thermal structure and seismicity of subducting lithosphere it has recently been suggested that the mechanical behaviour of the oceanic and continental uppermost mantle depends on temperature alone and that there is no strong evidence for compositional effects it seems in the majority of cases that if uppermost mantle temperatures exceed 600 c any deformation that occurs does so in the absence of significant seismicity this observation apparently applies irrespective of whether that mantle material underlies an ocean or a continent this paper investigates whether the same behaviour exists for earthquakes occurring in descending lithospheric slabs namely whether there is some limiting temperature above which deformation occurs without significant seismicity and whether that limiting temperature is consistent with the behaviour seen in the uppermost mantle beneath oceans and continents by re examining the thermal structure in subducting slabs accounting for the temperature dependence of the relevant physical parameters teleseismically recorded seismicity is shown to be limited to material having potential temperatures less than not vert similar570 600 c or homologous temperatures less than not vert similar0 42 the exceptions to this pattern occur in the subhorizontally subducting regions of the nazca plate in which temperatures are probably affected by the relatively cold overriding plate

earthquakes beneath the himalayas and tibet evidence for strong lithospheric mantle eleven intracontinental earthquakes with magnitudes ranging from 4 9 to 6 occurred in the mantle beneath the western himalayan syntaxis the western kunlun mountains and southern tibet near xigaze between 1963 and 1999 high resolution seismic waveforms show that some focal depths exceeded 100 kilometers indicating that these earthquakes occurred in the mantle portion of the lithosphere even though the crust has been thickened there the occurrence of earthquakes in the mantle beneath continental regions where the subduction of oceanic lithosphere ceased tens of millions years ago indicates that the mantle lithosphere is sufficiently strong to accumulate elastic strain

the relationship between crust lithosphere structures and seismicity on the southeastern edge of the tibetan plateau abstract the crust lithosphere structure especially the discontinuities such as moho and lithosphere asthenosphere boundary lab is important in the investigation of geodynamic process implications and tectonic evolution of the lithosphere and regional seismic activity we stacked the s receiver functions from 51 permanent broad band stations to investigate the crust lithosphere structures beneath the southeastern edge of the tibetan plateau and further discussed the mechanism of lower crust earthquakes the regional tectonics deformation characteristics and the relationship between the two discontinuities and seismicity in this region the results show that the moho depth increases from 40 to 52 km beneath the sichuan basin to 56 74 km on the both western side of the longmenshan lms fault and anninhe zemuhe fault the lab depth ranges from 130 to 170 km beneath the sichuan basin and a shallow belt ranges from 100 km to 130 km around the sichuan basin on the moho depth contour map 374 moderate earthquakes that accounted for 72 2 corresponding to the depth range of 44 52 km and 60 68 km on the both sides of a narrow strip longitude 102 104 on the lab depth contour map 321 earthquakes accounted for 62 15 in the depth range of 130 150 km the variation of lab corresponds to the increasing crustal thickness intensive lower crustal earthquakes hypocenters about 60 km and magnitude 4 0 ms 4 9 and surface abrupt elevations it implied that normal strong earthquakes generate the lower crustal earthquakes or aftershocks that resulted in the shear zones where fluid induced metamorphic transformations the eclogitization of dry granulite and further affect the crustal thickness and deformation to control the spatial distribution of deep faults and seismicity

a reduced relevance of vegetation change for alluvial aggradation in arid zones a basic tenet of arid region geomorphology is that a decrease in hillslope vegetation density coupled with increases in monsoonal precipitation after a change to a warmer and drier climate is an essential factor for an increase in sediment yield and fan aggradation over the last two decades vegetative cover change is increasingly cited as a prominent factor in triggering the onset of fan aggradation in deserts of southwestern north america especially in analysis of fan deposition during the late pleistocene holocene lph climatic transition to assess this assertion we compiled paleobotanical and alluvial fan stratigraphy histories from 18 published studies broadly representing four different areas in the mojave and northern sonoran deserts instead of focusing on chronology aggregation and comparison with global or regional climate change proxies forced by orbital parameters we discriminated by altitudinal regions looking for linkages between local vegetation change and alluvial deposition results indicate that the onset of extensive alluvial fan deposition 1 began well before changes in catchment vegetative cover and 2 can occur during several possible combinations of vegetation change the ambiguous relation between vegetation change and alluvial fan aggradation indicates that vegetation had a reduced role in lph aggradation other factors such as local storm intensity and water and sediment redistribution pathways on hillslopes need to be considered in this analysis given the important role that the combined hillslope alluvial system has on arid region ecosystem functions

thermal and tectonic consequences of india underthrusting tibet the tibetan plateau is the largest orogenic system on earth and has been influential in our understanding of how the continental lithosphere deforms beneath the plateau are some of the deepest 100 km earthquakes observed within the continental lithosphere which have been pivotal in ongoing debates about the rheology and behaviour of the continents we present new observations of earthquake depths from the region and use thermal models to suggest that all of them occur in material at temperatures of t 600 1c thermal modelling combined with experimentally derived flow laws suggests that if the indian lower crust is anhydrous it will remain strong beneath the entire southern half of the tibetan plateau as is also suggested by dynamic models in northwest tibet the strong underthrust indian lower crust abuts the rigid tarim basin and may be responsible for both the clockwise rotation of tarim relative to stable eurasia and the gradient of shortening along the tien shan

seismicity and one dimensional velocity structure of the himalayan collision zone earthquakes in the crust and upper mantle 1 earthquakes beneath the himalayan collision zone occur at depths between near surface and around 100 km below sea level after relocating earthquakes with two one dimensional 1 d velocity models we found a clear bimodal depth distribution for earthquakes in the himalayas of eastern nepal and the southern tibetan plateau and evidence that some earthquakes originate at upper mantle depths seismicity in nepal shows an accumulation of earthquakes along the front of the himalayan arc with a seismic gap between longitudes 87 3 e and 87 7 e although upper crustal seismicity along the topographic front of the high himalaya is consistent with a region of high strain accumulation associated with convergence on the main himalayan thrust fault microearthquakes do not necessarily occur on this fault instead they concentrate in the hanging wall seismic activity in the sub himalaya and the terai plains is almost exclusively limited to the vicinity of the location of the magnitude 6 5 20 august 1988 udayapur earthquake with most of the earthquakes in the lower crust and the upper mantle clusters of earthquakes in the lesser and high himalayas and south tibet tethyan himalayas mark very well defined zones of seismicity at depths between 50 and 100 km confirming the presence of earthquakes in the upper mantle in the region of continental collision the occurrence of earthquakes at sub moho depths favors the idea that the continental upper mantle deforms by brittle processes

accurate source depths and focal mechanisms of shallow earthquakes in western south america and in the new hebrides island arc synthetic seismograms are matched to long period p waveforms in order to obtain accurate depths of shallow earthquakes with known focal mechanisms accurate depths are obtained for a large sample of moderate sized 6 ms 7 events which produced relatively simple p waveforms and do not require intensive analysis the estimated error in the determination of depth is about 5 km or less for most of the events considered the procedure is applied to two suites of events one in south america 83 events and one in the new hebrides 61 events in these two areas of contrasting styles of subduction the accurate depths provide new information on the bending of descending plates near trenches the seismicity and tectonics of the upper plate and the geometry and structure of the zone of contact between the upper and descending plates depths of suboceanic earthquakes which occurred near the trench and which have either tensional and compressional horizontal stress axes agree well with chapple and forsyth s 1979 preferred model of a bending elastic perfectly plastic plate however an unusually deep event occurred beneath the fiji plateau in an area of young ocean floor supposed to have been formed by sea floor spreading processes only since late miocene time the calculated depth of the event 48 km places it below the lithosphere asthenosphere boundary derived from thermal models vertical cross sections through the shallow parts of the subduction zones show that most of the earthquakes with thrust faulting focal mechanisms can be interpreted to be interplate events located in a thin 10 km thick curved zone of contact dipping arcward or landward from the trench axis however a few events with focal mechanisms similar to the interplate events do not fit on the inferred thin zone of contact these events would be accommodated by an interplate zone 15 km thick in both subduction zones interplate events occur at depths between about 15 and 50 km however the downdip width of the south american interplate zone is 2 to 3 times larger than that in the new hebrides most of the intraplate events near the interplate boundary are located in the descending plate in the upper plate seismicity is more pronounced in the back arc regions of the two subduction zones thus our results support the generality of a relatively aseismic zone in the upper plate located trenchward of the magmatic arc depths of shallow earthquakes in the eastern andes and sub andean regions of south america range between about 9 and 36 km indicating that both the upper and middle crust are seismically active about 75 of the events occurred in the upper 25 km furthermore the focal mechanisms indicate that from ecuador to argentina the crust of the eastern side of the andean cordilleran is in a state of east west compression oriented nearly parallel to the direction of convergence between the nazca and south american plates the depths the orientations of the earthquake nodal planes and the close relationships to quaternary tectonics imply basement involved crustal shortening in much of the eastern andes and sub andean zones of peru and argentina

seismic polarization anisotropy beneath the central tibetan plateau sks and skks shear waves recorded on the indepth iii seismic array deployed in central tibet during 1998 1999 have been analyzed for the direction and extent of seismic polarization anisotropy the 400 km long nnw trending array extended south to north from the central lhasa terrane across the karakoram jiali fault system and banggong nujiang suture to the central qiangtang terrane substantial splitting with delay times from 1 to 2 s and fast directions varying from e w to ne sw was observed for stations in the qiangtang terrane and northernmost lhasa terrane no detectable splitting was observed for stations located farther south in the central lhasa terrane the change in shear wave splitting characteristics occurs at 32 n approximately coincident with the transcurrent karakoram jiali fault system but 40 km south of the surface trace of the banggong nujiang suture this location is also near the southernmost edge of a region of high sn attenuation and low upper mantle velocities found in previous studies the transition between no measured splitting and strong anisotropy 2 2 s delay time is exceptionally sharp 15 km suggesting a large crustal contribution to the measured splitting the e w to ne sw fast directions are broadly similar to the fast directions observed farther east along the yadong golmud highway suggesting that no large scale change in anisotropic properties occurs in the east west direction however in detail fast directions and delay times vary over lateral distances of 100 km in both the n s and e w direction by as much as 40 and 0 5 1 s respectively the onset of measurable splitting at 32 n most likely marks the northern limit of the underthrusting indian lithosphere which is characterized by negligible polarization anisotropy taken in conjunction with decades of geophysical and geological observations in tibet the new anisotropy measurements are consistent with a model where hot and weak upper mantle beneath northern tibet is being squeezed and sheared between the advancing indian lithosphere to the south and the tsaidam and tarim lithospheres to the north and west resulting in eastward flow and possibly thickening and subsequent detachment due to gravitational instability in northern tibet crustal deformation clearly follows this large scale deformation pattern

geodynamic models of continental subduction and obduction of overriding plate forearc oceanic lithosphere on top of continental crust continental subduction takes place in the final stage of subduction when all oceanic lithosphere is consumed and continental passive margin is pulled into the mantle when the overriding plate is oceanic dense forearc oceanic lithosphere might be obducted onto light continental crust forming an ophiolite tethyan style ophiolite obduction four dimensional dynamic analog subduction models have been constructed to evaluate the mechanical feasibility of continental subduction and forearc oceanic lithosphere obduction on top of continental crust the roles of continental crust thickness passive margin length subducting lithosphere thickness and overriding plate thickness were investigated to determine the maximum continental subduction depth maximum forearc obduction distance and forearc deformation during continental subduction our buoyancy driven experiments indicate that deep continental subduction occurs in most circumstances down to 560 km and that obduction of dense oceanic forearc lithosphere on top of light continental crust is mechanically feasible maximum obduction distances are relatively small 26 37 km but are sufficient to explain obduction of short ophiolite sheets such as observed in new caledonia when including the thin 5 10 km thick accretionary wedge of off scraped deep sea sediments oceanic crust and mantle then maximum obduction distances are much larger 60 160 km sufficient to account for the obducted northland allochthon in new zealand results indicate that increasing continental crust thickness decreases continental subduction depth whereas increasing passive margin length and subducting lithosphere thickness increases continental subduction depth notably during continental subduction backarc extension continues while forearc deformation shortening increases moderately compared to the preceding phase of normal oceanic subduction

processes leading to eclogitization densification of subducted and tectonically buried crust 1 the transformation of subducted and tectonically buried crustal rocks to denser eclogite plays a fundamental role in the dynamics of mountain building and crustal recycling however a complex of partially eclogitized granulites on the island of holsnoy in the western part of the norwegian caledonides reveals that such densification depends in part on the antecedent history of rock masses and that large bodies of untransformed rock can persist metastably even after long residence times in the lower crust the rocks on holsnoy suggest that conversion to eclogite was delayed until lower or subcrustal seismic events allowed aqueous fluids to enter the previously dry and unusually strong granulite complex metamorphism then occurred in a spatially heterogeneous manner with several distinct physical processes involving feedbacks between deformation fluid infiltration and chemical reactions operating simultaneously in different parts of the rock mass field relations show that the introduction of fluids and conversion to eclogite significantly weakened the rocks a reservoir flux systems model is used to represent the various processes contributing to eclogitization of the granulites which operated at vastly different rates over disparate intervals of time the model suggests that the metamorphic process may have been brief 1 myr and ultimately self limiting arrested by the change in rheology associated with the conversion of brittle granulite to ductile eclogite

seismic evidence for a moderately thick lithosphere beneath the siberian platform 1 we have built a sv wavespeed tomographic model for the upper mantle beneath the siberian platform and surrounding region derived from the analysis of more than 13 000 fundamental and higher mode regional waveforms the dense path coverage and rich higher mode content of the data allow building an upper mantle image with an horizontal resolution of a few hundred kilometers extending to 400 km depth the high velocity upper mantle lid or seismic lithosphere is 200 km thick beneath most of the siberian platform but may extend to 250 km depth beneath small areas a high velocity seismic lid also underlies a large region west of the siberian platform our observation of a 200 thick seismic lithosphere beneath the siberian platform on the slow moving eurasian plate similar to the thickness of the seismic lithosphere beneath precambrian terrains on the fast moving australian plate suggests that a moderately thick seismic lithosphere beneath precambrian terrains may be more common than previously supposed

interseismic slip rate of the northwestern xianshuihe fault from insar data 1 the xianshuihe fault is a highly active strike slip fault system near the eastern margin of the tibetan plateau we use a multi interferogram method to construct a map of line of sight deformation rate due to interseismic strain accumulation on the northwestern xianshuihe fault from a decade of sar data acquired by the ers 1 2 and envisat satellites the rate map shows a clear deformation gradient associated with the fault but slightly offset to the northeast from the surface trace joint inversion of insar rate map and gps data using monte carlo method we estimate a slip rate of 9 12 mm yr with a locking depth of 3 6 km at the 90 confidence level the slip rate agrees with the holocene fault slip rate and the historical earthquakes our results also suggest no significant across fault extension in the future insar data from both ascending and descending orbits may further constrain the 3 d fault slip rate on this fault

seismicity of the baikal rift system from regional network observations abstract in the paper we report the state of the art of seismicity study in the baikal rift system and the general results obtained at present the regional earthquake catalog for fifty years of the permanent instrumental observations consists of over 185 000 events the spatial distribution of the epicenters which either gather along well delineated belts or in discrete swarms is considered in detail for different areas of the rift system at the same time the hypocenters are poorly constrained making it difficult to identify the fault geometry clustered events like aftershock sequences or earthquake swarms are typical patterns in the region moreover aftershocks of m 4 7 earthquakes make up a quarter of the whole catalog the maximum magnitude of earthquakes recorded instrumentally is mlh7 6 for a strike slip event in the ne part of the baikal rift system and mlh6 8 for a normal fault earthquake in the central part of the rift system lake baikal basin predominant movement type is normal faulting on ne striking faults with a left lateral strike slip component on w e planes in conclusion some shortcomings of the seismic network and data processing are pointed out

thermosphere global time response to geomagnetic storms caused by coronal mass ejections we investigate for the first time with a spatial superposed epoch analysis study the thermosphere global time response to 159 geomagnetic storms caused by coronal mass ejections cmes observed in the solar wind at earth s orbit during the period of september 2001 to september 2011 the thermosphere neutral mass density is obtained from the champ challenge mini satellite payload and grace gravity recovery climate experiment spacecraft all density measurements are intercalibrated against densities computed by the jacchia bowman 2008 empirical model under the regime of very low geomagnetic activity we explore both the effects of the pre cme shock impact on the thermosphere and of the storm main phase onset by taking their times of occurrence as zero epoch times cme impact and interplanetary magnetic field bz southward turning for each storm we find that the shock impact produces quick and transient responses at the two high latitude regions with minimal propagation toward lower latitudes in both cases thermosphere is heated in very high latitude regions within several minutes the bz southward turning of the storm onset has a fast heating manifestation at the two high latitude regions and it takes approximately 3 h for that heating to propagate down to equatorial latitudes and to globalize in the thermosphere this heating propagation is presumably accomplished at least in part with traveling atmospheric disturbances and complex meridional wind structures current models use longer lag times in computing thermosphere density dynamics during storms our results suggest that the thermosphere response time scales are shorter and should be accordingly adjusted in thermospheric empirical models

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

an assessment of the accuracy of admittance and coherence estimates using synthetic data previous work has shown that estimates of the admittance between topography and free air gravity anomalies are often biased by spectral leakage even after the application of multiple prolate spheroidal wavefunction data tapers despite this a number of authors who have used the free air admittance method to estimate the weighted average effective elastic thickness of the lithosphere te and to identify topography supported by mantle convection have not tested their methods using synthetic data with a known relationship between topography and gravity in this paper i perform a range of such tests using both synthetic surface data and synthetic line of sight los accelerations of satellites orbiting around an extra terrestrial planet it is found that spectral leakage can cause the estimated admittance and coherence to be significantly in error but only if the box in which they are estimated is too small the definition of small depends on the redness of the gravity spectrum there is minimal error in the whole box weighted average estimate of te if the admittance between surface gravity and topography is estimated within a box at least 3000 km wide when the synthetic uniform te is less than 20 km and the coherence is high the errors in te are mostly 5 km for all box sizes greater than 1000 km on the other hand when the true te is greater than 20 km and the box size is 1000 km the best fitting te is likely to be at least 5 10 km less than the true te however even when the coherence is high it is not possible to use elastic plate admittance models to distinguish between real and spurious small fractions of internal loading when the boxes are smaller than 2000 km in width noise in the gravity introduces error and uncertainty but no additional bias into the estimates of the admittance it does however bias estimates of te calculated using the coherence between bouguer gravity and topography the admittance at wavelengths between 1000 and 4000 km which is of interest when studying mantle convection is only well resolved when the box size is larger than 2000 km or the synthetic te is less than 15 km and the true long wavelength admittance is greater than 30 mgal km 1 when the box size is 1000 km it is not possible to distinguish between real and spurious convective support of long wavelength topography at the altitude of most orbiting satellites 200 400 km gravity is much redder than it is at ground level therefore it is necessary to use much larger boxes when using los accelerations to estimate the admittance than it is when using surface measurements however gradual changes in the altitude of the satellite or even occasional sudden changes make almost no difference to the estimate of the admittance finally three short investigations are undertaken concerning the effect on the admittance of the application of irregular windows to the data of lateral variations in te and of the infilling of flexural moats by low density sediment it is found that the bias due to the application of an irregular window is not significantly greater than the bias due to the application of a regular window of similar size that the best fitting te is strongly biased by the value associated with the highest amplitude topography and that sediment infill raises the admittance at wavelengths corresponding to the width of the moat

thermal diffusivity and conductivity of olivine wadsleyite and ringwoodite to 20 gpa and 1373 k abstract we present results of lattice thermal diffusivity measurements on mg0 9fe0 1 2sio4 olivine and its high pressure polymorphs wadsleyite and ringwoodite under mantle conditions we used the angstrom method on cylindrical samples in multianvil apparatus at pressures up to 20 gpa and temperatures up to 1373 k because of the fine polycrystallinity of the specimens 30 40 m or less there is strong scattering absorption of light and suppression of radiative transport so that the lattice vibrational component is the dominant heat transfer mode lattice thermal conductivities were calculated from the thermal diffusivity results using heat capacity and equation of state data olivine thermal conductivities are consistent with previous results obtained at 1 atm e g j am ceramic soc 38 1954 107 j geophys res 77 1972 6966 science 283 1999 1699 thermal conductivity increases by approximately 30 at the transition from olivine to wadsleyite corresponding to the 410 km discontinuity and a further but smaller increase may occur at the transition from wadsleyite to ringwoodite for each of these phases lattice conductivity closely follows a t 1 2 dependence on temperature t phys rev 119 1960 507 j geophys res 79 1974 703 brown 2002 if such a dependence applies to other silicates and complex crystals there should be a useful way to estimate conductivities at high temperatures from room temperature measurements

new seismic model of the upper mantle beneath africa we present a seismic model of the upper 400 km of the mantle beneath africa and surrounding regions this model is constructed by inverse modeling of fundamental mode rayleigh wave phase velocities 40 200 s for about 8000 propagation paths among the most pronounced anomalies are high shear velocity structures as much as 6 higher than in the preliminary reference earth model beneath the west african congo and kalahari cratons that extend to about 250 km depth these structures have near vertical margins across which the shear velocity changes by as much as 6 over 500 km distance anomalous low shear velocities 3 4 lower than in the preliminary reference earth model structures are observed beneath the east african red sea and gulf of aden rifts and beneath the northwestern indian ocean these structures extend to a depth of at least 250 km our model cannot be reconciled with models that invoke a large number of plumes that have impinged on the base of the lithosphere nor does our seismic model indicate that high temperature low density material beneath the lithosphere is responsible for the uplift of southern africa

the geodynamic province of transitional lithosphere adjacent to magma poor continental margins two different types of transitional lithosphere have been documented along magma poor rifted margins one consists of apparently subcontinental mantle that has been exhumed brit tlely deformed and serpentinized during late stages of rifting a second is thinned 10 km con tinental crust which in some cases is known to have been supported near sea level at least early in the rift history and thus is interpreted to reflect depth dependent extension in both cases it is typically assumed that oceanic crust forms at the time that the brittle continental crust is breached or soon thereafter and thus that transitional lithosphere has relatively limited width here three representa tive cases of transitional lithosphere are examined one in the newfoundland iberia rift and one at goban spur both exhumed mantle and one off the angola congo margin thin continental crust flanked seaward by apparently exhumed lower continental crust exhumed mantle considering the geological and geophysical evidence it appears that depth dependent extension riftward flow of weak lower continental crust and or upper mantle may be a common phenomenon on magma poor margins and that this can result in a much broader zone of transitional lithosphere than has hitherto been assumed transitional lithosphere in this wide zone may consist of subcontinental mantle lower continental crust or some combination thereof depending on the strength profile of the pre rift continental lithosphere transitional lithosphere ceases to be emplaced i e final break up occurs only when emplacement of heat and melt from the rising asthenosphere becomes domi nant over lateral flow of the weak lower lithosphere this model implies a two stage break up first the rupture of the brittle continental crust and second the eventual separation of the ductile sub continental lithosphere which is coincident with emplacement of normal oceanic crust well defined magnetic anomalies can form in transitional lithosphere that consists of highly serpenti nized exhumed mantle and such anomalies therefore are not diagnostic of oceanic crust where present the anomalies can be helpful in interpreting and dating the rifting history

the timescale and mechanism of granulite formation at kurunegala sri lanka incipient charnockite formation at kurunegala in sri lanka is characterized by the growth of orthopyroxene at the expense of amphibole and biotite in an originally homogeneous gneiss mineral equilibria in the charnockite assemblage record pressure temperature p t conditions of 738 60 c and 6 9 1 2 kbar at 17 0 1 2 log fo2 and ah2o 0 18 0 16 wholerock trace element and isotopic measurements show that charnockite formation was accompanied by a systematic depletion of sm rb pb u sr nd with a fractionation of rb sr sm nd and th u ratios and crystallization of the charnockite assemblage at 535 5 ma major element fe mg ca and sm nd equilibration between minerals occurred at 524 9 ma whereas pb and rb sr underwent continued exchange to 501 5 ma and 486 1 ma respectively trace element data for both amphibolite and charnockite minerals show that depletion on a whole rock scale can be accounted for either by changes in mineral modes or trace element abundances within the immediate area of dehydration the fractionation of sm nd on a whole rock scale is controlled by the breakdown of amphibole without the growth of a major new host phase for sm in the charnockite rubidium and sr are dependent on the relative behaviour of biotite plagioclase and alkali feldspar modelling of dehydration melting involving the breakdown of amphibole biotite and alkali feldspar reproduces the observed sm nd and rb sr fractionation and indicates the loss of small melt fractions on a cm scale from the charnockite these observations suggest that partial melting is the most plausible means of effecting both the dehydration and depletion that accompanies charnockite formation

seismicity structure and rheology of the lithosphere in the lake baikal region this paper combines observations of seismicity gravity topography and thermal and velocity structures to investigate the rheological properties of the lithosphere in the lake baikal region we examine the seismogenic thickness ts using 25 earthquakes of mw 5 1 7 1 whose full source parameters have been determined by inversion of teleseismic waveforms 13 of which are presented here for the first time these 25 events plus six others mw 5 0 5 8 whose depths are well constrained show that moderate earthquakes occur at depths up to 30 km in the northeast baikal rift based on the teleseismic waveform modelling results and published relocations of microearthquakes using regional networks we conclude that the mantle is not a significant source of seismicity in the baikal region using the admittance between free air gravity and topography we estimate the effective elastic thickness te in the region to be between 5 and 20 km nowhere do the data require that te ts consistent with the simple interpretation that the long term strength of the lithosphere resides in its seismogenic layer a weak mantle in the baikal region can be explained by its high temperature which we estimate by combining local geotherm estimates with the regional upper mantle velocity structure obtained from fundamental and higher mode surface waves geotherms are fitted to pressure and temperature estimates from mantle nodules at four sites both within and outside the siberian shield in order to constrain the temperatures at the moho we estimated crustal thicknesses using teleseismic receiver functions moho temperatures are estimated to exceed 550 c beneath the siberian shield and are higher in the more recently deformed mountain belts to the south based on a reassessment of oceanic geotherms and seismicity it seems likely therefore that the mantle in the baikal region is too hot to be a source of long term strength this is consistent with the recent suggestion that the distribution of mantle seismicity in both the oceans and the continents is dependent on temperature alone finally we note that results from s wave tomography studies combined with the observed locations of rift related earthquakes lead us to suspect that the frequently published position of the edge to the siberian shield at the surface provides a poor description of that same boundary at depth

receiver function analysis of the crust and upper mantle from the north german basin to the archaean baltic shield summary two passive seismic experiments have been carried out across the trans european suture zone tesz from northern germany to southern sweden tor and across the proterozoicarchaean suture in finland svekalapko to improve our understanding of the processes involved in the creation of the european continent teleseismic earthquakes recorded by the two networks and stations of the grsn and geofon permanent networks have been used for studies of the crust mantle and upper mantle seismic discontinuities with the receiver function method along the tor network the depth to the moho increases from 30 km at the southern edge of the profile to 40 km at the elbe line between the elbe line and tesz the moho branches off and whereas the deeper branch continues at 40 km depth to the tesz a second branch appears at 30 35 km depth the upper branch descends north of the tesz to below 55 km under the northern end of the tor profile the crustal thickening north of the tesz is accompanied by an increase in average vp vs values appearance of intracrustal conversion zones and north dipping features which we interpret as remnants of the subduction and subsequent collision between avalonia and baltica in southern finland beneath the svekalapko network the moho starts in the south at the depth of 40 45 km plunges to about 65 km depth south of the archaean proterozoic suture this deepening of the moho is coincident with a north dipping intracrustal structure apparently related to the subduction and collision and of the proterozoic and archaean provinces in proterozoic north of the line of the suture the moho rises smoothly to 45 50 km depth in the archaean province along the tor profile 410 and 660 discontinuities were hard to detect however manyfold stacking of receiver functions revealed that the conversions from the two discontinuities arrive more or less according to iasp91 predicted time across the svekalapko network 410 and 660 discontinuities arrive markedly earlier than iasp91 theoretical arrival times in particular north of the archaean proterozoic suture in finland the 410 and 660 km conversions arrive about 2 s earlier indicating about 5 per cent higher average upper mantle velocities and lower temperatures than what iasp91 global model predicts

new views on the structure and rheology of the lithosphere over the last 10 years a series of developments have led to a new understanding of what controls the variations in lithosphere strength structure and evolution that produce dramatic contrasts between the geological histories of oceans ancient shields and young orogenic belts those developments involve a wide range of observations from a great diversity of geological geophysical and geochemical disciplines that none the less provide a mutually consistent and coherent overall picture this paper summarizes in one place the essential stages in the evolution of the relevant ideas and observations that have led to this situation

lithospheric thickness of the chinese continent abstract we invert for the upper mantle temperatures of the chinese continent in the depth range of 70 240 km from a recent s velocity model the depth where temperatures intersect a mantle adiabat with a potential temperature of 1300 c is in close correspondence with the top of the seismic low velocity zone for most regions this correspondence implies that seismic lithosphere estimated from short time scale seismic information may be equivalent to the long time scale geodynamical lithosphere defining the 1300 c adiabat as coinciding with the lithospheric base we estimate the seismic thermal lithosphere thickness the estimated thickness shows obvious dependence on the tectonic settings beneath eastern china which mainly belongs to the circum pacific tectonic domain it has a thickness of 100 km and beneath the qinghai tibet plateau and south to the tarim craton which belong to the tethyan tectonic domain it has a thickness of 160 220 km the lithospheric thicknesses of the three large para platforms cratons range from 170 km for the western yangtze 140 km for tarim and 100 km for sino korean the three cratons may have been reshaped by phanerozoic tectonic activities and are thinner than most cratons in other continents

earthquakes in the deep continental crust insights from studies on exhumed high pressure rocks summary recent studies on two exhumed anorthositic and gabbroic high pressure complexes in the bergen arcs and western gneiss region of norway demonstrate that deeply subducted dry rocks in the lowermost continental crust may resist metamorphic re equilibration for geologically significant periods of time this persistent metastability exercised a profound control on the mechanical properties of these rocks as evidenced by abundant pseudo tachylyte veins cataclasites and fractures documenting eclogite facies seismic failure at depths exceeding 60 km in the thickened caledonian crust our observations indicate that the brittle seismogenic state of some lower crustal rocks and their propensity to remain metamorphically metastable despite prevailing conditions of t 600 800 c and p 1 5 to 2 0 gpa is largely controlled by

a re assessment of focal depth distributions in southern iran the tien shan and northern india do earthquakes really occur in the continental mantle summary we investigate the depth distribution of earthquakes within the continental lithosphere of southern iran the tien shan and northern india by using synthetic seismograms to analyse p and sh body waveforms in the zagros mountains of southern iran earthquakes are apparently restricted to the upper crust depths of 20 km whereas in the tien shan and northern india they occur throughout the thickness of the continental crust to depths of 40 45 km we find no convincing evidence for earthquakes in the continental mantle of these regions in spite of previous suggestions to the contrary and question whether seismicity in the continental mantle is important in any part of the world in some regions such as iran the aegean tibet and california seismicity is virtually restricted to the upper continental crust whereas in others including parts of east africa the tien shan and northern india the lower crust is also seismically active although usually less so than the upper crust such variations cannot reliably be demonstrated from published catalogue or bulletin locations even from ones in which depth resolution is generally improved in contrast to the oceanic mantle lithosphere in which earthquakes certainly occur the continental mantle lithosphere is we suggest virtually aseismic and may not be significantly stronger than the lower continental crust these variations in continental seismogenic thickness are broadly correlated with variations in effective elastic thickness suggesting that the strength of the continental lithosphere resides in the crust and require some modification to prevalent views of lithosphere rheology

continental transforms a view from the alpine fault abstract continental transform faults are dominantly highly localized strike slip shear zones hundreds of kilometers long that accumulate tens to hundreds of kilometers of displacement from work on the alpine fault we pose the questions what is the deep structure of a continental transform and how does the displacement become localized we review research on the alpine fault and propose a model in which the fault partitions at depth into a steep zone extending into the mantle with largely fault parallel motion and a flat ductile decollement in the lower crust the fault localizes around two thirds of the plate motion within a 100 km wide zone of distributed deformation a review of other active continental fault systems suggests that variation between them may reflect their tectonic origins the nature of the crust in which they develop the presence of a significant oblique component of motion and the displacement rate all however have evidence for the development of a single principal fault zone that carries 50 of the total displacement and extends as a localized zone of shear into the upper mantle we review mechanisms of strain weakening and suggest that localization of a principal fault may be initiated in the seismogenic crust and through a series of positive feedbacks eventually extend through the lower crust into the upper mantle

a major step in the continental moho and its geodynamic consequences the taranaki ruapehu line new zealand summary receiver function analysis reveals a 7 km step like change in crustal thickness across the taranaki ruapehu line tr line of north island new zealand the tr line runs east west between the active andesite volcanoes of mt taranaki and ruapehu and marks the southernmost extent of subduction zone volcanism in new zealand north of the tr line receiver functions show a strong and sharp p to s ps conversion at 25 1 5 km depth which is interpreted as a shallow moho at the tr line the moho ps conversion deepens across a step to 32 km depth and weakens further south the moho deepens to 35 km most of the 7 km step change in crustal thickness occurs over a lateral distance of 8 km yet there is little surface or topographic manifestation of the line given fresnel zone considerations the dip of the moho offset could vary between 90 and 45 gravity seismic attenuation and electrical data all show that the tr line is not only a step in crustal thickness but also a profound lithospheric boundary as mantle properties such as attenuation qp 1 implied density and electrical resistivity change abruptly across the line an east west oriented cluster of earthquakes with hypocentral depths of 20 40 km is centred on the moho step we propose that the moho step the earthquakes and the rapid change in mantle properties across the tr boundary are causally related processes that could be responsible for the phenomena described here include the rapid removal of mantle lithosphere and lower crust to the north of the tr line

subsidence control on river morphology and grain size in the ganga plain the ganga plain represents a large proportion of the current foreland basin to the himalaya the himalayan sourced waters irrigate the plain via major river networks that support approximately 10 percent of the global population however some of these rivers are also the source of devastating floods the tendency for some of these rivers to flood is directly linked to their large scale morphology in general the rivers that drain the east ganga plain have channels that are perched at a higher elevation relative to their floodplain leading to more frequent channel avulsion and flooding in contrast those further west have channels that are incised into the floodplain and are historically less prone to flooding understanding the controls on these contrasting river forms is fundamental to determining the sensitivity of these systems to projected climate change and the growing water resource demands across the plain here we present a new basin scale approach to quantifying floodplain and channel topography that identifies areas where channels are super elevated or entrenched relative to their adjacent floodplain we explore the probable controls on these observations through an analysis of basin subsidence rates sediment grain size data and sediment supply from the main river systems that traverse the plain yamuna ganga karnali gandak and kosi rivers subsidence rates are approximated by combining basement profiles derived from seismic data with known convergence velocities results suggest a more slowly subsiding basin in the west than the east grain size fining rates are also used as a proxy for relative subsidence rates along the strike of the basin the results also indicate higher fining rates and hence subsidence rates for given sediment supply in the east by integrating these observations we propose that higher subsidence rates are responsible for a deeper basin in the east with perched low gradient river systems that are relatively insensitive to climatically driven changes in base level in contrast the lower subsidence rates in the west are associated with a higher elevation basin topography and entrenched river systems recording climatically induced lowering of river base levels during the holocene

a reassessment of outer rise seismicity and its implications for the mechanics of oceanic lithosphere we use body waveform modelling to constrain the source parameters of earthquakes occurring globally in oceanic lithosphere beneath the subduction zone outer rise and outer trench slope these data are then used to map the stress state in the lithosphere of the downgoing plate as it bends into the subduction zone our results provide new constraints on the faulting of oceanic lithosphere at the outer rise which is important for understanding the transmission of plate driving forces through the subduction system in all cases shallow normal faulting earthquakes are observed at the top of the plate and are separated in depth from any deeper thrust faulting earthquakes no temporal variation associated with large thrust faulting earthquakes on the subduction interface is seen in the depth extent of each type of faulting at the outer rise the transition depth from trench normal extension to compression is found to vary in agreement with models in which deformation is driven by the combination of in plane stresses and bending stresses resulting principally from slab pull combining the seismologically derived constraints on the thickness of the elastic core of the plate with estimates of the plate curvature we place upper bounds on the strength of the lithosphere at the outer rise which is required to be 300 mpa for a constant yield stress model or governed by an effective coefficient of friction of 0 3

microseismicity tectonics and seismic potential in the western himalayan segment nw himalaya india abstract the tectonics and seismic potential of the western himalayan segment 30 33 n 76 80 e of the nw himalayan india region have been determined in this study 423 earthquakes were located in the nw himalaya between 2004 and 2013 using more than 4495 p and 4453 s differential travel times to determine the moment tensors for 8 mw 4 0 of these earthquakes using their broadband regional waveforms the geometry of the main himalayan thrust mht plane which varies along the strike of the himalaya in flat and ramp segments with a dip ranging between 2 5 to 4 to 19 below the himalayan frontal thrust hft in the south to the south tibetan detachment std in the north has also been deduced in this study two crustal ramps were reported in this study with a depth variance below the main central thrust mct and to the south tibetan detachment std between 12 to 22 km and 28 to 40 km depth respectively the estimated earthquake potential prevailing in the western himalayan seismic gap lying between the epicentral zone of the 1905 kangra earthquake and the 1975 kinnaur earthquake reveals that the total amount of energy released since the last great event is only a fraction 3 5 of the accommodated energy i e 1 1e 28 dyne cm yr this suggests that if an earthquake hits this nw himalayan segment in the future its magnitude might be around mw 8 0

metastability mechanical strength and the support of mountain belts exhumed high pressure rocks from the caledonian root zone in norway provide analogues for processes occurring today under southern tibet allowing large scale geophysical observations from gravity and earthquakes to be linked with the mechanical properties of metastable rocks under high mountains metastability is essential for the survival of thick mountain roots and hence of high mountains and is in turn controlled by water if water is absent dry granulite formed from earlier melting episodes is both stable and strong and likely to survive in the eclogite stability field of deep root zones for hundreds of million years but if hydrous fluid is introduced the transformation of granulite to eclogite is relatively rapid and accompanied by a dramatic loss of strength in norway this transformation was initiated by water infiltration along fractures formed by earthquakes the same process as marked by deep earthquakes may be occurring today beneath southern tibet

imaging the indian subcontinent beneath the himalaya new seismic images reveal the geometry of the indian plate as it passes beneath the himalayan plateau several important subsurface features of vanished india are revealed including a shear zone above the base of the indian crust beneath the himalaya and conversion of the deep indian crust into denser rock the rocks of the indian subcontinent are last seen south of the ganges before they plunge beneath the himalaya and the tibetan plateau they are next glimpsed in seismic reflection profiles deep beneath southern tibet1 yet the surface seen there has been modified by processes within the himalaya that have consumed parts of the upper indian crust and converted them into himalayan rocks2 3 the geometry of the partly dismantled indian plate as it passes through the himalayan process zone has hitherto eluded imaging here we report seismic images both of the decollement at the base of the himalaya and of the moho the boundary between crust and mantle at the base of the indian crust a significant finding is that strong seismic anisotropy develops above the decollement in response to shear processes that are taken up as slip in great earthquakes at shallower depths north of the himalaya the lower indian crust is characterized by a high velocity region consistent with the formation of eclogite a high density material whose presence affects the dynamics of the tibetan plateau

the behavior of the lithosphere on seismic to geologic timescales the strength of the lithosphere and how it responds to loading and unloading are fundamental problems with wide implications flexure studies suggest that the elastic thickness a proxy for the strength of the lithosphere increases with plate age but decreases with load age the elastic thickness is significantly less than the seismic thickness of the lithosphere as indicated by the depth to the low velocity zone suggesting that the lithosphere is a strong structure at short seismic timescales and a weak one at long timescales the mechanism controlling this weakening is not known but it probably involves some form of load induced stress relaxation despite weakening the lithosphere is capable of retaining its strength and supporting loads such as volcanoes and sediments for long periods of geological time lithosphere relaxation should be included in geodynamical models especially as it has impacts on stratigraphy sea level change and dynamic topography

new evidence for active tectonics at the boundary of the kashi depression china from time series insar observations abstract kashi depression is one of the most complex active tectonic areas in the southern flank of tianshan china due to the lack of ground observations the boundary of basin mountain transition zone and the interseismic activity of the tianshan have not been clearly determined in this study 48 envisat advanced synthetic aperture radar asar imagery acquired from 2003 to 2010 are used to construct interferograms for measuring high resolution interseismic deformation in the kashi depression area a global atmospheric model era interim provided by the european center for medium range weather forecast ecmwf and a global network orbital correction are applied to remove atmospheric effect and the long wavelength orbital errors respectively for the interferograms interferometric sar time series with atmospheric estimation model insar ts aem are then used to obtain a deformation rate map for the kashi depression area the insar rate map indicates that the north part of south atushi fault has 3 mm year uplift relative to that of the south part this result manifests the main tectonic deformation potentially occurs along the southern atushi fault based on a simple edge dislocation model the dip angle of 31 0 6 slip rate of 2 3 0 1 mm year and locking depth of 10 6 0 4 km for the southern atushi fault between tianshan orogenic belt and the kashi depression are obtained this modeling result shows in good agreement with the insar derived rates our results show that the southern atushi fault is the main active fault in block boundary region between the south of tianshan and the tarim basin

characteristics and consequences of flow in the lower crust in some places there is strong evidence that the lower continental crust has flowed so as to smooth out variations in crustal thickness caused by differential crustal extension or shortening in order to better understand the processes involved we investigate the behavior of a fluid layer over a fluid half space to see how such a system responds to the deformation of its upper and lower boundaries this simple system can be used to study both the decay of crustal thickness contrasts and the behavior of a thin lithospheric sheet the changing response of the system to variations in density and viscosity contrasts and to different boundary conditions imposed on the fluid interface can easily be studied analytically the most important results are that variations in crustal thickness on a wavelength of a few times the thickness of the flowing channel will decay quickest and that large lateral variations in crustal thickness cause the fluid to develop a steep front which may cause a topographic step above it at the earth s surface deformation within the channel will be principally by simple shear the clear association of lower crustal flow with regions of thickened crust and magmatic activity suggests that both can reduce the viscosity of the lower crust to levels at which flow can occur the smoothing of crustal thickness contrasts leads to differential vertical motions and is thus a method by which substantial tilting can occur without faulting this differential uplift may be responsible for rotating and exhuming some of the detachment faults in metamorphic core complexes in the basin and range province of the western united states it is also a method of causing structural inversion in basins that does not require the reactivation of normal faults as thrusts or reverse faults

correlation between seismic anisotropy and bouguer gravity anomalies in tibet and its implications for lithospheric structures summary as a baseline measurement for understanding the himalayan tibetan orogen a product of continent continent collision between india and eurasia we analyse digital seismic data in order to constrain the seismic anisotropy of the indian shield based on spatially sparse data that are currently available in the public domain there is little shear wave birefringence for sks phases under the indian shield even though it is part of a fast moving plate in the hotspot frame of reference if most of the northern indian mantle has little transverse anisotropy the onset of significant anisotropy under tibet marks the northern terminus of intact indian lithosphere that is thrusting under the himalayan tibetan orogen beyond this terminus tectonic fabric such as that associated with the deforming lithospheric mantle of eurasia must be present in the upper mantle along the profile from yadong to golmud the only profile in tibet where a number of shear wave birefringence data are available the amount of birefringence shows two marked increases near 30 and 33 n between which a local high in bouguer gravity anomaly is observed such a correlation between patterns of shear wave birefringence and gravity anomalies is explained by the juxtaposition of indian lithosphere against the overlying eurasian lithosphere while the eurasian lithospheric mantle appears only to the north of 30 n the indian lithospheric mantle extends northwards to near 33 n

the influence of extension rate and crustal rheology on the evolution of passive margins from rifting to break up abstract extension rate and crustal strength are two important factors that influence the shape of initial rift basins and final passive margin architecture the extension rate will also affect crustal strength as the passive margin evolves therefore adding more complexity to the system we used thermomechanical numerical experiments to systematically explore the relationship of extension rate and initial crustal strength on final passive margin structure varying the initial amount of ductile crust from 33 to 70 and extension rates from 2 to 50 mm yr 1 results in passive margins of different widths shapes and evolution histories we find four distinct margin types type i narrow symmetric margins type ii wide symmetric margins type iii asymmetric margins and type iv rift jumping experiments with strong initial crust 33 ductile crust have crust mantle coupling of strain and produce narrow symmetric margins with high rift flank uplift type i margins ultraslow extension experiments 2 mm yr 1 undergo thermal cooling and produce wide symmetric margins with hyperextended crust type ii margins experiments with intermediate to fast extension rates 10 50 mm yr 1 and thick layers of ductile crust will produce initial rift basins that are symmetric but later rifting stages yield asymmetric conjugate margins with hyperextended domains 100 250 km wide type iii margins in special cases with intermediate extension rates and thick ductile crustal layers we observe a jump in rift location over 50 km and abandoned rifts with isolated crustal blocks type iv margins these results highlight how ductile crustal layers and extension rates influence passive margin formation during different stages of rifting

towards an improved understanding of the mechanical properties and rheology of the lithosphere an introductory article to rock deformation from field experiments and theory a volume in honour of ernie rutter ernie rutter s influential contribution to our understanding of rock deformation spans the whole continental crust from deformation just under the grass e g rutter green 2011 to melt migration in the lower crust e g rutter neumann 1995 ernie has also worked on many aspects of the deformation of oceanic crust rutter brodie 1988 however here we reflect on the wide range of deformation conditions ernie has studied by presenting key aspects of the strength and rheology of the continental lithosphere as we understand it today with the aim of providing the context for the contributions included within this volume the continental crust forms just over a third of the earth s surface and it is different from the crust of any other planet in our solar system it is rich in those elements that partition in a silicate melt and its formation and evolution determined the composition of the mantle and that of the atmosphere hawkesworth 2006 thanks to its physical and chemical fingerprint the earth s crust has sustained life uninterrupted for 3 8 ga and it is the source of major natural resources such as hydrocarbons and mineral deposits and others such as geothermal energy the continental crust is less dense than its oceanic counterpart and is therefore more buoyant it forms a substantial part of the cold thermal layer of the earth and its rheology from greek r1v rhe flow and logia logia study the study of the flow of matter is a key factor controlling plate tectonic processes at the earth s surface and in its interior davies 2011 beneath the continental crust lies the lithospheric mantle and together they form the continental lithosphere considerable variations in structure thickness and composition of the continental lithosphere mean that despite its importance we currently have limited understanding of the rheology of this complex system thatcher pollitz 2008 significant advances in our knowledge of continental rheology have been possible through

implications of shortening in the himalayan fold thrust belt for uplift of the tibetan plateau 1 recent research in the himalayan fold thrust belt provides two new sets of observations that are crucial to understanding the evolution of the himalayan tibetan orogenic system first u pb zircon ages and sm nd isotopic studies demonstrate that 1 greater himalayan medium to high grade metasedimentary rocks are much younger than true indian cratonic basement and 2 these rocks were tectonically mobilized and consolidated with the northern margin of gondwana during early paleozoic orogenic activity these observations require that greater himalayan rocks be treated as supracrustal material in restorations of the himalayan fold thrust belt rather than as indian cratonic basement in turn this implies the existence of greater himalayan lower crust that is not exposed anywhere in the fold thrust belt second a regional compilation of shortening estimates along the himalayan arc from pakistan to sikkim reveals that 1 total minimum shortening in the fold thrust belt is up to 670 km 2 total shortening is greatest in western nepal and northern india near the apex of the himalayan salient and 3 the amount of himalayan shortening is equal to the present width of the tibetan plateau measured in an arc normal direction north of the indus yalu suture zone this information suggests that a slab of greater indian lower crust composed of both indian cratonic lower crust and greater himalayan lower crust with a north south length of 700 km may have been inserted beneath the tibetan crust during the cenozoic orogeny we present a modified version of the crustal underthrusting model for himalayan tibetan orogenesis that integrates surface geological data recent results of mantle tomographic studies and broadband seismic studies of the crust and upper mantle beneath the tibetan plateau previous studies have shown that incremental mesozoic and early cenozoic shortening had probably thickened tibetan crust to 45 55 km before the onset of the main cenozoic orogenic event thus the insertion of a slab of greater indian lower crust with maximum thickness of 20 km tapering northward could explain the cenozoic uplift of the tibetan plateau the need for tibetan crust to stretch laterally as the greater indian lower crust was inserted may explain the widespread east west extension in the southern half of the plateau our reconstruction of the himalayan fold thrust belt suggests that indian cratonic lower crust of presumed mafic composition and high strength should extend approximately halfway across the tibetan plateau to the banggong suture from there northward we predict that the tibetan plateau is underlain by more felsic and therefore weaker lower crust of greater himalayan affinity two slab break off events are predicted by the model the first involved neotethyan oceanic lithosphere that foundered 45 35 ma and the second consisted of greater indian lithosphere most likely composed of greater himalayan material that delaminated and foundered 20 10 ma asthenospheric upwelling associated with the break off events may explain patterns of cenozoic volcanism on the tibetan plateau although the model predicts a northward migrating topographic front due solely to insertion of greater indian lower crust the actual uplift history of the plateau was complicated by early middle tertiary shortening of tibetan crust

age dependence of oceanic intraplate seismicity and implications for lithospheric evolution we have determined depths for 16 oceanic intraplate earthquakes using body wave modeling a data set composed of these and 11 other well constrained depths shows that the maximum depth of seismicity deepens with increasing lithospheric age and appears to be bounded by a 700 800 c isotherm the maximum depth to which intraplate earthquakes occur is approximately equal to the flexural elastic thickness but much less than the seismic thickness from surface wave dispersion the maximum faulting depth is consistent with the predictions of rapid weakening at high temperatures for dry olivine rheologies determined by experimental rock mechanics oceanic intraplate earthquakes can occur at greater depths and temperatures than has previously been reported for continental crustal events presumably because oceanic lithosphere is stronger at high temperatures both the number of earthquakes and the cumulative seismic moment decrease with lithospheric age the cause of this trend is unclear possibilities include nonrandom location of preexisting weak zones with respect to the age provinces fault strengthening with age or a decrease in the stress level a decrease in intraplate stress with age is opposite that expected from ridge push stresses which increase with age but may result either from thermoelastic stresses which are greatest in young lithosphere or near ridge stress concentrations due to local heterogeneities in the spreading process

thermobarometry of metamorphosed pseudotachylyte and associated mylonite constraints on dynamic co seismic rupture depth attending caledonian extension north norway abstract the exhumed post caledonian eidsfjord and fiskfjord extensional shear zones of northern norway exhibit evidence of coseismic rupture propagating into the ductile crust as evidenced by the presence of mylonitic and metamorphosed pseudotachylyte geothermobarometric calculations on garnet bearing mineral assemblages in mylonitic gneisses associated with mylonitic pseudotachylyte and in metamorphosed pseudotachylyte permit determination of the depth and ambient temperature of seismogenic low angle ductile extension average pressures from eidsfjord 570 115 mpa at ca 650 c and fiskfjord 1120 220 mpa at ca 650 c correspond to faulting depths of 21 4 km and 41 9 km respectively the fiskfjord results agree with previous thermobarometric calculations on mylonitic cpx grt bearing pseudotachylyte at fiskfjord the calculated depths are 5 25 km below the depth of the standard seismogenic zone these results demonstrate that low angle normal faults may cut through a large portion of continental crust this occurrence of mylonitic pseudotachylyte in an extensional crustal setting is most easily explained by dynamic downward rupture into the ductile regime and or unusually high shear stresses to account for coseismic rupture at such depths implying a direct connection with a seismogenic normal fault in the upper crust

a review of heterogeneous materials and their implications for relationships between kinematics and dynamics in continents 1 the variation of mechanical properties as a function of depth in the lithosphere controls the relationship between surface deformation and whole lithosphere deformation where mechanical competence elastic strength or viscous stiffness is a continuous function with depth surface deformation can be used to constrain either force balance or rheological parameters for the lithosphere however where competence is vertically discontinuous the surface deformation and deformation at depth may have a range of relationships including complete independence therefore both inversions and forward models designed to test either rheology or stress state are highly sensitive to the a priori choice of a mechanical model

analysis of seismic activity in the crust from earthquake relocation in the central tien shan using p and s wave arrivals from local earthquakes recorded by a temporary broadband seismic network ghengis and a kyrgyzstan broadband seismic network knet we determined the source parameters of 1938 earthquakes occurring from 1997 to 1998 in the central tien shan and adjacent areas based on one dimensional and three dimensional velocity models the results indicate that seismic activity is not restricted to only central tien shan but also penetrates into the edges of the kazakh platform and the tarim basin to the north and south respectively it implies that most range bounding faults in the central tien shan are active and they play a significant role in the tectonic activity of the mountain belt however seismic activity of the talas fergana fault in the study area is different its central segment seems more active than its eastern segment we infer that this part of the fault may be activated by neighboring faults whereas its eastern segment could be temporally locked the earthquakes at the depths of 30 40 km beneath the east central tien shan reveal an interaction between the upper and mid lower crust this suggests that brittle failure is possible to greater depth within the crust another possibility is that the seismic activity in the mid lower crust is affected by a positive buoyancy force created by mantle upwelling of this area these observations indicate that dynamic processes such as small scale mantle convection or mantle upwelling which resulted in lithospheric thinning or removal still occur beneath the central tien shan

new constraints on crustal structure and moho topography in central tibet revealed by sinoprobe deep seismic reflection profiling abstract from october 2009 to may 2010 through the support of the sinoprobe project a series of deep seismic reflection profiles were conducted in order to image deep crustal structure of central tibet these profiles extend from the northern lhasa terrane to the qiangtang terrane by crossing the bangong nujiang suture bns the moho depth varies from 75 1 km beneath the northmost lhasa terrane to 68 9 km beneath southmost qiangtang terrane there is a 6 2 km sharp moho offset across the bns within 25 km to the north of the bns the moho rises smoothly to 62 6 km distinct moho reflector lies at 62 6 67 3 km beneath the qiangtang terrane the boundary of the middle and upper crusts and that between the middle and lower crusts interpreted from indepth velocity model correspond to the strong reflection horizons at about18 8 km and 31 3 km depth beneath south qiangtang terrane respectively a north dipping series of reflection packages in the mid to lower crust may mark subduction of the lhasa terrane beneath the qiantang terrane the central portion of the reflection profile exhibits an antiformal structure at 14 1 km in the upper crust which corresponds with the blueschist bearing metamorphic belt

joint modeling of lithosphere and mantle dynamics sensitivity to viscosities within the lithosphere asthenosphere transition zone and d layers abstract although mantle rheology is one of the most important properties of the earth how a radial mantle viscosity structure affects lithosphere dynamics is still poorly known particularly the role of the lithosphere asthenosphere transition zone and d layer viscosities using constraints from the geoid plate motions and strain rates within plate boundary zones we provide important new refinements to the radial viscosity profile within the key layers of the lithosphere asthenosphere transition zone and d layer we follow the approach of the joint modeling of lithosphere and mantle dynamics ghosh and holt 2012 ghosh et al 2013b 2019 wang et al 2015 to show how the viscosities within these key layers influence lithosphere dynamics we use the viscosity structure sh08 steinberger and holme 2008 as a starting model the density variations within the mantle are derived from the tomography models which based on prior modeling had provided a best fit to the surface observables wang et al 2015 our results show that narrow viscosity ranges of moderately strong lithosphere 2 6 5 6 1022 pa s and moderately weak transition zone 5 9 3 1020 pa s as well as slightly large ranges of moderately weak asthenosphere 5 34 1019 pa s and d layer 4 8 18 1020 pa s are necessary to match all the surface observables we also find that a very strong lithosphere 8 6 1022 pa s along with a weak asthenosphere

basin inversion and contractional reactivation of inherited normal faults a review based on previous and new experimental models abstract compressional inversion of former extensional sedimentary basins is a particularly common phenomenon given that passive margins share the same destiny of being subsequently incorporated into fold and thrust belts during basin inversion pre existing faults may concentrate stress and localise future thrust ramps or may be reactivated recording reversal of movement from extension to reverse this process has attracted attention largely because of its economic implications related to petroleum and ore deposit prospects and seismic hazard assessment since more than two decades analogue modelling has been used to indentify and or test a number of parameters relevant for the inversion process this paper aims therefore to offer a reasoned review of the analogue modelling work done on the subject coupling such previous results with those of a new experimental series inspired to the northern apennines stratigraphy and evolution past models have explored the role of main factors governing fault reactivation susceptibility such as fluid pressure fault weakness fault steepness angle of shortening and sediment loading the new models have addressed the role played by geometry and strength of a basal ductile layer during inversion two sets of sand silicone models were first extended orthogonally at different velocities to produce dissimilar pre inversion internal geometries and then shortened coaxially at different velocities to vary the brittle ductile coupling the experimental results confirm the selectiveness of fault inversion and reveal strong similarities with structural styles of tectonic inversion reported from the northern apennines and various areas worldwide the preferred geological model inferred from this modelling involves contribution of 1 syn inversion rotation of pre existing faults to shallower dip and 2 rotation of principal stress axes which could explain the invariable reactivation of only one of the two oppositely dipping graben bounding faults these factors would thus interact with the other well established parameters controlling the fault reactivation process

the effective elastic thickness of the india plate from receiver function imaging gravity anomalies and thermomechanical modelling summary the range and the meaning of the effective elastic thickness eet in continental areas have been subject to controversy over the last two decades here we take advantage of the new data set from the hi climb seismological experiment to re estimate the eet of the india plate along a south north profile extending from the ganges basin to central tibet receiver functions give a high resolution image of the base of the foreland basin at 5 km depth and constrain the crustal thickness which increases northwards from 35 km beneath the indo gangetic plain to 70 km in southern tibet together with available data sets including seismic profiles seismological images from both indepth and himnt experiments deep well measurements and bouguer anomaly profiles we interpret this new image with 2 d thermomechanical modelling solutions using different type of crustal and mantle rheologies we find that 1 the eet of the india plate decreases northwards from 60 80 to 20 30 km as it is flexed down beneath himalaya and tibet due to thermal and flexural weakening 2 the only resistant layer of the india plate beneath southern tibet is the upper mantle which serves as a support for the topographic load and 3 the most abrupt drop in the eet located around 200 km south of the mft is associated with a gradual decoupling between the crust and the mantle we show that our geometrical constraints do not allow to determine if the upper and lower crust are coupled or not our results clearly reveal that a rheology with a weak mantle is unable to explain the geometry of the lithosphere in this region and they are in favour of a rheology in which the mantle is strong

lithosphere rheology and active tectonics in mongolia relations between earthquake source parameters gravity and gps measurements in this paper we use observations of earthquake source parameters and gravity to investigate the mechanical properties and the active faulting of the lithosphere in mongolia well determined earthquake centroid depths including 10 from inversions of p and sh waveforms that are presented here for the first time show that the seismogenic thickness ts within mongolia itself is less than 20 km however to both the east in parts of the lake baikal rift system and the west adjacent to the junggar basin and kazakhstan platform the seismogenic thickness is considerably greater and includes essentially the whole crust from the admittance between the free air gravity and the topography and also from profiles across a flexural foreland basin we determine the effective elastic thickness te in central mongolia to be 30 km in both mongolia and its surroundings te is comparable with ts and where it is well determined te ts these data are consistent with the view that the strength of the continental lithosphere resides in its seismogenic part which in mongolia is the upper crust but to both the east and west appears to be the whole crust the earthquake source parameters also allow us to ask how the active faulting in mongolia accommodates the velocity field revealed by gps measurements it is likely that the entire mongolian altay range in the west rotates counter clockwise relative to stable asia and is responsible for the distributed e w left lateral shear seen further east in central mongolia the admittance observations show no evidence at the present day for convective mantle support or a hotspot responsible for the elevated region of the hangay dome in central mongolia even though the geochemical data from nodules in late cenozoic basalts and seismic tomography studies suggest elevated temperatures at shallow depths 125 km and probably thinned lithosphere

an explanation for the age independence of oceanic elastic thickness estimates from flexural profiles at subduction zones and implications for continental rheology most properties of oceanic lithosphere are widely observed to be dependent on the age of the plate such as water depth heat flow and seismogenic thickness however estimates of the effective elastic thickness of oceanic lithosphere based on the deflection of the plate as it enters a subduction zone show little correlation with the age of the incoming lithosphere this paradox requires reconciliation if we are to gain a full understanding of the structure rheology and behaviour of oceanic lithosphere here we show that the permanent deformation of the plate due to outer rise faulting combined with uncertainties in the yield stress of the lithosphere the in plane forces transmitted through subduction zones and the levels of noise in bathymetric and gravity data prevents simple elastic plate modelling from accurately capturing the underlying rheological structure of the incoming plate the age independent estimates of effective elastic thickness obtained by purely elastic plate modelling are therefore not likely to represent the true rheology of the plate and hence are not expected to correspond to the plate age similar effects may apply to estimates of elastic thickness from continental forelands with implications for our understanding of continental rheology

seismic receiver functions and the lithosphere asthenosphere boundary abstract the lower boundary of the lithospheric plates has remained as an enigmatic boundary for seismologists since it is relatively poorly observed by seismic means there is traditionally a broad consensus that the asthenosphere is observable as a low velocity zone by seismic surface waves seismic techniques which use shorter period p to s or s to p converted body waves are now far enough developed to be successful in observing such a low velocity zone with a higher resolution the principle of this technique the so called receiver function technique is that a strong teleseismic mother phase e g p s pp or sks incident from below on any seismic discontinuity beneath a station produces a converted phase ps or sp which indicates its depth and properties we discuss details of this technique a sufficient number of such observations exist already to indicate that the top of the low velocity zone is a globally observable discontinuity and it is sharper than previously thought an intriguing observation is that in some cratons the new seismic data indicate that the low velocity zone exists already at shallower depths than obtained from surface waves this confirms earlier results from controlled source observations thybo and perchuc 1997 we discuss possible interpretations of this shallow low velocity zone in cratonic regions

southern african crustal evolution and composition constraints from receiver function studies 1 stacking of approximately 1500 radial receiver functions recorded at about 80 broadband seismic stations deployed in southern africa reveals systematic spatial variations in the ratio of crustal p and s wave velocities crustal thickness h and the amplitude of the converted moho phases r the eastern zimbabwe and the southern kaapvaal cratons are characterized by small h 38 km small 1 73 and large r 0 15 values suggesting that the relatively undisturbed archean crust beneath southern africa is separated from the mantle by a sharp moho and is felsic in composition the limpopo belt which was created by a collisional event at 2 7 ga displays large h 43 km but similar and r values relative to the cratonic areas the bushveld mafic intrusion complex and its surrounding areas show large 1 78 large h 43 km and small r 0 11 values reflecting the intrusion of mafic material into the original crust as a result of the bushveld event at 2 05 ga excluding the bushveld the spatially consistent and age independent low accentuate the difference between felsic crustal composition and more mafic island arcs that are thought to be the likely source of continental material within such an island arc model our data combined with xenolith data excluding mantle delamination in cratonic environments suggest that the modification to a felsic composition e g by the partial melting of basalt and removal of residue by delamination is restricted to have occurred during the collision between the arcs and the continent

active transverse faulting within underthrust indian crust beneath the sikkim himalaya summary deep focus earthquakes within the underthrust indian lower crust beneath the himalaya occur in very specific regions and have distinct source characteristics the study of the source mechanisms of these earthquakes provides valuable constraints on the kinematics of deformation of the underthrust indian plate and its influence on the active deformation of the overlying himalayan wedge one of the most significant regions of these deep focus earthquakes is beneath the sikkim and bhutan himalaya we study the source characteristics of the 2011 september 18 mw 6 9 deep focus sikkim main shock and its major aftershocks using global regional and local waveform data we determined the focal mechanism of the main shock using moment tensor inversion of global p and sh waveforms and ascertained the earthquake fault plane using rupture directivity from regional p wave spectra the main shock originated at 53 4 km depth and ruptured at least 20 km thickness of the underthrust indian lower crust faulting occurred on a near vertical dextral strike slip fault oriented nw se strike 127 d ip 81 and rake 167 oblique to the local strike of the himalayan arc the rupture initiated from the se end of the fault and propagated to the northwest the main shock was followed by 20 small to moderate aftershocks mb 3 0 which we relocated using phase arrival times we computed the focal mechanisms of the larger ones mb 3 5 using local waveform inversion we find that all aftershocks originated se of the main shock between depths of 12 and 50 km and have dominantly strike slip mechanisms our results combined with the source mechanisms of earthquakes from previous studies reveals that the entire underthrust indian crust is seismogenic and deforms by dextral strikeslip motion on oblique structures beneath the sikkim and bhutan himalaya these active oblique structures with transverse motion possibly mark the western boundary of the clockwise rotating microplates in northeast india observed from gps geodesy

grain size sensitive seismic wave attenuation in polycrystalline olivine 1 in order to investigate the processes responsible for the attenuation of seismic shear waves in the earth s upper mantle four olivine polycrystals ranging in mean grain size d from 3 to 23 m have been fabricated characterized and mechanically tested in torsion at high temperatures and seismic frequencies both the shear modulus which governs the shear wave speed vs and the dissipation of shear strain energy q 1 have been measured as functions of oscillation period to temperature t and for the first time grain size at sufficiently high t all four specimens display similar absorption band viscoelastic behavior adequately represented for 1000 t 1200 or 1300 c and 1 to 100 s by the expression q 1 a tod 1 exp e rt with a 7 5 102 s m 0 26 and e 424 kj mol 1 this mildly grain size sensitive viscoelastic behavior of melt free polycrystalline olivine is attributed to a combination of elastically and diffusionally accommodated grain boundary sliding the latter becoming progressively more important with increasing t and or to extrapolation to the larger mm cm grain sizes expected in the earth s upper mantle yields levels of dissipation comparable with those observed seismologically implying that the same grain size sensitive processes might be responsible for much of the observed seismic wave attenuation the temperature sensitivity of vs is increased substantially by the viscoelastic relaxation allowing the lateral variability of wave speeds to be associated with relatively small temperature perturbations

pressure temperature time paths of regional metamorphism i heat transfer during the evolution of regions of thickened continental crust the development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional scale thermal regimes these are the total radiogenic heat supply within the thickened crust the supply of heat from the mantle the thermal conductivity of the medium and the length and time scales of erosion of the continental crust the orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening during which little temperature change occurs in individual rocks followed by a lengthier phase of erosion at the end of which the crust is at its original thickness the principal features of pressure temperature time ptt paths followed by rocks in this environment are a period of thermal relaxation during which the temperature rises towards the higher geotherm that would be supported by the thickened crust followed by a period of cooling as the rock approaches the cold land surface the temperature increase that occurs is governed by the degree of thickening of the crust its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary for much of the parameter range considered the heating phase encompasses a considerable portion of the exhumation decompression part of the ptt path in addition to the detailed calculation of ptt paths we present an idealized model of the thickening and exhumation process which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path an important feature of these ptt paths is that most of them lie within 50 c of the maximum temperature attained for one third or more of the total duration of their burial and uplift and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm a companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes 1984 oxford university press

uplift of tibet from eclogites to granulites implications for the andean plateau and the variscan belt abstract using a simplified petrogenetic grid and a one dimensional thermal model to compute the evolution of the density of the crust with time the contribution to the uplift of tibet of metamorphic transitions in the lower crust is shown to be large the computations are made for a lower crust of either mafic or intermediate composition a mafic lower crust would have a mantle like density incompatible with seismological results this conclusion implies that any pre thickening gabbroic lower crust has been removed during the orogenic process a lower crust of intermediate composition would be affected by about 3 km of post thickening uplift over a few tens of millions of years this uplift is essentially the result of the eclogite to granulite transformation the resulting altitude matches the present altitude and does not require any delamination of the mantle lithosphere such a process would also explain why the neogene uplift of tibet to an average elevation of 5 km appears to have occurred without crustal shortening the same model may explain the uplift of the altiplano puna plateau which also occurred without surface shortening since about 10 ma a comparison is made with the now collapsed variscan high mountain belt for which there is evidence of a progressive increase of the temperature in the crust over tens of million years

mantle earthquakes crustal structure and gravitational instability beneath western north island new zealand a cluster of 30 52 km deep earthquakes and a 7 10 km step in the moho beneath western north island new zealand are both interpreted as manifestations of active delamination of the continental lower crust and mantle lithosphere these phenomena occur in the back arc region beneath the east west oriented taranaki ruapehu tr line which strikes at a high angle to the present day plate boundary through new zealand across the line there is an abrupt change in crustal and mantle lid thickness and in upper mantle seismic attenuation q p 1 showing that the mantle lithosphere has been highly thinned on the north side we show from a receiver function profile that nearly all of the deep earthquakes reside in the uppermost mantle on the northern side of the tr line a sum of earthquake moment tensors suggests strike slip motion either parallel or orthogonal to the tr line resulting in northwest southeast oriented horizontal extension active normal faults oriented northwest southeast and north south are seen at the surface on each side of the tr line but the surface is uplifting here at 0 4 mm yr this requires the mantle lithosphere to be thinning at a higher rate than the overlying crust consistent with a delamination process

pseudotachylytes generated during seismic faulting and eclogitization of the deep crust pseudotachylytes are typically interpreted to have formed by frictional melting during coseismic faulting within the upper to middle crust pseudotachylytes in the bergen arcs of western norway contain microlites including omphacite garnet plagioclase and quartz this eclogite facies assemblage is stable at temperatures of about 800 c and pressures of 18 to 19 kilobars corresponding to depths of 60 kilometers or more the pseudotachylytes are exposed in grenvillian granulites that locally underwent fluid induced eclogitization and corresponding volume reduction of approximately 10 percent during the caledonian continental collision the pseudotachylytes may have formed as a result of the rapid relaxation of stresses caused by the eclogitization process