gravitational instabilities triggered by fluid overpressure and downslope incision insights from analytical and analogue modelling fluid overpressure at the base of low permeability strata reduces effective stress allowing for gravitational sliding of the overlying cover the force driving sliding is the slope parallel component of the weight of the cover whereas the resisting forces are the friction at the base of the cover and the buttressing resistance to shortening which can be critically reduced by incision at the base of the slope we developed an analytical model and undertook a series of analogue experiments to better understand the evolution of a sedimentary cover sliding above a low permeability layer subjected to fluid overpressure where a downslope buttress was present the sliding sheet length decreased with increasing pore fluid pressure in the absence of such buttress the slide s length increased exponentially with increasing pore fluid pressure another important difference dealt with geometry and kinematics buttressed slides consisted of one large slope parallel mass rigidly translated and bounded by downslope thrusts and upslope normal faults with increasing pore fluid pressure the contractional structures propagated upslope by contrast non buttressed slides showed intense strain deformation started with normal faults forming near the incision then propagating upslope throughout the slide s evolution

mantle flow in regions of complex tectonics insights from indonesia 1 indonesia is arguably one of the tectonically most complex regions on earth today due to its location at the junction of several major tectonic plates and its long history of collision and accretion it is thus an ideal location to study the interaction between subducting plates and mantle convection seismic anisotropy can serve as a diagnostic tool for identifying various subsurface deformational processes such as mantle flow for example here we present novel shear wave splitting results across the indonesian region using three different shear phases local s sks and downgoing s to improve spatial resolution of anisotropic fabrics allows us to distinguish several deformational features for example the block rotation history of borneo is reflected in coast parallel fast directions which we attribute to fossil anisotropy furthermore we are able to unravel the mantle flow pattern in the sulawesi and banda region we detect toroidal flow around the celebes sea slab oblique corner flow in the banda wedge and sub slab mantle flow around the arcuate banda slab we present evidence for deep sub 520 km anisotropy at the java subduction zone in the sumatran backarc we measure trench perpendicular fast orientations which we assume to be due to mantle flow beneath the overriding eurasian plate these observations will allow to test ideas of for example slab mantle coupling in subduction regions

mid mantle seismic anisotropy around subduction zones abstract there is increasing evidence for mid mantle seismic anisotropy around subduction zones whose interpretation remains elusive in this study i estimate the strain induced mid mantle fabric and associated seismic anisotropy developing in 3d petrological thermo mechanical subduction models where the slab is either stagnating over the 660 km discontinuity or penetrating into the lower mantle the modelling of synthetic lattice preferred orientation lpo development of wadsleyite and perovskite has been calibrated with results from deformational experiments and ab initio atomic scale models and the single crystal elastic tensor of the different mineral phases is scaled by local p t conditions the lower transition zone ringwoodite garnet is assumed to be isotropic mid mantle fabric develops in proximity of the subducting slab where deformation and stresses are high except at depths where upwelling or downwelling material undergoes phase transformations yielding to lpo reset the upper transition zone wadsleyite garnet is characterized by weak transverse isotropy 2 3 with symmetry axes oriented and fast s wave polarized dip normal a slightly stronger transverse isotropy develops in the lower mantle perovskite periclase where the symmetry axes the polarization of the fast s wave and the maximum vp and dvs are parallel to the slab dip and subduction direction for stagnating slab models this translates into negative and positive radial anisotropy in the upper transition zone and lower mantle back arc respectively minimum delay times for vertically travelling shear waves and large shear wave splitting for waves propagating horizontally in the lower mantle these results may help in reconciling the seismic anisotropy patterns observed in some subduction zones with subduction induced deformation such as those measured in the mid mantle between the australian plate and the new hebrides tonga kermadec trenches that i interpret as related to stagnating portions of the subducted pacific plate

tectonic and gravity extensional collapses in overpressured cohesive and frictional wedges two modes of extensional collapse in a cohesive and frictional wedge of arbitrary topography finite extent and resting on an inclined weak decollement are examined by analytical means the first mode consists of the gravitational collapse by the action of a half graben rooting on the decollement and pushing seaward the frontal part of the wedge the second mode results from the tectonics extension at the back wall with a similar half graben kinematics and the landward sliding of the rear part of the wedge the predictions of the maximum strength theorem equivalent to the kinematic approach of limit analysis and based on these two collapse mechanisms not only match exactly the solutions of the critical coulomb wedge theory once properly amended but generalizes them in several aspects wedge of finite size composed of cohesive material and of arbitrary topography this generalization is advantageous to progress in our understanding of many laboratory experiments and field cases for example it is claimed from analytical results validated by experiments that the stability transition for a cohesive triangular wedge occurs with the activation of the maximum length of the decollement it is shown that the details of the topography for the particular example of the mejillones peninsula north chile is however responsible for the selection of a short length scale dynamic instability corresponding to a frontal gravitational instability a reasonable amount of cohesion is sufficient for the pressures proposed in the literature to correspond to a stability transition and not with a dynamically unstable state

deformation and stability of over pressured wedges insight from sandbox models 1 in the well known and widely used critical taper model of accretionary wedges pore pressure is introduced as a first order parameter despite the fact that the importance of the pore pressure on deformation is largely accepted the theoretical predictions of the critical model has never been experimentally confirmed for over pressured wedges to fill this gap and to understand how pore pressure may influence deformation within a wedge we performed scaled experiments under conditions close to the critical taper hypothesis the wedges were built with a non cohesive sand and lied on glass microbeads layer to insure a weak decollement as we applied horizontally varying fluid pressure at the base of the model the pore pressure ratio was almost constant in the whole wedge combining these experiments with an optical image correlation technique particle imaging velocimetry we followed the strain in the model during the shortening we highlighted three different modes of deformation which are strongly influenced by the pressure ratio and the wedge taper although we found a pretty good agreement between the experimental observations and the critical taper model predictions it appears that the transition between sub critical and super critical conditions is not sharp as described by the critical taper model but continuous the distance to the critical taper condition which depends on both the topographic angle and the pressure ratio seems to control the propagation rate of the decollement and the strain localization within the wedge

the depth distribution of mantle anisotropy beneath the tonga subduction zone abstract shear wave splitting recorded by a temporary deployment of broadband seismometers located above the tonga subduction zone yields strong constraints on the depth distribution of anisotropy in the mantle beneath the tonga back arc region splitting parameters obtained for local s and teleseismic sks phases were modeled using a ray based method that predicts shear wave splitting on individual phase paths and determines the best fitting values of anisotropic strength orientation and depth extent splitting in teleseismic sks phases is identical to that in s phases from local earthquakes that occur at the base of the transition zone demonstrating that there is no significant splitting due to anisotropy in the lower mantle splitting times for local s phases do not vary significantly with depth and for models in which the strength of anisotropy is laterally uniform they rule out anisotropy in the transition zone finally the observed splitting indicates that anisotropy of more than 0 8 exists in the upper mantle with a fast symmetry axis at 60 w roughly parallel to the absolute plate motion vector of the subducting pacific lithosphere

origin of overpressure and pore pressure prediction in the baram province brunei accurate pore pressure prediction is critical in hydrocarbon exploration and is especially important in the rapidly deposited tertiary baram delta province where all economic fields exhibit overpressures commonly of high magnitude and with narrow transition zones a pore pressure database was compiled using wireline formation interval tests drillstem tests and mud weights from 157 wells in 61 fields throughout brunei overpressures are observed in 54 fields both in the inner shelf deltaic sequences and in the underlying prodelta shales porosity vs vertical effective stress plots from 31 fields reveal that overpressures are primarily generated by disequilibrium compaction in the prodelta shales but have been generated by fluid expansion in the inner shelf deltaic sequences however the geology of brunei precludes overpressures in the inner shelf deltaics being generated by any conventional fluid expansion mechanism e g kerogen to gas maturation and we propose that these overpressures have been vertically transferred into reservoir units via faults from the prodelta shales sediments overpressured by disequilibrium compaction exhibit different physical properties to those overpressured by vertical transfer and hence different pore pressure prediction strategies need to be applied in the prodelta shales and inner shelf deltaic sequences sonic and density log data detect overpressures generated by disequilibrium compaction and pore pressures are accurately predicted using an eaton exponent of 3 0 sonic log data detect vertically transferred overpressures even in the absence of a porosity anomaly and pore pressures are reasonably predicted using an eaton exponent of 6 5

upper and mid mantle anisotropy beneath the tonga slab measurements of source side splitting in s waves fromevents within the tonga slab reveal anisotropy in the upperand mid mantle beneath the slab we observed splitting forevents originating at both upper mantle and transition zonedepths anisotropic fast directions are trench parallel orsub parallel for both upper mantle and transition zoneevents delaytimes dt decreasewithdepthforuppermantleevents the source of anisotropy for the upper mantle eventsis likely in the sub slab mantle and is likely indicative oftrench parallel flow due to slab rollback the source ofanisotropy for the deeper earthquakes is more difficult toconstrain but the pattern of splitting measurements arguesfor an uppermost lower mantle anisotropic source and slabinduced deformation may be responsible for this anisotropyas well additional constraints from mineral physics studiesare necessary to interpret the mid mantle anisotropy signalin terms of geodynamical processes

laboratory and seismological observations of lower mantle isotropy we have carried out seismological and mineral physics investigations to identify the extent and origin of elastic anisotropy in the lower mantle based on observations of shear wave splitting we conclude that the lower mantle is effectively isotropic this result is surprising since the lower mantle is composed of elastically anisotropic minerals silicate perovskite mgo sio 2 that have been extensively strained by geologic processes to reconcile the seismic observations we have measured texture development during deformation and high temperature recrystallization of mantle silicates we show that these experiments provide a direct explanation for the shear wave splitting measurements the lower mantle appears isotropic because silicate perovskite maintains an isotropic texture during deformation and recrystallization

olivine spinel transitions experimental and thermodynamic constraints and implications for the nature of the 400 km seismic discontinuity the sequence of high pressure phase transitions in olivine is traditionally used as a model for seismic velocity variations in the 200 to 650 km depth interval in a mantle of peridotitic bulk composition it has been proposed that the observed seismic velocity increase at 400 km depth is too sharp and of too small a magnitude to be attributable to the phase change and that the upper mantle must thus be chemically stratified with the 400 km discontinuity being due either to a combination of phase changes in a layer of pyroxene garnet rich piclogite composition or to a chemical boundary between such a piclogite layer and an overlying peridotitic layer using available calorimetric thermoelastic and synthesis data and their associated experimental uncertainties we have derived internally consistent high pressure phase relations for the mg2sio4 fe2sio4 join we find that the divariant transition which is generally regarded as occurring over a broad depth interval for mantle olivine compositions is in fact extremely sharp the seismic discontinuity corresponding to the transition in mg0 9fe0 1 2sio4 should occur over a depth interval isothermal of about 6 km at a depth of approximately 400 km the sharpness of this transition is quite insensitive to uncertainties in the constraining calorimetric thermoelastic and synthesis data in addition we have computed seismic velocity profiles for a model mantle consisting of pure olivine of mg0 9fe0 1 2sio4 composition comparison of these computed profiles to those derived from recent seismic studies indicates that the magnitude of the observed velocity increase at 400 km depth is consistent with a mantle transition zone composed of about 60 70 olivine we conclude that there is no need to infer the existence of pyroxene garnet rich compositions such as eclogite or piclogite in the transition zone since an upper mantle of homogeneous olivine rich peridotitic composition is consistent with the available seismic velocity data

frictional properties of shale reservoir rocks 1 we performed laboratory friction experiments on shale samples from three hydrocarbon reservoirs to assess compositional controls on fault slip behavior accompanying hydraulic stimulation the samples span a range of clay and total organic content from 10 to 60 by weight and demonstrate fine scale heterogeneity within each reservoir friction measurements demonstrate strong dependence of strength and rate state constitutive properties on shale composition shale samples with clay and organic content above 30 by weight show coefficient of friction 0 4 and consistently velocity strengthening behavior while those below this threshold show increased strength and velocity weakening behavior this transition in frictional strength and stability suggests a change in the shale grain packing framework from rigid clast supported to clay mineral and organic matter supported measurements of gouge dilatancy further support these relationships showing a negative correlation with clay and organic content critical slip evolution distance exhibits similar dependence on composition implying common micromechanical mechanisms for the observed transition in frictional behavior we performed microstructural characterization of the experimental samples and observe changes in the gouge load bearing framework and shear localization features consistent with these mechanical data while these results can be applied generally to infer slip stability on faults in clay bearing sedimentary rock we employ the experimental data to place constraints on microearthquake magnitudes and the occurrence of slow shear slip on preexisting faults during hydraulic stimulation in shale reservoirs

control of fluid pressures on the formation of listric normal faults abstract listric normal faults are widespread in the extension of the upper crust despite major advances in understanding the formation of listric faults through various experiments the mechanical conditions that allow their formation are highly debated in particular anderson s faulting theory predicts that newly formed normal faults are planar and are dipping at least at 45 and in practice at 60 65 for most rock types here we develop limit analysis to investigate the formation of a listric fault at the onset of slip linking a deep detachment to the topographic surface we find that listric normal faults can occur in the brittle upper crust without appealing to viscous or ductile behaviours nor to flexural stresses the disequilibrium compaction fluid pressures typically observed in many sedimentary basins are essential for the formation of listric faults the fluid pressure is hydrostatic down to the fluid retention depth z f r d and sustains a higher gradient below this depth parametric studies show that the surface slope is also essential for the formation of listric faults even with a gently dipping surface slope 4 whereas a flat topographic surface leads to a simple andersonian geometry at the onset of slip the method is applied to two field examples in order to determine fluid overpressures that best match the fault shapes interpreted from seismic data for the offshore niger delta the simulated normal faults match the observed listric faults with a very shallow z f r d 0 5 0 75 km and below z f r d the fluid pressure has a lithostatic gradient consistent with the observed fluid pressure profiles to reproduce a series of listric faults joining on one common low angle detachment in the nw gulf of mexico we demonstrate that a shallow z f r d 0 7 1 1 km is required below which the fluid pressure increases to the lithostatic pressure on the detachment in agreement with the fluid pressure observations

trench parallel flow beneath the nazca plate from seismic anisotropy shear wave splitting of s and sks phases reveals the anisotropy and strain field of the mantle beneath the subducting nazca plate cocos plate and the caribbean region these observations can be used to test models of mantle flow two dimensional entrained mantle flow beneath the subducting nazca slab is not consistent with the data rather there is evidence for horizontal trench parallel flow in the mantle beneath the nazca plate along much of the andean subduction zone trench parallel flow is attributale utable to retrograde motion of the slab the decoupling of the slab and underlying mantle and a partial barrier to flow at depth resulting in lateral mantle flow beneath the slab such flow facilitates the transfer of material from the shrinking mantle reservoir beneath the pacific basin to the growing mantle reservoir beneath the atlantic basin trenchparallel flow may explain the eastward motions of the caribbean and scotia sea plates the anomalously shallow bathymetry of the eastern nazca plate the long wavelength geoid high over western south america and it may contribute to the high elevation and intense deformation of the central andes

the coulomb critical taper theory applied to gravitational instabilities the critical coulomb wedge theory has been widely applied to the structural evolution of accretionary wedges and fold and thrust belts but it also predicts the spreading of a wedge under gravity this solution may be applied to the formation of gravitational spreading and gliding along passive margins where elevated pore fluid pressure is common following the initial hypotheses of the theory we provide an alternative expression of the exact solution for a noncohesive wedge better suited to slope instabilities our formulation allows a direct calculation of the fluid pressure required for the system to deform and predicts two kinds of gravitational deformation shallow slumping and or deep gravitational spreading rooting on the basal detachment to verify the predictions of the model we performed scaled experiments with pore pressure under conditions close to the critical taper hypotheses the good agreement between our experimental results and the theory confirms the applicability of the critical taper model to the formation of gravitational structures in sedimentary wedges subjected to pore fluid pressure

prediction of thrusting sequences in accretionary wedges 1 the main objective is to determine the three stages of the life of a thrust in an accretionary wedge which are the onset of thrusting along its ramp the development with the construction of the relief and the arrest because of the onset of another thrusting event a simple kinematics is proposed for the geometry of the developing thrust fold based on rigid regions separated by velocity discontinuities along which work is dissipated according to the coulomb criterion the evolution of the thrust fold satisfies mechanical equilibrium and is optimized at every time of the three stages to provide the least upper bound in tectonic force according to the maximum strength theorem the development of the thrust or its arrest because of the initiation of another thrust is decided by selecting the event which leads to the least upper bound in tectonic force the approach is first validated by proving that the critical slope angle c for the classical triangular wedge is properly captured it is shown that a perturbation in the form of an extra relief in this perfectly triangular geometry leads to the onset of thrusting with the ramp or the back thrust outcropping either at the back or to the front of the perturbation respectively for a range of slope dip close to the critical angle c the study of normal thrust sequences from the rear to the front in the wedge toe reveals that weakening of the ramp accounted for by changing its friction angle from an initial to a smaller final value is necessary for each thrust to have a finite life span this life span is longer with a larger relief buildup for more pronounced weakening decreasing the decollement friction angle results in an increase in the number of thrusts in the sequence each thrust creating milder relief the normal sequence is ended with the first out of sequence thrust which occurs earlier for smaller weakening over the ramp the proposed methodology is partly used to construct an inverse method proposed to assess the likeliness for the transfer of activity from the active to the incipient thrust in a section of nankai s accretionary wedge the inverse method provides the initial friction angle over the incipient ramp and the final friction angle over the fully active ramp from the geometry of the corresponding thrusts and the topography it is shown that the friction angle over the incipient ramp is most likely to be larger then the one over the active ramp justifying a key hypothesis needed to predict discrete sequences of thrusting

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

thrust wedges and fluid overpressures sandbox models involving pore fluids the well known model for the critical taper of an accretionary wedge includes overpressure as a first order parameter fluid overpressures reduce frictional resistance at the base of a wedge but they also act as body forces on all material particles of the wedge in addition to that of gravity by means of sandbox modeling many workers have tried to verify the predictions of the critical taper model but few of them have so far incorporated true fluid pressures we have used scaled experiments in which compressed air flows through sand packs so as to model the deformation of overpressured wedges a new apparatus provides for a horizontally varying fluid pressure for example a linear variation as in the critical taper model we have done three series of experiments involving horizontal shortening of homogeneous or multilayered sand models for various gradients of fluid pressure as predicted by the critical taper model the apical angle of the resulting wedge depends on the overpressure gradient in homogeneous sand at a high overpressure gradient deformation becomes diffuse and looks ductile in multilayered models detachments form beneath layers of low permeability so that thrusts propagate rapidly toward the undeformed foreland the efficiency of a detachment and its ability to propagate depend not only on the fluid pressure but also on the permeability ratios between the various layers

mechanical restoration of gravity instabilities in the brunei margin n w borneo in offshore brunei the baram delta displays pliocene to recent gravitational deformations on top of the overpressured setap shales we use the limit analysis implemented in slamtec to constrain their kinematics and propose first order values of compatible pore pressures using the critical coulomb wedge theory we confirm that extreme overpressure are needed to account for the gravitational deformations currently observed in the delta slamtec simulations quantify the role of the migration of the deltaic system on the deformations and suggest that this overpressure has existed since the initiation of the gravity driven system an inverse analysis is performed to identify the fluid overpressure and sedimentation pattern that best reproduces the forward structural model this inverse approach validates the proposed kinematics since we can reproduce quantities describing the structural style with overpressure coherent with the present day observations it points out the existence of key events explaining the structural interpretations which leads to propose new kinematic models enriched with compatible pore pressure and sedimentation rates this mechanical study highlights how mechanical modelling helps design a cross section with an appropriate kinematics based on mechanical solutions and discuss on the basis of objective mechanical criteria the ranges of pore pressure consistent with a prescribed structural evolution

sub slab anisotropy beneath the sumatra and circum pacific subduction zones from source side shear wave splitting observations understanding the dynamics of subduction is critical to our overall understanding of plate tectonics and the solid earth system observations of seismic anisotropy can yield constraints on deformation patterns in the mantle surrounding subducting slabs providing a tool for studying subduction dynamics while many observations of seismic anisotropy have been made in subduction systems our understanding of the mantle beneath subducting slabs remains tenuous due to the difficulty of constraining anisotropy in the sub slab region recently the source side shear wave splitting technique has been refined and applied to several subduction systems worldwide making accurate and direct measurements of sub slab anisotropy feasible and offering unprecedented spatial and depth coverage in the sub slab mantle here we present source side shear wave splitting measurements for the central america alaska aleutians sumatra ryukyu and izu bonin japan kurile subduction systems we find that measured fast splitting directions in these regions generally fall into two broad categories aligning either with the strike of the trench or with the motion of the subducting slab relative to the overriding plate trench parallel fast splitting directions dominate beneath the izu bonin japan and southern kurile slabs and part of the sumatra system while fast directions that parallel the motion of the downgoing plate dominate in the ryukyu central america northern kurile western sumatra and alaska aleutian regions we find that plate motion parallel fast splitting directions in the sub slab mantle are more common than previously thought we observe a correlation between fast direction and age of the subducting lithosphere older lithosphere 95 ma is associated with trench parallel splitting while younger lithosphere 95 ma is associated with plate motion parallel fast splitting directions finally we observe source side splitting for deep earthquakes transition zone depths beneath japan and sumatra suggesting the presence of anisotropy at midmantle depths beneath these regions

a regionalized upper mantle rum seismic model seismic velocity heterogeneity in the earth s mantle is strongly concentrated near its top the shallow heterogeneity of the mantle correlates strongly with surface tectonics we use these observations as constraints of a tomographic experiment aimed at building a regionalized upper mantle rum reference model we use a select set of teleseismic travel times to minimize the mapping of mislocation into structure the data selection emphasizes the robustness of individual picks the form of the rum model is a set of velocity profiles as functions of depth through the upper mantle for each of the different tectonic provinces of earth together the profiles constitute a three dimensional model which incorporates considerable structural detail but is described by only 90 parameters and has only about 22 degrees of freedom this is achieved by irregularly sampling a detailed regionalization of the globe by detailed mapping of subducted lithosphere in the mantle as defined by seismicity and by combining these structures in an irregular grid in which bookkeeping is efficiently handled the resulting rum model includes subducting slabs as sharp fast features in the upper mantle old continents are fast young oceans are slow models have been derived for both compressional and shear velocity the rum model is designed to represent as much of upper mantle heterogeneity as seen by body wave travel times as possible with a simple model it can be useful as a reference model for individual tectonic regions travel times are efficiently generated for the rum model mislocations of explosions of known location are significantly reduced when corrections for the rum model are applied to travel time residuals for a spherically symmetrical earth model

global azimuthal anisotropy in the transition zone surface wave dispersion measurements for love wave overtones carry evidence of azimuthal anisotropy in the transition zone of earth s mantle 400 to 660 kilometers deep a backus gilbert inversion of anisotropic phase velocity maps with resolution kernels mainly sensitive to the transition zone shows a robust long wavelength azimuthally anisotropic velocity structure this observation puts new constraints on the mineralogy and dynamics of the transition zone because this anisotropy may result from aligned minerals tilted laminated structures or even organized pockets of fluid inclusions

structural styles in the deep water fold and thrust belts of the niger delta the deep water niger delta includes two large fold and thrust belts products of contraction caused by gravity driven extension on the shelf that exhibit complex styles of thrusting these contractional structures formed above multiple detachment levels in the overpressured shales of the akata formation using the patterns of growth sedimentation fold shapes fault plane seismic reflections and combined conventional and shear fault bend folding theories we describe and model the structural styles and kinematics of the fault related folds and imbricate thrust systems that compose these belts individual fault related folds involving both forethrusts and backthrusts are characterized by long planar backlimbs that dip less than the associated fault ramps with upward shallowing of dips in growth strata above the backlimbs suggesting components of progressive limb rotation forelimbs are short compared to backlimbs but growth strata show more consistent dips that suggest a component of folding by kink band migration thus we employ a combination of classic and shear fault bend fold theories to describe these structures including the influence of a weak basal detachment zone in the overpressured shales we expand upon these theories to model the kinematics of imbricate thrust systems which display a complex history of thrusting related to spatial and temporal variations in deposition across the delta regional patterns of folded growth strata are used to define break forward break backward and coeval thrusting involving single and multiple detachment levels we define two main types of imbricate thrust systems type i system with a single basal detachment level and type ii imbricate system with multiple basal detachment levels which cause massive structural thickening of the akata formation and refolding of shallow thrust sheets through the sequential restoration of two regional cross sections across these systems we resolve the structural styles the timing and sequences of thrusting as well as the regional amounts of shortening all of which have important implications for hydrocarbon maturation and charge in the deep water niger delta

deep water niger delta fold and thrust belt modeled as a critical taper wedge the influence of elevated basal fluid pressure on structural styles we use critical taper wedge mechanics theory to show that the niger delta toe thrust system deforms above a very weak basal detachment induced by high pore fluid pressure the niger delta exhibits similar rock properties but an anomalously low taper sum of the bathymetric slope and dip of the basal detachment compared with most orogenic fold belts this low taper implies that the niger delta has a very weak basal detachment which we interpret to reflect elevated pore fluid pressure 0 90 within the akata formation a prodelta marine shale that contains the basal detachment horizon the weak basal detachment zone has a significant influence on the structural styles in the deep water niger delta fold belts the overpressured and thereby weak akata shales ductilely deform within the cores of anticlines and in the hanging walls of toe thrust structures leading to the development of shear fault bend folds and detachment anticlines that form the main structural trap types in the deep water fold belts moreover the low taper shape leads to the widespread development of backthrust zones as well as the presence of large relatively undeformed regions that separate the deep water fold and thrust belts this study expands the use of critical taper wedge mechanics concepts to passive margin settings while documenting the influence of elevated basal fluid pressures on the structure and tectonics of the deep water niger delta

strength anisotropy of shales deformed under uppermost crustal conditions conventional triaxial tests were performed on three sets of samples of tournemire shale along different orientations relative to bedding 0 45 90 experiments were carried out up to failure at increasing confining pressures ranging from 2 5 to 160 mpa at strain rates ranging between 3 10 7s 1 and 3 10 5s 1 this allowed us to determine the entire anisotropic elastic compliance matrix as a function of confining pressure results show that the orientation of principal stress relative to bedding plays an important role on the brittle strength with 45 orientation being the weakest we fit our results with a wing crack micromechanical model ashby and sammis 1990 and an anisotropic fracture toughness we found low values of internal friction coefficient and apparent friction coefficient in agreement with friction coefficient of clay minerals between 0 2 and 0 3 and values of kic comparable to that already published in the litterature we also showed that strain rate has a strong impact on peak stress and that dilatancy appears right before failure and hence highlighting the importance of plasticity mechanisms although brittle failure was systematically observed stress drops and associated slips were slow and deformation always remained aseismic no acoustic emission were detected this confirms that shales are good lithological candidates for shallow crust aseismic creep and slow slip events

the lateral strike slip domain in gravitational detachment delta systems a case study of the northwestern margin of the niger delta we use two and three dimensional seismic data to describe the structural geology of the lateral margin of a deep water delta lobe within the niger delta that has undergone basinward gravitationally driven translation we term this region the lateral strike slip domain deformation is characterized by a strike slip fault system that can be followed for a distance of approximately 75 km 47 mi from the shelf to the slope and toe of slope on the northwestern side of the fault system a fold and thrust belt that propagated north to northwest has developed within a large scale restraining area of 460 km2 180 mi2 on the southeastern side of the strike slip fault system widespread extension has occurred characterized by several graben and kilometer scale rollover structures lateral margins of gravitational collapses give key information on how they deformed we estimate a minimum horizontal displacement on the main strike slip fault of approximately 7 km 4 mi structural and kinematic evidence such as present day propagating strike slip faults for possible future lateral expansions of the lateral strike slip domain is described we expect to observe similar sets of deformation styles at the margins of other preserved gravitational collapse sliding over a detachment whose efficiency in causing downdip slip may vary laterally

controls on overpressure evolution during the gravitational collapse of the amazon deep sea fan abstract the amazon fan provides a natural laboratory to study the generation of overpressure due to rapid late cenozoic burial that has resulted in gravitational collapse above shale detachments here we examine collapse systems for the first time using the techniques of petroleum systems analysis we propose an integrated methodology based on numerical modeling constrained by the structural restoration of a seismic profile across the southwestern fan the results provide information on the evolution of pore pressure and temperature and their implications for the operation of the detachment and overlying extensional and compressional faults during the deposition of up to 6 km of sediment over the last 8 ma the modelled thermal history implies that fluid release by smectite to illite transformation has taken place within the thickening sedimentary succession but has not significantly contributed to pore pressures along the detachment modeling of hydrocarbon generation and migration from source rocks beneath the fan indicates gas accumulated in successions at depths of 102 103 m beneath the detachment without influencing pore pressures along it in contrast model results indicate that overpressures have varied in response to disequilibrium compaction fault activity within the collapse system took place during phases of higher sedimentation rates and ceased from 5 5 to 3 7 ma when sediment supply to the se fan decreased from 2 ma renewed sediment flux and shelf slope progradation led to a basinward migration both of overpressure along the detachment and of fault activity above it we conclude that gravity tectonics in the amazon fan over the last 8 ma have been mainly controlled by overpressures due to disequilibrium compaction with secondary contributions from clay mineral transformation present day pressure conditions show that the southeastern amazon fan is not at equilibrium and gravity driven deformation could occur at any time

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

origin of rollover rollover is the folding of hanging wall fault blocks by bending or collapse in response to slip on nonplanar commonly listric normal faults the shapes of rollover folds are controlled by a number of variables including 1 the shape of the fault 2 the total fault slip after a bed is deposited 3 the direction of relative particle motion in hanging wall collapse 4 the history of sedimentation rate relative to fault slip rate and 5 compaction the importance and role of each of these variables is illustrated by a two dimensional balanced structural modeling technique that treats continuously curved faults as though composed of a large number of straight fault segments in this modeling an active axial surface oriented parallel to the direction of relative p rticle motion in hanging wall collapse emanates from each fault bend and is the instantaneous locus of folding the quantitative correctness of this theory of rollover is tested by modeling natural structures from the gulf of mexico for which both fault shape and fold shape are known from high quality seismic and well sections the direction of hanging wall collapse commonly is in the antithetic or synthetic normal fault or coulomb shear orientations although sliding along weak bedding planes also is an important collapse mechanism in some regions collapse is in the antithetic shear direction for concave fault bends and in the synthetic shear direction for convex bends these collapse directions can be observed directly in some high quality seismic images as axial surfaces emanating from fault bends the shapes of rollovers within growth strata depend strongly on the sedimentation rate relative to fault slip rate as well as the total slip after a bed is deposited the crests of classic gulf coast rollovers are growth axial surfaces along which are abrupt changes in sedimentation rate within the growth st atigraphic interval these changes are produced by deformation of the sediment water interface along active axial surfaces compaction can substantially modify the relationship between fault shape and rollover shape however under certain common conditions the history of compaction can be neglected if the folding is modeled in the compacted state

seismic imaging of upper mantle structure with new evidence for a 520 km discontinuity stacked images of long period seismic data reveal many phases some not previously observed which are caused by reflections and conversions at discontinuities in the earth s upper mantle these images support the existence of discontinuities at 410 520 and 660km depth but no evidence is found for a significant interface at 220 km

discovery of bridgmanite the most abundant mineral in earth in a shocked meteorite meteorites exposed to high pressures and temperatures during impact induced shock often contain minerals whose occurrence and stability normally confine them to the deeper portions of earth s mantle one exception has been mgsio 3 in the perovskite structure which is the most abundant solid phase in earth here we report the discovery of this important phase as a mineral in the tenham l6 chondrite and approved by the international mineralogical association specimen ima 2014 017 mgsio 3 perovskite is now called bridgmanite the associated phase assemblage constrains peak shock conditions to 24 gigapascals and 2300 kelvin the discovery concludes a half century of efforts to find identify and characterize a natural specimen of this important mineral

salt tectonics driven by sediment progradation part i mechanics and kinematics using conceptual reasoning and results from physical models we describe the mechanical and kinematic characteristics of sediment wedges spreading seaward above a viscous evaporitic layer spreading can occur if the distal sediment overburden is thin and weak or if it comprises preexisting salt bodies spreading is accommodated by proximal extension and an associated rise of diapiric ridges by midslope seaward translation and by distal shortening rapid sediment progradation can lead to the reactivation of older distal folds by later extension either retrogradation or renewed sedimentation following a long depositional hiatus can reactivate older grabens and diapirs in shortening

half graben structures balanced models of extensional fault bend folds one common structural style in extensional regimes is a half graben bounded on one side by a master normal fault and on the other side by a domain of beds dipping toward the master fault this geometry is modeled as being caused by a bend in the master fault the hanging wall beds dipping toward the master fault are bounded by axial surfaces formed as the hanging wall moves past the bend resulting in an extensional fault bend fold the footwall beds remain undeformed and unrotated the major assumptions used in the derivation are that the geometry is area balanced and that the axial surfaces in the hanging wall have dips equal and opposite to the dip of the master fault above the bend important consequences of the model include a relationship between master fault dip above and below a bend and the amount of hanging wall dip the horizontal width of the dipping beds in the hanging wall is twice the heave on the master fault above the bend the asymmetry of the axial surfaces in the hanging wall requires that the rotated beds be strained hanging wall beds dipping toward the master fault are predicted for a fault bend wherein the dip decreases downward hanging wall beds dipping away from the master fault are predicted for a bend wherein the dip increases downward the model fits measurements taken from a published ramp and flat clay model by e cloos and is used to develop a balanced and restorable cross section of the schell creek fault a basin and range structure in nevada

fluid pressure control on splay fault activation in accretionary prism based on the maximum strength theorem with application to the nankai wedge abstract the maximum strength theorem is applied to determine the pressure conditions in accretionary wedges responsible for the activation of splay faults sf this theorem based on the limit analysis developed in soil mechanics selects among several modes of deformation the one prevailing for a given pressure distribution it applies for frictional rock properties and for an arbitrary wedge geometry in particular the decollement is assumed here to be partitioned into two mechanically distinct regions the internal region being part of the seismogenic zone the considered modes of deformation include the activation of a thrust fold rooting on the decollement and outcropping anywhere on the topography this mode if close to the transition between the seismic and the aseismic region of the decollement is in competition with the sf which constitutes also a potential mode of deformation the dominance of the various modes of deformation is presented in stability maps in the plane spanned by the pressure ratios of the two regions of the decollement pore pressure divided by lithostatic pressure both referenced to the sea floor several maps are proposed for the particular case of the kumano transect of nankai wedge south west japan varying the sf pressure ratio the position of the sf root on the decollement with respect to the transition between the seismic and aseismic regions as well as the respective length of these two regions in addition these stability maps are suggesting various scenarios to explain the changes through time in the prevailing mode of deformation in an accretionary wedges for the kumano transect three scenarios could explain the observed deactivation of the sf one of them being the seaward migration of the decollement transition point

evidence of midmantle anisotropy from shear wave splitting and the influence of shear coupled p waves 1 until recently the midmantle region of the earth the transition zone and the uppermost lower mantle has been considered isotropic recent work by wookey et al 2002 presented evidence of anisotropy in the midmantle region near the tonga kermadec subduction zone inferred from shear wave splitting in teleseismic s phases at australian seismic stations this data set is revisited to examine the possibility of contamination by shear coupled p waves using reflectivity modeling we explore the effects of these phases on the measurement of shear wave splitting contamination of such measurements is demonstrated for short epicentral distances 30 wave field decomposition is a simple technique which can be used to separate these phases from the main s wave arrival the tonga australia data set is reprocessed after wave field decomposition shear wave splitting ranging between 0 7 and 6 2 s is observed a range comparable to that observed by wookey et al 2002 as before the polarization of the fast shear wave is observed to be predominantly horizontal however the magnitudes of splitting for several events at short epicentral distances are significantly reduced suggesting some influence of shear coupled p waves in the original analysis the results are compared with possible models of mantle anisotropy and the results can be best explained by significant anisotropy in the midmantle region the best candidate location for this anisotropy is in the uppermost lower mantle and scenarios involving the alignment of lower mantle or subducted materials by regional dynamic processes are suggested to account for this

sandbox experiments on gravitational spreading and gliding in the presence of fluid overpressures whereas in previous analogue experiments on gravitational spreading and gliding detachment occurred on a ductile layer we have used a relatively new technique of injecting compressed air into sand packs so as to simulate the effects of fluid overpressures in sedimentary strata and to trigger slope instabilities in our experiments the governing equations yield scales for dimensions stresses and fluid pressure however the more transitory phenomena of production and decrease of overpressure cannot be suitably scaled by using layers of differing permeability we are able to produce sharp detachments in models made of sand alone the experiments involve gravity spreading or gravity gliding in gravity spreading propagation of the detachment and of extensional deformation depends on the fluid pressure for medium values of fluid overpressure normal faults are closely spaced numerous and bound rotated blocks they propagate progressively toward the back of the model for the highest pressures the deformation propagates very fast and faults bound non rotated blocks which slide on an efficient basal detachment fault dips are also controlled by fluid pressure and by frictional resistance at the base to model gravitational gliding required an apparatus with a more complex system of air injection we did a series of experiments using injection windows of various lengths and compared the results with predictions from a quasi 3d analytical model of sliding in contrast with predictions for an infinite slope sliding depends on 1 the fluid overpressure on the detachment 2 the fluid overpressure in the body of the sliding sheet and 3 the shape of the detachment surface in particular we show that frictional resistance at the lower edge is a primary control on the dynamics of gliding

seismic anisotropy in the mantle transition zone induced by shear deformation of wadsleyite abstract mantle flow in the earth s mantle transition zone between 410 and 660 km depth plays a key role to understand the nature of mantle convection which can be mapped by observed seismic anisotropy combined with crystallographic preferred orientations of mantle minerals although wadsleyite is the most important mineral to cause seismic anisotropy observed in the mantle transition zone there have been limited experimental data on its crystallographic preferred orientation because of experimental limitations we experimentally evaluated the preferred orientation of wadsleyite developed by shear deformation at pressure temperature conditions of the mantle transition zone 17 6 gpa and 1800 1900 k using a deformation dia apparatus the deformation experiments reveal that the 0 0 1 axis and the 0 1 0 plane of wadsleyite tend to be subparallel to the shear direction and the shear plane during deformation respectively these results demonstrate that polarization seismic anisotropy velocity contrast between horizontally polarized and vertically polarized s waves vsh vsv observed in the mantle transition zone might be attributed to the preferred orientation of wadsleyite caused by horizontal mantle flow

topography of the transition zone seismic discontinuities phase transformations in mantle mineralogies probably cause the transition zone seismic discontinuities nominally at 410 520 and 660 km depth thermodynamic principles govern phase transformations making them sensitive to changes in the mantle s ambient conditions through the thermodynamic response changes in temperature or composition shift the transformation to a different pressure creating topography on a level discontinuity with this use as an exploratory tool for the mantle in mind we review the basic seismological and phase equilibrium concepts underlying the detection and interpretation of the seismic wave arrivals associated with the transition zone discontinuities reviewing the evidence for and against the phase transition model we conclude that it is viable and describe how discontinuities have been and can be used to probe the physical and chemical state of the transition zone

predicted glide systems and crystal preferred orientations of polycrystalline silicate mg perovskite at high pressure implications for the seismic anisotropy in the lower mantle abstract we use first first principle methods and the peierls nabarro model to evaluate the resistance to glide characterized by the peierls stress of glide systems for end member mgsio 3 perovskite at mantle pressures 010 100 is the easiest glide system in mg perovskite at all pressures peierls stresses increase systematically with pressure for all systems except 001 010 indicating the importance of lattice friction at lower mantle pressures the ratio of the maximum peierls stress for each system relative to the 010 100 value defines their critical resolved shear stress crss these crss are used in a visco plastic self consistent homogenization model to predict the evolution of crystal preferred orientations cpo during deformation of polycrystalline mg perovskite in axial compression 100 tends to align with the compression direction in agreement with in situ observations in axial compression experiments in simple shear 010 concentrates near the shear direction and 100 although more dispersed tends to align near the shear plane consistent with the dominant activity of the easier 010 100 system the calculated seismic anisotropy for a 100 mg perovskite aggregate using the cpo in simple shear and the elastic constants of mgsio 3 perovskite at lower mantle pressures and temperatures is weak 3 for p waves with and 2 for s waves and decreases with increasing temperature and pressure p waves show the fastest propagation parallel to the lineation and s waves fast polarization is in the foliation at 38 gpa and normal to the lineation at 88 gpa this weak anisotropy is consistent with global seismological observations of a nearly isotropic lower mantle there are however two regions where strain induced mg perovskite cpo could contribute to anisotropy a low temperature regions in the uppermost lower mantle where the predicted s wave polarization anisotropy may attain 1 6 with a fast polarization parallel to the foliation b in high high temperature domains in the d layer where mg perovskite may be the major stable phase leading to polarization of fast s waves normal to the lineation for propagation directions at high angle to the lineation and an apparent isotropy for all other propagation directions

strain induced seismic anisotropy of wadsleyite polycrystals and flow patterns in the mantle transition zone 1 we use forward models based on recent high pressure experimental data on mantle minerals to predict the seismic anisotropy produced by plastic strain of orthorhombic wadsleyite the dominant mineral in the upper transition zone these models predict a weak seismic anisotropy for a polycrystal of pyrolitic composition 60 wadsleyite 40 garnet at transition zone conditions 2 for p and 1 for s waves for a shear strain of 1 both p and s wave anisotropy patterns show an orthorhombic symmetry p waves propagate faster at low angle to the shear direction and slower at high angle to the shear plane s wave anisotropy is characterized by faster propagation of waves polarized at low angle to the shear direction horizontal shearing results therefore in higher velocities for horizontally propagating p waves ph and horizontally polarized s waves sh as well as in weak azimuthal variation of sv and sh velocities on the other hand vertical flow leads to higher velocities for vertically propagating p waves pv and vertically polarized s waves sv and to a weak azimuthal variation of sv velocity but to a roughly constant sh velocity analysis of global observations of seismic anisotropy in the transition zone in the light of these models supports dominant horizontal flow in the uppermost transition zone in agreement with predictions of geodynamical models that explicitly introduce phase transitions

elastic properties of hydrous ringwoodite at high pressure conditions 1 the sound velocities and single crystal elastic moduli of hydrous mg2sio4 ringwoodite containing 2 3 wt of h2o have been measured by brillouin spectroscopy at high pressures to 23 4 gpa spanning the pressure range in earth s transition zone the resulting pressure derivatives of the adiabatic bulk modulus k s and shear modulus are 4 4 1 and 1 7 1 respectively compared with results for anhydrous ringwoodite the pressure derivatives of the elastic moduli are consistent with an increase due to hydration of as much as 7 for the k s and 30 for depending on the data sets used for comparison however the gradients of velocity as a function of pressure for hydrous ringwoodite are significantly less than the corresponding gradients in the earth s transition zone we conclude that transition zone seismic velocity gradients are not due to wet ringwoodite as previously speculated

heterogeneous seismic anisotropy in the transition zone and uppermost lower mantle evidence from south america izu bonin and japan summary measurements of seismic anisotropy are commonly used to constrain deformation in the upper mantle observations of anisotropy at mid mantle depths are however relatively sparse in this study we probe the anisotropic structure of the mid mantle transition zone and uppermost lower mantle beneath the japan izu bonin and south america subduction systems we present source side shear wave splitting measurements for direct teleseismic s phases from earthquakes deeper than 300km that have been corrected for the effects of upper mantle anisotropy beneath the receiver in each region we observe consistent splitting with delay times as large as 1 s indicating the presence of anisotropy at mid mantle depths clear splitting of phases originating from depths as great as 600km argues for a contribution from anisotropy in the uppermost lower mantle as well as the transition zone beneath japan fast splitting directions are perpendicular or oblique to the slab strike and do not appear to depend on the propagation direction of the waves beneath south america and izu bonin splitting directions vary from trench parallel to trench perpendicular and have an azimuthal dependence indicating lateral heterogeneity our results provide evidence for the presence of laterally variable anisotropy and are indicative of variable deformation and dynamics at mid mantle depths in the vicinity of subducting slabs

single crystal equation of state of phase d to lower mantle pressures and the effect of hydration on the buoyancy of deep subducted slabs 1 an understanding of the physical properties of the hydrous magnesium silicate phase d is important for the interpretation of the seismic anomalies observed in subducted slabs and to evaluate the effect of hydration on slab dynamics here we report the equation of state of phase d mg1 1si1 8h2 5o6 up to 65 gpa obtained from high precision single crystal x ray diffraction a single crystal of phase d was loaded in a diamond anvil cell using helium as pressure transmitting medium to ensure quasi hydrostatic conditions during the entire data collection the volume of phase d decreases smoothly over the entire pressure range without the anomalies in the compressibility reported at 40 gpa in previous powder diffraction studies if existing in phase d a hydrogen bond symmetrization transition as predicted by first principles calculation is therefore not associated with anomalies in the volume compression behavior the isothermal bulk modulus kt and its pressure derivative k t obtained from the fitting of the unit cell volumes using a third order birch murnaghan equation of state are kt 151 4 1 2 gpa and k t 4 89 0 08 respectively this bulk modulus is in good agreement with a recent single crystal brillouin scattering experiment the presence of 16 vol of phase d in hydrous peridotite lithologies reduces the density by up to 2 6 at upper most lower mantle pressure temperature conditions 1273 k and 30 gpa phase d is thus a potential candidate to influence the buoyancy of hydrated stagnant slabs below the transition zone

seismic anisotropy in the mantle transition zone beneath fiji tonga 1 we demonstrate a rare unequivocal case of anisotropy in the transition zone tz evident from many horizontal polarized shear waves sh that arrive up to 3 s earlier than vertical polarized shear waves sv the anisotropic region correlates with deep earthquakes away from the active subduction zone and known boundaries of lateral heterogeneities in compression and shear wave speeds we interpret the anisotropy as strong fabric associated with a large petrologic anomaly in the core of remnant lithosphere which is buoyant and stagnates in the tz fast subduction of cold slab actually favors such a configuration so slab penetration into the lower mantle does not uniformly occur

an experimental investigation of gravity driven shale tectonics in progradational delta abstract many deltas exhibit gravitational deformation of their sedimentary cover in these systems the decollement layers do not always consist of rock salt but sometimes of overpressured shale unlike salt the efficiency of detachment in shale depends on the magnitude of fluid overpressures and it varies through time and space as rapid sedimentary burial progrades into deeper water as a result the gravity deformational domains are progressively translated seaward sandbox models involving high air pore pressures were used to simulate such gravity driven shale tectonics in prograding deltas models were built with sand of various permeabilities and air was injected to simulate the mechanical effects of fluid overpressure our apparatus for the injection of air allowed us to control subsurface pressures in space and time during the experiments and it was used to simulate the advance of the front of the overpressured domain during the sedimentary progradation in our models sand kept obeying a frictional behavior for medium to high pore pressures and the detachment appeared as very thin shear bands compressional belts that formed during the experiment were dominated by asymmetric basinward verging fore thrusts as is often observed in deep water shale detached foldbelts where the value of fluid pressures approached that of the lithostatic stress sand was fluidized resulting in ductile strains analogous to what occurs in highly overpressured mobile shale during progradation ancient buried thrustbelts were reactivated thereby controlling later extension during the experiments sand volcanoes analogous to mud volcanoes formed in relation with tectonic structures some of them developed near normal faults but many of them formed directly above old buried thrusts

new insights into deformation mechanisms in the gravitationally driven niger delta deep water fold and thrust belt we use two and three dimensional seismic reflection data from the deep water niger delta fold and thrust belt to document evidence for two discrete postfaulting deformation mechanisms an early phase of thrust propagation folding is followed by folding caused by thickness changes within the basal shale detachment unit the later phase of folding is caused by a lateral redistribution of the strata within the basal detachment unit this example of late deformation occurred over a 4 5 m y period as a result of the displacement of approximately 590 km3 370 mi3 of the underlying strata within the detachment unit in another deformation event in the basal detachment unit about 160 km3 100 mi3 moved laterally creating a synform in the overburden and parallel onlap fill indicative of the relatively rapid creation of accommodation space on the basis of seismic reflection data from the delta and a consideration of the volumes and rates of movement of sedimentary rock we conclude that the poorly imaged succession commonly referred to as mobile shale cannot deform solely by ductile mechanisms as interpretations of shale tectonic provinces have commonly suggested but instead by brittle processes that involve thickening by thrust faulting and subseismic accommodation structures processes such as liquefaction where a complete loss of shear strength is observed had a minimal function therefore the term mobile shale in this setting is widely exaggerated an awareness of postfaulting deformation mechanisms will be important for the successful exploration of gravitationally driven fold and thrust belts

mantle anisotropy beneath northwest pacific subduction zones to assess the location strength and orientation of seismic anisotropy in the southern kuril japan and izu bonin subduction zones we analyzed shear wave splitting in local s phases and teleseismic sks phases fast directions from these phases are roughly parallel to the direction of absolute pacific plate motion wnw in izu bonin roughly parallel to the strike of the trench nne in central honshu and roughly n in the southern kuril back arc in the vicinity of sakhalin island assuming that the orientation and strength of anisotropy does not vary with depth modeling of splitting times from local s and teleseismic phases recorded at sakhalin island requires that the maximum depth of anisotropy lies between 480 km and 950 km in contrast splitting times for local s phases from events in izu bonin rule out anisotropy in the transition zone all the data are consistent with a model in which the lower transition zone 520 660 km and lower mantle are largely isotropic and in which anisotropy occurs intermittently in the upper transition zone 410 520 km possibly due to preferred orientation of spinel assuming that splitting in the upper mantle is produced by preferred orientation of olivine the observed fast directions indicate that the geometry of back arc strain varies systematically between subduction zones the relationship of fast directions and plate motions suggests that the subducting slab exerts significant control on back arc flow but that flow correlated with upper plate deformation must also exist

mechanism of thin skinned detachment in the amazon fan assessing the importance of fluid overpressure and hydrocarbon generation abstract since about 10 ma a wedge of clastic sediment up to 10 km in thickness has accumulated near the mouth of the amazon river forming the amazon fan thin skinned extensional and contractional structures root into a common basal detachment hundreds of kilometers long extensional faults have formed landward of the prograding delta slope and contractional folds and faults have formed seaward of it by plan view restoration we show that extension and contraction balance we therefore infer that slope instability has provided the driving force for thin skinned deformation on seismic data the detachment appears as a sharp surface within late cretaceous strata however there is no evidence at that level for soft layers such as evaporites or muds abnormal fluid pressure provides a more reasonable mechanism for the detachment at depths of several thousand meters deep wells have encountered fluid overpressures amongst the possible causes of abnormal fluid pressures are compaction and hydrocarbon generation although compaction may have contributed to fluid overpressures encountered in wells we argue that hydrocarbon generation provides the best explanation for detachment in cretaceous rocks which were old and lithified when it occurred in particular the broadly triangular area of detachment coincides with an inferred pod of cenomanian turonian source rock which is interpreted to have been in the gas window during detached deformation we have developed new techniques of sandbox modelling to investigate the geometries of fault patterns produced by fluid flow through porous media for the amazon fan we have been able to reproduce the structural pattern on the assumption that fluid overpressures existed over a broadly triangular area beneath the detachment

present day stress orientations and tectonic provinces of the nw borneo collisional margin received 23 september 2009 revised 17 may 2010 accepted 9 june 2010 published 16 october 2010 1 borehole failure observed on image and dipmeter logs from 55 petroleum wells across the nw borneo collisional margin were used to determine maximum horizontal stress sh orientations combined with seismic and outcrop data they define seven tectonic provinces the baram delta deepwater fold thrust belt exhibits three tectonic provinces its inner shelf inverted province sh is nw se margin normal its outer shelf extension province sh is ne sw margin parallel and its slope to basin floor compression province sh is nw se margin normal in the inverted province sh reflects inversion of deltaic normal faults the sh orientations in the extension and compression provinces reflect deltaic gravitational tectonics the shale and minibasin provinces have been recognized in offshore sabah in the shale province sh is n010 e which aligns around the boundary of a massif of mobile shale currently no data are available to determine sh in the minibasin province in the balingian province sh is ese wnw reflecting ese absolute sunda plate motions due to the absence of a thick detachment seen elsewhere in nw borneo the central luconia province demonstrates poorly constrained and variable sh orientations these seven provinces result from the heterogeneous structural and stratigraphic development of the nw borneo margin and formed due to complex collisional tectonics and the varied distribution and thicknesses of stratigraphic packages

effective media for transversely isotropic models based on homogenization theory with applications to borehole sonic logs sonic log records including measurements of wave speeds in boreholes provide critical input to the geological geophysical and petrophysical studies of a region under exploration 1d background models are routinely built based on sonic log records for applications such as seismic imaging of hydrocarbon reservoirs and microseismic source inversions smoothing or upscaling techniques are required to produce models in coarser scales than the very fine layers in the raw log data in this paper we follow the recently popular homogenization theory derive its application to the special case of 1d ti models for both p sv and sh waves and show that it is consistent with the backus averaging technique commonly used to upscale 1d fine layered models we examine a study case of sonic log data from a well in the horn river basin in northeastern british columbia a region known for its tight shale gas deposit we demonstrate the computational accuracy and efficiency gained by proper upscaling procedures for spectral element simulations of seismic wave propagation and discuss the effect of control parameters on wavefield recovery