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

petrology of high pressure granulites from the eastern himalayan syntaxis the eastern himalayan syntaxis situated at the eastern terminus of the himalayas is the least known segment of the himalayas recent research in this area has revealed that the syntaxis consists of the gangdise the yarlung zangbo and the himalayan units each of which is bounded by faults the himalayan unit the northernmost exposed part of the indian plate mainly contains amphibolite facies rocks marked by the assemblages staurolite kyanite plagioclase biotite muscovite sillimanite and garnet amphibole plagioclase in the south to the north low to medium pressure granulite grade pelitic gneisses and marbles are present and are characterized by the assemblages garnet sillimanite k feldspar plagioclase or antiperthite biotite quartz spinel cordierite orthopyroxene in gneisses and anorthite diopside wollastonite and plagioclase diopside quartz phlogopite calcite in marbles within this unit the namula thrust system is a series of moderately north dipping structures that displaced the granulite facies rocks southwards over the amphibolite facies rocks high pressure granulites occur as relics within these granulite facies rocks and contain garnet kyanite granulite and garnet clinopyroxenite the peak assemblage of the garnet kyanite granulite includes garnet core part kyanite ternary feldspar quartz rutile sillimanite garnet rim part k feldspar oligoclase ilmenite biotite and spinel albite biotite or spinel cordierite orthopyroxene which are coronas around sillimanite and garnet are retrograde products of this peak assemblage another peak assemblage includes very high ca garnet cao 32 34 wt alm10 grs 80 and diopside cao 22 24 wt scapolite meionite quartz and accessory al bearing titanite al2o3 4 4 5 wt the diopside has kink bands partial or complete breakdown of ca rich garnet during post peak metamorphism produced pseudomorphs and coronas consisting of fine grained symplectic intergrowths of hedenbergite and anorthite thermobarometric estimates in combination with reaction textures mineral compositions and recent experimental studies indicate that these peak assemblages were formed at p c 1 7 1 8 gpa t c 890 c and the retrograde assemblages experienced near isothermal decompression to p 0 5 0 1 gpa t 850 50 c the whole rock compositions indicate that marble and pelite are plausible candidates for the protoliths these facts suggest the following 1 sedimentary rocks were transported to upper mantle depths and equilibrated at those conditions to form these high pressure granulites which were then emplaced into the crust quickly during the rapid exhumation of these rocks the earlier high pressure assemblages were overprinted by the later low to medium pressure assemblages that is the high pressure granulite belt formed in the syntaxis 2 the namula thrust system is an important tectonic boundary in the syntaxis or even in the higher himalaya more generally

tethyan and indian subduction viewed from the himalayan high to ultrahigh pressure metamorphic rocks abstract the himalayan range is one of the best documented continent collisional belts and provides a natural laboratory for studying subduction processes high pressure and ultrahigh pressure rocks with origins in a variety of protoliths occur in various settings accretionary wedge oceanic subduction zone subducted continental margin and continental collisional zone ages and locations of these high pressure and ultrahigh pressure rocks along the himalayan belt allow us to evaluate the evolution of this major convergent zone 1 cretaceous 80 100 ma blueschists and possibly amphibolites in the indus tsangpo suture zone represent an accretionary wedge developed during the northward subduction of the tethys ocean beneath the asian margin their exhumation occurred during the subduction of the tethys prior to the collision between the indian and asian continents 2 eclogitic rocks with unknown age are reported at one location in the indus tsangpo suture zone east of the nanga parbat syntaxis they may represent subducted tethyan oceanic lithosphere 3 ultrahigh pressure rocks on both sides of the western syntaxis kaghan and tso morari massifs formed during the early stage of subduction exhumation of the indian northern margin at the time of the paleocene eocene boundary 4 granulitized eclogites in the lesser himalaya sequence in southern tibet formed during the paleogene underthrusting of the indian margin beneath southern tibet and were exhumed in the miocene these metamorphic rocks provide important constraints on the geometry and evolution of the india asia convergent zone during the closure of the tethys ocean the timing of the ultrahigh pressure metamorphism in the tso morari massif indicates that the initial contact between the indian and asian continents likely occurred in the western syntaxis at 57 1 ma west of the western syntaxis the higher himalayan crystallines were thinned rocks equivalent to the lesser himalayan sequence are present north of the main central thrust moreover the pressure metamorphism in the kaghan massif in the western part of the syntaxis took place later 7 m y after the metamorphism in the eastern part suggesting that the geometry of the initial contact between the indian and asian continents was not linear the northern edge of the indian continent in the western part was 300 to 350 km farther south than the area east of the nanga parbat syntaxis such en baionnette geometry is probably produced by north trending transform faults that initially formed during the late paleozoic to cretaceous gondwana rifting farther east in the southern tibet the collision occurred before 50 6 0 2 ma finally high pressure to ultrahigh pressure rocks in the western himalaya formed and exhumed in steep subduction compared to what is now shown in tomographic images and seismologic data

metamorphic response to collision in the central himalayan orogen abstract closure of the neotethys ocean and high angle continent continent collision between india and asia after about 55 ma resulted in low angle subduction of the indian plate below the tibetan plateau and by 30 ma established an arcuate 2300 km long shallow north dipping metamorphic fold thrust belt in the foreland this himalayan metamorphic front quickly established an upper plate lower plate paired metamorphic architecture centred on the median high himal thrust that largely controlled subsequent evolution the upper plate is a thick slab of high t moderate p high grade migmatized metamorphic rocks whereas the lower plate is an inverted series of moderate t high p schists in two crustal wedges the broad main central thrust zone and at lowest structural levels the footwall below the basal main central thrust spatial and temporal patterns of metamorphic response in the evolving himalayan metamorphic front has been characterized in a large scale integrated structural metamorphic study based on 8 profiles across eastern nepal metamorphic response at all structural levels is established using a large dataset n 160 of internally consistent quantitative pt determinations petrology of metapelite samples semi quantitative p t paths metamorphic mapping and metamorphic field gradients these results are integrated with previously published metamorphic studies structural profiles and metamorphic chronology from these datasets the architecture and evolution of the himalayan metamorphic front is constrained by rock kinematics metamorphic field gradients showing discontinuities and diachronous metamorphism with contrasting p t evolutions at different structural levels each of the three panels constituting the himalayan metamorphic front upper plate main central thrust zone and footwall experienced distinctly different tectono metamorphic histories crustal processes operating during metamorphism and exhumation differ between the upper and lower plates the upper plate experienced long lived metamorphism starting from at least 28 38 ma and tracking low p t clockwise p t paths that culminated at 19 27 ma in peak high grade condition with 27 31 c km thermal regimes protracted high grade conditions produced significant partial melt which facilitated gravity driven southward extrusion involving internal ductile flow processes southward extrusion of the upper plate was accommodated by coeval reverse movement on the high himal thrust and top down to the north normal reactivation of the south tibet detachment system between 22 10 ma transport of this thick slab to the south resulted in further prograde burial of the lower plate below culminating in peak metamorphism at the highest pressures attained in the lower plate rocks the lower plate consists of at least two deeply buried 7 5 9 4 kb crustal wedges both of which experienced steep p t hairpin clockwise p t paths with isothermal decompression the main central thrust zone experienced 20 26 c km metamorphism at 12 22 ma and was exhumed along the basal main central thrust after 11 ma whereas loading by the main central thrust zone gave rise to further prograde burial of the footwall which experienced 16 21 c km metamorphism at 4 10 ma and was exhumed along the main boundary thrust at 3 9 ma in contrast to the upper plate both crustal wedges were exhumed by a process of out wedging which involved upthrusting along basal and internal thrusts with coeval extensional slip in the hanging walls this three stage foreland propagating lateral exhumation history resulted in telescoping of the himalayan metamorphic front in concert with peak metamorphic events at different structural levels during main phase orogenesis and is not the result of superimposed retrograde reactivation of the belt

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

u pb zircon geochronology of coesite bearing eclogites from the southern dulan area of the north qaidam uhp terrane northwestern china spatially and temporally extensive uhp metamorphism during continental subduction coesite bearing eclogites from 100 km 2 in the southern dulan area north qaidam mountains nqm of western china contain zircon that records protolith crystallization and ultra high pressure uhp metamorphism sensitive high resolution ion microprobe mass spectrometer and laser ablation inductively coupled plasma mass spectrometry u pb analyses from cathodoluminescence cl dark zircon cores in a coesite bearing eclogite yield an upper intercept age of 838 50 ma and oscillatory zoned cores in a kyanite bearing eclogite gave a weighted mean 206 pb 238 u age of 832 20 ma these zircon cores yield steep heavy rare earth element hree slopes and negative eu anomalies that suggest a magmatic origin thus c 835 ma is interpreted as the eclogite protolith age unzoned cl grey or bright zircon and zircon rims from four samples yield weighted mean ages of 430 4 438 2 446 10 and 446 3 ma flat hree patterns without eu anomalies and contain inclusions of garnet omphacite rutile phengite and rare coesite these ages are interpreted to record 16 5 myr of uhp metamorphism these new uhp ages overlap the age range of both eclogite and paragneiss from the northern dulan area suggesting that all uhp rock types in the dulan area belong to the same tectonic unit our results are consistent with slow continental subduction but do not match oceanic subduction and diapiric exhumation uhp model predictions these new data suggest that similar to eclogites in other hp uhp units of the nqm and south altyn tagh protoliths of the eclogites in the dulan area formed in a continental setting during the neoproterozoic and then subducted to mantle depth together with continental materials during the early palaeozoic

thermal evolution of the gangdese batholith southern tibet a history of episodic unroofing the gangdese batholith southern tibet was part of an andean type arc at the southern margin of asia prior to the collision of india and asia at approximately 50 to 40 ma fission track and 40ar 39ar analyses of 28 rocks from 10 gangdese granitoid plutons along an 250 km length of the batholith in the lhasa region provide a detailed understanding of the age and the postcrystallization erosional and tectonic history of these rocks these data suggest a range of ages for these plutons of 94 to 42 ma with the majority being of tertiary age the postcrystallization cooling histories of all of these plutons are characterized by marked discontinuities we conclude that most of these discontinuities and all of them after 40 ma reflect tectonic changes that produced brief pulses of rapid erosion which were distributed in both space and time in addition to the initial cooling of hot magma against cold country rock all of the rocks we studied showed evidence for at least one subsequent episode of rapid cooling dropping many tens of degrees in a few million years conversely these plutons all experienced intervals during which they cooled very slowly or not at all these slow cooling intervals lasted from 5 to 50 million years our data indicate that since the collision between india and asia began response to continued convergence has been quite variable in even this relatively small area the data reported here are consistent with a recently proposed model of oligo miocene crustal shortening along the gangdese thrust system in this area

metamorphic p t path and sims zircon u pb dating of amphibolite of the namche barwa complex southeast tibet china abstract amphibolite from the core and the outer part of the namche barwa complex nbc southeast tibet experienced upper amphibolite facies metamorphism and records clockwise metamorphic p t paths prograde m1 peak m2 and retrograde m3 metamorphic mineral assemblages were identified in both amphibolite types the m2 p t conditions in the core and outer part of the nbc are similar zhibai amphibolite zba from the core 750 c 10 kbar paixiang amphibolite pxa from the outer part 670 750 c 8 5 10 5 kbar sims zircon u pb dating suggests that the protolith of pxa the outer part of the nbc formed at 90 80 ma similar to the mafic dykes within the tethyan himalaya this magmatism is probably related to tectonic events on the northern indian passive margin before its collision with the eurasian plate furthermore zircon records metamorphism from oligocene to late miocene 30 ma to 10 ma in the zba while pxa only documents a miocene metamorphic event 9 ma it is inferred that from oligocene to late miocene semi continuous advection of relatively hot material from the depth might have operated only in nbc core thus exhuming the zba to a shallower level while this material did not efficiently reach affect the pxa at late miocene 10 ma rapid exhumation started to propagate to the outer part of nbc and both units were amalgamated as a coherent block

exact timing of granulite metamorphism in the namche barwa eastern himalayan syntaxis new constrains from sims u pb zircon age zircon grains separated from 2 granulites from the eastern himalaya were investigated by raman spectroscopy cathodoluminescence imaging and secondary ion mass spectrometry these grains have a thin homogeneous rim and an oscillatory inner zone domain with or without a relict inherited core garnet kyanite and rutile inclusions were identified within only the rim domain of zircon grains indicating that the rim had formed during peak granulite facies metamorphism u pb zircon data record three distinct age populations 1 805 ma for the inherited core ca 500 ma oscillatory inner zone as well as 24 25 ma and ca 18 ma for the metamorphic rim these new precision ages suggest that the peak metamorphic age for the hp granulite is at ca 24 25 ma and subsequent amphibolite facies retrograde metamorphism occurred at ca 18 ma

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

erosion of finite thickness sediment beds by single and multiple circular jets sediment management in reservoirs with the help of water jets has motivated this work erosion caused by single and multiple submerged circular turbulent wall jets on a noncohesive sediment bed of finite thickness lying on a fixed boundary was studied with the help of laboratory experiments different combinations of jet diameter jet separation and sediment thickness to jet diameter ratio were tested results show a relation between dimensionless parameters characterizing the steady state bed profile and the densimetric particle froude number f0 given by the velocity at the nozzle and the effective diameter and submerged specific density of the sediment evolution of scour with time confirms previous studies where the erosion was found to initially grow with the logarithm of time up to a certain reference time t this time made dimensionless with a time scale tc involving the volume of sediment scoured and the rate of erosion was also related to the densimetric froude number a comparison with studi

geochronology and geochemistry of mesoproterozoic granitoids in the lhasa terrane south tibet implications for the early evolution of lhasa terrane abstract the early history of the lhasa terrane remains poorly constrained due to the poor exposure of the proterozoic rocks we report here u pb zircon ages geochemical and hf isotopic data for granite gneisses and biotite gneisses from the bomi complex in the eastern part of the lhasa terrane south tibet petrological and geochemical data suggest that the protoliths of the granite gneisses and the biotite gneisses could be granites and tonalites respectively la icpms u pb zircon analyses yielded ages of 1343 27 ma mswd 0 3 and 1276 22 ma mswd 0 4 for two granite gneisses and a consistent age of ca 1250 ma for two biotite gneisses these ages are interpreted as the magma crystallization time of both the gneisses protoliths and thus the bomi complex represents the oldest rocks found in the lhasa terrane our data indicate that the mesoproterozoic detrital zircons from the paleozoic metasedimentary rocks in the lhasa terrane could be derived from the lhasa terrane itself or the tethyan himalaya rather than necessarily from the albany fraser belt in the australia geochemical characteristics show that the granite gneisses have an aluminous a type granite affinity the two granite gneisses dated in this study have zircon hf t values between 4 0 and 1 8 and between 2 6 and 0 2 respectively they have identical two stage hf model ages of 2 0 ga we suggest that the protoliths of the granite gneisses were produced by protracted high temperature partial melting of a felsic intracrustal source in an extensional setting in contrast the biotite gneisses have similar geochemical characteristics to those of calc alkaline granitoids that probably formed in a subduction related environment zircons from the two dated biotite gneisses have relatively higher hf t values of 8 1 to 3 6 and 10 5 to 5 7 respectively indicating a juvenile mantle contribution to their magma source earlier magmatism at 1343 1276 ma may formed in a continental rift setting related to the final breakup of supercontinent columbia while subsequent magmatism of 1250 ma resulted from subduction of ocean slab during the assemblage of rodinia we thus infer that the bomi complex was related to the contact zone between the eastern ghats belt and the archean cratons in southeastern india during the mesoproterozoic

himalayan metamorphism and its tectonic implications the himalayan range exposes a spectacular assemblage of metamorphic rocks from the mid and deep crust that have fostered numerous models of how the crust responds to continental collisions recent petrogenetically based petrologic and geochronologic studies elucidate processes with unprecedented resolution and critically test models that range from continuum processes to one time events the pronounced metamorphic inversion across the main central thrust reflects continuum thrusting between ca 15 and 20 ma whereas exposure of ultrahigh pressure rocks in northwestern massifs and syntaxis granulites reflects singular early 45 ma and late 10 ma exhumation events multiple mechanisms including wedge collapse and flow of melt weakened midcrust are debated to explain pressure temperature trajectories patterns of thinning and thermal overprinting a geochronologic revolution is under way in which spatially resolved compositions and ages of accessory minerals are combined in a petrogenetically valid co

rapid eocene erosion sedimentation and burial in the eastern himalayan syntaxis and its geodynamic significance article i nfo the lower bomi group of the eastern himalayan syntaxis comprises a lithological package of sedimentary and igneous rocks that have been metamorphosed to upper amphibolite facies conditions the lower bomi group is bounded to the south by the indus yarlung suture and to the north by unmetamorphosed paleozoic sediments of the lhasa terrane we report u pb zircon dating geochemistry and petrography of gneiss migmatite mica schist and marble from the lower bomi group and explore their geological implications for the tectonic evolution of the eastern himalaya zircons from the lower bomi group are composite the inherited magmatic zircon cores display 206 pb 238 u ages from 74 ma to 41 5 ma indicating a probable source from the gangdese magmatic arc the metamorphic overgrowth zircons yielded 206 pb 238 u ages rang ing from 38 ma to 23 ma that overlap the anatexis time 37 ma recorded in the leucosome of the migmatites our data indicate that the lower bomi group do not represent precambrian basement of the lhasa terrane instead the lower bomi group may represent sedimentary and igneous rocks of the residual forearc basin similar to the tsojiangding group in the xigaze area derived from denudation of the hanging wall rocks during the india asia continental collision we propose that following the indian asian collision the forearc basin was subducted together with himalayan lithologies from the indian continental slab the minimum age of detrital magmatic zircons from the supracrustal rocks is 41 5 ma and their metamorphism had happened at 37 ma the short time interval b5 ma suggests that the tectonic processes associated with the eastern himalayan syntaxis encompassing uplift and erosion of the gangdese terrane followed by deposition imbrication and subduction of the forearc basin were extremely rapid during the late eocene 2012 international association for gondwana research published by elsevier b v all rights reserved

exhumation during crustal folding in the namche barwa syntaxis geological observation in the eastern end of the himalaya shows that the asia india suture is folded metamorphic rocks derived from india occur structurally below the suture in the core of a regional antiform isotopic and fission track dating establish cooling exhumation of rocks from c 30 km depth within the last 4 myr we argue that exhumation is caused by 10 mm yr 1 erosion coeval with crustal scale folding

tectono metamorphic evolution of the karakorum metamorphic complex dassu askole area ne pakistan exhumation of mid crustal ht mp gneisses in a convergent context the south karakorum margin east of the himalayan syntaxis consist of an e w elongated zone of young 10 3 ma high grade metamorphic rocks m2 and related migmatitic domes this late tectono metamorphic event post dates the palaeogene 55 37 ma phase of thickening of the belt featured by nw se structures and associated m1 amphibolite facies metamorphism 0 7 gpa 700 c this m2 metamorphism is characterised by low pressure high temperature conditions coeval with migmatite formation in response to a thermal increase of c 150 c compared to m1 culminating at a temperature of c 770 c and a pressure of 0 5 0 6 gpa rapid exhumation of migmatitic domes at a rate of 5 mm yr 1 was accommodated by vertical extrusion in the core of e w crustal scale folds these crustal scale folds formed in response to n s syn collisional shortening and were enhanced by thermal weakening of the migmatised continental crust m2 metamorphism is spatially and temporarily associated with granitoids showing a mantle affinity firmly suggesting that this could be the advective heat source for the granite and syenite generation and the subsequent migmatisation of the mid crustal level such relationships between a mantle related magmatism and a high temperature metamorphism in a convergent shortening context are suggestive of the breakoff of the subducted indian slab since 20 ma

a tectonic model reconciling evidence for the collisions between india eurasia and intra oceanic arcs of the central eastern tethys abstract despite several decades of investigations inferences on the timing and nature of collisions along the mesozoic cenozoic eurasian margin remain controversial we assimilate geological and geophysical evidence into a plate tectonic model for the india eurasia collision that includes continuously closing topological plate polygons constructed from a time dependent network of evolving plate boundaries with synthetic plates constructed for now subducted ocean floor including back arc basins that formed on the southern eurasian margin our model is regionally constrained and self consistent incorporating geophysical data from abyssal plains offshore west australia and east antarctica including jurassic age data from offshore northwest australia limiting much of northern greater india to a 1000 km long indenter originally reaching to the wallaby zenith fracture zone southern eurasia and southeast asia are riddled with dismembered oceanic arcs indicating long lived tethyan intra oceanic subduction this intra oceanic subduction system was well established from cretaceous time in the india eurasia convergence zone in the neotethys which was consumed during greater india s northward trajectory towards eurasia from the early cretaceous fragments of obducted oceanic crust within the yarlung tsangpo suture zone between india and eurasia predominantly date to the late jurassic or mid cretaceous barremian aptian the various ophiolites along strike and a hiatus in subduction related magmatism during the tithonian aptian suggest that there was at least one generation of intra oceanic arc formation whose plate boundary configuration remains uncertain paleomagnetic and magmatic studies suggest that the intra oceanic arc was at equatorial latitudes during the early cretaceous before subduction resumed further north beneath the eurasian margin lhasa terrane with another hiatus in subduction related magmatism along southern lhasa during 80 65 ma possibly as the back arc spreading centre approached the active andean style margin in our model greater india collided with the tethyan intra oceanic arc in paleocene eocene time finally closing the tethyan seaway from mid late eocene time which is consistent with the age of the youngest marine deposits found between india and eurasia geological evidence from the collision zone indicates an age of initial arc continent collision by 52 ma followed by the soft initial continent continent collision between india and eurasia by 44 2 ma this timing is supported by marine geophysical data where the spreading centres in the indian ocean record a drastic decline in seafloor spreading rates and changes in spreading directions first at 52 ma followed by another reorganisation at 43 ma the abandonment of spreading in the wharton basin and the onset of extrusion tectonics in asia by 36 ma are likely indicators of hard complete continent collision and highlight the multi stage collisional history of this margin our continuously evolving network of mid ocean ridge and subduction zone geometries and divergence convergence vectors through time provide a basis for future refinements to assimilate new data and or test alternative tectonic scenarios

sm nd and trace element characterization of shales from the abitibi belt labrador trough and appalachian belt consequences for crustal evolution through time nd isotopic compositions and sm nd li k rb sr ba ni and cr abundances are reported for 25 shale samples from the canadian shield late archean abitibi greenstone belt and the mid proterozoi

clear water local scour by submerged three dimensional wall jets effect of tailwater depth the present study deals with scour caused by three dimensional jets issuing from a square cross section nozzle onto a noncohesive sand bed experiments were conducted using two different nozzles and three tailwater depths the densimetric froude number was maintained below ten while the jet expansion ratio was held greater than ten the results indicate that the densimetric froude number tailwater depth and grain size to nozzle width ratio all have an influence on the extent of scour however each parameter has a dominant influence at different flow conditions for example at values of densimetric froude numbers less than five nondimensional tailwater depth and grain size to nozzle width ratio have no effect on the maximum depth of scour at higher densimetric froude numbers the effect of tailwater depth appears to be important at larger values of grain size to nozzle width ratio a set of scaling parameters based on nozzle hydraulic radius grain size and densimetric froude number provides for a better scaling of the time variation of the scour parameters

the onset of india asia continental collision early steep subduction required by the timing of uhp metamorphism in the western himalaya ultrahigh pressure uhp rocks in the nw himalaya are some of the youngest on earth and allow testing of critical questions of uhp formation and exhumation and the timing of the india asia collision initial collision of india with asia is widely cited as being at 55f1 ma based on a paleomagnetically determined slowdown of india s plate velocity and as being at ca 51 ma based on the termination of marine carbonate deposition even relatively small changes in this collision age force large changes in tectonic reconstructions because of the rapid india asia convergence rate of 134 mm a at the time of collision new u pb shrimp dating of zircon shows that indian rocks of the tso morari complex reached uhp depths at 53 3f0 7 ma given the high rate of indian subduction this dating implies that indian continental crust arrived at the asian trench no later than 57f1 ma providing a metamorphic age for comparison with previous paleomagnetic and stratigraphic estimates india s collision with asia may be compared to modern processes in the timor region in which initiation of collision precedes both the slowing of the convergence rate and the termination of marine carbonate deposition the indian uhp rocks must have traveled rapidly along a short hence steep path into the mantle early continental subduction was at a steep angle probably vertical comparable to modern continental subduction in the hindu kush despite evidence for modern day low angle subduction of india beneath tibet oceanic slab break off likely coincided with exhumation of uhp terranes in the western himalaya and led to the initiation of low angle subduction and leucogranite generation d 2005 elsevier b v all rights reserved

u pb zircon constraints on the tectonic evolution of southeastern tibet namche barwa area the eastern syntaxis of the himalayas is expressed in the crust as a pronounced southward bend in the orogen the change in strike of geologic features coincides with the high topography of the namche barwa region the exposure of granulite grade metamorphic rocks and a 180 degree bend in the yalu tsangpo we have conducted a geochronologic and geochemical investigation of several suites of granitoids collected from the namche barwa massif and subjacent terranes of southeastern tibet ranging from cm scale dikes and sills to larger outcrop scale intrusions u pb shrimp rg zircon ages establish at least five magmatic episodes 400 to 500 ma 120 ma 40 to 70 ma 18 to 25 ma and 3 to 10 ma these episodes broadly correlate to spatial patterns in sample localities as follows 400 to 500 ma ages occur in zircon cores collected from within the massif proper 120 ma granites related to early gangdese arc plutonism are primarily located northeast of namche barwa later 40 70 ma gangdese activity is expressed in granites west of namche barwa 18 to 25 ma granites occur both along the suture zone west of gyala peri and directly north of namche barwa along the area of the jiali fault zone and are attributed both to shearing within the jiali fault zone and to an early miocene gangdese thrust event exceptionally young 10 ma zircon ages are clustered near the core of the massif along the yalu tsangpo gorge trace element geochemical data indicates the presence of both fluid present and fluid absent melts with a fluid absent decompression melting regime dominating near the core of namche barwa

two stages of granulite facies metamorphism in the eastern himalayan syntaxis south tibet petrology zircon geochronology and implications for the subduction of neo tethys and the indian continent beneath asia the eastern himalayan syntaxis in southeastern tibet consists of the lhasa terrane high himalayan rocks and indus tsangpo suture zone the lhasa terrane constitutes the hangingwall of a subduction zone whereas the high himalayan rocks represent the subducted indian continent our petrological and geochronological data reveal that the lhasa terrane has undergone two stages of medium p metamorphism an early granulite facies event at c 90 ma and a late amphibolite facies event at 36 33 ma however the high himalayan rocks experienced only a single high p granulite facies metamorphic event at 37 32 ma it is inferred that the late cretaceous c 90 ma medium p metamorphism of the southern lhasa terrane resulted from a northward subduction of the neo tethyan ocean and that the oligocene 37 32 ma high p 1 8 1 4 gpa rocks of the high himalayan and coeval medium p 0 8 1 1 gpa rocks of the lhasa terrane represent paired metamorphic belts that resulted from the northward subduction of the indian continent beneath asia our results provide robust constraints on the mesozoic and cenozoic tectonic evolution of south tibet

precise determination of phanerozoic zircon pb pb age by multicollector sims without external standardization 1 zircon has long been recognized as the best geochronometer and the most important timekeeper in geosciences modern microbeam techniques such as sims and la icpms have been successfully applied to in situ u pb zircon age determinations at spatial resolutions of 20 30 m or less matrix matched calibration by external standardization of well characterized natural zircon references is a principal requirement for precise microbeam u pb zircon age determination due to fractionation effects between pb and u which usually result in an external age error exceeding 1 alternatively zircons with a closed u pb system can be directly dated by measurement of 207pb 206pb isotopic ratio without external standardization which has been a common practice for zircons older than 1 0 ga but not for relatively young 1 0 ga and particularly phanerozoic ones because of limitations of analytical precision we describe in this paper a method of 207pb 206pb measurement on phanerozoic zircons using a new generation of large radius magnetic sector multicollector cameca ims 1280 sims in combination with multicollector mode a nuclear magnetic resonance nmr magnet controller and oxygen flooding techniques we achieve precisions of 207pb 206pb ratio of 0 1 and 0 1 0 2 propagating to pb pb age errors 0 4 and 1 3 excluding u decay constant uncertainties for zircons of latest neoproterozoic and late paleozoic to mesozoic age respectively therefore the multicollector sims is capable of direct determination of zircon pb pb ages as young as mesozoic age with uncertainties of geological significance this technique is useful for direct dating of zircons in thin sections moreover it has significance for dating of some other u rich minerals i e baddeleyite and zirconolite that are not suitable for sims u pb dating by external standardization

initial geometry of western himalaya and ultrahigh pressure metamorphic evolution ultrahigh pressure metamorphic rocks on both sides of the western himalayan syntaxis show different p t t paths the kaghan unit was metamorphosed under the uhp conditions significantly later 46 ma than the tso morari unit 53 ma implying that the tso morari was subducted earlier 57 ma than the kaghan unit 52 ma the age difference likely reflects the initial shape of greater india with the kaghan unit located greater than 300 km south of the tso morari before the collision of two continents we calculate the dip of the subducting plate using two independent methods the results show gentle dipping subduction east of the western syntaxis and steep subduction west of the syntaxis since the time of india eurasia collision to the present time we propose that the steep subduction in the western part is likely related to the proto chaman and karakorum faults along which the indian plate moved northward in the eastern part the overlying eurasian plate extruded to east which allowed gentle dipping subduction of the indian continent although the main period of eastward extrusion of the eurasian continent occurred between 30 and 15 ma our results suggest that this was likely taking place since the early india asia collision using those geometrical constraints a 3d image of the slab is reconstructed in the western part showing the sharp bending of the western syntaxis along the proto chaman fault this bending resulted in the warping of the slab surface to form a conical fold with a north dipping axis located near the western syntaxis 2007 published by elsevier ltd

contrasting cw and ccw tectono metamorphic belts in the eastern himalayan syntaxis quantification of p t t paths and tectonic interpretation abstract in the eastern himalayan syntaxis the indus tsangpo suture zone separates the himalaya to the south from the southern lhasa terrane to the north in this study we combine petrology zircon u pb geochronology and phase equilibrium modeling to quantify the p t t paths of metapelites from the namche barwa complex part of the himalaya and the nyingchi complex part of the southern lhasa terrane the namche barwa complex is interpreted to record slow cooling across a hp granulite facies p t field at 12 kbar from 790 c to the solidus at 750 c between c 50 ma and c 17 ma followed by decompression with slight heating to a p t field at 5 5 8 kbar and 790 825 c at c 3 ma consistent with the post peak segment of a cw p t t path by contrast the nyingchi complex records a ccw p t t path characterized by compression through peak granulite facies p t conditions of 7 5 9 kbar at 760 780 c before re crossing the solidus at p of 11 kbar at c 35 31 ma based on the youngest published spot date from a detrital core in zircon from metapelite the maximum depositional age for the protoliths of the nyingchi complex is c 53 ma we propose a tectonic scenario in which the nyingchi complex ccw p t t path was buried in the interval c 53 35 ma synchronous with heating then compression during ongoing convergence related to the india asia collision by contrast the namche barwa complex cw p t t path which represents the subducted indian continental lithosphere records relatively fast exhumation from c 17 to c 3 ma that was initiated by crustal scale folding linked to orogen scale gravitational spreading and thrusting enhanced by surface processes

the eastern himalayan syntaxis major tectonic domains ophiolitic melanges and geologic evolution abstract geologic mapping in the eastern himalayan syntaxis confirmed the three regional tectonic elements outlined by previous geologic workers our studies however show that the indus yarlung tsangpo suture iys is a continuous melange zone that forms an inverted u in map view around the namche barwa antiform the namche barwa and nyainqentanglha crystalline complexes lie below and above the iys suture respectively and both were parts of the northern indian plate basement rocks with petrologically and geochronologically correlative protoliths both units were deformed metamorphosed and intruded at the end of the proterozoic the zhibai formation the lower part of the namche barwa group extends along the northwest slope of the himalaya and is mainly composed of highly deformed aluminous felsic gneiss containing sporadic boudins of high pressure granulite in the namche barwa antiform the upper part of the namche barwa group includes a calcareous rock assemblage characteristized by marble and diopside bearing calcsilicate rocks interlayed with felsic gneiss petrochemical studies show that the iys contains lenses of oceanic crustal rocks originated from fore arc trench island arc and back arc basin environments which implies they were derived from a ssz type supra subduction zone ophiolite our field mapping identifies the jiali parlung tsangpo remnant suture jps that lies north of the namche barwa antiform as a possible branch of the neo tethyan oceanic realm subduction of the mesozoic neo tethyan oceanic plate resulted in both mesozoic and cenozoic granite intrusions in the northwest trending gangdise magmatic belt along the southern edge of asia uplift and exhumation have been the most recent dominant tectonic processes in the late cenozoic for the high himalayan crystalline rocks namche barwa group in the core of the namche barwa antiform

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

age and origin of magmatism along the cenozoic red river shear belt china to decipher the geodynamic significance of cenozoic magmatism along the red river shear belt geochemical analyses u pb and rb sr dating and pb sr nd isotope tracing were undertaken zircon monazite titanite and a ti u oxide from foliated granitoid intrusions in the shear belt gneisses yield u pb emplacement ages of 33 1 0 2 2 31 9 0 3 25 8 0 2 and 24 7 0 2 ma and an age of 35 0 0 3 ma was obtained for the roughly 100 km long adjacent jinping phan si pang alkali granite together with our previous data the new ages suggest that magmatism and left lateral strike slip movements occurred coevally during latest eocene oligocene times from 33 to 22 ma the rb sr dating of muscovite and biotite from the northernmost gneisses indicates that cooling to 500 c occurred at 52 6 1 1 ma pre dating the onset of magmatism whereas further cooling to 300 c took place at 28 9 0 6 this shows that unroofing in the north took place almost 9 million years earlier than in the central gneiss segments of the shear zone geochemical data substantiate two types of magmas 1 amphibole bearing intrusions of alkaline trend which are derived from sources with isr 0 7065 0 7089 and i nd 3 7 to 6 6 2 leucogranitic layers and bodies having isr 0 7084 0 7354 and i nd 3 3 to 13 4 the former type of intrusion is found in both the gneisses and the adjacent unmetamorphosed cover rocks whereas leucogranites are restricted to the shear belt gneisses source signatures of the alkaline intrusions lie adjacent to the those of oib plotting at the lower end of the mantle array contamination of these melts by continental material seems to be very limited on the other hand the leucogranitic layers are essentially crustal derived but none of the them has country rock isotope signatures requiring melting of crust different from the actually exposed gneisses magma sources similar to those of ocean island basalt indicate magmatism to involve melting of light rare earth element and large ion lithophile element enriched mantle domains most likely present in the lithosphere underneath the region since lithospheric thickening or subduction can be ruled out to produce both types of magmas the presence of an important thermal anomaly is required which is coevally active with left lateral strike slip shear adiabatic decompression and melting within the rising anomaly is the most plausible mechanism to produce the mantle magmas which successively migrate through the crust to induce anatectic melting at 20 15 km crustal depth alkaline magmas largely dominate the volume of magmatism along the belt being continuously present in the shear zone for millions of years such lubrication potentially explains how very large amounts of displacement can be absorbed in surprisingly narrow shear zones such as the red river belt possibly also playing a role for where and when zones of plate scale lateral extrusion develop

eocene early miocene foreland basin development and the history of himalayan thrusting western and central nepal sedimentologic petrographic and u pb detrital zircon ages from middle eocene through early miocene sedimentary rocks in the lesser himalayan zone of western and central nepal indicate that a peripheral foreland basin system had developed in the eastern himalayan collision zone by middle eocene time the shallow marine eocene bhainskati formation accumulated in a back bulge depozone between a southward migrating forebulge and the indian craton migration of the forebulge through this region during eocene oligocene time produced a regional unconformity that spans 15 20 myr by early miocene time the forebulge unconformity was onlapped by the distal fringes of the southward migrating foredeep depozone represented by fluvial deposits of the dumri formation continued southward migration of the foredeep during the neogene accommodated the fluvial siwalik group light mineral provenance data and u pb detrital zircon ages suggest that the bhainskati was derived partly from tethyan sedimentary rocks of the tibetan himalayan zone during initial growth of the himalayan fold thrust belt the dumri was derived from metasedimentary and crystalline rocks of the greater himalayan zone during emplacement of the main central thrust and contemporaneous tectonic unroofing by normal faulting along the south tibetan detachment system the lesser himalayan crystalline thrust sheets were emplaced soon after deposition of the dumri formation 15 10 ma paleocurrent and lithofacies data from the dumri formation indicate deposition by west southwestward flowing rivers that drained into the indus portion of the himalayan foreland basin system during the early miocene thick channel sandstones in the lower dumri may represent the early miocene counterpart of the modern ganges river eastward diversion of the ganges drainage system to near its present location had occurred by 15 ma as the high standing aravalli range on the northern indian shield approached the front of the fold thrust belt assuming reasonable values for the flexural rigidity of indian lithosphere the time span of the forebulge unconformity yields a velocity of 14 33 mm yr for the southward migration of the fold thrust belt relative to india this range of values is consistent with neogene and present day estimates and suggests that only one third to one half of india eurasia convergence has been accommodated by shortening in the himalayan fold thrust belt since the onset of collision

tertiary deformation history of southeastern and southwestern tibet during the indo asian collision geologic mapping and geochronological analysis in southwest kailas area and southeast zedong area tibet reveal two major episodes of tertiary crustal shortening along the classic indus tsangpo suture in the yalu river valley the older event occurred between ca 30 and 24 ma during movement along the north dipping gangdese thrust the development of this thrust caused extensive denudation of the gangdese batholith in its hanging wall and underthrusting of the xigaze forearc strata in its footwall examination of timing of major tectonic events in central asia suggests that the initiation of the gangdese thrust was approximately coeval with the late oligocene initiation and development of north south shortening in the eastern kunlun shan of northern tibet the nan shan at the northeastern end of the altyn tagh fault the western kunlun shan at the southwestern end of the altyn tagh fault and finally the tian shan north of the tarim basin such regionally synchronous initiation of crustal shortening in and around the plateau may have been related to changes in convergence rate and direction between the eurasian plate and the indian and pacific plates the younger thrusting event along the yalu river valley occurred between 19 and 10 ma along the south dipping great counter thrust system equivalent to the locally named renbu zedong thrust in southeastern tibet the backthrust system in south central tibet and the south kailas thrust in southwest tibet the coeval development of the great counter thrust and the north himalayan granite gneiss dome belt is consistent with their development being related to thermal weakening of the north himalayan and south tibetan crust due perhaps to thermal relaxation of an already thickened crust created by the early phase of collision between india and asia or frictional heating along major thrusts such as the main central thrust beneath the himalaya

timing of granulite facies metamorphism in the eastern himalayan syntaxis and its tectonic implications abstract we present geochronological evidence in the eastern himalayan syntaxis namche barwa for high pressure hp granulite facies metamorphism and explain its importance for understanding both the deep continental subduction of the indian plate beneath asia and its subsequent exhumation the timing of peak and retrograde metamorphism in part constrains these processes but is debated we present zircon u pb and trace element data on granulite facies rocks zircon cores and rims from a weakly retrograded mafic granulite p 14 18 kbar t 800 c yield u pb ages of 24 0 0 3 ma and 18 8 0 3 ma respectively zircon cores and rims from an orthogneiss the host of the mafic granulite yield u pb ages of 490 3 ma and 24 2 0 4 ma respectively an amphibolitized mafic granulite gives a u pb zircon age of 17 0 0 4 ma combined with petrography zircon cl images th u ratios and ree patterns we suggest that the peak metamorphic age for the hp granulite is at 24 ma and subsequent moderate and low pressure retrograde metamorphism occurred at 19 17 ma indicating reasonably rapid exhumation for the hp granulite the ages of detrital zircons from a metasedimentary rock another host rock of the mafic granulite range from 0 6 to 2 0 ga with peak at 0 8 1 2 ga the protolith depositional ages for the metasedimentary rock are constrained to be between 490 and 600 ma our data suggest that the granulite terrane in the eastern himalayan syntaxis has an affinity with the greater himalayan series the hp granulite facies metamorphic events in the eastern himalayan syntaxis are distinct from ultrahigh pressure uhp metamorphic events in the western himalayan syntaxis in both age and depth of burial however the metamorphic history of the eastern and western himalayan syntaxes becomes similar after 24 ma the namche barwa granulites appear to result from the underthrusting of the indian plate lithologies beneath the lhasa block during progressive collisional processes followed by extrusion and or exhumation that result from a slab breakoff of the indian plate during the miocene

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

new garnets for old cautionary tales from young mountain belts major and trace elements in garnet may be used to characterize p t t paths in orogenic belts but they are also often used without age constraints to obtain p t estimates alone during investigations into the prograde phase of the alpine himalayan tertiary orogeny garnets with anomalously old sm nd ages were discovered in three different areas they occurred in identical parageneses to garnets that grew during tertiary prograde metamorphism ages 500 ma were determined for homogenized garnets from the garhwal himalaya while garnets with distinctive growth zonation from the betic cordillera s spain gave an age of 235 1 1 7 ma although the garhwal date coincides with a recognized magmatic and metamorphic event the 235 1 ma age from the betics at first sight appears one of the more unlikely times for garnet growth duplicate analyses precisely date growth of the garnet cores which x ray maps reveal to be entirely distinct from the rims implying a hiatus before resorption and rim development this initial garnet growth may be linked to early crustal extension during pangaea break up these results reaffirm the value of garnet in the integrated study of polymetamorphic orogenic belts at the same time the data show the potential for unravelling the history of these complex zones and strike a cautionary note for thermobarometry

constraining the age of high pressure metamorphism of paragneisses from the eastern himalayan syntaxis using zircon petrochronology and phase equilibria

linking growth episodes of zircon and metamorphic textures to zircon chemistry an example from the ultrahigh temperature granulites of rogaland sw norway in situ u th pb analyses by ion microprobe on zircon in intact textural relationships are combined with backscatter and cathodoluminescence imaging and trace element analyses to provide evidence for growth episodes of zircon this approach helps a to unravel the polymetamorphic history of aluminous migmatitic and granitoid gneisses of the regional contact aureole around the rogaland anorthosite norite intrusive complex and b to constrain the age of m 2 ultrahigh temperature uht metamorphism and the subsequent retrograde m 3 event all samples yield magmatic inherited zircon of c 1035 ma some an additional group at c 1050 ma this suggests that loss of pb by volume diffusion in non metamict zircon is not an important factor even under extreme crustal conditions furthermore the identical inheritance patterns in aluminous garnet cordierite osumilite bearing migmatites and orthogneisses indicate a metasomatic igneous instead of a sedimentary protolith for the migmatite results for the m 1 metamorphic event at c 1000 ma bp are consistent in all samples including those from outside the orthopyroxene in isograd the latter do not show evidence for zircon growth during the m 2 metamorphic episode zircon intergrown with or included within m 2 metamorphic minerals magnetite spinel orthopyroxene give an age of 927 7 ma 2 n 20 the youngest observed results are found in zircon outside m 2 minerals some overgrown by m 3 mineral assemblages late garnet coronas garnet quartz and orthopyroxene garnet symplectites and yield a slightly younger pooled age of 908 9 ma 2 n 6 these textures are relative time markers for the crystallization of zircon overgrowths during discrete stages of the uht event these youngest age groups are consistent with the emplacement age of the rogaland intrusive complex and the last magmatic activity tellnes dyke intrusion respectively this is direct and conclusive evidence for uht metamorphism in the regional aureole being caused by the intrusions and corrects earlier notions that the events are not linked trace element behaviour of zircon tb u and y content has been tracked through time in the samples and shows variations both within and between samples this heterogeneous behaviour at all scales appears to be common in metamorphic rocks and precludes the use of rules of thumb in the interpretation of zircon chemistry but chemical tracers are useful for recognition of zircon growth or recrystallization during metamorphism

local scour by a deeply submerged horizontal circular jet this study investigates local scour caused by a deeply submerged circular jet both a wall jet and an offset jet were used in the experiments with uniform cohesionless sediments making up the bed experiments conducted in both air and water show that the densimetric froude number f i do is the main characteristic scouring parameter in the correlation of the maximum equilibrium scour dimensions for an offset jet the offset distance iy which is the vertical distance between the undisturbed bed level and the center of the jet also affects the scour hole dimensions a higher iy causes the formation of a smaller scour hole because most of the energy is diffused into the flow instead of being used to erode the sediment for the same f i do value the size of the scour hole is consistently smaller for tests conducted in water than in air the reason is attributed to the formation of a sediment ridge at the downstream end of the scour hole due to the density difference between air and water the ridge height is markedly different in the two flow media

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

shrimp u pb in zircon geochronology of the chor granitoid evidence for neoproterozoic magmatism in the lesser himalayan granite belt of nw india abstract porphyritic and peraluminous granitoids of the lesser himalayan granite belt fringe the high himalaya from nepal to pakistan magmatic zircons in the chor granitoid in himchal pradesh define a shrimp u pb age of 823 5 ma this neoproterozoic age contrasts with cambro ordovician ages for other dated lesser himalayan granites but is coeval with granite magmatism in the south china craton neoproterozoic magmatism within the northern margin of gondwanaland may provide a more local source for detrital zircons in high himalayan metasediments during the neoproterozoic and early paleozoic than the recently postulated east african orogen

the zedong window a record of superposed tertiary convergence in southeastern tibet determining the timing magnitude and location of deformation due to the indo asian collision is widely acknowledged as an important step in understanding how the lithosphere responds during continental collision a puzzling result of geological investigations of the lhasa block over the past 2 decades has been the apparent lack of significant tertiary deformation there perhaps the most important structural feature of the lhasa block is the south directed gangdese thrust system which developed along its southern edge the thrust system which separates the andean type batholith of southern asia from rocks of indian affinity is obscured at most locations across southeastern tibet by back thrusts of the younger north directed renbu zedong thrust system the best documented site where both thrusts are exposed occurs near zedong zedong window systematic geochronologic analyses were conducted in this area u pb zircon dating of three samples of a synkinematically to postkinematically deformed hanging wall granitoid the yaja granodiorite cut by the gangdese thrust indicates a crystallization age of 30 4 0 4 ma 2 thus placing an upper bound on the initiation of the thrust u pb zircon dating of granitoid samples structurally higher in the gangdese hanging wall yields emplacement ages of 42 5 1 0 ma and 48 9 0 8 ma 2 similar to other magmatic complexes within the gangdese arc geochemical results are consistent with these plutons forming in the same environment as precollisional intrusions within the gangdese batholith suggesting a significant post collisional input of juvenile heat the 40ar 39ar thermochronology of samples from a vertical section through the hanging wall within the yaja granodiorite coupled with results of a numerical thermal model indicate an average slip rate along the gangdese thrust of 7 mm yr between 30 and 23 ma and a minimum displacement of 50 km farther east in the zedong window thermal effects produced by later north directed thrust sheets of the renbu zedong system appear to have obscured thermal history signatures in the gangdese hanging wall related to earlier south directed thrusting thermochronological results from this region indicate that thermal overprinting related to the north directed thrusting occurred between 25 and 10 ma consistent with previous estimates

temporal evolution of plunge pool scour the temporal development of plunge pool scour was investigated using a novel experimental approach longitudinal profiles along the scour hole were recorded with an optical method to allow its definition at any time from the initiation of scour to nearly the end scour condition the characteristics of the scour hole geometry were investigated namely the maximum scour hole depth the maximum ridge height and their locations relative to the scour hole origin it is demonstrated that the evolution is logarithmic similar to that found for bridge pier and abutment scour a distinction is further made between the developing and the developed scour hole phases the main issue of the present research was to define the developed scour hole characteristics because the developing scour phase is influenced by turbulence features that may be difficult to assess this work therefore allows for an appreciation of the temporal evolution of a scour process of engineering interest

new interpretation of tectonic model in south tibet we present a new interpretation of tectonic evolution in southern tibet that is drastically different from the existing models a detailed tectono sedimentary study crossing the yarlung zangpo ophiolite zone shows that many geological features are different from those commonly described in large subduction collision models for example no n s oriented shear zones are found between the ophiolitic sequence and country flysch strata whereas a conformable contact relationship is recognized between them a tectonic window exists inside the ophiolite body in the bailang region some intrusion like mafic ultramafic bodies occurred in the renbu region where the country strata are in sub concordant contact with these bodies and show contact metamorphic aureole toward the west the ophiolite zone was separated by flysch sequences into sub parallel branches in the lhasa region the sedimentary facies are similar on both sides of the zangpo valley and have preserved an intact mesozoic basin system instead of ophiolite rocks volcaniclastic deposits occurred in the corresponding location of the ophiolite in the zangpo valley consequently we conclude that the zangpo ophiolite zone has a tectonic affinity of back arc basin with its spasmodic growth of juvenile oceanic crust the real tectonic suture or the closure zone of the neotethys should be represented by the high himalaya central gneiss unit which shows a large scale strong shearing in same orientation high metamorphism and protracted re mobilization the oceanic crust subducted northward and split off the himalaya continental front arc created the zangpo back arc basin since late triassic the collapse of the zangpo back arc basin by supra subduction occurred since the eocene the final collision between india and the himalayan arc took place since late eocene with a re mobilized large shear system the major mylonitic zones migrated progressively southward with bulk of shear slip absorbing the crust of north india and south tibet

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

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

the namche barwa syntaxis evidence for exhumation related to compressional crustal folding abstract geological observation in the eastern end of the himalayas shows that the asia india suture is folded and continues southward into india and burma where the continuation of the transhimalayan plutonic complex can be identified metamorphic rocks derived from india occur structurally below the suture in the core of a regional antiform isotopic and fission track dating are consistent with the geological interpretation in addition they establish cooling exhumation of rocks from c 30 km depth within the last 4 ma rapid exhumation of deep crustal levels in mountain systems is generally related to extension subsequent to thermal softening of a thickened continental lithosphere however in the eastern himalayas we argue that exhumation is caused by c 10 mm yr 1 erosion coeval with crustal scale folding the general history of this syntaxis resembles the evolution of the western himalayas syntaxis in pakistan

prolonged partial melting of garnet amphibolite from the eastern himalayan syntaxis implications for the tectonic evolution of large hot orogens

tertiary structural evolution of the gangdese thrust system southeastern tibet structural and thermochronological investigations of southern tibet xizang suggest that intracontinental thrusting has been the dominant cause for formation of thickened crust in the southernmost tibetan plateau since late oligocene two thrust systems are documented in this study the north dipping gangdese system gts and the younger south dipping renbu zedong system rzt west of lhasa the gangdese thrust juxtaposes the late cretaceous forearc basin deposits of the lhasa block the xigaze group over the tethyan sedimentary rocks of the indian plate whereas east of lhasa the fault juxtaposes the late cretaceous eocene andean type arc the gangdese batholith over tethyan sedimentary rocks near zedong 150 km southeast of lhasa the gangdese thrust is marked by a 200 m thick mylonitic shear zone that consists of deformed granite and metasedimentary rocks a major south dipping backthrust in the hanging wall of the gangdese thrust puts the xigaze group over tertiary conglomerates and the gangdese plutonics north of xigaze and west of lhasa a lower age bound for the gangdese thrust of 18 3 0 5 ma is given by crosscutting relationships the timing of slip on the gangdese thrust is estimated to be 27 23 ma from 40 ar 39 ar thermochronology and a displacement of at least 46 9 km is indicated near zedong the age of the gangdese thrust gt is consistent with an upper age limit of 24 ma for the initiation of movement on the main central thrust in places the younger renbu zedong fault is thrust over the trace of the gt obscuring its exposure the rzt appears to have been active at circa 18 ma but had ceased movement by 8 1 ma the suture between india and asia has been complexly modified by development of the gts rzt and locally strike slip and normal fault systems

accretionary tectonics of burma and the three dimensional geometry of the burma subduction zone the geometry of the burma wadati benioff zone wbz has been determined by fitting a trend surface parameterized with eight effective degrees of freedom to 184 well located hypocenters the dip of this surface which passes through the middle of the wbz varies from about 50 in the north near the eastern himalayan syntaxis to about 30 in the bay of bengal area the eastern edge of the indo burman ranges closely follows the map projection of the 60 km depth contour of the wbz the curvature of the indo burman ranges is controlled by the geometry of the interface between the more steeply dipping part of the indian plate and the leading edge of the overriding burma platelet shallow earthquakes beneath the indo burman ranges are primarily confined to the underthrusting indian plate their focal mechanisms indicate strike slip faulting and north south shortening parallel to the eastern margin of the indian plate

tectonics and structural zonation of southern tibet china the yarlung zangbo suture zone is composed of several thrust sheets with distinct stratigraphies states of deformation and metamorphic grades successive deformations have been produced by persistent north south shortening since obduction of the ophiolites by late cretaceous palaeocene times a possible decoupling of the crust may be associated with the intracontinental thrust system which has formed in the subducted indian plate this article emphasizes the role of shear deformations in the building of a collision belt

evolution of scouring process downstream of grade control structures under steady and unsteady flows for many incised channels one of the most common strategies is to install some hard structures such as grade control structures gcss in the riverbed to resist further incision in this study a series of experiments including both steady and unsteady flow conditions were conducted to investigate the scouring process downstream of a gcs three distinct phases including the initial developing and equilibrium phases during the evolution of scour holes were identified in addition a semi empirical method was proposed to predict the equilibrium scour hole profile for the scour countermeasure design in general the comparisons between the experimental and simulated results are reasonably consistent as the studies on temporal variation of the scour depth at gcss caused by floods are limited the effect of flood hydrograph shapes on the scour downstream of gcss without upstream sediment supply was also investigated experimentally in this study based on the dimensional analysis and the concept of superposition a methodology is proposed to simulate the time evolution of the maximum scour depth downstream of a gcs for steady flows moreover the proposed scheme predicts reasonably well the temporal variations of the maximum scour depth for unsteady flows with both single and multiple peak copyright 2012 john wiley sons ltd

plate velocity exhumation of ultrahigh pressure eclogites in the pakistan himalaya u pb ages of zircon and allanite from the coesite bearing ultrahigh pressure uhp units in the kaghan valley northern pakistan demonstrate that peak uhp metamorphism along the northern margin of the indian plate occurred at 46 4 0 1 ma at peak pressure temperature conditions of 27 5 kbar 100 km and 720 770 c much lower pressure retrogressive growth of titanite took place between 46 4 and ca 44 ma indicating that the eclogites were exhumed to 35 km depth at or before 44 ma implying very rapid exhumation rates within the mantle of 30 80 mm yr or more comparable to rapid plate velocities once entrained in the base of the crust the eclogites followed a slower cooling history from 45 ma similar to the amphibolite facies gneisses of the pakistan himalaya

high temperature metamorphism in a major strike slip shear zone the ailao shan red river people s republic of china abstract petrographic and thermobarometric analysis provides constraints on thept path of the mylonitic gneisses of the left lateral ailao shan red river shear zone which has accommodated the lateral extrusion of indochina during the tertiary two different paragenesis p1 and p2 are coeval with this deformation and correspond respectively to the amphibolite and greenschist facies microprobe analysis reveals that p1 garnets bear a chemical zonation from core to rim this zonation indicates a temperature increase during garnet growth conditions of formation of garnet rims p1b which are estimated using biotite garnet and plagioclase garnet thermobarometers are close to the granitic solidus 710 70 c and4 5 1 5 kbar p2 conditions are estimated to be approximately 500 c and

scour caused by a propeller jet abstractthe scour hole induced by a propeller wash is experimentally investigated with noncohesive sediments the associated scour profile at the asymptotic condition which can be expressed by a combination of three polynomials comprises 1 a small scour hole beneath the propeller 2 a primary scour hole downstream of the small scour hole and 3 a deposition mound farther downstream of the primary scour hole the time dependent maximum scour depth induced by propeller jets is closely related to the densimetric froude number a reference time scale offset height relative to the propeller diameter and sediment size a semiempirical formula which shows good agreement with the experimental data is proposed to describe evolution of the maximum scour depth an equation for the determination of the time to initiate scour is also proposed moreover empirical equations for the maximum asymptotic scour depth induced by a propeller and three dimensional submerged offset jets agree well with the experimen

high pressure granulites formation recovery of peak conditions and implications for tectonics high pressure granulites are characterised by the key associations garnet clinopyroxene plagioclase quartz in basic rocks and kyanite k feldspar metapelites and felsic rocks and are typically orthopyroxene free in both basic and felsic bulk compositions in regional metamorphic areas two essential varieties exist a high to ultrahigh temperature group and a group representing overprinted eclogites the high to ultrahigh temperature type formerly contained high temperature ternary feldspar now mesoperthite coexisting with kyanite is associated with garnet peridotites and formed at conditions above 900 c and 1 5 gpa clinopyroxene in subordinate basic rocks is al rich and textural evidence points to a high pressure high temperature melting history the second variety contains symplectite like or poikilitic clinopyroxene plagioclase intergrowths indicating former plagioclase free i e eclogite facies assemblages this type of rock formed at conditions straddling the high pressure amphibolite high pressure granulite field at around 700 850 c 1 0 1 4 gpa importantly in the majority of high pressure granulites orthopyroxene is secondary and is a product of reactions at pressures lower than the peak recorded pressure in contrast to low and medium pressure granulites which form at conditions attainable in the mid to lower levels of normal continental crust high pressure granulites of nonxenolith origin mostly represent rocks formed as a result of short lived tectonic events that led to crustal thickening or subduction of the crust into the mantle short times at high temperature conditions are reflected in the preservation of prograde zoning in garnet and pyroxene high pressure granulites of both regional types although rare are known from both old and young metamorphic terranes e g c 45 ma namche barwa e himalaya 400 340 ma european variscides 1 8 ga hengshan china 1 9 ga snowbird saskatchewan and 2 5 ga jianping china this spread of ages supports proposals suggesting that thermal and tectonic processes in the lithosphere have not changed significantly since at least the end of the archean

metamorphic textures and p t evolution of high p granulites from the lelukuau terrane ne grenville province high pressure hp granulites and eclogitized metagabbro are exposed along an orogen parallel high p belt that was developed at c 1050 1020 ma in the ne grenville province among these rocks mafic granulites derived from a labradorian anorthosite suite of the lelukuau terrane contain garnet al na diopside and depending on bulk composition plagioclase and kyanite moreover the distribution of phases is influenced by the original igneous texture for instance in high xmgo leucocratic varieties garnet porphyroblasts nucleated together with kyanite in an rich cores of plagioclase domains whereas in low xmgo rocks garnet occurs together with clinopyroxene within formerly igneous ferromagnesian domains and kyanite is missing in contrast garnet pseudomorphs after igneous plagioclase in melanocratic varieties display evidence of earlier corona development metamorphic textures are consistent with a two stage evolution a development of garnet and al na diopside cpx1 under high p metamorphic conditions concomitant with elimination of plagioclase in the mesocratic to melanocratic varieties and b partial loss of al na from cpx1 resulting in production of new andesitic plagioclase and growth of new clinopyroxene cpx2 after garnet and quartz in leucocratic to mesocratic rocks consistent with decompression widespread equilibrium textures between garnet pl2 cpx2 and or reset cpx1 are consistent with development at the thermal peak estimated p t conditions for the presumed thermal peak fall in the range 1500 1800 mpa and 800 900 c and are comparable to those recorded by eclogitized gabbros from other parts of the high p belt of the ne grenville province low jadeite content of clinopyroxene from the hp granulites is attributed to the low bulk na2o na2o cao of these rocks relative to common basaltic compositions scarcity of apparent retrograde textural overprint in both the hp granulites and the eclogites suggests fast subsequent cooling consistent with extrusion of the high p belt towards the foreland shortly after the metamorphic peak

sm nd and u pb dating of high pressure granulites from the zlote and rychleby mts bohemian massif poland and czech republic in the orlica snie znik complex at the ne margin of the bohemian massif high pressure granulites occur as isolated lenses within partially migmatized orthogneisses sm nd different grain size fractions of garnet clinopyroxene and or whole rock and u pb isotope dilution thermal ionization mass spectrometry id tims single grain and sensitive high resolution ion microprobe shrimp ages for granulites collected in the surroundings of cervenydul czech republic and at stary gieraltow poland constrain the temporal evolution of these rocks during the variscan orogeny most of the new ages cluster at c 350 340 ma and are consistent with results previously reported for similar occurrences throughout the bohemian massif this interval is generally interpreted to constrain the time of high pressure metamorphism a more complex evolution is recorded for a mafic granulite from stary gieraltow and concerns the unknown duration of metamorphism single short lived metamorphic cycle or different episodes that are significantly separated in time the central grain parts of zircon from this sample yielded a large spread in apparent 206 pb 238 u shrimp ages c 462 322 ma with a distinct cluster at c 365 ma this spread is interpreted to be indicative for variable pb loss that affected magmatic protolith zircon during high grade metamorphism the initiating mechanism and the time of pb loss has yet to be resolved a connection to high pressure metamorphism at c 350 340 ma is a reasonable explanation but this relationship is far from straightforward an alternative interpretation suggests that resetting is related to a high temperature event not necessarily in the granulite facies and or at high pressures around 370 360 ma that has previously gone unnoticed this study indicates that caution is warranted in interpreting u pb zircon data of ht rocks because isotopic rejuvenation may lead to erroneous conclusions

dating the indian continental subduction and collisional thickening in the northwest himalaya multichronology of the tso morari eclogites multichronometric studies of the low temperature eclogitic tso morari unit ladakh india place timing constraints on the early evolution of the northwest himalayan belt several isotopic systems have been used to date the eclogitization and the exhumation of the tso morari unit lu hf sm nd rb sr and ar ar a ca 55 ma age for the eclogitization has been obtained by lu hf on garnet omphacite and whole rock from mafic eclogite and by sm nd on garnet glaucophane and whole rock from high pressure metapelites these results agree with a previously reported u pb age on allanite and together these ages constrain the subduction of the indian continental margin at the paleocene eocene boundary during exhumation the tso morari rocks underwent thermal relaxation at about 9 3 kbar characterized by partial recrystallization under amphibolite facies conditions ca 47 ma as dated by sm nd on garnet calcic amphibole and whole rock from metabasalt rb sr on phengite apatite and whole rock and ar ar on medium si phengite from metapelites ar ar analyses of biotite and low si muscovite from metapelites which recrystallized at 5 kbar toward the end of the exhumation show that the tso morari unit was at upper crustal levels ca 30 ma these results indicate variable exhumation rates for the tso morari unit beginning with rapid exhumation while the indian margin subduction was still active and later proceeding at a slower pace during the crustal thickening associated with the himalayan collision

interactions between deformation and fluids in the frontal thrust region of the nantroseize transect offshore the kii peninsula japan results from iodp expedition 316 sites c0006 and c0007 integrated ocean drilling program iodp expedition 316 sites c0006 and c0007 examined the deformation front of the nankai accretionary prism offshore the kii peninsula japan in the drilling area the frontal thrust shows unusual behavior as compared to other regions of the nankai trough drilling results integrated with observations from seismic reflection profiles suggest that the frontal thrust has been active since 0 78 0 436 ma and accommodated 13 to 34 of the estimated plate convergence during that time the remainder has likely been distributed among out of sequence thrusts further landward and or accommodated through diffuse shortening unlike results of previous drilling on the nankai margin porosity data provide no indication of undercompaction beneath thrust faults furthermore pore water geochemistry data lack clear indicators of fluid flow from depth these differences may be related to coarser material with higher permeability or more complex patterns of faulting that could potentially provide more avenues for fluid escape in turn fluid pressures may affect deformation well drained sand rich material under the frontal thrust could have increased fault strength and helped to maintain a large taper angle near the toe recent resumption of normal frontal imbrication is inferred from seismic reflection data associated decollement propagation into weaker sediments at depth may help explain evidence for recent slope failures within the frontal thrust region this evidence consists of seafloor bathymetry normal faults documented in cores and low porosities in near surface sediments that suggest removal of overlying material overall results provide insight into the complex interactions between incoming materials deformation and fluids in the frontal thrust region

crustal structure beneath namche barwa eastern himalayan syntaxis new insights from three dimensional magnetotelluric imaging the eastern terminations of the himalayan orogeny named namche barwa are considered a vital natural laboratory in the tibetan plateau for geodynamics due to its distinctive geological and geomorphological characteristics magnetotelluric mt data measured at 83 sites around the namche barwa are imaged by three dimensional 3d inversion to better reveal the crustal structure of the eastern himalaya the results show a complex and heterogeneous electrical structure beneath the namche barwa the electrical conductors distributed in the middle and lower crust around the namche barwa provide additional evidence for the crustal flow model if they are considered as some parts of the flow in a relatively large scale region the near surface resistivity model beneath the inner part of namche barwa conforms with the locations of hot spring and fluid inclusions the brittle ductile transition and the 300 c 400 c isotherm from previous hydrothermal studies relatively resistive upper crust 800 m is underlain by a more conductive middle to lower crust 80 m the electrical characteristics of the thermal structure at shallow depth indicate an accumulation of hydrous melting a localized conductive steep dipping zone for decompression melting consistent with the tectonic aneurysm model for explaining the exhumation mechanism of metamorphic rocks at namche barwa the results also imply that both surface processes and local tectonic responses play a vital role in the evolution of namche barwa an alternative hypothesis that the primary sustained heat source accounts for the local thermal rheological structure beneath namche barwa is also discussed

cenozoic structural and metamorphic evolution of the eastern himalayan syntaxis namche barwa combined geological and geochronological investigations of the eastern himalayan syntaxis in the namche barwa region of tibet reveal the first order elements of its cenozoic tectonic evolution the syntaxis is characterized by a northeast plunging antiform and is bounded by two northeast striking strike slip shear zones a left slip shear zone on the western side and a right slip shear zone on the eastern side these strike slip shear zones are linked by east westtrending thrusts and served either as 1 a roof thrust to a large duplex system or 2 transfer faults to a south directed thrust system that accommodated northward indentation of a folded indian plate an east west trending pop up structure in the core of the antiform juxtaposes a granulite bearing complex over sillimanite bearing gneisses of gangdese affinity to the north and of indian affinity to the south previous studies suggest that mafic granulites in the complex record at least two episodes of metamorphism at v800 c the first at high pressures 14 15 kbar followed by a second event at 8 10 kbar zircons from mafic granulites yield four populations of concordant u pb ion microprobe ages two groups are at v65 ma and v160 ma and likely crystallized during andean type gangdese magmatism prior to the indo asian collision a third cluster at v40 ma exhibits very low th u ratios and is interpreted to have crystallized in the presence of fluids associated with a high pressure granulite facies metamorphic event during the early stages of the indo asian collision subsequent to high pressure metamorphism in the western himalaya syntaxis between v50 and 43 ma a fourth cluster of zircons yields ages between 11 and 25 ma and th u ratios that decrease systematically with decreasing age we interpret the youngest zircon age v11 ma to represent the timing of moderatepressure high grade metamorphism with the older ages and higher th u ratios being a result of mixing with a restitic igneous component this interpretation coupled with a v 8m a 40 ar 39 ar age on hornblende from a metadiorite within the core of the antiform suggests that the namche barwa syntaxis has been characterized by rapid cooling and exhumation since at least late miocene time despite its contrasting structural setting the miocene and younger metamorphism and cooling history in the namche barwa syntaxis are strikingly similar to those of the nanga parbat syntaxis of the western himalaya fl 2001 elsevier science b v all rights reserved

timing of himalayan ultrahigh pressure metamorphism sinking rate and subduction angle of the indian continental crust beneath asia coesite relics were discovered as inclusions in clinopyroxene in eclogite and as inclusions in zircon in felsic and pelitic gneisses from higher himalayan crystalline rocks in the upper kaghan valley north west himalaya the metamorphic peak conditions of the coesite bearing eclogites are estimated to be 27 32 kbar and 700 770 c using garnet pyroxene phengite geobarometry and garnet pyroxene geothermometry respectively cathodoluminescence cl and backscattered electron bse imaging distinguished three different domains in zircon inner detrital core widely spaced euhedral oscillatory zones and thin broadly zoned outermost rims each zircon domain contains a characteristic suite of micrometre sized mineral inclusions which were identified by in situ laser raman microspectroscopy core and mantle domains contain quartz apatite plagioclase muscovite and rutile in contrast the rim domains contain coesite and minor muscovite quartz inclusions were identified in all coesite bearing zircon grains but not coexisting with coesite in the same growth domain rim domain 206 pb 238 u zircon ages reveal that the quartz bearing mantle domains and the coesite bearing rim were formed at c 50 ma and 46 2 0 7 ma respectively these facts demonstrate that the continental materials were buried to 100 km within 7 9 myr after initiation of the india asia collision palaeomagnetic data from the indian oceanic floor supports an initial india asia contact at 55 53 ma combination of the sinking rate of 1 1 1 4 cm year 1 with indian plate velocity of 4 5 cm year 1 suggests that the indian continent subducted to about 100 km depth at an average subduction angle of 14 19