boiling fluids in a region of rapid uplift nanga parbat massif pakistan abstract the nanga parbat massif of northern pakistan is currently undergoing rapid uplift 5 10 mm a 1 resulting in near surface elevated temperatures numerous quartz veins cut geologically young structures 100 c km 1 and the isotherms describe a giant pillar of heat above about 4 km fluid temperature is greater than conductive rock temperature in permeable zones which carry convecting boiling meteoric fluid

constant elevation of southern tibet over the past 15 million years the uplift of the tibetan plateau an area that is 2 000 km wide to an altitude of about 5 000 m has been shown to modify global climate and to influence monsoon intensity mechanical and thermal models for homogeneous thickening of the lithosphere make specific predictions about uplift rates of the tibetan plateau but the precise history of the uplift of the plateau has yet to be confirmed by observations here we present well preserved fossil leaf assemblages from the namling basin southern tibet dated to approximately 15 myr ago which allow us to reconstruct the temperatures within the basin at that time using a numerical general circulation model to estimate moist static energy at the location of the fossil leaves we reconstruct the elevation of the namling basin 15 myr ago to be 4 689 895 m or 4 638 847 m depending on the reference data used this is comparable to the present day altitude of 4 600 m we conclude that the elevation of the southern tibetan plateau probably has remained unchanged for the past 15 myr

lateral extrusion of lower crust from under high topography in the isostatic limit where there is isostasy the rocks of the lower continental crust are subject to an effective lateral pressure gradient equal to the gradient of topographic load whether compensation is in crustal roots or in the mantle the result is a poiseuille flow planar channel flow in the weak lower crust which removes crust from under mountains and smooths and levels the topography assuming cubic power law creep the flux of crust is proportional to the third power of the topographic gradient to the usual arrhenius term to the tenth power of the absolute temperature of the lower crust and to the negative fifth power of the geothermal gradient the result is that any initial condition with an isolated high tends eventually toward a state where the topography is pancake shaped with a flat central plateau and steeper flanks spreading outward when adjacent pancakes merge most of the relief is eliminated flow may either roughen or smooth the moho depending on whether there are lateral density contrasts in the mantle lithosphere or not although it is difficult to find analytic solutions for this evolution it is easily simulated numerically using various published flow laws for plausible lower crustal rocks lateral extrusion is shown to be insignificant under the oceans marginally significant under shields and platforms important under elevated plains and dominant beneath high plateaus and or hot delaminated regions in particular the basin and range province of the western united states cannot maintain short wavelength 100 km moho relief of more than 1 km for times greater than 10 20 m y at most if the tibetan plateau of china has been delaminated as recently suggested then it may flatten even faster reducing 100 km wavelength topographic features to 200 m relief in no more than 0 03 0 13 m y and leaving only features with wavelengths over 400 km at the present therefore the lower crust of tibet may resemble a hydraulic reservoir as previously suggested because the flatness of the moho is self maintained in these regions information on past tectonics is continually lost and the present moho shape cannot be used to balance cross sections for times in the past the flatness of the moho today is not a constraint on the mechanisms of extension or shortening in these regions nor is it evidence for underplating by intrusions it also follows that very intense and inhomogeneous dilational strains may have occurred locally as in the metamorphic core complexes of the basin and range province

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

surface deformation and lower crustal flow in eastern tibet field observations and satellite geodesy indicate that little crustal shortening has occurred along the central to southern margin of the eastern tibetan plateau since about 4 million years ago instead central eastern tibet has been nearly stationary relative to southeastern china southeastern tibet has rotated clockwise without major crustal shortening and the crust along portions of the eastern plateau margin has been extended modeling suggests that these phenomena are the result of continental convergence where the lower crust is so weak that upper crustal deformation is decoupled from the motion of the underlying mantle this model also predicts east west extension on the high plateau without convective removal of tibetan lithosphere and without eastward movement of the crust east of the plateau

coupling and decoupling of crust and mantle in convergent orogens implications for strain partitioning in the crust topographic relief strain partitioning and crust mantle coupling within convergent orogens may be systematically related to the distribution of strength as a function of depth within continental crust this result is obtained by solving analytically for basally driven three dimensional flow within an idealized crust with a laterally invariant depth dependent newtonian viscosity two end member flow states are shown to occur in the first crustal flow is directly coupled to the underlying mantle and is controlled by stresses transmitted vertically through the crust in the second upper crustal to midcrustal flow is decoupled from the underlying mantle and is controlled by stresses transmitted horizontally through the crust this model is applied to deformation of continental crust above plate like mantle that undergoes oblique convergence across a narrow suture if no significant low viscosity zone develops within the crust crustal deformation is coupled to motion of the underlying mantle and a triangular mountain range develops surface shortening occurs at the foot of the mountains normal to the trend of the range while strike slip and extension are partitioned along the axis of the orogen if a low viscosity zone is initially absent but develops during crustal thickening a steep sided flat topped plateau forms crustal deformation is coupled to mantle motions on the plateau flanks but may be strongly decoupled beneath the plateau surface shortening normal to the plateau occurs at the foot of the plateau while surface extension normal to the plateau occurs at the plateau edge if a low viscosity zone is present in the lower crust prior to convergence a wide orogen with low topographic relief develops crustal deformation is decoupled from the mantle except at the edges of the deforming region and surface shortening is partitioned uniformly across the belt this study shows that horizontal and vertical partitioning of strain occur even in the absence of lateral strength variations and that shortening and extension may exist simultaneously in different parts of an orogen similar morphology and strain partitioning can be observed in many real orogenic belts including the eastern alps the tibetan plateau and the lachlan tasman fold belt

core complex geometries and regional scale flow in the lower crust a simple flexural model that treats the lithosphere as a thin elastic plate overlying a fluid asthenosphere produces an excellent fit to the domal detachment fault geometries observed in typical core complexes of the basin and range province however this mode of isostatic compensation requires an unacceptably large reduction in local topographic elevation 3 5 km above core complex structures because topographic lows of this magnitude are not observed above core complexes today it requires mountain ranges 3 5 km higher than the surrounding areas to have existed at the site of each future core complex and requires these mountain ranges to have had large crustal roots that were subsequently uplifted to form a flat moho as the core complexes developed an alternate mode of isostatic compensation whereby compensation occurs primarily by regional scale flow of material within the lower crust beneath an elastic midcrust provides an equally good fit to the observed detachment fault geometries while requiring little reduction in local topographic elevation above developing core complex structures and providing an obvious means of maintaining a flat moho beneath these zones of upper crustal doming if this interpretation is correct then the effective elastic plate thickness below the basin and range province probably cannot exceed a maximum of about 4 km with a best guess estimate of about 0 5 to 1 km computation of the temperature structure associated with crustal flow during doming shows that an increase in temperature at the moho of about 100 c follows doming by several million years perhaps explaining the observed coincidence of core complexes with crustally derived igneous rocks subsequent cooling of the mid to lower crust beneath the core complex could result in freezing of the initially ductile lower crust within the core of the dome effectively locking the domal shape into the crust even after the detachment surface is cut by younger faults

lithospheric extension near lake mead nevada a model for ductile flow in the lower crust small variations in gravity anomalies and topographic elevation observed in areas that have undergone highly variable amounts of upper crustal thinning can be satisfactorily explained by ductile flow of lower crustal material under the proper conditions in this study we examine the boundary between the unextended colorado plateau and a strongly extended domain in the basin and range province in the lake mead nevada region bouguer gravity and topography data suggest that both present and preextensional variations in crustal thickness between the unextended and extended regions are small analytic channel flow models show that viscosities required for ductile flow in a lower crustal channel to reduce discontinuities in crustal thickness associated with variable amounts of extension are highly dependent on the channel thickness and on the length scale of flow required finite element modeling of newtonian flow and power law creep shows that flow over the length scale of the eastern basin and range 500 km or more corresponding to upper crustal extension by a factor of 1 4 3 over 10 m y requires effective viscosities less than 10 18 10 20 pa s for ductile channels 10 25 km thick flow over shorter length scales 150 km may be accommodated with effective viscosities as high as 10 21 pa s modeling suggests that these effective viscosities may be sustained by lower crustal material deforming at laboratory derived power law creep rates the longer scale flow may require elevated crustal temperatures more than 700 c depending on the composition and material properties assumed under the boundary conditions assumed in this study the linear viscous flow models yield a satisfactory approximation to deformation by power law creep this work suggests that flow in the lower crust may be a viable mechanism for producing small variations in total crustal thickness between strongly extended and less extended regions and thereby explaining the relative uniformity in gravity and topography between such regions

metamorphic response to collision in the central himalayan orogen abstract closure of the neotethys ocean and high angle continent continent collision between india and asia after about 55 ma resulted in low angle subduction of the indian plate below the tibetan plateau and by 30 ma established an arcuate 2300 km long shallow north dipping metamorphic fold thrust belt in the foreland this himalayan metamorphic front quickly established an upper plate lower plate paired metamorphic architecture centred on the median high himal thrust that largely controlled subsequent evolution the upper plate is a thick slab of high t moderate p high grade migmatized metamorphic rocks whereas the lower plate is an inverted series of moderate t high p schists in two crustal wedges the broad main central thrust zone and at lowest structural levels the footwall below the basal main central thrust spatial and temporal patterns of metamorphic response in the evolving himalayan metamorphic front has been characterized in a large scale integrated structural metamorphic study based on 8 profiles across eastern nepal metamorphic response at all structural levels is established using a large dataset n 160 of internally consistent quantitative pt determinations petrology of metapelite samples semi quantitative p t paths metamorphic mapping and metamorphic field gradients these results are integrated with previously published metamorphic studies structural profiles and metamorphic chronology from these datasets the architecture and evolution of the himalayan metamorphic front is constrained by rock kinematics metamorphic field gradients showing discontinuities and diachronous metamorphism with contrasting p t evolutions at different structural levels each of the three panels constituting the himalayan metamorphic front upper plate main central thrust zone and footwall experienced distinctly different tectono metamorphic histories crustal processes operating during metamorphism and exhumation differ between the upper and lower plates the upper plate experienced long lived metamorphism starting from at least 28 38 ma and tracking low p t clockwise p t paths that culminated at 19 27 ma in peak high grade condition with 27 31 c km thermal regimes protracted high grade conditions produced significant partial melt which facilitated gravity driven southward extrusion involving internal ductile flow processes southward extrusion of the upper plate was accommodated by coeval reverse movement on the high himal thrust and top down to the north normal reactivation of the south tibet detachment system between 22 10 ma transport of this thick slab to the south resulted in further prograde burial of the lower plate below culminating in peak metamorphism at the highest pressures attained in the lower plate rocks the lower plate consists of at least two deeply buried 7 5 9 4 kb crustal wedges both of which experienced steep p t hairpin clockwise p t paths with isothermal decompression the main central thrust zone experienced 20 26 c km metamorphism at 12 22 ma and was exhumed along the basal main central thrust after 11 ma whereas loading by the main central thrust zone gave rise to further prograde burial of the footwall which experienced 16 21 c km metamorphism at 4 10 ma and was exhumed along the main boundary thrust at 3 9 ma in contrast to the upper plate both crustal wedges were exhumed by a process of out wedging which involved upthrusting along basal and internal thrusts with coeval extensional slip in the hanging walls this three stage foreland propagating lateral exhumation history resulted in telescoping of the himalayan metamorphic front in concert with peak metamorphic events at different structural levels during main phase orogenesis and is not the result of superimposed retrograde reactivation of the belt

a possible lower crustal flow channel in the middle urals based on reflection seismic data a possible lower crustal flow channel in the middle urals based on reflection seismic data

indepth iii seismic data from surface observations to deep crustal processes in tibet 1 during the summer of 1998 active source seismic data were collected along a transect running 400 km nnw sse across the central tibetan plateau as the third phase of project indepth international deep profiling of tibet and the himalaya the transect extends northward from the central lhasa block across the jurassic bangong nujiang suture bns at 89 5 e to the central qiangtang block a seismic velocity model for the transect to 25 km depth produced by inversion of p wave first arrivals on 3000 traces shows 1 a 50 km wide region of low velocity at least 5 less than surrounding velocities extending to the base of the model at the bns 2 sedimentary cover for the southern qiangtang block that is 3 5 km thick 3 a distinct interface between sedimentary cover and qiangtang basement or underplated jurassic melange in the central qiangtang block and 4 evidence that the bangoin granite extends to a depth of at least 15 km the bns has little geophysical signature and appears unrelated to the 5 km northward shallowing of the moho which is associated with the bns in central tibet geophysical data along the main indepth iii transect show little evidence for widespread crustal fluids in contrast to the seismic bright spots found in southern tibet and to magnetotelluric evidence of fluid accumulations in eastern tibet a comparison between the global average and tibetan velocity depth functions offers constraints for models of plateau uplift and crustal thickening taken together with the weak geophysical signature of the bns these velocity depth functions suggest that convergence has been accommodated largely through pure shear thickening accompanied by removal of lower crustal material by lateral escape likely via ductile flow although we cannot resolve the details we believe lateral lower crustal flow has overprinted or destroyed evidence in the deep crust for the earlier assembly of tibet as a series of accreted terranes

a flow law for dislocation creep of quartz aggregates determined with the molten salt cell we have used the molten salt cell to conduct an experimental study on the rheology of a natural quartzite containing 0 15 wt water co axial deformation experiments were conducted at constant piston displacement rates approximating constant strain rates at low strain the strengths of our natural quartzite measured in the molten salt cell are approximately half those measured at the same conditions in solid media because unlike solid confining media molten salt does not contribute to the strength of the sample it reduces the friction on the moving piston and it allows clear identification of the hit point we have limited the experimental conditions to those required for dislocation creep and have used only steady state flow stresses measured during climb accommodated dislocation creep to calculate the flow law parameters two flow laws were determined one for samples containing minor amounts of melt 1 2 and one for melt free samples in both cases the power law stress exponent n is 4 0 0 9 which is greater than that previously reported in flow laws for dislocation creep of quartz aggregates determined in solid media the activation energy q is 137 34 kj mol 1 for samples with melt and 223 56 kj mol 1 for those without within the range of previously determined values for quartz aggregates containing 0 1 wt water the pre exponential term a is 1 1 10 4 2 mpa ns 1 for samples without melt and 1 8 10 8 2 mpa ns 1 for those with melt the lower strengths measured in the molten salt cell indicate that previous piezometer relations for quartz experimentally determined in solid media are not correct extrapolation of the flow law for melt free aggregates to natural strain rates predicts higher strengths than most previous quartz flow laws however accurate extrapolation requires determining the dependence of flow stress on fh2o and or ah

multi mineral geochronology insights into crustal behaviour during exhumation of an orogenic root under rare conditions reworked cratons and their margins preserve the orogenic roots of ancient mountain building events however based on the preservation of high temperature 800 c middle and lower crustal metamorphic assemblages present day exposure of these terrains is not simply a result of protracted denudation but also must reflect a multifaceted exhumational history in situ analysis within thin section preserves the textural setting of target minerals that can be used as thermochronometers such as u pb of zircon monazite titanite and apatite and sm nd of apatite in situ analyses of these chronometers has the potential to provide critical timing constraints on exhumation processes related to decompression melting and cooling across large metamorphic terrains the repulse bay block of the rae craton preserves a large composite amphibolite granulite area 50 000 km2 of archean orthogneiss migmatite and slivers of proterozoic metasediments that underwent high grade metamorphism partial melting ductile flow and finally exhumation during the paleoproterozoic trans hudson orogeny the granulite domain preserves dry granitoid assemblages whereas the amphibolite domain is dominated by hydrated migmatites and orthogneiss metasediments occur in both domains and preserve mineral assemblages that are consistent with having undergone tectonometamorphic conditions of 9 kbar 800 c during burial u pb thermochronometers document identical cooling histories of the granulite and amphibolite domains through the u pb closure temperatures of titanite 650 c and apatite 450 c this suggests that melt loss from the underlying granulite domain and melt gain to the amphibolite domain prior to cooling through 650 c are a controlling factor of the metamorphic assemblages across the composite granulite amphibolite terrains such as the repulse bay block rather than significant differences in burial history cooling history and or reorganization of the crust

mantle dynamics uplift of the tibetan plateau and the indian monsoon convective removal of lower lithosphere beneath the tibetan plateau can account for a rapid increase in the mean elevation of the tibetan plateau of 1000 m or more in a few million years such uplift seems to be required by abrupt tectonic and environmental changes in asia and the indian ocean in late cenozoic time the composition of basaltic volcanism in northern tibet which apparently began at about 13 ma implies melting of lithosphere not asthenosphere the most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere the initiation of normal faulting in tibet at about 8 3 ma suggests that the plateau underwent an appreciable increase in elevation at that time an increase due solely to the isostatic response to crustal thickening caused by india s penetration into eurasia should have been slow and could not have triggered normal faulting another process such as removal of relatively cold dense lower lithosphere must have caused a supplemental uplift of the surface folding and faulting of the indo australian plate south of india the most prominent oceanic intraplate deformation on earth began between about 7 5 and 8 ma and indicates an increased north south compressional stress within the indo australian plate a tibetan uplift of only 1000 m if the result of removal of lower lithosphere should have increased the compressional stress that the plateau applies to india and that resists india s northward movement from an amount too small to fold oceanic lithosphere to one sufficient to do so the climate of the equatorial indian ocean and southern asia changed at about 6 9 ma monsoonal winds apparently strengthened northern pakistan became more arid but weathering of rock in the eastern himalaya apparently increased because of its high altitude and lateral extent the tibetan plateau provides a heat source at midlatitudes that should oppose classical symmetric hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons for the simple case of axisymmetric heating no dependence on longitude of an atmosphere without dissipation theoretical analyses by hou lindzen and plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal monsoonlike circulation than such a source centered on the equator but only if heating exceeds a threshold whose level increases with the latitude of the heat source because heating of the atmosphere over tibet should increase monotonically with elevation of the plateau a modest uplift 1000 2500 m of tibet already of substantial extent and height might have been sufficient to exceed a threshold necessary for a strong monsoon the virtual simultaneity of these phenomena suggests that uplift was rapid approximately 1000 m to 2500 m in a few million years moreover nearly simultaneously with the late miocene strengthening of the monsoon the calcite compensation depth in the oceans dropped plants using the relatively efficient c4 pathway for photosynthesis evolved rapidly and atmospheric co2 seems to have decreased suggesting causal relationships and positive feedbacks among these phenomena both a supplemental uplift of the himalaya the southern edge of tibet and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the himalaya that in turn enhanced extraction of co2 from the atmosphere thus these correlations offer some support for links between plateau uplift a downdrawing of co2 from the atmosphere and global climate change as proposed by raymo ruddiman and froehlich mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts but might also play an important role in altering regional and global climates

ablative subduction a two sided alternative to the conventional subduction model subduction is conventionally modelled as a convergence and overlapping of two semirigid plates such a convergence is one sided in that one lithospheric plate descends while the other remains on the surface we suggest an alternative model one based on the dynamics of fluids in this model the viscous lower lithosphere flows downward and the brittle upper lithosphere deforms in passive response this process is potentially double sided for we find that even a buoyant plate can be dragged downward by a dense descending neighbor thus an apparent overriding plate may be worn away by a process of viscous ablation with the rate of ablation a function of plate buoyancy we call this process ablative subduction ablative subduction allows us to simply interpret observations concerning slab profiles interplate seismicity back arc tectonics and complex processes such as double subduction and subduction polarity reversal in performing experiments modelling the evolution of simple fluid slabs we find that slab profile is strongly influenced by ablation in the overriding plate when ablation is weak as when a buoyant continent borders the trench deformable slabs adopt shallow andean style profiles these profiles develop over time from an initially steep shape more vigorous ablation rates as might occur in an ocean ocean convergence yield steeper marianas style profiles thus differences between marianas style and andean style slabs may result from differences in ablation rate and subduction duration the occurrence of ablation might not be easily detected at depth because material from subducting and ablating plates adhere closely as a single slab these slabs show downdip deformations that are consistent with seismic focal mechanisms plate deformations associated with ablative subduction are also consistent with observed patterns of seismicity in particular the relative aseismicity of marianas style plate boundaries is consistent with the ablative model we may thus explain how plates can converge aseismically without requiring that these plates be decoupled ablation should be weakest in andean style subduction although pulses of rapid ablation might lead to episodes of crustal shortening in oceanic regions the manifestation of ablation may depend upon tectonic regime in an extensional area like the marianas ablation may be temporally self limiting and ablative cycles may explain conflicting observations of tectonic erosion and accretion in a compressional regime vigorous ablation might lead to observations of double subduction and subduction reversal in each case the behavior of the lower lithosphere is relatively simple what varies is the rate of ablation and the response of the upper lithosphere

channel profiles around himalayan river anticlines constraints on their formation from digital elevation model analysis 1 we present a comparison between measured and numerically modeled channel profiles of rivers in two important drainage basins of central nepal the kali gandaki and the arun drainage basins modeled channel profiles are based on a simple stream power approach using best fit exponents defining the nonlinearities in the relative contributions of local channel gradient and water flux to erosion rate our analysis of the stream power in the whole river network confirms the work of other authors that a 50 to 80 km wide zone roughly corresponding to the high himalayan topography is subjected to rapid rock uplift we suggest a model where the uplift of this zone is driven by erosion and isostatic response so that centers of maximum uplift are located within the main channels of the north south draining rivers we also suggest that the rate of uplift slows down with increasing distance to the main channels such a spatial distribution of the uplift leads ultimately to the formation of river anticlines as observed along all major himalayan rivers we propose that the formation of river anticlines along south draining himalayan rivers was accelerated by a sudden increase of the drainage area and discharge when the rivers captured orogen parallel drainages on the north side of the range this may follow successive headward cutting into the tibetan plateau the model is confirmed by differences between main channels and east west running tributaries time dependent numerical models predict that capture events cause strongly elevated erosion rates in the main channel

the steady state thermal structure of eroding orogenic belts and accretionary prisms an analytical expression derived in this paper for the steady state thermal structure of thrust belts and subduction systems shows that surface erosion rate basal accretion rate and upper plate heat production are extremely important in controlling the temperature and metamorphic history of orogenic belts and accretionary prisms especially where upper plate thickness h is large for example even at rapid erosion and accretion rates and with large upper plate heat productions temperatures near the toe of a thrust wedge e g at h 5 km are nearly the same as those that would be present in the absence of erosion accretion and upper plate heat production in thicker parts of the same wedge e g at h 30 km the combined effects of erosion accretion and upper plate heat production can be dramatic for sufficiently large magnitudes of upper plate heat production or sufficiently rapid erosion rates inverted negative geothermal gradients are produced in the lower part of the upper plate and the upper part of the lower plate although it is difficult to produce inverted geotherms where h 20 km the potential importance of erosion and accretion can be illustrated through two examples 1 beneath the himalayas increasing the upper plate radiogenic heat production from 0 to 2 5 mw m3 and increasing the rate of surface erosion from 0 to 1 mm yr raises the computed steady state temperatures on the main central thrust at h 30 km from 265 c to 600 c and produces a local temperature maximum of 635 c located 6 km above the fault and 2 within the upper plate of the peru trench increasing the rate of surface erosion and basal accretion from 0 to 2 4 mm yr or increasing the upper plate radiogenic heat production from 0 to 1 3 mw m3 increases the computed steady state surface heat flow at h 30 km from 10 to 33 mw m3 but only increases temperatures at the fault surface from 125 c to 170 c shear heating along thrust faults or subduction boundaries is not necessary to produce the high metamorphic temperatures or surface heat flows observed in these two examples and the generation of large amounts of heat by shear stresses on the faults more than a few tens of bars appears to be inconsistent with some of the observations made in these areas this study indicates that many of the major discrepancies between existing thermal models of overthrust belts and observed temperatures as inferred from field data can be attributed to neglect of horizontal and vertical advection of heat caused by surface erosion and by accretion of material from lower plate to upper plate across the fault surface

seismic imaging of the downwelling indian lithosphere beneath central tibet a tomographic image of the upper mantle beneath central tibet from indepth data has revealed a subvertical high velocity zone from 100 to 400 kilometers depth located approximately south of the bangong nujiang suture we interpret this zone to be downwelling indian mantle lithosphere this additional lithosphere would account for the total amount of shortening in the himalayas and tibet a consequence of this downwelling would be a deficit of asthenosphere which should be balanced by an upwelling counterflow and thus could explain the presence of warm mantle beneath north central tibet

orogeny and orography the effects of erosion on the structure of mountain belts a numerical model of the coupled processes of tectonic deformation and surface erosion in convergent orogens is developed to investigate the nature of the interaction between these processes crustal deformation is calculated by a two dimensional finite element model of deformation in response to subduction and accretion of continental crust erosion operates on the uplifted surface of this model through fluvial incision which is taken to be proportional to stream power the relative importance of the tectonic and erosion processes is given by a dimensionless erosion number relating convergence velocity rock erodibility and precipitation rate this number determines the time required for a system to reach steady state and the final topographic shape and size of a mountain belt fundamental characteristics of the model orogens include asymmetric topography with shallower slopes facing the subducting plate and an asymmetric pattern of exhumation with the deepest levels of exhumation opposite to subduction these characteristics are modified when the regional climate exhibits a dominant wind direction and orographically enhanced precipitation on one side of the mountain belt the two possible cases are dominant wind in the direction of motion of the subducting plate and dominant wind direction in the opposite direction of the subducting plate velocity models of the former case predict a broad zone of exhumation with maximum exhumation in the orogen interior models of the latter case predict a focused zone of exhumation at the margin of the orogen and at high erosion number a reversal in the topographic asymmetry natural examples of these two cases are presented the southern alps of new zealand exhibits the climate and exhumation asymmetry characteristic of wind in the direction opposite to motion of the subducting plate the asymmetry of topography suggests that erosion is not efficient enough to have reversed the topographic asymmetry the contrasting example of dominant wind in the direction of subduction motion is provided by the olympic mountains of washington state in this case exhumation of deep levels of the cascadia accretionary wedge shows a broad domal pattern consistent with the observed orographic precipitation

thermal and kinematic modeling of bedrock and detrital cooling ages in the central himalaya 1 we introduce a new method that convolves detrital mineral cooling ages with digital elevation models to test numerical models of erosion in collisional orogens along a trans himalayan transect in central nepal we develop a kinematic and thermal model to predict variations in bedrock cooling ages in modern himalayan topography the model assumes a thermal steady state and utilizes a simple ramp and flat style decollement representing the main himalayan thrust the model also assumes a topographic steady state such that overthrusting is balanced by erosion to maintain a constant topographic profile erosion rates display strong spatial variations as a function of the angle between the slope of the topographic surface and the trajectories of rock particles approaching the surface to predict the detrital cooling age signal we combine the distribution of bedrock cooling ages within a catchment with the rate of erosion and distribution of muscovite predicted cooling age distributions are compared with detrital 40ar 39ar muscovite data to assess varying tectonic and erosion scenarios such cooling age distributions are very sensitive to how much of the total plate convergence is expressed as erosion of the overthrusting plate the best fit model assigns 4 6 km myr 1 of overthrusting equivalent to as much as 1 5 2 km myr 1 of vertical erosion to the asian plate although a trade off exists between ramp geometry along the decollement and the best fit rate only a narrow range of ramp dips decollement depths and erosionally compensated overthrusting rates are compatible with the observed detrital ages

kinematics topography shortening and extrusion in the india eurasia collision we examine the problem of partitioning between shortening and extrusion in the india asia collision since 45 ma we compute the amount of shortening expected from the kinematics of india s motion with respect to eurasia using the reconstruction at collision time to put bounds on the possible amounts of surface loss within greater india and within eurasia we then compute the amounts of surface loss corresponding to the thickened crust of tibet and of the himalayas assuming conservation of continental crust the spatial distribution of the topography reveals a large systematic deficit of crustal thickening distributed rather uniformly west of the eastern syntaxis but an excess of shortening east of it this distribution indicates an important eastward crustal mass transfer however the excess mass east of the eastern syntaxis does not account for more than one third to one half of the deficit west of the eastern syntaxis the deficit may be accounted either by loss of lower crust into the mantle for example through massive eclogitization or by lateral extrusion of nonthickened crust a mass budget of the crust of the himalayas indicates that lower crust has not been conserved there but the deficit is so large that local loss in the mantle is unlikely to be the unique cause of the deficit alternatively following zhao and morgan 1985 lower crust may have been transferred below the tibetan crust we conclude that a combination of possible transfer of lower crust to the mantle by eclogitization and lateral extrusion has to account for a minimum of one third and a maximum of one half of the total amount of shortening between india and asia since 45 ma this conclusion leaves open the possibility that the partitioning between extrusion and loss of lower crust into the mantle on the one hand and shortening on the other hand has significantly changed during the 45 m y history of the collision

effect of a retreating subduction zone on deformation in simple regions of plate convergence plane strain two dimensional finite element models are used to calculate deformation patterns within model mountain belt systems where subduction zone retreat the process by which space is created between the subducting pro and overriding retro lithospheres either by motion of one or both is active the model uses coulomb plastic and thermally activated power law viscous rheologies overall deformation is determined by balancing the internal strength of the model layer against the sum of applied boundary stresses plus the gravitational stress induced by mass redistribution within the deforming model the effect of incorporating retreat in the subduction setting diffuses deformation as the moving retreating subduction zone deforms a larger cross sectional area of material for a shorter period of time than equivalent models without subduction zone retreat material is deformed while within the subduction zone but once the material has passed completely through the moving subduction zone it is passively appended onto the retrolithosphere and is no longer deformed thus relative to a convergent setting without subduction zone retreat the retreating convergent zone has an areally extensive zone of simple deformation a deformation factor is derived which characterizes the deformation shortening the material experienced within the subduction zone variable is the ratio of convergence rate rate at which material enters the subduction zone to subduction rate rate at which material is subducted variables and f the fraction of the layer that is subducted together determine the dominant style of deformation in the orogenic zone subduction zone retreat is shown to have significant and predictable effects on the model morphology e g basin or mountain formation and on the model geology e g its presence suggests lower topography lower metamorphic grade rocks at the surface and simpler deformational structures which agree with observations

neotectonics of the min shan china implications for mechanisms driving quaternary deformation along the eastern margin of the tibetan plateau the min shan region located along the eastern margin of the tibetan plateau north of the sichuan basin provides an important natural laboratory in which to study the rates and patterns of deformation and their relationship to mountain building at the margin of the plateau the topographic margin of the plateau is coincident with a north trending mountain range the min shan that stands nearly 2 km above the mean elevation of the plateau 3500 m in this region we exploit the preservation of a series of variably deformed quaternary sediments along the western flank of the range to investigate the pleistocene holocene deformation field within the min shan region mapping and field observations of remnant alluvial fans of late pleistocene age indicate that deformation within the min shan involved substantial 10 rapid down to the northwest tilting the geometry of the deposits and the partial preservation of an erosion surface beneath the basin suggest that much of the modern relief of the min shan relative to the tibetan plateau is a consequence of this late pleistocene tilting rates of tilting inferred from luminescence dating of interbedded loess have been remarkably rapid 10 8 rad yr similarly rapid rates of holocene differential rock uplift are inferred from tilted lacustrine sediments in the southwestern part of the range the range is bounded on the west by the min jiang fault zone an east vergent reverse fault however holocene alluvial terraces in headwaters of the min river are preserved across the fault in several places indicating that displacement rates on the min jiang fault are 1 mm yr active faulting only occurs along the eastern foot of the range huya fault for a short distance 60 km despite 3 km of relief on the eastern range front the relationship between these structures and the tilting observed in the min jiang basin is enigmatic the faults do not appear to exert a strong control on the rates and pattern of deformation within the basin a simple flexural model demonstrates that rates of tilting on the western flank of the min shan are too high to be simply attributed to an isostatic response to surficial loading and unloading of the lithosphere present day horizontal shortening across the min shan is geodetically determined to be less than 2 3 mm yr suggesting that only a small part of the observed tilting can be attributed to horizontal shortening thus tilting and concomitant differential rock uplift in the min shan appear to require an additional driving component we suggest that quaternary deformation along the western min shan may reflect the surface response to thickening of a weak lower crust at the margin of the tibetan plateau

north south extension within the convergent himalayan region recent work by burg et al 1984 indicates the presence of east west striking gently north dipping normal faults in the higher himalayas and southern tibet that formed during postcollisional convergence of india and tibet these faults extend for at least 800 km along strike we interpret these normal faults as probable late miocene extensional features with perhaps several tens of kilometres of down to the north displacement a simple elastic model suggests that these normal faults may have formed during gravitational collapse of the miocene topographic front between india and tibet in this interpretation gravitational collapse occurred by southward motion relative to india and tibet of a wedge of crustal rocks bounded above by gently north dipping normal faults and below by thrust faults that probably dip north north south extension produced in this way is probably confined to upper crustal levels only and does not reflect regional extension of the entire lithosphere

synconvergent ductile flow in variable strength continental crust numerical models with application to the western grenville orogen 1 we present results from numerical models for a convergent orogen with laterally variable lower crustal strength representing a simplified orogenic system in which a strong craton flanked by progressively weaker terranes collides with another continent with progressive convergence crustal thickening and thermal relaxation lower crust becomes decoupled from upper and middle crust forming a ductile orogenic infrastructure beneath a stronger superstructure collision with strong external crust results in uplift and expulsion of ductile nappes from the orogenic core creating allochthonous terranes overlying a lower crustal indentor the extent of transport and exhumation of lower crustal nappes over the indentor reflects the amount of convergence and the erosion rate the western grenville orogen displays across strike variations in age tectonic history and protolith association suggesting a systematic variation in precollision crustal strength the laurentian craton margin and accreted terranes were variably reworked at synorogenic depths of 25 35 km during the ottawan orogeny deformation propagated from younger monocyclic rocks in the southeast into older polycyclic rocks flanking the craton on the northwest a comparison between numerical model results and crustal scale cross sections from the georgian bay and montreal val d or transects shows close correspondence between crustal structure and model geometry this indicates that the models produce geologically realistic results and provides a context for interpreting the tectonic evolution of the western grenville orogen contrasts between the results of homogeneous channel flow models and the present ductile nappe models suggest that the effects of different styles of ductile flow can be distinguished in the geologic record

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

present day crustal deformation in china constrained by global positioning system measurements global positioning system gps measurements in china indicate that crustal shortening accommodates most of india s penetration into eurasia deformation within the tibetan plateau and its margins the himalaya the altyn tagh and the qilian shan absorbs more than 90 of the relative motion between the indian and eurasian plates internal shortening of the tibetan plateau itself accounts for more than one third of the total convergence however the tibetan plateau south of the kunlun and ganzi mani faults is moving eastward relative to both india and eurasia this movement is accommodated through rotation of material around the eastern syntaxis the north china and south china blocks east of the tibetan plateau move coherently east southeastward at rates of 2 to 8 millimeters per year and 6 to 11 millimeters per year respectively with respect to the stable eurasia

a slab breakoff model for the neogene thermal evolution of south karakorum and south tibet on the south karakorum margin neogene high temperature medium pressure ht mp gneisses define an east west trending thermal anomaly these rocks have been heated from 600 to 750 c during a slight pressure drop from 0 7 to 0 5 gpa their retrogressive path cross cuts the relaxed geotherm of tectonically thickened crust such a p t evolution occurs only if an advective source of heat is involved involvement of an advective heat source is also implied by the occurrence of neogene granitoids and lamprophyres within the ht mp gneiss area these rocks are strongly enriched in large ion lithophile elements relative to primitive mantle and show negative high field strength element anomalies we interpret these geochemical characteristics to be the result of melting of metasomatized asian lithospheric mantle the nd and sr isotopic compositions of the south karakorum neogene magmatic rocks nd 12 to 7 and 87sr 86sr 0 705 0 725 further suggest they could have originated from mixing between asian variously metasomatized mantle and precambrian crust by contrast the origin of the youngest magmatic rocks 10 myr here exemplified by the hemasil syenite and associated lamprophyres requires involvement of a depleted mantle the combined hf nd signature of these rocks hf 10 4 11 5 and nd 3 4 4 3 suggests that the source of the hemasil syenite could have been depleted mantle contaminated by oceanic sediments likely during the earlier subduction of the tethyan ocean neogene magmatic rocks with the same geochemical characteristics and evolution as those of south karakorum have previously been described in south tibet based on their location and the geochemical evolution of their source region we here propose that the neogene magmatic and metamorphic evolution of the south asian margin was controlled by slab breakoff of the subducting indian continental margin starting at about 25 ma this model is supported by available geophysical data from south karakorum and south tibet

glacial effects limiting mountain height the height of mountain ranges reflects the balance between tectonic rock uplift crustal strength and surface denudation tectonic deformation and surface denudation are interdependent however and feedback mechanisms in particular the potential link to climate are subjects of intense debate spatial variations in fluvial denudation rate caused by precipitation gradients are known to provide first order controls on mountain range width crustal deformation rates and rock uplift moreover limits to crustal strength are thought to constrain the maximum elevation of large continental plateaus such as those in tibet and the central andes there are indications that the general height of mountain ranges is also directly influenced by the extent of glaciation through an efficient denudation mechanism known as the glacial buzzsaw here we use a global analysis of topography and show that variations in maximum mountain height correlate closely with climate controlled gradients in snowline altitude for many high mountain ranges across orogenic ages and tectonic styles with the aid of a numerical model we further demonstrate how a combination of erosional destruction of topography above the snowline by glacier sliding and commensurate isostatic landscape uplift caused by erosional unloading can explain observations of maximum mountain height by driving elevations towards an altitude window just below the snowline the model thereby self consistently produces the hypsometric signature of the glacial buzzsaw and suggests that differences in the height of mountain ranges mainly reflect variations in local climate rather than tectonic forces

thermal constraints on the emplacement rate of a large intrusive complex the manaslu leucogranite nepal himalaya the emplacement of the manaslu leucogranite body nepal himalaya has been modelled as the accretion of successive sills the leucogranite is characterized by isotopic heterogeneities suggesting limited magma convection and by a thin 100 m upper thermal aureole these characteristics were used to constrain the maximum magma emplacement rate models were tested with sills injected regularly over the whole duration of emplacement and with two emplacement sequences separated by a repose period additionally the hypothesis of a tectonic top contact with unroofing limiting heat transfer during magma emplacement was evaluated in this latter case the upper limit for the emplacement rate was estimated at 3 4 mm year or 1 5 myr for 5 km of granite geological and thermobarometric data however argue against a major role of fault activity in magma cooling during the leucogranite emplacement the best model in agreement with available geochronological data suggests an emplacement rate of 1 mm year for a relatively shallow level of emplacement granite top at 10 km uninterrupted by a long repose period the thermal aureole temperature and thickness and the isotopic heterogeneities within the leucogranite can be explained by the accretion of 20 60 m thick sills intruded every 20 000 60 000 years over a period of 5 myr under such conditions the thermal effects of granite intrusion on the underlying rocks appear limited and cannot be invoked as a cause for the formation of migmatites

love wave phase velocity models of the southeastern margin of tibetan plateau from a dense seismic array abstract love wave dispersion maps across the southeastern margin of the tibetan plateau are obtained using earthquake data recorded by the temporary chinarray and permanent china digital seismic array fundamental mode love wave phase velocity curves are measured by inverting love wave amplitude and phase with the two plane wave method the phase velocity maps with resolution better than 150 km are presented at periods of 20 100 s which is unprecedented in the study area the maps agree well with each other and show clear correlations with major tectonic structures the love wave phase velocity could provide new information about structures in the crust and upper mantle beneath the southeast margin of tibetan plateau like the radial anisotropy

crustal channel flows 1 numerical models with applications to the tectonics of the himalayan tibetan orogen 1 plane strain thermal mechanical numerical models are used to examine the development of midcrustal channel flows in large hot orogens in the models radioactive self heating reduces the viscosity of tectonically thickened crust and increases its susceptibility to large scale horizontal flow channels can be exhumed and exposed by denudation focused on the high relief transition between plateau and foreland we interpret the himalaya to have evolved in this manner channel flows are poorly developed if the channel has a ductile rheology based on wet quartz flow laws and well developed if there is an additional reduction in viscosity to 1019 pa s this reduction occurs from 700 c to 750 c in the models and is attributed to a small percentage of in situ partial melt melt weakening model ht1 provides an internally consistent explanation for the tectonic evolution of many features of the himalayan tibetan orogenic system erosional exhumation exposes the migmatitic channel equivalent to the greater himalayan sequence ghs between coeval normal and thrust sense ductile shear zones corresponding to the south tibetan detachment and the main central thrust systems outward flow of unstable upper crust rotates these shears to low dip angles in the model both the ghs and the lesser himalayan sequence are derived from indian crust with the latter from much farther south similar models exhibit a range of tectonic styles including the formation of domes resembling north himalayan gneiss domes model results are relatively insensitive to channel heterogeneities and to variations in the behavior of the mantle lithosphere beneath the model plateau

heat production heterogeneity of the indian crust beneath the himalaya insights from the northern indian shield abstract crustal thermal structure of the himalaya depends partly on the thermal properties of the indian crust that has subducted beneath the himalayan range and partly accreted to it in most studies the indian crust is assigned uniform thermal properties mostly derived from the data from southern india we show here that this might actually not be a good assumption based on the in situ heat production measurements at 279 sites in the bundelkhand craton chottanagpur gneissic complex and singhbhum craton the dominant rocks granites and gneisses in the bundelkhand craton and chottanagpur gneissic complex are characterized by high heat production 3 93 6 52 w m 3 whereas the rocks in the singhbhum craton are low in heat production 1 41 3 27 w m 3 thereby suggesting a large variation of heat production in the northern indian shield the available surface heat flow values also show consistently large variation from 56 to 96 mw m 2 the crustal heat production heterogeneity within the indian crust has to be duly considered in the interpretation of thermobarometric and thermochronological data of the himalaya

the effects of lateral density gradients slopes and buoyancy on channel flow 1d analytical solutions and applications to the se canadian cordillera abstract we present 1d analytical solutions for channel flow in orogens driven by various types of pressure gradients our calculations demonstrate that lateral density gradients in the upper crust such as would occur across a suture zone separating arc rocks from pericratonic sediments provide a driving force for poiseuille flow as large as topographic gradients observed in modern mountain belts for cases for which the gradients are external topographic and lateral density gradients and internal e g partial melting of channel material to the channel inclination decreases and increases the poiseuille component of the average flow velocity within the channel by the cosine and sine of the slope respectively the magnitude of the pressure gradient consequent upon the buoyancy generated by partial melting of metapelites in a channel with a 30 slope such as would occur above an underthrusting basement ramp is similar to that of topographic or lateral density gradients channel flow up a ramp could thus constitute an important exhumation mechanism in large hot orogens our calculations indicate that mid crustal channel flow was a highly likely process in the late cretaceous paleocene setting of the southeastern canadian cordillera the flow was first driven by the lateral density contrast between pericratonic sediments and the arc related intermontane terrane then by combined effect of topographic gradient and melt induced buoyancy of the lower selkirk allochthon part of the shuswap complex flow up the underthrusting basement ramp resulted in exhumation from mid to upper crustal levels channel flow then migrated downward to involve basement and overlying cover sequence rocks our results indicate that syn convergent channel flow was a viable and very likely process in the southeastern canadian cordillera

the detrital record of orogenesis a review of approaches and techniques used in the himalayan sedimentary basins the sediment archive of material eroded from an active tectonic region and stored in adjacent basins can provide a valuable record of hinterland tectonism especially when information in the source region itself is obscured by later metamorphism or removed by tectonism or erosion using the sediment record to document tectonism is a well established approach but more recently there has been a burgeoning of the number of isotopic techniques which can be applied to detrital material in particular single grain analyses thus the scope for application of detrital studies to a number of different tectonic problems has widened considerably in this review the example of sediments eroded from the himalayan orogen and preserved in the suture zone basin foreland basin remnant ocean basins and deep sea fans is used to illustrate the approach techniques as diverse as petrography heavy mineral xrf and sr nd studies single grain dating by ar ar u pb and fission track methodologies and single grain sm nd and pb isotopic analyses are described the paper documents how the sediment record can be used to determine the thermal and tectonic evolution of the orogen constrain mechanisms of continental deformation exhumation rates and palaeodrainage

implications of crustal property variations for models of tibetan plateau evolution shear coupled teleseismic p waves sampling the interior of the tibetan plateau provide evidence of systematic variations in crustal structure the crust thins by up to 20 km from south to north with a concomitant increase in poisson s ratio from normal values in the south to unusually high values in the north this suggests that the crust of the northern plateau is partially melted due to high temperatures these changes imply spatial and perhaps temporal variations in the way the elevation of the high plateau is created and maintained

has focused denudation sustained active thrusting at the himalayan topographic front the geomorphic character of major river drainages in the himalayan foothills of central nepal suggests the existence of a discrete west northwest trending break in rock uplift rates that does not correspond to previously mapped faults the 40 ar 39 ar thermochronologic data from detrital muscovites with provenance from both sides of the discontinuity indicate that this geomorphic break also corresponds to a major discontinuity in cooling ages samples to the south are proterozoic to paleozoic whereas those to the north are miocene and younger combined these observations virtually require recent pliocene holocene motion on a thrust sense shear zone in the central nepal himalaya 20 30 km south of the main central thrust field observations are consistent with motion on a broad shear zone subparallel to the fabric of the lesser himalayan lithotectonic sequence the results suggest that motion on thrusts in the toe of the himalayan wedge may be synchronous with deeper exhumation on more hinterland structures in central nepal we speculate that this continued exhumation in the hinterland may be related to intense sustained erosion driven by focused orographic precipitation at the foot of the high himalaya

ductile extrusion of the higher himalayan crystalline in bhutan evidence from quartz microfabrics quartz textures measured from deformed quartz tectonites within the lesser himalaya and higher himalaya crystalline of bhutan show similar patterns orientation and distribution of the quartz crystallographic axes were used to confirm the regional shear sense the asymmetry of c axis and a axis patterns consistently indicates top to the south shearing the obliquity of the texture and the inferred finite strain plane strain to moderately constrictional suggest the strain regime had a combination of rotational and irrotational strain path in most of the samples from the bhutan himalaya the inferred deformation mechanisms suggest moderate to high temperature conditions of deformation that produced the observed crystallographic preferred orientation much higher temperature of deformation is indicated in the quartz veins from a leucogranite the observed ductile deformation is pervasively developed in the rocks throughout the investigated area the intensity of deformation increases only slightly in the vicinity of the main central thrust simultaneous southward shearing within a large part of the higher himalaya crystalline near and above the main central thrust and normal faulting across the south tibetan detachment is explained by the tectonically induced extrusion of a ductily deforming wedge the process of extrusive flow suggested here can be approximated quantitatively by channel flow models that have been used to describe subduction zone processes channel flow accounts for some observed phenomena in the himalayan orogen such as inverted metamorphic sequences near the main central thrust not related to an inversion of isotherms and the syntectonic emplacement of leucogranites into the extruding wedge locally leading to an inversion of isotherms due to heat advection

global positioning system measurements from eastern tibet and their implications for india eurasia intercontinental deformation we present and interpret global positioning system gps measurements of crustal motions for the period 1991 1998 for a network encompassing the eastern part of the tibetan plateau and its foreland relative to a eurasian frame defined by minimizing the velocities of 16 gps stations in europe central asia and siberia stations within all parts of the plateau foreland in south china move 6 10 mm yr east southeast indicating that the eastward movement within the plateau is part of a broader eastward movement that involves the plateau and its eastern and northern foreland north of the plateau foreland stations move northeastward at 10 mm yr indicating that the northern boundary of the deformation zone lies north of the plateau with this realization of a eurasian frame the velocity of the gps station at bangalore in southern india implies that the northward motion of india is 5 12 mm yr slower than that predicted from the nuvel 1a plate reconstruction viewed relative to the south china block stations of the northeast plateau bounded on the north by the qilian shan and the altyn tagh fault move nne to ne with velocities ranging from 19 mm yr within the plateau to 5 11 mm yr in its foreland the altyn tagh fault shows left lateral slip of 10 mm yr at 95 e and shortening across the fault of 5 mm yr stations south and west of the xianshuihe xiaojiang fault system define a crustal fragment rotating clockwise at 10 mm yr relative to the south china block around the eastern himalayan syntaxis the gps measurements indicate no significant shortening 3 mm yr within the longmen shan of the central eastern plateau and its adjacent foreland although the longmen shan rise over 6 km in 100 km horizontal distance geological studies indicate that the deformational field was established diachronously in late miocene to pliocene time was characterized by no east west shortening of tibetan crust and has an inhomogeneous style of deformation resulting from a balance of different tectonic processes

southward extrusion of tibetan crust and its effect on himalayan tectonics the tibetan plateau is a storehouse of excess gravitational potential energy accumulated through crustal thickening during india asia collision and the contrast in potential energy between the plateau and its surroundings strongly influences the modern tectonics of south asia the distribution of potential energy anomalies across the region derived from geopotential models indicates that the himalayan front is the optimal location for focused dissipation of excess energy stored in the plateau the modern pattern of deformation and erosion in the himalaya provides an efficient mechanism for such dissipation and a review of the neogene geological evolution of southern tibet and the himalaya shows that this mechanism has been operational for at least the past 20 million years this persistence of deformational and erosional style suggests to us that orogens like other complex systems can evolve toward steady state configurations maintained by the continuous flow of energy the capacity of erogenic systems to self organize into temporally persistent structural and erosional patterns suggests that the tectonic history of a mountain range may depend on local energetics as much as it does on far field plate interactions

injection of indian crust into tibetan lower crust a two dimensional finite element model study we propose a model in which the stronger indian crust injects into the weaker lower crust of tibet if the viscosity of the tibetan lower crust is below a critical value the continuous advancing of the indian crust beneath tibet would produce negligible variation in stresses on the bottom of the stronger brittle tibetan upper crust and would result in a spatially uniform uplift and a virtually undisturbed tibetan plateau surface this process is analogous to that of a piston rising in a hydraulic jack as additional fluid is forced into the chamber we find that if the viscosity of the tibetan lower crust is assumed to be constant throughout the computed critical viscosity is 6 1018 pa s for the present day lower crustal thickness of 58 km in addition to uniform viscosity two hypothetical depth dependent viscosities are also tested which give rise to essentially the same distribution of stress along the bottom of brittle upper crust our model also interprets the 4 km elevation contrast between the tibetan plateau and the tarim block a 1 160 viscosity contrast between the tibetan and tarim lower crust is required in order to maintain the current thickening rate of the tibetan crust 1 2 mm yr while keeping the thickness of the tarim crust unchanged

finite strain calculations of continental deformation 2 comparison with the india asia collision zone numerical experiments on a thin viscous sheet model for deformation of continental lithosphere subjected to an indenting boundary condition yield distributions of crustal thickness of stress and strain rate and of latitudinal displacements that may be compared with observations in the india asia collision zone a simple indenting boundary condition applied to initially laterally homogeneous sheets obeying a power law rheology produces results that are in broad agreement with the observations provided that the power law exponent is three or greater and the sheet can support vertically integrated stress differences of 2 1013 5 1012 n m 1 in regions in front of the indenter under these conditions the calculated deformation shows accommodation of convergence primarily by crustal thickening to produce a plateau in front of the indenter palaeomagnetic data from india and tibet and the observed distribution of topography suggest that much of the post eocene convergence of india with asia has been taken up by deformation within asia that involved crustal thickening the principal difference between calculation and observation is the absence from the calculated strain rate fields of east west extension of the plateau in front of the indenting boundary the calculations show that once such a plateau is formed the buoyancy force associated with the crustal thickness contrast inhibits further thickening and the plateau strains at less than half the rate of its immediate surroundings seismically determined regional strain rates exhibit a similar distribution with the tibetan plateau straining at about one quarter the rate of the tien shan and ningxia gansu regions calculated principal compressive stress orientations and regional strain rates agree with the seismically determined quantities in the mongolia baikal tien shan tibet and ningxia gansu regions of asia to within the uncertainty of the latter the vertically integrated stresses that are calculated for the viscous sheet are comparable with those that can be supported by a theologically stratified continental lithosphere obeying laboratory determined flow laws we suggest that the thin viscous sheet model described in this paper and its companion gives a simple and physically plausible description of the observed deformation in central asia in this description the predominant mechanism of accommodation of continental convergence is diffuse crustal thickening with shear on vertical planes playing a subsidiary role once large crustal thickness contrasts have been established

metamorphism transient mid crustal rheology strain localization and the exhumation of high grade metamorphic rocks 1 we present a series of three dimensional numerical models investigating the effects of metamorphic strengthening and weakening on the geodynamic evolution of convergent orogens that are constrained by observations from an exposed mid crustal section in the new england appalachians the natural mid crustal section records evidence for spatially and temporally variable mid crustal strength as a function of metamorphic grade during prograde polymetamorphism our models address changes in strain rate partitioning and topographic uplift as a function of strengthening weakening in the middle crust as well as the resultant changes in deformation kinematics and potential exhumation patterns of high grade metamorphic rock results suggest that strengthening leads to strain rate partitioning around the zone and suppressed topographic uplift rates whereas weakening leads to strain rate partitioning into the zone and enhanced topographic uplift rates deformation kinematics recorded in the orogen are also affected by strengthening weakening with complete reversals in shear sense occurring as a function of strengthening weakening without changes in plate boundary kinematics

the kumaun and garwhal lesser himalaya india part 2 thermal and deformation histories an integrated petrologic thermochronologic and numerical modeling study constrains the thermal evolution of the lesser himalayan sequence in the kumaun and garwhal regions of india south of the tons thrust peak metamorphic temperatures do not exceed 330 c on the northern side a transition toward higher temperatures adjacent to the main central thrust hanging wall is documented in the immediate footwall of the main central thrust peak temperatures average 550 c an inverted thermal field gradient of 30 c km beneath the vaikrita thrust is calculated an inverted thermal field gradient of 20 c km is also documented beneath a klippe of the main central thrust hanging wall 25 km south of the vaikrita thrust and a peak footwall temperature of 530 c is recorded close to the shear zone at the base of the klippe the mechanism for thermal metamorphism in the lesser himalayan sequence is interpreted to be conduction of heat from the main central thrust fault and or hanging wall to the footwall a localized thermal field gradient is also associated with the tons thrust the 40 ar 39 ar thermochronology reveals that the exposed rock in the kumaun and garwhal lesser himalaya underwent cooling below the closure temperature tc for white mica at different times in the late tertiary the proximal footwall of the munsiari thrust experienced cooling below the white mica tc at ca 8 5 ma south of the munsiari thrust in the proximal footwalls of the chaukori and askot klippen muscovites ages are ca 11 5 ma 3 ma older than samples to the north the age differences in samples from northern and southern exposures are interpreted to document cooling related to thermal relaxation following passage of the main central thrust hanging wall atop the lesser himalayan sequence results of thermal modeling achieve good fits to the data when scenarios involve an early miocene phase of overthrusting of a hot hanging wall over a downgoing footwall followed by the initiation of a duplex within lesser himalayan sequence rocks

thermal mechanical model for crustal thickening in the central andes driven by ablative subduction the thick up to 65 km crust of the high elevation regions of the andean orogenic belt results from structural thickening of the felsic upper crust of south america the great thickness implies shortening or loss of several hundred kilometers of lithospheric mantle and lower crust through numerical simulation of the thermal and mechanical processes of crustal thickening we test a model of lithospheric mantle removal by ablative subduction the model combines kinematic assumptions regarding mantle motion with dynamic calculations of crustal deformation predictions include the formation of an outwardly growing orogenic plateau analogous to the altiplano that results from high temperature low viscosity flow of the lower crust due to thickening of the high heat production upper crust

tectonic mode switches and the nature of orogenesis abstract the birth and death of many mountain belts occurs in lithosphere that over rides major subduction zones here the tectonic mode shortening versus extension can abruptly switch even during continuous and otherwise smooth convergence if the hinge line of the foundering slab rapidly retreats i e rolls back the foundering slab creates a gravitational potential well into which the orogen collapses this motion coupled with stress guides can pull the orogen apart a slowing of roll back or of hinge retreat means that the subduction flexure may subsequently begin to be pushed back or be pushed over by the advancing orogen the consequence of such changes in relative motion is that orogenic belts are affected by abrupt tectonic mode switches the change from push to pull leads to a sudden change from horizontal extension to horizontal shortening potentially throughout the entire mass of the orogenic lithosphere that over rides the subducting slab the sequencing of these tectonic mode switches affects the thermal evolution of the orogen and thus fundamentally determines the nature of orogenesis this insight led to us to our quite different views as to how orogens work it is evident that orogens affected by abrupt push pull mode switches are characterized by high pressure metamorphism whereas orogens affected by abrupt pull push mode switches are characterized by high temperature metamorphism magmatism and anatexis

inverted metamorphism and the main central thrust field relations and thermobarometric constraints from the kishtwar window nw indian himalaya following the early eocene collision of the indian and asian plates intracontinental subduction occurred along the main central thrust mct zone in the high himalaya in the kishtwar zanskar himalaya the mct is a 2 km thick shear zone of high strain distributed ductile deformation which emplaces the amphibolite facies high himalayan crystalline hhc unit south westwards over the lower greenschist facies lesser himalaya an inverted metamorphic field gradient mapped from the first appearance of garnet staurolite and kyanite index minerals is coincident with the high strain zone petrography and garnet zoning profiles indicate that rocks in the lower mct zone preserve a prograde assemblage whereas rocks in the hhc unit show retrograde equilibration thermobarometric results derived using thermocalc indicate a p t increase of c 180 c and c 400 mpa across the base of the mct zone which is a consequence of the syn to postmetamorphic juxtaposition of m1 kyanite grade rocks of the hhc unit on a cooling path over biotite grade footwall rocks which subsequently attain their peak m2 during thrusting inclusion thermobarometry from the lower mct zone reveals that m2 was accompanied by loading and peak conditions of 537 38 c and 860 120 mpa were attained m1 kyanite assemblages in the hhc unit which have not been overprinted by m2 fibrolitic sillimanite were not significantly affected by m2 and conditions of equilibration are estimated as 742 53 c and 960 180 mpa there is no evidence for dissipative or downward conductive heating in the mct zone instead the primary control on the distribution of peak assemblages represented by the index minerals is postmetamorphic ductile thrusting in a downward propagating shear zone polymetamorphism and diachroneity of equilibration are also important controls on the thermal profile through the mct zone and hhc unit

variations in the earth s oblateness during the past 28 years 1 analysis of satellite laser ranging slr data indicates that the earth s dynamic oblateness j2 has undergone significant variations during the past 28 years the dominant signatures in the observed variations in j2 are 1 a secular decrease with a rate of approximately 2 75 10 11 yr 1 2 seasonal annual variations with a mean amplitude of 2 9 10 10 3 significant interannual variations with timescales of 4 6 years and 4 a variation with period of 21 years and an amplitude of 1 4 10 10 with minimum in december 1988 two large interannual variations are related to the strong el nino southern oscillation events during the periods of 1986 1991 and 1996 2002 and it appears that another interannual cycle may have started in late 2002 the superposition of the decadal variation on the interannual signal makes the j2 fluctuation appear to be anomalously large during the 1996 2002 period contemporary models of the mass redistributions in the atmosphere ocean and surface water can explain a major part of the 4 to 6 year fluctuations however the cause of the decadal variation remains unknown

crustal structure of the himalayan orogen at 90 east longitude from project indepth deep reflection profiles international deep profiling of tibet and the himalaya indepth deep seismic reflection profiles show that the indian lithosphere is underthrusting the central himalaya along a gently north dipping decollement that is traceable northward beneath the tethyan belt to 28 6 n and to a depth of about 45 km the decollement carries in its hanging wall a near crustal thickness slice of internally deformed indian continental basement and cover represented in outcrop by the greater himalayan belt and structurally overlying tethyan belt geometric relationships suggest that the decollement probably ramps downward beneath the northern tethyan belt to near the base of the crust and that the hanging wall crustal slice was detached from indian mantle lithosphere that presently underlies southern tibet to the north the north himalayan anticlinorium kangmar dome appears to be a large duplex ramp anticline in the hanging wall of the decollement the south tibetan detachment appears to have been imaged in two areas in the wagye la area it appears to follow the tethyan belt greater himalayan belt contact northward in the subsurface and to have been folded over the kangmar dome in the zherger la area it appears to have been active relatively recently as a 30 north dipping normal fault that cuts deeply into the greater himalayan belt allochthon palinspastic reconstruction indicates a minimum of about 326 km of shortening of the indian basement across the himalaya since the initiation of the main central thrust hence indian continental lithosphere largely stripped of its overlying crust can extend northward beneath the tibetan plateau to at least 32 n where earthquake seismological observations show that the properties of the upper mantle change markedly e g mcnamara et al 1995 taken together the seismic reflection and earthquake seismological observations lend support for the view that the indian mantle lid mechanically underplates roughly the southern half of the tibetan plateau the indepth reflection data do not show that indian crust attached to this lithosphere extends north of southernmost tibet kangmar dome nor do they negate this possibility similarly the reflection data do not yield direct evidence for or against lower crustal subduction beneath the himalaya

mechanical model for the tectonics of doubly vergent compressional orogens a mechanical model of crustal shortening and deformation driven by the relative convergence of rigid underlying mantle plates explains many features of convergent orogens results based on numerical models and supported by sandbox models show that a coulomb crustal layer subject to basal velocity boundary conditions corresponding to asymmetric detachment and subduction of the underlying mantle passes through three stages of orogenic growth 1 block uplift bounded by step up shear zones 2 development of a low taper wedge over the underthrusting mantle plate and 3 development of a low taperwedge overlying the overthrusting mantle plate and verging in the opposite direction when modified by isostasy basal viscous flow surface erosion and denudation and sedimentation the resultant model orogens exhibit a variety of styles with characteristics in common with small rapidly denuded orogens large orogens with plateaus and extensional characteristics and active subduction margins with doubly vergent accretionary wedges and deformed fore arc basins

a closer comparison of early and late winter atmospheric trends in the northern hemisphere abstract decadal trends are compared in various fields between northern hemisphere early winter november december nd and late winter february march fm months using reanalysis data it is found that in the extratropics and polar region the decadal trends display nearly opposite tendencies between nd and fm during the period from 1979 to 2003 dynamical trends in late winter fm reveal that the polar vortex has become stronger and much colder and wave fluxes from the troposphere to the stratosphere are weaker consistent with the positive trend of the arctic oscillation ao as found in earlier studies while trends in nd appear to resemble a trend toward the low index polarity of the ao in the tropics the hadley circulation shows significant intensification in both nd and fm with stronger intensification in fm unlike the hadley cell the ferrel cell shows opposite trends between nd and fm with weakening in nd and strengthening in fm comparison of the observational results with general circul

large scale crustal deformation of the tibetan plateau the topography velocity and strain fields calculated from a three dimensional newtonian viscous model for large scale crustal deformation are generally in good agreement with results from geological geodetic and earthquake studies in and around the tibetan plateau provided that the model rheology incorporates a weak zone within the deep crust beneath the plateau equivalent to a viscosity of 1012 pa s within a 250 m thick channel or 1018 pa s within a 15 km thick channel model studies and observations show a plateau at 5 km elevation with steep topographic gradients across the southern and northern plateau margins and more gentle gradients across the southeastern and northeastern margins rapid shortening strain is concentrated along the lower portions of the northern and southern plateau margins at rates 20 mm yr model results show north south shortening 10 mm yr in reasonable agreement with gps data 5 8 mm yr of north south shortening across the northern two thirds of the plateau and east west stretching 10 15 mm yr across the eastern half of the high plateau in reasonable agreement with seismic geologic and gps data upper crustal material moves eastward from the plateau proper into a lobe of elevated topography that extends to the south and east clockwise rotation of material around the east himalayan syntaxis at rates up to 10 mm yr occurs partly as a result of dextral shear between indian and asian mantle at depth and partly as a result of gravitational spreading from the high plateau to the south and east there is little difference in model surface deformation for assumptions of moderately weak or extremely weak lower crust except in southern and northern tibet where margin perpendicular extension occurs only for the case of an extremely weak lower crust our results suggest that the tibetan plateau is likely to have gone through a two stage development the first stage produced a long narrow high orogen whose height may have been comparable to the modern plateau the second stage produced a plateau that grew progressively to the north and east east west stretching eastward plateau growth and dextral rotation around the east himalayan syntaxis probably did not begin until well into the second stage of plateau growth perhaps becoming significant after 20 m y of convergence

crustal channel flows 2 numerical models with implications for metamorphism in the himalayan tibetan orogen 1 results from a thermal mechanical model ht1 that includes midcrustal channel flow are compatible with many features of the himalayan tibetan system radioactive self heating and rheological weakening of thickened model orogenic crust lead to the formation of a hot low viscosity midcrustal channel and a broad plateau channel material corresponding to the greater himalayan sequence ghs flows outward from beneath the plateau in response to topographically induced differential pressure at the plateau flank it is exhumed by focused surface denudation and juxtaposed with cooler newly accreted material corresponding to the lesser himalayan sequence lhs the model channel is bounded by coeval thrust and normal sense ductile shear zones interpreted to represent the main central thrust mct zone and south tibetan detachment system respectively inverted metamorphism associated with the model mct zone results from distributed ductile shear along the mct and extrusion of the hot channel a variety of model p t t path styles resembling those observed in the ghs and lhs are produced for points traveling through contrasting tectonic regimes that coexist in different parts of the model predicted times of peak metamorphism cooling and erosion of metamorphic facies are generally compatible with observations although model ghs cooling ages are too young the times of m1 and m2 metamorphic events observed in the ghs correspond to model times of maximum burial and maximum heating respectively the results highlight the need to integrate tectonics and metamorphism in continental collision models and demonstrate the importance of lateral transport of both heat and material in large hot orogens

shortening budgets and the role of continental subduction during the india asia collision abstract the india asia collision provides a remarkable example of diffused deformation because the zone of compressive strain extends far into the hinterland of the himalaya molnar and tapponnier s science 189 1975 419 concept of india as a rigid indenter responsible for the diffused deformation has been widely accepted perhaps uncritically this review gives a reappraisal of landmark papers by dewey et al philos trans r soc london a 327 1988 379 eclogae geol helv 82 1989 717 which discussed the shortening budget in se asia and the evidence for the amount and the rate of the convergence between india and asia taking place after the initial collision of these continents at roughly 55 50 ma evidence for the total convergence comes from magnetic anomaly palaeomagnetic and volumetric balancing studies the results are consistent in showing about 1800 2100 km convergence between india and asia in the western sector 2475 km in the central and 2750 2800 km in the eastern sector the rate of convergence between india and asia over the period since initial collision is roughly 5 cm year however the possible long term rate of convergence within the himalaya is considerably less i e 1 5 2 0 cm year a fraction of the total convergence this shortfall can be made up by adding to the rate for the himalaya alone the convergence rates in other parts of the diffused zone of compressive strain i e tibet tien shan altai this works well for the post oligocene but not for the earlier post collisional history when the areal extent of thrust fold tectonics was much more limited possibly the apparent shortening deficit before the miocene was made up by strike slip faulting but with the notable exception of the red river fault evidence for widespread pre miocene strike slip faults is not convincing summation of the amount of shortening in thrust fold regimes in the himalaya and its hinterland falls short of that required by the total convergence figures quoted above the deficit arises firstly because of the likelihood that pre collisional crustal shortening of uncertain areal extent in tibet reduces tibet s contribution 40 according to dewey et al philos trans r soc london a 327 1988 379 eclogae geol helv 82 1989 717 to the shortening budget for the cenezoic revised shortening budgets are presented on the assumption that most of the lhasa block southern tibet was already thickened prior to collision this thickening is difficult to quantify secondly balanced cross sections from various parts of the himalaya give 60 70 shortening that is only about 500 km roughly half of the shortening proposed by dewey et al the conclusion of le pichon et al tectonics 11 1992 1085 that only indian mid upper crust is involved in the himalayan thrust sheets is supported but it needs to be reassessed in the light of the recognition of eocene oligocene thickening in the himalaya this early crustal thickening antedates the sequence of thrusts which started with the main central thrust and therefore the shortening estimates based on balanced cross sections probably underestimate the total shortening in the himalaya the prospect of substantial underthrusting of india beneath tibet minimizes the amount of horizontal shortening of tibetan lithosphere during the cenozoic the conclusion is that either the total convergence deduced from geophysical and volumetric work is too large or the deficit must be made up by shortening mechanisms other than thrust fold regimes i e strike slip faulting which serves to move rocks out of the way of india

tethyan subducted slabs under india abstract tomographic imaging of the mantle under tibet india and the adjacent indian ocean reveals several zones of relatively high p wave velocities at various depths under the hindu kush region in northeastern afghanistan and southern tajikistan a regional northward dipping slab is seen in the entire upper 600 km of the mantle and is apparently still attached to the lithosphere of the indian plate under northern pakistan this same slab shows a roll over structure with the deeper portion overturned and dipping southward as can also be seen in the distribution of earthquake hypocenters farther east southeast e g in the vicinity of nepal a well resolved anomaly below 450 km depth is connected to the slab under the hindu kush but seems to be separated from the lithosphere above 350 km these upper mantle anomalies are interpreted as the remnants of delaminated sub continental lithosphere that went down when greater india continued to converge northward with asia after 45 ma the deeper high velocity anomalies under the indian sub continent appear clearly separated from the shallower ones as well as from each other and are inferred to represent remnants of oceanic lithospheric slabs that have sunk into the lower mantle and were subsequently overridden by the indian plate they occur at depths between 1000 and 2300 km and occasionally descend down to the core mantle boundary the anomalies form three parallel wnw ese striking zones we interpret the two southern zones as remnants of oceanic lithosphere that was subducted when the neo tethys ocean closed between india and tibet in the cretaceous and earliest tertiary the northern deep mantle zone under northern afghanistan the himalayas and the lhasa block in southern tibet may represent the last subducted remnant of the paleo tethys ocean which is thought to have closed before the hauterivian stage of the early cretaceous the middle zone continues southeastward as a rather straight high velocity zone towards sumatra where it becomes convex southward and parallel to the subduction zone under the sunda arc comparison of this straight middle zone near india with the shallower upper 600 1000 km northern zone which displays a cusp like shape near the yunnan sw china syntaxis of the eastern himalayas supports the notion that the shallow northern zone represents later subduction than the deeper middle zone the suggestion of a counterclockwise rotation 20 of the indian plate during tertiary indentation of asia is supported by these features the present day latitudes of 5 35 n of the deep slabs under india and adjacent areas correspond to the approximate paleolatitudes of the cretaceous subduction zones the slab remnants in the middle mantle occur therefore near the ancient locations where they started their downward journey which implies that lateral movements in the deeper mantle were not large

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

evolution of orogenic wedges and continental plateaux insights from crustal thermal mechanical models overlying subducting mantle lithosphere the links between an early phase of orogenesis when orogens are commonly wedge shaped and a later phase with a plateau geometry are investigated using coupled thermal mechanical models with uniform velocity subductionboundary conditions applied to the base of the crust and simple frictional plastic and viscous rheologies models in which rheological properties do not change with depth or temperature are characterized by growth of back to back wedges above the subduction zone wedge taper is inversely dependent on the ramberg number rm gravity stress basal traction increasing convergence velocity or crustal strength produces narrower and thicker wedges models that are characterized by a decrease in crustal viscosity from c to b with depth or temperature leading to partial or full basal decoupling of the crust from the mantle display more complex behaviour for models with a moderate viscosity ratio b c 10 1 the crustal wedges have dual tapers with a lower taper in the central region and a higher taper at the edges of the deformed crust a redaction in the viscosity ratio b c 10 2 is sufficient to cause a transition of the central wedge region to a plateau this transition depends on the basal traction therefore the thickness of the weak basal layer also affects the transition further reduction of the viscosity ratio b c 10 4 leads to full basal decoupling and the development of plateaux in all cases considered in most models the plateaux grow laterally at constant thickness between characteristic edge peaks associated with the transitions from coupled to decoupled lower crust where the crust is fully decoupled large scale model geometries for both depth and temperature dependent rheologies are similar with gravity driven flow concentrated in the low viscosity region however strong lateral temperature gradients within these models controlled by the interaction of horizontal and vertical thermal advection diffusion and heterogeneous thickening of the radioactive crustal layer lead to differences in the velocity and deformation fields between the two cases particularly at the plateau margins the results suggest that simple depth dependent viscosity models may be reasonable approximations for describing the large scale geometry of fully developed plateaux but that they are not appropriate for describing the internal features of large orogenic systems or the transition from wedge to plateau geometry

himalayan tectonics explained by extrusion of a low viscosity crustal channel coupled to focused surface denudation recent interpretations of himalayan tibetan tectonics have proposed that channel flow in the middle to lower crust can explain outward growth of the tibetan plateau1 2 3 and that ductile extrusion of high grade metamorphic rocks between coeval normal and thrust sense shear zones can explain exhumation of the greater himalayan sequence4 5 6 7 here we use coupled thermal mechanical numerical models to show that these two processes channel flow and ductile extrusion may be dynamically linked through the effects of surface denudation focused at the edge of a plateau that is underlain by low viscosity material our models provide an internally self consistent explanation for many observed features of the himalayan tibetan system8 9 10

metamorphic processes a celebration of the career contribution of ron vernon this special issue is in celebration of the career contribution of ron vernon it arises from the ron vernon symposium which was held as part of the 15th australian geological convention in sydney in 2000 to date ron has published more than one hundred scientific papers several books and contributed widely to international conferences he is the doyen in the multidisciplinary field of microstructures which spans petrology and structural geology however ron s range is phenomenal and he has contributed to many aspects of metamorphism magmatism and rock deformation he has drawn on data from his work in these areas to contribute widely to understanding thermal regimes crustal evolution and tectonics as exemplified by his ideas about htlp metamorphism and emplacement of granites ron has followed the scientific method in his work it has led him to many innovative interpretations some of which undermined established dogma by example ron has been a positive role model for the younger generation however for the older generation he has been something of a bete noire the papers in this special issue cover the range from low to high grade metamorphism from metasomatism to melting from microstructures to tectonics and from fieldwork to experiments it is our hope that in reading them you will take time to think about what has been achieved in the past 40 years and use this as motivation for what remains to be done

complex 3d strain patterns in a synkinematic tonalite batholith from the aracuai neoproterozoic orogen eastern brazil evidence from combined magnetic and isotopic chronology studies abstract this work combines structural and geochronological data to improve our understanding of the mechanical behaviour of continental crust involving large amount of magma or partially melted material in an abnormally hot collisional belt we performed a magnetic and geochronological u pb study on a huge tonalitic batholith from the neoproterozoic aracuai belt of east brazil to determine the strain distribution through space and time anisotropy of magnetic susceptibility combined with rock magnetism investigations supports that the magnetic fabric is a good proxy of the structural fabric field measurements together with the magnetic fabrics highlight the presence in the batholith of four domains characterized by contrasted magmatic flow patterns the western part is characterized by a gently dipping orogen parallel ns magmatic foliation that bears down dip lineations in agreement with westward thrusting onto the sao francisco craton eastward the magmatic foliation progressively turns sub vertical with a lineation that flips from sub horizontal to sub vertical over short distances this latter domain involves an elongated corridor in which the magmatic foliation is sub horizontal and bears an orogen parallel lineation finally the fourth narrow domain displays sub horizontal lineations on a sub vertical magmatic foliation oblique n150 e to the trend of the belt u pb dating of zircons from the various domains revealed homogeneity in age for all samples this together with the lack of solid state deformation suggests that 1 the whole batholith emplaced during a magmatic event at 580 ma 2 the deformation occurred before complete solidification and 3 the various fabrics are roughly contemporaneous the complex structural pattern mapped in the studied tonalitic batholith suggests a 3d deformation of a slowly cooling large magmatic body and its country rock we suggest that the development of the observed 3d flow field was promoted by the low viscosity of the middle crust that turned gravitational force as an active tectonic force combining with the east west convergence between the sao francisco and congo cratons

interaction of metamorphism deformation and exhumation in large convergent orogens coupled thermal mechanical models are used to investigate interactions between metamorphism deformation and exhumation in large convergent orogens and the implications of coupling and feedback between these processes for observed structural and metamorphic styles the models involve subduction of suborogenic mantle lithosphere large amounts of convergence 450 km at 1 cm yr 1 and a slope dependent erosion rate the model crust is layered with respect to thermal and rheological properties the upper crust 0 20 km follows a wet quartzite flow law with heat production of 2 0 w m 3 and the lower crust 20 35 km follows a modified dry diabase flow law with heat production of 0 75 w m 3 after 45 myr the model orogens develop crustal thicknesses of the order of 60 km with lower crustal temperatures in excess of 700 c in some models an additional increment of weakening is introduced so that the effective viscosity decreases to 1019 pa s at 700 c in the upper crust and 900 c in the lower crust in these models a narrow zone of outward channel flow develops at the base of the weak upper crustal layer where t 600 c the channel flow zone is characterised by a reversal in velocity direction on the pro side of the system and is driven by a depth dependent pressure gradient that is facilitated by the development of a temperature dependent low viscosity horizon in the mid crust different exhumation styles produce contrasting effects on models with channel flow zones post convergent crustal extension leads to thinning in the orogenic core and a corresponding zone of shortening and thrust related exhumation on the flanks velocities in the pro side channel flow zone are enhanced but the channel itself is not exhumed in contrast exhumation resulting from erosion that is focused on the pro side flank of the plateau leads to ductile extrusion of the channel flow zone the exhumed channel displays apparent normal sense offset at its upper boundary reverse sense offset at its lower boundary and an inverted metamorphic sequence across the zone the different styles of exhumation produce contrasting peak grade profiles across the model surfaces however p t t paths in both cases are loops where pmax precedes tmax typical of regional metamorphism individual paths are not diagnostic of either the thickening or the exhumation mechanism possible natural examples of the channel flow zones produced in these models include the main central thrust zone of the himalayas and the muskoka domain of the western grenville orogen

mesozoic evolution of the hefei basin in eastern china sedimentary response to deformations in the adjacent dabieshan and along the tanlu fault this paper presents a study of jurassic early cretaceous sedimentary evolution of the hefei basin in eastern china and explores the relationship between clastic sedimentation and coeval deformation of the dabieshan to the south and the tanlu fault to the east the hefei basin experienced a two stage evolution the basin was initiated in the early jurassic and expanded in the middle and late jurassic the synsedimentary jinzhai normal fault is considered to be a border fault of the basin because it controlled middle to upper jurassic proximal alluvial deposits persistent n to ne directed paleo flows in the southern hefei basin indicate that sediments came from the dabieshan and the presence of triassic coesite bearing detrital zircon in lower jurassic sediments documents exhumation of ultrahigh pressure rocks of the dabieshan to the surface as early as the early jurassic occurrences of eclogite clasts in an upper jurassic unit indicate continued denudation of the dabieshan at that time thickening of jurassic clastic units to the southern and southeast parts of the basin suggests that basin subsidence and depositional loci were under the coupled control of the jinzhai normal fault on the south and the ne striking left lateral transtensional tanlu fault on the east jurassic extensional subsidence of the hefei basin is in marked contrast to the coeval development of a contractional foreland basin south of the dabieshan which combines to indicate contemporaneous extension and contraction on the north and south sides of the dabieshan respectively vigorous volcanism and uplift of the southern hefei basin characterized the second stage of development of the hefei basin in the early cretaceous and this led to a synchronous shift of its main depocenter to the north this younger depocenter is characterized by lacustrine and fluvial deltaic sedimentation where alluvial and fan deltaic coarse grained deposition mainly occurred along the eastern edge of the basin early cretaceous subsidence is attributed to e w extension across the middle segment of the tanlu fault and the zhangbaling massif on the east acted as a footwall and provided a source for sediment to the northern basin a model is accordingly advanced to account for how the hefei basin developed in response to the tectonic exhumation of the dabieshan and the deformation of the tanlu fault in the mesozoic it illustrates that the hefei basin initiated and evolved during jurassic time in an extensional setting that was triggered by southerly upward extrusion of ultra high pressure rocks of the dabieshan early cretaceous development of the basin was controlled by magmatism related uplift of the dabieshan on the south and orthogonal normal faulting of the middle segment of the tanlu fault on the east this study provides an independent constraint upon the exhumation processes of ultrahigh pressure rocks of the dabieshan

an arbitrary lagrangian eulerian formulation for creeping flows and its application in tectonic models summary this paper presents and analyses numerical techniques developed to investigate viscoplastic stokes flows within a model of lithospheric deformation in particular the techniques are related to a subduction model of compressional orogens the driving mechanism in the model corresponds to the near rigid convergence and subduction of one mantle lithosphere beneath another in plane strain and this boundary condition forces flow in an overlying viscoplastic model crust the numerical techniques use the arbitrary eulerian lagrangian formulation in which flows with free surfaces and large deformation are computed on an evolving eulerian finite element grid that conforms to the material domain a regridding algorithm allows the associated lagrangian motion and fields to be followed and in addition coupled back to the eulerian calculation of the flow mass flux boundary conditions are used so that the effects of erosion and deposition by surface processes and mass loss by subduction can be included in the model calculation the evolving model crustal layer is flexurally compensated using a general elastic beam formulation the applicability of the numerical techniques to problems ranging from accretionary wedges to crustal and lithospheric scale deformation is discussed simple flows a linear viscous subduction model a whirl flow and a quasi convection model are used to show that the mass conservation regridding and surface tracking errors are small the broader applicability of the modelling techniques is reviewed

distribution of active rock uplift along the eastern margin of the tibetan plateau inferences from bedrock channel longitudinal profiles 1 current models of long term river incision into bedrock suggest that the local rate of differential rock uplift should exert a primary control on the gradient of channel longitudinal profiles however discrimination of this effect from the influence of variations in substrate erodibility sediment flux precipitation and transient changes in profile shape has proved difficult in practice here we investigate the controls on the spatial distribution of bedrock channel gradients adjacent to the sichuan basin in an effort to assess the degree and nature of active deformation along this margin of the tibetan plateau analysis of river longitudinal profiles utilizing a channel steepness index a measure of profile gradient normalized for drainage area reveals a zone of anomalously steep channels adjacent to the topographic front of the plateau margin channel profiles are systematically less steep in their headwater reaches on the plateau and in their lower reaches east of the plateau margin comparison of steepness indices to mapped lithologic variations reveals that lithology has only a limited influence on channel gradient in this field area we observe no systematic relationship between steepness indices and upstream drainage area channels of all size are steeper near the plateau margin we argue that these systematic changes are not readily explained as a consequence of increased sediment flux or of orographic precipitation we are led to conclude that steep channel profiles along the topographic front of the plateau reflect active differential rock uplift between this region and the foreland

crustal volume balance during the india eurasia collision and altitude of the tibetan plateau a working hypothesis a working hypothesis is developed for the two stages of evolution of the tibetan plateau during the continental collision between india and eurasia the first phase was dominated by crustal shortening and thickening in what is now tibet while indochina was expelled southeastward away from eurasia at the end of this phase at 17 ma the estimated tibetan crustal thickness was 51 km the second phase has been dominated by thickening of tibet by the influx of crustal material from india with no associated crustal shortening the crustal thickening that is inferred to have occurred by this process is only feasible because leakage of crust from beneath the plateau is restricted by neighboring archean cratons that block lower crustal flow this leakage to the east and south from tibet to beneath yunnan is estimated as 20 30 of the influx at present but was much smaller at the start of this second phase of deformation it is suggested that at 11 ma the force exerted against the tibetan upper crust by this flowing material first exceeded the threshold that this brittle layer could resist and tibet began to extend eastward any satisfactory working hypothesis for the tibetan plateau must explain its dimensions its 17 ma ending of shortening and 11 ma start of extension and volcanism the eastward slip sense on its active normal and strike slip faults and the ratios of their displacements to present day slip rates which imply that their slip rates have gradually increased this working hypothesis accounts for these features and predicts a smooth and continuing increase in altitude

subduction of asian lithospheric mantle beneath tibet inferred from models of continental collision the relative northward motion of the indian subcontinent that followed the onset of continental collision with asia has produced extensive deformation of the earth s crust giving rise to the world s highest mountains in the himalayan chain and the world s largest high elevation region the tibetan plateau the formation of the broad mountain belt implies that contrary to the original tenets of plate tectonics the lithospheric plates have experienced widespread deformation far from the plate boundary1 several models have been proposed2 6 to explain the manner in which this post collisional deformation is distributed within the continental lithosphere of the indian and asian plates here we propose an alternative model in which subduction of the asian lithospheric mantle develops following the collision of india our model is supported by numerical calculations of crustal deformation and thickening and is consistent with available geological and geophysical data7 9 this picture suggests that lithospheric mantle is not deformed along with the crust and would imply that continental collision zones are more analogous to oceanic subduction zones than was previously believed