole thrust that probably is located within the upper jurassic stratigraphy this fan system generates an intercutaneous wedge with a well developed triangle zone at its leading edge the sole thrust of the upper imbricates forms the roof thrust of the underlying duplex in the northern part of the vicuna area the syntectonic sedimentary wedge of the foredeep consists of late cretaceous through tertiary siliciclastics that have been deformed and uplifted by passive back thrusting at the triangle zone the structural style in the foreland region shows three main subhorizontal detachment levels located within the sedimentary wedge as a result of the progressive transfer of slip from the thrust belt to the foreland minor blind thrusts produce stacked pop up and triangle structures that result in complex geometries in the cores of anticlines a forward breaking sequence of thrusting is interpreted during deformation the active foredeep wedge migrated at least 10 km northward balanced geological cross sections indicate approximately 60 30 km shortening for this part of the magallanes thrust belt

duplex geometry an example from the moine thrust belt abstract the geometry and microstructure of a small duplex formed in one bed from the moine thrust belt of northwest scotland is reported the structure is seen in oblique section within the cambrian pipe rock in an area of low strain a range of movement direction indicators are present in the structure an early grain shape fabric developed close to the roof thrust is taken as the best estimate of the overall movement direction towards 287 slickensides in the gouge developed on movement planes within the duplex show varied orientations on a given plane and are not considered useful indicators of thrust transport direction branch lines exposed converge and diverge suggesting little lateral continuity of the exposed structure the microstructures present within the structure indicate an increase in localised deformation and in cataclastic behavior as the duplex evolved early layer parallel shear is ubiquitous giving rise to an elongate grain shape fabric close to bedding surfaces in early formed horses a layer parallel oblate grain shape fabric which shows localised slip zones is overprinted by gouge formation later formed horses show only fracturing and gouge development this sequence is attributed to stick slip behavior in the propagation or displacement of the original fault now the floor thrust

mechanics of triangle zones and passive roof duplexes implications of finite element models abstract finite element models of passive roof duplexes simulate the evolution and distribution of fault slip and deformation as functions of boundary conditions friction on the roof fault cover strength and amount of overburden a foreland buttress is essential for backthrust displacement in the models this foreland buttress is analogous to a pin line between the cover and footwall forelandward of the triangle zone in a balanced cross section and could be caused by a stratigraphic pinchout of the upper detachment or a strong cover in the foreland with boundary conditions that prevent the cover from slipping toward the hinterland relative to the wedge incipient forethrust and backthrust ramps zones of high equivalent plastic strain cut through the cover in the models there is more backthrust displacement with a lower friction coefficient on the upper detachment and with a stronger cover because of pressure dependence both fault strength and material strength increase with increasing overburden therefore the effect of overburden on backthrust displacement is determined by the nature of the pressure dependencies for fault friction and material strength in models with multiple imbricate faults the distribution of equivalent plastic strain adjacent to each fault is similar to that in single fault models with differences resulting from bending strains in the wedge and cover the strain distribution in the cover of the models is similar to that above natural passive roof duplexes characterized by layer parallel shortening above the tip of each horse in the duplex

interpretation of the eastern sulaiman fold and thrust belt pakistan a passive roof duplex abstract surface geology seismic reflection and well data from the eastern sulaiman fold and thrust belt sftb were integrated to analyze its structural geometry deformation style foreland structure development and tectonic shortening the n s oriented fold belt s deformation front has a monoclinal expression over 350 km along the eastern edge of the sftb the cross section shows a 9 km thickness of platform sequences that are detached from the basement by an inferred decollement in paleozoic strata the base of cretaceous strata is 6 km deep in the foredeep syncline rising 7 km in the foreland in the absence of a major thrust and is interpreted as a thin skinned passive roof duplex style of deformation the duplex sequence of jurassic and older strata are separated from the cretaceous and younger roof sequence which is intact over foreland structures for 60 km a balanced structural cross section of this area restores to an original length of 172 km implying 112 km 65 of shortening since 6 2 ma with rates of 18 mm yr this is greater than the 50 shortening along a 359 km long n s balanced cross section across the sftb but is likely to occur along stacked duplexes with high relief along the eastern sftb

the structure of mountain fronts commonly the part of a mountain front which is visible at the surface consists of foreland dipping thrust belt rocks elevated above their regional structural position and overlain more or less conformably by molasse several explanations for their geometry are possible 1 major detachments exist within or beneath the molasse resulting in transport of the foreland basin examples of this geometry come from the swiss molasse plain the southern pyrenees and the mackenzie mountains of canada 2 displacement is lost on major backthrusts beneath the frontal monocline examples cited here are the rockies of alberta the sulaiman ranges of pakistan the mackenzies and the andes in peru 3 thrust sheets travelled over an old land surface and syntectonic molasse contemporaneously offlaps the topographic high of the thrust front this phenomenon occurs along the alpine thrust front in haute provence 4 the frontal fold represents deformation above a large scale thrust tip no unequivocal example of tip line strain at this scale has been recorded although this type of deformation may occur in the brooks range of alaska in many areas mountain fronts show a combination of these idealized geometries

a dynamical index for the east asian winter monsoon abstract a new index measuring the east asian winter monsoon is defined using the mean wind shears of upper tropospheric zonal wind based on the belief that the physical processes of both higher and lower latitudes and at both lower and upper troposphere should be considered to depict the variability of monsoon when the index is high low the westerly jet is strong weak the east asian trough is deep shallow the siberian high is strong weak and anomalous low level northerlies southerlies prevail over east asia as a result the surface and lower tropospheric temperature over east asia decreases increases and the cold surges over southeast asia and tropical western pacific are more less active the index which exhibits distinct interannual variations is also strongly correlated with the arctic oscillation and nino 3 4 sea surface temperature sst index compared to previous indexes this index takes into account more influencing factors and better elucidates the physical processes ass

structure of the high grade opatica belt and adjacent low grade abitibi subprovince canada an archaean mountain front plutonic gneisses in the archaean opatica belt in canada record crustal scale wsw vergent i e longitudinal high temperature d1 ductile shearing similar structures occur in adjacent greenstone belts but only in narrow amphibolite facies zones near the base metavolcanics in the greenschist facies portions do not record penetrative d1 structures the greenstone belts are interpreted as allochthonous on the gneisses they form the uppermost d1 thrust sheets sse vergent i e approximately transverse d2 thrusting imbricated the opatica belt and formed a large antiform either a culmination or an antiformal stack in the southern opatica the imbricated opatica belt wedged under the northern part of the abitibi subprovince and created a tight syncline with nnw vergent d2 thrusts interpreted as backthrusts on its northern limb the opatica belt abitibi subprovince contact therefore resembles some phanerozic mountain fronts further southwards propagating imbrication of the crust beneath the abitibi subprovince created a s vergent foreland fold and thrust belt in the overlying greenstones high grade opatica gneisses and adjacent low grade abitibi greenstones contain complementary structural patterns and may therefore represent the internal and foreland parts of an archaean mountain belt similar in many respects to phanerozoic examples

moist and the continuity of crustal reflector geometry along the caledonian appalachian orogen new aspects of the deep structure of the scottish caledonides are revealed by the moine and outer isles deep seismic reflection traverse moist the caledonian foreland is underlain by an easterly dipping strongly reflecting surface cutting through the moho and traceable to more than 45 km depth thrusts within the foreland basement and caledonian orogen have been reactivated as normal faults bounding half grabens filled with sediments of late palaeozoic to mesozoic age the moine thrust which carries rocks of the orogen over lewisian foreland dips at 20 25 to the e on moist and is either a the westernmost of a series of easterly dipping reflections thrusts which flatten or terminate at 17 20 km depth or b a thrust lying further e which structurally overlies these easterly dipping reflectors by comparison with easterly dipping reflections on cocorp lines in the northern and southern appalachians the latter interpretation is preferred implying that many of the easterly dipping reflections on moist correspond to off shelf metasedimentary rocks with slivers of basement stacked and imbricated against the lewisian basement edge regional seismic refraction and conductivity data from northern scotland suggest that regions of the caledonian orogen have lower mid crustal velocity gradients and higher conductivities than the foreland which might be explained by this interpretation of the easterly dipping reflections despite similarities in crustal reflector geometries there are important differences between the moist and cocorp lines including 1 a remarkably continuous relatively horizontal moho seen on moist data at about 25 km depth and 2 the sedimentary basins offshore from northern scotland which have formed by reactivation under crustal extension of the easterly dipping thrusts furthermore it is probable that the easterly dipping reflectors in these areas of the moist and cocorp lines were formed at different times in the early palaeozoic

fault related folding a review of kinematic models and their application abstract folding that is directly related to fault activity is an important deformation feature that occurs all over the world in mountain belts accretionary wedges fold and thrust belts and intra plate settings in either strike slip compressional or extensional regimes due to their widespread occurrence knowledge about the development of these structures is important to a broad spectrum of geoscience sub disciplines such as structural geology seismology geomorphology petroleum geology and quaternary geology fault related folding has been analyzed intensively over the last 140 years for the sake of this review we divide the folds according to the way the faults and the folds form that is into detachment fault bend and fault propagation folds all fault related folds are caused by changes in fault parameters the simplest method to produce folds is to transport material along faults that have stepped flat ramp flat geometries fault bend fold alternatively the slip can decrease along the length of the fault and depending on whether the fault remains within a detachment layer or steps up through mechanical stratigraphy either a detachment fold or a fault propagation fold is formed respectively detachment folding was first investigated in the early 20th century whereas the full significance of fault propagation folds was recognized quite late in the 1980s seminal work on fault related folding was carried out in the 1930s but quantitative kinematic models have only been available in the last 30 years these models are extremely valuable because they allow a comprehensive understanding of the evolution of fault related folds and lead to more accurate predictions of the sub surface structure from the mid 1990s onwards numerical simulations have been used to identify how fault parameters such as dip and fault bend angle propagation to slip ratio and shape of the trishear zone influence the geometry of the related folding this is directly applicable to the analysis of the shape of anticlines produced however this does not mean that fold geometry is uniquely related to fault geometry on the contrary different kinematic approaches can lead to a similar fold shape

tectonic wedge geometry at bardas blancas southern andes 36 s argentina abstract a tectonic wedge at the mountain front of the cordillera principal mendoza province argentina shows some geometrical features that depart from examples described elsewhere this wedge underthrusts the roof sequence beneath a blind thrust the wedge is deformed by imbricate backthrusting that branches into the hangingwall ramp of the leading horse this backthrust system contributed to the uplift and shortening of the mountain front

deformation of the northern sumatra accretionary prism from high resolution seismic reflection profiles and rov observations abstract following the 2004 great sumatran andaman m w 9 2 earthquake high resolution seismic reflection multibeam bathymetric and remotely operated vehicle data were acquired to investigate the tectonic framework of the rupture zone and search for evidence of seafloor and near surface displacement three distinct regions off northern sumatra were investigated 1 a portion of the sunda trench 2 the adjacent frontal deformation zone and 3 the seaward flank of the aceh forearc basin a thick 1 5 km sediment section within the sunda trench shows evidence of shallow normal faulting possibly representing early stages of assimilation into the accretionary wedge the frontal deformation zone consists of ridges of predominantly landward verging thrust folds seaward verging backthrust faults at or near the base of the steep slope commonly reach the seafloor we do not observe a single laterally extensive structural offset at the deformation front that might be interpreted as contributing to the 2004 tsunami rather a series of small offset tens of metres faults were noted across this broad zone of the frontal accretionary wedge the western boundary of the aceh forearc basin is the west andaman strike slip fault juxtaposing the accretionary complex s forearc high with basin fill sediments neither the seismic nor rov data show evidence of recent seafloor displacement along the fault trace basin infill demonstrates consistent along strike patterns of tilting and seaward subsidence during sedimentation while modern fill is flat lying and coherent across the entire basin intercalated chaotic layers interpreted as mass transport deposits may record a history of seismicity but recent examples of such deposits were not observed on the modern seafloor either seaward of the deformation front or in the aceh basin lack of any evidence of faulting offset or disruption of sediments within aceh basin suggests that there was little impact of the 2004 earthquake in this area distributed faults throughout the frontal deformation zone combined with observations of landward verging folds at the deformation front folding within piggy back basin sediments and lack of evidence of disruption along the west andaman fault zone and within the forearc basin all support strain partitioning across the margin a proposed strong wedge interior may act as a backstop during major thrust events constraining deformation to the frontal deformation zone and the slope apron tsunami generation in response to the 2004 event did not result from surficial displacements along a single fault or narrow fault zone at the toe of the deformation front but was more likely a result of vertical displacement across the entire outer forearc

mechanical stability of the triangle zone the backthrust wedge the triangle zone arrangement of faults that commonly develops at the foreland margin of fold and thrust belts is one of the few places in this structural setting that backthrusts of significant dimensions are observed even in this setting the backthrust may be a relatively ephemeral feature as it can be cannibalized by the foreland advance of thrusting in an active fold and thrust system one of the factors that determines the longevity of a specific backthrust and triangle zone in an active system is the mechanical stability of the backthrust wedge the wedge of rock that forms the hangingwall to the backthrust upper and lower stability limits for this backthrust wedge can be determined using the critical wedge concept for a cohesive coulomb material the configuration of the backthrust wedge developed in triangle zones along the southern and central alberta portion of the canadian cordillera agrees well with the stability limits derived for reasonable rock properties and normal fluid pressures in contrast a low taper triangle zone interpretation for northeastern british columbia does not fall within the calculated stability limits even when abnormally high fluid pressures and extreme rock properties are assumed however a similar low taper geometry may be achieved by the shingling of stable backthrust wedges upper crustal delamination and passive roof duplexing are both processes that serve to sustain a mechanically stable backthrust wedge configuration suggesting that they are both viable paths for the progressive advance of the mountain front triangle zone

early proterozoic collisional orogeny along the northern thelon tectonic zone northwest territories canada evidence from the foreland the subsidence history of kilohigok basin 1 9 ga n w t canada reflects the opening and closing of an ocean basin along the present thelon tectonic zone the basal sedimentary sequence represents a passive margin shelf that faced se toward the orogen following passive margin subsidence tectonic loading occurred resulting in rapid subsidence of the outer shelf margin and simultaneous uplift and erosion of the platform interior a flexural moat formed subparallel to the thelon tectonic zone the shelf was subsequently drowned and successively buried by deep water flysch shallow marine shelf sediments and fluvial molasse forming a foreland basin tapered to the nw direct structural evidence for the tectonic load is provided by the leading edge of a thin skinned thrust fold belt exposed in oblique cross section within the se extremity of the basin upright folds at high structural levels can be traced downward to recumbent tight folds and thrusts near the basement cover contact documenting increasing shear strain with depth overall northwest vergence and about 50 shortening is indicated by fold thrust geometry and finite stretching lineations above a brittle ductile decollement that roots within the thelon tectonic zone

passive roof duplexes under the rocky mountain foreland basin alberta seismic reflection data in the central alberta foothills near edson reveal the presence of small passive roof duplexes in upper cretaceous rocks within the otherwise undeformed foreland basin as much as 40 km northeast of the mountain front monocline the tops and bottoms of the duplexes are defined by backthrusts and sole thrusts which follow bedding planes within upper cretaceous strata overlying the structures is an essentially uncontracted 1 8 km thick section of upper cretaceous and tertiary rocks which is passively uplifted over the thickened duplexes the underlying autochthonous sequence of mesozoic and paleozoic rocks exhibits some minor folding but is also uncontracted our interpretation extends both the upper and lower detachments of the widely accepted triangle zone model more than 30 km farther under the foreland basin than has previously been supposed the seismic data illustrate relatively clearly the form of the leading edge of the last phase of rocky mountain thrusting we expect that similar features will be observed elsewhere in the rocky mountain foothills and probably at other mountain fronts worldwide

three dimensional horizon reconstruction from outcrop structural data restoration and strain field of the baisahi anticline western nepal abstract within many fold and thrust belts preservation of bed thickness is generally assumed during deformation a three dimensional 3d reconstruction method is proposed from outcrop structural data this technique accounts for along strike thickness variations by a series of 3d spatial projections of dip and strike data the thickness of strata from any point at the ground surface to reference horizons on stratigraphic columns can be calculated across strike and projected along the poles of the associated strata providing sets of dots in space which represent folded horizons the technique is applied to the baisahi antiform located on the frontal subhimalayan thrust an along strike deformation gradient is evidenced by two dimensional cross section balancing which shows a passive roof duplex structure the 3d geometry of the roof is reconstructed allowing other approaches such as unfolding to be done the residual strain field induced by restoration shows distortion features which are interpreted as internal strain due to the propagation of the fault

passive roof duplex geometry in the frontal structures of the kirthar and sulaiman mountain belts pakistan abstract exploration for hydrocarbons over the past few years has greatly improved our understanding of the geometry of frontal mountain belt structures in this study we introduce and discuss the concept of the passive roof duplex using as the main example the kirthar and sulaiman ranges in the baluchistan province of pakistan structures similar to those described here have been recognized previously in other mountain belts and they appear to exist as a common feature in many more frontal regions of mountain belts our example of a passive roof duplex which we describe from pakistan is compared briefly with similar structures reported by others the passive roof duplex is here defined as a duplex whose roof thrust has backthrust sense passive roof thrust and whose roof sequence those rocks lying above the roof thrust remains relatively stationary during foreland directed piggy back style propagation of horses within the duplex