paleoproterozoic tectonic transition from collision to extension in the eastern cathaysia block south china evidence from geochemistry zircon u pb geochronology and nd hf isotopes of a granite charnockite suite in southwestern zhejiang abstract the badu complex and associated paleoproterozoic granitoids are among the oldest known rocks in the cathaysia block in south china the paleoproterozoic units of the badu complex are dominantly composed of metapelitic rocks and meta greywackes here we report la ss icp ms laser ablation split stream inductively coupled plasma mass spectrometry zircon u pb data from a newly discovered garnet bearing granite which show an emplacement age of 1929 15 ma and metamorphism at 1872 34 ma we also report u pb ages of 1886 16 ma 1858 7 ma 1848 11 ma from a gneissic granodiorite and two charnockites respectively the garnet bearing granite is peraluminous with a cnk range from 1 1 to 1 3 the rock shows relatively high sio2 k2o and rb contents and low total ree sr cao and ferromagnesian components typical of leucogranites the whole rock nd two stage model age tdm2 nd of this rock is ca 2 7 ga zircon hf crustal model ages tdmc hf peak at about 2 7 ga and abundant inherited zircons occur with u pb ages in the range of 2044 to 2803 ma evidences from zircon u pb age and hf isotope compositions whole rock nd isotopes and whole rock major and trace elements suggest a metasedimentary protolith and that the garnet bearing granite leucogranite was derived by partial melting in a thickened crust at about 1 93 ga the gneissic granodiorite and charnockites show indistinguishable major and trace element features as well as zircon hf and whole rock nd isotope compositions indicating that they were generated from the same source rocks the gneissic granodiorite and charnockites are ferroan calc alkalic and metaluminous with a cnk range from 0 80 to 0 98 they display relatively low sio2 contents and ga al ratios suggesting their a type affinity their zircon tdmc hf age peak is 2 9 ga and whole rock tdm2 nd ages range from 2 8 to 2 9 ga these high temperature rocks were generated possibly through the partial melting of ancient amphibolites 2 9 ga with heat input from asthenosphere upwelling and magmatic underplating in an extensional setting at about 1 85 1 88 ga combined with data from previous studies we suggest three episodes of major crustal growth in this region at 1 8 1 9 ga 2 7 ga and 2 9 ga during the 1 8 1 9 ga event extensive reworking also took place from the 1 93 ga syn collision leucogranite garnet bearing granite the 1 88 1 85 ga post collision a type granites and charnockites the granite charnockite suite and the1 85 1 77 ga intraplate basaltic rocks li 1997 and xiang et al 2008 in the region a paleoproterozoic orogenic cycle can be deciphered in the cathaysia block the rapid transition of tectonic regime from collision to post collisional extension can be correlated to the assembly of the supercontinent columbia

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

thermo tectonic history of the tethyan himalayas deduced from the palaeomagnetic record of metacarbonates from shiar khola central nepal abstract palaeomagnetic measurements were carried out on low grade metamorphic carbonates of mesozoic age from the shiar area 85 1 e 28 6 n of the tethyan himalaya th in north central nepal two characteristic remanence components carried by pyrrhotite chrm 1 and magnetite chrm 2 could be identified by their unblocking temperature spectra of 270 340 and 430 580 c respectively fold tests are not significant due to the uniform bedding of all sites however according to results from other areas of the th the pyrrhotite component has been probably acquired as a secondary p trm during exhumation and cooling thus the age of remanence acquisition can be related to the last cooling event 25 17 ma in the surrounding areas the inclination of the magnetite component matches the value expected from the indian apwp this may the primary origin of the chrm 2 pyrrhotite site mean directions show a small circle distribution with a best fit parallel to the n s direction backtilting to the expected inclination i exp by intersection of the remanence small circle with the small circle of constant i exp yields a clockwise block rotation of 30 35 with respect to the indian plate characteristics of the pyrrhotite component small circle distribution of site means secondary origin p trm with unblocking temperatures below about 300 c allow the interpretation of the chronologic order of the thermo tectonic history i an earlier main folding phase at elevated temperatures ii a later event of cooling through about 300 c coinciding with the acquisition of chrm 1 iii clockwise block rotations with respect to the indian plate and iv long wave folding as the youngest tectonic event

fabric transitions in the saxony granulite terrain abstract quartz fabric transitions reported from the saxony granulite terrain and from the nearby ore mountains are interesting because of the changes in the pattern types and the variation in the opening angles of the fabric skeletons a short summary is presented of salient features in research documented in the east german literature between 1961 and 1978 the changes in fabric skeleton with decreasing metamorphic grade are proposed as mainly consequent to the deactivation of prism c glide systems as temperature decreases fabrics indicative of the appearance of the c burgers vector might be diagnostic for deformation in regimes of comparatively high temperatures or low strainrates or for deformation under conditions such that there is a relatively high kinetically effective hydroxyl content in the quartz structure

tectonic evolution of a continental collision zone a thermomechanical numerical model we model evolution of a continent continent collision and draw some parallels with the tectonic evolution of the himalaya we use a large scale visco plasto elastic thermomechanical model that has a free upper surface accounts for erosion and deposition and allows for all modes of lithospheric deformation for quartz olivine rheology and 60 mm yr convergence rate the continental subduction is stable and the model predicts three distinct phases during the phase 1 120 km or 6 of shortening deformation is characterized by back thrusting around the suture zone some amount of delaminated lower crust accumulates at depth during phase 2 120 km 420 km or 6 22 of shortening this crustal root is exhumed medium to high grade rocks along a newly formed major thrust fault this stage bears similarities with the period of coeval activity of the main central thrust and of the south tibetan detachment between 20 16 myr ago during phase 3 420 km or 22 of shortening the crust is scraped off from the mantle lithosphere and is incorporated into large crustal wedge deformation is localized around frontal thrust faults this kinematics should produce only low to medium grade exhumation this stage might be compared with the tectonics that has prevailed in the himalaya over the last 15 myr allowing for the formation of the lesser himalaya the experiment is conducted at constant convergence rate which implies increasing compressive force considering that this force is constant in nature this result may be equivalent to a slowing down of the convergence rate as was observed during the india asia collision

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

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

vertical profiles of suspended sand concentration and size from multifrequency acoustic backscatter vertical profiles of suspended sand concentration and size are obtained from multifrequency acoustic profiling data collected in 1989 during a nearshore experiment at stanhope lane beach prince edward island the data were acquired with acoustic sounders operating at 1 2 25 and 5 mhz independent estimates of concentration were made using optical backscatter sensors obss the algorithm for inversion of the three frequency backscatter data to particle size and concentration based on the ratios of the different signals was tested in laboratory experiments with an unconfined high reynolds number suspended sediment jet results from the stanhope beach experiment are presented for three different surface wave energy regimes with significant wave orbital velocities ranging from 0 3 to 0 8 m s and peak wave periods of 4 6 s the acoustic estimates of mean concentration are shown to be within 10 on average of those determined with the obs nearest the bottom at 5 to 10 cm height over time scales ranging from 6 5 min to 4 6 s one wave period the acoustic estimates of suspended sediment size near the bottom are within 5 20 of the bottom sediment mean size the statistical variability of the size estimates is high with standard deviations in the estimates ranging between 30 and 50 of the mean the time averaged concentration profiles exhibit an exponential decrease with height above an o 10 cm thick near bottom region of nonexpo nential decrease in contrast the time averaged mean size profiles decrease approximately linearly with height and rather slowly about 25 in 0 5 m

the pangong injection complex indian karakoram a case of pervasive granite flowthrough hot viscous crust in the karakoram range of nw india the tangtse gorge cuts across deep crustal rock sexhumed by transpression along two strands of the dextral karakoram fault crustal leucogranitic magmas pervasively intruded amphibolites and migmatites in the form of sheets that locally coalesced and expanded to form kilometre scale plutons sheets resulted from slow seeping of granite into foliation planes magma wedging rather than from dyking driven by magma buoyancy and possibly syn intrusive tectonic deformation high temperature of the country rocks at the time of intrusion is inferred from similar crystallization ages of the intrusive leucogranite and in situ partial melt in the migmatites and reflected in the ductile structures developed whereas low country rock viscosity inhibited dyking high temperature freed the magma from the constraints of freezing and permitted pervasive intrusion the injection complex exposed along the tangtse gorge rather than representing the final emplacement structures represent transient structures of the granite pathways on their upward journey to build the karakoram batholith which crops out a few kilometres north at structurally shallower levels we suggest that pervasive magma flow is a transitional step between magma segregation at the source and later rise through cold crust and it may be one of several factors controlling whether dyking or diapirism becomes dominant during late ascent

channel profiles around himalayan river anticlines constraints on their formation from digital elevation model analysis 1 we present a comparison between measured and numerically modeled channel profiles of rivers in two important drainage basins of central nepal the kali gandaki and the arun drainage basins modeled channel profiles are based on a simple stream power approach using best fit exponents defining the nonlinearities in the relative contributions of local channel gradient and water flux to erosion rate our analysis of the stream power in the whole river network confirms the work of other authors that a 50 to 80 km wide zone roughly corresponding to the high himalayan topography is subjected to rapid rock uplift we suggest a model where the uplift of this zone is driven by erosion and isostatic response so that centers of maximum uplift are located within the main channels of the north south draining rivers we also suggest that the rate of uplift slows down with increasing distance to the main channels such a spatial distribution of the uplift leads ultimately to the formation of river anticlines as observed along all major himalayan rivers we propose that the formation of river anticlines along south draining himalayan rivers was accelerated by a sudden increase of the drainage area and discharge when the rivers captured orogen parallel drainages on the north side of the range this may follow successive headward cutting into the tibetan plateau the model is confirmed by differences between main channels and east west running tributaries time dependent numerical models predict that capture events cause strongly elevated erosion rates in the main channel

zircon u pb ages and hf isotopes of the askot klippe kumaun northwest india implications for paleoproterozoic tectonics basin evolution and associated metallogeny of the northern indian cratonic margin

records of the evolution of the himalayan orogen from in situ th pb ion microprobe dating of monazite eastern nepal and western garhwal abstract in situ th pb monazite ages from rocks collected along two transects the dudh kosi everest eastern nepal and the bhagirathi river garhwal himalaya india perpendicular to the main central thrust mct suggest a striking continuity of tectonic events across the himalaya the youngest age reported in this study 5 9 0 2 ma mswd 0 4 from matrix monazite grains collected beneath the mct in the garhwal region is consistent with several age data from rocks at similar structural levels in central nepal providing support for widespread late miocene mct activity the lateral parallelism of orogenic events is further manifested by the 20 7 0 1 ma age of a high himalayan leucogranite from an injection complex that outcrops along the dudh kosi everest transect resembling ages of these bodies reported elsewhere the youngest monazite grain analyzed along the dudh kosi everest transect is 10 3 0 8 ma the absence of 7 3 ma monazite ages in eastern nepal may reflect a different nappe structure which obscures the reactivated ramp equivalent exposed in the garhwal and central nepal garnets from the mct hanging wall greater himalayan crystallines and footwall lesser himalaya display different major element zoning and the patterns are useful for constraining the location of the thrust system that separates the two lithologies pressure temperature paths for two upper lesser himalayan garnets that contain monazite inclusions indicate the utility of an in situ methodology to constrain the metamorphic evolution of the shear zone along the dudh kosi everest transect upper lesser himalayan monazite grains from three rocks record a clear signal at 14 5 0 1 ma mswd 8 4 and the 23 ma age that characterizes the hanging wall is notably absent monazites collected within a large scale greater himalayan crystallines fold yield the 14 ma age consistent with the structure forming due to mct related compression paleo mesoproterozoic 1407 35 ma matrix monazite grains are found within an augen gneiss unit located beneath the mct whereas cambro ordovician 436 8 548 17 ma inclusions are preserved within garnets of the greater himalayan crystallines the presence of 45 2 2 1 ma grains from lower structural levels of the greater himalayan crystallines indicates the unit realized conditions conducive for monazite growth during the eocene

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

porphyroclast systems as kinematic indicators porphyroclasts of relatively strong minerals in mylonites commonly have an internal monoclinic shape symmetry defined by tails of dynamically recrystallized material the geometry of a porphyroclast and its tails called a porhyroclast system can serve as a valuable indicator of the sense of vorticity porphyroclast systems have been divided into and types on the basis of the geometry of the tails types have wedge shaped recrystallized tails whose median lines lie on opposite sides of a reference plane parallel to the tails and containing the symmetry axis for the system types are further subdivided into a a types in which the porphyroclast is isolated in a relatively homogeneous matrix and b types in which the porphyroclast system is associated with a shear band foliation in the matrix types typically have narrow recrystallized tails whose median lines cross the reference plane adjacent to the porphyroclast consequently embayments of matrix material occur adjacent to the porphyroclasts and the tails display characteristic bends a porphyroclast system in a mylonite develops when the relatively weak dynamically recrystallized grain aggregate in the porphyroclast mantle changes its shape due to non coaxial flow in the adjacent matrix this behaviour has been modelled in shear box experiments passive marker lines around rigid cylinders embedded in silicone putty were subjected to simple shear the experiments were modified to simulate a change in recrystallization rate r with respect to rate of deformation by decreasing the diameter of the rigid cylinder during deformation at variable rates the ratio r appears to be one of the most important factors in determining which porphyroclast system will develop at high r values flow of recrystallized material away from the porphyroclast is continuously appended by the production of new grains and wedge shaped a type tails develop at low r values relatively few new grains are added to the tails which become thinned and deflected by drag due to the spinning motion of the porphyroclast in addition most porphyroclast systems at low shear strains are of a type or lack monoclinic symmetry whereas types are only developed at high shear strain values complex porphyroclast systems characterized by two generations of tails are observed in many of the natural and model shear zones studied and may form due to fluctuating r conditions that allow isolated a and type porphyroclast systems to be used as sense of vorticity indicators are the systems should have a monoclinic shape symmetry matrix grain size should be small with respect to porphyroclast size matrix fabric should be homogeneous deformation history should be simple and observations should be made on sections normal to the inferred bulk vorticity vector for the mylonite

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

ductile shearing to brittle thrusting along the nepal himalaya linking miocene channel flow and critical wedge tectonics to 25th april 2015 gorkha earthquake the 25th april 2015 magnitude 7 8 gorkha earthquake in nepal ruptured the main himalayan thrust mht for 140 km east west and 50 km across strike the earthquake nucleated at a depth of 15 18 km approximating to the brittle ductile transition and propagated east along the mht but did not rupture to the surface leaving half of the fault extent still locked beneath the siwalik hills coseismic slip shows that motion is confined to the ramp flat geometry of the mht and there was no out of sequence movement along the main central thrust mct below 20 km depth the mht is a creeping aseismic ductile shear zone cumulated deformation over geological time has exhumed the deeper part of the himalayan orogen which is now exposed in the greater himalaya revealing a tectonic history quite different from presently active tectonics there early miocene structures including the mct are almost entirely ductile with deformation occurring at temperatures higher than 400 c and were active between 22 16 ma kyanite and sillimanite grade gneisses and migmatites approximately 5 20 km thick in the core of the greater himalayan sequence ghs together with leucogranite intrusions along the top of the ghs were extruded southward between 22 15 ma concomitant with ages of partial melting thermobarometric constraints show that ductile extrusion of the ghs during the miocene occurred at muscovite dehydration temperatures 650 775 c and thus brittle thrusting and critical taper models for ghs deformation are unrealistic as partial melting and channel flow ceased at 15 ma brittle thrusting and underplating associated with duplex formation occurred along the lesser himalaya passively uplifting the ghs

thermal constraints on the emplacement rate of a large intrusive complex the manaslu leucogranite nepal himalaya the emplacement of the manaslu leucogranite body nepal himalaya has been modelled as the accretion of successive sills the leucogranite is characterized by isotopic heterogeneities suggesting limited magma convection and by a thin 100 m upper thermal aureole these characteristics were used to constrain the maximum magma emplacement rate models were tested with sills injected regularly over the whole duration of emplacement and with two emplacement sequences separated by a repose period additionally the hypothesis of a tectonic top contact with unroofing limiting heat transfer during magma emplacement was evaluated in this latter case the upper limit for the emplacement rate was estimated at 3 4 mm year or 1 5 myr for 5 km of granite geological and thermobarometric data however argue against a major role of fault activity in magma cooling during the leucogranite emplacement the best model in agreement with available geochronological data suggests an emplacement rate of 1 mm year for a relatively shallow level of emplacement granite top at 10 km uninterrupted by a long repose period the thermal aureole temperature and thickness and the isotopic heterogeneities within the leucogranite can be explained by the accretion of 20 60 m thick sills intruded every 20 000 60 000 years over a period of 5 myr under such conditions the thermal effects of granite intrusion on the underlying rocks appear limited and cannot be invoked as a cause for the formation of migmatites

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

trans hudson orogen of north america and himalaya karakoram tibetan orogen of asia structural and thermal characteristics of the lower and upper plates 1 the trans hudson orogen tho of north america and the himalaya karakoram tibetan orogen hkto of asia preserve a paleoproterozoic and cenozoic record respectively of continent continent collision that is notably similar in scale duration and character in tho the tectonothermal evolution of the lower plate involves 1 early thin skinned thrusting and barrovian metamorphism 2 out of sequence thrusting and high t metamorphism and 3 fluid localized reequilibration anatexis and leucogranite formation the crustal evolution of the indian lower plate in hkto involves 1 early subduction of continental crust to ultrahigh pressure uhp eclogite depths 2 regional barrovian metamorphism and 3 widespread high t metamorphism anatexis and leucogranite formation the shallow depths of the high t metamorphism in hkto are consistent with early to mid miocene ductile flow of an indian lower plate midcrustal channel from beneath the southern tibetan plateau to the greater himalaya melt weakening of the lower plate in tho is not observed at a similar scale probably due to the paucity of pelitic lithologies tectonothermal events in the upper plate of both orogens include precollisional accretion of crustal blocks emplacement of andean type plutonic suites and high t metamorphism syncollisional to postcollisional events include emplacement of garnet biotite muscovite leucogranites anatectic granites and sporadic metamorphism up to 90 myr following the onset of collision in tho comparing the type and duration of tectonothermal events for tho and hkto supports the notion of tectonic uniformitarianism for at least the later half of dated earth history and highlights the complementary nature of the rock record in an older exhumed orogen compared to one undergoing present day orogenesis

structural and thermochronological studies of the almora klippe kumaun nw india implications for crustal thickening and exhumation of the nw himalaya

towards pyrrhotite magnetite geothermometry in low grade metamorphic carbonates of the tethyan himalayas shiar khola central nepal abstract in mesozoic metacarbonates of the tethyan himalayas shiar khola area central nepal two characteristic remanent magnetisations chrm1 and chrm2 were identified by their unblocking temperature spectra the chrm1 is carried by pyrrhotite unblocking temperature 270 360 c and the chrm2 by magnetite unblocking temperature spectra 430 580 c the temperature related formation of pyrrhotite at the expense of primary magnetite during low grade metamorphism in marly carbonates allows the determination of thermal gradients by the pyrrhotite magnetite ratio this new method can be used as a geothermometer for t 300 c in low grade metamorphic carbonates where other methods are not available this method is applied for the first time in the tethyan himalayas of central nepal in the shiar khola valley systematic variations in the ferrimagnetic content of the metacarbonates along an e w profile were detected by the ratio of remanence intensity of pyrrhotite to magnetite derived from natural remanent magnetisation rpyr mag and saturation magnetisation spyr mag over a stretch of 10 km the rpyr mag and spyr mag increase from w to e from 0 42 to 0 91 and 0 48 to 1 0 respectively based on temperature estimates the eastern part experienced upper anchizone epizone 250 300 c conditions while the western part underwent only diagenesis 200 c the temperature gradient and the temperature ranges suggested are consistent with the findings of the calcite twin lamellae geothermometry which is a non magnetic method

acoustic measurements of boundary layer flow and sediment flux results are reported on an assessment of the application of coherent doppler and cross correlation techniques to measure nearbed boundary layer flow the approaches use acoustic backscattering from sediments entrained into the water column from the bed to obtain high resolution profiles of the nearbed hydrodynamics measurements are presented from a wave tunnel experiment in which sediment was entrained by unidirectional oscillatory and combined flows the data collected have been used to evaluate the capability of the two flow techniques to measure boundary layer mean turbulent and intrawave velocity profiles further the backscattered signal has been used to measure suspended sediment concentration profiles which have been combined with the velocity profiles to obtain high resolution measurements of boundary layer sediment flux

tectonic controls on metamorphism partial melting and intrusion timing and duration of regional metamorphism and magmatism in the nigde massif turkey protracted heating and melting of orogenic middle crust followed by emplacement of a syn extensional pluton during decompression and cooling are demonstrated for a metamorphic core complex in central turkey new geochronologic data for the nigde massif of the central anatolian crystalline complex document an interval of melting crystallization and pb loss at elevated ambient temperature 85 92 ma determined by u pb shrimp within a single sample of peraluminous granite crustal melting was syn to post peak metamorphism 725 c dated at 91 0 2 0 ma by shrimp analysis of zircon rims in sillimanite schist that is part of the country rock to the pluton metamorphic monazite in sillimanite schist yields a younger age of 84 7 0 7 ma for matrix grains and inclusions in garnet hornblende 40ar 39ar ages from amphibolites interlayered with the sillimanite schists coincide with the timing of emplacement of magma into metamorphic country rock that had experienced cooling and decompression to 12 km and 550 c the similarity of 40ar 39ar ages for biotite in the pluton 79 ma and hornblende 79 81 ma biotite 74 76 ma and muscovite 78 83 ma in the metamorphic country rock document rapid cooling 100 c my following intrusion the coincidence of the youngest u pb magmatic zircon ages and the oldest 40ar 39ar ages combined with structural evidence for normal sense shearing and tectonic unroofing in the late cretaceous suggests that intrusion was driven by extension we propose that crustal melting occurred in the middle crust during prograde metamorphism and or initial decompression and that ascent and emplacement of magma as a pluton were only triggered once extension occurred and removed upper crustal rocks

abstract structural and paleogeographic elements of the nanpanjiang basin guizhou guangxi and yunnan provinces south china a compilation from satellite images regional geologic maps and ground observations

the extent of equilibration between melt and residuum during regional anatexis and its implications for differentiation of the continental crust a study of partially melted metapelitic enclaves thisstudy presentsthefirst precise evaluation of the extent ofchem ical equilibrium between felsic melt and crystalline residuumduring the crustal anatexis of metasediments the high precision ofthese resultsstemsfrom thefactthat in thiscase the meltsare repre sented by pristine glasses that occur in the matrices of partiallymelted metapelitic enclaves and as abundant glassy melt inclusionstrapped by different minerals in the same enclaves in previous stu dies of anatexis the composition of the melt has been approximatedby that of leucosomes in migmatites which do not necessarily repre sent the composition of the melt very well the present study isbased on laser ablation analyses of 35 trace elements in the glassand principal minerals of enclaves included within neogene peralu minous dacites exposed at el hoyazo se spain the enclaves which contain the assemblage plagioclase biotite sillimanite garnet glass ilmenite graphite k feldspar cordieritequartz represent fragments of metapelitic continental crust thatwaspartiallymeltedatpressuresof5 7kbarduringaregionalmeta morphic event matrix melt and melt inclusions within the enclaveswere quenched to glass upon eruption of the dacite the glasses inthe melt inclusions are interpreted as the remains of melt producedby an initial melting reaction involving a muscovite rich assemblageat 700 7508c whereasthe matrixglassesare consistent with con tinuedmeltingand oralatermeltdominatedbytheincipientmeltingof biotite at 800 8508c the distribution of trace elements be tween residuum and melt duringanatexis of the enclaves isdifferentfrom that predicted by disequilibrium models that consider effectivedistribution coefficients close to unity instead the bulk melt andthe major minerals plagioclase biotite alkali feldspar cordierite were close to equilibrium except for the case of garnet duringthe generation of the melt inclusions and matrix glasses thequenchedmelts meltinclusionsandparticularlythematrixglasses were not in equilibrium with accessory zircon and monazite andare depleted in zirconium and the light rare earth elements recrystallization of mineralsand diffusion in the melt were import ant controls during the redistribution of trace elements betweenthe solids and the bulk melt local disequilibrium processes alsotook place at mineral melt interfaces during the crystallizationof peritectic minerals and the recrystallization of residual phases as shown by the depletion of compatible elements in the meltinclusions

comparison of field observations of the vertical distribution of suspended sand and its prediction by models models for the vertical distribution of suspended sediment under waves and currents are compared with field observations the models include a diffusion based model a convection based model and a combined convection diffusion model the field observations were carried out at vilano beach florida in a water depth of approximately 3 to 4 m with a median grain diameter of 0 14 mm the rms wave height ranged from approximately 0 4 to 1 2 m under low waves the seabed was covered with very small ripples and under the higher waves sheet flow conditions were present the diffusion based model was found to best describe the observed data under high wave conditions but the convection model was more accurate under low wave conditions over all conditions the combined model was the most accurate these results are consistent with the different mechanisms for mixing suspended sediment over flat versus rippled beds

s c mylonites two types of foliations are commonly developed in mylonites and mylonitic rocks a s surfaces related to the accumulation of finite strain and b c surfaces related to displacement discontinuities or zones of relatively high shear strain there are two types of s c mylonites type i s c mylonites described by berthe et al typically occur in deformed granitoids they involve narrow zones of intense shear strain which cut across mylonitic foliation type ii s c mylonites described here have widespread occurrence in quartz mica rocks involved in zones of intense non coaxial laminar flow the c surfaces are defined by trails of mica fish formed as the result of microscopic displacement discontinuities or zones of very high shear strain the s surfaces are defined by oblique foliations in the adjacent quartz aggregates formed as the result of dynamic recrystallization which periodically resets the finite strain clock these oblique foliations are characterized by grain elongations alignments of segments of the grain boundary enveloping surfaces and by trails of grains with similar c axis orientations examples of this aspect of foliation development in mylonitic rocks are so widespread that we suggest the creation of a broad class of s c tectonites and a deviation from the general tradition of purely geometric analysis of foliation and time relationships kinematic indicators such as those discussed here allow the recognition of kilometre scale zones of intense non coaxial laminar flow in crustal rocks and unambiguous determination of the sense of shear

tracing patterns of erosion and drainage in the paleogene himalaya through ion probe pb isotope analysis of detrital k feldspars in the indus molasse india abstract the indus molasse is a pre and syn tectonic sedimentary sequence situated in the indus suture zone of the western himalaya spanning in time the collision of india and asia this deposit is well placed to record the evolving uplift and erosion history of the early himalayan orogen nd isotope analyses from clay extracted from shales interbedded within the dominantly alluvial sequence indicate a low negative nd 1 64 to 0 72 in the basal paleocene chogdo formation slightly more negative than measured values from the transhimalaya and kohistan dras arc up section nd becomes more negative as low as 10 05 indicating influence of a different more enriched source ion microprobe pb isotopic analyses of single k feldspars help constrain this source as being either the lhasa or karakoram blocks with westward paleo current flow favoring the former 207pb 204pb ratios are too low to be consistent with known indian plate sources a conclusion supported by the lack of muscovite or garnet that would be indicative of a high himalayan contribution given the known age of rapid cooling of the high himalaya at 20 ma and the lack of exposure of suitable lithologies prior to that time an age of sedimentation prior to 20 ma is inferred the post collisional change in paleo flow and provenance is suggested to reflect the initiation of the indus river during the early eocene this study demonstrates the power of combined bulk sediment and single grain analyses in resolving provenance in tectonically complex settings

thrusting and back thrusting as post emplacement kinematics of the almora klippe insights from low temperature thermochronology abstract crystalline klippen over the lesser himalayan sequence lhs in the kumaon and garhwal regions of nw himalaya are the representative of southern portion of the main central thrust mct hanging wall these were tectonically transported over the juxtaposed thrust sheets berinag tons and ramgarh of the lhs zone along the mct these klippen comprise of nw se trending synformal folded thrust sheet bounded by thrusts in the south and north in the present study the exhumation histories of two well known klippen namely almora and baijnath and the ramgarh thrust sheet in the kumaon and garhwal regions vis a vis himalayan orogeny have been investigated using apatite fission track aft ages along a 60 km long orogen perpendicular transect across the almora klippe and the ramgarh thrust sheet 16 aft cooling ages from the almora klippe and 2 from the ramgarh thrust sheet have been found to range from 3 7 0 8 to 13 2 2 7 ma and 6 3 0 8 to 7 2 1 0 ma respectively from lhs meta sedimentary rocks only a single aft age of 3 6 0 8 ma could be obtained three aft ages from the baijnath klippe range between 4 7 0 5 and 6 6 0 8 ma aft ages and exhumation rates of different klippen show a dynamic coupling between tectonic and erosion processes in the kumaon and garhwal regions of nw himalaya however the tectonic processes play a dominant role in controlling the exhumation thrusting and back thrusting within the almora klippe and ramgarh thrust sheet are the post emplacement kinematics that controlled the exhumation of the almora klippe combining these results with the already published aft ages from the crystalline klippen and the higher himalayan crystalline hhc the kinematics of emplacement of the klippen over the lhs and exhumation pattern across the mct in the kumaon and garhwal regions of nw himalaya have been investigated

rb sr geochronology of granites and gneisses from the mount everest region nepal himalaya nineteen samples of granites orthogneisses and paragneisses from the high himalaya basement nappe s of the mount everest region have been dated by the rb sr method the post metamorphic tourmaline leucogranites of the upper imja drangka nuptse lhotse glacier have high initial sr87 sr86 characteristic of an anatectic origin from crustal material a whole rock isochron age of 52 m y early eocene has been obtained for the samples from the granite body of lhotse glacier apparently sr isotopic homogenization was not reached throughout the much larger nuptse granite

u pb zircon ages from the spontang ophiolite ladakh himalaya the spontang ophiolite in the ladakh himalaya consist of a thrust sheet of upper mantle and oceanic crustal rocks obducted onto the northern passive continental margin of india during the late cretaceous precise isotopic ages of himalayan ophiolites are presently unavailable u pb dating of zircons from a dioritic segregation in the high level gabbros of the spontang ophiolite shows that the complex formed at 177 1 ma mid jurassic and we suggest that it represents a fragment of neo tethyan oceanic crust this jurassic age contrasts with late cretaceous ages 91 95 ma previously reported for the formation of the troodos and oman ophiolite complexes further west along the neo tethyan suture zone and also predates the early to mid cretaceous masirah ophiolite c 150 ma and its alkaline volcanic seamounts 115 125 ma an andesitic rock from an arc sequence overlying the spontang ophiolite was also dated using as 88 5 ma this late cretaceous age constrains the minimum age of initiation of subduction beneath the spontang ophiolite the complex was emplaced during the turonian maastrictian some 80 ma after its formation when the indian continental margin collided with a cretaceous arc about 25 ma before the india asia collision the lack of a sub ophiolite metamorphic sheet beneath the spontang mantle sequence may be explained by the fact that the ophiolite was old and cold at the time of obduction

north almora fault a crucial missing link in the strike slip tectonics of western himalaya abstract geometry and kinematics of a hitherto ignored crustal scale tectonic discontinuity from the west central sector of the himalaya viz the north almora fault naf is discussed in this paper choice of the tectonic model for response of the himalayan south tibetan ensemble to the tertiary himalayan continental collision is largely dictated inter alia by the geometry and kinematics of the large orogen parallel strike slip faults in the region based on field laboratory and satellite imagery data it is concluded that the north almora fault thrust is not one but two closely parallel tectonic discontinuities viz i the north almora fault a steep south dipping high angle brittle fault with a dominant east slip and subordinate dip slip component and ii the north almora thrust a low angle south dipping thrust marking the northern boundary of the large almora jajarkot nappe the 500 km long dextral strike slip north almora fault mimicking the 800 km long karakoram fault is responsible for significant strain partitioning in the west central lesser himalaya the north almora fault assumes special salience for the himalayan tectonics as it partitions sizable strains in addition to the karakoram fault in the region and lends support to the oblique convergence model that best describes the himalayan extensional and strike slip deformation

leucogranites in lhozag southern tibet implications for the tectonic evolution of the eastern himalaya abstract petrogenesis of the himalayan leucogranite is strongly influenced by conditions which are associated with the tectonic evolution of himalayan orogen in this article we present petrological geochronological and geochemical results of the lhozag leucogranites that crop out alongside the south tibetan detachment system stds in the east of himalaya zircon u pb dating revealed three episodes of leucogranitic magmatism in lhozag at 17 8 0 1 ma 15 1 0 1 ma and 12 0 0 1 ma respectively the lhozag leucogranites show relatively low end t low zircon ehf t and high initial 87sr 86sr ratios which are similar to the high himalayan crystalline series hhcs indicating that they were derived from the hhcs the characteristics of relatively high na2o and rb contents high rb sr ratios and low cao mgo tfe2o3 tio2 and sr contents indicate that both the ca 18 ma lhozag tourmaline leucogranites and the ca 15 ma lhozag two mica granites were derived from fluid absent muscovite dehydration melting of metasediments the opposite geochemistry characteristics of the ca 12 ma khula kangri two mica granites imply that these granites are derived from fluid present melting of metasediments four khula kangri two mica granite samples with relatively lower tio2 tfe2o3 mgo and cao contents higher rb concentrations and rb sr ratios could be evolved from the khula kangri two mica granites with relatively lower rb sr ratios the melting behaviors of the lhozag leucogranites varied from fluid absent melting to fluid present melting implying that there were p t xh2o variations in the deep crust the tectonic evolution would give rise to variation of p t xh2o variation and subsequent transformation of melting behavior our new results display the transformation of melting behavior of the lhozag leucogranites which implies the tectonic evolution from earlier n s extension to later e w extension in the eastern himalaya at ca 12 ma

structural events and metamorphic consequences in almora nappe during himalayan collision tectonics abstract almora nappe in uttarakhand india is a lesser himalayan representative of the himalayan metamorphic belt that was tectonically transported over the main central thrust mct from higher himalaya the basal shear zone of almora nappe shows complicated structural pattern of polyphase deformation and metamorphism the rocks exposed along the northern and southern margins of this nappe are highly mylonitized while the degree of mylonitization decreases towards the central part where the rocks eventually grade into unmylonitized metamorphics mylonitized rocks near the roof of the basal shear zone show dynamic metamorphism m2 reaching upto greenschist facies 450 c 4 kbar in the central part of nappe the unmylonitized schists and gneisses are affected by regional metamorphism m1 reaching upper amphibolite facies 4 0 7 9 kbar and 500 709 c four zones of regional metamorphism progressing from chlorite biotite to sillimanite k feldspar zone demarcated by specific reaction isograds have been identified these metamorphic zones show a repetition suggesting that the zones are involved in tight f2 folding which has affected the metamorphics south of the almora town the regionally metamorphosed rocks have been intruded by almora granite 560 20 ma resulting in contact metamorphism the contact metamorphic signatures overprint the regional s2 foliation it is inferred that the dominant regional metamorphism in almora nappe is highly likely to be of pre himalayan precambrian age

kinematic model for the main central thrust in nepal comment and reply comment in the main central thrust zone of the central nepalese himalaya monazites decrease in age downsection between the main central thrust and main central thrust 1 mct 1 in fig 1 1 from ca 22 to 3 3 ma catlos et al 2001 2 this astonishingly systematic result kohn et al 2001 3 has

laboratory measurements of current flow using cross correlation on acoustic backscattering from suspended sediments the application of acoustic backscattering for measuring near bed high resolution profiles of suspended sediment concentration and particle size has advanced substantially in recent years to interpret these suspension observations and link them with the hydrodynamics similarly detailed flow observations would be advantageous in the present work backscattering from suspensions of marine sediments has been investigated to examine the potential of using the backscattered signal to also measure high resolution near bed vertical profiles of horizontal current flow the approach adopted has been to attempt to utilize the temporal coherence in the suspension field and to employ cross correlation of the backscattered signal between pairs of horizontally separated transducers to measure current profiles to investigate this technique a series of laboratory measurements have been conducted for a variety of experimental arrangements and the outcome from these measurements is reported in the present work an e

outward growth of the tibetan plateau during the cenozoic a review abstract the surface uplift history of the tibetan plateau tp offers a key testing ground for evaluating models of collisional tectonics and holds important implications for processes ranging from global cooling to the onset of the asian monsoon various models have been proposed to reveal the surface uplift history of the tp but controversies remain we evaluate these models using data from sedimentology and stratigraphy structural geology magmatism exhumation and paleoaltimetry studies structural analyses indicate that thrust belts which spread from the central tp outward toward its surrounding margins accommodated most of the india asia convergence and facilitated crustal shortening and thickening in the central tp eocene adakitic rocks located in the qiangtang and the lhasa blocks likely were generated by partial melting of an eclogitic source paleogene 50 30 ma potassic rocks only occur in the qiangtang block whereas late oligocene late miocene 26 8 ma potassic rocks occur both in the qiangtang and lhasa blocks low temperature thermochronologic ages in the central tp are older than 40 35 ma whereas those in the margins are younger than 20 ma mostly late miocene and pliocene pleistocene in age independent paleoaltimetry estimates suggest that the lhasa and qiangtang terranes attained their current elevations during the eocene most likely due to the initial collision between india and lhasa whereas the hoh xil basin area to the north and himalayas to the south were still low even below sea level in the latter case we argue for an inside out growth pattern for the tibetan plateau the tp grew southward and northward from a nucleus of high topography and is likely to continue expanding along the mazar tagh fault to the northwest the kuantai shan hei shan longshou shan to the northeast the longquan shan to the east and the shillong plateau to the south if the northward force of india would not diminished

post emplacement kinematics and exhumation history of the almora klippe of the kumaun garhwal himalaya nw india revealed by fission track thermochronology tectonically transported crystalline thrust sheet over the lesser himalayan meta sedimentary zone along the main central thrust mct is represented by almora baijnath askot and chiplakot crystalline klippen the almora dadeldhura klippe in the kumaun garhwal and western nepal himalaya is the witness and largest representative of these crystalline klippen south of mct here we investigate the post emplacement kinematics and exhumation history of the almora klippe the newly derived zircon fission track zft ages combined with published apatite fission track aft 40ar 39ar ages from the almora dadeldhura klippe and ramgarh thrust sheet to quantify the temporal variation in cooling ages and exhumation rates using 1 d numerical modelling approach we calculate the transient exhumation rates with respect to different time intervals new zft cooling ages along 50 km long orogeny perpendicular transect across the almora klippe range between 13 4 0 6 and 21 4 0 9 ma published aft ages and 40ar 39ar ages from the almora dadeldhura klippe range 3 7 0 8 13 2 2 7 and 18 20 25 69 ma respectively aft ages reported from ramgarh thrust sheet range 6 3 0 8 7 2 1 0 ma in kumaun region and 10 3 0 5 14 4 2 2 ma in western nepal the linear age trend along with youngest zft age 14 ma close to the north almora thrust nat in its hanging wall suggests rapid uplift close to the nat due to its reactivation as back thrust the transient exhumation rates of almora klippe agree that the erosion rate was rapid 0 58 mma 1 close to the nat in its hanging wall and relatively slow 0 31 mma 1 close to the sat in its hanging wall during 15 11 ma we interpret that the fission track ages and transient exhumation rate pattern of crystalline klippen show a dynamic coupling between tectonic and erosion processes in the kumaun garhwal himalaya however the tectonic processes play the major role in controlling the exhumation pattern

deformation mechanisms and inverted thermal profile in the north almora thrust mylonite zone kumaon lesser himalaya india abstract the thermal structure of a 2 2 km thick mylonite zone from the kumaon lesser himalaya india is interpreted based on deformation mechanisms and microstructures the mylonite zone lies along the north almora thrust nat a s dipping thrust that marks the northern flank of a large synformal crystalline klippe almora klippe approximately 17 km north of the town of almora the mylonite zone evolved during the upper eocene emplacement of the almora klippe part of a larger munsiari thrust sheet following the collision of india and eurasia in lower middle eocene mylonitic rocks near the top of the zone show dynamicrecrystallization of both feldspar and quartz indicating deformation at amphibolite grade 500 600 c near the center of themylonite zone the feldspars are fractured and quartz is dynamically recrystallized though to a lesser degree than near the top of the zone this evidence combined with the occurrence of secondary epidote chlorite biotite and calcite in these rocks suggests that mylonitization occurred at greenschist grade 500 400 c near the base of the mylonite zone biotite is present feldspars are fractured and altered to sericite and quartz shows limited plastic deformation in the form of undulose extinction and subgrain formation indicating that temperatures of 400 c or less were reached during deformation deformation mechanisms and metamorphic reactions in the mylonite zone rocks thus point to a decrease in the temperature of deformation from 500 300 c near the top of the thrust zone to about 400 c near its base suggesting an inverted thermal profile we interpret this inverted thermal profile in the mylonite zone to have resulted from gradual cooling of the zone by heat transfer to a cold footwall at the time of thrusting

sediment transport in oscillatory boundary layers in cases of rippled beds and sheet flow an experimental study was focused on the process of sediment transport in unsteady flow conditions due to wave action wave induced oscillatory flow conditions near the seabed were simulated at full scale 1 1 in a new large oscillating water tunnel two sets of experiments series a and b were carried out during series a bed forms and wave cycle averaged suspended sediment concentrations were measured under sinusoidal waves series b focused on measurement of the wave cycle averaged sediment transport rates under regular and random asymmetric oscillatory flows upper shoreface conditions and was aimed at the verification of quasi steady formulas for the description of cross shore sediment transport bedform dimensions appeared to decrease considerably under the influence of wave asymmetry and wave randomness only the measured ripple dimensions under asymmetric waves series b showed good agreement with nielsen s 1979 relations for most of the series b experiments the bed was plane sheet flow the net sediment transport was directed onshore and the measured transport rates showed a strong correlation with the velocity moment an empirical quasi steady transport model is proposed which is based on the new tunnel data and other existing data sets the limitations of the quasi steady model approach became clear in the rippled bed regime and through the presence of a consistent influence of the wave period in plane bed conditions in rippled bed conditions the suspended concentration profiles followed a negative exponential distribution for most of the experiments the measured concentration decay length showed a linear relation with the ripple height in plane bed sheet flow conditions the measured suspended concentration profiles followed a negative power function the power or concentration decay parameter was constant 2 1 for a wide range of velocity conditions it is suggested that the mobile bed sheet flow layer has a strong damping effect on the mixing of suspended sediments turbulence damping

mio pleistocene zanda basin biostratigraphy and geochronology pre ice age fauna and mammalian evolution in western himalaya article i nfo the pliocene 5 3 2 6 ma of tibet witnessed the drying of the northern tibetan plateau and the approach to the pleistocene ice age within the background of intensifying indian and east asian monsoons yet little is known about pliocene mammals living on the high tibetan plateau despite the fact that fossil mammals elsewhere consti tute an important knowledge base for terrestrial environments the late miocene to pleistocene zanda basin at the northern foothills of the himalayas affords a welcome opportunity to evaluate the biological response to environ mental change at high elevations abundant well preserved fossil mammals and fish from an 800 m continuous section of fine to coarse grained sediments thus open a rare window into a past biological world for example the discovery of an ancestral wooly rhino from zanda basin that was the precursor of its late pleistocene megafaunal descendants leads to our out of tibet hypothesis suggesting that the high tibetan plateau was a pliocene cradle for ice age cold adaptations in this paper we document in detail the mammalian biostratigraphy chronology and paleozoogeography based on zanda basin fossil mammals our high resolution biostratigraphy and biochronology offer for the first time independent constraints that both support and modify recent magnetostratigraphic correlations using charac teristic pliocene and pleistocene mammals particularly the small mammal assemblages in the lower part of the section and monodactylid equus from the upper section we propose a correlation to c1n to c3an 1r with an age range of 400 ka to 6 4 ma within the 800 m zanda section the lower 0 150 m is of latest miocene age spanning 6 4 5 3 ma sparsely fos siliferous the lower section has produced five taxa so far ochotona panthera qurliqnoria palaeotragus a nd hipparion all are consistent with a late miocene age the middle 150 620 m section spans the entire pliocene this section is by far the most fossiliferous including such typical pliocene small mammals as prosiphneus mimomys apodemus a ndtrischizolagus as well as large mammals such as coelodonta thibetana hipparion zandaense chasmaporthetes nyctereutes meles antilospira and others in the upper 620 800 m section the fos sils are rare but do include characteristic pleistocene taxa such as equus zoogeographically zanda basin mammals are a mixture from two major sources taxa such as mimomys prosiphneus trischizolagus chasmaporthetes nyctereutes meles a ndxenocyon are commonly found in north china or east asia in contrast several forms such as unique species of pikas ochotona squirrels aepyosciurus and ancestral tibetan antelope qurliqnoria seem to belong to an indigenous tibetan fauna evolved within the plateau a lack of shared taxa with the oriental realm suggests a formidable barrierby the himalayas despite a short distance 100 km between zanda basin and the indian subcontinent

pb isotopic variability in the modern pleistocene indus river system measured by ion microprobe in detrital k feldspar grains abstract the western himalaya karakoram and tibet are known to be heterogeneous with regard to pb isotope compositions in k feldspars which allows this system to be used as a sediment provenance tool we used secondary ion mass spectrometry to measure the isotopic character of silt and sand sized grains from the modern sutlej and chenab rivers together with thar desert sands in order to constrain their origin the rivers show a clear himalayan provenance contrasting with grains from the indus suture zone but with overlap to known karakoram compositions the desert dunes commonly show 207 pb 204 pb and 206 pb 204 pb values that are much higher than those seen in the rivers most consistent with erosion from nanga parbat this implies at least some origin from the trunk indus probably reworked by summer monsoon winds from the sw a hypothesis supported by bulk nd and u pb zircon dating further data collected from holocene and pleistocene sands shows that filled and abandoned channels on the western edge of the thar desert were sourced from himalayan rivers before and at 6 8 ka but that after that time the proportion of high isotopic ratio grains rose indicating increased contribution from the thar desert dunes prior to 4 5 ka when flow ceased entirely this may be linked to climatic drying northward expansion of the thar desert or changes in drainage style including regional capture channel abandonment or active local thar tributaries our data further show a himalayan river channel east of the present indus close to the delta in the nara river valley during the middle holocene while this cannot be distinguished from the indus it is not heavily contaminated by reworking from the desert the pb system shows some use as a provenance tool but is not effective at demonstrating whether these nara sediments represent a ghaggar hakra stream independent from the indus our study highlights an important role for eolian reworking of floodplain sediments in arid rivers such as the indus

mid eocene high sr y granites in the northern himalayan gneiss domes melting thickened lower continental crust abstract within the himalayan collisional belt granotoids occur along two sub parallel belts the northern himalayan gneiss domes nhgd and the high himalayan crystalline series hhcs in the yardoi area of nhgd two mica granite a new type granite occurs in the core of the yardoi gneiss dome ygd dala and quedang from north to south and extends at least 50 km long these granites have similar mineral composition elemental and radiogenic isotope geochemistry and age of formation shrimp zircon u pb dating indicates that the yardoi and the quedang two mica granites formed at 42 6 1 1 ma and 42 8 0 6 ma respectively similar to the dala pluton these two mica granites have 1 high sio2 68 wt al2o3 15 wt and a cnk 1 0 2 relatively high sr and lree but low y 40 and up to 250 and la yb 30 4 very weak or no eu anomalies and 5 as compared with those in the himalayan leucogranites low initial sr 87sr 86sr i

features and characterization of graphite in almora crystallines and their implication for the graphite formation in lesser himalaya india layers bands lenses and pockets of graphite are confined to mica schists garnet mica schist micaceous quartzites and black carbonaceous phyllites of the almora group forming graphite prospect in indian lesser himalaya the field and microscopic features of graphite reveal that the graphite in almora group was crystallized from metamorphism of the organic material before himalayan thrusting upto 100 m size nodular and 5 m size circular features points to the involvement of biogenic carbonaceous material in graphitization as also substantiated by 13c values and probably by the presence of v and p associated with this graphite impurities consisting si al mg o fe ti v ca and k are common with graphite these are derived from associated minerals such as muscovite quartz biotite garnet etc raman shifts obtained from this graphite suggest that it is an ordered graphite with its fine dust scattered in the host rocks the calculated degree of graphitization varies from 64 to 89 with no systematic spatial variation in the estimated temperatures cf 485 568 c of graphite formation within this range the temperatures for the intensely folded graphite are higher probably promoted by the himalayan tectonics although high end thermal records reported from the almora group are not evident in graphite however these temperatures obtained from graphite match with host rock thermometry implying that the graphite was formed during regional metamorphism

stable carbon isotope stratigraphy across the permian triassic boundary in shallow marine carbonate platforms nanpanjiang basin south china a distinct negative 13c excursion is documented in two permian triassic sections heping and taiping in shallow marine carbonate platform deposits in the nanpanjiang basin south china these sections span from the changhsingian to the dienerian and are characterized by a distinct marine boundary facies change from massive skeletal lime packstone in the changhsingian to distinctive calcimicrobial framestone in the griesbachian hindeodus parvus zone the 13corg and 13ccarb excursions occur directly after the onset of the calcimicrobial framestone herein termed the permian triassic boundary event and before the first occurrence of h parvus the isotope shifts are associated with a sharp drop in species abundance and diversity and coincide with a decrease in total organic carbon toc content the shift towards depleted values in 13corg and 13ccarb at the permian triassic boundary event together with low toc contents persists throughout the griesbachian h parvus zone these data document a corresponding negative shift of 13corg and 13ccarb values and low toc contents with the onset of growth of calcified microbial framestones a postextinction disaster facies immediately below the base of the griesbachian h parvus zone based on paleontological evidence the first occurrence of the disaster facies follows the extinction event which implies that the 13c depleted values above this facies postdate the event this suggests that two separate events had to account for the initiation of the extinction and the 13c excursion however the consequences that led to the negative isotopic shift might be linked to the intriguing recovery lag of early triassic ecosystems based on data from ptb sections worldwide of a greater 13c offset in high compared with low latitudes we propose that methane eruptions from thermal destabilization of high latitude clathrate deposits may have led to the negative 13c shift and may have caused long term adverse ecological conditions

oligocene initiation of the south tibetan detachment system constraints from syn tectonic leucogranites in the kampa dome northern himalaya abstract himalayan leucogranites provide a robust record of the thermal and tectonic processes that occurred during convergence of the eurasian and indian tectonic plates we present structural petrological mineralogical whole rock geochemical sr nd isotope and geochronological results for cenozoic leucogranites from the kampa dome northern himalaya the kampa leucogranites are typical syn tectonic granites and occur as a series of sills dikes and puddings in the kampa shear zone which is considered to be the exhumed portion of the south tibetan detachment system stds in the northern himalaya the kampa syn tectonic leucogranites provide information about the timing and nature of regional kinematics and record thermal conditions and the initiation of the stds whole rock geochemistry and sr nd isotope analyses show that the kampa leucogranites are characterized by the following 1 they are peraluminous and have high sio2 al2o3 and a cnk low tio2 mgo p2o5 and tfe2o3 and variable cao na2o k2o and total alkalis 2 they are enriched in large ion lithophile elements e g k and rb and depleted in high field strength elements e g zr and ti with negative eu anomalies and 3 they display variable initial 87sr 86sr compositions 0 7507580 826065 and their end i values range between 7 6 and 12 2 anatexis of graywackes and metapelites in the great himalayan crystalline series played an important role in the formation of the kampa leucogranites anatexis was controlled by fluid absent partial melting of muscovite mineralogical and petrological analyses show that zircons belong to an early crystallization phase whereas monazites are remnants of the final crystallization phase of the granitic melt zircon u pb dating reveals that anatexis in the deep himalayan crust occurred at ca 32 23 ma during the oligocene monazites yield homogeneous 208pb 232th ages of ca 23 5 ma and record rapid emplacement in the kampa shear zone these results indicate that movement on the stds was initiated during the early oligocene and became well developed during the late oligocene in the kampa region the oligocene was a unique period of counterbalance between continued plate convergence and intense extension in the himalayan orogen

impact of differential tectonic subsidence on isolated carbonate platform evolution triassic of the nanpanjiang basin south china the nanpanjiang basin of south china contains four exceptionally well exposed isolated triassic carbonate platforms detailed mapping of two dimensional transects and description of stratigraphic sections allow the reconstruction of facies architecture sequence stratigraphy and evolution of the platforms biostratigraphy magnetic susceptibility profiles and volcanic ash horizons allow chronostratigraphic correlation and thus a basinwide evaluation of mechanisms controlling platform evolution a comparison of platform architecture demonstrates that southerly platforms have substantially greater thickness backstepping geometry pinnacle development and earlier drowning that resulted from greater tectonic subsidence proximal to a probable convergent margin along the southern perimeter of the basin felsic volcanics thicken southward and contributed to the termination of the southernmost platform indicating the development of a volcanic arc along the southern margin of the south china tectonic block the northernmost isolated platform had greater longevity and lesser accumulation and lacks backstepping and pinnacle phases of development basin margin intertonguing relationships or lack thereof demonstrate that earlier siliciclastic influx into the basin to the south and concurrent starved basin conditions to the north impacted the evolution of platform margin geometries comparative analysis of platform evolution shows that the timing and rates of tectonic subsidence controlled the timing of platform termination by drowning backstep geometries pinnacle development and overall platform thickness the timing of siliciclastic basin fill dictated differences in platform margin geometries such as slope angle relief above basin floor and the presence or absence of basinward platform progradation despite the dramatic differences in platform architecture eustatic sea level fluctuations imparted a basinwide sequence stratigraphic signal

emplacement of himalayan leucogranites by magma injection along giant sill complexes examples from the cho oyu gyachung kang and everest leucogranites nepal himalaya abstract the upper part of the high himalayan slab in north central nepal is comprised of a thick layer parallel sheet of biotite muscovite tourmaline garnet sillimanite cordierite leucogranite up to 3 4 km thick and dipping north at 5 20 these strongly peraluminous magmas were emplaced into high temperature low pressure sillimanite and cordierite bearing gneisses calc silicates and rare amphibolites which were metamorphosed at temperatures of 600 650 c some time during the oligocene early miocene parallel stringers of black xenolithic gneisses within the leucogranites suggest passive magmatic intrusion along fractures parallel to the schistosity in the country rocks in the mountains of cho oyu gyachung kang pumori lingtren and the base of the everest massif these leucogranites form part of a single structural horizon bounded at the top by the lhotse detachment the lower of two n dipping normal faults of the south tibetan detachment std system and below by the khumbu thrust kt an out of sequence fault which was partly responsible for the uplift erosion and exhumation of the leucogranites a model for the emplacement of these leucogranites is proposed where they represent viscous minimum melts produced by melting of a pelitic protolith similar to the underlying sillimanite grade gneisses through muscovite breakdown either during fluid absent melting at

the significance of himalayan rivers for silicate weathering rates evidence from the bhote kosi tributary the significance of weathering by himalayan runoff for both the sr isotope marine record and the removal of atmospheric co2 through silicate dissolution has been examined by systematic sampling of dissolved loads and bedloads from the bhote kosi a tributary of the ganges that rises in tibet from tethyan sediment bedrock and traverses the major himalayan lithologies of eastern nepal before debouching onto the gangetic plains throughout the section the cation geochemistry of water samples is dominated by ca and mg ions suggesting that carbonates are the predominant lithology undergoing dissolution particularly within the lesser himalayas as the river transects the metasedimentary and granitic lithologies of the high himalayas the sr isotope ratio of the bedload rises rapidly closely reflecting the isotope geochemistry of the bedrock in contrast the 87sr 86sr ratio of the dissolved load remains roughly constant 0 719 0 723 downstream of the main central thrust where the river transects the carbonate bearing lithologies of the lesser himalayas the 87sr 86sr ratio of the dissolved load rises sharply 0 768 the relative contributions of silicate and carbonate weathering from each of the main himalayan units has been estimated from major cation sr concentration and sr isotope mass balance equations these calculations suggest that the high sr and high 87sr 86sr characteristics of riverine analyses arise initially from a component dissolved from the tibetan sedimentary series which is substantially enhanced by input weathering fluxes particularly as the river traverses the lesser himalayas whilst mechanical erosion is maximised within the high himalayan crystalline series as confirmed by 143nd 144nd ratios from the bedload at least 63 of the dissolved load is acquired by chemical weathering of bedrock lithologies and or of transported particulates within the lesser himalayas enhanced by higher ambient temperatures and slower discharge rates this may involve continued dissolution of the high himalayan crystalline series particulates in addition to lesser himalayan lithologies although himalayan rivers collectively have a major influence on the sr isotope marine record the high 87sr 86sr ratios of their dissolved load results from the mixing of a small component 90 of carbonate derived material some of which is characterised by a high 87sr 86sr ratio up to 0 8 elevated 87sr 86sr ratios in rivers are therefore not necessarily indicative of anomalously high dissolution rates of silicates

structure of the main central thrust zone and extrusion of the high himalayan deep crustal wedge kishtwar zanskar himalaya the main central thrust is a crustal scale ductile shear zone between 1 5 and 3 km wide which places the oligocene miocene metamorphic rocks of the high himalayan zone south or sw over the unmetamorphosed or weakly metamorphosed rocks of the lesser himalaya the high strain zone of the main central thrust is coincident with an inverted metamorphic field gradient from biotite to kyanite grade over a structural thickness of 1500 m kyanite grade rocks metamorphosed at 9 5 10 kbar were exhumed from depths of 33 37 km along the main central thrust hanging wall and emplaced over lesser himalayan rocks never buried deeper than 10 12 km exhumation of the deepest buried kyanite grade rocks occurred along the zone above the main central thrust zone approximately 45 km width 28 km structural thickness of the high himalaya exposes sillimanite grade gneisses migmatites and 19 5 21 5 ma old leucogranites formed at pressures between 4 5 and 7 kbar and depths of 16 25 km a ne dipping normal fault ductile shear zone at the top of the slab zanskar shear zone shows condensed but right way up isograds from sillimanite to chlorite grade over 2 400 metres structural thickness 40 ar 39 ar geochronology shows that most of the high himalayan slab had cooled below 350 c by 16 ma supporting models linking the two bounding faults of the high himalaya both kinematically and temporally there is no evidence of melting along the main central thrust zone and the himalayan leucogranites were generated 10 30 km structurally above the main central thrust frictional heating along the main central thrust could not therefore have played any role in generating the leucogranites thus far there is little evidence for late miocene reactivation along the main central thrust as seen elsewhere along the main central thrust in nepal

structural and geochronological evidence for the leading edge of the greater himalayan crystalline complex in the central nepal himalaya abstract the himalaya is commonly described as a three layer two fault stack namely a high grade crystalline core featuring an inverted metamorphic field gradient the greater himalayan crystalline complex ghc is separated from units above and below by shear zones the lesser himalayan sequence lhs underlies the ghc below the main central thrust and the tethyan himalayan sequence ths overlies it along the south tibet detachment however the southern main central thrust hanging wall consists of a lower unit dominated by a right way up metamorphic sequence of biotite garnet schists bhimphedi group and an upper unit with only anchizone metamorphism pulchauki group the bhimphedi group is commonly equated to the ghc while the pulchauki group is well correlated to the ths however no shear zone separates these units we present new structural and geochronological data along the boundary between the ghc and the bhimphedi group in the kathmandu region these data reveal an 200 m thick early middle miocene top north northeast shear zone that we term the galchi shear zone we correlate this shear zone to the south tibet detachment on the basis of lithological metamorphic structural and chronological criteria the galchi shear zone merges with the main central thrust to the south bounding the leading edge of the ghc this result combined with recent work in the western himalaya suggests that the locally preserved leading edge of the ghc is sub parallel to the arc of the orogen and the southern main central thrust hanging wall is dominated by ths rocks this orogenic architecture rules out wedge extrusion and channel flow focused denudation kinematic models for the himalayan orogen but is accommodated by tectonic wedging kinematic models including channel tunneling models with modified timing

acoustic velocity profiler for laboratory and field studies a monostatic acoustic doppler velocity profiler advp for use in hydraulic research is presented including signal treatment algorithms the 1 mhz advp used in the present study has an accuracy better than 2 mms s 1 with a range of up to 100 cm for velocities up to 290 cms s 1 and a resolution of 256 points in the profile results of advp measurements in uniform flow in two laboratory open channels with smooth and gravel beds compare favorably with established distribution laws for mean velocity variance and reynold s stress indicating that the turbulence scales are sufficiently resolved by the instrument measurements with the same instrument in a river and a lake illustrate that it is capable of producing excellent results in the field as well it was demonstrated that unaffected by water quality the instrument is a good research tool for nonintrusive and accurate instantaneous profile measurements under turbulent flow conditions