marine oxygenation lithistid sponges and the early history of paleozoic skeletal reefs abstract microbial carbonates were major components of early paleozoic reefs until coral stromatoporoid bryozoan reefs appeared in the mid ordovician microbial reefs were augmented by archaeocyath sponges for 15 myr in the early cambrian by lithistid sponges for the remaining 25 myr of the cambrian and then by lithistid calathiid and pulchrilaminid sponges for the first 25 myr of the ordovician the factors responsible for mid late cambrian microbial lithistid sponge reef dominance remain unclear although oxygen increase appears to have significantly contributed to the early cambrian explosion of marine animal life it was followed by a prolonged period dominated by greenhouse conditions as sea level rose and co2 increased the mid late cambrian was unusually warm and these elevated temperatures can be expected to have lowered oxygen solubility and to have promoted widespread thermal stratification resulting in marine dysoxia and hypoxia greenhouse conditions would also have stimulated carbonate platform development locally further limiting shallow water circulation low marine oxygenation has been linked to episodic extinctions of phytoplankton trilobites and other metazoans during the mid late cambrian we propose that this tendency to dysoxia hypoxia in shallow marine environments also limited many metazoan reef builders in contrast during the mid late cambrian the ability of lithistid sponges to withstand low oxygen levels allowed them to create a benthic association with microbial carbonates that dominated global reefs these conditions ameliorated during the ordovician as temperature decline promoted ocean ventilation the prolonged time gap occupied by low diversity reefs between the cambrian explosion and the great ordovician biodiversification event reflects elevated temperatures and reduced marine oxygenation that limited metazoan diversification in shallow marine environments

ophiolite melange zone records exhumation in a fossil subduction channel recent models propose that the exhumation of high pressure rocks occurs by means of return flow inside a low viscosity channel of serpentinite situated between the plates to test this hypothesis we investigated a serpentinite melange in the western alps which contains exotic mafic and metasedimentary tectonic blocks recording heterogeneous metamorphic evolutions and variable high pressure ages the peak metamorphic conditions range from eclogite to garnet blueschist facies the structural evidence and the pressure temperature paths of the different blocks suggest coupling between blocks and matrix at least in the blue schist facies 39 ar 40 ar dating indicates eclogite facies peak at ca 43 ma and blueschist facies peak at ca 43 and 40 ma in different blocks respectively these data point to diachronous metamorphic paths resulting from independent tectonic evolutions of the different slices we therefore propose that this melange formed during exhumation of subducted rocks equilibrated at different depths inside a subduction channel this mechanism can be extended to other serpentinite melanges in the alps and other orogens e g the cyclades the coast ranges of california for which a growing heterogeneity in the timing of metamorphic equilibration and of pressure temperature paths can be expected with further investigations

hotter mantle but colder subduction in the precambrian what are the implications abstract on contemporary earth subduction recycles mafic oceanic crust and associated volatile elements creating new silicic continental crust in volcanic arcs however if the mantle was hotter in the precambrian the style of subduction the depth of devolatilization and the formation of silicic continental crust may have been different consequently the generation of the tonalite trondhjemite granodiorite ttg suite which is characteristic of archean crust may not have been related to subduction here we use a two dimensional numerical magmatic thermomechanical model to investigate intraoceanic subduction for contemporary mantle conditions and at higher mantle temperatures as appropriate to the precambrian in each case we characterize the thermal structure of the subducting plate and investigate magma compositions and production rates we use these results to assess the potential growth of silicic continental crust associated with intraoceanic subduction at different mantle temperatures for the precambrian in a set of experiments with t 150 k and decreasing subducting plate velocity we find that the contemporary style of subduction was preceded by an arc free regime dominated by rapid trench rollback and vigorous upwelling of asthenospheric mantle into the space created above the retreating slab in this regime formation of magmas by fluid fluxed melting of the mantle is suppressed instead decompression melting of upwelling asthenospheric mantle results in the widespread development of voluminous plateau like basaltic magmas in addition retreating subduction at higher mantle temperature causes faster descent of the downgoing slab leads to colder thermal gradients similar to those associated with active subduction in the western pacific today and suppresses melting of the basaltic crust limiting production of silicic adakite like magmas with increasing maturity of the subduction system retreat of the subducting plate ceases the role of decompression melting strongly decreases and fluid fluxed melting of the mantle coupled with melting of the hydrated slab begins to produce basaltic and felsic arc volcanic rocks similar to those formed during contemporary subduction in an additional series of individual experiments at various t an increase of the mantle temperature above t 150 k leads to episodic and short lived subduction accompanied by limited production of silicic continental crust the results of our experimental study demonstrate that a hotter mantle in the precambrian changes dramatically both the slab dynamics and the processes of magma generation and crustal growth associated with intraoceanic subduction zones these changes may preclude growth of the early precambrian silicic continental crust by processes that were dominantly similar to those associated with contemporary subduction

controls on back arc basin formation the relationship between subduction and back arc spreading has been well known since the early days of plate tectonics however the reasons why back arc basins are associated with some subduction systems but not all has remained elusive we examine the kinematic controls on subduction and back arc basins for both the present day and cenozoic to differentiate between the major competing hypotheses for back arc basin formation and to explain their temporal and spatial distribution our new data set of subduction and back arc basin parameters uses a new set of paleo oceanic age grids muller et al 2005 associated with a moving atlantic indian ocean hot spot reference frame o neill et al 2005 the plate model includes detailed reconstructed spreading histories of back arc basins based on marine geophysical and satellite gravity data our combined rotation and oceanic paleo age model provides the age distribution of subducting lithosphere through space and time convergence rates and the absolute motion of the downgoing and overriding plates we find that back arc basins develop when the age of subducting normal oceanic lithosphere is greater than 55 million years additionally we establish an age dip relationship showing that the intermediate dip angle of the subducting slab is always greater than 30 with back arc spreading our results suggest that back arc basin formation is always preceded by an absolute motion of the overriding plate away from the subduction hinge thereby creating accommodation space between the overriding and subducting plates once back arc extension is established it continues regardless of overriding plate motion indicating back arc spreading is not a simple consequence of overriding plate behaviour the landward migration of the overriding plate as a precursor to back arc extension may indicate that extension on the overriding plate is influenced by the oceanward lateral flow of the mantle however once back arc extension is established rollback of the subduction hinge appears to be the primary force responsible for the continued creation of accommodation space our analysis indicates the driving mechanism for back arc extension is a combination of surface kinematics properties of the downgoing slab the effect of lateral mantle flow on the slab and mantle wedge dynamics

back arc strain in subduction zones statistical observations versus numerical modeling 1 recent statistical analysis by lallemand et al 2008 of subduction zone parameters revealed that the back arc deformation mode depends on the combination between the subducting vsub and upper vup plate velocities no significant strain is recorded in the arc area if plate kinematics verifies vup 0 5 vsub 2 3 cm a in the hs3 reference frame arc spreading shortening occurs if vup is greater lower than the preceding relationship we test this statistical law with numerical models of subduction by applying constant plate velocities far away from the subduction zone the subducting lithosphere is free to deform at all depths we quantify the force applied on the two converging plates to sustain constant surface velocities the simulated rheology combined viscous non newtonian and brittle behaviors and depends on water content the influence of subduction rate vs is first studied for a fixed upper plate after 950 km of convergence steady state slab pull the transition from extensional to compressive stresses in the upper plate occurs for vs 1 4 cm a the effect of upper plate velocity is then tested at constant subduction rate upper plate retreat advance with respect to the trench increases extension compression in the arc lithosphere and increases decreases the subducting plate dip our modeling confirms the statistical kinematic relationship between vsub and vup that describes the transition from extensional to compressive stresses in the arc lithosphere even if the modeled law is shifted toward higher rates of upper plate retreat using our set of physical parameters e g 100 km thick subducting oceanic plate and short term simulations our results make valid the choice of the hs3 reference frame for assessing plate velocity influence on arc tectonic regime the subduction model suggests that friction along the interplate contact and the mantle stokes reaction could be the two main forces competing against slab pull for upper mantle subductions besides our simulations show that the arc deformation mode is strongly time dependent

numerical simulation of metamorphic pressure temperature time paths and fluid production in subducting slabs in order to investigate metamorphic reactions and fluid production in subduction zones a 2 dimensional finite difference model has been constructed the model simulates heat transfer in a subduction zone with a 27 dip a spatial resolution of 1 km is used in the region of the subduction thrust in order to carefully monitor metamorphic reactions elsewhere the spatial resolution is 5 km in these calculations the oceanic crust is assumed to contain 2 wt bound volatiles distributed homogeneously within the 7 5 km thick crust fluids are released by continuous and discontinuous end members of three different dehydration models pressure sensitive temperature sensitive and amphibole negative dp dt dehydration subduction zone pressure temperature time p t t paths predicted by the model intersect the wet basaltic solidus only at the initiation of subduction in mature subduction zones and in the absence of significant frictional heating and induced mantle convection p t t paths encounter subsolidus conditions at depths of 100 150 km beneath magmatic arcs suggesting that slab dehydration reactions play an important role in arc magma genesis metamorphic dehydration reactions that consume 50 kj kg result in p t t paths that are only 5 k cooler than models that neglect such reactions in both the temperature and amphibole dehydration models the fluid production region moves to greater depths over time as the subduction zone cools within the oceanic crust fluid production starts at the top of the crust and migrates downward in the temperature and amphibole dehydration models this phenomenon coupled with the weakening effect of devolatilization reactions may result in the accretion of oceanic crust to the hanging wall of the subduction zone for a convergence rate of 3 cm yr average fluid fluxes out of the subducting slab range from 0 1 kg fluid m yr for the continuous reaction models to 1 kg fluid m yr for the discontinuous pressure and amphibole dehydration models vertical fluid fluxes on the order of 1 kg fluid m yr can substantially perturb the thermal structure of overlying rocks and can cause large scale hydration and metasomatism of the overlying mantle wedge in contrast the thermal effect of fluid flow parallel to the subduction thrust zone is insignificant compared to the thermal effect of subducting oceanic lithosphere

subduction dynamics as revealed by trench migration 1 new estimates of trench migration rates allow us to address the dynamics of trench migration and back arc strain we show that trench migration is primarily controlled by the subducting plate velocity vsub which largely depends on its age at the trench using the hot and weak arc to back arc region as a strain sensor we define neutral arcs characterized by the absence of significant strain meaning places where the forces slab pull bending and anchoring almost balance along the interface between the plates we show that neutral subduction zones satisfy the kinematic relation between trench and subducting plate absolute motions vt 0 5vsub 2 3 in cm a 1 in the hs3 reference frame deformation occurs when the velocity combination deviates from kinematic equilibrium balancing the torque components of the forces acting at the trench indicates that stiff old subducting plates facilitate trench advance by resisting bending

redox fluctuations in the early ordovician oceans an insight from chromium stable isotopes abstract chromium cr stable isotopes are a useful tracer of changes in redox conditions because changes in its oxidation state are accompanied by an isotopic fractionation recent co precipitation experiments have shown that cr vi is incorporated into the calcite lattice suggesting that carbonates are a potential sink for dissolved cr in the ocean and could effectively capture the cr isotope composition of ambient seawater thus the cr isotopic composition 53cr of carbonates has the potential to record changes in the oxygenation state of ancient oceans to test the potential of this system we measured 53cr values of limestones and dolostones from an early ordovician marine carbonate platform cerro la silla section of the precordillera san juan argentina an increasing number of studies support the hypothesis that the early ordovician oceans were stratified with respect to oxygen and that the global ocean experienced multiple episodes of anoxic expansion and contraction making this time period ideal for testing the usefulness of the cr isotope system the most prominent feature of the cerro la silla cr isotope profile is a positive excursion of 0 5 during the late tremadocian accompanied by exceptionally low cr concentrations

numerical modelling of melting processes and induced diapirism in the lower crust summary the processes of partial melting and magmatic diapirism within the lower crust are evaluated using a numerical underplating model fully molten basalt t 1200 c is emplaced at the moho beneath a solid granite t 750oc in order that a melt front grows into the granite if diapirism does not occur this melt front in the granite reaches a minimal depth in the crust before like in the molten basalt crystallization takes place the density contrast between the partially molten granite layer and the overlying solid granite can lead to a rayleigh taylor instability rti which results in diapiric rise of the partially molten granite assuming a binary eutectic system for both the granite and the underplating basalt and a temperature and stress dependent rheology for the granite we numerically solve the governing equations and find a that diapirism occurs only within a certain but possibly realistic range of parameters and b that if diapirs occur they do not rise to levels shallower than 15 or perhaps 12km the growth rate depends on the degree of melting and the thickness of the partially molten layer as well as the viscosity of the solid and the partially molten granite from a comparison of the growth rate with the velocity of a stefan front it is possible to predict whether a melt front will become unstable and result in diapiric ascent or whether a partially molten layer is created which remains at depth we carry out such a comparison using our thermodynamically and thermomechanically consistent model of melting and diapirism

burial of organic carbon and pyrite sulfur in sediments over phanerozoic time a new theory in present day marine sediments almost all of which are deposited in normal oxygenated seawater rates of burial of organic carbon c and pyrite sulfur s correlate positively and bear a constant ratio to one another c s 3 on a weight basis by contrast calculations based on the isotopic model of garrels and lerman 1981 indicate that at various times during the phanerozoic the worldwide burial ratio must have been considerably different than the present day value this ratio change is caused by the requirement that increases in the worldwide mass of organic carbon must be accompanied by equivalent decreases in the mass of sedimentary pyrite sulfur in order to maintain a roughly constant level of o2 in the atmosphere such apparently contradictory behavior can be explained if the locus of major organic carbon burial has shifted over time from normal marine environments as at present to non marine freshwater or to euxinic environments in the geologic past a shift to predominantly freshwater burial can help explain predicted high c s ratios in permo carboniferous sediments and a shift to euxinic environments can help explain predicted low c s ratios during the early paleozoic it is demonstrated that the three environments today exhibit distinguishably different average c s ratios

crustal structure and evolution of the mariana intra oceanic island arc a new high resolution velocity model of the mariana arc backarc system obtained from active source seismic profiling demonstrates velocity variations within the arc middle and lower crusts of intermediate to felsic and mafic compositions the characteristics of the oceanic island arc crust are a middle crust with velocity of 6 km s laterally heterogeneous lower crust with velocities of 7 km s and unusually low mantle velocities petrologic modeling suggests that the volume of the lower crust composed of restites and olivine cumulates after the extraction of the middle crust should be significantly larger than is observed suggesting that a part of the lower crust especially the cumulates is seismically a part of the mantle

numerical simulations of subduction zones effect of slab dehydration on the mantle wedge dynamics abstract in oceanic subduction zones dehydration of slab s minerals may favor asthenospheric flow in the mantle wedge by decreasing rocks strength this should enhance the upper plate base reheating and markedly alter its thermal structure to quantify this phenomenon we model slab subduction within a viscous mantle dehydration hydration processes and the rock strength dependence on water content we use accurate phase diagrams for a h2o satured mantle peridotite and a gabbroic crust to determine at each time step the amount of water released or absorbed by each unit of rock transition phases are supposed to be not metastable water is released from the oceanic crust and from the altered peridotite portion of the slab dehydration of the subducting lithosphere occurs first at 60 75 km depth when the crust is eclogitized and second deeper around 105 km when serpentine and chlorite in serpentinite layer below the crust become unstable for high convergence rates because of cold p t t paths in the slab serpentine can be transformed into the hydrated phase a and water is recycled by the slab until great depth however in all investigated cases the released water is sufficient to hydrate by dissolution the whole mantle wedge until 217 55 km away from the trench and as it goes up to form hydrated minerals in the overlying lithosphere over a significant volume the convergence rate slightly shifts dehydration fronts location and consequently widens or reduces the hydrated mantle wedge note that for the dynamic corner flow modelled here with a non newtonian rheology the slab surface is significantly warmer than for an isoviscous analytical corner flow model yielding plausible crust melting we assume a strength reduction associated to hydration larger for rocks containing nominally hydrous minerals than for rocks with only dissolved water the rock strength thus becomes quite uniform at the base of the hydrated upper plate and in the wedge this results in cooler temperatures along the slab top but in an enhanced corner flow effect on the upper plate thermal structure for large hydration strength reductions a strong thermal erosion of the overlapping lithosphere develops in less than 15 myr due to convective destabilization this weakens drastically the upper plate at a distance from 110 to 220 km away from the trench the geometry of the eroded region could correspond to the low velocity zone observed below the arc region

robust characteristics method for modelling multiphase visco elasto plastic thermo mechanical problems abstract we have extended our previous 2d method gerya t v yuen d a 2003 characteristics based marker in cell method with conservative finite differences schemes for modeling geological flows with strongly variable transport properties phys earth planet interiors 140 295 320 which is a combination of conservative finite differences with marker in cell techniques to include the effects of visco elasto plastic rheology self gravitation and a self consistently derived evolving curvilinear planetary surface this code is called i2elvis and can solve a new class of computationally challenging problems in geodynamics such as shear localization with large strains crustal intrusion emplacement of magmas bending of realistic visco elasto plastic plates and core formation by vigorous shell tectonics activities related to a global rayleigh taylor instability of a metal layer formed around silicate rich lower density primordial core during planetary accretion we discuss in detail the computational strategy required the rheological constraints to be satisfied at each time step and spatial location we show analytical benchmarks and examples drawn from comparing between numerical and analogue experiments in structural geology subducting slab bending with a visco elasto plastic rheology and equilibrium spherical configurations from self gravitation we have also tested possibilities of future applications by addressing 3d geometries based on multigrid method and including inertial effects in the momentum equation with tracers in order to simulate meteoritic impact events and eventually earthquake instabilities

structure and growth of the izu bonin mariana arc crust 1 seismic constraint on crust and mantle structure of the mariana arc back arc system 1 a high resolution seismic velocity model is presented for the crust and upper mantle of the mariana arc back arc system mabs based on active source seismic profiling the major characteristics are 1 slow mantle velocity of 8 km s 1 in the uppermost mantle especially and deep reflectors under the mariana arc ma and the west mariana ridge wmr 2 a deep reflector in the upper mantle beneath the relative thick crust of the mariana trough mt axis 3 distribution of lower velocity lower crusts 6 7 6 9 km s 1 beneath the volcanic front and adjacent to the mt and 4 high velocity lower crust 7 2 7 4 km s 1 beneath the boundary regions between the ma and mt and between the wmr and the parece vela basin pvb adding to structural characteristics of crust and upper mantle beneath the mabs of the characteristics described above characteristic 1 suggests that the origins of the slow mantle velocity and the deep reflectors be explained by transfer of the lower crustal residues to the upper mantle across the moho considering that the wmr is extinct arc currently on the other hand characteristic 2 suggests that the origin of deep reflectors beneath the mt axis might be lower velocity materials due to the diffractive signals with strong amplitudes characteristic 3 suggests that the lower velocity lower crust advanced crustal growth and characteristic 4 suggests that the high velocity lower crust beneath arc back arc transition zone is composed of mafic ultramafic materials created by extensive partial melting of mantle peridotites or last stage of the arc magmatism rather than serpentinized peridotite

the provenance of low calcic black shales the elemental concentration of sedimentary rocks depends on the varying reactivity of each element as it goes from the source through weathering deposition diagenesis lithification and even low rank metamorphism however non reactive components of detrital particles ideally are characteristic of the original igneous source and thus are useful in provenance studies to determine the source of detrital granitic and volcanic components of low calcic 1 caco3 marine black shales the concentrations of apparently non reactive i e unaffected by diagenetic redox and or low rank metamorphic processes trace elements were examined using standard trace element discrimination diagrams developed for igneous rocks the chemical data was obtained by neutron activation analyses of about 200 stratigraphically well documented black shale samples from the cambrian through the jurassic a la th sc ternary diagram distinguishes among contributions from the upper and bulk continental crust and the oceanic crust taylor and mclennan 1985 all the low calcic black shales cluster within the region of the upper crust th hf co ternary diagrams also are commonly used to distinguish among the upper and bulk continental crust and the oceanic crust taylor and mclennan 1985 as co is redox sensitive in black shale environments it was necessary to substitute an immobile element i e example rb in the diagram with this substitution of black shales all cluster in the region of the upper continental crust to determine the provenance of the granitic component pearce et al 1984 plots of ta vs yb and rb vs yb ta shows a cluster at the junction of the boundaries separating the volcanic arc granite vag syn collision granite syn colg and within plate granite wpg fields the majority fall within the vag field there are no occurrences of ocean ridge granite org the minimal contribution of basalts to marine black shales is confirmed by the ternary wood diagram th hf 3 ta wood et al 1979 the black shales plot in a cluster in a high th region outside the various basalt fields which suggests contribution from the continental crust

geodynamic regimes of subduction under an active margin effects of rheological weakening by fluids and melts the dynamics of subduction under an active margin are analysed by using a 2d coupled petrological thermomechanical numerical model of an oceanic continental subduction system the model includes spontaneous slab bending dehydration of the subducted crust aqueous fluid transport partial melting of both crustal and mantle rocks as well as melt extraction processes resulting in magmatic arc crust growth based on our models the following five geodynamic regimes of subduction which may potentially form at active margins are identified i stable subduction with no backarc spreading centre and without plumes in the mantle wedge ii retreating subduction with a focused backarc spreading centre and without plumes iii retreating subduction with distributed intra arc extension and trans lithospheric sedimentary plumes iv advancing subduction with underplating laterally extending sub lithospheric plumes and v stable to advancing subduction with stationary laterally limited sub lithospheric plumes transitions between these different regimes are mainly caused by the concurrence of rheological weakening effects of i aqueous fluids percolating from the subducting slab into the mantle wedge and ii melts propagating from the partially molten areas formed in the mantle wedge towards the surface the aqueous fluids mainly affect the forearc region strong fluid related weakening promotes plate decoupling and reduces subduction drag and thus results in stacking of sediments in the accretion prism in contrast reduced weakening by fluids results in strong coupling of the plates and leads to advancing collision like subduction with enhanced subduction erosion thickening of the overriding plate and large sedimentary plumes in the mantle wedge are the consequences on the other hand melts extracted from the hot regions above the slab rheologically weaken the lithosphere below the arc which thus controls overriding plate extension and shortening strong rheological weakening by melts in combination with weak plate coupling trigger retreating subduction with a pronounced backarc spreading centre also weakening of the continental lithosphere by melts extracted from trans lithospheric sedimentary plumes generate a weak channel through which these structures may be emplaced into subarc crust if there is insufficient melt related weakening plumes cannot ascend but extend horizontally and thus underplate the lithosphere

late glacial lake level changes in the lake karakul basin a closed glacierized basin eastern pamirs tajikistan abstract the lake level variations during the late glacial in the lake karakul basin a closed glacierized basin in the northernmost part of the eastern pamirs were reconstructed using geomorphological and sedimentological evidence with a chronology developed using luminescence ages from sand sized quartz and k feldspar in the lake sediments lake transgression started before 19 ka with the peak water level of 35 m above the present elevation occurring at 15 ka this was synchronous with a significant advance of the glacier in the catchment stepwise lake regression including a rapid lowering of the lake level 13 m at 12 ka persisted until at least 10 ka lake transgression and localized glacier expansion from 19 to 15 ka likely correlate with the more regional late glacial glacier advances across the semi arid western himalayan tibetan orogen and the eastern pamirs the longer term trend of this lake transgression was probably caused by colder and or wetter climatic conditions forcing a notable glacier advance

subduction versus accretion of intra oceanic volcanic arcs insight from thermo mechanical analogue experiments we perform thermo mechanical laboratory experiments designed to explore the behaviour of the volcanic arc during intra oceanic arc continent collision following oceanic subduction and subsequent back arc opening the overriding oceanic lithosphere is made of two layers representing the oceanic crust and the lithospheric mantle this lithosphere carries a volcanic arc and is thinned and weakened beneath both the arc and the back arc basin the subducting plate contains three parts one layer oceanic and two layer crust and mantle continental lithosphere with a continental margin between them when the continental margin reaches the trench and starts subducting the overriding plate undergoes growing horizontal compression and finally fails in the vicinity of the back arc spreading centre which is the weakest part of this plate the failure can result in subduction of the whole arc plate comprised between the trench and the back arc spreading centre during subduction of the arc plate the mantle part of this plate subducts completely while the behaviour of the arc crust depends on its thickness and strength which is a function of composition and temperature we tested four cases with different arc crust thicknesses and composition rheology with total lithosphere thickness in the arc being constant three types of tectonic evolution have been obtained complete arc subduction complete arc accretion and partial arc subduction accretion the result is largely controlled by the crustal thickness of the arc a thin arc thickness equivalent to v16 km in nature made of the same strong material as the oceanic crust subducts completely without leaving any trace at the surface on the contrary a thick arc equivalent to v26 km in nature made of the same material is scraped off and accreted to the overriding plate the lower crust of such an arc is hotter therefore its strength at moho depth and coupling between crust and mantle are small in addition the thick arc has a high isostatic relief and hence a greater mechanical resistance to subduction therefore the arc is scraped off if the arc is made of a weaker continental like material or contains a weak layer low friction interface it is completely or partially scraped off even if it is small when there is no back arc opening before collision no thin and weak lithosphere in the rear of the arc the overriding plate fails in the arc area which may result in a complete fore arc block subduction with the volcanic arc remaining at the surface the obtained models are compared with mountain belts with nearly no trace of arc activity oman with accreted arc kohistan and with small remnants of subducted arc southern tibet 2003 elsevier science b v all rights reserved

thermo mechanical controls of flat subduction insights from numerical modeling this study was supported by national basic research program of china 2014cb440901 the strategic priority research program b of cas xdb18020104 national science foundation of china 41190073 41372198 and 41304071 and nerc grant ne j021822 1

insights into the causes of arc rifting from 2 d dynamic models of subduction author s billen mi abstract 2017 american geophysical union all rights reserved back arc spreading centers initiate as fore arc or arc rifting events when extensional forces localize within lithosphere weakened by hydrous fluids or melting two models have been proposed for triggering fore arc arc rifting rollback of the subducting plate causing trench retreat or motion of the overriding plate away from the subduction zone this paper demonstrates that there is a third mechanism caused by an in situ instability that occurs when the thin high viscosity boundary which separates the weak fore arc from the hot buoyant mantle wedge is removed buoyant upwelling mantle causes arc rifting drives the overriding plate away from the subducting plate and there is sufficient heating of the subducting plate crust and overriding plate lithosphere to form adakite or boninite volcanism for spontaneous fore arc arc rifting to occur a broad region of weak material must be present and one of the plates must be free to respond to the upwelling forces

palaeoecology of the graptoloidea abstract graptoloids class graptolithina were colonial pterobranch hemichordates and formed a major component of the early paleozoic macrozooplankton by analogy with modern pterobranchs they filtered food particles from the water mass in which they lived by means of ciliated lophophores they are inferred to have been able to migrate vertically through the water column for feeding efficiency and predator avoidance as by modern euphausiids but their means of locomotion is still highly conjectural the rhabdosomes which housed the colonies grew by distal accretion and large examples are inferred to have taken up to several years to have reached full size the few studies of survivorship suggest a relatively constant mortality of individuals within the community tubular and blister like outgrowths of cortical tissue are seen rarely and are inferred to be a response to parasitism and rare faecal packages with crumpled rhabdosomes suggest ingestion by predators the function of the monospecific aggradations of conjoined colonies synrhabdosomes seen on rare bedding planes is uncertain but is unlikely to have been part of their normal life history and ecology monospecific assemblages of unconnected colonies on single bedding planes are not rare however and suggest localised populations of single species in the overlying water column in life other bedding planes have a range of species resulting from mass mortality from volcanic eruptions and suggest a diversity of species in the water column the onshore offshore and depth facies preference of some 200 ordovician species has led to an ecological model in which the graptoloid habitat is partitioned vertically and horizontally in waters of the neritic and pelagic realms one group of species group 1 is largely confined to a deep water biotope mesopelagic zone characterised by dysaerobic water where they are thought to have exploited high bioproductivity denitrification zones a second group group 2 occupies an epipelagic biotope and is confined to waters of the epipelagic zone aerobic water a third and less well defined group group 3 occupies the inner shelf waters of the epipelagic zone only the median duration of species in the deep water biotope is significantly shorter than that of species in the epipelagic biotope suggesting greater environmental stress and instability in the dysaerobic zone over geological time consistent with models of disruption of the density structure and circulation patterns of the oceans from climatic instability the diversity and abundance of species were enhanced in both the epipelagic and deep water biotopes in regions of marginal upwelling where nutrients were brought from the deep to the shallow water zones it was in these regions that the classical graptolitic shale developed

slab dynamics in the transition zone abstract seismic images of horizontal slab segments in across or below the transition zone invoke scenarios in which slabs are laid down in the mantle through progressive trench retreat however observations of subduction characteristics do not exhibit a clear correlation between trench retreat and slab dip in the transition zone instead analysis of a range of subduction characteristics demonstrates that while transition zone slabs include slabs of many ages and subduction velocities with both retreating and advancing trench motion they also have several enigmatic characteristics such as faster sinking rates and a larger range in slab dip than slabs that extend to either shallower or deeper depths comparison of subduction characteristics with several analytical and numerical models of subduction dynamics suggests that many of the possible mechanisms for trapping slabs in the transition zone e g positive buoyancy sources viscous resistance slab weakening are only viable if the slab is already shallow dipping two scenarios for the formation of stagnant slabs are proposed 1 trench retreat prior to slabs entering the transition zone or caused by the negative buoyancy forces associated with the wadsleyite and ringwoodite phase transitions and 2 slow lateral migration of slabs in stable subduction zones

effect of time evolving age and convergence rate of the subducting plate on the cenozoic adakites and boninites partial melting of subducting oceanic crust expressed as high mg volcanic rocks such as adakites and boninites has been actively studied for decades and lee and king 2010 reported that time evolving subduction parameters such as the age and the subduction rate of the converging oceanic plate play important roles in transient partial melting of the subducting oceanic crust e g aleutians however few subduction model experiments have considered time evolving subduction parameters posing problems for studies of transient partial melting of subducting oceanic crust in many subduction zones therefore we constructed two dimensional kinematic dynamic subduction models for the izu bonin mariana northeast japan kuril tonga java sunda and aleutian subduction zones that account for the last 50 myr of their evolution the models include the time evolving age and convergence rate of the incoming oceanic plate so the effect of time evolving subduction parameters on transient partial melting of oceanic crust can be evaluated our model calculations revealed that adakites and boninites in the izu bonin and aleutian subduction zones resulted from transient partial melting of oceanic crust however the steady state subduction model using current subduction parameters did not produce any partial melting of oceanic crust in the aforementioned subduction zones indicating that time evolving subduction parameters are crucial for modeling transient eruption of adakites and boninites our model calculations confirm that other geological processes such as forearc extension back arc opening mantle plumes and ridge subduction are required for partial melting of the oceanic crust in the mariana northeast japan tonga and southeastern java sunda subduction zones

subduction zone decoupling retreat modeling explains south tibet xigaze and other supra subduction zone ophiolites and their uhp mineral phases abstract the plate tectonic setting in which proto ophiolite oceanic lithosphere is created remains controversial with a number of environments suggested recent opinions tend to coalesce around supra subduction zone ssz forearc extension with a popular conceptual model in which the proto ophiolite forms during foundering of oceanic lithosphere at the time of spontaneous or induced onset of subduction this mechanism is favored in intra oceanic settings where the subducting lithosphere is old and the upper plate is young and thin we investigate an alternative mechanism namely decoupling of the subducting oceanic lithosphere in the forearc of an active continental margin followed by subduction zone trench retreat and creation of a forearc oceanic rift basin containing proto ophiolite lithosphere between the continental margin and the retreating subduction zone a template of 2d numerical model experiments examines the trade off between strength of viscous coupling in the lithospheric subduction channel and net slab pull of the subducting lithosphere three tectonic styles are observed 1 c continuous subduction without forearc decoupling 2 r forearc decoupling followed by rapid subduction zone retreat 3 b breakoff of subducting lithosphere followed by re initiation of subduction and in some cases forearc decoupling b r in one case ba b r where ba denotes backarc subduction zone retreat follows backarc rifting subduction zone decoupling is analyzed using frictional plastic yield theory and the stefan solution for the separation of plates containing a viscous fluid the numerical model results are used to explain the formation of xigaze group ophiolites southern tibet which formed in the lhasa terrane forearc likely following earlier subduction and not necessarily during subduction initiation either there was normal coupled subduction before subduction zone decoupling or precursor slab breakoff subduction re initiation and then decoupling rapid deep upper mantle circulation in the models during subduction zone retreat can exhume and emplace material in the forearc proto ophiolite from as deep as the mantle transition zone thereby explaining diamonds and other 10 15 gpa uhp phases in tibetan ophiolites

a new geochemical classification of sedimentary environments because of the relative constancy of ph in most subaqueous sediments and the general lack of measurability of eh these parameters are not practically useful for classifying sedimentary environments in their place a new classification is offered which derived from studies of modern sediments is based on the presence or absence of dissolved oxygen and dissolved sulfide in the sediments at the time of authigenic mineral formation sedimentary environments are first of all divided into oxic and anoxic depending upon the presence or absence of measurable dissolved oxygen anoxic environments in turn are divided into sulfidic and non sulfidic depending upon the presence of measurable dissolved sulfide anoxic nonsulfidic environments are further divided into postoxic or resulting from oxygen removal without sulfate reduction weakly reducing and methanic or resulting from complete sulfate reduction with consequent methane formation strongly reducing the environments are shown to succeed one another during early diagenesis in the order oxic post oxic sulfidic methanic iron and manganese minerals characteristic and stable in each environment are listed and discussed so that they may be used to distinguish the environments when studying the ancient geological record

paleoredox variations in ancient oceans recorded by rare earth elements in fossil apatite abstract rare earth element concentrations in biogenic apatite of conodonts fish debris and inarticulate brachiopods were determined in over 200 samples from cambrian to modern sediments tests for experimental bias caused by the chemicals used to separate fossils from the rock matrix and for interlaboratory interexperiment or interspecies related variations clearly show that no resolvable fractionation of ree occurs incorporation of ree in biogenic apatite of recent fish debris occurs near the sediment water interface soon after deposition and reflects characteristics of seawater the original ree signature apparently survives subsequent burial and diagenesis cerium variations in fish debris from modern environments are controlled by redox potential ce is fractionated by co precipitation with metallic oxides under oxidizing conditions this fractionation produces a negative ce anomaly ce anom in seawater that is reflected in the sedimented fish debris conversely no fractionation of ce occurs under reducing conditions resulting in ce concentrations that are normal to enriched in anoxic seawater and fish debris deposited under local or basinwide anoxic conditions extrapolation of observations of the redox control of ce anom in modern oceans to fossil apatite indicates that anoxic conditions were prevalent in lower paleozoic and lower triassic oceans whereas in the upper paleozoic the world ocean was generally oxidizing

control of seafloor aging on the migration of the izu bonin mariana trench article i nfo recent global kinematic studies reveal that most of the trenches roll back but a significant number of them advance toward the upper plate those advancing trenches are mostly located in the western pacific and correspond to the subduction of very old mesozoic oceanic lithosphere while retreating trenches are commonly explained by the slab pull action of the descending lithosphere the origin of advancing trenches is still debated since this relationship is dependent upon the adopted reference frame we select region where geological studies show the variability of trench migration style with time the izu bonin mariana ibm region represents a key example the detailed reconstruction of the trench migration of the ibm subducting system reveals that after a long episode of asymmetric rollback the ibm trench recently started advancing we propose that this change from retreating to advancing trench mode results from the subduction of progressively older and stiffer lithospheric material we test this hypothesis by means of two dimensional 2 d numerical models reproducing the effects of the lithospheric aging during subduction the result of our numerical tests shows that the entrance of old and stiff lithosphere forces the trench to advance because the increasing stiffness of the slab prevents the slab to unbend once it has subducted we adapt this physical result to the ibm evolution showing that the switch from trench retreat to trench advance can reconcile the shape of ibm slab as previously suggested finally we discuss the possibility that the subduction of the cretaceous lithosphere might have triggered the change of the motion of the pacific plate around 5 ma

petrology and geochemistry of ordovician k bentonites in new york state constraints on the nature of a volcanic arc altered volcanic ashes k bentonites in the late ordovician caradocian utica shale of new york state are the product of explosive arc volcanism most of the 30 k bentonites examined in this investigation contain fragmental crystals and rock fragments microliths up to 600 mu m in diameter that generally are neither detrital contamination from the surrounding black shale nor igneous phenocrysts the dominant phases are garnet two groups gr sub 15 20 alm sub 45 75 py sub 35 0 sp sub 2 5 gr sub 2 5 alm sub 55 85 py sub 10 40 sp sub 1 plagioclase feldspar an sub 80 10 alkali feldspar or sub 99 0 clinopyroxene wo sub 50 40 en sub 50 30 fs sub 0 30 and orthopyroxene wo sub 1 2 en sub 77 32 fs sub 32 66 accompanied by lesser quantities of hornblende aluminosilicate quartz sphene fe ti oxides apatite and zircon most of the fragmental crystals appear to be derived from the same source as the metamorphic microliths which possess minerals with similar compositions both crystals and microliths are interpreted as xenocrysts and xenoliths from the ancient continental crust on which the ordovician arc was constructed they became entrained in the volcanic plume during explosive eruptions a precambrian age acquired on k feldspars from one k bentonite using the sup 40 ar sub 39 ar method shows that these xenocrysts were derived from depths of less than 5 10 km in the microcontinent at the time of late ordovicianmore volcanism the occurrence of xenocrysts and xenoliths in these k bentonites underscores the importance of performing detailed petrology on ash layers prior to the onset of more sophisticated tasks e g isotopic age determinations regional stratigraphic correlations of k bentonites based upon chemical compositions less

dynamics of subduction initiation with different evolutionary pathways changes of plate motion may have induced subduction initiation si but the tectonic history of si is different from one subduction zone to another izu bonin mariana ibm si accompanied by strong backarc spreading and voluminous eruption of boninites contrasts with the aleutians which shows neither using finite element models we explore visco elasto plastic parameters and driving boundary conditions for si evolution with an imposed velocity we find three different evolutionary modes of si continuous without backarc spreading continuous with backarc spreading and a segmented mode with an increase in the coefficient of friction and a decrease in the rate of plastic weakening the amount of convergence needed for si increases from 20 to 220 km while the mode changes from segmented to continuous with backarc spreading and eventually to continuous without backarc spreading if the imposed velocity boundary condition is replaced with an imposed stress the amount of convergence needed for si is reduced and backarc spreading does not occur these geodynamic models provide a basis for understanding the divergent geological pathways of si first ibm evolution is consistent with subduction of an old strong plate with an imposed velocity which founders causing intense backarc spreading and boninitic volcanism second the new hebrides si is in the segmented mode due to its weak plate strength third the puysegur si is in the continuous without backarc spreading mode with no associated volcanic activities fourth the aleutians si has neither trench rollback nor backarc spreading because the slab is regulated by constant ridge push forces

palaeoceanographic linkage of geochemical and graptolite events across the silurian devonian boundary in bardzkie mountains southwest poland abstract the silurian devonian s d transition in the bardzkie mountains is developed as deep water graptolitic black shales the highest silurian transgrediens zone and the first devonian uniformis zone are separated by a thin 30 cm interval of a linograptid interregnum thirteen samples from the s d transition have been analysed for major and minor elements si al si al toc ga al 2 o 3 v v ni ratios and toc mn zr ti re mo cd cu and ree have been found to correlate with the habitat replacement of the graptolite assemblages the linograptid burst accompanied by changing sediment geochemistry across the s d boundary are attributed to short term shallowing of the upwelling system which introduced specific nutrient rich waters into the mixed layer

oceanic circulation as an element in palaeogeographical reconstructions the arenig early ordovician as an example a new conceptual palaeo oceanographical model is outlined in this paper the model differs from previous models by using shifts in hadley circulation caused by orbital variations and the rotation rate of the earth to locate the position of the planetary oceanic low and high pressure systems around which the planetary ocean surface currents flow adapting the model to the arenig early ordovician the temperate low pressure zones were found to be located at 50 latitude and the subtropical high pressure zones at 25 latitude traditionally most palaeozoic palaeogeographical recon structions are reconstructed using palaeomagnetic data supplemented with data from climate sensitive lithofacies and palaeo biogeographical distributions however as a new approach in palaeogeographical reconstructions the con ceptual palaeo oceanographical model is combined with palaeobiogeographical data for the arenig series comple menting the palaeomagnetic data and resulting in a new refined palaeogeographical reconstruction

evolution from fore arc oceanic crust to island arc crust a seismic study along the izu bonin fore arc 1 petrological studies have suggested that oceanic crust is formed during the initial stage of subduction however there is little geophysical evidence for the formation of oceanic crust in the fore arc region we conducted an active source seismic survey in the fore arc region of the izu bonin intraoceanic arc to examine processes of crustal formation associated with the initiation of subduction we used seismic refraction tomography to obtain a detailed seismic velocity model and diffraction stack migration of picked reflection travel times to derive reflectivity images these data show a remarkably thin crust 10 km thick along the northern half of the bonin ridge and abrupt crustal thickening to 20 km toward the south of the ridge comparison of velocity depth profiles of the thin fore arc crust of the bonin ridge with those of typical oceanic crusts showed them to be seismologically identical boninitic magmatism is evident in the area of thin crust and tholeiitic calcalkaline andesitic volcanism in the area of thick crust high precision dating of the volcanic rocks showed that the thin fore arc oceanic crust was created soon after initiation of subduction 48 45 ma and that the nonoceanic thick crust was created with tholeiitic calcalkaline andesitic magmatism after the boninitic magmatism was ceased our seismological image strongly supports the view that fore arc oceanic crust was formed by fore arc spreading during the initial stage of subduction along the izu bonin intraoceanic arc

episodic magmatism during the growth of a neoproterozoic oceanic arc anti atlas morocco abstract we present an integrated study combining detailed field geochronological and geochemical data of a neoproterozoic intra oceanic arc systems exposed in the pan african belt of the moroccan anti atlas the arc rock units exposed in bou azzer and sirwa inliers consist of a tectonic patchwork of back arc ophiolitic sequences to the north thrusted onto accreted arc complexes to the south arc complexes are composed of amphibolite granodioritic and granitic gneisses intruded by various undeformed hydrous ultramafic hornblendite mafic hornblende gabbro diorite and felsic granodiorite tonalite granite arc lithologies we show that these complexes are remnants of a long lived 120 myr neoproterozoic oceanic arc punctuated by three successive magmatic episodes 760 730 ma 710 690 ma 660 640 ma respectively interspersed with periods of magmatic quiescence the typical geochemical arc signature and positive ndt values for the igneous rocks emplaced during each magmatic episode medians at 7 1 5 4 and 5 7 from older to younger attest that their parental magmas derived from a depleted mantle source without substantial assimilation by the wac older crustal basement trace element geochemistry i e sr y la yb of intermediate to felsic arc rocks produced during each magmatic pulse suggests that the arc crust was thickened 30 35 km over a short time period between the first and second magmatic episodes 730 710 ma which coincides with an important regional shortening event soft docking of the oceanic arc on a buoyant transitional margin is invoked to explain tectonic inversion in overriding plate leading to shortening and related thickening of the arc crust concomitant magmatic shutdown resulting from a reorganization of subduction dynamics i e change in slab geometry flip in subduction polarity a non tectonic critical thickening of the arc crust is invoked to explain the second magmatic shutdown 680 660 ma by freezing the subarc mantle influx this lull period is followed by a third magmatic episode which is likely triggered by delamination of the dense lower crust and reactivation of subarc mantle flow this is supported by the bimodal chemical signature of evolved magmatic products suggesting two distinct sources partial melts from the foundered lower crust and new magmatic products which differentiated from a post delamination thinned crust

iridium abundances across the ordovician silurian stratotype chemostratigraphic analyses in the ordovician silurian boundary stratotype section bracketing a major extinction event in the graptolitic shale section at dob s linn scotland show persistently high iridium concentrations of 0 050 to 0 250 parts per billion there is no iridiumn concentration spike in the boundary interval or elsewhere in the 13 graptolite zones examined encompassing about 20 million years iridium correlated with chromium both elements showing a gradual decrease with time into the middle part of the lower silurian the chromium iridium ratio averages about 10 6 paleogeographic and geologic reconstructions coupled with the occurrence of ophiolites and other deep crustal rocks in the source area suggest that the high iridium and chromium concentrations observed in the shales result from terrestrial erosion of exposed upper mantle ultramafic rocks rather than from a cataclysmic extraterrestrial event

palaeo oceanography and biogeography in the tremadoc ordovician iapetus ocean and the origin of the chemostratigraphy of dictyonema flabelliforme black shales high concentrations of vanadium molybdenum uranium arsenic antimony with low concentrations of manganese iron and cobalt heretofore restricted to dictyonema flabelliforme bearing tremadoc black shales in balto scandia have been found in coeval black shales in the saint john new brunswick area prior palaeogeographic reconstructions place these areas about 400 km apart in high southern latitudes in the iapetus ocean with new brunswick in proximity to avalonia southeastern newfoundland these geochemical similarities are not found in coeval tremadoc black shales of bolivia new york quebec wales and belgium palaeo oceanographic reconstructions of iapetus support the proximity of balto scandia and the saint john area during the early tremadoc and gee sx 1981 suggestion that the signature is a feature of eastern iapetus furthermore first order modelling of the major surface currents and related primary productivity in the tremadoc iapetus ocean explain the apparent wide latitudinal range of d flabelliforme fortey 1984 and the anomalous trace metal content of certain black shales of that time variations in the elemental content of these black shales is produced by oceanographic and geologic conditions unique to the geographic site the distinctive balto scandic geochemical signature resulted from the coincidence of anoxic waters transgressing the shelf at latitudes of high organic productivity at the polar ekman planetary divergence this produces the conditions for concentrations of v u and mo in the shales metal enriched anoxic bottom waters produced by leaching of volcanics or through hydrothermal activity may be the source of the other enhanced signature elements such as as and sb the absence of this geochemical signature in younger non d flabelliforme tremadoc and later black shales in balto scandia and other areas suggests that the closing of iapetus moved the depositional sites into less productive oceanic areas

episodicity in back arc tectonic regimes abstract the evolution of back arc basins is tied to the development of the dynamics of the subduction system they are a part of we present a study of back arc basins and model their development by implementing 3d time dependant computer models of subduction including an overriding plate we define three types of episodicity pseudo quasi and hyper episodicity and find evidence of these in nature observations of back arc basin ages histories of spreading quiescence and compression in the overriding plate give us an understanding of the time development of these subduction zones and back arc basins across the globe today a number of trenches are advancing the izu bonin trench the mariana trench the japan trench the java sunda trench and the central portion of the peru chile trench the andes subduction zone the izu bonin mariana and japan all have established back arc basins while the others have documented episodes of spreading quiescence compression or a combination of these the combination of advancing and retreating trench motion places these subduction zones in the category of hyper episodicity quasi episodicity in which the back arc shifts between phases of rifting spreading and quiescence is the dominant form of episodic back arc development in the present we find this type of episodicity in models for which the system is dynamically consistent that we have allowed the subducting plate s velocity to be determined by the sinking slabs buoyancy quasi and hyper episodicity are only found in subduction zones with relatively high subducting plate velocities between 6 and 9 cm year finally those subduction zones for which the subducting plate is moving slowly such as in the mediterranean or the scotia sea experience only pseudo episodicity where the spreading moves linearly towards the trench but often does so in discrete ridge jump events

seismic imaging of a possible paleoarc in the izu bonin intraoceanic arc and its implications for arc evolution processes crustal evolution processes in intraoceanic arcs including crustal accretion and rifting have been long discussed to examine crustal evolution in the izu bonin intraoceanic island arc we conducted an active source wide angle seismic study along a north south profile 500 km long within a possible paleoarc in the rear arc i e the nishi shichito ridge about 150 km west of the present day volcanic front in this study the seismic velocity and reflectivity images are obtained using the wide angle seismic data for the seismic velocity imaging we applied refraction tomography in which 93 535 picks were used the overall root mean square rms misfit calculated from the initial model of the refraction tomography was 483 1 ms and those calculated from the final model were reduced to 66 7 ms the resultant seismic image shows marked variations of crustal thickness along the seismic profile thin crust 10 15 km thick in the northern part three discrete thick crustal segments 20 25 km thick in the central part and a moderately thick crust 15 km thick in the southern part these variations are mainly attributed to thickness variations of the middle crust having seismic velocity of 6 0 6 8 km s this variation of crustal thickness does not correlate with seafloor topography which is characterized by post miocene across arc seamount chains it does correlate well with crustal variations observed along the present day volcanic front of the izu bonin arc these findings suggest that the magmatic activity that created the across arc seamount chains had little effect on the rear arc crust and that the main part of the rear arc crust was created before the rear arc separated from the volcanic front by correlating the structural variations along the rear arc i e the variation of the average seismic velocity as well as the thickness of the middle crust and those along the present day volcanic front we found that the direction of rifting to separate the rear arc paleoarc from the present day volcanic front was north northeast

impact of arc continent collision on the conditions of burial and exhumation of uhp lt rocks experimental and numerical modelling abstract a 2 d physical and finite element numerical modelling of arc continent collision was performed to study the deformation and failure of the overriding lithosphere the experimental technique allowed us to model the whole subduction collision process from oceanic subduction to deep subduction of the continental crust with the numerical approach we have modelled the deformation of the overriding plate only through initial stages of its failure and studied the influence of different parameters on this process the results obtained by both techniques are coherent and mutually complementary they show that the failure of the overriding plate is physically quite plausible or even inevitable during subduction the conditions for such a failure the weakening of this plate are prepared during oceanic subduction the weakening occurs due to the interaction between the subducting lithosphere and the asthenosphere in the mantle corner between the two plates and due to back arc spreading in oceanic subduction zones with a compressional regime no back arc opening thick and strong back arc lithosphere the weakest zone is volcanic arc area when weakening becomes sufficient during subduction the lithosphere fails in this area the failure occurs along a fault dipping under the arc in either of two possible directions and results either in subduction reversal or subduction of the fore arc almost half of the presently active subduction zones are characterised by a tensional subduction regime with back arc spreading in such subduction zones the weakest place is not the volcanic arc but the back arc spreading centre when a subduction regime changes from tensional to compressional failure occurs in the vicinity of the extinct spreading centre this process can occur during oceanic subduction again along either a trench vergent or trenchward dipping fault but the formation of a trench verging fault is most likely in this latter case which is a principal subject of our study the failure is followed by partial subduction of the arc plate complete subduction occurs during arc continent collision subduction of the continental margin when tectonic compression of the lithosphere increases rapidly and becomes sufficient to push the arc plate into the mantle the arc itself can be subducted completely or be partially or completely scraped off and accreted a deeply subducted material including continental margin is preserved at relatively low temperatures between the lithospheric mantle and the cold subducted arc plate to about 150 km depth subduction of the arc plate is a major phenomenon which affects all processes associated with continental subduction from deep burial and hp lt metamorphism to exhumation of subducted material does this process occur in nature future investigations will allow us to answer this question in this paper we analyse the conditions of emplacement of a very young oceanic lithosphere samail ophiolite on the continental crust in oman in the late cretaceous and argue that this lithosphere formed in a back arc basin it reached and overthrust the arabian continent after complete subduction of the arc plate

definition of the tremadoc series and the series of the ordovician system in britain the tremadoc series and series of the ordovician system are widely used in international correlations and this review explains and summarizes how the base of each of these series is recognized in britain no type section in which the base of any one of them is displayed has ever been selected and formally designated hence the basal horizon of each series is a matter of general understanding rather than precise definition this general understanding is of particular biozones or of assemblages of fossils and their relative ages the geographical separation between type sections of series and differences in facies complicate the understanding of relative ages of biozones brachiopod and trilobite faunas of the llandeilo caradoc and ashgill series have been studied in detail in recent years and biozones proposed these biozones have proved difficult or impossible to recognize outside their type areas in britain and brachiopod trilobite faunas of the tremadoc arenig and llanvirn series are likewise highly provincial thus for correlation within and outside britain attention has been focused on the standard graptolite biozones particularly since such conodont faunas as have been found have not proved useful yet there are problems associated with these standard biozones for example the tetragraptus approximatus biozone is unknown in wales or england the relationship between the base of the glyptograptus teretiusculus biozone and the earliest llandeilo shelly faunas requires clarification and at what level the base of the caradoc series may lie in the nemagraptus gracilis biozone is unknown thus to bring precision to definitions of ordovician series in britain the identification of continuous fossiliferous sequences that will serve as stratotypes is required such a section for the basal portion of the tremadoc series has recently been proposed systematic studies related to particular biozones are needed for example early caradoc and early ashgill brachiopods and graptolites of the didymograptus bifidus and glyptograptus teretiusculus biozones the stratigraphical ranges and abundances of species in particular sections require documentation so that the type of biozone assemblage acme concurrent range etc may be recognized such work could lead towards chronostratigraphy the provision of a standard stratigraphical scale in britain for the tremadoc series and ordovician series

structural variations of arc crusts and rifted margins in the southern izu ogasawara arc back arc system 1 we carried out a reflection refraction seismic survey across the southern izu ogasawara arc back arc system covering three arcs with different crustal ages the oldest eocene arc has middle and lower crust with high velocities of 6 4 6 6 and 6 8 7 4 km s respectively suggesting denser crustal materials the current volcanic arc has middle and lower crust with lower velocities of 5 7 6 5 and 6 7 7 1 km s suggesting advanced crustal differentiation the crust mantle transition layer with a velocity of 7 5 8 0 km s is distributed beneath the current volcanic arc and the rear arc suggesting a pool of dense materials emanating from the crust through the crustal growth these structural differences between the eocene arc and current arc indicate a difference of crustal growth based on basaltic and andesitic magmas according to known petrologic studies commonly rifted crusts have lower crusts with high velocities of over 7 0 km s and the arc back arc transition zone also has a thinner more reflective crust that may have been affected by postrifting magmatism

missing oligocene crust of the izu bonin arc consumed or rejuvenated during collision the 50 myr old izu bonin mariana ibm arc consists mostly of oligocene middle and lower crust that underlies the upper crust these units are in turn covered by quaternary volcanic rocks seismic imaging forearc geology ocean drilling program drilling and magnetic anomalies suggest that most ibm arc crust was created in eocene oligocene times however remnants of this old crust have never been found at the northern end of the arc where it is colliding with the honshu arc izu collision zone two batholiths in this collision zone thetanzawa tonalites and the kofu granitic complex were emplaced during the miocene 4 17 ma major elements zr y rare earth element ratios and normalized abundance patterns and sr nd isotopic data indicate that these plutonic bodies are compositionally similar to the oligocene ibm volcanic rocks and that they are dissimilar to the miocene pliocene and quaternary ibm lavas and volcaniclastic rocks we suggest that the miocene plutonic rocks in the izu collision zone were derived from partially melted oligocene middle crust a model is proposed in which ibm arc middle crust in the collision zone was partially melted during the collision and then intruded into the overlying upper crust of the honshu and ibm arcs this resulted in the complete loss of chronological information related to their original source

the whole rock cerium anomaly a potential indicator of eustatic sea level changes in shales of the anoxic facies abstract the whole rock cerium anomaly tested for outer shelf upper slope stratigraphic sections from the middle ordovician through the lower silurian of scotland is proposed as an empirical technique to develop a eustatic 3rd order or finer scale sea level curve this interval was chosen as it straddles the well documented late ordovician glaciation and can be defined by graptolite zones the anomaly is calculated from neutron activation analysis of low carbonate phosphate free fossil free field identified shales of the graptolite facies by comparison of the normalized cerium content with the linearized trend of the normalized composition of other rare earth elements in order of atomic number for sections originally deposited in the main pycnocline below the surface mixed layer values of the anomaly for a given sample would indicate its position on the redox curves developed for the early paleozoic by wilde 1987 changes in the anomaly that are positive with time would indicate a lowering of sea level as the apparent depth on the redox curve would reflect more oxic conditions relative changes negative with time would indicate a rise in sea level as the apparent depth reflects more anoxic conditions depending on the vertical sample spacing and the time interval sampled resolution of the order of 1 m y might be achieved thus the vail et al 1977 curves of the 3rd order 1 to 10 m y or of finer scale could be obtained by this technique with the proper choice of section accordingly for the early through middle paleozoic when the main pycnocline was anoxic this geochemical technique could be used to develop eustatic sea level curves and additionally offer an independent calibration for seismic stratigraphy as well as an indicator of glacial interglacial climatic sequences or eustatic changes due to fluctuations in global ridge crest volumes

modeling the dynamics of subducting slabs cold dense subducting lithosphere provides the primary force driving tectonic plates at earth s surface the force available to drive the plates depends on a balance between the buoyancy forces driving subduction and the mechanical and buoyancy forces resisting subduction because both the buoyancy and rheology of the slab and mantle depend on temperature composition grain size water content and melt fraction unraveling which of these processes exert a first order control on slab dynamics and under what circumstances other processes become first order effects can be challenging laboratory and numerical models of slab dynamics provide a powerful method for testing the combined effects of buoyancy and strength changes that accompany the slab evolution in the upper mantle transition zone and lower mantle recent studies have focused on understanding how rheologic variations newtonian versus non newtonian viscosity or water content geometry 2d versus 3d and plate motions trench roll back or advance influence the evolution of slabs in the upper mantle and how they sink into the lower mantle these models suggest that spatial and temporal variations in slab strength and the history of subduction determine whether slabs sink directly into the lower mantle or are trapped in the transition zone

how collision triggers backarc extension insight into mediterranean style of extension from 3 d numerical models the formation and evolution of a backarc basin are linked to the dynamics of the subduction system the opening of the central mediterranean basins is a well documented example of backarc extension characterized by short lived episodes of spreading the underlying reasons for this episodicity are obscured by the complexity of this subduction system in which multiple continental blocks enter the subduction zone we present results from three dimensional numerical models of laterally varying subduction to explain the mechanism of backarc basin opening and the episodic style of spreading our results show that efficient backarc extension can be obtained with an along trench variation in slab buoyancy that produces localized deformation within the overriding plate we observe peaks in the trench retreating velocity corresponding first to the opening of the backarc basin and later to the formation of slab windows we suggest that the observed episodic trench retreat behavior in the central mediterranean is caused by the formation of slab windows

experimentally based water budgets for dehydrating slabs and consequences for arc magma generation phase diagrams of hydrous mid ocean ridge mor basalts to 330 km depth and of hydrous peridotites to 250 km depth are compiled for conditions characteristic for subduction zones a synthesis of our experimentally determined phase relations of chlorite lawsonite epidote zoisite amphibole paragonite chloritoid talc and phengite in basalts and of phase relations from the literature of serpentine talc chlorite amphibole and phase a in ultramafics permits calculation of h2o contents in hydrous phase assemblages that occur in natural compositions this yields the information necessary to calculate water budgets for descending slabs starting from low grade blueschist conditions 10 20 km depth with h2o contents between 5 and 6 wt for hydrated oceanic crust complete dehydration is achieved between 70 and 300 km depth as a function of individual slab geotherms hydrous phases which decompose at depth below volcanic arcs are lawsonite zoisite chloritoid and talc phengite in mafic compositions and chlorite and serpentine in peridotite approximately 15 35 of the initially subducted h2o are released below volcanic arcs the contribution of amphibole dehydration to the water budget is small 5 20 and occurs at relatively shallow depth 65 90 km in any predicted thermal structure dehydration is a combination of a stepwise and a continuous process through many different reactions which occur simultaneously in the different portions of the descending slab such a dehydration characteristic is incompatible with single phase dehydration models which focus fluid flow through a unique major dehydration event in order to explain volcanic fronts as a consequence of continuously progressing dehydration water ascending from the slab will be generally available to depth of ca 150 200 km the fluid rising from the subducting lithosphere will cause partial melting in the hot portion of the mantle wedge we propose that the volcanic front simply forms above the mantle wedge isotherm where the extent of melting is sufficient to allow for the mechanical extraction of parental arc magmas thermal models show that such an isotherm ca 1300oc locates below volcanic fronts slab surface depths below such an isotherm are compatible with the observed depths of the slab surface below volcanic fronts 1998 elsevier science b v all rights reserved

pyrite and organic matter in phanerozoic normal marine shales data have been obtained from our own chemical analyses and from the literature for the concentrations of organic carbon c and pyrite sulfur s in over 600 samples of cambrian to recent normal marine shales normal marine refers to deposition in oxygenated bottom water as evidenced by the presence of benthic fossils and or indicators of bioturbation all samples were selected to minimize 1 1 loss of c and or s due to weathering at the outcrop by emphasizing the use of drill core material 2 2 analytical errors and non diagenetic effects by avoiding sediments low in c or s content such as sandstones 3 3 fe limitation of pyrite formation by avoiding limestones cherts and euxinic shales 4 4 metamorphic loss of c relative to s by avoiding rocks obviously subjected to even low grade metamorphism our results indicate that 1 there is generally a good positive linear correlation between organic c and pyrite s for normal marine shales of all ages and 2 the mean cs ratio for normal marine shales has varied over time devonian to tertiary shales exhibit mean cs weight ratios 1 8 0 5 somewhat lower than quaternary sediments meancs 2 8 this we believe is due to preferential c loss relative to s during biogenic methanogenesis plus diagenetic heating distinctly lower mean c s values for the cambrian and ordovician 0 5 0 1 cannot be explained solely in terms of diagenetic c loss and instead must represent low original ratios the low original ratios we suggest were due mainly to the absence of bacterially refractory organic matter added to the marine environment by rivers at that time because the major source of such material vascular land plants had not yet evolved this along with a possibly lower degree of bioturbation contributed to enhanced pyrite formation and preservation and thus lower early paleozoic cs ratios

new seismological constraints on growth of continental crust in the izu bonin intra oceanic arc the process by which continental crust has formed is not well understood though such crust mostly forms at convergent plate margins today it is thus imperative to study modern intra oceanic arcs such as those common in the western pacific ocean new seismic studies along the representative izu bonin intra oceanic arc provide unique along strike images of arc crust and uppermost mantle to complement earlier cross arc lithospheric profiles these reveal two scales 1000 10 km scale of variations one at the scale of the izu versus bonin thick versus thin arc crust the other at the intervolcano 50 km scale these images show that 1 the bulk composition of the izu bonin arc crust is more mafic than typical continental crust 2 the middle crust with seismic velocities similar to continental crust is predominantly beneath basaltic arc volcanoes 3 the bulk composition beneath basaltic volcanoes changes little at thick and thin arc segments and 4 a process to return lower crustal components to the mantle such as delamination is required for an arc crust to evolve into continental crust continued thickening of the izu bonin crust accompanied by delamination of lowermost crust can yield velocity structure of typical continental crust

metamorphism of franciscan tectonostratigraphic assemblage pacheco pass area east central diablo range california coast ranges the pacheco pass area well known for the relatively high pressure low temperature mineral assemblages developed in a well ordered section of chiefly metaclastic strata was restudied in order to elucidate the geologic paragenetic and textural relationships among the various lithologic units the mapped 7 minute quadrangle extends from 4 km west of the range crest eastward to the ortigalita fault tectonic contact of the franciscan assemblage with coeval great valley strata semicontinuous metachert beds underlain by scraps of na cpx and or na amphibole bearing metabasaltic rock mark the base of terrigenous tectonostratigraphic units evidently decoupled from the downgoing oceanic crust capped plate underplating along newly recognized bedding plane faults has juxtaposed members of an apparently conformable franciscan sedimentary sequence four depositional units each consisting of monotonous metagraywacke and interlayered metashale are stacked within the accretionary section from top down these are tectonostratigraphic members a b c and d unit d differs from overlying members in possessing a greater abundance of bluestone greenstone pods minor serpentinite bodies and rare metaconglomerate lenses varying degrees of stratal disruption indicate that it has been arrested in the transformation to broken formation the ortigalita fault is nearly vertical strikes north northwest and exhibits apparent dextral offset it truncates the east west striking gonzaga fault and other franciscan structures the topographic low occupied by san luis reservoir is a cenozoic tectonic depression sited at a releasing bend on the ortigalita fault newly acquired textural chemical mineralogic and areal relations of albitic and jadeitic pyroxene bearing franciscan metagraywacke near pacheco pass demonstrate that 1 na cpx and na amphibole are metamorphic minerals not clastic materials 2 pumpellyite is a minor neoblastic phase in albite bearing metagraywacke west of pacheco pass and in the north western corner of the quadrangle but it does not occur in jadeitic pyroxene bearing metaclastic rocks 3 higher textural grade metagraywacke specimens generally contain higher modal proportions of jd 4 abundance textural grade and chemistry of na cpx are not related to tectonostratigraphic units and 5 because of sluggish reaction rates albite persisted metastably into the higher pressure jadeitic pyroxene quartz p t field production of na cpx resulted from a reaction of the sort ab chl jd iws qtz gln h 2 o irregular oscillatory zoning within single jadeitic pyroxene prisms and variable jd ab modes for rocks of the same bulk composition reflect growth governed by diffusion rather than recrystallization within a p t transition zone high pressure overstepping of the phase boundary explains variable modal proportions of ab and jd as well as chemical heterogeneity of the jd elevated p attended metamorphism of na cpx bearing rocks exposed in the pacheco pass area as indicated by the wide spread coexistence of quartz and naalsi 2 o 6 rich pyroxene inferred physical conditions of prograde metamorphism were 150 50 c at 7 8 kbar or more franciscan rocks in the east central diablo range underwent subduction zone metamorphism accompanying paleo pacific lithospheric plate descent during mid cretaceous convergence underplating contraction and sequestering of the tectonostratigraphic assembly at depths of 25 30 km during underflow followed by gradual rise took place under conditions of refrigeration due to protracted descent of the oceanic slab paleogene diapiric uplift and extension followed indicating more complete decoupling of the buoyant trench complex from the downgoing plate finally passage of the east pacific rise triple junction in neogene time initiated the present dextral slip regime in the california coast ranges

organic carbon losses during burial and thermal maturation of normal marine shales the percentage of organic carbon lost during the burial diagenesis of normal marine shales those originally deposited in oxygenated bottom waters can be estimated by using the ratio of organic carbon c to pyrite sulfur s and assuming that the mean initial burial value of c s for such sediments has been approximately constant since the devonian plots of mean c s vs vitrinite reflectance r sub 0 for some sedimentary basins of jurassic and carboniferous age show a monotonic decrease of c s with increasing r sub 0 macrofossils

exhumation rates of high pressure metamorphic rocks in subduction channels the effect of rheology 1 exhumation of high pressure metamorphic rocks can take place with typical plate velocities of cm year this is consistent with a model of forced flow in a subduction channel the micro structural record of exhumed metamorphic rocks indicates that stresses are generally too low to drive deformation of the bulk material by dislocation creep according to a power law rheology instead deformation appears to be localized in low strength shear zones and is dominated by dissolution precipitation creep or fluid assisted granular flow implying a newtonian rheology 1d modeling shows that the effective rheology of the material in the subduction channel has a significant influence on the rate of exhumation when the subduction flux either equals or exceeds the return flux the maximum exhumation rate for newtonian behavior of the material is at least twice as high 1 3 of the subduction burial rate compared to that for power law creep 1 6 of the subduction burial rate

faunal evidence for oceanic separations in the palaeozoic of britain the faunal criteria for the recognition of the relative separation of old continents are 1 the contrast between shallow not deep water faunas off opposite shorelines 2 the recognition of deeper water facies surrounding the edges of continents and 3 the recognition of the disposition of ancient climatic belts these criteria are defined and used to assess the relative position of the various parts of what is now britain in the palaeozoic the iapetus ocean is well characterized by faunal differences from the cambrian until the lower ordovician but from caradoc times onwards these differences dwindle until by the latest silurian only the distribution of the ostracods separates the two sides of the ocean on faunal grounds in the early ordovician s british faunas are comparable with those from bohemia france and elsewhere to the s indicating connection with gondwanaland and these faunas differ from those in the baltic area a distribution which we attribute to a true ocean tornquist9s sea between n and s europe tornquist9s sea probably closed in the upper ordovician and from ashgill times onwards the s british and baltic faunas remained similar during the middle and late silurian there is evidence of increasing faunal distinction between britain the baltic area and north america on the one hand and southern europe and gondwanaland on the other indicating an opening rheic ocean which appears to have persisted until late carboniferous times

spatial and temporal evolution of the subducting pacific plate structure along the western pacific margin 1 tomographic images of the subducting pacific plate beneath the izu bonin mariana arc illustrate a progression of geometries from shallow dipping to vertical from north to south along the arc recent advances in technology and inversion techniques have improved resolution of slab structure beneath the western pacific island arcs but reasons for the variation in geometry and morphology are still poorly understood by comparing high resolution tomographic images of the western pacific and updated paleogeographic reconstructions we are able to link the spatiotemporal evolution of the subducting pacific plate back to the mid miocene we have reconstructed tectonic motions along the kurile japan izu bonin mariana arc system and the ryukyu arc to provide an independent additional interpretation of the subducting pacific plate using the most current plate motion data we then investigate the plausibility of our model and three other proposed models based on the interpreted slab structure from tomographic images the new reconstruction agrees with the basic characteristics of former trench retreat models but illustrates the importance of the collision of the ogasawara plateau with the trench in the mid miocene and its subsequent effect on the slab structure at depth and the impact of other aseismic ridge collisions along the plate boundary the combination of evidence in changed physical properties imaged with tomography and the current interpreted slab morphology can be analyzed with the past plate motions to understand subduction zone processes

influences of the buoyancy of partially molten rock on 3 d plume patterns and melt productivity above retreating slabs using 3 d petrological thermo mechanical subduction models we investigate how the buoyancy of partially molten rock affects the development of thermal chemical plumes and melt productivity in the mantle wedge as a first order approximation we limit the positive buoyancy of partially molten rock compared to non molten rock which can be decreased due to rapid melt extraction and removal to the surface our simulations show that a large to moderate density contrast of 200 kg m

geodynamic regimes of intra oceanic subduction implications for arc extension vs shortening processes abstract 40 of the subduction margins of the earth are intra oceanic they show significant variability in terms of extension and shortening we investigated numerically the physical controls of these processes using a 2d petrological thermo mechanical intra oceanic subduction model with spontaneous volcanic arc growth and deformation we varied the fluid and melt related weakening the ages of both the subduction slab and the overriding plate the subducting plate velocities and the cohesive strength of rocks three main geodynamic regimes were identified retreating subduction with opening of a backarc basin stable subduction and advancing compressive subduction the main difference between these regimes is the degree of rheological coupling between plates which is governed by the intensity of rheological weakening induced by fluids and melts retreating subduction regimes require plate decoupling which results from strong weakening due to both fluids and melts spreading centers nucleate either in forearc or in intraarc regions episodic trench migration is often due to variations of plate coupling with time which is caused by fore arc deformation stable subduction regime with little variation in the trench position forms at an intermediate plate coupling and shows a transient behavior from the retreating to advancing modes the advancing subduction regime results from strong plate coupling at the mature stage this subduction mode is associated with both partial fragmentation and subduction of the previously serpentinized forearc region forearc subduction is typically associated with a magmatic pulse which is caused by dehydration of subducted serpentinized forearc fragments our models demonstrate distinct differences in thermal and lithological structure of subduction zones formed in these different geodynamic regimes results compare well with variations observed in natural intra oceanic arcs

chemical and mineralogical analysis of devonian black shale samples from martin county kentucky carroll and washington counties ohio wise county virginia and overton county tennessee u s a abstract core samples of devonian shales from five localities in the appalachian basin have been analyzed chemically and mineralogically the amounts of major elements are similar however the minor constituents organic c s phosphate and carbonate show ten fold variations in amounts trace elements mo ni cu v co u zn hg as and mn show variations in amounts that can be related to the minor constituents all samples contain major amounts of quartz illite two types of mixed layer clays and chlorite in differing quantities pyrite calcite feldspar and kaolinite are also present in many samples in minor amounts dolomite apatite gypsum barite biotite and marcasite are present in a few samples in trace amounts trace elements listed above are strongly controlled by organic c with the exception of mn which is associated with carbonate minerals amounts of organic c generally range from 3 to 6 and s is in the range of 2 5 amounts of trace elements show the following general ranges in ppm parts per million co 20 40 cu 40 70 u 10 40 as 20 40 v 150 300 ni 80 150 high values are as much as twice these values the organic c was probably the concentrating agent and the organic c and sulfide s together created an environment that immobilized and preserved these trace elements closely spaced samples showing an abrupt transition in color also show changes in organic c s and trace element contents several associations exist between mineral and chemical content pyrite and marcasite are the only minerals found to contain sulfide s in general the illite chlorite mixed layer clay mineral shows covariation with organic c if calcite is not present the enriched trace elements are not related to the clay types although the clay and organic matter are intimately associated as the bulk fabric of the rock

rheologic controls on slab dynamics several models have been proposed to relate slab geometry to parameters such as plate velocity or plate age however studies on the observed relationships between slab geometry and a wide range of subduction parameters show that there is not a simple global relationship between slab geometry and any one of these other subduction parameters for all subduction zones numerical and laboratory models of subduction provide a method to explore the relative importance of different physical processes in determining subduction dynamics employing 2 d numerical models with a viscosity structure constrained by laboratory experiments for the deformation of olivine we show that the observed range in slab dip and the observed trends between slab dip and convergence velocity subducting plate age and subduction duration can be reproduced without trench motion i e slab roll back for locations away from slab edges successful models include a stiff slab that is 100 1000 times more viscous than previous estimates from models of plate bending the geoid and global plate motions we find that slab dip in the upper mantle depends primarily on slab strength and plate boundary coupling with a small dependence on subducting plate age once the slab sinks into the lower mantle the primary processes controlling slab evolution are 1 the ability of the stiff slab to transmit stresses up dip 2 resistance to slab descent into the higher viscosity lower mantle and 3 subduction induced flow in the mantle wedge corner

brine pool deposition for the zn pb cu massive sulphide deposits of the bathurst mining camp new brunswick canada ii ocean anoxia during mineralisation abstract current models of massive sulphide ore genesis in the bathurst mining camp new brunswick involve settling of sulphide particles from a stagnating low salinity hydrothermal plume spreading laterally in an anoxic ocean layer with minimal sulphate content there is fragmentary evidence of ocean anoxia in the form of local fine lamination in the shales that host some of the deposits but the total organic carbon s fe u th ni co v ni and v cr relationships indicate deposition under oxic or dysoxic conditions vanadium and mn values range from oxic to anoxic and sulphate reducing to non sulphate reducing but mn may be anomalously low due to derivation by erosion of acidic volcanic rocks the somewhat equivocal physical and chemical data combined with the likely disturbing effects of penecontemporaneous volcanism considerably weaken the case for an anoxic bottom layer in a static ocean the presence of barite with ambient seawater 34 s values in brunswick no 12 ore and the abundance of sulphate in modern euxinic basin waters make a sulphate free layer unlikely even if anoxic sulphate bearing low salinity fluids mixing with seawater would lead to growth of barite bearing chimneys and baritic rubble mounds which are not observed a model involving brine pool deposition better explains the major features of the bathurst ores

geodynamics of trench advance insights from a philippine sea style geometry abstract for terrestrial parameter sets trench retreat is found to be nearly ubiquitous and trench advance quite rare largely due to rheological and ridge push effects recently updated analyses of global plate motions indicate that significant trench advance is also rare on earth being largely restricted to the marianas izu bonin arc thus we explore conditions necessary for terrestrial trench advance through dynamical models involving the unusual geometry of the philippine sea region in this subduction system a slab pull force from distal subduction is transmitted to the overriding plate at the pacific trench our 2d modeling demonstrates that trench advance can occur for terrestrial rheologies in such special geometries we observe persistent trench advance punctuated by two episodes of back arc extension characteristic features of the model such as time interval between extensional episodes high back arc heat flow and stress state of philippine plate correspond to processes recorded in the region

asthenospheric upwelling oceanic slab retreat and exhumation of uhp mantle rocks insights from greater antilles 1 exhumation of garnet bearing peridotites has been associated with low density continental rocks this association does not explain the occurrence of garnetbearing peridotites in the oceanic subduction zone of the greater antilles in hispaniola we use numerical models of intra oceanic subduction to explain exhumation of garnet peridotites without involvement of buoyant continental crust we demonstrate that rheological weakening of the mantle wedge takes place due to its strong hydration during subduction of serpentinized slow spreading ridge this weakening triggers upwelling of the hydrated peridotites and partially molten peridotites followed by upwelling of hot asthenosphere and subsequent retreat of the subducting slab according to numerical modelling of p t paths this process can explain exhumation of uhp 4gpa rocks in an intra oceanic setting citation gorczyk w s guillot t v gerya and k hattori 2007 asthenospheric upwelling oceanic slab retreat and exhumation of uhp mantle rocks insights from greater antilles geophys res lett 34 l21309 doi 10 1029 2007gl031059

revised age of aleutian island arc formation implies high rate of magma production radioisotopic dating of subaerial and submarine volcanic and plutonic rocks from the aleutian island arc provides insight into the timing of arc formation in the middle eocene twenty eight 40 ar 39 ar ages constrain the duration of arc magmatism to the last 46 m y basaltic lavas from the finger bay volcanics the oldest exposed rocks in the arc gave an isochron age of 37 4 0 6 ma which is 12 17 m y younger than a widely cited age of 55 50 ma three main pulses of arc wide magmatism occurred at 38 29 16 11 and 6 0 ma which coincide with periods of intense magmatism in other western pacific island arcs using the geochronology and volumetric estimates of crust generated and eroded over the last 46 m y we calculate a time averaged magma production rate for the entire arc that exceeds previous estimates by almost an order of magnitude