factors that control the angle of shear bands in geodynamic numerical models of brittle deformation abstract numerical models of brittle deformation on geological timescales typically use a pressure dependent mohr coulomb or drucker prager plastic flow law to simulate plastic failure despite its widespread usage in geodynamic models of lithospheric deformation however certain aspects of such plasticity models remain poorly understood one of the most prominent questions in this respect is what are the factors that control the angle of the resulting shear bands recent theoretical work suggest that both roscoe 45 coulomb angles 45 2 where is the angle of internal friction and arthur angles 45 4 where is the dilation angle as well as all intermediate angles are possible published numerical models however show a large range of shear band angles with some codes favoring arthur angles whereas others yield coulomb angles in order to understand what causes the differences between the various numerical models here i perform systematic numerical simulations of shear localization around an inclusion of given length scale both numerical element type geometrical and rheological viscoplastic versus viscoelastoplastic effects are studied results indicate that the main factor controlling shear band angle is the non dimensional ratio between the length scale of the heterogeneity d and the size of the numerical mesh x coulomb angles are observed only in cases where the inclusion is resolved well d x 5 10 and in which it is located sufficiently far from the boundary of the box in most other cases either arthur or roscoe orientations are observed if heterogeneities are one element in size coulomb angles are thus unlikely to develop irrespective of the employed numerical resolution whereas differences in element types and rheology do have consequences for the maximum obtainable strain rates inside the shear bands they only have a minor effect on shear band angles shear bands initiated from random noise or from interactions of shear bands with model boundaries or other shear bands result in stress heterogeneities with dimensionless length scales d x 1 2 such shear bands are thus expected to form roscoe or arthur orientations consistent with the findings in previous numerical models

making coulomb angle oriented shear bands in numerical tectonic models abstract localization of shear strain can be induced by strain weakening plasticity in continuum models and is often used to represent faults of various scales the orientation of the shear bands is thus required to be consistent with those observed from natural faults and with simple theories of brittle failure although the coulomb angle at which a shear band satisfies the coulomb failure criterion is widely used as an assumed fault orientation the currently available numerical techniques do not always produce shear bands oriented at this angle we demonstrate that under an associated plastic flow rule for which dilation and friction angles are equal the coulomb angle becomes a unique initial shear band orientation regardless of the numerical methods employed and model resolution the known problem of overly expanding shear bands in case of a constant dilation angle is preventable if dilation angle is reduced as shear strain along the shear band increases this treatment corresponds to natural processes reducing roughness of a fault plane

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

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

mechanics of blind thrusting comparison of numerical and physical modeling abstract physical models and numerical simulations offer alternative approaches to developing a mechanical understanding of deformation histories this contribution 1 compares results from two independent studies one employing the finite element method the other using dynamically scaled physical models of blind thrusting and 2 discusses the implications that these results have for the role that boundary conditions play in thrust system mechanics the finite element models were designed to assess roof sequence response to duplex emplacement whereas the physical modeling focused on triangle zone evolution despite this difference both studies address the same fundamental question how does a roof sequence respond to the propagation and displacement of blind thrust faults the emplacement of thrust horses leads to a deformation partitioning in the overlying roof sequence deformation is partitioned between translation and internal distortion our results show that one of the controls on deformation

benchmarking numerical models of brittle thrust wedges we report quantitative results from three brittle thrust wedge experiments comparing numerical results directly with each other and with corresponding analogue results we first test whether the participating codes reproduce predictions from analytical critical taper theory eleven codes pass the stable wedge test showing negligible internal deformation and maintaining the initial surface slope upon horizontal translation over a frictional interface eight codes participated in the unstable wedge test that examines the evolution of a wedge by thrust formation from a subcritical state to the critical taper geometry the critical taper is recovered but the models show two deformation modes characterised by either mainly forward dipping thrusts or a series of thrust pop ups we speculate that the two modes are caused by differences in effective basal boundary friction related to different algorithms for modelling boundary friction the third experiment examines stacking of forward thrusts that are translated upward along a backward thrust the results of the seven codes that run this experiment show variability in deformation style number of thrusts thrust dip angles and surface slope overall our experiments show that numerical models run with different numerical techniques can successfully simulate laboratory brittle thrust wedge models at the cm scale in more detail however we find that it is challenging to reproduce sandbox type setups numerically because of frictional boundary conditions and velocity discontinuities we recommend that future numerical analogue comparisons use simple boundary conditions and that the numerical earth science community defines a plasticity test to resolve the variability in model shear zones

a new strategy for discrete element numerical models 2 sandbox applications 1 here we present a series of numerical experiments using a new formulation of the discrete element method dem that improves performance in modeling faults and shear zones in the new method named the stress based discrete element method sdem which is introduced in the companion paper by egholm stress tensors are stored at each circular particle further sdem includes rotational resistivity of particles and elastoplastic constitutive rules for governing particle deformation when combining these new features the sdem is capable of reproducing the friction properties of rocks and soils without the need for the ad hoc calibration routines normally associated with dem in contrast to the conventional dem the friction properties of a sdem particle system are in agreement with the mohr coulomb constitutive model with friction angles specified on a particle level benchmark sandbox models show that unlike most commonly used numerical methods sdem faults and shear zones develop at angles in agreement with general observations from structural geology and analogue modeling studies citation egholm d l m sandiford o r clausen and s b nielsen 2007 a new strategy for discrete element numerical models 2 sandbox applications j geophys res 112 b05204 doi 10 1029 2006jb004558

bedding plane slip in initial stages of fault related folding abstract frictional slip along bedding planes contributes to fault related folding of layered rocks we use numerical experiments to investigate the deformation of frictional bedding planes near dipping faults under layerparallel contraction and extension within the numerical experiments contraction boundary conditions produce asymmetric anticlines and extension produces asymmetric synclines the fold shape may be used to infer dip of the underlying fault in situations where the fault may not be observable additionally sense of slip along bedding planes may indicate proximity to the fault tip under uniform remote tectonic strain fault slip induces deformation in both the hangingwall and the footwall at depths as shallow as 1 km there is no significant difference between fold amplitudes in the hangingwall and the footwall this result is contrary to many kinematic models currently in use kinematic models of fault related folds commonly include the development of flat ramp and flat ramp flat fault geometries which may be attributed to initial ramp thrusting and later flat development our mechanical models show that fault flats may be produced from fault ramps due to slip along frictional bedding planes near the thrust fault tips our numerical experiments also evaluate joint initiation joints perpendicular to bedding are promoted in extensional environments we compare the results of the mechanical model to kinematic models of fault propagation folds and conclude that mechanical models offer important insights to better understand the folding process

predicting orogenic wedge styles as a function of analogue erosion law and material softening 1 the evolution of a compressive frictional wedge on a weak frictional and planar decollement subjected to frontal accretion is predicted with a two step method called sequential limit analysis the first step consists in finding with the kinematic approach of limit analysis the length of the active decollement and the dips of the emerging ramp and of the conjugate shear plane composing the emerging thrust fold the second step leads to a modification of the geometry first because of the thrust fold development due to compression and second because of erosion erosion consists in removing periodically any material above a fictitious line at a selected slope as done in analogue experiments this application of sequential limit analysis generalizes the critical coulomb wedge theory since it follows the internal deformation development with constant frictional properties the deformation is mostly diffuse a succession of thrust folds being activated so that the topographic slope reaches exactly the theoretical critical value frictional weakening on the ramps results in a deformation style composed of thrust sheets and horses applying an erosion slope at the critical topographic value leads to exhumation in the frontal central or rear region of the wedge depending on the erosion period and the weakening erosion at slopes slightly above or below the critical value results in exhumation toward the foreland or the hinterland respectively regardless of the erosion period exhumation is associated with duplexes imbricate fans antiformal stacks and major backthrusting comparisons with sandbox experiments confirm that the thickness dips vergence and exhumation of thrust sheets can be reproduced with friction and erosion parameters within realistic ranges of values

predictive modelling of structure evolution in sandbox experiments in this paper a computational approach is presented that is able to forward model complex structural evolution with multiple intersecting faults that exhibit large relative movement the approach adopts the lagrangian method complemented by robust and efficient automated adaptive meshing techniques a constitutive model based on critical state concepts and global energy dissipation regularized by inclusion of fracture energy in the equations governing state variable evolution the efficacy of the approach is benchmarked by forward simulation of two extensional analogue experiments that exhibit the development of a roll over anticline with a series of superimposed crestal collapse graben systems these sandbox experiments are excellent benchmarks for computational models as both intersecting localisations with different rates of slip and large relative movement on localisations must be represented while other complex phenomena associated with structural evolution over geological timeframes are not present

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

sensitivity analysis of numerical scaled models of fold and thrust belts to granular material cohesion variation and comparison with analog experiments scaled analog and numerical brittle viscous shortening models are employed to evaluate how fold thrust structures evolve with changes in the cohesion of brittle materials a rather poorly constrained physical parameter at this scale of experiment the shortening models are characterized by various styles of shear zones and features resembling pop up structures the kinematics geometry and number of these structures are controlled by the viscous detachment layers in the models the finite deformation of the model wedges is fundamentally different in model sets with one or two viscous layers we demonstrate that the structural evolution of the numerical models is very sensitive to small changes in cohesion value this is especially pronounced in the experiments that incorporate two weak viscous layers the overall deformation of the numerical models is most similar to analog models when cohesion values are 70 80 pa

multivariate statistical analysis of geochemical data as indicative of the hydrogeochemical evolution of groundwater in a sedimentary rock aquifer system summary the study of groundwater hydrogeochemistry of the paleozoic basses laurentides sedimentary rock aquifer system in quebec produced a large geochemical dataset groundwater samples were collected at 153 sites over a 1500 km2 study area and analyzed for major and minor ions the large number of data can lead to difficulties in the integration interpretation and representation of the results two multivariate statistical methods hierarchical cluster analysis hca and principal components analysis pca were applied to a subgroup of the dataset to evaluate their usefulness to classify the groundwater samples and to identify geochemical processes controlling groundwater geochemistry this subgroup consisted of 144 samples and 14 parameters ca2 mg2 na k hco 3 cl so 4 2 fe2 mn2 br sr2 f ba2 hs seven geochemically distinct clusters c1 c7 resulted from the hca samples from clusters c3 c4 c6 and c7 are mostly located in preferential recharge areas the majority of these samples have ca mg hco3 recharge groundwater c3 c6 c7 and na hco3 evolved groundwater c4 samples from the other three clusters c1 c2 c5 are characteristic of an aquifer system under confined conditions the majority of these samples have na hco3 evolved groundwater c1 c5 and na cl ancient groundwater that exhibits elevated concentrations in br c2 in addition to recognizing the importance of hydrogeological conditions on groundwater geochemistry the distribution of clusters also showed the importance of the geological formations on minor and trace elements such as fe2 mn2 sr2 f and ba2 the first five components of the pca account for 78 3 of the total variance in the dataset component 1 is defined by highly positive loadings in na cl and br and is related to groundwater mixing with champlain sea water and solute diffusion from the marine clay aquitard the high positive loadings in ca2 and mg2 of component 2 suggest the importance of dissolution of carbonate rocks in this aquifer system from their characteristic loadings the first two components are defined as the salinity and hardness components respectively components 3 5 are related to more local and geological effects the integration of the hca and the pca with conventional classification of groundwater types as well as with the hydrogeological and geological contexts allowed the division of the region into four main geochemical areas providing an improved regional picture of the aquifer system dynamics and hydrogeochemical evolution of groundwater the following factors were recognized as influencing the evolution of groundwater identified in every geochemical area 1 geological characteristics including sedimentary rock type and till mineralogy 2 hydrogeological characteristics represented by the level of confinement and the hydraulic gradient and 3 the geological history including the latest glaciation and the champlain sea invasion with its integrated approach this hydrogeochemical study contributes to the characterization and understanding of complex groundwater flow systems and provides an example of the long term geochemical evolution of hydrogeological systems after a major perturbation in this case seawater invasion

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

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

analogue and numerical modeling of normal fault patterns produced due to slip along a detachment zone abstract in this paper we investigate normal fault patterns produced by the sliding motion along a gently dipping normal fault by using analogue model tests and numerical modeling the motivation for this study was offered by microseismic test data that indicate the existence of an active low angle shear zone at a depth of 9 11 km in the extensional region of high seismic activity of the gulf of corinth greece both modeling techniques seem to support the hypothesis that the system of high angle normal faults that are responsible for the final asymmetrical graben formation initiate at the tip of the active basal detachment nearest to the free surface the normal faults propagate upwards with progressive sliding of the inclined basal plane resulting in a first phase of symmetrical graben configuration that is delimited by a main synthetic fault and an antithetic fault forming a rankine zone subsequent sliding on the inclined base induces a family of secondary antithetic normal faults which are responsible for the asymmetry of the failure pattern and the diffusive character of deformation in that area shear deformation is more intense and localized along the synthetic normal fault than along the antithetic faults elaboration on the analogue test results has led to the phenomenological relations among four main parameters that describe the geometry of grabens namely i the width and ii the maximum subsidence of the graben iii the dip angles of the conjugate normal faults and iv the amount of sliding along the low angle normal fault however analogue models do not produce the system of synthetic faults that is observed in the gulf of corinth the effects of both friction angle variation along the detachment base and of the constitutive behavior of the model material on the configuration of the final structural pattern were also studied with a series of numerical continuum models it was found that a the fault pattern of the gulf of corinth may be reproduced with either a strain softening material with low elastic modulus or a constant strength material and b two consecutive grabens such as those of gulfs of corinth and evia may also be reproduced by an appropriate combination of variation of dip and frictional properties along the hypothesized detachment zone

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

analogue benchmarks of shortening and extension experiments we report a direct comparison of scaled analogue experiments to test thereproducibility of model results among ten different experimental modelling laboratories we present results for two experimen

is plane strain a valid assumption in non cylindrical fault cored folds many algorithms assume plane strain to construct model and restore fault cored folds using mechanical models that allow heterogeneous transport in three dimensions we explore the distribution and magnitude of out of plane transport in plunging fault cored anticlines and provide guidelines of where plane strain should and should not be applied we developed a new technique of incrementing infinitesimal elastic strains to produce folds with aspect ratios similar to natural folds map views of displacement vectors show that in general out of plane displacement is localized near the lateral fold tips cross sections show that out of plane transport is depth dependent with out of plane displacement increasing toward the surface flexural slip surfaces compartmentalize out of plane transport within distinct mechanical units with the maximum out of plane displacement near the tops of mechanical units two dimensional models with additional frictionally slipping bed contacts suggest that freely slipping contacts can approximate the deformation of many frictionally slipping contacts we show that out of plane transport is significant in the simplest non cylindrical folds and suggest that complex non cylindrical structures should not be modeled using plane strain exclusively we also show that flexural slip surfaces exert a significant control on the magnitude and structural position of out of plane transport in our models

statistical analysis of an experimental compressional sand wedge the quasi static deformation of dry sand is widely used as an analogue to the brittle deformation of the upper crust the quantitative comparison of analogue to natural tectonics or to mechanical predictions requires identifying sources of biases and estimating the intrinsic variability of the experimental results we develop experimental and statistical methods that fulfill these requirements we consider an initially perfect wedge resting on a flat layer made of a uniform dry sand in a rectangular glass box the box is shortened lengthwise by translating one of its end walls towards the other the lateral walls can remain fixed or be translated with the moving end wall upon shortening the wedge is thrusted above the flat layer forming classical fore and backthrusts as essentially plane strain structures lifetimes locations and dips of all thrusts constitute seven quantifiable output parameters called observables in addition to the shortening forces monitored at both end walls during shortening up to seventy measurements of each observable were performed in seven final state cross sections of ten experiments a three step statistical analysis allows us to prove that first the observables vary independently justifying their modeling with independent distributions second the ergodic hypothesis holds meaning that along strike variations can be used to infer the intrinsic experimental variability measurements can thus be repeated on successive cross sections in each experiment third our data set is free from bias due to friction on the lateral walls or due to the finite length of the box we then construct statistical models of each observable using either gauss or laplace distributions for example forethrusts dip at 38 3 2 and backthrusts at 41 3 3 we finally show how to apply these statistical models to experiments using a different initial geometry the statistical methods presented here are applicable to experiments with different setups materials and observables although the ergodic hypothesis is relevant only to plane strain experiments

is the earth lazy a review of work minimization in fault evolution abstract the principle of work minimization has been used in various forms to account for the development of active fault systems within a wide range of tectonic settings we review the successes challenges and implications learned from previous applications of work minimization examination of the energy budget provides insight into the competing influences of different processes within fault systems at a variety of scales we present each work budget component with considerations for numerical implementation additionally we demonstrate numerical implementation by solving for the energy budgets and finding the most efficient propagation paths of a joint and several faults consideration of the energy budget captures growth patterns that are consistent with both laboratory observations and theoretical predictions using the energy release rate g unlike using g work minimization is not limited to collinear growth in comparison with predictions using coulomb failure planes work minimization eliminates uncertainty in predicting fault propagation by 1 evaluating the trade off between tensile and shear failure ahead of the fault and 2 avoiding the ambiguity introduced by the two potential coulomb failure planes application of work minimization to faulting demonstrates the effectiveness of this approach in predicting both the orientation and timing of fault propagation

seismic source mechanism inversion from a linear array of receivers reveals non double couple seismic events induced by hydraulic fracturing in sedimentary formation abstract we developed a new method to invert source mechanisms of seismic events observed from a single vertical or near vertical array of receivers this method was tested on a synthetic dataset and applied to a seismic dataset acquired during hydraulic fracturing of a sedimentary formation we show that a crack opening seismic event recorded on a single vertical array can be inverted as a pure shear seismic event indicating that the best fitting shear mechanisms inverted from seismic data acquired along a vertical borehole provide misleading information we found that the majority of the analyzed source mechanisms of the induced events are not consistent with pure shear faulting as previously thought the non shear source mechanisms are required even when seismic noise location uncertainty and uncertainty in the attenuation structural model are considered

extensional fault systems in sedimentary basins a review of analogue model studies abstract scaled analogue models have provided graphic new insights into the progressive development of extensional fault systems this paper reviews recent results of an on going analogue modelling programme and compares the structural styles in the models with examples of natural extensional fault systems four fundamental model styles scaled to simulate deformation in brittle sedimentary rocks in the upper 10 km of the crust are reviewed extension above a basal detachment which undergoes stretching over a limited are produces an asymmetric rift graben bounded by planar faults and in which the internal deformation is accommodated by rotational domino faults these experimental results are directly comparable to structures found in intracontinental rifts extension above a basal detechment which undergoes extension under the whole model and which has no constraints at the ends produces domino style fault arrays in the pre rift sequence and listric growth faults in the syn rift sequence these results compare with highly extended rift basins and terranes such as the basin and range province usa extension above a simple listric fault produces a characteristic hangingwall roll over and crestal collapse graben system at high values of extension superposed crestal collapse grabens occur and a fan of listric growth faults is developed these structures bear striking similarities to growth faults in progradational delta systems ramp flat listric extensional fault systems using sand mica models produce a characteristic family of structures with an upper roll over crestal collapse system a ramp syncline and reverse fault fold zone and a lower roll over crestal collapse system an example of a ramp flat fault system from the gulf coast usa is presented as a comparison with the analogue model results preliminary results of three dimensional extensional model experiments are presented to illustrate the complexities in such fault systems in all of the experimental models it is found that the geometry of the underlying detachment system exerts fundamental control on structures produced in the hangingwall the analogue models illustrated in this review provide valid geometric and kinematic templates for the progressive evolution of extensional fault systems in sedimentary basins

3d modelling of rifting through a pre existing stack of nappes in the gulf of corinth greece a mixed analogue numerical approach the gulf of corinth is a young 1 ma active rift currently extending n00 which displays significant contrasts in structural style along strike a possible explanation for these variations is the presence of the phyllades nappe in the basement of the western part of the gulf previous 2d thermo mechanical models have shown that a strong strength contrast between this metamorphic unit and the rest of the basement can explain the kinematics and the spacing of the faults in the western part the rift however displays a wide variety of 3d features e g en echelon faulting n30 transverse normal faults that cannot be taken into account using 2d modelling to obtain 3d insights into the role of an inherited dipping weakness zone analogue sand and pdms experiments based on the results of the 2d numerical thermo mechanical model have been performed the analogue models show that a 308 discrepancy between the dipping direction of the weak nappe and the direction of extension leads to the formation of en echelon and n30 striking normal faults as observed in the gulf of corinth however fault spacing and graben width completely misfit both the data and the results of the thermo mechanical models on which the analogue experiments were based in order to understand those differences numerical mechanical benchmarks of the analogue experiments have been run to test different factors 3d lateral displacements values of the elastic parameters and bottom boundary conditions that could have affected the dynamics of the analogue model this approach highlights for our case study that the misfits are mostly related to the lack of isostatic compensation at the base of the analogue experiments

mechanics of non critical fold thrust belts based on finite element models abstract the mechanics of fold thrust belts and accretionary wedges is investigated using a two dimensional plane strain elastic plastic cohesive mohr coulomb mechanical model solved with the finite element method results show that when a layer with an initially non critical geometry is compressed from the rear it does not form a wedge that is at failure throughout as assumed in critical wedge theory rather the wedge consists of narrow plastic shear zones that propagate sequentially outward with time loading rocks ahead while unloading rocks behind not only are stress states within the wedge not everywhere at failure but principal stress orientations vary strongly in time and space particularly across shear zones near the basal detachment and in the hanging wall of active structures where local surface extension may be observed the reason the investigated wedges are not stressed to compressive failure throughout is related to strength reduction associated with strain localisation that enables material outside shear zones to unload and return to an elastic stress state this mechanism is intrinsic to elastic plastic materials and occurs regardless of any material degradation such as loss of cohesion even though the stress state of the investigated wedges is generally non critical the overall geometry may still be consistent with cohesionless critical wedge theory since the local surface slope is created when a particular part of the wedge is at a limit state prowedge tapers display non self similar growth through time but eventually evolve to the minimum critical taper retrowedges on the other hand may get caught within this initial transient state and thus may have tapers anywhere between the minimum and maximum critical taper however if the basal detachment is such that lateral propagation is not kinematically inhibited retrowedges are shown to also eventually evolve towards minimum critical tapers resulting in a symmetrical doubly vergent orogen because all of these wedges have non critical stress states both pro and retrowedges are predicted to display stable non equilibrium taper angles in the presence of external perturbations for example related to erosion or sediment deposition