kinematic model for the main central thrust in nepal we present a kinematic model for the himalayan thrust belt that satisfies structural and metamorphic data and explains recently reported late miocene pliocene geochronologic and thermochronologic ages from rocks in the main central thrust zone in central nepal at its current exposure level the main central thrust juxtaposes a hanging wall flat in greater himalayan rocks with a footwall flat in lesser himalayan rocks of the ramgarh thrust sheet which is the roof thrust of a large lesser himalayan duplex sequential emplacement of the main central early miocene and ramgarh middle miocene thrust sheets was followed by insertion of thrust sheets within the lesser himalayan duplex and folding of the main central and ramgarh thrusts during late miocene pliocene time thorium lead th pb ages of monazite inclusions in garnets from central nepal record the timing of coeval progressive metamorphism of lesser himalayan rocks in the footwall of the main central thrust although this model does not rule out minor late stage reactivation of the main central thrust major late miocene reactivation is not required

flexure of the india plate underneath the bhutan himalaya 1 we investigate flexural geometry and rheology of the india plate beneath the eastern himalaya from a new gravity data set acquired in bhutan compared to the well studied nepal himalaya the obtained bouguer anomaly profiles across the range show shorter wavelength flexure of the lithosphere with a narrower and shallower foreland basin this new data set and seismic moho constraints are interpreted together in terms of lithospheric flexure using a 2 d thermomechanical model our results suggest that the strongest layer of the continental lithosphere beneath bhutan is the upper mantle as it is beneath nepal the observed west to east decrease in flexural wavelength is associated with weakening mantle rheology the simulations show that this decrease can be related to ductile mantle behavior either hydrated wet dunite or dry and hot olivine rheology both models display decoupled lithospheric layers leading to an eastward decrease of flexural rigidity from 10 24 to 5 10 22 n m in nepal and bhutan respectively

age and cooling history of the manaslu granite implications for himalayan tectonics abstract the manaslu granite is one of the high himalayan leucogranites which are believed to be derived by partial melting of quartzofeldspathic gneissess of the high himalayan crystallines hhc and emplaced at the contact between the hhc and the tethyan sedimentary series we have analyzed a suite of muscovite 15 biotite 3 and alkali feldspar 10 by the 40 ar 39 ar method to help constrain the age and cooling history of the granite 40 ar 39 ar ages ranges are 18 4 to 13 3 ma 17 0 to 14 7 and 16 4 to 3 4 for muscovite biotite and k feldspar respectively based on the muscovite analyses and a re interpretation of a previously published u pb monazite age we conclude that the crystallization age of the manaslu granite is everywhere 20 ma the postcrystallization cooling history given by the micas and the feldspars suggests the presently exposed parts of the pluton were emplaced at depths of 8 to 15 km these data place a minimum age of movement on the mct in this area of 20 ma we estimate the time of magma segregation and transport to be no more than 5 ma

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

abnormal monsoon years and their control on erosion and sediment flux in the high arid northwest himalaya abstract the interplay between topography and indian summer monsoon circulation profoundly controls precipitation distribution sediment transport and river discharge along the southern himalayan mountain front shf the higher himalayas form a major orographic barrier that separates humid sectors to the south and arid regions to the north during the indian summer monsoon vortices transport moisture from the bay of bengal swirl along the shf to the northwest and cause heavy rainfall when colliding with the mountain front in the eastern and central parts of the himalaya precipitation measurements derived from passive microwave analysis ssm i show a strong gradient with high values at medium elevations and extensive penetration of moisture along major river valleys into the orogen the end of the monsoonal conveyer belt is near the sutlej valley in the nw himalaya where precipitation is lower and rainfall maxima move to lower elevations this region thus comprises a climatic transition zone that is very sensitive to changes in indian summer monsoon strength to constrain magnitude temporal and spatial distribution of precipitation we analyzed high resolution passive microwave data from the last decade and identified an abnormal monsoon year amy in 2002 during the 2002 amy violent rainstorms conquered orographic barriers and penetrated far into otherwise arid regions in the northwest himalaya at elevations in excess of 3 km asl while precipitation in these regions was significantly increased and triggered extensive erosional processes i e debris flows on sparsely vegetated steep hillslopes mean rainfall along the low to medium elevations was not significantly greater in magnitude this shift may thus play an important role in the overall sediment flux toward the himalayan foreland using extended precipitation and sediment flux records for the last century we show that these events have a decadal recurrence interval during the present day monsoon circulation hence episodically occurring amys control geomorphic processes primarily in the high elevation arid sectors of the orogen while annual recurring monsoonal rainfall distribution dominates erosion in the low to medium elevation parts along the shf

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

evolution of the methane cycle in ace lake antarctica during the holocene response of methanogens and methanotrophs to environmental change abstract post glacial ace lake vestfold hills antarctica which was initially a freshwater lake and then an open marine system is currently a meromictic basin with anoxic sulfidic and methane saturated bottom waters lipid and 16s ribosomal rna gene stratigraphy of up to 10 400 year old sediment core samples from the lake revealed that these environmentally induced chemical and physical changes caused clear shifts in the species composition of archaea and aerobic methanotrophic bacteria the combined presence of lipids specific for methanogenic archaea and molecular remains of aerobic methanotrophic bacteria 13c depleted 8 14 sterols and 16s rrna genes revealed that an active methane cycle occurred in ace lake during the last 3000 calendar years and that the extant methanotrophs were most likely introduced when it became a marine inlet 9400 y bp rdna sequences of known methanogens were only recovered from the oldest sediments deposited during the freshwater phase the latter sequences showed 100 sequence similarity with methanosarcinales species from freshwater environments and were the source of sn 2 and sn3 hydroxyarchaeols archaeal phylotypes related to uncultivated archaea associated with various marine environments were recovered from the present day anoxic water column and sediments deposited during the meromictic and marine period

control of detachment geometry on lateral variations in exhumation rates in the himalaya insights from low temperature thermochronology and numerical modeling 1 the himalayan range is commonly presented as largely laterally uniform from west to east however geological structures topography precipitation rate convergence rates and low temperature thermochronological ages all vary significantly along strike here we focus on the interpretation of thermochronological data sets in terms of along strike variations in geometry and kinematics of the main crustal detachment underlying the himalaya the main himalayan thrust mht we report new apatite fission track aft ages collected along north south transects in western and eastern central nepal at the latitudes of the annapurna and langtang massifs respectively aft ages are consistently young 3 ma along both n s transects in the high relief zone of the higher himalaya and increase 4 to 6 ma toward the south in the lesser himalaya we compare our new data to published low temperature thermochronological data sets for nepal and the bhutan himalaya we use the full data set to perform both forward and inverse thermal kinematic modeling with a modified version of the pecube code in order to constrain potential along strike variations in the kinematics of the himalayan range our results show that lateral variations in the geometry of the mht in particular the presence or absence of a major crustal scale ramp strongly control the kinematics and exhumation history of the orogen

temperature and strain rate dependent microfabric evolution in monomineralic mylonite evidence from in situ deformation of norcamphor norcamphor c7h10o was subjected to plane strain simple shear in a see through deformation rig at four different strain rate and temperature conditions two transient stages in the microfabric evolution to steady state are distinguished the grain scale mechanisms associated with the microstructural and textural evolution vary with the applied temperature strain rate and strain in high temperature low strain rate experiments computer integrated polarization microscopy reveals that the texture evolution is closely related to the crystallographic rotation paths and rotation rates of individual grains high c axis rotation rates at low to intermediate shear strains are related to the development of a symmetrical c axis cross girdle by the end of the first transient stage 1 5 to 2 during the second transient stage 1 5 to 6 the cross girdle yields to an oblique c axis single girdle as c axis rotation rates decrease and the relative activity of grain boundary migration recrystallization increases steady state 8 is characterized by a stable end orientation of the sample texture and the cyclic growth rotation and consumption of individual grains within the aggregate

deformation temperatures and flow vorticities near the base of the greater himalayan series sutlej valley and shimla klippe nw india abstract we report new deformation temperature and flow vorticity data from the base of the greater himalayan series ghs exposed in the sutlej valley and shimla klippe of nw india we focus on three groups of transects across the hanging wall of the main central thrust mct in order of relative foreland hinterland positions they are the shimla klippe western and eastern sutlej transects deformation temperatures indicated by quartz c axis fabric opening angles increase both from foreland to hinterland at a given structural distance above the mct and up structural section from the mct within individual transects deformation temperatures in the immediate hanging wall to the mct are estimated at 510 535 535 550 and 610 c on the shimla western sutlej and eastern sutlej transects respectively the steepest inferred field gradients in deformation temperatures are recorded adjacent to the mct and progressively decrease up structural section following a power law relationship comparison with temperature estimates based on multi mineral phase equilibria data suggests that penetrative shearing occurred at close to peak metamorphic conditions vorticity analyses indicate that shearing along the base of the ghs occurred under sub simple shear conditions wm values of 0 9 1 0 with a minor component of pure shear

lateral variation in structural style of mountain building controls of rheological and rift inheritance many mountain belts exhibit significant along strike variation in structural style with changes in the width of the orogen the geometry and kinematics of the crustal scale thrust system and the degree of partitioning between pro and retro wedge deformations although the main factors controlling first order structural style are understood the cause of these lateral variations remains to be resolved here we focus on the pyrenees characterized by significant lateral variation in structural style with a thrust system involving more and thinner thrust sheets in the eastern section than in the western part similarly the prior mesozoic rifting event was characterized by significant lateral variation in structure we integrate available geological and geophysical data with forward lithospheric scale numerical models we show that lateral variation in crustal strength attributed to inherited variscan crustal composition accentuated during mesozoic rifting explains the variation in structural style observed during pyrenean mountain building

transfer zones of deformation in thrust wedges an experimental study abstract although numerous transfer zones of deformation have been observed in thrust wedges their mechanisms and conditions of development are poorly understood in this paper three basic questions are addressed 1 1 what are the parameters controlling the genesis of transfer zones 2 2 what are the kinematics of thrust propagation during transfer zone development 3 3 which kinds of geological structure link the two compartments on either side of the transfer zone we attempted to solve these problems using a 3 d experimental approach sand models were used to simulate the foreland a rigid but mobile backstop was used to produced a thrust coulomb wedge which was studied during the various stages of its formation shortening led to the development of transfer zones in the wedge separating two domains of homogenous behaviour and connecting thrust planes in different domains the experimental results show that the development of transfer zones is controlled by mechanical and or geometrical parameters such as 1 1 variations in the sedimentary thickness in the foreland 2 2 the steepness of the backstop 3 3 domains with different basal friction in the foreland 4 4 kinked backstop geometry during deformation of the models two types of thrust sequence were identified alternative thrust propagations on both sides of the transfer zone and synchronous thrust propagations which are anastomosed within the transfer zone 3 d reconstructions revealed that the thrust deformations observed in the transfer zone are oblique ramps the frontal thrust bending of the barbados accretionary complex as well as the imbrication of thrust in the castellane arc and the oblique ramp in the zanskar range are discussed in relation to the mechanisms and structures revealed by the models the results of analogue modelling suggest that the development of transfer zones is directly linked to variations in the boundary conditions mechanical and or geometrical between two adjacent parts of a thrust wedge

transtensional deformation in the central himalaya and its role in accommodating growth of the himalayan orogen 1 field mapping structural analysis and geochronologic data from northwestern nepal reveal major normal right slip motion along a previously unrecognized west northwest striking system of shear zones that we term the gurla mandhata humla fault system gmh the gmh obliquely cuts across the greater himalayan crystalline sequence and into the lesser himalayan imbricate thrust belt via two right step over structures the average slip direction on the gmh parallels the strike of the himalayan orogen motion along this fault system has resulted in an apparent left separation of the south tibet detachment main central thrust zone and lesser himalayan imbricate thrust belt along a north striking segment of the fault system we estimate a minimum of 21 km of net slip on the southern branch of the gmh by restoring the trace of the main central thrust zone parallel to the average slip direction on the fault taking into account slip estimates from the northern branch of the gmh yields a minimum net slip estimate of 24 4 to 32 4 km for the gmh the 232th 208pb ion microprobe monazite ages from leucogranite bodies indicate that motion on the gmh occurred after 15 ma its initiation immediately followed crustal thickening between the main central thrust zone and indus yalu suture zone motion on the gmh is contemporaneous with arc normal contraction in the southernmost himalayan orogen these observations can be explained by a model that involves foreland propagating structural systems facilitating arc normal contraction in the foreland and arc parallel extension in the hinterland that work together to maintain the arcuate shape of the himalayan orogen

variations in the kinematics of deformation along the zagros inclined transpression zone iran implications for defining a curved inclined transpression zone abstract the combination of inclined collision and plate boundary shape can control the nature of deformation and the sense of shear along a transpression zone the present study investigated the effects of a boundary zone with curvilinear shape along a transpression zone on the kinematics of deformation the kinematics of the zagros transpression zone varies with the orientation of the zone boundary detailed structural and microstructural studies showed sinistral sense of shear on the southeastern part of the zagros inclined transpression zone fars arc but dextral sense of shear on the northwestern part of the zone it is inferred that the both senses of shear were developed coevally under a bulk general shear regional scale deformation along a curved inclined transpression miming the shape of the fras arc of the zagros and the reentrant of the bandar abbas syntaxis the zagros transpression zone formed by inclined continental collision between the afro arabian continent and iranian microcontinent

indian plate motion and shape constraints on the geometry of the himalayan orogen abstract sea floor palaeomagnetic data that reflect variations in rate and vector of indian plate movement and rotation suggest that initial collision between india and asia occurred at about 50 55 ma ago as the pre collisional indian plate was diamond shaped with the northern margin comprised of two oblique boundaries collision was earliest where these boundaries meet or in what is now the northwest himalaya oblique convergence along each of these two boundaries would generate rotation of thrust sheets as they climb on to the indian plate the oroclinal shape of the main himalayan chain to the east of the northwest himalayan syntaxes reflects a combination of the effects of oblique convergence post collisional anticlockwise rotation of the indian plate and the pinning of the main thrusts at their northwestern terminations by crust thickened during the earliest collisional stage the indian plate rotation enhances a strike slip component of movement along the western oblique margin with the transpressively sinistral chaman fault zone now acting as a continental escape structure

40ar loss in experimentally deformed muscovite and biotite with implications for 40ar 39ar geochronology of naturally deformed rocks abstract the effects of deformation on radiogenic argon 40ar retentivity in mica are described from high pressure experiments performed on rock samples of peraluminous granite containing euhedral muscovite and biotite cylindrical cores 15 mm in length and 6 25 mm in diameter were drilled from granite collected from the south armorican massif in northwestern france loaded into gold capsules and weld sealed in the presence of excess water the samples were deformed at a pressure of 10 kb and a temperature of 600 c over a period 29 of hours within a solid medium assembly in a griggs type triaxial hydraulic deformation apparatus overall shortening in the experiments was approximately 10 transmitted light and secondary and backscattered electron imaging of the deformed granite samples reveals evidence of induced defects and for significant physical grain size reduction by kinking cracking and grain segmentation of the micas infrared ir laser co2 heating of individual 1 5 2 5 mm diameter grains of muscovite and biotite separated from the undeformed granite yield well defined 40ar 39ar plateau ages of 311 2 ma 2 identical experiments on single grains separated from the experimentally deformed granite yield results indicating 40ar loss of 0 35 in muscovite and 2 3 40ar loss in biotite intragrain in situ ultraviolet uv laser ablation 40ar 39ar ages 4 10 1 of deformed muscovites range from 309 13 to 264 7 ma consistent with 0 16 40ar loss relative to the undeformed muscovite the in situ uv laser ablation 40ar 39ar ages of deformed biotite vary from 301 to 217 ma consistent with up to 32 40ar loss no spatial correlation is observed between in situ 40ar 39ar age and position within individual grains using available argon diffusion data for muscovite the observed 40ar loss in the experimentally treated muscovite can be utilized to predict average 40ar diffusion dimensions maximum 40ar 39ar ages obtained by uv laser ablation overlap those of the undeformed muscovite indicating argon loss of

effects of coarse woody debris and its removal on a channel affected by the 1980 eruption of mount st helens washington during the may 18 1980 eruption of mount st helens washington a pyroclastic surge introduced large volumes of coarse woody debris cwd and fine grained sediment to clearwater creek approximately 15 km northeast of the summit effects of controlled cwd removal on sediment storage substrate and pool frequency and volume were measured in four reaches each with three 200 m segments from 1982 to 1990 in each reach the upstream segment served as a control with no debris removal and cwd was totally and selectively removed from the other two segments unique among similar experiments are the large size and volume of cwd and the large inputs of fine grained sediment except for segments of two reaches that received debris torrents the clearwater channel thalweg scoured until 1985 in three reaches total debris removal caused additional scour and coarsening of the bed surface compared to segments with no or partial debris removal pools contracted from 1982 to 1985 and expanded afterward especially in control segments total debris removal apparently caused pools to become shallower and in segments of low sinuosity decreased the frequency of major pools habitat complexity decreased after total debris removal as indicated by a decrease in the standard deviation of residual depth and an increase in the size of substrate patches

the motion and active deformation of india measurements of surface displacements using gps constrain the motion and deformation of india and india eurasia plate boundary deformation along the himalaya the gps velocities of plate interior sites constrain the pole of the angular velocity vector of india with respect to eurasia to lie at 25 6 1 0 n 11 1 9 0 e approximately 6 west of the nuvel 1a pole of 3 ma plate motion the angular rotation rate of 0 44 0 03 myr 1 is 14 slower than the long term rate of 0 51 myr 1 insignificant velocities between plate interior sites indicate that the exposed indian plate is stable to within 7 10 9 yr 1 the observed contraction vector across the himalaya 20 mm yr veers from n20 e in the northwest himalaya to n25 w in east nepal consistent with east west extension of southern tibet

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

lateral ramps as an aid to the unroofing of deep crustal rocks seismic evidence from the grenville province we suggest that lateral ramps in the basement to orogenic wedges may be important agents in the rapid unroofing of deeply buried continental crust our suggestion is based on a study of the distribution of high pressure metamorphosed continental rocks in the mesoproterozoic grenvillian orogen in eastern quebec these rocks were buried to depths of more than 50 km through crustal thickening during a continental collision and were then rapidly unroofed they are currently exposed in a 100 km 60 km lobate embayment north of the manicouagan reservoir using a combination of surface structural data and seismic reflection data from lithoprobe line 55 we reconstruct the three dimensional form of the high pressure rocks our reconstruct ion methods are based on demonstrable correlations between the attitudes of gneissic layering at the surface and the attitudes of reflectors on the seismic profile they make extensive use of the fact that much of the seismic reflectivity on line 55 comes from outside the vertical plane of the seismic line because of the sinuous nature of the line and its obliquity to tectonic strike the validity of the reconstructions is demonstrated by the generation of a synthetic seismogram from the interpreted geometry that is very similar to the observed seismogram our reconstruction shows that a steep lateral ramp extends to the base of the crust immediately behind the broad lobe of high pressure rocks at the surface the lateral ramp probably provided a favorable pathway for the extrusion of deep seated rocks from the deep crust and their transport onto the foreland

leucogranite intruding the south tibetan detachment in western nepal implications for exhumation models in the himalayas the most popular models regarding the exhumation of the greater himalayan sequence ghs such as extrusion channel flow critical taper and wedge extrusion require prolonged activity of the two bounding shear zones and faults the main central thrust mct and the south tibetan detachment std we present the crystallization age of an undeformed leucogranite that intrudes both the ghs and the tethyan himalaya sequence ths zircon and monazite u pb ages in the leucogranite give ages between 23 and 25 ma constraining at that time the end of shearing along the std our results limit the contemporaneous activity of the mct and std to a short period of time 1 2 ma and thus argue against exhumation models requiring prolonged contemporaneous activity of the mct and std

limit of strain partitioning in the himalaya marked by large earthquakes in western nepal great earthquakes in the himalaya are thought to occur mostly along the range front field mapping and radiocarbon dating reveal earthquake rupture on a previously unrecognized fault in the interior of the western himalaya which forms part of a key structural component of the mountain range implying that seismic risk evaluations should be revised

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

monazite geochronology unravels the timing of crustal thickening in nw himalaya abstract greenschist to amphibolite grade haimanta metasediments of the nw himalaya preserve much of the prograde metamorphic history of eohimalayan crustal thickening which has been erased by oligo miocene migmatization elsewhere in the himalaya our zircon and monazite u th pb data unravel a multi stage prograde metamorphic evolution the earliest evidence of prograde barrovian metamorphic monazite growth is 41 ma peak metamorphic conditions 8 8 5 kbar 600 700 c were attained at 37 36 ma and followed by a prolonged evolution at high temperatures with at least three distinct episodes of monazite growth which may be related to the formation of the northern himalayan nappes e g shikar beh nappe nyimaling nappe rapid exhumation of the crystalline started at 26 ma and resulted in cooling through the muscovite 40 ar 39 ar closure temperature by 21 8 ma although a local continuation of the south tibetan detachment is not unambiguously identified in central himachal pradesh extrusion was likely facilitated by a system of several minor late oligocene early miocene top to the n to ne shear zones in contrast to the crystalline of zanskar and eastern himachal pradesh extrusion was not accompanied by widespread decompression melting

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

pressure temperature time path discontinuity in the main central thrust zone central nepal metapelites collected in central nepal reveal a discontinuity in metamorphic pressuretemperature time p t t paths near the base of the main central thrust zone despite an absence of obvious structural breaks garnets in the structurally lowest rocks grew with increasing t and p loading whereas garnets 1 3 km upsection grew with increasing t but decreasing p exhumation monazite grains in structurally lower rocks yield ionmicroprobe th pb ages of 8 9 ma structurally higher monazite grains range from 10 to 22 ma the p t t paths confirm previous interpretations that footwall metamorphism in part resulted from thrust reactivation ca 8 ma but also reflect thermal relaxation following older 20 ma or older thrust movement the main central thrust zone formed during pulses of movement that resulted in progressive transfer of material from the lower to upper plate

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

thermal model for the zanskar himalaya abstract crustal thickening along the northern margin of the indian plate following the 50 ma collision along the indus suture zone in ladakh caused widespread high temperature medium pressure barrovian facies series metamorphism and anatexis in the zanskar himalaya metamorphic isograds are inverted and structurally telescoped along the main central thrust mct zone at the base of the high himalayan slab along the zanskar valley at the top of the slab isograds are the right way up and are also telescoped along northeast dipping normal faults of the zanskar shear zone zsz which are related to culmination collapse behind the miocene himalayan thrust front between the mct and the zsz a metamorphic anatectic core within sillimanite grade rocks contains abundant leucogranite granite crustal melts of probable himalayan age a thermal model based on a crustal scale cross section across the zanskar himalaya suggests that m1 isograds developed during early himalayan barrovian metamorphism were overprinted during high grade mct related anatexis and folded around a large scale recumbent fold developed in the hanging wall of the mct

continental margins orogenic belts and intracratonic structures angular bends promontories and embayments in rifted and passive continental margins are interpreted to be the precursors of curves recesses and salients respectively in orogenic belts convergence at an irregularly shaped continental margin results in along strike diachroneity of closing and thus of orogeny and in a variable trajectory of stress into continental crust both timing and sense of movement of intracratonic structures apparently have complex relationships to the progress of convergence at irregular continental margins

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

tectonometamorphic discontinuities within the greater himalayan sequence in western nepal central himalaya insights on the exhumation of crystalline rocks abstract the core of the greater himalayan sequence in the mugu karnali area western nepal is affected by a thick shear zone with development of nearly 4 km of mylonites mangri shear zone it is a contractional shear zone showing a top to the sw and wsw sense of shear the shear zone developed during the decompression in the sillimanite stability field of rocks that previously underwent relatively high pressure metamorphism deformed under the kyanite stability field p t conditions indicate that the footwall experienced higher pressure 1 0 0 9 gpa than the hanging wall 0 7 gpa and similar temperatures 675 700 c u pb in situ dating of monazites indicate a continuous activity of the shear zone between 25 and 18 ma samples from the lower part of the greater himalayan sequence underwent similar ductile shearing at 17 13 ma these ages and the associated p t t paths revealed that peak metamorphic conditions were reached 5 7 ma later in the footwall of the shear zone with respect to the hanging wall pointing to a diachroneity in the metamorphism triggered by the shear zone itself mangri shear zone with the other recently documented tectonic and metamorphic discontinuities within the greater himalayan sequence point to the occurrence of a regional tectonic feature the high himalayan discontinuity running for more than 500 km along the strike of the central himalayas it was responsible of the exhumation of the upper part of the greater himalayan sequence starting from 28 ma well before the activation of the main central thrust and the south tibetan detachment our data point out that exhumation of the greater himalayan sequence was partitioned in space and time and different slices were exhumed in different times starting from the older in the upper part to the younger in the lower one

crustal structure of the himalayan orogen at 90 east longitude from project indepth deep reflection profiles international deep profiling of tibet and the himalaya indepth deep seismic reflection profiles show that the indian lithosphere is underthrusting the central himalaya along a gently north dipping decollement that is traceable northward beneath the tethyan belt to 28 6 n and to a depth of about 45 km the decollement carries in its hanging wall a near crustal thickness slice of internally deformed indian continental basement and cover represented in outcrop by the greater himalayan belt and structurally overlying tethyan belt geometric relationships suggest that the decollement probably ramps downward beneath the northern tethyan belt to near the base of the crust and that the hanging wall crustal slice was detached from indian mantle lithosphere that presently underlies southern tibet to the north the north himalayan anticlinorium kangmar dome appears to be a large duplex ramp anticline in the hanging wall of the decollement the south tibetan detachment appears to have been imaged in two areas in the wagye la area it appears to follow the tethyan belt greater himalayan belt contact northward in the subsurface and to have been folded over the kangmar dome in the zherger la area it appears to have been active relatively recently as a 30 north dipping normal fault that cuts deeply into the greater himalayan belt allochthon palinspastic reconstruction indicates a minimum of about 326 km of shortening of the indian basement across the himalaya since the initiation of the main central thrust hence indian continental lithosphere largely stripped of its overlying crust can extend northward beneath the tibetan plateau to at least 32 n where earthquake seismological observations show that the properties of the upper mantle change markedly e g mcnamara et al 1995 taken together the seismic reflection and earthquake seismological observations lend support for the view that the indian mantle lid mechanically underplates roughly the southern half of the tibetan plateau the indepth reflection data do not show that indian crust attached to this lithosphere extends north of southernmost tibet kangmar dome nor do they negate this possibility similarly the reflection data do not yield direct evidence for or against lower crustal subduction beneath the himalaya

structural control on along strike variation in the seismicity of the northwest himalaya an overview of seismicity along the northwest himalaya exhibit along strike segmentation intricately controlled by the subsurface topographic ridges on the underthrusting indian plate as well by rift and nappe structures in the overriding wedge of the himalaya the segmentation exists for thrust dominated large magnitude earthquakes m 6 seated on the active detachment beneath the outer and lesser himalaya segmentation also prevails for moderate and small magnitude earthquakes concentrated in a narrow himalaya seismic belt hsb straddling northern lesser himalaya and southern higher himalaya numerical calculations of stress distribution favour that the degree of seismicity in the hsb is a good proxy to the presence of mid crustal ramp connecting the locked section of active detachment to aseismically slipping detachment beneath higher himalaya further gap or diffused pattern in the concentrated seismicity in the hsb in agreement with mapped high electrical conductive structure suggests ramp structure may be absent where underthrusting delhi hardwar ridge interacts with the himalaya arc in the nappe dominated tectonics the accommodation of the accumulating strains on the listric thrust faults produces increased frequency of moderate magnitude earthquakes and thereby possibly influences stress level on the detachment this may explain relatively less frequent occurrences of large earthquake in the kangra chamba region compared to that in the garhwal himalaya the low level seismicity in the simla region may also be influenced by the active kaurik chango rift in the higher himalaya as the former casts a stress shadow on the latter

when can muscovite 40ar 39ar dating constrain the timing of metamorphic exhumation abstract metamorphic cooling rates determined using muscovite 40 ar 39 ar ages underpin many tectonic models but are based on several simplifying approximations we demonstrate that a number of these approximations especially those relating to the assignment of a single closure temperature might not hold for certain metamorphic scenarios and particularly for metamorphism during rapid orogenic cycles using numerical diffusion models that include a recently reported significant pressure dependence of ar diffusion in muscovite we systematically interrogate the approximations associated with linking 40 ar 39 ar dates to pressure temperature histories the results of the simulations are presented in a simple graphic form allowing evaluation of the pressure temperature pt regions in which ar diffusion is efficient and hence those in which the muscovite may yield true 40 ar 39 ar cooling ages we suggest that a robust method for determining whether metamorphic muscovite 40 ar 39 ar ages relate to the timing of cooling involves 1 the determination of the pt conditions and relative timing of muscovite crystallisation during the metamorphic cycle 2 the collection of high precision and high spatial resolution 40 ar 39 ar data from muscovite grains and 3 the comparison between analytical data and numerical diffusion models which test different post crystallisation pressure temperature histories

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

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

diachronous deformation along the base of the himalayan metamorphic core west central nepal geologic mapping combined with microstructural and geochronologic analyses from the lower himalayan metamorphic core in west central nepal record along strike similarity in flow style despite variability in the timing of metamorphism and deformation the main central thrust zone at the base of the himalayan metamorphic core varies in thickness tectonostratigraphy and metamorphic gradient along the 250 km of strike length studied in situ u th pb geochronology of monazite sampled from an along strike transect at the top of the high strain zone records eocene oligocene prograde metamorphism followed by miocene retrograde metamorphism the timing of prograde and retrograde metamorphism and the muscovite 40 ar 39 ar dates gradually decrease along strike from northwest to southeast this age trend is punctuated by an abrupt 3 8 m y decrease in the age of prograde and retrograde metamorphism and muscovite 40 ar 39 ar dates near the marsyandi river in central nepal quartz crystallographic preferred orientation fabrics from a parallel transect along the base of the high strain zone document similar flow style at 440 c in central nepal muscovite 40 ar 39 ar ages interpreted to approximate the age of deformation at this structural level decrease from ca 7 to 4 ma along strike from northwest to southeast diachronous deformation and metamorphism along strike in west central nepal demonstrate the necessity of incorporating more than a single transect into tectonic models along strike tectonometamorphic variability in west central nepal spatially corresponds to faults in the indian basement bounding the subsurface faizabad ridge highlighting the possible influence of inherited basement faults on the geometry of the basal himalayan detachment the main himalayan thrust as well as the tectonic evolution of the structurally overlying himalayan metamorphic core this study highlights the potential influence of inherited structures on the overlying orogenic wedge and the probability of along strike diachroneity of deformation in the himalaya

structural inheritance in mountain belts an alpine apennine perspective abstract the geological structure of continental lithosphere shows complex variety that is inherited into orogenic belts and influences the localization and amplification of contractional structures during mountain building in the alpine apennine sector together with other sectors of the tethyan orogenic system the pre orogenic crustal template can include arrays of extensional faults other faults can form adjacent to the evolving mountain belt and subsequently become incorporated as the thrust belts migrate into their forelands while in some areas these inherited features may simply reactivate under inversion more commonly faults show complex partial reactivation structures in volumes of distributed strain faults may serve to nucleate large scale buckle folds for example along basement cover interfaces these different patterns of basement reactivation may reflect spatially varying strength depth profiles in continental lithosphere that are themselves inherited from spatially distinct geological histories even when not themselves reactivating basement faults can control deformation in the overlying sedimentary cover by offsetting otherwise regionally extensive detachment horizons the 3d form of thrust systems can be strongly compartmentalized by pre existing cross faults such as the oblique lineaments of the apennines on a large scale the distribution of pre existing faults and other weaknesses may affect the propensity for orogenic contraction in basement and therefore directly control larger scale tectonic processes in the central mediterranean the evolution of slab roll back and the related growth of overlying extensional basins e g tyrrhenian sea may be strongly modulated by the distribution of rift related weak zones in the adjacent continental crust the subduction of continental crust will strongly depend on the inherited structure of this crust specifically the distribution of deep crust of basic composition this develops relatively higher densities associated with eclogite metamorphism which act in turn to reduce the buoyancy of thickened continental crust that otherwise serves to inhibit further shortening investigating all these aspects from the scale of bulk crustal compositions to the geometry timing and strength of earlier fault zones preserved in orogenic belts requires the integration of substantial multidisciplinary geological data sets the extent to which continental orogenic belts represent the amplification of inherited geological heterogeneities as opposed to self ordered phenomena modulated by the syntectonic environment remains unclear

underplating in the himalaya tibet collision zone revealed by the hi climb experiment we studied the formation of the himalayan mountain range and the tibetan plateau by investigating their lithospheric structure using an 800 kilometer long densely spaced seismic array we have constructed an image of the crust and upper mantle beneath the himalayas and the southern tibetan plateau the image reveals in a continuous fashion the main himalayan thrust fault as it extends from a shallow depth under nepal to the mid crust under southern tibet indian crust can be traced to 31 n the crust mantle interface beneath tibet is anisotropic indicating shearing during its formation the dipping mantle fabric suggests that the indian mantle is subducting in a diffuse fashion along several evolving subparallel structures

possible influence of subducting ridges on the himalayan arc and on the ruptures of great and major himalayan earthquakes abstract subduction of bathymetric features such as ridges seamounts fractures etc on the subducting plate influences the arc morphology and earthquake ruptures we analyse their effect on the development of the arcuate shape of the himalayan arc and on the ruptures of great and major himalayan earthquakes besides the two most prominent ridges in the indian ocean namely the chagos laccadive deccan ridge and the 90 e ridge which are assumed to extend up to the himalayan arc at least three major subsurface ridges have been mapped on the underthrusting indian plate under the indo gangetic plains it appears that the subduction of the two most prominent ridges contributed to the development of the arcuate shape of the himalayan arc the interaction and subduction of the other subsurface ridges probably influenced the himalayan arc morphology by causing a localised cusp in the frontal topography also these ridges probably acted as barriers to the ruptures of the major and great himalayan earthquakes

prism and basal plane parallel subgrain boundaries in quartz a microstructural geothermobarometer empirical data on quartz subgrain patterns from various metamorphic rocks show that at least up to 10 kbar in the stability field of low quartz prismatic subgrain boundaries are dominant whereas basal subgrain boundaries are not developed in the high quartz stability field both prismatic and basal subgrain boundaries occur and form typical rectangular chessboard patterns the likely reason behind the different occurrence of these subgrain patterns is that in high quartz prismatic glide becomes as easy as or probably even easier than basal glide the two types of subgrain patterns can be clearly distinguished by optical microscopy consequently the occurrence of chessboard subgrain patterns in quartz represents a practicable geothermobarometer the possibilities of its application are far reaching and include the specification of deformation conditions at high grade metamorphism the recognition of syntectonic intrusions and the distinction between pluton emplacement at lower and at higher crustal levels

relationships between displacement and distortion in orogens linking the himalayan foreland and hinterland in central nepal greater himalayan sequence rocks exposed in the manaslu himal chuli himalaya can be separated into distinct upper and lower parts deformation recorded in both parts occurred at temperatures ranging between 450 c and 640 c and is characterized by almost equal coaxial and noncoaxial components across the upper greater himalayan sequence peak metamorphic temperatures are essentially isothermal whereas corresponding metamorphic pressure estimates across the same section decrease downward with an apparent gradient of 620 bars km in the lower greater himalayan sequence however both metamorphic pressure and temperature decrease with structural depth the abnormal pressure gradient in the upper greater himalayan sequence is attributed to 50 vertical thinning during southward displacement while the inverted gradient in the lower portion is interpreted to be the result of coeval exhumation and downward expansion of the main central thrust shear zone and the progressive incorporation of more rock into the greater himalayan sequence deformation in the upper portion of the greater himalayan sequence was characterized by extending flow i e extension in the direction of flow whereas deformation in its lower portion was characterized by compressing flow i e compression in the direction of flow extending flow is a distinctive feature of displacement and distortion in deep orogenic hinterlands while compressing flow is emblematic of displacement and distortion in orogenic foreland regions the transition between the upper and lower parts of the greater himalayan sequence therefore represents a fundamental transition between hinterland style deformation involving processes such as lateral midcrustal flow and foreland style deformation involving critical taper thrust fold wedge development

heterogeneous 40ar distributions in naturally deformed muscovite in situ uv laser ablation evidence for microstructurally controlled intragrain diffusion abstract micas are commonly used in 40 ar 39 ar thermochronological studies of variably deformed rocks yet the physical basis by which deformation may affect radiogenic argon retention in mica is poorly constrained this study examines the relationship between deformation and deformation induced microstructures on radiogenic argon retention in muscovite a combination of furnace step heating and high spatial resolution in situ uv laser ablation 40 ar 39 ar analyses are reported for deformed muscovites sampled from a granitic pegmatite vein within the siviez mischabel nappe western swiss alps penninic domain brianconnais unit the pegmatite forms part of the variscan 350 ma alpine basement and exhibits a prominent alpine s c fabric including numerous mica fish that developed under greenschist facies metamorphic conditions during the dominant tertiary alpine tectonic phase of nappe emplacement furnace step heating of milligram quantities of separated muscovite grains yields an 40 ar 39 ar age spectrum with two distinct staircase segments but without any statistical plateau consistent with a previous study from the same area a single 3 5 mm muscovite porphyroclast fish was investigated by in situ uv laser ablation a histogram plot of 170 individual 40 ar 39 ar uv laser ablation ages exhibit a range from 115 to 387 ma with modes at approximately 340 and 260 ma a variogram statistical treatment of the 40 ar 39 ar results reveals ages correlated with two directions a highly correlated direction at 310 and a lesser correlation at 0 relative to the sense of shearing using the highly correlated direction a statistically generated kriging method age contour map of the 40 ar 39 ar data reveals a series of elongated contours subparallel to the c surfaces which where formed during tertiary nappe emplacement similar data distributions and slightly younger apparent ages are recognized in a smaller mica fish the observed intragrain age variations are interpreted to reflect the partial loss of radiogenic argon during alpine 35 ma greenschist facies metamorphism one dimensional diffusion modelling results are consistent with the idea that the zones of youngest apparent age represent incipient shear band development within the mica porphyroclasts thus providing a network of fast diffusion pathways during alpine greenschist facies metamorphism the incipient shear bands enhanced the intragrain loss of radiogenic argon the structurally controlled intragrain age variations observed in this investigation imply that deformation has a direct control on the effective length scale for argon diffusion which is consistent with the heterogeneous nature of deformation

initial plate geometry shortening variations and evolution of the bolivian orocline comparisons of newly published cross sections across the bolivian andes with existing cross sections through argentina emphasize significant along strike changes in crustal shortening a sharp decrease in the magnitude of crustal shortening from 530 km to 150 km north to south occurs at 23 s a 20 40 m y difference in the ages at which deformation was initiated accompanies the abrupt decrease in the magnitude of shortening extending the western margin of south america to account for 530 km of shortening in the bolivian andes and 150 km of shortening in argentina produces a central andean salient that is perpendicular to the nazca plate shortening direction from 60 to 26 ma during this same time interval the chilean coast south of 23 s was in an orientation sufficiently oblique to oceanic convergence to allow for predominantly strike slip offset and backarc extension deformation within the andean mountain chain may be a function of plate convergence where the oblique nature of convergence south of 23 s inhibited mountain building whereas north of 23 s normal convergence to a central andean salient facilitated contractional deformation the magnitude of deformation north of 23 s is a consequence of both plate convergence direction providing a longer period of contractional deformation from ca 70 ma to the present and a thick phanerozoic sedimentary package that permitted large magnitudes of thin skinned deformation significant along strike changes in the shape of the south american margin allowing for convergence to change from compression to extension along the strike of the orogen may help explain the dramatic differences in timing amount and style of deformation in the andes

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

a structural transect in the lower dolpo insights on the tectonic evolution of western nepal abstract we present the results of a structural transect in lower dolpo cross cutting the upper part of the lesser himalaya lh the higher himalayan crystallines hhc and the lower part of the tibetan sedimentary sequence tss the mct zone affects the upper part of the lh as well as the lower part of the hhc and shows a later brittle reactivation mean vorticity in the mct points to non coaxial deformation these data together with available kinematic data along the belt on the south tibetan detachment system stds and in the core of the hhc point to increasing simple shear toward the tectonic boundaries a top to the sw high temperature shear zone toijem shear zone is recognized in the middle part of the hhc at the boundary between units 1 and 2 it developed during the earlier stages of exhumation of the hhc enhancing the decompression of the hanging wall and the emplacement of leucogranite dykes and sills its development could be explained by a change in the velocity profile during the extrusion of the hhc triggered by first order changes in rock types of the tectonic unit the stds is marked by a wide zone of high strain and by a metamorphic jump from amphibolite facies in the carbonate rocks of the upper part of the hhc to greenschist facies marbles in the lower part of the tss the development of a pervasive foliation towards the bottom of the tss indicates increasing strain related to top down to the ne tectonic transport a low p metamorphic event marked by the growth of post d1 biotite porphyroblasts at the base of the tss is related to the conductive heating from the underlying hhc

new constraints on the age of the manaslu leucogranite evidence for episodic tectonic denudation in the central himalayas the manaslu leucogranite of central nepal transacts one segment of the south tibetan detachment system a major extensional feature that helped to moderate large topographic gradients in the himalayan orogen in middle tertiary time 40 ar 39 ar ages for hornblendes from the northeastern contact aureole of the pluton indicate that intrusion occurred prior to 22 23 ma providing a minimum age for formation of the detachment system in the manaslu region that is at least 2 m y older than previous estimates from other parts of the orogen 40 ar 39 ar mica ages from the aureole similar to previously published mica ages from the upper part of the leucogranite indicate an episode of rapid cooling at 19 16 ma we attribute this cooling event to tectonic denudation of the pluton by movement on structurally higher exten sional faults and we suggest that gravitational collapse of the orogenic front occurred episodically over the late oligocene early miocene interval in the manaslu area

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

the himalayan main central thrust pile and its quartz rich tectonites in central nepal as a consequence of the india asia collision a deeply rooted crustal slice has been moved southwards over the northern boundary of the indian plate at the present level of observation the main central thrust pile a rock pile of several kilometers thickness is affected by shear deformation which imprinted a flat foliation and a constant n s trending lineation these tectonites were deformed under a wide range of metamorphic conditions grading from lower greenschists up to the sillimanite field the m c t shear zone is studied from the view point of the quartz rich rocks over 100 samples helped to define a microstructural zonation roughly parallel to the thermal one 35 samples particularly rich in quartz have been used for the preferred orientation study of their c axes and a few of them for their a axes strong lattice orientations when undisturbed by mainly secondary recrystallization attest to large plastic strains in quartz the c axes are mostly distributed along unequally populated crossed girdles which may evolve into an inclined single girdle the a axes concentrate perpendicular to the more populated girdle concerning quartz the following is suggested 1 1 the deformation has a dominant shear component responsible for the dominant stretching lineation it could be simple shear in case of inclined single girdles 2 2 one set of a axes line up parallel to the shear direction slip on various planes in the a direction may be an important deformation mechanism 3 3 asymmetry in the lattice preferred orientations could be used to predict the sense of shear for the himalayan thrusting in more than 80 of cases

dislocation creep regimes in quartz aggregates abstract using optical and tem microscopy we have determined that three regimes of dislocation creep occur in experimentally deformed quartz aggregates depending on the relative rates of grain boundary migration dislocation climb and dislocation production within each regime a distinctive microstructure is produced due primarily to the operation of different mechanisms of dynamic recrystallization at lower temperatures and faster strain rates the rate of dislocation production is too great for diffusion controlled dislocation climb to be an effective recovery mechanism in this regime recovery is accommodated by strain induced grain boundary migration recrystallization with an increase in temperature or decrease in strain rate the rate of dislocation climb becomes sufficiently rapid to accommodate recovery in this regime dynamic recrystallization occurs by progressive subgrain rotation with a further increase in temperature or decrease in strain rate dislocation climb remains sufficiently rapid to accommodate recovery however in this regime grain boundary migration is rapid thus recrystallization occurs by both grain boundary migration and progressive subgrain rotation the identification of the three regimes of dislocation creep may have important implications for the determination of flow law parameters and the calibration of recrystallized grain size piezometers in addition the identification of a particular dislocation creep regime could be useful in helping to constrain the conditions at which a given natural deformation has occurred

textural chemical and isotopic insights into the nature and behaviour of metamorphic monazite monazite is a mineral of choice for dating metamorphism in amphibolite and granulite grade metapelites however there exist a number of difficulties that complicate the interpretation of monazite geochronological data and prevent its application to many geological problems the two main obstacles addressed in this contribution are firstly the minor but significant e g 1 30 ma dispersal in duplicate isotope dilution thermal ionisation mass spectrometry id tims u pb age data commonly recorded from a single rock and secondly the difficulty of attaching monazite age data to pressure and temperature information through a multidisciplinary approach utilising tims and laser ablation multicollector inductively coupled plasma mass spectrometry la mc icpms isotope data quantitative and qualitative emp chemical analyses of monazite and textural studies we assess the significance of pb loss older components and continuous and episodic monazite growth in the generation of dispersed age data three samples from the canadian cordillera and one sample from the himalaya of pakistan are examined each sample exhibits an age dispersion of between 1 and 12 ma for single crystal and multi grain tims u pb monazite age determinations consideration of the closure temperature for pb diffusion in monazite and the metamorphic temperatures experienced by these samples suggests diffusive pb loss did not play a significant part in generating this age dispersal the la mc icpms study indicates that an older component 100 ma older than the tims ages contributed to the age dispersal in three of the four samples in all the samples however chemical analyses identified that the majority of monazites examined exhibited significant intra crystalline zoning in y content the la mc icpms analysis of one sample that was constrained to zones of distinct y content indicates that these zones are of distinct age we suggest that monazite grown before the appearance of garnet and during garnet breakdown is relatively rich in y whereas monazite grown after garnet is relatively poor in y a combination of these chemical data with textural observations suggests that once monazite had entered the mineral assemblage it grew or recrystallised episodically throughout the prograde and retrograde paths of the metamorphic event this behaviour contributes to and in one of the samples controls the observed age dispersal this recognition allows the generation of pressure temperature time points by combining textural and chemical information of monazite with in situ age determinations and pressure temperature information from garnet thus the episodic growth of compositionally distinct monazite throughout a metamorphic event provides the geochronologist with a very valuable chronological tool