provenance of xigaze fore arc basin clastic rocks cretaceous south tibet the xigaze group of south tibet was deposited in a fore arc basin produced by northward subduction of the neotethys under eurasia during middle cretaceous time clasts of andesite from the gangdese magmatic belt to the north and limestone clasts from a shelf rimming the basin to the north are the near exclusive detrital components in the xigaze group in the eastern part of the studied region and in older albian deposits in the west younger cenomanian coniacian deposits from western and central parts of the xigaze group show increasing amounts of tonalite pebbles in conglomerates and quartz sand these trends reflect differential arc uplift and erosional dissection with higher rates in the west some arenites show evidence for sediment contribution from an additional source probably from an orogen to the north of the lhasa block the presence of this orogen indicates that crustal thickening of tibet is not restricted to cenozoic times the middle cretaceous gangdese belt apparently lacked acid volcanic rocks this suggests a low degree of crustal contamination and or differentiation of primitive magma probably because the crust beneath the arc area was thin a thin crust is in accordance with a low elevation of the arc deduced from continuous carbonate production on and marine excursions beyond the arc magma eruption rates of the arc estimated from the xigaze basin sediment volume were probably low 7 5 km 3 km arc segment per million years but within the range of modern arcs the varying sediment compositions of the xigaze group may be typical of fore arc basin deposits in front of continental magmatic arcs the lack of a correlation of trends in sediment composition with fining upward sedimentary supercycles in the xigaze group is taken as evidence in favor of a eustatic cycle control

geochronological and geochemical constraints on mesozoic suturing in east central tibet 1 this paper reports isotopic major and minor element geochemistry of igneous and metamorphic rocks from the kokoxili and yushu regions of central and eastern tibet the first region lies along the kunlun suture which separates the bayan har songpan ganze songpan terrane from the tarim and qaidam blocks two kokoxili granitoids yield u pb zircon dates of 217 10 and 207 3 ma late triassic which represent the time of emplacement and rb sr isochron dates of 195 3 and 190 3 ma early jurassic which are interpreted as cooling ages the geochemical signatures of these granitoids suggest that they are related to subduction continuing into the late triassic in the yushu area three samples help constrain the age of the jinsha suture which separates the songpan terranes from the qiangtang blocks a leucocratic granite and an orthogneiss in the suture zone yield u pb zircon dates of 206 7 and 204 1 ma respectively and a paragneiss south of it a u pb monazite date of 244 4 ma the existence of coeval magmatism in both the jinsha and kunlun sutures suggests that the two subduction zones were simultaneously active combining isotopic dating with structural evidence on subduction polarity and paleomagnetic reconstructions we propose that the kunlun and qinling block boundaries which were distinct in the permian subsequently formed a continuous late triassic northward subducting plate margin our data suggest that the jinsha suture correlates with the benzilan and nan uttaradit sutures which together belong to a major late triassic subduction zone

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

miocene volcanism in the lhasa block tibet spatial trends and geodynamic implications miocene ultrapotassic up shoshonitic p and calc alkaline ca volcanism is recognized widely over the tibetan plateau this volcanism occurred after the end of india s oceanic lithosphere subduction beneath the plateau ambiguities about the distribution and significance of this volcanism arise in part from a paucity of geochronologic and geochemical data we have sampled two volcanic fields in the lhasa block 1 the eastern edge of the zabuye salt lake central part of the block 2 the southwestern edge of the yangbajin graben major and trace elements display high k2o 6 06 6 54 over a wide range of sio2 75 03 55 30 na2o 1 32 3 30 and lree hree enrichment with a significant rare earth element fractionation 266 la yb n 6 42 as well as negative nb ta and ti anomalies these features indicate that our samples are chemically identical to the post collision miocene up to p volcanic rocks already identified all over the tibetan plateau and the southern plateau specifically but distinct from the ca rocks found in the lhasa block the miocene volcanics found in the lhasa block present greater chemical variation than the rest of the tibetan plateau 40 ar 39 ar ages on both sanidine and biotite gave ages ranging from 16 16 g 0 12 ma to 16 01 g 0 12 ma and 10 60 g 0 14 ma to 10 88 g 0 17 ma 2 c excluding systematic errors relative to fish canyon sanidine at 28 02 ma for zabuye salt lake and yangying geothermal field volcanics respectively these results are consistent with the range of previously published ages from this part of the tibetan plateau the presence of n s trending dikes and the location of the volcanic fields suggest that local e w extension in the lhasa block was coeval we propose that the synchronous up p and ca volcanism in the lhasa block between 23 and 8 ma is the result of progressively deepening northward subduction of indian continental lithosphere producing asthenospheric upwelling convective thinning delamination of lithospheric mantle fueled by the hot asthenosphere induced partial melting of an enriched sub continental mantle lithosphere as well as eclogitic lower crust this mechanism could also explain the local e w extension observed in the same period 18 10 ma we also propose that lateral migration of the deepening indian continental lithosphere mantle slab could explain the observed younger volcanism in the eastern part of the lhasa terrane

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

mantle dynamics uplift of the tibetan plateau and the indian monsoon convective removal of lower lithosphere beneath the tibetan plateau can account for a rapid increase in the mean elevation of the tibetan plateau of 1000 m or more in a few million years such uplift seems to be required by abrupt tectonic and environmental changes in asia and the indian ocean in late cenozoic time the composition of basaltic volcanism in northern tibet which apparently began at about 13 ma implies melting of lithosphere not asthenosphere the most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere the initiation of normal faulting in tibet at about 8 3 ma suggests that the plateau underwent an appreciable increase in elevation at that time an increase due solely to the isostatic response to crustal thickening caused by india s penetration into eurasia should have been slow and could not have triggered normal faulting another process such as removal of relatively cold dense lower lithosphere must have caused a supplemental uplift of the surface folding and faulting of the indo australian plate south of india the most prominent oceanic intraplate deformation on earth began between about 7 5 and 8 ma and indicates an increased north south compressional stress within the indo australian plate a tibetan uplift of only 1000 m if the result of removal of lower lithosphere should have increased the compressional stress that the plateau applies to india and that resists india s northward movement from an amount too small to fold oceanic lithosphere to one sufficient to do so the climate of the equatorial indian ocean and southern asia changed at about 6 9 ma monsoonal winds apparently strengthened northern pakistan became more arid but weathering of rock in the eastern himalaya apparently increased because of its high altitude and lateral extent the tibetan plateau provides a heat source at midlatitudes that should oppose classical symmetric hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons for the simple case of axisymmetric heating no dependence on longitude of an atmosphere without dissipation theoretical analyses by hou lindzen and plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal monsoonlike circulation than such a source centered on the equator but only if heating exceeds a threshold whose level increases with the latitude of the heat source because heating of the atmosphere over tibet should increase monotonically with elevation of the plateau a modest uplift 1000 2500 m of tibet already of substantial extent and height might have been sufficient to exceed a threshold necessary for a strong monsoon the virtual simultaneity of these phenomena suggests that uplift was rapid approximately 1000 m to 2500 m in a few million years moreover nearly simultaneously with the late miocene strengthening of the monsoon the calcite compensation depth in the oceans dropped plants using the relatively efficient c4 pathway for photosynthesis evolved rapidly and atmospheric co2 seems to have decreased suggesting causal relationships and positive feedbacks among these phenomena both a supplemental uplift of the himalaya the southern edge of tibet and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the himalaya that in turn enhanced extraction of co2 from the atmosphere thus these correlations offer some support for links between plateau uplift a downdrawing of co2 from the atmosphere and global climate change as proposed by raymo ruddiman and froehlich mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts but might also play an important role in altering regional and global climates

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

evidence for constriction and pliocene acceleration of east west extension in the north lunggar rift region of west central tibet 1 the active north trending north lunggar rift in west central southern tibet exposes an extensional metamorphic core complex bounded by an east dipping low angle normal fault apatite and zircon u th he thermochronology and thermal modeling of the north lunggar rift document a minimum timing for rift inception at 10 ma and rapid footwall exhumation between 5 and 2 ma miocene footwall cooling and exhumation rates were initially slow to moderate at 400 c ma 1 and 4 10 mm a 1 footwall isotherms were significantly compressed during rapid exhumation resulting in an elevated transient geothermal gradient between 50 and 90 c km 1 the minimum magnitude of horizontal extension for the north lunggar rift is 8 1 12 8 km maximum is 15 20 km less in the south at 10 km mean pliocene extension rate is 1 2 2 4 mm a 1 in the 120 direction results for the north lunggar rift are similar in magnitude rate and orientation of slip to the kinematically linked lamu co dextral strike slip fault to the north this suggests a state of constrictional strain during pliocene time along this stretch of the bangong nujiang suture from which the lamu co fault emanates the onset of extension in this region may be explained by crustal thickening and gravitational orogenic collapse followed by accelerated rifting resulting from localized crustal stretching and increased magmatic activity potentially driven by the position and northward extent of underthrusting indian lithosphere

late cretaceous high mg granitoids in southern tibet implications for the early crustal thickening and tectonic evolution of the tibetan plateau abstract this study presents new major and trace element plus sr nd and zircon u pb isotope data for the zhongcang granitic plutons which are located to the south of the yongzhu asuo ophiolite belt within the northwestern part of the central lhasa subterrane tibetan plateau these data provide new insights into the late cretaceous tectonic evolution of southern tibet the zhongcang plutons are dominated by granodiorites and granites that yield zircon u pb emplacement ages of 94 88 ma they can be further divided into metaluminous and peraluminous subtypes the metaluminous rocks have adakite like geochemical signatures including high sio 2 al 2 o 3 and sr concentrations and low yb and y concentrations and high sr y and la yb n ratios these rocks also have negative end t values 3 17 to 0 17 variable initial 87 sr 86 sr i ratios 0 705927 0 707668 and high k 2 o and th concentrations suggesting that they were not derived from the partial melting of subducted oceanic crust in an arc setting the zhongcang adakitic rocks have higher mgo and cr concentrations and mg values than do contemporaneous intrusive rocks derived from a region of thickened lower crust within the central lhasa subterrane these data suggest that the zhongcang adakitic rocks were generated by the partial melting of a delaminated thickened lower crust within a late cretaceous continental setting in comparison with the zhongcang adakitic rocks the peraluminous rocks have significant negative eu and sr anomalies and lower end t values 4 06 to 6 64 this combined with their high mg values and cr concentrations suggests that the peraluminous units formed from primitive magmas similar to those that formed the zhongcang adakitic rocks but modified by contamination with ancient crustal material and by fractional crystallization of plagioclase and apatite during uprising and or emplacement the zhongcang high mg granitoids provide robust evidence for late cretaceous crustal thickening prior to india asia collision

injection of tibetan crust beneath the south qaidam basin evidence from indepth iv wide angle seismic data 1 the international deep profiling of tibet and the himalaya phase iv indepth iv active source seismic profile in northeast tibet extends 270 km roughly north south across the songpan ganzi terrane the predominantly strike slip north kunlun fault along the kunlun suture the east kunlun mountains and the south qaidam basin refraction reflection and gravity modeling provide constraints on the velocity and density structure down to the moho the central qaidam basin resembles average continental crust whereas the songpan ganzi terrane and east kunlun mountains exhibit thickened lower velocity crust also characteristic of southern tibet the crustal thickness changes from 70 km beneath the songpan ganzi terrane and east kunlun mountains to 50 km beneath the qaidam basin this jump in crustal thickness is located 100 km north of the north kunlun fault and 45 km north of the southern kunlun qaidam boundary farther north than previously suggested ruling out a moho step caused by a crustal penetrating north kunlun fault the qaidam moho is underlain by crustal velocity material 6 8 7 1 km s for 45 km near the crustal thickness transition the southernmost 10 km of the qaidam moho are underlain by a 70 km reflector that continues to the south as the tibetan moho the apparently overlapping crustal material may represent songpan ganzi lower crust underthrusting or flowing northward beneath the qaidam basin moho thus the high tibetan plateau may be thickening northward into south qaidam as its weak thickened lower crust is injected beneath stronger qaidam crust

neogene quaternary magmatism and geodynamics in the carpathian pannonian region a synthesis in the carpathian pannonian region in neogene times westward dipping subduction in a land locked basin caused collision of two lithospheric blocks alcapa and tisia with the southeastern border of the european plate calc alkaline and alkaline magmatism was closely related to subduction rollback collision and extension from the spatial distribution of the magmatic activity four segments can be defined western segment magmatism occurring on the alcapa block central segment magmatism occurring on both alcapa and the tisia blocks south eastern segment and interior segment both on the tisia block most calc alkaline magmatism in the region resulted from melting of a heterogeneous asthenospheric mantle source modified by addition of fluids and sediment assimilation and fractionation processes at shallow crustal levels occurred in most of the segments strongly masking the deeper source processes long term subduction rollback and or delamination led to contamination of the asthenosphere beneath the western segment here large volume partial melts of the contaminated mantle caused underplating and crustal anatexis leading to mixing of mantle derived calc alkaline magmas with crustal melts in the central segment calc alkaline magmas were formed by subduction and rollback followed by back arc extension and slab breakoff avariable mantle source is indicated in the back arc setting and larger amount of fluid induced metasomatism source enrichment and assimilation nearer to the trench in the interior segment evolution from typical calc alkaline magmas to adakite like ones was related to extension due to fast rotations and transtensional tectonics here calc alkaline magmas formed by decompression melting of a heterogeneous crust mantle lithosphere while adakite like melts resulted from fliud dominated melting of the lithosphere along the south eastern segment slab breakoff was responsible for the generation of typical calcalkaline magmas but in the extreme south of the segment shallow level tearing of the slab followed breakoff strike slip tectonics allowed the rise of hot asthenosphere and generation of adakite like magmas via slab melting along the torn edge of the east european plate alkalic basaltic volcanism with an oib like asthenosphere source followed the calc alkaline stage

evolution of the bangong nujiang tethyan ocean insights from the geochronology and geochemistry of mafic rocks within ophiolites abstract the bangong nujiang suture zone bnsz is located in the central tibetan plateau and represents the relict of the bangong nujiang tethyan ocean bnto this paper presents zircon u pb ages and whole rock geochemical and sr nd isotope data for the ophiolitic rocks from the rutog dongco dongqiao amdo and dengqen areas from west to east in the bnsz zircon u pb ages obtained from five gabbros and one leucogabbro from the five areas are 169 2 167 2 187 2 184 2 177 3 and 164 2 ma respectively mafic rocks gabbros and basalts in the bnsz can be divided into morb like and oib type based on geochemical data the morb like rocks are tholeiitic and can be further divided into n and e morb subtypes marked by depleted and flat ree patterns respectively all morb like rocks show supra subduction zone ssz geochemical affinities such as mild enrichment in the large ion lithophile elements and depletion in the high field strength elements coupled with high and positive whole rock e nd t values 8 1 to 5 4 the oib type rocks which formed in the early cretaceous 132 108 ma are exposed in the dongco dongqiao and tarenben areas and they consist mostly of alkali basalts with within plate geochemical characteristics and positive e nd t values 5 9 to 4 5 on the basis of these new data combined with information from the literature we further support that the bnto was an intra oceanic arc backarc basin that existed between the north lhasa subterrane nlt and the south qiangtang subterrane sqt during the early middle jurassic this basin may have been active until the early cretaceous when the oib type basalts erupted

petrochemistry of the south marmara granitoids northwest anatolia turkey post collision magmatic rocks are common in the southern portion of the marmara region kapidag karabiga gonen yenice can areas and also on the small islands marmara avsa pasalimani in the sea of marmara they are represented mainly by granitic plutons stocks and sills within triassic basement rocks the granitoids have ages between late cretaceous and miocene but mainly belong to two groups eocene in the north and miocene in the south the miocene granitoids have associated volcanic rocks the eocene granitoids do not display such associations they are both granodioritic and granitic in composition and are metaluminous calc alkaline medium to high k rocks their trace elements patterns are similar to both volcanic arc and calc alkaline post collision intrusions and the granitoids plot into the volcanic arc granite vag and collision related granite areas colg of discrimination diagrams the have high 87sr 86sr 0 704 0 707 and low 143nd 144nd 0 5124 0 5128 during their evolution the magma was affected by crustal assimilation and fractional crystallization afc nd and sr isotopic compositions support an origin of derivation by combined continental crustal afc from a basaltic parent magma a slab breakoff model is consistent with the evolution of south marmara sea granitoids

present day crustal thinning in the southern and northern tibetan plateau revealed by gps measurements gps measurements from sites within the tibetan plateau show not only east southeast west northwest extension but also more importantly horizontal dilation throughout the interior of the plateau assuming conservation of volume vertical thinning strain rates equal horizontal dilation rates and they 8 9 0 8 nanostrain a 1 and 7 4 1 2 nanostrain a 1 in northern and southern tibet and 12 0 3 2 nanostrain a 1 in its southwestern part suggest no measureable difference principal extensional strain rates also are similar in magnitude and orientation if crustal thinning began at 10 15 ma and the current rates of horizontal dilation applied both to the entire crust and to that period the crust should have thinned by 5 5 8 5 km if isostatic equilibrium applied the mean elevation of the plateau would have dropped 1 km the similar rates for northern southern and southwestern tibet suggest that the processes dictating crustal extension normal faulting and crustal thinning in the three regions differ little from one another

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

basin formation in the high himalaya by arc parallel extension and tectonic damming zhada basin southwestern tibet 1 the late miocene pleistocene zhada basin in southwestern tibet provides a record of subsidence and basin formation within an active collisional thrust belt the 800 m thick basin fill is undeformed and was deposited along an angular unconformity on top of tethyan strata that were previously shortened in the himalayan fold thrust belt modal sandstone petrographic data conglomerate clast count data and detrital zircon u pb age spectra indicate a transition from detritus dominated by a distal northern source to a local southern source this transition was accompanied by a change in paleocurrent directions from uniformly northwestward to basin centric at the same time the depositional environment in the zhada basin changed from a large braided river to a closed basin lake sedimentation in the zhada basin was synchronous with displacement on the qusum and gurla mandhata detachment faults which root beneath the basin and exhume midcrustal rocks along the northwestern and southeastern flanks of the basin respectively these observations indicate that accommodation for zhada basin fill was produced by a combination of tectonic subsidence and damming as midcrustal rocks were evacuated from beneath the zhada basin in response to arc parallel slip on crustal scale detachment faults

anomalously fast convergence of india and eurasia caused by double subduction prior to collision with eurasia the indian plate rapidly accelerated northwards numerical simulations show that the combined pull of two slabs in two parallel north dipping subduction systems could have caused this pulse of rapid movement

a traverse through the western kunlun xinjiang china tentative geodynamic implications for the paleozoic and mesozoic the northern part of the western kunlun southern margin of the tarim basin represents a sinian rifted margin to the south of this margin the sinian to paleozoic proto tethys ocean formed south directed subduction of this ocean beneath the continental southern kunlun block during the paleozoic resulted in the collision between the northern and southern kunlun blocks during the devonian the northern part of the paleo tethys ocean located to the south of the southern kunlun was subducted to the north beneath the southern kunlun during the late paleozoic to early mesozoic this caused the formation of a subduction accretion complex including a sizeable accretionary wedge to the south of the southern kunlun a microcontinent or oceanic plateau which we refer to as uygur terrane collided with the subduction complex during the late triassic both elements together represent the kara kunlun final closure of the paleo tethys ocean took place during the early jurassic when the next southerly located continental block collided with the kara kunlun area from at least the late paleozoic to the early jurassic the tarim basin must be considered a back arc region the kengxiwar lineament which connects the karakorum fault in the west and the ruogiang xingxingxia altyn tagh fault zone in the east shows signs of a polyphase strike slip fault along which dextral and sinistral shearing occurred

contact metamorphism in the malashan dome north himalayan gneiss domes southern tibet an example of shallow extensional tectonics in the tethys himalaya combined petrographic structural and geochronological study of the malashan dome one of the north himalayan gneiss domes reveals that it is cored by a miocene granite the malashan granite that intruded into the jurassic sedimentary rocks of tethys himalaya two other granites in the area are referred to as the paiku and cuobu granites new zircon shrimp u pb and muscovite and biotite 40ar 39ar dating show that the paiku granite was emplaced during 22 2 16 2 ma average 19 3 3 9 ma and cooled rapidly to 350 400 c at around 15 9 ma whole rock granite chemistry suggests the original granitic magma may have formed by muscovite dehydration melting of a protolith chemically similar to the high himalayan crystalline sequence abundant calcareous metasedimentary rocks and minor garnet staurolite biotite muscovite andalusite schists record contact metamorphism by three granites that intruded intermittently into the jurassic sediments between 18 5 and 15 3 ma two stages of widespread penetrative ductile deformation d1 and d2 can be defined microstructural studies of metapelites combined with geothermobarometry and pseudosection analyses yield p t conditions of 4 8 0 8 kbar at 550 50 c during a non deformational stage between d1 and d2 and 3 1 4 1 kbar at 530 575 c during syn to post d2 the pressure estimates for the syn to post d2 growth of andalusite suggest relatively shallow depth of 15 2 km extensional ductile deformation that took place within a shear zone of the south tibetan detachment system close temporal association between intrusion of the malashan granite and onset of d2 suggests extension may have been triggered by the intrusion of the malashan granite

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

paleogene carbonate microfacies and sandstone provenance gamba area south tibet stratigraphic response to initial india asia continental collision abstract the paleogene marine strata in the gamba area of south tibet comprise carbonates of the zongpu formation and siliciclastic rocks of the enba and zhaguo formations documenting the final stages of marine deposition in the tethyan himalaya the 350 m thick zongpu formation was dated as late danian to ypresian based on larger benthic foraminifers thirteen distinct microfacies identify three sedimentary environments mudstone wackestone with udoteacean algae bioclastic peloidal packstone packstone with rotaliids and green algae floatstone with alveolina and orbitolites were deposited in restricted lagoonal environments bioclastic packstone and grainstone with rotaliids were deposited in high energy shoal environments floatstones with nummulitids or alveolinids were deposited in shallow open marine environments the zongpu formation was accumulated on a carbonate ramp it documents two deepening upward sequences separated by an unconformity corresponding to the palaeocene eocene boundary and marked by a conglomerate with limestone clasts the overlying enba formation comprises greenish grey calcareous shales intercalated with litho quartzose sandstones in the upper part and capped by subaerial litho quartzose red beds of the zhaguo formation petrographic analysis detrital zircon geochronology and hf isotopic data indicate that detritus in the enba and zhaguo formations deposited on the indian passive margin was derived from the asian active margin in the north these clastic units were thus deposited after the onset of the india asia continental collision in the early himalayan foreland basin major lithological and paleoenvironmental changes occurred at three stratigraphic levels the jidula zongpu boundary 62 ma the paleocene eocene boundary 56 ma and the zongpu enba boundary 51 ma our provenance study confirms that the india asia collision was already under way during the deposition of the enba member i e by 51 ma and along with facies analysis and general palaeogeographic considerations indicates that neo tethys was still wide open during the early middle paleocene it is thus argued consistently with previous studies that the paleocene eocene disconformity documented in the gamba area as in the northwestern tethyan himalaya is likely to record flexural uplift consequent to initial underthrusting of the indian continental margin beneath asia at or just a little earlier than 56 ma

india eurasia collision chronology has implications for crustal shortening and driving mechanism of plates the motion of the indian plate is determined in an absolute frame of reference and compared with the position of the southern margin of eurasia deduced from palaeomagnetic data in tibet the 2 600 900 km of continental crust shortening observed is shown to have occurred in three different episodes subduction of continental crust intracontinental thrusting and internal deformation and lateral extrusion the detailed chronology of the collision and plate reorganizations in the indian and pacific oceans supports the hypothesis that slab pull is a dominant driving mechanism of plate tectonics

new insight into the south tibetan detachment system not a single progressive deformation 1 low angle normal faults lanf typically regarded as accommodating crustal or lithospheric extension may also form during lithospheric shortening the best studied system of syn contractional lanfs is the south tibetan detachment system a network of low angle normal sense faults and shear zones that formed coevally with and parallel to south vergent thrusts during lithospheric shortening accompanying development of the himalayan orogen in the eastern himalaya there are several across strike exposures of the south tibetan detachment system we present new structural and thermometry data from the eastern himalaya that demonstrate that the south tibetan detachment system cannot have formed as a single progressive structure we characterize and distinguish two distinct structural and tectonic components within the currently recognized system 1 an extensive diffuse sheared layer that formed the boundary between strong upper crust and weak southward flowing middle crust and 2 a network of brittle ductile lanfs that locally exhume partly excise and overprint the earlier mylonite zone at the topographic break between the himalayan orogen and the tibetan plateau the sheared layer not a lanf formed the boundary between upper and middle crust during ductile flow of the middle crust and is extensively exposed in the himalaya at the base of klippen of upper crustal rocks preserved in bhutan along the crest of the himalaya where it has been excised and exhumed by the brittle ductile extrusion lanfs and bounding the cores of the north himalayan gneiss domes citation kellett d a and d grujic 2012 new insight into the south tibetan detachment system not a single progressive deformation tectonics 31 tc2007 doi 10 1029 2011tc002957

tectonic setting origin and obduction history of the spontang ophiolite ladakh himalaya nw india abstract the spontang ophiolite forms the highest tectonic thrust slice above the mesozoic early tertiary continental margin of the north indian plate in the ladakh zanskar himalaya detailed field mapping combined with geochemical analysis has defined two major units a full ophiolite sequence spontang ophiolite overlain by an upper unit consisting of 500 m thick basalt andesite volcanic and volcano sedimentary rocks of island arc affinity spong arc the spontang ophiolite comprises a harzburgitic mantle sequence gabbroic and ultramafic cumulates isotropic gabbros and highly tectonized sheeted dikes feeding pillow lavas with a few uncommon highly fractionated plagiogranites a separate lherzolitic peridotite unit affected by possible transform related shearing was thrust over the harzburgites in the west possibly during the early stages of subduction initiation beneath the ophiolite whole rock geochemistry and u pb geochronology show that the ophiolite formed at a normal mid ocean ridge sp

cretaceous tertiary structural evolution of the north central lhasa terrane tibet 1 in the north central lhasa terrane of tibet two distinct structural levels of an east west striking thrust system are exposed along the north trending late cenozoic xiagangjiang rift upper paleozoic strata deformed by the south directed langgadong la thrust and cretaceous strata involved in variably north and south directed thrusting characterize these lower and upper structural levels respectively these two structural levels are separated by the tagua ri passive roof thrust balanced cross section restoration suggests that the thrust system accommodated 103 km 53 shortening the 40ar 39ar results together with an interpretation of synthrust deposition of upper cretaceous strata suggest that the majority of shortening occurred during the late cretaceous paleocene cretaceous strata lie unconformable on permian rocks volcanic tuffs directly above the unconformity yield u pb zircon ages of 131 ma upper cretaceous strata record a change from shallow marine to nonmarine deposition indicating uplift above sea level during this time the overall south directed vergence of the thrust belt is consistent with substantial crustal thickening in central tibet by large scale northward underthrusting of lhasa terrane basement beneath the qiantang terrane prior to the indo asian collision the documented decoupling of contractional deformation at shallow crustal levels appears to be a regional characteristic of tibet from at least the bangong suture in the north to the tethyan himalaya to the south this style of deformation explains the absence of basement exposures and major denudation in this region despite substantial crustal shortening

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

zircon thermometry of high temperature rhyolites near volcanic associated massive sulfide deposits abitibi subprovince canada high temperature high silica rhyolites are commonly found near cu zn volcanic associated massive sulfide vms deposits in the archean abitibi subprovince they are characterized by anhydrous phenocrysts feldspar quartz in concentrations of generally 73 sio 2 high incompatible element contents rb sr 1 zr y

the evolution of yarlung tsangpo river constraints from the age and provenance of the gangdese conglomerates southern tibet abstract cenozoic conglomerates are exposed discontinuously along the length of the yarlung tsangpo suture zone on the southern margin of the gangdese arc these conglomerates the gangdese conglomerates herein record a crucial stage in the uplift and erosion histories of the southern tibet after the initial india asia collision in the mt kailas area the gangdese conglomerates strata consist of multiple sedimentary cycles and each cycle is a fining upward sequence that was deposited by alluvial fan braided river and delta systems whereas in the xigaze area the gangdese conglomerates strata comprise a coarsening upward sequence that was deposited by delta braided river and alluvial fan systems based on the detrital and igneous zircon u pb ages the depositional ages of the gangdese conglomerates are late oligocene to early pliocene ca 26 5 ma in the mt kailas area late oligocene to middle miocene ca 26 15 ma in the xigaze area and late oligocene to early miocene ca 26 19 ma in the zedong area paleocurrent measurements and provenance data i e conglomerate clast composition sandstone petrology and detrital zircon age indicate that the initial detritus of the gangdese conglomerates were entirely derived from the north mainly from the gangdese arc sediment resulting from denudation to the south the xigaze forearc basin the yarlung tsangpo suture zone and the northern margin of the indian plate first appeared by the early miocene ca 19 ma and subsequently increased in abundance gradually our new results together with previous data from the xigaze area reveal 3 major stages in the evolution of the yarlung tsangpo river system 1 the southward flowing stage ca 26 19 ma featured southward draining transverse rivers that transported materials from the gangdese arc southward southward paleocurrents in the gangdese conglomerates indicate a northern source 2 the westward flowing stage ca 19 15 ma developed due to the uplift of the suture zone and tethys himalaya to the south northward draining rivers began to develop and lakes resembling a string of beads formed and finally connected together initiating the westward flowing paleo yarlung tsangpo river westward paleoflows were recorded in the gangdese conglomerates 3 the eastward flowing stage ca 15 ma present resulted from differential uplift and denudation of the southern tibet which reversed the direction of the young yarlung tsangpo river by ca 15 ma the deposition of the gangdese conglomerates was controlled by eastward paleoflows at this point the modern eastward flowing yarlung tsangpo river system was established

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

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

middle to late miocene extremely rapid exhumation and thermal reequilibration in the kung co rift southern tibet 1 the kung co rift is an approximately nnw striking wsw dipping normal fault exposed in southern tibet and is part of an extensive network of active approximately ns striking normal faults exposed across the tibetan plateau detailed new and published u th he zircon and apatite thermochronometric data from the footwall of the early miocene kung co granite provide constraints on the middle miocene to present day exhumation history of the footwall to the kung co fault inverse modeling of thermochronometric data yield age patterns that are interpreted as indicating 1 initiation of normal fault slip at 12 13 ma and rapid exhumation of the footwall between 13 and 10 ma 2 acceleration of normal fault slip at rates of 21 9 6 9 mm yr at 10 ma 3 rapid thermal reequilibration between 10 and 9 ma and 4 slow exhumation and or quiescence from 9 ma to the present day hanging glacial valleys in the footwall and fault scarps that cut late quaternary till and moraine deposits indicate that fault slip continues today middle to late miocene initiation of extension across the kung co rift is broadly the same as the documented initiation of ew extension across the south central tibetan plateau eastward flow of middle or lower crust from beneath tibet accommodated by northward underthrusting of indian crust beneath tibet provides a plausible explanation for the onset of ew extension across the tibetan plateau

late cretaceous 81 ma high temperature metamorphism in the southeastern lhasa terrane implication for the neo tethys ocean ridge subduction abstract an integrated study of u pb zircon dating geochemical and sr nd hf isotopic compositions of garnet bearing granulite and marble from the granulite facies domain of the nyingchi complex eastern himalayan syntaxis has provided insights into the tectonic evolution of the southern lhasa terrane the peak metamorphism of the garnet bearing granulite is marked by a mineral assemblage of garnet orthopyroxene high ti amphibole plagioclase quartz rutile abundant exsolved rutile needles are observed within amphibole garnet and quartz the peak metamorphic temperatures are estimated at 803 924 c geochemical data from the garnet bearing granulites provide evidence for a basaltic protolith that formed in a continental margin arc setting sr nd hf isotopic compositions indicate that this protolith was sourced from partial melting of a depleted mantle la icp ms u pb zircon dating shows that the protolith age and metamorphic age of the garnet bearing granulites are 89 3 0 6 ma and 81 1 0 8 ma respectively the detrital magmatic zircons from the marble yield ages from 86 3 to 167 ma the age distribution and hf isotopic composition e hf t 5 9 to 17 5 of the detrital magmatic zircon in the marble are consistent with the isotopic data of zircons from the jurassic cretaceous gangdese batholiths suggesting that the clastic sediments were partially derived from these intrusives or associated volcanic rocks and deposited in the fore arc basin of the gangdese arc the metamorphic zircons in the marble yield a metamorphic age of 81 4 0 5 ma these results show that both the arc magmatic rocks and forearc sedimentary rocks underwent high temperature ht granulite facies metamorphism at 81 ma indicating anomalously high heat input in the forearc region a range of tectonic observations including a coeval hiatus in arc magmatism and a period of regional uplift indicate that ht metamorphism resulted from the subduction of the neo tethys ocean ridge beneath the southern lhasa terrane during the late cretaceous

simultaneous miocene extension and shortening in the himalayan orogen the south tibetan detachment system separates the high grade metamorphic core of the himalayan orogen from its weakly metamorphosed suprastructure it is thought to have developed in response to differences in gravitational potential energy produced by crustal thickening across the mountain front geochronologic data from the rongbuk valley north of qomolangma mount everest in southern tibet demonstrate that at least one segment of the detachment system was active between 19 and 22 million years ago an interval characterized by large scale crustal thickening at lower structural levels these data suggest that decoupling between an extending upper crust and a converging lower crust was an important aspect of himalayan tectonics in miocene time

seismotectonics of the nepal himalaya from a local seismic network the national seismological network of nepal consists of 17 short period seismic stations operated since 1994 it provides an exceptional view of the microseismic activity over nearly one third of the himalayan arc including the only segment between longitudes 78 e and 85 e that has not produced any m 8 earthquakes over the last century it shows a belt of seismicity that follows approximately the front of the higher himalaya with most of the seismic moment being released at depths between 10 and 20 km this belt of seismicity is interpreted to reflect interseismic stress accumulation in the upper crust associated with creep in the lower crust beneath the higher himalaya the seismic activity is more intense around 82 e in far western nepal and around 87 e in eastern nepal western nepal between 82 5 and 85 e is characterized by a particularly low level of seismic activity we propose that these lateral variations are related to segmentation of the main himalayan thrust fault the major junctions between the different segments would thus lie at about 87 e and 82 e with possibly an intermediate one at about 85 e these junctions seem to coincide with some of the active normal faults in southern tibet lateral variation of seismic activity is also found to correlate with lateral variations of geological structures suggesting that segmentation is a long lived feature we infer four 250 400 km long segments that could produce earthquakes comparable to the m 8 4 bihar nepal earthquake that struck eastern nepal in 1934 assuming the model of the characteristic earthquake the recurrence interval between two such earthquakes on a given segment is between 130 and 260 years

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

tectonic and sedimentary basin evolution of the eastern bangong nujiang zone tibet a reading cycle the ophiolite bearing bangong nujiang zone bnz traversing central tibet from east to west separates the qiangtang block in the north from the lhasa block in the south their stratigraphic development indicates that both blocks once formed a continuous continental platform until the late triassic following late paleozoic triassic rifting ocean crust formed between both blocks during the late triassic creating the dongqiao naqu basin dnb among other basins yu et al 1991 the analysis of the rift flank sequences reveals that rifting was dominated by transtension the basin was shortened by post mid cretaceous transpression thus the overall basin evolution represents a reading cycle despite some active margin processes which gave this cycle a special imprint major basin parts were preserved despite transpressional shortening suggesting that the eastern bnz represents a remnant basin our understanding of the dnb solves the prior problem of viewing the bnz as a mid late jurassic collisional suture although typical collision related deformation thickening mountain building as well as related molasse formation are lacking our model also explains the scattered linear ophiolite distribution by local transpression of remnant oceanic basin floor without having to consider problematic long range ophiolite thrusting

mesozoic cenozoic geological evolution of the himalayan tibetan orogen and working tectonic hypotheses the himalayan tibetan orogen culminated during the cenozoic india asia collision but its geological framework and initial growth were fundamentally the result of multiple previous ocean closure and intercontinental suturing events as such the himalayan tibetan orogen provides an ideal laboratory to investigate geological signatures of the suturing process in general and how the earth9s highest and largest orogenic feature formed in specific this paper synthesizes the triassic through cenozoic geology of the central himalayan tibetan orogen and presents our tectonic interpretations in a time series of schematic lithosphere scale cross sections and paleogeographic maps we suggest that north dipping subducting slabs beneath asian continental terranes associated with closure of the paleo meso and neo tethys oceans experienced phases of southward trench retreat prior to intercontinental suturing these trench retreat events created ophiolites in forearc extensional settings and or a backarc oceanic basins between rifted segments of upper plate continental margin arcs this process may have occurred at least three times along the southern asian margin during northward subduction of neo tethys oceanic lithosphere from 174 to 156 ma 132 to 120 ma and 90 to 70 ma at most other times the tibetan terranes underwent cordilleran style or collisional contractional deformation geological records indicate that most of northern and central tibet the hoh xil and qiangtang terranes respectively were uplifted above sea level by jurassic time and southern tibet the lhasa terrane north of its forearc region has been above sea level since 100 ma stratigraphic evidence indicates that the northern himalayan margin of india collided with an asian affinity subduction complex forearc arc system beginning at 60 ma both the himalaya composed of indian crust and tibet show continuous geological records of orogenesis since 60 ma as no evidence exists in the rock record for a younger suture the simplest interpretation of the geology is that india asia collision initiated at 60 ma plate circuit paleomagnetic and structural reconstructions however suggest that the southern margin of asia was too far north of india to have collided with it at that time seismic tomographic images are also suggestive of a second more southerly neo tethyan oceanic slab in the lower mantle where the northernmost margin of india may have been located at 60 ma the geology of tibet and the india asia suture zone permits an alternative collision scenario in which the continental margin arc along southern asia the gangdese arc was split by extension beginning at 90 ma and along with its forearc to the south the xigaze forearc rifted southward and opened a backarc ocean basin the rifted arc collided with india at 60 ma whereas the hypothetical backarc ocean basin may not have been consumed until 45 ma a compilation of igneous age data from tibet shows that the most recent phase of gangdese arc magmatism in the southern lhasa terrane initiated at 70 ma peaked at 51 ma and terminated at 38 ma cenozoic potassic adakitic magmatism initiated at 45 ma within a 200 km wide elliptical area within the northern qiangtang terrane after which it swept westward and southward with time across central tibet until 26 ma at 26 to 23 ma potassic adakitic magmatism swept southward across the lhasa terrane a narrow 20 km width orogen parallel basin developed at low elevation along the axis of the india asia suture zone the kailas basin and greater himalayan sequence rocks began extruding southward between the south tibetan detachment and main central thrust the kailas basin was then uplifted to 4 km elevation by 20 ma after which parts of the india asia suture zone and gangdese arc experienced 6 km of exhumation between 20 and 16 ma between 16 and 12 ma slip along the south tibetan detachment terminated and east west extension initiated in the northern himalaya and tibet potassic adakitic magmatism in the lhasa terrane shows a northward younging trend in the age of its termination beginning at 20 to 18 ma until volcanism ended at 8 ma we interpret the post 45 ma geological evolution in the context of the subduction dynamics of indian continental lithosphere and its interplay with delamination of asian mantle lithosphere

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

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

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

imaging the colliding indian and asian lithospheric plates beneath tibet 1 a high resolution image of the base of the lithosphere from s to p converted seismic waves revealed the collision architecture of the indian and asian continental plates beneath the tibetan plateau the base of the indian lithosphere dips northward from a depth of 160 km beneath the himalayas to a depth of 220 km just south of the bangong suture the base of the asian lithosphere is nearly horizontal at the depth of 160 180 km from central to northern tibet there is a vertical gap of about 50 km between indian and asian lithospheres our observation of a well defined thick lithosphere throughout the entire plateau is not consistent with models of wholesale convective instability of a thickened mantle lithosphere which would predict a very thin asian lithosphere the hypothesized sequential southward subduction of tibetan and asian continental lithospheres leading to the growth of the tibetan plateau if correct cannot be occurring below 180 km depth our results along with available geological and geophysical data strongly support that the plateau is predominantly formed by a relatively coherent north dipping subducted indian continental lithosphere in the south which presently can be traced to the middle of the plateau and a south dipping subducted asian lithosphere in the north at a shallower depth

paleocene early eocene uplift of the altyn tagh mountain evidence from detrital zircon fission track analysis and seismic sections in the northwestern qaidam basin

precollision tilt of crustal blocks in rifted island arcs structural evidence from the kohistan arc 1 we describe results from field and microstructural investigations in the unusually large chilas plutonic complex which was emplaced within the mesozoic kohistan island arc northern pakistan magmatic and deformational fabrics of the chilas gabbronorite are concordant with magmatic fabrics of associated gabbros diorites and tonalites and show that this reportedly huge intrusion is composed of juxtaposed plutonic bodies with predominantly ssw dipping flow structures over the northeastern part of the complex which turn to sw ne striking subvertical attitudes to the west evidence for multiple intrusions includes concordant and discordant intrusive and or mylonitic contacts between gabbronorites and associated rocks owing to structural and kinematic consistency we attribute transitions from magmatic to more localized mylonitic fabrics to higher finite strain and or strain rate produced from drag of magma against its margin we discuss this structural pattern and conclude that the chilas gabbronorite emplaced incrementally into generally south dipping listric faults that dominated arc extension about 85 ma in addition several sets of hornblende pegmatite indicate that brittle deformation occurred in the presence of residual melt during consolidation of the main body the latest pegmatite set is composed of variably striking vertical extensional veins that show no relationship to the magmatic fabric we argue that the original attitude of these veins along with that of the enclosing chilas complex has been mostly preserved we hypothesize that the northward dip of the southern side of the kohistan is inherited from late cretaceous rotation of crustal blocks in the hanging wall of the listric faults

reconciling himalayan midcrustal discontinuities the main central thrust system the occurrence of thrust sense tectonometamorphic discontinuities within the exhumed himalayan metamorphic core can be explained as part of the main central thrust system this imbricate thrust structure which significantly thickened the orogenic midcrustal core comprises a series of thrust sense faults that all merge into a single detachment the existence of these various structures and their potential for complex overprinting along the main detachment may help explain the contention surrounding the definition mapping and interpretation of the main central thrust the unique evolution of specific segments of the main central thrust system along the orogen is interpreted to be a reflection of the inherent basement structure and ramp position and structural level of exposure of the mid crust this helps explain the variation in the timing and structural position of tectonometamorphic discontinuities along the length of the mountain belt

a review of indian phanerozoic palaeomagnetism implications for the india asia collision abstract an overview is presented of the indian apparent polar wander path apwp for the phanerozoic and in particular for post late palaeozoic times this apwp is compiled on basis of data available at october 1981 from peninsular and extrapeninsular indo pakistan and from dsdp cores from the indian plate one of the more important and newly recognized features of this apwp is a large scale triassic jurassic loop this loop indicates a changeover from a late palaeozoic early mesozoic northwards and counter clockwise rotational movement with greater india reaching moderately low southern latitudes into a southwards and clockwise rotational movement during the early to middle jurassic recognizable likewise in apwp s from other gondwana continents this loop reflects the opening of the neotethys studies of extrapeninsular regions up to and north of the indus tsangpo suture zone have shown wide spread presence of magnetic overprints which delineate two regionally confined age groups younger overprints 20 40 m y predominate in the more external thrust zones older overprints 50 60 m y in contrast are found in the more internal zones both north and south of the indus tsangpo suture zone the latter are interpreted to reflect a late phase of relaxation in the early tertiary collision of greater india with south central asia or off shore island arcs which occurred at equatorial to low northern palaeolatitudes 0 10 n subsequent northwards movement over 2500 3000 km or more and impingement of greater india into southern asia resulted into large scale underthrusting of greater india along the main central thrust beneath southern tibet and to clockwise rotation of thrust units in the western himalaya a discrepancy between indian palaeomagnetic data and results available todate from southern tibet is discussed

constraints on the collision and the pre collision tectonic configuration between india and asia from detrital geochronology thermochronology and geochemistry studies in the lower indus basin pakistan abstract knowledge of the timing of india asia collision is a fundamental prerequisite for understanding the evolution of the himalayan tibetan orogen and its role in global climate oceanic chemistry and ecological evolution despite much active research the basic pre collision tectonic configuration and the timing of terminal india asia suturing remain debated for example debates regarding when and how the intervening kohistan ladakh arc was sutured with india and asia still remain elusive some models propose the arc collided with asia at about 100 ma with india asia collision at ca 55 ma whilst a newer model proposed the arc s collision with india at 50 ma and subsequently with asia at 40 ma another example is the recent proposition that an oceanic greater india basin separated the tethyan himalaya microcontinent from the remaining indian plate until 20 25 ma with the consumption of this oceanic basin marking the final collision at this time these controversies relate to whether the commonly documented 50 ma contact represents the terminal india asia suturing or the amalgamation between various arcs or microcontinents with india or asia here we present an integrated provenance study of geochronology thermochronology and geochemistry on the late cretaceous pleistocene sediments from the lower indus basin on the indian plate the detrital zircon u pb and fission track data show a reversal in sediment source from a pure indian signature to increasing inputs from the suture zone and the asian plate between the middle paleocene and early oligocene the nd and sr isotopes narrow down this change to 50 ma by revealing input of asian detritus and the establishment of a nd sr isotopic pattern similar to the present day indus fan by 50 ma with no significant variations up section contrary to what might be expected if later major collisions had occurred our isotopic data indicate that greater india was occupied by a fluvial deltaic system analogous to the present day indus and named as the paleo indus which has been transporting asian detritus southward across the suture zone and kohistan ladakh arc since 50 ma suggesting no other ocean basins intervened between india and asia after this time in this region our data require that in the west the india asia collision were accomplished by 50 ma

structural and stratigraphic development of the newark extensional basin eastern north america evidence for the growth of the basin and its bounding structures the newark basin is a half graben bounded on its northwestern margin by a system of mostly normal slip faults many of which are reactivated paleozoic thrust faults and were active at least sporadically during the deposition of the entire basin fill as a result of along strike variations in displacement on the border fault system subsidence increased from the lateral edges toward the center of the basin and from the hinged margin toward the border fault system some intrabasinal faults also formed during the deposition of units currently preserved in their hanging walls along strike variations in the density of intrabasinal faults probably reflect changes in the dip angle of the border fault system variations in the amount of extension accommodated on these intrabasinal faults resulted in the formation of a transfer fault along strike variations in fault displacement probably produced the transverse folds in the hanging walls immediately adjacent to border faults and major intrabasinal normal faults some folds formed syndepositionally progressively younger strata onlap basement rocks along the hinged margin of the basin and younger splays of the border fault system propagated into the footwall the basin therefore grew deeper wider and longer through time

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

the evolution of the bangong nujiang neo tethys ocean evidence from zircon u pb and lu hf isotopic analyses of early cretaceous oceanic islands and ophiolites abstract we conducted in situ u pb analyses of zircons from three basalts and one gabbro from the zhonggang oceanic island one basalt from the zhaga oceanic island and one gabbro from the kangqiong ophiolite all located in the middle segments of the bangong nujiang suture zone of tibetan plateau as well as in situ hf isotope analyses of zircons from one gabbro from the zhonggang oceanic island to constrain the tectonic evolution of the bangong nujiang neo tethys ocean all samples contain numerous inherited zircons and all the zircons contain magmatic oscillatory zoning and have th u ratios exceeding 0 4 moreover the average ree content of these zircons is less than 2000 ppm and they display clear negative eu and variable positive ce anomalies indicating a magmatic origin la icp ms u pb dating of the zircons revealed three clear peaks in the age distribution at 248 255 162 168 and 117 120 ma lu hf isotopic analyses of zircons from the gabbro of the zhonggang oceanic island yielded a 269 ma crust mantle separation age taking into account the regional geology previous data and our new analyses we infer that the middle and western segments of the bangong nujiang neo tethys ocean had initially opened in the late permian 254 269 ma and that the ocean opened substantially between the late permian and the early triassic 248 255 ma in addition we infer that the initiation of subduction of the bangong nujiang neo tethys ocean took place at 162 168 ma which is middle jurassic the 117 120 ma age is the time when the oceanic islands and ophiolites were formed indicating that the bangong nujiang neo tethys ocean was to some extent still open at that time

a cretaceous eocene depositional age for the fenghuoshan group hoh xil basin implications for the tectonic evolution of the northern tibet plateau the fenghuoshan group marks the initiation of terrestrial deposition in the hoh xil basin and preserves the first evidence of uplift above sea level of northern tibet the depositional age of the fenghuoshan group is debated as are the stratigraphic relationships between the fenghuoshan group and other terrestrial sedimentary units in the hoh xil basin we present new radiometric dates and a compilation of published biostratigraphic data which are used to reinterpret existing magnetostratigraphic data from the fenghuoshan group from these data we infer an 85 51 ma depositional age range for the fenghuoshan group u pb detrital zircon age spectra from this unit are compared to age spectra from tibetan terranes and mesozoic sedimentary sequences to determine a possible source terrane for fenghuoshan group strata we propose that these strata were sourced from the qiangtang terrane and may share a common sediment source with cretaceous sedimentary rocks in nima basin field relationships and compiled biostratigraphic data indicate that the fenghuoshan and tuotuohe groups are temporally distinct units we report late oligocene ages for undeformed basalt flows that cap tilted fenghuoshan group strata together our age constraints and field relationships imply exhumation of the central qiangtang terrane from the late cretaceous to earliest eocene followed by eocene oligocene deformation and shortening of the northern qiangtang and southern songpan ganzi terranes crustal shortening within the hoh xil basin ceased by late oligocene time as is evident from flat lying basaltic rocks which cap older deformed strata

assembly of the lhasa and qiangtang terranes in central tibet by divergent double subduction abstract integration of lithostratigraphic magmatic and metamorphic data from the lhasa qiangtang collision zone in central tibet including the bangong suture zone and adjacent regions of the lhasa and qiangtang terranes indicates assembly through divergent double sided subduction this collision zone is characterized by the absence of early cretaceous high grade metamorphic rocks and the presence of extensive magmatism with enhanced mantle contributions at ca 120 110 ma two jurassic cretaceous magmatic arcs are identified from the caima duobuza rongma kangqiong amdo magmatic belt in the western qiangtang terrane and from the along tso yanhu daguo baingoin daru tso magmatic belt in the northern lhasa terrane these two magmatic arcs reflect northward and southward subduction of the bangong ocean lithosphere respectively available multidisciplinary data reconcile that the bangong ocean may have closed during the late jurassic early cretaceous most likely ca 140 130 ma through arc arc soft collision rather than continent continent hard collision subduction zone retreat associated with convergence beneath the lhasa terrane may have driven its rifting and separation from the northern margin of gondwana leading to its accretion within asia

the geological evolution of the tibetan plateau the geological evolution of the tibetan plateau is best viewed in a context broader than the india eurasia collision zone after collision about 50 million years ago crust was shortened in western and central tibet while large fragments of lithosphere moved from the collision zone toward areas of trench rollback in the western pacific and indonesia cessation of rapid pacific trench migration 15 to 20 million years ago coincided with a slowing of fragment extrusion beyond the plateau and probably contributed to the onset of rapid surface uplift and crustal thickening in eastern tibet the latter appear to result from rapid eastward flow of the deep crust probably within crustal channels imaged seismically beneath eastern tibet these events mark a transition to the modern structural system that currently accommodates deformation within tibet

low elevation of the northern lhasa terrane in the eocene implications for relief development in south tibet the uplift history of tibet is crucial for understanding the geodynamic and palaeoclimatologic evolution of asia however the pattern of plateau uplift through space and time remains unresolved we use stable isotope data from palaeosol carbonates to reconstruct the eocene 46 ma elevation of the tangra yum co region northern lhasa terrane the difference between the oxygen isotopic compositions of the studied palaeosols and eocene surface water near the source of the indian monsoon indicates a palaeoelevation of 2590 730 910 m for tangra yum co present altitude 4600 m a similar palaeoaltitude is obtained by comparing our data with modern soils and taking past environmental changes into account combining our results with published palaeoelevation estimates for tibet suggests significant relief 2 km during the eocene when a low lying basin in the northern lhasa terrane was bounded by the qiangtang and gangdese mountain belts to the north and south respectively

late mesozoic tectonic evolution and growth of the tibetan plateau prior to the indo asian collision abstract the elevation of the tibetan plateau is a revolutionary event in the earth history which resulted in change not only of cenozoic regional and global climate but also of monsoon intensity a critical aspect for the development of the tibetan plateau that remains open to intense debate is whether the collision among the tibetan continental blocks or the subduction of the tethys prior to the indo asian collision in the cenozoic contributed to the plateau growth we propose here that an andean type orogen could have been present in southern tibet from the jurassic to early cretaceous but collapsed during the middle cretaceous based on a synthesis of existing structural sedimentary geochronological and geochemical data the orogeny is marked by north verging jurassic early cretaceous thrusting in the lhasa block a result of flat subduction of the yarlung zangpo neo tethys that is recorded by a narrow belt of 205 174 ma adakitic rocks along the southernmost margin of the lhasa block denudation of the orogen produced a thick up to 15 km sequence of jurassic turbidites and molasse type sediments on the northern lhasa block widespread middle cretaceous 135 100 ma postorogenic strongly peraluminous to calc alkaline magmatism and extensional deformation in the lhasa block and an extensive marine transgression in tibet indicate that the orogen broke down due to extension resulting from lithosphere delamination and asthenosphere upwelling meanwhile the north dipping subduction of the bangong meso tethys produced a wide jurassic middle cretaceous magmatic arc along the southern margin of the qiangtang block findings of 132 108 ma aged ophiolites rich with middle cretaceous radiolarians in central tibet indicate that the bangong meso tethys did not close until the late cretaceous repeated flat slab subduction of the yarlung zangpo neo tethys occurred during the late cretaceous 90 78 ma as indicated by the adakites in the southernmost lhasa block which together with the ongoing qiangtang lhasa collision could have contributed to the growth of the tibetan plateau

the giant shakhdara migmatitic gneiss dome pamir india asia collision zone 1 geometry and kinematics 1 cenozoic gneiss domes comprise one third of the surface exposure of the pamir and provide a window into the deep crustal processes of the india asia collision the largest of these are the doubly vergent composite shakhdara alichur domes of the southwestern pamir tajikistan and afghanistan they are separated by a low strain horst top to sse noncoaxial pervasive flow over the up to 4 km thick south pamir shear zone exhumed crust from 30 40 km depth in the 250 80 km shakhdara dome the top to nne alichur shear zone exposed upper crustal rocks in the 125 25 km alichur dome the gunt shear zone bounds the shakhdara dome in the north and records alternations of normal shear and dextral transpression it contributed little to bulk exhumation footwall exhumation along two low angle normal sense detachments resulted in up to 90 km syn orogenic n s extension extension in the southwestern pamir opposes shortening in a fold thrust belt north of the domes and in particular in the tajik depression where an evaporitic decollement facilitated upper crustal shortening gravitational collapse of the pamir plateau margin drove core complex formation in the southwestern pamir and shortening of the weak foreland adjacent to the plateau overall this geometry defines a vertical extrusion scenario comprising frontal and basal underthrusting and thickening and hanging gravitationally driven normal shear in contrast to the himalayan vertical extrusion scenario erosion in the pamir was minor preserving most of the extruded deep crust including the top of the south pamir shear zone at peak elevations throughout the dome

kinematics and vorticity in kangmar dome southern tibet testing midcrustal channel flow models for the himalaya 1 kinematic kinematic vorticity wm and deformation temperature analyses were completed to test the hypothesis that midcrustal rocks exposed in the core of the kangmar gneiss dome southern tibet record ductile deformation patterns of a frozen segment of a southward flowing midcrustal channel microscopic and mesoscopic kinematic indicators exhibit a downward transition from a subequal mix of top north and top south shear in garnet zone rocks to dominantly top north shear in staurolite kyanite zone and deeper rocks kinematic vorticity values indicate an increase in pure shear component with depth from 48 pure shear in chloritoid zone rocks through 62 in garnet zone to staurolite kyanite zone rocks to 68 pure shear in an orthogneiss the deepest exposed rocks deformation temperatures inferred from grain scale microstructures and quartz lattice preferred orientations increase from 300 c 400 c in chloritoid zone rocks to 600 c in the deepest exposed rocks these temperatures are equivalent to temperatures derived from garnet biotite thermobarometry indicating that wm was recorded during peak metamorphism this ductile deformation zone was cut by the brittle southern tibetan detachment system stds that juxtaposed metasedimentary rocks upon the orthogneiss on the basis of these relations midcrustal rocks in the core of kangmar dome record 1 general shear vertical thinning and n s horizontal extension with a component of top north shear during peak metamorphism within a ductile shear zone corresponding to the northern and deeper portion of the stds 2 an increase in pure shear with structural depth a consequence of an increase in lithostatic load and 3 displacement of the high temperature shear zone by the brittle stds our data are compatible with the deformation patterns predicted for the top part of a southward flowing midcrustal channel

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

variable shortening rates in the eastern himalayan thrust belt bhutan insights from multiple thermochronologic and geochronologic data sets tied to kinematic reconstructions 1 we present data on the burial displacement and exhumation history of the himalayan fold thrust belt in eastern bhutan these data document the magnitude and timing of displacement of large discrete structures and highlight temporal variability in shortening rates eight new40ar 39ar ages from white mica 32 new zircon u th he ages 7 new apatite fission track ages and 1 new u pb zircon la mc icp ms metamorphic rim growth age are combined with published cooling ages and deformation temperatures and incremental shortening magnitudes from restorations of two published balanced cross sections to illustrate the kinematic and temporal development of the bhutan thrust belt integrating these data from 23 ma to the present illustrates rapid horizontal shortening rates 28 35 mm yr between 23 20 ma and 15 10 ma separated by more moderate rates 10 23 mm yr shortening rates decrease significantly to 7 10 mm yr and possibly as low as 3 4 mm yr from 10 to 0 ma this decrease is interpreted to represent the onset of strain partitioning in the eastern part of the himalayan tibetan orogenic system between shortening in the bhutan thrust belt uplift of the shillong plateau and deformation and outward growth of the northern and eastern tibetan plateau within estimated error horizontal shortening rates during emplacement of the main central thrust sheet and during construction of the upper lesser himalayan duplex approached india asia tectonic velocities thus for periods of time between 23 20 ma and 15 10 ma the bhutan thrust belt may have absorbed nearly all india asian convergence at this longitude

implications of shortening in the himalayan fold thrust belt for uplift of the tibetan plateau 1 recent research in the himalayan fold thrust belt provides two new sets of observations that are crucial to understanding the evolution of the himalayan tibetan orogenic system first u pb zircon ages and sm nd isotopic studies demonstrate that 1 greater himalayan medium to high grade metasedimentary rocks are much younger than true indian cratonic basement and 2 these rocks were tectonically mobilized and consolidated with the northern margin of gondwana during early paleozoic orogenic activity these observations require that greater himalayan rocks be treated as supracrustal material in restorations of the himalayan fold thrust belt rather than as indian cratonic basement in turn this implies the existence of greater himalayan lower crust that is not exposed anywhere in the fold thrust belt second a regional compilation of shortening estimates along the himalayan arc from pakistan to sikkim reveals that 1 total minimum shortening in the fold thrust belt is up to 670 km 2 total shortening is greatest in western nepal and northern india near the apex of the himalayan salient and 3 the amount of himalayan shortening is equal to the present width of the tibetan plateau measured in an arc normal direction north of the indus yalu suture zone this information suggests that a slab of greater indian lower crust composed of both indian cratonic lower crust and greater himalayan lower crust with a north south length of 700 km may have been inserted beneath the tibetan crust during the cenozoic orogeny we present a modified version of the crustal underthrusting model for himalayan tibetan orogenesis that integrates surface geological data recent results of mantle tomographic studies and broadband seismic studies of the crust and upper mantle beneath the tibetan plateau previous studies have shown that incremental mesozoic and early cenozoic shortening had probably thickened tibetan crust to 45 55 km before the onset of the main cenozoic orogenic event thus the insertion of a slab of greater indian lower crust with maximum thickness of 20 km tapering northward could explain the cenozoic uplift of the tibetan plateau the need for tibetan crust to stretch laterally as the greater indian lower crust was inserted may explain the widespread east west extension in the southern half of the plateau our reconstruction of the himalayan fold thrust belt suggests that indian cratonic lower crust of presumed mafic composition and high strength should extend approximately halfway across the tibetan plateau to the banggong suture from there northward we predict that the tibetan plateau is underlain by more felsic and therefore weaker lower crust of greater himalayan affinity two slab break off events are predicted by the model the first involved neotethyan oceanic lithosphere that foundered 45 35 ma and the second consisted of greater indian lithosphere most likely composed of greater himalayan material that delaminated and foundered 20 10 ma asthenospheric upwelling associated with the break off events may explain patterns of cenozoic volcanism on the tibetan plateau although the model predicts a northward migrating topographic front due solely to insertion of greater indian lower crust the actual uplift history of the plateau was complicated by early middle tertiary shortening of tibetan crust

synchronous n s and e w extension at the tibet to himalaya transition in nw bhutan despite 50 myr of continuous continent continent collision contractional structures in the himalayan tibetan orogen are today limited to the northern and southern margins of the system while extension dominates much of the interior on the tibetan plateau cenozoic e w extension has been accommodated by strike slip faults and extensional grabens while n s extension at the tibet to himalaya transition has been accommodated by the south tibetan fault system stfs the genetic relationship between n s and e w extension is disputed although age constraints indicate temporal overlap of at least 7 myr in nw bhutan the two intersect where the stfs basal detachment is cut by the yadong cross structure ycs an extensional half graben that provides a rare opportunity to constrain relative timings we report u pb zircon dates from four stfs footwall leucogranites consistent with episodic magmatism during the middle late miocene and in situ u th pb monazite and xenotime dates from three metasedimentary rocks ranging from late oligocene to middle miocene we suggest that amphibolite facies footwall metamorphism was ongoing at the time the basal stfs detachment initiated as a ductile structure in the middle late miocene late stage granitic intrusions may reflect footwall melting during extensional exhumation along the stfs but post metamorphic and post intrusion fabrics suggest that most displacement occurred after emplacement of the youngest granites some of the oldest ycs related fabrics are found in a deformed 14 ma leucogranite implying middle miocene ductile deformation this observation along with evidence for subsequent brittle ycs deformation suggests that n s and e w extensional structures in the area had protracted and overlapping deformation histories

kinematic evolution of the eastern tethyan himalaya constraints from magnetic fabric and structural properties of the triassic flysch in se tibet anisotropy of magnetic susceptibility ams combined with structural analysis are used in this work with the aim to characterize the tectonic evolution of the triassic flysch within the eastern tethyan himalaya thrust belt in se tibet the attitude of the magnetic foliation and lineation are concordant with the planar and linear structures of tectonic origin defined by the preferred orientation of the iron bearing silicates two different tectonic domains can be defined a the southern domain is controlled by the eohimalayan tectonic foliation s1 recorded in the magnetic foliation which trends east west and dips to the north b the northern domain is dominated by the neohimalayan magnetic foliation with wnw ese strike and dips to the south opposite to the vergence of the main structures a slightly prolate magnetic ellipsoid has been found in between the two domains recording the intersection of s1 and the subtle development of the s2 tectonic foliation hinterland propagation of the deformation lead to the great counter backthrust generation pointed out by the ssw steeply plunging magnetic lineation furthermore different orientations of magnetic foliation may indicate an early miocene c 208 clockwise vertical

origin and pre cenozoic evolution of the south qiangtang basement central tibet abstract the qiangtang terrane in central tibet with its high pressure rocks is a key area to unravel the evolution of the paleo tethys we present a new 1 50 000 map of the rongma area in the central qiangtang terrane of which relatively little is known so far we show that the qiangtang metamorphic belt can be separated in a paleozoic autochthonous basement and an overlying allochthonous thrust stack of subduction melange that contains high pressure rocks and permian sediments detrital zircon ages youngest peak at 591 ma and an 470 ma granite intrusion age constrain the age of the qiangtang basement to be between the late precambrian to middle ordovician this is in agreement with the observed unconformity between basement and overlying mid late ordovician strata this unconformity and the zircon age spectra are comparable to the himalaya area the qiangtang terrane must thus have been part of the gondwana supercontinent during the early paleozoic the occurrence of the late triassic eclogite and glaucophane bearing schists in the central qiangtang terrane indicates the existence of a suture zone between the north and south qiangtang terranes before the late triassic this suture zone resulted from closure of the paleo tethys between the north and south qiangtang terranes late triassic syn collisional granites and rapidly exhumed high pressure rocks resulted from the closure of this ocean and final amalgamation of the re united qiangtang terranes with eurasia collision of the lhasa and qiangtang terranes is correlated with north dipping reverse faults in the south qiangtang terrane which were probably related to exhumation of the qiangtang basement and formation of the qiangtang culmination

lower edge of locked main himalayan thrust unzipped by the 2015 gorkha earthquake faults are unlocked by earthquakes analysis of seismic data from the 2015 nepal earthquake shows that only part of the main himalayan thrust fault was unzipped by the quake leaving much of the fault locked and ready to slip in a future event

accommodation of transpressional strain in the arabia eurasia collision zone new constraints from u th he thermochronology in the alborz mountains north iran the alborz range of n iran provides key information on the spatiotemporal evolution and characteristics of the arabia eurasia continental collision zone the southwestern alborz range constitutes a transpressional duplex which accommodates oblique shortening between central iran and the south caspian basin the duplex comprises nw striking frontal ramps that are kinematically linked to inherited e w striking right stepping lateral to obliquely oriented ramps new zircon and apatite u th he data provide a high resolution framework to unravel the evolution of collisional tectonics in this region our data record two pulses of fast cooling associated with sw directed thrusting across the frontal ramps at 18 14 and 9 5 7 5 ma resulting in the tectonic repetition of a fossil zircon partial retention zone and a cooling pattern with a half u shaped geometry uniform cooling ages of 7 6 ma along the southernmost e w striking oblique ramp and across its associated nw striking frontal ramps suggests that the ramp was reactivated as a master throughgoing n dipping thrust we interpret this major change in fault kinematics and deformation style to be related to a change in the shortening direction from ne to n nne the reduction in the obliquity of thrusting may indicate the termination of strike slip faulting and possibly thrusting across the iranian plateau which could have been triggered by an increase in elevation furthermore we suggest that 7 6 m y old s directed thrusting predated inception of the westward motion of the south caspian basin

vast early miocene lakes of the central tibetan plateau lacustrine strata of the wudaoliang group demonstrate that two vast lake complexes covered the central tibetan plateau during early miocene time the wudaoliang group which is characterized by dolostone limestone and marl as thick as 310 350 m lies horizontally or with a slight dip above oligocene reddish brown sandstone and is covered by upper miocene and pliocene mudstone and sandstone the extensive distribution of this group outlines interconnected lake basins of 5000 15 000 km 2 or larger in the north central tibetan plateau such as the wudaoliang beiluhe tuotuohe tongtianhe and east wenquan basins and lake basins 2000 km 2 or larger in the south central plateau such as the ando naqu bangoin lunpola and shuanghu basins these lake basins separated by mountain ranges and islands were linked by water passages in early miocene time when they apparently formed a huge lake complex that covered as much as 100 000 km 2 in the north central tibetan plateau and another one 50 000 km 2 in the south central part of the plateau such immense lakes existed for several million years between ca 23 5 and ca 13 5 ma after the tibetan plateau rose and their interconnected basins define the topography during early miocene time the fossils contained in these basins indicate a change from the warm dry climate of the oligocene to moderately cool and wet conditions at the beginning of the early miocene followed by progressive cooling and drying which suggests continuous uplift during deposition of the wudaoliang group

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

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

mountain building and mantle dynamics 1 mountain building at convergent margins requires tectonic forces that can overcome frictional resistance along large scale thrust faults and support the gravitational potential energy stored within the thickened crust of the orogen a general dynamic model for this process is still lacking here we propose that mountain belts can be classified between two end members first those of slab pull type where subduction is mainly confined to the upper mantle and rollback trench motion lead to moderately thick crustal stacks such as in the mediterranean second those of slab suction type where whole mantle convection cells conveyor belts lead to the more extreme expressions of orogeny such as the largely thickened crust and high plateaus of present day tibet and the altiplano for the slab suction type deep mantle convection produces the unique conditions to drag plates toward each other irrespective of their nature and other boundary conditions we support this hypothesis by analyzing the orogenic volcanic and convective history associated with the tertiary formation of the andes after 40 ma and himalayas after collision at 55 ma based on mantle circulation modeling and tectonic reconstructions we surmise that the forces necessary to sustain slab suction mountain building in those orogens derive after transient slab ponding from the mantle drag induced upon slab penetration into the lower mantle and from an associated surge of mantle upwelling beneath africa this process started at 65 55 ma for tibet himalaya when the tethyan slab penetrated into the lower mantle and 10 myr later in the andes when the nazca slab did this surge of mantle convection drags plates against each other generating the necessary compressional forces to create and sustain these two orogenic belts if our model is correct the available geological records of orogeny can be used to decipher time dependent mantle convection with implications for the supercontinental cycle

normal faulting in central tibet since at least 13 5 myr ago tectonic models for the evolution of the tibetan plateau interpret observed east west thinning of the upper crust to be the result of either increased potential energy of elevated crust1 or geodynamic processes that may be unrelated to plateau formation2 3 4 5 6 a key piece of information needed to evaluate these models is the timing of deformation within the plateau the onset of normal faulting has been estimated to have commenced in southern tibet between about 14 myr ago7 and about 8 myr ago8 and in central tibet about 4 myr ago9 here however we report a minimum age of approximately 13 5 myr for the onset of graben formation in central tibet based on mineralization ages determined with rb sr and 40ar 39ar data that post date a major graben bounding normal fault these data along with evidence for prolonged activity of normal faulting in this and other tibetan grabens support models that relate normal faulting to processes occurring beneath the plateau thinning of the upper crust is most plausibly the result of potential energy increases resulting from spatially and temporally heterogeneous changes in thermal structure and density distribution within the crust and upper mantle beneath tibet this is supported by recent geophysical and geological data10 11 12 13 14 15 16 17 which indicate that spatial heterogeneity exists in both the tibetan crust and lithospheric mantle

geodynamics of trench advance insights from a philippine sea style geometry abstract for terrestrial parameter sets trench retreat is found to be nearly ubiquitous and trench advance quite rare largely due to rheological and ridge push effects recently updated analyses of global plate motions indicate that significant trench advance is also rare on earth being largely restricted to the marianas izu bonin arc thus we explore conditions necessary for terrestrial trench advance through dynamical models involving the unusual geometry of the philippine sea region in this subduction system a slab pull force from distal subduction is transmitted to the overriding plate at the pacific trench our 2d modeling demonstrates that trench advance can occur for terrestrial rheologies in such special geometries we observe persistent trench advance punctuated by two episodes of back arc extension characteristic features of the model such as time interval between extensional episodes high back arc heat flow and stress state of philippine plate correspond to processes recorded in the region

geometry of the pamir hindu kush intermediate depth earthquake zone from local seismic data 1 we present new seismicity images based on a two year seismic deployment in the pamir and sw tien shan a total of 9532 earthquakes were detected located and rigorously assessed in a multistage automatic procedure utilizing state of the art picking algorithms waveform cross correlation and multi event relocation the obtained catalog provides new information on crustal seismicity and reveals the geometry and internal structure of the pamir hindu kush intermediate depth seismic zone with improved detail and resolution the relocated seismicity clearly defines at least two distinct planes one beneath the pamir and the other beneath the hindu kush separated by a gap across which strike and dip directions change abruptly the pamir seismic zone forms a thin approximately 10 km width curviplanar arc that strikes east west and dips south at its eastern end and then progressively turns by 90 to reach a north south strike and a due eastward dip at its southwestern termination pamir deep seismicity outlines several streaks at depths between 70 and 240 km with the deepest events occurring at its southwestern end intermediate depth earthquakes are clearly separated from shallow crustal seismicity which is confined to the uppermost 20 25 km the hindu kush seismic zone extends from 40 to 250 km depth and generally strikes east west yet bends northeast toward the pamir at its eastern end it may be divided vertically into upper and lower parts separated by a gap at approximately 150 km depth in the upper part events form a plane that is 15 25 km thick in cross section and dips sub vertically north to northwest seismic activity is more virile in the lower part where several distinct clusters form a complex pattern of sub parallel planes the observed geometry could be reconciled either with a model of two sided subduction of eurasian and previously underthrusted indian continental lithosphere or by a purely eurasian origin of both pamir and hindu kush seismic zones which necessitates a contortion and oversteepening of the latter

assembly of the pamirs age and origin of magmatic belts from the southern tien shan to the southern pamirs and their relation to tibet 1 magmatic rocks and depositional setting of associated volcaniclastic strata along a north south traverse spanning the southern tien shan and eastern pamirs of kyrgyzstan and tajikistan constrain the tectonics of the pamirs and tibet the northern pamirs and northwestern tibet contain the north facing kunlun suture the south facing jinsha suture and the intervening carboniferous to triassic karakul mazar subduction accretion system the latter is correlated with the songpan garze hoh xi system of tibet the kunlun arc is a composite early paleozoic to late paleozoic triassic arc arc formation in the pamirs is characterized by 370 320 ma volcanism that probably continued until the triassic the cryptic tanymas suture of the southern northern pamirs is part of the jinsha suture a massive 227 ma batholith stitches the karakul mazar complex in the pamirs there are striking similarities between the qiangtang block in the pamirs and tibet like tibet the regional structure of the pamirs is an anticlinorium that includes the muskol and sares domes like tibet the metamorphic rocks in these domes are equivalents to the karakul mazar songpan garze system granitoids intruding the qiangtang block yield 200 230 ma ages in the pamirs and in central tibet the stratigraphy of the eastern pshart area in the pamirs is similar to the bangong nujiang suture zone in the amdo region of eastern central tibet but a triassic ocean basin sequence is preserved in the pamirs arc type granitoids that intruded into the eastern pshart oceanic basin arc sequence 190 160 ma and granitoids that cut the southern qiangtang block 170 160 ma constitute the rushan pshart arc cretaceous plutons that intruded the central and southern pamirs record a long lasting magmatic history their zircons and those from late miocene xenoliths show that the most distinct magmatic events were cambro ordovician 410 575 ma triassic 210 250 ma likely due to subduction along the jinsha suture middle jurassic 147 195 ma subduction along rushan pshart suture and mainly cretaceous middle and late cretaceous magmatism may reflect arc activity in asia prior to the accretion of the karakoram block and flat slab subduction along the shyok suture north of the kohistan ladakh arc respectively before india and asia collided the pamir region from the indus yarlung to the jinsha suture was an andean style plate margin our analysis suggests a relatively simple crustal structure for the pamirs and tibet from the kunlun arc in the north to the southern qiangtang block in the south the pamirs and tibet likely have a dominantly sedimentary crust characterized by karakul mazar songpan garze accretionary wedge rocks the crust south of the southern qiangtang block is likely of granodioritic composition reflecting long lived subduction arc formation and cretaceous cenozoic underthrusting

kinematics and flow patterns in deep mantle and upper mantle subduction models influence of the mantle depth and slab to mantle viscosity ratio 1 three dimensional fluid dynamic laboratory simulations are presented that investigate the subduction process in two mantle models an upper mantle model and a deep mantle model and for various subducting plate mantle viscosity ratios sp m 59 1375 the models investigate the mantle flow field geometrical evolution of the slab sinking kinematics and relative contributions of subducting plate motion and trench migration to the total rate of subduction all models show that the subducting plate is always moving trenchward resulting from slab pull furthermore all deep mantle models show trench retreat as do upper mantle models in the initial stage of subduction before slab tip transition zone interaction upper mantle models with a low sp m 66 217 continue to show trench retreat after interaction upper mantle models with a high sp m 378 709 show a period of trench advance after interaction followed by trench retreat upper mantle models with a very high sp m 1375 show continued trench advance after interaction the difference in trench migration behavior and associated slab geometries is attributed to both sp m and the mantle depth to plate thickness ratio tm tsp which both affect the slab bending radius to mantle thickness ratio rb tm four subduction regimes can be defined regime i with rb tm 0 3 trench retreat slab draping and a concave trench regime ii with 0 3 rb tm 0 5 episodic trench migration slab folding and a concave trench regime iii with rb tm 0 5 trench advance slab rollover geometries and minor trench curvature and regime iv with rb tm 0 8 trench retreat slab draping and a rectilinear trench in all models slab parallel downdip motion induces poloidal mantle flow structures in addition trench retreat and rollback motion of the slab induce quasi toroidal return flow around the lateral slab edges toward the mantle wedge rollback induced poloidal flow around the slab tip is not observed in any of the experiments finally comparison between the slab geometries observed in the upper mantle models and slab geometries observed in nature imply that the effective viscosity ratio between slab and ambient upper mantle in nature is less than 103 and of the order 1 7 102 with a best estimate of 1 3 102

interseismic coupling on the main himalayan thrust we determine the slip rate and pattern of interseismic coupling on the main himalayan thrust along the entire himalayan arc based on a compilation of geodetic interferometric synthetic aperture radar and microseismicity data we show that convergence is perpendicular to the arc and increases eastwards from 13 3 1 7 mm yr to 21 2 2 0 mm yr these rates are comparable to geological and geomorphic estimates indicating an essentially elastic geodetic surface strain the interseismic uplift rate predicted from the coupling model closely mimics the topography suggesting that a small percentage of the interseismic strain is permanent we find that the fault is fully locked along its complete length over about 100 km width we don t find any resolvable aseismic barrier that could affect the seismic segmentation of the arc and limit the along strike propagation of seismic ruptures the moment deficit builds up at a rate of 15 1 1 10 19 n m yr for the entire length of the himalaya