structure of the north indian continental margin in the ladakh zanskar himalayas implications for the timing of obduction of the spontang ophiolite india asia collision and deformation events in the himalaya the collision of india and asia can be defined as a process that started with the closing of the tethyan ocean that during mesozoic and early tertiary times separated the two continental plates following initial contact of indian and asian continental crust the indian plate continued its northward drift into asia a process which continues to this day in the ladakh zanskar himalaya the youngest marine sediments both in the indus suture zone and along the northern continental margin of india are lowermost eocene nummulitic limestones dated at similar 54 ma along the north indian shelf margin southwest facing folded palaeocene lower eocene shallow marine limestones unconformably overlie highly deformed mesozoic shelf carbonates and allochthonous upper cretaceous shales indicating an initial deformation event during the latest cretaceous early palaeocene corresponding with the timing of obduction of the spontang ophiolite onto the indian margin it is suggested here that all the ophiolites from oman along western pakistan bela muslim bagh zhob and waziristan to the spontang and amlang la ophiolites in the himalaya were obducted during the late cretaceous and earliest palaeocene prior to the closing of tethys

paleomagnetic evidence for neogene rotational deformations in the aegean domain new paleomagnetic data are reported from the islands of skyros and lesbos in central aegea and the izmir foca region in western anatolia together with previous results obtained in northwestern greece and with results from northern aegea reported by others authors these data suggest that the lower miocene arc was almost rectilinear trending e w and that the curvature was acquired by opposed rotational deformations at each termination moreover the observed inclinations which are shallower than those expected on the basis of a geocentered dipole field suggest a large northward drift since middle miocene

back folds in the core of the himalayan orogen an alternative interpretation the hanging wall of the south tibetan detachment system in the central nepal himalaya is characterized by regional scale northeast verging folds classically interpreted as gravity induced structures developed during down to the north extensional shearing along the detachment system new structural observations and balanced cross sections of the tethyan sedimentary sequence in the kali gandaki area and new u pb geochronology support an alternative interpretation the northeast verging folds developed before ductile extensional shearing along the detachment system thereby recording some of the earliest contraction of this part of the orogen we propose a new model in which the northeast verging folds of the kali gandaki area represent the northern part of a late eocene to oligocene contractional fan structure

minimum age of initiation of collision between india and asia north of everest based on the subsidence history of the zhepure mountain section the collision of india with asia one of the most profound tectonic events to have occurred in past 100 m y is thought to have had geological geochemical and climatological consequences of global extent surprisingly the age of initiation of this collision remains poorly constrained estimates range from the late cretaceous 65 ma to latest eocene 40 ma with little consensus in between this paper reviews the stratigraphic section preserved on zhepure mountain on the north flank of everest mount qomolongma and its implied subsidence history zhepure mountain lies about 65 km south of the indus yarlung zangbo suture and contains the most complete and youngest passive margin shelf sediments in the tethyan himalayas on the basis of the subsidence history of the preserved section there is no evidence of acceleration of the subsidence up to the youngest rocks therefore collision related loading and accelerated subsidence must post date the youngest sediments preserved which date from the earl

zedong terrane revisited an intra oceanic arc within neo tethys or a part of the asian active continental margin abstract precise timing of the india asia collision is important to constrain the evolution history of both the himalayan orogen and the tibetan plateau it has been proposed that the indian plate first collided with an intra oceanic arc at 55 ma and then the composite terrane collided with the asian continent at 35 ma the zedong terrane has been suggested to represent the vestige of such an intra oceanic arc developed within the neo tethys ocean as some volcanic rocks with high k2o have been classified as shoshonites in this study we present detailed geochemical and geochronological data of various types of magmatic rocks including volcanic cumulate and granitic rocks widely exposed in the zedong terrane to constrain the formation age and tectonic setting of the zedong terrane we found that the zedong volcanic rocks belong to calc alkaline series rather than shoshonites and high k2o contents in some volcanic rocks resulted from alteration the basalts are highly enriched in lree and lile but strongly depleted in hfse indicating they were derived from a metasomatized mantle presence of hornblende phenocryst in both gabbros and hornblendites indicates that the cumulates were produced from hydrous basalts through crystallization the granitic rocks have adakite like compositional characteristics i e high sr y ratios but low y contents which were formed by melting of a thickened lower crust zircons from six samples including a volcanic rock an andesite three cumulates a hornblendites a hornblende bearing gabbro and a gabbro and two granitic a tonalite and a granodiorite rocks have been dated to yield identical ages of 155 160 ma this suggests that the volcanic eruption and plutonic emplacement were coevally developed in the zedong terrane zircons from both the andesite and the cumulates have similar positive ehf t values 11 6 to 16 7 indicating they were stemmed from similarly depleted mantle sources meanwhile zircons from the granitic rocks also have positive ehf t values of 12 6 to 15 2 implying their derivation from a juvenile lower crust therefore we proposed that the basalts in the zedong terrane were formed through partial melting of the mantle wedge metasomatized by slab released fluids melts a part of hydrous basalts were underplated in the thickened lower crust beneath the zedong terrane which gave rise to the cumulate and granitic rocks by comparison magmatic rocks in the zedong terrane show compositional similarities with the jurassic rocks exposed in the gangdese arc this suggests that the zedong terrane represents a slice of the active continental margin developed on the southern margin of the lhasa terrane as a result of the northward subduction of the neo tethys ocean during the late jurassic rather than the vestige of an intra oceanic arc

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

tectonometamorphic evolution of the himalayan metamorphic core between the annapurna and dhaulagiri central nepal the metamorphic core of the himalaya in the kali gandaki valley of central nepal corresponds to a 5 km thick sequence of upper amphibolite facies metasedimentary rocks this greater himalayan sequence ghs thrusts over the greenschist to lower amphibolite facies lesser himalayan sequence lhs along the lower miocene main central thrust mct and it is separated from the overlying low grade tethyan zone tz by the annapurna detachment structural petrographic geothermobaro metric and thermochronological data demonstrate that two major tectonometamorphic events charac terize the evolution of the ghs the first eohimalayan episode included prograde kyanite grade metamorphism during which the ghs was buried at depths greater than c 35 km a nappe structure in the lowermost tz suggests that the eohimalayan phase was associated with underthrusting of the ghs below the tz a c 37 ma 40ar 39ar hornblende date indicates a late eocene age for this phase the second neohimalayan event corresponded to a retrograde phase of kyanite grade recrystallization related to thrust emplacement of the ghs on the lhs prograde mineral assemblages in the mct zone equilibrated at average t 880 k 610 c and p 940 mpa 35 km probably close to peak of meta morphic conditions slightly higher in the ghs final equilibration of retrograde assemblages occurred at average t 810 k 540 c and p 650 mpa 24 km indicating re equilibration during exhumation controlled by thrusting along the mct and extension along the annapurna detachment these results suggest an earlier equilibration in the mct zone compared with higher levels as a consequence of a higher cooling rate in the basal part of the ghs during its thrusting on the colder lhs the annapurna detachment is considered to be a neohimalayan synmetamorphic structure representing extensional reactivation of the eohimalayan thrust along which the ghs initially underthrust the tz within the upper ghs a metamorphic discontinuity across a mylonitic shear zone testifies to significant late to post metamorphic out of sequence thrusting the entire ghs cooled homogeneously below 600 700 k 330 430 c between 15 and 13 ma middle miocene suggesting a rapid tectonic exhumation by movement on late extensional structures at higher structural levels

metamorphic response to collision in the central himalayan orogen abstract closure of the neotethys ocean and high angle continent continent collision between india and asia after about 55 ma resulted in low angle subduction of the indian plate below the tibetan plateau and by 30 ma established an arcuate 2300 km long shallow north dipping metamorphic fold thrust belt in the foreland this himalayan metamorphic front quickly established an upper plate lower plate paired metamorphic architecture centred on the median high himal thrust that largely controlled subsequent evolution the upper plate is a thick slab of high t moderate p high grade migmatized metamorphic rocks whereas the lower plate is an inverted series of moderate t high p schists in two crustal wedges the broad main central thrust zone and at lowest structural levels the footwall below the basal main central thrust spatial and temporal patterns of metamorphic response in the evolving himalayan metamorphic front has been characterized in a large scale integrated structural metamorphic study based on 8 profiles across eastern nepal metamorphic response at all structural levels is established using a large dataset n 160 of internally consistent quantitative pt determinations petrology of metapelite samples semi quantitative p t paths metamorphic mapping and metamorphic field gradients these results are integrated with previously published metamorphic studies structural profiles and metamorphic chronology from these datasets the architecture and evolution of the himalayan metamorphic front is constrained by rock kinematics metamorphic field gradients showing discontinuities and diachronous metamorphism with contrasting p t evolutions at different structural levels each of the three panels constituting the himalayan metamorphic front upper plate main central thrust zone and footwall experienced distinctly different tectono metamorphic histories crustal processes operating during metamorphism and exhumation differ between the upper and lower plates the upper plate experienced long lived metamorphism starting from at least 28 38 ma and tracking low p t clockwise p t paths that culminated at 19 27 ma in peak high grade condition with 27 31 c km thermal regimes protracted high grade conditions produced significant partial melt which facilitated gravity driven southward extrusion involving internal ductile flow processes southward extrusion of the upper plate was accommodated by coeval reverse movement on the high himal thrust and top down to the north normal reactivation of the south tibet detachment system between 22 10 ma transport of this thick slab to the south resulted in further prograde burial of the lower plate below culminating in peak metamorphism at the highest pressures attained in the lower plate rocks the lower plate consists of at least two deeply buried 7 5 9 4 kb crustal wedges both of which experienced steep p t hairpin clockwise p t paths with isothermal decompression the main central thrust zone experienced 20 26 c km metamorphism at 12 22 ma and was exhumed along the basal main central thrust after 11 ma whereas loading by the main central thrust zone gave rise to further prograde burial of the footwall which experienced 16 21 c km metamorphism at 4 10 ma and was exhumed along the main boundary thrust at 3 9 ma in contrast to the upper plate both crustal wedges were exhumed by a process of out wedging which involved upthrusting along basal and internal thrusts with coeval extensional slip in the hanging walls this three stage foreland propagating lateral exhumation history resulted in telescoping of the himalayan metamorphic front in concert with peak metamorphic events at different structural levels during main phase orogenesis and is not the result of superimposed retrograde reactivation of the belt

cenozoic anatexis and exhumation of tethyan sequence rocks in the xiao gurla range southwest tibet article i nfo in order to advance our understanding of the suturing process between continental landmasses a geologic and geochronologic investigation was undertaken just south of the india asia suture in southwestern tibet the focus of this study the xiao gurla range is located near the southeastern terminus of the active right lateral strike slip karakoram fault in southwestern tibet the range exposes metasandstone phyllite schist locally of sillimanite facies calc gneiss and marble paragneiss pyroxene quartzite metagranite and variably deformed leucogranite these metamorphic rocks are exposed in the footwall of a domal top to the west low angle normal detachment fault structurally beneath neogene quaternary basin fill and serpentinized ultramafic rocks of the kiogar jungbwa ophiolite the detachment is interpreted to be the northeastern continuation of the gurla mandhata detachment fault system that bounds metamorphic rocks of the gurla mandhata range 60 km to the southwest u pb geochronology on five detrital zircon samples of schist phyllite and quartzite yielded maximum depositional ages that range from 644 363 ma and age probability distributions that are more similar to tethyan sequence rocks than lesser himalayan sequence rocks a felsic gneiss yielded a metamorphic zircon age of 35 3 0 8 ma with a significant population of early paleozoic xenocrystic core ages the gneiss is interpreted to be the metamorphosed equivalent of the cambro ordovician gneiss that is exposed near the top of the greater himalayan sequence leucogranitic bodies intruding metasedimentary footwall rocks yielded two distinct u pb zircon ages of 23 ma and 15 ma locally rocks exposed in the hanging wall of this fault and of the southward dipping northward verging great counter thrust to the north consist of serpentinite bearing melange and conglomerate of inferred paleogene age dominated by carbonate clasts the melange is intruded by a 44 ma granite and the stratigraphically highest conglomerate unit yielded detrital zircon u pb ages similar to tethyan sequence rocks we attribute the middle eocene magmatism south of the suture to break off of the neo tethyan oceanic slab in addition our observations are consistent with the late eocene shortening and crustal thickening within the tethyan himalayan sequence early middle miocene leucogranite emplacement being related to underthrusting and melting of the greater and possibly lesser himalayan sequences and late miocene arc parallel extension in the hinterland of the southward propagating himalayan thrust belt

neogene foreland basin deposits erosional unroofing and the kinematic history of the himalayan fold thrust belt western nepal sedimentological and provenance data from the lower miocene pliocene dumri formation and siwalik group in western nepal provide new information about the timing of thrust faulting and the links between erosional unroofing of the himalaya and the cenozoic 87 sr 86 sr record of the ocean in western nepal the dumri formation is an 750 1300 m thick fluvial sandstone and overbank mudstone unit the siwalik group is 4200 m thick and consists of a lower member 850 m of 2 12 m thick fluvial channel sandstones and oxidized calcareous paleosols a middle member 2400 m of very thick 20 m channel sandstones and mainly organic rich histosols and an upper member 1000 m composed of gravelly braided river deposits paleocurrent data indicate that middle miocene pliocene rivers in western nepal flowed southward transverse to the thrust belt throughout deposition of the siwalik group no evidence was found for an axial fluvial trunk system i e the paleo ganges river in siwalik group sandstones a major increase in fluvial channel size is recorded by the transition from lower to middle siwalik members at 10 8 ma probably in response to an increase in seasonal discharge modal petrographic data from sandstones in the dumri formation and the siwalik group manifest an upsection enrichment in potassium feldspar carbonate lithic fragments and high grade metamorphic minerals modal petrographic analyses of modern river sands provide some control on potential source terranes for the miocene pliocene sandstones the dumri formation was most likely derived from erosion of sedimentary and low grade metasedimentary rocks in the tibetan tethyan himalayan zone during early miocene emplacement of the main central thrust the presence in dumri sandstones of plagioclase grains suggests exposure of crystalline rocks of the greater himalayan zone perhaps in response to tectonic unroofing by extensional detachment faults of the south tibetan detachment system during deposition of the lower siwalik group 15 11 ma emplacement of the dadeldhura thrust sheet one of the synformal crystalline thrust sheets of the southern himalaya on top of the dumri formation supplied abundant metasedimentary lithic fragments to the foreland basin a steady supply of plagioclase grains and high grade minerals was maintained by deeper erosion into the main central thrust sheet from 11 ma to the present k feldspar sand increased steadily suggesting that granitic source rocks became widely exposed during deposition of the upper part of the lower siwalik group this provenance change was caused by erosion of passively uplifted granites and granitic orthogneisses in the dadeldhura thrust sheet above a large duplex in the lesser himalayan rocks since the onset of deposition of the conglomeratic upper siwalik group 4 5 ma fault slip in this duplex has been fed updip and southward into the main boundary and main frontal thrust systems we obtained 113 u pb ages on detrital zircons from modern rivers and siwalik group sandstones that cluster at 460 530 ma 850 1200 ma 1 8 2 0 ga and 2 5 ga an abundance of cambrian ordovician grains in the siwalik group suggests sources of siwalik detritus in the granites of the dadeldhura thrust sheet and possibly the greater himalayan orthogneisses the older ages are consistent with sources in the greater and lesser himalayan zones an overall upsection increase in zircons older than 1 7 ga suggests increasing aerial exposure of lesser himalayan rocks none of the detrital zircons even in the modern river samples yielded a cenozoic age that might suggest derivation from the cenozoic greater himalayan leucogranites but this may be attributable to the inheritance problems that characterize the u pb geochronology of the leucogranites when compared with recent studies of the 87 sr 86 sr composition of paleosol carbonate nodules and detrital carbonate in paleosols from the siwalik group the provenance data suggest that erosion and weathering of metamorphosed carbonate rocks in the lesser himalayan zone and cambrian ordovician granitic rocks of the crystalline thrust sheets in central and eastern nepal may have played a significant role in elevating the 87 sr 86 sr ratio of middle miocene synorogenic sediments in the indo gangetic foreland basin and the bengal fan as well as global seawater

himalayan paleogene foreland basin its collision induced early volcanic history and failed rift initiation abstract the himalayan foreland basin was formed over a failed rift zone that produced early paleogene volcanism which was synchronous to indo asia collision the rift zone was located over acutely flexed leading edge of the indian continental crust the volcanism preceded upliftment of the proto himalaya the jammu himalaya is one of the few areas exposing the basal part of the foreland basin comprising a persistent horizon of volcani sedimentary rhyolitic chert breccia at the base of the subathu formation which unconformably overlies the precambrian jammu limestone contemporaneous felsic ash beds occur in the subathu kalka area in the simla hills at the base of the subathu formation unconformably overlying the precambrian simla group in arunachal pradesh in the eastern most himalaya early mid eocene foraminifera fauna bearing yinkiong formation overlies the abor volcanics the age of the latter is disputed whether it is early permian or eocene in age or it exposes a mixture of both components recent u pb shrimp zircon geochronological study indicates abor volcanics being part of the kerguelen mantle plume trail traversing southern tibet sylhet rajmahal bunbury and kerguelen plateau trace element geochemistry of the abor volcanics the exposed volcanic rocks around yinkiong and those exposed between pangin and rotung indicate rift signature in the frontal belt early eocene shallow marine sediments locally associated with mafic volcanics occur tectonically at the base of the early neogene foreland sequence the latter is locally thrust at mbt by the early permian meta sediments tectonically intermixed with marine eocene rocks early eocene sediments and rift related mafic volcanics from arunachal pradesh possibly occur at the tectonic base of the tertiary foreland basin sequence from northeastern himalaya the early paleogene mafic volcanics from the arunachal pradesh and coeval acid volcanics occurring at the base of the paleogene sequence in jammu simla himalaya might represent cogenetic bimodal volcanism the subathu formation with its coal and carbonaceous sediments is lithologically similar to the coal and mud rock bearing late paleocene patala formation or the hangu formation and early eocene ghazij formation from the adjacent indus foreland basin pakistan which overlie the mesozoic paleocene carbonate flat although the lithofacies and sedimentary tectonic setting of the patala hangu and the ghazij formations are similar to that of the subathu formation the former units typically lack the presence of syn sedimentary ash beds or any other volcanic rocks that are characteristically present at the base of the subathu formation mainly late paleocene coal bearing patala and the hangu formations in salt range and north potwar area pakistan although located close to the jammu sector of the himalayan foreland basin is free of any volcanic activity salt range and potwar areas are possibly not parts of the foreland fold and thrust belt of the himalaya as postulated by some workers geodynamically salt range and potwar basins belong to northern parts of the indus foreland basin because the depositional basin from potwar extending into the adjacent kohat area during the paleocene and the eocene time probably formed a collision zone between the india pakistan plate and a micro continent prior to the collision of the former with asia the micro continent possibly buffered intensity of collision thereby there was no tectono magmatic activity along this belt as distinct from that of direct collision between india and asia continent

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

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

subsidence history of the tethys himalaya abstract this article reconstructs the evolution of the passive northern margin of peninsular india facing the neotethys from the earliest rifting stages in the carboniferous to final collision with the active southern margin of asia in the early eocene classical techniques of basin analysis were applied to an extensive and coherent stratigraphic data base facies analysis biostratigraphic dating and palaeoenvironmental interpretation of an over 10 km thick sedimentary succession from eight main composite sections allowed us to draw accurate sedimentation and backstripped subsidence curves and to place quantitative constraints on the palaeotectonic scenario inferred from the sedimentary record in each of the investigated composite sections three major stratigraphic gaps are consistently recognised in the rift sequence the rift unconformity mississippian the carboniferous permian hiatus largely kasimovian to asselian and the break up unconformity mid sakmarian two or even all three gaps may merge into a single hiatus spanning up to 70 ma overall gaps are associated with rift shoulder uplift which after calculation of backstripped subsidence sums up to 600 m at least in three of the investigated sections thermal subsidence and low sedimentation rates are documented by the mid permian to middle triassic drift sequence deposited while the neotethys was expanding between northern gondwana and the detached peri gondwanan blocks a marked increase in accumulation rates is recorded in the carnian norian but calculations of uniform lithospheric stretching suggest that extension was minor and associated with tectonic processes affecting distant parts of gondwana the tethys himalayan margin was uplifted around the jurassic cretaceous boundary at the onset of a major volcanic episode finally uplift exceeding in magnitude all previous events is documented by the uppermost units of the stratigraphic column deposited during the earliest stages of collision between india and asia in the latest paleocene to early eocene

direct stratigraphic dating of india asia collision onset at the selandian middle paleocene 59 1 ma the collision of india with asia had a profound influence on cenozoic topography oceanography climate and faunal turnover however estimates of the time of the initial collision when indian continental crust arrived at the transhimalayan trench remain highly controversial here we use radiolarian and nannofossil biostratigraphy coupled with detrital zircon geochronology to constrain firmly the time when asian derived detritus was first deposited onto india in the classical sangdanlin section of the central himalaya which preserves the best paleocene stratigraphic record of the distal edge of the indian continental rise deep sea turbidites of quartzarenite composition and indian provenance are replaced upsection by turbidites of volcano plutoniclastic composition and asian provenance this sharp transition occurs above abyssal cherts yielding radiolaria of paleogene radiolarian zones rp 4 6 and below abyssal cherts containing radiolaria of zone rp6 and calcareous shales with nannofossils of the paleocene calcareous nannofossil zone cnp 7 constraining the age of collision onset to within the middle paleocene selandian the youngest u pb ages yielded by detrital zircons in the oldest asia derived turbidites indicate a maximum depositional age of 58 1 0 9 ma collision onset is thus mutually constrained by biostratigraphy and detrital zircon chronostratigraphy as 59 1 ma this age is both more accurate and more precise than those previously obtained from the stratigraphic record of the northwestern himalaya and suggests that within the resolution power of current methods the india asia initial collision took place quasi synchronously in the western and central himalaya

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

provenance of the lower tertiary murree redbeds hazara kashmir syntaxis pakistan and initial rising of the himalayas abstract the murree supergroup of northern pakistan is represented by an over 6 km thick succession of deltaic redbeds and intercalated impure foraminiferal limestones of latest palcocene to middle eocene ilerdian to lutetian age cropping out in the northern part of the hazara syntaxis and by redbeds of younger age early middle eocene to early miocene cropping out in the southern part of the syntaxis rawalpindi area the quartzolithic composition of the murree redbeds testifies to a collision orogen provenance detritus was derived from a suture belt including thrust sheets of metasedimentary and subordinately sedimentary rocks volcanic or volcaniclastic rocks and ophiolites phyllite rock fragments are most common in the ilerdian sandstones whereas volcanic and carbonate rock fragments are more abundant in the lutetian sandstones minor quantities of chert and serpentineschist indicating contribution from uplifted subduction complex sources as well as siltstone shale and limestone grains are invariably present detrital modes of sandstones and southwestward progradation of tertiary clastic wedges both testify to provenance from the proto himalayan chain located to the north and uplifted since the very first stages of the india asia continental collision at the end of the paleocene the coeval chulung la redbeds of the tethys himalaya instead consisting of volcanic and subordinate ophiolitic detritus testify to exclusive provenance from the obducting trans himalayan are trench system this major petrographic difference may be accounted for by the different structural setting of the chulung la piggy back and murree foreland basins these two distinct collisional basins have been separated probably since the onset of collision by a fold thrust belt beginning to rise in the position occupied today by the high and lesser himalayan structural domains throughout the murree supergroup main petrographic parameters do not vary greatly and volcanic sedimentary low grade metasedimentary and ophiolitic detritus persisted until the early miocene indicating slow progressive growth of the chain only during the middle miocene when the highly metamorphosed rocks of the high himalaya were carried southward along the main central thrust mct the mountain range began to rise to dramatic heights and huge amounts of detritus started to feed the siwalik foreland basin sandstones and the remnant ocean turbidites of the indus and bengal fans

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

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

present day kinematics of the middle east and eastern mediterranean the middle east and eastern mediterranean regions contain three major strike slip fault zones the dead sea fault zone dsfz and the north and east anatolian fault zones nafz and eafz these accommodate northward motion of the african and arabian plates relative to eurasia and westward motion of the small turkish plate this study deduces an internally consistent solution for the present day kinematics of these plates which is accurate to better than 20 and applicable throughout time since 5 ma by constraining the slip senses and rates on these strike slip fault zones the 7 0 5 mm yr 1 left lateral slip rate on the dsfz is associated with counterclockwise rotation of arabia relative to africa at 0 33 m y 1 around an eulcr pole near 33 n 23 e the nafz takes up counterclockwise rotation of the turkish plate relative to eurasia at 0 83 m y 1 13 around a pole at 31 n 35 5 e with right lateral slip rate 15 mm yr 1 the resulting prediction for the eafz involves 14 mm yr 1 left lateral motion and 2 mm yr 1 of shortening between arabia and turkey the triple junction of africa arabia and turkey is near the city of kahramanmaras this locality contains a narrow promontory bounded by ssw trending left lateral faults at the edges of the arabian and turkish plates which is loosely attached to the african plate moving westward relative to its interior at 4 mm yr 1

definition of low grade metamorphic zones using illite crystallinity abstract two major problems which exist in the use of illite crystallinity to define low grade metamorphic zones are the variety of values chosen for the zone boundaries and the persistent use of three different indices of crystallinity although measurement techniques which cause much of the interlaboratory variation can be standardized it is shown that there is nevertheless significant additional variation which demands calibration on standards the greatest variations are due to choices of different absolute values of crystallinity to define zone boundaries the problem of relating measurements between different indices is approached by fitting mathematical relationships to pairs of measurements from the same sample a power law relationship is a satisfactory fit to the kubler weaver and weaver weber pairs while the kubler weber indices are linearly related these relationships are used to transform definitions of the diagenetic zone anchizone and epizone from one index to the others although they apply strictly only to the data set from which they are derived this results in compatibility between the three zones and shows that previous definitions to the anchizone in different indices have been chosen at incompatible values the boundaries of kubler s anchizone 0 42 and 0 25 2 are 0 4 and 0 215 2 in this study which become 5 1 and 14 6 in the weaver index and 278 and 149 in the weber index an error analysis shows that percentage errors in both kubler and weaver indices increase with crystallinity the kubler measurements are marginally preferred at all grades

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

detrital zircon fission track ages from the lower cenozoic sediments nw himalayan foreland basin clues for exhumation and denudation of the himalaya during the india asia collision detrital zircon fission track ft ages from the lower cenozoic sub himalayan foreland basin reflect the progressive effects of crustal thickening and exhumation on the himalayan source rocks as a consequence of the india asia collision the oldest stratum the transgressive marine paleocene eocene subathu formation 57 41 5 ma contains ca 50 ma detrital zircon p1 peak which was derived from the indus tsangpo suture zone and the ladakh batholith of the asian plate a dominant 302 4 21 9 ma peak with a few 520 ma grains in this formation has been derived by erosion of the zircon partial annealing zone zpaz of 240 180 c as the first imprint of the collision this zone affected the himalayan proterozoic basement and its tethyan sedimentary cover since the detritus in the subathu has been derived both from the indian and asian plates the possible suturing of these plates took place during the subathu sedimentation a sudden change in the provenance is recorded in the detrital zircon ft cooling ages in the oligo miocene dagshai and kasauli formations which have dominant 30 and 25 ma p1 peaks respectively we interpret a distinct unconformity spanning 10 m y between the subathu and dagshai formations since ca 30 ma molassic sedimentation coincides with shifting of the source rocks to the himalayan metamorphic belt this belt has sequentially undergone three distinct cooling and exhumation pulses after the ultrahigh pressure high pressure uhp hp metamorphism 53 50 ma in the extreme north and two widespread m1 and m2 metamorphisms 40 30 and 25 15 ma in the middle parts these events appear to be largely responsible for the deposition of the ca 30 ma zircon himalayan peak and ca 25 and 15 ma young himalayan peaks respectively the latter appears within the lower siwalik subgroup 13 11 ma during the lower siwalik deposition pre himalayan peaks gradually decrease with the intensification of the himalayan events in source rocks in spite of uninterrupted fluvial sedimentation in the dagshai kasauli lower siwalik sequences since 30 ma breaks of 5 7 m y in the zircon ft ages reveal pulsative cooling and exhumation in the well identified source areas although cooling and exhumation of the himalayan source rocks remained almost uniform during the eocene source heterogeneity is reflected in fluvial sedimentation since 37 ma from pakistan to nepal in response to the india asia collision

distributed deformation in southern and western tibet during and after the india asia collision field and radiometric data are used to describe and date strain and stress states in southern longitude 88 o to 91oe latitude 28 o to 30on and western tibet longitude 79 o to 82oe latitude 30 o to 34on we factorize deformation into syncollisional and postcollisional and we present stretching lineation and displacement orientation maps two sections across the indian shelf sequence and stress orientations calculated from mesoscale fault slip data in southern tibet syncollisional stretching and displacement directions trend 90 46 o and displacement is top to south synkinematic low grade metamorphism is dated at 50 ma at one locality in the indian shelf sequence underlying the main mantle thrust of the indus yarlung suture this implies paleocene onset of continental collision for the investigated section postcollisional structures comprise a backthrust group which includes foreland and hinterland directed thrusts reverse and strike slip faults and folds it dominates postcollisional deformation is concentrated along the indus yarlung suture and portrays n s compression otrend of 8o 17 o o2 of 97o 17o a strike slip group consists of conjugate strike slip faults is concentrated in east trending narrow highly deformed zones and indicates that n s compression is locally compensated by e w extension oof 15o 29 o o3 of 103o 30o synkinematic muscovite dates postcollisional deformation as late early miocene 17 5 ma at one locality at the suture strike slip and oblique normal 03 of 600 23 o oof 144o 21 o and normal o3 114o 16 o faulting dated between late miocene and recent and including active deformation represents dominant e w and minor n s extension due to e w stretching of southern tibet and oroclinal bending along the himalayan arc restoring syncollisional and postcollisional deformation yields a minimum of 67 258 km shortening across the indian shelf sequence incorporating recently published contraction estimates across the eastern himalaya yields minimum shortening between undeformed india and the indus yarlung suture of 66 536 km the himalaya tibet orogenic system south of the indus yarlung suture had an initial width of 811 km in the southern tibetan section in western tibet imbrication of an ophiolite sequence of the bangong nujiang suture is top to south stretching lineation trend of 15o 18o and o3 of active deformation trends ese faulting along the shiquanhe fault zone which transfers displacement from the northern part of the karakorum fault to a system of rifts in western central tibet indicates dextral strike slip alternating with sinistral oblique normal faulting and block rotations around vertical axes during a prolonged shearing history the indian shelf sequence south of mount kailas shows top to south imbrication stretching lineation trend of 52o 60o both indian shelf rocks and oligocene miocene kailas conglo merates record backthrusting and backfolding oof 33 o and recent e w extension 03 of 85o 28o

the himalayan arc large scale continental subduction oroclinal bending and back arc spreading abstract palaeomagnetic results from the himalayan arc and southern tibet compared with simulated apparent polar wander paths for the indian plate show a consistent pattern of rotations of the himalayan arc relative to the indian shield varying gradually from 45 clockwise in the northwestern himalaya to slightly counterclockwise in the lhasa region this pattern is consistent with continental underthrusting of greater india beneath the tibetan plateau since the early miocene over at least 650 km at the longitude of western nepal and oroclinal bending since the latest miocene available palaeomagnetic observations support the steady state model for the formation of the himalayan arc with refinements as follows 1 collision between greater india s northern boundary and southern asia occurred at equatorial latitudes with progressive suturing from palaeocene in the northwestern himalaya until early eocene in the eastern himalaya 2 continuing convergence and indentation of greater india into southern asia over about 2000 km up to the early miocene resulted in southeastward extrusion of indochina and 3 neogene counterclockwise rotational underthrusting of greater india along the main central thrust with pliocene quaternary oroclinal bending of the himalayan arc

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

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

multicyclic history of the northern india continental margin northwestern himalaya 1 the geologic evolution of northern india is best recorded in the stratigraphic succession of the zanskar range northwestern himalaya which represents the most complete transect through this ancient continental margin sedimentary history began in the late proterozoic and recorded a late pan african orogenic event around the cambrian ordovician boundary when the gondwana supercontinent was eventually assembled the following long period of epicontinental deposition in shallow seas linked to palaeo tethys lasted until the early permian when a neo tethyan rift began to open between paleo india and the cimmerian microcontinents neo tethyan history can be subdivided into two sedimentary megasequences both recording a major tectonic and magmatic event in the lower part the first one began with breakup in the late permian and lasted until the end of the jurassic the second one started in the early cretaceous with the final detachment of india from gondwana and the opening of the indian ocean and ended with the india eurasia collision in the early eocene the two megasequences can be in turn subdivided into six transgressive regressive supersequences bounded by tectonically enhanced unconformities basal sandstone units of early permian late permian norian callovian early cretaceous and paleocene age are invariably associated with oolitic ironstones or reworked glauco phosphorites and mark the transgre sive part of each supersequence next condensed nodular carbonates or shales with pelagic fauna are typically overlain by thick shallowing upward marly units capped by regressive platformal carbonates the six tectono eustatic supercycles reflect successive rifting episodes which punctuated the progressive separation of india from the rest of gondwana and document the combination of plate microplate reorganizations and eustatic climatic and oceanographic changes in the tethyan domain after the onset of collision between india and asia close to the paleocene eocene boundary obduction of the remnants of the neo tethys ocean floor onto the indian margin began and the latter underwent multiphase deformation with fold thrust shortening followed by heating and extension after the main metamorphic event ophiolitic nappes were re thrusted and finally emplaced with their sedimentary sole on top of the passive margin succession

late cenozoic volcanic evolution of the northeastern aegean region abstract late cenozoic volcanism occurred in an extensional tectonic regime in the northeastern aegean activity commenced in northeastern greece during the early oligocene and in northwestern turkey during the late oligocene early miocene the beginning of activity was characterized by calc alkaline intermediate acid volcanism however alkaline intermediate basic volcanism became dominant from the late miocene until the pleistocene this change in nature of volcanism is not related to a change of regional tectonic regime as previously thought instead we propose that the volcanic evolution reflects an increasing asthenospheric contribution facilitated by a thinned extended lithosphere arising from the regional extensional tectonics

the transition from tethys to the himalaya as recorded in northwest pakistan early cenozoic sedimentary rocks exposed in the kohat plateau of northwestern pakistan record tectonic closure of the tethys sea and development of a restricted marine basin that formed during himalayan collision between india asia and a series of microplates in the paleocene initial subsidence of the basin was caused by the downward deflection of the indian plate in response to loading of the asian plate the western margin of the early eocene basin was dominated by deposition of shale sandstone and conglomerate derived from microplates located to the north and west of the indian continental margin the eastern margin of the basin was a carbonate shelf and sabkha flat salt deposition occurred subaqueously in the central parts of the basin in the late early eocene redbeds derived from the northwest were deposited by a fluvial and or deltaic system this influx of clastic sediments marks the earliest record of terrestrial foreland basin deposition in northwest pakistan during the middle eocene the basin was reflooded and a carbonate shelf developed relative sea level rise may reflect subsidence of the indian plate in response to continued crustal loading in the himalayan suture zone uplift and erosion occurred between the late eocene and miocene possibly related to a peripheral bulge south of the himalayan suture zone the main phase of himalayan foreland basin fluvial deposition began in the miocene renewed uplift related to final collision of india and afghanistan during the pliocene is recorded by a thick sequence of conglomerate in the western kohat plateau correlation with eocene sedimentary rocks from southern pakistan to northern india delineates the depositional systems that developed as the tethys sea closed during himalayan convergence

fracture and fluid flow paths analysis of an offshore carbonate reservoir using oil based mud images and petrophysical logs abstract hydrocarbon production and development of carbonate reservoirs mainly depend on fractures that cross wells therefore quantification of their characteristics has a crucial role in reservoir management in this research bedding planes faults and different types of fractures of a cenomanian turonian carbonate reservoir in the persian gulf were studied image logs including the oil based mud image obmi and the ultrasonic borehole image ubi were used the results from conventional petrophysical log suites were compared to the image logs results based on the image logs structural characteristics of bedding planes were reconstructed by estimating their strike dip dip direction and layer thickness fracture types and their distribution and the geometry in the well were determined also fault zone geometry and type of faults were evaluated the results indicate that majority of the bedding planes have a n58w strike with an average dip of 18 towards ne furthermore both tensile and shear fractures were distinguished although shear fractures dominated and the 328 36sw and 29 45nw were found to be the prevalent orientation of fractures based on petrophysical logs analyses four facies groups are distinguished and two statistical relationships are proposed for estimation of discontinuities the proposed index for evaluation of fractures using petrophysical logs shows good performance and it could be used for wells when no image log is available especially in offshore reservoirs lack of outcrop and unavailability of image logs of old wells lead to unknown fractures characteristics which will be addressed by the suggested equations

stratigraphy and sedimentary history of the nepal tethys himalaya passive margin abstract the sedimentary history of the nepal tethys himalaya began with deposition of thick carbonates in the cambro ordovician followed by a mixed siliciclastic carbonate epicontinental succession recording two major deepening events in the early silurian and late devonian fossiliferous carbonate ramp deposits in the tournaisian were disconformably followed by white quartzose sandstones and black mudrocks with locally intercalated diamictites derived from sedimentary rocks and deposited in asymmetric tectonic basins rift stage break up in the mid early permian locally associated with effusion of tholeiitic lava flows was followed by a transgressive sandy to shaly locally coal bearing or bioclastic unit capped by condensed pelagic carbonates in the middle to late permian juvenile ocean stage subsidence of the cooling stretched crust led close to bathyal water depths in the olenekian but then slowed down in the middle triassic to increase again sharply in the late triassic owing to renewed extensional tectonic activity and sediment loading during up and out building of the indian continental terrace deposition of tropical platform carbonates finally became widespread in the middle liassic mature passive margin stage the initial fragmentation of gondwana in the middle jurassic led to rejuvenation of the indian craton and deposition of quartzo feldspathic hybrid arenites capped by condensed oolitic ironstones deposited at warm subtropical latitudes in the late bathonian middle callovian next a discontinuous pelagic grey marly limestone unit was followed by the ammonoid rich offshore spiti shale in the late jurassic the final disintegration of gondwana began in the berriasian when quartzose siliciclastics derived again from the rejuvenated indian craton and partly from recycling of older clastic successions were followed by thick deltaic to shelf volcaniclastics documenting eruption of alkali basalts in the valanginian followed in the hauterivian to albian by more felsic differentiates such as the trachyandesites exposed in the lesser himalaya 120 km to the south a widespread drowning episode fostered by waning volcaniclastic supply during a global eustatic rise is documented by a major glauconitic horizon deposited at middle southern latitudes in the late albian overlain by scaglia like pelagic limestones in the latest albian the final part of sedimentary history during the rapid northward flight of india and its collision with eurasia is not documented anywhere in nepal due to later erosion of upper cretaceous to lower tertiary strata

tectonic evolution of the troodos ophiolite within the tethyan framework a new tectonic model reconciles conflicting structural and geochemical evidence for the origin of the troodos ophiolite a well preserved remnant of neotethyan oceanic crust grabens and normal faults within the sheeted dike complex and the extrusive sequence of the troodos ophiolite resemble those of oceanic spreading centers diverse intrusive and tectonic contact relationships between the sheeted dike complex and the underlying plutonic sequence indicate multiple and episodic intrusion of magma and along and across strike variation in volcanic and tectonic activity during development of oceanic crust coupled with the existence of the arakapas transform fault to the south these structural and intrusive relationships suggest origin at an intersection between a spreading center and a transform fault the arclike chemistry of sheeted dikes and related extrusive rocks and the inferred highly depleted and hydrous nature of the mantle source of the late stage intrusive and extrusive rocks argue however for generation of part of the ophiolite within a subduction zone environment regional reconstructions suggest that the mesozoic neotethys may have evolved as a marginal basin both to the afro arabian continent and the paleotethyan ocean over an active or recently active south dipping subduction zone the troodos ophiolite and other eastern mediterranean ophiolites whose magma compositions were affected by the subducted paleotethyan slab may have formed along east west trending spreading centers separated by north south trending transform faults within this marginal basin a rapid change in relative plate motion in late cretaceous time between eurasia and afro arabia created a regional compressive regime that may have resulted in plate boundary reorganizations within the neotethyan realm and in initiation of north dipping subduction zone s beneath the troodos and other ophiolites in the region the apparent forearc setting of the troodos ophiolite is a consequence of this intraoceanic displacement after its formation and thus is unrelated to its generation

evidence for early 44 ma himalayan crustal thickening tethyan himalaya southeastern tibet abstract comprehensive understanding of himalayan orogenesis is limited in part by a poor knowledge of the eohimalayan episode a phase of tectonic activity predating the better understood neohimalayan crustal thickening associated with coaxial deformation along the main central thrust the cambo ordovician to tertiary metasedimentary sequences outcropping within the tethyan himalaya are the structurally highest units of the himalayan fold and thrust belt they and are inferred to have been the first to have accreted to asia a compilation of data from transects along the length of the main himalayan arc shows that the tethyan sequences have experienced at least five deformation events although the timing of these episodes is poorly constrained emplacement of a series of undeformed granitoid bodies following the second of these events which accounts for the majority of crustal thickening in the tethyan himalayan units is constrained by u pb zircon dating to be older than 44 1 1 2 ma thus significant crustal thickening had occurred along the length of the proto himalayan arc by the mid eocene or within 10 to 20 myr of the initiation of himalayan orogenesis

counterclockwise rotation of the anatolian block abstract seismological and other data from the boundaries of anatolia are used to discuss the motion and plate tectonics of this block models which include a rotation of anatolia around a single pole are shown to be inconsistent with much of this data in particular a westward motion of anatolia which is the most commonly used model should be reconsidered models which use a single pole of rotation fail because they assume the north anatolian fault to be an ideal transform fault which describes all of the motion between anatolia and the black sea in effect internal deformation in the form of extension in western anatolia and for the most part strike slip faulting in eastern anatolia are important and account for some of the relative motion thus a more appropriate model for this region is one which not only stresses the dominance of the north anatolian fault but also recognises the importance of internal deformation and the limitation of classic plate tectonic models which this deformation implies the preferred model for the motion of anatolia is a nonuniform tight counterclockwise rotation approximated by the shape of the north anatolian fault as well as by the faults which splay from it to the south such a model is consistent with data from the boundaries of anatolia including the source mechanism of earthquakes and depth of benioff zone along the southern boundary of the block counterclockwise rotation of anatolia is the natural consequence of the collision of arabia with eastern anatolia coupled with subduction in the eastern mediterranean

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

tectonic evolution of the central annapurna range nepalese himalayas the metamorphic core of the himalayan orogen or greater himalayan sequence is a northward tapering prism bound at the bottom by a n dipping family of thrust faults the main central thrust system and at the top by a n dipping family of normal faults the south tibetan detachment system research in the central annapurna range of nepal demonstrates a close temporal and spatial association between contractional and extensional deformation on these bounding fault systems and within the metamorphic core throughout much of the early miocene the main central thrust system is represented here by a 2 to 3 km thick zone of high strain that developed during two or more episodes of movement most of its displacement was concentrated along the chomrong thrust a sharp late metamorphic discontinuity that places middle amphibolite facies rocks of the greater himalayan sequence on top of lower amphibolite facies rocks of the lesser himalayan sequence the earliest demonstrable movement on this thrust system occurred 22 5 ma the most recent movement may be as young as pliocene the oldest element of the south tibetan detachment system in this area is the deorali detachment which appears to have been active at the same time as the earliest shortening structures of the main central thrust system fabrics related to the deorali detachment are disrupted by a previously unrecognized sw vergent thrust structure the modi khola shear zone the effect of this structure which is constrained to be between 22 5 and 18 5 ma was to shorten rock packages that had been extended previously during movement on the deorali detachment transition back to a local extensional regime after 18 5 ma was marked by development of the machhupuchhare detachment and related splays geologic evidence for rapid two way transitions between contraction and extension in the annapurna range indicates that extensional deformation in convergent settings does not only represent gravitational collapse at the end of an orogenic cycle it also appears to be an important factor in mountain range development

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

kinematic evolution of the eastern tethyan himalaya constraints from magnetic fabric and structural properties of the triassic flysch in se tibet anisotropy of magnetic susceptibility ams combined with structural analysis are used in this work with the aim to characterize the tectonic evolution of the triassic flysch within the eastern tethyan himalaya thrust belt in se tibet the attitude of the magnetic foliation and lineation are concordant with the planar and linear structures of tectonic origin defined by the preferred orientation of the iron bearing silicates two different tectonic domains can be defined a the southern domain is controlled by the eohimalayan tectonic foliation s1 recorded in the magnetic foliation which trends east west and dips to the north b the northern domain is dominated by the neohimalayan magnetic foliation with wnw ese strike and dips to the south opposite to the vergence of the main structures a slightly prolate magnetic ellipsoid has been found in between the two domains recording the intersection of s1 and the subtle development of the s2 tectonic foliation hinterland propagation of the deformation lead to the great counter backthrust generation pointed out by the ssw steeply plunging magnetic lineation furthermore different orientations of magnetic foliation may indicate an early miocene c 208 clockwise vertical

paleomagnetic chronology fluvial processes and tectonic implications of the siwalik deposits near chinji village pakistan a 2800 m section of siwalik strata containing the stratotypes for both the chinji and nagri formations has been dated by magnetic polarity stratigraphy and the observed polarity zonation has securely been correlated with the chron 17 7 segment of the time scale the base of the section is the base of the kamlial formation which occurs near the top of chron 17 18 3 m y the kamlial chinji formation boundary occurs in the middle of chron 15 14 3 m y the chinji nagri boundary near the bottom of chron 10 10 8 m y and the nagri dhok pathan boundary at the chron 8 9 boundary 8 5 m y the siwalik deposits near chinji village consist of four distinct classes of fluvial cycles each with a characteristic periodicity a first order of 10 7 years a second order of 10 6 years a third order of 10 4 10 5 years and a fourth order of 10 0 10 4 years the river system responsible for siwalik sedimentation flowed from west to east and had properties much like the modern ganges river syste

paleogeographic maps of the continents bordering the indian ocean since the early jurassic paleomagnetic data from the african plate from the early jurassic onward are critically re evaluated and updated resulting in a set of 30 paleopoles from which an apparent polar wander path is derived using 20 ma intervals this path is then used in conjunction with kinematic models of the indian and atlantic oceans to place other continents bordering the indian ocean eurasia india islands and plateaus madagascar seychelles mascarene and blocks involved in the collision of india with eurasia south tibet indochina in an absolute paleogeographic frame fixed with respect to africa paleogeographic maps are presented for the early jurassic pre breakup the jurassic cretaceous boundary anomaly ml5 the early cretaceous m2 the late cretaceous a 34 the cretaceous tertiary boundary a 29 the early eocene a 24 the middle eocene a 21 and the oligocene miocene boundary a 6 observed and predicted paleomagnetic directions for landmasses other than africa i e for western europe china korea india and australia which are not used in deriving the maps are found to be in very satisfactory agreement the quality of reconstructions as measured by rms discrepancies between observed and predicted paleolatitudes is of the order of 5 the main features of the paleogeographic maps are discussed with the aim of relating oceanic and on land geodynamic events the constraints provided by tibetan data on the india asia collision and the 20 ma reorganization of geology and plate boundaries in a band extending from the south china sea through tibet to the red sea system are discussed in more detail we emphasize the episodic nature of the breakup of gondwanaland and formation of the indian ocean and subsequent interaction with eurasia in which six major discrete events are identified separated by 30 ma phases of rather uniform plate motion

cretaceous tertiary geology of the gangdese arc in the linzhou area southern tibet abstract knowledge of the cretaceous tertiary history of upper crustal shortening and magmatism in tibet is fundamental to placing constraints on when and how the tibetan plateau formed in the lhasa terrane of southern tibet the widely exposed angular unconformity beneath uppermost cretaceous lower tertiary volcanic bearing strata of the linzizong formation provides an excellent geologic and time marker to distinguish between deformation that occurred before vs during the indo asian collision in the linzhou area located 30 km north of the city of lhasa a 3 km thick section of the linzizong formation lies unconformably on cretaceous and older rocks that were shortened by both northward and southward verging structures during the late cretaceous the linzizong formation dips northward in the footwall of a north dipping thrust system that involves triassic jurassic strata and a granite intrusion in the hanging wall u pb zircon geochronologic studies show that the linzizong formation ranges in age from 69 ma to at least 47 ma and that the hanging wall granite intrusion crystallized at 52 ma coeval with dike emplacement into footwall cretaceous strata 40ar 39ar thermochronologic studies suggest slow cooling of the granite between 49 and 42 ma followed by an episode of accelerated cooling to upper crustal levels beginning at 42 ma the onset of rapid cooling was coeval with the cessation of voluminous arc magmatism in southern tibet and is interpreted be a consequence of either 1 tertiary thrusting in this region or 2 regional rock uplift and erosion following removal of overthickened gangdese arc lower crust and upper mantle or break off of the neo tethyan oceanic slab

age of initiation of collision between india and asia a review of stratigraphic data abstract the collision of india with asia is perhaps the most profound tectonic event to have occurred in past 100 ma it is responsible for the uplift of the himalayas and tibetan plateau and has been argued to have been responsible for geological geochemical and climatological consequences of global extent yet the age of initiation of this collision remains poorly constrained the literature is replete with estimates that range from the late cretaceous 65 ma to latest eocene

the metamorphism in the central himalaya all along the himalayan chain an axis of crystalline rocks has been preserved made of the higher himalaya crystalline and the crystalline nappes of the lesser himalaya the salient points of the metamorphism as deduced from data collected in central himalaya central nepal and kumaun are 1 the higher himalaya crystalline also called the tibetan slab displays a polymetamorphic history with a first stage of barrovian type overprinted by a lower pressure and or higher temperature type metamorphism the metamorphism is due to quick and quasi adiabatic uplift of the tibetan slab by transport along an mct ramp accompanied by thermal refraction effects in the contact zone between the gneisses and their sedimentary cover the resulting metamorphic pattern is an apparent diachronic inverse zonation with the sillimanite zone above the kyanite zone 2 conversely the famous inverted zonation of the lesser himalaya is basically a primary pattern acquired during a one stage prograde metamorphism its origin must be related to the thrusting along the mct with heat supplied from the overlying hot tibetan slab as shown by synmetamorphic microstructures and the close geometrical relationships between the metamorphic isograds and the thrust 3 thermal equilibrium is reached between units above and below the mct far behind the thrust tip there is good agreement between the maximum temperature attained in the hanging wall and the temperature of the tibetan slab during the second metamorphic stage but closer to the mct front the thermal accordance between both sides of the thrust is due to a retrogressive metamorphic episode in the basal part of the tibetan slab

laser 40ar 39ar dating of single detrital muscovite grains from early foreland basin sedimentary deposits in india implications for early himalayan evolution in india the dagshai and overlying kasauli formations represent the oldest exposed continental foredeep sediments eroded from the himalayan orogen 40ar 39ar dating of individual detrital white micas from these sedimentary units has provided maximum depositional ages of 28 ma for the dagshai formation at one locality and 25 ma at a second locality whereas deposition of the kasauli formation occurred after 28 ma at two localities and after 22 ma at a third locality this timing suggests that in india the start of substantial exhumation and erosion from the rising himalayan orogen was delayed until 28 ma

reconstructing the greater indian margin a balanced cross section in central nepal focusing on the lesser himalayan duplex across much of the himalaya rocks in the lesser himalayan duplex are covered by roof thrusts of other paleoproterozoic lesser himalayan rocks or greater himalayan rocks however in central nepal between the main central thrust and the main boundary thrust lesser himalayan rocks are exposed in structurally complex relationships within the lesser himalayan duplex we present two balanced cross sections with different stratigraphy involved in the duplex one with the basal kuncha formation and one without this unit to test stratigraphic assumptions both cross sections have roof thrust sheets consisting of the main central thrust ramgarh munsiari thrust and trishuli thrust folded over the hinterland dipping lesser himalayan duplex dissected by late faults cross section 1 has a shortening estimate from the main central thrust to the main boundary thrust including motion on the main central thrust of 359 km or 78 cross section 2 has a shortening estimate of 349 km or 76 over the same region because the amount and percentage of shortening are not significantly different between the two cross sections the different stratigraphic assumptions did not change the shortening results this similarity suggests that many of the choices made when constructing a cross section may be less important than researchers previously thought

dating the indian continental subduction and collisional thickening in the northwest himalaya multichronology of the tso morari eclogites multichronometric studies of the low temperature eclogitic tso morari unit ladakh india place timing constraints on the early evolution of the northwest himalayan belt several isotopic systems have been used to date the eclogitization and the exhumation of the tso morari unit lu hf sm nd rb sr and ar ar a ca 55 ma age for the eclogitization has been obtained by lu hf on garnet omphacite and whole rock from mafic eclogite and by sm nd on garnet glaucophane and whole rock from high pressure metapelites these results agree with a previously reported u pb age on allanite and together these ages constrain the subduction of the indian continental margin at the paleocene eocene boundary during exhumation the tso morari rocks underwent thermal relaxation at about 9 3 kbar characterized by partial recrystallization under amphibolite facies conditions ca 47 ma as dated by sm nd on garnet calcic amphibole and whole rock from metabasalt rb sr on phengite apatite and whole rock and ar ar on medium si phengite from metapelites ar ar analyses of biotite and low si muscovite from metapelites which recrystallized at 5 kbar toward the end of the exhumation show that the tso morari unit was at upper crustal levels ca 30 ma these results indicate variable exhumation rates for the tso morari unit beginning with rapid exhumation while the indian margin subduction was still active and later proceeding at a slower pace during the crustal thickening associated with the himalayan collision

provenance analysis of cretaceous peripheral foreland basin in central tibet implications to precise timing on the initial lhasa qiangtang collision abstract mesozoic strata along northern margin of the lhasa terrane near dingqing tibet provide a semi continuous record of the bangong nujiang ocean closure and the subsequent lhasa qiangtang collision in this study we present results of sandstone petrographic detrital zircon u pb and cr spinel geochemical data to determine the provenance of the mesozoic strata from the triassic quehala group and the mid jurassic to lower cretaceous xihu lagongtang and duoni formations which are young from 220 ma to 100 ma in this region thereby allowing for interpretation of their tectonic setting the similar 1200 900 age cluster from the lower xihu formation to that of the triassic quehala group and the distinct age peaks at 200 ma and 146 ma from the upper xihu formation suggest a lhasa terrane provenance to the south distinctive age clusters of 300 210 ma and 1800 ma and cr spinel composition analysis of the lagongtang formation indicate a provenance shift from the lhasa terrane to the qiangtang terrane and the bangong nujiang suture zone to the north the lagongtang formation was deposited in a bathyal abyssal to shallow slope environment from north to south that we interpret as the foredeep depozone and distal foredeep of a peripheral foreland basin system that developed due to flexural subsidence related to the lhasa qiangtang collision and terrane accretion the age of the lhasa qiangtang collision is constrained by a 140 ma tuffite u pb age at the base of the foreland basin strata an angular unconformity between the xihu and lagongtang formation which we interpret as the result of the lhasa qiangtang collision our results indicate that the lhasa qiangtang collision initiated around 140 ma in the dingqing region simultaneously with previous determinations 1200 km to the west near gaize area therefore we prefer quasi simultaneous onset of collision along strike to zippering collision models wherein the east collision age is older than the west

relation of alkaline volcanism and active tectonism within the evolution of the isparta angle sw turkey the isparta angle ia is formed along the boundary of the african and eurasian plates by ne and nw striking faults north of the antalya gulf in sw turkey the ne striking strike slip burdur fault bounds the ia to the west and is probably the continuation of the pliny system of the hellenic arc the nw striking aksehir fault bounds it to the east platform type parautochthonous mesozoic carbonate sequences such as the beydag and anamas akseki occur in the western and eastern parts of the ia respectively whereas allochthonous ophiolite nappes include the antalya beysehir hoyran and lycian nappes the ia and adjacent areas are divided into three areas the teke the antalya and the akseki fragments by ne nw and n striking active strike slip faults with normal components ia region volcanics are alkaline and hyperkaline in character potassic ultrapotassic and locally occur as subvolcanic stocks and dikes they can be traced between the afyon and isparta regions these volcanics consist mainly of