evolution of mica fish in mylonitic rocks abstract mineral fish are lozenge shaped porphyroclasts single crystals in a finer grained matrix which occur in ductile shear zones and which are commonly used as shear sense indicators mineral fish of biotite tourmaline k feldspar garnet hypersthene and quartz occur in mylonites but most common are white mica fish these mica fish can be subdivided into six morphological groups that develop by different mechanisms determined by different initial shapes and orientations the principal mechanisms of formation are intracrystalline deformation combined with rigid body rotation concomitant selective grain size reduction occurs by recrystallisation cataclastic separation pressure solution and diffusional mass transfer microboudinage has been proposed for the breakdown of large mica fish into smaller ones but many mica fish undergo shortening rather than extension along their long axes evidence is presented for an alternative process in which the tips of mica fish are isoclinally folded and then break off along the hinge of these microfolds all the presented fish shaped porphyroclasts or mineral fish have a specific shape preferred orientation with their long axis at a small antithetic angle with respect to the foliation they represent a special group of objects with a stable orientation due to flow partitioning into narrow shear bands

pacific and atlantic multidecadal variability in the kiel climate model pacific decadal variability pdv and atlantic multidecadal variability amv the two leading decadal modes of observed northern hemisphere sea surface temperature sst variability are investigated in a multi millennial control integration of the kiel climate model kcm it is shown that the two phenomena are independent modes in the model and can be easily separated by principal oscillation pattern pop analysis of model sst pdvrelated variability covers the whole north pacific with strong signals in both the mid latitude north pacific and the western tropical pacific strong signals are also simulated in the eastern indian ocean sector pdv s memory however resides in the north pacific and is linked to the subtropical gyre the amv mechanism is related to the atlantic meridional overturning circulation amoc a stochastic mechanism applies to both pdv and amv

triassic bedded cherts in central japan are not pelagic although the origin of bedded radiolarian cherts in orogenic belts has been widely debated1 15 their geochemistry is still poorly understood triassic bedded radiolarian cherts occur extensively in the japanese islands and are generally not associated with ophiolitic rocks but rather with clastic rocks we present here chemical data on the triassic bedded cherts of kamiaso in central japan the relationship of sio2 with al2o3 tio2 and zr shows that the bedded cherts have been formed by a simple mixing of biogenic silica derived from radiolarians and sponge spicules with lithogenic materials the ratios of several elements relative to tio2 strongly suggest that the shale partings and lithogenic portions of the cherts are consanguineous sedimentary structures as well as the geochemical evidence suggest that most kamiaso cherts are turbidity current deposits our geochemical and geological data compared with those of recent marine sediments from various environments demonstrate that the triassic cherts and associated shale partings are not deep pelagic in origin but formed in an offshore or marginal sea environment

palaeontology and u pb detrital zircon geochronology of upper triassic strata on the northern margin of the bangong co nujiang suture zone tibet constraints on the age of opening of the meso tethys abstract we present the results of field mapping paleontology and detrital zircon geochronology of newly found clastic rocks with intercalations of limestone in the gaerqin area of tibet gaerqin formation the gaerqin formation consists of sandstone siltstone and shale with intercalations of bioclastic limestone abundant late triassic norian fossils are found in the limestone beds the fossils include distichophyllia norica retiophyllia litangensis and mesozoic chaetetida in addition 273 detrital zircons were selected for u pb dating yielding the following age groups 210 300 500 600 800 1100 1600 1800 and 2400 2600 ma more than 80 of the detrital zircons are older than 550 ma and the most reliable youngest detrital zircon age for the gaerqin formation is 219 ma the results indicate that the gaerqin formation formed in a shallow marine to bathyal continental slope environment and represents the first reported upper triassic norian strata in the gaerqin area of tibet the findings also suggest that the bangong co nujiang tethys ocean was a bathyal basin by the late triassic

nao implicated as a predictor of northern hemisphere mean temperature multidecadal variability 1 the twentieth century northern hemisphere mean surface temperature nht is characterized by a multidecadal warming cooling warming pattern followed by a flat trend since about 2000 recent warming hiatus here we demonstrate that the north atlantic oscillation nao is implicated as a useful predictor of nht multidecadal variability observational analysis shows that the nao leads both the detrended nht and oceanic atlantic multidecadal oscillation amo by 15 20 years theoretical analysis illuminates that the nao precedes nht multidecadal variability through its delayed effect on the amo due to the large thermal inertia associated with slow oceanic processes an nao based linear model is therefore established to predict the nht which gives an excellent hindcast for nht in 1971 2011 with the recent flat trend well predicted nht in 2012 2027 is predicted to fall slightly over the next decades due to the recent nao decadal weakening that temporarily offsets the anthropogenically induced warming

crustal structure across the ne tibetan plateau and ordos block from the joint inversion of receiver functions and rayleigh wave dispersions abstract we investigated the crustal structure at 34 stations using the h stacking method and jointly inverting receiver functions with rayleigh wave phase and group velocities these seismic stations are distributed along a profile extending across the songpan ganzi terrane qinling qilian terranes and southwestern ordos basin our results reveal the variation in crustal thickness across this profile we found thick crust beneath the songpan ganzi terrane 47 59 km that decreases to 45 47 km in the west qinling and qilian terranes and reaches its local minimum beneath the southwestern ordos block 43 51 km at an average crustal thickness of 46 7 2 5 km a low velocity zone in the upper crust was found beneath most of the stations in ne tibet which may be indicative of partial melt or a weak detachment layer our observations of low to moderate vp vs 1 67 1 79 represent a felsic to intermediate crustal composition the shear velocity models estimated from joint inversions also reveal substantial lateral variations in velocity beneath the profile which is mainly reflected in the lower crustal velocities for the ordos block the average shear wave velocities below 20 km are 3 8 km s indicating an intermediate to felsic lower crust the thick ne tibet crust is characterized by slow shear wave velocities 3 3 3 6 km s below 20 km and lacks high velocity material vs 4 0 km s in the lower crust which may be attributed to mafic lower crustal delamination or and the thickening of the upper and middle crust

differences in geochemical features between permian and triassic cherts from the southern chichibu terrane southwest japan ree abundances major element compositions and sr isotopic ratios abstract ree abundances sr isotopic data and major element compositions are reported for the permian and triassic cherts from the kuma and saiki areas in the southern chichibu sambosan terrane the outer zone of southwest japan in order to elucidate the nature of sources depositional environment and any changes that occurred from the permian to the triassic a higher contribution of detrital components in the triassic sambosan cherts than in the permian ones is suggested by the ce anomaly seen in the ree pattern mno tio 2 and al 2 o 3 al 2 o 3 fe 2 o 3 ratios and initial 87 sr 86 sr ratio of these cherts small negative ce anomalies ce ce 0 75 0 95 are observed in the permian cherts from both kuma and saiki areas in contrast to small positive ce anomalies ce ce 1 1 1 3 for the triassic cherts from both areas it should be noted that the sambosan cherts show neither large negative ce anomalies ce ce 0 2 0 5 commonly observed in deep sea cherts nor large variation in ce anomalies ce ce 0 3 1 5 observed for cherts from the franciscan terrane of california al 2 o 3 al 2 o 3 fe 2 o 3 ratios are 0 5 0 8 for all sambosan cherts mno tio 2 ratios are larger than 0 5 for most of the permian kuma cherts and the early permian saiki cherts while the ratios are smaller than 0 5 for the triassic kuma cherts and late permian and triassic cherts from the saiki area initial 87 sr 86 sr ratios for the permian saiki cherts are 0 706 0 707 and are slightly lower than those for the triassic kuma cherts 0 707 0 709 these 87 sr 86 sr ratios for the permian and triassic sambosan cherts are close to the coeval marine 87 sr 86 sr ratios of burke et al 1982 and are clearly lower than that for cherts from the mino terrane 0 713 inner zone of southwest japan shibata and mizutani 1982 our geochemical and isotopic data of the sambosan cherts suggest a change from ocean basin floor deposition for the permian to hemipelagic for the triassic cherts however geochemical features showing deposition on a typical deep ocean floor are not observed even for the permian cherts some difference in depositional location between the kuma and saiki cherts in the sambosan terrane is also implied from the difference in temporal variation of geochemical features between these two cherts e g increase of ree content with age for the kuma cherts in contrast to a decrease for the saiki cherts in addition it is suggested that the permian and triassic sambosan cherts were deposited in a more open ocean environment than the triassic cherts in the mino terrane this study demonstrates that geochemical and isotopic data could be used to distinguish differences of depositional environment of cherts and those data from the permian to the triassic cherts suggest increasing proximity to land in their depositional environments

experimental compaction of quartz sand at low effective stress and temperature conditions experiments have been carried out on dry and fluid saturated quartz sands to investigate the behaviour during compaction under diagenetic conditions and to evaluate the importance of stress induced solution transfer 9pressure solution9 as a compaction mechanism the experiments were performed at temperatures t in the range 150 350 c applied effective stresses e up to 20 7 mpa and pore fluid pressures p 1 of 12 5 and 15 5 mpa using material with a grain size d in the range 20 100 m and an initial porosity of 45 52 dry quartz sands underwent significant compac tion during the loading stage but showed very little compaction creep once full load was achieved i e essentially time independent compaction in contrast fluid saturated material at constant applied effective stress showed substantial time dependent compaction i e creep with increasing temperature there is a decreasing number of grains in the wet compacted sand which show intragranular cracks and an increasing number of grains which show dissolution features at contacts with adjacent grains in addition there is a decreasing dependence of the volumetric strain rate on e with both increasing volumetric strain e v and increasing e and a decreasing dependence of on e v with increasing temperature these observations suggest that for wet quartz sand a gradual change might occur from compaction creep controlled by time dependent microcracking at t 250 300 c to compaction creep controlled by stress induced intergranular solution transfer at t 300 350 c

fragments of oceanic islands in accretion collision areas of gorny altai and salair southern siberia russia early stages of continental crustal growth of the siberian continent in vendian early cambrian time abstract the altai salair area in southern siberia is a caledonian folded area containing fragments of vendian early cambrian island arcs in the vendian early cambrian an extended system of island arcs existed near the paleo asian ocean siberian continent boundary and was located in an open ocean realm in the present day structural pattern of southern siberia the fragments of vendian early cambrian ophiolites island arcs and paleo oceanic islands occur in the accretion collision zones we recognized that the accretion collision zones were mainly composed of the rock units which were formed within an island arc system or were incorporated in it during the subduction of the paleo asian ocean under the island arc or the siberian continent this system consists of accretionary wedge fore arc basin primitive island arc and normal island arc the accretionary wedges contain the oceanic island fragments which consist of oib basalts and siliceous carbonate cover including top and slope facies sediments oceanic islands submerged into the subduction zone and later were incorporated into an accretionary wedge collision of oceanic islands and island arcs in subduction zones resulted in reverse currents in the accretionary wedge and exhumation of high pressure rocks our studies of the gorny altai and salair accretionary wedges showed that the remnants of oceanic crust are mainly oceanic islands and ophiolites therefore it is important to recognize paleo islands in folded areas the study of paleo islands is important for understanding the evolution of accretionary wedges and exhumation of subducted high pressure rocks

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

creep of wet gypsum aggregates under hydrostatic loading conditions abstract gypsum dominated evaporite sequences are known to play an important role in controlling the mechanics of foreland thrust and nappe tectonics as well as occasionally forming large scale halokinetic structures associated with oil and gas accumulations however relatively little is known regarding the relevant deformation mechanisms and mechanical behaviour of gypsum in this study attention is focused on determining the compaction creep behaviour of wet granular gypsum aggregates with the aim of assessing the importance of pressure solution hydrostatic compaction experiments were performed at room temperature and applied effective pressures in the range 0 4 3 9 mpa using grain sizes ranging from 15 to 125 m the microstructure of the wet tested samples showed that compaction occurred by a process of dissolution within grain contacts and precipitation on pore walls in addition for most of the range of conditions investigated the mechanical data were found to be broadly consistent with models for interface reaction controlled grain boundary diffusional pressure solution combining the mechanical data with kinetic data taken from the crystal growth dissolution literature it is suggested that for the bulk of conditions studied creep of our wet granular gypsum aggregates was probably rate limited by precipitation on the pore walls this is in sharp contrast to the diffusion controlled pressure solution creep behaviour recently reported for highly soluble salts such as nacl and nano3 under similar conditions

water films at grain grain contacts debye huckel osmotic model of stress salinity and mineralogy dependence abstract water film diffusion is one of the mechanisms proposed to explain the deformation of rocks by pressure solution during geological processes in the upper crust this mechanism assumes that matter is dissolved inside the contact between two grains the resulting solutes are transported in the pore fluid through diffusion in an adsorbed water film the main problem of this theory is that it requires the presence of a water film that is believed to be stable under large deviatoric stresses inside the contact between two grains in this paper we show that the electrically charged surface of a mineral can attract counter ions from the pore and by the related change of osmotic pressure keep water within the contact this is due to the counter ions in the water film that increase the salinity in the film relative to that in the pore this lowers the free energy of water in the contact zone to a degree that balances the increase in free energy of water due to the elevated pressure in the film these notions are made more precise by combining the theory of the debye huckle double layer with equations of osmotic pressure the resulting d h o theory predicts the dependence of the water film thickness on stress across the contact composition of the pore fluid and the identity of the minerals involved

compaction of granular quartz under hydrothermal conditions controlling mechanisms and grain boundary processes 1 we report isostatic compaction experiments performed on granular quartz under hydrothermal conditions 3 129 m of initial grain size 300 600 c 200 mpa of fluid pressure and 25 100 mpa of effective pressure from microstructural evidence it was determined that whereas microcracking controlled precompaction at room temperature pressure solution was the main mechanism during hydrothermal compaction although a role of microcracking could not be excluded entirely our mechanical data together with theoretical pressure solution rate models further indicated that pressure solution was controlled by interface kinetics dissolution being the most likely rate controlling mechanism an empirical relation of the form 10 7 8 0 10 0 3 4d 1 exp 105 000 rt was fitted to our data to describe experimental compaction rates electron backscatter diffraction ebsd analysis performed on one sample showed limited evidence for plastic deformation dauphine twinning and lattice bending at grain contact points under high stress contact microstructures formed during compaction were studied using quartz single crystal discs or a mica plate as reference surfaces this showed that in all contacts microstructures were rough with a micrometer scale roughness many contacts also showed internal microcracking and it is inferred that microcracking is likely an important mechanism for creating and maintaining rough contacts during quartz pressure solution at least under the present experimental conditions extrapolation of our empirical equation for compaction rates to natural conditions is consistent with previous observations that during burial and diagenesis of a sandstone pressure solution starts to operate at depths of about 1 5 2 km becoming the dominant compaction mechanism at depths greater than 2 5 3 km the extrapolation gives good agreement with porosity depth trends reported for natural arenitic sandstones

kinetics of pressure solution at halite silica interfaces and intergranular clay films pressure solution is widely regarded as a potentially important deformation mechanism along crustal faults and during aliagenesis yet the mechanisms and kinetics of this process remain highly controversial to better understand the fundamental factors controwing the rates of pressure solution at the grain to grain scale we conducted experiments in which convex halite lenses were pressed against fiats of fused silica in brine fluid pressures were maintained at 0 1 mpa temperatures and mean contact normal stresses ranged from 8 3 o to 90 2oc and 0 5 to 13 5 mpa respectively the geometry and growth rate of the contact spot between the two lenses and the rate at which the lenses approached one another convergence were monitored using reflected light interferometry and transmitted light photomicrography convergence occurred when halite and silica lenses were pressed together in brine halite silica experiments no undercutting was observed and dry control experiments indicated negligible dislocation creep convergence rates in experiments at 50 2oc ranged from 0 01 to 0 05 m d depending on mean normal stress and contact spot radius the data are consistent with intergranular pressure solution ips rate limited by diffusion through an intergranular film with a very high diffusion coefficient 10 5 10 7 cm2 s the data further suggest that the diffusion coefficient and or thickness of this film increases with decreasing normal stresses at least for normal stresses less than about 4 mpa as no island channel boundary structures were observed we propose that this film consists of a continuous layer of strongly adsorbed i e structured water that is maintained between the halite and silica lenses during deformation convergence rates in similar experiments conducted at a constant load of 0 11 n but at 8 3 o 50 2 o and 90 2oc were approximately constant at a given normal stress and contact spot size the cause of this temperature insensitivity is unknown but might result from changes in interphase boundary structure or thickness with increasing temperature that are sufficient to offset the expected thermal activation an experiment was also conducted in which a halite lens was pressed against a fused silica fiat coated with an 0 8 m thick film of na montmorillonite in brine this clay film produced an approximately fivefold increase in convergence rates over those observed in a halite silica experiment conducted without clay at the same load and temperature the strong sensitivity of ips rates both to contact spot radius and to the presence of second phases along grain boundaries suggests that fine grained clay rich fault gouges and multiphase granular aggregates should be particularly susceptible to pressure solution creep in the middle to upper crust

the production of north atlantic deep water sources rates and pathways updating an earlier account by dickson et al 1990 this paper reviews the initial development phase of north atlantic deep water nadw production from the points where the dense inflows from nordic seas cross the greenland scotland ridge to the point off south greenland where the buildup of new production appears almost complete in particular three long term current meter arrays totaling 91 instruments and set at 160 km intervals south from the denmark strait sill are used to validate earlier short term arrays by others and in combination with these earlier arrays to describe the downstream evolution of mean speed depth and entrainment the variability of the overflow current in space and time and the likely contribution of the other three main constituents of nadw production at densities greater than 27 8 from the points of overflow 5 6 sv the transport within this range increases by entrainment and confluence with other contributory streams to around 13 3 sv at cape farewell while recirculating elements prevent us from determining the net southgoing transport a nadw transport of this order appears consistent with recent estimates of net abyssal flow passing south through the north and south atlantic

principal oscillation pattern analysis of the 30 to 60 day oscillation in general circulation model equatorial troposphere a new technique is described for identifying time dependent patterns i e principal oscillation patterns or pops in a set of geophysical time series pops are defined as the normal modes of a linear dynamical representation of the data in terms of a first order autoregressive vector process with residual noise forcing pops associated with real eigenvalues represent nonpropagating nonoscillatory patterns which decay exponentially pops associated with complex eigenvalues occur in complex conjugate pairs and can represent standing wave structures if one pattern is much stronger than the other propagating waves if both patterns are periodic and have the same structure except for a quarter wavelength shift or in general an arbitrary amphidromal oscillation after the pops have been defined for a selected set of primary variables associated correlation or composite patterns may be derived for additional secondary fields to gain further insight into the structure of the interaction mechanisms the method is illustrated by analyzing the tropical variability structure of a 10 year numerical simulation with the t21 general circulation model gcm of the european centre for medium range weather forecasts reading england the pop analysis is applied to the 200 hpa horizontal divergence field along the equator for time scales shorter than 10 weeks the associated patterns are determined for a number of additional fields in the tropical belt between 30 s and 30 n one dominant pop pair is found its spatial scale corresponds to zonal wave number 1 the patterns travel eastward with an average period of 24 days and an e folding decay time of 10 days the maximum variance occurs in the area with maximum tropical precipitation between 100 e and the dateline these features correspond rather closely to the characteristics of the observed tropical 30 to 60 day wave except for the smaller period a frequency wave number analysis confirms that this 30 to 60 day wave is the most dominant regular oscillation in the tropical gcm troposphere the associated patterns of sea level pressure precipitation and other quantities exhibit a number of intriguing aspects namely 1 the phase velocity of the 30 to 60 day wave varies with longitude from 6 m s 1 in the indonesian area to more than 30 m s 1 over the eastern pacific small phase speeds being associated with large amplitudes and high phase speeds with small amplitudes 2 in the high amplitude regions rainfall and upper air velocity potential are in phase while in the low amplitude regions rainfall and velocity potential appear uncorrelated 3 at the surface a pattern strongly resembling gill s theoretical response to an equatorial heating source is found with a trough to the east and two off equatorial cyclones to the west of the heating the lack of meridional winds is probably related to the disregard of the seasonal asymmetries

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

influence of time temperature confining pressure and fluid content on the experimental compaction of spherical grains theoretical models of compaction processes such as for example intergranular pressure solution ips focus on deformation occurring at the contacts between spherical grains that constitute an aggregate in order to investigate the applicability of such models and to quantify the deformation of particles within an aggregate isostatic experiments were performed in cold sealed vessels on glass sphere aggregates at 200 mpa confining pressure and 350 c with varying amounts of fluid

a microphysical interpretation of rate and state dependent friction for fault gouge the evolution of fault strength during the seismic cycle plays a key role in the mode of fault slip nature of earthquake stress drop and earthquake nucleation laboratory based rate and state dependent friction rsf laws can describe changes in fault strength during slip but the connections between fault strength and the mechanisms that dictate the mode of failure from aseismic creep to earthquake rupture remain poorly understood the empirical nature of rsf laws remains a drawback to their application in nature here we analyze an extensive data set of friction constitutive parameters with the goal of illuminating the microphysical processes controlling rsf we document robust relationships between 1 the initial value of sliding or kinetic friction 2 rsf parameters and 3 the time rates of frictional strengthening aging we derive a microphysical model based on asperity contact mechanics and show that these relationships are dictated by 1 an activation energy that controls the rate of asperity growth by plastic creep and 2 an inverse relationship between material hardness and the activation volume of plastic deformation collectively our results illuminate the physics expressed by the rsf parameters and which describe the absolute value of frictional strength and its dependence on time and slip rate moreover we demonstrate that seismogenic fault behavior may be dictated by the interplay between grain properties and ambient conditions controlling the local shear strength of grain scale asperity contacts

multidecadal and multicentennial variability of the meridional overturning circulation the variability of the meridional overturning circulation moc simulated in a multimillenial control integration of the kiel climate model kcm displays enhanced variability relative to the red background at decadal and centennial timescales the multidecadal variability is the model s version of the atlantic multidecadal variability amv often referred to as atlantic multidecadal oscillation amo while multidecadal variability originates in the north atlantic multicentennial variability is driven in the southern ocean both multidecadal and multicentennial variability are associated with considerable changes in sea ice extent this may be important to understand the different evolution of sea ice cover in the northern and southern hemisphere during the most recent decades with a strong decline observed in the north and almost no trend in the south

origin and global tectonic significance of early archean cherts from the marble bar greenstone belt pilbara craton western australia abstract five sections of bedded chert in mafic ultramafic rocks of the archean warrawoona group in the marble bar greenstone belt pilbara craton were analyzed in order to understand their depositional environment and to provide some constraints on early archean tectonics the sections are divisible into two types based on their field occurrence mineralogy and geochemistry thicker ones a and b that overlie fe rich low k tholeiites and thinner ones c1 c2 and c3 overlying komatiitic basalts the thickest ferruginous section a 45 m thick is the marble bar chert of the towers formation and is interpreted to have been an in situ precipitate derived from a high t hydrothermal solution emanating from a mid oceanic ridge mor the following geochemical features are similar to those of modern hydrothermal iron rich sediments at a mor i p v zn and y are positively correlated with fe ii a positive eu anomaly normalized to chondrite decreases from 6 6 to 1 3 and the magnitude of a negative ce anomaly decreases from 0 6 to 1 0 as ree and lree hree increase the 13 m thick b section in the apex basalt dominated by sio2 and containing significant amounts of ba up to 4330 ppm originated from a low t mor hydrothermal solution this section is characterized by an association with massive black gray silica veins that were hydrothermal feeders in normal fault zones in the spreading center geochemical evidence from the greenstones underlying the b section indicates that they are of morb origin in contrast to a and b c1 c2 and c3 sections are intercalated conformably with komatiitic basalts of the apex basalt and are 3 6 m thick the geochemical signatures of these three sections suggest that they were most likely formed by low t weak hydrothermal activity that may have been associated with hot spot volcanism they show strong enrichment of cr and ni reflecting a significant contribution of komatiitic basaltic detritus during sedimentation the presence of volcaniclastic chert in their uppermost beds indicates a decrease of hydrothermal silica precipitation due to waning hydrothermal activity among the three sections the uppermost c3 exhibits greater enrichment in zr nb hf and th and higher th sc and la yb n relative to the lower c1 and c2 sections this could mean that the depositional environment of c3 was relatively closer to a continent or to island arcs composed of granitoids and or their volcanic equivalents siliceous mudstones and sandstones of the uppermost clastic rocks t section panorama formation have geochemical signatures analogous to those of felsic plutonic volcanic rocks high th sc negative eu anomalies and high ree in some siliceous mudstones from this unit imply that differentiated granitoids or volcanic equivalents were already exposed erupted at least locally in the pilbara at this time this study shows these warrawoona group cherts were deposited in a variety of environments ranging from a mid oceanic spreading center to a convergent plate boundary via a hot spot this variation was most likely due to horizontal plate motions which accordingly support the operation of plate tectonics in the early archean

remnants of a late triassic ocean island in the gufeng area northern tibet implications for the opening and early evolution of the bangong nujiang tethyan ocean abstract in this paper we present new major and trace element compositions of basaltic rocks in the gufeng ocean island gfoi area in the western segment of the bangong nujiang suture zone northern tibet our aim was to assess the genesis of these rocks and discuss the implications of this new dataset for the evolution of the bangong nujiang tethyan ocean an ocean island type double layer structure comprising a basaltic basement and an oceanic sedimentary cover sequence found within the gfoi provides direct evidence for the interpretation that the assemblage is a typical ocean island the basalts in the gfoi can be divided into three types named g1 g2 and g3 basalts and these basalts range in composition from morb to oib types which is typical of ocean islands the g1 basalts have morb type affinities possibly indicating the existence of morb oceanic crust under the gfoi the g2 basalts represent the early stage of formation of the gfoi and are produced by the interaction of rising oib type basaltic magma and the existing morb oceanic crust the g3 basalts are typical oib basalts and they are the products of the direct eruption of oib type basaltic magmas the g3 basalts have high la yb n 12 3 14 4 ce yb n 10 8 11 8 la sm n 2 39 2 76 and sm yb n 4 89 5 23 ratios indicating the presence of oceanic lithosphere below the gfoi with a thickness of 50 60 km geochemical analyses of the gfoi cherts show that they contain terrigenous material indicating the gfoi formed close to a continental margin norian conodont fossils within the gfoi limestones indicate the gfoi formed during the late triassic these data combined with geological evidence and a half space model of lithosphere cooling where the thickness of the oceanic lithosphere is determined from the age of the lithosphere indicate that the western segment of the bangong nujiang tethyan ocean opened initially in the late permian expanded rapidly during the early middle triassic and was a mature ocean during the late triassic

impact of the atlantic multidecadal oscillation on north pacific climate variability 1 in this paper we found that the atlantic multidecadal oscillation amo can contribute to the pacific decadal oscillation pdo especially the component of the pdo that is linearly independent of el nino and the southern oscillation enso i e the north pacific multidecadal oscillation npmo and the associated pacific north america pna pattern using a hybrid version of the gfdl cm2 1 climate model we show that the amo provides a source of multidecadal variability to the north pacific and needs to be considered along with other forcings for north pacific climate change the lagged north pacific response to the north atlantic forcing is through atmospheric teleconnections and reinforced by oceanic dynamics and positive air sea feedback over the north pacific the results indicate that a north pacific regime shift opposite to the 1976 77 shift might occur now a decade after the switch of the observed amo to a positive phase around 1995 citation zhang r and t l delworth 2007 impact of the atlantic multidecadal oscillation on north pacific climate variability geophys res lett 34 l23708

on multidecadal and quasi decadal north atlantic variability observed sea surface temperatures ssts in the north atlantic from 1958 through 2000 as well as data from an ocean model simulation driven with the atmospheric variability observed during the same period are examined using multichannel singular spectrum analysis the two leading oscillatory modes are associated with a multidecadal and a quasi decadal period the former is connected to a basinwide uniform sst pattern and changes in the deep north atlantic meridional overturning circulation the quasi decadal mode involves a tripolar sst anomaly pattern forced by atmospheric variability with a spatial structure resembling that of the north atlantic oscillation nao the upper ocean s dynamical response to this nao variability provides an instantaneous positive feedback to the sst pattern while a delayed negative feedback is due to shallow overturning circulation anomalies

atlantic meridional overturning circulation and the prediction of north atlantic sea surface temperature highlights north atlantic sea surface temperature exhibits high decadal predictability potential model bias hinders exploiting the decadal predictability potential an innovative method was developed to overcome some of the bias problem north atlantic sea surface temperature will stay anomalously warm until about 2030 abstract the atlantic meridional overturning circulation amoc a major current system in the atlantic ocean is thought to be an important driver of climate variability both regionally and globally and on a large range of time scales from decadal to centennial and even longer measurements to monitor the amoc strength have only started in 2004 which is too short to investigate its link to long term climate variability here the surface heat flux driven part of the amoc during 1900 2010 is reconstructed from the history of the north atlantic oscillation the most energetic mode of internal atmospheric variability in the atlantic sector the decadal variations of the amoc obtained in that way are shown to precede the observed decadal variations in basin wide north atlantic sea surface temperature sst known as the atlantic multidecadal oscillation amo which strongly impacts societally important quantities such as atlantic hurricane activity and sahel rainfall the future evolution of the amo is forecast using the amoc reconstructed up to 2010 the present warm phase of the amo is predicted to continue until the end of the next decade but with a negative tendency

new paleomagnetic results from middle jurassic limestones of the qiangtang terrane tibet constraints on the evolution of the bangong nujiang ocean

interdecadal variations of the thermohaline circulation in a coupled ocean atmosphere model abstract a fully coupled ocean atmosphere model is shown to have irregular oscillations of the thermohaline circulation in the north atlantic ocean with a time scale of approximately 50 years the irregular oscillation appears to be driven by density anomalies in the sinking region of the thermohaline circulation approximately 52 n to 72 n combined with much smaller density anomalies of opposite sign in the broad rising region the spatial pattern of see surface temperature anomalies associated with this irregular oscillation bears an encouraging resemblance to a pattern of observed interdecadal variability in the north atlantic the anomalies of sea surface temperature induce model surface air temperature anomalies over the northern north atlantic arctic and northwestern europe

magmatic hydrothermal evolution of the cretaceous duolong gold rich porphyry copper deposit in the bangongco metallogenic belt tibet evidence from u pb and 40ar 39ar geochronology abstract the duolong gold rich porphyry copper deposit was recently discovered and represents a giant prospect inferred resources of 4 5 mt fine cu with a grade of 0 72 cu 30 50 t fine gold with a grade of 0 23 g t au in the bangongco metallogenic belt tibet zircon shrimp and la icp ms u pb geochronology shows that the multiple porphyritic intrusions were emplaced during two episodes the first at about 121 ma bolong mineralized granodiorite porphyry bmgp and barren granodiorite porphyry bgp and the second about 116 ma duobuza mineralized granodiorite porphyry dmgp moreover the basaltic andesites also have two episodes at about 118 ma and 106 ma respectively one andesite yields an u pb zircon age of 111 9 1 9 ma indicating it formed after the multiple granodiorite porphyries by contrast the 40ar 39ar age of 115 2 1 1 ma hydrothermal k feldspar vein hosted in dmgp reveals the close temporal relationship of ore bearing potassic alteration to the emplacement of the dmgp the sericite from quartz sericite vein hosted in dmgp yields a 40ar 39ar age of 115 2 1 2 ma therefore the ore forming magmatic hydrothermal evolution probably persisted for 6 m y additionally the zircon u pb ages 106 121 ma of the volcanic rocks and the porphyries suggest that the neo tethys ocean was still subducting northward during the early cretaceous

mesozoic subduction accretion history in central tibet constrained from provenance analysis of the mugagangri subduction complex in the bangong nujiang suture zone

stochastically forced multidecadal variability in the north atlantic a model study observations show a multidecadal signal in the north atlantic ocean but the underlying mechanism and cause of its timescale remain unknown previous studies have suggested that it may be driven by the north atlantic oscillation nao which is the dominant pattern of winter atmospheric variability to further address this issue the global ocean general circulation model nucleus for european modelling of the ocean nemo is driven using a 2 000 years long white noise forcing associated with the nao focusing on key ocean circulation patterns we show that the atlantic meridional overturning circulation amoc and sub polar gyre spg strength both have enhanced power at low frequencies but no dominant timescale and thus provide no evidence for a oscillatory ocean only mode of variability instead both indices respond linearly to the nao forcing but with different response times the variability of the amoc at 30 n is strongly enhanced on timescales longer than 90 years while that of the spg strength starts increasing at 15 years the different response characteristics are confirmed by constructing simple statistical models that show amoc and spg variability can be related to the nao variability of the previous 53 and 10 winters respectively alternatively the amoc and the spg strength can be reconstructed with auto regressive ar models of order seven and five respectively both statistical models reconstruct interannual and multidecadal amoc variability well while on the other hand the ar 5 reconstruction of the spg strength only captures multidecadal variability using these methods to reconstruct ocean variables can be useful for prediction and model intercomparision

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

local and remote influences on the heat content of the labrador sea an adjoint sensitivity study the labrador sea is one of the few regions on the planet where the interior ocean can exchange heat directly with the atmosphere via strong localized wintertime convection with possible implications for the state of north atlantic climate and global surface warming using an observationally constrained ocean adjoint model we find that annual mean labrador sea heat content is sensitive to temperature salinity changes 1 along potential source water pathways e g the subpolar gyre the north atlantic current the gulf stream and 2 along the west african and european shelves which are not significant source water regions for the labrador sea the west african coastal shelf adjustment mechanism which may be excited by changes in along shelf wind stress involves pressure anomalies that propagate along a coastal waveguide towards greenland changing the across shelf pressure gradient in the north atlantic and altering heat convergence in the labrador sea we also find that non local in space and time heat fluxes e g in the irminger sea the seas south of iceland can have a strong impact on labrador sea heat content understanding and predicting the state of the labrador sea and its potential impacts on north atlantic climate and global surface warming will require monitoring of oceanic and atmospheric properties at remote sites in the irminger sea the subpolar gyre and along the west african and european shelf coast system among others

interdecadal variations in north atlantic sea surface temperature and associated atmospheric conditions abstract evidence is presented for a distinct pattern of ocean atmosphere relationship associated with interdecadal variability in the north atlantic region using a century of surface marine observations it is shown that middle and high latitude sea surface temperature sst display a long term fluctuation with negative anomalies before 1920 and during the 1970s and 1980s positive sst conditions prevailed from about 1930 to 1960 the pattern of interdecadal sst variability is constructed by subtracting the average field during 15 cold years from that during a similar interval of warm years the early century warming and the more recent cooling display a similar spatial pattern in both cases the pattern is basin scale and largely of one polarity with maxima in the vicinity of iceland in the labrador sea and northeast of bermuda the corresponding differences in surface atmospheric conditions are determined by averaging and subtracting in the same manner fields of sea level pressure slp and surf

a mechanism for atlantic multidecadal variability in the kiel climate model atlantic multidecadal variability amv is investigated in a millennial control simulation with the kiel climate model kcm a coupled atmosphere ocean sea ice model an oscillatory mode with approximately 60 years period and characteristics similar to observations is identified with the aid of three dimensional temperature and salinity joint empirical orthogonal function analysis the mode explains 30 of variability on centennial and shorter timescales in the upper 2 000 m of the north atlantic it is associated with changes in the atlantic meridional overturning circulation amoc of 1 2 sv and atlantic sea surface temperature sst of 0 2 c amv in kcm results from an out of phase interaction between horizontal and vertical ocean circulation coupled through irminger sea convection wintertime convection in this region is mainly controlled by salinity anomalies transported by the subpolar gyre spg increased decreased dense water formation in this region leads to a stronger weaker amoc after 15 years and this in turn leads to a weaker stronger spg after another 15 years the key role of salinity variations in the subpolar north atlantic for amv is confirmed in a 1 000 year long simulation with salinity restored to model climatology no low frequency variations in convection are simulated and the 60 year mode of variability is absent

experimental study of the compaction of phyllosilicate bearing sand at elevated temperature and with controlled pore water pressure relative to the time dependent compaction behavior of clean quartz sands little experimental work has been carried out on phyllosilicate rich siliciclastic rocks mudstones compaction is expected to be produced by intracrystalline plastic deformation of the phyllosilicate matrix coupled with pressure solution of embedded quartz grains and with silica reprecipitation in the adjacent pore spaces or loss from the system via an advective flux of pore water time dependence of compaction also arises in nature owing to the generation of excess pore pressure which lowers effective stress and hence compaction rate until such pressure excess becomes dissipated by permeation a suite of experiments was performed on synthetic mixtures of quartz sand in a matrix of fine grained illite muscovite hydrostatically compacted at a range of effective pressures between 10 mpa and 210 mpa with a constant pore water pressure of 70 mpa and at temperatures ranging between 300 and 450 c a small adjacent volume of porous quartz sand was provided as a precipitation sink for silica dissolved from the phyllosilicate quartz mixture volume strains were measured by a direct measurement of sample length and b measurement of amount of pore water expelled at constant pore pressure and interpreted with the aid of quantitative petrography on compacted samples control experiments were performed on i dry clean sand ii dry sand phyllosilicate and iii wet clean sand dry clean sand showed almost no compaction wet clean sand showed a degree of compaction similar to dry phyllosilicate sand but wet phyllosilicate sand showed dramatically more compaction compaction rate for the wet mixed phyllosilicate sand samples is described by formula 1 in which is effective stress mpa is porosity r is the gas constant and t is temperature k petrographic study showed interpenetration of grain contacts formation of quartz overgrowths a decrease in the amount of quartz embedded in the phyllosilicate matrix and an increase in mean grain size of quartz with progressive compaction these effects are attributed to solution and redeposition of quartz the empirical compaction law was integrated over a natural burial history the porosity evolution is extremely sensitive to the evolution of pore fluid pressure and relatively less to temperature gradient and burial rate there is reasonable qualitative agreement when compared with porosity depth relations reported in the literature 1 embed graphic 1 gif

early cretaceous morb type basalt and a type rhyolite in northern tibet evidence for ridge subduction in the bangong nujiang tethyan ocean abstract new zircon u pb ages major and trace element data and hf isotopic compositions are presented for bimodal volcanic rocks of the zhaga formation zf in the western middle segment of the bangong nujiang suture zone bnsz northern tibet the genesis of these rocks is described and implications for late stage evolution of the bangong nujiang tethyan ocean bnto are considered detailed studies show that the zf bimodal rocks which occur as layers within a typical bathyal to abyssal flysch deposit comprise morb type basalt that formed at a mid ocean ridge and low k calc alkaline a type rhyolite derived from juvenile crust the combination of morb type basalt calc alkaline a type rhyolite and bathyal to abyssal flysch deposits in the zf leads us to propose that they formed as a result of ridge subduction the a type zf rhyolites yield la icp ms zircon u pb ages of 118 112 ma indicating formation during the early cretaceous data from the present study combined with regional geological data indicate that the bnto underwent conversion from ocean opening to ocean closure during the late jurassic early cretaceous the eastern segment of the bnto closed during this period while the western and western middle segments were still at least partially open and active during the early cretaceous accompanied by ridge subduction within the bangong nujiang tethyan ocean

flattening of the sea floor depth age curve as a response to asthenospheric flow the flattening of sea floor depths from the age dependence predicted by considering the cooling plate as a growing thermal boundary layer is a fundamental constraint on the evolution of oceanic lithosphere1 previous explanations for the flattening have included reheating from convective instabilities that form beneath lithosphere older than 80 myr ref 2 thermal rejuvenation of lithosphere that passes over stationary hotspots3 5 or a whole mantle flow forced by two converging plates6 we suggest here that the flattening of old ocean floors can perhaps best be explained as a dynamic phenomenon reflecting flow in asthenosphere underlying the oceanic lithosphere applying the model to the pacific plate we show that a solution for flow in an asthenosphere low viscosity channel which is consumed by plate accretion and the subduction of lithosphere at trenches and replenished by near ridge upwelling and near ridge hotspots generates the observed 1 km of sea floor flattening for asthenospheric viscosities of 2 1018 pa s and an absolute pacific plate motion of 100 mm yr 1 our model can also explain the asymmetric subsidence of the south american and african plates away from the mid atlantic ridge

observation of decadal change in the atlantic meridional overturning circulation using 10 years of continuous transport data the meridional overturning circulation moc represents the main mechanism for the oceanic northward heat transport in the atlantic and fluctuations of this circulation are believed to have major impacts on northern hemisphere climate while numerical ocean and climate models and paleo records show large variability in this circulation the use of direct observations of the moc for detecting climate timescale changes has proven difficult so far this report presents the first observational record of moc measurements that is continuous and sufficiently long to exhibit decadal scale changes here a decrease by 20 over the observational period jan 2000 june 2009 and large interannual changes in the flow and its vertical structure data are from a mooring array at 16 n meridional overturning variability experiment move the observed change agrees with the amplitude of multi decadal natural fluctuations seen in numerical ocean and climate models knowledge of the existence and phasing of such internal cycles provides multi decadal climate predictability recently some numerical model simulations have produced results that show a weakening of the moc since the 1990 s and observational confirmation of this now is a high priority

provenance of mesozoic clastic rocks within the bangong nujiang suture zone central tibet implications for the age of the initial lhasa qiangtang collision abstract the bangong nujiang suture zone separating the lhasa and qiangtang blocks of the tibetan plateau is marked by remnants of the bangong nujiang oceanic basin in the gaize area of central tibet mesozoic sedimentary strata recording the evolution of the basin and subsequent collision between these two blocks include the upper triassic lower jurassic turbidites of the mugagangri group the upper jurassic lower cretaceous sandstone dominated wuga and shamuluo formations and the upper cretaceous molasse deposits of the jingzhushan formation the shamuluo and jingzhushan formations rest unconformably on the underlying mugagangri group and wuga formation respectively in this contribution we analyze petrographic components of sandstones and u pb hf isotopic compositions of detrital zircons from the wuga and jingzhushan formations for the first time based on the youngest detrital zircon ages the maximum depositional ages of the wuga and jingzhushan formations are suggested to be 147 150 ma and 79 91 ma respectively petrographic and isotopic results indicate that sediments in the wuga formation were mainly sourced from the accretionary complex preserved as the mugagangri group in the north while sediments in the jingzhushan formation have mixed sources from the lhasa block the qiangtang block and the intervening suture zone provenance analysis together with regional data suggests that the upper jurassic lower cretaceous wuga and shamuluo formations were deposited in a peripheral foreland basin and a residual sea basin respectively in response to the lhasa qiangtang collision whereas the upper cretaceous jingzhushan formation reflects continental molasse deposition during the post collisional stage the development of the peripheral foreland basin evidenced by deposition of the wuga formation reveals that the age of the initial lhasa qiangtang collision might be the latest jurassic 150 ma

experimental investigation into the microstructural and mechanical evolution of phyllosilicate bearing fault rock under conditions favouring pressure solution abstract mature crustal fault zones are known to be zones of persistent weakness this weakness is believed to result from microstructural modifications during deformation such as grain size reduction and foliation development around the brittle ductile transition phyllosilicates are expected to have a significant effect on fault strength in particular under conditions favouring pressure solution to study such effects we performed rotary shear experiments on brine saturated halite kaolinite mixtures aimed at investigating the relation between microstructural and mechanical evolution in a system where pressure solution and cataclasis dominate the results show significant strain weakening and a transition with progressive strain towards more rate sensitive and less normal stress sensitive behaviour this was accompanied by a microstructural evolution from a purely cataclastic microstructure to a mylonitic microstructure consisting of elongate asymmetric clasts in a fine grained foliated matrix the results demonstrate that strain weakening and the development of a typical mylonitic microstructure can occur as a consequence of grain size reduction by cataclasis and a transition to pressure solution accommodated deformation even in the absence of dislocation creep the data raise questions regarding the reliability of microstructures as rheology indicators as well as on the use of low strain monomineralic flow laws for modelling crustal dynamics

rare earth element partition coefficients from enstatite melt synthesis experiments enstatite en80fs19wo01 was synthesized from a hypersthene normative basaltic melt doped at the same time with la ce nd sm eu dy er yb and lu the rare earth element concentrations were measured in both the basaltic glass by empa and the enstatite by sims rare earth element concentrations in the glass were determined by electron microprobe analysis with uncertainties less than two percent relative rare earth element concentrations in enstatite were determined by secondary ion mass spectrometry with uncertainties less than five percent relative the resulting rare earth element partition signature for enstatite is similar to previous calculated and composite low ca pigeonite signatures but is better defined and differs in several details first the enstatite lree signature is steeper than for pigeonite second the signature has a negative eu anomaly which relates to oxygen fugacity and third the dependence of the partition coefficients on alumina content is linked to crystal structural constraints

opposing decadal changes for the north atlantic meridional overturning circulation hydrographic properties of the north atlantic ocean have changed significantly over the past decades a combination of changes in seawater density calculated from observed properties of sea water and a numerical ocean model reveals that the strength of the meridional overturning circulation has changed in different directions in the subpolar and subtropical basins respectively

reconstructing monitoring and predicting multidecadal scale changes in the north atlantic thermohaline circulation with sea surface temperature sea surface temperature sst observations in the north atlantic indicate the existence of strong multidecadal variability with a unique spatial structure it is shown by means of a new global climate model which does not employ flux adjustments that the multidecadal sst variability is closely related to variations in the north atlantic thermohaline circulation thc the close correspondence between the north atlantic sst and thc variabilities allows in conjunction with the dynamical inertia of the thc for the prediction of the slowly varying component of the north atlantic climate system it is shown additionally that past variations of the north atlantic thc can be reconstructed from a simple north atlantic sst index and that future anthropogenically forced changes in the thc can be easily monitored by observing ssts the latter is confirmed by another state ofthe art global climate model finally the strong multidecadal variability may mask an anthropogenic signal in the north atlantic for some decades

molybdenite re os and shrimp zircon u pb dating of duobuza porphyry copper deposit in tibet and its geological implications the duobuza porphyry copper deposit the first large size porphyry copper deposit discovered along the bangong nujiang suture zone is situated in the duobuza tectonic magmatic zone on the southern margin of the qiangtang sanjiang massif the main mineralized porphyry in the ore deposit is a granodiorite porphyry whose sio2 content varies from 61 37 to 67 73 65 16 on the average its ree content is 41 4 94 10 6 and the lree content is higher than the hree content suggesting the enrichment of lree as for trace elements the porphyry shows enrichment of rb k th ba la ce sr and depletion of nb ta p ti the major rare earth and trace elements of the mineralized porphyry have characteristics of volcanic rocks formed in the island arc the zircon shrimp dating 14 samples of duobuza granodiorite porphyry gave 206pb 238u concordant ages from 116 5 4 6 ma to 125 3 4 7 ma with a weighted mean age of 120 9 2 4 ma which indicates the age of the copper mineralized porphyry rock meanwhile the 187re and 187os content of molybdenite from the ore deposit was determined by inductively coupled plasma mass sperctrometry icp ms and the results show that the re os model ages of the deposit are from 117 6 1 3 to 118 5 1 4 ma and its isochron age 118 0 1 5 ma which have accurately determined the timing of mineralization the age of the ore deposit is consistent with the evolution of the bange nujiang archipelagic arc basin system in early cretaceous implying that the duobuza copper deposit was formed in an archipelagic are basin system similar to the system of southeast asia

impact of enso on east asian winter monsoon during interglacial periods effect of orbital forcing orbital forcing influences climate phenomena by changing incoming solar radiation in season and latitude here changes in the el nino southern oscillation enso s impact on the east asian winter monsoon eawm due to orbital forcing especially for three selected time periods in each of two interglacial periods the eemian 126 122 115 ka and holocene 9 6 0 ka are investigated there was a high negative correlation between enso and eawm when the obliquity was low the processional angle was large and especially when accompanied by large eccentricity which corresponds to a weaker monsoon period the correlation was also high when enso variability was high which interestingly corresponded to lower obliquity and higher degree precession periods therefore as both lower obliquity and higher degree precession such as during 115 ka and 0 ka cause the eawm to be weakened through higher winter insolation over northern hemisphere and the enso to be enhanced through an intensified zonal contrast of the equatorial sea surface temperature the relationship between the enso and eawm becomes tighter the opposite case i e during 126 and 9 ka is also true dynamically furthermore the sensitivity of boreal winter precipitation against sea surface temperature sst anomaly over the tropical pacific which depends on mean sst was positively correlated to the strength of the enso eawm correlation implying that the warmer mean ocean surface causes the strong response of atmosphere to change in the sst anomaly thereby enhancing the impact of enso on eawm warmer wintertime tropical sst is attributed to higher insolation over the tropics especially during 115 and 0 ka while cooler ssts occurred during 126 and 9 ka

influence of grain boundary structural evolution on pressure solution creep rates intergranular pressure solution is a well known rock deformation mechanism in wet regions of the upper crust and has been widely studied especially in the framework of compaction of granular materials such as reservoir sandstones and fault rocks several analytical models exist that describe compaction creep by stress induced mass transport and the parameters involved are relatively well constrained by laboratory experiments while these models are capable of predicting compaction behavior observed at relatively high porosities they often overestimate compaction rates at porosities below 20 by up to several orders of magnitude this suggests that the microphysical processes operating at low porosities are different and are not captured well by existing models the implication is that available models cannot be extrapolated to describe compaction of sediments and fault rocks to the low porosities often reached under natural conditions to address this problem we propose a new thermodynamic model that describes the decline of pressure solution rates within individual grain contacts as a result of time averaged growth of asperities or islands and associated constriction of the grain boundary diffusion path here termed grain boundary evolution the resulting constitutive equations for single grain grain contacts are then combined and solved semianalytically the compaction rates predicted by the model are compared with those measured in high strain compaction experiments on wet granular halite a significant reduction in compaction rate is predicted when grain boundary evolution is considered which compares favorably with the experimental compaction data

the north anatolian fault zone and the role of fluids in seismogenic deformation a study of the north anatolian fault zone turkey in dicates significant spatial variations of deformation mechanisms and fluid flow along and across a major active strike slip fault brittle deformation and later dissolution are mostly localized within the fault core whereas crystal plastic deformation and diffusive mass transfer are more widely distributed the fault underwent a three stage development i repeated episodes of frictional failure pressure solution fluid migration and subsequent healing cataclasites and veins were formed during this stage we propose that the episodic events are associated with open and closed fluid systems during seismogenic cycles the progressive brittle failure led to stage ii a more open system with increasing dilation connected with dissolution the dramatic increase of dissolution processes terminated the episodic character of deformation and marked the beginning of a stage iii mostly characterized by diagenetic processes the fluid regime is now an op

the evolution of the bangong nujiang neo tethys ocean evidence from zircon u pb and lu hf isotopic analyses of early cretaceous oceanic islands and ophiolites abstract we conducted in situ u pb analyses of zircons from three basalts and one gabbro from the zhonggang oceanic island one basalt from the zhaga oceanic island and one gabbro from the kangqiong ophiolite all located in the middle segments of the bangong nujiang suture zone of tibetan plateau as well as in situ hf isotope analyses of zircons from one gabbro from the zhonggang oceanic island to constrain the tectonic evolution of the bangong nujiang neo tethys ocean all samples contain numerous inherited zircons and all the zircons contain magmatic oscillatory zoning and have th u ratios exceeding 0 4 moreover the average ree content of these zircons is less than 2000 ppm and they display clear negative eu and variable positive ce anomalies indicating a magmatic origin la icp ms u pb dating of the zircons revealed three clear peaks in the age distribution at 248 255 162 168 and 117 120 ma lu hf isotopic analyses of zircons from the gabbro of the zhonggang oceanic island yielded a 269 ma crust mantle separation age taking into account the regional geology previous data and our new analyses we infer that the middle and western segments of the bangong nujiang neo tethys ocean had initially opened in the late permian 254 269 ma and that the ocean opened substantially between the late permian and the early triassic 248 255 ma in addition we infer that the initiation of subduction of the bangong nujiang neo tethys ocean took place at 162 168 ma which is middle jurassic the 117 120 ma age is the time when the oceanic islands and ophiolites were formed indicating that the bangong nujiang neo tethys ocean was to some extent still open at that time

order from chaos a field based estimate on bulk rheology of tectonic melanges formed in subduction zones abstract many tectonic hp lt melanges exhibit block in matrix structures in contrast to the pervasively deformed matrix the blocks of various lithologies and pt histories usually appear undeformed if we accept that an hp lt melange reflects persistent high strain deformation in a deep 30 km subduction channel the record of these rock associations can provide insight into stress state rheology and flow patterns along a plate interface the bulk rheology of a block in matrix structure is controlled by a the dominant deformation mechanism of the matrix b the volume fraction of blocks engulfed in that matrix and c the geometry of the blocks the microstructures of the matrix indicate distributed deformation by dissolution precipitation creep dpc while crystal plastic deformation is subordinate dpc is grain size sensitive controlled by the type of interfaces and requires an aqueous pore fluid at quasi lithostatic pressure the rheology of a rock body undergoing deformation by dpc is believed to be newtonian viscous matrix viscosities are estimated to be on the order of 10 19 pa s or less here we explore the effect of rigid blocks embedded in a weak matrix on the bulk rheology using stereology we determine the block size distributions as well as the total block volume fractions for block in matrix structures from several regions the volume fractions are found to range from 2 to 70 with typical values between 5 and 50 using published mixing laws we model the subduction channel as a suspension of rigid spheres with a linear viscous matrix and predict that for the typical values of 5 to 50 the viscosity increase is less than one order of magnitude consequently the influence of blocks on the bulk viscosity of a subduction channel is minor instead bulk rheology is primarily controlled by the deformation behaviour of the matrix material

a reversal of climatic trends in the north atlantic since 2005 the mid 1990s warming of the north atlantic subpolar gyre was probably related to strengthened overturning observations and numerical models suggest that a climate reversal to a cooling trend occurred around 2005

slowing of the atlantic meridional overturning circulation at 25 n the atlantic meridional overturning circulation carries warm upper waters into far northern latitudes and returns cold deep waters southward across the equator its heat transport makes a substantial contribution to the moderate climate of maritime and continental europe and any slowdown in the overturning circulation would have profound implications for climate change a transatlantic section along latitude 25 n has been used as a baseline for estimating the overturning circulation and associated heat transport here we analyse a new 25 n transatlantic section and compare it with four previous sections taken over the past five decades the comparison suggests that the atlantic meridional overturning circulation has slowed by about 30 per cent between 1957 and 2004 whereas the northward transport in the gulf stream across 25 n has remained nearly constant the slowing is evident both in a 50 per cent larger southward moving mid ocean recirculation of thermocline waters and also in a 50 per cent decrease in the southward transport of lower north atlantic deep water between 3 000 and 5 000 m in depth in 2004 more of the northward gulf stream flow was recirculating back southward in the thermocline within the subtropical gyre and less was returning southward at depth

petrogenesis and tectonic implications of late cretaceous highly fractionated i type granites from the qiangtang block central tibet abstract the tectonic evolution of the lhasa and qiangtang collision zone is poorly understood due to a lack of pivotal magmatic records and complex geology in central tibet in this study we present zircon u pb ages whole rock major and trace elements and sr nd pb isotopic data for the newly discovered chuburi pluton in the southern qiangtang block zircon u pb data reveal that the chuburi pluton was emplaced during the late cretaceous ca 73 ma bulk rock analyses show that this intrusion is characterized by high sio2 na2o and k2o but low mgo cao and p2o5 the granites are enriched with large ion lithophile elements and light rare earth elements with marked eu anomalies and there are few heavy rare earth elements and high field strength elements these geochemical features indicate that the chuburi pluton is a highly fractionated i type granite the nine granitic samples show high 87sr 86sr i ratios 0 7064 0 7089 negative end t values 1 94 to 5 59 and have nd isotopic model ages ranging from 0 76 to 0 97 ga the initial pb isotopic ratios of the chuburi pluton is uniform 206pb 204pb t 18 774 18 884 207pb 204pb t 15 695 15 713 208pb 204pb t 39 362 39 613 based on geochemical characteristics of the chuburi pluton and consideration of the region geological setting we suggest that the parent magma was generated by magmatic mixing of mantle and lower crust and then undergoing extensive fractional crystallization with removal of k feldspar plagioclase mafic mineral and minor accessory mineral the crust beneath the central tibet may have been significantly thickened following the final lhasa qiangtang amalgamation we argue the generation of the chuburi pluton can be interpreted as a result of regional lithospheric delamination

volume changes during diagenesis abstract the literature pertaining to volume change during diagenesis of clastic sediments is reviewed with respect to the problem of calculating pre and syn compaction thicknesses of sediments for basin reconstruction and stragraphic correlation studies four major mechanisms for volume change are identified mechanical compaction mechanical dissolution chemical dissolution and phase change the first two of these are found to be strongly dependent on the effective stress whilst the latter two show at least a pressure dependence quantification of the relationships between porosity and depth of burial of a sediment seems to be possible only for specific examples of the first of these processes at present this quantification is dealt with in the accompanying related publication

numerical modeling of crenulation cleavage development a polymineralic approach abstract the finite element method was used to investigate how the elastic interactions of quartz and muscovite minerals affect grain scale stress and strain distributions at different stages of crenulation cleavage development the polymineralic structure comprises individual grains that were each assigned their own 3d stiffness tensor and orientation gradients in mean stress and volumetric strain within quartz grains develop between the limbs and hinges of microfolds at the earliest stages of crenulation development with higher values in the microfold limbs these gradients decrease with development of the crenulation cleavage as the microfold limbs become phyllosilicate rich p domains and the hinges become quartz and feldspar rich qf domains crystallographic orientations of the quartz grains have a relatively minor effect on the mean stress and volumetric strain distributions our findings are broadly consistent with both pressure solution and strain driven dissolution models for crenulation cleavage development however because crenulation cleavage development typically involves metamorphic reactions we favor a model in which dissolution is driven by those reactions and mass transfer leading to development of the mineralogically segregated fabric is driven by pore fluid pressure gradients that follow gradients in volumetric strain local concentrations of stress and strain across mineral interfaces may identify sites of enhanced reaction

on the pacific decadal oscillation and the atlantic multidecadal oscillation might they be related 1 the nature of the pacific decadal oscillation pdo is investigated based upon analyses of sea surface temperature observations over the last century the pdo is suggested to be comprised of a 20 year quasi periodic oscillation and a lower frequency component with a characteristic timescale of 60 years the 20 year quasi periodic oscillation is clearly identified as a phase locked signal at the eastern boundary of the pacific basin which could be interpreted as the signature of an ocean basin mode we demonstrate that the 60 year component of the pdo is strongly time lag correlated with the atlantic multidecadal oscillation amo on this timescale the amo is shown to lead the pdo by approximately 13 years or to lag the pdo by 17 years this relation suggests that the amo and the 60 year component of the pdo are signatures of the same oscillation cycle

early jurassic and late cretaceous granites in the tongka micro block central tibet implications for the evolution of the bangong nujiang ocean abstract the tongka micro block located in the eastern section of the bangong nujiang suture and characterized by mesozoic granitoids and high grade metamorphic rocks holds the key to the evolution of the bangong nujiang ocean bno here we present a systematic petrologic geochemical zircon u pb hf and whole rock sr nd pb isotopic investigation on six new granitoids in tongka the results suggest these granitoids yielded two stages of crystallization 171 177 ma and 74 ma and two stages of metamorphism 201 215 ma and 170 175 ma combined with the existing work within bns the magmatism along the bns can be divided into five stages stage 1 240 250 ma stage 2 200 230 ma stage 3 150 190 ma stage 4 100 135 ma and stage 5 73 100 ma both the early jurassic and late cretaceous tongka granitoids in this study are high k calc alkaline series with enrichments in large ion lithophile elements e g cs rb and k and depletions of large ion lithophile elements e g nb ta and ti the variable ehf t values 17 3 to 7 6 end t values 12 08 to 11 15 and ancient tdm2 1 48 2 00 ga for the early jurassic granitoids and weakly negative ehf t values 6 3 to 1 5 end t values 8 75 to 8 60 and old tdm2 ages 1 07 1 33 ga for the late cretaceous granitoids suggest that they are both mainly derived from the melting of ancient crust with minor mantle material input however the formation of the early jurassic and late cretaceous granitoids in tongka is related to different tectonic settings roll back after the tongka micro block subducted beneath the southern qiangtang subterrane and extension collapse after the tongka micro block collided with the northern lhasa subterrane the initial subduction of the bno may start from 190 ma and the collision between the lhasa and qiangtang terranes may continute to the latest cretaceous

sedimentary successions and onset of the mesozoic qiangtang rift basin northern tibet southwest china insights on the paleo and meso tethys evolution abstract the mesozoic qiangtang basin is investigated to reconstruct the tectonic evolution of the paleo and meso tethys oceans we present a detailed sedimentary facies analysis of the nadi kangri and quemoco formations and the systematic zircon u pb ages of the volcanoclastic rocks in the nadi kangri formation to understand the onset and nature of the mesozoic qiangtang basin the nadi kangri formation consisting of volcanic volcanoclastic facies associated with alluvial diluvial facies association which represents continental volcanic succession the quemoco formation consisting of braided fluvial restricted platform and tidal flat facies associations represents continental siliciclastic and marine carbonate successions the deepening upward transgressive sequences reconstructed from the two formations are indicative of a rift basin zircon u pb ages of the volcanoclastic rocks in the bottom middle and upper sections of the nadi kangri formation in the fanghu area are 221 7 1 3 ma 217 0 1 5 ma and 207 1 1 3 ma respectively combined with the previous zircon ages of the nadi kangri formation we interpreted that 221 7 220 4 ma is the initial age for the onset of the mesozoic rift basin an evolution pattern for the tethys ocean and mesozoic qiangtang basin has been proposed based on the magmatism metamorphism sedimentary sequence and deformation data obtained from the kunlun klt hoh xil bayan har hbt north qiangtang nqs central uplift belt cub south qiangtang sqs and north lhasa subterranes terranes nls during the early triassic middle jurassic based on the new data we interpret a double sided subduction and subsequently a collision of the paleo tethys ocean beneath the klt to north and nqs to south probably occurring at ca 252 237 ma and 237 227 ma respectively this interpretation is consistent with the late triassic residual sea ocean model that attributed to the origin of the hoh xil bayan har basin the qt and nls experienced a strong extension between ca 225 and 201 ma owing to the rapid expansion of the bangong lake nujiang ocean meso tethys ocean which is consistent with the massive volcanic eruption and deposition of volcanic sediments in the qt and nls in addition the qt and nls experienced a stabilized subsidence during 201 166 ma due to the existence of the mature bangong lake nujiang ocean

a comparison between rate and state friction and microphysical models based on numerical simulations of fault slip rate and state friction rsf is commonly used for the characterisation of laboratory friction experiments such as velocity step tests however the rsf framework provides little physical basis for the extrapolation of these results to the scales and conditions of natural fault systems and so open questions remain regarding the applicability of the experimentally obtained rsf parameters for predicting seismic cycle transients as an alternative to classical rsf microphysics based models offer means for interpreting laboratory and field observations but are generally over simplified with respect to heterogeneous natural systems in order to bridge the temporal and spatial gap between the laboratory and nature we have implemented existing microphysical model formulations into an earthquake cycle simulator through this numerical framework we make a direct comparison between simulations exhibiting rsf controlled fault rheology and simulations in which the fault rheology is dictated by the microphysical model even though the input parameters for the rsf simulation are directly derived from the microphysical model the microphysics based simulations produce significantly smaller seismic event sizes than the rsf based simulation and suggest a more stable fault slip behaviour our results reveal fundamental limitations in using classical rate and state friction for the extrapolation of laboratory results the microphysics based approach offers a more complete framework in this respect and may be used for a more detailed study of the seismic cycle in relation to material properties and fault zone pressure temperature conditions

the middle triassic evolution of the bangong nujiang tethyan ocean evidence from analyses of oib type basalts and oib derived phonolites in northern tibet in this paper we present new major and trace element chemical data for the basalts and phonolites of the nare ocean island fragment naoi as well as zircon u pb age data and hf isotope compositions for the naoi phonolites in the middle segment of the bangong nujiang suture zone northern tibet our aim is to assess the genesis of these rocks and to reconstruct the middle triassic evolution of the bangong nujiang tethyan ocean bnto the naoi retains an ocean island type double layered structure comprising a basaltic basement and an oceanic sedimentary cover sequence conglomerate and limestone the latter accompanied by layers of erupted phonolite near the top of the sequence the basalts in the naoi are enriched in light rare earth elements and high field strength elements nb ta zr hf and ti and they exhibit chondrite normalized ree patterns and primitive mantle normalized trace element patterns similar to those of ocean island basalts taking into consideration their high dy yb sm yb and la sm ratios we conclude that the naoi basalts were derived from the partial melting of garnet peridotite in the mantle the naoi phonolites have lree enriched chondrite normalized ree patterns with negative eu anomalies eu eu 0 41 0 43 and primitive mantle normalized trace element patterns with enrichments in nb ta zr and hf and depletions in ba u sr p and ti given the high contents of nb 172 256 ppm ta 11 8 16 0 ppm zr 927 1117 ppm and hf 20 8 26 9 ppm and the very low contents of mgo 0 11 0 25 wt the very low mg values 5 10 and the near zero contents of cr 1 27 7 59 ppm ni 0 43 7 19 ppm and co 0 11 0 38 ppm and the small and homogeneously positive e hf t values 4 9 to 9 5 we infer that the naoi phonolites were formed by the fractional crystallization of an oib derived mafic parent magma the phonolites of the naoi contain zircons that yielded u pb ages of 239 and 242 ma indicating that the naoi formed during the middle triassic these data combined with data from modern ocean islands e g canary islands cape verde fernando de noronha tristan da cunha and gough in the atlantic ocean and society and austral cook in the pacific ocean lead us to infer that the bnto was open for a long time before the middle triassic and that the ocean had already developed into a mature ocean with a thick oceanic lithosphere by at least the middle triassic

north atlantic interdecadal variability oceanic response to the north atlantic oscillation 1865 1997 abstract in contrast to the atmosphere knowledge about interdecadal variability of the north atlantic circulation is relatively restricted it is the objective of this study to contribute to understanding how the north atlantic circulation responds to a forcing by the north atlantic oscillation nao on interdecadal timescales for this purpose the authors analyze observed atmospheric and sea surface temperature sst data along with the response of an ocean general circulation model to a realistic monthly surface flux forcing that is solely associated with the nao for the period 1865 1997 in agreement with previous studies it is shown that the relationship between the local forcing by the nao and observed sst anomalies on interdecadal timescales points toward the importance of oceanic dynamics in generating sst anomalies a comparison between observed and modeled sst anomalies reveals that the model results can be used to assess interdecadal variability of the north atlantic circulation the observed

interannual to decadal variability of outflow from the labrador sea a decade of weak convection in the labrador sea associated with decreasing water mass transformation in combination with advective and eddy fluxes into the convection area caused significant warming of the deep waters in both the central labrador sea and boundary current system along the labrador shelf break the connection to the export of deep water was studied based on moored current meter stations between 1997 and 2009 at the exit of the labrador sea near the shelf break at 5 3n more than 100 year long current meter records spanning the full water column have been analyzed with respect to high frequency variability decaying from the surface to the bottom layer and for the annual mean flow showing intra to interannual variability but no detectable decadal trend in the strength of the deep and near bottom flow out of the labrador sea

cretaceous structures in the duolong region of central tibet evidence for an accretionary wedge and closure of the bangong nujiang neo tethys ocean abstract this article focuses on the cretaceous structures in the duolong region northern tibet our aim was to constrain the metallogenic setting of the duolong giant porphyry cu au deposit and to discuss the implications for the closure of the bangong nujiang neo tethys ocean there are two tectonic stages in the duolong region during the early to late cretaceous the first stage is characterised by numerous large scale folds and faults with ne sw compression the 40ar 39ar plateau age of sericite from mylonite is 127 8 1 1 ma which represents the timing of tectonic emplacement of the jurassic mugagangri formation jm and ophiolite this stage corresponds to the northward subduction of the bangong nujiang neo tethys ocean the second stage is characterised by the duolong thrust nappe with a series of sinter deposits slip surfaces fault fracture zones klippes and tectonic windows basalt from the hanging wall yields a zircon u pb age of 110 4 1 7 ma which is older than the upper cretaceous abushan formation ka of the footwall a red layer obtained from drillcores is younger than the overlying jurassic strata on the basis of the youngest detrital zircon u pb age of 116 2 ma this stage is correlated with the closure of the bangong nujiang neo tethys ocean we suggest the upper triassic riganpeicuo formation jm and ophiolite successively accreted to the southern margin of the south qiangtang terrane during the late jurassic to early cretaceous these constitute the main body of the epicontinental accretionary wedge this accretionary wedge includes many early cretaceous epicontinental arc volcanic rocks and granites and the duolong cu au deposits were formed in an andean type active continental margin setting we also conclude that the bangong nujiang neo tethys ocean closed during the late early cretaceous to early late cretaceous and that continental collision continued until the end of the late cretaceous

compaction creep of quartz sand at 400 600 c experimental evidence for dissolution controlled pressure solution intergranular pressure solution ips is an important compaction and deformation mechanism in quartzose rocks but the kinetics and rate controlling process remain unclear the aim of the present study is to test microphysical models for compaction creep by ips against isostatic hot pressing experiments performed on quartz sand under conditions expected to favor pressure solution confining pressure 300 mpa pore water pressure 150 250 mpa temperature 400 600 c microstructural observations revealed widespread intergranular indentation features and confirmed that intergranular pressure solution was indeed the dominant deformation mechanism under the chosen conditions for porosities down to 15 the mechanical data agree satisfactorily with a microphysical model incorporating a previously determined kinetic law for dissolution of loose granular quartz suggesting that the rate limiting mechanism of ips was dissolution the model also predicts ips rates within one order of magnitude of those measured in previous experiments at 150 350 c and thus seem robust enough to model sandstone compaction in nature such applications may not be straightforward however as the present evidence for dissolution control implies that the compositional variability of natural pore fluids may strongly influence ips rates in sandstones

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

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

geochronology geochemistry and zircon hf isotopic compositions of mesozoic intermediate felsic intrusions in central tibet petrogenetic and tectonic implications abstract mesozoic intermediate felsic intrusions are widely distributed in the southern qiangtang terrane central tibet zircon u pb dating shows that these intrusions formed in two periods in the jurassic 169 150 ma and cretaceous 127 113 ma they mostly belong to the high k calc alkaline series and show strong enrichments in large ion lithophile elements e g cs rb and k depletions in nb ta and ti and negative ba anomalies on primitive mantle normalized diagrams p 2 o 5 contents decrease with increasing sio 2 content and th contents increase with increasing rb content consistent with the evolution trend of i type magmas these intrusions show a wide range of zircon ehf t values from 19 4 to 11 2 including in rocks with similar sio 2 contents suggesting variable contributions from mantle and qiangtang crustal sources fine grained mafic to intermediate igneous enclaves in jurassic intrusions have similar zircon u pb ages and similar or slightly higher zircon ehf t values to the host rocks suggesting that the enclave magmas were derived from mixed magmas at depth and injected into more evolved magmas in upper crustal magma chambers magma mixing is also supported by the wide range of zircon hf isotopic compositions ehf t 19 4 to 2 5 from within individual jurassic and cretaceous intrusions and jurassic enclaves the jurassic cretaceous magmas likely formed in a continental arc setting during subduction of the bangong nujiang ocean between 170 and 110 ma and evolved in the upper plate crust by mash processes

a microphysical model for strong velocity weakening in phyllosilicate bearing fault gouges 1 previous rotary shear experiments performed on a halite muscovite fault gouge analogue system have shown that the presence of phyllosilicates under conditions favoring the operation of cataclasis and pressure solution in the matrix phase can have major effects on the frictional behavior of gouges while 100 halite and 100 muscovite samples exhibit rate independent frictional brittle behavior the strength of mixtures containing 10 30 muscovite is both normal stress and sliding velocity dependent at high sliding velocities 1 m s 1 such mixtures show unusually marked velocity weakening along with the development of a structureless cataclastic microstructure in the present paper a micromechanical model is developed in an attempt to explain this behavior the model assumes a granular flow process involving competition between intergranular dilatation and compaction by pressure solution the predictions of the model agree favorably with the experimental results extension of the model to quartz mica systems implies that the presence of phyllosilicates plus the operation of pressure solution can strongly promote unstable velocity weakening behavior at rapid slip rates on natural faults under midcrustal conditions static stress drop predictions based on the model agree reasonably well with estimates from seismic observations our results may help explain the discrepancy between laboratory derived rate and state friction parameter values obtained for dry low strain and or single phase rock systems and the values for natural fault rocks inferred from seismological data