triggered deformation and seismic activity under mammoth mountain in long valley caldera by the 3 november 2002 mw 7 9 denali fault earthquake the 3 november 2002 mw 7 9 denali fault earthquake triggered defor mational offsets and microseismicity under mammoth mountain mm on the rim of long valley caldera california some 3460 km from the earthquake such strain offsets and microseismicity were not recorded at other borehole strain sites along the san andreas fault system in california the long valley offsets were recorded on borehole strainmeters at three sites around the western part of the caldera that includes mammoth mountain a young volcano on the southwestern rim of the caldera the largest recorded strain offsets were 0 1 microstrain at po on the west side of mm 0 05 microstrain at mx to the southeast of mm and 0 025 microstrain at bs to the northeast of mm with negative strain extensional high sample rate strain data show initial triggering of the offsets began at 22 30 utc during the arrival of the first rayleigh waves from the alaskan earthquake with peak to peak dynamic strain amplitudes of about 2 microstrain corresponding to a stress amplitude of about 0 06 mpa the strain offsets grew to their final values in the next 10 min the associated triggered seismicity occurred beneath the south flank of mm and also began at 22 30 utc and died away over the next 15 min this relatively weak seismicity burst included some 60 small events with magnitude all less than m 1 while poorly constrained these strain observations are consistent with triggered slip and intrusive opening on a north striking normal fault centered at a depth of 8 km with a moment of 10 16 n m or the equivalent of a m 4 3 earthquake the cumulative seismic moment for the associated seismicity burst was more than three orders of magnitude smaller these observations and this model resemble those for the triggered deformation and slip that occurred beneath the north side of mm following the 16 october 1999 m 7 1 hector mine california earthquake however in this case we see little post event slip decay reflected in the strain data after the rayleigh wave arrivals from the denali fault earthquake and onset of triggered seismicity did not lag the triggered defor mation by 20 min these observations are also distinctly different from the more widespread and energetic seismicity and deformation triggered by the 1992 m 7 3 landers earthquake in the long valley caldera thus each of the three instances of remotely triggered unrest in long valley caldera recorded to date differ in each case however the deformation moment inferred from the strain meter data was more than an order of magnitude larger than the cumulative moment for the associated triggered seismicity

advection of the salt wedge and evolution of the internal flow structure in the rotterdam waterway an analysis of field measurements recorded over a tidal cycle in the rotterdam waterway is presented these measurements are the first to elucidate the processes influencing the along channel current structure and the excursion of the salt wedge in this estuary the salt wedge structure remained stable throughout the measuring period the velocity measurements indicate decoupling effects between the layers and that bed generated turbulence is confined below the pycnocline the barotropic m4 overtide structure is imposed at the mouth of the estuary and the generation of m4 overtides within the estuary is found to be relatively small internal tidal asymmetry does not make a significant contribution to the m4 velocity frequency band instead the combination of barotropic and baroclinic forcing in conjunction with the suppression of turbulence at the interface provides the main explanation for the time dependence and mean structure of the flow in the rotterdam waterway this gives rise to the observed differences in the length of the flood and ebb in the magnitudes of the flood and ebb velocities in the length of the slack water periods and in the timing of the onset of slack water at the surface and near the bed it results in the formation of distinct exchange flow profiles at the head of the salt wedge around slack water and the creation of maximal velocities at the pycnocline during flood advection governs the displacement and structure of the salt wedge since turbulent mixing is suppressed the tidal displacement of the salt wedge controls the height of the pycnocline above the bed at a particular site hence it controls the height to which bed generated turbulence can protrude into the water column consequently the authors find asymmetries in the structure of the internal flow turbulent mixing and bed stresses that are not related to classical internal tidal asymmetry

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

lhasa terrane in southern tibet came from australia the u pb age and hf isotope data on detrital zircons from paleozoic metasedimentary rocks in the lhasa terrane tibet defi ne a distinctive age population of ca 1170 ma with e hf t values identical to the coeval detrital zircons from western australia but those from the western qiangtang and tethyan himalaya terranes defi ne an age population of ca 950 ma with a similar e hf t range the ca 1170 ma detrital zircons in the lhasa terrane were most likely derived from the albany fraser belt in southwest australia whereas the ca 950 ma detrital zircons from both the western qiangtang and tethyan himalaya terranes might have been sourced from the high himalaya to the south such detrital zircon connections enable us to propose that the lhasa terrane is exotic to the tibetan plateau system and should no longer be considered as part of the qiangtang greater india tethyan himalaya continental margin system in the paleozoic reconstruction of the indian plate as current models show rather it should be placed at the northwestern margin of australia these results provide new constraints on the paleogeographic reconstruction and tectonic evolution of southern tibet and indicate that the lhasa terrane evolved as part of the late precambrian early paleozoic evolution as part of australia in a different paleogeographical setting than that of the qiangtang greater india tethyan himalaya system

provenance of late triassic sediments in central lhasa terrane tibet and its implication abstract in southern tibet late triassic sequences are especially important to understanding the assembly of the lhasa terrane prior to indo asian collision we report new data relevant to the provenance of a late triassic clastic sequence from the mailonggang formation in the central lhasa terrane tibet petrographic studies and detrital heavy mineral assemblages indicate a proximal orogenic provenance including volcanic sedimentary and some ultramafic and metamorphic rocks in situ detrital zircon hf and u pb isotope data are consistent with derivation of these rocks from nearby triassic magmatic rocks and basement that comprise part of the newly recognized late permian triassic sumdo cuoqen orogenic belt the new data suggests correlation with the upper triassic langjiexue group which lies on the opposing southern side of indus yarlung ophiolite sediments from both the mailonggang formation and langjiexue group are interpreted to represent formerly contiguous parts of a sequence deposited on the southern flanks of the sumdo cuoqen belt

the timing of india asia collision onset facts theories controversies abstract the timing of initial collision between india and asia has remained controversial for half a century this paper attempts to review this crucial and hotly debated argument describing first the different methods used to constrain the age of collision and discussing next the rationale results inferences and problems associated with each we conclude that stratigraphy represents the best direct way to unravel collision chronology other methods focusing on the magmatic metamorphic or paleomagnetic record provide additional fundamental constraints but cannot provide a robust direct estimate of collision onset initial collision in the central eastern himalaya is dated directly at the middle paleocene 59 1 ma by the abrupt change in sediment provenance recorded in trench settings the quasi synchronous unconformities documented along both tethyan passive margin of india and active margin of asia from tibet to zanskar ladakh confirm that orogeny was underway at the close of the paleocene 56 ma well before the disappearance of marine seaways in the himalaya during the early middle eocene 50 45 ma sedimentary evolution and provenance changes in marine to fluvio deltaic successions are recorded synchronously within error from the western to the central eastern himalaya failing to provide conclusive evidence for diachronous collision these coherent observations are hard to reconcile with three widely cited hypotheses invoking either paleogene arc continent collision or late cretaceous ophiolite obduction or the protracted existence of a greater india basin which are all not favored after discussing the geological evidence critically point by point a scenario no more complex than the one involving solely the passive continental margin of india and the active continental margin of asia is needed to explain the geological evolution of the nascent himalaya the collision between the tethys himalaya and the transhimalayan arc trench system does represent the collision between india and asia because the yarlung zangbo ophiolite is the forearc basement of the asian active margin its obduction onto india could not have preceded the initial closure of neo tethys ophiolite obduction began when collision began in the middle paleocene

zircon u pb chronology of the nyingtri group southern lhasa terrane tibetan plateau implications for grenvillian and pan african provenance and mesozoic cenozoic metamorphism abstract the nyingtri group of the lhasa terrane in southern tibet consists dominantly of metasedimentary rocks and orthogneiss these rocks have a similar mineral paragenesis of plagioclase k feldspar biotite quartz sillimanite garnet staurolite muscovite amphibole indicating amphibolite facies metamorphic conditions inherited detrital zircons from the metasedimentary rocks show magmatic features and yield widely variable 206pb 238u ages ranging from 3300 to 50 ma the data define two prominent age populations 1200 1000 and 600 500 ma indicating that the source of the nyingtri group preserves the records of both grenville and pan african magmatic thermal events inherited magmatic zircon cores from the orthogneiss yield a crystallization age of 496 ma limiting the depositional age of the metasedimentary sequence to cambrian or older overgrowth rims on the detrital zircons from one metasedimentary rock yield a metamorphic age of 32 ma on the basis of these results together with the

evolution of the bangong nujiang tethyan ocean insights from the geochronology and geochemistry of mafic rocks within ophiolites abstract the bangong nujiang suture zone bnsz is located in the central tibetan plateau and represents the relict of the bangong nujiang tethyan ocean bnto this paper presents zircon u pb ages and whole rock geochemical and sr nd isotope data for the ophiolitic rocks from the rutog dongco dongqiao amdo and dengqen areas from west to east in the bnsz zircon u pb ages obtained from five gabbros and one leucogabbro from the five areas are 169 2 167 2 187 2 184 2 177 3 and 164 2 ma respectively mafic rocks gabbros and basalts in the bnsz can be divided into morb like and oib type based on geochemical data the morb like rocks are tholeiitic and can be further divided into n and e morb subtypes marked by depleted and flat ree patterns respectively all morb like rocks show supra subduction zone ssz geochemical affinities such as mild enrichment in the large ion lithophile elements and depletion in the high field strength elements coupled with high and positive whole rock e nd t values 8 1 to 5 4 the oib type rocks which formed in the early cretaceous 132 108 ma are exposed in the dongco dongqiao and tarenben areas and they consist mostly of alkali basalts with within plate geochemical characteristics and positive e nd t values 5 9 to 4 5 on the basis of these new data combined with information from the literature we further support that the bnto was an intra oceanic arc backarc basin that existed between the north lhasa subterrane nlt and the south qiangtang subterrane sqt during the early middle jurassic this basin may have been active until the early cretaceous when the oib type basalts erupted

the subduction accretion history of the bangong nujiang ocean constraints from provenance and geochronology of the mesozoic strata near gaize central tibet abstract the mesozoic strata within the bangong nujiang suture zone in central tibet recorded critical information about the subduction accretion processes of the bangong nujiang ocean prior to the lhasa qiangtang collision this paper reports detailed field observations petrographic descriptions sandstone detrital zircon u pb ages and hf isotopic analyses from an accretionary complex preserved as mugagangri group and the unconformably overlying shamuluo formation near gaize the youngest detrital zircon ages together with other age constraints from literature suggest that the mugagangri group was deposited during late triassic early jurassic while the shamuluo formation was deposited during late jurassic early cretaceous based on the differences in lithology age and provenance the mugagangri group is subdivided into the upper middle and lower subunits these units are younging structurally downward southward consistent with models of progressive off scrapping and accretion in a southward facing subduction complex the upper subunit comprising mainly quartz sandstone and siliceous mud shale was deposited in abyssal plain environment close to the qiangtang passive margin during late triassic with sediments derived from the southern qiangtang block the middle and lower subunits comprise mainly lithic quartz sandstone and mud shale containing abundant ultramafic ophiolitic fragments the middle subunit of late triassic early jurassic age records a transition in tectono depositional setting from abyssal plain to trench wedge basin with sudden influx of sediments sourced from the central qiangtang metamorphic belt and northern qiangtang magmatic belt the appearance of ultramafic ophiolitic fragments in the middle subunit reflects the subduction initiation the lower subunit was deposited in a trench wedge basin during early jurassic with influx of jurassic aged zircons originating from the newly active southern qiangtang magmatic arc the lower subunit records the onset of arc magmatism related to the northward subduction of the bangong nujiang ocean the shamuluo formation comprising mainly lithic feldspar sandstone with limestone interlayers was deposited in a post collisional residual sea or pre collisional trench slope basin with sediments derived entirely from the qiangtang block

the disappearance of a late jurassic remnant sea in the southern qiangtang block shamuluo formation najiangco area implications for the tectonic uplift of central tibet abstract located between the bangong nujiang suture zone and the qiangtang block in central tibet the najiangco area 5 km to the north of nima selinco contains an upper jurassic lower cretaceous sedimentary succession deposited during a period of marine regression the youngest marine sedimentary unit in the najiangco area is the upper jurassic shamuluo formation which consists of sandstone limestone siltstone and shale sedimentary facies analysis shows that tidal flat and subtidal lagoonal facies characterized the northern margin of the basin while delta front and prodelta facies dominated the middle part and carbonate shoal and patch reef facies prevailed along the southern margin provenance analysis including petrographic modal analysis of sandstones and u pb dating of detrital zircons shows that a recycled orogen in the central qiangtang to the north of najiangco area was the source of the sandstones in the shamuluo formation biostratigraphy and u pb zircon dating of a porphyritic granitoid dike 151 2 ma indicate that the shamuluo formation was deposited during the late jurassic oxfordian to kimmeridgian during middle jurassic time the southern qiangtang basin was dominated by shallow marine environments later during the late jurassic oxfordian to kimmeridgian the shallow marine facies retreated to the southern margin of the basin combined with regional paleogeographic data from central tibet two stages of southward retreat of the qiangtang remnant sea and three stages of topographic uplift of the qiangtang block can be recognized during late middle jurassic to early cretaceous time

a tectonic model reconciling evidence for the collisions between india eurasia and intra oceanic arcs of the central eastern tethys abstract despite several decades of investigations inferences on the timing and nature of collisions along the mesozoic cenozoic eurasian margin remain controversial we assimilate geological and geophysical evidence into a plate tectonic model for the india eurasia collision that includes continuously closing topological plate polygons constructed from a time dependent network of evolving plate boundaries with synthetic plates constructed for now subducted ocean floor including back arc basins that formed on the southern eurasian margin our model is regionally constrained and self consistent incorporating geophysical data from abyssal plains offshore west australia and east antarctica including jurassic age data from offshore northwest australia limiting much of northern greater india to a 1000 km long indenter originally reaching to the wallaby zenith fracture zone southern eurasia and southeast asia are riddled with dismembered oceanic arcs indicating long lived tethyan intra oceanic subduction this intra oceanic subduction system was well established from cretaceous time in the india eurasia convergence zone in the neotethys which was consumed during greater india s northward trajectory towards eurasia from the early cretaceous fragments of obducted oceanic crust within the yarlung tsangpo suture zone between india and eurasia predominantly date to the late jurassic or mid cretaceous barremian aptian the various ophiolites along strike and a hiatus in subduction related magmatism during the tithonian aptian suggest that there was at least one generation of intra oceanic arc formation whose plate boundary configuration remains uncertain paleomagnetic and magmatic studies suggest that the intra oceanic arc was at equatorial latitudes during the early cretaceous before subduction resumed further north beneath the eurasian margin lhasa terrane with another hiatus in subduction related magmatism along southern lhasa during 80 65 ma possibly as the back arc spreading centre approached the active andean style margin in our model greater india collided with the tethyan intra oceanic arc in paleocene eocene time finally closing the tethyan seaway from mid late eocene time which is consistent with the age of the youngest marine deposits found between india and eurasia geological evidence from the collision zone indicates an age of initial arc continent collision by 52 ma followed by the soft initial continent continent collision between india and eurasia by 44 2 ma this timing is supported by marine geophysical data where the spreading centres in the indian ocean record a drastic decline in seafloor spreading rates and changes in spreading directions first at 52 ma followed by another reorganisation at 43 ma the abandonment of spreading in the wharton basin and the onset of extrusion tectonics in asia by 36 ma are likely indicators of hard complete continent collision and highlight the multi stage collisional history of this margin our continuously evolving network of mid ocean ridge and subduction zone geometries and divergence convergence vectors through time provide a basis for future refinements to assimilate new data and or test alternative tectonic scenarios

cenozoic tectonics of western north america controlled by evolving width of farallon slab the basin and range province of western north america exemplified by the alternating mountain ridge and valley landscapes across nearly the entire u s state of nevada started to form 50 million years ago through a series of extensions of the continental crust prior to that massive mountain building collisions at the boundary between north america and the pacific ocean formed a subduction zone and compressed the continent by combining observations of global subduction zone velocities with numerical modeling schellart et al p 316 1 demonstrate that the thinning of the subducting portion of the oceanic plate controlled how and when the transition from compression to extension occurred even today where the much smaller remnant oceanic plate continues to slowly subduct below north america in the pacific northwest the width of the slab and not its age controls the velocity of subduction indeed this relation may explain the dynamics of other modern subduction zones from south america to japan 1 lookup doi 10 1126 science 1190366

rodingite from the beila ophiolite in the bangong nujiang suture zone northern tibet new insights into the formation ofophiolite related rodingite abstract the beila ophiolite is located in the middle part of the bangong nujiang suture zone on the northern tibetan plateau and rodingites are widespread within serpentinized peridotites in the ophiolite according to the mineral assemblages the beila rodingite can be divided into three types type i rodingites have a mineral assemblage of prehnite chlorite and minor diaspore type ii rodingites comprise diopside prehnite chlorite epidote and minor hydrogrossular and type iii rodingites underwent further rodingitization and contain more hydrogrossular and vesuvianite than the type ii rodingites zircon grains from two rodingite samples yielded shrimpu pb ages of 172 164 ma with positive values of hf t 9 0 to 16 3 and mantle like 18o values 4 31 to 6 82 indicating the beila ophiolite formed during the middle jurassic the rodingitization of the beila ophiolite took place in two stages the early stage involved the production of the type i and ii rodingites during metasomatism by ca rich fluids with high co2 h2o ratios and this stage is characterized by the formation of ca silicates diopside prehnite and hydrogrossular the second stage involved the production of type iii rodingites at the shallower depths in exhumed mantle under the influence of fluids with lower co2 h2o ratios estimates of the migration of elements during rodingitization e g ca alkalis fe cr ni nb ti rare earth elements and large ion lithophile elements combined with sr nd isotopic compositions indicate that the formation of the beila rodingite involved an extensive exchange of materials among the protoliths serpentinized peridotite and subduction related materials the results indicate that analyses of rodingites in ophiolites can contribute to our understanding of material exchange during oceanic plate subduction

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

effects of slab window alkaline volcanism and glaciation on thermochronometer cooling histories patagonian andes abstract southern patagonia is a prime example of ongoing oceanic ridge collision and slab window formation sustained over several million years the impact of these phenomena on the thermal structure and exhumation of the crust have been mainly assessed with low temperature thermochronology of bedrock samples here we infer thermal histories from new and existing thermochronological data from the region of most recent ridge collision in particular we evaluate the potential far reaching thermal effects of the evolving slab window which have previously been considered responsible for patterns of late miocene reheating associated with back arc alkaline volcanism our model results define protracted cooling since 15 ma and stepwise exhumation since the late miocene the pattern of stepwise exhumation closely matches the onset of patagonian glaciation at 7 ma and the successive pulse of glacial incision coeval with neotectonic activity since 3 4 ma that are also documented by independent geological and geomorphological evidence in the region importantly our findings challenge the recently suggested lack of glacial erosion and incision since 5 ma in this region furthermore in contrast to previous modelling studies we find that the available data do not evidence a previously proposed northward propagating heating event associated with alkaline volcanism we hypothesize that the anomalous alkaline volcanism in the patagonian back arc might be related to trench orthogonal tears aligned with transform faults in the subducting plate the substantial differences from the previous modelling procedure on some of the same samples is demonstrated to result from an important lack of convergence in model runs

tectonic control on rock uplift exhumation and topography above an oceanic ridge collision southern patagonian andes 47 s chile

late cretaceous volcanic rocks in the sangri area southern lhasa terrane tibet evidence for oceanic ridge subduction abstract late cretaceous magmatism in the southern lhasa subterrane tibet provides critical constraints on the subduction processes of the neo tethyan oceanic plate here we provide data on the whole rock geochemistry zircon u pb ages trace element contents and hf isotopes of early late cretaceous volcanic rocks from sangri county to sangye temple ss along the southern lhasa subterrane the ss volcanic rocks include basalts andesites and dacites sims and la icp ms zircon u pb data indicate that the ss dacites were erupted at ca 95 ma coeval with the early late cretaceous flare up event in the southern lhasa subterrane the ss volcanic rocks belong to the calc alkaline to high k calc alkaline series and have the geochemical features of subduction related volcanic rocks with negative nb ta and ti anomalies the basalts have similar rare earth element ree patterns to e morb and are characterized by high zr abundances 65 190 ppm and zr y ratios 3 3 6 5 they were possibly derived by high degree partial melting of mantle peridotite that had been metasomatized by subduction related fluids with an input of asthenospheric components positive whole rock end t 2 9 to 3 2 values for the andesites and their variable mg values 35 61 and mgo 2 29 7 32 wt cr 12 232 ppm and ni 10 127 ppm contents can be attributed to medium degree partial melting of mantle peridotite followed by fractional crystallization of clinopyroxene and olivine the calc alkaline ss dacites are similar to adakites with high sr 663 1188 ppm and low heavy rare earth element hree and y 10 7 11 5 ppm contents these adakitic dacites have positive whole rock end t 2 9 to 3 4 and zircon ehf t 8 4 to 14 9 values with relatively high values of mg 36 57 and high contents of compatible elements indicating that they were derived from the partial melting of subducted oceanic slab and with the ascending magma interacting with mantle wedge peridotite our new data indicate a distinct rock association of coeval asthenosphere component involved basalts and slab derived adakitic dacites such an association together with the coeval high temperature charnockites and high temperature granulite facies metamorphism of the charnockitic country rocks in the southern lhasa subterrane indicates an anomalous input of materials and heat during the early late cretaceous flare up event such an event can probably be attributed to the northwards subduction of a neo tethyan oceanic ridge that allowed for contributions from upwelling asthenosphere that differed from the typical results of normal angle subduction or low angle flat subduction and subsequent slab rollback

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

the bangong lake ophiolite nw tibet and its bearing on the tectonic evolution of the bangong nujiang suture zone abstract the jurassic bangong lake ophiolite nw tibet is a key element within the western part of the bangong nujiang suture zone which marks the boundary between the lhasa and qiangtang blocks it is a tectonic melange consisting of numerous blocks of peridotite mafic lavas and dikes the mantle peridotites include both clinopyroxene bearing and clinopyroxene free harzburgites the cpx bearing harzburgite contains al rich spinel with low cr s 20 25 resembling peridotites formed in mid ocean ridge settings on the other hand the cpx free harzburgite is highly depleted with cr rich spinel cr 69 73 typical of peridotites formed in subduction zone environments mafic rocks include lavas of n morb and e morb affinity and boninites the n morb rocks consist of pillow lavas and mafic dikes whereas the e morb rocks are brecciated basalts the boninites have high sio 2 53 2 57 9 wt mgo 6 5 12 5 wt cr 166 752 ppm and ni 63 213 ppm and low tio 2 0 22 0 37 wt and y 5 34 8 10 ppm and are characterized by having u shaped chondrite normalized ree patterns the n morb and e morb lavas probably formed by different degrees of partial melting of primitive mantle whereas the boninites reflect partial melting of depleted peridotite in a suprasubduction zone environment the geochemistry of the ophiolite suggests that it is a fragment of oceanic lithosphere formed originally at a mid ocean ridge mor and then trapped above an intraoceanic subduction zone ssz where the mantle peridotites were modified by boninitic melts the bangong nujiang suture zone is believed to mark the boundary between two blocks within gondwanaland rather than to separate gondwanaland from eurasia

petrogenesis of the majiari ophiolite western tibet china implications for intra oceanic subduction in the bangong nujiang tethys abstract the majiari ophiolite lies in the western bangong nujiang suture zone which separates the qiangtang and lhasa blocks in central tibet the ophiolite consists of peridotite gabbro diabase and basalt zircon u pb dating yielded an age of 170 5 1 7 ma for the gabbro whereas 40 ar 39 ar dating of plagioclase from the same gabbro yielded ages of 108 4 2 6 ma plateau age and 112 2 ma isochron age indicating that the ophiolite was formed during the middle jurassic and was probably emplaced during the early cretaceous zircons from the gabbro have e hf t values ranging from 6 9 to 10 6 and f lu hf values ranging from 0 92 to 0 98 mafic lavas plot in the tholeiitic basalt field but are depleted in nb ta and ti and enriched in rb ba and th in the n morb normalized trace element spider diagram these lavas have whole rock e nd t values of 5 9 to 6 6 suggesting that they were derived from a depleted mantle source which was probably modified by subducted materials the majiari ophiolite probably formed in a typical back arc basin above a supra subduction zone ssz mantle wedge intra oceanic subduction occurred during the middle jurassic and collision of the lhasa and south qiangtang terranes likely occurred in the early cretaceous thus closure of the bangong nujiang tethys ocean likely occurred before the early cretaceous

subduction zone infancy examples from the eocene izu bonin mariana and jurassic california arcs a new model for the earliest stages in the evolution of subduction zones is developed from recent geologic studies of the izu bonin mariana ibm arc system and then applied to late jurassic ophiolites of cailfornia the model accounts for several key observations about the earliest stages in the evolution of the ibm system 1 subduction nucleated along an active transform boundary which separated younger less dense lithosphere in the west from older more dense lithosphere to the east 2 initial arc magmatic activity occupied a much broader zone than existed later 3 initial magmatism extended up to the modern trench over a region now characterized by subnormal heat flow 4 early are magmatism was characterized by depleted tholeiitic and ultra depleted boninitic magmas indicating that melting was more extensive and involved more depleted mantle than is found anywhere else on earth 5 early arc magmatism was strongly extensional with crust forming in a manner similar to slow spreading ridges and 6 crust production rates were 120 to 180 km 3 km ma several times greater than for mature arc systems these observations require that the earliest stages of subduction involve rapid retreat of the trench we infer that this resulted from continuous subsidence of denser lithosphere along the transform fault this resulted in strong extension and thinning of younger more buoyant lithosphere to the west this extension was accompanied by the flow of water from the sinking oceanic lithosphere to the base of the extending lithosphere and the underlying asthenosphere addition of water and asthenospheric upwelling led to catastrophic melting which continued until lithosphere subsidence was replaced by lithospheric subduction application of the subduction zone infancy model to the late jurassic ophiolites of california provides a framework in which to understand the rapid formation of oceanic crust with strong arc affinities between the younger sierran magmatic arc and the franciscan subduction complex provides a mechanism for the formation and subsidence of the great valley forearc basin and explains the limited duration of high t high p metamorphism experienced by franciscan melanges

a review on forearc ophiolite obduction adakite like generation and slab window development at the chile triple junction area uniformitarian framework for spreading ridge subduction this paper aggregates the main basic data acquired along the chile triple junction ctj area 45 48 s where an active spreading center is presently subducting beneath the andean continental margin updated sea floor kinematics associated with a comprehensive review of geologic geochemical and geophysical data provide new constraints on the geodynamics of this puzzling area we discuss 1 the emplacement mode for the pleistocene taitao ridge and the pliocene taitao peninsula ophiolite bodies 2 the occurrence of these ophiolitic complexes in association with five adakite like plutonic and volcanic centers of similar ages at the same restricted locations 3 the inferences from the cooccurrence of these sub coeval rocks originating from the same subducting oceanic lithosphere evolving through drastically different temperature pressure p t path low grade greenschist facies overprint and amphibolite eclogite transition respectively 4 the evidences that document ridge jump events and associated microplate individualization during subduction of the scr1 and scr 1 segments the chonos and cabo elena microplates respectively the ridge jump process associated with the occurrence of several closely spaced transform faults entering subduction is controlling slab fragmentation ophiolite emplacement and adakite like production and location in the ctj area kinematic inconsistencies in the development of the patagonia slab window document an 11 km westward jump for the scr 1 spreading segment at 6 5 to 6 8 ma the scr 1 spreading center is relocated beneath the north patagonia icefield npi we argue that the deep seated difference in the dynamically sustained origin of the high reliefs of the north and south patagonia icefield npi and spi is asthenospheric convection and slab melting respectively the chile triple junction area provides the basic constraints to define the basic signatures for spreading ridge subduction beneath an andean type margin

triassic continental subduction in central tibet and mediterranean style closure of the paleo tethys ocean the qiangtang metamorphic belt qmb in central tibet is one of the largest and most recently documented high pressure hp to near ultrahigh pressure near uhp belts on earth lu hf ages of eclogite and blueschist facies rocks within the qmb are 244 223 ma indistinguishable from the age of uhp metamorphism in the qinling dabie orogen results of a u pb detrital zircon study suggest that protoliths of the qmb include upper paleozoic qiangtang continental margin strata and sandstones that were derived from a paleozoic arc terrane that developed within the paleo tethys ocean to the north we attribute qmb hp metamorphism to continental collision between the qiangtang terrane and a paleo tethys arc terrane this collision and the coeval south china north china collision may have slowed convergence between laurasia and gondwana derived terranes and initiated mediterranean style rollback and backarc basin development within much of the remnant paleo tethys ocean realm

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

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

mesozoic cenozoic geological evolution of the himalayan tibetan orogen and working tectonic hypotheses the himalayan tibetan orogen culminated during the cenozoic india asia collision but its geological framework and initial growth were fundamentally the result of multiple previous ocean closure and intercontinental suturing events as such the himalayan tibetan orogen provides an ideal laboratory to investigate geological signatures of the suturing process in general and how the earth9s highest and largest orogenic feature formed in specific this paper synthesizes the triassic through cenozoic geology of the central himalayan tibetan orogen and presents our tectonic interpretations in a time series of schematic lithosphere scale cross sections and paleogeographic maps we suggest that north dipping subducting slabs beneath asian continental terranes associated with closure of the paleo meso and neo tethys oceans experienced phases of southward trench retreat prior to intercontinental suturing these trench retreat events created ophiolites in forearc extensional settings and or a backarc oceanic basins between rifted segments of upper plate continental margin arcs this process may have occurred at least three times along the southern asian margin during northward subduction of neo tethys oceanic lithosphere from 174 to 156 ma 132 to 120 ma and 90 to 70 ma at most other times the tibetan terranes underwent cordilleran style or collisional contractional deformation geological records indicate that most of northern and central tibet the hoh xil and qiangtang terranes respectively were uplifted above sea level by jurassic time and southern tibet the lhasa terrane north of its forearc region has been above sea level since 100 ma stratigraphic evidence indicates that the northern himalayan margin of india collided with an asian affinity subduction complex forearc arc system beginning at 60 ma both the himalaya composed of indian crust and tibet show continuous geological records of orogenesis since 60 ma as no evidence exists in the rock record for a younger suture the simplest interpretation of the geology is that india asia collision initiated at 60 ma plate circuit paleomagnetic and structural reconstructions however suggest that the southern margin of asia was too far north of india to have collided with it at that time seismic tomographic images are also suggestive of a second more southerly neo tethyan oceanic slab in the lower mantle where the northernmost margin of india may have been located at 60 ma the geology of tibet and the india asia suture zone permits an alternative collision scenario in which the continental margin arc along southern asia the gangdese arc was split by extension beginning at 90 ma and along with its forearc to the south the xigaze forearc rifted southward and opened a backarc ocean basin the rifted arc collided with india at 60 ma whereas the hypothetical backarc ocean basin may not have been consumed until 45 ma a compilation of igneous age data from tibet shows that the most recent phase of gangdese arc magmatism in the southern lhasa terrane initiated at 70 ma peaked at 51 ma and terminated at 38 ma cenozoic potassic adakitic magmatism initiated at 45 ma within a 200 km wide elliptical area within the northern qiangtang terrane after which it swept westward and southward with time across central tibet until 26 ma at 26 to 23 ma potassic adakitic magmatism swept southward across the lhasa terrane a narrow 20 km width orogen parallel basin developed at low elevation along the axis of the india asia suture zone the kailas basin and greater himalayan sequence rocks began extruding southward between the south tibetan detachment and main central thrust the kailas basin was then uplifted to 4 km elevation by 20 ma after which parts of the india asia suture zone and gangdese arc experienced 6 km of exhumation between 20 and 16 ma between 16 and 12 ma slip along the south tibetan detachment terminated and east west extension initiated in the northern himalaya and tibet potassic adakitic magmatism in the lhasa terrane shows a northward younging trend in the age of its termination beginning at 20 to 18 ma until volcanism ended at 8 ma we interpret the post 45 ma geological evolution in the context of the subduction dynamics of indian continental lithosphere and its interplay with delamination of asian mantle lithosphere

late jurassic adakitic granodiorite in the dong co area northern tibet implications for subduction of the bangong nujiang oceanic lithosphere and related accretion of the southern qiangtang terrane abstract in this paper we present new major and trace element chemical data zircon u pb age data and zircon hf isotopic compositions for the dong co granodiorites dgs that were emplaced directly in the dong co ophiolites dos in the middle segment of the bangong nujiang suture zone bnsz northern tibet we use these new data to discuss the genesis of the dgs and implications for the evolution of the region the dgs have high contents of sio 2 al 2 o 3 na 2 o and sr high values of mg low contents of yb and y and markedly high sr y ratios indicating an adakitic affinity and we show that they were derived from the partial melting of subducted oceanic crust the dgs contain large numbers of inherited zircons that have similar shapes and peak ages to the detrital zircons in the accretionary wedge graywackes that surround the dgs suggesting that many of these sediments were assimilated during the formation of the dgs five dg samples yield the youngest zircon u pb ages of 155 160 ma indicating that the dgs formed during the late jurassic based on the present results and regional geological data we infer that the bangong nujiang oceanic lithosphere was subducted northwards beneath the southern qiangtang terrane during the formation of a continental arc system during the late jurassic the bangong nujiang oceanic lithosphere retreated rapidly after its initial subduction and the southern qiangtang terrane to the north of the bangong nujiang ocean acquired a massive amount of new oceanic material in the form of an accretionary wedge this wedge which included the dos and the mugagangri group became sufficiently large both in size and width for arc type magmas dgs to develop within it during the late jurassic

neogene kinematic history of nazca antarctic phoenix slab windows beneath patagonia and the antarctic peninsula article i nfo the patagonian slab window is a subsurface tectonic feature resulting from subduction of the nazca antarctic spreading ridge system chile rise beneath southern south america the geometry of the slab window had not been rigorously defined in part because of the complex nature of the history of ridge subduction in the southeast pacific region which includes four interrelated spreading ridge systems since 20 ma first the nazca phoenix ridge beneath south america then simultaneous subduction of the nazca antarctic and the northern phoenix antarctic spreading ridge systems beneath south america and the southern phoenix antarctic spreading ridge system beneath antarctica spreading ridge paleo geographies and rotation poles for all relevant plate pairs nazca phoenix antarctic south america are available from 20 ma onward and form the mathematical basis of our kinematic reconstruction of the geometry of the patagonia and antarctic slab windows through neogene time at approximately 18 ma the nazca phoenix antarctic oceanic ridge ridge ridge triple junction enters the south american trench we recognize this condition as an unstable quadruple junction heat flow at this junction and for some distance beneath the forearc would be considerably higher than is generally recognized in cases of ridge subduction from 16 ma onward the geometry of the patagonia slab window developed from the subduction of the trailing arms of the former oceanic triple junction the majority of the slab window s areal extent and geometry is controlled by the highly oblique near parallel subduction angle of the nazca antarctic ridge system and by the high contrast in relative convergence rates between these two plates relative to south america the very slow convergence rate of the antarctic slab is manifested by the shallow levels achieved by the slab edge b45 km thus no point on the antarctic slab is sufficiently deep to generate normal mantle derived arc type magmas upwelling beneath the region may have contributed to uplift and eastward transfer of extension in the scotia sea

subduction of a spreading ridge within the bangong co nujiang tethys ocean evidence from early cretaceous mafic dykes in the duolong porphyry cu au deposit western tibet abstract the intrusion of mafic dykes into a near trench accretionary prism and continental margin magmatism with characteristics that differ from those of adjacent arc magmatism are direct manifestations of the subduction of a spreading ocean ridge and the formation of a slab window in this paper we investigated mafic dykes intruded into the accretionary prism that hosts the duolong porphyry cu au deposit dcad of western tibet la icp ms analysis of u pb in zircon indicates that the dykes formed during the early cretaceous 126 127 ma the dykes are characterized by ehf t values from 2 44 to 11 8 twenty nine mafic dyke samples were divided into three groups based on their locations and geochemical compositions group i has nb 8 31 10 2 ppm nb la 0 71 1 20 and nb u 21 4 37 9 group ii has nb 40 5 52 6 ppm nb la 0 84 1 58 and nb u 18 8 47 8 and group iii has nb 65 7 105 ppm nb la 1 35 2 08 and nb u 36 5 73 8 group i is classified as nb enriched basalts enbs whereas groups ii and iii are classified as high nb basalts hnbs both the enbs and hnbs were derived from an adakite metasomatized mantle wedge that subsequently underwent crystallization of olivine and clinopyroxene the compositional variations of the studied dykes resulted mainly from mantle source heterogeneity the volume of the slab melts gradually increases from group i enbs to group ii hnbs and group iii hnbs leading to gradually increasing incompatible element concentrations considering their geochemical characteristics and field relationships as well as the unique characteristics of continental margin magmatism in the dcad we propose that the dykes emplaced in an extensional accretionary prism were derived from the northward subduction of a spreading ridge in the bangong co nujiang tethys ocean during the early cretaceous the ridge subduction event was also responsible for the generation of coeval adakites intermediate felsic intrusions the maierze bimodal volcanic rocks as well as the metallogenesis of the dcad

an early cretaceous slab window beneath central tibet sw china evidence from oib like alkaline gabbros in the duolong area

subduction of diverging plates and the principles of slab window formation consumption of an ocean basin by subduction commonly brings a sea floor spreading ridge toward a deep sea trench if plate divergence and convergence continue after the ridge intersects the subduction zone a slab window forms between the subducted parts of the diverging oceanic plates producing anomalous thermal physical and chemical effects in the surrounding asthenospheric mantle in turn these conditions alter the tectonic and magmatic evolution of the overriding plate usually disturbing ordinary fore arc and arc regimes differential lithospheric stresses on opposite sides of the triple junction contribute to disturbances in the overriding plate anomalous magmatism from fore arc to back arc may be accompanied by fore arc metamorphism strike slip faulting uplift extension and in extreme cases rifting the shape and size of the window are controlled mainly by the pre subduction ridge transform trench configuration slab dip angles and vectors of plate convergence subducted ridge segments expand into windows whose margins approximately parallel the motion vectors between the triple junction and the subducting plates subducted transform faults continue to be active usually as oblique slip faults until the plates separate as transform faults subduct they become longer on the plate which occupies the acute angle between ridge and trench and shorter on the other plate trains of isolated windows produced by subduction of a segmented ridge transform system progressively expand during descent commonly merging together to form a composite slab window oblique subduction of a highly segmented ridge is likely to produce two or more fraternal slab windows one at each site of ridge trench intersection above a slab window arc volcanism diminishes and may be replaced by volcanism of mid ocean ridge or rift affinity the change in chemical character reflects various processes including elevated heat flow decreasing hydration of the upper mantle juxtaposition of supra and sub slab mantle reservoris asthenospheric upwelling and melting of the trailing plate edges if the slab window migrates the anomalous magmatic regime may be replaced by renewed arc volcanism identifying the effects of slab windows in ancient convergent margin assemblages requires an understanding of slab window principles and implications

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

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

island arc elements and arc related ophiolites abstract all major structural elements in island arc systems fore arc magmatic arc back arc basins and remnant arcs are potential ophiolite sources and those features that allegedly characterise ophiolites of ocean ridge origin sheeted dyke complexes mantling pelagic rocks hydrothermal metamorphism and associated mineralization can also arise within arc settings age relations are critical in the interpretation of arc related ophiolites remnants of oceanic lithosphere identified by a pre arc initiation age are restricted to fore arc magmatic arc and remnant arc elements as are ophiolite masses generated at the inception of underthrusting the latter apparently common in ancient fore arc terrains form in nascent arc systems in which the rate of role back of the subduction hinge exceeds the rate of convergence spreading occurs above a foundering slab resulting in some arc like compositional features in simple arc systems later ophiolitic rocks have formed either in the active back arc basin or the magmatic arc only those ophiolites that have resided within or very close behind magmatic arcs should show calcalkaline or arc tholeiite magmatic affinities or be intruded or overlain by these rocks volcanic derived sediment or pelagic material may mantle ophiolites from all arc settings but pelagic rocks will generally dominate in stratigraphic sequences above remnant arcs and on back arc basin floors except adjacent to the magmatic arc ophiolites generated at major ocean ridges are unlikely to be immediately overlain by sediment with a significant volcanic component whereas such detritus may lie directly on arc inception arc and back arc ophiolites some arc derived ophiolites are preserved in their original tectonic position others can be identified from their internal features their relationship to other tectonic elements and the nature of associated rocks

slab tearing following ridge trench collision evidence from miocene volcanism in baja california mexico abstract neogene magmatic activity in central baja california underwent a major change at ca 12 5 ma when the pacific farallon active oceanic ridge collided with the trench east of vizcaino peninsula the calc alkaline magmatism which built the comondu volcanic arc vanished and was replaced by unusual volcanic associations which were erupted within six late miocene to quaternary volcanic fields jaraguay san borja san ignacio santa rosalia santa clara la purisima delineating a 600 km array along the baja california peninsula new fieldwork k ar datings and geochemical analyses on jaraguay and san borja lavas combined with previous data allow us to show that these associations include 1 adakites emplaced between 12 5 and 8 2 ma in the santa clara santa rosalia and jaraguay volcanic fields 2 niobium enriched basalts neb in santa clara and santa rosalia 11 2 to 7 4 ma 3 tholeiitic basalts and basaltic andesites 11 3 to 7 2 ma displaying a very weak subduction imprint in la purisima and san ignacio 4 alkali trachybasalts in mesas san carlos and santa catarina northwest of jaraguay 9 3 to 7 5 ma and finally 5 basalts and associated magnesian basaltic andesites and andesites locally referred to as bajaites the latter lavas display very specific geochemical characteristics including very high sr and ba contents they were emplaced in all the above mentioned volcanic fields between 14 6 and 5 3 ma the origin of these late miocene volcanics has been considered linked either to the opening of an asthenospheric window through which they ascended tholeiites and alkali trachybasalts or to the melting of its edges due to thermal erosion adakites and the subsequent reaction between adakitic melts and the supraslab mantle neb and bajaites however the identification of the remnants of the pacific farallon fossil ridge south of vizcaino peninsula indicates that active ridge subduction was not responsible for the opening of the asthenospheric window we propose that the emplacement of slab window related volcanic rocks between 13 and 7 ma from ca 25 n to 30 n was due to a process of slab tearing which started when the active ridge collided with the trench east of vizcaino peninsula the oldest part of the downgoing plate sunk into the mantle leading to the opening of a tear in the slab which likely started from the southern limit of the already existing southern california slab window it propagated parallel to the trench over about 600 km and developed into an asthenospheric window

sedimentary and tectonic evolution of the southern qiangtang basin implications for the lhasa qiangtang collision timing the mesozoic stratigraphic record of the southern qiangtang basin in central tibet records the evolution and closure of the bangong nujiang ocean to the south the jurassic succession includes toarcian aalenian shallow marine limestones quse formation aalenian bajocian feldspatho litho quartzose to feldspatho quartzo lithic sandstones shallow marine sewa formation and deep sea gaaco formation and bathonian outer platform to shoal limestones buqu formation this succession is truncated by an angular unconformity overlain by upper bathonian to lower callovian fan delta conglomerates and litho quartzose to quartzo lithic sandstones biluoco formation and callovian shoal to outer platform limestones suowa formation sandstone petrography coupled with detrital zircon u pb and hf isotope analysis indicate that the sewa and gaaco formations contain intermediate to felsic volcanic detritus and youngest detrital zircons 183 170 ma with ehf t ranging widely from 13 to 25 pointing to continental arc provenance from igneous rocks with mixed mantle and continental crust contributions an arc trench system thus developed towards the end of the early jurassic with the southern qiangtang basin representing the forearc basin above the angular unconformity the biluoco formation documents a change to dominant sedimentary detritus including old detrital zircons mainly 500 ma ages in the lower part of the unit with age spectra similar to those from paleozoic strata in the central qiangtang area a major tectonic event with intense folding and thrusting thus took place in late bathonian time 166 1 ma when the qiangtang block collided with another microcontinental block possibly the lhasa block

tectonic implications of fore arc magmatism and generation of high magnesian andesites alexander island antarctica alexander island situated off the west coast of the antarctic peninsula contains a suite of late cretaceous to early tertiary subduction related magmatic rocks the rocks occupy a fore arc position 100 200 km trenchward of the main arc antarctic peninsula and they become younger northward along the length of the island major and trace element geochemistry for 222 samples shows the suite to be a medium to high k calc alkaline series ranging in composition from picro basalt to rhyolite andesite samples show a large range in mgo and mg with nine samples representing high magnesian andesites sr and nd isotopic data indicate that the andesites range isotopically to more depleted mantle compositions than the associated basalts the dacite rhyolites can be related compositionally to the andesites by assimilation of typical pacific rim accretionary material to produce high magnesian andesite lavas it is necessary to introduce a suitable source of heat into the fore arc thus enabling partial melting of depleted sub fore arc hydrous peridotite a causative link with ridge subduction prior to magmatism is proposed with successive ridge trench collisions producing a temporal migration of the magmatism and high geothermal gradients in an anomalously hot fore arc region

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

early cretaceous sedimentary evolution of the northern lhasa terrane and the timing of initial lhasa qiangtang collision abstract lower cretaceous strata in the baingoin basin of the northern lhasa terrane record initial collision between the lhasa and qiangtang blocks followed by the early uplift of central tibet north south traverses across the baingoin basin highlight major differences between the duba formation in the north and the quasi coeval duoni formation in the south the duba formation documents upward transition from shallow shelf and deltaic environments to coarse grained siliciclastic fluvial sedimentation abundance of detrital zircons yielding jurassic cretaceous ages with ehf t values mainly between 2 and 10 occurrence of chert cr spinel and pyroxene grains together with southward paleocurrent directions indicate that the duba formation was sourced from the southern qiangtang terrane and bangong nujiang suture zone to the north the duoni formation in the south was deposited in shelfal to fan delta and fluvial environments abundant volcanic clasts detrital zircons yielding cretaceous ages with mainly negative ehf t values and northward paleocurrents indicate an active volcanic source located in the central lhasa terrane to the south with minor input from the northern lhasa terrane only the northern part of the baingoin basin was directly controlled by the lhasa qiangtang collision and may thus be considered a peripheral foreland basin whereas the southern part was mainly influenced by tectonic processes related to the northward subduction of neotethyan lithosphere and may thus be comparable to a retroarc foreland basin but these sedimentary features and the 139 79 ma baingoin plutonic intrusion do not fit well with classical foreland basin models zircon chronostratigraphy constrains the final consumption of bangong nujiang oceanic lithosphere and initial collision between the lhasa and qiangtang microcontinents to have taken place by 122 ma which has major implications for paleotectonic reconstructions of the tibetan plateau

a stable southern margin of asia during the cretaceous paleomagnetic constraints on the lhasa qiangtang collision and the maximum width of the neo tethys

northward subduction of bangong nujiang tethys insight from late jurassic intrusive rocks from bangong tso in western tibet abstract new zircon la icpms u pb age data whole rock major and trace elements and zircon hf isotopic data of intrusive rocks from larelaxin and caima plutons in the southern margin of the western qiangtang subterrane provide important evidence of northward subduction of the bangong nujiang tethyan ocean lithosphere host granitoids including quartz diorites granodiorites and syenogranites and associated mafic varieties including gabbroic enclaves dioritic dikes and enclaves and quartz dioritic enclaves are investigated in this study five host granitoid samples are dated at 163 160 ma and one dioritic dike and three dioritic enclave samples are dated at 162 158 ma which indicates that these rocks were contemporaneous the quartz diorites and granodiorites are normal calc alkaline i type granitoids the syenogranites are characterized by high sio 2 74 77 wt and differentiation index di 92 97 and marked depletion in ba nb ta sr p ti and eu thus indicating that they are highly fractionated i type granitoids the host granitoids exhibit uniform zircon e hf t values 1 4 to 1 9 considering the current data and their negative whole rock e nd t values 4 7 to 3 5 reported in recent studies these rocks can be interpreted to have resulted from the partial melting of ancient mafic lower crust with varying contributions from mantle derived or juvenile crust derived components one gabbroic enclave sample exhibits high al 2 o 3 18 3 wt low mgo 3 3 wt high tio 2 1 4 wt and high nb 24 ppm and is geochemically similar to high alumina basalt hab and high nb basalt hnb this indicates that the gabbroic enclave originated from the partial melting of mantle wedge peridotite that was metasomatized by slab melting and subsequently experienced significant fractional crystallization of olivine and clinopyroxene the dioritic dike and enclave samples yield e hf t values 1 6 to 3 1 similar to those of the host granitoids they most likely originated from magma mixing between mantle derived and crust derived melts at or close to the moho the intrusive rocks with geochemical diversity of the larelaxin and caima plutons from bangong tso in western tibet can be interpreted to have resulted from melting assimilation storage and homogenization a mash process above subduction zones thus indicating that the bangong nujiang tethyan ocean lithosphere was subducted northward beneath the western qiangtang subterrane during the late jurassic

late triassic paleogeographic reconstruction along the neo tethyan ocean margins southern tibet abstract sandstone petrographic and u pb detrital zircon analyses of upper triassic sedimentary rocks from the northern margin of india tethyan himalaya sequence and southern margin of eurasia lhasa terrane provide new constraints on the mesozoic paleogeography of neo tethyan ocean basins the upper triassic nieru formation of the tethyan himalaya sequence ths near lazi city 29 n 87 5 e is dominated by indian affinity precambrian detrital zircons which are typical of the majority of the ths however the upper triassic langjiexue formation of the ths exposed to the east at 90 93 e longitude includes significant populations of permian to early jurassic 291 184 ma detrital zircons for which there is no known indian source in addition the upper triassic nieru formation near kangma town 28 5 n 90 e located 200 km to the southeast of lazi city yielded detrital zircon age spectra that are similar to those of langjiexue formation based on detrital zircon age spectra comparisons we propose that both the langjiexue and nieru formations were derived from continental crustal fragments that were adjacent to the northwestern margin of australia furthermore we suggest that these ths units and age equivalent strata in northwest australia west sulawesi timor and west papua comprised a late triassic submarine fan along the northern australian shelf the upper triassic mailonggang formation in the southern lhasa terrane 35 km northeast of lhasa city 30 n 91 5 e is dominated by permian detrital zircons which were likely derived from proximal lhasa terrane sources the mailonggang formation differs from all age equivalent strata in the tethyan himalaya therefore we interpret that it was separated from greater india by the neo tethyan ocean

horizontal subduction zones convergence velocity and the building of the andes we discuss the relationships between andean shortening plate velocities at the trench and slab geometry beneath south america although some correlation exists between the convergence velocity and the westward motion of south america on the one hand and the shortening of the continental plate on the other hand plate kinematics neither gives a satisfactory explanation to the andean segmentation in general nor explains the development of the bolivian orocline in paleogene times we discuss the cenozoic history of horizontal slab segments below south america arguing that they result from the subduction of oceanic plateaus whose effect is to switch the buoyancy of the young subducting plate to positive we argue that the existence of horizontal slab segments below the central andes during eocene oligocene times and below peru and north central chile since pliocene resulted 1 in the shortening of the continental plate interiors at a large distance from the trench 2 in stronger interplate coupling and ultimately 3 in a decrease of the trenchward velocity of the oceanic plate present day horizontal slab segments may thus explain the diminution of the convergence velocity between the nazca and south american plates since late miocene

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