overpressure generation and evolution in a compressional tectonic setting the southern margin of junggar basin northwestern china overpressure is widespread in the southern margin of the junggar basin northwestern china pressure measurements in drillstem tests and repeated formation tests and estimates from wire line logs indicate contrasting overpressure values between permeable sandstones and adjacent low permeability mudrocks in addition excess pressure differs among anticlines with similar depth lithologies and geologic age indicating significant lateral changes of overpressure major factors controlling overpressure generation and distribution include rapid sediment deposition pressure compartmentalization by thick mudrocks tectonic stress faulting and folding clay transformation and hydrocarbon generation are believed to be insignificant in overpressure generation in the southern junggar basin numerical modeling of pressure generation and evolution suggests that faulting and stratal tilting associated with folding are the most significant factors in the overpressure generation of a permeable sandstone the extremely high overpressure pressure coefficient up to 2 43 may have been caused by hydraulic adjustment within permeable sandstones associated with structural deformation caused by post miocene intense tectonic activities

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

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

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

nappe tectonics extension and metamorphic evolution in the indian tethys himalaya higher himalaya se zanskar and nw lahul in the southern part of the indian tethys himalaya between se zanskar and nw lahul nappe tectonics with transport toward the wsw during the main deformation phase has been ascertained if the stratigraphy of the different units considered as nappes is combined it results in a composite section ranging from lower cambrian to middle cretaceous a metamorphism of amphibolite facies in the lowest unit is associated with the nappe stacking the lower miocene cooling ages of metamorphic biotite and muscovite suggest that the nappe emplacement starts at the oligocene or at the oligocene eocene boundary after the compressional phases an extension provokes the reactivation of older main thrusts as normal fault and the formation of a steeply dipping normal fault this interpretation is based on detailed mapping

synconvergence spreading of the higher himalaya crystalline in ladakh in ladakh suru valley nw himalaya regional fabrics of the higher himalaya crystalline result from a major deformation event that 1 is restricted to the indian crust above the main central thrust mct 2 postdates piling up of units above the mct 3 ended while thrusting was still continuing on the mct 4 developed under high temperature conditions following thickening 5 resulted in an asymmetric gneiss dome and 6 is associated with ene wsw to ew regional extension we suggest that extension was accommodated by spreading of the hot basement accompanied and followed by its uplift within the overlying cover along a major detachment zone the onset of the process appears closely related to strong thermal softening of the higher himalaya crystalline and is broadly synchronous with the formation of the himalayan leucogranites around 25 ma the regional pattern suggests that the mct zone could have acted as a decoupling zone between a thickening lower part and a thinning upper part of the indian crust during miocene times the extension direction is consistent with bulk himalayan kinematics involving nw se dextral wrenching along the western syntax and eastward extrusion of tibet we suggest that exhumation of gneissic ridges which characterize indian units above the mct all along the himalayan belt could reflect large scale crustal instability developed from thermally anomalous layers whose spreading ability can favor synconvergence extension

himalayan leucogranites and migmatites nature timing and duration of anatexis widespread anatexis was a regional response to the evolution of the himalayan tibetan orogen that occurred some 30 ma after collision between asia and india this paper reviews the nature timing duration and conditions of anatexis and leucogranite formation in the greater himalayan sequence ghs and compares them to contemporaneous granites in the karakoram mountains himalayan leucogranites and associated migmatites generally share a number of features along the length of the mountain front such as similar timing and duration of magmatism common source rocks and clockwise p t paths despite commonalities most papers emphasize deviations from this general pattern indicating a fine tuned local response to the dominant evolution there are significant differences in p t xh2o conditions during anatexis and timing in relation to regional decompression further to that some regions underwent a second event recording melting at low pressures zircon and monazite ages of anatectic rocks range between c 25 and 15 ma suggesting prolonged crustal melting typically a single sample may have ages covering most of this 10 ma period suggesting recycling of accessory phases from metamorphic rocks and early formed magmas recent studies linking monazite and zircon ages with their composition have determined the timing of prograde melting and retrograde melt crystallization thus constraining the duration of the anatectic cycle in some areas this cycle becomes younger down section towards the leading front of the himalayas whereas the opposite is true in other areas the relationship between granites and movement on the south tibetan detachment std reveals that fault motion took place at different times and over different durations requiring complex internal strain distribution along the himalayas the nature and fate of magmas in the ghs contrast with those in the karakoram mountains ghs leucogranites have a strong crustal isotopic signature and migration is controlled by low angle foliation leading to diffuse injection complexes concentrated below the std in contrast the steep attitude of the karakoram shear zone focused magma transfer feeding the large karakoram baltoro batholith anatexis in the karakoram involved a cretaceous calcalkaline batholith that provided leucogranites with more juvenile isotopic signatures the impact of melting on the evolution of the himalayas has been widely debated melting has been used to explain subsequent decompression or conversely decompression has been used to explain melting weakening due to melting has also been used to support channel flow models for extrusion of the ghs or alternatively to suggest it triggered a change in its critical taper in view of the variable nature of anatexis and of motion on the std it is likely that anatexis had only a second order effect in modulating strain distribution with little effect on the general history of deformation thus despite all kinds of local differences strain distribution over time was such that it maintained the well defined arc that characterizes this orogen this was likely the result of a self organized forward motion of the arc controlled by the imposed convergence history and energy conservation balancing accumulation of potential energy and dissipation independent of the presence or absence of melt

postglacial denudation of western tibetan plateau margin outpaced by long term exhumation the indus river one of asia s premier rivers drains the western tibetan plateau and the nanga parbat syntaxis these two areas juxtapose some of the lowest and highest topographic relief and commensurate denudation rates in the himalaya tibet orogen respectively yet the spatial pattern of denudation rates upstream of the syntaxis remains largely unclear as does the way in which major rivers drive headward incision into the tibetan plateau we report a new inventory of 10 be based basinwide denudation rates from 33 tributaries flanking the indus river along a 320 km reach across the western tibetan plateau margin we find that denudation rates of up to 110 mm k y 1 in the ladakh and zanskar ranges systematically decrease eastward to 10 mm k y 1 toward the tibetan plateau independent results from bulk petrographic and heavy mineral analyses support this denudation gradient assuming that incision along the indus exerts the base level control on tributary denudation rates our data show a systematic eastward decrease of landscape downwearing reaching its minimum on the tibetan plateau in contrast denudation rates increase rapidly 150 200 km downstream of a distinct knickpoint that marks the tibetan plateau margin in the indus river longitudinal profile we infer that any vigorous headward incision and any accompanying erosional waves into the interior of the plateau mostly concerned reaches well below this plateau margin moreover reported long term 10 6 yr exhumation rates from low temperature chronometry of 0 1 0 75 mm yr 1 consistently exceed our 10 be derived denudation rates with averaging time scales of 10 3 10 4 yr for our denudation data we report postglacial rates of downwearing in a tectonically idle landscape to counterbalance this apparent mismatch denudation rates must have been higher in the quaternary during glacial interglacial intervals

focusing simulations of synthetic aperture radar ocean images the sar ocean images obtained in the tower ocean wave and radar dependence experiment toward are carefully analyzed at different focus settings and compared with simulated results based on various theories for imaging surface waves the agreement between the experimental data and all of the sar simulations except one is found to be favorable there is also surprisingly close agreement among the different theoretical models themselves suggesting a closer fundamental similarity among the contending theories than was previously thought it is shown that the width of the focusing curve has an inverse dependence on the sar integration time for the toward conditions it is found that the image modulation due to azimuth traveling surface waves is greater than that due to range traveling waves

palaeoproterozoic metamorphism in the jeori wangtu gneissic complex jwgc western himalayas abstract the jeori wangtu gneissic complex jwgc exposed as a tectonic window in the lesser himalayas represents one of the oldest gneissic complex of the himalayas foliated granite and the metapelite constitute the dominant lithologies of the jwgc the western margin of the jwgc is bounded by a brittle shear while in the east the tectonic surface is a ductile shear zone kyanite schist chloritoid schist staurolite schist st 1 garnet schist and staurolite schist st 2 are present in a west to east sequence beginning from near to the jhakhri thrust and up to the contact with the jwgc granite mica schist is intermittently present and is the dominant metapelite low to medium grade regional metamorphic conditions has been inferred for these rocks calc silicate enclaves within the jwgc granite preserve the contact metamorphic effects these are reflected in development of narrow zones of disequilibrium assemblages of calcareous garnet grs 53 clinopyroxene k feldspar calcic plagioclase an86 quartz zoned sphene zoned allanite amphiboles calcite and epidote recording of contact metamorphic assemblage of 1 80 ga granite witin the enclave calc silicates and in the host metapelites over an earlier relict low to medium grade assemblage indicates that the jwgc preserves palaeoproterozoic metamorphic imprints

u pb zircon and microfabric data of meta granitoids of western cameroon constraints on the timing of pluton emplacement and deformation in the pan african belt of central africa abstract striking characteristics of the western neoproterozoic belt of cameroon nfbc are the large volume of granitoids and crustal scale shear zones new structural and geochronological data from this area are provided to put constraints on the tectonic evolution of this segment of the belt and to make further correlations between major shear zones exposed on both sides of the atlantic ocean three different complexes have been identified in the study area the migmatitic complex of foumbot mcf the metagranitoid complex of bangwa bc and the batie pluton bp the mcf was intruded by the bc while the bp cuts through the bc u pb zircon dating of metaleucogranite and metagranodiorite of the bc yielded concordant to subconcordant ages of 638 2 ma and 637 5 ma respectively a concordant u pb zircon age of 602 1 4 ma has been obtained from porphyrogranite of the bp these ages are interpreted as emplacement ages continuous deformation from magmatic to solid state flow along the bp margins and the sub parallelism of the steep solid state foliation in the bp margins with the foliation in the surrounding bc and mfc suggest synkinematic emplacement of the bp along crustal scale nne to ene trending strike slip shear zones subhorizontal foliations in migmatitic gneiss xenoliths found in the bc suggest that the major transcurrent motion was preceded by thrusting the new data confirm previous assumptions that the western nfbc is equivalent to parts of the borborema province of brazil there are geochronological correlations between the studied meta granitoids and brasiliano pre to syn transcurrent granitoids of the borborema province

zanskar shear zone northeast southwest extension within the higher himalayas ladakh india a complete geologic section through the northwestern himalayas is well exposed in ladakh india the collision between the indian shield and the tibetan platform caused large scale intracrustal thrusting and piling up of the himalayan nappes this compressional event was accompanied by prograde regional metamorphism the metamorphism ranges from low grade anchimetamorphism in the indus suture zone in the northeast to upper amphibolite grade in the higher himalaya tectonic unit in the southwest in the zanskar area a well exposed and morphologically prominent shear zone 2 25 to 6 75 km wide can be followed from the northwest to the southeast for at least 80 km in contrast to the main compressional tectonics this shear zone indicates an extensional event within the higher himalaya tectonic unit the zanskar shear zone involves granitic rocks leucogranites of miocene age and various metasediments and in general separates the late precambrian early cambrian sedimentary sequence phe formation from the underlying crystalline basement zanskar crystalline unit a normal sense of shear has been determined by using asymmetric macroscopic and microscopic structures such as pegmatite boudins feldspar augen and crystallographic fabrics the metamorphic isograds are very close within this shear zone as a result of shear deformation a gradual transition from upper amphibolite to lower greenschist facies occurs within 200 m movement in this zone occurred late in the metamorphic history under greenschist or lower metamorphic conditions the minimum horizontal extension and vertical displacement are on the order of 16 and 19 km respectively

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

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

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

magmatism and metamorphism in the ladakh himalayas the indus tsangpo suture zone abstract ladakh india provides a complete geological section through the northwestern part of the himalayas from kashmir to tibet within this section the magmatic metamorphic and geotectonic evolution of the northern himalayan orogeny has been studied using petrographic geochemical and isotope analytical techniques the beginning of the himalayan cycle was marked by large basaltic extrusions panjal trap of permian to lower triassic age at the northern margin of the gondwana continent indian shield these continental type tholeiitic basalts were followed by a more alkaline volcanism within the triassic to jurassic lamayuru unit of the gondwana continental margin lower jurassic to cretaceous oceanic crust and sediments ophiolitic melange s s accompany the triassic to cretaceous flysch deposits within the indus tsangpo suture zone the major structural divide between the indian shield high himalaya and the tibetan platform so far no relic of paleozoic oceanic crust has been found subduction of the tethyan oceanic crust during upper jurassic and cretaceous time produced an island arc represented by tholeiitic and calc alkaline volcanic rock series dras volcanics and related intrusives accompanied by volcaniclastic flysch deposits towards the tibetan continental margin subsequent to the subduction of oceanic crust large volumes of calc alkaline plutons trans himalayan or kangdese plutons intruded the tibetan continental margin over a distance of 2000 km and partly the dras island arc in the ladakh region the collision of the indian shield and tibetan platform started during the middle to upper eocene and caused large scale still active intracrustal thrusting as well as the piling up of the himalayan nappes the tectonically highest of these nappes is built up of oceanic crust and huge slices of peridotitic oceanic mantle spongtang klippe in the high himalayas the tectonic activity was accompanied and outlasted by a barrovian type metamorphism that affected triassic sediments of the kashmir nun kun synclinorium up to kyanite staurolite grade and the deeper seated units up to sillimanite grade cooling ages of micas are around 20 m y muscovite and 13 m y biotite towards the indus tsangpo suture zone metamorphism decreases with no obvious discontinuity through greenschist prehnite pumpellyite to zeolite grade remnants of possibly an eo himalayan blueschist metamorphism have been found within thrust zones accompanying ophiolitic melange in the suture zone

plate tectonic interpretation of the western himalaya abstract the western himalaya which includes the regions of kumaun garhwal himachal jammu and kashmir in india displays a complete cross section of the himalayan orogenic belt encompassing the principal tectonic zones of outer himalaya lesser himalaya higher himalaya tethys himalaya and trans himalaya the rock formations of the lesser himalaya which are correlatable to those of the northern part of the indian peninsular shield represent deposits on the northern edge of the indian craton the phanerozoic sequence of the tethys himalaya zone is interpreted as representing sedimentary deposits of the continental margin the central crystallines of the higher himalaya zone underlying the phanerozoic sequence of the tethys himalaya zone have been exposed in their present position as a result of uplift and southward movement along the main central thrust the trans himalaya zone consists of the indus suture ladakh magmatic arc shyok suture and karakoram subzones the nidar ophiolite and the shergol and zildat ophiolitic melanges of the indus suture are the tectonised remnants of the tethys ocean floor the ladakh plutonic complex the dras volcanics and other related cretaceous volcanics belong to the plutonic volcanic series of the ladakh magmatic arc the shyok suture is interpreted as a back arc basin and the palaeozoic mesozoic sequence of the karakoram subzone represents continental margin deposits of the karakoram block the molasse sedimentary deposits of the outer himalaya zone are interpreted as representing post collision sedimentation in a trough that developed in front of the rising himalaya the tectonic evolution of northwestern himalaya has been involved in the following sequence of events 1 a subduction of the indian oceanic plate under the karakoram tibet block in the cretaceous giving rise to the ladakh magmatic arc and arc trench gap sedimentation 2 b collision of the indian plate with the ladakh magmatic arc in the middle eocene forming the indus palaeosubduction zone 3 c closure of the shyok back arc basin and collision of the ladakh magmatic arc with the karakoram block 4 d suturing of the lithospheric blocks as a result of collision the penetrative and non penetrative deformation regional metamorphism generation of s type granites in the higher himalaya zone and the formation of nappes and thrusts owe their origin to crustal shortening in the oligocene miocene

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

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

metamorphic evolution 40ar 39ar chronology and tectonic model for the neelum valley azad kashmir ne pakistan

the contact between the higher himalaya crystallines and the tibetan sedimentary series miocene large scale dextral shearing space and time evolution of the synmetamorphic structures across the metamorphic pile have been studied in several areas along the himalayan belt east and central nepal garhwal zanskar from one area to the other the evolution is very similar 1 at the base of the pile in the main central thrust mct shear zone the stretching lineation penetrative and regularly oriented n0 e to n30 e indicates the mct transport direction very constant all along the belt from the eohimalayan main metamorphic development up to the late metamorphic movements 2 at the top of the pile at the contact between the crystalline unit and its sedimentary cover gravity driven structures are confirmed north vergent folds ductile normal faulting however there are numerous local indications of late miocene syn to late emplacement of the leucogranitic plutons dextral shearing 3 in between across the medium part of the pile the stretching lineation shows a conspicuous progressive regional clockwise rotation clearly indicated by the strain trajectories mapped in nepal and garhwal points 1 and 2 show that the crystalline sedimentary boundary despite its apparent structural and metamorphic continuity is not only a normal fault but also an important dextral shear zone which has acted since the upper miocene as the main southern limit of the eastward extruding tibetan block

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

synorogenic extension quantitative constraints on the age and displacement of the zanskar shear zone northwest himalaya structural thermobarometric and geochronological data place limits on the age and tectonic displacement along the zanskar shear zone a major north dipping synorogenic extensional structure separating the high grade metamorphic sequence of the high himalayan crystalline sequence from the overlying low grade sedimentary rocks of the tethyan himalaya a complete barrovian metamorphic succession from kyanite to biotite zone mineral assemblages occurs within the 1 km thick zanskar shear zone thermobarometric data indicate a difference in equilibration depths of 12 3 km between the lower kyanite zone and the garnet zone which is interpreted as a minimum estimate for the finite vertical displacement accommodated by the zanskar shear zone for the present day dip of the structure 20 a simple geometrical model shows that a net slip of 35 9 km is required to regroup these samples to the same structural level because the kyanite to garnet zone rocks represent only part of the zanskar shear zone and because its original dip may have been less than the present day dip these estimates for the finite displacement represent minimum values field relations and petrographic data suggest that migmatization and associated leucogranite intrusion in the footwall of the zanskar shear zone occurred as a continuous process starting at the barrovian metamorphic peak and lasting throughout the subsequent extension induced exhumation geochronological dating of various leucogranitic plutons and dikes in the zanskar shear zone footwall indicates that the main ductile shearing along the structure ended by 19 8 ma and that extension most likely initiated shortly before 22 2 ma

high pressure metamorphism in the high himalayan crystallines of the stak valley northeastern nanga parbat haramosh syntaxis pakistan himalaya

early palaeozoic orogenesis along the indian margin of gondwana tectonic response to gondwana assembly abstract shrimp u pb dating of zircons from a peralkaline s type lesser himalayan granite from the kathmandu region nepal indicate an age of emplacement of 475 ma the granites along with metasedimentary xenoliths show a similar signature of inherited detrital zircon ages ranging from archaean to early palaeozoic with prominent late mesoproterozoic grenvillian and neoproterozoic pan african age peaks and have a maximum age of 500 ma based on the youngest detrital grains deformation structures in xenoliths are truncated by the granite and along with the granites are assigned to a cambro ordovician orogenic event herein termed the bhimphedian orogeny that can be traced across the himalaya from pakistan to the eastern himalaya and possibly extends west into afghanistan we interpret the orogeny as being related to andean type orogenic activity on the northern margin of the indian continent following gondwana assembly the magmatic arc was associated with andesitic and basaltic volcanism and was active from c 530 to 490 ma the arc activity overlaps with and is succeeded by regional deformation crustal melting and s type granite emplacement that extended to 470 ma orogenic activity was driven by coupling across the plate margin either during on going subduction or through accretion of microcontinental ribbons possibly represented by the lhasa and qiangtang blocks it represents the termination of an orogenic cycle termed the north indian orogen that commenced with neoproterozoic rifting and passive margin development and terminated with the bhimphedian orogeny this was succeeded by a return to passive margin setting along the northern indian margin of gondwana which continued until the cenozoic himalayan orogeny

metamorphic evolution of the high himalayan crystallines in se zanskar india the high himalayan crystallines hhc of se zanskar consist of biotite paragneisses of orthogneisses that derive from early palaeozoic granitoids of minor metabasics and of post metamorphic leucogranites of miocene age two main metamorphic events have been documented in the hhc the first event occurred at p 12 0 0 5 kbar and t 750 50 c in rare metabasics intruded by early palaeozoic granitoids in the biotite paragneisses thermobarometric estimates of the first event point to comparable t at p 4 5 kbar lower the first event is followed by a pervasive syn tectonic crystallization characterized by lower p and t on the basis of the cooling ages of the metamorphic minerals and on the geological evidence the second event is referred to the tertiary himalayan crystallization further petrological and geochronological studies are necessary to prove whether a few mineral relics ascribed to the first event define a polyphase himalayan evolution or if they record the incomplete obliteration of an older history during the himalayan event the hhc of se zanskar show a decrease in metamorphic grade from the middle structural levels upward close to the kade unit and downward close to the lesser himalaya from sillimanite k feldspar biotite bearing assemblages to kyanite staurolite muscovite bearing assemblages this metamorphic zonation is probably a consequence of the polyphase history of intracontinental thrusts and of the tectonic emplacement of hot crustal slabs within shallower and colder thrust sheets at relatively late stages of the continental collision between india and eurasia

himalayan gneiss dome formation in the middle crust and exhumation by normal faulting new geochronology of gianbul dome northwestern india a general lack of consensus about the origin of himalayan gneiss domes hinders accurate thermomechanical modeling of the orogen to test whether doming resulted from tectonic contraction e g thrust duplex formation antiformal bending above a thrust ramp etc channel flow or via the buoyant rise of anatectic melts this study investigates the depth and timing of doming processes for gianbul dome in the western himalaya the dome is composed of greater himalayan sequence migmatite paleozoic orthogneiss and metasedimentary rock cut by multiple generations of leucogranite dikes these rocks record a major penetrative d2 deformational event characterized by a domed foliation and associated ne sw trending stretching lineation and they are flanked by the top down to the sw normal sense khanjar shear zone and the top down to the ne normal sense zanskar shear zone the western equivalent of the south tibetan detachment system monazite u th pb geochronology records 1 paleozoic emplacement of the kade orthogneiss and associated granite dikes 2 prograde barrovian metamorphism from 37 to 33 ma 3 doming driven by upper crustal extension and positive buoyancy of decompression melts between 26 and 22 ma and 4 the injection of anatectic melts into the upper levels of the dome neutralizing the effects of melt buoyancy and potentially adding strength to the host rock by ca 22 6 ma on the southwestern flank and ca 21 ma on the northeastern flank as shown by a northeastward decrease in 40 ar 39 ar muscovite dates from 22 4 to 20 2 ma ductile normal sense displacement within the zanskar shear zone ended by ca 22 ma after which the gianbul dome was exhumed as part of a rigid footwall block below the brittle zanskar normal fault tilting an estimated 5 10 sw into its present orientation

high p t polymetamorphism dehydration melting and generation of migmatites and granites in the higher himalayan crystalline complex sikkim india the higher himalayan crystalline complex hhc in sikkim india consists of pelitic migmatites interlayered with calc silicate rocks and minor metabasites microstructural relationships between the mineral phases and deformational fabric elements and zoning characteristics of garnet indicate a prolonged and complex polymetamorphic history for the hhc the pelitic rocks in the upper part of the hhc contain the assemblage plagioclase quartz garnet k feldspar biotite sillimanite and are devoid of muscovite most of the mineral phases grew syn to post tectonically mineral growth coincided with development of a pervasive fabric s 2 during prograde metamorphism m 2 in the early stages of the collisional event some garnet grains display texturally distinct cores and rims which are separated by calcic plagioclase this texture suggests an earlier metamorphic episode m l m may represent pre himalayan metamorphism and decompression of the hhc later the collisional event led to renewed burial of the hhc and m 2 reactions m 2 is reflected by dehydration melting of muscovite and biotite to form granitic melts which either crystallized in situ to form leucosomes or migrated from their source regions to form larger granitic bodies geothermobarometric estimates for peak m 2 conditions indicate p 10 12 kbar t 800 850 c a subsequent metamorphic event m 3 occurred because of 5 kbar of decompression m 3 is recorded by the breakdown of porphyroblastic garnet in all hhc lithologies higher temperature and pressure estimates come from progressively higher structural levels of the complex the thermal gradient of 5 5 c km is anomalous and may be a consequence of thermal buffering during melting however the pressure gradient of 0 25 kbar km resembles a normal lithostatic gradient which suggests that the hhc in sikkim represents an inverted barrovian sequence this inverted zonation of the hhc is probably the result of large scale structural inversion and or tectonic juxtaposition because of ductile shearing

origin of the inverted metamorphism of the lower himalayas central nepal abstract the southern slope of the central nepal himalayas is divided geologically by the main central thrust zone into the underlying midland metasediment zone lower himalayas in the south and the overlying himalayan gneiss zone higher himalayas in the north the former zone consists of a thick pile of various kinds of metasediments folded into an anticlinorium and the latter zone consists of high grade crystalline rocks with a north dipping homoclinal structure and is further overlain by the tethys himalayas composed of non or slightly metamorphosed tethyan sediments the main central thrust zone shows a characteristic lithology such as calcareous schist quartzite phyllitic schist and augen gneiss and is intensely mylonitized the mineral composition and paragenesis of the rocks in the present area indicate an inverted metamorphism from the midland metasediment to the main central thrust zones that is the metamorphic grade of the midland metasediment zone increases upward toward the thrust zone from the chlorite to the biotite zones of the greenschist facies and the greenschist amphibolite transitional zone is attained in the thrust zone the himalayan gneiss zone is apparently much higher in metamorphic grade than the thrust zone on the basis of the compositional zoning of garnets and muscovite compositions however it is concluded that the himalayan gneisses suffered a polymetamorphism the younger metamorphism resulted in a retrograde metamorphism of the himalayan gneisses together with the above mentioned inverted metamorphism and an anatectic melting in depth this was caused by shear heating due to the thrust movement along the main central thrust zone during the alpine tectonism the older metamorphism which seems to be precambrian in age was much higher in metamorphic grade than the younger one and formed the pyrope rich cores of garnets in the himalayan gneisses the maximum metamorphic temperature of 550 c estimated from the mineral paragenesis in the thrust zone is compatible with that caused by shear heating due to the thrusting with thrusting velocity of 5 10 cm yr maximum stress of 300 bars and thrusting distance of 250 km bird 1978