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

a new method of reconstituting the p t conditions of fluid circulation in an accretionary prism shimanto japan from microthermometry of methane bearing aqueous inclusions in paleo accretionary prisms and the shallow metamorphic domains of orogens circulating fluids trapped in inclusions are commonly composed of a mixture of salt water and methane producing two types of fluid inclusions methane bearing aqueous and methane rich gaseous fluid inclusions in such geological settings where multiple stages of deformation veining and fluid influx are prevalent textural relationships between aqueous and gaseous inclusions are often ambiguous preventing the microthermometric determination of fluid trapping pressure and temperature conditions to assess the p t conditions of deep circulating fluids from the hyuga unit of the shimanto paleo accretionary prism on kyushu japan we have developed a new computational code applicable to the h2o ch4 nacl system which allows the characterization of ch4 bearing aqueous inclusions using only the temperatures of their phase transitions estimated by microthermometry tmi the melting temperature of ice thyd the melting temperature of gas hydrate and th aq homogenization temperature this thermodynamic modeling calculates the bulk density and composition of aqueous inclusions as well as their p t isochoric paths in a p t diagram with an estimated precision of approximatively 10 we use this computational tool to reconstruct the entrapment p t conditions of aqueous inclusions in the hyuga unit and we show that these aqueous inclusions cannot be cogenetic with methane gaseous inclusions present in the same rocks as a result we propose that pulses of a high pressure methane rich fluid transiently percolated through a rock wetted by a lower pressure aqueous fluid by coupling microthermometric results with petrological data we infer that the exhumation of the hyuga unit from the peak metamorphic conditions was nearly isothermal and ended up under a very hot geothermal gradient in subduction or collision zones modeling aqueous fluid inclusions in the ternary h2o ch4 nacl system and not simply in the binary h2o nacl is necessary as the addition of even a small amount of methane to the water raises significantly the isochores to higher pressures our new code provides therefore the possibility to estimate precisely the pressure conditions of fluids circulating at depth

the 5 september 2012 nicoya costa rica mw 7 6 earthquake rupture process from joint inversion of high rate gps strong motion and teleseismic p wave data and its relationship to adjacent plate boundary interface properties 1 on 5 september 2012 a large thrust earthquake mw 7 6 ruptured a densely instrumented seismic gap on the shallow dipping plate boundary beneath the nicoya peninsula costa rica ground motion recordings directly above the rupture zone provide a unique opportunity to study the detailed source process of a large shallow megathrust earthquake using very nearby land observations hypocenter relocation using local seismic network data indicates that the event initiated with small emergent seismic waves from a hypocenter 10 km offshore 13 km deep on the megathrust a joint finite fault inversion using high rate gps strong motion ground velocity recordings gps static offsets and teleseismic p waves reveals that the primary slip zone slip 1 m is located beneath the peninsula the rupture propagated downdip from the hypocenter with a rupture velocity of 3 0 km s the primary slip zone extends 70 km along strike and 30 km along dip with an average slip of 2 m the associated static stress drop is 3 mpa the seismic moment is 3 5 1020 nm giving mw 7 6 the coseismic large slip patch directly overlaps an onshore interseismic locked region indicated by geodetic observations and extends downdip to the intersection with the upper plate moho at deeper depths below the upper plate moho seismic tremor and low frequency earthquakes have been observed most tremor locates in adjacent areas of the megathrust that have little coseismic slip a region of prior slow slip deformation to the southeast also has no significant coseismic slip or aftershocks an offshore locked patch indicated by geodetic observations does not appear to have experienced coseismic slip and aftershocks do not overlap this region allowing the potential for a comparable size rupture offshore in the future

thermal regime of the costa rican convergent margin 2 thermal models of the shallow middle america subduction zone offshore costa rica at the costa rica margin along the middle america trench along strike variations in heat flow are well mapped these variations can be understood in terms of either ventilated fluid flow where exposed basement allows fluids to freely advect heat between the crustal aquifer and ocean or insulated fluid flow where continuous sediment cover restricts heat advection to within the crustal aquifer we model fluid flow within the subducting aquifer using nusselt number approximations coupled with finite element models of subduction and explore its effect on temperatures along the subduction thrust the sensitivity of these models to the initial thermal state of the plate and styles of fluid flow either ventilated or insulated is explored heat flow measurements on cool crust accreted at the east pacific rise are consistent with ventilated hydrothermal cooling that continues with subduction these models yield much cooler temperatures than predicted from simulations initialized with conductive predictions and without hydrothermal circulation heat flow transects on warm crust accreted at the cocos nazca spreading center are consistent with models of insulated hydrothermal circulation that advects heat updip within the subducting crustal aquifer near the trench these models are warmer than conductive predictions and cooler than conductive predictions downdip of the trench comparisons between microseismicity and modeled isotherms suggest that the updip limit of microseismicity occurs at temperatures warmer than 100 c and that the downdip extent of microseismicity is bounded by the intersection of the subduction thrust with the base of the overriding crust

the effect of meltwater plumes on the melting of a vertical glacier face abstractfreshwater produced by the surface melting of ice sheets is commonly discharged into ocean fjords from the bottom of deep fjord terminating glaciers the discharge of the freshwater forms upwelling plumes in front of the glacier calving face this study simulates the meltwater plumes emanated into an unstratified environment using a nonhydrostatic ocean model with an unstructured mesh and subgrid scale mixing calibrated by comparison to established plume theory the presence of an ice face reduces the entrainment of seawater into the meltwater plumes so the plumes remain attached to the ice front in contrast to previous simple models ice melting increases with height above the discharge also in contrast to some simple models and the authors speculate that this overcutting may contribute to the tendency of icebergs to topple inwards toward the ice face upon calving the overall melt rate is found to increase with discharge flux only up to a critical value which depends on the channel size

runaway slip to the trench due to rupture of highly pressurized megathrust beneath the middle trench slope the tsunamigenesis of the 2011 tohoku earthquake off the east coast of northern japan abstract the gigantic 2011 march 11 mw 9 tohoku earthquake is examined from the viewpoint of the pre seismic forearc structure the seismic reflection properties of a megathrust around the usual up dip limit of the seismogenic zone the thermal state of a shallow subduction zone and the dehydration of underthrust sediments at the japan trench the pacific plate is subducting westward beneath northeast japan at a dip angle of 4 6 the middle and lower slopes of the landward side dip eastward at angles of 2 5 and 8 0 respectively the forearc prism beneath the middle and lower slopes is inferred to be in extensionally and compressively critical states respectively based on the presence of clear internal deformation features and on the occurrence of aftershock earthquakes the rapid uplift of the forearc that caused the 2011 tohoku tsunami may have been associated with this internal deformation of the prism the critical state of the prism indicates that the effective basal friction b of the plate boundary megathrust is 0 08 for the lower prism the megathrust especially under the middle slope is characterized by a prominent reflector with negative polarity one of multiple possible explanations is that the megathrust hosts highly pressurized fluids underthrust sediments in this part of the japan trench are dominated by pelagic and siliceous vitric diatomaceous silt with clay the dehydration kinetics of opal a to quartz the clay transformation of smectite illite and the thermal structure of the japan trench suggest that maximum dehydration of the sediments would take place at 50 60 km horizontally from the deformation front where the temperature along the megathrust is 100 120 c the zone of maximum dehydration coincides with the prominent seismic reflector that has negative polarity we hypothesize a possible free slip along this portion of the megathrust during the 2011 tohoku earthquake caused by anomalously high fluid pressure resulting from fluid accumulation over centuries

shear stresses on megathrusts implications for mountain building behind subduction zones 1 shear stresses on a subduction megathrust play an important role in determining the forces available for mountain building adjacent to a subduction zone in this study the temperatures and shear stresses on megathrusts in 11 subduction zones around the pacific rim hikurangi tonga izu ogasawara western nankai northeastern japan aleutians western alaska cascadia northern chile southern chile and se asia northern sumatra have been determined the main constraint is that vertical normal stresses beneath the highlands behind the subduction zone are nearly equal to horizontal normal stresses in the plane of a trench or arc normal section for a typical brittle and ductile megathrust rheology frictional shear stress gz for depth z and ductile shear stress a exp b rt at temperature t where a b are rheological parameters treated as constants rheological constants common to all the megathrusts crust mantle b are determined by simultaneously solving for the force balance in the overlying wedge and megathrust thermal structure using a simplex minimization algorithm taking account of the induced mantle corner flow at depth 65 15 km 2 and constant radiogenic heating 0 65 0 3 w m 3 2 throughout the crust the a constants are solved individually for each subduction zone assuming that the maximum depth of interplate slip earthquakes marks the brittle ductile transition the best fit solution shows two groupings of megathrusts with most subduction zones having a low mean shear stress in the range 7 15 mpa crust 0 032 0 006 mantle 0 019 0 004 and unable to support elevations 2 5 km for a typical frictional sliding coefficient 0 5 the low effective coefficients of friction suggest high pore fluid pressures at 95 lithostatic pressure tonga and northern chile require higher shear stresses with crust 0 095 0 024 mantle 0 026 0 007 suggesting slightly lower pore fluid pressures at 81 lithostatic ductile shear in the crust is poorly resolved but in the mantle appears to show a strong power law dependency with b 36 18 kj mol 1 amantle values are sensitive to the precise value of b but are in the range 1 20 kpa the power law exponent n for mantle flow is poorly constrained but is likely to be large n 4 the brittle ductile transition in the crust occurs at temperatures in the range 370 c 512 c usually close to the base of the crust and in the mantle at much lower temperatures 180 c 300 c possibly reflecting a marked change in pore fluid pressure or quasi ductile and subfrictional properties in subduction zones where the subducted slab is older than 50 ma a significant proportion of the integrated shear force on the megathrust is taken up where it cuts the mantle and temperatures are 300 c in much younger subduction zones the stress transmission is confined mainly to the crust the shear stresses particularly in the crust may be kept low by some sort of lubricant such as abundant water rich trench fill which lowers the frictional sliding coefficient or effective viscosity and or raises pore fluid pressure the unusual high stress subduction zone in northern chile lacks significant trench fill and may be poorly lubricated with a mean shear stress 37 mpa required to support elevations 4 km in the high andes however where the crust is thin in sediment starved and poorly lubricated subduction zones such as tonga the mean shear stress will still be low sediment may lubricate megathrusts accommodating underthrusting of continental crust such as in the himalayas or eastern central andes which have a low mean shear stress 15 mpa

tectonic erosion and consequent collapse of the pacific margin of costa rica combined implications from odp leg 170 seismic offshore data and regional geology of the nicoya peninsula the convergent margin off the pacific coast of the nicoya peninsula of costa rica exhibits evidence for subduction erosion caused by the underthrusting cocos plate critical evidence for efficacy of this process was recovered at the ocean drilling program odp drilling site 1042 leg 170 positioned 7 km landward of the middle america trench axis off the nicoya peninsula the primary drilling objective at this site was to identify the age and origin of a regionally extensive and prominent seismic discontinuity the so called base of slope sediment boss horizon or surface the boss horizon which can be traced landward from near the trench to the nicoya coastal area and parallel to it for hundreds of kilometers separates a low velocity 2 0 2 5 km s 1 sequence of slope sediment from an underlying sequence of much higher velocity 4 4 5 km s 1 rock site 1042 reached the acoustically defined boss horizon at a below sea level depth of 3900 m and yielded a carbonate cemented calcarenitic breccia of early middle miocene age sedimentological geochemical paleontological and cement paragenesis data document that the breccia accumulated in a shallow water depositional environment on the basis of coastal exposures the boss horizon as a margin wide geologic interface can be temporally and lithostratigraphically correlated to a regional angular unconformity this unconformity known as the mal pais unconformity separates neogene and younger shelf to littoral beds from the underlying mafic units of the mesozoic nicoya complex and cretaceous and early tertiary sedimentary sequences at site 1042 it is inferred that tectonism caused the vertical subsidence of the early neogene breccia from a shallow to a deep water setting the mal pais unconformity of the boss horizon thus connects the rock fabric of the outermost part of margin to that of coastal nicoya and implies that in the early neogene the nicoya shelf extended seaward to near the present trench axis this circumstance requires that the early neogene trench axis was at least 50 km seaward of where it is now located the long term effects of subduction erosion similar to those described for the scientifically drilled japan tonga and peru margins best account for offshore and onshore evidence for a post paleogene history of crustal thinning and landward trench migration of costa rica s pacific margin during the past 16 17 myr the calculated mass removal and landward migration rates are 34 36 km myr 1 km 1 of margin and 3 km myr 1 respectively these values are similar to those found for other pacific margins dominated by nonaccretionary subduction zone processes

modern and ancient seismogenic out of sequence thrusts in the nankai accretionary prism comparison of laboratory derived physical properties and seismic reflection data 1 to investigate characteristics of a seismogenic out ofsequence thrust oost imaged as a strong reflection on seismic profiles in the nankai accretionary prism we determined acoustic properties of discrete samples from an fossil nobeoka oost outcrop under confining pressures and compared the acoustic properties with those of an active oost in the nankai accretionary prism we observed anisotropy of velocity and attenuation in the hanging wall of nobeoka oost attributed to foliation of pelitic phyllite in contrast the footwall is composed of brittlely deformed chaotic shales and fine sandstones and velocities in the footwall are lower than those in the hanging wall amplitude variation with offset avo modeling utilizing contrasts in p and s wave velocities and densities between the hanging wall and footwall of the nobeoka oost indicates that fractures filled with overpressured fluid likely account for angle dependent reflection amplitudes of the active oost in the nankai trough citation tsuji t g kimura s okamoto f kono h mochinaga t saeki and h tokuyama 2006 modern and ancient seismogenic out ofsequence thrusts in the nankai accretionary prism comparison of laboratory derived physical properties and seismic reflection data geophys res lett 33 l18309 doi 10 1029 2006gl027025

chemistry of springs across the mariana forearc shows progressive devolatilization of the subducting plate abstract cold springs upwelling through large serpentinite mud volcanoes in the outer half of the mariana forearc provide a unique window into processes of devolatilization of the subducting pacific plate we have sampled upwelling pore waters with lower chlorinity than seawater from six sites on five serpentinite mud volcanoes by conventional gravity and piston coring by push coring from the rov jason by drilling on odp legs 125 and 195 and by manned submersible the sites range from 13 47 n to 19 33 n and 52 to 90 km from the mariana trench axis corresponding to approximate depths to the top of the downgoing plate of 16 to 29 km the composition of the springs varies systematically over this distance nearer the trench the upwelling waters have much higher ca and sr than seawater and much lower carbonate alkalinity sulfate na cl k rb and b farther from the trench the waters show the opposite trends relative to seawater chlorinity is consistently lower than in seawater and shows large variations that are not systematic with distance from the trench cs is consistently higher than in seawater and increases with distance from the trench all of the waters have high ph and are heavily depleted in mg si li f and 87sr 86sr relative to seawater they tend to be enriched in o18 o16 except for odp drilling none of the cores was long enough to produce an asymptotic compositional trend with depth we have inferred the end member compositions of the upwelling waters by extrapolation against mg at two sites we were able to compare data from gravity cores with data from drill cores or push cores collected at springs to estimate the effects of reactions that occur at shallow depth below the seafloor on mixing of the upwelling waters with seawater these effects are different for sites high in dissolved ca nearer the trench vs those high in alkalinity farther from the trench common to both are large losses from solution of 1 ca as caco3 and in exchange for na 2 mg in exchange for na or ca and as brucite 3 sulfate probably reduced by microbes or possibly precipitated as gypsum 4 sr ba si and f na is consistently leached from the solids into solution whereas k and o18 o16 are relatively unreactive we infer that the upwelling waters are uniformly saturated with caco3 and that the excess h2o and the trends in ca sr alkalinity and sulfate with distance from the trench result from introduction of h2o and dissolved carbonate and sulfate from an external source the sediment and altered basalt at the top of the subducting plate the concurrent trends in na cl b cs and especially k and rb indicate that these species originate from the top of the subducting plate in response to increasing temperature these systematic variations across the outer forearc imply that the solutions ascend more or less vertically from the source region and do not travel long distances laterally along the decollement before ascending based on leaching of k the 150 c isotherm is crossed approximately beneath big blue seamount at a depth of 22 km below the seafloor 70 km behind the trench by this point it appears that carbonate dissolution has joined dehydration as a significant process at the top of the subducting plate

history of metamorphic fluids along outcrop scale faults in a paleogene accretionary prism sw japan implications for prism scale hydrology 1 synkinematic quartz crystals collected from late stage syn to post cleavage faults in the paleogene shimanto accretionary complex contain fluids that provide constraints on fluid flow within the accretionary wedge in particular the trapped fluids provide a glimpse of intraprism flow during a phase of widespread prism thickening carbonic ch4 co2 rich and aqueous h2o rich inclusions occur intermixed within single crystals that nucleated primarily in releasing stepovers along faults that deform well indurated and or cleaved wallrock invariably the carbonic inclusions contain an aqueous liquid as a perimeter phase and the aqueous inclusions contain a carbonic vapor phase suggesting coeval trapping of unmixed phases from a miscible parent fluid these observations are interpreted to reflect crystal growth following deformation driven dilitation and consequent unmixing of the parent fluid at one location cape oyama the occurrence of ch4 and h2o in the absence of co2 enables explicit constraints to be placed on pressure and temperature conditions during crystal growth these conditions suggest geothermal gradients of 50 c km during late stage faulting our results suggest local intraprism flow that was driven by deformation the scale of flow that is suggested is quite restricted and this leads us to hypothesize that the paleogene shimanto accretionary prism acted largely as a barrier to fluid flow during late stage deformation as a consequence the paleogene rocks probably served to focus fluid escape elsewhere most likely trenchward and arcward the combination of the elevated geothermal gradient which would have contributed to increasing the relative strength of this package of rocks and the lack of fluid pathways within it would have favored the development of new out of sequence thrust faults outboard of the paleogene prism or the reactivation of older more landward structures

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

multiscale simulations of tropospheric chemistry in the eastern pacific and on the u s west coast during spring 2002 1 regional modeling analysis for the intercontinental transport and chemical transformation 2002 itct 2k2 experiment over the eastern pacific and u s west coast is performed using a multiscale modeling system including the regional tracer model chemical weather forecasting system cfors the sulfur transport and emissions model 2003 stem 2k3 regional chemical transport model and an off line coupling with the model of ozone and related chemical tracers mozart global chemical transport model co regional tracers calculated online in the cfors model are used to identify aircraft measurement periods with asian influences asian influenced air masses measured by the national oceanic and atmospheric administration noaa wp 3 aircraft in this experiment are found to have lower deltaacetone deltaco deltamethanol deltaco and deltapropane deltaethyne ratios than air masses influenced by u s emissions reflecting differences in regional emission signals the asian air masses in the eastern pacific are found to usually be well aged 5 days to be highly diffused and to have low noy levels chemical budget analysis is performed for two flights and the o 3 net chemical budgets are found to be negative net destructive in the places dominated by asian influences or clear sites and positive in polluted american air masses during the trans pacific transport part of gaseous hno3 was converted to nitrate particle and this conversion was attributed to noy decline without the aerosol consideration the model tends to overestimate hno3 background concentration along the coast region at the measurement site of trinidad head northern california high concentration pollutants are usually associated with calm wind scenarios implying that the accumulation of local pollutants leads to the high concentration seasonal variations are also discussed from april to may for this site a high resolution nesting simulation with 12 km horizontal resolution is used to study the wp 3 flight over los angeles and surrounding areas this nested simulation significantly improved the predictions for emitted and secondary generated species the difference of photochemical behavior between the coarse 60 km and nesting simulations is discussed and compared with the observation

on the mechanical role of smectite in subduction zones smectites a group of hydrated phyllosilicate minerals are often a main component of sediments entering a subduction zone they are common in hemipelagic sediments and consist of an authigenetic component that arises from the alteration of volcanic ash intercalated in the sediment and a detrital component smectite rich strata are structurally weak retain water and are highly porous a review of sediment lithology in subduction zones around the world suggests that decollement zones may preferentially form in smectite rich horizons one consequence of this observation is that the assignment of high frictional resistance to materials in a decollement may be unwarranted at deeper structural levels smectite is transformed to illite a mechanically stronger mineral estimates of the depth of this transition coincide with the depth at which earthquakes are first detected along plate interfaces i speculate that the smectite illite transition in subduction zones may be an important component of sediment consolidation processes that trigger interplate seismicity

long term evolution of an accretionary prism the case study of the shimanto belt kyushu japan the shimanto belt in sw japan is commonly described as a paleo accretionary prism whose structure is explained by continuous accretion like in modern accretionary prisms such as nankai we carried out a structural study of the cretaceous to miocene part of the shimanto belt on kyushu to test this hypothesis of continuous accretion most deformation structures observed on the field are top to the se thrusts fitting well the scheme of accretionary wedge growth by frontal accretion or underplating in particular the tectonic melange at the top of the hyuga group records a penetrative deformation reflecting burial within the subduction channel in contrast we documented two stages of extension that require modifying the traditional model of the belt as a simple giant accretionary wedge the first one in the early middle eocene is mostly ductile and localized in the foliated bases of the morotsuka and kitagawa groups the second one postdating the middle miocene is a brittle deformation spread over the whole belt on kyushu integrating these new tectonic features to existing data we propose 2 d reconstructions of the belt evolution leading to the following conclusions 1 erosion and extension of the margin in the early middle eocene resulted from the subduction of a trench parallel ridge 2 the late eocene to early miocene evolution is characterized by rapid growth of the prism followed by a middle miocene stage where large displacements occurred along low angle out of sequence thrusts such as the nobeoka tectonic line 3 from middle miocene the strain regime was extensional

underthrust sediments fluid migration paths and mud volcanoes associated with the accretionary wedge off costa rica middle america trench styles of deformation and tectonic responses resulting from the convergence of oceanic and continental plates are strongly dependent on fluids in the sediments we estimate the volume of fluids and sediment underthrust beneath the toe of an accretionary wedge and describe evidence of fluid migration observed in seismic data in the middle america trench near costa rica reduction in normal incident travel time of the underthrust oceanic plate section arcward of the deformation front may be related to fluid expulsion within 4 km of the deformation front where overburden is 800 m more than half of the total water in the pore spaces may have been expelled the rate of fluid transfer from the underthrust section is highest in the zone 0 4 km arcward of the deformation front the clarity of the reflection associated with the decollement is exceptional and shows a phase reversal relative to the seafloor this high amplitude phase reversed reflection ends about 4 km arcward of the deformation front suggesting a rapid reduction in the density and or velocity contrast between the dewatering underthrust section and overlying offscraped sediments at this position fluid migration within the wedge produces mud volcanoes or ridges in the midslope region having relief of about 50 m and extent of perhaps 500 m 1000 m a reflection traced beneath a seafloor mud volcano crosses through the accretionary wedge slope cover boundary and may mark a fluid conduit locally high amplitude reflections associated with the base of slope cover are coincident with closure of structure and indicate local traps to upward organized or diffusive fluid flow some fluids appear to migrate along pathways marked by reflections which extend deep within the accretionary wedge but not always extending into the underthrust section near the trench where fluid expulsion rates are highest water may exit to the seafloor seaward of the downslope pinch out of the slope section

origin and in situ concentrations of hydrocarbons in the kumano forearc basin from drilling mud gas monitoring during iodp nantroseize exp 319 abstract nantroseize exp 319 of the integrated ocean drilling program iodp was the first cruise in the history of scientific ocean drilling with drilling mud circulation through a riser drilling mud was pumped through the drill string and returned to the drill ship through the riser pipe during drilling of hole c0009a from 703 to 1604 mbsf meter below sea floor and hole enlargement from 703 to 1569 mbsf during riser drilling gas from returning drilling mud was continuously extracted sampled and analyzed in real time to reveal information on the gas composition and gas concentrations at depth hydrocarbons were the only formation derived gases identified in drilling mud and reached up to 14 vol of methane and 48 ppmv of ethane the chemical and isotopic compositions of hydrocarbons exhibit a microbial origin hydrocarbons released from drilling mud and cuttings correlate with visible allochthonous material wood lignite in drilling cuttings at greater depth addition of small but increasing amounts of hydrocarbons probably from low temperature thermal degradation of organic matter is indicated the methane content is also tightly correlated with several intervals of low poisson s ratio from vp vs observed in sonic velocity logs suggesting that the gas is situated in the pore space of the rock as free gas the gas concentrations in the formation determined from drilling mud gas monitoring reaching up to 24 lgas lsediment for methane in hole c0009a in line with gas concentrations from interpreted downhole sonic logs

subducted seamount imaged in the rupture zone of the 1946 nankaido earthquake the nankai trough is a vigorous subduction zone where large earthquakes have been recorded since the seventh century with a recurrence time of 100 to 200 years the 1946 nankaido earthquake was unusual with a rupture zone estimated from long period geodetic data that was more than twice as large as that derived from shorter period seismic data in the center of this earthquake rupture zone we used densely deployed ocean bottom seismographs to detect a subducted seamount 13 kilometers thick by 50 kilometers wide at a depth of 10 kilometers we propose that this seamount might work as a barrier inhibiting brittle seismogenic rupture

how weak is the subduction zone interface several lines of evidence suggest that subduction zones are weak and that the unique availability of water on earth is a critical factor in the weakening process we have evaluated the strength of subduction zone interfaces using two approaches i from empirical relationships between shear stress at the interface and subduction velocity deduced from laboratory experiments and ii from a parametric study of natural subduction zones that provides new insights on subduction zone interface strength our results suggest that subduction is only mechanically feasible when shear stresses along the plate interface are relatively low less than 35 mpa to account for this requirement we propose that there is a feedback mechanism between subduction velocity water released from the subducting plate and weakening of the fore arc mantle that may explain how relatively low shear stresses are maintained at subduction interfaces globally

pacific plate subduction beneath the central mariana and izu bonin fore arcs new insights from an old margin 1 multichannel seismic mcs profiles and bathymetric data from the central mariana and izu bonin subduction systems image the subducting pacific plate from the outer trench slope to beneath serpentinite seamounts on the outer fore arc subducting oceanic crust varies along the mariana margin from 5 3 to 7 km thick and is covered by 0 5 2 km thick sediments and numerous seamounts oceanic crustal thickness east of the izu bonin trench is 6 km faulting resulting from flexure of the incoming pacific plate begins up to 100 km east of the trench axis near the 6 km depth contour the plate is cut by normal faults that reactivate inherited tectonic fabric where that fabric strikes 25 incoming crust breaks along new faults with a trench parallel strike the mariana trench axis is commonly a graben that accommodates an abrupt change within 8 beneath the outer fore arc we infer that the plate fails there rather than simply bends under the applied loads along portions of the mariana margin subducting seamounts displace the trench axis westward and uplift the toe of the slope surprisingly west of the toe there is no geophysical evidence of disturbance of the upper plate in response to seamount subduction nor of significant subduction erosion or sediment underplating mcs profiles across the base of the mariana inner trench slope provide evidence for both complete subduction and small scale accretion of pacific plate sediments however we found no evidence for long term sediment accretion the subducting plate dips 9 12 beneath serpentinite seamounts on the izu bonin and mariana fore arcs along the mariana margin the majority of these seamounts are located 50 70 km west of the trench where the mantle wedge is 3 7 km thick between 8 10 km thick fore arc crust and the top of the subducting plate the apparent lack of significant deformation of the mariana fore arc crust by subducting seamounts may be the result of a weak serpentinized mantle wedge and or progressive fracturing as the subducting plate increases in dip as it passes through the trench graben

edge deep seismic reflection transect of the eastern aleutian arc trench layered lower crust reveals underplating and continental growth an edge deep crustal seismic reflection transect of the eastern aleutian arc trench traces oceanic crust and moho more than 200 km beneath the accretionary prism to depths of more than 30 km these horizons project beneath a prominent sequence of layered reflectors that extends from about 9 to 35 km beneath the mesozoic core of the prism earthquake hypocenters imply continuity of the downgoing lithosphere from the base of the layered reflectors to beneath and beyond the active augustine volcano rapid lateral growth of the prism in eocene oligocene time coincided with uplift of the mesozoic core of the prism during lateral growth maintenance of critical taper requires thickening either by internal deformation or underplating because exposed rocks show only modest postemplacement shortening thickening most likely occurred by underplating probably of the layered reflectors the overall geometry of the layered reflectors is reminiscent of nappe structures and their emplacement may represent crustal scale duplexing associated with underplating the edge reflection data and borehole results indicate that the self edge is marked by an active out of sequence thrust that separates the paleogene and neogene prisms this thrust apparently developed in response to the prism s need to maintain critical taper and demonstrates that contrasts inmore lithology can result from mechanisms other than terrane emplacement less

fluid regimes during late stages of a continental collision physical chemical and stable isotope measurements of fluid inclusions in fissure quartz from a geotraverse through the central alps switzerland fluid evolution during neo alpine metamorphism during late stages of the continental collision between europe and africa was studied by analyzing fluid inclusions in alpine fissure quartz collected in forty nine localities along a geotraverse through the central alps switzerland the methods employed include microthermometry micro raman spectroscopy k na thermometry and stable isotope analysis early fluid inclusions provide evidence of close to peak metamorphic temperatures of the late tertiary or neo alpine metamorphic event fluid composition evolved along the geotraverse from north to south as follows higher hydrocarbons were dominant in the low and medium grade diagenetic zones methane was the main volatile in the high grade diagenetic and low grade anchizone water dominated in the highgrade anchizone and low grade epizone with co2 10 mol in the high grade epizone and in the mesozone higher hydrocarbons and ch4 were the products of kerogen maturation and cracking of preexisting petroleum large water supplies originated from the dehydration of cooler metasedimentary rocks that were overthrust by crystalline basements of the lepontines aar and gotthard massifs carbon isotope analyses suggest that the co2 component was derived from oxidation of graphitic matter especially in the vicinity of sulfate bearing metasediments and from decarbonation reactions in the aar and gotthard massifs as well as in the helvetic axen nappe and its underlying north helvetic flysch high fluid pressures prevailed and favored nappe transport by contrast in the southern lepontine area very low early fluid pressures were probably related to dry rocks and scarce metasediments and to high geothermal gradients that resulted from intense uplift and erosion between 26 and 18 ma retrograde fluid evolution was recorded by a succession of fluid inclusion populations in each alpine fissure it was controlled by uplift and cooling and characterized by decreasing contents of volatiles and an increase in 18o of host quartz tectonic activity led to episodic pressure drops of at least 0 5 to 2 kbar and promoted fluid unmixing channelized flow and rapid growth of skeletal quartz channelized rather than pervasive fluid migration at temperatures 450 c and under conditions of brittle deformation is documented by episodic increases in salinity and by fluid flushing through the massifs there is stable isotope evidence for involvement of meteoric water only in late crystallizing quartz formation of alpine fissures and fissure minerals was the result of a unique coincidence of late continental collision 450 c fluid expulsion from overthrust metasediments uplift and erosion

hydrologic control of forearc strength and seismicity in the costa rican subduction zone the subduction zone beneath costa rica experiences infrequent large earthquakes in its northwestern part whereas slow slip dominates in the southeast seismic data reveal a disparity in fluid accumulation in the overriding continental crust that correlates with this change in seismic behaviour implying that spatial gradients in fluid content may control subduction zone seismicity

sediment accretion subduction and dewatering at the base of the trench slope off costa rica a seismic reflection view of the decollement sea beam bathymetric and water gun seismic reflection data off the west coast of costa rica image the base of the trench slope its underlying structure and the behavior of a decollement surface in the presence of relief on the subducting plate a low angle landward dipping reflection marks the master decollement above which only about 80 m of a 350 m oceanic sedimentary section is offscraped and accreted to the lower trench slope a small horst block at least 9 km long 1 5 km wide and 300 m high is being obliquely subducted beneath the landward trench slope on five seismic lines the decollement rises over the horst and then descends into the next incoming graben as the fault zone maintains a specific stratigraphic level the undulating geometry of the decollement implies that the subducting grabens are not sites for sediment entrapment and subduction and that the horsts are not causing tectonic erosion of the landward trench wall substantial dewatering within the subducting sediments is documented within about 4 km of the deformation front the dewatering may favor development of bedding parallel zones of low shear strength within the subducting sediments and the master decollement may seek one of these zones

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

structure and composition of the plate boundary slip zone for the 2011 tohoku oki earthquake the mechanics of great subduction earthquakes are influenced by the frictional properties structure and composition of the plate boundary fault we present observations of the structure and composition of the shallow source fault of the 2011 tohoku oki earthquake and tsunami from boreholes drilled by the integrated ocean drilling program expedition 343 and 343t logging while drilling and core sample observations show a single major plate boundary fault accommodated the large slip of the tohoku oki earthquake rupture as well as nearly all the cumulative interplate motion at the drill site the localization of deformation onto a limited thickness less than 5 meters of pelagic clay is the defining characteristic of the shallow earthquake fault suggesting that the pelagic clay may be a regionally important control on tsunamigenic earthquakes

warm fluid migration along tectonic melanges in the kodiak accretionary complex alaska analyses of methane water fluid inclusions from syntectonic quartz veins in melange boudins of the kodiak accretionary complex alaska indicate fluid temperatures ranging from 215 to 290 c at depths between 10 and 14 km these fluid temperatures are significantly higher than those predicted by conductive heat flow models of subduction zones given the available geological constraints this discrepancy is interpreted as the result of migration of warm fluids from deeper structural levels along active fault zones during subduction related underthrusting of these units this interpretation requires more careful consideration of how migrating fluids affect the general thermal structure of subduction zones

melange and its seismogenic roof decollement a plate boundary fault rock in the subduction zone an example from the shimanto belt japan 1 the mugi melange located in western shikoku of the shimanto belt shows systematic y p deformation fabrics formed by microshear and pressure solution that penetrate throughout the melange pile magnetic susceptibility ellipsoids obtained from the anisotropy of magnetic susceptibility ams are highly oblate maximum and minimum axes of the ellipsoids are consistent with the shear orientation of the melange and the mean pole of p surfaces respectively this agreement suggests that the mugi melange was formed as a result of underthrusting of trench filling sediment vitrinite reflectance ranges from 2 52 to 3 08 which corresponds to a maximum paleotemperature of 180 200 c pseudotachylyte evidence of a seismogenic slip was found in the upper boundary roof fault of the mugi melange however there is not a thermal gap between the melange and the overlying coherent piles and the temperature from vitrinite reflectance gradually rises downward from the coherent piles to the melange beyond the boundary fault which suggests that paleoisotherms parallel the boundary fault orientation the isotherms in the seismogenic zone are estimated as subparallel to the plate boundary decollement therefore the setting of the cataclastic boundary fault which includes pseudotachylyte appears to be a major plate boundary thrust or a subhorizontal splay fault a probable geologic setting that accounts for the mugi melange and the seismogenic roof fault is partitioning of the slip along the plate boundary fault in space and time interseismic slip in the melange and seismic slip along the roof fault

dynamic role of tectonic melange during interseismic process of plate boundary mega earthquakes abstract tectonic melange has a key role in subduction zones because its thick pile forms the plate boundary itself therefore the plate boundary process is nearly identical to the melange forming process we examined three tectonic melanges in the shimanto belt in southwest japan to decipher their progressive deformation process with subduction especially within the seismogenic zone here we report detailed observation of sandstone blocks and the strain history of the shale matrix of a sediment dominated tectonic melange the necessity of tectonic melange accompanying fossil seismogenic decollement is unveiled several deformation processes continue by turn until the depth of the down dip limit of the seismogenic zone is reached the results support the space time partition of deformation in terms of seismic behavior and suggest a possible candidate for a geological consequence of recently observed slow earthquakes

a new source of water in seismogenic subduction zones 1 seismogenic plate boundary faults at accretionary margins e g the nankai margin southwest japan may occur where the uppermost part of subducting oceanic crust composed of basaltic rocks is in contact with the overriding plate of a lithified accretionary prism the plate boundary faults in ancient accretionary complexes typically record high velocity slip under fluid rich conditions although previous studies have emphasized the mechanical significance of fluids in terms of dynamic slip weakening the source of fluid in seismogenic subduction zones remains poorly constrained in this work we focus on the hydrous smectite in the uppermost oceanic crust an alteration product of intact basalt before arrival at the trench axis a comparison between 1 new mineralogical data on basalt drillcore recovered by integrated ocean drilling program iodp expedition 322 at site c0012 a reference site for subduction input to the nankai trough and 2 mineralogical data on basalt within ancient oceanic crust embedded in a fossil accretionary complex of the shimanto belt southwest japan suggests that progressive smectite chlorite conversion would liberate bound fluids at a rate of 0 34 to 0 65 10 14 s 1 along the plate interface this rate of fluid production appears to be more than an order of magnitude greater than that from other possible sources including from overlying sediments via smectite illite conversion and the expulsion of pore fluids and may facilitate seismic slip along plate boundary faults

the 2010 mw 8 8 maule megathrust earthquake of central chile monitored by gps large earthquakes produce crustal deformation that can be quantified by geodetic measurements allowing for the determination of the slip distribution on the fault we used data from global positioning system gps networks in central chile to infer the static deformation and the kinematics of the 2010 moment magnitude m w 8 8 maule megathrust earthquake from elastic modeling we found a total rupture length of 500 kilometers where slip up to 15 meters concentrated on two main asperities situated on both sides of the epicenter we found that rupture reached shallow depths probably extending up to the trench resolvable afterslip occurred in regions of low coseismic slip the low frequency hypocenter is relocated 40 kilometers southwest of initial estimates rupture propagated bilaterally at about 3 1 kilometers per second with possible but not fully resolved velocity variations

persistence of partial melting in the southern north china craton evidence from paleoproterozoic migmatites of the taihua complex abstract the taihua complex is exposed in the southern north china craton ncc it has experienced a long and complicated history of multiple crustal growth magmatism metamorphism and deformation during the early precambrian abundant leucosomes are widely distributed within the migmatites of the taihua complex which occurred as irregular lenses blocks layers or dikes with several thickness of millimeters to tens of centimeters wide meanwhile former presence of melt and typical microstructures related to partial melting also can be recognized in the thin sections under the microscope those provide compelling evidences that the taihua complex was extensively suffered partial melting and can be regarded as the best target to decipher multistage anatexis during metamorphism as well as assembly of the whole ncc however the timing and nature of the partial melting in the taihua complex have not been well studied up to now in this contribution we present an integrated in situ zircon u pb ages and hf isotope compositions for the paleoproterozoic migmatites from the taihua complex in the southern ncc both the metasedimentary and metamorphized plutonic rocks of the taihua complex have extensively suffered partial melting the ca 2 50 ga magmatic zircons or inherited magmatic cores from the migmatites and leucosomes present similar hf isotope composition to those of the contemporaneous depleted mantle at ca 2 50 ga suggesting that ca 2 50 ga is a major period of crustal growth in the southern ncc as well as the whole ncc the ages of metamorphic zircon obtained from leucosomes melanosomes and migmatites show a large time interval of 1909 1798 ma indicating that the taihua complex has experienced a persistence of partial melting event during late paleoproterozoic that lasted more than 100 ma the crustal anatexis in the southern ncc probably occurred during the post peak upper amphibolite to granulite facies retrograde stage with a migmatization temperature of at least 652 753 c the decompression melting and widespread migmatization of the taihua compelx are probably due to extension related to exhumation of the previously thickened crust during paleoproterozoic collision between the eastern and western block or between the southern archean block with other micro continent the main collision related assembly and crustal thickening events in the southern ncc likely occurred at ca 1 95 ga rather than ca 1 85 ga as previously believed

seismogenic zone temperatures and heat flow anomalies in the to nankai margin segment based on temperature data from iodp expedition 333 and thermal model the location of the seismogenic zone across the to nankai margin segment has been widely investigated using geodetic tsunami seismologic and heat flow data as well as thermal models seafloor heat flow measurements show large scatter raising questions about the thermal state of the incoming and overriding plates in this study new temperature and conductivity measurements recorded in the philippine sea plate psp and in the accretionary prism during iodp expedition 333 are integrated with recent data on the margin structure and seismicity to improve constraints on the thermal structure of the incoming plate and the subduction zone as well as the thermal conditions within the seismogenic zone iodp expedition 333 measurements provide heat flow values that are lower than the average of nearby seafloor measurements thermal modeling for the psp suggests that hydrothermal warming is insignificant in the trench thus an observed widespread positive thermal anomaly in the shikoku basin that peaks at the kashinozaki knoll more likely results from vigorous thermal convection in the upper mantle in back arc context the modeled thermal structure of the margin from the trench to the mantle shows a significantly colder interplate contact than in previous studies the seismogenic zone is likely to be 30 170 km from the deformation front corresponding to the 100 340 c temperature range the 1944 co seismic slip zone extends farther southward where temperatures are about 60 c slow slip earthquakes are located in the downdip transition zone between 340 and 440 c across the intersection of the moho and the megathrust fault this improved thermal structure from the trench to the mantle results mainly from a better estimate of oceanic plate age slab dip and sedimentation rate variations in the trench this last parameter cools the interplate contact until a depth of 35 km shifting the thermally defined seismogenic zone landward by 20 km and significantly diminishing its influence near the corner flow area

earthquake slip weakening and asperities explained by thermal pressurization for the earth s crust to release accumulated stress in the form of an earthquake a fault must weaken and give way otherwise the movement would quickly grind to a halt due to the surrounding pressure this weakening is not really understood laboratory experiments simply don t match seismologists estimates of the distance a fault must slip before it becomes very weak a new study concludes that the missing ingredient is water geological studies and measurements show the slip zones to be narrow impermeable structures water already in the fault zone can quickly become pressurized by frictional heating as the rocks start to move catalysing large earthquakes from minor movements an earthquake occurs when a fault weakens during the early portion of its slip at a faster rate than the release of tectonic stress driving the fault motion1 2 this slip weakening occurs over a critical distance dc understanding the controls on dc in nature is severely limited however because the physical mechanism of weakening is unconstrained conventional friction experiments typically conducted at slow slip rates and small displacements have obtained dc values that are orders of magnitude lower than values estimated from modelling seismological data for natural earthquakes3 4 5 6 here we present data on fluid transport properties of slip zone rocks and on the slip zone width in the centre of the median tectonic line fault zone japan we show that the discrepancy between laboratory and seismological results can be resolved if thermal pressurization of the pore fluid7 8 9 10 is the slip weakening mechanism our analysis indicates that a planar fault segment with an impermeable and narrow slip zone will become very unstable during slip and is likely to be the site of a seismic asperity

locking of the chile subduction zone controlled by fluid pressure before the 2010 earthquake large subduction zone earthquakes are thought to occur where the down going and overriding tectonic plates are strongly locked analysis of geodetic and seismic data collected in the decade before the 2010 chile earthquake shows that variations in pore fluid pressure correlate with the degree of plate interface locking and may therefore control earthquake rupture

thermal maturity in the shimanto accretionary prism southwest japan with the thermal change of the subducting slab fluid inclusion and vitrinite reflectance study abstract the p t path of the cretaceous shimanto accretionary complex western shikoku southwest japan was determined by a combination of vitrinite reflectance data and homogenization temperatures of fluid inclusion analysis within the complex the p t paths of the upper cretaceous yokonami melange and surrounding coherent strata were compared no p t difference was found between the yokonami melange and surrounding strata at least two types of fluids methane rich and water rich fluids migrated at different stages during metamorphism within the complex the methane rich fluid was initially trapped in vein quartz under pressures of 260 mpa and a geothermal gradient of 24 c km the water rich fluid was included in calcite at 95 125 mpa and a geothermal gradient of 50 c km the complex sequence of occurrence of the vein minerals and radiometric age data suggest that the water rich fluid was trapped during the kula pacific ridge subduction into the shimanto accretionary complex the paleoheat flow values for the two stages of subduction are consistent with the changes in the inferred slab age the metamorphism of the shimanto accretionary complex was predominantly controlled by the thermal condition of the subducting plate

seismic and aseismic slip on the central peru megathrust slip on a subduction megathrust can be seismic or aseismic with the two modes of slip complementing each other in time and space to accommodate the long term plate motions although slip is almost purely aseismic at depths greater than about 40 km heterogeneous surface strain suggests that both modes of slip occur at shallower depths with aseismic slip resulting from steady or transient creep in the interseismic and postseismic periods thus active faults seem to comprise areas that slip mostly during earthquakes and areas that mostly slip aseismically the size location and frequency of earthquakes that a megathrust can generate thus depend on where and when aseismic creep is taking place and what fraction of the long term slip rate it accounts for here we address this issue by focusing on the central peru megathrust we show that the pisco earthquake with moment magnitude m w 8 0 ruptured two asperities within a patch that had remained locked in the interseismic period and triggered aseismic frictional afterslip on two adjacent patches the most prominent patch of afterslip coincides with the subducting nazca ridge an area also characterized by low interseismic coupling which seems to have repeatedly acted as a barrier to seismic rupture propagation in the past the seismogenic portion of the megathrust thus appears to be composed of interfingering rate weakening and rate strengthening patches the rate strengthening patches contribute to a high proportion of aseismic slip and determine the extent and frequency of large interplate earthquakes aseismic slip accounts for as much as 50 70 of the slip budget on the seismogenic portion of the megathrust in central peru and the return period of earthquakes with m w 8 0 in the pisco area is estimated to be 250 years

along strike variations in underthrust sediment dewatering on the nicoya margin costa rica related to the updip limit of seismicity 1 along the costa rican subduction zone offshore the nicoya peninsula an offset in the updip limit of seismicity coincides with a transition between subduction of warm crust generated at the cocos nazca spreading center and cool crust formed at the east pacific rise we evaluate whether the observed difference in thermal state of incoming crust would result in significant differences in sediment dehydration reaction progress along strike thought to be a control on the updip limit of seismicity we combine thermal models with models of dehydration reaction kinetics for opal and smectite to estimate the distribution of diagenetic fluid sources the modeled distribution of diagenetic fluid sources mimics the pattern of the updip limit of seismicity seismicity begins 15 20 km landward of most of the smectite to illite transition this suggests that the location of the updip limit of seismicity may be influenced by the dissipation of fluid overpressures landward of smectite to illite dehydration

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

mechanism of thin skinned detachment in the amazon fan assessing the importance of fluid overpressure and hydrocarbon generation abstract since about 10 ma a wedge of clastic sediment up to 10 km in thickness has accumulated near the mouth of the amazon river forming the amazon fan thin skinned extensional and contractional structures root into a common basal detachment hundreds of kilometers long extensional faults have formed landward of the prograding delta slope and contractional folds and faults have formed seaward of it by plan view restoration we show that extension and contraction balance we therefore infer that slope instability has provided the driving force for thin skinned deformation on seismic data the detachment appears as a sharp surface within late cretaceous strata however there is no evidence at that level for soft layers such as evaporites or muds abnormal fluid pressure provides a more reasonable mechanism for the detachment at depths of several thousand meters deep wells have encountered fluid overpressures amongst the possible causes of abnormal fluid pressures are compaction and hydrocarbon generation although compaction may have contributed to fluid overpressures encountered in wells we argue that hydrocarbon generation provides the best explanation for detachment in cretaceous rocks which were old and lithified when it occurred in particular the broadly triangular area of detachment coincides with an inferred pod of cenomanian turonian source rock which is interpreted to have been in the gas window during detached deformation we have developed new techniques of sandbox modelling to investigate the geometries of fault patterns produced by fluid flow through porous media for the amazon fan we have been able to reproduce the structural pattern on the assumption that fluid overpressures existed over a broadly triangular area beneath the detachment

the updip and downdip limits to great subduction earthquakes thermal and structural models of cascadia south alaska sw japan and chile this study examines thermal and structural controls of the updip and downdip rupture limits of great subduction thrust earthquakes data on past great earthquake seismic limits have been compiled for four continental subduction zones cascadia sw japan south alaska and chile these limits have then been compared to the predictions of several models for what constrains great earthquake rupture temperatures on the subduction thrusts have been estimated by finite element numerical models the landward limits of the observed updip aseismic zones correspond to the position where the thrust temperature reaches about 100 c that is depths of about 2 to 10 km for the subduction zones studied this temperature agrees with the dehydration of stable sliding smectite clays to illite chlorite the temperatures in this region are controlled mainly by the thickness of sediment on the incoming crust and by the crustal age and thus heat flow the downdip limits correspond to the depth on the thrust where either 1 the temperature reaches about 350 c which corresponds to thermally activated stable sliding behavior for crustal rocks with a transition to 450 c or 2 about 40 km depth if 350 c is reached at greater depth depths of about 40 km approximately correspond to the intersection of the thrust with the continental forearc moho and this downdip limit may be a consequence of stable sliding serpentinite or talc and other hydrated forearc mantle rocks the primary temperature controls on the downdip region are the age of the subducting oceanic plate and the thrust dip profile secondary control comes from the thickness of incoming sediment the convergence rate and the radioactive heat generation in the overlying forearc the 100 c updip limit occurs near the trench for young subducting plates with a thick sediment section such as cascadia 6 8 ma and up to 80 km landward for older oceanic crust such as south alaska 50 ma the 350 c downdip thermal limit is applicable for young oceanic plates e g cascadia and nankai whereas the forearc mantle limit applies for older plates e g south alaska and chile except near the chile rise for the margins studied that have experienced great earthquakes there is generally good agreement between the postulated thermal and moho limits and the rupture or seismogenic zone as defined by the distribution of aftershocks and by waveform tsunami and dislocation modeling the downdip limit of the interseismic locked zone from dislocation modeling is also in agreement with these limits

geopressure evolution and hydrocarbon generation in a north indonesian basin two dimensional quatitative modelling abstract a two dimensional fluid flow compaction model was used to simulate geopressure evolution and hydrocarbon evolution in a 5 million year old basin in indonesia data taken from two wells on the cross section analysed showed the occurrence of large overpressure mainly due to undercompaction driven by the high sedimentation rate 4000 5000 m of deposition the entire section is controlled by a major growth fault system which has been modelled by a staircase like set of vertical instantaneous faults appearing progressively in time after calibration of the dynamic parameters controlling 1 the deposition of the sediments and 2 the evolution of the compaction process it was possible to reproduce the high excess fluid pressure in accordance with present day observations the important role of anomalously high temperature developed in connection with excess pressure formation is shown through effects on the expansion of water a major heating event suggested by a previous one dimensional study was introduced around three million years before present which has a major influence on the onset of maturation for the high gas potential organic matter suggesting the onset of oil gas generation in the last million years or so

organic matter in the franciscan and cedros subduction complexes the problems of instantaneous maturation and missing petroleum in accretionary prisms abstract most accretionary complexes are submarine mountain belts consisting predominantly of sedimentary rock past studies of accretionary complexes have concluded that they have minimal petroleum potential although they do contain potential source rocks owing to their great bulk of sedimentary rock large amounts of petroleum must be generated in accretionary prisms and these hydrocarbons must be trapped in the complexes or vented into the ocean or on land to understand the nature and abundance of organic material in accretionary complexes organic matter in the shales limestones and melange matrix of the franciscan and cedros subduction complexes was studied mostly using rock eval pyrolysis the largest amounts of total organic carbon toc were found in the calera limestone from the san francisco area 1 6 toc terrigenous sedimentary rocks of probable trench or trench slope origin contain from

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

the origin of tectonic fabrics in undeformed clays the early stages of deformation in extensional sedimentary basins 1 an integrated approach was used to detect tectonic effects in apparently undeformed fine grained clay sediments filling extensional basins in southern italy results from low and high field anisotropy of magnetic susceptibility and mineral fabric deduced by neutron texture analysis show penetrative deformation on a grain scale the well defined tectonic magnetic lineation is parallel to the stretching direction and located at the intersection of the basal planes of phyllosilicates forming grain scale folds and crenulations this spatial configuration reflects basin scale deformation processes and suggests the existence of a nearly uniaxial stress field during the incipient extensional phases the integrated approach of magnetic and mineral fabric investigations is a valid tool to detect grain scale and regional deformation pattern in weakly deformed extensional basins where macroscopic evidence of deformation is not visible

physical characteristics of subduction interface type seismogenic zones revisited 1 based on global earthquake catalogs the hypocenters nodal planes and seismic moments of worldwide subduction plate interface earthquakes were extracted for the period between 1900 and 2007 assuming that the seismogenic zone coincides with the distribution of 5 5 m 7 earthquakes the subduction interface seismogenic zones were mapped for 80 of the trench systems and characterized with geometrical and mechanical parameters using this database correlations were isolated between significant parameters to identify cause effect relationships empirical laws obtained in previous studies were revisited in light of this more complete accurate and uniform description of the subduction interface seismogenic zone the seismogenic zone was usually found to end in a fore arc mantle rather than at a moho depth the subduction velocity was the first order controlling parameter for variations in the physical characteristics of plate interfaces determining both the geometry and mechanical behavior as such the fast subduction zones and cold slabs were associated with large and steep plate interfaces which in turn had large seismic rates the subduction velocity could not account for the potential earthquake magnitude diversity that was observed along the trenches events with mw 8 5 preferentially occurred in the vicinity of slab edges where the upper plate was continental and the back arc strain was neutral this observation was interpreted in terms of compressive normal stresses along the plate interface large lateral ruptures should be promoted in neutral subduction zones due to moderate compressive stresses along the plate interface that allow the rupture to propagate laterally