lithospheric uplift in eastern gondwana evidence for a long lived mantle plume system recent work has demonstrated that modern drainage systems on rifted continental margins may be the product of ancient lithospheric doming in areas of flood basalt magmatism this uplift is thought to reflect the existence of widespread mafic underplating and excess melt generation at rift zones here i show that uplift due to the initiation of a thermal anomaly plume head in the asthenosphere may be evidenced by the formation of discrete rift structures and persistent paleodrainage sediment dispersal patterns for up to 150 m y prior to the commencement of igneous activity and final continental breakup volcanism thus occurs as a comparatively late stage geochemical response to the long term residence of hot plume material beneath continental mantle lithosphere

from partial to full strain partitioning along the indo burmese hyper oblique subduction abstract the andaman nicobar trench and its onshore prolongation the indo burmese wedge is the least studied segment of the india australia subduction new offshore geological and geophysical data have recently been collected along the burma scarp during a marine survey conducted with the r v marion dufresne andaman cruise swath bathymetric mapping combined with shallow and deep seismic show that the dominant active tectonics is dextral strike slip faulting accompanied by various amounts of shortening the southern portion of the burma scarp is transpressive north andaman islands the central portion is pure strike slip on nne oriented segments southern myanmar the northern portion shows sedimentary wedge growth along nw segments both onshore and offshore arakan yoma prism further north the wedge bends to the west while dextral shear faults more or less parallel to the sagaing fault develop within the internal part of the wedge and elongated ns folds form in its external part abutting onto the shillong plateau we propose a simple kinematic model involving evolution from partial to full partitioning from south to north along the west burma scarp wbs and we test it quantitatively using the most recent geodetic results in the south half of the 3 5 cm year of india motion is taken at the trench itself the other half being accommodated onto a single shear fault the sagaing fault in myanmar in the north where the bangladesh fold system developed dextral strike slip faults are activated within the arakan yoma belt and at the accretionary prism backstop contact resulting in full partitioning there faults accommodating the oblique component of motion of india are progressively migrating in space from far field faults andaman sagaing to near trench faults arakan yoma belt and trench itself the burma sliver being buttressed by the eastern himalayas this kinematic extends back to 4 ma which is the time of initiation of the last pulse of oceanic accretion in the andaman basin

pre and post collisional depositional history in the upper and middle bengal fan and evaluation of deepwater reservoir potential along the northeast continental margin of india abstract the bengal fan is the largest submarine fan in the world that has formed as a result of high sediment transport from the himalaya by the ganga brahmaputra river system the himalaya was formed as a result of the collision between the eurasian and indian plates the initiation of this collision known as soft collision occurred around 59 ma whereas the major collision known as hard collision took place around 15 ma ago prior to the collision sediments into the bay of bengal were derived from the northwest by relatively smaller river system like mahanadi godavari the switching of river systems with time was not distinct but gradational in the post collision period the sediment input from the nw was masked in most instances because of rapid sediment supply from the himalaya to the north pre collisional sediment dispersal pattern from the nw was largely affected by pre existing basement high known as 85 e ridge this ridge was submerged during the post collisional period post collisional sediments are commonly referred to as the bengal fan sediments and show huge accumulation along the shelf and beyond high resolution 2d seismic data acquired along a corridor covering the upper middle and distal parts of the present day active bengal fan system indicates that the fan has prograded southward with time because of continuously increasing sediment supply and has therefore masked the effect of eustacy the present day geometry of the fan shows a single active canyon and an associated single active fan the active channel shows typical meandering pattern that shifts laterally with time the seismic facies analysis indicates that both the pre and post collision basin has significant hydrocarbon potential the thermogenic model is best suited for modeling source rock maturity in the pre collision basin whereas both biogenic and thermogenic models best explain source rock maturity in the post collision younger bengal fan the wedge out against the 85 e ridge is considered to be one of the important play types for hydrocarbon exploration in the deeper part of the basin on the other hand the channel levee complexes and frontal splay basin floor fan are the possible target areas for petroleum exploration in relatively younger bengal fan deposits

polyphase deformation in a fore arc back arc basin salin subbasin myanmar burma the salin subbasin of myanmar part of the larger central basin is a fore arc back arc basin couplet situated between an oblique subduction zone to the west and a major right lateral strike slip fault to the east surface and subsurface expressions of folds and faults suggest that the basin experienced north northwest directed extensional deformation in the miocene followed by east northeast directed pliocene pleistocene transpressional deformation resulting in a variety of structural styles including thrust faults oblique reverse faults strike slip faults and normal faults fault propagation folds cored by west dipping thrust faults in the basin center are located above steps in the top of basement surface most likely fault controlled unconformities at the base of and within the pliocene pleistocene synorogenic section indicate that the thrust faults were active during the pliocene pleistocene the southeastern region contains broad north northwest trending uplifts and east northeast striking normal faults associated with thickened miocene synorogenic deposits pliocene pleistocene sedimentary rocks lie unconformably above the miocene section and are folded above the uplifts reflecting pliocene pleistocene compression in the southern part of the basin miocene sedimentary rocks thicken dramatically over the 20 n uplift and the yedwet uplift suggesting that the north northwest striking faults that bound them represent pliocene pleistocene inversion of miocene normal faults during the miocene the burma plate acted as a fore arc sliver coupled with the india plate subducting obliquely underneath it and moved northward relative to asia along the sagaing fault normal faulting and local basin formation took place at this time when the northern part of the burma plate collided with eastern asia in the pliocene transpressional deformation predominated creating thrust and reverse faults positive flower structures and inverted basins

zircon hf isotopic constraints on the sources of the indus molasse ladakh himalaya india 1 india asia collision has resulted in the largest orogenic event on earth since at least 500 ma and this has had a profound influence on the drainage patterns of asia on the asian monsoon climate on global oceanography as well as on faunal extinctions despite this importance understanding of the paleogene evolution of this system is limited u pb and lu hf isotopic analyses of detrital zircons from the clastic sedimentary rocks of the indus molasse located within the indus suture of the ladakh himalaya india show that most have mesozoic cenozoic ages and juvenile hf isotopic compositions this suggests that the main source of these sediments is the transhimalayan batholith with minor flux from the lhasa karakoram terrane and indian plate the lower indus molasse chogdo formation is inferred to have been deposited at 60 49 ma whereas the upper choksti formation was deposited later than 45 41 ma and contains a greater contribution from the lhasa terrane the transhimalayan batholith was eroded throughout indus molasse sedimentation but increasing exhumation of lhasa karakoram terrane indicates its progressive uplift after initial collision and requires formation of apaleo indus river shortly after collision

timing of intraplate deformation northeastern indian ocean abstract seismic stratigraphy and dsdp and odp drilling have demonstrated that two unconformities of early eocene and latest miocene age can be traced over much of the northeastern indian ocean the eocene event was a major hiatus in sedimentation following collision of india with asia and was the time of termination of deposition of a continental rise off the east side of india and the initiation of proximal bengal fan deposition the miocene event was the onset of north south compression in the indian plate during rapid uplift in tibet and the himalayas

sequence stratigraphy and the evolution of the ganges brahmaputra delta complex 1 a seismic stratigraphic analysis of 2600 km of data covering part of the ganges brahmaputra delta has shown that it evolved in three stages 1 the proto ganges delta developed immediately after the breakup of gondwana at approximately 126 ma the indian plate then moved rapidly northward ultimately crossing more than 50 degrees of latitude because the catchment area was small sedimentation was relatively restricted and the early delta consists of relatively small scale prograding clastic bodies alternating with mixed carbonate clastic and carbonate platform assemblages 2 the transitional delta developed following a major eustatic sea level low at 49 5 ma initially sedimentation was relatively restricted but at about 40 ma the flux of water and clastic sedime ts dramatically increased and the delta grew rapidly with the development of major prograding clastic depositional sequences abundant evidence of lobe switching suggests that the delta was stream dominated at that time the abrupt change in the morphology of the delta appears to be a direct response to the collision of the indian plate with the eurasian plate and the rise of the himalayas the delta remained stream dominated some time after the major plate collision and until the bay of bengal became a well developed deep embayment at about 21 ma the bengal fan began to accumulate during this interval as turbidity currents carried the increased sediment load into deeper water 3 the modern delta began to appear in its present form following a major eustatic sea level low and erosio at 10 5 ma by this time the delta had become tidally dominated and had prograded well beyond the study area into the now deeply embayed bay of bengal

late miocene pleistocene evolution of india eurasia convergence partitioning between the bhutan himalaya and the shillong plateau new evidences from foreland basin deposits along the dungsam chu section eastern bhutan the shillong plateau is a unique basement cored uplift in the foreland of the eastern himalaya that accommodates part of the india eurasia convergence since the late miocene it was uplifted in the late pliocene to 1600 m potentially inducing regional climatic perturbations by orographically condensing part of the indian summer monsoon ism precipitations along its southern flank as such the eastern himalaya shillong plateau ism is suited to investigate effects of tectonics climate and erosion in a mountain range broken foreland system this study focuses on a 2200 m thick sedimentary section of the siwalik group strategically located in the lee of the shillong plateau along the dungsam chu at the front of the eastern bhutan himalaya we have performed magnetostratigraphy constrained by vitrinite reflectance and detrital apatite fission track dating combined with sedimentological and palynological analyses we show that 1 the section was deposited between 7 and 1 ma in a marginal marine deltaic transitioning into continental environment after 5 ma 2 depositional environments and paleoclimate were humid with no major change during the depositional period indicating that the orographic effect of the shillong plateau had an unexpected limited impact on the paleoclimate of the bhutanese foothills and 3 the diminution of the flexural subsidence in the basin and or of the detrital input from the range is attributable to a slowdown of the displacement rates along the main boundary thrust in eastern bhutan during the latest miocene pleistocene in response to increasing partitioning of the india eurasia convergence into the active faults bounding the shillong plateau

elevated arsenic in deeper groundwater of the western bengal basin india extent and controls from regional to local scale the deeper groundwater depending on definition of the bengal basin ganges brahmaputra delta has long been considered as an alternate safe drinking water source in areas with as enrichment in near surface groundwater the present study provides the first collective discussion on extent and controls of elevated as in deeper groundwater of a regional study area in the western part of the bengal basin deeper groundwater is defined here as non brackish potable cl 250 mg l groundwater available at the maximum accessed depth similar to 80 300 m the extent of elevated as in deeper groundwater in the study area seems to be largely controlled by the aquifer aquitard framework arsenic enriched deeper groundwater is mostly encountered north of 22 75 degrees n latitude where an unconfined to semi confined aquifer consisting of holocene to early neogene age gray sand dominates the hydrostratigraphy to 300 m depth below land surface aquifer sediments are not abnormally enriched in as at any depth but sediment and water chemistry are conducive to as mobilization in both shallow and deeper parts of the aquifer s the biogeochemical triggers are influenced by complex redox disequilibria results of numerical modeling and profiles of environmental tracers at a local scale study site suggest that deeper groundwater abstraction can draw as enriched water to 150 m depth within a few decades synchronous with the advent of wide scale irrigational pumping in west bengal india

regional tectonics of myanmar burma and adjacent areas analysis of 38 contiguous landsat multispectral scanner scenes acquired over myanmar burma reveals numerous large scale features associated with margins of the burman plate previously unidentified northeast southwest trending discontinuities important extensions of previously mapped fault trends and numerous structural features that appear favorable for petroleum exploration a mosaic of these scenes at 1 1 000 000 scale shows a large number of tectonic elements and their spatial relationships within the area of investigation are portions of the indian burman lhasa and shan thai plates and perhaps other smaller plates the himalayan front and indo burman ranges manifest effects of current and recently past plate movement the complexity of the kinematic history accounts for the diversity of structural features in the area the last major event in this long and violent saga which began in middle miocene approximately 11 ma time and continues to the present is the recent change from a collisional to a right lateral strike slip transform margin between the indian and burman plates the complexity of the structures visible is the product of multiple plate collisions rotation of the indian plate and parts of the asian plate and long continued convergence that changed velocity and direction tbrough time the most obvious evidence of this complexity which ismore immediately apparent on geologic maps or the landsat mosaic of the region is the almost right angle relationship of the folds of the indo burman ranges and the frontal thrusts and suture zones of the himalaya these two sets of compressive features imply maximum compressive stress axes that lie at right angles to each other the implications are either that the orientation of the stress field changes rapidly over a short distance or that the stress field has changed through time both occurrences seem to be true less

palaeomagnetism of cretaceous to tertiary sediments from southern tibet evidence for the extent of the northern margin of india prior to the collision with eurasia abstract palaeomagnetic studies have been carried out on mid cretaceous to latest palaeocene sediments from the tethyan himalaya on the northern margin of the indian plate near the localities of gamba 28 3 n 88 5 e and duela 28 0 n 89 2 e in southern tibet a total of 61 sites more than 700 cores have been sampled mainly from limestones zongshan formation jidula formation member ii zongpu formation sandstones jidula formation member iii and graywackes gamba group member ii with ages well defined from detailed lithological and biostratigraphic analyses carriers of characteristic remanences are magnetite in limestones and graywackes and hematite in sandstones reliable palaeodirections were obtained from 35 sites 356 specimens positive fold tests antiparallel normal and reverse polarities a northward trend of the palaeolatitudes with decreasing age and absence of any metamorphic overprint indicate a primary origin for the characteristic remanences the results have been compared with expected palaeodirections according to available data for the indian subcontinent based on its present day geometry the late maastrichtian sites fourteen sites 156 specimens and the middle late palaeocene sites fourteen sites 113 specimens yield palaeolatitudes of 5 7 s and 4 0 n for the sampling localities in southern tibet respectively these palaeolatitudes are much further north than expected and imply that crustal shortening between the tethyan himalaya and the indian shield since the time of india eurasia collision has occurred on the order of 1500 km at the longitude of gamba duela comparison of our observed palaeolatitudes with palaeolatitudes for the lhasa block during cretaceous and palaeocene times indicate that initial contact of the northern indian margin with the lhasa block was already established at 65 60 ma with suturing completed at about 55 50 ma

miocene rise of the shillong plateau and the beginning of the end for the eastern himalaya abstract a common feature of convergent plate boundaries is the self organization of strain exhumation and topography along discrete arcuate boundaries deviations from this geometry can represent first order changes in stress applied at a plate boundary that must affect how strain is partitioned within the interior of an orogen the simplicity of the himalayan fold and thrust belt seen along its central portion breaks down along the eastern extremity of the arc where the 400 km long shillong plateau has developed this change in strain partitioning affects nearly 25 of the arc and has not previously been considered to be important to the orogen s development new low temperature thermochronometry data suggest this structure initiated in mid to late miocene time significantly earlier than was previously estimated from the sedimentary record alone development of the shillong plateau may be linked to a number of kinematic changes within the himalayan and burman collision zones that occur at the same time these events include the onset of e w extension in central tibet eastward expansion of high topography of the tibetan plateau onset of rotation of crustal fragments in southeastern tibet and re establishment of eastward subduction beneath the indo burman ranges we suggest that the coincidence of these tectonic events is related to the dismemberment of the eastern himalayan arc signifying a change in regional stress applied along the india eurasia burma plate boundaries discrepancies between vertical long term faulting rates and geodetically derived far field convergence rates suggest that the collisional boundary in the eastern himalayan system may be poorly coupled due to introduction of oceanic and transitional crust into the eastern plate boundary the introduction of dense material into the plate boundary late in the orogen s history may explain regional changes in the strain field that affect not only the himalaya but also the deformation field more than 1000 km into the tibetan plateau

tectonic framework of the northern junggar basin part i the eastern luliang uplift and its link with the east junggar terrane abstract the luliang uplift preserves relatively complete carboniferous strata and is an important region to study the spatio temporal framework and tectonic evolution of the junggar basin here we report new shrimp sims u pb ages whole rock sr nd data and in situ zircon hf isotopic compositions for carboniferous strata samples in the eastern luliang uplift the gravity and magnetic data suggest that the carboniferous volcanics are distributed as nw trending bands and also bring out n s oriented parallel amalgamation the sedimentary units and structural deformation of the eastern luliang uplift suggest that the basin evolution involved late carboniferous extensional graben stage and latest carboniferous early permian compressional stage the carboniferous batamayineishan formation is divided into the lower volcanic unit followed up sedimentary unit and an upper volcanic unit as inferred from the lithology and unconformities the lower volcanic unit from the drill cores mainly consists of andesite dacite rhyolite and tuff intercalated with minor basalt the middle sedimentary unit is made up of sandstone and mudstone with tuff and tuffaceous sandstone and the upper volcanic unit is composed of basalt rhyolite tuff and breccia with minor clastic sedimentary rocks zircon shrimp u pb age 337 2 ma for the tuff and sims u pb age 308 6 ma for basalt suggest that the lower and upper volcanic units belong to lower carboniferous and upper carboniferous respectively the basalts from upper volcanic unit in the eastern luliang uplift show typical calc alkaline affinity with low nb y ratios 0 17 0 28 and sio2 contents 47 54 and moderate th abundances 0 45 3 74 ppm enrichment in lree and liles and depletion in nb ta and ti they have low initial 87sr 86sr ratios 0 703571 0 705122 and positive end t values 5 5 8 6 and constant sm yb ratios 1 56 2 03 and variable trace element ratios e g ba th ba la and ba nb suggesting that the parental magmas were mainly derived from 2 to 8 partial melting of a spinel iherzolite mantle source metasomatized by hydrous fluids and melts of subducted sediments furthermore relatively high tio2 1 52 3 76 and zr contents 172 414 ppm indicate that they were formed in an extensional setting taking into account the occurrence of coeval island arc related volcanics we propose an intra arc rift basin model for the eastern luliang uplift the eastern luliang uplift and east junggar terrane display different tectonic features during the closure of the junggar ocean during the late carboniferous continued northward subduction prevailed and generated a fault controlled intra arc basin in the eastern luliang uplift intra arc basin contraction in permian is probably indicative of the closure of junggar ocean in the northern junggar basin our study confirms the important role of subduction accretion process associated with the carboniferous crustal growth in the junggar terrane

morphological signatures of fault lines in an earthquake prone zone of southern baromura hill north east india a multi sources approach for spatial data analysis this paper aims to assess the morphological signatures of tectonic features faults on the basis of multi sources spatial data analysis for that purpose the present authors selected the southern part of baromura hill of tripura which has been discussed very little in the scientific literatures an extensive field work was done in the assumed faulted areas to understand the geomorphic signatures of faulting in the field gps was used for fixing the ground control points scanned survey of india soi topographical sheets spot pan and land sat band 5 was used for digital analysis an old dem on irs liss iii fcc was also consulted for final mapping two clear lineaments faults were detected from field observation on southern baromura hill apart from these another one was found in the outer part of this hill range an attempt was made to explain the morphological signatures of those faults by density slicing on radiometric data based on emittance of different physical components finally a geomorphic map with main tectonic features of baromura was prepared for future disaster management planning

diagenetic history of the surma group sandstones miocene in the surma basin bangladesh abstract this study examines the various diagenetic controls of the miocene surma group sandstones encountered in petroleum exploration wells from the surma basin which is situated in the northeastern part of the bengal basin bangladesh the principal diagenetic minerals cements in the surma group sandstones are fe carbonates with fe calcite dominating quartz overgrowths and authigenic clays predominantly chlorite illite smectite and minor kaolin the isotopic composition of the carbonate cement revealed a narrow range of 18o values 10 3 to 12 4 and a wide range of 13c value 1 4 to 23 1 the 13c vpdb and 18o vpdb values of the carbonate cements reveal that carbon was most likely derived from the thermal maturation of organic matter during burial as well as from the dissolution of isolated carbonate clasts and precipitated from mixed marine meteoric pore waters the relationship between the intergranular volume igv versus cement volume indicates that compaction played a more significant role than cementation in destroying the primary porosity however cementation also played a major role in drastically reducing porosity and permeability in sandstones with poikilotopic pore filling blocky cements formed in early to intermediate and deep burial areas in addition to fe carbonate cements various clay minerals including illite smectite and chlorite occur as pore filling and pore lining authigenic phases significant secondary porosity has been generated at depths from 2500 m to 4728 m the best reservoir rocks found at depths of 2500 3300 m are well sorted relatively coarse grained more loosely packed and better rounded sandstones having good porosities 20 30 and high permeabilities 12 6000 md these good quality reservoir rocks are however not uniformly distributed and can be considered to be compartmentalized as a result of interbedding with sandstone layers of low to moderate porosities low permeabilities owing to poor sorting and extensive compaction and cementation

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

fluvial facies architecture in small scale river systems in the upper dupi tila formation northeast bengal basin bangladesh abstract the late stage basin fill history of the fluvial dupi tila group plio pleistocene is described these rocks have been deposited in the sylhet trough a sub basin of the bengal basin in a foreland basin setting this outcrop study carried out in sylhet bangladesh presents the first detailed facies analysis of the upper dupi tila formation four facies have been identified trough cross bedded sandstone st ripple cross laminated sandstone sr finely laminated mud with ripples fl and massive mud with rootlets fm facies analysis supplemented with embedded markov chain analysis reveals small scale fining upward cycles average 4 5 m thick facies architectural elements include channel ch lateral accretion la sandy bedforms sb and overbank fines of with limited vertical and lateral connectivity of the sand bodies the average channel depth and width is 5 and 30 m respectively sand body geometry ranges from tabular to sheet to shoestring with a 0 45 net to gross ratio this study shows that the upper dupi tila formation is composed of small scale mudstone reach meandering river deposits in bangladesh the dupi tila formation is the main aquifer presently being utilized understanding of facies architecture and sand body geometry of this formation is crucial in examining the issue of arsenic and other contaminations of ground water in bangladesh

evolution of the eastern indian ocean since the late cretaceous constraints from geosat altimetry we propose a new model for the tectonic evolution of the eastern indian ocean from the late cretaceous to the present two types of data are used to improve previously published reconstructions first recent reinterpretations of seafloor magnetic anomalies between australia and antarctica and in the wharton basin provide new constraints on spreading rates and the timing of major reorganizations second vertical deflection profiles i e horizontal gravity anomaly derived from 22 repeat cycles of geosat altimeter data reveal the tectonic fabric associated with fracture zones these new geosat data provide tight constraints on paleospreading directions for example three prominent fracture zones can be traced from south of tasmania to the george v basin antarctica providing an important constraint on the relative motions of australia and antarctica through the late eocene in addition the geosat profiles are used to locate the conjugate continental margins and continent ocean boundaries of australia and antarctica as well as the conjugate rifted margins of kerguelen plateau and broken ridge based on a compilation of magnetic anomaly data from the crozet basin the central indian basin the wharton basin and the australian antarctic basin ten plate tectonic reconstructions are proposed reconstructions at chrons 5 11 ma 6 21 ma 13 36 ma and 18 43 ma confirm that the southeast indian ridge behaved as a single plate boundary since chron 18 the constraints from the geosat data provide an improvement in the fit of the kerguelen plateau and broken ridge at chron 20 46 ma to avoid overlaps between broken ridge and the kerguelen plateau prior to their breakup our model includes relative motions between the northern and southern provinces of the kerguelen plateau finally we examine the implications of our model for the relative motions of india australia and antarctica on the tectonic evolution of the kerguelen plateau and broken ridge and the adjacent labuan basin and diamantina zone as well as the emplacement of the ninetyeast ridge and the kerguelen plateau over a fixed hot spot

geologic influence on groundwater salinity drives large seawater circulation through the continental shelf observations of offshore freshened groundwater and saline groundwater discharge along continental shelves have important implications for water resources ecosystem function and the composition of the ocean but they cannot be explained by basic theory we show that these independent observations are linked and result from processes that drive variable density groundwater flow through the spatial heterogeneity that is ubiquitous in geologic formations we use lithologic data to develop geostatistical models that mimic the architecture of coastal aquifers simulation of groundwater flow and salt transport through these random realizations shows that heterogeneity produces spatially complex subsurface salinity distributions that extend tens of kilometers offshore even at steady state the associated density gradients drive high saline groundwater circulation rates that cannot be predicted by equivalent homogeneous models results suggest that these phenomena may be common along continental shelves potentially altering estimates of ocean chemical budgets and impacting coastal water management for future generations

structural features and formation conditions of mud diapirs in the andaman sea basin data from offshore oil and gas explorations have revealed that mud diapirs occur widely not only at continental margins but also in foreland basins and may have played an important role in the entrapment of oil and gas although the structural features and formation mechanism of salt diapirs have been extensively investigated mud diapirs are still not fully understood largely due to the difficulty of identifying them from seismic data in this paper the structural features and main controlling factors of mud diapirs in the andaman sea basin are investigated based on seismic profiles combined with drilling data and regional tectonic settings the results show that there are five types of mud diapir in the andaman sea basin turtleback mud diapir mud dome piercing mud diapir mud volcano and gas chimney like mud diapir turtleback mud diapirs mainly occur in the southern segment of the accretionary wedge of the andaman sea basin which is far from the bengal fan and characterized by low deposition rate and strong compression tectonic setting piercing mud diapirs exist mainly in the central segment of the accretionary wedge which is close to provenances of sediments and characterized by rapid sedimentation rates large mudstone thickness and transpressional tectonic setting mud domes and mud volcanoes mainly occur in the northern segment of the accretionary wedge which is characterized by rapid sedimentation rates large mudstone thickness and sedimentary wedge growth tectonic setting the gas chimney like mud diapirs only occur in the northern segment of the back arc depression close to the sagaing strike slip fault belt which is characterized by high deposition rate large mudstone thickness and high geothermal gradient these features suggest that thick mudstone deposit rapid sedimentation rates large geothermal gradient strong tectonic stress and gravitational spreading and sliding may have prompted the formation of mud diapirs in the andaman sea basin

cretaceous eocene sedimentation tectonism and biofacies in the bengal basin india abstract the bengal basin located in the eastern part of the indian subcontinent is one of the thickest sedimentary basins in the world the basin started downwarping in the early cretaceous and the sedimentation has since been continuous from an initial transgressive marine phase lasting almost up to end of the eocene the basin has evolved through a subsequent regressive marine and a later continental deltaic phase in the absence of detailed information available from the deeper basinal parts an analysis of the sedimentary and tectonic history and biofacies of the regions forming the western and northwestern shelves of the bengal basin pertaining to the initial phase of its evolution is attempted this is achieved through the synthesis of all the geological information available on the basis of recent exploration and research conducted on the sequences developed in west bengal meghalaya mikir hills and assam india the subsidence of the bengal basin was the result of differential adjustments of the crystalline basement along certain well established trends and was initiated soon after the gondwana tectonism sedimentation and volcanicity the earlier sediments were not typically marine but were more paralic to estuarine in nature the entire bengal basin except the area now comprising assam came under the influence of the first regional marine transgression during the late campanian maestrichtian the coarse clastic sediments were deposited under shallow coastal marine conditions a more widespread marine transgression took place during the early middle eocene which covered almost the entire shelf area including assam during this transgression the sediments belonging to the orthoquartzite carbonate association were formed in the shelf the sea started receding in the late eocene during which the entire bengal shelf came under the influence of heavy unloading of fine clastics the top of this fine clastic shaly sequence is an important sedimentary datum in the bengal shelf which marks the end of the first phase in the evolutionary history of the basin

cretaceous cenozoic tectonic events in the western myanmar burma assam region correlation of late mesozoic mafic and ophiolitic rocks suggests that in the late lower cretaceous a northeasterly facing mafic arc was emplaced onto southeast borneo western sumatra western myanmar burma then 1100 km south of its present position the mogok belt of myanmar and the denqen bangong co ophiolite zone in tibet all then situated on the southwestern margin of asia reversal in tectonic polarity and eastward subduction generated a late cretaceous magmatic arc in western myanmar and sumatra and its continuation through the mogok belt into the gandise batholith of tibet magmatism interrupted in the latest cretaceous probably resumed in the western myanmar tibet and sumatra arc segments in the early palaeogene it was again interrupted in the mid eocene when an east vergent ophiolite nappe overrode the indo burman ranges deforming lower eocene turbidites derived from the magmatic arc to the east following renewed northward and eastward subduction of ocean floor in the latest eocene india collided with northern myanmar and with tibet along a suture south of the jurassic triassic flysch and adjacent yarlung ophiolite in the mogok segment of the late cretaceous arc crust thickened in the early cretaceous orogeny underwent extension to form a metamorphic core complex as myanmar west of the sagaing fault moved northward more than half of this northward movement preceded the well documented 450 km of post early miocene dextral displacement on the sagaing fault and related spreading in the andaman sea generation of late cretaceous to early eocene i and s type granites in and east of the mogok belt was related either to crustal shortening or more probably to subsequent extension in the mogok belt

mesozoic anomalies in the bay of bengal abstract the analysis of 8200 line km of total magnetic intensity data in the bay of bengal northeastern indian ocean revealed the presence of approximately n30 e trending seafloor spreading type magnetic anomalies these anomalies resemble the mesozoic anomaly series m11 m0 reported elsewhere the oldest anomaly m11 132 5 ma identified close to the east coast of india is followed by the younger series of mesozoic anomalies towards the offshore some of the anomalies are offset by 60 80 km the configuration of the offsets of the isochrons allowed us to propose approximately n120 e trending oceanic fracture zones the mesozoic crust of 132 5 118 ma is estimated to evolve with an average half spreading rate of 3 5 cm yr except for the ocean floor between the m9 and m4 magnetic isochrons this part of the crust appears to be affected by the buried subsurface 85 e ridge

from gullies to mountain belts a review of sediment budgets at various scales this paper reviews the state of the art in the concept as well as in the application of sediment budgets in sedimentary research sediments are a product of mass dispersal at the earth surface and take part in global cycles sediment budgets aim at quantifying this mass transfer based on the principle of mass conservation and are the key to determine ancient fluxes of solid matter at the earth surface this involves fundamental questions about the interplay of uplift climate and denudation in mountain belts and transfer of sediments from the continents to the oceans as well as applied issues such as soil and gully erosion reservoir siltation and coastal protection first after introducing basic concepts relevant scales and methodologies the different components of quaternary routing systems from erosion in headwaters river systems glacial and paraglacial systems lakes deltas estuaries coasts shelves epicontinental seas and deep sea fans are discussed in terms of their sediment budget most suitable are sedimentologically closed or semi closed depositional environments e g alluvial fans lakes deltas and deep sea fans in a second step the dynamics of passive active and collisional tectonic settings and sediment budgets in related sedimentary basins are explored and new concepts of sediment portioning at large geodynamic scales are introduced ancient routing systems are more or less incomplete and may be intensively fragmented or destroyed in active tectonic settings in terms of sedimentary basin types rifts intracontinental and epicontinental settings are preferred objects of sediment budgets because of their persistence and relatively simple overall sedimentary architecture however closing basins such as foreland forearc retroarc piggy back and wedge top basins may provide excellent snapshots of orogenic sediment fluxes in a third step the large long lived routing systems of the amazon the ganges brahmaputra and the rhine are reviewed for each system estimates of either sediment volumes mass or sediment fluxes of continental and marine subsystems have been compiled in order to receive a complete routing in terms of mass conservation for specific time periods since the late glacial maximum as well as the cenozoic following lessons can be taken from these case studies i depositional centers and fluxes show strong shifts in space and time and call for caution when simply looking at subsystems ii the response times of these large systems are within the milankovich time interval thus lower than predicted from diffusion models iii cyclic routing of sediments in continental basins is much more dominated by climate human control than by eustacy and iv at long time scales ultimate sinks win over intermittent storage it is concluded from this review that the quantitative understanding of global sediment cycling over historic and geologic time and its response to allogenic forcing is still in its infancy and further research is needed towards a holistic view of sediment routing systems at various temporal and spatial scales and their coupling with global biogeochemical cycles this includes i to better determine response times of large routing systems by linking quaternary with cenozoic sediment budgets and continental with marine sub systems ii to combine advanced provenance techniques with sediment budgets in order to reconstruct ancient systems iii to study sediment partitioning at the basin scale iv to reconcile continental supply dominated sequence stratigraphy with the eustatic dominated marine concept and iv to account for non actualism of ancient systems with respect to their erosion and transport mode in particular during glaciations and pronounced arid intervals glacial and eolian sediment routing may cross over hydrologic boundaries of drainage basins thus challenging the principle of mass conservation

the detrital record of orogenesis a review of approaches and techniques used in the himalayan sedimentary basins the sediment archive of material eroded from an active tectonic region and stored in adjacent basins can provide a valuable record of hinterland tectonism especially when information in the source region itself is obscured by later metamorphism or removed by tectonism or erosion using the sediment record to document tectonism is a well established approach but more recently there has been a burgeoning of the number of isotopic techniques which can be applied to detrital material in particular single grain analyses thus the scope for application of detrital studies to a number of different tectonic problems has widened considerably in this review the example of sediments eroded from the himalayan orogen and preserved in the suture zone basin foreland basin remnant ocean basins and deep sea fans is used to illustrate the approach techniques as diverse as petrography heavy mineral xrf and sr nd studies single grain dating by ar ar u pb and fission track methodologies and single grain sm nd and pb isotopic analyses are described the paper documents how the sediment record can be used to determine the thermal and tectonic evolution of the orogen constrain mechanisms of continental deformation exhumation rates and palaeodrainage

magnetostratigraphy of the neogene siwalik group in the far eastern himalaya kameng section arunachal pradesh india abstract the siwalik group was deposited from the mid miocene to the pliocene in the foreland of the himalaya and records the unroofing history of the mountain belt in this study we provide the first magnetostratigraphic data for the eastern himalayan foreland basin we analyzed two sections of the lower to upper siwalik group along the kameng river in arunachal pradesh india magnetostratigraphic data were acquired from 395 sites within a 5 8 km thick molassic series thermal demagnetization and magnetic rock property analyses indicate a relatively low temperature 150 340 c characteristic remanent magnetization chrm yielding reliable primary directions carried by iron sulfides such as greigite the results show local counter clockwise rotation of the thrust sheets which is consistent with partitioning of arc normal and left lateral strain along the himalaya nineteen polarity zones have been identified in the two sections detrital fission track data from the top and bottom of the sections are used to constrain correlations with the geomagnetic polarity time scale gpts from chron c5ar 1n to chron c2an 2n from these results we propose that the siwalik formation in arunachal pradesh was deposited between 13 and 2 5 ma the transition between the lower and middle siwaliks is dated at about 10 5 ma and the middle to upper siwaliks transition is dated at 2 6 ma these results coupled with sedimentological observations suggest that the eastern himalayan chrono stratigraphic record is nearly synchronous with that analyzed in other parts of the neogene himalayan foreland basin nevertheless some differences in the evolution of the sedimentation rate and in the thickness of the middle siwaliks suggest that indian plate flexure in the east is different from that observed in other parts of the indian foreland basin and could be affected by the presence of the shillong plateau to the south

provenance and paleo weathering of tertiary accretionary prism forearc sedimentary deposits of the andaman archipelago india abstract in order to understand the provenance and tectono sedimentary processes occurring in the andaman subduction zone asz the late cretaceous to oligocene sedimentary records from the andaman islands have been studied these sedimentary records are considered to have preserved the history of the india asia collision evolution of the himalayas climatic development and palaeo drainage reorganizations on the indian and asian plates about 47 sandstones and mudstones shales and siltstones samples were analyzed for whole rock major trace and rare earth element compositions the geochemical results suggest mixing of sediments derived from the mafic igneous sources comprising local ophiolites and volcanic arc of the asz and an older archean to proterozoic age felsic cratonic source with compositions similar to average granodiorite or upper continental crustal sources the compositions were dominated by sources of the mafic arc during deposition of the mithakhari group whereas they were controlled by continental sources during deposition of the andaman flysch group the hope town conglomerate unit of the mithakhari group was mainly derived from weathering and erosion of the subaerially exposed local ophiolite thrust sheets whereas its namunagarh unit contains significant detritus from volcanic arcs the andaman flysch turbidites were deposited with a greater supply of sediments from first cycle active continental margin sources probably located in the tibetan and eastern myanmar region and recycled quartzose sedimentary sources within the nascent himalayas the sediments supplied to both the mithakhari and the andaman flysch groups were characterized by varying values of cia pia and w these variable values were either due to non steady state weathering conditions in the sources or the changing climatic conditions owing to the motion of indian plate with reference to the equator the uniformly high cia and w values in the andaman flysch rocks can be related to high precipitation and strong chemical weathering associated with the initiation of the indian monsoon

sedimentation and tectonics of the sylhet trough bangladesh the sylhet trough a sub basin of the bengal basin in northeastern bangladesh contains a thick fill 12 to 16 km of late mesozoic and cenozoic strata that record its tectonic evolution stratigraphic sedimentologic and petrographic data collected from outcrops cores well logs and seismic lines are here used to reconstruct the history of this trough the sylhet trough occupied a slope basinal setting on a passive continental margin from late mesozoic through eocene time subsidence may have increased slightly in oligocene time when the trough was located in the distal part of a foreland basin paired to the indo burman ranges oligocene fluvial deltaic strata barail formation were derived from incipient uplifts in the eastern himalayas subsidence increased markedly in the miocene epoch in response to western encroachment of the indo burman ranges miocene to earliest pliocene sediments of the surma group were deposited in a large mud rich delta system that may have drained a significant proportion of the eastern himalayas subsidence rates in the sylhet trough increased dramatically 3 8 times from miocene to pliocene pleistocene time when the fluvial tipam sandstone and dupi tila formation were deposited this dramatic subsidence change is attributed to south directed overthrusting of the shillong plateau on the dauki fault for the following reasons 1 pliocene and pleistocene strata thin markedly away from the shillong plateau consistent with a crustal load emplaced on the northern basin margin 2 the shillong plateau is draped by mesozoic to miocene rocks but pliocene and younger strata are not represented suggesting that the massif was an uplifted block at this time 3 south directed overthrusting of the shillong plateau is consistent with gravity data and with recent seismotectonic observations sandstone in the tioam has a marked increase in sedimentary lithic fragments compared to older rocks reflecting uplift and erosion of the sedimentary cover of the shillong plateau if the dauki fault has a dip similar to that of other himalayan overthrusts then a few tens of kilometers of horizontal tectonic transport would be required to carry the shillong plateau to its present elevation uplift of the shillong plateau probably generated a major 300 km westward shift in the course of the brahmaputra river

cenozoic plate reconstruction of southeast asia abstract the india eurasia collision alone set up a series of chain reactions and caused the formation and destruction of sedimentary basins within the domain of the collision belt changes in the rate and angle of convergence between the india and eurasia plates reflect different stages of tectonic development in southeast asia for example extrusion of the indochina block induced the consumption of the pre existing proto south china sea along northeastern kalimantan and led to the eventual opening of the south china sea along the south china margin subsequent motions of the sino burma thailand malay peninsula sumatra and kalimantan blocks have produced a succession of basins stretching from north sumatra to central thailand and on to the natuna area we present reconstructions of the southeast asia region at 60 ma 50 ma 40 ma 30 ma 20 ma 15 ma 10 ma and 5 ma it is clear from the reconstructions that the impact between greater india and southeast asia took place in the northwestern part of southeast asia the duration for the impact was probably from the middle eocene to early miocene this timing is in good agreement with the dating of the main red river fault motion wu et al 1989 scharer et al 1990 since the impact between greater india and southeast asia was basically west of the burma block there is no reason to assume that the sumatra malay peninsula and kalimantan should extrude to the southeast first along the left laterally displaced mae ping and three pagodas fault zones as suggested by peltzer and tapponnier 1988 if the opening of the central thailand basins the gulf of thailand and the malay basin are taken into consideration a dextral megashear zone is required to compensate the relative motion between indochina and the malay peninsula this dextral megashear zone might even extend into western kalimantan and serve as a boundary between the indochina block and kalimantan

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

post eocene biofacies palaeoenvironments and palaeogeography of the bengal basin india abstract the bengal basin is known for the extensive development of a thick cretaceous tertiary sedimentary sequence in the eastern part of india the basin has undergone several phases of transgressions regressions periodic uplifts and localised negative movements throughout the period of its evolution the sedimentation up to the end of the eocene was largely under the influence of a major transgressive phase which was later replaced by a regressive phase on account of the uplift and resulting marine retreat in this paper an analysis of the microfaunal distribution biofacies and sedimentary and tectonic history is made for reconstruction of the palaeoenvironmental conditions and palaeogeography of the bengal basin relating to the post eocene regressive phase of evolution during the oligocene early miocene the open marine conditions existing since the eocene were progressively modified on account of the regression into more brackish lagoonal and estuarine types a typical prograding delta started to spread over a large part of the marginal area of the basin the rocks of the barail group formed under the transitional environments of the delta are found to occur in almost all the major geo provinces of the basin the sedimentary environments and characteristics of the rocks were ultimately controlled by the configurations of the respective geo provinces the negative movements in the miocene along certain marginal areas were associated with the marine transgressive phase giving rise to the surma group this transgressive phase was however short lived and its impact was not equally felt in all areas the northeastern region was still uplifted which favoured an almost continuous existence of the deltaic phase by the end of miocene a greater part of the basin underwent a strong tectonic upheaval which was possibly the precursor of the main himalayan orogeny the intermontane depressions formed in the uplifted areas were the depositional sites for the tipam group and younger beds in plio pleistocene times the rapidly eroded debris from the highlands spread over a wide area towards the south under the influence of the river system of the ganga and brahmaputra which has transformed the original basinal areas into a large modern delta complex

the youngest marine deposits preserved in southern tibet and disappearance of the tethyan ocean abstract fossil ages as young as priabonian 38 34 ma are reported for the last marine sedimentary rocks in southern tibet correlation is based on examination of foraminifers and nannofossil biostratigraphy of youngest preserved sediments in sections at gamba zongpu tingri qumiba as well as a previously unreported section at yadong our results demonstrate that a marine seaway remained in existence south of the yarlung tsangpo suture zone until at least priabonian time notably this remains a maximum age estimate in this area as all sections are truncated by erosion or faulting we compare our results with sections throughout the himalaya region to demonstrate that shallow marine conditions existed widely during the eocene period in fact it seems likely that the marine conditions in the tethyan himalaya did not entirely disappear by the end of priabonian especially in the eastern himalaya the data presented in this study place direct constraints on the elimination of the tethyan ocean and thus have important implications for timing of the india eurasia collision

origins of microbial microstructures in the neoproterozoic beck spring dolomite variations in microbial community and timing of lithification abstract microbialite microstructures form at the scale of microbial processes and can preserve evidence of growth degradation and lithification in microbial communities understanding how these processes are preserved in microstructures will provide insights into microbial ecosystems and how microbial communities have evolved through time microbialites in the neoproterozoic beck spring dolomite southern california u s a contain microstructures that record variations in the morphology of microbial communities as well as the timing of degradation versus lithification in the communities these processes are represented by five end member microbial microstructures 1 distinct laminated microstructure 2 diffuse laminated microstructure 3 distinct clotted microstructure 4 diffuse and regular clotted microstructure and 5 diffuse and irregular clotted microstructure these microstructures are distinguished by fabrics defined by organic inclusions in distinct microstructures organic inclusions sharply define microbial features such as laminae that are laterally continuous for millimeters or 50 200 micron diameter microdots the contacts between distinct microstructures are always sharp in diffuse microstructures zones of organic inclusions lack distinct boundaries neighboring laminae and clots grade into each other within tens of microns and boundaries between microbial structures and encasing cements are indistinct diffuse microstructures grade into each other and into distinct microstructures in a systematic way that suggests that differences are due to variations in the timing of mineralization relative to growth and degradation of microbial communities sharply defined textures in distinct microstructures suggest that lithification started during growth of the microbial community and the close spatial association but lack of intergrading between distinct laminated and distinct clotted microstructures suggests that they formed from two distinct microbial communities that lived in the same environment by contrast diffuse microstructures formed where lithification occurred after the onset of decay and degradation of the primary growth structure the continuum from distinct microbial textures through more diffuse and irregular textures documents variations in the timing of lithification relative to growth and decay of the microbial community thus where petrographic preservation is extremely good such as in the beck spring dolomite sub millimeter scale microbial microstructures record a gradation between growth and degraded microbial structures depending on the relative timing of lithification and heterotrophic decay

zircon u pb ages geochemistry and sr nd pb hf isotopes of the nuri intrusive rocks in the gangdese area southern tibet constraints on timing petrogenesis and tectonic transformation abstract abundant magmatic rocks of various ages are exposed in gangdese southern tibet these rocks play an important role in understanding the tectonic transformation from the subduction of neo tethyan oceanic crust to the collision of the indian and asian continents based on zircon u pb ages geochemistry and sr nd pb hf isotopic data of the late cretaceous to early oligocene 96 30 ma intrusive rocks in the nuri cu w mo deposit we discuss the late cretaceous to early oligocene tectonic transformation of the region and the origin of oligocene cu w mo mineralization in southern gangdese the nuri intrusive rocks represent three magmatic episodes 96 91 56 52 and 33 30 ma the 96 91 and 56 52 ma rocks have relatively low 87sr 86sr i 0 7041 to 0 7060 and high end t 3 1 to 3 5 and ehf t values 3 7 to 15 the 33 30 ma rocks have relatively high 87sr 86sr i 0 7061 to 0 7063 and pb isotopes and low end t 3 8 to 1 8 and ehf t values 0 6 to 10 1 the three stages of intrusive rocks have geochemical characteristics that are similar to those of coeval rocks in gangdese the 96 91 and 33 30 ma rocks are adakitic whereas the 56 52 ma rocks have characteristics of arc calc alkaline magmatic rocks the 96 91 ma rocks were produced by the partial melting of neo tethyan basaltic oceanic crust and minor sediments whereas the 56 52 ma rocks were generated by the partial melting of juvenile crust and the 33 30 ma rocks were formed by the melting of indian plate lower crust contaminated with overlying mantle materials on the basis of the regional tectonic and magmatic characteristics we suggest that neo tethyan oceanic slab subduction and slab roll back occurred from 100 to 65 ma collision between the indian and asian continents occurred at 65 to 40 ma neo tethyan oceanic slab break off took place at 50 ma and the indian continent subducted northwards beneath the asian continent at 30 ma from the late cretaceous 96 91 ma to oligocene 30 ma the geodynamic setting of gangdese was changed from neo tethyan oceanic subduction to indian continental subduction in contrast to the miocene metallogenic rocks the magmatic source of oligocene metallogenic rocks contains old indian continental crust materials which result in the formation of intense unique cu w mo mineralization especially w mineralization in the southern gangdese

crustal architecture and moho topography beneath the eastern indian and bangladesh margins new insights on rift evolution and the continent ocean boundary the thick sedimentary cover 18 km along the eastern continental margin of india ecmi and over the bay of bengal has hindered understanding of the mechanisms of rifting that operated on the eastern indian and bangladesh margins analysis of multichannel seismic reflection data together with 3d gravity inversion and 2d gravity forward modelling illuminates basement configuration crustal thickness and moho topography thereby revealing the modes of rift evolution and the location of the continent ocean boundary cob along these margins the basement particularly in the shelf and slope regions of eastern peninsular india is traversed by nearly coast perpendicular graben and their seaward continuity is delineated up to 125 km from the coastline three different types of continental margin are present along the ecmi 1 sheared rift on the southern segment of the ecmi up to 14 n latitude offshore the southern granulite terrain 2 hyper extended rift in the central segment between 14 n and 17 n offshore the dharwar craton and 3 hypo extended rift on the northern segment between 17 n and 20 n offshore the eastern ghats mobile belt the graben terminations crustal thickness and moho topography of both the ecmi and offshore bengal basin clearly suggest that the cob runs nearly parallel to the coastline of peninsular india but takes an orthogonal turn offshore in the bengal basin and connects the palaeo continental shelf and the rajmahal sylhet line onshore in the bengal basin crustal thickness 8 10 km and depth to the moho discontinuity 28 km beneath the bangladesh margin are unusual because the crustal thickness increased through interaction of the kerguelen plume with the existing oceanic lithosphere the moho deepened largely due to the load of the bengal fan sediments the absence of rifted crustal blocks on the bangladesh margin and the continuity of the cob into the onshore bengal basin together imply that the present bangladesh region was under marine conditions at least until the beginning of bengal fan sedimentation i e no less than 23 ma

plateau pop up in the great 1897 assam earthquake the great assam earthquake of 12 june 1897 reduced to rubble all masonry buildings within a region of northeastern india roughly the size of england and was felt over an area exceeding that of the great 1755 lisbon earthquake1 hitherto it was believed that rupture occurred on a north dipping himalayan thrust fault propagating south of bhutan2 3 4 5 but here we show that the northern edge of the shillong plateau rose violently by at least 11 m during the assam earthquake and that this was due to the rupture of a buried reverse fault approximately 110 km in length and dipping steeply away from the himalaya the stress drop implied by the rupture geometry and the prodigious fault slip of 18 7 m explains epicentral accelerations observed to exceed 1g vertically and surface velocities exceeding 3 m s 1 ref 1 this quantitative observation of active deformation of a pop up structure confirms that faults bounding such structures can penetrate the whole crust plateau uplift in the past 2 5 million years has caused the indian plate to contract locally by 4 2 mm yr 1 reducing seismic risk in bhutan but increasing the risk in northern bangladesh

how deep was the early himalayan foredeep abstract the himalayan foredeep is a good archive of the events that occurred during different stages of the basin evolution the early part of this archive is used here to interpret depositional environments and paleobathymetry based on facies associations sedimentary structures and larger foraminifera three facies associations are recognized in the early himalayan foreland basin sequences facies association i containing chert breccia bauxite quartzose sandstone coal and black shale was deposited in short lived ephemeral streams swamps and inner parts of lagoons the limestones and shales of the facies association ii were deposited in a barrier lagoonal system where the former represent barrier and the latter represent lagoon deposits the limestone beds containing hummocks and wavy ripples were deposited under the influence of oscillatory storms and represent tempestites siltstone beds containing hummocky cross stratification and ripple cross laminae were deposited in the shoreface or foreshore zone where storm activities were dominant facies association iii comprises marl laminites and purple shale deposited in intertidal and supratidal zones of a tidal flat larger foraminifera such as nummulites and assilina flourished in the tropical climatic belt and accumulated in the coastal zone as a result of their transportation by waves and tides in general the sedimentation took place on a ramp similar to the present day persian gulf with a paleobathymetry of 55 m in strike continuity in pakistan the coeval sequences were deposited on shelves with a paleobathymetry

environmental effects of deccan volcanism across the cretaceous tertiary transition in meghalaya india the um sohryngkew section of meghalaya ne india located 800 1000 km from the deccan volcanic province is one of the most complete cretaceous tertiary boundary ktb transitions worldwide with all defining and supporting criteria present mass extinction of planktic foraminifera first appearance of danian species 13c shift ir anomaly 12 ppb and ktb red layer the geochemical signature of the ktb layer indicates not only an extraterrestrial signal ni and all platinum group elements pges of a second impact that postdates chicxulub but also a significant component resulting from condensed sedimentation p redox fluctuations as co fe pb zn and to a lesser extent ni and cu and volcanism from the late maastrichtian c29r into the early danian a humid climate prevailed kaolinite 40 60 detrital minerals 50 80 during the latest maastrichtian periodic acid rains carbonate dissolution cia index 70 80 associated with pulsed deccan eruptions and strong continental weathering resulted in mesotrophic waters the resulting super stressed environmental conditions led to the demise of nearly all planktic foraminiferal species and blooms 95 of the disaster opportunist guembelitria cretacea these data reveal that detrimental marine conditions prevailed surrounding the deccan volcanic province during the main phase of eruptions in c29r below the ktb ultimately these environmental conditions led to regionally early extinctions followed by global extinctions at the ktb

an early india asia contact paleomagnetic constraints from ninetyeast ridge odp leg 121 new paleomagnetic results from sedimentary rock and basement of the ninetyeast ridge ocean drilling program leg 121 sites 756 758 detail the northward movement of the indian plate for the past 80 m y analysis of the combined paleolatitude age profile indicates a distinct reduction in india9s northward movement rate at 55 ma interpreted as completion of suturing of greater india and asia india9s northward motion slowed from 18 19 5 cm yr to 4 5 cm yr for the location of site 758 comparison of this profile with paleomagnetic data from the wider himalayan region indicates that initial contact between northwestern greater india and southern asia was already established by cretaceous tertiary time this supports a possible causal link between the india asia convergence and the deccan traps extrusion

provenance study on eocene miocene sandstones of the rakhine coastal belt indo burman ranges of myanmar geodynamic implications the indo burman ranges ibr represent an accretionary wedge which is the result of subduction of the indian plate beneath the asian plate in the rakhine coastal belt it comprises a thick stack of cretaceous to neogene turbiditic sediments and localized thrust sheets of oceanic plate mafics and pelagic sediments we investigate eocene miocene sandstones aiming to reveal the provenance of the detrital material using modal framework grain heavy mineral and detrital zircon analysis u pb laser ablation icp ms dating hf isotope geochemistry and typology the results show a predominant derivation of the clastic material from i late cretaceous to oligocene igneous rocks which are often bimodal with a low number of zircons spanning the cretaceous palaeogene boundary and ii recycled orogenic terrane sources comprising ophiolitic rocks age corrected hf isotope ratios confirm subduction related mixed mantle crust sources we also observe minor reworking of older magmatic zircons by comparing our obtained petrographic parameters and zircon characteristics with potential himalayan indian continent and burman margin sources we conclude a burman margin and arc origin provenance with regard to hydrocarbon exploration in the ibr a forearc and trench basin system model linked with the burman arc appears more appropriate for evaluating the petroleum system

the configuration of asia prior to the collision of india cretaceous paleomagnetic constraints paleomagnetic data from central asia show that 1700 610 km of shortening of southern asia since cretaceous time have been absorbed by distributed deformation between southern tibet and the siberia craton this result is based on a compilation of cretaceous poles from the junggar tarim tibet indochina south china north china and mongolia blocks complementing the recent compilation of enkin et al 1992a we propose a paleogeographic reconstruction of asia in the cretaceous in which the position of siberia is derived from the synthetic apparent polar wander path of besse and courtillot 1991 the resulting map which likely represents asia as it remained throughout the cretaceous until the collision with india began features an unbent tibet with an east west trending andean margin at tropical latitudes and a rather continuous belt of continental red bed basins extending from sichuan to tarim through tibet the map allows one to estimate continental shortening and rotations between the blocks which are attributed to the collision despite large uncertainties these have amounts and senses which are in all cases compatible with some recent kinematic models such as that of avouac 1991 appendix is available with entire paper on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b93 007 2 50 payment must accompany order

lateral variations in the crustal structure of the indo eurasian collision zone we thank michael ritzwoller and two anonymous reviewers for constructive comments that have helped improve the manuscript the majority of the seismic data used in this study were downloaded from iris dmc data for the ngri stations in india were provided by s s rai and zahid ra provided the pmd pakistan data kajal borah provided the ambient noise cross correlations for the uttaranchal network figures were prepared using generic mapping tools gmt software wessel and smith 1998 we thank robert herrmann for making the computer programs in seismology freely available

geological structure of an arsenic contaminated aquifer at sonargaon bangladesh abstract continuous sediment core samples and groundwater were collected in the northern part of sonargaon central bangladesh to document the hydrogeological constraints on as contaminated aquifers the study area spans the alluvial plain of the old brahmaputra river and a pleistocene terrace the madhupur tract the quaternary sequence comprises plio pleistocene sand upper pleistocene mud and holocene sand units highly as contaminated groundwater 50 1000 g l is found in the upper aquifer corresponding to the holocene sand unit that underlies the alluvial plain and it plausibly appears to be closely related to the distribution of lenses of silt to fine sand as free 1 g l groundwater occurs only within sediments coarser than medium sand highly as contaminated groundwater is characterized by low concentrations of cl and documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepac

depositional environment and hydrocarbon source potential of the permian gondwana coals from the barapukuria basin northwest bangladesh article barapukuria coal basin is situated in dinajpur district of the northwestern part of bangladesh a total of eight coal samples have been collected from two different locations of the basin and analyzed using organic geo chemical and organic petrological methods the extracted organic matter of the studied samples is a mixture of type iii and type ii kerogen as evaluated by the source rock analyzer sra and pygc pyrograms the measured total organic carbon toc ranges from 61 to 74 wt and the recovered extractable organic matter eom varies from 27 561 to 41 389 ppm these results suggest that the coals are ranked as good quality source rock the hydrocarbon yield has been calculated which is also high and it ranges from 12 192 to 20 799 ppm the organic matter is thermally mature for hydrocarbon generation considering their tmax and measured mean vitrinite reflec tance values of 431 to 435 c and 0 72 to 0 81 ro respectively the hopane 22s 22s 22r moretane hopane ratio and sterane parameters are also in support of these thermal maturity assessment

structure and kinematics of the indo burmese wedge recent and fast growth of the outer wedge 1 the northern sunda subduction zone offshore burma and the associated indo burmese wedge mark the active eastern boundary of the burma platelet jammed between the india plate and the sunda plate the aim of this paper is to provide a structural and kinematics analysis of the indo burmese wedge based on seismic reflection geodetic and geological field data we found out that the indo burmese wedge is the place of diffuse strain partitioning right lateral shearing in the innermost part and e w shortening in the outermost part in the outer wedge thick skinned deformation overprints thin skinned deformation it can be explained by the required preservation of the critical taper after the fast westward propagation of the outer wedge above a very efficient clayey decollement layer the thick skinned deformation is here characterized by major right lateral strike slip faults the kaladan fault and the chittagong coastal fault and yet a typical internal deformation of the indo burmese wedge we suggest that the westward migration of internal right lateral shear deformation allows preservation of the strain partitioning ratios between internal n s trending right lateral shearing and external e w shortening some seismic lines show that the outer wedge deformation is not older than 2 ma its propagation could have been enhanced by the large amount of sediments filling up the sylhet flexural basin formed after the pliocene tectonic uplift of the shillong plateau

evolution of the himalayan paleogene foreland basin influence of its litho packet on the formation of thrust related domes and windows in the eastern himalayas a review abstract eocene oligocene foreland basin was formed in response to the india asia collision and ensuing himalayan orogenesis the initial collision during late paleocene to early eocene was broadly contemporaneous laterally the paleogene sediments although discontinuously exposed are remarkably similar in their character and organization in the eastern himalayas it is virtually concealed tectonically but is exposed close to and beneath the main boundary thrust mbt as narrow but laterally extensive thrust slivers of fossiliferous eocene sediments lateral continuity and similar marine fauna in them establish the presence of thicker eocene sediments beneath mbt further north several domes and windows in the eastern himalayan frontal belt have evolved with similar geometry the largest siang window exposes duplex antiform of paleogene sediments and volcanics at the core which have arched up the mbt roof thrust the latter underlies passively domed up late plaeozoic and proterozoic himalayan thrust packets the cores of other windows mainly expose low grade proterozoic rocks with or without the late paleozoic rocks a similar sequence of pre tertiary thrust packet is exposed to the south in the frontal belt which override the neogene sediments across mbt a buried basement indenture of the indian continent continues beneath the siang window located at the eastern himalayan syntaxis the cores of other windows are possibly arched up by the duplexes of pre tertiary and tertiary sediments during the process of southward movement of the crystalline and foreland sheets

crustal structure of the western bengal basin from joint analysis of teleseismic receiver functions and rayleigh wave dispersion teleseismic data recorded at the broadband seismological observatory of the indian institute of technology iit kharagpur are analyzed to determine the seismic characteristics of the crust beneath the western bengal basin receiver functions calculated from the teleseismic p waveform for a range of back azimuths show little variation in the moho ps arrival time indicating a nearly one dimensional crustal structure beneath kharagpur transforming the time domain receiver functions into the h v p v s domain for a stack of all receiver functions reveals a 38 2 km thick crust with a v p v s of 1 73 0 01 receiver functions stacked in narrow bins of back azimuth and great circle arc distance were inverted jointly with group velocity dispersion data for periods from 15 to 45 sec joint inversion results reveal a near surface sedimentary layer of 1 5 km thickness a 13 km thick upper crust and a 23 km thick lower crust with the moho at a depth of 37 5 km the average crustal s wave velocity is 3 7 km sec the top 0 5 km of the sedimentary layer has very low s wave velocity and a large impedance contrast with the underlying layer this represents the thin layer of unconsolidated sediments and weathered laterites near the surface as seen from deep seismic sounding dss studies across the region the s wave velocity structure of the crust beneath kharagpur is the first such result for the western bengal basin and is broadly similar to that of the indian shield crust a comparison of the crustal thickness and lower crustal velocities between kharagpur and agartala seperated by the eocene hinge zone shows remarkable differences crustal models beneath agartala reveal 36 km thick crust with the presence of a high velocity lower crust 6 km thick possibly oceanic in nature which is absent beneath kharagpur the hinge zone therefore separates the continental crust of the western bengal basin from the eastern deep basinal oceanic crust both being overlain by sediments derived from the himalayas

sedimentation and basin fill history of the neogene clastic succession exposed in the southeastern fold belt of the bengal basin bangladesh a high resolution sequence stratigraphic approach abstract the tertiary basin fill history of the bengal basin suffers from oversimplification the interpretation of the sedimentary history of the basin should be consistent with the evolution of its three geo tectonic provinces namely western northeastern and eastern each province has its own basin generation and sediment fill history related mainly to the indo burmese and subordinately to the indo tibetan plate convergence this paper is mainly concerned with facies and facies sequence analysis of the neogene clastic succession within the subduction related active margin setting oblique convergence in the southeastern fold belt of the bengal basin detailed fieldwork was carried out in the sitapahar anticline of the rangamati area and the mirinja anticline of the lama area the study shows that the exposed neogene succession represents an overall basinward progradation from deep marine through shallow marine to continental fluvial environments based on regionally correlatable erosion surfaces the entire succession 3000 m thick has been grouped into three composite sequences c b and a from oldest to youngest composite sequence c begins with deep water base of slope clastics overlain by thick slope mud that passes upward into shallow marine and nearshore clastics composite sequence b characteristically depicts tide dominated open marine to coastal depositional systems with evidence of cyclic marine regression and transgression repetitive occurrence of incised channel tidal inlet tidal ridge shoal tidal flat and other tidal deposits is separated by shelfal mudstone most of the sandbodies contain a full spectrum of tide generated structures e g herringbone cross bedding bundle structure mud couplet bipolar cross lamination with reactivation surfaces tidal bedding storm activities appear to have played a subordinate role in the mid and inner shelf region rizocorallium rosselia planolites and zoophycos are the dominant ichnofacies within the shelfal mudstone this paralic sedimentation of neogene succession in the study area can serve as a good point of reference for tide dominated regressive shelf depositional systems the top of the composite sequence b is marked by a pronounced erosion surface indicating the final phase of marine regression followed by the gradual establishment of continental fluvial depositional systems represented by composite sequence a in this composite sequence stacked channel bars of low sinuosity braided rivers gradually pass upsequence into high sinuosity meandering river deposits a sequence stratigraphic approach has been adopted to interpret the basin fill history with respect to relative sea level changes and to subdivide the rock record into several sequences and units systems tracts and parasequences based on identified bounding discontinuities such as transgressive erosion surface tes regressive erosion surface res marine flooding surface mfs and incised valley floor ivf this approach provides new insight for both exploration and exploitation strategy for hydrocarbon plays that may prove vital to the oil companies engaged in exploration activities in the bengal basin it is strongly recommended here that the traditional lithostratigraphic classification of this part of the basin which is based on the assam stratigraphy be abandoned or at least revised a tentative allostratigraphic scheme is presented and it is suggested that to formalize the scheme further study both surface and subsurface is needed

geology and depositional history of cenozoic sediments of the bengal basin of bangladesh abstract the bengal basin lies on the eastern side of the indian subcontinent and occupies the northern part of the bengal geosyncline sedimentation started with the breakup of gondwanaland cretaceous fluvial and deltaic sedimentation was slowly taken over by open marine and deep sea fan sedimentation by the mid eocene the basin was witnessing the maximum marine transgression and most of the stable shelf was in carbonate regime as part of the extended tethyan carbonate shelf the deeper part of the basin was dominated by deep sea fan sedimentation fed by clastic turbidites from the northeast with limited carbonate turbidites from the shelf the collision of the indian plate with the tibetan plate and with the burmese plate in the miocene resulted in a rapid switch in sedimentation pattern in the bengal basin the basin structure and sedimentation were both strongly influenced by the collision pattern of the plates and by the uplift of the himalayas with the miocene collision of the indian plate there was a change from flysch to molasse sedimentation the present day basin configuration with the ganges bramaputra delta on the north and the bengal deep sea fan on the south was established in the pliocene the stratigraphic and tectonic framework of the bengal basin suggest that the deposition of hydrocarbon source rocks was associated with the flysch sedimentation whereas formation of traps and deposition of most of the reservoir rocks in the basin was associated with mid pliocene tectonic movements and with the molasse sedimentation the pre mid miocene sediments now deeply buried in the deeper part of the bengal basin and in the eastern folded belt are the major source of gas of the eastern folded belt of the basin

influence of overpressure on formation velocity evaluation of neogene strata from the eastern bengal basin bangladesh abstract interpretation of sonic log data of anticlinal structures from eastern bangladesh reveals significant variations of acoustic velocity of subsurface strata the amount of variation in velocity is 32 from miocene to pliocene stratigraphic units in titas and bakhrabad structure whereas 21 in rashidpur structure velocity fluctuations are influenced by the presence of gas bearing horizons with velocities of gas producing strata 3 7 lower than laterally equivalent strata at similar depth average velocities of miocene boka bil and bhuban formations are respectively 2630 and 3480 m s at titas structure 2820 and 3750 m s at bakhrabad and 3430 and 3843 m s at the rashidpur structure from the overall velocity depth distribution for a common depth range of 915 3000 m the titas bakhrabad and rashidpur structures show a gradual increase in velocity with depth in contrast the sitakund anticline in se bangladesh reveals a decrease in velocity with depth from 3000 to 4000 m probably due to the presence of overpressured mudrocks of the bhuban formation tectonic compression associated with the indo burmese plate convergence likely contributed the most toward formation of subsurface overpressure in the sitakund structure situated in the chittagong tripura fold belt of the eastern bengal basin bangladesh

the paleogene record of himalayan erosion bengal basin bangladesh a knowledge of himalayan erosion history is critical to understanding crustal deformation processes and the proposed link between the orogen s erosion and changes in both global climate and ocean geochemistry the most commonly quoted age of india asia collision is 50 ma yet the record of paleogene himalayan erosion is scant either absent or of low age resolution we apply biostratigraphic petrographic geochemical isotopic and seismic techniques to paleogene rocks of the bengal basin bangladesh of previously disputed age and provenance our data show that the first major input of sands into the basin in the 1 km thick deltaic barail formation occurred at 38 ma our biostratigraphic and isotopic mineral ages date the barail formation as spanning late eocene to early miocene and the provenance data are consistent with its derivation from the himalaya but inconsistent with indian cratonic or burman margin sources detrital mineral lag times show that exhumation of the orogen was rapid by 38 ma the identification of sediments shed from the rapidly exhuming southern flanks of the eastern central himalaya at 38 ma provides a well dated accessible sediment record 17 myr older than the previously described 21 ma sediments in the foreland basin in nepal discovery of himalayan detritus in the bengal basin from 38 ma 1 resolves the puzzling discrepancy between the lack of erosional evidence for paleogene crustal thickening that is recorded in the hinterland 2 invalidates those previously proposed evidences of diachronous collision which were based on the tenet that himalayan derived sediments were deposited earlier in the west than the east 3 enables models of himalayan exhumation e g by mid crustal channel flow to be revised to reflect vigorous erosion and rapid exhumation by 38 ma and 4 provides evidence that rapid erosion in the himalaya was coincident with the marked rise in marine 87sr 86sr values since 40 ma whether 38 ma represents the actual initial onset of vigorous erosion from the southern flanks of the east central himalaya or whether older material was deposited elsewhere remains an open question

phanerozoic plate boundaries in mainland se asia the himalayas and tibet recently available geological data on tibet yunnan and burma facilitate recognition of phanerozoic subduction systems and two continental fragments of gondwanaland which extend through mainland se asia a cambrian subduction system and 5 mesozoic early caenozoic collision belts are identified in the region indochina eastern thailand and central tibet accreted to china in the early triassic western se asia and s tibet separated from gondwanaland in the permian or early triassic and collided with asia in the late triassic and the western burma island arc system collided with asia in the jurassic upper triassic flysch and schist in the eastern indoburman ranges were accreted to western burma in the jurassic early cretaceous during eastward subduction and formation of a back arc thrust belt following collision of greater india with arc complexes to the n in the cretaceous and with tibet in the early eocene western burma moved northwards about 450 km along the sagaing namyin ridge thrust transform fault formation of the 3 major belts of tin granites and the porphyry copper deposits in the region took place in specific tectonic settings within the subduction systems and collision belts

petrography and whole rock geochemistry of the tertiary sylhet succession northeastern bengal basin bangladesh provenance and source area weathering abstract detrital modes and the geochemical compositions of sandstones and mudrocks from the paleocene to plio pleistocene jaintia barail surma tipam and dupitila groups of the sylhet basin in bangladesh have been examined to identify their provenance and source area weathering conditions barail and surma group sandstones are quartz rich feldspar poor and are rich in sedimentary and metamorphic lithic fragments indicating a recycled orogen source geochemically the succession is divisible into three parts paleocene late eocene jaintia group mudrocks form separate trends and have very high chemical index of alteration cia values suggesting derivation from a stable cratonic source the second part the late eocene to miocene barail and surma groups forms single trends on variation diagrams maximum cia ratios decrease from the barail to the surma group suggesting rapid influx of detritus from a common felsic source undergoing active uplift compatible with weathering and erosion proxies from the bengal and indus fans average bulk geochemical and ree characteristics of these two groups are identical as are a cn k trends these features and comparison with potential himalayan sources support derivation of both the barail and surma groups from that orogen and hence supply of himalayan detritus to the sylhet basin as early as the late eocene rather than from the neogene the third part the non marine tipam late miocene pliocene and dupitila pliocene pleistocene groups forms slightly differing geochemical trends and has higher maximum cia values ree fractionation between sandstones and mudrocks is more marked but ree patterns and parameters remain unchanged suggesting that the overall source remained unchanged higher cia and greater ree fractionation in the tipam and dupitila groups may be a product of their fluvial deposition and more intense weathering on the floodplain during intensification of the south asian monsoon from 4 ma onwards

the diagenesis of neogene clastic sediments from the bengal basin bangladesh abstract the diagenetic history of the neogene clastic sediments of the bengal basin is described based on petrological analysis of samples from four boreholes in the shales evidence has been found of progressive illitization of the mixed layer illite smectite with increasing depth of burial unlike other studies notably of the tertiary gulf coast sediments there is no evidence of the illite smectite changing from a random to a regular mixed layer phase during diagenesis other diagenetic changes in the shales include the decrease in the proportions of kaolinite and calcite with increasing burial diagenesis in the sandstones principally involves the progressive development of various types of cements in the following order chlorite authigenesis siderite crystallization illite rim formation quartz overgrowths and late fe calcite cements after the formation of the cementation phases there was a dissolution phase creating secondary porosity via the dissolution of carbonate and feldspars and this was accompanied by the generation of a late kaolinite cementing phase the latter processes are thought to arise from the generation of acidic pore waters generated by the maturation of organic matter in the neogene and paleogene shales in some of the sandstones an early poikilotopic fe calcite cement formed and halted further diagenesis beyond the formation of the chlorite and early siderite crystallization

stress transfer and connectivity between the bhutan himalaya and the shillong plateau abstract within the northern indian plate the shillong plateau is a peculiar geodynamic terrane hosting significant seismic activity outboard the himalayan belt this activity is often used as an argument to explain apparent reduced seismicity in the bhutan himalayas although current geophysical and geodetic data indicate that the bhutan himalayas accommodate more deformation than the shillong plateau we aim to quantify the extent to which the two geodynamic regimes are connected and potentially interact through stress transfers we compiled a map of major faults and earthquakes in the two regions and computed co seismic stress transfer amplitudes our results indicate that the bhutan himalayas and the shillong plateau are less connected than previously suggested major earthquakes in either of the two regions mainly affect transverse faults connecting them causing up to 40 bar coulomb stress change however this effect is clearly less on thrust faults of the either region up to 1 bar only the mw 8 25 1897 assam earthquake that affected the shillong plateau did not cause a stress shadow on the main himalayan thrust in bhutan as previously suggested similarly the mw 8 0 5 1714 bhutan earthquake had negligible impact on stress accumulation on thrust faults bounding the shillong plateau furthermore the main process shaping the regional stress patterns continues to be interseismic loading with complex boundary conditions in a diffuse deformation field involving the bengal basin and indo burman ranges while both the bhutan himalayas and the shillong plateau exhibit a compressional regime their stress evolutions are more weakly connected than hypothesized although our modelling suggests lateral increase in stress interactions from west less to east more in the bhutan himalayas a clearer picture will only emerge with better constrained fault geometries slip rates crustal structure and seismicity catalogues in the entire region of distributed deformation

collision of the ganges brahmaputra delta with the burma arc implications for earthquake hazard abstract we take a fresh look at the topography structure and seismicity of the ganges brahmaputra delta gbd burma arc collision zone in order to reevaluate the nature of the accretionary prism and its seismic potential the gbd the world s largest delta has been built from sediments eroded from the himalayan collision these sediments prograded the continental margin of the indian subcontinent by 400 km forming a huge sediment pile that is now entering the burma arc subduction zone subduction of oceanic lithosphere with 20 km sediment thickness is fueling the growth of an active accretionary prism exposed on land the prism starts at an apex south of the gbd shelf edge at 18 n and widens northwards to form a broad triangle that may be up to 300 km wide at its northern limit the front of the prism is blind buried by the gbd sediments thus the deformation front extends 100 km west of the surface fold belt beneath the comilla tract which is uplifted by 3 4 m relative to the delta this accretionary prism has the lowest surface slope of any active subduction zone the gradient of the prism is only 0 1 rising to 0 5 in the forearc region to the east this low slope is consistent with the high level of overpressure found in the subsurface and indicates a very weak detachment since its onset the collision of the gbd and burma arc has expanded westward at 2 cm yr and propagated southwards at 5 cm yr seismic hazard in the gbd is largely unknown intermediate size earthquakes are associated with surface ruptures and fold growth in the external part of the prism however the possibility of large subduction ruptures has not been accounted for and may be higher than generally believed although sediment clogged systems are thought to not be able to sustain the stresses and strain weakening behavior required for great earthquakes some of the largest known earthquakes have occurred in heavily sedimented subduction zones a large earthquake in 1762 ruptured 250 km of the southern part of the gbd suggesting large earthquakes are possible there a large but poorly documented earthquake in 1548 damaged population centers at the northern and southern ends of the onshore prism and is the only known candidate for a rupture of the plate boundary along the subaerial part of the gbd burma arc collision zone

ref changing river courses in the western part of the ganga brahmaputra delta by kalyan rudra 2014 geomorphology 227 87 100 abstract research on river dynamics of the gb delta is in the early stages and it is a topic of multidisciplinary interest a narrow sectoral approach of research may lead to much confusion the discussion has a declared vision of offering corrected information against inconsistencies improper understanding and misrepresentation of facts in my paper but in reality the critics arguments are based on the wrong premise and those may create much confusion this reply is designed sequentially to address issues raised in the discussion

the elevation history of the tibetan plateau and its implications for the asian monsoon abstract the determination of the evolving palaeoaltitude of the tibetan plateau since the india eurasia collision underpins our understanding of how orography in central asia affects the intensity of the monsoon and hence global climate change palaeoaltitudes however cannot be measured directly and need to be inferred from proxy observations that are usually model dependent differing tectonic models for the behaviour of the lithosphere during continental collision have contrasting implications for the elevation of the plateau however two techniques recently employed for determining palaeo elevation are independent of tectonic models the first involving the variation with altitude of oxygen isotopes in precipitation and the second involving the change of leaf morphology with moist static energy of the atmosphere elevation studies have focused on southern tibet largely due to the relative ease of access to the region there is a remarkable unanimity amongst the diverse techniques applied that the altitude of the southern plateau has not significantly changed since at least the mid miocene ca 15 ma arguing for an onset of the monsoon system during or before the early miocene a range of tectonic studies suggest that the northern and eastern parts of the plateau are younger geomorphological features but there are few quantitative constraints of the timing of elevation from these regions of tibet since both the elevation and the surface area of the plateau impact on atmospheric circulation palaeoaltitude studies need to be extended to chart the increasing areas of elevated land surface through time

geologic geomorphic and hydrologic framework and evolution of the bengal basin india and bangladesh abstract the bengal basin the largest fluvio deltaic sedimentary system on earth is located in bangladesh and three eastern states of india sediment accumulates in the basin from the ganges brahmaputra and meghna gbm river systems and is dispersed into the bay of bengal forming the largest submarine fan in the world the basin is located in the himalayan foreland at the junction of the indian eurasian and burmese plates the basin is bounded by the indian craton on the west and the indo burmese fold belts on the east it can be broadly divided into a stable shelf and a foredeep separated by a deep seismic hinge zone basin sediments overlie gondwanan basement and vary in thickness from a few kilometers on the stable shelf to more than 16 km in the foredeep the basin was initiated at the breakup of gondwanaland in the late mesozoic and evolved through the formation of the proto gbm delta to the present delta starting around 10 5 ma the stratigraphy of the different parts of the basin differs considerably because of contrast in depositional history within the several sub basins that were produced by intra plate tectonic activities associated with ongoing himalayan orogeny the present day geomorphology is dominated by the extensive holocene gbm floodplain and delta the vertical succession of the deltaic plain can be classified into five units on the basis of differences in grain size which reflect differing depositional environments the initiation of the modern gbm delta at the onset of the pleistocene glacial maximum and its evolution to the present configuration are intricately related to holocene fluvio dynamic processes eustatic sea level changes and tectonic movements the sedimentology and mineralogy of the different parts of the basin reflect differences in sediment provenance the mineralogy is dominated by detrital quartz some feldspar and minor amounts of carbonates illite and kaolinite are the main clay minerals the basin has profuse groundwater resources but the architecture of the aquifers is not yet well resolved different classification schemes have been proposed on the basis of lithology regional groundwater flow follows a low hydraulic gradient from north to south but pumping for irrigation during the past several decades appears to have severely distorted regional flow recharge occurs primarily during the monsoon season groundwater chemistry is dominated by anoxic ca hco 3 type water with relatively high concentrations of fe mn and as carbonate dissolution silicate weathering feooh reduction and mixing with saline water are the primary processes controlling hydrochemistry

an overview of the sedimentary geology of the bengal basin in relation to the regional tectonic framework and basin fill history abstract the bengal basin in the northeastern part of indian subcontinent between the indian shield and indo burman ranges comprises three geo tectonic provinces 1 the stable shelf 2 the central deep basin extending from the sylhet trough in the northeast towards the hatia trough in the south and 3 the chittagong tripura fold belt due to location of the basin at the juncture of three interacting plates viz the indian burma and tibetan eurasian plates the basin fill history of these geo tectonic provinces varied considerably precambrian metasediments and permian carboniferous rocks have been encountered only in drill holes in the stable shelf province after precambrian peneplanation of the indian shield sedimentation in the bengal basin started in isolated graben controlled basins on the basement with the breakup of gondwanaland in the jurassic and cretaceous and northward movement of the indian plate the basin started downwarping in the early cretaceous and sedimentation started on the stable shelf and deep basin and since then sedimentation has been continuous for most of the basin subsidence of the basin can be attributed to differential adjustments of the crust collision with the various elements of south asia and uplift of the eastern himalayas and the indo burman ranges movements along several well established faults were initiated following the breakup of gondwanaland and during downwarping in the cretaceous by eocene because of a major marine transgression the stable shelf came under a carbonate regime whereas the deep basinal area was dominated by deep water sedimentation a major switch in sedimentation pattern over the bengal basin occurred during the middle eocene to early miocene as a result of collision of india with the burma and tibetan blocks the influx of clastic sediment into the basin from the himalayas to the north and the indo burman ranges to the east rapidly increased at this time and this was followed by an increase in the rate of subsidence of the basin at this stage deep marine sedimentation dominated in the deep basinal part while deep to shallow marine conditions prevailed in the eastern part of the basin by middle miocene with continuing collision events between the plates and uplift in the himalayas and indo burman ranges a huge influx of clastic sediments came into the basin from the northeast and east throughout the miocene the depositional settings continued to vary from deep marine in the basin to shallow and coastal marine in the marginal parts of the basin from pliocene onwards large amounts of sediment were filling the bengal basin from the west and northwest and major delta building processes continued to develop the present day delta morphology since the cretaceous architecture of the bengal basin has been changing due to the collision pattern and movements of the major plates in the region however three notable changes in basin configuration can be recognized that occurred during early eocene middle miocene and plio pleistocene times when both the paleogeographic settings and source areas changed the present basin configuration with the ganges brahmaputra delta system on the north and the bengal deep sea fan on the south was established during the later part of pliocene and pleistocene and delta progradation since then has been strongly affected by orogeny in the eastern himalayas pleistocene glacial activities in the north accompanied sea level changes in the bay of bengal

eocene ostracods from southern tibet implications for the disappearance of neo tethys abstract the yadong area was located in the residual neo tethyan basin during the paleogene it contains a record of the youngest marine deposition in the tethyan himalayas therefore biostratigraphic data from this area are essential to constrain the timing of elimination of the tethyan ocean and the india asia collision ostracods are one of the most diverse groups of living crustaceans inhabiting marine non marine and even some semi terrestrial habitats they provide important evidence to reconstruct paleoenvironment and biogeography this investigation examined ostracod faunas recovered from the paleogene zhepure formation in the gulupu section tuna yadong southern tibet the formation can be further divided into a lower limestone member and an overlying shale member which are conformable abundant diverse well preserved marine ostracods were recovered from the shale member we report 29 species from 15 genera the stratigraphic distribution of the eocene ostracod assemblages found reveals three ostracod zones and shows strong affinity to sections previously investigated throughout the neo tethyan region the zhepure formation also yields diverse planktonic and benthic foraminifers charophytes nannofossils and pteropods and indicates that open marine conditions existed widely during the eocene based on this study of little known eocene ostracods we suggest that final closure of the tethyan seaway occurred after early late eocene

a paleo brahmaputra subsurface lithofacies analysis of miocene deltaic sediments in the himalayan bengal system bangladesh abstract the bengal foreland basin contains a succession of up to 16 km of dominantly deltaic deposits eroded from the eastern himalayas and the indo burman ranges and carried by major river systems similar to the present day ganges and brahmaputra analysis of electric logs and core descriptions acquired during oil and gas exploration in bangladesh allows construction of lithofacies maps which constrain depositional patterns of miocene strata compilations of sand thickness and sand shale ratio of the miocene surma group show that lower to middle miocene strata of the bhuban formation accumulated in a large elongate trough sand thickness and percentage both decrease markedly away from this depocenter which describes a large scale bend running initially westward from rashidpur northeast bengal basin and curving southward toward the bengal fan middle to upper miocene strata of the boka bil formation show a similar geographic trend in deposition of coarsest and thickest sediment but the major depocenter had shifted northward relative to that of the bhuban formation by some 30 km passing near beani bazar these trends suggest that deltaic deposits of the surma group filled the sylhet trough of the northeast bengal basin from the east published seismic data from western bangladesh show that additional large channels also contributed materials to the bengal basin from the northwest during the late miocene but these channels resulted in very little accumulation in the northwestern part of the basin probably due to restricted subsidence of underlying continental crust this study suggests that there was a major drainage system similar to the modern brahmaputra river during miocene time which carried orogenic sediments eroded from the uplifted terranes of the eastern himalayas and indo burman ranges to the eastern bengal delta

eocene foraminifers of the naga hills of manipur indo myanmar range imr implications on age and basin evolution abstract the disang group incorporates the oldest cenozoic clastic sedimentary rocks that crop out in a series of nne ssw trending ridges and valleys throughout much of the naga hills the stratigraphic position of this unit is not well constrained due to a paucity of definitively recognisable in situ fossils we report forty four species attributable of twenty eight genera based on our new reported foraminiferal assemblages we refine the age and paleoenvironmental interpretations for the upper disang formation we recognize planktonic foraminiferal zones e14 16 correlative to upper middle eocene to upper eocene in the upper disang formation within the naga hills a basin model for this lithological unit involving an upper bathyal bathymetry rapidly shallowing in a restricted basin with slump deposits where anoxic conditions prevailed is proposed based on field observations foraminiferal assemblages and published data

the potential for giant tsunamigenic earthquakes in the northern bay of bengal the sumatra andaman earthquake and indian ocean tsunami of 2004 came as a surprise to most of the earth science community while it is now widely recognized that the risk of another giant earthquake is high off central sumatra just east of the 2004 earthquake there seems to be relatively little concern about the subduction zone to the north in the northern bay of bengal along the coast of myanmar formerly burma phil cummins suggests however that giant earthquake activity is possible in this region and that a large and vulnerable population may be facing a significant earthquake and tsunami hazard there is a high potential for giant earthquake activity along the coast of myanmar indications that this may be the case include the tectonic environment historical earthquake activity and stress and crustal strain observations these findings suggest that giant earthquakes do occur off the coast of myanmar and that a very large and vulnerable population is thereby exposed to a significant earthquake and tsunami hazard the great sumatra andaman earthquake and indian ocean tsunami of 2004 came as a surprise to most of the earth science community although it is now widely recognized that the risk of another giant earthquake is high off central sumatra1 just east of the 2004 earthquake there seems to be relatively little concern about the subduction zone to the north in the northern bay of bengal along the coast of myanmar here i show that similar indicators suggest a high potential for giant earthquakes along the coast of myanmar these indicators include the tectonic environment which is similar to other subduction zones that experience giant megathrust earthquakes stress and crustal strain observations which indicate that the seismogenic zone is locked and historical earthquake activity which indicates that giant tsunamigenic earthquakes have occurred there in the past these are all consistent with active subduction in the myanmar subduction zone and i suggest that the seismogenic zone extends beneath the bengal fan i conclude therefore that giant earthquakes probably occur off the coast of myanmar and that a large and vulnerable population is thereby exposed to a significant earthquake and tsunami hazard

assessment of gas hydrate accumulations using velocities derived from vertical seismic profiles and acoustic log data in krishna godavari basin india abstract the second indian national gas hydrate program expedition nghp 02 was executed in 2015 in four areas termed as areas a b c and e during this expedition twenty five research sites were drilled and or cored in krishna godavari kg and mahanadi basins eastern indian offshore during nghp 02 zero offset vertical seismic profile vsp data were acquired at three sites sites nghp 02 17 19 and 22 in area b of the kg basin in this study we focus on the three sites in area b of the kg basin where zero offset vsp and downhole acoustic log data are used to assess and characterize the gas hydrate deposits zero offset vsp data are correlated with wireline log surface seismic and synthetic seismic data to characterize and delineate gas hydrate accumulations in the kg basin low velocities ranging from 1500 to 1650 m s are observed in the unconsolidated shallow sedimentary section above gas hydrate bearing units whereas very high velocities are observed in the acquired acoustic log and vsp data in the gas hydrate bearing sedimentary sections the vsp derived interval velocities range from 2000 to 3000 m s in the depth interval between 267 and 287 m below sea floor mbsf at site nghp 02 17 between 1650 and 1750 m s in the depth interval between 306 and 366 mbsf at site nghp 02 19 and between 1700 and 1800 m s in the depth interval between 198 and 290 mbsf at site nghp 02 22 gas hydrates are distributed both as pore filling and fracture filling at all three sites however high concentrations are only observed as pore filling morphology we estimate the amount of gas hydrate considering both isotropic pore filling and anisotropic fracture filling acoustic reservoir models our estimations of gas hydrate saturation match well with the available pressure core data and suggest that high gas hydrate concentrations to nearly 85 of pore space are distributed in a load bearing morphology

deep structure and tectonics of the burmese arc constraints from earthquake and gravity data abstract active subduction of the indian plate is currently occurring beneath the burmese arc along an east dipping benioff zone which extends to a depth of about 180 km the overriding burma plate has an appearance of an inland seismic slab that is deflected downwards in the vicinity of the benioff zone a crustal seismic zone some 60 80 km east of the benioff zone correlates to backarc activity a triangular aseismic wedge in the top part of the crust outlines the central belt molasse basin east of the burmese foldbdt fault plane solutions show that the burmese benioff zone is characterized by shallow angle thrusting at its upper edge whereas down tip tensional events dominate its lower edge most of the backarc seismicity is accounted for by the sagaing transform or by the activity of the shan scarp normal fault zone at the margin of the asian plate a gravity anomaly pair with amplitude of 175 mgal coincides with the 1100 km long burmese arc lying in a north south direction the gravity anomalies along a profile in central burma and in adjacent areas of the bengal basin are interpreted in terms of plate subduction as well as near surface mass anomalies this suggests that sediments below the central belt may have an average thickness of the order of 10 km but may be as thick as 15 km at the subduction zone the oceanic crust underlying deeper parts of the bengal basin experiences phase transition at about 30 km depth in a benioff zone environment east of the burmese foldbelt several thrust planes are present within the folded and deformed cretaceous tertiary sediments of the fold belt these are often associated with ophiolites and basic to ultrabasic rocks a low density zone at least 60 km wide underlies the andesitic volcanic axis in the overriding plate

trench slope controlled deep sea clastics in the exposed lower surma group in the southeastern fold belt of the bengal basin bangladesh abstract deltaic to shallow marine neritic depositional settings have until now been the accepted interpretation for the neogene surma group of rocks exposed throughout the southeastern fold belt of the bengal basin the present study revises the earlier views and proposes some new insights on the basis of detailed field studies carried out in the sitapahar anticline rangamati area and in the mirinja anticline lama area it is here proposed that the surma group succession represents an overall basinward progradation from deep marine to coastal marine depositional settings sedimentological evidence strongly suggests that the lower part of the surma group represents a slope apron the growth of which is thought to have been governed by a westward migrating accretionary prism complex within the active margin setting of the indo burmese plate convergence thin packages of distinct turbidity current generated deposits together with some slump and debris flow deposits contained within thicker intervals of mudstone characterize the lowermost exposed unit of the surma group in the sitapahar anticline the overlying slope deposits are essentially mudstone dominated comparable deep sea clastics with thicker intervals of sandstone turbidites contained within a submarine canyon are present in the mirinja anticline this new model suggests that it may be timely to re evaluate the existing stratigraphic and tectonic framework of the eastern bengal basin

geologic implications of gas hydrates in the offshore of india krishna godavari basin mahanadi basin andaman sea kerala konkan basin gas hydrate resource assessments that indicate enormous global volumes of gas present within hydrate accumulations have been one of the primary driving forces behind the growing interest in gas hydrates gas hydrate volumetric estimates in recent years have focused on documenting the geologic parameters in the gas hydrate petroleum system that control the occurrence of gas hydrates in nature the primary goals of this report are to review our present understanding of the geologic controls on the occurrence of gas hydrate in the offshore of india and to document the application of the petroleum system approach to the study of gas hydrates national gas hydrate program of india executed the national gas hydrate program expedition 01 nghp 01 in 2006 in four areas located on the eastern and western margins of the indian peninsula and in the andaman sea these areas have experienced very different tectonic and depositional histories the peninsular margins are passive continental margins resulting from a series of rifting episodes during the breakup and dispersion of gondwanaland to form the present indian ocean the andaman sea is bounded on its western side by a convergent margin where the indian plate lithosphere is being subducted beneath southeast asia nghp 01 drilled logged and or cored 15 sites 31 holes in the krishna godavari basin 4 sites 5 holes in the mahanadi basin 1 site 2 holes in the andaman sea and 1 site 1 hole in the kerala konkan basin holes were drilled using standard drilling methods for the purpose of logging while drilling and dedicated wireline logging as well as through the use of a variety of standard coring systems and specialized pressure coring systems nghp 01 yielded evidence of gas hydrate from downhole log and core data obtained from all the sites in the krishna godavari basin the mahanadi basin and in the andaman sea the site drilled in the kerala konkan basin during nghp 01 did not yield any evidence of gas hydrate most of the downhole log inferred gas hydrate and core recovered gas hydrate were characterized as either fracture filling in clay dominated sediments or as pore filling or grain displacement particles disseminated in both fine and coarse grained sediments geochemical analyses of gases obtained from sediment cores recovered during nghp 01 indicated that the gas in most all of the hydrates in the offshore of india is derived from microbial sources only one site in the andaman sea exhibited limited evidence of a thermogenic gas source the gas hydrate petroleum system concept has been used to effectively characterize the geologic controls on the occurrence of gas hydrates in the offshore of india

14c dating of deep groundwater in the bengal aquifer system bangladesh implications for aquifer anisotropy recharge sources and sustainability summary environmental isotopes and 14 c dating were applied to estimate ages and recharge sources of deep groundwater 150 m bgl in south east bangladesh with one exception deep groundwater is shown to have been recharged more recently than 10 ka range 3 9 ka mean 7 6 ka under climatic conditions indicated by 18 o and 2 h as similar to the present day groundwater age distributions have been used to infer the scale of aquifer hydraulic anisotropy 2d groundwater flow modelling is able to reproduce the observed vertical profiles of groundwater age as determined at two locations when the aquifer is assigned a permeability anisotropy k x k z of at least 10 3 under these conditions deep groundwater originates as recharge in the hill regions at the eastern boundary of the basin recharge rates estimated from the groundwater ages are close to an estimate of the current rate of deep groundwater abstraction cautious development and careful monitoring are therefore necessary as excessive deep groundwater pumping could draw dissolved arsenic from the shallow levels of the bengal aquifer system bas and contaminate the deep groundwater resource