control of synorogenic sedimentation on back and out of sequence thrusting insights from analog modeling of an orogenic front outer carpathians southern poland 1 scaled sandbox models based on field and seismic data were used to simulate miocene evolution of the carpathian orogenic front the experiments investigated the influence of synorogenic deposition on the geometry and kinematics of growing tectonic structures the deposition introduced at the foreland of an active fold thrust wedge resting on top of a weak ductile detachment horizon generally results in transferring the deformation to the rear part of the model depending on the rate and across strike extent of synorogenic sedimentation and on the moment it starts a single major back thrust above the rear edge of the ductile layer can form or out of sequence foreland verging thrusts can be initiated alternatively older thrust faults can be reactivated at the rear part of the model thrust wedge leading to destruction of the earlier formed structures our experiments explain the origin and evolution of three different structural geometries known from the carpathian orogenic front involving major back thrusts related to triangle zones and out of sequence breaching forethrusts that substantially reorganized deformation patterns in a growing thrust wedge the development of these geometries was controlled by massive synorogenic sedimentation in the foreland from the modeling and natural examples we infer a number of typical structural situations that can occur at an orogenic front underlain by a weak ductile horizon depending on parameters controlling synorogenic sedimentation and tectonic push exerted by an approaching thrust wedge

duplex style and triangle zone formation insights from physical modeling duplexes are a common feature in thrust belts at many scales their geometries vary significantly from antiformal stacks with significant forethrusting in the cover e g southern pyrennes spain to triangle zones e g foreland canadian rockies to low displacement individually spaced ramp anticlines e g sub andean thrust belt bolivia we present a series of physical experiments demonstrating that the strength of the decollements relative to that of the intervening and overlying rock layers plays a significant role in controlling the duplex style the models comprise brittle layers made of dry quartz sand and decollements made of two types of viscous silicone polymers the strength of the decollements in the models is a function of the shortening rate applied to the model the relative strength of the decollements and surrounding rocks affects the development of active or passive roof duplexes triangle zones it also affects the amount of translation of individual thrust blocks and the spacing of thrust ramps which in turn determine if a duplex evolves into an antiformal stack or into individually spaced ramp anticlines model results indicate that specific associations of structural features form systematically under similar rheological and boundary conditions the presence of relatively strong decollements promotes local underthrusting of the cover individual ramp anticlines internal deformation of thrust sheets low early layer parallel shortening and sequential towards the foreland propagation of structures weak decollements promote forethrusting of the cover antiformal stacks coeval growth of structures and low internal strain with the exception of significant early layer parallel shortening no underthrusting at a regional scale occurred in any model

balanced cross sections and their implications for the deep structure of the northwest alps reply abstract the structure of the northwest external french alps around the internal belledonne and mont blanc massifs in the beaufortain area can be explained by foreland wnw propagating thrust tectonics the internal belledonne massif is interpreted as having developed by imbrication of an originally thin skin of basement with a mesozoic cover above this massif all thrusts tend to climb lateral sections to the south to converge with the frontal pennine thrust the southern termination of the mont blanc massif is merely a spectacular example of such a lateral ramp

role of decollement material with different rheological properties in the structure of the aljibe thrust imbricate flysch trough gibraltar arc an analogue modelling approach abstract the miocene aljibe thrust imbricate flysch trough in the external zone of the gibraltar arc shows large changes in structural style towards the foreland rheological variation of the decollement rocks triassic evaporitic rocks and mesozoic limestones appears to be the main factor controlling the contrasts in structural style that characterise adjacent wedge domains in order to test this hypothesis we carried out analogue experiments involving three dimensional variations of decollement material a plate of viscous material was placed in the central part of the models at the base of the sand multilayers different geometries of the viscous plate were used experimental results show a strong link between the rheological properties of the decollement material and the structure of the overlying thrust wedge these results compare well with the internal geometry and vergence distribution in the aljibe thrust imbricate indeed the tectonic style of the wedge can be explained as a consequence of variations of rock type along the basal decollement

epicenter distribution and magnitude of earthquakes in fold thrust belts insights from sandbox models scaled analogue models are used to illustratethe eect of basal friction and erosion on fault activity andhence on epicentre distribution and magnitude of earth quakes in the sedimentary cover of active fold thrust belts model results suggest that in fold thrust belts shortenedabove low friction ductile decollements rock salt or over pressured mudstone low to moderate magnitude earth quakes mw 5 3 5 6 distributed over a wide area occuralong several long lived thrust faults conversely in areasshortened above high friction decollements large magnitudeearthquakes mw 6 6 6 8 distributed over a narrowzone are likely to occur along few short lived thrust ramps calculated magnitude of earthquakes from models and theirdistribution are in agreement with recorded earthquake pat tern from the zagros mountain belt which is partially short ened above a ductile decollement of hormuz salt formation model results also showed that erosion reactivates older in active thrusts and promotes formation of out of sequencethrusts

passive roof thrusting and forelandward fold propagation in scaled brittle ductile physical models of thrust wedges a set of scaled analogue models was performed to investigate the role of a decollement layer in the structural style of fold and thrust belts silicone putty sgm 36 has been employed to represent the ductile decollement while frictional material quartz sand has been used to simulate the brittle behavior of roof and floor sequences situated above and below the decollement respectively in order to explore a wide combination of strength profiles models have been deformed under five distinct rates of compression 0 15 0 3 0 45 0 6 and 1 5 cm h 1 employing four different thicknesses for the silicone layer 0 1 0 2 0 4 and 0 8 cm experimental results indicate that roof sequences exhibit two distinct styles of deformation 1 passive roof duplex prd geometry e g banks and warburton 1986 and 2 outward propagation of folding ofp along the decollement e g davis and engelder 1985 in prd geometry the roof sequence remains relatively stationary being underthrust by a wedge shaped floor duplex while in ofp displacement along the floor thrusts is transferred horizontally along the decollement layer experiments suggest the occurrence of a genetic relationship between shear stresses at the base of the roof sequence b and within the ductile layer d and the development of prd or ofp geometry for a given strain rate the deformation pattern of roof sequences depends upon the b d ratio as well as the absolute b and d values the transition field between ofp and prd styles has been approximated by curvilinear regression of transition style experimental data points with a minimum value of d varying between about 112 and 160 pa to enter the prd field syntectonic sedimentation in the molasse basin is found to inhibit development of folding into the foreland but to promote prd style experiments simulate many of the principal characteristics of fold and thrust belts developed above a basal evaporite decollement as well as the development of passive roof duplexes at mountain fronts shear stress values inferred for many fold and thrust belts correlate well the deformation fields predicted by the present experimental study allowing to propose a simple model describing the mechanical behavior of natural roof sequences underlain by a decollement layer

the oligocene miocene tectonic evolution of the northern outer carpathian fold and thrust belt insights from compression and rotation analogue modelling experiments this paper presents the different analogue scenarios of the tectonic evolution of the northern outer carpathians i e the western and northern eastern outer carpathians which formed as an accretionary wedge in front of the east alpine carpathian pannonian alcapa block during oligocene miocene times currently this fold and thrust belt forms an arc which is asymmetrically convex to the north and wider in its eastern part palaeomagnetic investigations have suggested that the rocks of the arc underwent counter clockwise rotation along almost the whole arc which is difficult to explain as an effect of simple indentation of the triangular indenter in this case two branches of the arc should be rotated in the opposite directions the structural evolution of the western outer carpathians is characterized by superposition of two successive tectonic shortening events directed n s and ne sw the results of the presented analogue modelling suggest that two scenarios of the geodynamic evolution of the studied belt could explain the occurrence of such differently oriented shortening events 1 two phases of differently directed indentation first to the n then to the ne and 2 indenter movement to the ne with simultaneous counter clockwise rotation however the experiment in which the moving indenter is simultaneously rotated produces the most suitable model the counter clockwise rotation of the material is only possible in front of both sides of the convex indenter in this model the results of the analogue modelling also prove that rotation of the alcapa block started after formation of the magura nappe the innermost nappe of the western outer carpathians

experimental and field study of the effects of lithological contrasts on thrust related deformation 1 the influence of mechanical contrasts related to stratigraphy on thrust related deformation are investigated by sandbox modeling using combinations of granular materials possessing different mechanical properties and by structural analysis of a natural example from the spanish pyrenees the models used in this study consist of loose sand and glass microbeads in varied stratigraphic thicknesses and sequences and the natural example consists of a weakening upward sequence of sandstones and carbonates in both cases layers of different composition had a mechanical significance and accommodated shortening differently model sand and natural sandstone exhibited most shortening by thrust imbrication irrespective of stratigraphic position whereas glass microbeads and carbonates exhibited wider deformation zones and enhanced folding accompanied by displacement reduction along thrust surfaces cutting them transitions of structural style were guided by the stratigraphy rather than by a simple unidirectional propagation from a step up layer parallel shortening was accommodated by diverse mechanisms but did not vary significantly in layers which had different properties the magnitude with which mechanical contrasts were expressed in the models was influenced by the strength at the decollement zone the effect of mechanical contrast was more profound in models shortened above a low friction decollement

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

computerized x ray tomography analysis of sandbox models examples of thin skinned thrust systems computerized x ray tomography applied to analog sandbox experiments performed in a normal gravity field makes possible the analysis of the kinematic evolution as well as the three dimensional geometry of models that simulate tectonic deformations most of the plastic or viscous analog materials generally used in a normal gravity field for such models have x ray attenuations compatible with medical scanner images models using soft cohesionless granular materials were studied faults in granular materials corresponding to dilatant zones with lower by 30 50 x ray attenuation are thus clearly visible in the tomography images kinematic analysis was done on imbricate thrust propagation in materials with an internal friction angle of 30 thrust propagation is controlled mainly by a triangular pop up structure resulting from the nucleation of two conjugate thrusts at the basal decollement the pop up structure is created immediately in front of the previous thrust three dimensional geometry analysis was tested on lateral ramps parallel to the transport direction lateral ramps were induced by changing the frictional boundary conditions at the bottom of the model though the lateral ramps strike almost parallel to the transport direction their dip angle is comparable to the dip angle of frontal ramps

three dimensional geometry and kinematics of experimental piggyback thrusting the three dimensional geometry and kinematics of piggyback stacks of imbricate thrust sheets are illustrated and discussed using a single model shortened in a squeeze box strike parallel geometric elements simulated include lateral ramps eyed sheath folds splays and thrust thrust interference fine details of these structures were exposed by eroding a shortened wedge of sand using a newly developed vacuum eroding technique a kinematic analysis of the model shows a stepwise increase in imbricate thrust spacing and or a decrease in rate of nucleation of imbricate thrusts in the direction of thrust transport despite the steady forward advance of a rear wall the piggyback wedge accreted episodically recording different strain domains in longitudinal cross sections strain partitioning in single layers by bed length balancing showed an increase in layer shortening with volume loss and a corresponding decrease in imbricate thrusting and ramp folding with depth

critical taper behaviour and out of sequence thrusting on orogenic wedges an example of the eastern alpine molasse basin we demonstrate that increasing erosion during the kinematic evolution of a thrust wedge will lead to out of sequence thrusting as a result of backwards critical taper movement in sequence thrusting in the subalpine german molasse basin built a critical tapered foreland coulomb thrust wedge later out of sequence thrusts dissected all but the frontal duplex stacks the footwall hangingwall relation visible on seismic data proves the out of sequence nature of the latest thrusting stage establishing a stable drainage system leads to increased erosion in elevated areas of the thrust wedge resulting in flattening of the critical wedge in order to keep its predefined angle the critical wedge repositions and the tip of the taper moves towards the hinterland thus thrusting will also reposition and move towards the hinterland

modelling the geometry of coulomb thrust wedges internal geometry and detachment of coulomb thrust wedges is examined in a combined gravity and pressure force field under various boundary conditions and coulomb parameters frontally buttressed sand wedges accreted with narrow tapers above soles with negligible friction in these models serial imbricates stepped up in conjugate sets which operated synchronously or serially with increases in sole friction model coulomb wedges accreted in piggyback style with enhanced taper but narrower spacing of imbricates in this mode flat topped box fold anticlines nucleated at the frontal tip of a sole thrust prior to the fore limb failing as a ramp the initial ramp thrusts subsequently rotated and steepened to acquire concave upwards listric geometry when cohesion was a significant component of strength and the sand grains interlocked thoroughly ramp climb during piggyback thrusting occurred through the generation of a stack of backthrusts the taper of cohesionless sand wedges which lack any length scale was consistent with the critical cohesionless coulomb wedge taper in contrast the taper of experimental cohesive sand wedges which are scale dependent was consistent with the theoretical cohesive coulomb narrow taper only when the cohesive length scale was lr sr rar 10 6 this was constrained by comparing geometry of cohesive sand wedges shortened at different body force per unit mass

contrasting tectonic and sedimentary history of the central and eastern parts of the polish carpathian foredeep basin results of seismic data interpretation polish carpathian foredeep basin pcfb is located at the junction between western and eastern european platforms such location caused different evolution of various parts of this basin in the central part of the pcfb growth fault propagation folds related to latest badenian stages of thrusting in this segment of the carpathians developed only minor normal faulting related to miocene reactivation of older basement faults was detected paleovalleys mapped within the mesozoic basement were interpreted as palaeogene tectonically controlled erosional features developed after inversion of the polish trough in the central pcfb post evaporitic sediments were supplied into the foredeep basin from south from the eroded carpathians in the eastern part of the pcfb a system of large normal faults related to miocene flexure induced reactivation of inherited mesozoic faults was identified within the hangingwall of this system reverse faults rotated blocks and basement pop up structures were described their formation was related to combined effect of compressional stresses transferred from the carpathian collision zone and reactivation of older large scale fault caused by both flexural extension and strike slip movements during deposition of the miocene foredeep sediments differential compaction also played an important role gentle onlap of the middle badenian evaporites by post evaporitic sediments towards the nw was interpreted as a result of tectonically driven relative sea level rise and re establishment of marine connections with eastern paratethys

thermal maturity of miocene organic matter from the carpathian foredeep in the czech republic 1d and 3d models a three dimensional reconstruction of burial and palaeogeothermal conditions is presented for miocene sediments of the carpathian foredeep beneath the outer western carpathians fold and trust belt in the eastern part of the czech republic the sedimentary units involved include autochthonous paleozoic sequences miocene deposits of the carpathian foredeep and of the western carpathian nappe system reservoir rocks with economic oil and gas accumulations occur in the fractured crystalline basement and in the neogene carpathian foredeep the studied vizovice area is characterized by rocks representing both variscan and carpathian orogenic cycles the 3d thermal maturity and subsidence model presented allows the significance of both tectonic events to be evaluated the model calibrated by vitrinite reflectance from eight boreholes proved that eroded units related to the variscan orogeny approach in amount those eroded during the carpathian orogeny the thickness of the eroded rocks does not exceed 300 m in either case vitrinite reflectance data from representative core samples of the miocene organic matter show that rr values increase with depth from 0 36 to 0 58 a re evaluation of archival data on the quantity and quality of organic matter shows that total organic carbon ranges from 0 20 to 2 92 wt and residual hydrocarbons s2 from 0 04 to 8 48 mg hc g rock these results lead to the conclusion that neogene unit ii that was interpreted as coastline through to shallow marine deposition environment within the carpathian foredeep in the czech republic is potential source rock for hydrocarbon accumulations

prehispanic and colonial landscape change and fluvial dynamics in the chalco region mexico abstract the late quaternary deposits of the lakeshore plain and western edge of lake chalco in the southeastern basin of mexico were explored in order to understand the influence of alluvial sedimentation on archeological visibility and preservation as well as to seek out records that would permit direct comparison of erosion and sedimentation with the demographic record derived from the archeological survey the chalco lakeshore plain is traversed by two of the largest rivers in the region and the pattern of human settlement recorded here by archeological survey is partly an artifact of alluvial sedimentation where the most prominent settlements were observed in areas with little or very slow sedimentation detailed radiocarbon dating of the alluvial deposits indicates that prior to the establishment of a sedentary lifestyle alluvial sedimentation was slow catastrophic soil erosion is inferred from a period of widespread rapid alluviation and mass movements during late preceramic early and middle formative periods which were periods of rapid population growth mass movements appear to coincide also with periods of wetter climate no evidence of sedimentation was noted during the population collapse that occurred at the end of the formative the next phase of alluvial sedimentation occurred during the early classic to late toltec periods which were also periods of population expansion during a gradually drying climate the postclassic period which was the largest pre columbian demographic expansion was paradoxically accompanied by minor evidence of alluvial activity the last and most widespread period of alluvial activity occurs in the post conquest period and is difficult to date with precision but appears to occur primarily during the seventeenth and eighteenth centuries

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

influence of syntectonic sedimentation on thrust wedges in analogue models scaled analogue sandbox models have been used to simulate the growth of coulomb thrust wedges that include syntectonic sedimentation during thrusting increases in the syncontractional sedimentation rate dramatically change the shape of the coulomb thrust wedge decreasing the number of thrust slices the internal shortening and the critical taper of the wedge thrust trajectories change from ramp flat to sigmoidal with increased syncontractional sedimentation the analogue models show many similarities to thrust fault geometries in accretionary prisms and foreland fold and thrust belts in which thrust slices prograded into basins with active sedimentation

lower plate geometry controlling the development of a thrust top basin the tectonosedimentary evolution of the ofanto basin southern apennines the ofanto basin is a pliocene pleistocene thrust top basin that formed with an unusual east west orientation along the frontal part of the southern apennine allochthon during the latest stages of tectonic transport its tectonic and sedimentary evolution was studied integrating field surveys biostratigraphic analyses and the interpretation of a large seismic grid well data and seismic interpretation indicate that a large east west trending normal fault underlies the northern margin of the basin displacing the apulian carbonates that form the foreland and the footwall of the southern apennine allochthon in our reconstruction the ofanto basin formed at the rear of the bulge caused by buttressing of the southern apennine allochthon against this normal fault in a second stage of contraction the footwall of the southern apennine allochthon was involved in deformation with a different trend from the normal faulting and buttressing this caused eastward tilting of the basin and broad folding around its eastern termination good stratigraphic constraints permit the age of buttressing to be defined as early pliocene and that of the shortening in the apulian carbonates as early pleistocene this study highlights the importance of early orogenic normal faults in conditioning the evolution of the frontal parts of orogenic wedges supplementary material a 3d reconstruction of the base of the pliocene deposits of the ofanto basin based on seismic interpretation is available at www geolsoc org uk sup18559

thrust dip and thrust refraction in fault bend folds analogue models and theoretical predictions analogue models made of two layers of loose sand separated by a thin layer of micro glass beads were shortened by a rigid emerging ramp dipping at either 30 or 45 and possessing a high intermediate or null surface friction shortening resulted in formation of closely spaced back thrusts in the sand layers the dips of the back thrusts vary within a range of 30 depending on the ramp friction and 7 depending on its dip an increase in ramp friction or to a lesser extent in ramp dip decreases the thrust dips in the model the second important observation is that when friction is greater along the ramp than along the layer of glass beads then the glass beads layer acts as a separate upper ramp above which the back thrusts steepen the theory proposed to explain these observations predicts the thrust dips through a two step procedure first global equilibrium of forces in the two layers is required to yield the mean forces at stake along the ramps and thrusts second the total dissipation is minimized with respect to the dips of the back thrusts the relevant frictional properties of our analogue materials have been measured in stress conditions as close as possible to the experimental ones below 1 kpa and used with the theory to yield optimal back thrust dips that are all within 3 of the measured dips this is a surprisingly good fit when considering that we did not take into account geometric changes strain softening and dilatancy or compaction due to slip on the thrusts we conclude that this general two step theoretical procedure is validated in the context of analogue frictional materials we also propose a possible mechanism for thrust refraction and top to the foreland sense of shear observed in the hanging walls of lower flat to ramp transition in sedimentary piles that is based on the triggering of secondary upper ramps finally this mechanical approach can also be seen as complementary to the kinematic models of fold thrust structures which by definition cannot grasp the strong effects of friction on the kinematics

structural geometry and deformation mechanism of the longquan anticline in the longmen shan fold and thrust belt eastern tibet abstract the 2008 mw 7 9 wenchuan earthquake is a consequence of ongoing india tibet collision and reflects the growth of the longmen shan fold and thrust belt in this paper we present new constraints on the deformation mechanism of the longmen shan fold and thrust belt by comparing the physical models to the example of the longmen shan fold and thrust belt the result indicates that the deformation mechanism of the belt is mainly dominated by the pre sinian layer whereas locally is controlled by the lower triassic layer such as the longquan anticline in addition we discuss the deformation style of the longquan anticline various along strike based on the seismic reflection data interpretations of structural cross sections and field observations as well as physical modeling the sandbox modeling suggests that the deformation of the central segment of longquan anticline is likely controlled by higher displacement rate higher erosion and lower sedimentation which is in contrast with the southern and northern segment moreover the structural geometry of the central segment of longquan anticline is more complex than the end member models of fault related folds which is mainly controlled by pure shear wedge fault bend fold and bounded by west verging thrust fault combining the studies of the structural geometry deformation mechanism and previous studies we infer that the longquan anticline is active and potentially seismogenic therefore a quantitative re evaluation of seismic hazard in longquan anticline and adjacent area directly beneath the densely populated sichuan basin is urgently needed

erosion and exhumation in accretionary orogens experimental and geological approaches 1 this study examines the role of erosion on exhumation and deformation history of accretionary orogens through experimental and geological methods fault propagation kinematics and exhumation rates in an eroded thrust wedge are analyzed through scaled sandbox simulations by testing different parameters friction rheologic layering and sediment input output ratio a model coulomb wedge is submitted to erosion under flux steady state conditions that is the volume of eroded material remains equal to the volume of newly accreted material maintaining a constant surface slope during shortening results show that the way of exhumation depends on the internal dynamics of thrust wedges and conversely on how this dynamics is modified by erosion in the eroded thrust wedges the diversity of exhumation patterns is controlled by the mode of fault propagation depending on the basal friction high or low the vertical component of exhumation is generally higher for the wedges with high basal friction than for low friction wedges the uplift of material occurs along a cluster of subvertical thrusts in the middle part of the eroded thrust wedge with low basal friction the material is exhumed along a series of inclined 20 50 thrusts in the rear of the high friction wedge the zone of maximum exhumation is generally localized in the middle part of the thrust wedge and migrates toward the backstop with continued shortening the vertical exhumation rate increases with time and the material accreted later is rapidly transferred to the main exhumation zone compared to the material accreted during the early stages the exhumation occurs at rates 2 times faster in the wedges with half the thickness than in full thickness wedges the total quantity of eroded material at the end of convergence constitutes 36 50 of initial model area for the thrust wedges with low basal friction and 20 40 for the high basal friction wedges the extent of basal underplating increases with total shortening the area of basal underplated material constitutes up to 30 and 40 of the eroded thrust wedge area for the models with low and high basal friction respectively presence of decollements in the accreted series allows underplating of thrust units developing an anticlinal stack whose growth and location is favored by erosion our results are compared to different present day active and ancient convergent orogens himalaya taiwan southern alps of new zealand and cascadia orogen allowing the different specific exhumation processes to be characterized

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

deformation of brittle ductile thrust wedges in experiments and nature 1 even though the rheology of thrust wedges is mostly frictional a basal ductile decollement is often involved by comparison with purely frictional wedges such brittle ductile wedges generally display anomalous structures such as backward vergence widely spaced thrust units and nonfrontward sequences of thrust development laboratory experiments are used here to study the deformation of brittle ductile thrust wedges results are compared with natural systems in the jura mountains and the northern pakistan salt range and potwar plateau two series of three models are used to illustrate the effects of varying the basal wedge angle and shortening rate v these two parameters directly control variations in relative strength between brittle and ductile layers bd coupling wedges with strong bd coupling low and high v give almost regular frontward sequences with closely spaced thrust units and as such are not significantly different from purely frictional wedges weak bd coupling high and low v gives dominantly backward thrusting sequences intermediate bd coupling produces frontward backward oscillating sequences the spacing of thrust units increases as coupling decreases back thrusts develop in parts of a wedge where bd coupling is weak regardless of the thrust sequence wedges with weak bd coupling need large amounts of bulk shortening more than 30 to attain a state of equilibrium at which stable sliding along the base occurs on this basis we argue that a state of equilibrium has not yet been attained in at least some parts of the jura mountains and eastern salt range and potwar plateau thrust systems

modeling of thrust fronts above ductile and frictional detachments application to structures in the salt range and potwar plateau pakistan series of scaled sandbox models are used to simulate the development of thin skinned simultaneous shortening above adjacent ductile and frictional substrates these models simulate the evolution of the potwar plateau and salt range in pakistan where pale

episodic accretion and strain partitioning in a model sand wedge we model led thrust wedge accretion and deformation partitioning in a passively layered sand detached and shortened above a smooth rigid decollement the thrust wedge accretes in piggyback style during forward advance of a rear wall the passively layered sand accommodates the shortening penetratively by kinking ramp folding and by imbrication in single layers the style of compression changes with time and or stratigraphic position within the sand prism initial penetrative layer shortening above a slipped zone of decollement is succeeded by formation of monoclinal or conjugate shear bands at temporary terminations of decollement the shear bands subsequently narrow down and lock as thrust ramps the model sand wedge accrets episodically rather than steadily episodicity is controlled by the stick slip mode of decollement propagation and by volume loss and compaction of the wedge material in response to convergence this results in formation of a wedge with a convex upwards cross sectional topography

cannibal creek granite post tectonic ballooning pluton or pre tectonic piercement diapir a discussion groves d i eds recent advances in understanding precambrian gold deposits univ west australia geology dept and extension pub 11 and 1989 paleosols and their relevance to the precambrian atmospheric composition jour geology v 97 p 77 92 phillips g n 1988 widespread fluid infiltration during metamorphism of the witwatersrand goldfields generation of chloritoid and pyrophyllite jour meta geol v 6 p 311 332 meyers r e et al 1988 recent advances in the geology of the witwatersrand goldfields including the importance of postdepositional processes ext abs bicentennial gold 88 melbourne p 319 324

analogue sandbox models of thrust wedges with variable basal frictions abstract scaled analogue sandbox models have been used to simulate the growth of coulomb thrust wedges in isotropic cohesionless and anisotropic cohesionless materials variations in the basal coefficient of friction by using different materials as basal detachments combination of basal detachments and the anisotropy of the layered system have been investigated imbricate fans of dominantly foreland vergent thrust systems are developed in the way similar to those developed in accretionary prisms and in foreland fold and thrust belts critical taper wedges close to theoretically predicted geometries are developed for intermediate values of basal friction mb 0 47 whereas for the lower value of basal friction mb 0 37 low taper wedges are formed with slopes less than the theoretically predicted supra critical wedges are formed when the high basal friction mb 0 55 is either equal to or greater than the coefficient of friction of the deforming material in the wedges then the wedges have a high taper angle close to the angle of rest for the modelling material similar geometries developed when the experiments were carried out on combination of two materials with basal frictions as intermediate low low intermediate and low high the spacing of thrust faults increases with increased basal friction higher basal friction or anisotropy within the layered systems favours displacement along foreland vergent thrusts and eventually checks backward breaking thrusting

paleogene and neogene kinematics of the alpine carpathian fold thrust belt at the alpine carpathian transition abstract in this study we analyze a 300 km long segment of the alpine carpathian orogen using novel outcrop data and regional seismic profiles from the west carpathians to unravel the kinematics and timing of eocene to late miocene deformation of the fold thrust belt comparison with data from the eastern alps and the vienna basin leads to an updated tectonic model for the paleogene and neogene of the alpine carpathian transition which includes three stages 1 eocene to early oligocene nnw directed foreland propagating thrusting thrusts are directed perpendicular to the strike of the european foreland in the outer west carpathians owc and sub parallel to thrusts observed in the eastern alps west of the vienna basin 2 continued late oligocene to early miocene nnw directed foreland propagating thrusting simultaneous with sinistral strike slip reactivation of former nappe boundaries in the hinterland of the active thrusts this deformation corresponds to the early stage of eastward lateral extrusion of the eastern alps the ene striking sinistral faults in the owc are kinematic equivalents of the semp fault system in the eastern alps which was active at the same time the latest stage of thrusting is characterized by sse directed out of sequence thrusts in the hinterland of the owc and the reactivation of variscan basement thrusts in the european foreland as sse directed backthrusts 3 formation of n ne striking sinistral strike slip faults which are kinematically linked to n ne directed out of sequence thrusts post dating nnw directed thrusting the sinistral faults formed coeval with the opening of the vienna pull apart basin during the middle to late miocene stage of lateral extrusion unlike proposed in previous studies the west carpathians do not appear to be affected by oligocene to early miocene crustal scale wrenching of the pieniny klippen belt our data show that the deformation of the pkb is in line with the eocene to late miocene tectonic evolution reconstructed for the owc

out of sequence thrusts out of sequence thrusts oost are those thrusts which do not obey the foreland propagating or in sequence deformation style they include both isolated thrusts which develop hindward of the thrust front and sequences of break back thrusts which propagate from the foreland to the hinterland two end members of a series of oost types are recognized 1 older in sequence thrusts which are reactivated along their entire length and 2 completely new thrusts which propagate through already deformed thrust sheets between the two end members are thrusts composed partially of reactivated in sequence thrust sequences and partially of new entirely out of sequence segments oosts can be initiated for a variety of reasons including 1 keeping the orogenic wedge at critical taper 2 break back sequences from the suture zone in the overriding plate 3 ramping to overcome a sticking point which inhibits in sequence thrust propagation and 4 during simultaneous displacement along two stacked thrusts culminations which bow up segment of the upper thrust may be chopped through to permit continued displacement on the upper thrust many different types of thrust behavior including gravity sliding plucking and derivation of isolated horses from ramps may mimic some of the characteristics of oosts consequently it may be difficult to conclusively prove an oost origin for a complex thrust geometry

displacement fields and finite strains in a sandbox model simulating a fold thrust belt this thesis uses geodetic satellite data to measure present day crustal deformation in the zagros fold thrust belt sw iran geodetic type measurements are also used in down scaled models that simulate the surface deformations seen in convergent settings like the zagros fold thrust belt global positioning system gps measurements of three surveys between 1998 and 2001 indicate 9 3 mm yr and 5 3 mm yr shortening across the se and nw zagros respectively gps results show that in addition to the different rates and directions of shortening on either side of the ns trending kazerun fault local along belt extension occurs to the east differential sar interferograms of ers1 2 images between 1992 and 1999 detect 8 4 mm yr uplift rate across a newly recognized fault in sw qeshm island this can be attributed to a steep imbricate thrust that may still represent the local zagros deformation front the salt diapirs in the zagros rise from a source layer that acts as a low frictional decollement that decouples the deformation of the cover sediments from their basement in the eastern zagros whereas the cover to the west deforms above a high friction decollement physical models were prepared to simulate cover deformation in the zagros by shortening a sand pack above adjacent high and low frictional decollements represented by a ductile layer the strain distributions differed above the two types of decollements it was more heterogeneous above the salt where local extension in the shortening direction was dominant a separate work also investigated systematically the role of basal friction on cover deformation in convergent settings accurate height measurements of the model surface by laser scanner indicated a deformation front more distal than usual particularly in the low basal frictional models the volume reduction in our shortened sand models correlated directly with their basal friction

experimental insights on the geometry and kinematics of fold and thrust belts above weak viscous evaporitic decollement a discussion experimental insights on the geometry and kinematics of fold and thrust belts above weak viscous evaporitic decollement a discussion

synchronous and velocity partitioned thrusting and thrust polarity reversal in experimentally produced doubly vergent thrust wedges implications for natural orogens a sandbox analog modeling research program was used to study the detailed evolution of doubly vergent thrust wedges high resolution multilayers of thin alternating sand and mica layers were used in contractional coulomb wedge experiments to simulate deformation of anisotropic brittle upper crustal strata in doubly vergent orogens experiments incorporating syntectonic sedimentation in foreland basins and in piggyback basins were also carried out our laboratory models evolved in two main stages 1 initial high velocity thrusting in the retrowedge and high frequency together with low displacement folding and thrusting in the prowedge and 2 low frequency high displacement synchronous thrusting in the prowedge and low velocity thrusting in the retrowedge transition from stage i to stage ii occurred when the growing wedges reached the critical height at which they behaved as a backstop for further prowedge accretion addition of syntectonic sediments increased the persistence of stage i and triggered out of sequence thrusting in the axial zone of the experimental orogens thrust motion was stick slip retrovergent thrusting occurred along a long lived ramp whose lower tip was located at the subduction slot provergent kink bands nucleated at the subduction slot in stage i in contrast during the second stage of wedge evolution kink bands nucleated in a piggyback fashion in the foreland far from the subduction slot and then evolved into high displacement faults that remained active up until the end of the experiments at progressively decreasing rates of thrusting the axial zones of the model wedges were characterized by fast uplift rates during stage i due to backward translation of the belt along the retrovergent long lived ramp and due to the localization of deformation close to the subduction slot outward migration of the deformation front in the prowedge region during stage ii caused the progressive decrease in the rate of the wedge uplift until it eventually stopped analytical models to quantify the coulomb behavior in the wedges validated the reversal of thrust polarity with increasing shortening triggered by the buildup of the topographic load during deformation this thrust polarity reversal highlights problems with the classic concept of back thrusting as a reflection of a significant change in the deformation regime our results compare well with the kinematic evolution of natural accretionary prisms and of thrust and fold belts such as the lesser antilles arc system the mediterranean ridge and the pyrenees