growth and deformation of the ladakh batholith northwest himalayas implications for timing of continental collision and origin of calc alkaline batholiths abstract the calc alkaline ladakh batholith nw himalayas was dated to constrain the timing of continental collision and subsequent deformation batholith growth ended when collision disrupted subduction of the tethyan oceanic lithosphere and thus the youngest magmatic pulse indirectly dates the collision both u pb ages on zircons from three samples of the ladakh batholith and k ar from one subvolcanic dike sample were determined magmatic activity near leh the capital of ladakh occurred between 70 and 50 ma with the last major magmatic pulse crystallizing at ca documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand tex

magnitude of post middle jurassic bajocian displacement on the central altyn tagh fault system northwest china the altyn tagh fault system a key structural feature in the tectonic collage of central asia is an active left lateral strike slip fault system however its age of initiation kinematic history and total magnitude of displacement are not well known middle jurassic nonmarine sedimentary rocks that crop out along the central segment of the altyn tagh fault were deposited in a contiguous tarim qaidam foreland style basin that was geodynamically linked to mesozoic contraction in the tian shan bei shan qilian shan and kunlun shan this study documents the best evidence of a piercing point to date consisting of an offset lacustrine shoreline demarcated by aalenian bajocian open lacustrine strata to the west and fluvial and alluvial strata of comparable age to the east detrital compositions palynology paleocurrents and lithostratigraphy support correlation of these sections restoration of this jurassic facies boundary indicates 400 60 km of post bajocian left lateral separation on the altyn tagh fault consistent with this result restoration of the jurassic shoreline aligns felsic plutons across the western segment of the altyn tagh fault system that may have been left laterally offset by 360 km these two pairs of offset features suggest that the current best estimate for net magnitude of post bajocian left lateral separation on the altyn tagh fault is about 360 km

new u th pb constraints on timing of shearing and long term slip rate on the karakorum fault 1 zircons and monazites from 6 samples of the north ayilari dextral shear zone nasz part of the karakorum fault zone kfz have been dated with the u th pb method using both id tims and sims techniques the ages reveal 1 inheritance from several events spanning a long period between the late archean and the jurassic 2 an eocene oligocene magmatic event similar to 35 32 ma 3 an oligo miocene magmatic event similar to 25 22 ma at least partly synkinematic to the right lateral deformation and 4 a period of metamorphism metasomatism similar to 22 14 ma interpreted as thermal and fluid advection in the shear zone the labhar kangri granite located similar to 375 km farther southeast along the kfz is dated at 21 1 0 3 ma such occurrence of several oligo miocene granites along the kfz some of which show evidence for synkinematic emplacement suggests that the fault zone played an important role in the genesis and or collection of crustal melts we discuss several scenarios for the onset and propagation of the kfz and offset estimates based on the main sutures zones our preferred scenario is an oligo miocene initiation of the fault close to the na range and propagation along most of its length prior to similar to 19 ma in its southern half the averaged long term fault rate of the kfz is greater than 8 to 10 mm a in good agreement with some shorter term estimates based on the indus river course or quaternary moraines and geodesy our results show the kfz cannot be considered as a small transient fault but played a major role in the collision history

experimental resetting of the u th pb systems in monazite abraded fragments 200 400 m of a large chemically homogeneous and non metamict brazilian monazite crystal characterised by a concordant u pb ages of 474 1 ma 208pb 206pb 19 5 were hydrothermally treated at varying temperatures with solutions of different compositions product monazites were analysed with scanning electron microscope sem electron microprobe emp secondary ion mass spectrometer sims and isotope dilution thermal ionisation mass spectrometer id tims experiments with pure water over a temperature range of 800 1200 c at 700 mpa and durations ranging from 5 to 60 days showed that even at 1200 c any dissolution and recrystallization of new monazite is confined to the outermost surface of the grain neither pb diffusion at the emp scale nor significant discordancy were observed we performed experiments at 800 and 1000 c for different durations using different fluid compositions at quartz saturation a 10 wt cacl2 fluid a 10 wt srcl2 fluid a 10 wt nacl fluid and a fluid containing nbs 982 pb standard 208pb 206pb 1 for all runs emp traverses revealed no pb diffusion profiles significant overgrowths of newly formed monazite are documented by sem analyses they occurred only in the 1000 c experiments when either cacl2 or pb bearing fluids were present in the cacl2 experiment two zones could be distinguished within the crystal a core possessing the initial monazite composition and a rim consisting of newly formed monazite produced by dissolution precipitation which was enriched in ca and pb free id tims dating of single grains treated with srcl2 and cacl2 solutions at 1000 c are significantly discordant experiments employing the nbs pb standard produced sub concordant monazite for which the 207pb 206pb apparent age has become older than prior to the experiment 544 ma at 800 c and 495 ma at 1000 c the newly grown monazite rim had obviously incorporated pb from the fluid none of our reaction products contained a detectable diffusion profile the only resetting mechanism we detected involved dissolution precipitation the extent of the dissolution precipitation process depends on fluid composition and is a more efficient mechanism than diffusion for controlling the resetting of monazite in natural rocks

the pangong injection complex indian karakoram a case of pervasive granite flowthrough hot viscous crust in the karakoram range of nw india the tangtse gorge cuts across deep crustal rock sexhumed by transpression along two strands of the dextral karakoram fault crustal leucogranitic magmas pervasively intruded amphibolites and migmatites in the form of sheets that locally coalesced and expanded to form kilometre scale plutons sheets resulted from slow seeping of granite into foliation planes magma wedging rather than from dyking driven by magma buoyancy and possibly syn intrusive tectonic deformation high temperature of the country rocks at the time of intrusion is inferred from similar crystallization ages of the intrusive leucogranite and in situ partial melt in the migmatites and reflected in the ductile structures developed whereas low country rock viscosity inhibited dyking high temperature freed the magma from the constraints of freezing and permitted pervasive intrusion the injection complex exposed along the tangtse gorge rather than representing the final emplacement structures represent transient structures of the granite pathways on their upward journey to build the karakoram batholith which crops out a few kilometres north at structurally shallower levels we suggest that pervasive magma flow is a transitional step between magma segregation at the source and later rise through cold crust and it may be one of several factors controlling whether dyking or diapirism becomes dominant during late ascent

new insights into the origin of two contrasting himalayan granite belts the two parallel belts of miocene granite that extend along much of the himalaya differ in age petrogenesis and emplacement style we suggest that their origin is linked to shear heating on a continuously active decollement that cuts through previously metamorphosed indian supracrustal rocks that were transformed into basement during the initial stages of the indian asian collision numerical simulations assuming a shear stress of 30 mpa indicate that initiation of slip on the himalayan thrust at 24 ma could trigger discontinuous melting reactions leading to formation of the high himalayan granite chain from 24 to 20 ma and the north himalayan belt from 18 to 12 ma this model result agrees well with observation as do model predictions regarding emplacement style and location

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

relationships between right lateral shear along the karakoram fault and metamorphism magmatism exhumation and uplift evidence from the k2 gasherbrum pangong ranges north pakistan and ladakh the karakoram fault is a major nw se aligned dextral strike slip fault that has accommodated minor extrusion of thickened tibetan crust since the mid miocene controversy exists as to whether the fault is lithospheric in scale resulting in synkinematic metamorphism and granite melting or whether it is an upper crustal fault cutting through earlier metamorphic and granitic rocks structural metamorphic and geochronological data from the karakoram fault show that 1 metamorphism and granite genesis occurred prior to strike slip shearing meaning that u pb zircon and monazite ages from deformed granites cannot be used as proxies for dating strike slip shearing only as a maximum age constraint on initiation of ductile shearing 2 sheared miocene leucogranites along the fault are equivalent to the baltoro granite batholith offset 40 150 km along the fault and show a strain gradient from high ductile strain to undeformed away from the fault 3 40 ar 39 ar and fission track ages of k2 orthogneisses and gasherbrum diorite show that rocks adjacent to the fault cooled during cretaceous to mid tertiary time and were not reset during shearing indicating that shear heating was not important 4 the gasherbrum diorite adjacent to the fault has never been at depths greater than 6 km since the early cretaceous and less than 3 km of material has been eroded off since then we conclude that the karakoram fault has had no significant influence on regional metamorphism or melting at least at levels exposed today and is restricted to the upper middle crust

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

geological evidence against large scale pre holocene offsets along the karakoram fault implications for the limited extrusion of the tibetan plateau two end member models proposed to accommodate the convergence between india and asia north of the himalaya are 1 homogeneous crustal thickening of the tibetan plateau and 2 continental escape or extrusion of tibet and southeast asia away from the indenting indian plate foremost among the arguments supporting the latter would be large scale 1000 km offsets and high present day slip rates along the major strike slip faults bounding the postulated extruding crust notably the altyn tagh fault along the northern margin of tibet and the karakoram fault along the sw margin satellite photographic interpretation and field mapping in the karakoram mountains in pakistan the nubra siachen area of north ladakh and the pamirs in xinjiang show that although the karakoram fault is extremely active today geological offsets along the right lateral fault are probably less than 120 km the 21 0 5 ma baltoro monzogranite leucogranite batholith has been rotated clockwise about a vertical axis 35 40 into nw se alignment parallel with the karakoram fault east of the siachen glacier with a maximum offset of 90 km across the fault the bangong shyok suture zone similarly has a dextral offset of 85 km the course of the indus river which was antecedent to the rise of the ladakh karakoram and himalayan ranges has been offset dextrally by 120 km south of pangong lake if present day slip rates approximately 32 mm yr avouac and tapponnier 1993 are correct only 4 ma are required to obtain a 120 km offset there is no geological evidence for any larger scale pre holocene offsets and it is suggested that the karakoram fault cannot have accommodated major eastward lateral motion of tibetan crust the fault has also exerted little or no influence on surface topographic uplift cutting obliquely across the highest peaks of the karakoram dextral motion along the central part of the karakoram fault has been transferred in the north to the rangkul murghab and karasu transpressional faults in the central pamir north of tashkurgan the karakoram fault shows mainly normal motion around the kongur and mustagh ata gneiss domes metamorphic core complexes and the extensional muji graben minor dextral displacement has occurred along the shiquanhe fault but this motion cannot be linked to the jiale fault of east tibet the southern end of the karakoram fault merges with the indus yarlung suture zone near mount kailas and does not cut across the himalaya the lack of large scale geological offsets along the karakoram fault together with its very recent initiation 5 or 4 ma suggests that it is related to the pliocene quaternary northward indentation of the pamir and not to any long term extrusion of tibetan crust following the indian collision

rbsr dating of the kohistan arc batholith in the trans himalaya of north pakistan and tectonic implications the kohistan arc batholith in northern pakistan is situated between the indus suture and the northern suture it belongs to the trans himalayan belt which continues eastwards as the ladakh arc batholith in northwest india and the kangdese batholith in southern tibet in kohistan the island arc consists upward sequence of the chilas stratiform complex of norites noritic gabbros and chromite layered dunites which formed in the sub arc magma chamber plutons of tonalite and diorite one of which has a rbsr isochron age of 102 12 ma the chalt volcanics of basaltic tholeiites succeeded by andesites to rhyolites and the yasin group sediments which formed in overlying intra arc basins and which contain albian aptian faunas all these rocks were deformed by major fold structures which are correlated with the formation of the northern suture ne sw trending basic jutal nomal dykes north of gilgit cross cut all the above structures and have a39ar 40ar hornblende age of 75 ma thus the northern suture must have formed in the period 102 12 to 75 ma with the island arc attached to the eurasian plate further northward subduction of the tethyan plate gave rise to an andean type batholith two plutons of which have rbsr isochron ages of 54 4 ma and 40 6 ma the dir utror group of calc alkaline lavas and sediments with eocene fossils are remnants of the volcanic cover of the andean type batholith we suggest that late cretaceous blueschists formed during subduction under the active continental margin and that continental collision and formation of the indus suture was in the eocene the batholith was intruded by layered aplite pegmatite sheets at 34 14 ma and 29 8 ma rbsr ages in post collisional times the87sr 86sr initial ratios of all the dated rocks range between 0 7039 and 0 7052

the transhimalaya gangdese plutonism in the ladakh region a upb and rbsr study abstract the age and origin of the transhimalaya gangdese plutonic belt in the ladakh area has been studied by high resolution u pb analyses of accessory minerals zircon and monazite allanite and rb sr measurements on whole rock samples the ages determined of 101 2m y for a granodiorite and of 60 7 0 4m y for a granite substantiate that the transhimalaya plutonism was active at least in mid cretaceous albian and earliest tertiary palaeocene times a rb sr isochron of 73 4 2 4m y might be fortuitous because the rb sr systematics shows important heterogeneities on the whole rock scale the inherited radiogenic lead in zircon demonstrates that anatexis of continental crust was involved in magma genesis the thus recycled continental material was heterogeneous with respect to its primary ages and or metamorphic history the approximate minimum ages of the continental sources range from 350 to 590 m y the 101 2m y old transhimalaya granodiorite which intrudes a series of mainly basaltic island arc rocks dras series shows that this island arc was attached to the continental margin at that time this is consistent with the hypothesis that the ladakh tethys closed through two subduction regimes 1 an early cretaceous subduction which formed the dras island arc and 2 a late cretaceous to palaeocene subduction at the continental margin which caused the emplacement of the transhimalaya plutonic belt

revealing hidden structures the application of cathodoluminescence and back scattered electron imaging to dating zircons from lower crustal xenoliths abstract the use of cathodoluminescence cl and or back scattered electron bse imaging techniques with in situ ion probe analyses of zircons can help unravel complex crustal histories of metamorphic rocks that otherwise might remain elusive using these techniques we have imaged zircons from three lower crustal xenolith suites that have previously been dated by shrimp sensitive high resolution ion microprobe in all three cases the zircons are featureless in transmitted light but cl and bse reveal internal structures that correlate with distinct growth events generally cl and bse images reveal similar structures with cl showing finer detail neither imaging technique is capable of delineating all growth features in every sample and the best results are obtained using a combination of the two techniques igneous cores in zircons commonly emit a different color cl emission in many cases zoning in the cores is truncated indicating that the zircons either spent time in the supracrustal environment after their initial crystallization and prior to the granulite facies event s or that part of the core zircon was resorbed during the subsequent metamorphic event metamorphic rims when present are commonly 10 to 30 m thick and are nearly always unzoned and featureless igneous cores and metamorphic overgrowths commonly have distinctive cl emission spectra and trace element concentrations however the cl spectra can only be used to qualify chemical differences as a linear relationship has not been shown to exist between cl intensity and trace element concentration in natural zircons in many cases the hf y p u and the heavy rare earth elements hrees concentrations can be correlated to igneous and metamorphic growth using a combination of cl and bse imaging techniques and in situ trace element analyses with either the electron microprobe or pixe particle induced x ray emission

age and origin of magmatism along the cenozoic red river shear belt china to decipher the geodynamic significance of cenozoic magmatism along the red river shear belt geochemical analyses u pb and rb sr dating and pb sr nd isotope tracing were undertaken zircon monazite titanite and a ti u oxide from foliated granitoid intrusions in the shear belt gneisses yield u pb emplacement ages of 33 1 0 2 2 31 9 0 3 25 8 0 2 and 24 7 0 2 ma and an age of 35 0 0 3 ma was obtained for the roughly 100 km long adjacent jinping phan si pang alkali granite together with our previous data the new ages suggest that magmatism and left lateral strike slip movements occurred coevally during latest eocene oligocene times from 33 to 22 ma the rb sr dating of muscovite and biotite from the northernmost gneisses indicates that cooling to 500 c occurred at 52 6 1 1 ma pre dating the onset of magmatism whereas further cooling to 300 c took place at 28 9 0 6 this shows that unroofing in the north took place almost 9 million years earlier than in the central gneiss segments of the shear zone geochemical data substantiate two types of magmas 1 amphibole bearing intrusions of alkaline trend which are derived from sources with isr 0 7065 0 7089 and i nd 3 7 to 6 6 2 leucogranitic layers and bodies having isr 0 7084 0 7354 and i nd 3 3 to 13 4 the former type of intrusion is found in both the gneisses and the adjacent unmetamorphosed cover rocks whereas leucogranites are restricted to the shear belt gneisses source signatures of the alkaline intrusions lie adjacent to the those of oib plotting at the lower end of the mantle array contamination of these melts by continental material seems to be very limited on the other hand the leucogranitic layers are essentially crustal derived but none of the them has country rock isotope signatures requiring melting of crust different from the actually exposed gneisses magma sources similar to those of ocean island basalt indicate magmatism to involve melting of light rare earth element and large ion lithophile element enriched mantle domains most likely present in the lithosphere underneath the region since lithospheric thickening or subduction can be ruled out to produce both types of magmas the presence of an important thermal anomaly is required which is coevally active with left lateral strike slip shear adiabatic decompression and melting within the rising anomaly is the most plausible mechanism to produce the mantle magmas which successively migrate through the crust to induce anatectic melting at 20 15 km crustal depth alkaline magmas largely dominate the volume of magmatism along the belt being continuously present in the shear zone for millions of years such lubrication potentially explains how very large amounts of displacement can be absorbed in surprisingly narrow shear zones such as the red river belt possibly also playing a role for where and when zones of plate scale lateral extrusion develop

syn kinematic emplacement of the pangong metamorphic and magmatic complex along the karakorum fault n ladakh this paper investigates the age p t conditions and kinematics of karakorum fault kf zone rocks in the nw part of the himalaya karakorum belt granulite to greenschist facies assemblages were developed within the kf zone during strike slip shearing the granulites were formed at high temperature 800 c 5 5 kbar were subsequently retromorphosed into the amphibolite facies 700 750 c 4 5 kbar and the greenschist facies 350 400 c 3 4 kbar the tangtse granite emplaced syn kinematically at the contact between a lt and the ht granulite facies intrusion occurred during the juxtaposition of the two units under amphibolite conditions microstructures observed within the tangtse granite exhibit a syn magmatic dextral s c fabric compiled u pb and ar ar data show that in the central kf segment granulite facies metamorphism occurred at a minimum age of 32 ma subsequent amphibolite facies metamorphism at 20 18 ma further shearing under amphibolite facies 650 500 c was recorded at 13 6 0 9 ma and greenschist facies mica growth at 11 ma these data give further constrains to the age of initiation and depth of the karakorum fault the granulite facies conditions suggest that the kf accommodating the lateral extrusion of tibet could be at least a crustal or even a lithosphere scale shear zone comparable to other peri himalayan faults

u pb age of the baltoro granite northwest himalaya and implications for monazite u pb systematics the baltoro granite is a major late to post tectonic plutonic phase of the karakoram batholith of the northwest himalaya in northern pakistan u pb zircon analyses indicate both emplacement at 21 0 0 5 ma and significant precambrian zircon inheritance dates on monazite are 17 19 ma and are interpreted to have remained near their closure temperature of about 700 c for several million years after emplacement resulting in pb loss by diffusion we suggest that the granite was emplaced into rocks which were at high temperature and that they remained so until late miocene northeast tilting rapid uplift and or tectonic denudation and cooling

shoshonitic and ultrapotassic post collisional dykes from northern karakorum sinkiang china abstract high k calc alkaline shoshonitic and ultrapotassic post collisional dykes of neogene age have been found in the remote and little known region of northern karakorum located around the shaksgam valley north of the k2 gasherbrum range china the dykes derive from more or less comparable basic magma s and display rather unusual petrographic and geochemical characters with respect to the other k rich rocks the geochemical data are consistent with derivation of the basic magma s from small degrees of partial melting of garnet lherzolites previously enriched in incompatible elements of crustal origin possibly during the subduction of the indian plate beneath karakorum the spectrum of compositions reflects fractional crystallization governed by an early removal of clinopyroxene phlogopite plagioclase garnet amphibole followed by the precipitation of abundant alkali feldspar amphibole apatite quartz additionally assimilation of crustal rocks during magma ascent contributed to the unusual compositional characteristics and is suggested by the abundance of corroded quartz plagioclase xenocrysts and by the occasional presence of granitic xenoliths an apparent connection exists between magmatism and tectonism in the complex karakorum fault zone kfz it is suggested that during the neogene the strike slip kfz and some adjacent post metamorphic faults transiently behaved as extensional fault systems down to deep levels triggering ascent and emplacement of the k rich magma the subsequent re activation of a compressive and transcurrent regime determined the rapid and recent uplift of the more primitive lamprophyres occurring in the plutonites and metamorphites of the upper baltoro glacier and k2 gasherbrum range relative to the more fractionated and contaminated lamprophyres injecting the shallower rocks of the shaksgam valley area

westward propagation of the north anatolian fault into the northern aegean timing and kinematics we present new evidence for the propagation processes of the north anatolian fault folding in the dardanelles straits region allows us to document the timing of the deformation preceding and the finite displacement after the passage of the propagating tip of the fault the accuracy of the observations is due to interplay between deformation and the sea level changes in the mediterranean the well known messinian regression followed by the pliocene transgression the long term kinematics around the sea of marmara pull apart total displacement of about 85 km over the past 5 m y is similar to the present day kinematics deduced from space geodesy at a larger scale westward propagation of the north anatolian fault over nearly 2000 km in the past 10 m y appears to be associated with strain recovery suggesting that the continental lithosphere retains long term elasticity

kongur shan normal fault type example of mountain building assisted by extension karakoram fault eastern pamir the northernmost segment of the karakoram fault is an active normal dextral wrench fault that bounds the muji tashgurgan pliocene quaternary basin eastern pamir the kongurshan 7719 m and mustaghata 7546 m mountains are interpreted as twin crustal ramp anticlines formed en echelon on the eastern side of the karakoram fault by subduction of the tarim lithosphere under the pamir west dipping s c mylonitic gneisses that have downdip stretching lineations form the western flank of kongur mountain kinematic criteria consistently indicate a top to the west sense of shear the mylonitic foliation is cut by a steeper active normal fault which is marked by prominent triangular facets on west facing spurs the ductile normal shearing deformation occurred during the interval 5 1 ma therefore like the himalayas the pamir mountain range shows the coeval development of frontal thrusting and rear normal faulting the kongur fault thus appears to be an excellent example of synorogenic extension where normal faulting contributes to relief

gabbro and related rock emplacement beneath rifting continental crust upb geochronological and geochemical constraints for the galicia passive margin spain the thinned continental crust of the west galicia margin is bound by a belt of serpentinized peridotites peridotite ridge lying about 300 km off the coast in the north atlantic ocean from this ridge a gabbro and a chlorite rock were studied in an attempt to substantiate rift related subcontinental magmatism occurring prior to sea floor spreading u pb dating of 13 different zircon fractions yields a precise age of 122 1 0 3 ma 2 for the emplacement of the chlorite rock protolith from which more than 50 of si and alkali calc alkali elements were lost during greenschist facies tectonometamorphism sr and nd isotope signatures suggest that the gabbro and chlorite rock protoliths were derived from mantle sources that were moderately depleted in lile relative to a chondritic reservoir no evidence for the presence of continental material in the magma source regions can be observed from the new zircon age of 122 1 0 3 ma and earlier determined39ar40ar age of 122 0 0 6 ma for amphibole from the same locality it can be documented that magma formation solidification and unroofing of the mantle rocks occurred during a short period of time of about 3 4 ma which means that the peridotite ridge detached from the continent and rose to the surface immediately after or even coevally with mantle melting

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

quartz fabric transition in a cooling syntectonic granite hermitage massif france abstract the hermitage granite situated in the northwest massif central france is a syntectonic hercynian leucogranite emplaced along an active transcurrent shear zone during emplacement and cooling the progressive deformation is marked by the development of a primary homogeneous foliation gradually affected by ductile shear bands c s mylonites increase in strain heterogeneity during cooling corresponds to a change of dominant deformation mechanism of the quartz phase from grain growth and migration recrystallization to intracrystalline a slip and rotation recrystallization migration recrystallization is characterized by preferred orientations of c axes close to the principal extension direction we discuss relevant deformation mechanisms and rheological implications for syntectonic plutons in particular we argue that the transitions between homogeneous and heterogeneous accumulation of strain cannot generally be correlated with transition between magmatic and solid state flow

karakoram fault zone rocks cool in two phases the karakoram fault zone of ladakh india is a major strike slip boundary along which the tectonic evolution of tibet has been accommodated in this paper 40 ar 39 ar isotopic age data are integrated with structural and metamorphic data to infer an exhumation history for granites and low to intermediate grade metamorphic rocks exposed adjacent to two strands of the fault zone near tangtse leucogranites which crystallized c 17 ma ago are cut by the karakoram fault zone indicating that fault movement was initiated subsequent to 17 ma the 40 ar 39 ar data give temperature time histories which indicate that the two phases of rapid cooling of karakoram fault zone rocks are evident one at c 17 ma to c 13 ma and another following an intervening period of relatively slow cooling starting at c 8 ma and continuing until at least 7 ma uplift since c 17 ma via dextral oblique thrusting has resulted in exhumation from amphibolite facies metamorphic conditions possibly with a larger thrust component from 17 to 13 ma followed by a change to dominant strike slip motion at about 13 ma rapid cooling starting at c 8 ma at greenschist facies temperatures is probably the result of renewed oblique thrusting it is notable that the two episodes of rapid cooling recorded in karakoram fault zone rocks coincide with episodes of exhumation in the pakistani karakoram and also in southern tibet

exhumation of the ultrahigh pressure tso morari unit in eastern ladakh nw himalaya a case study exhumation processes of the ultra high pressure uhp tso morari dome nw himalaya are investigated using structural petrological and geochronological data the uhp tso morari unit is bounded by the low grade metamorphic indus suture zone to the ne and mata unit to the sw three deformation phases d1 d2 and d3 are observed only d3 is common to the uhp unit and the surrounding units in the uhp unit the first deformation phase d1 produced upright folds under eclogitic conditions 20 kbar 580 60 c d1 is overprinted by d2 structures related to a nw se trending open anticline this phase is characterized by blueschist mineral associations and corresponds to the quasi isothermal decompression from a depth of 90 km eclogitic conditions up to 30 40 km the final exhumation phase of the tso morari unit is dominated by tectonic denudation and erosion d3 associated with a slight temperature increase radiochronological analyses indicate that the uhp exhumation process began during the eocene exhumation was fast during d1 d2 and slowed down through d3 in oligocene time the change in the deformation style from d1 d2 to d3 in the tso morari unit coincides with changes in the exhumation rates and in the metamorphic conditions these changes may reflect the transition from an exhumation along the subduction plane in a serpentinized wedge to the vertical uplift of the tso morari unit across the upper crust

late cenozoic right lateral strike slip faulting in southern tibet field observations collected during the sino french himalaya tibet cooperation project 1980 1982 show that right lateral faulting between southern and northern tibet roughly along the chord joining the eastern and western syntaxes of the himalayan arc is a major component of the active tectonics north of the himalayas just north of the well developed rifts of southern tibet we studied three right lateral 100 200 km long strike slip faults forming an en echelon array along that chord evidence for earthquake surface breaks was found on each of these faults for the m 8 beng co or damxung strike slip earthquake of november 18 1951 rupture parameters could be constrained to be 8 2 m and l 90 km offset glacial and postglacial morphological markers 105 years and 104 years b p imply displacement rates of the order of 1 2 cm yr on the three faults the en echelon fault zone appears to extend westward 2000 km across the plateau to the karakorum fault and is thus referred to as the karakorum jiali fault zone kjfz fast rates of slip on it and on faults along the northern rim of tibet appear to contrast with the slower rates of deformations in the interior of the plateau our estimates of slip rates on the strike slip and normal faults of southeastern tibet suggest that eastward extrusion of north tibet between the right lateral kjfz and the left lateral altyn tagh and kun lun faults absorbs at least 30 of the present convergence between india and asia we propose an instantaneous velocity model of the active tectonics of asia compatible with such faulting rates and geometries evidence for tertiary right lateral strike slip movements in southern tibet and around the eastern syntaxis of the himalayas is also found in the structural record offsets of various large scale geological features imply that these movements have been large

twenty million years of continuous deformation along the karakorum fault western tibet a thermochronological analysis the role of the karakorum fault zone kfz is debated south of 33 n ongoing dextral oblique slip along the sw edge of the gar basin exhumes metamorphic and magmatic rocks of the ayilari range minerals have recorded a continuum of deformation from temperatures 600 400 c down to 20 ma of deformation along the fault greenschist facies deformation superimposed upon the medium to high grade deformation marks a kinematic change from pure dextral to dextral normal motion associated with the onset of rapid cooling at the regional scale the coexistence of transtension in the gar basin with transpression documented along the pangong range farther north suggests another example of the zipper tectonics model developed along the red river fault the kinematic shift induced the rise of the ayilari range starting at 16 12 ma and the incision of major river courses the indus river might have become captive of the relief at this time the river s 120 km of apparent offset implies dextral motion at a long term rate of ca 8 5 1 5 mm yr

mapping iron oxide cu au iocg mineral potential in australia using a knowledge driven mineral systems based approach abstract iron oxide copper gold iocg deposits are consequences of lithospheric to deposit scale earth processes and form where there was a coincidence of ore forming processes in space and time building on previous conceptualisations we view a mineral system as an ore forming geological system in which four components are required to have operated efficiently and coincidentally namely 1 available sources of ore metals i e copper gold uranium rare earth elements and hydrothermal fluids 2 energy sources to drive fluids in the ore forming system 3 active crustal and mantle lithospheric architecture representing hydrothermal fluid and or magma flow pathways and 4 physico chemical gradients along which ore metals were deposited to form ore bodies this holistic multi scale mineral systems framework has been used to develop a practical knowledge based yet data rich prospectivity mapping method applicable at regional to continental scales for hydrothermal and orthomagmatic ore forming systems we demonstrate how the mineral system components can be translated into mappable criteria and show how maps of mineral potential are generated by integrating diverse and rich input data sets the method enables prediction of mineral potential not only in brownfields areas but also in greenfields and covered terranes with no previously known mineralisation here we report the application of this methodology in regional scale mapping of the potential for iocg deposits in australia using examples from five studies over the last decade in northern queensland eastern south australia and southern and central eastern northern territory uncertainties in the results arising from assignment of weightings to input data layers were investigated by the application of monte carlo type probabilistic simulations the results of 500 iterations using randomly assigned weightings overall support the deterministic results but also show that modelled prospectivity is controlled mainly by variations in intrinsic values of the input geoscientific data sets e g highs and lows of gravity values rather than by the weightings the results of the knowledge driven data rich analyses of iocg potential have been validated against known iocg deposits not used directly in the analysis we find in all five studies queensland south australia and northern territory a good spatial correspondence with few exceptions statistical analysis of prospectivity mapping results from the tennant creek mt isa study area demonstrate that 15 of 16 iocg deposits occur in the highest modelled prospectivity areas within 4 2 of the study area representing an area reduction of 95 8 moreover several new discoveries of cu au mineralisation have been made within areas previously highlighted as highly prospective this success and validation support the utility of geoscience australia s approach as a decision support tool to assist exploration companies and governments in craton to regional scale area selection for discovery of iocg and other mineral systems

meteorological overview of the arctic boundary layer expedition able 3a flight series a meteorological overview of the arctic boundary layer expedition able 3a flight series is presented synoptic analyses of mid tropospheric circulation patterns are combined with isentropic back trajectory calculations to describe the long range 400 3000 km atmospheric transport mechanisms and pathways of air masses to the arctic and sub arctic regions of north america during july and august 1988 siberia and the northern pacific ocean were found to be the two most likely source areas for 3 day transport to the study areas in alaska transport to the barrow region was frequently influenced by polar vortices and associated short wave troughs over the arctic ocean while the bethel area was most often affected by lows migrating across the bering sea and the gulf of alaska as well as ridges of high pressure which built into interior alaska july 1988 was warmer and dryer than normal over much of alaska as a result the 1988 alaska fire season was one of the most active of the past decade airborne lidar measurements verified the presence of biomass burning plumes on many flights often trapped in thin subsidence layer temperature inversions several cases of stratosphere troposphere exchange were noted based upon potential vorticity analyses and aircraft lidar data especially in the barrow region and during transit flights to and from alaska

reply to comment on large scale geometry offset and kinematic evolution of the karakorum fault tibetˮ searle and phillips comment provides us with a welcome opportunity to discuss further a fundamental problem in continental collision zones namely whether deformation is better approximated by fluid flow or block tectonics the central issue being whether strain is localized or not on faults and shear zones estimates of geological offsets slip rates and ages of shear are the most crucial data searle and phillips question the new results we present claiming instead that 1 cosmogenic dating indicates slow late quaternary slip rates on asian faults a few mm year or less including the karakorum in conflict with early inferences 1 3 2 the leucogranites we

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

oligocene miocene tectonics and sedimentation along the altyn tagh fault northern tibetan plateau analysis of the xorkol subei and aksay basins abstract tertiary basins adjacent to the northern edge of the tibetan plateau include both giant intracontinental sedimentary basins such as tarim and qaidam and smaller basins directly related to the evolution of the altyn tagh strike slip fault system and nearby fold thrust belts three examples of these smaller basins that formed during early evolution of the altyn tagh fault system are the xorkol aksay and subei basins the xorkol basin was the site of nonmarine siliciclastic sedimentation in alluvial fan playa and fluvial environments from at least the mid oligocene through the miocene sediment was delivered to the basin from source areas uplifted by basin bounding thrust faults and from the opposite side of the altyn tagh fault the age of the aksay basin is more poorly constrained but it also contains oligocene miocene nonmarine clastic strata that were derived from the southern side of the altyn tagh fault and adjacent thrust fault bounded uplifts the subei basin is characterized by s

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

a new miniaturized extraction chromatography method for precise u pb zircon geochronology abstract in this paper we describe a new method for the concomitant separation of u and pb in zircons for radiometric dating purposes the proposed separation scheme is based on a blend of two extraction chromatographic materials referred to as tru spec and sr spec respectively these afford a straightforward separation of u and pb with high yield good purity and satisfactory blank levels on micro columns using very small volume 5 l of mixed resins owing to the selectivity and high capacity of the specific extraction chromatographic materials for u and pb this new technique could be further scaled down the validity of the method is illustrated by measurements of u pb isotopic ratios and determination of corresponding ages in 0 2 0 8 mg fragments of the zr91500 zircon standard