modeling mountain building and the seismic cycle in the himalaya of nepal a host of information is now available regarding the geological and thermal structure as well as deformation rate across the himalaya of central nepal these data are reconciled in a two dimensional mechanical model that incorporates the rheological layering of the crust which depends on the local temperature and surface processes over geological timescale 5 ma the 20 mm yr estimated shortening rate across the range is accommodated by localized thrust faulting along the main himalayan thrust fault mht the mht reaches the surface along the foothills where it is called the main frontal thrust fault mft the mht flattens beneath the lesser himalaya and forms a midcrustal ramp at the front of the higher himalaya consistent with the river incision and the anticlinal structure of the lesser himalaya farther northward the mht roots into a subhorizontal shear zone that coincides with a midcrustal seismic reflector aseismic slip along this shear zone is accommodated in the interseismic period by elastic straining of the upper crust increasing the coulomb stress beneath the front of the higher himalaya where most of the microseismic activity clusters negligible deformation of the hanging wall requires a low apparent friction coefficient less than 0 3 on the flat portion of the mht on the ramp might be as high as 0 6 sensitivity tests show that a rather compliant quartz rich rheology and a high radioactive heat production in the upper crust of 2 5 w m 3 is required erosion affects the thermal structure and interplays with crustal deformation a dynamic equilibrium is obtained in which erosion balances tectonic uplift maintaining steady state thermal structure topography and deformation field using a linear diffusion model of erosion we constrain the value of the mass diffusivity coefficient to 0 5 1 6 l0 4 m 2 yr this study demonstrates that the data are internally consistent and compatible with current understanding of the mechanics of crustal deformation and highlight the role of viscous flow in the lower crust and of surface erosion in orogeny processes on the long term as well as during interseismic period

flexure of the india plate underneath the bhutan himalaya 1 we investigate flexural geometry and rheology of the india plate beneath the eastern himalaya from a new gravity data set acquired in bhutan compared to the well studied nepal himalaya the obtained bouguer anomaly profiles across the range show shorter wavelength flexure of the lithosphere with a narrower and shallower foreland basin this new data set and seismic moho constraints are interpreted together in terms of lithospheric flexure using a 2 d thermomechanical model our results suggest that the strongest layer of the continental lithosphere beneath bhutan is the upper mantle as it is beneath nepal the observed west to east decrease in flexural wavelength is associated with weakening mantle rheology the simulations show that this decrease can be related to ductile mantle behavior either hydrated wet dunite or dry and hot olivine rheology both models display decoupled lithospheric layers leading to an eastward decrease of flexural rigidity from 10 24 to 5 10 22 n m in nepal and bhutan respectively

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

lateral uniformity of india plate strength over central and eastern nepal the current understanding of the himalayan lithosphere stems mostly from cross sections through the range at the longitude of the kathmandu basin in this paper we laterally extend the analyses of structures and rheology along the nepal himalayas between the pokhara valley and the arun river we take advantage of available information and a new data set including gravity measurements and a receiver function profile it appears that the geometry of the moho inferred from seismological profiles and long wavelength gravity anomalies does not exhibit major east west variations within the 350 km wide study area using thermomechanical modelling we show that the northward deepening of the moho observed along profiles perpendicular to the main thrust faults can be interpreted simply as the bending of a strong india plate this result suggests a gradual mechanical decoupling between the crust and the mantle leading to a northward decrease of the effective elastic thickness of the indian lithosphere from 75 km to 25 km beneath the ganga basin and the tibetan plateau respectively our results also confirm partially eclogitized lower indian crust beneath southern tibet at shorter wavelengths the observed gravity profiles exhibit some small lateral variations that can be interpreted in terms of east west variations of the thickness of subsurface geological structures such as the ganga basin and the tethyan sedimentary sequence

upper limits of power law creep of rocks at depths below a few kilometers most rocks probably flow in the steady state dominantly by diffusion assisted dislocation creep mechanisms whereby strain rate is proportional to a low power of deviatoric stress at high stress levels for metals ceramics and rocks the power law breaks down plb to some other strain rate stress functional relationship and this paper explores four methods by which the breakdown stress b may be estimated for metals alloys and ceramics b 10 3 for non silicate rocks b 5 10 3 and for silicates b 10 2 for silicates the low stress exponent power law breaks down at high stresses to an exponential dependence of strain rate upon stress or to a dorn law the governing flow mechanisms generally being lattice resistance controlled dislocation glide and mechanical twinning plb breakdown stresses for silicates are nearly strain rate insensitive and extrapolated values of b are so high as to suggest that low temperature high stress plasticity may be suppressed entirely in geological deformations plb for olivine rich rocks is discussed in some detail and new three dimensional deformation mechanism surfaces for wet and dry olivine polycrystals are presented

tectonic evolution of a continental collision zone a thermomechanical numerical model we model evolution of a continent continent collision and draw some parallels with the tectonic evolution of the himalaya we use a large scale visco plasto elastic thermomechanical model that has a free upper surface accounts for erosion and deposition and allows for all modes of lithospheric deformation for quartz olivine rheology and 60 mm yr convergence rate the continental subduction is stable and the model predicts three distinct phases during the phase 1 120 km or 6 of shortening deformation is characterized by back thrusting around the suture zone some amount of delaminated lower crust accumulates at depth during phase 2 120 km 420 km or 6 22 of shortening this crustal root is exhumed medium to high grade rocks along a newly formed major thrust fault this stage bears similarities with the period of coeval activity of the main central thrust and of the south tibetan detachment between 20 16 myr ago during phase 3 420 km or 22 of shortening the crust is scraped off from the mantle lithosphere and is incorporated into large crustal wedge deformation is localized around frontal thrust faults this kinematics should produce only low to medium grade exhumation this stage might be compared with the tectonics that has prevailed in the himalaya over the last 15 myr allowing for the formation of the lesser himalaya the experiment is conducted at constant convergence rate which implies increasing compressive force considering that this force is constant in nature this result may be equivalent to a slowing down of the convergence rate as was observed during the india asia collision

gravity anomalies flexure and the elastic thickness structure of the india eurasia collisional system wehaveusedbouguergravityanomalyandtopographydatatodeterminetheequivalentelasticthicknessofthelithosphere te in the region of the india eurasia collisional system comparison of observed and modelled gravity anomalies along 1dimensional profiles suggest there are significant variations in te along strike of the himalaya foreland estimates decrease from a high of 70 km in the central region to 30 50 km in the east and west we have verified these inferences of spatial variations using a 2 dimensional non spectral interative flexure and gravity anomaly modelling technique the himalaya foreland forms a high te 40bteb100 km rigid block with a well defined edge as shown by the localisation of faulting and deformation along its northern margin other highteblocks occur to the north beneath the qaidam and sichuan basins the tibetan plateau forms a low te 0bteb20 km weak region that extends from the central part of the plateau into south western china tectonic styles in the india eurasia collisional system therefore involve both drigidt and dnon rigidt blocks where high te rigid blocks are present the stylesdominatedbyunderthrustingofthemorerigidblock wherethecollisionalzoneisnotconstrainedbyrigidblocks however the style appears to be dominated by lower crustal flow and a more continuous style of deformation d 2005 elsevier b v all rights reserved

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

neotectonics of the central nepalese himalaya constraints from geomorphology detrital 40ar 39ar thermochronology and thermal modeling 1 the southern flanks of the central nepalese himalaya correspond to a sharp transition in landscape morphology and bedrock mineral cooling ages that suggests a change in rock uplift rate this transition can be explained by either 1 accretion of footwall material to the hanging wall across a ramp in the decollement separating india from eurasia thereby enhancing rock uplift rates above the zone of accretion or 2 out of sequence surface thrust faulting at the physiographic transition here we use geomorphic data 649 new detrital 40ar 39ar cooling ages and a simple thermokinematic model to evaluate which of these tectonic configurations is most appropriate for the central nepalese himalaya we first define and delineate the physiographic transition in central nepal using maps of knickpoints river steepness indices local relief and the distribution of thick alluvial fill deposits we then report new detrital 40ar 39ar data from two trans himalayan transects each of which suggests a rapid northward increase in the total amount of exhumation across the physiographic transition thermokinematic modeling suggests that either of the two developmental scenarios for the transition is plausible but that an accretion model is viable only under an extremely narrow range of conditions we contend that the physiographic and thermochronologic data in our study area are most simply explained by recent out of sequence surface thrusting within the lesser himalayan metasedimentary sequence approximately 15 30 km south of the mapped surface trace of the main central thrust system an important finding of this work is that there are substantial along strike variations in physiography and thermal history that reflect along strike changes in the degree and location of out of sequence surface thrusting

initiation of crustal scale thrusts triggered by metamorphic reactions at depth insights from a comparison between the himalayas and scandinavian caledonides active eclogitization has recently been inferred at depth beneath the himalaya from geophysical observations and the mechanical consequences of eclogitization can be observed in the field in the eroded and extended nappe stack of the scandinavian caledonides there proterozoic metastable granulites and igneous protoliths underwent partial eclogitization during the collision of baltica with laurentia the reaction began in pseudotachylites and veins and eventually formed a connected network of eclogite facies shear zones that localized deformation and weakened the lower crust of baltica during the collision with laurentia all these features can be compared with the seismic activity of the indian lower crust its strength loss beneath the himalayan ranges and its delayed density increase regarding its penetration in the eclogite facies the caledonian bergen arc eclogites and the himalayan ama drime eclogites are both derived from continental crust in both cases these eclogites were formed contemporaneously with the activation of the main thrusts responsible for the construction of the orogenic wedges the main central thrust in the himalayas and the main thrust below the jotun nappe complex in the caledonides the similarities in these two orogens which compare both in size and structure highlight the importance of eclogitization at depth as a mechanism for weakening of the lower crust and for decoupling of the crust and lithospheric mantle in collision zones

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

tracking basement cross strike discontinuities in the indian crust beneath the himalayan orogen using gravity data relationship to upper crustal faults the himalaya is the result of the on going convergence and collision of india and asia two plates with complex and protracted geological evolutions the internal configuration and processes that govern the rise of the himalaya and tibetan plateau are crucial to understand continental collision zones however knowledge of the prior configuration of the colliding plates is equally important since inherited preorogenic basement structures can undeniably influence the development of the orogenic architecture throughout the orogen s cycle of collision and eventual collapse three northeast trending palaeotopographic ridges of faulted precambrian indian basement underlie the ganga basin south of the himalaya oil and natural gas commission 1968 valdiya 1976 gahalaut kundu 2012 our study godin harris 2014 illustrates a crustal scale fault origin for these ridges and succeeds in determining how far north beneath the himalayan system they extend and how they ultimately govern the location of upper crustal faults in southern tibet spectrally filtered egm2008 bouguer gravity data and edges in its horizontal gradient at different source depths gravity worms over northern peninsular india the himalaya and southern tibet reveal several continuous himalayan cross strike discontinuities interpreted to represent crustal faults fig 1 gravity lineaments in peninsular india coincide with edges of the precambrian basement ridges and megakinks up to 100 km wide develop in foreland cover sequences between the interpreted basement faults the interpreted basement faults project northward beneath the himalayan system and southern tibet our results suggest that several active himalayan cross strike faults such as the ones related to many graben in southern tibet are rooted in the underplated indian lower crust or step en echelon along interpreted basement faults our interpretation thus suggests that south tibet graben are spatially related to deepseated crustal scale faults rooted in the underplated indian crust these major discontinuities partition the himalayan range into distinct zones and could ultimately contribute to lateral variability in tectonic evolution along the orogen s strike

elastic thickness of the himalayan tibetan orogen estimated from the fan wavelet coherence method and its implications for lithospheric structure abstract this paper is concerned with the spatial variation of the elastic thickness t e in the himalayan tibetan orogen and its implications for the understanding of lithospheric structure and deformation new t e maps of the himalayan tibetan orogen are generated from the new combined satellite terrestrial gravity eigen6c3stat and topography etopo1 models using the fan wavelet coherence technique the high t e values 60 km are observed in the indian shield and the tarim basin while the low t e values 40 km are prevailing over most of the tibetan plateau we also find that the t e distributions estimated using the high and low central wavenumber i e k 0 wavelets are different in southern tibet by combining constraints from the current seismic and thermo rheological modeling this difference may reflect that the strength of the lithosphere in southern tibet mainly resides in the lithospheric mantle within most parts of central and northern tibet the t e values are relatively low 30 km indicating that the entire lithosphere is weak our analysis finds that the t e variations are probably associated with changes in lithosphere structure we interpret the abrupt changes in t e between southern and northern tibet to indicate the location of the underthrusting strong indian lithosphere beneath tibet our results suggest that the northern limit of the underthrusting indian plate reaches as far north as the hindu kush and the southwestern margin of the tarim basin near 70 80 e and extends along the bangong nujiang suture in the central 80 87 e eastward between 87 e 93 e it may move to the indus tsangpo suture indicating a delamination of indian mantle lithosphere there further to the east it may underthrust northward beneath the eastern qiangtang block

modelling the spatial distribution of present day deformation in nepal how cylindrical is the main himalayan thrust in nepal summary the structure of the himalayas in nepal is characterized by underthrusting of the indian lithosphere beneath the chain along the main himalayan thrust mht the mht is affected by major earthquakes of m 8 that have ruptured segments several hundred kilometres in length the study of historical seismicity underlines the existence of a significant seismic gap between the 1905 kangra and the 1934 bihar nepal earthquake areas in western nepal this contribution presents a numerical model of the interseismic deformation using the adeli 2 d finite element code a parametric study has been performed to estimate the influence of the topography the geometry of the flat ramp flat transition of the mht and its rheology the results of these runs are compared along three cross sections of eastern central and western nepal using various recordings of the active deformation of nepal neotectonics microseismicity gps velocities vertical displacement rates deduced from comparative levellings the results of this study suggest 1 a small amount of aseismic slip along the southern part of the mht and 2 an abrupt change in geometry of the mht between central and western nepal the stress build up in eastern and central nepal is produced by coupling of a bend flat ramp transition of the mht and a brittle ductile transition at the foot of the ramp in western nepal on the other hand stress build up is only induced by a brittle ductile transition located along the low angle part of the mht this difference suggests the presence of a lateral ramp on the mht to allow transition between the central and western nepal geometries of the mht this change in mht geometries between central and western nepal is also clearly marked in the himalayan relief an abrupt decrease in altitude and incision between central and western nepal

crustal density structure from 3d gravity modeling beneath himalaya and lhasa blocks tibet abstract the himalaya and lhasa blocks act as the main belt of convergence and collision between the indian and eurasian plates their crustal structures can be used to understand the dynamic process of continent continent collision herein we present a 3d crustal density model beneath these two tectonic blocks constrained by a review of all available active seismic and passive seismological results on the velocity structure of crust and lower lithosphere from our final crustal density model we infer that the present subduction angle of the indian plate is small but presents some variations along the west east extension of the orogenic belt the dip angle of the moho interface is about 8 9 in the eastern and western part of the orogenic belt and about 16 in the central part integrating crustal p wave velocity distribution from wide angle seismic profiling geothermal data and our crustal density model we infer a crustal composition model which is composed of an upper crust with granite granodiorite and granite gneiss beneath the lhasa block biotite gneiss and phyllite beneath the himalaya a middle crust with granulite facies and possible pelitic gneisses and a lower crust with gabbro norite troctolite and mafic granulite beneath the lhasa block our density structure

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

constraints on the structure of the himalaya from an analysis of gravity anomalies and a flexural model of the lithosphere the intracontinental subduction of india beneath the himalaya presents several similarities to that occurring at island arcs we study one of those similarities by analyzing gravity anomalies across the himalaya assuming that the topography is supported by the indian elastic plate flexed under the weight of both the overthrust mountains and the sediments in the ganga basin we first examine in detail the effects of each of the following parameters on the configuration of the elastic plate and on the gravity anomalies the flexural rigidity the position of the northern end of the elastic plate the amount of underthrusting of such a plate beneath the range and the density contrasts between the crust and mantle and between the sediments and the crust a plate with a constant flexural rigidity of about 0 7 1025 n m between 0 2 and 2 0 1025 n m allows a good fit to the data from the lesser himalaya and the ganga basin such a plate however cannot underthrust the entire himalaya instead the gravity anomalies show that the moho steepens from only about 3 beneath the lesser himalaya to about 15 beneath the greater himalaya this implies a smaller flexural rigidity beneath the greater himalaya 0 1 to 1 0 1023 n m than beneath the ganga basin and the lesser himalaya even with a thin weak plate beneath the greater himalaya the weight of the mountains depresses the plate too much unless an additional force or moment is applied to the plate the application of a bending moment unit length to the end of the plate of about 0 6 1018 n m is adequate to elevate the indian plate and to bring the calculated gravity anomalies in agreement with those observed both the smaller flexural rigidity and the bending moment can be understood if we assume that part or all of the indian crust has been detached from the lower lithosphere that underthrusts the greater himalaya we study the tectonic implications of these results by means of a series of idealized balanced cross sections from the collision to the present that reproduce several important features of the geology of the himalaya and predict an amount of eroded material comparable to that in the ganga basin and the bay of bengal these cross sections include high grade metamorphic rocks near the main central thrust and a steeper dip of it there than in the lesser himalaya they predict rapid uplift only in the greater himalaya and at the foot of the lesser himalaya

effects of parameterized boundary layer structure on hurricane rapid intensification in shear abstractthis study investigates the role of the parameterized boundary layer structure in hurricane intensity change using two retrospective hwrf forecasts of hurricane earl 2010 in which the ver

gravity anomalies flexure and deformation of the converging indian lithosphere in nepal and sikkim darjeeling himalayas researchers ubiquitously noted that the common processes of partitioning oblique convergence in response to drag from the trench hanging plate simultaneously produce radial slips along strike translation and extension parallel to the deformation front here we focus on the area between nepal and sikkim darjeeling himalayas and carry out gravity and finite element stress modeling of the strike orthogonal converging indian lithosphere we delineate the geometries of different layers and their interfaces through gravity modeling the optimum model parameters along with rheological parameters of different layers are used for finite element modeling finite element modeling is done with boundary conditions of keeping the upper surface free and rigidly fixing the section of the northern boundary below the main himalayan thrust we impart on its frontal section an amount of 6 1012 n m force equivalent to resistive force of the himalayan tibet system and analyze the maximum and minimum compressive stress fields evolved in the lithosphere we testify our observations with earthquake database and other geophysical and geological studies we note that an increasing flexing of the indian lithosphere beyond the main boundary thrust becomes maxima between the main central thrust and south tibetan detachment in both the areas however more steepening of the moho boundary is identified in the sikkim darjeeling himalaya this abrupt change in lithospheric geometry beneath the greater himalaya is likely correlated with the sharp elevation changes in the topography although the highest seismicity concentration is dominant in this zone the lesser and the tethys himalayas in sikkim darjeeling area also record relatively fair seismic activity more compressive stress field in different layers right within the sharp bending zone supports this observation we thus propose that the sharp bending zone beneath the greater himalaya is suffering maximum deformation and the deformation is continued in the mantle too we also identify both right lateral shear and radial vergence slip which are presumably associated with the general dynamics and kinematics of the himalaya

deep seismic reflection evidence for continental underthrusting beneath southern tibet the himalaya and adjacent tibetan plateau constituting earth s largest region of elevated topography and anomalously thick crust formed as a consequence of cenozoic collision between india and asia itself considered the archetypal continent continent collision1 3 here we report the first results from an attempt to image the structure of the crust beneath this region using deep seismic reflection profiling our 100 km long profile acquired in the tethyan himalaya shows a mid crustal reflection that prob ably marks the active thrust fault along which the indian plate is underthrusting southern tibet upper crustal reflections with geo metries suggestive of large scale structural imbrication of the upper crust and moho reflections from the base of the double normal thickness crust underlying the region these results lend substantial support to the view that crustal thickening beneath southernmost tibet was accomplished by wholesale underthrusting of indian continental crust beneath the structurally imbricated upper crust comprising the tethyan himalaya

erosional control on the dynamics of low convergence rate continental plateau margins s u m m a r y over the last decade thermomechanical models have revealed the control of both tectonics and erosion on the morphology of continental plateaus margins however unravelling the specific effects of these two coupled processes has been difficult in practice here to assess the control of erosion we investigate the dynamics of the eastern and the northern borders of the tibetan plateau which are characterized by a low convergence rate and a steep topographic escarpment adjacent to the sichuan and tarim basins respectively thermomechanical modelling of continental lithosphere coupled with fluvial denudation reveals that important crustal deformation with large scale horizontal displacements can occur without any convergence as a response to mass transfer due to gravitational collapse and to erosional unloading these processes are sensitive to crustal structure geothermal gradient as well as surface erosion at a timescale of several million years our results suggest that this denudation triggered deformation exerts a primary control on the evolution of those plateau margins by counterbalancing the mass removal due to erosion and stabilizing the topographic escarpment this finding supports a possible explanation for the morphology of the longmen shan eastern tibet in which the paradoxical combination of persistent high topographic gradients close to the foreland and low convergence rates can be related to the influence of erosion on deformation patterns

evidence for mechanical coupling and strong indian lower crust beneath southern tibet how surface deformation within mountain ranges relates to tectonic processes at depth is not well understood the upper crust of the tibetan plateau is generally thought to be poorly coupled to the underthrusting indian crust because of an intervening low viscosity channel here however we show that the contrast in tectonic regime between primarily strike slip faulting in northern tibet and dominantly normal faulting in southern tibet requires mechanical coupling between the upper crust of southern tibet and the underthrusting indian crust such coupling is inconsistent with the presence of active channel flow beneath southern tibet and suggests that the indian crust retains its strength as it underthrusts the plateau these results shed new light on the debates regarding the mechanical properties of the continental lithosphere and the deformation of tibet

mass accumulation rates in asia during the cenozoic summary this work establishes estimates of mass accumulation rates in 18 mostly offshore sedimentary basins in asia since the beginning of the cenozoic 66 ma the estimates were derived from isopach maps cross sections and drill holes or stratigraphic columns assuming regional similarity of the strata average solid phase volumes and accumulation rates were calculated for nine epochs approximately corresponding to geological periods palaeocene 66 58 ma eocene 58 37 ma oligocene 37 30 and 30 24 ma miocene 24 17 17 11 and 11 5 ma pliocene 5 2 ma and quaternary 2 0 ma these rates shed new light on the geological history of asia since the onset of the collision of india with asia 50 ma the overall average accumulation rates curve for asian sedimentary basins since the beginning of the tertiary shows an exponential form with slow accumulation rates less than 0 5 106 km3 myr 1 until the beginning of the oligocene more than 15 myr after the onset of the collision from the oligocene onwards rates increase quickly in an exponential manner reaching their maximum values in the quaternary more than 1 5 106 km3 myr 1 from these observations we suggest that extrusion and crustal shortening are complementary processes that have been successively dominant throughout the india eurasia collision history at smaller scales one may distinguish between independent histories at the subcontinental and basin scales this permits a comparison of the relative importance of tectonic and climatic erosion processes affecting the different mountain belts of asia during the cenozoic

topographic relief driven by variations in surface rock density earth s surface topography is generated by tectonically induced variations in crustal thickness combined with erosion and to a lesser degree by vertical stresses caused by convection in the underlying mantle rock hardness and resistance to erosion are also commonly thought to influence topography because hard rocks such as granites and basalts usually form topographic highs in the landscape here we use analytical and numerical models to simulate the erosion induced isostatic rebound of rocks we find that the isostatic rebound that accompanies erosion causes denser rocks to occupy higher elevations in the landscape thereby creating topographic relief that is proportional to surface rock density differences rather than rock hardness we quantify this effect taking into account the flexural strength of the continental lithosphere we show that in a steady state erosional setting density dependent isostatic rebound can cause the densest rocks to be exhumed at double the rate of surrounding less dense rocks and has a stronger effect than typical rock hardness variations the results imply that denser rock formations should erode faster and therefore be characterized by younger thermochronological ages thermochronological data sets from the kinabalu granite in borneo and the shakhdara alichur gneiss domes in pamir confirm this counter intuitive result our findings imply that lateral variations in surface rock density have significant control on the shaping of the large scale features of earth s surface the high elevation in earth s topography of hard rocks such as granites and basalts was thought to be caused by their inherent resistance to erosion numerical modelling now demonstrates counterintuitively that erosion induced isostatic rebound of rocks which is density dependent causes granites and basalts to occupy high elevations because they are more dense than surrounding rocks

flexure of the continental lithosphere with multilayered rheology summary in this paper a model of flexure of the continental lithosphere is derived taking into account crustal and mantle rheologies bending of the continental lithosphere is modelled with a double yield stress envelope three layers brittle elastic and ductile for the crust and three analogous layers for the mantle portion the deformation of the layers is controlled by the rheological properties of quartz rich crustal rocks and olivine rich mantle rocks the influence of various factors such as the depth of moho strain rates thermal structure of the lithosphere boundary conditions and topographic load is examined results show that the mechanical strength of the continental lithosphere in the horizontal and vertical directions is primarily controlled by the present thermal structure of the plate boundary forces and moments and the applied topographic load this explains why mountainous regions may be more locally compensated than adjacent regions we also thus are able to explain why many continental plates have apparent effective rigidities much smaller than those predicted on the basis of their geological ages the model is then applied to the tien shan tarim area central asia and original topography and gravity data are used to constrain parameters of the model we found that the model satisfactorily matches the data and is also able to predict the thermal state of the plate and the location of the deep seismicity

crustal structure and variability of the himalayan border of tibet reversed critical wide angle reflection profiles of the crust mantle boundary south of the yarlung zangbo suture in tibet reveal a deep 70 km moho extending north from the high himalayas whilst to the south the moho is 15 km higher even further south reflections from 35 km depth may correspond to the moho of peninsular india

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

crustal configuration of the northwest himalaya based on modeling of gravity data the gravity response and crustal shortening in the himalayan belt are modeled in detail for the first time in the nw himalaya the bouguer gravity anomaly along a 450 km long projected transect from the sub himalaya in the south to the karakoram fault in the north across the indus tsangpo suture zone is modeled using spectral analysis wavelet transform and forward modeling the spectral analysis suggests three layer interfaces in the lithosphere at 68 34 and 11 km depths corresponding to the moho the conrad discontinuity and the himalayan decollement thrust respectively the coherence admittance and cross spectra suggest crustal shortening because of convergence compensated by lithospheric folding at 536 and 178 km wavelength at the moho and the upper crustal level an average effective elastic thickness of around 31 km is calculated using the coherence method the gravity data are modeled to demarcate intracrustal to subcrustal regional thrust fault zones the geometrical constraints of these faults are obtained in the space scale domain using the wavelet transform showing good correlation with the major tectonic boundaries the crustal configuration along the transect shows how the moho depth increases from 45 to 80 km towards the north with the locus of flexure of the indian crust beneath the higher himalayan zone the combination of forward modeling and wavelet analysis gives insight into the subsurface extent and geometry of regional structures across the nw himalaya

lithospheric and upper mantle structure of the indian shield from teleseismic receiver functions receiver functions derived from teleseismic p waveforms of 297 earthquakes recorded at broadband station hyderabad hyb are analysed to investigate the lithospheric and upper mantle structure in the vicinity of the station located in the northeastern part of the archaean dharwar craton the internal structure of the 33 2 km thick crust is simple it can be approximated by a single layer with s velocity linearly increasing with depth from 3 4 to 4 0 km s the average being 3 7 0 2 km s the crustal poisson s ratio is 0 25 0 01 a seismic discontinuity at 90 km depth is inferred from a signal particularly prominent on the sh component and possibly indicative of a layer of depth localized anisotropy in the lithospheric mantle the two principal mantle discontinuities are at depths of 406 and 659 km which are close to the global averages

geophysical constraints on the link between cratonization and orogeny evidence from the tibetan plateau and the north china craton abstract understanding the geodynamic process of orogeny and cratonization and their transition is among the key topics of research in evaluating the link between plate tectonics and continental dynamics the tibetan plateau and the north china craton ncc two key areas in mainland china offer excellent laboratories to understand continental tectonics over a broad span of earth history particularly the deep structure of the lithosphere as imaged from geophysical data on the tibetan plateau and the ncc provide important clues in understanding orogeny and cratonization the tibetan plateau is the largest and highest plateau on earth in terms of mean altitude and is an important region for understanding the mechanisms of continent continent collision and cenozoic plateau uplift the ncc is an archean craton that underwent lithospheric disruption during the mesozoic here we reconstruct the main features of the structure of the crust and upper mantle from surface wave tomography and gravity modeling in tibet and its neighboring regions in order to understand the modality of the convergence and collision process between the indian and eurasian plates and the influence of this process on the uplift of the plateau in the ncc geological geochemical geophysical and tectonic investigations demonstrate that lithospheric destruction mainly occurred in the eastern block the crustal structure of the ncc is reconstructed from ambient noise surface wave tomography and the different possible disruption mechanisms are evaluated the vs shear wave velocity tomography results and the density structure of the crust and upper mantle to about 350 km depth demonstrate the lateral variation of the thickness of the metasomatic lid between the south and north of the bangong nujiang suture bns and the west and east of tibet which suggest that the leading edge of the subducting indian slab reaches the bns the subduction angle of indian plate indicates a transition from steep to shallow from the west to east tibet sections depicting the gravitational potential energy suggest that mantle flow contributes to the subduction of the indian plate as far as the bns and the transition from the asthenospheric layer to the metasomatic lid overlaps with the transition from north south shortening in south tibet to eastward tectonic escape in north tibet qiangtang and songpan ganzi blocks both vs and models suggest the following 1 north southward lower crust flow beneath the eastern ncc and interaction between the westward mantle flow and eastward escape flow beneath the central ncc in addition to the earlier proposed mechanisms of delamination and thermal erosion played important roles in the lithospheric disruption of the archean craton 2 mantle flow plays an important role in the continental tectonic transition between neighboring tectonic blocks and within the cycle between orogeny and cratonization

electrical structure of the himalaya of central nepal high conductivity around the mid crustal ramp along the mht twelve broadband magnetotelluric mt soundings were performed across the himalaya of central nepal in 1996 in order to determine the electrical structure of the crust and its relation to geological structures and active tectonics the mt impedance tensors were obtained for frequencies between 0 001 and 500 hz the 2 d section derived from joint inversion of te and tm mode after rri and groom bailey decomposition shows high conductivity in the foreland basin 30 m that contrasts with the resistive indian basement 300 m and lesser himalaya 1000 m in addition our mt sounding reveals a major conductive feature beneath the front of the higher himalaya also characterized by intense microseismic activity and the position of a mid crustal ramp along the major active thrust fault mht this high conductivity zone probably reflects metamorphic fluids released during underthrusting of the indian basement and pervading well connected microcracks induced by interseismic stress build up or distributed brittle deformation around the ramp

on the use of dislocations to model interseismic strain and stress build up at intracontinental thrust faults creeping dislocations in an elastic half space are commonly used to model interseismic deformation at subduction zones and might also apply to major intracontinental thrust faults such as the main himalayan thrust here we compare such models with a more realistic 2 d finite element model that accounts for the mechanical layering of the continental lithosphere and surface processes and that was found to fit all available constraints on interseismic and long term surface displacements these can also be fitted satisfactorily from dislocation models the conventional back slip model commonly used for subduction zones may however lead to a biased inference about the geometry of the locked portion of the thrust fault we therefore favour the use of a creeping buried dislocation that simulates the ductile shear zone in the lower crust a limitation of dislocation models is that the mechanical response of the lithosphere to the growth of the topography by bending of the elastic cores and ductile flow in the lower crust cannot be easily introduced fortunately these effects can be neglected because we may assume to first order a stationary topography moreover we show that not only can dislocation models be used to adjust surface displacements but with some caution they can also provide a physically sound rationale to interpret interseismic microseismicity in terms of stress variations

evidence from gravity and topography data for folding of tibet bouguer gravity and topography data from tibet suggest that the surface of the plateau and the subsurface density interfaces are warped into two series of ridges and troughs trending parallel to the collision zone with wavelengths of 150 and 500 km these folds are superimposed on an overall state of isostatic compensation owing to crustal thickening such folding is predicted from models of compressional shortening of a rheologically layered plate the results thus suggest the presence of a weak decoupling zone between the tibetan crust and upper mantle

postseismic deformation associated with the 2015 mw 7 8 gorkha earthquake nepal investigating ongoing afterslip and constraining crustal rheology abstract the 2015 mw 7 8 gorkha earthquake has not only imposed effective constraints on the geometrical structures friction behaviours and seismogenic patterns of the nepal himalaya thrust systems but has also provided valuable insights into the uplift mechanism and lithosphere rheology of the tibetan plateau here 1 6 year gps observations are used to reveal the postseismic deformation characteristics following the gorkha earthquake investigate the ongoing aseismic afterslip on the main himalayan thrust mht fault and constrain the crustal rheology of the southern tibetan plateau first afterslip is considered to be solely responsible for the postseismic deformation afterslip only model the results show that afterslip is anticorrelated with peak coseismic slip areas one high afterslip concentration area with a peak of 24 cm is distributed downdip of the coseismic rupture as well as in two other regions one partially overlapping the mainshock rupture and the other next to the mw 7 3 aftershock area second the gps postseismic observations are inverted to jointly investigate afterslip and viscoelastic deformation multiple mechanism model the afterslip inversion results of the above two models are highly consistent indicating the dominant contribution of afterslip to surface deformation during the 1 6 year postseismic period considering the interseismic fault coupling and historical seismicity no appreciable fault slip associated with the gorkha earthquake is found to occur both updip and west of the mainshock rupture areas this reveals that the gorkha earthquake only unzipped the lower edge of the locked portion of the mht leaving the shallow portion and western segment of the seismogenic zone still locked and the nepal region under high seismic risk the viscoelastic mechanism contributes minorly to surface deformation during the 1 6 year postseismic period the middle lower crust is assumed to comprise maxwell material beneath an elastic 25 km thick upper crust and the optimal viscosity is conservatively estimated to be 1 6 1019 pa s beneath the southern tibetan plateau which should be robustly constrained with more long term observations more effective spatial constraints and more detailed crustal models

study of plate deformation and stress in subduction processes using two dimensional numerical models two dimensional finite element modeling is used to model subduction of an oceanic lithospheric plate beneath continental lithosphere the subduction process is initiated along a preexisting inclined fault and continues until reaching 400 km of total convergence the lithosphere is assumed to be underlain by an in viscid asthenosphere different rheological laws have been considered for the lithosphere including elasticity and elastoplasticity the modeling shows that both the stress system in the plates and the surface topography are strongly dependent on two main parameters the density contrast between lithosphere and asthenosphere l a and the coefficient of friction along the subduction plane varying these two parameters allows explanation of the main characteristics of real subduction zones and results in two major regimes manifested by extension or compression in the arc back arc system extension and back arc rifting corresponds to a positive density contrast and a low coefficient of friction while negative values and or high friction leads to a compressional regime the coexistence of trench arc compression and back arc tension is only possible for a coefficient of friction lower than 0 1 the results of the numerical experiments agree with those of experimental modeling conducted under similar physical assumptions

lithospheric mantle deformation beneath the indian cratons abstract the nature of deformation of the deep continental roots beneath the archean early proterozoic terrains opens the question whether these ancient terrains have had stable roots since the precambrian or whether recent plate motions have deformed them in view of this we make an attempt to study the thermal structure beneath the cratonic regions of the indian shield which vary in lithospheric thickness from 65 km in the singhbhum craton to 148 km in the archean dharwars the average depth of 104 km to the top of the underlying asthenosphere is consistent with other termination methods and is in fact less than half the 200 400 km depth found in other stable areas of the earth similarly the average reduced heat flow of about 35 mw m2 and moho temperature of about 550 c range 400 730 c for the indian cratons are also much higher than their counterparts elsewhere our study indicates a large scale deformation of the cratonic mantle lithosphere beneath the indian shield since the mesoproterozoic c

incorporating metamorphism in geodynamic models the mass conservation problem summary geodynamic models incorporating metamorphic phase transformations almost invariably assume the validity of the boussinesq approximation that violates conservation of mass in such models metamorphic density changes take place without volumetric effects we assess the impact of the boussinesq approximation by comparing models of orogeny accompanied by lower crustal eclogitization both with and without the approximation our results demonstrate that the approximation may cause errors approaching 100 per cent in characteristic measures of orogenic shape mass conservation errors in boussinesq models amplify with model time mass conservative models of metamorphism are therefore essential to understand long term tectonic evolution and to assess the importance of the different geodynamic processes

rheology of the lithosphere selected topics we review recent results concerning the rheology of the lithosphere with special attention to the following topics 1 the flexure of the oceanic lithosphere 2 deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins 3 the rheological stratification of the continents 4 strain localization and shear zone development and 5 strain induced crystallographic preferred orientations and anisotropies in body wave velocities we conclude with a section citing the 1983 1986 rock mechanics literature by category

gravity anomalies flexure of the indian plate and the structure support and evolution of the himalaya and ganga basin bouguer gravity anomalies along four profiles across the western himalaya and ganga basin show large deviations from local isostatic equilibrium a deficit of mass characterizes the ganga basin and an excess underlies the lesser himalaya both can be understood if the indian plate is flexed down by the distributed load of part of the mountains the cross sectional shape of the ganga basin seems to be controlled by the deflection of the indian plate which we compute assuming the indian plate to overlie an inviscid fluid from the shapes of both bouguer anomaly profiles and the basement topography we place bounds on the flexural rigidity of such a plate if the ganga basin is a steady state feature then the age of the basal sediments in a given locality should be proportional to the distance of that locality from the southern edge of the basin if the rate of convergence of india and the himalaya were constant that rate should equal the distance divided by the corresponding age we find a rate of 10 to 15 mm a for the last 15 to 20 ma which is consistent with a large part of the 50 mm a rate of convergence between india and eurasia being absorbed by the eastward extrusion of parts of tibet profiles of bouguer gravity anomalies show only a small peak or plateau over the southern edge of the lesser himalaya implying that the boundary between the light sediments of the ganga basin and the heavier crustal rocks of the lesser himalaya is not sharp and that there exists some light material beneath the range we infer that some sediment deposited in the ganga basin has been underthrust beneath the lesser himalaya but the quantity is small most of this sediment probably is scraped off the indian plate to make the foothills of the range we find that the load of the high himalaya is too large to be supported solely by elastic stress in the indian plate if the flexural rigidity of the plate is constant and if no other external forces act on the plate the observed gradient in bouguer gravity anomalies increases from an average of about 1 mgal km over the ganga basin and lesser himalaya to about 2 mgal km over the high himalaya this increase in the gravity gradient implies that the moho dips more steeply 10 15 beneath the high himalaya than beneath the lesser himalaya 2 3 we interpret this steepening of the moho to be due to a weakening of the plate which allows it to bend more sharply beneath the high himalaya than farther south with the inclusion of a weak segment of indian plate beneath the high himalaya calculated anomalies show a somewhat increased gradient beneath the high himalaya but when the weight of the entire himalaya is used as a load on the plate calculated anomalies are more negative than observed therefore an external force system is needed to support much of the weight of the high himalaya as well as to bend the plate sufficiently beneath the high himalaya the magnitudes of the bending moment and the force per unit length that must be applied to the end of the plate are compatible with their sources being gravity acting on a part of india s mantle lithosphere stripped of its crust and underthrust beneath southern tibet

seismic structure of the crust and the upper mantle beneath the himalayas evidence for eclogitization of lower crustal rocks in the indian plate 1 variations in the seismic velocity structure of the himalayan collision zone include significant differences between its north and south portions with transitions in physical properties across the greater himalaya we combined p and s wave traveltimes from a temporary broadband seismic network in eastern nepal and southern tibet with arrival times at the permanent station network of the department of mines and geology of nepal to determine the seismic velocity structure across the himalaya using local earthquake tomography and traveltimes of regional earthquakes the p to s velocity ratio vp vs structure marks the difference between the indian plate and the overlying materials with the vp vs ratios being high for the former and low for the latter we also found a significant increase in the uppermost mantle seismic velocities from south to north reaching p wave velocities vp over 8 4 km s north of the greater himalaya these high vp values do not seem to be the result of biases due to anisotropy in the upper mantle beneath the greater and tethyan himalayas instead we suggest that rocks in the lower crust of the underthrusting indian plate undergo metamorphism to eclogite as they plunge to greater depth beneath the mountain range explaining the high seismic velocities

lithospheric structure of the central himalaya from 3 d tomographic imaging abstract the high himalaya are one of the most spectacular consequences of the ongoing collision between india and eurasia yet the role that the lithosphere beneath them play in this collision is not well understood to illuminate the influence of the himalaya on collisional processes we produced tomographic images of the subsurface beneath them from arrival times of p and s waves generated by 551 teleseisms and 1051 local regional events and recorded by the himalayan nepal tibet broadband seismic experiment himnt from 2001 to 2002 compared to those in initial 1d models the rms p and s arrival time misfits in our final 3d model were reduced by as much as 46 from 0 45 s to 0 24 s and 38 from 0 97 s to 0 6 s respectively our results show strong crustal variations perpendicular to but no significant lateral variation along the trend of the high himalaya within our study area in the ns direction the crust thickens from about 60 km under the lesser himalaya to 70 80 km under the tethys himalaya there is no localized crustal thickening beneath the high himalaya wavespeeds and seismicity patterns beneath the tethys himalaya suggest that the middle crust at 30 50 km depth is ductile and are consistent with the presence of a mid crustal reflector associated with the stronger gradients in the lower crust correlation between relocated earthquakes and the xainza dinggye rift indicates that the rift is not cutting through the crust at the present time shear wave splitting from core phases indicates that the upper mantle beneath the tethys himalaya is nearly isotropic the combination of isotropy and higher seismic wavespeed leads us to suggest that eclogites included in the lowermost crust and the uppermost mantle are responsible for the mid crustal reflector under the tethys himalaya as well as for a slightly high wavespeed anomaly just beneath the moho this interpretation is supported by the existence of granulitic eclogite in the ama drime massif 50 km northeast of mt everest

numerical modelling of erosion processes in the himalayas of nepal effects of spatial variations of rock strength and precipitation the interplay between tectonics and erosion has a predominant control on the evolution of the morphology of mountain belts here we investigate the modalities of deformation in central nepal on a c 100 ka time scale in response to tectonic and external forcings through the use of a finite element thermomechanical model coupled with an integrative denudation formulation that accounts for fluvial incision and hillslope landsliding we study the complex coupling existing between tectonics and erosion with special emphasis on the influences of rock strength and rainfall distributions our results underline the key role played by lithologic variations in the elevation of both rivers and mean topography we show that the location of the main frontal thrust is mainly controlled by the low erodability of the unconsolidated sandstone in the siwaliks hills as previously suspected burbank et al 2003 our simulations demonstrate that the pattern of uplift in nepal is mainly dependent on both erodability and fault geometry rather than on rainfall distribution

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

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

mapping the descent of indian and eurasian plates beneath the tibetan plateau from gravity anomalies the collision of india with asia has produced a complicated continental continental plate boundary involving folding and faulting of variable trends and styles within and along the margins of the tibetan plateau numerous lines of evidence including the development of two scales of folding in tibet suggest that the lowermost crust is behaving in a ductile fashion this weak lower crust might then decouple the fundamental plate tectonic motions in the uppermost mantle from the complex pattern of surface faulting in this study we use bouguer gravity anomalies to map out the geometry of indian and eurasian plate interactions in the mantle beneath the plateau based on both the inferred geometry of the moho and lateral variations in lithospheric strength determined from mechanical modeling in our preferred model the lithosphere beneath tibet consists of two distinct units 1 the underthrust to the north indian plate which sutures with the eurasian plate in the upper mantle below the yarlung zangpo suture or the gangdese igneous belt 200 400 km north of the main boundary thrust and 2 the underthrust to the south eurasian plate a subducting slab of indian upper mantle extends about 200 km into the asthenosphere north from the mantle suture and exerts a bending moment of about 3 5 1017 n on the indian plate thus the mantle lithosphere appears to be behaving in the simple fashion of converging oceanic plates while the more buoyant continental crust deforms under high gravitational potential in a complex pattern controlled by its lateral and vertical strength heterogeneity

the granulite eclogite facies transition a comparison of experimental work and a natural occurrence in the bergen arcs western norway a unique segment of the deeper parts of the caledonian continent continent collision zone is exposed in one of the thrust sheets in the bergen arcs of western norway it demonstrates that precambrian granulite facies rocks underwent fluid controlled eclogitization on a regional scale the transition from granulite to eclogite facies is sharp it occurs over distances of centimetres to metres and follows shear zones and infiltration fronts major anastomosing shear zones typically 50 to 100 m thick with well banded eclogite facies rocks crosscut the lithological boundaries they can be followed continuously for several kilometres breccias where metre size blocks of granulite float in an eclogite matrix outcrop over areas of 0 5 km2 areas comprising more than 80 eclogite occupy several square kilometres this mixture of metamorphic facies indicates the metastable occurrence of granulite facies rocks in the lower crust of modern and ancient orogenic belts in contrast to earlier experimental work such field relations imply that the granulite eclogite facies boundary in nature can be sharp and independent of lithological boundaries a partly eclogitized lower crust of the kind exposed in the bergen arcs can explain the 7 2 7 8 km s vp layer present in many crust mantle transition zones anastomosing shear zones and areas of almost complete eclogitization represent potential lower crustal reflectors

isostatic response and anisotropy of the eastern himalayan tibetan plateau a reappraisal using multitaper spectral analysis 1 we estimate the variation in strength of the eastern himalayan tibetan lithosphere from 2d gravity and topography coherence functions calculated with the thomson slepian multitaper technique the coherence shows evidence of structural weakness of the plate in the n s direction aligned with the direction of maximum compression the tibetan plateau is characterized by an effective elastic thickness te that is much lower than would be expected from its large crustal thickness and the additional strength contributed by a coherently folded mantle inferred from shear wave splitting the seismogenic thickness indicates that only the upper crust is contributing to the elastic strength and not the mantle the predominant weakness of the lithosphere in the n s direction revealed from the anisotropy in the coherence function suggests that the mantle may not be directly coupled to crustal deformation beneath the tibetan plateau

thermal structure and exhumation history of the lesser himalaya in central nepal the lesser himalaya lh consists of metasedimentary rocks that have been scrapped off from the underthrusting indian crust and accreted to the mountain range over the last 20 myr it now forms a significant fraction of the himalayan collisional orogen we document the kinematics and thermal metamorphism associated with the deformation and exhumation of the lh combining thermometric and thermochronological methods with structural geology peak metamorphic temperatures estimated from raman spectroscopy of carbonaceous material decrease gradually from 520 550 c below the main central thrust zone down to less than 330 c these temperatures describe structurally a 20 50 c km inverted apparent gradient the ar muscovite ages from lh samples and from the overlying crystalline thrust sheets all indicate the same regular trend i e an increase from about 3 4 ma near the front of the high range to about 20 ma near the leading edge of the thrust sheets about 80 km to the south this suggests that the lh has been exhumed jointly with the overlying nappes as a result of overthrusting by about 5 mm yr for a convergence rate of about 20 mm yr this implies underthrusting of the indian basement below the himalaya by about 15 mm yr the structure metamorphic grade and exhumation history of the lh supports the view that since the mid miocene the himalayan orogen has essentially grown by underplating rather than by frontal accretion this process has resulted from duplexing at a depth close to the brittle ductile transition zone by southward migration of a midcrustal ramp along the main himalayan thrust fault and is estimated to have resulted in a net flux of up to 150 m 2 yr of lh rocks into the himalayan orogenic wedge the steep inverse thermal gradient across the lh is interpreted to have resulted from a combination of underplating and post metamorphic shearing of the underplated units

mechanical analysis of controls on strain partitioning in the himalayas of central nepal we present a mechanical analysis of the problem of slip partitioning between the major thrust systems in a collisional range we focus on two structures in the himalayas of central nepal the main himalayan thrust mht and the main central thrust mct we use finite element modeling to test the influence of various parameters such as friction coefficients and surface processes and we investigate how they affect the distribution of deformation between these two faults we observe that reproduction of the late quaternary kinematic pattern across the range with our model requires strict conditions on the friction coefficients such that the mht is very weak whereas the mct is significantly stronger the most important parameter that controls slip partitioning appears to be the dip angle of the mct with a gentler or steeper mct promoting or inhibiting slip respectively we also show that transient loading and unloading through focused glacial erosion in the higher part of the range can unclamp the mct and allow a significant increase in slip rates the results of this mechanical sensitivity investigation have important implications for the dynamics of the himalayan wedge and point toward along strike structural variations as a first order control on slip partitioning

thrust tectonics crustal shortening and the structure of the far eastern nepal himalaya balanced and restored structural sections across the far eastern nepal himalaya have been constructed in order to determine the structure and evolution of the himalayan orogenic wedge and the amount of tectonic shortening the region has undergone since the initiation of thrusting along the main central thrust mct the far eastern nepal himalaya is comprised of three distinct thrust bound tectonic packages the higher himalayan crystalline thrust sheet the lesser himalayan metasediment thrust package and the sub himalayan imbricate fan the higher himalayan crystallines consisting of kyanite and sillimanite bearing gneisses intruded by the miocene jannu leucogranites have been thrust over the lesser himalayan metasediments along the mct for a distance of 140 km to 175 km the lesser himalayan metasediments are a 12 km thick unit consisting primarily of phyllites metaquartzites and mylonitic augen gneisses in which garnet biotite and chlorite metamorphic zones are exposed in progressively deeper structural levels the lesser himalayan metasediment thrust package is underlain by a decollement the main detachment fault mdf which lies at a calculated depth of between 6 and 10 km underneath the mahabharat lekh and at a calculated depth of 20 to 25 km north of the tamar khola dome the tamar khola dome overlies a footwall ramp along the mdf where the mdf cuts upsection through the lesser himalayan metasediments the lesser himalayan thrust package probably has an internal structure aproximating a hinterland dipping duplex with the mct and the mdf corresponding to the roof and floor thrusts respectively both the tamar khola thrust an out of sequence breach thrust and the main boundary thrust mbt are splay thrusts off of the mdf the sub himalaya consisting of nonmetamorphosed sedimentary rocks displays an emergent imbricate fan geometry and is underlain by the southern continuation of the mdf which lies at a depth of 5 5 km to 6 km beneath the siwalik hills folding and thrusting within the lesser himalayan thrust package and the sub himalayan imbricate fan have accomodated 45 to 70 km of tectonic shortening total north south shortening across the higher lesser and sub himalaya of far eastern nepal south of the tibetan plateau has been of the order of 185 km to 245 km and has occurred at an average rate of 7 4 mm to 15 3 mm per year since the initiation of the mct between 16 and 25 ma

kinematic thermal and petrological model of the central alps lepontine metamorphism in the upper crust and eclogitisation of the lower crust abstract seismic and seismological studies as well as gravimetric models indicate that a slab of european lithospheric mantle and lower crust is currently underthrust below the apulian crust we assume a simple kinematic model in which the lower and upper subducted european crusts are decoupled along a decollement the lower crust goes into subduction without deformation the upper crust deforms by pure shear with a horizontal compressional axis the total erosional flux is adjusted to balance upper crust input so that the belt keeps the same geometry different distributions of erosion being used the computed temperature field is steady state if the kinematic model applies during a minimum time of 40 myr for a convergence rate of 8 mm yr equilibrium mineral assemblages and densities are determined from the computed p t conditions for a granodioritic chemical composition of the upper crust and an andesitic composition of the lower crust assuming local isostasy the density model fits the average topographic profile across the central alps the p t t paths obtained for the part of the upper crust initially at depths 10 to 16 km are compatible with the medium pressure oligocene metamorphism in the lepontine dome the peak calculated temperature for the deepest non subducted crustal rocks is 600 c for a pressure of 0 8 gpa near the lower limit of high pressure amphibolites we thus propose that the lepontine metamorphism corresponds to the steady state thermal regime however either faster erosion rates in the internal part of the belt or tectonic denudation are required for exhumation of the deeper portion of the belt the computed temperature field implies eclogitisation of the lower crust at a depth of 55 to 60 km we conclude that the moho limiting the deepest part of the root may correspond to the eclogitisation phase change lower crust eclogites have a density comparable to or higher than that of the mantle depending on their chemical composition 3 37 for andesitic eclogites 3 56 for gabbroic eclogites thus andesitic eclogites may stay in gravitational equilibrium in the mantle below the root whereas gabbroic eclogites are gravitationally unstable and should sink

isostasy equivalent elastic thickness and inelastic rheology of continents and oceans the equivalent elastic thickness eet is used to estimate lithospheric strength expressed in response to loading by topography and subsurface loads the data on eet allow comparisons between different plates and detection of thermal events in oceans the eet corresponds to the mechanical core of the lithosphere i e a geotherm 400 600 c in continents the eet has no relation to any depth this has led to doubts in applicability of a unique approach to the continents and oceans and in the utility of estimates of eet for continents rheological data suggest that most rocks are inelastic in the long term 0 1 m y this requires interpretation of the eet in terms of real rheology we propose an analytical model that gives rheological interpretation of both the oceanic and continental eet it also allows estimates of the mechanical thickness of the lithosphere the eet depends upon three parameters geotherm age crustal thickness and flexural plate curvature any one of these values can be estimated if the others are known comparisons of model predictions with the observed eet suggest that most continental plates have a weak lower crust allowing mechanical decoupling between the upper crust and the mantle lithosphere such decoupling leads to strong reduction in the eet and thus can be easily detected flow of rocks in the weak lower crust may have a significant influence on the temporal evolution of relief mountain building erosion differences in the mechanical behavior of oceans and continents can be explained by domination of different parameters geotherm age has a major control in the oceans whereas in the continents crustal thickness is equally important additional local variations of the eet result from weakening by flexural stresses

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

gravity anomalies crustal structure and thermo mechanical support of the himalaya of central nepal summary we use two gravity profiles that we measured across central nepal in conjunction with existing data to constrain the mechanical behaviour and the petrological structure of the lithosphere in the himalayan collision zone the data show 1 overcompensation of the foreland and undercompensation of the higher himalaya as expected from the flexural support of the range 2 a steep gravity gradient of the order of 1 3 mgal km x1 beneath the higher himalaya suggesting a locally steeper moho and 3 a 10 km wide hinge in southern tibet we compare these data with a 2 d mechanical model in which the indian lithosphere is flexed down by the advancing front of the range and sedimentation in the foreland the model assumes brittle coulomb failure and nonlinear ductile flow that depends on local temperature which is computed from a steadystate thermal model the computed moho fits seismological constraints and is consistent with the main trends in the observed bouguer anomaly it predicts an equivalent elastic thickness of 40 50 km in the foreland the flexural rigidity decreases northwards due to thermal and flexural weakening resulting in a steeper moho dip beneath the high range residuals at short wavelengths over distances of 20 30 km are interpreted in terms of 1 sediment compaction in the foreland dr 150 kg m x3 between the lower and middle siwaliks 2 the contact between the tertiary molasse and the meta sediments of the lesser himalaya at the mbt dr 220 kg m x3 and 3 the palung granite intrusion in the lesser himalaya dr 80 kg m x3 finally if petrological transformations expected from the local p t are assumed a gravity signature of the order of 250 mgal is predicted north of the lesser himalaya essentially due to eclogitization of the lower crust which is inconsistent with the gravity data we conclude that eclogitization of the indian crust does not take place as expected from a steady state local equilibrium assumption we show however that eclogitization might actually occur beneath southern tibet where it could explain the hinge observed in the gravity data we suspect that these eclogites are subducted with the indian lithosphere

characterization of geological boundaries using 1 d wavelet transform on gravity data theory and application to the himalayas we investigate the use of the continuous wavelet transform for gravity inversion the wavelet transform operator has recently been introduced in the domain of potential fields both as a filtering and a source analysis tool here we develop an inverse scheme in the wavelet domain designed to recover the geometric characteristics of density heterogeneities described by simple shaped sources the 1 d analyzing wavelet we use associates the upward continuation operator and linear combinations of derivatives of any order in the gravity case we first demonstrate how to localize causative sources using simple geometric constructions both the upper part of the source and the whole source can be studied when considering low or high altitudes respectively the homogeneity degree of the source is deduced without prior information and allows us to infer its shape introducing complex wavelets we derive analytically the scaling behavior of the wavelet coefficients for the dyke and the step sources the modulus term is used in an inversion procedure to recover the thickness of the source the phase term provides its dip this analysis is performed on gravity data we measured along a profile across the himalayas in nepal good agreement of our results with well documented thrusting structures demonstrates the applicability of the method to real data also deeper less constrained structures are characterized

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

numerical modeling of mountain building interplay between erosion law and crustal rheology 1 coupling between erosion and tectonics is thought to play a determinant role in orogenic evolution here we investigate the interplay in this coupling between the assumed erosion law and the crustal rheology at the margin of a collisional plateau like the himalaya of central nepal lithospheric deformation is calculated over a time scale of 100 kyr by a 2d finite element model that incorporates the rheological layering of the crust and the main features of the convergence across the range for the upper boundary condition two surface processes were tested a linear diffusion model and a 1d1 2 integrative model including fluvial incision along the fluvial network and hillslope erosion by landsliding model results and their sensitivity to the chosen combinations of erosion law and crustal properties are discussed in light of the constraining geologic and geomorphologic observations in contrast with the conclusions of cattin and avouac 2000 where a compliant quartz rich crustal rheology and diffusion law were required we combine a composite quartz diabase rheology for the crust with fluvial incision erosion law to account for erosion and elevation profiles across the himalaya of central nepal more generally it is proposed that because of the interplay between the dominant denudation conditions and the rheology of the crust both well documented erosion rates and processes can provide significant constraints on crustal properties within an active orogen

the equivalent elastic thickness te seismicity and the long term rheology of continental lithosphere time to burn out creme brulee insights from large scale geodynamic modeling abstract depending on the conditions and time scale the lithosphere exhibits elastic brittle plastic or viscous ductile properties as suggested by rock mechanics experiments a large part of the long term lithospheric strength is supported in the ductile regime unfortunately these data validated for strain rates 10 6 s 1 small scales few cm and simplified conditions cannot be univocally interpolated to geological time and spatial scales strain rates 10 17 10 13 s 1 100 1000 km spatial scales complex conditions without additional parameterization an adequate parameterization has to be based on real time observations of large scale deformation indeed for the oceanic lithosphere the goetze and evan s brittle elastic ductile yield strength envelopes derived from data of experimental rock mechanics were successfully validated by a number of geodynamic scale observations such as the observations of plate flexure and the associated t e equivalent elastic thickness estimates yet for continents the uncertainties of flexural models and of the other data sources are much stronger due to the complex structure and history of continental plates for example in one continental rheology model dubbed jelly sandwich the strength mainly resides in the crust and mantle while in another dubbed creme brulee the mantle is weak and the strength is limited to the upper crust these models have arisen because of conflicting results from distributed earthquake elastic thickness t e and rheology data we address these problems by examining the plausibility of each rheological model from general physical considerations we review the elastic thickness t e estimates and their relationship to the seismogenic layer thickness t s to show that these two quantities have no direct physical relation we also show that some of small t e must be artifacts of inconsistent formulation of the mechanical problem in some free air anomaly admittance models we point out that this does not necessarily detract from the admittance method itself but refers to its incorrect application in the continental domain we then explore by analytical and numerical thermo mechanical modeling the implications of a weak and strong mantle for tectonic structural styles we conclude that rheological models such as creme brulee which invoke a weak lithosphere mantle are generally incompatible with observations the jelly sandwich is in better agreement and we believe provides a useful first order explanation for the long term support of the earth s main surface features

subduction initiation and continental crust recycling the roles of rheology and eclogitization abstract a mechanical model with viscous ductile and plastic brittle rheologies is used to investigate the effect of eclogitization on the dynamics of convergence density increases by 300 600 km m3 during eclogitization of crustal rocks and continental lower crust and oceanic crust reach higher density than mantle we explore cases of intracontinental deformation subduction and continental collision we consider a wide range of parameters for friction activation energy and initial thermal state and cases with or without eclogitization the style of deformation appears to be primarily controlled by the presence or absence of weak zones simulations are run with a constant convergence velocity 1 5 or 4 cm year and the evolution of the compressive force through time is thus a critical test of the model viability for intracontinental deformation when the brittle crust is decoupled from the mantle the mantle deforms by symmetrical bending or subduction and a variable amount of lowermost crust is entrained with the sinking mantle in these cases the compressive force remains in the 1012 3 1013 n m acceptable range oceanic subduction only occurs if a low friction shear zone is specified in the brittle realm during the transition from oceanic subduction to collision the oceanic crust in this shear zone remains trapped between colliding crusts eclogitization has no influence on the initial mode of deformation thus it is probably not an important factor in the initiation of subduction in all simulations the influence of eclogitization increases progressively with time in proportion of the amount of lower continental crust and oceanic crust entrained into the mantle the structural evolution of the orogen model depends on eclogitization if initial decoupling occurs at mid crustal level the simulations thus indicate that eclogitization promotes convergence and also in some cases enables the recycling of the whole continental lower crust into the mantle we also suggest that eclogitization could regulate the long term convergence rate in orogens

seismicity in the kingdom of bhutan 1937 2003 evidence for crustal transcurrent deformation we investigate moderate sized historic to recent earthquakes in the bhutan himalaya spanning the years from 1937 to 2003 we find that few moderate sized earthquakes occurred in the region during this time period in order to better characterize the seismicity we relocate all earthquakes and estimate focal mechanisms for events for which we have adequate data we use first motion data for older events and waveform model digital seismograms for three earthquakes that occurred in 1980 m w 6 4 1995 m w 5 4 and 2003 m w 5 4 for the modern events we utilize three techniques for focal mechanism and depth determination regional full waveform time domain modeling of the two moderate m w 5 4 events and surface wave spectral method and body wave depth modeling approach for the m w 6 4 event we find that the first motion and digital data focal mechanisms are mostly strike slip with midcrustal to deep crustal depths although no recent great event has occurred along the main himalayan thrust the indian plate is undergoing significant midcrustal to deep crustal transcurrent deformation likely due to oblique convergence of the indian asian collision in this region unlike oceanic continent oblique subduction strike slip and thrust partitioning occurs throughout the crust not in distinct zones the geological and geophysical nature of the bhutan himalaya appears significantly different than other himalayan regions due to the strike slip nature of many of the events an out of sequence thrust fault and the implications of deformation in the shillong plateau

erosive water level regime and climatic variability forcing of beach dune systems on south western vancouver island british columbia canada increases in the frequency and magnitude of extreme water levels and storm surges are correlated with known indices of climatic variability cv including the el nino southern oscillation enso and the pacific decadal oscillation pdo along some areas of the british columbia coast since a shift to a positive pdo regime in 1977 the effects of enso events have been more frequent persistent and intense teleconnected impacts include more frequent storms higher surges and enhanced coastal erosion the response of oceanographic forcing mechanisms i e tide surge wave height wave period to cv events and their role in coastal erosion remain unclear particularly in western canada as a first step in exploring the interactions between ocean atmosphere forcing and beach dune responses this paper assembles the historic erosive total water level twl regime and explores relations with observed high magnitude storms that have occurred in the tofino ucluelet region wickaninnish bay on the west coast of vancouver island british columbia canada extreme events where twl exceeded an erosional threshold i e elevation of the beach foredune junction of 5 5 m acd are examined to identify dominant forcing mechanisms and to classify a regime that describes erosive events driven principally by wave conditions 61 5 followed by surge 21 8 and tidal 16 7 effects furthermore teleconnections between regional cv phenomena extreme storm events and by association coastal erosion are explored despite regional sea level rise eustatic and steric rapid crustal uplift rates have resulted in a falling relative sea level and in some sedimentary systems shoreline progradation at rates approaching 1 5 m a 1 over recent decades foredune erosion occurs locally with a recurrence interval of approximately 1 53 years followed by rapid rebuilding due to high onshore sand supply and often in the presence of large woody debris and rapidly colonizing vegetation in the backshore copyright 2012 john wiley sons ltd

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