building the pamirs the view from the underside the pamir mountains are an outstanding example of extreme crustal shortening during continental collision that may have been accommodated by formation of a thick crust much thicker than is currently thought and or by continental subduction we pre sent new petrologic data and radiometric ages from xenoliths in miocene volcanic rocks in the southeastern pamir mountains that suggest that gondwanan igneous and sedimentary assemblages were underthrust northward buried to 50 80 km during the ear ly stage of the india asia collision and then heated and partly melted during subsequent thermal relaxation before finally being blasted to the surface these xenoliths the deepest crustal samples recovered from under any active collisional belt provide direct evidence for early cenozoic thickening of the pamirs and lower crustal melting during collision the xenoliths also suggest that the present mountain range was a steady state elevated plateau for most of the cenozoic

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

magmatism and metamorphism in the lhasa block tibet a geochronological study u pb systematics in small fractions of zircon monazite and sphene 5 to 20 grains each have been analyzed in order to decipher the age and origin of magmatism in the lhasa block and to date the tectono metamorphic events additionally we analyzed a few minerals for rb sr the results show that the granites and an ignimbrite were formed essentially through anatexis of continental crust the magma source regions of which consisted of palaeozoic to precambrian materials up to 2 b y in age the following episodes can be distinguished 1 an early tertiary plutonism volcanism in the southern lhasa block yangbajing region strongly related to intracontinental tectono metamorphism 2 a late jurassic to early cretaceous plutonism in the central and northern lhasa block and 3 a middle jurassic low grade metamorphism affecting the basement at the northern margin 171 6 m y ago the early tertiary magmatism and metamorphism in the south are probably a direct result of tethys subduction indian plate

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

neogene cooling and exhumation of upper amphibolite facies whiteschists in the southwest pamir mountains tajikistan abstract the pamir mountains of central asia are the product of accretion events during the paleozoic and mesozoic and have been further impacted by the continental collision between india and asia during the cenozoic deformation continues today in what is now a mountain belt within a continental interior the geology of the pamir is characterized by several arcuate shaped structurally bound regions with a general east west trend the southwest pamirs are noteworthy for the extensive development of whiteschists talc kyanite rocks and other ultramagnesian metamorphic assemblages in the goran series our interpretation of the goran series is that upper amphibolite facies metamorphism at temperatures above 650 c and pressures near 700 mpa kyanite talc kyanite potassium feldspar was followed by decompression to below 400 mpa sillimanite cordierite late stage andalusite is found only at kuhi lal available u pb k ar and rb sr data gave 2130 ma as a minimum age for the amphibolite facies metamorphism our 40ar 39ar cooling ages for biotite phlogopite and hornblende from kuhi lal darai stazh and mulvoj indicate miocene cooling for the goran series biotites from mica schist gave good plateau ages of 8 9 0 3 10 2 0 3 and 18 3 0 2 ma and phlogopite associated with sapphirine forsterite and enstatite gave a 16 9 0 3 ma plateau hornblende from amphibolite gave a plateau age of 22 2 1 3 ma and an age of 17 ma from a saddle shaped release spectrum our 17 22 ma ages are comparable to previously reported 18 23 ma rb sr isochron ages on the pamir shugnan complex of gneissic granite exposed 15 km northeast of the northernmost locality kuhi lal and thus could be related to heating associated with this plutonism our preferred interpretation is that the ages represent regional cooling due to exhumation related to tectonic activity in the miocene

tectonics of western tibet between the tarim and the indus a tarim to indus traverse provides insight into the tectonics of western tibet the kunlun was the site of a mid paleozoic collision at least three phases of post paleozoic accretion have thickened the blanket of sediments that covers western tibet sizeable parts of western qiangtang have remained stable however since the mid mesozoic since the neogene deformation and volcanism have been localized near the edges of the plateau strike slip motion along the karakorum and altyn tagh faults has been coeval with overthrusting in the himalayas and kunlun such slip partitioning and the volcanism appear to result simply from northward subduction of india and southward subduction of the tarim as tibet is extruded eastwards by india s penetration into asia

indentation of the pamirs with respect to the northern margin of tibet constraints from the tarim basin sedimentary record the pamirs represent the indented westward continuation of the northern margin of the tibetan plateau dividing the tarim and tajik basins their evolution may be a key factor influencing aridification of the asian interior yet the tectonics of the pamir salient are poorly understood we present a provenance study of the aertashi section a paleogene to late neogene clastic succession deposited in the tarim basin to the north of the nw margin of tibet the west kunlun and to the east of the pamirs our detrital zircon u pb ages coupled with zircon fission track bulk rock sm nd and petrography data document changes in contributing source terranes during the oligocene to miocene which can be correlated to regional tectonics we propose a model for the evolution of the pamir and west kunlun wkl in which the wkl formed topography since at least 200 ma by 25 ma movement along the pamir bounding faults such as the kashgar yecheng transfer system had commenced marking the onset of pamir indentation into the tarim tajik basin this is coincident with basinward expansion of the northern wkl margin which changed the palaeodrainage pattern within the kunlun progressively cutting off the more southerly wkl sources from the tarim basin an abrupt change in the provenance and facies of sediments at aertashi has a maximum age of 14 ma this change records when the pamir indenter had propagated sufficiently far north that the north pamir was now located proximal to the aertashi region

neogene extension and volcanism in the kunlun fault zone northern tibet new constraints on the age of the kunlun fault 1 the geology of the jingyu basin north tibet provides constraints on the relationship between localized e w extension in the qiangtang block and strike slip motion along the kunlun fault eocene to oligocene sw ne compression formed topographic relief in the present western kunlun shan pull apart basins started forming in the western kunlun ranges around 15 ma possibly coincident with the initiation of subduction of the tarim qaidam lithosphere under the kunlun shan extensional tectonism is still active and the jingyu basin is extending as a normal fault connected with the kunlun fault was recently active localized extension allows tertiary and quaternary shoshonitic magmas to reach the surface from crustal depths of 50 to 60 km miocene strike slip faulting indicates that the initiation of the kunlun strike slip fault is early neogene in age and that the total amount of horizontal movement that occurred along the fault has to be reconsidered

cenozoic evolution of the eastern pamir implications for strain accommodation mechanisms at the western end of the himalayan tibetan orogen detailed field mapping geochronologic and thermochronologic analyses and petrologic investigations conducted along the southern segment of the late cenozoic kongur shan extensional system provide new information on the cenozoic tectonic evolution of the eastern pamir at the western end of the himalayan tibetan orogen field relations and cooling age patterns in the hanging wall and footwall of the active faults show a southward decrease in the magnitude of east west extension along the southern kongur shan extensional system from 20 km or less along the muztaghata massif in the north to l3 km along the tashkorgan fault in the south these results in conjunction with previously published work on the northern segment of the kongur shan extensional system show a general southward decrease in east west extension along the entire length of the extensional system consistent with models of extension primarily driven by oroclinal bending or radial thrusting of the pamir petrologic data 40 ar 39 ar cooling ages and monazite th pb ages from schists and gneisses in the footwall of the southern kongur shan normal fault along the muztaghata massif record two tectonic events that immediately preceded late miocene initiation of east west extension 1 high grade schists and gneisses experienced upper amphibolite facies metamorphic conditions 9 10 kbar 700 750 c dated as late oligocene to middle miocene by in situ ion microprobe analyses of monazite inclusions in garnet and 2 high grade schists and gneisses were subsequently rapidly exhumed to shallow crustal levels in the late miocene with 40 ar 39 ar biotite cooling ages of 7 5 9 ma rapid exhumation was accommodated in part by the east west striking south dipping shenti normal fault field relations and regional geologic correlations indicate that this exhumation event was related to the formation of the central pamir gneiss domes and the antiformal muztaghata massif is the eastward continuation of the sares dome of the central pamir these observations suggest that the antiformal gneiss domes of the central pamir have not been offset across the karakorum right slip fault from the qiangtang anticlinorium in tibet instead we propose that the development of the central pamir gneiss domes may have been related to oligocene miocene northward underthrusting and thickening of crust beneath the pamir

xainxa ultramafic rocks central tibet china tectonic environment and geodynamic significance the xainxa ultramafic rocks 200 km south of the donqiao ophiolite bangong nu jiang suture zone central tibet form a 1 km thick sheet in tectonic contact with paleozoic sediments to the south and with mesozoic sediments to the north through roughly east west south dipping reverse faults the ultramafic rocks consist of depleted harzburgites with minor dunite patches and 1 m sized deposits of podiform chromite mafic rocks represented by a few isotropic gabbros and dolerites occur in only minor amounts these rocks which may have underlined a suture zone within the lhasa block have lithologies and tectonics very similar to those occurring in the donqiao area along the bangong nu jiang suture zone and therefore can be considered as a klippe of this ophiolite the xainxa ultramafic rocks were emplaced in early cretaceous time the main tectonic event observed in that area northward folding and overthrusting occurred after late early cretaceous time

arcuate pamir in the paleogene insights from a review of stratigraphy and sedimentology of the basin fills in the foreland of ne chinese pamir western tarim basin abstract the tectonic evolution of the pamir bears essential information for understanding intracontinental tectonic processes within the context of india asia convergence however it is unclear whether the arcuate shape of the pamir is entirely the result of the india asia collision or may have been in part an inherited pre cenozoic feature in this study we address the evolution of the ne chinese pamir during the paleogene by reviewing the stratigraphy of twelve sections in the western tarim basin and the establishment of sedimentary facies the results indicate three episodes of deposition of alluvial coarse sediments in the foreland regions of ne chinese pamir during the 1 early paleocene 2 late paleocene to early eocene and 3 middle eocene to oligocene respectively these are correspondingly attributed to 1 retroarc deformation in an andean type margin of the southern asia during subduction of the neo tethys ocean 2 the immediate response in the pamir to the india asia collision and 3 continued india asia convergence the alluvial conglomerates with grain size up to 5 10 cm imply a transport distance

old origin for an active mountain range geology and geochronology of the eastern hindu kush pakistan prior to accretion of the kohistan island arc during the late cretaceous and final suturing of india with asia at ca 50 ma the hindu kush mountains along the border of afghanistan and northwest pakistan were situated on the active southern margin of asia geology and geochronology of the eastern hindu kush range in pakistan demonstrate that localized crustal melting and leucogranite intrusion took place in the gharam chasma area at ca 24 ma more regionally developed and widespread deformation metamorphism and magmatism took place before collision between both india and the kohistan island arc with asia ca 195 ma u pb monazite ages on a deformed leucogranite dike from the upper lutkho valley indicate an early jurassic phase of crustal melting u pb monazite ages of 135 126 ma on a staurolite schist from near gharam chasma are interpreted as a minimum age for staurolite grade metamorphism within the tirich mir fault zone pegmatite dikes crosscut the staurolite schists u pb dating of uraninites from one of these pegmatite dikes reveals an age of 114 2 ma monazites from the same rock give ages of 125 121 ma possibly due to inheritance of older cores these jurassic early cretaceous constraints on metamorphism and magmatism relate to subduction and accretion processes perhaps including the suturing of the karakoram and hindu kush terranes along the tirich mir fault in general high temperature low pressure metamorphism and subduction related granitoid magmatism in the eastern hindu kush suggest a high thermal gradient in an active margin setting from early jurassic to cretaceous time

episodic exhumation and related tectonic controlling during mesozoic in the eastern tian shan xinjiang northwestern china abstract the eastern tian shan ets which is located in the southern margin of the central asian orogenic belt experienced multiple cooling episodes during the mesozoic however the topography and cooling pattern of the region have been extensively affected by complex structural deformations here we combined a detailed structural analysis with new and existing low temperature thermochronological data from the eastern ets moqingwula range to constrain the process of deformation and exhumation during the mesozoic our results indicate that two phases deformation occurred in the late jurassic and the late cretaceous to the paleocene that transformed the landscape of the moqingwula range the first phase tilted the northern moqingwula range coinciding with folding and thrusting the second phase of deformation disrupted planation surfaces which dominantly formed in the early cretaceous and were preserved on the summit of the ranges our new data reveals three prominent cooling episodes during the 222 ma 138 113 ma and 68 57 ma as well as another subdued cooling phase during 113 92 ma in addition through integration of the regional cooling history and topographic evolution we discuss the geodynamics of the deformations and exhumations associated with tectonism within the central asian orogenic belt

tectonic environment and geodynamic significance of the neo cimmerian donqiao ophiolite bangong nujiang suture zone tibet the bangong nujiang suture zone is underlain by a discontinuous and roughly ew ophiolite belt around donqiao gyanco ophiolitic and metasedimentary palaeozoic series form nappes thrusted over jurassic flysch series all these series have undergone a late symmetrical folding accompanied by generally northward reverse faulting although strongly dismembered a complete ophiolitic sequence comprising harzburgites with large podiform type chromite deposits wehrlites cumulate gabbros sheeted dolerite dykes foliated garnet bearing amphibolites and pillow lavas has been found

timing of seawater retreat from proto paratethys sedimentary provenance and tectonic rotations in the late eocene early oligocene in the tajik basin central asia abstract the tajik basin is now located to the west of the pamirs but it formerly connected with the tarim basin occupied by a large epicontinental sea proto paratethys sea once extended from europe to central asia however the northward indentation and collision of the pamirs into the tian shan separated the tajik and tarim basins creating manifold implications and consequences consequently timing of the final seawater retreat from the tajik basin is important for understanding the evolution of the proto paratethys sea the effect of the northward indentation of the pamirs on the sea regression and the influence of seawater retreat on the cenozoic aridification in the tajik basin here we present a multidisciplinary study of upper paleogene strata from the central tajik basin our results demonstrate that the final seawater retreat from the center of the tajik basin occurred at 38 6 ma which is earlier than a previous age of 37 4 ma from the northeastern tajik basin such a late eocene sea retreat was quasi simultaneous with that in the tarim basin this final marine regression was mainly a consequence of the northward indentation and growth of the pamirs which was an intracontinental response to the india asia collision our new detrital zircon ages indicate that the provenance of upper eocene lower oligocene strata was from the pamirs rather than tian shan implying that the reactivation of the tian shan was later than the early oligocene our paleomagnetic data indicate a 30 counterclockwise rotation of the central tajik basin that is much bigger than the previous estimate of 17 and we propose that such a counterclockwise rotation mostly occurred after the late miocene finally different from the previous reported aridfication at 39 ma or 37 ma our evidence demonstrates that drought intensified since the start of the oligocene at 34 ma being related to reduced water vapor transport to central asia caused by global cooling induced sea level drop the retreating paratethys sea and weakened sea surface evaporation

u pb dating on single detrital zircon grains from the triassic songpan ganze flysch central china provenance and tectonic correlations the songpan ganze flysch belt central china covers a huge triangular area of more than 200 000 km and is bounded by the continental blocks of south china north china and the tibetan plateau detrital zircons extracted from three flysch samples collected in the central part of the belt were analyzed grain by grain using the u pb method two samples of middle triassic sandstones collected at different locations in the belt provide identical results which suggests similar source regions the detrital zircons yield a wide range of ages and indicate their principal derivation from mid proterozoic 1 8 2 0 ga source rocks with minor contribution from late archean ca 2 5 2 6 ga material the discordance and pb loss patterns from low u zircons indicate disturbances during a subsequent event which may be of caledonian age as suggested by concordant zircon grains at ca 420 and 450 ma one sample collected within the palang shan pass zone provides concordant zircon grains at around 230 ma 231 1 ma and 233 1 ma these triassic ages are synchronous to flysch deposition and suggest intense geological activity calc alkaline volcanism at that time in the area close to the basin the data support an origin of the clastic material mainly from a northeastern landmass corresponding to the southern margin of the sino korean craton to a lesser degree inputs from the yangtze craton and possibly from the northern margin of the basin kunlun arc are also detected the age spectrum from the upper triassic sandstone is significantly different and shows predominance of sinian ca 760 ma grains probably derived from the yangtze craton this change in the source region is interpreted as reflecting the tectonic evolution of this area and in particular as being linked to the late triassic collision between south china and north china in the middle triassic while subduction of the songpan sea northward beneath the north china plate was still taking place continental subduction of south china in the dabie region was responsible for uplift of the overriding plate i e the sino korean craton which supplied large volumes of sediments during the late triassic clockwise rotation of the south china block uplifted the indo sinian part of the qinling belt and closed the basin as the accretionary wedge was thickening along the southern margin of north china detritus derived from this source region were unable to reach the flysch basin the age spectrum presented by detrital zircons indicates

u th pb systematics and ages of himalayan leucogranites south tibet abstract the age and origin of five leucogranites from the high and tethys himalaya and two country rock gneisses were investigated by u pb dating of zircon fractions and single grains and fractions of monazite additionally th u concentrations in whole rock powders and isotopic compositions of pb in leached k feldspars were determined monazites yield ages of 16 8 0 6 m y for the nialam migmatite granite 15 1 0 5 m y for the lhagoi kangri granite 14 3 0 6 m y for a granite from mt everest and 9 8 0 7 m y and 9 2 0 9 m y for two varieties of the maja granite these data together with monazite ages of 21 9 0 2 and 24 0 0 4 m y determined earlier on the makalu granite 1 substantiate a period of intracontinental granite emplacements from 24 to 9 m y ago i e from uppermost oligocene to late miocene times such a period of plutonic activity is consistent with the view that all these granites result from intracrustal melting following the collision of india with eurasia furthermore the individual ages together with structural relationships between granites and country rocks suggest that granite formation and tectono metamorphism occurred as alternating and strongly related processes with a periodicity of 7 to 9 m y inherited lead components present in all granite zircons point to large proportions of precambrian material in the magma source regions up to 2200 m y old th u systematics between monazite and country rocks indicate that u has been leached from most of the granites after crystallisation of monazite zircon dating of the kangmar granite gneiss which occurs in a window through the tethys himalayan sediments shows that this pluton transformed to a gneiss during the alpine orogeny crystallised in lowermost palaeozoic times 562 4 m y ago

the karakorum block in central asia from ordovician to cretaceous abstract the karakorum block records a predominantly marine ordovician to cretaceous sedimentary history six major sedimentary cycles are recognised the oldest sediments early ordovician transgress over a crystalline basement the 1 km thick ordovician silurian mostly shaly succession contains rare carbonate intercalations in the early devonian a wide peritidal platform spread over the craton sedimentation rates were low 10 20 m ma and upper devonian probable lower carboniferous carbonates and clastics rest unconformably on the early devonian carbonates no evidence has been found for either late carboniferous or glaciogenic deposits sedimentation rates increased during the permian up to 50 m ma with thicknesses between 1 and 2 5 km three steps are identified 1 a huge alluvial to marginal marine terrigenous prism aggraded during the asselian sakmarian 2 the artinskian murgabian is characterised by local emergence and erosion linked to extension with block rotations 3 greater differentiation occurred from late permian to middle triassic when a peritidal carbonate flat developed in the southwest facing a deeper basin to the northeast carbonate sediments prevailed throughout the carnian norian the permo triassic evolution is interpreted as the passive margin stage of the karakorum block which previously belonged to the perigondwanian fringe when like mega lhasa it drifted northward on the tethyan transit plate mega lhasa is considered as a collage of blocks possibly separated by thinned crust or short lived seas with ocean crust quartzo lithic sandstones with grains of mafic volcanics and serpentinite overlain with gentle unconformity by pliensbachian toarcian red sandstones with sedimentary and metasedimentary clasts record the eo cimmerian deformation in the karakorum this orogenic episode was over by the aalenian when a shallow water carbonate ramp aggraded onto the previously emergent area the pre barremian upper jurassic and lower cretaceous are poorly documented in post barremian times the sedimentary succession of n karakorum was severely deformed in huge thrust sheets including slabs of crystalline basement a coarse conglomerate of mid cretaceous age sealed the thrust sheet edifice finally isolated campanian pelagic mudstones are recorded anticlockwise rotation of mega lhasa followed the late triassic docking of the iranian spur using the latter as pivot the rotation was completed during the middle jurassic when se pamir shaksgam and karakorum joined the qiangtang more closely assembling mega lhasa as it approached the asian margin

a slab breakoff model for the neogene thermal evolution of south karakorum and south tibet on the south karakorum margin neogene high temperature medium pressure ht mp gneisses define an east west trending thermal anomaly these rocks have been heated from 600 to 750 c during a slight pressure drop from 0 7 to 0 5 gpa their retrogressive path cross cuts the relaxed geotherm of tectonically thickened crust such a p t evolution occurs only if an advective source of heat is involved involvement of an advective heat source is also implied by the occurrence of neogene granitoids and lamprophyres within the ht mp gneiss area these rocks are strongly enriched in large ion lithophile elements relative to primitive mantle and show negative high field strength element anomalies we interpret these geochemical characteristics to be the result of melting of metasomatized asian lithospheric mantle the nd and sr isotopic compositions of the south karakorum neogene magmatic rocks nd 12 to 7 and 87sr 86sr 0 705 0 725 further suggest they could have originated from mixing between asian variously metasomatized mantle and precambrian crust by contrast the origin of the youngest magmatic rocks 10 myr here exemplified by the hemasil syenite and associated lamprophyres requires involvement of a depleted mantle the combined hf nd signature of these rocks hf 10 4 11 5 and nd 3 4 4 3 suggests that the source of the hemasil syenite could have been depleted mantle contaminated by oceanic sediments likely during the earlier subduction of the tethyan ocean neogene magmatic rocks with the same geochemical characteristics and evolution as those of south karakorum have previously been described in south tibet based on their location and the geochemical evolution of their source region we here propose that the neogene magmatic and metamorphic evolution of the south asian margin was controlled by slab breakoff of the subducting indian continental margin starting at about 25 ma this model is supported by available geophysical data from south karakorum and south tibet

tectonic evolution of the early mesozoic blueschist bearing qiangtang metamorphic belt central tibet this is the published version copyright 2003 american geophysical union all rights reserved

mesozoic and cenozoic tectonic evolution of the shiquanhe area of western tibet 1 in the shiquanhe area of far western tibet mid cretaceous strata lie unconformable on ophiolitic melange and jurassic flysch associated with the bangong nujiang suture zone on the basis of our mapping and geochronologic studies we suggest that these cretaceous strata were shortened by 57 over a north south distance of 50 km during late cretaceous early tertiary time the late cretaceous narangjiapo thrust placed permian strata 20 km over ophiolitic melange and cretaceous strata north of the narangjiapo thrust 40 km of shortening was accommodated by the late cretaceous early tertiary south directed jaggang thrust system that involves jurassic flysch and cretaceous strata and roots into a decollement within ophiolitic melange the most recent shortening was accommodated to the south of the narangjiapo thrust along the north dipping shiquanhe thrust the shiquanhe thrust cuts flat lying 22 6 0 3 ma volcanic rocks and underlying folded tertiary nonmarine strata in its footwall and was likely active during slip along the oligocene gangdese thrust system of southern tibet ophiolitic melange and structurally overlying jurassic flysch near shiquanhe are interpreted to represent remnants of a subduction accretion complex and forearc basin respectively that were obducted southward onto the margin of the lhasa terrane during late jurassic early cretaceous closure of the bangong nujiang ocean subsequent imbrication of the obducted sheet could have produced the two east west trending belts of ophiolitic melanges separated by 100 km in western tibet late cretaceous early tertiary thin skinned shortening may have been accommodated in the deeper crust by northward underthrusting and duplexing of lhasa terrane rocks beneath the obducted ophiolitic melange and the qiangtang terrane to the north

reconstruction of the altyn tagh fault based on u pb geochronology role of back thrusts mantle sutures and heterogeneous crustal strength in forming the tibetan plateau 1 reconstructing deformation along the northwestern margin of the tibetan plateau is critical for evaluating the relative importance of microplate versus continuum models of the indo asian collision questions regarding this margin s evolution are as follows 1 what is the total offset along the sinistral altyn tagh strike slip system 2 how has that offset been absorbed in the western kunlun shan 3 why does the n s width of the plateau vary along strike ion microprobe u pb zircon apparent ages of 17 plutons from nw tibet together with regional geologic observations define a discrete e w trending boundary between two tectonic belts that has been offset along the altyn tagh system by 475 70 km kinematic arguments indicate that this offset cannot be the result of north directed thrusting in the western kunlun shan therefore we propose that south directed faulting in the tianshuihai thrust belt both offset the tectonic boundary and produced the asymmetry in the plateau shortening appears to have been absorbed in the upper crust by thin skinned thrusting in the middle lower crust by east directed ductile flow and or subduction and in the mantle by north dipping subduction factors controlling the formation of the south directed thrust system appear to be the contrast between the rigid tarim and the weaker songpan ganzi flysch belt and a mantle suture inherited from late paleozoic subduction the evolution of western tibet leads to a view of continental deformation that integrates elements of the microplate model e g plate like mantle and crust mantle decoupling with aspects of the continuum model weak crustal flow beneath the plateau

thermochronological insights into the structural contact between the tian shan and pamirs tajikistan gilby jepson stijn glorie dmitry konopelko jack gillespie martin danisik noreen j evans yunus mamadjanov alan s collins

on the seasonal variability of the oyashio extension fronts previous study reported that the annual mean eastern oyashio extension oe front shifts northward while the western oe front has no obvious poleward shift during 1982 2017 by wu et al geophys res lett 45 9042 9048 2018 here we revisit this topic and focus on the seasonal variability and shifts of the oe fronts from 1982 to 2018 with observational reanalysis data and a 1 5 layer reduced gravity model simulation in winter both the western and eastern oe fronts demonstrate consistent northward movement while in summer the eastern oe front still moves northward but the western oe front has no obvious and even southward shift it is shown that the trade wind s expansion during 1982 2018 favours the northward shift of the oe fronts for both winter and summer however there is a local cold ekman heat transport anomaly along the western oe front in summer which surpasses the effect of trade wind expansion and prohibits northward movement of the front this cold ekman advection is due to a westerly wind anomaly induced firstly by the atlantic multi decadal oscillation amo and secondly by the pacific decadal oscillation pdo in winter the local ekman heat transport is less effective than in summer in changing the oe front position because of the deep mixed layer our study demonstrates the seasonality of the oe front shift and highlights the importance of local ekman heat transport associated with the amo our results also partly explain the rainfall changes in both winter and summer in the western pacific ocean in the past 37 years since the rainband east of japan is affected by the sea surface temperature and its front

triassic continental subduction in central tibet and mediterranean style closure of the paleo tethys ocean the qiangtang metamorphic belt qmb in central tibet is one of the largest and most recently documented high pressure hp to near ultrahigh pressure near uhp belts on earth lu hf ages of eclogite and blueschist facies rocks within the qmb are 244 223 ma indistinguishable from the age of uhp metamorphism in the qinling dabie orogen results of a u pb detrital zircon study suggest that protoliths of the qmb include upper paleozoic qiangtang continental margin strata and sandstones that were derived from a paleozoic arc terrane that developed within the paleo tethys ocean to the north we attribute qmb hp metamorphism to continental collision between the qiangtang terrane and a paleo tethys arc terrane this collision and the coeval south china north china collision may have slowed convergence between laurasia and gondwana derived terranes and initiated mediterranean style rollback and backarc basin development within much of the remnant paleo tethys ocean realm

zircon ages and nd isotopic and chemical compositions of orthogneisses from the black forest germany evidence for a cambrian magmatic arc single zircon u pb dating combined with 207pb 206pb ages obtained by the evaporation method constrains the emplacement of tonalitic trondhjemitic and granodioritic orthogneisses of the moldanubian zone in the black forest between 500 and 510 ma two detrital zircon populations of 1 9 and 1 6 ga indicate early middle proterozoic material in the former setting of the basement the initial end values range from 0 1 to 3 4 and mean crustal residence ages of 1 0 1 4 ga are consistent with involvement of early middle proterozoic crust and a subordinate juvenile component probably originating from subduction related melting of the mantle the orthogneisses have fractionated ree patterns and slightly higher k2o na2o ratios than typical low k tonalite trondhjemite granite suites the chemical data are interpreted as evidence for melting of amphibolite and contributions from evolved crust the emplacement of the orthogneisses was superceded by a high temperature metamorphic event at 480 ma which we interpret as a result of lithospheric thinning in a marginal basin behind a cambrian magmatic arc

controls on the apex location of large deltas the majority of sediment transport to the world9s oceans is routed via large deltas we examine controls on delta apex location using a database of 84 of the world9s largest deltas of the dataset 94 of apices are controlled by either bedrock valleys 80 or pleistocene alluvial valleys 14 suggesting that the principal control on modern apex development is valley exit and or bedslope mediated avulsion and not hydrodynamic backwater length valley exit control on large delta apex location may have been as important in the rock record as it is today and should be considered as a key control on delta development supplementary material tabulated data on backwater length and apex type for studied deltas available at https doi org 10 6084 m9 figshare c 3469770

blueschist bearing metamorphic core complexes in the qiangtang block reveal deep crustal structure of northern tibet a 500 km long belt of metamorphic exposures in the qiangtang block provides an opportunity to study the internal structure of northern tibetan crust metamorphic rocks exposed at two widely separated areas along this belt consist of blueschist bearing melange and are bounded by late triassic early jurassic domal low angle normal faults we propose that this melange was underplated to the qiangtang block and was subsequently exhumed by detachment faulting both the underplating and the exhumation occurred during early mesozoic southward subduction of oceanic lithosphere along the jinsha suture this model predicts that the deeper crust of much of northern tibet consists of accretionary melange in contrast to the continental crystalline crust of southern tibet and may account for north south variations of cenozoic tectonism in tibet

the temporal evolution of the active margin along the southeast anatolian orogenic belt se turkey evidence from u pb ar ar and fission track chronology abstract the southeast anatolian orogenic belt saob resulted from the north dipping subduction of the southern neotethyan oceanic lithosphere in late mesozoic and early cenozoic however the timing and the rate of the continental collision are still under debate here we present new u pb ar ar and fission track ages from the i type calc alkaline granitoids esence dogansehir and baskil cutting the goksun ispendere and komurhan ophiolites and also the malatya keban metamorphics representing the tauride active continental margin along the saob in this study high to low temperature thermochronological methods applied to the granitoids are used to understand the temporal and geodynamic evolution of the tauride active continental margin and continental collision between tauride and arabian platforms in the frame of the neotethyan convergence during cretaceous and miocene the u pb zircon ages range from 83 to 88 ma for the baskil and from 81 to 83 ma for the esence granitoids the 39 ar 40 ar ages indicate that these late cretaceous granitoids cooled below 300 c in 6 10 ma the formation ages and the timing of the gradual cooling of the late cretaceous granitoids are similar to metamorphism age and the timing of the exhumation of the hp lt bitlis metamorphics during this stage the combined field geochemistry and geo thermochronological data suggest that the first continental collision event occurred between bitlis puturge micro continents to the south and tauride platform to the north in an oblique subduction zone between 84 and 74 ma the granitoids continued uplifting during early middle eocene together with the exhumation of hp uht berit metaophiolite in an extensional regime related to the opening of the maden back arc basin this period also let the intrusion of the dogansehir arc magmatism intruding the puturge and malatya keban metamorphics berit metaophiolite and maden complex the eocene dogansehir granitoid has similar u pb and 39 ar 40 ar ages suggesting a fast cooling the apatite fission track aft ages for all granitoid bodies suggest that they were mainly cooled or uplifted in two episodes where the first one is in the eocene in an extensional setting the aft data marked the final continental collision between the taurides and the arabian platform first in oligocene and the break off of the subducted slab and the delamination process caused fast uplift of the eastern anatolia during middle to late miocene

a view into crustal evolution at mantle depths abstract crustal foundering is an important mechanism in the differentiation and recycling of continental crust nevertheless little is known about the dynamics of the lower crust the temporal scale of foundering and its role in the dynamics of active margins and orogens this particularly applies to active settings where the lower crust is typically still buried and direct access is not possible crustal xenoliths derived from mantle depth in the pamir provide a unique exception to this the rocks are well preserved and comprise a diverse set of lithologies many of which re equilibrated at high pressure conditions before being erupted in their ultrapotassic host lavas in this study we explore the petrological and chronological record of eclogite and felsic granulite xenoliths we utilized accessory minerals zircon monazite and rutile for coupled in situ trace element analysis and u th pb chronology by laser ablation split stream inductively coupled plasma mass spectrometry each integrated analysis was done on single mineral zones and was performed in situ in thin section to maintain textural context and the ability to interpret the data in this framework rutile thermo chronology exclusively reflects eruption 11 17 0 06 ma which demonstrates the reliability of the u pb rutile thermo chronometer and its ability to date magmatic processes conversely zircon and monazite reveal a series of discrete age clusters between 55 11 ma with the youngest being identical to the age of eruption matching age populations between samples despite a lack of overlapping ages for different chronometers within samples exhibit the effectiveness of our multi mineral approach the ree systematics and age data for zircon and monazite and ti in zircon data together track the history of the rocks at a million year resolution the data reveal that the rocks resided at 30 40 km depth along a stable continental geotherm at 720 750 c until 24 20 ma and were subsequently melted densified and buried to 80 90 km depth 20 km deeper than the present day moho at 930 35 c the material descended rapidly accelerating from 0 9 1 7 mm yr 1 to 4 7 5 8 mm yr 1 within 10 12 myr and continued descending after reaching mantle depth at 14 13 ma the data reflect the foundering of differentiated deep crustal fragments 2 9 3 5 g cm 3 into a metasomatized and less dense mantle wedge through our new approach in constraining the burial history of rocks we provided the first time resolved record of this crustal recycling process foundering introduced vestiges of old evolved crust into the mantle wedge over a relatively short period c 10 myr the recycling process could explain the variability in the degree of crustal contamination of mantle derived magmatic rocks in the pamir and neighboring tibet during the cenozoic without requiring a change in plate dynamics or source region

tectonics of the qinling central china tectonostratigraphy geochronology and deformation history abstract the qinling orogen preserves a record of late mid proterozoic to cenozoic tectonism in central china high pressure metamorphism and ophiolite emplacement songshugou ophiolite assembled the yangtze craton including the lower qinling unit into rodinia during the 1 0 ga grenvillian orogeny the lower qinling unit then rifted from the yangtze craton at 0 7 ga subsequent intra oceanic arc formation at 470 490 ma was followed by accretion of the lower qinling unit first to the intra oceanic arc and then to the sino korea craton subduction then imprinted a 400 ma andean type magmatic arc onto all units north of the northern liuling unit oblique subduction created silurian devonian wnw trending sinistral transpressive wrench zones e g lo nan shang dan and late permian early triassic subduction reactivated them in dextral transpression lo nan shang xiang shang dan and subducted the northern edge of the yangtze craton exhumation of the cratonal edge formed the wudang metamorphic core complex during dominantly pure shear crustal extension at 230 235 ma post collisional south directed shortening continued through the early jurassic cretaceous reactivation of the qinling orogen started with nw se sinistral transtension coeval with large scale early cretaceous crustal extension and sinistral transtension in the northern dabie shan it presumably resulted from the combined effects of the siberia mongolia sino korean and lhasa west burma qiangtang indochina collisions and pacific subduction regional dextral wrenching was active within a ne sw extensional regime between 60 and 100 ma an early cretaceous andean type continental magmatic arc with widespread early cretaceous magmatism and back arc extension was overprinted by shortening related to the collision of yangtze indochina block with the west philippines block strike slip and normal faults associated with eocene half graben basins record paleogene nne ssw contraction and wnw ese extension the neogene is characterized by normal faults and nne trending sub horizontal extension pleistocene quaternary nw se extension and ne sw contraction comprises sinistral strike slip faults and is part of the nw se extension imposed across eastern asia by the india asia collision

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

did the indo asian collision alone create the tibetan plateau it is widely believed that the tibetan plateau is a late cenozoic feature produced by the indo asian collision however because tibet was the locus of continental accretion and subduction throughout the mesozoic crustal thickening during that time may also have contributed to growth of the plateau this portion of the geologic history was investigated in a traverse through the central lhasa block southern tibet together with earlier studies our mapping and geochronological results show that the lhasa block underwent little north south shortening during the cenozoic rather our mapping shows that 60 crustal shortening perhaps due to the collision between the lhasa and qiangtang blocks occurred during the early cretaceous this observation implies that a significant portion of southern tibet was raised to perhaps 3 4 km elevation prior to the indo asian collision

early paleozoic to devonian multiple accretionary model for the qilian shan nw china abstract the qilian shan located in the northern tibetan plateau of nw china has an excellent record of early to middle paleozoic subduction accretion that resulted from convergence between the alax and qaidam blocks but there is no consensus about its detailed tectonic history this paper summarizes its tectonic divisions and discusses its tectonic evolution from the cambrian to devonian the belt has the following tectonic divisions in the far north the southern passive margin of the alax block is juxtaposed against an early cambrian to ordovician marianan type intra oceanic arc north qilian the north qilian arc is separated from a japanese type arc central qilian to the south by a high pressure metamorphic belt composed mostly of ophiolitic slices and oceanic crusts the central qilian is bounded to the south by a wide melange zone south qilian consisting of ophiolitic slices and continental margin sequences farther to the south lies the oulongbuluk microcontinent that is separated from the qaidam block and farther south by an ultrahigh pressure metamorphic belt tectonostratigraphic analysis together with geochemical geochronological and geophysical data indicates a complex evolution by subduction accretion processes from the cambrian to the devonian before final amalgamation and docking to the northern alax block this model solves the long lasting discussion on the polarity of subduction in paleozoic time this multiple subduction accretion history sheds light on the continuity of paleozoic sutures along the qilian shan and the nature of the altyn tagh fault and thus contributes to an improved understanding of the tectonic architecture of central asia

restoration of cenozoic deformation in asia and the size of greater india a long standing problem in the geological evolution of the india asia collision zone is how and where convergence between india and asia was accommodated since collision proposed collision ages vary from 65 to 35 ma although most data sets are consistent with collision being underway by 50 ma plate reconstructions show that since 50 ma 2400 3200 km west to east of india asia convergence occurred much more than the 450 900 km of documented himalayan shortening current models therefore suggest that most post 50 ma convergence was accommodated north of the indus yarlung suture zone we review kinematic data and construct an updated restoration of cenozoic asian deformation to test this assumption we show that geologic studies have documented 600 750 km of n s cenozoic shortening across and north of the tibetan plateau the pamir hindu kush region accommodated 1050 km of n s convergence geological evidence from tibet is inconsistent with models that propose 750 1250 km of eastward extrusion of indochina approximately 250 km of indochinese extrusion from 30 to 20 ma of indochina suggested by se asia reconstructions can be reconciled by dextral transpression in eastern tibet we use our reconstruction to calculate the required size of greater india as a function of collision age even with a 35 ma collision age the size of greater india is 2 3 times larger than himalayan shortening for a 50 ma collision the size of greater india from west to east is 1350 2600 km consistent with robust paleomagnetic data from upper cretaceous paleocene tethyan himalayan strata these estimates for the size of greater india far exceed documented shortening in the himalaya we conclude that most of greater india was consumed by subduction or underthrusting without leaving a geological record that has been recognized at the surface

carboniferous triassic subduction and accretion in the western kunlun china implications for the collisional and accretionary tectonics of the northern tibetan plateau a newly defined 250 km by 500 km carboniferous triassic subduction accretion complex the mazar accretionary prism in the western kunlun comprises two subduction complexes and a forearc basin succession 1 the bazar dara subduction complex contains imbricated blocks of sandstone arenite limestone and metavolcanic rocks in a matrix of weakly metamorphosed triassic deep sea turbidites the metavolcanic rocks include basalt diabase spilite and andesitic porphyry trace element geochemistry shows that pillow and amygdaloidal basalts are oceanic island tholeiites this zone has ordovician to permian fossils and is situated on the older more highly deformed and metamorphosed side of the prism adjacent to the sailiyak magmatic arc 2 the heweitan subduction complex is composed of blocks of limestone turbidite and radiolarite in a slate phyllite matrix intercalated with calc alkalic volcanic rocks this complex has permian to triassic fossils and is situated on the younger less deformed and metamorphosed side of the prism adjacent to the suture zone 3 the qitai forearc basins are infilled with turbidites late triassic intercalated with carbonates these rocks overlie the accretionary prism the accretion related structure is dominated by large scale northeast dipping thrusts and subvertical cleavage stitched by 215 190 ma granites there is an overall decrease in metamorphic grade and deformation intensity from the arc to the suture zone across the bazar dara and heweitan subduction complexes the mazar accretionary prism formed by subduction accretion processes during closure of the paleotethyan ocean and the final docking of the gondwanan karakoram qiangtang block to the cathaysian eurasian kunlun block

cenozoic volcanism in tibet evidence for a transition from oceanic to continental subduction geochronological k ar or ar ar major and trace element sr nd pb isotopic and mineral chemical data are presented for newly discovered cenozoic volcanic rocks in the western qiangtang and central lhasa terranes of tibet alkali basalts of 65 45ma occur in the western qiangtang terrane and represent primitive mantle melts as indicated by high mgnumbers 100 mg mg fe 54 65 cr 204 839 ppm and ni 94 218 ppm contents and relatively low ratios of sr sr 0 7046 0 7061 pb pb 18 21 18 89 pb pb 15 49 15 61 and pb pb 38 42 38 89 and high ratios of nd nd 0 5124 0 5127 in contrast younger volcanic rocks in the western qiangtang terrane 30ma and the central lhasa terrane 23 13 and 8ma are potassic to ultrapotassic and interpreted to have been derived from an enriched mantle source they are characterized by very high contents of incompatible trace elements negative ta nb and ti anomalies and radiogenic pb isotopic compositions pb pb 18 43 19 10 pb pb 15 64 15 83 pb pb 39 14 39 67 sr sr 0 7088 0 7092 and nd nd 0 5122 ratios of the western qiangtang terrane potassic lavas are similar to those of 45 29ma potassic volcanic rocks in the north central qiangtang terrane whereas sr sr 0 7167 0 7243 and nd nd 0 5119 ratios of central lhasa terrane lavas are similar to those of 25 16ma ultrapotassic volcanic rocks in the western lhasa terrane the 65 45ma alkali basalts in the western qiangtang terrane along with widespread calc alkaline volcanic rocks of this age in the lhasa terrane may be related to roll back of a previously shallow north dipping slab of tethyan oceanic lithosphere beneath tibet subduction as opposed to convective thinning of continental lithosphere is favored to explain potassic volcanism in tibet because of its occurrence in distinct east west trending belts 45 29ma in the qiangtang terrane 25 17ma in the northern lhasa terrane 16 8ma in the southern lhasa terrane and temporal and spatial relationships with major thrust systems

whole grain evaporation for 207 pb 206 pb age investigations on single zircons using a double filament thermal ion source a technique has been developed and tested to analyse 207pb 206pb apparent ages by thermal evaporation of radiogenic lead directly from untreated whole zircon grains 0 3 mm the evaporation analyses are performed in the double filament arrangement of a thermal ion mass spectrometer thims the method is a powerful tool to distinguish between different lead components occurring in the same grain because differing activation energies of the competing lead components cause their sequential evaporation from the zircons the evaporation of test samples results in 207pb 206pb apparent ages in good agreement with u pb ages known from literature single zircons from a granite of the marble mountains california yield an age of crystallization of 1 410 30 ma ceylon zircons from heavy mineral bearing gravels yield 560 40 ma as age of crystallization of the pegmatitic gravel sources individuals from a heterogeneous zircon population of a diatexite from the southern schwarzwald sw germany indicate metamorphic zircon formation around 500 ma and the existence of middle proterozoic relics 1 95 0 05 ga

volcanic rocks of the 1985 tibet geotraverse lhasa to golmud volcanic rocks encountered during the tibet geotraverse have been studied in the field in thin section and by major and trace element geochemistry in order to determine their most probable original eruptive environment rocks from a total of eleven distinct volcanic provinces were studied in this way they provide evidence for an active continental margin or post collision province of probable devonian early carboniferous age in the northern kunlun mountains an active continental margin of late carboniferous age in the southern lhasa terrane permian continental rifts in the central qiangtang and central kunlun terranes triassic volcanic arcs in the southern lhasa and northern qiangtang terranes a triassic active continental margin dyke swarm in the northern kunlun mountains a jurassic post collision or back arc rifting province in the southern qiangtang terrane a jurassic island arc in the northern lhasa terrane a cretaceous post collision province in the northern lhasa terrane possibly extending into the southern qiangtang terrane and a palaeogene active continental margin in the southern lhasa terrane an oligocene trachyte plug in the northern qiangtang terrane was the only evidence encountered during the geotraverse of volcanism post dating the palaeogene india eurasia collision however the composition of this plug coupled with new and published analyses from miocene volcanics in the southern lhasa terrane and from the pliocene recent volcanic province of northwest tibet places important constraints on models for post collision underplating of tibet by continental lithosphere any underplating is likely to have been a much later than the start of collision b directed beneath tibet from the north as well as the south and c limited in extent

detrital zircon geochronology of the northeastern tibetan plateau u pb geochronologic analyses have been conducted on 413 detrital zircon grains collected from 16 samples in the altun shan nan shan and qilian shan the samples come primarily from quartz arenites and metaturbidites of middle to late proterozoic age and from feldspathic and volcanic clast rich sandstones of early paleozoic age zircon grains in proterozoic strata resting on tarim basement yielded mainly 2 0 1 9 ga ages whereas proterozoic strata of the qaidam and qilian terranes yielded mainly ca 930 820 ma and ca 1 9 1 1 ga ages the younger grains were apparently shed from local igneous rocks whereas the grains older than 1 1 ga were shed from an undetermined continental source grains in the lower paleozoic strata are mainly ca 500 430 ma and were shed from nearby plutonic and possibly volcanic rocks that formed in a magmatic arc setting our detrital zircon ages are consistent with a model first proposed by e r sobel and n arnaud in which early paleozoic magmatism occurred within a single northeast facing magmatic arc that was constructed across an assemblage of middle to late proterozoic accretionary complexes remnants of magmatic arcs and shallow marine strata this arc system was accreted to the tarim and sino korean cratons during silurian devonian time the resulting suture has been reactivated as tertiary thrust faults that currently define the structural and topographic margin of the tibetan plateau our data also provide two new estimates for the offset along the eastern altyn tagh fault a belt of middle proterozoic shallow marine strata is offset by 400 km whereas a belt of 490 480 ma magmatic arc rocks is offset by 370 km these values are generally similar to the 350 400 km offset reported in most previous studies

the dynamothermal aureole of the donqiao ophiolite northern tibet metamorphic rocks found at the base of the jurassic donqiao ophiolite of northern tibet are interpreted as a basal dynamothermal aureole produced during obduction of the massif the rocks form a sequence some 8 m thick varying from high grade amphibolites at the contact with overlying harzburgites to greenschist facies metasedimentary rocks lower down the mineral paragenesis is similar to other such aureoles and indicates that temperatures in excess of 750 c may have been reached during metamorphism the lack of high pressure minerals suggests that the rocks were produced by subcretion in a relatively shallow dipping subduction zone ar ar geochronology on amphibole separates provides dates of 175 180 ma for the displacement of the ophiolite significantly older than the age of emplacement estimated from stratigraphie relationships the ophiolite was clearly obducted very soon after its formation in a suprasubduction zone environment

geological development and phanerozoic crustal accretion in the western segment of the southern tien shan kyrgyzstan uzbekistan and tajikistan abstract the tien shan form a high intracontinental mountain belt lying north of the main india asia collision mountains and consist of re activated paleozoic orogens the western segment of the southern tien shan lies northwest of the pamir and west of the talas fergana fault the stratigraphy lithology igneous and metamorphic petrology and geochemistry of this segment indicate that it was formed by the assembly of lower paleozoic arcs which developed into microcontinents with upper paleozoic mature shelf and slope clastic and carbonate sediments precambrian continental crust is confined to two small blocks along its southern margin the bulk of the southern tien shan consists of vendian to silurian oceanic and slope clastic rocks resting on oceanic lithosphere and overlain by thick passive margin devonian to mid carboniferous mature shelf clastics and carbonates these are unconformably overlain by syn and post orogenic immature clastic sediments derived from mountains on the north formed by closure of a carboniferus southern tajik and a northern vendian to carboniferous turkestan ocean with the southern tien shan microcontinent sandwiched between associated with these collisions are late carboniferous to permian intrusives which form three south to north though overlapping suites a southern calc alkaline granodiorite granite suite an intermediate gabbro monzodiorite granite suite and a northern alkaline monzodiorite granite alaskite suite the gabbro monzodiorite granite suite forms the earliest subduction related magmatism of the southern tien shan rare earth element patterns are consistent with derivation from a primitive or slightly enriched mantle the other suites show more crustal contamination rb and sr vary with depth and degree of partial melting and are consistent with progressive involvement of crustal material in partial melts during collision the gradual change in composition within each complex lasting in some cases from 295 to 250 ma the entire permian may be explained by a consecutive shift in the melting sedimentary cover of the subducting plate from oceanic crust through transitional crust to marginal continental crust like the central asian orogenic belt the main focus of igcp 420 the tien shan represent a net addition of continental crust during the phanerozoic very little of the belt has any precambrian precursor

north atlantic sea floor spreading rates implications for the tertiary development of inversion structures of the norwegian greenland sea the tertiary development of the norwegian continental margin was dominated by the opening of the arctic north atlantic ocean the correct identification of magnetic anomalies and their ages and the analysis of spreading rates during the formation of this ocean are important in understanding the development of the region and specifically the history of its passive margins three ocean domains the aegir reykjanes and mohns regions were investigated in an effort to understand the lateral changes in structural development of the passive margin after continental break up spreading rates generally slowed down from 2 cm a 1 after early eocene initiation of sea floor spreading to values around 0 5 cm a 1 in oligocene time an increase in spreading rates to around 1 cm a 1 coincided with the positioning of the iceland hotspot under the north atlantic mid ocean ridge at the same time the european plate changed its absolute plate motion from a north directed drift to a motion more towards the east the location of inversion structures in the voring and faeroes basin rather than in the more basin is related to differences in spreading rates the mohns and the reykjanes ridges produced more ocean floor than the aegir kolbeinsey ridges asymmetric ocean floor formation in the aegir ridge led to differential stress at the base of the lithosphere which probably explains the absence of inversion features in the more basin less mantle drag furthermore upper plate margins such as the voring basin and possibly the faeroe basin have a lower compressional strength than lower plate margins such as the more basin and therefore preferentially developed inversion structures along the transform boundaries separating the domains additional stress probably built up along extension of the transform zones into the extended continental crust this additional stress probably also assisted initiation of the inversion structures in the voring basin and the faeroes area the amplification of the inversion structures in the voring basin and the faeroes basin was subsequently caused by a variety of processes related to sedimentation and uplift erosion

rapid emplacement of the oman ophiolite thermal and geochronologic constraints in understanding of ophiolite emplacement requires knowledge of the elapsed time between igneous crystallization and intraoceanic thrusting and the rate and duration of that thrusting hornblende 4oar 39ar ages demonstrate that the igneous oceanic crust in oman crystallized and cooled to 825 k in 1 2 m y hornblende ages from metamorphic rocks and from cross cutting dikes require that the basal metamorphic thrust fault beneath the ophiolite also cooled below 825 k in 1 2 m y motion along the sole thrust accounted for 200 km of displacement at a rate of 100 200 mm yr on the basis of age relationships and thermal considerations we favor a two stage model for the initial stages of samall ophiolite emplacement intraoceanic thrusting over 2 m y old lithosphere at 150 km m y parallel to a spreading ridge for 1 2 m y followed by equally rapid and brief thrusting over cold and old lithosphere preservation of the samail ophiolite is a direct result of its young age and positive buoyancy at the time of ocean closure and we propose that all ophiolites that originated near spreading centers and were emplaced onto continents were young at the time of intraoceanic thrusting

seismic imaging of subducting continental lower crust beneath the pamir abstract exhumation of ultra high pressure metamorphic rocks testifies that the continental crust can subduct to significant depth into the mantle despite its buoyancy however direct observation of ongoing subduction of continental crust is rare the pamir is regarded as a possible place of active continental subduction because of the intermediate depth seismicity crustal xenoliths and estimates of crustal shortening versus convergence rates here we present for the first time receiver function images from a passive source seismic array traversing the tien shan and the pamir plateau showing southward subduction of eurasian continental crust in the eastern pamir we observe a southerly dipping 10 15 km thick low velocity zone lvz that extends from 50 km depth near the base of the crust to more than 150 km depth with a dip angle increasing to subvertical while the upper and mid crustal material seems to be shortened and incorporated into the pamir the lower eurasian crust detaches and subducts in its deeper part 80 km the lvz envelopes the intermediate depth earthquakes our observations imply that the complete arcuate intermediate depth seismic zone beneath the pamir traces a slab of subducting eurasian continental lower crust

middle cretaceous back arc formation and arc evolution along the asian margin the shyok suture zone in northern ladakh nw himalaya abstract the shyok suture zone of the ladakh palaeovolcanic arc is made of cretaceous volcanosedimentary formations intruded by granodioritic plutons field observations show a tectonic stacking of litho units from different volcanic arc and back arc environments in the western part skardu area the shyok suture zone can be subdivided into two groups 1 the northern group which consists of olistolith basaltic blocks and tuffs the basalts are lree depleted with a lile enrichment and a slight nb depletion suggesting a back arc basin origin 2 the southern group which consists predominantly of andesites that show lree enrichment a flat hree pattern strong nb ta depletion and lile enrichment the volcanic rocks of the southern group appear to have island arc tholeiite iat to calc alkaline affinities in the eastern part of the suture zone nubra shyok area albian to cenomanian age silicoclastic sediments predominate these sediments correspond to a large detrital platform built on the northern part of the ladakh arc at the top these sediments interlayer with basaltic to andesitic lavas these lavas appear to be more differentiated and calc alkaline in nature than the skardu southern group lavas but show similar volcanic arc affinities no evidence of a back arc basin was found in this area our data from these two areas shows a northwest southeast evolution from back arc to arc formations in northern ladakh opening of this back arc basin occurred on the northwestern side of the ladakh arc this back arc was progressively closing eastward the arc itself was more mature in the east resembling the southern tibetan continental arc and was characterised by more continental sedimentation subsequent himalayan tectonometamorphic evolution closure of the back arc basin and deformation along the shyok suture reflects an early asymmetrical geometry along the asian margin and kohistan ladakh arc