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

growth and collapse of the reunion island volcanoes this work presents the first exhaustive study of the entire surface of the reunion island volcanic system the focus is on the submarine part for which a compilation of all multibeam data collected during the last 20 years has been made different types of submarine features have been identified a coastal shelf debris avalanches and sedimentary deposits erosion canyons volcanic constructions near the coast and seamounts offshore criteria have been defined to differentiate the types of surfaces and to establish their relative chronology where possible debris avalanche deposits are by far the most extensive and voluminous formations in the submarine domain they have built four huge submarine bulges to the east north west and south of the island they form fans 20 30 km wide at the coastline and 100 150 km wide at their ends 70 80 km offshore they were built gradually by the superimposition and or juxtaposition of products moved during landslide episodes involving up to several hundred cubic kilometers of material about 50 individual events deposits can be recognized at the surface the landslides have recurrently dismantled piton des neiges les alizes and piton de la fournaise volcanoes since 2 ma about one third are interpreted as secondary landslides affecting previously emplaced debris avalanche deposits on land landslide deposits are observed in the extensively eroded central area of piton des neiges and in its coastal areas analysis of the present day topography and of geology allows us to identify presumed faults and scars of previous large landslides the submarine bulges are dissected and bound by canyons up to 200 m deep and 40 km long filled with coarse grained sediments and generally connected to streams onshore a large zone of sedimentary accumulation exists to the north east of the island it covers a zone 20 km in width extending up to 15 km offshore volcanic constructions are observed near the coast on both piton des neiges and piton de la fournaise volcanoes and are continuations of subaerial structures individual seamounts are present on the submarine flanks and the surrounding ocean floor a few seem to be young volcanoes but the majority are probably old eroded seamounts this study suggests a larger scale and frequency of mass wasting events on reunion island compared to similar islands the virtual absence of downward flexure of the lithosphere beneath the island probably contributes to this feature the increased number of known flank failure events has to be taken into consideration when assessing hazards from future landslides in particular the probability of landslide generated tsunamis

thermal model of oceanic transform faults a thennal model of the oceanic transform fault which includes effects of the lateral heat conduction across the fault and the shear heating along the fault has been developed the results show that shear heating along the transform fault significantly raises the temperature along the fault the depths to isotherms of the thermal model were compared to the depths of earthquakes along transform faults by including the shear heating the limiting isotherm of the focal depths of atlantic transform earthquakes is 700 c 600 c if not including the shear heating which is close to the limiting temperature of the intraplate oceanic earthquakes if the transform earthquakes are limited by the 750 c isotherm then the transform earthquakes should be shallow events as predicted by our thermal model the model predicts the maximum depth of the 750 c isotherm increases with the length of the transform fault and decreases with the half slip rate both the effects of the lateral heat conduction across the transform fault and the shear heating along the fault generate topographic and geoid anomalies in a 30 to 60 km wide zone at the fault with magnitudes of the order of a few hundred meters and tens of centimeters respectively given the thermal structure of the transform fault the shear traction along the fault can be estimated the average shear traction per unit length of the transform fault is of the order of 1012 n m which is comparable to the magnitude of the major plate driving forces

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

magmatism and metamorphism in the lhasa block tibet a geochronological study u pb systematics in small fractions of zircon monazite and sphene 5 to 20 grains each have been analyzed in order to decipher the age and origin of magmatism in the lhasa block and to date the tectono metamorphic events additionally we analyzed a few minerals for rb sr the results show that the granites and an ignimbrite were formed essentially through anatexis of continental crust the magma source regions of which consisted of palaeozoic to precambrian materials up to 2 b y in age the following episodes can be distinguished 1 an early tertiary plutonism volcanism in the southern lhasa block yangbajing region strongly related to intracontinental tectono metamorphism 2 a late jurassic to early cretaceous plutonism in the central and northern lhasa block and 3 a middle jurassic low grade metamorphism affecting the basement at the northern margin 171 6 m y ago the early tertiary magmatism and metamorphism in the south are probably a direct result of tethys subduction indian plate

constraints on the lithospheric structure of venus from mechanical models and tectonic surface features radar images of the surface of venus show numerous structures that are interpreted as having formed due to horizontal compression and extension of the lithosphere many of these features exhibit characteristic scales widths or spacings of deformation one of 10 20 km and another of 100 300 km for a range of simple models we test the hypothesis that these length scales are controlled by dominant wavelengths arising from unstable compression or extension of the venus lithosphere results show that the existence of tectonic features that exhibit both length scales can be explained if at the time of deformation the lithosphere consisted of a crust that was relatively strong near the surface and weak at its base and an upper mantle that was stronger than or of nearly comparable strength to the upper crust the spacings of these features imply crustal thicknesses in the approximate range 5 30 km and a thermal gradient not greater than 25 k km 1 features that exhibit only the smaller scale of deformation can be explained by either a lithosphere with a thick crust that overlies a weak mantle or a lithosphere with a strong mantle but with small internal strength contrasts for a broad range of parameters the models require not only that smaller scale compressional and extensional features have similar spacings but also that the widths and spacings of larger scale compressional features be greater than those formed in extension this is consistent with observed differences in the characteristic length scales of larger scale surface features as well as the similarities in spacings of observed smaller scale features

lithospheric flexure at fracture zones bathymetric profiles across six major fracture zones fz s in the north pacific are used to demonstrate the absence of vertical slip on the fossil fault planes the scarp heights on these fz s are constant with age and equal to the initial vertical offsets at the ridge transform fault fz intersections because of the frozen in scarp and the differential subsidence of lithosphere far from the fz the lithosphere bends in the vicinity of the fz this flexure results in a characteristic ridge trough topographic fz signature the flexural amplitude which is the difference between the scarp height and the overall change in depth across the fz increases with age good fits to the bathymetric profiles across the mendocino and pioneer fz s are obtained by modelling the topography as the flexure of a thin elastic plate with an age dependent effective elastic thickness results of the modelling indicate that the base of the elastic lithosphere is approximately defined by the 450 c isotherm maximum bending stresses at fz s are on the order of 100 mpa substantially less than the stresses encountered at subduction zones because the mendocino and pioneer fz s are separated by less than a flexural wavelength they are elastically coupled

depth to curie temperature or bottom of the magnetic sources in the volcanic zone of la reunion hot spot we present an innovative study to generalize curie point depth cpd determinations at the scale of oceanic volcanic islands an approach which has previously focused largely on continental areas in order to determine the validity of this technique in oceanic environments we first tested the approach on sets of sea floor spreading anomalies assuming that magnetic anomalies are concentrated within the oceanic crust and uppermost mantle the curie depth should deepen as oceanic lithosphere increases in age and thickness away from spreading centers the calculated depths to the magnetic bottom are in agreement with this general pattern on the basis of this test we then applied the method to la reunion island and surrounding oceanic lithosphere the calculated extent of magnetic sources lies at depths between 10 and 30 km and exhibits a complex topography presumably caused by a combination of various magmatic and tectonic lithospheric structures these calculations indicate that magnetic sources extend well below the crust mantle interface at this location to the first order the bottom of the magnetic surface shallows beneath reunion and mauritius islands due to the thermal effect of the hot spot and deepens away from la reunion edifice on the scale of the mascarene basin several discontinuities in the cpd correlate well with major fracture zones

heat flow over reunion hot spot track additional evidence for thermal rejuvenation of oceanic lithosphere we present 99 new heat flow determinations obtained with a regular sampling 2 3 km along two seismic profiles across the mascarene ridge indian ocean this aseismic ridge is volcanic relief due to the activity of a hot spot now responsible of the active volcanism of reunion island our sampling technique allows us to identify short wavelength processes such as slope driven water flows that affect heat flow on the ridge with the exception of these perturbed sites heat flow appears to be very uniform across the ridge and significantly higher 6 8 mw m 2 than the reference heat flow predicted for the seafloor age although this anomaly is low it confirms other evidence for thermal rejuvenation of the lithosphere based on gravity and bathymetry the set of information now available for the mascarene ridge seems to indicate a mixed mode of compensation with dynamical support below the front of the swell track and thermal support behind for the older part of the swell

a note on the correction of ocean floor depths for sediment loading the measured physical properties of sediments recovered in a number of deep sea drill holes in the north atlantic have been used to derive expressions for the correction to basement depths required to account for loading by sediments the average density and porosity over a given thickness of sediment show an approximate linear dependence on sediment thickness this behavior can be explained as primarily the result of compaction if the compaction of the sediments is neglected in correcting basement depths for sediment loading errors of a few hundred meters in the corrected depths can be made a sediment loading correction based on the observed variation of density with depth can give corrected depths to better than 100 m with remaining uncertainties due to variablity in sediment properties and rebound effects on the measured densities a relation between sediment thickness and vertical travel time was obtained from sonic velocity measurements on samples from drill holes sonobuoy wide angle reflection experiments and direct correlation of seismic reflectors with downhole lithology the sediment loading correction can then be expressed directly in terms of the vertical travel time measured along seismic reflection profiles the loading correction obtained here for the north atlantic may not be applicablemore in areas with different sedimentary enviroments when direct measurements of physical properties in drill holes are not available in such areas it is shown that it is preferable to assume a compaction curve for the appropriate lithology rather than to use observed seismic velocities together with a velocity density relationship less

are oceanic fracture zones locked and strong or weak new evidence for volcanic activity and weakness traditionally oceanic fracture zones and other lithospheric and crustal faults have been viewed as zones of weakness that are vulnerable to seismic and volcanic activity this notion has recently been questioned on the basis of evidence showing that initial depth offsets scarp height across some pacific fracture zones are preserved even after 100 m y of differential subsidence this evidence suggests that some parts of oceanic fracture zones are locked shortly after they drift away from ridge transform intersections because the fracture zones seem to be strong welds differential cooling stresses cause flexure in the lithosphere on both sides of the fracture in this paper we briefly review the seemingly contradictory evidence for the thermomechanical characteristics of oceanic fracture zones locked and strong vs slipped and weak and present new evidence indicating that parts of some fracture zones are volcanically active volcanism is an indication of relative weakness because strain and failure are required to open volcanic conduits this and other evidence for the inherent weakness of fracture zones can be reconciled with evidence for strength if different segments of fracture zones exhibit contrasting thermomechanical behavior more data are needed to determine whether this is so and whether there are systematic patterns of distribution for weak and strong parts of individual fracture zones

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

contrasting modes of rifting the benue trough and cameroon volcanic line west africa the benue trough of west africa is commonly believed to be a rift feature that originated in the cretaceous at about the time that africa and south america began to separate bouguer gravity and available geological data in the trough indicate that its formation was probably the result of regional horizontal stresses in the lithosphere causing crustal extension and surface subsidence by contrast the data for the adjoining cameroon volcanic line suggests that the associated tensional stresses relate to mantle upwarp causing thinning of the lithosphere and regional crustal uplift similar to that associated with the kenya rift thus the association of passive and active rifts seen in the afro arabia rift system is also a feature of the cretaceous rift system in west africa

influence of late cenozoic mountain building on ocean geochemical cycles in a steady state ocean input fluxes of dissolved salts to the sea must be balanced in mass and isotopic value by output fluxes for the elements strontium calcium and carbon rivers provide the primary input whereas marine biogenic sedimentation dominates removal dissolved fluxes in rivers are related to rates of continental weathering which in turn are strongly dependent on rates of uplift the largest dissolved fluxes today arise in the himalayan and andean mountain ranges and the tibetan plateau during the past 5 m y uplift rates in these areas have increased significantly this suggests that weathering rates and river fluxes may have increased also the oceanic records of carbonate sedimentation level of the calcite compensation depth and delta sup 13 c and delta sup 87 sr in biogenic sediments are consistent with a global increase in river fluxes since the late miocene the cooling of global climate over the past few million years may be linked to a decrease in atmospheric co sub 2 driven by enhanced continental weathering in these tectonically active regions

regional variations in subsidence rate of oceanic plates a global analysis a global analysis of the dependence of seafloor depth on crustal age has been performed for each large oceanic plate independently each plate has been divided into regions bounded by major tectonic features a total of 32 regions have been considered bathymetry corrected for sediment loading has been plotted as a function of crustal age for each region except for three regions western pacific between 10 s and 40 n north american plate between 24 n and 38 n and african plate between 10 n and 24 n where ocean floor older than 80 100 ma flattens with age in other regions depth increases linearly with age1 2 subsidence rate over each region has been computed by linear regression large variations up to a factor 2 are reported from one region to another asymmetrical subsidence is also observed in a number of areas however the largest asymmetries are less than the regional variations in subsidence rate occurring on individual plates regional variations in subsidence rate appear linearly related to regional variations in ridge crest topography shallow ridge subsiding quickly and deep ridge subsiding slowly hence seafloor depth subsides at a rate modulated by the value of the initial axial depth this observation suggests that regional variations in subsidence rate reflect the thermal regime of the mantle beneath mid ocean ridges rather than lateral temperature variations inside the asthenosphere involving small scale convection

continental lithosphere folding in central asia part ii constraints from gravity and topography abstract periodical sub parallel fold like structures observed in the western gobi have two characteristic dominant wavelengths of approximately 50 and 300 360 km we explain these observations in terms of independent quasi viscous folding of the crustal and upper mantle parts of the lithosphere due to horizontal transpressional stresses in central asia induced by the collision with the indian plate we derive a model for the mechanical response of the continental lithosphere to horizontal stresses for a rheologically layered plate with non newtonian power law rheology overlying a low viscosity asthenosphere the differentiation in the effective viscosity and thickness of the strong upper crust and upper mantle lithosphere along with the presence of a low viscosity lower crust between them leads to their partial decoupling during compressional deformation these features result in the appearance of different wavelengths of the folds in the western gobi region we derive simple semi analytical estimates for the dominant wavelengths and rates of growth of surface undulations constrained by data from experimental rock mechanics topography and gravity

digital isochrons of the world s ocean floor we have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self consistent set of global isochrons and associated plate reconstruction poles the age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models we have constructed an age grid with error estimates for each grid cell as a function of 1 the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid 2 the distance of a given grid cell to the nearest magnetic anomaly identification and 3 the gradient of the age grid i e larger errors are associated with high age gradients at fracture zones or other age discontinuities future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere the heat loss of the earth ridge push forces through time asymmetry of spreading and providing constraints for seismic tomography and mantle convection models

how flat is tibet high resolution digital topography three arc second grid for most of tibet provides new information to characterize the relief of the highest and largest plateau on earth the arid to semiarid central and northern part of the plateau interior has low relief average slopes of over 250 m windows and a mean elevation of 5023 m above sea level at moderate wavelengths of m relief is or less for most of tibet as opposed to the much higher relief of up to 6 km on the plateau edges where glacial and fluvial dissection is greater because of higher levels of precipitation the only faults manifesting significant topographic relief are the relatively small scale generally north trending graben systems primarily in southern tibet and several large scale fault systems near the edges of tibet the flatness of tibet implies that 1 there has been little deformation especially shortening of the uppermost crust north of the graben systems during the late cenozoic and 2 shallow crustal isostatic compensation has been acting to level the surface of the plateau

on the anatomy of magma chamber and caldera collapse the example of trachy phonolitic explosive eruptions of the roman province central italy abstract textural and compositional features of pyroclastic products erupted during caldera forming events often reveal the tapping of different portions of variably zoned magma chambers due to changing geometries of the conduit vent systems here we report on ultrapotassic trachytic phonolitic explosive eruptions of the roman province central italy which show remarkable changes of textural features and glass compositions in the juvenile material even if the bulk chemical composition is essentially constant in each example the lower eruption sequence contains whitish crystal poor leucite free highly vesicular pumice emplaced by early plinian fallout and or pyroclastic currents upsection the eruption sequence contains black low porphyritic sanidine leucite bearing moderately vesicular scoria or spatter emplaced by major pyroclastic flows red tuff with black scoria and associated co ignimbrite coarse lithic rich breccias this suggests a shift from a central feeder conduit tapping the central part of the magma chamber to a ring fracture vent system tapping the peripheral portions of the magma chamber during caldera collapse key features of these evacuating magma chambers are the thermal and volatile concentration xvol gradients that produce the observed textural and compositional spectrum of trachy phonolitic rock types in particular the degrees of freedom during the crystallization of these ultrapotassic magmas are increased by the variation of the leucite stability field at different ph2o conditions both leucite free and leucite bearing differentiated ultrapotassic rock types can be produced in the course of individual eruptions as a result of pre eruptive conditions in the feeder magma with no need to invoke different differentiation suites related to mantle source heterogeneities of parental magmas

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

fossil hot spot ridge interaction in the musicians seamount province geophysical investigations of hot spot volcanism at volcanic elongated ridges the musicians seamount province is a group of volcanic elongated ridges vers and single seamounts located north of the hawaiian chain a 327 trending seamount chain defines the western part of the province and has been interpreted as the expression of a cretaceous hot spot beneath the northward moving pacific plate to the east elongated e w striking ridges dominate the morphology in 1999 wide angle seismic data were collected across two 400 km long vers we present tomographic images of the volcanic edifices which indicate that crustal thickening occurs in oceanic layer 2 rather than in layer 3 this extrusive style of volcanism appears to strongly contrast with the formation processes of aseismic ridges where crustal thickening is mostly accommodated by intrusive underplating high resolution bathymetry was also collected which yields a detailed image of the morphology of the vers from the occurrence of flat top guyots and from the unique geomorphologic setting two independent age constraints for the pacific crust during the cretaceous quiet zone are obtained allowing a tectonic reconstruction for the formation of the musicians vers hot spot ridge interaction leads to asthenosphere channeling from the plume to the nearby spreading center over a maximum distance of 400 km the musicians vers were formed by mainly extrusive volcanism on top of this melt generating channel the proposed formation model may be applicable to a number of observed volcanic ridges in the pacific including the tuamotu isles the eastern portion of the foundation chain and the western termination of the salas y gomez seamount chain

uplift of tibetan plateau a history of the elevation and crustal thickness of the tibetan plateau since the continental collision at 45 ma has been computed using a kinematic model based on plate tectonic reconstructions and conservation of crustal volume in our reconstructions the indochina block was attached to the southern margin of eurasia before being extruded to the southeast as evidenced by the opening of the south china sea this opening affected the mass input into tibet so that tibet did not uplift to an elevation higher than 1 km until early miocene when spreading of the south china sea ceased the model then gives a steady rise of tibet from middle miocene to its present elevation

tectonic reconstruction of the clipperton and siqueiros fracture zones evidence and consequences of plate motion change for the last 3 myr bathymetry side looking sonar and magnetics data from the northern east pacific rise have been analyzed to determine the tectonic history of the clipperton fracture zone cfz and the siqueiros fracture zone sfz over the last 2 3 m y results of tectonic reconstructions indicate a series of counterclockwise changes in spreading direction at 2 5 ma 1 2 1 5 ma 1 2 and 0 5 ma 4 5 along the right stepping clipperton transform the most recent change in spreading direction has resulted in fracture zone normal compression which we propose has created a median ridge and transform parallel troughs along the active transform fault volcanic intersection highs located near the ridge transform intersections rti of the cfz are interpreted to be the result of fracture zone normal extension that has created pathways for magma emplacement into onto the older lithosphere across from the rti along the left stepping sfz we propose the changes in spreading direction have generated extension across the transform and have resulted in the formation of intratransform spreading centers and flexural transverse ridges tectonic reconstructions indicate a single euler pole is unable to describe the motion of the pacific cocos plate boundary between the cfz and sfz during periods of spreading direction change since 3 ma transtensional transforms will adjust to a new spreading direction almost immediately while transpressional transforms will experience compression for a period of time comparable to half the age offset of the compressional transform

variations of elastic plate thickness at continental thrust belts after examining 15 reliable estimates of the elastic thickness of the lithosphere where it underthrusts mountain belts in europe asia and the americas we conclude that the strength of continental lithosphere as a function of depth must be very different from that of oceanic lithosphere depending on the state of stress continental lithosphere may be either very much stronger or very much weaker than would be predicted for oceanic lithosphere of the same thermal age we explain this observation as the result of two competing effects a thicker thermal plate beneath the continents which allows a deeper elastic ductile transition for plates more than 100 million years old and a low activation energy for ductile creep in the continental crust which leads to massive failure of the upper elastic lithosphere at high bending stresses the 100 km values for elastic plate thickness of billion year old lithosphere require a thermal plate thickness of at least 250 km the magnitude of apparent thinning of the elastic plate at high stress is consistent with that calculated from a simple yield envelope which includes a ductile zone in the lower crust the low yield strength of continental materials leads to correlations between effective elastic plate thickness and all of the following dip of the underthrust plate radius of curvature of the thrust front in map view and total length of the mountain belt using insight derived from laboratory experiments on the buckling of spherical shells and drawing analogies with the geophysical characteristics of oceanic subduction zones we arrive at the following sequence of causes and effects the primary variable is the dip of the downgoing plate presumably prescribed by its own excess mass and pressure exerted by dynamic flow in the mantle where this dip is steep the rigidity of the plate is weakened due to inelastic yielding the weakened plate can then buckle at small wavelengths into short arcuate mountain belts thus we believe that the mantle ultimately controls the pattern of surface tectonics but that the nonelastic rheology of the lithosphere is the mechanism by which that control is imposed

numerical modeling of intraplate deformation simple mechanical models of continental collision we study a model of continental collision in which one of the continents acts as a rigid die indenting the other plate which flows as an incompressible viscoplastic medium we consider two extreme cases of plane deformation 1 plane strain which corresponds to an infinitely thick lithospheric plate and 2 plane stress corresponding to a very thin plate deformation of the lithosphere a thick plate should be intermediate between those extremes we found that the flow in the plane strain case is quite similar to that obtained by slip line theory the plane stress results are quite different since in this case most of the plate shortening is taken up by the thickening of the lithosphere we also explored the role of boundary conditions on the flow in particular the role of the side walls containing the flow of the lithosphere in the case of a free lateral boundary the main feature is a flow of matter toward this free wall and a s like pattern for the horizontal stress field for a rigid wall on the other hand the plane strain and the plane stress results are quite different in the first case there is a large return flow on the sides of the punch the material being extruded along the only free surface available in the plane stress case the return flow disappears and the material displaced by the penetration of the die tends to thicken the plate the role of a nonlinear constitutive relation is studied for power law creep as the power of the flow limit increases the flow retains its general features but the deformation localizes creating sharper contrasts between high and low strain rate areas in plane stress the effect of nonlinearity is to increase the contrasts in vertical motion available data for asia are discussed in the light of the new results

comparison of the shapes and sizes of seafloor volcanoes on earth and pancake domes on venus the fact that flat topped pancake domes observed on venus resemble flat topped volcanoes on earth s seafloor suggests that volcanic processes on the seafloor and on venus may operate to construct volcanic forms with similar characteristics to test this the shapes and size distributions of terrestrial submarine volcanoes seamounts from several magmatic and tectonic provinces are quantitatively compared with those of venusian pancake domes seamount heights and diameters are highly correlated suggesting that there may be a universal mechanism for their growth the 2014 point sample mean of the height to diameter ratio is d 0 10 0 04 in contrast heights and diameters of 52 pancake domes exhibit no obvious trend and values of d are as much as three orders of magnitude smaller with a 52 point sample mean of d 0 02 0 03 seamounts with basal diameters db 10 km have flatness values ratio of top to bottom diameter that span the entire range between 0 pointed cone and 1 truncated cone larger sized seamounts tend to have small values of flatness and more pointed tops this is not true for pancake domes which show large flatnesses at all sizes additionally seamounts have distinctive profiles with their flank slopes increasing in gradient toward the summit flank slopes of pancake domes however commonly increase in gradient toward the base with respect to sizes the diameter distributions of the different seamount populations are consistent with a model in which the number of seamounts decreases exponentially with increasing diameter the diameter distribution of 126 pancake domes by contrast is more uniform its mean is db 22 13 km this size is at least an order of magnitude larger than the estimated characteristic diameters of the seamount populations based on the above results i conclude that seamounts have significantly different shapes and size distributions than venusian pancake domes that have been identified and measured to date this suggests that one or more of the variables e g magma composition crystallinity magma volume effusion rate cooling rate pre existing topography governing their formation differs in terms of shapes and size distributions the small db 20 km volcanic edifices first identified from venera data are good morphologic matches to seamounts and should be the focus of future work investigating the formation of comparable volcanic features in these two different planetary environments

evidence from gravity anomalies for interactions of the marion and bouvet hotspots with the southwest indian ridge effects of transform offsets abstract the ultra slow spreading southwest indian ridge swir presents a unique environment to study the interactions between hotspots and ridges with highly segmented geometry using recently available satellite free air gravity and shipboard bathymetry data we obtain mantle bouguer mba and residual mantle bouguer anomalies rmba by removing from free air gravity the attractions of seafloor topography sediment thickness variations a reference crust and theoretically predicted effects of lithospheric cooling the bouvet hotspot previously observed to be associated with anomalous bathymetry and geochemistry near the bouvet triple junction has an mba axial gravity low of 100 mgal implying pronounced localized crustal thickening off axis the rmba lows along previously calculated bouvet hotspot tracks are variable in amplitude suggesting the possibilities that bouvet flux varies in time or that hotspot magmatism is enhanced by proximity to a spreading center along axis geophysical anomalies suggest that the marion hotspot has a significant effect on accretionary processes in the central portion of the swir importantly the marion axial anomaly appears to be compartmentalized between the andrew bain and discovery ii fracture zones implying that transform offsets play a significant role in governing the distribution of plume material in a highly segmented ultra slow spreading system

apparent polar wander paths for the major continents 200 ma to the present day a palaeomagnetic reference frame for global plate tectonic reconstructions summary synthetic apparent polar wander apw paths for north america south america eurasia india central africa australia and antarctica for the last 200 myr are proposed computation of these apw paths is based upon the latest version 4 5a of the global paleomagnetic database gpmdb a revised global plate tectonic model since the early jurassic and a new technique for generating smoothed apw paths the smoothing technique includes the following steps 1 pre selection of palaeopoles including pre filtering parameters number of sites number of samples per site 95 per cent confidence circle about mean direction cleaning procedure and time uncertainty 2 generation of palaeolatitude and declination plots for a reference site on each continent that combines palaeopoles via a global plate tectonic circuit 3 independent spline regression analyses of the palaeolatitude and declination plots 4 removal of palaeolatitude or declination data that deviate by more than 10 from the regression curves post filtering process 5 generation of synthetic apw paths from the resulting palaeolatitude and declination plots these synthetic apw paths are then rotated into african coordinates to determine the best fit apw path and a global palaeomagnetic reference frame four representative plate tectonic reconstructions and global plate velocity fields are presented for the three time intervals that correspond to globally synchronous changes in plate motion

archeomagnetism of piton de la fournaise bearing on volcanic activity at la reunion island and geomagnetic secular variation in southern indian ocean historical and other recent lavas from piton de la fournaise la reunion island are studied using the large sample archeomagnetic method which provides here paleodirections of the earth magnetic field with confidence cones between 0 9 and 2 5 thus offering a precise record of the geomagnetic field and potential dating constraints ages of the lavas are known thanks to historical chronicles from 1708 or 14c dating and their analysis give information about the directional secular variation sv in a region where observatory measurements are scarce and not available before the end of the 19th century for the past 250 yr we find a high value of inclination 50 to 55 with respect to the geographic latitude 21 s connected with a very restricted directional sv conversely the older volcanic units present a larger sv with magnetic inclinations of between 31 and 46 and declinations from 16 w to 10 e these results which are in reasonable agreement with instrumental measurements made on ships in the vicinity of la reunion allow us to infer that undated lavas of the northern part of the caldera emanated from eruptions during the second half of the 1700s other volcanic products e g mare longue flow piton chisny cone and flows and pointe langevin despite their fresh morphology are necessarily older than about 1500 ad further knowledge of the path of the sv in the more distant past and therefore further archeomagnetic dating is hampered by the lack of a precise chronology through radiometric or other similar methods available models of the geomagnetic field during the last millennium which suffer from the scarcity of data in the southern hemisphere are discussed in the light of our results

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

a new model for the evolution of the volcanic island of reunion indian ocean the island of reunion has been studied using data from airborne and shipborne magnetic surveys the subaerial history of reunion spans the last 2 1 million years the brunhes matuyama geomagnetic reversal enables differentiation of volcanic rocks older and younger than 0 78 ma the lower submarine flanks are poorly magnetized and are interpreted as landslide deposits the core of the island is composed of highly magnetized rocks piton des neiges volcano is composed mostly of rocks older than 0 78 ma only its western flank and central area include thick piles of younger rocks piton de la fournaise is a highly and normally magnetized edifice but its northern and eastern flanks are underlain at shallow depth by reversely magnetized formations the latter are regarded as remnants of les alizes volcano associated with the grand brule hypovolcanic complex at the matuyama brunhes transition the island was composed of at least two main volcanoes piton des neiges and les alizes and perhaps also of a third old volcano takamaka at the center north of the island piton de la fournaise grows on the flank of les alizes and is a relatively young focus of volcanism these volcanoes have had successive phases of construction and destruction the analysis of magnetic anomalies over reunion was decisive in defining a new coherent model for the evolution of the island

reunion hotspot activitity through tertiary time initial results from the ocean drilling program leg 115 abstract leg 115 of the ocean drilling program recovered basaltic rocks from four sites along the ancient trail of the reunion hotspot the age of volcanism determined from biostratigraphic data at the four sites increases to the north and records the motion of india away from the reunion hotspot through tertiary time hotspot activity began with the eruption of the deccan flood basalt flows at the time of the cretaceous tertiary boundary the reunion hotspot has been stationary with respect to other hotspots in the atlantic and indian ocean basins through tertiary time the geochemical compositions of the drilled basalts differ according to the relative contributions of magmas from hotspot and morb mantle sources

shortening of analogue models of the continental lithosphere new hypothesis for the formation of the tibetan plateau initial stages of compression produce periodic buckling of models analogous to a four layer continental lithosphere with further shortening amplification of the buckles occurs by thrust faulting at inflection points in both brittle layers of the upper crust and upper mantle the rescaled results suggest that a continental lithosphere under compression will buckle with a first order wavelength of about 200 km and a second order wavelength of 20 30 km continued shortening results in the amplification of undulations troughs sink and become compressional basins while deformation keeps going on major thrusts within the upper mantle thus giving rise to an irregular topography of the moho fifty percent shortening results in a 60 km thick continental crust we argue that this model history may explain the elevation and deep crustal structure of the tibetan plateau

la reunion an example of channeled hot spot plume la reunion a large intraplate volcanic system in the mascarene basin indian ocean is widely considered as the expression of a mantle plume or hotspot it lies in a region of the mascarene basin bounded by 2 major transform faults and an extinct rift the bathymetry suggests the presence of a positive topographic anomaly around la reunion we have compared the bathymetry with a theoretical bathymetry based on the thermal subsidence of the oceanic lithosphere as a function of its age a bathymetric anomaly of several hundreds of meters is shown but it is confined to the la reunion zone analogue models have been carried out to explore the deformation resulting from a mantle plume rising beneath a faulted lithosphere they show that the plume and the associated deformation are channeled into the area enclosed by the transform faults and the paleo rift the deformation is virtually nil outside the zone fault bounded zone we have analyzed the surface features and the available gravity and magnetic data in order to find evidence of the deformation predicted by the presence of the bathymetric anomaly and by the analogue models no clear pattern of deformation has been recognized but we demonstrate the complex organization of the oceanic crust in the la reunion zone the fact that the plume head focuses on the restricted area of the zone may amplify the dynamic support of the lithosphere beneath la reunion where not flexure is observed we also suspect that some seamounts near la reunion could correspond to a recent magmatic activity distinct from that of the island the mantle plume lithosphere interaction model presented here is a new to the oceanic domain and it will have to be confirmed by subsequent observations in the la reunion area and possibly in other similar sites on earth

reconstruction of madagascar and africa evidence from the davie fracture zone and western somali basin new seismic reflection gravity and magnetic data from offshore east africa allow the davie fracture zone to be traced from 11 s to its intersection with the kenyan coast at 2 s constraining the relative motion of madagascar and africa seasat derived free air gravity anomalies and slope rise positive magnetic anomalies observed in shipboard data help to locate the continent ocean boundaries cob off the shore of east africa and madagascar seismic reflection data further document a diapirprovince off madagascar presumably conjugate to that observed off kenya and somalia the dhow and very large crude carrier vlcc basement ridges are complex features and do not appear to be simple fracture zones owing their existence entirely to the separation of madagascar and africa from these data we determine a predrift fit of madagascar and africa involving a 14 2 rotation of madagascar to africa about a pole at 10 n 150 e the geometry of the reconstruction adheres to seismic and potential field data indicating the oceanic nature and extent of the comoros basin and of the somali basin between kenya and the seychelles and it does not conflict with onshore or offshore stratigraphy timing of the opening of the western somali basin is constrained by mesozoic marine magnetic anomalies and extrapolation to the interpreted cob and occurred between approximately 165 and 130 ma

age spreading rates and spreading asymmetry of the world s ocean crust we present four companion digital models of the age age uncertainty spreading rates and spreading asymmetries of the world s ocean basins as geographic and mercator grids with 2 arc min resolution the grids include data from all the major ocean basins as well as detailed reconstructions of back arc basins the age spreading rate and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments the age uncertainties for grid cells coinciding with marine magnetic anomaly identifications observed or rotated to their conjugate ridge flanks are based on the difference between gridded age and observed age the uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges resulting in ridge jumps toward hot spots we also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age depth relationships the new set of grids helps to investigate prominent negative depth anomalies which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow a combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems

age progressive volcanism in the comores archipelago western indian ocean and implications for somali plate tectonics abstract the comores islands together with the tertiary volcanic province of northern madagascar form a sublinear trend of alkali olivine basalt shield volcanoes across the northern entrance of the mozambique channel potassium argon dating of shield building lavas confirms an eastward increase in age of volcanism along the chain consistent with a hotspot origin for the lineament the velocity of the somali plate over the mantle magma source is 50 mm yr we use the distribution of ages along the comores madagascar chain in conjunction with existing age data for the reunion mascarene plateau hotspot track to model the absolute motion of the somali plate for the last 10 m y we calculate the relative motion across the east african rift by subtracting the somali plate absolute motion from african plate absolute motion during this period the model predicts 320 km of total separation across the east african rift during the past 10 m y which is greater than has been estimated from surface geological evidence the geometry of older portions of the comores and reunion trends indicates that there was no significant relative motion between the african and somali plates prior to about 10 m y ago

constraints on lithospheric thermal structure for the indian ocean from depth and heat flow data models for the thermal evolution of oceanic lithosphere are primarily constrained by variations in seafloor depth and heat flow with age these models have been largely based on data from the pacific and atlantic ocean basins we construct seafloor age relations for the indian ocean which we combine with bathymetric sediment isopach and heat flow data to derive curves for depth and heat flow versus age comparison of these curves with predictions from three thermal models shows that they are better fit by the shallower depths and higher heat flow for the gdh1 model which is characterized by a thinner and hotter lithosphere than previous models

characteristics of seamounts near hawaii as viewed by gloria abstract using images and data acquired from the gloria sonar system 390 seamounts within the u s hawaiian exclusive economic zone heez off hawaii have been studied their diameters range from 1 to 57 km with most less than 15km seamount abundance increases exponentially with decreasing size the areal density of observed seamounts having diameters greater than 1 km is 182 10 6 km 2 the theoretical abundance of seamounts of all sizes normalized to a unit area is 309 17 10 6 km 2 about an order of magnitude less than other surveyed areas of the pacific this may reflect a lower abundance of cretaceous seamounts in this region the covering of small seamounts by sediment or discrepancies from the use of different data sets to derive the abundance statistics the seamounts have morphologies ranging from steep sided flat topped structures to cones to more amorphous structures they are similar to volcanoes found elsewhere on the seafloor a suite of secondary features associated with the seamounts includes summit craters summit mounds coalesced boundaries landslides and graben several seamount chains are aligned parallel to cretaceous fracture zones consistent with an origin close to the ancestral east pacific rise others are aligned parallel to the necker ridge suggesting that they formed contemporaneously with necker in the plate interior this observation together with high abundances of seamounts where other intraplate igneous processes have occurred suggests some seamounts formed since leaving the spreading center

evolution of the carlsberg ridge between 60 and 45 ma ridge propagation spreading asymmetry and the deccan reunion hotspot despite the availability of numerous geophysical data no convincing magnetic anomaly identification and structural map of a major part of the arabian and eastern somali basins have been published to date i propose a new interpretation of the available magnetic data based on the identification of tiny wiggles i e singular second order patterns of the magnetic signal only one fracture zone and a set of propagating rifts existed between the owen fracture zone and the chagos laccadive ridge between anomalies 26 and 20 at anomalies 26 25 at least two eastward propagating rifts are identified while at anomalies 24 20 seven westward propagating rifts are confidently recognized the major consequence of this systematic ridge propagation is a tremendous regional asymmetry between anomalies 26 and 25 about 65 of the crust formed at the carlsberg ridge was accreted to the african plate while at anomalies 24 20 more than 75 benefited to the indian plate i suggest that these asymmetries result from the relative position of the carlsberg ridge and the nearby deccan reunion hotspot the ridge tending to remain near the hotspot

structure and morphology of submarine volcanism in the hotspot region around reunion island western indian ocean the form of the deccan maldives mascarene reunion hotspot trace suggests that it has at least in part been strongly controlled by crustal structures especially fracture zones this makes it difficult to assess the present day or past location of the hotspot and thus complicates the interpretation of african plate motion reconstruction we present here results of a cruise to the reunion area of which the aims were a to determine the extent of present day volcanism associated with the reunion hotspot in the region and b to examine the role of pre existing oceanic crustal structures in controlling the location of present day volcanism additionally we examined the morphology and geology of the important extinct spreading centre southwest of reunion abandoned when spreading jumped to separate seychelles from india during the deccan flood basalt episode some 60 65 ma ago the extensive bathymetrie seismic and geological investigation shows that significant present day hotspot volcanism is confined to the piton de la fournaise edifice on reunion island itself apparently the location of recent reunion volcanism has not been controlled by a crustal fracture and the major fracture zones on both sides of the island are not acting as magma conduits for plate motion reconstruction and plume flux calculation purposes piton de la fournaise must be taken as the present location of the reunion hotspot accretion at the extinct spreading centre progressively ceased at the time of anomaly a27 63 ma and was associated with marked propagation of the rift tips

formation of submarine flat topped volcanic cones in hawai i high resolution bathymetric mapping has shown that submarine flat topped volcanic cones morphologically similar to ones on the deep sea floor and near mid ocean ridges are common on or near submarine rift zones of kilauea kohala or mauna kea mahukona and haleakala volcanoes four flat topped cones on kohala were explored and sampled with the pisces v submersible in october 1998 samples show that flat topped cones on rift zones are constructed of tholeiitic basalt erupted during the shield stage similarly shaped flat topped cones on the northwest submarine flank of ni ihau are apparently formed of alkalic basalt erupted during the rejuvenated stage submarine postshield stage eruptions on hilo ridge mahukona hana ridge and offshore ni ihau form pointed cones of alkalic basalt and hawaiite the shield stage flat topped cones have steep 25 sides remarkably flat horizontal tops basal diameters of 1 3 km and heights 300 m the flat tops commonly have either a low mound or a deep crater in the center the rejuvenated stage flat topped cones have the same shape with steep sides and flat horizontal tops but are much larger with basal diameters up to 5 5 km and heights commonly greater than 200 m the flat tops have a central low mound shallow crater or levees that surrounded lava ponds as large as 1 km across most of the rejuvenated stage flat topped cones formed on slopes 10 and formed adjacent semicircular steps down the flank of ni ihau rather than circular structures all the flat topped cones appear to be monogenetic and formed during steady effusive eruptions lasting years to decades these and other submarine volcanic cones of similar size and shape apparently form as continuously overflowing submarine lava ponds a lava pond surrounded by a levee forms above a sea floor vent as lava continues to flow into the pond the lava flow surface rises and overflows the lowest point on the levee forming elongate pillow lava flows that simultaneously build the rim outward and upward but also dam and fill in the low point on the rim the process repeats at the new lowest point forming a circular structure with a flat horizontal top and steep pillowed margins there is a delicate balance between lava heat supply to the pond and cooling and thickening of the floating crust factors that facilitate construction of such landforms include effusive eruption of lava with low volatile contents moderate to high confining pressure at moderate to great ocean depth long lived steady eruption years to decades moderate effusion rates probably ca 0 1 km3 year and low but not necessarily flat slopes with higher effusion rates sheet flows flood the slope with lower effusion rates pillow mounds form hawaiian shield stage eruptions begin as fissure eruptions if the eruption is too brief it will not consolidate activity at a point and fissure fed flows will form a pond with irregular levees the pond will solidify between eruptive pulses if the eruption is not steady lava that is too volatile rich or that is erupted in too shallow water will produce fragmental and highly vesicular lava that will accumulate to form steep pointed cones as occurs during the post shield stage the steady effusion of lava on land constructs lava shields which are probably the subaerial analogs to submarine flat topped cones but formed under different cooling conditions

evidence from geoid data of a hotspot origin for the southern mascarene plateau and mascarene islands indian ocean a high resolution geoid map of the madagascar and mascarene basins is constructed from geos 3 and seasat satellite altimetric data with an accuracy of 0 7 m large positive anomalies are due to topographic highs and their regional compensation the deflection of the lithosphere under volcanic loads and its effect on the geoid are modeled for the southern mascarene plateau mauritius reunion and rodriguez islands assuming a purely elastic lithospheric model the effective flexural rigidity is estimated for the four zones rigidity varies from 1 10 21 to 2 10 23 n m increasing from north to south the highest rigidity occurs under reunion island and suggests a recent origin for the volcanism 3 5 ma the southern mascarene plateau shows the lowest rigidity suggesting a near ridge origin rodriguez island shows the same trend but in another tectonic framework the topographic and geoid swell is studied as an effect of a thermal mantle plume using a conductive model of lithospheric heating we confirm the assumption of hotspot migration for the origin of the southern mascarene plateau and the mascarene islands

spatial distribution of hotspot material added to the lithosphere under la reunion from wide angle seismic data wide angle seismic lines recorded by ocean bottom and land seismometers provide a pseudo three dimensional investigation of the crust and upper mantle structure around the volcanically active hotspot island of la reunion the submarine part of the edifice has fairly low seismic velocities without evidence for intrusives an upper unit with a velocity depth gradient is interpreted as made of material erupted subaerially then transported and compacted downslope between this unit and the top of the oceanic plate imaged by normal incidence seismic reflection a more homogeneous unit indicated by shadow zones on several wide angle sections may correspond to lavas of a different nature extruded underwater in the earlier phase of volcanism coincident wide angle and normal incidence reflections document that the oceanic plate is not generally downwarping toward the island but doming instead toward its southeastern part with limited evidence for some intracrustal intrusion deeper in the lithosphere the presence of a layer of intermediate velocity between the crust and mantle is firmly established it is interpreted as resulting from the advection of hotspot magmatic products possibly partially molten and of a composition for which the crust is a density barrier the extensive wide angle coverage constrains the extent of this body it does not show the elongated shape expected from plate drift above a steady hotspot supply alternative propositions can hence be considered for example that la reunion is caused by a solitary wave of hotspot material or by a young hotspot the size of the underplate 140 km wide and up to 3 km thick corresponds to less than half the volume of the edifice on top of the plate

evidence from en echelon cross grain ridges for tensional cracks in the pacific plate sea floor topography in the pacific is mainly aligned with original spreading directions1 but is overprinted by alignments created by mid plate processes spreading produces abyssal hills and fracture zones and mid plate volcanism generates seamounts isolated or in chains a different category of topography the cross grain discovered in geoid height data collected by the seasat radar altimeter2 comprises linear troughs and swells spaced 200 km apart oblique to fracture zones and abyssal hills but parallel to the hawaiian chain three models have been proposed for the cross grain small scale convection organized into longitudinal rolls by the shear of the pacific plate2 compressive buckling3 and lithospheric boudinage resulting from plate wide tensile stresses4 5 none of the previously available data ruled out any of these models here we report multi beam bathymetric data revealing long narrow en echelon ridges along the cross grain interpreted as evidence of tension cracks in the pacific plate

flood basalts and large igneous provinces from deep mantle plumes fact fiction and fallacy abstract flood basalts large igneous provinces and all intraplate magmatism are almost unanimously ascribed to features called mantle plumes in current geodynamics literature in the starting plume model flood basalts and any associated hotspot tracks have been ascribed to the melting of a large plume head and a narrow tail respectively and the deccan flood basalt province of india and the laccadives maldives chagos reunion island volcanic chain in the indian ocean are widely thought to be an excellent example supporting this model however the existing geodynamic thermal petrological geochemical and geochronological constraints on deccan volcanism do not require or support a mantle plume origin a new interpretation made here rules out any role for the reunion plume in deccan volcanism and even questions the notion that the reunion hotspot is the manifestation of an underlying reunion plume the laccadives reunion hotspot track is viewed here as caused by a southward propagating fracture based on the published radiometric age data and the ultimate cause of deccan volcanism is argued to be mega scale lithospheric rifting the appreciation of the great original areal extent of the deccan nearly as far as the southern tip of peninsular india solves the hitherto unsolved problem of the cause of the uplift of the 1500 km long western ghats escarpment this cause being combined surface erosion and magmatic underplating hypothesized mantle plumes do not appear responsible for most large igneous provinces instead their very existence is questionable no geological evidence of any kind geochemical petrological thermal topographic requires mantle plumes the self contradictions and special pleading that proliferate in plume explanations are unjustified and unnecessary if one accepts based on various grounds that a globe encircling enriched mantle layer the perisphere may overlie a deeper depleted mantle which is the source of morb the systematic geochemical variations along the laccadive reunion chain can be explained by this model and are not consistent with a plume tail experiencing entrainment from surrounding morb mantle as in the current plume model hotspot tracks may not reflect plate motion above stationary deep mantle upwellings but the stress state of the lithosphere as originally proposed decades ago the locations of large igneous provinces hotspots and hotspot tracks are strongly controlled by lithospheric architecture and history they are not placed randomly on the globe indifferent to the surface geology neither are large igneous provinces sudden chance events within the vastness of geological time which plume theories typically allege

the forgotten slaves of tromelin indian ocean new geoarchaeological data abstract here we present geoarchaeological results from tromelin a small 1 km 2 atoll in the indian ocean looking to elucidate human environment interactions following the wrecking of the l utile in 1761 abandoned by their french crew around 60 madagascans were enforced to form a micro society on this remote island for a 15 year period little however is known about the cultural and environmental adaptations the population made during their stay this study focuses on sediment archives from site 5 where gran inrap archaeological work in 2008 brought to light three rooms attributed to the survivor s living quarters particular emphasis is placed on the impacts of cyclonic storm surges natural hazards that presented serious environmental risks to the long term survival of the slaves the small size of tromelin renders it a particularly discrete unit of analysis for such a study we have identified four chronostratigraphic units at the site 1 unit 4 constitutes coarse white sand incorporating rounded gravels and reworked head and branch corals no anthropogenic material is attested we attribute this unit to a storm layer preceding the 1761 occupation of tromelin 2 unit 3 yielded a multiplicity of archaeological material and corresponds to the 18th century cultural layer the unit varies in thickness from 10 to 40 cm 3 unit 2 constitutes a 20 to 40 cm sand sheet across much of site 5 that we attribute to a powerful cyclone event it forms an unconformable contact above unit 3 and can be divided into two distinct sub units 2a and 2b sub unit 2a is characterised by an inverse grading of the sediments constituting coarse sands at the base grading into rounded biogenic gravels in the upper part this sedimentology is typical of increasing flow velocity and is diagnostic of a storm setup flow the transition to sub unit 2b marks the onset of the waning flow characterised by a sharp fall in relative flow velocities these overwash deposits buried much of the archaeological remains and have been critical to the preservation of the vestiges through to present day 4 unit 1 corresponds to a layer of infill incorporating blocks of coral and cultural objects dating from the 19th to 20th centuries

role of the structural inheritance of the oceanic lithosphere in the magmato tectonic evolution of piton de la fournaise volcano la reunion island la reunion island is located east of madagascar on the eastern rim of the tectonically inactive mascarene basin this island is composed of three shield volcanoes of which only piton de la fournaise is currently active although the magmatic activity is restricted to piton de la fournaise a scattered seismicity occurs on the whole 200 km wide volcanic edifice and in the underlying oceanic crust we carried out a multiscale analysis to understand 1 the origin of the seismicity in the geodynamic context and 2 the role of the oceanic lithosphere in the deformation of piton de la fournaise and la reunion island analysis of the magmatic system suggests that the magma ascent is controlled by large n25 30 and n125 130 fracture zones located below the enclos depression we also show that the orientation difference between the eruptive fissures and the related dykes result from a rotation of the main principal stress s 1 from vertical to downslope through the surface combining a digital elevation model dem analysis field observations and the geophysical data reveals that the volcano is affected by large fault zones the fault distribution indicates the predominance of a main n70 80 trend magnetic data show the same n80 orientation characterizing the remnant part of the alizes volcano such parallel alignment suggests a control exerted by the underlying alizes volcano on piton de la fournaise furthermore the alignment between the crustal orientations and the structures determined on the island suggests a control of the crustal structures in la reunion s volcano tectonic activity contrary to several volcanic islands such as hawaii and tenerife la reunion volcanoes lie on an upbending crust then we interpret the reactivation of the crustal faults as resulting from a crustal uplift related to the thermal erosion of the base of the lithosphere and or to strong underplating the upward deformation may prevent the spreading of the volcanoes as no evidence of such a mechanism is observed in the bathymetry and the seismic data around the island

sheet intrusions and deformation of piton des neiges and their implication for the volcano tectonics of la reunion abstract to understand the volcano tectonic history of piton des neiges the dormant volcano of la reunion we measured in the field the orientation of sheeted intrusions and deformation structures and interpreted the two datasets separately with a paleostress inversion results show that the multiple proposed rift zones may be simplified into three trends 1 a n30 e 5 km wide linear rift zone running to the south of the edifice active in the shield building 2 48 0 43 ma and terminal stages 190 22 ka 2 a curved n110 to n160 e rift zone widening from 5 km to 10 km toward the nw flank essentially active during the early emerged shield building 1 3 ma and 3 two sill zones 1 km thick in total in the most internal parts of the volcano active in the shield building and terminal stages in parallel deformation structures reveal that the tectonics of the edifice consisted in three end member stress regimes sharing common stress axes 1 nw se extension affecting in priority the south of the edifice near the n30 e rift zone 2 nne ssw extension on the northern half of the volcano near the n110 160 e rift zone 3 compression occurring near the sill zones with a ne sw or nw se maximum principal stress these three stress regimes are spatially correlated and mechanically compatible with the injection trends combined together our data show that the emerged piton des neiges underwent sector spreading delimited by perpendicular rift zones as observed on piton de la fournaise the active volcano of la reunion analogue experiments attribute such sector spreading to brittle edifices built on a weaker substratum we therefore conclude that la reunion volcanoes are both brittle as opposed to hawaiian volcanoes or mount etna whose radial spreading is usually attributed to a ductile body within the edifices

velocities and propagation characteristics of pn and sn beneath the himalayan arc and tibetan plateau possible evidence for underthrusting of indian continental lithosphere beneath tibet new seismological observations on velocities and propagation characteristics of pn and sn waves beneath tibet can be interpreted although not uniquely to indicate the shallow angle underthrusting of the greater indian continental lithosphere beneath the tibetan plateau this is inferred from available geological and geophysical data and is now supported by new seismological observations the most significant observation is that high frequency sn waves shear waves that travel in the mantle lithosphere propagate efficiently in the uppermost mantle beneath the tibetan plateau except beneath the north central part of tibet the chang thang terrane furthermore sn propagates efficiently across the stable block of the tarim basin along the tien shan and himalayan mountains and across the indian shield apparent velocities of pn and sn waves that traverse the uppermost mantle beneath the tibetan plateau are 8 42 and 4 73 km s respectively these velocities are very similar to those beneath the himalayan mountains and the indian shield the efficient propagation and the velocities of pn and sn waves beneath tibet are also similar to those commonly observed beneath shield and stable continental regions these results are consistent with the underthrusting model and do not favor the class of alternative models in which the tibetan plateau is formed by shortening and thickening of hot crustal and uppermost mantle material in response to the convergence of india and asia

observations of ridge hotspot interactions in the southern ocean the evolution of ridge hotspot systems is not well understood in this investigation satellite derived marine gravity data are used in conjunction with underway bathymetric and magnetic anomaly profiles to investigate the nature of ridge hotspot interaction at four sparsely explored systems in the southern ocean these systems illustrate three different stages of ridge hotspot interaction in which a migrating spreading center approaches a hotspot pacific antarctic louisville passes over or is captured by the hotspot mid atlantic shona discovery and ultimately migrates away from the hotspot southeast indian kerguelen all of these systems show some evidence of discrete ridge jumps in the direction of the hotspot as the spreading center attempts to relocate toward the hotspot by asymmetric spreading interestingly these ridge jumps show no evidence of propagating offsets as have been seen on many other ridge hotspot systems a simple model predicts that typical plume excess temperatures can weaken the lithosphere sufficiently to promote asymmetric spreading and possibly allow a discrete ridge jump the presence of previously uncharted obliquely oriented aseismic ridges and gravity lineations between the ridge and the hotspot supports the notion of asthenospheric flux from the plume to the spreading center both before and after the time when the hotspot is ridge centered the azimuths of the aseismic ridges cannot be explained by plate kinematics alone they consistently extend from the ends toward the centers of the adjacent spreading segments suggesting some interaction between plume derived asthenospheric flux and local lithospheric structure the features discussed here also indicate that the transfer of asthenospheric material from the plume to the spreading center is influenced by the local plate boundary configuration and interaction with transform offsets

local lithospheric relief associated with fracture zones and ponded plume material 1 buoyant plume material ponds at the base of oceanic lithosphere it is a natural probe for the relief on the base of the lithosphere owing to plate age because it flows laterally along an upside down drainage pattern geomorphic and geochemical studies of near ridge and on ridge hot spots indicate as expected that the oceanic lithosphere thickens as it ages corridors of thin lithosphere channel plume material along the axis while the thick lithosphere at the end of ridge segments dams this along axis flow channel hot spots extend from plumes toward ridge axes however a good correlation between local lithospheric thickness and local age is not evident at fracture zones within the old ocean basins such a correlation would be expected if the lithosphere cooled only by conduction then pressure release melting and volcanoes would occur preferentially near fracture zones where plume material cascades from thick old to thin young lithosphere this concentration of volcanic edifices does not occur this lack of correlation between lithosphere thickness and plate age is an expected consequence of small scale convection at the base of the lithosphere scaling relationships and numerical models indicate that modestly vigorous convection of several milliwatt per meter squared is sufficient to perturb local lithospheric relief beneath fracture zones without changing the gross correlation of lithospheric thickness with plate age typically the step in lithospheric thickness moves toward the old side of the fracture zone

perturbation to the lithosphere along the hotspot track of la reunion from an offshore onshore seismic transect a 250 km long ne sw lithospheric transect spanning the 40 km wide island of la reunion and its submarine edifice is derived from lines of air gun shots at sea on either side along the assumed hotspot trace seismic records were obtained from an array spanning the whole transect and including sea bottom and land receivers providing a system of reversed and overlapping observations low seismic velocity and hence density is found on average for the whole edifice above the oceanic plate we attribute high velocity anomalies within the edifice to an intrusive core confined under the central northern quarter of the island crossing segment unexpectedly the main seismic interfaces top and bottom of the prevolcanic crust do not show significant flexural downwarping under the island in addition clear multipathing in the recorded wave field requires the presence of a body with a seismic velocity intermediate between the prevolcanic crustal material and the normal mantle this lithospheric structure provides the first example where underplating occur s beneath an active volcanic island suggesting a genetic relationship the underplated body could represent residues of the evolution of primary picritic melts that yield erupted basalts evidence for reflectors deeper in the lithosphere may indicate further related heterogeneity in the plate hotspot model commonly assumed the structural variation along the transect could be interpreted as a variation with time of the amount and physical state of underplated material

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