structure of the vakhsh fold and thrust belt tadjik ssr geologic mapping on a landsat image base an enhanced landsat image of the northeastern tadjik depression in the soviet union was combined with field data to produce a 1 250 000 geologic map of the vakhsh fold and thrust belt and adjacent autochthon mapping of the relatively continuous geologic structure in this region first required recognizing marker beds and contacts at or near the resolution of the landsat sensor and then defining mappable units satellite image mapping was complemented by field study which was necessary in this recently deformed area both to verify the image mapping and to obtain more accurate structural dips deformation of the cretaceous and tertiary strata of the fold and thrust belt is in the western half of the mapped area conspicuously absent north of a buried basement fault that marks the hinge zone of the late mesozoic passive margin in the eastern half of the mapped area however the major thrusts displacing rocks northward have moved coherent sheets over the block fault these sheets now lie flat atop the autochthon to the north tectonically these observations indicate that 1 the crustal structure inherited from the mesozoic extensional phase has strongly influenced the late cenozoic pattern of deformation 2 the thrust sheets that are displaced over the paleo hinge zone actively load the basement rocks to the north and 3 the development of the fold and thrust belt which is markedly asymmetric has included the progressive overlapping of thrusts the later thrusts apparently formed internal to the older thrusts and subsequently overrode them

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

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

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

transient drying during the paleocene eocene thermal maximum petm analysis of paleosols in the bighorn basin wyoming abstract the paleocene eocene thermal maximum petm was a transient episode of global warming that occurred at the paleocene eocene boundary the event is marked by a sharp decline in carbon isotope values that is globally recognized from marine and continental sections the negative isotope excursion has been attributed to the release of methane hydrates from oceanic sediments although studies concur that temperatures rose during the petm changes in precipitation are not well understood alluvial paleosols in the bighorn basin that span the petm interval contain a continuous and highly resolved record of climate including information on precipitation they show a significant but transient decrease in precipitation at the onset of the petm but a gradual return to pre petm levels by the end of the interval the paleosols also show additional although less dramatic wet dry cycles within the petm interval that may correspond to precessional cycles that have been identified in the marine record of the petm this study counters interpretations of increased precipitation for wyoming at this time and shows the importance of detailed case studies of continental strata to test climatic generalizations and models that have been developed for petm precipitation patterns

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

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

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

pedogenic and diagenetic influences on void coating formation in lower cretaceous paleosols of the mill creek formation southwestern alberta canada abstract pedogenic and diagenetic influences on void coating formation were studied in two paleosols from the lower cretaceous albian mill creek formation in southwestern alberta canada using detailed micromorphological mineralogical and geochemical methods the paleosols consist of alternating decimetre scale beds of red grey green and variegated mudstone intermediate horizons red and grey green variegated mudstones contain associations of pale yellow clay coatings dark red clay coatings fe depletion hypo coatings and fe oxide quasi coatings sem edx ray analyses indicate that the dark reddish coatings are dominated by si al and fe with subordinate amounts of k na ca mg and ti pale yellow clay coatings have a higher si content and lower al k na ca and fe contents relative to reddish coatings within the paleosol profiles the distribution of clay minerals indicates 1 an increase in discrete clay minerals downward 2 a decrease in mixed layer illite smectite downward and 3 an increase in chlorite upward x ray diffraction indicates that the pale yellow and dark red clay void coatings are polymineralic pale yellow clay coatings have a higher proportion of mixed layer illite smectite and kaolinite less chlorite and are less crystalline than dark reddish coatings despite a diagenetic overprint including development of diffusion gleyans degradation of organic matter burial reddening and probably some smectite illitization both compositional and morphological gradients exist in the paleosol profiles and void coatings these gradients are most likely related to depositional variations and to pedogenic development in the original soils under variable eh ph conditions in the presence of organic matter therefore although the void coatings are altered diagenetic features their morphology mineralogy and perhaps even some of their microchemistry is attributable to the presence of pedogenic precursors initially formed in soils similar to modern temperate alfisols and glossic soils

india eurasia collision chronology has implications for crustal shortening and driving mechanism of plates the motion of the indian plate is determined in an absolute frame of reference and compared with the position of the southern margin of eurasia deduced from palaeomagnetic data in tibet the 2 600 900 km of continental crust shortening observed is shown to have occurred in three different episodes subduction of continental crust intracontinental thrusting and internal deformation and lateral extrusion the detailed chronology of the collision and plate reorganizations in the indian and pacific oceans supports the hypothesis that slab pull is a dominant driving mechanism of plate tectonics

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

micromorphology of seasonally wet soils on the illinoian till plain u s a abstract two seasonally wet soils from southwestern ohio in the north central region of the u s a were studied in order to relate micromorphology to pedogenic processes associated with fluctuating oxidation reduction conditions the soils formed in approximately 70 cm of late winconsinian loess overlying a composite paleosol developed in early wisconsinian loess and illinoian till alternating oxidation reduction conditions in the upper 150 cm were accompanied by abundant albic neoskeletans fe mn nodules and partially decomposed grainy argillans conditions were most reduced along saturated macropores between ped faces in about the upper 80 cm where thick albic neoskeletans occurred horizons from 50 80 cm were extensively degraded and were characterized by many fe mn nodules and gray eluvial zones albic neoskeletans surrounding islands of brown sesquioxidic material having inundulic plasmic fabrics argillic horizon degradation is one of the most important genetic processes in these soils embedded grain ferriargillans enriched in goethite were abundant in the lower paleosols from approximately 150 250 cm neogoethans and neomangans were evident at a depth of approximately 2 m which is the lowest point to which the water table falls in these soils each year

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

genesis morphology and mineralogy of planosols developed from different parent materials in southern brazil abstract the genesis of planosols is a complex process since it can involve multiple cycles of soil formation deposition of allochthonous materials and also past climate changes furthermore this subject is still poorly studied in the humid tropics thus hindering a proper appraisal of the potentials and limitations of planosols the objective of this research was to study pedogenetic processes and properties of planosols formed from different parent materials in southern brazil four soils were sampled p1 hypereutric luvic albic planosol developed on lagoon plain sediments p2 orthoeutric luvic albic planosol on holocene alluvial deposits p3 hypereutric luvic albic planosol on granite and p4 orthoeutric luvic albic planosol on gneiss all planosols presented sandy horizons a and e with low ph

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

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

use of micromorphology for palaeoenvironmental interpretation of complex alluvial palaeosols an example from the mill creek formation albian southwestern alberta canada abstract field observations are often not sufficient for process based interpretations of palaeosols particularly where they form parts of thick aggradational pedocomplexes within alluvial successions under such conditions micromorphology provides genetic temporal and spatial information on soil forming processes that is critical to an understanding of past environmental conditions thick alluvial successions of the albian mill creek formation contain abundant evidence of pedogenesis but few well developed palaeosol profiles and therefore provide an ideal case study in which to demonstrate the usefulness of micromorphological data for palaeoenvironmental interpretation the micromorphological features of greatest interpretive value are types of clay coatings and ferruginous segregations structure and fabric papules evidence of bioactivity and ferruginous concretions provide information on geomorphic surface stability and assist in reconstructing temporal changes in drainage conditions while individual features can provide some palaeoenvironmental information the relationships of features to one another and assemblages of features provides additional information when analysed hierarchically to establish a sequence of sedimentologic and pedogenic events a common recurring sequence of palaeoenvironmental events subject to local variations can be recognized throughout the mill creek formation the presence of illuvial clay requires that water percolated through the soil and that the soil periodically dried out so that the translocated clay was retained dark reddish clay coatings indicate clay illuviation under freely drained conditions while pale yellow and silty clay coatings suggest that phases of free drainage alternated with phases of poorly drained or saturated soil conditions the presence of iron depletion coatings iron nodules and quasiferrans indicates that these units were at least periodically saturated and the occurrence of multiple overlapping phases within single thin sections demonstrates that redox conditions fluctuated strongly suggesting development in the vadose zone recent soils containing similar assemblages of features develop under warm temperate seasonal climates alternating phases of well drained and saturated conditions on the mill creek floodplains are attributed to changing sediment supply and local palaeogeomorphology rather than to any major regional climate change this type of process based micromorphological analysis should have broad application in other complex pedogenically modified alluvial successions and similar studies would lead to a more detailed understanding of ancient palaeoenvironments

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

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

earthquake focal mechanisms deformation state and seismotectonics of the pamir tien shan region central asia this paper presents a review of the seismotectonics of the pamir tien shan collision zone in the garm region tajikistan based on geological structure seismicity and focal mechanism solutions the region is dominated by horizontal compression manifested by imbricate low angle thrust faults that separate the upper crust into a series of tectonic sheets these thrust systems verge northward from the tajik depression toward the southern tien shan and southward toward the northern pamir the pattern of seismicity across the region suggests that similar low angle thrust faults exist within the crystalline basement as well in order to reconstruct the present day stress strain state of the region we used data from over 15 000 earthquake focal mechanism solutions for small earthquakes m 1 0 gathered over the 27 year period 1963 1989 the method of reconstruction of the seismotectonic deformation std field involves dividing the data set into small spatial windows and summing the individual focal mechanism solutions to form an average earthquake mechanism tensor for each cell the std state of that cell can then be represented by the orientation of the principal axes of compression and tension the intensity or relative uniformity of the average mechanism and the lode nadai coefficient which defines the relation between the magnitudes of the principal strain components we observe throughout the study area a general predominance of subhorizontal compression manifested in a mixture of thrust and strike slip deformation normal fault deformation is observed in only two small portions of the study area the orientation of principal compression varies from nearly n s in the northern pamir and the southern tien shan to nw se in the peter the first range in general the std field has a clearly developed mosaic structure defined by spatial groupings of nearly uniform orientations of compression and tension axes the boundaries of these groups cannot in most cases be directly correlated with known geological contacts the std structure also shows some variation with depth with more or less uniform std orientation in the upper crust 0 14 km depth changing significantly at greater depths examination of various subsets of the focal mechanism catalog shows the spatial structure of the std field to be largely stable with respect to both time and magnitude the reliability of the reconstructed std field was verified using three approaches 1 comparison of individual focal mechanism determinations for a large subset of the data using both soviet and u s algorithms 2 comparison of std reconstruction results using the two independent focal mechanism catalogs and 3 examination of the effect of spatial sampling on the results the dominance of generally north oriented subhorizontal compression is interpreted to be primarily the result of convergence between the pamir and tien shan ranges in turn caused by the ongoing collision of india and eurasia

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

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

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

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

morphology and weathering trends of the sangamon soil complex in south central indiana in relation to paleodrainage abstract three paleodrainage groups are proposed for profiles of the farmdale sangamon soil complex in south central indiana the groups good intermediate and poor are differentiated on the basis of matrix colors and color patterns genetic support for the groupings is provided by morphological trends still evident following 100 000 yr of pedogenesis and burial by late wisconsinan loess depth of carbonate leaching solum thickness and argillic horizon thickness all decrease with poorer drainage clay mineralogy also reflects paleodrainage illite degradation is intense in all profiles but profiles with good drainage have poorly crystalline interstratified expandable minerals while well crystalline smectites dominate in profiles with poor drainage remanent aggregation in former a horizons is stronger in more poorly drained profiles while the effects of structural overprinting from the modern soil increase with better modern drainage soil morphology mineralogy and parent material paleolandscape position of sangamon profiles are all internally consistent with color development under soil hydrological conditions during the last interglacial to glacial transition the occurrence frequency of each drainage group indicates that the sangamon soilscape was better drained than now and morphology and clay mineral evidence suggests that areas with poor drainage did not experience the extreme seasonal groundwater shifts that the modern landscape experiences

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

paleohydrologic response to continental warming during the paleocene eocene thermal maximum bighorn basin wyoming abstract geologically rapid global warming occurred during the paleocene eocene thermal maximum petm 56 ma several studies have argued that important changes occurred in the hydrological cycle during the petm but results have been inconsistent ranging from global increases in humidity to drier conditions changes in paleosols during the petm in the southeastern bighorn basin document major drying during the body of the event paleosol changes also suggest transitional episodes of climate change that both preceded and followed the petm qualitative semi quantitative and fully quantitative analyses of a 70 m thick interval of paleosols provide a high resolution record of changes in soil moisture and precipitation those changes are compared to changes in temperature determined from 18o values of tooth enamel from the mammal coryphodon a distinct shift to drier soils occurred just prior to the petm a conclusion that is consistent with previous observations that warming began before the onset of the negative carbon isotope excursion associated with the petm paleosols show a progressive drying trend into the lower part of the petm and become even drier in the upper part of the body of the petm purple red paleosols that appear during the recovery phase of the petm indicate wetter soils although they are better drained than paleosols below the onset the purple red paleosols continue for 15 m above the recovery and indicate that wetter soil conditions persisted after the recovery it is not clear whether changes in the paleosols that preceded and followed the petm reflect global forcing factors like orbital cycles or release of carbon that lacks an isotopic label however such mechanisms would provide a unifying explanation for shifts seen in continental and marine environments

evaluating the use of weathering indices for determining mean annual precipitation in the ancient stratigraphic record abstract different quantitative methods have been developed to determine paleo rainfall using paleosols of particular interest are methods that use bulk geochemistry to calculate a weathering index and from that mean annual precipitation twenty three paleosols that formed during a time of significant climate change the paleocene eocene thermal maximum petm were analyzed to evaluate the relative merits of two geochemical proxies that have been developed for estimating paleoprecipitation one depends on the chemical index of alteration without potash cia k and the other uses the calmag weathering index which depends on cao and mgo the paleosols are located in the paleogene fort union and willwood formations in the bighorn basin wy morphologic differences among the paleosols indicate that they formed under varied soil moisture conditions a morphology index was developed from pedogenic features that are sensitive to soil drainage and thus to rainfall to assess that variability quantitative mean annual precipitation map values were calculated for each paleosol using the cia k index and the calmag index which was developed specifically for vertisols those results were compared to the morphology index for 23 paleosols the results indicate that increases and decreases in map calculated from both methods correlate relatively well to changes in the soil morphology index however the results from the calmag weathering index show a stronger correlation the results suggest that the calmag proxy provides a more robust map estimate for vertisols in addition to comparing map estimates to the soil morphology index we compared the map values to paleorainfall estimates determined from paleofloras at the same stratigraphic levels as three of the paleosols that comparison confirms that the calmag method is reliable for reconstructing map from ancient vertisols the results also show that in the bighorn basin vertisols with a b horizon

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

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

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

faulting associated with large earthquakes and the average rate of deformation in central and eastern asia we determine approximate average rates of deformation in portions of eastern and central asia for the last 80 years these rates are based on estimates of source dimensions average displacements seismic moments and orientations of faults of all large earthquakes m 7 5 and as many other major earthquakes m 7 0 as possible in asia since 1900 a major part of this study is the compilation of these data for separate parts of asia we combine seismic moment tensors to estimate the average rotational strain or deformation tensors for these regions for the last 80 years we estimate that shortening across the tien shan has occurred at 11 mm yr between 5 and 19 mm yr left lateral shear on easterly trending planes in mongolia in northwestern tibet gansu and ningxia and on the xianshuihe fault in western sichuan has been very rapid more than 10 mm yr in each region and apparently as rapid as 40 mm yr in northwestern tibet gansu and ningxia slower rates of a few to 10 mm yr appear to characterize east west extension and conjugate north south shortening of tibet the earthquake history of the himalaya is consistent with rates of underthrusting of more than 10 mm yr but is probably less than the convergence rate of india with eurasia of about 50 mm yr the overall strain field is consistent with a large part of india s penetration into eurasia being absorbed by the extrusion of material out of india s way as a result southeast china moved at about 21 mm yr east southeast with respect to eurasia in the last 80 years with an uncertainty of about a factor of 2 in this estimated rate

significance of different modes of rhizolith preservation to interpreting paleoenvironmental and paleohydrologic settings examples from paleogene paleosols bighorn basin wyoming u s a abstract different modes of preservation of root traces rhizoliths provide information on soil moisture regimes in alluvial paleosols in the paleogene fort union and willwood formations of the bighorn basin wyoming u s a this paper links different styles of rhizolith preservation to paleosols whose other pedogenic attributes provide information on ancient soil drainage consequently the paleodrainage significance of different rhizolith preservation patterns is established more confidently rhizoliths consisting of elongate gray mottles rhizohaloes with red rims are common in relatively well drained red paleosols the gray zones are depleted of iron fe whereas the color of the rims indicates hematite accumulation these features are typical of surface water gley processes that caused fe and manganese mn to move from the root channel outward to the soil matrix calcareous rhizocretions either calcareous tubular concretions or micro accumulations of carbonate within gray rhizotubules are also common in moderately well drained red paleosols more poorly drained purple paleosols also have rhizoliths consisting of fe depletion zones however the rhizohaloes are surrounded by yellow brown goethite rims rather than red hematitic rims in many paleosols the red and yellow brown accumulation rims were partly removed by continued depletion producing abundant irregular shaped mottles the nature of the rhizohalo and the color of the fe oxide rim provide information about the intensity of gleying that a paleosol underwent the poorly drained paleosols also show rhizoliths preserved as goethite accumulations contained within gray depletion zones indicating movement of reduced fe from the matrix towards the root channel probably as a result of groundwater gley processes the goethite accumulations are intermingled locally with lignite representing preservation of the root organic material some rhizoliths are preserved as tube shaped concentrations of small 0 1 0 2 mm diameter black spheres whose color indicates a mix of mn and fe oxides these are most common in paleosols with low chroma gray matrix colors indicating very poor drainage the very poorly drained paleosols also show rhizoliths preserved in jarosite which is an oxidation product of pyrite careful observational and geochemical analysis of rhizoliths which are common in continental deposits can help produce a clearer and more thorough interpretation of ancient drainage conditions information about degree of ancient soil wetness or moisture is important for understanding past climatic conditions and for reconstructing terrestrial paleolandscapes

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

genesis and relationship of macromorphology and micromorphology to contemporary hydrological conditions of a welded argixeroll from the palouse in idaho abstract soils formed in distal loess of the palouse region have welded profiles that make assessment of their genesis and pedostratigraphic relationships difficult these soils typically have a mollic cambic horizon sequence that is underlain by a well expressed albic e horizon and a strongly developed argillic horizon in this study we utilized hydrological monitoring combined with micromorphological analysis and new stratigraphic data to help establish the genesis of a representative argixeroll of the region and the relationship of its morphology to ancient and contemporary hydrological conditions monitoring results indicate that the argixeroll is seasonally episaturated for approximately 5 6 months per year ferrimanganiferous nodules segregations hypocoatings and depletion mottles are observed in thin section throughout the pedon but dominate within just above and below the present day e horizon measured soil eh is sufficiently low for fe and mn reduction to occur for 1 3 months during the period of episaturation suggesting that at least some of these redoximorphic features are likely to be contemporary other compound redoximorphic features are clearly relict however as they are coated by illuvial clay which is unaffected by fe mn redistribution processes thick illuvial clay coatings on interpedal fissures of the argillic horizon postdate pockets of holocene tephra within deeply bioturbated aggregates while other channel argillans in the same part of the pedon are clearly older this indicates that at least two distinct phases of clay accumulation have occurred the most recent being contemporaneous with loess aggradation during the holocene our results suggest that these welded profiles consist of a mollic cambic horizon sequence that has formed primarily in holocene loess and an albic argillic sequence that has formed in wisconsinan loess the modern day e horizon was initiated as the clay eluvial surface horizon of the regionally recognized washtucna paleosol that continued forming into the late wisconsinan cold phase and upon burial by holocene loess it has been transformed into an albic horizon and even enlarged as a result of redoximorphic processes associated with seasonal saturation

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

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