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

40ar 39ar multigrain dating of detrital white mica of sandstones of the surma group in the sylhet trough bengal basin bangladesh abstract the sylhet trough a subbasin of the bengal basin in northeastern bangladesh is bordered by two orogens the himalaya to the north and the indo burman ranges to the east detrital white micas from six sandstone samples of the neogene subsurface surma group were analysed by the 40ar 39ar method the ages of the individual steps in the ar ar release diagrams range from 25 up to 92 ma these ages represent cooling ages rather than formation ages because they originate from large mica flakes the low temperature step ar ar ages of detrital white mica with plateau type character are diagnostic for the youngest cooling events and cluster between 25 and 36 ma except one age of 51 ma the medium and high temperature steps of the ar release diagrams are characterized by staircase type pattern these various age discordances are interpreted as a mixture of micas with different ages the 40ar 39ar results point to heterogeneous ages of the source region which seems to be typical of the early stages of a rapidly uplifting collision orogen the narrow age range of the low temperature steps can be correlated with early cooling ages known from the high himalayan crystalline basement the ar ar data indicate that the major sediment input in the sylhet trough during sedimentation of the surma group has been derived from the rising himalaya orogen

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

cenozoic history of the himalayan bengal system sand composition in the bengal basin bangladesh stratigraphic sequences preserved in the bengal basin provide detrital information that documents a significantly older history of the eastern himalaya than that available from ocean drilling program and deep sea drilling project cores recovered from the bengal fan the bengal basin formed as a result of the himalayan collision is located at the juncture of the indian craton to the west the shillong massif and the himalayan belt to the north and the indo burman ranges to the east modal analyses of eocene and oligocene sandstones of the cherra kopili and barail formations document compositions that are dominated by subangular monocrystalline quartz grains with scarce to no feldspar grains and few lithic fragments cherra and kopili qt 99 f 1 l 0 barail qt 90 f 3 l 7 qt total quartz f feldspar l lithic fragments these compositions are similar to sands derived from the indian craton suggesting that they underwent intense chemical weathering likely in a source with low relief and considerable transport himalayan tectonism during this time was probably significantly more distant from the bengal basin than at present sandstones of the miocene surma group bhuban and boka bil formations are rich in feldspar grains argillite and very low grade metamorphic lithic fragments bhuban qt 66 f 15 l 19 boka bil qt 57 f 23 l 20 relative to older sandstones suggesting onset of uplift and erosional unroofing in the eastern himalaya and initiation of stream systems supplying orogenic detritus to the bengal basin sands of the upper miocene to pliocene tipam group and the pliocene pleistocene dupi tila sandstone contain abundant argillitic and low to medium grade metamorphic lithic fragments and feldspar grains tipam qt 61 f 19 l 19 dupi tila qt 70 f 13 l 17 suggesting continued orogenic unroofing these younger sands are rich in potassium feldspar p f 0 30 0 20 relative to plagioclase p rich bhuban and boka bil sandstones p f 0 66 and 0 48 suggesting a granitic source probably the miocene leucogranites of the high himalayan crystalline terrane these results document for the first time contrasts in orogenic history recorded in the bengal system vs western himalayan foreland basins sands deposited in the bengal basin are generally more quartzose and less lithic than those from the western himalayan foreland basins and pre miocene strata in the bengal system show little to no evidence of orogenic activity in part this probably reflects west to east progression of the himalayan collision but it probably also results from sedimentary systems propagating southward ahead of the advancing mountain belt as it has been consuming the remnant ocean basin trapped between the indian craton and the burmese block

age of initiation of the india asia collision in the east central himalaya abstract we document the stratigraphy and provenance of the lower tertiary terrigenous sections in the zhepure shan region of the tethyan himalaya southern tibet using petrographic and geochemical whole rock and single grain techniques the cretaceous early tertiary shelf deposits of shallow marine carbonates and siliciclastics of the former indian passive margin near the western end of the zhepure shan are conformably overlain by lower tertiary clastic rocks sandstones in the jidula formation paleocene mostly contain monocrystalline quartz grains of cratonic origin in contrast significant amounts of immature framework grains with a distinct ophiolitic and volcanic arc influence are present in the youxia early eocene and shenkeza post early eocene formations major trace and rare earth element concentrations in both sandstones and shales complement the petrographic data and indicate that the source of the jidula formation consisted primarily of quartzose basement rocks probably of indian con

controls on the erosion of cenozoic asia and the flux of clastic sediment to the ocean rates of continental erosion may be reconstructed from variations in the rate of accumulation of clastic sediment most of which lies offshore global rates of marine sedimentation are usually considered to have reached a maximum after 3 4 ma driven by enhanced erosion in a variable glacial interglacial climate however a new compilation of seismic data from the marginal seas of asia now shows that only the red river reached its historic peak after 4 ma sediment flux from asia first peaked in the early middle miocene 24 11 ma well before the initiation of a glacial climate indicating that rock uplift and especially precipitation are the key controls on erosion at least over long periods of geologic time reconstructions of weathering in east asia show that faster erosion correlates with more humid warm climates in the early middle miocene changing to less erosive drier climates after 14 ma when antarctic glaciation begins average rates of sedimentation on most east asian continental margins since 1 8 ma are 5 6 times less than the modern fluvial flux implying that the flux to the oceans varies sharply on short timescales and is not always buffered over timescales of 10 4 yr by storage in flood plains

the timing of india asia collision onset facts theories controversies abstract the timing of initial collision between india and asia has remained controversial for half a century this paper attempts to review this crucial and hotly debated argument describing first the different methods used to constrain the age of collision and discussing next the rationale results inferences and problems associated with each we conclude that stratigraphy represents the best direct way to unravel collision chronology other methods focusing on the magmatic metamorphic or paleomagnetic record provide additional fundamental constraints but cannot provide a robust direct estimate of collision onset initial collision in the central eastern himalaya is dated directly at the middle paleocene 59 1 ma by the abrupt change in sediment provenance recorded in trench settings the quasi synchronous unconformities documented along both tethyan passive margin of india and active margin of asia from tibet to zanskar ladakh confirm that orogeny was underway at the close of the paleocene 56 ma well before the disappearance of marine seaways in the himalaya during the early middle eocene 50 45 ma sedimentary evolution and provenance changes in marine to fluvio deltaic successions are recorded synchronously within error from the western to the central eastern himalaya failing to provide conclusive evidence for diachronous collision these coherent observations are hard to reconcile with three widely cited hypotheses invoking either paleogene arc continent collision or late cretaceous ophiolite obduction or the protracted existence of a greater india basin which are all not favored after discussing the geological evidence critically point by point a scenario no more complex than the one involving solely the passive continental margin of india and the active continental margin of asia is needed to explain the geological evolution of the nascent himalaya the collision between the tethys himalaya and the transhimalayan arc trench system does represent the collision between india and asia because the yarlung zangbo ophiolite is the forearc basement of the asian active margin its obduction onto india could not have preceded the initial closure of neo tethys ophiolite obduction began when collision began in the middle paleocene

forced regression across the marine to continental transition in jammu sub basin implication to the oligo miocene unconformity in the himalayan foreland abstract similar to the other sub basins marine to continental transition in jammu foreland also shows the presence of a characteristic greenish gray quartz arenite unit locally termed as white sandstone unit process based sedimentological and sequence stratigraphic work carried out across this white sandstone unit suggest that this unit was deposited in a shoreface settings during a forced regression the sharp base of the white sandstone unit represents a surf diastem a kind of regressive surface of marine erosion and does not signify a 10 myr unconformity the gradational top of the unit showing various degree of exposure and soil development has been interpreted as type 1 sequence boundary the white sandstone unit present along the entire stretch of the western sub himalaya also shows forced regressive nature in the subathu sub basin situated 350 km se of jammu so the forced regression across the marine to continental transition in the entire western himalayan foreland is a regional phenomena and not a local one contrary to the earlier works which proposed the presence of an unconformity spanning 10 myr encompassing the oligo miocene period the present study showed that the unconformity if present is of small duration possibly spanning

geochronological constraints on evolution of singhbhum mobile belt and associated basic volcanics of eastern indian shield abstract the singhbhum mobile belt smb of the eastern indian shield represents a roughly east west trending arcuate belt of folded supracrustals overlying the granite greenstone basement of the singhbhum orissa craton along its northern eastern and western margins and is bounded by the chotanagpur gneissic complex to further north the radiometric ages of the basement singhbhum and equivalent granites and the intrusive anorogenic mayurbhanj granite pluton constrain the time of evolution of this mobile belt between 3 12 and 3 09 ga hence the smb supracrustals also known as singhbhum group is late mesoarchaean in age and not proterozoic as thought earlier the evolution of the smb was followed by emplacement of some major basic igneous rocks within or adjacent to the supracrustals these include simlipal volcanics at 3 09 ga on the smb mayurbhanj gabbro along with mayurbhanj granite at 3 09 ga along the marginal part of the craton near the smb and the dalma volcanics on the smb along with the dhanjori volcanics adjacent to smb at 2 80 ga the 2 80 ga old basic volcanics is also associated with emplacement of some small granite plutons occurring along the marginal part of the craton one of them the tamperkola granite intrudes the smb the 3 09 ga onward igneous activities along the marginal part of singhbhum orissa craton took place essentially under anorogenic tectonic setting before being affected by a major metamorphism at 2 50 ga which is recorded on the dalma volcanics and on some small granite pluton occurs along the marginal part of the craton the jagannathpur and stratigraphically equivalent malangtoli volcanics occurring within the singhbhum orissa craton at the west were erupted at 2 25 ga the boundary between the smb supracrustals and the singhbhum orissa craton is demarked by a prominent shear zone known as the singhbhum shear zone which shows multiple reactivation the oldest being at 3 09 ga followed by subsequent reactivation during palaeo and mesoproterozoic periods at 2 2 1 8 1 6 1 5 1 4 and 1 0 ga respectively the singhbhum group and the adjacent chotanagpur gneissic complex appear to have evolved from a near shore syn rift and a distal post rift stable shelf sedimentary assemblages respectively which were deposited without any stratigraphic break in a marine basin existed in the present north of the singhbhum orissa craton both of these assemblages were deformed and metamorphosed together during proterozoic at 2 5 to 2 3 ga 1 6 ga and 1 0 ga

sr isotope evolution of seawater the role of tectonics we use a high resolution seawater sr isotopic evolution curve for the last 100 m y in conjunction with modern riverine sr flux measurements and also geologic tectonic and geochronological data to make the case for a close relationship between seawater sr isotopic composition and the india asia continental collision using a simple seawater sr budget model we begin by showing that the sr flux associated with alteration of seafloor basalts is too small and does not have the right time evolution to account for much of the seawater sr isotopic curve of the last 100 m y the flux of dissolved sr carried by rivers originating in the himalaya tibet region on the other hand is presently a significant fraction of the global sr budget we calculate how this riverine flux would have had to change with time in order to match the observed seawater sr isotopic curve and find that the riverine flux remains relatively constant prior to the collision of india with asia but then increases very significantly after collision we note that the period of most rapid change in seawater sr isotopic ratio from 20 ma to 15 ma is also a period of exceptionally high erosion in parts of the himalayas and the tibetan plateau as further evidence that sr derived from the collision of india with asia plays a major role in the sr isotopic evolution of seawater we show that the total amount erosion of the himalaya tibetan plateau since collision which we calculate separately represents a total amount of sr that is very nearly the same as the cumulative amount required by the sr isotopic change of seawater since collision the relationship between erosion and riverine sr flux allows us to use the sr isotopic evolution of seawater to reconstruct a history of erosion since collision and we find that the erosion rate accelerates with time since collision with the present having the largest rate when we apply the sr budget model to the entire phanerozoic using a new compilation of deformed continental area versus time we find that we can account for the large scale structure of the seawater sr isotopic curve but fail to reproduce several local maxima and minima especially in the period 100 300 ma the present high s7sr s6sr of seawater and similar highs in the devonian and cambrian do correlate with extensive deformation on the continents

jurassic to miocene magmatism and metamorphism in the mogok metamorphic belt and the india eurasia collision in myanmar situated south of the eastern himalayan syntaxis at the western margin of the shan thai terrane the highgrade mogok metamorphic belt mmb in myanmar occupies a key position in the tectonic evolution of southeast asia the first sensitive high resolution ion microprobe u pb in zircon geochronology for the mmb shows that strongly deformed granitic orthogneisses near mandalay contain jurassic 170 ma zircons that have partly recrystallized during 43 ma high grade metamorphism a hornblende syenite from mandalay hill also contains jurassic zircons with evidence of eocene metamorphic recrystallization rimmed by thin zones of 30 9 plus or minus 0 7 ma magmatic zircon the relative abundance of jurassic zircons in these rocks is consistent with suggestions that southern eurasia had an andean type margin at that time mid cretaceous to earliest eocene 120 to 50 ma i type granitoids in the mmb myeik archipelago and western myanmar confirm that prior to the collision of india an up to 200 km wide magmatic belt extended along the eurasian margin from pakistan to sumatra metamorphic overgrowths to zircons in the orthogneiss near mandalay date a period of eocene 43 ma high grade metamorphism possibly during crustal thickening related to the initial collision between india and eurasia at 65 to 55 ma this was followed by emplacement of syntectonic hornblende syenites and leucogranites between 35 and 23 ma similar syntectonic syenites and leucogranites intruded the ailao shan red river shear belt in southern china and vietnam during the eocene oligocene to miocene and the wang chao and three pagodas faults in northern thailand that most likely link with the mmb were also active at this time the complex history of eocene to early miocene metamorphism deformation and magmatism in the mmb provides evidence that it may have played a key role in the network of deformation zones that accommodated strain during the northwards movement of india and resulting extrusion or rotation of indochina

stratigraphy structure and tectonic evolution of the himalayan fold thrust belt in western nepal regional mapping stratigraphic study and 40ar 39ar geochronology provide the basis for an incremental restoration of the himalayan fold thrust belt in western nepal tectonostratigraphic zonation developed in other regions of the himalaya is applicable with minor modifications in western nepal from south to north the major structural features are 1 the main frontal thrust system comprising the main frontal thrust and two to three thrust sheets of neogene foreland basin deposits 2 the main boundary thrust sheet which consists of proterozoic to early miocene lesser himalayan metasedimentary rocks 3 the ramgarh thrust sheet composed of paleoproterozoic low grade metasedimentary rocks 4 the dadeldhura thrust sheet which consists of medium grade metamorphic rocks cambrian ordovician granite and granitic mylonite and early paleozoic tethyan rocks 5 the lesser himalayan duplex which is a large composite antiformal stack and hinterland dipping duplex and 6 the main central thrust zone a broad ductile shear zone the major structures formed in a general southward progression beginning with the main central thrust in late early miocene time eocene oligocene thrusting in the tibetan himalaya north of the study area is inferred from the detrital unroofing record on the basis of 40ar 39ar cooling ages and provenance data from synorogenic sediments emplacement of the dadeldhura thrust sheet took place in early miocene time the ramgarh thrust sheet was emplaced between 15 and 10 ma the lesser himalayan duplex began to grow by 10 ma simultaneously folding the north limb of the dadeldhura synform the main boundary thrust became active in latest miocene pliocene time transport of its hanging wall rocks over an 8 km high footwall ramp folded the south limb of the dadeldhura synform thrusts in the subhimalayan zone became active in pliocene time the minimum total shortening in this portion of the himalayan fold thrust belt since early miocene time excluding the tibetan zone is 418 493 km the variation depending on the actual amounts of shortening accommodated by the main central and dadeldhura thrusts the rate of shortening ranges between 19 and 22 mm yr for this period of time when previous estimates of shortening in the tibetan himalaya are included the minimum total amount of shortening in the foldthrust belt amounts to 628 667 km this estimate neglects shortening accommodated by small scale structures and internal strain and is therefore likely to fall significantly below the actual amount of total shortening

mantle derived spinel lherzolite xenoliths from the deccan volcanic province india implications for the thermal structure of the lithosphere underlying the deccan traps abstract petrography mineral and host rock chemistry of the mantle derived spinel lherzolite xenoliths in the nephelinites of kutch a late variant of the deccan traps province india are reported the xenoliths register equilibration temperatures between 920 70 c and 1060 50 c and an ascent time between 19 and 168 days representing a palaeogeotherm of minimum heat flow between 60 and 70 mw m 2 the xenoliths possibly came from the oceanic lithosphere under the deccan field following the cretaceous eocene fragmentation of gondwanaland

the os isotopic composition of himalayan river bedloads and bedrocks importance of black shales re os data are presented for bedrocks and bedloads of selected rivers from the himalayan range in central nepal and bangladesh these data demonstrate that the os isotopic composition of most of the bedrocks is similar to that of the average continental crust notable exceptions to this are the black shales from the lesser himalaya lh which contain large quantities of radiogenic osmium in the black shales the os is not concentrated in the sulfides and may instead be associated with organic matter bedloads collected from rivers which include lh black shales in their catchment basins have notably high os ratios 187os 188os 3 this radiogenic signature persists into the ganga river in contrast the brahmaputra river is less radiogenic suggesting an ophiolite input in the headwater region simple physical mixtures of lh black shales with eroded products from the high range require unreasonably high proportions of black shale to explain the high os ratios observed at the outflow of the range assuming that our bedrock and bedload samples are representative of the source region and the eroded mixture respectively this observation suggests that erosion may not be occurring at steady state in addition mass balance calculations indicate that about 20 of the os and 60 of the re are carried in dissolved form implying that chemical erosion plays an important role in the osmium budget finally the contrast in os isotope ratio between ganga bedloads and bulk sediments in the bengal fan suggests that radiogenic os carried by particulate matter and later remobilized may significantly influence the os composition of seawater

unroofing history of the eastern himalaya and the indo burman ranges heavy mineral study of cenozoic sediments from the bengal basin bangladesh abstract upper eocene to neogene fill of the bengal basin provides an earlier unroofing history of the eastern himalaya and indo burman ranges than that provided by drilling of the bengal fan and analysis of heavy minerals in these sequences provides useful provenance constraints quartzose sandstones of the eocene kopili and oligocene barail formations contain only 0 2 heavy minerals comprising abundant opaque minerals and stable minerals such as tourmaline garnet rutile and zircon this assemblage suggests sediment sources dominated by low to intermediate grade metamorphic silicic igneous and metasedimentary rocks the low heavy mineral content and abundance of stable minerals suggest intense chemical weathering these sediments may have been derived in part from incipient uplifts of the proto himalaya and or the indo burman ranges but a more likely source is the indian craton immediately to the west miocene sandstones of the surma group contain more abundant and more diverse heavy mineral assemblages than oligocene sandstones these include abundant opaque minerals and garnet mostly almandine and moderate to minor amounts of tourmaline kyanite zircon calcic amphibole rutile chlorite zoisite staurolite epidote sillimanite and clinopyroxene indicating a broad range of mostly metamorphic source rocks upper strata of the surma group also contain abundant blue green amphibole orthopyroxene and sparse chromite suggesting exhumation of arc and ophiolitic rocks sands and sandstones from the upper miocene to pliocene tipam group and the pliocene to pleistocene dupi tila formation contain assemblages similar to those of the underlying miocene sandstones along with more abundant orthopyroxene and sillimanite in dupi tila sands temporal variations in heavy mineral assemblages in the bengal basin suggest that orogenic detritus first appears in the eastern himalayan foredeep much later early miocene than in the western part of the foredeep eocene the appearance of high pressure phases in upper miocene strata reflects continued unroofing of deeper crustal metamorphic and silicic to ultramafic plutonic rocks

207pb 206pb zircon ages and the evolution of the singhbhum craton eastern india an ion microprobe study1this paper is dedicated to prof a k saha one of the senior co authors who passed away after submission of the paper 1 abstract we used an ion microprobe to measure 207 pb 206 pb ages of single zircons from four key lithologies from the singhbhum craton of eastern india detrital zircons from the oldest recognized sedimentary sequence of the craton the older metamorphic group yielded 3 5 3 6 ga ages indicating that crustal formation had already been initiated by 3 6 ga in this region this data also established an older limit of 3 5 ga for the age of sediment deposition the first appearance of extensive continental crust in the singhbhum region marked by the emplacement of the older metamorphic tonalite gneisses is dated at 3 44 ga our data from both of these units reveal that a 3 2 ga event induced overgrowths within each of these pre existing zircon populations we determine an age of 3328 7 ma for the singhbhum granite phase ii the most widespread of this granite s three phases the emplacement of the two phases of the mayurbhanj granite marking the youngest major episode of acid plutonism in this craton took place in rapid succession at 3 1 ga marking the broad scale stabilization of this terrain

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

mass accumulation rates in asia during the cenozoic summary this work establishes estimates of mass accumulation rates in 18 mostly offshore sedimentary basins in asia since the beginning of the cenozoic 66 ma the estimates were derived from isopach maps cross sections and drill holes or stratigraphic columns assuming regional similarity of the strata average solid phase volumes and accumulation rates were calculated for nine epochs approximately corresponding to geological periods palaeocene 66 58 ma eocene 58 37 ma oligocene 37 30 and 30 24 ma miocene 24 17 17 11 and 11 5 ma pliocene 5 2 ma and quaternary 2 0 ma these rates shed new light on the geological history of asia since the onset of the collision of india with asia 50 ma the overall average accumulation rates curve for asian sedimentary basins since the beginning of the tertiary shows an exponential form with slow accumulation rates less than 0 5 106 km3 myr 1 until the beginning of the oligocene more than 15 myr after the onset of the collision from the oligocene onwards rates increase quickly in an exponential manner reaching their maximum values in the quaternary more than 1 5 106 km3 myr 1 from these observations we suggest that extrusion and crustal shortening are complementary processes that have been successively dominant throughout the india eurasia collision history at smaller scales one may distinguish between independent histories at the subcontinental and basin scales this permits a comparison of the relative importance of tectonic and climatic erosion processes affecting the different mountain belts of asia during the cenozoic

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

orthogonal intersections of megatrends in the western pacific ocean basin a case study of the mid pacific mountains abstract present day tectonic concepts of events on the western paleo pacific lithosphere must be assessed relative to new data data collected by the newer techniques of geophysical surveying reveal leaky fracture zones trending nnw sse and wsw ene non sequential in line ages on most seamount chains and orthogonal intersections of fracture zones the fracture zones meander braid merge splay start and stop at any place and are generally aligned with or contain linear chains of seamount the combination of these in line features is called megatrends when the geosat data are compared to the available bathymetry this seemingly jumbled tectonic structure is verified as the pole of rotation changes the stress field changes alignment to agree with the chandler wobble of earth younger megatrends propagating ever eastward cross the older already imprinted megatrends during the cretaceous the voluminous outpouring of igneous rock created the large pacific plateaus and rises where the megatrends active and inactive orthogonally intersected the magma floods at the intersections flowed outwardly and the outward flooding accounts for the fanning magnetic lineations around the manihiki magellan shatskiy and hess rises a case study of the mid pacific mountains mpm lying in the north central pacific ocean basin shows that the mpm formed about 125 110 ma by overprinting the orthogonal intersections of fracture zones at the molokai and easter krusenstern emperor megatrends and the murray and tubai mamua megatrends the mpm have been undergoing distortion into a vortex structure a feature which has been confirmed by updated bathymetry geosat altimetry data and drillsite information

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

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

sand petrology and focused erosion in collision orogens the brahmaputra case abstract the high relief and tectonically active himalayan range characterized by markedly varying climate but relatively homogeneous geology along strike is a unique natural laboratory in which to investigate several of the factors controlling the composition of orogenic sediments coupling of surface and tectonic processes is most evident in the eastern namche barwa syntaxis where the tsangpo siang brahmaputra river draining a large elevated area in south tibet plunges down the deepest gorge on earth here composition of river sands changes drastically from lithic to quartzofeldspathic after confluence with the lohit river draining the transhimalayan equivalent mishmi arc batholiths sediment composition remains remarkably constant across assam indicating subordinate contributions from himalayan tributaries independent calculations based on petrographical mineralogical and geochemical data indicate that the syntaxis representing only 4 of total basin area contributes 35 6 to the total brahmaputra sediment flux and 20 of total detritus reaching the bay of bengal such huge anomalies in erosion patterns have major effects on composition of orogenic sediments which are recorded as far as the bengal fan in the brahmaputra basin in spite of very fast erosion and detrital evacuation chemical weathering is not negligible sand sized carbonate grains are dissolved partially in mountain reaches and completely in monsoon drenched assam plains where clinopyroxenes are selectively altered plagioclase instead is preferentially weathered only in detritus from the shillong plateau which is markedly enriched in microcline most difficult to assess is the effect of hydraulic sorting in bangladesh where quartz garnet and epidote tend to be sequestered in the bedload and trapped on the coastal plain whereas cleavable feldspars and amphiboles are concentrated in the suspended load and eventually deposited in the deep sea high resolution petrographic and dense mineral studies of fluvial sands provide a basis for calculating sediment budgets for tracing patterns of erosion in mountain belts and for better understanding the complex dynamic feedback between surface processes and crustal scale tectonics

zircon u pb and hf isotope constraints on the mesozoic tectonics and crustal evolution of southern tibet the first in situ hf and u pb isotope analyses of zircon separates from mesozoic granites in southern tibet identify a significant previously unknown stage of magmatism igneous zircons n 34 from a granite within the gangdese batholith show a weighted mean 206 pb 238 u age of 188 1 1 4 ma and e hf t the parts in 10 4 deviation of initial hf isotope ratios between the zircon sample and the chondritic reservoir values between 10 4 and 16 8 suggesting predominantly early jurassic intrusive activity with a juvenile mantle contribution of 40 inherited zircons from two cretaceous s type granites in the northern magmatic belt 23 delineate a slightly older 206 pb 238 u age cluster between 188 and 210 ma these zircons have e hf t values from 3 9 to 13 7 yielding crustal hf model ages from ca 1 4 to 2 1 ga suggesting a major episode of crustal growth in proterozoic time and remelting of this crust in early jurassic time combining these with literature data we interpret the jurassic gangdese magmatism as an early product of the neo tethyan subduction that played a long lasting role in the tectonic evolution of southern tibet prior to the india asia collision

eocene early miocene foreland basin development and the history of himalayan thrusting western and central nepal sedimentologic petrographic and u pb detrital zircon ages from middle eocene through early miocene sedimentary rocks in the lesser himalayan zone of western and central nepal indicate that a peripheral foreland basin system had developed in the eastern himalayan collision zone by middle eocene time the shallow marine eocene bhainskati formation accumulated in a back bulge depozone between a southward migrating forebulge and the indian craton migration of the forebulge through this region during eocene oligocene time produced a regional unconformity that spans 15 20 myr by early miocene time the forebulge unconformity was onlapped by the distal fringes of the southward migrating foredeep depozone represented by fluvial deposits of the dumri formation continued southward migration of the foredeep during the neogene accommodated the fluvial siwalik group light mineral provenance data and u pb detrital zircon ages suggest that the bhainskati was derived partly from tethyan sedimentary rocks of the tibetan himalayan zone during initial growth of the himalayan fold thrust belt the dumri was derived from metasedimentary and crystalline rocks of the greater himalayan zone during emplacement of the main central thrust and contemporaneous tectonic unroofing by normal faulting along the south tibetan detachment system the lesser himalayan crystalline thrust sheets were emplaced soon after deposition of the dumri formation 15 10 ma paleocurrent and lithofacies data from the dumri formation indicate deposition by west southwestward flowing rivers that drained into the indus portion of the himalayan foreland basin system during the early miocene thick channel sandstones in the lower dumri may represent the early miocene counterpart of the modern ganges river eastward diversion of the ganges drainage system to near its present location had occurred by 15 ma as the high standing aravalli range on the northern indian shield approached the front of the fold thrust belt assuming reasonable values for the flexural rigidity of indian lithosphere the time span of the forebulge unconformity yields a velocity of 14 33 mm yr for the southward migration of the fold thrust belt relative to india this range of values is consistent with neogene and present day estimates and suggests that only one third to one half of india eurasia convergence has been accommodated by shortening in the himalayan fold thrust belt since the onset of collision

timing of prograde metamorphism in the zanskar himalaya the timing of the prograde metamorphism of continental crust buried during the collision of india and eurasia is of importance to our understanding of the mechanics of mountain building and crustal metamorphism our knowledge of the himalaya indicates that continental collision at 55 50 ma culminated in crustal melting and leucogranitic magmatism at 24 18 ma however the timing of events in the intervening 30 m y is poorly understood here we present sm nd isotope analyses of garnets from the zanskar himalaya that also preserve information on crustal and orogenic evolution during that period the prograde p t t paths indicate that heating and burial from 6 to 10 kbar continued until at least 25 30 ma and imply peak temperatures 700 c close to those necessary to cause the crustal melting in the himalaya at ca 20 ma without a requirement for additional heat sources such as shear heating on major crustal shear zones the persistence of high temperatures as late as 20 ma in the himalaya may be related to postcollisional thrusting south of the suture zone and within the indian plate at 40 35 ma

tracing patterns of erosion and drainage in the paleogene himalaya through ion probe pb isotope analysis of detrital k feldspars in the indus molasse india abstract the indus molasse is a pre and syn tectonic sedimentary sequence situated in the indus suture zone of the western himalaya spanning in time the collision of india and asia this deposit is well placed to record the evolving uplift and erosion history of the early himalayan orogen nd isotope analyses from clay extracted from shales interbedded within the dominantly alluvial sequence indicate a low negative nd 1 64 to 0 72 in the basal paleocene chogdo formation slightly more negative than measured values from the transhimalaya and kohistan dras arc up section nd becomes more negative as low as 10 05 indicating influence of a different more enriched source ion microprobe pb isotopic analyses of single k feldspars help constrain this source as being either the lhasa or karakoram blocks with westward paleo current flow favoring the former 207pb 204pb ratios are too low to be consistent with known indian plate sources a conclusion supported by the lack of muscovite or garnet that would be indicative of a high himalayan contribution given the known age of rapid cooling of the high himalaya at 20 ma and the lack of exposure of suitable lithologies prior to that time an age of sedimentation prior to 20 ma is inferred the post collisional change in paleo flow and provenance is suggested to reflect the initiation of the indus river during the early eocene this study demonstrates the power of combined bulk sediment and single grain analyses in resolving provenance in tectonically complex settings

the indus clastics forearc basin sedimentation in the ladakh himalaya india abstract the mid cretaceous to early tertiary clastic sediments of the indus group represent the forearc basin succession of the transhimalayan arc trench system in the late albian to early eocene a shallowing upward megasequence tar turbidites to nummulitic limestones and alluvial to deltaic sediments basgo and temesgam formations were deposited respectively on the southern and northern flanks of the basin clastic petrography points to provenance from the largely undissected volcanic cover of the massif with more important plutonic detritus recorded in the northern alluvial units after collision with the indian passive margin in the early eocene sedimentation in the whole basin was characterized by huge alluvial fans nurla formation and then by lacustrine deposits formed in an intermontane depression nimu formation a rapid change in petrographic and geochemical composition is recorded in post early eocene units with sharp increase in plutonic detritus and trace elements typically associated with ophiolites testifying to the uplift of both arc massif and subduction complex sources after collision with the indian passive margin the petrography of the indus basin clastics and the evolution of detrital modes compare well with volcano plutonic suites deposited in modern and ancient forearc basins the composition of sandstone units contained within the accretionary prism instead suggests provenance from an undissected magmatic arc and deposition in abyssal plain and trench settings nindam volcaniclastic turbidites reworking of arc derived sandstones took place in small basins perched on the accretionary trench slope the asian continental margin of the neotethys ocean was thus an active pacific type arc trench system until the eocene when it collided with india underwent polyphase fold thrust deformation and was finally uplifted several kilometers to form the indus suture zone of the himalayas

crustal flow modes in large hot orogens abstract crustal scale channel flow numerical models support recent interpretations of himalayan tibetan tectonics proposing that gravitationally driven channel flows of low viscosity melt weakened middle crust can explain both outward growth of the tibetan plateau and ductile extrusion of the greater himalayan sequence we broaden the numerical model investigation to explore three flow modes homogeneous channel flow involving laterally homogeneous crust heterogeneous channel flow involving laterally heterogeneous lower crust that is expelled and incorporated into the mid crustal channel flow and the hot fold nappes style of flow in which mid lower crust is forcibly expelled outward over a lower crustal indentor to create fold nappes that are inserted into the mid crust the three flow modes are members of a continuum in which the homogeneous mode is driven by gravitational forces but requires very weak channel material the hot fold nappe mode is driven tectonically by for example collision with a strong crustal indentor and can occur in crust that is subcritical for homogeneous flows the heterogeneous mode combines tectonic and gravitationally driven flows preliminary results also demonstrate the existence and behaviour of mid crustal channels during advancing and retreating dynamical mantle lithosphere subduction an orogen temperature magnitude t m diagram is proposed and the positions of orogens in t m space that may exhibit the flow modes are described together with the characteristic positions of a range of other orogen types

whitecap features induced by submarine sand waves in stereo optical imagery under high wind speed conditions waves break on the sea surface producing whitecaps which can be recorded in optical satellite images however the breaking of waves is also influenced by submarine topography in this study we investigate the whitecap features induced by submarine sand waves using optical stereo images we determine the whitecap coverage in images and estimate the probabilities of wave breaking the observed and estimated results are in agreement with some previous investigation results a physical model is used to discuss the mechanisms of wave breaking along sand wave crests the physical model is able to qualitatively explain the differences between the observed whitecap coverage and the estimated probability of wave breaking between the sand wave area swa and the background area bga further analysis shows that the periodic sand wave system may play the role of a frequency control device and may result in more broken waves in the swa than in the bga this study demonstrates that it is possible to observe submarine topography under high wind conditions by using whitecap features

himalayan architecture constrained by isotopic tracers from clastic sediments we report the first geochemical traverse to integrate u pb ages and hf data on single detrital zircons with bulk rock sm nd rb sr isotopic measurements across the breadth of the himalayan orogen in northwest india our dataset from key tectonostratigraphic units in the sutlej valley provides detailed insights into both the provenance of the sedimentary formations and their tectonic history whole rock nd data provide information on the mean age of source areas contributing detritus to the sedimentary units in the mountain belt regardless of their current metamorphic grade u pb ages of detrital zircons from the same units identify the age of melt formation in the source rocks while lu hf isotope data from zircons resolve discrete crust formation ages in the multiple eroding terrains complex whole rock sr systematics hint at the extent of pre himalayan thermal events our data confirm the geochemical distinction between the older lesser himalayan formations and those of the depositionally younger units including the high himalayan crystalline series that has been recorded along the strike of the himalaya from the western syntaxis to nepal palaeoproterozoic metasediments of the jutogh group and rampur formation yield end 500 values below 17 whereas neoproterozoic and younger metasediments yield end 500 above 13 the latter group encompasses the vaikrita group gneisses of the high himalayan crystallines and the overlying haimanta group but also includes a number of units in thrust sheets north of the main boundary thrust in the lesser himalayan zone designated the douter lesser himalayat to distinguish them from the palaeoproterozoic dinner lesser himalayat zircon populations from the inner lesser himalaya are characterised by palaeoproterozoic late archaean ages 2 6 1 8 ga whereas the depositionally younger units contain populations both of this age and a younger period meso to neo proterozoic 1 1 0 8 ga detrital zircon ages are younger than their respective hf isotope derived crustal formation ages by 0 7 2 1 ga indicating that the source regions of the detrital zircons consisted of older terranes with considerable amounts of reworking and renewed magmatism the

the palung granite himalaya high resolution upb systematics in zircon and monazite abstract u pb analyses of fractions of zircon and monazite 3 8 grains each and of single zircon grains resolve a lower ordovician age of 470 4m y for the palung granite which occurs in the high himalayan nappes south of kathmandu its thrusting during the alpine orogeny under lower greenschist facies conditions did not affect the u pb systems in zircon and monazite the granite crystallized from a magma which was mainly generated by anatexis of precambrian continental crust the magma was heterogeneous with respect to primary ages and or metamorphic histories of the magma source rocks this indicates either a derivation from meta sediments or an intense mixing of different crustally derived magmas the genesis of the palung granite is possibly related to an orogeny which affected the indian shield in lower palaeozoic times the detected inherited radiogenic lead in the palung zircons occurs in perfectly homogeneous transparent crystals i e this radiogenic excess lead is not related to the presence of old microscopically visible overgrown zircon cores the minimum ages of the inherited lead components range from about 800 to 1700 m y

provenance of eocene foreland basin sediments nepal constraints to the timing and diachroneity of early himalayan orogenesis in contrast to eocene himalayan foreland basin sedimentary deposits in india and pakistan coeval sedimentary rocks of the bhainskati formation in nepal contain scant petrographic evidence of orogenic input such data have been used as evidence to promote models of diachroneity of india asia collision in this paper we document orogenic input into the eocene foreland basin rocks of nepal from fission track analyses of detrital zircons our data provide evidence that significantly reduces the possible duration of any diachroneity of collision and brings the interpretation of the sedimentary record into better agreement with ages of early thrusting and metamorphism in the orogen we also use our detrital fission track data to bolster previous age determinations of the overlying dumri formation confirming the basin wide occurrence of a major unconformity comparison of our data set with that from coeval along strike rocks in india suggests that there is no evidence of diachroneity in early stages of the orogen s exhumation

mechanical model for the tectonics of doubly vergent compressional orogens a mechanical model of crustal shortening and deformation driven by the relative convergence of rigid underlying mantle plates explains many features of convergent orogens results based on numerical models and supported by sandbox models show that a coulomb crustal layer subject to basal velocity boundary conditions corresponding to asymmetric detachment and subduction of the underlying mantle passes through three stages of orogenic growth 1 block uplift bounded by step up shear zones 2 development of a low taper wedge over the underthrusting mantle plate and 3 development of a low taperwedge overlying the overthrusting mantle plate and verging in the opposite direction when modified by isostasy basal viscous flow surface erosion and denudation and sedimentation the resultant model orogens exhibit a variety of styles with characteristics in common with small rapidly denuded orogens large orogens with plateaus and extensional characteristics and active subduction margins with doubly vergent accretionary wedges and deformed fore arc basins

sediment volume and mass beneath the bay of bengal rates of sediment accumulation and the amount of sedimentary fill in depocenters lying downstream of erosion in the himalayas and tibet can provide some insight into tectonics and geological history the objective of this paper is to put on record the best estimates which are possible with existing data of the volume and mass of sediments sedimentary rock and metasedimentary rock beneath the sea floor of the bay of bengal the sedimentary section in the bay of bengal is divided into two parts 1 eocene through holocene sediments and sedimentary rocks which post date the initial india asia collision volume 12 5 106 km3 mass 2 88 1016 t this is most of the bengal fan including its eastern lobe the nicobar fan plus some of the outer bengal delta 2 early cretaceous through paleocene pre collision sedimentary and metasedimentary rocks volume 4 36 106 km3 mass 1 13 to 1 18 1016 t these are interpreted as continental rise and pelagic deposits

crustal channel flows 2 numerical models with implications for metamorphism in the himalayan tibetan orogen 1 results from a thermal mechanical model ht1 that includes midcrustal channel flow are compatible with many features of the himalayan tibetan system radioactive self heating and rheological weakening of thickened model orogenic crust lead to the formation of a hot low viscosity midcrustal channel and a broad plateau channel material corresponding to the greater himalayan sequence ghs flows outward from beneath the plateau in response to topographically induced differential pressure at the plateau flank it is exhumed by focused surface denudation and juxtaposed with cooler newly accreted material corresponding to the lesser himalayan sequence lhs the model channel is bounded by coeval thrust and normal sense ductile shear zones interpreted to represent the main central thrust mct zone and south tibetan detachment system respectively inverted metamorphism associated with the model mct zone results from distributed ductile shear along the mct and extrusion of the hot channel a variety of model p t t path styles resembling those observed in the ghs and lhs are produced for points traveling through contrasting tectonic regimes that coexist in different parts of the model predicted times of peak metamorphism cooling and erosion of metamorphic facies are generally compatible with observations although model ghs cooling ages are too young the times of m1 and m2 metamorphic events observed in the ghs correspond to model times of maximum burial and maximum heating respectively the results highlight the need to integrate tectonics and metamorphism in continental collision models and demonstrate the importance of lateral transport of both heat and material in large hot orogens

the nature of mesoproterozoic central indian tectonic zone with exhumed and reworked older granulites abstract the ese wnw trending central indian tectonic zone citz divides the indian subcontinent into southern and northern crustal provinces its narrow northern belt consists of closed early proterozoic mahakoshal rift zone which is presently confined between two moho reaching faults that characterize the son narmada sona lineament the mesoproterozoic sausar mobile belt smb the chhotanagpur granite gneiss complex cggc and the gneissic complex of northeast india represent the wider southern belt the mahakoshal rift belt has a thick pile of mafic ultramafic rocks and possibly oceanic components the basin closed due to a south directed subduction creating a magmatic arc that was sutured to its southern boundary during 1 8 1 7 ga a south dipping ductile shear zone known as the son narmada south fault affects both its supracrustals and intrusive granitoids the son narmada south fault generally delimits the northern boundary of the gondwana basins whereas the son narmada north fault delineates the northern boundary of the mahakoshal mobile belt and the southern boundary of the meso neoproterozoic vindhyan basin the smb representing the southern belt of the citz in the central sector comprises strongly folded and metamorphosed non volcanic and mn rich sausar sediments intermixed with reworked basement gneisses migmatites and granulites representing the pre sausar assemblage the sausar orogeny closed during terminal mesoproterozoic ca 1 0 ga on the other hand metamorphism in the basement gneiss migmatites and northern and southern granulite belts pre dates the sausar orogeny the earliest structure of the smb truncates and rotates the n s oriented structures of the southward located sakoli and the dongargarh belts of neoarchaean palaeoproterozoic age the northern granulite belt of the smb and the cggc and the high grade supracrustals from the sonapahar area in ne india all indicate very high p t conditions during 1 7 1 5 ga peak metamorphism which was followed by decompressional cooling the setting resembles continental collision which was followed by tectonic exhumation and denudation on the other hand the southern granulite belt from the smb documents a late archaean 2 6 ga event and a later 1 4 ga event the dalma volcanic belt located close to the southern tectonized boundary of the cggc belt and possibly the assemblage of copious mafic enclaves within the ca 550 ma granite plutons from ne india represent back arc like setting during 1 6 1 5 ga the setting is caused due to the southward subduction of the north indian block beneath the southern block leading to continental collision the metasedimentary belt located further to the south of the dalma volcanic belt and flanking the archaean singbhum craton to the north on the other hand is palaeoproterozoic in age but has been metamorphosed during ca 1 6 ga because of the northern collision related orogeny reworking and incorporation of basement gneisses and granulites from the mid crust level to the younger sausar cover sediments denote renewed spell of thrust tectonics that remobilized the collision zone rocks of the earlier orogeny the tectonic transport was from the north to the south with north dipping thrusts thus there was a reversal in the direction of convergence from the preceding orogeny the sausar orogeny closed during ca 1 0 ga it caused anatectic event producing granitoid plutons migmatites and charnockites in the cggc belt and migmatites in the gneissic complex in ne india

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

the indus river system india pakistan major ion chemistry uranium and strontium isotopes the indus river is one of the large river systems draining the himalaya we report in this paper the major ion chemistry sr and u isotope systematics of the indus system particularly its headwaters the results show that 1 on an average about a third of the cations in the waters can be from silicate weathering however most of the ca mg is likely to be from the weathering of carbonates and evaporites 2 the 87sr86sr of the waters ranges between 0 7085 and 0 7595 the higher values 0 72 are typical of the tributaries draining the precambrian granite gneissic terrains the 87sr86sr of the indus main channel throughout its entire stretch shows only minor variations 0 7104 0 7116 the indus transports 8 8 106 mol sr to the arabian sea annually with a 87sr86sr of 0 7111 if our results of sr concentration and 87sr86sr measured at thatta can be considered typical of the indus throughout the year and 3 the u concentration in the indus and its tributaries is generally high 0 37 10 3 g 1 1 the source for the high uranium can be the weathering of granites zones of uranium mineralisation and black shales the indus results when compared with our earlier data on the ganga brahmaputra show that in all these three river systems carbonate weathering is the dominant source of ca mg and hco3 and that u concentrations are high in the case of 87sr86sr the indus waters are less radiogenic however its tributaries draining the precambrian granites gneisses have 87sr86sr in excess of 0 72 similar to that in the rivers of the ganga system the low 87sr86sr of the indus 0 7111 suggests that its contribution to the sr isotope evolution of oceans since the cenozoic is less significant than that of the ganga brahmaputra

upb geochronology of gangdese transhimalaya plutonism in the lhasa xigaze region tibet abstract a series of different plutons from the lhasa xigaze segment of the gangdese transhimalaya belt has been studied by high resolution u pb analyses of zircon using zircon fractions of 5 100 grains each selected upon specific grain characteristics for two diorites located east of xigaze dazhuka the zircons yield concordant ages of 93 4 1 0 and 94 2 1 0 m y respectively also concordant ages of 41 1 0 4 and 41 7 0 4 m y have been obtained for two granodiorites collected southwest of lhasa qushui the precision on the ages of two granites from the xigaze and lhasa area is limited by two factors the presence of inherited radiogenic lead and the occurrence of subsequent lead loss however some concordant zircons detected in both granites define approximate ages of about 67 and 53 m y respectively the inherited lead components show that melting of precambrian material was involved in magma genesis the u pb ages substantiate a magmatic activity lasting from mid cretceous cenomanian to eocene lutetian time such a period of plutonism at the southern margin of eurasia as well as the occurrence of magma generation from continental crust suggest that the gangdese range results from the subduction of tethys oceanic lithosphere indian plate underneath eurasia eurasian plate if this model is true the collision of india with eurasia along the lhasa xigaze sector postdates the emplacement of the 41 m y old gangdese granodiorites i e the collision occurred after lutetian time

new constraints on the india asia collision the lower miocene gangrinboche conglomerates yarlung tsangpo suture zone se tibet abstract lower miocene conglomerates crop out along the length of the yarlung tsangpo suture zone on the southern margin of the lhasa terrane these conglomerates known by various local names are correlated herein as the gangrinboche conglomerates all units exhibit broadly similar stratigraphic histories and a basal depositional contact upon an eroded surface of rocks of the lhasa terrane is ubiquitous at most localities the tops of sections are either removed by erosion or truncated by north directed thrusts these conglomeratic molasse units developed in response to the india asia collision and record aspects of its development in all units initial clast derivation was from the lhasa terrane on the northern margin of the yarlung tsangpo suture zone up section the first appearance of clasts derived from terranes within the suture zone and the northern margin of india all of which lie to the south of any outcrops of gangrinboche conglomerates is observed although these units were previously thought to be eocene analysis of fossil and structural constraints indicates early miocene deposition as development of the gangrinboche conglomerates records a significant phase in the evolution of the india asia collision understanding of their age and stratigraphic evolution has wider implications for regional tectonic models

re os isotope systematics of sediments of the brahmaputra river system re os analyses were performed on suspended loads and coarser grained bank sediments of the brahmaputra river system re and os concentrations of these sediments vary from 7 to 1154 ppt and from 3 to 173 ppt respectively 187os 188os ratios range from 0 178 to 6 8 and thus vary from nearly mantle to very radiogenic crustal values nevertheless most of the sediments have 187os 188os ratios less than 1 5 and nearly all of the samples of the brahmaputra main channel have ratios less than 1 2 thus as previously suggested the brahmaputra is much less radiogenic than the ganga the siang river the northern extension of the brahmaputra is quite radiogenic in os despite receiving sediments from the tsangpo river which flows along a suture zone with ultramafic outcrops the brahmaputra main channel has a fairly constant 187os 188os ratio even though its tributaries contribute sediments with very heterogeneous os isotopic compositions these data along with the corresponding nd isotopic compositions suggest that about 60 90 of the sediment in the brahmaputra system is derived from himalayan formations higher himalaya and lesser himalaya whereas 10 40 comes from ophiolite bearing sequences perhaps eastern equivalents of those of the transhimalayan plutonic belt os data also confirm previously published sr and nd results indicating that about half of the sediments delivered to the brahmaputra are supplied by the siang river while the himalayan and the eastern tributaries account for 40 and 10 respectively the lower 187os 188os of the brahmaputra river compared to that of the ganga is due to two factors one is the more limited presence of the lesser himalaya and hence the lower black shale content of the eastern himalaya the other is the non radiogenic os supplied by the eastern and southern tributaries reflecting the presence of mantle derived lithologies in this region despite the lower sediment supply from these tributaries they contribute greatly to the os budget of the brahmaputra river this study indicates that the brahmaputra river has little effect on the present day seawater os budget however reconsideration of this budget suggests that the ganga which provides the most radiogenic os of major rivers studied to date may have significant impact on the marine os isotopic composition the indo asian collision cannot be excluded as an important cause of the increase in the marine 187os 188os over the past 16 million years until the contributions of all of the rivers draining the himalayan tibetan plateau are known

paleocene eocene record of ophiolite obduction and initial india asia collision south central tibet 1 uppermost cretaceous to eocene marine sedimentary sequences occur both to the south and north of the yarlung zangbo suture in south central tibet they consist of indian margin strata of the northern tethyan himalaya and asian margin strata of the gangdese forearc both assemblages are characterized by major changes in depositional environment and sedimentary provenance at 65 ma and an appearance of detrital chromium rich spinel of ophiolite affinity tio2 generally 0 1 wt during the paleocene ophiolitic melange exposed along the suture could have provided a source for detrital spinel the melange occurs in the hanging wall of a north dipping south directed mylonitic shear zone which includes a tectonic sliver of mafic schist amphibole from the schist yields 40ar 39ar ages of 63 ma which we attribute to cooling during slip along the shear zone and southward obduction of the melange melange obduction was coeval with the development of an angular unconformity within the gangdese forearc basin to the north between late maastrichtian time and 62 ma upper paleocene to middle eocene sandstones in the northern tethyan himalaya yield 200 120 ma u pb detrital zircon ages and 190 170 ma 40ar 39ar detrital mica ages these detrital grains were most likely sourced from regions north of the yarlung zangbo suture suggesting that onset of india asia collision in south central tibet is middle eocene or older in age collectively our results support previous suggestions that oceanic rocks were obducted onto the northern margin of india during latest cretaceous earliest tertiary time coeval changes in gangdese forearc sedimentation raise the possibility that this obduction event marks onset of tectonic interaction between india and asia at 65 ma alternatively in concert with the conventional view of eocene collision initiation the obducted oceanic rocks may be of intraoceanic origin while coeval changes in gangdese forearc sedimentation may be a consequence of an increase in the rate of ocean continent convergence following the demise of the intraoceanic subduction zone

geochemical and petrological evidence for subduction accretion processes in the archean eastern indian craton the composition of sandstones often provides key evidence about the tectonic weathering and transport processes operating on the surface at the time of deposition petrologic and geochemical analyses of little metamorphosed middle late archean sandstones from the eastern indian craton show that the sedimentary rocks were derived from dominantly 3 3 ga old amphibolites of the older metamorphic group omg and tonalites of the older metamorphic tonalite gneisses omtg the two oldest lithologic units of this craton chondrite normalized rare earth element ree patterns of the sandstones show a light ree enriched signature with la sm n varying from 5 2 to 6 7 with no eu anomaly while the heavy rees display flat patterns with gd lu n values of 0 9 1 6 primitive mantle normalized incompatible and compatible trace element plots of these sandstones demonstrate an overall similarity with global proterozoic archean sandstones including strong nb ta negative anomalies in an fsm nd vs ond 0 diagram the sandstones plot precisely between the regional amphibolites and tonalites we infer from the ree abundances and the fsm nd vs ond 0 plot that the sandstones represent a bimodal mechanical mixture of omg and omtg the low ce pb ratios of these rocks of 1 4 indicate a variably pb enriched archean crust and that the ce pb ratio acquired the continental crustal signature distinctly different from those of the bulk silicate earth and mantle values at least as early as mid archean strong nb ta depletion relative to the primitive mantle suggests the sandstones were derived from subduction related magmatic arc sources this latter suggestion is strongly supported by the low nb ta and high zr sm ratios of these sandstones identical to archean tonalite trondhjemites that require based on recent trace element partitioning results their protoliths to have formed by subduction melting of lowmagnesium amphibolites or metamorphosed hydrous basalts 1 2 the average nd model ages of the sandstones are greater than the sm nd crystallization ages of the omg and omtg at v3 3 ga the geochemical data presented here can be collectively interpreted to suggest the presence of subduction accretion processes operational in the midarchean eastern indian craton because the omg and omtg the source rocks of the sandstones formed in a subduction related arc setting the basement rocks upon which this arc was constructed must have been older this observation and the depleted mantle nd model ages tdm of the sandstones ranging from 3 6 to 4 0 ga strongly indicate the presence of continental crust in this eastern indian craton older than 3 3 ga and possibly as old as

the exhumation of the indo burman ranges myanmar abstract the indo burman ranges ibr are a mountain range comprised of mesozoic cenozoic rocks which run the length of western myanmar extending into india and bangladesh to the west lies the indian ocean and to the east lies the central myanmar basin cmb along which the irrawaddy river flows the ibr are considered to be an accretionary prism developed at the juncture of the indian and sunda plates and a number of hypotheses have been proposed for their evolution however in order for these hypotheses to be evaluated the timing of ibr evolution needs to be determined we undertook a two pronged approach to determining the timing of uplift of the ibr 1 we present the first low temperature thermochronological age elevation profiles of the ibr using zft aft and zhe techniques our data show a major period of exhumation occurred around the time of the oligo miocene boundary we tentatively suggest subject to further verification an additional period of exhumation at or before the late eocene 2 we carried out a detailed multi technique provenance study of the sedimentary rocks of the ibr and arakan coastal region to their west and compared data to coeval rocks of the cmb we determined that during eocene times rocks of the cmb and ibr were derived from similar local provenance that of the myanmar arc to the east therefore at this time there was an open connection from arc to ocean by contrast by miocene times provenance diverged rocks of the cmb were deposited by a through flowing irrawaddy river with detritus derived from its upland source region of the mogok metamorphic belt and cretaceous paleogene granites to the north such a provenance is not recorded in coeval rocks of the ibr indicating that the ibr had uplifted by this time providing a barrier to transport of material to the west to the previously published list of viable proposals to explain the exhumation of the range we add a new suggestion the period of exhumation around the time of the oligo miocene boundary could have been governed by a change to wedge dynamics instigated by a major increase in the thickness of the incoming bengal fan sediment pile

petrologic case for eocene slab breakoff during the indo asian collision the greater himalayan sequence is the metamorphic core of the himalaya and has been a focus of considerable study yet its petrologic evolution remains controversial pre oligocene metamorphism was nearly obliterated by miocene metamorphism and melting which many workers ascribe to shear heating unusually high concentrations of radioactive elements accompanying burial and or several kilometers of miocene exhumation due to extensional faulting sparse oligocene to eocene ages are often assigned to a tectonically unspecified event an alternative slab breakoff model subduction of greater himalayan rocks to 100 km depth in the eocene followed by buoyant extrusion due to decoupling of the oceanic lithosphere has also been proposed based on rare eocene eclogites but without explanation of miocene melting or metamorphic petrogenesis we argue that slab breakoff readily explains the eocene eclogites miocene partial melts and late eocene k rich magmas in southeastern tibet and that metamorphic and plutonic ages help define the timing and rates of breakoff and extrusion this model implies that 1 much of the greater himalayan sequence was subducted to depths greater than commonly considered 2 fluid absent decompression melting at 30 35 km depth was the consequence of as much as 100 km of extrusion rather than radioactive heating or a smaller crustal level extensional or erosional event and 3 eruption of eocene k rich tibetan plateau lavas has no implication for topography of the tibetan plateau

reconstructing early himalayan tectonic evolution and paleogeography from tertiary foreland basin sedimentary rocks northern india the latest paleocene middle eocene subathu formation and the oligocene miocene dagshai and kasauli formations of the indian foreland basin record the early evolution of the himalayan fold thrust belt sandstone petrography of the subathu formation shows a predominantly recycled sedimentary source with a distinct ophiolitic and volcanic influence that was drastically reduced by the time of deposition of the dagshai formation sandstones in the dagshai and kasauli formations consist predominantly of metapelitic detritus the metamorphic grade of metapelitic lithic grains increases with time from dominantly very low grade at the base of the dagshai formation to dominantly low grade in the kasauli sandstones mudstone geochemistry documents the presence of a mafic ultramafic source during the time of deposition of the subathu formation that becomes significantly less important by the time of deposition of the dagshai formation compositions of subathu formation detrital spinels show they were either derived from both mid ocean ridge basalt type and arc type ophiolites or from an ophiolite of composite origin and that southerly derivation from the deccan trap continental flood basalts is unlikely kasauli formation garnet compositions suggest derivation from medium grade metamorphic rocks to amphibolite facies subathu formation composition reflects provenance influence from the indus suture zone during latest paleocene middle eocene time indicating initiation of continent continent collision and development of the foreland basin by this time suture zone influence was drastically reduced by the time of deposition of the dagshai formation when the embryonic thrust belt provided a barrier sufficient to partially separate the suture zone from the himalayan foreland basin the first appearance of himalayan metamorphic detritus occurs in the dagshai formation at the close of oligocene time whereas the kasauli formation records erosion to deeper metamorphic levels during earliest miocene time the occurrence of garnet and higher grade metamorphic lithic grains during early neogene time is coincident with the timing of displacement along the main central thrust and south tibetan detachment zone composition of early foreland basin sediments from pakistan balakot murree and kamlial formations to nepal bhainskati and dumri formations and bangladesh kopili barail formations surma group indicates diachronous arrival of ophiolitic to low grade metamorphic detritus derived respectively from the indus suture and early himalayan thrust sheets in the north this is consistent with progressively later closure of neotethys along the suture from latest paleocene time in the west to eocene time or even later in the east

detrital modes and provenance of the paleogene khojak formation in pakistan implications for early himalayan orogeny and unroofing different tectonic settings have characteristic detrital modes and sediment dispersal patterns detrital modes and sediment dispersal patterns of the siliciclastic khojak formation in the katawaz basin pakistan suggest that its sand was derived from the early himalayan orogen and longitudinally transported to the katawaz remnant ocean where it was deposited as a delta submarine fan complex modal analysis of the khojak formation suggests composition that is dominated by subangular quartz with abundant lithic fragments and minor feldspar i e qt 60 f 9 l 31 qt total quartz f feldspar l lithic fragments the predominance of quartz sedimentary and low grade metamorphic lithic fragments suggests early derivation from a collision orogen scarcity of detrital feldspar and volcanic lithic fragments precludes a magmatic arc as the main source the decrease in monocrystalline quartz concomitant increase in total lithic percentages and relative abundance of low grade metamorphic lithic fragments from the bottom to the top of the khojak formation reflect progressive erosional history of the early himalaya this history is part of a previously known major unroofing trend collectively depicted by the detrital modes of the murree formation siwalik group and the modern indus fan in the indian ocean these detrital modes are also related in time and space

onset of mid crustal extensional flow in southern tibet evidence from u pb zircon ages new ion microprobe u pb dates from zircon in deformed orthogneiss and migmatite and an undeformed granite in mabja dome are the first to constrain the timing of peak metamorphism and onset and duration of mid crustal ductile extension in southern tibet at 35 0 0 8 ma and 12 19 million years the structural metamorphic and intrusive his tories in mid crustal rocks exposed in these north himalayan gneiss domes are similar to those recorded in the greater himalayan sequence suggesting that middle crust was continuous from beneath southern tibet southward to the high himalaya strain compatibility indicates that 35 ma ductile extension in mid crustal rocks of southern tibet was accommodated to the south at shallow crustal levels via normal slip along the southern tibetan detachment system the oldest age estimate for slip along this normal fault zone and extrusion of its footwall if gravitational collapse is an additional important process driving extension then southernmost tibet may have been at or near maximum elevation by late eocene early oligocene time

a time slice approach to the history of cenozoic sedimentation in the indian ocean abstract results from drilling at over 100 sites have been combined with a new plate tectonic model to produce a series of palinspastic maps which represent snap shots of the changing patterns of sediment accumulation in the indian ocean through the cenozoic the maps clearly delineate the main sedimentary depocenters and show how the sedimentary record has responded on an oceanwide scale to major changes in paleogeography tectonics paleocirculation and or climate paleogene sediment accumulation rates were generally low with significant accumulation in only limited areas suggesting weak ocean circulation and stable well stratified conditions by contrast the vigorous thermohaline circulation of the neogene resulted in substantial widespread sedimentation accumulation rates were generally high and displayed a wide range of values following rapid growth of the east antarctic ice sheet in the mid miocene carbonate sedimentation has been progressively restricted to the shallow ridges and plateaus of the western indian ocean but accumulates there with locally high rates the link between tectonics and terrigenous sedimentation is exemplified by the major deep sea fans off east africa zambezi fan and india indus and bengal fans which developed in response to uplift and erosion beginning in late oligocene time in the east african rift system and the himalayas respectively although maps from only five widely spaced times are shown the methodology is such that a continuous series of maps at intervals as close as 1 m y can readily be generated ultimately the combination of such time slice maps with the results of detailed time series studies will provide a powerful tool for understanding past oceanic environments

age of the baula pge mineralization orissa india and its implications concerning the singhbhum archaean nucleus abstract two types of platinum group element pge mineralization are identified in the mafic facies of the baula complex intruding the gabbro anorthosite unit of the singhbhum archaean nucleus the first pt dominated is magmatic the pgm are included in the magmatic silicates of a gabbro norite intrusion the bangur gabbro cutting the complex s ultramafic facies and are found preferentially where the gabbro incorporates clasts of dunite and chromitite near the gabbro ultramafics contact the second pd dominated is hydrothermal the pgm are associated with hydrous silicates located in the gabbro matrix of a magmatic breccia in which the blocks are from the baula ultramafic facies the originating fluids for the second type of mineralization were exsolved from the magma and concentrated pge and volatile elements we see no evidence of remobilization or external fluid contamination a structural study has demonstrated that a the breccia was provoked by the intrusion of the upper part of the bangur gabbro and b the breccia matrix corresponds to the bangur gabbro consequently both types of pge mineralization are associated with the same magmatic event a zircon age of 3122 5 ma has been obtained for the main bangur gabbro intrusion and similar ages 3123 7 and 3119 6 ma have been obtained on the pge mineralized gabbro matrix of the breccia this supports the hypothesis of a genetic link between the bangur gabbro with its pt dominated pge and the breccia with its pd dominated pge giving a mean weighted age of 3121 3 ma for the mineralization sm nd data on eight gabbro samples of the baula complex yielded a similar age of 3205 280 ma the dating of these events places them close to the broad scale stabilization of the singhbhum archaean nucleus estimated at 3 1 ga and defines one of the oldest pge mineralizing events ever recorded the bangur gabbro shows many affinities with the gabbro anorthosite unit of the singhbhum archaean nucleus in terms of age of emplacement and isotopic composition it may be therefore that the bangur gabbro actually forms part of the gabbro anorthosite unit and that a fundamental control of its two types of pge mineralization is the presence of ultramafic clasts

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

the petrology and mineralogy of mt popa volcano and the nature of the late cenozoic burma volcanic arc mt popa is situated on a line of widely spaced calc alkaline volcanic centres in the central lowlands of burma latites rhyodacites and ignimbrites are interbedded with strongly folded arenaceous sediments of topmost miocene and pliocene age these are overlain unconformably by a composite cone of basalt and basaltic andesite lavas and pyroclastic deposits of pleistocene or early recent age the two groups of lavas are regarded as a single petrographic suite in which several continuous and discontinuous mineral reaction series are recognized phenocryst phases analysed by microprobe are olivine bronzite hypersthene biotite diopsidic augite salite magnesio hastingsite magnesio hornblende titaniferous magnetite bytownite oligoclase and sanidine whole rock analyses are characterized by relatively high k ba sr rb and zr high mg mg fe ratios of 0 85 to 0 65 in both phenocrysts and whole rock high k rb 767 and low sr 87 sr 86 0 70431 and rb sr 0 0155 ratios suggest that the basalts are relatively primitive mantle derived compositions modified slightly by fractionation with negligible crustal contamination the more siliceous lavas may have been derived by fractional crystallization of such magmas at crustal levels this high k calc alkaline suite is typical of a continental margin orogenic environment analyses of contemporaneous lavas from elsewhere in burma define three n s volcanic lines which become more alkaline from w to e the magmatism is attributed to the waning stages of orogeny as eastward subduction of the indian plate beneath china gave way to strike slip movement associated with the andaman sea spreading centre within this tectonic model magmas were probably generated by hydrous mantle melting above dehydrating oceanic crust of the subduction zone

when and where did india and asia collide timing of the collision between india and asia is the key boundary condition in all models for the evolution of the himalaya tibetan orogenic system thus it profoundly affects the interpretation of the rates of a multitude of associated geological processes ranging from tibetan plateau uplift through continental extrusion across eastern asia as well as our understanding of global climate change during the cenozoic although an abrupt slowdown in the rate of convergence between india and asia around 55 ma is widely regarded as indicating the beginning of the collision most of the effects attributed to this major tectonic episode do not occur until more than 20 ma later refined estimates of the relative positions of india and asia indicate that they were not close enough to one another to have collided at 55 ma on the basis of new field evidence from tibet and a reassessment of published data we suggest that continent continent collision began around the eocene oligocene boundary 34 ma and propose an alternative explanation for events at 55 ma

sedimentology of the indus group ladakh northern india implications for the timing of initiation of the palaeo indus river the upper reaches of the indus river form a longitudinal network that drains a large portion of the western himalaya it plays an important role as a sediment routing system feeding the indus delta and submarine fan and has played a controlling role in the denudational history of the western himalaya given the rivers long term significance its timing of initiation remains poorly constrained the facies palaeocurrents and provenance of the post early eocene indus group preserved along the upper reaches of the modern indus reveal a history of fluvial deltaic and deep water sedimentation in an intermontane basin dominated by internal drainage hence the indus group is not considered to represent the deposits of a palaeo indus river as previously thought illite crystallinity values and apatite fission track dating of the indus group suggest that the succession was buried to temperatures of 155 280 c and that unroofing started in early miocene times and proceeded at 0 1 0 4 mm a 1 to the present the youngest sedimentary rocks preserved along the indus basin are early miocene in age from the tectonostratigraphically equivalent deposits around kargil to the west of the study area the transition from sediment accumulation to erosional unroofing in early miocene times coincides with accelerated regional unroofing of the high himalayas to the south and the initiation of the indus delta submarine fan to the west differential uplift between the northward thrusting of the zanskar and indus sedimentary succession against the undeformed ladakh batholith provides a mechanism for post early miocene initiation of a major longitudinal river hence early miocene times is believed to represent the earliest possible age of initiation of the palaeo indus river following this course

constraints on the exhumation and erosion of the high himalayan slab nw india from foreland basin deposits petrography sr nd isotope compositions and single grain laser 40ar 39ar ages of detrital white mica from early middle miocene molasse of the dharamsala and lower siwalik formations of northern india dated by magnetostratigraphy determine the sediment sources their metamorphic grade and exhumation rates in the himalayan palaeo hinterland deposition of the lower dharamsala member 21 17 ma occurred during the period of rapid isothermal decompression and crustal anatexis 24 18 ma of the metamorphic core this episode of decompression is thought to be coeval with thrusting on the main central thrust and normal faulting on the south tibetan detachment system the sediment composition and detrital mica ages indicate erosion from the rapidly exhumed metamorphic slab of the high himalayan crystalline series deposition of the upper dharamsala member 17 13 ma and basal siwalik group 13 12 5 ma spanned the period in which thrusting transferred south from the main central thrust the sediment composition and detrital mica ages contrast strongly with those of the lower dharamsala indicating erosion from sedimentary and low grade rocks the isotopic composition indicates that these rocks were part of the high himalayan series unaffected by tertiary metamorphism i e from upper structural levels of the high himalayan slab this suggests that a major reorganisation of the orogenic wedge occurred at 17 ma involving forward propagation of the mct and cessation of rapid exhumation of the metamorphic slab