landward dipping reflectors in accretionary wedges active dewatering conduits landward dipping seismic reflectors ldrs within the accretionary wedge at convergent plate margins have generally been interpreted as tilted bedding or thrust fault surfaces e g mexico northern japan this interpretation accords with models in which trench axis sediments are scraped off the descending plate to form a series of imbricate thrust sheets however intensely disturbed melanges with a regionally extensive landward dipping foliation are found beneath slope sediments in many ancient and active subduction complexes this observation combined with the fact that large volumes of fluid are released during dewatering of subducting and previously accreted sediments leads to the proposal that some of the seismic reflectors are porous laterally extensive fractureways filled with dewatering fluids derived from deeper underthrust water rich sediments these fractureways trend subparallel to the regional foliation of the accreted melange wedge the contrast in acoustic impedance between the high porosity fluid filled fractureways and the melange wall rocks is enhanced whenever methane and or carbon dioxide exsolve from rising waters ancient ldrs may be recognized where mineralized veins trend subparallel to the regional foliation of melange belts generated during subduction accretion 42 references 2 figures

backthrusting in accretionary wedges backthrusting of the rear of the accretionary wedge arcward over the rocks of the forearc basin is a common aspect of accretionary tectonics an older assumption that backstops of accretionary wedges dip back toward the arc and that accreted material is emplaced below such backstops is contradicted by observations from many active forearcs that indicate 1 backthrusting is common 2 forearc basins are nearly ubiquitous associates of accretionary wedges and 3 forearc basement where imaged appears to diverge from the sedimentary package dipping under the wedge while the overlying sediments are often lifted up against it backthrusting may be favored where relief is high between the crest of the wedge and the surface of the forearc basin because the relief must be supported by shear stress along the backthrust we find ratios of lt lb averaging 2 64 0 8 where lt is the horizontal distance from the crest of the wedge to the front of the toe and lb is the distance from the crest of the wedge to the surface projection of the backthrust this ratio is predicted to increase with total convergence and with sedimentation because the toe undergoes higher deformation rates than the backthrust and sedimentation in the trench favors outbuilding of the toe whereas sedimentation in the forearc basin tends to cancel the gravitational component of the driving stress for the backthrust

the alternative interpretation of chronology of flood events in the mid warta river valley record of early holocene alluviation in the kolo basin central poland abstract the detailed research of organic deposits with remains of riparian forest dated to the allerod younger dryas transition undertaken at the ko min las site enables the discussion of the evolution of river system and flood events in the kolo basin central poland results of former studies showed the intense accumulation of silty sandy overbank deposits during the younger dryas results of recent research in the ko min las site including results of 14c dating of organic deposits indicate significant increase of flooding in the early holocene in consequence this interpretation indicates the existence of a meandering river pattern during allerod and younger dryas and the development of a multichannel river system in the early holocene in the kolo basin including the mid warta river

offscraping accretion of jurassic chert clastic complexes in the mino tamba belt central japan abstract detailed structural and biostratigraphical analysis of the jurassic inuyama sequence a coherent chert clastic complex in the mino tamba belt central japan clarifies the evolution of accretionary processes at shallow structural levels the inuyama sequence is characterized by a series of stacked thrust sheets each sheet consists of an early triassic to middle jurassic oceanic plate stratigraphy composed of four lithologic units which are in ascending order siliceous claystone ribbon chert siliceous mudstone and clastic rocks the structural features of the inuyama sequence demonstrate a four stage progressive deformation 1 a decollement was initiated within the siliceous mudstone when this sequence was just seaward of the deformation front clastic dikes and sills formed at the horizon just above the decollement at this time 2 the stratigraphic section above the decollement was imbricated by in sequence thrusting in the frontal part of the wedge this initial stage of thrusting and imbrication was followed by 3 the formation of duplex structures with fault related folds within the lower stratigraphic section as the decollement stepped down section to the lowest siliceous claystone interval finally 4 these thrust packages were overprinted by secondary prism thickening in the form of out of sequence thrust faulting

methane derived authigenic carbonates formed by subduction induced pore water expulsion along the oregon washington margin authigenic magnesian calcite dolomite and aragonite are precipitated in the uppermost terrigenous sediments of the washington oregon accretionary prism by subduction induced dewatering these distinctive carbonates are methane derived and occur at sites of concentrated pore water expulsion unique biologic communities that subsist at least indirectly on methane suess et al 1985 are also found at some of these sites the methane which is dominantly biogenic is carried to the uppermost sediments of the prism by fluids and is oxidized by sulfate reducers before being incorporated into a carbonate cement carbonate precipitation occurs below the oxic layer probably no deeper than several centimetres to a few metres below the seabed cementation may be induced by three factors 1 increased carbonate alkalinity resulting from microbial sulfate reduction 2 decreased co 2 solubility resulting from a pressure decrease when the pore water escapes the prism and or 3 the addition of ca 2 and mg 2 ions from sea water near the sediment water interface the convergent margin setting engenders precipitation of authigenic carbonates in several ways compressive stresses induce anomalously rapid compaction and dewatering rates and they may cause overpressuring in migrating pore water thereby delaying precipitation of carbonates until pressure is released near the sediment water interface structural deformation of the accretionary prism creates pathways such as fault zones secondary fracture porosity and dipping permeable layers often exposed by mass movement for efficient advection and expulsion of methane enriched pore water these characteristic conditions which lead to the precipitation of methane derived carbonates may be found at other convergent margins

fluid venting activity within the eastern nankai trough accretionary wedge a summary of the 1989 kaiko nankai results fluid flow manifestations within the outer 20 km of the eastern nankai accretionary wedge between the water depths of 4000 and 2000 m are concentrated near the outcrop of a shallow detachment fault immediately behind the frontal thrust near a depth of 3800 m and along a probable strike slip fault on the top of a 2000 m deep pop up structure 20 km inside the wedge using subbottom temperature measurements we estimate darcy upward flow velocities below clams colonies at the 3800 m site and show that one can associate a narrow range of velocities with the different types of colonies then combining a statistical survey of the distribution of colonies with the velocity estimates we compute a yearly outflow budget of 200 m 3 per metre of width of the wedge in the toe area this is forty times larger than the amount expected from steady state compactive dewatering we propose however that 80 90 of this flow is produced by shallow seawater convection because variations in sea bottom temperatures were too large we could not estimate fluid outflow budgets in the 2000 m venting area however 20 000 and 130 000 year old shells fossilized within widespread carbonate concretions indicate that fluid flow has been active in the 2000 m venting area since at least 130 000 years ago probably in a sporadic fashion chemical evidence indicates that a significant part of the fluid is coming from a depth of at least several hundred metres subbottom temperature measurements over a period of two months in the 3800 m venting site show fluctuations of 20 30 in the venting velocity below an active colony that have an apparent period of two weeks although the fluid circulation appears to be controlled by active faults at least in the shallow part of the wedge not every active fault corresponds to an aquifer in the absence of good high resolution seismic data and drill holes we cannot describe this plumbing system in any detail and are not able to show why some active faults correspond to fluid outlets and some do not

tubular structures of northern wairarapa new zealand as possible examples of ancient fluid expulsion in an accretionary prism evidence from field and petrographical observations

structural characteristics of shallowly buried accretionary prism rapidly uplifted neogene accreted sediments on the miura boso peninsula central japan 1 the upper miocene misaki and nishizaki formations on the miura and boso peninsulas in central japan preserve the deformation features of an off scraped accretionary prism the spatial distribution geometry and style of accretion related deformation with paleotemperature and burial depth estimation are elucidated in this study the deformation structures and textures are similar to those of modern accretionary prisms the low maximum paleotemperature 50 c and high preserved porosity of the sediments 30 50 imply a maximum burial depth of less than 1000 m on the basis of the mode of deformation this off scraped body is divided into an imbricate thrust a thrust unit and an upper coherent unit in ascending order the imbricate thrust corresponds to a branch from the basal decollement zone and is subdivided into a brecciated zone a main gouge zone and a shear band zone the thrust unit hosts a concentration of thrust systems that form various orders of duplex structures while the upper coherent unit is characterized by gravitational instability and earthquake induced deformation without thrust faulting the duplex distribution shear strain and fluid migration associated with the off scraping processes are clearly localized within the imbricate thrust and thrust unit this accretion related deformation occurred before lithification of the sediment and under high fluid pressure induced by shear deformation thickening of the sedimentary sequence and earthquake induced liquefaction these processes are inferred to control the effect on the mode of deformation and the location of thrusting during early deformation of the accretionary prism

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

thermal effects of metamorphic fluids in subduction zones in subduction zones large volumes of fluid are released from the subducting slab during prograde metamorphism of hydrated oceanic crust once released metamorphic fluids tend to migrate upward owing to buoyancy fluid migration can affect the thermal structure of a subduction zone by 1 advecting heat and 2 causing retrograde hydration carbonation reactions in the hanging wall under most p t conditions the volumetric heat capacities of h 2 o and rocks are similar therefore the thermal effects of fluid migration and uplift are comparable calculations suggest that advective heat transport by fluid flow causes relatively minor thermal effects pervasive upward fluid flow results in thermal effects analogous to regional uplift rates on the order of 0 2 mm yr fluid flow channelized along the subduction shear zone translates the local thermal structure of the shear zone updip at 1 mm yr a more important thermal effect results from exothermic retrograde reactions in the hanging wall that release 30 kj mol of h 2 o consumed retrograde reactions significantly retard cooling of the hanging wall just as prograde reactions retard warming of the subducting slab

role of the tectonic facies concept in orogenic analysis and its application to tethys in the eastern mediterranean region abstract the purpose of this paper is to advocate the use of a tectonic facies concept which aids systematic and objective analysis of orogenic belts the tectonic facies concept as discussed here focuses on the recognition of discrete tectonic settings mainly those exposed at high crustal levels in orogenic belts application of the tectonic facies concept can help field geologists to relate plate scale processes to outcrop geology tectonic facies are recognised based on knowledge of the modern oceans and margins and well constrained land settings tectonic facies analogous to sedimentary facies can be identified from integrated sets of field data and related laboratory studies a number of tectonic facies models can be recognised in this way their identification aids interpretation and comparisons of orogenic belts palinspastic reconstructions require additional information particularly age kinematic and palaeomagnetic data in this paper the tectonic facies concept is tested by application to a well documented mainly high level orogenic segment tethys in the eastern mediterranean area and is found to be an effective way both of the describing and interpreting this region other methods of tectonic analysis e g recognition of tectono stratigraphic terranes may also be found useful in certain settings and can be used in conjunction with the tectonic facies concept

fluid flow compartmentalization in the sicilian fold and thrust belt implications for the regional aqueous fluid flow and oil migration history abstract the fluid flow history in the frontal part of the sicilian fold and thrust belt ftb has been reconstructed using an integrated structural petrographic geochemical and microthermometric approach the study focused on comparing fluid flow during progressive deformation along major thrust horizons and in pelagic sediments occurring in the associated thrust sheets foot and hanging wall a fluid flow model is constructed for the frontal part of the sicilian ftb syn deformational quartz and calcite have been precipitated along decollement horizons in the iudica scalpello study area the microthermometric analysis of fluid inclusions in the quartz and calcite indicated migration of low saline high temperature aqueous fluids 1 5 h the fluid flow history in the thrust sheets can be subdivided into two stages calcite of types 1 and 2 has identical light orange cathodoluminescence as the surrounding mudstone furthermore its isotope signature 2 13 c 18 o m h between 80 and 140 c type 3 calcite has less depleted 18 o values compared to calcite of types 1 and 2 and the 18 o from calcite in faults is even positive during the final stages of fluid flow with precipitation of calcite type 3 the fluid flow model invokes infiltration of overpressured fluids that migrated along the decollement zone these fluids only infiltrate the thrust sheet in the hanging wall of the thrust leading to a compartmentalized fluid flow pattern an identical fluid flow pattern with migration of low and hot saline fluids associated with hydrocarbons was observed in other parts of the sicilian fold and thrust belt with identical structural deformation and fluid flow patterns the study confirms the suitability of the frontal part of the sicilian fold and thrust belt as an outcrop analog for deeply submerged accretionary prisms the study thus offers unique insights in expected fluid flow patterns in thrust sheets of deeply submerged accretionary complexes

structural style and kinematics of an underplated slate belt kodiak and adjacent islands alaska the kodiak formation composed of coherent maastrichtian turbidites is a slate belt whose dominant structures developed during underplating to an accretionary wedge in the latest cretaceous it consists of about 80 coherent landward dipping thrust packets zones of disrupted sandstone associated with a scaly argillite matrix constitute the remainder about half of these disrupted sandstone zones are related to pre accretion deformation and the rest formed along late stage strike slip faults that postdate development of slaty cleavage the formation is divided into three structural belts the landward and seaward belts include steeply dipping structures and the central belt contains shallowly dipping structures and rocks that have experienced the highest strain the central belt probably acted as a low angle southeast verging floor thrust zone beneath the landward belt the structural history of the kodiak formation includes 1 early soft sediment disruption 2 tectonic stratal disruption 3 thrust faulting slaty cleavage s 1 development and folding f 1 4 intrusion of granodioritic plutons dikes and sills and associated normal faulting 5 development of crenulations f 2 and crenulation cleavage s 2 6 thrust faulting and 7 development of right lateral strike slip faults event 3 produced the dominant northwest dipping structural grain and caused the greatest amount of shortening the timing of event 7 relative to events 5 and 6 is not certain the dominant structures of the kodiak formation are interpreted as developing during underthrusting underplating and intra wedge shortening during latest cretaceous time within an accretionary wedge the maximum time from deposition to emplacement was about 12 m y and the timing is constrained by the maximum depositional age 74 m y and the age of plutonic rocks 62 m y that crosscut the dominant fabric pre cleavage zones of stratal disruption reflect deformation on the lower plate during underthrusting slaty cleavage thrusts and f 1 folds developed during underplating that also resulted in the formation of duplexes crenulations are probably related to post underplating subhorizontal shortening of sediments within the accretionary wedge syn deformation dynamic recrystallization of quartz minimum syn accretion metamorphic temperatures between 205 and 250 c and pressures of at least 2 65 kb recorded during the earliest stages of deformation suggest underplating at 10 km

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

friction on subduction megathrust faults beyond the illite muscovite transition abstract previous experimental studies addressing subduction megathrust friction have focused on shallow fault gouges dominated by smectite illite and quartz here we aim to determine the effect of the transition to muscovite rich fault rock at depths beyond the illite dominated region of the seismogenic zone to achieve this rotary friction experiments were performed on simulated muscovite quartz 65 35 gouge at 100 600 c an effective normal stress of 170 mpa pore pressures of 100 and 200 mpa and sliding velocities of 1 100 m s we isolated the effects of muscovite by shearing pure muscovite gouge at 200 600 c under otherwise identical conditions at low shear strains 1 muscovite quartz gouge showed a friction coefficient of 0 4 0 5 while pure muscovite gouge exhibited 0 3 both gouges showed an increase in with and temperature the muscovite quartz gouge was characterised by velocity strengthening or neutral behaviour at 100 350 c velocity weakening at 350 500 c and velocity strengthening at 500 600 c in contrast pure muscovite gouge showed predominantly velocity strengthening or neutral behaviour demonstrating a key role of quartz in producing velocity weakening the velocity weakening regime in the mixed gouge extends that of 250 400 c recently reported for illite quartz gouge up to 500 c taking into account the observed effect of sliding velocity on a b the data for illite quartz and muscovite quartz together help explain the depth extent of seismogenesis on subduction megathrusts interestingly comparison with previous microphysical models suggests that the mechanism causing velocity weakening in phyllosilicate quartz gouges is one of granular flow involving competition between shear induced dilation and compaction through thermally activated quartz deformation

fluid overpressure along an oligocene out of sequence thrust in the shimanto belt sw japan abstract out of sequence thrusts osts exposed in ancient accretionary prisms are considered as fossil analogs of present day megasplay faults in subduction margins and can provide direct information about the conditions of deformation during thrust activity in modern as well as in ancient accretionary prisms first order megasplay faults or osts truncate or merge with faults of lesser importance called second order osts structural analysis of the makinokuchi fault a branch of an oligocene to lower miocene second order ost in the tertiary shimanto belt of central kyushu sw japan brings information about the conditions of deformation at the time of thrusting the studied exposure shows that the fault footwall and to a much lesser extent the fault hanging wall consist of quartz cemented syntectonic dilatant hydraulic breccias testifying to pore fluid pressures larger than the least principal stress component the footwall sandstones are crossed by several centimeters thick quartz veins that merge with the footwall breccias the continuity between the veins and the breccias suggest that the veins acted as conduits which likely collected fluids from the footwall side sandstones upward and toward the fault fluid inclusions indicate that the quartz cementing the breccias and that filling the feeder veins crystallized from similar fluids and under similar pressure and temperature conditions 245 285 c and 5 8 km depth these similarities suggest that the fluids responsible for syn tectonic hydraulic brecciation were collected from the footwall through the conduits the fluid inclusion trapping temperatures are close to the temperatures expected to be reached along the seismogenic zone our analysis shows that fluid overpressures can play a key role in the growth and activity of second order osts in accretionary prisms and suggests that fluids collected along second order osts or splay faults may flow upward along first order osts or megasplay faults

a test of alternative caribbean plate relative motion models two contrasting models have been proposed to describe the present day motions between the caribbean plate and neighboring plates one model based on both north america caribbean and cocos caribbean data assumes that north america caribbean motion is reflected by the spreading rate approximately 2 cm yr inferred from magnetic anomalies at the cayman spreading center and the azimuths of nearby transforms jordan 1975 this geometry was used by minster and jordan 1978 in deriving global plate motion model rm2 the other model based only on north america caribbean data uses rates and azimuths inferred from the geometry of the lesser antilles wadati benioff zone sykes et al 1982 the cayman spreading center data were discounted assuming that they underestimate the full north america caribbean motion owing to the complex tectonics of the greater antilles the latter model assumes plate convergence at the lesser antilles at a rate about twice as fast as and an azimuth differing by 25 from the jordan model the two models also differ in their predictions of south america caribbean motion the jordan model predicts oblique convergence whereas sykes et al predict oblique divergence we use the nuvel 1 global relative motion data set which incorporates recent data not used in earlier studies to discriminate between the alternative models we find that the jordan geometry provides a better fit to the direction of north america caribbean motion moreover we find that models based on this geometry better fit the direction of cocos caribbean motion observed in earthquake slip vectors along the middle america trench since the prediction of this direction depends strongly on the rate of north america caribbean motion we conclude that the jordan model better describes caribbean plate motions the discrepancy between the two models results from the different methods of estimating plate motions sykes et al 1982 report that they determined a caribbean north america convergence rate of approximately 4 cm yr from the length of the lesser antilles wadati benioff zone we employ a similar procedure and obtain a rate closer to 2 cm yr we also find that the method they report using to determine the convergence direction is unable to distinguish between the alternative azimuths we thus conclude that estimates of the rate and direction of plate motion from wadati benioff zone configurations are insufficiently robust and that such estimates are better derived using rates from magnetic anomalies and azimuths from transform faults and slip vectors we further conclude that successful estimates of relative motion require internal consistency between the euler vectors of all relevant plates as demonstrated by the powerful cocos caribbean constraint not incorporated in the sykes et al model

critical taper model of fold and thrust belts and accretionary wedges the fold and thrust belts and submarine accretionary wedges that lie along compressive plate boundaries are one of the best understood deformational features of the earth s upper crust although there is considerable natural variation among the many fold and thrust belts and accretionary wedges that have been recognized and explored several features appear to be universal in cross section fold and thrust belts and accretionary wedges occupy a wedge shaped deformed region overlying a basal detachment or decollement fault the rocks or sediments beneath this fault show very little deformation the decollement fault characteristically dips toward the interior of the mountain belt or in the case of a submarine wedge toward the island arc the topography in contrast slopes toward the toe or deformation front of the wedge deformation within the wedge is generally dominated by imbricate thrust faults verging toward the toe and related fault bend folding two north american fold and thrust belts that exhibit these features are shown in figure 1 neither of these two examples is tectonically active today the southern canadian fold and thrust belt was active during the late jurassic and cretaceous 150 100 ma whereas the southern appa lachians were deformed during the late carboniferous to permian alle ghenian orogeny 300 250 ma figure 2 shows two examples that are currently active the taiwan fold and thrust belt produced by the sub duction of the eurasian plate beneath the philippine sea plate suppe 1981 1987 and the barbados accretionary wedge produced by the sub

variation in deformation of the south panama accretionary prism response to oblique subduction and trench sediment variation migrated single channel seismic lines and sea marc ii side scan and bathymetry data document an active accretionary prism along the obliquely convergent margin of south panama a region previously believed to be a transform margin the eastern flank of the coiba ridge is being subducted in this region creating the requisite geometry for along strike variation in trench sediment thickness and type the regional east dip of the downgoing plate causes the depth of the oceanic crust along the trench to drop approximately 1800 m from west to east in the western region of the study area the incoming sedimentary section consists of approximately 600 m of pelagic and hemipelagic sediments a wedge of trench turbidites that overlies the incoming hemipelagic sediments thickens from less than 100 m in the western region to more than 900 m in the eastern region of the study area the eastward increase in sediment thickness correlates with the following changes in the accretionary prism 1 decrease in initial surface slope 2 broadening of the inner trench slope 3 increase in thrust spacing 4 steepening of frontal thrusts and 5 fold development each of the responses reflects in varying degrees the increase in volume and changing physical properties of the accreted material as well as the changing shear strength of the decollement the increase in overburden pressure and compaction in the deeper sediments together with the change in lithology from oceanic to trenchfill sediments should produce an eastward increase in sediment shear strength both within the wedge and along its base because the decollement will tend to form where the pore fluid pressure overburden ratio is at a maximum the shear strength within the wedge should increase relative to basal shear strength decreased surface slopes will result from the increasing contrast in shear strength between the wedge and its base

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

coupled fluid flow and deformation modeling of the frontal thrust region of the kumano basin transect japan implications for fluid pressures and decollement downstepping 1 results of coupled fluid flow and deformation modeling illustrate the influence of frontal thrust slip on excess pore pressures and decollement downstepping in the kumano basin transect offshore japan although permeable trench sediments and a buried sandstone layer provide potential fluid escape pathways the numerical modeling suggests that excess pore pressure could build beneath the frontal thrust within hemipelagic clay of the shikoku basin facies simulated excess pore pressures range between 45 and 77 of the overburden weight 3 km landward of the deformation front and are very sensitive to the permeability of the shikoku basin clay and the coefficient of friction of the fault zone excess pore pressures increase as the fault coefficient of friction is increased suggesting that volume change due to shear within the footwall sediments is an important contributor to excess pore pressures raising the coefficient of friction also increases shear within the sediments beneath the fault zone suggesting downward migration of the decollement zone this shearing focuses in the weaker shikoku basin clays rather than an underlying turbidite unit suggesting that lithologic differences play an important role in controlling deformation location

lithoprobe southern vancouver island cenozoic subduction complex imaged by deep seismic reflections reply the lithoprobe seismic reflection project on vancouver island was designed to study the large scale structure of several accreted terranes exposed on the island and to determine the geometry and structural characteristics of the subducting juan de fuca plate in this paper we interpret two lithoprobe profiles from southernmost vancouver island that were shot across three important terrane bounding faults leech river san juan and survey mountain to determine their subsurface geometry and relationship to deeper structures associated with modem subduction the structure beneath the island can be divided into an upper crustal region consisting of several accreted terranes and a deeper region that represents a landward extension of the modern offshore subduction complex in the upper region the survey mountain and leech river faults are imaged as northeast dipping thrusts that separate wrangellia a large mesozoic paleozoic terrane from two smaller accreted terranes the leech river schist mesozoic rock

tectonic melange as fault rock of subduction plate boundary abstract an assemblage of quantitative data sets is examined to evaluate tectonic melange as a plate boundary fault rocks in subduction zone the research object is the latest cretaceous mugi melange in the shimanto belt southwest japan systematic age younging from pelagic to terrigenous through hemipelagic sediments is well documented even though original stratigraphy is disrupted systematic shear fabric consistent with ancient plate convergence is reconstructed the melange was formed at temperatures of 130 200 c by cataclastic comminution of sandstone layers accompanied by tensile cracking and plastic deformation and the dehydration of clayey shale matrix with subsequent peeling off and underplating of the oceanic basement the temperature setting for the mugi melange indicates around the up dip limit of the seismogenic zone therefore includes various fault rocks suggestive of earthquake fault pseudotachylyte fluidized ultracataclasite with heating evidence amorphous silica and so on these suggest that fluid induced lubrication was dominated localized cataclastic shear which is a candidate of small earthquake or very low frequency earthquake is also recognized especially in sandstone blocks dominated portion in melange these observations are consistent with the melange being a fault rock along the plate boundary that records various types of earthquakes in a subduction zone the quantitative examination of the mugi melange suggests several criteria to define the tectonic melange of the plate boundary fault in subduction zone from other melanges in orogenic belt

deformation mechanism paths for oceanic rocks during subduction and accretion the mesozoic forearc of alexander island antarctica the lemay group of alexander island antarctica is a mesozoic accretionary prism that contains slivers of ocean floor and ocean island material accreted under a range of conditions and depths it provides a rare opportunity to compare the deformation mechanism paths between oceanic and trench fill lithologies during subduction and accretion by offscraping or underplating ocean floor slivers consist of a bedded basalt volcaniclastite chert rock association overlain by trench fill sedimentary rocks the oceanic lithologies initially deformed by cataclasis combined with particulate flow in the volcaniclastic rocks at shallow levels the clastic trench fill sedimentary rocks deformed by independent particulate flow which changed to cataclastic flow at depth at those depths crystal plasticity affected the cherts and jaspers at the deepest levels achieved in the subduction zone crystal plasticity was the dominant deformation mechanism in the clastic sedimentary rocks whilst cataclasis continued in the lavas deformation was synchronous with metamorphism up to transitional greenschist blueschist facies all lithologies were affected by pressure solution after accretion with the formation of a bedding parallel or sub parallel cleavage the deformation mechanism paths indicate that 1 there is a crystal plastic deformation field at deep levels in the subduction zone and 2 that the base of the pressure solution field is concordant with the decollement a frontally accreted ocean island is exposed in the lully foothills and consists of a basalt volcaniclastite tuff rock association deformation is represented only by a patchily distributed bedding sub parallel pressure solution cleavage in the tuffs and volcaniclastites and is probably related to loading metamorphism does not exceed prehnite pumpellyite facies the accreted ocean island indicates that such features can be incorporated into accretionary prisms at shallow levels without significant segmentation

large scale fluid infiltration metasomatism and re equilibration of archaean basement granulites during palaeoproterozoic thrust belt construction ungava orogen canada abstract the metamorphic history of the archaean superior province crystalline basement in the palaeoproterozoic ungava orogen attests to the importance of structural and geohydrological controls on a retrograde amphibolite granulite transition two distinct metamorphic suites separated in age by nearly one billion years are recognized in extensively exposed tonalitic to dioritic metaplutonic gneisses the older suite comprises c 2 7 ga granulite facies assemblages orthopyroxene clinopyroxene hornblende plagioclase ilmenite biotite quartz that record moderate pressures 5 kbar and high temperatures 800 c a younger c 1 8 ga suite resulted from amphibolitization of the granulites and is characterized by regionally extensive amphibolite facies mineral zones that broadly parallel the basal decollement of the overlying proterozoic cape smith thrust belt deformation mineral growth relationships in the amphibolitized basement indicate that extensive hydration and re equilibration of the archaean granulites occurred during thrust belt deformation the transition from granulite facies to amphibolite facies assemblages is characterized by the growth of garnet hornblende quartz cummingtonite coronas between plagioclase and orthopyroxene clinopyroxene as well as titanite coronas on ilmenite multi equilibrium thermobarometry on the coronitic assemblages documents re equilibration of the granulitic gneiss to 7 7 kbar at 644 c in the south and 9 8 kbar at 700 c in the north the variably deformed amphibolite facies domain sandwiched between the coronitic garnet zone and the basal decollement is marked by significant metasomatic changes in major element concentrations within tonalite these changes are compatible with equilibrium flow of an aqueous chloride fluid down a temperature gradient the source of fluids for basement hydration metasomatism is interpreted to be dehydrating clastic rocks in the overlying thrust belt with fluid flow probably focused along the basal decollement

structural evolution of underthrusted sediments kodiak islands alaska the late cretaceous to early tertiary sediments of the kodiak accretionary complex preserve the earliest deformation associated with underthrusting along a thickly sedimented convergent margin because of the thick trench fill this early deformation resulted in the formation of two distinct structural terranes 1 coherent terranes of layered turbidites and 2 melange terranes the earliest deformation d1 in the coherent terranes is marked by normal faults mineralized layer perpendicular veins in sandstones and microfaults in shales these features suggest that 1 was vertical and that 3 was horizontal and nw trending during d1 we interpret d1 structures as hydrofractures that formed in flat lying sediments with high fluid pressures caused by rapid tectonic loading in contrast the melanges appear to record a more protracted and complex history that culminated in structures similar to the d1 structures above we have recognized three successive transitional stages in the melanges i breakup of sand beds along cataclastic shear zones with dispersal of sand inclusions within a relatively ductile shale matrix resulting in the blocks in matrix character of the melange ii development of ductile shear bands that crosscut the melange fabric at a low angle and iii hydrofracturing with the formation of calcite and quartz filled veins in sandstone inclusions and microfaults in the shales the sequence of microstructures associated with stages i iii records increased cohesion of the sediments during deformation dominated by layer parallel shear intense mineralization and microfaulting during stage iii suggest that the melanges are zones of concentrated fluid flow with a fracture dominated permeability stage iii structures are texturally indistinguishable and contemporaneous with the less pervasive d1 structures in the coherent terranes based on these observations we believe that the melanges are major shear zones that formed at the top of the subducting pile and immediately below a major decollement the coherent terranes represent the structurally and stratigraphically lowest sediments and they bypassed the stage i and ii deformations of the melanges because they were the greatest distance from the decollement when they were deformed moreover both the melange and coherent terranes appear to have bypassed the shallow zone of intense horizontal shortening that dominates the toe of the overriding accretionary prism

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

tectonic regime of the southern kurile trench as revealed by multichannel seismic lines the analysis of the two multichannel seismic reflection records p 855 and p 856 reveals the complexity of the tectonic structure of the kurile margin the absence of trench fill and the relatively constant thickness of the subducted sediments on top of the oceanic basement shows that little or no accretion is occurring at present the lower slope is characterized by active thrusting within a small accretionary prism the toe of the middle slope is a more consolidated unit that constitutes an efficient buttress a piggy back basin possibly filled by sediments that have drifted down slope has formed on top of this unit a narrow high marks the transition to the edge of the continental terrace furthermore the upper slope reveals an unconformity marking the top of the basement the similarity of this unconformity and its sedimentary cover with the well documented data acquired around dsdp legs 56 57 suggests that northern japan and the southern kurile forearcs may have experienced a similar cenozoic vertical history 1 regional setting the southwestern part of the kurile arc in the vicinity of the so called hokkaido corner underwent a polyphase tectonic evolution during the tertiary kurile subduction is believed to have begun in the early jolivet 1986 or late kimura and tamaki 1985 cretaceous after the accretion of the okhotsk terrane to the continent of siberia both the nemuro peninsula and the is

nankai trough and zenisu ridge a deep sea submersible survey abstract eight submersible dives between 3000 and 4200 m water depth were made off southern japan in the eastern nankai subduction zone benthic communities associated with chemosynthetic processes were discovered along the 800 m wide active tectonic zone at the toe of the accretionary prism a benthic community was also discovered along a zone of active compression at the foot of zenisu ridge 30 km south of nankai trough temperature measurements within the sediments below the benthic communities confirm that upward motion of interstitial water occurs there studies of water samples indicate advection of methane and light hydrocarbons specimens of the benthic community have been shown to have included in their shells carbonate resulting from methane consumption thus the benthic communities are related to overpressure driven fluid advection along tectonic zones with active surface deformation a 300 m high active scarp at the toe of the accretionary prism is related to relative motion in a 280 direction which is close to the 305 average direction of subduction in this area the dives establish further that compressive deformation is presently occurring at the foot of zenisu ridge the previous interpretation of the zenisu ridge as a zone of recent north south intraplate shortening 40 km south of the nankai trench is confirmed we conclude that tectonic evolution might well lead to future detachment of the zenisu ridge and overthrusting of this large piece of oceanic crust over the continental margin such a process might be an efficient one to emplace ophiolites over continents

detection of fluid flow variations at the nankai trough by electric and magnetic measurements in boreholes or at the seafloor detection of changes in the flow rate of expelled fluids in accretionary prisms by monitoring of electric and magnetic fields is discussed a numerical model of the electric and magnetic fields associated with fluid flow variations at the nankai trough is presented which gives a numerical solution of the coupled system of electric convection currents and conduction currents that directly determines the magnetic anomaly itself measurements in a borehole located between two vents are shown to be well adapted to detection of fluid flow variations using the vertical gradient of the electric potential and the horizontal magnetic field the vertical electric field is about 10 mv km up to 500 m depth where there is a lithologic reflector and about 50 mv km below this reflector the horizontal gradient of the magnetic field is 2 nt km at the seafloor modelization with a lower fault conductivity and a larger decollement thickness has also been modeled the vertical gradient of the horizontal magnetic field is 5 to 15 nt km a variation of 3 mv and 1 5 to 3 nt at 600 m depth in a borehole could reveal a fluid flow rate variation of 20 which is a reasonable fluid flow change based on some observations at short scale time since a 1 5 to 3 nt anomaly seems easier to detect than a 3 mv anomaly it is likely that the variation of the magnetic field would more sensitively reveal fluid flow variations when monitoring the magnetic field at the seafloor a change of 0 4 nt km in the horizontal gradient could reveal a fluid flow rate variation of 20

deformation at the toe of an active accretionary prism synopsis of results from odp leg 131 nankai sw japan toes of accretionary prisms record the initial deformation of wet sediments during accretion at a convergent plate margin odp leg 131 succeeded in coring the toe of the nankai prism sw japan together with the basal decollement the underthrust sediments and the basaltic ocean basement an unprecedented structural geological inventory has been compiled gravitational deformation is important in the slope apron deposits but tectonic stresses too are transmitted to these levels core scale deformation bands due to heterogeneous bulk shortening of the prism are exceptionally well developed at nankai and range through kink bands shear zones and faults anomalous frequencies indicate localized high strain zones stress inversion analysis gives a principal compression close to the plate convergence vector the frontal thrust of the prism is a 26 m thick zone of breccia scaly fabric with a 309 m net slip it may well be currently active and accommodating almost half of the present convergence strain the basal decollement a 19 m thick breccia zone separates the deforming overconsolidated sediments of the prism from the virtually undeformed materials below these are highly underconsolidated and probably overpressured anomalous fluid pressures localize several of the prism structures unlike some other prisms drainage of the nankai prism is dominantly pervasive rather than controlled by major structures

the influence of an embedded viscoelastic plastic layer on kinematics and mass transport pattern within accretionary wedges abstract the major aim of this study was to examine the influence of an embedded viscoelastic plastic layer at different viscosity values on accretionary wedges at subduction zones to quantify the effects of the layer viscosity we analysed the wedge geometry accretion mode thrust systems and mass transport pattern therefore we developed a numerical 2d sandbox model utilising the discrete element method starting with a simple pure mohr coulomb sequence we added an embedded viscoelastic plastic layer within the brittle undeformed sediment package this layer followed burger s rheology which simulates the creep behaviour of natural rocks such as evaporites this layer got thrusted and folded during the subduction process the testing of different bulk viscosity values from 1 1013 to 1 1014 pa s revealed a certain range where an active detachment evolved within the viscoelastic plastic layer that decoupled the over and the underlying brittle strata this mid level detachment caused the evolution of a frontally accreted wedge above it and a long underthrusted and subsequently basally accreted sequence beneath it both sequences were characterised by specific mass transport patterns depending on the used viscosity value with decreasing bulk viscosities thrust systems above this weak mid level detachment became increasingly symmetrical and the particle uplift was reduced as would be expected for a salt controlled forearc in nature simultaneously antiformal stacking was favoured over hinterland dipping in the lower brittle layer and overturning of the uplifted material increased hence we validated that the viscosity of an embedded detachment strongly influences the whole wedge mechanics both the respective lower slope and the upper slope duplex shown by e g the mass transport pattern

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

structural anatomy of the early paleozoic laohushan ophiolite and subduction complex implications for accretionary tectonics of the proto tethyan north qilian orogenic belt northeastern tibet

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

the 3 6 ma aridity and westerlies history over midlatitude asia linked with global climatic cooling midlatitude asia mla strongly influenced by westerlies controlled climate is a key source of global atmospheric dust and plays a significant role in earth s climate system however it remains unclear how the westerlies mla aridity and dust flux from this region evolved over time here we report a unique high resolution eolian dust record covering the past 3 6 ma retrieved from the thickest loess borehole sequence 671 m recovered to date at the southern margin of the taklimakan desert in the mla interior the results show that eolian dust accumulation which is closely related to aridity and the westerlies indicates existence of a dry climate desert area and stable land surface promoting continuous loess deposition since at least 3 6 ma this region experienced long term stepwise drying at 2 7 1 1 and 0 5 ma coeval with a dominant periodicity shift from 41 ka cyclicity to 100 ka cyclicity between 1 1 ma and 0 5 ma these features match well with global ice volume variability both in the time and frequency domains including the mid pleistocene transition highlighting global cooling forced aridity and westerlies climate changes on these timescales numerical modeling demonstrates that global cooling can dry mla and intensify the westerlies which facilitates dust emission and transport providing an interpretive framework increased dust may have promoted positive feedbacks e g decreasing atmospheric co2 concentrations and modulating radiation budgets contributing to further cooling unraveling the long term evolution of mla aridity and westerlies climate is an indispensable component of the unfolding mystery of global climate change

tectonically driven fluid flow in the kodiak accretionary complex alaska sandstone and chert boudins that have syntectonic extensional veins are dominant structural elements in melanges of the kodiak accretionary complex alaska detailed analyses of cogenetic methane rich and water rich fluid inclusions in syntectonic quartz veins indicate that fluid pressures dropped by 20 to 45 during crystal growth this drop occurred faster than uplift of the rock and probably represents the local effect of fluid escaping to shallow levels in the subduction complex during melange deformation possibly along an interconnected fracture network an important implication of channelized fracture flow to higher levels in a subduction complex is that fluids may transport heat and solutes thereby generating thermal and chemical anomalies along shallow faults

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

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

mud diapirism and subcretion in the barbados ridge accretionary complex the role of fluids in accretionary processes mud diapirism has recently been recognized in several modern accretionary wedges it provides an important means of dewatering accretionary wedges and should be regarded as an important process for producing the melanges found in both modern and ancient accretionary terranes mud diapirism affects a large area of the barbados ridge accretionary complex the distribution of the mud diapirs appears to be primarily controlled by the presence of underconsolidated terrigenous submarine fan deposits that are being accreted to the complex the frequency of diapir occurrence decreases northward as the fan becomes thinner mud diapirs are absent from the very eastern most part of the complex formed from sediments accreted at its toe with the exception of a few mud volcanoes on the ocean floor in front of the complex the initiation of diapirism appears to be spatially coincident with the onset of subcretion or underplating of sediment to the base of the complex at a ramp between two levels of decollement it is proposed that the release of mud and pore water from the subcreted sediments is a direct or indirect cause of most of the mud diapirism in the accretionary complex there is a range of diapiric form dependent on the viscosity of the mud from mud volcanoes fed by low viscosity mud to higher viscosity mud ridges the diapirs in the eastern areas of the complex are generally mud volcanoes with narrow conduits feeding a surface mound mud ridges are prominent in the western parts of the complex this is interpreted as reflecting a general westward decrease in the fluid content of the accretionary complex bottom simulating seismic reflectors formed by gas hydrate are commonly developed in the areas of mud volcano occurrences the presence of the hydrate indicates that large volumes of methane are being generated at depth in these regions the generation of methane may be contributing to zones of overpressuring in the wedge methane may also be partly responsible for driving the diapiric material to the surface to form mud volcanoes ridges in the subducting oceanic crust beneath the accretionary complex locally enhance diapirism above their crests and southern flanks faults formed later in the development of the complex are more commonly associated with diapirism than those resulting from accretion at the toe of the wedge these later faults play an important role in controlling the sites of individual mud volcanoes chains of mud volcanoes and mud ridges

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

extensive underthrusting of undeformed sediment beneath the accretionary complex of the lesser antilles subduction zone along the eastern margin of the caribbean plate which overrides the atlantic oceanic lithosphere at 20 km myr 1 in an easterly direction1 east of the lesser antilles island arc lies a wide accretionary complex barbados ridge complex 260 km wide at the latitude of barbados and 20 km thick beneath barbados3 4 it increases in width and thickness from north to south2 fig 1 the style of deformation varies considerably along the eastern margin of the complex2 3 5 11 and is related to the thickness and type of sediment on the atlantic ocean floor a common feature is that a proportion of undeformed sediment from the ocean floor is brought beneath the deformed sediments of the accretionary complex and in some areas a clearly defined seismic reflector separates the deformed and undeformed sediments which has been interpreted as a decollement surface5 12 13 a seismic section from an investigation of the accretionary complex conducted from rss discovery in 1980 using 12 channel seismic reflection and long range side scan sonar gloria shows undeformed sedimentary rocks carried horizontally 74 km beneath deformed rocks forming the accretionary wedge we report here that the low shear stresses required on the decollement between the deformed and undeformed rocks are made possible by porewater at pressures close to that of the lithostatic load

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

observations of shear coupled pl waves from shallow earthquakes in the sumatra and java subduction zones summary s phases generated by shallow earthquakes in the sumatra subduction zone and recorded at the geoscope station paf on the kerguelen isles in the southern indian ocean are often followed by a long and particularly large wave train on the longitudinal component the wave train has a dominant period of 25 30 s and lasts usually for more than 100 s it shows slight normal dispersion and has a prograde elliptical motion in the vertical propagation plane these characteristics suggest that the observed waves are shear coupled pl spl waves similar wave trains are not observed for events at comparable distances in other source regions a computational scheme based on the reflectivity method and allowing for different crust and upper mantle structures beneath source and receiver has been used to model the spl waveforms for six of the events located beneath the accretionary domain and outer high of the sumatra subduction zone for the source area a model representing the structure seaward of the trench has been used comprising a 5 5 km water layer and 600 m of sediments by placing the sources in the sedimentary layer the spl wave train on the longitudinal component is reproduced quite well these findings suggest that wave energy is guided into the water and sedimentary layers seaward of the trench where p and s wave reverberations are generated and leak as sv waves into the mantle the guidance can be explained by trapping of wave energy within the wedge shaped accretionary domain and outer high or in oceanic sediments which are subducted with the oceanic plate

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

stress and fluid control on decollement within competent limestone abstract the larra thrust of the pyrenees is a bedding parallel decollement located within a competent limestone unit it forms the floor of a thrust system of hectometric scale imbrications developed beneath a synorogenic basin the fault rock at the decollement is a dense stack of mainly bedding parallel calcite veins with variable internal deformation by twinning and recrystallization veins developed as extension fractures parallel to a horizontal maximum compressive stress cemented by cavity type crystals conditions during vein formation are interpreted in terms of a compressional model where crack arrays develop at applied stresses approaching the shear strength of the rock and at fluid pressures equal to or less than the overburden pressure the cracks developed in response to high differential stress which was channelled in the strong limestone and high fluid pressure in or below the thrust plane ductile deformation although conspicuous cannot account for the kilometric displacement of the thrust which was mostly accommodated by slip on water sills constituted by open cracks a model of cyclic differential brittle contraction stress reorientation slip and ductile relaxation at a rheological step in the limestone is proposed as a mechanism for episodic decollement movement the model accounts for the peculiar microstructural character of the fault zone for alternating sequences of bedding parallel shortening leading to crack dilation and bedding parallel shear leading to decollement slip and for hanging wall imbrication consequent upon decollement slip

tectonic context of fluid venting at the toe of the eastern nankai accretionary prism evidence for a shallow detachment fault during the kaiko nankai diving cruise the peak of the venting activity was located near the top of the very first anticline the most prominent morphological feature between the mid slope 3870 m and the apex of the fold 3770 m is a 20 m high cliff cutting through subhorizontal massive mudstones affected by numerous joints the trend of this scarp is oblique to the fold axis and structurally controlled along two sharply defined nne sse and e w directions fresh talus and blocks found locally suggest active tectonics and recent erosion intense deformation is evident from strongly tilted strata restricted to the base of the cliff that we interpret as an upslope thrust at the scale of seabeam mapping this thrust can be followed eastward for more than 5 km along the 3820 m isobath two seismic lines recorded during one of the pre site surveys show deformation at shallow depth including small scale folding and thrusting affecting only the wedge shaped top sequence deeper layers can be traced continuously below this sequence we conclude that the boundary between the piggy back basin and the frontal fold turbidites acts as a shallow detachment fault and interpret the base of the cliff as the outcrop of the fault dense colonies ofcalyptogena clams and strongly nonlinear thermal gradients locate the major peak of fluid activity at the edge of the plateau above the main cliff scattered biological colonies as well as white bacterial mats and cemented chimneys were also found in a narrow belt along the base of the cliff fluid activity is thus closely related to the shallow detachment fault fluid being expelled both at the outcrop of the fault and above it through the overlying strata possibly using the very dense joint network

isotopic evidence for channeled fluid flow in low grade metamorphosed jurassic accretionary complex in the northern chichibu belt western shikoku japan the low grade metamorphosed jurassic accretionary complex of the northern chichibu belt hijikawa area western shikoku is divided into two units the hijikawa and kanogawa units that are separated by an out of sequence thrust oost the ozu kawabegawa fault the kanogawa unit south of the ozu kawabegawa fault consists mainly of sandstone shale broken formation of alternating sandstone and shale greenstone chert and pelitic melange while the hijikawa unit is characterized by a stack of subunits separated by small scale thrusts the subunits are mainly made up of basic pelitic and psammitic semischists schistose pelitic melange and chert petrological and mineralogical constraints suggest peak metamorphic conditions of 204 247 c at 1 3 kbar in the kanogawa unit and 228 289 c at 3 5 6 kbar in the hijikawa unit quartz and quartz calcite veins are widely developed in the study area especially in the hijikawa unit regional variations in stable isotopic data show that the 18oquartz and 18ocalcite values in veins tend to increase towards the ozu kawabegawa fault the 18owhole rock values are remarkably high in some subunits close to oosts within the hijikawa unit oxygen isotopic compositions from vein quartz indicate that a higher 18o fluid migrated upward from depth along the ozu kawabegawa fault within the accretionary prism during prehnite pumpellyite facies metamorphism the fluid source is inferred to be pelitic rocks at higher temperatures and greater depths within the accretionary prism

pseudotachylytes in an ancient accretionary complex and implications for melt lubrication during subduction zone earthquakes abstract pseudotachylyte bearing fault zones found in the shimanto accretionary complex southwest japan developed during subduction or underplating at seismogenic depths the pseudotachylytes occur in narrow dark veins less than a few millimeters thick that are sharply bounded by foliated cataclasites that originated from a melange the microstructures of pseudotachylytes are represented by a fragment laden glass supported texture resulting from the rapid cooling of the frictional melt transmission electron microscopy reveals the presence of glass in which euhedral microcrystals of mullite are locally developed the compositions of the pseudotachylyte matrix and the characteristics of the unmelted grains and microlites in the matrix suggest that frictional melting occurred in an illite rich slip zone with a minimum melting temperature of 1100 c the viscosities of the frictional melt were calculated from the pseudotachylyte matrix composition as well as from the volume fraction and aspect ratio of the unmelted grains the viscosities at 1100 c range from 85 to 290 pa s and the corresponding shear resistance along a 1 mm thick melt layer at a slip rate of 1 m s was 0 1 0 3 mpa the formation of a melt layer in an illite rich slip zone can possibly induce large stress drops increase the slip rate and enhance rupture propagation which together could affect the earthquake magnitude in a subduction zone

along arc structural variation of the plate boundary at the japan trench margin implication of interplate coupling 1 at the japan trench convergent margin many large interplate earthquakes of greater than m7 5 frequently occur their epicenters have uneven distribution mostly located in the northern area to investigate the relationship between this distribution and tectonic structures we have conducted multichannel seismic surveys since 1996 our data show two kinds of interplate sedimentary units a wedge shaped unit and a channel like unit both units have a lower p wave velocity than the basal part of the overriding island arc crust the wedge shaped unit having a velocity of 2 3 km s is widely distributed over the forearc region in the northern area its thickness decreases with depth becoming several hundred meters at a depth of 12 km the channel like unit having a velocity of 3 4 km s is observed in the southern area extending in the downdip direction its thickness reaches 2 km at a depth of 12 km if the low velocity of these units results from the existence of fluid as many authors assume the units being thick implies higher fluid content assuming constant porosity considering that fluid reduces basal friction and with an assumption that fluid available at a specific interface is proportional to the total fluid content in the sediment the thickness variation of the units would cause different degrees of coupling at the plate boundary along the arc this may provide one explanation for the regional disparity in the interplate earthquake occurrence in the margin furthermore we attempt to call attention to the possibility that the channel like sediment works as a shear stress releaser

implications of measured thermal gradients for water movement through the northeast japan accretionary prism thermal gradients of 24 36 c km have been found in the slope cover sediments of the accretionary prism off northeast japan deep sea drilling project dsdp sites 438a 439 and 440 these gradients lead to the prediction of temperatures in excess of 400 c at depths of 15 km using an increase in thermal conductivity with depth due to the decrease in porosity such temperatures conflict with the predictions of temperatures of 150 c at even greater depths from conductive thermal models and petrologic studies of low temperature blueschists at similar ancient margins one dimensional steady state conduction and advection via darcian water flow of heat up through the prism was simulated numerically the measured thermal gradients can be reconciled with temperatures less than 125 c at the prism base by water flowing upward at rates of the order of 10 10 m s such velocities imply that the total volume of water available from compaction of sediments subducted beneath the northeast japan prism would flow up through a 10 km width of the prism perpendicular to the trench and would require the dsdp sites to have been located fortuitously over the localized zone s of water flow smaller velocities will reproduce the measured gradients and thus greater widths through which water flow occurs are predicted if the temperature at the prism base is higher and or if the prism is thinner and a subduction channel several kilometers high separates the accreted sediments from the descending plate where water flow is significant as likely wherever abundant water rich sediments are subducted thermal gradients below a few kilometers in accretionary prisms may be much lower than near surface values this has profound implications for the thermal structure of the prism off northeast japan and elsewhere

precursory localization and development of microfractures along the ultimate fracture plane in amphibolite under triaxial creep in a triaxial creep experiment in amphibolite we clearly found a precursory localization and development of microfractures along the final fracture plane using an ae acoustic emission source location technique the precursory localization of ae hypocenters first nucleated near a pre existing macroscopic defect and then extended gradually along the final fracture plane prior to failure on the other hand no significant precursory localization of ae hypocenters on the final fracture plane before failure has been reported in rock samples free of pre existing macroscopic defects this difference in ae occurrence patterns before failure could be explained by the difference in the degree of damage in the portion of the rock surrounding the localization zone when it nucleates

precambrian to early cretaceous rocks of the strandja massif northwestern turkey evolution of a long lasting magmatic arc1 the strandja massif northwestern turkey forms a link between the balkan zone of bulgaria which is correlated with the variscan orogen in europe and the pontides where cimmerian structures are

the tectonic fabric of the barbados ridge accretionary complex abstract a map showing near surface structure and other surficial geological features on the barbados ridge accretionary complex has been constructed from gloria long range side scan sonographs detailed bathymetric surveys using seabeam and single and multichannel seismic reflection sections the map shows major variations in the size orientation and style of structures across the accretionary complex that are principally related to the increasing thickness of the complex and the sediment drape upon it and variations along the complex that derive from the northward thinning of sediment accreted from the ocean floor and the effects of ridges in the oceanic basement the development of crossfaulting is mainly associated with basement ridges mud diapirism is abundant in the southern part of the complex and is characteristic of that part of the complex into which turbidites from the orinoco submarine fan are accreted slumping on a variety of scales is another feature of the complex the map is presented at a scale of 1 1063415 4 inches per degree of longitude and also shows bathymetry at 200 m contour intervals

a magnetotelluric sounding across vancouver island detects the subducting juan de fuca plate magnetotelluric1 mt and geomagnetic2 depth sounding measurements have been made at 18 sites 7 with remote reference3 across vancouver island fig 1 beneath which the juan de fuca plate is underthrusting4 5 vancouver island is part of the old accretionary wrangellia terrane6 and is located in the central portion of a 350 km forearc region that extends from the sediment filled trench7 to the garibaldi volcanic belt8 gravity9 and seismic refraction studies10 suggest that the descending plate has a shallow northeasterly dip of 8 16 with perhaps higher dips beneath northeastern vancouver island a high resolution seismic reflection survey11 along lines 1 and 3 fig 1 located a zone of strong acoustic reflections the e horizon11 near the top of the subducting juan de fuca plate at depths of 23 34 km our model of the broad band mt data defines a sloping highly conducting zone also near the top of the subducting plate at the same depths as the seismic e horizon the high electrical conductivity in this zone the e conductor is the result of saline fluid within the pore spaces of sedimentary and mafic materials of the upper oceanic crust that have been subducted to those depths beneath vancouver island these are the first data which clearly define a dipping conductive layer associated with the boundary region between converging plates they have significant implications for thrust earthquakes and metamorphic reactions that occur in subduction zones

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

hydrogeologic properties of a thrust fault within the oregon accretionary prism two sets of hydrogeologic tests conducted at ocean drilling program odp hole 892 on the oregon accretionary prism provided the opportunity to determine hydrogeologic properties of an active accretionary prism fault zone the first set of tests consisted of shipboard packer tests conducted during odp leg 146 fall 1992 while the second set of tests were constant drawdown and constant discharge tests conducted in fall 1993 using the submersible alvin pressure response during the first set of tests suggests that fractures remained open until excess fluid pressure relative to hydrostatic dropped below 0 315 to 0 325 mpa 0 53 to 0 54 where pore pressure hydrostatic lithostatic hydrostatic analysis of the packer test data suggested an apparent background pressure of 0 25 mpa 0 42 to 0 50 because the borehole had been open for 12 hours prior to the packer tests formation pore pressures may have exceeded this value prior to drilling of the borehole these overpressures dissipated by the time the second set of tests were conducted one possible explanation for this decay is that the borehole may provide a vertical conduit between the overpressured zone and overlying or underlying sediments that had previously been hydraulically separated from the overpressured zone the second set of tests were conducted at pressures 0 019 mpa or 0 03 below that estimated to maintain open fractures and yielded transmissivities 1 to 2 orders of magnitude less than estimated for the packer tests when fractures were open constraints on fluid flow rate along the fault are provided by observed displacement in a bottom simulating reflector bsr at its intersection with the fault zone the closed fracture transmissivities are insufficient to produce flow rates capable of displacing the bsr therefore open fracture transmissivities under conditions of elevated pore pressure are inferred to be necessary for the observed bsr displacement in addition calculated rates of specific discharge through the fault zone are 2 to 3 orders of magnitude lower than discharge measured at an associated seafloor vent site fluid flow must become spatially or temporally focused as it moves up the fault zone toward the seafloor

continental margin tectonics forearc processes recent studies of convergent plate margins and the structural development of forearc terranes are summarized in a critical review of u s research from the period 1987 1990 topics addressed include the geometry of accretionary prisms coulomb wedge taper and vertical motion in response to tectonic processes offscraping vs underplating or subduction the response to oblique convergence fluids in forearc settings the thermal framework and the effects of fluid advection and serpentinite seamounts also included is a comprehensive bibliography for the period

tectonic and geological framework for gas hydrates and cold seeps on the hikurangi subduction margin new zealand the imbricated frontal wedge of the central hikurangi subduction margin is characteristic of wide ca 150 km poorly drained and over pressured low taper not vert similar 4 thrust systems associated with a relatively smooth subducting plate a thick trench sedimentary sequence not vert similar 3 4 km weak basal decollement and moderate convergence rate not vert similar 40 mm yr new seismic reflection and multibeam bathymetric data are used to interpret the regional tectonic structures and to establish the geological framework for gas hydrates and fluid seeps we discuss the stratigraphy of the subducting and accreting sequences characterize stratigraphically the location of the interplate decollement and describe the deformation of the upper plate thrust wedge together with its cover sequence of miocene to recent shelf and slope basin sediments we identify approximately the contact between an inner foundation of deforming late cretaceous and paleogene rocks in which widespread out of sequence thrusting occurs and a 65 70 km wide outer wedge of late cenozoic accreted turbidites although part of a seamount ridge is presently subducting beneath the deformation front at the widest part of the margin the morphology of the accretionary wedge indicates that frontal accretion there has been largely uninhibited for at least 1 2 myr this differs from the offshore hawkes bay sector of the margin to the north where a substantial seamount with up to 3 km of relief has been subducted beneath the lower margin resulting in uplift and complex deformation of the lower slope and a narrow 10 20 km active frontal wedge five areas with multiple fluid seep sites referred to informally as wairarapa uruti ridge omakere ridge rock garden and builders pencil typically lie in 700 1200 m water depth on the crests of thrust faulted anticlinal ridges along the mid slope uruti ridge sites also lie in close proximity to the eastern end of a major strike slip fault rock garden sites lie directly above a subducting seamount structural permeability is inferred to be important at all levels of the thrust system there is a clear relationship between the seeps and major seaward vergent thrust faults near the outer edge of the deforming cretaceous and paleogene inner foundation rocks this indicates that thrust faults are primary fluid conduits and that poor permeability of the cretaceous and paleogene inner foundation focuses fluid flow to its outer edge the sources of fluids expelling at active seep sites along the middle slope may include the inner parts of the thrust wedge and subducting sediments below the decollement within anticlinal ridges beneath the active seep sites there is a conspicuous break in the bottom simulating reflector bsr and commonly a seismically resolvable shallow fault network through which fluids and gas percolate to the seafloor no active fluid venting has yet been recognized over the frontal accretionary wedge but the presence of a widespread bsr an extensive protothrust zone 200 km by 20 km in the hikurangi trough and two unconfirmed sites of possible previous fluid expulsion suggest that the frontal wedge could be actively dewatering there are presently no constraints on the relative fluid flux between the frontal wedge and the active mid slope fluid seeps article outline

dynamics of orogenic wedges and the uplift of high pressure metamorphic rocks subduction accretion complexes can be approximated as wedge shaped continua with a rigid buttress behind and a subducting litho spheric slab beneath thick wedges undergoing prograde metamorphism have a negligible long term yield strength and are likely to exhibit a complex nonlinear viscous rheology such a wedge will tend to deform internally until it reaches a stable configuration in which the gravitational forces generated by the wedge geometry balance the traction exerted on its underside by the subducting slab accretion of material at the wedge front will lengthen the wedge and cause it to shorten internally to regain the stable geometry this shortening will be expressed as late out of sequence thrusting backthrusting and folding conversely underplating of sediment or crustal slices will thicken the wedge which may need to extend internally to regain stability extension will cause listric normal faults that may merge downward into zones of ductile extension continued underplating at depth and compensating extension above provides a mechanism for bringing high p low t metamorphic rocks to upper levels in the rear of the wedge where they are commonly observed many major tectonic boundaries in convergent orogens such as the coast range thrust in the franciscan complex major nappe contacts in the alps and the contact between the nevado filabride and higher betic nappe complexes in the betic cordillera show abrupt increases in metamorphic grade downward across them this is consistent with their origin or reactivation as uplift related extensional structures

tectonics and geology of spreading ridge subduction at the chile triple junction a synthesis of results from leg 141 of the ocean drilling program an active oceanic spreading ridge is being subducted beneath the south american continent at the chile triple junction this process has played a major part in the evolution of most of the continental margins that border the pacific ocean basin a combination of high resolution swath bathymetric maps seismic reflection profiles and drillhole and core data from five sites drilled during ocean drilling program odp leg 141 provide important data that define the tectonic structural and stratigraphic effects of this modern example of spreading ridge subduction a change from subduction accretion to subduction erosion occurs along strike of the south american forearc this change is prominently expressed by normal faulting forearc subsidence oversteepening of topographic slopes and intensive sedimentary mass wasting overprinted on older signatures of sediment accretion overthrusting and uplift processes in the forearc data from drill sites north of the triple junction sites 859 861 show that after an important phase of forearc building in the early to late pliocene subduction accretion had ceased in the late pliocene since that time sediment on the downgoing oceanic nazca plate has been subducted site 863 was drilled into the forearc in the immediate vicinity of the triple junction above the subducted spreading ridge axis here thick and intensely folded and faulted trench slope sediments of pleistocene age are currently involved in the frontal deformation of the forearc early faults with thrust and reverse kinematics are overprinted by later normal faults the chile triple junction is also the site of apparent ophiolite emplacement into the south american forearc drilling at site 862 on the taitao ridge revealed an offshore volcanic sequence of plio pleistocene age associated with the taitao fracture zone adjacent to exposures of the pliocene aged taitao ophiolite onshore despite the large scale loss of material from the forearc at the triple junction ophiolite emplacement produces a large topographic promontory in the forearc immediately after ridge subduction and represents the first stage of forearc rebuilding

off scraping accretionary process under the subduction of young oceanic crust the shimanto belt of okinawa island ryukyu arc the eocene kayo formation exposed at the southwestern end of the shimanto belt in the ryukyu arc records the progressive development of off scraping accretion accompanied by increase in lithification and burial as well as synaccretionary thermal imprint the formation consists almost wholly of coherent trench turbidites supplied by axial currents detailed structural analysis has revealed that two successive deformation stages are recognized in the formation the early deformation stage 1 occurred by horizontal shortening related to the initial stage of off scraping accretion in this stage sands were water rich and nearly unlithified and the competence contrast between sandstone and shale was low the late deformation stage 2 is marked by the formation of seaward verging imbricate fold and thrust system within the accretionary wedge compared with stage 1 sands in stage 2 were more dewatered and lithified and the competence contrast between sandstone and shale increased in accordance with this deformation history structures caused by high pore fluid pressure indicate increased cohesion of sands during off scraping accretion studies of the vitrinite reflectance and the illite crystallinity clearly suggest that the formation became buried and suffered thermal imprint pressure solution cleavage developed as an axial planar to the stage 2 folds thermal imprint on the formation is characterized by high paleotemperatures at approximately 250 c 300 c in a region of the prehnite pumpellyite facies metamorphism this thermal event took place during and slightly after the stage 2 probably associated with subduction of a young and hot oceanic plate

duplexing and intraprism deformation of the paleogene shimanto supergroup in western shikoku southwest japan the kurusuno formation belongs to the late eocene to early oligocene shimanto belt and crops out in the hata peninsula shikoku japan detailed structural analysis reveals that the kurusuno formation was deposited as a trench fan formed mainly by transverse currents part of these thick trench sediments was underthrust and underplated to form a foreland dipping duplex steeply plunging folds were formed at a relatively early stage of the deformation and a pervasive slaty cleavage formed after the folding temperatures locally exceeded 350 c during slaty cleavage development abnormally high geothermal gradients may have been caused by a relatively young oceanic plate subducting at about 30 ma underplating of younger trench fill sediments below previously accreted material is probably an important mechanism for the uplift of the shallow level rock bodies in an accretionary complex and high temperature low pressure metamorphism occurs in the lower part of an accretionary complex when young oceanic lithosphere is subducting

tectonostratigraphy of the glen app area southern uplands scotland anatomy of an ordovician accretionary complex the stratigraphy and structure of the glen app coastal area northwestern southern uplands are characterized by structural repetition of both chaotic and coherent units of the middle ordovician caradocian tappins group the chaotic unit the currarie formation comprises blocks including basalt chert and terrigenous sedimentary rocks set in folded and in places sheared red to grey siliceous mudstone the coherent unit is turbiditic and is divided into the dalreoch formation in the north and the corsewall formation in the south the former is characterized by inclined isoclinal folds and thrusts whereas the latter shows a homoclinal dip although both are largely overturned northwards small scale duplexes are observed within the coherent turbidites of the dalreoch formation and chaotic units larger duplexes identified on the basis of mapping are oriented at an oblique angle to the general trend of regional structures the chemistry of basalt blocks within the currarie formation shows a within plate type suggesting an origin as seamounts the tectono stratigraphy of the area is interpreted as the result of accretion of a substantial seamount at a subduction zone during mid ordovician time thrust interleaving of chaotic and coherent units is inferred to have taken place in and above the decollement zone while thick turbidite beds without strong folding were probably preserved below the decollement and emplaced below a large scale nappe interpretation of the glen app area as part of an accretionary complex confirms that an ocean basin remained open in part of the iapetus system at least until caradocian times

structure inferred mechanical properties and implications for fluid transport in the decollement zone costa rica convergent margin faults in a variety of tectonic settings can act as both conduits for and barriers to fluid flow sometimes simultaneously documenting the interaction between hydrologic and tectonic processes in active faults in situ is the key to understanding their mechanical behavior and large scale fluid transport properties we present observations of the plate boundary decollement zone at the middle america trench off costa rica showing that it is structurally divisible into an upper brittle fracture dominated domain overlying a lower ductile domain pore water geochemical evidence shows that along fault flow is occurring specifically in the upper brittle domain but is hydrologically isolated from fluids in the underlying footwall sediments we propose a model for the mechanics of these contrasting domains in which differing stress paths coexist in the upper and lower parts of the decollement zone the data suggest a mechanically controlled permeability anisotropy at a scale of several meters to 10 m across the decollement zone this documentation of separate yet simultaneously active mechanical and hydrologic subregimes within a decollement provides a relatively simple explanation for enhanced along fault permeability coexisting with reduced cross fault permeability without requiring matrix scale permeability anisotropy

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

duplex accretion and underplating in an ancient accretionary complex kodiak islands alaska the landward belt of the kodiak formation represents a sequence of lower maastrichtian turbidites that was underplated to the kodiak accretionary complex by duplex accretion evidence for duplexes exists on the mesoscopic and map scale at the microscopic scale incremental strain histories are consistent with early shear during underthrusting abrupt landward rotation of individual thrust slices over fault ramps and locking of thrust packets in steeper orientations duplex geometries at the mesoscopic scale are represented by high angle reverse faults bounded either above or below by low angle southeast verging detachments at the map scale different structural belts can be related to a low angle detachment zone central belt flooring a more landward duplex landward belt duplex underplating can result in the uplift and thickening of accretionary prisms without necessarily deforming the overlying previously accreted material and slope sediments

consolidation state and strength of underthrust sediments and evolution of the decollement at the nankai accretionary margin results of uniaxial reconsolidation experiments 1 uniaxial reconsolidation experiments conducted on ocean drilling program drill cores along the muroto transect of the nankai accretionary margin demonstrate complex yield and postyield behavior and provide evidence for enhanced strengths within sediments beneath the decollement zone tests were conducted on samples collected from similar stratigraphic levels below the decollement and its seaward projection consolidation state of the samples increased landward and with depth and was tracked closely by sediment yield stress for all but one of the samples the sediments however exhibited substantial postyield strength up to 2 8 times the predicted in situ effective vertical stress beneath the protothrust zone this enhanced strength results from diagenesis that leads to matrix cementation during stable effective stress conditions within the underthrust section the close correspondence between yield stress and predicted in situ effective stress suggests that despite the cemented state the sediment matrix remains sensitive to in situ stress conditions the low yield stress of one sample collected within 40 m of the decollement fault at site 808 may reflect diagenesis under reduced effective stress conditions due to postconsolidation increases in pore pressure along the decollement due to their cemented state the strong underthrust sediments resist decollement downcutting beneath the toe of the prism leading to preferential incorporation of weaker continuously deforming accreted sediments during shear seismogenic slip along the decollement at depth may create stresses in excess of sediment strength causing shear failure and rapid release of trapped pore fluids feeding high pore fluid pressures along the decollement zone