extension during continental convergence with application to the tibetan plateau the tibetan plateau is the product of crustal thickening caused by the collision between india and asia and is the largest active example of extensional tectonics in a zone of continental collision throughout most of the tertiary the tectonics of the plateau were dominated by north south shortening a significant proportion of which took place on east west striking thrust faults for the last 5 m y or so the plateau has been thinning by the mechanism of extension on north south trending normal faults previous investigations of the collision have been able to account for the large scale features of the tertiary deformation but have failed to explain the transition in the late tertiary to quaternary strain rate field of the plateau from north south compression to east west extension the transition could in principle be effected by a reduction in the rate of convergence between india and asia or by a uniform reduction in strength of the whole continental lithosphere of asia it could not however be effected by a reduction in strength of the elevated region alone this produces increased compressional strain rates in the weakened zone an alternative explanation for the transition to extension comes from considering the thermal evolution of thickened continental lithosphere the lower part of the lithosphere consists of a thermal boundary layer which when thickened by horizontal shortening is colder and denser than its surroundings convective instability of the thickened thermal boundary layer and its replacement by hot asthenosphere would rapidly raise the surface elevation and gravitational potential energy of the overlying part of the lithosphere the convective instability would happen in a time brief compared with the collision time scale 50 m y in the case of india and asia but would only occur after there had already been substantial thickening of the lithosphere numerical experiments show that for a range of lithospheric parameters the increase in surface height as much as 2 km and of potential energy 5 to 10 1012 n m 1 resulting from convective instability of the lower lithosphere are sufficient for east west extension to replace north south compression as the dominant feature of the stress field of the tibetan plateau

late cretaceous early palaeogene tectonic development of se asia abstract the late cretaceous early palaeogene history of the continental core of se asia sundaland marks the time prior to collision of india with asia when se asia from the tethys in the west to the palaeo pacific in the east lay in the upper plate of subduction zones in myanmar and sumatra subduction was interrupted in the aptian albian by a phase of arc accretion woyla and mawgyi arcs and in java eastern borneo and western sulawesi by collision of continental fragments rifted from northern australia subsequent resumption of subduction in the myanmar thailand sector explains 1 early creation of oceanic crust in the andaman sea in a supra subduction zone setting 95 ma 2 the belt of granite plutons of late cretaceous early palaeogene age starting 88 ma in western thailand and central myanmar and 3 amphibolite grade metamorphism between 70 and 80 ma seen in gneissic outcrops in western and central thailand and 4 accretionary prism development in the western belt of myanmar until glancing collision with the ne corner of greater india promoted ophiolite obduction deformation and exhumation of marine sediments in the early palaeogene the ranong strike slip fault and other less well documented faults were episodically active during the late cretaceous palaeogene time n to nw directed subduction of the palaeo pacific ocean below southern china vietnam and borneo created a major magmatic arc associated with rift basins metamorphic core complexes and strike slip deformation which continued into the late cretaceous the origin and timing of termination of subduction has recently been explained by collision of a large luconia continental fragment either during the late cretaceous or palaeogene evidence for such a collision is absent from the south china sea well and seismic reflection record and here collision is discounted instead relocation of the subducting margin further west possibly in response of back arc extension which created the proto south china sea is preferred lying between the two subduction related arcs the khorat basin is of predominantly late jurassic early cretaceous age but stratigraphic and apatite fission track data also indicates deposition of 1 2 km of late cretaceous sediments the synformal basin geometry probably arose due to the dynamic topography created by converging tethyan and palaeo pacific subduction zones the aptian albian slowing of basin subsidence and onset of evaporite deposition coincides with collision of the mawgyi and woyla island arcs extensive palaeogene deformation and exhumation 3 km in places affected all margins of the khorat plateau deformation includes folds of the phu phan uplift and strike slip faults thrusts and folds on the southern and eastern margins south of the khorat plateau outcrop and seismic reflection data from the ton le sap basin cambodia and the gulf of thailand indicate syn depositional fault controlled subsidence was important during cretaceous deposition the hot thickened crust developed during the late cretaceous palaeogene events follows the weak indosinian crustal scale inthanon and sukhothai zones which persistently guided the location of later structures including cenozoic extensional and post rift basins and influenced the widespread occurrence of low angle normal faults metamorphic core complexes and eastern gulf of thailand super deep post rift basins

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

dynamics of the india eurasia collision zone we present simple new dynamic calculations of a vertically averaged deviatoric stress field over a depth average of 100 km for asia from geodetic geologic topographic and seismic data a first estimate of the minimum absolute magnitudes and directions of vertically averaged deviatoric stress is obtained by solving force balance equations for deviatoric stresses associated with gravitational potential energy differences within the lithosphere plus a first order contribution of deviatoric stresses associated with stress boundary conditions this initial estimate of the vertically averaged deviatoric stress field is obtained independent of assumptions about the rheology of the lithosphere absolute magnitudes of vertically averaged deviatoric stresses vary between 5 and 40 mpa assuming bulk viscous behavior for the lithosphere the magnitudes of deviatoric stresses together with the magnitudes of strain rates inferred from quaternary fault slip rate and gps data yield vertically averaged effective viscosities for tibet of 0 5 5 1022 pa s compared with 1 2 5 1023 pa s in more rigid areas elsewhere in the region a forward modeling method that solves force balance equations using velocity boundary conditions allows us to refine our estimates of the vertically averaged effective viscosity distribution and deviatoric stress field the total vertically averaged deviatoric stress and effective viscosity field are consistent with a weak lower crust in tibet they are consistent with some eastward motion of tibet and south china lithosphere relative to eurasia and they confirm that gravitational potential energy differences have a profound effect on the spatially varying style and magnitude of strain rate around the tibetan plateau our results for the vertically averaged deviatoric stress argue for a large portion of the strength of the lithosphere to reside within the seismogenic upper crust to get deviatoric stress magnitudes there to be as high as 100 300 mpa in accord with laboratory and theoretical friction experiments indicating that stress drops in earthquakes are small fractions of the total deviatoric stress

himalayan metamorphism and its tectonic implications the himalayan range exposes a spectacular assemblage of metamorphic rocks from the mid and deep crust that have fostered numerous models of how the crust responds to continental collisions recent petrogenetically based petrologic and geochronologic studies elucidate processes with unprecedented resolution and critically test models that range from continuum processes to one time events the pronounced metamorphic inversion across the main central thrust reflects continuum thrusting between ca 15 and 20 ma whereas exposure of ultrahigh pressure rocks in northwestern massifs and syntaxis granulites reflects singular early 45 ma and late 10 ma exhumation events multiple mechanisms including wedge collapse and flow of melt weakened midcrust are debated to explain pressure temperature trajectories patterns of thinning and thermal overprinting a geochronologic revolution is under way in which spatially resolved compositions and ages of accessory minerals are combined in a petrogenetically valid co

new paleomagnetic results from the lhasa block revised estimation of latitudinal shortening across tibet and implications for dating the india asia collision knowing how and when the tibetan plateau formed is of global concern as high topography modifies climate circulation patterns and alters species migration paths previous paleomagnetic work suggested that intracontinental shortening from deformation related to the india asia collision played a major role in building the tibetan plateau however these studies were derived from red bed facies which are often afflicted by inclination shallowing leading to overestimates of continental shortening we report new paleomagnetic data from the lhasa block the southern continental margin of asia before the india asia collision a high temperature characteristic remanent magnetization chrm was isolated from 43 sites of late cretaceous red beds 32 sites of late cretaceous lava flows and nine sites of eocene tuffs all the chrm directions are significantly different from the present day geomagnetic field and the mean directions of red beds lava flows and tuffs pass the fold test the distribution patterns of chrm directions derived from volcanic rocks are consistent with a secular variation model suggesting that secular variation was likely averaged out in contrast the red bed chrm directions exhibit an east west elongated distribution pattern indicative of inclination flattening the mean chrm inclination of the red beds 23 5 degrees 2 5 degrees is consistent with previous results but significantly shallower than that of 41 9 degrees 4 4 degrees carried by the coeval lava flows an elongation inclination analysis of the red bed chrm directions yielded an unflattened mean inclination of 39 7 degrees with 95 confidence limits between 37 7 degrees and 41 9 degrees the paleolatitudes of the late cretaceous volcanic rocks and inclination corrected sediments suggest about 10 degrees less shortening occurred between lhasa and siberia than previous estimates with total shortening lying between 3 3 degrees 2 6 degrees and 7 3 degrees 5 4 degrees since the late cretaceous the mean chrm inclination derived from the eocene tuffs 51 8 degrees 5 2 degrees is indistinguishable from the expected reference direction our new results imply that india asia convergence was accommodated mainly through consumption of the india plate and yield a much younger age ca 43 ma for the initial india asia collision than previously estimated by paleomagnetic data c 2010 elsevier b v all rights reserved

non destructive multi method internal analysis of multiple inclusions in a single diamond first occurrence of mackinawite fe ni 1 xs a single gem lithospheric diamond with five sulfide inclusions from the udachnaya kimberlite siberia russia has been analyzed non destructively to track the growth conditions of the diamond sulfides are the most abundant mineral inclusions in many lithospheric diamond crystals and are the most favorable minerals to date diamond crystals by re os isotope systematics our investigation used non destructive micro techniques combining x ray tomography x ray fluorescence x ray powder diffraction and raman spectroscopy this approach allowed us to determine the spatial distribution of the inclusions their chemical and mineralogical composition on the microscale and finally the paragenetic association leaving the diamond host completely unaffected the sample was also studied by x ray diffraction topography to characterize the structural defects of the diamond and to obtain genetic information about its growth history the x ray topographic images show that the sample investigated exhibits plastic deformation one set of 111 slip lamellae corresponding to polysynthetic twinning affects the entire sample chemical data on the inclusions still trapped within the diamond show they are monosulfide solid solutions of fe ni and indicate a peridotitic paragenesis micro x ray diffraction reveals that the inclusions mainly consist of a polycrystalline aggregate of pentlandite and pyrrothite a thorough analysis of the raman data suggests the presence of a further fe ni sulfide never reported so far in diamonds mackinawite the total absence of any oxides in the sulfide assemblage clearly indicates that mackinawite is not simply a late alteration of pyrrhotite and pentlandite due to secondary oxidizing fluids entering diamond fractures after the diamond transport to the surface instead it is likely formed as a low temperature phase that grew in a closed system within the diamond host it is possible that mackinawite is a more common phase in sulfide assemblages within diamond crystals than has previously been presumed and that the percentage of mackinawite within a given sulfide assemblage could vary from diamond to diamond and from locality to locality

low energy phase boundary pairs and preferred crystallographic orientations of olivines in nanometer sized ultrapotassic fluid inclusions of aykhal diamond abstract the healed internal conjugated cleavages at the core of aykhal octahedral diamond sample ah2 were decorated with 111 dia facetted ultrapotassic fluid melt inclusion pockets containing nanosized graphite phlogopite and olivine fo92 inclusions these olivines are either rounded in pockets with ample fluid or facetted by the 111 dia mold in the pockets with a fluid film transmission electron microscopy revealed two distinct crystallographic characteristics of olivine inclusions 1 pronounced crystallographic texture of olivines grouped in specific diamond domain and 2 frequent parallelism or sub parallelism of specific low energy faces of the two phases mainly 010 ol 120 ol 001 ol and 111 dia 110 dia 100 dia in the order of decreasing preference forming prominent 010 ol 111 dia 010 ol 110 dia 001 ol 110 dia 120 ol 111 dia and 120 ol 110 dia low energy phase boundaries with thin liquid film of 1 2 nm in between these findings not only testify to the extremely low adhesion energies of olivine diamond boundary pairs but also imply that in the presence of a fluid phase the interfacial energetics and the energetically favored crystallographic orientations of olivine inclusions in diamond can be controlled simply by the settlement attachment of low energy facets of olivine crystals precipitating from the parental fluid upon the low energy 111 dia or 110 dia surfaces of diamond such interfacial energetics control and the resultant low energy boundary pairs are characteristically distinct from the common topotaxy or epitaxy between oxide silicate mineral pairs but are in a sense like the van der waals heteroepitaxy in artificial systems

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

peneplain formation in southern tibet predates the india asia collision and plateau uplift the uplift history of tibet is crucial for understanding the geodynamic and paleoclimatologic evolution of asia however it remains controversial whether tibet attained its high elevation before or after india collided with asia 50 m y ago here we use thermochronologic and cosmogenic nuclide data from a large bedrock peneplain in southern tibet to shed light on the timing of the uplift the studied peneplain which was carved into cretaceous granitoids and jurassic metasediments is located in the northern lhasa block at an altitude of 5300 m thermal modeling based on u th he ages of apatite and zircon and apatite fission track data indicate cooling and exhumation of the granitoids between ca 70 and ca 55 ma followed by a rapid decline in exhumation rate from 300 m m y to 10 m m y between ca 55 and ca 48 ma since then the peneplain has been a rather stable geomorphic feature as indicated by low local and catchment wide erosion rates of 6 11 m m y and 11 16 m m y respectively which were derived from cosmogenic 10 be concentrations in bedrock grus and stream sediment the prolonged phase of erosion and planation that ended ca 50 ma removed 3 6 km of rock from the peneplain region likely accomplished by laterally migrating rivers the lack of equivalent sediments in the northern lhasa block and the presence of a regional unconformity in the southern lhasa block indicate that the rivers delivered this material to the ocean this implies that erosion and peneplanation proceeded at low elevation until india9s collision with asia induced crustal thickening surface uplift and long term preservation of the peneplain

a review of crust and upper mantle structure beneath the indian subcontinent this study has been supported by a grant from the ministry of earth sciences moes iitkgp ckh

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

how does the mid crust accommodate deformation in large hot collisional orogens a review of recent research in the himalayan orogen abstract the presence of hot weak crust is a central component of recent hypotheses that seek to explain the evolution of continent continent collisions and in particular may play an important role in accommodating the 3000 km of convergence within the himalaya tibetan collision over the last 55 myr models that implicate flow of semi viscous midcrustal rocks south toward the front of the himalayan orogen channel flow are able to account for many geologic observations in the himalaya while alternative models of collision particularly thrust wedge taper demonstrate that much of the observed geology could have formed in the absence of a low viscosity mid crustal layer several recent studies synthesized here have prompted a shift from initial assumptions that channel flow and thrust wedge taper processes are by definition mutually exclusive these new studies reveal the presence of several tectonometamorphic discontinuities in the midcrust that appear to reflect a continuum of deformation in which both channel and wedge type processes operate in spatially and temporally distinct domains within the orogen and further that the system may migrate back and forth between these types of behavior this continuum of deformation styles within the collisional system is of crucial importance for explaining the evolution of the himalayan orogen and hence for understanding the evolution of earth s many continent continent collision zones

influence of mantle dynamics on the topographic evolution of the tibetan plateau results from numerical modeling 1 we investigate numerically the evolution of crustal and lithospheric thickness thermal structure topography and strain rate of the tibetan plateau through time using the thin viscous sheet approach we show that lithospheric mantle must have been removed from beneath tibet to explain the present surface elevation and lack of regional surface slope in the absence of this removal the modeled topography reaches a maximum elevation of 4000 m for weak rheology or the surface slopes significantly northward for strong rheology the crust must have been warmed and weakened by an increase of radiogenic heat production at depth due to crustal thickening in the absence of this warming viscous stresses associated with plate convergence exceed stresses produced by topography and the present pattern of vertical thinning and east west extension would not have developed continuous removal of lithosphere by delamination or convection does not allow sufficient crustal warming and fails to reproduce either the present topography or the pattern of active deformation on the plateau in a reasonable time geologically rapid removal of the lithospheric root beneath the thickened crust of tibet successfully explains the current elevation of the plateau its lack of surface slope the steep south and north margins and the pattern of the present deformation including vertical thinning e w extension and extrusion and vertical axis rotation on the eastern margin our modeling suggests that this removal took place within the last 12 m y

seismic images of crust and upper mantle beneath tibet evidence for eurasian plate subduction seismic data from central tibet have been combined to image the subsurface structure and understand the evolution of the collision of india and eurasia the 410 and 660 kilometer mantle discontinuities are sharply defined implying a lack of a subducting slab beneath the plateau the discontinuities appear slightly deeper beneath northern tibet implying that the average temperature of the mantle above the transition zone is about 300 c hotter in the north than in the south there is a prominent south dipping converter in the uppermost mantle beneath northern tibet that might represent the top of the eurasian mantle lithosphere underthrusting the northern margin of the plateau

tibetan plate overriding the asian plate in central and northern tibet at the southern boundary between india and tibet the indian tectonic plate subducts northwards beneath the tibetan plateau seismic imaging shows that at the northern boundary of tibet the asian plate is also subducting southwards beneath the plateau and the tibetan lithosphere is separate

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

on the origin of orogens in order to understand how orogens work a quantitative approach demonstrating proof of concept is essential our goal is to reconcile the diverse array of tectonic features observed in natural orogens in the context of working numerical models that are consistent with both the underlying physics and first order geological constraints we present a simple conceptual temperature magnitude t m framework for orogenesis in terms of the progression from small cold to large hot orogens and we use forward numerical models to test hypotheses corresponding to specific stages along the t m spectrum small cold orogens are analyzed using crustal scale singularity s point models in which suborogenic mantle lithosphere is kinematically subducted beneath crust that deforms by critical wedge mechanics the transition from oceanic subduction to continental collision and the subsequent evolution of large hot orogens has been investigated using both crustal and upper mantle scale models the latter including dynamic subduction of suborogenic mantle lithosphere large hot orogens with thick crust are characterized by elevated plateaus with a strong superstructure underlain by hot weak lower crustal infrastructure beneath plateaus tectonic processes are dominated by ductile flow of weak crust in response to differential pressure while plateau flanks form external thrust sense wedges we discuss four topical issues in orogenic tectonics including the response of the suborogenic mantle lithosphere to convergence the interaction of climate and tectonics the current debate concerning wedge versus channel flow models to explain the himalayan tibetan system and the interpretation of metamorphic architecture in terms of orogenic processes we conclude that collisional orogenesis is driven largely by subduction and accretion of material at convergent margins accompanied by shortening thickening and heating of deformed crust

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

a thin viscous sheet model for continental deformation summary for the purposes of describing its large scale and long term deformation the continental lithosphere is regarded as a continuum obeying a newtonian or a power law rheology the flow of a thin sheet of power law material overlying an inviscid substrate is studied under the assumption that vertical gradients of the horizontal velocity are negligible a numerical model is used to investigate the deformation of such a sheet under conditions approximating those of continent continent collision the material flows in response to forces applied to its boundaries for example the indenting of one continent by another and to forces in its interior arising from gradients in crustal thickness the horizontal divergence of the flow produces changes in the crustal thickness and hence a time dependent form to the flow itself for a given set of boundary conditions the flow depends on the stress exponent in the power law rheology n and on the argand number which is a measure of the ratio between the stress arising from crustal thickness contrasts and the stress required to deform the material at the ambient strain rates when the effective viscosity of the medium is very high ar o crustal thickness variations do not influence the flow if the material is newtonian n l the deformation associated with an influx of material approximating an indenter is of much greater lateral dimension than the width of the indenter whereas when material has a power law rheology n 3 5 are used the deformation is confined to a region of lateral extent comparable to that of the indenter as ar increases the forces arising from crustal thickness contrasts exert more influence on the flow and the maximum crustal thickness that can be sustained by a given influx of material is related by a simple expression to the effective viscosity of the medium at the ambient strain rates in the limit of a very weak medium ar 10 the lithosphere is unable to sustain appreciable crustal elevation contrasts the results of these numerical experiments show that systems in which the effective viscosities are such that the maximum deviatoric stresses are between 1 kbar

diamond and its olivine inclusions a strange relation revealed by ab initio simulations abstract the study of diamond and its solid inclusions is of paramount importance to acquire direct information on the deepest regions of the earth however although diamond is one of the most studied materials in geology the diamond inclusion relationships are not yet understood do they form simultaneously syngenesis or are inclusions pre existing objects on which diamond nucleated protogenesis here we report for the first time adhesion energies between diamond d and forsterite fo to provide a crucial contribution to the syngenesis protogenesis debate the following interfaces were investigated at quantum mechanical level i 001 d 001 fo ii 001 d 021 fo and iii 111 d 001 fo our data along with the ones recently obtained on the 110 d 101 fo interface revealed an unexpected thermodynamic behaviour all interfaces showing almost equal and low adhesion energies accordingly diamond and olivine have an extremely low chemical affinity and cannot develop preferential orientations even during an eventual epitaxial growth combining these results with those of our previous work concerning the morphology constraints of diamond on its inclusions we can state that the two main arguments used so far in favour of diamond inclusions syngenesis cannot be longer considered valid at least for olivine

the indo asian continental collision a 3 d viscous model abstract the large scale physical process of the indo asian continental collision and the formation of the himalayan tibetan plateau have been simulated in various viscous thin sheet models but the thin sheet simplification also kept some important issues from being fully explored among these issues are the role of strike slip fault zones in facilitating large scale lateral translation of lithospheric blocks the escaping tectonics during the collision and the speculated lateral flow of the ductile middle lower crust under the tibetan plateau here we present a fully three dimensional finite element model to simulate the indo asian continental collision the model includes major boundary faults to simulate the escaping tectonics and vertically variable rheological structures to model lower crustal flow the collisional process is constrained by the history of the indo eurasian plate convergence and the present crustal thickness and topography of the tibetan plateau our results indicate that the restrictive boundaries of the tibetan plateau including the rigid tarim and south china blocks largely control the spatiotemporal patterns of crustal deformation in the collision zone as the indian indenter moves toward the tarim block higher strain rates and topography developed in the western part of the collision zone than in the eastern part causing the northward migration of the deformation front to gradually change to eastward migration in the past 10 20 myr broadly consistent with the initiation of widespread e w extension in the plateau these restrictive boundary blocks also force the crustal and mantle materials in the collision zone to flow coherently hence providing an alternative explanation for the apparently vertically coherent deformation in tibet assuming that the crust weakens as it thickens our model predicts the lateral expansion of the tibetan plateau an important feature of the tibetan tectonics that is missing in previous models with constant rheology

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

first evidence of hydrous silicic fluid films around solid inclusions in gem quality diamonds abstract diamonds form from fluids or melts circulating at depth in the earth s mantle analysis of these fluids is possible if they remain entrapped in the diamond during its growth but this is rarely observed in gem quality stones we provide the first evidence that typical mineral inclusions in gem quality diamonds from the siberian and kaapvaal cratons are surrounded by a thin film of hydrous silicic fluid of maximum thickness 1 5 m the fluid contains si 2 o oh 6 si oh 4 and molecular h 2 o and was identified using confocal micro raman spectroscopy and synchrotron based x ray tomographic microscopy as the solid mineral inclusions have both peridotitic and eclogitic affinities and occur in two cratonic regions our results demonstrate the strong connection between water rich fluids and the growth of gem quality lithospheric diamonds the presence of the fluid films should be taken into account for a proper evaluation of h 2 o contents in the mantle based on h 2 o contents in solid inclusions and for a robust assessment of diamond formation pressures based on the residual pressures of the inclusions

oblique stepwise rise and growth of the tibet plateau two end member models of how the high elevations in tibet formed are i continuous thickening and widespread viscous flow of the crust and mantle of the entire plateau and ii time dependent localized shear between coherent lithospheric blocks recent studies of cenozoic deformation magmatism and seismic structure lend support to the latter since india collided with asia 55 million years ago the rise of the high tibetan plateau likely occurred in three main steps by successive growth and uplift of 300 to 500 kilometer wide crustal thrust wedges the crust thickened while the mantle decoupled beneath gently dipping shear zones did not sediment infilling bathtub like of dammed intermontane basins formed flat high plains at each step the existence of magmatic belts younging northward implies that slabs of asian mantle subducted one after another under ranges north of the himalayas subduction was oblique and accompanied by extrusion along the left lateral strike slip faults that slice tibet s east side these mechanisms akin to plate tectonics hidden by thickening crust with slip partitioning account for the dominant growth of the tibet plateau toward the east and northeast

paleogeographic and tectonic evolution of southern tibet since middle cretaceous time new paleomagnetic data and synthesis during the second french chinese paleomagnetic campaign in tibet in the summer of 1981 we sampled the middle cretaceous red beds of the takena formation and the cenozoic volcanics of the lingzizong formation a detailed analysis of about 400 specimens of red beds reveals a stable component of normal magnetization n in the north of the lhasa block north of the nyainqentanglha range and two stable components in the south lt ht where the red beds are more strongly folded and are unconformably overlain by the lingzizong volcanics all three components are carried by hematite blocking temperatures range from 400 to 600 c for lt and up to the neel temperature of hematite for both n and ht lt is shown to postdate part of the folding of the takena whereas n and ht both indicate a positive fold test at the 99 probability level in addition a small less than 10 of the natural remanent magnetization antiparallel component is superimposed in most samples and may be consistent with a model of long term acquisition of a chemical remanent magnetization for the red beds analysis of the volcanics reveals a single stable component with mixed polarities carried either by hematite or magnetite the paleomagnetic directions indicate that 1 the lhasa block stood at 12 5 3 n by upper cretaceous and at 13 5 6 5 n in the paleocene eocene thus confirming the limited displacement of the margin of eurasia between albian aptian deposition of the red beds and eocene onset of the india asia collision these values imply crustal shortening by as much as 1900 850 km north of the block since the upper cretaceous and by 1400 1200 km south of the block since the onset of the collision 2 southern tibet formed a single tectonic unit by middle cretaceous time and 3 differential rotations are observed within the block and are related to the collision these rotations are discussed in the frame of a recent propagating extrusion tectonics model this study confirms the close paleogeographic relationship between indochina khorat plateau and southern tibet prior to the india asia collision

finite strain calculations of continental deformation 1 method and general results for convergent zones a finite element method is used to calculate stresses and strain rates in a thin viscous sheet representing the continental lithosphere that is bordered in one part by an indenting boundary we calculate solutions for velocity fields crustal thickness distributions strain rates rotation time integrated vorticity and finite strain ellipsoids to show the dependence of these quantities on the controlling parameters the stress strain exponent n and the strength of the sheet relative to the gravitational forces which may be expressed as a dimensionless number the argand number ar for n 3 and ar between 1 and 10 a plateau of thickened crust is formed in front of the indenter the strain histories of individual elements within the viscous sheet may be quite complex and the finite strain at the end of deformation may bear no obvious relation to the principal stress orientations during the deformation the strain rate fields are interpreted in terms of the style of faulting that would be expected in the brittle upper crust if it were coupled to the ductile but stronger upper mantle in general the length scale of deformation increases with ar and time since the start of the collision and decreases with n for ar 3 the increase in length scale is accompanied by a change in the style of deformation from predominantly crustal thickening with thrust faulting in the brittle layer to predominantly transcurrent deformation strike slip faulting in the brittle layer

development of topography in 3 d continental collision models understanding the formation and evolution of high mountain belts such as the himalayas and the adjacent tibetan plateau has been the focus of many tectonic and numerical models here we employ 3 d numerical simulations to investigate the role that subduction collision and indentation play on lithosphere dynamics at convergent margins and to analyze the conditions under which large topographic plateaus can form in an integrated lithospheric and upper mantle scale model distinct dynamics are obtained for the oceanic subduction side trench retreat slab rollback and the continental collision side trench advance slab detachment topographic uplift lateral extrusion we show that slab pull alone is insufficient to generate high topography in the upper plate and that external forcing and the presence of strong blocks such as the tarim basin are necessary to create and shape anomalously high topographic fronts and plateaus moreover scaling is used to predict four different modes of surface expression in continental collision models i low amplitude homogeneous shortening ii high amplitude homogeneous shortening iii alpine type topography with topographic front and low plateau and iv tibet himalaya type topography with topographic front and high plateau results of semianalytical models suggest that the argand number governs the formation of high topographic fronts while the amplitude of plateaus is controlled by the initial buoyancy ratio of the upper plate applying these results to natural examples we show that the alps belong to regime iii the himalaya tibet to regime iv whereas the andes altiplano fall at the boundary between regimes iii and iv

the carbon isotopic composition and nitrogen content of lithospheric and asthenospheric diamonds from the jagersfontein and koffiefontein kimberlite south africa asthenospheric diamonds from the jagersfontein kimberlite south africa have a 13c range from 8 9 to 24 4 vs pdb deep diamonds from the nearby koffiefontein mine have 13c values between 2 9 and 6 1 at these two locations asthenospheric diamonds are more frequently nitrogen type ii than type i if they are type i the nitrogen content is low and its state of aggregation is high diamonds from the jagersfontein kimberlite have a bimodal 13c distribution on the basis of the carbon isotopic composition of the hosts and their mineral inclusion composition one can suggest that as many as six distinct diamond source regions were sampled by the kimberlite in the mantle beneath jagersfontein the relative abundance of low 13c diamonds may be higher in the asthenosphere than in the lithosphere in the koffiefontein sample suite asthenospheric and lithospheric diamonds are indistinguishable in 13c the differences in the nature of the relationships between isotope and inclusion chemistry demonstrated by deep diamonds from two diatremes which are very closely associated in space and time indicate a high degree of complexity in mantle carbon geochemistry even within the asthenosphere

subduction dynamics and the origin of andean orogeny and the bolivian orocline the building of the andes results from the subduction of the oceanic nazca plate underneath the south american continent however how and why the andes and their curvature the bolivian orocline formed in the cenozoic era 65 5 million years myr ago to present despite subduction continuing since the mesozoic era 251 0 65 5 myr ago is still unknown three dimensional numerical subduction models demonstrate that variations in slab thickness arising from the nazca plate s age at the trench produce a cordilleran morphology consistent with that observed the age dependent sinking of the slab in the mantle drives traction towards the trench at the base of the upper plate causing it to thicken thus subducting older nazca plate below the central andes can explain the locally thickened crust and higher elevations here we demonstrate that resultant thickening of the south american plate modifies both shear force gradients and migration rates along the trench to produce a concave margin that matches the bolivian orocline additionally the varying forcing along the margin allows stress belts to form in the upper plate interior explaining the widening of the central andes and the different tectonic styles found on their margins the eastern and western cordilleras the rise of the central andes and orocline formation are directly related to the local increase of nazca plate age and an age distribution along the margin similar to that found today the onset of these conditions only occurred in the eocene epoch this may explain the enigmatic delay of the andean orogeny that is the formation of the modern andes

a hierarchical approach for evaluating fluvial systems architectural analysis and sequential evolution of the high net sand content middle wasatch formation uinta basin utah well exposed three dimensional fluvial outcrops of the high net sand content middle wasatch formation in three canyon uinta basin utah were used to create and develop a new methodology for describing the architecture of fluvial systems the methodology builds on the works of campbell jackson allen and miall and addresses sedimentary processes scale and temporal context for reservoir and non reservoir bodies the methodology developed herein is a three level hierarchical framework that classifies meso and macroscale architecture of fluvial systems the three level hierarchy contains from smallest to largest stories elements and archetypes eight story types provide the foundational building blocks of this framework and account for sedimentation in both channel belt and floodplain belt elements including 1 downstream accreting 2 laterally accreting 3 erosionally based fine grained fill 4 fine grained fill associated with laterally accreting 5 levee 6 splay 7 crevasse or overbank channels and 8 floodplain fines two types of elements are recognized 1 channel belt and 2 floodplain belt an archetype consists of a channel belt element and its genetically related floodplain belt elements two distinct upward stacking patterns differentiate braided and meandering archetypes in deconstructing the evolution of archetypes three distinct associations between channel belt elements and their adjacent splays are documented 1 unassociated splays 2 associated coeval splays and 3 associated non coeval splays width and thickness for stories channel belt elements and archetypes are documented providing dimensional constraints for analog high net sand content fluvial systems additionally this methodology provides object based models with shape defined reservoir and nonreservoir geobodies that realistically compare to fluvial systems

greater india basin hypothesis and a two stage cenozoic collision between india and asia cenozoic convergence between the indian and asian plates pro duced the archetypical continental collision zone comprising the himalaya mountain belt and the tibetan plateau how and where india asia convergence was accommodated after collision at or be fore 52 ma remains a long standing controversy since 52 ma the two plates have converged up to 3 600 35 km yet the upper crustal shortening documented from the geological record of asia and the himalaya is up to approximately 2 350 km less here we show that the discrepancy between the convergence and the shortening can be explained by subduction of highly extended continental and oceanic indian lithosphere within the himalaya be tween approximately 50 and 25 ma paleomagnetic data show that this extended continental and oceanic greater india promontory resulted from 2 675 700 km of north south extension between 120 and 70 ma accommodated between the tibetan himalaya and cratonic india we suggest that the approximately 50 ma india asia collision was a collision of a tibetan himalayan microconti nent with asia followed by subduction of the largely oceanic greater india basin along a subduction zone at the location of the greater himalaya the hard india asia collision with thicker and contiguous indian continental lithosphere occurred around 25 20 ma this hard collision is coincident with far field deforma tion in central asia and rapid exhumation of greater himalaya crys talline rocks and may be linked to intensification of the asian monsoon system this two stage collision between india and asia is also reflected in the deep mantle remnants of subduction imaged with seismic tomography

erosion as a driving mechanism of intracontinental mountain growth in nature mountains can grow and remain as localized tectonic features over long periods of time 10 m y by contrast according to current knowledge of lithospheric rheology and neglecting surface processes any intracontinental range with a width that exceeds that which can be supported by the strength of the lithosphere should collapse within a few tens of millions of years for example assuming a quartz dominated crustal rheology the relief of a range initially 3 km high and 300 400 km wide is reduced by half in about 15 m y as a result of lateral spreading of its crustal root we suggest that surface processes might actually prevent such a subsurface collapse removal of material from topographic heights and deposition in the foreland oppose spreading of the crustal root and could eventually drive a net influx of material toward the orogeny we performed a set of numerical experiments in order to validate this hypothesis a section of a lithosphere with a brittle elasto ductile rheology initially loaded by a mountain range is submitted to horizontal shortening and to surface processes if erosion is intense material is removed more rapidly than it can be supplied by crustal thickening below the range and the topography is rapidly smoothed for example a feature 3 km high and 300 400 km wide is halved in height in about 15 m y for an erosion coefficient k 10 3 m 2 yr the erosion rate is of the order of a few 0 1 mm yr this regime might be called erosional collapse if erosion is not active enough the crustal root spreads out laterally and subsurface collapse occurs in the third intermediate regime removal of the material by erosion is dynamically compensated by isostatic rebound and inward flow in the lower crust so that the range can grow in this mountain growth regime the range evolves toward a characteristic graded shape that primarily depends on the erosion law the erosion rate may be high e g 0 5 0 9 mm yr close to the rate of tectonic uplift e g 0 7 1 1 mm yr and few times higher than the rate of topographic uplift 0 15 0 2 mm yr these experiments show that surface processes can favor localized crustal shortening and participate in the development of an intracontinental mountain surface processes must therefore be taken into account in the interpretation and modeling of long term deformation of continental lithosphere conversely the mechanical response of the lithosphere must be accounted for when large scale topographic features are interpreted and modeled in terms of geomorphologic processes

crystallographic orientations of olivine inclusions in diamonds abstract in this work we report for the first time the crystallographic orientations of olivine inclusions trapped in diamonds from the kaapvaal craton south africa determined by single crystal x ray diffraction and analyze them together with all available data in the literature the overall data set indicates no preferred orientation of the olivine inclusions with respect to their diamond hosts however diamonds containing multiple olivine inclusions sometimes show clusters of olivines with the same orientation in the same diamond host we conclude that such clusters can only be interpreted as the remnants of single olivine crystals pre dating the growth of the host diamonds

early cenozoic faulting of the northern tibetan plateau margin from apatite u th he ages models to explain the distributed nature of continental deformation predict the propagation of strain and high topography away from the plate boundary yet a growing body of evidence in the tibetan orogen suggests that deformation occurred at the far northern extent of the modern plateau early in the orogen s history and thus our current mechanical understanding of orogenic plateau development is incomplete new apatite u th he ages from four elevation transects document periods of rapid exhumation related to erosion pulses in hanging wall rocks of major thrust structures accelerated erosion is used as a proxy of fault timing and is interpreted in a larger context of structural data and sediment accumulation in adjacent foreland basins helium results are synthesized with published geologic thermochronometric and sedimentologic data from which a growing picture of regional compressional deformation in middle to late eocene time in northern tibet emerges we relate the early cenozoic period of deformation to the initiation of collision between india and eurasia despite the fact that the plate boundary was located 3000 km to the south regardless of whether or not high topography was built simultaneously as a result of this deformation early cenozoic strain signifies that the modern limit of the orogen has been relatively stationary since continental collision began and that deformation has not significantly propagated farther away from the plate boundary in time

underplating in the himalaya tibet collision zone revealed by the hi climb experiment we studied the formation of the himalayan mountain range and the tibetan plateau by investigating their lithospheric structure using an 800 kilometer long densely spaced seismic array we have constructed an image of the crust and upper mantle beneath the himalayas and the southern tibetan plateau the image reveals in a continuous fashion the main himalayan thrust fault as it extends from a shallow depth under nepal to the mid crust under southern tibet indian crust can be traced to 31 n the crust mantle interface beneath tibet is anisotropic indicating shearing during its formation the dipping mantle fabric suggests that the indian mantle is subducting in a diffuse fashion along several evolving subparallel structures

outward growth of the tibetan plateau during the cenozoic a review abstract the surface uplift history of the tibetan plateau tp offers a key testing ground for evaluating models of collisional tectonics and holds important implications for processes ranging from global cooling to the onset of the asian monsoon various models have been proposed to reveal the surface uplift history of the tp but controversies remain we evaluate these models using data from sedimentology and stratigraphy structural geology magmatism exhumation and paleoaltimetry studies structural analyses indicate that thrust belts which spread from the central tp outward toward its surrounding margins accommodated most of the india asia convergence and facilitated crustal shortening and thickening in the central tp eocene adakitic rocks located in the qiangtang and the lhasa blocks likely were generated by partial melting of an eclogitic source paleogene 50 30 ma potassic rocks only occur in the qiangtang block whereas late oligocene late miocene 26 8 ma potassic rocks occur both in the qiangtang and lhasa blocks low temperature thermochronologic ages in the central tp are older than 40 35 ma whereas those in the margins are younger than 20 ma mostly late miocene and pliocene pleistocene in age independent paleoaltimetry estimates suggest that the lhasa and qiangtang terranes attained their current elevations during the eocene most likely due to the initial collision between india and lhasa whereas the hoh xil basin area to the north and himalayas to the south were still low even below sea level in the latter case we argue for an inside out growth pattern for the tibetan plateau the tp grew southward and northward from a nucleus of high topography and is likely to continue expanding along the mazar tagh fault to the northwest the kuantai shan hei shan longshou shan to the northeast the longquan shan to the east and the shillong plateau to the south if the northward force of india would not diminished

tibetan and indian lithospheres in the upper mantle beneath tibet evidence from broadband surface wave dispersion broadband seismic experiments over the last two decades have produced dense data coverage across tibet yet the mechanism of the india asia lithospheric convergence beneath it remains a puzzle with even its basic features debated and with very different end member models advocated today we measured highly accurate rayleigh and love wave phase velocity curves in broad period ranges up to 5 200 s for a few tens of pairs and groups of stations across tibet combining in each case hundreds to thousands of interstation measurements made with cross correlation and waveform inversion methods robust shear velocity profiles were then determined by extensive series of nonlinear inversions of the data designed to constrain the depth dependent ranges of isotropic average shear speeds and radial anisotropy temperature anomalies in the upper mantle were estimated from shear velocities using accurate petrophysical relationships our results reveal strong heterogeneity in the upper mantle beneath tibet very large high velocity anomalies in the upper mantle are consistent with the presence of underthrust beneath southwestern tibet and subducted beneath central and eastern tibet indian lithosphere the corresponding thermal anomalies match those estimated for subducted indian lithosphere in contrast to the indian lithosphere tibetan lithosphere and asthenosphere display low to normal shear speeds tibetan lithosphere is thus warm and thin radial anisotropy in the upper mantle is weak in central and strong in northeastern tibet possibly reflecting asthenospheric flow above the subducting indian lithospheric slab

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

constraints on the early uplift history of the tibetan plateau the surface uplift history of the tibetan plateau and himalaya is among the most interesting topics in geosciences because of its effect on regional and global climate during cenozoic time its influence on monsoon intensity and its reflection of the dynamics of continental plateaus models of plateau growth vary in time from pre india asia collision e g 100 ma ago to gradual uplift after the india asia collision e g 55 ma ago and to more recent abrupt uplift 7 ma ago and vary in space from northward stepwise growth of topography to simultaneous surface uplift across the plateau here we improve that understanding by presenting geologic and geophysical data from north central tibet including magnetostratigraphy sedimentology paleocurrent measurements and 40ar 39ar and fission track studies to show that the central plateau was elevated by 40 ma ago regions south and north of the central plateau gained elevation significantly later during eocene time the northern boundary of the protoplateau was in the region of the tanggula shan elevation gain started in pre eocene time in the lhasa and qiangtang terranes and expanded throughout the neogene toward its present southern and northern margins in the himalaya and qilian shan

a synoptic view of the distribution and connectivity of the mid crustal low velocity zone beneath tibet 1 based on 1 2 years of continuous observations of seismic ambient noise data obtained at more than 600 stations in and around tibet rayleigh wave phase velocity maps are constructed from 10 s to 60 s period a 3 d vsv model of the crust and uppermost mantle is derived from these maps the 3 d model exhibits significant apparently inter connected low shear velocity features across most of the tibetan middle crust at depths between 20 and 40 km these low velocity zones lvzs do not conform to surface faults and significantly are most prominent near the periphery of tibet the observations support the internal deformation model in which strain is dispersed in the deeper crust into broad ductile shear zones rather than being localized horizontally near the edges of rigid blocks the prominent lvzs are coincident with strong mid crustal radial anisotropy in western and central tibet and probably result at least partially from anisotropic minerals aligned by deformation which mitigates the need to invoke partial melt to explain the observations irrespective of their cause in partial melt or mineral alignment mid crustal lvzs reflect deformation and their amplification near the periphery of tibet provides new information about the mode of deformation across tibet

crystallographic orientations of magnesiochromite inclusions in diamonds what do they tell us we have studied by x ray diffractometry the crystallographic orientation relationships cors between magnesiochromite mchr inclusions and their diamond hosts in gem quality stones from the mines udachnaya siberian russia damtshaa botswana and panda canada in total 36 inclusions in 23 diamonds in nearly half of the cases n 17 111 mchr is parallel within error to 111 diamond but the angular misorientation for other crystallographic directions is generally significant this relationship can be described as a case of rotational statistical cor in which inclusion and host share a single axis 1 df the remaining mchr diamond pairs n 19 have a random cor 2 df the presence of a rotational statistical cor indicates that the inclusions have physically interacted with the diamond before their final incorporation of all possible physical processes that may have influenced mchr orientation those driven by surface interactions are not considered likely because of the presence of fluid films around the inclusions mechanical interaction between euhedral crystals in a fluid rich environment is therefore proposed as the most likely mechanism to produce the observed rotational cor in this scenario neither a rotational nor a random cor can provide information on the relative timing of growth of mchr and diamond some multiple iso oriented inclusions within single diamonds however indicate that mchr was partially dissolved during diamond growth suggesting a protogenetic origin of these inclusions