giant plagioclase basalts continental flood basalt induced remobilization of anorthositic mushes in a deep crustal sill complex giant plagioclase basalts rich in plagioclase megacrysts commonly 5 cm long form widespread lava flows and dikes in the deccan traps continental flood basalt province they also occur in the scottish hebrides columbia river and emeishan continental flood basalt provinces and closely similar rocks are known from archean and proterozoic terrains and oceanic settings the giant plagioclase basalts have usually been interpreted as end products of magmatic cycles operating in high level chambers and taken as the boundaries of stratigraphic formations a new model is proposed in which the plagioclase megacrysts grow over long 15 k y time periods in a deep crustal possibly moho level sill complex which forms tens to hundreds of thousands of years before the continental flood basalt eruptions megacryst flotation produces anorthositic mushes containing ferrodioritic interstitial liquids giant plagioclase basalts form when these components and any melt from the crust heated by the sill complex are remobilized by a newly injected tholeiitic magma batch and brought to the surface or high crustal levels by dikes giant plagioclase basalts are thus mixed heterogeneous rocks from the deep continental crust the new model explains a range of geological petrological mineralogical and geochemical data on the giant plagioclase basalts formation and remobilization of anorthositic mushes appear to be fundamental processes producing highly feldspar phyric basalts over a wide range of tectonic settings an important corollary of the new model is that anorthosites have continued to form well after the precambrian on a smaller scale during phanerozoic continental flood basalt events therefore continental flood basalt provinces should be recognized as sites of anorthosite formation in the deep continental crust

stratigraphy composition and form of the deccan basalts western ghats india in the western ghats between latitudes 18 20 n and 19 15 n 7000 km2 of deccan basalt have been mapped with the primary objective of establishing a flow stratigraphy as a guide to the volcanic history of the flood basalts using over 70 measured vertical sections major and trace element analyses of nearly 1200 samples and rare earth and87sr 86sr determinations for over 60 samples we divide the basalt into three subgroups and ten formations in this paper we describe the seven principal formations in the area and the most prominent individual flows

geology and geochemistry of the sangamner mafic dike swarm western deccan volcanic province india implications for regional stratigraphy abstract numerous large ne sw to e w trending mafic dikes outcrop around sangamner in the western deccan volcanic province this area is part of a broader region postulated to be a shieldlike feature and a major eruption center a combination of field geochemical and isotopic sr and nd characteristics is used here to understand the relationship of this dike swarm with the associated lava flows and their position in the established deccan stratigraphy many dikes are compositionally similar to the khandala and poladpur formations belonging to the lonavala and wai subgroups respectively while one dike is similar to the ambenali formation one dike has a composition distinct from all other dikes in this area as well as from most stratigraphic units although there are many similarities in composition with the bushe formation as well as the boyhare member of the khandala formation while several dikes are geochemically similar to specific flows members within certain formations their isotopic composi

estimating volcanic plume heights from depositional clast size 1 tephra deposits retain a considerable amount of information about the nature of volcanic eruptions with plume height commonly inferred from maximum clast size measurements however current methods for inferring plume height from maximum clast size lose some of the accuracy in measurements made in the field and have limited application when trying to constrain the uncertainty in these parameters here a predictive numerical model is used to determine plume height from maximum clast size found in a deposit plume height is an essential parameter for inferring eruption magnitude as it is explicitly related to mass eruption rate we investigate the effects of different atmospheric and wind profiles allowing the input conditions to be location specific for each eruption the use of a predictive model reduces the uncertainty in determining plume height as it retains the detail of measurements made in the field and incorporates the uncertainty in these measurements in the results in the form of a probability distribution we have applied this approach to fall deposits from the 1991 eruption of pinatubo one of the few plinian eruptions in which the true height of the plume is well known the predicted plume heights are in good agreement with those found from satellite measurements this new approach provides a method for determining plume height from poorly preserved fall deposits from which a limited amount of data can be sampled leading to large uncertainties in the field measurements

elemental and nd sr pb isotope geochemistry of flows and dikes from the tapi rift deccan flood basalt province india abstract the deccan traps are a large rift associated continental flood basalt province in india parts of which have been studied extensively in terms of geochemistry palaeomagnetism and stratigraphy however the basalts of the tapi rift in the central part of the province have been little studied thus far two ene wsw trending tectonic inliers of the deccan basalts in this region forming ridges rising from younger alluvium are made up of basalt flows profusely intruded by basaltic dikes both of these ridges lie along a single lineament although they are not physically continuous the flows are aphyric plagioclase phyric and giant plagioclase basalts and the dikes are aphyric or plagioclase phyric we consider the two inliers to have been originally continuous from the presence of bouldery remnants of a major dioritic gabbro dike along both samples of this dike from both ridges have previously yielded typical deccan ages of 65 6 0 5 ma and 65 6 0 6 ma by the 40 ar 39 ar incremental heating technique initial 87 sr 86 sr ratios and end t values and present day pb isotopic ratios of most dikes indicate that they are isotopically similar to lavas of the mahabaleshwar and panhala formations of the western ghats about 450 km to the south their mantle normalized trace element patterns have small pb and ba peaks one dike has a strong bushe formation affinity and a nd sr isotopic composition more extreme than that of any other deccan rock yet sampled with end t 20 2 and 87 sr 86 sr t 0 72315 its mantle normalized element pattern shows large pb th and u peaks and large nb ta troughs its elemental and isotopic chemistry reflects substantial continental contamination the flows cut by the mahabaleshwar type dikes are isotopically similar to the poladpur formation lavas of the western ghats their mantle normalized element patterns show modest peaks at rb ba and pb and rather low nb and ta relative to la indicating that they have been contaminated to intermediate degrees the mantle normalized element patterns of all the flows and dikes show enrichment in the light rare earth elements with small or no eu anomalies the entire flow dike sequence is similar to the wai subgroup of the western ghats in terms of its elemental and isotopic chemistry and stratigraphic relationships wai subgroup like lavas i e some of the younger magma types originally identified from the southern part of the western ghats are previously known from the central northern and northeastern deccan and many have been thought to be far travelled flows erupted in the southwestern deccan although at least the dikes and probably the giant plagioclase basalt flows of our study area are locally generated and emplaced our new data extend the known outcrop area of these widespread magma types substantially and these magma types indeed appear to have a nearly province wide distribution

a ā lava flows in the deccan volcanic province india and their significance for the nature of continental flood basalt eruptions newly identified a lava flows outcrop intermittently over an area of 110 km2 in the western deccan volcanic province dvp india they occur in the upper thakurvadi formation in the region south of sangamner the flows one of which is compound are 15 25 m thick and exhibit well developed basal and flow top breccias the lavas have microcrystalline groundmasses and are porphyritic or glomerocrystic and contain phenocrysts of olivine clinopyroxene or plagioclase feldspar they are chemically similar to compound p hoehoe flows at a similar stratigraphic horizon along the western ghats petrographic and geochemical differences between a flows at widely spaced outcrops at the same stratigraphic horizon suggest that they are the product of several eruptions potentially from different sources their presence in the dvp could suggest relative proximity to vents this discovery is significant because a lavas are generally scarce in large continental flood basalt provinces which typically consist of numerous inflated compound p hoehoe lobes and sheet lobes their scarcity is intriguing and may relate to either their occurrence only in poorly preserved or exposed proximal areas or to the flat plateau like topography of flood basalt provinces that may inhibit channelization and a formation or both in this context the a flow fields described here are inferred to be the products of eruptions that produced unusually high effusion rate lavas compared to typical flood basalt eruptions whether these phases were transitional to lower intensity sustained eruptions that fed extensive low effusion rate p hoehoe flow fields remains unclear

structure and emplacement of the nandurbar dhule mafic dyke swarm deccan traps and the tectonomagmatic evolution of flood basalts flood basalts such as the deccan traps of india represent huge typically fissure fed volcanic provinces we discuss the structural attributes and emplacement mechanics of a large linear tholeiitic dyke swarm exposed in the nandurbar dhule area of the deccan province the swarm contains 210 dykes of dolerite and basalt 1 km in length exposed over an area of 14 500 km2 the dykes intrude an exclusively basaltic lava pile largely composed of highly weathered and zeolitized compound pahoehoe flows the dykes range in length from 1 km to 79 km and in thickness from 3 to 62 m almost all dykes are vertical with the others nearly so they show a strong preferred orientation with a mean strike of n88 because they are not emplaced along faults or fractures they indicate the regional minimum horizontal compressive stress 3 to have been aligned n s during swarm emplacement the dykes have a negative power law length distribution but an irregular thickness distribution the latter is uncommon among the other dyke swarms described worldwide dyke length is not correlated with dyke width using the aspect ratios length thickness of several dykes we calculate magmatic overpressures required for dyke emplacement and depths to source magma chambers that are consistent with results of previous petrological and gravity modelling the anomalously high source depths calculated for a few dykes may be an artifact of underestimated aspect ratios due to incomplete along strike exposure however thermal erosion is a mechanism that can also explain this whereas several of the nandurbar dhule dykes may be vertically injected dykes from shallow magma chambers others particularly the long ones must have been formed by lateral injection from such chambers the larger dykes could well have fed substantial 1 000 km3 and quickly emplaced a few years flood basalt lava flows this work highlights some interesting and significant similarities and contrasts between the nandurbar dhule dyke swarm and regional tholeiitic dyke swarms in iceland sudan and elsewhere

range dependent seabed characterization by inversion of acoustic data from a towed receiver array the mapex2000 experiments were conducted in the mediterranean sea in march 2000 to determine seabed properties using a towed acoustic source and receiver array towed systems are advantageous because they are easy to deploy from a ship and the moving platform offers the possibility for estimating spatially variable range dependent seabed properties in this paper seabed parameters are determined using a matched field geoacoustic inversion approach with measured towed array data previous research has successfully applied matched field geoacoustic inversion techniques to measured acoustic data however in nearly all cases the inverted data were collected on moored vertical receiver arrays results here show that seabed parameters can also be extracted by inverting acoustic measurements from a towed array of receivers and these agree with those inverted using data received simultaneously on a vertical array these findings imply that a practical technique could be developed to map range dependent seabed parameters over large areas using a towed acoustic system an example of such a range dependent inversion is given using measurements from the mapex2000 experiments

petrogenetic significance of ferro enstatite orthopyroxene in basaltic dikes from the tapi rift deccan flood basalt province india abstract some basaltic dikes from the tapi rift in the deccan flood basalt province india contain abundant clusters of prismatic quench crystals of ferro enstatite orthopyroxene fe 39 42 mg 53 57 ca 2 4 such profuse orthopyroxene clusters are not yet known from any other flood basalt province in the world based on petrography mineral chemistry and whole rock chemistry we conclude that these crystals are formed due to assimilation of shale by deccan basalt magma this interpretation is consistent with the known bedrock geology of the area these quench crystals constitute the first direct petrographic evidence for crustal contamination in the deccan province the liquidus temperature for the dikes is estimated at 1220 10 c with the crystallization temperature of the orthopyroxene at 1080 c or less

results from an autonomous underwater vehicle towed hydrophone array experiment in nantucket sound shallow water experiments have been conducted in nantucket sound with an autonomous underwater vehicle towed hydrophone array system in an area proximate to that of a previous experiment frisk and lynch j acoust soc am 86 1928 1939 1989 transmission loss was measured for frequencies between 220 and 1228 hz in an ocean waveguide under conditions of an isovelocity water column with an approximate depth of 13 m over a sandy silty bottom results obtained at 415 hz show classic isovelocity waveguide phenomena that include interference effects mode stripping due to modal attenuation and transmission loss proportional 15 to 10 log 10 r

volcanological and tectonic control of stratigraphy and structure in the western deccan traps many of the world s flood basalt provinces form elevated plateaux at the margins of continents although in most cases their present large elevation is not the result of mountain building processes several explanations have recently been put forward to explain such occurrences of epeirogeny the deccan trap basalt province forms one such elevated plateau and results are presented here showing how the epeirogenic uplift in this region combined with crustal subsidence probably associated with the rifting of the indian continental margin has affected the structure of the basalt sequence trace element analytical data are used for samples from numerous vertical sections through the deccan traps lava series along and around the western ghats ridge in india the results reinforce the previously defined stratigraphy of the mahabaleshwar area and extend it over a region covering some 36 000 km2 reaching as far south as belgaum and the trap basement contact these results show that the lava pile is not flat lying but forms a very low amplitude anticlinal fold structure plunging southwards by up to 0 3 over most of the area although in the south there is evidence of a reversal of this plunge the fold is interpreted as being the result of two tilting processes 1 westward tilting near the coast due to the foundering of the passive continental margin and 2 epeirogenic uplift along the whole west coast of india producing the observed topography and the peninsula wide drainage patterns and also the easterly component of dip variations in the magnitude of the latter effect along the western continental margin may also be important in generating the plunge of the fold although the possibility of some component of depositional dip may also be important this latter possibility can be modelled using a simple computer program the results of this modelling show that a migrating linear volcanic edifice fits the observations best

the impact of water column variability on horizontal wave number estimation and mode based geoacoustic inversion results the influence of water column variability on low frequency shallow water geoacoustic inversion results is considered the data are estimates of modal eigenvalues obtained from measurements of a point source acoustic field using a horizontal aperture array in the water column the inversion algorithm is based on perturbations to a background waveguide model with seabed properties consistent with the measured eigenvalues water column properties in the background model are assumed to be known as would be obtained from conductivity temperature and depth measurements the scope of this work in addressing the impact of water column variability on inversion is twofold range dependent propagation effects as they pertain to eigenvalue estimation are first considered it is shown that mode coupling is important even for weak internal waves and can enhance modal eigenvalue estimates second the effect of the choice of background sound speed profile in the water column is considered for its impact on the estim

yellow shark spring 1995 inversion results from sparse broadband acoustic measurements over a highly range dependent soft clay layer in may 1995 saclantcen performed broadband 200 800 hz acoustic measurements in the giglio basin off the coast of italy as part of the yellow shark inversion experiments in this paper inversion of sparse broadband transmission loss tl measurements is investigated to determine bottom properties in strongly range dependent rd situations commonly encountered in shallow water the data are from the elba formiche transect where water depth varies from 65 m at the acoustic projector location to approximately 120 m at the 4 element vertical arrays deployed at ranges of 8 16 24 32 and 40 km along the transect the experimental site has a soft clay layer bottom which varies in thickness from 3 to 10 m with a sound speed less than the water column a modal analysis including coupling effect is given to explain the frequency bands for which high tl was observed as a function of range and depth the tl measurements were inverted by matching the rd fields with model results using parabolic equation modeling bottom geoacoustic parameters were varied in a marching search to fit the tl measured at the five ranges the experimental results demonstrate that rd bottom properties such as sound speed and thickness of the slow clay layer can be obtained from broadband tl measurements sparsely distributed in range and depth

geology and geochemistry of pachmarhi dykes and sills satpura gondwana basin central india problems of dyke sill flow correlations in the deccan traps many tholeiitic dyke sill intrusions of the late cretaceous deccan traps continental flood basalt province are exposed in the satpura gondwana basin around pachmarhi central india we present field petrographic major and trace element and sr nd pb isotope data on these intrusions and identify individual dykes and sills that chemically closely match several stratigraphically defined formations in the southwestern deccan western ghats some of these formations have also been identified more recently in the northern and northeastern deccan however the pachmarhi intrusions are significantly more evolved lower mg numbers and higher tio2 contents than many deccan basalts with isotopic signatures generally different from those of the chemically similar lava formations indicating that most are not feeders to previously characterized flows they appear to be products of mixing between deccan basalt magmas and partial melts of precambrian indian amphibolites as proposed previously for several deccan basalt lavas of the lower western ghats stratigraphy broad chemical and isotopic similarities of several pachmarhi intrusions to the northern and northeastern deccan lavas indicate petrogenetic relationships distances these lava flows would have had to cover if they originated in the pachmarhi area range from 150 to 350 km the pachmarhi data enlarge the hitherto known chemical and isotopic range of the deccan flood basalt magmas this study highlights the problems and ambiguities in dyke sill flow correlations even with extensive geochemical fingerprinting

geochemical stratigraphy of deccan flood basalts of the bijasan ghat section satpura range india abstract the 240 m thick bijasan ghat section exposes deccan basalt flows and dykes that have major and trace element compositions suggestive of variable amounts of contamination by continental material most flows and a dyke are chemically similar to lavas of the poladpur formation in the southwestern deccan one flow and one dyke are similar to lavas of the bushe formation in the southwest and one dyke is similar to lavas of the mahabaleshwar formation in the southwest the type sections of these formations are located 350 km to the south of bijasan ghat but many poladpur type lavas a few mahabaleshwar like lavas and dykes and a few broadly bushe like lavas and dykes have recently been identified in the satpura region lavas resembling the thick ambenali fm of the southwest are absent from bijasan ghat and nearby areas our data further extend the known outcrop area of poladpur like flows estimated at 3 105 km2 one of the most widespread deccan magma types seven giant plagioclase basalt flows encountered in seven different sections in this region cannot unambiguously be correlated at present either physically or chemically and therefore are of little use in deciphering the complicated stratigraphic and structural make up of the regional lava pile

were the deccan flood basalts derived in part from ancient oceanic crust within the indian continental lithosphere abstract deep mantle plumes supposedly incorporate deeply subducted eclogitized oceanic crust and continental flood basalts cfbs are now thought by some to be derived from such eclogite bearing peridotite plumes eclogite peridotite mixtures have much lower solidi and produce much greater melt fractions for a given temperature than peridotite fe rich eclogite or pyroxenite bearing sources have been inferred for many cfbs however plumes with considerable amounts of eclogite should have difficulty in upwelling owing to the high density of eclogite besides cfbs are always located along pre existing lithospheric structures suture zones edges of thick cratons and commonly associated with lithospheric rifting and continental breakup india s major late mesozoic cfb the deccan traps erupted through rift zones and a new continental margin that had developed along ancient suture zones traversing the subcontinent many deccan basalts are too fe rich to have been in equilibrium with a peridotite mantle source and have commonly been considered to be significantly fractionated derivatives of picritic liquids however it is possible to view them as relatively less evolved liquids derived from a source with extra fertility i e an fe rich source a new non plume plate tectonic model for icelandic hotspot volcanism involves melting of a shallowly recycled slab of eclogitized iapetus oceanic crust formerly trapped along the caledonian suture the model explains the geochemical petrological characteristics of icelandic basalts and is consistent with passive upper mantle upwelling under iceland inferred from recent seismic tomography based on the petrological and geochemical features of the deccan flood basalts of the type section in the western ghats i propose that old eclogitized oceanic crust trapped in the ancient indian suture zones could have produced voluminous basaltic melts during the deccan event

coupled c s fe geochemistry in a rapidly accumulating marine sedimentary system diagenetic and depositional implications in the present study we have investigated the c s fe systematics in a sediment core md161 13 from the krishna godavari k g basin bay of bengal the core covers the late holocene period with high overall sedimentation rate of 573 cm kyr 1 pore fluid chemical analyses indicate that the depth of the present sulfate methane transition zone smtz is at 6 mbsf the ta ca mg so42 ratios suggest that both organoclastic degradation and anaerobic oxidation of methane aom drive sulfate reduction at the study site the positive correlation between total organic carbon content toc and chromium reducible sulfur crs content indicates marked influence of organoclastic sulfate reduction on sulfidization coupled occurrence of 34s enriched iron sulfide pyrite with 12c enriched authigenic carbonate zones is the possible records of paleo sulfate methane transition zones where aom driven focused sulfate reduction was likely fueled by sustained high methane flux from underlying gas rich zone aluminum normalized poorly reactive iron fepr al and la yb ratios suggest increasing contribution from deccan basalts relative to that of archean proterozoic granitic complex in sediment flux of krishna godavari basin during the last 4 kyr

the deccan tholeiite lavas and dykes of ghatkopar powai area mumbai panvel flexure zone geochemistry stratigraphic status and tectonic significance abstract mumbai city situated on the western indian coast is well known for exposures of late stage deccan pillow basalts and spilites pyroclastic rocks rhyolite lavas and trachyte intrusions these rock units and a little studied sequence of tholeiitic flows and dykes in the eastern part of mumbai city constitute the west dipping limb of a regional tectonic structure called the panvel flexure here we present field petrographic major and trace element and sr nd isotopic data on these tholeiitic flows and dykes best exposed in the ghatkopar powai area the flows closely resemble the mahabaleshwar formation of the thick western ghats sequence to the east in sr nd isotopic ratios and multielement patterns but have other geochemical characteristics e g incompatible trace element ratios unlike the mahabaleshwar or any other formation the flows may have originated from a nearby eruptive center possibly offshore of mumbai two dykes resemble the ambenali formation of the western ghats in all geochemical characteristics though they may not represent feeders of the ambenali formation lavas most dykes are distinct from any of the western ghats stratigraphic units some show partial e g sr nd isotopic similarities to the mahabaleshwar formation and these include several dykes with unusual concave downward ree patterns suggesting residual amphibole and thus a lithospheric source the flows and dykes are inferred to have undergone little or no contamination by lower continental crust most dykes are almost vertical suggesting emplacement after the formation of the panvel flexure and indicate considerable east west lithospheric extension during this late but magmatically vigorous stage of deccan volcanism

the determination of geoacoustic models in shallow water a technique for determining geoacoustic models in shallow water is described for a horizontally stratified ocean and bottom the method consists of measuring the magnitude and phase versus range of the pressure field due to a cw point source and numerically hankel transforming these data to obtain the depth dependent green s function versus horizontal wavenumber in shallow water the green s function contains prominent peaks at horizontal wavenumbers corresponding to the eigenvalues for any trapped and virtual modes excited in the waveguide a geoacoustic model for the bottom is obtained by computing the theoretical green s function for various values of the bottom parameters and determining the parameter set which provides the best agreement with the experimental green s function particularly in the positions and relative magnitudes of the modal peaks comparisons are also made between the measured and theoretically computed pressure field magnitudes the technique is demonstrated using experimental data at 140 hz and 220 hz

joint estimation of water column and sediment acoustic properties from broadband towed array data using modal inverse method broadband data collected on a towed horizontal array of length 2375m during a geophysical survey are used to perform joint inversion for the water column and sediment acoustic parameters using modal inverse technique s d rajan j f lynch and g v frisk j acoust soc am 82 998 1017 1987 to perform the inversion based on modal wave numbers we first obtain the pressure field as a function of range at a discrete set of frequencies from the broadband data the sources of errors and their impact on the estimation of modal wave number from such data are discussed due to the possibility of large errors in the estimation of the modal wave numbers of the higher order modes the inversion was performed using the modal wave numbers of the dominant lower order modes the resolution estimates indicate that the wave speeds in the water column and top three sediment layers have resolution lengths that are approximately 1 5 times the layer thicknesses with respect to deeper sediment layers the resolu

geochemistry of flood basalts of the toranmal section northern deccan traps india implications for regional deccan stratigraphy unpublished data 1998 andthe southeastern deccan particularly the poladpur andambenali formations mitchell widdowson 1991 bilgrami 1999 for as much as 350 km from their south western type sections in addition lavas isotopically andchemicallyequivalenttothoseoftheambenaliformation fm arepresentinthenortheastern sectoroftheprovinceas far as 900 km from their southwesternmost counter parts mahoney 1988 deshmukh

dynamics and deposits generated by the kos plateau tuff eruption controls of basal particle loss on pyroclastic flow transport the explosive eruption of voluminous silicic magmas often produces widespread and massive deposits formed from pyroclastic density currents while these punctuated events dramatically alter the landscape and have potential climate altering impact our understanding of the internal structure and transport dynamics of these eruptions is hampered by a lack of direct observations we utilize the natural boundary conditions encountered by the eruption of the kos plateau tuff to probe its internal structure as well as constrain the neotectonic activity in the region and eruption duration of this moderate to large 60 km3 event at the time of the eruption 161 ka the lower sea level in the mediterranean may have resulted in flows that traversed mostly land to the north of the eruptive vent while flows to the south may have encountered an expanse of water steep topography and overwater transport can impede the transport of the dense basal portions of the flow where particles make multiple or sustained contact with the bed we use an eulerian eulerian lagrangian computational approach coupled with overwater and overland boundary conditions including topography to determine the role of bed load versus suspended load in the transport of these flows we find that a ring vent structure and eruptive fluxes greater than 2 106 m3 s are required for the spatial distribution of the kpt the maximum grain size and deposit locations of the first voluminous ignimbrite unit d can be explained by suspended flow to the south consistent with overwater transport and bed load and suspended load transport to the north consistent with overland transport however the maximum lithic size for the largest and last ignimbrite unit e requires some bed load transport in all directions we propose that the boundary conditions were significantly altered during the course of the eruption through either the in filling of a shallow sea to the south or the development of a thick pumice raft to aid saltation on the basis of the inferred eruptive flux we estimate that the duration of the eruption climax in which most of the material was erupted likely only lasted from a few hours to a day

source and differentiation of deccan trap lavas implications of geochemical and mineral chemical variations the basalt stratigraphy of the deccan trap between mahabaleshwar ghat and belgaum over steps the basement from north to south sr isotope and zr nb ratios and sr rb and ba concentrations correlate portions of the post poladpur stratigraphy over 250 km along the western ghats thereby confirming a southerly component of dip of 0 06 at the southwestern margin the stratigraphy extends upwards from the compositionally uniform ambenali formation cox hawkesworth 1984 into a sequence of grossly heterogeneous flow units which have been allocated to the mahabaleshwar and panhala formations lightfoot hawkesworth 1988 the mahabaleshwar formation is represented only by a sequence of highly fractionated flows termed the kolhapur unit with similar 87sr 86sr0 to the mahabaleshwar 0 7045 but with sr 2 25 succeeding the kolhapur unit are a series of flows with high 87sr 86sr0 0 7045 0 705 zr nb 13 and low sr 230 but trace element concentrations similar to the mahabaleshwar formation these have been allocated to the desur unit of the panhala geochemical variations in flows overlying the ambenali define two distinct trends one is attributed to gabbro fractionation and the other to variations in the compositions of the parental magmas and arguably their source regions there is little evidence for significant crustal contamination in these flows and the degree of fractionation and the composition of the phase extract are shown to vary along strike within the mahabaleshwar formation the high tio2 content of kolhapur unit flows is shown to be the result of shallow level gabbro fractionation rather than the presence of a primitive high ti magma mahabaleshwar formation basalts exhibit a broad negative correlation between the degree of fractionation and sr isotopic composition the endmember with lower 87sr 86sr0 has different zr y from the ambenali basalts and would appear to have been generated by lower degrees of melting of a similar source the other endmember has more radiogenic sr lower zr nb similar zr y but higher mg number the simplest interpretation is that these magmas were more primitive and hence hotter and more able to interact with the lithosphere en route to the surface and that they then mixed to produce the mahabaleshwar array the panhala formation basalts plot on the sr nd array defined by the mahabaleshwar formation and the desur unit basalts plot on an extension of this array this suggests that the source characteristics are also lithospheric the absolute elemental abundances may then be a function of melting and fractionation we are impressed by the apparent switch from crustal lithospheric contributions to mantle lithospheric contributions through the stratigraphy and suggest that this together with the more protracted fractionation of the magma reflects a change in the availability of the lithospheric components accompanying the southerly migration of the volcanic edifice

sr nd and pb isotopic and chemical compositions of central deccan traps lavas and relation to southwestern deccan stratigraphy abstract sr nd and pb isotopic compositions and major and trace element abundances of lavas in five central deccan traps sections are generally very similar to those of lavas in the southwestern deccan western ghats escarpment the combination of strong isotopic and chemical similarities between lavas of the two regions indicates that they shared a closely similar petrogenesis our results indicate that unlike many lavas in the northeastern deccan most of the lavas studied in the central deccan sections can be grouped into different stratigraphic members and chemical types cts belonging to four formations thakurvadi bhimashankar khandala and poladpur present in the type sections of the western ghats bhimashankar and thakurvadi type lavas are found in a section near outram whereas only khandala type lavas make up the mhaishmal ellora and ajanta sections to the east and only poladpur type lavas are present in the easternmost section at lonar this west east sequence is the same as that seen going upward in the western ghats stratigraphy except that bushe type lavas which are located stratigraphically between the khandala and poladpur formations in the western ghats have not been found in this study overall our results indicate that the khandala and poladpur formations extend over west east distances of more than 300 km and some individual members e g the dhak dongar may extend over as much as 500 km the regional dip from igatpuri in the southwest to lonar in the east central deccan is less than 0 5 to the east which is similar to the southward regional dip in the southwestern deccan near horizontal flows in the central and southwestern deccan areas suggest that post deccan tectonic activity has had limited effects on the studied area large volumes of basaltic lavas derived from eruptive centers in the western deccan may have covered both the central and southwestern deccan areas

a geological map of the southern deccan traps india and its structural implications a geological map of an area of over 125 000 km 2 within the southern deccan traps india is presented which is based upon variations in the basalt geochemistry it represents a synthesis of new data and earlier work the use of large data sets has allowed stratigraphic boundaries to be established with greater accuracy over a much larger area than before the geochemical sections demonstrate that the southward overstepping stratigraphic style encountered along the ghats ridge has an essentially north to south polarity and that there appears to be detectable overstep in an east to west sense however the stratigraphy defines a pronounced coastward dipping monoclinal flexure across the western ghats ridge between 16 n and 18 n the data are consistent with two models of eruption the first considers the deccan eruptive focus to have been fixed throughout the volcanic episode producing an essentially annular flow pattern the second considers the flow pattern to have been generated from a southerly migrating eruptive centre resulting from the northerly drift of india over a fixed hotspot both hypotheses are consistent with recent quantitative models which suggest that rapid and voluminous flood basalt eruptions are the result of continental rifting events associated with plume generated thermal 9highs9 until a causal link between established plume hotspot trails and impact sites can be satisfactorily demonstrated we find it unnecessary to invoke a cataclysmic impact event as an explanation for the deccan eruptions

the mount pavagadh volcanic suite deccan traps geochemical stratigraphy and magmatic evolution abstract the patterns of eruption and dispersal of flood basalt lavas on the surface or as magmas in dykes and sills within the crust determine the volcanological and stratigraphic development of flood basalt provinces this is a geochemical and sr isotopic study of lavas of varied compositions that outcrop around mount pavagadh 829 m deccan traps an important outlier north of the main basalt outcrop most of the 550 m thick exposed section at pavagadh is made up of subalkalic basalts rich in the incompatible elements particularly nb ba and sr picrite and rhyolite dacite flows also occur the latter capping the sequence the relatively high initial 87sr 86sr ratios up to 0 7083 and chemical characteristics of the rhyolitic rocks of pavagadh are consistent with a small but significant involvement of the granitic basement crust in their genesis an assimilation fractional crystallization afc model involving the picrite lava and either a southern indian or a western indian granite as the contaminant explains the geochemical and sr isotopic variation in the basalts and the rhyolites quite well a systematic comparison of the basaltic lavas with binary plots normalized multielement patterns and discriminant function analysis to the well established lava stratigraphy of the western ghats 400 500 km to the south precludes any chemical genetic relationships between the two basalts exposed in sections closer to pavagadh 150 200 km in the toranmal navagam and barwani mhow areas have several flows with some similar chemical characteristics however the pavagadh sequence is significantly different from all of these sequences geochemically petrogenetically and in magnetic polarity to be considered independently built this result is significant in terms of eruptive models for the deccan traps as it is increasingly apparent that there were separate but possibly coeval eruptive centers with their own distinctive chemistries developed in various areas of this vast province

shallow water waveguide characterization using the hankel transform a coherent processing technique is proposed for the characterization of shallow water channels for a horizontally stratified ocean and bottom the method consists of measuring the magnitude and phase versus range of the pressure due to a cw point source and hankel transforming this data to obtain the depth dependent green s function versus horizontal wave number the green s function contains all the information about the channel necessary to solve the forward problem including the nature of the discrete and continuous spectra and the plane wave reflection coefficient of the bottom characteristics of the green s function can also be used to infer acoustic properties of the bottom results for simple examples are presented using synthetic data and implications for the nonstratified case are discussed

pressure evolution during explosive caldera forming eruptions abstract caldera forming eruptions of silicic magmas result from a complex coupling of the mechanics and fluid dynamics of the associated magma chamber field studies of caldera forming eruption products suggest that great pressure variations occur inside the magma chamber and associated conduits during these eruptions pressure evolution during explosive caldera forming eruptions is investigated through a simple model that describes the first order quantitative behaviour of the chamber we consider a piston like model that assumes a coherent block subsiding along circular sub vertical ring faults into the magma chamber this subsidence occurs after significant decompression of the chamber by an initial central vent eruption we assume that the initial pressure distribution in the chamber is magmastatic once collapse has begun the chamber roof is supported by the magma so that magma pressure at the chamber roof increases to lithostatic we suggest that pressure variations during caldera forming eruptions are mainly controlled by variations in magma volatile content regardless of what induces the formation of ring faults the model suggests that the occurrence of explosive caldera forming events depends on the strength of the chamber walls and the depth water content and aspect ratio of the magma chamber no significant differences exist between model results for a cylindrical or a more realistic elliptical magma chamber geometry of comparable aspect ratio assuming a constant strength of the host rock the mass fraction of magma that must be erupted during the central vent phase in order to trigger caldera collapse ranges for deep gas poor chambers from a few percent up to 40 for shallow gas rich chambers the model suggests that zoned chambers tend to collapse earlier than homogeneous chambers dike shaped chambers will erupt less magma than sill like chambers before caldera collapse initiates although dike like geometries are not associated with stress fields appropriate to create ring faults the model suggests that once initiated caldera collapse will tend to force out most or all of the volatile rich magma from the chamber for volatile rich magma chambers the total volume of erupted magma during caldera forming event is of the same order as the chamber volume the model also explains the variation in the erupted mass during the different phases of explosive caldera forming eruptions and is in good agreement with natural examples

inversion for range dependent water column sound speed profiles on the new jersey shelf using a linearized perturbative method the environment of the new jersey shelf is characterized by high spatial and temporal variability of water column properties caused by intrusions of warm salty water from the continental slope these intrusions cause fluctuations in the water column sound speed profile which can have significant effects on acoustic propagation in shallow water in this work a linearized perturbative inverse technique is applied to estimate range dependent water column sound speed profiles this method utilizes estimates of horizontal wave numbers to determine sound speed as a function of depth this technique is appropriate for the range dependent shallow water environment as horizontal wave numbers can be measured semilocally 1 2 km aperture and their values are a direct measurement of the local environmental parameters difficulty is encountered in application of the perturbative inverse technique because the wave number data are insensitive to some portions of the waveguide and as a result the solution can deviat

highly heterogeneous precambrian basement under the central deccan traps india direct evidence from xenoliths in dykes abstract crustal or mantle xenoliths are not common in evolved tholeiitic flood basalts that cover huge areas of the precambrian shields yet the occasional occurrences provide the most direct and unequivocal evidence on basement composition few xenolith occurrences are known from the deccan traps india and inferences about the deccan basement have necessarily depended on geophysical studies and geochemistry of deccan lavas and intrusions here we report two basalt dykes rajmane and talwade dykes from the central deccan traps that are extremely rich in crustal xenoliths of great lithological variety gneisses quartzites granite mylonite felsic granulite carbonate rock tuff because the dykes are parallel and only 4 km apart and only a few kilometres long the xenoliths provide clear evidence for high small scale lithological heterogeneity and strong tectonic deformation in the precambrian indian crust beneath measured 87sr 86sr ratios in the xenoliths range from 0 70935 carbonate to 0 78479 granite mylonite the rajmane dyke sampled away from any of the xenoliths shows a present day 87sr 86sr ratio of 0 70465 and initial at 66 ma ratio of 0 70445 the dyke is subalkalic and fairly evolved mg no 44 1 and broadly similar in its sr isotopic and elemental composition to some of the lavas of the mahabaleshwar formation the xenoliths are comparable lithologically and geochemically to basement rocks from the archaean dharwar craton forming much of southern india as several lines of evidence suggest the dharwar craton may extend at least 350 400 km north under the deccan lava cover this is significant for precambrian crustal evolution of india besides continental reconstructions

climatic and tectonic controls on weathering in south china and indochina peninsula clay mineralogical and geochemical investigations from the pearl red and mekong drainage basins 1 results of clay mineralogy major element geochemistry and sr and nd isotopes in 93 argillaceous samples collected from drainage basins of the pearl red and mekong rivers reveal different degrees of chemical weathering in southeast asia despite similar climate conditions across these regions the kaolinite illite ratio illite chemistry index and illite crystallinity can be used as indicators of chemical weathering intensity these mineralogical proxies combined with the k2o na2o cao molar ratio chemical index of alteration cia and weathering trends observed from major element results indicate intensive silicate weathering in the pearl river basin moderate to intensive in the mekong river basin and moderate in the red river basin although a significant modification of epsilon nd 0 values in our riverine sediments during chemical weathering and transport is unlikely sr 87 sr 86 ratios are controlled by various states of chemical weathering of high sr minerals such as plagioclase rich in na and ca with a linear decrease trend from the pearl mekong to red river basins our results suggest that it is not the warm climate with heavy monsoon precipitation but tectonics playing the most significant role in controlling weathering and erosion processes in south china and indochina peninsula strong physical erosion caused by tectonic activities and river incision along the eastern margin of the tibetan plateau and along the red river fault system is responsible for high contents of primary minerals in the lowlands of red and mekong river basins

origin of deccan trap lavas evidence from combined trace element and sr nd and pb isotope studies abstract geochemical and isotope results are presented from a new study of the most southern basalts in the deccan trap india three chemical formations are recognised two of which can be correlated with the established stratigraphy in mahabaleshwar and imply a regional southerly dip of 0 06 over a distance of 250 km in detail sr isotope variations within the ambenali and mahabaleshwar formations can be shown to reflect three distinct end members which provide new constraints for petrogenetic models pb isotope data for selected basalts exhibit a wide range with 206 pb 204 pb 16 87 22 45 and a linear correlation on a pb pb diagram the least contaminated ambenali basalts plot within the pb array and interaction with mantle lithosphere involves a shift to less radiogenic pb whereas contamination with crust is characterised by more radiogenic pb unlike the karoo and parana continental flood basalt provinces only four flow units within the southern deccan appear to contain a significant contribution from mantle lithosphere the mahabaleshwar and ambenali formation basalts exhibit a striking negative pb sr isotope trend which is presently regarded as one of the features of interaction with shallow level lithospheric mantle it further suggests that basalts from the walvis ridge kerguelen and ninetyeast ridge all remobilised such shallow level material and that the deccan basalts which were not affected by crustal contamination reflect interaction between asthenospheric material similar to t type morb but related to the reunion hotspot and continental mantle lithosphere of the indian plate

modelling pyroclastic density currents from a subplinian eruption at la soufriere de guadeloupe west indies france we have used a three dimensional non equilibrium multiphase flow numerical model to simulate subplinian eruption scenarios at la soufriere de guadeloupe lesser antilles france initial and boundary conditions for computer simulations were set on the basis of independent estimates of eruption source parameters i e mass eruption rate volatile content temperature grain size distribution from a field reconstruction of the 1530 ce subplinian eruption this event is here taken as a reference scenario for hazard assessment at la soufriere de guadeloupe a parametric study on eruption source parameters allowed us to quantify their influence on the simulated dynamics and in particular the increase of the percentage of column collapse and pyroclastic density current pdc intensity at constant mass eruption rate with variable vent diameter numerical results enabled us to quantify the effects of the proximal morphology on distributing the collapsing mass around the volcano and into deep and long valleys and to estimate the areas invaded by pdcs their associated temperature and dynamic pressure significant impact temperature 300 c and dynamic pressure 1 kpa in the inhabited region around the volcano is expected for fully collapsing conditions and mass eruption rates 2 107 kg s we thus combine this spatial distribution of temperature and dynamic pressure with an objective consideration of model related uncertainty to produce preliminary pdc hazard maps for the reference scenario in such a representation we identify three areas of varying degree of susceptibility to invasion by pdcs very likely to be invaded and highly impacted susceptible to invasion and moderately impacted and unlikely to be invaded or marginally impacted the study also raises some key questions about the use of deterministic scenario simulations for hazard assessment where probability distributions and uncertainties are difficult to estimate use of high performance computing techniques will in part allow us to overcome such difficulties but the problem remains open in a scientific context where validation of numerical models is still necessarily an incomplete and ongoing process nevertheless our findings provide an important contribution to the quantitative assessment of volcanic hazard and risk at la soufriere de guadeloupe particularly in the context of the current unrest of the volcano and the need to prepare for a possible future reawakening of the volcano that could culminate in a magmatic explosive eruption supplementary information the online version of this article 10 1007 s00445 020 01411 6 contains supplementary material which is available to authorized users

a post k t boundary early palaeocene age for deccan type feeder dykes goa india basaltic dykes exposed along the coast of goa represent the youngest phase of a number of different mafic suites intruding the complex precambrian terrain that forms the pre deccan basement of peninsular india these dykes crop out 50 80 km beyond the southern limits of the deccan continental flood basalt cfb province four of seven sampled dykes were dated using the whole rock 40ar 39ar step heating technique three 40ar 39ar determinations yield palaeocene plateau and isochron ages that clearly indicate an early post cretaceous i e danian age and as a weighted mean average of all four samples yields an age of 62 8 0 2 ma these intrusions are clearly significantly younger than the current estimates for the age range of deccan traps volcanism 65 69 ma the trace element signatures rare earth element patterns and 87sr 86sr ratios of these dykes are consistent with those of the nearby deccan basalts more specifically their geochemical signatures are similar to those of the basalt flows composing the volumetrically important ambenali mahabaleshwar formations which form the uppermost succession wai subgroup of the sw deccan these geochemical and age data demonstrate that deccan type magmatism continued to affect western india for at least 1 2 my after the k t boundary 65 0 0 1 ma and therefore their emplacement places important constraints upon the timing duration and evolution of this much debated cfb province the importance of these dykes is examined within the context of the deccan eruptions and the associated rifting of the western indian margin

basalts of the northeastern deccan traps india isotopic and elemental geochemistry and relation to southwestern deccan stratigraphy the northeastern deccan traps include flows with strong isotopic and elemental affinities to lavas in the southwest as far as 900 km away however many of the northeastern flows show a negative correlation of 206pb 204pb with end t and positive correlation of 206pb 204pb with 87sr 86sr t distinct from any of the isotopic fields for the southwestern formations this array from end t 5 87sr 86sr t 0 7041 206pb 204pb 17 9 to end t 0 87sr 86sr 0 7068 206pb 204pb 20 5 overlaps the ambenali formation fm field and runs toward that of the common signature peng et al 1994 that appears to have been an important magma type in several of the lower formations of the southwest this array appears to represent mixing between an ambenali type and 1 a common signature end member or 2 an end member with even higher 206pb 204pb and 87sr 86sr and lower end than the common signature such mixing has been postulated but not documented previously for the deccan lavas isotopically and chemically indistinguishable from the ambenali fm form the tops of sections near jabalpur and chikaldara beneath them are flows that chemically resemble the poladpur fm which sits immediately below the ambenali in the southwest and several lavas similar to the khandala fm beneath the poladpur and intervening bushe fm in the southwest given the general stratigraphic correspondence with the southwestern sequence many of these northeastern flows may be petrogenetically related to the respective southwestern formations lavas with broadly poladpurand khandala like elemental compositions are also abundant in a thick section south of mhow several other mhow area flows resemble the distinctive bushe fm however nearly all of the poladpur and khandala like northeastern deccan lavas analyzed have higher 206pb 204pb man any southwestern poladpur or khandala fm basalts if they indeed are related petrogenetically to these formations many of the northeastern magmas interacted less with low 206pb 204pb continental material and thus must have erupted from different feeder systems

geology of the saucer shaped sill near mahad western deccan traps india and its significance to the flood basalt model an 22 m thick saucer shaped sill occurs near mahad and is exposed as a curvilinear miniature ridge within the deccan traps the sill has variable dips 42 55 it has a 7 1 km long axis and 5 3 km short axis aspect ratio of 1 4 and is larger than the mv sill of the golden valley sill complex south africa and the panton sill australia the sill has distinct glassy upper and lower chilled margins with a coarse grained highly jointed core the samples from the margin are invariably fractured and iron stained because of deuteric alteration the rock from the sill is plagioclase phyric basalt at least three thick sill like apophyses emanate from the base of the main sill the apophyses change direction because of bending and thinning from a horizontal concordant sheet at the top to a discordant inclined form that bends again to pass into a lower horizontal concordant sheet we interpret such features as nascent saucer shaped sills that did not inflate to form nested sills geochemically the sill consists of poorly differentiated tholeiitic basalt that has a restricted geochemical range critical trace element ratios and primitive mantle normalised trace and ree patterns indicate that the sills have geochemical affinities to the poladpur chemical type and that the pahoehoe flow they intrude belongs to the bushe formation calculated magmatic overpressures during sill emplacement range from 8 4 to 11 3 mpa for young s modulus e 5 gpa and 16 7 to 22 5 mpa for e 10 gpa and depth to magma chamber ranges from 8 5 to 11 5 km e 5 gpa and 17 1 to 22 9 km e 10 gpa consistent with petrological and gravity modelling the volume of the mahad sill is approximately 276 km3 and is constant irrespective of the variations in the values of host rock young s modulus in 1980 cox j petrol 21 629 650 1980 proposed a conceptual model of the crust mantle section beneath the karoo cfb which is considered as the fundamental model for flood basalt volcanism our paper confirms the presence of a sill plus the inferred substructure beneath mahad that are compatible with predictions of that model in lips saucer shaped sills are formed in areas experiencing extensional tectonics where processes such as the cook gordon delamination and dundurs elastic extensional mismatch between layered sedimentary rocks or lava flows are responsible for the deflection of dykes into sills a similar process is envisaged for the formation of the mahad sill