isotopic constraints on the structural relationships between the lesser himalayan series and the high himalayan crystalline series garhwal himalaya nd and sr isotope systematics may provide important constraints on the location of major thrust systems that separate lithologically similar sedimentary sequences the potential of the technique is illustrated by this isotopic study of the main central thrust system of the himalaya nd isotope data from the garhwal himalaya indicate that metasedimentary rocks from the vaikrita group nd 14 to 19 correlate closely with those from the high himalayan crystalline series which constitutes the hanging wall lithologies of the main central thrust in contrast metasedimentary rocks from the munsiari group nd 23 to 28 show marked similarities to the lesser himalayan series in the footwall of the main central thrust sr isotopes support the correlations in that the vaikrita group shows partial reequilibration at 500 ma whereas the munsiari group has not undergone sr isotope homogenization since 1800 ma thus the vaikrita thrust that juxtaposes these two formations is recognized as the main central thrust in garhwal himalaya the thrust coincides approximately with the location of the kyanite isograd confirming that inverted metamorphism is characteristic of both hanging wall and footwall of the main central thrust along the tons thrust known locally as the srinagar thrust 50 km south of the main central thrust low grade quartzarenites with nd sr isotope and trace element characteristics typical of lesser himalayan formations have been emplaced on phyllites and siltstones with geochemical characteristics of the high himalayan crystalline series the field relationships most probably result from out of sequence thrusting in which lesser himalayan series rocks to the north were emplaced over low grade equivalents of the high himalayan crystalline series preserved in the external part of the orogen this study establishes the value of isotope data for lithostratigraphic correlations within orogenic belts

isotope studies reveal a complete himalayan section in the nanga parbat syntaxis many models of orogenesis invoke simple anatomies for mountain belts comprising a small number of major tectonic provinces separated by major faults in the himalayan arc three main tectonic provinces the lesser himalayan high himalayan and tethyan series have been recognized over 2500 km along strike from bhutan to kashmir however their extension westward to the nanga parbat syntaxis remains equivocal we have supplemented detailed field work in the area with isotopic analysis aimed at revealing distinct signatures for each of the three main provinces using sr isotopic data to refine previous nd based discrimination we demonstrate that the three main tectonic provinces of the central himalaya also occur in the western syntaxis of northern pakistan these three units are thus continuous along the entire orogenic arc however their metamorphic grade is generally higher in the syntaxis than in the central himalaya challenging the validity of distinctions commonly drawn on this basis elsewhere in the mountain belt the corollary is that these high grade units probably continue beyond the syntaxis into northwestern pakistan which suggests that although the precollisional materials may be identical the western syntaxis marks a change in tectonic style from the main orogen this conclusion in turn requires that the burial and exhumation history in the western himalaya be radically different from that in the central himalaya and thus necessitates a reexamination of models for india asia collision

correlation of lithotectonic units across the eastern himalaya bhutan clastic sediments deposited in foreland basins and offshore fans allow the evolution of an orogen to be reconstructed provided their source regions are properly characterized isotopic data from the lithotectonic units of the eastern himalaya bhutan indicate clear isotopic differences directly comparable with the equivalent units from the central himalaya zircons from metaquartzites of the high himalayan series that range in age from 980 to 1820 ma and an orthogneiss intruded at 825 9 ma bracket deposition to between 816 and 980 ma model ages derived from nd isotopes of associated metapelites range from 1700 to 2200 ma in contrast zircons from a metaquartzite from the lesser himalayan series range from 1850 to 2550 ma a paleoproterozoic deposition age ca 1750 ma is inferred from the age of a metarhyolite associated with the sediments model nd ages of metapelites from the lesser himalayan series range from 2500 to 2600 ma the bhutanese himalayan units can be correlated more than 1000 km westward along strike from catchments of the brahmaputra to the headwaters of the sutlej on the basis of their structural position provenance and isotopes because current discharge from the brahmaputra carries the dominant sedimentary flux into the bengal fan results from this work validate the interpretation of isotopic variations observed in offshore deposits in terms of the unroofing history of the component lithologies of the himalaya

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

early himalayan exhumation isotopic constraints from the indian foreland basin nd and sr isotopic compositions of palaeogene foreland basin sediments are used to provide insights into early himalayan evolution particularly the timing of exposure of high 87sr 86sr units erosion of which may have caused the late tertiary increase in oceanic sr isotopic ratios during the late palaeocene early eocene erosion was from mixed sources including suture zone rocks exhumation of the high himalaya was occurring by the time of deposition of alluvial sediments after mid oligocene times and this source has dominated himalayan sediments from at least this time until the present day the transition is interpreted to reflect exhumation of basement rocks of the indian plate when the high himalaya became a sufficient topographic barrier to separate suture zone rocks from the foreland basin the marked rise in seawater 87sr 86sr from 40 ma is consistent with the erosion of a himalayan source with a high 87sr 86sr ratio

spatial coherence and rough bottom scattering in shallow water shallow water acoustic data collected off mallorca in march 1993 are analyzed for spatial coherence and their relation to rough bottom scattering is investigated bistatic reverberation data were collected by a horizontal receiver array with equispaced hydrophones suspended 1 m above the bottom a flextentional transducer towed behind the ship was used as a source generating a 209 db source level at center frequency of 400 hz with 5 ms signal length and 10 s interval the ship made a hexagonal run with the horizontal receiver array inside the track a theoretical source model has been designed to approximate the actual source pulse and is used to match filter the data the modal arrivals are clearly identified in the resulting impulse responses allowing for mode by mode analysis the total field is decomposed into a mean field coherent field and scattered field incoherent field by a stacking procedure the spatial coherence is then calculated using a correlation and a coherence function the mag

bounded influence magnetotelluric response function estimation author posting blackwell 2004 this article is posted here by permission of blackwell for personal use not for redistribution the definitive version was published in geophysical journal international 157 2004 988 1006 doi 10 1111 j 1365 246x 2004 02203 x

metamorphic reactions related to decompression and synkinematic intrusion of leucogranite high himalayan crystallines bhutan the high himalayan crystallines hhc of bhutan were penetratively deformed intruded by leucogranite and metamorphosed during the collision of the indian and asian plates metamorphic reaction textures in the hhc show that it experienced decompression while maintaining a laterally heterogeneous and locally inverted internal temperature range of c 600 750c this thermal structure was produced by thrusting hot migmatitic rocks over lower grade rocks within the hhc and by the advection of heat from the intrusion of leucogranite dykes and sills during decompression a variable velocity field within the hhc during exhumation and extrusion between india and tibet caused the inversion of top to the south sense of shear present throughout most of the hhc to top down to the north shear near its top

detrital sediment fluxes from continents to oceans abstract detrital sediment is carried from land to the sea by three agents rivers glaciers and winds the shoreline is an arbitrary boundary within the detrital sediment transport system which extends from a site of origin across areas of temporary storage to a site of long term deposition the most important of the agents moving sediment across the land is river transport estimated to be in the order of 20 10 12 kg of sediment annually at present analysis of drainage basins indicates that relief and runoff are the most important factors in determining the sediment load of rivers the competence of rivers to transport sediment is governed by the volume flow gradient and the sediment load itself today most large rivers are fed by snowmelt in highland areas runoff from rainfall in the drainage basin and groundwater inflow along the river course water is lost to evaporation and groundwater infiltration river courses can often be divided into two segments a degradational section in which the gradient is relatively steep and little temporary storage of sediment takes place and an aggradational section where the gradient is sharply reduced through meandering and where large scale temporary sediment storage forms a flood plain lakes trap sediment inland and prevent its transport to the sea today many high and mid latitude rivers are interrupted by lakes of glacial origin there are also some large areas of internal drainage that deliver no sediment to the sea the load carried by rivers has been markedly altered by human activity and may have doubled over the past few thousand years only to be reduced in the past century by the widespread construction of dams the ancient use of fire in hunting and its subsequent use in clearing land has increased erosion extensive deforestation and cultivation processes have also increased the sediment supply dam construction is a relatively new factor and affects the sediment transport system by trapping sediment before it can reach the sea the resulting lower sediment supply from rivers is at least in part compensated by increased coastal erosion glacial erosion is difficult to estimate there is an ongoing controversy whether ice sheets are effective erosive agents or not estimates of the present global flux of glacial detritus range from 0 8 50 10 12 kg annually with the lower value most probable the dust flux is in the order of 0 5 to 0 9 10 12 kg annually but may vary greatly with time

mode coupling due to interaction with the seabed a simple mode coupling model is developed to investigate acoustic propagation in an ocean environment in which the water depth is range dependent the model is applied to the case of a constant sloping bottom and the results are compared with those obtained using other propagation loss models

how steep are the himalaya characteristics and implications of along strike topographic variations differences in the topography of central nepal and bhutan are characterized by map and profile patterns of elevation slope relief and stream gradients nepal exhibits a narrow hinterland zone of extreme relief steep slopes and channels and deep fluvial dissection and a wide foreland zone of lower relief active shortening on the main frontal thrust occurs far outboard of the steep topography in contrast bhutan has two high relief zones separated by a narrow low relief step in the topographic and river profiles the southern zone has the higher relief rising abruptly from the main frontal thrust to a series of 4000 m peaks whereas the northern zone consists of low relief high elevation plateau like terrain cut by widely spaced steep walled valleys spatial relationships between these geomorphic features and geologic contrasts are used to examine scenarios of tectonic and surface process interactions the observations favor a model of along strike segmentation in which timing differences in the varying balance of uplift and erosion produce spatial differences of topographic and geologic patterns although other interpretations are possible

electrical conductivity imaging of the philippine sea upper mantle using seafloor magnetotelluric data abstract we performed a three year seafloor electromagnetic survey in the philippine sea including the western edge of the pacific ocean to image electrical features of a deep mantle slab stagnating in the transition zone and the surrounding mantle in three dimensions 3 d the project iterated one year deployment of ocean bottom electromagnetometers obems using a total of 37 instruments installed at 18 sites the data obtained have been analyzed in the order of their recovery based on a magnetotelluric mt method in this study we attempt to obtain a one dimensional 1 d electrical conductivity model beneath the philippine sea and the pacific region separately that can be used as a reference model in the first step toward the 3 d analysis the resultant 1 d models show three main features 1 the conductivity in the shallower 200 km of the upper mantle depths of the two regions contrasts sharply which is qualitatively consistent with the large difference in lithospheric age 2 the conductivity at 200 300 km depth in both regions is more or less the same at approximately 0 3 s m 1 3 the conductivity just below 400 km depth is higher for the philippine sea mantle than for the pacific mantle the conductivity structure can be interpreted in terms of the thermal structure mantle hydration and existence of partial melt using experimental results for the conductivity of mantle minerals if the conductivity is interpreted simply as the effect of temperature the mantle beneath the philippine sea could be hotter than the dry solidus of mantle peridotite and thus partially molten however beneath the pacific region the present analysis suggests that the partial melting is not required under the assumed peridotitic composition even if we consider mantle hydration

three dimensional numerical modeling of geoacoustic scattering from seafloor topography a three dimensional 3 d second order finite difference method was used to create synthetic seismograms for wave propagation in heterogeneous media in order to investigate the scattering of elastic and acoustic energy due to topography on the seafloor the method uses a fully staggered grid in cartesian coordinates as developed by virieux geophysics 51 889 901 1986 numerical results were generated for two models a linear fault scarp on the seafloor and a flat seafloor containing a rectangular channel wave front snapshots allow the scattering and focusing of different wave modes with direction to be visualized compressional and shear wave backscattering from the sides of the features can be seen together with the trapped compressional wave energy propagating inside the channel the results illustrate the effects of out of the plane scattering due to simple seafloor topographic features

isotopic constraints on the age and provenance of the lesser and greater himalayan sequences nepalese himalaya the main central thrust of the central himalaya juxtaposes the lesser himalayan sequence footwall with the greater himalayan sequence hanging wall both of which are regarded as components of the northern passive margin of india considerable uncertainty surrounds their respective ages and sedimentary derivation u pb and sm nd isotopic studies of samples from the langtang area central nepal demonstrate important distinctions between the two rock packages the greater himalayan sequence had a sedimentary provenance that included a major source of 0 8 1 0 ga zircons implying a late proterozoic age the source of the precambrian part of the lesser himalayan sequence contained 1 87 2 60 ga zircons and its depositional age is probably middle proterozoic nd isotopic characteristics of the two sequences are different at 21 ma the greater himalayan rocks had end values between 14 6 and 18 5 while their lesser himalayan counterparts had values between 21 4 and 25 9 giving rise to major differences in their model ages the lesser and greater himalayan sequences at langtang are interpreted as correlative to the lower and upper parts of the vindhyan supergroup currently exposed south of the gangetic plain in northern india on the basis of available age data the exposed hanging wall of the main central thrust in central and eastern nepal contains no archean early proterozoic basement if the fault does root into the basement at depth it must do so farther north than recently published cross sections indicate this implies a total displacement on the fault that is substantially greater than the 140 210 km required by tectonic overlap in the eastern himalayas

proterozoic crustal evolution in the nw himalaya india as recorded by circa 1 80 ga mafic and 1 84 ga granitic magmatism abstract single zircon dating of the rampur metabasalts of the larji kullu rampur window in the lesser himalayas yielded an evaporation age of 1800 13 ma the zircon age is considerably younger than the previously published whole rock sm nd age of 2510 90 ma suggesting that the sm nd age may be geologically meaningless and that the sm nd whole rock array may have resulted from mixing in the nw himalaya there is also evidence for extensive silicic melt generation in the paleoproterozoic zircons from a metarhyodacite in the larji kullu rampur window yielded an evaporation age of 1840 16 ma which we interpret as the minimum age of magmatism the main central thrust granitic mylonites are interpreted as the basement of the neoproterozoic haimanta group metasediments together with the granitic rocks from the lesser himalaya they were derived from pre existing continental crust prior to 1 84 ga the nd depleted mantle model ages are in the range of 2 6 2 4 ga suggesting a contribution of archean crust a recycled archean component is also documented by a 2 9 ga domain in one of the zircons

neogene himalayan weathering history and river87sr86sr impact on the marine sr record abstract clastic sediments in the bengal fan contain a neogene history of erosion and weathering of the himalaya we present data on clay mineralogy major element stable and radiogenic isotope abundances from lower miocene pleistocene sediments from odp leg 116 nd and sr isotope data show that the himalayan provenance for the eroded material has varied little since 17 ma however from 7 to 1 ma smectite replaces illite as the dominant clay while sediment accumulation decreased implying an interval of high chemical weathering intensity but lower physical erosion rates in the ganges brahmaputra gb basin o and h isotopes in clays are correlated with mineralogy and chemistry and indicate that weathering took place in the paleo gangetic flood plain the87sr86sr ratios of pedogenic clays vermiculite smectite record the isotopic composition of sr in the weathering environment and can be used as a proxy for87sr86sr in the paleo gb basin the sr data from pedogenic clays shows that river87sr86sr values were near 0 72 prior to 7 ma rose rapidly to 0 74 in the pliocene and returned to 0 72 in the middle pleistocene these are the first direct constraints available on the temporal variability of87sr86sr in a major river system the high87sr86sr values resulted from intensified chemical weathering of radiogenic silicates and a shift in the carbonate silicate weathering ratio modeling of the seawater sr isotopic budget shows that the high river87sr86sr values require a ca 50 decrease in the sr flux from the gb system in the pliocene the relationship between weathering intensity 87sr86sr and sr flux is similar to that observed in modern rivers and implies that fluxes of other elements such as ca na and si were also reduced increased weathering intensity but reduced sr flux appears to require a late miocene pliocene decrease in himalayan erosion rates followed by a return to physically dominated and rapid erosion in the pleistocene in contrast to the view that increasing seawater87sr86sr results from increased erosion mio pliocene to mid pleistocene changes in the seawater sr budget were the result of reduced erosion rates and sr fluxes from the himalaya

the kinematic evolution of the nepalese himalaya interpreted from nd isotopes neodymium nd isotopes from the himalayan fold thrust belt and its associated foreland basin deposits are useful for distinguishing between himalayan tectonostratigraphic zones and revealing the erosional unroofing history as controlled by the kinematic development of the orogen neodymium isotopic data from the himalayan fold thrust belt in nepal n 35 reveal that the lesser himalayan zone consistently has a more negative nd 0 value than the greater and tibetan himalayan zones our data show the average nd 0 value in the lesser himalayan zone is 21 5 whereas the greater and tibetan himalayan zones have an average nd 0 value of 16 these consistently distinct values throughout nepal enable the use of nd isotopes as a technique for distinguishing between lesser himalayan zone and greater himalayan zone rock the less negative nd 0 values of the greater himalayan rocks support the idea that the greater himalayan zone is not indian basement but rather a terrane that accreted onto india during early paleozoic time neodymium isotopic data from eocene through pliocene foreland basin deposits n 34 show that sediment provenance has been dominated by greater and tibetan himalayan detritus across nepal the nd t values in the synorogenic rocks in western and central nepal generally show an up section shift toward more negative values and record the progressive unroofing of the different tectonostratigraphic zones at 10 ma in khutia khola and 11 ma in surai khola a shift in nd t values from 16 to 18 marks the erosional breaching of a large duplex in the northern part of the lesser himalayan zone this shift is not seen in eastern nepal where the nd t values remain close to 16 throughout miocene time because there has been less erosional unroofing in this region

tippers at island observatories can we use them to probe electrical conductivity of the earth s crust and upper mantle 1 for decades time series of hourly mean values of the geomagnetic field measured on a global network of observatories have been routinely used to recover the electrical conductivity distribution in midmantle depths nowadays most observatories provide data in the form of minute means this allows for analysis of short period geomagnetic variations which in principle contain information about geoelectric structures in the crust and upper mantle however so far these data have been ignored for induction studies of the earth due to a theoretical preconception in this paper we demonstrate that short period responses tippers at island observatories being large owing to the ocean effect are also sensitive to 1 d structures and thus can be used for probing the earth this means that a huge amount of data that was not exploited hitherto for induction studies should be reconsidered as a useful source of information about geoelectric structures in oceanic regions where our knowledge is still very limited

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

isotopic and structural constraints on the location of the main central thrust in the annapurna range central nepal himalaya five isotope enhanced geologic transects in the southern annapurna range of central nepal elucidate structural geometries near the main central thrust whole rock nd isotopes and u pb ages of detrital zircons unambiguously distinguish greater himalayan hanging wall and lesser himalayan footwall metasedimentary rocks nd 0 values for lower lesser himalayan rocks typically range from 20 to 26 whereas greater himalayan rocks usually have nd 0 values of 19 to 12 lower lesser himalayan rocks yield detrital zircons with an age peak at ca 1880 ma and no detrital zircons younger than ca 1550 ma in contrast greater himalayan rocks yield detrital zircons with a prominent broad peak of ages at ca 1050 ma and no detrital zircons younger than ca 600 ma the protolith boundary between greater and lesser himalayan rocks is up to 1 km farther south than usually mapped on the basis of lithology field and microstructural observations indicate the presence of a top to the south ductile shear zone superimposed on this boundary confirming this shear zone as the main central thrust no evidence exists for large scale structural mixing of greater and lesser himalayan rocks along the main central thrust in the annapurna range

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

lithostratigraphic correlations in the western himalaya an isotopic approach we present the results of a whole rock nd isotopic study of two contrasting regions of the western himalaya using the neodymium model age approach on the scale of a single orogen high grade metasedimentary rocks from zanskar yield model ages t dm that are similar to those of the high himalayan crystalline series t dm 1 2 2 0 ga nd 6 to 16 and distinct from values from the lesser himalaya t dm 2 3 3 4 ga nd 18 to 27 hence these two lithological sequences can be recognized for 2000 km along the strike of the orogen data for the basement of the nanga parbat massif at the western extremity of the himalaya t dm 2 3 2 8 ga nd 18 to 30 suggest that these rocks are not equivalent to the high himalaya as previously supposed but have affinities with the lesser himalaya a thin metasedimentary cover sequence on the margins of the nanga parbat massif is isotopically indistinguishable from the high himalaya t dm 1 6 1 8 ga nd 10 to 14 the prior misidentification of the provenance of the massif stems from its high metamorphic grade characteristic of the high himalaya but in this case related to the unique neogene history of the nanga parbat massif which has exhumed a higher grade equivalent of the lesser himalaya that is not seen elsewhere

propagation of the thrust system and erosion in the lesser himalaya geochemical and sedimentological evidence centre de recherches pe trographiques et ge ochimiques bp 20 54501 vandoeuvre les nancy franceabstractsedimentological and nd isotope data of two sections of the sub himalaya of westernnepal are used as new constraints for understanding the erosion history of the himalaya throughout the deposition of the middle and upper members of the siwalik group thelesser himalaya contribution to the total detrital input progressively increased from lessthan 20 to 40 the increasing proportion of lesser himalaya sediments started at ca 10 8 ma and is associated with a coarsening of the maximum grain size at both micro scopic and macroscopic scales thin skinned tectonics of the lesser himalaya thrust sys tem would have controlled the exhumation of the lesser himalaya rocks and would havebegun at 12 10 ma taking into account the delay for denudation together with otherstudies these data restrict the onset of movement on the lesser himalaya thrust systemto less than 3 m y along more than 1750 km this short time frame implies a ratio oflateral propagation rate to shortening rate far too high for the propagation of a singlecrustal thrust thus we suggest instead the simultaneous initiation of several thrusts aheadof the main central thrust at ca 12 ma we suggest that a rapid rise of the tibetanplateau at this time has transformed the himalaya to an overcritical thrust wedge thathas propagated forward to return to a stable state this regional rising could be the primecause of the increase of sediment in ux at ca 11 ma around the himalaya keywords siwalik formations neodymium grain size erosion thrust sheets himalaya

chemical erosion in the eastern himalaya major ion composition of the brahmaputra and δ13c of dissolved inorganic carbon abstract major ion composition of waters 13 c of its dic dissolved inorganic carbon and the clay mineral composition of bank sediments in the brahmaputra river system draining india and bangladesh have been measured to understand chemical weathering and erosion and the factors controlling these processes in the eastern himalaya the time series samples collected biweekly at guwahati from the brahmaputra mainstream were also analyzed for the major ion composition clay mineralogy and chemical index of alteration cia of sediments suggest that weathering intensity is relatively poor in comparison to that in the ganga basin this is attributed to higher runoff and associated physical erosion occurring in the brahmaputra basin the results of this study show for the first time spatial and temporal variations in chemical and silicate erosion rates in the brahmaputra basin the subbasins of the brahmaputra watershed exhibit chemical erosion rates varying by about an order of magnitude the eastern syntaxis basin dominates the erosion with a rate of 300 t km 2 y 1 one of the highest among the world river basins and comparable to those reported for some of the basaltic terrains in contrast the flat cold and relatively more arid tibetan basin undergoes much slower chemical erosion 40 t km 2 y 1 the abundance of total dissolved solids tds 102 203 mg l in the time series samples collected over a period of one year shows variations in accordance with the annual discharge except one of them cause for which is attributable to flash floods na na corrected for cyclic component shows a strong positive correlation with si indicating their common source silicate weathering estimates of silicate cations na sil k sil ca sil mg sil suggest that about half of the dissolved cations in the brahmaputra are derived from silicates a proportion higher than that for the ganga system the co 2 consumption rate due to silicate weathering in the brahmaputra watershed is 6 10 5 moles km 2 y 1 whereas that in the eastern syntaxis subbasin is 19 10 5 moles km 2 y 1 similar to the estimates for some of the basaltic terrains this study suggests that the eastern syntaxis basin of the brahmaputra is one of most intensely chemically eroding regions of the globe and that runoff and physical erosion are the controlling factors of chemical erosion in the eastern himalaya

predictability of acoustic intensity and horizontal wave numbers in shallow water at low frequencies using parabolic approximations accuracy of relative intensity and horizontal wave number predictions from parabolic approximation models of shallow water low frequency less than 100 hz acoustic propagation problems is examined effects of input parameter uncertainties and basic approximations in parabolic equation models are investigated environmental parameters and uncertainties considered correspond generally to those from a recent new jersey shelf experiment parameter uncertainties are found to impose the greatest limitations on prediction accuracy with sediment sound speed uncertainties causing the largest restrictions issues associated with modeling the sediment sound speed such as using isospeed layers to approximate sound speed variations are also resolved weak range dependence in the channel depth and sound speed of similar type as at the experiment site is found to be well approximated by range independent models comparisons of the experimental results with model predictions illustrate how parameter uncertainties

himalayan tectonics explained by extrusion of a low viscosity crustal channel coupled to focused surface denudation recent interpretations of himalayan tibetan tectonics have proposed that channel flow in the middle to lower crust can explain outward growth of the tibetan plateau1 2 3 and that ductile extrusion of high grade metamorphic rocks between coeval normal and thrust sense shear zones can explain exhumation of the greater himalayan sequence4 5 6 7 here we use coupled thermal mechanical numerical models to show that these two processes channel flow and ductile extrusion may be dynamically linked through the effects of surface denudation focused at the edge of a plateau that is underlain by low viscosity material our models provide an internally self consistent explanation for many observed features of the himalayan tibetan system8 9 10

a coupled mode solution for acoustic propagation in a waveguide with stepwise depth variations of a penetrable bottom a coupled mode solution is formulated for the problem of acoustic propagation in a cylindrically symmetric ocean divided in range into a finite number of adjoining pekeris waveguides of differing water depths attenuation is included in the bottom and the problem is discretized by assigning a pressure release boundary condition at a depth which is sufficiently far removed to prevent significant energy from returning to the water this formulation includes backscatter from the depth variations of the water column and full coupling between a finite number of modes propagating in the water column and in the bottom numerical results based on an implementation of this solution are presented

the late oligocene early miocene himalayan belt constraints deduced from isotopic compositions of early miocene turbidites in the bengal fan abstract we present a mineralogical and isotopic study of early miocene turbidites from the bengal fan which are the oldest sediments recovered during odp leg 116 clay fractions are dominated by illite and chlorite and have low 18o 11 16 the clays are dominantly derived from a metamorphic precursor by physical erosion and have undergone only minor low t water rock exchange and chemical weathering sr and nd isotope analysis of silt and clay fractions and coarse muscovite biotite and feldspar fractions are remarkably similar to those of the presently exposed high himalaya crystalline hhc sequence the data show that a close analogue of the hhc was already subaerially exposed to active erosion during early miocene rb sr data on the biotite separates has been used to evaluate the time elapsed between cooling through the closure temperature for sr in biotite in the hhc ca 325 c and final sedimentation in the bengal fan despite the large uncertainties in the initial 87 sr 86 sr values of sedimentary mixtures our estimates of the elapsed time between cooling and deposition are short 4 14 ma assuming a transport time scale 1 ma deduced exhumation rates are in the same range of modern values 1 3 mm yr since ca 30 ma given that a the rocks exposed at the surface b the style of weathering and c rates of exhumation were similar to the present we infer that the factors controlling erosion of the early miocene himalayan belt were also similar to the present the data suggest that the early miocene himalayan belt had significant relief and induced orographic effects on precipitation similar to the modern mountain range it appears that the thrusting along the main central thrust mct or similar structures has uplifted metamorphic basement analogous to the present hhc at sufficiently high rate to maintain steep relief throughout miocene time despite high average erosion rates

acoustic backscattering at low grazing angles from the ocean bottom part ii statistical characteristics of bottom backscatter at a shallow water site analyses of the statistical characteristics of bottom backscatter measured in shallow water off san diego ca t g goldsberry s p pitt and r a lamb j acoust soc am suppl 1 72 s74 1982 are presented an experimental sonar operating at 30 khz mounted on the sea bottom was used to gather data over a wide sector of the bottom within its operating range the bottom was comprised of areas of coarse and fine sand the distribution function and probability of false alarm function of the detected envelope of a widebeam and a narrowbeam signal were measured some spatial and temporal correlation functions of the signal amplitudes were measured a limited attempt was made to compare the results with existing theoretical models work supported by navsea 63r and norda code 113

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