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

tectonics and foreland basin development at the leading edge of the humber arm allochthon western newfoundland canadian appalachians the thrust front of the northern appalachians involves ediacaran rift related structures of the laurentian margin that were re worked in taconic ordovician salinic mainly silurian and acadian mainly devonian deformation events much of the thrust front is concealed under the gulf of st lawrence but port au port peninsula in western newfoundland provides a cross section through the laurentian passive margin overlying foreland basins and the humber arm allochthon a large area of foreland basin strata is present in areas formerly mapped as humber arm allochthon abrupt thickness changes and an abundance of fault scarp derived limestone conglomerate indicate that deposition was influenced by block faulting and differential subsidence in a basin undergoing active flexural extension a coarsening upward trend in the goose tickle group represents a transition from a distal to a proximal sediment source as the humber arm allochthon was emplaced westwards goose tickle group separates two packages of allochthonous rocks the west bay thrust sheet the lower package represents the leading edge of the taconic allochthon its timing of emplacement is constrained by darriwilian 3 graptolites both above and below the thrust sheet its present day configuration results from a combination of processes including thrusting extension by gravity spreading gravity gliding and subsequent erosion of material that was deposited in the goose tickle group the structurally higher lourdes thrust sheet is an out of sequence structure associated with acadian devonian orogenesis high angle faults show a protracted history of movement that includes early taconic flexural extension acadian inversion and later carboniferous or mesozoic strike slip motion the leading edge of the humber arm allochthon was influenced by both thin skinned and thick skinned tectonics throughout the development of the appalachian orogen

high resolution 3d numerical modeling of thrust wedges influence of decollement strength on transfer zones 1 the mechanics and dynamics of thin skinned compressible thrust wedges with prescribed offsets in the backstop i e transfer zones are investigated using a three dimensional finite difference numerical model with a visco brittle plastic rheology the main questions addressed are as follows i what is the influence of the initial length of the backstop offset and ii what is the effect of the frictional strength of the main decollement on the structural evolution of the brittle wedges along such transfer zones results show that the shorter the backstop offset the earlier these two thrust planes connect forming a curved frontal thrust along the entire width of the model younger in sequence thrusts are formed parallel to this curved shape long backstop offsets produce strongly curved thrust faults around the indenting corner simulations with a weak basal friction evolve toward almost linear frontal thrusts orthogonal to the bulk shortening direction increased basal drag in models with a strong decollement favors propagation of the backstop offset into a transfer zone up to the frontal thrust these simulations revealed that surface tapers of the wedge in front of the backstop promontory are larger than what the critical wedge theory predicts whereas the tapers on the other side of the transfer zone are smaller than analytical values this difference is amplified with increasing length of the backstop offset and or strength of the decollement modeled surface elevation schemes reproduce well the topographic patterns of natural orogenic systems such as the topographic low along the minab zendan transform transfer fault between the zagros and makran

seismic refraction profiles in the gulf of saint lawrence and implications for extent of continuous grenville lower crust near the exposed grenville craton a two layer crust with velocities of 6 2 km s 1 and 6 65 km s 1 and low gradient is measured the 6 65 km s 1 velocity is observed at the shallow depth of 15 km and the moho at greater than 40 km depth towards the appalachian orogen a southwards dipping ramp is developed in the mid crust on the top of the 6 65 km s 1 layer a collinear reflection profile confirms the southwards dipping ramp the shallowing moho and the lateral changes in the lower crust a published refraction profile that crosses this reflection profile near the southern end confirms the termination of the 6 65 km s 1 layer and its replacement by a slower velocity with the publishing of wide angle reflection refraction war profile 88 2 two pairs of lines are now available in the gulf that cross the craton into the orogen with both war and deep seismic reflection coverage the combined data sets are used to distinguish the grenville lower crustal block based on patterns of reflectivity and associat

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

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

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

effect of basal friction on surface and volumetric strain in models of convergent settings measured by laser scanner abstract this paper uses measurements by a high accuracy laser scanner to investigate the role of basal friction on surface and volumetric strains in sandbox models simulating fold thrust belts and accretionary wedges we monitor progressive deformation wedge growth and strain distribution in three models with similar initial boundary conditions but with different basal frictions our analyses show that in addition to influencing the kinematics and geometry of model wedges basal friction also governs both the surface and volumetric strains of the wedge after 16 3 bulk shortening the volume decreased 5 0 5 9 5 0 5 and 12 5 0 5 in the models shortened above low intermediate and high friction decollements respectively applied to nature our model results suggest that more compaction and penetrative strain is expected in convergent settings with a high friction decollement than those shortened above a low friction decollement or a weak basal bed like the salt formation under parts of the zagros fold thrust belt this volume decrease probably reduces the porosity in the deformed lithologies

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

the seismic velocity structure of northern appalachian crust around western newfoundland the crustal velocity structure beneath western newfoundland is constrained by a reanalysis of older regional refraction profiles together with an analysis of one new profile initial interpretation of the older data gave inconsistent and discontinuous structures that are difficult to reconcile with more recent deep reflection profiles we also show that traveltimes predicted by the earlier models often do not yield acceptable fits to the original observations our reinterpretation reveals a simpler pattern in which the crust is characterized by a persistent high velocity lower crustal hvlc layer this layer has velocities of 7 0 7 9 kmls and thicknesses of 5 23 km it is thickest beneath the grenville crustal block east of the appalachian structural front and thins or is possibly absent within the central block analysis of the new much higher resolution profile off western newfoundland confirms the existence of the hvlc layer with a velocity of 7 2 kmls and thicknesses of 11 19 km increasing to

robust characteristics method for modelling multiphase visco elasto plastic thermo mechanical problems abstract we have extended our previous 2d method gerya t v yuen d a 2003 characteristics based marker in cell method with conservative finite differences schemes for modeling geological flows with strongly variable transport properties phys earth planet interiors 140 295 320 which is a combination of conservative finite differences with marker in cell techniques to include the effects of visco elasto plastic rheology self gravitation and a self consistently derived evolving curvilinear planetary surface this code is called i2elvis and can solve a new class of computationally challenging problems in geodynamics such as shear localization with large strains crustal intrusion emplacement of magmas bending of realistic visco elasto plastic plates and core formation by vigorous shell tectonics activities related to a global rayleigh taylor instability of a metal layer formed around silicate rich lower density primordial core during planetary accretion we discuss in detail the computational strategy required the rheological constraints to be satisfied at each time step and spatial location we show analytical benchmarks and examples drawn from comparing between numerical and analogue experiments in structural geology subducting slab bending with a visco elasto plastic rheology and equilibrium spherical configurations from self gravitation we have also tested possibilities of future applications by addressing 3d geometries based on multigrid method and including inertial effects in the momentum equation with tracers in order to simulate meteoritic impact events and eventually earthquake instabilities

kinematics of a sigmoidal fold and vertical axis rotation in the east of the zagros makran syntaxis southern iran paleomagnetic magnetic fabric and microtectonic approaches abstract the zagros simple fold belt is characterized by elongated curved or sigmoidal folds the trend of these structures together with the structural style change suddenly across the zagros makran syntaxis which separates the continental collision domain of zagros from the oceanic subduction one in makran this work focuses on the minab anticline outcropping in the easternmost part of zagros in order to understand the kinematics of a sigmoidal fold and underscore possible vertical axis rotations in the eastern side of the syntaxis we performed a joint study of magnetic fabric microtectonics and paleomagnetism of the northern termination of this fold the two limbs have been sampled 7 sites 134 samples along three cross sections corresponding to three different orientations of the fold axis the rocks are weakly deformed fine grained mio pliocene reddish siltstones the shortening directions deduced from both magnetic fabric analysis and microtectonic observations are consistent with each other they are horizontal and roughly perpendicular to the local fold axis following the torsion of the fold hinge line and indicating a tectonic origin of the magnetic fabric rockmagnetic analyses thermomagnetic curves hysteresis loops point to the presence of magnetite in the psd and md ranges as the main magnetic carriers together with a minor contribution from hematite apart from a post tilting sub actual vrm and or crm component a paleomagnetic analyses yield mainly two pre tilting magnetization components component b is carried by magnetite spanning the intermediate to high unblocking temperature range 300 c t ubs 580 c component c has unblocking temperatures characteristic of hematite 580 c t ubs 680 c both are ante folding based on positive reversal and fold tests inside each of the cross section but also for the three sections together however because component c is biased by some inclination flattening only component b is taken into account afterwards after full tectonic correction the site mean direction of component b calculated for 6 out of the 7 sites is not statistically different from the individual site mean directions the intersection direction determined by small circle analysis in these six sites is d 18 0 i 33 7 k 482 alpha 95 2 6 compared to the mio pliocene direction expected in this area for africa it corresponds to an overall clockwise rotation of the whole structure of some 18 3 5 1 consistent with the regional geodynamic context the absence of differential paleomagnetic rotation from one cross section to the other and the fact that the magnetic lineation together with the shortening direction deduced from microtectonic analysis follows the fold curvature demonstrate that the torsion of this fold is not secondary but rather coeval with the fold amplification the torsion of the fold would stem from underlying inherited mesozoic structures and the rheological response during the deformation of two incompetent decollement layers undergoing differential velocities and a change of thickness in a constant stress field direction

a geodynamic framework for interpreting crustal scale seismic reflectivity patterns in compressional orogens summary 2 d plane strain finite element models are used to calculate crustal deformation patterns within model compressional orogens and the results are compared with natural examples the fundamental driving mechanism for model deformation is provided by asymmetric detachment and underthrusting of mantle lithosphere with deformation being rooted at the stress singularity where the mantle detaches deformation steps up into the overlying crust and spreads laterally according to the rhcological properties assumed for the crust the crust is modelled using plastic frictional and viscous rheologies and incorporates the effects of compositional layering and variable geothermal gradients overall deformation is determined by balancing the internal strength of the crust against the sum of applied boundary stresses plus the gravitational stress induced by mass redistribution within the deforming orogen the amount of convergence in the models is limited to 150 km or less and so the results are not strictly comparable with larger orogens involving greater convergence nevertheless larger orogens seem to evolve through stages that may be understood in terms of the models provided that consideration is confined to smaller scale components that are limited in time and space the continuum deformation calculated by the finite element models is qualitatively converted to equivalent pattcrns of discrete faults and shear zones by associating these discrete features with the areas of highest cumulative strain using the results of sandbox experiments as guides significant styles of deformation include 1 coupling between step up shears and subhorizontal detachments at one or more levels 2 localization of upper crustal deformation by subduction of the lower crust and 3 pinning of deformation by surface denudation a partial catalogue of such model styles is presented in which the strength of basal coupling between crust and mantle the number of rhcological layers within the crust the thickness of subducted lower crust and the extent of surface denudation are varied systematically compressional orogens with a wide range of ages are characterized by crustal scale seismic reflectivity similar to patterns of fault shear zones represented in this catalogue of styles this similarity suggests that crustalscale seismic reflection data can be used to constrain the dynamic processes involved in compressional orogcnesis

devonian carboniferous tectonics and basin deformation in the cabot strait area eastern canada the magdalen basin in the cabot strait bay st george area was a major depocenter for devonian carboniferous sediments in eastern canada structure within the basin is complex and is characterized by northeast and east trending mainly dextral strike slip faults associated with the bend in the appalachian orogenic belt known as the st lawrence promontory under the cabot strait two linear grabens parallel the major fault trends and preserve up to 6 km of devonian carboniferous sedimentary rocks comprising mainly coarse grained terrigenous clastics with a lower carboniferous transgressive episode recorded by marine carbonates evaporites and fine grained clastics several unconformities within the sedimentary succession record deformation associated with movement alo g the regional strike slip faults minor local unconformities are interpreted within the upper horton early visean upper barachois late westphalian and post pennsylvanian successions a major unconformity cuts out most of the namurian and westphalian section in the study area and can be correlated on a regional scale kinematic strain partitioning along the master cabot fault led to the development of doubly vergent compressional structures within a wrench borderland this was complicated by transpressive deformation at a restraining bend in the master fault and by deformation associated with overstepped ends of splays of the regional fault system the magdalen basin probably formed in an early phase of post acadian extension and was later reactivated by regional strike slip faults the presence of thick source and reservoir rocks as well as diverse trapping possibilities and a favorable burial and maturation history indicate that the cabot strait area has good potential for accumulating and preserving petroleum

three dimensional numerical modeling of upper crustal extensional systems 1 we focus on understanding the evolution and structural style of crustal extension in three dimensions using state of the art forward dynamic models to date few 3 d models exist that follow the evolution of tectonic processes into large deformation modes with sufficient resolution to resolve individual faults and shear zones we use an arbitrary lagrangian eulerian fully parallel finite element code that solves for viscoplastic flows in three dimensions plastic materials weaken with accumulating strain to localize deformation a weak seed region is introduced at the base of a plastic model extended by velocity boundary conditions controls on the geometry and spacing of three dimensional plastic shear zones are investigated the sensitivity of varying the offset between weak seeds and the sensitivity of strain weakening parameters on the linkage between offset rift zones and on the efficiency of rift propagation are tested the model results indicate the primary controls of strain dependent plastic rheology and rift offset on the efficiency of rift propagation and the style of rift segment interaction the amount and onset of strain weakening also play a large role in the degree to which the primary segments link and propagate toward each other resulting in a trade off effect between the amount of offset between the initial grabens and the strain weakening ratio the three dimensional models indicate three main rift modes for linkage between two upper crustal rift segments in three dimensions 1 small offset grabens with a single relay zone 2 intermediate offset grabens with one or more secondary step over graben segments and 3 large offset grabens with limited to no significant segment interaction

post taconian shortening inversion and strike slip in the stephenville area western newfoundland appalachians deformed terrigenous and carbonate sedimentary rocks representing the early paleozoic laurentian continental margin form a series of elongate fault bounded blocks that plunge north beneath the humber arm allochthon in the stephenville area west newfoundland appalachians the continental shelf succession was folded and thrust faulted after emplacement of the humber arm allochthon in the west of the area table mountain is cut by a pop up structure bounded by downward converging reverse faults structures at the east margin of table mountain indicate both dextral and reverse slip the phillips brook structure farther east contains multiple fault bounded carbonate slices one slice carried by the west blanche brook fault is thrust over the humber arm allochthon the western edge of the indian head massif consisting of grenville basement is also a thrust contact these reverse faults and thrusts which cut the carbonate succession and postdate emplacement of the humber arm allochthon are in turn

basin evolution in western newfoundland new insights from hydrocarbon exploration the humber zone is the most external zone of the appalachian orogen in western newfoundland it records multiphase deformation of the cambrian ordovician passive margin and of the ordovician to devonian foreland basins by the taconian salinian and acadian orogenic events the recent phase of exploration drilling has provided new evidence for structural stratigraphic reservoir and source rock maturation models of western newfoundland the first well port au port 1 supported the hypothesis that the round head thrust had an earlier extensional history prior to the acadian compressional inversion that created the present day structural high of the port au port peninsula the well tested a small anticline formed in a footwall shortcut fault of the round head thrust the second well long point m 16 was drilled at the northern tip of long point to test a triangle zone identified by previous workers this well demonstrates that the frontal monocline at the western edge of the triangle zone is elevated by a stack of imbricate thrusts composed of rocks of the taconian allochthon and compressional basement involved faults that have uplifted the cambrian ordovician carbonate platform the structural model developed in the port au port area with the aid of these wells has been extended throughout the humber zone in western newfoundland changes in structural style illustrated by regional cross sections suggest that prospective trap geometries are only developed in the southern and central parts of the region the reservoir model proposed invokes exposure and karsting of the footwalls of extensional faults formed as the carbonate platform collapsed during a middle ordovician hiatus the st george unconformity structural relief became more pronounced as extensional collapse continued through the middle ordovician begin page 394 these structurally high fault footwalls became the foci for dolomitizing and mineralizing fluids that used major faults as fluid conduits during the devonian fluids deposited sulphide ores and created zebra and sparry dolomite and some sucrosic hydrothermal dolomites in the st george group and the table point formation the reservoir model maturity and source rock data and the structural models have been combined with seismic and onshore surface geology this enables the prospectivity of the western newfoundland cambrian ordovician play trend to be evaluated for further exploration

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

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

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

dynamical instability produces transform faults at mid ocean ridges transform faults at mid ocean ridges one of the most striking yet enigmatic features of terrestrial plate tectonics are considered to be the inherited product of preexisting fault structures ridge offsets along these faults therefore should remain constant with time here numerical models suggest that transform faults are actively developing and result from dynamical instability of constructive plate boundaries irrespective of previous structure boundary instability from asymmetric plate growth can spontaneously start in alternate directions along successive ridge sections the resultant curved ridges become transform faults within a few million years fracture related rheological weakening stabilizes ridge parallel detachment faults offsets along the transform faults change continuously with time by asymmetric plate growth and discontinuously by ridge jumps

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

influence of syntectonic sedimentation on thrust wedges in analogue models scaled analogue sandbox models have been used to simulate the growth of coulomb thrust wedges that include syntectonic sedimentation during thrusting increases in the syncontractional sedimentation rate dramatically change the shape of the coulomb thrust wedge decreasing the number of thrust slices the internal shortening and the critical taper of the wedge thrust trajectories change from ramp flat to sigmoidal with increased syncontractional sedimentation the analogue models show many similarities to thrust fault geometries in accretionary prisms and foreland fold and thrust belts in which thrust slices prograded into basins with active sedimentation

low damage anomaly of the 1976 tangshan earthquake an analysis based on the explosion ground motions abstract during the 1976 tangshan earthquake m 7 8 a low earthquake damage anomaly was reported in yutian area this low damage area has not been explained satisfactorily because of insufficient ground motion records in this area the japan china joint research project conducted during 1987 to 1988 provides us with a unique opportunity to investigate this problem in this study the ground motion data generated by explosions are used to explain the origin of the low earthquake damage anomaly four recording profiles are arranged for this purpose and each profile contains about 20 stations the arrival times and waveform characteristics are different along profile b b wave trains with small amplitudes in the north are related to a thin sedimentary layer and an up dip underground structure in the south the wave trains have longer coda waves that can be linked with a thicker sedimentary layer site effects on ground motions can be identified in the west of yutian based on the small amplitudes observed during three explosions the distributions of small amplitude also depend on the locations of explosions this reveals that the path effects also contribute to the seismic ground motion the regions with small peak velocities are very similar to the low earthquake damage area from the 1976 tangshan earthquake the cause of low damage anomaly during the tangshan earthquake can be attributed to both the site and path effects

facies geometry and geological significance of late ordovician early caradocian coral bioherms lourdes formation western newfoundland late ordovician coral bioherms in the lourdes formation of western newfoundland exhibit a complex mixing of architectural elements including framework boundstone and suspension deposits the bioherms occur within a narrow 16 m stratigraphic interval and a prominent unconformity truncates the interval of bioherm growth and tops of many of the bioherms the buildups developed along a carbonate ramp they occur isolated and in groups individuals in groups are aligned in parallel orientation the sizes of the bioherms range from small 50 100 cm coral piles to columnar and dome shaped masses 1 15 m however topographic relief was never more than 1 m bioherm construction reflects i stacking of the tabulate coral labyrinthites chidlensis and less common stromatoporoids ii accumulation of microbial stromatoporoid boundstone and suspension deposits within shelter cavities between corals and iii detrital bioherm flank skeletal grainstone beds trypanites borings are common in the tops of coral heads the bioherms exhibit three growth development stages i seafloor stabilization wherein rare abraded coral colonies lie scattered within pelmatozoan skeletal grainstone lenses ii colonization wherein corals l chidlensis rare stromatoporoids labechia sp and other biota bryozoans produced a bioherm overlying the basal sediment base and iii diversification which is marked by a more diverse range of fauna and flora as well as occurrence of shelter cavity deposits the diversification stage usually makes up more than 70 of a bioherm structure and in some defines multiple periods of start up and shut down of bioherm growth the latter is defined by bored omission surfaces and or deposition of inter bioherm sediment the lourdes bioherms have a similar ecological structure biotic diversity and depositional environment to patch reefs in the equivalent carters limestone in tennessee the mixture of coral stacking and boundstone as architectural elements identify an early palaeozoic transition of reef design development along shallow water platforms that began to displace the muddy boundstone bafflestone carbonate buildups more typical of the early and middle ordovician time

laboratory results indicating complex and potentially unstable frictional behavior of smectite clay a central problem in explaining the apparent weakness of the san andreas and other plate boundary faults has been identifying candidate fault zone materials that are both weak and capable of hosting earthquake like unstable rupture our results demonstrate that smectite clay can be both weak and velocity weakening at low normal stress 30 mpa our data are consistent with previous work which has focused on higher normal stress conditions 50 mpa and greater and found only velocity strengthening if natural fault zones contain significant smectite one key implication of our results is that localized zones of high pore pressure which reduce effective normal stress could be important in controlling potential sites of earthquake nucleation our experiments indicate that friction of smectite is complex and depends upon both sliding velocity and normal stress this complexity highlights the need for detailed experiments that reflect in situ conditions for fault gouges

initiation development and benefits of lithoprobe shaping the direction of earth science research in canada and beyondthis article is one of a series of papers published in this special issue on the theme lithoprobe parameters processes and the evolution of a continent lithoprobe contribution 1480 lithoprobe is canada s national collaborative multidisciplinary earth science research project established to develop a comprehensive understanding of the evolution of northern north america it is regarded internationally as one of the most successful national geoscience projects ever undertaken part of lithoprobe s success derives from its history and organizational structure a one year phase i program 1984 1985 was highly successful phases ii to v carried the project forward until 2005 when funding terminated lithoprobe was organized as a decentralized research network of multidisciplinary scientific studies and collaborating scientists about 1500 scientific publications were generated lithoprobe s legacy also includes economic and social benefits new and improved understanding of earth history provides petroleum and mining companies with an enhanced knowledge base lithoprobe demonstrated the applicability of high resolution seismic reflection studies to mineral exploration development of a portable seismic refraction recorder and a long period magnetotelluric system led to technology transfer to canadian companies very high resolution seismic reflection studies applied first by lithoprobe are proving effective for exploration for uranium deposits in saskatchewan in the cratonic areas of canada lithoprobe seismic and magnetotelluric studies have provided significant new information relevant to exploration for diamonds on the west coast of canada lithoprobe studies provided data and a framework for better understanding the mega thrust earthquake hazard in the region lithoprobe established a highly effective public outreach strategy that involved the print and electronic media as well as material for educational purposes unequivocally lithoprobe has enhanced the already strong reputation of the earth sciences in canada

structural and tectonic evolution of the humber zone western newfoundland 2 a regional model for acadian thrust tectonics the humber zone of the western newfoundland appalachians represents a cambrian ordovician passive continental margin which was deformed in taconian mid ordovician and acadian silurian devonian orogenic events a deformation front is imaged in seismic reflection data offshore of western newfoundland structures associated with this deformation front are exposed on port au port peninsula where silurian rocks are strongly deformed but mississippian strata are flat lying indicating that latest thrusting was acadian a gravity low in the gulf of st lawrence corresponds to a sediment filled acadian foreland basin previous models suggest that the on land shelf succession is autochthonous to parautochthonous however two lithoprobe seismic reflection transects show sub horizontal reflections between 2 and 5 s two way travel time which extend up to 85 km east of the thrust front these are interpreted as autochthonous platform and basement in this model shallower reflectors and outcropping units include both allochthonous platform and basement comprising the acadian port au port allochthon the taconian humber arm allochthon was carried westward as a high structural slice during thrusting of this allochthon no major structural discontinuity exists between grenville age crystalline rocks of the long range massif and platform rocks interpreted as allochthonous in the northern seismic line a monocline at the southern extremity of the long range probably represents an oblique or lateral hanging wall ramp above the basal detachment within the long range thrust zone at the western margin of the massif the long range thrust shows only a few kilometers of displacement however the parsons pond thrust which we interpret to run offshore at green point juxtaposes contrasting successions with different structural and thermal histories it probably carries a much larger amount of the total displacement the basal decollement of the port au port allochthon is therefore interpreted to pass beneath the southern part of the long range massif

development of a folded thrust stack humber arm allochthon bay of islands newfoundland appalachians in the humber arm area of the western newfoundland appalachians four distinct stratigraphic successions derived from the laurentian continental margin are exposed each succession is believed to be characteristic of a separate thrust sheet the platform sheet represents the ancient laurentian shelf and its foreland basin cover the watsons brook sheet is characterized by a succession including shelf margin carbonates overlying foreland basin clastics the corner brook sheet comprises continental slope and rise clastic and carbonate sedimentary rocks of the humber arm supergroup and the woods island sheet includes clastics of the blow me down brook formation that overlie mafic volcanics sheets are subdivided by thrusts into tectonic slices disrupted units and melange with scaly s1 foliation are found along the boundaries of some slices thrust sheets and related structures have been deformed by f2 folds with axial planar s2 cleavage s1 scaly foliations are transposed into parallelism with s2 there

source rock characterization and oil to source rock correlation of a cambrian ordovician fold and thrust belt petroleum system western newfoundland abstract in this study we use biomarker and isotope data to address the complexities of petroleum system analysis in fold thrust systems specifically related to hydrocarbon generation and migration such as variable thermal maturities of source rocks complex migration pathways and mixing source rocks extracts and oil samples are taken from paleozoic rocks along the appalachian structural front in western newfoundland oil seeps along this fold and thrust belt have motivated episodic exploration efforts over the last 150 years however economic development has been unsuccessful to date in part because of the complex nature of oil to source relationships pyrolysis analyses identify promising source intervals in lower ordovician floian formations with an excellent source potential containing type i ii organic matter toc up to 9 35 hi up to 840 a second good source interval identified within the late cambrian furongian continental slope and rise sediments contains type ii iii organic matter toc up to 2 34 hi 380 source rock samples from outcrops are marginally mature to mature with tmax values between 436 and 447 geochemical analyses of source rock extract imply a clastic shale dominated source rock with a minor contribution from carbonate source rocks chemometric analyses yield 3 extract groups groups 1 and 2 contain lower ordovician samples with organic matter derived from oxidized micro plankton with varying amounts of gleocapsomorpha prisca extract group 3 originates from cambrian samples and shows evidence of a bacterial derived organic matter similar genetic relationships among 10 oil samples from natural oil seeps and abandoned well sites are indicated by high resolution biomarker analyses seven oil samples originated from ordovician source rocks that produced high api oils with low bisnorhopane hopane tricyclic hopane ratios and high sterane concentrations supporting interpretation of a source containing algal derived organic matter three oils originated from cambrian shales these high api oils show high bisnorhopane hopane and tricyclic hopane ratios low sterane concentrations and low carbon isotope values all indicative of organic matter derived from bacteria generated over a wide range of maturities 0 98 1 26 ro thermal maturity sensitive biomarkers and naphthalene ratios show that source rocks generated oils with a wide range of maturities related to their dipping character in the imbricated thrust stack the cambrian source generated oils that locally experienced secondary cracking during acadian inversion whereas the ordovician source generated oil with low maturities after the acadian inversion oil operaters exploring fold and thrust belt petroleum systems may expect extreme changes of oil properties for example api gravity in spacially restricted areas

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

structure of the appalachian deformation front in western newfoundland implications of multichannel seismic reflection data petroleum industry multichannel marine seismic data in the public domain do not support the traditional interpretation of the acadian deformation front in the appalachians of western newfoundland they show that the late ordovician to latest silurian long point clam bank succession is folded in a northwest facing monocline whereas the underlying cambrian ordovician platform succession dips southeast we interpret the intervening region as a structural triangle zone similar to those at the foreland edges of other thrust belts the contact between the long point group and the underlying humber arm allochthon previously interpreted as unconformable on port au port peninsula is the southeast vergent upper detachment surface of the triangle zone the platform succession on the peninsula is probably within the triangle zone and is therefore allochthonous at least 30 km of acadian silurian devonian northwest transport is implied the triangle zone continues to the northeast offshore the bay of islands suggesting that grenvillian basement of the long range massif is also allochthonous

flexural modeling of paleozoic foreland basin subsidence offshore western newfoundland evidence for substantial post taconian thrust transport multichannel seismic reflection data gathered by the petroleum industry offshore of western newfoundland constrain the absolute thicknesses of the middle ordovician and post middle ordovician portions of the anticosti foreland basin immediately adjacent to the appalachian structural front these seismic lines which include 1970s vintage data in the public domain and recently acquired proprietary data indicate that the middle ordovician taconian portion of the foreland basin is more than an order of magnitude thinner than the preserved post middle ordovician salinian acadian portion the mechanical lithospheric coupling of foreland basins and their adjacent overthrust belts is a well understood phenomenon a thin taconian foreland succession and a thick salinian acadian succession imply that the overthrust loads responsible for flexural depression of the anticosti basin were much more distant and or much smaller during the middle ordovician taconian orogeny than they are today we quantify thi

origin of the odd twins anomaly magnetic effect of a unique stratigraphic marker in the appalachian foreland basin gulf of st lawrence the odd twins magnetic anomaly is a pair of linear asymmetric positive anomalies located in the gulf of st lawrence over the post taconian pre acadian foreland basin fill related to the development of the appalachians a marine magnetic survey allowed the anomaly to be traced close to the coast of newfoundland and an on land survey identified both peaks within the area of outcrop of the late ordovician long point group sandstones of high susceptibility sampled from sparse outcrops close to the locations of the anomaly peaks contain up to 0 58 magnetite models involving dipping sheets of sandstone having similar and slightly higher susceptibility can explain both the onshore and offshore anomaly the magnetite is of detrital origin and represents a paleoplacer heavy mineral concentration formed in a marginal marine environment the anomaly provides a unique tie point between the known onshore stratigraphy and the succession in the foreland basin known only from industry seismic profiles this tie

friction experiments on saturated sediments and their implications for the stress state of the nankai and barbados subduction thrusts the nankai and barbados forearcs have low stress subduction thrusts the sediments entering the subduction zone and namely the material in the decollement zones have been well characterized by numerous deep sea drilling legs and studies of the recovered cores nankai has high heat flow and significant amounts of illite while barbados is a smectite dominated system based on results from ring shear 2 mpa normal stress and direct shear 30 mpa tests on marine sediments and mineral standards this translates into a residual frictional resistance of r 0 25 and r 0 11 in clay horizons respectively such values agree with theoretical estimates from critical wedge theory nankai b 0 16 0 26 and barbados b 0 06 0 09 and fault spacing geometries from seismic profiles nankai b 0 12 0 23 and barbados b 0 11 0 19 maximum pore pressure ratios of 0 85 and 0 73 for nankai and barbados respectively allow us to estimate effective shear stresses as a function of friction coefficient and density of the sediment gouge to reach only 10 mpa or less in the frontal 50 km of the decollement zone respectively our data support the contention that fluid pressure transients and sediment composition contribute equally to the weakness along plate boundary faults down to the seismogenic zone with the first probably dominating the shallow decollement shear velocity stepping tests show that the clay dominated gouges strengthen velocity irrespective of the clay mineralogy and hence suggest that clay transformation does not affect the onset of seismogenesis

thrust mechanics in highly overpressured accretionary wedges rapid tectonic loading of wet sediment in accretionary wedges is likely to cause the fluid pressure to rise until it is sufficient to cause dilatant fracturing dewatering of sediment that has been underthrust and accreted beneath the wedge can produce a large steady supply of such highly overpressured fluid dilatant fracturing will create escape routes so the fluid pressure is likely to be buffered at the value required for the transition between shear and oblique tensile dilatant fracture which is slightly in excess of the load pressure if the maximum compression is nearly horizontal this in turn buffers the strength of the wedge at the cohesive strength which is not pressure dependent and will not vary greatly throughout the wedge near the wedge front the strength is likely to be that of the cohesion on existing thrust faults in the wedge the shear resistance on the base of the wedge will also be fairly constant and related to the cohesive strength of the weak sediment layer that acts as the basal detachment these assumptions allow the application of a simple plastic continuum model which successfully predicts the observed gently convex taper of accretionary wedges they also allow the use of a mechanical model for frontal imbrication which predicts the spacing of imbricate thrusts at the front of the wedge from the strength and thickness of the imbricated layer the ramp angle and the factional resistances on the bounding faults and decollement surfaces reconstruction of thrust geometries from seismic data across the makran and other accretionary wedges allows the determination of the geometrical parameters and the thrust imbrication model allows the mechanical parameters to be calculated in terms of an assumed cohesive strength for the incoming sediment layer the calculated values are reasonable and successful forward modelling of the profile of the makran shows that the postulate of a buffered value of fluid pressure in excess of the load pressure is consistent with observation

accommodation of arabia eurasia convergence in the zagros makran transfer zone se iran a transition between collision and subduction through a young deforming system 1 at iranian longitude the arabian plate is moving northward relative to eurasia 20 mm yr 1 according to gps to the east this relative motion is accommodated by northward subduction under the e w makran emerged accretionary prism to the west it is accommodated partly by the zagros fold and thrust belt and partly by the alborz kopet dagh deforming zones further north this work investigates the nnw striking transition zone that connects zagros and makran the minab zendan fault system satellite images and structural and geomorphic field observations show a distributed deformation pattern covering a wide domain five north to nw trending major faults were identified they exhibit evidence for late quaternary reverse right lateral slip and correspond to two distinct fault systems the western one transferring the zagros deformation to the makran prism and the eastern one northward transferring the deformation to the alborz kopet dagh tectonic study and fault slip vector analyses indicate that two distinct tectonic regimes have occurred successively since the miocene within a consistent regional ne trending compression 1 an upper miocene to pliocene tectonic regime characterized by partitioned deformation between reverse faulting and en echelon folding 2 a ne trending 1 axis transpressional regime homogeneously affecting the region since upper pliocene the change is contemporaneous with major tectonic reorganization regionally recorded therefore this study provides evidence for active deformation that is not localized but distributed across a wide zone it accommodates the convergence and transfers it from collision to subduction by transpressional tectonics without any partitioning process in the present day period

out of sequence thrusts out of sequence thrusts oost are those thrusts which do not obey the foreland propagating or in sequence deformation style they include both isolated thrusts which develop hindward of the thrust front and sequences of break back thrusts which propagate from the foreland to the hinterland two end members of a series of oost types are recognized 1 older in sequence thrusts which are reactivated along their entire length and 2 completely new thrusts which propagate through already deformed thrust sheets between the two end members are thrusts composed partially of reactivated in sequence thrust sequences and partially of new entirely out of sequence segments oosts can be initiated for a variety of reasons including 1 keeping the orogenic wedge at critical taper 2 break back sequences from the suture zone in the overriding plate 3 ramping to overcome a sticking point which inhibits in sequence thrust propagation and 4 during simultaneous displacement along two stacked thrusts culminations which bow up segment of the upper thrust may be chopped through to permit continued displacement on the upper thrust many different types of thrust behavior including gravity sliding plucking and derivation of isolated horses from ramps may mimic some of the characteristics of oosts consequently it may be difficult to conclusively prove an oost origin for a complex thrust geometry

structural evolution of a three dimensional finite width crustal wedge abstract we present the results of three dimensional numerical experiments designed to study the response of a layer of crustal material subjected to convergence through an imposed basal velocity discontinuity and to surface processes erosion sedimentation we focus on the three dimensional response of the system arising from the finite width of the imposed velocity discontinuity in particular we describe the complex structures that develop around the wedge and their interactions with the loading unloading produced by the surface processes we show that the pro and retro shear zones that develop in a doubly vergent two dimensional wedge curve around the end of the velocity line discontinuity to merge into the strike slip structure that naturally develop i e as a consequence of the imposed boundary conditions along the edge of the wedge along the retro shear zone the stress orientation rotates along a vertical axis which implies that the retro shear zone is a pure thrust along all of its curved length whereas along the pro shear zone stresses rotate along a horizontal axis which in turn implies that the pro shear zone progressively evolves towards an oblique thrust in its curved section furthermore the outward motion i e perpendicular to the direction of imposed shortening along the curved section of the retro shear zone is accommodated by oblique extension along a secondary kinked structure antithetic to the retro shear zone we also show the complex evolution of the wedge when ductile flow and ductile strain softening is activated by increasing the imposed basal temperature in this situation the wedge is broader as structures develop at finite distances on either side of the line discontinuity and its dynamics resembles more that of a vise like orogen at the surface a flat plateau forms that accumulates sediment from the surrounding actively deforming mountain ranges until a channel breaks through one of the sides and flushes the inward draining basin of its sedimentary content the complex numerical models provide a detailed insight into the geometry and complex three dimensional structures within and at the margins of an orogen and how they can be affected by erosional processes at the surface

melanges and disrupted rocks at the leading edge of the humber arm allochthon w newfoundland appalachians deformation under high fluid pressure abstract the humber arm allochthon was structurally emplaced onto the laurentian margin in western newfoundland during taconian ordovician and acadian devonian deformation on port au port peninsula disrupted allochthonous rocks previously mapped as melange and scaly shale include three mappable variably disrupted stratigraphic units in addition mixed rocks constitute melange with much smaller area than previously mapped at outcrop scale a qualitative assessment of disruption distinguishes broken but coherent stratigraphy from a more disrupted and mixed melange unit within coherent regions three generations of folds are probably related to taconian acadian and carboniferous deformation events more disrupted regions show an average of 24 blocks to 76 matrix with block sizes 0 5 158 cm a new sampling technique allowed recovery of oriented melange samples for thin section multiple orientations of extensional fractures suggest approximately coaxial extension abundant carbonate and less common bitumen filled veins suggest that high fluid pressure played a role in the emplacement of the allochthon high fluid pressure was probably also responsible for dewatering structures sandstone dykes and partially brecciated carbonate beds map relationships outcrop and thin section scale observations lead to a reinterpreted structural history for western newfoundland in which an early taconian west bay thrust sheet was rapidly emplaced onto the laurentian margin during emplacement debris flows initially contributed igneous blocks to the allochthon but the majority of fragmentation took place in an environment of horizontal tectonic extension promoted by high fluid pressure that encouraged brittle fracture the west bay thrust sheet was subsequently overridden by the out of sequence lourdes thrust parts of the allochthon were probably re imbricated in later events but because of previous disruption an organized imbricated thrust belt was not developed at the tip of an advancing thrust wedge a clear distinction between tectonic and surficial processes of melange formation may not be possible

basin development and inversion at the appalachian structural front port au port peninsula western newfoundland appalachians in the humber zone of the newfoundland appalachians cambro ordovician shelf and foreland basin successions are affected by middle ordovician taconian orogeny and devonian acadian orogeny deformation on port au port peninsula the presence of the late ordovician to late silurian long point clam bank succession allows these episodes to be separated the taconian foreland basin stratigraphy on port au port peninsula is highly variable on the west coast platform carbonates are overlain by megaconglomerates of the cape cormorant formation which record progressive exposure of 1 km of the platform succession the conglomerates are restricted to a narrow zone consistent with derivation from a fault scarp originally immediately west of the outcrops in palinspastic restoration farther east at victors brook the cape cormorant formation is absent but the overlying almost undeformed goose tickle group contains conglomerate derived both from the upper part of the platform succession and from the tacon

two sided orogen collision and erosion from the sandbox to the southern alps new zealand in continental collision zones the indentor is of approximately the same vertical dimension as the material being deformed thereby allowing material to move over the indentor and out of the deforming orogen this simple constraint on the height of the indentor differs from that imposed by the bulldozer model of wedge deformation and results in the formation of an orogen with two opposite dipping topographic slopes these two wedges are mechanically coupled and the mechanics are strongly influenced by the erosion conditions imposed on the wedge where the growing mountains perturb prevailing moisture laden winds the erosion pattern is highly asymmetric erosional work being concentrated at the toe of the steep wedge facing the indentor the inboard wedge concentration of erosional energy results in concentration of mechanical energy in the same region and produces the distinctive geological and geophysical patterns associated with continental collision these include rapid uplift and exposure of deep crustal assemblages adjacent to the indentor accompanied by high crustal heat flow and anomalously low seismicity the outcrop pattern along the inboard wedge evolves from that of a crustal cross section currently exposed in the southern alps to one dominated by repetitive nappe structures consisting predominantly of lower crustal units as in the western alps of europe the other wedge the outboard wedge undergoes relatively little erosion is broad with a more gentle surface slope and maintains an outcrop pattern consisting of upper crustal material

numerical simulation of rayleigh taylor instability for single and multiple salt diapirs abstract the diapiric ascent of light rocks through a denser oberburden is simulated using a new numerical method based in part on finite differences rocks are modelled as viscous newtonian liquids and the internal interfaces are located with the help of passive markers these markers are relocated using a new algorithm equivalent to a cellular automaton rule this allows the markers to be redistributed very rapidly this new method appears to be well adapted to quickly resolving geological problems simulations of the growth of a single dome are performed under various viscosity ratios for the salt and the overburden the effect of asymmetrical initial deformations of the salt layer is to produce diapiric structures that slowly return to symmetry as the diapir matures and rises in an isotropic medium simulations involving continuous sedimentation of the overburden are also performed an initial geometry on a basin margin in which multiple diapirs eventually develop is studied diapirs grow relatively rapidly near the centre while on the margins the salt layer flows upward following the basement simulations with salt that is more viscous than sediments show the development of previously unnoticed asymmetrical structures

steady state erosion of critical coulomb wedges with applications to taiwan and the himalaya 1 orogenic structure appears to be partially controlled by the addition to and removal of material from the mountain belt by tectonic accretion and geomorphic erosion respectively we developed a coupled erosion deformation model for orogenic wedges that are in erosional steady state and deform at their coulomb failure limit erosional steady state is reached when all material introduced into the wedge is removed by erosion that is limited by the rate at which rivers erode through bedrock we found that the ultimate form of a wedge is controlled by the wedge mechanical properties sole out depth of the basal decollement erosional exponents basin geometry and the ratio of the added material flux to the erosional constant as this latter ratio is increased wedge width and surface slopes increase we applied these models to the taiwan and himalayan orogenic wedges and found that despite a higher flux of material entering the former the inferred ratio was larger for the latter calculated values for the erodibility of each wedge showed at least an order of magnitude lower value for the himalaya relative to taiwan these values are consistent with the lower precipitation regime in the himalaya relative to taiwan and the exposure of crystalline rocks within the himalayan orogenic wedge independently determined rock erodibility estimates are consistent with the accretionary wedge sediments and metasediments and the crystalline and high grade metamorphic rocks exposed within taiwan and the himalaya respectively therefore differences in rock type and climate apparently lead to key differences in the erosion and hence orogenic structure of these two mountain belts

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

structural and tectonic evolution of the humber zone western newfoundland 1 implications of balanced cross sections through the appalachian structural front port au port peninsula the appalachian structural front in western newfoundland which is principally a submarine feature that comes ashore on port au port peninsula is interpreted to be a structural triangle zone or tectonic wedge rocks within the triangle zone which are therefore transported include the taconian middle ordovician humber arm allochthon taconian foreland clastic sediments and the cambro ordovician platform succession the transported clastic sediments and platformal rocks as well as structurally involved grenville crystalline basement exposed east of the peninsula compose the acadian siluro devonian port au port allochthon which carried the humber arm allochthon as a high structural slice acadian transport of tens of kilometers westward is suggested this interpretation contrasts markedly with the traditional and widely accepted interpretation of the taconian and acadian orogenies in western newfoundland here we reinforce our earlier arguments for an allochthonous interpretation by presenting a series of six closely spaced balanced cross sections across the port au port peninsula these sections illustrate our interpretation of complex and noncylindrical structures within the triangle zone as well as the envisioned structural linkage between exposures on port au port peninsula and the geometry of the triangle zone interpreted from offshore seismic data 23 km along strike to the northeast the southeast vergent tea cove thrust which formed an early upper detachment to the triangle zone is offset and overturned by the north northwest vergent round head thrust the round head thrust is interpreted to flatten into a second southeast vergent thrust the red brook detachment which formed a late upper detachment to the triangle zone some map scale structures are interpreted as fault bend folds above oblique ramps on the red brook detachment allochthonous crystalline basement within the triangle zone beneath its attached platformal cover is interpreted as a result of thrust reactivation of a preexisting basement cutting normal fault which may have formed during rifting of the early paleozoic iapetus ocean this interpretation is suggested by the stratigraphy and geometry of spatially restricted coarse conglomeratic units on port au port peninsula and is supported by the restored cross sections

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