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

regional tectonics and fracture patterns in the fall river formation lower cretaceous around the black hills foreland uplift western south dakota and northeastern wyoming

joint development in perturbed stress fields near faults abstract field evidence is presented for complex spatial and temporal perturbations of an otherwise systematic joint pattern around faults from well exposed faulted rock platforms joints propagating in perturbed stress fields will curve to follow the directions of the stress field trajectories a progressive change in joint direction is observed from unperturbed regions away from faults to strongly perturbed zones adjacent to faults this indicates that the joint pattern can reflect perturbations of the regional stress field around faults in the examples the stress field perturbations are probably due to points of high friction on the fault plane which concentrate stress and distort the stress field in the surrounding rock the corresponding joints converge at these points and are sub parallel to the fault along the remainder of the fault plane the possibility that a fault plane acts as a free surface contained within an elastic body is considered in this situation the fault plane induces a rotation of the principal stress axes to become either perpendicular or parallel to the fault the free surface model seems to explain the metre scale curvature of joints in the vicinity of existing joints but at the kilometre scale of a large fault plane the model becomes unrealistic unless the fault is open at the earth s surface two examples are investigated from the lias of great britain at nash point and robin hood s bay both comprise sub horizontal strata of relatively homogeneous lithology and bed thickness which provide striking examples of joints developed near faults

kinematic analysis of a large scale leading edge fold lost river range idaho abstract the mahogany creek buck creek structure within the lost river range idaho is a large scale leading edge fold which developed in late paleozoic outer shelf stratigraphy during the sevier orogeny incremental strain histories determined from antitaxial fibrous pressure shadows are used to quantify temporal variations in the magnitude and orientation of elongation as a function of structural position around the leading edge of this northward and eastward propagating blind thrust plane strain is indicated by coaxial up dip extension on cleavage planes on the backlimb of the structure the bluebird mountain formation a calcareous sandstone has deformed by flexural flow whereas the forelimb of the structure records spin through a fixed steeply plunging incremental extension direction the flat limb ahead of the anticline exhibits top to the foreland simple shear in contrast the underlying surrett canyon formation a thick bedded limestone has experienced flexural flow localized within thin zones on both limbs of the anticline samples from the anticlinal hinge zone suggests kinematic partitioning with pin lines which were variably distributed within each unit and which were temporally transient finite elongations are greatest in the hinge and above the foreland flat in the bluebird mountain formation but are negligible in the surrett canyon formation except within the mechanically active interbeds of the hanging wall and within the footwall adjacent to the fault along the northernmost exposures strain histories in the hanging wall do not vary significantly along strike while the footwall varies from undeformed in the south to penetratively deformed in the north thus the strain data are consistent with self similar along strike fold development in the hanging wall during strike parallel ramp propagation forward thrust propagation may have been blunted by a combination of fold accommodated shortening and footwall deformation to the north once the fold had tightened and fold shortening required greater differential stress forward fault propagation resumed transporting the anticline onto the upper flat with little change in hanging wall fold geometry or strain distribution

mechanical constraints on the chronology of fracture activation in folded devonian sandstone of the western moroccan anti atlas the three dimensional meter scale fracture networks observed on exposed folds between the towns of tata and akka western moroccan anti atlas consist mostly of planar discontinuities which are sub perpendicular to the bedding and partitioned in three main sets the chronology of their activation is proposed in five stages since the hercynian orogeny stage 1 predates folding and involves the horizontal compression of the emsian sandstone it involves fracture set i composed of systematic joints parallel to the direction of compression stages 2 4 correspond to the folding and are marked in the outer arc by the activation of fracture set ii composed mainly of joints parallel to the fold axial plane stage 5 is a regional shear event during which sets i and iii separated by an angle close to 608 are activated in a conjugate manner to throw light on the recurrent difficulty in discriminating between activation of inherited and new fractures an elasto plastic model is used to construct a stress path in the pervasively fractured medium idealized as a continuum each fracture set obeys the mohr coulomb criterion truncated in tension to describe both sliding and opening activations finite element simulations of a simple buckling event accounting for the field fracture sets are presented it is shown that set i cannot be generated by folding and thus does belong to stage 1 set ii is activated at a later stage of folding than expected from the field interpretation set iii cannot be activated during stage 2 confirming its role in stage 5 the advantages and limitations of the proposed modeling are finally discussed q 2002 elsevier science ltd all rights reserved

mechanical analysis of the geometry of forced folds a mechanical model of forced folding comprised of an anisotropic cover overlying displaced rigid basement blocks is used to investigate the influence of various parameters on theoretical fold form shape and dip of basement fault strength of basement cover contact and degree of anisotropy of the cover we show that the degree of anisotropy in the cover largely influences the geometry of the forelimb of the forced fold folds produced in isotropic cover display forelimbs that taper from large dip angles near the basement cover contact to low dip angles at the ground surface in contrast dips in the forelimbs of folds in anisotropic cover are nearly uniform with depth we show that the basement cover contact and the shape of the basement fault largely influence the geometry of the backlimb backlimb rotation occurs in cover welded to the basement and in cover underlying curved basement faults in addition the kinematic features of the theoretical folds are compared with the fold geometry generated by parallel kink and trishear models folds in isotropic cover overlying straight basement faults closely resemble the fold forms produced by the trishear kinematic model while fold forms in anisotropic cover more closely resemble folds produced by parallel kink geometric constructions

pseudo 3 d modeling of trishear fault propagation folding abstract basement structures to which trishear fault propagation models have most successfully been applied are commonly three dimensional folds formed at the tip line of a fault we present here a pseudo 3d trishear model in which various parameters are permitted to vary along strike and oblique slip can be modeled these variations may be combined in an infinite number of ways facilitating the simulation of many real structures a thrust changing from blind to emergent can be produced by a change in the slip or propagation to slip p s along strike folds with forelimbs changing from overturned to upright along strike can be modeled either by changing the slip p s or trishear angle also some minor folds perpendicular or oblique to the main structure can result from changes in the trishear angle or fault angle along the strike models including growth strata show that it is practically impossible to distinguish between growth and pre growth strata using the map patterns as a field test we have modeled the oblique slip east kaibab monocline demonstrating a good fit between the field observations and model predictions

the effect of non parallel thrust fault interaction on fold patterns the geometry of two subsurface non parallel thrust faults are systematically altered within mechanical models to determine the range of resulting fold map patterns and the effect of fault interaction on these fold patterns and ultimately to infer fault geometry from complex fold shapes multiple faults can create both complex fold patterns with more than one fold axis and single folds without evidence of a second fault fold length may not correlate to fault length when remote contraction is oblique to the faults thus contraction direction should be determined before fault length is inferred the influence of fault interaction is studied by comparing the fold pattern created by interacting faults to the fold pattern created from the superposition of folds produced by equivalent but isolated faults under small strains larger faults tend to have greater interaction but even interacting faults may not produce fold patterns notably distinct from the superposed fold patterns this suggests that fault interaction need not necessarily be considered when inferring fault geometry from fold shape in the field however under non coaxial strain conditions fold patterns may differ from those presented here

numerical modeling of trishear fault propagation folding in contrast to kink band migration modeling methods trishear numerical models produce fault propagation folds with smooth profiles and rounded hinges modeled fold hinges tighten and converge downward within a triangular zone of distributed deformation which is focused on the fault tip such features have been reported from field studies and are also seen in analogue models of compressional deformation however apart from its initial application to laramide folds little quantitative work has been undertaken on trishear fault propagation folding in other settings in addition no study has been undertaken into the growth strata which might be associated with such structures this paper uses an equivalent velocity description of the geometric model of trishear together with models of erosion and sedimentation to investigate trishear fault propagation folding of both pregrowth and growth strata the trishear model is generalized to include a variety of fault propagation to slip ratios and fault propagation from a flat decollement the models show continuous rotation of the forelimb with the characteristic development of cumulative wedges within growth strata when total slip on a structure is high the model predicts overturned pregrowth and growth strata during the initial stages of deformation beds in the forelimb thicken but later thin when they become steep or overturned the effect of variations in fault propagation to slip ratios on two dimensional finite strain in the models is assessed by the use of initially circular strain markers high fault propagation to slip p s ratios lead to narrow zones of high finite strain while lower p s ratios lead to more ductile deformation and broader zones of high strain in all cases hanging wall anticlines and footwall synclines originate as early ductile folds which are later cut by the propagating fault modeled structures are compared with natural examples

origin by fractional crystallization of transitional basalt for the asela ziway pantellerites crustal control in the genesis of plio quaternary bimodal magmatism of the main ethiopian rift mer geochemical and isotopic sr nd pb evidence by trua et al 1999

elastic stress control on the pattern of tensile fracturing around a small fault network at nash point uk abstract complex trajectories of fault related opening mode fractures at nash point in south wales uk are consistent with elastic stress fields computed for the observed fault network subject to north south compression the linear elasticity theory is used to calculate local stress increments produced by slip on the fault network faults are assumed to be free of shear stress and to slip in response to some remote stress load brittle failure of rock within this stress field is determined according to a combined griffith coulomb criterion discrete fracture networks are simulated according to simple growth rules where fractures initiate according to the state of brittle failure and propagate along a surface normal to the direction of most tensile local stress results demonstrate the dominant opening mode fracture were caused by a uniform body force e g unloading and are just locally perturbed by the pre existing fault network fault heights of 2 3 times the mean fault block width and regional contraction along an azimuth of 180 10 are required to reproduce the observed fracture pattern independent kinematic analysis of nearby faults yield estimates of the azimuth of regional contraction of 170 180 some features in the observed fracture pattern are not well reproduced but these are small scale and related to small scale irregularities on the fault planes

backlimb trishear a kinematic model for curved folds developed over angular fault bends abstract numerous analog models and some natural examples display smoothly curving backlimb fold hinges over sharp angular bends in the underlying thrust surface we present a new kinematic model that can reproduce this geometry by defining a triangular zone focused on the fault bend the model presupposes incompressible flow in the triangular zone and makes common assumptions about velocities on either side of the fault bend either symmetric or asymmetric triangular zones can be defined with parallel kink folding and similar folding being special cases of the two respectively where the apical angle of the triangular zone is zero the results of the numerical model compare well to analog experiments the model is conceptually analogous to forelimb trishear at the tip of a propagating fault and hence we refer to it as backlimb trishear

displacement and deformation associated with a lateral thrust termination southern golden gate range southern nevada u s a abstract the golden gate thrust offers an excellent opportunity to study displacement and deformation at a lateral thrust tip slip across this e vergent thrust is uniformly about 2 4 km for the southern 2 km of exposed strike length and dies out to zero in the northern 5 km constant displacement and uniform hangingwall structure along the southern one third of exposed strike length suggest that along strike displacement variation and consequent deformation are localized near the lateral tip structures located at and north of the lateral tip are consistent with transpression and later uplift as slip accumulated on the thrust fold and fault orientations and calcite twinning strain record possible transpression north of the lateral tip e w striking normal faults at the thrust tip record extension that is interpreted to be the result of the uplift and translation of a structurally continuous rigid block located north of the normal faults this block is connected to and was uplifted with the crest of the hangingwall anticline and was separated by the normal faults from thrust related folding to the south the main hangingwall structure is an anticline that in southern exposures trends n s parallel to the thrust but bends westward and opens into a box fold to the north the hangingwall anticline superficially resembles a classic fault propagation fold formed by a migrating ductile bead however we are forced to reject the ductile bead hypothesis for the fold because 1 the geometry of the hangingwall anticline far from the thrust tip cannot have evolved from the geometry at the thrust tip and 2 twinned calcite strain data from along the thrust suggest that strain was homogeneous and coaxial rather than inhomogeneous and progressively non coaxial as predicted for a migrating ductile bead the three dimensional structural and kinematic relations at the lateral tip of the golden gate thrust appear to be the result of deformation around a pinned tip although the reason that the tip was pinned at its present location is unclear we interpret the thrust to have propagated relatively quickly to its ultimate extent with little initial displacement indicating that it was easier to fault the rocks than to fold them

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 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

syn tectonic sedimentation associated with three dimensional fault bend fold structures a numerical approach abstract syn tectonic sediments growth strata in compressional structures are three dimensional 3d features whose geometry reflects both the style of deformation and the distribution of accommodation space in the system previously numerical modelling of growth strata in compressional settings has generally been done in two dimensions in the plane of the transport direction in contrast in this paper we examine growth strata associated with a 3d fault bend fold that exhibits variations in slip along strike a forward numerical modelling approach is used to investigate the 3d geometry of syn tectonic sediments under a variety of conditions we consider a fault bend fold in which slip decreases linearly from the centre of the structure to zero displacement at the edges in a direction perpendicular to transport we make use of kink band kinematics to describe the growth of the structure two specific models are presented which consider both 1 background regional sedimentation and 2 local erosion transport and sedimentation the first model approximates structures developed in submarine environments where structural growth may lead to the development of a bathymetric high the model shows considerable lateral changes in the geometries of growth strata along the structure with unconformities being limited to regions of high displacement and with growth triangles being variably developed but in some circumstances difficult to recognise the subaerial model approximates structures developed in settings where sediments simply aggrade to the local base level in this model unconformities are much more prominent as growth sedimentation is often limited to regions of lower displacement the results presented here emphasise the utility of using 3d information on stratigraphic architectures to decipher the growth style and deformational history of natural structures in particular we show that cross sections oblique to the transport direction may contain complex and highly variable pre and syn tectonic geometries that reflect both kink band migration and limb rotation the use of oblique two dimensional 2d information to interpret 3d structures could produce misinterpretations if the complexities of stratigraphic configurations in three dimensions are not taken into account

the giant shakhdara migmatitic gneiss dome pamir india asia collision zone 2 timing of dome formation 1 cenozoic gneiss domes exposing middle lower crustal rocks cover 30 of the surface exposure of the pamir western india asia collision zone they allow an unparalleled view into the deep crust of the asian plate we use titanite monazite and zircon u th pb mica rb sr and 40ar 39ar zircon and apatite fission track and zircon u th he ages to constrain the exhumation history of the 350 90 km shakhdara alichur dome southwestern pamir doming started at 21 20 ma along the gunt top to n normal shear zone of the northern shakhdara dome the bulk of the exhumation occurred by nnw ward extrusion of the footwall of the crustal scale south pamir normal shear zone along the southern shakhdara dome boundary footwall extrusion was active from 18 15 ma to 2 ma at 10 mm yr slip and with vertical exhumation rates of 1 3 mm yr it resulted in up to 90 km n s extension coeval with n s convergence between india and asia erosion rates were 0 3 0 5 mm yr within the domes and 0 1 0 3 mm yr in the horst separating the shakhdara and alichur domes and in the southeastern pamir plateau rates were highest along the dome axis in the southern part of the shakhdara dome incision along the major drainages was up to 1 0 mm yr thermal modeling suggests geothermal gradients as high as 60 c km along the trace of the south pamir shear zone and their strong n s variation across the dome the gradients relaxed to 40 45 c km since the end of doming

effect of initial fault geometry on the development of fixed hinge fault propagation folds abstract this paper describes how a model of fixed hinge basement involved fault propagation folds may be adapted to apply to thin skinned thrust faults to generate footwall synclines fixed hinge fault propagation folding assumes that the fold axial surfaces diverge upwards fold hinges are fixed in the rock the fault propagated through the forelimb thickness changes occur in the forelimb and the forelimb progressively rotates with increasing displacement on the underlying fault the original model for fixed hinge fault propagation folds was developed for the case of a planar fault in basement with a tip line that was at the interface between basement and the overlying sedimentary cover rocks the two geometries applicable to thin skinned thrusts are for the cases where a fixed hinge fault propagation fold develops above an initial bedding parallel detachment and an initial fault ramp of constant dip which flattens down dip into a bedding parallel detachment

two dimensional simulation of controls of fracture parameters on fracture connectivity two dimensional fracture simulation is conducted to analyze the controls of different fracture parameters variations in fracture orientation density and length on fracture network connectivity three different scenarios which are commonly encountered in natural fracture systems are analyzed 1 a single fracture set 2 two fracture sets with one primary through going set and 3 two fracture sets with approximately equal parameters the modeling reveals that certain parameters are more dominant in controlling the connectivity for each of the settings for a single set of fractures increases in length and dispersion and a decrease in spacing all result in higher fracture parallel connectivity but the decrease in spacing is the most important in increasing fracture normal connectivity especially where the dispersion in fracture strike is very low simulations of two sets of fractures reveal that the density length and angle between the two sets are important factors in producing complete connectivity in cases where one set of fractures is a systematic throughgoing set a critical combination of length of the second set and the angle between the two sets results in complete connectivity where both sets of fractures have varying length and density the influence of increasing density of one set has a great effect on connectivity when the other set is short and a more subtle to insignificant change when the other set is long the network also shows higher connectivity with increasing angles up to 90 between the two sets

fracture patterns in successive folding in the western sichuan basin china abstract a detailed field survey of fractures was carried out in the pingluoba and adjacent anticlines in the western sichuan basin to discuss fracture development during progressive folding including the relative timing of fracture formation the successive deformation in a rock layer the distribution of fracture sets in folds and the variation of fractures along with the subsequent deformation of rock layers based on a comparison of the diverse fracture patterns in folded strata at different stages of fold growth horizontal low angle dipping high angle dipping and vertical layers a model of fracture development related to fold evolution was built up it is suggested that the strata were initially under uniform compression and two sets of planar conjugate shear fractures were formed the orientations of which would vary along with the subsequent folding of strata then during progressive folding the deformation of rheologically hard layers was controlled by tangential longitudinal strain inducing the development of a set of tensile fractures that struck parallel to the fold axis layer parallel shear was predominant in the soft layers and two sets of bedding oblique dextral shear fractures intersecting with the bedding at angles of 25 40 and 160 were developed in the progressive folding process the shear fractures were developed later than the tensile fractures a polar stereonet of fractures was generated and applied to improve the efficiency and accuracy of fracture identification in the field providing precise evidence for the exact analysis of fracture patterns and rock deformation

the role of bed parallel slip in the formation of blind thrust faults abstract a finite element code is used to model the mechanical behaviour of elastoplastic sedimentary layers that are deformed by the movement of a faulted deformable basement the propagation of the blind thrust fault is analysed in terms of the evolution of strain localization in the elastoplastic layers of strain softening and strain hardening materials the role of layer thickness bedding parallel slip and fault dip are also studied we found that if no interlayer slip is allowed blind thrusts tend to propagate with constant dip regardless of the rheological properties of the sediment layers however if bedding parallel slip is present a mechanical decoupling appears between layers allowing stress transfers along large distances and favouring the formation of backthrusts fault dip also has significant influence on the qualitative behaviour when interlayer slip is possible a steep fault will tend to split

predicting the orientation of joints from fold shape results of pseudo three dimensional modeling and curvature analysis we treat layers of sedimentary rock as elastic plates and predict the orientations of joints by assuming that they open parallel to the maximum instantaneous stretch of a layer because the direction of maximum instantaneous stretch is parallel to the maximum curvature of a surface we hypothesize that joints will trend parallel to the minimum curvature of an elastically deformed layer after constructing pseudo three dimensional trishear models of laramide style uplifts that grow self similarly we calculated the direction of minimum curvature axes during the evolution of the fold our analysis of minimum curvature axes in evolving folds suggests several important characteristics for fold related joint sets 1 joints that are neither parallel nor perpendicular to the fold axis may be induced by local fold related strains 2 at any time during folding joint orientations may vary according to the structural position on a fold 3 at any location on a fold joint orientation may depend on when a joint forms during the evolution of the fold and 4 joint patterns in trishear folds may vary with stratigraphic position natural folds that evolve along simple geometric pathways may develop fold related joint sets the orientation dominance abutting relationships spacing and continuity of which will vary systematically throughout the structure this variation in joint system architecture may reflect the history of fold growth

activation of diffuse discontinuities and folding of sedimentary layers 1 folding of sedimentary layers is often accommodated by the opening or sliding of inherited and new discontinuities which are assumed here to be diffuse so that a continuum description applies at the fold scale the rock rheology is then described with an elastoplasticity model for which the permanent deformation is of simple shear sliding or dilation opening with respect to specific orientations of the new or inherited diffuse discontinuities to illustrate the relation between folding and activation of diffuse discontinuities a three dimensional layer under compression in the two horizontal directions and sustaining the overburden lithostatic pressure is studied cylindrical buckling occurs either before elastic or after the diffuse discontinuities have been activated if buckling is elastic inherited vertical discontinuities striking obliquely to the fold geometrical axes are activated in a sliding mode in the outer arc leading to a rotation of the principal stress directions opening is then detected across new vertical planes striking obliquely to the fold axis the activation of inherited or new vertical discontinuities can be suppressed if sliding takes place along weak bedding interfaces alternatively early and homogeneous layer parallel shortening marked by a reverse fault mode drastically reduces the critical buckling load compared to the euler load and modifies the final geometry of buckling which is then more of a circular dome shape the switching in buckling mode results in the fold limbs in a change from the early reverse fault to a strike slip fault sliding and to opening across diffuse planes oriented consistently with the final circular structure

combining outcrop data and three dimensional structural models to characterize fractured reservoirs an example from wyoming combining a detailed outcrop characterization of fracture and fault occurrence with attributes from a three dimensional model of an anticlinally folded clastic reservoir body we determine which characteristics of the structural form and evolution are most closely related to the development of important reservoir scale structures our example reservoir body studied is the frontier formation 1 sandstone in oil mountain an asymmetric anticline on the western flank of casper arch in central wyoming the three dimensional model of the structure was constructed using an iterative scheme designed to maximize interpretation accuracy and precision the model was analyzed to determine the spatial variance in morphologic and kinematic attributes using a quantitative testing approach we found that the intensity of tectonically produced fractures is closely related spatially to rate of dip change and total curvature with the former having the strongest correlation this folding is a low strain process compared to tear faulting which has the strongest spatial correlation to larger strains the location and magnitude of these higher strain areas can be adequately predicted by three dimensional restoration and forward modeling of the upper bounding surface of the reservoir body we use these results to build a predictive model for fault and fracture distribution at oil mountain and to discuss how this approach can aid in the exploitation of analogous producing reservoirs

the role of fractures in the structural interpretation of sheep mountain anticline wyoming the development of fractures in the sedimentary layers of sheep mountain anticline a laramide asymmetric fault cored fold of the bighorn basin is documented and interpreted to constrain the kinematic evolution of the fold the fracture pattern is interpreted to identify five main fracture sets a fracture set striking 110 set i oblique to the future fold trend is interpreted as a regional fracture set that predates laramide compression a joint set set ii striking 045 and present in the hinge and backlimb is related to the ne oriented laramide compression just prior to and during initial anticline growth joints striking 135 set iii parallel to the fold trend are found within the hinge and are interpreted to have developed in response to the bending of layers the two youngest fracture sets are attributed to a late stage of fold growth a joint set set iv in the backlimb striking parallel to the set i fractures but vertical and a fracture set in the forelimb consisting of set i fractures reactivated as reverse faults the relative chronology mode of formation opening vs shearing and structural locations of these fractures suggest the following kinematics of folding at sheep mountain anticline little or no lateral fold propagation no hinge migration and limb rotation with only minor limb flexure and stretching

characterization of stress perturbations near major fault zones insights from 2 d distinct element numerical modelling and field studies jura mountains abstract we consider and discuss the presence of discontinuities in the crust as a major source of stress perturbations based on 2 d distinct element modelling we reconstruct the local stress field around a vertical discontinuity in various geological contexts the resulting stress distribution reveals that major directional stress changes occur near the tips of the discontinuity so that stress deviations can reach values as large as 50 we establish simple relationships controlling stress changes around a pre existing fault zone as a function of 1 the remote differential stress magnitude 1 3 2 the friction coefficient on the discontinuity and 3 the strike of the discontinuity relative to the far field stress as a geological example we present the morez fault zone in the internal jura paleostress reconstruction in forty two sites indicates that the trends of the mio pliocene compression are n110 on average near the fault whereas they are n130 in the surrounding areas a comparison between the results of the tectonic study and those of theoretical modelling suggests that the 20 counterclockwise deviation is directly related to the reactivation of this large weak zone we thus evaluate the role of mechanical decoupling along pre existing zones of weakness especially with consideration to the accommodation of the alpine deformation in the jura belt

elastic modeling of fault driven monoclinal fold patterns abstract the traces of buried dip slip faults are often reconstructed by assuming that they follow the plan view patterns of the overlying monoclines and by interpolating between seismic profiles to place constraints on such reconstructions the uplift pattern produced by prescribed slip on a set of buried reverse faults is calculated assuming that the overall plan view fold pattern is determined by faulting induced elastic strain while inelastic relaxation and gravitation only modify the details of the finite cross sectional geometry of the folds faults are represented by dislocation planes embedded in an elastic medium which is otherwise continuous and uniform these assumptions are applied to the monoclines of the negev israel driven by high angle reverse faults a first model closely follows the existing structural map which is based on field mapping and seismic profiles and is characterized by long continuous faults slip is taken as uniform and proportional to fault length such a model does not produce either the pattern or the relative structural elevation of the overlying monoclines and in particular the observed fold axis variations in a second model we introduce fault en echelon discontinuities and slip gradients at fault terminations relative slip on faults underlying various structures is proportional to their observed structural elevations these features produce a reasonable approximation of the overall observed structural configuration of the monoclines it can thus be concluded that 1 the overall patterns of fault driven folds are determined by the coseismic elastic strain field 2 fault segmentation is a major cause of observed axis undulations of fault driven folds 3 finite dip slip offset at depth is proportional to the observed structural elevation and 4 along strike slip gradients account for variations of the structural elevation in that direction

little sand draw field big horn basin wyoming a hybrid dual porosity and single porosity reservoir in the phosphoria formation discussion little sand draw field produces hydrocarbons from an asymmetric anticline located on the southwestern margin of the big horn basin this 479 ac 32 well field has produced 10 4 million bbl of oil most production is from the ervay member of the permian phosphoria formation on the basis of gamma ray logs and textural evidence the dolomitized 90 100 ft 27 5 30 5 m thick ervay member can be subdivided into six reservoir zones zones are bounded by marine flooding surfaces and each zone is laterally consistent throughout the field lower ervay zones which were deposited under low energy open marine conditions are predominantly mud supported rocks higher energy yet more restricted marine conditions occurred during deposition of the upper ervay and resulted in a domin nce of grain supported rocks moldic and micro intercrystalline porosity occurs in four zones moldic pores dominate total porosity and have significant hydrocarbon storage capacity intercrystalline pores control matrix permeability which is generally less than 20 md such permeabilities cannot account for observed production rates of several hundreds to thousands of barrels of fluid per day open fractures are present in all zones and contribute significantly to reservoir drainage fracture intensity is controlled by lithology and structural position silicified strata are the most highly fractured structurally wells with the greatest fracture intensity are aligned along the northwest trending hingeline of the fold however in much of the reservoir flow behavior is dominated by east northeast trending fract res which are recognized on borehole imaging logs and as directional permeabilities from pressure interference tests most but not all wells are at or near virgin reservoir pressure this along with high water production rates suggests strong aquifer support for reservoir pressure a comparison of pressure and water cut data shows that those wells with depleted reservoir pressure also had water cuts that rose relatively slowly geographically these wells define dual porosity compartments in which matrix porosities and permeabilities as well as fractures affect flow elsewhere in the field fractures are the dominant control on flow in what appears to be single porosity behavior

the foreland deformation in the south western basque cantabrian belt spain abstract in the south western portion of the cenozoic basque cantabrian belt structural data collected in pre orogenic upper cretaceous limestones were combined with aerial images interpretation in flat lying areas observations about fracture strike type orientation and overprinting relationships both at the macro and mesoscales allow to constrain the pre thrusting and pre folding deformational sequence that affected the area when it was forming part of the foreland basin wnw ese striking extensional structures represent the oldest elements developed during the very late stages of the mesozoic rifting associated with the opening of the bay of biscay the onset of convergence is marked by the development of extensional structures striking roughly nnw sse and wsw ene which are associated with belt parallel stretching in the foredeep and outer arc extension in the peripheral bulge respectively and developed in a non fluid assisted environment these elements are postdated by a pre folding layer parallel shortening pattern which includes wsw ene striking pressure solution cleavages and reverse faults both of them frequently reactivating inherited early orogenic wsw ene striking extensional elements and right lateral reactivation of wnw ese striking pre orogenic extensional structures this stage was followed by a syn to post folding e w oriented belt parallel compression wnw ese striking left lateral faults being the most outstanding elements associated with this stage the presence of such elements in upper cretaceous sediments provides support to the idea that left lateral shear zones of the area related with a cenozoic intrabelt lateral extrusion tectonics our results indicate coherently with previously published works in the pyrenean orogen that most of the mesoscale deformation has been acquired before layer s tilting

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

analysis of microstructures in the meade plate of the idaho wyoming foreland thrust belt u s a abstract kinematic histories of several imbricate slices of the meade plate in the idaho wyoming foreland thrust belt were determined using analyses of e lamellae in calcite f lamellae in dolomite and sub basal i deformation lamellae in quartz supporting these data were analyses of stylolites strained fossils and other markers and vein filled extension fractures compression was always parallel or sub parallel to bedding on upright fold limbs and on overturned fold limbs was either sub parallel or more commonly at a high angle to bedding extension in both limbs was generally either perpendicular to bedding or parallel to the fold axes observations in the hinge zones of folds were inconsistent while near thrust faults the microstructures record compression at a high angle to the nearby thrust surface measurement of different minerals in the same sample as well as cross cutting stylolite vein relations indicate that early compression was parallel to bedding while later compression was at a high angle to bedding these data suggest that regional shortening and probably compressive stress was horizontal or sub horizontal throughout the deformation and that most of the strain within thrust plates is directly related to folding and not thrusting the following structural history is proposed 1 1 initial layer parallel compression 2 2 buckle folding soon yielding to more complex folding in a three dimensional strain field 3 3 locking of folds and 4 4 locking of thrusts the fold locking angle as well as fold geometry indicates that these structures might be characterized as kink folds but the asymmetry is not consistent with theoretical analyses of kink folds strain due to basal traction along the fault surface is only important within 50 200 m of that surface depending on lithology

an alternative model for the growth of faults conventional growth models suggest that faults become larger due to systematic increases in both maximum displacement and length we propose an alternative growth model where fault lengths are near constant from an early stage and growth is achieved mainly by increase in cumulative displacement the model reconciles the scaling properties of faults and earthquakes and predicts a progressive increase in fault displacement to length ratios as a fault system matures this growth scheme is directly applicable to reactivated fault systems in which fault lengths were inherited from underlying structure and established rapidly the model may also apply to some non reactivated fault systems near constant fault lengths during subsequent growth are attributed to retardation of lateral propagation by interaction between fault tips the model is validated using kinematic constraints from growth strata which are displaced by a system of reactivated normal faults in the timor sea nw australia

mesostructural development in detached folds an example from west virginia an alleghanian duplex of three rootless anticlines in the central appalachians containing oriskany sandstone provides an example of mesostructural development in detached folds these mesostructures which include stylolites veins vein arrays joints and faults developed before during and after duplex formation non alleghanian mesostructures are the most ubiquitous joint sets began forming during the late pennsylvanian in response to burial stresses and fluid overpressure subsequent pre alleghanian n s compression reactivated the joint trends to form stylolites and vein arrays again in the presence of large fluid pressures during alleghanian deformation mesostructures developed only locally in the folds in response to layer rotation interlayer slip and hydrofracturing during thrusting during alleghanian and post alleghanian uplift and erosion thermal elastic relaxation and arc length changes reactivated a joint trend to form new joints the resulting varied distribution of mesostructures