de glauconiarum origine the glauconitic facies is widespread on present day continental shelves from 50 s to 65 n and at water depths between 50 and 500 m and is in particularly great abundance on the upper slope and outer shelf between 200 and 300 m it is also common in many ancient rocks of post late precambrian age it occurs as sand to pebble sized essentially green particles granular facies but also as a surface coating on particles and hardgrounds and as a diffuse impregnation film and diffuse facies we suggest the replacement of the term glauconite which has been interchangeably used to designate a morphological form and a specific mineral by glaucony facies and glauconitic smectite and glauconitic mica as end members of the glauconitic mineral family the widely accepted model of burst and hower for glauconitization requires a degraded micaceous 2 1 layer lattice structure parent clay mineral however detailed analysis of numerous samples of recent glaucony reveals that such a parent substrate is exceptional the model therefore requires modification generally the parent material is carbonate particles argillaceous kaolinitic faecal pellets infillings of foraminiferal tests various mineral grains and rock fragments that pass gradually into the commonly occurring green grains we show that the process of glauconitization is achieved by de novo authigenic growth of automorphous crystallites in the pores of the substrate accompanied by progressive alteration and replacement of the substrate it is this two fold evolution that causes the verdissement of granular substrates macrofossils and hardgrounds the authigenic mineral is an iron rich and potassium poor glauconitic smectite while new smectites are growing into the remaining pore space the earlier smectites are modified by incorporation of potassium producing decreasingly expandable minerals with a non expandable glauconitic mica as the end member this mineralogical diversity of the glauconitic mineral family explains the highly variable physical and chemical properties of glaucony four categories nascent little evolved evolved and highly evolved glaucony are distinguished glauconitization appears to be controlled by a delicate balance between degree of physical confinement of a particle and the amount of ionic exchange between the micro environment and ambient open marine sea water the optimum conditions for glauconitization are those of semi confinement as a result the interior of a grain is more glauconitized than its less confined periphery similarly for identical substrate types large grains 500 m provide more favourable substrates for glauconitization than lesser confined small grains on a larger scale the formation of glaucony is governed by the availability of iron and potassium and the balance between detrital influx and winnowing low accumulation rates expose grains to the open marine environment for sufficiently long times 105 106 years for highly evolved glaucony

ganges basin geometry records a pre 15 ma isostatic rebound of himalaya the tertiary continental strata of the himalayan foreland basin are subdivided into two groups but the meaning of this subdivision was previously unclear from the analysis of drill holes seismic lines dated sections outcrops and balanced cross sections we find that the southward migration rate of the depositional pinch out of the younger group is 19 5 mm yr and equals the himalayan shortening rate this equality shows that the flexural foreland basin development is mainly controlled by the motion of the thrust load the long term pinch out migration rate was slower for the older synorogenic group erosion locally occurred at the end of its deposition due to tectonic reactivation of lineaments of the indian shield we suggest that this change in the basin development is linked to the detachment of the subducted indian lithosphere that decreased the slab pull and increased the mean compressive stress within the indian plate whereas the plate motion remained constant the most important implication of our work is that the associated isostatic rebound could have increased the himalayan elevation prior to 15 ma

provenance evolution of collapse graben fill in the himalaya the miocene to quaternary thakkhola mustang graben nepal abstract this study focuses on the detailed provenance evolution of young syn to post orogenic extensional grabens in orogens like the himalaya to trace the tectonic history of such late stage basins using the neogene thakkhola mustang graben as a case study the graben is situated within the tibetan tethys zone and is filled with 870 m of continental deposits of miocene to holocene age based on logged sections within the predominantly alluvial to coarse grained fluvial fill of the graben we investigated paleocurrent data and the petrology of sandstones and conglomerates including heavy minerals studies to interpret provenance and source areas in detail significant changes are recorded by slight differences in heavy mineral and pebble compositions the sandstones can be classified as lithic greywackes lithic arkoses and feldspathic litharenites sandstone mudstone quartzite and some granite clasts are dominant in conglomerates of the central part of the graben tetang formation conglomerates of miocene age comprise mostly clasts of mesozoic rocks with an eastern provenance consistent with measured paleocurrent directions all paleocurrent data and compositional analyses of imbricated conglomerates of the miocene pliocene thakkhola formation in the northeast of the graben suggest that clasts were derived from eastern source areas comprising mainly mesozoic rocks whereas paleozoic clasts of a western to northern source area predominate in the centre of the graben heavy mineral analysis indicates that tourmaline staurolite zircon garnet and apatite constitute a significant proportion of the assemblages of all formations through time whereas epidote andalusite kyanite chloritoid hornblende chrome spinel rutile and amphiboles are less common these assemblages reflect in general stable minerals and low to high grade metamorphic source rocks and are principally controlled by reworking of older passive margin sediments of the tibetan tethys zone as indicated by provenance discrimination diagrams three successive stages in provenance evolution were recognized 1 the miocene tetang formation characterized by higher kyanite values corresponding to the himalayan foreland evolution 2 the thakkhola formation characterized by granite clasts and significantly higher amounts of andalusite indicating source area expansion and erosion of the mustang mugu granites to the northwest 3 the upper pleistocene holocene kaligandaki formation bearing higher amounts of epidote klinozoisite and ophiolite and high pressure low temperature detritus as indicated by chrome spinel and blue amphiboles derived from the north lying indus tsangpo suture zone the change in source areas from the miocene pliocene to the late pleistocene holocene is interpreted as a result of the evolution from an initial stage of high angle normal faulting and collapse basin formation to a low angle extensional detachment basin system

topography relief and trmm derived rainfall variations along the himalaya 1 along the southern himalayan topographic front the indian summer monsoon modulates erosive processes and rates to investigate the influence of topography and relief on rainfall generation and resultant erosion we processed satellite rainfall amounts for the last 8 years 1998 2005 from the tropical rainfall measurement mission trmm based upon a spatial resolution of 5 5 km for the himalaya we identify 1 the spatial distribution of rainfall and 2 the large scale relationships between topography relief and rainfall locations our results show two distinct rainfall maxima along strike in the himalaya the first outer rainfall peak occurs along the southern margin of the lesser himalaya within a narrow band of mean elevation 0 9 0 4 km and mean relief 1 2 0 2 km the second discontinuous inner band typically occurs along the southern flank of the greater himalaya elevation and relief both 2 1 0 3 km

miocene burial and exhumation of the india asia collision zone in southern tibet response to slab dynamics and erosion the india asia collision zone in southern tibet preserves a record of geodynamic and erosional processes following intercontinental collision apatite fission track and zircon and apatite u th he data from the oligocene miocene kailas formation within the india asia collision zone show a synchronous cooling signal at 17 1 ma which is younger than the ca 26 21 ma depositional age of the kailas formation constrained by u pb and 40 ar 39 ar geochronology and requires heating burial after ca 21 ma and subsequent rapid exhumation data from the gangdese batholith underlying the kailas formation also indicate miocene exhumation the thermal history of the kailas formation is consistent with rapid subsidence during a short lived phase of early miocene extension followed by uplift and exhumation driven by rollback and northward underthrusting of the indian plate respectively significant removal of material from the india asia collision zone was likely facilitated by efficient incision of the paleo indus river and paleo yarlung river in response to drainage reorganization and or intensification of the asian monsoon

fundamentals of fission track thermochronology fission track ft analysis has developed into one of the most useful techniques used throughout the geologic community to reconstruct the low temperature thermal history of rocks over geological time the ft method is based on the accumulation of narrow damage trails i e fission tracks in uranium rich mineral grains e g apatite zircon titanite and natural glasses which form as a result of spontaneous nuclear fission decay of 238u in nature price and walker 1963 fleischer et al 1975 the time elapsed since fission tracks began to accumulate is estimated by determining the density of accumulated tracks in a particular material in relation to the uranium content of that material chemical etching can be used to enlarge fission tracks that have formed within a mineral in order to make them readily observable under an ordinary optical microscope price and walker 1962 if a host rock is subjected to elevated temperatures fission tracks that have formed up to that point in time are shortened progressively and eventually erased by the thermal recovery i e annealing of the damage fleischer et al 1975 because thermal diffusion basically governs the annealing process the reduction in ft length is a function of heating time and temperature importantly fission tracks are partially annealed over different temperature intervals within different minerals this characteristic allows for the construction of time temperature paths of many different rock types by a plotting ft and other isotopic ages from different minerals versus their closure temperatures which is applicable in the case of a monotonous cooling history e g wagner et al 1977 zeilter et al 1982 and or by b the inverse modeling of observed ft age and confined track length data e g corrigan 1991 lutz and omar 1991 gallagher 1995 ketcham et al 2000 see also ketcham 2005

shisha pangma leucogranite south tibetan himalaya field relations geochemistry age origin and emplacement the shisha pangma pluton forming most of the xixabangma 8027 m massif in south tibet is one of the 20 larger leucogranite intrusives that mark the highest structural levels of the himalayan metamorphic core the pluton occurs immediately below the shisha pangma detachment a strand of the south tibetan detachment std system a low angle 30 north dipping normal fault placing paleozoic black slates atop sillimanite grade pelites and calc silicate rocks k feldspar augen gneisses containing fibrolite and sillimanite paragneisses along the footwall show strong internal s c fabrics indicative of down to the north extension the shisha pangma leucogranite is a heterogeneous polyphase intrusion with an earlier foliated biotite rich phase and a later tourmaline muscovite rich phase typically containing the assemblage kfs pl qtz ms tur gt bt sil ap the highly peraluminous granites have high 87sr 86sr ratios 0 738 0 750 typical of pelite derived anatectites nd depleted mantle model

indian punch rifts tibet the strikes of late cenozoic normal faults in tibet fan systematically from a northwesterly to northeasterly direction from west to east they exhibit an axis of symmetry that bisects the himalayan arc and trends subparallel to india asia motion the two dimensional stress field inferred from the spatial pattern of normal fault orientations can be explained solely by collisional stresses localized along a southern part of the himalayan arc continued insertion of indian crust into tibet can explain the persistence of extension and may be increasing plateau crustal thickness and elevation coeval with rifting

multiple episodes of continental subduction during india asia convergence insight from seismic tomography and tectonic reconstruction abstract high wavespeed tomographic anomalies shallower than 1100 km beneath the india asia collision zone are interpreted as continental slabs subducted during collision combining anomaly positions with paleogeographic reconstructions of india we constrain the spatio temporal evolution of multiple episodes of continental subduction likely related to these anomalies this study highlights the different evolution at lithospheric scales of the western and eastern parts of the collision zone the evolution of the western part is characterized by two episodes of steep subduction of the northern margin of india the first episode involving an area with a lateral extent as large as 1500 km started at about 40 30 and ended by a slab break off process at 15 ma the second episode consists on subduction beneath the hindu kush mountains since 8 ma to the east of the collision zone no anomaly related to steep subduction along the northern edge of india is found we interpret two tomographic anomalies beneath the eastern border of the indian plate beneath burma and beneath the andaman sea as the result of two successive episodes of southeastward extrusion followed by subduction we suggest that both extruded portions were initially located along the northern margin of india and that they slid around the eastern syntaxis then southward along the eastern boundary of indian plate both portions subducted along the eastern border of india south of the eastern syntaxis we provide a rough estimate of the amount of indian lithosphere consumed during these subduction and extrusion episodes by comparing this amount with the total amount of indian lithosphere at the onset of collision we conclude that these processes accommodated most of the india asia convergence during collision

particle interaction in fjord suspended sediment abstract glacial flour enters the surface layer of howe sound a fjord in southwestern british columbia as a sediment plume which moves quickly down inlet while gradually mixing with the sea water flocculation occurs in the lower brackish waters of the surface layer although mixing and diffusion are the dominant processes for sediment to enter the marine water once in the underlying marine water zooplankton pelletization and biologic agglomeration of inorganic floccules predominate this generates a response in the order of days between the two layers in terms of sedimentation of particulate matter most particles settling in the fjord environment fall as 1 sand and silt grains containing attached clay particles 2 clay clasts possibly related to river mudballs 3 mineral bearing fecal pellets from pelagic zooplankton 4 large grain inorganic and colloidal floccules and 5 inorganic biogenic agglomerates analysis of sediment trap data has led to the following conclusions 1 water turbidity cannot be used to infer the downward flux of particles 2 size distributions of sediment deposited on the sea bed are a function of non lognormal size distributions from particles composing sub laminae

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

cenozoic low temperature cooling history of the northern tethyan himalaya in zedang se tibet and its implications abstract major structures and climatic change during the cenozoic especially miocene interacted to shape the giant tibetan plateau whose formation was triggered by convergence of indian and asian plates in this work we report a low temperature thermochronology data set from the northern tethyan himalaya and indus yarlung suture zone in the zedang area where structures are defined by two parallel thrusts the zhongba gyantse thrust to the south and great counter thrust to the north eighteen samples from the region constrain their low temperature cooling and exhumation histories and further elucidate the evolution of the southern tibetan plateau thermal history modelling reveals eocene and miocene cooling episodes based on the temporal and spatial patterns eocene cooling is probably linked to crustal thickening resulting in the eocene anatexis in the northern tethyan himalaya the rapid pronounced cooling episode between ca 20 ma and 15 ma in the luobusha area reflects the activity of the great counter thrust gct the ca 14 10 ma cooling event of the yardoi dome can probably be linked to rapid erosional denudation during that period and pronounced cooling commencing in the middle miocene was probably caused by incision of branches of the yarlung zangbo river a further cooling episode commencing in late miocene time ca 5 ma in luobusha area and yardoi dome may reflect initial activity of the cona graben cutting through the region

evidence for constriction and pliocene acceleration of east west extension in the north lunggar rift region of west central tibet 1 the active north trending north lunggar rift in west central southern tibet exposes an extensional metamorphic core complex bounded by an east dipping low angle normal fault apatite and zircon u th he thermochronology and thermal modeling of the north lunggar rift document a minimum timing for rift inception at 10 ma and rapid footwall exhumation between 5 and 2 ma miocene footwall cooling and exhumation rates were initially slow to moderate at 400 c ma 1 and 4 10 mm a 1 footwall isotherms were significantly compressed during rapid exhumation resulting in an elevated transient geothermal gradient between 50 and 90 c km 1 the minimum magnitude of horizontal extension for the north lunggar rift is 8 1 12 8 km maximum is 15 20 km less in the south at 10 km mean pliocene extension rate is 1 2 2 4 mm a 1 in the 120 direction results for the north lunggar rift are similar in magnitude rate and orientation of slip to the kinematically linked lamu co dextral strike slip fault to the north this suggests a state of constrictional strain during pliocene time along this stretch of the bangong nujiang suture from which the lamu co fault emanates the onset of extension in this region may be explained by crustal thickening and gravitational orogenic collapse followed by accelerated rifting resulting from localized crustal stretching and increased magmatic activity potentially driven by the position and northward extent of underthrusting indian lithosphere

late cretaceous to middle tertiary basin evolution in the central tibetan plateau changing environments in response to tectonic partitioning aridification and regional elevation gain located in the bangong suture zone between the lhasa and qiangtang terranes of central tibet the nima basin records cretaceous late miocene sediment accumulation during a period of drastically changing paleogeography and paleoelevation the bangong suture zone originally formed during late jurassic early cretaceous time as the lhasa terrane collided with the qiang tang terrane during early to middle cretaceous time the region lay at northern near equatorial paleolatitudes near sea level by aptian time the nima basin was above sea level and was strongly influenced by nearby volcanic activity and crustal shortening in the reactivated bangong suture zone in the southern nima basin an 50 m y late cretaceous eocene depositional hiatus correlates with major crustal shortening and ensuing voluminous ignimbrite eruptions in the lhasa terrane in the northern nima basin deposition continued during latest cretaceous time recording arid paleoclimate in evaporitic lacustrine and eolian dunefield deposits by oligocene time the nima basin comprised two independent depocenters that accumulated coarse grained alluvial fluvial lacustrine evaporitic and fan delta deposits in close association with reactivated thrusts in the bangong suture zone carbon and oxygen isotope data from oligocene paleosol carbonate reported elsewhere indicate that regional paleoelevation during the late oligocene was 4 6 km as it is today overall the depositional record of the nima basin combined with ongoing structural and geochronologic studies demonstrates that the bangong suture zone was reactivated during middle cretaceous and middle tertiary time that the intervening 50 m y interval was a time of regional upper crustal shortening in the lhasa terrane followed by regional ignimbrite eruptions and that arid paleoclimate and high paleoelevation were established by the late cretaceous and late oligocene respectively within the context of other data sets from the lhasa terrane the record of deposition in the nima basin is consistent with low angle subduction of neotethyan oceanic lithosphere and reactivation of the bangong suture zone during the early cretaceous followed by shortening within the gangdese retroarc and northern lhasa terrane thrust belts during the late cretaceous lithospheric delamination dripping and regional magmatic flare up during latest cretaceous through early tertiary time and underthrusting of indian lower crust and lithosphere as far north as the bangong suture zone during late oligocene time

exhumation history of the gangdese batholith southern tibetan plateau evidence from apatite and zircon u th he thermochronology abstractto test previously suggested exhumation histories of the gangdese batholith in the central part of the transhimalayan plutonic belt we conducted paired apatite and zircon u th he thermochronological investigations of the yarlung zangbo gorge in the central part of the batholith age elevation relationships and multisystem thermochronometers showed three periods of accelerated exhumation 46 48 22 18 and 11 8 ma combining these data with previously published thermochronological ages and synthesizing these ages with regional geological events provides an entire exhumation history the cretaceous early paleogene exhumation of the gangdese batholith was probably caused by both the northward subduction of the neo tethys and the collision between the lhasa and qiangtang blocks the early miocene rapid exhumation might be a response to shortening caused by the gangdese thrust or erosion driven by dynamic uplift following lithospheric delamination in contrast the late miocene exhumation is coi

role of large particles in the transport of elements and organic compounds through the oceanic water column abstract during the past decade data from a variety of sources have been obtained which show conclusively that the relatively rare large particles sinking through the water column are responsible for the majority of the downward vertical mass flux in the sea this finding has important implications for understanding the transfer distribution and fate of elements and organic compounds in marine waters the large 100 m detrital particles responsible for vertical flux are primarily biogenic and range in size and composition from small discrete fecal pellets and plankton hard parts to large amorphous aggregates or snow which contain both organic and inorganic constituents depending on size shape and density these particles sink at rates ranging from 1000 m day 1 several methods have been developed for sampling these particles of which in situ sediment trapping has probably furnished the most comprehensive qualitative and quantitative information on the role large particles play in material transport flux studies have highlighted the importance of marine heterotrophs in packaging fine suspended particulate matter into large rapidly sinking particles which accelerate the movement of incorporated materials to depth large particle production via biological packaging is not restricted to the euphotic zone but can occur at all depths and information is now accruing on rates of production of large particles in the water column chemical analyses of sedimenting particles collected in sediment traps and those sampled by other means have allowed quantifying vertical fluxes and residence times of elements radionuclides and organic compounds natural and anthropogenic in various oceanic regimes pertinent studies dealing with the above aspects are reviewed and several areas for future research are suggested

contact metamorphism in the malashan dome north himalayan gneiss domes southern tibet an example of shallow extensional tectonics in the tethys himalaya combined petrographic structural and geochronological study of the malashan dome one of the north himalayan gneiss domes reveals that it is cored by a miocene granite the malashan granite that intruded into the jurassic sedimentary rocks of tethys himalaya two other granites in the area are referred to as the paiku and cuobu granites new zircon shrimp u pb and muscovite and biotite 40ar 39ar dating show that the paiku granite was emplaced during 22 2 16 2 ma average 19 3 3 9 ma and cooled rapidly to 350 400 c at around 15 9 ma whole rock granite chemistry suggests the original granitic magma may have formed by muscovite dehydration melting of a protolith chemically similar to the high himalayan crystalline sequence abundant calcareous metasedimentary rocks and minor garnet staurolite biotite muscovite andalusite schists record contact metamorphism by three granites that intruded intermittently into the jurassic sediments between 18 5 and 15 3 ma two stages of widespread penetrative ductile deformation d1 and d2 can be defined microstructural studies of metapelites combined with geothermobarometry and pseudosection analyses yield p t conditions of 4 8 0 8 kbar at 550 50 c during a non deformational stage between d1 and d2 and 3 1 4 1 kbar at 530 575 c during syn to post d2 the pressure estimates for the syn to post d2 growth of andalusite suggest relatively shallow depth of 15 2 km extensional ductile deformation that took place within a shear zone of the south tibetan detachment system close temporal association between intrusion of the malashan granite and onset of d2 suggests extension may have been triggered by the intrusion of the malashan granite

paleocene eocene foreland basin evolution in the himalaya of southern tibet and nepal implications for the age of initial india asia collision siliciclastic sedimentary rocks derived from the southern lhasa terrane sitting depositionally upon rocks of the northern indian passive continental margin provide an estimate of the age of initial contact between the continental parts of the indian and asian plates we report sedimentological sedimentary petrological and geochronological data from upper cretaceous paleocene strata in the sangdanlin section located along the southern flank of the indus yarlung suture zone in southern tibet this is probably the most proximal and therefore the oldest record of the india asia collision these strata were deposited by high density turbidity currents or concentrated density flows and suspension settling of pelagic biogenic debris in a deep marine setting an abrupt change from quartz arenitic to feldspatholithic sandstone compositions marks the transition from indian to asian sediment provenance the abrupt compositional change is accompanied by changes in u pb ages of detrital zircons diagnostic of a sediment provenance reversal from indian to asian sources the timing of the transition is bracketed between 60 ma and 58 5 0 6 ma by detrital zircon u pb ages and zircon u pb ages from a tuffaceous bed in the upper part of the section in the context of a palinspastically restored regional paleogeographic framework data from the sangdanlin section combined with previously published data from the northern tethyan himalaya and the frontal nepalese lesser himalaya and subhimalaya suggest that a flexural wave migrated 1300 km southward across what is now the himalayan thrust belt from paleocene time to the present

new insight into the south tibetan detachment system not a single progressive deformation 1 low angle normal faults lanf typically regarded as accommodating crustal or lithospheric extension may also form during lithospheric shortening the best studied system of syn contractional lanfs is the south tibetan detachment system a network of low angle normal sense faults and shear zones that formed coevally with and parallel to south vergent thrusts during lithospheric shortening accompanying development of the himalayan orogen in the eastern himalaya there are several across strike exposures of the south tibetan detachment system we present new structural and thermometry data from the eastern himalaya that demonstrate that the south tibetan detachment system cannot have formed as a single progressive structure we characterize and distinguish two distinct structural and tectonic components within the currently recognized system 1 an extensive diffuse sheared layer that formed the boundary between strong upper crust and weak southward flowing middle crust and 2 a network of brittle ductile lanfs that locally exhume partly excise and overprint the earlier mylonite zone at the topographic break between the himalayan orogen and the tibetan plateau the sheared layer not a lanf formed the boundary between upper and middle crust during ductile flow of the middle crust and is extensively exposed in the himalaya at the base of klippen of upper crustal rocks preserved in bhutan along the crest of the himalaya where it has been excised and exhumed by the brittle ductile extrusion lanfs and bounding the cores of the north himalayan gneiss domes citation kellett d a and d grujic 2012 new insight into the south tibetan detachment system not a single progressive deformation tectonics 31 tc2007 doi 10 1029 2011tc002957

sedimentary history of the tethyan basin in the tibetan himalayas after an epicontinental phase the sedimentary rocks in the tibetan himalayas document a complete wilson cycle of the neo tethyan tethys ill evolution between the gondwana supercontinent and its northward drifting margin lhasa block from the late permian to the eocene during the triassic rift stage the basin was filled with a huge clastic dominated sediment wedge with up to 5 000 m of flysch in the northern zone widespread deltaic clastics and shallow water carbonates of late norian to earliest jurassic age in the southern zone mark in conjunction with decreasing tectonic subsidence the transition to the drift stage some 4 500 m of jurassic and early cretaceous shallow water carbonates and siliciclastics accumulated on the tethyan indian passive margin deepening upward sequences with condensed beds at their tops alternate with repeated progradational packages of shelf sediments extensive abyssal sediments with basaltic volcanics in the northern deep water zone reflect continued ocean spreading and thermal subsidence paleomagnetic data gained separately for the northern indian plate and the lhasa block indicate that the neo tethys reached its maximum width about 110 ma ago with a spreading rate of 4 8 cm year before it commenced to close again during the remnant basin stage in the late cretaceous and paleogene a shallowing upward megasequence capped by a carbonate platform developed in the southern inner shelf realm in the northern slope basin plain zone turbidites and chaotic sediments derived from both the acretionary wedge and the steepening slope of the passive margin accumulated the depositional center of the remnant basin shifted southward as a result of flexural subsidence and southward overthrusting the sediments from the triassic to the paleogene are tentatively subdivided into five mega sequences which are controlled mainly by regional tectonics climatic influence e g carbonate deposition due to northward plate motion is partially subdued by terrigenous input and or increased water depth during the oligocene and miocene crustal shortening led to rapid uplift and the deposition of fluvial molasse in limited basins

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

uplift driven climate change at 12 ma a long δ18o record from the ne margin of the tibetan plateau carbonates from fluvial and lacustrine sediments were sampled from multiple measured sections in the linxia basin of western china based on textural and mineralogical evidence lacustrine carbonates are primary precipitates from lake water a 29 million year record of the oxygen isotope composition of meteoric water is inferred from the n 18 o values of these carbonates this inference is based on the most negative n 18 o values in the lake carbonates which represent lake waters that have experienced the least evaporative enrichment carbonate n 18 o values a proxy for rainfall n 18 o are v310 5x throughout the interval of 29 12 ma at 12 ma there is a shift to 39x a value that remains into the pliocene this implies a major reorganization of atmospheric circulation patterns and a shift to more arid conditions at the ne margin of the tibetan plateau with the post 12 ma system similar to that of today the 12 ma event may represent the time at which the tibetan plateau achieves sufficient elevation to block the penetration of moisture from the indian ocean or south pacific into western china the period of greatest aridity is from 9 6 to 8 2 ma a time interval which agrees well with other climate records 2003 elsevier b v all rights reserved

dominance of tectonics over climate in himalayan denudation landscape denudation in actively deforming mountain ranges is controlled by a combination of rock uplift and surface runoff induced by precipitation whereas the relative contribution of these factors is important to our understanding of the evolution of orogenic topography no consensus currently exists concerning their respective infl uences to address this question denudation rates at centennial to millennial time scales were deduced from 10be concentrations in detrital sediments derived from 30 small basins 10 600 km2 in an 200 km wide region in central nepal along a northward strike perpendicular transect average denudation rates sharply increase from 0 5 mm yr in the lesser himalayas to 1 mm yr when crossing the physiographic transition and then accelerate to 2 3 mm yr on the southern flank of the high peaks in the greater himalayas despite a more than five fold increase in denudation rate between the southern and northern parts of this transect the corresponding areas display similar precipitation rates the primary parameter that presents a signifi cant co variation with denudation is the long term rockuplift rate that is interpreted to result from the ramp fl at transition along the main himalayan thrust we propose that in this rapidly uplifting mountain range landscapes adjust quickly to changing climatic conditions such that denudation is mainly limited by the rate at which material is pushed upward by tectonic processes and made available for removal by surface processes in this particular context variations in precipitation appear to have only a second order role in modulating the denudation signal that is primarily set by the background rock uplift rate 2014 geological society of america

the south tibetan detachment system facilitates ultra rapid cooling of granulite facies rocks in sikkim himalaya 1 the eastern himalaya is characterized by a region of granulites and local granulitized eclogites that have been exhumed via isothermal decompression from lower crustal depths during the india asia collision spatially most of these regions are proximal to the south tibetan detachment system an orogen parallel normal sense detachment system that operated during the miocene suggesting that it played a role in their exhumation here we use geo and thermochronological methods to study the deformation and cooling history of footwall rocks of the south tibetan detachment system in northern sikkim india these data demonstrate that the south tibetan detachment system was active in sikkim between 23 6 and 13 ma and that footwall rocks cooled rapidly from 700 to 120 c between 15 13 ma while active the south tibetan detachment system exhumed rocks from mid crustal depths but an additional heat source such as strain heating advected melt and or crustal thinning is required to explain the observed isothermal decompression cessation of movement on the south tibetan detachment system produced rapid cooling of the footwall as isotherms relaxed a regional comparison of temperature time data for the eastern south tibetan detachment system indicates a lack of synchronicity between the sa er sikkim yadong section and the nw bhutan section to accommodate this requires either strike slip tear faulting or local out of sequence thrusting in the younger segment of the orogen

evolution of north himalayan gneiss domes structural and metamorphic studies in mabja dome southern tibet field structural and metamorphic petrology investigations of mabja gneiss dome southern tibet suggest that contractional extensional and diapiric processes contributed to the structural evolution and formation of the domal geometry the dome is cored by migmatites overlain by sillimanite zone metasedimentary rocks and orthogneiss metamorphic grade diminishes upsection and is defined by a series of concentric isograds evidence for three major deformational events two older penetrative contractional and extensional events and a younger doming event is preserved metamorphism migmitization and emplacement of a leucocratic dike swarm were syntectonic with the extensional event at mid crustal levels metamorphic temperatures and pressures range from 500 8c and 150 450 mpa in chloritoid zone rocks to 705 65 8c and 820 100 mpa in sillimanite zone rocks we suggest that adiabatic decompression during extensional collapse contributed to development of migmatites diapiric rise of low density migmatites was the driving force at least in part for the development of the domal geometry the structural and metamorphic histories documented in mabja dome are similar to kangmar dome suggesting widespread occurrence of these events throughout southern tibet q 2004 elsevier ltd all rights reserved

middle to late miocene extremely rapid exhumation and thermal reequilibration in the kung co rift southern tibet 1 the kung co rift is an approximately nnw striking wsw dipping normal fault exposed in southern tibet and is part of an extensive network of active approximately ns striking normal faults exposed across the tibetan plateau detailed new and published u th he zircon and apatite thermochronometric data from the footwall of the early miocene kung co granite provide constraints on the middle miocene to present day exhumation history of the footwall to the kung co fault inverse modeling of thermochronometric data yield age patterns that are interpreted as indicating 1 initiation of normal fault slip at 12 13 ma and rapid exhumation of the footwall between 13 and 10 ma 2 acceleration of normal fault slip at rates of 21 9 6 9 mm yr at 10 ma 3 rapid thermal reequilibration between 10 and 9 ma and 4 slow exhumation and or quiescence from 9 ma to the present day hanging glacial valleys in the footwall and fault scarps that cut late quaternary till and moraine deposits indicate that fault slip continues today middle to late miocene initiation of extension across the kung co rift is broadly the same as the documented initiation of ew extension across the south central tibetan plateau eastward flow of middle or lower crust from beneath tibet accommodated by northward underthrusting of indian crust beneath tibet provides a plausible explanation for the onset of ew extension across the tibetan plateau

assessing quaternary reactivation of the main central thrust zone central nepal himalaya new thermochronologic data and numerical modeling we study the recent dynamics of the central nepal himalaya focusing on possible reactivation of the footwall of the main central thrust which is marked by an abrupt topographic transition different tectonic mechanisms such as overthrusting of a major crustal ramp underplating or out of sequence thrusting have been suggested to explain the morphology and exhumation patterns in this area we present 25 new apatite fission track ages collected along a north south transect in central nepal as well as two age elevation profiles ages are consistently younger than 3 ma old in the main central thrust zone and increase continuously to 4 6 ma old in the south no jump in apatite fission track ages is observed across the topographic transition apparent exhumation rates from age elevation relationships vary from 0 46 0 13 0 09 km ma in the palung granite south of kathmandu to 4 4 4 8 1 5 km ma in the main central thrust zone the latter rate is probably overestimated by a factor of two due to topographic effects as shown by a new numerical model these strongly varying exhumation rates can be explained by overthrusting of a crustal ramp which exerts a primary control on age patterns and do not require out of sequence reactivation of thrusts in the main central thrust zone

east west extension and miocene environmental change in the southern tibetan plateau thakkhola graben central nepal east west extensional basins are distributed across the southern half of the tibetan plateau at an elevation of 4 km these basins have generated much interest because of their potential implications for the regional tectonics and force distribution in the plateau this study documents the sedimentology of the miocene pliocene thakkhola graben fill in order to reconstruct basin evolution and paleoenvironment analysis of depositional systems paleodrainage patterns and conglomerate clast provenance of the 1 km thick graben fill sets limits on the timing of activity of the basin bounding faults and the development of southward axial drainage in the basin during the deposition of the oldest basin fill tetang formation ca 11 9 6 ma probably in a restricted basin minor motion occurred on the basin bounding fault systems an angular unconformity separates the tetang and overlying thakkhola formations where this contact can be observed in the southern part of the basin southward axial drainage was established by ca 7 ma with the onset of deposition of the thakkhola formation several episodes of damming of this drainage system are recorded by widespread lacustrine deposits in the southern part of the basin facies distribution and the progressive rotation of strata in the thakkhola formation indicate that the dangardzong fault on the western edge of the basin was active at this time and drainage ponding may have been related to displacement on normal faults associated with the south tibetan detachment system to the south of thakkhola graben contrasts between deposits of the tetang and thakkhola formations provide evidence for environmental change in the basin in the tetang formation the abundance of lacustrine facies the pollen record and the absence of paleosol carbonate suggest that conditions were more humid than during subsequent deposition of the thakkhola formation environmental change in the thakkhola graben coincided with environmental change observed in the siwalik foreland basin sequence arabian sea and bay of bengal at ca 8 7 ma although this climate change event has been previously attributed to intensification of the asian monsoon in response to uplift of the tibetan plateau paleoaltimetry data indicate that this region had already attained a high elevation by ca 11 ma thus the thakkhola graben stratigraphic record suggests that uplift of the southern tibetan plateau and the onset of the asian monsoon as inferred from paleoclimatic indicators were not directly related in a simple way

thermomechanical modelling of slab detachment abstract slab detachment or breakoff is appreciated as an important geological process as shown by recent tomographic imaging using a 2 d upper mantle model 660 km deep and 2000 km wide we have investigated with a 2 d finite difference and marker in cell numerical technique the multi resolutional character of the thermomechanical phenomena related to this complex geological process our experiments show that this process can be initiated in form of slab necking by a prolonged 8 30 my period of slab weakening due to thermal diffusion 60 c my due to strong positive thermal feedback from shear heating detached slab fragments sink rapidly with a tendency for coherent rotation the influence of temperature and pressure dependent thermal conductivity on the process of thermal weakening of the slab is quite significant this supports the idea that the breakoff process is triggered by thermal diffusion on a time scale linearly dependent on heat conductivity rapid topographic changes and increasing volcanic activities due to the melting of subducted oceanic crust are possible scenarios of this vigorously driven geodynamic process

detrital thermochronology and sediment petrology of the middle siwaliks along the muksar khola section in eastern nepal abstract sediment derived from erosion of the himalaya during the miocene pliocene was deposited in the himalayan foreland basin and is now exposed in the siwalik formation between the main boundary thrust and main frontal thrust these sediments hold important information on orogenic exhumation during this time in this preliminary study we sampled the middle siwalik formation in eastern nepal along the muksar khola section for thermochronologic and sediment petrologic analysis detrital zircon fission track thermochronology shows that the age signal of the middle siwalik formation of eastern nepal has some similarities but also differences with published data for western central nepal the muksar khola section samples show two static peaks at about 14 and 140 ma which is similar to the siwalik formation of western central nepal in contrast a signal of consistently fast exhumation was not observed in the upper middle siwaliks in eastern nepal this could be related to a position of the main central thrust further south to overall less erosional exhumation of higher himalayan rocks or to a reduced exposure of the underlying lesser himalayan units in comparison with western central nepal based on our apatite fission track data we propose that above the section dated by ojha et al 2009 with magnetostratigraphy about 1000 m of overlying upper siwalik formation exist

decoupling of long term exhumation and short term erosion rates in the sikkim himalaya abstract understanding the relative strengths of tectonic and climatic forcing on erosion at different spatial and temporal scales is important to understand the evolution of orogenic topography to address this question we quantified exhumation rates at geological timescales and erosion rates at millennial timescales in modern river sands from 10 sub catchments of the tista river drainage basin in the sikkim himalaya northeast india using detrital apatite fission track thermochronology and cosmogenic 10be analyses respectively we compare these rates to several potential geomorphic or climatic forcing parameters our results show that millennial erosion rates are generally higher and spatially more variable than long term exhumation rates in sikkim they also show strongly contrasting spatial patterns suggesting that the processes controlling these rates are decoupled at geological timescales exhumation rates decrease from south to north with rates up to 1 2 0 6 mm yr recorded in southwest sikkim and as low as 0 5 0 2 mm yr in the northernmost catchment long term exhumation rates do not correlate with any geomorphic or climatic parameter we suggest they are tectonically controlled high rates in southwest sikkim may be linked to the building of the lesser himalaya rangit duplex whereas low rates in north sikkim are consistent with cessation of extensional exhumation along the south tibetan detachment after 13 ma the highest apparent erosion rates recorded by cosmogenic nuclides 5 mm yr occur in catchments spanning the main central thrust zone but these appear to be strongly influenced by recent landsliding high millennial erosion rates 1 2 mm yr also occur in north sikkim and may be climatically driven through strong glacial inheritance of the landscape as attested by high channel steepness values close to the maximum extent of glaciers during the last glacial maximum in contrast variations in rainfall rate do not seem to strongly influence either millennial erosion or long term exhumation rates in sikkim

e w extension and block rotation of the southeastern tibet unravelling late deformation stages in the eastern himalayas nw bhutan by means of pyrrhotite remanences abstract in the himalayan chain the collision of india into eurasia has produced some of the most complex crustal interactions along the himalayan alpine orogen in nw bhutan middle to late miocene deformation has been partitioned between conjugate strike slip faulting e w extension along the yadong gulu graben and kilometre scale folding to better understand the late deformation stages and their implications for the evolution of the eastern himalayas the palaeomagnetism in the erosional remnant of the tethyan himalayan rocks outcropping in nw bhutan has been studied their position to the south of the trace of the inner south tibetan detachment to the south of the tibetan plateau offers a unique possibility to study the tertiary rotation of the himalayas pyrrhotite is the carrier of the characteristic magnetisation based on 270 325 c unblocking temperatures the age of the remanence is ca 13 ma indicated by illite 40 k 40 ar cooling ages and a negative fold test small circle intersection method applied to the pyrrhotite components shows a ca 32 clockwise rotation with respect to stable india since 13 ma we suggest that this clockwise rotation is related to strain partitioning between ne directed shortening sinistral slip along the lingshi fault and east west extension this represents a field based explanation and a minimum onset age for present day eastward motion of the upper crust of se tibet and ne himalayas

cretaceous to cenozoic evolution of the northern lhasa terrane and the early paleogene development of peneplains at nam co tibetan plateau abstract highly elevated and well preserved peneplains are characteristic geomorphic features of the tibetan plateau in the northern lhasa terrane north northwest of nam co the peneplains were carved in granitoids and in their metasedimentary host formations we use multi method geochronology zircon u pb and u th he dating and apatite fission track and u th he dating to constrain the post emplacement thermal history of the granitoids and the timing and rate of final exhumation of the peneplain areas la icp ms u pb geochronology of zircons yields two narrow age groups for the intrusions at around 118 ma and 85 ma and a third group records paleocene volcanic activity 63 58 ma in the nam co area the low temperature thermochronometers indicate common age groups for the entire nam co area zircon u th he ages cluster around 75 ma apatite fission track ages around 60 ma and apatite u th he ages around 50 ma modelling of the thermochronological data indicates that exhumation of the basement blocks took place in latest cretaceous to earliest paleogene time by middle eocene time the relief was already flat documented by a thin alluvial sediment sequence covering a part of the planated area the present day horst and graben structure of the peneplains is a late cenozoic feature triggered by e w extension of the tibetan plateau the new thermochronological data precisely bracket the age of the planation to early eocene i e between ca 55 and 45 ma the erosional base level can be deduced from the presence of early cretaceous zircon grains in eocene strata of bengal basin the sediment generated during exhumation of the nam co area was transported by an early cenozoic river system into the ocean suggesting that planation occurred at low elevation

deformation of leucogranites of the crystalline main central sheet in southern tibet china abstract towards the top of the main central sheet in southern tibet china leucogranites intrude metamorphic rocks related to rocks of the main central thrust in the himalayas our study shows that the leucogranites suffered a northward shear deformation the significance of this event is discussed in terms of gravity driven decollement of the 10 000 m thick tethyan sedimentary pile over its crystalline basement

temperature and age constraints on the metamorphism of the tethyan himalaya in central nepal a multidisciplinary approach abstract metasediments of devonian to triassic age of the tethyan himalaya th from several areas in central and western nepal between western dolpo and marsyandi valley were sampled for thermo metamorphic studies illite and chlorite crystallinity vitrinite reflectance calcite dolomite and chlorite chloritoid geothermometers k ar dating on illite rich fractions and zircon fission track thermochronology this paper reports for the first time the occurrence of chloritoid in the th out of the range of granite contact aureoles it also presents the first zircon fission track dating performed on th metasediments the peak temperatures of metamorphism have been around 250 300 c 320 350 c 330 370 c and 400 450 c in the western dolpo hidden valley manang and marpha areas respectively in the manang and hidden valley areas illite k ar data are interpreted as ages of recrystallized k white micas newly formed during metamorphism at around 25 30 ma in the marpha area illite k ar and zircon fission track ages 12 15 ma are consistent representing cooling ages after metamorphic overprinting of higher grade than in the other areas the joint investigation of the organic maturation and zircon ft chronology yields insight on the thermal calibration of zircon reset the rmax of 5 7 to 8 0 indicates a temperature range of ca 315 325 c which is in the partial annealing zone of the zircon ft thermochronometer our results might be explained by the presence of a thick thrust sheet once existing above the study area

numerical inversion of deformation caused by pressure sources application to mount etna italy summary the interpretation of geodetic data in volcanic areas is usually based on analytical deformation models although numerical finite element fe modelling allows realistic features such as topography and crustal heterogeneities to be included the technique is not computationally convenient for solving inverse problems using classical methods in this paper we develop a general tool to perform inversions of geodetic data by means of 3 d fe models the forward model is a library of numerical displacement solutions where each entry of the library is the surface displacement due to a single stress component applied to an element of the grid the final solution is a weighted combination of the six stress components applied to a single elementsource the pre computed forward models are implemented in a global search algorithm followed by an appraisal of the sampled solutions after providing extended testing we apply the method to model the 1993 1997 inflation phase at mt etna documented by gps and edm measurements we consider four different forward libraries computed in models characterized by homogeneous heterogeneous medium and flat topographic free surface our results suggest that the elastic heterogeneities of the medium can significantly alter the position of the inferred source while the topography has minor effect

timing of india asia collision geological biostratigraphic and palaeomagnetic constraints 1 a range of ages have been proposed for the timing of india asia collision the range to some extent reflects different definitions of collision and methods used to date it in this paper we discuss three approaches that have been used to constrain the time of collision the time of cessation of marine facies the time of the first arrival of asian detritus on the indian plate and the determination of the relative positions of india and asia through time in the qumiba sedimentary section located south of the yarlung tsangpo suture in tibet a previous work has dated marine facies at middle to late eocene by far the youngest marine sediments recorded in the region by contrast our biostratigraphic data indicate the youngest marine facies preserved at this locality are 50 6 52 8 ma in broad agreement with the timing of cessation of marine facies elsewhere throughout the region double dating of detrital zircons from this formation by u pb and fission track methods indicates an asian contribution to the rocks thus documenting the time of arrival of asian material onto the indian plate at this time and hence constraining the time of india asia collision our reconstruction of the positions of india and asia by using a compilation of published palaeomagnetic data indicates initial contact between the continents in the early eocene we conclude the paper with a discussion on the viability of a recent assertion that collision between india and asia could not have occurred prior to 35 ma

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

the himalayan passive margin from precambrian to cretaceous times abstract the himalaya passive margin should be separated into the indian himalaya and the pakistan himalaya divided by the nanga parbat haramosh basement uplift in the northwestern syntaxes the indian passive margin shows a complex history involving splitting of microcontinents off the northern gondwana margin from early paleozoic times until the jurassic in contrast the pakistan himalaya formed part of the stable indian shelf until separation from africa started in the jurassic each margin has a distinct stratigraphic history until the mid mesozoic after which they show similar histories until the tertiary the himalaya mountains in india consist of three distinct tectonic units juxtaposed during the neogene by southward thrusting of the northern indian precambrian to mesozoic passive margin the lesser himalaya on the south forms a late precambrian passive margin usually directly overlain by late cretaceous to tertiary clastics its stratigraphy is closest to that of the indian shield in contrast the high himalaya has a thin ordovician to carboniferous shelf sequence deposited after early ordovician deformation and granite intrusion above this thick permian and mesozoic shelf sequences mark the separation of continental blocks off the northern indian margin and the opening of the neotethys ocean the north himalaya formed the slope and basin of this ocean and consists of reactivated paleozoic gneiss domes overlain by thick mesozoic sediments which pass in the northeastern himalaya into an enormously thick sedimentary sequence resting on oceanic crust paleomagnetic and structural evidence indicates at least 500 km of southward thrusting along the main central thrust between the lesser and high himalaya this thrusting together with a further 250 km of erosion at the front of the nappes has removed the entire inner shelf of the himalaya passive margin and can explain the startling contrast between the high and lesser himalaya stratigraphic sequences but there are still discrepancies particularly in the eastern himalaya where precambrian basement is juxtaposed with the north himalaya mesozoic slope these discrepancies can be partly resolved by strike slip movements roughly parallel to the himalaya which have removed parts of the northern indian passive margin such faults of the requisite orientation displacement and age occur in southeast asia where their cumulative displacements can add up to several thousand kilometres similar faults of undoubted pre tertiary age occur in the pakistan himalaya here the passive margin shows a much simpler history of jurassic to miocene subsidence

sedimentation in a glacier fed lake the role of pelletization on deposition of fine grained suspensates abstract bow lake is a small alpine lake in western alberta canada mainly fed by glacial meltwater suspended sediment loads high only during summer months are transported through the lake dominantly by interflows within the epilimnion consideration of interflow velocities and particle settling rates suggests that most clay supplied by stream inflow should be transported out of the lake during summer when thermal stratification is best developed field observations however show that clay comprises a persistent and significant component of lake bottom sediment deposited during summer this apparently anomalous clay settling behavior can be attributed to production of fecal pellets found in both suspended sediment traps and bottom surficial deposits the pellets probably produced by pelagic copepods are ovoidal with lengths typically ranging between 100 and 250 m and are composed of clay and silt measured settling rates in water at 5 c ranged between 80 and 400 m day pellet production then accelerates the downward transfer of suspended detritus resulting in deposition of much of the fine sediment that would otherwise escape the lake in interflows this confuses patterns of bottom sediment texture and mineralogy which would ordinarily be expected from settling behavior of unpelletized suspensates

himalayan metamorphic sequence as an orogenic channel insight from bhutan the bhutan himalayas differ from the rest of the himalayas in two major ways i low grade sedimentary rocks lie above the greater himalayan sequence as klippen i e erosional remains of the south tibetan detachment and ii an out of sequence thrust the kakhtang thrust lies structurally above the klippen and it doubles the exposed thickness of the greater himalayan sequence our field observations and geochronological data constrain the main kinematic events in the bhutan himalayas crystallisation ages of leucogranite dykes deformed by the main central thrust and the south tibetan detachment indicate that these two structures operated together between 16 and 22 ma the out of sequence kakhtang thrust was active at 10 14 ma and was concurrent with reactivation of the south tibetan detachment restoration of the bhutan himalayas prior to the out of sequence thrusting shows that the greater himalayan sequence was the core of a long low viscosity crustal channel extending under the tibetan plateau we propose that the gravity driven southward extrusion of the channel material from underneath the tibetan plateau caused the inverted metamorphic sequence in the lesser himalayan sequence and in the greater himalayan sequence this process also led to occurrences of present day surface rocks that were derived from variable distances down dip but from similar crustal depths such an exhumation pattern can explain the similar peak pressures for the greater himalayan sequence along the length of the himalayas

the interaction of plankton and suspended sediment in fjords abstract the interaction of plankton with suspended sediment affects both the plankton and the sediment trace metals organics and nutrients that are adsorbed onto suspended particles affect the supply of these materials to phytoplankton and zooplankton ingestion of suspended sediment by zooplankton exposes the particulate minerals to physical and chemical breakdown occasionally strong enough to produce changes in particle mineralogy fecal pellets produced from undigested material package fine particulate material and thus increase their rate of sedimentation this is particularly important in fjords an aqueous environment that is dominated by the vertical flux of suspended sediment pelletization also allows the transport of undigested food to greater depths providing an important source of organic material to benthic sediments and food to deep water organisms

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

sedimentary structures on the bengal shelf a multi scale approach to sedimentary fabric interpretation abstract sedimentary structures on the bengal shelf are examined using x radiographic thin section and scanning electron microscopic sem techniques which reveal contrasting depositional processes for the inner and middle shelf as well as for either side of a major submarine canyon indenting the western shelf margin the inner shelf is characterized by tidally produced interlaminated silts and muds that reveal a neap spring cyclicity the cyclicity is more pronounced for the winter season when high discharge monsoon conditions do not obscure the tidal signature on the middle shelf alternating beds of silts and muds probably represent high discharge and low discharge conditions respectively silty beds on the western margin of the submarine canyon are thinner than those found on the eastern margin the western margin receives predominantly fine grained ganges sediments in contrast to the eastern area which receives both ganges and coarser brahmaputra sediments these observations indicate that the submarine canyon presently intercepts westward along shelf sediment transport muddy beds and laminae that appear internally homogeneous in the x radiographs and exhibit faint bedding in thin sections often are composed of resistant protruding and less resistant recessed microlaminae as observed with the sem on the inner shelf resistant microlaminae are composed of fine grained polymineralic equant particles the less resistant microlaminae are composed of large platy illite grains rimmed by smaller platy or lath like illite particles resistant and less resistant microlaminae within middle shelf muddy sediments have the same textural characteristics observed on the inner shelf however both types essentially are monomineralic on the middle shelf the smallest particles within the less resistant microlaminae exhibit an open flocculated structure that may result from higher salinity and or decreased bottom shear stress associated with tidal currents biological structures predominantly occur on the western inner and middle shelf particularly in intervals of high smectite concentrations this relationship may exist because of a preferential organic affinity for expandable minerals providing a favorable substrate for benthic organisms

sedimentary processes of fine grained material and the effect of seawall construction in the daeho macrotidal flat nearshore area northern west coast of korea abstract sedimentary processes on the lower tidal flat of the reclaimed seosan bay daeho tidal flat at the west coast of korea have undergone significant changes since 1984 when seawalls over 8 km long were constructed at the bay mouth comparison of pre and post construction tidal flow models indicates that in general tidal currents have increased considerably in front of the daeho tidal flat with tidal flow directions having changed from shore normal to shore parallel the 12 h measurements of currents and suspended sediment concentrations at four anchored hydrographic stations show nearshore suspended sediments drifting southward throughout the year bypassing the daeho tidal flat cyclic seasonal variations occur prominently in the grain texture of the tidal flat sediments and associated sedimentation rates in summer fine grained sediments deposit at a rate of 2 6 mm month particularly in sheltered places during the winter monsoon erosional processes are greatly enhanced by the interplay of tidal currents and waves up to 30 mm month this sedimentary seasonality is reflected in the variations in the concentration of nearshore suspended matter the concentration of suspended matter is much higher in winter 60 80 mg l because of vigorous resuspension activities compared with the summer concentration 10 30 mg l the direct effect of seawall construction coupled with strong natural seasonality can be seen from the continuous monitoring of sedimentation rates on the tidal flat the 2 year 1993 1995 measurements indicate that the entire daeho tidal flat was eroded during that period at the abnormally high average rate of 10 cm year the causal blocking of bayward sediment and water movements by any artificial structure might lead to similar erosional conditions to many other bays facing the yellow sea on the both chinese and korean coastlines subjected to energetic winter wave attack

biodegradation of clay minerals laboratory experiments and results from wadden sea tidal flat sediments laboratory experiments indicate that clay particles of the 2 um fraction can be considerably degraded by the bivalve mytilus edulis dickite kaolinite smectite chlorite and illite particles are partially dissolved and rounded by the digestive processes structural changes and a decrease in crystallinity of kaolinites dickites and illites are striking investigations of clay minerals from tidal flat sediments highly populated by marine invertebrates from marine suspensions and samples from profiles across mytilis edulis beds on the north sea coast of germany show however that these materials have a uniform mineral composition this uniformity of clay mineralogy can be explained by the hydrodynamic conditions in the nearshore area where tidal currents and waves cause a periodic resuspension and transport of sediments introduced into the wadden sea from different sources glacial fluviatile processes of bioturbation also redistribute the sedimentary material these mixing processes effectively erase any evidence of local enrichment of the biogenically degraded clays however the disordered clays should react more sensitively i e have higher adsorption capacities for organic substances hydroxides and metals in sediments than untreated materials so that biodegradation is of basic importance for clay diagenesis during these processes pb v ni and fe are dissolved from the digested particles

the malashan gneiss dome in south tibet comparative study with the kangmar dome with special reference to kinematics of deformation and origin of associated granites

controls on cenozoic exhumation of the tethyan himalaya from fission track thermochronology and detrital zircon u pb geochronology in the gyirong basin area southern tibet the gyirong basin southern tibet contains the record of miocene pliocene exhumation drainage development and sedimentation along the northern flank of the himalaya the tectonic controls on basin formation and their potential link to the south tibetan detachment system stds are not well understood we use detrital zircon zft and apatite aft fission track analysis together with detrital zircon u pb dating to decipher the provenance of gyirong basin sediments and the exhumation history of the source areas results are presented for nine detrital samples of gyirong basin sediments aft zft and u pb two modern river sediment samples zft and aft and six bedrock samples zft from transect across the gyirong fault bounding the basin to the east the combination of detrital zircon u pb and fission track data demonstrates that the gyirong basin sediments were sourced locally from the tethyan sedimentary sequence this provenance pattern indicates that deposition was controlled by the gyirong fault active since 10 ma whose vertical throw was probably 5000 m rather than being controlled by normal faults associated with the stds the detrital thermochronology data contain two prominent age groups at 37 41 and 15 18 ma suggesting rapid exhumation at these times a 15 18 ma phase of rapid exhumation has been recorded widely in both southern tibet and the himalaya a possible interpretation for such a major regional exhumation event might be detachment of the subducting indian plate slab during the middle miocene inducing dynamic uplift of the indian plate overriding its own slab

effects of inherited cores and magmatic overgrowths on zircon u th he ages and age eu trends from greater himalayan sequence rocks mount everest region tibet previous constraints on the timing and rate of exhumation of the footwall of the south tibetan detachment system stds north of mount everest suggest rapid miocene cooling from 700 c to 120 c between 14 17 ma however 25 new single grain zircon he ages from leucogranites intruding greater himalayan sequence rocks in the footwall of the stds are between 9 9 and 15 ma with weighted mean ages between 10 and 12 ma zircon grains exhibit a positive correlation between age and effective uranium eu laser ablation zircon u pb geochronology detailed sem observations and laser ablation depth profiling of these zircons reveal low eu 0 5 2 5 ga inherited cores overgrown by high eu 17 22 ma rims this intragranular zonation produces ages as much as 32 too young when a standard alpha ejection correction assuming uniform eu distribution is applied modeling of the effects of varying rim thickness and rim eu concentration on the bulk grain eu and alpha ejection correction suggests that zonation also exerts the primary control on the form of the age eu correlation observed application of grain specific zonation dependent age corrections to our data yields zircon he ages between 14 and 17 ma in agreement with aft and 40ar 39ar ages growth of magmatic rims followed by cooling to 120 c within 1 6 million years supports rapid tectonic exhumation associated with slip along the stds in the miocene this study highlights the importance of characterizing parent nuclide zonation in zircon he studies which seek to understand the timing of exhumation along exhumed crustal blocks

yadong cross structure and south tibetan detachment in the east central himalaya 89 90 e the yadong cross structure ycs occurring at 89 30 east longitude in the himalaya is the largest across strike discontinuity in the geologic structure and topography of the high himalaya between the himalayan syntaxes it is manifest by a plan view left offset of the topographic crest of the range and a coincident apparent left strike offset of the south tibetan detachment system stds of about 70 km new field mapping indicates that the stds intersects the west side of the ycs at zherger la and further suggests that the ycs is the surface expression of a large north northeast striking west facing lateral ramp in the himalayan thrust system the greater himalayan allochthon is apparently draped across this lateral ramp resulting in a north northeast striking monoclinal flexure of the allochthon manifest in the chomolhari range superimposed steep west northwest directed normal shear appears to have obliterated earlier northerly directed stds shear fabric along along the ycs segment of the tethyan belt greater himalayan belt contact recent seismicity suggests that the greater himalayan allochthon is still moving southward along the lateral ramp u pb monazite dates on leucogranites in the footwall of the stds on opposite sides of the ycs are suggestive of a south to north decrease in the crystallization age of these granites the age pattern is consistent with southward extrusion of the greater himalayan allochthon and suggests the possibility of determining an average slip rate for the stds by dating granites in the immediate footwall of the stds that are substantially separated in north south section an average slip rate of about 7 mm yr between 23 and 12 ma is suggested by the data presented here

episodic rapid uplift in the himalaya revealed by 40ar 39ar analysis of detrital k feldspar and muscovite bengal fan detrital k feldspar and muscovite samples from ocean drilling program leg 116 cores have been dated by the sup 40 ar sup 39 ar technique and have depositional ages from 0 to 18 ma from 4 to 13 individual k feldspars and 1 to 12 individual muscovites have been dated from 7 stratigraphic levels in every level at least one k feldspar and one muscovite yielded a minimum age identical within uncertainty to the age of deposition these results indicate that a significant portion of the material in the bengal fan is first cycle detritus derived from the himalaya therefore the substantial amount of sediment deposited in the distal fan in early to middle miocene time can be ascribed to a significant pulse of uplift and erosion in the collision zone at this time moreover these data indicate that throughout the neogene some part of the himalayan orogen was undergoing rapid erosion 1 to 10 mm yr this erosion must have been less than or equal to uplift relative to sea level the lack of granulite facies rocks in the eastern himalaya and tibetan plateau suggests to us that very rapid uplift must have been distributed in brief pulses over different parts of the mountain belt these datamore are incompatible with tectonic models in which the himalaya and tibetan plateau are uplifted either uniformly over the past 40 m y or mostly within the past 2 to 5 m y less

sediment character and provenance in a complex fjord howe sound british columbia howe sound is a complex fjord in terms of geomorphology and circulation pattern sediment is presently being deposited in its deep basins from four distinct sources 1 land derived sediment from the squamish river 2 suspended sediment originating from surface waters influenced by discharges from the nearby fraser river 3 reworked pleistocene lag deposits found on some shore platforms and sills throughout the sound and 4 mine tailings from a defunct cu zn mine a less distinct source is sediment deposited via gravity flows from fjord wall slopes including side entry fanglomerate deltas and the major squamish river delta grain size data have been used to differentiate depositional versus nondepositional environments in addition surficial sediment samples collected from depositional basins could be modelled in terms of their size frequency distribution and distance from source those that deviate from a predicted grain size distribution were found to be bimodal partitioning of their probabili