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

metamorphic history and geodynamic significance of high grade metabasites from the ophiolitic melange beneath the yarlung zangbo ophiolites xigaze area tibet abstract blocks of highly foliated amphibolite are locally embedded within a serpentinite melange underlying the yarlung zangbo ophiolites in the xigaze area of southern tibet the ophiolites are remnants of an early cretaceous back arc basin within the permo cretaceous tethys ocean which are exposed along in the yarlung zangbo suture zone yzsz these amphibolites are interpreted as fragments of a dismembered dynamothermal sole three types of amphibolite are present 1 common amphibolite with assemblages of hbl pl ep ap ttn 2 clinopyroxene bearing amphibolite with hbl pl cpx ep ttn qtz ap and 3 garnet clinopyroxene bearing amphibolite characterized by the assemblages hbl cpx grt pl rt and grt hbl pl corona assemblage in all three types plagioclase is pseudomorphed by late albite prehnite retrograde cataclastic veins containing assemblages of prh ab ep chl are also present p t estimates indicate that the amphibolites reached peak metamorphic conditions of 13 15 kbar and 750 875 c partial replacement of pyrope rich up to 35 mole garnet by al tschermakite al2o3 up to 21 wt reflects a high pressure 18 kbar 600 c metamorphic event followed by rapid exhumation soon after exhumation the amphibolites were intruded by very fine grained diabase dykes that were then hydrothermally altered the field relationships and metamorphic history of the amphibolites indicate formation during inception of subduction within a back arc basin prior to obduction of the ophiolites onto the indian passive margin

relicts of the early cretaceous seamounts in the central western yarlung zangbo suture zone southern tibet abstract despite more than 30 years studies in the yarlung zangbo suture zone yzsz southern tibet is still a region of discovery and the geological evolution of the neo tethys here remains controversial in this paper we present the new field observation petrography zircon u pb dating and geochemistry of the zhongba mafic rocks in the western segment of the yzsz zircon u pb analyses from a diabase using sims secondary ion mass spectrometry yielded a concordant age of 125 7 0 9 ma all the zhongba mafic rocks exhibit lree enrichment lan ybn 4 7 15 8 without eu anomalies eu 0 97 1 01 their primitive mantle normalized trace element patterns are similar to those of the average oib and hawaii alkaline basalts as well as other oib type rocks already reported in the central western yzsz these observations lead us to suggest the existence of a series of seamounts within the neo tethys during the early cretaceous these oib type rocks could be originated from 1 hotspot mantle 2 near ridge seamounts by melting induced mixing of two distinct mantle sources 3 late stage magmatism fed by melts that originated from an asthenospheric window due to slab delamination combining literature data from the entire yzsz which include studies of the morb and iab type rocks and mantle peridotites we prefer the third explanation that the oib type rocks represent a late stage magmatic activity above an intra oceanic supra subduction zone within the neo tethys

geology of the higher himalayan crystallines in khumbu himal eastern nepal abstract in this paper we present the current geological knowledge and the results of new geological and structural investigations in the cho oyu sagarmatha makalu region eastern nepal and southern tibet the tectonic setting of the middle and upper part of the higher himalayan crystallines hhc and tibetan sedimentary sequence is characterized by the presence of pervasive compressive tectonics with south verging folds and shear zones overprinted by extensional tectonics in the middle and upper part of the hhc two systems of folds f2a and f2b have been recognized affecting the s1 high grade schistosity causing kilometer scale upright antiforms and synforms the limbs of these upright folds are affected by f3 collapse folds top to se extensional shear zones and extensional crenulation cleavages linked to extensional tectonics the uppermost portion of the hhc and the lower part of the tibetan sedimentary sequence is affected by two major extensional fault zones with a top to ne direction of movement the lower ductile extensional shear zone brings into contact the north col formation with the high grade gneisses and micaschists of the hhc it is regarded as the main feature of the south tibetan detachment system the upper low angle fault zone is characterized by ductile brittle deformation and thin levels of cataclasites and brings the slightly metamorphosed ordovician limestones into contact with the north col formation extensional tectonics continued with the formation of e w trending high angle normal faults three metamorphic stages of himalayan age are recognized in the hhc of the sagarmatha makalu region the first stage m1 is eclogitic as documented by granulitized eclogites collected at the top of the main central thrust zone in the kharta region of southern tibet the second event recorded in the kharta eclogites m2 was granulitic with medium p 0 55 0 65 gpa and high t 750 770 c and was followed by recrystallization in the amphibolite facies of low pressure and high t m3 the first event has also been recorded in the overlying barun gneiss where m1 was followed by decompression under increasing t the m2 event producing the dominant mineral assemblage garnet sillimanite biotite and then by strong decompression under high t with growth of andalusite cordierite and green spinel also changes in phase compatibilities suggest an increase in metamorphic temperature t coupled with a decrease in metamorphic pressure p in some of the thrust sheets of the mct zone a telescoped metamorphic zonation ranging from the sillimanite to the staurolite and biotite zones is characteristic of the ductile extensional shear zone which is the lower part of the stds in the sagarmatha region evidence for decompression under increasing temperature anatexis and leucogranite emplacement accompanying extension in the hhc was found throughout the whole ductile shear zone particularly in metapelites both below and above the makalu leucogranite and in micaschists of the staurolite zone

fabric kinematics of the ultrahigh pressure metamorphic rocks from the main borehole of the chinese continental scientific drilling project implications for continental subduction and exhumation abstract the 5158 m deep main borehole of the chinese continental scientific drilling project ccsd mh penetrated granitic gneisses paragneisses eclogites retrograde eclogites amphibolites and ultramafic rocks in the sulu ultrahigh pressure uhp metamorphic terrane eastern china the ccsd mh consists of four petro structural units separated by three se dipping ductile shear zones dfa 835 1280 m dfb 2010 2280 m and dfc 2920 3225 m which are correspondent with the regional shear zones in the northern sulu uhp supracrustal zone using the electron backscatter diffraction ebsd technique we investigated the lattice preferred orientations lpos of omphacite diopside and quartz in core samples from the ccsd mh omphacite from eclogites and diopside from garnet pyroxenites display very strong lpos which are characterized by the maximum concentration of 001 axes parallel to the lineation and 010 poles normal to the foliation quartz in para and granitic mylonites gneisses from the shear zones dfa dfb and dfc developed multiple slip systems 40ar 39ar dating of biotite in para and granitic gneisses from the ccsd mh yields 223 202 ma which constrains the formation ages of the quartz high temperature prism slip systems m and m c the asymmetric lpos of omphacite diopside olivine and quartz with respect to the structural frame reveal three deformation phases in the sulu terrane in the middle triassic the northward subduction of the yangtze plate to depths 100 km produced a top to the south shear sense in lpos of omphacite diopside and olivine and a nearly n s striking foliation and a subhorizontal n s trending lineation in eclogites and ultramafic rocks in the late triassic the uhp rocks were exhumed to the lower crust and quartz developed high temperature slip systems with a top to the nw shear sense which is consistent with the regional se dipping foliation and se plunging lineation in the ductile shear zones in the cretaceous the uhp rocks were exhumed to the middle crust when the migmatization and granitic intrusion formed a ne striking antiform structure as a result the activation of quartz low temperature basal slip 0001 is characterized by a top to the se shear sense in the south but a top to the nw shear sense in the north

tethyan and indian subduction viewed from the himalayan high to ultrahigh pressure metamorphic rocks abstract the himalayan range is one of the best documented continent collisional belts and provides a natural laboratory for studying subduction processes high pressure and ultrahigh pressure rocks with origins in a variety of protoliths occur in various settings accretionary wedge oceanic subduction zone subducted continental margin and continental collisional zone ages and locations of these high pressure and ultrahigh pressure rocks along the himalayan belt allow us to evaluate the evolution of this major convergent zone 1 cretaceous 80 100 ma blueschists and possibly amphibolites in the indus tsangpo suture zone represent an accretionary wedge developed during the northward subduction of the tethys ocean beneath the asian margin their exhumation occurred during the subduction of the tethys prior to the collision between the indian and asian continents 2 eclogitic rocks with unknown age are reported at one location in the indus tsangpo suture zone east of the nanga parbat syntaxis they may represent subducted tethyan oceanic lithosphere 3 ultrahigh pressure rocks on both sides of the western syntaxis kaghan and tso morari massifs formed during the early stage of subduction exhumation of the indian northern margin at the time of the paleocene eocene boundary 4 granulitized eclogites in the lesser himalaya sequence in southern tibet formed during the paleogene underthrusting of the indian margin beneath southern tibet and were exhumed in the miocene these metamorphic rocks provide important constraints on the geometry and evolution of the india asia convergent zone during the closure of the tethys ocean the timing of the ultrahigh pressure metamorphism in the tso morari massif indicates that the initial contact between the indian and asian continents likely occurred in the western syntaxis at 57 1 ma west of the western syntaxis the higher himalayan crystallines were thinned rocks equivalent to the lesser himalayan sequence are present north of the main central thrust moreover the pressure metamorphism in the kaghan massif in the western part of the syntaxis took place later 7 m y after the metamorphism in the eastern part suggesting that the geometry of the initial contact between the indian and asian continents was not linear the northern edge of the indian continent in the western part was 300 to 350 km farther south than the area east of the nanga parbat syntaxis such en baionnette geometry is probably produced by north trending transform faults that initially formed during the late paleozoic to cretaceous gondwana rifting farther east in the southern tibet the collision occurred before 50 6 0 2 ma finally high pressure to ultrahigh pressure rocks in the western himalaya formed and exhumed in steep subduction compared to what is now shown in tomographic images and seismologic data

brittle creep damage as the seismic signature of dyke propagations within basaltic volcanoes contemporary to nine dyke intrusions on piton de la fournaise etna and miyakejima volcanoes we recover stationary seismicity rate and energy release over time whether the dyke reaches the surface or not this generic seismicity pattern for the dyke propagation of low viscosity magma argues for the fluid driven crack propagation to be a scale independent stationary process this prevents any prediction of the time to eruption during the dyke propagation phase using seismicity rate alone the seismic signature of the volcano deformation triggered by dyke injections corresponds to brittle creep damage in a strain driven setting whether mechanical properties of host rock structure or geometrical effects influence this generic stationary response is not resolved by the seismic data because a few if any aftershocks are resolved contemporary to dyke intrusion the seismicity is purely driven by the dyke dynamics that is a proxy for the dyke volumetric growth

transpressive shear related to arc magmatism the paleoproterozoic storsjon edsbyn deformation zone central sweden 1 the polyphase 15 30 km wide and 200 km long storsjon edsbyn deformation zone sedz apparently separates two major paleoproterozoic provinces in central sweden four main phases of deformation have been recognized along the zone associated high strain rocks are characterized by their mineralogy microstructures kinematic patterns magnetic signatures and relative ages the mineral and stretching lineations are consistently steeply to moderately plunging analysis of mesoscopic shear zone populations of different generations suggests that most displacements are compatible with north south to northeast southwest bulk regional shortening the structural analysis suggests that the main ductile deformation along the sedz was due to dextral transpression with a steep coaxial component field evidence combined with published age data constrain the main deformation to the time interval circa 1 7 1 3 ga synkinematic titanite from a ductile protomylonite in the sedz yields an u pb age of 1674 6 ma the combined results imply transpressive deformation broadly synchronous with and spatially related to volcanism and plutonism in a continental margin magmatic arc setting

ophiolite genesis and global tectonics geochemical and tectonic fingerprinting of ancient oceanic lithosphere ophiolites and discussions on their origin and significance in earth s history have been instrumental in the formulation testing and establishment of hypotheses and theories in earth sciences the definition tectonic origin and emplacement mechanisms of ophiolites have been the subject of a dynamic and continually evolving concept since the nineteenth century here we present a review of these ideas as well as a new classification of ophiolites incorporating the diversity in their structural architecture and geochemical signatures that results from variations in petrological geochemical and tectonic processes during formation in different geodynamic settings we define ophiolites as suites of temporally and spatially associated ultramafic to felsic rocks related to separate melting episodes and processes of magmatic differentiation in particular tectonic environments their geochemical characteristics internal structure and thickness vary with spreading rate proximity to plumes or trenches mantle temperature mantle fertility and the availability of fluids subduction related ophiolites include suprasubduction zone and volcanic arc types the evolution of which is governed by slab dehydration and accompanying metasomatism of the mantle melting of the subducting sediments and repeated episodes of partial melting of metasomatized peridotites subduction unrelated ophiolites include continental margin mid ocean ridge plume proximal plume distal and trench distal and plume type plume proximal ridge and oceanic plateau ophiolites that generally have mid ocean ridge basalt morb compositions subduction related lithosphere and ophiolites develop during the closure of ocean basins whereas subduction unrelated types evolve during rift drift and seafloor spreading the peak times of ophiolite genesis and emplacement in earth history coincided with collisional events leading to the construction of supercontinents continental breakup and plume related supermagmatic events geochemical and tectonic fingerprinting of phanerozoic ophiolites within the framework of this new ophiolite classification is an effective tool for identification of the geodynamic settings of oceanic crust formation in earth history and it can be extended into precambrian greenstone belts in order to investigate the ways in which oceanic crust formed in the archean

xainxa ultramafic rocks central tibet china tectonic environment and geodynamic significance the xainxa ultramafic rocks 200 km south of the donqiao ophiolite bangong nu jiang suture zone central tibet form a 1 km thick sheet in tectonic contact with paleozoic sediments to the south and with mesozoic sediments to the north through roughly east west south dipping reverse faults the ultramafic rocks consist of depleted harzburgites with minor dunite patches and 1 m sized deposits of podiform chromite mafic rocks represented by a few isotropic gabbros and dolerites occur in only minor amounts these rocks which may have underlined a suture zone within the lhasa block have lithologies and tectonics very similar to those occurring in the donqiao area along the bangong nu jiang suture zone and therefore can be considered as a klippe of this ophiolite the xainxa ultramafic rocks were emplaced in early cretaceous time the main tectonic event observed in that area northward folding and overthrusting occurred after late early cretaceous time

propagation of seismic slip from brittle to ductile crust evidence from pseudotachylyte of the woodroffe thrust central australia abstract the woodroffe thrust central australia is a 1 5 km wide mylonitized shear zone marked by large volumes of mm to cm scale pseudotachylyte veins the pseudotachylytes display typical melt origin features including rounded and embayed clasts spherulitic and dentritic microlites and flow structures within a fine grained matrix three types of pseudotachylyte are identified on the basis of deformation texture vein morphology and host rock lithology cataclasite related c pt mylonite related m pt and ultramylonite related um pt the m pt and um pt veins intrude into mylonite and ultramylonite and are themselves overprinted by subsequent mylonitization these pseudotachylytes contain an internal foliation defined by flattened porphyroclasts and layering of the fine grained vein matrix and the foliation is generally oriented parallel to foliation in the surrounding mylonite and ultramylonite these observations constrain the timing and environment of m pt and um pt pseudotachylyte formation to a protracted period of deformation and mylonitization within the ductile regime of the crust the m pt and um pt veins as well as the host mylonite are overprinted by cataclasis and multiple generations of late stage c pt veins that were generated in the brittle dominated regime of the upper crust during uplift and exhumation of the shear zone the coexistence of multiple generations of voluminous c pt m pt and um pt veins indicates that the pseudotachylyte veins represent a large number of large earthquakes and accompanying seismic slip over an extended period of seismicity on the woodroffe thrust the timing and distribution of pseudotachylyte indicate that the earthquakes nucleated at the base of the brittle dominated seismogenic zone and propagated down through the brittle ductile transition into the ductile dominated regime of the crust

c slip in quartz from subsolidus deformed granite abstract the slip behaviour of quartz from a granite and granitic veins during subsolidus high temperature conditions 700 800 c of deformation is studied by lattice preferred orientation lpo and microstructural techniques lpos with a point maximum of c axes parallel to the shear direction which is close to the stretching lineation are presented and interpreted with the aid of light and electron microscopy in terms of c as the dominant slip direction during plastic deformation microscopy of such samples reveals the presence of 0001 tilt boundaries and free 0001 burgers vector dislocations in edge orientations a change in lpo in a decreasing temperature environment from c axes parallel to the shear direction to c axes normal to the shear direction documents a fabric transition from the rare c slip to the more common a slip fabric it is concluded that conditions of high temperature and partial pressure of water are required to initiate dominant c slip during deformation at geological strain rates

crustal structure of the indus tsangpo suture zone and its ophiolites in southern tibet abstract the 2000 km long indus tsangpo suture zone itsz in southern tibet contains remnants of neotethyan oceanic lithosphere ophiolitic melanges flysch units and continental rocks and has been interpreted as the boundary between the india and eurasia plates we have used field based structural analyses and geological mapping in five key ophiolite massifs luobusa zedang xigaze jiding and dongbo and petrofabric studies of core samples recovered through the luobusa scientific drilling project to document the structural architecture and the deformation history of the itsz the structural and kinematic analyses of the suture zone units indicate the occurrence of superposed deformation fabrics which developed first during the ophiolite emplacement and then during the subsequent india asia continental collision the suprasubduction zone ophiolites were emplaced southward onto the northern margin of the proto indian plate in the late cretaceous the continental collision between the ophiolite laden proto indian margin and the lhasa terrane of the eurasian plate in the early paleogene resulted in backthrusting and inversion of the structural order of the itsz the ophiolite massifs ophiolitic melanges and flysch units were thrust northward as nappe sheets over the eocene and younger forearc basin strata the itsz shows therefore a complex structural anatomy characterized by both south and north directed thrust faults and ductile and brittle deformation fabrics

revision of the cretaceous paleogene stratigraphic framework facies architecture and provenance of the xigaze forearc basin along the yarlung zangbo suture zone abstract new field investigation structural analysis review of biostratigraphy stratigraphic framework facies architecture provenance and flow direction of the cretaceous paleogene sedimentary strata in southern tibet resulted in reinterpretation of the evolution of the xigaze forearc basin xfb the xfb is comprised by flysch dominant xigaze group consisting of ngamring chongdoi and sangzugang formations and of the shallow marine cuojiangding group padana qubeiya quxia and gyalaze formations with their ages herein revised as the albian coniacian and the santonian ypresian respectively the deep sea flysch represented by the ngamring formation constitutes the early sedimentary fill of the xfb it was subdivided into five megasequences ms1 ms5 in order the early submarine fan facies stage channel submarine fan stage the mature submarine fan stage the highstand submarine fan stage and outer fan pelagic stage the paleocurrent flows were eastwards and westwards during the early and late stages of basin development and northwards and southwards during its middle stage indicating typical forearc basin sedimentation with longitudinal supply and lateral filling provenance studies point to multiple sources with andesitic debris sourced from the jurassic lower cretaceous yeba formation and sangri group formed in the volcanic island arc chains in the southern gangdese the four periods of the xfb development are 1 the formation of the xfb basement by the xigaze ophiolite and completed before the aptian the coeval deposits on the shelf are the sangzugang formation carbonate 2 the earliest xfb deposits of the earliest aptian age ca 125 120 ma represented by the chongdoi formation related to the residual forearc 3 composite forearc developed represented by the late albian late cenomanian flysch megasequences ms1 ms2 and the turonian coniacian flysch megasequences ms3 ms5 respectively and 4 the final stage of the xfb development started the terminal forearc stage represented by the late late cretaceous early paleogene shallow marine cuojiangding group and terminated during middle ypresian when the gyalaze formation was deposited

chemical and oxygen isotope composition of natural and artificial pseudotachylyte role of water during frictional fusion abstract pseudotachylyte samples from the long ridge fault north carolina the homestake shear zone colorado the fort foster fault maine and experimentally produced frictional melt were analyzed for major elements and o isotopes using an electron microprobe and laser microprobe respectively the pseudotachylytes display a 18o range of 5 9 10 7 standard mean ocean water smow and are both isotopically lighter and heavier than their respective host rocks in general the isotopic composition of bulk pseudotachylyte pt in the absence of water rock interaction can be modeled as 18 o bulk pt x frag 18 o frag x min 18 o min where xfrag refers to the volume fraction of unmelted mineral fragments and xmin refers to the volume fraction of minerals that undergo melting mineral melt isotopic fractionation is considered to be small and less than 1 the majority of pseudotachylyte isotopic compositions can be explained as a disequilibrium partial melt of the protolith rock comprising melted feldspar and micas plus a small proportion 10 of unmelted quartz clasts in contrast the long ridge fault samples are substantially depleted in 18o by up to 4 relative to their host rock which is best explained by closed system water rock interaction of a pore fluid at 100 c with a melt at 1000 c and a water rock weight ratio of 0 2 this fluid is also inferred to have caused oxidation of the pseudotachylyte melts producing the hematite characteristic of this location

the indus yarlung zangbo ophiolites from nanga parbat to namche barwa syntaxes southern tibet first synthesis of petrology geochemistry and geochronology with incidences on geodynamic reconstructions of neo tethys the purpose of this first synthesis is to summarize findings on the yarlung zangbo suture zone yzsz ophiolites and discuss still remaining problems the yzsz studied for almost 30 years and is the youngest of the sutures recognized on tibet plateau it is now acknowledged that the yzsz is a complex assemblage of sedimentary metamorphic and igneous rocks created during and shortly after the collision between india and eurasia the ages of the various lithologies span a time interval from the jurassic to the middle miocene with some permian and devonian exotic blocks from melange zone the yzsz is characterized by ophiolitic complexes and ophiolitic melange the ophiolites are of two types non dismembered and dismembered sections the non disturbed sections although tectonically reworked are observed along the segment from dazhuqu to jiding in xigaze area and spontang ophiolite the dismembered sequences are found in various locations such as nidar kiogar jungbwa saga sangsang xigugabu and luobusa the incomplete stratigraphic log could be connected to intraoceanic or orogenetic origins the ophiolites are distributed into two groups of ages the luobusa zedang and kiogar sequences being jurassic lower cretaceous whereas all other sequences are of lower cretaceous age compilation of geochronological data suggest that some ophiolite sequences might have evolved for over more than 70 my from their initial genesis to obduction which occurred around 70 90 my ago ophiolites differ in terms of petrological and geochemical aspects however they were all generated in suprasubduction zone and more specifically in arc few fore arc and back arc settings synthesis of more than 700 geochemical analyses show variable mixing of components from n morb type to iat cab and to oib end members the jurassic ophiolites show the maximum of arc component while the lower cretaceous ones show little to strong mixing in addition most ophiolites were created in short lived 30 my basins and generated close to the eurasiatic continental margin we propose that ladakh tibet ophiolites were generated in a suprasubduction context similar to mariana arc interarc and back arc or tonga lau system the variable arc signature of these ophiolites is directly related to their initial position within the suprasubduction system

pseudotachylyte generated in the semi brittle and brittle regimes bench canyon shear zone central sierra nevada pseudotachylyte cuts cretaceous plutonic and volcanic rocks in the bench canyon shear zone central sierra nevada california as documented by optical and scanning electron microscopy the pseudotachylyte displays evidence for a friction melt origin including vesicles amygdules crystallites flow fabrics and embayed crystal fragments based on fault related rock association vein morphology and inferred crustal level of generation two distinct types of pseudotachylyte are documented in the shear zone pseudotachylyte associated with cataclasite was generated at shallow crustal levels 1 10 km via unstable velocity weakening abrasive wear friction pseudotachylyte associated and interlayered with mylonite was generated at deeper crustal levels 10 15 km most likely via stable velocity strengthening adhesive wear friction this pseudotachylyte underwent ductile deformation in the solid state interlayered pseudotachylyte and mylonite records cyclic aseismic seismic slip at a transitional crustal level between the semi brittle field of quartzofeldspathic rocks and the base of the seismogenic zone the cyclicity is attributed to strain rate fluctuation effected by strain resistant phacoids pore fluid pressure variation ductile instabilities or downward fault propagation the mode of deformation within the shear zone progressed from i production of mylonite during amphibolite to greenschist facies conditions to ii generation of pseudotachylyte by intermittent seismic faulting within a background of aseismic shear in the upper realm of the semi brittle field 300 400 c to iii generation of pseudotachylyte through seismic events in the brittle 300 c during uplift exhumation and cooling some of the latter may have been generated at shallow enough crustal levels 4 km to allow vesicle formation the exhumation of a crustal level exposing such an assemblage provides a rare opportunity to study the enigmatic nature of the transitional change from semi brittle flow to seismogenic rupturing

ductile shear as a cause of inverted metamorphism example from the nepal himalaya a discussion hubbard 1996 postulated the cause of well known development of kyanite as well as other index minerals representing dominant metamorphism was metamorphic inversion in a section of the eastern nepal himalaya due to the intense ductile shearing either preor synchronous with but outlasted the deformation because of paucity of growth of metawithin the main central thrust mct zone and the overlying tibetan slab on the basis of geologimorphic minerals across the main tectonic fabric hubbard 1996 p 496 cal and textural data cf hubbard 1989 the main thesis of her recent paper revolves around the fact the metamorphic belts of the nw himalaya and the eastern nepal are characterized by i ductile that the distribution of inverted metamorphic isograds is confined to intensely sheared 5 km thick shear zones of high strain revealing s c fabric and other shear structures having a top to south southzone of the mct which either post dated or outlasted predominant metamorphic assemblages p west vergence not only in the basal parts but also in the tibetan slab at higher structural levels 495 conceptually right way up metamorphic iso ii growth of metamorphic minerals is dominantly preto synchronous but occasionally outlasting grads in the young orogenic belts like the himalaya can be very efficiently inverted due to intense the main deformation iii lack of large scale overturned or recumbent folds in many sections except shearing in a broad ductile shear zone we wish to point out that the model proposed by hubbard zanskar searle et al 1992 and the beas valley himachal pradesh frank et al 1977 and iv pres 1996 for the eastern nepal is the same in principle as ours for the northwestern himalayas pubence of inverted metamorphism in the lower and middle sequences and a normal sequence in the lished in geology in 1993 earlier hubbard 1989 fixed the upper boundary upper parts near the tethyan sedimentary cover numerous structures having predominantly topof the 3 5 km thick mct zone from this area arbitrarily as she has documented that the upper and to southwest sense of ductile shearing have been frequently documented across many tectonic units lower boundaries of the mct zone are not distinct so for the sake of discussions the author defines of the lesser himalaya hhc and tethyan sedimentary zone brunel 1986 treloar and rex 1990 boundaries that include the majority of high strain zone p 20 ductile shear zones were also obpatel et al 1993 and can therefore be correlated with the intense ductile shearing irrespective of the served by her in the overlying tibetan slab where gneisses of the tibetan slab are foliated though presence of the discrete thrusts or wide thrust zones jain and anand 1988 foliation is somewhat less penetrative than probable coeval mylonitic fabric within the mct zone the ductile shear model postulated by hubbard 1996 bears strong structural similarities to the p 21 indicating synchrony of two dominant planar structures in this region within the mct zone one given earlier by jain and manickavasagam 1993 as it is evident from the simplified version she recorded that the apparent metamorphic grade across the mct zone gradually increases toof our model where effects of post metamorphic thrusting folding and other zones of high strain wards structurally higher levels p 20 where fibrolite and sillimanite are developed along the folihave not been considered figure 1 keeping in mind the regional geological and tectonic variaation as well as in the tibetan slab hubbard 1989 thus an apparent structural and metamorphic grations that may be expected over the distance of about 1000 km from the nw himalaya it is likely dation persists near the upper contact of the mct zone with the tibetan slab within the mct zone that the growth of metamorphic minerals may be diachronous within the higher himalayan shear zone one of the very basic fact of the nw hima1 manuscript received february 1 1997

precollision tilt of crustal blocks in rifted island arcs structural evidence from the kohistan arc 1 we describe results from field and microstructural investigations in the unusually large chilas plutonic complex which was emplaced within the mesozoic kohistan island arc northern pakistan magmatic and deformational fabrics of the chilas gabbronorite are concordant with magmatic fabrics of associated gabbros diorites and tonalites and show that this reportedly huge intrusion is composed of juxtaposed plutonic bodies with predominantly ssw dipping flow structures over the northeastern part of the complex which turn to sw ne striking subvertical attitudes to the west evidence for multiple intrusions includes concordant and discordant intrusive and or mylonitic contacts between gabbronorites and associated rocks owing to structural and kinematic consistency we attribute transitions from magmatic to more localized mylonitic fabrics to higher finite strain and or strain rate produced from drag of magma against its margin we discuss this structural pattern and conclude that the chilas gabbronorite emplaced incrementally into generally south dipping listric faults that dominated arc extension about 85 ma in addition several sets of hornblende pegmatite indicate that brittle deformation occurred in the presence of residual melt during consolidation of the main body the latest pegmatite set is composed of variably striking vertical extensional veins that show no relationship to the magmatic fabric we argue that the original attitude of these veins along with that of the enclosing chilas complex has been mostly preserved we hypothesize that the northward dip of the southern side of the kohistan is inherited from late cretaceous rotation of crustal blocks in the hanging wall of the listric faults

geochemical and isotopic constraints on the age and origin of the nidar ophiolitic complex ladakh india implications for the neo tethyan subduction along the indus suture zone abstract the nidar ophiolite complex is exposed within the indus suture zone in eastern ladakh india the suture zone is considered to represent remnant neo tethyan ocean that closed via subduction as the indian plate moved northward with respect to the asian plate the two plates ultimately collided during the middle eocene the nidar ophiolite complex comprises a sequence of ultra mafic rocks at the base gabbroic rocks in the middle and volcano sedimentary assemblage on the top earlier studies considered the nidar ophiolite complex to represent an oceanic floor sequence based on lithological assemblage however present study based on new mineral and whole rock geochemical and isotopic data on bulk rocks and mineral separates indicate their generation and emplacement in an intra oceanic subduction environment the plutonic and volcanic rocks have nearly flat to slightly depleted rare earth element ree patterns the gabbroic rocks in particular show strong positive sr and eu anomalies in their ree and spidergram patterns probably indicating plagioclase accumulation depletion in high field strength elements hfse in the spidergram patterns may be related to stabilization of phases retaining the hfse in the subducting slab and or fractional crystallization of titano magnetite phases the high radiogenic nd and low radiogenic sr isotopic ratios for these rocks exclude any influence of continental material in their genesis implying an intra oceanic environment nine point mineral whole rock sm nd isochron corresponds to an age of 140 32 ma with an initial 143nd 144nd of 0 513835 0 000053 end t 7 4 this age is consistent with the precise early cretaceous age of hauterivian 132 2 to 127 1 6 ma to aptian 121 1 4 to 112 1 1 ma for the overlying volcano sedimentary radiolarian bearing chert sequences based on well preserved radiolarian fossils kojima s ahmad t tanaka t bagati t n mishra m kumar r islam r khanna p p 2001 early cretaceous radiolarians from the indus suture zone ladakh northern india in news of osaka micropaleontologists nom spec vol 12 257 270 and cooling ages of 110 130 ma based on 39ar 40ar for nidar spontang ophiolitic rocks maheo g berttrand h guillot s villa i m keller f capiez p 2004 the south ladakh ophiolites nw himalaya india an intra oceanic tholeiitic arc origin with implications for the closure of the neo tethys chem geol 203 273 303 as these gabbroic and volcanic rocks are interpreted to be arc related the new sm nd age data may indicate that intra ocean subduction in the neo tethyan ocean may have started much before 140 32 ma as this date is interpreted as the age of crystallization of the arc magma present and published age data on the arc magmatic rocks from the indus suture zone may collectively indicate episodic magmatism with increasing maturity of the arc from more basic during 140 32 ma when the arc was immature through intermediate andesitic granodioritic at 100 ma to more felsic rhyolitic dioritic magmatism at 50 45 ma when the indian and the asian plates collided

provenance of the liuqu conglomerate in southern tibet a paleogene erosional record of the himalayan tibetan orogen abstract the liuqu conglomerate exposed along the yarlung zangbo suture zone in southern tibet has been interpreted as a paleogene molasse deposited during india asia collision or the product of collision between india and an intra oceanic arc detailed petrographic studies and analyses of u pb and hf isotopes of detrital zircon indicate multiple sources for the conglomerate including the sedimentary succession within the xigaze forearc basin to the north of the suture the yarlung zangbo ophiolite within the suture itself and the langjiexue group and tethyan himalayan sedimentary rocks to the south of the suture the occurrence of clasts of both indian and asian origin suggests that the conglomerate was deposited after india asia collision the present data suggest that the onset of himalayan tibetan orogenic erosion occurred as early as the middle eocene when the yarlung zangbo suture zone and xigaze forearc succession were uplifted and became sources for the liuqu conglomerate thus the results are inconsistent with the proposal that the provenance and age of the liuqu conglomerate support the hypothesis that india asia collision did not occur until the oligocene it is further suggested that eocene neotethyan slab breakoff which may result in accelerated exhumation and uplift of the orogen provided a deep level dynamic control on the deposition of the liuqu conglomerate

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

fluid assisted fracturing cataclasis and resulting plastic flow in mylonites from the moresby seamount detachment woodlark basin the moresby seamount detachment msd in the woodlark basin offshore papua new guinea is a large active low angle detachment excellently exposed at the seafloor and cutting through mafic metamorphic rocks hydrothermal infiltration of quartz followed by that of calcite occurred during cataclastic deformation subsequent deformation of these a priori softer minerals leads to mylonite formation in the msd this study aims at a better understanding of the deformation mechanism switch from cataclastic to plastic flow deformation fabrics of the fault rocks were analyzed by light optical microscopy rheologically critical phases were mapped to determine distributions and area proportions and ebsd was used to measure crystallographic preferred orientation cpo strong calcite cpos indicate dominant dislocation creep quartz cpos however are weak and more difficult to interpret suggesting at least some strain accommodation by diffusion creep mechanisms when quartz aggregates are intermixed with the polymineralic mylonite matrix diffusion creep grain boundary sliding may be dominant the syntectonic conversion from mafic cataclasites to more siliceous and carbonaceous mylonites induced by hydrothermal processes is a critical weakening mechanism enabling the msd to at least intermittently plastic flow at low shear stresses this is probably a crucial process for the operation of low angle detachments in hydrated and dominantly mafic crust

on a possible connection between three major earthquakes in california and oil production abstract the 1983 coalinga m 6 5 the 1985 kettleman north dome m 6 1 and the 1987 whittier narrows m 5 9 earthquake sequences each occurred beneath major producing oil fields and caused an aftershock sequence whose epicentral extent coincides approximately with that of the overlying oil field despite this coincidence a causal relationship between these sequences and oil production has been discounted because the earthquakes were located at depths of the order of 10 km whereas oil production is confined to the topmost several km moreover because all three oil fields coalinga kettleman north dome and montebello are situated on anticlines that have undergone recent uplift as a consequence of horizontal tectonic compression of the crust natural earthquakes are not unexpected a mechanical connection between oil production and the earthquake sequence is suggested however by the observation that in each case the total seismic deformation was just that required to offset the force imbalance caused by oil production specifically for each of the three sequences m 0 2 m c where m 0 is the sum of the seismic moments is the modulus of rigidity m is the mass of the fluid removed due to oil production is the fraction of the upper crust that is seismogenic and c is the average upper crustal density in essence net extraction of oil and water reduces slightly the average density of the upper crust causing an isostatic imbalance the ductile lower crust deforms in response to this imbalance thus increasing the load on the seismogenic layer which fails seismically to thicken the crust so as to restore static equilibrium locally accordingly earthquakes near the base of the upper crust may be an expected outcome of major oil production from growing anticlines irrespective of the depths of the producing formations

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

viscosity determinations of some frictionally generated silicate melts implications for fault zone rheology at high strain rates analytical scanning electron microscopy has been used to determine the major element compositions of some natural and artificial silicate glasses and their microcrystalline equivalents derived by the frictional melting of intermediate to acid protoliths the data show that the matrices of the friction melts which cool to form pseudotachylytes are relatively basic and hydrous even when their protoliths are intermediate to acid this is because frictional fusion involves the selective comminution and nonequilibrium melting of minerals based on their individual mechanical properties and melting points not the formation of minimum melts through equilibrium mineral interaction this means that hydrous ferromagnesian minerals e g micas and amphiboles melt preferentially to form the liquid matrix while feldspars and especially quartz more readily survive as clasts pseudotachylytes generated by frictional melting are therefore not bulk melts and as clast melt suspensions they cannot be considered as simple newtonian fluids the calculated viscosities of the friction melts are low for example at 1200 c most friction melts possess zero shear suspension viscosities of 102 104 dpa s 1 dpa s 1 p this is equivalent to the viscosities of tholeiitic and alkaline basaltic magmas at the same temperature these viscosities are maximum determinations because as clast melt suspensions friction melts may undergo shear thinning and exhibit pseudoplasticity at high shear rates i e during slip on a fault surface contrary to earlier suggestions where the bulk melting of intermediate to acid protoliths was believed to result in the generation of viscous friction melts that could act to inhibit continued sliding this work shows that most pseudotachylytes are partial melts possessing low viscosities the formation of highly fluid suspensions during slip may have profound effects on the dissipation of stored strain energy in the rocks surrounding a fault interface lubrication could facilitate an increase in the slip rate and the rate of energy dissipation this would be manifest as an increase in high frequency seismic wave radiation and vibrational

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

u pb zircon ages for yarlung tsangpo suture zone ophiolites southwestern tibet and their tectonic implications ophiolite complexes preserved along the yarlung tsangpo suture zone ytsz and obducted onto the northern continental margin of india in southern tibet represent the remnants of the once extensive permian mesozoic neo tethyan ocean that separated india from asia complete ophiolite successions are preserved near xigaze whereas the rest of the belt is essentially represented by mantle rocks with subordinate disrupted lower crustal rocks u pb zircon la mc icp ms geochronology on two gabbro samples from the luobusa ophiolite yielded concordant data with mean 206pb 238u ages of 149 9 1 4 2 ma and 150 0 5 0 ma these ages are in contrast to a younger age of 131 8 1 0 ma obtained from a pegmatitic gabbro in xigaze five u pb zircon tims ages from gabbroic samples in the western portion of the ophiolite belt reveal that the dangxiong ophiolite formed between 126 7 0 4 ma and 123 4 0 8 ma zircons from the jungbwa ophiolite have similar ages of 123 4 0 8 ma and 123 9 0 9 ma a single zircon analysed from a gabbro in kiogar has an age of 159 7 0 5 ma geochronological data reported here show ytsz ophiolites formed in association with intra oceanic subduction zone systems and are related a significant tectonic episode within the tethyan ocean during late jurassic to early to mid cretaceous times