an outline of the plate tectonics of china china is principally a part of the eurasian plate but the margins of the indian and philippine sea plates are involved in the himalayas and in the coastal range of taiwan respectively within the eurasian plate the cathaysian paleoplate is separated from the angaraian paleoplate by the junggar hegen suture which contains paleozoic ophiolites and rare blueschists the three microplates of the cathaysian paleoplate consist of precambrian cratons and or phanerozoic accretionary fold belts these coalesced precambrian cratons record at least six stages of intense orogeny before cratonization the paleozoic to cenozoic accretionary fold belts of china can be correlated with similar events now found in west pacific andean and atlantic type active continental margins ophiolites occupying many of these tectonic zones provide evidence for the age and igneous history of oceanic crust formed during the paleozoic to cenozoic the presence of blueschist in some of these chinese sutures reveals evidence of large scale subduction and tectonic exhumation during consolidation of the eurasian plate cenozoic collision of the eurasian and indian plates produced deformation and uplift of the himalayas strongly influencing the tectonics of western china in contrast mesozoic tertiary evolution of eastern china is typical basin range geology similar to that of the western united states which included development of deep sedimentary basins along with calc alkaline plutonic and volcanic activity associated with crustal thinning and high heat flow the complicated tectonic evolution of china is greatly illuminated by the presence of ophiolites and blueschists in proterozoic to tertiary convergent boundaries these petrotectonic assemblages provide evidence of an extremely mobile history of plate movement in china

tectonic underplating versus out of sequence thrusting beneath the lesser himalaya insights from the analogue modeling of the nepal himalaya fold and thrust belt abstract the nepal himalaya fold and thrust belt is a classical two decollement thrust fold system the belt has been extensively studied by analogue and numerical modeling but the formation mechanism of the mid crustal duplex structure beneath the northern part of the lesser himalaya remains debated the formation of this thrust duplex has been attributed either to tectonic underplating along new ramps in response to the subducting indian plate or to out of sequence thrusting along preexisting ramps we conducted a series of experiments using analogue models employing a model setup that was faithful to the geological setting of the nepal himalaya we used industrial computed tomography ct and particle image velocimetry piv to generate detailed deformation maps the analogue models successfully reproduce the main structure of the nepal himalaya fold and thrust belt showing that the two competing processes i e underplating along new ramps and out of sequence thrusting along preexisting ramps operate alternately to form the duplex structure the models also show that underplating is associated with extensive foreland sedimentation analogous to the foreland molassic sedimentation in the sub himalaya which leads to a systematic recession of the deformation front and a smaller horse finally the recognition that out of sequence thrusting occurs in the interval between major underplating events provides important insights into the origin of the 2015 m w 7 8 gorkha earthquake

cyclical behavior of thrust wedges insights from high basal friction sandbox experiments scaled sandbox experiments with high basal friction simulating the growth of accretionary wedges display cycles alternating between frontal imbricate thrusting and underthrusting of long undeformed sheets by contrast low basal friction experiments with otherwise similar and constant initial conditions produce a classic frontal imbricate fan through repeated failure along frontal thrusts the cyclical behavior observed in high basal friction experiments is expressed by three quantities 1 the average spacing between frontal thrusts 2 the advance and retreat of the deformation front and 3 the frontal slope alpha of the actively deforming wedge as a long sheet is underthrust the front is steepened through slumping until the maximum critical angle is reached then frontal thrusting resumes and the accretion of imbricate slices builds the wedge forward thereby lowering the taper to the minimum critical angle at shallow tapers a long unit is underthrust and subsequently uplifts shortens and steepens the overlying wedge through backthrust deformation thus completing the cycle underthrusting of long units offers a simple mechanism for underplating overlying units it also provides a possible explanation for temporally and spatially varying wedge geometries in nature when basal frictions attain 80 90 of the internal friction

lateral variation of crustal structure and composition in the cathaysia block of south china and its geodynamic implications abstract in order to clarify lateral variations in the crustal structure and composition of the cathaysia block in south china which was affected by violent mesozoic and cenozoic tectono magmatic activities we studied the crustal thickness and the poisson s ratio in this region using the h stacking method of teleseismic receiver functions our results show that the poisson s ratio varies between 0 20 and 0 29 and the crustal thickness ranges from 26 to 34 km in the cathaysia block the crustal thickness and poisson s ratio show considerable variations across the lishui haifeng fault the southeast coast magmatic belt is characterized by poisson s ratios 0 25 and a crustal thickness of 28 33 km reflecting probably intermediate to mafic compositions of the crust the southwestern part of the cathaysia fold belt exhibits a crustal thickness of 27 30 km and poisson s ratios

effects of lateral friction on the structural evolution of fold and thrust belts insights from sandbox experiments with implications for the origin of landward vergent thrust wedges in cascadia various factors are inferred to influence the structural evolution of fold and thrust belts or thrust wedges but the effects of lateral friction such as that exerted by the opposite block of a strike slip fault in nature or by sidewalls in analogue models have rarely been given sufficient attention and hence are still far from being fully understood up to now seaward vergent and dually vergent thrust wedges have been satisfactorily explained by the classic coulomb wedge theory and can be well reproduced in analogue and numerical models in contrast landward vergent thrust wedges which are uncommon in nature e g those occurring in the cascadia subduction zone have not yet been adequately explained by the classic coulomb wedge theory and can only be reproduced in a few analogue models under particular conditions in order to look closer at the effects of lateral friction on the structural evolution of fold and thrust belts and to explore the origin of landward vergent thrust wedges we conducted a series of sandbox models under different conditions of lateral friction and various ratios of initial average lateral frictional stress to basal shear stress s d results of our models revealed that in the presence of a weak or ductile basal decollement the direction of lateral friction can dictate not only the deformation propagation direction and the surface slope direction but also the structural vergence of fold and thrust belts fold and thrust belts normally show a structural vergence in the same direction as the direction of lateral friction but a deformation propagation direction and surface slope direction opposite to the direction of lateral friction according to the results of models and the relationships between structural vergence and lateral friction direction occurring both in cascadia and the analogue models we propose that an extremely high s d ratio due to an extremely weak basal decollement and landward directed lateral friction exerted by the opposite blocks of basement involved strike slip faults are probably the two key factors responsible for the origin of landward vergent thrust wedges in cascadia this work illustrates that effects of lateral friction should be properly taken into account both in the kinematic analysis and designs of models to simulate fold and thrust belts in the presence of weak or ductile basal decollements

growth of a thrust fault array in space and time an example from the deep water niger delta abstract the temporal and spatial evolution of thrust fault arrays is currently poorly understood and marine fold and thrust belts at the toe of passive margin gravitational systems imaged by commercial 3d seismic reflection datasets afford a unique opportunity to investigate this problem in three dimensions using an extensive 3d seismic data set and age data the total cumulative strain shortening and interval strain rates have been calculated for 11 thrust related folds mapped in the toe thrust region of the southern lobe of the niger delta for the first time the sequence of thrust nucleation propagation and linkage through time at a scale of 10 s km both along and across strike is documented short thrust segments had nucleated throughout the entire study area by 15 ma they then grew largely by lateral growth and linkage increasing the fault trace length and generating asymmetric strain distance plots for the first 50 of their history thereafter growth continued by shortening with minimal along strike increase in fault length changes in shortening distance data between adjacent structures across strike suggest that the change in growth mode occurred once the thrusts had linked in 3d through the common underlying detachment over the entire thrust array the strain rate varies through time starting slowly

sandbox modelling of thrust wedges with fluid assisted detachments abstract we have used granular materials to model the development of thrust wedges where migrating pore fluids assisted in the formation of detachments the governing equations yield practical scales for linear dimensions stresses and time using compressed air as a pore fluid models a few centimetres thick were deformed in about half an hour model materials were sands of three different grain sizes and a loess they had suitable values of density permeability cohesion and internal friction fluid flow obeyed darcy s law at yield the materials satisfied a coulomb criterion for effective stresses models with various sequences of layers were submitted to horizontal shortening in a rectangular box compressed air entered through a sieve at the base the fluid pressure was uniform over the basal boundary in models made from a single material the style of deformation depended on the fluid pressure for no fluid flow the thrust wedge was short and high the surface slope attained large angles 30 and internal structures were mainly forethrusts for fluid pressures approaching lithostatic values thrust wedges were longer and lower and surface slopes attained smaller angles in models containing basal layers of small permeability detachments formed beneath them and the structural style was dominated by interacting forethrusts and backthrusts in multilayered models thin skinned detachments formed beneath less permeable layers in the sequence to understand how fluid flow controlled the first stages of detachment we calculated ideal vertical profiles of fluid pressure vertical normal stress effective stress and horizontal shear stress for multilayered models in the undeformed state the profiles are segmented because material properties vary from layer to layer sharp drops in shear strength occur at positions where detachments were observed in the sandbox models we infer that detachments resulted from large fluid pressures beneath relatively impermeable layers

cross equalization and prestack seismic inversion of time lapse land seismic data central thrace basin turkey abstract the thrace basin of turkey has sandstone and carbonate reservoirs of eocene and oligocene age mapping undrained sweet spots from seismic data is currently a challenge especially because there is no water drive after production time lapse 4d land seismic data was used to investigate undrained liquid gas pockets and to help with the planning of gas storage into the depleted reservoirs two vibroseis 3d baseline and monitor surveys were acquired in 2002 and 2011 respectively the 4d seismic data has low repeatability and required conditioning to reduce the nonrepeatable 4d noise that is not related to production cross equalization is applied to the base and the monitor data to bring out similarities noise cancellation by subtracting seismic volumes and the inverted rock properties primarily show changes due to production we cross equalized the corresponding angle band stack cubes prior to prestack inversion as the available cross equalization software was implemented for poststack seismic

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

inversion of field data in fault tectonics to obtain the regional stress iii a new rapid direct inversion method by analytical means summary a new method for determining the reduced stress tensor with four degrees of freedom the orientations of the three principal stress axes as well as the ratio of principal stress differences using fault slip data or focal mechanisms of earthquakes is presented from a computational point of view the inversion of fault slip data is made in a direct way by purely analytical means as a result the determination process is extremely fast and adaptable on small microcomputers from a physical point of view the method aims at simultaneously i minimizing the angles between theoretical shear stress and actual slip vector and ii having relative magnitudes of shear stress large enough to induce slip despite rock cohesion and friction examples of application to actual fault slip data sets with good or poor variety of fault slip orientations are shown the double significance of the basic criterion adopted results in a more realistic solution of the inverse problem than the single minimization of the shear stria angle

the effect of decollement dip on geometry and kinematics of model accretionary wedges we conducted a series of sand box models shortened asymmetrically above a frictional plastic decollement to study the influence of amount and sense of the decollement dip on the geometry and kinematics of accretionary wedges model results illustrate that the amount and direction of decollement dip strongly influence the geometry and mode of deformation of the resulting wedge in general for models having similar decollement frictional parameters the resulting wedge is steeper grows higher and is shorter when shortened above a decollement that dips toward the hinterland at 42 bulk shortening the length height ratio of wedges formed above a 5 dipping decollement was equal to 2 4 whereas this ratio was equal to 3 for wedges shortened above a horizontal decollement moreover models with a hinterland dipping decollement undergo larger amounts of layer parallel compaction lpc and area loss than models shortened above a non dipping decollement the effect of decollement dip on wedge deformation is most pronounced when basal friction is relatively high b 0 55 whereas its effect is less significant in models where the basal decollement has a lower friction b 0 37 model results also show that increasing basal slope has a similar effect to that of increasing basal friction the wedge grows taller and its critical taper steepens

combined control of decollement layer thickness and cover rock cohesion on structural styles and evolution of fold belts a discrete element modelling study abstract a series of numerical experiments based on the discrete element method were performed to simulate the formation of fold belts with varied decollement layer thickness and cover rock cohesion to investigate their controls on structural styles of fold belts each model consists of a weak unbonded basal decollement layer with a relatively low to high thickness and a bonded cover with a relatively low to high cohesive strength horizontal shortening of the particle assemblage was achieved by horizontal motion of a vertical boundary wall resulting in deformations in the system the results show that shortening was mainly accommodated by detachment folds sinusoidal and box folds fault related folds and opening mode fractures in the models the combination of a lower cohesion and a thinner decollement resulted in more distributed strain and a larger number of folds whilst the combination of a higher cohesion and a thicker decollement led to more pronounced strain localisations in fewer folds surface uplift and fold amplitude diapir height are mainly positively affected by the decollement thickness i e the thicker the decollement is the greater the surface uplift and fold amplitude are the propagation rate of deformation is predominately controlled by the cover rock cohesion a lower cohesion led to a higher propagation rate of deformation the model with a relatively low cohesion and decollement thickness produced regularly spaced box folds which are comparable to those in the southern zagros fold and thrust belt the models presented demonstrate that the combined effect of decollement layer thickness and cover rock cohesion can play a critical role in the structural styles and kinematic evolution of fold belts

submarine landslides at subduction margins insights from physical models abstract submarine landslides which commonly develop along subduction margins at the front of accretionary prisms have recently been recognized as one of the most serious geohazards to investigate the mechanical processes involved in the development of these landslides a series of sandbox experiments were performed and the results were analyzed using the digital image correlation dic technique numerous slope failures which may be analogous to natural submarine landslides were produced on the model surface the slope failures can be classified as follows type i failures which are relatively small but are frequently developed on the lower parts of the slope and type ii failures which are relatively large and cover the entire slope but occur less frequently detailed observations reveal that the failure process can be divided into four stages 1 pre failure 2 steepening of the lower slope 3 type i failure and 4 type ii failure the minor type i slides act as precursors of major type ii failures each slope failure produces a topographic undulation triggering events in adjacent areas and a lateral migration of discrete slope failures although such experiments provide useful insight into slope failure processes a detailed 3d analysis of the failed sediment is vital if we are to understand the precise history of thrust activity at subduction margins recent and miocene accretionary prisms along the nankai trough japan display the geometric and lithologic characteristics of type i and type ii failures in their slide deposits

fluid pressure and effective stress in sandbox models abstract we have done a series of simple tests on sandpacks involving upward flow of compressed air through the pores and its effect on the yield strength the ultimate objective is to model deformation coupled with fluid flow in sedimentary basins for all tests we used a single batch of fontainebleau sand sieved to a grain size between 0 200 and 0 315 mm poured into a cylindrical container and then fluidized the density of this sand was 1 585 g cm3 irrespective of sand thickness the lithostatic pressure was proportional to the thickness of the sandpack a yield envelope was obtained by shearing the sandpack horizontally compressed air entered the base of the sandpack and flowed upwards through the pore spaces for 69 measurements on air flow without shearing a plot of discharge velocity versus gradient of fluid pressure is close to a straight line verifying darcy s law and yielding an intrinsic permeability of about 1 7 darcy for the sand for 72 tests on simultaneous shearing and fluid flow the estimated effective stress lithostatic stress minus estimated pore fluid pressure ranged from 0 to 1600 pa and the pore fluid ratio between air pressure and lithostatic pressure from 0 0 to 1 0 a plot of shear stress versus effective stress at failure is almost linear verifying terzaghi s principle of effective stress the best fit slope coefficient of internal friction is about 0 55 and the intercept on the shear stress axis cohesion is less than 85 pa the tests show that it is feasible to use compressed air within sandpacks as a means of modelling deformation coupled with fluid flow the next step will be to build sandbox models of layered sequences and to investigate detachments caused by abnormal fluid pressures

petrogenesis of granitoids and associated xenoliths in the early paleozoic baoxu and enping plutons south china implications for the evolution of the wuyi yunkai intracontinental orogen abstract the early paleozoic wuyi yunkai orogen was associated with extensive felsic magmatic activities and the orogenic core was mainly distributed in the yunkai and wugong domains located in the western cathaysia block and in the wuyi domain located in the central part of the cathaysia block in order to investigate the evolution of the wuyi yunkai orogen elemental and sr nd isotopic analyses were performed for granites from the baoxu pluton in the yunkai domain and from the enping pluton in the central part of the cathaysia block the baoxu pluton consists of biotite granite with abundant xenoliths of gneissic granite granodiorite and diorite and the enping pluton is mainly composed of massive granodiorite biotite granites 441 5 ma and gneissic granite xenolith 443 4 ma of the baoxu pluton are all weakly peraluminous a cnk 1 05 1 10 they show high sr y and la yb ratios and have negative bulk rock end t values 7 0 to 4 4 which are similar to coeval gneissic s type granites in the yunkai domain and were probably derived from dehydration melting of a sedimentary source with garnet residue in the source granodiorites 429 3 ma from enping and granodiorite xenolith 442 4 ma from baoxu are metaluminous and have ree patterns with enriched light ree and flat middle to heavy ree possibly generated by the dehydration melting of an igneous basement at middle to lower crustal level diorite xenolith from baoxu is ultrapotassic k2o 4 9 wt has high contents of mgo 7 0 wt cr 379 ppm and ni 171 ppm and shows pronounced negative nb ta and ti anomalies this xenolith also has negative end t value 3 6 and low rb ba and high ba sr ratios and is thus interpreted to be derived from an enriched lithospheric mantle with the breakdown of phlogopite early paleozoic i and s type granites in the wuyi yunkai orogen mostly have negative end t values and do not have juvenile components consistent with genesis by an intracontinental orogenic event these early paleozoic granites occur near the ancient suture zone between the yangtze and cathaysia blocks and have high la yb and sr y ratios likely due to the existence of residual garnet in the source suggesting the thickened crust at ca 440 ma the 450 440 ma gneissic s type granites near the suture zone are earlier than those in the central part of the cathaysia block 430 ma the crustal thickening along the ancient suture zone at 440 ma propagated into the central part of the cathaysia block as evidenced by the 430 ma granites early paleozoic i type granites near the suture zone clearly show involvement of significant mantle derived materials in contrast to granites in the central part of the cathaysia block the ancient suture zone may have acted as channels for the emplacement of mafic magmas during the collapse of an intracontinental orogen

cretaceous pacific plate movement beneath se china evidence from episodic volcanism and related intrusions abstract extensive late mesozoic igneous rocks in se china have been widely considered to be generated under the paleo pacific tectonic regime the plate subduction model however remains controversial this study focuses on the cretaceous volcanic rocks in northwestern zhejiang province zircon u pb age determination indicates that cretaceous volcanism in northwestern zhejiang took place at three episodes of 140 130 130 127 and 123 118 ma in good agreement with the coeval lower volcanic series in southeastern zhejiang but lacking the episode at 110 88 ma corresponding to the upper volcanic series the cretaceous volcanic rocks in zhejiang therefore show an oceanward younging trend in situ zircon hf isotope analyses of three episodes of volcanics yield e hf t values of 11 2 to 8 7 4 8 to 2 4 and 4 4 to 2 2 respectively the entire sequences display typical isotopic features of magma mixing implying progressive involvement of juvenile component based on systematical researches on the cretaceous volcanic rocks and a series of granitoid plutons in zhejiang it is also identified that the juvenile component involvement gradually occurred from the inland to the coast under an enhanced lithospheric extensional tectonic setting all the observations in this study indicate the northwestward paleo pacific plate subduction with episodic slab rollback which triggered the arc system to retreat towards the pacific ocean rather than the southwestward subduction related to the ridge subduction along the lower yangtze river belt

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

estimation of vertical stochastic scale parameters in the earth s crystalline crust from seismic reflection data 1 its been shown that the statistical properties of a reflected seismic wavefield can be related to the statistical properties of the crust in this paper we report on a method to invert a reflected seismic wavefield for vertical stochastic parameters the method is founded on a deconvolution thresholding and numerical integration procedure to estimate the earth s bi modal velocity perturbation we then fit a von karman autocorrelation function to the autocorrelation of the estimated velocity perturbation and record the von karman parameters which result in the minimum misfit tests show that our scheme is successful at recovering the vertical characteristic length in synthetic seismic reflection data

computerized x ray tomography analysis of sandbox models examples of thin skinned thrust systems computerized x ray tomography applied to analog sandbox experiments performed in a normal gravity field makes possible the analysis of the kinematic evolution as well as the three dimensional geometry of models that simulate tectonic deformations most of the plastic or viscous analog materials generally used in a normal gravity field for such models have x ray attenuations compatible with medical scanner images models using soft cohesionless granular materials were studied faults in granular materials corresponding to dilatant zones with lower by 30 50 x ray attenuation are thus clearly visible in the tomography images kinematic analysis was done on imbricate thrust propagation in materials with an internal friction angle of 30 thrust propagation is controlled mainly by a triangular pop up structure resulting from the nucleation of two conjugate thrusts at the basal decollement the pop up structure is created immediately in front of the previous thrust three dimensional geometry analysis was tested on lateral ramps parallel to the transport direction lateral ramps were induced by changing the frictional boundary conditions at the bottom of the model though the lateral ramps strike almost parallel to the transport direction their dip angle is comparable to the dip angle of frontal ramps

mesozoic overthrust tectonics in south china whether south china was a platform or a para platform after the mid paleozoic deformation or whether the mountains of huanan south china form a mesozoic collision type orogen depends upon whether the ophiolite melange in the yangtze region is precambrian basement or was overthrust onto a foreland thrust belt during the mesozoic this paper presents evidence of tectonic windows under the melange and basement klippes above the melange in the yangtze region

crustal plumbing system of post rift magmatism in the northern margin of south china sea new insights from integrated seismology abstract the origin and evolution of magmatism in rifting margins are a fundamental geological subject and remain the focus of intense studies unlike the classical magma poor and magma rich rifting margins the northern margin of the south china sea scs exhibits a strong post rift magmatism but the evolution and mechanism of this behavior are still poorly understood in this work we investigate the detailed structural features of the post rift magmatism in the crust of the scs northern margin our analysis is based on wide angle and multichannel reflection seismic data combined with previous seismological results our results show a wide distribution of shallow igneous sills and intrusions these features are spatially and tectonically linked to the lower crustal high velocity bodies hvbs and submarine volcanism indicating a causative connection between the three features considering the existence of an obvious low velocity branch of the hainan plume in the northern scs and uplift of the lithosphere asthenosphere boundary we propose that the hvbs reflect consolidated mafic intrusions which formed the lower crustal reservoirs feeding the overlying igneous sills and intrusions in the scs northern margin where abundant extensional and detachment faults can act as magmatic channels such a dynamic process might cause post rift volcanism our results suggest that the crustal magmatic system of post rift volcanism has a multilevel upward migration mode and the hvbs in the lower crust could be the product of post rift magmatism further indicating that the scs northern margin had a magma poor property at the rifting phase but has undergone a strong magmatic rejuvenation by the subsequent mantle plume

discrete element modelling of contractional fault propagation folding above rigid basement fault blocks many studies have shown that discrete blind faults at depth are commonly linked to more distributed deformation in particular folding at higher levels one category of fault related folds forced folds is common where there is a distinct mechanical contrast between faulted basement and sedimentary cover outcrop numerical and analogue modelling studies indicate that such folds form as upward widening zones of distributed deformation monoclines above discrete faults at depth with increasing displacement the folds are often cut by faults as they propagate upwards into the cover while the trishear kinematic model of fault propagation folding appears to approximately represent the geometric development of such structures comparatively little is known of the mechanical controls on their development here we present a 2d discrete element model of sedimentary cover deformation above a contractional fault in rigid basement the elements consist of a series of soft spheres that obey newton s equations of motion and initially interact with elastic forces under the influence of gravity particles are bonded until the separation between them exceeds a defined breaking strain at which time the bond breaks simulated by the transition from repulsive attractive forces to solely repulsive forces the model is used to investigate the influence of basement fault dip and sedimentary cover strength on the geometry of the folds developed and the rate of fault propagation in all cases an upward widening monocline occurs above the basement fault we find that shallow basement fault dips produce homogenous thickening of the monocline limb while steeper dips produce contemporaneous thinning and thickening within the monocline thinning and thickening within the monocline are accommodated by a combination of small scale faulting and folding with decreasing cover strength the zone of deformation becomes wider localization does not occur on a single fault and fold geometries resemble trishear fold profiles with low propagation to slip ratios p s 1 in contrast a stronger cover produces a narrower zone of deformation localization on a single fault and more rapid fault propagation similar to trishear fold profiles where p s 2 3 the fault propagates into the cover at approximately the same angle as the basement fault the model reproduces well many of the features observed in analogue modelling and reported from outcrop and seismic studies

an overview of the earth crust under china abstract we assess the results of a number of deep seismic soundings performed in china over the last few decades and study the variations in crustal structure in 18 tectonic units comprised of three platforms and 15 fold systems thickness data on 344 mesozoic cenozoic sedimentary basins as well as data on moho depth are collected in order to discuss the relationship between the thickness of the basins and the average thickness of the consolidated crust in each tectonic unit the degree of mirror image symmetry between mesozoic cenozoic sedimentary basins and the uplifting topmost parts of the mantle is herein analyzed using deep geophysical data on sedimentary basins by applying standard methods of least squares analysis to both datasets we have obtained both the average thickness of the consolidated crust and the mirror image symmetry factor for every platform and tectonic fold system thereby allowing us to explore the correlation between the depths of the bottom of the sedimentary basins and the top of the uplifting mantle the thickness of the consolidated crust in china is found to be between 20 and 63 km following a pattern of gradual thickening from east to west expressed in terms of spatial seismicity and the concentration of seismic energy and according to the sharing out of earthquake hypocenters in the top 80 km of the earth the rheology of the area does not appear to suit the widely accepted jelly sandwich model for the continents which does not seem to be entirely valid in china the findings on the mirror image symmetry factor show that this parameter varies mainly in the range 0 5 to 1 8 compared with each other tectonic unit most of the tectonic systems in china appear to be isostatically compensated tibet is an exception in that the symmetry factor is positive for the gangdise nyainqentanglha 1 0 and himalayan 0 1 fold systems implying that these tectonic zones are far from being in the isostatic equilibrium of the other regions we have also analyzed the zoning characteristics through the logarithmic relationship ln r ln h a using the average thickness of the consolidated crust and the absolute value of the symmetry factor the key finding is that regardless of geographical location all the values of r h a for those sedimentary basins in which oil gas reservoirs have to date been found fall into the narrow range of values between 19 38 and 37 40 there is some appeal in a possible relationship involving the ratio of crustal thickness to symmetry factor more so when the results obtained appear to suggest a prognostic tool for exploratory practice in relation to oil gas reservoirs

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

nd sr isotopes and elemental geochemistry of surface sediments from the south china sea implications for provenance tracing abstract the mineralogy major and trace elements and neodymium and strontium isotopes of surface sediments in the south china sea scs are documented with the aim of investigating their applicability in provenance tracing the results indicate that mineralogical compositions alone do not clearly identify the sources for the bulk sediments in the scs the nd isotopic compositions of the scs sediments show a clear zonal distribution the most negative end values were obtained for sediments from offshore south china 13 0 to 10 7 while those from offshore indochina are slightly more positive 10 7 to 9 4 the nd isotopic compositions of the sediments from offshore borneo are even higher with end ranging from 8 8 to 7 0 and the sediments offshore from the southern philippine arc have the most positive end values from 3 7 to 5 3 this zonal distribution in end is in good agreement with the nd isotopic compositions of the sediments supplied by river systems that drain into the corresponding regions indicating that nd isotopic compositions are an adequate proxy for provenance tracing of scs sediments sr isotopic compositions in contrast can only be used to identify the sediments from offshore south china and offshore from the southern philippine arc as the 87sr 86sr ratios of sediments from other regions overlapped similar zonal distributions are also apparent in a la th sc discrimination diagram sediments from the west margin of the scs such as those from beibuwan bay offshore from hainan island offshore from indochina and from the sunda shelf plot in the same field while those offshore from the northeastern scs offshore from borneo and offshore from the southern philippine arc plot in distinct fields thus the la th sc discrimination diagram coupled with nd isotopes can be used to trace the provenance of scs sediments using this method we re assessed the provenance changes of sediments at ocean drilling program odp site 1148 since the late oligocene the results indicate that sediments deposited after 23 8 ma above 455 mcd meters composite depth were supplied mainly from the eastern south china block with a negligible contribution from the interior of the south china block sediments deposited before 26 ma beneath 477 mcd were supplied mainly from the north palawan continental terrane which may retain the geochemical characteristics of the materials covered on the late mesozoic granitoids along the coastal south china for that the north palawan continental terrane is presently located within the southern philippine arc but was located close to odp site 1148 in the late oligocene the weathering products of volcanic material associated with the extension of the scs ocean crust also contributed to these sediments the rapid change in sediment source at 26 23 8 ma probably resulted from a sudden cessation of sediment supply from the north palawan continental terrane we suggest that the north palawan continental terrane drifted southwards along with the extension of the scs ocean crust during that time and when the basin was large enough the supply of sediment from the south to odp site 1148 at the north slope may have ceased

characteristics of a tsunamigenic megasplay fault in the nankai trough 1 slip on the shallow part of a megasplay fault that is an out of sequence thrust and branch of the main subduction plate boundary can cause devastating tsunamis after earthquakes we analyzed the three dimensional geometry including dip amount and azimuths roughness distributions and thickness variations of the shallow part of a megasplay fault in the nankai trough using a three dimensional seismic data set the fault is divided into three zones based on its geometry thick smooth and simply convex in the east complexly curved in the middle and thin and kinked in the west results of scientific drilling indicate that the eastern region of the fault is most active and local heterogeneities in fault geometry including roughness and thickness may control the slip on this part of the fault the present findings can be used to evaluate the risk of future tsunamis arising from movement on shallow thrust faults at subduction margins

large intraplate earthquakes and static stress changes in the south china coastal region abstract a number of moderate earthquakes occurred in the south china continental edge or coastal area several large earthquakes are sparsely distributed along the seismic belt with the northeast direction between the south china and the continental shelf of the south china sea coulomb stress changes of the 1969 yangjiang main shock ms 6 4 with the following events of the 1986 ml 5 4 the 1987 ml 5 4 and the 2004 ml 5 2 in the yangbian bay area are studied the complex stress patterns of the main shock caused by the dextral pinggang strike slip fault imported on the planes of the normal fault strike slip fault and thrust fault of three events indicate that the distribution of aftershocks mainly located in the areas of positive stress changes and part of aftershocks fall into the stress shadow area due to the reactivity of the local faults the 1969 yangjiang event occurred between the intraplate and the broad plate boundary zone setting when we apply on the empirical relationship between the aftershock durations and the fault loading rate the intersection fault model is proposed and could explain the stress changes of the yangjiang earthquake sequences in south china coastal area the future hazard assessment requires to take account of the stress increase area along the tip of the main shock source fault and off fault stress increase area

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

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

fluid pressure in the frontal thrust of the oregon accretionary prism experimental constraints seismic reflection profiles of many accretionary prisms including the oregon prism exhibit high amplitude reversed polarity reflections from the decollement and other thrusts it has been suggested that these reflectors image fault zones with enhanced fluid content due to dilation by very high fluid pressure we present measurements of velocity as a function of effective stress on samples of the oregon frontal thrust that show that velocity decreases by up to 15 as fluid pressure rises from hydrostatic to lithostatic conditions under constant confining stress in fault zone and wall rock alike synthetic seismic modeling shows that the frontal thrust reflector at this location is the result of a thin low velocity zone in the plane of the fault 100 300 m s slower than the walls combining models with experimental results we conclude that fluid pressure of 86 to 98 of lithostatic stress reduces velocity enough to generate the reflections

geometries of frontal fold and thrust belts insights from discrete element simulations abstract this study investigated the evolution of frontal structures of fold and thrust belts with 2d numerical simulations using the discrete element method dem of specific interest is the occurrence of triangle zones and how mechanical stratigraphy affects the formation of the triangle zones in our simulations we varied the strength of a two unit stratigraphy as well as the decollement horizons to determine the controlling parameters of different types of frontal structures for models with homogenous stratigraphy deformation was concentrated along forethrusts which involved both the upper and lower units with the addition of a mechanically weak upper unit deformation along forethrusts and popup structures largely occurred in the upper unit with relatively little deformation of the lower unit with thicker decollement surfaces and strong upper and lower units decoupling was enhanced and allowed for the formation of triangle zones in front of the deforming wedge the triangle zones were uplifted along lower forethrusts until a new frontal triangle zone formed results compared favorably with triangle zones found in the canadian cordillera in southern alberta

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

effects of oceanic ridge subduction on accretionary wedges experimental modeling and marine observations sandbox modeling is used to study the deformation of accretionary wedges caused by the subduction of oceanic ridges the first experiment incorporates a massive ridge within a sand wedge the wedge thickens and shortens when the forward propagation of the basal decollement ceases the wedge thickening results in taper change reactivation of preexisting thrusts and retreat of the frontal part of the sand wedge similar mechanisms may have affected some margins that have undergone ridge subduction such as the tonga margin after the subduction of the oblique louisville oceanic ridge the second experiment shows the effects of an active basement thrust slice as it enters a subduction zone this process may have happened in the eastern nankai accretionary wedge initially the wedge in this experiment behaved similarly to that of the first experiment rapidly the topographic slope changed the wedge thickened above the basement slice generating a slope break in the topography a deeper propagating accretionary wedge again characterized by a small taper developed these results when compared with observations made in the eastern nankai trench are in agreement with the past subduction of a basement thrust slice in this area

crustal structure and contact relationship revealed from deep seismic sounding data in south china abstract a 400 km long wide angle seismic profile in the south china crosses three tectonic domains southeast continental margin of yangtze block northwest continental margin of cathaysia block and south china suture zone separating these two blocks the crustal velocity model constructed from traveltime fitting shows that the crust thickness thins from the northwest to the southeast along the profile it also reveals the crustal contact relationship between the yangtze and cathaysia blocks where the wuchuan sihui fault is the boundary between them combining this profile with two more profiles in the continental south china and three profiles in the northern margin of south china sea constructs a continent ocean transition section this cross section reveals that the moho depth shallows gradually along the cross section from the yangtze block to the cathaysia block and the northern margin of south china sea but shallows abruptly in the continent ocean transition to the south china sea

control of internal structure and fluid migration pathways within the barbados ridge decollement zone by strike slip faulting evidence from coherence and three dimensional seismic amplitude imaging the application of three dimensional seismic reflection and coherence imaging to the study of the decollement zone of the barbados ridge accretionary complex has provided new insights into the relationships among internal structure fluid flow and previously unrecognized strike slip faulting combined coherence and seismic amplitude imaging of the decollement zone reveal anomalous northeast trending lineaments parallel to and abutting zones of high amplitude negative polarity reflections analysis of these lineaments shows them to be penetrative structures dipping southeast with apparent reverse dip slip offset isopach mapping of the accretionary wedge indicates significant right lateral displacement across these structures these faults apparently channel fluid flow within the decollement zone and the prominent northeast trending conduits so formed are readily visible as high amplitude negative polarity reflections additionally north northeast trending zones of variable coherence and high positive amplitude are inferred barriers to up structure fluid migration pathways movement along strike slip structures probably alternates with displacement along the decollement zone northeast trending strike slip faults extend for 13 km crossing the length of the survey area and into the incoming oceanic plate active arc oblique strike slip faulting of the decollement zone beneath the barbados ridge accretionary wedge implies a stress regime in that 1 is fixed and 2 and 3 either transpose with time or are nearly equal this state of stress may be a common occurrence in forearc tectonism and may have led to the formation of many as yet unrecognized arc oblique strike slip faults at convergent margins

borehole image analysis of the nankai accretionary wedge odp leg 196 structural and stress studies electrical images recorded with resistivity at bit rab from two sites drilled during ocean drilling program odp leg 196 were analyzed to study the effects of subduction at the nankai margin for the first time in the history of scientific deep sea drilling in odp in situ complete borehole images of the decollement zone were obtained analyses of all drilling induced fracture data indicated that the maximum horizontal compressive stress shmax axes have an azimuth of 303 and analyses of breakout data from rab images indicated an azimuth of 310 these azimuths approximate the convergence direction of the philippine sea plate towards the eurasian plate the frontal thrust at site 808 was encountered at about 389 mbsf density porosity resistivity and gamma ray data change across the frontal thrust the decollement zone at the deformation front was identified between 937 and 965 mbsf the base of the decollement is sharply defined as the maximum extent of conductive fracturing and is marked by abrupt changes in physical properties mikada h becker k moore j c klaus a austin g l bangs n l bourlange s broilliard j bruckmann w corn e r davis e e flemings p b goldberg d b gulick s s hansen m b hayward n hills d j hunze s ienaga m ishiguro h kinoshita m macdonald r d mcneill l obana s hong o s peacock s pettigrew t l saito s sawa t thaiprasert n tobin h j tsurumi h 2002 proc odp initial rep 196 college station tx ocean drilling program the upper boundary of the decollement is marked by several sets of conductive fractures and by high variability in physical properties the decollement zone is characterized by intense brittle fracturing these fractures are considered to be the consequence of cyclic stresses and high fluid pressures in this zone we analyzed fracture dips and their orientations at both sites and found that they are all consistent with a unique stress field model surrounding the two sites

crustal structure across the three gorges area of the yangtze platform central china from seismic refraction wide angle reflection data abstract we present active source seismic data recorded along a 300 km long profile across the three gorges area of the western yangtze platform central china from west to east the profile crosses the zigui basin huangling dome and jianghan basin the derived crustal p wave velocity structure changes significantly across the tongchenghe fault that lies at the transition from the huangling dome to the jianghan basin west of the tongchenghe fault beneath the zigui basin and the huangling dome we observe a 42 km thick crust of relatively low average velocity 6 3 6 4 km s in contrast east of the tongchenghe fault beneath the jianghan basin the crust is only 30 km thick and has a high average velocity 6 6 6 7 km s a west east variation in crustal composition along the tongchenghe fault is also inferred west of the fault p wave velocities suggest a felsic composition with an intermediate layer at the base of the crust whilst east of the fault felsic intermediate and mafic crustal layers are apparent our results suggest that the crust beneath the jianghan basin has been thinned by rifting accompanied by intrusion of the lower crust by mafic dikes and sills the west to east division of the crust in the three gorges area coincides with first order geophysical contrasts in gravity topography crustal and lithospheric thickness

rupture nucleations in the 1995 hyogo ken nanbu earthquake and its large aftershocks summary local seismograms of the hyogo ken nanbu earthquake m 7 2 and its largest aftershocks show that a slow rising initial phase precedes the main p phase high dynamic range short period seismograms of the foreshocks mainshock and aftershocks were recorded by the japan meteorological agency jma network and the net hyogo using these seismograms we investigate the initial rupture process of the mainshock and large aftershocks using an empirical green s function method the deconvolved moment rate function of the mainshock shows that it begins with a small pre event mw 4 0 then the moment releases gradually and after about 0 6 s it increases rapidly the first 0 7 s of the deconvolved moment rate functions are inverted to obtain the spatio temporal images of the initial rupture process the results show that at the hypocentral area less than 1 km from the hypocentre slip velocity rises slowly with a low peak in the surrounding area 1 km east and downward from the hypocentre slip velocity rises rapidly with a high peak it is also found that during the initial rupture process the slip velocity pulse grows continuously these observations are in accord with a theoretical model of the transition process from nucleation to high speed dynamic rupture in which slip velocity rises slowly at the nucleation zone we also analyse the initiation and the main dynamic rupture of large aftershocks spatio temporal images of the largest aftershocks m 5 1 and m 4 8 show that there is a clear initial rupture process with a low moment release and after about 0 2 s slip velocity increases rapidly we compare the initial rupture process of the mainshock with those of the large aftershocks m 4 0 5 1 the duration 0 6 s of the initial phase of the mainshock is longer than those 0 02 0 3 s of the large aftershocks furthermore the slip velocity of the initial rupture process of the mainshock is several times larger than those of the two largest aftershocks these results suggest that the mainshock starts in the region where the stress drop and fracture energy is higher

formation and tectonic evolution of southeastern china and taiwan isotopic and geochemical constraints abstract the southern part of china consists of the proterozoic yangtze craton and the phanerozoic south china foldbelts including the himalayan foldbelt of taiwan models for continental growth have been many and controversial isotopic and geochemical data from mesozoic and younger granitoids and sediments are used here to place constraints on the tectonic evolution of southeastern china and to evaluate whether the young phanerozoic foldbelts are representative of old rejuvenated landmass whether they characterize crustal accretion through successive subduction processes hence suggesting a net growth of continental mass or whether they represent some intermediate situation available sm nd isotopic data for phanerozoic granitoids and metasediments from the south china foldbelts and taiwan invariably show proterozoic model ages t dm ranging from 1 to 2 5 ga with a mean of 1 54 0 30 1 ga all rocks have negative nd t values 2 to 15 suggesting variable but important contributions of old continental materials to the sources of the phanerozoic rocks obviously they do not represent wholesale growth of continental mass the granitoids of the south china foldbelts have multiple origins as viewed from their chemical and isotopic characteristics secular geochemical variation has been established for inland granitoids of proterozoic to mesozoic age their high i sr values 0 710 0 737 suggest that most granitoids are essentially anatectic products of ancient continental crust this is compatible with remelting via microcontinental collision or terrane accretion repeated intracrustal reprocessing by partial fusion and differentiation has undoutedly contributed to important mineralization mesozoic granitoids excluding a type alkaline granites in the coastal region and taiwan show relatively low i sr 0 705 0 710 and high nd values implying that greater amounts of mantle components have been added to ancient continental material in the generation of these rocks this means that mesozoic magmatism in the coastal region resulted from subduction of a paleo pacific plate kula beneath the asian continent the overall geochemical evidence seems to be consistent with a model of serial collisions between microcontinental blocks for the bulk of the south china foldbelts the latest arc continent collision began about 4 ma ago in eastern taiwan

north palawan block philippines its relation to asian mainland and role in evolution of south china sea the island of mindoro the northern part of palawan island and the reed bank area southwestern philippines together constitute a continental fragment the north palawan block lying within an island arc oceanic setting the permian to paleogene rocks of these areas indicate a geologic origin and history for the block contrasting with that of the rest of the philippine archipelago these rocks also suggest that the north palawan block once occupied a pre drift position contiguous with the south china mainland four prominent pre neogene regional unconformities are recognized both onshore and offshore the china mainland in taiwan and in the north palawan block the synchrony of these unconformities and the facies relation of the unconformity bounded sedimentary units strongly suggest a common pre neogene history for all these areas by contrast an important regional unconformity occurring at the top of the middle miocene throughout the palawan area is absent from the asian mainland extension in the south china sea basin since the mesozoic which has separated the north palawan block from the asian mainland has been approximately uniform from west to east however there is strong evidence to suggest that this extension has been achieved by temporally separated phases of continental crustal attenuation and more recent sea floor spreading from the foregoing observations and using the most recent magnetic spreading anomaly data for the south china sea a suite of palinspastic reconstructions has been prepared which shows the evolution of the south china sea area from the late triassic to the pliocene the reconstructions illustrate 1 the convergent continental margin setting of the north palawan block during much of the jurassic and cretaceous 2 the late cretaceous inception of the philippine island arc system 3 the subsequent counter clockwise rotation of the arc system from the late eocene onward 4 the paleocene to middle miocene opening of the south china sea and 5 the early to middle miocene collision between the north palawan block and the palawan subduction system

lithospheric structure in the cathaysia block south china and its implication for the late mesozoic magmatism abstract mesozoic granitoids are widely distributed in south china but the mechanism for generating the felsic magmas is still in debate to understand the possible origin of the granitoids we use joint inversion methods and forward modelling of receiver functions to reveal the lithospheric structure in the cathaysia block the joint inversion of receiver functions and surface wave dispersion with p velocity constraints is applied to a dense array in the cathaysia block and the specific crustal vp vs ratios obtained from a generalized h stacking method are introduced to the traditional joint inversion for passive seismic stations the detailed structures provide valuable knowledge of the geological processes 1 the present day lithosphere is 60 70 km thick which has been thinned since the late mesozoic with reference to geochemical data 2 there is a lack of high vp and vs in the lower crust in this region the results may have limited resolution on the localized magmatic underplating but they do not support the extensive magmatic underplating at the late mesozoic other factors could be the mechanism for the granitoid formation in this region

tectonics of the middle triassic intracontinental xuefengshan belt south china new insights from structural and chronological constraints on the basal decollement zone in orogenic belts a basal decollement zone often develops at depth to accommodate the shortening due to folding and thrusting of the sedimentary cover in the early mesozoic intracontinental xuefengshan belt of south china such a decollement zone is exposed in the core of anticlines formed by the emplacement of the late orogenic granitic plutons our detailed multi scale structural analysis documents a synmetamorphic ductile deformation in the basal decollement the neoproterozoic pelite and sandstone and the intruding early paleozoic granites were deformed and metamorphosed into mylonites and orthogneiss respectively the metamorphic foliation contains a nw se stretching lineation associated with top to the nw kinematic indicators the ductile shearing of these high strained rocks can be correlated with nw verging folds and thrusts recognized in the neoproterozoic to early triassic sedimentary cover monazite u th pbtot chemical dating and zircon sims u pb dating provide age constraints of the ductile shearing between 243 and 226 ma and late orogenic granite emplacement around 235 215 ma in agreement with recent geochronological data these new results show that the xuefengshan belt is an early mesozoic orogen dominated by the nw directed shearing and thrusting at the southeastern boundary of the xuefengshan belt the chenzhou linwu fault separates the early mesozoic domain to the nw from the early paleozoic domain to the se the tectonic architecture of this belt was possibly originated from the continental underthrusting to the se of the south china block in response to northwest directed subduction of the paleo pacific plate

rifting and magmatism in the northeastern south china sea from wide angle tomography and seismic reflection imaging we present a new travel time tomography velocity model and seismic reflection images that delineate the rift architecture and magmatic features of the rifted margin in the northeastern south china sea these data reveal moderately stretched crust 25 km thick along the continental shelf and thin but laterally variable crustal thickness in the distal margin along the continental slope crust rapidly thins to 4 km in a basin characterized by tilted fault blocks that sole into a low angle detachment strain was localized to a degree within the highly stretched basin but failed to progress to breakup and seafloor spreading crust in the distal margin is 12 15 km thick few extensional structures are apparent in the distal margin but seismic velocities are suggestive of highly thinned and magmatically intruded continental crust the magmatic features we interpret include volcanic zones at the top of the basement that deform or disrupt overlying postrift strata sills intruded into the postrift sedimentary section and a high velocity 6 9 7 5 km s lower crustal layer that we take to be magmatic underplating or pervasive lower crustal intrusions these features primarily occur in the distal margin and may have been emplaced during postrift seafloor spreading the postrift magmatism may have been induced by convective removal of continental lithosphere following breakup and the onset of seafloor spreading in the south china sea

evolution of the nankai trough decollement from the trench into the seismogenic zone inferences from three dimensional seismic reflection imaging we mapped the amplitude of the nankai trough subduction thrust seismic reflection from the trench into the seismogenic zone with three dimensional seismic reflection data the decollement thrust forms within the lithologically homogeneous lower shikoku basin facies along an initially nonreflective interface the reflection develops from a porosity contrast between accreted and underthrust sedimentary material because of accretionary wedge consolidation and rapid loading and delayed consolidation of the underthrust section a decollement amplitude map shows a significant decline from high amplitudes at the trench to barely detectable levels 25 30 km landward three other observations coincide with the amplitude decline 1 the decollement initially steps down to deeper stratigraphic levels 2 the wedge taper increases dramatically and 3 the thrust becomes seismogenic the amplitude decline and the coincident decollement and accretionary wedge tectonic and seismogenic behavior are attributed to the loss of fluids and potentially loss of excess fluid pressures downdip along the subduction thrust

seismic imaging of a mid crustal low velocity layer beneath the northern coast of the south china sea and its tectonic implications abstract low velocity layer lvl within the continental crust is usually closely associated with tectonic events thus it may reflect ongoing processes or offer a chance to backtrack the evolutional history mid crustal lvl has been reported in the northern coast of the south china sea scs by several geophysical studies but the 3d distribution of lvl in the whole coastal area is still poorly known here we perform a receiver function study based on 67 broadband seismological stations we use the common conversion point method to construct structures of the moho as well as intra crustal interfaces we image sharp intra crustal negative velocity gradient broadly distributed in the eastern cathaysia block with depth increasing from 10 km in the nw to 12 16 km in the se the corresponding amplitude of the negative interface shows variable values from 0 03 to 0 1 with an evidential trend of strengthening eastwards we interpret the negative gradient as top of the lvl produced by accumulation of fluids from dehydration of the paleo pacific subduction plate the mid crustal lvl tends to act as weak domains during continental rift and the lateral variation of the lvl may be one possible reason explaining the along strike variation in crustal stretching in the northern scs rifted margin

effect of increased shear stress along a plate boundary fault on the formation of an out of sequence thrust and a break in surface slope within an accretionary wedge based on numerical simulations abstract we investigated the effect on accretionary wedge structure of increased shear stress which describes the frictional sliding resistance along a decollement arising from an increase in material friction or reduction in pore pressure to clarify the nature of the effect we performed numerical simulations using two models a stable friction model and an increased friction model the stable friction model produced a low angle smooth surface slope and an in sequence thrust whereas the increased friction model produced a break in surface slope scarp and an out of sequence thrust ost that cuts through the thrust sheet the ost formed via the connection of segments of two adjacent thrusts and its formation resulted in a change in the thickening mode of the wedge from thrust sheet rotation and back thrust activity to underplating this contrast in thickening mode between the landward high friction zone and seaward low friction zone resulted in the formation of a clear break in slope as the landward zone is steeper than the seaward zone consistent with critical taper theory the subduction of a basement slice or seamount can produce similar structures arising from an increase in resistance to basal shear sliding however the distinctive structures arising in an accretionary wedge as a result of increased shear sliding resistance include a flat basal plane and absence of slope failure sediments beneath the ost these structural features are observed in accretionary wedges of the nankai trough off muroto japan the sunda strait and the barbados ridge

cdp mapping to obtain the fine structure of the crust and upper mantle from seismic sounding data an example for the southeastern china abstract we present an efficient and general approach of common depth point cdp mapping to image the fine structure of the crust and upper mantle we use velocity model and reflector geometries which are the results of conventional interpretation as a reference model to construct our cdp mapping of crust and upper mantle after three steps the calculation of cdp points and reflecting traveltimes the determination of cdp mapping trajectories and the implementation of cdp imaging reflection images were created by cdp mapping technique in which each time element of a seismic trace beginning with the direct arrival is mapped to reflection point our test of the method presented in this paper shows that the algorithm can quickly accurately image the subsurfaces of the given model the result of cdp mapping in the southeastern china shows that the depth of moho is about 34 km there are numerous reflection interfaces within the upper crust and the moho is a transition zone with a thickness of 2 3 km the cdp mapping approach presented here can easily be extended to three dimensions and complex anisotropic media the method can also be used to develop seismic attribute images of the crust and upper mantle

application of geometric models to inverted listric fault systems in sandbox experiments paper 2 insights for possible along strike migration of material during 3d hanging wall deformation abstract fault geometry is a primary control on hanging wall deformation in this study a series of positive inversion analogue experiments was conducted using rigid fault surfaces of true 3d geometry with consistent listric geometry along the transport direction the deformed geometry of the top horizon of the syn extension sequence on vertical serial sections was examined with a conventional 2d geometric restoration technique to calculate the inclined antithetic shear angle that best approximates the actual fault shape and to estimate the amount of apparent horizontal shortening during contraction the apparent shear inclination and the estimated apparent shortening show a systematic change along strike corresponding to the plan geometry of the master detachment surface at the hanging wall above embayments in the fault geometry the uplift is highest the apparent shear inclination is gentlest and the apparent horizontal shortening is greatest in contrast the apparent shear inclination is steepest and the estimated shortening is smallest above salients in the master detachment geometry where the uplift is lowest these changes suggest that the hanging wall displacement had an along strike component during deformation the hanging wall material moves from the regions above salients to those above embayments in the detachment surface during contraction average of the estimated apparent shortening was smaller than the actual amount of the experiments probably due to tectonic compaction this study shows that the geometry of the master detachment in plan view has the primary control on the lateral variation of the hanging wall deformation the data presented in this paper help understand 3d geometric relations between the hanging wall deformation and the underlying detachment surfaces

crustal structure inferred from gravity anomalies in south china abstract the bouguer gravity anomaly map of the mainland of south china and the south china sea compiled from the most recent 1 500 000 and 1 1 000 000 gravity data is presented the distribution of faults including the secondary faults which can be located by a method based on cross correlation between the theoretical anomaly due to a vertical step and the second vertical derivative of the bouguer anomalies large strike slip faults without vertical displacement or with a small throw which are difficult to identify on the bouguer gravity map and can be inferred from the distribution of the secondary faults and major faults inferred from the zones of gravity gradients on the bouguer gravity map is presented also comparison of the regional gravity anomalies with known density interfaces deduced from seismic data and geological maps shows that the main geological factors causing gravity anomalies are granites low velocity layers in the upper crust the moho discontinuity and the upper crustal thickness variations the broad trend of gravity anomalies depends mainly on the moho and the upper crustal thickness variations we have attempted to isolate approximately the anomalies due to the upper crustal thickness variations by subtracting the anomalies due to the moho from the regional anomalies and then the upper crustal thickness in south china can be estimated by inversion of the gravity anomalies

a crustal structure profile across the northern continental margin of the south china sea an ocean bottom seismometer obs geophysical experiment was carried out across the northern continental margin of the south china sea two dimension kinematic ray tracing was used to construct synthetic records and to define the geological model along the obs profile according to the calculation and interpretation of refracted p wave phases the crustal thickness decreases from 22 km obs 1 in littoral area to 8 km obs 15 in deep sea the lower slope structured by volcanism changes sharply in moho depth from 22 to 9 km the volcanoes over the lower slope were markers of ocean continental transition zone a high velocity layer hvl with an average thickness of 4 km was found in the base of the lower crust over the shelf and slope the south china sea was originated by the passive rifting of south china continent and seafloor spreading however geological and geophysical data collected in the northern margin of the south china sea show a small amount of magma intruded into or erupted onto the crust during pull apart and breakup further analysis shows that the hvz in the base of the thinned continental crust was related to magmatism that occurred after the cessation of seafloor spreading of the south china sea magmatism during the crustal extension and breakup is very weak therefore the northern continental margin of the scs is classified as non volcanic passive margin the southern margin of the south china sea has a crust with a steady thickness of 18 24 km which is different from the nor them margin of the south china sea with a gradually thinned crust they constitute an asymmetrically conjugated pair of passive margins c 2001 elsevier science b v all rights reserved

deep penetration seismic soundings across the northern margin of the south china sea twenty reversed two ship expanding spread profiles esps with maximum source receiver offsets of 100 km were collected in three transects across the rifted northern margin of the south china sea source receiver offset versus two way travel time x t data were mapped into the intercept time versus ray parameter p domain and velocity depth solutions were obtained by a combination of sum inversion in the p domain and ray tracing in both the p and x t domains arrivals from the moho were detected on 17 of the esps the depths to moho determined for individual esp interpretations have reproducibilities of 0 1 km to 3 km in most cases the moho depth has been determined to within 1 5 km moho depths determined in this investigation represent a significant improvement over previous estimates of moho along the margin from gravity data variations in present day crustal thickness measured from top of prerift basement to moho are one measure of the amount and nature of the crustal thinning associated with the rifting of continental crust preceding the formation of the adjacent south china sea basin the esp interpretations reveal that across the eastern portion of the south china margin the crust appears to thin more or less continuously toward the continent ocean boundary in the west esp interpretations also show a general trend of seaward crustal thinning but in addition indicate at least two instances of focused localized crustal thinning crustal velocities and the relative proportion of upper crust vp 6 4 km s are used to identify areas of the south china margin with similar and contrasting crustal structures variations in these properties are believed to result primarily from contrasting prerift crustal structure across the margin however magmatic underplating during rifting depth dependent extension and pleistocene igneous intrusions may also have contributed to the variations in present crustal structure reliable information about variations in crustal thickness and velocity structure across and along the south china margin is an important prerequisite to understanding better the nature of the spatially variable rifting processes which dominated the formation of this margin

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

isotopic and paleomagnetic constraints on the mesozoic tectonic evolution of south china in order to better constrain the paleogeographic evolution of south china we measured sm nd and rb sr isotopic compositions for 23 mesozoic granites that crop out throughout the area tightly grouped neodymium depleted mantle model ages 1 4 0 3 ga suggest the region is underlain by relatively homogeneous proterozoic crust and fail to define crustal provinces neither the isotopic nor geologic data suggest that a mesozoic suture exists however granites possessing anomalously high sm 8 ppm and nd 45 ppm concentrations relatively high initial epsilon neodymium 4 to 8 and high but variable initial 87sr 86sr 0 759 to 0 713 form a northeast trending zone that coincides with two prominent mesozoic basins southeast of the zone lie the majority of mesozoic intrusives and upper triassic to lower cretaceous extensional basins found in south china mesozoic paleomagnetic poles are well clustered northwest of the zone pre cretaceous poles southeast of it are discordant with respect to those from the northwest the only recognized tectonostratigraphic terrane in south china lies southeast of the zone the terrane is bordered by a northeast trending sinistral fault that was active in the mesozoic other faults in south china have similar attitudes ages and sense of shear together the observations suggest that the mesozoic tectonic regime in south china consisted of strike slip activity plus concomitant rifting as terranes or fragments of similar crust were transported north along sinistral faults the zone defined by the granites enriched in nd and sm demarcates displaced terranes to the southeast from relatively stable land to the northwest

extensional tectonics within a subduction type orogen the case study of the wugongshan dome jiangxi province southeastern china abstract in southeastern china the wugongshan massif consists of metamorphic rocks and granites generally considered as basement rocks formed during an early paleozoic orogeny our structural and radiometric reappraisal of this massif shows that it results from a mesozoic doming in the metamorphic rocks the foliation defines an e w elliptical shape and bears a n s stretching lineation the axial zone of the dome is occupied by oriented plutons the outer part of the dome consists of devonian quartzite carboniferous permian limestone and triassic jurassic sandstone ductile deformation observed in the permian carbonates and older units develops contrasting kinematics the northern and southern flanks present top to the north and top to the south motions respectively the same divergent pattern of motion is recorded from brittle structures up to the jurassic sandstone the late cretaceous red beds are undeformed in the axial zone coaxial flow indicated by symmetric pressure shadows and quartz fabrics predominates along the n and s flanks of the dome post folial recumbent folds are symmetrically overturned to the n and s respectively but at the two extremities of the dome the fold axes are parallel to the stretching lineation 40 ar 39 ar ages of biotite and muscovite from mylonite and granite suggest a late triassic age for the ductile deformation and an early cretaceous age for the final doming the wugongshan dome is similar to a metamorphic core complex formed in an extensional tectonic regime a model which emphasizes plutonism is discussed from permian to early cretaceous southeastern china was a subduction type orogen the crust of which was thickened by accretion of mantle derived magmas the emplacement of a large amount of mesozoic granitoids in southeastern china promoted thermal softening and gravitational instability that allowed the crust to extend the formation of the wugongshan dome and other metamorphic core complexes appears to be a the direct consequence of plutonic activity and crust rheology

geochemical and sm nd isotopic study of amphibolites in the cathaysia block southeastern china evidence for an extremely depleted mantle in the paleoproterozoic abstract geochemical and sm nd isotopic results are reported for late paleoproterozoic mafic amphibolites from sw zhejiang and nw fujian parts of the cathaysia block of se china two suites of contemporaneous amphibolites are distinct in their geochemical characteristics group 1 samples from nw fujian have chemical compositions of transitional and alkali basalts show lree enriched patterns and plot mainly in the field of within plate basalt on a number of trace element discrimination diagrams group 2 rocks from sw zhejiang have tholeiitic compositions and are characterized by flat to lree depleted patterns and fall into the morb field all the amphibolite samples have high end t values of 5 6 to 8 5 t 1766 19 ma a positive correlation between end t and nb th suggests possible mixing of a mantle derived magma and a crustal component with the least contaminated samples having very high end t values 8 8 5 and nb th ratios of 13 the geochemical and isotopic characters and close temporal relationship of these two suites of amphibolites suggest that their magmatic precursors were likely formed in an environment similar to an ensialic rift developing into a proto oceanic basin e g the gulf of tadjoura the exceptionally high end t values of up to 8 5 for some of the amphibolites suggest the presence of a time integrated extremely depleted mantle source beneath cathaysia during the paleoproterozic

an unstable kinematic state of the himalayan tectonic wedge evidence from experimental thrust spacing patterns abstract the frontal tectonic wedge in the nw himalayas shows a sequence of imbricate thrusts with increasing spacing in the foreland direction we used analogue model experiments to analyze the thrust pattern in relation to the kinematics of wedge evolution experimental findings reveal two kinematic states of the foreland ward propagation of a wedge unstable state and stable state the unstable state is characterized by continuous vertical growth of the wedge with progressive horizontal contraction on the other hand in the stable state the wedge ceases to grow vertically but propagates laterally in the foreland direction in the experimental runs thrust wedges remained in the unstable state even after large horizontal shortening 50 when the basal friction was high 0 46 our analysis suggests that successive thrusting occurs always with increasing spacing in unstable wedges and the rate of increase is larger for larger basal friction it can maintain uniform spacing only when the wedges turn into the stable state using finite element models we determined the stress distribution in the deforming wedge to find the potential locations of new thrusts in front of a wedge which also show larger spatial distances with increasing vertical thickness of the wedge both physical and numerical models suggest that the tectonic wedge in the nw himalayan frontal belt has evolved in an unstable state

kwangsian crustal anatexis within the eastern south china block geochemical zircon u pb geochronological and hf isotopic fingerprints from the gneissoid granites of wugong and wuyi yunkai domains abstract gneissoid granites were traditionally thought to be the components of the precambrian basement in the eastern south china block but twenty four gneissoid granite samples from the wugong northern wuyi southern wuyi and yunkai domains gave zircon u pb ages of 424 456 ma 410 457 ma 426 438 ma and 415 450 ma respectively the existence of the abundant kwangsian gneissoid granites which have incorrectly been assigned to the precambrian basement indicates that the so called precambrian stratigraphic sequence of the cathaysia block should be termed complex rather than group these gneissoid granites are peraluminous granites with relatively high al 2 o 3 mgo tio 2 feot and cao na 2 o but low cao al 2 o 3 mgo feot and rb sr ratios they exhibit strongly negative ba sr nb p and ti and positive pb anomalies in primitive mantle normalized diagrams and have initial 87 sr 86 sr ratios ranging from 0 70924 to 0 72935 and negative end t values from 6 4 to 11 4 similar to those of paragneissic enclaves and precambrian sedimentary rocks in the eastern south china block the zircons crystallized in the kwangsian gneissoid granites gave ehf t values ranging from 2 4 to 19 4 with the peak at 4 almost all grains clustering at 1 to 12 and hf model ages of 1 1 2 1 ga the synthesis of these geochemical and in situ zircon hf isotopic data indicates that the kwangsian gneissoid granites dominantly originated from proterozoic metapelitic and meta igneous components with insignificant input of juvenile mantle derived magmas a model of two stage crustal anatexis is proposed for the kwangsian granitic magma in the eastern south china block with 1 the formation of 460 430 ma granite through the breakdown of hydrous minerals under the condition of the doubly thickened crust and 2 the generation of the 430 400 ma granite accompanying promoted melting along a path of isothermal decompression due to the increasing thermal weakening for the collapse of the thickened crust the crustal anatexis has probably occurred in an intracontinental tectonic regime that was geodynamically linked to the far field response to the assembly of the australian indian plate with the cathaysia block during middle paleozoic kwangsian time

tectonics of south china key to understanding west pacific geology abstract south china is not a post caledonian platform but a composite of orogenic belts this late proterozoic mesozoic orogen is a collage of three continental fragments the three blocks are yangzi huanan and dongnanya and they are separated by the banxi nanpanjiang formerly xianggangzhe and gunanhai suture zones yangzi was separated from gondwanaland during late precambrian when an open ocean called banxi was present between the two continents tectonic processes at an active margin during sinian and early paleozoic time led to the genesis of an accretionary wedge complex the banxi melange and the huanan flysch nappes on the northern margin of huanan which was then the northern margin of the gondwana continent reorganization of plates during the devonian suspended subduction at the huanan active margin huanan was uplifted and unconformably overlain by transgressive deposits of devonian and or carboniferous age a remnant ocean the nanpanjiang sea still existed between yangzi and huanan and deep water sedimentation continued both at the southern margin of yangzi and at the northwestern margin of huanan the latter again became an active margin during late paleozoic when permian and triassic flysch sediments were deposited in foredeeps ahead of advancing nappes huanan and yangzi collided during the triassic and resulted in the deformation of the passive margin of the latter to form the yangzi deformed belt huanan was separated from a more southerly continent dongnanya in the devonian by seafloor spreading which created a late paleozoic and early mesozoic ocean gunanhai ocean a continuously deposited sequence ranging in age from devonian to triassic was laid down on the southern passive margin of huanan on the other side of the ocean the dongnanya permian strata include glacial marine deposits of gondwanaland affinity dongnanya became separated from gondwanaland during the late permian when it marched northward to be reunited with huanan the huanan passive margin sequence was deformed by folding and overthrusting after the late mesozoic collision of huanan and dongnanya scattered outcrops of the suture melange resulting from this collision occur at a few localities in coastal fujian dongnanya in our model was the mesozoic continent of southeast asia peripheral to mainland asia this may or may not have been the eastern continuation of the microcontinent sibumasu siam thailand burma malaysia sumatra the collision of dongnanya and asia gave rise not only to the gunanhai melange of coastal fujian but also to the ophiolite melanges parts of which are present in taiwan tailuko the philippines and western malaysia

application of geometric models to inverted listric fault systems in sandbox experiments paper 1 2d hanging wall deformation and section restoration abstract fault geometry is a primary control on hanging wall deformation in order to examine their geometrical relationships a positive inversion analogue experiment was conducted using a rigid fault surface of listric geometry the hanging wall deformation observed on a representative vertical section was examined with conventional 2d geometric models and was restored to its pre inversion phase with two techniques these results suggest that the deformation can be best approximated by inclined simple shearing iss the iss model can determine the inclination of the apparent shear plane and the amount of apparent horizontal shortening which is equivalent to that calculated with the conventional depth to detachment method this estimated apparent shortening was generally smaller than the actual amount of the experiments probably due to tectonic compaction

computerized x ray tomography analysis of three dimensional fault geometries in basement induced wrench faulting abstract computerized x ray tomography applied to sandbox experiments makes it possible to analyze the kinematic evolution as well as the three dimensional geometry of faults in basement controlled wrench faulting with increasing basement displacement the evolution of shear bands developed in the model sand bed is observed as follows 1 the right stepping shears that have a cirque or shell or ship body shape develop on both sides of the basement fault 2 the shears on the one side of the basement fault join those on the other side and therefore each shear surface forms a helicoidal shape shears reach the surface of the sand near or above the basement fault thereby en echelon riedel shears are observed at the surface of the sand 3 lower angle shears and p shears lying within the zone defined by the first riedels have steeper dips and a less pronounced helicoidal aspect than the riedels such three dimensional shape and kinematic evolution of shears in the fault experiments agree well with the similar faults which propagate up through the unconsolidated sediments from a wrench fault in bedrock as observed in the excavated trenches at some active faults in the fault experiments the strike of the riedel shear to the basement fault in dense sand is larger than that in loose sand in reasonable agreement with the directions predicted by the coulomb criterion in terms of the angle of internal friction likewise the geometries of the riedel shears are considered to depend on the overburden materials in natural cases of wrench faulting

investigating new materials in the context of analog physical models abstract to broaden the availability of granular materials that are suitable for the analog modeling of upper crustal deformation we investigated the mechanical behaviors of pure quartz sand and two sand mixtures quartz sand powdered barite and quartz sand mica crystals using ring shear tests and simple convergent sandbox experiments the ring shear test results indicate that the three materials have similar peak friction angles between 39 25 and 36 02 but the magnitude of the shear strain and the shear strength required to cause their failure are different the differences between the analog models are identified by distinct fault kinematics and different grain flows which are primarily related to differences in the plastic elasto frictional rheology we conclude that the use of the quartz mica mixture which showed the strongest distributed plastic deformation can improve analog models where different materials are required to simulate crystalline basement sand and supracrustal rocks sand mica mixture this is a common situation in extension and inversion tectonics such as for example in inversion tectonics when a granitic basement block acts as a buttress

out of plane motion of a thrust sheet during along strike propagation of a thrust ramp a distinct element approach abstract using a 3d distinct element mechanical modeling program we examined the nature and magnitude of out of plane motion during along strike propagation of a simple thrust ramp and attendant fault related folding our numerical model was composed of about 83 000 spherical elastic particles that were bonded together to create an initial solid numerical rock mass capable of macroscopic fracture and faulting in response to displacement loading the algorithim is fully dynamic and as such allowed for the storage and sudden release of elastic strain energy in the form of seismic events we induced progressive along strike fault growth and attendant non cylindrical fold development by varying the along strike friction within a seeded planar zone of weakness within the aggregate a zone that was to become the fault the result was a well developed plunging ramp anticline that grew in amplitude and along strike length as the macroscopic fault propagated from lower to higher friction regions lateral fault propagation was relatively rapid compared with shortening with a ratio of thrust displacement to propagation length k of 6 5 incremental motion of the particles reflected episodic slip at both the laterally propagating fault tip and along the existing fault these episodes were characterized by local clusters of outward radiating velocity vectors that propagated at seismic velocities marking the model equivalents of earthquakes the finite displacement vectors in the hanging wall were approximately parallel to the far field transport direction with a strike parallel component that ranged from one to two orders of magnitude less than the strike perpendicular component furthermore about two thirds of the particles had strike parallel motion in the direction of fault propagation and fold plunge because the far field shortening boundary conditions were uniformly perpendicular to fault strike this component of motion in the plunge direction was apparently due to a topographically induced plunge parallel shear stress the along strike motion involved slip on the main fault as well as transient strike slip on minor vertical sometimes conjugate zones apparently induced by the increased topographic load

reprint of p wave velocities and anisotropy of typical rocks from the yunkai mts guangdong and guangxi china and constraints on the composition of the crust beneath the south china sea abstract in order to provide constraints on the interpretation of seismic data of the crust beneath the south china sea scs and its continental margins we have measured p wave velocities and anisotropy as a function of hydrostatic confining pressure up to 650 mpa for 31 representative samples i e granite diorite felsic gneiss mylonite and ultramylonite amphibolite schist and marble from the yunkai mts guangdong and guangxi provinces china that represent the crystalline basement beneath the continental margins of the scs the intrinsic velocity of each crack free rock increases with increasing density which is linearly dependent on the chemical composition increases with increasing mgo cao feo fe 2 o 3 and al 2 o 3 contents but decreases with increasing contents of sio 2 and na 2 o k 2 o most of the rocks have small m f of 41 43 and the ratio shows a general increase from the continental margin to the central basin the high velocity 7 0 7 6 km s materials in the lower crust could be either the former lower crustal mafic rocks that were present before rifting which have experienced less extensional thinning than the felsic upper crust or the materials crystallized from mafic magma which underplated the lower crust from the partially molten upper mantle during rifting

petrogenesis and tectonic significance of a mesozoic granite syenite gabbro association from inland south china article i nfo a granite syenite gabbro association from inland south china has been studied for zircon u pb ages and hf isotopic compositions as well as whole rock elemental and sr nd isotopic compositions to constrain their sources and tectonic settings these rocks have distinctive crystallization ages the longyuanba biotite granites have a mean age of 240 ma the longyuanba two mica granites pitou alkali feldspar granites and amphibole bearing alkali feldspar granites tabei and huangbu syenites and chebu and chenglong gabbros have a similar age of 178 ma whereas the dafengnao syenite gives a mean age of 165 ma the longyuanba biotite granites and two mica granites exhibit s type characteristics probably derived from a neoproterozoic crustal source the chenglong gabbro has oib like trace element and highly depleted isotope compositions ehf t 10 0 1 3 end t 5 2 suggesting its parental melt to be of asthenospheric origin with insignificant crustal assimilation however crustal contamination is required to explain the isotopic compositions of the chebu gabbro the syenites are shoshonitic in composition and have depleted sr nd hf isotopic signatures which we interpret to have resulted from a mixed source of asthenospheric mantle and metasomatized lithospheric mantle as expected the signals of crustal assimilation are conspicuous in the petrogenesis of the more evolved syenites the pitou alkali feldspar granite and amphibole bearing alkali feldspar granite exhibit i type and a type characteristics respectively they are isotopically more depleted than the s type longyuanba granites we suggest that they may have formed through magma mixing of predominantly mantle derived melts with the neoproterozoic crust the jurassic granite syenite gabbro association was the product of asthenosphere lithosphere crust interactions which records the primary role of asthenospheric mantle upwelling in magma generation both in the mantle and in the crust in the early yanshanian time in south china they probably occurred in an intraplate rift like environment as a tectonic response to far field stress at plate margins during the early stage of the paleo pacific plate subduction the tectonic transition from the tethys orogenic regime to the paleo pacific regime was accomplished in the early jurassic and the early yanshanian magmatism should be genetically associated with the paleo pacific tectonic regime

crust upper mantle seismic velocity structure across southeastern china abstract one in line wide angle seismic profile was conducted in 1990 in the course of the southeastern china continental dynamics project aimed at the study of the contact between the cathaysia block and the yangtze block this 380 km long profile extended in nw se direction from tunxi anhui province to wenzhou zhejiang province five in line shots were fired and recorded at seismic stations with spacing of about 3 km along the recording line we have used two dimensional ray tracing to model p and s wave arrivals and provide constraints on the velocity structure of the upper crust middle crust lower crust moho discontinuity and the top part of the lithospheric mantle p wave velocity s wave velocity and v p v s ratio are mapped the crust is 36 km thick on average albeit it gradually thins from the northwest end to the southeast end offshore of the profile the average crustal velocity is 6 26 km s for p waves but 3 6 km s for s waves a relatively narrow low velocity layer of about 4 km of thickness with p and s wave velocities of 6 2 km s and 3 5 km s respectively marks the bottom of the middle crust at a depth of 23 km northwest and 17 km southeast at the crust mantle transition the p and s wave velocity change quickly from 7 4 to 7 8 km s northwest and 8 0 to 8 2 km s southeast and from 3 9 to 4 2 km s northwest and 3 9 to 4 5 km s southeast respectively this result implies a lateral contrast in the upper mantle velocity along the 140 km sampled by the profile approximately the average v p v s ratio ranges from 1 68 1 8 for the upper crust to 1 75 for the middle and 1 75 1 85 for lower crust with the interpretation of the wide angle seismic data jiangshan shaoxin fault is considered as the boundary between the yangtze and the cathaysia block

boundary conditions in sandbox models of crustal extension an analysis using distinct elements abstract the geometries of faults that form in the brittle hanging wall above large low angle detachments has been an active topic of research in petroleum geology and geophysics for the last decade in order to understand the progressive development of the geologic structures and to identify possible petroleum traps experimentalists have used sandbox models to try to gain insight into the kinematics and mechanics of the deformation of overburden above detachments although the structures observed in some of the models resemble those observed in nature boundary conditions and constitutive properties of these models have rarely been measured precisely so the experiments are often difficult to interpret we use an entirely numerical approach based on the distinct element method dem in which the overburden is represented by hundreds of individual elements that interact mechanically through contact forces the faults that form in an assemblage of elements resemble those observed in nature the boundary conditions are prescribed as are the material reactions of individual elements so the numerical experiments are more easily interpreted we suggest that dem models complement sandbox models and can be used to interpret the progressive development of geologic structures

initiation of plate boundary slip in the nankai trough off the muroto peninsula southwest japan 1 to investigate variations in physical properties accompanying the development of the decollement we used a neural network analysis of seismic attributes derived from 3d seismic reflection data gathered in the nankai trough convergent margin off the muroto peninsula of southwest japan this analysis resulted in a detailed map of the geological features along the decollement and revealed a drastic change in physical properties at the sole of the proto thrust ocean drilling program odp data suggest that the decollement physical property change coincides with the initiation of slip furthermore changes in the structure of the accretionary prism and reorganization of the decollement geometry also attend the initiation of plate boundary slip