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

paleolatitudes of the kerguelen hotspot new paleomagnetic results and dynamic modeling abstract the kerguelen plateau a large igneous province in the southern indian ocean was formed as a product of the kerguelen hotspot in several eruptive phases during the last 120 myr we obtained new paleolatitudes for the central and northern kerguelen plateau from paleomagnetic investigations on basalts which were drilled during odp leg 183 to the kerguelen plateau broken ridge the paleolatitudes coincide with paleolatitudes from previous investigations at the kerguelen plateau and ninetyeast ridge the track of the kerguelen hotspot and indicate a difference between paleolatitudes and present position at 49 s of the kerguelen hotspot we show that true polar wander the global motion between the mantle and the rotation axis cannot explain this difference in latitudes we present numerical model results of plume conduit motion in a large scale mantle flow and the resulting surface hotspot motion a large number of models all predict southward motion between 3 and 10 for the kerguelen hotspot during the last 100 myr which is consistent with our paleomagnetic results

time variation in coarse materials from lake bottom sediments and secular paleoclimatic change variations in the percentage of materials coarser than 4 5 phi in sediment from lake biwa are closely related to global climatic change and are indicative of large fluctuations in sediment discharge into the lake coarse materials increased markedly during the interglacials postglacial and last interglacial periods the variations also are linked with solar insolation and have distinct periods of about 20 000 and 40 000 years which correspond to the periods of the obliquitical and processional parameters periods of 60 000 and 120 000 years are major in the variations

the use of the common pb isotope composition of detrital k feldspar grains as a provenance tool and its application to upper carboniferous paleodrainage northern england k feldspar is a common detrital mineral in sandstones it contains little u or th and hence the common pb isotope composition of unaltered k feldspar is that of the source rock pb isotope variations in igneous and metamorphic crustal rocks define broad spatial patterns that make the pb signature of detrital k feldspar grains a useful provenance tool however it is unclear how robust this signal is and to what extent it can be modified by weathering transport burial diagenesis and or exposure to hydrothermal fluids this study reports an evaluation of the technique using two granite arkose pairs from helmsdale northern scotland and shap northwest england where the composition of the detrital grains can be compared to feldspars in the granite from which they were derived the results of this test have informed a pilot study using the pb isotope composition of detrital k feldspar in upper carboniferous fluvial sedimentary rocks of northern england to constrain the sediment routing pb isotope compositions have been measured using an in situ high resolution single grain technique that can distinguish original composition heterogeneity and subsequent alteration within individual sand grains the results for the granite arkose pairs show that the common pb isotope signature of the detrital k feldspar grains matches the in situ feldspars in the source granites the signal is independent of grain size medium grained sand to granules but alteration along a paleo regolith at helmsdale has resulted in perturbations in pb composition and an increase in the errors on the analyses the application of the technique to arkosic carboniferous serpukhovian bashkirian millstone grit group sandstones in the pennine basin reveals two discrete populations of k feldspars that cannot be distinguished petrographically matching of the compositions to potential sourcelands to the north and northeast of the basin suggest a far traveled 500 km source of k feldspar from the archean paleoproterozoic lewisian rocks of northwest scotland or their extension to the north but also an equally important proximal source emanating from the southern uplands belt of scotland in what would have been the lower part of the hinterland drainage basin of the millstone grit

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

an exotic mesoarchean microcontinent the coorg block southern india abstract sandwiched between the dharwar craton in the north and the neoarchean proterozoic crustal blocks to the south the coorg block in southern india is composed dominantly of a suite of arc magmatic rocks including charnockites ttg tonalite trondhjemite granodiorite related granitoid suite and felsic volcanic tuffs together with minor accreted oceanic remnants along the periphery of the block coeval mafic and felsic magmatism with magma mixing and mingling in an arc setting is well represented in the block here we present the petrology geochemistry zircon u pb geochronology and lu hf isotopes of all the major lithologies from this block computation of metamorphic p t conditions from mineral chemical data shows consistent granulite facies p t conditions of 820 870 c and up to 6 kbar our geochemical data from major trace and ree on representative samples of the dominant rock types from the coorg block corroborate an arc related signature with magma generation in a convergent margin setting the zircon data yield weighted mean 207pb 206pb ages of 3153 4 9 to 3184 0 5 5 ma for syenogranites 3170 3 6 8 ma for biotite granite 3275 5 1 ma for trondhjemite 3133 12 to 3163 8 6 9 ma for charnockites 3156 10 to 3158 3 8 2 for mafic enclaves 3161 16 ma for diorite and 3173 16 ma for felsic volcanic tuff an upper intercept age of 3363 59 ma and a lower intercept age of 2896 130 ma on zircons from a charnockite as well as an evaluation of the th u values of the zircon domains against respective 207pb 206pb ages suggest that the mesoarchean magma emplacement which probably ranged from 3 3 to 3 1 ga was immediately followed by metamorphism at ca 3 0 to 2 9 ga the ages of magmatic zircons from the charnockites and their mafic granulite enclaves as well as those from the volcanic tuff and biotite granite are all remarkably consistent and concordant marking ca 3 1 ga as the peak of subduction related crust building in this block within the tectonic milieu of an active convergent margin the majority of zircons from the coorg rocks show hf isotope features typical of crystallization from magmas derived from juvenile sources their hf crustal model ages suggest that the crust building might have also involved partial recycling of basement rocks as old as ca 3 8 ga the crustal blocks in the southern granulite terrane in india preserve strong imprints of major tectonothermal events at 2 5 ga 2 0 ga 0 8 ga and 0 55 ga associated with various subduction accretion collision or rifting events however the coorg block is exceptional with our data suggesting that none of the above events affected this block importantly there is also no record in the coorg block for the 2 5 ga pervasive regional metamorphism that affected all the other blocks in this region the geochronological data raise the intriguing possibility that this block is an exotic entity within the dominantly neoarchean collage in the northern domain of the southern granulite terrane of india the mesoarchean arc related rocks in the coorg block suggest that the magma factories and their tectonic architecture in the early earth were not markedly different from those associated with the modern style plate tectonics

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

40ar 39ar and k ar geochronology of the dykes from the south indian granulite terrain abstract 40ar 39ar and conventional k ar data on mafic dykes and a felsic dyke together with previous data have been used to constrain the ages of the dyke magmatism in the south indian granulite terrain the age of the mainly ene wsw trending agali anaikatti swarm is estimated to be 1980 25 ma with a very limited occurrence of upper cretaceous 80 90 ma dykes in the area the nw se dharmapuri swarm is dated as at least 1800 ma possibly 1855 9 ma both the nw se and the ne sw dyke swarms of tiruvannamalai are dated at 1650 10 ma only a few dykes in north kerala are of middle proterozoic ca 1700 1650 ma age furthermore the 40ar 39ar results of dharmapuri dykes exhibit age spectrum characteristic of partial degassing they provide strong evidence of latest proterozoic overprinting by a thermal event probably related to the pan african orogeny most of the phanerozoic dykes are confined to the coastal region in kerala province the dolerites of central kerala and the majority of north kerala dolerites intruded during the 70 65 ma period whereas a large gabbro and a felsic dyke in central and north kerala respectively are of upper cretaceous age ca 85 ma and the mean 81 1 5 ma age from these dykes may represent cooling age the oldest phanerozoic upper jurassic 144 6 ma dyke intrusions have a restricted occurrence in south kerala very close to the southern tip of india the proterozoic dykes are interpreted as being lateral intrusions from under plating igneous bodies beneath the eastern shield region the geological record of the terrain does not show indications of significant pre magmatic extension and rifting but dyke magmatism may have led to extension rifting and the consequent development of the india antarctica sri lanka proterozoic high grade terrain the data are also interpreted to suggest that the south indian granulite terrain is an early proterozoic tectono metamorphic terrane with strong thermal imprints of neoproterozoic early palaeozoic age it is difficult to reconcile its extensions into proterozoic sri lanka or antarctica where tectono metamorphism is dated to be of neoproterozoic age the late phanerozoic dykes are predominantly related to the deccan trap event and are associated with mantle plume decompressional tectonics that gave rise to ocean floor spreading between the seychelles and the indian continent the few older ca 145 ma dykes in south kerala may be related to the rift tectonics that preceded the separation of australia antarctica and india

maturity of central madagascar s landscape low temperature thermochronological constraints abstract low temperature thermochronological data from two profiles across central madagascar give apatite fission track and apatite u th he ages ranging between 258 ma and 176 ma and from 239 ma to 48 ma respectively thermal models derived from these data as well as modelling of basement denudation and the sedimentary record indicate that first order topography of central madagascar developed mainly due to flexural uplift during mesozoic times this was in response to successive erosion and depositional loading associated with the sedimentation in the morondava and majunga basins both of which are now exposed along the western margin of madagascar our data suggest that the eastern margin of the island had a similar denudation history and was probably at a similar topographic level before the late cretaceous break up of madagascar and the india seychelles block cretaceous normal faulting without major amounts of denudation led to the development of the present east coast topography defined by a tectonically juvenile escarpment in the centre of the island cenozoic tectonics and volcanism has had a minor and localised influence on the landscape of central madagascar

the drift history of adria and africa from 280 ma to present jurassic true polar wander and zonal climate control on tethyan sedimentary facies article i nfo the orogenic belts surrounding the undeformed adriatic sea represent the margins of an area known as adria the african promontory we have undertaken a critical appraisal of paleomagnetic data from regions of adria considered parautochthonous relative to africa obtained either from biostratigraphically dated sed imentary rocks corrected for inclination shallowing or from igneous rocks that are regarded as free from any inclination shallowing bias paleomagnetic directions were used to calculate paleomagnetic poles for compar ison with coeval and inclination flattening free paleomagnetic poles from stable africa visual coherence of paleopoles for several time slices from the early permian to the eocene supports the construction of a com posite apparent polar wander path apwp valid for parautochthonous adria and stable africa this compos ite apwp is compared to previous apwps finding good agreement with the global apwp of kent and irving 2010 both apwps show a remarkable and rapid polar shift of 40 in the jurassic that other apwps tend to underestimate we interpret this shift to represent a major episode of true polar wander tpw from 183 ma in the early jurassic to 151 ma in the late jurassic using a simple zonal climate model the drift motion of adria attached to africa appears to be consistent with the distribution of permian cretaceous sed imentary facies on adria

neoarchean continental growth through arc magmatism in the nilgiri block southern india the formation and growth of continental crust in the archean have been evaluated through models of subduction accretion and mantle plume the nilgiri block in southern india exposes exhumed neoarchean lower crust uplifted to heights of 2500 m above sea level along the north western margin of the peninsula major lithologies in this block include charnockite with or without garnet anorthosite gabbro suite pyroxenite amphibolite and hornblende biotite gneiss ttg all these rock types are closely associated as an arc magmatic suite with diffuse boundaries and coeval nature the charnockite and hornblende biotite gneisses ttg show sio2 content varying from 64 to 73 wt the hornblende biotite gneisses ttg are high al type with al2o3 15 wt whereas the charnockites show al2o3 15 wt the composition of charnockite is mainly magnesian and calcic to calc alkaline the mafic ultramafic rocks show composition close to that of tholeiitic series the low values of k 2 o 3 wt k rb k2o 500 zr ti and trace element ratios like la yb n sr y y nb y nb rb y ta rb yb ta indicate a volcanic arc signature for these rocks the geochemical signature is consistent with arc magmatic rocks generated through oceanic plate subduction the primitive mantle normalized trace element patterns of these rocks display enrichment in large ion lithophile elements lile and comparable high field strength elements hfse in charnockite and hornblende biotite gneisses ttg consistent with subduction related origin primitive mantle normalized ree pattern displays an enrichment in lree in the chamockite and hornblende biotite gneisses ttg as compared to a flat pattern for the mafic rocks the chondrite normalized ree patterns of zircons of all the rock types reveal cores with high hree formed at ca 2700 ma and rims with low hree formed at 2500 2450 ma log transformed la th nb th sm th yb th discrimination diagram for the mafic and ultramafic rocks from nilgiri displays a transition from mid oceanic ridge basalt morb to island arc basalt iab suggesting a morb source the u pb zircon data from the charnockites mafic granulites and hornblende biotite gneisses ttg presented in our study show that the magma generation during subduction and accretion events in this block occurred at 2700 2500 ma together with the recent report on neoarchean supra subduction zone ophiolite suite at its southern margin the nilgiri block provides one of the best examples for continental growth through vertical stacking and lateral accretion in a subduction environment during the neoarchean c 2014 elsevier b v all rights reserved

compositional evolution of siliciclastic sediments recording the tectonic stability of a pericratonic rift mesozoic kutch basin western india abstract this study presents a combined petrographical and geochemical investigation to track the compositional changes in the mesozoic sedimentary record during the evolution of the pericratonic rift basin of kutch the mesozoic sequence of the proven petroliferous basin of kutch representing 3000 m thick siliciclastic and carbonate deposition within a time frame of 70 ma consists of jhurio jhumara jhuran and bhuj formations in ascending order of succession the dominance of arkosic sandstones and the abundance of zircon and monazite indicate predominantly felsic source rocks the concentration of individual trace elements such as th sc zr v ni and ratios like th sc cr v y ni th co la sc support the predominantly felsic source rock the lree enrichment and the negative eu anomaly corroborate the predominantly felsic source however concentrations of v ni and ti reveal the mafic input in older stratigraphic units while gdn ybn indicates dominant post archean source across the mesozoic succession younger stratigraphic units reveal contributions from the archean rocks from nagar parkar and aravalli highland regions the cia values suggest intermediate to intense weathering while the icv data record a shift from immature sediments in the older formations to mature and recycled types in the younger formations the high value of zr sc as well a high content of hf reflects the addition of zircon by recycling or by erosional unroofing of older continental rocks major oxide sio2 na2o and k2o and trace elements la th and hf bear the passive margin setting for the kutch basin during the mesozoic the decrease in feldspar content in sandstones increase in the concentration of zr and hf in shale and a decrease in values of icv from older to younger stratigraphic units corresponds to the compositional evolution related to increased stability of the kutch basin during the transition from early syn rift to post rift

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

paleomagnetic evidence for motion of the hawaiian hotspot during formation of the emperor seamounts the bend in the hawaiian emperor chain is the best example of a change in plate motion recorded in a fixed hotspot frame of reference alternatively the bend might record primarily differences in motion of the hawaiian hotspot relative to the pacific lithosphere new paleomagnetic data from the emperor chain support the latter view althouth the rate of motion is difficult to constrain because of uncertainties posed by true polar wander and limited sampling of the chain the best available paleomagnetic data suggest pacific hotspots may have moved at rates comparable to those of lithospheric plates 30 mm yr 1 in late cretaceous to early tertiary times 81 43 ma if correct this requires a major change in how we view mantle dynamics and the history of plate motions in the early to mid cretaceous 128 95 ma hotspots in the atlantic moved at similar rates these episodes during which groups of hotspots appear to move rapidly are separated by times of much slower motion such as the past 5 m y

40ar 39ar age of the st mary s islands volcanics southern india record of india madagascar break up on the indian subcontinent abstract the felsic volcanics rhyolites and rhyodacites of the st mary s islands smi southern india 13 n were originally interpreted as a distant outlier of the 65 ma deccan volcanic province of west central india comprising dominantly flood basalts later the smi volcanics were dated at 93 ma by the k ar technique however this k ar age was dubious being merely an average of five out of six widely varying dates and arbitrary data selectivity being involved in this averaging our first 40ar 39ar dating of the smi volcanics yields excellent plateau and isochron ages and their weighted mean isochron age is 85 6 0 9 ma 2 interestingly the southern indian precambrian terrain is intruded by numerous mafic doleritic dyke swarms ranging in age from proterozoic to the latest cretaceous 69 65 ma deccan related and indeed two regional dykes a leucograbbro and a felsite from the kerala region of southwestern india remain previously dated at 85 ma but again with the k ar technique however this age for the smi volcanics also corresponds excellently with 40ar 39ar ages of 89 85 ma weighted mean isochron age 87 6 1 2 ma 2 equivalent to 88 1 1 2 ma corresponding to mmhb 1 age of 523 1 2 6 ma for the madagascar flood basalt province together therefore the madagascar flood basalt province the smi volcanics and possibly the kerala dykes could represent volcanic activity associated with the break up of greater india india plus seychelles and madagascar thought to have occurred in the upper cretaceous at 88 ma

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

u pb geochronology and isotope geochemistry of the archean and proterozoic rocks of north central madagascar abstract new u pb zircon ages and sm nd and rb sr isotopic data are presented for orthogneisses from north central madagascar including ile sainte marie alaotra beforona maevatanana and ambatolampy ambatomarina a migmatite tonalite gneiss from ile sainte marie is dated precisely at documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape 3187 pm 2 end document ma and has a sm nd model age tdm of 3204 ma thereby establishing a middle archean age for the oldest juvenile gneisses in northeast madagascar dated orthogneisses int

origin of the rajmahal traps and the 85 e ridge preliminary reconstructions of the trace of the crozet hotspot the 85 e ridge is a buried aseismic ridge approximately parallel to and west of the ninetyeast ridge in the northeastern indian ocean it was previously shown to be of probable volcanic origin emplaced on very young oceanic crust but no satisfactory model of emplacement of the rocks was offered we propose a model of origin of the rajmahal traps of northeastern india the 85 e ridge and afanasy nikitin seamount as the trace of the hotspot that now lies beneath the crozet islands in the southern indian ocean this reconstruction places the kerguelen hotspot which formed the ninetyeast ridge at the triple junction between greater india australia and antarctica before the breakup of eastern gondwana

statistical analysis of grain size distribution in pleistocene sediments from lake biwa japan abstract time series of grain size distributions from pleistocene sediments deposited in lake biwa during the past 550 millennia show dominant periods of 40 000 and 20 000 yr that are very close to those predicted by the milankovitch theory as well as a period of about 70 000 yr not directly predicted by this theory the 70 000 yr period is strongest followed by the 20 000 yr period the sequences also show that coarser particles were deposited in general during strong solar insolation whereas finer particles were deposited during weak insolation

the cretaceous igneous province of madagascar geochemistry and petrogenesis of lavas and dykes from the central western sector 1999 similarly the buchan 1997 crops out in the area between thecontributions of hotspot usually taken to be plume mailaka limestones and the contact between the permo mantle and non hotspot asthenosphere to ood basalt triassic sedimentary rocks of the morondava basin andmagmas appear to vary greatly from province to province the precambrian basement fig 1b the dykes reach e g kent

zircon u pb geochronology and hf isotope of felsic volcanics from attappadi southern india implications for neoarchean convergent margin tectonics abstract the attappadi area on the south western flanks of the archean dharwar craton in southern india is located along the e w trending bhavani shear zone which marks the trace of a neoarchean suture zone the dominant rock types in the area include meta ultramafics amphibolites ttg tonalite trondhjemite granodiorite gneisses metapelites and sulphidic banded iron formation bif here we report the occurrence of felsic volcanic rocks preserving primary textures from the anaikatti area in eastern attappadi the felsic volcanics are interbanded with the bif amphibolite metapelite metapyroxenite and hornblende gneisses the felsic volcanics are divided into two types based on their textures the type 1 rock is medium grained with gneissic texture and lack unequivocal primary volcanic textures type 2 felsic volcanics are thinly laminated fine grained and at places preserve relict soft sediment deformation structures they also contain relict volcanic clasts or lapilli and are interpreted as felsic tuff geochemically the attappadi felsic volcanics are rhyolitic in composition and have arc related trace element signatures they also possess a calc alkaline volcanic affinity we report la icpms u pb ages from zircons in four samples of the felsic tuffs which show weighted mean ages of 2567 18 ma and mswd 1 4 2499 19 ma mswd 0 57 2555 24 ma mswd 1 7 and 2576 64 ma mswd 5 8 the late neoarchean early paleoproterozoic ages obtained in our study correlate well with the zircon u pb ages reported in recent studies from ophiolites and other suprasubduction suites from attappadi and surrounding regions the zircon ehf values range from 11 1 to 7 6 suggesting heterogeneous source material involving both juvenile and older reworked components we build a tectonic model for the sw margin of the dharwar craton with an oceanic realm characterized by island arcs and widespread submarine tholeiitic as well as komatiitic ultramafic and mafic volcanism during the neoarchean this predominantly mafic volcanism on the ocean floor is represented by primitive komatiitic lavas oxide and sulphide facies bif the birth of volcanic arc at the convergent margin is marked by felsic volcanism and the deposition of felsic volcanics and volcano sedimentary successions in the final stage of ocean closure the ocean plate and continental arc assemblages were brought in juxtaposition including the accretion of the ophiolitic fragments our study confirms the recent models of arc arc and arc continental accretion to the southern margin of the dharwar craton and major continental growth at the end of the archean

sand supply to the lake albert basin uganda during the miocene pliocene a multiproxy provenance approach a multi proxy provenance approach heavy mineral analysis u pb zircon geochronology and pb isotopic analysis of k feldspar has constrained sediment supply within the upper nile drainage system in the miocene pliocene provenance data from sandstones were obtained from three exploration wells two situated on the north eastern margin and one on the eastern flank of the lake albert basin nw uganda data suggest that high to low grade metamorphic rocks and granitoids have variably supplied the heavy mineral assemblages around the lake albert basin during the miocene pliocene with contributions from the isotopically heterogeneous archean cratons including the local ugandan craton tanzanian and congo cratons and the pan african rocks the mozambique belt with possible contributions from the neoproterozoic and paleoproterozoic rocks these data also highlight clear differences between supply to the eastern basin margin compared with the northeast which is reconcilable with current models for miocene pliocene drainage in the region supply to northeastern lake albert during the miocene pliocene appears to have been through a proto albert nile draining from ne to sw and from a proto victoria nile or similarly oriented palaeo river systems draining from the east in contrast the eastern flanks of the basin were likely supplied via the palaeo nkusi river tapping local hinterland sources and more distal basement to the far east mozambique belt this study highlights the importance of utilizing a multi proxy approach in provenance analysis as no one signal is capable of distinguishing the different source lands and constraining the evolving drainage patterns

post palaeozoic evolution of weathered landsurfaces in uganda by tectonically controlled deep weathering and stripping a model for the evolution of weathered landsurfaces in uganda is developed using available geotectonic climatic sedimentological and chronological data the model demonstrates the pivotal role of tectonic uplift in inducing cycles of stripping and tectonic quiescence for cycles of deep weathering it is able to account for the development of key landforms such as inselbergs and duricrust capped plateaux which previous hypotheses of landscape evolution that are based on climatic or eustatic controls are unable to explain development of the ugandan landscape is traced back to the permian following late palaeozoic glaciation a trend towards warmer and more humid climates through the mesozoic enabled deep weathering of the jurassic mid cretaceous surface in uganda during a period of prolonged tectonic quiescence uplift associated with the opening south atlantic ocean terminated this cycle and instigated a cycle of stripping between the mid cretaceous and early miocene deep weathering on the succeeding miocene to recent african surface has occurred from miocene to present but has been interrupted in the areas adjacent to the western rift where development of a new drainage base level has prompted cycles of stripping in the miocene and pleistocene

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 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

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

cretaceous oceanic anoxic event 2 triggered by a massive magmatic episode oceanic anoxic events oaes were episodes of widespread marine anoxia during which large amounts of organic carbon were buried on the ocean floor under oxygen deficient bottom waters oae2 occurring at the cenomanian turonian boundary about 93 5 myr ago is the most widespread and best defined oae of the mid cretaceous although the enhanced burial of organic matter can be explained either through increased primary productivity or enhanced preservation scenarios the actual trigger mechanism corresponding closely to the onset of these episodes of increased carbon sequestration has not been clearly identified it has been postulated that large scale magmatic activity initially triggered oae2 refs 4 5 but a direct proxy of magmatism preserved in the sedimentary record coinciding closely with the onset of oae2 has not yet been found here we report seawater osmium isotope ratios in organic rich sediments from two distant sites we find that at both study sites the marine osmium isotope record changes abruptly just at or before the onset of oae2 using a simple two component mixing equation we calculate that over 97 per cent of the total osmium content in contemporaneous seawater at both sites is magmatic in origin a approximately 30 50 fold increase relative to pre oae conditions furthermore the magmatic osmium isotope signal appears slightly before the oae2 as indicated by carbon isotope ratios suggesting a time lag of up to approximately 23 kyr between magmatism and the onset of significant organic carbon burial which may reflect the reaction time of the global ocean system our marine osmium isotope data are indicative of a widespread magmatic pulse at the onset of oae2 which may have triggered the subsequent deposition of large amounts of organic matter

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

ar ar geochronology of ultrahigh pressure metamorphism in central china the dabie shan of china contain the largest areal exposure of ultrahigh pressure regional metamorphic rock known on earth the thermal history of these unusual rocks is central to understanding the tectonic processes responsible for their creation preservation and exhumation published ages of ultrahigh pressure metamorphism and subsequent cooling range from archean to jurassic by analyzing 21 dabie hornblende phengite and biotite samples by the 40ar 39ar method we find that 1 cooling from peak metamorphic temperatures to 300 c occurred between about 206 and 178 ma 2 widespread cretaceous ages reflect reheating by a postultrahigh pressure magmatic extensional episode 3 40ar 39ar and k ar ages older than 230 ma and perhaps 210 ma are geologically meaningless due to the incorporation of excess 40ar and 4 dabie ultrahigh pressure metamorphic rocks are temporally related to blueschist facies rocks farther west in the suture zone

geodynamic implications of moving indian ocean hotspots abstract the reconstructed positions of many of the indian ocean intraplate volcanic features are inconsistent with the present positions of the hotspots to which they have been ascribed the discrepancy can be explained by the past motion of the hotspots themselves and in the errors introduced in an absolute plate model based on fixed hotspots previous models for the motion of hotspots in the indian ocean e g steinberger and o connell in the history and dynamics of global plate motions 2000 pp 377 398 and geophys j int 132 1998 412 434 antretter et al earth planet sci lett 203 2002 635 650 have had success in matching paleolatitudes of the kerguelen hotspot but not the marion or reunion hotspots we calculate the motion of the indian ocean hotspots using a method described in steinberger and o connell op cit employing a plate motion model revised using identification of mesozoic magnetic anomalies in the enderby basin gaina et al in the evolution and dynamics of the australian plate in press we find a motion of 7 south for the kerguelen plume similar to previous models antretter et al op cit this motion alleviates the discrepancies in the position of the kerguelen plume and its purported involvement in the formation of the ninetyeast ridge rajmahal traps bunbury basalts and assorted volcanics of the western australian margin we find a motion of 5 north and 7 south for the reunion and marion plumes respectively the motion of the reunion hotspot is consistent with paleolatitude estimates the apparent fixity of the marion hotspot torsvik et al earth planet sci lett 164 1998 221 232 is not well constrained but may be due to the combination of hotspot motion and true polar wander acting in opposing directions we present revised absolute finite rotations for the indian plate for the last 65 ma based on the motion of these hotspots which place india farther north in the past than for fixed hotspot models

oldest rocks from peninsular india evidence for hadean to neoarchean crustal evolution abstract the evolution of continental crust during hadean and archean and related geodynamic processes provides important clues to understand the early earth history here we report evidence for hadean and eoarchean crust from the fringe of the coorg block one of the oldest crustal blocks in peninsular india we present geological petrological and geochemical data together with zircon u pb ages and lu hf isotopes from a suite of metaigneous granitoids diorite charnockite metavolcanics and metasedimentary quartz mica schist calcareous schist ferruginous quartzite and bif rocks petrological and geochemical studies indicate that the igneous suite formed from subduction related arc magmatism and that the sedimentary suite represents an imbricated accretionary package of continental shelf sequence and pelagic components mineral thermometry suggests metamorphism under temperatures of 710 730 c and pressures up to 8 kbar magmatic zircons in the metaigneous suite show oscillatory zoning and high th u contents up to 3 72 and record multiple pulses of magmatism at ca 3 5 3 2 2 7 and 2 5 2 4 ga the metasedimentary rocks accreted along the margins of the coorg block show multiple zircon population with mean 207pb 206pb ages at 3 4 3 2 3 1 2 9 2 7 2 6 2 5 2 2 2 0 and 1 3 ga and overprinted by younger thermal event at ca 0 8 0 7 ga zircons in the 3 5 ga dioritic gneiss show positive ehf t values ranging from 0 0 to 4 2 and hf crustal model ages tdmc of 3517 to 3658 ma suggesting that the parent magma was derived from eoarchean juvenile sources the zircons in the 3 2 ga charnockite display ehf t values in the range of 3 0 to 2 9 and hf crustal model ages tdmc of 3345 to 3699 ma the neoarchean metagranites diorites and felsic tuff show both positive and negative ehf t values and a range of tdmc values from 2904 to 3609 ma suggesting magma derivation from meso to eoarchean juvenile and reworked components the tdmc values of the dominant zircon population in the metasedimentary suite range from 3126 to 3786 ma with the oldest value 4031 ma recorded by zircon grain in a ferruginous quartzite the ehf t values of detrital zircons also show both positive and negative values with a dominant crustal source our zircon u pb and lu hf data suggest vestiges of neohadean primordial continental crust in peninsular india with episodic crustal growth during eoarchean mesoarchean and neoarchean building the continental nuclei the results contribute to the understanding of crustal evolution in the early history of the earth

making ends meet in gondwana retracing the transforms of the indian ocean and reconnecting continental shear zones interpretation of the detailed patterns of ocean floor transforms revealed by satellite altimetry enables the creation of the indian ocean to be described quantitatively as four consecutive plate tectonic regimes separated at 200 136 89 and 43 ma each regime is reversed in turn by keeping transform termini coincident and colinear until conjugate points on the margins of pre existing plates regain their pre regime integrity progressive elimination of the indian ocean demonstrable as a smooth computer animation http www kartoweb itc nl gondwana leads to a refined re assembly of the continental fragments of central gondwana that is substantiated by new geological data a sequence of euler interval poles that describes the dispersal of the gondwana fragments time calibrated against available magnetic anomaly data is given the model requires a mid cretaceous position for india s southern tip about 1000 km south of madagascar prior to india s rapid northward migration

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

madagascar basalts tracking oceanic and continental sources extensive upper cretaceous volcanism in southern madagascar was fed in part by mantle sources resembling those expressed today in the indian ocean at marion and prince edward islands and on the central southwest indian ridge in addition very low end t to 17 4 high 87sr 86sr t to 0 72126 tholeiites in southwestern madagascar were variably but highly contaminated by ancient continental material broadly like that affecting the bushe and poladpur formations of the later deccan traps in india alkalic dikes in southwestern madagascar have a rough analogue in the mahabaleshwar formation of the deccan in that they document the influence of a low 206pb 204pb negative end relatively low 87sr 86sr reservoir a very similar reservoir is manifested at present in mid ocean ridge basalts on the central southwest indian ridge near 40 e the original location of this end member appears likely to have been in the madagascan lithospheric mantle a portion of which may have been removed in the middle cretaceous by the action of the marion hotspot or the rifting of indo madagascar an origin within the hotspot itself also may be possible however recent products of the hotspot appear to lack completely the necessary low 206pb 204pb low end signatures

the late cretaceous paleomagnetic field in north america a south american perspective determining the late cretaceous paleomagnetic pole for north america has been difficult because of the lack of suitable rocks of that age in cratonic areas to provide the necessary data as an alternative different studies have appealed to paleomagnetic data from rocks in western north america using paleopoles from stable areas in neighboring continents it is suggested that the available late cretaceous paleomagnetic record in western north america should be analyzed in terms of rigid body deformations rather than be used to represent the cratonic reference field

paleomagnetic evidence for a stationary marion hotspot additional paleomagnetic data from madagascar abstract the island of madagascar experienced widespread magmatism at ca 90 ma due to its interaction with the marion hotspot previous paleomagnetic data from igneous rocks in the southwestern and northwestern regions of the island indicated that the marion hotspot has remained fixed for the past 90 ma we report paleomagnetic data from northeastern madagascar d analava complex samples were collected from basalts rhyolites gabbros and a dolerite dyke sixty samples from 5 sites yield a paleomagnetic pole at 66 7 s 43 5 e a95 10 7 and a grand mean pole gmp calculated from 10 different studies covering the entire island of madagascar falls at 68 9 s 49 0 e a95 4 4 this pole translates to a paleolatitude for the volcan de l androy focal point of the hotspot at 45 2 6 5 s compared to the current location of the marion hotspot at 46 s our results confirm and expand upon previous studies that argue for the fixity of the marion hotspot for the past 90 ma

shear velocity structure of the crust and upper mantle of madagascar derived from surface wave tomography the crust and upper mantle of the madagascar continental fragment remained largely unexplored until a series of recent broadband seismic experiments an island wide deployment of broadband seismic instruments has allowed the first study of phase velocity variations derived from surface waves across the entire island late cenozoic alkaline intraplate volcanism has occurred in three separate regions of madagascar north central and southwest with the north and central volcanism active until 1 ma but the sources of which remains uncertain combined analysis of three complementary surface wave methods ambient noise rayleigh wave cross correlations and two plane wave illuminate the upper mantle down to depths of 150 km the phase velocity measurements from the three methods for periods of 8 182 s are combined at each node and interpolated to generate the first 3 d shear velocity model for sub madagascar velocity structure shallow upper 10 km low shear velocity regions correlate well with sedimentary basins along the west coast upper mantle low shear velocity zones that extend to at least 150 km deep underlie the north and central regions of recent alkali magmatism these anomalies appear distinct at depths 100 km suggesting that any connection between the zones lies at depths greater than the resolution of surface wave tomography an additional low shear velocity anomaly is also identified at depths 50 150 km beneath the southwest region of intraplate volcanism we interpret these three low velocity regions as upwelling asthenosphere beneath the island producing high elevation topography and relatively low volume magmatism

long term climato limnological oscillation during the past 2 5 million years printed in lake baikal sediments the physical properties of long sediment cores bdp96 obtained from the academician ridge of lake baikal eastern siberia show that the changes in the climato limnological environment of the continental interior during the past 2 5 m y clearly reflected global climatic change oscillation in water content and grain size which are closely related to biogenic productivity coincides with marine 18o variation the values for water content and grain size are large during interglacial periods and small during glacial periods milankovitch parameters were also imprinted in the sediments over the past 2 5 m y a 400 ky period as well as a 100 ky period due to eccentricitical parameters is found in addition to other orbital parameter a 40 ky period of obliquitical and a 20 ky period of precessional parameter this indicates that change in solar insolation was closely related to long term variations in climato limnological regime in this area

eocene paleomagnetic pole for south america northward continental motion in the cenozoic opening of drake passage and caribbean convergence 1 a paleomagnetic study of eocene volcanic rocks in patagonia yields high unblocking temperature and high coercivity magnetizations combining these results with those of a previous study on patagonian eocene basalts yields a high precision high quality pole located at latitude 81s longitude 337 4e a95 5 7 critically this paleopole is indistinguishable from that of the late cretaceous circa 85 65 ma pole position of south america indicating that the plate was essentially motionless with respect to the spin axis for a period of 45 m y the pole position places south america at higher 5 than present day latitudes during the eocene indicating that northward continental motion toward present day latitudes must have been accomplished sometime since the late eocene paleomagnetic and tectonic correlation admits the hypothesis that cenozoic northward drift was associated with oligocene miocene extension in the southern continental edge leading to the opening of the drake passage and it agrees with the timing of foredeep formation and development of fold thrust belts in the northern continental edge this positive correlation between the paleomagnetically predicted drift of a major continent with extension at its trailing edge and convergence at its leading edge during times for which seafloor tectonic fabric and the geological record are particularly well preserved illustrates the utility of paleomagnetism in constraining paleogeographic and tectonic reconstructions for pre cretaceous times

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

orbit related long term climate cycles revealed in a 12 myr continental record from lake baikal quaternary records of climate change from terrestrial sources such as lake sediments1 2 and aeolian sediments3 4 in general agree well with marine records5 6 but continuous records that cover more than the past one million years were essentially unavailable until recently when the high sedimentation rate site of lake baikal was exploited1 2 7 8 because of its location in the middle latitudes lake baikal is highly sensitive to insolation changes9 and the entire lake remained uncovered by ice sheets throughout the pleistocene epoch making it a valuable archive for past climate here we examine long sediment cores from lake baikal that cover the past 12 million years our record reveals a gradual cooling of the asian continental interior with some fluctuations spectral analyses reveal periods of about 400 kyr 600 kyr and 1 000 kyr which may correspond to milankovitch periods reflecting orbital cycles our results indicate that changes in insolation were closely related to long term environmental variations in the deep continental interior over the past 12 million years

seismicity and seismotectonics of madagascar revealed by the 2011 2013 deployment of the island wide macomo broadband seismic array abstract the seismicity and seismotectonics of madagascar have been studied using an island wide distribution of broadband seismic stations the 28 station madagascar comoros mozambique macomo array was deployed for a 23 month period between 2011 and 2013 macomo data were supplemented by seven temporary stations from the seismological signatures in the lithosphere asthenosphere system of southern madagascar selasoma project ten temporary stations from the reunion hotspot and upper mantle reunions unterer mantel project rhum rum and 11 permanent stations a total of 695 earthquakes with magnitudes between ml1 and ml5 3 located within madagascar were recorded a new local magnitude scale was developed and focal mechanisms were determined for 23 well recorded events most of the seismicity is clustered within central madagascar forming nw se trends in the ankaratra region and ne sw trends in the alaotra and ankay regions that roughly align with extensional tectonic features the northern and southern parts of madagascar also show seismicity clusters that align parallel to existing tectonic features primarily precambrian shear zones focal mechanisms exhibit a wide orientation of nodal planes show predominantly normal faulting throughout madagascar and provide no evidence for a sharp east west striking plate boundary between the lwandle and somalian plates in the middle of the island however a diffuse plate boundary cannot be excluded we suggest that topographically generated extensional stresses is the cause of most seismicity given that significantly fewer earthquakes are located in the lower elevation areas of the island compared to regions of higher elevations the frequency magnitude distribution has a b value of 1 2 and indicates that events with magnitude ml5 or greater should occur with an average repeat time of 1 4 years however no earthquake exceeding ml6 has been recorded in the last century suggesting that the frequency magnitude distribution could be truncated

periodic variations of grain size in pleistocene sediments in lake biwa and earth orbital cycles a time sequence of grain size distributions from the pleistocene sediments in lake biwa located in the central part of japan 35 n 136 e has dominant periodicities of 41 000 23 000 and 19 000 years which are very close to those predicted from the astronomical theory of paleoclimate introduced by milankovitch the time variation of the grain size is very similar to that of insolation of caloric summer in the northern hemisphere

a simple model for 100 k year oscillations in glaciation abstract a simple model is presented where response to forcing with components at 20k 40k and 100k yr where forcing however is strongly dominated by 20k yr is primarily at 100k yr the main features of the model are a very sensitive response of the snow and sea ice line to solar input a threshold to transitions between large snow and sea ice coverage to 53 latitude and very little snow and sea ice coverage and a glaciation cycle forced by the snow and sea ice fine position

a neoarchean dismembered ophiolite complex from southern india geochemical and geochronological constraints on its suprasubduction origin abstract ophiolites the remnants of ancient oceanic lithosphere have been described from collisional sutures of various ages with only few examples from archean terranes here we report the discovery of a neoarchean ophiolite suite from the southern margin of the dharwar craton in india tectonically intercalated within a neoproterozoic suture zone the metamorphosed and variably dismembered ophiolite suite exposed around devanur comprises altered ultramafic units websterite gabbros mafic dykes amphibolites trondhjemites and pegmatites associated with ferruginous metachert structural and petrographic studies indicate that the rocks represent a highly sheared and metamorphosed suite emplaced as a thrust sheet the major and trace element geochemistry of the mafic dykes indicate derivation from basaltic andesitic magmas with tholeiitic to calc alkaline characteristics the rocks display negative nb anomalies with enrichment of lile k rb ba th and depletion in hfse ti nb hf tb the tectonic discrimination of these rocks based on various geochemical plots suggests that they were generated in a suprasubduction zone setting we present new shrimp zircon u pb data for two trondhjemite samples from this complex which yield 238u 206pb ages of 2528 61 and 2545 56 ma the neoarchean age from the trondhjemites obtained in our study is closely comparable to similar ages obtained in recent studies from magmatic zircons in charnockites and orthogneisses in the area the suprasubduction zone assemblages and arc magmas suggest a neoarchean ocean closure along the southern margin of the dharwar craton

middle miocene to pleistocene sedimentary record of rift evolution in the southern albert rift uganda this study presents an almost complete middle miocene to pleistocene sequence of synrift sediments in the western branch of the east african rift the studied succession is exposed in several patches on an eastward tilted block between the northern tip of the rwenzori block and the eastern shoulder of the albert rift in this position it reaches a maximum thickness of 600 m of which 350 m have been logged systematically by analysing lithofacies and sediment architecture stratigraphic subdivision of the succession relies on published biostratigraphic data of endemic mollusc associations and their correlation across east africa the synrift sediments encountered are siliciclastics ranging from clay to coarse gravel with gypsum and ferrugineous interlayers or impregnations lithofacies and architectural analysis indicate alluvial plain delta plain nearshore delta front or lacustrine depositional environments based on the vertical stacking pattern prograding and retrograding trends of the depositional environments and climatic indicators e g conservation of feldspar gypsum and or iron hydroxide precipitation four evolutionary phases can be distinguished i a first phase between ca 14 5 and 10 0 ma is characterised by bedload dominated fluvial environment with massive sandy to gravelly bedforms feldspar rich sands rare iron impregnations and relatively low accommodation space this phase is interpreted as pre and early synrift sedimentation under a semiarid climate ii from ca 10 0 to 4 5 ma predominantly fine grained siliciclastics were deposited in a distal fluvial plain to lacustrine setting characterised by limited accommodation space fluctuation of thin beds dominance of clay and frequent iron impregnations point to a more humid climate with seasonality and weak tectonic activity iii during the third phase between 4 5 and 2 0 ma delta plain and nearshore deposits with frequent ferrugineous impregnations and rich mollusc associations occurred indicating a humid period with lake level highstands and accelerated subsidence iv during the final sedimentary interval between 2 0 and 1 5 ma gravel units reoccurred with less iron but more carbonate and gypsum impregnations and arkosic sandstones this phase recorded a general aridisation trend most probably caused by the upcoming rain barrier of the rwenzori mountains together with accelerated rift flank uplift and strong subsidence of the rift floor the results of this study are of particular importance for delineating key controls on sedimentation in the albert rift

polarity inversion in the rajmahal lavas north east india trap emplacement near commencement of the cretaceous normal superchron summary to evaluate suggestions from earlier work that a reversal of magnetization is present in the rajmahal traps of mid cretaceous probable aptian age in north eastern india we have sampled a 140 m thick section comprising a minimum of nine flow units at 25 sites in the north western part of the outcrop a coherent nrn magnetostratigraphy is identified with evidence for transitional behaviour at the top and bottom of the succession the mean characteristic remanence direction of the reversed sites d i 145 7 63 5 95 6 0 is approximately antiparallel to that of the normal polarity sites d i 300 2 55 7 95 12 4 and the group mean confirms a palaeomagnetic pole position near 125 e 9 s a95 10 5 for the time of eruption of the traps from 40ar 39ar evidence that the rajmahal bengal traps were emplaced over a short interval at ca 117 ma we interpret the brief reversed episode identified near the base of the succession as a record of the jsea event the polarity evidence therefore supports the view that the rajmahal traps were fed by the crozet hotspot at the beginning of the cretaceous normal superchron 119 83 ma

conduit diameter and buoyant rising speed of mantle plumes implications for the motion of hot spots and shape of plume conduits 1 mantle plumes are expected to be affected by large scale flow in the earth s mantle related to plate motions subducted slabs and possibly large scale upwellings motion of plume conduits will depend on both large scale flow and buoyant rising speed of the conduit through the mantle here we present a model of depth dependent plume conduit temperature viscosity radius and buoyant rising speed and use it to compute plume and hot spot motion results support a temperature anomaly of about 500 k at the plume base in this case sublithospheric temperature anomaly is about 150 k for the iceland plume transition zone anomaly is between 150 and 200 k for iceland about 250 k for samoa and about 300 k for hawaii thermal plume radii are about 100 km in the upper mantle increasing to about 200 km in the lower mantle beneath the lithosphere viscosity in the vicinity of plumes is substantially reduced compared to the underlying mantle and fast moving plates deflect plumes by 200 km or less corresponding to a plume conduit buoyant rise time of about 3 myr between 400 and 100 km depth where most of the shear flow due to plate motions occurs during 100 myr plume conduits rise buoyantly from about 1500 2000 km depth in many cases computed hot spot motion agrees with previous computations south southeastward motion of the hawaiian hot spot during the past 100 myr louisville hot spot motion in a similar direction but at slower speed during the past 50 myr and westward motion of the easter hot spot relative to hawaii and louisville however the previously computed substantial southward motion of the kerguelen hot spot is not confirmed if plume sources in the lowermost mantle are assumed to move with large scale flow they are predicted to be displaced toward the two large scale upwellings beneath africa and the pacific relative to hot spot surface locations for fixed sources predicted tilts of the lower part of conduits tend to be opposite to that

paleomagnetism and rock magnetism of early cretaceous rajmahal basalts ne india implications for paleogeography of the indian subcontinent and migration of the kerguelen hotspot abstract we present here new paleomagnetic and rock magnetic results from rajmahal traps rt india the results are comprising of 350 standard specimens collected from 18 sites 90 samples the detailed rock magnetic studies indicate that a single domain sd magnetite fe3o4 is the dominant magnetic carrier with ilmenite pyrrhotite and maghemite in the accessory form the calculated characteristic remanent magnetization chrm directions yield a mean declination of 317 51 mean inclination of 61 92 95 3 6 k 93 39 the obtained virtual geomagnetic pole vgp position of the rt at 117 ma is 11 37 n 297 58 e 62 42 w with a95 3 5 k 99 99 dp 4 33 dm 5 58 and the calculated paleolatitude is at 43 1 s indistinguishable from that of earlier works this paleomagnetic pole of rt basalts has minimal effect of secular variation and comply with the geocentric axial dipole gad model the vgp is plotted on synthetic apparent polar wander path apwp for india constrained better and agrees well with the previously reported poles the postulation of migration of mantle plume and the global mantle circulations are tested and the present study results are in accord with this suggesting the southward migration of the kerguelen hotspot mantle plume by at least 6

the influence of tectonic setting on the hydrological characteristics of deeply weathered terrains evidence from uganda abstract deeply weathered landscapes occur throughout equatorial regions and are transformed by cycles of deep weathering and stripping these processes are driven by rainfall that infiltrates the subsurface as groundwater recharge or runs across the landsurface recent work in uganda demonstrates that long term 10 5 10 6 a cycles of deep weathering and stripping are tectonically controlled the influence of tectonics on the present day hydrological regime and hence contemporary geomorphic process is examined in this study the hydrological characteristics of two humid basins in uganda were determined using soil moisture balance techniques hydrograph separation methods borehole hydrographs and stable isotope 2 h 18 o tracers these were in turn related to the dominant geomorphic process deep weathering or stripping suggested by the local tectonic history in central uganda where deep weathering of the landsurface has occurred since the miocene soil moisture balance calculations monitoring well records and stable isotope data show that groundwater recharge is derived from monsoon rainfall is in the order of 120 mm a 1 and greatly exceeds surface runoff of 3 4 mm a 1 basin output occurs as evapotranspiration from a wetland that runs centrally through the catchment consistency in this discharge despite highly seasonal rainfall reflects a large basin storage that is permitted by a thick weathered mantle in south western uganda where stripping of the landsurface has occurred since the mid pleistocene soil moisture balance calculations and hydrograph separation techniques indicate groundwater recharge is restricted to years of exceptionally high rainfall annual surface runoff 34 mm a 1 generated primarily from monsoon rainfall events exceeds the recharge flux 8 15 mm a 1 streamflow discharge varies considerably from 0 2 to 14 m 3 s 1 and occurs along well incised drainage channels the comparatively smaller basin storage results from a thinner partially stripped weathered mantle the influence of tectonics is indicated by the conclusion that deep weathering associated with prolonged tectonic stability in central uganda is facilitated by a recharge dominated hydrological regime whereas stripping prompted by mid pleistocene tectonic uplift in south western uganda continues to occur under a runoff dominated regime

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

changes in hydrological conditions in the central qing zang tibetan plateau inferred from lake bottom sediments abstract grain particle and chemical analyses of core samples obtained from siling co a lake on the central tibetan plateau and a sedimentation model for the lake with 14 c dating show fluctuations in the hydrological environment of the area during the past 15 000 yr the area of the lake was small and the water level low in the late part of the last glacial period thereafter the lake became larger and the water level high because of meltwater and rainfall this hydrological environment continued until 4000 yr b p at about which time the level of the lake fell sharply and the lake area shrank after a short period of low level and aridity the water level increased markedly but did not reach the previous high level after the large water inflow for a short period the climate became again arid at about 3500 yr b p the change in this interval is closely related to global change statistical analyses of the datasets of the grain particle components of bottom sediments preserved in an aquatic condition over the past 13 000 yr show that most datasets for grain size and chemical parameters had a common period of 2000 2200 yr during this interval

a 750 yr record of autumn snowfall and temperature variability and winter storminess recorded in the varved sediments of bear lake devon island arctic canada abstract the varve record from high arctic proglacial bear lake reveals a regionally coherent hydroclimatic signal as well as complexities due to changing hydroclimatic and limnologic conditions varve formation is strongly dependent on underflows that exhibit variability in strength during the past 750 yr periods with reduced underflow sedimentation and accumulation rates fail to produce varves in the distal part of the lake isolated coarse silt and sand grains occur in 80 of the varves and are interpreted to be niveo aeolian in origin coarse 500 m sand grains deposited on the lake ice by strong winter winds are notably less common since a d 1850 likely due to reduced storminess regression of the varve thickness record with meteorological records indicates high correlations with autumn september and october temperatures and total monthly snowfall these correlations are best at times when underflow activity is sufficiently strong to produce varves throughout the lake the close association with warmer temperatures and snow bearing synoptic systems moving north in baffin bay suggests that the primary climate signal in the varves is varying autumn snow pack that controls nival discharge in the following year the similarity between the other records of melt season temperature and sea ice cover and the bear lake record suggests that summer and autumn conditions were generally similar across the baffin bay region through much of the last millennium

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

erosion patterns in the changjiang yangtze river catchment revealed by bulk sample versus single mineral provenance budgets abstract the changjiang the fourth longest river on earth and the largest in eurasia has a complex sediment routing system presently interrupted by the three gorges dam the world s largest hydroelectric engineering project to study sediment generation processes in the huge catchment and compare the different erosion patterns obtained by different methodological approaches high resolution petrographic and heavy mineral analyses were performed on sands from the trunk river and its major tributaries the frequency distributions of diverse groups of detrital amphiboles were also investigated rigorous statistical methods were used to define end members evaluate mineralogical variability assess similarities among samples and eventually calculate the relative contributions from each major tributary to the trunk river by forward end member modelling of integrated compositional data the litho quartzose sand with few heavy minerals generated in tibetan headwaters evolves downstream to feldspatho litho quartzose with medium rank metamorphic rock fragments and moderately rich amphibole epidote suites sand across the sichuan basin and as far as the three gorges dam is enriched in mafic volcanic clinopyroxene and carbonate grains eroded from permian basalts and paleozoic strata of the south china block the final yangtze tract is characterized by litho feldspatho quartzose sand with moderately poor amphibole dominated suites with epidote clinopyroxene and garnet the orogenic compositional signature acquired in the upper part of the basin is thus carried all the way to the chinese passive margin as observed also for the yellow river in the north even after long distance transport across wide continental areas detrital modes thus reveal the tectonic character of the source rather than the geodynamic environment of the sink quantitative provenance analysis indicates that left bank tributaries draining the longmen and qinlin mountains supply most of the sand reaching the east china sea contributions from tibetan headwaters are negligible as those from right bank tributaries in the middle lower part of the basin sediment budgets and erosion patterns inferred from our integrated petrographic and heavy mineral data set contrast with those based on detrital zircon geochronology but agree with stream profile analysis revealing close correspondence with slope steepness precipitation stream power tectonic activity and frequency of major earthquakes sediment budgets based on amphibole groups are poorly constrained we conclude that provenance budgets based on single mineral species are typically unrobust particularly because they suffer from inaccurate estimates of mineral concentration in the end member sources exploring in detail a millesimal part of the sediment typically 1 5000 in the case of zircon is a risky strategy that may return a distorted picture of reality

adakite ttg connection and fate of mesoarchaean basaltic crust of holenarsipur nucleus dharwar craton india abstract holenarsipur nucleus hn is one of the oldest part 3 5 3 1 ga of the continental crust of india it is made up of mafic ultramafic komatitic 3 35 ga greenstone belts migmatitic ttg tonalite trondhjemite granodiorite 3 3 ga and diapiric trondhjemites 3 1 ga new major trace and ree data on these rocks presented here provide constraints on adakite ttg connection model and petrogenesis of these rocks of hn in addition to recycling of the early mesoarchaean basaltic crust and evolution of mesoarchaean continental crust although of two different ages the chemical characteristics of the migmatitic ttgs and diapiric trondhjemites of the hn resemble with the archaean and cenozoic high silica adakites hsa and the adakites from two neoarchaean greenstone belts of dharwar craton they also display similarities with the early mesoarchaean ttgs from different cratons based on the compositional variations observed in ttg and diapiric trondhjemites of hn we propose that plume fed basaltic and or basaltic komatiitic oceanic crust slab flatly shallow angle subducted and melted under high archaean geothermal regime to produce ttgs identification of hsa in 2 7 ga old greenstone belt subduction complexes that are compositionally similar to ttg suggests the possibility of continuation of this process into neoarchaean and supports a genetic connection between ttg and adakites

time variations of erosional force and grain size in pleistocene lake sediments abstract theoretical investigations on models of the developmental processes of erosional features show that the time variations in erosional force have more sensitive influence on the time fluctuation of eroded material such as sediments in lakes than that of the quantity representing erosional features such as volume of mountains time sequences of grain size distributions from the pleistocene sediments in lake biwa central japan indicate dominant periodicities of about 40 000 and 20 000 yr very close to those predicted from the milankovitch theory the time variations of the grain size parameters which may be indicators of erosional forces are very similar to that of the caloric summer insolation in the northern hemisphere

early late cretaceous ca 93 ma norites and hornblendites in the milin area eastern gangdese lithosphere asthenosphere interaction during slab roll back and an insight into early late cretaceous ca 100 80 ma magmatic flare up in southern lhasa tibet abstract at more than 500 km in length the mainly jurassic early eocene gangdese batholith is one of the most important constituents of the southern lhasa sub block and provides an ideal site for study of tibetan orogenesis recent studies on gangdese intermediate felsic intrusive rocks mainly granites demonstrate that remarkable crustal growth as well as an early late cretaceous ca 100 80 ma magmatic flare up event occurred in southern tibet however the mechanism that drove this magmatic event and its relationship to crustal growth event are not yet clear here we report detailed petrological geochronological geochemical and sr nd hf isotopic data for recently identified norites and hypersthene bearing hornblendites in the milin area southern lhasa sub block these mafic rocks are dated at early late cretaceous ca 93 ma and are characterized by relatively uniform sr nd hf isotopic compositions 87sr 86sr i 0 7042 to 0 7047 end t 2 9 to 3 6 and ehf t zircon 10 9 to 17 0 suggesting that they evolved from similar parental magmas the milin norites and hornblendites are likely to be the products of mineral fractionation and accumulation from a common parental magma during the early and late stages of the magma evolution respectively thermometric calculations indicate that pyroxenes from the milin norites have high crystallization temperatures 1240 1349 c the parental magmatic compositions calculated from pyroxene trace element compositions in the milin norites show slightly flat to enriched light rare earth element lree patterns la yb n 2 9 3 4 la gd n 2 1 2 9 with variable negative nb anomalies nb la n 0 18 0 81 indicating their dual or hybrid geochemical characteristics we suggest that their parental magmas may have been generated by the interaction of upwelling asthenospheric and metasomatized lithospheric mantle taking into account the spatial and temporal distribution of the mesozoic magmatic rocks and regional paleomagnetic data we further suggest that the early late cretaceous magmatic flare up in the southern lhasa sub block was also triggered by the asthenospheric upwelling which resulted from roll back of subducted tethyan oceanic slab

the oslo rift new palaeomagnetic and 40ar 39ar age constraints summary 40ar 39ar whole rock and alkali feldspar ages demonstrate that dioritic to monzonitic dykes from boverbru and lunner belong to the youngest recorded magmatic activity in the oslo rift region southeast norway these dykes represent the terminal phase of rift and magmatic activity in the oslo graben at the dawn of the triassic 246 238 ma the boverbru and lunner dyke ages are statistically concordant however the palaeomagnetic signature of the boverbru dyke is complex and directions from the margins and the interior of the dyke diver in polarity therefore the new early triassic palaeomagnetic pole for baltica eurasia is exclusively based on the less complex lunner dykes and contacts palaeomagnetic pole latitude 52 9 n longitude 164 4 e dp dm 4 5 7 3 the early triassic palaeomagnetic pole mean age 243 5m a 2s is slightly diverent from the upper carboniferous permian 294 274 ma and kiaman aged poles from the oslo rift

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

thermal and exhumation history of the central rwenzori mountains western rift of the east african rift system uganda the rwenzori mountains mtns in west uganda are the highest rift mountains on earth and rise to more than 5 000 m we apply low temperature thermochronology apatite fission track aft and apatite u th sm he ahe analysis for tracking the cooling history of the rwenzori mtns samples from the central and northern rwenzoris reveal aft ages between 195 0 8 4 ma and 85 3 5 3 ma and ahe ages between 210 0 6 0 ma to 24 9 0 5 ma modelled time temperature paths reflect a protracted cooling history with accelerated cooling in permo triassic and jurassic times followed by a long period of constant and slow cooling than succeeded by a renewed accelerated cooling in the neogene during the last 10 ma differentiated erosion and surface uplift affected the rwenzori mtns with more pronounced uplift along the western flank the final rock uplift of the rwenzori mtns that partly led to the formation of the recent topography must have been fast and in the near past pliocene to pleistocene erosion could not compensate for the latest rock uplift resulting in oligocene to miocene ahe ages

cryogenian alkaline magmatism in the southern granulite terrane india petrology geochemistry zircon u pb ages and lu hf isotopes abstract the southern granulite terrane sgt in india preserves the records of the formation and recycling of continental crust from mesoarchean through paleoproterozoic to neoproterozoic and cambrian involving multiple subduction accretion collision associated with major orogenic cycles a chain of unmetamorphosed and undeformed alkaline magmatic intrusions occurs along the northern margin of the sgt aligned along paleo suture zones here we investigate two representative plutons from this suite the angadimogar syenite am and the peralimala alkali granite pm through field petrological geochemical zircon u pb and lu hf studies magma mixing and mingling textures and mineral assemblages typical of alkaline rocks are displayed by these plutons the whole rock major and trace element data characterize their alkaline nature in trace element discrimination diagrams the am rocks straddle between the vag volcanic arc granites and wpg within plate granites fields with most of the samples confined to the vag field whereas the pm rocks are essentially confined to the wpg field the diversity in some of the geochemical features between the two plutons is interpreted to be the reflection of source heterogeneities most zircon grains from the am and pm plutons display oscillatory zoning typical of magmatic crystallization although some grains particularly those from the pm pluton show core rim structures with dark patchy zoned cores surrounded by irregular thin rims resulting from fluid alteration the weighted mean 206 pb 238 u ages of the magmatic zircons from three samples of the am syenite are in the range of 781 8 3 8 ma to 798 3 6 ma and those from two samples of the pm alkali granite yield ages of 797 5 3 7 ma and 799 6 2 ma a mafic magmatic enclave from the am pluton shows weighted mean 206 pb 238 u age of 795 3 3 ma the am and pm plutons also carry rare xeneocrystic zircons which define upper intercept concordia ages of 3293 13 ma and 2530 22 ma correlating with the ages of the basement rocks from these areas the initial 176 hf 177 hf isotope ratios of the zircon grains from the am syenite fall in the range between 0 281771 and 0 282284 with moderately negative ehf t values between 5 9 and 0 1 similarly the initial 176 hf 177 hf isotope ratios for the zircon grains of pm ultrapotassic granite range between 0 281197 and 0 281970 albeit with more negative ehf t values in the range between 22 7 and 0 3 average ehf t value 18 8 the lu hf data suggest the involvement of variable extent of older crust with distinct crustal residence times either in the form of assimilation during magma emplacement or crustal recycling during magma genesis based on the geochemical and isotopic systematics a possible petrogenetic model would be asthenospheric upwelling in an extensional setting melting of enriched lithosphere and interaction of the magmas with lower crustal domains with subduction related components of various ages the disposition of these alkali plutons along two paleo sutures that weld the meso neoarchean crustal blocks in the northern periphery of sgt suggests that the zones of emplacement might represent an aborted rift the paleo sutures probably served as a weak zone along which extension occurred broadly coeval with the cryogenian subduction further south

late cretaceous magmatism in madagascar palaeomagnetic evidence for a stationary marion hotspot late cretaceous basaltic volcanics in the morondava basin sw madagascar possess high quality and pre fold palaeomagnetic data declination 353 5 inclination 54 8 95 2 4 the palaeomagnetic data are all of normal magnetic polarity and remanence acquisition is linked to the terminal stages of the cretaceous normal superchron 83 ma this is sustained by an 40ar 39ar age of 83 6 1 6 ma from one of the tested basaltic flows a precise u pb zircon baddeleyite age from northeast madagascar demonstrates magmatism at least back to 91 6 0 3 ma thus reliable isotope ages for the madagascar cretaceous igneous province span a range of 8 million years late cretaceous palaeomagnetic data obtained from volcanics and dolerites all over madagascar are directionally concordant and the combined palaeomagnetic pole latitude 68 5 n longitude 230 3 e a95 5 5 n 8 studies sampling age range ca 84 90 ma represents one of the best late cretaceous poles for the former gondwanan elements the collective palaeomagnetic data yield a palaeolatitude of 45 3 s 5 3 4 7 for the proposed focal point of the marion plume volcan de l androy southeast madagascar during the late cretaceous this is in perfect agreement with hotspot controlled reconstructions that place the marion hotspot 46 0 s beneath southeast madagascar during the late cretaceous setting true polar wander aside hotspot movements in the indian ocean do not appear to exceed ca 0 75 cm yr and the marion hotspot appears stationary within the resolution power of palaeomagnetic data

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

periodic variations in late pleistocene speleothem abundance in britain abstract temporal change in the growth of speleothems in the united kingdom during the past 150 000 years is shown to be related to the insolation variation by the milankovitch theory the speleothem data compiled by gordon et al 1989 quarternary research 31 14 26 have two milanokovitch frequencies the ca 40 000 yr period related to change in the earth s obliquity and the ca 20 000 yr period related to the precession of the equinoxes the abundance of speleothem growth was in general large during the last interglaciation and small during the last glaciation in both periods however speleothem abundance was greater during periods of strong insolation and less during weak insolation

anomalous bathymetry and palaeobathymetric models of the mozambique basin and riiser larsen sea abstract in this study we introduce a palaeobathymetric model for the conjugate mozambique basin and riiser larsen sea built by employing backstripping techniques compensating for dynamic topography and plate motions the model is presented at 0 2 0 2 grid resolution making it suitable for future oceanographic and climate simulation model experiments aimed at a better understanding of the climatic and oceanographic relevance of oceanic gateways in the southern ocean at the present day the seafloor next to the mozambican continental margin is around 300 m shallower and that in the central mozambique channel is almost 1300 m shallower than their conjugate areas or the predictions of oceanic thermal subsidence models the cause of this anomalous depth is difficult to determine confidently because of sparse data in particular concerning sediment thickness and because of the wide range of amplitudes in modelled present day dynamic topography the distribution of shallow seafloor suggests that it might be attributed to the presence of thicker than usual oceanic crust which in turn can be attributed to the paleogene passage of the quathlamba plume beneath the basin we portray these effects in our palaeobathymetric models in contrast the riiser larsen sea has experienced fairly stable subsidence since its formation in jurassic times with only slight observable changes attributable to the onset of antarctic glaciation and during the middle miocene climate transition both basins display flexure over half wavelengths of 60 80 km with amplitudes of 1500 m towards their continental margins this plays an important role in models of palaeobathymetry for times older than 100 ma near the margins isolated areas of transitional or debatable crustal composition including beira high and gunnerus ridge are depicted to subside in a similar fashion to oceanic crust further into the indian ocean oceanic lithosphere younger than 100 ma on both plates has subsided to depths that are typical of thermal subsidence models finally the new palaeobathymetry had distinct consequences for the current systems in the young southern ocean during the time periods the onset of coast parallel bottom currents and associated contourite deposition in the mozambique channel at palaeo water depths of 3500 4000 m may be a consequence of either an opening of a deep water passage into the south atlantic between southwest indian ridge and agulhas plateau or into the tethys ocean in the late cretaceous

global plate motion frames toward a unified model 1 plate tectonics constitutes our primary framework for understanding how the earth works over geological timescales high resolution mapping of relative plate motions based on marine geophysical data has followed the discovery of geomagnetic reversals mid ocean ridges transform faults and seafloor spreading cementing the plate tectonic paradigm however so called absolute plate motions describing how the fragments of the outer shell of the earth have moved relative to a reference system such as the earth s mantle are still poorly understood accurate absolute plate motion models are essential surface boundary conditions for mantle convection models as well as for understanding past ocean circulation and climate as continent ocean distributions change with time a fundamental problem with deciphering absolute plate motions is that the earth s rotation axis and the averaged magnetic dipole axis are not necessarily fixed to the mantle reference system absolute plate motion models based on volcanic hot spot tracks are largely confined to the last 130 ma and ideally would require knowledge about the motions within the convecting mantle in contrast models based on paleomagnetic data reflect plate motion relative to the magnetic dipole axis for most of earth s history but cannot provide paleolongitudes because of the axial symmetry of the earth s magnetic dipole field we analyze four different reference frames paleomagnetic african fixed hot spot african moving hot spot and global moving hot spot discuss their uncertainties and develop a unifying approach for connecting a hot spot track system and a paleomagnetic absolute plate reference system into a hybrid model for the time period from the assembly of pangea 320 ma to the present for the last 100 ma we use a moving hot spot reference frame that takes mantle convection into account and we connect this to a pre 100 ma global paleomagnetic frame adjusted 5 in longitude to smooth the reference frame transition using plate driving force arguments and the mapping of reconstructed large igneous provinces to core mantle boundary topography we argue that continental paleolongitudes can be constrained with reasonable confidence

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

large scale linked drainage systems in the nw european triassic insights from the pb isotopic composition of detrital k feldspar pb isotopic data from k feldspars in middle triassic anisian sandstones in the wessex basin onshore sw uk and the east irish sea basin some 350 km to the north show that the same grain populations are present this indicates that the drainage system the budleighensis river feeding these basins originated from the same source s most probably the remnant variscan uplands to the south fluvial and aeolian sandstones have the same provenance suggesting that if water and wind driven sands were originally derived from different sources this has been obscured through reworking prior to final deposition significant recycling of feldspar from arkosic sandstones in earlier sedimentary basins can be ruled out the provenance data agree with previous depositional models indicating transport distances in excess of 400 km with a drainage pattern that linked separate basins this supports the idea that the regional fluvial system was driven by topography and episodic flooding events of sufficient magnitude to overcome evaporation and infiltration over hundreds of kilometres importantly this drainage system appears to have been isolated and independent from those operating contemporaneously to the nw of the irish and scottish massifs where the remnant variscan uplands apparently exerted no influence on drainage or sand supply

neoarchean suprasubduction zone arc magmatism in southern india geochemistry zircon u pb geochronology and hf isotopes of the sittampundi anorthosite complex abstract the sittampundi anorthosite complex sac in southern india is one of the well exposed archean layered anorthosite gabbro ultramafic rock associations here we present high precision geochemical data for the various units of sac coupled with zircon u pb geochronology and hf isotopic data for the anorthosite the zircon ages define two populations the older yield a concordia age of 2541 13 ma which is interpreted as the best estimate of the magmatic crystallization age for the sittampundi anorthosite a high grade metamorphic event at 2461 15 ma is suggested by the upper intercept age of the younger zircon population a neoproterozoic event at 715 180 ma resulted in pb loss from some of the metamorphic zircons the magmatic age of the anorthosite correlates well with the timing of crystallization of the arc related 2530 ma magmatic charnockites in the adjacent salem block while the metamorphic age is synchronous with the regional metamorphic event the geochemical data suggest that the rocks were derived from a depleted mantle source sub arc mantle metasomatism of slab derived fluids and subsequent partial melting produced hydrous aluminous basalt magma the magma fractionated at depth to produce a variety of high alumina basalt compositions from which the anorthositic complex with its chromite rich and amphibole rich layers formed as cumulates within the magma chamber of a supra subduction zone arc the coherent initial176hf 177hf ratios and positive ehf values 1 7 4 5 of the magmatic zircons in the anorthosite are consistent with derivation of a rather homogeneous juvenile parent magma from a depleted mantle source our study further confirms that the southern part of the dharwar craton was an active convergent margin during the neoarchean with the generation and emplacement of suprasubduction zone arc magmas which played a significant role in continental growth

provenance of proterozoic garnet biotite gneiss recovered from elan bank kerguelen plateau southern indian ocean at elan bank of the kerguelen plateau in the southeast indian ocean leg 183 of the ocean drilling program recovered clasts of garnet biotite gneiss in a fluvial conglomerate intercalated with basalt flows u pb and pb pb dates of zircons and monazites in these clasts and an overlying sandstone range from 534 to 2547 ma which is much older than the surrounding indian ocean seafloor these dates show that old continental crust resides in the shallow crust of the oceanic kerguelen plateau and that the breakup of gondwana dispersed continental fragments into the nascent indian ocean lithosphere

african climate change and faunal evolution during the pliocene pleistocene abstract environmental theories of african faunal evolution state that important evolutionary changes during the pliocene pleistocene interval the last ca 5 3 million years were mediated by changes in african climate or shifts in climate variability marine sediment sequences demonstrate that subtropical african climate periodically oscillated between markedly wetter and drier conditions paced by earth orbital variations with evidence for step like 0 2 ma increases in african climate variability and aridity near 2 8 ma 1 7 ma and 1 0 ma coincident with the onset and intensification of high latitude glacial cycles analysis of the best dated and most complete african mammal fossil databases indicates african faunal assemblage and perhaps speciation changes during the pliocene pleistocene suggesting more varied and open habitats at 2 9 2 4 ma and after 1 8 ma these intervals correspond to key junctures in early hominid evolution including the emergence of our genus homo pliocene pleistocene shifts in african climate vegetation and faunal assemblages thus appear to be roughly contemporary although detailed comparisons are hampered by sampling gaps dating uncertainties and preservational biases in the fossil record further study of possible relations between african faunal and climatic change will benefit from the accelerating pace of important new fossil discoveries emerging molecular biomarker methods for reconstructing african paleovegetation changes tephra correlations between terrestrial and marine sequences as well as continuing collaborations between the paleoclimatic and paleoanthropological communities

age dating and the orbital theory of the ice ages development of a high resolution 0 to 300 000 year chronostratigraphy using the concept of orbital tuning a continuous high resolution deep sea chronostratigraphy has been developed spanning the last 300 000 yr the chronology is developed using a stacked oxygen isotope stratigraphy and four different orbital tuning approaches each of which is based upon a different assumption concerning the response of the orbital signal recorded in the data each approach yields a separate chronology the error measured by the standard deviation about the average of these four results which represents the best chronology has an average magnitude of only 2500 yr this small value indicates that the chronology produced is insensitive to the specific orbital tuning technique used excellent convergence between chronologies developed using each of five different paleoclimatological indicators from a single core is also obtained the resultant chronology is also insensitive to the specific indicator used the error associated with each tuning approach is estimated independently and propagated through to the average result the resulting error estimate is independent of that associated with the degree of convergence and has an average magnitude of 3500 yr in excellent agreement with the 2500 yr estimate transfer of the final chronology to the stacked record leads to an estimated error of 1500 yr thus the final chronology has an average error of 5000 yr