reconciling subduction dynamics during tethys closure with large scale asian tectonics insights from numerical modeling we use three dimensional numerical models to investigate the relation between subduction dynamics and large scale tectonics of continent interiors the models show how the balance between forces at the plate margins such as subduction ridge push and far field forces controls the coupled plate margins and interiors evolution removal of part of the slab by lithospheric break off during subduction destabilizes the convergent margin forcing migration of the subduction zone whereas in the upper plate large scale lateral extrusion rotations and back arc stretching ensue when external forces are modeled such as ridge push and far field forces indentation increases with large collisional margin advance and thickening in the upper plate the balance between margin and external forces leads to similar convergent margin evolutions whereas major differences occur in the upper plate interiors here three strain regimes are found large scale extrusion extrusion and thickening along the collisional margin and thickening only when negligible far field forces ridge push and larger far field forces respectively add to the subduction dynamics the extrusion tectonics develops a strong asymmetry toward the oceanic margin driven by large scale subduction with no need of preexisting heterogeneities in the upper plate because the slab break off perturbation is transient the ensuing plate tectonics is time dependent the modeled deformation and its evolution are remarkably similar to the cenozoic asian tectonics explaining large scale lithospheric faulting and thickening and coupling of indentation extrusion and extension along the asian convergent margin as a result of large scale subduction process

shear wave splitting and subcontinental mantle deformation splitting observations are interpreted in terms of the strain induced lattice preferred orientation of mantle minerals especially olivine we consider three hypotheses concerning the origin of the continental anlsotropy 1 strain associated with absolute plate motion as in the oceanic upper mantle 2 crustal stress and 3 the past and present internal deformation of the subcontinental upper mantle by tectonic episodes it is found that the last hypothesis is the most successful namely that the most recent significant episode of internal deformation appears to be the best predictor of q for stable continental regions this is interpreted as fossil anisotropy whereas for presently active regions such as alaska the anisotropy reflects present day tectonic activity in the stable portion of north america there is a good correlation between delay time and lithospheric thickness this is consistent with the anisotropy being localized in the subcontinental lithosphere and suggests that intrinsic anisotropy is approximately constant the acceptance of this hypothesis has several implications for subcontinental mantle deformation first it argues for coherent deformation of the continental lithosphere crust and mantle during orogenies this implies that the anisotropic portion of the lithosphere was present since the deformational episode and rules out the addition of undeformed material to this layer by subsequent underplating or conductive growth of the thermal boundary layer one of the most important issues in the study of orogenies is the need to reconcile the formation of thickened lithosphere with the paradoxically high mantle temperatures often associated with orogenic episodes most efforts to date have focussed on modes of deformation whereby the cold lithospheric mantle is removed by convective instability or delamination and replaced by warm asthenosphere these models however are incompatible with the evidence for preserved coherent lithospheric deformation rather the deformed mantle appears to have been heated in place we suggest that the elevated mantle temperatures may be due to the strain heating accompanying the deformation

seismic anisotropy of south african upper mantle xenoliths abstract seismic properties of six upper mantle xenoliths from south africa were determined both by laboratory measurements at elevated pressure and numerical calculations from petrofabric analyses in order to provide constraints on the structural interpretation of the field seismic data obtained in recent years the samples studied include harzburgite garnet harzburgite mica harzburgite lherzolite and garnet lherzolite average anisotropies calculated from petrofabrics are 5 4 for v p and 4 4 for v s ultrasonic measured anisotropies from xenolith rock cores are approximately 1 lower than calculated values this difference is likely due to simplifications in modal mineralogy used in the calculations and small amounts of alteration in the samples used in the velocity measurements comparison of laboratory data with the sks and p n data yields possible structural orientations in the upper mantle beneath the south africa continent our results suggest that foliation in the upper mantle is likely horizontal and the stretching lineation olivine a axis is oriented n30 e which coincides with the absolute plate motion apm of the south african continent it is apparent from our data that the fast s wave polarization depends on the propagation direction with respect to petrofabric orientations and does not always indicate the stretching lineation

lithospheric stress states predicted from long term tectonic models influence of rheology and possible application to taiwan abstract the long term stress state of the lithosphere results from a combination of boundary forces surface loads and tractions from mantle flow here we have employed a numerical technique which allows for the self consistent treatment of the interaction between the lithosphere and mantle and takes into account the deformation of the free surface we have used this to study lithospheric dynamics in order to gain insights on the present day conditions of stress and deformation of the lithosphere our numerical approach takes the observed topography and far field plate velocities as input and computes the resultant deformation as a function of the rheological stratification our model application is inspired by taiwan tectonics stress data and earthquake focal mechanisms indicate that the upper crust in taiwan may be predominantly under compression the recent pingtung earthquake in 12 26 2006 however suggests that the sub moho mantle lithosphere under southern taiwan may be under extension if this earthquake indeed represents the local state of stress this raises the question as to why extension and compression can occur simultaneously in this region for addressing this issue we have performed a number of numerical simulations in which we studied the effects of rheological stratification boundary conditions and geometry on the stress state and lithospheric dynamics we have employed both layered viscosity models and models in which we use laboratory based non newtonian and temperature dependent creep laws results show that the rheological stratification plays an important role in lithospheric deformation the relatively hot lithospheric thermal state in taiwan makes it likely that part of the lower crust is relatively weak which results in decoupling from surface near deformation with deformation at depth bending of a subducting slab underneath taiwan causes an extensional stress state in the sub moho mantle lithosphere whereas the collision of the philippine sea plate with the eurasian margin is responsible for a compressional state of stress in the crust it is demonstrated that models with a weak lower crust are also in reasonable agreement with available gps data as well as with constraints on uplift rates from long term geodynamic modelling studies

shear wave splitting in the upper mantle wedge above the tonga subduction zone summary intermediate period seismograms of shear waves from deep earthquakes are examined for shear wave splitting diagnostic of seismic velocity anisotropy in the upper mantle wedge between the subducting tonga slab and stations in the fiji islands earthquakes with near vertical incidence angles are selected to minimize complications of converted phases and the free surface horizontal component seismograms for 25 deep earthquakes recorded at station lak on the lau ridge show splitting of up to 0 9 s with an average of 0 45 0 25 s between fast and slow s wave components anisotropy in the upper mantle is the most likely cause of the splitting the ese orientation of the fast velocity direction at station lak corresponds to an axis of symmetry of the anisotropic mantle and is approximately perpendicular to magnetic lineations in the lau basin and the northeastern portion of the south iiji basin the observations are consistent with a model of flow alignment of olivine 100 and orthopyroxene 001 axes parallel to the direction of basin extension seismograms for eight deep earthquakes at station sva on viti levu show up to 1 0 s splitting with an average of 0 65 0 29 s and a weak nne preferred orientation of fast velocity polarizations of shear wave particle motions corrected for anisotropy are within 36 5 26 of polarixations predicted for nine moment tensor solutions the magnitude of shearwave splitting and estimates of the thickness of the lithosphere in the fiji region imply that the anisotropy cannot be contained in the lithosphere alone either the anisotropy is distributed in both the lithosphere and asthenosphere or the asthenosphere alone is anisotropic

evidence for mechanically coupled lithosphere in central asia and resulting implications the recent dramatic increase in seismic anisotropy and surface global positioning system gps data for central asia permits a comprehensive examination of the mantle s role in mountain building a joint analysis of 178 shear wave splitting and 2000 gps observations using a new technique reveals that the crust and lithospheric mantle deform coherently arguing for crust mantle mechanical coupling during deformation the observed spatial variations in anisotropy refl ect the large scale pattern of lithospheric deformation as well as a change in deformational style from simple shear on the tibetan plateau transitioning to pure shear in surrounding regions

anisotropy beneath an active collision orogen of taiwan results from across islands array observations 1 we examined shear wave splitting in sks phases from a large event recorded by a temporary array across southern taiwan the span extends from the western plain to the east coast we applied particle motion and cross correlation methods to estimate its polarization directions and delay times analysis shows clear evidence of splitting except stations on the east coast the fast split shear wave along this transect are approximately parallel to the strike of the mountain belt ne sw the delay times show a short wavelength variation and are well correlated to the surface geology the largest split time is about 1 6 sec located at eastern central mountain range and indicates its possible mantle origin implications suggest that anisotropy is related to the collision tectonics which built the taiwan islands and that this tectonic compression involved the lithosphere and is characterized by a strong coherent deformation of the upper mantle and the crust

the taiwan ryukyu subduction collision complex folding of a viscoelastic slab and the double seismic zone 1 the termination of the ryukyu trench against eurasia and the oblique subduction of the philippine sea plate create a subduction collision complex offshore taiwan which has not previously been elucidated in detail we combine traveltime data from the seismic networks in taiwan and japan to better illuminate how the subducting ryukyu slab deforms in this subduction collision zone more than 5000 events recorded by both networks were relocated with the double difference method using an optimal regional one dimensional velocity model the offshore seismicity indicates that the double seismic zone with a gap of 15 20 km exists in the subducting slab in the depth range of 40 80 km focal mechanisms suggest that the double seismic zone is caused by east west compression resulting from oblique convergence the improved hypocentral locations for the first time reveal folding of the slab into a horizontal curvature larger in magnitude than and opposite in sign to that of the ryukyu trench in the depth range 50 100 km the anomalous curvature together with the focal mechanisms suggests that the slab folds against the eurasian lithosphere and that this deformation cannot be fully elastic we model this deformation mode as the developing instability of a viscoelastic maxwell layer embedded in a viscous medium the characteristic wavelength of the instability i e 250 km is consistent with folding of a slab whose viscosity is 100 times higher than that of the surrounding mantle for an along strike elastic membrane strain as small as 0 01 or more than 3 orders of magnitude higher if 5 elastic strain is allowed

geological and industrial implications of extensive dilatancy anisotropy extensive dilatancy anisotropy is the hypothesized distribution of stress aligned fluid filled microcracks pervading most rocks in the earth s crust the geometry of the cracks and the aligning stress field can be monitored by analysing the waveforms of shear waves propagating through the rockmass the ability to estimate some of the parameters of the crack and stress geometry by analysing shear wave particle displacements has widespread implications

upper mantle seismic anisotropy at a strike slip boundary south island new zealand new shear wave splitting measurements made from stations onshore and offshore the south island of new zealand show a zone of anisotropy 100 200 km wide measurements in central south island and up to approximately 100 km offshore from the west coast yield orientations of the fast quasi shear wave nearly parallel to relative plate motion with increased obliquity to this orientation observed farther from shore on the eastern side of the island fast orientations rotate counterclockwise to become nearly perpendicular to the orientation of relative plate motion approximately 200 km off the east coast uniform delay times between the fast and slow quasi shear waves of nearly 2 0 s onshore continue to stations approximately 100 km off the west coast after which they decrease to 1 s at 200 km stations more than 300 km from the west coast show little to no splitting east coast stations have delay times around 1 s simple strain fields calculated from a thin viscous sheet model representing distributed lithospheric deformation with strain rates decreasing exponentially to both the northwest and southeast with e folding dimensions of 25 35 km approximately 75 of the deformation within a zone 100 140 km wide match orientations and amounts of observed splitting a model of deformation localized in the lithosphere and then spreading out in the asthenosphere also yields predictions consistent with observed splitting if at depths of 100 130 km below the lithosphere typical grain sizes are 6 7 mm

shear wave splitting at the edge of the ryukyu subduction zone abstract intraslab events recorded by ocean bottom seismometers in the okinawa trough provide an extended depiction of shear wave splitting in the southwest section of the ryukyu subduction zone at 100 200 km from the western edge of the subduction system we observed trench normal fast polarization direction in the back arc compatible with 2d slab or rifting driven corner flow towards the edge the fast directions are sub parallel to the trench in the arc back arc region and rotate to trench normal within 50 km of the edge splitting constrained by land stations with paths mostly in the mantle wedge exhibits similar trench normal fast directions in the subduction edge zone further inland the dominant component of fast directions becomes roughly parallel to the taiwan orogenic fabric splitting of p to s phases converted at the moho of the okinawa trough and of s phases from shallow events suggest that crustal anisotropy may affect the measured splitting but the observed pattern reflects predominantly mantle anisotropy the variation in splitting along the okinawa trough cannot be explained by a b type a type olivine fabric transition in the mantle wedge it may indicate the presence of an along arc flow in the mantle wedge towards the edge where it is blocked and deflected by the eurasian lithosphere this scenario bolsters previous studies suggesting a significant impact of eurasian lithosphere on the dynamics of the ryukyu subduction system

reconciling surface plate motions with rapid three dimensional mantle flow around a slab edge a central axiom of the theory of plate tectonics is that the surface plates are coupled to and carried along by convection in the earth s mantle the situation is rather different at subduction zones however margarete jadamec and magali billen now present calculations that quantify this variance they use three dimensional numerical models of buoyancy driven deformation with realistic slab geometry for the alaska subduction transform system to determine the surrounding mantle flow field they show that near a subduction zone edge mantle flow velocities can have magnitudes of greater than ten times the surface plate motions and that the complex mantle flow field is consistent with observations from seismic anisotropy these results imply local rapid transport of geochemical signatures through subduction zones the direction of tectonic plate motion at the earth s surface and the flow field of the mantle inferred from seismic anisotropy are well correlated globally implying large scale coupling between the mantle and surface plates but the fit is poor at subduction zones here three dimensional numerical models of buoyancy driven deformation for the alaska subduction transform system are used to find the origin of the regional decoupling of flow the results imply local rapid transport of geochemical signatures through subduction zones the direction of tectonic plate motion at the earth s surface and the flow field of the mantle inferred from seismic anisotropy are well correlated globally suggesting large scale coupling between the mantle and the surface plates1 2 the fit is typically poor at subduction zones however where regional observations of seismic anisotropy suggest that the direction of mantle flow is not parallel to3 4 5 6 7 and may be several times faster than6 plate motions here we present three dimensional numerical models of buoyancy driven deformation with realistic slab geometry for the alaska subduction transform system and use them to determine the origin of this regional decoupling of flow we find that near a subduction zone edge mantle flow velocities can have magnitudes of more than ten times the surface plate motions whereas surface plate velocities are consistent with plate motions8 and the complex mantle flow field is consistent with observations from seismic anisotropy5 the seismic anisotropy observations constrain the shape of the eastern slab edge and require non newtonian mantle rheology the incorporation of the non newtonian viscosity9 10 results in mantle viscosities of 1017 to 1018 pa s in regions of high strain rate 10 12 s 1 and this low viscosity enables the mantle flow field to decouple partially from the motion of the surface plates these results imply local rapid transport of geochemical signatures through subduction zones and that the internal deformation of slabs decreases the slab pull force available to drive subducting plates

sks skks splitting and taiwan orogeny 1 we determine 58 sets of splitting parameters fast direction and delay time t from broadband sks skks waves recorded at temporary and permanent stations in taiwan these and earlier results from s and scs as well as sks show an island wide pattern of alignment with n nne structural trends in southern and central taiwan and then turn smoothly to ew following the structural bight in northern taiwan remarkable differences in t for eastern and southwestern events are found resulting probably from a more anisotropic upper mantle in the east under the central range with t an average of 1 3 seconds our observations are consistent with the hypothesis that the orogeny involves shear or flow in the upper mantle in central and southern taiwan in northern taiwan near the ends of the collision and subduction zones the extrusion and the hydration of olivine may contribute to the orientation of

correlated crust and mantle strain fields in tibet vertical to subvertical planes of shear within the active crustal deformation field in tibet align with fast directions of shear wave polarization this observation suggests that the present day velocity gradient tensor field within the crust of tibet correlates with the lithospheric mantle velocity gradient tensor field beneath tibet this inference requires the following 1 the 100 axes of olivine are aligned within established zones of lithospheric mantle fabric or anisotropy built up through finite strain 2 present day shear in the mantle lithosphere is occurring within these established zones of fabric parallel to the direction of olivine 100 axis alignment 3 the zones of mantle lithospheric fabric or zones of mantle shear are aligned with zones of crustal shear faults the correlation of crustal and mantle strain fields most simply results from the fact that both crust and mantle lithosphere are under the influence of similar velocity boundary conditions furthermore the observations confirm the distributed nature of lithospheric mantle deformation beneath east central tibet and suggest that left lateral shear has been the dominant component of finite shear there

the dynamics of western north america stress magnitudes and the relative role of gravitational potential energy plate interaction at the boundary and basal tractions summary we investigate the forces involved in driving long term large scale continental deformation in western north america and quantify the vertically averaged deviatoric stress field arising from internal buoyancy forces and the accommodation of relative plate motions in addition we investigate the ability of regional models to resolve the level of tractions acting at the base of the lithosphere we directly solve force balance equations for vertically averaged deviatoric stresses associated with differences in values of 1 lithospheric thickness times the gravitational potential energy per unit area gpe the gpe values are inferred using both the etopo5 topographic data set and the crust2 0 crustal thickness model deviatoric stresses associated with basal tractions are calculated globally with inputs determined from an isoviscous upper mantle 10 21 pa s 3 d large scale convection model in which mantle density variations were inferred from tomographic data and the history of subduction in a 211parameter iterative inversion we then solve for a stress field boundary condition by fitting stress field indicators i e the directions and relative magnitudes of the principal axes of kinematic strain rates magnitudes of the total vertically averaged deviatoric stress field sum of gpe solution with the boundary condition solution range from 5 to 10 mpa within a 100 km thick lithosphere these magnitudes are calibrated by the gpe differences along with the spatial variation in deformation style there is a trade off between the scaling of the basal traction deviatoric stress field and the boundary condition solution however the combined boundary conditions plus basal traction solution is robust in both magnitude and style and when added to the contribution from gpe differences provides a global minimum of misfit between the total deviatoric stress solution and the stress field indicators gpe variations account for 50 per cent of the deviatoric stress magnitudes driving deformation while boundary condition stresses account for the remaining 50 per cent of deviatoric stress magnitude by comparing possible end member strength profiles with our vertically averaged deviatoric stresses we infer that the bulk of the strength within the lithosphere in western north america lies within the brittle seismogenic layer

subsurface imaging taiger experiments and tectonic models of taiwan abstract the seismicity deformation rates and associated erosion in the taiwan region clearly demonstrate that plate tectonic and orogenic activities are at a high level major geologic units can be neatly placed in the plate tectonic context albeit critical mapping in specific areas is still needed but the key processes involved in the building of the island remain under discussion of the two plates in the vicinity of taiwan the philippine sea plate psp is oceanic in its origin while the eurasian plate eup is comprised partly of the asian continental lithosphere and partly of the transitional lithosphere of the south china sea basin it is unanimously agreed that the collision of psp and eu is the cause of the taiwan orogeny but several models of the underlying geological processes have been proposed each with its own evolutionary history and implied subsurface tectonics taiger taiwan integrated geodynamics research crustal and mantle imaging experiments recently made possible a new round of testing and elucidation the new seismic tomography resolved structures under and offshore of taiwan to a depth of about 200 km in the upper mantle the steeply east dipping high velocity anomalies from southern to central taiwan are clear but only the extreme southern part is associated with seismicity toward the north the seismicity disappears the crustal root under the central range is strongly asymmetrical using 7 5 km s as a guide the steep west dipping face on the east stands in sharp contrast to a gradual east dipping face on the west a smaller root exists under the coastal range or slightly to the east of it between these two roots lies a well delineated high velocity rise spanning the length from hualien to taitung the 3 d variations in crustal and mantle structures parallel to the trend of the island are closely correlated with the plate tectonic framework of taiwan the crust is thickest in the central taiwan collision zone and although it thins toward the south the crust is over 30 km thick over the subduction in the south in northern taiwan the northward subducting psp collides with taiwan and the crust thins under northern taiwan where the subducting indenter reaches 50 km in depth the low vp vs ratio of around 1 6 at a mid crustal depth of 25 km in the central range indicates that current temperatures could exceed 700 c the remarkable thickening of the crust under the central range its rapid uplift without significant seismicity its deep exhumation and its thermal state contribute to make it the core of orogenic activities on taiwan island the expanded network during the taiger deployment captured broadband seismic data yielding enhanced s splitting results with mainly sks skks data the polarization directions of the fast s waves follow very closely the structural trends of the island supporting the concept of a vertically coherent taiwan orogeny in the outer few hundred kilometers of the earth

origin of crustal anisotropy shear wave splitting studies in japan shear wave splitting manifested as leading shear wave polarization that is parallel alignment of leading shear wave particle motions from a variety of sources has been observed at a number of seismograph stations in japan detected on shear wave seismograms from crustal earthquakes over a wide range of source zones and source receiver azimuths the shear wave splitting can be attributed to crustal anisotropy this paper discusses the relation between leading shear wave polarization directions and tectonic features of japan to explain the observed shear wave splitting the author proposes that at least three phenomena should be taken into account stress induced microcracks primarily aligned in vertical or subvertical planes cracks or fractures in the vicinity of active faults having their orientation parallel to the fault planes and intrinsic rock anisotropy resulting from preferred orientation of minerals travel time differences between leading and slower split shear waves from crustal and upper mantle earthquakes analyzed for about one third of the stations suggest that the crustal anisotropy which causes the observed shear wave splitting may be limited to the upper 15 25 km this implies that the density of nonhorizontally aligned cracks or fractures below 15 25 km and into the upper mantle is muchmore smaller than that in the crust above 15 25 km less

evidence for early cretaceous oceanic crust trapped in the philippine sea plate abstract the huatung basin is a small oceanic basin located east of taiwan previous age estimates from magnetic lineation studies indicated an eocene age for the basin and formation from the central basin spreading center of the west philippine basin during the last phase of spreading in middle eocene new ar ar ages obtained on gabbros dredged on oceanic basement highs of the huatung basin are early cretaceous these old ages are consistent with early cretaceous ages determined on radiolarian assemblages from lanyu island luzon arc we have performed magnetic anomalies modeling for an early cretaceous oceanic crust our results are in good agreement with new ar ar ages determinations the best fit is indeed obtained with an opening of the huatung basin during the early cretaceous from 131 to 119 ma with a half spreading rate varying between 25 and 30 mm yr the spreading center appears to be located south of the actual basin the abnormal depth 5500 m instead of 5900 m and thickness 12 km instead of 6 km of the crust beneath the basin indicate that there was probably an excess supply of magma during its formation we propose that the basin is a fragment of the former proto south china sea or possibly the new guinea basin that has been trapped by the philippine sea plate

trench parallel flow in the southern ryukyu subduction system effects of progressive rifting of the overriding plate 1 the okinawa trough in the ryukyu subduction system is one of a few actively extending back arc basins within the continental lithosphere recent shear wave splitting measurements show variable fast directions along the trough suggesting a complex three dimensional mantle flow field in this study we use numerical subduction models to demonstrate that the thickness variations of the continental lithosphere bounding the edge of a subduction zone can result in complicated mantle circulation and regional dynamics the model results for the southern ryukyu show a combination of two effects along the okinawa trough the increasing thickness of the overriding eurasian plate toward taiwan due to a gradually diminishing rifting induces pressure gradients that drive trench parallel flow to the edge in the shallow portion of the mantle wedge and the thick eurasian lithosphere to the west effectively blocks and diverts this along arc flow and limits the toroidal flow around the slab edge below it the toroidal flow thus enters the mantle wedge at depths of more than about 100 km opposite in direction with and largely below the along arc flow these combined geometry effects of the eurasian lithosphere create an intricate three dimensional flow structure at the southern edge of the ryukyu subduction zone model predictions for lattice preferred orientations of olivine aggregates show rotation patterns that agree with the observed shear wave splitting patterns this three dimensional scenario echoes the geochemical and seismological evidence worldwide that indicates complex depth varying mantle circulations in subduction systems

tomography of the westernmost ryukyu subduction zone and the serpentinization of the fore arc mantle 1 three dimensional tomographic images of the subducting philippine sea slab and the fore arc mantle beneath ne taiwan and the westernmost ryukyu were generated in this study more than 5600 events recorded simultaneously by seismic networks in taiwan and japan were relocated for the inversion for variations in vp vs and vp vs analysis of the trade off between the data variance reduction and model variance helps to determine the appropriate strictness for regularization to avoid either overinterpretatioin or underinterpretation of data the regularization parameters were also chosen to ensure suppression of artificial vp vs anomalies the subducting slab is characterized by high vp high vs and intermediate to low vp vs notable in the mantle wedge is the high vp vs anomalies that abut the surface of the subducting slab at depths of 30 80 km the previously identified positive vp vs channel connecting the slab and the arc volcano interpreted to be a melt pathway is not reproduced in this study we convert vs and vp vs to perturbations of temperature and serpentinization of the mantle the slab is cooled by 200 c 400 c relative to the mantle in accord with the estimates from theoretical modeling of subduction the serpentinization reaches 15 or 2 water content at 50 km depth in the fore arc mantle we interpret the peak serpentinization as hydrated by the water released from the basalt eclogite metamorphic reaction in the oceanic crust of the subducting philippine sea plate the spatial limitation of the present seismic networks in this region with respect to subduction zone events hinders a full description of the pattern of melting in much of the mantle wedge resolution tests of the tomographic inversion provide a basis to guide our interpretation to better resolved regions

dynamic interaction of cold anomalies with the mid ocean ridge flow field and its implications for the australian antarctic discordance abstract negative thermal anomalies beneath a mid ocean ridge are dynamically isolated from the ambient upwelling and diverging flow field in the asthenosphere whose viscosity is on the order of 5 10 19 pa s or less this study examines on what condition a near ridge cold anomaly ascends with the upwelling passive flow and spread off axis three dimensional numerical modeling demonstrates that for a given magnitude of the cold anomaly the viscosity of the asthenosphere the spreading rate and the interference from continental rifting are the dominant controlling factors to the ascent descent of the anomaly to overcome the weight of such an anomaly and couple it with the upwelling either the spreading rate or the asthenospheric viscosity has to be high in a low viscosity asthenosphere the cold anomaly also ascends during the early stage of continental rifting due to the enhanced upwelling induced by the thick continental lithosphere the dynamic interaction between the cold anomaly and the ambient flow renders a transient nature of the subsidence of the seafloor which may lead to exaggerated temperature variation estimated by using a conduction model alone the scenarios examined are employed to place a constraint on dynamic models recently proposed for the australian antarctic discordance in which the source of the negative anomaly is hypothesized to be deeply rooted in the upper mantle with the asthenospheric viscosity less than 10 20 pa s the upwelling of the cooler material from great depths which causes a significant topographic low at the discordance is made possible only by rifting of the australian continent off the gondwanaland

evidence of a slab of subducted lithosphere beneath central taiwan from seismic waveforms and travel times abstract knowing whether there is a slab of subducted lithosphere beneath central taiwan is important for our understanding of the regional tectonic evolution in this study we aim at resolving this issue by investigating the seismic effects of a potential near receiver slab using data from a local broadband array rays from deep and intermediate depth earthquakes in the tonga kermadec subduction zone to central taiwan stations exhibit reduced amplitudes and travel times relative to kmnb in the 0 1 0 4 hz band with a correlation between the degree of reduction and earthquake latitudes the overall amplitude reduction observed at central taiwan stations is not observed for earthquakes from the hindu kush our analysis suggests that the observations are not caused by crustal structure but by receiver side heterogeneous mantle or more specifically an eastern dipping aseismic slab beneath central taiwan the extent of the slab is determined by jointly modeling amplitude and travel time observations at sslb and tpub using a 2 d pseudospectral method

investigating the lithospheric velocity structures beneath the taiwan region by nonlinear joint inversion of local and teleseismic p wave data slab continuity and deflection the interaction between two flipping subduction systems shapes the complicated lithospheric structures and dynamics around the taiwan region whether and in what form the eurasian plate subducts deforms under taiwan island is critical to the debate of tectonic models although an east dipping high velocity anomaly down to a depth below 200 km has been reported previously its detailed morphology remains uncertain and leads to different interpretations with a two step strategy of nonlinear joint inversion the slab images of the eurasian plate were retrieved in a geometry that is hyperthin in the south becoming massive and steeper in the central and severely deformed in the north the possible depth and dimension of a slab break were also investigated through synthetic tests of whether the slab had torn moreover the slab deflection found at 23 2 n latitude seems to correspond to where the nonvolcanic tremors and recent nw se striking structures have occurred in southern taiwan

controls on the evolution of passive margin salt basins structure and evolution of the salina del bravo region northeastern mexico we would like to thank cgg and tgs for permission to use seismic data nancy cottington drafted the diagrams cari breton constructed the geographic information system database eva silverfine ott edited the text the paper was greatly improved by reviews from chris jackson and an anonymous reviewer the project was funded by the applied geodynamics laboratory consortium consisting of the following companies anadarko bhp billiton bp cgg chevron condor ecopetrol emgs eni equinor exxonmobil fieldwood hess ion geophysical midland valley murphy nexen noble petrobras petronas pgs repsol rockfield saudi aramco shell spectrum geo stone talos tgs total west erngeco and woodside additional funding came from the jackson school of geosciences juan i soto also acknowledges the financial support provided by the ministry of education in spain with a salvador de madariaga fellowship publication was authorized by the director of the bureau of economic geology jackson school of geosciences the university of texas at austin

significant and vertically coherent seismic anisotropy beneath eastern tibet 1 shear wave splitting measurements using teleseismic sks and skks phases recorded by the indepth iv arrays has revealed a strong upper mantle anisotropic fabric in northeastern tibet with large delay times of up to 2 2 s suggesting that anisotropy exists in both the lithospheric and asthenospheric mantle the coherence among fast polarization orientations of split core phases and the left lateral slip on eastern striking southern striking faults in eastern tibet and the surface deformation fields calculated from both gps observations and quaternary fault slip rates support the idea that left lateral shear strain is the predominant cause of the orientation of the upper mantle petrofabrics we suggest the bending of the eastern himalayan syntaxis around the foundering burma andaman sumatra slab also contributes to the observed seismic anisotropy in the eastern himalayan syntaxis region two plausible competing processes are proposed for the flow of asthenosphere in eastern tibet in the first the deforming lithosphere glides over the passive asthenosphere inducing flow in the asthenospheric mantle in the second the asthenosphere beneath northeastern tibet is squeezed between the advancing indian continental lithosphere and the thick tarim and qaidam lithospheric blocks to the north a westward retreat of the burma slab from eurasia may induce flow that is toroidal and located exclusively around the northern edge of the slab the rotation of fast orientations for stations in the eastern himalayan syntaxis region are consistent with the toroidal flow pattern as well as the rotational deformation of the overlying lithosphere

hudson bay river sediments and regional glaciation ii comparison of carbonate mineralogy of size fractions for ice movement inference comparison of the carbonate mineralogy of sand coarse silt fine silt and coarse clay fractions of river sediments from the hudson bay region demonstrates that dolomite is more abundant than calcite in the coarse silts but is less abundant than calcite in other size fractions in coarse silts dolomite commonly occurs as single crystals in the shape of well defined rhombs whereas calcite is present as equant to subequant grains few rhomb shaped crystals dolomite grains appear to have greater resistance to abrasion than calcite grains after particles have been reduced to sizes lying in the coarse silt range although differing physical properties may play a role it appears likely that the primary control on the particle size distributions of the carbonate minerals is their original crystal size in the source bedrock the calcite dolomite ratio for dolomite enriched coarse silt parallels the regional distribution observed for sand fine silt and coarse clay with highest values occurring in the cape

a community benchmark for subduction zone modeling we have developed a suite of benchmarks to facilitate the comparison of numerical models for the dynamics and thermal structure of subduction zones the benchmark cases are based on a thermomechanical approach in which the slab is prescribed kinematically and the wedge flow is computed dynamically we propose various cases to investigate the influence of boundary conditions and rheology on wedge flow and resulting thermal structure a comparison between the codes suggest that accurate modeling of the thermal field requires a good implementation of the velocity discontinuity along the seismogenic zone and high resolution in the thermal boundary layers a minor modification to the boundary conditions of the wedge flow is also necessary to avoid a pressure singularity that exists in analytical solutions of the cornerflow model

isostatic response of the australian lithosphere estimation of effective elastic thickness and anisotropy using multitaper spectral analysis gravity and topography provide important insights regarding the degree and mechanisms of isostatic compensation the azimuthally isotropic coherence function be tween the bouguer gravity anomaly and topography evolves from high to low for increasing wavenumber a diagnostic that can be predicted for a variety of lithospheric loading models and used in inversions for flexural rigidity thereof in this study we investigate the isostatic response of continental australia we consider the effects of directionally anisotropic plate strength on the coherelce the anisotropic coherence function is calculated for regions of australia that have distinctive geological and geophysical properties the coherence estimation is performed by the thomson multiple slepian taper spectral analysis method extended to two dimensional fields our analysis reveals the existence of flexural anisotropy in central australia indicative of a weaker n s direction of lower te this observation is consistent with the suggestion that the parallel faults in that area act to make the lithosphere weaker in the direction perpendicular to them it can also be related to the n s direction of maximum stress and possibly the presence of e w running zones weakened due to differential sediment burial rates we also demonstrate that the multitaper method has distinct advantages for computing the isotropic coherence function the ability to make many independent estimates of the isostatic response that are minimally affected by spectral leakage results in a coherence that is more robust than with modified periodogram methods particularly at low wavenumbers our analysis elucidates the reasons for discrepancies in previous estimates of effective elastic thickness te of the australian lithosphere in isotropic inversions for te we obtain values that are as much as a factor of 2 less than those obtained in standard inversions of the periodogram coherence using bouguer gravity and topography but greater than those obtained by inversions that utilize free air rather than bouguer gravity and ignore the presence of subsurface loads however owing to the low spectral power of the australian topography the uncertainty on any estimate of te is substantial

two station measurement of rayleigh wave phase velocities for the huatung basin the westernmost philippine sea with obs implications for regional tectonics summary a broad band ocean bottom seismometer obs deployed 180 km east of taiwan provides a first glimpse into the upper mantle beneath the westernmost section of the philippine sea or the huatung basin hb we measured interstation phase velocities of rayleigh waves between the obs and stations on the eastern coast of taiwan the phase velocities show smooth variations from 3 8 to 3 9 km s 1 for periods of 25 40 s in this short period range phase velocities are comparable to those characterizing the 15 30 ma parece vela basin of the philippine sea modelling of the finite frequency effect proves the validity of the measurement for the average hb the shear wave velocity models inverted from the 25 to 40 s dispersion show a velocity at lithospheric depths about 0 1 km s 1 lower than that of the west philippine sea which agrees with the age effect derived from the pacific pure path model inversions incorporating the less reliable data above 40 s yield a shear velocity 4 0 km s 1 below 150 km an unrealistic value even for a hotspot plume environment the seismological evidence together with the correlation in seafloor depth suggests that the hb and the parece vela basin may have a similar age this is at odds with the previous geochronological study suggesting an early cretaceous age for the hb thermal rejuvenation of the lithosphere was examined as a potential solution to reconciling the two age models

layered deformation in the taiwan orogen the underthrusting of continental crust during mountain building is an issue of debate for orogens at convergent continental margins we report three dimensional seismic anisotropic tomography of taiwan that shows a nearly 90 rotation of anisotropic fabrics across a 10 to 20 kilometer depth consistent with the presence of two layers of deformation the upper crust is dominated by collision related compressional deformation whereas the lower crust of taiwan mostly the crust of the subducted eurasian plate is dominated by convergence parallel shear deformation we interpret this lower crustal shearing as driven by the continuous sinking of the eurasian mantle lithosphere when the surface of the subducted plate is coupled with the orogen the two layer deformation clearly defines the role of subduction in the formation of the taiwan mountain belt

seismic shear wave splitting in upper crust characterized by taiwan tectonic convergence summary this study conducts a comprehensive investigation of crustal seismic anisotropy over varied geological regimes of taiwan with a large amount of earthquake data the lateral variation of seismic shear wave splitting sws is fully examined in terms of crustal deformation process as the well known vigorous orogeny subjected to the eurasian philippine plate collision tectonic convergence of taiwan is presumably propagating from east to west the acquired sws waveform data cover areas from the slightly deformed western plains to the intermediate to high metamorphic western foothills and central mountain ranges by means of waveform cross correlation the sws parameters the fast wave polarization orientation and delay time infer that the mechanism of lithologic deformation of taiwan convergence can be classified into two domains the convergence parallel laminating west of the deformation front and the convergence perpendicular striking east of the deformation front the convergence parallel sws measurement presents the internal fabrics consisting of microfractures subject to lateral compression before the yielding of the lithologic strength whereas the convergence perpendicular measurements reveal the lateral accommodation of deformation as the stress strain surpass the yielding strength of rock where the predominant sws polarization is in the ne sw direction similar to the general trend of taiwan s mountain ranges it is remarkable that there is no correlation between metamorphic degrees with sws parameters the geological province which corresponds to higher metamorphism is not consistent with large sws parameters this may be because of anisotropic weakness caused by multiple tectonic processes at considerable metamorphic zone furthermore comparison of the sws delay times with corresponding focal depths suggests that seismic anisotropy in the upper crust may come from multiple layers and the fabric lamination causing the anisotropy may be confined only within the shallow crust

split s waveforms observed in northern taiwan implications for crustal anisotropy short period stations ent and nsk in northern taiwan have frequently recorded split s waveforms from earthquakes in the underlying subduction zone to determine splitting parameters together with objective straightforward error estimations we employ a waveform cross correlation method the error for each measurement is estimated by translating the lower limit of the 95 confidence interval of the cross correlation coefficient into azimuth and time only measurements with 95 confidence regions excluding zero delay time are accepted as splitting data we also use an energy minimization method to constrain the solutions with minor splitting we interpret the splitting as caused by stress aligned cracks in the upper crust the average delay times are 0 03 0 04 s for ent and 0 07 s for nsk the fast directions at the two stations are predominantly n s which roughly match the stress trajectories predicted by a model simulating the arc continent collision in the taiwan area however at ent there is a subset of data with fast directions oriented in roughly e w the nearly 90 rotation of the inferred crack orientation occurs in a horizontal distance of 10 15 km in the upper crust both the predicted heterogeneous stress regime and the local structural setting in northeastern taiwan favor a lateral variation with a similar length scale

defining a proxy for the interpretation of seismic anisotropy in non newtonian mantle flows seismic anisotropy can provide unique insights on convection in the upper mantle here we study the link between seismic anisotropy and mantle flow using a non newtonian rheology consistent with deformation by dislocation creep using analytical first order flow models underneath a ridge and in subduction zones we find that finite strain ellipsoid fse is a robust proxy of seismic anisotropy both in terms of orientation and strength for natural strains smaller than 1 at larger strains anisotropy aligns with the infinite strain axis isa defined as the orientation of the long axis of the fse in the limit of infinite strain and its percentage reaches a plateau anisotropy aligns with the flow direction only when the product of the inverse strain rate with the timescale of isa rotation within the flow is smaller than 0 1

shear wave anisotropy beneath the taiwan orogen abstract shear wave splitting in regional and teleseismic shear waves scs and s has been analyzed for fast polarization directions and delay times at 33 short period and 17 broad band permanent and temporary seismic stations in the taiwan region the orientations of anisotropy for the taiwan stations are generally parallel to the strike of the mountain belt with a clockwise rotation following the trend of regional geologic fabric in north taiwan the delay times on the other hand vary greatly from 0 5 to 2 1 s the largest values are from the southwestern taiwan foothills and an abrupt change in delay times is found in south taiwan the main source of shear wave splitting in taiwan is compatible with an anisotropic region between 25 and 230 km depths the observed splitting parameters are consistent with upper mantle anisotropy resulting from the collisional tectonics that built the taiwan orogen

vertical coherence of deformation in lithosphere in the eastern himalayan syntaxis using gps quaternary fault slip rates and shear wave splitting data we present 59 new sks skks and combine them with 69 previously published data to infer the mantle deformation field in se tibet the dense set of anisotropy measurements in the eastern himalayan syntaxis ehs are oriented along a ne sw azimuth and rotate clockwise in the surround regions we use gps measurements and geologic data to determine a continuous surface deformation field that is then used to predict shear wave spitting directions at each station comparison of splitting observations with predictions yields an average misfit of 11 7 illustrating that deformation is vertically coherent consistent with previous studies within the central ehs in areas directly surrounding the namche barwa metamorphic massif the average misfit of 11 stations increases to 60 8 and vertical coherence is no longer present the complexity of the mantle anisotropy and surface observations argues for local alteration of the strain fields here associated with recent rapid exhumation of the indian crust

amphibole and lower crustal seismic properties abstract measurements of seismic anisotropy frequently offer insights of the deformation kinematics in situ within the earth in the continental crust seismic anisotropy is commonly ascribed to regionally aligned lattice preferred orientation lpo of mica crystals however determinations of composition suggest that the deep continental crust contains rather little mica in this contribution the role of amphibole a far more plausible constituent especially for deep continental crust of basic composition is assessed a field analogue approach is adopted where the lpo are measured correlated with the strain state of the rocks and used to calculate the seismic properties the seismic properties may be up scaled through the specimen using the modal proportions of the constituent mineral phases to examine the role of composition in controlling the intensity of seismic anisotropy seismic properties are computed using an array of modal compositions so called rock recipes the case history comes from the central block of the lewisian of nw scotland where a regional basic dyke swarm the scourie dykes is locally deformed within amphibolite facies shear zones conditions that are representative of the deep crust shear strain profiles across a 100 m wide shear zone using deflected banding in the host gneisses yield values of 0

seismic anisotropy beneath the continents probing the depths of geology seismic anisotropy beneath continents is analyzed from shear wave splitting recorded at more than 300 continental seismic stations anisotropy is found to be a ubiquitous property that is due to mantle deformation from past and present orogenic activity the observed coherence with crustal deformation implies that the mantle plays a major if not dominant role in orogenies no evidence is found for a continental asthenospheric decoupling zone suggesting that continents are coupled to general mantle circulation

the fracture criticality of crustal rocks summary the shear wave splitting observed along almost all shear wave ray paths in the earth s crust is interpreted as the effects of stress aligned fluid filled cracks microcracks and preferentially oriented pore space once away from the free surface where open joints and fractures may lead to strong anisotropy of 10 per cent or greater intact ostensibly unfractured crustal rock exhibits a limited range of shear wave splitting from about 1 5 to 4 5 per cent differential shear wave velocity anisotropy interpreting this velocity anisotropy as normalized crack densities a factor of less than two in crack radius covers the range from the minimum 1 5 per cent anisotropy observed in intact rock to the 10 per cent observed in heavily cracked almost disaggregated near surface rocks this narrow range of crack dimensions and the pronounced effect on rock cohesion suggests that there is a state of fracture criticality at some level of anisotropy between 4 5 and 10 per cent marking the boundary between essentially intact and heavily fractured rock when the level of fracture criticality is exceeded cracking is so severe that there is a breakdown in shear strength the likelihood of progressive fracturing and the dispersal of pore fluids through enhanced permeability the range of normalized crack dimensions below fracture criticality is so small in intact rock that any modification to the crack geometry by even minor changes of conditions or minor deformation particularly in the presence of high pore fluid pressures may change rock from being essentially intact below fracture criticality to heavily fractured above fracture criticality this recognition of the essential compliance of most crustal rocks and its effect on shear wave splitting has implications for monitoring changes in any conditions affecting the rock mass these include monitoring changes in reservoir evolution during hydrocarbon production and enhanced oil recovery and in monitoring changes before and after earthquakes amongst others