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

mechanical modelling of folding versus faulting in brittle ductile wedges abstract the question of whether layered sequences of sedimentary rock subjected to layer parallel compression will fold or fault is investigated by means of mechanical modelling using the finite element method the model consists of a rigid basement a weak visco elasto plastic decollement horizon and a relatively competent elastic plastic cover sequence syntectonic erosion and sedimentation on the surface are included using a diffusion model numerical results show that folding is the dominant mode of deformation when the decollement horizon is relatively thick and has a low viscosity when the upper layer has a relatively high elastic shear modulus and when the total thickness of the sequence is relatively small if any one of these conditions is not satisfied faulting dominates other parameters such as the angle of internal friction and cohesion influence the style of deformation but have little influence on the boundary between folding and faulting results are interpreted in terms of competing deformation instabilities if fold amplification rates are large folding dominates over faulting if on the other hand fold amplification rates are small faulting dominates because stresses intersect the failure surface before significant fold amplification can occur

the impact of cover rock rheology on the style of folding in the zagros fold thrust belt abstract the present day morphology of the zagros fold thrust belt is dominated by magnificent exposures of nw se trending folds these folds differ in their size and geometry and these differences are related mainly to the rheological profile of the cover rock the cover rock succession of the zagros consists of a sequence of competent and incompetent units which vary both along and across the belt field based study combined with the use of satellite images reveals that the thickness and facies distribution of the cover rock succession has a significant impact on the style of deformation during the shortening linked to the current convergence of the arabian and iranian plates the incompetent units act as detachment horizons which localise thrusting and which act as decollement above which detachment folds form in addition where these incompetent units are thick e g 1 km they allow the deformation above and below them to become completely decoupled enabling disharmonic folding to occur as a result the folds above and below the incompetent units in the central part of the zagros folded belt have significantly different geometries and wavelengths as the zagros folds host the majority of the hydrocarbon reserves in iran and iraq an understanding of the processes that influence their geometry and spatial organization at different levels in the cover rock is crucial for the future exploration in the region

a review of brittle compressional wedge models abstract fold and thrust belts and accretionary wedges form by compression of sedimentary sequences and basement rocks into forward and backward thrusts folds nappes and duplexes for over a century models have been used to investigate the essential characteristics of such brittle wedges here i review model studies of brittle thrust wedges in orthogonal compression focussing on critical taper theory analogue and numerical techniques for modelling brittle behaviour and the most commonly investigated variations in wedge model studies those in basal dip basal strength internal strength and surface processes many model results can be placed in the context of critical taper theory which provides analytical solutions for wedge taper angle and slip line orientations for a homogeneous material on the verge of failure throughout dynamic forward models have confirmed critical taper predictions of decreasing surface dip for increasing basal dip wider wedges with steeper forward thrusts and shallower backward thrusts for decreasing basal strength narrower wedges and enhanced exhumation for surface erosion and decreasing surface dip for increasing internal strength but analogue and numerical models have been able to take these results a step further by investigating the evolution of non critical thrust wedges and the effects of non homogeneous materials with decollement layers and strain weakening shear zones these results have highlighted the strong impact of heterogeneous materials on the internal structures of thrust wedges and raise the question whether the complexity of structures that are observed in many natural fold and thrust belts requires that lithological layering inherited faults or a wide range of rheologies need to be included in forward models of brittle thrust wedges

do faults trigger folding in the lithosphere a number of observations reveal large periodic undulations within the oceanic and continental lithospheres the question if these observations are the result of large scale compressive instabilities i e buckling remains open in this study we support the buckling hypothesis by direct numerical modeling we compare our results with the data on three most proeminent cases of the oceanic and continental folding like deformation indian ocean western gobi central asia and central australia we demonstrate that under reasonable tectonic stresses folds can develop from brittle faults cutting through the brittle parts of a lithosphere the predicted wavelengths and finite growth rates are in agreement with observations we also show that within a continental lithosphere with thermal age greater than 400 my either a bi harmonic mode two superimposed wavelengths crustal and mantle one or a coupled mode mono layer deformation of inelastic folding can develop depending on the strength and thickness of the lower crust

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

numerical models of alpine type cover nappes we investigate the dynamics of alpine cover nappes derived from basins with inherited geometries using a two dimensional finite element code model studies are based on a natural analogue the klippen nappe in the swiss alps which detached from the basement over a basal evaporite horizon and was transported onto the foreland in a thin skinned tectonic style finite element models are used to examine the influence of specific features on the structural style developing in the models experiments dealing with the influence of wedge geometry and mechanical properties show modifications in wavelength patterns and structures in detail but do not change the general trends the control that different layer thicknesses multilayered rocks and lateral heterogeneities in the underlying basement and within the sediments have on the style of deformation is investigated results suggest that the development of fold versus thrust structures in a cover nappe is controlled by the thickness of the weak detachment horizon and the relative thickness of individual layers thickness ratio n thickness of weak thickness of strong layer if n 1 folds with variable and irregular wavelengths form if 0 5 n 1 regularly spaced detachment folds develop imbricate thrusting is observed for values of n 0 5 lateral heterogeneities in both basement and rock formations cause stress concentrations control the geometry of the evolving folds and thrusts significantly and inhibit the growth of dominant wavelengths predicted by viscosity contrasts and thickness ratios

mechanical stratigraphy and fault fold interaction absaroka thrust sheet salt river range wyoming abstract a distinct contrast in deformation style from tightly folded and faulted cambrian through ordovician strata to broadly folded and faulted mississippian and younger strata is displayed in the hanging wall of the absaroka thrust salt river range wyoming based on style the paleozoic section can be divided into two main structural lithic units each unit is characterized by a relatively weak ductile anisotropic lower section and a relatively strong brittle more isotropic upper section where the two units were stacked within the thrust sheet inverted fault propagation folds formed in the center of each unit and the overall transition upward from close to wide spaced folds and imbricate faults developed in the multilayer where the upper unit was isolated deformation was dominated by imbricate faulting with little associated folding and inverted fault propagation folds did not form observations illustrate that the mechanical interaction between the two units and boundary conditions imposed on them were significant to defining the deformation response it is note worthy that the boundary between the two structural lithic unts in part defined by the roof thrust of the stewart passive roof duplex occurs within a dolostone rather than in overlying shale of the darby formation i propose that the roof thrust and other detachments in dolostone reflect the relatively low frictional strength of dolostone under shallow crustal conditions cross cutting relations suggest that the absaroka sheet was cut by large imbricate faults during late stage movement over a major dip ramp a high deviatoric and low mean compressive stress state just beyond the upper ramp flat corner promoted break back imbrication and allowed tectonic thickening of the thrust wedge necessary to reestablish critical taper and for forward translation

tectonic geomorphological investigations of antiforms using differential geometry permam anticline northern iraq modern differential geometric methods are applied to compute and analyze curvature quantities of the permam anticline which is part of the zagros fold and thrust belt in northern iraq northeast of the city of erbil because this particular anticline is composed of among others weathering resistant limestones the surface topography strongly resembles the antiformal fold shape this makes it an ideal area where numerical curvature analysis applied to shuttle radar topography mission digital elevation models dems allows drawing not only geomorphological but also tectonic conclusions the curvature analysis is based on the computation of the gaussian and mean curvatures and is used to classify the folded surface into eight geologically relevant shapes antiform synform plane dome basin and three types of saddles the performed curvature analysis investigates in detail the effects of two adaptable parameters the cutoff wavelength of the low pass filter that is applied to the dem before curvature calculation and the curvature threshold that is applied to the principal curvature values before the calculation of the gaussian and mean curvatures the analysis demonstrates that these two parameters strongly influence each other and that they together determine the information content and interpretability of the results by choosing appropriate parameter combinations geomorphological oriented studies and tectonic oriented studies are viable using the same dem how tectonic oriented studies can be used to determine fracture patterns or densities in folded layers is discussed further suggestions are made to combine curvature analysis of dems with similar studies applied to seismically mapped surfaces in three dimensional seismic data sets where erosion is absent

at what stress level is the central indian ocean lithosphere buckling internal contrasts in strength are responsible for lithospheric buckling these are quantified by comparing the indian ocean data to two dimensional visco elasto plastic numerical models where the material properties depend on temperature and pressure the central indian basin is known for its intraplate seismicity and long wavelength undulations of the sea floor and associated gravity signatures to simulate the amplitudes of undulations that reach 1 km within about 11 ma of compression and 60 km of shortening the required mean yield strength of the lithosphere is 400 mpa if either a hydrostatic fluid pressure is considered in the crust or a mechanical decoupling at the depth of the moho small crustal wavelengths are superimposed on the long wavelength deformation in agreement with observations it is then possible to match the alternative indication that buckling commenced only 4 ma ago with a total amount of shortening of 30 km and with a required yield strength of 200 mpa about 10 of homogeneous thickening accompanies buckling taking into account variable thermal conductivity demonstrates that a change in the geotherm is sufficient to increase buckling amplitudes by 10 within 5 ma fl 2000 elsevier science b v all rights reserved

numerical investigation of deformation mechanics in fold and thrust belts influence of rheology of single and multiple decollements 1 thin skinned fold and thrust belts related to convergence tectonics develop by scraping off a rock sequence along a weaker basal decollement often formed by water saturated shale layers or low viscosity salt horizons a two dimensional finite element model with a viscoelastoplastic rheology is used to investigate the structural evolution of fold and thrust belts overlying different types of decollements in addition the influence of multiple weak layers in the stratigraphic column is studied model shale decollements are frictional with lower friction angles as the cover sequence model salt layers behave linear viscous due to a lower viscosity as the cover sequence or with a power law rheology single viscous decollement simulations have been compared to an analytical solution concerning faulting versus folding results show that fold and thrust belts with a single frictional basal decollement generate thrust systems ramping from the decollement to the surface spacing between thrust ramps depends on the thickness of the cover sequence the structural evolution of simulations with an additional low frictional layer depends on the strength relationship between the basal and the intersequential decollement tectonic underplating and antiformal stacking occur if the within sequence decollement is weaker in the frontal part of models deformation is restricted to the upper part and imbrication occurs with a wavelength depending on the depth of the intermediate weak layer salt decollement with a viscosity of 10 18 pa s leads to isolated box folds detachment folds multiple salt layers 10 18 pa s result in long wavelength folding our results for both frictional and viscous decollements are in bulk agreement with the mohr coulomb type critical wedge theory citation ruh j b b j p kaus and j p burg 2012 numerical investigation of deformation mechanics in fold and thrust belts influence of rheology of single and multiple decollements tectonics 31 tc3005 doi 10 1029 2011tc003047

fault propagation folds above thrusts with constant dip abstract previous models of fault propagation folds have been extended to include the case where folding initiates above a thrust ramp of constant dip the model may be useful in analyses of fold thrust structures in thin and thick skinned compressional terrains it may be most applicable to fault propagation folds that develop as a fault propagates from a thick massive brittle unit into a thinly layered anisotropic unit

the role of salt in fold and thrust belts davis d m and engelder t 1985 the role of salt in fold and thrust belts in n l carter and s uycda editors collision tectonics deformation of continental lithosphere tectonophysics 119 67 88 the style of deformation in thin skinned fold and thrust belts is critically dependent upon the resistance to sliding along the detachment between the mass of deforming sediments and the underlying rocks evaporites can provide an extremely weak horizon within which a basal detachment can form and along which only a relatively small shear traction can be supported fold and thrust belts that form atop a salt layer such as the appalachian plateau the franklin mountains in northwestern canada and the jura of the alps among others share several readily observable characteristics as predicted by a simple mechanical model for fold and thrust belts a detachment in salt permits a thrust belt to have an extremely narrow cross sectional taper in addition predicted orientations of the principal stress axes over a salt dtcollement are consistent with the commonly observed lack of a consistently dominant vergence direction of structures within the thrust belt other common attributes of salt basal thin skinned deformation include the presence of several widely but regularly spaced folds and abrupt changes in deformational style at the edge of the salt basin

3d effects of strain vs velocity weakening on deformation patterns in accretionary wedges abstract shear zones are weaker than surrounding rocks whether weakening depends on accumulated strain or strain rate is debated we used a three dimensional numerical code with a visco plastic brittle rheology to investigate the influence of strain and strain rate weakening often referred to as velocity weakening on the evolution of thin skinned fold and thrust belts two model setups are used i a uniform setup to recognize the effects of each weakening mode on the structural evolution and dynamics of fold and thrust belts and ii a dual setup with two adjacent domains of decollement strength to investigate the structural response of laterally varying systems strain weakening and velocity weakening lead to remarkably different structural patterns in particular strain weakening favours out of sequence shear zones whilst some amount of finite plastic strain has to be achieved before the structural development initiates weakening in contrast velocity weakening starts with the onset of high strain rate bands and does not favour out of sequence deformation we also tested separately the influences of shortening velocity cover sequence thickness and weakening style and amount on the structural evolution of the dual systems the shortening velocity has a minor effect in strain weakened models velocity weakening and a thinner cover sequence enhance the occurrence of strike slip shear zones we conclude that the weakening mode strongly influences the fault patterns and the dynamics of thrust wedges inversely natural fault patterns are keys to discriminate weakening processes

the kinematics of break thrust folds abstract the kinematics of asymmetric open to close often overturned thrust related folds are commonly explained with fault propagation or detachment fold models field observations of thrust related folds exposed in tennessee virginia wyoming and montana that exhibit this geometry indicate that models of fault propagation and detachment folds do not adequately describe the kinematics of these structures existing models of fault propagation and detachment folding employ migrating kink shaped hinges and relate fold geometry entirely to fault geometry slip and to the thickness of the basal detachment layers the models exclude the relation between fold shape and amplitude and they do not predict a correlation between competent layer thickness and fold wavelength as expected from buckling theory thrust related folds examined in this study exhibit unique distributions of fabrics in the hinges and forelimbs which demonstrate a lack of hinge migration furthermore the ratios of dominant member thickness to fold wavelenght agree with those expected for folds controlled by a single layer and they are within the expected range for folds with multiple evenly spaced harmonically deforming layers our analyses suggest these folds evolved from an initial brief stage of sinusoidal buckling to a later stage of fixed hinge kinking and thrusting thrust related folds which exhibit a geometry and mesofabric distribution suggesting this type of evolution are defined as break thrust folds

episodic growth of fold thrust belts insights from finite element modelling abstract the sequential development of a fold thrust belt was investigated using 2d finite element modelling fem the new model results show that a thrust system is typically composed of three distinct regions the thrust wedge pre wedge and undeformed region the thrust wedge involves growth that repeats episodically and cyclically a cycle of wedge building starts as frontal accretion occurs which is accompanied by a rapid increase in wedge width reducing the taper angle below critical in response to this the wedge interior tracked here by the 50 m displacement position rapidly propagates forwards into a region of incipient folding the taper angle progressively increases until it obtains a constant apparent critical value 10 during this period the wedge experiences significant shortening after a new thrust initiates at the failure front leading to a decrease in wedge width successive widening of the wedge and subsequent shortening and thrusting maintain a reasonably constant taper angle the fold thrust belt evolves cyclically through a combination of rapid advancement of the wedge and subsequent gradual slow wedge growth the new model results also highlights that there is clear although minor deformation 0 10 m horizontal displacement in front of the thrust wedge

spatial arrangement of decollements as a control on the development of thrust faults abstract we used two dimensional finite element models to explore different configurations of weak layers in undeformed sedimentary sequences to investigate the occurrence of three characteristic types of thrust configurations ramp flat imbricate and duplex in our models we embedded two low friction weak layers with a finite spatial extent in a sequence of stronger rock these two weak layers were initially horizontal were separated vertically by 1 km and were arranged in three different relative positions to each other when the models were deformed and these weak layers developed into decollements they interacted to produce one of the three types of thrust faults as a function of their initial configurations when the tips of weak layers were separated by a large gap 10 km only the lower level decollement became active producing imbricate thrusts when the two weak layers overlapped for a large distance 10 km they simultaneously became active decollements producing duplexes in the overlapped zone when the gap or overlap was small

interaction of folding and thrusting during fold and thrust belt evolution insights from numerical simulations and application to the swiss jura and the canadian foothills abstract we use two dimensional numerical simulations to investigate the interaction of folding buckling and thrusting localized shearing during the structural evolution of fold and thrust belts ftb we model layer parallel shortening of mechanically strong layers resting above a weaker layer referred to as detachment we consider a viscoplastic rheology with linear viscosity and frictional plastic drucker prager yield criterion the detachment thickness is larger in one model half and only this geometrical thickness perturbation triggers folding and thrusting in the simulations we conduct a systematic analysis to quantify the impact of model parameters such as magnitude of the thickness perturbation or detachment rheology frictional or viscous on the ftb evolution the magnitude of the perturbation determines whether or not folding is favoured over thrusting and hence controls the progressive interaction of folding and thrusting during ftb evolution the asymmetry of modelled anticlines is proportional to the relative height of the initial perturbation we also adapt the model configuration to specific mechanical stratigraphies of two regions in natural ftb namely the haute chaine of the jura mountains switzerland and the foothills of alberta canada the modelled fold and thrust geometries agree with different types of observed anticlinal structures such as symmetric detachments folds and asymmetric thrust folds our results suggest that an initial stage of low amplitude buckling likely occurred prior to the main folding and thrusting phase we argue that folding and thrusting are equally important and interacting processes during the structural evolution of many natural ftb

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

thrust initiation and propagation during shortening of a 2 layer model lithosphere analogue modelling was used to investigate thrust initiation and propagation in the lithosphere the model comprised a top layer of light sand model brittle crust overlying a layer of heavy silicone putty model ductile lower crust both resting upon higher density and lower viscosity natural honey model ductile mantle the experiments show the following 1 thrust initiation may occur in two main modes fold first or fault first they seem to be the result of presence or absence of an initial perturbation close to the piston respectively the fault first mode eventually goes into a fold first mode after a certain amount of deformation 2 in both modes thrusts localized around the inflection points of both fore and back limbs of buckles as a result of tectonic horizontal gravitational vertical and elastic forces 3 thrust propagation in the fault first mode occurred by thrust wedge thickening and downward flexure due to isostatic compensation which led to forward formation of bulges and subsequent amplification by buckling continued shortening increased the stresses that reached the brittle yield of sand and new thrusts originated 4 bulging ahead of thrusts tended to develop into asymmetric buckles with the shorter limb back limb steeper towards the piston 5 as a whole thrust initiation and propagation seem to depend mostly on how stresses evolve and propagate through the model and how material strength varies with progressive deformation the main stresses at play are born at the moving piston model horizontal tectonic force from thickening and isostatic compensation in the model gravitational forces and from elastic resistance to buckling

the structure of the helvetic nappes and its relation to the mechanical stratigraphy abstract this paper discusses structure and variations in structural style due to changes in mechanical stratigraphy of the helvetic nappes based on five cross sections the ratio n of the thickness of mechanically weak to the thickness of adjacent mechanically strong layers is of particular importance a low value of n favours imbricate thrusting and harmonic folding a high value favours detachment and disharmonic folding the transition at around n 0 5 is characterized by polyharmonic folding stratigraphic thickness variations due to local basin subsidence resulted in arcuate fold hinges and important plunges upon basin inversion lateral variations in the overall structure of the helvetic nappes is largely controlled by the initial irregular arrangements of the various small basins one more or less coherent thrust sheet the cretaceous stockwerk of the wildhorn drusberg santis nappe can be traced along the entire fold and thrust belt

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

control of folding by gravity and matrix thickness implications for large scale folding 1 we show that folding of a non newtonian layer resting on a homogeneous newtonian matrix with finite thickness under influence of gravity can occur by three modes 1 matrix controlled folding dependent on the effective viscosity contrast between layer and matrix 2 gravity controlled folding dependent on the argand number the ratio of the stress caused by gravity to the stress caused by shortening and 3 detachment folding dependent on the ratio of matrix thickness to layer thickness we construct a phase diagram that defines the transitions between each of the three folding modes our priority is transparency of the analytical derivations e g thin plate versus thick plate approximations which permits complete classification of the folding modes involving a minimum number of dimensionless parameters accuracy and sensitivity of the analytical results to model assumptions are investigated in particular depth dependence of matrix rheology is only important for folding over a narrow range of material parameters in contrast strong depth dependence of the viscosity of the folding layer limits applicability of ductile rheology and leads to a viscoelastic transition our theory is applied to estimate the effective thickness of the folded central asian upper crust using the ratio of topographic wavelength to moho depth phase diagrams based on geometrical parameters show that gravity does not significantly control folding in the jura and the zagros mountains but does control folding in central asia applicability conditions of viscous and thin sheet models for large scale lithospheric deformation derived in terms of the argand number have implications for the plate like style of planetary tectonics

transition from thin to thick skinned tectonics and consequences for nappe formation numerical simulations and applications to the helvetic nappe system switzerland abstract we use two dimensional numerical simulations to study the deformation style and the formation of nappes during the shortening of basement cover systems with half grabens we perform simulations with only linear and power law viscous rheology and also thermo mechanical simulations with viscoelastoplastic rheology and temperature dependent viscosities the accumulated area averaged strain is calculated for the sediments es and for the basement eb to quantify the deformation style the results show that the transition from thin skinned dominated es eb 1 to thick skinned dominated es eb 1 deformation is controlled by two effective viscosity ratios the ratio of viscosity at the top of the basement to viscosity at the bottom of the basement and the ratio of viscosity at the top of the basement to viscosity of the sedimentary cover a higher basement internal viscosity ratio favors thick skinned deformation whereas a higher basement cover viscosity ratio favors thin skinned deformation simulations with only viscous rheology show the same first order results as the thermo mechanical simulations the results further show that fold nappe formation is associated with thick skinned dominated deformation and thrust nappe formation with thin skinned dominated deformation fold nappes are formed by shearing and extrusion of sediments during the deformation and closure of half grabens whereas thrust nappes are formed due to the shearing off of sediments from half grabens which are nearly undeformed the results are applied to the morcles fold nappe and the helvetic nappe system switzerland

impact of the late triassic dashtak intermediate detachment horizon on anticline geometry in the central frontal fars se zagros fold belt iran abstract integration of 2 d seismic lines well data and field studies allow us to determine the geometry variations of anticlines in the highly prolific central frontal fars region in the se zagros fold belt these variations are directly related to changes in thickness of the principal evaporitic intermediate detachment level located along the late triassic dashtak formation anticlines of short wavelength contain a significant over thickening of the evaporitic detachment level in their crestal domain that may reach 1900 m from an original thickness of 550 800 m folds containing thick dashtak evaporites show decoupling across the detachment level and thus a shift of the anticline crest in the underlying permo triassic carbonates of the dehram group which form the major gas reservoir in the central frontal fars four main parameters control the extent and distribution of the decoupled anticlines in the study area a original large thickness of the late triassic evaporitic basin b coinciding larger amounts of anhydrites with increasing total thickness of formation c parallel occurrences of abnormally high fluid pressures and d shortening variations across and along the strike of specific folds the present work relating the different parameters of the dashtak evaporites with the anticline geometry allows a better understanding of the fold geometry variations with depth which is applicable to oil and gas exploration in the se zagros and other similar hydrocarbon provinces characterised by intermediate detachment horizons

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

petrogenesis of the early cretaceous intermediate and felsic intrusions at the southern margin of the north china craton implications for crust mantle interaction abstract new major and trace element whole rock sr and nd isotopes and zircon u pb ages and hf isotope data are presented for rocks from the early cretaceous tianqiaogou dioritic and taishanmiao granitic plutons at the southern margin of the north china craton ncc in order to investigate their petrogenesis and geological evolution la icp ms u pb analyses for zircons from these two plutons yield similar 206 pb 238 u ages of 122 ma and 115 125 ma respectively monzodiorites from the tianqiaogou pluton have whole rock e nd t values ranging from 6 2 to 1 3 and zircon e hf t values from 2 9 to 6 2 they are variably enriched in ra ba and sr and depleted in nb ta zr hf and ti indicating that they were derived from a depleted mantle and underwent subsequent magma differentiation and crustal contamination the taishanmiao pluton is composed of metaluminous to peraluminous highly fractionated i type granites that have high sio 2 na 2 o k 2 o rb th and u and low p ba sr ti and eu contents the granites have strong negative whole rock e nd t values 16 1 to 7 5 and zircon e hf t values 20 9 to 6 1 their nd t dm ages 1 19 to 2 01 ga and zircon hf t c dm ages 1565 to 2490 ma are much younger than the basement rocks beneath the southern margin of the ncc suggesting derivation from an ancient crustal source with minor involvement of mantle derived components therefore rocks from the tianqiaogou dioritic pluton were partial melts of the mantle source underplating of the mafic magmas initiated partial melting of the ancient continental crust resulting in the formation of the taishanmiao granitic pluton their complex petrogenesis reflects a strong crust mantle interaction process related to lithospheric thinning beneath the southern margin of the ncc in early cretaceous

building the zagros collisional orogen timing strain distribution and the dynamics of arabia eurasia plate convergence he zagros mountains are the result of the arabia eurasia collision initiated at 35 ma as the rifted arabian lithosphere was underthrusted beneath the iranian plate due to its negative buoyancy the onset of crustal thickening started at 25 ma as recorded by the hinterland exhumation and foreland clastic deposition deformation throughout the arabia eurasia collision zone and the uplift of the iranian plateau occurred after 15 12 ma as a result of shortening thickening of the thin iranian crust we emphasize that only 42 of the post 35 ma convergence is partitioned by shortening within central iran tomographic constraints show ongoing slab steepening or breakoff in the nw zagros whereas underthrusting of the arabian plate is observed beneath central zagros the current subduction dynamics can be explained by the original lateral difference in the buoyancy of the distal margin that promoted slab sinking in nw zagros and underthrusting in central zagros critical wedge approach applied to the zagros favors the hypothesis of strong brittle crust detached above a viscous lower crust in contrast the weak sedimentary cover deforms by buckling of a thick multilayered cover thrust faulting associated with folding occurs in the competent layers and is responsible for most of the earthquakes there is evidence that the role of the slab pull force in driving the arabian plate motion was reduced after 12 ma large scale mantle flow induced by mantle upwelling at the afar plume appears to be the main driver of the arabia plate motion we stress that the main kinematic change in the zagros region occurred at 15 12 ma as the zagros uplifted before the arabian slab detached the zagros appears key to investigate coupling between continental rheology plate driving forces and mountain building in which the role of rift inheritance appears to be central

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

changing mechanical response during continental collision active examples from the foreland thrust belts of pakistan abstract we have used data from teleseismic seismic reflection and field geologic studies along with both geomechanical and gravity modeling to contrast the tectonics of four active orogenic wedges in pakistan the kashmir himalaya the salt range potwar plateau foldbelt the sulaiman range and the makran accretionary wedge in makran oceanic crust is still being subducted and a thick pile of sediments is being accreted and underplated undercompaction and excess pore pressures can explain the narrow cross sectional taper and frontal aseismicity of this wedge beneath the sulaiman wedge continental crust is just starting to be underthrust indirect evidence suggests that fine grained carbonate rocks found in abundance deep in the stratigraphic section may be deforming ductilely at the base of the sulaiman wedge and provide a zone of ductile detachment the collision has proceeded to a much more mature stage in the salt range potwar plateau foldbelt and the kashmir himalaya isostatic response to underthrusting of continental crust has kept the sedimentary pile quite thin in both of these wedges so in that respect the two foldbelts are similar however thick eocambrian salt beneath the salt range and potwar plateau permits that foldbelt to be much wider in map view with a thinner cross sectional taper and a mixture of thrust vergence directions a major normal fault in basement causes the salt range to rise in front of the mildly deformed molasse basin of the southern potwar plateau much of the diversity among these mountain belts can be understood in terms of differences in the maturity of the collision process in each area the resulting thickness of the sedimentary pile encountered at the deformation front and the presence or absence of large contrasts in strength between the various layers of the stratigraphic section and basement relief

influence of mechanical stratigraphy on folding vs faulting abstract the competing processes of folding and faulting during shortening of a layered sequence are investigated using elastic plastic models in order to determine the influence of mechanical stratigraphy on the development of fold belts vs thrust belts the models consist of a strong middle layer and weaker decollement and cover layers the mechanical behaviour of each layer is determined by its plastic hardening modulus h and its elastic shear modulus g with bonded contacts fold amplification at low shortening strain relatively large h is greatest for a thin cover layer and a thick decollement layer at higher shortening strain h g the middle layer may reach its peak strength before the other layers causing it to become relatively weaker in shortening and causing fold amplification to be greatest for a thick middle layer free slip contacts between the layers enhance folding and fold amplification is greatest if the slip surfaces are roughly equally distributed throughout the sequence a relatively weak decollement layer enhances folding consistent with the fact that fold belts usually include a salt decollement layer dominant wavelengths increase with increasing strength contrasts between the layers and with increasing thicknesses of the decollement and strong layers

fold thrust structures where have all the buckles gone special publication title folding and fracturing of rocks 50 years of research since the seminal text book of j g ramsay we dedicate the paper to the memory of martin casey 1948 2008 who did much through good humored argument to ensure that buckling ideas were not lost to what he called the ramping club the thrust belt community the fold thrust research group has been funded by interoil oilsearch and santos we thank paul griffiths and anonymous referee for comments together with hermann lebit for scientific editing the views expressed here of course remain those of the authors

structural evolution of orogenic wedges interplay between erosion and weak decollements orogenic wedges commonly display an inner wedge where crystalline units have been exhumed and an outer wedge formed by imbricated sedimentary units detached from the basement analog experiments have shown that similar structures can emerge naturally in the presence of weak decollements due to the interplay between erosion and deformation in this study we further investigate this hypothesis using two dimensional visco elasto plastic numerical models our experiments assume a basal and an intermediate decollement within the wedge experiments with a frictional strength of the basal decollement lower or equal to that of the intermediate decollement show a structural evolution of fold and thrust belts dominated by out of sequence thrusting conversely when the intermediate decollement is weaker than the basal decollement distinct outer and inner wedges are formed this process leads to episodic migration of midcrustal ramps tectonic underplating and antiformal stacking facilitated by erosion comparison between our models and the himalayan wedge suggests a low effective friction 0 10 which is probably due to dynamic weakening during large m8 himalayan earthquakes the deeper decollement along which the lower plate thrusts beneath the high himalaya must be a thermally activated ductile shear zone with an apparent friction of 0 18 fold and thrust belts worldwide exhibit various architectures in which different decollement levels might be activated thus our study provides a framework to help assess under which conditions a variety of structures observed in orogenic systems can arise

diatom evidence of climatic change in holsteinsborg dyb west of greenland during the last 1200 years diatom assemblages from holsteinsborg dyb on the west greenland shelf were analysed with high temporal resolution for the last 1200 years a high degree of consistency between changes in frequency

decollement folding as a mechanism for thrust ramp spacing the hypothesis that early formed decollement folds localize thrust ramps explaining the regular spacing of major thrust ramps observed in many mountain belts is examined the model for folding instability consists of a rigid basement a weak decollement zone and a stiff thrust sheet the model incorporates the effects of topographic stresses and syntectonic erosion and redeposition and uses a linearized power law rheology for the thrust sheet this approximation is valid for small angles of fold limb dip 15 and is appropriate for analysis of the wave length selection of folds at the onset of deformation the spacing of major thrust ramps and the stratigraphic thicknesses of the decollement zone and thrust sheet from the wyoming portion of the sevier fold and thrust belt are used to test the model the ratio of the fold dominant wavelength to thrust sheet thickness ld h2 increases with the ratio r of the viscosity of the stiff layer to that of the decollement zone and decreases with the power law stress exponent n2 the fold amplification factor increases with both n2 and r as the topographic decay ratio d increases ld h2 increases at fixed n2 and r acceptable models for the absaroka and darby thrust sheets have n2 3 only when r 1000 and significant erosion and redeposition of sediments accompany the folding of the thrust sheet d 103 does the required r fall below a reasonable limit thus a model in which decollement folding is accompanied by syntectonic erosion and redeposition precedes thrusting is consistent with the available observational constraints

changes in dip and frictional properties of the basal detachment controlling orogenic wedge propagation and frontal collapse the external central betics case thin skinned fold and thrust belts ftbs have been extensively studied through both field examples and modelling the overall dynamics of ftbs are therefore well understood one less understood aspect is the combined influence of across strike changes in the detachment properties and the basement topography on the behaviour of an orogenic wedge in this paper we use field data together with reflection seismic interpretation from the external zones of the central betics ftb s spain to identify a significant increase in the wedge basal dip a basement threshold coinciding with the pinch out of a weak substrate this induced both changes to the wedge geometry and to the basal friction which in turn influenced the wedge dynamics the changing dynamics led to a transient stagnation of the ftb propagation topographic build up and subsequent collapse of the ftb front this in turn fed an important langhian depocenter made up of mass transport deposits coevally with the ftb propagation extension took place both parallel and perpendicular to the orogenic trend this case study illustrates how across strike changes in wedge basal properties can control the detailed behaviour of a developing ftb front but questions remain regarding the time space interaction and relative importance of the basal parameters

toward 4d modeling of orogenic belts example from the transpressive zagros fold belt abstract latest developments in computer power and computational solutions open new ways to envision complex natural systems advances in high resolution 3d geodynamic modeling in particular allow investigating tectonic processes such as transpression which involve normal and tangential kinematics with respect to the bulk orogenic trends we demonstrate that implementing the 4 dimensions space and time provides and constrains new answers to long lasting discussions as case study we have chosen the zagros simply folded belt in iran which exhibits spatial variations of structural patterns from the homogeneously shortened wide fold belt in the fars to the narrower higher elevated area in the izeh high resolution 4d numerical modeling shows how a low viscosity decollement becoming frictional toward the dezful embayment influences the exposed fold patterns results also emphasize the importance of an oblique backstop to produce en echelon arranged folds where convergence related and backstop controlled folds are mingled in the transpressional orogen