mechanics of oblique convergence the distribution and magnitude of the strike parallel component of velocity in an obliquely converging thrust wedge or accretionary prism are determined by the geometry and mechanical properties of the wedge a mechanical analysis based on the assumption of a critical or stable geometry of the wedge for which the rate of cross strike deformation is zero leads to the following conclusions for different bulk rheologies 1 in a linear viscous wedge the strike parallel motion relative to the underthrust slab decreases exponentially away from the rear and is effectively concentrated in a shear zone with a width comparable to the thickness of the wedge at the rear the wedge also deforms by corner flow producing a circulation in the cross strike plane the strike parallel and corner flow velocities depend on the thickness and viscosity of the wedge and on the shear stresses applied to its lower and rear boundaries convergence at the wedge front is normal to strike 2 a critically tapered perfect plastic wedge moves coherently without internal deformation for low and moderate obliquities of the convergence vector the wedge moves at the same velocity as the backstop upper plate for high angles of obliquity the wedge moves laterally relative to the underthrust slab at a maximum velocity dependent on its dimensions and the stress conditions on its boundaries so that it is separated from the upper plate by a strike slip fault defining a forearc sliver no geometrical configuration exists that allows the strike parallel motion to be distributed through the wedge 3 a noncohesive coulomb wedge behaves in much the same way as a plastic wedge but the geometry and velocity depend only on its mechanical properties and the shear stresses on its boundaries and they are independent of scale

p t t paths of collisional orogens pressure temperature time p t t paths summarize the changes in pressure and temperature imposed on a metamorphic rock during orogenesis they provide a convenient framework for the interpretation of complex metamorphic histories and also offer insight into the thermal and tectonic factors controlling metamorphism in collisional orogens p t t data are acquired through a combination of textural observations thermobarometry and thermochronometry and assembled into a p t t path using geological constraints one dimensional p t t models assuming instantaneous deformation and thermal relaxation by conduction are flexible and useful for testing tectonic models particularly where geochronological constraints are available two dimensional models allow more sophisticated deformation geometries and allow the effects of advection to be incorporated analysis of collisional orogens in terms of critical wedge theory can yield p t t paths that reflect coupling between thickening uplift exhumation erosion and convergence where rates of erosion approach rates of tectonic uplift as is currently happening in the southern alps of new zealand high grade metamorphic rocks can be exhumed rapidly from considerable depth alternatively rapid exhumation may reflect gravity driven extension in an over steepened or thermally weakened orogen

viscous flow and deformation of regional metamorphic belts at convergent plate boundaries 1 viscous flow and deformation of a newtonian fluid between rigid boundaries have been modeled for investigating deformation of a forearc wedge and a regional metamorphic belt as a weak deforming zone between the plates at convergent boundaries three types of analytic formulations together with numerical formulations for a newtonian fluid with a constant viscosity are used for investigating various flows and deformation 1 flows induced by squeezing between two parallel boundaries 2 flows induced by squeezing and heterogeneous mass influx in the wedge between two oblique boundaries e g accretionary wedge over the subducting plate with the continental plate as a backstop and 3 flows induced by dragging along the boundaries and by homogeneous mass influx in the wedge as in type 2 all cases include strike slip movements of the boundaries which allows us to investigate three dimensional 3 d deformation e g 3 d corner flow the corresponding flow deformation of infinitesimal and finite elements and the timescale of flow are calculated for each configuration with various mechanical boundary conditions the model results show that configuration and mechanism of the flow can be inferred from the spatial variation of finite deformation e g deformed radiolarians as a strain marker and geometry of folding as finite deformation of a large block in particular prolate strain nearly parallel to the strike of subduction zone together with a large scale folding can be a good indicator for deformation in the forearc wedge associated with 3 d corner flow induced by oblique subduction deformation observed in the cretaceous regional metamorphic belts in southwest japan can be explained by this mechanism

the influence of climate on the tectonic evolution of mountain belts simple physical arguments analogue experiments and numerical experiments all suggest that the internal dynamics of actively deforming collisional mountain ranges are influenced by climate however obtaining definitive field evidence of a significant impact of climate on mountain building has proved challenging spatial correlations between intense precipitation or glaciation and zones of rapid rock uplift have indeed been documented in numerous mountain ranges and are consistent with model predictions more compelling evidence such as tectonic changes in response to rather than just coincident with climate change has however rarely been documented triggered by a climate driven increase in erosion rate friction dominated mountain ranges are expected to show a number of simultaneous responses a decrease in the width of the range a temporary increase in sediment yield a persistent increase in the rate of rock uplift and a reduction in the subsidence rate of surrounding basins the most convincing field evidence for such a coordinated response of a mountain range to climate change comes from the european alps and the st elias range of alaska field studies and experimental research during the past two decades have provided considerable evidence for a significant influence of climate on tectonics recent advances suggest that model predictions can guide future fieldwork aimed at substantiating this view

exhumation of eclogites insights from depth time path analysis abstract literature data on geochronology and pressure temperature estimates have been used to assess the depth time paths for high pressure units in the alps and the variscan belt these depth time paths consistently show that exhumation rates decrease through time the first stages of the exhumation are rapid 1 mm yr 1 and last less than a few million to a few tens of million years in contrast the final 10 20 km ascent to the surface is at least one order of magnitude slower for the deepest and most important part of the exhumation trajectory the chronology of the retrogressive metamorphism excludes gravitational collapse as the major driving force material balance and structural arguments also exclude isostatically assisted erosion processes prism dynamics and tectonically assisted erosion are the most likely mechanisms accounting for the kinetics of exhumation isostasy assisted erosion and shallow level extension are nevertheless required to eventually bring the high pressure rocks to exposure

from foreland to forearc domains new multichannel seismic reflection survey of the mediterranean ridge accretionary complex eastern mediterranean abstract the mediterranean ridge of the eastern mediterranean sea is a huge accretionary wedge emplaced as a consequence of the africa aegean plate convergence that has a rate estimated at 40 mm yr processing of recent multichannel seismic data and integration with previous results seismic profiling swath bathymetry sonar have facilitated the study of the overall deformation pattern of the area which includes three distinct major structural domains facing various forelands 1 1 the outer domain is bounded southward by the present day wedge toe and towards the north by topographic slope breaks and clear changes in deformational style beneath the ionian and levantine mediterranean ridge branches the base of messinian evaporites seems to act as a major decollement level 2 2 the central or crestal domain the shallowest one shows evidence of mud diapiric and mud volcano activity and is overthrusting northward over the inner domain facing the libyan margin this area which displays spectacular mud volcano fields appears much wider and shallower 3 3 the less deformed inner domain is bounded northward by the hellenic trench system beneath its ionian inner area seismic velocities characteristic of continental crust have been recorded while within its levantine region it is made of a series of disconnected and uplifted basement blocks strabo seamounts along the matapan and pliny trenches structures which could be caused by transpressive deformation are locally observed we believe that both the north south structural arrangement and the clear west east lateral variations of the mediterranean ridge relate to its progressive collision against the libyan margin promontory this event which may have initiated 5 6 myr ago may have induced a a differentiated paleogeography of messinian evaporites later available for tectonic accretion leading to the further lateral structural variations of the outer domain and b strain partitioning between the central mediterranean ridge and the hellenic trench system

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

offscraping and underthrusting of sediment at the deformation front of the barbados ridge deep sea drilling project leg 78a on leg 78a we drilled sites 541 and 542 into the seaward edge of the barbados ridge complex and site 543 into the adjacent oceanic crust the calcareous ooze marls and muds at sites 541 and 542 are lithologically and paleontologically similar to the upper strata at site 543 and are apparently offscraped from the down going plate a repetition of miocene over pliocene sediments at site 541 documents major thrust or reverse faulting during offscraping the hemipelagic to pelagic deposits offscraped in the leg 78a area include no terrigenous sand beds but they contain numerous neogene ash layers derived from the lesser antilles arc hence this sequence is quite unlike the siliciclastic dominated terranes on land that are inferred to be accretionary complexes the structural fabric of the offscraped deposits at sites 541 and 542 is disharmonic probably along a decollement with an underlying acoustically layered sequence suggesting selective underthrusting of the latter the acoustically layered sequence correlates seismically with pelagic strata cored at site 543 on the incoming oceanic plate cores recovered from the possible decollement surface at both sites 541 and 542 show scaly foliation and stratal disruption approximately lithostatic fluid pressure measured in the possible decollement zone probably facilitates the underthrusting of the pelagic sediments beneath the offscraped deposits in the incoming section a transition from smectitic to radiolarian mud with associated increases in density and strength probably controls the structural break between offscraped and underthrust strata in the leg 78a area the underthrust pelagic section can be traced seismically at least 30 km arcward of the deformation front beneath the barbados ridge complex

dynamics of the polish and eastern slovakian parts of the carpathian accretionary wedge insights from palaeostress analyses the arcuate outer carpathian accretionary wedge formed in front of the east alpine carpathian pannonian alcapa megablock during the eocene sarmatian the wedge accreted sediments of the subducting remnants of the carpathian flysch basin a large oceanic tract left in front of the alpine orogen the palaeostress data for the orogenic hinterland particularly the data related to the early miocene extension that was expanding towards the ne combined with coeval subduction related volcanism that was expanding towards the ne indicate that the uneven roll back of the subduction zone was the main mechanism controlling the development of the northern west carpathian arc the palaeostress data for the tertiary accretionary wedge from the same time period are characterized by outward fanning 1 trajectories that changed gradually during the wedge development in contrast the palaeostress data for the hinterland are characterized by preferred directional stress events that changed abruptly during the wedge development these palaeostress results are in accordance with the behaviour of the wedge and the hinterland as the wedge behaved as a weak continuum and the hinterland behaved as a block mosaic with weak boundaries the fault traces of the northern west carpathian arc converge to both ends of the arc and suggest that the pre existing basin was the factor that controlled the arc location these fault trace patterns are asymmetric indicating a slightly oblique overall convergence in a ne sw direction in accordance with this convergence the palaeostress data for the accretionary wedge indicate that the western part of the wedge which is characterized by nw se oriented maximum principal compressional stress 1 was undergoing sinistral transpression meanwhile the eastern part which is characterized by ne sw oriented 1 was undergoing compression apparently the dynamics of the accretionary wedge was further influenced by the shape of an elongated ne sw trending alcapa megablock which was located behind the wedge and advanced in the direction of the general early miocene convergence during the most pronounced stages of the wedge development this megablock served as the local indenter as its strength surpassed that of the accretionary wedge located to its front further dynamic complexities were added because of the complex shape of the magura unit which was located in the most proximal portion of the wedge and was stronger than the units in front of it wedge outcrops indicate that the large scale shortening which is characterized by the development of detachments and ramps was preceded by an initial layer parallel shortening this is indicated by scaly fabrics and minor reverse faults that rotated into locked positions during the later accretion several outcrops with a wedge detachment fault indicate that there was a relatively low amount of friction during its development the decollement zone is several hundred metres thick and shows evidence of transient fluid flow that was driven by pressure gradients this is documented by frequent hydrofracturing sandstone dykes and fibrous veins that opened against the weight of the whole wedge all of which indicate cycles of higher pore fluid pressures that lowered the basal friction

seismic velocities from the barbados ridge complex indicators of high pore fluid pressures in an accretionary complex three wide aperture 0 5 km offset seismic profiles were shot perpendicular to the strike of the barbados ridge accretionary complex at 16 12 n line 465 14 23 n line 480 and 13 20 n line 484 to image the deep structure of the accretionary complex and obtain seismic velocity data velocity analyses on full fold common depth point cdp gathers were used to provide velocity depth functions at 1 km separation along each line to map the variation in velocity with depth and distance across the accretionary complex the sediments within the complex show consolidation increasing laterally with greater age of the complex as indicated by the increase in seismic velocity and the inferred decrease in porosity the velocity at the base of the wedge along line 480 is 2 4 km s at the toe the velocity at the wedge base at 60 km from the toe where the wedge thickness has increased from 1 8 km to 5 0 km has increased to greater than 3 0 km s however the velocities within the leading margins and within undeithrust sequences in each of the three sections are low as much as 1 0 km s less than velocities found in undeformed sedimentary sequences of equivalent thickness the low velocity regions are interpreted as sediments with high porosity where porosity is inferred to be high pore fluid pressures are also inferred to exceed hydrostatic and pore fluids actually support the increased lithostatic load produced by recent thrusting and thickening of the complex the correlation between the most rapidly thickened regions and the velocity anomalies suggests that a rapid rate of loading relative to the rate of pore fluid expulsion produced these overpressured sediments velocities in the underthrust sequences where the differences between loading histories of wedge sediments and the underthrust sequences are largest are sufficiently low to produce velocity inversions low velocities also occur in regions adjacent to the complex where the pore fluid pressures may be elevated by the pressure produced in the adjacent wedge west of the wedge in the forearc basin along line 480 and near a seismically imaged mud diapir velocities at a depth of 3 5 km below the seafloor are 0 5 km s less than what is expected in a more normal sedimentary section immediately seaward of the complex the deepest undeformed sequences have a velocity anomaly of 0 5 km s velocities remain low for at least 12 km seaward of the complex but they become closer to a more normal sediment section farther from the wedge

the geometry and kinematics of the main boundary thrust and related neotectonics in the darjiling himalayan fold and thrust belt west bengal india abstract the main boundary thrust mbt is well exposed in a section of the tista river valley in the darjiling himalayan fold and thrust belt the fault trace trends e w across the n s flowing tista and thrusts gondwana permian age sandstones over lower siwalik miocene age sandstones the mbt has been folded into a fault propagation synform antiform pair by tectonic activity along a younger south kalijhora thrust skt in its footwall that transports lower siwalik rocks over middle siwalik sandstones the skt cuts through the fault propagation structure preserving the mbt fault propagation fold pair in its hanging wall the motion along the skt rotated the bedding and the limbs of the folded mbt in its hanging wall footwall imbrication in front of the skt further rotated the folded mbt footwall imbrication continued to about 30 km south of the study area in the middle siwalik and younger section the region has been subsequently reactivated as evident from uplifted gravel beds and strath terraces along the folded trace of the mbt fault zone and also in the footwall of the skt the reactivation resulted in the formation of connective splays off the existing faults probably forming horses of a connecting splay duplex in the sub surface within the siwalik section the motion along these splays passively folded overlying beds and formed uplifted gravel beds and strath terraces along the local drainage the rotations resulting from footwall imbrication and reactivated splays probably caused the southern limb of the fault propagation synform to become overturned the duplexing also raised the river bed and arrested the flow of the river tista causing the river to pond and deposit its load resulting in the formation of a bar upstream of the mbt trace the tectonic reactivation of the area points to taper building activities in a subcritical wedge probably in response to monsoon induced excessive erosion in the middle of the darjiling sikkim tibet himalayan thrust wedge

emergence of the canadian rockies and adjacent plains a comparison of physiography between end of laramide time and the present day abstract the landscape of the canadian rockies in southern alberta is not a direct result of constructional processes that is the ridges and peaks have not been pushed into the positions in which we see them today tectonic activity provided original elevation but not mountains at the end of laramide time what are now the front ranges and foothills of the rockies comprised a high elevation upland of relatively low relief the present mountain physiography is the result of 55 60 million years of post orogenic differential erosion in which more resistant rocks have been left at higher elevations than less resistant rocks the canadian rockies and the foothills are developed in a thin skinned thrust and fold belt created during the laramide orogeny the adjacent interior plains cut across foreland basin sediments derived from the mountains the mountains currently consist of large parts of ridges of well indurated paleozoic and locally proterozoic rock alternating with valleys developed in soft mesozoic clastic rock in the foothills where the soft mesozoic rock is at the surface relief is subdued but ridges of more resistant sandstone rise above shaley lowlands the plains are relatively flat but also contain erosional outliers of higher paleo plains surfaces numerous lines of evidence suggest that the mountains and foothills have lost several kilometers of overburden since the end of the laramide orogeny while the western plains have lost at least 2 km requiring that the local relief of the mountains and foothills that we see is erosional in origin local physiography is adjusted to lithology the mountains have high relief because the exposed sub mesozoic rocks can hold up high steep slopes whereas the foothills have low relief because the underlying cretaceous rocks cannot hold up high steep slopes the east facing escarpment at the mountain front is a fault line scarp along a low angle thrust mesozoic rocks involved in the deformation originally extended all the way across the thrust and fold belt and physiography of the belt at the end of laramide time 60 55 ma depended mainly on whether mesozoic or paleozoic proterozoic rocks were exposed at the surface at that time a reconstruction using critical taper theory generally agrees with reconstructions from earlier stratigraphic and paleothermometry studies what are now the front ranges at the eastern edge of the rocky mountains were mostly or perhaps entirely covered with mesozoic rocks and despite that high elevation had a hilly not mountainous character the main ranges in the central rocky mountains were in part stripped of mesozoic cover by then and more mountainous treeline was higher then and the thrust belt may have been largely or entirely vegetated generation of modern relief in the front ranges including the escarpment at the mountain front had to await stripping of mesozoic rocks and incision of rivers into harder substrates in post laramide time the interior plains are an erosional surface that was cut 1 to 3 km below the aggradational top of the foreland basin sediments although some of the present low local relief of the plains results from weakness of underlying cretaceous tertiary rocks the low relief is probably largely related to the process of denudation

deformation of the nankai trough inner accretionary prism the role of inherited structures funding for this research was provided by the national science foundation grant oce 0451790 and a u s science support program post expedition award

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

modeling of large scale accretionary wedge deformation instantaneous stress and velocity fields within an active accretionary wedge can be calculated using plastic slip line theory given the assumptions of a perfectly plastic rheology and plane strain conditions after selecting a plastic yield stress and an average density a stress field slip line field can be computed for a wedge of known cross sectional configuration from known stress boundary conditions on the upper surface basal surface tractions are derived without basal boundary conditions the geometry of the computed slip line field can then be used to constrain the lower limit of the yield stress the known velocity boundary conditions are insufficient for calculation of a velocity field because the coupling between the wedge and the subducting plate is unknown consequently basal velocity distributions must be assumed to permit the construction of velocity fields these assumed distributions are constrained by known rates of surface uplift and rates of tilt as well as inferred patterns of instantaneous strain rates within a wedge the sunda accretionary wedge west of central sumatra is used to illustrate the plasticity approach a minimum average yield stress of 20 30 mpa and a weaker basal layer of variable effective strength are indicated for this example surface uplift rates may be affected significantly by regions of high basal strain rate finite velocity discontinuities and underplating the known pattern of uplift rates near nias island located on the outer arc ridge is consistent with the constraint that 90 of the total incremental shortening of the wedge is concentrated within 20 km of the trench given a sufficient number of uplift and tilt rate observations across a particular wedge the relative influence of underplating tectonic erosion and basal strain rate variations could be assessed using the perfectly plastic model

the interplay between climate and tectonics during the upward and outward growth of the qilian shan orogenic wedge northern tibetan plateau abstract mountains grow upward and outward when the accretion of material outpaces its removal mountain building is synergistically influenced by geodynamics internal structures crustal and lithospheric and climate the northward growth of the qilian shan which occupies a transition zone between the 4500 m high central tibetan plateau and the adjacent

mechanics of wedge shaped fault blocks 2 an elastic solution for extensional wedges listric planar low angle and high angle normal faults are common in hanging walls of detachment faults an elastic model has been developed to evaluate the role of basal friction wedge geometry pore fluid pressure within the wedge and boundary conditions applied along the wedge rear in controlling the stress distribution in an extensional fault wedge this model assumes a stress free condition on the top and fractional sliding on the base of the wedge respectively a linear variation of stress components as a function of depth along the wedge rear and a uniform horizontal stress applied on the wedge toe the model predicts that 1 for the surface slope equal to zero a thin wedge favors development of high angle and planar faults whereas a thick wedge favors development of low angle and listric normal faults variation of pore fluid pressures along the basal detachment fault hardly affects the predicted fault geometry pore fluid pressure within the wedge is critical in controlling the state of stress in the wedge higher values of the internal pore fluid pressure promote low angle normal faulting and locally high angle reverse faulting 2 variation of surface slope and the uniform horizontal normal stress applied at the wedge toe does not affect the predicted fault geometry appreciably although the distribution of deviatoric stress magnitude changes for different cases and 3 listric normal faults are predicted in all computations and the fault curvature increases as the vertical gradient of the horizontal normal stress along the wedge rear the wedge angle surface slope dip angle and the internal pore fluid pressure increase the model provides a simple conceptual guide to deciphering the formation of complex fault geometries and cross cutting relationships as a function of mechanical parameters related to geologic processes and settings for example it provides an explanation for why low angle normal faults cut high angle normal faults in hanging walls of some detachment fault systems in the u s cordillera

continental tectonics selected topics cause and effect relations between plate boundary processes and orogeny were investigated using new and refined plate models for the mesozoic and cenozoic terrane accretion and large components of horizontal shear were shown to be fundamental aspects of orogen evolution deep crustal thrust systems with large lateral transport and vertical stacking of slices of crystalline basement were documented in the cordillera and appalachians new geomechanical models of crystalline thrust sheets and foreland thrust belts emphasized the dominance of tectonic rather than gravitational stresses lithospheric thickening was shown to cause buoyancy forces that control the lateral spread of deformation outward from the interior of an orogen and may modify regional stresses in the vicinity of topographic fronts resulting in extension within the upper part of the lithosphere coeval with shortening at depth

dynamic interactions between erosion deposition and three dimensional deformation in compressional fold belt settings 1 this study investigates whether the three dimensionality of erosion and deposition can influence three dimensional folding even when compression is purely convergent this problem is studied by employing a three dimensional mathematical model incorporating dynamic thin plate deformation coupled with fluvial and hillslope sediment transport the approach taken is to impose several fundamentally different sediment routing patterns by changing the initial regional topographic slope and the ratio of the characteristic timescales for deformation to fluvial surface processes r and to investigate how deformation responds numerical results indicate that when and or r are small i e initial topography is regionally flat and or rates of surface processes are slow compared to rate of imposed deformation folding is two dimensional and unaffected by surface processes since loads created by surface processes are imposed after deformation this situation favors development of small scale drainage networks transporting sediment from anticlines to adjacent basins when and or r are relatively large the development of a large scale transverse drainage network which can be maintained during folding is able to strongly amplify and localize folding this leads to interesting features where the largest transverse rivers coincide with the greatest structural and topographic amplitude of doubly plunging folds which is caused by erosion enhanced deformation for intermediate parameter values folding and surface processes interact in a complex manner giving rise to three dimensional en echelon structures that migrate in a transient manner this study demonstrates that under some circumstances one can anticipate major two way interaction between surface processes and three dimensional deformation

prediction of faulting from the theories of elasticity and plasticity what are the limits elasticity rigid plasticity and elasto plasticity are the simplest constitutive models used to describe the initiation and evolution of faulting however in practice the limits of their application are not always clear in this paper we test the behaviour of these different models using as examples tectonic problems of indentation of a die compression with basal shear bending of a plate and normal faulting around a dike by comparing the results of these tests we formulate some guidelines that may be useful for the selection of an appropriate constitutive model of faulting the theory of elasticity reasonably predicts the initiation of the fault pattern but gives erroneous results for large strains the theory of rigid plasticity is more appropriate for large deformations where the geometry of faults can be found by the method of characteristics this method works well for zones of failure that are not severely constrained by elastic material outside e g when faults are connected to the free surface a viscous substratum or a zone of weakness non associated elasto plasticity is the most complete theory among those considered in this paper it describes the evolution of faults from the initiation of localized deformations to the formation of a complicated fault network

critical taper model of fold and thrust belts and accretionary wedges the fold and thrust belts and submarine accretionary wedges that lie along compressive plate boundaries are one of the best understood deformational features of the earth s upper crust although there is considerable natural variation among the many fold and thrust belts and accretionary wedges that have been recognized and explored several features appear to be universal in cross section fold and thrust belts and accretionary wedges occupy a wedge shaped deformed region overlying a basal detachment or decollement fault the rocks or sediments beneath this fault show very little deformation the decollement fault characteristically dips toward the interior of the mountain belt or in the case of a submarine wedge toward the island arc the topography in contrast slopes toward the toe or deformation front of the wedge deformation within the wedge is generally dominated by imbricate thrust faults verging toward the toe and related fault bend folding two north american fold and thrust belts that exhibit these features are shown in figure 1 neither of these two examples is tectonically active today the southern canadian fold and thrust belt was active during the late jurassic and cretaceous 150 100 ma whereas the southern appa lachians were deformed during the late carboniferous to permian alle ghenian orogeny 300 250 ma figure 2 shows two examples that are currently active the taiwan fold and thrust belt produced by the sub duction of the eurasian plate beneath the philippine sea plate suppe 1981 1987 and the barbados accretionary wedge produced by the sub

slip line model for southern california crustal kinematics we propose a simple slip line model to explain the faulting and seismicity patterns of southern california we assume the sierra nevada great valley sngv block to be rigid in view of its low seismicity and other regions to undergo plastic deformation from this point of view the complex deformation patterns observed in southern california including the borderland region result from the plowing of the eastern margin of the pacific plate by the nondeforming sngv block a simple analogy can then be made between regional tectonics in southern california and two dimensional flow solutions obtained in metallurgy for the extrusion of rigid plastic metals we use a simplified geometry based on the geology and seismicity of southern california to calculate a two dimensional slip line field which predicts the shape of the san andreas fault near the big bend surprisingly well the largely aseismic high topography western mojave can then be interpreted as an analogue to the nondeforming dead metal zone which arises for certain metal extrusion geometries the slip field agrees quite well with the strikes and slip orientation of major dextral faults in southern california including the san jacinto elsinore and newport inglewood faults it also predicts correctly the strikes and slip orientation of sinistral cross faults found in some areas of the region in the transverse ranges where thrust faulting dominates the slip lines are no longer potential fault planes but a simple argument predicts that they should lie at 45 on the strikes of thrust faults in fair agreement with the observations

a fluid model for the shape of accretionary wedges in this paper the accretionary wedges landward of most oceanic trenches are modelled assuming a newtonian fluid rheology it is shown that lubrication theory is applicable and that the addition of material to the wedges can be neglected in determining the shape of the wedges the predicted shape of the wedges is in good agreement with bathymetric profiles across the kurile ryukyu and aleutian wedges from the shape of the above wedges it is deduced that accretionary sediments have viscosities between 1018 and 1019 poise

estimating basal friction in accretionary wedges from the geometry and spacing of frontal faults elastic theory applied to the deformation in accretionary wedges is used to calculate the condition for slip along an active frontal fault and the basal decollement the equations for calculating the stresses can be solved for the coefficient of basal friction in the situation of the formation of a new frontal thrust fault this allows us to calculate the efficient coefficient of basal friction which includes the weakening effect of pore fluid pressure from geometric parameters and material properties only the geometric parameters like fault dip and layer thickness can be derived from high resolution seismic cross sections application of our analysis to the makran and the nankai accretionary wedge allows us to estimate the upper limit of the effective coefficient of basal friction wbw0 16 and wbw0 2 in these two areas respectively fl 2001 elsevier science b v all rights reserved

impact of sedimentation on evolution of accretionary wedges insights from high resolution thermomechanical modeling syntectonic sedimentation history is a potential cause of differentiated accretionary wedge structures along the subduction margin recent efforts to model the role of sedimentation on wedge evolution have highlighted the importance of spatio temporal history of sedimentation on the evolution of the wedge moreover reconstruction of deformation history of the accretionary wedges using reflection seismic and borehole data have further substantiated the impact of sedimentation on wedge evolution we conduct several numerical experiments using a high resolution dynamic 2d thermo mechanical plate subduction model to systematically investigate and quantify different effects of sedimentation on accretionary wedge evolution models with sedimentation suggest migration of deformation to parts of the wedge lying outside the sedimentation zone leading to emergence reactivation of out of sequence thrusts oosts frequency and length of new thrust sheets are correlated with sedimentation in the trench models undergo a transition period of 1 5 myr following the onset of sedimentation after which they continue to grow under a new steady state stabilization of the wedge and increased load on the oceanic plate due to sedimentation create conditions in which smaller wedge top basins combine to form a large and flat forearc basin last but not the least emergence of oost in models of accretionary wedges undergoing sedimentation provide important insights in to evolution of potentially tsunamigenic oosts like the megasplay fault seaward of the kumano forearc basin

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

characterization of iron in lake towuti sediment abstract sediments collected from lake towuti an ultramafic hosted lake in indonesia preserve a visible alternating pattern of red and green sediments due to variations in clay mineral and fe oxide composition and abundance consistent with changes in iron oxidation state through time spectral mineralogical and chemical analyses on soils river and sediment samples from across the lake and its catchment were carried out to better understand the starting composition of these sediments and the processes that affected them before and after deposition despite high fe abundances in all samples and abundant fe oxides in lateritic source regions mineralogical analyses x ray diffraction xrd and mossbauer spectroscopy of the modern lake sediment show almost no well crystalline iron oxides in addition sequential fe extractions suggest an increasing proportion of easily extractable poorly crystalline x ray amorphous material with burial depth xrd bulk chemistry and visible near infrared vnir spectral reflectance measurements demonstrate that clay mineralogy and bulk chemistry can be inferred from vnir data these results provide evidence for variations in fe mineralogy and crystallinity based on location in this source to sink system understanding how the mineralogy and chemistry of sediments within a ferruginous lake basin are affected by transport chemical alteration physical alteration and deposition from source to sink on earth and the degree to which these trends and underlying processes can be inferred from chemical and spectral properties may provide useful direction in assessing paleoenvironmental conditions in other terrestrial lakes as well as ancient lacustrine environments preserved in the stratigraphic record of mars

fold thrust belt evolution expressed in an internal thrust sheet sevier orogen the role of cataclastic flow large hinterland thrust sheets are the dominant structures in retroarc fold thrust belt ftb wedges these internal sheets remain active throughout the history of an ftb and strongly influence the kinematics of the external portions of the ftb and the overall behavior of the entire tectonic wedge their emplacement and deformation account for a significant portion of the total mechanical energy budget in ftb evolution based on estimates using deformation features preserved within such sheets moreover the deformation within dominant internal sheets can be used as a means to evaluate the tectonic history of the wedge as a whole the principal deformation mechanisms in different parts of an ftb wedge are strongly controlled by variations in the geothermal gradient retroarc ftbs typically show a steeper geothermal gradient in the hinterland than in the foreland resulting in a foreland ward slope of the brittl ductile transition therefore a change from plastic to elastico frictional ef mechanisms is expected not only with decreasing depth but also across strike as a dominant internal thrust sheet is transported toward the foreland and undergoes exhumation by synorogenic erosion however evidence for cataclasis overprinting early plastic deformation features is often not preserved in ancient ftbs because of extensive erosion of the wedge top even where evidence for lat stage cataclastic behavior exists in either ancient or modern i e active orogens it is often not recognized or studied our work in the canyon range central utah segment of the sevier ftb shows that unraveling the ef mechanisms such as lat stage cataclastic flow is critical to understanding the total deformation history of an internal thrust sheet after initial emplacement under plastic deformation conditions the cr thrust sheet was continuously reactivated under ef conditions thereby helping to maintain critical taper of the ftb wedge during its development structures preserved in the cr thrust sheet therefore record a significant portion of the deformation history of the ftb as a whole the canyon range cr syncline part of the cr sheet underwent fold tightening within the ef regime and the cataclasized rocks are well preserved the geometry and progressive deformation patterns of the cr syncline reveal four distinct to partly overlapping deformation stages the history of the rocks in the cr thrust sheet has been tracked by incrementally retro deforming the ftb and by unraveling details of the deformation within the cr syncline at various scales for each stage we use the cr syncline as a type example to show that unraveling ef mechanisms is critical to 1 understanding the state of a ftb wedge during its evolution 2 calculating the energy involved in ftb emplacement and 3 developing accurate restorations placing constraints on the depth of the rocks for successive stages of deformation is essential for developing retrodeformable balanced cross sections and in turn geometric and kinematic models for wedge evolution

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

a kinematic model for evolution of island arc trench systems summary plate subduction zones are typically characterized by the patterns of surface topography and gravity anomalies consisting of island arc high trench low and outer rise gentle high these patterns are stable on the time scale of 106 107 yr at some subduction zones regardless of its age steady uplift of marine terraces formed by eustatic sea level changes during the last lo5 yr can be also observed the stable patterns of topography and gravity anomalies and the steady uplift of marine terraces seem to contradict each other we constructed a kinematic model which could explain the evolution process of island arc trench systems and demonstrated that this puzzle could be solved by considering the effect of accretion at plate boundaries in our model the lithosphere asthenosphere system is represented by a stratified visco elastic half space under gravity which consists of a high viscosity surface layer and a low viscosity substratum the rheological properties of both layers are assumed to be a maxwell fluid in shear and an elastic solid in bulk interaction between oceanic and continental plates is represented by the steady increase of discontinuity in tangential displacement across the plate boundary the other essential factors considered in our model are accretion of oceanic sediments at plate boundaries erosion on land and sedimentation on inner trench wall we computed the evolution process of island arc trench systems by using the kinematic model and obtained the following results the island arc trench systems grow at nearly constant rates in the early stage of plate subduction therefore at young subduction zones we can generally expect the steady uplift of marine terraces the stable patterns of topography and gravity anomalies are formed within several million years after the initiation of plate subduction when the accretion process continuously proceeds at plate boundaries these stable patterns gradually migrate seaward as a whole in such a case we can expect the steady uplift of marine terraces even at old subduction zones we demonstrate that our model can consistently explain the observed surface topography gravity anomalies and uplift rates of marine terraces at subduction zones

the obliquely convergent plate boundary in the south island of new zealand implications for ancient collision zones abstract the alpine fault of new zealand forms the western boundary of a zone of distributed deformation formed by the oblique convergence of continental crust belonging to the pacific and australian plates structural and geodetic data from the alpine fault show that a large proportion of the total plate displacement is accommodated by rapid oblique slip on the fault the remaining displacement is distributed over a 200 km wide zone to the east the collision may be modelled as a two sided deforming orogen with the partitioning of deformation being controlled by erosional differences between the narrow high strain inboard and broad low strain outboard sides in ancient collision zones little evidence may remain of the nature and amount of displacement on the inboard side partitioning of deformation among pre existing structures complicates interpretation of the outboard zone radiometric ages may post date collision by several tens of millions of years and indicate slow isostatic uplift and unroofing collision ages if preserved may be recognized by high uplift rates calculated from muscovite biotite pairs

orogeny and metamorphism a model for deformation and pressure temperature time paths with applications to the central and southern appalachians a model is presented for deformation uplift and exhumation in convergent orogens based on the following premises 1 foreland basin subsidence monitors the distribution in time and space of overthrust loads added to an orogen 2 the presence of an inherited rifted margin plays an important role in the subsequent history of an orogen by forming a basement ramp or buttress that must be surmounted in order for thrusts to advance on the craton on the seaward side of this transition up to 20 km of overthrusts can be accumulated without creating topography 3 critical shape theory can be applied to orogens as a whole allowing predictions to be made concerning dynamic balance between mass addition and loss uplift exhumation and growth of topography and 4 pressure temperature time p t t data can be used to reconstruct particle paths within the deforming orogen three states of overthrust orogens can be defined based on the relative rates of mass loss and mass addition as monitored by the foreland rapid exhumation of metamorphic terranes is most likely in the steady state where erosion balances accretion a constructive state where mass addition is greater than mass loss results in crustal thickening and initially little exhumation a destructive state where mass loss outweighs mass addition results in slow exhumation or subsidence of metamorphic rocks in the orogen depending on whether mass is removed by erosion or by extension p t t data on peak metamorphism pluton emplacement uplift exhumation and cooling can be used in conjunction with data from the foreland to distinguish among these various states the topography and strength of the inherited rifted margin play an important role in localizing zones of exhumation and uplift within the orogen application of this model to the central and southern appalachians suggests that during the constructive phases of the taconian and acadian orogenies overthrust loads accumulated mainly on the seaward side of the ocean continent transition as marked by the bouguer gravity gradient between the acadian and alleghanian orogenies the appalachian orogen was probably in a steady state with uplift and exhumation of rocks in the orogen but little subsidence of the foreland or advance of thrusts toward the craton the constructive stage of the alleghanian orogeny achieved the critical topography necessary to drive crystalline thrust sheets over the craton this resulted in widespread uplift and westward transport of older metamorphic rocks post orogenic unroofing of the southern appalachians is compatible with the predictions of flexural models the post orogenic unroofing history of the central appalachians and ouachitas probably reflects mesozoic extension in these areas

growth of the south pyrenean orogenic wedge a six step reconstruction of the south pyrenean foreland fold and thrust belt in spain delineates the topographic slope basal dtcollement angie internal deformation and thrust front advance from the early eocene until the end of contractional deformation in the late oligocene style of thrust front advance dip of the basal dtcollement slope of the upper surface and internal deformation are decoupled and not simply related internal deformation increased decreased and maintained surface slope angle at different stages from the onset to the cessation of deformation the basal dtcollement angle decreased overall suggesting translation of the thrust belt onto stronger crust with time taper angle of the pyrenean thrust wedge was fundamentally controlled by the flexural rigidity of the lower plate the relative rate of creation of structural relief in the rear versus the front of the wedge the extent of deposition of eroded material within the deforming wedge and the taper of the pretectonic stratigraphic wedge

three dimensional numerical models with varied material properties and erosion rates implications for the mechanics and kinematics of compressive wedges received 19 march 2008 revised 29 january 2009 accepted 13 february 2009 published 21 april 2009 1 we develop three dimensional mechanical models of a compressive wedge and investigate how the form and kinematics of the outboard wedge are affected by variation in initial topography material properties and erosion rate inclusion into the wedge of weaker less dense material affects the form of the wedge producing a region of steeper average slopes and higher topography while depriving the region further inboard of material enhanced erosion has a similar effect the wedge attempts to replace the eroded material by focusing deformation the result is a stepped region of lower topography within the outboard of the orogen we observe that uplift velocities at three points in the orogen vary cyclically from near zero to 3 times the average uplift rate over cycles lasting on the order of 15 200 000 model years our models along with analog models and some well dated examples from active orogens suggest that transient accommodation of strain may be common the cycles observed responded rapidly to changes in the amount of erosion imposed our models suggest that orogens may be driven by strong coupling between erosion and strain on temporal scales of 10 4 10 5 years and spatial scales comparable to the scale of the erosional perturbation we compare our models to the puli embayment of west central taiwan a region of anomalous low topography and suggest that it presence may reflect the presence of weaker and more erodible sediments than those present along strike in the orogen

thrust fault mechanics and dynamics during a developmental stage of a foreland belt abstract studies of continental seismogenic zones indicate that deformation during an orogenic stage is episodic and spasmodic down to mid crustal depths and that seismic as well as aseismic modes of deformation need to be incorporated in quantitative theories of foreland belts such a theory is developed here it includes a method to help decipher the development of structures such as imbricate fans frontal ramps and ramp anticlines as illustrated for the hogsback frontal ramp and its ramp anticline in the kemmerer region of the wyoming salient the structure of the hogsback ramp is consistent with a shear fracture origin limit equilibrium mechanics indicate that shear failure would have proceeded incrementally possibly in response to recurring fault slip on the basal decollement contrasts in elastic constants and rock strength are critical e g current data indicate that shear failure development of the ramp would have begun 1 fourth fifth s distance down the ramp in competent madison carbonates if they compared in strength with the marianna limestone or 2 higher in the section if the madison compared in strength with the hasmark or blair dolostones whether the initial development of the ramp anticline involved seismic modes is not clear yet but elastic stiffness data indicate that subsequent imbrication of the anticline did involve seismic as well as aseismic modes of deformation

extensive underthrusting of undeformed sediment beneath the accretionary complex of the lesser antilles subduction zone along the eastern margin of the caribbean plate which overrides the atlantic oceanic lithosphere at 20 km myr 1 in an easterly direction1 east of the lesser antilles island arc lies a wide accretionary complex barbados ridge complex 260 km wide at the latitude of barbados and 20 km thick beneath barbados3 4 it increases in width and thickness from north to south2 fig 1 the style of deformation varies considerably along the eastern margin of the complex2 3 5 11 and is related to the thickness and type of sediment on the atlantic ocean floor a common feature is that a proportion of undeformed sediment from the ocean floor is brought beneath the deformed sediments of the accretionary complex and in some areas a clearly defined seismic reflector separates the deformed and undeformed sediments which has been interpreted as a decollement surface5 12 13 a seismic section from an investigation of the accretionary complex conducted from rss discovery in 1980 using 12 channel seismic reflection and long range side scan sonar gloria shows undeformed sedimentary rocks carried horizontally 74 km beneath deformed rocks forming the accretionary wedge we report here that the low shear stresses required on the decollement between the deformed and undeformed rocks are made possible by porewater at pressures close to that of the lithostatic load

controls on the geometry and evolution of thin skinned fold thrust belts and applications to the makran accretionary prism and indo burman ranges the formation of fold thrust belts at convergent margins is a dynamic process accretion of weak sediments to the front of the overriding plate results in crustal thickening and continued flexural subsidence of the underthrusting plate fold thrust belts are often treated as a coulomb wedge having self similar geometries with a critical taper and either a rigid or isostatically compensated base in this paper we build upon this work by developing a new dynamic model to investigate both the role of the thickness and material properties of the incoming sediment and the flexure in the underthrusting plate in controlling the behaviour and evolution of fold thrust belts our analysis shows that the evolution of fold thrust belts can be dominated by either gravitational spreading or vertical thickening depending on the relative importance of sediment flux material properties and flexure we apply our model to the makran accretionary prism and the indo burman ranges and show that for the makran flexure must be considered in order to explain the dip of the sediment basement interface from seismic reflection profiles in the indo burman ranges we show that incoming sediment thickness has a first order control on the variations in the characteristics of the topography from north to south of the shillong plateau

effect of the brittle ductile transition on the topography of compressive mountain belts on earth and venus the coulomb critical taper model has been very successful in explaining the large scale topography of a number of terrestrial accretionary wedges however this model is limited to cases of purely brittle frictional deformation in this paper we extend the range of applicability of the critical taper model by explicitly including the effects of temperature dependent ductile deformation the new model includes temperature dependent power law flow an assumed velocity field and linear thermal gradients in the atmosphere and within the crust flexural isostasy is also incorporated so that the decollement geometry is computed as a response to the applied load of the wedge material we assume that ductile deformation within the decollement zone is controlled primarily by diffusion flow whereas ductile deformation within the wedge itself is controlled by dislocation creep the topographic profiles predicted by the model are very similar to those of a number of fold and thrust belts on both earth and venus a typical wedge profile includes three distinctive topographic regions a narrow taper toe where both the wedge and the decollement zone deform in a brittle frictional manner a region of relatively steep slope where the wedge base deforms ductilely and the decollement zone is still frictional and a flat plateau region where both the wedge base and the decollement zone are deforming by ductile flow we have applied the model to two fold and thrust belts on venus maxwell montes and uorsar rupes and to the andes on earth and we find good agreement between observed and predicted topography using reasonable parameter values the model accounts for the observed positive correlation between relief and elevation of venusian fold and thrust belts on the basis of different thermal environments at different elevations it is also able to explain the first order differences between terrestrial and venusian fold and thrust belts fundamentally this difference is due to a combination of the lower temperatures and the presence of water on earth

mechanical role of backstops in the growth of forearcs we use numerical and laboratory modeling to analyze the mechanical role of backstops in the overriding plate at subduction zones a backstop is defined as a region within a forearc that has significantly greater shear strength than the sediment lying farther trenchward it can be thought of as the bulldozer behind an accretionary wedge we calculate the stress and displacement fields within forearcs for various backstop models using the finite element method and we simulate deformation over a backstop using a small scale laboratory model in this way we model the effects of the mechanical properties and geometry of a backstop on forearc structures we find that the growth of an outer arc high the development of an inner deformation belt with landward vergence and the seemingly paradoxical presence of an undeformed forearc basin within an otherwise highly deformed forearc can be explained by assuming the existence of a geometrically simple backstop with geologically reasonable contrasts in mechanical properties compared to the sediments just trenchward of it the structures produced in our numerical models are quite insensitive to the rheology boundary conditions and exact mesh geometry employed the general types of structures observed in the laboratory models depend only weakly upon the strength and the geometry of the backstop these results suggest that a detailed picture of an underlying backstop cannot be determined from surface information alone backstops in which the contact with the accretionary wedge dips arcward rather than trenchward however should produce only slightly different forearc structures with less development of both the outer arc high and the landward vergent inner deformation belt although natural forearcs are far more complex than our simple models they exhibit many of the same features indicating that relatively simple backstop mechanics may be a very important factor in the overall growth of forearcs

continental margin tectonics submarine accretionary prisms submarine accretionary prisms are sites of incipient mountain building where sediments and rocks of the deep ocean are uplifted deformed and ultimately transformed to continental crust accretionary prisms develop at sites of lithospheric convergence that is subduction zones commonly accretionary prisms form along continental margins although accretionary prisms may also occur adjacent to island arcs which are ultimately coalesced into continental landmasses here we review the studies of submarine accretionary prisms during the past four years with primary focus on the contributions of geoscientists from the united states

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

three dimensional critical wedges tectonics and topography in oblique collisional orogens in orogens deforming in oblique compression both the long wavelength mean topography of averaged slopes and the higher frequency topography first harmonic of valleys and ridges are related to the mechanics of deformation in three dimensions and cannot be adequately modeled in two dimensions a method of analysis of the principal stresses failure planes and topography is presented for three dimensional critical wedges in a critical coulomb orogen which is at failure throughout the components of the stress tensor are interrelated through the failure condition defined in terms of the principal stresses variation of any of the cartesian components must be accommodated by adjustment of one or more of the other components so that the orogen remains at failure this coupling of cartesian components to maintain the wedge at failure permits exploration of the behavior of a three dimensional critical orogen in which shear stresses arise outside of the plane of convergence internal adjustment of shear stresses to externally imposed stress conditions results in the formation of characteristic orogen shapes for an oblique collision zone such as the southern alps of new zealand the resultant mountain belt is one of an obliquely deforming outboard fold thrust belt with subdued relief rising abruptly into a steep backslope to the main divide which falls steeply to the indentor forming the inboard slope the outboard topographic slope is reduced in relation to that of the two dimensional orogen the rigid indentor has a fixed strike and therefore imposes a failure orientation adjacent to the indentor that differs from that of the ideal three dimensional critical wedge solution this produces several different slip orientations and minor partitioning of strain near the indentor first harmonic topography of valleys and ridges arises due to nonlinear heterogeneous behavior of both erosional and mechanical processes and is characterized by uplift controlled ridges in the outboard and incision controlled valleys in the inboard

accretionary margin of north western hispaniola morphology structure and development of part of the northern caribbean plate boundary abstract broad range side scan sonar gloria images and single and multi channel seismic reflection profiles demonstrate that the margin of north western hispaniola has experienced compression as a consequence of oblique north american caribbean plate convergence two principal morphological or structural types of accretionary wedges are observed along this margin the first type is characterized by a gently sloping 4 sea floor and generally margin parallel linear sets of sea floor ridges that gradually deepen towards the flat hispaniola basin floor to the north the ridges are caused by an internal structure consisting of broad anticlines bounded by thrust faults that dip southwards beneath hispaniola anticlines form at the base of the slope and are eventually sheared and underthrust beneath the slope in contrast the second type of accretionary wedge exhibits a steeper 6 16 sea floor slope characterized by local slumping and a more abrupt morphological transition to the adjacent basin the internal structure appears chaotic on seismic reflection profiles and probably consists of tight folds and closely spaced faults we suggest that changes in sea floor declivity and internal structure may result from variations in the dip or frictional resistance of the decollement or possibly from changes in the cohesive strength of the wedge sediments the observed pattern of thickening of hispaniola basin turbidites towards the insular margin suggests differential southwards tilting of the hispaniola basin strata probably in response to north america caribbean plate interactions since the early tertiary based upon indirect age control from adjacent parts of the northern caribbean plate boundary we infer a late eocene to early miocene episode of transcurrent motion i e little or no tilting an early miocene to late pliocene period of oblique convergence i e increased tilt during which the accretionary wedge began to be constructed and a late pliocene to recent episode of increased convergence i e twice the miocene to pliocene tilt which has led to rapid uplift and erosion of sediment sources on the margin and on hispaniola generating a submarine fan at the base of the insular slope

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

mapping and interpretation of sinlap crater on titan using cassini vims and radar data 1 only a few impact craters have been unambiguously detected on titan by the cassini huygens mission among these sinlap is the only one that has been observed both by the radar and vims instruments this paper describes observations at centimeter and infrared wavelengths which provide complementary information about the composition topography and surface roughness several units appear in vims false color composites of band ratios in the sinlap area suggesting compositional heterogeneities a bright pixel possibly related to a central peak does not show significant spectral variations indicating either that the impact site was vertically homogeneous or that this area has been recovered by homogeneous deposits both vims ratio images and dielectric constant measurements suggest the presence of an area enriched in water ice around the main ejecta blanket since the ku band sar may see subsurface structures at the meter scale the difference between infrared and sar observations can be explained by the presence of a thin layer transparent to the radar an analogy with terrestrial craters in libya supports this interpretation finally a tentative model describes the geological history of this area prior during and after the impact it involves mainly the creation of ballistic ejecta and an expanding plume of vapor triggered by the impact followed by the redeposition of icy spherules recondensed from this vapor plume blown downwind subsequent evolution is then driven by erosional processes and aeolian deposition

the influence of backstop dip and convergence velocity in the growth of viscous doubly vergent orogenic wedges insights from thermomechanical laboratory experiments abstract we investigate the effects of convergence velocity and backstop geometry on the evolution of experimental viscous doubly vergent orogenic wedges experiments are performed in a thermomechanical shortening box using the newtonian properties of 52 54 en type paraffin as analogue to the natural strength distribution in the continental crust a velocity discontinuity is imposed at the subduction slot where the base plate is pulled below a rigid backstop the mobile base plate dips 5 two different backstop dips 60 and 30 are used in combination with constant convergence velocities of 2 and 10 cm h 1 results point out that backstop geometry exerts a first order control on the evolution of doubly vergent experimental wedges steeply dipping backstops cause the stationary uplift of the wedge axial region above the subduction slot whereas shallow dipping backstops allow its retroward translation in concomitance with a more pronounced deformation in the retrowedge convergence velocity controls the uplift in the wedge axial region and the amount of the outward migration of the deformation front in both the prowedge and retrowedge regions for fast convergence velocity and shallow dipping backstop deformation in the retrowedge is favoured an increase in the backstop dip and or a decrease in the convergence velocity favour deformation in the prowedge both prowedge and retrowedge tend to grow in self similar conditions with an aspect ratio depending upon the backstop dip and convergence velocity these results may have important consequences for the evolution and deformation style of ductile dominated orogenic wedges suggesting that location of the axial region and deformation progression in natural orogens can be strongly controlled by boundary and kinematic conditions

porosity modeling and stress evaluation in the barbados ridge accretionary complex based on sediment physical property data from dsdp odp legs 78a and 110 barbados ridge complex a simple palinspastic reconstruction scheme was employed to study porosity changes during accretion undoing the compactional effects caused by imbricate stacking of wedge slices a synthetic pre accretion porosity depth profile was developed which bears strong resemblence to a characteristic profile from a reference drillhole in front of the barbados accretionary complex differences between synthetic and reference profile are interpreted in terms of a semiquantitative estimate of the relative impact of the horizontal stress component on gravitational compaction in accretionary wedge environments an exponential relationship between depth and porosity divergence for distinct lithologic units is evident the defined relationship facilitates the analysis of deformational behavior of accreted sediments in general

critical taper wedge mechanics of fold and thrust belts on venus initial results from magellan fold and thrust belts exist on venus at the margins of crystal blocks such as plateaus tesserae and coronae and as ridge belts within the plains these fold belts display a number of key features that are consistent with their formation by critical taper wedge mechanics a mechanics that is well known for fold and thrust belts and accretionary wedges on earth for example an analysis of fold geometry at the toe of the artemis chasma fold belt indicates fault bend folding above a regionally extensive decollement horizon at a depth of about 1 5 km near surface deformation on venus is interpreted to be brittle and is anticipated to be dominated by cohesive strength in the upper 1 2 km critical taper wedge mechanics under anticipated venus conditions suggests that brittle wedges should have maximum surface slopes in the range 10 20 which is similar to some estimated slopes in the steep parts of the fold belts the low taper toes of fold belts may be cohesion dominated toes on either brittle or plastic decollement horizons once the base of the wedge undergoes the brittle plastic transition the surface slope is expected to flatten to near horizontal in qualitative agreement with many topographic profiles of fold and thrust belts on venus the estimated depth of the brittle plastic transition is uncertain based on rock mechanics data but is expected to be close enough to the surface to be affected by the atmospheric temperature gradient the relief of fold belts measured between the toe of the wedge and the flat crest displays a remarkable linear dependence on absolute elevation figure 15 ranging from 6 km for maxwell montes at an elevation of 10 km to a few hundred meters at the lowest planetary elevations 0 to 2 km this remarkable phenomenon appears to reflect an absolute elevation dependence of the depth of the brittle plastic transition possibly controlled by an isostatic coupling of elevation lithospheric thickness and geothermal gradient

the estimation of diffuse strains in the toe of the western nankai accretionary prism a kinematic solution arcward changes in the vertical spacing of seismically defined stratigraphic reflectors and sediment porosities determined from drill cores and seismic interval velocities suggest a compact kinematic solution for estimating diffuse deformation within accretionary prism toes the method involves the solution of the lagrangian form of the conservation of mass equation for the two dimensional finite deformation gradient field within discrete deforming domains we apply this technique to the unfaulted protothrust zone ptz along a western transect across the nankai accretionary prism in the vicinity of deep sea drilling project sites 582 and 583 the results yield horizontal shortening estimates of about 1 7 km or about 20 this shortening appears to be accommodated through different strain responses depending on the consolidation state of the sediments at the time of accretion shallow sediments respond to horizontal shortening dominantly by expelling pore fluids producing volume strains up to 30 while deeper sediments accommodate horizontal shortening dominantly through vertical extension reaching 70 in the footwall of the frontal thrust horizontal shear strains within the ptz are not well resolved by this solution due to uncertainties in the porosity distribution but they generally appear to be quite small

tectonic dewatering and mechanics of protothrust zones example from the cascadia accretionary margin recent analyses of multichannel seismic data for the cascadia margin off central oregon show a systematic landward increase in velocity increase in sediment dewatering within a protothrust zone about 10 km wide seaward of the toe of the accretionary wedge cochrane et al 1994 here we analyze the mechanical conditions for the transmission of compression from the rear of the accretionary wedge to the area seaward of the frontal thrust to create the protothrust zone and to cause sediment dewatering the analysis assumes that sediments are elastoplastic coulomb material i e they behave elastically up to a yield limit at which failure occurs we show that the necessary conditions for the transmission of compression from the accretionary wedge to the seaward area are 1 the presence of incipient decollements seaward of the frontal thrust and 2 the presence of high pore pressures within the incipient decollement we also show that the magnitude of pore pressure within the incipient decollement may control the width of the protothrust zone we suggest that the latter determines the location of the next frontal thrust and hence the ramp spacing between the imbricated thrusts within accretionary wedges thus pore pressures within incipient decollements may significantly influence the evolution of the internal structures of accretionary wedges

contrasts in thermal maturity within terranes and across terrane boundaries of the franciscan complex northern california the franciscan subduction complex of northern california includes several tectonostratigraphic units we have used the technique of vitrinite reflectance to document levels of thermal maturity for the central belt and the yager coastal and king range terranes spatial variations in thermal maturity help define the thermal structure of each franciscan terrane and each terrane displays a different geometry of thermal structure these differences are controlled by the style of structural deformation the overprinting effects of deformation following metamorphism and thermal overprinting caused by hydrothermal circulation anomalies along and contrasts across terrane boundaries help establish the relative timing and the structural mechanisms of terrane amalgamation for example localized anomalies are evident along the eel river fault central yager boundary and these increases in thermal maturity may be due to shear heating hydrothermal discharge or conductive heat transfer across the fault cross c

sequential thrusting model for mountain building constraints from geology and heat flow of taiwan in this study we analyze the thermal evolution of the taiwan fold and thrust belt based on the available geological information a finite element program is used to simulate the complex kinematics of the observed sequential thrusting and the interplay of uplift erosion and sedimentation processes the discrete sequential thrusting model successfully reproduces the observed decrease in heat flow at the deformation front in northern taiwan this reveals that the heat flow drop may be due to the recent stacking of cold thrust sheets at the deformation front this inference is consistent with the high seismicity and dense quaternary fault distribution in western taiwan the model also successfully reproduces the observed high heat flow in the central ranges suggesting that the observed high heat flow may be a result of tectonic unroofing due to active underplating uplift and erosion these conclusions are insensitive to uncertainties in the frictional coefficient on the thrust plane

seismic modeling of the decollement zone at the base of the barbados ridge accretionary complex wide aperture seismic reflection profiling of the northern barbados ridge accretionary complex at 16 12 n has imaged the decollement horizon across more than 100 km of the accretionary complex revealing prominent changes in the amplitude of the decollement reflection synthetic seismograms from models of the variation of velocity and density with depth were compared with selected seismic traces between 2 5 km seaward of the toe of the accretionary complex and 20 km landward of the toe to determine the changes in velocity density properties along the decollement thrust zone modeling requires a thin low velocity layer to develop landward along the decollement beneath the complex while no such layer exists at the corresponding stratigraphic boundary in the undeformed sediments seaward of the complex two dimensional modeling that incorporated lateral variations in topography velocity and density was performed on a 7 km section of the decollement 20 km landward of the deformation front the best fitting model includes a 20 m thick low velocity layer at the decollement horizon with a relative velocity decrease in the layer of 0 2 km s changes in the reflection amplitude along this 7 km section could be produced by either a change in the velocity anomaly within the layer or a thinning of the decollement layer to less than 4 m the shape of the reflected wavelet from the decollement indicates that the principal cause of the amplitude variation is a change in the velocity contrast rather than a change in thickness the low velocity layer can be explained by a decollement shear zone that has high porosity maintained by elevated pore fluid pressure high fracture permeability within the decollement permits dewatering of sediment attached to the oceanic crust and thrust beneath the accretionary complex

geodynamic patterns of ophiolites and marginal basins in the indonesian and new guinea regions analysis of spatial temporal geological and geochemical patterns of ophiolites in the indonesian and new guinea region indicates a strong correlation with marginal basin development and closure the spatial distribution of ophiolites is mostly linked with marginal basin producing zones of oblique convergence and collision strain partitioning in these zones creates a series of ephemeral plate boundaries between several independently moving lithospheric blocks repeated disruption of the diffuse boundaries between the blocks by changes in plate motion and collision induced mantle extrusion creates space that is rapidly filled by new ocean basins in the upper plate of subduction zones suprasubduction zone ssz spreading of these basins is enhanced by episodic extrusion of asthenosphere escaping collisional suture zones various closure events and global plate motion changes are reflected in the temporal distribution of marginal basin and ophiolite ages most ophiolite slabs in the indonesian and new guinea region represent fragments of oceanic lithosphere with a subduction zone component as indicated by the common refractory petrochemistry of the mantle sequence and occurrence of boninite age and compositional heterogeneity may indicate that some ophiolite bodies are composite terranes collisions with buoyant lithosphere transfo parts of these ocean basins into ophiolites the connection between ophiolites and marginal basins is strongest where parts of actively spreading ssz basins are partially represented as ophiolites in collision zones many of the various tectonic models proposed for the origin and tectonic evolution of ophiolites are supported by plate interactions in the indonesian and new guinea region dewey bird 1970 silver smith 1983 moores et al 1984 searle stevens 1984 dilek moores 1990 harris 1992 dickinson et at 1996 however the geolo gical associations of these interactions remain poorly resolved allowing different parts of the region to be used as modern analogues for almost any tectonic scenario simple explanations based on plate kinematics alone fail to account for the increasing levels of complexity observed in the indonesian and new guinea region new ideas and supporting evidence for mantle extrusion mechanisms e g flower et at 2001 also rely heavily on descriptions of plate interactions in this region doglioni et at 1999 and raise new questions about how these processes may relate to the spatial and temporal pattern of ophiolites this paper provides a synthesis of the temporal and spatial patterns and tectonic associations of marginal basins and ophiolites throughout the indonesian and new guinea region it explores the relations between various plate boundary processes and ophiolite genesis and emplacement such as the role of strain partitioning trench rollback subduction polarity asymmetry astheno spheric flow and plate kinematics

critical wedges in three dimensions analytical expressions from mohr coulomb constrained perturbation analysis we present simple analytical expressions for evaluating three dimensional stress fields in critical wedges coulomb failure throughout that are deforming within an oblique convergent zone we assume that the load and geometry variations in the lateral direction and the topographic slope are relatively small the stress state of a two dimensional critical wedge is perturbed by admitting a small lateral shear which is accommodated to maintain criticality by a reduction in the dominant normal compressive stress cohesionless cases are further simplified with taylor series expansions in the topographic slope the analytical expressions distinguish the behavior of strong base wedges associated with steep slopes from weak base wedges with more modest slopes and are reliable for lateral shear values as large as half the lithostatic load in the weak base case where the vertical shear is small the orientations of the principal stresses are very sensitive to the perturbation as the lateral shear increases through relatively small values the roles of the lateral and vertical coordinate axes are interchanged with a resultant transition from predominantly thrust failure to strike slip failure transition to strike slip in the strong base case is more gradual the particular value of horizontal shear for which transition occurs also depends on the difference of the minor normal stresses we produce an analytical expression predicting displacement patterns compatible with those observed in natural and analog orogens displacement partitioning is predicted for weak base wedges due to minor changes in lateral boundary conditions associated with variable orogen geometry

magma in forearcs implication for ophiolite generation abstract forearc areas non volcanic arcs of contemporary island arcs at convergent plate boundaries contain magmatic rocks geological evidence seismic profiles heat flow data density considerations and petrological and geochemical arguments suggest that a forearc tholeiitic association fat containing high mg calc alkaline andesites is present in non volcanic arcs at some stage of island arc development the fractionated as well as primitive magma is unable to penetrate low density sediments and underplates thick piles of unconsolidated accreting rocks the underplating causes upwelling the occurrence of magma in forearcs provides an alternative interpretation for the tectonic setting of some ophiolitic masses rather than ocean ridge formation and later obduction it offers an autochthonous island arc bound and geologically substantiated interpretation for the ophiolite suite

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

plate boundary reorganization in the active banda arc continent collision insights from new gps measurements abstract new gps measurements reveal that large sections of the se asian plate are progressively accreting to the edge of the australian continent by distribution of strain away from the deformation front to forearc and backarc plate boundary segments the study was designed to investigate relative motions across suspected plate boundary segments in the transition from subduction to collision the oblique nature of the collision provides a way to quantify the spatial and temporal distribution of strain from the deformation front to the back arc the 12 sites we measured from bali to timor included some from an earlier study and 7 additional stations which extended the epoch of observation to ten years at many sites the resulting gps velocity field delineates at least three sunda arc forearc regions around 500 km in strike length that shows different amounts of coupling to the australian plate movement of these regions relative to se asia increases from 21 to 41 to 63 eastward toward the most advanced stages of collision the regions are bounded by the deformation front to the south the flores wetar backarc thrust system to the north and poorly defined structures on the sides the suture zone between the nw australian continental margin and the sunda banda arcs is still evolving with more than 20 mm yr of movement measured across the timor trough deformation front between timor and australia