provenance of glacial marine conglomerates in the permian lagar formation of southern tibet evidence for affinity of the lhasa terrane with australia abstract paleogeographic reconstructions of the lhasa terrane of the southern tibetan plateau are uncertain in paleozoic and investigating the paleogeographic location of the terrane via the current geographical configuration is difficult however gravels contained within glacial sediments offer an opportunity to solve this problem the lagar formation in the central lhasa sub terrane contains glacial marine deposits that incorporate gravel clasts composed of granitoids sandstone quartzite limestone and rhyolite these diamictites were derived from continental glaciation and were deposited mainly near a grounding line or ice front in littoral neritic to neritic environments we present new zircon u pb isotopic and whole rock geochemical data for the tonalitic conglomerates of the lagar formation the results indicate that the tonalites crystallized during the paleoproterozoic 1 76 ga and show a geochemical affinity with i type granite the tonalites typically have negative eu anomalies eu 0 31 0 37 high concentrations of rare earth elements depletion in high field strength elements enrichment in th marked positive zircon ehf t values 6 46 14 68 end t values of 1 24 to 1 76 and tdm2 ages of 2 37 2 13 ga all of these data indicate that the tonalites were produced by the partial melting of modified mafic crust or underplate in a subduction related environment on the basis of a comprehensive analysis of all 1 8 1 7 ga magmatism in east gondwana we propose that the most likely provenance of the tonalitic conglomerates is southern arunta and mt isa in northern australia therefore the central lhasa sub terrane may have been located near australia during the paleozoic

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

deep reflection surveying in central tibet lower crustal layering and crustal flow summary in the summer of 1998 a series of multichannel deep seismic reflection tests were conducted in central tibet around 89 e as part of the third phase of project indepth a multidisciplinary study of the structure of the core of the tibetan plateau short seismic reflection sections from four locations spanning the jurassic banggong nujiang suture show several features thin 2 km sedimentary cover sequences an unreflective upper crust down to approximately 25 km depth strongly reflective lower crust down to 65 km depth 22 s traveltime and a distinct change in seismic character from layered reflectivity to unreflective that we interpret as marking the reflection moho these profiles are unlike those in parts of the yadong gulu rift to the south where few deep reflections exist and reflection profiles are dominated by highamplitude reflection bright spots at 6 s which have been interpreted as fluid bodies either water or magma analyses of seismic amplitude decay on individual shot records confirm sourcegenerated energy at traveltimes beyond 22 s thus indicating that the change in reflection character at the moho is not the result of a loss of signal penetration the depths to the top and bottom of the lower crustal reflectivity match both a mid crustal velocity increase and the moho determined for this area from refraction profiling lower crustal reflectivity is consistent with current ductile deformation as suggested by models for middle and lower crustal flow in tibet however cretaceous age magmatic intrusions may explain at least part of the observed reflectivity

metamorphic rocks in central tibet lateral variations and implications for crustal structure insights about lateral variations in the age composition and structure of the central tibetan crust are provided by geologic investigations of metamorphic rocks in the qiangtang terrane previous studies have shown that a tectonic melange of triassic age with blueschist and eclogite bearing blocks within a greenschist facies matrix is exposed over an e w distance of 600 km in the central qiangtang terrane new mapping shows that the melange extends over a n s distance of 150 km nearly to the trace of the early mesozoic jinsha suture in the north the melange exposed structurally beneath upper paleozoic to mesozoic strata in the footwalls of early mesozoic normal faults is composed mostly of paleozoic metasedimentary and crystalline rocks these findings support the hypothesis that a large part of the central and northern qiangtang terrane crust is composed of supracrustal rocks the duguer range

garnet biotite geothermometry revised new margules parameters and a natural specimen data set from maine the garnet biotite geothermometer has been recalibrated using recently obtained margules parameters for iron magnesium calcium garnet mn interactions in garnet and al interactions in biotite as well as the fe oxidation state of both minerals fe mg and dwal margules parameters for biotite have been retrieved by combining experimental results on 6 al free and 6 al bearing biotite using statistical methods margules parameters per mole of biotite are bt w 5 40 719 2 30t j mole mgfe

rapid eocene erosion sedimentation and burial in the eastern himalayan syntaxis and its geodynamic significance article i nfo the lower bomi group of the eastern himalayan syntaxis comprises a lithological package of sedimentary and igneous rocks that have been metamorphosed to upper amphibolite facies conditions the lower bomi group is bounded to the south by the indus yarlung suture and to the north by unmetamorphosed paleozoic sediments of the lhasa terrane we report u pb zircon dating geochemistry and petrography of gneiss migmatite mica schist and marble from the lower bomi group and explore their geological implications for the tectonic evolution of the eastern himalaya zircons from the lower bomi group are composite the inherited magmatic zircon cores display 206 pb 238 u ages from 74 ma to 41 5 ma indicating a probable source from the gangdese magmatic arc the metamorphic overgrowth zircons yielded 206 pb 238 u ages rang ing from 38 ma to 23 ma that overlap the anatexis time 37 ma recorded in the leucosome of the migmatites our data indicate that the lower bomi group do not represent precambrian basement of the lhasa terrane instead the lower bomi group may represent sedimentary and igneous rocks of the residual forearc basin similar to the tsojiangding group in the xigaze area derived from denudation of the hanging wall rocks during the india asia continental collision we propose that following the indian asian collision the forearc basin was subducted together with himalayan lithologies from the indian continental slab the minimum age of detrital magmatic zircons from the supracrustal rocks is 41 5 ma and their metamorphism had happened at 37 ma the short time interval b5 ma suggests that the tectonic processes associated with the eastern himalayan syntaxis encompassing uplift and erosion of the gangdese terrane followed by deposition imbrication and subduction of the forearc basin were extremely rapid during the late eocene 2012 international association for gondwana research published by elsevier b v all rights reserved

cenozoic volcanism and tectonic evolution of the tibetan plateau abstract cenozoic volcanism on the tibetan plateau which shows systematic variations in space and time is the volcanic response to the india asia continental collision the volcanism gradually changed from na rich k rich to potassic ultrapotassic adakitic compositions along with the india asia collision shifting from contact collision i e soft collision or syn collision to all sided collision i e hard collision the sodium rich and potasium rich lavas with ages of 65 40 ma distribute mainly in the lhasa terrane of southern tibet and subordinately in the qiangtang terrane of central tibet the widespread potassic ultrapotassic lavas and subordinate adakites were generated from 45 to 26 ma in the qiangtang terrane of central tibet subsequent post collisional volcanism migrated southwards producing ultrapotassic and adakitic lavas coevally between 26 and 8 ma in the lhasa terrane then potassic and minor adakitic volcanism was renewed to the north and has become extensive and semicontinuous since 20 ma in the western qiangtang and songpan ganze terranes such spatial temporal variations provide important constraints on the geodynamic processes that evolved at depth to form the tibetan plateau these processes involve roll back and break off of the subducted neo tethyan slab followed by removal of the thickened lhasa lithospheric root and consequently northward underthrusting of the indian lithosphere the tibetan plateau is suggested to have risen diachronously from south to north whereas the southern part of the plateau may have been created and maintained since the late oligocene the northern plateau would have not attained its present day elevation and size until the mid miocene when the lower part of the western qiangtang and songpan ganze lithospheres began to founder and detach owing to the persistently northward push of the underthrust indian lithosphere

nyainqentanglha shan a window into the tectonic thermal and geochemical evolution of the lhasa block southern tibet 1 in the nyainqentanglha nqtl massif southern tibet a late cenozoic se dipping normal fault exhumed an oblique section of crust in its footwall u th pb dating of zircon and monazite from footwall exposures reveals a collage of felsic intrusions including cretaceous early tertiary and miocene granitoids ages of the latter span 10 m y suggesting semicontinuous or episodic miocene magmatism geochemical and isotopic analyses show a gangdese arc affinity indicating significant mantle heat and mass transfer in their formation and semicontinuous calc alkaline magmatism throughout the cenozoic indo asian collision the undeformed nature of the footwall cretaceous and miocene granitoids suggests that mesozoic cenozoic lhasa block deformation was thin skinned being concentrated in supracrustal assemblages this coupled with the lack of migmatites exposed in the nqtl implies the exposed crust was not a partial melt zone nor involved in large scale channel flow some 40ar 39ar thermochronologic studies of footwall k feldspars reveal that samples collected within several kilometers below the normal fault cooled prior to emplacement of young leucogranites indicating little perturbation of the background thermal structure since 15 ma this plus high melting temperatures and the lack of penetrative granitoid deformation requires that the melts formed at lower crustal levels and were emplaced rapidly to the midcrust seismic reflection results showing high bright spot anomalies in the midcrust along the nqtl rift may have imaged the youngest magmatic episode or its associated hydrothermal system

xiongcun tibet a telescoped system of veinlet disseminated cu au mineralization and late vein style au ag polymetallic mineralization in a continental collision zone abstract the xiongcun cu au deposit occurs in the western part of the gangdese porphyry and skarn cu au mineralization belt in the himalayan tibetan orogen and was formed in an intracontinental orogenic environment related to the india asia collision sericite 40 ar 39 ar dating of the xiongcun deposit indicates that ore formation occurred around 38 11 0 90 ma much earlier than the main ore forming ages 20 to 14 ma of the gangdese cu au metallogenic belt the potassium feldspar k ar age of the sulfide bearing pegmatoid dyke in the xiongcun district is 47 62 0 70 ma these ages indicate that the mineralization in xiongcun is related to relaxation ca 52 to 40 ma and subsequent uplift 40 to 38 ma of the orogen in the xiongcun deposit mineralization hosted by jurassic pyroclastic rocks and controlled by nw striking faults can be divided into two styles i e veinlet disseminated cu au mineralization and vein style au ag polymetallic mineralization the former is associated with local remnant k silicate alteration extensive phyllic alteration and peripheral widespread propylitization the latter overprinting the former is associated with strong silicification chloritization and kaolinization the early veinlet disseminated cu au mineralization is similar to porphyry type mineralization and has a mineral assemblage of quartz sericite pyrite chalcopyrite bornite magnetite pyrrhotite sphalerite rutile anhydrite the late vein style au ag polymetallic mineralization characterized by a mineral assemblage of quartz pyrite pyrrhotite sphalerite rich in fe chalcopyrite galena native gold electrum chlorite kaolinite barite calcite siderite hematite belongs to a sulfide rich low sulfidation type epithermal system the h o and s isotopic compositions and high salinities of fluid inclusions indicate a magmatic contribution and increasing contents of meteoric water in the mineralized system from early to late stage the wide range of homogenization temperatures 380 to 120 c homogenization pressures 45 92 to 1 94 bar and salinities 36 61 to 1 23 wt nacl equiv suggest rapid uplift and are consistent with the geological setting because of uplift the hydrothermal system of xiongcun evolved from a porphyry system to an epithermal system resulting in an immature porphyry system and the telescoping of late vein style au ag polymetallic mineralization onto the early veinlet disseminated cu au mineralization

preliminary conclusions of the royal society and academia sinica 1985 geotraverse of tibet the 1985 chinese british expedition to the tibetan plateau attempted to solve the question of the origin of the very thick crustal rocks in this region continuing northwards movement of the indian plate over the past 38 myr has given rise to severe folding and thrust faulting causing crustal thickening by internal deformation previous collisions of microplate terranes derived from gondwanaland occurred during mesozoic times but the kun lun terrane of northern tibet was already part of laurasia by the carboniferous

mesozoic crustal evolution of southern tibet constraints from the early jurassic igneous rocks in the central lhasa terrane abstract phanerozoic growth of continental crust on our planet is one of the important research themes in earth science here we present the results of a systematic study of newly found and previously reported mesozoic igneous rocks including diorite cumulate granodiorite cumulate mafic magmatic enclaves mme and host granitoids in the central lhasa terrane southern tibet these igneous rocks give zircon u pb crystallization ages of 199 189 ma based on constituent mineral and bulk rock compositions the cumulates are best understood as resulting from amphibole plagioclase and titanite crystallization from a mafic andesitic magma the host granitoids also show compositional systematics consistent with amphibole plagioclase fractional crystallization from andesitic magma the mmes share many characteristics with their host granitoids in common including identical crystallization age similar mineralogy mineral chemistry and zircon isotopic compositions representing earlier cumulate derived from the same magmatic system as their host rocks the magma parental to the studied early jurassic igneous rocks is best explained as resulting from partial melting of hydrated ocean crust together with varying continental material the increasing zircon ehf t values of multiple plutons in the central lhasa terrane with time during 215 170 ma indicate its gradual increase in mantle contribution we present the early mesozoic crustal evolution in the lhasa terrane

geophysical constraints on the link between cratonization and orogeny evidence from the tibetan plateau and the north china craton abstract understanding the geodynamic process of orogeny and cratonization and their transition is among the key topics of research in evaluating the link between plate tectonics and continental dynamics the tibetan plateau and the north china craton ncc two key areas in mainland china offer excellent laboratories to understand continental tectonics over a broad span of earth history particularly the deep structure of the lithosphere as imaged from geophysical data on the tibetan plateau and the ncc provide important clues in understanding orogeny and cratonization the tibetan plateau is the largest and highest plateau on earth in terms of mean altitude and is an important region for understanding the mechanisms of continent continent collision and cenozoic plateau uplift the ncc is an archean craton that underwent lithospheric disruption during the mesozoic here we reconstruct the main features of the structure of the crust and upper mantle from surface wave tomography and gravity modeling in tibet and its neighboring regions in order to understand the modality of the convergence and collision process between the indian and eurasian plates and the influence of this process on the uplift of the plateau in the ncc geological geochemical geophysical and tectonic investigations demonstrate that lithospheric destruction mainly occurred in the eastern block the crustal structure of the ncc is reconstructed from ambient noise surface wave tomography and the different possible disruption mechanisms are evaluated the vs shear wave velocity tomography results and the density structure of the crust and upper mantle to about 350 km depth demonstrate the lateral variation of the thickness of the metasomatic lid between the south and north of the bangong nujiang suture bns and the west and east of tibet which suggest that the leading edge of the subducting indian slab reaches the bns the subduction angle of indian plate indicates a transition from steep to shallow from the west to east tibet sections depicting the gravitational potential energy suggest that mantle flow contributes to the subduction of the indian plate as far as the bns and the transition from the asthenospheric layer to the metasomatic lid overlaps with the transition from north south shortening in south tibet to eastward tectonic escape in north tibet qiangtang and songpan ganzi blocks both vs and models suggest the following 1 north southward lower crust flow beneath the eastern ncc and interaction between the westward mantle flow and eastward escape flow beneath the central ncc in addition to the earlier proposed mechanisms of delamination and thermal erosion played important roles in the lithospheric disruption of the archean craton 2 mantle flow plays an important role in the continental tectonic transition between neighboring tectonic blocks and within the cycle between orogeny and cratonization

crustal deformation of the lhasa terrane tibet plateau from project indepth deep seismic reflection profiles international deep profiling of tibet and the himalaya indepth deep reflection data in the yangbajain damxung graben of southern tibet yield evidence of magmatism and deformation beneath the southern lhasa terrane and yarlung zangbo suture shallow reflections and low velocity first arrivals indicate a thin quaternary graben fill of generally less than a few hundred meters thickness underlying stratified reflections extending to a maximum of about 11 5 km depth likely originate from deformed paleozoic cenozoic supracrustal strata of the lhasa terrane a prominent undulatory band of reflections within the crystalline basement extends beneath the length of the yangbajain damxung graben depth ranges from 12 km to 18 km this horizon has been interpreted to mark the top of a midcrustal partial melt zone underlying southern tibet the undulatory character of this horizon additionally suggests that it may be tectonically deformed a somewhat deeper subhorizontal wide angle reflection extends southward beneath the outcrop of the yarlung zangbo suture indicating that the suture is cut off or superposed by a younger structure at depth three gently north dipping reflections at 19 km 24 km and 27 km depth beneath the gangdese batholith are suggestive of a midcrustal duplex and may mark the northward downdip extension of the late oligocene early miocene gangdese thrust system a prominent 40 north dipping reflection imaged in the deep crust beneath the gangdese batholith between 40 and 60 km depth might mark the downdip expression of the yarlung zangbo suture or alternatively a younger reverse fault in the lower crust or both viewed in aggregate the reflection data are suggestive of moderate postcollisional shortening of the upper crust of the lhasa terrane accompanied by melting of the middle crust although the data are permissive of wholesale underthrusting or fluid injection of indian continental crust beneath the lhasa terrane they show no direct evidence for this having occurred

marine electromagnetic methods a new tool for offshore exploration electromagnetic survey methods used for decades mainly for mining exploration on land have burst upon the offshore hydrocarbon exploration scene over the past five years this move driven by the costs and challenges associated with the deepwater exploration environment has left many companies scrambling to understand new and unfamiliar technologies and has resulted in a sudden expansion of the portfolios of the few nonseismic geophysicists left in the oil industry of the two main methods magnetotelluric mt surveys are at least somewhat familiar from their use on land as a reconnaissance tool and for problem solving in areas of poor seismic performance and to a large extent can be applied to the marine environment in a similar way to their terrestrial counterparts the other dominant technology controlled source electromagnetic csem sounding has featured much less in land exploration but more importantly behaves so very differently in the deepwater marine environment that it might as well

triple oxygen isotope composition of dissolved o2 in the equatorial pacific a tracer of mixing production and respiration 1 as a contribution to the study of equatorial pacific biogeochemistry we measured the o2 ar ratio and the triple isotope composition 18o 17o and 16o of o2 along six meridional lines in the equatorial pacific 8 n 8 s at 95 w 110 w 125 w 140 w 155 w and 170 w o2 ar ratios and 18o were close to equilibrium values within the mixed layer and followed the general trend of increasing 18o with decreasing o2 ar at greater depths the 17 17o 0 5 18o constrains the fraction of photosynthetic o2 17 was slightly elevated with respect to equilibrium within the mixed layer due to local photosynthetic production in aphotic zone waters above 250 m depth the average 17 values were higher than in the mixed layer there are four sources of this photosynthetic signal in the dark ocean production in the euphotic zone prior to subduction in the distant source regions production below the mixed layer during travel to the equatorial zone diapycnal mixing with shallower waters bearing photosynthetic o2 and accumulation of photosynthetic o2 produced at very low rates below the 1 light level our results also constrain biological production rates within the mixed layer at several locations along 95 w and 110 w our average rate of 14c production 53 34 mmol c m 2 d 1 agreed well with other estimates in the equatorial pacific while our average rate of net c production 6 9 6 2 mmol c m 2 d 1 and f ratio 0 12 0 11 were somewhat lower than other estimates adding 18o and 17 as tracers to three dimensional biogeochemical ocean gcms and comparing results with observations will extend our understanding of metabolic rates in the study region

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

cretaceous plutonism in central tibet an example of post collision magmatism the magmatic province of the northern lhasa terrane includes an early cretaceous 120 130 ma plutonic event and a late cretaceous 80 110 ma volcanic event the plutonic association constitutes an older suite of granodiorites monzogranites and tonalites and a younger peraluminous leucogranite facies plutonism occurred about 20 ma after obduction of the banggong ophiolite following closure between the lhasa and qiantang terranes the earlier suite is of broadly calc alkaline in composition but differs from arc related magmas in that only more evolved compositions are represented sio2 58 and rb zr ratios are elevated relative to the gangdese batholith to the south trace element and isotopic constraints are consistent with derivation from a late proterozoic amphibole bearing crustal source requiring temperatures 950 c during anatexis the leucogranites require a pelitic source which is tentatively identified as the nyaingentanglha basement exposed south of the plutonic province unlike the high himalaya leucogranites trace elements and field relations require a high degree of melting at source 50 suggesting fluid absent melting at temperatures 850 c such high crustal temperatures indicate convective heat transfer from the mantle thermal constraints together with a tectonic setting of post emplacement uplift followed by a marine transgression in the northern lhasa terrane can not be reconciled with a model of tectonically thickened crust but are consistent with post collision attenuation of the lithosphere

late jurassic early cretaceous episodic development of the bangong meso tethyan subduction evidence from elemental and sr nd isotopic geochemistry of arc magmatic rocks gaize region central tibet china abstract the bangong meso tethys plays a critical role in the development of the tethyan realm and the initial elevation of the tibetan plateau however its precise subduction polarity and history still remain unclear in this study we synthesize a report for the late jurassic early cretaceous two phase magmatic rocks in the gaize region at the southern margin of the qiangtang block located in central tibet these rocks formed during the late jurassic earliest cretaceous 161 142 ma and early cretaceous 128 106 ma peaking at 146 ma and 118 ma respectively the presence of inherited zircons indicates that an archean component exists in sediments in the shallow qiangtang crust and has a complex tectonomagmatic history geochemical and sr nd isotopic data show that the two phase magmatic rocks exhibit characteristics of arc magmatism which are rich in large ion incompatible elements liies but are strongly depleted in high field strength elements hfses the late jurassic earliest cretaceous magmatic rocks mixed and mingled among mantle derived mafic magmas subduction related sediments or crustally derived felsic melts and fluids formed by a northward and steep subduction of the bangong meso tethys ocean crust the magmatic gap at 142 128 ma marks a flat subduction of the meso tethys the early cretaceous magmatism experienced a magma mash melting assimilation storage and homogenization process among mantle derived mafic magmas or crustally derived felsic melts and fluids as a result of the meso tethys oceanic slab roll back which triggered simultaneous back arc rifting along the southern qiangtang block margin

synthetic biotite oxidation under hydrothermal conditions the oxidation of synthetic ferruginous biotite in hydrothermal systems was studied for two ideal compositions annite kfe3a1si30iq 0h and siderophyllite kfe2 5a12si2 soiq 0h 2 these biotite samples were annealed in shaw type vessels under controlledfh2at a total pressure of 1 kbar the ratio of fe2 to total fe determined by wet chemical analysis is directly proportional to the fh2 regardless of temperature variations for fh2 values of i bar to 102 bars this ratio varies from 0 72 to 0 90 for annite and from 0 76 to 0 96 for siderophyllite the h contents of the biotite do not vary with feh content for either composition and result in 94 and 100 of the oh sites being occupied in annite and siderophyllite respectively a consistent decrease in total octahedral cation content with increasing feh content for both annite and siderophyllite suggests that the fe vacancy substitution 3fe2 2feh 6jvacplays a dominant role in the oxidation of these synthetic biotite samples

metamorphic rocks of the 1985 tibet geotraverse lhasa to golmud two examples of uplifted basement have been studied in the lhasa terrane of the tibetan plateau the nyainqentanglha orthogneisses are bounded by staurolite garnet schists to the north which record prograde metamorphism at 5 0 1 3 kbar 610 70 degrees c garnet sillimanite xenoliths within the orthogneiss suggest that peak temperatures reached at least 700 70 degrees c at 5 1 2 5 kbar these p t fields reflect high t low p metamorphism during eocene subduction and indicate that the syntectonic nyainqentanglha orthogneiss was emplaced at depths greater than 10 km sillimanite bearing assemblages from the amdo gneisses in the northern lhasa terrane provide evidence of crustal anatexis at temperatures 680 degrees c this event is poorly constrained in time but is probably cambrian or earlier within the kunlun terrane biotite and garnet isograds north of the xidatan fault indicate an increase in metamorphic grade from north to south reaching peak metamorphism at 470 30 degrees c 4 3 1 5 kbar synchronous with the emplacement of the triassic batholith regional metamorphism was followed by uplift of at least 2 km before emplacement of post tectonic early jurassic granites

correction of seafloor magnetotelluric data for topographic effects during inversion 1 the large contrast in electrical conductivity between seawater and the underlying seafloor accumulates boundary electric charges which can severely distort observed electric and magnetic fields for marine magnetotelluric mt studies correcting this topographic effect is critical to obtaining accurate conductivity models for the mantle previously correction for topography was based on the thin sheet approximation which breaks down at periods under 1000 s in the deep ocean this paper introduces an analysis method for seafloor mt data which combines removal of three dimensional 3 d topographic effects with inversion of the data for 2 d structure the observed mt impedance is first corrected to a flat lying seafloor datum using the observed bathymetry without invoking the thin sheet approximation the corrected mt response is then inverted in a flat seafloor model space because of coupling between topographic effects and deeper structure the correction and inversion steps are iterated until changes in each become small the procedure is verified using synthetic and real data tests for synthetic 3 d topography over a half space show that the method closely recovers the true half space model after a few iterations the procedure is also applied to real data collected in the mantle electromagnetic and tomography melt experiment on the east pacific rise at 17 s

forgotten major elements hydrogen and oxygen variation in biotite from metapelites variations in the h sup and o sup 2 minus content of minerals have been virtually ignored since the advent of the electron microprobe in the mid 1960s such studies do not serve as a basis for consideration of the variation of other major constituents such as fe sup 3 h sup and o sup 2 minus the authors report results of a comprehensive study of all major element compositional variation in biotite using samples from a suite of metapelites ranging from lower garnet to sillimanite k feldspar grades the data show that fe sup 3 remains constant within analytical error while percentages by weight of h sub 2 o and o decrease and increase respectively with increasing metamorphic grade h sup and o sup 2 minus contents of biotite appear to be highly sensitive to temperature and pressure samples from low metamorphic grades contain bulk h sup contents 4 h sup formula unit excess h sup probably occupies vacancies in the interlayer sites as either h sub 2 o or h sub 3 o these results will have an impact on geoscience disciplines that make assumptions about stoichiometry of hydrous phases for example the temperature range of reactions involving the breakdown of hydrous phases with more or less h sup than predicted will change more in some cases dramatically and any calculations involving water activities must be modified to consider h sup variation partitioning of h sup among coexisting hydrous phases delta d studies may be affected by both amount of h sup and its location and bonding interlayer h sub 2 o or h sub 3 o vs oh sup minus ar diffusivities calculated for sup 40 ar sup 39 ar work will be influenced by the h sup occupancy in the interlayer sites cation substitution models for h sup bearing phases need to consider exchanges involving both h sup and o sup 2 minus less

mapping offshore sedimentary structure using electromagnetic methods and terrain effects in marine magnetotelluric data summary marine magnetotelluric mt and marine controlled source electromagnetic csem soundings can be used to study sedimentary structure offshore in an example of this application we collected mt and csem data in the 1 km deep water of the san diego trough california the trough is a pull apart basin and part of the complex pacific north american tectonic plate boundary and is flanked by the thirtymile bank to the west and the coronado bank to the east our mt data are highly distorted by seafloor topography and the coast effect which is largely 2 d and can be modelled using 2 d finite element codes the distortion includes a strong several orders of magnitude static depression of tm mode resistivities electric field perpendicular to structure upward cusps in the te mode resistivities electric field parallel to structure and negative te mode phases the depressed tm mode resistivity is a well known consequence of galvanic interruption of coastperpendicular electric fields the te mode distortion is an inductive effect associated with currents flowing along the edge of the deep ocean basins steepening the magnetic field and even causing a phase reversal in the horizontal field used for mt impedance calculations and thus generating negative phases the land side enhanced vertical magnetic field is well known as the geomagnetic coast effect but the ocean side consequences have been less well documented although the mt data are dominated by coast effect and topographic distortion inclusion of accurate bathymetry in the inversion model s finite element mesh allows the subseafloor geological structure to be recovered this shows the trough sediments to be about 3 km thick bounded to the west by resistive basement but to the east by conductive clastic sediments forming coronado bank amplitudes and phases of five frequencies of csem data from 0 1 to 1 0 hz collected along the axis of the trough are well fit with a simple 1 d layered model indicating that sediment resistivities increase with depth from 1 5 to 2 3 m and are no more than 3300 m thick thinning to the north in good agreement with the mt model an existing density model generated by fitting surface and deep towed gravity is in good agreement with the em interpretations in particular combining sediment densities and csem resistivities allows us to estimate pore water conductivity and temperature which follows a geothermal gradient of 25 4 8kk m 1

reworked precambrian metamorphic basement of the lhasa terrane southern tibet zircon titanite u pb geochronology hf isotope and geochemistry abstract due to the paucity of exposure the formation and evolution of the precambrian basement of the lhasa terrane remain poorly known here we report zircon and titanite in situ u pb ages bulk rock geochemical and zircon hf isotopic data on the orthogneisses from the dongjiu area of the southern lhasa subterrane slt southern tibet geochemical data suggest that the protoliths of the biotite amphibole gneiss and biotite gneiss are granodiorite and granite respectively inherited magmatic zircon cores from these orthogneisses give protolith crystalline ages of 1520 1506 ma whereas the overgrown zircon rims give metamorphic ages of 605 590 ma the mesoproterozoic granitic rocks have bulk rock end t values of 3 6 to 0 1 and zircon core ehf t values of 4 5 to 2 6 which give similar tdm2 ages of 2 35 2 05 ga and 2 54 2 10 ga respectively suggesting their derivation from partial melting of paleoproterozoic crustal material the granitic rocks are also local provenance for the mesoproterozoic detrital zircons in the paleozoic strata in the lhasa terrane titanite in situ u pb ages further indicate that the dongjiu orthogneiss experienced more recent metamorphism at 26 ma the mineral assemblage and thermobarometry calculations indicate that the oligocene metamorphism occurred under medium pressure mp amphibolite facies conditions 5 4 7 2 kbar 691 765 c we propose that the dongjiu gneisses represent the precambrian metamorphic basement of the lhasa terrane but have been intensively reworked by metamorphism in the slt in response to the continued india asia convergence since the collision

early mesozoic deep crust reworking beneath the central lhasa terrane south tibet evidence from intermediate gneiss xenoliths in granites abstract understanding the rheological behavior of the tibetan plateau and its response to geodynamic processes requires a clear knowledge of the composition evolution and lithological properties of the deep crust here we present u pb hf isotopes of zircons bulk rock geochemistry and mineral compositions for seven intermediate gneiss xenoliths and their host early mesozoic granites 205 6 ma in the central lhasa terrane to probe the deep crust beneath southern tibet the xenoliths contain plagioclase amphibole titanite allanite quartz biotite and muscovite with accessory fe ti oxides apatite and zircon bulk rock and mineral geochemistry suggests that these xenoliths have a magmatic origin and experienced deformation and amphibolite facies metamorphism equilibration at pressures of 0 46 0 83 gpa and temperatures of 650 c before they were captured by the host granite at 205 ma zircons in these xenoliths show complex microstructures including inherited cores magmatic or metamorphic bands and high u th hydrothermal rims inherited zircon cores record u pb ages from 2277 ma to 517 ma igneous zircons show a range of concordant u pb ages suggesting a protracted magmatism from 236 ma to 203 ma metamorphic zircon zones record the timing of amphibolite facies metamorphism from 224 to 192 ma while the high u th hydrothermal rims show a subsequent fluid activity until 150 ma unradiogenic hf isotopic compositions of both xenoliths and host granites xenolith e hf t 11 2 to 0 host granite e hf t 17 3 to 3 3 indicate that the early mesozoic deep crust in the central lhasa terrane originated mainly from ancient i e proterozoic crust with little or no interaction with juvenile magmas this study suggests a possible continental differentiation mechanism during crustal reworking progressive melting may initiate from the lower mafic crust at ca 236 ma and gradually migrate into the sediment rich upper crust until ca 203 ma the reworking results in the transition from small fluxes of intermediate magmas to voluminous peraluminous s type granite in a convergent depth

u pb zircon age geochemical and lu hf isotopic constraints of the southern gangma co basalts in the central qiangtang northern tibet abstract a recent study suggests that the central qiangtang is a key locality to investigate the evolution of the opening and closure of the paleo tethys ocean basalts are commonly associated with supercontinent fragmentation and they have the potential to indicate the tectonic environment into which they were erupted in this study laser ablation inductively coupled plasma mass spectrometry la icp ms u pb zircon dating whole rock geochemistry and zircon hf isotope analysis were used to study newly discovered basalts in the southern gangma co area of central qiangtang dating of magmatic zircons from three basalt samples indicates that the basalts erupted in the late devonian to early carboniferous early tournaisian at 360 350 ma the basalts are geochemically similar to within plate basalts and formed from a depleted mantle source we suggest that the basalts may have been a product of the breakup of the northern margin of gondwana in the late devonian to early carboniferous 360 350 ma which may be linked to the initial rifting and opening of the paleo tethys ocean in the qiangtang area

large scale basement involved landslides california continental borderland large seafloor relief shallow metamorphic basement and seismic activity in the california continental borderland combine to produce major submarine slides capable of generating local tsunamis the catalina schist basement complex of the borderland contains melange units faults between metamorphic facies and foliation derived from a history of deep subduction underthrusting and subsequent tectonic exhumation along regional low angle fault systems neogene volcanic and sedimentary rocks covered the exhumed metamorphic basement forming an interface with a strong contrast in material properties neogene transtensional block faulting formed steep escarpments that cut through and elevated the low angle detachment surface providing slip surfaces with free faces susceptible to failure along the rheological boundary two basement involved slope failure examples west of san diego are described including large block glides and progressive rotational slumps along the steep northeast facing thirtymile bank escarpment and along the southwest flank of fortymile bank

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

metagabbros of the gangdese arc root south tibet implications for the growth of continental crust abstract the gangdese batholith of the southern tibet is regarded as a principal component of the pre himalayan collision andean type continental margin however the gangdese arc root remains poorly understood because most of previous studies concentrated on granitoids from the middle and upper crust of the arc in this paper we report new petrochemical and geochronological characteristics of metagabbros occurred as a part of the late cretaceous lilong batholith in the eastern gangdese arc the gabbros with protolith ages of 82 95 ma underwent slightly late 68 77 ma granulite facies metamorphism and nearly coeval anatexis typical of lower arc crusts the metagabbros and diorites of the lilong batholith show geochemical features of juvenile crust in continental magmatic arcs and an original magmatic differentiation trend with gabbros formed as the cumulates whereas diorites as the crystalline products of evolved magmas the lilong batholith forms an arc crustal section with a possible thickness of 20 30 km and may have an overall andesitic composition providing a support for the andesite model of continental crust growth

himalayan metamorphism and deformations in the north himalayan belt southern tibet china abstract the kangmar massif which has been attributed to the north himalayan belt of two mica granites in southern tibet china is shown to be an orthogneiss in the core of a dome which displays an intermediate high pressure type metamorphism himalayan in age 40 to 20 ma and above the main central sheet this metamorphism took place during high shear strain deformation and it is not recorded to the west where north himalayan plutons have developed contact metamorphism

the tula uplift northwestern china evidence for regional tectonism of the northern tibetan plateau during late mesozoic early cenozoic time geologic mapping combined with petrographic and geochronologic studies in the tula uplift of western china provides insights into the tectonic evolution of the northern edge of the tibetan plateau the mesozoic and early cenozoic history of the area is preserved in the tula uplift which includes basin strata now exposed in a large syncline pre mesozoic metamorphic basement and cretaceous plutons petrographic analyses of upper jurassic through paleogene syntectonic sandstones show that rocks in the area were derived from lithologically diverse source terranes consisting of sedimentary metasedimentary and igneous rocks these relationships imply that uplift in the tula area began in late jurassic time and that uplift of the tibetan plateau s northern edge may have been initiated long before the early tertiary indiaasia collision continued orogenic activity in the tula area is recorded by intrusion of ca 74 ma granitoid bodies latest cretaceous to paleogene shortening uplift of precambrian basement rock the syntectonic nature of cretaceous and paleogene sandstones and folding of all the basin strata into a regional north vergent syncline the northern range bounding thrust of the tula uplift has been recently active suggesting that uplift and thickening continues in the northern tibetan plateau

activation of the nyainqentanghla shear zone implications for uplift of the southern tibetan plateau neogene extension of the tibetan plateau is manifested as a series of north south trending graben the most prominent of which is the yadong gulu rift the nyainqen tanghla shan a ne sw trending mountain range 100 km nw of lhasa bounds the western margin of the yangbajian graben the central segment of the yadong gulu rift the eastern edge of the nyainqentanghla massif is marked by a low angle 25 o detachment fault shear zone of amphibolite grade mylonites the noar 3 ar thermal history results from samples collected along two deeply incised valleys within the massif reveal that a rapid cooling event propagated from 8 ma in the core of the range to 4 ma within the high strain zone at the eastern boundary assuming that faulting initiated at high angle 60o thermal histories were fit to a numerical simulation of slip on a normal fault to yield estimates of both the age of fault initiation and the slip rate history the form of the isotopically derived thermal histories are similar to general form predicted by the thermal model and suggests that significant movement began at 8 1 ma in the southern valley goring la and proceeded at an average slip rate of 3 mm yr between 8 and 3 ma a more complex history is required to fit the data from the northern valley balum chun but the timing of initiation and average slip rate are similar to the goring la result numerical simulations in which the fault angle is varied indicate that the isotopically derived temperature histories are inconsistent with slip occurring at low angle 40o because the extension direction of the yangbajian graben is representative of most rifts on the southern tibetan plateau our data suggest that crustal thickness and elevation reached close to their present values by 8 1 ma a carbon isotopic shift in pedogenic carbonates from the siwalik formation at about 7 5 ma appears to reflect intensification of the asian monsoon and by inference that the plateau had attained an important threshold elevation by that time formation of a diffuse plate boundary in the indian oceanic lithosphere beginning at 7 5 8 0 ma is also consistent with this history we suggest that the plateau had attained a threshold area and elevation by 8 1 ma sufficient to trigger these three independent manifestations

tibetan basement rocks near amdo reveal missing mesozoic tectonism along the bangong suture central tibet the u pb and 4 0 ar 3 9 ar studies of a unique exposure of crystalline basement along the jurassic early cretaceous bangong suture of central tibet reveal previously unrecognized records of mesozoic metamorphism magmatism and exhumation the basement includes cambrian and older orthogneisses that underwent amphibolite facies metamorphism coeval with extensive granitoid emplacement at 185 170 ma the basement cooled to 300 c by 165 ma and was exhumed to upper crustal levels in the hanging wall of a south directed thrust system during early cretaceous time we attribute jurassic metamorphism and magmatism to the development of a continental arc during bangong ocean subduction and early cretaceous exhumation to northward continental underthrusting of the lhasa terrane beneath the qiangtang terrane we speculate that a jurassic arc extended regionally along the length of the bangong suture but in all other places in tibet has been buried either depositionally or structurally beneath supracrustal assemblages

gangdese retroarc thrust belt and foreland basin deposits in the damxung area southern tibet abstract geologic mapping and u pb detrital zircon geochronologic studies of meta sedimentary rocks in the damxung area 90 km north of lhasa of the southern lhasa terrane in tibet provide new insights into the history of deformation and clastic sedimentation prior to late cenozoic extension cretaceous nonmarine clastic rocks 10 km southeast of damxung are exposed as structural windows in the footwall of a thrust fault the damxung thrust that carries paleozoic strata in the hanging wall to the north of damxung in the southern part of the northern nyainqentanglha range nnqtl metaclastic rocks of previously inferred paleozoic age are shown to range in depositional age from late cretaceous to eocene the metaclastic rocks regionally dip southward and are interpreted to have been structurally buried in the footwall of the damxung thrust prior to being tectonized during late cenozoic transtension along the northern flank of the nnqtl lower eocene syncontractional redbeds were deposited in a triangle zone structural setting all detrital zircon samples of cretaceous eocene strata in the damxung area include early cretaceous grains that were likely sourced from the gangdese arc to the south we suggest that the that newly recognized late cretaceous to early eocene meta clastic deposits and thrust faults represent the frontal and youngest part of a northward directed and propagating gangdese retroarc thrust belt and foreland basin system that led to significant crustal thickening and elevation gain in southern tibet prior to india asian collision

characteristics and genesis of gangdese porphyry copper deposits in the southern tibetan plateau preliminary geochemical and geochronological results porphyry type cu mo au deposits have been discovered along the gangdese magmatic arc in the southern tibetan plateau extensive field investigations and systematic studies of geochemistry s pb isotopic tracing together with re os and 40ar 39ar isotopic dating indicate that the mineralisation of the copper belt is genetically related to emplacement of late orogenic granitic porphyries during the post collisional crustal relaxation period of the late himalayan epoch these porphyries are petrochemically k enriched and belong to shoshonitic to high k calc alkaline series they display enrichment of large ion lithophile elements lile rb k u th sr pb and depletion of high field strength elements hfse nb ta ti and the heavy rare earth elements hree and y without eu anomalies these characteristics demonstrate that subduction played a dominant role in their petrogenesis and residual garnet was left in the magma sources pb isotope data show a linear correlation in the plumbotectonic framework diagram ranging from orogenic pb in the eastern segment of the copper belt to mantle pb in the western segment these constitute a mixing line of the indian oceanic morb with indian oceanic sediments and suggest that the porphyry magmas were dominantly derived from partial melting of subducted oceanic crusts mixed with a minor quantity of sediments and mantle wedge components the gangdese porphyry copper polymetallic belt has alteration characteristics and zonation typical of porphyry type copper deposits which include potassic alteration k feldspathisation and biotitisation silicification sericitisation and propylitisation mineralisation mainly occurs in strongly altered granitic cataclasite at the exo contact with veinlet disseminated textures the porphyries themselves are weakly mineralised with disseminated pyrite and chalcopyrite the copper deposits contain simple ore mineral associations consisting of chalcopyrite pyrite bornite molybdenite sphalerite and oxidised minerals of malachite covellite and molybdite during supergene oxidation primary ores underwent secondary enrichment to form economic orebodies with cu grade ranging from 1 to 5 ore sulphides of the copper belt display s and pb isotopic compositions identical to the ore bearing porphyries their 34s values vary between 3 8 and 2 4 and are typical of mantle sulphur the 206pb 204pb 207pb 204pb and 208pb 204pb ratios vary in the ranges 18 106 to 18 752 15 501 to 15 638 and 37 394 to 39 058 respectively and yielded radiogenetic lead enriched signatures twelve molybdenite samples from the copper belt yielded isochron ages of 14 76 0 22 ma and 13 99 0 16 ma for the nanmu and chongjiang deposits and model ages of 13 5 to 13 6 ma for the lakang e deposit meanwhile 40ar 39ar isotopic dating of two biotite phenocrysts from the chongjiang and lakang e deposits give plateau ages of 13 5 1 0 ma and 13 42 0 10 ma respectively during the geodynamic evolution of the gangdese collision orogenic belt intrusion of the ore bearing porphyries took place just before the rapid uplift and e w extension of the southern plateau and the ore forming process may have occurred simultaneously with the uplift and extension 14 0 1 ma

inversion of magnetotelluric data for 2d structure with sharp resistivity contrasts we have developed a linearized algorithm to invert noisy 2 d magnetotelluric data for subsurface conductivity structures represented by smooth boundaries defining sharp resistivity contrasts we solve for both a fixed number of subsurface resistivities and for the boundary locations between adjacent units the boundary depths are forced to be discrete values defined by the mesh used in the forward modeling code the algorithm employs a lagrange multiplier approach in a manner similar to the widely used occam method the main difference is that we penalize variations in the boundary depths rather than in resistivity contrasts between a large number of adjacent blocks to reduce instabilities resulting from the breakdown of the linear approximation we allow an option to penalize contrasts in the resistivities of adjacent units we compare this boundary inversion method to the smooth occam inversion for two synthetic models one that includes a conductive wedge between two resistors and another that includes a resistive wedge between two conductors the two methods give good agreement for the conductive wedge but the solutions differ for the more poorly resolved resistive wedge with the boundary inversion method giving a more geologically realistic result application of the boundary inversion method to the resistive gemini subsalt petroleum prospect in the gulf of mexico indicates that the shape of this salt feature is accurately imaged by this method and that the method remains stable when applied to real data

structure and metamorphism of a tectonically thickened continental crust the yalu tsangpo suture zone tibet three tectonic units the active palaeomargin the palaeo oceanic domain and the passive palaeomargin are now juxtaposed and constitute the yalu tsangpo suture zone between india and eurasia in southern tibet we examine the deformation metamorphic and uplift histories of each of these units our observations are accounted for in terms of the processes whereby the crust was thickened to about 70 km we finally discuss the implications for ancient continental collision orogens

a new geobarometer for the assemblage biotite muscovite chlorite quartz the equilibrium thermodynamics of the reaction 4kmga1si40 o oh mg5a1 si30 o oh muscovlte chlorite muscovite biotite ka13si 0 oh 3kmg3aisi 0 0 oh 7si0 4h o quartz fluid can be formulated as a geobarometer thermo dynamic data for the mg a1 celadonite end member of muscovite is derived from the reversed mineral equilibria experiments of velde i 965 using the calculation method of powell 1978 giving afh 1298 5823 5 kj s 1298 0 3060kjk v 1298 13 8kjkbar

inversion of long offset tem soundings near the borehole munsterland 1 germany and comparison with mt measurements summary data from a long offset transient electromagnetic lotem survey in northwest germany around the borehole munsterland 1 are interpreted using 1 d inversion special emphasis is given to the fact that the best fitting model is not necessarily the most realistic one we therefore investigate different inversion algorithms in order to obtain a more geologically meaningful result the 1 d inversion results using a second order marquardt algorithm show a three layer structure with a conductor embedded between two more resistive layers this is in good agreement with the known lithology and the well logs profile inversion uses the result from one site as the starting model for the next the application of the technique yields a smoother section than the individual inversions and corresponds more closely to the presumably layered structure in the area if desired using a parameter weighting scheme makes it possible to force consistency at the cost of a larger misfit yielding a smooth section even in a part of the area where the data are influenced by a 3 d anomaly occam inversion is a smoothness constrained technique which is independent of the starting model the result of the occam inversion shows the same features as the marquardt inversion including effects which are attributed to 3 d distortions in the data joint inversions of the lotem data were carried out with magnetotelluric data from the same area in that case the lotem electric field is recognized as crucial for the resolution of the resistive layer in the section