past forearc deformation in nicaragua and coupling at the megathrust interface evidence for subduction retreat 1 deformation at the boundary between the cocos and caribbean plates in nicaragua is currently focused in the volcanic arc in the form of arc parallel shearing in the middle miocene however there was widespread subsidence with local uplift in the outer forearc and inner forearc broad uplift to understand this complex deformation we use numerical modeling to investigate the effect of inhomogeneous friction at the megathrust fault on strain localization in the upper plate a good fit is obtained when 1 the interface between the slab and upper plate is not situated at the current wadati benioff zone but rather is moved 50 km landward to where subduction may have been active in the early tertiary and 2 there is high stress accumulation between 15 and 25 km depths and low stress accumulation updip and downdip of the thrust interface the results are consistent with a high velocity block being the remnant of an oceanic plateau that subducted in the eocene and was later incorporated into the upper plate we suggest that this geometry persisted into the middle miocene before the slab broke off and the thrust interface jumped seaward

a numerical model of deformation and fluid flow in an evolving thrust wedge abstract to investigate deformation and fluid flow in an actively deforming tectonic wedge we model the evolution of a large two dimensional 100 km long 5 km thick mechanically and hydrologically homogeneous and isotropic pile of sedimentary strata that is deformed to become a thrust wedge we compare both dry and wet cases in order to illustrate 1 the relative importance of fluids on wedge evolution and 2 the effect of brittle deformation on fluid flow we use an elastic plastic constitutive relation including a mohr coulomb failure criterion and a non associated flow rule and coupled fluid flow with bulk rock properties that approximate typical foreland sedimentary strata simulations are made both with and without dilation the model is fully dynamic but inertial forces remain small results show that deformation within the wedge is accommodated by reverse slip movement on shear bands which migrate in both directions through the wedge as both fore and back thrusts the model has features predicted by the critical taper theory 1 overall wedge geometry 2 crudely self similar growth during evolution 3 more intense deformation toward the rear of the wedge the models show strong overall in sequence faulting behavior with major thrusts isolating relatively undeformed packages which are moved in a piggyback manner upon the active thrusts intermittent out of sequence faulting does however occur in order to maintain the wedge taper fluid flow in the deforming wedge is dominated by topography but is also strongly affected by dilational plastic deformation in all simulations there is focused fluid flow within fault zones when mechanical time stepping is shut off uncoupled flow systems evolve to steady state where inflow equals outflow by subtracting the two states we isolate the mechanical fluid response from the total coupled system response the mechanical fluid response is manifest as contours of head and pressure difference and focusing of fluid flow in areas of active deformation in non dilational simulations these zones are quite localized and are a result of the contrasting stresses in and adjacent to a brittle shear zone in simulations with dilation the zones of head and pressure difference are broad and regional in extent they are ten times greater in magnitude than in non dilation simulations and they track a rock damage front that moves through the wedge

chemistry and mineralogy of some plio pleistocene tuffs from the shungura formation southwest ethiopia abstract the shungura formation of southwestern ethiopia has yielded many tens of thousands of vertebrate fossils including hominids and microvertebrates and in addition has also yielded fossil wood pollen and invertebrates widespread tuffs have made subdivision and detailed mapping of the formation possible have provided material for potassium argon dating and have allowed direct lithostratigraphic correlation with the koobi fora formation in northern kenya the basis for correlation between the two formations is the distinctive chemistry of the tuffs but systematic chemical variation within some tuffs invalidates some statistical correlation techniques here chemical analysis of glass separates and minerals from tuffs of the shungura and usno formations are presented which may allow further ties to be established when data become available on other tuffs of the koobi fora formation the tuffs consist primarily of glass but also contain phenocrysts of anorthoclase hedenbergitic pyroxene sodic amphibole ilmenite titanomagnetite chevkinite quartz zircon and rarely orthopyroxene and plagioclase the glasses show evidence of alkali loss during hydration and are not now peralkaline although it is likely that they were initially the source volcanoes were most likely situated within the ethiopian rift valley or on its margins

an adaptive staggered grid finite difference method for modeling geodynamic stokes flows with strongly variable viscosity 1 here we describe a new staggered grid formulation for discretizing incompressible stokes flow which has been specifically designed for use on adaptive quadtree type meshes the key to our new adaptive staggered grid asg stencil is in the form of the stress conservative finite difference constraints which are enforced at the hanging velocity nodes between resolution transitions within the mesh the new asg discretization maintains a compact stencil thus preserving the sparsity within the matrix which both minimizes the computational cost and enables the discrete system to be efficiently solved via sparse direct factorizations or iterative methods we demonstrate numerically that the asg stencil 1 is stable and does not produce spurious pressure oscillations across regions of grid refinement which intersect discontinuous viscosity structures and 2 possesses the same order of accuracy as the classical nonadaptive staggered grid discretization several pragmatic error indicators that are used to drive adaptivity are introduced in order to demonstrate the superior performance of the asg stencil over traditional nonadaptive grid approaches furthermore to demonstrate the potential of this new methodology we present geodynamic examples of both lithospheric and planetary scales models

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

material transfer in accretionary wedges from analysis of a systematic series of analog experiments the structural evolution of accretionary wedges was modeled in a series of analog experiments with systematically varying initial boundary conditions the two principle parameters investigated were basal friction and thickness of sediment input relative to subducted sediment output material transfer in terms of degree and location of accretion and erosion is quantified as a function of these two parameters low basal friction produces long thin wedges by continuous frontal accretion of imbricate thrusts simultaneously substantial basal erosion may occur with little influence on the frontal dynamics the formation of an outer arc high with an accompanying forearc basin depends upon the relative amount of basal erosion the opening of the subduction window spacing of thrust faults is proportional to the layer thickness confirming the principle of self similar wedge growth from critical wedge theory high basal friction typically generates steep wedges marked by an erosional mass wasting frontal slope in the cases i o this is due to underthrusting and or subduction of the entire sedimentary section the case i o also features the late development of normal faulting and an extensional forearc basin for both low and high basal friction the material balance affects the wedge slope erosive conditions i o for low basal friction the primary modes of material transfer are frontal accretion and basal erosion for high basal friction the primary modes of material transfer are underthrusting underplating and frontal erosion applying the experimental results to active convergent margins parallels may be drawn thus the barbados ridge complex is interpreted to be a good example of a low basal friction wedge with an accretionary material balance while the japan and mariana margins provide examples of an erosive material balance

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

from passive continental margin to mountain belt insights from analytical and numerical models and application to taiwan abstract the physics of deformation at plate boundaries is insufficiently understood taiwan is particularly interesting in this context since geological and geophysical arguments indicate that passive margin sediments were exhumed into an active mountain belt over a relatively short period of time classically taiwan has been described as a critical wedge orogeny in which deformation is entirely restricted to the crust more recent data however indicate that deeper parts of the lithosphere also participate in deformation in order to study the physical feasibility of various endmember hypothesis we have performed two dimensional 2 d thermo mechanical numerical experiments in which both mantle lithospheric and surface processes are taken into account and in which crustal deformation evolves as a function of mantle flow our setup consists of ocean ocean subduction followed by passive continental margin deformation results indicate that in most cases the subducting slab becomes close to vertical upon arrival of continental material to the trench whereas crustal rheology deformation and surface processes do not modify mantle flow significantly they are important for crustal scale processes relatively little crustal exhumation occurs if the crust is homogeneous and erosion rates are small significant exhumation in a dome like structure is obtained if a weak lower crust and large erosion rates are present the presence of an oceanic arc in the overriding oceanic lithosphere modifies but does not control crustal deformation synthetic metamorphic facies maps and thermochronological data computed from the numerical models are in reasonable agreement with observations in southern taiwan if exhumation occurs in a dome like structure further physical insight in the doming mechanisms is obtained by a combination of analytical techniques and crustal scale numerical models this analysis indicates that the dome like exhumation mechanism can be attributed to a compressional mechanical instability facilitated by both a weak lower crust and large rates of erosion both conditions are likely to be satisfied in taiwan

crustal deformation dynamics and stress evolution during seamount subduction high resolution 3 d numerical modeling funding was received from the people programme marie curie actions of the european union s seventh framework programme fp7 2007 2013 under rea grant agreement 604713 zip zooming in between plates v s and c r r acknowledge financial support received through the hades project ctm2011 30400 c01 and ctm2011 30400 c02 funded by spanish mineco

composite trough evolution of selected west pacific extratropical cyclones abstract the observed vertical structures of the trough axes for 27 extratropical cyclones are presented this study is motivated by results from a simple theoretical model two observing times during the cyclone life cycle are shown prior to development and during the mature but still amplifying stage prior to development upper and lower troughs are present and separate each has little or no tilt the upper one is typically prominent down to 4 km elevation and the separation between the lower and the upper features varies depending on where the approaching upper trough happens to be at the observing time at the mature stage upper and lower features are connected a uniform tilt typically develops through the entire troposphere the tilt is typically due west with height and the tilt may have a preferred slope an empirical orthogonal function eof analysis finds that two modes account for more than 97 of the variance the equivalent barotropic eof has the most variance by far though the frac

the impact of analogue material properties on the geometry kinematics and dynamics of convergent sand wedges abstract simulation of geodynamic processes in sandbox experiments requires analogue materials with a deformation behaviour that reproduces the deformation mechanisms of typical crustal rocks we present data on the frictional strength of different sand types employing static and dynamic shear tests the sand types analysed are characterised by an elastic frictional plastic mechanical behaviour with a transient strain hardening and strain softening phase prior to transition to stable sliding this is in conflict with the standard assumption of an ideal cohesionless coulomb material with constant frictional properties the influence of the identified transient material properties on the kinematics growth mechanisms and internal deformation patterns of convergent sand wedges results in characteristic wedge segments which vary depending on material compaction between wedges with well defined segments i e frontal deformation zone frontal imbrication zone and internal accumulation zone with straight slopes and wedges with a continuous convex topographic profile for most materials only the frontal part of the wedge is critical during experimental runs taper and strength of the wedge segments can be shown to be controlled by the frictional properties of active faults wedge segmentation is controlled by a bulk strength increase toward the rear of the wedge due to fault rotation in mechanically less favourable orientations and plastic material hardening the limit between the frontal critical parts of a wedge and internal stable parts is largely controlled by a critical state of stress upon which either renewed failure or fault inactivation occurs on this basis we suggest that critical taper analysis of wedges must be restricted to specific kinematic segments comparison of the experimental results with the nankai accretionary wedge suggests that our interpretation also applies to natural convergent wedges moreover we provide constraints for the selection of adequate granular analogue materials to simulate typical crustal rocks in natural convergent wedges

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

stratigraphical significance of the tulu bor tuff of the koobi fora formation the tulu bor tuff a major marker horizon in the koobi fora formation has been misidentified in previous studies identification of this tuff in sediments previously mapped as the kubi algi formation requires re evaluation of the existing stratigraphical framework of the koobi fora region

the nature of magnetic single domains in volcanic glasses of the kbs tuff magnetic and optical variations in volcanic glasses of the east african rift have been investigated by means of experimental and analytical study the results add to the formal understanding of magnetism in volcanic glasses and give us new perspectives on the nature origin and significance of fine grained magnetic minerals and on the magnetic petrology of glassy rocks the samples that we have studied in detail glass shards from the kbs tuff of northern kenya contain a superparamagnetic precipitate of microcrystals 2 to 10 nm in size the precipitate is likely to have formed at elevated temperature during or subsequent to eruption about 1 9 m y ago we have studied these microcrystals by means of transmission electron microscopy tem ac and dc magnetic methods electron spin resonance esr mossbauer spectroscopy and optical spectroscopy our efforts have concentrated on determining the magnetic mineralogy and understanding the mechanism s responsible for the optical and magnetic behavior of these glasses the particle size determination for the larger of these microcrystals has been achieved using tem imaging and mossbauer spectroscopy the mineralogy is nonstoichiometric magnetite based on mossbauer x ray diffraction and electron diffraction results the ratio of saturation remanent magnetization to saturation magnetization for the larger microcrystals is very nearly that predicted by single domain theory for a uniaxial anisotropy the origin of which may lie with stress or surface effects we interpret magnetic and esr results to infer the possible existence of microcrystals or amorphous clusters of fe atoms 1 nm or smaller in kbs glasses that appear homogeneous in high resolution tem images results from optical absorption experiments show that the color variations in kbs shards arise due to optical absorption by the precipitate a result consistent with classical electromagnetic theory

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

lithospheric pressure depth relationship in compressive regions of thickened crust we investigate the pressure distribution with depth in regions undergoing horizontal shortening and experiencing crustal thickening both analytically and numerically our results show that in a convergent tectonic setting pressure can be considerably higher than lithostatic the pressure resulting from the weight of the overburden increases in pressure with respect to lithostatic conditions result from both the contribution of horizontal stresses and the flexural vertical loads the latter generated by the deflection of the upper crust and of the mantle because of the presence of topographic relief and a root respectively the contribution of horizontal stresses is particularly relevant to the upper crust and uppermost mantle where rocks are thought to deform brittlely in these domains pressure gradients twice lithostatic can be achieved the contribution of horizontal stresses is less important in the ductile domains as diverential stresses are progressively relaxed nevertheless the evects are still noteworthy especially close to the brittle ductile transition flexural vertical loads generated by the deflection of the upper crust and lithospheric mantle are relevant for rocks of the weaker lower crust as a result of the combination of the two mechanisms the pressure gradient varies vertically through the lithosphere ranging from negative inverted gradients to gradients up to several times the lithostatic gradient the pressure values range from one to two times the lithostatic values 1rgz to 2rgz

numerical models of the evolution of accretionary wedges and fold and thrust belts using the distinct element method a 2 d numerical model is used to investigate the evolution of accretionary wedges and fold and thrust belts the numerical method is based on the distinct element method dem unlike many continuum numerical models demallows localization to occur even after substantial amounts of deformation the method is used to study the evolution of simple accretionary wedges and thrust belts with a rigid backstop and base experiments are done with a large range of coefficients of interelement friction e and element wall friction b two modes of deformation which depend mainly on b are observed for the weak base case low b the dominant mode is frontal accretion by pop up structures at or near the toe of the wedge for the strong base case high b uplift is concentrated near the back of the wedge and is accompanied by underthrusting along a flat ramp flat or staircase thrust fault structure at intermediate values of b the wedge oscillates between the two modes of deformation during periods of frontal accretion normal faulting sometimes occurs in regions where the material has thickened considerably the transition between the two modes of deformation is found to be a strong function of b but a weak function of e a simple explanation of the experimental results is made using the principle of work minimization comparisons between the results and some accretionary wedges fold and thrust belts are also made

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

discrete element modelling of hanging wall deformation along the d 1 fault system danish north sea with implications for gas chimney interpretations abstract this study uses 3d seismic mapping nncc attribute analysis and 2d sdem modelling of soft sediment deformation to analyse hanging wall deformation related to faulting across a listric fault detaching along the top of a major salt roller the computational model experiments used the overall fault and salt structure evolution as boundary conditions to predict the distribution of faults within the hanging wall the modelling successfully reproduced geometries similar to the syn and antithetic faults that are visible on seismics but it also showed the presence of well defined vertical deformation zones both in the hanging wall and the foot wall of the listric fault in the model salt movement controls the formation of the deformation zones the similarity of nncc inferred vertical chimneys and the modelled deformation zones suggests that the subtle vertical chimneys identified in the 3d seismic data might actually be related to vertical to sub vertical deformation zones caused by the presence and movement of the salt structure

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

dynamics of outer rise faulting in oceanic continental subduction systems 1 during subduction bending of downgoing oceanic lithosphere gives rise to normal faulting due to the extensional stress state generated in the upper plate as deformation patterns inherently reflect a material s state of stress and rheology extensive global observations of outer rise faulting patterns and subduction dynamics provide a unique opportunity to examine the factors controlling outer rise deformation despite a wide range of observed oceanic plate ages convergence rates and slab pull magnitudes across modern subduction systems however measured outer rise faulting patterns show effectively no correlation to variations in these parameters this lack of correlation may reflect that outer rise faulting patterns are strongly sensitive to all of these parameters are dependent on additional parameters such as downgoing overriding plate coupling or that existing faulting measurements require additional analysis in order to provide a basis for future analysis of outer rise faulting patterns we build on previous thermo mechanical numerical models of outer rise deformation and explore the relationship between outer rise faulting patterns subduction dynamics and brittle rheology in an oceanic continental subduction system analysis of time averaged outer rise faulting patterns indicates that downgoing plate age and velocity downgoing overriding plate coupling and slab pull all significantly affect faulting patterns while variations in brittle rheology have a significantly smaller impact these relationships reflect that the sensitivity of outer rise faulting patterns to the frictional properties of the oceanic crust and mantle is small compared to variations in the overall stress state and deformation rate of subduction systems in order to gain additional insight into the origin outer rise faulting patterns future numerical studies should focus on specific regions in order to place constraints on the structure of the downgoing plate and dynamics of the subduction system

new single crystal 40ar 39ar ages improve time scale for deposition of the omo group omo turkana basin east africa six tuffaceous beds within the omo group of the omo turkana basin have been dated using the 40 ar 39 ar single crystal total fusion method on anorthoclase yielding eruption ages the omo group constitutes up to 800 m of subaerially exposed sediments surrounding lake turkana within the east african rift system in northern kenya and southern ethiopia rhyolitic explosive eruptions produced tuffs and pumice clasts that are considered to have been deposited shortly after eruption the new age data on feldspars from the pumice clasts range from 4 02 0 04 ma for the naibar tuff of the koobi fora formation to 1 53 0 02 ma for tuff k of the shungura formation the orange tuff in the kbs member of the koobi fora formation was dated at 1 76 0 03 ma providing good control in this part of the sequence where formerly there was a 200 ka gap data are consistent with earlier measurements and significantly improve age resolution within the omo group which has yielded many vertebrate fossils including hominin fossils comprising a number of species we suggest new age estimates for a limited number of hominin specimens supplementary material eleven tables and nine figures are available at www geolsoc org uk sup18506

numerical models of crustal scale convection and partial melting beneath the altiplano puna plateau we employ 2 d thermo mechanical modelling to study possible mechanisms for generating large scale crustal magmas in the altiplano puna region of the central andes the peak of ignimbrite activity in the late miocene and pliocene is associated in space and time with tectonic shortening and plateau uplift a seismic low velocity zone and other geophysical observations indicate that partial melting in the mid crust is still present under the ignimbrite province today we show that neither radiogenic heat production in a thickening crust nor shear heating due to tectonic shortening nor heat brought by intrusions of arc magmas into the mid crust can heat the mid crust to the degree and within the time frame suggested by the geologic petrologic and geophysical observations a viable mechanism to achieve high temperatures within the time constraints is convective heat and mass transfer by partially molten lower crust which itself was heated by enhanced mantle heat flow possibly associated with delamination of the mantle lithosphere during tectonic shortening and intensified magmatic arc activity the thermo mechanical models explain the mid crustal low velocity zone and the high and strongly variable surface heat flow observed in the altiplano convection by bulk flow of the crust appears when the middle and lower crust are mechanically weak quartz dominated rheology the basal heat flow from the mantle is high s 60 mw m 2 and tectonic shortening is active we argue that these conditions are satisfied in the central andes orogen 6 2002 elsevier science b v all rights reserved

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

a sedimentation device to produce uniform sand packs abstract in physical modelling using sand the construction of a sand pack is one of the main experimental difficulties existing solutions are generally based either on delicate manipulations pouring and scraping or on motorized sedimentation devices and the qualities of packs thus produced are known to affect experimental outcomes however uniformity planarity reproducibility and efficiency can be achieved with a simple sedimentation device without motors a rectangular reservoir pierced with holes rests on a support plate pierced with wider holes sand flows when the reservoir is displaced so that its holes match those of the support plate sand jets are diffused by planar horizontal sieves below which the sand sets into the experimental box tests on a 250 m median grain size sand show that the density is at its maximum value reproducible and uniform the spatial variations are only of 0 4 of the mean density thickness of the sand layers shows spatial variations around 2 of the mean thickness and 6 near the side walls very fine grains 90 m median grain size produce less uniform and less planar packs because of their greater sensitivity to air currents caused by the sedimentation according to direct shear tests the sand pack has a well defined static friction coefficient decreasing to a lower dynamic value after about 3 mm of slip with dilatancy in contrast poured sand packs which are initially less dense develop only the dynamic friction coefficient no peak shear stress during slip without dilatancy hole diameters hole spacing and the number and openings of the sieves are the parameters controlling the qualities of the sand pack for a given grain size distribution

4d analogue modelling of transtensional pull apart basins abstract scaled sandbox models were used to investigate the 4d evolution of pull apart basins formed above underlapping releasing stepovers in both pure strike slip and transtensional basement fault systems serial sectioning and 3d volume reconstruction permitted analysis of the full 3d fault geometries results show that very different pull apart basins are developed in transtension compared to pure strike slip both types of models produced elongate sigmoidal to rhomboidal pull apart systems but the transtensional pull apart basins were significantly wider and uniquely developed a basin margin of en echelon oblique extensional faults dual opposing depocentres formed in the transtensional model whereas a single central depocentre formed in pure strike slip in transtension a distinct narrow graben system formed above the principal displacement zones pdzs cross basin fault systems that linked the offset pdzs formed earlier in the transtensional models sequential model runs to higher pdz displacements allowed the progressive evolution of the fault systems to be evaluated in cross section transtensional pull aparts initiated as asymmetric grabens bounded by planar oblique extensional faults with increasing displacement on the pdzs basin subsidence caused these faults to become concave upwards and lower in dip angle due to fault block collapse towards the interior of the basin in addition strain partitioning caused fault slip to become either predominantly extensional or strike slip the models compare closely with the geometries of natural pull apart basins including the southern dead sea fault system and the vienna basin austria

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

noncohesive critical coulomb wedges an exact solution active fold and thrust belts or submarine accretionary complexes can be modeled as critically tapered wedges of material on the verge of coulomb failure everywhere overlying a basal decollement where frictional sliding is occurring ignoring cohesion the four strength parameters needed to describe a critical coulomb wedge are its internal and basal coefficients of friction and b and its internal and basal hubbert rubey fluid pressure ratios and b an exact relation between surface slope and basal dip of a noncohesive critical wedge with uniform properties is derived the state of stress within such a wedge has the same orientation everywhere and is constant if is and vice versa a coefficient of internal friction 1 1 is consistent with the known surface slope basal dip and pore fluid pressures in the active fold and thrust belt of western taiwan assuming that byerlee s law b 0 85 is valid on the base the wide variety of tectonic styles observed to occur along convergent margins including subduction erosion active accretion subduction without accretion and even extension and normal faulting may be controlled by relatively small spatial or temporal variations in either b or b

an application of two dimensional finite element modelling for studying the deformation of the variscan fold and thrust belt belgium abstract this paper presents 2d finite element simulations of fold and thrust belts with a focus on the variscan foreland thrust belt in belgium the numerical models are limited to the deformation of a 5 km thick sedimentary layer separated from a backstop by a deformable zone the sedimentary layer obeys an elastic plastic law and can include one detachment horizon with some ramps our study deals with the effects of rheological and geometrical parameters on the structural characteristics of the belt for regular basin geometries rectangular and wedge shapes results of numerical simulations show that deformation continuously propagates and decreases towards the foreland in agreement with sandbox experiments the basal friction appears to be a major parameter controlling the tectonic style of a fold and thrust belt our analysis has also shown that the strain propagation in time and space is strongly controlled by the presence of a weak intra detachment and of ramps which initiate shear bands for the rhenohercynian fold and thrust belt a major decollement is usually considered within the palaeozoic sediments and the existence of steps in the basin geometry is proven by numerous geological data showing the activity of pre orogenic normal faulting during the variscan orogeny this irregular detachment horizon influenced the regional tectonics simulation results show that ramps create strongly deformed anticlinioria separated by weakly deformed synclinoria which is the basic structure observed in the western and northern rhenohercynian belt

dynamics of the tengchong volcanic region in the southeastern tibetan plateau a numerical study abstract the constraints of surface heat flow and lithospheric structure were used to investigate the dynamics of the tengchong volcanic region based on thermal mechanical modeling three different boundary conditions representing three types of tectonic movements were modeled to discuss the thermal state and lithospheric deformation the compressive model shows that the subduction of the indian plate causes the upwelling of asthenosphere beneath the tengchong block in this process upwelling of the asthenosphere causes thinning of the lithosphere beneath the tengchong block however the surface heat flow of this model is not consistent with the observations the regional extension model shows that under the extension the asthenosphere upwells along the preexisting weak suture zone in the southeastern tibetan plateau the thickness of lithosphere is the thinnest along the suture zone and the surface heat flow is the highest however the extension time was found to generally need 10 15 myr to conform to the surface heat flow however this is not consistent with accepted geochronological data when a boundary condition from compression to extension is imposed both the surface heat flow and the lithospheric structure fit well with the observations the results show that the subduction of the indian plate leads to the upwelling of the asthenosphere which causes a thinned lithosphere beneath the tengchong block since the late miocene 8 5 ma the extension or extrusion has been predominant the thermal erosion of the lithosphere caused by upwelling of the asthenosphere renders the lithosphere even thinner along the bangong nujiang suture zone the upwelling of the asthenosphere provides an upward driving force and high temperature environment for the tengchong volcanic region these findings suggest that the origin of the tengchong volcanic region is caused by both the subduction of the indian plate and regional extension across the bangong nujiang suture zone

the benefits of using a consistent tangent operator for viscoelastoplastic computations in geodynamics strain localization is ubiquitous in geodynamics and occurs at all scales within the lithosphere how the lithosphere accommodates deformation controls for example the structure of orogenic belts and the architecture of rifted margins understanding and predicting strain localization is therefore of major importance in geodynamics while the deeper parts of the lithosphere effectively deform in a viscous manner shallower levels are characterized by an elastoplastic rheological behavior herein we propose a fast and accurate way of solving problems that involve elastoplastic deformations based on the consistent linearization of the time discretized elastoplastic relation and the finite difference method the models currently account for the pressure insensitive von mises and the pressure dependent drucker prager yield criteria consistent linearization allows for resolving strain localization at kilometer scale while providing optimal that is quadratic convergence of the force residual we have validated our approach by a qualitative and quantitative comparison with results obtained using an independent code based on the finite element method we also provide a consistent linearization for a viscoelastoplastic framework and we demonstrate its ability to deliver exact partitioning between the viscous the elastic and the plastic strain components the results of the study are fully reproducible and the codes are available as a subset of m2di matlab routines

numerical investigation of the geodynamic mechanism for the late jurassic deformation of the ordos block and surrounding orogenic belts abstract orogenic belts have developed along the edges of the stable ordos block in northern china three main geodynamic models have been proposed to explain the formation of these orogenic belts they have included the collision between the north china and south china blocks subduction of the pacific plate and stresses transmitted over long distances from the closure of the mongolia okhotsk ocean however these explanations are still controversial and not universally accepted leaving the mechanisms that formed the orogenic belts poorly understood to address these fundamental questions we developed a 3d numerical model using the finite element method to explore the geodynamic mechanism for the late jurassic deformation of the ordos block and its surrounding orogenic belts we investigated the effect of different dynamic regimes on the late jurassic deformation of this region our primary results suggest that strong and stable ordos block remains undeformed despite its location at the center of a region of deformation east west trending fold and thrust belts would have developed along the north and south edges of the ordos block during the closure of the mongolia okhotsk ocean or the collision between the north china and south china blocks north south trending fold and thrust belts would have developed along the east and west edges of the ordos block due to subduction of the pacific plate however the paleo stress field in the late jurassic indicates that the orientations of the maximum compressive principle stress were nearly perpendicular to the edge of the ordos block and the compressive deformation around it was coeval it is difficult to explain the distribution of belts of deformation with a single stress regime our numerical model reveals that multi direction convergence pattern with time during the transformation of these three regimes can be used to interpret the formation and deformation styles of ringed mountains around the ordos block during the late jurassic this proposed mechanism results in many insights into the mesozoic tectonic deformation and mountain building of this region

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

controls of inherited lithospheric heterogeneity on rift linkage numerical and analog models of interaction between the kenyan and ethiopian rifts across the turkana depression inherited rheological structures in the lithosphere are expected to have large impact on the architecture of continental rifts the turkana depression in the east african rift connects the main ethiopian rift to the north with the kenya rift in the south this region is characterized by a nw se trending band of thinned crust inherited from a mesozoic rifting event which is cutting the present day n s rift trend at high angle in striking contrast to the narrow rifts in ethiopia and kenya extension in the turkana region is accommodated in sub parallel deformation domains that are laterally distributed over several hundred kilometers we present both analog experiments and numerical models that reproduce the along axis transition from narrow rifting in ethiopia and kenya to a distributed deformation within the turkana depression similarly to natural observations our models show that the ethiopian and kenyan rifts bend away from each other within the turkana region thus forming a right lateral stepover and avoiding a direct link to form a continuous n s depression the models reveal five potential types of rift linkage across the pre existing basin three types where rifts bend away from the inherited structure connecting via a 1 wide or 2 narrow rift or by 3 forming a rotating microplate 4 a type where rifts bend towards it and 5 straight rift linkage the fact that linkage type 1 is realized in the turkana region provides new insights on the rheological configuration of the mesozoic rift system at the onset of the recent rift episode

erosional control of the kinematics and geometry of fold and thrust belts imaged in a physical and numerical sandbox 1 we investigate the effect of systematically varying erosion intensity k on the geometry and kinematics of fold and thrust belts modeled using equivalent conditions in physical and numerical experiments similar material properties and boundary conditions were used to compare numerical experiments to those modeled in the sandbox using an erosion rule that removes mass from the scaled sand wedges according to rates expected from bedrock fluvial incision the geometry of both modeling approaches is quantitatively compared with that expected from analytical theory the fold and thrust belt growth rate is inversely proportional to k and is well predicted by theory except for the high erosion case in which both the numerical and the experimental sandbox grows supercritically a direct relationship exists between the erosion intensity and the number of fore shear bands and shear strain magnitude the number of fore shear bands decreases and their strain increases with erosion intensity the results indicate that realistic and mechanistic erosion rules can be applied to physical experiments to model mass removal and this approach opens up the possibility of calibrating strain history to erosion intensities for predictions in natural settings quantitative comparisons between the physical and numerical sandbox experiments indicate that deformation style and geometry of the thrust and fold belts are similar when identical rheologies and boundary conditions are used this benchmarking suggests that numerical experiments realistically model conditions observed in the simple cohesionless rheologies commonly employed in sandbox experiments this suggests that numerical simulation may reliably model experimental deformation in geological situations that are difficult to represent in scaled experiments because of the different scales and rheologies of natural orogenic wedges

the seismic cycle at subduction thrusts 2 dynamic implications of geodynamic simulations validated with laboratory models 1 the physics governing the seismic cycle at seismically active subduction zones remains poorly understood due to restricted direct observations in time and space to investigate subduction zone dynamics and associated interplate seismicity we validate a continuum visco elasto plastic numerical model with a new laboratory approach paper 1 the analogous laboratory setup includes a visco elastic gelatin wedge underthrusted by a rigid plate with defined velocity weakening and strengthening regions our geodynamic simulation approach includes velocity weakening friction to spontaneously generate a series of fast frictional instabilities that correspond to analog earthquakes a match between numerical and laboratory source parameters is obtained when velocity strengthening is applied in the aseismic regions to stabilize the rupture spontaneous evolution of absolute stresses leads to nucleation by coalescence of neighboring patches mainly occurring at evolving asperities near the seismogenic zone limits consequently a crack or occasionally even pulse like rupture propagates toward the opposite side of the seismogenic zone by increasing stresses ahead of its rupture front until it arrests on a barrier the resulting surface displacements qualitatively agree with geodetic observations and show landward and from near the downdip limit upward interseismic motions these are rebound and reversed coseismically this slip increases adjacent stresses which are relaxed postseismically by afterslip and thereby produce persistent seaward motions the wide range of observed physical phenomena including back propagation and repeated slip and the agreement with laboratory results demonstrate that visco elasto plastic geodynamic models with rate dependent friction form a new tool that can greatly contribute to our understanding of the seismic cycle at subduction zones

correlation of pliocene and pleistocene tephra layers between the turkana basin of east africa and the gulf of aden abstract electron microprobe analyses of glass shards from volcanic ash in pliocene and pleistocene deep sea sediments in the gulf of aden and the somali basin demonstrate that most of the tephra layers correlate with tephra layers known on land in the turkana basin of northern kenya and southern ethiopia previous correlations are reviewed and new correlations proposed together these data provide correlations between the deep sea cores and to the land based sections at eight levels ranging in age from about 4 to 0 7 ma specifically we correlate the moiti tuff 4 1 ma with a tephra layer at 188 6 m depth in dsdp hole 231 and with a tephra layer at 150 m depth in dsdp hole 241 the wargolo tuff with a tephra layer at 179 7 m in dsdp hole 231 and with a tephra layer at 155 3 m depth in dsdp hole 232 the lomogol tuff defined here with a tephra layer at 165 m in dsdp hole 232a the lokochot tuff with a tephra layer at 140 1 m depth in dsdp hole 232 the tulu bor tuff with a tephra layer at 160 8 m depth in dsdp hole 231 the kokiselei tuff with a tephra layer at 120 m depth in dsdp hole 231 and with a tephra layer at 90 3 m depth in dsdp hole 232 the silbo tuff 0 74 ma with a tephra layer at 35 5 m depth in dsdp hole 231 and possibly with a tephra layer at 10 9 m depth in dsdp hole 241 we also present analyses of other tephra from the deep sea cores for which correlative units on land are not yet known the correlated tephra layers provide eight chronostratigraphic horizons that make it possible to temporally correlate paleoecological and paleoclimatic data between the terrestrial and deep sea sites such correlations may make it possible to interpret faunal evolution in the lake turkana basin and other sites in east africa within a broader regional or global paleoclimatic context

crustal structures of the rhinegraben and the massif central grabens an experimental approach two of the most improtant segments of the west european rift the rhinegraben and the massif central grabens show in plan and cross section a very different crustal structure

distinct element modeling of structures formed in sedimentary overburden by extensional reactivation of basement normal faults fault and fold patterns in sedimentary rocks have been modeled using a computer code based on the distinct element method dem this numerical method considers the relative motion of disconnected elastic elements governed by newton s laws and frictional sliding criteria forces are transmitted between elements at contact points by changing element size ratios assemblages of elements can simulate different kinematic behavior from localized faulting to distributed flow as an application of the method we investigate folding and faulting patterns that develop in the sedimentary overburden above basement normal faults the number position orientation and sense of slip on faults as well as the fold geometry depend on the basement fault dip and basement rotation the dem model results have practical implications for predicting the evolving characteristics of structural traps and their petroleum potential

shear test results for cohesion and friction coefficients for different granular materials scaling implications for their usage in analogue modelling abstract laboratory tests have been carried out on dry granular materials such as quartz sand glass microspheres and sugar with different grain size rounding and sphericity the measurements have been made with a simple shear test machine for different values of normal stress 50 900 pa shear stress has been plotted against normal stress in order to determine the cohesion and coefficient of internal friction for the investigated materials resulting values of cohesion and coefficient of internal friction are mainly dependent on rounding and sphericity while grain size has a less significant influence further the behaviour of the materials for very small normal stresses 0 400 pa is more complex than previously assumed the fracture envelopes for all materials investigated are convex outward for this small range and converge towards a straight failure envelope with increasing normal stress finally in extensional faulting experiments there is no significant change in fault dip with increasing depth therefore the non linear behaviour for small normal stresses is best described as a dependence of the cohesion on the normal stress and not as a dependence of the coefficient of internal friction on the normal stress values for cohesion increase from 0 pa 15 pa at zero normal stress to 137 247 pa 15 pa for normal stresses greater than 250 400 pa the results show that well rounded spherical material is better suited to model brittle behaviour of rocks in crustal and lithospheric scale analogue models than less well rounded material since it has a smaller cohesion and a coefficient of internal friction which is closer to values of natural rocks

origin of convex accretionary wedges evidence from barbados the surface slopes of submarine accretionary wedges generally decrease away from the toe or defor mation front toward the arc this paper presents evidence that this characteristic convex cross sectional shape is a direct mechanical response to the lithification of sediments during accretion the recently accreted sediments near the toe which are typically unconsolidated and therefore very weak must deform until a critical taper is attained at which they are stable in the presence of the horizontal tectonic compression this critical wedge taper and thus the bathymetric slope of the accretionary prism is relatively high for the porous weak sediments near the toe in contrast the compacted sediments farther back in the wedge are stronger so they require a relatively modest critical taper and therefore a lower bathymetric slope to be stable under the same tectonic compression the result is a convex cross sectional wedge shape we investigate several possible causes of the increase in sediment strength within the wedge and conclude that the primary cause is an increase in cohesive strength with decreasing porosity a straightforward generalization of the exact critical taper model is combined with empirical cohesion porosity velocity relations to infer the distribution of porosity and p wave velocity within the barbados accretionary wedge we estimate that the porosity decreases from its near toe value of 70 to approximately 30 at a depth of 3 km below the seafloor and a distance of 60 km arcward from the front of the wedge

a benchmark comparison for mantle convection codes summary we have carried out a comparison study for a set of benchmark problems which are relevant for convection in the earth s mantle the cases comprise steady isoviscous convection variable viscosity convection and time dependent convection with internal heating we compare nusselt numbers velocity temperature heat flow topography and geoid data among the applied codes are finite difference finite element and spectral methods in a synthesis we give best estimates of the true solutions and ranges of uncertainty we recommend these data for the validation of convection codes in the future

the seismic cycle at subduction thrusts insights from seismo thermo mechanical models 1 the underestimation of the size of recent megathrust earthquakes illustrates our limited understanding of their spatiotemporal occurrence and governing physics to unravel their relation to associated subduction dynamics and long term deformation we developed a 2 d continuum viscoelastoplastic model that uses an eulerian lagrangian finite difference framework with similar on and off fault physics we extend the validation of this numerical tool to a realistic subduction zone setting that resembles southern chile the resulting quasi periodic pattern of quasi characteristic m8 m9 megathrust events compares quantitatively with observed recurrence and earthquake source parameters albeit at very slow coseismic speeds without any data fitting surface displacements agree with gps data recorded before and during the 2010 m8 8 maule earthquake including the presence of a second order flexural bulge these surface displacements show cycle to cycle variations of slip deficits which overall accommodate 5 of permanent internal shortening we find that thermally and stress driven creep governs a spontaneous conditionally stable downdip transition zone between temperatures of 350 c and 450 c ruptures initiate above it and below the forearc moho propagate within it interspersed by small intermittent events and arrest below it as ductile shearing relaxes stresses ruptures typically propagate upward along lithological boundaries and widen as pressures drop the main thrust is constrained to be weak due to fluid induced weakening required to sustain regular subduction and to generate events with natural characteristics fluid pressures of 75 99 of solid pressures the agreement with a range of seismological geodetic and geological observations demonstrates the validity and strength of this physically consistent seismo thermo mechanical approach

nonlithostatic pressure during sediment subduction and the development and exhumation of high pressure metamorphic rocks a subduction shear zone can be modeled as a long narrow channel with the flow of subducted metasedimentary rocks in the channel driven by two sets of forces the downward shearing force exerted by the subducting slab and the gradient in the hydraulic potential which combines the effect of both pressure and buoyancy if the channel walls are effectively rigid very slight narrowing or broadening of the channel convergence angles 2 gpa in the channel at only 40 km depth the model is consistent with a horizontal balance of forces across the plates and with a reasonable value for the thickness of subducted sediment 650 m the practical limit for overpressures attainable in subduction zones is determined by the strength and permeability of the channel walls at 40 km depth the channel is effectively confined on both sides by cold lithospheric mantle which should be strong enough to support a significant tectonic overpressure episodic failure of the upper plate to produce great earthquakes at 30 40 km focal depth could vent overpressured fluid from the channel allowing a cyclical buildup and release of both rock and fluid pressure topography on the subducting plate e g seamounts and thinned continental crust may lead to an anvil like jamming of the channel and local high overpressures tectonic erosion by topography on the lower plate of slivers from overlying continental crust and the compression of these slivers between the topography and the narrowing channel walls could produce high overpressures in continental rocks a decrease in the convergence rate or cessation of subduction with a consequent general warming within the channel and associated viscosity decrease promotes exhumation by buoyant reverse flow the most rapid reverse flow occurs in the region of previously greatest overpressure since the exhumation distance is shorter than for a simple lithostatic pressure distribution and any increase in temperature is coupled with a strong increase in the rate of exhumation preservation of high pressure assemblages at the surface in fossil subduction zones is promoted for such a model

episodic accretion and strain partitioning in a model sand wedge we model led thrust wedge accretion and deformation partitioning in a passively layered sand detached and shortened above a smooth rigid decollement the thrust wedge accretes in piggyback style during forward advance of a rear wall the passively layered sand accommodates the shortening penetratively by kinking ramp folding and by imbrication in single layers the style of compression changes with time and or stratigraphic position within the sand prism initial penetrative layer shortening above a slipped zone of decollement is succeeded by formation of monoclinal or conjugate shear bands at temporary terminations of decollement the shear bands subsequently narrow down and lock as thrust ramps the model sand wedge accrets episodically rather than steadily episodicity is controlled by the stick slip mode of decollement propagation and by volume loss and compaction of the wedge material in response to convergence this results in formation of a wedge with a convex upwards cross sectional topography

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

diapirism and topography summary a new numerical technique using markers and a deformable lagrangian mesh is used to study the deformation of the surface above a rising diapir this method allows modelling of a free surface boundary in a self consistent way the method makes it possible to investigate many different problems with non regular geometry the codes were verified through comparison with analytical solutions for the initial stages and with other numerical codes for mature stages our simulations led to the following conclusions 1 the growth rate of the diapir is more strongly influenced by the viscosity contrast than the wavelengths this is due to the strong growth rate difference during the initial stage 2 the maximum elevation above the diapir linearly increases with increasing wavelength and is approximately the same for different viscosity contrasts 3 the elevation will be considerably higher for a low viscosity diapir at a given depth than for an isoviscous diapir comparisons for different thicknesses of the buoyant layer showed that the highest topography is produced when the two layers have equal thickness we showed the dependence of the topographic behaviour on the parameter r the ratio of the density difference between two layers to the density of the upper layer it was found that topography behaves linearly up to r 0 15 it indicates that a posteriori estimation of topography for traditional calculations using the free slip boundary condition works well within this limit

predicting pore pressure in active fold thrust systems an empirical model for the deepwater sabah foldbelt abstract measurements related to mudrock shale and siltstone porosity such as acoustic velocity density or electrical resistivity have traditionally been used to predict pore pressures in extensional stress settings the underlying assumption is that burial and vertical effective stress ves which is the overburden minus the pore pressure controls the compaction of these rocks through porosity loss the dataset presented here compares ves and acoustic velocity of similar composition mudrocks in both an extensional and a compressional stress setting in the extensional stress environment the mudrocks follow a typical compaction trend with a porosity loss and increase in acoustic velocity that can be related to ves in an active fold thrust belt the compressive stresses further reduce the porosity and increase the acoustic velocity of the mudrocks first a layer parallel shortening compacts sediments beyond what is observed for the ves this additional compaction is further enhanced near thrust faults and in anticlinal forelimbs presumably due to additional shear stress in these areas the mudrocks located in folds that are buried by additional sedimentation do not compact again until the tectonic compaction is overridden by enough new burial after that the mudrocks follow the observed extensional setting compaction trend in the fold thrust belt the observed reduction in porosity by stresses other than burial leads to an under prediction of pore pressure using traditional methods to account for this we present a correction that can be applied to the acoustic velocity or porosity using two parameters a proximity to thrust faults and anticlinal forelimbs and b the amount of burial after fold formation with these corrections the extensional velocity ves compaction trend can be used to accurately predict pore pressure within the active fold thrust belt the correction is calibrated with well data and is empirical none the less it is a first step toward understanding the magnitudes of the compressive lateral stresses involved in generating the observed porosity loss within the fold thrust belt

a continuum mechanics model for normal faulting using a strain rate softening rheology implications for thermal and rheological controls on continental and oceanic rifting abstract we use a strain rate dependent visco plastic rheology in a finite element model to investigate the effects of crustal rheology and thermal structure on the development of normal faults in 2 d extending lithosphere strain rate softening in the brittle regime is used to simulate the rate dependence of frictional strength observed in laboratory studies results of numerical experiments show that efficient strain rate softening can result in localized zones of high strain rate which develop in response to the rheology and boundary conditions and are not imposed a priori we argue that these zones of localized shearing are analogous to faults and use the calculated deformation field to predict the preferred location of fault formation for a range of thermal and rheologic conditions when no regional temperature gradient is imposed deformation is predicted to be distributed between several sets of conjugate normal faults however in the presence of a horizontally varying temperature field faulting is calculated to focus where the lithosphere is thinnest forming rift like topography model results predict that when crustal thickness is sufficiently small that no ductile layer is present in the lower crust deformation is mantle dominated and rift half width is controlled primarily by the vertical geothermal gradient in contrast when crustal thickness is large enough that the stress accumulation in the upper crust becomes much greater than the stress accumulation in the upper mantle deformation becomes crust dominated and the calculated rift half width is a function of both crustal rheology and the vertical geothermal gradient this implies that deformation in the brittle upper crust does not behave independently of a strong upper mantle even in situations where ductile flow occurs in the lower crust

a community benchmark for viscoplastic thermal convection in a 2 d square box numerical simulations of thermal convection in the earth s mantle often employ a pseudoplastic rheology in order to mimic the plate like behavior of the lithosphere yet the benchmark tests available in the literature are largely based on simple linear rheologies in which the viscosity is either assumed to be constant or weakly dependent on temperature here we present a suite of simple tests based on nonlinear rheologies featuring temperature pressure and strain rate dependent viscosity eleven different codes based on the finite volume finite element or spectral methods have been used to run five benchmark cases leading to stagnant lid mobile lid and periodic convection in a 2 d square box for two of these cases we also show resolution tests from all contributing codes in addition we present a bifurcation analysis describing the transition from a mobile lid regime to a periodic regime and from a periodic regime to a stagnant lid regime as a function of the yield stress at a resolution of around 100 cells or elements in both vertical and horizontal directions all codes reproduce the required diagnostic quantities with a discrepancy of at most 3 in the presence of both linear and nonlinear rheologies furthermore they consistently predict the critical value of the yield stress at which the transition between different regimes occurs as the most recent mantle convection codes can handle a number of different geometries within a single solution framework this benchmark will also prove useful when validating viscoplastic thermal convection simula tions in such geometries

constraints on friction coefficients by an inverse analysis of sand box thrust dips abstract we measured the dips of thrusts forming in a uniform sand layer under horizontal shortening by a rigid emerging ramp in conditions of full erosion various ramp dips and four types of ramp material were used throughout the 12 experiments in three experiments the ramp was shortcut by a less steep thrust forming a spontaneous ramp in the sand the forward modelling proposed to predict these observations involves i computing the total dissipation in the structure by using global force equilibrium and a parameterised kinematic framework including the possibility of shortcut development and ii minimising the total dissipation of mechanical work with respect to the kinematic parameters i e the thrust dips for which two analytic solutions are found the inverse problem involves computing probability densities over the parametric space of friction coefficients including the internal friction angle of the sand and friction angles of the four materials used on the rigid ramps practically this involves computing a misfit using the l2 norm between the observed and predicted thrust dips for every possible value of the friction angles the resulting probability density provides all the quantitative information that can be retrieved from the experiments in the light of our forward modelling ramp friction angles exhibit sharp probability changes at values that trigger the occurrence of shortcuts thus yielding non trivial bounds on the ramp friction values that are directly related to the observations of ramp and back thrust geometry also the probability distribution of the internal sand friction and those of the ramp frictions is in good agreement with independent measurements this approach is designed to be applicable to field studies of brittle deformation where geometries are observed and rheological parameters are sought

mechanical validation of balanced cross sections the case of the mont terri anticline at the jura front nw switzerland abstract close to north west front of the jura thrust belt in switzerland the mont terri anticline is located at a disruption of the muschelkalk decollement due to a former normal fault numerous geological data are available thanks to a motorway tunnel cross cutting the fold and an underground laboratory original field data two geological maps a borehole and a highway tunnel constrain the inner structure the anticline is characterized by an overturned frontal limb a steep back limb and a ramp cross cutting older normal faults three geological cross sections and three 2d kinematic models are proposed as various combinations of fault bend folding fault propagation folding and detachment folding the main motivation of this study is to illustrate how geometrical constructions can be constrained by the kinematic approach of limit analysis which determines the optimal deformation including faults that verifies mechanical equilibrium and the coulomb criterion matching some of the important kinematic steps assumed in each three interpretations to the optimal deformation leads us to define the compatible ranges of values of the frictional parameters these ranges allow us to assess the likeliness of the important kinematic steps of each kinematic model the present analysis provides a quantitative link between the assumed evolution of the internal structure and the concomitant evolutions of the topography and of the frictional parameters

melange a viscoelastic lattice particle model applicable to the lithosphere 1 this article introduces the software melange a 3d lattice particle hybrid model the software was specifically designed in order to simulate ductile visco elasto plastic deformation and can be used to study tectonic processes in the lithosphere from the micro to the macro scale melange is under an open source license the code takes both relevant yield mechanisms for the deformation of lithospheric material into account dynamic brittle failure and ductile creep where ductile creep is modeled as viscoelasticity the software considers effects of the local geology of the inherent disorder of geomaterials of rheological layering of the lithosphere and applies repulsion when the material fractures driving forces are the externally applied strain and the gravitational load melange applies an elastically isotropic regular 3d lattice with hcp geometry and next nearest neighbor interactions young s modulus viscosity material density and system size can be freely chosen poisson s ratio is restricted to values 0 25 the most important innovation of the software is a physically consistent scheme to model viscoelasticity in a lattice particle model the scheme couples the local lattice geometry to the time dependent volume conservative viscoelastic deformation of particles the procedure achieves the typical viscoelastic stress strain relationship the article incorporates a number of benchmark simulations where the model output is being tested and evaluated the results obtained with melange show strong similarities with the benchmark values e g stress fields in crack vicinity geologically relevant examples include structures associated with the brittle ductile transition growth of wing cracks and brittle graben formation

mafic injection as a trigger for felsic magmatism a numerical study 1 the origin of crustal scale silicic magmatism remains a matter of debate and notable uncertainty exists concerning the physical mechanisms that drive ascent and emplacement of felsic magmas in upper crustal regions a 2 d numerical model demonstrates that injection of mantle derived mafic magma into a partially molten hot zone in the lower crust can drive felsic magma ascent and intrusion into upper crustal levels the injection of mafic magma induces overpressure in the reservoir which increases crustal stresses and triggers development of brittle plastic shear zones and can drive significant surface uplift the emerging topography causes a nonuniform overpressure distribution in the reservoir and can trigger felsic magma ascent along crustal shear zones based on systematic numerical experiments we investigate the influence of crustal strength and injection rate the initial upper crustal strength controls the degree of crustal faulting and surface uplift and therefore whether felsic magma ascent can be initiated or not the final upper crustal strength influences the depth and final style of felsic intrusion the injection rate of mafic magma determines the time scale of overpressure growth and surface uplift stage in contrast the duration of the subsequent felsic ascent and intrusion emplacement stages remains nearly constant our results imply that mafic underplating and intrusion into the lower crust may not only be a prime control for the generation of felsic magmas in the lower crust but may also be an important physical driving mechanism for felsic magma ascent and intrusion into upper crustal levels

two dimensional numerical modeling of fracturing and shear band development in glacier fronts in this contribution we present a two dimensional numerical model of a deforming glacier front the model is based on a hybrid lattice spring network approach where particles in the model can deform in a volume conservative visco elastic manner but at the same time they can be compressed elastically and fracture by discrete failure we restrict ourselves to a simple setting where the glacier sits on a frictionless slope that dips with 5 10 the ice block is fixed on one side and has a free surface on the other the glacier varies in viscosity and can flow at the base whereas it is brittle at the top results show that the head of the glacier is unstable failure happens as a combination of extension fractures crevasses at the top surface of the glacier and shear fractures that are dipping toward the glacier head once the shear fractures intersect with the free side wall of the glacier a triangular ice block is carving from the glacier head during successive flow of the glacier the failure is stepping backwards into the glacier and large shear planes develop that connect the sliding ice at the base with crevasses at the top variations of overall viscosity of the glacier indicate that higher viscosities and thus a more brittle glacier lead to larger spacing of shear surfaces and thus to larger ice blocks that are carving from the head of the glacier in addition the geometry of the deformation structures within the glacier does not vary significantly with the height of the ice indicating that larger glaciers carve larger blocks a higher tilt of the ground surface however leads to tighter spacing of shear surfaces and a more pronounced crevasse development this indicates that glacier heads that lie on steeper slopes will carve smaller blocks than glacier heads that lie on shallower slopes failure and carving of ice from the model glaciers is a combination of early developing closely spaced extension fractures crevasses and later developing wider spaced and more localized shear fractures or shear zones

effect of ductile and frictional decollements on style of extension abstract scaled analogue models were used to study the effect of frictional and ductile detachments on thin skinned extension models consisted of two halves one half is the ductile and the other has a frictional detachment extension occurred above two different basal configurations a stretchable rubber sheet and a folded banded sheet intended to produce homogeneous and heterogeneous extension respectively model parameters varied systematically and included the brittle ductile thickness ratio rheologies and bulk strain structures in the two halves are compared in profiles and plan views a series of graben developed above both halves of models extended above a banded sheet although there were differences in style propagation rate and width of the deformation zone between the two halves different rates of propagation of structures in the two halves led to the formation of an accommodation or transfer zone parallel to the extension direction most relay ramps and inflection of normal faults in this zone indicate differential extension between the two halves in contrast in models extended above a stretchable rubber sheet extensional structures such as horst and graben developed only above the ductile detachment model results indicate that heterogeneous mechanical stratigraphy and displacement rate have no effect on extensional structure above a rubber sheet however above 20 bulk extension deformation becomes heterogeneous along multiple sets of conjugate faults oblique to the extension direction

modelling of structural complexity in sedimentary basins the role of pre existing faults in thrust tectonics abstract analogue and numerical models have been used to study the role of pre existing faults in compressive regimes from a theoretical point of view reactivation is mainly controlled by fault attitude stress regime and frictional properties of fault planes in scaled down sandbox experiments precut faults are introduced in the homogeneous granular media with a nylon wire which is forced through the sand cake producing a thin planar disturbed zone systematic experiments of thrust inversion with various dips and strikes of such planar discontinuities have been modelled comparison of experimental results with theoretical diagrams indicate that disturbed zones have a friction angle which is 10 20 lower than the homogeneous sand and that the compressive regime in the sandbox has a shape factor close to 0 4 the static analysis of fault reactivation is in accordance with the experimental observations except for pre existing faults dipping at very low angle however numerical modelling using the udec code shows that low angle faults can be reactivated as a result of stress concentration in the lower part of the fault in addition sandbox experiments indicate that in thrust systems reactivation of pre existing faults is not only dependent on their attitude but also on their spacing and location relative to the thrust system

localization patterns in sandbox scale numerical experiments above a normal fault in basement abstract the finite element program elfen is used to study the effect of basement fault dip on the evolution of shear band patterns in unconsolidated sand the material properties and boundary conditions of the model were chosen to correspond to generic sandbox experiments model results reproduce the range of structural styles found in corresponding sandbox experiments with a basement fault dip of 60 and lower a graben structure is formed composed of a synthetic shear band followed by one or more antithetic shear bands with a basement fault dip of 70 and steeper a reverse precursor shear band forms first followed by a synthetic normal shear band that accommodates all further displacement the dip of the synthetic shear band is close to the basement fault dip for basement fault dips between 60 and 70 we observe a transition in localization patterns an analysis of the stress fields and velocity vectors in the model explains the first order aspects of the relationships observed we consider the observed precursor dominated and graben dominated structural domains to be important components of normal fault systems in which the first order structural style and deformation patterns are only weakly dependent on the details of the rheology of the model materials and explore the interesting problem of the change in structural style from precursor dominated to graben dominated structural domains above a normal fault in basement we find similar structural domains in sandbox experiments for the same set of boundary conditions but with slightly different material properties suggesting that the modeled patterns are robust within these two structural domains i e will occur over a range of similar material properties and boundary conditions the results of this study contribute to our ability to validate numerical models against experiments in order to finally better simulate natural systems

episodic imbricate thrusting and underthrusting analog experiments and mechanical analysis applied to the alaskan accretionary wedge seismic reflection profiles from the sediment rich alaska subduction zone image short frontally accreted imbricate thrust slices and repeated sequences of long underthrust sheets rapid landward increases in wedge thickness backthrusting and uplift of the forearc are observed suggesting underthrusting beneath the wedge these features and a widely varying frontal wedge morphology are interpreted to be caused by different modes of accretion active concurrently along the trench at different locations episodic wedge growth is observed in high basal friction experiments using sand as an analog material two phases of an accretionary cycle can be distinguished frontal accretion of short imbricate thrust slices alternating with underthrusting of long undeformed sheets the phase is shown experimentally to depend upon the surface slope of the wedge mechanical analysis of the forces at work predicts these two modes of deformation due to the varying frictional forces and yield strengths for a temporally varying wedge geometry maximum length of thrust slices is calculated for experimental conditions and confirmed by the observations for a steep frontal slope at the upper limit of the mohr coulomb taper stability field the overburden is too great to permit underthrusting and failure occurs repeatedly at the wedge front producing short imbricate slices the wedge grows forward lowering the surface angle to the minimum critical taper for a shallow frontal slope the reduced overburden along an active roof thrust permits sustained underthrusting causing frontal erosion and backthrusting steepening the wedge and thus completing the cycle

an arbitrary lagrangian eulerian formulation for creeping flows and its application in tectonic models summary this paper presents and analyses numerical techniques developed to investigate viscoplastic stokes flows within a model of lithospheric deformation in particular the techniques are related to a subduction model of compressional orogens the driving mechanism in the model corresponds to the near rigid convergence and subduction of one mantle lithosphere beneath another in plane strain and this boundary condition forces flow in an overlying viscoplastic model crust the numerical techniques use the arbitrary eulerian lagrangian formulation in which flows with free surfaces and large deformation are computed on an evolving eulerian finite element grid that conforms to the material domain a regridding algorithm allows the associated lagrangian motion and fields to be followed and in addition coupled back to the eulerian calculation of the flow mass flux boundary conditions are used so that the effects of erosion and deposition by surface processes and mass loss by subduction can be included in the model calculation the evolving model crustal layer is flexurally compensated using a general elastic beam formulation the applicability of the numerical techniques to problems ranging from accretionary wedges to crustal and lithospheric scale deformation is discussed simple flows a linear viscous subduction model a whirl flow and a quasi convection model are used to show that the mass conservation regridding and surface tracking errors are small the broader applicability of the modelling techniques is reviewed

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

finite thickness of shear bands in frictional viscoplasticity and implications for lithosphere dynamics permanent deformations in the lithosphere can occur in the brittle as well as in the ductile domain for this reason the inclusion of viscous creep and frictional plastic deformation is essential for geodynamic models however most currently available models of frictional plasticity are rate independent and therefore do not incorporate an internal length scale which is an indispensible element for imposing a finite width of localized shear zones therefore in computations of localization either analytical or numerical a width of the shear zone is obtained which tends to zero in numerical computations this manifests itself in a severe mesh sensitivity moreover convergence of the global iterative procedure to solve the non linear processes is adversely affected which negatively affects the reliability and the quality of predictions the viscosity which is inherent in deformation processes in the lithosphere can in principle remedy this mesh sensitivity however elasto visco plastic models which are commonly used in geodynamics assume a series arrangement of rheological elements maxwell type approach which does not introduce an internal length scale here we confirm that a different rheological arrangement which put a damper in parallel to the plastic slider kelvin type approach introduces an internal length scale as a result pressure and strain and strain rate profiles across the shear bands converge to finite values upon decreasing the grid spacing we demonstrate that this holds for non associated plasticity with constant frictional properties and with material softening with respect to cohesion finally the introduction of kelvin type viscoplasticity also significantly improves the global convergence of non linear solvers

synchronous and velocity partitioned thrusting and thrust polarity reversal in experimentally produced doubly vergent thrust wedges implications for natural orogens a sandbox analog modeling research program was used to study the detailed evolution of doubly vergent thrust wedges high resolution multilayers of thin alternating sand and mica layers were used in contractional coulomb wedge experiments to simulate deformation of anisotropic brittle upper crustal strata in doubly vergent orogens experiments incorporating syntectonic sedimentation in foreland basins and in piggyback basins were also carried out our laboratory models evolved in two main stages 1 initial high velocity thrusting in the retrowedge and high frequency together with low displacement folding and thrusting in the prowedge and 2 low frequency high displacement synchronous thrusting in the prowedge and low velocity thrusting in the retrowedge transition from stage i to stage ii occurred when the growing wedges reached the critical height at which they behaved as a backstop for further prowedge accretion addition of syntectonic sediments increased the persistence of stage i and triggered out of sequence thrusting in the axial zone of the experimental orogens thrust motion was stick slip retrovergent thrusting occurred along a long lived ramp whose lower tip was located at the subduction slot provergent kink bands nucleated at the subduction slot in stage i in contrast during the second stage of wedge evolution kink bands nucleated in a piggyback fashion in the foreland far from the subduction slot and then evolved into high displacement faults that remained active up until the end of the experiments at progressively decreasing rates of thrusting the axial zones of the model wedges were characterized by fast uplift rates during stage i due to backward translation of the belt along the retrovergent long lived ramp and due to the localization of deformation close to the subduction slot outward migration of the deformation front in the prowedge region during stage ii caused the progressive decrease in the rate of the wedge uplift until it eventually stopped analytical models to quantify the coulomb behavior in the wedges validated the reversal of thrust polarity with increasing shortening triggered by the buildup of the topographic load during deformation this thrust polarity reversal highlights problems with the classic concept of back thrusting as a reflection of a significant change in the deformation regime our results compare well with the kinematic evolution of natural accretionary prisms and of thrust and fold belts such as the lesser antilles arc system the mediterranean ridge and the pyrenees