interpretation of porosity and lwd resistivity from the nankai accretionary wedge in light of clay physicochemical properties evidence for erosion and local overpressuring in this study we used porosity to assess the compaction state of the nankai accretionary wedge sediments and any implications for stress and pore pressure however hydrous minerals affect porosity measurements and accounting for them is essential toward defining the interstitial porosity truly representative of the compaction state the water content of sediments was measured in core samples and estimated from logging data using a resistivity model for shale we used the cation exchange capacity to correct the porosity data for the amount of water bound to clay minerals and to correct the porosity estimates for the surface conductivity of hydrous minerals the results indicate that several apparent porosity anomalies are significantly reduced by this correction implying that they are in part artifacts from hydrous minerals the correction also improves the fit of porosity estimated from logging while drilling lwd resistivity data to porosity measured on cores low overall porosities at the toe of the accretionary wedge and in the splay fault area are best explained by erosion and we estimated the quantity of sediments eroded within the splay fault area by comparing porosity effective stress relationships of the sediments to a reference curve additionally a comparison of lwd data with core data resistivity and p wave velocity obtained at site c0001 landward of the mega splay fault area suggested a contribution from the fracture porosity to in situ properties on the formation

the structure and fault activity of the makran accretionary prism the makran subduction zone has the highest incoming sediment thickness up to 7 5 km of any subduction zone these sediments have formed a wide accretionary prism 400 km seismicity in the makran is generally low however the margin experienced an mw 8 1 earthquake in 1945 which generated a significant regional tsunami seismic reflection data and swath bathymetry data from offshore pakistan are used to analyze the structure and fault activity of the outer accretionary prism the outer prism has a simple structure of seaward verging imbricate thrust faults many continuous for over 100 km along strike fault activity is analyzed using basin stratigraphy and fault geometry revealing a frontal continuously active zone a central intermittently active zone and a landward inactive zone over 75 of the faults in the seaward 70 km of the prism show evidence for recent activity the decollement occurs within the lower sediment section but steps onto the top basement surface in regions of elevated basement topography fault spacing 6 km and taper 4 5 are comparable to other margins such as s hikurangi cascadia and nankai suggesting that high sediment input is not leading to an unusual prism structure the decollement is unreflective which is unexpected considering other prism characteristics predicting a weak surface and may suggest a potentially stronger decollement than previously predicted this study provides a significant advance in our understanding of the structure of an end member convergent margin and demonstrates that systematic analyses of accretionary prism structure can help to elucidate subduction zone dynamics with ultimate relevance to seismogenic potential

dayside magnetic topology at the earth s magnetopause for northward imf 1 when the interplanetary magnetic field is northward magnetic reconnection occurs at the earth s high latitude magnetopause in regions poleward of the earth s magnetospheric cusps observations at the high latitude magnetopause appear to support reconnection poleward of both northern and southern hemisphere cusps forming closed field lines in the magnetosheath however observations at the dayside low latitude magnetopause and in the low altitude cusp do not support this reconnection scenario using plasma and magnetic field data from the cluster spacecraft signatures of reconnection poleward of both cusps were investigated as a function of latitude it was found that reconnection in at least one hemisphere occurs essentially all the time for northward imf this investigation also confirmed the apparent discrepancy between high and low latitude observations a scenario for reconnection in both cusps whereby the field line reconnected poleward of one cusp convects away from the dayside magnetopause before reconnecting in the opposite hemisphere is consistent with observations at all latitudes

cold dense magnetopause boundary layer under northward imf results from themis and mhd simulations 1 a layer of nearly stagnant cold dense plasma is observed by themis spacecraft in a closed field region immediately inside the dayside magnetopause near the low latitude boundary layer on 3 june 2007 using the openggcm global mhd magnetosphere numerical model we successfully reproduce this observed cold dense plasma layer in the simulation the simulation results show that reconnection first occurs poleward of the cusp in the northern hemisphere creating new open field lines extending southward and forming an open field layer then subsequently occurs in the other hemisphere creating new closed field lines that capture the magnetosheath plasma and form the dayside cold dense plasma layer in this event the open layer and the skin of the cold dense plasma layer have a southward tangential flow while the inner part of the cold dense plasma layer has a more stagnant and more turbulent flow

analysis of temperature versus density plots and their relation to the llbl formation under southward and northward imf orientations the paper reports a clear dependence of a basic structure of two sublayers of the low latitude boundary layer llbl on northward and southward interplanetary magnetic field imf orientations regardless of different processes responsible for a formation and evolution of the entire llbl the outer part of the llbl is significantly influenced by the sign of the imf bz component under northward imf conditions this layer is present whereas it is missing during a southward pointing imf this behavior can be understood in terms of a motion of reconnection spots due to the changes of the orientation of the magnetosheath magnetic field in the vicinity of the magnetopause our case and statistical studies demonstrate that the changes of the llbl structure can be observed in the subsolar region as well as on flanks near the dawn dusk meridian moreover the study emphasizes a role of magnetosheath magnetic field variations on the boundary layer formation

the rates of plate creation and consumption summary an alternative vector derivation of the expression for the rates of plate creation and consumption is given from which it can be seen that the rates are proportional to the scalar product of the edges of the geohedron and plate polyhedron corresponding to the same boundary the plate polyhedron is defined by the set of triple junction vectors this is used together with a recent determination of the angular velocity vectors and a number of auxiliary plate boundary vectors to revise the rates of plate creation and consumption along sections of plate boundaries all plates which have a substantial proportion of their boundary connected to a downgoing slab are found to be decreasing in area the total rate of plate generation is used in estimates of the area of the ocean floor and the total heat loss from the oceans the rate of consumption within a relatively small area around new guinea implies a large mass flux away from this region knowledge of the rates of plate creation and consumption is used in a number of different areas calculating the contribution of plate creation to the total heat loss of the earth calculating geochemical fluxes out of the mantle such as for the rare gases and other volatiles and return fluxes back into the mantle resulting from reactions between sea water and the ocean crust during hydrothermal circulation calculating the area of the ocean floor as a function of age mass fluxes related to the distribution of the rates of creation and destruction along the plate boundaries are important boundary conditions on the flow pattern in the mantle previous estimates of these rates deffeyes 1970 chase 1972 garfunkel 1975 have applied a simple expression derived for spreading ridges deffeyes 1970 using angular distances from the poles of rotation measured graphically on a globe an alternative derivation of the expression for rates of plate creation and consumption which is independent of assumptions about the nature of the boundary is given below in terms of the framework used to describe plate motions by mckenzie parker 1967 1974 this derivation emphasizes that the rates depend on two sets of vectors one the instantaneous rotation vectors defining the relative plate motions the other a set of triple junction and auxiliary vectors defining the plate boundaries

reconnection at the high latitude magnetopause during northward interplanetary magnetic field conditions the polar spacecraft had a prolonged encounter with the high latitude dayside magnetopause on may 29 1996 this encounter with the magnetopause occurred when the interplanetary magnetic field was directed northward from the three dimensional electron and ion distribution functions measured by the hydra instrument it has been possible to identify nearly all of the distinct boundary layer regions associated with high latitude reconnection the regions that have been identified are 1 the cusp 2 the magnetopause current layer 3 magnetosheath field lines that have interconnected in only the northern hemisphere 4 magnetosheath field lines that have interconnected in only the southern hemisphere 5 magnetosheath field lines that have interconnected in both the northern and southern hemispheres 6 magnetosheath that is disconnected from the terrestrial magnetic field and 7 high latitude plasma sheet field lines that are participating in magnetosheath reconnection reconnection over this time period was occurring at high latitudes over a broad local time extent interconnecting the magnetosheath and lobe and or plasma sheet field lines in both the northern and southern hemispheres newly closed boundary layer field lines were observed as reconnection occurred first at high latitudes in one hemisphere and then later in the other these observations establish the location of magnetopause reconnection during these northward interplanetary magnetic field conditions as being at high latitudes poleward of the cusp and further reinforce the general interpretation of electron and ion phase space density signatures as indicators of magnetic reconnection and boundary layer formation

kinematic models of dewatering accretionary prisms steady state two dimensional analytical solutions are derived for sediment velocity and rate of fluid expulsion in a thickening accretionary prism with smooth upper and lower surfaces two different kinematic assumptions lead to two similar sets of solutions in one case the sediment flow lines are assumed to diverge uniformly landward in the other case the horizontal velocity of any given sediment column is assumed uniform by further assuming zero horizontal darcy velocity in the prism expressions can be obtained for the approximate vertical fluid velocity and pore fluid pressure fields results from applying these solutions to the cascadia accretionary prism off vancouver island show the importance of modeling the effects of lateral as well as vertical porosity variation on the hydrogeology of the prism these solutions provide a means to estimate fluid flow and pore pressure distribution for prisms with realistic geometry the rate of fluid expulsion can also be incorporated into numerical models as the fluid source term

arc continent collisions sediment recycling and the maintenance of the continental crust author posting geological society of london 2009 this is the author s version of the work it is posted here by permission of geological society of london for personal use not for redistribution the definitive version was published in geological society london special publications 318 2009 75 103 doi 10 1144 sp318 3

seismic velocities from the barbados ridge complex indicators of high pore fluid pressures in an accretionary complex three wide aperture 0 5 km offset seismic profiles were shot perpendicular to the strike of the barbados ridge accretionary complex at 16 12 n line 465 14 23 n line 480 and 13 20 n line 484 to image the deep structure of the accretionary complex and obtain seismic velocity data velocity analyses on full fold common depth point cdp gathers were used to provide velocity depth functions at 1 km separation along each line to map the variation in velocity with depth and distance across the accretionary complex the sediments within the complex show consolidation increasing laterally with greater age of the complex as indicated by the increase in seismic velocity and the inferred decrease in porosity the velocity at the base of the wedge along line 480 is 2 4 km s at the toe the velocity at the wedge base at 60 km from the toe where the wedge thickness has increased from 1 8 km to 5 0 km has increased to greater than 3 0 km s however the velocities within the leading margins and within undeithrust sequences in each of the three sections are low as much as 1 0 km s less than velocities found in undeformed sedimentary sequences of equivalent thickness the low velocity regions are interpreted as sediments with high porosity where porosity is inferred to be high pore fluid pressures are also inferred to exceed hydrostatic and pore fluids actually support the increased lithostatic load produced by recent thrusting and thickening of the complex the correlation between the most rapidly thickened regions and the velocity anomalies suggests that a rapid rate of loading relative to the rate of pore fluid expulsion produced these overpressured sediments velocities in the underthrust sequences where the differences between loading histories of wedge sediments and the underthrust sequences are largest are sufficiently low to produce velocity inversions low velocities also occur in regions adjacent to the complex where the pore fluid pressures may be elevated by the pressure produced in the adjacent wedge west of the wedge in the forearc basin along line 480 and near a seismically imaged mud diapir velocities at a depth of 3 5 km below the seafloor are 0 5 km s less than what is expected in a more normal sedimentary section immediately seaward of the complex the deepest undeformed sequences have a velocity anomaly of 0 5 km s velocities remain low for at least 12 km seaward of the complex but they become closer to a more normal sediment section farther from the wedge

forearc basins types geometries and relationships to subduction zone dynamics forearc basins are one of the fundamental sedimentary basins and they preserve various geological phenomena arising from dynamic interactions between subducting and overriding plates along convergent plate boundaries previous studies have proposed descriptive classification schemes for such basins and performed analog experiments and numerical simulations examining basin formation and evolution however the great variability of forearc basins means that a comprehensive understanding of these features remains elusive this paper proposes a new classification scheme for forearc basins from the viewpoints of material transfer between the two plates accretionary or nonaccretionary and the long term strain field in the basin compressional or extensional by using characteristics of 41 modern forearc basins geometric characteristics were also analyzed in order to reveal the relationship to subduction zone dynamics including material transfer at the plate boundary accretion erosion plate convergence configurations and kinematics and the mechanics of strain and its partitioning compressional accretionary type basins which are characterized by landward tilting strata and the landward migration of depocenters show a constant width thickness ratio indicating self similar growth of the basin as the outer wedge builds up extensional accretionary type basins develop normal faults in the basin probably due to a gravitational instability of the outer wedge an increase of sediment flux at the trench may grow the accretionary prism and thicken the subduction channel faster leading to a reduction of basal and internal shear strengths of the outer wedge subsidence caused by crustal thinning characterizes the nonaccretionary type basins which have thin trench fill sediments large forearc slopes steady widths of the outer wedge and variable width thickness ratios of the basins from these results the sediment flux at the trench and the plate boundary can be considered as a dominant factor that determines accretion or erosion of the outer wedge and uplift or subsidence of the outer arc high and that is also responsible for the style of deposition in the forearc basin an increase or decrease of the sediment flux may change the type geometry and deformation manner of the forearc basin the results of this study provide important clues to further our understanding of dynamic interactions between forearc basins and subduction zone processes

tectonics and hydrogeology of accretionary prisms role of the decollement zone abstract at convergent margins the decollement zone comprises the plate boundary and is marked by profound structural disharmony by changes in stress orientation and by a discontinuity in plate velocity the decollement zone initiates in a weak sediment layer typically a low permeability hemipelagic mud lying below a more rapidly deposited stronger more permeable trench turbidite sequence at the deformation front trench turbidites tend to be offscraped whereas the finer grained hemipelagic and pelagic sediments are more likely to be underthrust offscraped materials may undergo only limited burial whereas underthrust deposits can be deeply buried undergo high pressure metamorphism and are more likely to be preserved in the stratigraphic record burial rates associated with underthrusting are high exceeding 20 km my with high rates of underthrusting sediment descending below the decollement zone is probably buried faster than it can dewater resulting in emplacement of relatively high porosity deposits at depth fluids flowing from modern decollement zones have migrated substantial distances laterally and tapped deep sources the episodicity of flow suggests pumping of fluid by cycles of dilation and flow to the decollement zone followed by failure and fluid expansion along the decollement zone possible pressure gradients below the decollement zone allow flow upward into it while maintaining minimum effective stress along it the thickness of sediment underthrust beneath the decollement zone determines whether these deposits are emplaced by diffusive underplating with stratal disruption and efficient dewatering or by coherent underplating with the formation of macroscopic duplexes and transfer of fluid to the base of the accretionary prism deformation mechanisms affecting accreted sediment depend on depth of entrainment into the decollement zone and duration of residence

high altitude cusp flow dependence on imf orientation a 3 year cluster statistical study 1 we report on the statistical properties of the plasma flows measured by the cluster spacecraft in the high altitude cusp region of the northern hemisphere as a function of the interplanetary magnetic field imf orientation with selected clock angle intervals the technique uses a magnetic field model taking into account the actual solar wind conditions and level of geomagnetic activity in order to model the magnetopause and cusp displacements as a function of these conditions the distributions of the magnetic field vector show a clear consistency with the imf clock angle intervals chosen and demonstrate that the technique used here fixes the positions of the cusp boundaries adequately the antisunward convection observed in the exterior cusp suggests that this region is statistically quite convective under southward imf while for northward imf the region appears more stagnant the presence of large parallel downward flows at the equatorward edge of the cusp shows that plasma penetration occurs preferentially at the dayside low latitude magnetopause for southward imf conditions in contrast under northward imf the results are suggestive of plasma penetration from the poleward edge of the cusp combined with a substantial sunward convection but no flows are observed at all at the dayside boundary with the plasma sheet the comparison of the measured flow speed with the alfven speed suggests that the magnetosheath adjacent to the external boundary is more sub alfvenic even for high magnetic latitudes under northward imf than under southward imf this result is consistent with the preference for the plasma depletion layer to develop under such conditions the transverse plasma convection in the exterior cusp appears to be controlled by the imf by component as well for dawnward duskward imf orientations the convection is preferentially directed toward dusk dawn these results are interpreted as strong arguments in favor of the cusp being structured at large scales by the occurrence of magnetic reconnection at the high latitude magnetopause for northward imf and at the low latitude magnetopause for southward imf

modeling of fluid flow and pore pressure at the toe of oregon and barbados accretionary wedges fluid venting was recently found on the central oregon accretionary wedge and evidence of fluid flow along the decollement were obtained in the barbados wedge the chemistry of hydrocarbons contained in the migrating fluids is different in these two areas and indicates that the main source is deep for the barbados decollement and shallow for the oregon vents we apply a finite element model with imposed thrusting kinematics and nonlinearities in the compaction equation to the toe of both wedges no deep fluid source is included thus the main source is the footwall just below the frontal thrust the model indicates the following 1 pore pressure build up would be essentially controlled by kn the equivalent permeability in the direction normal to the strata in spite of very different sediment lithologies kn is of the order of 10 17 m2 for both oregon turbidites horath 1989 and barbados muds taylor and leonard 1990 and no high pore pressures can be obtained inside the offscraped sediments 2 permeability anisotropy greatly modifies fluid flow paths a fault with a permeability anisotropy of 103 would channel fluids efficiently but leave the footwall pressure almost unchanged a stratigraphic permeability anisotropy of 102 as documented for the oregon turbidites would redirect the flow of pressurized fluids in the footwall along strata into the trench if clogging of the frontal thrust occured fluids expelled from the footwall would move across the frontal thrust migrate along permeable strata in the hanging wall and then be vented near the top of the eroded anticline as observed in oregon 3 comparison of published estimates from data of fluid explusion fluxes with the modeling results suggests that a deeper and larger fluid source is needed at the barbados site but is not required for the oregon site

porosity of sediments in accretionary prisms and some implications for dewatering processes rapid dewatering largely through loss of porosity is one of the most striking changes affecting deformed sediments in accretionary prisms analysis of porosity from sample and seismic data confirms that porosity in accretionary prisms is significantly less than that in basinal strata of similar lithology more specifically the porosity depth gradient increases arcward across the prism until porosities less than 10 occur within a few kilometers of the surface in deeper parts of the prism dewatering and volume reduction occur not only during loss of porosity but also during dehydration and metamorphic reactions calculations involving volume change lead us to define a function which reflects total volume reduction the distribution of the relative intensity of dewatering within the prism can be calculated from the porosity contours and trajectories of sediment elements an initial attempt indicates that in the toe region the rate of dewatering is accelerated at depth relative to the basinal section and may be greatest near the base of the prism toward the rear of the prism the maximum rate of dewatering occurs at progressively shallower depths several lines of evidence suggest that in addition to gradual diffuse dewatering localized zones of strong dewatering are associated with zones of intense shear this model of shear dewatering based on critical state soil mechanics suggests that sediments adjacent to the basal decollement should be strongly dewatered

nature of the plate contact and subduction zones diversity abstract in recent studies we showed that the nature of the plate contact in subduction zones is an important physical feature in both oceanic lithospheric subduction and continental collision we investigated two fundamental states of the plate contact one based on a fault and the other based on a subduction channel using geodynamic modeling we determined the speci c signatures of both states of the subduction contact we established that the nature of the plate contact influences the dynamic response of the overriding and subducting plate and is one of the controlling factors whether back arc extension develops or not in the present study we combine results of our previous numerical experiments with a re analysis of published observations overall our synthesis connects seismic moment release with back arc deformation and tectonic processes at the margin it leads us to identify four classes of subduction zones the rst two classes result directly from our numerical experiments in class 1 subduction zones are characterized by a plate contact that is largely fault like with an accretionary margin in class 2 the plate contacts are largely channel type and have an erosive margin class 3 where the plate contact is entirely channel like consists of accretionary margins with a high sediment supply subduction zones of class 4 mostly characterized by an erosive convergent margin northern chile peru honshu and kuril are more complicated they can be explained by incorporating regional observations

electron densities in the ionosphere of mars a comparison of marsis and radio occultation measurements radio occultation electron densities measurements from the mariner 9 and viking spacecraft which orbited mars in the 1970s have recently become available in a digital format these data are highl

joint ray born least squares migration and simulated annealing optimization for target oriented quantitative seismic imaging a seismic processing workflow based on iterative ray born migration inversion and target oriented postprocessing of the migrated image is developed for fine scale quantitative characterization of reflectors the first step of the workflow involves linear iterations of the ray born migration inversion the output of the first step is a true amplitude migrated image parameterized by velocity perturbations in a second step postprocessing of the migrated image is performed through a random search with a very fast simulated annealing vfsa algorithm the forward problem of the global optimization is a simple convolutional model that linearly relates a vertical profile of the band limited migrated image after depth to time conversion to a 1d velocity model composed of a stack of homogeneous layers of arbitrary velocity and thickness the aim of the postprocessing is to eliminate the limited bandwidth effects of the source from the migrated image for resolution improvement and enhanced geological interpre

on the formation of the high altitude stagnant cusp cluster observations received 24 february 2005 revised 4 may 2005 accepted 16 may 2005 published 16 june 2005 1 on 16 march 2002 cluster moved from nightside to dayside across the high altitude northern cusp during an extended period of relatively steady positive imf by and bz combined cluster and superdarn data imply the existence of two reconnection sites in the high latitude northern hemisphere dusk and southern hemisphere dawn sectors within the cusp cluster encounters 3 distinct plasma regions first injections of magnetosheath like plasma associated with dawnward and sunward convection suggest cluster crosses newly reconnected field lines related to the dusk reconnection site second cluster observes a stagnant exterior cusp sec characterized by nearly isotropic and stagnant plasma finally cluster crosses a region with significant antifield aligned flows we suggest the observed sec may be located on newly re closed field lines reconnected first poleward of the northern hemisphere cusp and later reconnected again poleward of the southern hemisphere cusp we discuss how the cluster observations correspond to expectations of double reconnection model citation bogdanova y v a marchaudon c j owen m w dunlop h u frey j a wild a n fazakerley b klecker j a davies and s e milan 2005 on the formation of the high altitude stagnant cusp cluster observations geophys res lett 32 l12101 doi 10 1029 2005gl022813

the low latitude boundary layer and the boundary plasma sheet at low altitude prenoon precipitation regions and convection reversal boundaries the dayside zone of soft precipitation can be divided into four distinct types of plasma regimes each corresponding to the respective magnetospheric source region the cusp the mantle the low latitude boundary layer llbl and the dayside extension of the bps based on a detailed spectral study including comparisons with nonsimultaneous isee 1 satellite llbl data we identify regions of llbl type plasma in the dmsp data set and compare these plasma boundaries with convection reversal boundaries crbs as determined by either sondrestrom or the drift meter instrument on board the dmsp f9 spacecraft the nine cases considered are all in the prenoon local time sector we find that in eight of the nine cases the crb occurs within the llbl as expected generally near to but not coincident with the equatorward edge of the llbl type plasma in our sample set chosen for cases with latitudinally wide easily identifiable llbl signatures the average latitudinal width was 1 85 magnetic latitude the crb defined as the onset of steady antisunward convection occurred about 30 of this width beyond the equatorward onset of llbl type particles the most equatorward portion of the region with llbl type plasma usually had near zero or erratic convection and may correspond to the stagnation region reported from isee observations the potential drop observed across the low altitude llbl is roughly estimated to be typically 5 kev a summary is given on how the various high altitude sources can be identified when plasma regions are observed at low altitude in the dayside auroral oval

coulomb wedge model applied to the subduction of seamounts in the japan trench results from the french japanese kaiko exploration program of the japan trench and complementary data are used to propose a model of subduction of seamounts examples in two seamount chains one at the junction between the japan and kuril trenches and the other in the southern part of the japan trench off inubo cape illustrate the stages of subduction the japan trench margin evolved as a homogeneous wedge of deformable noncohesive coulomb material during seamount subduction this is illustrated by the subduction of the daiichi kashima seamount in the southern japan trench our model predicts that the first consequence of seamount subduction is a compressive thickening of the toe of the wedge above the advancing flank of the seamount because of the oversteepening of the rigid base of the wedge this agrees with our observations in the daiichi kashima area this thickening shifts landward across the wedge with the advancing flank of the seamount erosion dominates the wedge above the trailing flank of the seamount because of the corresponding understeepening of the rigid base of the wedge a reentrant of the toe of the wedge is thus created it is largest when the oceanic flank of the seamount coincides with the base of the landward slope at this time the trench axis will probably have an abnormally large amount of sediments as illustrated at the junction between the japan and kuril trenches after the subduction of the seamount the edge of the wedge should progressively move back to its initial location frontal accretion is absent or very limited

high pore pressure generation in sediments in front of the barbados ridge complex dsdp data for the barbados ridge complex show the existence of high pore pressures in sediments both oceanward and arcward of oceanic trenches we model the evolution of pore pressure by using a finite element procedure three consecutive stages are recognized 1 in the oceanic basin sedimentation rate is low there is little excess pore pressure 2 approaching the trench but before being incorporated into the accretionary prism sediments are subjected to increasing tectonic compression as well as greater sediment overburden thus high pore pressure can be created an important result is that the lowest effective stress occurs within the sedimentary layer instead of at its base and a decollement may develop along this weakened zone 3 arcward of the trench sediments above the decollement are offscraped while those beneath it are subducted fast sedimentation combined with tectonic compression can create pore pressures so high as to cause porosity to increase with depth this facilitates the formation of mud volcanoes even oceanward of the deformation front

geological record of fluid flow and seismogenesis along an erosive subducting plate boundary a fossil erosive subduction channel the shear zone marking the plate boundary preserved in the northern apennines of italy is identified the fossil zone records the presence of two decollements which seem to have been simultaneously active at the top and base of the subduction channel in modern subduction zones the onset of seismic activity is believed to occur at 150 c however in the fossil channel the onset seems to have occurred at colder temperatures tectonic erosion of the overriding plate by the downgoing slab is believed to occur at half the earth s subduction zones1 2 in situ investigation of the geological processes at active erosive margins is extremely difficult owing to the deep marine environment and the net loss of forearc crust to deeper levels in the subduction zone until now a fossil erosive subduction channel the shear zone marking the plate boundary3 has not been recognized in the field so that seismic observations have provided the only information on plate boundary processes at erosive margins here we show that a fossil erosive margin is preserved in the northern apennines of italy it formed during the tertiary transition from oceanic subduction to continental collision and was preserved by the late deactivation and fossilization of the plate boundary the outcropping erosive subduction channel is 500 m thick it is representative of the first 5 km of depth with its deeper portions reaching 150 c the fossil zone records several surprises two decollements were simultaneously active at the top and base of the subduction channel both deeper basal erosion and near surface frontal erosion occurred at shallow depths extension was a key deformation component within this erosive convergent plate boundary and slip occurred without an observable fluid pressure cycle at depths greater than about 3 km a fluid cycle is clearly shown by the development of veins and the alternation of fast co seismic and slow inter seismic slip in the deepest portions of the outcropping subduction channel extension is finally overprinted by compressional structures in modern subduction zones the onset of seismic activity is believed to occur at 150 c but in the fossil channel the onset occurred at cooler palaeo temperatures

control of internal structure and fluid migration pathways within the barbados ridge decollement zone by strike slip faulting evidence from coherence and three dimensional seismic amplitude imaging the application of three dimensional seismic reflection and coherence imaging to the study of the decollement zone of the barbados ridge accretionary complex has provided new insights into the relationships among internal structure fluid flow and previously unrecognized strike slip faulting combined coherence and seismic amplitude imaging of the decollement zone reveal anomalous northeast trending lineaments parallel to and abutting zones of high amplitude negative polarity reflections analysis of these lineaments shows them to be penetrative structures dipping southeast with apparent reverse dip slip offset isopach mapping of the accretionary wedge indicates significant right lateral displacement across these structures these faults apparently channel fluid flow within the decollement zone and the prominent northeast trending conduits so formed are readily visible as high amplitude negative polarity reflections additionally north northeast trending zones of variable coherence and high positive amplitude are inferred barriers to up structure fluid migration pathways movement along strike slip structures probably alternates with displacement along the decollement zone northeast trending strike slip faults extend for 13 km crossing the length of the survey area and into the incoming oceanic plate active arc oblique strike slip faulting of the decollement zone beneath the barbados ridge accretionary wedge implies a stress regime in that 1 is fixed and 2 and 3 either transpose with time or are nearly equal this state of stress may be a common occurrence in forearc tectonism and may have led to the formation of many as yet unrecognized arc oblique strike slip faults at convergent margins

a summary of cenozoic tectonic history along the ipod japan trench transect along the international program of ocean drilling ipod japan trench transect a mid cenozoic period of tectonism without arc volcanism separated an earlier cretaceous to paleogene and a later neogene period of plate convergence and subduction the cretaceous to early paleogene period of convergence is evidenced by andesitic volcanism associated with a large syncline probably a forearc basin the seaward flank of the basin is a tectonically thickened sediment sequence inferred to be an accretionary complex the arc the syncline and the thickened sediment sequence resemble an arc trench system of the size and structure of the great valley forearc basin and franciscan accretionary complex of california and they were probably continuations of the yezo geosyncline and associated sequences exposed to the north on hokkaido the arc trench system is contemporaneous with the adjacent shimanto arc trench system southwest of tokyo bay in the early paleogene volcanism ceased and part of the thickened sediment sequence of the cretaceous paleogene margin was emergent as a 160 km wide landmass which is recognized in seismic records as a later paleogene sediment source at the end of the paleogene this landmass began to subside and volcanism began again but it was located first briefly on the east and then on the west of the cretaceous arc along the present arc of northern honshu the land mass erosion surface is clearly visible as an angular unconformity in multichannel seismic reflection records the sequence of neogene sediment explosive volcanism and benthic foraminiferal assemblages recording subsidence from subareal conditions to the present bathyal depths have been studied in deep sea drilling project dsdp cores cretaceous subduction appears to have resulted in a very extensive accretionary complex whereas the neogene subduction period appears to have been marked by little net accretion in the forearc area but massive subsidence and some erosion of the front of the convergent margin

interplate patchiness and subduction erosion mechanisms evidence from depth migrated seismic images at the central ecuador convergent margin prestack depth migrated seismic lines provide accurate images of the convergent ecuadorian margin where the southern flank of the carnegie ridge subducts the margin is fronted by a 5 7 km wide compressional sediment prism beneath the slope margin thinning is accomplished by gradual thinning of the lower part of the basement accompanied by pervasive seaward dipping normal faulting indicating basal subduction erosion at a low friction plate interface along the plate boundary a thin subduction channel locally thickens to form low velocity 600 m thick lenses of underthrusted fluid rich sediment supplied by slope mass wasting the contrast between the fluid rich sediment and the surrounding thinner and drier sections of the subduction channel creates a three dimensional patchiness across the plate boundary implying variations in mechanical interplate coupling the subduction channel patchiness modulates subduction erosion processes by alternately favoring margin basement weakening and material removal basement weakening would occur 1 at the base of the upper plate where enhanced reflectivity indicates basement damage by overpressured fluids from the subduction channel and 2 at the basement apex where basement breakup is caused by superposition of compressional and extensional fault systems resulting from time variable interplate mechanical coupling the detachment of rock debris in the higher coupled sections and the subsequent dragging of the fragments into the subduction channel cause tectonic erosion deeper in the subduction the subduction channel patchiness may influence processes like earthquake nucleation and rupture propagation and material recycling in the mantle

fluid flow along a decollement layer a model applied to the 16 n section of the barbados accretionary wedge a seismic section across the whole barbados accretionary complex at 16 n recently published by westbrook et al 1988 shows that the reflection marking the decollement fades between 40 and 60 km arcward of the deformation front but is present again further inland this interruption appears to be related to a thrust cutting through the wedge one dimensional steady state models of water flow along the decollement suggest a mechanism responsible for decollement disruption a local increase of the slope either of the wedge or of the decollement causes the subducted layer to compact and locally increases the friction along the decollement if the thrust is used as a conduit for fluids to escape from the decollement the friction along the decollement toeward of the thrust is further increased the model also shows that the permeability of the decollement should be of the order of 10 14 m2 but may be higher near the deformation front for short periods if fluid expulsion mainly occurs in transient bursts associated with hydraulic fracturing as a consequence of the non linearity of the compaction equation pore pressure at depth in the decollement depends on the water flux on the lithostatic pressure gradient along the decollement and on the parameters from which the physical properties of the sediment are calculated however it is almost independent on the pore pressure below the toe

accretion and erosion in subduction zones the role of fluids convergence occurs today along about 60 000 km of plate boundaries parsons 1981 in this paper we consider frontal subduction which occurs along about 40 000 km of convergent plate boundaries using the kine matic solution of de mets et al 1990 and including 0 5 km 2 yr1 of marginal basin openings otsuki 1989 we find that 2 95 km 2 per year disappear in frontal subduction at an average rate of 7 3 cm yr the upper overriding plate is significantly modified not only through the addition of a large amount of magmatic material markhinin 1968 but also through tectonic processes which result either in outgrowth of the margin through addit ion of sediments silver 1969 or in recession through frontal and subcrustal erosion which appears to be linked to sediment subduction coats 1962 gilluly 1963 the various phenomena observed must in some way be related to variations in mechanical coupling between the two plates uyeda kanamori 1979 had proposed that the transition from erosion to accretion results from increasing mechanical coupling the marianas trench was presented as typical of an crosional margin and the chile trench of an accreting margin ruff kanamori 1980 and kana mori 1986 showed that the magnitude of the subduction earthquakes increases with the convergence velocity and the inverse of the age of the subducted lithosphere this correlation was qualitatively explained by the control of age on the d ip of the platc and of convergence velocity on the

structure of the nankai trough accretionary zone from multichannel seismic reflection data new multichannel seismic reflection data collected over the nankai trough image the accretionary complex in two areas the international program of ocean drilling leg 87 transect area western area and the region of upcoming ocean drilling program leg 131 eastern area the incoming shikoku basin sedimentary section consists of hemipelagic clays and thin terrigenous turbidites the basin section is overlain by a trench wedge that is 12 16 km wide and 350 750 m thick at the thrust front accretionary deformation begins in a protothrust zone that is characterized by thickening and seaward tilting of the trench wedge the zone in the western area is 4 5 km wide and is characterized by kink folds the zone in the eastern area is only 2 5 km wide and does not exhibit such folds the frontal thrusts in each area are imaged as fault plane reflections and ramp upward from within the basin hemipelagic section the overthrusting sediments form fault bend folds over these ramps thrust spacing at the toe of the slope is 1 5 2 5 km the second thrust cuts up from an inferred decollement within the shikoku basin sedimentary section in the eastern area a reflection marking the top of the basin pelagic sediment section changes from normal to reversed polarity about 6 3 km seaward of the thrust front and underlies the entire protothrust zone this reflector continues with reversed polarity under the accretionary complex and is at the level of the basal decollement the underlying basin pelagic section is apparently thrust undisturbed beneath the accretionary prism the reversal of polarity indicates a change in reflection coefficient that is due to a combination of decreasing seismic velocity and density across the interface this decrease in velocity and density may indicate that the decollement is a zone of high porosity due to fluid expulsion from deeper within the accretionary prism the reflections from the first and second thrusts are also reversed polarity possibly indicating that they also are pathways of fluid expulsion the critical wedge taper of the western area is greater than that of the eastern area an observation that is consistent with the existence of an overpressured decollement in the eastern area

empirical relationships among the convergence rate of plates rollback rate of trench axis and island arc tectonics laws of convergence rate of plates abstract the data on 27 recent arcs which were subducted by normal oceanic lithosphere were compiled among the compression extension rates for island arcs v a compression is defined as positive and the kinematic components of absolute plate motions parallel to the direction of backarc compression extension v on oceanward velocity of the overriding plate v sn continentward velocity of the subducting plate and v tn rollback rate of the trench axis the equations v a v on v sn 7 2 cm y 1 v tn 7 2 cm y v sn 2 are found to hold as for the convergent zones associated with a wadati benioff zone deeper than 200 km equation 1 suggests that backarc spreading and crustal shortening play the role of compensating the deviation of the convergence rate from the critical value of 7 2 cm y the equation 2 means that any subducting oceanic lithosphere has the potentiality of making the trench axis rollback at the rate of 7 2 cm y and that the virtual rollback rate v tn is determined by the balance between the potential rollback rate 7 2 cm y and v sn the present author calls the kinematic relationships represented by equations 1 and 2 laws of convergence rate of plates the convergent zones of plates associated with the wadati benioff zone shallower than 200 km also show a tendency that the greater and smaller values of v on v sn correspond to the backarc compression and extension however the critical value is 3 4 cm y instead of 7 2 cm y

deciphering the late cretaceous cenozoic structural evolution of the north peruvian forearc system the link between plate tectonics and the evolution of active margins is still an ongoing task to challenge since the acceptance of plate tectonic paradigm this paper aims at deciphering the structural architecture and uplift history of the north peruvian forearc system to better understand the evolution and the mechanics that govern the late cretaceous cenozoic building of this active margin in this study we report surface structural geology data interpretation of seismic reflection profiles apatite fission track data and the construction of two offshore onshore crustal scale balanced cross sections the structure of the north peruvian forearc system is dominated by an accretionary style with northwestward propagation of thrust related structural highs involving continental oceanic basement rocks and off scrapped sediments the thrust systems bound thick thrust top forearc depocenters mainly deformed by crustal normal to strike slip faults and thin skinned gravitational instabilities the sequential restoration of the margin calibrated with apatite fission track data suggests a correlation between uplift shortening and plate convergence velocity during late cretaceous and miocene pliocene quaternary shortening and uplift of the coastal zone is rather related to the subduction of asperities during convergence rate decrease the development of crustal normal to strike slip faulting and subsidence zones might be the consequence of slab flexure local basal erosion along subduction fault and or oblique subduction associated with sediment loading control we conclude that the evolution of the north peruvian forearc system was controlled by subduction dynamics strong sediment accumulation and recent ridge subduction and it recorded the orogenic loading evolution of the andes over the cenozoic

detection and location of earthquakes in the central aleutian subduction zone using island and ocean bottom seismograph stations a network of eight university of texas ocean bottom seismographs obs operated for 6 weeks in 1978 about 50 km offshore of adak island alaska and nearby islands in 1979 a similar network of nine instruments was deployed for 7 weeks farther offshore within and up to 100 km seaward of the aleutian trench for shallow earthquakes on the outer trench slope for shallow earthquakes in the thrust zone and for intermediate depth events we have analyzed the obs and island based network data and evaluated the island network s capabilities for earthquake detection and location and for focal mechanism determination our three major conclusions are presented the first concerns shallow earthquakes on the outer trench slope in 1979 about 30 earthquakes occurred within the aleutian trench and up to 60 km seaward of the trench axis the island network located none of these events and detected p phases for only three of them ray tracing shows that the islands lie in a geometric shadow zone for events on the outer trench slope the best located events are shallower than 20 km and exhibit first motions consistent with normal faulting several authors have suggested that these events are caused by bending of the oceanic lithosphere at the outer rise prior to subduction if so then the event locations reported here show that the bending stresses exceed the strength of lithosphere only in a narrow zone extending about 10 km landward and 60 km seaward of the trench axis the second conclusion concerns shallow earthquakes in the thrust zone epicenters determined by island stations alone are virtually identical to epicenters determined using data from both island and obs stations the locations are similar whether they are determined using flat layered velocity models or a more realistic model and a ray tracing scheme nearly all the earthquakes detected by the obs network are also recorded by island stations three composite focal mechanism solutions that use data from both island and obs stations have p axes parallel to the direction of plate convergence the thrust zone events are separated from events on the outer trench slope by a gap in seismicity about 50 km wide situated on the inner trench slope the third conclusion concerns earthquakes deeper than 70 km epicenters determined using island network stations alone lie 10 to 80 km south of those determined using obs and island stations with the differences between epicenters depending both on event depth and on the velocity model used ray tracing with a realistic velocity model shows that for events at 200 km depth locations are about 80 km north of locations determined from island data alone these results do not support the observed increase in the dip of the benioff zone beneath 100 km depth that is suggested by locations determined from island network data alone using a flat layered velocity model

contrasting geothermal regimes of the barbados ridge accretionary complex three e w transects of closely spaced heat flow measurements across the barbados accretionary complex are reported one transect about 100 km in length at 14 20 n and a shorter 15 km line at 14 35 n show that heat flow decreases arcward roughly in proportion to the thickening of the wedge over the actively accreting zone this decrease is due to the combined thermal effects of the underthrusting oceanic lithosphere and thickening of sediments that have been ingested into the wedge except for anomalously high heat flow over a mud volcano on the eastern end of the 100 km transect fluid expulsion appears to have only a secondary effect on the seafloor heat flow in this region an increase of heat flow is observed at the western end of the 100 km transect where high angle arcward vergent thrusting is occurring we attribute this increase primarily to a decrease in the arcward thickening of wedge material and heat generated by friction on the decollement in strong contrast a short line of stations at 15 30 n the deep sea drilling project ocean drilling project dsdp odp drilling transect found anomalously high heat flow a 40 increase over the heat flux from the lithosphere with large variability over the most recently accreted material and also in the adjacent basin the thermal regime in the vicinity of the dsdp odp transect is dominated by a strong flow of pore fluids along the decollement and upward flow along fault planes in the toe of the wedge and adjacent basin sediments heat transport by pore fluids also appears to dominate the subseafloor thermal regime in a zone near 13 50 n the thermal effects of dewatering of the sediments by upward diffusive flow were examined using a simple one dimensional model which showed that the resulting effects are too small to produce detectable heat flow anomalies consequently the flow of fluids must be channelized in subseafloor conduits to produce the observed anomalies in heat flow

heat flow tectonics and fluid circulation at the toe of the barbados ridge accretionary prism detailed patterns of near surface tectonics and heat flow were investigated at two locations at the toe of the barbados ridge accretionary prism at the two locations in the thickly sedimented southern portion of the prism near 12 20 n and in the northern area drilled during ocean drilling program odp leg 110 near 15 32 n relative highs of heat flow were associated with zones of thrusting we attribute them to upward advection of water rising from the decollement along faults through the sediments in the odp area a significant heating plate effect from warm fluids flowing along the decollement and thrusts is required to account for the high regional heat flow this effect implies high rates of fluid flow at the decollement which can only be non steady state or would require massive lateral transport of fluids along strike

temperatures heat flux and frictional stress near major thrust faults we examine the two dimensional advective and conductive transport of heat in a region of thrust faulting both simple theoretical considerations and numerical experiments show that the steady state temperatures near the fault are reduced by a divisor s 1 b z vsin below what they would be with the same heat sources but in the absence of advection in this expression z is the depth to the fault v is the slip rate is the component of the dip of the fault in the direction of underthrusting is thermal diffusivity and b is a dimensionless factor that is essentially equal to one for most forms of heating initial changes in temperatures near the fault are given by t z t t z 0 12vsin t z 0 z these simple formulae are successful because of the neglible influence of lateral conduction of heat at least for slip rates of a few mm yr or more two time constants govern the transition from the initial change in temperature to steady state t1 u v where u is the distance along the fault in the direction of underthrusting from the surface to the depth in question and t2 z 2 2 where z is the depth to the fault when elapsed times exceed the sum of these two time constants temperatures differ from their steady state values by only about 10 percent the numerical experiments indicate that the simple formulae are sufficiently accurate that sophisticiated numerical modeling of temperatures in specific regions is unwarranted putain an application of these simple formulae to measurements of conductive heat flow at island arcs implies that shear stresses at island arcs approach 100 mpa and are greater than 30 mpa calculations of temperatures appropriate for the himalaya suggest that shear stresses of 100 mpa on the main central thrust probably are required to account for the tertiary granites of the region if melting took place after slip began on the thrust similarly the cut off in seismicity at a depth of about 15 km in the himalaya if due to temperatures exceeding 350 to 450 c implies a deviatoric stress close to 100 mpa

mechanism of decollement formation in subduction zones summary the mechanism of decollement formation was investigated through a particle based simulation model assuming homogeneity e g no weak layer or pore fluid a decollement like structure appeared as a spontaneously localized shear deformation near the bottom of the sediment when the thickness of the sediment was sufficient to balance the gravitational force and tectonic loading in contrast no such decollement like structure was formed when the sediment was too thin in this case the entire prism was deformed because of plate motion these results are consistent with various observations in real subduction zones a precise analysis of the stress state evolution during accretion reveals that the formation of a decollement like structure is controlled by the spatio temporal distribution of isotropic compression states

fluid circulation in the depths of accretionary prisms an example of the shimanto belt kyushu japan accretionary prisms constitute ideal targets to study fluid circulation and fluid rock interactions at depths beyond the reach of active margin deep drilling the highest grade rocks from the shimanto belt on kyushu were buried under 3 5 kbars at 300 c toriumi and teruya 1988 they contain abundant quartz veins formed throughout burial and exhumation and variably affected by brittle and ductile deformation cathodoluminescence cl reveals the existence of two distinct types of quartz characterized by a blue and brown color respectively cl blue quartz fills macro veins width 10 m while cl brown quartz is present in micro veins width 1 10 m and ductilely recrystallized domains on the basis of microstructures the fluids associated with the cl blue and cl brown quartz are interpreted as external and local respectively quartz growth rims of alternating cl colors as well as mutually cross cutting veins show that the two fluids cyclically wetted the host rock from fluid inclusions analysis the fluid associated with cl blue quartz has a salinity similar to seawater while the fluid associated with cl brown quartz is less saline in addition cl blue quartz is richer in aluminum than the cl brown one in contrast to the salinity aluminum signature the 18o isotopic signature of both quartz types is similar and buffered by host rock the difference between the preservation of the salinity signature of the fluid and the loss of its 18o signature is explained by quicker exchange kinetics and larger host rock buffering capacity for isotopic reequilibration the local fluid associated with cl brown quartz reflects the dilution of pore water by the pure water produced by prograde dehydration reactions of clay minerals the external fluid associated with cl blue quartz is interpreted as seawater or pore water from shallow depth 1 2 km below seafloor sediments we propose that downward percolation of shallow water to depths 10km is a transient process associated with mega earthquakes

interpretation of temperature measurements from the kaiko nankai cruise modeling of fluid flow in clam colonies during the kaiko nankai detailed submersible survey numerous measurements of the temperature gradient inside the sediment were performed on the deepest active zone of fluid venting which is situated on the anticline related to the frontal thrust using the ifremer t naut temperature probe operated from the submersiblenautile we thus obtained the temperature structure below different types of clam colonies associated with fluid venting we used the finite element method to model the thermal structure and fluid flow pattern of these vents and to determine the velocity of upward fluid flow through the colonies on a biological basis four types of clam colonies are defined each biological type has distinctive thermal characteristics and corresponds to a particular fluid flow pattern darcian flow velocity in the most active type of colony type a is of the order of 100 m a the total amount of fluid flowing through colonies in the studied area is estimated to be 200 m3 a 1 per metre width of subduction zone most of the flow is vented through type a colonies this value is more than one order of magnitude too high to be compatible with the amount of water available from steady state compaction of sediments in the whole wedge thermal arguments suggest that downwelling of seawater occurs around type a colonies and that seawater is then mixed with upcoming fluids at a depth of 1 or 2 metres furthermore finite element modeling shows that a salinity difference of a few parts per mil between the upcoming fluids and seawater is sufficient to drive convection around the colonies as water samples from a few vents indicate that the fluid source should actually be significantly less saline than seawater we propose that the very high fluid flows measured are a consequence of the dilution of the fluid of deep origin with seawater by a factor of 5 to 10

finding a realistic velocity distribution based on iterating forward modelling and tomographic inversion tomography is like a photograph that was taken by a camera with blurred and defective lenses that deform the shapes and colours of objects reporting quantitative parameters derived from tomographic inversion is not always adequate because tomographic results are often strongly biased to quantify the results of tomographic inversion we propose a forward modelling and tomographic inversion fm ti approach that aims to find a more realistic solution than conventional tomographic inversion the fm ti scheme is based on the assumption that if two tomograms derived from the inversion of observed and synthetic data are identical the synthetic structure may appear to be closer to the real unknown structure in the ground than the inversion result however the manual design of the synthetic velocity distribution is usually time consuming and ambiguous in this study we propose an approach that automatically searches for a probabilistic model in this approach a synthetic model is iteratively updated while taking into account the bias of the model in previous stages of the fm ti performance here we present an example of synthetic modelling and real data processing for an active source refraction data set corresponding to a marine profile across the subduction zone in chile at about 32 s latitude a key feature of the model is a low velocity channel above the subducted oceanic crust which was defined in the synthetic model and expected in the real case the conventional first arrival traveltime tomography was barely able to resolve this channel however after several iterations of the fm ti modelling we succeeded in reconstructing this channel clearly in the paper we briefly discuss the nature of this low velocity subduction channel and we compare the results with other studies

thick strong sediment subduction along south central chile and its role in great earthquakes abstract the south central chile margin experienced the largest and sixth largest earthquakes ever recorded the 1960 mw 9 5 valdivia and 2010 mw 8 8 maule megathrust earthquakes respectively in early 2017 we conducted a seismic survey along 1 000 km of south central chile to image these rupture zones using a 15 15 km long multi channel seismic streamer we processed these data using pre stack depth migration which provides the best look at the shallow part of the south central chile margin to date relative to other sediment dominated subduction zones where sediment is typically accreted at the toe an unusually large percentage of the thick trench sediments are consistently subducted beneath the slope with little thrust faulting or deformation analysis of the sediment p wave velocities and structure in the trench and outer wedge leads us to conclude that most of south central chile contains well drained strong sediments an exception in the vicinity of the subducting mocha fracture zone mfz has trench sediments that appear to experience localized delayed compaction thus lowering their strength and allowing the development of protothrusts similar to what is seen in other accretionary subduction zones the very shallow decollement along the south central chile allows more sediment to pass beneath the lower slope than almost all other subduction zone many of which have much thicker trench sections we conclude that subduction of the strong well drained thick sediment layer beneath the lower slope is typical for nearly all of the south central chile comparison to other thick sedimented subduction zones worldwide reveals that the subduction of such a large fraction of the trench sediment is particularly unusual this strong thick subducting sediment is likely a determining factor for developing a smooth plate interface located well above the subducted crust topography that ultimately becomes a broad megathrust with high homogeneous frictional properties and generates particularly large earthquakes along the south central chile margin

the role of compaction contrasts in sediments in decollement initiation in an accretionary prism abstract to understand how decollements develop into the pristine sedimentary succession entering subduction zones we have performed mechanical tests on samples from the sediment column entering the nankai accretionary prism japan odp site 1173 both poroelastic compliance and plastic shrinkage upon application of a large effective pressure sharply decrease with depth in a 100 m thick domain in the upper section of the lower shikoku basin unit i e in a domain stratigraphically close to the actual location of the decollement near the toe of the prism these property contrasts provide a potential explanation for the outward migration of the decollement into the incoming sediments when approaching the deformation front a given material particle is affected by an increase in stress which has a component of vertical loading due to the deposition of overburden trench sediment and also a component of lateral compression transmitted from the accretionary wedge depending on its initial mechanical state the amount of lateral shortening in the incoming nankai sediment column varies with depth and causes horizontal velocity gradients that concentrate into the mechanical transition zone upper section of the lower shikoku basin at appx 450 550 m depth into which the decollement eventually propagates future work has to assess the role of this plastic deformation relative to other governing factors such as friction coefficient and excess pore pressure both at nankai and along other active margins

major and trace element and sr isotope constraints on fluid circulation in the barbados accretionary complex part ii circulation rates and fluxes abstract 87 sr 86 sr ratios of pore fluids recovered from diapiric structures east of the deformation front of the barbados accretionary prism remain quasi constant from the surface to depths of 8 m the sr isotopic ratios are non radiogenic compared with seawater which implies that non local fluid was expelled at such a high rate that its sr isotopic composition was dominated by the source fluid and not by pore fluid or seawater chloride depletion recorded in the fluids corresponds to a five fold dilution in the deepest sample from the atalante diatreme this also strongly suggests upward flow the comparison of measured chloride contents and sr isotope depth profiles with theoretical values obtained using a steady state advection diffusive model gives some constraints on the fluid circulation rate in the two diatremes atalante and cyclope and in one mud diapir c the velocities are variable but the diatremes which have the largest thermal and chemical anomalies yield the highest estimated advection rates the fluid circulation rate in the centre part of the largest diatreme atalante 0 47 km 2 total surface is estimated at about 1 m yr the fluid flux from the deepest to shallowest levels of the atalante diatreme was estimated on the basis of the fluid circulation rate calculations fluid release is clearly favoured if active conduits are present as they are in the diatremes but not in the mud diapirs however even though this steady state advection diffusion model provides a rough estimation of flow rates modification of the fluid pressure and therefore fluid volume during earthquakes may superimpose large but transient fluid expulsions

fluid venting along japanese trenches tectonic context and thermal modeling abstract large benthic chemosynthetic communities have been observed at four main locations during the kaiko submersible dives in the japanese trenches they appear to be associated with venting along fractures the first site for our observation was along the japan and kuril trenches where the continental margin is eroded by the subducting plate and collapses into the trench the benthic communities there seem to be related to tension gashes parallel to the subduction vector the other communities were found on the toe of the nankai accretionary prism along the frontal thrust and tension gashes the temperature anomaly associated with one of the communities is modeled to constrain the upward flow of interstitial water as the anomaly has a small spatial extent and as the peak thermal gradient is high the best fitting model is to be found in a vertical upward flow at a velocity of 100 m yr in a cylindrical conduit leading out of an underlying shallow thrust

inferences on sediment strength and fault friction from structures at the aleutian trench the structural relations of conjugate thrust faults and the master decollement imaged in a research level processed multichannel seismic reflection record across the eastern aleutian trench allow quantitative estimates of the sediment strength and friction across thrust boundaries of the subduction zone these estimates are derived from geometric relations between faults and from the overall geometry of accreted prisms they suggest the presence of high pore fluid pressures that result in low levels of friction along a master decollement and on the main bounding thrust faults in the accreted sediment therefore coupling in the immediate trench area apparently does not contribute significantly to the generation of horizontal compression upslope