effects of topography on the curvature of fold and thrust belts during shortening of a 2 layer model of continental lithosphere abstract we have used analogue experiments to investigate the effects of surface topography on the curvature of fold and thrust belts under conditions of 1 initial relief but no erosion and 2 no initial relief but differential erosion sedimentation and transport in experiments where a 2 layer model lithosphere shortened and thickened in front of an advancing straight piston the geometry of the developing thrust wedge was very sensitive to variations in surface topography in models with an initially flat horizontal surface and in the absence of erosion and sedimentation thrusts were straight propagated forwards and nucleated at buckle folds far in front of an advancing piston around an initial topographic high plateau or cone thrusts tended to be arcuate forming salients towards the foreland initial plateaux and cones tended to behave rigidly while arcuate thrust slices formed around them to accommodate differential slip transfer zones developed on both sides of initial highs fault blocks rotated about vertical axes and thrusts moved in oblique slip within transfer zones in models with initially horizontal surfaces which were subject to differential erosion sedimentation and transport thrusts initially were straight but then progressively rotated around non eroded thickened and stronger areas these worked as indenters in front of which new thrusts nucleated at curved buckle folds these thrusts were also curved their apices being in front of the thickened non eroded areas in nature arcuate structural patterns are to be found around the altiplano of the central andes and around the tromen volcanic ridge in the neuquen basin of northern patagonia we infer that these areas behaved in quasi rigid fashion protected as they were by their high elevations and that differential erosion at the scale of the entire andes may have contributed to oroclinal bending

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

evaluating sand and clay models do rheological differences matter abstract dry sand and wet clay are the most frequently used materials for physical modeling of brittle deformation we present a series of experiments that shows when the two materials can be used interchangeably document the differences in deformation patterns and discuss how best to evaluate and apply results of physical models extension and shortening produce similar large scale deformation patterns in dry sand and wet clay models indicating that the two materials can be used interchangeably for analysis of gross deformation geometries there are subtle deformation features that are significantly different 1 fault propagation and fault linkage 2 fault width spacing and displacement 3 extent of deformation zone and 4 amount of folding vs faulting these differences are primarily due to the lower cohesion of sand and its larger grain size if these features are of interest the best practice would be to repeat the experiments with more than one material to ensure that rheological differences are not biasing results dry sand and wet clay produce very different results in inversion models almost all faults are reactivated in wet clay and few if any are significantly reactivated in sand models fault reactivation is attributed to high fluid pressure along the fault zone in the wet clay a situation that may be analogous to many rocks sand inversion models may be best applied to areas where most faults experience little to no reactivation while clay models best fit areas where most pre existing normal faults are reactivated

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

charleston transverse zone wasatch mountains utah structure of the provo salient s northern margin sevier fold thrust belt the charleston transverse zone of north central utah is an east west trending corridor of faults and folds that forms the boundary between the provo salient of the sevier fold thrust belt and the uinta cottonwood arch the zone trends nearly 90 to the regional structural grain of the fold thrust belt structural analysis and mapping in american fork canyon near the western margin of the charleston transverse zone demonstrates that the zone contains an array of low angle thrust faults and high angle reverse and normal faults crosscutting relations indicate that the low angle thrust faults formed first followed by the reverse faults and possibly selected normal faults and then by the majority of normal faults locally normal faults reactivate reverse faults the overall pattern of faulting and folding in the charleston transverse zone suggests that the zone is a left lateral strike slip flower structure superimposed on an imbricate thrust fan trends of normal faults in the transverse zone suggest that many formed during a later phase of oblique extension that had a component of right lateral shear our results support a tectonic model in which the charleston transverse zone initiated during the sevier orogeny to accommodate along strike changes in the geometry of the thrust belt caused in turn by along strike changes in thickness of the stratigraphic sequence involved in thrusting thrust sheets in the provo salient to the south of the charleston transverse zone moved farther toward the foreland than did thrust sheets to the north of the zone so the charleston transverse zone evolved into a left lateral strike slip accommodation zone preliminary paleomagnetic data support this proposal they suggest that fault blocks within the charleston transverse zone locally rotated counterclockwise about a vertical axis laramide uplift of the uinta cottonwood arch tilted structures of the charleston transverse zone to the south and may have locally reactivated faults within the zone subsequent extensional tectonism reactivated the charleston transverse zone with an oblique component of right lateral shear

oroclines thick and thin an orocline is a thrust belt or orogen that is curved in map view due to it having been bent or buckled about a vertical axis of rotation two distinct types of oroclines are recognized progressive and secondary progressive oroclines are restricted to the scale of a thrust sheet to thrust belt are thin skinned and develop during thrust sheet emplacement secondary oroclines are larger occurring at the scale of an orogen and are plate scale features that affect crust and lithospheric mantle unlike progressive oroclines which develop during initial orogenesis and in response to the same orogen perpendicular stress responsible for thrust sheet emplacement secondary oroclines are extra orogenic developing after initial orogenesis and in response to an orogen parallel principal compressive stress that is oriented at a high angle to the stress responsible for orogen development we present case studies of the wyoming salient a progressive orocline that characterizes the sevier thrust belt of the western united states and the coupled cantabrian and central iberian oroclines which are linked secondary oroclines affecting the variscan orogen of iberia the vertical axis rotations involved in progressive and secondary orocline formation are most readily quantified through paleomagnetic analysis detailed three dimensional palinspastic restoration that incorporates translation rotation and strain can distinguish the role if any of primary curvature in progressive oroclines the use of tectonic vectors such as paleocurrent directions provides a means of recognizing and characterizing the initial geometry of secondary oroclines because secondary oroclines involve the entire lithosphere detailed studies of coeval metamorphism and magmatism provide a means of constraining the fate of the mantle lithosphere during oroclinal buckling

evolution of experimental thrust wedges accreted from along strike tapered silicone floored multilayers the possible effect of pre deformational along strike sedimentary tapers on the structural architecture of thrust wedges accreted above viscous materials has been commonly underestimated or neglected we illustrate results of an experimental programme specifically designed to study the impact of this parameter on thrust wedge evolution using sandbox analogue modelling two series of experiments were performed the first series was designed to investigate the sensitivity of wedge growth to the along strike sedimentary taper variability the second was designed to investigate the sensitivity of a growing along strike tapered wedge to the convergence rate our experimental results indicate that even gentle pre deformational along strike thickness variations in foredeep clastic sequences overlying viscous materials such as evaporites or overpressured shales can produce dramatic changes in the first order structural architecture and kinematic evolution of accreting thrust wedges

mechanics of fold and thrust belts and accretionary wedges cohesive coulomb theory a critically tapered fold and thrust belt or submarine accretionary wedge is one that is on the verge of coulomb failure everywhere including its base where frictional sliding along a decollement is assumed to be occurring cohesion within a wedge can add significantly to the overall strength near the toe the effect of this is to decrease the near toe taper leading to a critical topographic profile that is concave upward if the decollement is planar we obtain an approximate self consistent solution for the state of stress within a thin skinned cohesive critical coulomb wedge and determine the relationship between the wedge taper and its strength and basal friction the theory is then applied to the presently deforming fold and thrust belt of western taiwan fitting of theoretical critical wedge shapes to topographic profiles and measurements of the step up angles of thrust faults from the basal decollement are used to constrain the taiwan wedge strength parameters an attractive assertion fully consistent with all the observations is that the mechanics of fold and thrust belts and accretionary wedges is governed by normal frictional and fracture strengths of rocks measured in the laboratory in particular if byerlee s law b 0 85 is adopted as the coefficient of sliding friction on the base we find a coefficient of internal friction 0 9 1 0 in the wedge and a wedge cohesion so 5 20 mpa other solutions having strengths and ambient stresses up to 4 times lower than this can also however satisfy the data

fracture network fluid pathways and paleostress at the tolhuaca geothermal field abstract in this study we examine the fracture network of the tolhuaca geothermal system located in the southern andean volcanic zone that may have acted as a pathway for migration and ascent of deep seated fluids under the far local stress field conditions of the area we collected the orientation slip data and mineralogical content of faults and veins recovered on a ca 1000 m deep borehole tol 1 located in the nw flank of the tolhuaca volcano tol 1 is a non oriented vertical borehole that recovered relatively young 1 axis 083 74 and a subhorizontal ns trending 3 axis 184 03 within the topmost 400 m of the borehole faults and veins are randomly oriented whereas below 400 m depth faults and veins show preferential ne to ew strikes and steep 50 dips the ew striking veins are compatible with the calculated local stress field whereas ne striking veins are compatible with the regional stress field the morphological elongation of volcanic centers alignments of flank vents and dikes orientation our results demonstrate that the paleomagnetic methodology proved to be reliable and it is useful to re orient vertical boreholes such as tol 1 furthermore our data show that the bulk transpressional regional stress field has local variations to a tensional stress field within the ne striking fault zone belonging to the liquine ofqui fault system favoring the activation of both nw and ne striking pre existent discontinuities especially the latter which are favorably oriented to open under the prevailing stress field the vertical 1 and ns trending subhorizontal 3 calculated in the tgs promote the activation of ew striking extensional veins and both ne and nw striking hybrid faults constituting a complex fluid pathway geometry of at least one kilometer depth

late cenozoic tectonic evolution of the southern chinese tian shan structural sedimentological magnetostratigraphic and 40ar 39ar thermochronological investigations were conducted in the southern chinese tian shan on the basis of our own mapping and earlier investigations in the area the late cenozoic southern tian shan thrust belt may be divided into four segments based on their style of deformation from west to east they are 1 kashi aksu imbricate thrust system 2 the baicheng kuche fold and thrust system 3 the korla right slip transfer system and 4 the lop nor thrust system the westernmost kashi aksu system is characterized by the occurrence of evenly spaced 12 15 km imbricate thrusts the baicheng kuche and korla systems are expressed by a major north dipping thrust the kuche thrust that changes its strike eastward to become a nw striking oblique thrust ramp the korla transfer zone the lop nor system in the eastern most part of the southern chinese tian shan consists of widely spaced thrusts all involved with basement rocks geologic mapping and cross section construction suggest that at least 20 40 km of crustal shortening with a horizontal shortening strain of 20 30 has occurred in the southern chinese tian shan during the late cenozoic these estimates are minimum because of both conservative extrapolation of the thrust geometries and partial coverage of the thrust belt by the cross sections the timing of initial thrusting is best constrained in the kuche basin where crustal shortening may have occurred at 21 24 ma the time of a major facies transition between lacustrine and braided fluvial sequences constrained in general by biostratigraphy and in detail by magnetostratigraphy this estimate represents only a minimum age as development of thrusts in the southern chinese tian shan may have propagated southward toward the foreland thus the sedimentary record only represents the southernmost and therefore youngest phase of thrusting if our estimate of timing for the thrust initiation 21 24 ma is correct using the estimated magnitude of shortening 20 40 km and shortening strain 20 30 the averaged rates of late cenozoic horizontal slip and shortening strain are 1 1 9 mm yr 1 and 2 9 4 5 10 16 s 1 respectively our reconnaissance 40ar 39ar thermochronological analysis in conjunction with earlier published results of apatite fission track analysis by other workers in the chinese tian shan suggests that the magnitude of cenozoic denudation is no more than 10 km most likely less than 5 km we demonstrate via a simple airy isostasy model that when the thermal effect on changes in surface elevation is negligible determination of the spatial distribution and temporal variation of both horizontal shortening strain and denudation becomes a key to reconstructing the elevation history of the tian shan using this simple model the loosely constrained magnitude of crustal shortening strain and denudation in the southern chinese tian shan implies that it may have been elevated 1 0 2 0 km since the onset of cenozoic thrusting

transfer zones of deformation in thrust wedges an experimental study abstract although numerous transfer zones of deformation have been observed in thrust wedges their mechanisms and conditions of development are poorly understood in this paper three basic questions are addressed 1 1 what are the parameters controlling the genesis of transfer zones 2 2 what are the kinematics of thrust propagation during transfer zone development 3 3 which kinds of geological structure link the two compartments on either side of the transfer zone we attempted to solve these problems using a 3 d experimental approach sand models were used to simulate the foreland a rigid but mobile backstop was used to produced a thrust coulomb wedge which was studied during the various stages of its formation shortening led to the development of transfer zones in the wedge separating two domains of homogenous behaviour and connecting thrust planes in different domains the experimental results show that the development of transfer zones is controlled by mechanical and or geometrical parameters such as 1 1 variations in the sedimentary thickness in the foreland 2 2 the steepness of the backstop 3 3 domains with different basal friction in the foreland 4 4 kinked backstop geometry during deformation of the models two types of thrust sequence were identified alternative thrust propagations on both sides of the transfer zone and synchronous thrust propagations which are anastomosed within the transfer zone 3 d reconstructions revealed that the thrust deformations observed in the transfer zone are oblique ramps the frontal thrust bending of the barbados accretionary complex as well as the imbrication of thrust in the castellane arc and the oblique ramp in the zanskar range are discussed in relation to the mechanisms and structures revealed by the models the results of analogue modelling suggest that the development of transfer zones is directly linked to variations in the boundary conditions mechanical and or geometrical between two adjacent parts of a thrust wedge

the impact of analogue material properties on the geometry kinematics and dynamics of convergent sand wedges abstract simulation of geodynamic processes in sandbox experiments requires analogue materials with a deformation behaviour that reproduces the deformation mechanisms of typical crustal rocks we present data on the frictional strength of different sand types employing static and dynamic shear tests the sand types analysed are characterised by an elastic frictional plastic mechanical behaviour with a transient strain hardening and strain softening phase prior to transition to stable sliding this is in conflict with the standard assumption of an ideal cohesionless coulomb material with constant frictional properties the influence of the identified transient material properties on the kinematics growth mechanisms and internal deformation patterns of convergent sand wedges results in characteristic wedge segments which vary depending on material compaction between wedges with well defined segments i e frontal deformation zone frontal imbrication zone and internal accumulation zone with straight slopes and wedges with a continuous convex topographic profile for most materials only the frontal part of the wedge is critical during experimental runs taper and strength of the wedge segments can be shown to be controlled by the frictional properties of active faults wedge segmentation is controlled by a bulk strength increase toward the rear of the wedge due to fault rotation in mechanically less favourable orientations and plastic material hardening the limit between the frontal critical parts of a wedge and internal stable parts is largely controlled by a critical state of stress upon which either renewed failure or fault inactivation occurs on this basis we suggest that critical taper analysis of wedges must be restricted to specific kinematic segments comparison of the experimental results with the nankai accretionary wedge suggests that our interpretation also applies to natural convergent wedges moreover we provide constraints for the selection of adequate granular analogue materials to simulate typical crustal rocks in natural convergent wedges

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

geometrical modeling of fault related folds a pseudo three dimensional approach abstract area balanced cross sections of ramp anticlines and tip line folds in thin skinned fold thrust belts can be quantitatively constructed using plane strain fault bend and fault propagation fold models cross sections alone however are somewhat inadequate for interpreting along strike changes in thrust related fold geometry complex interplay between along strike changes in fault slip and fault shape determines a range of geometric configurations which are difficult to visualize without the aid of quantitative three dimensional models we extend conventional two dimensional folding theories into the third dimension by allowing for continuous variations in fault slip and fault shape along strike geometric equations relating map view angles between fold axial surface traces are derived for a case assuming a uniform fault geometry and an along strike displacement gradient the resulting folds possess no unique fold axis but rather are characterized by multiple fold hingelines each corresponding to bends in the fault for typical displacement gradients observed in the field analysis of the models suggests that discernible differences exist between map patterns of fault bend folds and fault propagation folds whereas little difference exists between map patterns of similar and parallel fault bend folds map view angles between axial surface traces for parallel fault bend folds and fault propagation folds are a function of both ramp cutoff angle and fault slip whereas these same angles for similar fault bend folds are solely a manifestation of fault slip examples assuming uniform shortening and continuously varying fault geometry along strike are also presented fault geometry is varied along strike to simulate oblique and lateral ramps depending upon model type and fault shape parameter modified resulting folds exhibit either cylindrical or non cylindrical fold geometries closure is generated by a decrease in fold amplitude where there is a decrease in ramp cutoff angle along strike or where a fault laterally cuts up section in either instance closure develops without varying shortening along strike model map patterns resemble actual thrust related fold geometries and can be compared to maps of real structures to help predict subsurface geology

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

origin of the uinta recess sevier fold thrust belt utah influence of basin architecture on fold thrust belt geometry abstract structural trends in the sevier fold thrust belt define a pronounced concave to the foreland map view curve the uinta recess in north central utah this recess separates two convex to the foreland curves the provo salient on the south and the wyoming salient on the north the two limbs of the recess comprise transverse zones fault zones at a high angle to the regional trend of the orogen that border the flanks of the east west trending uinta cottonwood arch our structural analysis indicates that the transverse zones formed during the sevier orogeny and that they differ markedly from each other in structural style the charleston transverse zone ctz on the south side of the arch initiated as a complex sinistral strike slip fault system that defines the abrupt northern boundary of the provo salient the mount raymond transverse zone mrtz on the north side of the arch represents the region in which the southeast verging southern limb of the gently curving wyoming salient was tilted northwards during the laramide phase of uplift of the uinta cottonwood arch in effect the mrtz represents an oblique cross section through a thrust belt the contrasting architecture of these transverse zones demonstrates how pre deformation basin geometry influences the geometry of a fold thrust belt analysis of isopach maps indicates that at the time the sevier fold thrust belt formed the area just north of the present site of the uinta cottonwood arch was a basement high with a gently dipping north flank and a steeply dipping south flank thus predeformational sediment thickened abruptly to the south of the high and thickened gradually to the north of the high as illustrated by sandbox models the distance that a fold thrust belt propagates into the foreland depends on the thickness of the sedimentary layer being deformed so the shape of the salient mimics the longitudinal cross sectional shape of the sedimentary basin where basins taper gradually along strike the thrust belt curves gently but where basins taper abruptly along strike the thrust belt curves so tightly that it disarticulates and becomes bounded laterally by a strike slip accommodation zone the geometry of the uinta recess provides a field example of this concept differential movement of sevier thrusts led to formation of gradually curving thrusts on the north side of the high because of the gradual slope of the high s north flank but led to the along strike disarticulation of thrusts on the south side of the high because of the steep slope of the high s south flank in effect therefore thrust belt map view geometry provides insight into predeformational basin geometry

the origin of bore core remanences mechanical shock imposed irreversible magnetizations summary repeated laboratory induced weak mechanical shocking c 0 57 kg m s 1 of marine sandstone samples showing drilling induced remanence from commercial bore cores from the north sea and prudhoe bay causes increases in their low field susceptibility x and their ability to acquire an isothermal remanent magnetization irm these enhancements are reduced by some 20 per cent by af demagnetization in 100 mt doubling the intensity of the shock doubles the susceptibilities and irms acquired the susceptibility increase ceases after 300 to 400 shocks for the north sea samples and 20 to 30 shocks for those from prudhoe bay while the irm saturates after 800 1000 and 30 50 shocks respectively continental haematite bearing sandstones from commercial bore cores with no drilling induced remanence subjected to the same shocks do not show these evects diverences in the magnetic mineralogy of shocked and unshocked marine samples suggest that the magnetic enhancement is predominantly due to the creation of pyrrhotite by shock induced irreversible crystallographic changes in iron bearing sulphides when shocked during commercial drilling these new ferromagnetic minerals acquire strong chemical crystalline remanences associated with a wide spectrum of grain sizes in the magnetic field of the drill string and these are resistant to both thermal and af demagnetization similar processes are likely in any situation involving the shock of physically metastable iron bearing minerals particularly anoxic sediments a 5 cm non magnetic collar between the drill stem and crown should drastically reduce the magnetic intensity of this evect under commercial conditions but would not prevent its occurrence

load and unload as interference factors on cyclical behavior and kinematics of coulomb wedges insights from sandbox experiments modes of accretion in orogenic wedges are strongly controlled by mass balance and the efficiency of basal detachment sandbox models of accretionary wedges have demonstrated that fault systems grow episodically via cycles of alternating wedge thickening and lengthening generally a new thrust plane generates within the footwall of the previous one following a piggy back mode of accretion whereas redistribution of the mass balance in the wedge is associated to underthrusting and the reactivation of previous thrusts we present the results of five sandbox experiments that model the interaction between tectonic accretion sedimentation in foredeep area and erosion in the axial zone in order to analyze the influence of these events on the cyclical behavior of a growing wedge modifications of the initial setup were made to analyze the effect on wedge development of syntectonic denudation syntectonic sedimentation and coeval sedimentation and erosion which markedly altered mass transfer within the wedge in particular lowering the surface slope by syntectonic erosion triggered synchronous accretion and underthrusting modes by contrast a sudden syntectonic sediment load in the prowedge region promoted prolonged phases of underthrusting retarding the accretion of new imbricates at the prowedge toe whether wedge volume was increased or not the high length thickness ratio of the model 120 cm 2 5 cm allowed us to monitor a complete cycle of recovery of the system after each episode of sedimentation and or erosion observing how the wedge reacted to modification both altering its own cyclicity and progressively recovering it

critical taper model of fold and thrust belts and accretionary wedges the fold and thrust belts and submarine accretionary wedges that lie along compressive plate boundaries are one of the best understood deformational features of the earth s upper crust although there is considerable natural variation among the many fold and thrust belts and accretionary wedges that have been recognized and explored several features appear to be universal in cross section fold and thrust belts and accretionary wedges occupy a wedge shaped deformed region overlying a basal detachment or decollement fault the rocks or sediments beneath this fault show very little deformation the decollement fault characteristically dips toward the interior of the mountain belt or in the case of a submarine wedge toward the island arc the topography in contrast slopes toward the toe or deformation front of the wedge deformation within the wedge is generally dominated by imbricate thrust faults verging toward the toe and related fault bend folding two north american fold and thrust belts that exhibit these features are shown in figure 1 neither of these two examples is tectonically active today the southern canadian fold and thrust belt was active during the late jurassic and cretaceous 150 100 ma whereas the southern appa lachians were deformed during the late carboniferous to permian alle ghenian orogeny 300 250 ma figure 2 shows two examples that are currently active the taiwan fold and thrust belt produced by the sub duction of the eurasian plate beneath the philippine sea plate suppe 1981 1987 and the barbados accretionary wedge produced by the sub

effect of lateral thickness variation of an intermediate decollement on the propagation of deformation front in the lurestan and izeh zones of the zagros fold thrust belt insights from analogue modeling although the role of various basal decollement levels on structural style and deformation propagation is well documented in many folded belts the effect of lateral variation in intermediate decollements is poorly constrained this work uses results of three scaled sand box models shortened from one end to study the variation in structural development between areas with a ductile intermediate decollement and areas without or with a thinner intermediate decollement combined results of scaled models with field observations are used to argue that the presence of mechanically different intermediate decollement horizons within the zagros stratigraphy has resulted in deformation partitioning between the lurestan and izeh zones a thick intermediate decollement facilitates a faster propagation of deformation front and a lower taper in comparison with a thinner or non existing intermediate decollement during compression however the effect of lateral thickness variation in the intermediate decollement on propagation of deformation is less profound than the effect of mechanical differences in basal decollements

structural setting and synplutonic fault kinematics of a cordilleran cu au mo porphyry mineralization system bingham mining district utah cu au mo porphyry type and cu skarn type mineralization in the bingham mining district of utah are temporally and spatially related to a suite of monzonite and quartz monzonite porphyry intrusions comprising the bingham complex structural control on mineralization at the bingham canyon mine has been described at the scale of individual large scale folds and fracture sets but no integrated pattern for the synmineralization deformation structures has emerged from earlier research in the work reported here we have investigated hitherto largely unrecognized components of deformation translations rotations and strains associated with emplacement of many cubic kilometers of granitic rocks in the intrusions of the bingham district we have used both the local three dimensional static mine model and the regional context in digital kinematic two and three dimensional forward and reverse modeling to identify a geometrically valid and admissible fault framework for the bingham district and a kinematic solution for this framework through time the results indicate that porphyry intrusions and associated mineralization were emplaced during reactivation of a basement dictated linked system comprising two sets of northwest and north northeast trending strike slip faults both sets operated as transfer faults during extensional collapse of the sevier orogen in the eocene each fault set is characterized by overstep geometries with relay ramps breached by faults re activated in extension strike slip was accompanied by progressive dilation of the extensional faults in fault oversteps to permit emplacement of composite stocklike low aspect ratio intrusions the kinematic fault framework identified provides a new structural context for porphyry and skarn type mineralization at the bingham canyon mine with potential for linking mineralizing fluid flow to three dimensional structure and the development of that structure through time

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

paleocurrent analysis of pleistocene turbidite sediments in the forearc basin inferred from anisotropy of magnetic susceptibility and paleomagnetic data at the gas hydrate production test site in the eastern nankai trough abstract to understand the paleocurrent directions and depositional processes in the forearc basin along the active convergent margin of the eastern nankai trough analysis of the anisotropy of magnetic susceptibility ams has been applied to pleistocene turbidite sediments in a borehole core obtained in the vicinity of the gas hydrate production test site sixty one core specimens mainly sampled from silty and sandy sediments were measured and the ams results show generally oblate magnetic fabrics characterized by girdle distributed weakly clustered maximum and intermediate susceptibility axes diagnostic magnetic fabric parameters show that the sediments generally preserve primary sedimentary structures without significant sediment disturbance due to events such as coring and bioturbation the characteristic remanent magnetization was also measured to determine the reorientation of each core paleocurrent directions estimated from the offset of the minimum susceptibility axes in the stratigraphic coordinates show two predominant directions one toward the southwest sw and the other toward the northwest nw on the basis of the interpretations of 3d seismic data the principal sw direction is generally parallel to the trough shaped basin axis this suggests that the sediments were carried by axial turbidity currents within the channel the subordinate nw direction corresponds to the downslope direction of the daini atsumi knoll this may be due to a sediment supply from the southeast associated with a submarine slope failure and landslide evolution or it may be due to flow reflections from the slope toward the basin center the results of the paleocurrent analysis suggest that the pleistocene depositional system around the daini atsumi knoll is affected by the basin configuration these results are in good agreement with previous studies of the coeval depositional system for the adjacent forearc minibasin around the eastern nankai trough

experimental modelling of orogenic wedges a review abstract experimental modelling applied to the study of orogenic wedge dynamics has been a subject of fruitful research for more than 30 years although the technique dates back as far as the early xix th century on one hand several first order parameters controlling the structural evolution of mountain belts have been intensively investigated using the classic tectonic sandbox models the main parameters are the properties of the basal decollement the deforming material the backstop and fluxes kinematics and surface processes on the other hand the morphological evolution of a mountain relief subjected to changing tectonic or climatic forcing has been addressed using another kind of approach called geomorphic models nowadays the literature is extremely rich particularly for the sandbox technique so that it becomes difficult to have an exhaustive view of the effects of the above parameters on mountain evolution in this article we propose a detailed review of the main results obtained using both tectonic and geomorphic approaches our goal is to provide an almost complete state of the art in the experimental study of relief dynamics to guide present and future researchers in their understanding of mountain belt evolution

the effect of basement step topography on the geometry of the zagros fold and thrust belt sw iran an analog modeling approach systematic analog models are run to study the variation in deformation across basement steps in the zagros fold thrust belt our model results demonstrate that basement configuration topography influences the sedimentation thickness and hence the kinematics and geometric evolution of the fold and thrust belt the greater the difference in thickness between the adjacent cover units across a basement step the sharper and clearer will be the offset of the deformation front based on model results we conclude that in a fold thrust belt where basement step topography is covered by a layer of ductile salt acting as a decollement the effect of the salt decollement on the evolution of the belt is far greater than the effect of thickness variation of the cover units

a simple method for orienting drill core by correlating features in whole core scans and oriented borehole wall imagery abstract assessing the regional significance of fractures in drill cores requires the collection of oriented core intervals direct orientation of cores during drilling is possible but is commonly precluded because of expense and time requirements a simple and accurate method of core reorientation is presented where high resolution imagery of drill cores whole core scans are directly compared with oriented borehole imagery core intervals are reoriented by aligning features i e fractures bedding and clasts in whole core scans with correlative oriented features in borehole wall imagery unlike other core orientation techniques the direct side by side comparison of core scans and borehole wall imagery can identify core segments that were mismatched due to undetected rotation between two portions of core the combined analysis of core based fracture data whole core scans and borehole imagery in this method optimizes data integration to improve structural interpretations

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

palaeomagnetism of granitic intrusives from the precambrian basement under eastern kansas orienting drill cores using secondary magnetization components summary the precambrian basement under east central kansas was drilled at two circular aeromagnetic positives one at osawattamie and one at big springs the core retrieved from these sites is a coarse to medium grained granite which has been dated by u pb to be 1350 ma old the palaeomagnetism of these azimuthally unoriented cores was studied to see if a technique which uses low coercivity low temperature magnetization components to orient the cores would allow an independent confirmation of the core s mid proterozoic age orthogonal projection plots of the alternating field af and thermal demagnetization data show that the magnetization of these cores is relatively simple having only two components a low temperature low coercivity magnetization with steep positive inclinations and a shallow negative inclination characteristic magnetization for the osawattamie core or a positive moderate inclination characteristic magnetization for the big springs core if the declination of the low temperature low coercivity component is aligned parallel to the present field declination the characteristic directions may be azimuthally oriented this allows the calculation of palaeomagnetic poles for the big springs core lat 4 5 s long 29 9 e and the osawattamie core lat 20 2 n long 39 3 e which are consistent with irving s apparent polar wander path for laurentia at about 1300 1400 ma comparison of anhysteretic remanent magnetization arm viscous remanent magnetization vrm and isothermal remanent magnetization af demagnetization curves with a natural remanent magnetization nrm demagnetization curve suggests that the osawattamie core probably acquired a piezoremanent magnetization prm parallel to the core axis during drilling

differential transport and continuity of thrust sheets abstract structural geologists long have strived to establish basic tenets describing the interaction between thrust displacement and lateral extent of thrust sheets individual non metamorphic thrust sheets can be partitioned into segments that exhibit linear variations in displacement along strike the differential transport between endpoints of each segment define an along strike bulk shear angle which describes the spatial rate of change in displacement along the length of the fault empirical measurements of differential transport angles for continuous thrust sheets from different north american fold thrust belts yield bulk shear angles that do not exceed 35 40 without formation of tear faults these values might thus be considered a limit for coherent behavior of continuous non metamorphic thrust sheets thereby providing constraints on three dimensional interpretations of fold thrust belts

comment on diffraction of solitary kelvin waves at cape mendocino discussion d un article de dorman evidence of kelvin waves in california s marine layer and related eddy generation paru ibid 1985 113 p 827 839

structural evolution of a fold and thrust belt generated by multiple decollements analogue models and natural examples from the northern apennines italy this study analyses the role played by the mechanical proprieties of the rocks involved in a thrust system focusing on the problem of multiple decollements and of related compressional structures integration of geophysical seismic reflection profiles and geological data and scaled sandbox models are used to study the deformation style of two areas of the northern apennines italy the po plain and the umbria marche apennines both areas are characterised by a complex stratigraphy consisting of decollements located at different depths that influence the geometry and kinematics of the thrust system the main characteristic of the models presented here is the presence of two decollement horizons situated at the base and in an intermediate level of the models during deformation of the models these two decollement horizons generate two sets of structures with different geometrical characteristics and significance through the joint analysis of geophysical and geological data and model results wavelengths of the compressional structures analysed show that the structural style of the two analysed areas is almost similar moreover model results prove that the final configuration of the thrust system follows the general rules of evolution of a wedge deformed above a weak decollement and is largely governed by the larger deep seated structures

erosional control of the kinematics and geometry of fold and thrust belts imaged in a physical and numerical sandbox 1 we investigate the effect of systematically varying erosion intensity k on the geometry and kinematics of fold and thrust belts modeled using equivalent conditions in physical and numerical experiments similar material properties and boundary conditions were used to compare numerical experiments to those modeled in the sandbox using an erosion rule that removes mass from the scaled sand wedges according to rates expected from bedrock fluvial incision the geometry of both modeling approaches is quantitatively compared with that expected from analytical theory the fold and thrust belt growth rate is inversely proportional to k and is well predicted by theory except for the high erosion case in which both the numerical and the experimental sandbox grows supercritically a direct relationship exists between the erosion intensity and the number of fore shear bands and shear strain magnitude the number of fore shear bands decreases and their strain increases with erosion intensity the results indicate that realistic and mechanistic erosion rules can be applied to physical experiments to model mass removal and this approach opens up the possibility of calibrating strain history to erosion intensities for predictions in natural settings quantitative comparisons between the physical and numerical sandbox experiments indicate that deformation style and geometry of the thrust and fold belts are similar when identical rheologies and boundary conditions are used this benchmarking suggests that numerical experiments realistically model conditions observed in the simple cohesionless rheologies commonly employed in sandbox experiments this suggests that numerical simulation may reliably model experimental deformation in geological situations that are difficult to represent in scaled experiments because of the different scales and rheologies of natural orogenic wedges

late laramide dolomite recrystallization of the husky rainbow a hydrocarbon devonian reservoir northwestern alberta canada paleomagnetic and geochemical evidence the rainbow field is in reefal carbonates of the middle devonian keg river formation in the western canada sedimentary basin alternating field and thermal step demagnetization was done on specimens from unoriented core from a vertical an inclined and a horizontal well core in the dolomite reservoir although they had no viscous remanent magnetization component to use for orientation most specimens had a well defined characteristic remanent magnetization chrm that resides in single to pseudosingle domain magnetite by rotating the mean chrm direction around its core axis a small circle can be generated for each core and the small circles intersect in the true chrm direction of d 168 i 73 5o 95 5 8 k 32 2 its paleopole of 164 e 83 n a95 10 defines a tertiary age with one sigma limits of middle eocene to middle miocene age petrographic examination defines four generations of dolomite matrix dolomite has 60 100 m diameter crystals that were later recrystallized to 200 400 m

late cenozoic tectonics of the kepingtage thrust zone interactions of the tien shan and tarim basin northwest china the kepingtage kalpin thrust zone northwest china is an actively deforming part of the india asia collision system it lies south of the tien shan has an area of 16 000 km2 and consists of arcuate emergent imbricates overall vergence is toward the interior of the tarim basin to the south thrust sheets typically expose upper cambrian to permian platformal strata thrusting is largely thin skinned thin upper cambrian evaporites are likely to be the main decollement horizon the kepingtage thrust zone is the only margin of the tarim basin to deform in this style in the cenozoic it is replaced to the east and west by thrust zones which have propagated shorter distances into the interior of the tarim basin thick up to 10 km mesozoic and cenozoic clastic successions are present and deformed in these regions such successions are not present in the kepingtage thrust zone or the adjacent bachu uplift within the tarim basin because of episodic activity on steep northwest southeast trending thrusts which define the margins of the bachu uplift these mesozoic cenozoic strata may have suppressed the ability of regions along strike from the kepingtage thrust zone to deform by thin skinned thrusting utilizing the upper cambrian decollement the along strike variation in active thrusting at the southern margin of the tien shan is an example of syntectonic sedimentation controlling thrust belt deformation style a balanced section across the thrust zone indicates 28 shortening equivalent to 35 km this is equivalent to an average slip rate of 1 8 mm yr 1 and a strain rate of 4 4 10 16 s 1 assuming deformation began at circa 20 ma active deformation is focused along the frontal thrust the kepingtage fault the northern boundary of the kepingtage thrust zone is formed by the south tien shan fault this major north dipping thrust is seismically active with published earthquake focal depths at midcrustal levels 14 and 18 km it juxtaposes upper carboniferous sedimentary rocks of different facies and different paleozoic deformation histories speculatively it represents a reactivation of a late paleozoic thrust which originated at the boundary between the platformal interior of the tarim block and a deeper water foreland basin to its north

mechanical behaviour of granular materials used in analogue modelling insights from grain characterisation ring shear tests and analogue experiments abstract the mechanical behaviour of several dry granular materials is investigated through ring shear tests grain characterisation and simple analogue experiments analysed by x ray computed tomography an improved knowledge of granular materials is essential to determine their suitability as analogues for upper crustal rocks in experimental models and to compare analogue and numerical experiments the ring shear tests show that the granular materials have an elastic frictional plastic behaviour with strain hardening preceding failure at peak strength followed by strain softening until a dynamic stable value is reached this is similar to the behaviour exhibited by experimentally deformed rocks the physical characteristics of the grains determine the amount of diffuse deformation before failure the percentage of strain softening and act on the thickness of the shear zones before broadening initial shear zone width in extensional and contractional experiments is between 11 and 16 times the mean grain size the angle of internal friction defining one of the mechanical properties of granular materials and thus fault dip is not only related to physical characteristics of the grains and to the handling technique used e g sieving or pouring but also to the overburden and to the experimental setup used

evaluating the role of syn thrusting sedimentation and interaction with frictional detachment in the structural evolution of the sw tarim basin nw china insights from analogue modeling abstract the structural evolution of the fold and thrust belt in sw tarim basin nw china is poorly understood as the region is covered by thick pliocene quaternary conglomerate which significantly decreases the quality of seismic images we here improve the understanding of the thrust belt evolution with an investigation using analogue modeling on the deformation mechanism based on a case of real geological section in which we focus on the impact of syn thrusting sedimentation on the structural style and evolution the results show that syn thrusting sedimentation significantly increases the distance and rate of foreland propagation resulting in a much broader fold and thrust belt the modeling results also suggest that the fold and thrust belt exhibits different structural patterns under different syn thrusting sedimentary conditions in the case of thick syn thrusting sediments a frictional detachment layer of mudstone acts as decollement and the fold and thrust belt develops a duplex pattern in the hinterland in the case of thin syn thrusting deposits the belt develops a pop up structure with backthrusts in the foreland region which cut through the detachment layer the striking similarity between the model with thick syn thrusting sediments and the real geologic section implies that the overlying thick pliocene quaternary sediments play a major role in shaping the structure pattern in the fold and thrust belt of sw tarim basin

influence of synkinematic sedimentation in a thrust system with two decollement levels analogue modelling abstract compressive systems in foreland domains are characterised by fold and thrust belts linked to the presence of one or several ductile layers in depth acting as a decollement level the main parameters controlling the structural evolution are the presence of a decollement level the amount and rate of shortening and the amount of synkinematic sedimentation the effect of these parameters has only been studied on a thrust belt scale furthermore only the effect of synkinematic sedimentation on a simple system with one decollement level has been studied at the scale of a single structure the aim of this study was to use analogue modelling to test the effect of shortening rate velocity and the localization of sedimentation on a single system characterised by the presence of two prekinematic decollement levels the main results showed variations in the structural vergence folding geometry symmetric or asymmetric the evolution of the deformation horizontal propagation versus vertical uplift and the decoupling of the lower and upper brittle structures in relation with the main parameters shortening rate and mass transfer the results of the experiments were then compared to natural examples from the sub andean thrust belt

structural features of northern tarim basin implications for regional tectonics and petroleum traps the rhombus shaped tarim basin in northwestern china is controlled mainly by two left lateral strike slip systems the northeast trending altun fault zone along its southeastern side and the northeast trending aheqi fault zone along its northwestern side in this paper we discuss the northern tarim basin s structural features which include three main tectonic units the kalpin uplift the kuqa depression and the north tarim uplift along the northern margin of the tarim basin structural mapping in the kalpin uplift shows that a series of imbricated thrust sheets have been overprinted by strike slip faulting the amount of strike slip displacement is estimated to be 148 km by restoration of strike slip structures in the uplift the kuqa depression is a mesozoic cenozoic foredeep depression with well developed flat ramp structures and fault related folds the baicheng basin a quaternary pull apart basin developed at the center of the kuqa depression subsurface structures in the north tarim uplift can be divided into the mesozoic cenozoic and the paleozoic lithotectonic sequences in seismic profiles the paleozoic litho tectonic sequence exhibits the interference of earlier left lateral and later right lateral strike slip structures many normal faults in the mesozoic cenozoic litho tectonic sequence form the negative flower structures in the north tarim uplift these structures commonly directly overlie the positive flower structures in the paleozoic litho tectonic sequence the interference regions of the northwest trending and northeast trending folds in the paleozoic tectonic sequence have been identified to have the best trap structures our structural analysis indicates that the tarim basin is a transpressional foreland basin rejuvenated during the cenozoic

effect of overburden thickness on thrust belt geometry and development laboratory sand models simulating development of thrust belts indicate that 1 distance between the backstop and the first formed thrust is a linear function of initial sand layer thickness 2 spacing between the first two thrusts is most likely a nonlinear function of initial sand layer thickness 3 initial dip of successive ramps decreases progressively toward the foreland because of variations in principal stress directions with depth and or changes of mechanical properties of sand with increasing overburden 4 faults to the hinterland steepen and become less active as hinterland thrust sheets undergo penetrative shear during sand wedge growth and 5 new ramp initiation occurs to the hinterland of the basal detachment tip line these observations can be interpreted both in context of area balance considerations and soil mechanics theory if initial sand layer thicknesses in our models represent syntectonic overburden in natural thrust belts i e distance between the ground surface and the detachment then our results suggest that locations of some thrust belt salients are controlled by along strike variations in stratigraphic thicknesses and that initial thrust spacing decreases progressively to the foreland if the thrust belt involves a foreland tapering basin furthermore our sand model observations suggest that the concept of a fixed rigid backstop in the hinterland of thrust belts may be misleading as a consequence of thickening the hinterland portion of the thrust belt becomes stabilized and effectively becomes the backstop itself

the magnetic fabric of early tertiary sediments from the rockall plateau northeast atlantic ocean abstract magnetic fabric anisotropy of magnetic susceptibility measurements have been made on mudstones of paleocene to eocene age from the southwest margin of the rockall plateau these sediments are from deep sea drilling project dsdp sites 403 404 and 405 and were cored by d v glomar challenger during leg 48 non horizontal magnetic foliation planes in these sediments are interpreted as representing paleoslopes at sites 403 and 404 a deltaic sequence is identified as having prograded towards the southeast or east indicating that it was derived from greenland rather than rockall this can be confirmed from other geological evidence and from paleo geographic reconstructions southerly dipping depositional surfaces have been identified in a submarine fan complex at site 405 in agreement with interpretation of seismic reflection records this supports the view that this fan has been built out southwards from the east west trending transform fault scarp which controls the rockall margin in this area

thrust wedges with decollement levels and syntectonic erosion a view from analog models abstract analog sandbox models have been set up to study the impact of syntectonic erosion on thrust wedges with one and two decollement levels different accretion mechanisms are activated depending on interactions between surface processes and wedge mechanics frontal accretion backthrusting underthrusting and underplating due to decollement induced duplex formation at depth these mechanisms may function simultaneously being located at different parts across the wedge for all the experiments a high friction is imposed at the base of models and the volume of eroded material remains equal to the volume of newly accreted material maintaining a constant surface slope during the shortening erosion limits the forward propagation of thrust wedges and favors the underthrusting of basal layers allowing duplex formation erosion promotes development of major backthrusts in the thrust wedges without or with one decollement but no backthrusts was formed in the wedges with two decollements slow erosion allows lower extent of basal underthrusting in comparison with regular rate erosion variations in the erosion taper lead to changes in duplex geometry and exhumation rate in thrust wedges with one or two decollements the 6 erosion taper promotes formation of antiformal stack at the rear part of thrust wedge high rate of basal underthrusting and high extent of erosional removal the cover layers are nearly completely eroded above the antiformal stack and form the synformal klippe in frontal part of thrust wedges the 8 erosion taper allows development of individual ramp anticlines and active forward thrusting of cover layers above the decollement and low rate of basal underplating below it with consequent low extent of erosional removal the results of our experiments support the observations on structural evolution and erosion in the alberta foothills of the canadian rockies

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

temporal constraints and pulsed late cenozoic deformation during the structural disruption of the active kashi foreland northwest china 1 in response to the ongoing indo asian collision structural deformation has encroached into the tian shan foreland in western china since the early miocene in order to reconstruct a detailed history of foreland deformation along the southern margin of the tian shan we have synthesized extensive mapping analysis of seismic sections magnetostratigraphy of the foreland fill and associated growth strata apatite fission track dating changes in sediment accumulation rates and geodetic surveys of deformed fluvial terraces across an area exceeding 6500 km2 that spans the kashi foreland in the northwestern tarim basin we use these data to place temporal constraints on individual structures that delineate at least 4 distinct stages of deformation stage 1 involved initial uplift of hinterland structures beginning at 20 25 ma stage 2 began at 16 3 ma when the basement involved deformation front stepped south to the kashi basin thrust that bounds the foredeep strata stage 3 occurred from 13 5 to 4 ma as the deformation front migrated south in episodic steps above thin skinned detachment and ramp flat folding finally stage 4 began after 4 ma with growth of the keketamu anticline followed by rapid southward migration of the deformation front to the atushi and then kashi detachment anticlines on the basis of the deformed tertiary strata and available seismic data calculations of the minimum magnitude of shortening yield totals ranging from 10 to 32 km since the early miocene of which 7 12 km occurred since 4 ma although average shortening rates since 16 3 ma were 0 8 1 3 mm a rates were faster from 16 3 to 13 5 ma 1 1 3 2 mm a after which they decreased to 0 5 mm a between 13 5 and 4 ma before increasing to 2 2 2 7 mm a after 4 ma similarly the rate of southward propagation of the deformation front was initially 3 7 mm a for the first 10 ma of basin development slowed to 1 5 mm a between 13 5 and 4 ma and increased to more than 10 mm a since 4 ma these changes in shortening and structural migration rates over time suggest that the loci of orogenic deformation shifted in and out of the foreland during the neogene surveyed stream terraces suggest that although most current deformation is located along the southern front of the foreland modest deformation and shortening continue in the hinterland structural style within the foreland has also changed over time from imbricate thrust faults with hanging wall folds along the northern margin to ramp flat faults and detachment folds along the southern margin where the basement rocks are previously deformed such as in the hinterland of the kashi foreland thick skinned fault ramps and related fault propagation folds dominate the structural style as syntectonic sedimentation within the foreland produced thick subhorizontal stratigraphy the structural style evolved to detachment and fault bend folding above these footwall flats within the neogene strata

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

structural evolution of the piqiang fault zone nw tarim basin china abstract the piqiang fault is a prominent strike slip tear fault that laterally partitions the keping shan thrust belt in the nw tarim basin china in satellite images the piqiang fault appears as a sharp nw trending lineament that can be traced for more than 70 km it is oblique to the general structural trend of the thrust belt and subparallel to the thrust transport direction this paper presents a structural analysis of the piqiang fault based on satellite image interpretation and field data a net loss of late paleozoic sediment across the fault zone implies that it was initiated as a major normal fault during the early permian and corresponds to widespread extension and magmatism during this period differential erosion across the fault resulted in the subsequent removal of sediment from the east relative to the west during the middle to late cenozoic contraction of the nw tarim basin and the formation of the keping shan thrust belt resulted in reactivation of the piqiang fault as a strike slip tear fault the fault has accommodated lateral differences in thrust density and spacing which have arisen due to the abrupt pre existing change in stratigraphic thickness across it the piqiang fault provides an insight into the formation of oblique strike slip tear faults in contractional belts and demonstrates the importance of inherited basement structures in such settings

kelvin waves and internal bores in the marine boundary layer inversion and their relationship to coastally trapped wind reversals abstract detailed observations of a coastally trapped disturbance or wind reversal on 10 11 june 1994 along the california coast provide comprehensive documentation of its structure based on aircraft wind profiler radio acoustic sounding system and buoy measurements unlike the expectations from earlier studies based on limited data which concluded that the deepening of the marine boundary layer mbl was a key factor the 1994 data show that the perturbation was better characterized as an upward thickening of the inversion capping the mbl as the event propagated over a site the reversal in the alongshore wind direction occurred first within the inversion and then 3 4 h later at the surface a node in the vertical structure defined here as the altitude of zero vertical displacement is found just above the inversion base with up to 200 m upward displacements of isentropic surfaces above the node and 70 m downward displacements below although this is a single event it is shown that the vertic

the effect of overburden thickness on deformation mechanisms in the keping fold thrust belt southwestern chinese tian shan mountains insights from analogue modeling abstract the structural evolution of the keping fold thrust belt in the south chinese tian shan nw china is poorly understood here we present the results of a series of scaled sandbox models based on the structural architecture of the keping fold thrust belt in our investigation of the effect of overburden thickness on deformation mechanisms in fold thrust belts these models simulated the evolution of the keping fold thrust belt where the western segment was shortened with a thicker overburden relative to the eastern segment model results confirm that fewer thrust faults with wider spacing develop in thicker cover units whereas more thrust faults with narrower spacing develop in thinner cover units transfer zones also form as a result of faster and farther propagation in a segment with thicker overburden consistent with the prediction of the critical taper theory our modeling further shows that due to the episodic accretion of thrust wedges and non simultaneous formation of thrust faults the deformation front may locally propagate farther in the thinner overburden alternatively there may not be so much difference between two parts of sand layers with different thicknesses an observation that is inconsistent with the theoretical prediction the similarities between the model and real geological observations suggest that the variations in the overburden thicknesses across the piqiang fault transfer zone strongly influenced the development of the structural architecture of the keping fold thrust belt they further indicate that the growth of fold thrust belts in nature is episodic and that the propagation of deformation front cannot be simply predicted by the critical taper theory the overburden thickness variations across the piqiang fault transfer zone were produced by high angle reverse faulting along the pre existing piqiang selibuya fault during the late pliocene and associated crustal uplift and erosion of the hanging wall strata

measurements of friction coefficients and cohesion for faulting and fault reactivation in laboratory models using sand and sand mixtures abstract simple laboratory tests have been made on dry quartz sand and on mixtures of sand and clay and sand and cement tests were made on both low and high density specimens of each mixture as determined by physical handling techniques the tests included measurements of density estimates of the coefficient of internal friction based on fault orientation and estimates of the friction coefficients and cohesion values for fault initiation and reactivation in a shear frame apparatus which was also used to test artificial fault zones made from pure clay glass microspheres and wheat flour the results suggest that density is determined more by handling technique than by composition and that friction coefficients and perhaps cohesion both increase with increasing density the shear testing revealed a consistent drop in cohesion for fault reactivation in all densely packed mixtures tested although small this drop may be significant for some types of laboratory models

role of lateral thickness variations on the development of oblique structures at the western end of the south pyrenean central unit abstract the montsec unit is one of the most important detached south verging nappes within the south pyrenean central unit spcu southern pyrenees a n s cross section of its western sector based on seismic reflection profiles shows a hangingwall ramp geometry in mesozoic strata overlain by a syntectonic series of lower eocene sediments with growth geometry the geometry of growth strata constrains the age of its movement between the paleocene and the middle eocene the geometry of the western oblique ramp of the south pyrenean central unit is defined by a series of n s folds in some cases associated with underlying west verging thrusts as indicated by seismic reflection profiles and field data in this paper we propose that the geometry of the thrust wedge of mesozoic units progressively thinning from east to west strongly contributed to constrain the location and geometry of the western termination of the montsec thrust the hypothesis proposed is checked by a series of experimental wedges developed in a sandpack with lateral and three dimensional thickness variations oblique structures form as thrusting progresses at the tip of the sand wedge

models of strain variation in nappes and thrust sheets a review abstract strain within nappes and thrust sheets can be modelled using simple boundary conditions an original rectangular prism of material resting on a thrust plane is considered to be subject to various pure and simple shear components operating parallel to the thrust deformation gradient tensors are determined for the models and solutions found for the principal strains a two dimensional model considers heterogeneous simple shear combined with a stretch in the direction of shear simple shear zones arise as a special case where 1 recumbent fold nappes characteristic of gravitational collapse can be modelled with 1 and thrust sheets with layer parallel shortening and folding with differential transport of nappes gives rise to additional wrench type shears solutions for the principal strains in this type of model are found and used to interpret side wall ramps and steep zones parallel to nappe movements by including a stretch in the transport direction these differential transport models produce prolate strains 1 and oblate strains finally a brief consideration of bending strains produced in a sheet passing over a ramp indicates reversals in the shear direction during the strain history these predict possible crenulation of early thrust fabrics at ramps

sandbox models of relay ramp structure and evolution relay ramps are a common feature formed during the growth of normal fault systems we performed analogue experiments to investigate the structure and evolution of relay ramps an extending rubber sheet induces extension at the base of a sand pack brittle crust analogue silicone bars between the rubber and the sand control the location of fault nucleation we tested the role of fault spacing fault length overlap length and fault strike in the evolution of relay ramps the modelled relay ramps evolved in three stages characterized by the growth of the normal faults their interaction and linkage interaction and linkage occurred only when the combined length of the two interacting faults was larger than eight times their spacing the length to width ratio of the relay ramps during the interaction stage showed preferred geometries clustering around three the propagation of the fault tips was observed both before and after the linkage stage overlap length and spacing relations of the modelled relay ramps are similar to those in nature at different scales and can be explained using existing mechanical models nevertheless the further propagation of the fault tips after linkage has not been described previously

on the generation and propagation of the southern african coastal low the time height variability of the lower marine atmosphere during the eastwards propagation of coastal lows around the coast of southern africa is presented these coastal lows are shallow mesoscale disturbances that are trapped vertically by a strong subsidence inversion and horizontally through coriolis effects against the steep escarpment that fringes the subcontinent prior to the event warm offshore flow descending off this escarpment at about the inversion level provides an input of buoyancy which together with the cyclonic vorticity acquired during the descent acts to generate a coastal low rather than a high analysis of the data shows that coastal lows are characterized by a marked lowering of the inversion level a switch in the direction of the winds below the inversion and a drop in the near surface temperature which are all consistent with the hypothesis that the coastal low is an internal kelvin wave propagating in the marine layer good agreement between the observed propagation speed of the coastal low and the theoretical phase speed of such a wave reinforces this hypothesis a model of the vertical buoyancy advection into the marine atmosphere that results from the warm offshore flow is presented it is illustrated that an internal kelvin wave in the form of a coastal low results from this buoyancy advection comparisons are made between coastal low dynamics and those characteristics of other mesoscale coastally trapped disturbances which propagate in the marine layers of california and south eastern australia

depth shifting and orientation of core data using a core log integration approach a case study from odp iodp hole 1256d abstract studies of the evolution and architecture of the oceanic crust s basement are significantly hindered by the lack of sample orientation with respect to geographic coordinates generally the length of cores recovered in oceans is not equal to the drilled length recovery percentage 100 thus data such as structural measurements are either lost or not suitable to reliably interpret many aspects of the geological environment core log integration enables the correction of each core piece to the appropriate in situ depth position and reorientation of core structures with respect to magnetic north by correlating core measurements to downhole high resolution resistivity images and acoustic images this technique was tested on results from the shallowest section of odp iodp hole 1256d that drilled through a complete in situ section of upper ocean crust formed at ultra high spreading rate

magnetochronology of the upper cenozoic strata in the southwestern chinese tian shan rates of pleistocene folding and thrusting abstract the southwestern chinese tian shan of central asia is an actively deforming part of the indian asian collision system paleomagnetic investigations of two plio pleistocene terrestrial successions provide the first detailed magnetostratigraphy for the upper cenozoic strata in this region paleomagnetic samples were collected from 358 sites within the atushi formation and xiyu formation across the atushi talanghe anticline near atushi thermal demagnetization behavior reversal and fold tests of paleomagnetic stability indicate that the characteristic remanence directions were acquired before tilting and folding of the strata a composite magnetostratigraphic section for this sequence correlates with the mammoth subchron to the jaramillo subchron between 3 3 and 1 07 myr of the geomagnetic polarity timescale of cande and kent j geophys res 100 1995 6093 6095 the mean declination 349 3 in the boguzihe section indicates a counterclockwise vertical axis rotation 11 2 during the past 1 4 myr the conglomeratic xiyu formation is time transgressive along its progradational contact with the underlying atushi formation across a map distance of 6 km the basal contact ranges from less than 1 0 myr in the ganhangou section to 1 9 myr in the boguzihe section to the southwest and to 2 8 myr on the northwest limb of the anticline northeastward lateral propagation and growth of the atushi talanghe anticline initiated at 1 4 myr in the boguzihe and 1 2 myr in the ganhangou the cross section in the boguzihe provides a conservative maximum estimate of shortening rate of 3 3 4 4 mm yr and uplift rate of 3 mm yr this rate represents about half of the geodetically determined shortening rate between the northern tarim basin and the kyrgyz tian shan wang et al acta seismol sin 22 2000 263 270 reiger et al earth planet sci lett 191 2001 157 165 during fold growth over the past 1 4 myr the crest of the atushi talanghe anticline eroded at an average rate of 2 6 2 7 mm yr

tectonic evolution of a crustal scale oblique ramp hellenides thrust belt greece abstract this study investigates the tectonic evolution of the omalos transverse zone which served as a crustal scale oblique ramp in the external hellenides thrust belt on crete island the omalos oblique ramp developed above an inherited mesozoic fault zone that strikes ne sw oblique to the regional ssw directed tectonic transport during the early miocene pleistocene evolution of the thrust belt the oblique ramp was repeatedly reactivated localizing deformation above the inherited structure geological and structural mapping combined with kinematic analysis of ductile and brittle structures suggest that the omalos oblique ramp generated a local kinematic field which deviated significantly from the regional kinematic pattern in the thrust belt the most conspicuous feature in the tectonic evolution of the oblique ramp is a change from a ductile wrench dominated to a brittle primarily reverse faulting regime across the brittle ductile transition followed by brittle wrench deformation after the final exhumation of high pressure hp rocks deflections of transport and compression orientations from the regional pattern are attributed to buttressing against basement cover offsets produced by the pre existing fault zone to oblique ramping and to transfer faulting respectively our findings are potentially applicable to other examples of crustal scale oblique thrust ramps in various tectonic settings

paleocurrent analysis for the late pleistocene holocene incised valley fill of the yangtze delta china by using anisotropy of magnetic susceptibility data abstract anisotropy of magnetic susceptibility ams analysis has been conducted on samples from borehole core cm 97 from the yangtze river changjiang incised valley fill china to determine the paleocurrent directions to help in reconstructing sedimentary paleoenvironments borehole cm 97 consists of fluvial unit 1 estuarine units 2 6 and deltaic units 7 9 sediments after the last glacial maximum in ascending order the ams results show that the paleocurrent directions for the tide dominated estuarine and deltaic sediments were westerly or northwesterly directed flood tide dominated but give no definite trend for the fluvial sediments comparison between the paleocurrent directions inferred from primary sedimentary structures and in situ ams data shows that they are in good agreement confirming the applicability of ams as a good paleocurrent indicator for sediments deposited in coastal tide dominated environments considering that these sediments were strongly tide influenced and the tidal pattern since 12 kyr bp has not changed significantly we think that the westerly or northwesterly current direction most probably resulted from flood tidal currents and the sedimentary paleoenvironment was a flood tide dominated estuary or delta additionally it has been found that downhole changes of some ams parameters including the mean magnetic susceptibility k the corrected anisotropy degree p j and the magnetic foliation f clearly mark the defined stratigraphic boundaries in the borehole this further extends the validity of ams as a good stratigraphic marker in addition to its long known credibility as a sensible paleocurrent recorder

thrust geometry controlled by erosion and sedimentation a view from analogue models analogue sandbox models have been set up to study thrust systems formed above two decollements and affected by erosion and sedimentation the deformation was imaged by x ray scanner three erosion sedimentation conditions were tested for all the experiments a duplex develops between the two decollement levels for the experiment without erosion and without sedimentation ramp anticlines form in the upper series for the model with syntectonic sedimentation and without erosion the shortening in the upper series is first characterized by fault related folds and then by a forward propagation of the decollement that induces the development of a piggy back basin for the experiment with erosion and sedimentation a passive roof duplex is formed relationships between structural style and syntectonic superficial conditions in classical thin skinned thrust belts are similar to those found in the analogue models the ramp anticlines of the jura developed with low erosion and low sedimentation and the large sub andean thrust belt is controlled by syntectonic sedimentation whereas the passive roof duplex of pakistan is synchronized with intense erosion this study outlines the crucial effect of the superficial mass transport phenomena in the tectonic style of thrust belts

lateral variations and linkages in thrust geometry the traligill transverse zone assynt culmination moine thrust belt nw scotland abrupt lateral changes in thrust geometry occur in many fold and thrust belts along so called transverse zones commonly related to pre existing basement faults however the causative structures are usually concealed we analyse here the traligill transverse zone in the assynt culmination of the caledonian moine thrust belt nw scotland this transverse zone trends sub parallel to the wnw transport direction and is associated with en echelon faults cutting thrusts discontinuity of thrust architecture and oblique fold and thrust structures thick thrust sheets north of the transverse zone contain thick basement slices thrust sheets to the south are thin and involve a thin bedded sequence the traligill transverse zone developed above the loch assynt fault a basement cross fault and reactivated proterozoic ductile shearzone piercing point analysis shows that the cross fault was active both before and after thrusting thrusting thus affected strata that were already disrupted by steep faults the amplitude of the disturbance in fold and thrust architecture along the traligill transverse zone is much greater than the vertical displacement along the fault this is attributed to localised transpressional thrust stacking other basement cross faults and their relationship with lateral variations within the moine thrust belt and in other thrust belts are discussed

controls on the geometry of fold thrust belt salients salients convex to the foreland curves in the map traces of orogens typically involve fold thrust belts formed along the foreland margin of orogens an examination of the map view shape degree of asymmetry dimensions and structural trend line patterns i e the variation in fold trend and fault strike along the length of the belt of 20 fold thrust belt salients worldwide illustrates relationships between salient geometry and the geologic setting in which the salient formed in order to understand these relationships we used simple sandbox models to simulate the formation of three of the several types of salients specifically we examined indenter controlled salients the geometry of which reflects interaction of a fold thrust belt with a hinterland indenter basin controlled salients the geometry of which reflects lateral variations in the thickness of predeformational basin fill and detachment controlled salients the geometry of which reflects lateral variations in the strength of a detachment surface our models demonstrate that thrusts in these three types of salients initiate with a curved shape and that rocks within thrust sheets on the limbs of salients can undergo a vertical axis rotation in response to simple shear without requiring major rotation of the fault trace furthermore trend line patterns distinguish basin controlled from indenter controlled salients specifically structural trend lines in basin controlled salients converge at the end points of the curve while in indenter controlled salients they diverge at the end points of the curve the asymmetry of a salient correlates with the asymmetry of the predeformational basin and or of the indenter and with the convergence direction thus our results explain geometric characteristics of individual salients and address controversies concerning paleomagnetic data in salients they also may help prioritize petroleum exploration targets in salients foreland basin oil fields concentrate at the apex of basin controlled salients because the geologic factors that generate the salient also contribute to localizing oil fields