kinematic model for the main central thrust in nepal we present a kinematic model for the himalayan thrust belt that satisfies structural and metamorphic data and explains recently reported late miocene pliocene geochronologic and thermochronologic ages from rocks in the main central thrust zone in central nepal at its current exposure level the main central thrust juxtaposes a hanging wall flat in greater himalayan rocks with a footwall flat in lesser himalayan rocks of the ramgarh thrust sheet which is the roof thrust of a large lesser himalayan duplex sequential emplacement of the main central early miocene and ramgarh middle miocene thrust sheets was followed by insertion of thrust sheets within the lesser himalayan duplex and folding of the main central and ramgarh thrusts during late miocene pliocene time thorium lead th pb ages of monazite inclusions in garnets from central nepal record the timing of coeval progressive metamorphism of lesser himalayan rocks in the footwall of the main central thrust although this model does not rule out minor late stage reactivation of the main central thrust major late miocene reactivation is not required

calibrating the end permian mass extinction the end permian mass extinction was the most severe biodiversity crisis in earth history to better constrain the timing and ultimately the causes of this event we collected a suite of geochronologic isotopic and biostratigraphic data on several well preserved sedimentary sections in south china high precision u pb dating reveals that the extinction peak occurred just before 252 28 0 08 million years ago after a decline of 2 per mil in 13c over 90 000 years and coincided with a 13c excursion of 5 that is estimated to have lasted 20 000 years the extinction interval was less than 200 000 years and synchronous in marine and terrestrial realms associated charcoal rich and soot bearing layers indicate widespread wildfires on land a massive release of thermogenic carbon dioxide and or methane may have caused the catastrophic extinction

climate warming in the latest permian and the permian triassic mass extinction high resolution oxygen isotope records document the timing and magnitude of global warming across the permian triassic p tr boundary oxygen isotope ratios measured on phosphate bound oxygen in conodont apatite from the meishan and shangsi sections south china decrease by 2 in the latest permian translating into low latitude surface water warming of 8 c the oxygen isotope shift coincides with the negative shift in carbon isotope ratios of carbonates suggesting that the addition of isotopically light carbon to the ocean atmosphere system by siberian traps volcanism and related processes resulted in higher greenhouse gas levels and global warming the major temperature rise started immediately before the main extinction phase with maximum and harmful temperatures documented in the latest permian meishan bed 27 the coincidence of climate warming and the main pulse of extinction suggest that global warming was one of the causes of the collapse of the marine and terrestrial ecosystems in addition very warm climate conditions in the early triassic may have played a major role in the delayed recovery in the aftermath of the permian triassic crisis

grain size effects on the closure temperature of white mica in a crustal scale extensional shear zone implications of in situ 40ar 39ar laser ablation of white mica for dating shearing and cooling tauern window eastern alps abstract in situ 40ar 39ar laser ablation dating of white mica grains was performed on samples from the footwall of a crustal scale extensional fault katschberg normal fault knf that accommodated eastward orogen parallel displacement of alpine orogenic crust in the eastern part of the tauern window this dating yields predominantly cooling ages ranging from 31 to 13 myr with most ages clustering between 21 and 17 myr folded white micas that predate the main katschberg foliation yield within error the same ages as white mica grains that overgrow this foliation however the absolute ages of both generations are older at the base 20 myr where their grain size is larger 300 500 m than at the top and adjacent to the hangingwall 17 myr of this shear zone where grain size is smaller

a model for the simulation of combined major and trace element liquid lines of descent abstract variations of major and trace element partition coefficients as functions of temperature and composition have severely limited the range of composition for quantitative modeling of the differentiation of natural magmatic systems application of a two lattice melt model greatly reduces compositional dependence for trace elements this allows the temperature dependence of isobaric trace element partitioning to be evaluated independently thus making it possible to model an expanded range of conditions and compositions the first goal of this investigation was to construct a model to simulate major and trace element systematics of natural differentiation processes by combining trace element partitioning with a major element phase equilibrium model nielsen 1985b the second goal was the application of this model to the simulation of magma chamber recharge assimilation eruption and fractional crystallization these simulations indicate that the proportions of minerals fractionating from an open system are not necessarily the same as those in a closed system recharge or assimilation will drive a magma composition off the cotectic along which it is evolving however fractionation will drive the liquid back towards the cotectic in a recharged magma chamber as the recharge flux increases the fractionating mineral proportions will be increasingly biased towards those phases which appear first from the primitive magma the bulk partition coefficient for an open system can be considerably different 5 10 or more for some elements compared to a closed system paired assimilation and fractional crystallization are often cited as the mechanisms responsible for the origin of high al basalts and andesites however the addition of an al rich assimilant into a fractionating basaltic magma will increase the proportion of plagioclase to the extent that al in the derived liquids will be effectively buffered or even decrease the assimilation of forsteritic olivine by reaction keleman and ghiorso 1986 increases the proportions of pyroxene and olivine thus reducing the fe enrichment and al depletion characteristic of low pressure basalt fractionation

metallogenesis in tibetan collisional orogenic belt i mineralization in main collisional orogenic setting metallogenesis of continents is an important research frontier in the present regional metallogeny and its theoretic framework might be built up by deepening the understanding of metallogenesis in the collisional orogenic belt however metallogenesis in the collisional orogenic belt is rather poorly understood in comparison with metallogenesis in the plate tectonic setting the himalayan tibetan orogenic belt which is characterized by the existence of large scale intense and young mineralizations as well as various kinds of large size and weakly reformed deposits is regarded as an ideal field for understanding the ore forming processes in the collisional orogenic belt based on three years systematical work on metallogeny in the tibetan collisional orogenic belt the authors have summarized the main ore forming events and their temporal spatial distribution analyzed four important metallogenic belts and representative deposits as well as their plutonic processes and tectonic control factors and on such a basis set up a metallogenic model in the main collisional orogenic setting the main collisional event lasted about 25 ma from 65 ma to 41 ma resulting in the formation of the principal part of the tibetan plateau orogenic belt characterized by the south tibet foreland thrust belt the gangdese main collisional tectonic magmatic belt and the north tibet fold thrust belt in company with the collisional orogenic event many important magmatic events were developed as evidenced by the existence of the crust derived muscovite granite potassic calc alkaline granite assemblage 66 50 ma the positive e nd granite gabbro assemblage 52 47 ma and the mantle derived basaltic subvolcanics diabase assemblage as well as widespread and thick linzizong volcanics 64 43 ma the development of above magmatic activities suggests that the deep part of tibetan plateau experienced in turn continental collision 65 52 ma breakoff of continental plate 52 42 ma and low angle underthrust of continental plate 40 ma at least four important ore forming events occurred during the main collisional orogenic epoch which included the sn and rare metal event related to crust derived granite forming the tengchong sn and rare metal ore concentration area in east tibetan plateau the copper gold molybdenum event related to crust mantle derived granite forming a cu au ore belt 100 km long in south gangdese the shear zone gold event related to collisional orogeny forming the potential au belt along the yarlung zangbo suture zone and the copper gold event related to crust uplift forming porphyry epithermal transitional type cu au deposits represented by the xiongcun cu au deposit

along strike magma mixing beneath mid ocean ridges effects on isotopic ratios abstract the effects of mixing processes on the isotopic variability of mid ocean ridge basalts are studied the processes considered are porous flow dispersion and convective mixing in magma chambers porous flow dispersion is capable of mixing magmas over distances of only a few tens of meters convective mixing on the other hand is found to produce mixing over scales of kilometers to hundreds of kilometers calculations of convective mixing are carried out for continuous magma chambers where mixing is limited by convective processes and for discontinuous chambers where mixing is limited by chamber size preliminary comparison of our calculations with observations along the mid ocean ridges shows that the calculations are consistent with the existence of a correlation between bathymetry and isotopic ratio at long but not at short wavelengths they are also capable of explaining a decrease in isotopic variability with increasing spreading rate

petrogenesis and assembly of the don manuel igneous complex miocene pliocene porphyry copper belt central chile the 4 0 3 6 ma don manuel igneous complex dmic central chile provides a window into igneous processes involved in magma genesis associated with porphyry style copper mineralization this study uses petrographic petrological geochemical and isotopic data to examine the evolution of magmas from the mid to lower crustal source region to shallow emplacement the data provide evidence for progressive oxidation of magma during differentiation and ascent fractionation of cl from s through degassing and the late stage near solidus removal of cl from the system magmas of basaltic andesite to rhyolite composition were produced by polybaric differentiation of hydrous parental mafic magmas variations in crustal differentiation depths led to variable suppression of plagioclase saturation that is recorded in distinctive strontium versus anorthite evolution patterns hydrous derivative magmas generated over a wide range of pressures were episodically emplaced into the shallow crust at depths between 3 5 and 5 km intermediate porphyry dikes closely associated with copper mineralization contain diverse crystal cargoes indicating significant magma mixing these crystal cargoes represent samples of crystal mush entrained from different depths as well as crystals originating in different magmas and crystals grown in situ from hybridized magmas mafic enclaves containing plagioclase and amphibole compositions that match those of the basaltic andesites occur within biotite tonalite testifying to magma mingling during ascent sulfur and chlorine contents of apatite within the different dmic units record variable degassing and decoupling of volatile components with sulfur showing variations of three orders of magnitude compared with one order of magnitude for chlorine the hypabyssal nature of the dmic affords a detailed integrated record of magmatic differentiation processes occurring within trans crustal magmatic systems of the sort thought to characterize many crustal arc settings and play a fundamental role in driving porphyry style copper mineralization

griesbachian earliest triassic palaeoenvironmental changes in the salt range pakistan and southeast china and their bearing on the permo triassic mass extinction abstract facies and faunal analysis from pakistan and china show that the permo triassic mass extinction of marine invertebrate faunas was associated with a spectacularly rapid griesbachian transgression which lead to the widespread establishment of deep water anoxic and dysoxic conditions the extinction event was thus caused by habitat loss due to the extensive development of inhospitable conditions the initial griesbachian transgression in pakistan produced extensive shallow normal marine conditions in which permian holdover taxa were able to survive until the development of dysaerobic facies in the late griesbachian the exceptionally complete sections of china show a three phased deepening and extinction event beginning in the latest permia by the late griesbachian a variety of dysaerobic and anaerobic facies were developed in all the regions studied several of these contain evidence for minimal sulphate reducing activity suggesting that marine productivity and thus organic matter flux to the sediments was very low in early triassic seas

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

a delayed end permian extinction in deep water locations and its relationship to temperature trends bianyang guizhou province south china new collecting at a biostratigraphically highly resolved deep water section in south china reveals a brief a few tens of thousands of years but measurable delay in extinction timing relative to contemporaneous shallower water sections foraminifers and conodonts in the bianyang section show a sharp extinction at the top of hindeodus changxingensis zone whilst in shallower sections such as the well known meishan gssp location these taxa have a major extinction pulse at the top of the preceding clarkina yini zone the main end permian extinction coincides with the onset of a negative carbon isotope excursion whilst the deep water extinctions occur around the lowpoint of the carbon isotope curve rapidly increasing surface water temperatures and the effects of deep water anoxia were all important during the crisis although it is noteworthy that regression and extinction also show a close temporal link

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

deep tow and sea beam studies of dueling propagating ridges on the east pacific rise near 20 40 s nontransform ridge axis discontinuities such as propagating rifts overlapping spreading centers oscs and saddle points occur frequently along the mid ocean ridge system we used sea beam and deep tow to study an example of a relatively large nontransform offset near 20 40 s on the east pacific rise epr the two spreading center tips are 15 km apart and separated by a diffuse shear zone a broad v shaped wake of disturbed topography and magnetics occurs on the flanks of the epr near the offset on a fine scale the offset is tectonically much more complex than either the typical osc or propagating rift structures for two reasons first it is a dueling propagator in which structures produced by successive short term episodes of northward and southward propagation have overprinted each other with migration toward the south prevailing over the long term second its offset distance is such that its tectonic behavior may be transitional from that of a propagating rift to that of an osc system the offset s rate of migration to the south has been 20 mm yr over the last 2 m y far less than total opening rate of 162 mm yr distinct propagation events may have been much more rapid however surging north and south at rates of 160 230 mm yr in the process several fossil spreading ridges 30 50 km long have been abandoned and rafted to the side we propose a fine scale tectonic evolution in which a propagating spreading center either curves inside or runs outside of a soon to be abandoned propagating rift tip we call this process self decapitation dueling propagation at 20 40 s may be caused by competing magmatic sources at shallow points along the epr one to the north near 20 s and one to the south near 21 30 s the 20 40 s dueling propagator occurs at a long wavelength low in the axial depth profile between these competing sources in general the division of spreading centers into segments separated by ridge axis discontinuities occurs at low points along the axis and may be caused by the pattern of upper mantle convection and the factors governing flow of magma along the strike of the ridge

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

abrupt warming in the latest permian detected using high resolution in situ oxygen isotopes of conodont apatite from abadeh central iran abstract climate warming probably as a result of massive degassing of greenhouse gases from the siberian traps magmatism has often been acclaimed as a major cause of the end permian mass extinction indeed several studies have documented a sudden rise in seawater temperatures during the latest permian earliest triassic as evidenced by oxygen isotopic records measured on conodont apatite however whether such a rapid increase in seawater temperatures occurred before during or after the mass extinction remains controversial moreover the pattern of this rise in seawater temperatures and its timing relative to the latest permian earliest triassic carbon cycle disruption mass extinction as well as the siberian traps magmatism still need to be rigorously examined in various regions in this study we present high resolution oxygen isotopic records of conodont apatite 18oapatite from the upper permian lowermost triassic interval at the abadeh section central iran that are analyzed with in situ secondary ion mass spectrometry sims method the 18oapatite results from abadeh demonstrate a clear pattern consisting of three phases 1 from the lower wuchiapingian clarkina dukouensis zone to the end permian mass extinction horizon 18oapatite values are relatively stable fluctuating in the range of 18 28 20 15 with an average of 19 44 2 18oapatite value remains high as 19 26 at the mass extinction horizon above this horizon a sudden decrease occurs in the clarkina hauschkei zone and reaches a low value of 17 05 close to the permian triassic boundary 3 in the lowermost triassic 18oapatite values maintain a low baseline in the range of 16 92 17 39 with an average of 17 11 overall the most dramatic change in 18oapatite values i e a decrease of 2 converting into an abrupt warming of 10 c occurred above the mass extinction horizon and below the permian triassic boundary at abadeh the abadeh 18oapatite record is consistent with previous results documented in south china iran and armenia in terms of the timing and magnitude of a substantial warming and therefore represents a global signature if applying the high precision temporal framework established in the well dated meishan gssp section to abadeh the abrupt warming of 10 c took only a maximum duration of 37 kyr thousand years by projecting the carbon cycle change temperature rise mass extinction at the abadeh and meishan sections and the temporal evolution of the siberian traps magmatism onto a unified timescale the temporal correlation strongly suggests that the switch from dominantly extrusive eruptions to widespread sill intrusions is probably the most annihilating phase of the siberian traps magmatism and is temporally consistent with the end permian mass extinction

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

permian triassic boundary ptb in the lower yangtze region southeastern china a new discovery of deep water archive based on organic carbon isotopic and u pb geochronological studies abstract the lower yangtze region of southeastern china is an important area for the study of permian triassic boundary ptb in this region many well known ptb sections e g meishan have been investigated widely and extensively however these sections are dominantly in shallow water settings and data from deep water areas are lacking to fill the gap we conducted a field survey recently and found a deep water ptb section with relatively continuous deposition using integrated organic carbon isotopic and u pb geochronological studies this well exposed and fresh section is located at niushan xuancheng city anhui province 90 km west of the well known meishan section which is the ptb global boundary stratotype section and point the organic carbon isotope curve of the section contains three positive excursions pcies and two negative excursions ncies which can be well correlated with other ptb sections as well as with wuchiapingian changhsingian boundary wcb sections worldwide they archive the original ptb and wcb signatures further u pb dating of magmatic zircons in volcanic ashes validates this new finding i e the ptb interval between 252 49 0 76 ma and 251 74 0 77 ma carbon isotopes of ptb sections in south china reflect the regional paleogeography the two ncies ncie 2 and ncie 3 across the ptb transition are most likely caused by widespread large scale volcanic activity in contrast the ncie 1 near the wcb is possibly related to an increase in the productivity of organic matter as a result of transgression and a change in the organic matter type from terrigenous to marine

geochemistry of tholeiites from lanai hawaii lanai is the third smallest of the fifteen principal subaerial shield volcanoes of the hawaiian hotspot this volcano apparently became extinct during the shield building stage of volcanism as shown by the absence of both alkalic cap and post erosional lavas major and trace element analyses of 22 new samples collected primarily from 3 stratigraphic sections show that lanai tholeiites span a large range in composition some lanai lavas are unique geochemically among hawaiian tholeiites in having the lowest abundances of incompatible trace elements of any hawaiian lavas and well developed positive eu anomalies the geochemical characteristics of these low abundance lanai tholeiites are not the result of alteration differences in mantle source modal mineralogy the presence of residual accessory mantle phases or fractional crystallization of such phases assimilation of depleted morb wall rock or accumulation resorption of phenocrysts or xenocrysts incompatible trace element ratios e g nb la nb th la th la hf ce pb in lanai tholeiites span considerable ranges and form coherent trends with each other and with absolute abundances of these elements large variations in la sm la yb and absolute ree abundances at constant mgo suggest that lanai tholeiites formed by variable amounts of partial melting however large ranges in incompatible element ratios cannot be explained solely by variations in partial melting of a geochemically homogeneous source but must reflect geochemical heterogeneities in the lanai source partial melting modeling indicates that the mixed lanai source is probably lree enriched i e la yb cn 1 one component in the lanai source exemplified by the low abundance tholeiites has markedly lower ree hfse th hfse alkali hfse and ce pb ratios than other lanai or hawaiian tholeiites and may indicate the presence of recycled residual subduction zone materials in the hawaiian plume source the positive eu anomalies that characterize the low abundance lanai tholeiites are not the result of plagioclase accumulation or assimilation but are a feature of this source component progressive temporal geochemical variations in lanai tholeiites from 2 stratigraphic sections indicate that the source composition of these lavas probably evolved over time this change could have resulted from a progressive decrease in the extent of partial melting of the lanai source the compositional variability of lanai tholeiites suggests that geochemical heterogeneities in their source are larger than the scale of partial melting lanai tholeiites could not have formed by smaller degrees of partial melting of plume material than did the larger volume hawaiian shields therefore volume differences between hawaiian shields must be controlled primarily by differences in the volume of supplied plume material rather than by differences in the degree of partial melting the premature cessation of eruptive activity at lanai may be attributed to relatively large degrees of partial melting of a small plume

geochronological and geochemical constraints on the origin of the yunzhug ophiolite in the shiquanhe yunzhug namu tso ophiolite belt lhasa terrane tibetan plateau abstract the formation of the shiquanhe yunzhug namu tso ophiolite melange zone snmz within the lhasa terrane tibetan plateau is key to understanding the mesozoic tectonic evolution of this terrane which remains controversial we show that the yunzhug ophiolite in the central segment of the snmz formed at 150 ma based on u pb dating of zircons from a gabbroic sample in a well developed sheeted dike complex geochemically these mafic rocks are dominated by e morb type compositions along with minor amounts of rocks with p morb type compositions the samples also exhibit high end t values and lack negative nb and ta anomalies data for all the samples plot within the morb array on a th yb nb yb diagram therefore these mafic rocks most likely formed in either a slow spreading oceanic setting or an embryonic ocean and not in a back arc basin as has been previously assumed taking into account the regional geology we propose that the yunzhug ophiolite is part of a distinct ophiolitic belt and represents material formed in an embryonic ocean within the lhasa terrane which provides new insights into the jurassic tectonic evolution of the lhasa terrane

paleomagnetic constraints on the paleolatitude of the lhasa block during the early cretaceous implications for the onset of india asia collision and latitudinal shortening estimates across tibet and stable asia abstract interbedded volcano sedimentary sequences are well exposed in the northern part of the lhasa block in southern tibet zircon u pb dating results from two samples indicate that the emplacement age of the duoni formation volcanic flows is 120 2 0 5 ma paleomagnetic results from 235 progressively demagnetized volcanic rock samples 25 sites and 41 sandstone samples 5 sites indicate that the dominant remanence carriers are ti poor titanomagnetite and ti poor titanohematite in the volcanic samples and ti rich titanomagnetite in the sandstone samples rock magnetic investigations systematic demagnetization behavior positive fold test results and direct petrographic identifications all indicate that the paleodirections recorded by the chemically stable magnetic particles are primary thermal remanent magnetization in the volcanic flows and primary detrital remanent magnetization in the sandstones the tilt corrected chrm mean direction is d i 356 4 16 4 with 95 6 3 n 19 corresponding to a paleopole position of p 66 9 n p 281 2 e with a95 6 1 combined with previously published results the geochronological dating and paleomagnetic analysis indicate a paleolatitude of 13 1 2 7 n for the southern margin of the lhasa block during the early cretaceous therefore the southernmost margin of the eurasian continent likely remained at the low middle paleolatitude of 13 1 2 7 n between the early cretaceous and the paleocene based on comparisons to results from the tethyan himalayan block and the reference poles from stable india and eurasia the low middle paleoposition of the lhasa block during the early cretaceous through early paleocene suggests that the initial contact between india and asia occurred at ca 59 3 ma under the assumption of a rigid eurasian plate this timing implies that a total collision related latitudinal convergence of 1450 400 km 13 1 3 7 n has been accommodated by folding thrust faulting normal faulting crustal thickening intracontinental subduction in tibet and central asia and southeastward continental extrusion of the indo china block from the eastern syntaxis between the lhasa block and stable asia

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

uppermost permian to lower triassic conodont successions from the enshi area western hubei province south china abstract we present recent study results of the lower triassic conodont succession from the ganxi and jianshi areas of enshi city western hubei province south china which were situated on a carbonate ramp at the northeastern margin of the upper yangtze platform therein the uppermost permian to lower triassic succession is well exposed and yielded abundant conodonts a total of nine conodont zones are differentiated 1 clarkina yini zone 2 clarkina meishanensis zone 3 hindeodus changxingensis zone 4 hindeodus parvus zone 5 hindeodus postparvus zone 6 clarkina planata zone 7 neoclarkina aff discreta zone 8 neospathodus dieneri zone and 9 novispathodus waageni zone the ns dieneri m1 ns dieneri m2 and ns dieneri m3 subzones have also been distinguished within the ns dieneri zone and the nv waageni eowaageni and nv waageni waageni subzones are recognizable within the nv waageni zone in jianshi most of the zones occur in succession but the c planata zone is a local ecozone reflecting deeper water biofacies during h postparvus zone time in the jianshi section the permian triassic boundary is placed at bed 136 2 based on the appearance of hindeodus parvus immediately above the disappearance of changhsingian taxa while the induan olenekian boundary iob is drawn at bed 225 40 cm based on the first appearance of nv waageni eowaageni within the upper range of the ns dieneri zone indices the enshi conodont zones overall correlate well with those recognized elsewhere in south china and worldwide

extreme isotopic homogeneity among basalts from the southern east pacific rise mantle or mixing effect basaltic lavas which have erupted along the great oceanic ridge system are characterized by their overall chemical and isotopic homogeneity nevertheless they show evidence for variability in the chemical and isotopic composition of the ocean ridge basalt orb source whose scale and distribution remain uncertain isotopic compositions of sr nd pb and hf and ratios of some highly incompatible elements have been used as tracers of orb source compositions usually permitting source characteristics to be distinguished from petrogenetic effects the mid atlantic ridge mar in particular shows correlations in the isotopic compositions of several elements between isotopic compositions and topography and between these parameters and incompatible element concentrations and ratios1 3 we report here isotopic measurements for basalts dredged between approximately 30 and 34 s on the east pacific rise and adjacent maddingly rise which show highly uniform sr and nd isotopic compositions possibly resulting in part from mixing associated with a high rate of magma generation and rapid spreading 170 mm yr 1 however a ridge segment of comparable length between 18 and 22 s and with only a slightly lower spreading rate shows much greater isotopic variability hence we are apparently sampling small but detectable regional differences in mantle composition the isotopic homogeneity among the basalts from 30 to 34 s implies that any heterogeneity in the underlying upper mantle is uniformly distributed on a scale much smaller than the volume sampled in individual magma batches and is thus averaged out in the resulting lavas

geochemical characteristics of harizha granite diorite porphyry in east kunlun and their geological implications the harizha cu mo poly metallic deposit is one of the typical large size porphyry deposits in the eastern kunlun metallogenic belt with the la icp ms u pb age of its mother rock granite diorite porphyry being 234 5 4 8 ma mswd 0 24 detailed petrographic and geochemical studies show that the granite diorite porphyry in harizha belongs to high k calc alkaline series rich in sio2 70 94 76 08 and k2o 4 91 6 07 and poor in cao 0 16 2 01 and na2o 0 24 0 78 the granite diorite porphyry is characterized by total content of ree varying greatly from 124 69 10 6 to 178 68 10 6 rich in lree and low hree with eu values from 0 26 0 30 trace elements show enrichment of lile such as rb u la and nd and depletion of hfse like ba sr nb and y granite diorite porphyry resulted from the syn collisional tectonic setting

controls on facies architecture of a large triassic carbonate platform the great bank of guizhou nanpanjiang basin south china abstract the great bank of guizhou gbg is an exceptionally well exposed isolated triassic platform in the nanpanjiang basin of south china the platform is exhumed with its depositional profile preserved and is dissected by a faulted syncline that exposes a complete and uncomplicated cross section providing a unique opportunity to evaluate mechanisms involved in its birth evolution and demise the gbg formed near the southern margin of the yangtze platform during a deepening event that expanded the nanpanjiang basin and drowned the region surrounding the gbg in the latest permian initial accumulation of the gbg began in the latest permian with small reef mounds and open marine skeletal packstones following the end permian extinction cyanobacterial boundstones grew over the bank top during the early triassic the gbg developed a low relief bank profile with mobile oolite shoals at the margin shallow subtidal and peritidal deposits in the interior and gentle slopes dominated by pelagics debris flow deposits and turbidites at the basin margin in the middle triassic anisian the gbg developed a progressively steepening profile rimmed with massive tubiphytes reefs the platform was flat topped with tidal flat deposits across the interior basin margin deposition was dominated by turbidites and debris flow deposits but eventually shifted to avalanche and rock fall deposits as the slopes steepened to the angle of repose in the middle triassic ladinian an erosional escarpment up to 1700 m high developed at the margin platform margin strata are bedded packstones similar to interior strata whereas breccias at the basin margin contain coral boundstone clasts suggesting erosion of reefs from the escarpment a restricted subtidal lagoon formed in the interior producing an atoll like morphology later a flat topped profile was restored as tidal flats spread across the interior in the beginning of the late triassic deepening contributed to termination of the gbg before siliciclastic turbidites and shales were deposited over the platform in contrast with the well known platforms of the dolomites of northern italy the gbg contains abundant muddy carbonates and a progressively steepening bank to reef rimmed and escarpment architecture the italian platforms contain little mud and have angle of repose pinnacle geometries the gbg s larger size increased mud production and protected it from extensive winnowing of mud which in turn resulted in off bank shedding of muddy sediments that were stable on relatively gentle basin margin slopes which progressively steepened and ultimately led to avalanche deposits and a high relief erosional escarpment in contrast the lesser mud content of the dolomites platforms forced avalanche and talus deposition to dominate throughout deposition of basin margins which in turn produced their angle of repose geometries

seismic structure of the southern east pacific rise seismic data from the ultrafast spreading 150 to 162 millimeters per year southern east pacific rise show that the rise axis is underlain by a thin less than 200 meters thick extrusive volcanic layer seismic layer 2a that thickens rapidly off axis also beneath the rise axis is a narrow less than 1 kilometer wide melt sill that is in some places less than 1000 meters below the sea floor the small dimensions of this molten body indicate that magma chamber size does not depend strongly on spreading rate as predicted by many ridge crest thermal models however the shallow depth of this body is consistent with an inverse correlation between magma chamber depth and spreading rate these observations indicate that the paradigm of ridge crest magma chambers as small sill like midcrustal bodies is applicable to a wide range of intermediate and fast spreading ridges

the three dimensional seismic velocity structure of the east pacific rise near latitude 9 30 n three dimensional images of crustal seismic structure beneath the east pacific rise show pronounced axial heterogeneity over distances of a few kilometres a linear high velocity anomaly approximately 1 2 km in width and restricted to the uppermost 1 km of the crust is centred on the rise axis an axial low velocity anomaly at depths of 1 3 km varies in amplitude along the axis consistent with a zone of higher crustal temperatures midway between two discontinuities in the morphology of the rise axis this apparent thermal segmentation along axis is consistent with injection of mantle derived melt midway along a locally linear 12 km long segment of the rise

87sr86sr and ree variations along the easter microplate boundaries south pacific application of multivariate statistical analyses to ridge segmentation abstract we report 87 sr 86 sr ree sc v cr and co analyses on 48 basaltic glasses dredged at 47 different localities along the spreading boundaries of the easter microplate emp the latitudinal 87 sr 86 sr variation along the east and west rifts closely parallels that of the pb isotope variation previously published and confirms the strong abnormality of the mantle underlying this part of the east pacific rise epr the data also further support the binary mantle mixing model previously proposed we evaluate the potential of multivariate statistical analyses in studying this and previously published geochemical data obtained on these very same 48 basaltic glasses the data base includes major and trace elements ranging from compatible e g cr to highly incompatible e g la and the pb and sr isotope ratios multivariate classification of these geochemical variables in q mode sample space allows to clearly distinguish major element grouping from those of moderate to highly incompatible elements and the isotope ratios reflecting mantle source heterogeneities present in the region the power of this multivariate geochemical classification is best represented in two dimensional space by combining principal components analysis pca with cluster analysis ca illustrated in the form of a dendrogram the method is further illustrated by classifying elements of the lanthanide series at different statistical level of affinities using the variance covariance matrix and correlation matrix in r mode we further demonstrate a close relationship between segmentation revealed by the multiple variation of the four isotope ratios and the actual tectonic segmentation both discriminant analysis and pca further indicate a close relationship between the geochemical anomaly located on the east rift of the emp and sala y gomez and easter island to a lesser extent the tuamotu chain whereas there is no close affinity between the emp and the more distant juan fernandez san felix or society islands in the isotopic space considered this provides further support that the geochemical anomaly located on the east rift of the emp may be related to the influence of a plume located either beneath the east rift or in the vicinity of sala y gomez or easter island in the last two models the plume would be laterally deflected at shallow depth towards the epr emp spreading complex an analysis of statistical generalized distances against geographical distances rules out that such influence is caused by radial dispersion of this plume in the upper mantle the flow between the plume and the east rift of the emp must be more confined or channelled the possibility that the plume would be located directly beneath the east rift cannot be ruled out but appears less likely

characteristic mineralogy of arc related cumulate gabbros implications for the tectonic setting of gabbroic plutons and for andesite genesis several mineralogically distinct suites of gabbroic cumulates occur as xenoliths in basalt and andesite from active arc volcanos and in plutons exposed in deeply eroded arcs calcic an85 100 plagioclase and moderately fe rich fo60 80 olivine commonly occur together in arc cumulate gabbros but not in cumulate gabbros from mid ocean ridges ocean islands or tholeiitic layered intrusions compositions of coexisting olivine and plagioclase in gabbro cumulates can establish the tectonic setting of ancient volcano plutonic complexes in deeply eroded arcs gabbroic cumulates typically occur in zoned plutons that also contain gabbros diorites and quartz diorites of basaltic to andesitic composition in many cases it is clear that the cumulate gabbros are chemically and physically related to the gabbro diorite basalt andesite fractionation trends seen in the zoned plutons zoned plutons in deeply eroded arcs provide physical evidence that crystal fractionation plays an important role in andesite petrogenesis

comment on quantitative biochronology of the permian triassic boundary in south china based on conodont unitary associations by brosse et al 2016 recently brosse et al 2016 have proposed the use of conodont unitary associations zones uazs to substantially modify the biostratigraphy of the permian triassic transition and to redefine the permian triassic boundary ptb however in our opinion the uaz analysis presented by brosse et al 2016 is based on unreliable taxonomic data sets with unjustified taxonomic re assessments no evidence shows that the uaz approach improves the biozone biostratigraphy currently used to date the ptb

reconstructing in space and time the closure of the middle and western segments of the bangong nujiang tethyan ocean in the tibetan plateau when and how the bangong nujiang tethyan ocean closed is a highly controversial subject in this paper we present a detailed study and review of the cretaceous ophiolites ocean islands and flysch deposits in the middle and western segments of the bangong nujiang suture zone bnsz and the cretaceous volcanic rocks late mesozoic sediments and unconformities within the bnsz and surrounding areas our aim was to reconstruct the spatial temporal patterns of the closing of the middle and western segments of the bangong nujiang tethyan ocean our conclusion is that the closure of the ocean started during the late jurassic and was mainly complete by the end of the early cretaceous the closure of the ocean involved both longitudinal diachronous closure from north to south and transverse diachronous closure from east to west the spatial temporal patterns of the closure process can be summarized as follows the development of the bangong nujiang tethyan oceanic lithosphere and its subduction started before the late jurassic after the late jurassic the ocean began to close because of the compressional regime surrounding the bnsz along the northern margin of the bangong nujiang tethyan ocean collisions involving the arcs back arc basins and marginal basins of a multi arc basin system first took place during the late jurassic early early cretaceous resulting in regional uplift and the regional unconformity along the northern margin of the ocean and in the southern qiangtang terrane on the northern side of the ocean however the closure of the bangong nujiang tethyan ocean cannot be attributed to these arc arc and arc continent collisions because subduction and the development of the bangong nujiang tethyan oceanic lithosphere continued until the late early cretaceous the gradual closure of the middle and western segments of bangong nujiang tethyan ocean was diachronous from east to west starting in the east in the middle early cretaceous and being mainly complete by the end of the early cretaceous the bnsz and its surrounding areas underwent orogenic uplift during the late cretaceous

quantitative stratigraphic correlation of the lower triassic in south china based on conodont unitary associations abstract unitary association method uam analyses of conodont faunas from 28 sections spanning the biggest phanerozoic end permian mass extinction and significant global environmental and ecosystem perturbations during the succeeding early triassic are presented based on 72 conodont species 26 unitary association zones uazs are established for the latest permian to earliest middle triassic of south china these uazs provide quantitative high resolution tools to correlate sequences in the early triassic of south china and to compare and test high resolution conodont biostratigraphy based on interval conodont zones developed over the past three decades our quantitative analyses provide insights on ongoing debates relating to the first appearance datum fad of the conodont hindeodus parvus which is used to place and subsequently correlate the golden spike defining the base of the triassic permian triassic boundary at the base of bed 27c at the gssp meishan d section previous proposals that suggested potential earlier occurrences of h parvus below its fad at meishan section are not supported by our results in deep water sections the first occurrence fo of h parvus lies at the base of uaz 5 at the bianyang section while it lies within uaz 6 at the meishan section this indicates that the earliest occurrence of h parvus in south china is in the bianyang section but this conclusion needs further testing due to the reliance on spot data by the uam anomalously high occurrences of hindeodus in the mingtang section close to the bianyang section further suggests that the bianyang mingtang area may have provided a temporally extended habitable zone for anchignathodontid conodonts stage boundaries currently proposed or established using interval conodont zones locate within or between uazs and are difficult to correlate with carbon isotope curves uazs are useful in helping to define gssps by recognizing which correlations are most robust and in the selection of the most appropriate species and level for gssps

implications of the infra and inter trappean biota from the deccan india for the role of volcanism in cretaceous tertiary boundary extinctions the fossil record shows a remarkable similarity between the biota that existed before and after the initiation of deccan volcanic activity virtually all the available palaeontological evidence such as the fauna and flora of the freshwater infra and inter trappean beds and the planktonic foraminifera from the subsurface infra and inter trappean beds of the southeastern coast do not favour the initiation of deccan volcanism as the cause of mass extinctions at the cretaceous tertiary k t boundary instead a combination of events such as an extended period of volcanism changes in sea level and related climatic changes and certain local factors may have played a significant role in selectively eliminating various groups of organisms this view is further reinforced by the biotic changes across the k t iridium layer in the um sohryngkew river section of meghalaya northeastern india

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

geochemical and nd pb isotopic characteristics of the tethyan asthenosphere implications for the origin of the indian ocean mantle domain abstract it is unclear why the pb nd and sr isotopic composition of the modern mid ocean ridge basalts morb from the indian ocean is different from that of the north atlantic and pacific oceans a possible explanation for this is that the indian morb type isotopic signature is a long lived regional feature of the mantle as evidently shown by the isotopic composition of the 350 ma morb like mian lue northern ophiolite which was formed in the same region presently occupied by the indian ocean however this hypothesis is in conflict with the lack of indian morb type isotopic signature in a number of 150 ma tethyan and indian ocean crusts to further constrain the origin of the indian morb type isotopic signature we analyze the geochemical and pb nd and sr isotopic composition of representative mafic rocks from four tethyan ophiolites ranging in age from 90 to 360 ma the sr isotopic composition of the samples is unreliable due to alteration but the age corrected nd and pb isotopic ratios and geochemical data indicate that these tethyan rocks were derived from a geochemically depleted asthenospheric source that had a clear indian morb type isotopic signature we therefore conclude that the bulk of the indian suboceanic mantle was most probably inherited from the tethyan asthenosphere a few regions in both the tethyan and indian oceans however are most probably underlain by pacific and north atlantic morb type mantle and vice versa because of the flow of the asthenosphere in response to tectonic plate reorganizations that lead to openings and closures of ocean basins the indian morb type isotopic signature of the western pacific marginal basin crusts could be due to either flow of the indian ocean mantle into the western pacific or to endogenous production of such an isotopic signature from delaminated east asian sublithospheric materials during closure of the tethys ocean

submersible observations at the axis of the ultra fast spreading east pacific rise 17 30 to 21 30 s abstract manned submersible observations conducted with the diving saucer cyana at the axis of the east pacific rise at four sites near 21 30 21 10 18 30 and 17 30 s demonstrate that the ridge evolves between two end members the volcanic end member is characterized by a non rifted or only slightly rifted topographic ridge with widespread but diffuse water venting the tectonic end member is marked by a well defined axial graben whose floor can be the locus of intense sulfide deposition at one site a small stockwork mineralization exposed through normal faulting was observed at the base of the graben wall the dives illustrate the discontinuous nature of volcanic tectonic and hydrothermal activities even in the case of the fastest accretion found in the ocean

correlations of mid ocean ridge basalt chemistry with the geoid abstract the major element composition of mid ocean ridge basalts morb shows geochemical variations controlled by the temperature pressure and source composition produced by convection in the mantle beneath the spreading centers the long wavelength signal of the geoid reflects deep mantle processes in this paper we characterize the correlations between the major element chemistry of morb and the geoid anomalies at different wavelengths to determine the extent to which the major element composition of the morb reflects deep mantle circulation the chemical effects of the low pressure fractionation have been corrected using a new method based on the feot mgo ratio the fractionation trends modeled by a linear trend in the geochemical parameter space normalized by the mgo denominator converge to a common point with 8 mgo the separation vector d between the fractionation trends and this common point defines a new parameter whose sodium component shows a negative correlation significant at the 99 level with the axial depth for both the mid atlantic ridge mar and the east pacific ridge epr and with the geoid only for the mar the latter correlation suggests that the major element composition of the morb is clearly related to deep mantle convection by material and thermal coupling between the lower and upper mantle the lack of correlation at long or medium wavelengths between the mantle enrichment inferred from the k2o tio2 ratio and the geoid indicates that either source composition does not influence morb chemistry or that hotspot material is injected along relatively narrow conduits with little interaction with the surrounding upper mantle

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

high resolution δ13ccarb chemostratigraphy from latest guadalupian through earliest triassic in south china and iran large carbon cycle perturbations are associated with the end permian mass extinction and subsequent recovery but late permian lopingian carbon cycle dynamics prior to the mass extinction event remain poorly documented here we present a high resolution 13ccarb chemostratigraphic framework from latest guadalupian to earliest triassic time calibrated with high resolution conodont biostratigraphy and high precision geochronology we observe two large negative excursions in 13ccarb the first in uppermost guadalupian strata and the second at the end of the changhsingian stage and between these events distinctive excursions from the middle wuchiapingian to the early changhsingian the end changhsingian excursion represents a major reorganization of the global carbon cycle associated with the end permian mass extinction however the extent to which the end guadalupian and wuchiapingian changhsingian boundary excursions result from local versus global controls remains unresolved regardless of their underlying causes these three excursions provide chemostratigraphic markers for global correlation of lopingian strata

reply to the comment on quantitative biochronology of the permian triassic boundary in south china based on conodont unitary associations 1 introduction in their comment jiang et al 2016 claim that the discordance between our zonation brosse et al 2016 and the interval zones does not rest on the use of the unitary association method guex 1991 and guex et al 2015 per se but on our failure to use the most recent published conodont ranges from some key chinese sections they add that our analysis is based on unreliable taxonomic data sets with unjustified taxonomic re assessments and hence that we did not demonstrate that the unitary association method performs better than traditional interval zones additionally the comment by jiang et al 2016 contains misunderstandings pertaining to the method used in our study brosse et al 2016 our goal was to apply the unitary association method on a dataset of conodont distributions from south china around the permian triassic boundary ptb in order to reassess the quality of the corresponding data and to provide a discrete and robust alternative zonation to the continuous first occurrence based interval zones such interval zones are routinely utilized in permian and triassic conodont biostratigraphy regardless of their abundant internal contradictions three categories of points of contention can be extracted from the comment of jiang et al 2016 method selection of data and taxonomy and illustrations each of these is addressed separately below 2 method fig 1 complies with the recommendation of jiang et al 2016 and takes into account all the recent literature summarizing the most recent published conodont ranges from the relevant sections the reader will immediately observe that the sequences of first occurrences fos used in the most recently published interval zones occupy contradictory positions between the different sections this is precisely the main criticism expressed in our work this problem cannot be solved by simply standardizing the taxonomy of the considered taxa many contradictions do persist after taxonomic homogenization as conceded by jiang et al 2016 fos are prone to diachronism but contrary to what jiang et al 2016 suggest sampling effort is not the only nor the main reason for such diachronism and ptb sections from south china are no exceptions in this respect fig 9 of brosse et al 2016 demonstrates the contrary ironically enough jiang et al 2016 acknowledge that last occurrences los can be diachronous because of local ecological differences but they exclude that fos can be affected in reality these authors unduly equate every local fo with an alleged instantaneous spreading of a species across all sections incidentally such an unwarranted assumption also deliberately ignores that speciation is an intrinsically space restricted evolutionary process

geochemical and structural studies of the lamont seamounts seamounts as indicators of mantle processes abstract a chain of seamounts located just west of the east pacific rise epr axis at 09 55 n herein called the lamont seamounts have been investigated using sea marc i and sea beam sonar surveys alvin submersible dives deep sea camera tows and dredging the most recently active volcanic vents on the summits of these volcanoes are located at the periphery of each caldera or crater rim or at the margins of caldera floors the distribution of recent volcanic vents and the morphology of the summit plateaus suggest the possible existence of ring fractures on these seamounts ring fractures could be important in focusing post caldera collapse eruptions around summit craters and building summit plateaus structural and morphological data and the petrology of recovered lavas suggest that calderas in the lamont seamounts probably formed by collapse associated with magma withdrawal into large primary vertical magma conduit systems rather than by repeated inflation deflation cycles associated with steady state shallow level magma chambers calderas in the lamont seamounts have evolved through coalescence of adjoining craters and the plan view shape and morphology of each caldera suggest that primary magma conduits have migrated beneath the summit of each seamount through time compared to lavas from the adjacent epr the majority of lamont seamount and cone lavas are more primitive depleted in incompatible elements and compositionally heterogeneous variations in light rare earth elements relative to major elements e g tio 2 na 2 o na 2 o cao and radiogenic isotopes cannot be ascribed to differences in the amount of partial melting of a single n type morb source rather it appears that lamont seamount lavas were derived from heterogeneous mantle sources variably depleted in highly incompatible elements comparison with lavas from the adjacent ridge crest suggests that magma plumbing and ascent mechanisms beneath the lamont seamounts are separate from those along the epr in particular seamount magmas appear to reflect near primary melting events in the mantle whereas epr lavas are the products of shallow level crystallization and mixing in crustal magma chambers absence of long lived magma chambers within the lamont seamounts may account for the low level of hydrothermal activity observed throughout the chain furthermore because of the tight control on sample location and the recovery of lavas having distinct parents from individual volcanoes as well as the lamont chain as a whole we propose that the scale of mantle chemical heterogeneities is less than 5 km

tectonic activities in dongshangen polymetallic ore district eastern kunlun mountains qinghai tibet plateau evidences from fission track thermochronology abstract the dongshangen polymetallic ore district is located in east section of the eastern kunlun mountains the south eastern margin of the qaidam basin and is one of the most important polymetallic mining regions however there is a relatively lower research degree on the district especially lack of thermochronology data using zircon and apatite fission track thermochronology this article reveals the tectonic activities experienced in dongshangen polymetallic ore district it is shown that the 6 age groups of geological thermal events i e 197 ma 182 170 ma 160 110 ma 110 80 ma 80 23 ma and 23 0 ma combining with the geological characteristics we consider that the 6 age groups belong to 6 periods of tectonic activities in the dongshangen ore district the first age group of 197 ma mainly resulted from the south kunlun fault strike slip and the qiangtang block collided with the kunlun block the second age group of 182 170 ma was related to the songpan ganzi bayan har basin closure the third age group of 160 110 ma was bound up with the left lateral strike slip took place in the north kunlun fault and the squeezing and collaging activities of both the qiangtang block and the lhasa block toward the eurasian plate the event during the 110 80 ma responded to northward subduction of the neo tethys ocean and strong strike slip in the altyn tagh fault zone the late cretaceous to miocene 80 23 ma was affected by the northward napping of the central kunlun terrane and the northward subduction of neo tethys ocean continued from 23 ma to present the collision of the indian plate toward the eurasian plate occurred leading to the qinghai tibet plateau uplifting as a whole synthesizing the geological characteristics and the tectonothermal events we discriminate 4 metallogenic periods of 197 ma 182 170 ma 155 112 ma and 109 88 ma in the district better reflecting the characteristics of multi stage metallogenic events

variability of apatite fission track annealing kinetics ii crystallographic orientation effects a method is presented that permits the length of any horizontal confined fission track inclined at a specified angle to the crystallographic c axis in apatite to be converted to an equivalent track length parallel to the crystallographic c axis the model is based on the results of annealing experiments for six selected apatites five calcian fluorapatites and durango apatite representing a subset of the 15 total apatite specimens studied an iterative process of calculation is required to project fission track lengths onto the c axis and computer source code implementing the solution to this problem is presented this method of projecting apatite fission track lengths onto the crystallographic c axis is shown to remove effectively fission track length variation within single fission track populations due to anisotropic track length reduction for all 15 apatites studied in addition a model is developed that offers predictions that closely reproduce published experimental data concerning the relationship between fission track density etched fission tracks per unit area of apatite surface and the arithmetic mean fission track length finally it is shown that natural fission track populations exhibit fission track length anisotropy similar to that of fission track populations created and annealed in the laboratory this observation implies that the same process by which apatite fission tracks anneal in the laboratory is responsible for annealing of apatite fission tracks in the geological environment

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

oceanic ridge crest processes oceanic ridge crest processes constitute a vigorous frontier of multidisciplinary research with global ramifications in the earth atmospheric and biologic sciences this deep ocean frontier is gradually opening in response to investigations which are elucidating the role of these processes in the lithosphere hydrosphere atmosphere and biosphere and the application of advanced instrumental and interpretive techniques which are increasing the quality and quantity of data these techniques include crustal drilling with borehole logging deep sea drilling project reports high resolution bathymetry sea beam etc side scan sonar seamarc i and ii gloria etc imagery with video and photography multichannel seismics determination of water column properties ctd with rosette sampler etc and submersible applications

erosional truncation of uppermost permian shallow marine carbonates and implications for permian triassic boundary events on shallow marine carbonate buildups in south china turkey and japan uppermost permian skeletal limestones are truncated by an erosional surface that exhibits as much as 10 cm of topography including overhanging relief sedimentary facies microfabrics carbon isotopes and cements together suggest that erosion occurred in a submarine setting moreover biostratigraphic data from south china demonstrate that the surface postdates the uppermost permian sequence boundary at the global stratotype section and truncates strata within the youngest known permian conodont zone the occurrences of similar truncation surfaces at the mass extinction horizon on carbonate platforms across the global tropics each overlain by microbial buildups and their association with a large negative excursion in 13 c further suggest a causal link between erosion of shallow marine carbonates and mass extinction previously proposed to account for marine extinctions the hypothesis of rapid carbon release from sedimentary reservoirs or the deep ocean can also explain the petrographic observations rapid unbuffered carbon release would cause submarine carbonate dissolution accounting for erosion of uppermost permian skeletal carbonates and would be followed by a pulse of high carbonate saturation explaining the precipitation of microbial limestones containing upwardgrowing carbonate crystal fans models for other carbon release events suggest that at least 5 10 18 g of carbon released in 100 k y would be required of previously hypothesized permian triassic boundary scenarios thermogenic methane production from heating of coals during siberian traps emplacement best accounts for petrographic characteristics and depositional environment of the truncation surface and overlying microbial limestone as well as an associated carbon isotope excursion and physiologically selective extinction in the marine realm

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

evolution of the northward subduction of the neo tethys implications of geochemistry of cretaceous arc volcanics in qinghai tibetan plateau abstract the subduction history of the neo tethys is critical for understanding the early stage tectonic evolution of the qinghai tibetan plateau arc volcanics which usually record the subduction process thus can help trace the evolution of the neo tethys in this paper we report for the first time zircon la icp ms u pb ages whole rock major and trace element geochemistry as well as sr nd hf isotopic data for the volcanic rocks from the danshiting formation at the southern edge of the lhasa terrane two samples collected from the danshiting formation volcanic rocks yield zircon u pb ages of 97 1 0 8 ma and 90 1 0 7 ma respectively the volcanic rocks of the danshiting formation are composed dominantly of basalts and andesites characterized by depletion of high field strength elements low 87sr 86sr i ratios 0 7042 0 7045 high end t 2 1 to 2 7 and high ehf t values mean 11 6 these rocks show geochemical affinity of arc volcanic rocks the volcanic rocks of the danshiting formation may have formed during the northward subduction of the neo tethys this paper combined with the previous data briefly divide the northward subduction of the neo tethys into four periods the jurassic 195 150 ma when the magmatic activity was controlled by the northward high angle subduction of the neo tethys that produced volcanics of the yeba and bima formations the late jurassic to early cretaceous 150 110 ma when the neo tethys subducted at low angles the late cretaceous 110 80 ma when subduction was of the andean type and produced volcanic rocks of the danshiting formation the paleogene 70 40 ma when there was transition from ocean to continent subduction to continent continent collision that produced volcanic rocks of the linzizong group

geochronology of the initiation and displacement of the altyn strike slip fault western china abstract 40ar 39ar dating studies have been carried out along the dangjin pass transect across the altyn strike slip fault assf the samples gave ages of 445 2 454 3 ma in the northern belt 164 3 178 4 ma in the mesozoic shear zone and 26 3 36 4 ma in the cenozoic shear zone using the piercing point of the bashikaogong fault and the cangma heihe fault an offset of 350 400 km along the assf has been estimated the 40ar 39ar dating of the syntectonic growth or syntectonic resetting minerals from the samples within the assf belt and offset estimations from different age piercing points suggest that the assf should be initiated in the middle jurassic 178 4 160 ma combined with previously reported ages our studies show that the assf is characterized by multi phase re activation during 85 100 25 40 and 8 10 ma following its initiation in the middle jurassic in the regional tectonic setting of convergence between the indian and eurasian continents

the end permian regression in south china and its implication on mass extinction abstract records of more than 20 permian triassic boundary ptb sections from south china show that widespread end permian regression took place in whole south china timing of the low ebb of this regression was pinpointed to the clarkina meishanensis and hindeodus changxingensis zones biostratigraphic and paleogeographic studies at conodont zonation level reveal that the yangtze carbonate platform and isolated small carbonate platforms in the hunan guizhou guangxi hgg basin experienced sedimentary hiatus during these two zones which lasted about 50 100 kya meanwhile the basinal areas namely the northern marginal basin of yangtze platform and the hunan guizhou guangxi basin have continuous conodont zonation with silicious and ashbed deposits the basinal areas show two negative 13c shifts during the ptb interval from clarkina yini zone to isarcicella isarcica zone while on the platforms there is usually only one negative shift because the early shift at c meishanensis zone was lost during hiatus the end permian regression and successive rapid transgression taking place at the ptb casted important effects to the pattern and process of the ptb mass extinction the depositional hiatus hitherto underestimated will necessitate reassessment of some interpretations on the causes and results of that extinction

uncertainties and implications of the late cretaceous and tertiary position of north america relative to the farallon kula and pacific plates we present updated global plate reconstructions and calculated uncertainties of the pacific kula and farallon vancouver plates relative to north america for selected times since 68 ma improved magnetic data from the indian ocean decrease the uncertainties in the global plate circuit approach these uncertainties are now considerably smaller than those inherent in equivalent reconstructions based on the assumption of fixed hotspots major differences between these results and those of others are due to our use of more detailed africa north america reconstructions separate vancouver and farallon plate reconstructions and the assumption of a rigid antarctica plate during cenozoic time the uncertainties in the relative positions of the pacific and north america plates since the time of anomaly 7 26 ma range up to 100 km in position or from 1 to 3 m y in time if the mendocino triple junction initiated at about 28 5 ma its position would have been at 31 3 n 130 km relative to fixed north america unacceptable overlap of oceanic crust of the pacific plate with continental crust of western north america in the anomaly 10 30 ma reconstruction is a minimum of 340 200 km along an azimuth of n60 e and may be accounted for by basin and range extension pacific north america displacement in the past 20 ma is found to be considerably less than that calculated by fixed hotspot reconstructions farallon vancouver north america convergence velocity decreased greatly between the times of anomalies 24 and 21 56 to 50 ma prior to the 43 ma age of the hawaiian emperor bend and the often quoted 40 ma end of the laramide orogeny a change in direction of farallon north america convergence occurred sometime between 50 and 42 ma and also may not correlate with the time of the hawaiian emperor bend the lack of data from subducted parts of the farallon and kula plates permits many possibilities regarding the position of the kula farallon ridge the age of subducted crust or the position of oceanic plateaus during the laramide orogeny leaving open the question of the relationship between plate tectonic scenarios and tectonic style during laramide time displacements of points on the various oceanic plates along the west coast of an arbitrarily fixed north america during the interval between anomalies 30 31 and 18 68 to 42 ma are found to be pacific plate 1700 200 km northward farallon plate 3200 400 km northeastward vancouver plate 3000 400 km northeastward kula plate if attached to the pacific plate after a24 time 2500 400 km northward

chemical and isotopic variations along the superfast spreading east pacific rise from 6 to 30 s chemical data of 39 fresh basaltic glasses from the east pacific rise epr between 6 and 30 s and pb sr and nd isotopic compositions of 12 basalt glasses are presented major and trace element data indicate a wide compositional range including primitive basalts mg 0 67 and highly evolved feti basalts mg 0 34 molmg mg fe2 the compositional range can be attributed to low pressure fractional crystallization fractionation corrected major element concentrations provide evidence for varying mantle melting conditions calculations of the melting conditions suggest melt generation in a rising upper mantle column between 20 and 10 kbar at temperatures between 1430 and 1280 c and total degrees of partial melting between 17 and 20 by weight leached and hand picked basalt glasses display large variations in 87sr 86sr 0 70235 0 70270 143nd 144nd 0 51312 0 51323 and 206pb 204pb 18 064 18 665 but are similar to other n type morb from the epr the isotopic ratios of basalts from 13 to 23 s show strong correlations and delineate two systematic trends from 23 to 17 s 87sr 86sr and pb isotope ratios increase and 143nd 144nd decrease in agreement with previous results mahoney et al 1989 a reverse trend is indicated by basalts from 17 to 13 s however k ti and la sm n continuously increase from 23 to 13 s this opposite behavior indicates a recent decoupling of isotopic and minor element ratios in the mantle between 13 and 17 s north of 13 5 s garrett fracture zone isotopic data show no systematic variation with ridge location and display an overall weaker covariation the results suggest that the isotopic variations and ridge segmentation appear to be unrelated and that major ridge offsets apparently coincide with changes in mantle melting conditions p t f f degrees of melting there is no evidence for a systematic relationship between calculated melting conditions and second order ridge segmentation our isotopic data provide further evidence for regionally confined chemical variations in the mantle at 5 to 30 s we interpret the isotopic trends as reflecting melting of distinct smallvolume and old enriched mantle components in contrast variations in trace elements are attributed to young mantle differentiation processes

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

the miocene gangdese porphyry copper belt generated during post collisional extension in the tibetan orogen abstract the mid miocene gangdese orogenic belt in southern tibet contains porphyry cu deposits that developed after the indian asian continental collision in the early cenozoic field work confirms that these porphyry cu deposits are temporally and spatially associated with mid miocene monzogranitic and quartz monzonitic stocks which intruded mesozoic cenozoic gangdese granitoid batholiths and surrounding triassic tertiary volcano sedimentary sequences in the lhasa terrane and formed a 50 km wide 400 km long e w belt magmatism occurred during the interval 11 2 to 19 7 ma peaking at 16 ma for the porphyry stocks this is consistent with the age 10 to 18 ma of the east west extension and subsequent ns striking normal fault systems 13 5 ma in the tibetan orogen the stocks typically occur as multiple small diameter intrusions 50 km breakdown of hornblende in the juvenile lower crust source during melting is regarded as fundamental for the formation of the fertile adakite and porphyry cu systems in south tibet the gangdese belt contains four large and three intermediate and small sized porphyry cu mo deposits and shows a continuous spectrum from typical porphyry type cu mo to skarn type zn pb mineralization twenty three samples collected from four deposits yielded a limited range of molybdenite re os isochron ages from 15 2 to 16 0 ma similar to previously reported re os ages 13 8 to 14 9 ma for other porphyry cu mo deposits in the belt suggesting that cu mo mineralization in the belt occurred in a post collisional extension environment the characteristics of porphyry cu mo deposits in the collisional zone are similar in many aspects to those in arc settings i e mineralization style alteration zoning and metal association preliminary fluid inclusion studies indicate that the early hydrothermal fluids are preserved as high temperature 450 c high salinity 40 to 47 wt nacl equiv and high pressure 1100 to 1500 bar fluid inclusions these fluids are interpreted to represent an orthomagmatic fluid that boiled episodically due to fluid overpressuring and hydrofracturing resulting in potassic alteration and precipitation of chalcopyrite molybdenite assemblages on the basis of comparison with porphyry cu deposits in arc settings we proposed that melting of thickened newly formed basaltic crust resulted in generation of cu bearing magmatic systems in the tibetan collision zone and both orogen transverse normal faults and orogen parallel thrust zones during the miocene post collisional extension constrain the temporal and spatial localization of the porphyry cu systems in the collision zone

integrative timescale for the lopingian late permian a review and update from shangsi south china abstract the lopingian is the uppermost series of the paleozoic and it is bracketed by two major biological events including the pre lopingian crisis and the end permian mass extinction epme a high resolution temporal framework is essential to understand the patterns and causes of the extinction lopingian strata of south china have been intensively studied because three gssps have been defined in the region based on the review and updates to data from the shangsi section as well as correlation with the meishan section the time framework for the lopingian is revised including biostratigraphy chemostratigraphy magnetostratigraphy cyclostratigraphy and geochronology the temporal framework is constrained by both precise geochronologic data and a high resolution conodont succession and provides the possibility that the current high resolution marine international standard can also be applied to nonmarine strata by means of geochronology magnetostratigraphy and cyclostratigraphy the entire lopingian high resolution conodont succession is for the first time since the lopingian series was adopted as the international standard recognized at a single section in south china and the succession correlates very well with the lopingian gssp sections at penglaitan and meishan the permian triassic boundary ptb of the shangsi section is constrained to the basal 6 cm interval of bed 28b in view of ammonoids bivalves conodonts u pb ages and the epme the mass extinction interval is between bed 27 and lower bed 28a according to the epme pattern at the shangsi section and the alternative interval between extinct permian species and new triassic species is from upper bed 28a to lower bed 28b unitary association ua analysis of lopingian conodonts reveals 15 unitary association zones uazs based on seven important lopingian sections of south china most of the uazs correlate well with the standard biozonation except for uaz 3 to uaz 5 and uaz 12 the correspondence between uazs and standard conodont biozones at the shangsi section provides a practical example to understand controls on conodont uaz determination the lopingian conodont succession is temporally calibrated by geochronologic ages identified 405 kyr eccentricity cycles and a monte carlo analysis at the shangsi and meishan sections updated ages for the base of the lopingian and base of the changhsingian are provided

late cenozoic magmatic inflation crustal thickening and 2 km of surface uplift in central tibet chinese acad sci guangzhou inst geochem state key lab isotope geochem guangzhou 510640 guangdong peoples r china

pyrite framboid study of marine permian triassic boundary sections a complex anoxic event and its relationship to contemporaneous mass extinction size analysis of pyrite framboids has been undertaken on epicontinental permian triassic boundary sections throughout the world in order to evaluate the intensity and duration of anoxia mid paleolatitude sections from the margins of the boreal spitsbergen greenland and neotethyan oceans western australia reveal intense anoxia throughout the permian triassic boundary interval with euxinic conditions frequently developing and dysoxia encountered even in relatively shallow water settings above storm wave base at equatorial paleolatitudes weakly oxygenated dysoxic conditions are widely developed in a broad range of water depths including those shallow enough to produce oolite deposition although euxinia was rare western and eastern tethyan locations re veal a complex and unstable redox history anoxia in the hindeodus praeparvus zone was replaced by oxygenated facies in the permian triassic boundary interval h changxingensis to h parvus zones oxygen poor deposition returned during the succeeding isarcicella isarcica zone the more persistent and intense development of oxygen restriction in cooler water mid paleolatitude sections argues against warming and dissolved oxygen decline as the key cause of permian triassic bound ary anoxia in higher paleolatitudes the ben thic invertebrate extinctions occurred during a prolonged phase of oxygen poor deposition while in equatorial tethyan locations benthic losses occurred at the end of the fi rst anoxic phase in the late h praeparvus zone

revised phosphate water fractionation equation reassessing paleotemperatures derived from biogenic apatite oxygen isotopes of biogenic apatite have been widely used to reassess anomalous temperatures inferred from oxygen isotope ratios of ancient biogenic calcite more prone to diagenetic alteration however recent studies have highlighted that oxygen isotope ratios of biogenic apatite differ dependent on used analytical techniques this questions the applicability of the phosphate water fractionation equations established over 25 years ago using earlier analytical techniques to more recently acquired data in this work we present a new phosphate water oxygen isotope fractionation equation based on oxygen isotopes determined on fish raised in aquariums at controlled temperature and with monitored water oxygen isotope composition the new equation reveals a similar slope but an offset of about 2 to the earlier published equations this work has major implications for paleoclimatic reconstructions using oxygen isotopes of biogenic apatite since calculated temperatures have been underestimated by about 4 to 8 c depending on applied techniques and standardization of the analyses

early late cretaceous ca 93 ma norites and hornblendites in the milin area eastern gangdese lithosphere asthenosphere interaction during slab roll back and an insight into early late cretaceous ca 100 80 ma magmatic flare up in southern lhasa tibet abstract at more than 500 km in length the mainly jurassic early eocene gangdese batholith is one of the most important constituents of the southern lhasa sub block and provides an ideal site for study of tibetan orogenesis recent studies on gangdese intermediate felsic intrusive rocks mainly granites demonstrate that remarkable crustal growth as well as an early late cretaceous ca 100 80 ma magmatic flare up event occurred in southern tibet however the mechanism that drove this magmatic event and its relationship to crustal growth event are not yet clear here we report detailed petrological geochronological geochemical and sr nd hf isotopic data for recently identified norites and hypersthene bearing hornblendites in the milin area southern lhasa sub block these mafic rocks are dated at early late cretaceous ca 93 ma and are characterized by relatively uniform sr nd hf isotopic compositions 87sr 86sr i 0 7042 to 0 7047 end t 2 9 to 3 6 and ehf t zircon 10 9 to 17 0 suggesting that they evolved from similar parental magmas the milin norites and hornblendites are likely to be the products of mineral fractionation and accumulation from a common parental magma during the early and late stages of the magma evolution respectively thermometric calculations indicate that pyroxenes from the milin norites have high crystallization temperatures 1240 1349 c the parental magmatic compositions calculated from pyroxene trace element compositions in the milin norites show slightly flat to enriched light rare earth element lree patterns la yb n 2 9 3 4 la gd n 2 1 2 9 with variable negative nb anomalies nb la n 0 18 0 81 indicating their dual or hybrid geochemical characteristics we suggest that their parental magmas may have been generated by the interaction of upwelling asthenospheric and metasomatized lithospheric mantle taking into account the spatial and temporal distribution of the mesozoic magmatic rocks and regional paleomagnetic data we further suggest that the early late cretaceous magmatic flare up in the southern lhasa sub block was also triggered by the asthenospheric upwelling which resulted from roll back of subducted tethyan oceanic slab

mantle contributions to crustal thickening during continental collision evidence from cenozoic igneous rocks in southern tibet the tibetan crust is twice as thick as average continental crust crustal compression and shortening as a result of indian asian collision isoftenconsideredtobetheprimarycauseforthecrustalthickening inthispaper weshowthatmagmaticcontributionisalsoimportant we come to this conclusion by documenting the paleogene linzizong volcanic succession lvs its coeval granitoid batholiths and the miocene adakitic rocks along the gangdese magmatic belt in southern tibet it has been widely accepted that the indian asian collision proceeded from a soft phase at 65 70 ma to a hard phase at 45 40 ma followed by continued post collisional convergence to the present in response to the collision and post collision convergence are a series of tectono magmatic events recorded in the gangdese magmaticbelt theseinclude 1 thesyn collisionallvsvolcanism 65 40ma andtheemplacementofsoutherngangdesebatholiths a peakageof 50ma 2 aperiod 40mato25ma thatismagmaticallyquiescent yettectonicallydominatedbyactivecompressionand crustal shortening and 3 the emplacements of post collisional adakitic rocks 25 12 ma potassic ultrapotassic volcanics 25 10 ma and peraluminous muscovite bearing granites between 24 and 18 ma these three major events contribute in different ways to the crustal thickness phase i formation of the lower juvenile crust from 65 ma to 50 ma with crustal thickening largely concentrated at 50 40maviainputofmantle derivedmagmas phaseii crustalthickeningbytectonicshorteningat 40 25ma andphaseiii retaining crustal thickness but thinning of the lithospheric mantle since 25 ma in response to crustal extension and upwelling and lateral flow of asthenosphericmantle weemphasizethatcollision inducedcrustalthickeningtookplacemainlyintheperiodof 50 40maand 25ma i e the period between the late stage of the lvs volcanism and the beginning of the adakitic rock emplacement most of the lvs rocks and the collision related granitoids in southern gangdese have end 0 attesting to the significance of mantle input most likely through remelting of mantle derived basaltic rocks including the subducted neo tethyan ocean crust the petrologic and geochemical characteristics of the miocene potassic adakitic rocks support the idea that the lower portion of the thickened tibetan crust is mafic and is genetically associated with the earlier lvs magmatism we estimate that the mantle material input contributed about 30 of the total thickness of the present day tibetan crust by assuming a pre collision crustal thickness of 35 km then the tectonic contribution would be about 20 km

tectonics of the easter plate abstract we present a new model for the easter plate in which rift propagation has resulted in the formation of a rigid plate between the propagating and dying ridges we use the distribution of earthquakes eleven new focal mechanisms and existing bathymetric and magnetic data to describe the tectonics of this area both the easter nazca and easter pacific euler poles are sufficiently close to the easter plate to cause rapid changes in rates and directions of motion along the boundaries the east and west boundaries are propagating and dying ridges the southwest boundary is a slow spreading ridge and the northern boundary is a complex zone of convergent and transform motion the easter plate may reflect the tectonics of rift propagation on a large scale where rigid plate tectonics requires boundary reorientation we use simple schematic models to illustrate the general features and processes which occur at plates resulting from large scale rift propagation

uzth isotopic systematics at 13 n east pacific ridge segment fresh basaltic glasses have been analyzed for uzth disequilibrium systematics as part of an extensive study on the east pacific rise epr at 12 45 n these samples are well described in terms of major and trace elements as well as in nd pb and sr isotopes our results show significant heterogeneities in the mantle source at a small scale and show heterogeneities at larger scales also when compared to other epr data u and th concentration and isotopic data rule out fractional crystallization as a main process and support a mixing model in agreement with the marble cake model developed by alle gre and turcotte and constrained by trace elements and nd sr and pb isotopes on the same samples by prinzhofer et al based on the high 230th 232th isotopic ratios on recent tholeiites especially the th u values inferred for their sources clearly show that the upper mantle th u has decreased with time

two pulses of extinction during the permian triassic crisis over 90 of marine species were lost during the end permian extinction fossil data show that the crisis in china was marked by two distinct phases of marine extinction separated by a 180 000 year recovery period

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

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

multi stage tectono magmatic events of the eastern kunlun range northern tibet insights from u pb geochronology and u th he thermochronology article i nfo to understand the magmatic and tectonic events in the eastern kunlun range we conducted systematic zircon u pb geochronology and paired zircon zhe and apatite ahe u th he themchronology investigations in the central and eastern segments of this range zircon u pb ages show two stage magmatic events of late silurian and latest permian early jurassic zhe and ahe ages and multi system thermochronometers reveal multi stage rapid cooling events from a combination of these data with previously published geochronological and thermochronological ages and regional geological setting we confirm several important phanerozoic tectono magmaticeventsinthisregion whereasthesiluriangranitesmightberelatedtoasmallpaleo oceansub duction and subsequent collision the latest permian early triassic granitoids were produced by northward sub duction of the songpan ganzi paleo tethyan ocean the late triassic earliest jurassic granitoids and late triassic early jurassic exhumation are interpreted as response to the collision between kunlun qaidam and qiangtang samples close to the kunlun fault show rapid late oligocene early miocene exhumation 1 3 1 6 km ma whereas those from the northern part of central segment display extremely low exhumation rates 0 02 0 05 km ma this heterogeneous denudation during 30 20 ma between northern and southern part of the central segment requires that crustal thickening was completed by lower crustal underthrusting with littleex humation or crustal thickening in pre cenozoic times the activity of the kunlun fault and associated normal faulting component were probably responsible for this stage of rapid exhumation in the southern part the com plex tectono magmatic evolution of the eastern kunlun range is controlled by pre cenozoic paleo ocean subduc tions and subsequent continent collisions and paleogene india asia collision

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