tectonic interactions between india and arabia since the jurassic reconstructed from marine geophysics ophiolite geology and seismic tomography gondwana breakup since the jurassic and the northward motion of india toward eurasia were associated with formation of ocean basins and ophiolite obduction between and onto the indian and arabian margins here we reconcile marine geophysical data from preserved oceanic basins with the age and location of ophiolites in nw india and se arabia and seismic tomography of the mantle below the nw indian ocean the north somali and proto owen basins formed due to 160 133 ma n s extension between india and somalia subsequent convergence destroyed part of this crust simultaneous with the uplift of the masirah ophiolites most of the preserved crust in the owen basin may have formed between 84 and 74 ma whereas the mascarene and the amirante basins accommodated motion between india and madagascar east africa between 85 and circa 60 ma and 75 and circa 66 ma respectively between circa 84 and 45 ma oblique arabia india convergence culminated in ophiolite obduction onto se arabia and nw india and formed the carlsberg slab in the lower mantle below the nw indian ocean the nne ssw oriented slab may explain the anomalous bathymetry in the nw indian ocean and may be considered a paleolongitudinal constraint for absolute plate motion nw india asia collision occurred at circa 20 ma deforming the sulaiman ranges or at 30 ma if the hindu kush slab north of the afghan block reflects intra asian subduction our study highlights that the nw india ophiolites have no relationship with india asia motion or collision but result from relative india africa arabia motions instead key points we present a new tectonic model for the evolution of nw indian ocean subducted slab under the carlsberg ridge resulted from arabia india convergence

discovery of the palaeo indus delta fan complex the indus river carries sediments from the western himalaya and deposits some of these as channel and floodplain sediments or molasse the rest of its load forms the indus delta at the margin of the indian ocean the indus delta passes some of its sediments to be deposited as indus submarine fan turbidites thus as elsewhere himalayan molasse delta and fan deposition are related hi tune and space however when we examine fluvial and marine age range data of the older indus deposits in terms of this sedimentary assemblage a major portion of its marine record is missing the oldest known molasse along the indus suture zone and hi the foredeep are middle eocene and late palaeocene hi age respectively a recent synthesis of sedimentation in the northern indian ocean however shows that turbidite sedimentation started around early miocene in the modern indus fan and even later in the bengal fan where are the palaeogene indus delta and fan sediments we suggest herein that these are preserved as the palaeogene siliciclastic khojak formation hi the katawaz basin and eastern makran the modern indus river is 2900 km long coumes kolla 1984 and has 30 000 m 3 s 1 discharge milliman et al 1984 in summer fig 1 twice that of the mississippi river the annual suspended sediment load wells coleman 1984 varies between 395 and 435 x 10 3 t almost equal to that of the mississippi the existence of the middle eocene to middle miocene indus molasse along the

formation and tectonic evolution of the cretaceous jurassic muslim bagh ophiolitic complex pakistan implications for the composite tectonic setting of ophiolites the muslim bagh ophiolitic complex balochistan pakistan is comprised of an upper and lower nappe and represents one of a number of ophiolites in this region which mark the boundary between the indian and eurasian plates these ophiolites were obducted onto the indian continental margin around the late cretaceous prior to the main collision between the indian and eurasian plates the upper nappe contains mantle sequence rocks with numerous isolated gabbro plutons which we show are fed by dolerite dykes each pluton has a transitional dunite rich zone at its base and new geochemical data suggest a similar mantle source region for both the plutons and dykes in contrast the lower nappe consists of pillow basalts deep marine sediments and a me lange of ophiolitic rocks the rocks of the upper nappe have a geochemical signature consistent with formation in an island arc environment whereas the basalts of the lower nappe contain no subduction component and are most likely to have formed at a mid ocean ridge the basalts and sediments of the lower nappe have been intruded by oceanic alkaline igneous rocks during the northward drift of the indian plate the two nappes of the muslim bagh ophiolitic complex are thus distinctively different in terms of their age lithology and tectonic setting the recognition of composite ophiolites such as this has an important bearing on the identification and interpretation of ophiolites where the plate tectonic setting is less well resolved

global marine gravity from retracked geosat and ers 1 altimetry ridge segmentation versus spreading rate 1 three approaches are used to reduce the error in the satellite derived marine gravity anomalies first we have retracked the raw waveforms from the ers 1 and geosat gm missions resulting in improvements in range precision of 40 and 27 respectively second we have used the recently published egm2008 global gravity model as a reference field to provide a seamless gravity transition from land to ocean third we have used a biharmonic spline interpolation method to construct residual vertical deflection grids comparisons between shipboard gravity and the global gravity grid show errors ranging from 2 0 mgal in the gulf of mexico to 4 0 mgal in areas with rugged seafloor topography the largest errors of up to 20 mgal occur on the crests of narrow large seamounts the global spreading ridges are well resolved and show variations in ridge axis morphology and segmentation with spreading rate for rates less than about 60 mm a the typical ridge segment is 50 80 km long while it increases dramatically at higher rates 100 1000 km this transition spreading rate of 60 mm a also marks the transition from axial valley to axial high we speculate that a single mechanism controls both transitions candidates include both lithospheric and asthenospheric processes

effects of latent heat release at phase boundaries on flow in the earth s mantle phase boundary topography and dynamic topography at the earth s surface abstract mantle flow models that do not consider the effects of latent heat on phase boundaries typically predict dynamic surface topography too large to be compatible with observations here these effects were implemented in a mantle flow model and resulting changes in dynamic topography and topography of phase boundaries were computed inclusion of these effects was found to reduce the rms amplitude of dynamic topography by about 50 60 m still leaving a substantial misfit from observations in most cases considered generally it also leads to a slight improvement of correlation with dynamic surface and phase boundary topography constrained by observations results thus indicate that the model implemented here is applicable to the earth but that other effects need to be considered as well in order to fully explain phase boundary and dynamic surface topographies

breakup and early seafloor spreading between india and antarctica summary we present a tectonic interpretation of the breakup and early seafloor spreading between india and antarctica based on improved coverage of potential field and seismic data off the east antarctic margin between the gunnerus ridge and the bruce rise we have identified a series of ene trending mesozoic magnetic anomalies from chron m9o 130 2 ma to m2o 124 1 ma in the enderby basin and m9o to m4o 126 7 ma in the princess elizabeth trough and davis sea basin indicating that india antarctica and india australia breakups were roughly contemporaneous we present evidence for an abandoned spreading centre south of the elan bank microcontinent the estimated timing of its extinction corresponds to the early surface expression of the kerguelen plume at the southern kerguelen plateau around 120 ma we observe an increase in spreading rate from west to east between chron m9 and m4 38 54 mm yr 1 along the antarctic margin and suggest the tectono magmatic segmentation of oceanic crust has been influenced by inherited crustal structure the kinematics of gondwanaland breakup and the proximity to the kerguelen hotspot a high amplitude e w oriented magnetic lineation named the mac robertson coast anomaly mca coinciding with a landwards step down in basement observed in seismic reflection data is tentatively interpreted as the boundary between continental transitional zone and oceanic crust the exposure of lower crustal rocks along the coast suggests that this margin formed in a metamorphic core complex extension mode with a high strength ratio between upper and lower crust which typically occurs above anomalously hot mantle together with the existence of the mca zone this observation suggests that a mantle temperature anomaly predated the early surface outpouring steady state magmatic production of the kerguelen lip an alternative model suggests that the northward ridge jump was limited to the elan bank region whereas seafloor spreading continued in the west enderby basin and its sri lankan conjugate margin in this case the mca magnetic anomaly could be interpreted as the southern arm of a ridge propagator that stopped around 120 ma

multiple episodes of continental subduction during india asia convergence insight from seismic tomography and tectonic reconstruction abstract high wavespeed tomographic anomalies shallower than 1100 km beneath the india asia collision zone are interpreted as continental slabs subducted during collision combining anomaly positions with paleogeographic reconstructions of india we constrain the spatio temporal evolution of multiple episodes of continental subduction likely related to these anomalies this study highlights the different evolution at lithospheric scales of the western and eastern parts of the collision zone the evolution of the western part is characterized by two episodes of steep subduction of the northern margin of india the first episode involving an area with a lateral extent as large as 1500 km started at about 40 30 and ended by a slab break off process at 15 ma the second episode consists on subduction beneath the hindu kush mountains since 8 ma to the east of the collision zone no anomaly related to steep subduction along the northern edge of india is found we interpret two tomographic anomalies beneath the eastern border of the indian plate beneath burma and beneath the andaman sea as the result of two successive episodes of southeastward extrusion followed by subduction we suggest that both extruded portions were initially located along the northern margin of india and that they slid around the eastern syntaxis then southward along the eastern boundary of indian plate both portions subducted along the eastern border of india south of the eastern syntaxis we provide a rough estimate of the amount of indian lithosphere consumed during these subduction and extrusion episodes by comparing this amount with the total amount of indian lithosphere at the onset of collision we conclude that these processes accommodated most of the india asia convergence during collision

seismic data geometry evolution and shortening in the active sulaiman fold and thrust belt of pakistan southwest of the himalayas surface and subsurface data from the sulaiman fold and thrust belt are integrated to analyze the deep structure tectonic shortening and kinematics of the sulaiman fold and thrust belt at the western margin of the indian subcontinent seismic reflection data show that nearly all the 10 km thick sequence of dominantly platform 7 km and molasse strata is detached at the deformation front these strata thicken tectonically to about 20 km in the hinterland without significant thrust faults in the foreland a balanced structural cross section suggests that structural uplift in the sulaiman fold and thrust belt is a result of a thin skinned passive roof duplex style of deformation the duplex sequence consists of jurassic and older strata it is separated from the roof equence by a passive roof thrust in thick cretaceous shales that has a backthrust sense of displacement over foreland propagating duplexes the roof sequence with incipient out of sequence thrusting is intact for about 150 km northwards where it emerges along a passive backthrust sequential restoration of the balanced section reveals a series of structural and geometrical features including 1 development of low amplitude broad concentric detachment folds sui and loti at the tip of the decollement 2 increase in amplitude of a detachment fold to a critical level for development of ramp and duplex structures and 3 out of sequence thrusting to create required critical taper for an outward translation of the foreland fold and thrust belt a balanced structural cross section 349 km long from the sulaiman fold and thrust belt restores to an original length of 727 km suggesting a maximum of 378 km of shortening since 21 ma in the cover strata of the indian subcontinent calculation of displacement rates over the sulaiman fold and thrust belt 18 mm yr added to the resolved rate of the chaman fault vector for the component parallel to the plate convergence direction 15 mm yr are close to the current india asia plate convergence rate 37 mm yr

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

towards absolute plate motions constrained by lower mantle slab remnants the movement of tectonic plates before the cretaceous period is poorly understood a global mantle tomography model suggests that the longitude of oceanic subduction zones up to 300 million years ago was offset by up to 18 compared with tectonic reconstructions for the same period

locating earliest records of orogenesis in western himalaya evidence from paleogene sediments in the iranian makran region and pakistan katawaz basin a combination of sediment petrography detrital zircon u pb and fission track dating is used to show that provenance of the paleogene sedimentary rocks exposed in the makran region of southern iran and the katawaz basin of pakistan is consistent with a source from the nascent western himalaya and associated magmatic arc results from this important archive show that paleogene erosion was focused mainly on the arc and northern indian margin and in contrast to the east central himalaya we have not detected widespread rapid exhumation indicative of strong forcing by climate driven erosion at that time

cenozoic subsidence and uplift of continents from time varying dynamic topography time varying global dynamic topography has been computed for the entire cenozoic the spherical viscous flow model is driven by a distribution of density heterogeneity based on mesozoic and cenozoic subduction by using the calculated topography the uplift and subsidence histories of five regions fixed to the north american indian australian indonesian and south american continents are computed predicted vertical motions are in qualitative agreement with observed values except for india and south america for australia and north america the predicted periods of uplift and subsidence are out of phase with respect to observations but display the correct overall trend the latest phase of uplift for north america is directly related to the cessation of kula plate subduction after 48 ma dynamic topography depends on the viscosity structure of the mantle a lower mantle that is 50 times more viscous than the upper mantle yields the best agreement between modeled and predicted vertical motions the presence of a low viscosity channel beneath the lithosphere has only a small influence on our results compared to dynamic topography the time varying geoid is insignificant in controlling relative sea level

ophiolite genesis and global tectonics geochemical and tectonic fingerprinting of ancient oceanic lithosphere ophiolites and discussions on their origin and significance in earth s history have been instrumental in the formulation testing and establishment of hypotheses and theories in earth sciences the definition tectonic origin and emplacement mechanisms of ophiolites have been the subject of a dynamic and continually evolving concept since the nineteenth century here we present a review of these ideas as well as a new classification of ophiolites incorporating the diversity in their structural architecture and geochemical signatures that results from variations in petrological geochemical and tectonic processes during formation in different geodynamic settings we define ophiolites as suites of temporally and spatially associated ultramafic to felsic rocks related to separate melting episodes and processes of magmatic differentiation in particular tectonic environments their geochemical characteristics internal structure and thickness vary with spreading rate proximity to plumes or trenches mantle temperature mantle fertility and the availability of fluids subduction related ophiolites include suprasubduction zone and volcanic arc types the evolution of which is governed by slab dehydration and accompanying metasomatism of the mantle melting of the subducting sediments and repeated episodes of partial melting of metasomatized peridotites subduction unrelated ophiolites include continental margin mid ocean ridge plume proximal plume distal and trench distal and plume type plume proximal ridge and oceanic plateau ophiolites that generally have mid ocean ridge basalt morb compositions subduction related lithosphere and ophiolites develop during the closure of ocean basins whereas subduction unrelated types evolve during rift drift and seafloor spreading the peak times of ophiolite genesis and emplacement in earth history coincided with collisional events leading to the construction of supercontinents continental breakup and plume related supermagmatic events geochemical and tectonic fingerprinting of phanerozoic ophiolites within the framework of this new ophiolite classification is an effective tool for identification of the geodynamic settings of oceanic crust formation in earth history and it can be extended into precambrian greenstone belts in order to investigate the ways in which oceanic crust formed in the archean

nappe stacking resulting from subduction of oceanic and continental lithosphere below greece we quantitatively investigate the relation between nappe stacking and subduction in the aegean region if nappe stacking is the result of the decoupling of upper crustal parts 5 10 km thick from subducting lithosphere then the amount of convergence estimated from balancing the nappe stack provides a lower limit to the amount of convergence accommodated by subduction the balanced nappe stack combined with the estimated amount of completely subducted lithosphere indicates 700 km of jurassic and 2400 km of post jurassic convergence from seismic tomographic images of the underlying mantle we estimate 2100 2500 km of post jurassic convergence we conclude that 1 the imaged slab represents the subducted lithosphere that originally underlay the nappes 2 since the early cretaceous subduction in the aegean has occurred in one single subduction zone and 3 the composition of the original basement of the nappes indicates that at least 900 km of sub upper crust continental lithosphere subducted in the aegean

cenozoic kinematic history of the kohistan fault in the pakistan himalaya detailed mapping suggests a new scenario for the kinematic history of the late cretaceous kohistan arc complex and for the cause of cenozoic metamorphism and exhumation along the indian continental margin in pakistan the earliest deformational phases resulted in early cenozoic amphibolite facies metamorphism superposed tight to isoclinal f1 f2 folds and development of strong penetrative rock fabric within the indian plate in response to southwestward and then southeastward overthrusting of ophi olitic melange in the indus suture zone and west pakistan fold belt respectively this deformation included the development of the banna and malakand thrust faults prograde metamorphism ended following this deformational phase and the early structures were deformed along n trending open upright f3 folds and domes this was followed by ese to e directed thrusting of the kohistan arc complex onto the indian continent along the dominantly brittle kohistan fault thrusting ended in the late oligocene early miocene when the kohistan arc assumed its present position along the northern margin of india south verging structures began to form on the indian plate for the first time in the late oligocene early miocene deformation had expanded to the foreland by the middle to late miocene with development of the panjal khairabad thrust neogene structures in the metamorphic zone include n trending east side up high angle fault zones that deform or reactivate the kohistan fault and nw trending east side up reverse faults both of which are seismically active in the tectonic scenario outlined here shortening along the kohistan fault occurred subsequent to metamorphism and was directed toward the ese e throughout its history the southern termination of the fault is considered to be a right slip sidewall ramp such that the kohistan arc never advanced much father south on the indian plate than its present position broad flexure folding and erosion in front of and south of the actively advancing kohistan arc complex contributed to exhumation of the indian plate this differs from interpretations that state that kohistan was thrust in a southward direction that underthrusting beneath kohistan was the primary cause of early cenozoic metamorphism on the indian plate and that top side north extensional reactivation of the kohistan fault contributed to exhumation

petrology of the dykes from the waziristan ophiolite nw pakistan abstract the waziristan ophiolite is located in the suture zone between the indian plate to the east and afghan block to the west it is highly dismembered and divisible into three main sheets or nappes which from east to west are the vezhda sar nappe entirely comprised of pillow basalts the boya nappe made up of ophiolitic melange with an intact section in its basal part and the datta khel nappe consisting mainly of sheeted dykes with smaller proportions of other components faunal evidence suggests that the ophiolite is of tithonian valanginian age it was thrust over the mesozoic shelf slope sediments of the indian plate to the east during the paleocene and is unconformably overlain by sedimentary rocks of early to middle eocene age to the west beside the sheeted dykes best exposed in the hanging wall of the datta khel thrust ene of datta khel the ophiolite also contains isolated dykes these are doleritic and basaltic in composition the dykes contain high na 2 o contents and high feo t mgo and lile hfse ratios and low tio 2 2 o contents non depletion of nb and high lile hfse ratio negate respectively an island arc or mid ocean ridge setting for these dykes enrichment in the lile suggests the involvement of a crustal component driven by fluids along the subduction zone several geochemical parameters suggest that the dykes of waziristan ophiolite have transitional characteristics between mid ocean ridge basalt and island arc tholeiite it is therefore proposed that these dykes may have originated in a back arc basin tectonic setting

lithofacies depositional environments regional biostratigraphy and age of the chitarwata formation in the bugti hills balochistan pakistan abstract the oligocene early miocene chitarwata formation records a critical interval of terrestrial sedimentation that predates the siwalik deposits on the potwar plateau of north central pakistan this oligocene early miocene time interval has long been considered as lacking in the entire indo pakistan region the chitarwata formation is widely exposed in the sulaiman range but has never been described in detail in the sulaiman lobe where the famous fossiliferous strata called bugti bone beds have been known for over a century and half the chitarwata formation represents coastal delta at the base and plain and fluvial environments at the top lithofacies and sedimentary structures of the chitarwata formation in the bugti area are described in detail and show a clearly distinct lithologic pattern different from that reported from the zinda pir area the chitarwata formation also records an important transition in the evolution of the drainage systems in the area during the late paleogene and early neogene this transition from the west flowing paleo indus fluvial system to the development of the ancestral indus drainage system may explain the numerous hiatuses that characterize the chitarwata formation the abundance of fossil mammals from the chitarwata and overlying vihowa formation in the bugti hills provides critical biochronologic information that sheds new light on biostratigraphic correlation with the zinda pir area and for the entire sulaiman range

bela oceanic lithosphere assemblage and its relation to the reunion hotspot the bela ophiolite of pakistan contains a complete ophiolite accretionary wedge trench sequence emplaced onto the indian continental margin during the northward drift of india seychelles over the active reunion hotspot a structurally higher ophiolite overlies an accretionary prism which is thrust over a foreland basin shear sense determinations in peridotite mylonites in the ophiolite footwall and imbrication structures in the underlying accretionary wedge indicate an ese emplacement sedimentary rocks in the accretionary wedge indicate aptian albian pillow lavas initially deep water conditions and increasing influence from the continent until the maastrichtian the ophiolite emplacement was predated and accompanied by fe tholeiitic and alkaline magmatism related to the reunion hotspot and continuous incorporation of trench sediments into the accretionary wedge 39ar 40ar dating shows that the ophiolite formed around 70 ma intraoceanic subduction initiated between 70 and 65 ma obduction onto the indian passive margin occurred during the formation of the deccan traps at 66 ma and final thrusting onto the continental margin ended in the early eocene 50 ma the ophiolite emplacement occurred during the counterclockwise separation of madagascar and india seychelles which caused shortening and consumption of oceanic lithosphere between the african arabian and the indian seychelles plates

late cretaceous india madagascar fit and timing of break up related magmatism a u pb zircon age of 91 2 0 2 myr from western india st mary islands confidently links india with the late cretaceous magmatic province in madagascar 84 92 ma and the u pb age is within analytical error of the u pb age of the analalava gabbro pluton 91 6 0 3 myr in northeastern madagascar palaeomagnetic data from india and madagascar allow us to postulate a new india madagascar fit euler latitude 14 24 longitude 38 8 and rotation angle 69 2 this fit is applicable to the late cretaceous directly prior to and during the early phase of madagascar india separation in our late cretaceous reconstruction south west india runs roughly subparallel with the first known break up related magnetic anomaly a34 it maintains a close connection between mada gascar and india but places india slightly rotated compared to the eastern margin of madagascar and more northerly compared with some reconstructions st mary magmatism is linked to the initial break up between india and madagascar and magmatism probably resulted from rift related extensional processes initially induced by the marion hotspot underlying southern madagascar during the late cretaceous

the tectonic setting of the seychelles mascarene and amirante plateaus in the western equatorial indian ocean abstract a granitic crustal fragment occurs within a system of the marine plateaus in the western equatorial indian ocean the plateaus form an arcuate series of wide shallow banks with small islands in places in spite of the geographical proximity and the morphological similarity the geological structure and origin of the seychelles bank amirante arc and mascarene plateau contrast sharply the seychelles bank is founded on precambrian granite basement the mascarene plateau from saya de malha bank to mauritius island is founded on paleocene basalts and upper cretaceous basalts prevail in the amirante arc geological and geophysical data suggest that the precambrian granite of the seychelles bank is a fragment separated from the indian continental plate during the paleocene this break up was associated with the volcanic activity that built the mascarene plateau the occurrence of middle eocene syenite diorite and microgranite in the western part of the seychelles bank could indicate that crustal accretion across the northwestern indian ocean ridge apparently drove the seychelles block southwestwards causing it to collide with the northeastern edge of the mascarene basin this collision could have led to an early tertiary subduction and subsequently to the development of the amirante trough the absence of post eocene igneous suites in the seychelles could indicate that a change in the spreading pattern of the northwestern indian ocean during the late eocene probably terminated the amirante subduction

a new global marine magnetic anomaly data set the geophysical data system geodas stores more than 20 million magnetic measurements acquired over oceans and seas since the 1950s usually the original total field tf and magnetic anomaly values are both stored the anomaly results from the subtraction of the core and external magnetic field estimates from tf values the most recent international geomagnetic reference field models available at the time of the surveys were used to estimate the core field component these models were revised later external fields were estimated from magnetic observatory data however most of the measurements were not corrected for the external fields here we use comprehensive models to properly remove the core and external magnetic fields from all original tf measurements stored in the geodas besides a track by track analysis of each data is necessary mainly to correct or to remove many shifted values as well as to reduce the noise in some track lines two additional processes are applied to obtain a data set coherent over the world it includes an adjustment of long wavelength magnetic anomalies using the national geophysical data center ngdc 720 model plus a line leveling method which mainly reduced some inconsistencies between different surveys the root mean square of the crossover differences was reduced from 179 6 to 35 9 nt comparisons of magnetic anomaly maps before and after our treatment also highlight an improvement in the quality and the coherence of the data set this study will serve to build a new world digital magnetic anomaly map

4 d evolution of se asia s mantle from geological reconstructions and seismic tomography abstract how the collision between india and asia is related to processes deeper in the mantle is unclear here we compare geological reconstructions of block motions within asia since 50 ma with the tomographically imaged three dimensional 3 d morphology of subducted lithosphere to obtain insight into the spatiotemporal evolution of mantle structure past positions of the convergent margin show remarkable similarities with slab geometry at specific depths the striking change in slab geometry from a linear structure beneath 1100 km to an increasingly distorted shape at depths of less than 700 km results from collision the slab contours match the progressive deformation of asia s margin including india s indentation and sundaland s extrusion ever since the onset of collision the indian plate appears to have overridden its own sinking mantle and it does not seem at present to underthrust tibet significantly north of the zangbo suture if correct this observation would provide further evidence against models of plateau build up involving indian lithosphere the tomographic images beneath india confirm that asian deformation has absorbed at least 1500 km of convergence since collision began from the match between the southeastward motion of sundaland between 40 and 20 ma and the principal change in slab structure between 700 and 1100 km depths we infer that lateral advection in the mantle is small and that the sinking rate beneath sunda was 2 cm yr in the lower mantle and 5 cm yr above the transition zone

a mesozoic time scale we present an integrated geomagnetic polarity and stratigraphic time scale for the triassic jurassic and cretaceous periods of the mesozoic era with age estimates and uncertainty limits for stage boundaries the time scale uses a suite of 324 radiomenc dates including high resolution 40 ar 39 ar age estimates this framework involves the observed ties between 1 radiometric dates biozones and stage boundaries and 2 between biozones and magnetic reversals on the seafloor and in sediments interpolation techniques include maximum likelihood estimation smoothing cubic spline fitting and magnetochronology

cretaceous sedimentary rocks on the masirah ophiolite sultanate of oman evidence for an unusual bathymetric history autochthonous sedimentary rocks on the masirah ophiolite preserve a record of vertical movements of their oceanic basement the masirah ophiolite was formed in the uppermost jurassic and subsequently underwent thermal subsidence for about 20 ma as documented by pelagic sediments submarine alkali basaltic volcanism in the lower barremian marks the onset of rapid basement uplift to or above sea level carbonate platforms grew in barremian mid aptian time on highs of oceanic basement the uplift is interpreted as the result of transpressive tectonism along a large oceanic fracture zone the platforms submerged in the mid aptian and reached the ccd in the albian ribbon cherts were deposited during the albian to santonian time the first influx of siliciclastic detritus shed from the arabian craton is recorded in coniacian sandstones ophiolitic conglomerates interfinger in the campanian to mid maastrichtian with continent derived sandstones reflecting regional compressive tectonism crystalline exotics deposited in the late maastrichtian are derived most likely from the precambrian basement of arabia whilst the origin of the sedimentary exotics dated triassic to cretaceous remains unclear the masirah ophiolite was overthrust at the cretaceous tertiary boundary by an upper ophiolite nappe and emplaced onto the margin of southeast oman

india eurasia collision chronology has implications for crustal shortening and driving mechanism of plates the motion of the indian plate is determined in an absolute frame of reference and compared with the position of the southern margin of eurasia deduced from palaeomagnetic data in tibet the 2 600 900 km of continental crust shortening observed is shown to have occurred in three different episodes subduction of continental crust intracontinental thrusting and internal deformation and lateral extrusion the detailed chronology of the collision and plate reorganizations in the indian and pacific oceans supports the hypothesis that slab pull is a dominant driving mechanism of plate tectonics

modeling the evolution of continental subduction processes in the pamir hindu kush region several geological and geophysical studies suggest the presence of two converging subduction zones in the western syntaxis of the india eurasia collision zone with steep northward subduction of indian lithosphere beneath the hindu kush and southward subduction of asian lithosphere under the pamir we investigate the geometry and the timing of these subduction processes seismic tomography images are used to constrain the geometry of the two slabs under the pamir and the hindu kush by measuring the tomographically inferred length of the indian slab under the hindu kush region and by comparison with paleomagnetic reconstructions we estimate that the process of indian slab break off most likely occurred at the early stage of the collision at ca 44 48 ma we infer that after slab break off western india continued its northward motion until its northern boundary reached the present hindu kush region where it began to subduct we estimate an age for the initiation of subduction in this region of about 8 ma we apply 2d thermo kinematic and rheological numerical modeling to compute the temperature distribution and brittle field along two vertical sections across the hindu kush and pamir regions for the hindu kush we introduce parameters reproducing fast and near vertical subduction whereas in the pamir region we use the results from published studies to reproduce slower subduction modeling results indicate that faster subduction under the hindu kush results in a deeper brittle region which is consistent with observed deeper seismicity than in the pamir region

formation and emplacement ages of the masirah ophiolite sultanate of oman hornblende separates from gabbros and biotite separates from crosscutting potassic granites of the masirah ophiolite have been analyzed by conventional k ar techniques the gabbros have late jurassic early cretaceous ages 126 158 ma and the granites have broadly similar ages 124 146 ma because the gabbros are likely to give ophiolite formation ages and the granites ophiolite emplacement ages it can be concluded that the masirah ophiolite was emplaced very soon after formation the late jurassic early cretaceous ages contrast with the middle cretaceous ages 90 95 ma of the semail ophiolite confirming earlier suggestions that the masirah ophiolite cannot simply be a right laterally displaced fragment of the semail ophiolite but is instead an uplifted block of indian ocean crust the formation and emplacement of the masirah ophiolite are discussed in the context of the late mesozoic evolution of the western indian ocean

the breakup of east gondwana assimilating constraints from cretaceous ocean basins around india into a best fit tectonic model published models for the cretaceous sea oor spreading history of east gondwana result in unlikely tectonic scenarios for at least one of the plate boundaries involved and or violate particular constraints from at least one of the associated ocean basins we link east gondwana spreading corridors by integrating magnetic and gravity anomaly data from the enderby basin off east antarctica within a regional plate kinematic framework to identify a conjugate series of east west trending magnetic anomalies m4 to m0 126 7 120 4 ma the mid ocean ridge that separated greater india from australia antarctica propagated from north to south starting at 136ma northwest of australia and reached the southern tip of india at 126 ma sea oor spreading in the enderby basin was abandoned at 115 ma when a ridge jump transferred the elan bank and south kerguelen plateau to the antarctic plate our revised plate kinematic model helps resolve the problem of successive two way strike slip motion between madagascar and india seen in many previously published reconstructions and also suggests that sea oor spreading between them progressed from south to north from 94 to 84 ma this timing is essential for tectonic ow lines to match the curved fracture zones of the wharton and enderby basins as greater india gradually began to unzip from madagascar from 100 ma in our model the 85 east ridge and kerguelen fracture zone formed as conjugate anks of a leaky transform fault following the 100ma spreading reorganization our model also identi es the afanasy nikitin seamounts as products of the conrad rise hotspot

extremely thin oceanic crust in the proto indian ocean evidence from the masirah ophiolite sultanate of oman the masirah ophiolite is a good example of thin oceanic crust below pillow lavas and a sheeted dike complex with a relatively normal thickness of 1 1 5 km the gabbroic lower crust barely exceeds 500 m in thickness in spite of this reduced thickness the oceanic crust preserves all members of a model ophiolite in a coherent lithostratigraphic sequence the crust was formed during the uppermost jurassic circa 150 ma when the indian madagascar plate separated from the african arabian plate and is therefore related to the opening of the coeval somali basin geological relationships indicate that this portion of oceanic crust was formed at a ridge transform intersect the peculiarly reduced thickness of the gabbro layer is interpreted as the result of a weak magma supply at the edge of a ridge segment rather than the consequence of a tectonic thinning the cooling effect due to the vicinity of two large continental lithospheric blocks indian madagascar and african arabian plates during this initial stage of the oceanization might have been an additional factor contributing to the reduction of the crustal thickness

cosmogenic 3he production rates revisited from evidences of grain size dependent release of matrix sited helium measurements of the cosmogenic 3 he 3 hec content of various size aliquots of exposed olivines show that the fine fraction 140 m has 3 hecconcentrations between14and100 lowerthanthatofthecoarsefractions 0 14 1mm suchdifferencesattest to a grain size dependent partial release of 3 hec from the phenocrysts matrix during the preliminary in vacuo crushing this result mighthave importantimplications since most 3 hecmeasurements haveusedfor 20yrastandardroutine basedonthe fusion ofbulk powdered phenocrysts whatever their grain size a suite of new data obtained from coarse olivine grains yielded a mean sea level high latitude 3 hec production rate slhl p3 of 128 5 and 136 6 at g 1 yr 1 depending on the scaling factors used this new value which is 15 higher than previously published rates is obtained from 5 ropy flow surfaces of mt etna 38 n and hawaiian 19 n volcanoes at elevations between sea level and 870 m and ranging in age from 1 47 0 05 to 149 23 ka according to independent 14 c or k ar dating 3 he loss during the crushing step might account for the discrepancy between the standard reference valueof110 115at g 1 yr 1 andthehigherslhlp3proposedhere moregenerally removal ofthe powderedfraction beforefusion is an important point to consider in further studies in order to avoid any 3 hec systematic underestimates

early cretaceous migmatitic mafic granulites from the sabzevar range ne iran implications for the closure of the mesozoic peri tethyan oceans in central iran terra nova 22 26 34 2010 abstract the ophiolitic melange of the sabzevar range northern iran is a remnant of the mesozoic oceanic basins on the northern margin of the neotethys that were consumed during the arabia eurasia convergence history occurrence of km scale dismembered mafic hp granulitic slices is reported in this study granulites record an episode of amphibole dehydratation melting and felsic tonalite throndhjemite melt segregation at c 1 1 gpa and 800 c in situ u th pb geochronology of zircon and titanite grains hosted in melt segregations points to an early cretaceous albian age for the metamorphic climax results of this study i impose reconsideration of the current palaeotectonic models of the neothetyan convergent margin during the early cretaceous and ii argue that punctuated events of subduction of short lived back arc oceanic basins accompanied the long lasting history of the neotethyan subduction in the region

the indus yarlung zangbo ophiolites from nanga parbat to namche barwa syntaxes southern tibet first synthesis of petrology geochemistry and geochronology with incidences on geodynamic reconstructions of neo tethys the purpose of this first synthesis is to summarize findings on the yarlung zangbo suture zone yzsz ophiolites and discuss still remaining problems the yzsz studied for almost 30 years and is the youngest of the sutures recognized on tibet plateau it is now acknowledged that the yzsz is a complex assemblage of sedimentary metamorphic and igneous rocks created during and shortly after the collision between india and eurasia the ages of the various lithologies span a time interval from the jurassic to the middle miocene with some permian and devonian exotic blocks from melange zone the yzsz is characterized by ophiolitic complexes and ophiolitic melange the ophiolites are of two types non dismembered and dismembered sections the non disturbed sections although tectonically reworked are observed along the segment from dazhuqu to jiding in xigaze area and spontang ophiolite the dismembered sequences are found in various locations such as nidar kiogar jungbwa saga sangsang xigugabu and luobusa the incomplete stratigraphic log could be connected to intraoceanic or orogenetic origins the ophiolites are distributed into two groups of ages the luobusa zedang and kiogar sequences being jurassic lower cretaceous whereas all other sequences are of lower cretaceous age compilation of geochronological data suggest that some ophiolite sequences might have evolved for over more than 70 my from their initial genesis to obduction which occurred around 70 90 my ago ophiolites differ in terms of petrological and geochemical aspects however they were all generated in suprasubduction zone and more specifically in arc few fore arc and back arc settings synthesis of more than 700 geochemical analyses show variable mixing of components from n morb type to iat cab and to oib end members the jurassic ophiolites show the maximum of arc component while the lower cretaceous ones show little to strong mixing in addition most ophiolites were created in short lived 30 my basins and generated close to the eurasiatic continental margin we propose that ladakh tibet ophiolites were generated in a suprasubduction context similar to mariana arc interarc and back arc or tonga lau system the variable arc signature of these ophiolites is directly related to their initial position within the suprasubduction system

global sediment thickness data set updated for the australian antarctic southern ocean we present a new 5 min sediment thickness grid for the australian antarctic region 60 e 155 e 30 s 70 s new seismic reflection and refraction data have been used to add detail to the conjugate australian and antarctic margins and intervening ocean floor where regional sediment thickness patterns were poorly known previously on the margins sediment thickness estimates were computed from velocity depth functions from sonobuoy refraction velocity solutions ground truthed against seismic reflection data for the southeast indian ridge abyssal plains sediment thickness contours from geli et al 2007 were used the new regional minimum sediment thickness grid was combined with the global national geophysical data center ngdc sediment grid to create an updated global grid even using the minimum estimates sediment accumulations on the extended australian and antarctic continental margins are 2 km thicker across large regions and up to 9 km thicker in the ceduna basin compared to the global ngdc compilation of sediment thickness data

comment on dynamic surface topography a new interpretation based upon mantle flow models derived from seismic tomography by a m forte w r peltier a m dziewonski and r l woodward in a recent paper in this journal forte et al 1993 have proposed a model of global dynamic topography with an amplitude of three kilometers six kilometers peak to peak if north america australia and other continents were dynamically depressed by the one to two kilometers predicted then these continents would today be below sea level and nearly covered by epeiric seas since the continents are nearly entirely exposed and have become progressively exposed since the late mesozoic the amplitude of the forte et al model is clearly much too large

a palaeomagnetic reconnaissance of northeastern baluchistan pakistan a total of 560 samples from three areas in northeastern baluchistan the southern sulaiman range the central loralai range and the northern kirthar range were analyzed using thermal demagnetization techniques thirteen formations of late palaeozoic to early tertiary age were studied palaeomagnetically inclinations of the obtained results show a general affinity with the indian apparent polar wander path deviating declinations from the loralai range indicate a clockwise rotation over 50 with respect to the indian shield secondary magnetization components probably of late palaeocene to early eocene age were consistently present in the samples from the kirthar range and the sulaiman range but were not observed in samples from the loralai range acquisition of these components is attributed to crustal upwarping during the palaeocene which is tentatively related to initial collision of continental indo pakistan with an island arc off south central asia the baluchistan data support recent palaeomagnetic results from the indus tsangpo suture zone in ladakh nw himalaya which indicate that such an initial collision took place at very low northern palaeolatitudes

subduction history of the tethyan region derived from seismic tomography and tectonic reconstructions 1 in the mantle underneath the tethyan suture zone large volumes of positive velocity anomalies have been imaged by seismic tomography and interpreted as the present day signature of subducted tethyan lithosphere we investigate the mesozoic cenozoic subduction history of the region by integrating independent information from mantle tomography and tectonic reconstructions three different subduction scenarios for the tethyan oceanic lithosphere representative for the available tectonic reconstructions are used to predict the present thermally anomalous volumes associated with the lithospheric surface subducted since the late mesozoic next these predicted thermal volumes and their expected positions are compared to the relevant anomalous volumes derived from seismic tomographic images in this analysis we include among others the possible effects of ridge subduction and slab detachment after the cenozoic continental collisions absolute plate motion and slab thickening in the mantle our preferred subduction model comprises the opening of large back arc oceanic basins within the eurasian margin the model points to slab thickening by a factor of 3 in the mantle in which case the estimated volumes allow for active oceanic spreading 1 2 5 cm yr in the tethyan lithosphere during convergence our results further indicate the occurrence of early oligocene slab detachment underneath the northern zagros suture zone followed by both westward and eastward propagation of the slab tear and diachronous eocene to miocene slab detachment below the eastern to western himalayas free sinking rates of the detached material of 2 cm yr in the lower mantle provide the best fit between the tomographic mantle structure and our tethyan subduction model

oceanic style subduction controls late cenozoic deformation of the northern pamir orogen abstract the northern part of the pamir orogen is the preeminent example of an active intracontinental subduction zone in the early stages of continent continent collision such zones are the least understood type of plate boundaries because modern examples are few and of limited access and ancient analogs have been extensively overprinted by subsequent tectonic and erosion processes in the pamir it has been assumed that most of the plate convergence was accommodated by overthrusting along the plate bounding main pamir thrust mpt which forms the principal northern mountain and deformation front of the pamir however the synopsis of our new and previously published thermochronologic data from this region shows that the hanging wall of the mpt experienced relatively minor amounts of late cenozoic exhumation the pamir orogen as a whole is an integral part of the overriding plate in a subduction system while the remnant basin to the north constitutes the downgoing plate with the bulk of the convergence accommodated by underthrusting herein we demonstrate that the observed deformation of the upper and lower plates within the pamir alai convergence zone resembles highly arcuate oceanic subduction systems characterized by slab rollback subduction erosion subduction accretion and marginal slab tear faults we suggest that the curvature of the north pamir is genetically linked to the short width and rollback of the south dipping alai slab northward motion indentation of the pamir is accommodated by crustal processes related to this rollback the onset of south dipping subduction is tentatively linked to intense pamir contraction following break off of the north dipping indian slab beneath the karakoram

subduction of the indian lithosphere beneath the tibetan plateau and burma abstract subduction of the indian lithosphere under eurasia plays an important role in the tectonic evolution of the tibetan plateau and surrounding regions to improve our knowledge of pertinent mantle structures through tomographic imaging we combine p wave arrival time data from temporary arrays in tibet and stations of the chinese seismograph network with reprocessed data from the international seismological centre the new images reveal considerable spatial variations in mantle structure along the collision zone and the horizontal distance over which presumed continental indian lithosphere slides northward beneath the plateau decreases from west where it underlies the himalayas and the entire plateau to east where no indication is found for present day underthrusting beyond the himalayan block and indus tsangpo suture p wavespeed appears low in the shallow mantle beneath much of central and eastern tibet these observations suggest that indian lithosphere underlies only the southwestern part of the plateau and that the central and northeastern part is underlain by lithosphere of asian origin the continental parts of the indian plate that currently underthrust or subduct below the plateau appear generally detached from the oceanic slab fragments that subducted longer ago and that have been detected deeper in the mantle except perhaps in the central part of the collision zone 80 90 e this suggests that ongoing india eurasia collision must increasingly be driven by other forces such as subduction of the indian plate further west and east e g beneath indochina our images reveal structures associated with eastward subduction along the burmese arc southeast of the eastern himalayan syntaxis the tengchong volcanic complex in southwest china is marked by slow wave propagation to 150 km depth suggesting a causal link to subduction along the burmese arc the low velocities beneath the red river fault region extend to greater depth and may be related to upper mantle processes further southeast

in situ evidence for dextral active motion at the arabia india plate boundary the arabia india plate boundary also called theowen fracture zone is perhaps the least known boundary among large tectonic plates1 6 although it was identified early on as an example of a transform fault converting the divergent motion along the carlsberg ridge to convergent motion in the himalayas7 its structure and rate of motion remains poorly constrained here we present the first direct evidence for active dextral strike slip motion along this fault based on seafloor multibeam mapping of the arabia india somalia triple junction in the northwest indian ocean there is evidence for 12km of apparent strike slip motion along the mapped segment of the owen fracture zone which is terminated to the south by a 50 km wide pull apart basin bounded by active faults by evaluating these new constraints within the context of geodetic models of global plate motions we determine a robust angular velocity for the arabian plate relative to the indian plate that predicts 2 4mmyr 1 dextral motion along the owen fracture zone this transformfault was probably initiated around 8 million years ago in response to a regional reorganization of plate velocities and directions8 11 which induced a change in configuration of the triple junction infrequent earthquakes of magnitude 7 and greater may occur along the arabia india plate boundary unless deformation is in the formof aseismic creep

the transition between the sheba ridge and owen basin rifting of old oceanic lithosphere summary magnetic quiet zones are present along the margins of the entire length of the gulf of aden to the owen fracture zone this includes the easternmost 300 km between the eastern edges of arabia and africa to the owen fracture zone where old oceanic lithosphere was rifted to form the sheba ridge within this easternmost region the boundary between the quiet zone and the old oceanic lithosphere is marked by ridge complexes the sharbithat ridge complex to the north and the error ridge complex to the south these ridge complexes which lack a magnetic signature occupy a structural position similar to the hinge zones at the continental margins to the west they appear to have formed early in the opening of the gulf of aden or perhaps to have been pre existing features the boundary between sheba ridge and the northern magnetic quiet zone is often marked by an abrupt end of the sheba ridge seafloor spreading magnetic anomaly pattern and a sharp basement deepening to the north the boundary between the east sheba ridge and northern magnetic quiet zone becomes less distinct near the owen fracture zone this is also accompanied by changes in the east sheba ridge specifically a decrease in magnetic anomaly amplitudes increase in ridge flank depths and a loss of the cooling curve ridge flank shape this may be the result of lower mantle temperatures in the vicinity of the owen fracture zone the sediments within the magnetic quiet zone can be divided into a lower disturbed unit and an upper unit consisting of flat lying reflectors the disturbance of the lower sediments may have resulted from a period of diffuse extension in the magnetic quiet zone prior to the establishment of the sheba ridge spreading centre the similarity of the easternmost quiet zone to quiet zones at rifted continental margins leads to the suggestion that these regions were formed by diffuse extension of old oceanic lithosphere using a two layer lithospheric attenuation model and assuming extension during a period of 15 myr followed by 10 myr of cooling the basement depths and heat flow measurements can be adequately matched the modelling implies 45 per cent crustal extension in the quiet zone the amount of extension calculated is compatible with documented motion between arabia and africa the old oceanic lithosphere thus must have been substantially thinned to a thickness similar to 10 14 myr old ocean before seafloor spreading was initiated at the sheba ridge

changes in paleocurrents during the development of an obliquely convergent plate boundary sulaiman fold belt southwestern himalayas west central pakistan abstract pre orogenic cretaceous shelf sandstones at the northern and southern ends of the sulaiman range in west central pakistan show basically westward dispersal which is off the craton however paleocene marine sandstones in the south spread east southeast and early eocene deltaic sandstones in the north spread southeast and north these are mostly onto the craton and indicate slope reversal of the cretaceous shelf post oligocene fluvial molasse shows mostly southward drainage a 3852 m section of molasse in the northern sulaiman range comprises a lower 2378 m of claystones and upwardly fining sandstones that represent meandering rivers and an upper 1474 m that are dominated by conglomerates that represent gravelly braided rivers paleocurrents in the lower meandering part of the section were generally southward but very variable whereas those in the upper braided part were more uniform and show eastward flow a 3146 m section of molasse in the southern sulaiman range comprises 1 a basal 1160 m sequence of claystones and sandstones meandering streams with very diverse flow directions overall to the southwest 2 a middle 1603 m sequence dominated by pebbly sandstone with large bedforms big sandy braided rivers with less diverse flow to the southwest and 3 an upper 383 m conglomeratic sequence gravelly braided streams that still flow uniformly southwest rivers in the early molasse paralleled the modern sulaiman range in the north and flowed obliquely into it in the south which suggests no uplift of the modern front of the range until the onset of modern transverse eastward drainage and the associated influx of conglomerates in present and admittedly uncertain lithostratigraphic terms uplift of the modern front of the range apparently began about 700 m lower in the north than in the south the northern and southern molasse sections otherwise show broadly similar histories of river development

owen fracture zone the arabia india plate boundary unveiled we surveyed the owen fracture zone at the boundary between the arabia and india plates in the nw indian ocean using a high resolution multibeam echo sounder owen cruise 2009 for search of active faults bathymetric data reveal a previously unrecognized submarine fault scarp system running for over 800 km between the sheba ridge in the gulf of aden and the makran subduction zone the primary plate boundary structure is not the bathymetrically high owen ridge but is instead a series of clearly delineated strike slip fault segments separated by several releasing and restraining bends despite an abundant sedimentary supply by the indus river flowing from the himalaya fault scarps are not obscured by recent deposits and can be followed over hundreds of kilometres pointing to very active tectonics the total strike slip displacement of the fault system is 10 12 km indicating that it has been active for the past 3 to 6 ma if its current rate of motion of 3 1 mm yr 1 has remained stable we describe the geometry of this recent fault system including a major pull apart basin at the latitude 20 n and we show that it closely follows an arc of small circle centred on the arabia india pole of rotation as expected for a transform plate boundary

the geotectonic history of the makran and adjacent areas of southern iran abstract the makran mountain range in southern iran has been entirely covered by regional mapping on a scale of 1 250 000 but in contrast to the mapping of oman on the other side of the gulf the recent mapping is little known the cenozoic accretionary prism in the south fronts onto a mesozoic melange zone representing earlier subduction and to the north of this is an eastwards tapering microcontinental sliver which is followed by a zone of largely intact mesozoic ophiolite complexes to the north of this lies the jaz murian depression a desert expanse with no bedrock outcrop the sanandaj sirjan bajgan dur kan microcontinental sliver which extends from turkey through the zagros to the makran is one of a number of microcontinents which detached from gondwana no later than the early triassic nw iran yazd tabas lut birk helmand farah during the mesozoic these were enclosed by tracts of ocean now represented by ophiolites and the sectors closed at different times ranging from early cretaceous to end palaeogene an analogy is drawn with the present day mediterranean the sistan ocean in the east was infilled with an immense thickness of abyssal turbidites during the eocene before it closed the field evidence indicates that there was a second cenozoic accretionary prism here but sedimentation ceased after the eocene on its collision with the back of the main makran prism this eastern sistan trough now completely occluded is compared with the still open tract separating the makran from oman at the present time and its northern extension which has been similarly occluded by collision beneath the zagros some residual very shallow inland seas persisted inland from the makran through the neogene the occurrence of blueschists in the inner ophiolite belt of the makran is discussed abyssal turbidite sedimentation continued in the main southern zone of the makran into the early miocene to be succeeded by thick shallow water sequences including reefal limestones intense tectonic dislocation at the end of the miocene was accompanied by a shift of the subduction front to its present position out in the indian ocean and uplift leading to the deposition of spectacular pliocene pleistocene fanglomerates

lower cretaceous xigaze ophiolites formed in the gangdese forearc evidence from paleomagnetism sediment provenance and stratigraphy the india asia suture zone of southern tibet exposes lower cretaceous xigaze ophiolites and radiolarian cherts and time equivalent asian derived clastic forearc sedimentary rocks xigaze group these ophiolites have been interpreted to have formed in the forearc of the north dipping subduction zone below tibet that produced the gangdese magmatic arc around 15 20 n or in the forearc of a sub equatorial intra oceanic subduction zone to better constrain the latitude of the ophiolites we carried out an integrated paleomagnetic geochronologic and stratigraphical study on epi ophiolitic radiolarites chongdui and bainang sections and xigaze group turbiditic sandstones unconformably overlying the ophiolite s mantle units sangsang section detrital zircon u pb geochronology of tuffaceous layers from the chongdui section and sandstones of the xigaze group at the sangsang sectionprovides maximum depositional ages of 116 5 3 1maand128 8 3 4ma respectively for the chongdui sectionand an asian provenance signature for the xigaze group paleomagnetic analyses integrated with rock magnetic experiments indicate significant compaction related inclination shallowing of the remanence within the studied rocks two independent methods are applied for the inclination shallowing correction of the paleomagnetic directions from the sangsang section yielding consistent mean paleolatitudes of 16 2 n 13 n 20 9 n and 16 8 n 11 1 n 23 3 n respectively these results are indistinguishable from recent paleolatitude estimates for the gangdese arc in southern tibet radiolarites from the chongdui and bainang sections yield low paleomagnetic inclinations that would suggest a sub equatorial paleolatitude but the distribution of the paleomagnetic directions in these rocksstrongly suggests a low inclination bias by compaction our data indicate that spreading of the xigaze ophiolite occurred in the gangdese forearc and formed the basement of the forearc strata

retrodeforming the arabia eurasia collision zone age of collision versus magnitude of continental subduction the arabia eurasia collision has been linked to global cooling the slowing of africa mediterranean extension the rifting of the red sea an increase in exhumation and sedimentation on the eurasian plate and the slowing and deformation of the arabian plate collision age estimates range from the late cretaceous to pliocene with most estimates between 35 and 20 ma we assess the consequences of these collision ages on the magnitude and location of continental consumption by compiling all documented shortening within the region and integrating this with plate kinematic reconstructions shortening estimates across the orogen allow for 350 km of neogene upper crustal contraction necessitating collision by 20 ma a 35 ma collision requires additional subduction of 400 600 km of arabian continental crust using the oman ophiolite as an analogue ophiolitic fragments preserved along the zagros suture zone permit 180 km of subduction of the arabian continental margin plus overlying ophiolites wholesale subduction of this more dense continental margin plus ophiolites would reconstruct 400 500 km of postcollisional arabia eurasia convergence consistent with a ca 27 ma initial collision age this younger arabia eurasia collision suggests a noncollisional mechanism for the slowing of africa and associated extension

tectonics and history of the andaman sea region abstract the andaman sea is an active backarc basin lying above and behind the sunda subduction zone where convergence between the overriding southeast asian plate and the subducting australian plate is highly oblique the effect of the oblique convergence has been formation of a sliver plate between the subduction zone and a complex right lateral fault system the late paleocene collision of greater india and asia with approximately normal convergence started clockwise rotation and bending of the northern and western sunda arc the initial sliver fault which probably started in the eocene extended through the outer arc ridge offshore from sumatra through the present region of the andaman sea into the sagaing fault with more oblique convergence due to the rotation the rate of strike slip motion increased and a series of extensional basins opened obliquely by the combination of backarc extension and the strike slip motion these basins in sequence are the mergui basin starting at 32 ma the conjoined alcock and sewell rises starting at 23 ma east basin separating the rises from the foot of the continental slope starting at 15 ma and finally at 4 ma the present plate edge was formed alcock and sewell rises were separated by formation of the central andaman basin and the faulting moved onshore from the mentawai fault to the sumatra fault system bisecting sumatra

gravity and tectonic model across the sulaiman fold belt and the chaman fault zone in western pakistan and eastern afghanistan abstract gravity data from the western boundary of the indian subcontinent have been analysed to infer the gross crustal structure across the indian afghan collision zone seismic reflection profiles reveal the gross structural duplex geometry of the sulaiman fold belt these data show that the wedge which is 10 km thick at the deformation front thickens northwestward to attain a tectonic thickness thickness due to deformation of about 20 km in the hinterland gravity modeling depicts the depth of the moho between 33 and 35 km at the deformation front of the sulaiman fold belt the moho depth shows an upward convexity along an e w profile it decreases northward with a gentle gradient of 1 1 20 m km below the sulaiman fold belt and then deepens abruptly with a gradient of about 7 8 136 m km across the chaman fault zone attaining a depth of about 57 km in eastern afghanistan the model suggests that the sulaiman fold belt is underlain by transitional crust 15 27 km thick in contrast to the continental crust 38 km thick underneath the fold belt of the himalayan collision zone in northern pakistan the about 57 km thick crystalline crust in eastern afghanistan may be due to 1 underplating by crust of the indian subcontinent and 2 structural thickening within the afghan block

structure and early evolution of the arabian sea and east somali basin summary the laxmi ridge is a large scale basement high buried beneath the sediments of the indus fan the location of the ocean continent transition oct on this margin has previously been proposed at either the southern edge of the laxmi ridge or beyond it towards the india pakistan shelf the former explains the margin parallel laxmi basin as thinned continental crust the latter as a failed rift of earlier seafloor spreading to examine the structure of this margin a reassessment of marine magnetic data has detailed seafloor spreading magnetic anomalies prior to anomaly 24 in both the arabian and east somali basins the previously identified anomaly 28 is not interpreted as a seafloor spreading anomaly but as a magnetized basement feature adjacent to and merging with the ridge the laxmi spur new gravity models across the laxmi ridge and adjacent margin using ship and satellite data corroborate the existence of underplated crust beneath the laxmi ridge and basin and the location of the oct at the southern edge of the ridge the results are not compatible with the existence of a pre anomaly 28 phase of seafloor spreading although large scale intrusions may be the origin of some of the basement features in the laxmi basin the models also identify the laxmi spur as a low density feature with a natural remanent magnetization nrm compatible with serpentinization the laxmi ridge is mapped to the southeast where it appears to terminate at a point coinciding with the appearance of e w magnetic lineations and gravity anomalies at 15 5 n thereafter it becomes indistinct this is interpreted as necessary in the reconstruction to the mascarene plateau to avoid continental overlap

contrasting late cretaceous palaeocene lithostratigraphic successions across the bibai thrust western sulaiman fold thrust belt pakistan their significance in deciphering the early collisional history of the nw indian plate margin abstract the sulaiman fold thrust belt is a significant tectonic element in northwest pakistan located on the nw margin of the indian plate having a rift drift collision plate tectonic history late cretaceous palaeocene successions in the western sector of this belt conventionally comprise strata above the widespread pelagic parh limestone albian campanian and below the fluvio deltaic ghazij formation early eocene these successions display marked lithostratigraphic variations across the bibai thrust long known to be an important structural feature in the western sulaiman belt the successions on the foot wall to the east and southeast of the bibai thrust quetta spera ragha and chinjun areas exhibit the most variable lithostratigraphy were deposited exclusively in shallow marine conditions and include volcanics associated with a local submarine edifice they also display several internal disconformities locally accompanied by oxidised units that attest to intermittent emergence by contrast coeval successions on the hanging wall to the north and northwest of the bibai thrust urghargai mazu ghar and kach ziarat areas are the products of deposition in deeper marine conditions and also include important volcanigenic contributions these contrasting lithofacies accumulated respectively in widely separated proximal and distal sectors of the north western margin of the indian plate and their current proximity is attributed to tectonic juxtaposition by relative southward translation of the distal facies along the bibai and related thrusts involvement of late eocene limestones in these thrusts and their sealing by mio pliocene fluvial sediments demonstrates that active translation on these thrusts continued at least into late eocene or oligocene times

ridge push mantle plumes and the speed of the indian plate summary the buoyancy of lithospheric slabs in subduction zones is widely thought to dominate the torques driving plate tectonics in late cretaceous and early paleogene times the indian plate moved more rapidly over the mantle than freely subducting slabs sink within it this signal event has been attributed to arrival of the deccan rmantle plume beneath the plate but itisunknowninwhichproportionstheplumeactedtoalterthebalanceofexistingplatedriving torques and to introduce torques of its own our plate kinematic analysis of the mascarene basin yields a detailed indian plate motion history for the period 89 60ma plate speed initially increases steadily until a pronounced acceleration in the period 68 64ma after which it abruptly returns to values much like those beforehand this pattern is unlike that suggested to result from the direct introduction of driving forces by the arrival of a thermal plume at the base of the plate a simple analysis of the gravitational force related to the indian plate s thickening away from its boundary with the african plate suggests instead that the sudden acceleration and deceleration may be related to uplift of part of that boundary during a period when it was located over the plume head in this instance torques related to plate accretion and subduction may have contributed in similar proportions to drive plate motion

formation and evolution of the masirah ophiolite constrained by paleomagnetic study of volcanic rocks abstract the extrusive rocks of the lower ophiolitic nappe of masirah island oman were paleomagnetically studied four mean directions of magnetization were isolated one at low temperature the others at high temperature the low temperature component found in most samples corresponds to a recent viscous and or chemical remagnetization the high temperature component found at three sites in the centre area is interpreted as a possible primary magnetization acquired during extrusion or more probably as an early remagnetization related to subsequent hydrothermal alteration and indicates formation of the masirah ophiolite around a paleolatitude of 40 s close to the present position of the west somali basin a second high temperature component found in three other sample sites in the hakl and in the thumi areas is probably a chemical remagnetization related to the emplacement of the upper ophiolite nappe in late maastrichtian to paleocene times finally a last high temperature component found only in the two sites from naft area could be a post tectonic remagnetization acquired in oligocene or miocene times or represents differential tilting of pillows and sedimentary blocks the high temperature components are usually fairly scattered due to the very complex tectonic history of the area nevertheless the relation between the defined paleopoles and the indian polar wander is unambiguous and the evolution of the masirah ophiolite from formation up to the emplacement of the second ophiolitic nappe is clearly related to the northward movement of the indian plate this new paleomagnetic study rules out the possibility of a common or related origin for the masirah and the semail ophiolites

major intrusive stages in afghanistan typology age and geodynamic setting afghanistan straddles the gondwana eurasia boundary most of its major structural areas comprise large plutonic belts based on some 700 major element analyses the typology of the eleven main plutonic belts has been determined these belts differ from each other on the whole they display a very broad range of petrographic types from nepheline syenites and gabbros to leucocratic muscovite tourmaline granites and a wide diversity of magmatic associations from cafemic or alumino cafemic associations of various types tholeiitic calc alkaline subalkaline i e monzonitic alkaline peralkaline to various aluminous ones taking into account the typology of the belts and chronological data four major intrusive stages have been recognized and related to the geodynamic evolution of afghanistan and central asia 1 aluminous lower palaeozoic ca 500 ma stage related to linear crustal thinning at the gondwana scale 2 cafemic and alumino cafemic triassic ca 230 ma stage related to southward dipping subduction of oceanic crust beneath an island arc involving a large continental strip detached from the southern asia margin 3 cafemic and alumino cafemic cretaceous ca 100 ma stage dominantly related to subduction of oceanic crust under marginal gondwana fragments 4 eocene oligocene stage ca 35 ma induced by the india eurasia collision including two quite different groups of intrusives a a cafemic and alumino cafemic group related to extensional phenomena dominantly associated with the westward extrusion of central afghanistan along the herat fault these phenomena occurred on both sides of the gondwana palaeozoic asia boundary b an aluminous group resulting from crustal thickening in marginal gondwana fragments

late cretaceous early tertiary convergence between the indian and arabian plates recorded in ophiolites and related sediments abstract remnants of ocean floor forming the eastern ophiolite belt in oman and the western ophiolite belt in pakistan have a common plate tectonic history culminating in emplacement at the cretaceous tertiary boundary fragments of ocean floor in these two belts have ages between 150 and 65 ma and recorded tectonic events in the early indian ocean at 150 ma 130 120 ma 110 100 ma and 70 65 ma new radiometric and chronostratigraphic ages and paleomagnetic and sedimentary information are used to relocate these ophiolites in the frame of the evolving indian ocean which was characterised by the stepwise breakup of gondwana at 158 ma east and west gondwana 130 ma southern atlantic breakup of east gondwana 95 84 ma madagascar and india seychelles 65 ma india and seychelles and finally at 40 ma rifting between africa and arabia the 150 ma old oceanic rocks of masirah island in oman originally formed the extension of the basins now preserved along the eastern edge of the afro arabian plate masirah drifted together with india seychelles when a new ridge formed at approximately 130 ma separating microplates e g kabul block from the northern edge of greater india parts of this new ocean were later emplaced to form the western ophiolite belt in pakistan consumption of oceanic crust and emplacement of ophiolites during the closure of the neotethys occurred not only north of india seychelles but also along two or three subduction zones between the afro arabian plate and india seychelles documented by the formation of metamorphic soles beneath ophiolites relics of ocean floor formed approximately 65 70 ma ago now form the upper units in the western ophiolite belt of pakistan where they overlie an accretionary prism of pillow lavas of dominantly aptian albian age and sedimentary rocks the breakup of india and the seychelles at 65 ma was associated with the eruption of flood basalts deccan and seychelles and enhanced counter clockwise movement of india this breakup possibly caused the late jurassic early cretaceous fragments of ocean floor e g masirah to be thrust onto the eastern edge of oman the ophiolites which were emplaced onto the western edge of india drifted as part of this plate farther north where collision with eurasia and the accreted microplates occurred in the eocene 55 ma

dynamic surface topography a new interpretation based upon mantle flow models derived from seismic tomography the very long wavelength i e in the degree range l 1 8 dynamic topography at the earth s surface estimated by isostatic reduction of the observed topography is shown to be in good agreement with the topography predicted by a model of viscous flow in the mantle this flow model was previously derived by independent consideration of the observed nonhydrostatic geoid and is based upon a recent model of large scale seismic shear velocity heterogeneity in the mantle our estimate and prediction of the long wavelength dynamic surface topography reveals significant depressions 2 3 km of the continental shields relative to their hydrostatic reference positions

sandstone detrital modes in the makran accretionary wedge southwest pakistan implications for tectonic setting and long distance turbidite transportation detrital modes of early miocene to early pliocene sandstones from the makran accretionary wedge in southwest pakistan show a mainly quartzolithic composition with an evolution from the transitional recycled to quartzose recycled the lithic types however indicate two distinct petrofacies accreted abyssal plain turbidites have qp11lvm27lsm62 and lm39lv27ls34 showing a predominant supply from sedimentary and metasedimentary source terranes whereas slope and shelf facies sediments deposited on the accretionary wedge have qp7lvm47lsm47 and lm22lv48ls30 due to an increase of volcanic detritus the detrital modes of the abyssal plain sediments suggest a recycled orogenic source probably the himalayan collision zone the facies and longitudinal dispersal pattern suggest deposition in an oligo miocene analogue of the present indus fan the sediment must have been transported across strike parallel to the transform structure linking the makran wedge to the himalayas chaman ornach nal fault system and fed into the fan at the western end of the subduction zone the detrital modes also show an increase in volcanic detritus with time lv l 0 27 for the early miocene abyssal plain sediments to 0 47 for the slope sequences this may have been derived from late mesozoic volcanic terrains in northern baluchistan or the ladakh himalayas or more probably from the early to middle miocene andesitic volcanic centre in the northern makran

cretaceous to present kinematics of the indian african and seychelles plates an iterative inverse model of seafloor spreading data from the mascarene and madagascar basins and the flanks of the carlsberg ridge describes a continuous history of indian african plate divergence since 84 ma visual fit modelling of conjugate magnetic anomaly data from near the seychelles platform and laxmi ridge documents rapid rotation of a seychelles plate about a nearby euler pole in palaeocene times as the euler pole migrated during this rotation the amirante trench on the western side of the plate accommodated first convergence and later divergence with the african plate the unusual present day morphology of the amirante trench and neighbouring amirante banks can be related to crustal thickening by thrusting and folding during the convergent phase and the subsequent development of a spreading centre with a median valley during the divergent phase the model fits fracture zone trends in the north arabian and east somali basins suggesting that they formed in india africa plate divergence seafloor fabric in and between the basins shows that they initially hosted a segmented spreading ridge that accommodated slow plate divergence until 71 69 ma and that upon arrival of the deccan re union plume and an increase to faster plate divergence rates in the period 69 65 ma segments of the ridge lengthened and propagated ridge propagation into the indian continental margin led first to the formation of the laxmi basin which accompanied extensive volcanism onshore at the deccan traps and offshore at the saurashtra high and somnath ridge a second propagation episode initiated the ancestral carlsberg ridge at which seychelles india and india africa plate motions were accommodated with the completion of this propagation the plate boundaries in the mascarene basin were abandoned seafloor spreading between this time and the present has been accommodated solely at the carlsberg ridge

timing of india asia collision geological biostratigraphic and palaeomagnetic constraints 1 a range of ages have been proposed for the timing of india asia collision the range to some extent reflects different definitions of collision and methods used to date it in this paper we discuss three approaches that have been used to constrain the time of collision the time of cessation of marine facies the time of the first arrival of asian detritus on the indian plate and the determination of the relative positions of india and asia through time in the qumiba sedimentary section located south of the yarlung tsangpo suture in tibet a previous work has dated marine facies at middle to late eocene by far the youngest marine sediments recorded in the region by contrast our biostratigraphic data indicate the youngest marine facies preserved at this locality are 50 6 52 8 ma in broad agreement with the timing of cessation of marine facies elsewhere throughout the region double dating of detrital zircons from this formation by u pb and fission track methods indicates an asian contribution to the rocks thus documenting the time of arrival of asian material onto the indian plate at this time and hence constraining the time of india asia collision our reconstruction of the positions of india and asia by using a compilation of published palaeomagnetic data indicates initial contact between the continents in the early eocene we conclude the paper with a discussion on the viability of a recent assertion that collision between india and asia could not have occurred prior to 35 ma

40ar 39ar dating of the emplacement of the muslim bagh ophiolite pakistan abstract the obduction related basal part of the muslim bagh ophiolite baluchistan pakistan and the underlying metamorphic sequence were studied structurally which demonstrated a wsw ene trending thrusting sequence for the initial obduction 40ar 39ar measurements on amphiboles and plagioclase from the subophiolitic metamorphic rocks and on plastically deformed and recrystallized dolerite samples from the base of the sheeted dyke complex give apparent ages between 70 7 5 0 and 65 1 4 1 ma interpreted as cooling ages dating approximately the formation of the plastic deformation and obduction the results indicate that the muslim bagh ophiolite represents a segment of ocean floor from the small and slow spreading ocean branch of the neo tethys located between the indo pakistani and the afro arabian plates the wsw ene oriented obduction of the muslim bagh ophiolite onto the indo pakistani continental margin occurred with the convergence of the neo tethys branch during the late cretaceous and before the tertiary collision of the indo pakistani plate with the eurasian plate

tethyan subducted slabs under india abstract tomographic imaging of the mantle under tibet india and the adjacent indian ocean reveals several zones of relatively high p wave velocities at various depths under the hindu kush region in northeastern afghanistan and southern tajikistan a regional northward dipping slab is seen in the entire upper 600 km of the mantle and is apparently still attached to the lithosphere of the indian plate under northern pakistan this same slab shows a roll over structure with the deeper portion overturned and dipping southward as can also be seen in the distribution of earthquake hypocenters farther east southeast e g in the vicinity of nepal a well resolved anomaly below 450 km depth is connected to the slab under the hindu kush but seems to be separated from the lithosphere above 350 km these upper mantle anomalies are interpreted as the remnants of delaminated sub continental lithosphere that went down when greater india continued to converge northward with asia after 45 ma the deeper high velocity anomalies under the indian sub continent appear clearly separated from the shallower ones as well as from each other and are inferred to represent remnants of oceanic lithospheric slabs that have sunk into the lower mantle and were subsequently overridden by the indian plate they occur at depths between 1000 and 2300 km and occasionally descend down to the core mantle boundary the anomalies form three parallel wnw ese striking zones we interpret the two southern zones as remnants of oceanic lithosphere that was subducted when the neo tethys ocean closed between india and tibet in the cretaceous and earliest tertiary the northern deep mantle zone under northern afghanistan the himalayas and the lhasa block in southern tibet may represent the last subducted remnant of the paleo tethys ocean which is thought to have closed before the hauterivian stage of the early cretaceous the middle zone continues southeastward as a rather straight high velocity zone towards sumatra where it becomes convex southward and parallel to the subduction zone under the sunda arc comparison of this straight middle zone near india with the shallower upper 600 1000 km northern zone which displays a cusp like shape near the yunnan sw china syntaxis of the eastern himalayas supports the notion that the shallow northern zone represents later subduction than the deeper middle zone the suggestion of a counterclockwise rotation 20 of the indian plate during tertiary indentation of asia is supported by these features the present day latitudes of 5 35 n of the deep slabs under india and adjacent areas correspond to the approximate paleolatitudes of the cretaceous subduction zones the slab remnants in the middle mantle occur therefore near the ancient locations where they started their downward journey which implies that lateral movements in the deeper mantle were not large

mesozoic ophiolites sutures and arge scale tectonic movements in afghanistan abstract the tectonic history of afghanistan appears to be the result of successive accretion of fragments of gondwana to the active margin of laurasia since the end of the paleozoic the margin in afghanistan lies along the present herat and panjshir faults south of hindu kush swings around the central pamirs and can presumably be traced along the present western altyn tagh and kunlun faults in tibet north of this boundary paleozoic rocks have been deformed in the upper paleozoic whereas south of it there is no trace of the hercynian orogeny the first collision of gondwanian fragments with laurasia probably occurred in the early mesozoic along the hindu kush and kunlun to the south ophiolites along the panjao and pangong nu chiang sutures respectively in central afghanistan and central tibet testify for another suturing event in the upper jurassic or lower cretaceous the indus tsangpo suture between india and tibet corresponds in eastern afghanistan to two ophiolite subbelts near kabul and khost both ophiolite complexes have been emplaced between maestrichtian and lower eocene by choking of two northward dipping subduction zones after complete contact between the indian and asian continents was achieved presumably in the end of eocene the penetration of india into asia caused large scale intra continental shortening a large part of the shortening was accommodated by strike slip faulting along mesozoic and more ancient sutures central afghanistan in particular was extruded to the west along the herat suture by the protrusion of the pamir wedge it subsequently collided with the lut block

newly discovered paleogene deltaic sequence in katawaz basin pakistan and its tectonic implications in the katawaz basin pakistan the deltaic and turbidite facies of the khojak formation are the paleogene analogue of the modern indus river emptying into the arabian sea to form the indus delta fan system facies identified include upper continental slope to prodelta distal to proximal distributary mouth bar distributary channel interdistributary bay estuary fluvial channel natural levee and flood plains of lower and upper delta plains the proposed model is a fluvial dominated wave modified delta that axially fed khojak submarine fan turbidites exposed in the southwestern part of the basin these sediments were eroded from the early himalayan orogenic highlands and transported southwestward down the axis of the katawaz remnant ocean basin

zircon u pb and hf isotopic constraints from eastern transhimalayan batholiths on the precollisional magmatic and tectonic evolution in southern tibet abstract before the indian collision with asia northward subduction of the neo tethyan oceanic lithosphere produced an andean type convergent margin in south asia characterized by arc magmatism starting from the early jurassic and lasting until the eocene the magmatic arc is now represented by widespread intrusive bodies or the so called transhimalayan batholiths in the lhasa terrane of southern tibet that have been divided into two main magmatic suites i e the northern plutonic belt and the southern gangdese batholith their temporal distribution concerning how exactly the magmatic suites correlate eastwards and then southeastwards around the eastern himalayan syntaxis however remains poorly constrained here we report the first combined zircon u pb and hf isotopic study of the transhimalayan batholiths from the eastern part of the lhasa terrane 95 97 5 e and 28 5 30 n se tibet zircon u pb dating results of 24 granitoids indicate that the rocks were emplaced principally in the early cretaceous 133 110 ma and subordinately in the paleocene 66 57 ma long after an older granite intrusion in the earliest jurassic 198 ma the zircon ehf t values range from 5 to 20 yielding hf crustal model ages tdmc between 0 8 and 2 4 ga that peak at 1 7 ga suggesting a major episode of crustal growth in the proterozoic and a predominantly or even exclusively crustal source for the granitoid petrogenesis these zircon u pb and hf isotopic constraints together with whole rock geochemical characteristics allow us to correlate them to s type granitoids in the northern plutonic belt rather than to the i type gangdese batholith from the central part of the southern lhasa terrane in contrast to the gangdese magmatism that formed with a significant juvenile mantle input related to the neo tethyan subduction these s type granitoids show geochemical similarities to the north american cordilleran interior batholiths that did not form directly from a subduction setting thus we attribute the petrogenesis of the widespread early cretaceous granitoids in the northern belt and eastern himalayan batholiths to a postcollisional regime due to the late jurassic early cretaceous continental collision between the lhasa and qiangtang terranes under this framework the s type magmatism was generated in response to collision induced crustal thickening which may have capabilities of causing crustal anatexis by itself in the region however continued interplay with the neo tethyan subduction zone processes such as back arc extension could also have played a crucial and long lasting role in the magma generation

the african plate a history of oceanic crust accretion and subduction since the jurassic we present a model for the jurassic to present evolution of plate boundaries and oceanic crust of the african plate based on updated interpretation of magnetic gravity and other geological and geophysical data sets location of continent ocean boundaries and age and geometry of old oceanic crust jurassic and cretaceous are updated in the light of new data and models of passive margin formation a new set of oceanic palaeo age grid models constitutes the basis for estimating the dynamics of oceanic crust through time and can be used as input for quantifying the plate boundary forces that contributed to the african plate palaeo stresses and may have influenced the evolution of intracontinental sedimentary basins as a case study we compute a simple model of palaeo stress for the late cretaceous time in order to assess how ridge push slab pull and horizontal mantle drag might have influenced the continental african plate we note that the changes in length of various plate boundaries especially trenches do not correlate well with absolute plate motion but variations in the mean oceanic crust age seem to be reflected in acceleration or deceleration of the mean absolute plate velocity

bela ophiolite zone of southern pakistan tectonic setting and associated mineral deposits northwest of karachi the bela ophiolite zone is the southernmost exposure in a string of three major ophiolitic occurrences in the axial fold thrust belt which extends generally south and southwest from northern pakistan to the arabian sea allochthonous and para allochthonous jurassic rock units are tectonically associated with this ophiolite zone these units contain massive deposits of lead zinc sulfides and barite whereas the ophiolite zone hosts chromite manganese massive copper sulfide and related mineral deposits this study describes the regional tectonic setting and relationship to local geological features of the mineral deposits using gravity and aeromagnetic data these geophysical studies provide a basis for interpreting the emplacement models and deformation history of the bela ophiolite zone and associated jurassic rocks these models and their supporting geologic data suggest that hydrothermal solutions containing barite and massive sulfides rich in zinc and lead rose on the sea floor through crustal faults developed in areas of divergent tectonism and deposited these minerals in the jurassic sedimentary sequence on the western margin of indo pakistan subcontinental plate the cretaceous neotethyan spreading center however gave rise to metallic mineralizations the tertiary convergence between the rifted western margin of the indian subcontinent and the neotethyan oceanic plate a segment of the arabian oceanic plate obducted the bela ophiolite zone consisting of e g chromite manganese massive copper sulfide along the overriding edge of the subcontinental plate which contained massive sulfides rich in zinc lead and barite these convergent phases through paleocene to pleistocene time intensely deformed the divergent products and created a geologically complex region

restoration of cenozoic deformation in asia and the size of greater india a long standing problem in the geological evolution of the india asia collision zone is how and where convergence between india and asia was accommodated since collision proposed collision ages vary from 65 to 35 ma although most data sets are consistent with collision being underway by 50 ma plate reconstructions show that since 50 ma 2400 3200 km west to east of india asia convergence occurred much more than the 450 900 km of documented himalayan shortening current models therefore suggest that most post 50 ma convergence was accommodated north of the indus yarlung suture zone we review kinematic data and construct an updated restoration of cenozoic asian deformation to test this assumption we show that geologic studies have documented 600 750 km of n s cenozoic shortening across and north of the tibetan plateau the pamir hindu kush region accommodated 1050 km of n s convergence geological evidence from tibet is inconsistent with models that propose 750 1250 km of eastward extrusion of indochina approximately 250 km of indochinese extrusion from 30 to 20 ma of indochina suggested by se asia reconstructions can be reconciled by dextral transpression in eastern tibet we use our reconstruction to calculate the required size of greater india as a function of collision age even with a 35 ma collision age the size of greater india is 2 3 times larger than himalayan shortening for a 50 ma collision the size of greater india from west to east is 1350 2600 km consistent with robust paleomagnetic data from upper cretaceous paleocene tethyan himalayan strata these estimates for the size of greater india far exceed documented shortening in the himalaya we conclude that most of greater india was consumed by subduction or underthrusting without leaving a geological record that has been recognized at the surface

the jurassic history of the africa antarctica corridor new constraints from magnetic data on the conjugate continental margins abstract finding the best fit for east and west gondwana requires a detailed knowledge of the initial jurassic movements between africa and antarctica this study presents results of systematic and densely spaced aeromagnetic measurements which have been conducted in 2009 2010 across the astrid ridge antarctica and in the western riiser larsen sea to provide constraints for the early seafloor spreading history between both continents the data reveal different magnetic signatures of the northern and southern parts of the astrid ridge which are separated by the astrid fracture zone the southern part is weakly magnetised corresponding to the low amplitude anomaly field of the southwestern riiser larsen sea in contrast the northern astrid ridge bears strong anomalies of positive value furthermore several sets of trends are visible in the data in the mozambique channel we extended the existing magnetic spreading anomaly identifications combined with the spreading anomalies in the conjugate riiser larsen sea they were used to establish a new model of the relative movements of africa and antarctica after the breakup of gondwana in jurassic times a detailed model for the emplacement of the mozambique ridge is now incorporated the model postulates a tight fit between africa and antarctica and two stages of breakup the first of which lasting until 159 ma m33n during this stage antarctica rotated anticlockwise with respect to africa the grunehogna craton cleared the coastal plains of mozambique to a position east of the mozambique fracture zone the southern astrid ridge is interpreted to consist of oceanic crust also formed during this first stage prior to the riiser larsen sea during the second stage antarctica moved southward with respect to africa forming the mozambique basin and the conjugate riiser larsen sea the mozambique ridge and the northern natal valley were formed at different spreading centres being active subsequently

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

rapid emplacement of young oceanic lithosphere argon geochronology of the oman ophiolite 40 ar 39 ar dates of emplacement related metamorphic rocks beneath the samail ophiolite in oman show that cooling to

oblique convergence and the lobate mountain belts of western pakistan the thin skinned structures of the pakistani convergent margin have formed as a consequence of the relative motion between india and eurasia most of the resultant motion is being accommodated along or near the current edge of the eurasian plate the southwest northeast striking chaman fault zone it has been observed at oblique margins that the total plate motion is resolved into a component parallel to the margin accommodated through strike slip faulting and a component normal to the margin taken up as contraction however the orientations of structures along the pakistani convergent margin in and around the sulaiman lobe and sulaiman range cannot be explained simply by resolving the plate motion vector into components normal and parallel to the plate boundary our modeling suggests that the complex juxtaposition of strike slip faults with thrust faults of various orientations can be explained by the presence of a block centered upon the katawaz basin that translates along the southwest northeast structural barrier of the chaman fault zone moving with respect to both eurasia and india as this relatively rigid block moves northeastward relative to asia it causes deformation of the sedimentary cover and is responsible for much of the structural complexity in the pakistani foreland our simple model explains several first order features of this oblique margin such as the eastward facing sulaiman range the strike slip kingri fault located between the sulaiman lobe and sulaiman range and the reentrant at sibi this leads us to conclude that very complex structural and geometric relationships at oblique convergent plate boundaries can result from the accommodation of strain with simple initial geometric constraints

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

new age constraints for the geodynamic evolution of the sistan suture zone eastern iran abstract the sistan suture zone ssz in eastern iran extends as a n s trending belt over more than 700 km along the border area between iran and afghanistan the ssz formed as a result of eastward directed subduction of a neotethyan ocean basin beneath the afghan block and includes a tectonic melange consisting of disrupted meta ophiolitic rocks within a low grade matrix of ultramafic mafic and pelitic schists some melange blocks were affected by eclogite blueschist and or epidote amphibolite facies p t conditions understanding of the petrological and geochronological record of these rocks plays a key role in unravelling the geodynamic evolution of the ssz the main aim of the present study was to assess the geological significance of previously published 40ar 39ar ages c 116 139 ma which have not provided robust age constraints for geodynamic reconstructions on a regional scale for this purpose samples were collected within a nnw sse trending belt spanning a distance of c 120 km that exposes the major occurrences of high pressure low temperature rocks and epidote amphibolites multi point rb sr mineral isochrons indicate a regional consistent pattern with ages ranging between c 83 and c 87 ma for eight samples representing different metamorphic grade and widely separated locations weighted mean 85 7 0 8 ma additional 40ar 39ar dating for five of these samples yielded in most cases ages that are identical to the rb sr results within analytical uncertainty for one sample the 40ar 39ar age of 81 3 1 3 ma is about 3 myr younger than the corresponding rb sr age a still younger rb sr age of 78 9 0 5 ma was determined for a retrograde epidote biotite assemblage in an overprinted domain of an eclogite ion probe u pb zircon ages for two eclogites and two meta acidic rocks from the melange yielded weighted mean 206pb 238u ages of 87 3 1 4 ma 86 1 1 5 ma 88 7 1 4 ma and 87 2 1 2 ma respectively the results of this study do not support previous interpretations suggesting 125 ma high pressure low temperature metamorphism and amphibolite facies overprinting but instead document the importance of late cretaceous subduction zone processes for the geodynamic evolution of the ssz

biotic versus abiotic calcite formation on prehistoric cave paintings the arcy sur cure grande grotte yonne france case abstract the grande grotte cave at arcy sur cure yonne france with its prehistoric paintings shows important calcite concretions two types of calcite have been observed on the wall translucent yellowish layers and opaque white or grey layers that completely obstruct the paintings other calcite types are present in the lakes of the cave floating calcite rafts at the surface of the lake and soft calcite at the bottom of the lake the morphology of the different calcites was observed at different scales by optical microscopy with normal and polarized light scanning electron microscopy sem and transmission electron microscopy tem the elemental composition was measured by using particle induced x ray emission micro pixe and the structure by x ray diffraction xrd infrared ft ir and raman spectroscopy the bacterial diversity and its role in calcite formation were assessed by culture and 16s sscp in order to distinguish and to assess various abiotic and biotic formation mechanisms the investigation of calcite characteristics enables conclusions on the formation mechanism and on a biotic or abiotic origin of the calcites the change of calcite types on the walls reveals changes of the environmental cave parameters in addition interactions of calcites with the prehistoric paint layer could be evaluated

early india australia spreading history revealed by newly detected mesozoic magnetic anomalies in the perth abyssal plain the seafloor within the perth abyssal plain pap offshorewestern australia is the only section of crust that directly records the early spreading history between india and australia during the mesozoic breakup of gondwana however this early spreading has been poorly constrained due to an absence of data including marine magnetic anomalies and data constraining the crustal nature of key tectonic features here we present new magnetic anomaly data from the pap that shows that the crust in the western part of the basin was part of the indian plate the conjugate flank to the oceanic crust immediately offshore the perth margin australia we identify a sequence of m2 and older anomalies in the west pap within crust that initially moved with the indian plate formed at intermediate half spreading rates 35 mm yr consistent with the conjugate sequence on the australian plate more speculatively we reinterpret the youngest anomalies in the east pap finding that the m0 age crust initially formed on the indian plate was transferred to the australian plate by a westward jump or propagation of the spreading ridge shortly afterm0 time samples dredged from the gulden draak and batavia knolls at the western edge of the pap reveal that these bathymetric features are continental fragments rather than igneous plateaus related to broken ridge these microcontinents rifted away from australia with greater india during initial breakup at 130ma then rifted from india following the cessation of spreading in the pap 101 103ma

passive roof duplex geometry in the frontal structures of the kirthar and sulaiman mountain belts pakistan abstract exploration for hydrocarbons over the past few years has greatly improved our understanding of the geometry of frontal mountain belt structures in this study we introduce and discuss the concept of the passive roof duplex using as the main example the kirthar and sulaiman ranges in the baluchistan province of pakistan structures similar to those described here have been recognized previously in other mountain belts and they appear to exist as a common feature in many more frontal regions of mountain belts our example of a passive roof duplex which we describe from pakistan is compared briefly with similar structures reported by others the passive roof duplex is here defined as a duplex whose roof thrust has backthrust sense passive roof thrust and whose roof sequence those rocks lying above the roof thrust remains relatively stationary during foreland directed piggy back style propagation of horses within the duplex