geology of panamint valley saline valley pull apart system california palinspastic evidence for low angle geometry of a neogene range bounding fault geological studies support the interpretation that northern panamint valley and saline valley southeast california form paired pull apart basins on opposite sides of the right slip hunter mountain fault zone eight to ten km of late cenozoic net slip can be established on the hunter mountain fault zone palinspastic reconstruction of northern panamint valley indicates that the valley was formed by movement on a shallow crustal low angle normal fault of 0 15 degree west dip during the last 3 0 ma this interpretation appears to contradict the notions that little extension is accomodated in the uppermost crust by low angle faulting and that the most recent extension in the basin and range province is accomodated exclusively by high angle faulting saline valley however is interpreted to have formed by extension on closely spaced rotated planar normal faults thus within one geometric system of paired pull apart basins extension appears to have been accommodated in the shallow crust in two different ways

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

thrust faulting and hydrocarbon generation discussion following the discovery of large volumes of hydrocarbons in the cordilleran thrust belt of southwestern wyoming and northern utah angevine and turcotte 1983 edman and surdam 1984 and furlong and edman 1984 discussed the role of thrust faulting in hydrocarbon generation the authors concluded that overthrusting plays an important or even dominant role in the process of hydrocarbon generation in overthrust terrains these conclusions may be in error because they are based on a mathematical analysis of thermal and geologic models that have little resemblance to actual thrust belt geology in this discussion the authors discuss shortcomings of the thermal model used in the papers referred to above emphasize the differences between their model and the geologic facts as they exist in thrust belts in general and in western wyoming in particular and caution readers about applying the conclusions based on such a model in the search for hydrocarbons they contend that the temperature effects of thrust faulting were rather insignificant in the process of hydrocarbon generation in the wyoming thrust belt

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

making mountains with lithospheric drips the rigid underpinnings of several western states may have dribbled away raising mountains as the drips passed by

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

tectonic evolution of fault bounded continental blocks comparison of paleomagnetic and gps data in the corinth and megara basins greece 1 we report on new paleomagnetic and anisotropy of magnetic susceptibility ams data from plio pleistocene sedimentary units from corinth and megara basins peloponnesus greece paleomagnetic results show that megara basin has undergone vertical axis cw rotation since the pliocene while corinth has rotated ccw during the same period of time these results indicate that the overall deformation in central greece has been achieved by complex interactions of mostly rigid rotating fault bounded crustal blocks the comparison of paleomagnetic results and existing gps data shows that the boundaries of the rigid blocks in central greece have changed over time with faulting migrating into the hanging walls sometimes changing in orientation the megara basin belonged to the beotia locris block in the past but has now been incorporated into the peloponnesus block possibly because the faulting in the gulf of corinth has propagated both north and east paleomagnetic and gps data from megara and corinth basins have significant implications for the deformation style of the continental lithosphere in areas of distributed deformation the continental lithosphere behaves instantaneously like a small number of rigid blocks with well defined boundaries this means that these boundaries could be detected with only few years of observations with gps however on a larger time interval the block boundaries change with time as the active fault moves paleomagnetic studies distinguishing differential rotational domains provide a useful tool to map how block boundaries change with time index terms 1208 geodesy and gravity crustal movements intraplate 8110 1518 geomagnetism and paleomagnetism magnetic fabrics and anisotropy 1525 geomagnetism and paleomagnetism paleomagnetism applied to tectonics regional global 8107 tectonophysics continental neotectonics 9335 information related to geographic region europe

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

continental rifting at pre existing lithospheric weaknesses continental rifting is the poorly understood thermomechanical process by which continents break up and new ocean basins form two dominant styles of continental rifting are recognized1 crevice style rifts are characterized by initial graben formation local uplift of the graben shoulders and little or no volcanism arch volcanic rifts exhibit an initial doming of the crust extensive volcanism and late stage graben formation the prevailing expla nation for the two styles is that crevice rifts form in response to regional horizontal stresses principally derived from the inter action of the lithospheric plates along their edges whereas arch volcanic rifts are thought to be the result of mantle convection currents which upwell directly beneath the rift axis2 we propose an alternative working model of continental rifting in which both styles initiate in response to regional horizontal stresses the different surface expressions of the two styles are explained in terms of differences in the mode of failure at different kinds of pre existing weaknesses in the continental lithosphere rather than differences in the nature of the driving forces involved

timing of deformation in overthrust belt and foreland of idaho wyoming and utah a review of the timing and displacement evidence of the major structures of the western wyoming overthrust belt and foreland shows there is a progression in thrust displacement apparent duration of motion and palinspastic position of thrust traces from west to east those toward the west moved farther for an apparently longer period of time and are more widely spaced in their restored positions than those toward the east however average thrust velocities are all on the order of 0 5 0 5 cm yr 0 2 in yr foreland events are in part synchronous with thrust belt events and had an effect on them although dating precision varies widely on major normal faults present evidence does not contradict the generally held view that all normal faults postdate the youngest thrusting

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

stresses beneath a ramping thrust sheet abstract there is a systematic progression of both the timing of motion of thrust sheets and the spacing of thrust ramps during the development of some thrust dominated foreland fold and thrust belts on the basis of these observations we suggest that a regional mechanical process that controls frontal ramp formation may dominate local effects such as stratigraphic pinchouts and interaction with basement to investigate this process we have developed elastic plane strain models of the state of stress in the footwall of the last formed or most forward thrust ramp these models include the load of an overriding thrust sheet and a synorogenic sedimentary cover the effect of emplacing a thrust sheet or developing a synorogenic sedimentary cover above an incipient thrust sheet is to stabilize the footwall toward the hinterland with respect to failure and thereby move failure more to the foreland failure is localized where there is a favorable trade off between the spatial rates at which mean stress and differential stress decrease toward the foreland the models also predict that frontal ramps nucleate well above the basal decollement and propagate upward and downward eventually linking with the regional decollement fault trajectories predicted by using a coulomb criterion are subhorizontal in the hinterland portion of the model and curve upward to the foreland resulting in a curved flat and ramp geometry these analyses show that each successive thrust sheet would be thinner as well as shorter than the previous thrust sheet based on these results we propose a model for the formation of the first thin skinned thrust sheet in the external portions of orogenic belts the synorogenic sediments that form during uplift of the internal zones of mountain belts will cover the preorogenic strata to the foreland as deformation progresses to the foreland and deformation becomes more shallow and more brittle in nature the preorogenic strata will interact with the synorogenic strata and failure will occur creating the first brittle thrust sheet the location of this first thrust will be controlled by the mechanisms outlined in our models to the foreland of this point each sucessive thrust fault will localize as a result of the combined effects of an overriding thrust sheet and synorogenic deposits

regional structural synthesis wyoming salient of western overthrust belt surface geologic mapping regional and high density reflection seismic data and information from approximately 370 wells have been combined to describe geometrically that area of the western overthrust belt between the snake river plain and the uinta uplift particular care has been taken to verify two dimensional interpretations of multiple thrust sheets by linear restoration by establishing equivalence of pre thrust lengths of affected beds one can gain confidence in interpreted structural geometries as well as generate data in regard to internal shortening and deformation intensity because of irresoluable geometric problems within sections 47 cross sections were developed at roughly 6 mi 10 km spacing to help verify changes in displacement or placement of key st uctural elements displacements of 0 to 32 mi 0 to 51 km have been demonstrated jurassic to paleocene sevier thrusts were active across a previously deformed cratonic shelf terrane and interacted with active structural elements such as the uinta uplift gros ventre range and moxa arch thrusts are progressively younger to the east except for the darby prospect pair in which the darby system is younger

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

kinematic evolution of a tectonic wedge above a flat lying decollement the alpine foreland at the interface between the jura mountains northern alps and the upper rhine graben we estimate strain rates of three parameters that describe the buildup of a tectonic wedge jura front france the uplift rate on frontal ramps decreases with the slip on the ramps and the coeval increase of vertical loads the migration rate of the tip of the sole thrust appears to be episodic and faster than the translation rate of the backstop the long term tilting rate appears to be constant whereas the short term rate is slightly slower we propose a model of wedge growth that proceeds without underthrusting in a prefractured medium the wedge would activate the frontal ramp with a weaker frictional resistance than the forward decollement an instantaneous forward jump of the deformation front would occur when the resistance on the ramp equals the resistance of the frontal decollement this model differs from those proposed for accretionary zones and relates to the backstop of a single tapered orogenic belt

rheology of the lithosphere selected topics we review recent results concerning the rheology of the lithosphere with special attention to the following topics 1 the flexure of the oceanic lithosphere 2 deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins 3 the rheological stratification of the continents 4 strain localization and shear zone development and 5 strain induced crystallographic preferred orientations and anisotropies in body wave velocities we conclude with a section citing the 1983 1986 rock mechanics literature by category

detachment faulting and the evolution of passive continental margins major detachment faults play a key role in the lithospheric extension process in the basin and range province and may also be important in other continental extension terranes such detachment faulting leads to an inherent asymmetry of extensional structure and of uplift subsidence patterns detachment models developed for the formation of metamorphic core complexes can also be applied to the formation of passive continental margins we therefore suggest the existence of upper plate and lower plate passive margins these give rise to a complementary asymmetry of opposing margins after continental breakup transfer faults offset marginal features and allow margins to switch from upper plate to lower plate characteristics along strike

thrust fault geometry of the snake river range idaho and wyoming the snake river range is located at the north end of the absaroka fault system in the idaho wyoming utah thrust belt and it adjoins the teton mountains nine imbricate sheets of the absaroka system form a shingled array of overlapping thrusts most fault nomenclature in the range dates from early discontinuous reconnaissance studies wherein names were applied from the top thrust down whether or not the thrust sheets separated by this method are or ever were connected as mechanical blocks fault names are revised here to consider the lateral continuity of sheets as the major criteria for defining the block to which a name should be applied the absaroka thrust and st john thrust are recognized to be equivalent different parts of a major transfer zone using the continuity of sheets as the main criteria the map pattern of the range is dominated by major re entrants to the west in the traces of the st john and absaroka thrusts the re entrants in both major faults are caused by folding in their foot walls related to lateral ramping of each thrust upward to the southeast the folding of the st john thrust above the indian creek culmination also folds the thrust sheets above it helping to mark the emplacement of the imbricate sheets as being from top to bottom and west to east the absaroka system thrusts formed from 20 to 40 km west of their present position they are folded by underlying faults of the jackson or farther south the darby system indicating that the absaroka system was emplaced into its present position by motion on these underlying faults discussions concerning mechanical interaction between the thrust belt and a foreland buttress ancestral teton or targhee uplift concentrate on intensities of deformation and changes in geometry adjacent to the proposed buttress the individual imbricate geometries and regional changes in trend are more related to a changing stratigraphic package and the lateral thrust ramps than to geographic proximity to any proposed buttress

a synthetic stratigraphic model of foreland basin development the processes of erosion and deposition must be included in foreland basin models to predict correctly basin geometry and stratigraphy we present a synthetic stratigraphic model of the development of nonmarine foreland basins that predicts progressive geometry topography and facies patterns in the model steady crustal shortening occurs according to a wedgethickening model erosion and deposition follow a diffusive process and the lithosphere is compensated elastically erosion and deposition are controlled by the transport coefficients r of the diffusion equation for a range of thrust velocities and lithospheric rigidities transport coefficients are of order 102 103 m yr in the mountain belt these values are much higher than those derived from the study of scarp degradation in the sedimentary basin transport coefficients of order 104 m2 yr are appropriate and are compatible with previous studies of fluvial and deltaic deposition rapid thrusting results in a narrow underfilled basin while slow thrusting results in a wide overfilled basin in addition by varying the erosional and depositional transport coefficients while holding other parameters constant we generate both overfilled and underfilled basins these results suggest that changes in the rate of thrust loading the climate or the source rock lithology can create stratigraphic signatures that have been interpreted to record viscoelastic relaxation of the lithosphere clearly to understand either the long term behavior of the lithosphere or to interpret orogenic history from preserved foreland strata the manner in which a basin was filled must be considered we apply the model to the evolution of the modern sub andean foreland and find that an erosional transport coefficient of 3000 m2 yr and a depositional transport coefficient of 20 000 m2 yr successfully predict the observed basin geometry

structural geology of stewart peak culmination idaho wyoming thrust belt a structural culmination is the highest point along a structural axis or fold system away from which the folds plunge structural culminations are an important and predictable component of foreland fold thrust belts and are prime targets for oil and gas accumulations the stewart peak culmination is a doubly plunging anticlinorium in the absaroka thrust sheet of western wyoming located in the northern salt river range major thrust faults in the stewart peak culmination comprise a large scale duplex fault zone the roof fault of the duplex is the absaroka thrust the floor fault is the basal or regional decollement and the intervening murphy and firetrail thrusts are trailing edge imbricate faults in the darby plate that have locally uplifted and deformed the absaroka into an asymmetric breached anticlinorium or culmination in addition these major thrust faults subdivide the culmination into fault bound structural horses each with a characteristic style of deformation as a function of its stratigraphic and structural level in the duplex a recent exploratory test of the culmination provided data which suggest that the absaroka thrust fault locally cuts downsection in the transport direction this may be explained by initial folding and breaching of the firetrail anticline followed by main motion on the absaroka thrust which overrode the fold the available data suggest that the stewart peak culmination is the result of 1 polyphase uplift or arching of the absaroka thrust sheet by the murphy and firetrail thrust sheets 2 lateral imbrication and ramping and 3 arching of the precambrian basement complex beneath the northern salt river range in addition a basic kinematic model of deformation applicable to structural culminations includes emplacement of an allochthonous mass over a parautochthonous substratum closely followed by or contemporaneous with deformation and translation of the footwall section to produce a duplex fault zone that folds the overlying allochthon the stewart peak culmination bears many similarities in structural style and origin to polyphase folded fault domes in the canadian rocky mountains specifically the stewart peak area is compared to the savanna creek and moose mountain culminations

influence of surface processes and initial topography on lateral fold growth and fold linkage mode elongation of randomly distributed fold segments and their potential linkage are important for hydrocarbon exploration because it can greatly influence the morphology of the reservoir and both migration and accumulation of hydrocarbons in antiformal traps here we study the effects of surface processes and the presence of a topographic slope on the different linkage modes that can occur and how these parameters affect the required horizontal offset for perturbations to link the proposed numerical model represents a sedimentary cover detached over a much weaker basal decollement layer the upper surface is modified by mass redistribution which is achieved by a combination of fluvial and hillslope processes several series of simulations were performed 1 without surface processes or regional slope 2 with regional slope only 3 with fluvial incision and hillslope processes and 4 with hillslope processes only model results show that the presence of a regional slope reduces the critical distance required for the transition between linkage and no linkage modes whereas erosion and redeposition of sediments on the contrary increase this distance the location of the saddle point where fold segments link and its vertical distance to the crests of the anticlines are different compared to the case without erosion or initial topographic slope which potentially can affect the morphology of hydrocarbon traps moreover both erosion and redeposition of sediments enhance the fold elongation growth along the fold axis once the erosion velocity exceeds the folding velocity model results have been compared to the zagros fold belt

ramp initiation and spacing in a homogeneous thrust wedge 1 fold and thrust belts grow largely through sequential initiation of thrust faults and subsequent motion on them from hinterland to foreland i e away from the plate boundary while perturbations in basal geometry stratigraphy excess fluid pressure folding and surface slope have all been suggested as mechanisms for thrust ramp nucleation we show that preexisting inhomogeneities are not necessary we present a mechanically homogeneous finite element wedge model to assess the effects of variations in yield stress dilation angle and basal friction angle on foreland fold and thrust belt evolution computer experiments produce a broad foreland dipping plastic strain band that marks the topographic inflection produced by the previous ramp this band migrates toward the rigid base where the plastic strain is preferentially concentrated in a thrust ramp subsequent ramps develop toward the foreland in a self similar fashion the ramp formation process is systematic ramps form at the same location with respect to the wedge origin regardless of significant variations in yield stress dilation angle or basal friction however the backstop displacement required to initiate a new ramp is dependent on these variables we conclude that the wedge geometry is the primary factor in determining both thrust ramp location and spacing syntectonic erosion and deposition or any other process that changes wedge geometry may in some cases control the location of thrust ramps

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

fault related folding in sandbox analogue models of thrust wedges abstract the nucleation and growth of thrust fault related folds has been simulated using high resolution sandbox coulomb wedge type analogue models thrust related anticlines grew progressively from initial layerparallel shortening to decollement folding thrust tip folding and eventually thrust ramp folding thrust ramps nucleated in the central sector of the overturned forelimb of decollement folds as a consequence of limb thinning thrust tip folding occurred during outward migration of the ramp tips the fault propagation rate was greater at the lower ramp tip which quickly joined the basal thrust flat while folding continued beyond the upper tip the development of the analogue model thrust fault related folds by progressive changes in kinematics from decollement folding to thrust tip folding and finally thrust ramp folding compares well with analyses of naturally occurring thrust fault related folds

preferential rifting of continents a source of displaced terranes lithospheric rifting while prevalent in the continents rarely occurs in oceanic regions to explain this preferential rifting of continents we compare the total strength of different lithospheres by integrating the limits of lithospheric stress with depth comparisons of total strength indicate that continental lithosphere is weaker than oceanic lithosphere by about a factor of 3 also a thickened crust can halve the total strength of normal continental lithosphere because the weakest area acts as a stress guide any rifting close to an ocean continent boundary would prefer a continental pathway this results in the formation of small continental fragments or microplates that once accreted back to a continent during subduction are seen as displaced terranes in addition the large crustal thicknesses associated with suture zones would ironically make such areas likely locations for future rifting episodes this results in the tendency described as the wilson cycle of new oceans to open along the suture where a former ocean had closed

factors that control the development of fault bend versus fault propagation folds insights from mechanical models based on the discrete element method dem abstract we investigate the role and relative importance of a range of geometric and mechanical factors in the development of contractional fault related folds with an emphasis on defining the factors that promote the development of fault bend and fault propagation folds we construct a series of discrete element mechanical models in order to test the effects of fault dip bulk material strength mechanical layer anisotropy and spacing sedimentation rate and boundary conditions on the style of fault related fold that develops we find that fault bend folding is most favored at low fault ramp dips with thinly spaced mechanical layers and strong layer strength contrasts in contrast conditions that inhibit slip on a potential upper detachment surface such as increased friction and a fixed foreland boundary encourage the development of fault propagation folds additionally steeper fault dips more widely spaced mechanical layers and decreased layer strength contrast favor the increased localization of shear during the growth of structures this leads to structures that deform by a mixture of fault bend and fault propagation folding styles observations of the distortional strains that develop in the model provide insight into the relationship between the different deformation mechanisms such as flexural slip and localized shear which accommodate structural growth and ultimately determine fault related folding style thus these models provide a context for understanding how rock and fault properties influence whether structures evolve as fault bend or fault propagation folds or as combinations of these end members we apply these insights to interpret two natural examples from the offshore niger delta outer fold and thrust belt that exhibit changes in structural style through time as a result of changes in fault properties and syntectonic sedimentation

deformation localization in orogens spatiotemporal expression and thermodynamic constraint abstract orogens are spatiotemporal expressions of instabilities in materials under load constrained by thermodynamics and preserved in the cold outer shell of the planet their pressure temperature time histories are consistent with the predictions of differential grade 2 dg 2 materials in pure shear we place the statistically invariant shear localization mechanism of these materials in a coherent thermodynamic context using an analysis of strained elastic materials this prototype system exhibits non classical thermodynamic symmetry breaking where the potentials are all functions of a single variable and the distinction between heat and work fades from view consequently internal energy must be described by a monotonically decreasing function of the entropy in order for heat capacity and absolute temperature to be positive the entropy itself exhibits an inverse dependence on length these constraints are satisfied by the overall shape and slope of the distributed deformation threshold d for dg 2 materials and its noted 1 length correlation with naturally observed folds as a function of thermomechanical competence we predict that temperature in this non linear elastic material will vary in proportion to the slope of d being high at low competence and low at high competence similar constraints apply to a self gravitating body where the energy function varies inversely with radius assigning zero pressure at the surface of the body we also predict that pressure the tensor trace of its stress energy density will vary inversely with radius thus the body force of gravity will be expressed in this elastic self gravitating system through the interplay of elastic and thermal lengths deformation localization in dg 2 materials arises due to the dynamic rescaling of lengths in response to a spike in the intrinsic energy i at while the intrinsic i and localization l thresholds are monotonically decreasing for they exhibit positive slopes at lower competence signaling a return to classical thermodynamics and joule heating in this transitional domain numerous structural and tectonic observations can be correlated using this remarkably simple model beginning with the thickness and mechanical character of the brittle crust and oceanic lithosphere in effect this model projects the kinematic theory of plate tectonics into four dimensional spacetime

kinematic evolution of thrusts wedge and erratic line length balancing insights from deformed sandbox models kinematic evolution of fold thrust structures has been investigated by analogue models that include syntectonic sedimentation different decollement dips and basement thicknesses produced different wedge geometries and propagating characteristics a model with one decollement level was characterized by a closely spaced thrust system during early stages of shortening as compared to the late stages the frequency of fault nucleation was rapid during the early stages of deformation conversely the frequency of fault nucleation was low and thrust spacing was significantly wider in a model with two decollement levels individual faults became locked at steep dips and deformation stepped forward as a new fault nucleated in sequence in front of the older locked structure once the thrust system was established up to 27 overall shortening an overlying bed was introduced to simulate syntectonic deformation model sand wedge did not grow self similarly but rather its length and height increased episodically with deformation restoration of deformed models show that layer parallel shortening accommodated for approximately half of the total model shortening across the multilayers calculated error in apparent layer shortening from the restored layers revealed a direct relation with depth of the layers in the models the experimental results are comparable to a natural example from the northern apennines fold and thrust belts

is extension in death valley accommodated by thinning of the mantle lithosphere beneath the sierra nevada california while relatively rapid and large scale extension occurred in the death valley region of california during neogene time little or no shallow extension occurred in the adjacent sierra nevada this contrast in tectonic history has often been extrapolated to include the entire lithosphere but geophysical and geologic observations indicate that more extension of the mantle lithosphere has occurred under the sierra than under the death valley region upper mantle seismic velocities observed beneath the high sierra are lower than those observed in other regions with comparable surface heat flow this discrepancy could be resolved if the mantle lithosphere beneath the high sierra had become warmer presumably by tectonic thinning in the last 10 m y upper mantle seismic velocities averaged topography and bouguer gravity anomalies all are consistent with the presence of thinner mantle lithosphere beneath the high sierra than beneath the california portion of the basin and range province to the east this suggests that extension of the crust near death valley might be accommodated at a deeper level by thinning of the mantle lithosphere beneath the sierra nevada the extension in the crust of the california basin and range province and the thinning of the mantle lithosphere under the high sierra appear to share the same bounds in time and space the uplift of the high sierra occurred over the past 9 m y which coincides with most of the extension that occurred in the california basin and range province because the orientation of extension in the california basin and range province is inferred to be approximately n60 w from geologic geodetic and in situ stress measurements the northern and southern edges of the death valley extensional subprovince may extend n60 w from the inferred northern and southern limits of west dipping low angle normal faults of the death valley region pronounced changes in the averaged topography and bouguer gravity anomaly across these two bounds both in the basin and range province and in the sierra nevada support a connection between the tectonics of both regions the geomorphic history of the southern sierra suggests an up to the north warp of the sierra across this southern bound during latest cenozoic time hence extension near death valley may be localized in the crust and may be laterally connected to thinning of the mantle lithosphere beneath the sierra nevada this geometry requires