minimum age of initiation of collision between india and asia north of everest based on the subsidence history of the zhepure mountain section the collision of india with asia one of the most profound tectonic events to have occurred in past 100 m y is thought to have had geological geochemical and climatological consequences of global extent surprisingly the age of initiation of this collision remains poorly constrained estimates range from the late cretaceous 65 ma to latest eocene 40 ma with little consensus in between this paper reviews the stratigraphic section preserved on zhepure mountain on the north flank of everest mount qomolongma and its implied subsidence history zhepure mountain lies about 65 km south of the indus yarlung zangbo suture and contains the most complete and youngest passive margin shelf sediments in the tethyan himalayas on the basis of the subsidence history of the preserved section there is no evidence of acceleration of the subsidence up to the youngest rocks therefore collision related loading and accelerated subsidence must post date the youngest sediments preserved which date from the earl

onset timing of significant unroofing around qaidam basin northern tibet china constraints from 40ar 39ar and ft thermochronology on granitoids 40ar 39ar and fission track thermochronological results from two granitoid suites in the mountain ranges along the northern and southern edges of qaidam basin constrain the timing of a tertiary denudation event and their tectonic association in northern tibet the cooling histories based upon 40ar 39ar and fission track ages suggest a rapid cooling event 7 5 10 7 c ma during oligocene early miocene time this cooling event is thought to reflect an increasing denudation rate associated with the rapid uplift and denudation of the north qaidam and kunlun shan mountain ranges with consideration to the field evidence sedimentation rate of the qaidam basin and the propagation rate along the altyn tagh fault the rapid uplifting and denudation are probably due to crustal thickening associated with cenozoic propagation of the altyn tagh fault in accommodating the collision of india with asia

new insights into the timing of the india asia collision from the paleogene quxia and jialazi formations of the xigaze forearc basin south tibet abstract the xigaze forearc basin provides information on subduction evolution and magmatic growth of the gangdese arc as well as on the india asia continental collision recently obtained sedimentological biostratigraphic petrographic geochemical and geochronological data on cretaceous to paleogene strata in the cuojiangding area zhongba county south tibet shed new light on the tectonic evolution of the southern margin of the lhasa block during closure of neotethys and initial collision with india the uppermost cretaceous padana and qubeiya formations deposited in deltaic to inner shelf environments and representing the final filling of the xigaze forearc basin were unconformably overlain by the quxia and jialazi formations deposited in fan delta environments during the paleocene earliest eocene petrographic data and u pb ages of detrital zircons document the progressive unroofing of the gangdese arc which remained the dominant source of detritus throughout the late cretaceous to paleogene detrital cr spinels in the quxia and jialazi formations are geochemically similar to those in cretaceous xigaze forearc strata but different from those hosted in yarlung zangbo ophiolites suggesting that the latter were not exposed to erosion in the considered time window sandstone petrography cr spinel geochemistry u pb age spectra and hf isotopic ratios of detrital zircons in the quxia and jialazi formations match those in paleogene sediments deposited on the distal sangdanlin and zheya formations and proximal indian margin enba and zhaguo formations suggesting that the quxia and jialazi formations documents syncollisional fan deltas deposited on top of the nascent himalayan orogenic belt in this scenario the onset of the india asia collision predates deposition of the quxia and jialazi formations and is thus constrained as younger than 66 ma and older than 58 ma

paleogene clockwise tectonic rotation of the xining lanzhou region northeastern tibetan plateau 1 to help understand the deformational history of the northeastern tibetan plateau paleomagnetic samples were collected from 177 sites and two magnetostratigraphic sections at 16 localities distributed among upper jurassic lower cretaceous to pliocene sedimentary and subordinate volcanic rocks within the xining lanzhou region 34 37 n 101 105 e a total of 127 sites at 12 localities yielded primary magnetizations confirmed by fold reversal and conglomerate tests age control on sedimentary rocks is provided by regional synthesis of chronostratigraphic data and our own biostratigraphic and magnetostratigraphic analysis presented in the companion paper by horton et al 2004 analysis of paleomagnetic declination combined with results from previous studies yield a remarkably consistent trend of vertical axis tectonic rotations across the studied region whereas 19 0 7 2 to 37 8 10 6 clockwise rotations are recorded consistently in all paleomagnetic localities in lower cretaceous to eocene rocks all paleomagnetic localities in oligocene to pliocene rocks have recorded minor to insignificant rotations indicating that the xining lanzhou region has undergone a wholesale regional clockwise rotation during late paleogene time consistent with regional chronostratigraphic and thermochronologic results this late paleogene tectonic rotation confirms that deformation reached regions of the northern tibetan plateau shortly after the initial collision of india with asia when compared to rotational paleomagnetic results from adjacent regions several mechanisms can be proposed to explain the clockwise rotation on the basis of consistency with geologic data we prefer a model involving clockwise rotation of the xining lanzhou region through right lateral shear and associated shortening related to northward indentation of the qaidam basin index terms 1525 geomagnetism and paleomagnetism paleomagnetism applied to tectonics regional global 8102 tectonophysics continental contractional orogenic belts 9320 information related to geographic region asia 9604 information related to geologic time cenozoic keywords tectonics paleomagnetism tibetan plateau

an early unroofing in northeastern tibet constraints from 40ar 39ar thermochronology on granitoids from the eastern kunlun range qianghai nw china abstract in order to better constrain the mechanisms accommodating the india asia collision along the whole kunlun range northern tibet which is essential to better understand the evolution of the entire deformations area a 40 ar 39 ar thermochronological study has been carried out on a suite of granitoids collected in the vicinity of golmud eastern kunlun range biotite and k feldspar have been analysed and the cooling histories of the latter have been modelled the ranges of 40 ar 39 ar data bt 128 257 ma kf 102 228 ma are similar to previous rb sr and k ar data indicating both a middle paleozoic unit and a mesozoic unit including a triassic series and a jurassic cretaceous series this suggests that the two geological units previously reported in the western kunlun are also present in the eastern part of the range suggesting that eastern and western kunlun ranges might have been left laterally offset by as much as 500 km along a strike slip fault presently parallel to the altyn tagh fault the cooling histories based upon k feldspar modelling show that samples from the southern part of the range recorded an important cooling event 9 15 myr around 30 ma which is thought to reflect an increasing rate of denudation related to the tectonic activity in this area minimum denudation rates of 0 2 0 3 km ma have been estimated in contrast samples from the northern part of the range did not record any variation in their cooling history this heterogeneous denudation across the range is thought to be related to ramp stacking and normal faulting with associated uplift mainly affecting the southern part of the range suggesting that the eastern kunlun underwent an important crustal thickening period 30 ma ago this crustal thickening event is suggested to be related to the motion along the great asian strike slip faults in a way which remains problematic but possibly associated to the cenozoic propagation of the altyn tagh fault finally unroofing in the eastern kunlun range appears to predate that in the western kunlun range where it is suggested to have started around 20 ma ago this diachronous denudation could be related to a higher denudation rate in western kunlun range and it is thus highly probable that the important unroofing event outlined around 20 ma ago in the western kunlun has also affected the eastern part of the range even though the signature is not available at the moment

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

mesozoic and cenozoic tectonics of the northern edge of the tibetan plateau fission track constraints abstract fission track analysis on zircons and apatites yields new information about the timing of deformation of the northern tibetan plateau ages on zircons ranging from 221 22 to 96 4 ma are indicative of a general late triassic early jurassic cooling probably driven by the collision between the qiantang and kunlun blocks mid jurassic slow cooling is recorded also in the apatites in regions not affected by later cenozoic deformation this jurassic denudation was followed by a period of sedimentation during the cretaceous except along the altyn tagh fault atf zone and in some restricted areas of the western and eastern qilian shan this long and relatively quiet period ended at about 40 10 ma along the major altyn tagh and kunlun strike slip fault zones which were activated by the india asia collision this first movement along lithospheric faults resulted in the eastward extrusion of the tibet plateau which was followed in late oligocene miocene times by a major compression event initiating the formation of the high relief of north tibet a final compressional event took place at 9 5 ma and is well correlated with high sedimentation rates in the basins of this region this compression induced continental subduction in the kunlun ranges the altun shan belt and possibly the qilian shan belt

constraints on the configuration of andaman nicobar subduction zone from em modeling abstract the electrical conductivity structures using long period magneto telluric data at three sites nbg nabagram hav havelock in andaman and cby campbell bay in nicobar region are investigated after obtaining 1d responses 2d forward models of electrical structure are constructed along profiles across north andaman middle andaman and nicobar islands the initial configuration along each profile is constrained from models based on seismic reflection data the responses obtained after iterative forward computations match the observed data at respective sites reasonably well new structures within the forearc high are discerned from north to south we delineate a moderately conductive body beneath north andaman which is associated with the presence of ophiolites around this location in the middle andaman a highly conductive body below the island is interpreted as presence of fluids along the active diligent fault facilitated by the barrier of the invisible bank a complex geometry of resistive material embedded in a highly conductive body inferred beneath at great nicobar at cby is commensurate with the presence of a layered magmatic body possibly linked to the volcanic seamount to the east near the tear of the subducting plate

brittle modification of triassic architecture in eastern tibet implications for the construction of the cenozoic plateau abstract in the songpan ganzi fold belt east tibetan plateau post triassic deformation is localised within northwest trending sinistral and southwest trending dextral brittle fault zones between these zones large tracts of the fold belt preserving late triassic indosinian architecture have been transported effectively intact this transport was almost exclusively strike slip and did not accommodate significant differential exhumation or uplift right dihedra paleostress reconstructions from fault plane kinematic data indicate a rotation of the principal compressive stress from ne sw trending in the north to a dominant e w trend throughout the eastern songpan ganzi fold belt exceptions occur adjacent to competent proterozoic basement complexes and other rigid bodies these stress solutions are consistent with a compressive stress regime that radiates about the eastern himalayan syntaxis a secondary n s compressive paleostress identified in southern regions may record late southwards transportation of the songpan ganzi fold belt relative to the eastern himalayan syntaxis the lack of pervasive cenozoic recent upper crustal shortening in this region suggests that thickening of the east tibetan crust was predominantly accommodated by viscous deformation at depth we envisage decoupling of the lower and upper crust and partitioning of an e w compressive stress between 1 a laterally mobile thin skinned veneer and 2 a homogeneously thickened tectonic basement

evidence for mesozoic shear along the western kunlun and altyn tagh fault northern tibet china the strike slip faults of north tibet accommodate part of the cenozoic convergence between india and asia along the karakax valley south of yecheng and near the xidatan trough south of golmud the active traces of the altyn tagh and kunlun faults follow narrow belts of metamorphic rocks the deformation recorded in those mylonites is sinistral strike slip rb sr and 40ar 39ar ages of deformation from syntectonic fabrics formed at 350 400 c 120 ma argon loss suggests that deformation was associated to a 250 300 c thermal pulse that lasted 5 20 ma after the onset of movement unroofing occurred much later around 25 ma when sudden cooling suggests a component of thrusting or more likely normal faulting the cretaceous shear may be related to collision between the qiantang and the lhasa blocks the karakax and xidatan shear zones may have formed a unique continuous boundary in the cretaceous which was later reused by the tertiary strike slip faults leading to potentially calculable offsets along the altyn tagh fault

structure and evolution of the himalaya tibet orogenic belt the 1981 french chinese expedition to tibet focused on the lhasa block extending earlier coverage 400 km north of the tsangpo suture the lhasa block stood between 10 and 15 n latitude over most of the upper cretaceous and eocene and if gondwanian in origin had detached from gondwana by early permian seismic profiles reveal a complex moho topography resulting both from multiple continental thrusting and large scale strike slip faulting subduction related granitoids representing mixtures of mantle and crustal components and anatectic granitoids have been analysed and dated this study emphasizes the role of smaller blocks in the accretion of the continental mosaic

cenozoic tectonic evolution of the elashan range and its surroundings northern tibetan plateau as constrained by paleomagnetism and apatite fission track analyses abstract magnetostratigraphic studies of sediments in the western gonghe and eastern qaidam basins together with apatite fission track aft analyses in the elashan range suggest that the elashan range and its surroundings experienced a two stage evolution a protracted uplift from the cretaceous to the early tertiary which explains the absence of the paleogene strata in the region and an accelerated growth of the elashan range since the latest oligocene caused by extensively distributed thrust faults in the region this later phase of uplift resulted in the subsequent thick sediments within the both easternmost qaidam and gonghe basins basal ages of 21 22 ma in the wulan section of 20 ma in the southern gonghe section and of l2 ma in the chaka section the two phases of thrust motion and range growth mean that the qaidam and gonghe basins were never united during cenozoic times the strike slip is likely to have occurred at l0 ma as shown by the paleomagnetic declination data which show a rapid transition in rotation sense from counterclockwise to clockwise at that time this time is also similar to the 9 ma age of initiation of the wenquan fault determined by backward calculation based on its late quaternary slip rate eventually the qinghai nan shan thrust belt began to develop at 6 ma

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

the origin and pre cenozoic evolution of the tibetan plateau different hypotheses have been proposed for the origin and pre cenozoic evolution of the tibetan plateau as a result of several collision events between a series of gondwana derived terranes e g qiangtang lhasa and india and asian continent since the early paleozoic this paper reviews and reevaluates these hypotheses in light of new data from tibet including 1 the distribution of major tectonic boundaries and suture zones 2 basement rocks and their sedimentary covers 3 magmatic suites and 4 detrital zircon constraints from paleozoic metasedimentary rocks the western qiangtang amdo and tethyan himalaya terranes have the indian gondwana origin whereas the lhasa terrane shows an australian gondwana affinity the cambrian magmatic record in the lhasa terrane resulted from the subduction of the proto tethyan ocean lithosphere beneath the australian gondwana the newly identified late devonian granitoids in the southern margin of the lhasa terrane may represent an extensional magmatic event associated with its rifting which ultimately resulted in the opening of the songdo tethyan ocean the lhasa northern australia collision at similar to 263 ma was likely responsible for the initiation of a southward dipping subduction of the bangong nujiang tethyan oceanic lithosphere the yarlung zangbo tethyan ocean opened as a back arc basin in the late triassic leading to the separation of the lhasa terrane from northern australia the subsequent northward subduction of the yarlung zangbo tethyan ocean lithosphere beneath the lhasa terrane may have been triggered by the qiangtang lhasa collision in the earliest cretaceous the mafic dike swarms ca 284 ma in the western qiangtang originated from the panjal plume activity that resulted in continental rifting and its separation from the northern indian continent the subsequent collision of the western qiangtang with the eastern qiangtang in the middle triassic was followed by slab breakoff that led to the exhumation of the qiangtang metamorphic rocks this collision may have caused the northward subduction initiation of the bangong nujiang ocean lithosphere beneath the western qiangtang collision related coeval igneous rocks occurring on both sides of the suture zone and the within plate basalt affinity of associated mafic lithologies suggest slab breakoff induced magmatism in a continent continent collision zone this zone may be the site of net continental crust growth as exemplified by the tibetan plateau c 2012 international association for gondwana research published by elsevier b v all rights reserved

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

mesozoic cenozoic evolution of the xining minhe and dangchang basins northeastern tibetan plateau magnetostratigraphic and biostratigraphic results 1 accurate stratigraphic ages are crucial to understanding the deformation history of the tibetan plateau prior to and during the indo asian collision efforts to quantify mesozoiccenozoic ages are hindered by limited fossils and a paucity of volcanic horizons and regionally correlative strata magnetostratigraphic and biostratigraphic results from the xining minhe longzhong basin complex and dangchang basin provide an improved chronology of nonmarine basin development over a large region of the northeastern tibetan plateau 34 37 n 101 105 e analyses of 171 magnetostratigraphic levels and 24 palynological assemblages 120 species indicate late jurassic early cretaceous to mid tertiary deposition although magnetic polarity zonation is incomplete independent palynological age control partially restricts possible correlations to the geomagnetic polarity timescale the sediment accumulation record basin provenance structural geology and published thermochronological data support a history of jurassic exhumation late jurassic early cretaceous fault related basin initiation and cretaceous paleogene reduced accumulation these patterns which are compatible with late jurassic early cretaceous extension and cretaceous paleogene postrift thermal subsidence were disrupted at about 40 30 ma when shortening related to the indo asian collision induced localized range uplift vertical axis rotation and amplified subsidence indexterms 1520 geomagnetism and paleomagnetism magnetostratigraphy 8102 tectonophysics continental contractional orogenic belts 9320 information related to geographic region asia 9604 information related to geologic time cenozoic 9609 information related to geologic time mesozoic keywords tectonics magnetostratigraphy tibetan plateau cenozoic mesozoic sedimentary basins

continental erosion and large scale relief a worldwide investigation of continental erosion is carried out by the study of large drainage basins on the basis of hydrological data environmental factors and basin relief distribution inside each basin mean geochemical and mechanical denudation rates are defined a multicorrelation analysis shows that the mechanical denudation rates ds are uncorrelated with environmental factors and correlated with mean basin elevation h while chemical denudation rates dd are insensitive to relief but correlated with mean annual precipitation furthermore two linear relationships between h and ds are detected 1 ds m 10 yr 419 10 6 h m 0 245 with v explained variance 95 1 this law concerns basins related to orogenies younger than 250 ma the negative intercept is interpreted as a continental sedimentation rate of 245 m m y an alternative model in which one invokes a critical elevation separating erosion from sedimentation is equally successful and leads to lower sedimentation rates 60 110 m m y for both models one derives from the slope of the adjustments erosion time constants on the order of 2 5 m y 2 ds m 10 yr 61 10 6 h m with v 86 5 this law concerns basins related to older orogenies the null intercept suggests the lack of continental storage because of the more important dispersion of the data the erosion time constant is calculated separately for each basin it ranges from 15 to 360 m y the tectonic implications of these results are discussed in particular the short time constant 2 5 m y agrees with orogenic uplift rates on the order of 1 mm yr observed in active mountain chains

temperature and pressure field in the tertiary succession of the western qaidam basin northeast qinghai tibet plateau china abstract based on pressure and temperature data a comprehensive analysis was carried out for the pressure and temperature regimes in four tertiary formations of the western qaidam basin northeast qinghai tibet plateau the pressure data were obtained from well pressure tests and the temperature data from systematic steady state temperature measurements in 10 wells temperature tests in over 200 wells and calculation for 60 theoretical calculated points in general the geothermal gradient in the four oligocene pliocene formations decreases with depth the pliocene upper youshashan formation has the highest gradient of 33 c km the miocene oligocene upper and lower ganchaigou formations have lower gradients of 25 27 5 c km in most areas but also several lower gradients down to 20 c km abnormally high pressures were found at depths of 2650 4350 m beneath the regional seal in the western qaidam basin the abnormal high pressure is mainly confined to the lower ganchaigou formation with a pressure coefficient up to 1 9 most parts of the pliocene upper and lower youshashan formations are in normal and low abnormal high pressure r 4 mg l indicates the formation was in a closed chemical environment of fluid flows over the majority of geological time the reservoirs in this formation all occur in an abnormally high pressure zone with a pressure coefficient of more than 1 5

magnetostratigraphy of cenozoic sediments from the xining basin tectonic implications for the northeastern tibetan plateau 1 the xining subbasin of the longzhong basin holds the longest continuous cenozoic stratigraphic record at the margin of the northeastern tibetan plateau despite a rich biostratigraphic content including the xiejia mammal fauna the tectonic evolution of the basin is largely unconstrained in this study we present stratigraphic biostratigraphic and magnetostratigraphic results that provide a basis for reconstructing the cenozoic tectonic evolution of the xining basin with respect to adjacent regions of the northeastern tibetan plateau magnetostratigraphic analysis from three red bed sections in the xining basin indicates continuous deposition at low and nearly constant accumulation rates average 2 2 cm kyr from 52 0 to 17 0 ma we interpret this result to indicate that no major regional tectonic event implying large sediment accumulation variations has affected the xining basin deposition during this considerable time window in detail accumulation rate variations outline a three stage evolution with 1 8 cm kyr from 52 0 ma to 34 5 ma 4 1 cm kyr from 34 5 to 31 0 ma and 2 3 cm kyr from 31 0 to 17 0 ma the second order increase between 34 5 and 31 0 ma can be interpreted to result from either a distal tectonic event or to be of climatic origin although the region was tectonically quiescent for much of the cenozoic tectonic activity occurred during basin initiation or reactivation at circa 55 0 52 5 ma and during intense basin deformation after 17 0 ma

ophiolites of the kunlun mountains china and their tectonic implications abstract three ophiolite belts ranging in age from cambrian to triassic provide valuable data on the tectonic evolution of the kunlun mountains which lie along the northern margin of the tibetan plateau all of the ophiolites contain dismembered but nearly complete assemblages of peridotite mafic and ultramafic cumulates pillow and massive lavas and radiolarian cherts 1 the oldest belt contains ophiolites of cambrian age but the initial spreading of the ocean basin most likely began in the late proterozoic these ophiolites outcrop in the central part of the eastern kunlun extend for at least a few hundred kilometers and are believed to reflect a small ocean basin closure of this ocean led the accretion of south kunlun block to the north kunlun block and tarim craton ophiolitic lavas in this belt have island are affinities 2 a second belt of early carboniferous ophiolites extends nearly 600 km along the northern margin of the western kunlun lavas in this belt are basalt basaltic andesite and andesite all of which have compositions characteristic of volcanic arcs this ocean basin possibly developed on the basis of the early paleozoic oceanic basin closure of this ocean basin by southward subduction in the early permian produced a suite of calc alkaline volcanic rocks in what is now the central part of the western kunlun 3 the third belt extends nearly 1200 km along the southern margin of the eastern kunlun and contains numerous ophiolites of early permian to middle triassic age these ophiolites are highly tectonized containing volcanic rocks with the geochemical characteristics of mid ocean ridges oceanic islands and volcanic arcs this belt is tentatively interpreted as the suture zone between gondwana and eurasia

apatite fission track evidence for neogene uplift in the eastern kunlun mountains northern qinghai tibet plateau china abstract forty one apatite fission track ages aft were determined on samples collected along a n s section of the eastern kunlun mountains across the middle and south kunlun faults between buqingshan and dulan measured aft ages lie between 25 2 and 130 10 ma and all are significantly younger than their host rock formation or sediment deposition ages modelling the aft data identifies a two stage regional cooling history that spans the last 100 myrs the earliest cooling phase occurred between the late jurassic and early tertiary and involved a moderate level of cooling between 20 and 40 c equivalent to average exhumation rates of 15 m myr the second phase of cooling took place from 20 ma with cooling rates increasing tenfold average exhumation rates for this period are estimated to be in the range of 100 150 m myr the first stage of protracted cooling is consistent with regional evidence from the qiantang and lhasa terrans where previous studies have noted low rates of denudation in relation to a back arc extensional setting the more recent acceleration in cooling seen in the kunlun data coincides with an increase in sedimentation rates in the adjacent qaidam basin this points to a phase of neogene uplift and increased erosion of the kunlun range although contemporaneous monsoon strengthening may also have had a role

oblique stepwise rise and growth of the tibet plateau two end member models of how the high elevations in tibet formed are i continuous thickening and widespread viscous flow of the crust and mantle of the entire plateau and ii time dependent localized shear between coherent lithospheric blocks recent studies of cenozoic deformation magmatism and seismic structure lend support to the latter since india collided with asia 55 million years ago the rise of the high tibetan plateau likely occurred in three main steps by successive growth and uplift of 300 to 500 kilometer wide crustal thrust wedges the crust thickened while the mantle decoupled beneath gently dipping shear zones did not sediment infilling bathtub like of dammed intermontane basins formed flat high plains at each step the existence of magmatic belts younging northward implies that slabs of asian mantle subducted one after another under ranges north of the himalayas subduction was oblique and accompanied by extrusion along the left lateral strike slip faults that slice tibet s east side these mechanisms akin to plate tectonics hidden by thickening crust with slip partitioning account for the dominant growth of the tibet plateau toward the east and northeast

multiple tectonothermal events in the granulite blocks of southern india revealed from epma dating implications on the history of supercontinents abstract we report age data on zircon monazite uraninite and huttonite from a suite of 29 samples covering four major granulite blocks in southern india using an electron microprobe technique the rocks analysed in this study cover all of the major lithounits in these terrains and include garnet bearing and garnet free charnockites garnet biotite gneisses khondalites calc silicate rocks and a suite of orthogneisses biotite gneiss biotite hornblende gneiss two pink metagranites representing the magmatic phase were also analysed zircons from the madras block yield well defined isochrons at 2 5 2 6 ga core to rim analyses of single zircon grains show age zoning with 2 6 2 9 ga igneous cores mantled by 2 4 2 5 ga rims detrital zircons show age up to ca 3 2 ga monazites in this block have cores and rims with 2 5 2 3 ga a suite of 19 samples from the madurai block brings out the multiple tectonothermal events in this terrain zircons from an orthogneiss yield well defined isochrons at 1 7 0 1 ga 0 82 0 05 ga and 0 58 0 04 ga from core inner rim and rim portions respectively zircon grains in other rocks preserve either core or secondary growth ages at 0 8 1 0 ga zircons in a pink metagranite from this block show sharply defined isochrons of 0 68 0 03 ga for the core and 0 57 0 01 ga for the secondary portion a late pan african overprint is observed throughout this block with zircon rims monazite uraninite and huttonite yielding age values in the range of 0 45 0 60 ga zircons from both the trivandrum and nagercoil blocks show a major tectonothermal event at 0 55 ga with faint indications of previous tectonothermal events during 0 8 1 0 and 1 7 2 0 ga monazite data from both the trivandrum and nagercoil blocks are essentially similar to those from the madurai block except for presence of relic monazite in the former our study confirms the notion that the palghat cauvery shear zone marks the major terrain boundary between an archean craton in the north and proterozoic terrains in the south it also strengthens the view of paleoproterozoic accretion and pan african reworking of blocks south of this shear zone the ages of production accretion and reworking in the terrains of southern india yield important information for the histories of columbia 1 8 1 5 ga rodinia 1 1 1 0 ga and gondwana 0 6 0 5 ga supercontinents the southern indian terrains formed part of a worldwide network of orogenic belts that is centered around 1 8 ga and outlines the configuration of columbia accretion of terrains in the madurai block to archean rocks north of the palghat cauvery shear zone at this time is consistent with the ages suggested to be the oldest metamorphic event in the eastern ghats orogen of eastern india and the rayner belt of coastal east antarctica our data confirm earlier evidence that all of southernmost india underwent resetting of isotopic systems during the final accretion of gondwana at the time of the pan african orogeny 0 5 ga the possibility that the trivandrum and nagercoil terrains accreted to the madurai block after 0 7 ga suggests that this pan african zone may also have involved accretion and closure of ocean basins if that happened then this zone may be the long sought suture between east and west gondwana

tethys tectonic evolution and its bearing on the distribution of important mineral deposits in the sanjiang region sw china abstract the sanjiang region in se tibet plateau and nw yunnan is known to have formed by amalgamation of gondwana derived continental blocks and arc terranes as a result of oceanic subduction followed by continental collision from paleozoic to mesozoic in this paper we provide a synthesis of tectonic evolution magmatism and metallogeny in the region based on data from literatures early paleozoic ophiolites 473 439 ma in the changning menglian belt indicate the existence of a proto tethys ocean in this region two episodes of subduction related magmatism in the early paleozoic one occurred in the baoshan and tengchong blocks at 502 455 ma and the other occurred in the simao block at 421 401 ma are regarded as evidence for two different events of subduction of the proto tethys ocean at different locations the proto tethys was succeeded in early devonian by the paleo tethys which comprised the main ocean and three branches ailaoshan jinshajiang and garze litang the changning menglian main ocean existed from middle devonian to middle triassic the remnants of the oceanic crust are preserved in a few places in the longmu tso shuanghu suture as well as in the changning menglian ophiolite belt the eastward subduction of the main oceanic plate from early permian to early triassic formed a prominent arc terrane stretching 1500 km from yunnan to eastern tibet from the waning stage of subduction to post subduction numerous s type granite plutons with ages varying between 230 and 219 ma such as the lincang batholith in yunnan were emplaced at or close to the suture this event produced several hydrothermal w sn deposits in the region the tectonic evolution and associated magmatism of the jinshajiang and ailaoshan branch oceans are generally comparable to those of the main ocean however the branch oceans were subducted westward instead the garze litang branch ocean also underwent westward subduction from middle devonian to late triassic arc related high sr y porphyry intrusions and associated porphyry skarn cu mo au deposits are common in the jinshajiang ailaoshan region especially in the yidun arc which formed prior to jurassic the vms deposits in the sanjiang region formed in diverse tectonic settings including middle silurian back arc basins carboniferous oceanic islands paleozoic subduction zones and triassic post subduction rifting environments the mesozoic and early cenozoic evolution of the baoshan and tengchong blocks was largely influenced by eastward oceanic subduction of the meso and neo tethys from late permian to middle cretaceous and from late cretaceous to 50 ma respectively abundant early cretaceous granitoids and associated skarn type pb zn and sn fe deposits in the baoshan and tengchong blocks were produced in the background of the shan boundary oceanic slab subduction to the west and the break off of the nujiang bitu oceanic slab to the north the subduction of the neo tethys oceanic plate beneath the tengchong block from late cretaceous to paleogene formed abundant s type granitoids and many skarn type and greisen type sn w deposits granitoids formed at 105 to 81 ma and contemporaneous hydrothermal w mo ag and au deposits which temporally coincided with the subduction of the neo tethys are common in the yidun arc terrane

tectonics of the northern himalaya since the india asia collision abstract the india asia collision resulted in the construction of the vast himalayan orogen the northern himalayan domain began to experience extensional tectonics since eocene oligocene transition eot when the himalayan orogeny was still in progress major structures generated by the extension in the northern himalaya include the south tibet detachment system stds the north himalayan gneiss domes nhgd and the north south trending rifts nstr the earliest emplacement of syn deformational leucogranite at 36 32 ma along the stds marks the initial transformation from thickening to thinning in the northern himalayan domain at eot the thickening before eot caused crustal partial melting which formed the weak root of the thickened orogen or the so called channel flow this channel flow triggered the collapse of the orogen extrusion of the greater himalayan crystalline complex ghc and the onset of extensional tectonics of the stds the similarities in tectono thermal history and geochemistry of rocks between the stds and nhgd suggest that the formation of the nhgd has a direct relationship to the activity of the stds the extension of stds and the resultant thinning caused further partial melting of the crust leading to the larger scale emplacement of leucogranite during miocene 27 13 ma diapirism of these plutons shaped the domes in nhgd exposing the ghc and the shear zones of the stds in the northern himalaya in gyirong another tectonic transform from extension to shortening occurred after 18 7 ma in dinggye region the stds was offset by the nstr culminating the deformation at 13 ma when the nstr began to be active this indicates another tectonic transform in the northern himalaya at 13 ma when the n s extension of the stds ceased and gave way to e w extension of the nstr marking the end of the peak granitic magmatism in northern himalaya multiple episodes of deformation in the major structures such as the stds mct and nstr and superposition of different structures indicate a multiphase orogenic process in the himalayas in which mountain building and collapse occurred alternately with the formation of nstr and conjugate shear zones in tibet this tectonic scenario was possibly controlled by the india asia convergent rate we propose that the extension was an integral part of the orogenic process rather than a simple marker of the culmination of the orogeny

large scale crustal deformation of the tibetan plateau the topography velocity and strain fields calculated from a three dimensional newtonian viscous model for large scale crustal deformation are generally in good agreement with results from geological geodetic and earthquake studies in and around the tibetan plateau provided that the model rheology incorporates a weak zone within the deep crust beneath the plateau equivalent to a viscosity of 1012 pa s within a 250 m thick channel or 1018 pa s within a 15 km thick channel model studies and observations show a plateau at 5 km elevation with steep topographic gradients across the southern and northern plateau margins and more gentle gradients across the southeastern and northeastern margins rapid shortening strain is concentrated along the lower portions of the northern and southern plateau margins at rates 20 mm yr model results show north south shortening 10 mm yr in reasonable agreement with gps data 5 8 mm yr of north south shortening across the northern two thirds of the plateau and east west stretching 10 15 mm yr across the eastern half of the high plateau in reasonable agreement with seismic geologic and gps data upper crustal material moves eastward from the plateau proper into a lobe of elevated topography that extends to the south and east clockwise rotation of material around the east himalayan syntaxis at rates up to 10 mm yr occurs partly as a result of dextral shear between indian and asian mantle at depth and partly as a result of gravitational spreading from the high plateau to the south and east there is little difference in model surface deformation for assumptions of moderately weak or extremely weak lower crust except in southern and northern tibet where margin perpendicular extension occurs only for the case of an extremely weak lower crust our results suggest that the tibetan plateau is likely to have gone through a two stage development the first stage produced a long narrow high orogen whose height may have been comparable to the modern plateau the second stage produced a plateau that grew progressively to the north and east east west stretching eastward plateau growth and dextral rotation around the east himalayan syntaxis probably did not begin until well into the second stage of plateau growth perhaps becoming significant after 20 m y of convergence

geology geochronology and geochemistry of the saishitang cu deposit east kunlun mountains nw china constraints on ore genesis and tectonic setting abstract the saishitang cu deposit located on the eastern area of the eastern kunlun orogenic belt ekob and on the northern margin of the tibetan plateau is one of the most important copper deposits in qinghai province china skarn and cu orebodies mainly occur as stratoids or lenses along the contact zone between the quartz diorite and lower to middle triassic metamorphosed tuff and marble in this study we use new zircon u pb ages molybdenite re os ages granitoid geochemical compositions and zircon hf isotope data to constrain the ore genesis and the tectonic setting of the saishitang cu deposit the quartz diorite is composed of inner medium to fine grained quartz diorite phase and an outer porphyritic quartz diorite phase u pb dating of zircons by laser ablation inductively coupled plasma mass spectrometry la icp ms yield ages of 222 7 2 3 ma and 222 6 2 4 ma for the inner and outer phase of quartz diorite respectively molybdenite separated from ore bearing quartz veins yields a re os isochron age of 223 4 1 5 ma indicating a close temporal link between magmatism and mineralization geochemically the quartz diorite shows high contents of sio 2 59 63 68 21 wt al 2 o 3 15 57 18 13 wt and na 2 o 3 07 3 56 wt with a cnk values between 0 90 and 1 01 and mg between 46 and 52 classified as the calc alkaline i type granite the trace element patterns are characterized by high lree lree hree 7 36 13 03 weak negative eu anomalies eu 0 69 0 86 relative enrichment of rb th k and pb and depletion of nb ta p and ti the quartz diorite has variable e hf t values 2 5 to 4 87 and mesoproterozoic two stage model ages 997 to 1387 ma based on petrological and geochemical features we infer that the quartz diorite is derived from the partial melting of thickened lower crust with an additional input of mantle components according to the geological geochemical and mineralogical features we propose that the saishitang cu deposit is a skarn deposit with minor porphyry type mineralization and the porphyry skarn mineral system is related to the crust mantle interaction based on the tectonic evolution of the paleo tethys ocean during the late triassic we infer that the intrusions and associated cu mineralization were related to regional extension of the ekob in a post collision environment the results also show that most porphyry skarn cu deposits in the ekob have similar mineralization ages 240 220 ma and the late triassic granitoids in the ekob present significant potential for the prospecting of porphyry skarn cu deposits

timing of india asia collision geological biostratigraphic and palaeomagnetic constraints 1 a range of ages have been proposed for the timing of india asia collision the range to some extent reflects different definitions of collision and methods used to date it in this paper we discuss three approaches that have been used to constrain the time of collision the time of cessation of marine facies the time of the first arrival of asian detritus on the indian plate and the determination of the relative positions of india and asia through time in the qumiba sedimentary section located south of the yarlung tsangpo suture in tibet a previous work has dated marine facies at middle to late eocene by far the youngest marine sediments recorded in the region by contrast our biostratigraphic data indicate the youngest marine facies preserved at this locality are 50 6 52 8 ma in broad agreement with the timing of cessation of marine facies elsewhere throughout the region double dating of detrital zircons from this formation by u pb and fission track methods indicates an asian contribution to the rocks thus documenting the time of arrival of asian material onto the indian plate at this time and hence constraining the time of india asia collision our reconstruction of the positions of india and asia by using a compilation of published palaeomagnetic data indicates initial contact between the continents in the early eocene we conclude the paper with a discussion on the viability of a recent assertion that collision between india and asia could not have occurred prior to 35 ma

early cenozoic faulting of the northern tibetan plateau margin from apatite u th he ages models to explain the distributed nature of continental deformation predict the propagation of strain and high topography away from the plate boundary yet a growing body of evidence in the tibetan orogen suggests that deformation occurred at the far northern extent of the modern plateau early in the orogen s history and thus our current mechanical understanding of orogenic plateau development is incomplete new apatite u th he ages from four elevation transects document periods of rapid exhumation related to erosion pulses in hanging wall rocks of major thrust structures accelerated erosion is used as a proxy of fault timing and is interpreted in a larger context of structural data and sediment accumulation in adjacent foreland basins helium results are synthesized with published geologic thermochronometric and sedimentologic data from which a growing picture of regional compressional deformation in middle to late eocene time in northern tibet emerges we relate the early cenozoic period of deformation to the initiation of collision between india and eurasia despite the fact that the plate boundary was located 3000 km to the south regardless of whether or not high topography was built simultaneously as a result of this deformation early cenozoic strain signifies that the modern limit of the orogen has been relatively stationary since continental collision began and that deformation has not significantly propagated farther away from the plate boundary in time

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

preservation and exhumation history of the harizha halongxiuma mining area in the east kunlun range northeastern tibetan plateau china abstract the extent to which ore bodies are preserved in orogenic belts remains a poorly understood area of ore deposit research using zircon and apatite fission track analysis together with apatite u th he dating we constrained the erosion history of the ore bodies in the harizha halongxiuma mining area of the east kunlun range northern tibetan plateau china apatite fission track ages range from 114 8 to 87 6 ma with mean track lengths varying from 11 4 1 9 to 12 9 2 0 m zircon fission track ages vary from 205 14 to 142 7 ma in addition apatite u th he dating yielded ages of 60 56 ma the thermal history of jiapigou was modelled based on the apatite fission track data including ages and track lengths with constraints of zircon fission track ages and u th he ages the exhumation history of the harizha halongxiuma mining area was reconstructed with these age data revealing that since the early mesozoic the area has undergone three cooling stages 1 rapid cooling from 175 30 ma to 100 10 ma with a cooling rate and inferred exhumation of 2 0 0 8 c myr and 4 3 1 7 km respectively 2 a relatively stable stage from 100 10 ma to 40 10 ma with a cooling rate and inferred exhumation of 0 3 0 1 c myr and 0 5 0 2 km respectively and 3 rapid cooling since 40 10 ma with a cooling rate and inferred exhumation of 1 2 0 6 c myr and 1 4 0 4 km respectively this exhumation history is consistent with the subduction process of pacific plate and the strike slip movements of dunmi fault the total exhumation after main mineralization is calculated to be 7 6 3 2 km suggesting that ore bodies in the harizha halongxiuma mining area remain partially preserved

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

direct dating of eocene reverse faulting in northeastern tibet using ar dating of fault clays and low temperature thermochronometry abstract paired together fault gouge dating and low temperature thermochronometry overcome the limitations and assumptions inherent in each independent technique here we establish timing of brittle faulting along the west qinling fault of northeastern tibet by dating several size fractions of fault gouge clay that represent variable populations of illite polytypes results show that the authigenic or fault generated component of illite formed at 50 8 ma and that the detrital component formed at 236 7 ma indicating a middle eocene age of faulting and a middle triassic age of the wall rocks comparing this dataset with published thermochronology from hanging wall rocks supports the interpretation that the west qinling fault initiated at 50 ma and continued until at least middle miocene time and that authigenic clay growth occurred at ambient temperatures of 110 c lack of overprinting of younger clay ages at this site may indicate that rocks were out of the thermal window for authigenic clay formation during later faulting episodes the potential for temperature to control illite growth has implications for interpretation of authigenic illite ages and their relationship to deformation episodes within fault zones

age of initiation of collision between india and asia a review of stratigraphic data abstract the collision of india with asia is perhaps the most profound tectonic event to have occurred in past 100 ma it is responsible for the uplift of the himalayas and tibetan plateau and has been argued to have been responsible for geological geochemical and climatological consequences of global extent yet the age of initiation of this collision remains poorly constrained the literature is replete with estimates that range from the late cretaceous 65 ma to latest eocene

improved measurement of fission track annealing in apatite using c axis projection apatite fission track length data are used extensively for thermal history inversion however several studies have documented instances of poor reproducibility of length data we address this problem by using c axis projection to normalize track lengths for crystallographic angle in the extensive laboratory annealing data set acquired by barbarand et al 2003a 2003b a new simplification reduces the c axis projection model from six to four fitted parameters normalizing for track angle using c axis projection improves every aspect of length measurement reproducibility examined it accelerates convergence of mean length in single analyses increases consistency among replicate measurements by a single analyst enhances consistency of measurements of the same mounts by different analysts and improves the match between analyses conducted with and without cf irradiation c axis projection is also shown to enhance the thermal sensitivity of length data based on these results we assert that c axis projection is a good means of compensating for observer bias although it does not overcome differences caused by experimental error

geochronology and tectonic evolution of karimnagar and bhopalpatnam granulite belts central india abstract the karimnagar granulite belt kgb and the bhopalpatnam granulite belt bgb occur along both flanks of the pranhita godavari pg rift basin we present a state of the art overview on the geochronological and tectonic aspects of these belts and surrounding geologic domains and report new age data on zircon monazite and uraninite recovered from granulite facies assemblages from kgb and bgb based on electron microprobe analyses epma zircons from kgb charnockites show core ages of up to 3 1 ga mantled by rims of 2 6 ga zircons from bgb have 1 9 ga cores mantled by 1 7 ga rims zircons with core ages of 1 6 to 1 7 ga in bgb rocks suggest new growth at this time monazites and uranitite from kgb show clear peaks with well defined ages in the narrow range between 2 42 0 08 ga and 2 47 0 03 ga rims of monazite show mean age of 2 21 0 08 ga monazites from bgb define sharp linear trend in pbo vs tho2 diagram delineating a clear isochron with age of 1 59 0 03 ga age data from kgb and bgb presented in this report negate current models linking these terrains to godavari granulite belt and considering them as single and contemporaneous entity the mid archaean to early palaeoproterozoic signature recognized from kgb is totally missing in bgb on the other hand kgb rocks do not record any evidence for major mesoproterozoic thermal regime the two granulite belts shouldering the pg rift basin have therefore evolved in different times under distinct p t conditions and thermal regimes our results have important implications in evaluating models of supercontinent assemblies particularly the older assemblies of ur columbia and rodinia while tectonothermal events in kgb broadly match with those of east dharwar we propose that bgb represents a 1 6 ga collisional mobile belt between the bastar and the dharwar cratons the 1 6 ga collisional mobile belt at the southern margin of the bastar craton was superposed by rift activity along the pg basin at 1 5 ga this sequence of events goes against the existence of a 3 0 ga old contiguous assembly of ur but closely matches with the history of accretion and break up of the columbia further parts of the pg basin located away from the influence of the eastern ghats mobile belt neither recorded any grenville ages 1 0 ga corresponding to the rodinia accretion nor late pan african ages ca 550 ma relating to the gondwana amalgamation indicating that the region did not witness any of these younger tectonic events

constraints on the early uplift history of the tibetan plateau the surface uplift history of the tibetan plateau and himalaya is among the most interesting topics in geosciences because of its effect on regional and global climate during cenozoic time its influence on monsoon intensity and its reflection of the dynamics of continental plateaus models of plateau growth vary in time from pre india asia collision e g 100 ma ago to gradual uplift after the india asia collision e g 55 ma ago and to more recent abrupt uplift 7 ma ago and vary in space from northward stepwise growth of topography to simultaneous surface uplift across the plateau here we improve that understanding by presenting geologic and geophysical data from north central tibet including magnetostratigraphy sedimentology paleocurrent measurements and 40ar 39ar and fission track studies to show that the central plateau was elevated by 40 ma ago regions south and north of the central plateau gained elevation significantly later during eocene time the northern boundary of the protoplateau was in the region of the tanggula shan elevation gain started in pre eocene time in the lhasa and qiangtang terranes and expanded throughout the neogene toward its present southern and northern margins in the himalaya and qilian shan

cretaceous tertiary shortening basin development and volcanism in central tibet the geologic map pattern of the qiangtang terrane in central tibet defines a 600 km long and up to 270 km wide east plunging structural culmination it is characterized by early mesozoic blueschist bearing melange and upper paleozoic strata in the core and mainly triassic jurassic strata along the limbs in the western qiangtang terrane 84 e the culmination is unconformably overlain by weakly deformed mid cretaceous volcanic flows and tuffs along the bangong suture to the south 32 n 84 e mid cretaceous nonmarine red beds and volcanic rocks lie unconformably on jurassic suture zone rocks these relationships demonstrate that west central tibet was above sea level during the mid cretaceous and experienced significant denudation prior to mid cretaceous time growth of the qiangtang culmination is inferred to have initiated during southward emplacement of a thrust sheet of early mesozoic melange and upper paleozoic strata during the early cretaceous lhasa qiangtang collision the north south width of the inferred thrust sheet provides a minimum slip estimate of 150 km at 84 e decreasing eastward to 70 km at 87 e paleogene deformation in the qiangtang terrane is characterized by widely distributed mainly north dipping thrust faults that cut eocene oligocene red beds and volcanic rocks in their footwall along the bangong suture the north dipping shiquanhe gaize amdo thrust system cuts 64 and 43 m y old volcanic tuffs in its footwall and accommodated 40 km of post mid cretaceous shortening the tertiary south dipping gaize siling co backthrust bounds the southern margin of the bangong suture and marks the northernmost limit of mid cretaceous marine strata in central tibet cretaceous deformation and denudation in central tibet is attributed to northward underthrusting of the lhasa terrane beneath the qiangtang terrane along the bangong suture this model implies that 1 cretaceous strata along the bangong suture and in the northern lhasa terrane were deposited in a flexural foreland basin system and derived at least in part from the qiangtang terrane and 2 the central tibetan crust was thickened substantially prior to the indo asian collision although its magnitude is poorly known tertiary shortening in the qiangtang terrane is more prevalent than in the lhasa terrane this difference may be attributed to the presence of underthrust melange in the deeper central tibetan crust which would have made it weaker than the lhasa terrane during the indo asian collision

a users guide to fission track dating calibration abstract misconceptions regarding the fission track dating method prompt the description of five handling procedures and the derivation of commonly used system calibrations problems of registration geometry inhomogeneous uranium distribution accumulated radiation damage and anisotropic etching largely invalidate all but the population and external detector methods and impose strict limitations upon even their routine application complexities of unique absolute thermal neutron dosimetry are illustrated by the calibration and use of the nbs pre irradiated glass srm 962 the 20 disparity in measured f decay constant values cannot be isolated from neutron fluence measurement and calibration frequently a calibration ratio f is evaluated against an age standard and then split into its component parts to illustrate this interdependence of f and zircon fission track ages in agreement with independent k ar ages are obtained by calculating the same track count data with each of the preferred values of f f 7 03 10 17 yr 1 and8 46 10 17 yr 1 together with appropriate selected neutron dosimetry schemes an alternative approach is presented formally relating unknown ages of samples to known ages of standards either by direct comparison of standard and sample track densities or by the repeated calibration of a glass against age standards practical recommendations are given for the reporting and critical assessment of fission track data

uplift of tibet from eclogites to granulites implications for the andean plateau and the variscan belt abstract using a simplified petrogenetic grid and a one dimensional thermal model to compute the evolution of the density of the crust with time the contribution to the uplift of tibet of metamorphic transitions in the lower crust is shown to be large the computations are made for a lower crust of either mafic or intermediate composition a mafic lower crust would have a mantle like density incompatible with seismological results this conclusion implies that any pre thickening gabbroic lower crust has been removed during the orogenic process a lower crust of intermediate composition would be affected by about 3 km of post thickening uplift over a few tens of millions of years this uplift is essentially the result of the eclogite to granulite transformation the resulting altitude matches the present altitude and does not require any delamination of the mantle lithosphere such a process would also explain why the neogene uplift of tibet to an average elevation of 5 km appears to have occurred without crustal shortening the same model may explain the uplift of the altiplano puna plateau which also occurred without surface shortening since about 10 ma a comparison is made with the now collapsed variscan high mountain belt for which there is evidence of a progressive increase of the temperature in the crust over tens of million years

lhasa block and bordering sutures a continuation of a 500 km moho traverse through tibet a north south wide angle fan profile through the yarlung zangbo suture across the lhasa block and through the bangong nujiang suture shows several significant and sharp changes in crustal thickness both sutures appear as vertical 20 km steps in the moho and may have been the loci for eastward strike slip motion of the tibetan lithosphere