evidence for relative motions between the indian and australian plates during the last 20 m y from plate tectonic reconstructions implications for the deformation of the indo australian plate we use plate tectonic reconstructions to establish whether motions between india and australia occurred since chron 18 43 ma we test the africa antarctica australia india plate circuit closure at chrons 5 10 ma 6 21 ma and 13 36 ma using a compilation of magnetic anomalies and fracture zone traces from the southeast southwest central indian and the carlsberg ridges additional reconstructions at chrons 23 55 ma and 26 61 ma are used to estimate the overall motion between india and australia relative motions between the indian and australian plates are estimated using the plate circuit india africa australia a new statistical approach based on spherical regression analyses is used to assess the uncertainty of the best fitting finite rotations from the uncertainties in the data the uncertainty in a rotation is described by a covariance matrix directly related to the geometry of the reconstructed plate boundary to the distribution and estimated errors of the data points along it our parameterization of the rotations allows for simple combination of the rotation uncertainties along a plate circuit path results for chron 5 are remarkably consistent with present day kinematics in the indian ocean except that the arabian and indian plates are found to be separate plates comparisons of the motions between the indian and african plates across the carlsberg ridge with that between the australian and african plates across the central indian ridge evidence a significant counterclockwise rotation of the australian plate relative to the indian plate about a pole located in the central indian basin the determinations are consistent for chrons 26 13 6 and 5 determination at chron 23 is different but questionable due to the small number of available data we propose two alternative solutions that both predict convergence within the wharton and central indian basins and extension in the vicinity of the chagos laccadive ridge the first solution assumes that all the deformation in the equatorial indian ocean started 7 ma ago as found by ocean drilling program leg 116 hence all the determinations at different times represent the total motion between india and australia the averaged india australia euler vector chrons 5 6 13 and 26 11 1 s 78 0 3 54 lies within the central indian basin and yields a n s contraction of 46 52 km at 85 and 80 63 km at 90 however the difference of the india australia euler vectors at chrons 5 and 6 suggests that the india australia convergence started between 10 and 21 ma following the continent continent collision of india with asia in the early miocene the second averaged solution chrons 6 13 and 26 5 2 s 74 3 e 5 93 predicts a total n s contraction of 123 73 km at 85 e and 178 91 km at 90 e both models are compatible with the deformation pattern observed in the equatorial indian ocean

a previously unreported type of seismic source in the firn layer of the east antarctic ice sheet we identify a unique type of seismic source in the uppermost part of the east antarctic ice sheet recorded by temporary broadband seismic arrays in east antarctica these sources termed firnquakes are characterized by dispersed surface wave trains with frequencies of 1 10 hz detectable at distances up to 1000 km events show strong dispersed rayleigh wave trains and an absence of observable body wave arrivals most events also show weaker love waves initial events were discovered by standard detection schemes additional events were then detected with a correlation scanner using the initial arrivals as templates we locate sources by determining the l2 misfit for a grid of potential source locations using rayleigh wave arrival times and polarization directions we then perform a multiple filter analysis to calculate the rayleigh wave group velocity dispersion and invert the group velocity for shear velocity structure the resulting velocity structure is used as an input model to calculate synthetic seismograms inverting the dispersion curves yields ice velocity structures consistent with a low velocity firn layer 100 m thick and show that velocity structure is laterally variable the absence of observable body wave phases and the relative amplitudes of rayleigh waves and noise constrain the source depth to be less than 20 m the presence of love waves for most events suggests the source is not isotropic we propose the events are linked to the formation of small crevasses in the firn and several events correlate with shallow crevasse fields mapped in satellite imagery

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

a precambrian microcontinent in the indian ocean ridges of thick raised crust on the indian ocean floor were thought to be mostly volcanic seamounts formed above the reunion mantle plume dating of zircon minerals in mauritian lavas however indicates that fragments of an ancient microcontinent may be preserved beneath the seamounts contributing to the thickened crust

early pre 8 ma fault activity and temporal strain accumulation in the central indian ocean the diffuse deformation zone in the central indian ocean is the classical example of distributed deformation of the oceanic lithosphere where shortening between the indian and capricorn plates is manifest as reverse faulting 5 10 km spaced faults and long wavelength 100 300 km folding the onset of this deformation is commonly regarded as a key far field indicator for the start of major uplift of the himalayas and tibet some 4000 km further to the north due to increased deviatoric stresses within the wider india asia area there has been disagreement concerning the likely timing for the onset of deformation between plate motion inversions and seismic reflection based studies in the present study fault displacement data from seismic reflection profiles within the central indian ocean demonstrate that compressional activity started much earlier than previously thought at around 15 4 13 9 ma from reconstructions of fault activity histories we show that 12 of the total reverse fault population had been activated and 14 of the total strain accumulated prior to a sharp increase in the deformation rate at 8 0 7 5 ma there is no evidence for any regional unconformity before 8 0 7 5 ma early shortening was accommodated by activity on single isolated fault blocks total strain estimates derived are more variable and complex than those predicted from plate inversion and they do not show simple west to east increase

hotspots mantle plumes flood basalts and true polar wander persistent long lived stationary sites of excessive mantle melting are called hotspots hotspots leave volcanic trails on lithospheric plates passing across them the global constellation of fixed hotspots thus forms a convenient frame of reference for plate motions through the orientations and age distributions of volcanic trails left by these melting anomalies hotspots appear to be maintained by whole mantle convection in the form of upward flow through narrow plumes evidence suggests that plumes are deflected little by horizontal flow of the upper mantle mantle plumes are largely thermal features and arise from a thermal boundary layer most likely the mantle layer just above the core mantle boundary experiments and theory show that gravitational instability drives flow beginning with the formation of diapirs such a diapir will grow as it rises fed by flow through the trailing conduit and entrainment of surrounding mantle the structure thus develops a large spherical plume head and a long narrow tail on arrival at the base of the lithosphere the plume head flattens and melts by decompression producing enormous quantities of magma which erupt in a short period these are flood basalt events that have occurred on continents and in ocean basins and that signal the beginning of major hotspot tracks the plume supported hotspot reference frame is fixed in the steady state convective flow of the mantle and is independent of the core generated axial dipole paleomagnetic reference frame comparison of plate motions measured in the two frames reveals small but systematic differences that indicate whole mantle motion relative to the earth s spin axis this is termed true polar wander and has amounted to some 12 since early tertiary time the direction and magnitude of true polar wander have varied sporadically through the mesozoic probably in response to major changes in plate motions particularly subduction zone location that change the planet s moments of inertia

location of the africa australia india triple junction and motion between the australian and indian plates results from an aeromagnetic investigation of the central indian and carlsberg ridges accepted 1994 june 7 received 1994 may 23 in original form 1993 may 24 summary prior studies have proposed and examined the hypothesis that india and australia are separate rigid plates separated by a wide near equatorial e w striking plate boundary attempts to place narrow limits on the location of the africa australiaindia triple junction have been hindered however by the lack of useful magnetic profiles crossing the eastern carlsberg ridge and northern central indian ridge herein we present near ridge portions of new profiles from an aeromagnetic survey of the carlsberg ridge east of 66 e and of the central indian ridge north of 19 s these new data are used to estimate 35 new spreading rates averaged from the middle of chron 2a 3 03 ma to the present all other plate motion data along the central indian and carlsberg ridges are also analysed to investigate the present kinematics of the indian ocean especially the motion and boundary between the indian and australian plates unlike prior efforts we objectively estimate uncertainties in the strikes of transform faults along the carlsberg and central indian ridges

a tectonic model reconciling evidence for the collisions between india eurasia and intra oceanic arcs of the central eastern tethys abstract despite several decades of investigations inferences on the timing and nature of collisions along the mesozoic cenozoic eurasian margin remain controversial we assimilate geological and geophysical evidence into a plate tectonic model for the india eurasia collision that includes continuously closing topological plate polygons constructed from a time dependent network of evolving plate boundaries with synthetic plates constructed for now subducted ocean floor including back arc basins that formed on the southern eurasian margin our model is regionally constrained and self consistent incorporating geophysical data from abyssal plains offshore west australia and east antarctica including jurassic age data from offshore northwest australia limiting much of northern greater india to a 1000 km long indenter originally reaching to the wallaby zenith fracture zone southern eurasia and southeast asia are riddled with dismembered oceanic arcs indicating long lived tethyan intra oceanic subduction this intra oceanic subduction system was well established from cretaceous time in the india eurasia convergence zone in the neotethys which was consumed during greater india s northward trajectory towards eurasia from the early cretaceous fragments of obducted oceanic crust within the yarlung tsangpo suture zone between india and eurasia predominantly date to the late jurassic or mid cretaceous barremian aptian the various ophiolites along strike and a hiatus in subduction related magmatism during the tithonian aptian suggest that there was at least one generation of intra oceanic arc formation whose plate boundary configuration remains uncertain paleomagnetic and magmatic studies suggest that the intra oceanic arc was at equatorial latitudes during the early cretaceous before subduction resumed further north beneath the eurasian margin lhasa terrane with another hiatus in subduction related magmatism along southern lhasa during 80 65 ma possibly as the back arc spreading centre approached the active andean style margin in our model greater india collided with the tethyan intra oceanic arc in paleocene eocene time finally closing the tethyan seaway from mid late eocene time which is consistent with the age of the youngest marine deposits found between india and eurasia geological evidence from the collision zone indicates an age of initial arc continent collision by 52 ma followed by the soft initial continent continent collision between india and eurasia by 44 2 ma this timing is supported by marine geophysical data where the spreading centres in the indian ocean record a drastic decline in seafloor spreading rates and changes in spreading directions first at 52 ma followed by another reorganisation at 43 ma the abandonment of spreading in the wharton basin and the onset of extrusion tectonics in asia by 36 ma are likely indicators of hard complete continent collision and highlight the multi stage collisional history of this margin our continuously evolving network of mid ocean ridge and subduction zone geometries and divergence convergence vectors through time provide a basis for future refinements to assimilate new data and or test alternative tectonic scenarios

waveform tomography reveals channeled flow at the base of the oceanic asthenosphere understanding the relationship between different scales of convection that drive plate motions and hotspot volcanism still eludes geophysicists using full waveform seismic tomography we imaged a pattern of horizontally elongated bands of low shear velocity most prominent between 200 and 350 kilometers depth which extends below the well developed low velocity zone these quasi periodic fingerlike structures of wavelength 2000 kilometers align parallel to the direction of absolute plate motion for thousands of kilometers below 400 kilometers depth velocity structure is organized into fewer undulating but vertically coherent low velocity plumelike features which appear rooted in the lower mantle this suggests the presence of a dynamic interplay between plate driven flow in the low velocity zone and active influx of low rigidity material from deep mantle sources deflected horizontally beneath the moving top boundary layer

periodic deformation of oceanic crust in the central indian ocean new seismic reflection profiles of 5370 km running through the ocean drilling program leg 116 sites and deep sea drilling project sites 215 and 218 were obtained to investigate the spatial extent timing and nature of the tertiary deformation of the equatorial central indian ocean analyses of the data revealed basement and sedimentary structures and structural unconformities that resulted from the release of compressional forces long wavelength 150 300 km anticlinal basement structures with 1 2 km relief and tight folding and high angle faulting 5 20 km long of oceanic basement and overlying sediments exist between the afanasy nikitin seamount and the ninety east ridge the deformation zone extends from 10 s to the north up to 7 n latitude the changes in the regional basement trend along 87 e longitude between latitudes 11 s and 15 n coincide with the significant tectonic events of the first major plate reorganization occurred in the eastern indian ocean at 90 5 ma and changes in the indian plate motion at 65 5 ma the basement deformation at selected places a widespread unconformity of the upper miocene and subsequent younger unconformities lower pliocene and upper pleistocene indicate that the deformation activity might have begun earlier than the generally believed age of 7 5 ma and appears to be periodic the upper miocene and upper pleistocene deformational unconformities are in general observed south of 1 s while the basement deformation and lower pliocene deformational unconformity are mostly present in the area north of 1 s it is surmised that the compressional stresses built up since the hard collision of india with eurasia may have released for a short period prior to the early miocene time and deformed the oceanic basement in the northeastern indian ocean thereafter the stress regime seems to have transferred to the north of the indian shield and caused deformation of the continental lithosphere later the activity reoccurred during the late miocene 7 5 ma early pliocene 4 ma and late pleistocene 0 8 ma with a cyclicity of 3 5 m y and deformed the oceanic crust and sedimentary strata in the central indian ocean

seismic reflection images of intraplate deformation central indian ocean and their tectonic significance multichannel seismic reflection profiles collected from the intraplate deformation area in the central indian ocean basin are used to describe brittle structures produced under compressive stress reflectors within oceanic basement are divided into four types north dipping and south dipping reverse faults that are interpreted as reactivations of structures formed at the spreading centre as outward and inward dipping faults respectively lower angle north dipping reflectors that probably represent new faults or faults just initiating and sub horizontal reflectors within the uppermost crust that are interpreted as hydrothermal alteration fronts within the sedimentary cover upwards fault propagation shows the faults steepening from c 40 just above basement to near vertical and is preceded by sediment folding a fractal analysis of faulting suggests two fault populations possibly reflecting different criteria for brittle failure measurements of north south crustal shortening indicate a shortening rate of 2 5 0 9 mm a 1 which is at the lower end of predictions from plate motions but significant enough to recognize this area as a diffuse plate boundary the formation of long wavelength basement undulations and the reactivation of fracture zones and ridge parallel fault fabrics are linked in a unified tectonic model driven by the high level of intraplate compressive stress in the area there is little evidence from the seismic profiles for intraplate deformation starting before the widespread unconformity dated as 7 ma

reconciling plate kinematic and seismic estimates of lithospheric convergence in the central indian ocean the far field signature of the india asia collision and history of uplift in tibet are recorded by sediment input into the indian ocean and the strain accumulation history across the diffuse plate boundary between the indian and capricorn plates we describe the history of india capricorn convergence from updated estimates of india somalia capricorn plate rotations and observations derived from seismic reflection data new india capricorn plate rotations for the past 20 m y are consistent with slow north south convergence from 18 ma about a stationary or nearly stationary pole near the eastern edge of the chagos laccadive ridge simpler than predicted by previous models based on many fewer data the new rotations suggest that convergence began between 18 and 14 ma consistent with marine seismic evidence for an onset of deformation at 15 4 13 9 ma they further show that convergence rates doubled at 8 ma in agreement with a sharp increase in fault activity at 8 7 5 ma seen on seismic reflection profiles a discrepancy between the total strain estimated from kinematic and seismic reflection data can be reconciled if pervasive reverse faulting within the diffuse plate boundary is accompanied by block rotations of 1 3

statistical tests for closure of plate motion circuits the standard method of testing whether data describing the relative motion of three plates meeting at a triple junction are consistent with rigid plate tectonics uses the condition of plate circuit closure the euler vectors or angular velocities describing the relative motion of each plate pair must sum to zero deviations from closure suggest that the data are systematically in error or that the plate boundaries have been identified incorrectly or that the plates are non rigid we examine two methods one based on a chi square test and the other based on an f ratio test of testing whether plate motion circuits close we find the latter test is useful but the former is of limited value because of the common practice of systematically overestimating errors in plate motion data we apply the f ratio test to minster and jordan s data for the boundaries dividing the pacific cocos and nazca plates which prove consistent with plate circuit closure application of the test to their data for the boundaries dividing the australian antarctic and african plates shows nonclosure even after deletion of data from the carlsberg ridge which recently has been suggested to divide the african plate from an indo arabian plate this nonclosure suggests systematic errors in the data or internal deformation of one or more of the plates meeting at the indian ocean triple junction

indian ocean floor deformation induced by the reunion plume rather than the tibetan plateau the central indian ocean is considered the archetypal diffuse oceanic plate boundary data from seismic stratigraphy and deep sea drilling indicate that the contractional deformation of the indian ocean lithosphere commenced at 15 4 13 9 ma but experienced a sharp increase at 8 7 5 ma this has been maintained through to the present day with over 80 of the shortening accrued over the past 8 myr here we build on previous efforts to refine the form timing and magnitude of the regional plate motion changes by mitigating the noise in reconstructed indian and capricorn plate motions relative to somalia our noise mitigated reconstructions tightly constrain the significant speed up of the capricorn plate over the past 8 myr and demonstrate that the history of the indian ocean floor deformation cannot be explained without this plate motion change we propose that the capricorn plate motion change is driven by an increase in the eastward directed asthenospheric flow associated with the adjacent reunion plume and quantitatively demonstrate the viability of this hypothesis our inference is supported by volcanic age distributions along the reunion hotspot track the anomalously high residual bathymetry of the central indian ridge full waveform seismic tomography of the underlying asthenosphere and geochemical observations from the central indian ridge these findings challenge the commonly accepted link between the deformation of the indian ocean floor and the tibetan plateau s orogenic evolution and demonstrate that temporal variations in upwelling mantle flow can drive major tectonic events at the earth s surface deformation of the indian ocean floor over the past 8 million years was caused by a change in plate motions linked to flow of the reunion mantle plume according to analyses of forces upon plates

long wavelength convection poiseuille couette flow in the low viscosity asthenosphere and the strength of plate margins summary mixed heated 3 d mantle convection simulations with a low viscosity asthenosphere reveal relatively short and long wavelength regimes with different scalings in terms of surface velocity and surface heat flux and show that mantle flow in the lithosphere asthenosphere region is a poiseuille couette flow the poiseuille couette velocity magnitude ratio d u allows us to characterize solid state flow in the asthenosphere and to predict the regime transition the transition from dominantly pressure driven poiseuille flow at shorter wavelengths to dominantly shear driven couette flow at long wavelengths depends on the relative strength of lithosphere and asthenosphere and is associated with a switch in the dominant resistance to convective motion in the poiseuille regime significant resistance is provided by plate bending whereas in the couette regime most resistance is due to vertical shear in the bulk mantle the couette case corresponds to classical scaling ideas for mantle convection whereas the poiseuille case with asthenospheric velocities exceeding surface velocities is an example of a sluggish lid mode of mantle convection that has more recently been invoked for thermal history models of the earth our simulations show that both modes can exist for the same level of convective vigour i e rayleigh number but at different convective wavelengths additional simulations with temperature and yield stress dependent viscosity show consistent behaviour and suggest an association of the regime crossover with the relative strength of plate margins our simulations establish a connection between the strength of plate margins solid state flow in the asthenosphere and the wavelength of mantle convection this connection suggests that plate tectonics in the sluggish lid mode is wavelength dependent and potentially more robust than previously envisioned

constraints on indian plate motion since 20 ma from dense russian magnetic data implications for indian plate dynamics we use more than 230 000 km of russian marine magnetic and bathymetric data from the carlsberg and northern central indian ridges comprising one of the most geographically extensive dense shipboard surveys anywhere in the ocean basins to describe in detail seafloor spreading since 20 ma along the trailing edge of the indian plate india somalia plate rotations for 1 myr intervals over the past 20 myr are derived from inversions of more than 6600 crossings of 20 magnetic reversals and 1400 crossings of fracture zones that offset these two ridges statistical analysis of the numerous data indicates that outward displacement of reversal boundaries due to finite seafloor emplacement widths and correlated noise for anomaly crossings from individual spreading segments constitute two distinct sources of systematic bias in the locations of magnetic anomaly crossings contrary to the often made assumption that random gaussian distributed noise dominates the error budget seafloor spreading rates slowed gradually by 30 from 20 ma to 10 1 ma about a relatively stationary pole of rotation from 11 ma to 9 ma the rotation axis migrated several angular degrees toward the plate boundary modestly increasing the spreading gradient along the plate boundary india somalia kinematic data for times since 9 ma are consistent with remarkably steady motion with no evidence for a change in either the rotation pole or rate of angular opening within the few percent precision of our data the timing and nature of changes in india somalia motion since 20 ma closely resemble those for the capricorn somalia plate pair indicating that india and capricorn plate motions are strongly coupled we speculate that the slowdown in seafloor spreading at the trailing edges of the indo capricorn composite plate from 20 ma to 10 1 ma resulted from the increasing amount of work that was needed to build topography in the himalayan collisional zone the transition to stable india somalia and capricorn somalia seafloor spreading at 10 9 ma corresponds well with the onset at 8 ma of folding and faulting across an equatorial plate boundary separating the indian and capricorn plates suggesting that the latter may have played a fundamental role in restoring equilibrium between the torques that were driving and resisting the northward motions of the indian and capricorn plates

plate tectonics and net lithosphere rotation over the past 150 my we have developed an improved model of global digital palaeo plate boundaries and plate motion to describe the distribution and history of plates since the late jurassic from this history we computed net lithospheric rotation nr through time confirming the so called westward drift but only for the past 30 myr the nr has significantly smaller magnitudes 0 13 my past 5 my than for some other plate models it averages to 0 11 0 03 my for the past 50 my with a small but systematic increase toward the present the westward drift seen only for the past 30 my is attributed to the increased dominance of a steadily growing and accelerating pacific plate nr shows peaks with time but only an early tertiary peak of 0 33 my when the indian plate was undergoing the largest known acceleration deceleration can be interpreted with some confidence we find a linear decreasing trend in net rotation over the past 150 my but attribute this trend to increasing reconstruction uncertainties back in time as subduction consumed more than half of the oceanic crust since the jurassic after removing a linear time trend we find a nr average of about 0 12 my for the past 150 my

evidence for long term diffuse deformation of the lithosphere of the equatorial indian ocean the presence of large earthquakes east west striking folds and thrust faults in sediments and east west striking undulations of wavelength 200 km in topography and gravity shows that the equatorial indian ocean is the locus of unusual deformation1 2 3 4 5 6 7 8 this deformation has been interpreted as a diffuse boundary between two tectonic plates9 10 11 12 13 seismic stratigraphy and deep sea drilling at two locations in the bengal fan indicate that the deformation began 7 5 8 0 myr ago3 14 15 here however we show using plate reconructions that motion across this diffuse oceanic plate boundary began more than 10 myr earlier than previously inferred and that the amount of north south convergence across the boundary through the central indian basin has been significantly greater than the convergence estimated from seismic profiles the relative plate velocity accommodated across the central indian basin has varied with time and has been as fast as 6 mm yr 1 about half the separation rate of earth s slowest spreading mid ocean ridge the earliest interval of measurable motion which began more than 18 myr ago may coincide with rapid denudation of the tibetan plateau from 21 myr to 15 17 myr ref 16 the present motion across the central indian basin began no earlier than 11 myr following an earlier interval of slower motion from 18 to 11 myr and may have begun at 8 myr when the tibetan plateau is thought to have attained its maximum elevation16 17

the role of poiseuille flow in creating depth variation of asthenospheric shear summary asthenospheric flow accommodates differential shear between plate and mantle motions couette flow and hosts additional flow driven by horizontal pressure gradients poiseuille flow that may be associated with mantle upwelling and subduction large uncertainties in the upper mantle flow field and its rheological structure have thus far hindered our ability to constrain the relative importance of couette and poiseuille flows in the asthenosphere however quantifying the relative contributions of asthenospheric couette and poiseuille flows and determining the pattern of their distribution around the globe could help discriminate among competing theories of asthenospheric origin and shed light on thermal history of the earth we propose a new method to quantify asthenospheric poiseuille flow using observations of the depth dependence of azimuthal seismic anisotropy which can be obtained from frequency dependent surface wave tomography models in particular we employ a simple 1 d couette poiseuille flow model and analytically solve for depth profiles of the strain axis orientations which approximates the orientations of azimuthal seismic anisotropy we show that couette poiseuille flow induces rotation of azimuthal seismic anisotropy with depth provided that the horizontal pressure gradient has a component transverse to plate motion we then construct an algorithm that uses depth rotations of azimuthal anisotropy to invert for horizontal pressure gradients everywhere in the asthenosphere and test it on a global numerical mantle flow model a comparison of pressure gradients predicted using our method with those computed directly from the numerical model shows that our algorithm is stable and accurate unless the pressure gradient is nearly parallel to plate motion applying this method to seismic data will require additional constraints on asthenospheric geometry and viscosity structure in the numerical model we establish that poiseuille flow drives 40 per cent of the total flow velocity amplitude in the asthenosphere which indicates that pressure gradients from mantle convection may be an important component of asthenospheric dynamics that can in principle be constrained seismically

the motion and boundary between the capricorn and australian plates the motions between the somalian antarctic and australian plates the three plates believed to meet at the rodrigues triple junction in the indian ocean are inconsistent with the assumption that all three plates are rigid the discrepancy is best explained if the australian plate contains two component plates thus the traditionally defined indo australian plate consists of three component plates and multiple diffuse plate boundaries the pattern of present deformation indicates that the boundaries between the three component plates are two unconnected zones accommodating divergence and a larger zone which we interpret as three diffuse convergent plate boundaries and a diffuse triple junction

non hotspot volcano chains produced by migration of shear driven upwelling toward the east pacific rise while most oceanic volcanism is associated with the passive rise of hot mantle beneath the spreading axes of mid ocean ridges mor volcanism occurring off axis reflects intraplate upper mantle dynamics and composition yet is poorly understood off the south east pacific rise sepr volcanism along the pukapuka hotu matua and sojourn ridges has been attributed to various mechanisms but none can reconcile its spatial temporal and geochemical characteristics our three dimensional numerical models show that asthenospheric shear can excite upwelling and decompression melting at the tip of low viscosity fingers that are propelled eastward by vigorous sublithospheric flow this shear driven upwelling is able to sustain intraplate volcanism that progresses toward the mor spreads laterally close to the axis and weakly continues on the opposite plate these predictions can explain the anomalously fast eastward progression of volcanism and its spatial distribution near the sepr moreover for a heterogeneous mantle source involving a fertile component the predicted systematics of volcanism can explain the geochemical trend along pukapuka and the enriched anomaly of sepr mid oceanic ridge basalt at 16 20 5 s our study highlights the role of horizontal asthenospheric flow and mantle heterogeneity in producing linear chains of intraplate volcanism independent of a deep rooted buoyancy source

acceleration and deceleration of india asia convergence since the cretaceous roles of mantle plumes and continental collision 1 a strong 50 35 ma decrease in india asia convergence is generally ascribed to continent continent collision however a convergence rate increase of similar magnitude occurred between 65 50 ma an earlier increase occurred at 90 ma both episodes of accelerated convergence followed upon arrival of a mantle plume below and emplacement of a large igneous province lip on the indian plate we here first confirm these convergence rate trends reassessing the indo atlantic plate circuits then using two different numerical models we assess whether plume head arrival and its lateral asthenospheric flow may explain the plate velocity increases and whether decreased plume flux and increasing continent plume distance may explain deceleration even without continental collision the results show that plume head arrival can indeed lead to absolute indian plate motion accelerations on the order of several cm yr followed by decelerations on timescales similar to the reconstructed fluctuations the 90 ma increase could potentially be explained as response to the morondova mantle plume alone the 65 50 ma convergence rate increase however is larger than can be explained by plume head spreading alone we concur with previous hypotheses that plume induced weakening of the indian continental lithosphere asthenosphere coupling and an increased slab pull and ridge push efficiency are the most likely explanations for the large convergence rate increase the post 50 ma decrease is best explained by orogeny related increased trench resistivity decreased slab pull due to continental subduction and possibly restrengthening of lithosphere asthenosphere coupling upon plume demise

slow downs and speed ups of india eurasia convergence since 20 ma data noise uncertainties and dynamic implications india somalia and north america eurasia relative motions since early miocene 20 ma have been recently reconstructed at unprecedented temporal resolution o 1 myr from magnetic surveys of the carlsberg and northern mid atlantic ridges these new datasets revamped interest in the convergence of india relative to eurasia which is obtained from the india somalia nubia north america eurasia plate circuit unless finite rotations are arbitrarily smoothed through time however the reconstructed kinematics i e stage euler vectors appear to be surprisingly unusual over the past 20 myr in fact the euler pole for the india eurasia rigid motion scattered erratically over a broad region while the associated angular velocity underwent sudden increases and decreases consequently convergence across the himalayan front featured significant speed ups as well as slow downs with almost no consistent trend arguably this pattern arises from the presence of data noise which biases kinematic reconstructions particularly at high temporal resolution the rapid and important india eurasia platemotion changes reconstructed since early miocene are likely to be of apparent nature because they cannot result even from the most optimistic estimates of torques associated for instance with the descent of the indian slab into earth s mantle our previous work aimed at reducing noise in finiterotation datasets via an expanded bayesian formulation which offers several advantages over arbitrary smoothing methods here we build on this advance and revise the india eurasia kinematics since 20 ma accounting also for three alternative histories of rifting in africa we find that india eurasia kinematics are simpler and most importantly geodynamically plausible upon noise reduction convergence across the himalayan front overall decreased until 10 ma but then systematically increased albeit moderately towards the present day we test with global dynamic models of the coupled mantle lithosphere system how a previously proposed uplift of the tibetan plateau by 1 2 km as well as the break up of the indo australian plate into the india and australia units impact the kinematic record from 20 to 10 ma our modelling results indicate that the convergence slow down until 10 ma resulted chiefly from the indo australian break up however the tectonic scenario yielding the best fit to kinematic reconstructions is indeed one where the tibetan mean elevation increased by 1 2 km

palaeo altimetry of the late eocene to miocene lunpola basin central tibet the elevation history of the tibetan plateau provides direct insight into the tectonic processes associated with continent continent collisions here we present oxygen isotope based estimates of the palaeo altimetry of late eocene and younger deposits of the lunpola basin in the centre of the plateau which indicate that the surface of tibet has been at an elevation of more than 4 kilometres for at least the past 35 million years we conclude that crustal but not mantle thickening models combined with plate kinematic solutions of india asia convergence are compatible with palaeo elevation estimates across the tibetan plateau

feedback between mountain belt growth and plate convergence while it is generally assumed that global plate motions are driven by the pattern of convection in the earth s mantle the details of that link remain obscure bouyancy forces associated with subduction of cool dense lithosphere at zones of plate convergence are thought to provide significant driving force but the relative magnitudes of other driving and resisting forces are less clear as are the main factors controlling long term changes in plate motion the ability to consider past as well as present plate motions provides significant additional constraints because changes in plate motion are necessarily driven by changes in one or more driving or resisting forces which may be inferred from independent data here we present for the first time a model that explicitly links global mantle convection and lithosphere models to infer plate motion changes as far back as miocene time by accurately predicting observed convergence rates over the past 10 m y we demonstrate that surface topography generated at convergent margins is a key factor controlling the long term evolution of plate motion specifically the topographic load of large mountain belts and plateaus consumes a significant amount of the driving force available for plate tectonics by increasing frictional forces between downgoing and overriding plates

india asia collision and the cenozoic slowdown of the indian plate implications for the forces driving plate motions the plate motion of india changed dramatically between 50 and 35 ma with the rate of convergence between india and asia dropping from 15 to 4 cm yr this change is coincident with the onset of the india asia collision and with a rearrangement of plate boundaries in the indian ocean on the basis of a simple model for the forces exerted upon the edges of the plate and the tractions on the base of the plate we perform force balance calculations for the precollision and postcollision configurations we show that the observed euler poles for the indian plate are well explained in terms of their locations and magnitudes if 1 the resistive force induced by mountain building in the himalaya tibet area is 5 6 10 12 n m 2 the net force exerted upon the indian plate by subduction zones is similar in magnitude to the ridge push force 2 5 10 12 n m and 3 basal tractions exert a resisting force that is linearly proportional to the plate velocity in the hot spot reference frame the third point implies an asthenospheric viscosity of 2 5 10 19 pa s assuming a thickness of 100 150 km synthetic euler poles show that crustal thickening in the tibetan plateau was the dominant cause of the cenozoic slowdown of the indian plate

indian and african plate motions driven by the push force of the reunion plume head mantle plumes are thought to play an important part in the earth s tectonics yet it has been difficult to isolate the effect that plumes have on plate motions here we analyse the plate motions involved in two apparently disparate events the unusually rapid motion of india between 67 and 52 million years ago and a contemporaneous transitory slowing of africa s motion and show that the events are coupled with the common element being the position of the indian and african plates relative to the location of the reunion plume head the synchroneity of these events suggests that they were both driven by the force of the reunion plume head the recognition of this plume force has substantial tectonic implications the speed up and slowdown of india the possible cessation of convergence between africa and eurasia in the palaeocene epoch and the enigmatic bends of the fracture zones on the southwest indian ridge can all be attributed to the reunion plume an analysis of the plate motions involved in two apparently disparate tectonic events the unusually rapid motion of india between 67 million and 52 million years ago and a transitory slowing of africa s motion at around the same time has uncovered a possible common cause for the two the reunion volcanic hotspot beneath the indian ocean has been active for about 67 million years and the close coupling of the motion of india and africa strongly suggests that it is the waxing and waning of the reunion plume head that is driving these motions

analytic models for the dynamics of diffuse oceanic plate boundaries summary some plates deform across zones that are many hundreds to thousands of kilometres wide much broader than traditional boundaries such as mid ocean ridges deep sea trenches and oceanic transform faults across which most deformation is concentrated in a zone just a few kilometres wide these wide zones of deformation commonly referred to as diffuse plate boundaries occur in both continental and oceanic lithosphere composite plates are composed of two or more rigid or nearly rigid component plates separated by one or more diffuse plate boundaries royer gordon 1997 such complete diffuse boundaries are terminated at both ends by triple junctions although there are other diffuse boundaries that transform into narrow boundaries here we consider the dynamics of complete diffuse oceanic plate boundaries by constructing simple analytical models on a flat earth and on a spherical earth assuming that the viscous force resisting deformation is described by either a linear newtonian law or a high exponent power law we investigate the observed tendency for the pole of relative motion between component plates separated by a diffuse plate boundary to lie within the diffuse boundary itself we show that this tendency is due to geometrical effects that make it unlikely that the total torque acting between plates at a diffuse boundary could be oriented such that relative rotation occurs between the plates about a pole lying outside the boundary this is demonstrated for both flat and spherical earth cases assuming that resistance to strain along the diffuse boundary increases linearly with stress newtonian rheology we further show that the pole of rotation is even more likely to lie in the diffuse plate boundary if the viscous force resisting deformation is described by a high exponent power law rather than a newtonian law

a diffuse plate boundary model for indian ocean tectonics it is suggested that motion along the virtually aseismic owen fracture zone is negligible so that arabia and india are contained within a single indo arabian plate divided from the australian plate by a diffuse boundary the boundary is a zone of concentrated seismicity and deformation commonly characterized as intraplate the rotation vector of australia relative to indo arabia is consistent with the seismologically observed 2 cm yr of left lateral strike slip along the ninetyeast ridge north south compression in the central indian ocean and the north south extension near chagos

remotely triggered earthquakes in south central tibet following the 2004 mw 9 1 sumatra and 2005 mw 8 6 nias earthquakes