organic matter cracking a source of fluid overpressure in subducting sediments the pressure of deep fluids in subduction zones is a major control on plate boundary strength and earthquake genesis the record by methane rich fluid inclusions of large 50 100 mpa and instantaneous pressure variations in the shimanto belt japan points to the presence of large fluid overpressure at depth 300 500 mpa 250 c to further analyze the connection between methane and fluid overpressure we determined with rock eval the potential for a worldwide selection of deep seafloor sediments to produce methane as a result of organic matter om cracking due to temperature increase during subduction the principal factor controlling the methanogenesis potential of sediments is om proportion while om nature is only a subordinate factor in turn om proportion is mainly controlled by the organic terrigenous input considering a typical sediment from ocean continent subduction zones containing 0 5 wt of type iii om cracking of om has two major consequences 1 methane is produced in sufficient concentration as to oversaturate the pore filling water the deep fluid in accretionary prisms is therefore a mechanical mixture of water rich and methane rich phases 2 ch4 production can generate large fluid overpressure of the order of several tens of mpa the conditions for these large overpressure are a low permeability of the upper plate 10 km where om thermal cracking occurs at these depths om thermal cracking appears as a source of overpressure larger than the last increments of smectite to illite reaction such large overpressures play potentially a role in facilitating slip along the plate interface conversely the scarcity of earthquakes in ocean ocean subduction zones such as marianna or barbados may be related to the low influx of detrital om and the limited methane overpressure generation at depth

seismic evidence for overpressured subducted oceanic crust and megathrust fault sealing water and hydrous minerals play a key role in geodynamic processes at subduction zones however there are few seismological constraints on the presence of water within the subducting plate audet et al model converted teleseismic waves to constrain the seismological properties of subducted oceanic crust from the cascadia continental margin to its intersection with the forearc mantle their observations suggest that water is pervasively present in fluid form at high pore pressures indicating that the megathrust is a low permeability boundary these results may hold important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip this study models converted teleseismic waves to constrain the seismological properties of subducted oceanic crust from the cascadia continental margin to its intersection with the forearc mantle the observations suggest that water is pervasively present in fluid form at high pore pressures indicating that the megathrust is a low permeability boundary these results may hold important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip water and hydrous minerals play a key part in geodynamic processes at subduction zones1 2 3 by weakening the plate boundary aiding slip and permitting subduction and indeed plate tectonics to occur4 the seismological signature of water within the forearc mantle wedge is evident in anomalies with low seismic shear velocity marking serpentinization5 6 7 however seismological observations bearing on the presence of water within the subducting plate itself are less well documented here we use converted teleseismic waves to obtain observations of anomalously high poisson s ratios within the subducted oceanic crust from the cascadia continental margin to its intersection with forearc mantle on the basis of pressure temperature and compositional considerations the elevated poisson s ratios indicate that water is pervasively present in fluid form at pore pressures near lithostatic values combined with observations of a strong negative velocity contrast at the top of the oceanic crust our results imply that the megathrust is a low permeability boundary the transition from a low to high permeability plate interface downdip into the mantle wedge is explained by hydrofracturing of the seal by volume changes across the interface caused by the onset of crustal eclogitization and mantle serpentinization these results may have important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip

empirical relationships among seismic velocity effective pressure porosity and clay content in sandstone we use a multivariate analysis to investigate the influence of effective pressure pe porosity and clay content c on the compressional velocity vp and shear velocity vs of sandstones laboratory measurements on water saturated samples of 64 different sandstones provide a large data set that was analyzed statistically for each sample relationships between effective pressure and vp and vs have been determined all samples were well fit by relationships that have an exponential increase in velocity at low pe tapering to a linear increase with pe for pe greater than 0 2 kbar there are differences in the pressure dependences of velocity for different rocks particularly at very low pressures however the differences cannot be attributed to or c for the combined set of measurements from all samples the best fitting formulations are vp 5 77 6 94 1 73c 0 446 pe e 16 7pe and vs 3 70 4 94 1 57c 0 361 pe e 16 7pe while this is admittedly a very simplified parameterization it is remarkable how well

tectonic incorporation of the upper part of oceanic crust to overriding plate of a convergent margin an example from the cretaceous early tertiary mugi melange the shimanto belt japan abstract a tectonic melange exposed on land is examined to reveal relationships between melange formation underplating and deformation mechanisms focusing on the deformation of basaltic rocks the studied mugi melange of the shimanto belt is composed of a shale matrix surrounding various blocks of sandstone pelagic sediments and basalts the melange was formed during late cretaceous to early tertiary times in a subduction zone under p t conditions of 150 200 c and 6 7 km depth as estimated from vitrinite reflectance and quartz veins fluid inclusions the melange represents a range of deformation mechanisms pressure solution with micro scale cataclasis in the shale matrix brittle tension cracking in the blocks and ubiquitous strong cataclasis in the basal portion of basaltic layers the cataclastic deformation in the basalts suggests a breakage of a topographic high in the seismogenic depth

new insights into deformation and fluid flow processes in the nankai trough accretionary prism results of ocean drilling program leg 190 moore g f taira a klaus a becker l boeckel b cragg b a dean a fergusson c l henry p hirano s hisamitsu t et al 2001 new insights into deformation and fluid flow processes in the nankai trough accretionary prism results of ocean drilling program leg 190 geochemistry geophysics geosystems 2 article no 2001gc000166

the role of frictional strength on plate coupling at the subduction interface 1 at a subduction zone the amount of friction between the incoming plate and the forearc is an important factor in controlling the dip angle of subduction and the structure of the forearc in this paper we investigate the role of the frictional strength of sediments and of the serpentinized peridotite on the evolution of convergent margins in numerical models we vary thickness of a serpentinized layer in the mantle wedge 15 to 25 km and the frictional strength of both the sediments and serpentinized mantle friction angle 1 to 15 or static friction coefficient 0 017 to 0 27 to control the amount of frictional coupling between the plates with plastic strain weakening in the lithosphere our numerical models can attain stable subduction geometry over millions of years we find that the frictional strength of the sediments and serpentinized peridotite exerts the largest control on the dip angle of the subduction interface at seismogenic depths in the case of low sediment and serpentinite friction the subduction interface has a shallow dip while the subduction zone develops an accretionary prism a broad forearc high a deep forearc basin and a shallow trench in the high friction case the subduction interface is steep the trench is deeper and the accretionary prism forearc high and basin are all absent the resultant free air gravity and topographic signature of these subduction zone models are consistent with observations we believe that the low friction model produces a geometry and forearc structure similar to that of accretionary margins conversely models with high friction angles in sediments and serpentinite develop characteristics of an erosional convergent margin we find that the strength of the subduction interface is critical in controlling the amount of coupling at the seismogenic zone and perhaps ultimately the size of the largest earthquakes at subduction zones

microfracturing paleostress and the growth of faults abstract we have analyzed the microfractures in samples taken near one thrust fault and five normal faults of known displacement to test the dependence of microfracture density and distribution on fault slip we have also recorded the orientation of microfractures as a function of distance from the associated fault surface in order to control as many parameters as possible shallow faults with similar lithologies were chosen they showed the same kind of deformation namely microfracturing without block rotation recrystallization extensive granulation or crystal plasticity the results of over 8000 microfracture measurements suggest that maximum microfracture density is independent of net slip we interpret this relationship as indicating that the preponderance of microfracturing occurs in proximity to the propagating fault tip the angular relationship between the shear plane and microfractures was commonly found to be between 5 and 20 significantly less than the 30 expected between the far field maximum principal stress and the shear surface this low angle between microfracture orientation and shear plane orientation is predicted for a local stress field associated with a propagating mode ii fault tip other possible explanations for the small angle for tensional microfractures and the shear surface are 1 the effective confining pressure was very low or 2 shear strength at failure was much larger than expected

elastic piezosensitivity of porous and fractured rocks understanding the stress dependencies of seismic velocities is important for interpreting a variety of seismic data ranging from amplitude versus offset avo and velocity analysis to overpressure prediction and 4d seismic monitoring of reservoirs sometimes rather complex forms of these dependencies based on specific models of porous space geometry are used for example spherical contacts models duffy and mindlin 1957 merkel et al 2001 duffy merkel 2001 and crack contacts models gangi and carlson 1996 carcione and tinivella 2001 have been used in different studies gangi96 however usually the pore pressure velocity dependence along with the velocity dependence on the confining pressure are phenomenologically described by the following simple relationship zimmerman et al 1986 eberhart phillips et al 1989 freund 1992 jones 1992 prasad and manghnani 1997 khaksar et al 1999 carcione and tinivella 2001 kirstetter and macbeth 2001

frontal accretion an internal clock for bivergent wedge deformation and surface uplift 1 we propose a conceptual model of frontal accretion within bivergent wedges which is based on two dimensional sandbox simulations and the analysis of particle displacement fields each frontal accretion cycle consists of a thrust initiation an underthrusting phase and a reactivation phase the location and magnitude of deformation within a bivergent wedge and its associated surface uplift vary systematically with the phase of the frontal accretion cycle and are thus predictable therefore the frontal accretion cycle can be considered as an internal clock for wedge scaled deformation and surface uplift we further demonstrate that the geometry of the deformation front and the spatial distribution of surface uplift can be used to infer the currently active phase within a frontal accretion cycle surface uplift of the axial zone and the retrowedge may reach up to half of the thickness of the incoming sedimentary layer during a frontal accretion cycle we estimate cycle duration to range between 104 and 105 years which is thus similar to the time frame of climatic cycles the proposed conceptual model provides an explanation for the commonly observed transience of deformation and surface uplift and for the discrepancy between geodetic paleoseismologic and geologic estimates of fault slip

subducting slab ultra slow velocity layer coincident with silent earthquakes in southern mexico great earthquakes have repeatedly occurred on the plate interface in a few shallow dipping subduction zones where the subducting and overriding plates are strongly locked silent earthquakes or slow slip events were recently discovered at the down dip extension of the locked zone and interact with the earthquake cycle here we show that locally observed converted sp arrivals and teleseismic underside reflections that sample the top of the subducting plate in southern mexico reveal that the ultra slow velocity layer usl varies spatially 3 to 5 kilometers with an s wave velocity of 2 0 to 2 7 kilometers per second most slow slip patches coincide with the presence of the usl and they are bounded by the absence of the usl the extent of the usl delineates the zone of transitional frictional behavior

deformation and fluid flow of a major out of sequence thrust located at seismogenic depth in an accretionary complex nobeoka thrust in the shimanto belt kyushu japan 1 nobeoka thrust in kyushu southwest japan was investigated to understand the relationship between the seismogenic out of sequence thrust ost and fluid flow in accretionary prisms the nobeoka thrust is a fossilized ost being active at seismogenic depth the hanging wall exhibits a penetrative plastic deformation while a brittle cataclastic melange like occurrence characterizes the footwall although both of them have same shale and sandstone dominant protolith vitrinite reflectance analyses indicate that the maximum temperatures of the hanging wall and footwall are approximately 320 and 250 c respectively this thermal gap across the thrust corresponds to 8 6 14 4 km of displacement assuming a 28 47 c km geothermal gradient the brittle damage zone of the thrust is asymmetric only 2 m for hanging wall side and 100 m for footwall three types of mineral veins quartz and carbonate are well developed especially in the damaged footwall the tension crack filling vein the fault filling vein and postmelange one the first is harmonious with fabric perpendicular to the p surface fluid inclusion geothermobarometry indicates the p t of fluid in the intensively damaged zone of the footwall is 300 c 230 250 mpa higher than that from vitrinite reflectance which suggests that hydrothermal fluid flow is associated with deformation the same type vein in the lowest portion of the footwall damaged zone includes a much lower p t fluid this difference suggests that continuous underplating caused the damaged zone to propagate downward with cooling and shallowing which differs from faults characterized by shear localization and might be unique to aquiferous ost in accretionary complexes

accretion in the wake of terrane collision the neogene accretionary wedge off kenai peninsula alaska subduction accretion and repeated terrane collision shaped the alaskan convergent margin the yakutat terrane is currently colliding with the continental margin below the central gulf of alaska during the neogene the terrane s western part was subducted after which a sediment wedge accreted along the northeast aleutian trench this wedge incorporates sediment eroded from the continental margin and marine sediments carried into the subduction zone on the pacific plate prestack depth migration was performed on six seismic reflection lines to resolve the structure within this accretionary wedge and its backstop the lateral extent of the structures is constrained by high resolution swath bathymetry and seismic lines collected along strike accretionary structure consists of variably sized thrust slices that were deformed against a backstop during frontal accretion and underplating toward the northeast the lower slope steepens the wedge narrows and the accreted volume decreases notwithstanding a doubling of sediment thickness in the trench in the northeasternmost transect near the area where the terrane s trailing edge subducts no frontal accretion is observed and the slope is eroded the structures imaged along the seismic lines discussed here most likely result from progressive evolution from erosion to accretion as the trailing edge of the yakutat terrane is subducting

mechanical decoupling and basal duplex formation observed in sandbox experiments with application to the western mediterranean ridge accretionary complex sandbox experiments of accretionary wedges were performed incorporating a thin weak layer of micro glass beads the impact of heterogeneous sedimentary input on wedge mechanics evolution and mass transfer was investigated we report the first experimentally documented growth of basal duplexes these occurred for high basal friction conditions with restricted output of the lower section the upper and lower sections were completely decoupled due to the intervening layer of glass beads with frontal accretion occurring in the upper section simultaneously with basal duplex formation and underplating of subsequent generations of duplexes imerse multichannel seismic reflection data from the western mediterranean ridge wmr image tertiary clastics beneath a thick section of messinian evaporites the base of the evaporites is identified as the primary decollement for deformation in the frontal part of the accretionary complex constriction of the channel of subducting tertiary sediments as well as internal deformation observed as arcward dipping reflectors argue for basal underplating and or two different active decollements we propose an evolution of the wmr in accordance with the sandbox experimental results a weak mid level detachment base of evaporites combined with a strong basal detachment produce mechanical decoupling and basal accretion of toeward verging duplexes

waveform tomography imaging of a megasplay fault system in the seismogenic nankai subduction zone abstract we apply frequency domain waveform tomography to form quantitative high resolution p wave velocity images of a megasplay fault system within the central nankai subduction zone offshore of southwest japan using controlled source ocean bottom seismometer obs data originally acquired in 2004 the waveform tomography method exploits recorded seismic waveforms beyond their first arrivals and thus achieves a much higher resolution of the order of a wavelength than that of conventional traveltime tomography methods frequency domain waveform tomography facilitates a multi scale approach to stabilize the inversion in which initial traveltime tomography results are sequentially improved on by first fitting low frequency components of the seismic records starting at 2 25 hz higher frequency components up to 8 5 hz are then introduced progressively our final waveform tomography image allows velocity anomalies as small as 700 m horizontally and 350 m vertically to be discerned and interpreted as confirmed by checkerboard modeling tests the improved explanatory power of the final images is verified by observing that synthetic waveforms calculated from the final results yield much better fit to the observed waveforms than those estimated from the original traveltime tomography image apparent lithological boundaries from waveform tomography agree well with corresponding reflections on seismic migration images providing further confidence in the validity of the results the megasplay fault is evident on the final waveform tomography image as a sharp velocity discontinuity delineating the upper surface of a velocity reduction of nearly 1 km s with respect to the regional 1d velocity trend the megasplay fault can be traced continuously throughout the entire image from a nearly horizontal section at the landward extent moving seaward through to a steeper angle penetrating the old accretionary prisms with several additional splays appearing to branch in the shallow subsurface the location of the low velocity zones imaged by our waveform tomography method coincides with two previously identified low velocity zones the image reveals a low velocity zone that is continuous from deeper to shallower portions of the subsurface suggesting that pore fluids may be transported from the inner wedge to the transition zone and to the surface through fluid conduits associated with the megasplay fault system

mud volcanoes the most important pathway for degassing deeply buried sediments abstract this paper discusses the nature of the phenomenon of mud volcanism with respect to degassing of deeply buried sediments mud volcanoes are defined as geological structures and their main elements are described based on the nature of activity mud volcanoes are grouped in to three main types and the relationship between each type and corresponding morphological expression is discussed the presented up to date data of the world geographical distribution of mud volcanoes show that they approximate to 1800 individuals a detail overview of geological environments in which they occur helps to deduce some necessary conditions for mud volcano formation the basic mechanisms of mud volcano formation are discussed specifically the fluid pressure hypothesis and some triggering events are identified the most common investigation approach and some criteria for recognizing of a submarine mud volcano on side scan sonar records and seismic sections are given mud volcanism in the mediterranean ridge an accretionary complex and black sea abyssal plane a back ark basin with tectonic regime of extension are described as case studies to show variety in morphology and common factors in development of mud volcanoes the manifestation of continuous gas migration through mud volcanoes is clarified and the attempt made to quantify the gas release shows that mud volcanoes are one of the significant natural sources of atmospheric methane emitting a total amount of about 10 3 to 12 6 tg annually

high vp vs ratio saturated cracks or anisotropy effects 1 we measured vp vs ratios of thermally cracked westerly granite thermally cracked carrara marble and 4 porosity fontainebleau sandstone for an effective mean pressure ranging from 2 to 95 mpa samples were fluidsaturated alternatively with argon gas and water 5 mpa constant pore pressure the experimental results show that at ultrasonic frequencies vp vs ratio of water saturated specimen never exceeded 2 15 even at effective mean pressure as low as 2 mpa or for a lithology for which the poisson s ratio of minerals is as high as 0 3 calcite in order to check these results against theoretical models we examine first a randomly oriented cracked medium with dispersion but without anisotropy and second a medium with horizontally aligned cracks with anisotropy but without dispersion the numerical results show that experimental data agree well with the first model at high frequency vp vs ratios range from 1 6 to 1 8 in the dry case and from 1 6 to 2 2 in the saturated case the second model predicts both vp sv and vp sh to vary from 1 2 to 3 5 depending on the raypath angle relative to the crack fabric in addition perpendicular to the crack fabric a high vp vs ratio is predicted in the absence of shear wave splitting from these results we argue the possibility that high vp vs ratio 2 2 as recently imaged by seismic tomography in subduction zones may come from zones presenting important crack anisotropy the cumulative effects of crack anisotropy and high pore fluid pressure are required to get vp vs ratios above 2 2 citation wang x q a schubnel j fortin e c david y gueguen and h k ge 2012 high vp vs ratio saturated cracks or anisotropy effects geophys res lett 39 l11307 doi 10 1029 2012gl051742

electrical conductivity of some minerals at high temperature and for extended times abstract the influence of the time factor on the results of laboratory measurements of high temperature electrical conductivity has been studied the electrical conductivity of tourmaline muscovite plagioclase orthoclase almandine and titanomagnetite was measured in a dry regime examples of decrease increase or no change of conductivity are given for temperatures kept constant for a long time interpretation of some changes of electrical conductivity is discussed

common evolution of mechanical and transport properties in thermally cracked westerly granite at elevated hydrostatic pressure increasing the damage and crack porosity in crustal rocks can result in significant changes to various key physical properties including mechanical strength elastic and mechanical anisotropy and the enhancement of transport properties using a non interactive crack effective medium nic theory as a fundamental tool we show that elastic wave dispersion can be inverted to evaluate crack density as a function of temperature and is compared with optically determined crack density further we show how the existence of embedded microcrack fabrics in rocks also significantly influences the fracture toughness kic of rocks as measured via a suite of tensile failure experiments chevron cracked notch brazilian disk finally we include fluid flow in our analysis via the gueguen and dienes crack porosity permeability model using the crack density and aspect ratio recovered from the elastic wave velocity inversion we successfully compare permeability evolution with pressure with the laboratory measurements of permeability

significance of mud volcanism 1 mud volcanism and diapirism have puzzled geoscientists for 2 centuries they have been described onshore and offshore in many places on earth and although they occur in various tectonic settings the majority of the features known to date are located in compressional tectonic scenarios this paper summarizes the main thrusts in mud volcano research as well as the various regions in which mud volcanism has been described mud volcanoes show variable geometry up to tens of kilometers in diameter and several hundred meters in height and a great diversity regarding the origin of the fluid and solid phases gas predominantly methane water and mud may be mobilized at subbottom depth of only a few meters but in places can originate from several kilometers depth with minor crustal or mantle input the possible contribution of mud extrusion to global budgets both from quiescent fluid emission and from the extrusive processes themselves is important in regions where mud volcanoes are abundant such as the collision zones between africa and eurasia fluid flux through mud extrusion exceeds the compaction driven pore fluid expulsion of the accretionary wedge also quiescent degassing of mud volcanoes may contribute significantly to volatile budgets and hence to greenhouse climate

electrical conductivity measurement of gneiss under mid to lower crustal p t conditions abstract we conducted electrical conductivity measurements perpendicular and parallel to mineral foliation in dry gneiss at up to 1000 k and a constant pressure of 1 gpa the analyzed gneisses were obtained from the higo metamorphic belt kyushu japan as the metamorphic conditions of these rocks have been well determined by previous studies we were able to select samples that were representative of the middle to lower crust prior to the conductivity measurements the samples were maintained at the maximum temperature for a long period until the electrical conductivity had stabilized our experiment results reveal linear and reproducible conductivity data between temperatures of 600 and 1000 k conductivity measured perpendicular and parallel to foliation differ by an order of magnitude over the same temperature window a plausible explanation for this discrepancy in conductivity is the contrasting configuration of minerals in the two sample orientations as observed by backscattered electron image bei and electron probe microanalysis epma we evaluated the conductivity and computed activation energy for each of the samples and compared the results with those of previous studies our results are consistent with the conductivity values reported for other types of rocks we also compared the experiment results with data derived from electromagnetic em soundings electrical conductivity measurements undertaken perpendicular to foliation can account for the subsurface conductivity structure beneath central kyushu japan

opposing gradients of permanent strain in the aseismic zone and elastic strain across the seismogenic zone of the kodiak shelf and slope alaska an industry acquired seismic reflection line from the aleutian arc to the alaska trench was prestack depth migrated and further constrained with wide angle ocean bottom hydrophone seismic velocity data and swath mapping from the seismic reflection image individual thrust slices the backstop and the forearc basin were resolved sufficiently well to balance the section tectonic shortening and age estimates provided rates of permanent contraction in five discrete segments over 150 km during the past 3 myr about 70 of the plate convergence is measured as contraction across the 40 km wide neogene accretionary prism which composes less than 10 of the forearc permanent contraction is minimal above the 1964 alaska earthquake aftershock zone indicating dominently elastic strain since elastic strain is poorly recorded in the geological record it was approximated from the historical record of the 1964 alaska earthquake the 1964 coseismic vertical deformation displays an inverse pattern to the permanent strain consistent with the earthquake cycle concept elastic deformation is dominant beneath the shelf and upper slope rapid permanent deformation is concentrated beneath the lower slope and a transition between them occurs beneath the middle slope

nature of the low velocity zone in cascadia from receiver function waveform inversion abstract a dipping low velocity zone lvz is a ubiquitous structural element of subduction zones worldwide in this study we map seismic attributes characterizing the lvz beneath the cascadia subduction zone from northern vancouver island to northern california using receiver function waveform inversion throughout this region the lvz is characterized by high v p v s ratios mean 2 77 strong s velocity contrasts 50 and thicknesses averaging 3 37 km the lvz is immediately underlain by a second weaker layer exhibiting moderate v p v s ratios mean 1 85 with mean thickness of 4 62 km we interpret the combined structure in terms of subducting oceanic crust based on classical structural petrological descriptions and constraints from previous studies of ophiolites and ocean drill cores the lvz is identified with pervasively hydrated high porosity pillow basalts and sheeted dikes of layer 2 with possible contributions from sediments layer 1 fluids released from metamorphic dehydration reactions are maintained near lithosphere fluid pressures through an impermeable plate boundary above and a low porosity gabbroic mafic cumulate dominated layer 3 below

high pore fluid pressure may cause silent slip in the nankai trough silent slip events have been detected at several subduction zones but the cause of these events is unknown using seismic imaging we detected a cause of the tokai silent slip which occurred at a presumed fault zone of a great earthquake the seismic image that we obtained shows a zone of high pore fluid pressure in the subducted oceanic crust located down dip of a subducted ridge we propose that these structures effectively extend a region of conditionally stable slips and consequently generate the silent slip

slip weakening as a mechanism for slow earthquakes slow earthquakes form part of a spectrum of fault behaviour between steady creep and fast rupture during a normal earthquake laboratory simulations of slow slip in rock samples taken from the nankai subduction zone japan reveal similar characteristics to fast earthquakes implying that some slow slip events could be prematurely arrested earthquakes

fluid venting in the eastern aleutian subduction zone fluid venting has been observed along 800 km of the alaska convergent margin the fluid venting sites are located near the deformation front are controlled by subsurface structures and exhibit the characteristics of cold seeps seen in other convergent margins the more important characteristics include 1 methane plumes in the lower water column with maxima above the seafloor which are traceable to the initial deformation ridges 2 prolific colonies of vent biota aligned and distributed in patches controlled by fault scarps over steepened folds or outcrops of bedding planes 3 calcium carbonate and barite precipitates at the surface and subsurface of vents and 4 carbon isotope evidence from tissue and skeletal hard parts of biota as well as from carbonate precipitates that vents expel either methane or sulfide dominated fluids a biogeochemical approach toward estimating fluid flow rates from individual vents based on oxygen flux measurements and vent fluid analysis indicates a mean value of 5 5 0 7 l m 2 d 1 for tectonics induced water flow wallmann et al 1997b a geophysical estimate of dewatering from the same area von huene et al 1997 based on sediment porosity reduction shows a fluid loss of 0 02 l m 2 d 1 for a 5 5 km wide converged segment near the deformation front our video guided surveys have documented vent biota across a minimum of 0 1 of the area of the convergent segment off kodiak island hence an average rate of 0 006 l m 2 d 1 is estimated from the biogeochemical approach the two estimates for tectonics induced water flow from the accretionary prism are in surprisingly good agreement

teleseismic constraints on the geological environment of deep episodic slow earthquakes in subduction zone forearcs a review abstract more than a decade after the discovery of deep episodic slow slip and tremor or slow earthquakes at subduction zones much research has been carried out to investigate the structural and seismic properties of the environment in which they occur slow earthquakes generally occur on the megathrust fault some distance downdip of the great earthquake seismogenic zone in the vicinity of the mantle wedge corner where three major structural elements are in contact the subducting oceanic crust the overriding forearc crust and the continental mantle in this region thermo petrological models predict significant fluid production from the dehydrating oceanic crust and mantle due to prograde metamorphic reactions and their consumption by hydrating the mantle wedge these fluids are expected to affect the dynamic stability of the megathrust fault and enable slow slip by increasing pore fluid pressure and or reducing friction in fault gouges resolving the fine scale structure of the deep megathrust fault and the in situ distribution of fluids where slow earthquakes occur is challenging and most advances have been made using teleseismic scattering techniques e g receiver functions in this paper we review the teleseismic structure of six well studied subduction zones three hot i e cascadia southwest japan central mexico and three cool i e costa rica alaska and hikurangi that exhibit slow earthquake processes and discuss the evidence of structural and geological controls on the slow earthquake behavior we conclude that changes in the mechanical properties of geological materials downdip of the seismogenic zone play a dominant role in controlling slow earthquake behavior and that near lithostatic pore fluid pressures near the megathrust fault may be a necessary but insufficient condition for their occurrence

backstop geometry and accretionary mechanics of the sunda margin the convergent sunda margin off indonesia displays all geological features characteristic of an accretion dominated subduction zone a combined interpretation of prestack depth migrated seismic reflection data and velocity information gained from refraction studies is supplemented by high resolution bathymetric data and for the first time allows the exact mapping of backstop regimes initially the outer high evolved as material was pushed against a static rigid arc framework backstop underlying a forearc basin increasing material strength of the outer high due to lithification formed a dynamic backstop which controls accretion today an out of sequence thrust marks the transition from the recent active frontal accretionary prism to the outer high and may be traced in the seismic and bathymetric data over the whole extent of the study area the existence of a static as well as a dynamic backstop controls the forearc geometry and is associated with the segmentation of the forearc which is observed in regimes of frontal as well as of oblique subduction mass balance calculations which account for porosity changes and metamorphism indicate a subduction history dominated by accretionary processes since the late eocene accretion is associated with the low values of basal friction inferred for the sunda margin structural investigations of conjugate fault planes indicate a very weak basal detachment effective stress analyses reveal that intrinsically weak material causes the high strength ratio of the detachment to the overlying sediments whereas overpressuring within the frontal accretionary prism is negligible

morphotectonics and mechanics of the central makran accretionary wedge off pakistan abstract swath bathymetry images and 3 5 khz data acquired during rv sonne cruise 123 in autumn 1997 are the base for analysing the morphology and deformation styles of the huge makran accretionary wedge the continental slope off pakistan is characterised by three morphological provinces a the lower slope is built of a sequence of imbricate thrust slices which form long and narrow accretionary ridges with flanks locally as steep as 20 and between 10 and 20 on average while the regional slope is only 1 2 b the nearly flat mid slope terrace is present in the central east part of the margin narrows significantly towards the east and is absent east of 63 45 and c the upper slope up to the shelf has a regionally uniform slope as steep as about 8 and comprises a very rough topography with many gullies and canyons and evidence for mass wasting two meandering canyons crossing the entire margin down to the abyssal plain at about 63 15 63 30 can be correlated with onshore rivers they have been formed by episodic erosion through turbidity flows and are characterised by erosional and depositional portions sinuosity of the canyons is exclusively caused by the accretionary ridges functioning as obstacles for flow to be directly downhill application of coulomb rheology to the frontal part of the makran wedge implies that the mid level decollement is intrinsically extraordinary weak with a great strength contrast to the overlying sediments comparison with nankai cascadia and the western mediterranean ridge reveals new insights in the parameters controlling the shape of an accretionary wedge and the role of a mid level decollement

seismicity of the kodiak island region 1964 2001 and its relation to the 1964 great alaska earthquake earthquake relocation first motion and waveform modeling studies of m 5 5 earthquakes are used to examine the seismicity associated with the kodiak island region since the 1964 great alaska earthquake our results indicate that the most intense seismicity and moment release in this region occurred within the pacific plate and along the plate interface of the southwestern edge of the kodiak segment where slip during the 1964 mainshock was less than 5 m few earthquakes have occurred within the north american plate the intense seismicity correlates well with the northern edge of a zone of high plate interface coupling detected by global positioning system geodesy studies our studies suggest that during 1964 1974 faulting within the pacific plate was characterized by normal and normal oblique faulting however since 1974 reverse oblique faulting has become more common in the pacific plate seismic moment release has been over 6 times greater in the kodiak region than in the prince william sound cook inlet region during the past 37 years unlike the prince william sound region there is also a lack of down dip migration of seismicity since 1964 manuscript received 23 october 2001

nonlinear variations of the physical properties along the southern ecuador subduction channel results from depth migrated seismic data abstract we use two high quality pre stack depth migrated multichannel seismic profiles acquired to quantify physical properties variations of underthrust sediments along the first 30 km of subduction off the erosional southern ecuadorian margin seismic data show three zones along the subduction channel referred to as zones i ii and iii characterized by distinct velocity and velocity derived physical properties which are in agreement with values estimated from experimental results of deformation in granular media these three zones result from transformational changes of underthrust sediments governed by fundamentally different physical processes that control their mechanical behavior at increasing confining pressures based on our observations and its comparison with experimental results we argue that the transformations undergone by underthrust sediments as they dip into the subduction zone are the following within zone i progressively increasing velocity and decreasing velocity derived porosity indicates continuous sediment compaction which must be accompanied by effective fluid drainage along the decollement and or across the accretionary wedge the underthrust material is here unconsolidated from a mechanical point of view laboratory experiments indicate that the dominant processes at this range of pressures are grain rolling particle rotation and frictional slip at grain contacts within zone ii velocity and porosity remains constant for 16 km sis 72 and 12 km sis 18 this suggests undrained conditions resulting in growing fluid overpressure at the subduction channel grain deformation is similar to zone i within zone iii velocity increases and porosity falls rapidly indicating sediment compaction and subsequent release of over pressured fluids where grain deformation is likely to be elastic this might be the dominant process until the grains attain their crushing strength resulting in granular cataclasis and eventually in the collapse of the system we suggest that over pressured fluid release may induce hydrofracturation and it is likely to increase inter plate coupling down from zone iii

continental margin tectonics submarine accretionary prisms submarine accretionary prisms are sites of incipient mountain building where sediments and rocks of the deep ocean are uplifted deformed and ultimately transformed to continental crust accretionary prisms develop at sites of lithospheric convergence that is subduction zones commonly accretionary prisms form along continental margins although accretionary prisms may also occur adjacent to island arcs which are ultimately coalesced into continental landmasses here we review the studies of submarine accretionary prisms during the past four years with primary focus on the contributions of geoscientists from the united states

peeling oceanic crust in subduction zones thin slabs of oceanic pillow basalt formed at ridge axes with associated pelagic sediments are common features in ancient accretionary complexes estimates of the average thicknesses of scraped off basaltic sections of oceanic crust in accretionary prisms vary from a few tens of metres to 300 m with a mean thickness of 80 100 m in the complexes studied the permeability contrast within the uppermost part of the oceanic crust is an important factor governing the thickness of delaminated oceanic crust this mechanism is an effective means of separating oceanic crust altered at low temperatures from the remainder of the downgoing slab and thus controlling geochemical budgets in subduction zones

rock uplift at the transition from flat slab to normal subduction the kenai mountains southeast alaska abstract the process of flat slab subduction results in complex deformation of overlying forearcs yet how this deformation decays with distance away from the zone of underthrusting is not well understood in south central alaska flat slab subduction of the yakutat microplate drives shortening and rock uplift in a broad coastal orogenic belt defined limits of the zone of underthrusting allow testing how orogenesis responds to the transition from flat slab to normal subduction to better understand forearc deformation across this transition apatite u th he low temperature thermochronometry is used to quantify the exhumation history of the kenai mountains that are within this transition zone measured ages in the northern kenai mountains vary from 10 20 ma and merge with the exhumation pattern in the chugach mountains to the northeast where high exhumation occurs due to flat slab related deformation in the southern kenai mountains however ages increase to 30 50 ma across a transition near seward alaska above the zone from flat slab to normal subduction these ages are relatively old in comparison to ages determined in other studies in southern alaska and suggest minimal exhumation furthermore transitions in topographic expression of the coastal orogen also occur at the margin of yakutat underthrusting these observations suggest that either deformation associated with flat slab subduction requires tens of kilometers to decay with distance away from the zone of underthrusting or that orogenesis in the kenai mountains is driven by a distinct tectonic cause a potential driver of deformation is underplating of thick sediments specifically the surveyor submarine fan along the aleutian megathrust analogous to the tectonic mechanism responsible for the emergence of the kodiak island forearc if correct this may represent a recent tectonic transition in the region given the minimal exhumation of the rugged kenai mountains despite the presence of an erosion conducive glacial climate

transition of accretionary wedge structures around the up dip limit of the seismogenic subduction zone the nankai accretionary prism is divided into three segments outer and inner wedges and their transition zone these wedges reflect different aspects of wedge taper internal deformation and basal plate boundary fault the outer wedge is characterized by narrow critical taper internal deformation by in sequence fold and thrust and aseismic decollement the inner wedge represents a stable narrow taper weakly deformed internal structure and seismogenic plate boundary fault along its base the transition zone between the two wedges shows large critical taper with steep surface slope internal structure of out of sequence thrust and step down of decollement onto the sediment oceanic basement interface the trench slope break and oceanward margin of forearc basin is located around the landward edge of this transition zone these common aspects might be related to the lithification of both accreted and underthrust sediments and the resultant switch of the plate boundary fault deformation and lithification process recorded in exhumed on land melange of accretionary complexes suggest that the step down of the plate boundary decollement occurs around the up dip limit of seismogenic subduction zone

upward delamination of cascadia basin sediment infill with landward frontal accretion thrusting caused by rapid glacial age material flux the cascadia convergent margin is a first order research target to study the impact of rapid sedimentation processes on the mechanics of frontal subduction zone accretion the near trench part of the accretionary prism offshore washington is affected by strongly increased glacial age sedimentation and fan formation that led to an outstanding quaternary growth rate with landward vergent thrust faulting that is rarely observed elsewhere in accretionary wedges multichannel seismic reflection data acquired on the orwell project allows us to study the structure and dynamics of the atypical frontal accretion processes we performed a kinematical and mechanical analysis of the frontal accretion structures and developed a dynamic coulomb wedge model for the landward verging backthrust formation backthrusting results from heterogeneous diffuse strain accumulation in the mechanically heterogeneous cascadia basin sediment succession entering the subduction zone and strain partitioning along a midlevel detachment that is activated by gravitational loading caused by rapid glacial age sedimentation these complex deformation processes cause the passive upward delamination of the upper turbidite beds from the basal pelagic carbonate section similar to triangle zone formation and passive backthrust wedging in foreland thrust belts caused by rapid burial beneath syntectonic sediment deposits the deformation mechanism at the tectonic front of the cascadia margin is an immediate response to the strongly increased late pleistocene sediment flux rather than to atypical physical boundary conditions as generally thought

relation between the subducting plate and seismicity associated with the great 1964 alaska earthquake tectonic studies of the great 1964 alaska earthquake have underappreciated the nature of the subducted plate in influencing seismicity we compare seismological observations in the prince william and kodiak areas that ruptured during this earthquake with the corresponding morphology and structure of the subducting plate the upper plate geology prince william terrane and velocity structure are the same in both areas in the prince william area where the yakutat terrane subducted the energy released and coupling were stronger than above the kodiak subduction zone where thick trench sediment subducts the conjecture that lower plate character or the amount of subducted sediment affects coupling helps explain variability in seismology geodetic inversions and the horizontal velocity of gps stations

wedge geometry and detachment strength in deepwater fold thrust belts deepwater fold thrust belts dwftbs have been classified into three groups according to the present day stress regime and tectonic setting near field stress driven dwftbs are usually associated with passive margins they are thin skinned gravitational deformation with thick salt or overpressured shale detachments dwftbs formed by far field stresses are primarily associated with continent convergent zones and active margins mixed near and far field stress driven dwftbs have aspects of both thin skinned and thick skinned tectonics detachment strength is the primary control on the first order wedge geometry of a dwftbs and is controlled by lithology and overpressure based on analysis of fault strength calculated from critical taper measurements it is found that detachments beneath different classes of dwftbs are not analogously weak near field stress driven dwftbs at passive margins generally have smaller wedge taper angles and weaker detachments far field stress driven and mixed near and far field stress driven dwftbs at continent convergent zones or active margins generally have larger wedge taper angles and stronger detachments differences in the way overpressures are generated and dissipated between the different types of dwftb are thought to be primarily responsible for the systematic variations in shale prone basal detachment weakness found in this study clear lithology related differences in dwftb critical taper are shown between shale higher taper and salt lower taper dominated basal detachments

alaska megathrust 2 imaging the megathrust zone and yakutat pacific plate interface in the alaska subduction zone we image the slab underneath a 450 km long transect of the alaska subduction zone to investigate 1 the geometry and velocity structure of the downgoing plate and their relationship to slab seismicity and 2 the interplate coupled zone where the great 1964 earthquake mw 9 2 exhibited the largest amount of rupture the joint teleseismic migration of two array data sets based on receiver functions rfs reveals a prominent shallow dipping low velocity layer at 25 30 km depth in southern alaska modeling of rf amplitudes suggests the existence of a thin layer vs of 2 1 2 6 km s that is 20 40 slower than underlying oceanic crustal velocities and is sandwiched between the subducted slab and the overriding plate the observed megathrust layer with vp vs of 1 9 2 3 may be due to a thick sediment input from the trench in combination with elevated pore fluid pressure in the channel our image also includes an unusually thick low velocity crust subducting with a 20 dip down to 130 km depth at 200 km inland beneath central alaska the unusual nature of this subducted segment results from the subduction of the yakutat terrane crust our imaged western edge of the yakutat terrane aligns with the western end of a geodetically locked patch with high slip deficit and coincides with the boundary of aftershock events from the 1964 earthquake it appears that this sharp change in the nature of the downgoing plate could control the slip distribution of great earthquakes on this plate interface

extreme efficiency of mud volcanism in dewatering accretionary prisms drilling results from two mud volcanoes on the mediterranean ridge accretionary complex as well as bottom sampling and the wealth of geophysical data acquired recently have provided fundamental knowledge of the 3d geometry of mud extrusions mud volcanism is generally related to buoyancy density inversion and is triggered by the collision of the african and eurasian blocks forcing undercompacted clayey sediments to extrude along faults in the central and hinterlandward parts of the prism volumetric estimates of extruded mud in several well studied areas were based on pre stack depth migrated seismic profiles across the entire up to 150 km wide prism the resulting volumes of mud were combined with ages from mud dome drilling so that rates of mud extrusion were obtained subtracting the solid rock mass from the bulk mud volume using physical property data fluid flux as a function of mud volcanism alone has been quantified for the first time the volume of fluid extruding with the mud is found to be variable but reaches up to 15 km3 fluid per km trench length and ma along cross sections with abundant mud volcanoes such large fluid quantities in a region some 50 150 km behind the deformation front exceed estimates from those elsewhere where undoubtedly the majority of the interstitial fluid is lost due to compaction such fluids near the backstop are likely to result predominantly from mineral dehydration and diagenetic reactions at depth and consequently provide a window to understand deeper processes along the deep decollement more importantly the enormous rates with which such fluids and liquified mud escape along the out of sequence faults alter fluid budget calculations in subduction zones drastically

interactions between deformation and fluids in the frontal thrust region of the nantroseize transect offshore the kii peninsula japan results from iodp expedition 316 sites c0006 and c0007 integrated ocean drilling program iodp expedition 316 sites c0006 and c0007 examined the deformation front of the nankai accretionary prism offshore the kii peninsula japan in the drilling area the frontal thrust shows unusual behavior as compared to other regions of the nankai trough drilling results integrated with observations from seismic reflection profiles suggest that the frontal thrust has been active since 0 78 0 436 ma and accommodated 13 to 34 of the estimated plate convergence during that time the remainder has likely been distributed among out of sequence thrusts further landward and or accommodated through diffuse shortening unlike results of previous drilling on the nankai margin porosity data provide no indication of undercompaction beneath thrust faults furthermore pore water geochemistry data lack clear indicators of fluid flow from depth these differences may be related to coarser material with higher permeability or more complex patterns of faulting that could potentially provide more avenues for fluid escape in turn fluid pressures may affect deformation well drained sand rich material under the frontal thrust could have increased fault strength and helped to maintain a large taper angle near the toe recent resumption of normal frontal imbrication is inferred from seismic reflection data associated decollement propagation into weaker sediments at depth may help explain evidence for recent slope failures within the frontal thrust region this evidence consists of seafloor bathymetry normal faults documented in cores and low porosities in near surface sediments that suggest removal of overlying material overall results provide insight into the complex interactions between incoming materials deformation and fluids in the frontal thrust region

historical seismicity 1918 1964 of the kodiak island region i have relocated 35 earthquakes and conducted waveform modeling studies of 12 events located primarily within the kodiak portion of the 1964 great alaska earthquake rupture zone and the northeasternmost portion of the 1938 semidi earthquake rupture zone these results show that there is considerable similarity between earthquakes occurring before the 1964 mainshock pre 1964 mainshock seismicity and after the 1964 mainshock post 1964 mainshock seismicity persistent seismicity has occurred for the past 85 years at the southwestern end of the 1964 rupture zone a region where global positioning system gps geodesy studies indicate the plate interface is currently locked earthquakes also occurred frequently within the kennedy entrance region where the transition between the kodiak and kenai block of the subducting pacific plate occurs the seismic moment rate for post 1964 mainshock seismicity is about two times larger than for premainshock seismicity this is in contrast to the prince william sound portion of the 1964 rupture zone where post 1964 mainshock moment rates are orders of magnitude lower than pre 1964 mainshock rates and the pattern of seismicity has changed considerably with time these observations reinforce previous studies that indicate the behavior of plate subduction beneath the kodiak island region is much different from that beneath the prince william sound region

episodic imbricate thrusting and underthrusting analog experiments and mechanical analysis applied to the alaskan accretionary wedge seismic reflection profiles from the sediment rich alaska subduction zone image short frontally accreted imbricate thrust slices and repeated sequences of long underthrust sheets rapid landward increases in wedge thickness backthrusting and uplift of the forearc are observed suggesting underthrusting beneath the wedge these features and a widely varying frontal wedge morphology are interpreted to be caused by different modes of accretion active concurrently along the trench at different locations episodic wedge growth is observed in high basal friction experiments using sand as an analog material two phases of an accretionary cycle can be distinguished frontal accretion of short imbricate thrust slices alternating with underthrusting of long undeformed sheets the phase is shown experimentally to depend upon the surface slope of the wedge mechanical analysis of the forces at work predicts these two modes of deformation due to the varying frictional forces and yield strengths for a temporally varying wedge geometry maximum length of thrust slices is calculated for experimental conditions and confirmed by the observations for a steep frontal slope at the upper limit of the mohr coulomb taper stability field the overburden is too great to permit underthrusting and failure occurs repeatedly at the wedge front producing short imbricate slices the wedge grows forward lowering the surface angle to the minimum critical taper for a shallow frontal slope the reduced overburden along an active roof thrust permits sustained underthrusting causing frontal erosion and backthrusting steepening the wedge and thus completing the cycle

mass and fluid flux during accretion at the alaskan margin prestack depth migration of a seismic reflection transect across the alaskan convergent margin clarified structure and provided an undistorted tectonic cross section between the trench floor and a backstop of eroded eocene miocene rock the accretionary prism displays three tectonic styles seaward of the deformation front trench sediment is cut by incipient faults confined to the subsurface and distributed deformation in the 10 km landward of the deformation front thrust faults develop into imbricate structures above a lower detachment in the remaining 20 km longer and more steeply dipping thrust units form a classical imbricate prism above an upper detachment a velocity model at the scale of the thrust slices was derived from depth processing and seismic refraction data the resulting true scale tectonic cross section was balanced to estimate horizontal contraction a porosity field was derived through the velocity porosity relation in five nearby drill holes it indicates porosity reduction patterns characteristic of each tectonic style seaward of the deformation front the strain rate is high and pore fluid pressure elevates but porosity reduction is minimal in the 10 km landward of the deformation front tectonism and dewatering are most rapid sea floor venting was observed only here in the classic accretionary zone strain rates decrease and dewatering is moderate here an upper detachment propagates across the lower parts of imbricate slices and doubles the underthrust mass one third of the trench sediment section is underthrust at the deformation front whereas two thirds of the sediment section is subducted beneath the backstop

a fluvial record of long term steady state uplift and erosion across the cascadia forearc high western washington state six late quaternary river terraces preserved along the clearwater river in northwestern washington state provide a 140 ka record of long term incision and uplift across the western side of the cascadia forearc high terrace ages are constrained by weathering rind and radiocarbon dating and by correlation to dated coastal glacio fluvial deposits and the global eustatic curve the terraces overlie flat bedrock surfaces called straths which represent uplifted segments of the river channel bedrock incision is measured by the height of a strath relative to the adjacent modern river channel the straths along the clearwater show an upstream increase in bedrock incision ranging from 0 at the coast to a maximum of 110 m in the headwaters the incision at any point along the profile increases systematically with strath age the calculated incision rates range from 10 ky the clearwater river valley has maintained a steady state profile defined by a long term balance in the rates of incision and rock uplift upstream divergence of terraces is best explained by an increase in the rate of rock uplift from the coast toward the central part of the range these results are consistent with other evidence indicating a long term steady state balance between the accretionary influx and the erosional outflux for this part of the cascadia subduction wedge since 14 ma these results help show how terrace deposits form in tectonically active landscapes the dominantly fluvial clearwater drainage was forming straths while alpine glaciers were advancing in adjacent drainages in turn the straths were buried during the transition to interglacial times because of increased sediment supply due to local deglaciation and because of eustatic highstands that forced aggradation in the lower reach of the drainage and across the continental shelf as well the fluvial system shows strong forcing by the glacial climate cycle even so the river appears to have returned to the same valley profile during each cycle of strath cutting thus bedrock incision is clearly unsteady at time scales shorter than the glacial climate cycle 100 ky but appears to be relatively steady when averaged over longer time scales a simple kinematic model is used to examine how uplift of the cl our analysis indicates that the accretionary flux into the wedge occurs mainly by frontal accretion and not by underplating if accretion occurred entirely at the front of the wedge the present west coast should be moving to the northeast at 3 m ky relative to a fixed puget sound this prediction is in good agreement with offset of a 122 ka sea cliff preserved at the southwest side of the clearwater valley profile in this case the long term margin perpendicular shortening would account for 20 to 35 percent of the geodetically measured northeast southwest shortening across the olympic mountains

structure of the lesser antilles subduction zone backstop and its role in a large accretionary system 1 the role of a backstop in subduction zones has been the subject of numerous laboratory and numerical modeling studies however few field observations exist revealing how backstops control deformation in subduction zones and accretionary wedge construction a seismic reflection and refraction survey acquired in 1998 with the r v maurice ewing reveals the geometry of the forearc igneous crust accretionary wedge and forearc basin structure of the northern guadeloupe area of the lesser antilles forearc an accreted block of buoyant crust accreted in the late miocene forms the toe of the overriding arc crust and forms the backstop we imaged the top of this surface beneath the forearc basin to its seaward edge where it meets the subducting oceanic crust the toe of the backstop was thrust upward and forms a steep buttress in contact with the lower half of the accretionary wedge the steep buttress produces a narrow inner deformation zone with minimal backthrusting of the accretionary complex landward over the backstop and a narrow 10 km transition between accreted and forearc basin sediment seismic reflections from the subducting crust and the decollement appear beneath the entire accretionary wedge and below the backstop toe separating the decollement and the subducting crust is an interval usually between 500 and 750 m of underthrust sediment carried underneath the accretionary wedge and subducted 15 km landward and beneath the toe of the backstop we speculate that the upturned geometry of the toe of the backstop and a weak fluid rich decollement may facilitate sediment subduction beneath the backstop and potentially into the mantle

evidence for fluid circulation overpressure and tectonic style along the southern chilean margin the southern chilean convergent margin between 50 and 57 s is shaped by the interaction of the three main plates antarctic south america and scotia north of 53 s the convergence between antarctic and south america plates is close to orthogonal to the continental margin strike here the deformational style of the accretionary prism is mainly characterized by seaward verging thrusts and locally by normal faults and fractures a very limited lateral extension of prism a very shallow dip 6 decollement and subduction of a thick and relatively undeformed trench sedimentary sequence south of 53 s convergence is oblique to the margin locally the trench sediments are proto deformed by double vergence thrusting and the front of the prism grows through landward verging thrusting the decollement is sub horizontal and deep involving most of the sediment over the oceanic crust in the accretionary process building a comparatively wide and thicker prism a bottom simulating reflector is present across the whole prism to the abyssal plan suggesting the presence of gas in the sediments the analysis of p and s wave velocity reflectivity sections derived by amplitude versus offset technique avo detailed velocity information and the velocity derived sediment porosity have been integrated with the structural analysis of the accretionary prism of two selected pre stack depth migrated seismic lines aiming to explain the relation between fluid circulation and tectonics accretion along double vergence thrust faults may be associated with the presence of overpressured fluid which decreases the effective shear stress coefficient along the main decollement and within the sediments and modify the rheolgical properties of rocks the presence of an adequate drainage network represented by interconnected faults and fractures affecting the entire sedimentary sequence can favour the escape of pore fluid toward the sea bottom while less permeable and not faulted sediments can favour fluid accumulations gravitational and tectonic dewatering and stratigraphy could control the consolidation and the pore overpressure of sediments involved in subduction along the trench the results of our analysis suggest the existence of a feedback between tectonic style and fluid circulation

microfractures within the fault damage zone record the history of fault activity 1 for faults without sedimentary covers there is no robust method to obtain paleoseismic data that is crucial for the prediction of future damaging earthquake events sudden failure along faults during earthquakes induces off fault damage surrounding such basement faults we showed that the quaternary active fault has the damage zone characterized by a fracture density that decays exponentially with distance from the fault for both healed and open microfractures in contrast quaternary inactive faults contain only healed microfracture damage zone cross cutting relationships between microfractures and a minimum healing temperature of 100 c suggest that healed microfractures formed before and at deeper levels than did unhealed microfractures the damage zone defined by open microfractures reflects the recent fault movement during exhumation associated with erosion and regional uplift from the maximum depth at which microfractures may remain unhealed microfracture analysis can therefore be used to examine the history of basement fault activity