seismotectonic and seismicity of makran a bimodal subduction zone se iran abstract the present study combined spatial distribution and mechanism of seismicity seismicity parameters geomorphology in satellite imagery and tectonic features in makran inland to show that the makran low angle thrust fault is divided into two separate segments depth dispersal of earthquakes especially relocated events shows that the sinking plate is steeper in the west than in the east of makran significant facts were identified from the variation of moho depth between west and east slip rates of the inland faults and crustal shortenings in the north of makran the sistan suture zone plays a significant role in dividing the overriding plate into a non deforming helmand block and a less deforming lut block these blocks have different resistances against the northward subduction large and great events such as the 1989 chabahar mw 5 5 and 1945 gwadur mw 8 1 occurred in the south of subduction fronts of the western and eastern makran respectively they occurred in shallow depth and were the pieces of evidence of weak and strong coupling between the oceanic and continental crusts on a low angle and a very low angle reverse faults respectively spectral analysis and depth determination of the instrumental and pre instrumental earthquakes respectively helped to separate some plate boundary and intraplate events this separation geometry and non characteristic behavior of the subduction zone revealed from spatial and temporal distribution of the earthquakes probably dedicate a mature aleutian type for the subduction zone in the east missing great m 6 1 earthquakes less slip rate of the continental faults and steeper dipping of the sinking plate in the west probably indicate a kurile type for the iranian part of subduction makran is a bimodal subduction because geomorphological and seismic pieces of evidence indicate a nnw surficial and right lateral fracture zone in the continental crust and a nw deep and left lateral fracture zone in the oceanic crust respectively the fractures and geometry of subductions showed that the eastern makran has seismotectonically a wider area for potential coupling of a larger earthquake than the western makran

great thrust earthquakes and aseismic slip along the plate boundary of the makran subduction zone the makran subduction zone of iran and pakistan exhibits strong variation in seismicity between its eastern and western segments and has one of the world s largest forearcs we determine the source parameters for 14 earthquakes at makran including the great mw 8 1 earthquake of 1945 the only instrumentally recorded great earthquake at makran we determine the loci of seismic and aseismic slip along the plate boundary and we assess the effects of the large forearc and accretionary wedge on the style of plate boundary slip we apply body waveform inversions and for small magnitude events use first motions of p waves to estimate earthquake source parameters for the 1945 event we also employ dislocation modeling of uplift data we find that the earthquake of 1945 in eastern makran is an interplate thrust event that ruptured approximately one fifth the length of the subduction zone nine smaller events in eastern makran that are also located at or close to the plate interface have thrust mechanisms similar to that of the 1945 shock seaward of these thrust earthquakes lies the shallowest 70 80 km of the plate boundary we find that this segment and the overlying accretionary wedge remain aseismic both during and between great earthquakes this aseismic zone as in other subduction zones lies within that part of the accretionary wedge that consists of largely uconsolidated sediments seismic velocities less than 4 0 km s the existence of thrust earthquakes indicates that either the sediments along the plate boundary in eastern makran become sufficiently well consolidated and de watered about 70 km from the deformation front or older lithified rocks are present within the forearc so that stick slip sliding behavior becomes possible this study shows that a large quantity of unconsolidated sediment does not necessarily indicate a low potential for great thrust earthquakes in contrast to the east the plate boundary in western makran has no clear record of historic great events nor has modem instrumentation detected any shallow thrust events for at least the past 25 years most earthquakes in western makran occur within the downgoing plate at intermediate depths the large change in seismicity between eastern and western makran along with two shallow events that exhibit right lateral strike slip motion in central makran suggest segmentation of the subduction zone two paleozoic continental blocks dominate the overriding plate the boundary between them is approximately coincident with the transition in seismicity although relative motion between these blocks may account for some of the differing seismic behavior the continuity of the deformation front and of other tectonic features along the subduction zone suggests that the rate of subduction does not change appreciably from east to west the absence of plate boundary events in western makran indicates either that entirely aseismic subduction occurs or that the plate boundary is currently locked and experiences great earthquakes with long repeat times evidence is presently inconclusive concerning which of these two hypotheses is most correct the presence of well defined late holocene marine terraces along portions of the coasts of eastern and western makran could be interpreted as evidence that both sections of the arc are capable of generating large plate boundary earthquakes if that hypothesis is correct then western makran could produce a great earthquake or it could rupture as a number of segments in somewhat smaller magnitude events alternatively it is possible that western makran is significantly different from eastern makran and experiences largely aseismic slip at all times a knowledge of the velocity structure and nature of the state of consolidation or lithification of rocks at depth in the interior portion of the forearc of western makran should help to ascertain whether that portion of the plate boundary moves aseismically or ruptures in large to great earthquakes a resolution of this question has important implications for seismic hazard not only for western makran but also for other margins such as the cascadia subduction zone of western north america where historical thrust events have not occurred

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

heat flow and age of the gulf of oman we present the results of twenty heat flow stations in the gulf of oman which are used to infer the first reliable age estimates for the basin a mean surface heat flux of 42 6 3 6 mw m 2 exhibits no significant regional variation after correction for thick and rapidly deposited sediments this yields an age of 70 to 100 ma according to oceanic thermal models this age is also consistent with the sediment corrected basement depths of 5 5 6 0 km and with formation during the cretaceous quiet zone the latter can explain the absence of magnetic sea floor spreading lineations heat flow measurements are also used to confirm the presence of gas hyrdate layers the measured thermal gradient yields a depth for the solid to free gas phase transition which is the same as that deduced from bright spots seen on seismic reflection profiles

structure of the makran subduction zone from wide angle and reflection seismic data abstract makran is one of the largest accretionary wedges on the globe formed by the convergence between the eurasian and the arabian plates it is characterised by an extremely high sediment input of 7 km and a shallow subduction angle we present seismic velocity models from four wide angle seismic lines that image the wedge sediments and the subducted oceanic crust three strike lines show complex but rather one dimensional structures where the observed seismic phases have been studied with the reflectivity method a 160 km long dip line was surveyed coincident with a mcs line collected by cambridge university in 1986 prestack depth migration of this data with the new wide angle velocities shows improved images compared to earlier results interpretation of the wide angle and the mcs data indicates that a decollement is developed along a bright reflector within the turbiditic sequence more than 3 km of sediment bypasses the first accretionary ridges underthrusting and is transported to greater depth this might help to explain the sparse earthquake activity associated with subduction here in the strike lines low velocity layers occur landward of the deformation front in zones where active thrusting or underplating takes place the oceanic crust is characterised by a strong velocity contrast between layer ii and layer iii

sediment accretion against a buttress beneath the peruvian continental margin at 12 s as simulated with sandbox modeling reflection seismic data from the peruvian continental margin at 12 s clearly reveal an accretionary wedge and buttress sandbox experiments applying the physical concept of the coulomb theory allow the systematic investigation of the growth and deformation of such an accretionary structure the style of deformation of the buttress and the internal structure of the wedge is observed in the sandbox models the possibility of underplating material beneath the buttress and the amount of tectonic erosion depend on the physical properties of the materials mainly internal friction cohesion and basal friction boundary conditions such as the height of the subduction gate and the thickness of incoming sand also constrain the style of growth of the model accretionary structure the configurations of two experiments were closely scaled to reflection seismic depth sections across the peruvian margin a deformable buttress constructed of compacted rock powder is introduced to replicate the basement rock which allows deformation similar to that in the seismic data with the sandbox models it is possible to verify a proposed accretionary history derived from seismic and borehole data the models also help in understanding the mechanisms which control the amount of accretion subduction and underplating as a function of physical properties boundary conditions and the duration of convergence

laterally varying structure and kinematics of the molasse fold and thrust belt of the central eastern alps implications for exploration the molasse deposits of the central eastern alps are partly incorporated into a fold and thrust belt that recently has come into exploration focus the structural style and timing of deformation varies significantly alongstrike regional three dimensional seismic and well data interpretation indicate three different structural segments from east to west 1 the sierning imbricates have a decollement close to the base of the molasse sequence and consist of varying numbers of thrust sheets alongstrike early miocene shortening of the molasse is at least 6 2 km 3 9 mi overthrusting of the internal penninic and helvetic wedge since the oligocene accommodated at least 25 km 15 5 mi of additional shortening 2 the regau segment is dominated by one to two small thrust sheets above a shallow detachment this segment is dominated by overthrusting of the alpine wedge 3 the perwang imbricates consist of an oligocene wedge with complex deformed thrust sheets above a detachment horizon in upper cretaceous marls minimum shortening in the imbricates is 18 5 km 11 5 mi with overthrusting 33 3 km 20 7 mi all shortening estimates have an uncertainty of approximately 20 to 35 the laterally varying thrust belt architecture results from predeformational conditions e g sediment thickness mechanical stratigraphy and basement dip in the sierning imbricates hydrocarbon trap definition and charge issues are exploration risks in the regau segment exploration is focused on the subthrust play the perwang imbricates have hydrocarbon shows but no economic discoveries charge and seal issues are the main risks the petroleum systems in the context of the structural evolution are not yet fully understood

geomorphological response of fluvial and coastal terraces to quaternary tectonics and climate as revealed by geostatistical topographic analysis geostatistical topographic analysis is widely recognized as a useful tool for the statistical reconstruction of planar geomorphic markers from relict surfaces this work is aimed at improving the geostatistical approach used in previous works and developing a method for evaluating the incision rates of rivers in their lower catchments during the late quaternary we chose the major valleys of the adriatic foothills central italy affected since late miocene by a differential tectonic uplift which is still active in particular i we applied the geostatistical analysis to reconstruct the original top surfaces of fluvial to coastal terrace bodies at the metauro river and cesano river mouths ii we performed correlations between the height distribution of the alluvial terrace sequences and the quaternary climatic curve to estimate the average long term fluvial incision rates in the lowermost reaches of the metauro cesano misa and esino rivers the obtained averaged incision rates have been interpreted also in the light of the stream length gradient index sl index steepness index ks and concavity index as proxies of the stream power per unit length results confirm that geostatistical and terrain analysis of topographic and geometric arrangements of fluvial and coastal terraces is an effective tool in detecting geomorphic and tectonic factors inducing perturbations on planar geomorphic markers in particular we better delineated the surface geometry and boundaries of well developed coastal fans at the mouths of the metauro and cesano rivers already recognized in previous works through sedimentological morphostratigraphic and chronological data moreover we found evidence for cut and fill phases that took place during and immediately after the river aggradation of the late quaternary glacial periods despite the slope area analysis evidenced a widespread influence of the regional differential uplift on single river basin configuration we observed some space and time variability of averaged incision rates for adjacent valleys mainly explained by physiographic configuration and dynamics of drainage network copyright 2011 john wiley sons ltd

mud volcanoes at the front of the makran accretionary complex pakistan abstract conical mounds 1 1 5 km in diameter and up to 65 m high were mapped at the foot of the active makran continental margin the mounds developed seaward of the accretionary front in a relatively planar zone where the beginning of build up of tectonic pressure initiates deformation based on shallow high resolution 4 khz sediment echosounding the sedimentary sequence in this area is generally well stratified as indicated by closely spaced horizontal reflections however in the vicinity of the mounds the sediment is characterised by many acoustically transparent zones which are 100 300 m in diameter and cut near vertically through the horizontal reflectors two sediment cores from the top of the largest cone and a neighbouring acoustically transparent zone reveal small scale post depositional deformation in a stratified sequence and methane concentrations up to 40 000 ng g this deformation and disruption of potential reflectors provides a clue to explain the acoustic transparency we interpret it as caused by the rise of charged fluids and mud leading initially to the slight disturbance of the generally good acoustic reflectors and eventually to the formation of conical mud mounds mud volcanoes mcs data showing a buried mound in an analogous structural position support the idea of tectonically induced mud fluid expulsion seaward of the accretionary front

large scale sediment underplating in the makran accretionary prism southwest pakistan field relations in the emergent part of the makran accretionary prism show that a mid miocene to early pliocene slope and shelf sedimentary sequence was deposited directly on abyssal plain turbidites without any detectable stratigraphic or structural discordance sedimentological evidence for rapid shoaling however indicates that the underlying sediment column was tectonically thickened by a factor of between two and three during this period this can be explained by large scale underthrusting and underplating of sediment a mode of accretion that is also favored by mass balance considerations the visible deformation in the coastal makran occurred when the region was 70 100 km north of the contemporary prism front

original structural color preserved in an ancient leaf beetle greenish blue metallic colored fossil elytra of the leaf beetle plateumaris sp have been discovered in the peat sediment of the middle pleistocene ca 600 ka hirabaru formation kyushu japan scanning and transmission electron microscopy revealed the smooth outer surface of the fossil elytra and five alternating electron dense and electron lucent layers in the epicuticle by applying the matrix method to the epicuticle three reflective peaks of wavelengths 388 nm 544 nm and 656 nm were computed these peaks correspond to the reflective color observed under white light thus the coloration of the fossils was caused by multilayer reflectors of the epicuticle of the elytra pyrolysis gas chromatography mass spectrometric analysis revealed that the fossil elytra have preserved some of their original macromolecules chitin protein and amino acids which are similar to those of a related holocene species the high porosity matrix of the peat contains many fragments of diatoms indicating the high productivity of the water column but a reducing bottom environment slight acidity of the interstitial water also seems to contribute to the preservation of some original macromolecules of fossil insects through geologic time

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

internal deformation and compaction of the makran accretionary wedge the makran accretionary wedge is one of the largest on earth a 7 km thick column of sands and quartzolithic turbidites are incorporated into this wedge in a series of deformed thrust sheets we present the results of prestack depth migration and focusing error analysis migration velocity analysis performed on a profile across the makran wedge the depth section shows the deformation style of the accreted sediments and the migration velocities allow us to estimate porosity variations in the sediments the thrust sheets show evidence of fault propagation folding with a long wavelength of deformation 12 km and secondary thrusting in the kink bands of the folds such that the central part of each thrust sheet is elevated to form an additional ridge this deformation style and the 15 steep surface slope of the first ridge suggest a high degree of consolidation porosities were calculated from the seismic migration velocities and the ratio of fluid pressure to lithostatic pressure was estimated for 5 locations along the profile rather than being undercompacted and overpressured as in most accretionary wedges the sedimentary input is normally compacted exponential porosity decay throughout almost the whole wedge however a slight increase in porosity and at depth with respect to the normal compaction curve indicates that the turbiditic sequence might be overpressured landward of the deformation front

controls on terrigenous sediment supply to the arabian sea during the late quaternary the makran continental slope the input of terrigenous sediment along the tectonically active makran continental margin off south western pakistan gulf of oman northern arabian sea is studied on the basis of sediment cores distributed along a transect from the upper slope to the abyssal plain spatial and temporal variations in sediment composition sedimentation rate and turbidite frequency in late pleistocene holocene time last 20 14c ka will be discussed and related to changes in sea level and climate and tectonic activity end member modelling of the grain size distributions of the hemipelagic and turbiditic sediments indicate that the sediments are adequately described as mixtures of three end members which represent turbidite sand turbidite silt or eolian dust and fluvial mud the geochemical and mineralogical compositions of the hemipelagic sediments indicate that the eolian dust was dominantly supplied from the northern arabian peninsula and the persian gulf region and that the fluvial input is from the makran margin the ratio of contributions of eolian and fluvial sediment in the hemipelagic intervals is used as an indicator of continental aridity i e summer monsoon intensity highest holocene turbidite frequencies and sedimentation rates are recorded at the deepest coring sites they are related to the proximity of the deformation front of the accretionary prism turbidite sedimentation on the upper continental slope was most frequent during the last glacial period of sea level lowstand and continued during the entire deglaciation and sea level rise infrequent turbidite sedimentation occurred during the holocene highstand of sea level turbidite sedimentation during the period of late sea level rise and the holocene sea level highstand is inferred to be due to the strong impact of episodic events i e flash floods and earthquakes and because of the narrow shelf of the active makran continental margin

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

coulomb theory applied to accretionary and nonaccretionary wedges possible causes for tectonic erosion and or frontal accretion based on observations from both modem convergent margins and sandbox modeling we examine the possible conditions favoring frontal accretion and or frontal and basal tectonic erosion mean characteristic parameters t t and are used to discuss the mechanical stability of 28 transects across the frontal part of convergent margins where the coulomb theory is applicable natural observations reveal that typical accretionary wedges are characterized by low tapers with smooth surface slope and subducting plate low convergence rates and thick trench sediment while nonaccretionary wedges display large tapers with irregular surface slopes and rough subducting plate high convergence rates and almost no trench fill sandbox experiments were performed to illustrate the effects of seamounts ridges in the subduction zone on the deformation of an accretionary wedge these experiments show that a wedge of sand is first trapped and pushed in front of the seamount which acts as a moving bulldozer this is followed by a tunnelling effect of the subducting seamount through the frontal wedge material which results in considerable sand reworking at an advanced subduction stage the d6collement jumps back from a high level in the wedge to its former basal position we conclude that a high trench sedimentation rate relative to the convergence rate leads to frontal accretion in contrast several conditions may favor tectonic erosion of the upper plate first oceanic features such as grabens seamounts or ridges may trap upper plate material during their subduction process second destabilization of the upper plate material by internal fluid overpressuring causing hydrofracturing is probably another important mechanism

untangling the complex origin of turbidite activity on the calabrian arc ionian sea over the last 60ka abstract the ionian sea due to the presence of two accretionary prisms the calabrian arc and the mediterranean ridge associated with the subduction of the nubia plate and the eurasia plate is the witness of large and catastrophic turbidite events these events are associated with high magnitude earthquakes and tsunamis they lead an important gravity driven sedimentary deposit in the ionian basin in this study we analysed turbidite deposits in order to establish a calendar of the palaeoseismicity and volcaniclastic events from etna on the calabrian arc two gravity cores collected in a slope basin on the calabrian arc and in the ionian abyssal plain record more than two hundred turbidites over the last 60 000 years these turbidites were dated with a multi proxy approach radiometric dating tephrochronology and sapropels the origin of the turbidites was studied with a sedimentary approach grain size lithology thin section and geochemistry of volcanic glass the results suggest that three triggers are responsible for the deposits of turbidites there are siliciclastic turbidites triggered by earthquakes and tsunamis waves and volcaniclastic turbidites triggered by the etna volcano eruptions or flank collapses co seismic turbidites present different recurrence times depending on the location of the core on the calabrian arc recurrence times vary between 450 and 1000 years according to the periods in time on the abyssal plain recurrence times are shorter and more regular around every 240 years

natural gas hydrates on the continental slope off pakistan constraints from seismic techniques summary in october 1997 new vertical incidence wide angle reflection and refraction data were obtained on the convergent continental margin ov pakistan the most prominent feature across the entire margin into the gulf of oman is a bottom simulating reflector bsr this reflection generally coincides with the depth predicted for the base of the gas hydrate stability field on the accretionary prism the bsr is a clear reflection with inverse polarity and strata beneath the bsr show enhanced reflectivities suggesting that gas is trapped between stratigraphic layers joint inversion of wide angle and refraction data yielded a 200 m thick low velocitiy zone at bsr depth using the born expansion band limited impedance logs were calculated from deconvolved and scaled single channel data suggesting that velocity at the bsr drops by 200 m s 1 some 40 km towards the coast cdp4400 of minshull white 1989 yielded a velocity inversion of about 600 m s 1 indicating that bsr characteristics vary significantly across the continental margin ov pakistan beneath the gulf the bsr was sampled at a similar subbottom depth where seismic bright spots occur however its reflection amplitude is unusually low and seismic stratigraphy beneath the bsr did not show enhanced reflectivities joint traveltime inversion of wide angle and refraction data revealed continuously increasing velocities suggesting that an adequate supply of methane for forming free gas and gas hydrate is mainly confined to sediments incorporated into the accretionary prism we believe that these variations are related to an evolutionary process due to tectonic uplift and ongoing sedimentation as sediments are incorporated into the accretionary prism sedimentation and uplift produce an upward migration of isotherms and therefore lead to a dissociation of gas hydrates which releases methane gas and thus causes an enhancement of the bsr our survey imaged bsr properties of the abyssal plain and within the first slope basin at water depths of 3300 m and 2490 m respectively cdp4400 was located farther landwards at a depth of 1730 m within a buried and uplifted and hence more evolved slope basin we therefore suggest that in the accretionary wedge tectonic uplift and ongoing sedimentation cause hydrate recycling by upward migration of the stability field in addition tectonic dewatering may accumulate hydrate at the base of the stability field by advection of methane from depth

comparison of measured and bsr derived heat flow values makran accretionary prism pakistan abstract during the german research cruise so 124 on rv sonne fall 1997 on the makran accretionary wedge off pakistan geophysical investigations were carried out to study the thermal regime at a gas hydrate bearing sediment in a tectonically deformed accretionary wedge on a transect perpendicular to the strike of the deformation front 42 heat flow measurements were carried out accompanied by seismic reflection experiments the investigations start in the south in the abyssal plain and cover the continental slope up to 2300 m water depth the aim of this study is to compare the bsr derived heat flow denoted as estimated heat flow with the values from measurements at the seafloor this requires the calculation of sediment physical properties at depth using empirical relationships between velocity and porosity the value measured and corrected for sedimentation of 47 mw m 2 south of the deformation front is slightly higher than values reported by hutchison et al earth planetary sci lett 56 1981 252 262 in all basins the estimated heatflow is significantly higher than the measured values as a result temperatures at the bsr extrapolated from seafloor measurements are 5 6 k lower than those taken from gas hydrate stability considerations as an overall trend the estimated as well as the measured heat flow show a small decrease from the deformation front to the northward thickening prism a similar observation was made at other accretionary wedges and described by wang et al j geophys res 98 b3 1993 4121 4142 and ferguson et al j geophys res 98 b6 1993 9975 9984 within the slope basins heat flow values show little variation indicating predominantly conductive heat transport fluid flow might occur at the bounding faults where we have little control the effect of rapid sedimentation on the dynamic behavior of the bsr might also have a significant influence on the estimated heat flow values our data set shows clearly that detailed seismic surveys and good control of the subsurface velocity are absolute necessities for the comparison of measured and bsr derived heat flow values however the uncertainty of the velocity porosity relationship together with a high and only approximately established sedimentation rate represent crucial but missing constraints which can be gained only by drilling

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

the submarine canyons of kamchatka abstract a system of large submarine canyons on the continental slope off the kamchatka peninsula has been studied these are the kamachatsky storozh tyushevsky olga kronotsky zhupanovsky and avachinsky canyons and they are generally controlled by fault zones and cut the pre miocene basement structural elements in the cross strike direction this means that they probably are partly the result of subaerial erosion the canyons appear to have been excavated again in miocene to quaternary time by turbidity currents that debouched onto the base of the continental slope and partially filled the kuril kamchatka deep sea trench turbidity currents which move through the canyons are thought to have been generally triggered by earthquakes which dislodged unstable sediments that had accumulated on the upper continental slope and shelf

interaction between accretionary thrust faulting and slope sedimentation at the frontal makran accretionary prism and its implications for hydrocarbon fluid seepage 1 using high resolution seismic profiles and other geophysical data collected during r v meteor cruise m74 2 we investigate the distribution patterns of shallow sediments their structure and deformation processes and their role in the migration accumulation and seepage of hydrocarbon rich fluids here we show that rapid syn kinematic sedimentation at the frontal makran accretionary prism provides a mechanism by which emerging imbricated thrust packets override the footwall at the seafloor without significant mass wasting and destruction of fault related anticline in the hanging wall these anticlines may rise high above the seafloor to form plate boundary parallel ridges and distinguish from simple thrust blocks seen at convergent margins elsewhere with the fast burial of many thrust faults by the syn kinematic sediments anticlinal growth structures form in these syn kinematic sediments by continuous thrust activity the anticlinal structures preserved within the cores of the ridges or formed from these syn kinematic sediments act as structural traps for migrating hydrocarbon rich fluids above which fluid escape structures are generated leading to seafloor seeps most of the discovered hydrocarbon seeps around sixth ridge are sourced from these traps despite the compressional environment and the rapid syn kinematic sedimentation rates shallow subsurface of the frontal makran is a normally pressured regime in which the buoyancy of hydrocarbons may account for the fluid migration in this important respect the makran accretionary prism differs from many other convergent margins and accretionary prisms where fluid flow is largely driven by tectonically induced overpressure

evidence for a thick free gas layer beneath the bottom simulating reflector in the makran accretionary prism abstract seismic reflection data from the makran continental margin indicate the presence of a strong and widespread bottom simulating reflector bsr we apply a nonlinear full waveform inversion technique to multichannel reflection data from this area to investigate the detailed velocity structure and hence the origin of the bsr our result shows an abrupt decrease in the compressional wave velocity from 2 2 to 1 3 km s at a depth of 500 m below the sea bed the low velocity zone is unusually thick 200 350 m and may contain large quantities of free gas similar to some of the recently drilled blake ridge sites of odp leg 164 the voluminous free gas may have been generated by the dissociation of gas hydrates as a consequence of the upward movement of the base of gas hydrate stability field relative to the sediment column due to uplift and sedimentation in the accretionary wedge

the mud volcanoes of pakistan marine geologic investigations on the arabian sea by bundesanstalt fur geowissenschaften und rohstoffe bgr in 1995 and 1998 and land expeditions in 1998 and 1999 to the coastal regions of the makran desert pakistan have extended the knowledge of the aerial distribution of mud volcanoes these structures rise from under compacted formations within the regional accretionary prism which is built by the subduction of the oceanic crust of the arabian sea and its km thick sedimentary load the occurrence of mud volcanoes is limited to the abyssal plain near the accretionary front to the coastal region of the makran desert and to a region in the interior of the desert to the south to southeast of the so called hinglay synform the location of mud volcanoes in pakistan is clearly tied to fault systems mud volcanoes are conspicuously absent on the lower slope of the accretionary prism where thick gas hydrate layers have developed the presence of large gas plumes emerging from the seafloor landward of the gas hydrate stability zone at water depths of less than 800 m points to a redirection of fluids from depth which might explain the absence of mud volcanoes along the lower slope

climatic and tectonic uplift evolution since 7 ma in gyirong basin southwestern tibet plateau clay mineral evidence climate evolution associated with the uplift in gyirong southern margin of the tibetan plateau was investigated based on the clay mineralogy of the gyirong sediments using x ray diffraction and scanning electron microscopy methods the chlorite illite kaolinite smectite ratio and the illite crystallinity index suggest the trend of paleoclimate evolution in gyirong basin during 7 1 7 ma bp with a warm and humid climate in 7 0 5 8 ma a relative cool and dry climate in 5 8 5 5 ma a relative warm and humid climate in 5 5 2 5 ma and a much warmer climate in 2 5 1 7 ma respectively the fluctuating kaolinite smectite index and calcite content and the frequent occurrence of gypsum in the episode of 6 7 5 5 ma suggest intense climatic fluctuations between humid and more seasonal humid conditions five peaks exhibit at 5 8 4 9 4 3 3 6 and 2 5 ma in the curves of illite and chlorite content and illite crystallinity the intense uplift and tectonic forced cooling at 5 8 and 4 3 ma were indicated by the high illite crystallinity index and illite and chlorite content a remarkable increase in illite and chlorite content associated with only little increase in illite crystallinity at 3 6 ma indicates a rapid uplift but no significant climate change and a sharp increase in illite crystallinity while small change in illite and chlorite content at 2 5 ma suggests that no remarkable relief was produced by the uplift which may take place in the whole south tibet plateau while cooling at 4 9 ma is probably a record of the global climate cooling

the chagai porphyry copper belt baluchistan province pakistan although porphyry copper gold mineralization in the region has been known for approximately 40 years the chagai belt has only recently revealed its true potential with the discovery of a world class deposit at h14 h15 at reko diq the 300 km long east west trending chagai belt comprises several superimposed magmatic arcs and corresponding porphyry copper gold molybdenum mineralization generated in consecutive events at 43 to 37 ma middle late eocene 24 to 22 ma early miocene 18 to 16 ma early miocene 13 to 10 ma middle miocene and 6 to 4 ma late miocene early pliocene the tectonomagmatic evolution of the region was marked by major continental scale events associated with 1 the northward drift of india and its interaction and final collision with the southern margin of eurasia and 2 the collision of arabia with central iran resulting in final closure of the neo tethyan ocean initial precollisional stages of arc evolution between the middle jurassic and paleocene formed kilometer thick submarine volcanic sequences with important flyschlike packages and intercalations of biohermal limestone the subalkaline basalt and basaltic andesite of this stage possess a tholeiitic lineage of oceanic island arc affiliation small scale submarine volcanogenic copper mineralization of manto and kuroko type formed during the late mature stages of this arc just prior to collision early contact of the leading edge of india and the island arcs in the late paleocene 55 ma resulted in a reorganization of the arc in the chagai belt with the consequent extrusion of subaerial andesite followed by emplacement of magnetite series calc alkaline composition batholiths during the late eocene this plutonism accompanied the first manifestations of porphyry type alteration and mineralization at 43 ma in a rapidly emerging arc of andean type the belt is defined by 48 porphyry systems with copper gold molybdenum mineralization associated with calc alkaline biotite and amphibole bearing porphyry stocks of predominantly quartz diorite to granodiorite composition hydrothermal alteration includes potassic propylitic sericite clay chlorite sericitic and advanced argillic assemblages the last in transitional epithermal environments calcic potassic assemblages are developed locally at the expense of more mafic commonly dioritic rocks widespread porphyry copper and copper gold mineralization during the miocene at 24 to 22 and 18 to 16 ma formed in a subaerial subvolcanic environment during moderate tectonic uplift most notably during the early miocene at saindak and tanjeel faster rates of regional uplift and exhumation in conjunction with a period of conspicuous volcanic quiescence characterized large scale porphyry copper gold mineralization at h14 h15 at reko diq during the middle miocene between 13 and 10 ma regional tectonically triggered uplift and consequent exhumation may have been the result of collision of arabia with central iran in general mature cumulative supergene enrichment and oxidation zones seem to be absent in the porphyry copper deposits of the chagai belt primarily due to the high neutralization potentials and low pyrite contents of the ore related potassic alteration nevertheless a supergene chalcocite blanket formed at tanjeel during the pliocene at 4 ma in sericitic alteration that accompanied the hypogene mineralization the tectonomagmatic characteristics of the chagai porphyry copper belt including the multimillion year history of subduction along an andean type margin together with conspicuous short lived contraction events and concomitant volcanic quiescence during arc construction and evolution are similar to those in contractional belts at convergent margins containing large scale porphyry copper deposits elsewhere superposition of the chagai magmatic arcs and corresponding porphyry copper events occurred during the last 55 m y a situation that implies that the focus of magma generation above the subducting slab remained essentially stationary possible mechanisms responsible for this include seaward migration of the trench axis during much of the tertiary especially since the oligocene and consequent flattening of the subducted slab

successive shifts of the india africa transform plate boundary during the late cretaceous paleogene interval implications for ophiolite emplacement along transforms abstract the arabian sea in the nw indian ocean is a place where two major transform boundaries are currently active the owen fracture zone between india and arabia and the owen transform between india and somalia these transform systems result from the fragmentation of the india africa transform boundary which initiated about 90 myrs ago when the india seychelles block separated from madagascar to move towards eurasia therefore the geological record of the arabian sea makes it possible to investigate the sensitivity of a transform system to several major geodynamic changes here we focus on the evolution of the india africa transform system during the 47 90 ma interval we identify the late cretaceous 90 65 ma transform plate boundary along chain ridge in the north somali basin from 65 to 42 47 ma the india africa transform is identified at the chain fracture zone which crossed both the owen basin and the north east oman margin finally the transform system jumped to its present day location in the vicinity of the owen ridge these shifts of the india africa boundary with time provide a consistent paleogeographic framework for the emplacement of the masirah ophiolitic belt which constitutes a case of ophiolite emplaced along a transform boundary the successive locations of the india africa boundary further highlight the origin of the owen basin lithosphere incoming into the makran subduction zone

new seismic images of the cascadia subduction zone from cruise so108 orwell abstract in april and may 1996 a geophysical study of the cascadia continental margin off oregon and washington was conducted aboard the german r v sonne this cooperative experiment by geomar and the usgs acquired wide angle reflection and refraction seismic data using ocean bottom seismometers obs and hydrophones obh and multichannel seismic reflection mcs data the main goal of this experiment was to investigate the internal structure and associated earthquake hazard of the cascadia subduction zone and to image the downgoing plate coincident mcs and wide angle profiles along two tracks are presented here the plate boundary has been imaged precisely beneath the wide accretionary wedge close to shore at ca 13 km depth thus the downgoing plate dips more shallowly than previously assumed the dip of the plate changes from 2 to 4 at the eastern boundary of the wedge on the northern profile where approximately 3 km of sediment is entering the subduction zone on the southern profile where the incoming sedimentary section is about 2 2 km thick the plate dips about 0 5 to 1 5 near the deformation front and increases to 3 5 further landwards on both profiles the deformation of the accretionary wedge has produced six ridges on the seafloor three of which represent active faulting as indicated by growth folding the ridges are bordered by landward verging faults which reach as deep as the top of the oceanic basement thus the entire incoming sediment package is being accreted at least two phases of accretion are evident and the rocks of the older accretionary phase s forms the backstop for the younger phase which started around 1 5 ma ago this documents that the 30 to 50 km wide frontal part of the accretionary wedge which is characterized by landward vergent thrusts is a pleistocene feature which was formed in response to the high input of sediment building the fans during glacial periods velocities increase quite rapidly within the wedge both landward and downward at the toe of the deformation front velocities are higher than 4 0 km s indicating extensive dewatering of deep oceanic sediment further landward considerable velocity variation is found which indicates major breaks throughout the accretionary history

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

new arabian sea records help decipher orbital timing of indo asian monsoon abstract a recent study suggested that indian monsoonal proxies commonly used in the arabian sea in general productivity proxies could be impacted by changes in the atlantic overturning rate amoc throughout a control on the nutrient delivery into the euphotic zone this oceanic mechanism could lead to a misunderstanding between the indian summer monsoon sm and orbital forcing and could confuse a direct comparison with other archives derived from other monsoonal sub systems such as east asian or african records here we analyze three independent proxies bromine foraminifera assemblages and grain size extracted from a marine sediment core md04 2861 covering the last 310 ka and retrieved in the northern arabian sea near the makran margin an area influenced by summer and winter indian monsoon the grain size proxy deals with the regional continental climate through fluvial and eolian processes it cannot be linked to changes in nutrient content of amoc and present the same phase relationship timing than the other sm proxies this demonstrates that the productivity signals bromine in the northern arabian sea are mainly controlled by sm dynamics and not amoc modulated nutrients at orbital scale changes we thus build a multi proxy record of sm variability i e sm stack using statistical tools principal component analysis further compiled on an age model constructed independently from orbital tuning we find that strong sm lag by 9 1 ka the nh summer insolation maximum minimum of precession june 21 perihelion and obliquity maximum in the precession band and by 6 1 3 ka in the obliquity band these results are consistent with previous studies based on marine and terrestrial records in both indian and asian regions except asian speleothems our study supports the hypothesis that internal climate forcing decreased ice volume together with the increase of latent heat export from the southern indian ocean set the timing of strong indo asian summer monsoons within both the precession and obliquity cycle the external forcing direct sensible heating initiate monsoonal circulation strong indian winter monsoon wm occurs between ice maxima and northern hemisphere sensible heat minima indicating that both act to strengthen wm circulation the summer and winter monsoons are in antiphase in the precession band suggesting that the two systems are dynamically linked

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

newly identified strike slip plate boundary in the northeastern arabian sea the first regional swath bathymetry survey of the makran accretionary wedge revealed a sinistral strike slip fault named herein the sonne fault obliquely crossing the wedge and continuing into the abyssal plane this fault separates the western part of the makran subduction zone where plate boundary events are absent from the eastern part that does show plate boundary seismicity most events are concentrated along the sonne fault little murray ridge a basement high and related magnetic anomalies are offset along the sonne fault together these observations identify the newly discovered sonne strike slip fault as a plate boundary that has been active 2 m y this finding suggests that what has been considered the northeasternmost part of the arabian plate is actually a separate microplate named herein the ormara plate the formation of which resulted from tearing of the arabian plate along the sonne fault with this concept the different dips of the downgoing plate below the western and eastern parts of the makran margin and the related different distances between the trench and quaternary arc volcanic centers can be unequivocally explained

morphology of san antonio submarine canyon on the central chile forearc a multibeam survey was conducted over san antonio submarine canyon near valparaiso chile in april and may 1993 using the seabeam 2000 system on the r v melville the bathymetric data from this survey reveal a canyon with an overall sinuosity of 1 25 a broad roughly u shaped cross section along most of its length and an almost constant channel slope above the forearc structural high the course of the canyon is deflected to the north by a prominent structural high opposite the town of san antonio seabeam 2000 side scan sonar data reveal high backscatter material on the floor of the canyon with a longitudinal fabric reminiscent of stream braiding and point bars formed on the inside of channel bends we interpret this high backscatter material to be coarse sediment transported down the canyon as turbidity currents the source of these turbidity currents is probably the rio maipo which enters the ocean near the head of the canyon

shallower structure and geomorphology of the southern puerto rico offshore margin oblique convergence between the north american and caribbean plates along the eastern greater antilles island arc has yielded the asymmetric muertos thrust belt in the backarc region offshore south of puerto rico this thrust belt disappears and is replaced by a succession of neesw and eew trending deep basins and steep ridges that characterize the western anegada passage resulting in a complex deformation pattern using new high resolution multibeam bathymetry and seismic reflection profiles we studied the geomorphology and shallower structure of the southern puerto rico offshore margin we have identified four morphotectonic provinces the puerto rican sub basin and muertos trough the muertos margin the insular shelf and the western anegada passage the muertos margin province shows two distinct slope sub provinces the active muertos thrust belt e which includes lower and upper thrust belts with distinct deformational styles and lateral continuity e and the shelf slope highly incised by a dense canyon network this network is disrupted by the investigator fault zone consisting of a 130 km long eew trending band of active extensional deformation the investigator fault zone shows differential surface expression caused by along strike changes in the magnitude and distribution of the deformation though this deformation is driven by a nes oriented extension in the western anegada passage province the whiting basin and whiting and grappler ridges are formed by large dip slip normal faults driven by a nwese oriented extensional regime the western st croix rise shows a complex structure where the neesw trending nw dipping normal faults observed at the summit of the rise predate the eew bounding faults that could accommodate the extensional deformation at the present this study provides detailed observations on the active tectonic and sedimentary processes to help future studies assessing the natural resources and the seismic and tsunamigenic hazard in the puerto rico region

high interseismic coupling in the eastern makran pakistan subduction zone estimating the extent of interseismic coupling along subduction zone megathrusts is essential for quantitative assessments of seismic and tsunami hazards up to now quantifying the seismogenic potential of the eastern makran subduction zone at the northern edge of the indian ocean has remained elusive due to a paucity of geodetic observations furthermore non tectonic processes obscure the signature of accumulating elastic strain historical earthquakes of magnitudes greater than 7 have been reported in particular the 1945 mw 8 1 earthquake resulted in a significant tsunami that swept the shores of the arabian sea and the indian ocean a quantitative estimate of elastic strain accumulation along the subduction plate boundary in eastern makran is needed to confront previous indirect and contradictory conclusions about the seismic potential in the region here we infer the distribution of interseismic coupling on the eastern makran megathrust from time series of satellite interferometric synthetic aperture radar insar images acquired between 2003 and 2010 applying a consistent series of corrections to extract the low amplitude long wavelength deformation signal associated with elastic strain on the megathrust we find high interseismic coupling i e the megathrust does not slip and elastic strain accumulates in the central section of eastern makran where the 1945 earthquake occurred while lower coupling coincides spatially with the subduction of the sonne fault zone the inferred accumulation of elastic strain since the 1945 earthquake is consistent with the future occurrence of magnitude 7 earthquakes and we cannot exclude the possibility of a multi segment rupture mw 8 however the likelihood for such scenarios might be modulated by partitioning of plate convergence between slip on the megathrust and internal deformation of the overlying actively deforming accretionary wedge

large holocene tsunamis in the northern arabian sea abstract tsunamis have repeatedly hit the shores of oman northern arabian sea in historical times e g 1945 2013 these events had small

sediment compaction and fluid migration in the makran accretionary prism the makran continental margin in the gulf of oman forms the seaward extremity of an accretionary sediment prism which extends several hundred kilometers inland a recently acquired multichannel seismic reflection profile shot across the margin imaged the structure of the prism in greater detail than was previously possible and allowed us to investigate the relationship between deformation and pore fluid motion in the region velocity analyses of the common midpoint gathers reveal a marked change in velocity structure at the toe of the accretionary wedge as seen in previous sonobuoy wide angle data accreted sediments show significantly higher vertical velocity gradients than those of sediments entering the prism this change is interpreted as due to porosity reduction as pore fluids are squeezed out of the compacting sediment a prominent bottom simulating reflector appears 500 800 m beneath the sea bed several lines of evidence suggest that this reflector represents the base of a gas hydrate zone underlain by widespread free gas which may be exsolved from pore water migrating from deep within the sediment pile up permeable fault planes imaged in the profile the hydrate reflector appears to shallow in the region of some faults suggesting a temperature anomaly due to the presence of warm pore fluids a heat flow profile derived from the depth of the hydrate reflector does not show the expected landward decrease as the sediment pile thickens simple thermal modeling suggests that advective heat flow within the prism may explain this anomaly the inferred presence of overpressured pore fluids in the makran suggests that accreted sediments have a low permeability the seismic evidence suggests a two stage compaction process with rapid initial dewatering through intergranular permeability as sediment enters the prism followed by a buildup of pore pressure as the permeability decreases and fluid migration is restricted to fault zones

morphotectonic and morphometric analysis of the nazca plate and the adjacent offshore peruvian continental slope implications for submarine landscape evolution we use new swath bathymetry data acquired during the rv sonne cruise geopeco and complement them with swath data from adjacent regions to analyse the morphotectonics of the peruvian convergent margin the nazca plate is not covered with sediments and therefore has a rough surface along the entire peruvian trench the styles of roughness differ significantly along the margin with linear morphological features trending in various directions most of them oblique to the trench and roughness magnitudes of a few to several hundred meters the lower slope is locally very rough and at the verge of failure throughout the entire peruvian margin as a result of subduction erosion causing the lower slope to over steepen using curvature attributes to quantitatively examine the morphology in the yaquina and mendana areas revealed that the latter shows a larger local roughness both seaward and landward of the trench however the amplitude of morphological roughness is larger in the yaquina area we identified a 125 km2 large slump on the lima middle slope morphometric dating suggests an age of 74500 years within 35 to 40 error estimated incision rates on the upper slope are between 0 1 and 0 3 mm per year suggesting that landscape evolution on the peruvian submarine continental slope is similarly slow than that in the atacama desert

authigenic carbonates derived from oxidized methane vented from the makran accretionary prism off pakistan we report the first discovery and sampling of a methane hydrogen sulfide rich cold seep from the makran accretionary prism off pakistan arabian sea a variety of cm to m scale pockmarks and gas seepage structures were identified from underwater tv photo sled profiles crossing the oxygen minimum zone omz related to highresolution seismic parasound records from the seeps isotopically light bacterially formed methane is emanating that is partly oxidized to hco3 in the bacterial sulfate reduction zone this results in the precipitation of irregular dark gray to black crusts of indurated authigenic carbonates mainly cryptocrystalline magnesian calcite and ca rich dolomite near the sediment seawater interface within the omz downcore the crusts grade into hemipelagic carbonate poor silty clays with transitional lithologies in between similar to the pockmark carbonates from the oregon margin and in the north sea the authigenic carbonates are extremely depleted in 13c 13ccarbonate 40 this suggests that they were derived from bacterial methane 13cmethane 77 8 that was oxidized under anaerobic conditions the authigenic carbonates are associated with white fluffy mats of chemoautotrophic h2s oxidizing bacteria small pockmarks appear to be concentrated at small scale escarpments suggesting focussed fault controlled pore fluid expulsion due to the tectonic dewatering and degassing of the accretionary prism whereas diffuse discharge of pore fluids is inferred from the widespread occurrence of tiny gas bubble tubes

geology of the onshore makran accretionary wedge synthesis and tectonic interpretation abstract this contribution is a synthesis of studies carried out since 2004 in the makran accretionary wedge exposed in se iran these studies were intended to improve our understanding of the behaviour of accretionary wedges and of the significance of the makran wedge in the frame of tethys tectonics reassessing the stratigraphy and structure of the iranian makran has allowed distinguishing four main litho tectonic units the lithological content of these four units is briefly described to discuss the sedimentary environments which record the general filling of an oceanic basin with clastic sediments derived from the north the petrology of ophiolites exposed in the backstop region to the north of the accretionary wedge itself demonstrates the existence of an oceanic basin formed in the jurassic late cretaceous to the north of continental slivers which had separated from central iran there is petrological evidence for a subduction related magmatic arc during the late cretaceous which sourced much of the clastic material in the cretaceous lower miocene turbidites present throughout the wedge the literature survey shows that the late cretaceous to present evolution of the pakistan part of the makran fold and trust belt is in its major attributes similar to that of the iranian makran these observations and results are discussed in a tectonic model that integrates knowledge on the oman mountains on the other side of the current oman subduction

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

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

coseismic strengthening of the shallow portion of the subduction fault and its effects on wedge taper 1 according to the dynamic coulomb wedge model coseismic strengthening of the shallowest part of the subduction interface can cause permanent deformation of the overlying outer wedge so that the cumulative effects of great interplate earthquakes control the geometry of the wedge in this work we use a numerical model which is a hybrid of the frictional contact model and the classical crack model to study how stress is coseismically transferred from the seismogenic zone to the strengthening updip zone to cause wedge deformation in this static model the critical strengthening required to prevent the rupture from breaking the trench depends on the force drop of the seismogenic zone defined as the product of the average shear stress drop and the area of the seismogenic zone in a simple model of uniform material properties with a few megapascals average stress drop over a seismogenic zone of 120 km downdip width the critical strengthening for a 30 km wide updip zone is an increase in the effective friction coefficient by about 0 05 using the coulomb wedge theory we demonstrate that this level of strengthening can readily push the overlying wedge into a critical state of failure with much greater strengthening the rupture is able to extend into the updip zone only slightly causing localized wedge compression in the area of slip termination we examined wedge geometry of 22 subduction zones in the light of the model results we found that the surface slope of these wedges is generally too high to be explained using the classical wedge theory but can be explained using the dynamic coulomb wedge model including coseismic strengthening of the shallow portion of the megathrust

material transfer in accretionary wedges from analysis of a systematic series of analog experiments the structural evolution of accretionary wedges was modeled in a series of analog experiments with systematically varying initial boundary conditions the two principle parameters investigated were basal friction and thickness of sediment input relative to subducted sediment output material transfer in terms of degree and location of accretion and erosion is quantified as a function of these two parameters low basal friction produces long thin wedges by continuous frontal accretion of imbricate thrusts simultaneously substantial basal erosion may occur with little influence on the frontal dynamics the formation of an outer arc high with an accompanying forearc basin depends upon the relative amount of basal erosion the opening of the subduction window spacing of thrust faults is proportional to the layer thickness confirming the principle of self similar wedge growth from critical wedge theory high basal friction typically generates steep wedges marked by an erosional mass wasting frontal slope in the cases i o this is due to underthrusting and or subduction of the entire sedimentary section the case i o also features the late development of normal faulting and an extensional forearc basin for both low and high basal friction the material balance affects the wedge slope erosive conditions i o for low basal friction the primary modes of material transfer are frontal accretion and basal erosion for high basal friction the primary modes of material transfer are underthrusting underplating and frontal erosion applying the experimental results to active convergent margins parallels may be drawn thus the barbados ridge complex is interpreted to be a good example of a low basal friction wedge with an accretionary material balance while the japan and mariana margins provide examples of an erosive material balance

mud diapirism within indus fan sediments murray ridge gulf of oman summary diapiric structures showing striking spatial periodicity have been widely observed at shallow levels on single and multichannel seismic reflection profiles within indus fan sediments the study area is some 200 to 400 km from the mouth of the indus at the fan s northern margin in the region of the murray ridge neighbouring diapirs are generally associated with the same acoustically transparent layer they form at depths less than 500 in below the seafloor and have the form of gentle waves on the upper surface of this parent layer the acoustic appearance of the features may be controlled by the presence of methane which has been widely reported in the area the diapirs are formed by gravitational instability of overpressured muds within the rapidly deposited sedimentary sequence simple 2 d two layer gravity instability modelling of diapir wavelengths and growth rates has determined values of approximately 104 to 106 for the ratio of equivalent viscosity of the overburden sediments to that of the diapiric layer and a maximum equivalent viscosity of 1011 to 1012 pa s for the diapiric layer itself

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

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