continental rifting as a function of lithosphere mantle strength abstract the role of the uppermost mantle strength in the pattern of lithosphere rifting is investigated using a thermo mechanical finite element code in the lithosphere the mantle crust strength ratio s m s c that decreases with increasing moho temperature t m allows two strength regimes to be defined mantle dominated s m s c and crust dominated s m s c the transition between the two regimes corresponds to the disappearance of a high strength uppermost mantle for t m 700 c 2d numerical simulations for different values of s m s c show how the uppermost mantle strength controls the style of continental rifting a high strength mantle leads to strain localisation at lithosphere scale with two main patterns of narrow rifting coupled crust mantle at the lowest t m values and deep crustal decollement for increasing t m values typical of some continental rifts and non volcanic passive margins the absence of a high strength mantle leads to distributed deformations and wide rifting in the upper crust these numerical results are compared and discussed in relation with series of classical rift examples

integration of ice core marine and terrestrial records for the australian last glacial maximum and termination a contribution from the oz intimate group the degree to which southern hemisphere climatic changes during the end of the last glacial period and early holocene 30 8 ka were influenced or initiated by events occurring in the high latitudes of the northern hemisphere is a complex issue there is conflicting evidence for the degree of hemispheric teleconnection and an unresolved debate as to the principle forcing mechanism s the available hypotheses are difficult to test robustly however because the few detailed palaeoclimatic records in the southern hemisphere are widely dispersed and lack duplication here we present climatic and environmental reconstructions from across australia a key region of the southern hemisphere because of the range of environments it covers and the potentially important role regional atmospheric and oceanic controls play in global climate change we identify a general scheme of events for the end of the last glacial period and early holocene but a detailed reconstruction proved problematic significant progress in climate quantification and geochronological control is now urgently required to robustly investigate change through this period copyright c 2006 john wiley sons ltd

the energy source for the coastal trapped waves in the australian coastal experiment region abstract the sea level on the southern australian coast is examined for the source of the coastal trapped wave energy observed during the australian coastal experiment sea level adjusted for atmospheric pressure and atmospheric pressure are observed to propagate eastward at about 10 m s 1 at the lowest frequency examined 24 day period some energy travels south along the west coast of tasmania but does not reach the east coast of mainland australia while some energy travels through bass strait to reach the east coast of mainland australia at the most energetic frequency 8 day period adjusted sea levels are coherent over the 3700 km of coastline from southern australia to the east coast and much of the wind forced coastal trapped wave energy appears to travel through bass strait to the mainland east coast we have not identified a mechanism for energy transfer through bass strait and we do not know what fraction of the coastal trapped wave energy incident on western bass strait actually reach

porous fluid flow enables oceanic subduction initiation on earth 1 although most of the presently active intra oceanic subduction zones are relatively young and initiated during the cenozoic subduction initiation process remains poorly understood previous models of subduction initiation assumed excessive weakening of tectonic plate boundaries that does not reconcile with laboratory rock strength measurements the weakening was assumed to be caused by fluids present along tectonic fractures however no self consistent solid fluid model of subduction initiation has been developed so far here we present new numerical hydro thermo mechanical model of spontaneous intra oceanic subduction initiation where solid rock deformation and fluid percolation are fully coupled based on 2 d numerical experiments we demonstrate that although subduction fails to initiate under fluid absent conditions it can naturally start when porous fluid is present inside oceanic crust and along the plate boundaries fluid percolation is localized along spontaneously forming faults where high fluid pressure compensates lithostatic pressure thus dramatically decreasing friction along the incipient subduction zone through the parametric study we conclude that the most important parameter for subduction initiation is the solid matrix permeability paradoxical at first lowering the permeability indeed favors subduction initiation by maintaining high fluid pressure and thus decreasing friction along active faults

coastal upwelling by wind driven forcing in jervis bay new south wales a numerical study for 2011 abstract the princeton ocean model pom was used to investigate an upwelling event in jervis bay new south wales se australia with varying wind directions and strengths the pom was adopted with a downscaling approach for the regional ocean model one way nested to a global ocean model the upwelling event was detected from the observed wind data and satellite sea surface temperature images the validated model reproduced the upwelling event showing the input of bottom cold water driven by wind to the bay its subsequent deflection to the south and its outcropping to the surface along the west and south coasts nevertheless the behavior of the bottom water that intruded into the bay varied with different wind directions and strengths upwelling favorable wind directions for flushing efficiency within the bay were ranked in the following order n 0 northerly nne 30 northeasterly nw 315 northwesterly ne 45 northeasterly ene 60 northeasterly increasing wind strengths also enhance cold water penetration and water exchange it was determined that wind driven downwelling within the bay which occurred with nne ne and ene winds played a key role in blocking the intrusion of the cold water upwelled through the bay entrance a northerly wind stress higher than 0 3 n m 2 was required for the cold water to reach the northern innermost bay

a modeling study of the climatological current field and the trajectories of upwelled particles in the east australian current the climatological current field off the coast of new south wales australia is investigated using results from a field experiment and a diagnostic numerical modeling study in particular the flow dynamics are examined near smoky cape 308559s in the vicinity of the east australian current separation point modeled velocity fields compare favorably with observed velocity measurements that were obtained in this region during november 1998 an investigation of the role of advection in the alongshore momentum equation reveals that uvx and vvy change sign about the separation point representing a shift in the flow regime from onshore advection and southward acceleration north of the separation to seaward advection and weaker southward currents south of the separation bottom stress increases on the continental shelf downstream of the separation point coincident with high horizontal divergence throughout the region south of smoky cape the near bottom flow is characterized by a low burger number coincident with a subcritical richardson number these characteristics imply that the timescale in which bottom boundary layer shutdown occurs will be lengthened enabling a persistent flow of potentially nutrient rich water into the coastal region a series of lagrangian particle tracking experiments designed to investigate the fate of the upwelled water are presented motivated by an observed chlorophyll a maximum along the 25 25 su isopycnal water of this density is used as a proxy for nutrient enriched water it is found that upwelled water originates at depth to the north of smoky cape and at times well offshore south of smoky cape coastal recirculations are evident suggesting that upwelled water could be trapped in eddy fields adjacent to the coast and thus explaining why phytoplankton blooms are prevalent in the region south of smoky cape

rapid conversion of an oceanic spreading center to a subduction zone inferred from high precision geochronology where and how subduction zones initiate is a fundamental tectonic problem yet there are few well constrained geologic tests that address the tectonic settings and dynamics of the process numerical modeling has shown that oceanic spreading centers are some of the weakest parts of the plate tectonic system gurnis m hall c lavier l 2004 geochem geophys geosys 5 q07001 but previous studies have not favored them for subduction initiation because of the positive buoyancy of young lithosphere instead other weak zones such as fracture zones have been invoked because these models differ in terms of the ages of crust that are juxtaposed at the site of subduction initiation they can be tested by dating the protoliths of metamorphosed oceanic crust that is formed by underthrusting at the beginning of subduction and comparing that age with the age of the overlying lithosphere and the timing of subduction initiation itself in the western philippines we find that oceanic crust was less than 1 my old when it was underthrust and metamorphosed at the onset of subduction in palawan philippines implying forced subduction initiation at a spreading center this result shows that young and positively buoyant but weak lithosphere was the preferred site for subduction nucleation despite the proximity of other potential weak zones with older denser lithosphere and that plate motion rapidly changed from divergence to convergence

rheological stratification of the lithosphere abstract the rheology of the lithosphere is estimated on the basis of pore pressure dependent frictional failure in the brittle regime and power law steady state creep in the ductile regime different petrological models of the continental and oceanic lithosphere combined with different geotherms and thicknesses are used to generate models covering a variety of geodynamic situations results show that the depth dependence of lithospheric rheology varies with the tectonic province and that in many instances the continental lithosphere has one at the bottom of the crust or two the previous one plus another at mid crustal levels soft ductile layers sandwiched between brittle layers the depth distribution of geophysical parameters seismicity seismic wave velocity and electrical conductivity matches the theological predictions well the lithospheric rheological profiles are used to analyze observed variations in the structural style of deformed continental margin and oceanic terranes in the southeastern canadian cordillera the large scale characteristics of the system show a satisfactory agreement with the inferred rheological structure sub horizontal decollements should be the rule rather than the exception where the lithospheric rheology is strongly stratified

initiation of subduction zones along transform and accreting plate boundaries triple junction evolution and forearc spreading centres implications for ophiolitic geology and obduction

petrological evidence for stepwise accretion of metamorphic soles during subduction infancy semail ophiolite oman and uae metamorphic soles are tectonic slices welded beneath most large scale ophiolites these slivers of oceanic crust metamorphosed up to granulite facies conditions are interpreted as forming during the first million years of intra oceanic subduction following heat transfer from the incipient mantle wedge towards the top of the subducting plate this study reappraises the formation of metamorphic soles through detailed field and petrological work on three key sections from the semail ophiolite oman and united arab emirates based on thermobarometry and thermodynamic modelling it is shown that metamorphic soles do not record a continuous temperature gradient as expected from simple heating by the upper plate or by shear heating as proposed in previous studies the upper high temperature metamorphic sole is subdivided in at least two units testifying to the stepwise formation detachment and accretion of successive slices from the down going slab to the mylonitic base of the ophiolite estimated peak pressure temperature conditions through the metamorphic sole from top to bottom are 850 c and 1 gpa 725 c and 0 8 gpa and 530 c and 0 5 gpa these estimates appear constant within each unit but differing between units by 100 to 200 c and 0 2 gpa despite being separated by hundreds of kilometres below the semail ophiolite and having contrasting locations with respect to the ridge axis position metamorphic soles show no evidence for significant petrological variations along strike these constraints allow us to refine the tectonic petrological model for the genesis of metamorphic soles formed via the stepwise stacking of several homogeneous slivers of oceanic crust and its sedimentary cover metamorphic soles result not so much from downward heat transfer ironing effect as from progressive metamorphism during strain localization and cooling of the plate interface the successive thrusts originate from rheological contrasts between the sole initially the top of the subducting slab and the peridotite above as the plate interface progressively cools these findings have implications for the thickness the scale and the coupling state at the plate interface during the early history of subduction obduction systems this article is protected by copyright all rights reserved

an expression of philippine sea plate rotation the parece vela and shikoku basins abstract the philippine sea plate located between the pacific eurasian and australian plates is the world s largest marginal basin plate the motion of the philippine sea plate through time is poorly understood as it is almost entirely surrounded by subduction zones and hence previous studies have relied on palaeomagnetic analysis to constrain its rotation we present a comprehensive analysis of geophysical data within the parece vela and shikoku basins two oligocene to miocene back arc basins which provide independent constraints on the rotational history of the philippine sea plate by means of their seafloor spreading record we have created a detailed plate model for the opening of the parece vela and shikoku basins based on an analysis of all available magnetic gravity and bathymetric data in the region subduction along the izu bonin mariana trench led to trench roll back arc rupture and back arc rifting in the parece vela and shikoku basins at 30 ma seafloor spreading in both basins developed by chron 9o 28 ma and possibly by chron 10o 29 ma as a northward and southward propagating rift respectively the spreading orientation in the parece vela basin was e w as opposed to ene wsw in the shikoku basin the spreading ridges joined by chron 6by 23 ma and formed a r r r triple junction to accommodate the difference in spreading orientations in both basins at chron 6no 20 ma the spreading direction in the parece vela basin changed from e w to ne sw at chron 5ey 19 ma the spreading direction in the shikoku basin changed from ene wsw to ne sw this change was accompanied by a marked decrease in spreading rate cessation of back arc opening occurred at 15 ma a time of regional plate reorganisation in se asia we interpret the dramatic change in spreading rate and direction from e w to ne sw at 20 1 3 ma as an expression of philippine sea plate rotation and is constrained by the spacing between our magnetic anomaly identifications and the curvature of the fracture zones this rotation was previously thought to have begun at 25 ma as a result of a global change in plate motions our results suggest that the philippine sea plate rotated clockwise by about 4 between 20 and 15 ma about a pole located 35 n 84 e this implies that the majority of the 34 clockwise rotation inferred to have occurred between 25 and 5 ma from paleomagnetic data may have in fact been confined to the period between 15 and 5 ma

on the initiation of subduction estimates of shear resistance associated with lithospheric thrusting and convergence represent lower bounds on the force necessary to promote trench formation three environments proposed as preferential sites of incipient subduction are investigated passive continental margins transform faults fracture zones and extinct ridges none of these are predicted to convert into subduction zones simply by the accumulation of local gravitational stresses subduction cannot initiate through the foundering of dense oceanic lithosphere immediately adjacent to passive continental margins the attempted subduction of buoyant material at a mature trench can result in large compressional forces in both subducting and overriding plates this is the only tectonic force sufficient to trigger the nucleation of a new subduction zone the ubiquitous distribution of transform faults and fracture zones combined with the common proximity of these features to mature subduction complexes suggests that they may represent the most likely sites of trench formation if they are even marginally weaker than normal oceanic lithosphere

mantle flow as a trigger for subduction initiation a missing element of the wilson cycle concept the classical wilson cycle concept describing repeated opening and closing of ocean basins hypothesizes spontaneous conversion of passive continental margins into subduction zones this process however is impeded by the high strength of passive margins and it has never occurred in cenozoic times here using thermomechanical models we show that additional forcing provided by mantle flow which is induced by neighboring subduction zones and midmantle slab remnants can convert a passive margin into a subduction zone models suggest that this is a long term process thus explaining the lack of cenozoic examples we speculate that new subduction zones may form in the next few tens of millions of years along the argentine passive margin and the u s east coast mantle suction force can similarly trigger subduction initiation along large oceanic fracture zones we propose that new subduction zones will preferentially originate where subduction zones were active in the past thus explaining the remarkable colocation of subduction zones during at least the last 400 myr

geodynamic mechanism and classification of basins in the earth system abstract the sedimentary basin is a basic geological unit in the earth system its formation and evolution are controlled mainly by plate tectonics and geodynamics which have produced a variety of basin types the development and evolution of various basins depend not only on the tectonic effect of lithospheric plate but also on sublithospheric and earth surface actions basin classification schemes based on the theories of platform geosyncline or plate tectonics could neither include all basin types nor reflect the complex processes of basin formation and evolution from the viewpoint of earth system we proposed a new basin classification scheme based on geodynamic mechanism that is lithospheric plate sublithospheric and earth surface actions in our geodynamic classification we employed a series of geodynamic settings of basin formation and evolution including the lithospheric plate sublithospheric and earth surface environments the dynamic regimes of basin formation and evolution such as tectonic setting extension compression and shearing the thermal processes and gravity effects the various natures of basin basement and crustal types continental oceanic or transitional crust and the history of basin sedimentary filling and tectonic paleogeography successively as the controlling factors for basin classification this new geodynamic classification scheme highlights both the role of the lithosphere and the effects of the sublithospheric and the earth surface actions on controlling the basin formation and evolution the basic dynamic types of basin classification include the prototype basin forming under a single tectonic thermal or gravity regime and the superimposed basin as well as residual basin forming under multistage thermal and or tectonic regimes thus this classification scheme could provide an evolving understanding of lithosphere crust mantle interaction and geodynamic processes as well as of hydrocarbon accumulation and metallogenic mechanisms in sedimentary basins and would help in the synthetic exploration of various resources

on the east australian current variability encroachment and upwelling 1 observations from an intensive oceanographic field program which took place in 1998 1999 about the separation point of the east australian current eac show significant spatial and temporal variability of the eac upstream of the separation point southward flowing currents are strong with subinertial velocities of up to 130 cm s 1 in the near surface waters whereas downstream currents are highly variable in both strength 1 70 cm s 1 and direction upwelling is observed to occur through both wind driven and current driven processes with wind effects playing a lesser role by contrast the encroachment of the eac upon the coast has a profound effect on the coastal waters accelerating the southward alongshore currents and decreasing the temperature in the bottom boundary layer bbl by up to 5 degreesc as the axis of the jet moves onshore negative vorticity increases in association with an increase in nonlinear acceleration during this time bottom friction is increased the burger number is reduced and the bbl shut down time lengthens the observed upwelling is attributed to enhanced onshore ekman pumping through the bbl resulting from increased bottom stress as the southerly flow accelerates when the eac encroaches across the continental shelf

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

shear heating induced lithospheric scale localization does it result in subduction abstract even though it is a well established fact that the earth is currently in a plate tectonics mode the question on how to break lithospheric plates and initiate subduction remains a matter of debate here we focus on shear heating as a potential mechanism to cause lithospheric shear localization and subsequent subduction initiation in oceanic plates it is shown that shear heating under some conditions i facilitates the formation of a lithospheric scale shear zone and ii is capable of stabilizing a lithospheric scale shear zone thus creating the necessary condition for subduction initiation to occur furthermore we demonstrate that not only the localization process is of importance but also the post localization stage where rapidly growing convective instabilities might prevent an incipient subduction zone from developing we develop scaling laws for both the localization and the post localization stages in the case of lithospheric localization the characteristic length scale equals a quarter of the dominant folding wavelength in the lithosphere thus of the order of the competent layer thickness this shows that oceanic lithosphere develops an intrinsic perturbation length scale that is largely independent of smaller scale heterogeneities we compare the subduction initiation potential of wet olivine and dry olivine rheologies and find that for earth like conditions a dry olivine rheology is more likely to result in subduction initiation a large plate age does not always increase the potential for subduction initiation as it increases the potential for convective instabilities to occur instead an optimal plate age exists for shear heating induced subduction initiation which is around 40 ma for a wet olivine rheology

variation in sea temperature and the east australian current in the solitary islands region between 2001 2008 abstract improved understanding of the east australian current eac and sea temperature patterns within the solitary islands region of northern new south wales an area where tropical and temperate faunas overlap is an essential step in explaining cross shelf gradients in biotic patterns sea temperature at 10 m was logged using thermistors at seven stations every 30 minutes between january 2001 and december 2008 stations were replicated in three distance from shore categories 6 km from the coast corresponding with predominant assemblage patterns of reef fish daily monthly seasonal and yearly sea temperature patterns were compared between and within stations and distance from shore categories sst images were examined to determine the role of the eac in producing short period 2 to 4 days temperature anomalies sea temperatures ranged between 16 6 27 5 c and were highest offshore and lowest inshore offshore sites experienced average temperatures 1 c higher than nearshore sites over the 8 year study there was considerable variation in sea temperature between years with 2002 and 2006 being the warmest and 2007 the coolest these patterns correspond with strong inter annual variability of the eac at the scale of the solitary islands region the eac influenced shelf waters most strongly during late spring summer when temperatures were also most variable over the smallest temporal scales hours days short period anomalies between and within stations could largely be explained by variable encroachment of the eac across the shelf and or colder intrusions of water forming adjacent to the coastline previous assumptions that the eac strongly influences gradients in the distribution of tropical species in this nearshore region are strongly supported

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

controls of the structure of subducted slabs numerical simulations of subducting slabs are formulated in which the shape and dip of the slab are determined by the dynamics of the flow rather than imposed a priori the dip of slabs is a function of the time since the initiation of subduction slabs fold develop a kink in dip and thicken on entry into a high viscosity lower mantle comparison of the simulations with seismic observations suggest that the lower mantle is at least 10 30 times more viscous than the upper mantle

subinertial currents on the southern califoinia shelf abstract subinertial currents on the southern california shelf are investigated using observations from a current meter array deployed near san diego during the summer fall and winter of 1978 79 this region is characterized by weak winds order 1 2 m s 1 and thus other driving mechanisms for the subinertial currents may be important a simplified depth averaged longshore momentum equation consisting of a balance between current accelerations the adjusted sea level asl gradient and the wind and bottom stress is examined the asl gradient is estimated using sea level and atmospheric pressure observations separated by 350 km a linear parameterization of the bottom stress is used with a drag coefficient of 5 10 4 m s 1 during fall and winter the first order balance of terms in the longshore momentum equation over the inner shelf 15 m depth is between the wind and bottom stress over the outer shelf 60 m the primary balance is between the asl gradient and the bottom stress at midshelf 30 m bo

dynamics of intraoceanic subduction initiation 1 oceanic detachment fault inversion and the formation of supra subduction zone ophiolites subduction initiation is a critical link in the plate tectonic cycle intraoceanic subduction zones can form along transform faults and fracture zones but how subduction nucleates parallel to mid ocean ridges as in e g the neotethys ocean during the jurassic remains a matter of debate in recent years extensional detachment faults have been widely documented adjacent to slow spreading and ultraslow spreading ridges where they cut across the oceanic lithosphere these structures are extremely weak due to widespread occurrence of serpentine and talc resulting from hydrothermal alteration and can therefore effectively localize deformation here we show geochemical tectonic and paleomagnetic evidence from the jurassic ophiolites of albania and greece for a subduction zone formed in the western neotethys parallel to a spreading ridge along an oceanic detachment fault with 2 d numerical modeling exploring the evolution of a detachment ridge system experiencing compression we show that serpentinized detachments are always weaker than spreading ridges we conclude that owing to their extreme weakness oceanic detachments can effectively localize deformation under perpendicular far field forcing providing ideal conditions to nucleate new subduction zones parallel and close to or at spreading ridges direct implication of this is that resumed magmatic activity in the forearc during subduction initiation can yield widespread accretion of suprasubduction zone ophiolites at or close to the paleoridge our new model casts the enigmatic origin of regionally extensive ophiolite belts in a novel geodynamic context and calls for future research on three dimensional modeling of subduction initiation and how upper plate extension is associated with that

thermo mechanical modeling of the obduction process based on the oman ophiolite case obduction emplaces regional scale fragments of oceanic lithosphere ophiolites over continental lithosphere margins of much lower density for this reason the mechanisms responsible for obduction remain enigmatic in the framework of plate tectonics we present two dimensional 2d thermo mechanical models of obduction and investigate possible dynamics and physical controls of this process model geometry and boundary conditions are based on available geological and geochronological data and numerical modeling results are validated against petrological and structural observations of the oman semail ophiolite our model reproduces the stages of oceanic subduction initiation away from the arabian margin the emplacement of the oman ophiolite on top of it and the domal exhumation of the metamorphosed margin through the ophiolitic nappe a systematic study indicates that 350 400 km of bulk shortening provides the best fit for both the maximum pressure temperature conditions of the metamorphosed margin 1 5 2 5 gpa 450 600 c and the dimension of the ophiolitic nappe 170 km width our results confirm that a thermal anomaly located close to the arabian margin 100 km is needed to initiate obduction we further suggest that a strong continental basement rheology is a prerequisite for ophiolite emplacement

the experimental deformation of dunite deformation experiments have been carried out on two dunites anita bay of 100 m grain size and aheim of 900 m grain size at strain rates from 10 3 to 10 6 s 1 and temperatures from 1000 c to 1300 c in a gas medium deformation apparatus at 300 mpa confining pressure most of the tests were under wet conditions defined by the presence of small amounts of water from hydrous minerals initially present constant strain rate and relaxation experiments covering ranges of flow stress down to about 70 mpa and 7 mpa respectively show that there is a change in flow law in going below about 100 mpa differential stress and that the coarser grained rock is stronger than the finer grained one power law parameters above the transition are n 4 48 0 31 and q 498 38 kj mol 1 for aheim dunite and n 3 35 0 17 and q 444 24 kj mol 1 for anita bay dunite while below the transition relaxation tests on anita bay dunite give n 2 44 0 18 and q 386 27 kj mol 1 it is concluded that there is a weakening effect of water that this effect is mainly in the grain boundaries and that grain boundary sliding is probably a significant deformation mechanism at the lower stresses under wet conditions

baroclinic response of sydney shelf waters to local wind and deep ocean forcing a study of the forcing processes responsible for upwelling events in the coastal ocean of sydney australia has been performed using data collected over the summer of 1994 from a shore normal aligned mooring array and a numerical model analyses of the data show that vertical displacements of fluid in the nearshore zone responded principally to the local wind stress during the experimental period however intrusions of mesoscale east australian current features are shown to significantly influence the vertical structure of the water column over the middle and outer shelf regions numerical simulations are performed to investigate the internal processes occurring during strong steady intrusion events of the east australian current onto the sydney shelf these simulations suggest that ekman transport in the bottom boundary layer underlying an intrusion event is not an efficient mechanism for advecting colder water into the nearshore zone on the sydney shelf preconditioning of shelf waters by the east australian current and concurrent forcing by local winds is suggested as an efficient process by which upwelling states may be achieved in the sydney coastal ocean

observations of divergence and upwelling around point loma california 1 historical records of near surface water temperatures in the southern californian bight often show a preferential cooling in the lee of headlands such as point dume palos verdes and point loma at times this cooler water is associated with an increase in chlorophyll a as is evident in satellite images of ocean color from the region here we combine hydrographic data from a 1 day cruise aboard the rv roger revelle a precursor to the 0304 california cooperative oceanic fisheries investigations calcofi cruise with high frequency hf radar codar measurements satellite images and long term thermistor records of near surface temperature to identify a small scale isolated upwelling in the lee of point loma 32 5 n associated with the more saline water downstream of the headland are higher nutrient concentrations an increase in chlorophyll a concentration and a bloom of chain forming diatoms indicative of a mature upwelling system it is suggested that this upwelling is not primarily due to local or remote wind forcing but rather to the divergence of the prevailing southerly flow as it passes the point loma headland time series of surface vorticity calculated from hf radar measurements of sea surface velocity show that as the flow separates from the headland relative vorticity increases offshore of the cape inshore the time series of divergence convergence shows a tendency toward divergence at the surface indicating a preferential upwelling which appears to raise the thermocline thus resulting in a flux of cold nutrient rich water to the surface in the presence of high nutrients and light photosynthetic organisms bloom in these upwelled waters as they are advected away from the headland and offshore by the prevailing surface currents

dynamics of subduction initiation at preexisting fault zones we show that a new subduction zone can initiate at a preexisting dipping fault zone with reasonable plate forces consistent with what is observed in the western pacific the dynamics of subduction initiation within a viscoelastic medium has been systematically explored with the finite element method we investigate the compression of oceanic lithosphere with both force and velocity boundary conditions and track the thermal structure in which heat is transported by both advection and diffusion the viscosity of the medium is non newtonian and temperature dependent we also examine the influence of a perfectly plastic yield stress a new method to model the preexisting weak zone as a graded fault with remeshing is introduced the first 400 km of plate convergence is examined during which time there are profound changes in plate boundary dynamics initially topography across the plate boundary is characterized by over 1000 m of uplift close to the trench subsequently a bathymetric depression develops on the overriding plate with an initial subsidence rate of 125 m myr for a slab subducting at 2 cm yr tectonic subsidence of the overriding plate is primarily dependent upon the depth of slab penetration for a plate force of 4 10 12 n m subduction initiates even with a fault shear stress of 3 mpa the resistance most consistent with seismological observations however subduction will not initiate when this fault shear stress is 5 mpa our results are consistent with plate reconstructions which predict the initiation of subduction across preexisting weak zones within oceanic basins

on the formation of oceanic detachment faults and their influence on intra oceanic subduction initiation 3d thermomechanical modeling abstract extensional detachment faults which have been widely documented in slow spreading and ultraslow spreading ridges on earth can effectively localize deformation due to their weakness after the onset of oceanic closure these weak oceanic detachments may directly control the nucleation of a subduction zone parallel to the former mid ocean ridge as is suggested for the neotethys in middle jurassic times so far this hypothesis has only been tested by 2d numerical models whereas the geometry of detachment faults is intrinsically three dimensional here we conducted a series of 3d numerical thermomechanical experiments in order to investigate the formation of detachment faults in slow oceanic spreading systems and their subsequent response upon inversion from oceanic spreading to convergence numerical results show that during the oceanic spreading stage the formation of detachment faults strongly depends on the magnitude of the healing rate of faulted rocks in the oceanic lithosphere that reflects the stability of hydrated minerals along fractured rocks the detachment faults formed in our 3d numerical models deviate from the rolling hinge model of oceanic detachment faulting where fault footwalls are rotated and oceanic core complexes are thereby formed our results accentuate that the controlling physical parameters for the development of oceanic core complexes and detachment faults can differ and that their coupled development in nature remains a key target for future research upon modeled transition to compression previously formed asymmetric spreading patterns are prone to asymmetric inversion where one oceanic plate thrusts under the other our results suggest that detachment faults accommodate significant amounts of shortening during the initiation of oceanic closure but in contrast to the previously proposed simple conceptual model no direct inversion of a single detachment fault into an incipient subduction zone is observed instead a widespread interaction of multiple detachment faults occurs after the onset of convergence ultimately the nascent subduction zone cuts through the base of several pre existing detachment faults thereby forming an initial accretionary wedge in the incipient fore arc

continental interior and edge breakup at convergent margins induced by subduction direction reversal a numerical modeling study applied to the south china sea margin the dynamics of continental breakup at convergent margins have been described as the results of back arc opening caused by slab rollback or drag force induced by subduction direction reversal although the rollback hypothesis has been intensively studied our understanding of the consequence of subduction direction reversal remains limited using thermo mechanical modeling based on constraints from the south china sea scs region we investigate how subduction direction reversal controls the breakup of convergent margins the numerical results show that two distinct breakup modes namely continental interior and edge breakup edge refer to continent above the plate boundary interface may develop depending on the maturity of the convergent margin and the age of the oceanic lithosphere for a slab age of 15 45 ma increasing the duration of subduction promotes the continental interior breakup mode where a large block of the continental material is separated from the overriding plate in contrast the continental edge breakup mode develops when the subduction is a short duration event and in this mode a wide zone of less continuous continental fragments and tearing of the subducted slab occur these two modes are consistent with the interior relic late mesozoic arc and edge relic fore arc rifting characteristics in the western and eastern scs margin suggesting that variation in the northwest directed subduction duration of the proto scs might be a reason for the differential breakup locus along the strike of the scs margin besides a two segment trench associated with the northwest directed subduction is implied in the present day scs region

the role of deformation in the formation of metamorphic gradients ridge subduction beneath the oman ophiolite two tectonic scenarios have been proposed for genesis and emplacement of the oman ophiolite one suggests that the ophiolite was generated at a spreading center the other suggests generation within an intraoceanic arc an integrated thermal and kinematic model of the temperature stress rock type and displacement fields during early stages of the emplacement of the oman ophiolite was developed to test these two possibilities the thermal evolution was calculated by a finite difference algorithm for heat conduction considering heats of metamorphic reactions deformational heating heat advection by flowing rock mantle heat flow and radioactive heating the stress and displacement fields were calculated by an analytical model using a velocity boundary condition power law constitutive relations and a brittle frictional sliding relationship field observations in oman can be satisfied with the spreading center model but not with the arc model moreover simulations indicate that the ophiolite was probably 2 m y old at the time of intraoceanic thrusting and that shear stresses of 100 mpa were attained during thrusting in addition to satisfying currently available field constraints these simulations also indicate specific further work that will help resolve the controversy over thevolution of this unique orogen future work should be directed toward 1 using thermobarometry and thermochronology to constrain the spatial variation in ptt paths of the metamorphic sole 2 dating the igneous genesis of the ophiolite and 3 using quartzite and dunite recrystallized grain size paleopiezometers to infer stresses during thrusting these simulations point out the important effect synmetamorphic deformation can have on the formation of metamorphic field gradients in contractional fault zones metamorphic field gradients may be normal or inverted and contracted or extended relative to the initial thermobarometric gradient and adjacent fault zone rocks may display crossing pt paths unusually contracted or extended metamorphic field gradients may also develop in the footwalls of normal fault zones where portions of the heating hangingwall accrete to the uplifting cooling footwall

tectonic inheritance and continental rift architecture numerical and analogue models of the east african rift system 1 the western branch of the east african rift is composed of an arcuate succession of elongate asymmetric basins which differ in terms of interaction geometry fault architecture and kinematics and patterns of uplift subsidence and erosion sedimentation the basins are located within proterozoic mobile belts at the edge of the strong tanzanian craton surface geology suggests that the geometry of these weak zones is an important parameter in controlling rift development and architecture although other processes have been proposed in this study we use lithosphere scale numerical models and crustal scale analogue experiments to shed light on the relations between preexisting structures and rift architecture results illustrate that on a regional scale rift localization within the mobile belts at the curved craton s western border results in an arcuate rift system which implies that under a constant extensional stress field part of the western branch experienced orthogonal extension and part oblique extension largest depocenters are predicted to form mostly orthogonal to the extension direction and smaller depocenters will form along the oblique parts of the rift the varying extension direction along the rift zone furthermore results in lengthwise varying rift asymmetry segmentation characteristics and border fault architecture trend length and kinematics analogue models predict that discrete upper crustal fabrics may influence the location of accommodation zones and control the architecture of extension related faults at a local scale models support that fabric reactivation is responsible for the oblique slip kinematics on faults and for the development of z shaped or arcuate normal faults typically documented in nature

response of an oceanic bottom boundary layer on a slope to interior flow part i time independent interior flow abstract the behavior of oceanic boundary layers on a sloping bottom in the presence of stratification is investigated by the method of direct numerical simulations the navier stokes equations are decomposed into mean and turbulent components with the mean equations involving a slope burger number s n2sin2 f2 where n is the buoyancy frequency is the bottom slope angle and f is the coriolis parameter the influence of the turbulent fluctuations is parameterized as eddy coefficients of viscosity and diffusivity two regimes are considered the first is for cases where the eddy coefficients are constant in time but variable in space and the interior flow is allowed to adjust to the boundary mixing the numerical results agree with analytic theory for two limiting cases the first case is when the mixing extends into the fluid interior and the second is when the mixing is limited to a slab region the second regime involves cases in which the eddy coefficients are determined from a gradient richard

cross shelf dynamics in a western boundary current regime implications for upwelling abstractthe cross shelf dynamics up and downstream of the separation of the south pacific ocean s western boundary current wbc are studied using two years of high resolution velocity and temperature measurements from mooring arrays the shelf circulation is dominated by the east australian current eac and its eddy field with mean poleward depth integrated magnitudes on the shelf break of 0 35 and 0 15 m s 1 up and downstream of the separation point respectively the high cross shelf variability is analyzed though a momentum budget showing a dominant geostrophic balance at both locations among the secondary midshelf terms the bottom stress influence is higher upstream of the separation point while the wind stress is dominant downstream this study investigates the response of the velocity and temperature cross shelf structure to both wind and eac intrusions despite the deep water up to 140 m the response to a dominant along shelf wind stress forcing is a classic two layer ekman structure du

near ridge initiation of intraoceanic subduction effects of inheritance in 3d numerical models of the wilson cycle abstract despite its global significance much of the wilson cycle which infers kinematic inversion from plate divergence to convergence remains conjectural in particular for what concerns subduction initiation we present a high resolution 3d thermomechanical numerical model of the wilson cycle evolving from continental rifting through breakup and oceanic spreading to convergence and self consistent subduction initiation in the models intra oceanic subduction initiates off ridge and is an intrinsically three dimensional process its location is controlled by convergence induced ridge swelling and the subduction geometry is controlled by three main factors 1 the inherited compositional and thermal heterogeneity of the ridge and oceanic lithosphere from rifting and seafloor spreading 2 the curvature obliquity of the ridge with respect to the convergence direction and 3 the duration of transition from plate divergence to convergence the modelled mechanisms are consistent with geological records for example in the oman subduction obduction system and are undervalued elements of the wilson cycle

accelerated subglacial erosion in response to stick slip motion subglacial stick slip motion speeds erosion by hydrofracturing and in other ways as determined from analysis of the growing body of field data microearthquake monitoring commonly detects subglacial earthquakes likely mostly from stick slip motion of debris laden ice over bedrock source parameters show that many quakes cause enough motion to greatly lower water pressure in cavities on the lee sides of bedrock steps we calculate that the resulting expansion of higher pressure water in nearby cracks promotes hydrofracturing with even relatively small cracks growing unstably under thick glaciers and all cracks growing faster than for aseismic behavior this mechanism also helps generate the step like topography favoring block plucking this stick slip glacier erosion hypothesis suggests that the erosion rate will increase with ice thickness as well as basal shear stress ice flow velocity and water supply

low temperature high stress deformation of olivine under water saturated conditions abstract low temperature high stress rheology of olivine was investigated under water saturated conditions by large strain shear deformation experiments using the griggs type apparatus samples at t 1273 1373 k and p 2 gpa under water saturated conditions deformed under a significantly lower applied stress than expected from the power law relation and the strain rate increases more rapidly with stress these observations suggest that an exponential flow law operates under the present experimental conditions in which the stress dependence of activation enthalpy is characterized by the peierls stress the peierls stress is determined between 1 6 and 2 9 gpa depending on the choice of the form of stress dependence of the activation enthalpy these values are significantly smaller than the peierls stress under dry conditions 9 1 gpa we suggest that water hydrogen incorporated in the olivine lattices decreases the peierls stress and hence the rate of deformation in the exponential creep is significantly enhanced by the presence of water the reduction of the peierls stress by water results in the highly anisotropic weakening effect of water that causes the fabric transitions in olivine the transition between the power law and the exponential creep occurs at relatively low stress 100 mpa at a geologically relevant strain rate of 10 15 s 1 when temperature is lower than 1273 k and under water rich conditions hence the exponential creep might be dominated in olivine rich rocks at low temperature and high stress under water rich conditions and such regions could be widespread in the subduction zone upper mantle where the cold lithospheric plates sink into the mantle

what can crystal size distributions and olivine compositions tell us about magma solidification processes inside kilauea iki lava lake hawaii abstract lava lakes offer the opportunity to investigate magma solidification and can be considered as a proxy for small magma chambers here we present olivine compositions and crystal size distributions csds from scoria and drill core samples from kilauea iki lava lake which formed during the 1959 eruption of kilauea volcano hawaii three chemically distinct olivine populations were distinguished in the basis of their forsterite fo content 1 a high fo population fo86 90 2 an intermediate fo population fo78 82 and 3 a minor low fo population fo74 78 populations 1 and 2 both have deformed and undeformed crystals the third population may be the result of rejuvenation olivine in the lower 60 m of lake has a less fo rich composition and more crystals are deformed the csd analysis yields estimates of the average olivine residence time 1 60 years the shape of the olivine csds is fairly uniform with respect to depth curved csds are considered to be evidence of hybrid populations partly or totally involving crystal or magma mixing the turndown at the smallest sizes of most foundered crust and lake csds may be the result of coarsening making this process active both before and after eruption our csd modelling does not support significant crystal settling and overall convection in the lava lake although small advective currents are known to have occurred the olivine vertical stratification cannot be an original feature which is consistent with supposed strong stirring of the lake magma due to intense activity over the 17 eruptive phases it is also possible that independent basal feeding of the lake during the eruption may be needed to explain fully features of the chemical and mineralogical stratification

from continental rifting to seafloor spreading insight from 3d thermo mechanical modeling abstract continental rifting to seafloor spreading is a continuous process and rifting history influences the following spreading process however the complete process is scarcely simulated using 3d thermo mechanical coupled visco plastic numerical models we investigate the complete extension process and the inheritance of continental rifting in oceanic spreading our modeling results show that the initial continental lithosphere rheological coupling decoupling at the moho affects oceanic spreading in two manners 1 coupled model a strong lower crust mechanically couples upper crust and upper mantle lithosphere generates large lithospheric shear zones and fast rifting which promotes symmetric oceanic accretion i e oceanic crust growth and leads to a relatively straight oceanic ridge while 2 decoupled model a weak ductile lower crust mechanically decouples upper crust and upper mantle lithosphere generates separate crustal and mantle shear zones and favors asymmetric oceanic accretion involving development of active detachment faults with 3d features complex ridge geometries e g overlapping ridge segments and curved ridges are generated in the decoupled models two types of detachment faults termed continental and oceanic detachment faults are established in the coupled and decoupled models respectively continental detachment faults are generated through rotation of high angle normal faults during rifting and terminated by magmatism during continental breakup oceanic detachment faults form in oceanic crust in the late rifting early spreading stage and dominates asymmetric oceanic accretion the life cycle of oceanic detachment faults has been revealed in this study

evolving force balance during incipient subduction nearly half of all active subduction zones initiated during the cenozoic all subduction zones associated with active back arc extension have initiated since the eocene hinting that back arc extension may be intimately associated with an interval several tens of myr following subduction initiation that such a large proportion of subduction zones are young indicates that subduction initiation is a continuous process in which the net resisting force associated with forming a new subduction zone can be overcome during the normal evolution of plates subduction initiation is known to have occurred in a variety of tectonic settings old fracture zones transform faults and extinct spreading centers and through polarity reversal behind active subduction zones although occurring within different tectonic settings four known subduction initiation events izu bonin mariana ibm along a fracture zone tonga kermadec along an extinct subduction boundary new hebrides within a back arc and puysegur fiordland along a spreading center were typified by rapid uplift within the forearc followed by sudden subsidence other constraints corroborate the compressive nature of ibm and tonga kermadec during initiation using an explicit finite element method within a two dimensional domain we explore the evolving force balance during initiation in which elastic flexure viscous flow plastic failure and heat transport are all considered in order to tie theory with observation known tectonic settings of subduction initiation are used as initial and boundary conditions we systematically explore incipient compression of a homogeneous plate a former spreading center and a fracture zone the force balance is typified by a rapid growth in resisting force as the plate begins bending reaching a maximum value dependent on plate thickness but typically ranging from 2 to 3 1012 n m for cases that become self sustaining this is followed by a drop in stress once a shear zone extends through the plate the formation of a throughgoing fault is associated with rapid uplift on the hanging wall and subsidence on the footwall cumulative convergence not the rate of convergence is the dominant control on the force balance viscous tractions influence the force balance only if the viscosity of the asthenosphere is 1020 pa s and then only after plate failure following plate failure buoyancy of the oceanic crust leads to a linear increase with crustal thickness in the work required to initiate subduction the total work done is also influenced by the rate of lithospheric failure a self sustaining subduction zone does not form from a homogeneous plate a ridge placed under compression localizes subduction initiation but the resisting ridge push force is not nearly as large as the force required to bend the subducting plate the large initial bending resistance can be entirely eliminated in ridge models explaining the propensity for new subduction zones to form through polarity reversals a fracture zone fz placed in compression leads to subduction initiation with rapid extension of the overriding plate a fz must be underthrust by the older plate for 100 150 km before a transition from forced to self sustaining states is reached in fz models the change in force during transition is reflected by a shift from forearc uplift to subsidence subduction initiation is followed by trench retreat and back arc extension moderate resisting forces associated with modeled subduction initiation are consistent with the observed youth of pacific subduction zones the models provide an explanation for the compressive state of western pacific margins before and during subduction initiation including ibm and tonga kermadec in the eocene and the association of active back arcs with young subduction zones on the basis of our dynamic models and the relative poles of rotation between pacific and australia during the eocene we predict that the northern segment of the tonga kermadec convergent margin would have initiated earlier with a progressive southern migration of the transition between forced and self sustaining states

slope control in western boundary currents an analytic solution is presented for the steady state depth averaged western boundary current flowing over the continental slope by combining three highly idealized models the stommel model the munk model and the arrested topographic wave model the main vorticity balance over the slope is between planetary vorticity advection and the slope induced bottom stress torque which is proportional to ry h21 x where r is the rayleigh friction coefficient h is the water depth and y is the meridional velocity this slope induced torque provides the necessary source of vorticity for poleward flow over the slope its simple interpretation being that vorticity is produced because the bottom stress has to act over the seaward deepening water column the character of the solution depends on the slope a as well as on the assumed bottom drag coefficient and the length scale of the boundary current is it is further shown that if the depth averaged velocity flows along isobaths 2r ba then the stretching of water columns associated with cross isobath geostrophic flow which compensates bottom ekman transport is identical to the slope induced torque by the geostrophic velocities

topographically induced upwelling off eastern australia abstract a high resolution numerical study of an idealized western boundary current flow over variable topography is presented with application to the east australian current eac the results indicate that alongshelf topographic variations off australia s east coast cause the eac to accelerate over the narrowing continental shelf near cape byron this acceleration is sufficient to hinder the geostrophic adjustment in the bottom boundary layer bbl which would usually cause the eac driven bbl to shut down consequently a region of persistent high bottom stress was established off cape byron which was responsible for driving an upwelling bbl it is shown that the enhanced vertical mixing associated with a low richardson number flow beneath the eac reduced the local stratification consequently the burger number is decreased resulting in a long shutdown timescale of the bbl which enables a nearshore thermal to be established and maintained such fronts are commonly observed in the region as a p

initiation of subduction by small scale convection 1 the possibility of initiation of subduction by sublithospheric small scale convection is investigated with the help of fowler s theory numerical constraints on the lithospheric stresses generated by convection and direct numerical simulations of subduction the derived scaling laws show that the critical yield stress for initiation of subduction by small scale convection is of the order of 30 mpa this value is comparable with the critical yield stress required for initiation of subduction by plate forces and with the estimates of stresses in the subduction zone this implies that small scale convection can play an important role in the initiation of subduction on the present day earth this also implies that small scale convection can initiate subduction in the absence of already occurring plate tectonics and thus can be a trigger mechanism for plate tectonics on the early earth and other terrestrial planets the critical yield stress for initiation of subduction by either mechanism is at least one order of magnitude smaller than the estimates of the lithospheric strength suggested by laboratory experiments on rock deformation such a low lithospheric strength is most likely caused by a relatively high abundance of water in the interior of the earth since the critical yield stress depends only weakly on planetary parameters water seems to be the key condition required for plate tectonics to occur on a terrestrial planet

seasonality of sporadic physical processes driving temperature and nutrient high frequency variability in the coastal ocean off southeast australia physical processes forced by alongshore winds and currents are known to strongly influence the biogeochemistry of coastal waters combining in situ observations moored platforms hydrographic surveys and satellite data sea surface wind and sea surface height we investigate the transient occurrence of wind driven upwelling downwelling and current driven upwelling events off southeast australia remote sensed indices are developed and calibrated with multiannual time series of in situ temperature and current measurements at two shelf locations based on archives up to 10 years long climatological analyses of these indices reveal various latitudinal regimes with respect to seasonality magnitude duration of events and their driving mechanisms wind or current generally downwelling favorable winds prevail in this region however we demonstrate that up to 10 wind driven upwelling days per month occur during spring summer at 28 33 5 degrees s and up to 5 days in summer further south current driven upwelling upstream of the east australian current separation zone approximate to 32 degrees s occurs twice as often as downstream using independent in situ data sets we show that the response of the coastal ocean is consistent with our climatology of shelf processes upwelling leads to a large range of temperatures and elevated nutrient concentrations on the shelf maximized in the wind driven case while downwelling results in destratified nutrient poor waters the combination of these sporadic wind and current driven processes may drive an important part of the high frequency variability of coastal temperature and nutrient content our results suggest that localized nutrient enrichment events of variable magnitude are favored at specific latitudes and seasons potentially impacting coastal ecosystems key points multisensor analysis of shelf processes combining in situ and satellite data spatio temporal variability of transient wind and current driven up downwelling cold nutrient rich water intrusions favoured at specific locations seasons

origin of the palau and yap trench arc systems summary this paper attempts to account for several unique morphological and geological characteristics of the palau and yap trench arc systems in the northwestern pacific ocean using a new hydrodynamic model postulated herein the model is a simple mathematical treatment of a stationary viscous flow pattern in the asthenosphere that causes an appreciable amount of mechanical resistance against a vertically subducted rigid slab moving horizontally together with the flat plate a couple of thrusting forces comprising uplifting pressure generated in the frontal edge of the overriding lid and a new type of tectonic erosion along the surface of the vertical slab seem to reasonably explain apparently contradictory features of the palau and yap trench arcs long term gps measurements show that the palau and yap trench arcs are moving over the viscous asthenosphere toward the nww with velocities as great as 10 cm yr 1 stresses along the trench arcs predicted by the model would be sufficiently large to maintain their present morphology 1 water depths greater than 8000 m in spite of the extremely small plate convergence rates and relatively young ages of subducted plate 2 v shaped cross section of the axial bottom of these trenches implying little cover of slumped deposits in spite of very steep landward slopes 3 serpentinized peridotites gabbros basalts and limestones are exposed on the landward slope with neither sediment cover nor ferromanganese hydro oxide coatings 4 abnormally close proximity of the palau and yap islands to the trench axis 5 predominant distribution of barrier reefs in their western periphery apparently opposite to the easterly to northeasterly prevailing wind

three dimensional thermomechanical modeling of oceanic spreading initiation and evolution abstract this work employs high resolution 3d thermomechanical numerical models of the incipient oceanic spreading to investigate nucleation and long term evolution of ridge transform spreading patterns the eulerian lagrangian visco plastic model allows for large strains and accounts for plate cooling by both heat conduction and hydrothermal circulation as well as for partial melting of the asthenosphere and oceanic crust growth by magmatic accretion according to the numerical experiments the oceanic spreading pattern depends strongly on the initial offset of spreading centers and the magnitude of fracture healing rate three different characteristic long term spreading modes are obtained 1 ridge transform patterns 2 single ridges and 3 spreading centers with an intermediate plate ridge transform oceanic spreading patterns form gradually from moderate initial offsets of 40 60 km and become fully established several million years after the plate breakup moreover it is demonstrated on the basis of simple analyses that the ridge transform system is a long term plate growth pattern that is generally different from an initial plate rifting pattern geometry of the ridge transform system is governed by geometrical requirements 180 rotational symmetry for open space occupation for simultaneous accretion and displacement of new plate material within two offset spreading centers connected by a sustaining rheologically weak transform fault according to these requirements the characteristic spreading parallel orientation of oceanic transform faults is the only thermomechanically consistent steady state orientation results of numerical experiments compare well with both incipient and mature ridge transform systems observed in nature