zonal extent of oceans high latitude fresh water supplies and the thermohaline circulation one of the most distinctive features of the present day global thermohaline circulation is the absence in the north pacific ocean and the presence in the north atlantic ocean of deep water formation in this paper the different zonal extents and the associated high latitude fresh water supplies of the pacific and the atlantic when under a zonally uniform fresh water flux forcing are examined as a possible cause for the asymmetry a series of numerical experiments using the bryan cox semtner ocean general circulation model is performed in order to achieve a true independence the model configuration and the model forcing are so constructed that zonal extent is the only built in difference between the two oceans three configurations are used they are a an idealized atlantic ocean narrow zonal extent alone b an idealized pacific ocean wider zonal extent alone and c a connected pacific atlantic system upon spin up the model ocean is forced by a zonally uniform schopf boundary condition for temperature and a zonally uniform fresh water flux for salinity both forcings are diagnosed from a spin up all diagnosed fresh water fluxes have net precipitation in polar regions and net evaporation in low latitude regions as observed this feature means that the high latitude model pacific is subject to a fresh water flux twice that in the high latitude model atlantic this causes the northern sinking cell in the model pacific of the spin up state to collapse a southern sinking cell settles in the process is similar to that in many model studies examining the response of the thermohaline circulation to a high latitude freshening by contrast the northern sinking cell in the north atlantic of the spin up state persists in the connected pacific atlantic system these produce the conveyor belt type circulation and the asymmetry between the north atlantic and the north pacific it is found that the high latitude fresh water supply is the crucial parameter and that the difference in basin width is not directly important it is also found that the connection by the antarctic circumpolar current is not important a large number of experiments is performed in the model pacific atlantic system to test the sensitivity to initial conditions the system shows a clear tendency to favour the conveyor belt circulation although the strength of the north atlantic northern sinking cell shows considerable variations thus although in nature there are many factors that can contribute to the asymmetry the difference in total available fresh water at the northern high latitudes between the two oceans as a result of the width difference may have played a significant role

contact metamorphism and depth of emplacement of the manaslu granite central nepal implications for himalayan orogenesis abstract the manaslu massif central nepal provides a well exposed example of a deeply eroded pluton its contact aureole can be followed from the base to the top along medium to low grade mesozonal to anchizonal tethyan metasediments contact metamorphic mineral assemblages and thermobarometric estimations suggest that the granite was emplaced at 18 21 km for the base and 9 13 km for the roof calculated temperatures within the aureole at 550 40 c are compatible with the intrusion temperature of a leucogranitic magma microstructural evidence shows that the temperature remained high 500 c at the base of the massif during and after granite emplacement whereas towards the top of the granite deformation proceeded rapidly at lower temperature heating of the abundant calcareous rocks in the contact aureole released a local co2 rich fluid whereas a h2o and boron rich fluid seems to have pervaded the whole zone this fluid is probably exsolved from the granitic melt during its crystallization the depth of emplacement of the massif has an important implication for the reconstruction of the himalayan geodynamic evolution implying a burial of the tethyan metasediments by a major refolding of the sedimentary cover or more probably by extensive development of the north himalayan nappes towards the south for more than 70 km before granite emplacement i e before the miocene the young age of the granite and its depth of emplacement suggest a rapid tectonic denudation in the order of 1 mm a 1 probably by normal faulting north of the massif during its cooling

change from late tertiary compression to quaternary extension in southern tibet during the india asia collision field studies in southern tibet show that subsequent to folding resulting from the india asia collision 50 40 ma reverse faults and backthrusts formed as a result of nne ssw and nnw sse trending compressions they affect conglomerates of oligocene miocene age and are probably contemporaneous with shearing on the himalayan main central thrust and folding of paleogene basins along the northern border of southern tibet 25 16 ma rift systems postdate compressional structures they were active during the pleistocene and probably during the pliocene 5 2 ma change from a compressional to an extensional regime in tibet probably occurred during the late miocene 10 5 ma this change is tentatively correlated with uplift inferred by chinese workers from paleogene to present day variations of the tibetan vegetation recent and active faulting in the high plateau results from a wnw ese striking extension this is in agreement with tibetan focal mechanisms the himalayan lowlands are subjected to a nne ssw trending compression as demonstrated by superficial deformations and seismicity therefore the horizontal maximum compressive stress trajectory is roughly parallel to india asia convergence hmax being 1 in the lowlands and 2 in the high plateau this change results from the increase of the vertical principal stress value due to the weight of the elevated mass zz is 3 in the lowlands and 1 in the high plateau thus the third principal stress axis is 2 in the lowlands and 3 in the high plateau allowing extension to occur in the wnw ese direction extensional tectonics are significantly different in southern and northern tibet southern tibet being constrained between the himalayan syntaxes the e w extensional principal deviatoric stress value 3 is of rather small magnitude and the n s trending 2 value is extensional as demonstrated by analysis of recent faults normal components prevail along all the azimuths in the field this favors formation of rift systems of large axial extension on the contrary north of the himalayan syntaxes northern tibet flows easily eastward the e w extensional principal deviatoric stress value 3 is of higher magnitude and the n s stress 2 is compressional as shown by inversion of focal mechanism data this favors strike slip components on faults which deviate from the n s direction as expressed in the field by shorter n s trending graben linked with normal strike slip faults these faults permit some n s shortening in northern tibet without thickening in response to india asia collision in high plateaus which are subjected to lateral compression such as the andean altiplano and tibet the extensional direction is basically controlled by the convergence direction being roughly orthogonal to the latter

etude petro structurale des chevauchements ductiles himalayens sur la transversale de l everest makalu nepal oriental tectonic and microtectonic data in eastern nepal indicate that the major observed thrusting 100 km on the main central thrust mct postdates the barrovian metamorphism of the high himalaya gneisses this result at variance with the famous reverse metamorphism model better explains the abnormal metamorphic superpositions in the himalayas and accounts for the lack of high pressure assemblages under the thick allochtonous high himalaya tibetan slab pressure and temperature estimates by microprobe analysis on plagioclase biotite garnet kyanite sillimanite and cordierite assemblages are presented for samples collected along the mct shear zone and across the gneiss slab in the everest makalu area since there is very little difference in pressure at the front of the slab kathmandu klippe and its root these estimates support the existence of important late metamorphic thrusting the decrease of pressure towards the top of the gneiss pile combined with a small temperature increase explains the k

interdecadal variability in an ocean model driven by a small zonal redistribution of the surface buoyancy flux abstract an idealized three dimensional model of size comparable to the atlantic is used to study interdecadal variability of the thermohaline circulation thc in most of the model experiments salinity is kept uniform and constant the model being driven by surface heat flux only when the model is driven by the surface heat flux diagnosed from a restoring spinup experiment no oscillations occur driving the model by a time independent surface heat flux obtained by applying a small zonal redistribution to the diagnosed flux leads to strong interdecadal oscillations small means that the modification to the diagnosed flux is within the error bars on estimates of surface heat flux based on observations the model sea surface temperature sst anomalies are similar to the observed pattern of sst anomalies in the north atlantic and to the sst anomalies associated with the interdecadal oscillation in the gfdl fully coupled ocean atmosphere model for redistributions that weaken the east west varia

an early pliocene thermal disturbance of the main central thrust central nepal implications for himalayan tectonics since the beginning of the collision between india and asia at about 50 ma the convergence in the himalaya has largely been taken up along major thrust zones in this study samples of the lesser himalaya formations up to 10 km below the main central thrust mct and the greater himalaya sequence up to 12 km above the mct have been analyzed by the 40ar 39ar and u pb methods to investigate the thermal history of the mct the ages can be summarized as follows 1 the youngest ages from muscovites 3 1 ma biotites 3 4 ma and hornblendes 4 1 ma all come from within 1 km of the mct 2 there is a marked asymmetry of ages between the footwall and the hanging wall of the mct the maximum mica age in the hanging wall greater himalaya sequence is 13 ma whereas a muscovite 5 km below the mct in the footwall has an age spectrum with a gradient from 400 to 1400 ma 3 five k feldspars from 6 2 to 11 9 km above the mct all give minimum ages in the range 3 0 6 4 ma and 4 a 206pb 238u age on a brannerite from the greater himalayan sequence is 4 8 ma structural and petrologic observations preclude the possibility that this age distance distribution reflects faulting within the greater himalaya sequence and production of the necessary thermal energy by shear heating requires unrealistically high shear stresses infiltration of hot fluids through the mct zone appears to be the best hypothesis to explain these data simple numerical simulations which account for heat transfer by advection within the fluid flow zone and by conduction outside it indicate that the observed age distribution could have been produced by infiltration of hot fluids through the mct zone at circa 5 to 4 ma within the following range of conditions the fluids heated rocks to temperatures in the range 470 to 510 c for less than 1 million years in a region narrower than the entire mct zone the temperature of the thermal disturbance inferred from the 40ar 39ar data is consistent with petrologic data from albitic alteration assemblages in the greater himalaya sequence and the u pb age of the brannerite is similar to the youngest 40ar 39ar hornblende age this scenario hydrothermal heating culminating at about 4 ma is similar to the model of le fort 1981 for the generation of the high himalayan leucogranites in the late oligocene early miocene by dehydration of the footwall rocks of an active thrust in the younger instance we interpret that fluids derived from the footwall rocks of the main boundary thrust migrated upward through the lesser himalaya formations and were subsequently channeled along the mct producing the thermal disturbance

the role of ocean atmosphere reorganizations in glacial cycles abstract a case is made that glacial to interglacial transitions involve major reorganizations of the ocean atmosphere system such reorganizations constitute jumps between stable modes of operation which cause changes in the greenhouse gas content and albedo of the atmosphere only in this way can the rapidity of glacial terminations the hemispheric synchroneity and symmetry of mountain glaciation and the large polar air temperature and dustiness variations be accounted for if these reorganizations are driven in some fashion by orbitally induced seasonal insolation changes then the connection between insolation and climate is most likely through impacts of fresh water transport on the ocean s salinity distribution

correlation between melting deformation and fluid interaction in the continental crust of the high himalayas langtang valley nepal combined metamorphic stable isotope and structural studies from the high himalayan crystalline sequence in the langtang valley of north central nepal reveal a strong positive correlation between distance above the base of the section the main central thrust the amount of melt material and evidence of prolonged fluid and deformation histories thus suggesting that these processes are strongly interdependent kyanite grade rocks at the base of the section are unmelted and have undergone little syn or post metamorphic internal deformation and little prograde or retrograde fluid rock interaction by contrast sillimanite grade rocks higher in the section contain progressively larger volumes of melt have suffered increasingly complex syn and post metamorphic deformation and show increasing evidence for the presence of fluids although the factors that initiated these processes remain problematic it is suggested that fluid distributions within the langtang section have been passively controlled by the movement of melts these melts may have provided a primary control on deformation during both the magmatic stage and subsequently through the exsolution of exsolved aqueous fluids

fluid events and exhumation history of the main central thrust zone garhwal himalaya india mineral thermobarometric and fluid inclusion studies were carried out on the low to medium grade metamorphics which occur at different tectonic levels of the main central thrust mct zone in garhwal himalaya mct i in north mct ii in the centre and the southern most mct iii the augen gneiss and pelitic schists present in this zone show an increase in the grade of metamorphism from chlorite to kyanite towards the north the pressure calculated from garnet biotite muscovite plagioclase phase equilibria increases from 1 9 to 8 kbar and the temperature obtained through garnet biotite and chlorite geothermometer varies between 360 and 5628c the mineral assemblages and these thermobarometric estimates reveal that the grade of metamorphism in the mct zone increases from south to north considering the inclusion types it is apparent that the carbonic fluid increases towards the mct i which is related to increase in p t conditions whereas the aqueous phase is more pronounced near the mct iii reflecting fluid assisted retrogression this observation is also substantiated by the systematic increase in co2 density from 0 75 g cm 3 in co2 h2o inclusions near mct iii to 1 01 g cm 3 in pure co2 inclusions near mct i the pressure and temperature estimated using fluid isochores varies from 1 9 to 4 8 kbar and 360 to 5628c when compared with mineral phase thermobarometry these p t conditions suggest a post peak metamorphic nature of the fluids a smooth trend of mineral and fluid phase p t conditions is observed in the mct zone based on combined mineral p t data and fluid isochores a decompressional uplift path is suggested the decrease in p t and the evidence of movement along well distributed shear fabric across the mct zone indicate that exhumation from north to south occurred under decreasing p t conditions q 2001 elsevier science ltd all rights reserved

the crystalline units of the high himalayas in the lahul zanskar region northwest india metamorphic tectonic history and geochronology of the collided and imbricated indian plate in the high himalayan belt of northwest india crustal thickening linked to palaeogene collision between india and eurasia has led to the formation of two main crystalline tectonic units separated by the syn metamorphic miyar thrust the high himalayan crystallines sensu stricto hhc at the bottom and the kade unit at the top these units are structurally interposed between the underlying lesser himalaya and the very low grade sediments of the tibetan nappes they consist of paragneisses orthogneisses minor metabasics and chiefly in the hhc leucogranites the hhc registers a polyphase metamorphism with two main stages designated as m1 and m2 a metamorphic zonation with high temperature recrystallization and migmatization at middle structural levels and medium temperature assemblages at upper and lower levels in contrast the kade unit underwent a low temperature metamorphism rb sr and u th pb isotope data point to derivation of the orthogneisses from early palaeozoic granitoids while the leucogranites formed by anatexis of the hhc rocks and were probably emplaced during miocene time most of the complicated metamorphic setting is related to polyphase tectonic stacking of the hhc with the cooler kade unit and lesser himalaya during the himalayan history however a few inconsistencies exist for a purely himalayan age of some ml assemblages of the hhc as regards the crustal derived leucogranites the formation of a first generation mixed with quartzo feldspathic leucosomes was possibly linked to melt lubricated shear zones which favoured rapid crustal displacements at upper levels they intruded during stage m2 and the latest movements along the syn metamorphic miyar thrust but before juxtaposition of the tibetan nappes along the late metamorphic zanskar fault

tectonic evolution of kishtwar window with respect to the main central thrust northwest himalaya india abstract the crystalline rocks of the himalaya high himalayan crystallines central crystallines etc are bounded to the north by the south tibetan detachment zanskar shear zone trans himadri fault system at the top and to the south by the main central thrust mct at the base to the south and tectonically below the mct lies the lesser himalaya comprising low grade meta sedimentary thrust sheets and the crystallines klippen north and tectonically above the zanskar shear zone lies the sedimentary succession of the tethys himalaya however there is a different geological set up to the northwest of beas valley which defies this normal set up i e the tethyan rocks either lie in contact with the lesser himalayan thrust sheets or they to a large extent surround the high himalayan crystallines hhc this unique set up is documented around areas comprising chamba kishtwar and zanskar regions in the northwest himalaya the lesser himalayan imbricated sequence extending progressively into deeper structural level beneath and crops out as kishtwar window within the hhc this tectonic contact along the window was described as the mct however the present study of the window indicates that its western side is bounded by a high angle ne dipping reverse fault kishtwar thrust the structural analyses reveal that this fault is younger than the mct and is interpreted here as breach thrust present observations suggest that the rocks of the window and the northeasterly lying hhc have been folded and exhumed together as an antiform along the kishtwar thrust the hhc occurring south of the kishtwar window have actually been thrust over the lesser himalayan thrust sheet this contact is observed and marked for the first time and is accordingly described as the main central thrust as per convention along the length of the himalayan belt northeast and in the hangingwall of the mct the repetition of the contact of the lesser himalayan rocks and the hhc is interpreted here as the deeper part of the mct brought to the surface by kishtwar thrust

geochronologic and thermobarometric constraints on the evolution of the main central thrust central nepal himalaya the main central thrust mct juxtaposes the high grade greater himalayan crystallines over the lower grade lesser himalaya formation an apparent inverted metamorphic sequence characterizes the shear zone that underlies the thrust garnet bearing assemblages sampled along the marysandi river and darondi khola in the annapurna region of central nepal show striking differences in garnet zoning of mn ca mg and fe above and below the mct thermobarometry of mct footwall rocks yields apparent inverted temperature and pressure gradients of 18 c km 1 and 0 06 km mpa 1 respectively pressure temperature p t paths calculated for upper lesser himalaya samples that preserve prograde compositions show evidence of decompression during heating whereas garnets from the structurally lower sequences grew during an increase in both pressure and temperature in situ i e analyzed in thin section ion microprobe ages of monazites from rocks immediately beneath the greater himalayan crystallines yield ages from 18 to 22 ma whereas late miocene and pliocene monazite ages characterize rocks within the apparent inverted metamorphic sequence a lesser himalayan sample collected near the garnet isograd along the marysandi river transect contains 3 3 0 1 ma monazite ages p 0 72 gpa t 535 c this remarkably young age suggests that this portion of the mct shear zone accommodated a minimum of 30 km of slip over the last 3 ma i e a slip rate of 10 mm yr 1 and thus could account for nearly half of the convergence across the himalaya in this period the distribution of ages and p t histories reported here are consistent with a thermokinematic model in which the inverted metamorphic sequences underlying the mct formed by the transposition of right way up metamorphic sequences during late miocene pliocene shearing

depositional environments architecture and controls of early cretaceous non marine successions in the northwestern part of kyongsang basin korea abstract in the northwestern part of kyongsang basin southeast korea non marine successions of interbedded sandstone and mudstone are divided into successive stratigraphic units on the basis of facies assemblages and sandstone architecture parts of the successions sinpyong anpyong and jotap units formed in southward or southeastward draining fluvial systems are documented in detail in terms of macroform scale to stratigraphic level architecture for the assessment of fluvial styles and evolution of non marine depositional environments the sinpyong anpyong unit consists of thick sandstone bodies 2 47 m thick thin sandstone bodies generally the jotap unit overlying the sinpyong anpyong unit comprises thick sandstone bodies 2 10 m thick interbedded sandstone siltstone bodies and siltstone dominated bodies the thick sandstone bodies are dominated by the deposits of small bars and dunes with common lenticular beds of purple siltstone and internal scour surfaces suggestive of bedform dominated channels with ephemeral discharges the channels are estimated at generally a few meters in depth and a few tens to hundreds of meters in width the interbedded sandstone siltstone bodies comprise thin sheet like and lenticular sandstone beds along with the marked change in fluvial styles the successions show systematic variations in the proportion and connectedness of channel sandstone bodies and the distribution of floodplain lake facies the basal and uppermost parts of the sinpyong anpyong unit are characterized by the sourceward expansion of distal poorly drained floodplain shallow lake facies and the low proportion and connectedness of channel bodies following these gray mudstone dominated sequences the middle part of sinpyong anpyong unit and jotap unit show increases in the proportion connectedness and grain size of channel bodies and basinward expansion of proximal well drained floodplain facies such a stratigraphic architecture can be attributed to the fluctuations in the ratio of accommodation space sediment supply regulated by repeated basin subsidence

effects of diffusional modification of garnet growth zoning on p t path calculations the effect of intragranular diffusion on chemical zoning in garnet and on p t paths calculated from that zoning was evaluated using a numerical model of multicomponent diffusion in combination with simulations of garnet growth syn and post growth diffusion of mg fe mn ca species in garnet was calculated for a model pelitic assemblage over a range of temperatures from 485 to 635 c compositions from zoned garnet as modified by diffusion hypothetical inclusions of plagioclase within garnet and matrix phases were used to reconstruct pressure temperature p t paths from isobaric and polybaric model histories p t path calculations based on numerical simulations conducted over an input isobaric heating path that reached peak temperatures between 585 and 635 c show that relaxation of garnet compositional gradients by diffusion can induce modest to appreciable curvature in the inferred paths retrieved paths also indicated somewhat smaller overall temperature changes relative to the actual temperature difference of the input path the magnitude of these distortions is shown to depend upon the heating and cooling rate and garnet crystal size as well as the actual peak temperature condition the effect of diffusion on path trajectories in simulations with thermal histories that also included cooling were comparable to heating only models that reached peak temperatures approximately 15 30 c higher compositions of garnets with radii less than 1 mm that reached actual peak temperatures of 605 c along temperature time histories characteristic of regional metamorphism experienced sufficient diffusional relaxation to introduce errors of hundreds of bars to in excess of one kilobar in path trajectories path distortions were significant at heating cooling rates less than 10 c ma but rapidly diminished for rates faster than this in polybaric simulations diffusion effects were least noticeable when the actual pressure temperature conditions changed in a clockwise sense i e convex to higher p and higher t but apprecciable modification was seen in path models that underwent counterclockwise changes in p and t reequilibration of garnet rim compositions occurred during cooling on all paths and temperature maxima obtained from garnet biotite geothermometry underestimated actual peak conditions by 40 to 70 c calculations suggest that p t path trajectories calculated from garnets of at least 1 mm size and that experienced actual thermal maxima below 585 c are not likely to be distorted by diffusional effects during regional metamorphism however p t path reconstructions based on garnet zonation with smaller grains or higher temperatures may lead to misinterpretation of crystallization history the partitioning record of peak metamorphic temperatures may be destroyed by diffusional reequilibration of garnet rim compositions during cooling seriously complicating the task of quantitatively estimating diffusion effects on path calculations

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

india eurasia collision chronology has implications for crustal shortening and driving mechanism of plates the motion of the indian plate is determined in an absolute frame of reference and compared with the position of the southern margin of eurasia deduced from palaeomagnetic data in tibet the 2 600 900 km of continental crust shortening observed is shown to have occurred in three different episodes subduction of continental crust intracontinental thrusting and internal deformation and lateral extrusion the detailed chronology of the collision and plate reorganizations in the indian and pacific oceans supports the hypothesis that slab pull is a dominant driving mechanism of plate tectonics

the p t evolution of the middle koli nappe complex scandinavian caledonides 68 n and its tectonic implications near 68 n the scandinavian caledonides are composed of 3 tectonic domains each of which has a different tectonostratigraphy the lower 2 domains can be related stratigraphically to scandinavia prior to caledonian deformation whereas the highest domain the middle koli nappe complex mknc represents a fore arc accretionary complex that was accreted to scandinavia during caledonian deformation subsequent to accretion the flyschoid sediments that dominate the mknc were metamorphosed to the amphibolite facies in the area covered by this study the mknc is composed of two nappes a lower langvatn nappe and an upper marko nappe each of which has a unique early metamorphic history pelitic mineral assemblages in the marko nappe constrain the peak p t to be 625 7 0 kbars whereas ultramafic mineral assemblages in the lower langvatn nappe constrain its peak temperature to be 580 c p t estimates from garnet biotite and garnet plagioclase geothermobarometry for both nappes overlap ranging from 528 c and 6 6 kbars to 620 c and 8 8 kbars with an average of 567 32 c and 8 0 0 9 kbar

u th pb systematics and ages of himalayan leucogranites south tibet abstract the age and origin of five leucogranites from the high and tethys himalaya and two country rock gneisses were investigated by u pb dating of zircon fractions and single grains and fractions of monazite additionally th u concentrations in whole rock powders and isotopic compositions of pb in leached k feldspars were determined monazites yield ages of 16 8 0 6 m y for the nialam migmatite granite 15 1 0 5 m y for the lhagoi kangri granite 14 3 0 6 m y for a granite from mt everest and 9 8 0 7 m y and 9 2 0 9 m y for two varieties of the maja granite these data together with monazite ages of 21 9 0 2 and 24 0 0 4 m y determined earlier on the makalu granite 1 substantiate a period of intracontinental granite emplacements from 24 to 9 m y ago i e from uppermost oligocene to late miocene times such a period of plutonic activity is consistent with the view that all these granites result from intracrustal melting following the collision of india with eurasia furthermore the individual ages together with structural relationships between granites and country rocks suggest that granite formation and tectono metamorphism occurred as alternating and strongly related processes with a periodicity of 7 to 9 m y inherited lead components present in all granite zircons point to large proportions of precambrian material in the magma source regions up to 2200 m y old th u systematics between monazite and country rocks indicate that u has been leached from most of the granites after crystallisation of monazite zircon dating of the kangmar granite gneiss which occurs in a window through the tethys himalayan sediments shows that this pluton transformed to a gneiss during the alpine orogeny crystallised in lowermost palaeozoic times 562 4 m y ago

two mica and tourmaline leucogranites from the everest makalu region nepal tibet himalayan leucogranite genesis by isobaric heating in the higher himalaya of the region from cho oyu to the arun valley northeast of makalu the miocene leucogranites are not hosted only in the upper high himalayan crystallines hhc a network of dykes also cuts the lower hhc and the lesser himalayan crystallines lhc the plutons and dykes are mainly composed of two mica muscovite biotite tourmaline cordierite andalusite sillimanite leucogranite with tourmaline 2 6 and biotite 1 5 modal and tourmaline muscovite tourmaline biotite sillimanite garnet kyanite andalusite spinel corundum leucogranite with tourmaline 2 2 and biotite 1 5 modal both leucogranite types were produced by partial melting in the andalusite sillimanite facies series under lp ht conditions constrained by the occurrence of peritectic andalusite and cordierite the geochemical features of the leucogranites suggest that tourmaline leucogranite was produced by muscovite dehydration melting in muscovite rich metapelites at p 350 mpa and t 640 c whereas two mica leucogranite was produced by biotite dehydration melting in biotite rich metapelites at p 300 mpa and t 660 710 c melting in fertile muscovite rich metapelites of the top of both the hhc and lhc produced magmas which were emplaced at the same structural level in which they had been generated melting in the biotite rich gneiss of both the hhc and lhc produced hotter magmas which were transported upwards by dyking and eventually coalesced in the plutons of the upper hhc a similar process also produced a network of two mica granite at the top of the lhc in the ama drime nyonno ri range northeast of makalu the prograde character of leucogranite melt producing reactions in the everest makalu area suggests that here the generation of miocene leucogranites took place in a regime of nearly isobaric heating following nearly adiabatic decompression

on preparatory causal factors initiating the prehistoric tsergo ri landslide langthang himal nepal abstract the most probable series of causal factors and their complexity make analysis and synthesis of the tsergo ri landslide langthang himal central north nepal restricted especially in this case of a prehistoric event with a spread of age data interpreting morphologic lithologic structural and engineering geologic aspects in relation to preparatory causal factors and subsequent processes will therefore be only an approach the solid basement and the broken crest of the landslide as well as the surrounding areas of the upper section of langthang valley are to be included in the high himalayan crystalline leucogranitic dikes dipping towards the southwest intruded gneisses and migmatites primary foliation towards the northeast formation of these dikes caused one of many factors or preexisting structures leading to the development of the landslide in addition subsequent tectonic movements and related seismicity forced specific deformation of metamorphic rocks ultramylonites two generations of faults with striated slickensides and pseudotachylites degraded to substantial horizons of weakness the spatial orientation enabled predetermination of subsequent morphologic processes e g slope destabilising forces trend and dip of sliding in association with leucogranitic intrusions a disseminated mineralized extensive sulphidic ore structure outcrops at the broken crest of the landslide due to brittle rheologic behaviour and poor weathering resistance this ore bearing horizon parallel to the main sliding plane also made the slopes susceptible to sliding material displaced from the scarp by landsliding was mixed up with rocks from undisturbed ground and subsequently cemented by secondary ore mineralization thus forming ore breccias at the top of the surface of rupture neotectonic structures caused by stress release associated with erosional processes and similarly oriented as the existing zones of weakness may have acted as a preparatory and or triggering causal factor for slope destabilisation too hyalomylonite from primary and secondary sliding surfaces as well as gneiss from the solid basement of the landslide analysed by means of scanning electron microscopy and energy dispersive x ray analysis indicate deformational patterns attributed to tectonics and landslide mechanisms the deformation feature is distinguished clearly from those created by shock wave events i e impact triggered kofels landslide tyrolean alps austria thus seismic activity might have been the triggering causal factor for the tsergo ri landslide event

manaslu leucogranite a collision signature of the himalaya a model for its genesis and emplacement the manaslu pluton is one of 10 leucogranites that formed in the overthrusted higher himalaya after the indo eurasian collision field and analytical data indicate that the underlying migmatites of the tibetan slab may be where the leucogranitic melts were generated the himalayan crustal thrusting of a hot slab over a rather cold volcano sedimentary pile le fort 1975a provides the necessary release of fluids these fluids cross the main central thrust mct induce the partial anatexis of the overheated tibetan slab and produce a leucogranitic magma the emplacement of the magma at first is located along the main disharmonic plane above the mct between the infrastructure and the superstructure there it generates a convective hydrothermal system extending very far laterally according to the stratification of permeabilities progressive emplacement of the granite proceeds as the convected fluids including the fluids released by the saturated magma dissolve the mainly calcareous host rocks of tibetan sedimentaries a frozen image of this caving out progression is given by the extensive network of granitic dikes outside the pluton the two quite independant fluid cycles of the generation and of the emplacement were triggered or guided by the tectonics due to collision this two fold model dominated by fluid activity may be of importance for other leucogranites and granites

northern margin of the indian plate some litho tectonic constraints abstract the indus tsangpo suture zone represents crustal expression of a subduction zone in the himalaya all along its extension in kohistan ladakh and tibet the suture zone from south to north is constituted of litho tectonic assemblages referable to trench zone fore arc basin island arc and back arc basin these assemblages lie between the platform sediments deposited at the northern margin of indian and southern margin of tibet the trench assemblage consisting of ophiolitic assemblages exhibits high pressure glaucophanitic metamorphism the flysch sequence of the indus group represents fore arc sediments deposited in front of the ladakh magmatic arc comprising dras volcanics the ladakh plutonic complex and the khardung volcanics to the north of the magmatic arc the back arc basin assemblage represented by the shyok group consists of acid and basic volcanics and volcanoclastics intruded by granitic plutons the rocks of the arc and back arc region exhibit low pressure high temperature metamorphism the high pressure and low pressure metamorphic belts lying to the south and north respectively of the magmatic arc constitute paired metamorphic belts detailed analysis of tectonic and lithological characters of various litho tectonic units indicate that these developed at the northern margin of the indian plate during the mesozoic and early tertiary the miocene pliocene molassic rocks overlying various litho units unconformably were deposited after the collision of the indian and tibetan plates

crustal generation of the himalayan leucogranites abstract detailed studies of the himalayan two mica leucogranites such as the manaslu pluton indicate that they have very uniform mineralogical petrological and structural characteristics one can relate their occurrence to the thickest zones of the underlying tibetan slab in these zones migmatization attains its greatest development and vertical extension the granite is emplaced at first along the main disharmonic plane above the main central thrust mct at the top of the tibetan slab infrastructure ductile deformation of the granite is variable the granite being syn to late kinematic with regard to the functioning of the mct major elements are very homogeneous except for na and k implying that p t conditions of melting were relatively uniform the melted material was of a similar composition over a vast volume and the percentage of melting was small 10 15 trace elements are highly variable some are characteristic for very evolved material ta rb cs u or show the link with the tibetan slab ba sr whilst others are problematic th ree ree and th abundances being much less in the granite than in the tibetan slab imply that they have been extracted during one of the main stages of formation possibly by monazite radiogenic pb sr nd and stable o isotopes are consistent with the origin of the granite from the tibetan slab however the heterogeneous sr isotopic ratios make age dating difficult and imply poor mixing or little fluid interaction during its evolution h isotope data indicate that magmatic compositions of the main body of the manaslu granite have been preserved late or post magmatic alterations are extremely local in the main pluton to the north another belt of two mica granites occurs whose characteristics are very similar to the high himalaya belt they were probably generated in a similar way during this recent intracontinental evolution

the kangmar dome a metamorphic core complex in southern xizang tibet the kangmar metamorphic igneous complex is one of the most accessible examples of an enigmatic group of gneiss domes the north himalayan belt that lies midway between the greater himalaya and the indus tsangpo suture in southern tibet structural analysis suggests that the domal structure formed as a consequence of extensional deformation much like the tertiary metamorphic core complexes in the north american cordillera unlike its north american counterparts the kangmar dome developed in an entirely convergent tectonic setting the documentation of metamorphic core complexes in the himalayan orogen supports the emerging concept that extensional processes may play an important role in the evolution of compressional mountain belts

younger hanging wall rocks along the vaikrita thrust of the high himalaya a model based on inversion tectonics abstract in sharp contrast to the common observed characteristic of areas of thrust tectonics where older rocks are thrust over younger along the vaikrita thrust in the high himalaya younger hanging wall rocks i e vaikrita group late mesoproterozoic to early neoproterozoic lie above the older footwall rocks i e munsiari formation paleoproterozoic the phenomenon is explained by an inversion tectonics based model where normal faulting and metamorphism were followed by thrusting in which the thrust displacement was less than the displacement during the earlier normal faulting the present day hanging wall tilt towards north may have been caused by a later thrust initiated as a piggy back sequence accompanied by folding and himalayan metamorphism

thrust geometries and deep structure of the outer and lesser himalaya kumaon and garhwal india implications for evolution of the himalayan fold and thrust belt the kumaon garhwal region of the himalaya lies near the center of the himalayan fold and thrust belt we have drawn two balanced cross sections 100 km apart through the outer and lesser himalaya the cross sections incorporate all the surface well log and earthquake seismic data currently available from the region two branch line maps showing trailing and leading branch lines and cutoff lines of the major thrusts in the region are also drawn the three dimensional deep structure of the outer and lesser himalaya is interpreted based on the balanced cross sections and the branch line maps deep structure of the higher and tethyan himalaya is extrapolated based on surface geology and is subject to revision as more surface and seismic data become available from these areas a sequential evolutionary model for the kumaon himalaya along the eastern pindari section is proposed according to this model the kumaon himalaya evolved by an overall forelandward progression of thrusting with some reactivation along the munsiari thrust mt the main boundary thrust mbt and the main central thrust mct we use structural stratigraphic and radiometric criteria to place time constraints on the motion of these thrusts earliest motion along the mbt may have occurred in early middle paleocene but the main episode probably started in late eocene and may still be continuing emplacement of the mt had occurred by middle late eocene whereas the mct shows activity around 20 ma thus exhibiting break back thrusting with respect to the mt shortening estimates are obtained from the pindari section minimum shortening in the sedimentary thrust sheets of the outer and lesser himalaya is 161 km or 65 as a first approximation we have also restored the crystalline sheets in order to obtain shortening estimates for the entire himalaya minimum shortening for the himalaya after restoring the mct sheet varies from 354 76 to 421 km 79 these estimates were further combined with published data from the area between the mct sheet and the indus tsangpo suture zone itsz minimum shortening between the indo ganga foreland and the itsz thus obtained lies in the range 687 754 km or 69 72 we compare our shortening estimates with those available from the pakistan and nepal himalaya

north pacific deep water formation during the latest glaciation for at least 20 years palaeoceanographers have speculated about the existence of a source of young nutrient depleted deep water in the north pacific npdw during glaciations proof of its existence has eluded researchers because the present deep north pacific is very corrosive to calcium carbonate thus it has been difficult to obtain a long time series of oxygen isotope data on benthic foraminifera for dating the sediment and carbon isotope data for use as a proxy for deep water nutrient content i report here on a stable isotope record from a western subarctic pacific core taken at 3 km water depth the carbon isotope ratios in the benthic foraminifera cibicidoides show no evidence of nutrient depleted npdw from glacial to holocene time

thermobarometric and40ar 39ar geochronologic constraints on eohimalayan metamorphism in the dinggye area southern tibet mineral assemblages in the dinggye area of southern tibet 28 n 88 e provide new insights regarding the poorly understood eohimalayan metamorphic event in the eastern himalayan orogen major element partitioning thermobarometry of pelitic rocks indicates temperatures of 750 830 k at depths of 14 3 km consistent with the presence of kyanite sillimanite and andalusite schists in the area laser and resistance furnace40ar 39ar analyses of hornblendes from intercalated amphibolites yield closure ages of 25 ma overlap between the probable range of ar closure temperatures for these hornblendes and the metamorphic temperatures estimated through thermobarometry suggests that eohimalayan metamorphism in the dinggye area occurred in late oligocene time no more than about 10 million years before the main or neohimalayan phase of metamorphism in early to middle miocene time muscovite biotite and k feldspar40ar 39ar ages indicate an important episode of rapid cooling between 16 and 13 ma which is interpreted as a signature of tectonic denudation related to movement on n dipping extensional structures of the south tibetan detachment system

role of inversion tectonics in structural development of the himalaya abstract analysis of surface and subsurface structures variation of shortening amounts obtained by restoration of deformed cross sections and occurrence of younger hangingwall rocks over the older footwall rocks across the vaikrita thrust in the higher himalaya suggests reactivation of early normal faults as thrusts based on this an inversion tectonics model is proposed for structural development of the himalaya the model explains the geometrical shape of the himalaya as primary arcuation and helps in resolving superimposed deformation in the region

the gangotri granite garhwal himalaya laccolithic emplacement in an extending collisional belt the gangotri miocene leucogranite is composed of several laccoliths 6 7 km long 1 5 2 km thick which can be divided into two sets of lenses the southern lenses intruded in the lower part of the high himalaya sedimentary cover and the northern lenses intruded in an older porphyritic biotite bearing granite in both cases the magmatic fabric is commonly weak with no dominant stretching direction although a rough e w trend is present in the northern lenses the magnetic fabric is characterized by the fact that drastic changes in the direction in the magnitude of the magnetic parameters or in the fabric type planar or linear may take place over very short distances measurements of preferred orientations in thin sections indicate that the strain regime was largely dominated by a coaxial component for both sets of lenses in agreement with the large dispersion observed in both the field and magnetic lineations the comparison of the granite and host rock structures shows that the leucogranite emplacement dates the onset of the extensional tectonism in the high himalaya range and is not related to a southward directed thrust event associated with the main central thrust this is exemplified by the vertical attitude of the feeder dikes that intrude the metasedimentary rocks beneath die southern lenses the presence of these dikes indicates in turn that magma ascent occurred by fracture propagation the spatial disposition of the southern lenses could have resulted either from the disruption of a single laccolithic intrusion by crustal scale boudinage due to a northern gravity backslide of the top of the tibetan slab or from the intrusion of independent laccoliths the use of the elastic bending theory of pollard and johnson 1973 shows that in both hypotheses the current laccolith sizes are compatible with a laccolithic mode of magma emplacement however neither the density contrast between the magma and its enclosing rocks nor the lithological boundary between the tibetan slab and the overlying tibetan metasedimentary series controlled the level of magma emplacement rather flat lying collapse structures which intersected the upward propagating magma dikes are the most likely causes of magma arrest such a mechanism was favored by the schist rich lithology of the metasedimentary host rocks in addition field relationships indicate that the melt supply through the dike system was a continuous rather than pulsed process existing numerical treatments on the rates of magma transport through fractures show that in such a case the laccoliths could have been built in less than 100 years this short time of emplacement the small size of the laccoliths and their peripheral disposition relative to the badrinath granite suggest that the gangotri lenses may represent the initial stage of pluton accretion in the high himalaya which ultimately gave rise to a much larger massif such as the manaslu granite

pressure temperature deformation time constraints on the south tibetan detachment system in the garhwal himalaya nw india a peculiar feature of the himalaya is the occurrence of a system of low angle normal faults and shear zones the south tibetan detachment system stds at the mountain crests the stds was active during syn convergent tectonics we describe the stds related sheared rocks along the dhauli ganga valley in the garhwal himalaya nw india where the malari granite reported as an undeformed igneous body cross cutting the stds occurs a detailed multidisciplinary study integrating field based microstructural petrographic and geochronological analyses was carried out on rocks along this valley we demonstrate how the non coaxial ductile portion of the stds affected the upper part of the greater himalaya sequence migmatite which experienced peak pressure p temperature t conditions of 0 9 1 1 gpa and 750 c at 24 ma this migmatite has been reworked structurally upwards leading to the formation of high t sillimanite bearing mylonites further upward medium t shearing deformed the malari granite and leucogranite dykes forming medium t mylonites ductile shearing was temporally constrained based on new in situ monazite datings and previously published ar ar geochronology between 20 15 ma we demonstrate that a preserved ductile to brittle spatial and temporal transition of the stds deformation exists with the brittle features overprinting ductile ones our data shed new light on the geological evolution of the stds in the nw himalaya with implications for the relationship and relative timing of partial melting granite emplacement and deformation along low angle normal faults

granite migmatite genetic link the example of the manaslu granite and tibetan slab migmatites in central nepal in central nepal the tibetan slab is made up of biotite gneisses metapelites and metagreywackes orthogneisses metagranites and migmatites melanosomes are generally biotite muscovite bearing but locally they may be tourmaline rich when associated with boron rich granitic material leucosomes occur as lenses conformable with the foliation veins patches or as fillings in shear zones and extensional structures field relationships and mineralogical and chemical data show that three processes may have contributed to the formation of the tibetan slab leucosomes metamorphic differentiation or disequilibrium partial melting low zr tonalitic leucosomes in situ equilibrium partial melting high zr leucosomes and some granitic leucosomes and injection of externally derived melts most granitic and some tonalitic leucosomes the manaslu pluton belongs to the high himalayan leucogranite belt and was emplaced at the top of the tibetan slab it corresponds to a muscovite biotite leucogranite that has been assumed to derive from melting of the tibetan slab gneisses formation i phase relationships a more magnesian chemistry of the ferromagnesian minerals from the tibetan slab migmatites as compared to the manaslu leucogranite the microtextures of accessory phases and trace element compositions lower u li f and higher sr eu y yb contents in the migmatite leucosomes show that the in situ himalayan migmatites at the crustal level presently exposed have not been produced under the same p t xh2o conditions as the manaslu leucogranite magma while the formation i was the probable source for the manaslu granite migmatites within the formation are not the remanants of a melting process from which the manaslu granite was derived both the tibetan slab migmatites and the manaslu leucogranite may be considered as evidence of dehydration and melting at deeper crustal levels and of percolation of melts and hydrothermal fluids through the crust

an evaluation of the inverted metamorphic gradient at langtang national park central nepal himalaya the crystalline core of the himalayan orogen in the langtang area of nepal located between the annapurna manaslu region and the everest region contains middle to upper amphibolite grade pelitic gneisses and schists these rocks are intimately associated with the main central thrust mct one of the major compressional structures in the northern indian plate which forms a 3 7 km wide zone containing rocks of both footwall and hangingwall affinity an inverted metamorphic gradient is noticeable from upper footwall through hangingwall rocks where metamorphic conditions increase from garnet grade near the mct zone to sillimanite k feldspar grade in the upper hangingwall petrographic data distinguish two metamorphic episodes that have affected the area a high pressure moderate temperature episode m1 and a moderate pressure high temperature episode m2 comparison with appropriate reaction boundaries suggests that conditions for m1 in the hangingwall were approximately 900 1200 mpa and 425 525 c thermobarometric results for 24 samples from the footwall mct zone and hangingwall reflect p t conditions during the m2 phase of 400 1200 mpa and 490 660 c the decrease in estimated palaeopressures from footwall to hangingwall approximate a lithostatic gradient of 27 mpa km 1 with slight fluctuations in the mct zone reflecting structural discontinuities in contrast to the palaeopressures palaeotemperatures are indistinguishable across the entire area sampled although field evidence suggests the presence of the inverted palaeothermal gradient well known in the himalaya quantitative thermobarometry indicates that temperatures of final equilibration were all within error of each other across 17 km of section at langtang change in pressure is responsible for the presence of the sequence of index minerals through the section i interpret these data to reflect diachronous attainment of equilibrium temperature conditions in a lithostatic palaeopressure profile after ductile faulting of the sequence

fluid inclusions tectonic indicators abstract during the first half of the 20th century fluid inclusions have been studied in ore geology in order to determine the chemistry and physical parameters density of the mineralising fluids apart from a few pioneering works in the middle of the century fluid inclusions were not used as structural markers before the late 1980s during the same time experiments as well as observations of natural samples have shown that the volume shape and density of fluid inclusions may change when submitted to pressure temperature conditions different from their trapping isochores in view of these experiments and based on natural examples from the higher himalaya metamorphic rocks it is evident how fluid inclusions can be used as thermobarometric structural and tectonic markers points of interest for further studies on fluid inclusions in nature and experiments are also stressed

some thermotectonic aspects of the tibetan plateau abstract advective heat transfer plays a prominent role in the thermal evolution of tectonically active regions such as the tibetan plateau different tectonic histories lead to different thermal evolutions so that a study of the present and ancient thermal signatures can improve our understanding of thermotectonic histories we use an upwind finite element method to calculate the thermal consequences of two tectonic models suggested by previous studies and compare the calculated results with observations many factors that may affect the thermal structure are investigated including advection frictional shear heating latent heat of melting and heat refraction an imbricated thrust model yields a thermal structure that agrees best with observations of heat flow crustal low velocity zones leucogranites and inverted metamorphism we suggest that a heterogeneous thermal structure exists beneath tibet southern tibet has a relatively hot mid crust and cold mantle resulting from frictional shear heating and advective cooling that are due to active thrusting whereas northern tibet has a relatively cool crust and hot mantle resulting from thermal relaxation in a region of thickened crust after the cessation of thrusting

multiple equilibria of the global thermohaline circulation abstract a general circulation model with a highly idealized geometry is used to investigate which fundamentally different equilibria of the global thermohaline circulation may exist the model comprises two identical basins representing the atlantic and pacific oceans which are connected by a circumpolar channel in the south the model circulation is driven in addition to wind forcing by restoring the sea surface temperature to prescribed values and specified freshwater fluxes in the surface salinity budget mixed boundary conditions the boundary conditions are symmetric with respect to the equator and identical for both oceans four fundamentally different stable steady states are found under the same set of boundary conditions two of the equilibria show both oceans in the same state with high altitude deep water formation occuring either in both northern or in both southern oceans respectively two additional equilibria exist in which the thermohaline circulations of the basins differ fundament

thermal evolution of the greater himalaya garhwal india the hanging wall of the main central thrust mct in garhwal india roughly 79 n 80 e 30 n 31 n exhibits an inverted metamorphic gradient sillimanite potassium feldspar assemblages near the top of the hanging wall or greater himalayan sequence are underlain by kyanite grade rocks near the fault textural relationships in pelitic samples from the alaknanda and dhauli river valleys indicate that the inversion is the product of two distinct metamorphic events an early harrovian event m1 which affected the entire greater himalayan sequence and a later buchan event m2 the effects of which are most obvious in the upper part of the sequence rim thermobarometry garnet inclusion thermobarometry and thermodynamic modeling of garnet zoning reveal that the basal portions of the metamorphic sequence experienced peak m1 conditions of 900 k and 960 mpa roughly 36 km depth before following an erosion controlled uplift path e g england and richardson 1977 m2 metamorphic temperatures in the upper part of the sequence also exceeded 900 k but maximum pressures 317 523 mpa indicate paleodepths of only 12 19 km calculated pressure temperature paths indicate that m2 was characterized by temperature increases of 80 k and roughly 5 km of tectonic burial we attribute m1 to tectonic burial of the greater himalayan sequence during the early stages of india eurasia collision we believe that the uplift and cooling path of the sequence was interrupted in late oligocene miocene time by a second burial and heating event m2 related to thrust imbrications in southern tibet this burial was coincident with the generation of leucogranites which are abundant near the top of the greater himalayan sequence but are virtually absent near the mct field relations do not constrain whether the leucogranites were derived from some presently unexposed portion of the greater himalayan sequence and were injected at their present structural level or were melted in situ if the granites were injected then they may have provided some of the heat necessary for m2 metamorphism although our data suggest no direct relationship between the main central thrust as mapped in garhwal and metamorphism in the greater himalaya anatectic melting of an unexposed portion of the greater himalayan sequence could have been associated with movement along a blind thrust with characteristics similar to the mapped mct in central nepal cf le fort 1981 if the granites were produced by in situ m2 melting then we must appeal to a heat source within the upper part of the greater himalayan sequence such as locally high concentrations of heat producing elements cf pinet and jaupart 1987

k ar and ar ar geochronology of the himalayan collision in nw pakistan constraints on the timing of suturing deformation metamorphism and uplift mica and hornblende k ar and ar ar data are presented from each of the three crustal components of the himalayan collision zone in north pakistan the asian plate the kohistan island arc and the indian plate together with u pb and rb sr data published elsewhere these new data 1 date the age of suturing along the northern suture which separates kohistan from asia at 102 85 ma 2 establish that the basic magmatism in kohistan which postdates collision along the northern suture predates 60 ma and that the later granite magmatism spanned a range of 60 25 ma 3 show that uplift amounts within kohistan are greater toward the nanga parbat syntaxis than away from it and that rate of uplift near the syntaxis increased over the last 20 ma to a current figure of about 5 5 mm a year 4 show that much of southern kohistan had cooled to below 500 c by 80 ma and that the major deformation which imbricated kohistan probably predated 80 ma and certainly predated 60 ma and was related to the kohistan asia collision rather than the kohistan india one 5 imply that uplift along the hunza shear in the asian plate together with imbrication of the metamorphics in its hanging wall took place at about 10 ma and was associated with breakback thrusting in the hanging wall of the main mantle thrust 6 suggest that the indian plate has a lengthy pre himalayan history with an early metamorphism at about 1900 ma major magmatism at 500 550 ma and early jurassic lithospheric extension or inversion and 7 show that the indian plate rocks were metamorphosed shortly after the collision within kohistan which occurred at circa 50 ma and subsequently cooled back through 500 c at circa 38 ma and 300 c at 30 35 ma with ages of cooling through 200 and 100 c as determined by fission track data locally controlled by nanga parbat related uplift tectonics

stability of north atlantic deep water formation in a global ocean general circulation model abstract a global ocean general circulation model is forced using mixes boundary conditions i e a restoring condition on the upper level temperature but using a fixed specified surface salt flux freshwater flux anomalies lasting 5 years are then applied over the western half of the subpolar gyre in the northern north atlantic the current climate is found to be stable to anomalies that have salt deficits equivalent to about seven times that estimated for the great salinity anomaly of 1968 1982 although this value is a function of the duration over which the anomaly is imposed above this level the thermohaline circulation collapses to a state in which the zonally averaged overturning associated with north atlantic deep water formation is only about half its original value the sea surface temperatures over the north atlantic are lowered and both the subpolar and subtropical gyres have weakened horizontal transports various atmospheric feedbacks on the momentum and salt flux are then applied und

pressure temperature and time constraints on himalayan metamorphism from eastern kashmir and western zanskar himalayan metamorphism in the kashmir and zanskar sector of the high himalaya resulted from thrust and fold related crustal thickening within the indian plate following the collision of india and asia at c 50 ma interbanded metapelites marbles calcareous schists amphibolites and quartzites represent metamorphosed equivalents of the palaeozoic lower mesozoic continental margin rocks granitic rocks include pre collision k feldspar megacrystic and biotite granites and post collision two mica garnet tourmaline granites average pressure temperature conditions of equilibration using the self consistent thermodynamic data set of holland powell 1990 are presented across a 45 km traverse of the eastern kashmir western zanskar high himalaya kyanite is the stable aluminosilicate phase across 35 km outcrop width in the middle of the slab complex microstructures indicate that prograde metamorphism up to kyanite grade and fabric development in the upper structural nappes is early and unrelated to the main central thrust diachronous metamorphism propagated southwards with the overall structural evolution early isograds and thrust fold structures were carried passively in the hanging wall of the main central thrust peak metamorphism based on 40 ar 39 ar hornblende ages occurred pre 30 7 2 0 ma at the top of the slab and in the middle of the slab was pre 22 1 0 ma probably 25 28 ma the lower structural levels in zanskar record peak metamorphic conditions around 700 750 c and 8 kbar reflecting depths of burial of 28 30 km a new younger schistosity which is not present in the higher structural levels was developed under kyanite grade conditions the regional distribution of high temperatures recorded by thermo barometry does not support the concept of additional heat being supplied by frictional heating along the main central thrust or magmatic heat resulting from anatexis exhumation of the himalayan metamorphic rocks was achieved by three major processes erosion as a result of crustal thickening uplift of the rocks along the hangingwall of the main central thrust above a major frontal ramp in the kishtwar window area and extensional unroofing along the footwall of a large scale ne dipping normal fault at upper crustal levels probably synchronous with compression at depth

multiple equilibria of an asymmetric two basin ocean model abstract an ocean general circulation model is used to examine the role of model geometry and surface buoyancy and wind stress forcing in the asymmetry of the global thermohaline circulation the model domain is a highly idealized atlantic and pacific linked by a circumpolar ocean in the south and the integrations are performed under mixed boundary conditions diagnosed from spinups under various temperature and salinity profiles constructed from the present day climatology the model exhibits a tendency to favor either a conveyor type circulation with sinking in the northern north atlantic and upwelling in the north pacific or a southern sinking state with deep sinking in the antarctic only this bias is not dictated solely by the hydrological cycle nor apparently by the greater northern extension of the atlantic basin but presumably by the overall asymmetry of the geometry equilibria with northern sinking in both basins can appear however when the winds in the southern ocean are reduced or th

isopycnal mixing in ocean circulation models abstract a subgrid scale form for mesoscale eddy mixing on isopycnal surfaces is proposed for use in non eddy resolving ocean circulation models the mixing is applied in isopycnal coordinates to isopycnal layer thickness or inverse density gradient as well as to passive scalars temperature and salinity the transformation of these mixing forms to physical coordinates is also presented

morphotectonic analysis and gnss observations for assessment of relative tectonic activity in alaknanda basin of garhwal himalaya india abstract alaknanda basin in the garhwal himalaya india is a tectonically active region owing to ongoing crustal deformation erosion and depositional processes active in the region active tectonics in this region have greatly affected the drainage system and geomorphic expression of topography and provide an ideal natural set up to investigate the influence of tectonic activity resulting from the india eurasia collision we evaluated active tectonics by using high resolution digital elevation model dem based on eight geomorphic indices stream length gradient index valley floor width to height ratio hypsometric integral drainage basin asymmetry transverse topography symmetry factor mountain front sinousity index bifurcation ratio and basin shape index and seismicity in eight subbasins of alaknanda basin the integrated product relative tectonic activity index tai map was classified into three classes such as highly active with values ranging up to 2 0 moderately active with values ranging from 2 0 to 2 25 and less active with values 2 25 further the results were compared with relatively high crustal movement rate of 41 10 mm y computed through high precession global navigation satellite system gnss based continuous operating reference station cors data thus we concluded that this new quantitative approach can be used for better characterization and assessment of active seismotectonic regions of the himalaya and elsewhere

tectonic and polymetamorphic history of the lesser himalaya in central nepal abstract the lesser himalaya in central nepal consists of precambrian to early paleozoic low to medium grade metamorphic rocks of the nawakot complex unconformably overlain by the upper carboniferous to lower miocene tansen group it is divided tectonically into a parautochthon two thrust sheets thrust sheets i and ii and a wide shear zone main central thrust zone from south to north by the bari gad kali gandaki fault the phalebas thrust and the lower main central thrust respectively the lesser himalaya is overthrust by the higher himalaya along the upper main central thrust umct the lesser himalaya forms a foreland propagating duplex structure each tectonic unit being a horse bounded by imbricate faults the umct and the main boundary thrust are the roof and floor thrusts respectively the duplex is cut off by an out of sequence fault at least five phases of deformation d 1 d 5 are recognized in the lesser himalaya two of which d 1 and d 2 belong to the pre himalayan pre tertiary orogeny petrographic microprobe and illite crystallinity data show polymetamorphic evolution of the lesser and higher himalayas in central nepal the lesser himalaya suffered a pre himalayan probably early paleozoic anchizonal prograde metamorphism m 0 and a neohimalayan syn to post umct diagenetic to garnet grade prograde inverted metamorphism m 2 the higher himalaya suffered an eohimalayan pre or early umct kyanite grade prograde metamorphism m 1 which was in turn overprinted by neohimalayan syn umct retrograde metamorphism m 2 the isograd inversion from garnet zone in the lesser himalaya to kyanite zone in the higher himalaya is only apparent due to post metamorphic thrusting along the umct both the lesser and higher himalayas have undergone late stage retrogression m 3 during exhumation

fluid enhanced melting during prograde metamorphism anatexis is a commonly recognized feature of high grade metamorphism but segregated melts are generally ascribed to anatexis during peak metamorphic conditions and little is known about melting along the prograde path a suite of small volume deformed two mica leucogranites has been recognized within the high himalayan crystalline series of the garhwal himalaya these granites are consistently more siliceous than minimum melt granite compositions and are characterized by low rb sr ratios high ba low abundances of hfs elements and positive eu anomalies such trace element characteristics contrast strongly with the geochemistry of the well studied early miocene leucogranites of the high himalaya derived from fluid absent melting sm nd garnet dating of one deformed granite indicates a crystallization age of 39 3 ma c 15 ma before the emplacement of the more voluminous high himalayan leucogranites whilst some entrainment of restitic phases cannot be excluded trace element signatures suggest a low temperature 2 o activity positive eu anomalies and unusually low rb sr ratios are indicative of rapid disequilibrium melting fluid enhanced melting may be a common feature of prograde upper amphibolite facies metamorphism of orogenic belts predating peak metamorphism by at least 15 ma these melts will only crystallize within this period if they segregate from their protoliths subsequent dating of long lived melts would indicate erroneously young ages for the prograde melting events however melts formed in this way may be recognized by their distinctive trace element chemistry the persistence of early formed melts within an orogen provides insights into the prograde heating path and may be critical in controlling the rheology of the middle crust and hence its deformational history

microstructures in quartz and feldspars of the bomdila gneiss from western arunachal himalaya northeast india implications for the geotectonic evolution of the bomdila mylonitic zone abstract microstructures of the bomdila gneiss bg that intruded the metapelites of the lesser himalayan crystallines lhc in the western arunachal himalaya were studied to understand the deformation mechanisms and temperatures during deformation the lhc is delimited at the base by the bomdila thrust bt and the main central thrust mct at the top the microstructural features of quartz and feldspar in the bg indicate that the rocks underwent an early stage of deep level ductile deformation superimposed by shallow level brittle deformation during a later stage of deformation grain boundary migration and subgrain rotation recrystallization are the main deformation mechanisms for recrystallization of quartz and feldspars pressure solution and solution transfer were also important at the base of the bomdila mylonitic zone both the feldspars plagioclase and k feldspar and quartz show dynamic recrystallization with growth of myrmekite at the upper level while the development of quartz subgrains and fractured in feldspars are dominate at the lower level the deformation microstructures of bg suggest that the bomdila mylonitic zone shows an inverted thermal profile towards the mct i e increase in deformation temperature from 400 c at the lower zone to 500 600 c at the upper zone of the gneiss the microstructural variations and decreasing temperatures of deformation from west to east indicate that the lhc initially buried below higher himalayan crystallines hhc along the mct have been exhumed along the bt shear criteria indicates top to the se displacement in agreement with the kinematics of the lhc along the bt producing of an inverted thermal profile in the study area may be related to imbrication and post metamorphic shearing

structural evolution and sequence of thrusting in the high himalayan tibetan tethys and indus suture zones of zanskar and ladakh western himalaya reply abstract the timing of motion on major thrusts in the western himalaya shows an extremely complex sequence that spans approximately 70 ma from the latest cretaceous throughout the tertiary three major phases of thrusting can be distinguished the earliest phase t1 is associated with emplacement of tethyan basin thrust sheets lamayuru sediments and spontang ophiolite south and south westwards onto the submerged northern passive margin of india 75 60 ma collision between india and asia occurred at 50 36 ma and was followed immediately by the major phase t2 of crustal shortening involving large scale south and south westward directed thrusting of the complete palaeozoic mesozoic and late tertiary tibetan tethys zone rocks preliminary balanced cross sections show a minimum shortening of 126 km of these rocks across the zanskar range the late collision phase t3 involved re thrusting of the previously stacked pile breaching or leap frog thrusting reversing the earlier stacking order in places and widespread steepening overturning and backthrusting along the whole northern margin of the tibetan tethys zone and throughout the indus suture zone

linked fluid and tectonic evolution in the high himalaya mountains nepal fluid inclusions were studied in a quartz lens from the structurally highest unit of the himalaya mountains in nepal from a textural geometrical chemical and isotopic point of view six types of fluid inclusions were distinguished one of these types consists of annular inclusions this shape is attributed to a confining pressure increase in a non isotropic stress field two successive stress fields were deduced from the orientation of the inclusion planes relative to the schistosity the bulk composition of the fluid was dominated by co2 84 mol and h2o the composition remained constant during the whole history of the sample indicating that it was buffered by the carbonaceous host rock and or that one single fluid was reworked in situ by decrepitation stable isotope of fluids and minerals indicate 1 that fluids were buffered by surrounding rocks for o and c and 2 that at least two types of water metamorphic and meteoric were involved finally a p t t e path is proposed for the sample taking into account the southward thrusting along the main central thrust the northward tectonic denudation of the himalaya mountains inducing tectonic burying below the annapurna range and lastly rapid uplift

crustal velocity structure and earthquake processes of garhwal kumaun himalaya constraints from regional waveform inversion and array beam modeling abstract in order to understand present day earthquake kinematics at the indian plate boundary we analyse seismic broadband data recorded between 2007 and 2015 by the regional network in the garhwal kumaun region northwest himalaya we first estimate a local 1 d velocity model for the computation of reliable green s functions based on 2837 p wave and 2680 s wave arrivals from 251 well located earthquakes the resulting 1 d crustal structure yields a 4 layer velocity model down to the depths of 20 km a fifth homogeneous layer extends down to 46 km constraining the moho using travel time distance curve method we then employ a multistep moment tensor mt inversion algorithm to infer seismic moment tensors of 11 moderate earthquakes with m w magnitude in the range 4 0 5 0 the method provides a fast mt inversion for future monitoring of local seismicity since green s functions database has been prepared to further support the moment tensor solutions we additionally model p phase beams at seismic arrays at teleseismic distances the mt inversion result reveals the presence of dominant thrust fault kinematics persisting along the himalayan belt shallow low and high angle thrust faulting is the dominating mechanism in the garhwal kumaun himalaya the centroid depths for these moderate earthquakes are shallow between 1 and 12 km the beam modeling result confirm hypocentral depth estimates between 1 and 7 km the updated seismicity constrained source mechanism and depth results indicate typical setting of duplexes above the mid crustal ramp where slip is confirmed along out of sequence thrusting the involvement of tons thrust sheet in out of sequence thrusting indicate tons thrust to be the principal active thrust at shallow depth in the himalayan region our results thus support the critical taper wedge theory where we infer the microseismicity cluster as a result of intense activity within the lesser himalayan duplex lhd system

thermal controls on early tertiary short lived rapid regional metamorphism in the nw himalaya pakistan abstract during tertiary collision in the nw himalaya the leading edge of the indian plate was subducted beneath the kohistan island arc along the main mantle thrust mmt metamorphism within indian plate cover sediments was synchronous with ductile shearing and took place along a path of increasing pressure during subduction beneath the island arc initial collision cannot have pre dated 65 ma and probably shortly pre dated 50 ma radiometric data constrain the metamorphic peak as shortly post dating 50 ma as firstly initially subducted units are now probably located beneath tibet secondly the subduction thrust separating the kohistan arc terrane from the indian plate was probably cooled by continued underthrusting and thirdly the heat producing indian plate cover sediments were delaminated from the basement during collision metamorphism was more rapid than can be predicted by purely conductive models of thermal relaxation although dissipative shear heating along the mmt doubtless contributed to early stages of heating of the footwall rocks the temperatures attained in the footwall are too high to support the shear stresses required to generate them solely through shear heating a model is derived to account for both the rapid regional metamorphism and the equally rapid post metamorphic cooling dissipative shear heating along the mmt generated an early inverted thermal profile in the upper units of the indian plate as the hanging wall mafic rocks have a low thermal conductivity they would have acted as a thermal reflector and the heat would have been conducted away only slowly as footwall temperatures increased through the brittle ductile transition the role of dissipative shear heating decreased and continued heating became a function of internal heat generation within the footwall rocks together with hanging wall thermal reflectivity the metamorphic inversion was reinforced by imbrication of the metamorphic stack as it accreted onto the mmt footwall during early stages of uplift and exhumation dissipative shear heating within thrust systems provides a potentially important mechanism by which areas of large regional extent can undergo regional metamorphism over short timescales in the absence of magmatic heating

high latitude salinity effects and interhemispheric thermohaline circulations a general circulation model for the ocean is used to investigate the interaction between the global scale thermohaline circulation and the salinity distribution it is shown that an equatorially asymmetric circulation can be maintained even under equatorially symmetric basin geometry and surface forcing multiple equilibrium solutions are obtained for the same forcing by perturbing the high latitude salinity field in an otherwise equatorially symmetric initial condition the timescale of the transition from the symmetric circulation to an asymmetric circulation depends critically on the sign of the initial salinity perturbation

chronology of tectonic events in the crystalline core of the himalaya langtang national park central nepal the main central thrust mct is an important intracontinental subduction zone that accommodated a significant proportion of shortening between india and asia during tertiary himalayan orogenesis argon 40 argon 39 geochronology indicates at least two distinct periods of thrust movement on the mct in the langtang national park region of central nepal ductile deformation and associated mylonitization characterizing the earlier event are constrained by 40ar 39ar dating of muscovites to have occurred sometime before 5 8 ma a later period of brittle deformation resulting in the juxtaposition of rocks of different lithology within the mct zone occurred at approximately 2 3 ma based on 40ar 39ar dating of neoblastic muscovites from the brittle fault zones the hanging wall of the mct contains amphibolite to upper amphibolite grade gneisses and small leucogranite bodies assigned to the greater himalayan sequence argon 40 argon 39 cooling ages of muscovite and biotite from the gneisses range from 4 6 to 9 7 ma these dates contrast with previously obtained 16 21 ma u pb monazite and zircon ages for metasedimentary rocks from the same structural levels parrish et al 1992 indicating relatively slow cooling over the early to middle miocene interval for much of the mct hanging wall however one 19 3 ma biotite 40ar 39ar cooling age for a sample from the uppermost portion of the hanging wall is only slightly younger than u pb monazite ages for nearby anatexites possibly suggesting rapid cooling of the uppermost greater himalayan sequence by tectonic denudation associated with the structurally higher south tibetan detachment system the general consistency of 40ar 39ar ages throughout the 11 km thick greater himalayan sequence suggests rapid cooling in late miocene time probably due to an increase in erosion rate related to ramping on the structurally lower main boundary thrust an alternative possibility would be massive hydrothermal resetting in late miocene time of the entire sequence

marine transgression and regression in miocene sequences of northern pegu bago yoma central myanmar abstract neogene strata of the northern part of the pegu bago yoma range central myanmar contain a series of shallow marine clastic sediments with stratigraphic ages ranging from the early to late miocene the studied succession around 750 m thick is composed of three major stratigraphic units deposited during a major regression and four major transgressive cycles in the early to late miocene the transgressive deposits consist of elongate sand bars and broad sand sheets that pass headward into mixed flats of tidal environments marine flooding in transgressive deposits is associated with coquina beds and allochthonous coral bearing sandy limestone bands major marine regressions are associated with lowstand progradation of thick estuary point bars passing up into upper sand flat sand bodies encased within the tidal flat sequences and lower shoreface deposits with local unconformities the succession initially formed in a large scale incised valley system and was later interrupted by two major marine transgressions in the generally regressive or basinward stepping stratigraphic sequences successive sandbodies were formed during a sea level lowstand and early stage of the subsequent relative rise of sea level in a tide dominated estuary system in the eastern part of the central myanmar tertiary basin during early to late miocene times

zoned poikiloblastic garnets p t paths and syn metamorphic uplift through 30 km of structural depth wopmay orogen canada

a thermal model for the distribution in space and time of the himalayan granites abstract in southern tibet crustal thickening due to the india asia collision has led to the formation of two granite belts one is located at the southern edge of the accretionary wedge of tethyan sedimentary rocks close to the contact with basement gneisses of the tibetan slab the other is found within the wedge itself close to the kangmar thrust trace available ages suggest that the granites appeared first in the southern belt and then in the kangmar belt this sequence seems to violate the chronology of thrusting another feature of the himalayas is that melting started only about 20 ma after the onset of thickening which is much less than the thermal time constant of thick crust we give a thermal model based on the assumption of conductive heat transfer which explains these features the model relies on the geometry of a sedimentary accretionary wedge bounded by low angle thrust faults and on the existence of a thermal conductivity contrast between old basement and young sedimentary rocks the wedge of sedimentary rocks acts as an insulating cap and its southern edge heats up along the contact with basement rocks on a horizontal cross section there is a temperature maximum along this southern edge which explains why melting starts there the early thermal evolution is sensitive to local conditions and granites first appear in the vicinity of the most radiogenic parts of the basement the distribution of granites in space and time is seemingly random reflecting different melting events in different radiogenic environments in the heterogeneous basement this model predicts a relationship between radioactivity and age which is compatible with available data the results emphasize that there are large horizontal temperature variations across a thickened region and that granite ages are not related simply to the timing of tectonic phases