crustal structure and tectonics of the ninetyeast ridge from seismic and gravity studies seismic reflection and refraction gravity and bathymetric data across and along the central part of the ninetyeast ridge were analyzed to determine the crustal structure of the ridge and to understand its tectonics the ridge in the study area rises to 3 5 km above the seafloor of adjacent ocean basins basement is covered by sediments up to 0 8 km thick but exposed at some locations seismic refraction and gravity models of the ridge show that the crustal thickness is 22 km greater than the average thickness 7 km of oceanic crust estimated in the central indian and wharton basins the ridge is compensated by flexure of layers 2a 2b and 3a to the extent of 2 km and by a 12 km thick deep crustal body layer 3b of underplated material upper crustal velocities layer 2a beneath the ridge are lower up to 3 4 km s 1 than in adjacent basins because of weathering process and hydrothermal circulation of the waters in the tectonized blocks while velocities in deep crustal body layer 3b are higher up to 7 8 km s 1 and seem to be resulted from magmatic rocks accreted within the lithosphere steep down ward faulting of 2 km along the eastern flank of the ninetyeast ridge possibly originated because of compressional and extensional stresses applied along the 89 e fracture zone fz the stresses are interpreted as resulting from multiple ridge jumps of wharton spreading centers during the late cretaceous and middle eocene the velocity depth model suggests that the rocks of layers 2a and 2b and layer 3a are exposed on the eastern flank of the ninetyeast ridge and 90 e fz respectively they are consistent with basalt dolerite and gabbro rocks collected from the fault scarps the central part of the ridge between 1 n and 11 s has uniformly deepened by 0 7 km and is bounded by major sublatitudinal deep seated fractures we interpret that the deepened structural block and associated fractures resulted from the activity of the intense deformation of the central indian ocean

weakening of rock salt by water during long term creep the rheological properties of rock salt are of fundamental importance in predicting the long term evolution of salt based radioactive waste repositories and strategic storage caverns and in modelling the formation of salt diapirs and associated oil traps1 2 the short term high stress rheology of rock salt is well known from laboratory experiments however extrapolation to appropriately low stresses fails to predict the rapid flow seen in certain natural structures furthermore experiments have failed to reproduce the recrystallized microstructure of naturally deformed salt here we report experiments indicating that the above discrepancies can be explained by taking into account the influence of trace amounts of brine trace brine is always present in natural salt but sometimes escapes during experiments our tests on dry dilated salt show more or less conventional dislocation creep behaviour but brine bearing samples show marked weakening at low strain rates this is associated with dynamic recrystallization and a change of deformation mechanism to solution transfer creep because natural rock salt always contains some brine these results cast substantial doubt on the validity of presently accepted dislocation creep laws for predicting the long term rheological behaviour of salt in nature

quaternary glaciations and glacial landform evolution in the tailan river valley tianshan range china abstract the tailan river originates on the southern slope of tumur peak the largest center of modern glaciation in the tianshan range five moraine complexes and associated fluvioglacial deposits in this valley record a complex history of quaternary glacial cycles and landform evolution electron spin resonance esr dating of glacial sediments was carried out using germanium ge centers in quartz grains which are sensitive to both sunlight and grinding based on the dating results as well as geomorphic and stratigraphic data the piyazilike end moraines the second moraine complex were deposited during neoglaciation the largest glacial advances during the last 3 4 ka in western china and an early holocene glacial advance the third set of moraines was deposited in marine isotope stage mis 2 4 and the glacial landforms of the tailan glaciation the fourth moraine complex which include hummocky moraines on the piedmont were formed in mis 6 penultimate glaciation the end moraines of the innermost moraine complex the first moraine complex are inferred to have been deposited during the little ice age lia thus the landforms and dates indicate compound valley glaciers from the lia to mis 2 4 and piedmont glaciers during mis 6 the oldest tills studied belong to the kokdepsang glacial stage and occur on a high plateau based on similar glacial landforms glacial deposits on a high plateau and a high glacial terrace and their esr ages in adjacent valleys on the southern slope of tumur peak the kokdepsang glacial stage is assigned to mis 12

reconstruction of the central indian ocean abstract sixteen plate tectonic reconstructions are presented illustrating the development of the central indian ocean from the latest cretaceous anomaly 29 to the present the history of seafloor spreading in the indian ocean can be divided into five distinct tectonic epochs 1 1 latest cretaceous early paleocene a29 a27 2 2 middle paleocene early eocene a27 a22 3 3 middle eocene late eocene a22 a18 4 4 late eocene early miocene a18 a8 and 5 5 early miocene recent a8 a5 during each of these tectonic epochs subtle changes took place in the directions and rates of spreading along the central southeast and southwest indian ridges the most dramatic changes in plate motion occurred between a21 and a18 time due to the collision of india and eurasia

abandoned paleocene spreading center in the northeastern indian ocean evidence from magnetic and seismic reflection data abstract magnetic data from the northeastern indian ocean reveal east west trending magnetic lineations 28 through 34 an abandoned spreading center asc and part of the cretaceous magnetic quiet zone cmqz which are used to reconstruct past tectonic processes involved in the evolution of the ocean offsets in the magnetic lineations outline the existence of four n s trending fracture zones 80 e indira 84 5 e and 86 e the 84 5 e fracture zone fz acts as a boundary separating oceanic crust with different magnetic isochron patterns the pairs of magnetic lineations 30 through 32n 2 between the 86 e fz and the ninetyeast ridge reveal an asc of about 65 ma age parallel to 0 5 s latitude the structure of the asc is well identified in seismic reflection data as an undulating basement topographic rise covered by up to 2 km of bengal fan sediments the spreading center might have initiated its activity along with other spreading centers of the wharton ridge and india antarctica ridge after the first major plate reorganization of the indian ocean about 95 5 ma but ceased shortly after formation of anomaly 30 about 65 ma then it jumped southward between anomalies 32n 2 and 33 the jump captured the oceanic crust of anomalies 30 through 32n 2 that initially formed on the antarctica plate but were later transferred to the indian plate the spreading centers in the vicinity of the ninetyeast ridge jumped several times southward to maintain a close proximity to the kerguelen hotspot

the variety of subaerial active salt deformations in the kuqa fold thrust belt china constrained by insar abstract surface salt bodies in the western kuqa fold thrust belt of northwestern china allow study of subaerial salt kinematics and its possible correlations with weather variations ephemeral subaerial salt exposure during the evolution of a salt structure can greatly impact the subsequent development and deformation of its tectonic setting here we present a quantitative time lapse survey of surface salt deformation measured from interferometric synthetic aperture radar insar using envisat radar imagery acquired between 2003 and 2010 time series analysis and inspection of individual interferograms confirm that the majority of the salt bodies in western kuqa are active with significant insar observable displacements at 3 of 4 structures studied in the region subaerial salt motion toward and away from the satellite at rates up to 5 mm yr with respect to local references rainfall measurements from the tropical rainfall measuring mission trmm and temperature from a local weather station are used to test the relationship between seasonality and surface salt motion we observe decoupling between surface salt motion and seasonality and interpret these observations to indicate that regional and local structural regimes exert primary control on surface salt displacement rates

tectonic speed limits from plate kinematic reconstructions abstract the motion of plates and continents on the planet s surface are a manifestation of long term mantle convection and plate tectonics present day plate velocities provide a snapshot of this ongoing process and have been used to infer controlling factors on the speeds of plates and continents however present day velocities do not capture plate behaviour over geologically representative periods of time to address this shortcoming we use a plate tectonic reconstruction approach to extract time dependent plate velocities and geometries from which root mean square rms velocities are computed resulting in a median rms plate speed of 4 cm yr over 200 myr linking tectonothermal ages of continental lithosphere to the rms plate velocity analysis we find that the increasing portions of plate area composed of continental and or cratonic lithosphere significantly reduces plate speeds plates with any cratonic portion have a median rms velocity of 5 8 cm yr while plates with more than 25 of cratonic area have a median rms speed of 2 8 cm yr the fastest plates 8 5 cm yr rms speed have little continental fraction and tend to be bounded by subduction zones while the slowest plates 2 6 2 8 cm yr rms speed have large continental fractions and usually have little to no subducting part of plate perimeter more generally oceanic plates tend to move 2 3 times faster than continental plates consistent with predictions of numerical models of mantle convection the slower motion of continental plates is compatible with deep keels impinging on asthenospheric flow and increasing shear traction thus anchoring the plate in the more viscous mantle transition zone we also find that short lived up to 10 myr rapid accelerations of africa 100 and 65 ma north america 100 and 55 ma and india 130 80 and 65 ma appear to be correlated with plume head arrivals as recorded by large igneous province lips emplacement by evaluating factors influencing plate speeds over the mesozoic and cenozoic our temporal analysis reveals simple principles that can guide the construction and evaluation of absolute plate motion models for times before the cretaceous in the absence of hotspot tracks and seafloor spreading histories based on the post pangea plate motions one principle that can be applied to pre pangea times is that plates with less than 50 continental area can reach rms velocities of 20 cm yr while plates with more than 50 continental fraction do not exceed rms velocities of 10 cm yr similarly plates with large portions of continental or cratonic area with rms velocities exceeding 15 cm yr for more than 10 myr should be considered as potential artefacts requiring further justification of plate driving forces in such scenarios

evolution of the eastern indian ocean since the late cretaceous constraints from geosat altimetry we propose a new model for the tectonic evolution of the eastern indian ocean from the late cretaceous to the present two types of data are used to improve previously published reconstructions first recent reinterpretations of seafloor magnetic anomalies between australia and antarctica and in the wharton basin provide new constraints on spreading rates and the timing of major reorganizations second vertical deflection profiles i e horizontal gravity anomaly derived from 22 repeat cycles of geosat altimeter data reveal the tectonic fabric associated with fracture zones these new geosat data provide tight constraints on paleospreading directions for example three prominent fracture zones can be traced from south of tasmania to the george v basin antarctica providing an important constraint on the relative motions of australia and antarctica through the late eocene in addition the geosat profiles are used to locate the conjugate continental margins and continent ocean boundaries of australia and antarctica as well as the conjugate rifted margins of kerguelen plateau and broken ridge based on a compilation of magnetic anomaly data from the crozet basin the central indian basin the wharton basin and the australian antarctic basin ten plate tectonic reconstructions are proposed reconstructions at chrons 5 11 ma 6 21 ma 13 36 ma and 18 43 ma confirm that the southeast indian ridge behaved as a single plate boundary since chron 18 the constraints from the geosat data provide an improvement in the fit of the kerguelen plateau and broken ridge at chron 20 46 ma to avoid overlaps between broken ridge and the kerguelen plateau prior to their breakup our model includes relative motions between the northern and southern provinces of the kerguelen plateau finally we examine the implications of our model for the relative motions of india australia and antarctica on the tectonic evolution of the kerguelen plateau and broken ridge and the adjacent labuan basin and diamantina zone as well as the emplacement of the ninetyeast ridge and the kerguelen plateau over a fixed hot spot

crustal evolution and sedimentation history of the bay of bengal since the cretaceous multichannel seismic reflection gravity magnetic and bathymetric investigations were made along five latitudinal profiles from the eastern shelf of india to the andaman shelf and four nw se profiles in the western basin off madras seismics on three latitudinal profiles in the bay of bengal the trend of the fracture zones the locations of the magnetic chron 34 and the cretaceous magnetic quiet zone suggest that greater india separated from antarctica after a period of transform motion in the early cretaceous that is about polarity chron m0 120 ma and drifted northwestward negative gravity anomalies are associated with basement rises including the 85 e ridge between the continental margin of india and the ninetyeast ridge magnetic reversals and northward trend of the 85 e ridge support a hotspot origin of the ridge and its emplacement most likely after the cretaceous quiet period juxtaposition of high amplitude hyperbolic reflections down faulted continental blocks buried under thick sediments and associated gravity and magnetic anomalies mark the boundary between continental and oceanic rocks at the foot of continental slope about 80 km seaward of the present continental shelf edge eight seismic sequences as thick as 8 5 km overlie the early cretaceous oceanic basement and include four unconformities lower eocene upper oligocene upper miocene and upper pleistocene which have been correlated to the major geologic tectonic events we also interpret the late cretaceous early tertiary features on the eastern flank of the buried 85 e ridge as carbonate reefs the observation of steep subduction of older cold indian plate beneath the burmese plate near the andaman islands suggests the sunda arc in this region as low to intermediate stress subduction zone

new constraints on the tectonic evolution of the eastern indian ocean abstract from marine magnetic anomaly studies a fossil spreading ridge is identified beneath the nicobar fan in the northwestern wharton basin several north south trending transform faults offset this ridge left laterally east of the 86 e transform fault our findings show that this ridge which was part of the plate boundary between the indian and australian plates ceased its spreading shortly after formation of magnetic anomaly 20 45 6m y b p since the breakup of australia and antarctica probably occurred sometime between 110 and 90 m y b p we suggest that the indian australian and antarctic plates were moving relative to one another from about 90 to 45 m y b p a triple junction would have existed in the southeastern indian ocean during that period of time at anomaly 19 time 45m y b p the junction became inactive and australia and india became a single plate the northwest southeast trending southeast indian ridge was formed by connecting the india antarctica spreading center with the australia antarctica spreading center its activity has continued to the present time

internal structure of the 85 e ridge bay of bengal evidence for multiphase volcanism abstract the 85 e ridge located in the bay of bengal of the northeastern indian ocean is an enigmatic geological feature as it possesses unusual geophysical signatures the ridge s internal structure and mode of eruptions are unknown due to lack of deep seismic reflection and borehole data control here we analyze 10 km of long streamer seismic reflection data to unravel the ridge s internal structure and thereby to enhance the understanding of how the ridge was originated and grew over a geologic time seismic facies analysis reveals the ridge structure consisting of volcanic vent and several stratigraphic units including packs of prograding clinoforms the clinoform sequences are interpreted as volcanic successions and led to the formation of lava delta fronts interpreted features of lava fed deltas and intervening erosional surfaces and mass flows along ridge flanks suggest that the 85 e ridge is a volcanic construct and was built by both subaqueous and multiphase sub marine volcanism during the late cretaceous approximately from 85 to 80 ma at later time from oligocene miocene 23 ma onwards the ridge was buried under the thick sediments of the bengal fan system

stress reconstruction and lithosphere dynamics along the sumatra subduction margin abstract gravity modeling and stress reconstructions over the sumatra subduction margin between indian australian and asian plates provide insights into the geodynamic setting and kinematics of the area based on ridge trench interaction plate obliquity plate age and seismicity the area was divided into two sectors northern and southern for gravity modeling along two profiles the stress data with magnitude mw 5 0 and focal depth 100 km for the subducting plate were compiled for six zones i vi and were isolated under different time domains for high resolution study the reconstructed gravity models were used to isolate the events for subducting oceanic indian australian and overriding continental asian plates the data sets for the subducting lithosphere were used for stress inversion to determine the predominant stress regimes for pre earthquake and post earthquake phases of the sumatra earthquake of december 2004 the results were interpreted in terms of operative intra plate stress fields and for reconstruction of kinematics and dynamics along the sumatra subduction margin analysis shows both sparse and intense seismic activities identified over different time segments in the six zones under pre and post december 2004 phases with respect to the incidence of mw 9 2 off sumatra mega earthquake of 2004 the stress regime of the 2004 mega event and subsequent release of strain energy through cyclic changes of the stress obliquities in the northern part of the study area account for heterogeneous coupling caused by rough topography of the subducting indian australian plate reduction of compressive stress regime in the deeper segment of the subducting plate in the northern sector triggered twin great strike slip dominated earthquakes in 2012 in the floor of the northeastern indian ocean the weak zones of a spreading ridge subducting below the asian plate possibly suffered strain localization and weakening the less transmission of co and post seismic strain energy from northern to southern part of the margin was possibly prohibited near the zone of ridge trench interaction the trench orthogonal tearing slab window at the edge of the subducting plate in this area cannot be ruled out the slab window caused by the subduction of the young buoyant ridge and the trench ward movement of the upper plate has transformed the central segment into compressional regime

oceanic microplate formation records the onset of india eurasia collision abstract mapping of seafloor tectonic fabric in the indian ocean using high resolution satellite derived vertical gravity gradient data reveals an extinct pacific style oceanic microplate mammerickx microplate west of the ninetyeast ridge it is one of the first pacific style microplates to be mapped outside the pacific basin suggesting that geophysical conditions during formation probably resembled those that have dominated at eastern pacific ridges the microplate formed at the indian antarctic ridge and is bordered by an extinct ridge in the north and pseudofault in the south whose conjugate is located north of the kerguelen plateau independent microplate rotation is indicated by asymmetric pseudofaults and rotated abyssal hill fabric also seen in multibeam data magnetic anomaly picks and age estimates calculated from published spreading rates suggest formation during chron 21o 47 3 ma plate reorganizations can trigger ridge propagation and microplate development and we propose that mammerickx microplate formation is linked with the india eurasia collision initial soft collision the collision altered the stress regime at the indian antarctic ridge leading to a change in segmentation and ridge propagation from an establishing transform fast indian antarctic spreading that preceded microplate formation and kerguelen plume activity may have facilitated ridge propagation via the production of thin and weak lithosphere however both factors had been present for tens of millions of years and are therefore unlikely to have triggered the event prior to the collision the combination of fast spreading and plume activity was responsible for the production of a wide region of undulate seafloor to the north of the extinct ridge and w shaped lineations that record back and forth ridge propagation microplate formation provides a precise means of dating the onset of the india eurasia collision and is completely independent of and complementary to timing constraints derived from continental geology or convergence histories

tectonics of the ninetyeast ridge derived from spreading records in adjacent oceanic basins and age constraints of the ridge 1 analysis of new and existing geophysical data for the central indian and wharton basins of the indian ocean were used to understand the formation and evolution of the ninetyeast ridge ner especially its relationship to the kerguelen hot spot and the wharton spreading ridge satellite gravity data and magnetic anomalies 34 through 19 define crustal isochrons and show fracture zones striking n5 e one of these at 89 e crosses the n10 e trending ner impacting the ner morphology from 77 to 43 ma the ner lengthened at a rate of 118 km myr twice that of the 48 58 km myr accretion rate of adjacent oceanic crust this difference can be explained by southward jumps of the wharton spreading ridge toward the hot spot which transferred portions of crust from the antarctic plate to the indian plate lengthening the ner magnetic anomalies document a small number of large spreading ridge jumps in the ocean crust immediately to the west of the ner especially two leaving observable 65 and 42 ma fossil spreading ridges in contrast complex magnetic anomaly progressions and morphology imply that smaller spreading ridge jumps occurred at more frequent intervals beneath the ner comparison of the ner dates and magnetic anomaly ages implies that the hot spot first emplaced ner volcanoes on the indian plate at a distance from the wharton ridge but as the northward drifting spreading ridge approached the hot spot the two interacted keeping later ner volcanism near the spreading ridge crest by spreading center jumps

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

mesozoic anomalies in the bay of bengal abstract the analysis of 8200 line km of total magnetic intensity data in the bay of bengal northeastern indian ocean revealed the presence of approximately n30 e trending seafloor spreading type magnetic anomalies these anomalies resemble the mesozoic anomaly series m11 m0 reported elsewhere the oldest anomaly m11 132 5 ma identified close to the east coast of india is followed by the younger series of mesozoic anomalies towards the offshore some of the anomalies are offset by 60 80 km the configuration of the offsets of the isochrons allowed us to propose approximately n120 e trending oceanic fracture zones the mesozoic crust of 132 5 118 ma is estimated to evolve with an average half spreading rate of 3 5 cm yr except for the ocean floor between the m9 and m4 magnetic isochrons this part of the crust appears to be affected by the buried subsurface 85 e ridge

tectonic model for the evolution of oceanic crust in the northeastern indian ocean from the late cretaceous to the early tertiary bathymetry and magnetic studies part of the trans indian ocean geotraverse investigations in the northeastern indian ocean revealed seafloor topographic features magnetic lineations 19 through 32b and abandoned spreading centers the seafloor topography of the ninetyeast ridge is relatively wider and shallower south of 15 s the magnetic anomalies indicate nine fracture zones two of them are newly identified some of the fracture zones are reflected in the bathymetry abandoned spreading centers between 86 e fracture zone fz and 92 e fz are interpreted as the western extensions of the wharton ridge they ceased spreading along with other spreading centers in the wharton basin soon after the formation of magnetic anomaly 19 around 42 ma and merged the indian and australian plates as single indo australian plate the pattern of magnetic lineations between 86 e fz and 90 e fz indicate a series of southerly ridge jumps at anomalies 30 26 royer et al 1991 and other workers and 19 these ridge jumps transferred portions of the antarctic plate to the indian plate the captured portions and offset along 86 e fz between india antartica ridge and wharton ridge resulted in an anomalous extra oceanic crust between 86 e fz and ninetyeast ridge spanning 11 in latitude

mid ocean ridge jumps associated with hotspot magmatism abstract hotspot ridge interaction produces a wide range of phenomena including excess crustal thickness geochemical anomalies off axis volcanic ridges and ridge relocations or jumps ridges are recorded to have jumped toward many hotspots including iceland discovery galapagos kerguelen and tristan de cuhna the causes of ridge jumps likely involve a number of interacting processes related to hotspots one such process is reheating of the lithosphere as magma penetrates it to feed near axis volcanism we study this effect by using the hybrid finite element code flac to simulate two dimensional 2 d cross section viscous mantle flow elasto plastic deformation of the lithosphere and heat transport in a ridge setting near an off axis hotspot heating due to magma transport through the lithosphere is implemented within a hotspot region of fixed width to determine the conditions necessary to initiate a ridge jump we vary four parameters hotspot magmatic heating rate spreading rate seafloor age at the location of the hotspot and ridge migration rate our results indicate that the hotspot magmatic heating rate required to initiate a ridge jump increases non linearly with increasing spreading rate and seafloor age models predict that magmatic heating itself is most likely to cause jumps at slow spreading rates such as at the mid atlantic ridge on iceland in contrast despite the higher magma flux at the galapagos hotspot magmatic heating alone is probably insufficient to induce a ridge jump at the present day due to the intermediate ridge spreading rate of the galapagos spreading center the time required to achieve a ridge jump for fixed or migrating ridges is found to be on the order of 105 106 years simulations that incorporate ridge migration predict that after a ridge jump occurs the hotspot and ridge migrate together for time periods that increase with magma flux model results also suggest a mechanism for ridge reorganizations not related to hotspots such as ridge jumps in back arc settings and ridge segment propagation along the mid atlantic ridge

analysis of satellite gravity and bathymetry data over ninety east ridge variation in the compensation mechanism and implication for emplacement process 1 we investigate the mode of compensation emplacement history and deep density structure of the ninety east ridge indian ocean using spectral analyses and forward modeling of satellite gravity and bathymetry data we find that the northern 0 10 n and the southern 20 30 s parts of the ridge are flexurally compensated with an effective elastic thickness 15 km whereas the central part 0 20 s is locally compensated furthermore we find that for a part of central block 10 20 s over osborn knoll the compensation depth is unreasonably very high 30 40 km therefore we favor a model with subsurface loading and interpret this to be due to underplating of mafic material at the base of the crust a hypothesis that is supported by seismic results and direct modeling of gravity data along some profiles these results suggest that the northern and southern parts of ninety east ridge were emplaced off to a ridge axis compared to the central one which might have been emplaced on or near a spreading center locally compensated large topography thick underplated crust in the central part near osborn knoll might result from an interaction of a hot spot with the extinct wharton spreading ridge

thrust and fold tectonics and the role of evaporites in deformation in the western kuqa foreland of tarim basin northwest china the kuqa foreland between the tarim basin and the tianshan mountains is rich in oil and gas based on field work and seismic profiles the structural styles and their formation mechanisms were determined and the role of evaporites in the deformation was demonstrated the main structural styles in the overburden are detachment folds large scale nappes triangle zones gentle and wide synclines fault propagation folds and pop ups the main structures in the substrate are small scale thrust faults duplexes pop ups and fault bend and fault propagation folds and formed mainly at the end of the pliocene under north south compression the evaporite layer in the lower section of the paleogene is the decollement zone for the disharmonic deformations in the overburden and in the substrate the detachment along the evaporite layer made it possible for compressive stresses to be transmitted farther in the overburden than in the substrate deformation in the overburden is more extensive than in the substrate at the leading edge of deformation at the trailing edge of deformation the structural highs in the overburden closely correspond to those in the substrate which is of significance for petroleum exploration in the western kuqa foreland

insar mapping and modelling of an active iranian salt extrusion abstract interferometric synthetic aperture radar insar imaging is a powerful technique that is increasingly used to extract detailed 3d information on earth surface structures including exposed diapiric surfaces we have used insar to map for the first time the cumulative surface deformation in a 6 km 6 km region surrounding an active salt diapir syahoo and its associated surface salt flow or namakier exposed in the zagros mountains of southern iran images provided by the european space agency were acquired in 12 increments ranging in length between 35 and 1248 days over a 14 year interval between july 1992 and may 2006 the deformation of the salt surface is non steady with extrapolated rates of displacement varying between surficial uplifts of 1 4 mm day 1 511 mm a 1 and subsidence of 2 2 mm day 1 803 mm a 1 growth of a central topographic dome occurs following short wet intervals to create a salt fountain morphology which then slowly decays during the intervening long dry periods salt associated with dynamic bulging9 of the central dome during wet intervals may flow laterally via gravity spreading into the surrounding salt sheet resulting in deflation and subsidence of the dome which is counteracted by growth and inflation of the adjacent namakiers salt bulges9 that migrate down the namakier resulting in local inflationary and deflationary cycles of the surface may be regarded as episodic pulses of gravity spreading areas of inflation and deflation are also observed to commonly reverse during dry to wet periods as the overall salt system effectively self regulates as it continually strives for dynamic equilibrium as long as the source of salt remains undepleted gravity spreading of the dome ultimately results in more buoyancy driven salt flowing up the diapiric neck to replenish and feed the extrusion and maintain the gross fountain morphology

a new geomagnetic polarity time scale for the late cretaceous and cenozoic we have constructed a magnetic polarity time scale for the late cretaceous and cenozoic based on an analysis of marine magnetic profiles from the world s ocean basins this is the first time since heirtzler et al 1968 published their time scale that the relative widths of the magnetic polarity intervals for the entire late cretaceous and cenozoic have been systematically determined from magnetic profiles a composite geomagnetic polarity sequence was derived based primarily on data from the south atlantic anomaly spacings in the south atlantic were constrained by a combination of finite rotation poles and averages of stacked profiles fine scale information was derived from magnetic profiles on faster spreading ridges in the pacific and indian oceans and inserted into the south ariantic sequence based on the assumption that spreading rates in the south atlantic were smoothly varying but not necessarily constant a time scale was generated by using a spline function to fit a set of nine age calibration points

revisiting the structure age and evolution of the wharton basin to better understand subduction under indonesia understanding the subduction processes along the sunda trench requires detailed constraints on the subducting lithosphere we build a detailed tectonic map of the wharton basin based on reinterpretation of satellite derived gravity anomalies and marine magnetic anomalies the wharton basin is characterized by a fossil ridge dated 36 5 ma offset by n s fracture zones magnetic anomalies 18 to 34 38 84 ma are identified on both flanks although a large part of the basin has been subducted we analyze the past plate kinematic evolution of the wharton basin by two plate india australia and three plate india australia antarctica reconstructions despite the diffuse plate boundaries within the indo australian plate for the last 20 ma we obtain finite rotation parameters that we apply to reconstruct the subducted wharton basin and constrain the thickness buoyancy and rheology of the subducting plate the lower subductability of younger lithosphere off sumatra has important consequences on the morphology with a shallower trench forearc islands and a significant inward deviation of the subduction system this deviation decreases in the youngest area where the wharton fossil spreading center enters subduction the discontinuous magmatic crust and serpentinized upper mantle consequences of the slow spreading rates at which this area was formed weaken the mechanical resistance to subduction and facilitate the restoration of the accretionary prism deeper effects include the possible creation of asthenospheric windows beneath the andaman sea in relation to the long offset fracture zones and east of 105 e as a result of subduction of the spreading center

two stage eastward diachronous model of india eurasia collision constraints from the intraplate tectonic records in northeast indian ocean abstract the onset of timing and the model of the india eurasia collision are strongly debated based on continental works without systematic marine tectonic constraints two aseismic ridges the laccadives maldives chagos ridge lmcr formed after the deccan volcanism in western india and the ninetyeast ridge ner formed after the emplacement of the rajmahal traps in eastern india are prominent features in the northeast indian ocean the onset of a major and steady increase in magma production rates along the aseismic ridge associated with the significant slowdown of the northward moving indian plate is a likely indicator of the onset of the india eurasia collision using gravity derived crustal thickness we calculated the magma production rates along the lmcr and the ner respectively the steady increasing magma production rate along the lmcr and a southwestward jump of the central indian ridge constrained the soft india island arc and hard india eurasia collisions between western india and eurasia to 50 52 ma and 41 ma respectively the steady increasing magma production rate along the ner and the formation of the mammerickx microplate constrained the soft collision between eastern india and eurasia to 47 49 ma and the extinction of the wharton ridge constrained the hard collision between eastern india and eurasia to 38 ma both the soft and hard collisions between western india and eurasia are 3 myr earlier than the soft and hard collisions between eastern india and eurasia respectively with systematic marine tectonic constraints a two stage eastward diachronous model of the india eurasia collision was proposed

seafloor age dependence of rayleigh wave phase velocities in the indian ocean variations in the phase velocity of fundamental mode rayleigh waves across the indian ocean are determined using two inversion approaches first variations in phase velocity as a function of seafloor age are estimated using a pure path age dependent inversion method second a two dimensional parameterization is used to solve for phase velocity within 1 25 1 25 grid cells rayleigh wave travel time delays have been measured between periods of 38 and 200 s the number of measurements in the study area ranges between 4139 paths at a period of 200 s and 22 272 paths at a period of 40 s at periods 100 s the phase velocity variations are strongly controlled by seafloor age and shown to be consistent with temperature variations predicted by the half space cooling model for a mantle potential temperature of 1400 c the inferred thermal structure beneath the indian ocean is most similar to the structure of the pacific upper mantle where phase velocities can also be explained by a half space cooling model the thermal structure is not consistent with that of the atlantic upper mantle which is best fit by a plate cooling model and requires a thin plate removing age dependent phase velocity from the 2 d maps of the indian ocean highlights anomalously high velocities at the rodriguez triple junction and the australian antarctic discordance and anomalously low velocities immediately to the west of the central indian ridge

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

effect of time evolving age and convergence rate of the subducting plate on the cenozoic adakites and boninites partial melting of subducting oceanic crust expressed as high mg volcanic rocks such as adakites and boninites has been actively studied for decades and lee and king 2010 reported that time evolving subduction parameters such as the age and the subduction rate of the converging oceanic plate play important roles in transient partial melting of the subducting oceanic crust e g aleutians however few subduction model experiments have considered time evolving subduction parameters posing problems for studies of transient partial melting of subducting oceanic crust in many subduction zones therefore we constructed two dimensional kinematic dynamic subduction models for the izu bonin mariana northeast japan kuril tonga java sunda and aleutian subduction zones that account for the last 50 myr of their evolution the models include the time evolving age and convergence rate of the incoming oceanic plate so the effect of time evolving subduction parameters on transient partial melting of oceanic crust can be evaluated our model calculations revealed that adakites and boninites in the izu bonin and aleutian subduction zones resulted from transient partial melting of oceanic crust however the steady state subduction model using current subduction parameters did not produce any partial melting of oceanic crust in the aforementioned subduction zones indicating that time evolving subduction parameters are crucial for modeling transient eruption of adakites and boninites our model calculations confirm that other geological processes such as forearc extension back arc opening mantle plumes and ridge subduction are required for partial melting of the oceanic crust in the mariana northeast japan tonga and southeastern java sunda subduction zones

regional trends in active diapirism revealed by mountain range scale insar time series 1 we construct an orogen wide insar time series over the zagros mountains and eastern markan subduction zone in southern iran from 19 envisat tracks spanning 2003 2010 we observe active salt diapirism of the infra cambrian hormuz salt at 20 locations and identify several diapirs that are not moving all active diapirs reach the surface within the asmari limestone or older rock units while we do not observe any active diapirism occurring within younger exposures reflecting a complex vertical accumulation of the basal salt and reactivation of diapirs by erosion diapir reactivation of this sort may be a critical factor affecting the feasibility of sequestering nuclear waste and co2 in evaporite sequences over geologic time scales the distribution of active diapirism is indifferent to increases in mean elevation suggesting a relatively uniform thickness of the zagros fold belt above the basal salt with steps in topography driven by basement thickening we do not observe aseismic uplift of anticlines because rates are likely below the detection threshold of our time series this suggests that uplift is accommodated at small rates 2 3 mm yr over numerous structures instead of focused at the deformation front

an early india asia contact paleomagnetic constraints from ninetyeast ridge odp leg 121 new paleomagnetic results from sedimentary rock and basement of the ninetyeast ridge ocean drilling program leg 121 sites 756 758 detail the northward movement of the indian plate for the past 80 m y analysis of the combined paleolatitude age profile indicates a distinct reduction in india9s northward movement rate at 55 ma interpreted as completion of suturing of greater india and asia india9s northward motion slowed from 18 19 5 cm yr to 4 5 cm yr for the location of site 758 comparison of this profile with paleomagnetic data from the wider himalayan region indicates that initial contact between northwestern greater india and southern asia was already established by cretaceous tertiary time this supports a possible causal link between the india asia convergence and the deccan traps extrusion

repeat ridge jumps associated with plume ridge interaction melt transport and ridge migration 1 repeated shifts or jumps of mid ocean ridge segments toward nearby hot spots can produce large long term changes to the geometry and location of the tectonic plate boundaries ridge jumps associated with hot spot ridge interaction are likely caused by several processes including shear on the base of the plate due to expanding plume material as well as reheating of lithosphere as magma passes through it to feed off axis volcanism to study how these processes influence ridge jumps we use numerical models to simulate 2 d in cross section viscous flow of the mantle viscoplastic deformation of the lithosphere and melt migration upward from the asthenospheric melting zone laterally along the base of the lithosphere and vertically through the lithosphere the locations and rates that magma penetrates and heats the lithosphere are controlled by the time varying accumulation of melt beneath the plate and the depth averaged lithospheric porosity we examine the effect of four key parameters magmatic heating rate of the lithosphere plate spreading rate age of the seafloor overlying the plume and the plume ridge migration rate results indicate that the minimum value of the magmatic heating rate needed to initiate a ridge jump increases with plate age and spreading rate the time required to complete a ridge jump decreases with larger values of magmatic heating rate younger plate age and faster spreading rate for cases with migrating ridges models predict a range of behaviors including repeating ridge jumps much like those exhibited on earth repeating ridge jumps occur at moderate magmatic heating rates and are the result of changes in the hot spot magma flux in response to magma migration along the base of an evolving lithosphere the tendency of slow spreading to promote ridge jumps could help explain the observed clustering of hot spots near the mid atlantic ridge model results also suggest that magmatic heating may significantly thin the lithosphere as has been suggested at hawaii and other hot spots

systematic insar tropospheric phase delay corrections from global meteorological reanalysis data 1 despite remarkable successes achieved by differential insar estimations of low tectonic strain rates remain challenging in areas where deformation and topography are correlated mainly because of the topography related atmospheric phase screen aps in areas of high relief empirical removal of the stratified component of the aps may lead to biased estimations of tectonic deformation rates here we describe a method to correct interferograms from the effects of the spatial and temporal variations in tropospheric stratification by computing tropospheric delay maps coincident with sar acquisitions using the era interim global meteorological model the modeled phase delay is integrated along vertical profiles at the era i grid nodes and interpolated at the spatial sampling of the interferograms above the elevation of each image pixel this approach is validated on unwrapped interferograms we show that the removal of the atmospheric signal before phase unwrapping reduces the risk of unwrapping errors in areas of rough topography

large fault fabric of the ninetyeast ridge implies near spreading ridge formation 1 ninetyeast ridge ner is a linear volcanic ridge in the indian ocean thought to have formed by hotspot volcanism on the northward drifting indian plate geological data from the ridge are sparse so its tectonic evolution is poorly known we studied satellite derived gravity data seismic reflection profiles and multibeam bathymetry to examine ner structure gravity data show that the ridge displays a series of nearly e w trending lineations with average spacing 0 4 45 km in seismic and bathymetry data these lineations correlate with horsts and grabens that probably formed near the time of ridge emplacement from their extensional nature and trends we infer that these faulted structures formed near the spreading ridge that separated the indian and antarctic plates and their ubiquity implies the hotspot was never far from this spreading ridge

late cretaceous cenozoic deformation of northeast asia abstract the plate tectonic paradigm implies rigid plates and narrow plate boundaries in contrast diffuse plate boundaries are common both in the oceans and continents r g gordon annu rev earth planet sci 26 1998 615 642 and their history is difficult to constrain especially in remote tectonically complex areas such as northeast asia m e chapman s c solomon j geophys res 81 1976 921 930 here we show how extensive north atlantic marine magnetic r macnab et al eos 76 1995 449 458 and gravity data d t sandwell w h f smith j geophys res 102 1997 10039 10054 can be used to unravel with tight confidence limits successive periods of deformation over 80 million years along the diffuse continental eurasian north american plate boundary a period of compression in the late cretaceous 14 mm yr in the laptev sea to 20 mm yr in kamchatka led to thrusting in the verkhoyansk mountains and was followed by extension from 68 to 40 ma when 400 km of extension was accommodated by the formation of a series of grabens including the moma rift system since 40 ma time varying compression and transpression along the moma rift system created strike slip faults thrusts and folds at rates up to 6 3 mm yr in the laptev sea region 600 km of extension from latest late cretaceous to present created the laptev sea and lena rift systems the deformation predicted by our model fits most geological features formed in the laptev sea and central northeast asia during late cretaceous cenozoic times the most recent deformation late miocene pliocene is not very well constrained since our model lacks data younger than 11 ma the deformation that occurred in kamchatka reflects a complex tectonic setting and our model s predictions are only tentative

sunda java trench kinematics slab window formation and overriding plate deformation since the cretaceous the kinematics and time dependence of back arc extension or compression is one of the most poorly understood aspects of plate tectonics and has nearly exclusively been studied from snapshots of present day observations here we combine absolute and relative plate motions with reconstructions of now subducted ocean floor to analyse subduction kinematics and upper plate strain from geological observations since 80 ma along the 3200 km long sunda java trench one of the largest subduction systems on earth combining plate motions and slab geometries enables us to reconstruct a time dependent slab window beneath sundaland formed through wharton spreading ridge subduction we find that upper plate advance and retreat is the main influence on upper plate strain but subduction of large bathymetric ridges and slab window effects also play a significant and at times dominant role compression in the sundaland back arc region can be linked to advance of the upper plate extension of the sundaland backarc region correlates with two patterns of upper plate motion a retreat of the upper plate and b advance of the upper plate combined with more rapid advance of the sundaland margin due to hinge rollback subduction of large bathymetric ridges causes compression in the upper plate especially wharton ridge subduction underneath sumatra over the period 15 0 ma our reconstructions unravel the evolving geometry of a slab window underlying the java south sumatra region and we propose that decreased mantle wedge viscosities associated with this slab window exacerbated palaeogene extension in the java sea region via active rifting and enabled sumatran continental extension to continue at 50 35 ma when upper plate advance would otherwise have led to compression

evolution of the late cretaceous crust in the equatorial region of the northern indian ocean and its implication in understanding the plate kinematics summary analysis of 3100 km of newly acquired marine magnetic data constrained by satellite and shipborne free air gravity anomalies in the corridor between the 86 e fracture zone and ninetyeast ridge north of the equator reveals the evolutionary history of the late cretaceous crust characterised by anomaly 34 through 31 83 5 68 7 ma under complex tectonic settings seafloor spreading model studies suggest that the crust particularly between the chrons 33r and 33 79 0 73 6 ma was formed with variable and slightly higher half spreading rates 4 8 7 1 cm yr than the crust of similar age either in the regions west of 86 e fracture zone east of the ninetyeast ridge or the southern crozet basin further the interpretation of magnetic anomalies suggests the presence of fossil spreading ridge segments and extra oceanic crust on the indian plate that has been transferred from the antarctica plate by discrete southward ridge jumps these ridge jumps are caused by thermal instability of the spreading center as the indian plate moved northward over the kerguelen mantle plume the present study indicates that the spreading ridge plume interaction is the prime mechanism for these ridge jumps which have occurred since 75 8 ma the newly identified magnetic anomalies 34 through 31 and the inferred n3 e trending fracture zones refined the plate reconstruction models for that period

on the derivation of coseismic displacement fields using differential radar interferometry the landers earthquake we present a map of the coseismic displacement field resulting from the landers california june 28 1992 earthquake derived using data acquired from an orbiting high resolution radar system we achieve results more accurate than previous space studies and similar in accuracy to those obtained by conventional field survey techniques data from the ers 1 synthetic aperture radar instrument acquired in april july and august 1992 are used to generate a high resolution wide area map of the displacements the data represent the motion in the direction of the radar line of sight to centimeter level precision of each 30 m resolution element in a 113 km by 90 km image our coseismic displacement contour map gives a lobed pattern consistent with theoretical models of the displacement field from the earthquake fine structure observed as displacement tiling in regions several kilometers from the fault appears to be the result of local surface fracturing comparison of these data with global positioning system and electronic distance measurement survey data yield a correlation of 0 96 thus the radar measurements are a means to extend the point measurements acquired by traditional techniques to an area map format the technique we use is 1 more automatic 2 more precise and 3 better validated than previous similar applications of differential radar interferometry since we require only remotely sensed satellite data with no additional requirements for ancillary information the technique is well suited for global seismic monitoring and analysis

gps velocity field for the tien shan and surrounding regions 1 measurements at 400 campaign style gps points and another 14 continuously recording stations in central asia define variations in their velocities both along and across the kyrgyz and neighboring parts of tien shan they show that at the longitude of kyrgyzstan the tarim basin converges with eurasia at 20 2 mm yr nearly two thirds of the total convergence rate between india and eurasia at this longitude this high rate suggests that the tien shan has grown into a major mountain range only late in the evolution of the india eurasia collision most of the convergence between tarim and eurasia within the upper crust of the tien shan presumably occurs by slip on faults on the edges of and within the belt but 1 3 mm yr of convergence is absorbed farther north at the dzungarian alatau and at a lower rate with the kazakh platform to the west the tarim basin is thrust beneath the tien shan at 4 7 mm yr with respect to eurasia the ferghana valley rotates counterclockwise at 0 7 myr 1 about an axis at the southwest end of the valley thus gps data place a bound of 4 mm yr on the rate of crustal shortening across the chatkal and neighboring ranges on the northwest margin of the ferghana valley and they limit the present day slip rate on the right lateral talas ferghana fault to less than 2 mm yr gps measurements corroborate geologic evidence indicating that the northern margin of the pamir overthrusts the alay valley and require a rate of at least 10 and possibly 15 mm yr

modeling of thrust fronts above ductile and frictional detachments application to structures in the salt range and potwar plateau pakistan series of scaled sandbox models are used to simulate the development of thin skinned simultaneous shortening above adjacent ductile and frictional substrates these models simulate the evolution of the potwar plateau and salt range in pakistan where pale

active faulting on the ninetyeast ridge and its relation to deformation of the indo australian plate 1 the 4500 km long ninetyeast ridge ner in the northeastern indian ocean crosses a broad zone of deformation where the indo australian plate is fracturing into three smaller plates india capricorn australia separated by diffuse boundaries whose extents are poorly defined new multichannel seismic reflection profiles image active faults along the entire length of the ner and show spatial changes in the style of deformation along the ridge the northern ner 0 n 5 n displays transpressional motion along wnw ese faults observed fault patterns confirm strike slip motion at the western extent of the april 2012 wharton basin earthquake swarm in the central ner 5 s 8 s deformation on wnw ese trending thrust faults implies nearly n s compression an abrupt change in fault style occurs between 8 and 11 s with modest extension characterizing the southern ner 11 s 27 s although extension is dominant narrow zones of faults with strike slip or compressional character also occur in the southern ner suggesting a complex combination of fault motions at all sites active faulting is controlled by the reactivation of original spreading center formed normal faults implying that deformation is opportunistic and focused along existing zones of weakness even when original fault trend is oblique to the direction of relative plate motion observed faulting can be interpreted as india australia deformation in the northern ner and capricorn australia deformation in the southern ner the india capricorn boundary is directly adjacent to the northern ner and this juxtaposition combined with a different style of faulting to the east of the ner imply that the ridge is a tectonic boundary

a little spin in the indian ocean plate circuit abstract divergence of the australian and east antarctic plates is well understood from the late jurassic onset of half graben development on the australian continental shelf and from post mid eocene chron 20 45 ma seafloor spreading isochrons further offshore relative plate motion between these times is less confidently interpretable from magnetic reversal anomalies landwards of isochron 20 and localised evidence for mid to late cretaceous subsidence and growth strata from the continental shelf and rise south of australia a new test of this history examines it within the post 34y 84 ma indian ocean plate circuit built using seafloor spreading data from the wharton and central indian ocean basins the australian antarctic jurassic onset rift system is interpreted to have been abandoned before 34y because motion in the circuit is inconsistent with an active plate boundary during 34y 26y 84 58 ma starting 26y the model depicts plate divergence distances and azimuths that after 25y 57 ma can be independently confirmed by reassessment of the pre chron 20 magnetic reversal anomaly pattern previous studies have identified evidence for mantle exhumation and focused magmatism in basement marginwards of these anomalies these processes are not directly or confidently dated but mantle exhumation is inconsistent with the circuit model s fast plate divergence at 26y 25y hence plate motion during the immediate build up to post 57 ma seafloor spreading may have been accommodated by focused magmatism whilst mantle exhumation may mark the conclusion of the jurassic onset rift phase during a slower pre 84 ma period of plate divergence using the new model to make tectonic reconstructions results in a large overlap between tasmania and victoria land that can be explained with reference to eocene strike slip faulting and transtension in recently discovered subglacial basins of wilkes and george v lands and terre adelie

structure and origin of the 85 e ridge the submerged 85 e ridge in the bay of bengal trends approximately n s between 19 n and 6 n latitudes off the southeast coast of sri lanka it takes an arcuate shape and seems to terminate with the northward extension of the afanasy nikitin seamounts situated around 2 s latitude the ridge is characterized by positive magnetic 100 400 nt and negative free air gravity 60 mgal anomalies with variable widths of 100 180 km magnetic model studies revealed that the rocks of the 85 e ridge are magnetized with reversed polarity the well defined geophysical anomalies and lack of magnetic polarity reversals together with the deep burial nature of the ridge may not favor a hotspot origin two alternative processes for the ridge emplacement have been suggested ridge emplacement may be 1 due to shearing of the lithosphere caused by stretching and compressional forces associated at the time of major plate reorganization immediately after the evolution of the early cretaceous crust in the bay of bengal more precisely at m0 isochron or during the middle albian reversals within the cretaceous long normal polarity k t superchron epoch when the earth s magnetic polarity changed from normal to reversed polarity and or 2 due to sagging followed by deformation produced by the buckling instability of the oceanic plate caused by horizontal compressional forces on the passive continental margin however more marine geophysical data are required to support the postulated coincidence of the ridge with a reversed polarity magnetic anomaly and their associated model further the rajmahal traps normally polarized and the 85 e ridge reversely polarized appear to be associated with two different episodes of eruption that might have been triggered by the kerguelen mantle plume the 85 e ridge seems to extend into the onshore west bengal basin as a subsurface ridge and merges with the reported nne ssw trending zone of strong geophysical anomalies east of rajmahal traps up to 25 n latitude

a revised identification of the oldest sea floor spreading anomalies between australia and antarctica we propose that magnetic anomalies south of australia and along the conjugate margin of antarctica that were originally identified as anomalies 19 to 22 may be anomalies 20 to 34 the original anomaly identification has two troublesome aspects 1 it does not account for an extra anomaly between anomalies 20 and 21 and 2 it provides no explanation for the rough topography comprising the diamantina zone with our revised identification there is no extra anomaly and the diamantina zone is attributed to a period of very slow spreading 4 5mm yr half rate between 90 and 43 m y the ages bounding the interval of slow spreading 90 and 43 m y correspond to times of global plate reorganizations our revised identification opens up the possibility that part of the magnetic quiet zone south of australia formed during the cretaceous long normal polarity interval breakup of australia and antarctica probably occurred sometime between 110 and 90 m y b p the breakup unconformity identified by falvey in the otway basin may correspond to a eustastic sea level change

detailed structure and plate reconstructions of the central indian ocean between 83 0 and 42 5 ma chrons 34 and 20 the central indian ocean namely the central indian crozet and madagascar basins formed by rifting and subsequent drifting of india now capricorn antarctica and africa now somalia we gathered a comprehensive set of sea surface magnetic anomaly profiles over these basins and revisited location and identification of magnetic isochrons between c34ny 83 0 ma and c20ny 42 536 ma using the objective analytic signal technique we present high resolution magnetic isochrons for 29 periods based on 1 400 magnetic anomaly picks from the conjugate sets of picks we derive two plate finite rotation parameters for both the capricorn antarctica and capricorn somalia motions these finite rotations are compared to three plate reconstructions of the plate boundaries between the capricorn antarctica and somalia plates constrained by the closure of the indian ocean triple junction in general the three plate reconstructions slightly overrotate the reconstructed isochrons with respect to the capricorn antarctica and capricorn somalia two plate reconstructions conversely the two plate reconstructions for somalia antarctica slightly underrotate the isochrons compared to the capricorn antarctica somalia three plate reconstructions we suspect that the discrepancies between the two plate and three plate methods result from the recent seafloor deformation in the capricorn india diffuse plate boundary and or from the contrasted nature and geometry of magnetic isochrons at different spreading rates i e magnetic structure of the three spreading centers three plate reconstructions better constrain the closure of the triple junction but spread any misfit among all three plate boundaries when enough quality data are available two plate reconstructions may lead to more realistic plate motion estimates from which additional geological problems can be identified and solved

spatial variations in isostatic compensation mechanisms of the ninetyeast ridge and their tectonic significance the ninetyeast ridge ner one of the longest linear volcanic features on the earth offers an excellent opportunity of understanding the isostatic response to the interactions of mantle plume with the migrating mid ocean ridge bathymetry geoid and gravity ship borne and satellite data along 72 closely spaced transects and 17 overlapping grids on the ner are analyzed and modeled to determine the effective elastic thickness te beneath the entire ridge the results of 2 d and 3 d flexural modeling of the ner show large spatial variations in te values ranging from 4 to 35 km suggesting that the ridge was compensated along its length by different isostatic mechanisms the southern south of 22 s latitude and northern north of 2 n latitude parts of the ner have te values of 10 and 23 km respectively revealing that the southern part was emplaced on a lithosphere of intermediate strength possibly on flank of the indian plate whereas the northern part was emplaced in an intraplate setting in contrast in the central part of the ner between latitudes 22 s and 2 n highly variable te values 4 22 km are estimated the scattered te values in the central ner suggest that this part may have evolved due to the occurrence of more frequent ridge jumps caused by the interaction of kerguelen hot spot with rapid northward migration of the wharton spreading ridge residual mantle bouguer anomaly rmba map of the ner and adjacent basins reveals that the entire length of the ner is associated with a significant negative anomaly up to 200 mgal indicating the presence of thickened crust or less dense mantle beneath the ridge 3 d crustal thickness map of the ner generated by inversion of the rmba data shows a thick crust ranging from 15 to 19 km the present study clearly shows that ner possesses a highly segmented isostatic pattern with the occurrence of sub crustal underplating or sub surface loading

twenty years of deep seismic reflection profiling in germany a contribution to our knowledge of the nature of the lower variscan crust near vertical reflection studies in germany have concentrated on the following objectives 1 the nature of crustal reflectors 2 a detailed velocity structure 3 zones of increased tectonic activity e g faults approximately 20 years ago the identity between reflectors in the near vertical and the wideangle range was established wide angle events being strongly influenced by velocity gradients a pronounced but limited low velocity body below the urach geothermal anomaly was detected by using an especially long spread length an alternating polarity of deep crustal reflectors could be demonstrated a deep fault zone cutting the whole crust at steep angles and shallow angle overthrust structures were detected in the variscan area there is generally a strong reflectivity in the variscan lower crust between the conrad level at 6 s two way travel time i e 18 km and the moho at 9 10 s i e 27 30 km depth the lower crust seems to consist of thin lenses or lamellae with alternating velocity they were probably generated by an extensive synand post orogenic melting process leaving behind mafic and ultramafic splinters in the lower crust and a compact ultramafic residue in the uppermost mantle the moho being formed as a new and uniform boundary the poorly reflective upper crust is probably dominated by vertically oriented plutons in a rigid environment

the early opening between broken ridge and kerguelen plateau abstract reconstructions to total closure of the australia antarctic continents causes an unacceptable overlap of broken ridge and kerguelen plateau this has been partially resolved in the past by supposing that the northern part of kerguelen that is principally involved in the overlap is younger than the remainder we have revised the early reconstructions using a newly proposed breakup chronology of australia and antarctica which suggests that opening began at least 90 m y b p at an initially slow rate this eliminates the overlap problem without invoking major age differences within the kerguelen plateau we also suggest that the northeastern flank of kerguelen plateau may be underlain by the missing westward continuation of the diamantina zone it may have been isolated on the antarctic plate by a ridge crest jump at about anomaly 24 time that also formed the ob trench

the shaping of salt diapirs abstract six parameters shape the geometry of passive diapirs associated with stiff overburden rates of salt supply s dissolution d sediment accumulation a erosion er extension e and shortening sh these parameters change in space and time and hence influence the geometry of the structure as it forms a complex six parameter plot representing the evolution history of a salt diapir can be simplified into three separate graphs this study recommends adding to plots of s against a a second of rate of salt supply s this time with extension rate e and a third of rate of sediment accumulation a plotted against extension rate e integrating these three plots on a single diagram results in a complete description of the evolution history of a diapir when it is applied to three dimensional data however if applied to a profile the plot shows only the two dimensional evolution history of the diapir lateral forces extension or compression have a significant role in moulding the geometry of a salt diapir by influencing the space which it occupies by incorporating extension in the moulding plots of salt diapirs this study introduces the rate of sediment accumulation multiplied by extension e a as a significant factor in moulding salt diapirs by using this rate against the rate of salt supply this study redefines the conventionally accepted interpretation of upward narrowing upward widening and columnar diapirs upward narrowing diapirs form when the rate of salt supply is less than the rate of sediment accumulation multiplied by extension upward widening diapirs form when the salt supply is greater than the rate of sediment accumulation multiplied by extension columnar diapirs form when the rate of salt supply is equal to the rate of sediment accumulation multiplied by extension this new relationship explains the absence of columnar and upward widening diapirs in passive margins where thin skinned extension dominates and emphasizes the significance of lateral movement extension and shortening in moulding the geometry of salt diapirs

emplacement of afanasy nikitin seamount based on transfer function analysis of gravity and bathymetry data abstract the nature of compensation of the afanasy nikitin seamount in the central indian ocean lithosphere is examined through transfer function analysis of gravity and bathymetry data along five profiles across the seamount the seamount appears to be regionally compensated by the bending of an elastic plate of only 2 5 km thickness from this low elastic thickness it is inferred that the seamount is formed near a spreading axis transform junction

geoid and gravity anomaly data of conjugate regions of bay of bengal and enderby basin new constraints on breakup and early spreading history between india and antarctica 1 timing of breakup of the indian continent from eastern gondwanaland and evolution of the lithosphere in the bay of bengal still remain as ambiguous issues geoid and free air gravity data of bay of bengal and enderby basin are integrated with shipborne geophysical data to investigate the early evolution of the eastern indian ocean geoid and gravity data of the bay of bengal reveal five n36 w fracture zones fzs and five isolated ne sw structural rises between the eastern continental margin of india ecmi and the 85 e ridge 86 e fz the fzs meet the 86 e fz at an angle of 39 the rises are associated with low gravity and geoid anomalies and are oriented nearly orthogonal to the fzs trend the geoid and gravity data of the western enderby basin reveal a major kerguelen fz and five n4 e fzs the fzs discretely converge to the kerguelen fz at an angle of 37 we interpret the fzs identified in bay of bengal and western enderby basin as conjugate fzs that trace the early cretaceous rifting of south ecmi from enderby land structural rises between the fzs of bay of bengal may either represent fossil ridge segments possibly have extinct during the early evolution of the bay of bengal lithosphere or may have formed later by the volcanic activity accreted the 85 e ridge two different gravity signatures short wavelength high amplitude negative gravity anomaly and relatively broader low amplitude negative gravity anomaly are observed on south and north segments of the ecmi respectively the location of continent ocean boundary cob is at relatively far distance 100 200 km from the coastline on north ecmi than that 50 100 km on the south segment on the basis of geoid gravity and seismic character and orientation of conjugate fzs in bay of bengal and western enderby basin we believe that transform motion occurred between south ecmi and enderby land at the time of breakup which might have facilitated the rifting process in the north between combined north ecmi elan bank and macrobertson land and in the south between southwest sri lanka and gunnerus ridge region of east antarctica approximately during the period between the anomalies m1 and m0 and soon after detachment of the elan bank from north ecmi the rifting process possibly had reorganized in order to establish the process along the entire eastern margins of india and sri lanka