new global marine gravity model from cryosat 2 and jason 1 reveals buried tectonic structure gravity models are powerful tools for mapping tectonic structures especially in the deep ocean basins where the topography remains unmapped by ships or is buried by thick sediment we combined new radar altimeter measurements from satellites cryosat 2 and jason 1 with existing data to construct a global marine gravity model that is two times more accurate than previous models we found an extinct spreading ridge in the gulf of mexico a major propagating rift in the south atlantic ocean abyssal hill fabric on slow spreading ridges and thousands of previously uncharted seamounts these discoveries allow us to understand regional tectonic processes and highlight the importance of satellite derived gravity models as one of the primary tools for the investigation of remote ocean basins

a new mesozoic isochron chart of the northwestern pacific ocean paleomagnetic and tectonic implications a new mesozoic isochron chart of the northwestern pacific ocean has been completed by utilizing all the currently available data from 357 cruises our chart provides the first whole view of the mesozoic magnetic anomaly lineations in the northwestern pacific ocean and yields essential information for understanding the problems of the mesozoic magnetostratigraphic time scale and the evolutional history of the pacific plate the mesozoic magnetostratigraphic time scale must be recalibrated using not only the hawaiian lineation set but also other mesozoic lineation sets the lack of lineation m0 in the lineation sets other than the hawaiian lineation set indicates that a reorganization of the ridges except the pacific farallon ridge occurred between chrons m1 and m0 nearly 120 ma the eruption rate of the shatsky hotspot seems to be comparable to that of the reunion hotspot at their initial stage the growth of the pacific plate began in a small triangular region at about 190 ma

deep structures and surface boundaries among proto tethyan micro blocks constraints from seismic tomography and aeromagnetic anomalies in the central china orogen abstract the qinling qilian orogen preserves the records of early paleozoic convergence among the proto tethyan micro blocks in this study we analyze the seismic velocity structure and the aeromagnetic anomalies in the qinling qilian orogen and its adjacent areas showing that the northernmost boundaries of these proto tethyan micro blocks are defined by the guyuan longshoushan fault in the qilian orogen and by the luonan luanchuan fault in the qinling orogen respectively the lithosphere north of the qinling qilian orogen subducted southward under the qinling qilian orogen the boundaries of the micro blocks in the qinling qilian orogen of the proto tethys domain are mostly ew or ne trending combined with early paleozoic geological records our results indicate that the south dipping low velocity anomaly under the alxa block and the south dipping high velocity ones under the north qilian orogen and the north china block might indicate that the alxa block the north qilian oceanic slab and the north china block subducted southward the south dipping high velocity anomaly under the north qilian orogen is interpreted as the remnant of the proto tethys ocean in that area however the north dipping low velocity anomaly under the south qinling and the north qinling orogens the qaidam block and the north dipping high velocity one under the yangtze and the bikou blocks suggest that the south qinling and the north qinling orogens the qaidam the yangtze and the bikou blocks subducted northward the present spatial framework of the qinling qilian orogen is related to early paleozoic convergence among the proto tethyan micro blocks

microplate tectonics along a superfast seafloor spreading system near easter island there is a growing easter microplate between the pacific and nazca plates the eastern and western boundaries of the microplate are actively spreading and propagating north and south respectively the northern and southern boundaries are broad transform zones most of the interior of the microplate contains the sort of oblique sheared structures characteristic of zones of transferred lithosphere between propagating and failing spreading centres

systematics of ridge propagation south of 30 s abstract new high resolution geosat altimetry data south of 30 s reveal numerous propagating ridge wakes along intermediate and slow spreading ridges these new examples provide a large enough database to establish systematics of ridge propagation almost all active propagating ridges propagate down a regional along axis gravity or bathymetry gradient the sense of the propagating ridge offset right lateral vs left lateral is related to recent changes in spreading direction we find there is a significant difference between the propagation of ridges with an axial high morphology which propagate at greater than 50 of their full spreading rate and ridges with a median valley morphology which usually propagate at 25 of their spreading rate the axial high propagators leave behind an asymmetric wake the outer pseudofault appears as a continuous linear trough step while the sheared zone appears as a chain of small gravity bumps while we clearly see the propagating ridge wakes from offsets greater than 10 km at slow and intermediate spreading ridges at ridges spreading faster than 75 mm yr the amplitude of the wake topography decreases to the point where we no longer see these wakes in geosat altimetry data the systematics seen in this new data set support a fracture mechanics model for the dynamics of ridge propagation

non hotspot volcano chains originating from small scale sublithospheric convection 1 some oceanic volcano chains violate the predictions of the hotspot hypothesis for geographic age progressions one mechanism invoked to explain these observations is small scale sublithospheric convection ssc in this study we explore this concept in thermo chemical 3d numerical models melting due to ssc is shown to emerge in elongated features 750 km parallel to plate motion and not just at a fixed spot therefore volcanism occurs in chains but not with hotspot like linear age progressions the seafloor age at which volcanism first occurs is sensitive to mantle temperature as higher temperatures increase the onset age of ssc because of the stabilizing influence of thicker residue from previous mid ocean ridge melting mantle viscosity controls the rate of melt production with decreasing viscosities leading to more vigorous convection and volcanism calculations predict many of the key observations of the pukapuka ridges and the volcano groups associated with the line cook austral and marshall islands

a precambrian microcontinent in the indian ocean ridges of thick raised crust on the indian ocean floor were thought to be mostly volcanic seamounts formed above the reunion mantle plume dating of zircon minerals in mauritian lavas however indicates that fragments of an ancient microcontinent may be preserved beneath the seamounts contributing to the thickened crust

microplate tectonics new insights from micro blocks in the global oceans continental margins and deep mantle abstract any plate has a growth process from small to large the micro blocks or micro plates are sometimes the precursors of large plates the origin growth aborting extinction and residual process of micro blocks are of great significance for the study of plate tectonics and pre plate tectonics the micro block can be divided into continental oceanic and mantle micro blocks according to their compositions in this paper the micro blocks in the global oceans have been summarized according to the following five environments mid ocean ridge system subduction system transform fault system deep sea intraplate system and extension rift system we first propose a genetic classification of micro blocks comprising detachment derived rifting derived transform derived propagation derived ridge jumping derived subduction derived accretion derived collision derived and delamination derived micro blocks the different types of micro block boundaries such as active or fossil detachment fault subduction zone mid ocean ridge transform fault fracture zone transfer fault accommodation zone lithosphere scale strike slip fault pseudofault intra oceanic convergent zone overlapping spreading centre non transform offset rheological crustal or mantle discontinuity are systematically discussed for different micro blocks thus the number of triple junctions will be more than the 16 in the traditional plate tectonic theory a stability analysis of these triple junctions is the key to understanding the causes of micro blocks however the micro block has no ultimate cause so it is unnecessary to pursue one ultimate cause or initiation of plate tectonics these micro blocks within oceanic basins along oceanic margins or within the deep mantle can not only be used to develop deep ocean fine structural analysis and plate tectonic reconstruction but also to explain the causes of micro blocks in some orogens this will enrich the research of more detailed pre orogenic or syn orogenic evolution of orogenic belts and even extend to the study of early precambrian pre plate tectonic mechanisms the micro block can be a transition among microplate plate and terrane under a plate tectonic regime it can also be formed in inter sphere tectonic processes it helps better our understanding of regimes of cratonic basin formation and intra continental deformation which are the difficulties faced by the plate tectonics theory we speculate that the micro block tectonics theory is a unified tectonic theory of cross layer cross phase cross space time scale and cross planet

westward migration of the izu bonin trench northward motion of the philippine sea plate and their relationships to the cenozoic tectonics of japanese island arcs abstract the essential elements in the understanding of the cenozoic island arc tectonics of the japanese islands came from reconstructing the paleo position of the plate boundaries and estimating the change in the philippine sea plate motion by using the 2nd law of the convergence rate of plates it was estimated that the izu bonin trench wandered around 400 km east from its present position during the paleogene and migrated westward thereafter island arc tectonism is related to the convergence rate of plates 1st law of the convergence rate of plates hence the changes in the philippine sea plate motion was examined by compiling paleomagnetic data as a result the main events of the cenozoic tectonics of japan were well explained by the change in the position of the plate boundaries and the change in motion of the philippine sea plate

northern east pacific rise evolution from 25 m y b p to the present the northeast pacific topography and magnetic lineations 25 m y b p to the present record the traces of three major spreading reorganizations only one spreading center is observed today but there is evidence for several ephemeral episodes of twin spreading accompanying the evolution from an extensive pacific guadalupe plate boundary to a much shortened pacific cocos and pacific rivera plate boundary the 25 m y b p plate reorganization culminated with the formation of the guadalupe plate bound by the murray fracture zone to the north and the cocos nazca spreading ridge to the south between 25 and 12 5 m y b p spreading continued while the plate retained the same general outline the 12 5 11 m y b p reorganization resulted in the creation of a much shortened pacific cocos plate boundary located in its early stages over the mathematician seamounts and a much reduced cocos plate the last reorganizaton 6 5 3 5 m y b p resulted in the abandonment of the mathematician spreading ridge as a pacific cocos plate boundary in favor of the east pacific rise

terrane wrecks coupled oroclines and paleomagnetic inclination anomalies abstract three major orogenic belts that are host to coupled oroclines terrane wrecks are likewise characterized by or directly associated with paleomagnetic inclination anomalies 1 the north american cordillera 2 the western european variscan belt and 3 the eastern european alpine system implied paleolatitudinal discrepancies in each orogen are of similar magnitude to the translation required by orogen parallel shortening during oroclinal formation the alaskan coupled oroclines accommodated 3000 km of orogen parallel shortening in the north american cordillera where primary remanent paleomagnetic inclinations in late cretaceous rocks require the bulk of the belt to have been translated 2000 to 3000 km northward relative to cratonic north america the iberian coupled oroclines accommodated 1100 km of orogen parallel shortening in the western european variscan belt within which mid paleozoic paleomagnetic inclinations are shallow with respect to stable gondwana the implied existence of a 3500 km wide paleotethys ocean between the variscan autochthon and the north gondwana margin is seemingly at odds with paleontological constraints the carpathian balkan oroclines accommodated 1000 km of orogen parallel shortening in the eastern alpine system directly north northeast of an eastern mediterranean region where primary remanent inclinations in late oligocene to miocene rocks require northward translation between 500 and 1000 km palinspastic restoration of the alaskan and carpathian balkan oroclines fully accounts for the translations recorded by the cordilleran and eastern mediterranean inclination anomalies respectively restoration of the iberian coupled oroclines confirms the existence of paleotethys but only accounts for one third of the paleolatitudinal discrepancy recorded by the variscan inclination anomaly fully resolving the variscan inclination anomaly requires 1 a paleomagnetic data set of greater temporal and spatial extent 2 refinement of the geometric and geological continuity of the variscan belt across western europe

oceanic microplate formation records the onset of india eurasia collision abstract mapping of seafloor tectonic fabric in the indian ocean using high resolution satellite derived vertical gravity gradient data reveals an extinct pacific style oceanic microplate mammerickx microplate west of the ninetyeast ridge it is one of the first pacific style microplates to be mapped outside the pacific basin suggesting that geophysical conditions during formation probably resembled those that have dominated at eastern pacific ridges the microplate formed at the indian antarctic ridge and is bordered by an extinct ridge in the north and pseudofault in the south whose conjugate is located north of the kerguelen plateau independent microplate rotation is indicated by asymmetric pseudofaults and rotated abyssal hill fabric also seen in multibeam data magnetic anomaly picks and age estimates calculated from published spreading rates suggest formation during chron 21o 47 3 ma plate reorganizations can trigger ridge propagation and microplate development and we propose that mammerickx microplate formation is linked with the india eurasia collision initial soft collision the collision altered the stress regime at the indian antarctic ridge leading to a change in segmentation and ridge propagation from an establishing transform fast indian antarctic spreading that preceded microplate formation and kerguelen plume activity may have facilitated ridge propagation via the production of thin and weak lithosphere however both factors had been present for tens of millions of years and are therefore unlikely to have triggered the event prior to the collision the combination of fast spreading and plume activity was responsible for the production of a wide region of undulate seafloor to the north of the extinct ridge and w shaped lineations that record back and forth ridge propagation microplate formation provides a precise means of dating the onset of the india eurasia collision and is completely independent of and complementary to timing constraints derived from continental geology or convergence histories

structural pattern of the galapagos microplate and evolution of the galapagos triple junctions a 13 000 km2 microplate intervenes to prevent the junction of the nazca cocos and pacific plates in the eastern equatorial pacific ocean west of the galapagos islands the microplate is rotating clockwise at 6 m y about an absolute pole 400 km southeast of its center so that although its northern margin has a similar motion to adjacent parts of the cocos plate its southern margin is 50 slower the perimeter of the galapagos microplate is best defined by bathymetry and has been surveyed with sea beam and examined by deep tow two of the three triple junctions on its margins are ridge ridge ridge rrr junctions 160 km apart on the crest of the east pacific rise epr the most like a classic rrr junction is the pacific nazca galapagos intersection at 1 10 n where the epr axis is joined by a narrow volcanic ridge 70 km long and 1 km high the cocos nazca galapagos junction at the west end of the cocos nazca spreading center is inferred to be of the rff type but the transform fault branches are represented by large nontransform offsets lack of stable transform faults is related to rapid westward migration of the junction caused by lengthening of the cocos nazca axis at 60 70 mm yr this rate which was once required to maintain a stable nazca cocos pacific rrr junction has persisted even after birth of the microplate about 1 ma continuing propagation of the cocos nazca axis has opened up the hess deep rift basin in the eastern epr flank in advance of the eruptive propagator tip this rift valley has a relief of more than 3 km from its uplifted rims to the floor where severe attenuation exposes basal layers of the epr crust the similar dietz deep rift basin at the southeastern margin of the microplate 70 km from hess deep was formed when a lengthening galapagos nazca spreading axis rifted through another part of the epr flank

edge driven microplate kinematics it is known from plate tectonic reconstructions that oceanic microplates undergo rapid rotation about a vertical axis and that the instantaneous rotation axes describing the microplate s motion relative to the bounding major plates are frequently located close to its margins with those plates close to the tips of propagating rifts we propose a class of edge driven block models to illustrate how slip across the microplate margins block rotation and propagation of rifting may be related to the relative motion of the plates on either side an important feature of these edge driven models is that the instantaneous rotation axes are always located on the margins between block and two bounding plates according to those models the pseudofaults or traces of disrupted seafloor resulting from the propagation of rifting between microplate and major plates may be used independently to approximately trace the continuous kinematic evolution of the microplate back in time pseudofault geometries and matching rotations of the easter microplate show that for most of its 5 m y history block rotation could be driven by the drag of the nazca and pacific plates on the microplate s edges rather than by a shear flow of mantle underneath

subduction controls the distribution and fragmentation of earth s tectonic plates the theory of plate tectonics describes how the surface of the earth is split into an organized jigsaw of seven large plates 1 of similar sizes and a population of smaller plates whose areas follow a fractal distribution 2 3 the reconstruction of global tectonics during the past 200 my 4 suggests that this layout is probably a long term feature of our planet but the forces governing it are unknown previous studies 3 5 6 primarily based on statistical properties of plate distributions were unable to resolve how the size of plates is determined by lithosphere properties and or underlying mantle convection here we demonstrate that the plate layout of the earth is produced by a dynamic feedback between mantle convection and the strength of the lithosphere using 3d spherical models of mantle convection with plate like behaviour that match the plate size frequency distribution observed for earth we show that subduction geometry drives the tectonic fragmentation that generates plates the spacing between slabs controls the layout of large plates and the stresses caused by the bending of trenches break plates into smaller fragments our results explain why the fast evolution in small back arc plates 7 8 reflects the dramatic changes in plate motions during times of major reorganizations our study opens the way to use convection simulations with plate like behaviour to unravel how global tectonics and mantle convection are dynamically connected

geophysical characteristics of the southern mariana trough 11 50 n 13 40 n despite slow opening rates generally inferred for the mariana trough the southernmost part of the basin has fast spreading geophysical and morphologic characteristics that are unlike the features of the basin to the north a side scan sonar and geophysical survey maps the eastern part of the basin and the seafloor spreading center between 11 50 n and 13 40 n and identifies the following characteristics the ridge across axis profile forms a triangular to rounded high with relief of 100 to 500 m and cross sectional area variations of 1 to 7 km2 the along axis mantle bouguer gravity gradient is 0 2 mgal km axial segmentation occurs as overlapping axes and small deviation in trend no transform fault offsets exist despite significant changes in the trend of the spreading center characteristics of the surrounding basin include shallower overall depth than in the north no well developed frontal arc high in the southernmost trough the close proximity of submarine arc type volcanoes to the spreading center and tectonic fabric that is at a high angle to the trend of the spreading center on the eastern flank but is concordant on the western flank these characteristics imply different tectonic and magmatic conditions in the southern trough from the rest of the basin we propose that these effects are related to 1 the geometry of trench rollback in the southern trough leading to trench parallel extension generating inward radiating extensional faults 2 decoupling of the trench parallel extensional strain by the spreading center so that it primarily affects the eastern flank of the basin and 3 augmentation of the spreading center s magmatic budget by arc magmatic sources contributing to its fast spreading character although these effects may be accentuated in the southern mariana trough by the geometry of trench rollback and position of the slab which here underlies the spreading center they reflect distinct volcanic and tectonic processes which are varyingly expressed in back arc systems but are not normally found at mid ocean ridges

analogue modeling of arc and backarc deformation in the new hebrides arc and north fiji basin in most backarc basins extension is perpendicular to the arc thus individual spreading ridges extend approximately parallel to the arc in the north fiji basin however several ancient and active spreading ridges strike 70 90 to the new hebrides arc these high angle spreading ridges relocated southward during the asymmetric opening of the north fiji basin we have simulated the structural development of the north fiji basin and the new hebrides arc with scaled analogue models and the results have inspired us to come to several tentative conclusions we interpret the orientation of the high angle spreading ridges to be related to the asymmetric opening of the backarc basin around a hinge where they form close to the hinge relocation of these spreading ridges is most likely related to subduction of the west torres plateau along the new hebrides trench this resulted in localized collision retarded rollback of the subducting slab along the northwest corner of the trench and reduced extension and shearing in the northwest corner of the north fiji basin backarc extension continued in the rest of the north fiji basin owing to continued rollback of the southern part of the subducting slab here active extension was separated from the slightly or nonextending northwest corner by a zone striking at high angle to the new hebrides arc i e the hazel holme extensional zone moreover impingement of the d entrecasteaux ridge into the overriding plate led to local deformation and fragmentation of the arc

metamorphic amphiboles composition and coexistence amphiboles have long been key players in metamorphic petrology the concept of facies was developed for rocks of basaltic composition eskola 1920 1939 fyfe et al 1958 and within this concept amphiboles had a leading role the first appearance of amphiboles the presence of amphiboles of certain compositions or last appearance of amphiboles that are replaced by pyroxene bearing assemblages mark a number of major facies boundaries the transition from greenschist to amphibolite facies is marked in part by hornblende replacing actinolite and the granulite facies is defined by the breakdown of hornblende to orthopyroxene bearing assemblages the blueschist facies is defined by the appearance of glaucophane while the transition from blueschist facies to eclogite facies conditions is indicated by the breakdown of glaucophane to omphacitic pyroxene bearing assemblages references to amphibole names actinolite glaucophane or rocks characterized by certain amphiboles amphibolite hornblende figure prominently in six facies names turner 1981 liou et al 1985 oh et al 1988 evans 1990 peacock 1993 a large part of the literature that is devoted to metamorphic petrology of amphiboles concerns reactions which produce amphiboles from more hydrated mafic igneous rocks or lead to the destruction of amphiboles as they break down to form anhydrous or nearly anhydrous mineral assemblages consequently amphiboles can be thought of as occupying the wide intermediate portion of p t xfluid space in which metamorphic rocks are found the number of citations of the published amphibole nomenclature serves as a yardstick meterstick of the interest in amphiboles and a minimum estimate of the amount of literature devoted to this mineral two versions of the nomenclature of amphiboles have appeared 1978 and 1997 leake 1978 leake et al 1997 and they have been cited a over 2300 times in the past 28 years this works out to a minimum

reconstruction of the deformed collision zone between india and asia by backward motion of lithospheric blocks 1 on the basis of a synthesis of tectonic data available on the india asia collision we present a first attempt to reconstruct the evolution of the collision zone assuming that the deformation of the lithosphere is localized along narrow shear zones and that the interiors of mantle blocks in between remain relatively undeformed we define block contours from the fault pattern and move back the blocks along their boundary faults along convergent or extensional boundaries the crust is assumed to shorten or stretch coherently step by step we go backward in time to finally reach the collision onset for each time step we find a solution compatible with the data set available and the position of the adjacent blocks for each block the search for compatibility at the scale of the entire collision zone allows for solving the kinematics of regions with fewer data and suggests plausible scenarios for regions where data is lacking for each step we calculate large scale displacement maps and determine euler poles for each block for the most recent time step the map proposed is compared to gps motions the deformation budget implies that extrusion absorbed 30 of the convergence between india and siberia during the entire collision span but varied with time accounting for as little as 3 or as much as 60 of this convergence at different epochs

juvenile versus recycled crust in the central asian orogenic belt implications from ocean plate stratigraphy blueschist belts and intra oceanic arcs abstract new or juvenile crust forms and grows mainly through mafic to andesitic magmatism at pacific type or accretionary type convergent margins as well as via tectonic accretion of oceanic and island arc terranes and translation of continental terranes during the last decades the juvenile or recycled nature of crust has been commonly evaluated using whole rock isotope and hf in zircon isotope methods however evidence for the accretionary or pacific type nature of an orogenic belt comes from geological data for example from the presence of accretionary complexes ac intra oceanic arcs ioa oceanic plate stratigraphy units ops and morb oib derived blueschist belts bsb the central asian orogenic belt caob represents the world s largest province of phanerozoic juvenile crustal growth during ca 800 m y between the east european siberian north china and tarim cratons from geological point of view the caob is a typical pacific type belt as it hosts numerous occurrences of accretionary complexes intra oceanic arcs ops units and morb oib derived blueschist belts in spite of its accretionary nature supported by positive whole rock nd isotope characteristics in caob granitoids the hf in zircon isotope data reveal a big portion of recycled crust such a controversy can be explained by presence of accreted microcontinents isotopically mixed igneous reservoirs and by the tectonic erosion of juvenile crust the most probable localities of tectonic erosion in the caob are the middle and southern tienshan and southern transbaikalia because these regions comprise a predominantly recycled crust based on isotope data but the geological data show the presence of intra oceanic arcs blueschist belts and accreted ops with oceanic island basalts oib and tectonically juxtaposed coeval arc granitoids and accretionary units this warrants combination of detailed geological studies with isotopic results as on their own they may not reflect such processes as tectonic erosion of juvenile crust and or arc subduction

formation of continental fragments the tamayo bank gulf of california mexico we present a model for microcontinent formation that is based on the structure of the tamayo trough in the gulf of california mexico potential field modeling of a transect through the tamayo trough and tamayo bank suggests that the crust of the tamayo trough is oceanic and that the tamayo bank is a detached continental fragment the oceanic crust that separates the tamayo bank from the mainland of mexico is thin 5 km so oceanic spreading was probably magma starved before it ceased such a thin crust has also been described on the aegir ridge in the north atlantic which became extinct after the jan mayen microcontinent separated from greenland in our model for the origin of microcontinents the locus of plate spreading jumps to the weakened continental margin when the spreading ridge becomes amagmatic and the force required for continued extension at the ridge increases a microcontinent is formed when the ridge jumps into a continental margin and an asymmetric ocean basin or microplate is formed when the ridge jumps within oceanic crust

geochemistry and origin of cordierite orthoamphibole gneiss and associated rocks at an archaean volcanogenic massive sulphide camp manitouwadge ontario canada abstract cordierite orthoamphibole gneiss amphibolites quartz biotite schist and calc silicate rocks occur in spatial association with archaean metamorphosed volcanogenic massive sulphide vms deposits at manitouwadge ontario trace element characteristics particularly those of the high field strength elements hfse support field relationships that most of the cordierite orthoamphibole gneisses represent metamorphosed equivalents of chlorite rich rocks that were derived hydrothermally from tholeiitic basalts as an integral part of the syngenetic ore forming processes on the archaean seafloor however a garnet rich cordierite orthoamphibole dike that is characterized by low tio2 content and high concentrations of zr y nb and ree and has single zircon pb evaporation 207 pb 206 pb ages of 2715 2743 ma may have been derived from a synvolcanic felsic dike evidently the seafloor hydrothermal alteration at manitouwadge resulted in similar alteration products from contrasting protoliths and therefore was a fluid dominant system the amphibolite lenses within the cordierite orthoamphibole gneiss also have been affected by the seafloor hydrothermal alteration and some of them most likely represent metamorphosed equivalents of calcsilicate alteration products formed on the seafloor mass balance calculations indicate that hfse and ree were mobilized during the seafloor hydrothermal alteration the mobility of hfse and ree was controlled by both their host minerals and the hydrothermal fluids in particular a significant portion of igneous zircon in the garnet rich dike has survived the seafloor hydrothermal alteration and the subsequent high grade regional metamorphism the regional metamorphism did not change the bulk rock ree characteristics but was important in redefining the mineralogy of ree trace element geochemistry of mafic metavolcanic rocks at manitouwadge are similar to those of the adjacent schreiber hemlo greenstone belt and are consistent with a volcanic island arc setting however further work is required to determine whether the manitouwadge and adjacent greenstone belts of the wawa subprovince were originally a larger continuous terrane

the western central asian orogenic belt a window to accretionary orogenesis and continental growth abstract the architecture of accretionary orogens is a key to understand continental growth here we present an overview of the orogenic components and their amalgamation in the western central asian orogenic belt caob the caob records the convergence and interactions among various types of orogenic components including the japan type mariana type and alaska aleutian type arc systems as well as the active marginal sequences of the siberia craton which incorporated wide accretionary complexes and accreted arcs and terranes during construction of the caob the kazakhstan arc chain was characterized by multiple subduction whereas the northern fringe of the tarim craton remained mostly as a passive margin the multiple convergence and accretions among these various orogenic components generated huge orogenic collages in the late paleozoic and even in the early triassic involving parallel amalgamation circum microcontinent amalgamation and oroclinal bending the preservation of trapped basins played a significant role in orogenesis with some parts of the oceanic plate being subducted and others behaving as rigid units the orogenesis in the caob was long lived lasting for more than 800 m y involving multiple subduction and long continuous accretion and featuring the complexity of accretionary orogenesis and continent growth

area balancing as a test of models for the deep structure of mountain belts with specific reference to the alps abstract basic concepts of structural restoration are applied to crustal cross sections through mountain belts to explore large scale tectonic models and deep structure however restored sections should account for variations in pre orogenic crustal thicknesses crustal balancing approaches are reviewed and applied to two alpine sections coinciding with deep seismic experiments nrp 20 east central alps and ecors crop western alps existing studies assume large 300 km orogenic contraction and only moderately thinned pre orogenic crust the resulting restored sections contain more crust than is imaged beneath the present day alps the missing crust generally assumed to be subducted two kinematic modifications reduce the requirement for subduction thinning and buoyancy driven return flow of ultra high pressure metamorphic rocks during orogenesis and pre orogenic hyperextension using large stretching factors for the pre orogenic crust negates crustal subduction on both alpine transects if the lower crust was approximately rigid restorations of the central alps require strongly depth heterogeneous stretching of upper and lower crust during mesozoic rifting relaxing this requirement allows uniform lithospheric stretching a corollary consistent with published subsidence estimates restorations make implicit statements on the form of pre orogenic basins and the structure of continental margins incorporated into mountain belts that can in turn provide tests of tectonic models

mesozoic basins in eastern china and their bearing on the deconstruction of the north china craton abstract mesozoic basins occur widely in the eastern block and the neighboring area of the north china craton including the bohai bay the jiaolai the hefei and the north yellow sea in the north and the jianghan and the subei south yellow sea basins to the south their spatial temporal framework is the consequence of the indosinian and yanshanian tectonic regimes in eastern china and record the events related to mesozoic deconstruction of the north china craton our results demonstrate that the mesozoic tectonic evolution of the eastern north china craton was related to both sub crustal delamination and intra crustal extrusion or escape tectonics thus we propose that the mechanism of uplift of the yanshanian north china plateau and related lithosphere thinning in the eastern north china craton were related to sub crustal delamination at depth however the different distribution patterns of the basins on both sides of the tan lu fault system as well as the co existence of both compressional and extensional basins in the mesozoic indicate that these were controlled by escape tectonics in different tectonic parts of the crust

geochemistry and geochronology of the delinggou intrusion implications for the subduction of the paleo asian ocean beneath the north china craton abstract the formation and evolution of the central asian orogenic belt caob are intimately related to the opening and closure of the paleo asian ocean the final closure led to amalgamation or collision between the caob and north china craton however the issue about the subduction polarity and final closure of the paleo asian ocean has long been debated in order to resolve this issue we carried out a detailed geochemical and age study on the delinggou intrusion from the siziwangqi area of central inner mongolia at the northern margin of the north china craton the delinggou intrusion consists predominantly of diorite and quartz diorite with minor gabbroic diorite u pb zircon dating indicates that the intrusion was emplaced during early carboniferous 330 to 343 ma geochemically the rocks are metaluminous and belong to the calc alkaline series they are enriched in large ion lithophile elements lile e g rb sr and ba and depleted in high field strength elements hfse e g zr nb hf ta and ti the delinggou rocks were probably generated by partial melting of an enriched subcontinental lithospheric mantle further contaminated by crustal material during the magmatic differentiation in a continental arc setting we conclude that the paleo asian ocean underwent southward subduction beneath the north china craton and the subduction initiated in the carboniferous 343 330 ma but the final ocean closure time is not yet constrained

experimentally determined femg exchange between synthetic staurolite and garnet in the system mgofeoal2o3sio2h2o the fe mg exchange between synthetic staurolite and garnet was determined experimentally as a function of pressure temperature and bulk composition in the range of 5 25 kbar 600 750 c on the basis of the reaction12 pyrope 14 fe staurolite 13 almandine 14 mg staurolite eq 1 the results of the exchange runs clearly indicate that the fe mg fractionation between staurolite and garnet does not depend much on pressure but on temperature and bulk composition the compositional data on coexisting grtss and stst as determined by microprobe were processed by a nonlinear least squares procedure using different mixing models for staurolite solid solutions and the asymmetric solution model for garnet hackler and wood 1989 in all the models staurolite solid solutions show large negative deviation from ideality the experimental results are best described by a temperature dependent symmetric mixing model with wsst 79 j k one site model such a large negative ws value however is rather unrealistic especially considering that only a relatively small temperature interval has been investigated therefore a symmetric mixing model with wgst 11539 830 j one site model is recommended for use in thermodynamic calculations the expression for the derived geothermometer is based on the temperature dependent mixing parameters because these describe the fe mg fractionation between staurolite and garnet very well the geothermometer is valid for staurolites with xfe 0 6 and is t 18 02 0 044 p rt in fegrt rt in mggrt 87 78 0 057 p xmgst2 xmgst 2 12 55 xcagrt xmngrt r 10 3 in kd 0 021 0 079 xfest 2 xmgst 2 where pressure p is in kbar and temperature in c

conjugate volcanic rifted margins seafloor spreading and microcontinent insights from new high resolution aeromagnetic surveys in the norway basin we have acquired and processed new aeromagnetic data that cover the entire oceanic norway basin located between the more volcanic rifted margin and the jan mayen microcontinent jmmc the new compilation allows us to revisit the structure of the conjugate volcanic rifted margins and the spreading evolution of the norway basin from the early eocene breakup time to the late oligocene when the aegir ridge became extinct the volcanic margins in a strict sense that formed before the opening of the norway basin have been disconnected with the previous jurassic mid cretaceous episode of crustal thinning we also show evidence of relationships between the margin architecture the breakup magmatism distribution along the continent oceanic transition and the subsequent oceanic segmentation the norway basin shows a complex system of asymmetric oceanic segments locally affected by episodic ridge jumps the new aeromagnetic compilation also confirms that a fan shaped spreading evolution of the norway basin was clearly active before the cessation of seafloor spreading and extinction of the aegir ridge an important mid eocene kinematic event at around magnetic chron c21r can be recognized in the norway basin this event coincides with the onset of diking and increasing rifting activity and possible oceanic accretion between the proto jmmc and the east greenland margin it led to a second phase of breakup and microcontinent formation in the norwegian greenland sea 26 myrs later in the oligocene

fossil slabs attached to unsubducted fragments of the farallon plate as the pacific farallon spreading center approached north america the farallon plate fragmented into a number of small plates some of the microplate fragments ceased subducting before the spreading center reached the trench most tectonic models have assumed that the subducting oceanic slab detached from these microplates close to the trench but recent seismic tomography studies have revealed a high velocity anomaly beneath baja california that appears to be a fossil slab still attached to the guadalupe and magdalena microplates here using surface wave tomography we establish the lateral extent of this fossil slab and show that it is correlated with the distribution of high mg andesites thought to derive from partial melting of the subducted oceanic crust we also reinterpret the high seismic velocity anomaly beneath the southern central valley of california as another fossil slab extending to a depth of 200 km or more that is attached to the former monterey microplate the existence of these fossil slabs may force a reexamination of models of the tectonic evolution of western north america over the last 30 my

seismotectonics of the fiji plateau and lau basin abstract focal mechanism solutions of shallow earthquakes in the fiji plateau and lau basin provide significant contributions to the understanding of the recent tectonic movements beneath these marginal seas left lateral shear zones are identified on the linear shallow seismic zone extending from north of the fiji islands to the northern end of the lau basin in the central area of the fiji plateau and in the northern part of the lau basin nearly nw se trending opening tectonics is suggested nearly e w striking compressional tectonics is inferred near the western edge of the hazel holme fracture zone

cenozoic faulting of the bohai bay basin and its bearing on the destruction of the eastern north china craton abstract the cenozoic bohai bay basin is located at the center of the eastern block of the north china craton the structural architecture of this basin provides important clues on the deep seated lithosphere thinning of the north china craton the cenozoic regional stress field is characterized by nw oriented extension however the various cenozoic structural patterns of normal faulting and related transverse folding in the bohai bay basin are controlled not only by cenozoic stress field but also by strain field and mesozoic basement fault assemblages in this area regionally the cenozoic tectonic features and the dynamic evolution of the eastern north china craton are dominated by two lithosphere penetrating fault systems including the sinistral tan lu fault system and the dextral lan liao fault system to the west of the lan liao fault system cenozoic extensional tectonics includes nne trending listric normal faults that controlled half grabens however between these two fault systems are wnw trending half grabens which show basement involved faulting in the north and overlapping relations between sedimentary cover and basement in the south to the east of the tan lu fault the north yellow sea basin is a wnw trending fault depression with faulting in the south and overlapping relations in the north these structural features are inherited from the mesozoic tectonic framework of this area whose tectonic characteristics were completely controlled by two opposite strike slipping faults the trans extensional or oblique rifting in the paleogene followed by extensional faulting and subsequent subsidence furthermore the culmination of the decratonization of the north china craton was also related to an eastward jump of cenozoic subduction of the pacific plate and the far field effect of eastward extrusion of cenozoic subduction of the indian plate and was not essentially restricted to the early mesozoic processes therefore the cenozoic especially 25 ma marks the time of cessation of the processes that led to lithosphere thinning and destruction of the eastern block of the north china craton

the tectonic fabric of the ocean basins 1 we present a global community data set of fracture zones fzs discordant zones propagating ridges v shaped structures and extinct ridges digitized from vertical gravity gradient vgg maps we use a new semi automatic fz tracking program to test the precision of our hand digitized traces and find a mean absolute deviation of less than 3 4 km from the raw vgg minima that most clearly delineate each feature and less than 5 4 km from the fz location predicted by fitting model profiles to the vgg data that represent the morphology of the individual fzs these offsets are small considering gravity data only provide an approximation for the underlying basement morphology we further investigate the origin of non fz seafloor fabric by combining published abyssal hill heights computed from gravity anomalies with global half spreading rates a residual abyssal hill height grid with spreading rate effects removed combined with our interpreted tectonic fabric reveals several types of seafloor fabric distinct from typical abyssal hills where discordant zones do not overprint abyssal hill signals residual abyssal hill height anomalies correspond to seafloor that accreted near mantle thermal anomalies or zones of melt depletion our analysis reveals several areas where residual abyssal hill height anomalies reflect pseudo faults and extinct ridges associated with ridge propagation and or microplate formation in the southern pacific ocean

microplate boundaries as obstacles to pre earthquake strain transfer in western turkey inferences from continuous geochemical monitoring abstract warm and hot spring water chemistry changes as well as soil gas radon release patterns have been monitored in western turkey alongside regional seismicity providing a multi disciplinary approach from january 2009 to may 2011 33 earthquakes with ml between 4 0 and 6 0 occurred in this seismically very active region the ml 6 0 earthquake occurred on 19 may 2011 in simav town of kutahya province at a location midway between dense multidisciplinary monitoring networks of marmara region mr and the aegean extensional province aep we previously reported on noteworthy precursory anomalies prior to several earthquakes ml 4 in the mr and aep but no precursory anomaly was detected prior to the ml 6 0 event on 19 may 2011 in simav kutahya province midway between dense multidisciplinary monitoring mr and aep networks although these networks operate within the theoretical strain radii of this earthquake dobrovolsky et al 1979 no reliable anomaly were found geodetic studies based on gps data have identified crustal blocks in this region the epicentral area of the simav event is located within a block tectonically separated from aep and mr thus we speculate that pre earthquake strain accumulation within the simav block did not effectively transfer to adjacent blocks where the mr and aep networks are located thereby providing an explanation for the absence of detectable anomalies moreover prior to some earthquakes quadrant features of geochemical transients have been found suggesting that soil radon anomalies appear in compressional quadrant s of pre earthquake strain distribution

an experimental determination of the effect of bulk composition on phase relationships in metasediments at near solidus conditions sediments of terrigenous origin represent a conspicuouspart of the cover of the oceanic and continental crust taylor mclennan 1985 wedepohl 1991 pelitesand greywackes are involved in a variety of processes from deep mass transfer in subduction environments e g chopin 1984 smith 1984 sobolev shatsky 1990 reinecke 1991 responsible for dehydration and fluidrelease during dehydration schmidt etal 2004 to partialmelting in continental collision zones which is at theorigin of intracrustal differentiation clemens vielzeuf 1987 the evolution of metapelites and metagreywackescan be reconstructed if the dependence of phase relation ships on pressure temperature and bulk composition iswell known and under h

intratransform origins of mid ocean ridge microplates we present evidence that the easter and juan fernandez microplates originated by rift propagation from within transform fault zones at locations far from ridge transform intersections an intratransform spreading center may have served as the focal point for each new propagation both events appear coincident with a circum pacific plate reorganization dated at 5 6 ma which may have been related to a change in pacific absolute plate motion and a counterclockwise rotation of pacific nazca relative plate motion this intratransform model may be applicable to the generation of other microplates and may thus serve as a general model for microplate origins

tectonic evolution of the northern pacific plate and pacific farallon izanagi triple junction in the late jurassic and early cretaceous m21 m10 abstract magnetic lineations in the northwestern pacific near the shatsky plateau were interpreted to trace the tectonic history of the northern pacific plate from the time of anomalies m21 to m10 at m22 time the pacific farallon izanagi p f i triple junction was located southwest of the plateau and migrating north northwest with respect to the pacific plate shortly thereafter the stress field acting on the three plates changed significantly and the pacific lzanagi ridge reacted by pivoting 24 clockwise from m21 to m19 time several ridge propagation events occurred while the ridge adjusted to the new spreading regime to the east of the m22 location of the junction it appears that this reorganization triggered a series of at least three ridge jumps along the pacific farallon ridge the geometry of magnetic isochron m21 suggests that several triple junctions existed simultaneously during the reorganization bordering one or more microplates differences in the spreading rate of the japanese magnetic lineations north of the plateau imply that one microplate which we call the shatsky microplate existed until about m15 m13 time it was probably captured by either the farallon or the izanagi plate and subsequently destroyed by subduction after m15 the p f i triple junction was reformed on seafloor that is now beneath the shatsky plateau the junction drifted generally northeast either in a ridge ridge ridge or in a ridge fault fault configuration perhaps alternating between the two from m11 m10 time the stress field on the three plates changed again and the pacific farallon ridge southeast of the plateau reoriented 15 clockwise at this time the p f i junction assumed a ridge ridge ridge geometry at the northeast end of the plateau

an unrecognized major collision of the okhotomorsk block with east asia during the late cretaceous constraints on the plate reorganization of the northwest pacific abstract interactions at plate boundaries induce stresses that constitute critical controls on the structural evolution of intraplate regions however the traditional tectonic model for the east asian margin during the mesozoic invoking successive episodes of paleo pacific oceanic subduction does not provide an adequate context for important late cretaceous dynamics across east asia including continental scale orogenic processes significant sinistral strike slip faulting and several others the integration of numerous documented field relations requires a new tectonic model as proposed here the okhotomorsk continental block currently residing below the okhotsk sea in northeast asia was located in the interior of the izanagi plate before the late cretaceous it moved northwestward with the izanagi plate and collided with the south china block at about 100 ma the indentation of the okhotomorsk block within east asia resulted in the formation of a sinistral strike slip fault system in south china formation of a dextral strike slip fault system in north china and regional northwest southeast shortening and orogenic uplift in east asia northeast striking mountain belts over 500 km wide extended from southeast china to southwest japan and south korea the peak metamorphism at about 89 ma of the sanbagawa high pressure metamorphic belt in southwest japan was probably related to the continental subduction of the okhotomorsk block beneath the east asian margin subsequently the north northwestward change of motion direction of the izanagi plate led to the northward movement of the okhotomorsk block along the east asian margin forming a significant sinistral continental transform boundary similar to the san andreas fault system in california sanbagawa metamorphic rocks in southwest japan were rapidly exhumed through the several kilometer wide ductile shear zone at the lower crust and upper mantle level accretionary complexes successively accumulated along the east asian margin during the jurassic early cretaceous were subdivided into narrow and subparallel belts by the upper crustal strike slip fault system the departure of the okhotomorsk block from the northeast striking asian margin resulted in the occurrence of an extensional setting and formation of a wide magmatic belt to the west of the margin in the campanian the block collided with the siberian margin in northeast asia at about 77 ma a new oceanic subduction occurred to the south of the okhotomorsk block ending its long distance northward motion based on the new tectonic model the abundant late archean to early proterozoic detrital zircons in the cretaceous sandstones in kamchatka southwest japan and taiwan are interpreted to have been sourced from the okhotomorsk block basement which possibly formed during the late archean and early proterozoic the new model suggests a rapidly northward moving okhotomorsk block at an average speed of 22 5 cm yr during 89 77 ma it is hypothesized that the okhotomorsk east asia collision during 100 89 ma slowed down the northwestward motion of the izanagi plate while slab pull forces produced from the subducting izanagi plate beneath the siberian margin redirected the plate from northwestward to north northwestward motion at about 90 89 ma

amphibolites with staurolite and other aluminous minerals calculated mineral equilibria in ncfmash amphibolite facies mafic rocks that consist mainly of hornblende plagioclase and quartz may also contain combinations of chlorite garnet epidote and more unusually staurolite kyanite sillimanite cordierite and orthoamphiboles such assemblages can provide tighter constraints on the pressure and temperature evolution of metamorphic terranes than is usually possible from metabasites because of the high variance of most of the assemblages the phase relationships in amphibolites depend on rock composition in addition to pressure temperature and fluid composition the mineral equilibria in the na2o cao feo mgo al2o3 sio2 h2o ncfmash model system demonstrate that aluminium content is critical in controlling the occurrence of assemblages involving hornblende with aluminous minerals such as sillimanite kyanite staurolite and cordierite except in aluminous compositions these assemblages are restricted to higher pressures the iron to magnesium ratio x fe and to a lesser extent sodium to calcium ratio have important roles in determining which if any of the aluminous minerals occur under particular pressure temperature conditions where aluminous minerals occur in amphibolites the p t x dependence of their phase relationships is remarkably similar to that in metapelitic rocks the mineral assemblages of fe rich amphibolites are typically dominated by garnet and staurolite bearing assemblages whereas their more mg rich counterparts contain chlorite and cordierite assemblages involving staurolite hornblende can occur over a wide range of pressures 4 10 kbar at temperatures of 560 650 degrees c however except in the more aluminous iron rich compositions they occupy a narrow pressure temperature window thus although their occurrence in typical amphibolites may be indicative of relatively high pressure metamorphism in more aluminous compositions their interpretation is less straightforward

micro plate tectonics and kinematics in northeast asia inferred from a dense set of gps observations abstract the plate tectonics of northeast asia are very complex with diffuse and sparse seismicity in the broad plate deformation zones embedded by a number of micro plates particularly the controversial amurian plate now the increasingly dense gps networks in this area provide an important tool to investigate plate tectonic kinematics and to identify the approximate plate tectonic geometries in this paper we have processed gps data 1998 2005 collected by an extensive gps network china and south korea with more than 85 continuous sites and about 1000 campaign gps stations the kinematics of northeast asia is studied by modeling gps derived velocities with rigid block rotations and elastic deformation we find that the deformation in northeast asia can be well described by a number of rotating blocks which are independent of the eurasian plate motion with statistical significance above the 99 confidence level the tectonic boundary between the north china and amuria plates is the yin shan yan shan mountain belts with about 2 4 mm yr extension along the boundary between north china and south china the qinling dabie fault is moving left laterally at about 3 1 mm yr the amuria and south korea blocks are extending at about 1 8 mm yr the baikal rift between the amurian and eurasian plates is spreading at about 3 0 mm yr the 9 17 mm yr relative motion between the amuria and okhotsk blocks is accommodated at the east sea japan trench zone localized deformation near the qinling dabie fault and yin shan yan shan mountain belts may be elastic strain accumulation due to interseismic locking of faults

the rrr triple junction at the southern end of the pacific cocos east pacific rise abstract a sea beam survey of the pacific cocos galapagos triple junction at 2 40 n on the east pacific rise epr establishes that it is a ridge ridge ridge junction with the epr axis making a small but sharp bend where it is intersected by a slow spreading 15 mm yr fast propagating rift at the tip of the narrow triangle of crust accreted at this slow spreading axis is volcanic ridge built on epr crust tholeiitic lavas erupted along the slow spreading axis here and at the rather similar pacific nazca galapagos junction differ in minor element and rare earth element geochemistry from lavas on adjacent segments of the epr axis being slightly enriched in incompatible elements

the formation of pangea abstract the making of pangea is the result of large scale amalgamation of continents and micro continents which started at the end of the neoproterozoic with the formation of gondwana as pieces were added to gondwana on its south american antarctica and australia side ribbon like micro continents were detached from its african and south chinese side cadomia in the late neoproterozoic avalonia and hunia in the ordovician galatia in the devonian and cimmeria in the permian cadomia was re accreted to gondwana but the other ribbon continents were accreted to baltica north china laurussia or laurasia finding the origin of these numerous terranes is a major geological challenge recently a global plate tectonic model was developed together with a large geological geodynamic database at the lausanne university covering the last 600 ma of the earth s history special attention was given to the placing of gondwana derived terranes in their original position using all possible constraints we propose here a solution for the variscan terranes another paper deals with the altaids the galatian super terrane was detached from gondwana in the devonian during the opening of paleotethys and was quickly separated into four sub terranes that started to by pass each other the leading terranes collided at the end of the devonian with the hanseatic terrane detached from laurussia in the carboniferous gondwana started to impinge onto the amalgamated terranes creating the variscan chain and the pangean super continent east of spain paleotethys remained opened until the triassic subducting northward under laurasia roll back of the paleotethyan slab triggered the collapse of most of the european variscan orogen which was replaced by series of permian rifts some of them becoming oceanized back arc basins during the triassic major force changes at the pangean plate limits at the end of the triassic provoked its break up through the opening of the proto caribbean central atlantic alpine tethys oceanic seaways

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

the dynamics of big mantle wedge magma factory and metamorphic metasomatic factory in subduction zones abstract the east asian continental margin is underlain by stagnant slabs resulting from subduction of the pacific plate from the east and the philippine sea plate from the south we classify the upper mantle in this region into three major domains a metasomatic metamorphic factory mmf subduction zone magma factory szmf and the big mantle wedge bmw whereas the convection pattern is anticlockwise in the mmf domain it is predominantly clockwise in the szmf and bmw along a cross section from the south here we define the mmf as a small wedge corner which is driven by the subducting pacific plate and dominated by h 2 o rich fluids derived by dehydration reactions and enriched in large ion lithophile elements lile which cause the metasomatism the szmf is a zone intermediate between mmf and bmw domains and constitutes the main region of continental crust production by partial melting through wedge counter corner flow large hydrous plume generated at about 200 km depth causes extensive reduction in viscosity and the smaller scale hydrous plumes between 60 km and 200 km also bring about an overall reduction in the viscosity of szmf more fertile and high temperature peridotites are supplied from the entrance to this domain the domain extends obliquely to the volcanic front and then swings back to the deep mantle together with the subducting slab the bmw occupies the major portion of upper mantle in the western pacific and convects largely with a clockwise sense removing the eastern trench oceanward sporadic formation of hydrous plume at the depth of around 410 km and the curtain flow adjacent to the trench cause back arc spreading we envisage that the heat source in bmw could be the accumulated ttg tonalite trondhjemite granodiorite crust on the bottom of the mantle transition zone the ongoing process of transportation of granitic crust into the mantle transition zone is evident from the deep subduction of five intra oceanic arcs on the subducting philippine sea plate from the south in addition to the sediment trapped subduction by the pacific plate and philippine sea plate the dynamics of mmf szmf and bmw domains are controlled by the angle of subduction a wide zone of mmf in sw japan is caused by shallow angle subduction of the philippine sea plate and the markedly small mmf domain in the mariana trench is due to the high angle subduction of pacific plate the domains in ne japan and kyushu region are intermediate between these two during the tertiary a series of marginal basins were formed because of the nearly 2000 km northward shift of the subduction zone along the southern margin of tethyan asia which may be related to the collision of india with asia and the indentation the volume of upper mantle under asia was reduced extensively on the southern margin with a resultant oceanward trench retreat along the eastern margin of asia leading to the formation of a series of marginal basins the western pacific domain in general is characterized by double sided subduction from the east by the oldest pacific plate and from the south by the oldest indo australian plate the old plates are hence hydrated extensively even in their central domains and therefore of low temperature the cracks have allowed the transport of water into the deeper portions of the slab and these domains supply hydrous fluids even to the bottom of the upper mantle thus a fluid dominated upper mantle in the western pacific drives a number of microplates and promote the plate boundary processes

transtensional rifting in the proto gulf of california near bahia kino sonora mexico continental rifts require focused strain to rupture and form an ocean basin in oblique rifts such as the gulf of california focused transtensional strain associated with strike slip faulting may serve as a catalyst for rupture to test this hypothesis we analyzed structural mapping geochronology paleomagnetism and fault kinematics of pre and synrift rocks exposed in an 200 km 2 coastal mountain belt flanking the eastern rift margin of the northern gulf of california this coastal sonora region hosts the onshore portion of the transform boundary between the upper tiburon and adair tepoca marine basins two early formed oblique rift segments extension commenced here between 11 5 ma and 7 ma resulting in 25 40 of e ne tilting initiation of clockwise vertical axis rotation of fault bounded blocks and minor basin sedimentation rates of deformation prior to 7 ma are unconstrained due to a lack of exposed syntectonic deposits deformation after 7 ma was associated with rapid tilting and the majority of observed clockwise vertical axis rotation and strike slip faulting nonmarine sedimentary basins accumulated coarse sediments above an unconformity eroded across older tilted strata by 5 6 ma deformation in coastal sonora must have largely ceased and migrated westward into the upper tiburon marine basin we document up to 120 total extension and total clockwise block rotations up to 53 in portions of the study area extension and rotation were supplanted by strike slip faulting as deformation proceeded we develop a tectonic model for this coastal sonora fault zone which is bounded by major nw striking transform faults with 10 km of displacement internal to the coastal sonora fault zone the majority of an estimated 6 2 1 1 km of dextral deformation associated with up to 5 7 km of wnw directed extension occurred over the final 1 2 m y of its life span at a strain rate approaching 10 14 s 1 this activity occurred as the plate boundary localized along nascent pull apart basins in the northern gulf of california consistent with the hypothesis that late proto gulf of california dextral shear zones such as the coastal sonora fault zone acted to focus lithospheric scale strain and promoted continental rupture in the wide rift setting of the mexican basin and range

a change in the geodynamics of continental growth 3 billion years ago models for the growth of continental crust rely on knowing the balance between the generation of new crust and the reworking of old crust throughout earth s history the oxygen isotopic composition of zircons for which uranium lead and hafnium isotopic data provide age constraints is a key archive of crustal reworking we identified systematic variations in hafnium and oxygen isotopes in zircons of different ages that reveal the relative proportions of reworked crust and of new crust through time growth of continental crust appears to have been a continuous process albeit at variable rates a marked decrease in the rate of crustal growth at 3 billion years ago may be linked to the onset of subduction driven plate tectonics

permian geodynamic setting of northeast china and adjacent regions closure of the paleo asian ocean and subduction of the paleo pacific plate abstract northeast china and adjacent regions are located in the central east asian continent and consist tectonically of both the paleo asian and paleo pacific orogens between the siberian platform and sino korean north china block this paper discusses some hotly debated issues concerning the permian geodynamic setting of these regions based on a comprehensive analysis of available geological geochemical paleobiogeographical and paleomagnetic data spatial and temporal distribution of ophiolites and associated continental marginal sequences permian sedimentary sequences spatial distribution and geochemistry of permian magmatic rocks and the evolution of paleobiogeographical realms imply 1 that the permian marine basins in northeast china and adjacent regions include remnants of the paleo asian ocean in southeastern inner mongolia and central jilin province and active continental margins of the paleo pacific ocean 2 that the suture between the siberian and sino korean paleoplates was finally emplaced in the permian and is located in areas from suolunshan solonker eastwards through regions north to the xar moron river in southeastern inner mongolia and then central jilin province to the yanji area and 3 that the permian crustal evolution of northeast china and adjacent regions as well as parts of the siberian paleoplate was influenced by subduction of the paleo pacific oceanic plate finally the permian tectonic framework and paleogeography of northeast china and adjacent regions in central east asia are discussed briefly and early and late permian palinspastic reconstruction maps are provided

amount of asian lithospheric mantle subducted during the india asia collision abstract body wave seismic tomography is a successful technique for mapping lithospheric material sinking into the mantle focusing on the india asia collision zone we postulate the existence of several asian continental slabs based on seismic global tomography we observe a lower mantle positive anomaly between 1100 and 900 km depths that we interpret as the signature of a past subduction process of asian lithosphere based on the anomaly position relative to positive anomalies related to indian continental slab we propose that this anomaly provides evidence for south dipping subduction of north tibet lithospheric mantle occurring along 3000 km parallel to the southern asian margin and beginning soon after the 45 ma break off that detached the tethys oceanic slab from the indian continent we estimate the maximum length of the slab related to the anomaly to be 400 km adding 200 km of presently asian subducting slab beneath central tibet the amount of asian lithospheric mantle absorbed by continental subduction during the collision is at most 600 km using global seismic tomography to resolve the geometry of asian continent at the onset of collision we estimate that the convergence absorbed by asia during the indentation process is 1300 km we conclude that asian continental subduction could accommodate at most 45 of the asian convergence the rest of the convergence could have been accommodated by a combination of extrusion and shallow subduction underthrusting processes continental subduction is therefore a major lithospheric process involved in intraplate tectonics of a supercontinent like eurasia

a new index of glacier area change a tool for glacier monitoring since the mid 19th century most glaciers have been losing area and volume this loss of area has not been homogeneous in time and space and direct observations are sparse making regional and global estimates of glacier change difficult this paper focuses on developing a single index for monitoring glacier change one that would be particularly useful for remote sensing applications we combine the results of direct glacier mass balance observations b total glacier area s and accumulation area sc derived from maps or remotely sensed images using the accumulation area ratio aar sc s we note the differences between observed aar time averaged aar hi and the equilibrium state aar0 as determined by its value at b 0 from a regression of b aar we suggest that d aar hi aar0 aar0 quantifies the difference between the currently observed state of glaciers and their equilibrium state and measures the delay in the dynamic response of s relative to the climatic response of sc using all available observations for the period 1961 2004 d 65 for tropical glaciers which implies their rapid shrinkage as s continues to decrease and catch up with sc during the same period mid latitude and polar glaciers show less negative values of d of 86 glaciers from all latitudes and regions only 11 show positive d at any time between 1961 and 2004 averaged over 1961 2004 d is 15 1 2 2 and b hi is 360 42 mm a 1 w e values for aar0 range between about 40 and 80 but the bulk of the equilibrium values are between 50 and 60 the average aar0 is 57 9 0 9 and has remained stable over time the equilibrium aar has not changed with climate overall the observed negative d suggests a committed retreat of glaciers and their continuing contribution to sea level even if global temperature is held constant

tectonics and evolution of the juan fernandez microplate at the pacific nazca antarctic triple junction in this study magnetic and bathymetric data from an r v endeavor survey are combined with earthquake mechanism studies to produce a tectonic model for the juan fernandez microplate at the pacific nazca antarctic triple junction magnetic lineations along the east ridge of the microplate fan to the south indicating that the nazca juan fernandez pole is near the northern end of this feature relative motions of the juan fernandez pacific nazca antarctic four plate system are determined using relative plate motion inversions the resulting tectonic model predicts that the northern boundary of the juan fernandez microplate is a zone of compression and that the west ridge and southwestern boundary are spreading obliquely the southeastern boundary is predicted to be obliquely divergent but present data fail to constrain its geometry we also present a schematic evolutionary model which relates the formation of the juan fernandez microplate to differential spreading rates at the triple junction and discuss related complexities the future evolution of the fourplate system depends on whether one or both ridges remain active

the chile ridge a tectonic framework a new chile ridge tectonic framework is developed based on satellite altimetry data shipboard geophysical data and primarily 38 500 km of magnetic data gathered on a joint u s chilean aeromagnetic survey eighteen active transforms with fossil fracture zones fzs including two complex systems the chile fz and valdivia fz systems have been mapped between the northern end of the antarctic nazca plate boundary chile ridge at 35 s and the chile margin triple junction at 47 s chile ridge spreading rates from 23 ma to present have been determined and show slowdowns in spreading rates that correspond to times of ridge trench collisions the valdivia fz system previously mapped as two fzs with an uncharted seismically active region between them is now recognized to be a multiple offset fz system composed of six fzs separated by short ridge segments 22 to 27 km long at chron 5a 12 ma the chile ridge propagated from the valdivia fz system northward into the nazca plate through crust formed 5 myr earlier at the pacific nazca ridge evidence for this propagation event includes the friday and crusoe troughs located at discontinuities in the magnetic anomaly sequence and interpreted as pseudofaults this propagation event led to the formation of the friday microplate which resulted in the transferal of crust from the nazca plate to the antarctic plate and in a 500 km northward stepwise migration of the pacific antarctic nazca triple junction rift propagation microplate formation microplate extinction and stepwise triple junction migration are found to occur during large scale plate motion changes and plate boundary changes in the southeast pacific

reaction textures in al rich metabasite implication for metamorphic evolution of the eastern border of the middle austroalpine basement units abstract al rich metabasite from the speik complex in the eastern alps contains coexisting kyanite staurolite garnet and calcic amphibole compositional zoning and inclusions trails in minerals suggest a prograde metamorphic history to high pressure amphibolite facies conditions the formation of kyanite and staurolite was controlled by bulk rock composition mainly by high al content in some metabasites textural relations and thermodynamic calculations indicate isothermal decompression from 1 1 1 2 gpa 580 c to 0 5 gpa 580 c that resulted in formation of ca rich plagioclase and new amphibole replacing garnet and locally corundum substituting for clinozoisite and kyanite the replacement of amphibole by biotite and chlorite and plagioclase by margarite and the formation of prehnite and pumpellyite were due to isobaric cooling to very low grade conditions