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

india madagascar paleo fit based on flexural isostasy of their rifted margins the present study contributes new constraints on and definitions of the reconstructed plate margins of india and madagascar based on flexural isostasy along the western continental margin of india wcmi and the eastern continental margin of madagascar ecmm we have estimated the nature of isostasy and crustal geometry along the two margins and have examined their possible conjugate structure here we utilize elastic thickness te and moho depth data as the primary basis for the correlation of these passive margins we employ the flexure inversion technique that operates in spatial domain in order to estimate the spatial variation of effective elastic thickness gravity inversion and flexure inversion techniques are used to estimate the configuration of the moho crust mantle interface that reveals regional correlations with the elastic thickness variations these results correlate well with the continental and oceanic segments of the indian and african plates the present study has found a linear zone of anomalously low te 1 5 km along the wcmi similar to 1680 km which correlates well with the low te patterns obtained all along the ecmm we suggest that the low te zones along the wcmi and ecmm represent paleo rift inception points of lithosphere thermally and mechanically weakened by the combined effects of the marion hotspot and lithospheric extension due to rifting we have produced an india madagascar paleo fit representing the initial phase of separation based on the te estimates of the rifted conjugate margins which is confirmed by a close fit correlation of moho geometry and bathymetry of the shelf margins the matching of tectonic lineaments lithologies and geochronological belts between india and madagascar provide an additional support for the present plate reconstruction c 2014 international association for gondwana research published by elsevier b v all rights reserved

structure and isostatic compensation of the comorin ridge north central indian ocean summary bathymetry gravity and magnetic data about 9200 lkm of the comorin ridge north central indian ocean were investigated using the transfer function and forward model techniques to understand the mode of isostatic compensation and origin of the ridge the ridge extends for about 500 km in nnw sse direction and associates with low amplitude gravity anomalies ranging from 25 to 30 mgal compared to the ridge relief suggesting that the anomalies are compensated at deeper depths from admittance analysis an airy model or local compensation with an elastic plate thickness t e of about 3 km and crust thickness t of 15 20 km are suggested for the southern part of the comorin ridge south of 5 n whereas for the northern part a flexural plate model with an elastic thickness of about 15 km is obtained admittance analysis together with the results from gravity forward modelling reveal that the south part was emplaced on relatively weak oceanic crust with both surface and subsurface loading while the north part was emplaced on the continental crust based on present studies and published plate kinematic models we interpret that the comorin ridge was evolved at about 90 ma during the rift stage of madagascar from the southwest of india we have also demarcated the continent ocean boundary cob west of sri lanka and southern tip of india which runs across the strike of the ridge placing the northern part of the ridge on continent and southern part on oceanic crust on the southern part of the ridge eastern flank is steep faulted up to 0 6 km and is controlled by the 79 e fz and then by cob

recurrent early cretaceous indo madagascar 89 86 ma and deccan 66 ma alkaline magmatism in the sarnu dandali complex rajasthan 40 ar 39 ar age evidence and geodynamic significance abstract the sarnu dandali alkaline complex in rajasthan northwestern india is considered to represent early pre flood basalt magmatism in the deccan traps province based on a single 40 ar 39 ar age of 68 57 ma rhyolites found in the complex are considered to be 750 ma malani basement our new 40 ar 39 ar ages of 88 9 86 8 ma for syenites nephelinite phonolite and rhyolite and 66 3 0 4 ma 2 melanephelinite provide clear evidence that whereas the complex has deccan age 66 ma components it is dominantly an older by 20 million years alkaline complex with rhyolites included basalt is also known to underlie the early cretaceous sarnu sandstone sarnu dandali is thus a periodically rejuvenated alkaline igneous centre active twice in the late cretaceous and also earlier many such centres with recurrent continental alkaline magmatism sometimes over hundreds of millions of years are known worldwide the 88 9 86 8 ma 40 ar 39 ar ages for sarnu dandali rocks fully overlap with those for the indo madagascar flood basalt province formed during continental breakup between india plus seychelles and madagascar recent 40 ar 39 ar work on the mundwara alkaline complex in rajasthan 120 km southeast of sarnu dandali has also shown polychronous emplacement over 45 million years and 84 80 ma ages obtained from mundwara also arguably represent post breakup stages of the indo madagascar flood basalt volcanism remnants of the indo madagascar province are known from several localities in southern india but hitherto unknown from northwestern india 2000 km away additional equivalents buried under the vast deccan traps are highly likely

sedimentary mercury enrichments as a marker for submarine large igneous province volcanism evidence from the mid cenomanian event and oceanic anoxic event 2 late cretaceous oceanic anoxic event 2 oae 2 during the cenomanian turonian transition 94 ma was the largest perturbation of the global carbon cycle in the mid cretaceous and can be recognized by a positive carbon isotope excursion in sedimentary strata although oae 2 has been linked to large scale volcanism several large igneous provinces lips were active at this time e g caribbean high arctic madagascan ontong java and little clear evidence links oae 2 to a specific lip the mid cenomanian event mce 96 ma identified by a small 1 positive carbon isotope excursion is often referred to as a prelude to oae 2 however no underlying cause has yet been demonstrated and its relationship to oae 2 is poorly constrained here we report sedimentary mercury hg concentration data from four sites three from the southern margin of the western interior seaway and one from demerara rise in the equatorial proto north atlantic ocean we find that in both areas increases in mercury concentrations and hg toc ratios coincide with the mce and the oae 2 however the increases found in these sites are of a lower magnitude than those found in records of many other mesozoic events possibly characteristic of a marine rather than atmospheric dispersal of mercury for both events combined the new mercury data presented here are consistent with an initial magmatic pulse at the time of the mce with a second greater pulse at the onset of oae 2 possibly related to the emplacement of lips in the pacific ocean and or the high arctic

the basement of the deccan traps and its madagascar connection constraints from xenoliths paleogeographic reconstructions of india and madagascar before their late cretaceous rifting juxtapose the antongil block of madagascar against the deccan traps of india indicating that the western dharwar craton extends below the deccan lavas some recent studies have suggested that the south maharashtra shear zone along the northern konkan coast of india limits the northern extent of the western dharwar craton implying that the craton does not extend below the deccan traps raising a question mark on paleogeographic reconstructions of india and madagascar the continuity of the western dharwar craton north of the south maharashtra shear zone below the deccan traps or its lack thereof is critical for validating tectonic models correlating madagascar with india in this study zircons in tonalitic basement xenoliths hosted in deccan trap dykes were dated in situ using the u pb isotope system the data furnish u pb ages that define three populations at 2527 6 2456 6 and 2379 9 ma the 2527 6 ma ages correspond to the igneous crystallization of the tonalites whereas the 2456 6 and 2379 9 ma ages date metamorphic overprints the results help to establish for the first time that the basement is a part of the neoarchean granitoid suite of the western dharwar craton which extends northward up to at least talvade in central and kihim beach in the western deccan by implication the south maharashtra shear zone cannot be the northern limit of the western dharwar craton the granitoids are correlated with the neoarchean felsic intrusions 2 57 2 49 of the masaola suite in the antongil block of madagascar supporting the existence of a neoarchean greater dharwar craton comprising the western dharwar craton and the antongil masora block

morphotectonic characteristics distribution and probable genesis of bathymetric highs off southwest coast of india abstract the western continental margin of india and the adjacent deep ocean basins were formed by break up and separation among india seychelles and madagascar since the late cretaceous the initial india madagascar separation and the subsequent india seychelles separation are believed to have been caused by the marion hotspot at 90 ma and the reunion hotspot at 68 5 ma respectively these geodynamic events resulted in the formation of several bathymetric highs that probably represent imprints of these volcanic events in the present study these bathymetric high features were mapped comprehensively to understand their morphotectonic characteristics using a fresh set of multibeam bathymetry sea surface gravity and magnetic anomalies complemented by the available multichannel seismic reflection sections a high resolution bathymetric map of the southwestern continental margin of india and the adjoining deep offshore regions has been generated to decipher detailed morphological configuration and distribution of prominent undersea bathymetric features we also carried out detailed morphometric analysis of these features to deduce the morphological parameters a total of 33 individual bathymetric high features were identified and classified as seamounts hills knolls guyots and plateaus based on the standardization of undersea feature names published by intergovernmental oceanographic commission ioc and international hydrographic organization iho in 2013 multichannel seismic reflection sea surface gravity and magnetic data were used to describe the sub seafloor configuration and qualitative interpretation of the geophysical signatures associated with the bathymetric highs interpretation of the multichannel seismic reflection sections suggest that some of these identified features are extrusive in nature while others are intrusive these features are associated with characteristic gravity highs superimposed over regional negative anomalies and complex negative and positive magnetic anomalies the study results suggest that the genesis of the bathymetric highs mapped in the study area could be attributed to the hotspot volcanism caused by the marion or reunion hotspots we infer that the features in the southwestern continental margin of india closer to the alleppey trivandrum terrace complex might have been created by the marion hotspot volcanism while those in the laccadive basin and eastern sector of the laccadive plateau might have been formed by reunion hotspot volcanism

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

relative hotspot motions versus true polar wander abstract the fixity of hotspots and mantle plume locations has long been axiomatic if the assumption of fixed hotspots is granted absolute plate motions and movements of the spin axis with respect to the hotspot framework defined by some as true polar wander tpw can be determined however this assumption can be tested by paleomagnetic data and such tests are gradually raising some doubts about the fixity of hotspots the result is that discrepancies between cretaceous and tertiary hotspot and paleomagnetic reference frames are now beginning to be interpreted as the result of plume drift within a convective mantle in the indo atlantic hotspots have remained relatively stationary with respect to the spin axis for the last 95 million yr however the pacific hotspots notably hawaii appear to have undergone large scale southward drift with respect to the spin axis during the early tertiary global paleomagnetic data do not indicate that any tpw occurred during the late cretaceous or tertiary although the early cretaceous paleomagnetic and hotspot frames for the indo atlantic realm can be interpreted as slow tpw direct estimates of paleolatitude and hotspot motion in particular the kerguelen hotspot challenge tpw as a global phenomenon at present we consider that the large early cretaceous discrepancy between hotspot and paleomagnetic data is best explained by southward drift of the atlantic hotspots prior to 95 ma

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

an insight into the breakup of gondwana identifying events through low temperature thermochronology from the basement rocks of madagascar 1 fission track analysis was applied to the precambrian suites of madagascar in order to identify the lower temperature cooling histories and their relationships to the phanerozoic events that affected the island apatite ages range from 431 to 68 ma and zircon ages range from 452 to 238 ma thermochronologically the island can be divided into a southern central and northern region each with a subdivision on an east west basis the southern region is sharply separated from the central region by strongly contrasting apparent apatite ages over the northwest southeast striking ranotsara shear zone rsz the change in apparent ages over the rsz is indicative of later reactivation along younger brittle faults the central region has the oldest ages of the island and has a diffuse contact to the third region northward along the entire western margin of the precambrian basement initial paleozoic exhumation was followed by heating burial by sediments during jurassic and cretaceous times a decrease in ages along the eastern margin from 119 to 68 ma coincides with the predicted positions of the marion hot spot after effects of erosion are considered on the other hand these ages may represent progressive opening of the margin in a southward direction together with associated denudation of the rift shoulder the eastern part of the central region has remained very stable since at least devonian times undergoing only long term very slow exhumation at rates of 1 5 m myr

does deccan volcanic sequence contain more reversals than the three chron n r n flow magnetostratigraphy a palaeomagnetic evidence from the dyke swarm near mumbai

the terrace like feature in the mid continental slope region off trivandrum and a plausible model for india madagascar juxtaposition in immediate pre drift scenario abstract bathymetry of the southwestern continental margin of india reveals the presence of an anomalous terrace like feature in the mid continental slope region off trivandrum the genesis of this terrace of large areal extent 9000 sq km is yet to be established based on exercises with several existing paleogeographic reconstruction models and updated compilation of identified offshore tectonic elements this study attempts to identify a plausible model of india madagascar juxtaposition in immediate pre drift scenario which provides idea about genesis of this terrace it is inferred that the terrace off trivandrum and an anomalous bathymetric notch located in the northern madagascar ridge are conjugate features related to india madagascar separation and the rifted and sheared segments of the pre drift plate boundary have shaped their outlines the drifting of india from madagascar is suggested to have commenced at about 86 5 ma

magnetostratigraphy and 40ar 39ar geochronology of the malwa plateau region northern deccan traps central western india significance and correlation with the main deccan large igneous province sequences abstract basaltic lava flows of malwa plateau together with the mandla area and the toranmal section constitute the northern deccan province in central western india with the present outcrop area of 80 000 km2 the malwa flows form an extensive but relatively thin sequence we sampled deccan basalts along two 100 km each n s and e w transects along the western and southern rims of the malwa plateau the 67 12 0 44 ma 40ar 39ar age obtained in the present study clearly shows that the eruption of the malwa flows started during the magnetochron c30n by mapping the magnetic c30n c29r transition on the western and southern edge of the plateau the total volume and eruption rate can be calculated 42 000 km3 0 03 km3 a furthermore the results point towards the narmada tapi rift zone as the possible source area for the malwa lava flows the malwa sequence is older and partly synchronous with the thick sequences of the western ghats since their eruption centers are spatially independent they should not be correlated with younger although geochemically similar formations of the western ghats other northern deccan flows show clear similarities with the malwa flows and can be magnetostratigraphically and geochemically correlated we therefore suggest a new subgroup for the northern deccan province the narmada subgroup defined on the geochemical and magnetic characteristics of the malwa mandla and toranmal flows the introduction of this new subgroup clarifies the position of the northern deccan province in the deccan stratigraphy and is a step towards a better understanding of the deccan eruption history

geochemical provinciality in the cretaceous basaltic magmatism of northern madagascar mantle source implications the northern part of the late cretaceous flood basalt province of madagascar is formed by lava flows dykes and intrusive complexes these range from tholeiitic to weakly alkaline and have composition from basalts to basaltic andesites with rare more evolved rocks the magmatic evolution is dominated by low pressure crystal fractionation of olivine cr spinel plagioclase and clinopyroxene the erupted rocks form two subprovinces in terms of incompatible element concentrations or ratios and geographical position the mafic and intermediate samples from the western subprovince have low abundances of high field strength elements e g nb 2 9 ppm zr 50 230 ppm negative nb anomalies on mantle normalized diagrams and a wide range of initial 88 ma 143 nd 144 nd 0 51293 0 51197 e nd 7 9 to 10 9 these elemental and isotopic variations are compatible with melting of predominantly n morb like mantle and variable low pressure crustal contamination the mafic samples of the eastern subprovince are mildly enriched in incompatible elements e g nb 6 25 ppm zr 109 330 ppm with the highest values found in tholeiites from the east mahajanga basin tampoketsa kamoreen and tamatave they also have a comparatively narrow range of initial 143 nd 144 nd 0 51276 0 51258 e nd 5 1 to 1 0 consistent with derivation from incompatible element enriched mantle sources and minor subsequent crustal contamination the marked differences in the chemical and isotopic characteristics of the samples imply variable degrees of partial melting of hotter than normal chemically heterogeneous mantle sources at pressures where spinel to garnet are residual phases no clear evidence is seen for a chemical component similar to that represented at present by the marion hotspot lavas

correlations between silicic volcanic rocks of the st mary s islands southwestern india and eastern madagascar implications for late cretaceous india madagascar reconstructions abstract the st mary9s islands southwestern india expose silicic volcanic and sub volcanic rocks rhyolites and granophyric dacites emplaced contemporaneously with the cretaceous igneous province of madagascar roughly 88 90 ma ago the st mary9s islands rocks have phenocrysts of plagioclase clinopyroxene orthopyroxene and opaque oxide moderate enrichment in the incompatible elements e g zr 580 720 ppm nb 43 53 ppm la yb n 6 9 7 2 relatively low initial 87 sr 86 sr 0 7052 0 7055 and near chondritic initial 143 nd 144 nd 0 51248 0 51249 they have mineral chemical whole rock chemical and isotopic compositions very close to those of rhyolites exposed between vatomandry ilaka and mananjary in eastern madagascar and are distinctly different from rhyolites from other sectors of the madagascan province we therefore postulate that the st mary9s and the vatomandry ilaka mananjary silicic rock outcrops were adjacent before the late cretaceous rifting that split madagascar from india if so they provide a valuable tool to check and aid traditional cretaceous india madagascar reconstructions based on palaeomagnetism matching precambrian geological features and geometric fitting of continental shelves supplementary material mineral analyses mass balance calculations and locality information are available at http www geolsoc org uk sup18332

the karnataka late cretaceous dykes as products of the marion hot spot at the madagascar india breakup event evidence from 40ar 39ar geochronology and geochemistry two late cretaceous mafic dykes with an ene strike that is orthogonal to the west coast of india and located nearly 200 km inland around huliyardurga karnataka state yield 40ar 39ar plateau ages of 90 0 1 0 and 87 5 0 9 ma these fe ti enriched tholeiites are essentially co eval with at least four other igneous suites widely scattered in southern india namely the south and north kerala dykes the agali anaikatti dykes of central kerala tamil nadu and lavas of st mary islands off the west coast of india the karnataka cretaceous dykes are also co eval and compositionally very similar to the fe ti enriched tholeiitic lavas and dykes around mananjary a major phase of late cretaceous magmatism along the eastern rifted margin of madagascar which are believed to be products of the marion hot spot that extruded at 88 ma synchronous with the india madagascar break up event the age and compositional similarities between these late cretaceous magmatic rocks from india and madagascar constitute a clear evidence for extension of the thermal manifestations deep into the indian peninsula

transient basin as indicator of tectonic expressions in bedrock landscape approach based on matlab geomorphic tool transient profiler abstract the tectonic and erosional processes sculpture the earth s surface producing a landscape that preserves the evolutionary history of rock uplift and incision the bedrock channels act as an efficient gradation agent where the river profile adjusts to changing tectonic climatic or topological e g river capture boundary conditions and develops a transient basin a suite of slope break knickpoints at comparable elevations in the channel profiles of the transient basin act as important markers we developed a set of comprehensive and interactive matlab tools called a transient profiler to extract the geomorphic indices from the digital elevation model for a basin characterization and landscape evolution period the paleo channel reconstruction from the relict reach in the three studied transient drainage basins palar umiam and dibang suggest a varying surface uplift from 514 1246 and 1981 m with a respective relief gain of 158 230 and 95 respectively we found that the horizontal knickpoint retreat rate is faster in the himalayan thrust system as compared to the shillong pop up system whereas the vertical knickpoint retreat rate is faster in the latter

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

mantle conveyor beneath the tethyan collisional belt abstract collisional belts are generated by the arrival of continental lithosphere into a subduction zone the tethyan suture from the bitlis to the himalayas is a prime example where the arabian and indian plates collided with eurasia during the cenozoic while the kinematics of this process are well established its dynamics are more uncertain india and arabia intriguingly keep advancing in spite of large collisional resisting forces and in the absence of a substantial upper mantle slab driving force at present day we perform global mantle circulation computations to test the role of deep mantle flow as a driving force for the kinematics of the tethyan collisional belt evaluating different boundary conditions and mantle density distributions as inferred from seismic tomography or slab models our results show that mantle drag exerted on the base of the lithosphere by a large scale convective conveyor belt with an active upwelling component is likely the main cause for the ongoing indentation of the indian and arabian plates into eurasia

latest cretaceous himalayan tectonics obduction collision or deccan related uplift abstract diverging interpretations and incompatible scenarios have been proposed for the early stages of himalayan history numerous researchers have postulated that northern india was involved in ophiolite obduction arc continent or continent continent collision during the late cretaceous or early paleocene but firm geological evidence was never produced in this article we argue against orogenic events predating the late paleocene when the neotethys ocean was still open the tethys himalayan sedimentary record testifies to anorogenic evolution primarily controlled by dynamic uplift of the passive margin prior to the massive outburst of deccan lavas and eventually followed by thermal subsidence major stratigraphic gaps in pelagic sediments suggest that such tectono magmatic episode started to affect the base of the indian plate in the campanian or possibly even in the santonian 10 to 20 ma before the climax of deccan flood basalt eruptions the abrupt increase in siliciclastic supply and accumulation rates recorded in sedimentary basins all around the indian subcontinent during the maastrichtian was followed by progradation of coastal quartzarenites along the northern indian margin in the early paleocene sandstones derived from the rejuvenated craton and uplifted inner continental margin in the south are dominantly but not exclusively quartzose felsitic volcanic rock fragments and cr spinels many of which with the same geochemical fingerprint as deccan spinels and newly found throughout the maastrichtian to danian succession resisted the combined effect of subequatorial weathering and subsequent diagenesis and testify that detritus from deccan basalts reached the indian passive margin as far as south tibet at the close of the early paleocene india drifted away from the seychelles block and thermal subsidence led to widespread carbonate deposition along the tethys himalaya this article illustrates how mega events of magmatic upwelling followed by lithospheric cooling may control passive margin sedimentation and stratigraphic patterns as occurred in northern india first in the early cretaceous and next in the latest cretaceous paleocene

the crustal structure of the southern davie ridge offshore northern mozambique a wide angle seismic and potential field study abstract some of the oldest surviving oceanic basins in the world the mozambique and west somali basins were created during the breakup of gondwana starting around 180 ma between the two basins relative movements of west gondwana and east gondwana including madagascar created a shear zone the davie fracture zone dfz with a topographic elevation davie ridge dr marking its centre the crustal composition of the dfz and dr is a subject of speculation and debate in this study we present seismic refraction data across the prominent topography of the southern dr ray tracing of the wide angle data as well as additional seismic amplitude modelling and 2 5d density modelling constrain its crustal structure and architecture the data indicate that in the mozambique channel the dr consists of fragments of continental crust with a thickness of 10 to 12 km an oceanic crust indenter extends northward from the mozambique basin into the area between the dr and the east african margin at 16 5 s northeast of the dr at 41 8 w 14 5 s the somali basin is probably floored by 6 km thick oceanic crust hence the continental dr separates oceanic crust of the somali and mozambique basins the transitional crustal area at the central mozambican margin is underlain by high velocity lower crust hvlc the hvlc has velocities up to 7 3 km s and extents along the margin vanishing northward between 16 5 and 14 5 s at the madagascan side of the dr at 16 5 s the highly intruded stretched continental crust is 9 km thick and possibly underlain with a smaller hvlc of 2 9 km thickness and an e w extent of 120 km the oceanic crust at 14 5 s represents the oldest part the somali basin which formed after the initial nw se rifting between east and west gondwana

phanerozoic surface history of southern peninsular india from apatite u th sm he data quantifying bedrock cooling history is crucial for understanding the long term landform evolution across passive margins and its control onto the sediment routing system to constrain the low temperature cooling history and its relationships to the phanerozoic tectonic events of southern peninsular india we present new apatite u th sm he ahe analyses of 39 precambrian basement samples the new ahe ages range from 38 1 6 8 to 364 2 44 6 ma they are younger than 50 ma in the palghat gap region and older than 200 ma in the interior of the deccan plateau thermal modeling based on ahe data indicates enhanced cooling and exhumation in the interior of the deccan plateau by permian triassic times followed by gradual cooling up to the present this discrete episode of permian triassic cooling is associated with continental extension that preceded the early jurassic breakup of gondwana bedrock cooling and exhumation on the southeastern and southern limits of the deccan plateau was likely accomplished by late cretaceous drainage reorganization the distribution of old 200 ma ahe ages over the 2600 m high nilgiri plateau reflects very low erosion exhumation rates and adds to examples of long lived postorogenic topography the relatively younger ahe ages from the 30 km wide low mountain pass palghat gap within the western ghat mountains attest for intense cenozoic erosion likely facilitated by the erodible lithological backbone of the neoproterozoic shear zone ahe ages across the western coastal plain challenge the widely hold notion of 3 km of post breakup isostatic rebound in response to erosion of the margin instead the new ahe data are more compatible with less than 1 1 5 km of crustal denudation along the coastal strip

thermochronological dating of brittle structures in basement rocks a case study from the onshore passive margin of sw sri lanka 1 the brittle structural inventory of southern and southwestern sri lanka has been studied by kinematic mineralogical and thermochronological techniques thermochronological analyses of faults comprise apatite and zircon fission track ft data and apatite u th sm he ages from fault planes and undisturbed host rocks and range between 65 and 230 ma the ages of both settings are undistinguishable for topographic altitudes below 100 m while fault planes from higher elevations are significantly younger than the corresponding host rocks thermal history modeling and qualitative interpretation of the thermochronological data identify at least five episodes of thermal overprint associated with faulting activity occurring at 159 18 144 14 120 10 94 8 and 70 10 ma the kinematic mineralogical and thermochronological data collectively show that sri lanka was subjected to major n s oriented extension subsequent to the gondwana breakup the resulting brittle structures exert primary control on the island s geomorphology especially on the southern escarpment of the sri lankan highlands

crustal architecture and isostatic compensation of the comorin ridge central indian ocean implications for the breakup of east gondwana abstract the comorin ridge located off the southwest margin of sri lanka is an important geological feature with significant relevance to understand the breakup of eastern gondwanaland earlier geophysical studies over this ridge provided several contrasting views due to which its origin and nature of emplacement could not be established very well in this study an integrated interpretation of bathymetry gravity geoid and ship borne magnetic data is carried out to bring out a well constrained evolution model for the ridge results of 3 d gravity inversion of crustal bouguer anomalies and joint gravity magnetic modeling indicate that the southern part of the ridge is underlain by 7 12 km thick crust intruded with volcanic rocks and the presence of 13 18 km of anomalously thicker crust below the northern part of the ridge further crustal models spatial variations in geoid topography ratio gtr and the effective elastic thickness te value obtained in this study suggest a two stage emplacement model for the evolution of the ridge the observed lower te values 7 8 km in the southern part 0 5 n 3 5 n and its proximity to the c34ny spreading anomaly suggest its emplacement on the oceanic lithosphere due to marion plume activity along a fracture zone during 88 84 ma on the other hand relatively higher te values 12 5 15 km in the northern part 3 5 n 6 5 n with absence seafloor spreading magnetic anomalies suggests that it could be a marginal basement high formed at the continent ocean boundary cob during the second phase of rifting between india and sri lanka 94 ma

timeline of the south tibet himalayan belt the geochronological record of subduction collision and underthrusting from zircon and monazite u pb ages the exact timing of collision between india and eurasia is a recurring theme careful dating is critical for all tectonic events with the example of the south tibet himalaya collision system

distribution and origin of clay minerals in surficial shelf sediments western north island new zealand abstract western shelf surficial sediments are mainly terrigenous muds and variably muddy fine and very fine sands of mixed modern relict heritage overall their clay fraction is dominated by illite and smectite lesser amounts of chlorite and mixed layer clays and some kaolinite the close similarity of shelf clay mineral assemblages with those in adjacent river and harbour sediments demonstrates that offshore north south regional trends in clay mineralogy parallel regional changes in onshore geology involving both western north island and more distant northwestern south island sources the same detrital clay mineral patterns on the shelf have persisted during the sea level rise following the last glaciation and are complicated only by local concentrations of authigenic smectite from volcanogenic material on the basis of the distribution and origin of the major clay minerals five north south clay petrologic zones occur on the shelf 1 hamilton shelf abundant smectite and common poorly crystalline illite primarily detrital from oligocene mudstones some smectite authigenic from quaternary volcanogenic material 2 north taranaki shelf abundant crystalline illite and common mixed layer clays and crystalline chlorite primarily detrital from miocene mudstones 3 central taranaki shelf abundant crystalline illite and common smectite and chlorite smectite is authigenic from egmont derived andesitic material contemporaneous with deposition of detrital illite and some chlorite derived from northwestern south island via the westland current and from adjacent shelf zones 4 south taranaki shelf abundant crystalline illite and chlorite detrital via the westland current from northwestern south island rivers that drain an actively eroding granitic schistose provenance and 5 north cook strait basin abundant crystalline illite and common mixed layer clays and crystalline chlorite detrital from pliocene quaternary mudstones of the south taranaki wanganui region as well as from northwestern south island sources via the d urville current

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

refining gondwana and pangea palaeogeography estimates of phanerozoic non dipole octupole fields summary reliable phanerozoic paleopoles have been selected from the stable parts of the gondwana continents and upon appropriate reconstruction have been combined in an apparent polar wander apw path which can be compared with a previously compiled path for laurussia this comparison once again confirms that pangea a reconstructions for late palaeozoic and early mesozoic times cannot be reconciled with the available paleomagnetic data unless these data are corrected for latitudinal errors caused by non dipole octupole field contributions or by inclination shallowing because the discrepancies persist even when only paleopoles from igneous rocks are used inclination shallowing cannot be the sole cause of the problem there is an apparent decrease in the percentage of octupole field contributions needed as a function of time for mesozoic and younger time gondwana laurussia comparisons require on average lower ratios of octupole dipole fields than for palaeozoic time however the gondwana paleopoles for the palaeozoic include a much greater proportion of results derived from sedimentary rocks than do those for the mesozoic so that this apparently diminishing octupole field contribution may be an artefact we have also examined whether the clustering of coeval gondwana poles improves with optimal g3 contributions but found that while there are improvements they are not systematic and not statistically significant a combined apw path has been constructed for pangea for times since the mid carboniferous which accounts for octupole fields or equivalently inclination shallowing we argue that this global path is an improvement over previous constructions as it represents a self consistent plate tectonic model and does not violate widely accepted pangea a reconstructions

impact of tectonic and volcanism on the neogene evolution of isolated carbonate platforms sw indian ocean understanding the impact of tectonic activity and volcanism on long term i e millions years evolution of shallow water carbonate platforms represents a major issue for both industrial and academic perspectives the southern central mozambique channel is characterized by a 100 km long volcanic ridge hosting two guyots the hall and jaguar banks and a modern atoll bassas da india fringed by a large terrace dredge sampling geo physical acquisitions and submarines videos carried out during recent oceanographic cruises revealed that sub marine fl at top seamounts correspond to karsti fi ed and drowned shallow water carbonate platforms largely covered by volcanic material and structured by a dense network of normal faults microfacies and well constrained stratigraphic data indicate that these carbonate platforms developed in shallow water tropical envi ronments during miocene times and were characterized by biological assemblages dominated by corals larger benthic foraminifera red and green algae the drowning of these isolated carbonate platforms is revealed by the deposition of outer shelf sediments during the early pliocene and seems closely linked to 1 volcanic activity typi fi ed by the establishment of wide lava fl ow complexes and 2 to extensional tectonic deformation associat ed with high offset normal faults dividing the fl at top seamounts into distinctive structural blocks explosive vol canic activity also affected platform carbonates and was responsible for the formation of crater s and the deposition of tuff layers including carbonate fragments shallow water carbonate sedimentation resumed during late neogene time with the colonization of topographic highs inherited from tectonic deformation and volcanic accretion latest carbonate developments ultimately led to the formation of the bassas da india modern atoll the geological history of isolated carbonate platforms from the southern mozambique channel represents a new case illustrating the major impact of tectonic and volcanic activity on the long term evolution of shallow water car bonate platforms

re examination of geophysical data off northwest india implications to the late cretaceous plate tectonics between india and africa abstract the gop and laxmi basins lying off northwest india have been assigned ambiguous crustal types and evolution mechanisms the chagos laccadive ridge clr complex lying along the southwest coast of india has been attributed to different evolutionary processes late cretaceous seafloor spreading between india and africa formed the mascarene basin and the plate reconstruction models depict unequal crustal accretion in this basin re interpretation of magnetic data in the gop and laxmi basins suggests that the underlying oceanic crust was accreted contemporaneously from 79 ma at slow half spreading rates 0 6 to 1 5 cm yr separating the seychelles laxmi ridge complex from india the spreading ridge became extinct at 71 ma in the central region between 19 20 n and 65 5 67 e and at 68 7 ma in the gop basin extinction progressed southwards with time until 64 1 ma at 14 5 n in the laxmi basin this spreading probably limited the seafloor spreading in the northern mascarene basin while normal to fast spreading continued in the south the fracture zone fz2 acted as a major boundary between the northern and southern mascarene basin and may have also restricted the southward propagation of the laxmi basin spreading ridge plate reconstruction models generated in this study accommodate several microplates between madagascar and india and trace their movements over time within the india africa spreading corridor furthermore these models suggest the formation of the seychelles laxmi ridge microplate by 79 ma in a complex three plate system this microplate may have existed up to 64 1 ma and experienced an anticlockwise rotation of about 50 under the influence of the deccan hotspot activity the laxmi basin spreading ridge jumped southwestward splitting the seychelles laxmi ridge complex and initiating the carlsberg ridge the presence of about 160 km of oceanic crust in the gop and laxmi basins partially reduces the space problem faced in the earlier plate reconstruction models

how to identify oceanic crust evidence for a complex break up in the mozambique channel off east africa abstract the identification of oceanic crust at rifted margins plays a crucial role in academic research understanding rifting mechanisms and the architecture of continent ocean boundaries and is also important for hydrocarbon exploration extending into deeper water in this paper we provide a workflow for the determination of the crustal nature in the mozambique channel east of davie ridge by presenting a compilation of several geophysical attributes of oceanic crust at divergent margins previous reconstructions locate the davie ridge at the trace of a transform fault along which madagascar drifted to the south during the breakup of gondwana this implies a sharp transition from continental to oceanic crust seaward of davie ridge using new multichannel seismic profiles offshore northern mozambique we are able to identify distinct portions of stretched basement east of davie ridge two phases of deformation affecting the basement are observed with the initial phase resulting in the formation of rotated fault blocks bounded by listric faults half grabens are filled with wedge shaped syn extensional sediments overlain by a prominent unconformity that northward merges with the top of highly reflective mildly deformed basement interpreted as oceanic crust the second phase of deformation is associated with wrench faulting and probably correlates with the southward drift of madagascar which implies that the preceding phase affected basement generated or modified prior to the opening of the west somali basin we conclude that the basement is unlikely to consist of normal oceanic crust and suggest that the first extensional phase corresponds to rifting between madagascar and africa we find evidence for a wide area affected by strike slip deformation in contrast to the earlier proposed major single transform fault in the vicinity of davie ridge and suggest that the mozambique channel area to the north of madagascar may be classified as an oblique rather than sheared margin

spatial variability of 10be derived erosion rates across the southern peninsular indian escarpment a key to landscape evolution across passive margins abstract the persistence of significant topography in ancient tectonically inactive orogenic belts remains one of the outstanding questions in geomorphology in southern peninsular india the impressive topographic relief of the western ghat mountains in tectonic quiescence since at least ca 65 ma has raised important questions concerning the long term mechanism of topographic evolution quantifying the distribution of erosion in space and time is critical to understanding landscape evolution although the long term erosion rates are reasonably well known the short term erosion rates and the relative importance of factors controlling erosion in southern peninsular india are less well constrained we present a new suite of catchment averaged and local erosion rates using in situ produced 10be concentrations in river sediments and exposed bedrock samples in southern peninsular india catchment averaged erosion rates vary from 9 6 0 8 m ma 1 in the highlands to 114 3 13 8 m ma 1 on the escarpment side bedrock erosion rates range from 2 4 0 2 m ma 1 in the ridge top to 143 4 25 4 m ma 1 in active channel beds of the highlands catchment averaged erosion rates derived from the across escarpment westward draining catchments are significantly higher than those derived from the eastward draining over highland catchments the difference indicates that long term down wearing of the highland proceeds at lower rates than in the escarpment zones catchment averaged erosion rates are moderately correlated with mean hillslope angles and local relief whereas they are strongly correlated with catchment averaged channel steepness index this suggests that topographic steepness is the major control on the spatial variability of erosion while strong rainfall gradient is of minor importance in this area 10be derived average erosion rates in highlands are consistent with previous long term erosion rate estimated from thermochronometry these results collectively point to large scale steady state topography only decaying slowly with time steady state likely reflects the balance between erosion and isostatically driven uplift of the southern peninsular india

gondwana to asia plate tectonics paleogeography and the biological connectivity of the indian sub continent from the middle jurassic through latest eocene 166 35 ma abstract using the most up to the date information available we present a considerably revised plate tectonic and paleogeographic model for the indian ocean bordering continents from gondwana s middle jurassic break up through to india s collision with asia in the middle cenozoic the landmass framework is then used to explore the sometimes complex and occasionally counter intuitive patterns that have been observed in the fossil and extant biological records of india madagascar africa and eastern eurasia as well those of the more distal continents although the paleogeographic model confirms the traditional view that india became progressively more isolated from the major landmasses during the cretaceous and paleocene it is likely that at various times minor physiographic features principally ocean islands provided causeways and or stepping stone trails along which land animals could have migrated to from the sub continent aside from a likely link albeit broken by several marine gaps to africa for much of this time it is notable that the present day recent biota of madagascar indicates that the ancestors of five land mammal orders plus bats crossed the 400 km wide mozambique channel at different times in the cenozoic it is possible that the kerguelen plateau connected india and australia antarctica in the mid cretaceous approximately 115 90 ma later the seychelles mascarene plateau and nearby elevated sea floor areas could have allowed faunas to pass between southern india and madagascar in the late cretaceous from around 85 65 ma with an early cenozoic extension to this path forming as a result of the reunion hot spot trace islands growing on the ocean floor to the ssw of india the modelling also suggests that india s northward passage towards asia with eventual collision at 35 ma involved the ne corner of the sub continent making a glancing contact with sumatra followed by burma from 57 ma late paleocene onwards a scenario which is compatible with the fossil record indicating that india asia faunal exchanges began occurring at about this time finally we contend that a number of biologically based direct terrestrial migration routes that have been proposed for last 15 m y of the cretaceous asia to india antarctica to madagascar and or india can probably be dismissed because the marine barriers likely varying from 1000 up to 2500 km were simply too wide

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

the ezhimala igneous complex southern india possible imprint of late cretaceous magmatism within rift setting associated with india madagascar separation abstract the gabbro granophyre granite complex of ezhimala emplaced along the western rifted continental margin of india preserves evidence for bimodal magmatism with related magma mixing and mingling processes here we report petrological geochemical zircon u pb geochronological and lu hf isotopic data from the ezhimala igneous complex eic that provide insights into the late cretaceous magmatic activity field investigations and petrographic observations in zircon u pb data from the granophyres show emplacement ages of 93 21 0 6 ma and 94 26 0 92 ma the evolved lu hf isotopic systematics for these rocks are indicative of the involvement of older crustal material during magma genesis the geochemical systematics together with isotopic data suggest magma generation in a rift related setting and interaction with or melting of neoproterozoic basement rocks the timing of magmatism broadly correlates with the late cretaceous marion hotspot activity which is considered to be responsible for the break up of india and madagascar we thus interpret the eic to be one of the rare signatures in southern india for the final phase of rifting of gondwana

moho depth variations over the maldive ridge and adjoining arabian and central indian basins western indian ocean from three dimensional inversion of gravity anomalies abstract analysis of high resolution satellite derived free air gravity data has been undertaken in the greater maldive ridge gmr maldive ridge deep sea channel northern limit of chagos bank segment of the chagos laccadive ridge and the adjoining arabian and central indian basins a complete bouguer anomaly cba map was generated from the indian ocean geoidal low removed free air gravity hereinafter referred to as fag iogl data by incorporating bullard a b and c corrections using the parker method moho topography was initially computed by inverting the cba data from the cba the mantle residual gravity anomalies mrga were computed by incorporating gravity effects of sediments and lithospheric temperature and pressure induced anomalies further the mrga was inverted to get moho undulations from which the crustal thickness was also estimated it was found that incorporating the lithospheric thermal and pressure anomaly correction has provided substantial improvement in the computed moho depths especially in the oceanic areas but along the gmr there was not much variation in the moho thickness computed with and without the thermal and pressure gravity correction implying that the crustal thickness of the ridge does not depend on the oceanic isochrones used for the thermal corrections the estimated moho depths in the study area ranges from 7 km to 28 km and the crustal thickness from 2 km to 27 km the moho depths are shallower in regions closer to central indian ridge in the arabian basin i e the region to the west of the gmr is thinner compared to the region in the east central indian basin the thickest crust and the deepest moho are found below the n s trending gmr segment of the chagos laccadive ridge along the gmr the crustal thickness decreases from north to south with thickness of 27 km below the maldives ridge reducing to 9 km at 3 s and further increasing towards chagos bank even though there are similarities in crustal thickness between maldive ridge and other regions like mascarene plateau which was recently interpreted as underlain by continental crust much more geoscientific work including drilling has to be undertaken to finally confirm the exact nature of the ridge

subsidence history crustal structure and evolution of the somaliland yemen conjugate margin 1 we have used biostratigraphic data from deep exploration wells to determine the tectonic subsidence history of the somaliland northwestern somalia yemen conjugate margin a poorly known margin in the central part of the gulf of aden bathymetry and magnetic anomaly data suggest the gulf of aden is a young feature that formed following the rifting apart and breakup of the african and arabian plates 32 ma our tectonic subsidence data suggest however that the present day gulf of aden developed on an earlier mesozoic rift system the oldest episode of rifting initiated at 156 ma and lasted for 10 ma and had a nw se trend we interpret the rift as a late stage event associated with the breakup of gondwana and the separation of africa and madagascar at 80 ma there is evidence of an intermediate rift event which correlates with a rapid increase in spreading rate on the ridges separating the african and indian and african and antarctica plates and a contemporaneous slowing down of africa s plate motion the combined effect of all three rifting events has been to thin the crust and upper mantle by up to a factor of 2 the amount of thinning deduced from the wells is in accord with the crustal structure inferred from available seismic refraction data and process oriented gravity and flexure modeling the margin is asymmetric with a steeper gradient in the moho on the yemen side than the somaliland side the main discrepancy is on the yemen side where the gravity derived moho is 10 km deeper than the well derived moho we attribute the discrepancy to the addition of material at the base of the crust since rifting possibly magma sourced from the afar plume

geochemistry and paleomagnetism of late cretaceous mafic dikes in kerala southwest coast of india in relation to large igneous provinces and mantle plumes in the indian ocean region new geochemical and paleomagnetic results are presented on two late cretaceous dikes of the 85 90 ma leucogabbroic and doleritic dikes and the 65 70 ma dolerites in kerala india the dikes are rich in incompatible elements have fractionated patterns with light rare earth element enrichment and are akin geochemically to cretaceous basalts on the east coast of madagascar the magmas were formed at garnet lherzolite depths above the marion plume constituting part of a large igneous province in madagascar in contrast the 65 70 ma dolerites are moderately depleted in incompatible elements with almost flat rare earth element patterns and resemble the upper formations of the deccan traps and the tholeiitic dikes of the seychelles these dolerites were formed by melting of spinel lherzolite over the reunion plume paleomagnetic data from the dikes and the other coeval igneous units from south india provide the 90 ma pole latitude 24 longitude 293 a 95 5 9 n 18 sites for india the 65 70 ma dolerites possess both normal and reverse polarities and the mean pole latitude 36 longitude 283 a 95 5 7 n 10 sites compares well with the deccan superpole paleolatitude estimates indicate 5 southward migration for the marion plume and a northward migration for the reunion plume in conformity with global mantle circulation models however distinguishing migration of the reunion plume from the effects of true polar wander is difficult furthermore the geodynamic reconstructions extending the shear zones of southern madagascar into south india are not tenable

petrology geochemistry and magnetic properties of sadara sill evidence of rift related magmatism from kutch basin northwest india abstract mafic volcanic rocks of the mesozoic kutch basin represent the earliest phase of deccan volcanic activity an olivine clinopyroxene plagioclase phyric undersaturated basalt occurs as a sill near sadara in the pachham upland northern kutch the sadara sill is deformed and emplaced along faults the sill is alkaline in character and is transitional between basalt and basanite compared to primitive mantle the sadara sill is enriched in sr ba pb and lree but depleted in nb cr y cs and lu fractional crystallization of olivine and clinopyroxene from an alkaline mafic melt generated by low degree partial melting of mantle peridotite can explain the observed chemical variation in the sill irm and l f test experiments and mineral analyses show titano magnetite as the major remanence carrying magnetic mineral af and thermal demagnetizations of the sadara sill yielded a mean chrm direction as d 315 6 i 43 0 95 9 78 k 25 38 and the corresponding vgp at 25 s 114 6 e dp dm 6 58 11 6 the sadara sill pole is significantly different from those of the deccan 65 ma and the rajmahal traps 118 ma but is close to the cretaceous poles of 85 91 ma rock units from southern india this suggests a pre deccan age for the sill

plume scar in the mantle lithosphere beneath the seychelles revealed by seismic imaging abstract continental flood basalts are commonly thought to form by direct melting of lithospheric mantle either during rifting episodes or when a mantle plume impinges on the base of the lithosphere if correct then significant alteration of the chemical and physical properties of the lithospheric mantle should be observable beneath such provinces to date however this hypothesis remains unproved through lack of direct evidence here we present the results from a seismic experiment conducted in the seychelles a region that separated from india at the time of the eruption of the deccan traps the analysis shows a stratified upper mantle with exceptionally low s wave velocities at depths between 50 and 190 km it is difficult to explain these anomalies without invoking the presence of melt however the depth extent of the low velocities rules out the presence of silicate melt we argue that the most probable interpretation is that these layers contain sulphide melt trapped from earlier interactions of the lithosphere with episodes of continental flood basalt generation the presence of this melt over a depth interval of 130 km suggests that it is stagnant and has remained coupled with the seychelles lithosphere

an integrated geophysical study east of the southern chagos laccadive ridge complex central indian ocean basin implications for the rodriguez triple junction dynamics in the late cretaceous abstract the central indian ocean basin east of the southern chagos laccadive ridge complex has evolved due to seafloor spreading in the n s direction since the late cretaceous the trace of the rodriguez triple junction rtj has been inferred in this basin but its exact location and dynamics prior to c29o are not known about 20 000 km of total magnetic intensity data along with satellite gravity mosaic were used to understand the dynamics of the rtj in the late cretaceous synthetic seafloor spreading models endorse the presence of late cretaceous to early palaeogene magnetic anomalies c34y to c24o in the study area half spreading rates are initially low 32 mm yr between c34y to c33y and increase up to 78 mm yr by c28y thereafter a decreasing trend is seen 57 mm yr towards the west in the study area higher spreading rates 120 mm yr between c33y and c32ay and distorted anomalies between chrons c32ay and c30y are observed detailed geophysical analysis suggests that the higher spreading rates are due to the presence of transferred crust and the distorted anomalies are caused by the presence of fragmented tectonic blocks depicting microplate formation and rotation plate reconstruction models suggest that the rtj at c34y time lay southwest of comorin ridge which coincided with the northern edge of the conrad rise until c33y the rtj left a linear trace parallel to the fracture zone trend on both the indian and antarctic plates while it gradually moved eastwards on the african plate the rtj migrated westwards until c31y causing the transfer of a considerable amount of crust from the african plate to both the indian and antarctic plates thereafter the rtj rapidly reversed its migration direction towards the east until c29o causing the formation and rotation of a microplate on the indian plate and lengthening of the southwest indian ridge the present study suggests that this episodic migration of the rtj may be due to ridge propagation under the influence of the marion hotspot which was in the vicinity during that period this rtj migration resulted in microplate formation lithospheric transfer and thickened lithosphere similar to the inferences made at many triple junctions in the pacific ocean thus the origin of anomalous structures such as the crozet bank may be related to triple junction dynamics under the influence of marion hotspot further the counter clockwise rotation of the southwest indian ridge between c33y and c29y may have a causal link with the formation and rotation of the microplate which occurred during the same period the present study thus facilitated the inference of the dynamics of the rtj trace between c34y and c29o on the three plates

input of river derived sediment to the new zealand continental shelf ii mineralogy and composition abstract clay mineralogies and elemental compositions of river borne sediments from new zealand were examined as indicators of likely sources of the lithogenous component of sediments on the continental shelf relative occurrences of kaolin and smectite enabled the rivers to be fitted into three main groups these correspond closely with geography as follows 1 most of the rivers from north island 2 all the rivers from the north east coast of south island 3 most of the rivers from the remainder of south island elemental analyses of their sediments also led to a geographical grouping of rivers six main groups resulted from a q mode cluster analysis of elemental compositions these were the northernmost river wairoa river the east coast of north island the west coast of north island the east coast of south island the south east and west coasts of south island the west coast of south island there is substantial overlap between the groupings by clay mineralogy and by composition the results can be interpreted mainly in terms of the geologies of source areas exceptions to regional trends reflect either an unusual geology in the source areas or low sediment loads where soils contribute significantly to the sediments the terrigenous component of offshore sediments and new zealand riverine sediments have similar compositions suggesting that the former are derived in a relatively unmodified form from riverine sediments

alteration and submergence of basalts in kachchh gujarat india implications for the role of the deccan traps in the india seychelles break up

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

morphotectonic architecture of an india madagascar breakup related anomalous submarine terrace complex on the southwest continental margin of india abstract an anomalous lateral bathymetric protrusion alleppey trivandrum terrace complex attc in the form of two contiguous terrace like features exists in the mid continental slope off southwest coast of india this bathymetric protrusion and a bathymetric notch in the northern madagascar ridge nmr have been postulated as conjugate features related to india madagascar separation in late cretaceous however geoscientific studies aimed to understand the crustal structure and genesis of the attc or its postulated conjugate region are meagre and ambiguous in this context the present study based on recently acquired as well existing bathymetry gravity magnetic and multi channel seismic reflection data was carried out to understand the morphotectonic architecture of the attc the bathymetric data suggest an escarpment chain kairali escarpment cke to demarcate the westward limit of the attc and another escarpment quilon escarpment qe to demarcate the boundary between the southerly larger trivandrum terrace tt and northerly smaller alleppey terrace at the multichannel seismic reflection data suggests a block faulted basement and presence of a nearly n s trending wide basement high in the central part of the trivandrum terrace forward modelling of gravity data constrained by seismic reflection information suggests that the crustal structure of the attc region is comparable to thinned continental crust forward modelling of prominent high amplitude magnetic anomalies suggest that the thinned continental crust of the attc region also contains volcanic bodies as intrusives extrusives the cke is inferred to be a sheared continental margin segment along which the se coast of madagascar glided past india and the volcanic emplacements in the attc region perhaps relates to the marion hotspot volcanism

remagnetization of mesozoic limestones from the jaisalmer basin nw india summary high coercivity but low unblocking lb temperature components 80 100 c demonstrate that goethite is the principal carrier of the natural remanent magnetization nrm in jurassic and cretaceous limestone from the jaisalmer basin rajasthan nw india goethite bearing components declination 359 9 inclination 46 0 95 4 0 n 10 sites plot close to the present day field direction high unblocking hb components declination 172 0 inclination 45 6 95 9 1 n 7 sites are carried by haematite mostly with reverse polarity directions but undoubtedly not primary this remagnetization event mostly within a period of reverse magnetic polarity occurred after 35 ma and before 780 000 yr the last known reversal only palaeomagnetic data from a single cretaceous site declination 302 0 inclination 47 2 95 14 0 n 10 samples has potentially preserved a primary magnetization carried by normal polarity magnetite if correct this site substantiates a mid southerly palaeolatitude for india during mid aptian times 117 ma

effective elastic thickness along the conjugate passive margins of india madagascar and antarctica a re evaluation using the hermite multitaper bouguer coherence application abstract gondwana correlation studies had rationally positioned the western continental margin of india wcmi against the eastern continental margin of madagascar ecmm and the eastern continental margin of india ecmi against the eastern antarctica continental margin eacm this contribution computes the effective elastic thickness te of the lithospheres of these once conjugated continental margins using the multitaper bouguer coherence method the results reveal significantly low strength values te 2 km in the central segment of the wcmi that correlate with consistently low te values 2 3 km obtained throughout the entire marginal length of the ecmm this result is consistent with the previous te estimates of these margins and confirms the idea that the low te segments in the central part of the wcmi and along the ecmm represents paleo rift inception points of the lithospheric margins that was thermally and mechanically weakened by the combined action of the marion hotspot and lithospheric extension during the rifting the uniformly low te value 2 km along the eacm indicates a mechanically weak lithospheric margin probably due to considerable stretching of the lithosphere considering the fact that this margin remained almost stationary throughout its rift history in contrast the ecmi has comparatively high te variations 5 11 km that lack any correlation with the regional tectonic setting using gravity forward and inversion applications we find a leading order of influence of sediment load on the flexural properties of this marginal lithosphere the study concludes that the thick pile of the bengal fan sediments in the ecmi masks and has erased the signal of the original load induced topography and its gravity effect has biased the long wavelength part of the observed gravity signal the hence uncorrelated flat topography and deep lithospheric flexure together contribute a bias in the flexure modeling which likely accounts a relatively high te estimate

40ar 39ar dating of the mumbai tholeiites and panvel flexure intense 62 5 ma onshore offshore deccan magmatism during india laxmi ridge seychelles breakup

mountain building and mantle dynamics 1 mountain building at convergent margins requires tectonic forces that can overcome frictional resistance along large scale thrust faults and support the gravitational potential energy stored within the thickened crust of the orogen a general dynamic model for this process is still lacking here we propose that mountain belts can be classified between two end members first those of slab pull type where subduction is mainly confined to the upper mantle and rollback trench motion lead to moderately thick crustal stacks such as in the mediterranean second those of slab suction type where whole mantle convection cells conveyor belts lead to the more extreme expressions of orogeny such as the largely thickened crust and high plateaus of present day tibet and the altiplano for the slab suction type deep mantle convection produces the unique conditions to drag plates toward each other irrespective of their nature and other boundary conditions we support this hypothesis by analyzing the orogenic volcanic and convective history associated with the tertiary formation of the andes after 40 ma and himalayas after collision at 55 ma based on mantle circulation modeling and tectonic reconstructions we surmise that the forces necessary to sustain slab suction mountain building in those orogens derive after transient slab ponding from the mantle drag induced upon slab penetration into the lower mantle and from an associated surge of mantle upwelling beneath africa this process started at 65 55 ma for tibet himalaya when the tethyan slab penetrated into the lower mantle and 10 myr later in the andes when the nazca slab did this surge of mantle convection drags plates against each other generating the necessary compressional forces to create and sustain these two orogenic belts if our model is correct the available geological records of orogeny can be used to decipher time dependent mantle convection with implications for the supercontinental cycle

mid cretaceous polar standstill of south america motion of the atlantic hotspots and the birth of the andean cordillera article i nfo a paleomagnetic study on mid cretaceous rocks from the san bernardo foldbelt patagonia yields high unblocking temperature and high coercivity magnetizations the results indicate absence of relative vertical axis rotations during development of the foldbelt with the associated pole position being highly concordant with coeval poles from brazil and patagonia taken together mid cretaceous poles derived from studies in widely distributed localities provide supportive evidence that south america was essentially motionless with respect to the paleomagnetic axis from ca 125 to at least 100 ma the paleolatitudes of south america are not consistent with the occurrence of mid cretaceous true polar wander suggesting that the previously observed discrepancy between the paleomagnetic and the fixed indo atlantic hotspot reference frames be related to motion of the atlantic hotspots in agreement with this the discrepancy is diminished by half when the cretaceous poles of the americas are observed in a moving hotpots reference frame with the residual offset being comparable to that seen for younger time intervals the south american paleopoles and the moving hotspot framework provide a kinematic scenario that allows relating the extensional tectonics in the early stages of andean evolution with episodic divergence between the trench and the continental interior likewise the beginning of contractional events correlates with model predicted westward acceleration of south america in the late cretaceous suggesting that the continent episodically overrode the andean trench by those times we argue that this change in andean tectonic regime is associated to major plate reorganization at ca 95 ma

active concomitant counterclockwise rotation and northwards translation of africa during the albian campanian time a paleomagnetic study on the wadi natash alkaline volcanic province 104 78ma south eastern desert egypt abstract africa was a key tectonic unit in the geologic evolution of the tethyan realm in the middle late cretaceous during the rifting of the south equatorial atlantic ocean this research sheds light on the tectonic evolution of africa during the albian campanian time the two end members of the wadi natash alkaline volcanic province wnavp 104 78 ma in the south eastern desert of egypt were studied paleomagnetically the wnavp 24 5 n 34 25 e is made up of a thick succession of differentiated lava flows grading from alkali olivine basalt aob followed by hawaiite mugearite and benmoreite flows with abundant sandstone interbeds all intruded by a large number of trachyte phonolite tr ph circular lava domes and ring dykes forming conspicuous landmarks in the province the oldest flows of the aob 104 7 ma and the youngest tr ph plugs and ring dykes 86 78 ma as well as the interbedded turonian taref sandstone formation previously known as nubian sandstone were sampled all over the province which covers more than 500 km 2 the isothermal remanent magnetization irm study pointed to magnetite family as the major remanence in wadi natash volcanics with little contribution from goethite hematite minerals on the other hand goethite and or hematite are the dominant remanence carriers in the interbedded taref sandstone after the progressive stepwise thermal demagnetization of all samples the characteristic remanence chrm direction for each rock type was visually isolated and its best fit line was calculated followed by the site means rock unit means and the paleomagnetic poles this paleomagnetic study revealed that 1 in the tilt corrected coordinates the mean chrm of the oldest aob flows n 12 sites was dec inc 340 10 k 54 95 6o yielding a north pole at 55 n 250 e the chrm of the youngest tr ph ring dykes and plugs n 10 sites was dec inc 354 3 5 k 60 95 6 3o yielding a north pole at 66 5 n 229 e 2 on the other hand the site mean chrms of the taref sandstone interbeds n 8 sites in the present day in situ coordinates are clustered around the present day geomagnetic field with a mean dec inc 3 33 k 37 95 9 3 yielding a north pole at 83 n 191 e the two paleomagnetic poles of the aob and the tr ph are in accordance with albian campanian poles from africa including madagascar and those rotated from the main cratons such as the north american craton europe including greenland and iberia south america and india using the published plate kinematics and paleomagnetic euler pole pep rotation parameters paleogeographically the two obtained poles of wadi natash volcanics place the african plate at two different azimuths and paleolatitudes during albian cenomaniam and santonian middle campanian tectonically these two paleomagnetic poles point to an active synchronous northwards translation associated with a counterclockwise rotational convergence of africa towards eurasia during the albian campanian time between the time of the aob eruption 104 7 ma and the intrusion of the tr ph 86 78 ma africa concomitantly rotated counterclockwise about 14 and translated northwards towards eurasia about 6 of latitude

eclogites from the czech part of the erzgebirge multi stage metamorphic and structural evolution a structural and petrological study of saxothuringian eclogites in the central erzgebirge czech republic revealed their polyphase thermal and tectonic evolution the first stage of deformation is related to eclogite facies metamorphism of mafic rocks at pressures of 26 kbar and temperatures of 650 700 c the clinopyroxene crystallographic preferred orientation is consistent with high temperature flow the second stage of deformation is characterized by transport of eclogitic rocks at the base of a crustal nappe and their large scale folding during this stage hp ht metamorphic assemblages developed in the surrounding metapelites 14 5 kbar 600 c while eclogite assemblages were preserved in passively transported eclogitic boudins heat advection and similar p t conditions in the surrounding crustal material are responsible for the stability of mineral assemblages and preclude closure of isotopic systems in the eclogites therefore the published isotopic ages of the eclogites do not reflect the subduction stage but the age of cooling under crustal conditions subsequent large scale d3 folding is responsible for reorientation of previously flat lying earlier fabrics d1 d2 the final stage of deformation is marked by the development of a heterogeneous network of ductile retrograde shear zones the thermometry and geochemical study of amphibolitized eclogites in the cores of shear zones indicates thermal conditions not exceeding those in surrounding metapelites and fluid influx the fluid influx is interpreted to be the result of water producing prograde reactions in the footwall metasediments and fluid chanelling into the base of the nappe during brittle deformation of the eclogites