sediment deformation and hydrogeology of the nankai trough accretionary prism synthesis of shipboard results of odp leg 131 the main objective of leg 131 was to provide data on the deformational processes and associated hydrogeology of the nankai prism toe drilling succeeded for the first time in the history of ocean drilling in penetrating the complete sedimentary sequence to basaltic basement reaching 1327 mbsf metres below seafloor with good core recovery 55 excellent correlation of the lithology and structure including the frontal thrust and the decollement with seismic reflection images was also determined bedding dips faults and shear bands analyzed in the cores confirm the pattern of deformation to be mainly due to nw se shortening as expected from the plate tectonic convergence vector below the decollement no significant deformation features were observed indicating that the decollement is a sharp discontinuity in stress transmission physical properties data show major discontinuities at the decollement notably an increase in porosity below the later this may indicate excess pore pressure in the subducted section and decollement zone a less marked increase in porosity below the frontal thrust may reflect the youthfulness of this feature attempts to make downhole measurements were severely hampered by unstable hole conditions but useful constraints have been placed on the thermal regime and some calibration of laboratory physical properties toin situ conditions has been provided andin situ stress and pore pressure were measured in the uppermost sediments evidence of channelized fluid flows is inconclusive no sharp geochemical signatures or unequivocal geochemical anomalies indicative of channelized fluid flow were found thermal measurements are not significantly different from those predicted by a purely conductive heat flow model a signature of low chloride pore water near the decollement may partly be related to smectite diagenesis but may also be due to episodic fluid flow events we conclude that dewatering probably occurred dominantly through diffuse flow throughout the accreted sediments at this site

linear and nonlinear relations between relative plate velocity and seismicity relationships between relative plate velocity and seismicity differ by plate boundary class we test the null hypothesis of linearity of earthquake rates with velocity in each of seven classes a linear relationship is expected if earthquake rate is proportional toseismicmoment rate whichis proportional torelative platevelocity to reducebiasbyaftershocksandswarms weestimateindependenceprobabilitiesofearth quakes and use them as weights we assign shallow earthquakes to boundary steps and classes thensortboundarystepswithineachclassbyvelocityandplotcumulativeearth quakesagainstcumulativemodelmomentrate weusetwomeasuresofnonlinearityand 10 4 stochastic simulations to assess significance in subduction zones the relationship betweenseismicityandvelocityisnonlinearwith99 9 confidence slowersubduction at 66 mm a producing35 oftectonicmomentunderthenullhypothesis produces only 20 of subduction earthquakes continental convergent boundaries display simi larnonlinearity p 0 001forthenullhypothesis oceanspreadingridgesshowseis micity decreasing with velocity oceanic transform faults and oceanic convergent boundaries show marginal nonlinearity p 0 01 p 0 05 continental rifts and continental transform faults follow the null hypothesis three effects may contribute to velocity dependence in subduction 1 the brittle ductile transition at a critical tem peratureisadvecteddeeperbyfasterunderthrusting 2 subductedsedimentisviscous so lower stresses in slower boundaries discourage earthquakes 3 pore pressures increase with velocity encouraging frictional failure mechanism 1 has only minor effects on earthquake productivity but mechanisms 2 and 3 could be important online material earthquake subcatalogs and cumulative distribution tables

smectite swelling in the miura boso accretionary prism possible cause for incipient decollement zone formation abstract an analysis of the mineralogy of the miura boso accretionary prism which consists of off scraped upper miocene strata exposed on the southern parts of the miura and boso peninsulas in central japan revealed homogeneous mineral composition throughout all levels of silt layers hosting deformation structures associated with sediment accretion xrd analyses showed that the sediment consists of randomly ordered mixed layer illite smectite i s with low illite content 40 wt quartz 30 wt illite 10 20 wt plagioclase 10 wt kaolinite chlorite 5 wt and calcite 0 10 wt the observed mineralogical assemblage indicates that location of the decollement formation was not constrained by mechanical heterogeneities associated with mineralogical variations but was controlled by spatial variations in effective pressure within the over consolidated sediment located above the incipient decollement zone the occurrence of abundant smectite with a shape preferred orientation gave rise to smectite swelling producing pressures in the order of 3 0 mpa the combined effect of high swelling pressure and high pore pressure resulted in an overpressured zone at the base of the over consolidated zone facilitating the development of a decollement zone within the prism

origin of smectite and illite smectite in the barbados accretionary complex oxygen isotopic evidence oxygen isotopic and mixed layer compositional data are used to infer modes of formation of smectite and illite smectite from the barbados accretionary complex nearly pure smectites have 18 o values between 26 and 30 and apparently formed authigenically by alteration of volcanic ash illite smectites are thought to have formed in situ at temperatures less than 40 c although a detrital origin cannot be ruled out for several samples the unusually low temperature of transition of smectite to illite indicates that fluid generation by mineral dehydration and consequent structural effects can occur at shallow depths in accretionary complexes

stress induced smectite dehydration ramifications for patterns of freshening and fluid expulsion in the n barbados accretionary wedge abstract presented is new experimental evidence that smectites can partially dehydrate from 18 5 a to 15 4 a hydrates when they are subjected to effective mineral framework stresses above 1 3 0 3 mpa we show here that this process can substantially freshen saline pore fluids and impact our understanding of the hydrogeologic processes in convergent margins where smectites are abundant the general anomalous freshening of pore fluids in the n barbados accretionary wedge was previously proposed to relate to the widespread lateral expulsion of fluids derived from mineral dehydration reactions at great depth in contrast our analysis suggests that a significant part of the widespread anomalous freshening relates to stress induced smectite dehydration that occurred during the ocean drilling program odp pore water extraction process clear evidence for long distance lateral flow appears to be limited to localized chloride and isotopic anomalies developed in a 20 m thick locally mineralized region at the top of decollement horizon at site 948 our reinterpretation is consistent with the fluid flow in the decollement zone being 1 highly transient on time scales of several hundreds of years and 2 channeled laterally into heterogeneous finger like flow paths that are apparently not intersected and or sampled by drilling in most regions at the toe of the wedge the incipient stress induced dehydration of smectite may also be buffering in situ fluid pressures in the wedge maintaining the effective stresses at 1 3 mpa

paleosol stratigraphy of the eskridge formation early permian pedogenesis and climate in southeastern nebraska abstract at least five aridisol and vertisol like paleosols are developed within the eskridge formation lower permian in richardson county nebraska these soils are recognized by their prominent soil structure peds nodules and filled cracks micromorphology oriented clay fabrics and iron cutans and well developed carbonate enriched horizons eskridge paleosols represent significant decreases or cessations in terrestrial sedimentation during long term subaerial exposure direct evidence for drowning of three out of the five paleosols by subsequent marine transgressions indicates strong marine influence on the duration of pedogenesis stratigraphic fluctuations in the 13c and 18o values of eskridge pedogenic micrites are potentially compatible with climatic fluctuations between wetter and drier conditions during the permian based on comparisons with isotope values from cenozoic paleosols occurrence of fossil plants and coals along with evaporites as well as the different paleosol types tends to support an interpretation of varying climate recognition of paleosols within the eskridge improves the understanding of cyclic sedimentation and sea level change by recording several episodes of low terrestrial sedimentation and geomorphic stability these episodes probably correspond to peak marine regressions

permeability and frictional strength of cation exchanged montmorillonite 1 smectites such as montmorillonite are abundant throughout the upper crust and are commonly found in fault gouge they are known for their weak frictional strength and low permeability smectites are swellable clays in which the interlayer cation can easily be exchanged in this study we measure permeability and frictional strength of montmorillonite exchanged with na k ca2 and mg2 using a triaxial shear apparatus we find that the interlayer cation influences both permeability and the friction coefficient k montmorillonite is significantly stronger 0 26 than na montmorillonite 0 15 and ca or mg montmorillonite 0 11 permeability for all four clays is between 10 21 and 10 23 m2 in the pressure range between 10 mpa and 100 mpa with k montmorillonite the most permeable clay gouge mg ca and na montmorillonite show relative similar permeabilities of which na montmorillonite is least permeable we relate the higher frictional strength and permeability of k montmorillonite to reduce interlayer water content caused by differences in cation hydration and adsorption the results show that the rapid cation exchange process in montmorillonite can influence macroscale parameters such as permeability and strength which can contribute to changes in local pressure conditions and frictional strength of shallow fault zones

offscraping and underthrusting of sediment at the deformation front of the barbados ridge deep sea drilling project leg 78a on leg 78a we drilled sites 541 and 542 into the seaward edge of the barbados ridge complex and site 543 into the adjacent oceanic crust the calcareous ooze marls and muds at sites 541 and 542 are lithologically and paleontologically similar to the upper strata at site 543 and are apparently offscraped from the down going plate a repetition of miocene over pliocene sediments at site 541 documents major thrust or reverse faulting during offscraping the hemipelagic to pelagic deposits offscraped in the leg 78a area include no terrigenous sand beds but they contain numerous neogene ash layers derived from the lesser antilles arc hence this sequence is quite unlike the siliciclastic dominated terranes on land that are inferred to be accretionary complexes the structural fabric of the offscraped deposits at sites 541 and 542 is disharmonic probably along a decollement with an underlying acoustically layered sequence suggesting selective underthrusting of the latter the acoustically layered sequence correlates seismically with pelagic strata cored at site 543 on the incoming oceanic plate cores recovered from the possible decollement surface at both sites 541 and 542 show scaly foliation and stratal disruption approximately lithostatic fluid pressure measured in the possible decollement zone probably facilitates the underthrusting of the pelagic sediments beneath the offscraped deposits in the incoming section a transition from smectitic to radiolarian mud with associated increases in density and strength probably controls the structural break between offscraped and underthrust strata in the leg 78a area the underthrust pelagic section can be traced seismically at least 30 km arcward of the deformation front beneath the barbados ridge complex

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

smectite dehydration its relation to structural development and hydrocarbon accumulation in northern gulf of mexico basin a comparison of clay diagenesis data obtained from a study of tertiary shales from the brazos colorado river system of texas the mississippi river system of louisiana and the niger river system of nigeria illustrates significant differences in temperature intervals over which smectite diagenesis occurs the threshold temperature required to initiate diagenesis ranges from about 160 f 71 c in mississippi river sediments to more than 300 f 150 c in the niger delta water expelled from smectite into the pore system of the host shale during the process of diagenesis may migrate out of the shale early or may be totally or partially trapped and released slowly through time in either situation the water can act as a vehicle for hydrocarbon migration p ovided hydrocarbons are present in a form and in sufficient quantities to be transported observations from the northern gulf of mexico basin indicate a close relation between buildup of high fluid pressure and the smectite illite transformation process abnormal pressures exert partial control on the type and quantity of hydrocarbons accumulated because pressure potential determines the direction of fluid flow and overpressuring partly controls the geometry of growth faults and related folds in basins where shale structures are the dominant type formed the depths to which growth faults can penetrate and the angle of dip that these faults assume at depth are largely dependent on fluid pressure in the sedimentary section at the time of faulting dips of some faults in texas have been observed to change abruptly within the interval of smectite diagenesis and some faults formed in the overpressured miocene and younger sections become beddingplane types at depths where the temperature is near that required for thermal generation of petroleum although these faults may be important for fluid redistribution in shallow sandstone shale sections they are a minor factor in moving hydrocarbons out of shale below the faults in much of the texas offshore area fluid movement upward through microfracture systems in the deeply buried overpressured section overlying and extending upward from fault trends in the sub tertiary section is proposed as a mechanism for flushing hydrocarbons from the deeper portion of the northern gulf of mexico basin this flushing process would be enhanced by smectite diagenesis because water derived from smectite that was trapped during basin subsidence would cause the flushing process to continue for longer periods of time and to extend to greater depths than could be attained if only remnants of original pore water were present shale tectonism is also the primary mechanism for structural development in the tertiary section of the niger delta however seismic data indicate that the rate of dip change of seaward dipping listric growth faults is commonly less than that observed in texas where dips as low as 10 15 can occur at depths as shallow as 10 000 15 000 ft 3 048 4 572 m syndepositional faults in nigeria are formed in sandstone shale sections where the clay composition of shale is primarily kaolinite and where little water of smectite diagenesis has been added to the pore system of the host sedimentary section subtle differences in structural styles in the tertiary sections of texas and nigeria are probably the result of differences in clay composition of the shaly sections being deformed

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

frictional properties of shale reservoir rocks 1 we performed laboratory friction experiments on shale samples from three hydrocarbon reservoirs to assess compositional controls on fault slip behavior accompanying hydraulic stimulation the samples span a range of clay and total organic content from 10 to 60 by weight and demonstrate fine scale heterogeneity within each reservoir friction measurements demonstrate strong dependence of strength and rate state constitutive properties on shale composition shale samples with clay and organic content above 30 by weight show coefficient of friction 0 4 and consistently velocity strengthening behavior while those below this threshold show increased strength and velocity weakening behavior this transition in frictional strength and stability suggests a change in the shale grain packing framework from rigid clast supported to clay mineral and organic matter supported measurements of gouge dilatancy further support these relationships showing a negative correlation with clay and organic content critical slip evolution distance exhibits similar dependence on composition implying common micromechanical mechanisms for the observed transition in frictional behavior we performed microstructural characterization of the experimental samples and observe changes in the gouge load bearing framework and shear localization features consistent with these mechanical data while these results can be applied generally to infer slip stability on faults in clay bearing sedimentary rock we employ the experimental data to place constraints on microearthquake magnitudes and the occurrence of slow shear slip on preexisting faults during hydraulic stimulation in shale reservoirs

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

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

landward vergence and oblique structural trends in the oregon margin accretionary prism implications and effect on fluid flow abstract the central oregon margin spans a regional transition in accretionary structures from seaward verging in the south to landward verging in the north new multichannel seismic mcs data image both landward and seaward vergent provinces along the central and northern oregon margin landward vergence is characterized by a deep decollement with deformation distributed across a broad lower continental slope in a coherent structural style in the landward vergent area virtually all 4 km of incoming trench sediments overthrust the preceding thrust sheet forming fault bend folds and a distinctive ridge trough morphology this style of landward vergence is not explained by existing models in contrast seaward vergence correlates with a shallower decollement approximately 1 4 km above the oceanic crust and a more intense style of deformation within a narrower slope initial thickening of the trench sediments occurs across a well developed protothrust zone the frontal thrust forms a ramp anticline that is cut by a prominent backthrust previously observed seafloor vent sites in both regions correlate with thrusts that exhibit high amplitude reversed polarity reflections suggestive of enhanced porosity along the faults potential fluid sources and migration paths are strongly influenced by changes in the level of the decollement and vergence along the margin abrupt changes in structural style occur both along strike and updip and are bounded by two sets of oblique slip faults three nw striking left lateral faults are imaged in both mcs and seabeam data plunging anticlines developed along the nw striking faults are venting fluids and were previously interpreted as mud volcanoes the deformation front is locally disrupted where these faults intersect the prism but they appear to have limited influence on the structural evolution of the prism in contrast the ne striking right lateral faults are confined to the deforming sediments of the upper plate these faults interact with the thrusts within the prism forming a rhomboidal pattern of three dimensional deformation

two potential sources for holocene clay sedimentation in the caribbean basin the lesser antilles arc and the south american continent abstract clay sedimentation in the caribbean basin is the result of the mixing of two types of materials of different origin 1 those derived from weathered volcanic products of the lesser antilles martinique island arc and identified in the bay of fort de france are made up of ferriferous montmorillonites and beidellites rich in mg and poor in si k trace elements and halloysite materials derived from the meteoric weathering of volcanic rocks andesites dacites are transported to deeper facies by turbidity and bottom currents the contribution of these materials to the sedimentation of the grenada basin and the eastern flank of the aves ridge is relatively important 2 south american terrigenous materials identified in the venezuela basin consist of illites kaolinites and ferriferous beidellites they are poor in mg and relatively rich in si k and trace elements these materials are formed of weathered metamorphic and magmatic acid rocks from the northern provinces of the south american continent brought into the grenada basin by guyana and equatorial surface currents the contribution of these materials to sedimentation on the island arc platform of the lesser antilles is relatively significant this work shows that during the transport of fine grained materials variations in the composition of clays took place only on the insular platform

plumbing accretionary prisms effects of permeability variations fault zones focus fluid expulsion in the muddy northern barbados ridge accretionary prism with fault parallel permeabilities about 1000 times greater than intergranular permeabilities in the adjacent sediment in the oregon prism the low bedding perpendicular permeability due to mudstones inhibits intergranular dewatering however intergranular flow is concentrated where submarine erosion breaches high permeability sandy layers even so faults can capture fluid flow from these exposed sandy layers suggesting the faults have a still higher permeability such observations coupled with laboratory measurements permeabilities suggest that faults off oregon may have fault parallel permeabilities at least 10 10000 times greater than the adjacent sediments results from barbados and oregon suggest fluid flow is concentrated along the most active faults at the toe of prisms the fault zones are being progressively loaded by the thickening wedge and are undergoing compaction preliminary experiments show that permeability decreases relative to the surrounding wall rocks along faults within this compactive deformation regime we believe that these faults must undergo dilation perhaps linked to transient increases in pore pressure if they are to be preferential fluid conduits farther upslope erosion exposes rocks that are more consolidated commonly more cemented and generally of lower intergranular permeability than rocks of equivalent burial further seaward because of their lithification and overconsolidation these rocks dilate during faulting locally enhancing fracture permeability in such dilative regimes faults become evermore focused zones of fluid expulsion relative to occluded intergranular pathways

hydrogeochemistry of the northern barbados accretionary complex transect ocean drilling project leg 110 detailed studies of the major element geochemistry oxygen and hydrogen isotope ratios of pore fluids and the 87sr 86sr ratio of dissolved strontium have made it possible to unravel physical and chemical processes that affect the pore fluid chemistry in a transect of drill holes across the northern barbados accretionary complex these processes include 1 alteration of volcanic ash buried in the pleistocene pliocene sediment column 2 alteration of underlying basalts of layer 2 of the oceanic crust 3 movement of fluids from deep in the accretionary complex along fault zones particularly the decollement and minor permeable layers these fluids from deeper in the complex are characterized by low chloride concentrations and increased 18o h2o values presumably as a result of dehydration of smectite interlayers and 4 mixing processes involving the migrating fluids cause incongruities in the geochemical anomalies of these fluids

mechanisms of subduction accretion along the central aleutian trench we have used migrated 24 fold seismic records to study mechanisms of subduction accretion along the central aleutian trench two major structural units underlie the landward trench slope and are separated by an acoustically defined decollement surface that can be traced at least 20 km landward of the trench axis the upper unit consists of three to four structural blocks of deformed trench fill each block possesses a consistent internal structure and is bound by major landward dipping thrust faults the structures within the blocks include monoclinal sequences antiforms synforms and faulted folds beneath the decollement nearly undeformed trench fill 1 km thick is being under thrust along with the underlying oceanic lithosphere this basal section is an intervening unit between the offscraped sediment and the rough igneous oceanic basement and may prevent the oceanic crust from being incorporated into the subduction complex at least within 20 km of the front of the subduction zone owing to bending stresses the oceanic crust fails along normal faults beneath and at least 15 km landward of the trench axis these normal faults although poorly constrained are believed to propagate upwards producing offsets within both the subducting and the accreting sediment in addition normal faulting enhances the irregular surface of the oceanic crust structural associations imaged on all of the seismic reflection profiles although possibly fortuitous suggest that the subduction of these irregular basement highs results in the temporary displacement of the overlying fault bound sediment packets and promotes the development of the structures found within the accretionary prism tectonic riffling of the accreted sediment by the passage of the basement relief may also provide a mechanism by which underthrusting sediments are decoupled from the subducting lithosphere and underplated to the base of the aleutian accretionary complex

porosity corrections for smectite rich sediments impact on studies of compaction fluid generation and tectonic history porosity is a fundamental parameter that must be correctly determined in order to relate physical property hydrologic and chemical flux studies to natural systems traditional porosity determinations generated from physical property or seismic data can greatly overestimate the true porosity of sediments in which hydrated minerals such as smectite are abundant to produce a true porosity distribution such data must be corrected to account for the h 2 o residing in smectite interlayers which can make up to 25 of the total hydrated mineral mass such h 2 o is easily removed from the mineral by oven drying and or exposure to low humidities standard physical property measurements can be corrected provided the weight percent smectite in the sediment is known and the interlayer h 2 o content of the mineral can be estimated we illustrate the significant consequences of this correction by comparing profiles of reported and corrected porosities for smectite rich barbados abyssal plain and accretionary wedge sediments from ocean drilling program leg 110 sites 671 and 672

influence of kinetics on the smectite to illite transition in the barbados accretionary prism low chloride pore fluids observed along faults in clay rich accretionary complexes are commonly attributed to the release of interlayer water during the smectite to illite transformation however to date there has been no thorough analysis of the location and quantity of fluids that may be generated by this mechanism to address this problem a temperature and time dependent rate expression describing the dehydration reaction was coupled to a kinematic model of the northern barbados accretionary complex temperatures in the complex were estimated by modeling heat flow through the prism sediments as they thicken arcward the sediments temperature time histories were computed using a model for the velocities of sediment motion through the complex in this model the prism sediments follow uniformly diverging paths from the toe while the underthrust sediments undergo uniaxial strain the model predictions are validated against clay mineralogy data from barbados island mudstones our results show that the location of the peak dehydration rate is 20 km farther arcward in the underthrust sediments than in the prism complex the fresh water produced by the reaction results in pore waters that are 10 30 fresher at a distance of 50 70 km from the prism toe this constraint indicates a probable fluid migration path of more than 50 km from the reaction zone to the sites where freshened pore fluids have been observed the peak rate of fluid production when expressed as the volume of fluid per volume of sediment per second is 2 l0 15

fluid pressure in the frontal thrust of the oregon accretionary prism experimental constraints seismic reflection profiles of many accretionary prisms including the oregon prism exhibit high amplitude reversed polarity reflections from the decollement and other thrusts it has been suggested that these reflectors image fault zones with enhanced fluid content due to dilation by very high fluid pressure we present measurements of velocity as a function of effective stress on samples of the oregon frontal thrust that show that velocity decreases by up to 15 as fluid pressure rises from hydrostatic to lithostatic conditions under constant confining stress in fault zone and wall rock alike synthetic seismic modeling shows that the frontal thrust reflector at this location is the result of a thin low velocity zone in the plane of the fault 100 300 m s slower than the walls combining models with experimental results we conclude that fluid pressure of 86 to 98 of lithostatic stress reduces velocity enough to generate the reflections

relation between permeability and effective stress along a plate boundary fault barbados accretionary complex in situ bulk permeability was measured in a borehole that intersected the decollement zone a low angle detachment fault between the north american and caribbean plates permeability measurements were made at a variety of fluid pressure conditions defining a quantitative relation between bulk permeability and effective stress for this plate boundary fault zone the bulk permeability in this zone changed by several orders of magnitude as a consistent function of fluid pressure this relation may help to explain the dynamics of fluid fault interactions and the transient nature of hydrologic processes during deformation at convergent margins

scene characterization using subaperture polarimetric sar data in synthetic aperture radar sar polarimetry the measured polarimetric signatures are used to analyze physical scattering properties of the imaged media it is generally assumed that the sensor has a fixed orientation with respect to the objects however sar sensors operating at lower frequencies like l and p band have a wide azimuth beamwidth i e during the formation of the synthetic aperture multiple squint angles are integrated to build the full resolution sar image variations in the polarimetric properties with the azimuthal look angle remain unconsidered in this paper a fully polarimetric subaperture analysis method is introduced using deconvolution synthesized sar images are decomposed into subaperture datasets which correspond to the scene responses under different azimuthal look angles a statistical analysis of the polarimetric parameters permits to clearly discriminate media showing a nonstationary behavior during the sar integration finally a method is proposed which eliminates the influence of azimuthal backscattering variations in conventional polarimetric sar data analysis the effectiveness of the new methods is demonstrated on fully polarimetric sar data acquired by the german aerospace center dlr airborne experimental sar sensor e sar at l band

fluids and thrusting abstract recent studies of the structure of the continental crust by cocorp and birps have shown that major thrust structures in the lower crust are ubiquitous plate tectonic processes of continental crust destruction or construction at sites where subduction or collision occurs the himalayas or the andes involve thrusting of continental materials or ocean crust beneath continents and local overthrusting may accompany transcurrent motion in all such cases massive fluid flow must occur as first stressed by m k hubbert and w w rubey thrusting mechanisms require that fluid pressures are close to or greater than lithostatic pressures when thrusting occurs compaction of the underplate must occur rapidly expelling pore water crustal thickening leads to gradual heating of the underplate with prograde metamorphism near the top and even melting at the base if the overthrust plate is thick and hot at the base retrograde metamorphism will occur at the base of the overthrust plate and rising fluids will encounter inverted thermal gradients in such a region veins need not occur but leaching phenomena may dominate if an underthrust plate is hydrated and or contains sedimentary aquifers large fluid volumes are expelled through shear zones into the overthrust plate the scale of fluid release processes can be large thus in thin skinned tectonics where the overthrust plate is 10 15 km thick induced fluid release can easily reach 4 109 g m 2 of thrust surface the flow of such fluids will be controlled by the thrust surface and lithology both of which will influence hydraulic fracture mechanisms and spacing of fractures the chemistry of thrust derived fluids may be highly variable depending on lithology and the time constants of thrusting vertical thermal and redox environments will be similarly dependent in the subduction process impressive quantities of fluids must pass back up the thrust surface and such fluids have recently been directly observed during drilling dewatering of spilites serpentinites and sediment layers of the underthrust oceanic lithosphere must produce massive fluid flow back to the surface but some of these fluids may form hydrated minerals in the overlying mantle and at great depth flux mantle melting extreme metasomatism in blueschist belts must result from such fluids a case of particular interest involves the cessation of subduction when a high level slab reaches thermal equilibrium in general flow regimes in the thrust and fault zones associated with collisions follow a sequence from conditions of high t p with locally derived fluids at low water rock ratios during initiation of the structures to high fluxes of reduced metamorphic fluids along conduits as the structures propagate and intersect hydrothermal reservoirs later in the tectonic evolution and at shallower crustal levels there may be incursion of oxidizing near surface fluid reservoirs into the faults these fluids may have extremely low 18o values where mountain ranges form on rebound faults and high altitude depleted fluids penetrate down the rebound structure extensive mineralization may be associated with such thrust derived fluids examples which will be discussed include the mother lode au deposits of california u s a and the large u deposits of lagoa real brazil

crystallographic preferred orientation of talc determined by an improved ebsd procedure for sheet silicates implications for anisotropy at the slab mantle interface due to si metasomatism

residual strength of kaolin and bentonite the influence of their constituent pore fluid the residual strength of clays is affected by their mineral composition and by the nature of their constituent pore fluid in principle both factors may influence either strength parameters or stresses but it is not yet completely clear how this influence works previous publications have elucidated some aspects of clay residual strength behaviour this paper investigates further the roles of minerals and pore fluid composition on the residual strength of clayey soils it describes and analyses tests on kaolin bentonite and their mixtures exposed to distilled water and sodium choride solutions with given concentrations the results show that the shear strength of kaolin is not affected by the solutions used whereas the residual strength of bentonite varies greatly because of the inward salt diffusion towards the clay further for the clay mixtures with any of the pore fluids considered the weaker component has the greater influence on the behaviour of the mixture la resistance residuelle des argile

accretion and recent deformation of sediments along the northern cascadia subduction zone new multichannel seismic reflection profiles acoustic imagery and swath bathymetric data from the vancouver island continental margin reveal the surface morphology cross sectional geometry and deformation style of this part of the cascadia accretionary prism the incoming 3 to 5 km thick sediment section fails most commonly along landward dipping thrusts that penetrate to or very close to the top of the underlying oceanic crust the faults are listric with approximately circular cross sections and hanging wall sediments are back tilted in a planar manner high near lithostatic pore pressures in the deeper part of the section are inferred from the flattening of the faults near the decollement and from the long distances over which the decollement has propagated seaward the faults dip more steeply at 40 45 near the surface where pleistocene turbidites are displaced similar folds have developed where the turbidites are particularly thick 4 km and near hydrostatic pore pressures are inferred to occur in the turbidite part of the section an increase in pore pressure with depth is expected as a consequence of sedimentation and tectonic loading particularly because the deeper hemipelagic sediments are probably far less permeable than are the overlying turbidites the top of the downgoing juan de fuca plate which defines the base of the accreted sedimentary wedge beneath the margin is well imaged in the new seismic reflection profiles and is further defined by land reflection data seismic refraction and earthquake hypocenters the landward dipping continental backstop to the prism is also well located if these constraints are used for the geometry of the accretionary prism a general balance is found between the present prism volume and the estimated total sediment supply over the approximately 42 m y accretion history this result along with the observation that deformation front faults penetrate most or all of the sediment section suggests that little or no sediment has been subducted since initiation of the present phase of subduction in the eocene application of the critically tapered wedge model of davis and others to the observed prism geometry suggests that high average pore pressures occur in the prism particularly the seawardmost part beneath the lower slope beneath the continental shelf the dip of the subducting slab increases to greater than 11 a dip that according to the critically tapered wedge model is too steep to allow upward growth of an accretionary prism this provides a simple explanation for ongoing subsidence of the tofino sedimentary basin over the older part of the accretionary complex to the southeast the juan de fuca plate dips much more gently beneath the olympic peninsula of northern washington allowing the accretionary prism to grow upward well above sea level to form the olympic mountains

effects of simulated clay gouges on the sliding behavior of tennessee sandston abstract the effects of simulated fault gouge on the sliding behavior of tennessee sandstone are studied experimentally with special reference to the stabilizing effect of clay minerals mixed into the gouge about 30 specimens with gouge composed of pure clays of homogeneously mixed clay and anhydrite or of layered clay and anhydrite along a 35 precut are deformed dry in a triaxial apparatus at a confining pressure of 100 mpa with a shortening rate of about 5 10 4 sec and at room temperature pure clay gouges exhibit only stable sliding and the ultimate frictional strength is very low for bentonite mont morillonite intermediate for chlorite and illite and considerably higher for kaolinite anhydrite gouge shows violent stick slip at 100 mpa confining pressure when this mineral is mixed homogeneously with clays the frictional coefficient of the mixed gouge determined at its ultimate frictional strength decreases monotonically with an increase in the clay content the sliding mode changes from stick slip to stable sliding when the frictional coefficient of the mixed clay anhydrite gouge is lowered down below 90 95 of the coefficient of anhydrite gouge the stabilizing effect of clay in mixed gouge is closely related to the ultimate frictional strength of pure clays that is the effect is conspicuous only for a mineral with low frictional strength only 15 20 of bentonite suppresses the violent stick slip of anhydrite gouge in contrast violent stick slip occurs even if the gouge contains as much as 75 of kaolinite the behavior of illite and chlorite is intermediate between that of kaolinite and bentonite bentonite anhydrite two layer gouge exhibits stable sliding even when the bentonite content is only 5 thus the presence of a thin clay rich layer in a fault zone stabilizes the behavior much more effectively than do the clay minerals mixed homogeneously with the gouge this result brings out the mechanical significance of internal structures of a fault zone in understanding the effects of intrafault materials on the fault motion based on the present experimental results incorporated with some other experimental data it is argued that although the stabilizing effect of montmorillonite and vermiculite is indeed remarkable at room temperature the effect should be much less pronounced at elevated temperatures due perhaps to the dewatering of the clays in most geological environments where shallow earthquakes occur the stabilizing effect of clays is probably not so conspicuous as to completely suppress the unstable motion of a fault

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

consolidation patterns during initiation and evolution of a plate boundary decollement zone northern barbados accretionary prism borehole logs from the northern barbados accretionary prism show that the plate boundary decollement initiates in a low density radiolarian claystone with continued thrusting the decollement zone consolidates but in a patchy manner the logs calibrate a three dimensional seismic reflection image of the decollement zone and indicate which portions are of low density and enriched in fluid and which portions have consolidated the seismic image demonstrates that an underconsolidated patch of the decollement zone connects to a fluid rich conduit extending down the decollement surface fluid migration up this conduit probably supports the open pore structure in the underconsolidated patch

barbados ridge hydrogeologic tests implications for fluid migration along an active decollement hydrogeologic tests were conducted at a sealed borehole penetrating the decollement of the barbados ridge accretionary complex at low excess pore pressures p p p h p l p h 0 0 to 0 36 where p p pore pressure p h hydrostatic pressure and p l lithostatic pressure estimated permeabilities were comparable to those of similar unfractured sediment these tests complement shipboard packer tests completed at higher fluid pressures 0 5 to 1 0 during ocean drilling program odp leg 156 together the test results suggest a 4 to 5 order of magnitude permeability increase as fluid pressure varied from hydrostatic 0 to lithostatic 1 however unlike the results of the shipboard packer tests the test results presented here exhibit no evidence of a relationship between permeability and pore pressure the combined findings from the two sets of hydrogeologic tests indicate that significant permeability increases can occur within the decollement at pore pressures below lithostatic pressure

fluid history related to the early eocene middle miocene convergent system of the northern apennines italy constraints from structural and isotopic studies 1 the late eocene middle miocene erosive plate boundary between the european and the adriatic plates is exhumed in the northern apennines of italy the fossil fault zone is 500 m thick and the outcropping portion exposes the first 5 km of its depth at this plate boundary basal and frontal tectonic erosion incorporated unlithified fluid rich sediments into the fault zone the deformation and nature of the material along the plate boundary define a fossil subduction channel here we couple a detailed structural analysis of the apennine subduction channel focusing in particular on calcite veins with a stable isotope analysis to characterize the fluid regime along an active subduction channel the 13c and 18o composition of calcite vein and host rock samples within the fault zone indicates that there is a deep metamorphic source of fluids migrating upward along the subduction channel in addition to locally derived fluid components dewatering of subducting turbidites contributes significantly only in the shallowest part of the channel structural observations indicate fluid flow along and across the subduction channel at deep levels fluid flow is associated with discrete deformation events on shear faults offset by dilational jogs filled with implosion breccias at intermediate levels deformation is still cyclic and associated with repeated crack and seal events at the shallowest levels deformation occurred while portions of the subducting material were still unlithified here the deformation was quasicontinuous without associated vein development both isotope and structural analyses indicate that this erosive subduction channel behaved as a weak fault with a vertical maximum principal stress

importance of mechanochemical effects on fault slip behavior during earthquakes 1 recent analyses of natural faults and laboratory rock friction experiments have revealed that physical transformations and chemical reactions of minerals and fluids occur in faults during earthquakes however exactly how these physicochemical processes affect fault weakening and earthquake energetics is not well understood we present quantitative experimental evidence of mechanochemical changes in a clay mineral illite during frictional slip and evaluate the effect of those changes on slip behavior friction distorts the crystal structure of illite and dramatically decreases the activation energy ea of the dehydroxylation reaction numerical modeling shows that the lower ea allows the dehydroxylation to occur at relatively low temperatures 200 300 c which in turn drastically reduces fault strength by pressurization of released water from the mineral thus dynamic thermo chemo mechanical processes can strongly affect earthquake instability especially along clay rich faults such as those in the shallow part of subduction zone plate boundaries

deformation and fluid pressure variation during initiation and evolution of the plate boundary decollement zone in the nankai accretionary prism 1 the plate boundary decollement zone in the muroto region of the nankai accretionary prism records deformation and consolidation histories that have been affected by temporal changes in fluid pressure microstructural observations and chemical analysis demonstrate that the decollement zone initiated in an interval of porous clayey sediments characterized by cementation due to intergranular bonding of authigenic clays crosscutting relations of microstructures indicate that the decollement zone records two compactive deformations the early compactive deformation involved destruction of porous cemented structure probably caused by fluid pressure fluctuation the late compactive deformation was characterized by clay particle rotation and porosity collapse along the sets of slip surfaces resulting in zones of preferred orientation of clay particles these compactive deformations led to significantly higher bulk densities within the decollement zone compared to the compaction trend of the overlying prism sediments elevated fluid pressure following compactive deformations induced an overconsolidated state within the decollement zone with fluid filled dilatant fractures bulk density abruptly decreases at the top of the underthrust sediments but there is no microstructural evidence for cementation fluids in the dilated fractures and underconsolidated underthrust sediments are potential sources for the elevated fluid pressure in and below the decollement zone resulting in mechanical decoupling of the accretionary prism from underthrust sediments the fault fluid interactions in the muroto region may be applicable to other convergent plate margins where high temperature associated with the subduction of a spreading ridge or hot young oceanic crust enhances diagenesis and cementation

behavior of the decollement at the toe of the middle america trench seismic reflection data from three areas of the middle america trench provide insights into the behavior of the decollement that separates subducted and offscraped sediment the range of responses observed along this single subduction system provides clues as to how the decollement forms and is influenced by local conditions the location of the decollement and whether or not the subducting basement topography influences the decollement are apparently controlled by the type and thickness of sediment in the trench where subducting basement topography and oceanic plate hemipelagic sediments are buried by sandy terrigenous turbidites such as in the trench axis off mexico the decollement is localized near the base of the turbidite section subducting normal fault blocks in the oceanic crust control thrust ramps and hanging wall anticlines form above the ramps

hydrogeological modeling of porous flow in the oregon accretionary prism venting of pore fluid was recently reported near the front of the oregon accretionary prism and was directly measured at one of the venting sites during alvin dives in this study the authors model the flow of pore fluid in the oregon accretionaly prism they use seismic reflection sections to constrain subsurface structures drill hole results from deep sea drilling project leg 18 to constrain sediment properties and regional tectonic information to constrain the timing and the rate of plate convergence and sediment imbrication and thickening the results of two dimensional time element finite element modeling show that high pore pressures are generated beneath the decollement due to the load of imbricated and thickened sediments diffusive flow at a rate of several millimetres per year is predicted within the accretionary prism if fault zone permeability is discounted the presence of a decollement or permeable fault penetrating the accretionary prism as imaged on the seismic reflection section can drastically alter the patterns of fluid flow and produce focused venting of fluid as observed on the other hand fluid venting along faults may not significantly alter the high pore pressures below the decollement

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

abundance of smectite and the location of a plate boundary fault barbados accretionary prism isolating the respective roles of factors responsible for the initiation and localization of fault zones remains one of the more important goals of research in neotectonics the frontal decollement zone of the northern barbados accretionary prism provides several important clues as to how the interwoven variables of clay mineralogy fluid flow chemical interactions and sediment physical properties affect strain localization this plate boundary fault is centered at a lithologic contact between miocene claystone and fine grained oligocene turbidites the fault zone is nearly 40 m thick at ocean drilling program site 948 and its upper part passes through smectite rich deposits a sharp minimum in percent smectite and a maximum in percent illite mark the base of the decollement there is a consistent increase in percent smectite with distance above the base of the decollement but the top of the decollement is poorly defined by clay mineralogy the intrinsic mechanical weakness of strata with abundant smectite group clays probably influences where the fault tip propagates into the undeformed stratigraphy of the atlantic abyssal plain a second inherited parameter is the local abundance of radiolarians which contribute to higher than normal porosities sediment shear strength also decreases because pore pressure within the fault zone is significantly greater than hydrostatic the principal cause of excess pore pressure seems to be updip fluid advection in theory however decreases in pore fluid salinity and porosity collapse should increase the amount of physicochemical stress generated by expandable clay minerals the imported fluid is unusually low in salinity because it has migrated from zones of deeper seated dehydration reactions if fresher pore water migrates to the propagating tip of the decollement its arrival should increase smectite swelling and reduce the shear strength of the mudstone even more the location and evolution of the decollement therefore are controlled by a complicated interplay of static factors inherited from the abyssal atlantic stratigraphy and dynamic factors associated with episodic fluid flow and changing fluid chemistry

heterogeneous hydrofracture development and accretionary fault dynamics ocean drilling program observations at many accretionary wedges indicate that fluid flow in low permeability sediments is focused along faults rich in clay minerals recent ring shear experiments show that shear failure generally reduces permeability in muddy faults to values well below the surrounding sediments and that very little increase occurs in the fault permeability even when continued failure proceeds under steadily reduced effective loads it is thus unlikely that observed focused flow occurs through the matrix of muddy faults and we suggest that focused flow can only be explained by the development of open fracture systems the lithostatic fluid pressures necessary for hydrofracturing in weak sediments cannot however exist throughout the imbricate thrust system and basal decollement zone because a zero basal shear stress condition would result in regional instability and accretionary wedge collapse we propose that the regional hydrofracture networks must propagate heterogeneously leaving relatively strong asperity regions at sublithostatic fluid pressures to balance forces in the wedge in three dimensions

friction experiments on saturated sediments and their implications for the stress state of the nankai and barbados subduction thrusts the nankai and barbados forearcs have low stress subduction thrusts the sediments entering the subduction zone and namely the material in the decollement zones have been well characterized by numerous deep sea drilling legs and studies of the recovered cores nankai has high heat flow and significant amounts of illite while barbados is a smectite dominated system based on results from ring shear 2 mpa normal stress and direct shear 30 mpa tests on marine sediments and mineral standards this translates into a residual frictional resistance of r 0 25 and r 0 11 in clay horizons respectively such values agree with theoretical estimates from critical wedge theory nankai b 0 16 0 26 and barbados b 0 06 0 09 and fault spacing geometries from seismic profiles nankai b 0 12 0 23 and barbados b 0 11 0 19 maximum pore pressure ratios of 0 85 and 0 73 for nankai and barbados respectively allow us to estimate effective shear stresses as a function of friction coefficient and density of the sediment gouge to reach only 10 mpa or less in the frontal 50 km of the decollement zone respectively our data support the contention that fluid pressure transients and sediment composition contribute equally to the weakness along plate boundary faults down to the seismogenic zone with the first probably dominating the shallow decollement shear velocity stepping tests show that the clay dominated gouges strengthen velocity irrespective of the clay mineralogy and hence suggest that clay transformation does not affect the onset of seismogenesis

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

seismically inferred dilatancy distribution northern barbados ridge decollement implications for fluid migration and fault strength a 5 x 25 km three dimensional seismic survey of the lower part of the northern barbados ridge accretionary prism creates a three dimensional image of a major active decollement fault the fault is usually a compound negative polarity reflection modeled as a low velocity high porosity zone less than 14 m thick this thickness is significantly less than that defined by drilling of a 40 m zone of deformation at ocean drilling program odp site 671b located within the surveyed area we infer that the seismically defined fault is a thin high porosity zone and is thus an undercompacted high fluid pressure dilatant section if these inferences are correct then map view variations in seismic reflection waveform and amplitude illustrate complex patterns of fault zone fluid content and fluid migration paths the amplitude map suggests kilometre wide channels of locally high porosity and thus focused fluid flow these paths are only subparallel to the expected minimum head as inferred from the shape of the overlying sediment wedge other factors must modify fluid concentrations and ultimately migration several areas of positive polarity fault reflections define square kilometre sized regions inferred to be lower porosity sections producing strong asperities in an otherwise weak fault one coincident with site 671b may explain the success of drilling through the fault here all other holes drilled in the area were within the negative polarity regions and were unsuccessful in penetrating through the entire fault zone possibly because of instability associated with high fluid pressures and a weak fault odp leg 156 planned for 1994 will test inferences related to fault permeability and fluid pressures

geochemical and geothermal evidence for fluid migration in the barbados accretionary prism odp leg 110 measured geochemical and geothermal effects of fluid migration in the northern barbados accretionary prism indicate that 1 fluid flows laterally along low angle active faults 2 the entire length of the leg 110 transect is a fluid discharge zone 3 migration results in heating of sediments across the transect but most strongly at the deformation front and 4 migration results in dilution of pore water cl and sup 18 o enrichment more in the arcward than the seaward side of the prism suggesting that the cl poor sup 18 o rich fluids that intrude the prism are derived from smectite illite reactions occurring arcward these conclusions have strengthened those derived from the initial appraisal of shipboard data modified ideas about where fluids are heated most and where the geochemistry is strongly altered and provided new ideas about the sources of fluids and their relative contributions

generation of high pore pressures in accretionary prisms inferences from the barbados subduction complex recent geological and geophysical exploration of several accretionary complexes reveals that pore pressures inside them are much higher than the hydrostatic values indirect inferences further suggest that such high pore pressures may be present inside most accretionary complexes in this study the mechanisms for generating excess pore pressures is investigated by focusing on the barbados ridge complex which is an end member of sediment rich accretionary prisms modeling is carried out using a two dimensional finite element procedure with constraints provided by the following data deep sea drilling project results on stratigraphy and lithology seismic results on sediment structure and thickness regional geology on tectonic history and laboratory measurements on mechanical properties of sediments coupling among the equations for fluid flow heat transfer and sediment deformation is made throughout the computation the results of the modeling predict that the principal decollement beneath the barbados ridge complex is a zone of high pore pressure high porosity and reversal in seismic velocity these characteristics may be responsible for the mechanical weakness of the decollement which allows continuous subduction of sediments below it and for its seismic signatures results of the modeling further suggest that tectonic overburden may be the most important mechanism for the generation of high pore pressures inside accretionary prisms the direct effect of tectonic compression is of secondary importance and the effect due to temperature increase with subduction is of little significance thrust faulting near the toe of accretionary prisms can disturb pore pressure patterns and produce localized highs rich sediment supply low permeability high subduction angle and high subduction velocity are all favorable factors in creating high pore pressures inside accretionary prisms

comparison of frictional strength and velocity dependence between fault zones in the nankai accretionary complex accretionary complexes host a variety of fault zones that accommodate plate convergence and internal prism deformation including the decollement imbricate thrusts and out of sequence thrusts or splays these faults especially the decollement and major splay faults are considered to be candidates for hosting slow slip events and large magnitude earthquakes but it is not clear what modes of slip should be expected at shallow levels or how they are related to fault rock frictional properties we conducted laboratory experiments to measure the frictional properties of fault and wall rock from three distinct fault zone systems sampled during integrated ocean drilling program expedition 316 and ocean drilling program leg 190 to the nankai trough offshore japan these are 1 a major out of sequence thrust fault termed the megasplay site c0004 2 the frontal thrust zone a region of diffuse thrust faulting near the trench site c0007 and 3 the decollement zone sampled 2 km from the trench site 1174 at 25 mpa effective normal stress at slip rates of 0 03 100 m s and in the presence of brine as a pore fluid we observe low friction 0 46 for all of the materials we tested however the weakest samples 0 30 are from the decollement zone material from the megasplay fault is significantly weaker than the surrounding wall rocks a pattern not observed in the frontal thrust and decollement all samples exhibit primarily velocity strengthening frictional behavior suggesting that earthquakes should not nucleate at these depths a consistent minimum in the friction rate parameter a b at sliding velocities of 1 3 m s 0 1 0 3 m d is observed at all three sites suggesting that these shallow fault zones may be likely to host slow slip events

pore pressure generation in sedimentary basins overloading versus aquathermal the principal mechanism for abnormally high pore pressure generation has been an important matter of debate for more than a decade among the various mechanisms proposed mechanical overloading and aquathermal pressuring have probably received the greatest attention most previous discussions however have been based upon qualitative conceptual models here we propose a quantitative analysis by first examining the basic principles of porous flow and the fundamental properties of sediments and second by comparing the relative importance of the overloading versus the aquathermal mechanisms the coupling between pore pressure and compaction as well as the nonlinear and path dependent material properties are found to be critical in the present analysis we find that although changes in stress and temperature both contribute to pressure generation mechanical overloading is the main mechanism under normal geological conditions and is capable of producing the observed pore pressure profiles

transient fluid flow through the toe of the barbados accretionary complex constraints from ocean drilling program leg 110 heat row studies and simple models thirty four sediment and mud line temperatures were collected from six drill holes on ocean drilling program odp leg 110 near the toe of the barbados accretionary complex when combined with thermal conductivity measurements from sediment cores and results from earlier surveys these data delineate the complicated thermal structure on the edge of this convergent margin heat flow values at the seafloor of 92 192 mw m sup 2 are 80 300 higher than those predicted by standard heat flow versus age models for oceanic crust but are compatible with earlier seafloor measurements made in this area at the same latitude heat flow tends to decrease downhole at four sites suggesting the presence of heat sources within the sediments these results are consistent with the flow of warm fluids through the complex along high permeability conduits including thrust faults the major decollement zone and sandy intervals simple calculations suggest that this fluid flow is transient occurring on time scales of tens of thousands of years fluid flow velocities along the decollement zone estimated with a simple thermal model are about 10 sup minus 7 m s 100 times faster than predicted by numerical steady state analyses the estimated maximum sediment permeability within the decollement zone which was basedmore on the above fluid flow velocity is about 10 sup minus 12 m sup 2 also 100 times higher that that calculated numerically high heat flow in the vicinity of 15 degree 30 prime n and not elsewhere along the deformation front suggests that the leg 110 drill sites may be situated over a prism water discharge zone with dewatering more active here than elsewhere along the accretionary complex less

meaning measurement and field application of swelling pressure of clay shales the meaning and laboratory measurement of swelling pressure are examined by swelling and swelling pressure tests on undisturbed specimens of taylor shale from san antonio texas swelling pressure response of a shale element is defined in terms of a void ratio against swelling pressure relationship rather than a single value of swelling pressure and is interpreted by means of constant volume free swell and controlled load or controlled deformation unloading tests a concept of unsaturated fissured shale is introduced and is used to explain the magnitude and time rate of swelling pressures measured in laboratory tests and mobilized in the field the magnitude of swelling pressure that a shale surface exposed in an excavation mobilizes against a stiff support is expressed in terms of an expansion of the shale mass adjacent to the excavation the expansion of shale is mainly through the opening of joints and fissures during excavation and before installation of the lining la signification et la mesure

a simplified analysis of parameters controlling dewatering in accretionary prisms abstract one of the many dynamic geologic processes taking place at subduction zones is the compaction driven dewatering of seafloor sediments as they are accreted to the overriding plate the rate of dewatering is equal to the divergence of the sediment matrix velocity field this rate can be estimated analytically if simplifying assumptions are made about the geometry of the prism and motion of the sediments the analytical expression depends only on the sediment accretion velocity thickness of the accreted section prism taper angle and sediment porosity distribution a sensitivity analysis of the solution shows that the fluid production distribution is relatively insensitive to the sediment porosity distribution however the solution is very sensitive to the taper angle of the wedge high angle wedges expel almost all of the incoming water within 20 km while low angle wedges may retain a significant fraction of the incoming water for 50 or more kilometers thickness of the incoming section is also important dewatering of thin accreted sections is more concentrated near the toe of the wedge analyses of transects through the northern barbados makran vancouver and nankai accretionary prisms illustrate a range of dewatering rates and spatial distributions of dewatering

implications for mechanical properties of brittle faults from observations of the punchbowl fault zone california field observations of the punchbowl fault zone an inactive trace of the san andreas are integrated with results from experimental deformation of naturally deformed punchbowl fault rocks for a qualitative description of the mechanical properties of the fault and additional information for conceptual models of crustal faulting the punchbowl fault zone consists of a single continuous gouge layer bounded by zones of extensively damaged host rock fault displacements were not only localized to the gouge layer but also to discrete shear surfaces within the gouge deformation in the exposure studied probably occurred at depths of 2 to 4 km and was dominated by cataclastic mechanisms textural data also suggest that significant amounts of pore fluids were present during faulting and that fluid assisted mechanisms such as dissolution diffusion and precipitation were operative the experimental data on specimens collected from the fault zone suggest that there is a gradual decrease in strength and elastic modulus and an increase in relative ductility and permeability toward the main gouge zone the gouge layer has fairly uniform mechanical properites and it has significantly lower strength elastic modulus and permeability than both the damaged and the undeformed host rock for the punchbowl fault and possibly other brittle faults the variations in loading of the gouge zone with time are primarily governed by the morphology of the fault and the mechanical properties of the damaged host rock in addition the damaged zone acts as the permeable unit of the fault zone and surrounding rock it appears that the gouge primarily governs whether displacements are localized and it therefore may have a significant influence on the mode of slip

strain decoupling across the decollement of the barbados accretionary prism the interrelation between deformation styles and behavior of fluids in accretionary prisms is under debate particularly the possibility that overpressuring within the basal decollement may enable mechanical decoupling of the prism from the subducting material anisotropy of magnetic susceptibility ams data from sediments spanning the basal decollement of the barbados accretionary prism show a striking progression across this structure that strongly supports the hypothesis that it is markedly overpressured in the accretionary prism above the decollement the minimum ams axes are subhorizontal and oriented nearly east west whereas the maximum ams axes are oriented nearly north south and shallowly inclined at the top of the decollement the minimum ams axes orientations abruptly change to nearly vertical this orientation is maintained throughout the decollement and in the underthrust sediments below the ams orientations in the prism sediments above the decollement are consistent with lateral shortening due to regional tectonic stress as the minimum axes generally parallel the convergence vector of the subducting south american plate and the maximum axes are trench parallel because the orientations of the ams axes in deformed sediments usually parallel the orientations of the principal strains the ams results indicate that the incremental strain state in the barbados prism is one dominated by subhorizontal shortening in contrast the ams axes within and below the decollement are consistent with a strain state dominated by vertical shortening compaction this abrupt change in ams orientations at the top of the decollement at site 948 is a direct manifestation of mechanical decoupling of the off scraped prism sediments from the underthrust sediments the decoupling horizon occurs at the top of the decollement zone coinciding with the location of flowing high pressure fluids

seismic modeling of the decollement zone at the base of the barbados ridge accretionary complex wide aperture seismic reflection profiling of the northern barbados ridge accretionary complex at 16 12 n has imaged the decollement horizon across more than 100 km of the accretionary complex revealing prominent changes in the amplitude of the decollement reflection synthetic seismograms from models of the variation of velocity and density with depth were compared with selected seismic traces between 2 5 km seaward of the toe of the accretionary complex and 20 km landward of the toe to determine the changes in velocity density properties along the decollement thrust zone modeling requires a thin low velocity layer to develop landward along the decollement beneath the complex while no such layer exists at the corresponding stratigraphic boundary in the undeformed sediments seaward of the complex two dimensional modeling that incorporated lateral variations in topography velocity and density was performed on a 7 km section of the decollement 20 km landward of the deformation front the best fitting model includes a 20 m thick low velocity layer at the decollement horizon with a relative velocity decrease in the layer of 0 2 km s changes in the reflection amplitude along this 7 km section could be produced by either a change in the velocity anomaly within the layer or a thinning of the decollement layer to less than 4 m the shape of the reflected wavelet from the decollement indicates that the principal cause of the amplitude variation is a change in the velocity contrast rather than a change in thickness the low velocity layer can be explained by a decollement shear zone that has high porosity maintained by elevated pore fluid pressure high fracture permeability within the decollement permits dewatering of sediment attached to the oceanic crust and thrust beneath the accretionary complex

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

quantitative modeling of fault fluid discharge and fault dilation induced fluid pressure variations in the seismogenic zone the rise of fluid pressure along a fault plane can trigger an earthquake as shear strength decreases recent models have thus suggested that the rate of interseismic fluid pressure increase may control earthquake recurrence in electric analog model simulations incorporating new experimental data on rock properties within the lower seismogenic zone resultant fluid pressure fluctuations are restricted to the fault plane and are strongly attenuated during their slow propagation into the country rock fluid pressure drops triggered by coseismic fault dilation require days to hundreds of years to propagate over distances 1 m into low permeability country rock 10 17 m2 the equilibration of fluid pressure between the faulted and the intact country rock requires tens to hundreds of years if the fault seals prior to this equilibration the fluid pressure recovers instantaneously to near the prefailure value these results may imply that if elevated fluid pressure weakens transcrustal fault zones they are likely to remain weak after earthquakes

water and argon permeability of phyllosilicate powders under medium to high pressure 1 phyllosilicates are abundant throughout the upper crust they are commonly found in upper crustal faults influence of the mineralogy on the low permeability of phyllosilicate rich rocks is poorly characterized we measured water and argon permeability of eight phyllosilicate powders at room temperature talc pyrophyllite kaolinite illite montmorillonite particle size 2 m and muscovite phlogopite and biotite particle size 20 m tests were performed under hydrostatic conditions with a constant pore pressure of 10 mpa water or argon and varying confining pressure up to 160 mpa results show phyllosilicate powders have very low permeabilities 10 19 m2 to 10 24 m2 depending on mineral pore fluid and effective pressure lowest permeabilities were found for montmorillonite and talc while muscovite was the most permeable permeability decreases with increased compaction of powders and for fully compacted powders after 10 pressure cycles pressure dependency of permeability follows an exponential relationship with a pressure sensitive gradient of 0 004 mpa 1 to 0 015 mpa 1 water permeability is always lower up to 1 8 orders of magnitude than for argon the difference in argon water permeability can be related to the hydrophobicity and hydrogen bonding surface properties of minerals tested talc the most hydrophobic mineral tested showed the smallest difference between argon and water permeability while montmorillonite showed the largest permeability might also be affected by packing controlled by frictional strength of particles as well as by particle size and shape results indicate that type of phyllosilicate mineral pore water chemistry and grain size distribution will strongly affect permeability in phyllosilicate rich regions of the upper crust such as faults

depth dependence of the frictional behavior of montmorillonite fault gouge implications for seismicity along a decollement zone to understand the seismogenic potential of shallow plate boundary thrust faults decollements in relatively warm subduction zones water saturated na montmorillonite gouges were sheared at a pore fluid pressure of 10 mpa effective normal stresses neff of 10 70 mpa temperatures t of 25 150 c and axial displacement rates of 0 03 3 m s the na montmorillonite gouges were frictionally very weak at all conditions tested steady state friction coefficient ss 0 05 0 09 at t 60 c na montmorillonite showed a transition from velocity strengthening to velocity weakening behavior with increasing neff whereas at t 90 c it was largely velocity neutral or velocity strengthening irrespective of neff the rates of frictional healing showed extremely low values mostly 0 001 at all temperatures our results suggest that the existence of na montmorillonite in the decollement zone at costa rica and nankai promotes aseismic slip particularly at shallow depths forming weakly coupled regions

elevated fluid pressure and fault zone dilation inferred from seismic models of the northern barbados ridge decollement in 1992 a large volume of three dimensional seismic reflection data were acquired in a 5 25 km area across the toe of the barbados accretionary complex that covers the deep sea drilling project leg 78a and ocean drilling program legs 110 and 156 drilling sites these data are used to examine the acoustic character of the decollement seismic reflection and to qualitatively and quantitatively characterize fluid pressures within the fault zone seismic models have been constructed across a 6 km region of the decollement where it has been mapped as a moderate to bright polarity reversed reflection the models show that this segment of the decollement reflection is caused by a low velocity interval usually 12 16 m thick the top of the low velocity interval appears to be a sharp boundary that requires a decrease in velocity from 1 8 km s to between 1 7 and 1 65 km s with some localized bright reflections with an even lower velocity of 1 6 km s the base of the low velocity layer is less certain from modeling the base consists of either a velocity increase that is usually approximately half the velocity contrast at the top of the layer or the velocity increase is equal to the contrast at the top of the layer but distributed over a 10 m thick interval comparison of these results to laboratory experiments on the relationship between fluid pressure and seismic velocity indicates that in this interval of the decollement fluid pressure is at or near lithostatic furthermore the reflection coefficients of the decollement are sufficiently large that some dilation of the fault zone is required the dilation should lead to high fracture zone permeability and explain the observation of a laterally consistent decollement reflection along a 5 km segment of the decollement it is within these segments of the fault that fluid pressure approaches lithostatic and significantly reduces fault strength