identification of inherited radiogenic pb in monazite and its implications for u pb systematics the simplest interpretation of uranium lead mineral ages in igneous rocks embodies the assumption that melting of pre existing rocks leads to homogenization of the lead isotopes thus resetting the radiometric clock zircons often violate this assumption by only partially dissolving and preserving a component of lead inherited from before the melting episode but until now such behaviour has not been observed in monazite here we report u pb data for zircons and monazites from a tertiary granite from the himalayas in which zircons and monazites contain inherited radio genic lead an analysis of a single entirely igneous zircon crystal plots on concordia at 19 5 0 4 myr eight monazite fractions lie on a line from an upper intercept of 471 10 myr to the youngest fraction which plots slightly above concordia at 21 myr the presence of excess 206pb in the youngest monazite fractions indi cates that the monazite data represent a mixing line between young magmatic monazite and an older inherited component rather than a resetting of 471 myr monazites at 20 myr these data suggest a closure temperature for pb in monazite significantly higher than previously published estimates

provenance of lower cretaceous wolong volcaniclastics in the tibetan tethyan himalaya implications for the final breakup of eastern gondwana abstract lower cretaceous volcanic lithic arenites widely distributed in the tethyan himalaya provide insights into the continental breakup of eastern gondwana in southern tibet they are represented by the wolong volcaniclastics the volcanic rocks that contributed clastic material to the lower parts of this unit were predominantly alkali basalts whereas rhyolitic dacitic volcanism becomes the predominant source of the upper strata geochemical analyses of basaltic grains and of detrital cr spinels from the wolong volcaniclastics demonstrate the alkaline character of the volcanism and suggest within plate tectonic setting for the volcanism zircon u pb ages confirm that this volcanism continued from 140 ma to 119 ma hf isotope data on these early cretaceous zircons indicate that their parental magmas were mantle derived but in the later stage of magmatic activity mantle derived magmas were mixed with partial melts derived from the continental crust the lower cretaceous volcaniclastics occur along a broad belt paralleling the northern margin of greater india the onset of volcaniclastic deposition in the himalayas appears to become progressively younger toward the west but it ended synchronously during the late albian 102 ma the low volume of volcanic rocks and their intra plate tectonic setting suggest that they are the result of decompressional melting along extensional deep seated fractures cross cutting the continental crust and reflect changes in regional intra plate tectonic stresses when greater india began to separate from the australia antarctica supercontinent

rb sr sm nd and pb isotope systematics of pyrite implications for the age and genesis of lode gold deposits lode gold deposits commonly consist of pyrite and lesser amounts of lead and zinc sulfides accompanied by quartz and calcite their exact origin remains controversial mainly because of the scarcity of reliable geochronological data here we present direct rb sr dating of pyrite from a lode gold deposit and propose the method as a useful geochronological technique for gold mineralization for which age data are poor a positive correlation between present day 87 sr 86 sr and 87 rb 86 sr ratios of pyrite fes 2 subsamples from the linglong gold mine of china corresponds to an age of 122 123 ma which dates the age of gold mineralization the sr nd and pb isotopic compositions of pyrites and the associated rocks suggest that the hydrothermal fluids responsible for the pyrite and gold mineralization were probably derived from a mixed source i e degassing of mafic magmas and meteoric water that had leached the country rocks

lower crustal processes leading to mesozoic lithospheric thinning beneath eastern north china underplating replacement and delamination abstract our recent studies show that the lower crust and lithospheric mantle in the eastern north china craton ncc was thinned in the mesozoic the lower crust played a key role in the tectonic mechanism that led to lithospheric thinning geochemical data from volcanic rocks and xenoliths support the idea that delamination of the lower crust and lithospheric mantle took place only along the northern and perhaps the southern margins of the ncc and this was linked to post collisional orogenic processes however it is difficult for the delamination process to act on the whole eastern ncc and remove 80 120 km of the lithosphere lower crustal xenoliths in mesozoic and cenozoic basalts within the ncc are of two kinds one is granulite facies metamorphosed two pyroxene gabbros and eclogite facies metamorphosed garnet pyroxenites formed at 140 120 ma the other is precambrian granulites that were strongly overprinted by mesozoic metamorphism large scale lower crustal replacement took place in the mesozoic beneath the ncc in other words some or perhaps most part of the present lower crust of the ncc is not precambrian the present lowermost crust is mostly composed of mesozoic meta gabbros and pyroxenites abundant late jurassic high sr granites in the ncc which are compositionally similar to a dakite probably formed by partial melting of the lowermost crust leaving a residue of eclogite or garnet amphibolite a hot upwelling mantle was necessary to partially melt the pre existing lowermost crust and underplate magma in order to form the present lower crust although eclogites could sink into the mantle it is hard to imagine that there was a thick enough eclogite layer that could drag 80 120 km of the lithosphere into the asthenosphere beneath the ncc magmatic underplating seems to be a better alternative mechanism thus the formation of the present lower crust through the underplating and replacement of the pre existing one was closely related to lithospheric thinning and represented a continuation of the same geodynamic process

triassic shoshonitic dykes from the northern north china craton petrogenesis and geodynamic significance zircon u pb ages major and trace element geochemistry and sr nd and pb isotope compositions of diorite and diorite porphyry dykes from the jinchanggouliang jcgl gold ore field on the northern margin of the north china craton ncc were studied to investigate their sources petrogenesis and geodynamic significance la icp ms zircon u pb dating reveals three major age groups of 2500 ma n 2 253 7 ma n 5 and 227 1 ma n 9 the inherited ages of 2500 ma contemporary with the archaean ncc continental growth imply that crustal material was involved in the magma source the igneous zircons with a concordia age of 227 1 ma may record the emplacement age of the jcgl dykes both diorite and diorite porphyry exhibit a wide range of sio 2 and mgo contents and are characterized by high concentrations of na 2 o k 2 o and al 2 o 3 and low abundances of p 2 o 5 and tio 2 they are enriched in large ion lithophile elements and light rare earth elements without significant eu anomalies and depleted in high field strength elements all are categorized as shoshonitic rocks all samples show a narrow range of sr isotope compositions with initial 87 sr 86 sr ratios from 0 70394 to 0 70592 variable e nd t values 1 1 to 12 0 and t dm2 ages of 913 1972 ma their pb isotope compositions form continuous variation trends and plot in the fields between enriched mantle 1 em1 and lower continental crust lcc the above results suggest that the jcgl dykes studied could have been derived from mixing of lower crust lithospheric mantle of the ncc and ascending asthenospheric melt in a post orogenic extensional geodynamic setting these shoshonitic dykes together with the geochronological data of regional ene trending retrograded eclogites ophiolites continental arc magmatic belt a type granite alkaline intrusions and metamorphic core complex from the northern ncc and central asian orogenic belt caob suggest that closure of the palaeo asian ocean i e stage of pre collision to collision had completed during latest permian to earliest triassic time and that the caob was subsequently tectonically dominated by post orogenic extensional regimes the involvement of asthenospheric melt in the magma source implies that the sub continental lithospheric mantle sclm of the ncc had been modified and the onset of lithospheric destruction and thinning beneath the northern ncc may have occurred in middle late triassic time as a result of post orogenic subducting slab detachment and lithospheric delamination

geologic correlation of the himalayan orogen and indian craton part 2 structural geology geochronology and tectonic evolution of the eastern himalaya despite being the largest active collisional orogen on earth the growth mechanism of the himalaya remains uncertain current debate has focused on the role of dynamic interaction between tectonics and climate and mass exchanges between the himalayan and tibetan crust during cenozoic india asia collision a major uncertainty in the debate comes from the lack of geologic information on the eastern segment of the himalayas from 91 e to 97 e which makes up about one quarter of the mountain belt to address this issue we conducted detailed field mapping u pb zircon age dating and 40ar 39ar thermochronology along two geologic traverses at longitudes of 92 e and 94 e across the eastern himalaya our dating indicates the region experienced magmatic events at 1745 1760 ma 825 878 ma 480 520 ma and 28 20 ma the first three events also occurred in the northeastern indian craton while the last is unique to the himalaya correlation of magmatic events and age equivalent lithologic units suggests that the eastern segment of the himalaya was constructed in situ by basement involved thrusting which is inconsistent with the hypothesis of high grade himalaya rocks derived from tibet via channel flow the main central thrust in the eastern himalaya forms the roof of a major thrust duplex its northern part was initiated at ca 13 ma while the southern part was initiated at ca 10 ma as indicated by 40ar 39ar thermochronometry crustal thickening of the main central thrust hanging wall was expressed by discrete ductile thrusting between 12 ma and 7 ma overlapping in time with motion on the main central thrust below restoration of two possible geologic cross sections from one of our geologic traverses where one assumes the existence of pre cenozoic deformation below the himalaya and the other assumes flat lying strata prior to the india asia collision leads to estimated shortening of 775 km 76 strain and 515 km 70 strain respectively we favor the presence of significant basement topography below the eastern himalaya based on projections of early paleozoic structures from the shillong plateau i e the central shillong thrust located 50 km south of our study area since northeastern india and possibly the eastern himalaya both experienced early paleozoic contraction the estimated shortening from this study may have resulted from a combined effect of early paleozoic and cenozoic deformation

the south tibetan detachment system facilitates ultra rapid cooling of granulite facies rocks in sikkim himalaya 1 the eastern himalaya is characterized by a region of granulites and local granulitized eclogites that have been exhumed via isothermal decompression from lower crustal depths during the india asia collision spatially most of these regions are proximal to the south tibetan detachment system an orogen parallel normal sense detachment system that operated during the miocene suggesting that it played a role in their exhumation here we use geo and thermochronological methods to study the deformation and cooling history of footwall rocks of the south tibetan detachment system in northern sikkim india these data demonstrate that the south tibetan detachment system was active in sikkim between 23 6 and 13 ma and that footwall rocks cooled rapidly from 700 to 120 c between 15 13 ma while active the south tibetan detachment system exhumed rocks from mid crustal depths but an additional heat source such as strain heating advected melt and or crustal thinning is required to explain the observed isothermal decompression cessation of movement on the south tibetan detachment system produced rapid cooling of the footwall as isotherms relaxed a regional comparison of temperature time data for the eastern south tibetan detachment system indicates a lack of synchronicity between the sa er sikkim yadong section and the nw bhutan section to accommodate this requires either strike slip tear faulting or local out of sequence thrusting in the younger segment of the orogen

petrogenesis of the dengzhazi a type pluton from the taihang yanshan mesozoic orogenic belts north china craton abstract the voluminous mesozoic monzonitic to monzogranitic rocks in the north china craton ncc mostly show high k calc alkaline and i type granitoids features the dengzhazi granitic pluton however shows features typical of a type granites the a type pluton was emplaced in the taihang yanshan orogenic belts of the northern margin of the ncc with zircon u pb ages of around 140 ma the dengzhazi a type granites are characterized by high sio 2 70 2 77 7 wt k 2 o na 2 o zr nb ga zn and y contents as well as high ga al ratios and extremely low cao ba sr in addition they show high zircon saturation temperatures 870 950 c low water and low oxygen fugacity all these features are consistent with the a type affinity of the pluton in situ hf isotopic analyses for the dated zircons show relatively small range of e hf t 13 to 17 they also have homogeneous initial nd isotopic compositions with e nd t ranging from 15 1 to 16 3 the hf and nd isotopic data suggest that the dengzhazi a type granites originated from a homogeneous crustal source probably the archean mafic intermediate granulites taking into account the high temperatures the low h 2 o and f o 2 of the magma system we believe that partial melting of the granulites should have been triggered by underplating of mantle derived magmas at the base of the mafic lower crust in an extensional regime the dengzhazi a type granite is the oldest pluton of the taihang yanshan mesozoic magma belts signifying the commencement of extensive underplating of mafic magmas and thus of lithospheric thinning in the northern ncc

constraining the area of rapid and deep seated exhumation at the st elias syntaxis southeast alaska with detrital zircon fission track analysis the chugach st elias orogen southeast alaska southwest yukon formed in response to the ongoing yakutat microplate subduction collision with the north american plate due to heavy glaciation the region is a prime location to study active convergent orogenesis and climate tectonic interactions this study focuses on the long term distribution of deformation in the st elias syntaxis area where dextral motion along the plate bounding fairweather fault transitions into convergence we present 2718 new zircon fission track single grain ages from 26 glaciofluvial outwash samples the grain ages range from 293 ma to 0 2 ma and each sample contains two to five age populations with peaks between 267 64 ma and 1 2 0 7 ma 1 the rocks of the yakutat microplate are dominated by latest cretaceous and eocene fission track ages while the rocks of the north american plate in the syntaxis region reveal two exhumation phases at 5 1 ma and 2 7 ma the spatial pattern of 5 ma cooling ages shows that the area of rapid and deep exhumation at the st elias syntaxis is more extensive than previously known and confined to the south by the fairweather fault to the west by the seward glacier catchment and to the east possibly by the inferred connector fault the area seems to be 4800 km2 large and may extend farther to the north the new zircon fission track data further suggest a transpressional plate boundary since 30 ma and the onset of plate collision 15 12 ma

detection of inherited monazite in the manaslu leucogranite by 208pb232th ion microprobe dating crystallization age and tectonic implications abstract although leucogranites are among the least petrologically variable of all igneous rocks ironically they are among the most difficult to reliably date the high himalayan leucogranites have been the subject of numerous geochronological investigations because of their interrelationship with the most significant tectonic features of that mountain belt for a variety of reasons linked to the minimum melt composition of these leucogranites these dating studies have not been entirely successful we report results of a new ion microprobe dating method based on the decay of 232 th to 208 pb in monazite that has directly revealed the presence of inherited pb in monazite from the manalsu granite casting doubt on its previously accepted age monazite ages obtained from this leucogranite yield two distinct populations a large number of ages with a normal distribution and mean age of 22 4 0 5 ma 2 s e that we interpret to be the crystallization age and a smaller inherited fraction with an age of ca 600 ma because formation of the granite is thought to be related to slip on the main central thrust both this date and a second less precise result from a structurally similar pluton near mt everest indicate that the main central thrust was active in the interval 24 22 ma but do not constrain its initiation together with crosscutting relationships these data require that movement on the north himalayan fault occurred prior to 22 ma at both locations

exhumation of the main central thrust from lower crustal depths eastern bhutan himalaya geothermometry and mineral assemblages show an increase of temperature structurally upwards across the main central thrust mct however peak metamorphic pressures are similar across the boundary and correspond to depths of 35 45 km garnet bearing samples from the uppermost lesser himalayan sequence lhs yield metamorphic conditions of 650 675 c and 9 13 kbar staurolite kyanite schists about 30 m above the mct yield p t conditions near 650 c 8 10 kbar kyanite bearing migmatites from the greater himalayan sequence ghs yield pressures of 10 14 kbar at 750 800 c top to the south shearing is synchronous with and postdates peak metamorphic mineral growth metamorphic monazite from a deformed and metamorphosed proterozoic gneiss within the upper lhs yield u pb ages of 20 18 ma staurolite kyanite schists within the ghs a few metres above the mct yield monazite ages of c 22 1 ma we interpret these ages to reflect that prograde meta morphism and deformation within the main central thrust zone mctz was underway by c 23 ma u pb crystallization ages of monazite and xenotime in a deformed kyanite bearing leucogranite and kyanite garnet migmatites about 2 km above the mct suggest crystallization of partial melts at 18 16 ma higher in the hanging wall south verging shear bands filled with leucogranite and pegmatite yield u pb crystallization ages for monazite and xenotime of 14 15 ma and a 1 2 km thick leuco granite sill is 13 4 0 2 ma thus metamorphism plutonism and deformation within the ghs con tinued until at least 13 ma p t conditions at this time are estimated to be 500 600 c and near 5 kbar from these data we infer that the exhumation of the mct zone from 35 to 45 km to around 18 km occurred from 18 to 16 to c 13 ma yielding an average exhumation rate of 3 9 mm year 1 this process of exhumation may reflect the ductile extrusion by channel flow of the mctz from between the overlying tibetan plateau and the underthrusting indian plate coupled with rapid erosion

geochronological and geochemical constraints on the subduction modified lithospheric origin of the early cretaceous volcanic rocks in the western north huaiyang belt of dabie orogen china geochronological elemental and sr nd hf isotopic data for intermediate acid volcanic sequences from the western north huaiyang belt of the dabie orogen china provide new constraints for the nature of the lithosphere and the potential involvement of subducted yangtze crust these rocks are calc alkaline andesite trachyandesite and dacite they yield ages of 129 127 ma and show a geochemical affinity to arc volcanic rocks 87sr 86sr i ratios and end t values range from 0 7065 to 0 7080 and 16 9 to 13 2 respectively zircon in situ ehf t values range from 29 4 to 20 2 and hf model ages from 3 03 to 2 46 ga these volcanic rocks were likely derived from a hybridized source involving enriched lithosphere of the north china craton ncc and deep recycled continental crust of yangtze block our data along with geological observations from early cretaceous igneous rocks suggest an east west extensional magmatic pulse at 130 120 ma along the entire north huaiyang belt the extent of involvement of the yangtze continental crust with the lithosphere of the ncc decreases from east to west this reflects along strike changes in triassic northward deep subduction and subsequent exhumation of the yangtze crust further lithospheric extension in the early cretaceous led to unrooting of the thickened lithosphere and generation of cretaceous volcanic rocks along the north huaiyang belt supplementary material microprobe data for all samples are available at https doi org 10 6084 m9 figshare c 4611911

eocene neotethyan slab breakoff in southern tibet inferred from the linzizong volcanic record abstract formation of the linzizong volcanic successions in the lhasa terrane southern tibet has long been related to northward subduction of the neotethyan oceanic slab under asia here we report new 40ar 39ar age results for the volcanic rocks recovered from a large area 29 n to 32 n and 85 e to 93 e that together with literature data delineate two discrete stages of volcanism these are a widespread cretaceous stage and an intense but spatially confined paleogene stage the latter occurring only in the southern part of the lhasa terrane resulted in the linzizong volcanic successions our data furthermore suggest southward migration and intensification of the volcanism in the lhasa terrane with magmatic flare ups at ca 50 ma while the volcanic successions consist dominantly of calc alkaline rocks typical of arc lava geochemistry those formed during the flare up period show significant compositional variations from low k tholeiitic through calc alkaline to shoshonitic magma suites these observations enable us to interpret the volcanic southward migration and following flare ups as the consequences of rollback and breakoff of the subducted neotethyan slab that occurred ahead and in the early stage respectively of the india asia collision our interpretation that involves a major eocene tectonomagmatic activity and concomitant topographic uplift in southern tibet is consistent with himalayan metamorphic constraints regional sedimentary records and seismic tomography

temperature and age constraints on the metamorphism of the tethyan himalaya in central nepal a multidisciplinary approach abstract metasediments of devonian to triassic age of the tethyan himalaya th from several areas in central and western nepal between western dolpo and marsyandi valley were sampled for thermo metamorphic studies illite and chlorite crystallinity vitrinite reflectance calcite dolomite and chlorite chloritoid geothermometers k ar dating on illite rich fractions and zircon fission track thermochronology this paper reports for the first time the occurrence of chloritoid in the th out of the range of granite contact aureoles it also presents the first zircon fission track dating performed on th metasediments the peak temperatures of metamorphism have been around 250 300 c 320 350 c 330 370 c and 400 450 c in the western dolpo hidden valley manang and marpha areas respectively in the manang and hidden valley areas illite k ar data are interpreted as ages of recrystallized k white micas newly formed during metamorphism at around 25 30 ma in the marpha area illite k ar and zircon fission track ages 12 15 ma are consistent representing cooling ages after metamorphic overprinting of higher grade than in the other areas the joint investigation of the organic maturation and zircon ft chronology yields insight on the thermal calibration of zircon reset the rmax of 5 7 to 8 0 indicates a temperature range of ca 315 325 c which is in the partial annealing zone of the zircon ft thermochronometer our results might be explained by the presence of a thick thrust sheet once existing above the study area

paleocene eocene record of ophiolite obduction and initial india asia collision south central tibet 1 uppermost cretaceous to eocene marine sedimentary sequences occur both to the south and north of the yarlung zangbo suture in south central tibet they consist of indian margin strata of the northern tethyan himalaya and asian margin strata of the gangdese forearc both assemblages are characterized by major changes in depositional environment and sedimentary provenance at 65 ma and an appearance of detrital chromium rich spinel of ophiolite affinity tio2 generally 0 1 wt during the paleocene ophiolitic melange exposed along the suture could have provided a source for detrital spinel the melange occurs in the hanging wall of a north dipping south directed mylonitic shear zone which includes a tectonic sliver of mafic schist amphibole from the schist yields 40ar 39ar ages of 63 ma which we attribute to cooling during slip along the shear zone and southward obduction of the melange melange obduction was coeval with the development of an angular unconformity within the gangdese forearc basin to the north between late maastrichtian time and 62 ma upper paleocene to middle eocene sandstones in the northern tethyan himalaya yield 200 120 ma u pb detrital zircon ages and 190 170 ma 40ar 39ar detrital mica ages these detrital grains were most likely sourced from regions north of the yarlung zangbo suture suggesting that onset of india asia collision in south central tibet is middle eocene or older in age collectively our results support previous suggestions that oceanic rocks were obducted onto the northern margin of india during latest cretaceous earliest tertiary time coeval changes in gangdese forearc sedimentation raise the possibility that this obduction event marks onset of tectonic interaction between india and asia at 65 ma alternatively in concert with the conventional view of eocene collision initiation the obducted oceanic rocks may be of intraoceanic origin while coeval changes in gangdese forearc sedimentation may be a consequence of an increase in the rate of ocean continent convergence following the demise of the intraoceanic subduction zone

tectonic evolution of the central annapurna range nepalese himalayas the metamorphic core of the himalayan orogen or greater himalayan sequence is a northward tapering prism bound at the bottom by a n dipping family of thrust faults the main central thrust system and at the top by a n dipping family of normal faults the south tibetan detachment system research in the central annapurna range of nepal demonstrates a close temporal and spatial association between contractional and extensional deformation on these bounding fault systems and within the metamorphic core throughout much of the early miocene the main central thrust system is represented here by a 2 to 3 km thick zone of high strain that developed during two or more episodes of movement most of its displacement was concentrated along the chomrong thrust a sharp late metamorphic discontinuity that places middle amphibolite facies rocks of the greater himalayan sequence on top of lower amphibolite facies rocks of the lesser himalayan sequence the earliest demonstrable movement on this thrust system occurred 22 5 ma the most recent movement may be as young as pliocene the oldest element of the south tibetan detachment system in this area is the deorali detachment which appears to have been active at the same time as the earliest shortening structures of the main central thrust system fabrics related to the deorali detachment are disrupted by a previously unrecognized sw vergent thrust structure the modi khola shear zone the effect of this structure which is constrained to be between 22 5 and 18 5 ma was to shorten rock packages that had been extended previously during movement on the deorali detachment transition back to a local extensional regime after 18 5 ma was marked by development of the machhupuchhare detachment and related splays geologic evidence for rapid two way transitions between contraction and extension in the annapurna range indicates that extensional deformation in convergent settings does not only represent gravitational collapse at the end of an orogenic cycle it also appears to be an important factor in mountain range development

tracking the growth of the himalayan fold and thrust belt from lower miocene foreland basin strata dumri formation western nepal u s national science foundation nsf tectonics ear 1140068 conoco fellowship scholarship 2016 chevron texaco geology fellowship 2016 university of arizona galileo circle scholarship 2017 u s national science foundation nsf nsf ear 1338583 ear1649254

magma mixing revealed from in situ zircon u pb hf isotope analysis of the muhuguan granitoid pluton eastern qinling orogen china implications for late mesozoic tectonic evolution in this paper we present zircon u pb age and hf isotope data to document the significance of magma mixing in the formation of late jurassic granitoid intrusions in the eastern qinling orogen china the muhuguan granitoid pluton from this orogen consists of monzogranite and lesser biotite granite and granodiorite all containing abundant hornblende rich cumulates dioritic xenoliths and mafic magmatic enclaves mmes the monzogranite and granodiorite are intruded by a number of lamprophyre dykes both a cumulate and a dioritic xenolith samples have concordant zircon u pb ages of ca 161 1 ma but possess contrasting hf isotopic compositions the cumulate has more radiogenic zircon hf isotopes with negative ehf t values 7 9 to 2 5 and tdm1 ages of 0 9 1 1 ga indicating its derivation likely from basaltic rocks of the neoproterozoic to early paleozoic kuanping group in the area the dioritic xenolith has much lower zircon ehf t values of 19 5 to 8 8 and tdm2 ages of 2 4 1 7 ga consistent with a juvenile paleoproterozoic crust source presumably represented by the metabasic rocks of the qinling group in the area individual samples of the monzogranite mme and a lamprophyre dyke have u pb ages of 150 1 152 1 and 152 1 ma respectively demonstrating coeval mafic and felsic magmatism in the late jurassic the lamprophyre dyke has homogeneous highly negative zircon ehf t values 29 8 to 24 8 and archean tdm2 ages 3 0 2 7 ga and its genesis is interpreted as partial melting of an ancient enriched subcontinental mantle source zircons from the fine grained mme show a large range of ehf t between 29 1 and 9 8 overlapping values of the monzogranite and lamprophyre dyke samples zircon u pb age and hf isotopes of the mmes are consistent with their formation by mixing of crustal and enriched mantle derived magmas the main group of zircons from the monzogranite has ehf t values 17 9 to 9 3 and tdm2 ages 2 3 1 8 ga that are compatible with the dioritic xenoliths indicating that the former was produced by partial melting of paleoproterozoic crustal source with involvement of mantle derived magmas mafic magmatism revealed from the muhuguan pluton indicates that the eastern qinling orogen was dominated by lithospheric extension during the late jurassic compilation of existing geological and geochronological data suggests that this extensional event started in late jurassic ca 160 ma and persisted into the early cretaceous until ca 110 ma the jura cretaceous extension may have resulted from the late mesozoic westward subduction of the pacific plate beneath the east asian continental margin

magnetic polarity stratigraphy of the neogene siwalik group at khutia khola far western nepal the middle miocene pliocene siwalik group was deposited in the himalayan foreland basin in response to uplift and erosion in the himalayan fold thrust belt results of thermal demagnetization experiments on samples from the siwalik group in central and western nepal demonstrate that laminated siltstones yield paleomagnetic data useful for tectonic and magnetostratigraphic studies sandstones and paleosols of the siwalik group however generally display highly erratic paleomagnetic behavior during thermal demagnetization on the basis of these observations siltstones from a well exposed 2423 m thick section of the siwalik group in khutia khola far western nepal were sampled for magnetic polarity stratigraphy the siwalik group is composed of informal lower middle and upper members correlation of the resulting polarity stratigraphy with the geomagnetic polarity timescale indicates that the exposed section spans 13 30 to 7 65 ma the lower middle siwalik boundary occurs at 11 05 ma near the beginning of chron c5n the rate of sediment accumulation increases upsection similar to rate changes previously observed in the pakistan siwalik group and probably in response to increasing proximity of the himalayan thrust belt in the khutia khola section a discordant declination indicates that this region has rotated about a vertical axis 16 6 counterclockwise with respect to the indian subcontinent measurements of 13 c in paleosol carbonate indicate the predominance of c 3 plants until 7 65 ma and the clear presence of c 4 plants higher in the undated portion of the section

geochemistry of late mesozoic lamprophyre dykes from the taihang mountains north china and implications for the sub continental lithospheric mantle many late mesozoic lamprophyre dykes occur in the taihang mountains north china these lamprophyres are rich in large ion lithophile elements e g rb sr ba and k and light ree and have highly differentiated ree patterns they show large but regular variations in chemical and nd sr isotopic compositions all these suggest that the lamprophyres were produced from differentiation of a parental magma coupled with contamination by lower crust the parental magma was derived from melting of a long term enriched sub continental lithospheric mantle source as is indicated by the highly enriched isotopic signatures of the lamprophyres mantle enrichment in the area was probably produced by interaction of volatile rich melts released from the asthenosphere with the above lithosphere in middle proterozoic times

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

tectonics exhumation and drainage evolution of the eastern himalaya since 13 ma from detrital geochemistry and thermochronology kameng river section arunachal pradesh the exhumation history of the central himalaya is well documented but lateral variations in exhumation remain poorly constrained in this study we identify sediment source areas and examine the late neogene exhumation history of the eastern himalaya from the synorogenic sedimentary record of its foreland basin we present nd and hf isotopic data as well as apatite and zircon fission track analyses from the miocene pliocene siwalik group along the recently dated kameng river section in arunachal pradesh northeastern india our isotopic data show that siwalik group sediments deposited between 13 7 and 2 6 ma in arunachal pradesh were mainly derived from higher himalayan source rocks in contrast sediments deposited between ca 7 and 3 ma have far less negative end and ehf values that require involvement of the gangdese batholith and yarlung suture zone source areas via the brahmaputra river system consequently these sediments should also record incision of the namche barwa massif by this river source area exhumation rates of himalayan derived sediments determined from detrital zircon fission track data were on the order of 1 8 km m y in the fastest exhuming areas these rates are very similar to those calculated for the central himalaya and have been relatively constant since ca 13 ma our results do not support the hypothesis of a major change in exhumation rate linked to either local or regional climate change or to shillong plateau uplift during the miocene as reported elsewhere the zircon fission track data further suggest that exhumation of the namche barwa massif between 7 and 3 ma was much slower than the 10 km m y rate recorded in the recent past detrital apatite fission track data indicate deformation of the siwaliks due to forward propagation of the frontal thrust since around 1 ma

miocene burial and exhumation of the india asia collision zone in southern tibet response to slab dynamics and erosion the india asia collision zone in southern tibet preserves a record of geodynamic and erosional processes following intercontinental collision apatite fission track and zircon and apatite u th he data from the oligocene miocene kailas formation within the india asia collision zone show a synchronous cooling signal at 17 1 ma which is younger than the ca 26 21 ma depositional age of the kailas formation constrained by u pb and 40 ar 39 ar geochronology and requires heating burial after ca 21 ma and subsequent rapid exhumation data from the gangdese batholith underlying the kailas formation also indicate miocene exhumation the thermal history of the kailas formation is consistent with rapid subsidence during a short lived phase of early miocene extension followed by uplift and exhumation driven by rollback and northward underthrusting of the indian plate respectively significant removal of material from the india asia collision zone was likely facilitated by efficient incision of the paleo indus river and paleo yarlung river in response to drainage reorganization and or intensification of the asian monsoon

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

indian continental subduction and slab break off during tertiary collision high wavespeed seismic anomalies in the transition zone and uppermost lower mantle beneath the india asia collision zone imaged by body wave seismic tomography have been interpreted as subducted fragments of continental material in this study we focus on the prominent anomaly located beneath india between depths of about 450 and 900 km by combining the location of this anomaly with palaeogeographical positions of india we constrain the timing of the subduction event probably related to this anomaly we infer that a large portion of the north western margin of india initiated subduction at 35 5 ma along a 1500 km long wnw ese striking zone and ended with a progressive slab break off process this break off started most probably around 25 ma at the western end of the slab and propagated eastwards until complete break off around 15 ma this study helps to constrain better the amount of convergence between india and asia absorbed by continental subduction

archean crustal evolution in china the taishan complex and evidence for juvenile crustal addition from long term depleted mantle abstract the main framework for crustal evolution of the late archean taishan complex in the shandong province of china is constructed using data from field investigation petrographic examination geochemical analysis and isotopic age determination the history of the complex began with emplacement of basic and tonalitic magmas about 2 7 2 75 ga ago which were metamorphosed designated as metamorphism i to amphibolites and grey gneisses shortly after their emplacement these rocks constitute a metamorphic basement termed the wangfushan gneisses the basement gneisses were later intruded by a plutonic association including 1 alkali and light rare earth element lree enriched dioritic rocks and their differentiates 2 6 ga ago 2 granites and trondhjemites collectively termed the hushan granites also 2 6 ga ago and 3 the aolaishan type granites sensu stricto 2 4 2 5 ga ago the plutonic association in conjunction with the older basement gneisses was later subjected to a second period of deformation and metamorphism metamorphism ii 2 ga ago as inferred from published k ar biotite dates the chronological order of all the above events has been determined by a multichronometric approach and the age results in ma with 2 uncertainties are summarized as follows the relatively low i sr initial 87 sr 86 sr ratio and high positive nd t values for the wangfushan gneisses and the diorite group provides strong evidence for continental growth by addition of juvenile material ultimately separated from long term depleted mantle sources by contrast the sr and nd isotopic data for the aolaishan type granites coherently suggest that they are likely to be derived by anatexis of the basement gneisses at shallow depths crustal evolution of the taishan complex offers an additional example supporting the concept of a continental accretion differentiation superevent that during such a worldwide tectonothermal episode particularly in the archean important differentiation of the upper mantle has contributed to the growth of the continental crust by addition of juvenile material

apatite triple dating and white mica 40ar 39ar thermochronology of syntectonic detritus in the central andes a multiphase tectonothermal history we applied apatite u pb fission track and u th he triple dating and white mica 40 ar 39 ar thermochronology to syntectonic sedimentary rocks from the central andean puna plateau in order to determine the source area geochronology and source sedimentary basin thermal histories and ultimately the timing of multiple tectonothermal events in the central andes apatite triple dating of samples from the eocene geste formation in the salar de pastos grandes basin shows late precambrian devonian apatite u pb crystallization ages eocene apatite fission track aft and eocene miocene u th he ca 8 47 ma cooling ages double dating of cobbles from equivalent strata in the arizaro basin documents early eocene 46 2 3 9 ma and cretaceous 107 6 7 6 109 5 7 7 ma aft and eocene oligocene ca 55 30 ma u th he ages thermal modeling suggests relatively rapid cooling between ca 80 and 50 ma and reheating and subsequent diachronous basin exhumation between ca 30 ma and 5 ma the 40 ar 39 ar white mica ages from the same samples in the salar de pastos grandes area are mainly 400 350 ma younger than apatite u pb ages suggesting source terrane cooling and exhumation during the devonian early carboniferous together these data reveal multiple phases of mountain building in the paleozoic and cenozoic basin burial temperatures within the plateau were limited to

zircon shrimp u pb ages and in situ hf isotopic analysis for the mesozoic intrusions in south taihang north china craton evidence for hybridization between mantle derived magmas and crustal components abstract the north china craton ncc was stabilized in paleo proterozoic times and the eastern part of it was remobilized in the mesozoic as is indicated by the presence of voluminous mesozoic magmas in eastern ncc in particular the lithospheric mantle beneath eastern ncc was significantly thinned during the intense mesozoic tectono magmatic activity which makes the ncc different from most other ancient cratons the mesozoic magmas may thus provide crucial evidence regarding the processes that transformed the ncc from craton to a tectonically active region the south taihang magmatic complex lies in the western part of the mesozoic magmatic zone in the ncc the complex falls into three main plutonic rocks the hongshan syenite the xishu gabbroic diorite monzonite and the wu an monzonitic rocks zircon u pb dating reveals that the three plutonic rocks were emplaced contemporaneously at 132 ma in situ hf isotopic analyses for the dated zircons indicate that the hongshan syenites show homogeneous hf t values 10 to 13 which are slightly lower than those 9 0 for the enriched subcontinental lithospheric mantle sclm as revealed by zircons from mafic rocks that originated from melting of sclm this suggests that the parental magma to the syenites was likely derived from melting of the enriched sclm and contaminated slightly by old lower continental crustal material during ascent the xishu and wu an gabbroic diorites to monzonitic rocks however show significantly varied hf isotopic compositions with hf t 9 2 to 16 and 15 5 to 24 respectively the wide range of hf t values for a single sample suggests that the xishu and wu an plutons formed through a process of hybridization between enriched mantle derived mafic magma and crustal components in variable proportions this is supported by the textural and compositional disequilibrium of plagioclase phenocrysts from the two plutons the wu an monzonitic rocks show lower hf t values than the xishu gabbroic diorite monzonite suggesting more incorporation of crustal components in the source the sudden surge of magmatic activity in the south taihang orogen took place as a result of lithospheric extension that caused partial fusion of the enriched portions of sclm generating voluminous mafic magmas underplating of the mafic magmas in the lower crust consequently caused partial melting of the old basement rocks to produce granitic melts which was followed by mixing between the mafic and granitic magmas

the detrital record of orogenesis a review of approaches and techniques used in the himalayan sedimentary basins the sediment archive of material eroded from an active tectonic region and stored in adjacent basins can provide a valuable record of hinterland tectonism especially when information in the source region itself is obscured by later metamorphism or removed by tectonism or erosion using the sediment record to document tectonism is a well established approach but more recently there has been a burgeoning of the number of isotopic techniques which can be applied to detrital material in particular single grain analyses thus the scope for application of detrital studies to a number of different tectonic problems has widened considerably in this review the example of sediments eroded from the himalayan orogen and preserved in the suture zone basin foreland basin remnant ocean basins and deep sea fans is used to illustrate the approach techniques as diverse as petrography heavy mineral xrf and sr nd studies single grain dating by ar ar u pb and fission track methodologies and single grain sm nd and pb isotopic analyses are described the paper documents how the sediment record can be used to determine the thermal and tectonic evolution of the orogen constrain mechanisms of continental deformation exhumation rates and palaeodrainage

ductile extrusion of the higher himalayan crystalline in bhutan evidence from quartz microfabrics quartz textures measured from deformed quartz tectonites within the lesser himalaya and higher himalaya crystalline of bhutan show similar patterns orientation and distribution of the quartz crystallographic axes were used to confirm the regional shear sense the asymmetry of c axis and a axis patterns consistently indicates top to the south shearing the obliquity of the texture and the inferred finite strain plane strain to moderately constrictional suggest the strain regime had a combination of rotational and irrotational strain path in most of the samples from the bhutan himalaya the inferred deformation mechanisms suggest moderate to high temperature conditions of deformation that produced the observed crystallographic preferred orientation much higher temperature of deformation is indicated in the quartz veins from a leucogranite the observed ductile deformation is pervasively developed in the rocks throughout the investigated area the intensity of deformation increases only slightly in the vicinity of the main central thrust simultaneous southward shearing within a large part of the higher himalaya crystalline near and above the main central thrust and normal faulting across the south tibetan detachment is explained by the tectonically induced extrusion of a ductily deforming wedge the process of extrusive flow suggested here can be approximated quantitatively by channel flow models that have been used to describe subduction zone processes channel flow accounts for some observed phenomena in the himalayan orogen such as inverted metamorphic sequences near the main central thrust not related to an inversion of isotherms and the syntectonic emplacement of leucogranites into the extruding wedge locally leading to an inversion of isotherms due to heat advection

the kinematic evolution of the nepalese himalaya interpreted from nd isotopes neodymium nd isotopes from the himalayan fold thrust belt and its associated foreland basin deposits are useful for distinguishing between himalayan tectonostratigraphic zones and revealing the erosional unroofing history as controlled by the kinematic development of the orogen neodymium isotopic data from the himalayan fold thrust belt in nepal n 35 reveal that the lesser himalayan zone consistently has a more negative nd 0 value than the greater and tibetan himalayan zones our data show the average nd 0 value in the lesser himalayan zone is 21 5 whereas the greater and tibetan himalayan zones have an average nd 0 value of 16 these consistently distinct values throughout nepal enable the use of nd isotopes as a technique for distinguishing between lesser himalayan zone and greater himalayan zone rock the less negative nd 0 values of the greater himalayan rocks support the idea that the greater himalayan zone is not indian basement but rather a terrane that accreted onto india during early paleozoic time neodymium isotopic data from eocene through pliocene foreland basin deposits n 34 show that sediment provenance has been dominated by greater and tibetan himalayan detritus across nepal the nd t values in the synorogenic rocks in western and central nepal generally show an up section shift toward more negative values and record the progressive unroofing of the different tectonostratigraphic zones at 10 ma in khutia khola and 11 ma in surai khola a shift in nd t values from 16 to 18 marks the erosional breaching of a large duplex in the northern part of the lesser himalayan zone this shift is not seen in eastern nepal where the nd t values remain close to 16 throughout miocene time because there has been less erosional unroofing in this region

eohimalayan fold and thrust belt implications for the geodynamic evolution of the nw himalaya india 1 in the himalayas the eocene deformation phase is called the eohimalayan event by means of a forward and backward balanced cross section along a classical traverse through the tethyan himalaya we constrain eohimalayan sw vergent folding and sw directed thrusting in a belt above a basal thrust at 10 km depth this thrust is located at the structural position of the southern tibetan detachment system with a ne directed normal sense displacement suggesting miocene reactivation of a major eocene contractional fault stepwise palinspastic restoration demonstrates that this eocene contractional fault has been subsequently openly folded due to southwestward propagation of the himalayan thrust system

a fully coupled numerical modeling for regional unsaturated saturated water flow summary it is a long lasting challenge in subsurface hydrologic modeling to develop numerically efficient algorithm for coupling unsaturated and saturated flow especially in regional scale modeling in this study a new scheme is developed for coupled numerical simulation of unsaturated saturated water flow at the regional scale the modeling domain is divided into sub areas in horizon according to spatially distributed inputs flow characteristics and topography conditions the unsaturated zone of each sub area is represented by individual one dimensional soil column water balance analysis method is employed to formulate the three dimensional groundwater model the unsaturated and saturated zones are implicitly coupled in space and time through the vertical flow between the unsaturated soil columns and the saturated aquifers in that the heads in the unsaturated and saturated zones are integrated in a single matrix equation the coupling scheme is verified and computational efficiency is evaluated in several hypothetical examples by comparing the simulation results with those of widely used software including hydrus1d swms2d feflow and hydrogeosphere in the real world application numerical results show that the coupling model can obtain satisfactory simulation results with fairly little computational cost compared with existing models the new numerical scheme is more suitable to regional scale modeling with complex domain geometry and alternating recharge or discharge fluxes however due to the assumptions involved in the method development the coupling method has its intrinsic limitations and should be used with caution in cases where the lateral flow is predominant in the unsaturated zone

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

cretaceous tertiary carbonate platform evolution and the age of the india asia collision along the ladakh himalaya northwest india abstract the india asia collision resulted in the formation and uplift of the himalaya and the enhanced uplift of the tibetan plateau the transition from marine to continental facies within the indus yarlung tsangpo suture zone and along the northern margin of the indian plate provides the most accurate method of dating the closure of the tethys ocean separating the indian and asian plates other indirect methods of dating the collision such as paleomagnetism dating the uhp metamorphism along the north margin of india dating the youngest subduction related granites along the southern margin of asia and dating the postorogenic indus molasse group deposits within the suture zone cannot provide such a precise or reliable age of collision ophiolite obduction onto the indian passive margin occurred during the latest cretaceous and predated initial collision of the two continental plates unconformities occur beneath the late maastrichtian marpo formation and beneath the danian stumpata formation on the

a short duration pulse of ductile normal shear on the outer south tibetan detachment in bhutan alternating channel flow and critical taper mechanics of the eastern himalaya in easternmost bhutan the south tibetan detachment std is a ductile shear zone that juxtaposes the radi or sakteng klippe of the tethyan sedimentary series from underlying high grade greater himalayan rocks in situ la icpms u th pb analysis of metamorphic monazite from the immediate footwall and hanging wall of the std within the shear zone at the base of the klippe constrains north vergent normal shear to between 25 and 20 ma coeval thrusting on the main central thrust during this time supports a phase of channel flow viscous wedge model activity lasting only ca 3 ma geochronologic data from the eastern himalaya indicate alternating mechanisms for extrusion of the metamorphic core of the orogen from the late oligocene through to the late miocene switching from channel flow viscous wedge behavior to critical taper frictional wedge behavior each phase lasting approximately only 2 to 5 ma the tectonic evolution of the eastern himalaya is comparable to central and western himalayan tectonics during the early miocene but during the middle miocene metamorphism and magmatism in the eastern himalaya migrated towards the orogenic hinterland a process not widely documented elsewhere in the himalaya thus highlighting the need for an orogenic model in three spatial dimensions

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

miocene to holocene exhumation of metamorphic crustal wedges in the nw himalaya evidence for tectonic extrusion coupled to fluvial erosion the himalayan crystalline core zone exposed along the sutlej valley india is composed of two high grade metamorphic gneiss sheets that were successively underthrusted and tectonically extruded as a consequence of the foreland directed propagation of crustal deformation in the indian plate margin the high himalayan crystalline sequence hhcs is composed of amphibolite facies to migmatitic paragneisses metamorphosed at temperatures up to 750 c at 30 km depth between eocene and early miocene during early miocene combined thrusting along the main central thrust mct and extension along the sangla detachment induced the rapid exhumation and cooling of the hhcs whereas exhumation was mainly controlled by erosion since middle miocene the lesser himalayan crystalline sequence lhcs is composed of amphibolite facies para and orthogneisses metamorphosed at temperatures up to 700 c during underthrusting down to 30 km depth beneath the mct the lhcs cooled very rapidly since late miocene as a consequence of exhumation controlled by thrusting along the munsiari thrust and extension in the mct hanging wall this renewed phase of tectonic extrusion at the himalayan front is still active as indicated by the present day regional seismicity and by hydrothermal circulation linked to elevated near surface geothermal gradients in the lhcs as recently evidenced in the himalayan syntaxes active exhumation of deep crustal rocks along the sutlej valley is spatially correlated with the high erosional potential of this major trans himalayan river this correlation supports the emerging view of a positive feedback during continental collision between crustal scale tectono thermal reworking and efficient erosion along major river systems

timing quantification and tectonic modelling of pliocene quaternary movements in the nw himalaya evidence from fission track dating abstract variable exhumation rates deduced from the pliocene quaternary ft zircon apatite ages from the himalayan metamorphic belt hmb of the nw himalaya along the sutlej valley in himachal pradesh have been modelled in the tectonic framework of fast exhumed lesser himalayan windows which caused lateral extensional sliding of the metamorphic nappe cover along the well known main central thrust mct and differential movements along thrust zones as well in the northern belt of the higher himalayan crystallines hhc two distinct clusters of the ft apatite ages have been deciphered apatite ages having a weighted mean of 4 9 0 2 ma 1 in basal parts on the hanging wall of the mct and 1 49 0 07 ma 1 in the hanging wall of a newly recognized ne dipping chaura thrust further north fast exhumation of the chaura thrust hanging wall has been inferred at a rate of 4 82 0 55 mm yr from the zircon apatite cogenetic pairs during 1 54 ma and 0 97 ma and 2 01 0 35 mm yr since 1 49 ma in comparison its foot wall has been exhumed at a much slower rate of 0 61 0 10 mm yr since 4 9 ma the overlying vaikrita thrust zone rocks reveal an exhumation rate of 1 98 0 34 mm yr from 2 70 0 40 ma to 1 31 0 22 ma and 2 29 0 66 mm yr since 1 31 0 22 ma using these data a vertical displacement of ca 2 08 0 68 km has been calculated along the chaura thrust between 4 9 and 1 50 ma on an average rate of 0 6 mm yr it is of the order of 1 18 km from 2 70 ma to 1 54 ma along the vaikrita thrust and 0 78 mm yr from 1 31 ma to 0 97 ma and has behaved as an extensional normal fault during these periods tectonic modelling of the exhumation rates in the nw himalaya reveals fastest uplifting himalayan domes and windows like the nanga parbat in pakistan suru and chisoti domes in zanskar and kishwar kulu rampur window axis in se kashmir and himachal pradesh during pliocene quaternary these windows appear to have caused lateral extensional sliding of the himalayan metamorphic nappes in the lower parts the middle parts of the hhc belt have witnessed both overthrusting and extensional faulting due to complex and variable exhumation patterns within the hanging and foot walls of the mct and vaikrita thrust along the sutlej valley thus causing movement of upthrust crustal wedge between the extensional ones thus ft zircon apatite ages provide evidence for the presence of a number of crustal wedges having distinct tectonothermal history within the hhc

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

geochronology geochemistry and hf isotope of late triassic magmatic rocks of qingchengzi district in liaodong peninsula northeast china abstract the initiation timing and mechanism of lithospheric thinning of the north china craton ncc was still controversial late triassic igneous rocks especially mantle derived mafic rocks would provide constrains on early mesozoic lithospheric mantle geodynamics and initiation of lithospheric thinning this paper reports late triassic magmatic rocks including lamprophyre diorite dykes and biotite monzogranite cropped out in qingchengzi district of liaodong peninsula northeastern ncc la icpms zircon u pb dating yield ages of 210 227 ma and 224 ma for lamprophyres and biotite monzogranite respectively lamprophyre is ultrapotassic strongly enriched in ree and liles depleted in hfses and negative hf isotopes which are discriminating signatures of crustal source but distinguishingly high compatible element contents indicate the primary magma originated from mantle source a fertile one lamprophyre derived from partial melting of an enriched lithospheric mantle which was modified by slab derived hydrous fluids melts associated with deep subduction between the yangtze craton and the ncc the diorite displays distinct features with relatively enriched nb ta hree and depleted th u which suggest it derived from a relatively depleted source the depletion was caused by break off of the yangtze slab during deep subduction introducing asthenospheric mantle into the source the biotite monzogranite shows adakitic affinity and originated from partial melting of the thickened lower crust with addition of small proportion of mantle material the recognition of late triassic magmatism implies extensional tectonic settings in liaodong peninsula and suggests initiation of lithospheric thinning of north china craton in eastern segment might begin early in late triassic

laser 40ar 39ar dating of single detrital muscovite grains from early foreland basin sedimentary deposits in india implications for early himalayan evolution in india the dagshai and overlying kasauli formations represent the oldest exposed continental foredeep sediments eroded from the himalayan orogen 40ar 39ar dating of individual detrital white micas from these sedimentary units has provided maximum depositional ages of 28 ma for the dagshai formation at one locality and 25 ma at a second locality whereas deposition of the kasauli formation occurred after 28 ma at two localities and after 22 ma at a third locality this timing suggests that in india the start of substantial exhumation and erosion from the rising himalayan orogen was delayed until 28 ma

post cratonization deformation processes and tectonic evolution of the north china craton abstract the timing of events during the tectonic evolution of the north china craton ncc and the east china region is debated extension compression transpression and shearing occurred throughout the ncc since 320 ma these main structural stages and the temporal evolution of the structural framework and kinematics are summarized we present here data from multiple structural and magmatic stages related to temporally varying kinematics and deformation mechanisms during the evolution of the post cratonization of the ncc the timing of each stage is constrained using isotopic dating along with the timing of magmatic intrusion dyke intrusion and basin formation associated with tectonic transformation of the ncc age data indicate distinct structural and magmatic stages at 1600 1500 1300 1100 445 315 270 200 170 155 130 110 75 65 and 25 20 ma relating to basin formation and continental uplift or collapse these stages define the tectonic evolution and structural transformation of the ncc and associated de rooting reactivation reworking and orogenic growth the tectonic setting and movement direction of adjacent plates changed multiple times during the various stages the stages are as follows separation of the ncc from the gondwana supercontinent and northwards drift formation of the central asian orogenic belt caob and northwards subduction of the yangtze plate resulting in closure of the qinling ocean the rapid initial formation of the western pacific plate and subsequent subduction and final closure of the mongol okhotsk ocean combined with closure of the bangong nujiang suture zone and southwards movement of the siberian continent as well as thinning of the north china cratonic lithosphere moreover temporal and spatial changes in deformation magmatism and basin formation as well as kinematic and mechanical transformations occurred in response to the changing tectonic setting and movement direction of surrounding plates these various factors resulted in a complex structural evolution of the ncc since 1 8 ga

geologic and u pb geochronologic evidence for early paleozoic tectonism in the dadeldhura thrust sheet far west nepal himalaya abstract the dadeldhura thrust sheet inm western nepal consists of proterozoic lower paleozoic sedimentary and plutonic rocks and their metamorphic equivalents that rest structurally on proterozoic strata of the lesser himalayan sequence although regional metamorphism and ductile deformation were widespread during tertiary thrust emplacement relicts of early paleozoic tectonism are preserved locally new field and geochronologic studies together with the findings of previous workers indicate that this early paleozoic tectonism included 1 regional metamorphism to at least garnet grade 2 regional folding of a thick metamorphic sequence into a broad east west trending syncline 3 outcrop scale folding of metasedimentary rocks 4 emplacement of cambro ordovician granitic bodies during and after the metamorphism and deformation 5 uplift and erosion of the metamorphic sequence with garnet grade rocks locally exposed at the surface and 6 derivation of ordovician conglomeratic sandstones from the early paleozoic orogen similar records of metamorphism deformation and uplift erosion have been found in other regions of the himalaya indicating that rocks of the dadeldhura thrust sheet were originally involved in a regionally extensive orogenic system future tectonic models of himalayan orogenesis must accommodate this early paleozoic event

neotectonics of the thakkhola graben and implications for recent activity on the south tibetan fault system in the central nepal himalaya the thakkhola graben is one of many north trending rifts that define the neogene structural pattern of the southern tibetan plateau lying at the southern margin of the plateau and extending to the crest of the himalaya the graben provides an opportunity to evaluate the kinematic relationships between east west extensional strain in southern tibet and north south extensional strain in the himalaya neotectonic and structural mapping of the dangardzong fault along the western margin of the graben reveals a southward decreasing component of normal slip coupled with a southward increasing component of right lateral slip that affects pleistocene basin fill sediments we present 14c ages for river terraces in the thakkhola graben that provide a ca 17 2 ka minimum age on the latest stage of dangardzong fault movement near the southern termination of the graben the dangardzong fault apparently offsets the annapurna detachment an early miocene strand of the east striking south tibetan fault system however the dangardzong fault itself terminates against a young i e younger than ca 17 2 ka strand of the south tibetan fault system the dhumpu detachment structural relationships among the dangardzong annapurna and dhumpu faults suggest that the dangardzong structure is a tear fault in the south tibetan allochthon that accommodates differential amounts and rates of displacement along the south tibetan fault system thus although the south tibetan fault system first developed as part of the structural architecture of the himalaya in miocene time at least some strands have been active as recently as the pleistocene in a regional context the south tibetan fault system serves to accommodate the strain gradient between extension in tibet and shortening in the himalaya

zoned olivine xenocrysts in a late mesozoic gabbro from the southern taihang mountains implications for old lithospheric mantle beneath the central north china craton zoned olivine grains are abundant in the late mesozoic shatuo gabbro southern taihang mountains central north china craton olivine cores are rich in mgo and nio rims are rich in feo and mno and both cores and rims have very low cao contents the cores invariably have a high mg no 92 94 similar to olivine xenocrysts from palaeozoic kimberlites in eastern china the compositional features of these olivines imply that they are xenocrysts rather than phenocrysts namely disaggregates of mantle peridotites at the time of intrusion the compositional similarity of olivine cores to xenocrysts from palaeozoic kimberlites suggests that the lithospheric mantle beneath the central north china craton is ancient and refractory in nature and quite different from eastern china where the mantle is mainly composed of newly accreted materials resulting from large scale lithospheric removal and replacement the contrasting features of the lithospheric mantle beneath the eastern and central north china craton imply that the large scale lithospheric removal in phanerozoic times was mainly confined to the eastern north china craton

u pb zircon geochronology of major lithologic units in the eastern himalaya implications for the origin and assembly of himalayan rocks models for the origin and deformation of himalayan rocks are dependent upon geometric and age relationships between major units we present field mapping and u pb dating of igneous and detrital zircons that establish the lithostratigraphic architecture of the eastern himalaya revealing that 1 the south tibet detachment along the bhutan china border is a top to the north ductile shear zone 2 late triassic and early cretaceous sedimentary samples from the northern indian margin show a similar age range of detrital zircons from ca 3500 ma to ca 200 ma but the late triassic rocks are distinguished by a significant age cluster between ca 280 and ca 220 ma and a well defined age peak at ca 570 ma 3 an augen gneiss in the south tibet detachment shear zone in southeast tibet has a cambrian ordovician crystallization age 4 main central thrust hanging wall paragneiss and footwall quartzites from the far western arunachal himalaya share similar provenance and late proterozoic maximum depositional ages and 5 main central thrust footwall metagraywacke from the central western arunachal himalaya has a paleoproterozoic maximum depositional age indicated by a single prominent age peak of ca 1780 ma recent work in the eastern himalaya demonstrates that in the early middle miocene the himalayan crystalline core here was emplaced southward between two subhorizontal shear zones that merge to the south a proposed subsequent middle miocene brittle low angle normal fault accomplishing exhumation of these rocks along the range crest can be precluded because new and existing mapping demonstrates only a ductile shear zone here the ca 280 220 ma detrital zircons of the late triassic strata are derived from an arc developed along the northern margin of the lhasa terrane detritus from this arc was deposited on the northern margin of india during india lhasa rifting along strike heterogeneity in main central thrust footwall chronostratigraphy is indicated by detrital zircon age spectrum differences from central western to far western arunachal nonetheless the late proterozoic rocks in the main central thrust hanging wall and footwall in far western arunachal can be correlated to each other and to previously analyzed rocks in the south tibet detachment hanging wall to the west and in the indian craton to the south these findings are synthesized in a reconstruction showing late triassic india lhasa rifting and cenozoic eastern himalayan construction via in situ thrusting of basement and cover sequences along the north indian margin

the age and rate of displacement along the main central thrust in the western bhutan himalaya abstract in western bhutan the main central thrust mct is broadly folded creating multiple exposures of the fault surface over a 70 km across strike distance this unusual map pattern presents a unique opportunity to map the mct and document both the magnitude and age of displacement in situ th pb sims and la icp ms geochronology of metamorphic monazite from the immediate hanging wall of the mct indicates that prograde monazite growth in greater himalayan gh rocks continued until 20 8 1 1 ma whereas crystallization of in situ melts characterized by high y monazite overgrowths occurred during cooling from ca 15 10 ma prograde monazite growth at 15 ma in lesser himalaya lh rocks in the immediate footwall requires that lh footwall strata began to be buried at this time and the mct had reached its southernmost exposed extent by combining prograde monazite ages in the immediate hanging wall and footwall the duration of mct displacement is bracketed between 20 8 1 1 and 15 0 2 4 ma immediately north of our study area a published estimate of shearing along the outer south tibetan detachment std argues for displacement between 20 and 15 ma coeval with the age range for mct displacement that we document in this study however retrograde monazite grains as young as 10 ma suggest that gh rocks were cooling until 10 ma 5 myr later than motion on the outer std immediately to the north this cooling was either the result of continued displacement on the mct or growth of a duplex that passively folded the mct using a sequential reconstruction we estimate a total displacement of 230 km which is the sum of displacements on the mct and the structurally lower paro thrust over a duration of 5 8 2 6 myr this indicates a horizontal shortening rate of 4 0 3 2 1 3 cm yr which exceeds present rates estimated from geodetic measurements across the himalaya and mct displacement rates c 2 cm yr inferred from petrologic and thermal models in central nepal but is indistinguishable from plate convergence rates calculated for eastern bhutan between 23 and 20 ma 3 3 0 7 cm yr our study highlights that displacement on the mct alone achieved plate velocity rates in western bhutan and that the age and rate of mct displacement varied significantly across the himalaya

paleocene eocene foreland basin evolution in the himalaya of southern tibet and nepal implications for the age of initial india asia collision siliciclastic sedimentary rocks derived from the southern lhasa terrane sitting depositionally upon rocks of the northern indian passive continental margin provide an estimate of the age of initial contact between the continental parts of the indian and asian plates we report sedimentological sedimentary petrological and geochronological data from upper cretaceous paleocene strata in the sangdanlin section located along the southern flank of the indus yarlung suture zone in southern tibet this is probably the most proximal and therefore the oldest record of the india asia collision these strata were deposited by high density turbidity currents or concentrated density flows and suspension settling of pelagic biogenic debris in a deep marine setting an abrupt change from quartz arenitic to feldspatholithic sandstone compositions marks the transition from indian to asian sediment provenance the abrupt compositional change is accompanied by changes in u pb ages of detrital zircons diagnostic of a sediment provenance reversal from indian to asian sources the timing of the transition is bracketed between 60 ma and 58 5 0 6 ma by detrital zircon u pb ages and zircon u pb ages from a tuffaceous bed in the upper part of the section in the context of a palinspastically restored regional paleogeographic framework data from the sangdanlin section combined with previously published data from the northern tethyan himalaya and the frontal nepalese lesser himalaya and subhimalaya suggest that a flexural wave migrated 1300 km southward across what is now the himalayan thrust belt from paleocene time to the present

insights on the kinematics of the india eurasia collision from global geodynamic models the eocene india eurasia collision is a first order tectonic event whose nature and chronology remains controversial we test two end member collision scenarios using coupled global plate motion subduction models the first conventional model invokes a continental collision soon after 60 ma between a maximum extent greater india and an andean style eurasian margin the alternative scenario involves a collision between a minimum extent greater india and a neotethyan back arc at 60 ma that is subsequently subducted along southern lhasa at an andean style margin culminating with continent continent contact at 40 ma our numerical models suggest the conventional scenario does not adequately reproduce mantle structure related to tethyan convergence the alternative scenario better reproduces the discrete slab volumes and their lateral and vertical distribution in the mantle and is also supported by the distribution of ophiolites indicative of tethyan intraoceanic subduction magmatic gaps along southern lhasa and a two stage slowdown of india our models show a strong component of southward mantle return flow for the tethyan region suggesting that the common assumption of near vertical slab sinking is an oversimplification with significant consequences for interpretations of seismic tomography in the context of subduction reference frames

detrital zircon fission track ages from the lower cenozoic sediments nw himalayan foreland basin clues for exhumation and denudation of the himalaya during the india asia collision detrital zircon fission track ft ages from the lower cenozoic sub himalayan foreland basin reflect the progressive effects of crustal thickening and exhumation on the himalayan source rocks as a consequence of the india asia collision the oldest stratum the transgressive marine paleocene eocene subathu formation 57 41 5 ma contains ca 50 ma detrital zircon p1 peak which was derived from the indus tsangpo suture zone and the ladakh batholith of the asian plate a dominant 302 4 21 9 ma peak with a few 520 ma grains in this formation has been derived by erosion of the zircon partial annealing zone zpaz of 240 180 c as the first imprint of the collision this zone affected the himalayan proterozoic basement and its tethyan sedimentary cover since the detritus in the subathu has been derived both from the indian and asian plates the possible suturing of these plates took place during the subathu sedimentation a sudden change in the provenance is recorded in the detrital zircon ft cooling ages in the oligo miocene dagshai and kasauli formations which have dominant 30 and 25 ma p1 peaks respectively we interpret a distinct unconformity spanning 10 m y between the subathu and dagshai formations since ca 30 ma molassic sedimentation coincides with shifting of the source rocks to the himalayan metamorphic belt this belt has sequentially undergone three distinct cooling and exhumation pulses after the ultrahigh pressure high pressure uhp hp metamorphism 53 50 ma in the extreme north and two widespread m1 and m2 metamorphisms 40 30 and 25 15 ma in the middle parts these events appear to be largely responsible for the deposition of the ca 30 ma zircon himalayan peak and ca 25 and 15 ma young himalayan peaks respectively the latter appears within the lower siwalik subgroup 13 11 ma during the lower siwalik deposition pre himalayan peaks gradually decrease with the intensification of the himalayan events in source rocks in spite of uninterrupted fluvial sedimentation in the dagshai kasauli lower siwalik sequences since 30 ma breaks of 5 7 m y in the zircon ft ages reveal pulsative cooling and exhumation in the well identified source areas although cooling and exhumation of the himalayan source rocks remained almost uniform during the eocene source heterogeneity is reflected in fluvial sedimentation since 37 ma from pakistan to nepal in response to the india asia collision

early cretaceous subduction modified lithosphere beneath the eastern qinling orogen revealed from the daying volcanic sequence in central china abstract new geochronological geochemical and sr nd hf isotopic data for the early cretaceous daying volcanic sequence eastern qinling orogen provides a probe into the nature of and tectonic process operating in the underlying mesozoic mantle the volcanic rocks in the sequence are composed of basaltic trachyandesite pyroxene andesite trachyandesite and trachyte they yield zircon u pb and whole rock 40ar 39ar plateau ages of 134 124 ma roughly synchronous with the cretaceous igneous rocks throughout the eastern qinling and dabie orogens clinopyroxene and plagioclase phenocrysts of the volcanic rocks exhibit simple normal zonation with higher mg number and an content in the core relative to the rim the amphibole is characterized by tschermakite and tschermakitic hornblende with mg number of 50 67 the daying volcanic rocks have variable sio2 54 27 63 44 wt al2o3 15 39 20 34 wt feot 5 23 9 94 wt and mgo 1 05 4 03 wt contents with low mg number 26 49 they exhibit arc like ree and pm normalized patterns with significant negative nb ta anomalies and highly enriched sr nd isotopic composition with 87sr 86sr i ratios of 0 7066 0 7078 and end t values of 15 5 11 0 their zircons exhibit strongly negative ehf t values ranging from 27 3 to 17 2 and two staged model ages from 2 90 ga to 2 28 ga such geochemical signatures suggest that the daying volcanic rocks were derived from a hybridized source of the north china craton enriched mantle with the input of some yangtze crustal materials created by triassic deep subduction digestion they also share similar eruption time as well as elemental and sr nd hf isotopic compositions with those in the southern margin of the north china craton suggesting a northward subduction of the yangtze block it is proposed that the volcanic rocks are petrogenetically related to an early cretaceous extensional event along the qinling dabie orogenic belt in response to unrooting of the thickened lithosphere

a comparative study of detrital mineral and bedrock age elevation methods for estimating erosion rates traditional age elevation method using detrital and bedrock 40 ar 39 ar muscovite data sets from a single river catchment in the annapurna range nepal himalaya a nominal erosion rate estimate of 0 6 km myr for the 5 0 2 5 ma period was calculated from the bedrock data presented here for the first time this result agrees with the 0 7 km myr maximum estimate from the detrital data set which was derived from the dmt variant that emphasizes the range of single grain ages for a detrital sample in this case 11 2 5 ma however the other dmt variant which emphasizes the mean of the sample age distribution yields an erosion rate estimate of 2 3 km myr the simplest explanation for this discrepancy is that erosion rate increased significantly after 2 5 ma a scenario that is supported by apatite fission track data from the catchment

preliminary stratigraphic and structural architecture of bhutan implications for the along strike architecture of the himalayan system article i nfo preliminary mapping and stratigraphic correlation of lesser himalayan rock in eastern bhutan using field characteristics u pb detrital zircon dating and nd geochemistry define the first order stratigraphic architecture of the indian passive margin sequence in the eastern himalaya we use this new image of the lateral and vertical relationships of the original stratigraphy to determine the structural framework of the eastern himalayan fold thrust belt in bhutan we propose that lesser himalayan rock in bhutan can be divided into lower lesser himalayan rocks with a paleoproterozoic detrital zircon signal and upper lesser himalayan rocks with detrital zircon signals of 1000 500 ma the 500 ma detrital grains are from rocks in the frontal portions of the fold thrust belt north of the main boundary thrust as well as directly in the footwall of the main central thrust in the hinterland our preliminary stratigraphic study coupled with mapping allows us to construct a composite balanced cross section through the kuru chu valley in eastern bhutan which provides the first image of the geometry and amount of shortening through bhutan the main frontal thrust tilts a 6 km of thick section of neogene foreland basin deposits these deposits are separated from folded and faulted upper lesser himalayan rocks by the main boundary thrust north of the main boundary thrust we propose two duplex systems the southernmost duplex contains 9 repeated sections of upper lesser himalayan units including the permian gondwana sequence and the cambrian or younger baxa group the northernmost duplex is located in the footwall of the main central thrust and is comprised of two repeated sections of the proterozoic shumar and daling formations the southern boundary of this northern duplex is the shumar thrust which acts as a roof thrust for the southern duplex system and may correlate to the ramgarh thrust in nepal and india we propose that the development of the duplex systems passively folded overlying tethyan and greater himalayan rocks minimum shortening for this part of the himalayan fold thrust belt is 359 km all of which has occurred from 22 ma to present defining a long term shortening rate of 16 mm yr

intercalibration of 40ar 39ar age standards nl 25 hb3gr hornblende ga1550 sb 3 hd b1 biotite and bmus 2 muscovite abstract 40 ar 39 ar dating requires precise and accurate age data of standard fluence monitors besides primary calibration of k and 40 ar concentrations i e the ages are calculated with precise k ar dating intercalibration of the different monitors in use provides important cross checks of the validity of standard ages and implicitly reliability and uncertainty of chronological studies that rely on these standards here we report intercalibration of 5 primary international 40 ar 39 ar standards and one internal laboratory monitor a major goal of this study was to check the ages of the nl 25 hornblende standard and of the muscovite laboratory standard bmus 2 against the international standards hd b1 ga1550 sb 3 and hb3gr in replicate measurements of three irradiation series 40 ar 39 ar ages for the fluence monitors were found which agree within 1 error limits with the reported age values of 2660 9 ma for nl 25 hb3gr 1073 6 4 6 ma bmus 2 328 5 1 1 ma sb 3 162 9 0 8 ga1550 98 79 0 54 and 24 21 0 32 for hd b1 for the hornblende nl 25 fluence monitor a new slightly different 40 ar 39 ar age of 2657 4 ma is suggested for the age of the muscovite bmus 2 laboratory standard we recommend 328 5 1 1 ma and for biotite hd b1 a more precise value of 24 18 0 09 ma

estimating transmissivity fields and their influence on flow and transport the case of champagne mounts 1 this paper presents a flexible and general methodology that combines hydrogeological and geostatistical modeling techniques to estimate a set of transmissivity fields and their influence on flow and transport the methodology may be applied to any case with only hydraulic head observations even if most of them are concentrated inside a small part of the entire domain of interest it is applied here to the case of the champagne chalk aquifer france where it is shown to be very efficient the methodology is decomposed in three independent parts first a reference head distribution is constructed by kriging in order to constrain the inverse problem as hydraulic heads and elevations are correlated a smoothed digital elevation model is used as external drift the inverse problem is then solved by using a simplified pilot point method with an efficient and easy to use minimization algorithm finally geostatistical simulations combined with flow simulations lead to a set of acceptable transmissivity fields the induced uncertainty is evaluated by calculating tracer concentrations pointing out areas where flow behavior is uncertain and where new borings would be advantageously drilled

stratigraphy and sedimentary history of the nepal tethys himalaya passive margin abstract the sedimentary history of the nepal tethys himalaya began with deposition of thick carbonates in the cambro ordovician followed by a mixed siliciclastic carbonate epicontinental succession recording two major deepening events in the early silurian and late devonian fossiliferous carbonate ramp deposits in the tournaisian were disconformably followed by white quartzose sandstones and black mudrocks with locally intercalated diamictites derived from sedimentary rocks and deposited in asymmetric tectonic basins rift stage break up in the mid early permian locally associated with effusion of tholeiitic lava flows was followed by a transgressive sandy to shaly locally coal bearing or bioclastic unit capped by condensed pelagic carbonates in the middle to late permian juvenile ocean stage subsidence of the cooling stretched crust led close to bathyal water depths in the olenekian but then slowed down in the middle triassic to increase again sharply in the late triassic owing to renewed extensional tectonic activity and sediment loading during up and out building of the indian continental terrace deposition of tropical platform carbonates finally became widespread in the middle liassic mature passive margin stage the initial fragmentation of gondwana in the middle jurassic led to rejuvenation of the indian craton and deposition of quartzo feldspathic hybrid arenites capped by condensed oolitic ironstones deposited at warm subtropical latitudes in the late bathonian middle callovian next a discontinuous pelagic grey marly limestone unit was followed by the ammonoid rich offshore spiti shale in the late jurassic the final disintegration of gondwana began in the berriasian when quartzose siliciclastics derived again from the rejuvenated indian craton and partly from recycling of older clastic successions were followed by thick deltaic to shelf volcaniclastics documenting eruption of alkali basalts in the valanginian followed in the hauterivian to albian by more felsic differentiates such as the trachyandesites exposed in the lesser himalaya 120 km to the south a widespread drowning episode fostered by waning volcaniclastic supply during a global eustatic rise is documented by a major glauconitic horizon deposited at middle southern latitudes in the late albian overlain by scaglia like pelagic limestones in the latest albian the final part of sedimentary history during the rapid northward flight of india and its collision with eurasia is not documented anywhere in nepal due to later erosion of upper cretaceous to lower tertiary strata

inverted metamorphism in the pre siwalik foreland basin sediments beneath the crystalline nappe western nepal himalaya abstract one of the pre siwalik foreland basin sedimentary units the dumri formation is tectonically covered by the lesser himalayan crystalline nappe and the kuncha naudanda thrust sheet it is narrowly distributed in the eastern margin of the karnali klippe along the nne ssw trending chakure fault the whole sequence of the fluvial dumri formation attaining 1500 m in thickness is weakly metamorphosed to muscovite phyllite and foliated phyllitic sandstone the metamorphic grade decreases stratigraphically downward and underlying nummulitic limestone of the middle eocene bhainskati formation is converted into a slaty limestone no metamorphic mica is detected from the late cretaceous to paleocene amile formation below the bhainskati formation these facts indicate that the tansen group has undergone inverted metamorphism a 40 ar 39 ar plateau age of 25 69 0 13 ma was obtained from garnetiferous biotite gneiss in the lower part of the crystalline nappe another 40 ar 39 ar age spectrum from muscovite phyllite of the dumri formation suggests that metamorphism occurred at 16 17 ma the origin of the inverted metamorphism limited to the uppermost part of the lesser himalayan autochthon can be attributed to heat from the hot crystalline nappe and shearing along the sole thrust of the kuncha naudanda thrust sheet the depositional age of the dumri formation is estimated to be 26 17 ma provenance of the dumri formation is considered to be from the naudanda quartzite the kuncha formation and the tibetan tethys sediments because the sandstone contains orthoquartzite pebbles phyllitic lithic fragments and a sparry calcite cement the sedimentary facies indicates deposition by meandering rivers on flood plains in the distal part of the foreland basin no proximal facies such as alluvial fan and pebbly braided river deposits could be detected from the formation though it is near the main central thrust mct the northern continuation of the foreland basin sediments must be concealed beneath the higher himalayan crystalline judging from the present distribution of the dumri formation from the south of the main boundary thrust mbt to near the mct and from the shortening of the lesser himalayan sediments by thrusts and folds the width of the foreland basin where the dumri formation was deposited is estimated to have been more than 300 km

propagation of the thrust system and erosion in the lesser himalaya geochemical and sedimentological evidence centre de recherches pe trographiques et ge ochimiques bp 20 54501 vandoeuvre les nancy franceabstractsedimentological and nd isotope data of two sections of the sub himalaya of westernnepal are used as new constraints for understanding the erosion history of the himalaya throughout the deposition of the middle and upper members of the siwalik group thelesser himalaya contribution to the total detrital input progressively increased from lessthan 20 to 40 the increasing proportion of lesser himalaya sediments started at ca 10 8 ma and is associated with a coarsening of the maximum grain size at both micro scopic and macroscopic scales thin skinned tectonics of the lesser himalaya thrust sys tem would have controlled the exhumation of the lesser himalaya rocks and would havebegun at 12 10 ma taking into account the delay for denudation together with otherstudies these data restrict the onset of movement on the lesser himalaya thrust systemto less than 3 m y along more than 1750 km this short time frame implies a ratio oflateral propagation rate to shortening rate far too high for the propagation of a singlecrustal thrust thus we suggest instead the simultaneous initiation of several thrusts aheadof the main central thrust at ca 12 ma we suggest that a rapid rise of the tibetanplateau at this time has transformed the himalaya to an overcritical thrust wedge thathas propagated forward to return to a stable state this regional rising could be the primecause of the increase of sediment in ux at ca 11 ma around the himalaya keywords siwalik formations neodymium grain size erosion thrust sheets himalaya

triassic granitoids of the qinling orogen central china genetic relationship of enclaves and rapakivi textured rocks abstract granitoids varying in composition from biotite amphibole granodiorite to monzogranite with rapakivi textures have recently been recognized in the qinling orogen central china the granitoids are magnesian calc alkalic and metaluminous a cnk 0 71 0 95 they contain many mafic to intermediate microgranular enclaves the enclaves are quenched mafic melts in granitic host magma as evidenced by fine grained sinuous margins against or transitional contacts with the host granitoids as well as disequilibrium textures and mineral assemblages u pb zircon dating of the granitoids yields 209 2 ma and 212 2 ma and the enclaves 210 2 ma and 211 2 ma respectively the host granitoids and enclaves show obvious silica gap but similar ree and trace element patterns the whole rock 87sr 86sr t end t and zircon ehf t values for the granitoids vary from 0 70516 to 0 70634 2 3 to 4 4 and 0 7 to 7 6 and for the enclaves from 0 70467 to 0 70600 1 1 to 4 0 and 1 2 to 8 8 respectively showing overlapped end t and zircon ehf t values all these suggest that the two magmatic systems represented by the host rock and enclave are probably derived from distinct sources with sufficient interaction or common origin but underwent different degrees of crustal contamination mineralogical and geochemical data point to enclave dissolution in silicic magma as a major process accounting for the origin of the rapakivi texture the possible bimodal magmatism suggested by the coeval granitoids and lamprophyre dykes combined with the structural pattern geochemical features and regional tectonics suggest a post collision setting for the qinling rapakivi textured granitoid plutons

a double medium approach to simulate groundwater level variations in a fissured karst aquifer abstract karst aquifers are known for their complex flow mechanisms produced by the high spatial heterogeneity of void distribution this study aims at confirming and enhancing the understanding of these flow mechanisms in a fissured karst aquifer using flow simulation with a double medium approach groundwater level gwl variations of the studied site a 17 km2 topographic hill located in burgundy france are monitored hourly through 18 boreholes 3 karstic resurgences and the outlet river discharge of the catchment basin 40 km2 the gerke and van genuchten double porosity model is implemented in the metis code and used to simulate gwl variations with a matrix and a fracture medium model parameters are calibrated by fitting a simulated fracture medium gwl to measured gwl for four boreholes that are representative of the topographic hill hydrodynamics the double medium approach succeeds in reproducing measured fast gwl responses after recharge events that are not reproduced by a single medium approach investigating the model parameters impact on simulated gwl variations and the exchange flow between the two media allows the conceptualization of the physical structure and the hydrodynamic mechanisms of the fissured karst aquifer the spatial organization of parameters is observed and indicates areas of higher fracture density producing rapid gwl responses in contrast to lower fracture density areas which produce slower and higher gwl responses the fracture medium hydraulic parameters and the exchange parameter controlling the matrix fracture interface properties are the most influential parameters for the simulated fracture medium gwl and the exchange flow between the matrix and the fracture medium during a recharge event calibrated fracture medium hydraulic conductivity and effective porosity vary respectively from 6 3e to 05 m s 1 and 0 3 to 3 2e 04 m s 1 and 2 4 and the exchange parameter varies from 2 9e 10 m 1 s 1 to 6 5e 10 m 1 s 1 matrix medium hydraulic conductivity variations have little impact on the simulation results while matrix medium effective porosity affects slightly simulated gwl values vary from 11 to 17 finally this study shows the gerke and van genuchten double porous medium validity for the simulation of groundwater level in a fissured karst aquifer

timescales of melt generation and the thermal evolution of the himalayan metamorphic core everest region eastern nepal in the everest region of the nepalese himalaya 40ar 39ar and u pb geochronology provide evidence for a complex thermal history marked by multiple episodes of granite intrusion the oldest mobilized melt formed syn deformational granitic sills that have u pb crystallization ages of 21 33 0 03 and 21 80 0 05 ma preserved in these same granites is a record of earlier magmatic crystallization of xenotime zircon and monazite between ca 26 ma and ca 23 ma this pattern of accessory phase crystallization is interpreted to reflect incremental melting and crystallization in the source region of the sills before ultimate melt migration and provides the earliest evidence for anatexis in the everest region the beginning of crustal melting in the everest region predates the earliest known movement on both the main central thrust and the south tibetan fault systems but is temporally associated with the implied pressure decrease between eohimalayan and neohimalayan metamorphism

origin of mafic enclaves from the taihang mesozoic orogen north china craton abstract the origin of mafic enclaves in intermediate and felsic granitoids has long been debated in this paper petrological and geochemical analyses whole rock geochemistry nd sr and zircon in situ hf isotopic compositions are used to address this controversial issue mafic enclaves in the taihang felsic plutons north china craton are fine grained and show typical magmatic textures they occur as rounded to ellipsoidal globules and are sometimes highly elongated without solid state deformation due to stretching within the partially crystallized convective felsic magma such occurrence suggests that the mafic enclaves and host rocks represent two co existing compositionally distinct magmas the mafic enclaves contain abundant hydrous phases hornblende and biotite and show the reaction relationship of clinopyroxene being rimmed by hornblende they also contain plagioclase xenocrysts showing compositional and textural disequilibrium in terms of nd hf isotopes mafic enclaves have end t values of 10 to 15 and host rocks have lower values as a whole 13 to 18 zircons from a single enclave show a large variation in hf isotopic compositions ehf t 5 to 20 a model is proposed whereby an evolved basaltic magma first mixed with a granitic magma of crustal origin at depth and then the mixed magma broke up into discrete globules upon entering the felsic magma chamber forming enclaves by convective motion or forceful injection in the host felsic magma this was followed by a double mechanical transfer of plagioclase crystals at the contact of the two magmas and by the diffusion of mobile elements e g si alkalis p y yb pb from felsic magma to enclave forming magma

late miocene environmental change in nepal and the northern indian subcontinent stable isotopic evidence from paleosols neogene sediments belonging to the siwalik group crop out in the himalayan foothills along the length of southern nepal carbon and oxygen isotopic analyses of siwalik paleosols from four long siwalik sections record major ecological changes over the past 11 m y the carbon isotopic composition of both soil carbonate and organic matter shifts dramatically starting ca 7 0 ma marking the displacement of largely c3 vegetation probably semi deciduous forest by c4 grasslands by the beginning of the pliocene all the flood plains of major rivers in this region were dominated by monsoonal grasslands the floral shift away from woody plants is also reflected by the decline and final disappearance of fossil leaves and the decrease in coal logs in the latest miocene a similar carbon isotopic shift has been documented in the paleosol and fossil tooth record of pakistan and in terrigenous organic matter from the bengal fan showing that the floral shift was probably continentwide the latest miocene also witnessed an average change of 4 in the oxygen isotopic composition of soil carbonate as observed previously in pakistan the reasons for this are unclear if not diagenetic a major environmental change is indicated perhaps related to that driving the carbon isotopic shift recently described pollen and leaf fossils from the surai khola section show that evergreen forest was gradually displaced by semi deciduous and dry deciduous forest between 11 and 6 ma the gradual nature of this floral shift which culminated in the rapid expansion of c4 grasses starting 7 0 m y ago is difficult to explain by a decrease in atmospheric pco2 alone cerling et al 1993 but fits well with a gradual onset of monsoonal conditions in the late miocene in the northern indian subcontinent himalayan uplift driving both monsoonal intensification and consumption of co2 through weathering may be the common cause behind major late miocene environmental change globally however the decline of effective moisture associated with monsoon development has probably slowed not increased the rate of consumption of co2 by chemical weathering of himalayan sediments

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

quasi 3d modeling of water flow in vadose zone and groundwater summary the complexity of subsurface flow systems calls for a variety of concepts leading to the multiplicity of simplified flow models one habitual simplification is based on the assumption that lateral flow and transport in unsaturated zone are not significant unless the capillary fringe is involved in such cases the flow and transport in the unsaturated zone above groundwater level can be simulated as a 1d phenomenon whereas the flow and transport through groundwater are viewed as 2d or 3d phenomena a new approach for a numerical scheme for 3d variably saturated flow using quasi 3d richards equation and finite difference scheme is presented the corresponding numerical algorithm and the quasi 3d computer code were developed results of the groundwater level simulations were compared with transient laboratory experimental data for 2d data constant flux infiltration quasi 3d hydrus modflow numerical model and a full 3d numerical model using richards equation hypothetical 3d examples of infiltration pumping and groundwater mound dissipation for different spatial time scales are presented water flow simulation for the alto piura aquifer peru demonstrates the quasi 3d model application at the regional scale computationally the quasi 3d code was found to be more efficient by an order of 10 300 while being accurate with respect to the benchmark fully 3d variable saturation code when the capillary fringe was considered

age of initiation of the india asia collision in the east central himalaya abstract we document the stratigraphy and provenance of the lower tertiary terrigenous sections in the zhepure shan region of the tethyan himalaya southern tibet using petrographic and geochemical whole rock and single grain techniques the cretaceous early tertiary shelf deposits of shallow marine carbonates and siliciclastics of the former indian passive margin near the western end of the zhepure shan are conformably overlain by lower tertiary clastic rocks sandstones in the jidula formation paleocene mostly contain monocrystalline quartz grains of cratonic origin in contrast significant amounts of immature framework grains with a distinct ophiolitic and volcanic arc influence are present in the youxia early eocene and shenkeza post early eocene formations major trace and rare earth element concentrations in both sandstones and shales complement the petrographic data and indicate that the source of the jidula formation consisted primarily of quartzose basement rocks probably of indian con

muscovite 40ar 39ar ages help reveal the neogene tectonic evolution of the southern annapurna range central nepal we present new muscovite 40 ar 39 ar ages from thirteen greater himalayan rocks and one lesser himalayan rock collected from four north trending transects across the southern annapurna range combining the new data with previously published ages leads to the following new insight into the tectonic development of the southern annapurna muscovite cooling ages from greater himalayan rocks are c 16 10 ma in the western annapurna and c 6 2 ma in the eastern annapurna revealing a decrease of 4 14 ma from west to east similarly the muscovite cooling age from one lesser himalayan rock in the west is c 7 ma and ages from several samples in the east are c 5 2 ma indicating a decline of 2 5 ma towards the east earlier cooling in the western annapurna can be explained by along strike differences in the geometry of the frontal ramp on the underlying thrust that carries these greater and lesser himalayan rocks and or by a ne striking fault that cut these rocks in greater himalayan rocks from the modi river valley one sample yielded muscovite 40 ar 39 ar ages of 18 0 0 7 and 16 2 0 5 ma for grain sizes of approximately 750 and 200 m respectively in contrast a sample collected 200 m structurally lower produced ages of 12 6 0 2 and 9 9 0 1 ma for these two grain sizes the north dipping bhanuwa fault has been proposed between these samples with different authors arguing for normal or thrust sense motion our newfound pattern of an older muscovite 40 ar 39 ar age pair in the hanging wall supports arguments for the existence of the bhanuwa fault and suggests normal sense motion

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

a new kinematic evolutionary model for the growth of a duplex an example from the rangit duplex sikkim himalaya india abstract the lesser himalayan duplex lhd is a prominent structure through much of the lesser himalayan fold thrust belt in the darjeeling sikkim himalaya a component of the lhd is exposed in the rangit window as the rangit duplex rd the rd consists of ten horses of the upper lesser himalayan sequence gondwana buxa upper daling the duplex varies from hinterland dipping in the north through an antiformal stack in the middle to foreland dipping in the south the ramgarh thrust rt is the roof thrust and based on a balanced cross section the main himalayan sole thrust is the floor thrust at a depth of 10 km and with a dip of 3 5 n retrodeformation suggests that the rd initiated as a foreland dipping duplex with the early ramgarh thrust as the roof thrust and the rt as the floor thrust the rt became the roof thrust during continued duplexing by a combination of footwall imbrication and concurrent rt reactivation this kinematic history best explains the large translation of the overlying mct sheets the restoration suggests that rd shortening is 125 km and the original gondwana basin extended 142 km northward of its present northernmost exposures within the window

exhumation history of eastern ladakh revealed by 40ar 39ar and fission track ages the indus river tso morari transect nw himalaya fission track and 40 ar 39 ar ages place time constraints on the exhumation of the north himalayan nappe stack the indus suture zone and molasse and the transhimalayan batholith in eastern ladakh nw india results from this and previous studies on a north south transect passing near tso morari lake suggest that the sw directed north himalayan nappe stack comprising the mata tetraogal and tso morari nappes was emplaced and metamorphosed by c 50 45 ma and exhumed to moderately shallow depths c 10 km by c 45 40 ma from the mid eocene to the present exhumation continued at a steady and slow rate except for the root zone of the tso morari nappe which cooled faster than the rest of the nappe stack rapid cooling occurred at c 20 ma and is linked to brittle deformation along the normal ribil zildat fault concomitant with extrusion of the crystalline nappe in the south data from the indus molasse suggest that sediments were still being deposited during the miocene

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

the siwaliks of western nepal i geometry and kinematics abstract the siwalik group which forms the southern zone of the himalayan orogen constitutes the deformed part of the neogene foreland basin situated above the downflexed indian lithosphere it forms the outer part of the thin skinned thrust belt of the himalaya a belt where the faults branch off a major decollement md that is the external part of the basal detachment of himalayan thrust belt this decollement is located beneath 13 ma sediments in far western nepal and beneath 14 6 ma sediments in mid western nepal i e above the base of the siwalik group unconformities have been observed in the upper siwalik member of western nepal both on satellite images and in the field and suggest that tectonics has affected the frontal part of the outer belt since more than 1 8 ma several north dipping thrusts delineate tectonic boundaries in the siwalik group of western nepal the main dun thrust mdt is formed by a succession of 4 laterally relayed thrusts and the main frontal thrust mft is formed by three segments that die out laterally in propagating folds or branch and relay faults along lateral transfer zones one of the major transfer zones is the west dang transfer zone wdtz which has a north northeast strike and is formed by strike slip faults sigmoid folds and sigmoid reverse faults the width of the outer belt of the himalaya varies from 25 km west of the wdtz to 40 km east of the wdtz the wdtz is probably related to an underlying fault that induces a a change of the stratigraphic thickness of the siwalik members involved in the thin skinned thrust belt and particularly of the middle siwalik member b an increase from west to east of the depth of the decollement level and c a lateral ramp that transfers displacement from one thrust to another large wedge top basins duns of western nepal have developed east of the wdtz the superposition of two decollement levels in the lower siwalik member is clear in a large portion of the siwalik group of western nepal where it induces duplexes development the duplexes are formed either by far travelled horses that crop out at the hangingwall of the internal decollement thrust id to the south of the main boundary thrust or by horses that remain hidden below the middle siwaliks or lesser himalayan rocks most of the thrusts sheets of the outer belt of western nepal have moved toward the s sw and balanced cross sections show at least 40 km shortening through the outer belt this value probably under estimates the shortening because erosion has removed the hangingwall cut off of the siwalik series the mean shortening rate has been 17 mm yr in the outer belt for the last 2 3 ma

detrital thermochronology and sediment petrology of the middle siwaliks along the muksar khola section in eastern nepal abstract sediment derived from erosion of the himalaya during the miocene pliocene was deposited in the himalayan foreland basin and is now exposed in the siwalik formation between the main boundary thrust and main frontal thrust these sediments hold important information on orogenic exhumation during this time in this preliminary study we sampled the middle siwalik formation in eastern nepal along the muksar khola section for thermochronologic and sediment petrologic analysis detrital zircon fission track thermochronology shows that the age signal of the middle siwalik formation of eastern nepal has some similarities but also differences with published data for western central nepal the muksar khola section samples show two static peaks at about 14 and 140 ma which is similar to the siwalik formation of western central nepal in contrast a signal of consistently fast exhumation was not observed in the upper middle siwaliks in eastern nepal this could be related to a position of the main central thrust further south to overall less erosional exhumation of higher himalayan rocks or to a reduced exposure of the underlying lesser himalayan units in comparison with western central nepal based on our apatite fission track data we propose that above the section dated by ojha et al 2009 with magnetostratigraphy about 1000 m of overlying upper siwalik formation exist

jurassic magmatism related pb zn w mo polymetallic mineralization in the central nanling range south china geochronologic geochemical and isotopic evidence from the huangshaping deposit abstract the nanling range in south china is characterized by extensive mesozoic magmatism and coeval nonferrous and rare metal mineralization huangshaping is a world class pb zn w mo polymetallic skarn deposit in the central nanling range magmatic rocks occurring in this ore district include quartz porphyry granite porphyry granophyre dacite porphyry and aplite with only the first three granitoids genetically associated with polymetallic mineralization most of the orebodies are constrained within the contact zones as skarn and veins between these granitic stocks and the carbonate wall rocks since the age of the quartz porphyry is still controversial and studies of the dacite porphyry and aplite are absent we focus on these magmatic rocks first la icp ms zircon u pb dating suggests that the crystallization ages of the quartz porphyry dacite porphyry and aplite are 154 3 1 9 ma 158 1 0 8 ma and 148 4 3 4 ma respectively combined with previously published age data we infer the evolutionary sequence of magmatic rocks should be dacite porphyry quartz porphyry granite porphyry granophyre aplite the quartz porphyry dacite porphyry and aplite yield high contents of high field strength elements zr nb ce y 255 440 ppm high ratios of 10 000 ga al 2 6 3 2 and prominent depletions in ba sr eu p and ti indicating their crustal affinities to a type granites they have negative end t values 9 4 to 7 0 and high initial pb isotopic ratios 206pb 204pbi 18 307 18 644 207pb 204pbi 15 689 15 742 208pb 204pbi 38 589 38 986 suggesting that they were probably derived by partial melting of ancient granulitic crustal materials the sulfide minerals exhibit a wide range of 34sv cdt values from 22 6 to 24 2 with 206pb 204pb of 17 669 19 708 207pb 204pb of 15 492 15 714 and 208pb 204pb of 37 880 39 789 indicating that sulfur lead and other associated metals were derived from a mixture of magmatic components and the carboniferous wall rocks fluid inclusions in pyrrhotite sphalerite and marmatite samples have 3he 4he ratios of 0 12 to 1 53 ra with calculated mantle helium proportions of 1 3 to 18 9 indicating a predominantly crustal origin for the ore fluids with minor inputs from the mantle the huangshaping deposit is a typical example of the genetic relationship both spatially and temporally between jurassic magmatism and polymetallic metallogeny in the nanling range

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

geometry and crustal shortening of the himalayan fold thrust belt eastern and central bhutan we present a new geologic map of eastern and central bhutan and four balanced cross sections through the himalayan fold thrust belt major structural features from south to north include 1 a single thrust sheet of subhimalayan rocks above the main frontal thrust 2 the upper lesser himalayan duplex system which repeats horses of the neoproterozoic cambrian baxa group below a roof thrust shumar thrust carrying the paleoproterozoic daling shumar group 3 the lower lesser himalayan duplex system which repeats horses of the daling shumar group and neoproterozoic ordovician jaishidanda formation with the main central thrust mct acting as the roof thrust 4 the structurally lower greater himalayan section above the mct with overlying tethyan himalayan rock in stratigraphic contact in central bhutan and structural contact above the south tibetan detachment in eastern bhutan and 5 the structurally higher greater himalayan section above the kakhtang thrust cross sections show 164 267 km shortening in subhimalayan and lesser himalayan rocks 97 156 km structural overlap across the mct and 31 53 km structural overlap across the kakhtang thrust indicating a total of 344 405 km of minimum crustal shortening 70 75 our data show an eastward continuation of lesser himalayan duplexing identified in northwest india nepal and sikkim which passively folded the overlying greater himalayan and tethyan himalayan sections shortening and percent shortening estimates across the orogen although minima do not show an overall eastward increase which may suggest that shortening variations are controlled more by the original width and geometry of the margin than by external parameters such as erosion and convergence rates

a new quasi 3d unsaturated saturated hydrogeologic model of the plateau de saclay france summary a new hydrogeologic model is developed for the plateau de saclay 20 km south west of paris france which covers an area of 74 km 2 it is based on a 2d model of the fontainebleau sands aquifer combined with a 1d model of the unsaturated zone taking into account the spatial variability of the groundwater permeability field and the thickness of the unsaturated zone the paper focuses on the estimation of a triplet of parameters permeability infiltration and effective porosity based on transient flow simulations first the permeability is obtained by inversion of mean hydraulic head data for different values of infiltration then infiltration and effective porosity are determined by using transient flow simulations and fitting the hydraulic head measurements at several piezometers over the 1970 2014 period the infiltration is estimated at between 125 and 150 mm yr the mean permeability between 2 10 5 and 6 10 5 m s and the effective porosity between 20 and 30 furthermore the role of the unsaturated zone is quantified and the induced delay is estimated at about 20 years in the case of the transport of a tracer

crust mantle interaction in the central north china craton during the mesozoic evidence from zircon u pb chronology hf isotope and geochemistry of syenitic monzonitic intrusions from shanxi province abstract in situ zircon u pb ages hf isotopic compositions and whole rock geochemical and sr nd isotopic compositions are presented for three mesozoic syenitic monzonitic intrusions from shanxi province central north china craton zircons from these intrusions all show core rim structures in that the oscillatory rims recorded their intrusive ages whereas the cores are interpreted as xenocrysts the u pb age data reveal that the northernmost dishuiyan monzonite was emplaced at 241 ma while the huyanshan and erfengshan syenitic monzonitic complexes were emplaced at 130 ma and 128 134 ma respectively the dishuiyan monzonite is petrologically and geochemically uniform it shows lree enrichment and hfse depletion and exhibits enriched sr and nd isotopic compositions with 87 sr 86 sr i 0 7065 and end t 8 3 7 9 the zircon rims in the dishuiyan monzonite show highly varied hf isotopic compositions with ehf t 27 8 6 4 the varied hf isotopic compositions and enriched sr nd isotopic compositions together with the ubiquitous xenocrystic zircon cores suggest the dishuiyan monzonite was produced by the mixing of melts from enriched lithospheric mantle and lower crust the monzonite and syenite from the huyanshan complexes exhibit different geochemical features the end t values of syenite which are higher than those of monzonite resemble the enriched lithospheric mantle and together with the absence of zircon in the syenite we propose that it was originated by partial melting of enriched lithospheric mantle monzonites from huyanshan and erfengshan share similar petrological and geochemical characteristics being enriched in lree and depleted in hfse and they show low end t values between 18 2 and 13 9 similar to zircon rims in the dishuiyan monzonite those in the huyanshan and erfengshan monzonites also exhibit highly varied hf isotopic compositions the consistent ages between the xenocrystic zircons in monzonites and the lower crustal basement rocks of the ncc reveal a significant contribution from the lower crust in the formation of the monzonites we propose that the monzonites from huyanshan and erfengshan were formed through hybridization of enriched lithospheric mantle derived and lower crust derived magmas the mesozoic syenitic monzonitic intrusions in shanxi province were produced under extensional environments which linked to different geodynamic settings the triassic dishuiyan monzonite is a product of post collisional lithospheric extension related to the subduction of the paleo asian oceanic plate under the ncc and subsequent slab break off in the north while the cretaceous huyanshan and erfengshan syenite monzonite was produced under a back arc extensional regime linked to the subduction of the paleo pacific slab beneath east asia