strain heating as a mechanism for partial melting and ultrahigh temperature metamorphism in convergent orogens implications of temperature dependent thermal diffusivity and rheology 1 we explore the importance of strain heating in ductile shear zones for production of leucogranites and high temperature metamorphism in collisional orogens with temperature dependent thermal diffusivity d and rock rheology new measurements on metamorphic rocks from the bohemian massif show that d is 1 3 mm2 s at 25 c and decreases exponentially to as low as 0 4 mm2 s at 600 c this temperature dependence causes model lithospheric geotherms to be straighter compared to geotherms calculated with a constant d using power law parameters for rheology we show that deformation of quartzite at strain rate of 3 10 13 s 1 produces 100 w m3 at 550 c and 7 w m3 at 800 c deformation of stronger clinopyroxenite at this strain rate produces 8 w m3 even at 1050 c when strain heating is introduced by a 3 km thick ductile shear zone with quartz rheology at the depth of 35 km in a lithosphere that has 70 km thick crust and 60 km thick lithospheric mantle the schist solidus is reached by 8 myr in the vicinity of the shear zone deforming at the strain rate of 3 10 13 s 1 for clinopyroxenite rheology ultrahigh temperature uht metamorphic conditions are reached in 40 myr after initiation of strain heating two dimensional models that replicate the exhumation of the greater himalaya crystalline rocks above the main central thrust produce extensive partial melting and an inverted metamorphic field gradient occurrences of leucogranites within crustal shear zone systems may be evidence of the coupling between deformation and heat production in the crust

thermo rheological shear heating model for leucogranite generation metamorphism and deformation during the proterozoic trans hudson orogeny black hills south dakota abstract this paper evaluates thermotectonic models for metamorphism and leucogranite generation during the proterozoic trans hudson orogeny as recorded in rocks exposed in the black hills sd intrusion of the harney peak granite and associated pegmatites at 1715 ma occurred at the waning stages of regional deformation and staurolite grade regional metamorphism published consortium for continental reflection profiling cocorp results indicate that proterozoic sedimentary rocks were thrust over the archean wyoming province during the trans hudson collision isotopic compositions of the harney peak granite suggest that the exposed proterozoic and archean metasedimentary rocks in the black hills represent source rocks of the granites numerical simulations of the regional metamorphism and harney peak granite generation assuming crustal thickening by thrusting coupled with erosion show the following 1 doubling of the crust with normal distribution of radioactive elements does not yield sufficiently high temperatures to cause anatexis anywhere in the crust or growth of garnet in the now exposed part of the crust 2 a 35 km drop off length for internal heat production can yield sufficient temperature for garnet growth at the current erosion level it is however insufficient to produce staurolite and melting can occur only in the deepest parts of the crust 3 temperatures in crust with stable 70 km thickness for 40 ma and 35 km drop off length for heat production could become sufficient to produce staurolite at the current erosion level and subsequent rapid denudation of the crust could potentially trigger decompression melting of lower crustal rocks although this model could potentially explain the observed temporal relationship between regional metamorphism and leucogranite generation it is inconsistent with melting of upper crustal proterozoic source rocks that is indicated by isotopic compositions of the granites with lack of evidence for rapid denudation of the trans hudson orogen and with confinement of the leucogranites to the deformed proterozoic metapelitic rocks production of the harney peak granite and its relationship to regional metamorphism of the country rocks are best explained by shear heating at the interface between the wyoming province and overthrusted sedimentary rocks we suggest that with reasonable rheologic properties of metapelites and rates of plate convergence shear heating sufficiently perturbs locally the geotherms to cause anatexis in a deep shear zone system and growth of staurolite in the overlying crust modeling rheology of the lithologically stratified thickened crust with granitic basement and metapelitic upper plate shows that the currently exposed part of the crust and the granite source region were ductile through much of the orogeny which explains regional folding of the schists and predicts ductile shear zones in the granite source region because of the lithologic stratification the granitic basement is likely to become significantly weaker during crustal thickening than the upper crust dominated by schists a weak basement under a folded upper crust is likely to contribute to the observed relatively flat topography of high plateaus over thickened orogens

crustal deformation of the eastern tibetan plateau revealed by magnetotelluric imaging the ongoing collision of the indian and asian continents has created the himalaya and tibetan plateau through a range of deformation processes these include crustal thickening detachment of the lower lithosphere from the plate delamination and flow in a weakened lower crust 1 6 debate continues as to which of these processes are most significant 7 in eastern tibet large scale motion of the surface occurs but the nature of deformation at depth remains unresolved a large scale crustal flow channel has been proposed as an explanation for regional uplift in eastern tibet 6 8 9 but existing geophysical data 10 11 do not constrain the pattern of flow magnetotellurics uses naturally occurring electromagnetic waves to image the earth s subsurface here we present magnetotelluric data that image two major zones or channels of high electrical conductivity at a depth of 20 40 km the channels extend horizontally more than 800 km from the tibetan plateau into southwest china the electrical properties of the channels imply an elevated fluid content consistent with a weak crust 12 13 that permits flow on a geological timescale these findings support the hypothesis that crustal flow can occur in orogenic belts and contribute to uplift of plateaux our results reveal the previously unknown complexities of these patterns of crustal flow many previous studies of the indiaasia

evidence from earthquake data for a partially molten crustal layer in southern tibet earthquake data collected by the indepth ii passive source experiment show that there is a substantial south to north variation in the velocity structure of the crust beneath southern tibet north of the zangbo suture beneath the southern lhasa block a midcrustal low velocity zone is revealed by inversion of receiver functions rayleigh wave phase velocities and modeling of the radial component of teleseismic p waveforms conversely to the south beneath the tethyan himalaya no low velocity zone was observed the presence of the midcrustal low velocity zone in the north implies that a partially molten layer is in the middle crust beneath the northern yadong gulu rift and possibly much of southern tibet

structure of the crust in the vicinity of the banggong nujiang suture in central tibet from indepth magnetotelluric data 1 magnetotelluric data from a 150 km long profile crossing the banggong nujiang suture bns central tibet acquired as part of the international deep profiling of tibet and the himalaya indepth project have been examined for crustal and upper mantle structure strike and dimensionality analyses demonstrate that regional scale electrical structures are two dimensional and oriented approximately parallel to surface geological strike as seen elsewhere in tibet the double thickness crust is generally characterized by resistive upper crust hundreds to thousands of ohm meters overlying conductive middle and lower crust tens to hundreds of ohm meters but in detail there are lateral variations at all levels regionally a northward transition from thick 45 km to thin 15 km resistive upper crust coincides with 1 the surface trace of the bns 2 a prominent strand of the karakorum jiali fault system 3 northward decrease in upper mantle seismic velocities and increase in attenuation and 4 pronounced northward onset of seismic polarization anisotropy the latter two seismological features have been taken to mark the northern limit of indian mantle lithosphere thrust beneath southern tibet on the basis of our electrical model we speculate that 1 the resistive upper crustal root beneath the neogene lunpola and duba basins was produced by crustal shortening localized along the northern edge of the lhasa terrane 2 the low midcrustal resistivity beneath the bns reflects enhanced neogene melting and or metamorphic dewatering of relatively fertile subduction zone complex rocks 3 observed steep upper crustal low resistivity anomalies are produced by hydrothermal fluids within active faults localized within and adjacent to the bns and 4 these strike slip and extensional fault arrays are surface manifestations of lithosphere penetrating shear localized along the northern edge of the underthrust indian plate

heat sources for tertiary metamorphism and anatexis in the annapurna manaslu region central nepal following the collision of india with southern tibet crustal rocks of the leading edge of india 1 underwent regional metamorphism to upper amphibolite grade 2 melted locally to produce anatectic granitoids and 3 were sheared and thrust onto lower grade rock along the main central thrust yielding an inverted metamorphic sequence this sequence is exemplified in the annapurna manaslu region we use simple physical calculations to examine the heat sources involved in the different phases of metamorphism the regional metamorphism apparently is due to the burial of the northern edge of india beneath the accretionary prism along the southern edge of tibet the observed temperatures and pressures of this first phase of metamorphism are consistent with the thermal relaxation during the 10 30 m y before slip on the main central thrust began of thickened continental lithosphere whose original surface heat flux was between 50 and 70 mw m 2 the release of water from the footwall of the main central thrust apparently facilitated melting of the overlying crust in the second phase such melting could have occurred in the first million years or so of thrusting if warm 550 650 c crust in the footwall contained the necessary water if melting did not occur in the earliest stages of slip on the main central thrust dissipative heating with shear stresses of 10 to 100 mpa is required for temperatures near the main central thrust to have remained high enough to generate melting above the fault during the underthrusting of cold material the thickness 6 8 km of the zone of inverted isograds associated with the fault if undisrupted and due solely to thermal diffusion implies that the time required to carry the rocks preserving the inverted metamorphism from the surface to depths of 30 40 km was 4 8 m y the apparent inverted temperature gradients about 10 25 c km in this zone can be understood as the combined result of two processes diffusion of heat from hot rock thrust over cold rock expunged the original temperature gradient near the fault and could have created an inverse gradient of 10 20 c km the peak temperatures in such a zone however would not have exceeded about 350 c without an additional source of heat dissipative heating at shear stresses of about 100 mpa can account for peak temperatures in excess of 600 c during slip on the fault and would have contributed as much as 13 c km to the inverse gradient although inversion of metamorphic isograds could have occurred as a result of deformation within the main central thrust zone the high temperatures during slip on this zone still require dissipative heating unless the duration of slip exceeded 25 m y

the onset of india asia continental collision early steep subduction required by the timing of uhp metamorphism in the western himalaya ultrahigh pressure uhp rocks in the nw himalaya are some of the youngest on earth and allow testing of critical questions of uhp formation and exhumation and the timing of the india asia collision initial collision of india with asia is widely cited as being at 55f1 ma based on a paleomagnetically determined slowdown of india s plate velocity and as being at ca 51 ma based on the termination of marine carbonate deposition even relatively small changes in this collision age force large changes in tectonic reconstructions because of the rapid india asia convergence rate of 134 mm a at the time of collision new u pb shrimp dating of zircon shows that indian rocks of the tso morari complex reached uhp depths at 53 3f0 7 ma given the high rate of indian subduction this dating implies that indian continental crust arrived at the asian trench no later than 57f1 ma providing a metamorphic age for comparison with previous paleomagnetic and stratigraphic estimates india s collision with asia may be compared to modern processes in the timor region in which initiation of collision precedes both the slowing of the convergence rate and the termination of marine carbonate deposition the indian uhp rocks must have traveled rapidly along a short hence steep path into the mantle early continental subduction was at a steep angle probably vertical comparable to modern continental subduction in the hindu kush despite evidence for modern day low angle subduction of india beneath tibet oceanic slab break off likely coincided with exhumation of uhp terranes in the western himalaya and led to the initiation of low angle subduction and leucogranite generation d 2005 elsevier b v all rights reserved

velocity fields in deforming asia from the inversion of earthquake released strains average strain rates in sectors of deforming asia are matched by a fifth order polynomial function and that function integrated to obtain the relative velocities and rotations occurring within east tibet western sichuan yunnan and south china the method was applied to strain rates obtained from moment tensor summation of both modern and historic earthquakes but can be applied as well to strains obtained from quaternary slip rates on major faults if south china has negligible motion relative to siberia then the velocity results indicate that nearly all of the expected motion between india and the south china portion of eurasia has in the last 85 years been accommodated by distributed intraplate deformation in east burma yunnan western sichuan and east tibet calculations indicate that these regions constitute a zone of distributed right lateral shear that accommodates an overall north south sense of relative motion between east tibet and south china and india and south china line elements parallel to both right lateral and left lateral faults in east burma and western yunnan are rotating clockwise relative to south china with the line elements parallel to left lateral faults rotating most rapidly 2 0 0 5 m y in eastern tibet and the gansu ningxia nw se trending left lateral faults give rotation clockwise relative to south china 1 2 5 m y in central tibet and western sichuan right lateral faults give slight counterclockwise rotation rates relative to south china 0 5 0 75 m y instantaneous rotation rates within the deforming region extrapolated over a 20 40 m y time period are in rough agreement with the paleomagnetic rotations measured in cretaceous aged rocks

crustal structure and deformation in the northeast india asia collision zone constraints from receiver function analysis summary crustal structure across the india asia collision zone tidding suture in the northeast himalaya bounded by the eastern himalayan syntaxis ehs is investigated using the p wave receiver function rf method the analysed data included three component waveforms of teleseismic earthquakes recorded by a linear array of 11 broad band seismic stations the rfs and inverted shear wave velocity models reveal azimuthally varying crustal structure the rfs for earthquakes from the northeast back azimuths are conspicuous by absence of p to s converted phase at the moho discontinuity inverted velocity model ascribe this to absence of the typical step velocity jump at the moho in a narrow section of the ehs bordering the indenting indian plate and pierced by travelling waves in contrast teleseismic waves arriving from southeast back azimuth sample different litho tectonic blocks of the himalayan collision zone and inverted models show northeast dipping moho across the tidding suture compared to an overall thickness of 70 km in the northwest and central himalaya the crust across the tidding suture is only about 55 km thick this is attributed to a slower rate of convergence in this part of the collision zone the moho structure beneath indian plate and southeastern tibetan plateau reveal opposite dip directions with their colliding margin placed just east of the walong thrust the inverted shear wave velocity models show evidence of intracrustal low velocity layer whose strength varies across the tidding suture the magnitude of velocity reduction beneath lohit plutonic complex trans himalaya favour partial melt as a possible mechanism whereas south of the tidding suture where velocity reduction is comparatively less fluids generated by dehydration reactions appear to be the source for velocity reduction

heat generation associated with collision of two plates the himalayan geothermal belt abstract an analysis of heat discharged by thermal fluids along the c 3000 km long himalayan geothermal belt hgb shows that heat transfer is concentrated along 30 to 50 km wide heat bands which are associated with at least 600 geothermal systems the bands have been interpreted as segments of major concentric slip lines caused by plastic deformation of the ductile crust within the asian plate resulting from plate collision assuming that this crust behaves like an ideal plastic medium the heat transfer within and along a slip line can be estimated it amounts to c 55 mw m2 for a 40 km wide band estimates of present day heat discharges point to 20 35 mw m2 for convective and 10 30 mw m2 for anomalous conductive losses less well constrained for a heated crustal strip in the greater lhasa area computed geotherms indicate that partial melting can develop at c 30 to 50 km depth within heat bands the spatial asymmetry of heat discharged in the hgb heat bands in the eastern himalaya transfer about 2 3 times more heat than does the entire western himalaya can be explained by a counterclockwise rotational penetration of the indian into the asian plate

tomographic imaging of the underthrusting indian slab and mantle upwelling beneath central tibet abstract to better understand the pattern of convergence between the indian and eurasian plates we determine a high resolution p wave tomography of the crust and upper mantle under southern to central tibet using a large number of high quality data of local earthquakes and teleseismic events collected by the antilope ii and hi climb projects a significant low velocity zone is detected above the northward underthrusting indian slab beneath the indus tsangpo suture which may reflect fault zones or incomplete fragmentation as well as melts and or fluids associated with the dehydration of the underthrusting indian slab the variations in the diving depth and extending distance of the indian slab under the antilope ii and hi climb seismic profiles may be caused by the differences in the viscosity contrast between the indian slab and the surrounding mantle as well as strong structural heterogeneities in the upper mantle under the study region both high and low velocity anomalies are revealed in the mid to lower crust under southern tibet which may reflect the complex pattern of crustal structure the rifts in southern tibet exhibit different velocity images suggesting that different structures exist beneath those rifts

mud volcanoes in the alboran sea evidence from micropaleontological and geophysical data during the basacalb ttr9 cruise of the r v professor logachev 1999 two mud volcanoes called marrakech and granada were discovered in the southern sector of the mud diapir province in the west alboran basin wab this paper presents micropaleontological and geophysical data on these mud volcanoes from gravity core samples sidescan sonar okean images and high resolution seismic lines mud breccia recovered from the granada mud volcano is matrix supported with well consolidated clasts of limestone marlstone claystone siltstone sandstone and mudstone whereas mud breccia from the marrakech mud volcano contains unconsolidated clasts the mud breccia matrix contains abundant miocene calcareous nannofossils cn together with pliocene pleistocene species and reworked late cretaceous and paleocene eocene species cn dating indicates that clasts in the mud breccia derive from late cretaceous paleocene eocene and probable miocene sediments these data suggest that the mud diapirs and mud volcanoes in the wab can be derived from the olistostromes of unit vi the basal stratigraphic sequence in the alboran sea basin unit viconsists of lower miocene sediments that incorporated late cretaceous and paleocene eocene materials and basement derived rock fragments the mud volcanic deposits are covered by a thin drape of pelagic marls suggesting that these two volcanoes are currently inactive structures determined on highresolution seismic profiles across mud volcanoes and surrounding diapirs correspond to the late stage pliocene toquaternary diapir development this stage is thought to have developed during a compressional tectonic setting that produced folding and wrench tectonics throughout the basin mud ascent at that time resulted in active diapirism and mud volcanoes on the seafloor 2002 elsevier science b v all rights reserved

sr and 87sr 86sr in waters and sediments of the brahmaputra river system silicate weathering co2 consumption and sr flux abstract the brahmaputra is a major river system draining the himalaya the concentration of sr and its 87 sr 86 sr have been measured in dissolved and particulate phases of the brahmaputra and its tributaries in india to trace the sources of dissolved sr dissolved sr ranges from 250 to 1050 nm with 87 sr 86 sr from 0 71298 to 0 75975 the sr data along with the available concentrations of major ions in the samples show that major contributors of sr in the brahmaputra system are the silicates and carbonates of the himalaya and the transhimalaya and carbonates and evaporites of the tibetan basin silicate sr in the brahmaputra river system ranges from 20 to 80 with an average of 45 the silicate component of sr in the brahmaputra system is similar or marginally higher than that in the ganga system due to contribution from the transhimalayan calc alkaline plutonic rocks which have higher concentration of sr evaporites of the tibetan sub basin and hot springs along the indus tsangpo suture could be the other significant contributors to the dissolved sr budget of the brahmaputra contribution from evaporite dissolution decouples the total dissolved sr flux from the tibetan basin from atmospheric co 2 consumption radiogenic sr of the brahmaputra system is derived from the himalayan silicates whereas other sr sources tend to dilute its radiogenic signature the 87 sr 86 sr of the dissolved phase shows significant correlation with indices of silicate weathering indicating that 87 sr 86 sr can serve as a proxy of silicate weathering at their outflow the brahmaputra is less radiogenic 0 72 compared to the ganga 0 73 however the flux of 87 sr from the brahmaputra is similar to that of the ganga

partial melt or aqueous fluid in the mid crust of southern tibet constraints from indepth magnetotelluric data summary the indepth project has applied modern geophysical techniques to the study of the crustal structure and tectonic evolution of the tibetan plateau in the lhasa block seismic reflection surveys in 1994 detected a number of bright spots at 15 20 km depths that indicate zones of crustal fluids aqueous fluids or partial melt coincident magnetotelluric mt data collected in 1995 detected a major zone of high electrical conductivity at the same depth as the brightspots using constrained inversion the mt data require a minimum crustal conductance of 6000 s this abnormally high electrical conductance can be best explained by a layered model with fluids partial melt aqueous fluids or a combination of partial melt and aqueous fluids the non uniqueness of the mt method means that a wide range of melt fraction thickness combinations for the above models could all explain the 6000 s conductance to distinguish between these three models other geophysical and geological data are required reflection seismic data suggest that a high fluid content 15 per cent is present at the top of the layer the amplitude versus offset data suggest that the top of this layer may be aqueous fluids rather than partial melt passive seismic data imaged a 20 km thick layer of lower fluid content that is probably partial melt petrological studies suggest that concentrations of aqueous fluids above 0 1 per cent at mid crustal depth cannot be sustained taken together these data show that the high conductivity in southern tibet is most probably the result of a relatively thin layer of aqueous fluids 100 200 m overlying a thicker zone of partial melt 10 km

hydrochemical signatures of deep groundwater circulation in a part of the himalayan foreland basin groundwater samples were analyzed from 71 springs and wells as part of a larger study in a region of compressional tectonic regime the study site covers the peshawar basin and surroundings in the himalayan foreland of pakistan the northern portion is mountainous and the water table is discontinuous in different intermontane valleys with abundant springs with normal and anomalous temperatures and composition the southern part is divided into isolated basins with a number of drilled deep and dug shallow wells hydrochemical signatures of elevated strontium sr sio2 boron b and the geothermometric signatures all indicate a deep circulation of the emerging groundwater moreover for several of the sample sites water chemical compositions measured spring and water well temperatures and reservoir temperatures calculated for spring waters all point to origin from deep horizons within the basin remarkable proximity of all the thermal and hydrochemical anomalies to major faults suggests that the waters ascended along these faults from greater depths the area is a natural western extension of the himalayan geothermal belt described in earlier literature for the eastern and central himalayas

granite gabbro and mafic microgranular enclaves in the gejiu area yunnan province china a case of two stage mixing of crust and mantle derived magmas geochronological geochemical whole rock sr nd and zircon hf isotopic analyses were carried out on the jiasha gabbro mafic microgranular enclaves mme and host longchahe granite samples from the gejiu area in the southeast yunnan province sw china with the aim of characterizing their petrogenesis compositional zoning is evident in the gabbro body as the cumulate textures and mineral proportions in the gabbro interior are distinct from the gabbro margin the longchahe granite largely comprises metaluminous quartz monzonite with distinctive k feldspar megacrysts but also contains a minor component of peraluminous leucogranite the mme have spheroidal to elongated lenticular shapes with sharp crenulated and occasionally diffuse contacts with the host granite which we attribute to the undercooling and disaggregation of mafic magma globules within the cooler host felsic magma field observations geochronology geochemistry sr nd and zircon hf isotopic compositions point to a complex petrogenesis for this granite mme gabbro association zircon 206pb 238u ages determined by la icp ms for a mafic enclave its host granite and the gabbro body are 83 1 0 9 ma 83 1 0 4 ma and 83 2 0 4 ma respectively indicating coeval crystallization of these igneous rock units crystal fractionation processes can explain much of the compositional diversity of the jiasha gabbro the geochemical features of the gabbro such as high mg up to 70 and cr up to 327 ppm enrichment in liles e g rb ba k2o and lrees and depletion in hfse e g nb ta ti together with initial 87sr 86sr ratios of 0 708 0 709 and negative end t values 5 23 to 6 45 indicate they were derived from a mantle source that had undergone previous enrichment possibly by subduction components the longchahe granite has a large range of sio2 59 87 74 94 wt is distinctly alkaline in composition and has sr nd hf isotopic compositions 87sr 86sr i 0 712 end t 6 93 to 7 62 and ehf t 5 8 to 9 9 that are indicative of derivation from a crustal source however the most primitive rocks of longchahe granite are compositionally distinct from any feasible crustal melt we interpret the spectrum of rock types of the longchahe granite to have formed via mixing between crustally derived peraluminous leucogranite magma and mantle derived magma of similar heritage to the jiasha gabbro we speculate that this mixing event occurred early in the magmatic history of these rocks at relatively high temperature and or deep in the crust to allow efficient physical mixing of magmas saturation and accumulation of k feldspar and zircon in the mixed magma is invoked to explain the megacrystic k feldspar and elevated k2o and zr content of some of the granitic rocks a later episode of magma mixing mingling is preserved as the mme that have geochemical and isotopic compositions that for the most part are intermediate between the granite and the gabbro the mme are interpreted to be fractionated melts of mafic magma related to gabbro that were subsequently injected into the cooler partly crystalline granitic magma mingling and mixing processes within the convectively dynamic upper crustal magma chamber resulting in a hybrid mme magma during this second mixing episode element interdiffusion rather than bulk physical mixing is interpreted to be the dominant mixing process

viscous dissipation pattern in incompressible newtonian simple shear zones an analytical model an analytical model of shear heating in an inclined simple shear zone with newtonian rheology under a reverse shear sense and an upward resultant pressure gradient is presented neglecting a number of secondary factors the shear heat is expressed as a function of the total slip rates at the boundaries pressure gradient dip and thickness of the shear zone and density viscosity and thermal conductivity of the rock a quartic temperature profile develops with a point of maximum temperature near the bottom part of the shear zone in general the profile is parabolic if pressure gradient vanishes leading to a couette flow the profile attains a bell shape if there is no slip at the boundaries i e a true pouseille flow the present model of shear heating is more applicable in subduction channels and some extruding salt diapirs where the rheology is newtonian viscous and pressure gradient drives extrusion

west east variation in crustal thickness in northern lhasa block central tibet from deep seismic sounding data 5 30 km depth 5 0 vp 6 4 km s a middle crust 33 45 km depth 6 5 vp 6 8 km s and a lower crust depths below 48 km 7 0 vp 7 4 km s the west to east increase in crustal thickness is accomplished by a 50 thickening of the middle and lower crust larger vertical velocity gradients separate these three layers and bound them above surficial and sedimentary rocks and below moho transition zone the most notable low velocity zone in the crust lies at the base of the upper crustal layer s wave velocity structure is less well constrained but parallels the p wave structure except that vp vs ratios may decrease from west to east in the lower crust our data suggest considerable variation in structure along strike of the tibetan plateau and show that interpretations of tibet as a purely two dimensional orogen are overly simplistic the west east increase in crustal thickness may occur across the karakorum jiali fault system and be an indicator of lateral tectonic escape of the qiangtang terrane

fluid geochemistry and geothermometry applications of the kangding high temperature geothermal system in eastern himalayas abstract high temperature geothermal systems hold an enormous capacity for generating geothermal energy the kangding area is a typical high temperature geothermal field in the himalayan geothermal belt hydrogeochemical gas geochemical and isotopic investigations were performed to identify and qualify the main hydrogeochemical processes affecting thermal water composition including mixing and degassing and then to estimate a reliable reservoir temperature nine water samples and four geothermal gas samples were collected and analysed for chemical and isotopic components the results demonstrate the alkaline deep geothermal water is the mixtures of approximately 75 snow melt water and 25 magmatic water it is enriched in na k f li and other trace elements indicating the granite reservoir nature the shallow geothermal water is the mixtures of approximately 30 upward flow of deep geothermal water and 70 meteoric cold water high concentrations of ca mg and hco3 indicate the limestone reservoir nature there is no remarkable oxygen isotope shift in the geothermal water since the rapid circulation is difficult to trigger off strong water rock interaction co2 is the predominant geothermal gas accounting for more than 97 of total gases in volume percentage the concentration of co2 degassing ranged from 0 4 mol l 1 to 0 8 mol l 1 via geothermometrical modelling as a result the geothermal water ph increased from 6 0 to 9 0 and approximately 36 of the total sio2 re precipitate the sources of co2 are the metamorphism of limestone and magmatic degassing based on the composition of carbon isotope the appropriate geothermometers of na k and na li yield reservoir temperature of 280 c the geothermometrical modelling developed to eliminate the effects of co2 degassing yields temperature of 250 c the silica enthalpy mixing model yields temperature of 270 c with no steam separation before mixing

gps measurements of present day convergence across the nepal himalaya the high elevations of the himalaya and tibet result from the continuing collision between india and asia which started more than 60 million years ago1 4 from geological and seismic studies of the slip rate of faults in asia5 it is believed that approximately one third of the present day convergence rate between india and asia 58 4mmyr 1 is responsible for the shortening uplift and moderate seismicity of the himalaya great earthquakes also occur infrequently in this region releasing in minutes the elastic strain accumulated near the boundary zone over several centuries and accounting for most of the advance of the himalaya over the plains of india the recurrence time for these great earthquakes is determined by the rate of slip of india beneath tibet which has hitherto been estimated indirectly from global plate motions6 from the slip rates of faults in asia7 8 from seismic productivity9 and from the advance of sediments on the northern ganges plain10 here we report geodetic measurements using the global positioning system gps of the rate of contraction across the himalaya which we find to be 17 52 2 mm yr 1 from the form of the deformation field we estimate the rate of slip of india beneath tibet to be 20 5 2 mmyr 1 strain sufficient to drive one or more great himalayan earthquakes with slip similar to that accompanying the magnitude 8 1 bihar nepal 1934 earthquake may currently be available in western nepal

crustal structure of northern and southern tibet from surface wave dispersion analysis 1 group and phase velocities of fundamental mode rayleigh waves in the period range of 10 to 70 s are obtained for southern and northern tibet significant variations in crustal velocity structure are found the group velocity minimum for tibet occurs at 33 s and the minimum is 0 12 km s lower for southern tibet than for northern tibet at periods greater than 50 s however group velocities are up to 0 2 km s faster in southern tibet the group and phase velocities are inverted for layered s wave models the dispersion observations in southern tibet can only be fit with a low velocity layer in the middle crust in contrast the velocity models for northern tibet do not require any lowvelocity zone in the crust the s wave velocity of the lower crust of southern tibet is 0 2 km s faster than the lower crust of northern tibet in southern tibet the sub moho velocity increases with a positive gradient that is similar to a shield while there is no velocity gradient beneath northern tibet the high velocity lower crust of southern tibet is consistent with the underthrusting of indian continental lithosphere the most plausible explanation of the mid crustal low velocity zone is the presence of crustal melt resulting from h2o saturated melting of the interplate shear zone between the underthrusting indian crust and overflowing asian crust the lack of a pronounced crustal low velocity zone in northern tibet is an indication of a relatively dry crust the low s wave velocity in the lower crust of northern tibet is interpreted to be due to a combination of compositional differences high temperatures presumably caused by a high mantle heat flux and possibly small amounts of partial melt combined with all available observations in tibet the new surface wave results are consistent with a hot and weak upper mantle beneath northern tibet index terms 7205 seismology continental crust 1242 7218 seismology lithosphere and upper mantle 7255 seismology surface waves and free oscillations 8102 tectonophysics continental contractional orogenic belts keywords tibet crustal velocity structure surface wave rayleigh waves continental collision

modeling shear zones in geological and planetary sciences solid and fluid thermal mechanical approaches abstract shear zones are the most ubiquitous features observed in planetary surfaces they appear as a jagged network of faults at the observable brittle surface of planets and in geological exposures of deeper rocks they turn into smoothly braided networks of localized shear displacement leaving centimeter wide bands of mylonitized reduced grain sizes behind the overall size of the entire shear network rarely exceeds kilometer scale at depth although mylonitic shear zones are only visible to the observer when uplifted and exposed at the surface they govern the mechanical behavior of the strongest part of the lithosphere below 10 15 km depth mylonitic shear zones dissect plates thus allowing plate tectonics to develop on the earth we review the basic multiscale physics underlying mylonitic ductile shear zone nucleation growth and longevity and show that grain size reduction is a symptomatic cause but not necessarily the main reason for localization we also discuss a framework for analytic and numerical modeling including the effects of thermal mechanical couplings thermal elasticity the influence of water and void volatile feedback the physics of ductile shear zones relies on feedback processes that turn a macroscopically homogenously deforming body into a heterogeneously slipping solid medium positive feedback can amplify strength heterogeneities by cascading through different scales we define basic intrinsic length scales of strength heterogeneity such as those associated with plasticity grain size fluid inclusion and thermal diffusion length scale for an understanding ductile shear zones we need to consider the energetics of deformation shear heating introduces a jerky flow phenomenon potentially accompanied by ductile earthquakes additional focusing due to grain size reduction only operates for a narrow parameter range of cooling rates for the long time scale deformational energy stored inside the shear zone through plastic dilation or crystallographic and shape preferred orientation consumes only a maximum of 10 of energy dissipated in the shear zones but creates structural anisotropy shear zones become long living features with a long term memory a special role is attributed to the presence of water in nominally anhydrous minerals we show that water directly affects the mechanical equation of state and has the potential to synchronize viscous and plastic flow processes at geological time scale we have shown that fully coupled finite element calculations using mechanical data from the laboratory can reproduce the basic mode of deformation of an entire mylonitic shear zone the next step of modeling lies in benchmarking basic feedback mechanism in field studies and zooming into the braided network of shear zone structure without losing the large scale constraint numerical methods capable of fulfilling the goal are emerging these are adaptive wavelet techniques and hybrid particle finite element codes which can be run over a computational grid across the net

measuring the seismic properties of tibetan bright spots evidence for free aqueous fluids in the tibetan middle crust seismic bright spots are commonly interpreted to mark fluid concentrations but their nature melt or aqueous is usually inferred only from circumstantial evidence of the geologic setting a band of bright spot reflections has been imaged by project indepth international deep profiling of tibet and the himalayas at about 15 km depth along 150 km of the northern yadong gulu rift southern tibet we use indepth three component wide angle seismic data to measure seismic velocities at the bright spot reflector and theoretical rock physics bounds to constrain the nature of the fluids merging of data from multiple bright spots allows us to use a one dimensional approximation travel time modeling yields average p and s velocities for the upper crust above the bright spots of 5 3 0 2 and 3 2 0 2 km s 1 respectively reflection amplitude variation with offset avo modeling constrains the p and s velocities of the bright spots to 3 0 0 8 and 1 6 0 8 km s 1 respectively multiple modeling procedures suggest these velocities are not model dependent our results imply that of the order of 10 volume of free aqueous fluids in the tibetan middle crust produces the observed bright spot reflections the presence of relatively large quantities of free aqueous fluids presumably mostly saline supercritical h2o does not preclude the presence of melt but does constrain the maximum temperature at the bright spots to the wet granite solidus about 650 c and thus the maximum surface heat flow to 110 mw m 2 the observed bright spots can alternatively be explained as a result of transient flow of aqueous fluids through a lower temperature and lower heat flow southern tibetan crust

right lateral shear and rotation as the explanation for strike slip faulting in eastern tibet the convergence of two continents can be accommodated by crustal thickening and by lateral transport of crust out of the path of the converging continents1 3 the principal features suggesting that lateral transport is important are major strike slip faults striking roughly orthogonally to the orientation of convergence between the continents displacement on such faults is imagined to allow lateral transport of material with respect to the converging continents possibly accounting for a large fraction of the convergence between the two continents the most spectacular example of such strike slip faulting is in the eastern part of the tibetan plateau where three major left lateral faults with slip rates of 10 mm yr 1 and several smaller roughly parallel faults dominate the regional strain field2 3 cobbold and davy4 suggested that these faults may be rotating in a clockwise fashion here we put bounds on that rate of rotation and conclude that the image of lateral transport on such faults2 known also as continental escape extrusion or expulsion is an illusion and that instead the left lateral slip on east striking planes in eastern tibet is a manifestation of north striking right lateral simple shear if this conclusion is correct the east striking left lateral faults and the crustal blocks between them are rotating clockwise at 1 2 myr 1 the east west dimension of eastern tibet is shortening at 10 20 mm yr 1 and little material is moving eastward out of india s path into eurasia by left lateral simple shear

cutting of the european continental lithosphere plasticity theory applied to the present alpine collision presently active european alpine collision has a history of more than 100 myr convergence we investigate the collisional contact problem of an apparently rigid african promontory currently the italy adria block iab steadily penetrating at 1 cm yr over at least the past 45 myr into the deformable european plate we assume that neither continental subduction nor delamination occurs presently in the 300 km wide contact zone enabling use of the indenter indentee approach a key parameter is the aspect ratio h a defined by the width of the deformable foreland over the width of the indenter itself this parameter controls the activation of two fundamentally different modes of deformation 1 classical indentation mode for h a 4 4 described by previous authors and having effect only in a portion of the deformable plate in the vicinity of the indenter and 2 cutting mode for h a 4 4 giving plane strain longitudinal splitting of the entire deformable plate and plastic separation of the two halves we use a matrix operator method to solve for slip lines in the mixed boundary value problem obliquity velocity friction stress of the european alpine case implications are as follows 1 plane stress vertical lithospheric thickening is the first indentation mode of continental collision and 2 for the present ratio of h a 3 deformation is in the cutting mode and slow plane strain intraplate extension rifting is predicted this mode explains the intraplate stress field obtained from borehole and earthquake data considering that the paleoindenter probably has been larger than the iab it is inferred that h a has been below 4 4 since at least the end of the eocene and that consequently the intraplate graben system lower rhine rhine bresse limagne is due to the cutting of the european plate by alpine collision

crustal flow pattern beneath the tibetan plateau constrained by regional lg wave q tomography abstract as a prominent geophysical anomaly unusually high seismic wave attenuation is observed in the crust and upper mantle of the tibetan plateau particularly along its northern area theoretical and laboratory investigations show that the strong seismic attenuation can indicate high temperatures and partial melting which may decrease the viscosity of the material and cause it to flow thus seismic attenuation distribution may provide useful constraints to the crust flows if they exist using lg wave q tomography we construct a 0 05 10 0 hz broadband high resolution crust attenuation model for the tibetan plateau and its surrounding regions the maximum spatial resolution is approximately 1 0 1 0 in well covered areas and for frequencies between 0 05 and 1 5 hz this attenuation model reveals an apparent low qlg belt stretching along the northern and eastern tibetan plateau combining the lg wave q model with other geophysical data two possible crust flow channels are found in the tibetan plateau the main flow channel is from north to east and then turns to southeastern tibet along the western edge of the rigid sichuan basin while a second channel starts from southern tibet and crosses the eastern himalayan syntaxis