earliest sea floor spreading magnetic anomalies in the north arabian sea and the ocean continent transition summary magnetic and gravity data collected during a gloria survey of the indus fan provide new information on the earliest sea floor spreading history of the arabian sea a negative gravity anomaly correlates with the buried laxmi ridge this ridge is interpreted here to be a sliver of continental crust adjacent to the oceancontinent transition which bounds thinned probably intruded transitional crust to the ne the oldest sea floor spreading anomaly is anomaly 28 65 66 ma breakup occurring at the time of the deccan traps volcanic event the earliest oceanic crust formed from two phases of rift propagation which accommodates the angular disparity between the e w trending anomalies in the western arabian sea and the ne sw trending western part of the laxmi ridge flow line projection shows that the laxmi ridge forms the conjugate structure to the northern mascarene plateau margin

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

structural highs on the western continental slope of india implications for regional tectonics abstract integrated geological and geophysical explorations over the last two decades along the central western continental margin of india reveal from east to west a terrace like flat shelf an advancing shelf break a nw se oriented wide shelf margin basin the upper slope ridge the prathap ridge and the laccadive ridge the basin on the continental slope extends between the shelf break and the eastern slope of the laccadive ridge and is divided longitudinally into a relatively narrow eastern and a wide western basin by the prominent upper slope ridge detailed multibeam swath mapping now over an area of little more than 20 300 km 2 within the shelf margin basin located on the upper continental slope now reveals the occurrence of several isolated structural highs of variable height four of these highs are located over the upper slope ridge and a similar number on the prathap ridge of these four highs have summit height 1000 m two have heights between 500 and 999 m while the remaining two have heights less than 499 m the digital terrain model developed from the processed multibeam swath bathymetry data offers a three dimensional perspective of these structural highs the tectonic understanding from multibeam bathymetry findings and satellite gravity data supported by other geophysical information suggest that these structural highs are likely to be the peaks of the upper slope ridge and the prathap ridge we discuss the possible origin and mode of emplacement of these structural highs and ridges in the light of regional tectonics and suggest that both the upper slope and prathap ridges may have been formed during the separation of india from madagascar during the middle cretaceous

on the genesis of archean granite through two stage melting of the quetico accretionary prism at a transpressional plate boundary regional geologic relationships and geochemical data support the idea that late stage biotite granite of the lac la croix type may have formed by partial melting of previously migmatized metasedimentary rocks in the quetico subprovince of the superior craton the proposed parent material is an intimately layered rock composed of approximately half to two thirds biotite schist and half to one third granitoid leucosome the bulk chemical attributes of such a material were estimated from detailed mapping and drilling of two large outcrops near pearl lake minnesota previous work has demonstrated that the granitoid leucosome of the layered migmatite is the intrusively injected product of partial melting of a metasedimentary protolith therefore the proposed derivation of the younger lac la croix type granite from layered migmatite in effect represents a second distillation of granitic liquid from meta sedimentary rocks that had been enriched ingranitic components in the first cycle of partial melting and melt migration it is postulated that the implied two stage melting of metasedimentary rocks was related to thermal consequences within the quetico accretionary complex as tectonism along the quetico wabigoon boundary alternated between subduction and transform kinematics during late archean time

petrogenesis of the dog lake granite chain quetico basin superior province canada implications for neoarchean crustal growth abstract the neoarchean dog lake granite chain consists of six intrusions shabaqua silver falls trout lake barnum lake white lily and penassen lake which parallel the tectonic boundary between the abitibi wawa terrane to the south and the quetico basin to the north it is a single composite body at depth with derivative branches extending higher into the crust and consists of three phases monzodiorite syenite quartz monzonite and granite emplaced at approximately 2 671 ga 2 663 2 669 ga and 2 670 ga respectively the metaluminous monzodiorites with abundant angular igneous mafic xenoliths enriched zircon hf t values ranging from 0 52 to 4 20 and enriched nd t 0 37 to 1 74 were generated by assimilation fractional crystallization of a mafic magma generated by partial melting of enriched mantle metasomatized by melts of subducted oceanic sediments the metaluminous syenite quartz monzonite phase has similar mineral assemblages trace element and nd hf isotopic characteristics to the monzodiorite and was probably produced by assimilation fractional crystallization of a monzodiorite magma the granite hosted zircons have prominent positive ce anomalies negative eu anomalies and mostly positive zircon hf t 0 12 to 4 25 and enriched nd t values 1 70 to 0 71 suggesting a mixed source of immature sedimentary rocks and metasomatically enriched mantle derived magma the subduction of oceanic lithosphere under the abitibi wawa arc during the collision of the abitibi wawa arc and the wabigoon terrane before 2 7 ga generated a metasomatically enriched mantle and associated volcanic rocks the volcanic rocks underwent rapid weathering and erosion to form the immature sedimentary rocks of the quetico accretionary prism lithospheric inversion caused partial melting of the metasomatically enriched mantle generating a mafic magma the mafic magma ascended and underwent assimilation fractional crystallization forming the monzodiorite and syenite quartz monzonite mafic magma was emplaced at the base of the archean immature sedimentary rocks triggering melting to generate felsic magma mixing of these felsic and mafic magmas generated the dlgc granite in the shallow crust the neoarchean arc continent collision preserved in the quetico basin is similar to that in modern arc systems where intra oceanic arc crust is added to continental margins suggesting it was an important mechanism for recycling juvenile crust generating compositional differentiation and cratonization

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

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

seamount statistics in the pacific ocean we apply the wide beam sampling technique of jordan et al 1983 to approximately 157 000 km of wide beam profiles to obtain seamount population statistics for eight regions in the eastern and southern pacific ocean population statistics derived from wide beam echograms are compared with seamount counts from sea beam swaths and with counts from bathymetric maps we find that the average number of seamounts with summit heights h h is well approximated by the exponential frequency size distribution h oe h the exponential model for seamount sizes characterized by the single scale parameter 1 is found to be superior to a power law self similar model which has no intrinsic scale in describing the average distribution of pacific seamounts and it appears to be valid over a size spectrum spanning 5 orders of magnitude in abundance large scale regional variations in seamount populations are documented we observe significant differences in seamount densities across the murray fracture zone in the north pacific and the eltanin fracture zone system in the south pacific the eltanin discontinuity is equally evident on both sides of the pacific antarctic ridge in the south pacific regions symmetrically disposed about the ridge axis have very similar seamount densities despite the large difference between pacific plate and antarctic plate absolute velocities evidently any differences in the shear flows at the base of the pacific and antarctic plates do not affect seamount emplacement systematic variations in o and are observed as a function of lithospheric age with the number of large seamounts increasing more rapidly than small seamounts these observations have been used to develop a simple model for seamount production under the assumptions that 1 an exponential size frequency distribution is maintained 2 production is steady state and 3 most small seamounts are formed on or near the ridge axis the limited data available from this study appear to be consistent with the model but they are insufficient to provide a rigorous test of the assumptions or determine accurately the model parameters however the data from the south pacific indicate that the off axis production of large seamounts probably accounts for the majority of seamounts with summit heights greater than 1000 m

two forms of volcanism implications for mantle flow and off axis crustal production on the west flank of the southern east pacific rise seamarcii side scan sonar and bathymetric data on the west flank of the east pacific rise 16 18 s reveal a large number of seamounts organized primarily in chains the easternmost ends of the chains near the ridge axis appear to be active as indicated by fresh lava flows in addition areas of unusually high reflectivity representing recent lava flows were found as far as 60 to 80 km from the spreading axis the flows are sometimes dammed by abyssal hill scarps formed near the ridge axis that have maximum relief of the order of 100 m in other cases the scarps appear to be buried by flows the disappearance of scarps can be used to estimate the extent of old lava flows or volcanic debris that are not detected acoustically in the sonar image under this criterion lava flows and volcanic debris surrounding the seamounts cover 40 50 of the survey area of about 18 000 km2 with an estimated average thickness of about 100 m the volume of lava flows and volcanic debris is about 0 8 0 3 of that of the total crust the volume of the seamounts is estimated by isolating the seamounts from the background topography with an anisotropic median filtering technique the volume of seamount edifices is 1 05 0 05 of the volume of the crust thus the total volume of volcanic material extruded off axis is about 1 5 to 2 2 of the volume of the crust the decreasing volcanic activity away from the ridge axis suggests that near axis seamounts may be associated with the upwelling system beneath the ridge perhaps by preferential melting of embedded passive heterogeneities however the decrease in volcanism with increasing distance from the axis also could be caused by a decrease in lithospheric vulnerability to penetration by magma since no apparent east west faults and no simultaneous volcanic activity along the length of a seamount chain are observed north south extension and ridge perpendicular convection are unlikely to be major causes of seamount formation in the study area the existence of closely spaced linear chains and the tendency for fresh flows to be found at near ridge ends of chains suggest that there are discrete sources in the upper mantle active for extended periods over 1 8 m y passive heterogeneities embedded in the upwelling mantle would have to be highly elongated along streamlines to produce persistent nearly stationary melting anomalies a variety of mechanisms may be responsible for the off axis volcanism but the single hypothesis most consistent with all observations is a mini hotspot origin although fresh lavas 80 km off axis are consistent with broad mantle upwelling beneath the ridge the possibility of narrow upwelling cannot be ruled out because the seamounts could be caused by independent buoyant upwelling of mini plumes that can penetrate thin lithosphere more easily near the axis

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

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

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

deep seismic reflection evidence for continental underthrusting beneath southern tibet the himalaya and adjacent tibetan plateau constituting earth s largest region of elevated topography and anomalously thick crust formed as a consequence of cenozoic collision between india and asia itself considered the archetypal continent continent collision1 3 here we report the first results from an attempt to image the structure of the crust beneath this region using deep seismic reflection profiling our 100 km long profile acquired in the tethyan himalaya shows a mid crustal reflection that prob ably marks the active thrust fault along which the indian plate is underthrusting southern tibet upper crustal reflections with geo metries suggestive of large scale structural imbrication of the upper crust and moho reflections from the base of the double normal thickness crust underlying the region these results lend substantial support to the view that crustal thickening beneath southernmost tibet was accomplished by wholesale underthrusting of indian continental crust beneath the structurally imbricated upper crust comprising the tethyan himalaya

geomorphology of the oceans abstract we present the first digital seafloor geomorphic features map gsfm of the global ocean the gsfm includes 131 192 separate polygons in 29 geomorphic feature categories used here to assess differences between passive and active continental margins as well as between 8 major ocean regions the arctic indian north atlantic north pacific south atlantic south pacific and the southern oceans and the mediterranean and black seas the gsfm provides quantitative assessments of differences between passive and active margins continental shelf width of passive margins 88 km is nearly three times that of active margins 31 km the average width of active slopes 36 km is less than the average width of passive margin slopes 46 km active margin slopes contain an area of 3 4 million km2 where the gradient exceeds 5 compared with 1 3 million km2 on passive margin slopes the continental rise covers 27 million km2 adjacent to passive margins and less than 2 3 million km2 adjacent to active margins examples of specific applications of the gsfm are presented to show that 1 larger rift valley segments are generally associated with slow spreading rates and smaller rift valley segments are associated with fast spreading 2 polar submarine canyons are twice the average size of non polar canyons and abyssal polar regions exhibit lower seafloor roughness than non polar regions expressed as spatially extensive fan rise and abyssal plain sediment deposits all of which are attributed here to the effects of continental glaciations and 3 recognition of seamounts as a separate category of feature from ridges results in a lower estimate of seamount number compared with estimates of previous workers

bathymetric highs in the mid slope region of the western continental margin of india structure and mode of origin analysis of the multi and single beam bathymetric seismic magnetic and free air gravity ship borne and satellite derived data from the western continental margin of india between 12 40 n and 15 n had revealed the presence of several flat topped nnw sse trending bathymetric highs of considerable areal extent 256 to 1165 km 2 area each at base they are interspersed by bathymetric lows thereby forming a chain of horst and graben structures covered by thick sediments 2 d modeling of the gravity and magnetic anomalies constrained by seismic results reflection and refraction had indicated 23 km thick crust consisting of 2 to 3 km thick sediments and a thin basaltic and granitic layer the model has brought into focus the presence of mantle based volcanic plugs their geomorphological and geophysical expressions and structural pattern and associated deep seated faults led to suggest continental crust these features are inferred as rifted fragments of the western continental margin of india formed during its northward movement they were subjected to sub aerial erosion during the paleogene due to lowered sea levels resulting in flat tops subsequently they were submerged to the present depths in the neogene the basic tectonic process appears to be regional

rayleigh wave phase velocities small scale convection and azimuthal anisotropy beneath southern california 1 we use rayleigh waves to invert for shear velocities in the upper mantle beneath southern california a one dimensional shear velocity model reveals a pronounced low velocity zone lvz from 90 to 210 km the pattern of velocity anomalies indicates that there is active small scale convection in the asthenosphere and that the dominant form of convection is three dimensional 3 d lithospheric drips and asthenospheric upwellings rather than 2 d sheets or slabs several of the features that we observe have been previously detected by body wave tomography these anomalies have been interpreted as delaminated lithosphere and consequent upwelling of the asthenosphere beneath the eastern edge of the southern sierra nevada and walker lane region sinking lithosphere beneath the southern central valley upwelling beneath the salton trough and downwelling beneath the transverse ranges our new observations provide better constraints on the lateral and vertical extent of these anomalies in addition we detect two previously undetected features a high velocity anomaly beneath the northern peninsular range and a low velocity anomaly beneath the northeastern mojave block we also estimate the azimuthal anisotropy from rayleigh wave data the strength is 1 7 at periods shorter than 100 s and decreases to below 1 at longer periods the fast direction is nearly e w the anisotropic layer is more than 300 km thick the e w fast directions in the lithosphere and sublithosphere mantle may be caused by distinct deformation mechanisms pure shear in the lithosphere due to n s tectonic shortening and simple shear in sublithosphere mantle due to mantle flow

rayleigh wave tomography of the northeastern margin of the tibetan plateau abstract the convergence between the indian continent and the southern margin of eurasia has eventually led to the growth of the tibetan plateau eastward various models have suggested that a large scale flow in the lower crust or and the asthenosphere is responsible for the uplift of eastern tibet it has also been suggested that the continental flow may move around the thick lithosphere of the ordos plateau and the sichuan basin in order to investigate the validity of the crust mantle flow and its possible path we have used rayleigh wave tomography with sensitivity kernels to estimate both the phase velocity structure 20 143 s and the shear wave velocity structure 0 200 km across the northeastern margin of the tibetan plateau from the surface to 100 km our shear wave velocity model shows a prominent low velocity anomaly beneath northeastern tibet suggestive of a warm lithosphere its boundary lies at 105 e longitude which borders the western ordos plateau and sichuan basin from 125 km to 200 km our shear wave velocity model shows a low velocity channel beneath the qilian qinling orogen that lies between the ordos and sichuan blocks this channel may represent the asthenosphere given the fact that the lithosphere is 120 150 km thick in northeastern tibet and the qinling orogen we also inverted for the anisotropy parameters in the study area simultaneously with the velocity parameters the dominant fast direction is nww see generally consistent with the sks splitting results the calculated shear strain from gps and the fault slip rate measurements furthermore the fast directions in our anisotropic model vary only slightly with periods and the anisotropy magnitudes at long periods 100 143 s are large and exceed those at short medium periods 20 80 s indicating potentially although not necessarily vertically coherent deformation from the crust to the asthenosphere and possibly large deformation in the asthenosphere the deformation seems to be linked to the southeastward extrusion tectonics caused by the collision boundary force

palaeomagnetism and 36ar40ar vs 39ar40ar isotope correlation ages of dyke swarms in central kerala india tectonic implications palaeomagnetic results and whole rock 36ar40ar vs 39ar40ar isotope correlation ages are presented for dyke swarms occurring within the granulite terrain in central kerala west coast of india palaeomagnetic data show that the nnw sse leucogabbro dykes sampled are normally magnetised d 307 and i 57 whereas most nw se dolerite dykes are reversely magnetised d 163 and i 61 one dolerite dyke shows normal polarity the present 40ar39ar isotope ages for dolerite dykes and previously reported k ar ages for leucogabbros suggest 81 2 ma for acquisition of the leucogabbro directions and 69 1 ma for acquisition of the reverse magnetic directions within the dolerites based on palaeomagnetic and isotopic age data the dolerite dykes appears to represent feeder systems for the deccan volcanic sequences we suggest that the deccan eruptions were widespread and probably extended southward at least into central kerala the entire magmatic episode may have occurred between ca 70 and 65 ma during the 31r period probably corresponding to the cretaceous tertiary boundary the leucogabbros may correspond to the 34n superchron and represent a plume related magmatic episode precursory to the main volcanic event these data suggest that an early intraplate plume tectonic setting since ca 80 ma preceded lithospheric thinning asthenospheric upwelling and consequent massive flood volcanism during the period 70 65 ma this culminated in ocean floor spreading between the seychelles mascarene and continental india

near ridge seamount chains in the northeastern pacific ocean high resolution bathymetry and side scan data of the vance president jackson and taney near ridge seamount chains in the northeast pacific were collected with a hull mounted 30 khz sonar the central volcanoes in each chain consist of truncated cone shaped volcanoes with steep sides and nearly flat tops several areas are characterized by frequent small eruptions that result in disorganized volcanic regions with numerous small cones and volcanic ridges but no organized truncated conical structure several volcanoes are crosscut by ridge parallel faults showing that they formed within 30 40 km of the ridge axis where ridge parallel faulting is still active magmas that built the volcanoes were probably transported through the crust along active ridge parallel faults the volcanoes range in volume from 11 to 187 km3 and most have one or more multiple craters and calderas that modify their summits and flanks the craters 1 km diameter range from small pit craters to calderas as large as 6 5 8 5 km although most are 2 4 km across crosscutting relationships commonly show a sequence of calderas stepping toward the ridge axis the calderas overlie crustal magma chambers at least as large as those that underlie kilauea and mauna loa volcanoes in hawaii perhaps 4 5 km in diameter and 1 3 km below the surface the nearly flat tops of many of the volcanoes have remnants of centrally located summit shields suggesting that their flat tops did not form from eruptions along circumferential ring faults but instead form by filling and overflowing of earlier large calderas the lavas retain their primitive character by residing in such chambers for only short time periods prior to eruption stored magmas are withdrawn probably as dikes intruded into the adjacent ocean crust along active ridge parallel faults triggering caldera collapse or solidified before the next batch of magma is intruded into the volcano probably 1000 10 000 years later the chains are oriented parallel to subaxial asthenospheric flow rather than absolute or relative plate motion vectors the subaxial asthenospheric flow model yields rates of volcanic migration of 3 4 3 3 and 5 9 cm yr 1 for the vance president jackson and taney seamounts respectively the modeled lifespans of the individual volcanoes in the three chains vary from 75 to 95 kyr these lifespans coupled with the geologic observations based on the bathymetry allow us to construct models of magma supply through time for the volcanoes in the three chains

the tarava seamounts a newly characterized hotspot chain on the south pacific superswell the tarava seamount chain also known as the savannah seamounts on some previous marine charts is located 200 km south of the society islands and was surveyed in 1996 it is 700 km long and comprises 18 seamounts two of which have been dredged and dated this chain exhibits two main branches a western branch parallel to pacific plate motion before 43 ma and an eastern one parallel to plate motion since 43 ma to first order k ar ages obtained on two dredged samples 43 5 2003 elsevier science b v all rights reserved

hotspots in the southern oceans an absolute frame of reference for motion of the gondwana continents abstract the geometry and geochronology of aseismic ridges and oceanic islands in the southern oceans provide a good test of the proposition that hotspots remain fixed over long periods of time that is motion of an order of magnitude less than the relative motion between plate pairs in most cases it is concluded that inter hotspot movement cannot be discerned for the period 100 m y to present and that widely distributed hotspots in the atlantic and indian oceans provide a frame of reference for plate motions following the disintegration of gondwanaland which is independent of paleomagnetism this frame of reference is absolute in that it gives the motion of the lithosphere with respect to the mantle hotspots the absolute motion model indicates that africa and antarctica are now moving only very slowly that there has been significant relative movement between east and west antarctica since the cretaceous and prescribes the relative motion between the somali and african plates

geochemical and structural studies of the lamont seamounts seamounts as indicators of mantle processes abstract a chain of seamounts located just west of the east pacific rise epr axis at 09 55 n herein called the lamont seamounts have been investigated using sea marc i and sea beam sonar surveys alvin submersible dives deep sea camera tows and dredging the most recently active volcanic vents on the summits of these volcanoes are located at the periphery of each caldera or crater rim or at the margins of caldera floors the distribution of recent volcanic vents and the morphology of the summit plateaus suggest the possible existence of ring fractures on these seamounts ring fractures could be important in focusing post caldera collapse eruptions around summit craters and building summit plateaus structural and morphological data and the petrology of recovered lavas suggest that calderas in the lamont seamounts probably formed by collapse associated with magma withdrawal into large primary vertical magma conduit systems rather than by repeated inflation deflation cycles associated with steady state shallow level magma chambers calderas in the lamont seamounts have evolved through coalescence of adjoining craters and the plan view shape and morphology of each caldera suggest that primary magma conduits have migrated beneath the summit of each seamount through time compared to lavas from the adjacent epr the majority of lamont seamount and cone lavas are more primitive depleted in incompatible elements and compositionally heterogeneous variations in light rare earth elements relative to major elements e g tio 2 na 2 o na 2 o cao and radiogenic isotopes cannot be ascribed to differences in the amount of partial melting of a single n type morb source rather it appears that lamont seamount lavas were derived from heterogeneous mantle sources variably depleted in highly incompatible elements comparison with lavas from the adjacent ridge crest suggests that magma plumbing and ascent mechanisms beneath the lamont seamounts are separate from those along the epr in particular seamount magmas appear to reflect near primary melting events in the mantle whereas epr lavas are the products of shallow level crystallization and mixing in crustal magma chambers absence of long lived magma chambers within the lamont seamounts may account for the low level of hydrothermal activity observed throughout the chain furthermore because of the tight control on sample location and the recovery of lavas having distinct parents from individual volcanoes as well as the lamont chain as a whole we propose that the scale of mantle chemical heterogeneities is less than 5 km

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

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

the geological evolution of the tibetan plateau the geological evolution of the tibetan plateau is best viewed in a context broader than the india eurasia collision zone after collision about 50 million years ago crust was shortened in western and central tibet while large fragments of lithosphere moved from the collision zone toward areas of trench rollback in the western pacific and indonesia cessation of rapid pacific trench migration 15 to 20 million years ago coincided with a slowing of fragment extrusion beyond the plateau and probably contributed to the onset of rapid surface uplift and crustal thickening in eastern tibet the latter appear to result from rapid eastward flow of the deep crust probably within crustal channels imaged seismically beneath eastern tibet these events mark a transition to the modern structural system that currently accommodates deformation within tibet

upper mantle seismic velocities and anisotropy in china determined through pn and sn tomography 1 we have obtained velocity images of the uppermost mantle beneath china by performing tomographic inversion of both pn and sn traveltimes from the annual bulletin of chinese earthquakes 99 139 pn arrivals and 43 646 sn arrivals were selected pn anisotropy was also obtained simultaneously with pn velocity average pn and sn velocities are 8 05 and 4 55 km s respectively and maximum velocity perturbations are about 3 4 the pn and sn velocities are low in eastern china and high in western china particularly high velocities are associated with old basins for example tarim junggar turpan hami qaidam and sichuan and stable craton for example ordos low sn velocities are found mainly throughout north china in addition velocities are relatively low beneath the central tibetan plateau and the north south seismic zone along 103 e in tarim and central china where we observe strong anisotropy the fast pn velocity directions are consistent with the directions of maximum principal compressive stress as well as directions of crustal movement determined from global positioning system beneath the india eurasia collision zone the pn anisotropy direction is parallel to the collision arc and nearly perpendicular to both the direction of maximum compression and crustal movement resulting from pure shear deformation both the velocity variations and anisotropy indicate that the tibetan plateau was extruded and the mantle material beneath the plateau has flowed around the east himalaya syntax while the remaining material has diverted northwestward beneath the tarim basin

the evolution of craters and calderas on young seamounts insights from sea marc i and sea beam sonar surveys of a small seamount group near the axis of the east pacific rise at 10 n a northwest trending group of seamounts located just west of the east pacific rise epr axis has been mapped using high resolution sea marc i side scan sonar and sea beam multibeam bathymetry these seamounts have 9 13 km basal diameters and the shapes of the volcanos change from conical domes near the epr axis to truncated cones and larger elliptical truncated cones farther away from the rise axis the detailed resolution of constructional volcanic features on these seamounts and the progressive development of summit depressions craters to large complex calderas and summit plateaus suggest to us a succession of evolutionary stages through which seamounts grow our evolutionary scheme for constructing these volcanos calls for the development of satellite extrusive centers around the base of a growing seamount which serve to drain off magmas and precipitate summit collapse the creation of a summit depression on a seamount will structurally constrain the style of eruptive volcanism to be principally along ring fractures which localize vents around the margin of a crater or caldera the concentration and channelization of volcanic eruptions around the top of the volcano lead to a progradation of the summit area and the establishment of a summit plateau the steep sides of a seamount reflect growth of the summit area out over the upper flanks of the volcano caused by continued eruption of lavas around the margin of the caldera the steep constructional slopes are sites for vigorous mass wasting that shapes and modifies flank terrain as a seamount evolves the crestal topography is the product of many volcanic cycles characterized by filling and subsequent draining of the caldera and expansion of caldera area through progressive collapse and migration of primary magmatic conduits the final phase of volcanism may be a calderafilling episode that builds a rough conical mount over the summit plateau

morphological characteristics and emplacement mechanism of the seamounts in the central indian ocean basin abstract the morphotectonic features of the central indian ocean basin ciob provide information regarding the development of the basin multibeam mapping of the ciob reveals presence of abundant isolated seamounts and seamount chains sub parallel to each other and major fracture zones along 73 e 79 e and 75 45 e morphological analyses were carried out for 200 seamounts that occur either as isolated edifies or along eight sub parallel chains the identified eight parallel seamount chains that trend almost north south and reflecting the absolute motion of the indian plate probably originated from the ancient propagative fractures inspite of the differences in their height the seamounts of these eight chains are morphologically correlatable in the study area the seamounts are clustered north and south of 12 s latitude interestingly in the area north of 12 s area ii 9 12 s the seamounts are distinctly smaller 400 m height whereas the area south of 12 s area i 12 15 s has a mixed population of seamounts the normalized abundance of the ciob seamount is 976 seamounts 10 6 km 2 but on a finer scale this value varies from 500 to 1600 seamounts 10 6 km 2 which is less than the seamount concentrations of the pacific and atlantic oceans 9000 to 16 000 seamounts 10 6 km 2 three categories of seamounts are present in the ciob e g 1 single peaked 2 multi peaked and 3 composite the study indicate that single peaked seamounts are dominant 89 while multi peaked is less 8 and composite ones are rare 3 in the ciob the progressive northward movement of the indian continent caused collision between india and asia at around 62 ma ago a majority of the near axis originated seamounts in the ciob seemed to have formed as a consequence of the temporally widespread cretaceous 65 ma to late eocene

global distribution of seamounts inferred from gridded geosat ers 1 altimetry using a forward modeling approach based on an axisymmetric gaussian seamount i characterize the global seamount distribution by locating circular maxima in the gridded vertical gravity gradient field derived from altimetry collected by the geosat and ers 1 satellite missions the global seamount distribution is long tailed and resembles a power law distribution for seamounts in the height range 2 7 km smaller seamounts are not well isolated by my technique nor are they well resolved in the gridded data several factors are likely to influence the height of volcanic seamounts such as melt availability magma driving pressure and plate thickness the observed relationship between seamount gravimetric amplitudes and the age of the underlying seafloor implies that there is an upper limit on seamount heights whether a seamount will reach that height depends most likely on supply side factors such as melt availability and magma driving pressure but the limiting height itself seems more likely to be controlled by the strength of the oceanic plate specifically compressional stresses directly beneath the seamount as a consequence of the lithosphere s flexural response to loading may eventually exceed the magma driving pressure and prevent magma from reaching the surface thus limiting the growth of the seamount because oceanic plate strength primarily is controlled by plate age the limit on seamount height is inferred to be a simple function of plate age at the time of seamount emplacement using analytical solutions i present a simple flexural model that predicts the observed global height age relationship

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

analyses of multichannel seismic reflection gravity and magnetic data along a regional profile across the central western continental margin of india abstract analyses of multichannel seismic reflection gravity magnetic and bathymetry data along a regional profile across the central western continental margin of india have revealed the depositional pattern of sediments crustal structure and tectonics the four most distinct and varied crustal regions of the margin are palaeo shelf edges shelf margin basin prathap and laccadive ridges and the arabian basin the shelf margin basin is carpeted by 4 5 km maximum thick aggraded and prograded paleocene to holocene sediments six major seismic sequences of the sediments of the margin are identified and their ages are assigned on correlation with drill well results development of the sequence boundaries is attributed to the events of rifting of western india eustatic sea level changes indian and eurasian plate collision and himalayan orogeny tilted fault blocks half grabens located almost equi distance from the igneous construct of the prathap ridge in the shelf margin basin suggest a failed rift associated with stretched continental crust of the basin 2 d model studies of gravity and magnetic anomalies constrained by the seismic results have revealed 6 to 27 km thick crust across the margin the laccadive ridge crust limited by two volcanic intrusives and a steep scarp at its western end is 16 km thick it gradually thins towards offshore and juxtaposed with early tertiary normal oceanic crust 6 km thick of the arabian basin the crustal thickness and velocity and density structure of the ridge are comparable to that of the laxmi ridge a continental sliver the inferences and abrupt change in magnetic and gravity anomaly signatures across the western end of the laccadive ridge mark the zone of transition from continental to oceanic crust

a structural reappraisal of the beardmore geraldton belt at the southern boundary of the wabigoon subprovince ontario and implications for gold mineralization the beardmore geraldton belt occurs along the southern margin of the archean wabigoon subprovince superior province ontario the belt consists of shear bounded interleaved metasedimentary and metavolcanic units the units were imbricated from 2696 to 2691 ma during d1 thrusting and accretion of the wabigoon quetico and wawa subprovinces post accretion d2 deformation produced regional f2 folds that transposed lithological units parallel to the axial plane s2 cleavage of the folds during d3 deformation the folds were overprinted by a regional s3 cleavage oriented anticlockwise of f2 axial planes and lithological contacts and s2 cleavage were reactivated as planes of shear within dextral regional shear zones that generally conform to the trend of the belt d3 is a regional dextral transpression event that also affected the quetico and wawa subprovinces south of the beardmore geraldton belt gold mineralization at the leitch and macleod cockshutt mines the two richest past producing gold mines in the beardmore geraldton belt is associated with d3 shear zones and folds overprinting regional f2 folds the plunge of the ore zones is parallel to f3 fold axes and to the intersection of d3 shear zones with f2 and f3 folds

deformation across the western quetico subprovince and adjacent boundarl regions in minnesota north to northwest directed crustal shortening across the western quetico subprovince and its boundary regions produced a complex deformation sequence within the quetico belt and resulted in concentrated zones of dextral ductile shear in the boundary regions within the adjacent greenstone granite terranes in this paper we review and introduce new data on the regional deformation features and their geometries and discuss the history of generation of these features we attribute the deformation sequence to differential partitioning of shortening and shear strains during dextral transpression associated with oblique convergence and accretion along the southern margin of the superior province the turbiditic wacke in the western quetico subprovince now typically amphibolite facies schist and migmatite underwent an early deformation stage that included recumbent folding f1 and the generation of an s1 bedding parallel foliation this event is most evident along the northern and southern boundaries of the

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