effects of lateral friction on the structural evolution of fold and thrust belts insights from sandbox experiments with implications for the origin of landward vergent thrust wedges in cascadia various factors are inferred to influence the structural evolution of fold and thrust belts or thrust wedges but the effects of lateral friction such as that exerted by the opposite block of a strike slip fault in nature or by sidewalls in analogue models have rarely been given sufficient attention and hence are still far from being fully understood up to now seaward vergent and dually vergent thrust wedges have been satisfactorily explained by the classic coulomb wedge theory and can be well reproduced in analogue and numerical models in contrast landward vergent thrust wedges which are uncommon in nature e g those occurring in the cascadia subduction zone have not yet been adequately explained by the classic coulomb wedge theory and can only be reproduced in a few analogue models under particular conditions in order to look closer at the effects of lateral friction on the structural evolution of fold and thrust belts and to explore the origin of landward vergent thrust wedges we conducted a series of sandbox models under different conditions of lateral friction and various ratios of initial average lateral frictional stress to basal shear stress s d results of our models revealed that in the presence of a weak or ductile basal decollement the direction of lateral friction can dictate not only the deformation propagation direction and the surface slope direction but also the structural vergence of fold and thrust belts fold and thrust belts normally show a structural vergence in the same direction as the direction of lateral friction but a deformation propagation direction and surface slope direction opposite to the direction of lateral friction according to the results of models and the relationships between structural vergence and lateral friction direction occurring both in cascadia and the analogue models we propose that an extremely high s d ratio due to an extremely weak basal decollement and landward directed lateral friction exerted by the opposite blocks of basement involved strike slip faults are probably the two key factors responsible for the origin of landward vergent thrust wedges in cascadia this work illustrates that effects of lateral friction should be properly taken into account both in the kinematic analysis and designs of models to simulate fold and thrust belts in the presence of weak or ductile basal decollements

low friction along the high slip patch of the 2011 mw 9 0 tohoku oki earthquake required from the wedge structure and extensional splay faults we analyze the mechanical properties needed to account for the large shallow slip during the 2011 tohoku oki earthquake and the activation of landward normal faulting within the forearc we show that the morphology and internal structure of the forearc follows closely the prediction of the critical coulomb wedge in horizontal compression implying a high internal pore pressure ratio 0 7 0 14 0 48 and a low effective basal friction eff b 0 14 0 18 0 04 we then show that the activation of the normal fault requires a lower effective basal friction beneath the outer wedge than beneath the inner wedge outer 0 015 possibly due to transient dynamic weakening associated to the seismic rupture forearc normal faults could be considered as evidence for very efficient dynamic weakening along the megathrust and typify megathrust with high tsunamigenic potential

tsunamigenic potential due to frontal rupturing in the sumatra locked zone abstract the sumatra subduction zone is one of the most seismically active subduction zones although there have been three m w 8 4 earthquakes in the region including the disastrous 2004 m w 9 2 sumatra andaman earthquake a 500 km long patch around mentawai islands is still locked and could produce a large megathrust earthquake if the rupture propagates to the subduction front as it most likely occurred during the 2004 earthquake it may lead to a devastating tsunami here we present high resolution reflection seismic data from the sumatra locked zone that shows the subduction interface down to 20 km depth the seismic data also show that the wedge is composed of two layers a shallow layer formed by mixed to landward vergent thrusts termed as pop ups and a deeper layer showing sub horizontal reflectors the lower layer is most probably formed by duplexes whose roof serves as a pseudo decollement for the mixed to landward thrust systems based on the seismic results we perform mechanical modeling in order to understand the formation of these structures and to retrieve the associated frictional properties we first show that the activation of the pseudo decollement requires 1 either a sudden increase of effective friction along the plate interface or an irregular geometry of the plate interface 2 a lower effective friction along the pseudo decollement than along the plate interface we then show that low effective frictional values 0 1 are required to reproduce the observed frontal structures the low effective friction along the pseudo decollement could either be due to the presence of a long term high pore pressure layer or to dynamic weakening associated with earthquakes since similar structures are present in the 2010 tsunami earthquake area we favor the dynamic weakening hypothesis according to the mechanical modeling if the next rupture propagates up to the toe rupturing the three most frontal pop up structures we could expect at least 5 5 to 9 2 m of frontal horizontal displacement and a frontal uplift of 2 to 6 6 m along the frontal thrusts this would amplify the uplift of the water column and as a consequence generate a large tsunami

geologic thermobarometry of retrograded metamorphic rocks an indication of the uplift trajectory of a portion of the northern scandinavian caledonides kyanite grade pelitic schists from the efjord area 68 15 n 16 45 e northern norway show abundant textural evidence of retrograde reequilibration during late orogenic uplift and cooling of this part of the scandinavian caledonides graphical analysis of mineral compositions in seven samples that show a range of retrograde textures indicates that they reached final chemical equilibrium under nonuniform pressure temperature conditions three element partitioning geothermometers and geobarometers yield consistent pt estimates in individual samples which together define a trend in pt space the sample showing the fewest signs of retrogression yields the highest pressure and temperature estimates whereas samples exhibiting abundant reequilibration textures give lower pt estimates we suggest that the trend indicated by these samples reflects a portion of the pt trajectory assumed by the efjord area during uplift thus the judicious use of element partitioning geothermometers and geobarometers on samples showing signs of retrograde metamorphism may yield important new information concerning the late tectonic evolution of metamorphic terrains

intraplate extensional tectonics of the eastern basin range inferences on structural style from seismic reflection data regional tectonics and thermal mechanical models of brittle ductile deformation the general lack of a correlation between earthquakes and surface faulting in the interior of the western u s cordillera has motivated our efforts to evaluate the geometry structural style and mechanism of normal faulting characteristic of this region of intraplate extension to address the problem we have interpreted over 1500 km of seismic reflection data and constructed detailed cross sections of the upper crust rheological models of the continental crust were calculated to examine the possibility of shallow quasi plastic flow and its influence on faulting in the eastern basin range the western colorado plateau and the middle rocky mountains our data and interpretations have revealed the following styles of cenozoic deformation 1 steep to low angle dip normal faulting along the wasatch fault 2 low angle dip and listric normal faulting possibly associated with movement on preexisting thrusts 3 the occurrence of asymmetric mostly eastward tilted tertiary basins that are bounded by low to moderate dipping planar and listric faults and 4 at least three vertically stacked en echelon low angle reflections in the mid to upper crust that dip gently westward from 3 km beneath the wasatch plateau to over 15 km at the utah nevada border these reflections are interpreted as normal detachment faults the structural style of the pervasive low to moderate angle dipping faults cannot be easily reconciled with classic brittle failure theory but the interpreted termination of normal faults at or above the deeper low angle reflections suggests the presence of shallow zones of ductile deformation that may have accommodated slip an important observation based on interpretations of seismic reflection profiles is that normal fault zones dip more gently in the subsurface than their associated scarps in unconsolidated surficial deposits segment boundaries of the wasatch fault zone apparently coincide with the positions of east trending displacement transfer structures in thrust sheets and the position of the precambrian uinta aulacogen suggesting that preexisting crustal structure partly controls the geometry of extension to examine the influence of ductile deformation quasi plastic flow was modeled for appropriate geotherms of the basin range and colorado plateau crusts these models provide constraints on the depth to the frictional quasi plastic transition that occurs as shallow as 8 km in the eastern basin range this depth also corresponds to the approximate depth above which most accurately determined earthquake foci for small earthquakes m 5 5 appear to nucleate at 10 15 km in or near the brittle ductile transition the rheological modeling suggests that a thermally deforming continental lithosphere plays an important role in the evolution of this extending intraplate region large scale low angle reflections may represent decollements and normal faults at the top of upper crustal zones of reduced shearing strength and higher ductility

seismic velocities from the barbados ridge complex indicators of high pore fluid pressures in an accretionary complex three wide aperture 0 5 km offset seismic profiles were shot perpendicular to the strike of the barbados ridge accretionary complex at 16 12 n line 465 14 23 n line 480 and 13 20 n line 484 to image the deep structure of the accretionary complex and obtain seismic velocity data velocity analyses on full fold common depth point cdp gathers were used to provide velocity depth functions at 1 km separation along each line to map the variation in velocity with depth and distance across the accretionary complex the sediments within the complex show consolidation increasing laterally with greater age of the complex as indicated by the increase in seismic velocity and the inferred decrease in porosity the velocity at the base of the wedge along line 480 is 2 4 km s at the toe the velocity at the wedge base at 60 km from the toe where the wedge thickness has increased from 1 8 km to 5 0 km has increased to greater than 3 0 km s however the velocities within the leading margins and within undeithrust sequences in each of the three sections are low as much as 1 0 km s less than velocities found in undeformed sedimentary sequences of equivalent thickness the low velocity regions are interpreted as sediments with high porosity where porosity is inferred to be high pore fluid pressures are also inferred to exceed hydrostatic and pore fluids actually support the increased lithostatic load produced by recent thrusting and thickening of the complex the correlation between the most rapidly thickened regions and the velocity anomalies suggests that a rapid rate of loading relative to the rate of pore fluid expulsion produced these overpressured sediments velocities in the underthrust sequences where the differences between loading histories of wedge sediments and the underthrust sequences are largest are sufficiently low to produce velocity inversions low velocities also occur in regions adjacent to the complex where the pore fluid pressures may be elevated by the pressure produced in the adjacent wedge west of the wedge in the forearc basin along line 480 and near a seismically imaged mud diapir velocities at a depth of 3 5 km below the seafloor are 0 5 km s less than what is expected in a more normal sedimentary section immediately seaward of the complex the deepest undeformed sequences have a velocity anomaly of 0 5 km s velocities remain low for at least 12 km seaward of the complex but they become closer to a more normal sediment section farther from the wedge

role of crustal flexure in initiation of low angle normal faults and implications for structural evolution of the basin and range province the mechanics of low angle normal faulting has proved difficult to establish despite clear geological evidence for the existence of such faults gently dipping shear stress trajectories are necessary for nucleation and propagation of low angle faults in a structurally isotropic medium such trajectories are not produced by simple horizontal extension but do arise in the presence of flexural stresses we model stress conditions in a cross section through the strong upper crust using airy stress functions for a two dimensional elastic medium numerical modeling indicates that high ratios of flexural stress to extensional stress produce conditions favorable for low angle normal fault initiation at intermediate depths in an elastic medium we propose that the gentle dip of cenozoic low angle normal faults in the basin and range province resulted from flexural stresses that were produced by isostatically uncompensated surface and especially moho relief resurgent mid cenozoic magmatism and the consequent reduction of the thermally dependent flexural strength of the mantle lithosphere transferred flexural stresses to the upper crust causing or promoting initiation of low angle normal faults temporal changes in structural style from dominantly low angle to dominantly high angle normal faulting may reflect reduction of the magnitude of flexural stresses in the upper crust due to flattening of the moho and cooling of the mantle lithosphere

the snake range decollement an exhumed mid tertiary ductile brittle transition the snake range decollement srd in east central nevada separates supracrustal rocks extended by normal faulting from ductilely deformed igneous and metamorphic rocks a well known stratigraphy unaffected by earlier faulting permits analysis of both upper and lower plate strain leading to a better understanding of how vastly different rock types and deformational styles are juxtaposed along low angle faults in metamorphic core complexes middle cambrian to permian upper plate rocks are cut by two generations of ne trending east directed normal faults both generations were initiated as high angle 60 planar faults that flattened abruptly into the srd and rotated domino style to low angles yielding a total rotation of bedding of about 80 90 faulting is tertiary in age as 35 m y old volcanic rocks are involved and resulted in about 450 500 extension in a n55w s55e direction the srd developed as a subhorizontal surface 6 7 km deep at the top of the cambrian pioche shale lower plate granitic rocks and their late precambrian cambrian metamorphic country rocks were involved in progressive ductile to brittle extension at low greenschist grade forming a penetrative subhorizontal foliation and n55 70w lineation that increases in intensity eastward and upward toward the srd stretching and thinning in the lower plate is coaxial and comparable in magnitude to upper plate extension and is interpreted as synchronous k ar ages ranging from 20 to 40 m y in the lower plate suggests the n snake range represents a tertiary thermal anomaly we conclude that the srd developed as a ductile brittle transition zone at 6 7 km depth gravity data suggests that the gently domed srd is cut by younger basin and range faults but the geology of adjacent ranges suggests that the srd does not continue more than 60 km in any given direction the lack of stratigraphic omission across the srd rules out large amounts of movement on a surface that originally cut downsection and we suggest that extensional detachment faults such as the srd can be developed locally as boundaries between brittlely extended rocks and underlying ductile extension and intrusion

work optimization predicts accretionary faulting an integration of physical and numerical experiments we employ work optimization to predict the geometry of frontal thrusts at two stages of an evolving physical accretion experiment faults that produce the largest gains in efficiency or change in external work per new fault area wext a are considered most likely to develop the predicted thrust geometry matches within 1 mm of the observed position and within a few degrees of the observed fault dip for both the first forethrust and backthrust when the observed forethrust is active the positions of the second backthrust and forethrust that produce 90 of the maximum wext a also overlap the observed thrusts the work optimal fault dips are within a few degrees of the fault dips that maximize the average coulomb stress slip gradients along the detachment produce local elevated shear stresses and high strain energy density regions that promote thrust initiation near the detachment the mechanical efficiency wext of the system decreases at each of the two simulated stages of faulting and resembles the evolution of experimental force the higher wext a due to the development of the first pair relative to the second pair indicates that the development of new thrusts may lead to diminishing efficiency gains as the wedge evolves the numerical estimates of work consumed by fault propagation overlap the range calculated from experimental force data and crustal faults the integration of numerical and physical experiments provides a powerful approach that demonstrates the utility of work optimization to predict the development of faults

predicting orogenic wedge styles as a function of analogue erosion law and material softening 1 the evolution of a compressive frictional wedge on a weak frictional and planar decollement subjected to frontal accretion is predicted with a two step method called sequential limit analysis the first step consists in finding with the kinematic approach of limit analysis the length of the active decollement and the dips of the emerging ramp and of the conjugate shear plane composing the emerging thrust fold the second step leads to a modification of the geometry first because of the thrust fold development due to compression and second because of erosion erosion consists in removing periodically any material above a fictitious line at a selected slope as done in analogue experiments this application of sequential limit analysis generalizes the critical coulomb wedge theory since it follows the internal deformation development with constant frictional properties the deformation is mostly diffuse a succession of thrust folds being activated so that the topographic slope reaches exactly the theoretical critical value frictional weakening on the ramps results in a deformation style composed of thrust sheets and horses applying an erosion slope at the critical topographic value leads to exhumation in the frontal central or rear region of the wedge depending on the erosion period and the weakening erosion at slopes slightly above or below the critical value results in exhumation toward the foreland or the hinterland respectively regardless of the erosion period exhumation is associated with duplexes imbricate fans antiformal stacks and major backthrusting comparisons with sandbox experiments confirm that the thickness dips vergence and exhumation of thrust sheets can be reproduced with friction and erosion parameters within realistic ranges of values

active normal faulting and crustal extension a world wide review of fault plane solutions and focal depths for large normal faulting earthquakes on the continents shows that the overwhelming majority of such earthquakes nucleate in the depth range 6 15 km on faults dipping between 30 and 60 in the few cases where levelling or seismic data are good enough these normal faults are shown to be approximately planar from the surface to their nucleation depth at the base of the brittle crust there is evidence that in some cases as a result of the enormous transitory increase in strain rate during large earthquakes rupture continues into the normally ductile lower crust on surfaces with substantially gentler dips these low angle surfaces may be analogous to some of the detachments seen in metamorphic core complexes of the western usa but the nature of the motion on them depends on strain rate as well as on rheological contrasts between the detachments and the blocks on either side such contrasts however are unlikely to introduce substantial curvature to an originally planar shear zone if the observed spread in active normal fault dips is caused by rotation of the faults and the blocks they bound during extension then a maximum value of 1 7 can be accommodated by seismic activity on a single generation of normal faults with continued extension either a new generation of steeper faults cutting the rotated first faults is likely to form or the deformation will continue aseismically on faults dipping at less than 30 large earthquakes have not been observed to nucleate on very low angle 20 normal faults within the continental crust anywhere in the world such faults can of course move aseismically but are unlikely to do so on a large scale within the upper crust in areas where steep normal faults are seismically active thus extensional models that require concentrated simple shear on large sub horizontal faults within the brittle upper crust will also require a spatial separation between aseismic very low angle faulting and seismic highangle faulting since seismogenic high angle faults dominate the topography of extending regions upper crustal very low angle faulting if it occurs on a large scale presumably does so in areas that are relatively fiat

seismic structure of the southern cascadia subduction zone and accretionary prism north of the mendocino triple junction four multichannel seismic reflection profiles collected as part of the mendocino triple junction seismic experiment image the toe of the southern cascadia accretionary prism today 250 600 m of sediment is subducting with the gorda plate and 1500 3200 m is accreting to the northern california margin faults imaged west and east of the deformation front show mixed structural vergence a north south trending 20 km long portion of the central margin is landward vergent for the outer 6 8 km of the toe of the prism this region of landward vergence exhibits no frontal thrust is unusually steep and narrow and is likely caused by a seaward dipping backstop close to the deformation front the lack of margin wide preferred seaward vergence and wedge taper analysis suggests the prism has low basal shear stress the three southern lines image wedge shaped fragments of oceanic crust 1 1 7 3 km in width and 250 700 m thick near the deformation front these wedges suggest shortening and thickening of the upper oceanic crust discontinuities in the seafloor west of the prism provide evidence for mass wasting in the form of slump blocks and debris fans the southernmost profile extends 75 km west of the prism imaging numerous faults that offset both the gorda basin oceanic crust and overlying sediments these high angle faults bounding basement highs are interpreted as strike slip faults reactivating structures originally formed at the spreading ridge northeast or northwest trending strike slip faults within the basin are consistent with published focal mechanism solutions and are likely caused by north south gorda pacific plate convergence

coseismic slip and afterslip of the great mw 9 15 sumatra andaman earthquake of 2004 we determine coseismic and the first month postseismic deformation associated with the sumatra andaman earthquake of 26 december 2004 from near field global positioning system gps surveys in northwestern sumatra and along the nicobar andaman islands continuous and campaign gps measurements from thailand and malaysia and in situ and remotely sensed observations of the vertical motion of coral reefs the coseismic model shows that the sunda subduction mega thrust ruptured over a distance of about 1500 km and a width of less than 150 km releasing a total moment of 6 7 7 0 10 22 n m equivalent to a magnitude m w 9 15 the latitudinal distribution of released moment in our model has three distinct peaks at about 4 n 7 n and 9 n which compares well to the latitudinal variations seen in the seismic inversion and of the analysis of radiated t waves our coseismic model is also consistent with interpretation of normal modes and with the amplitude of very long period surface waves the tsunami predicted from this model fits rela tively well the altimetric measurements made by the jason and topex satellites neither slow nor delayed slip is needed to explain the normal modes and the tsunami wave the near field geodetic data that encompass both coseismic deformation and up to 40 days of postseismic deformation require that slip must have continued on the plate interface after the 500 sec long seismic rupture the postseismic geodetic moment of about 2 4 10 22 nm mw 8 8 is equal to about 30 5 of the coseismic moment release evolution of postseismic deformation is consistent with rate strengthening frictional afterslip online material summary of geodetic data used in this study

controls on overpressure evolution during the gravitational collapse of the amazon deep sea fan abstract the amazon fan provides a natural laboratory to study the generation of overpressure due to rapid late cenozoic burial that has resulted in gravitational collapse above shale detachments here we examine collapse systems for the first time using the techniques of petroleum systems analysis we propose an integrated methodology based on numerical modeling constrained by the structural restoration of a seismic profile across the southwestern fan the results provide information on the evolution of pore pressure and temperature and their implications for the operation of the detachment and overlying extensional and compressional faults during the deposition of up to 6 km of sediment over the last 8 ma the modelled thermal history implies that fluid release by smectite to illite transformation has taken place within the thickening sedimentary succession but has not significantly contributed to pore pressures along the detachment modeling of hydrocarbon generation and migration from source rocks beneath the fan indicates gas accumulated in successions at depths of 102 103 m beneath the detachment without influencing pore pressures along it in contrast model results indicate that overpressures have varied in response to disequilibrium compaction fault activity within the collapse system took place during phases of higher sedimentation rates and ceased from 5 5 to 3 7 ma when sediment supply to the se fan decreased from 2 ma renewed sediment flux and shelf slope progradation led to a basinward migration both of overpressure along the detachment and of fault activity above it we conclude that gravity tectonics in the amazon fan over the last 8 ma have been mainly controlled by overpressures due to disequilibrium compaction with secondary contributions from clay mineral transformation present day pressure conditions show that the southeastern amazon fan is not at equilibrium and gravity driven deformation could occur at any time

the madden julian oscillation and the angular momentum balance in a barotropic ocean model surface wind stresses associated with the madden julian oscillation transfer angular momentum between the atmosphere and the underlying ocean the resulting anomalies in oceanic angular momentum related to either changes in zonal currents or latitudinal redistribution of mass and the torques coupling the ocean and the solid earth are examined for simple zonal stress patterns representing idealized 40 to 50 day fluctuations of pacific winds because the contribution of baroclinic variability to the angular momentum budget is in general small when vertically integrated quantities are considered ponte 1990 only the barotropic response is studied numerical solutions of the shallow water equations with linear bottom friction are found for a constant depth basin centered on the equator and extending over 140 in longitude the angular momentum exchanges between the ocean and solid earth in the model involve the continental torque i e body forces on eastern and western walls due to coastal sea level anomalies and the bottom friction torque solutions yield a dominant balance between the wind and continental torques with the bottom friction torque being negligible the amplitude of the total torque acting on the ocean and the corresponding angular momentum fluctuations are very small compared with the amplitude of the applied wind torque the ocean delivers to the solid earth with no significant time delays the angular momentum anomalies received from the atmosphere in general sea level anomalies are nearly uniform along the eastern boundary and the respective meridionally integrated torque is much larger than the one exerted on the western wall solutions also suggest that coastal barotropic sea level fluctuations as small as 0 5 1 5 cm are sufficient to cause variations in the length of day comparable to observed values in the 40 to 50 day band

noncohesive critical coulomb wedges an exact solution active fold and thrust belts or submarine accretionary complexes can be modeled as critically tapered wedges of material on the verge of coulomb failure everywhere overlying a basal decollement where frictional sliding is occurring ignoring cohesion the four strength parameters needed to describe a critical coulomb wedge are its internal and basal coefficients of friction and b and its internal and basal hubbert rubey fluid pressure ratios and b an exact relation between surface slope and basal dip of a noncohesive critical wedge with uniform properties is derived the state of stress within such a wedge has the same orientation everywhere and is constant if is and vice versa a coefficient of internal friction 1 1 is consistent with the known surface slope basal dip and pore fluid pressures in the active fold and thrust belt of western taiwan assuming that byerlee s law b 0 85 is valid on the base the wide variety of tectonic styles observed to occur along convergent margins including subduction erosion active accretion subduction without accretion and even extension and normal faulting may be controlled by relatively small spatial or temporal variations in either b or b

contrasting decollement and prism properties over the sumatra 2004 2005 earthquake rupture boundary styles of subduction zone deformation and earthquake rupture dynamics are strongly linked jointly influencing hazard potential seismic reflection profiles across the trench west of sumatra indonesia show differences across the boundary between the major 2004 and 2005 plate interface earthquakes which exhibited contrasting earthquake rupture and tsunami generation in the southern part of the 2004 rupture we interpret a negative polarity sedimentary reflector 500 meters above the subducting oceanic basement as the seaward extension of the plate interface this predecollement reflector corresponds to unusual prism structure morphology and seismogenic behavior that are absent along the 2005 rupture zone although margins like the 2004 rupture zone are globally rare our results suggest that sediment properties influence earthquake rupture tsunami hazard and prism development at subducting plate boundaries

spatial correlation of interseismic coupling and coseismic rupture extent of the 2011 mw 9 0 tohoku oki earthquake 1 imaging the extent to which the rupture areas of great earthquakes coincide with regions of pre seismic interplate coupling is central to understanding patterns of strain accumulation and release through the earthquake cycle both geodetic and seismic estimates of the coseismic rupture extent for the march 11 2011 mw 8 9 9 0 earthquake tohoku oki earthquake may be spatially correlated 0 26 0 05 to 0 82 0 05 with a region estimated to be partially to fully coupled in the interseismic period preceding the earthquake though there is substantial variation in the estimated distribution and magnitude of coseismic slip the 400 km long region estimated to have slipped 4 m corresponds to an area of the subduction zone interface that was coupled at 30 of long term plate convergence rate with peak slip near a region coupled 80 the northern termination of rupture is collocated with a region of relatively low 20 interseismic coupling near the epicenter of the 1994 mw 7 6 sanriku oki earthquake and near a region of potential long term low coupling or ongoing slow slip slip on the subduction interface beneath the coastline 40 50 km depth is best constrained by the land based gps data and least constrained on the shallowest portion of the plate interface due to the 230 km distance between geodetic observations and the japan trench

on the role of isostasy in the evolution of normal fault systems the footwalls of west dipping normal faults that separate the west central colorado plateau from the basin and range province record at least 5 7 km and perhaps as much as 15 20 km of west side up neogene uplift with an axis just 10 20 km west of undeformed plateau strata the uplift is expressed as folding and steep faulting in pre tertiary cratonic and disconformably overlying neogene strata forming a basement cored anticline and coincident topographic high on the western margin of the plateau the authors interpret the uplift as a nonelastic response of the crust to buoyancy forces accompanying the tectonic denudation of the plateau margin profound isostatically driven deformation of the footwalls of major normal faults may be common in extensional terrains calling into question several assumptions fundamental to existing models of the evolution of normal fault systems

megathrust friction determined from mechanical analysis of the forearc in the maule earthquake area the seismogenic potential of a given fault depends essentially on its frictional properties and on the mechanical properties of the medium determining the spatio temporal variations of frictional properties is therefore a key issue in seismotectonics this study aims to characterize the friction on the south america megathrust in the 2010 mw 8 8 maule earthquake area from mechanical analysis of the forearc structure and morphology based on the critical taper theory we first show that the rupture area of the maule earthquake also shown to be locked in the interseismic period coincides with the stable part of the wedge in the surrounding area the wedge is critical a finding consistent with various evidence for active deformation there this is in particular true for the arauco peninsula area which seems to have arrested the maule earthquake s rupture to the south this observation lends support to the view that the seismic rupture is inhibited when propagating beneath a critical area the geometry of the critical portion of the wedge suggests a standard internal friction int 0 7 0 13 and a hydrostatic pore pressure within the wedge the effective friction beneath the critical outer wedge is estimated to be eff b 0 27 0 03 this could be related to intrinsically low friction minerals clay or high pore pressure along the megathrust we next use the limit analysis approach to constrain the variation of the effective friction along the megathrust based on the location and geometry of internal faulting within the forearc a low effective friction is found within the rupture area eff b 0 14 to explain the reactivation of thrust fault such as the santa maria updip of the coseismic rupture or the activation of normal splay faults downdip of the rupture area the low effective friction found there could reflect strong dynamic weakening

rupture to the trench frictional properties and fracture energy of incoming sediments at the cascadia subduction zone abstract the mechanical properties of sediment inputs to subduction zones are important for understanding rupture propagation through the accretionary prism during megathrust earthquakes clay minerals strongly influence the frictional behavior of fault gouges and the clay content of subduction input materials varies through a stratigraphic section as well as for subduction margins globally to establish the frictional properties of the shallow cascadia subduction zone and place the results in a global context we conducted high velocity rotary shear experiments on odp core samples retrieved from cascadia input sediments 35 45 clay and a suite of individual clay species we compared our results to a compilation of published high velocity experiments conducted on samples of wet gouge dry gouge and intact rock for each sample type three trends were identified with increasing normal stress 1 the stress drop p s s increases linearly 2 the characteristic thermal weakening distance d t h decreases as a power law function except for wet clay rich gouges and 3 the fracture energy w b shows no dependence however fracture energy does vary with sample type clay rich gouges under wet conditions have the lowest fracture energy and fracture energy for both dry and wet gouges is at least an order of magnitude lower than estimates from intact rocks therefore when clay rich lithologies are present they may minimize spatial variations in frictional behavior allowing earthquakes to propagate to the trench for cascadia input sediments there is little variation in the fracture energy between lithologies but the fracture energy of cascadia sediments is around an order of magnitude higher than input sediments from other subduction margins the high fracture energy of cascadia sediments relative to other subduction margins may inhibit large amounts of shallow earthquake slip and dynamic overshoot

comparison of fold thrust belts driven by plate convergence and gravitational failure abstract deepwater fold thrust belts ftbs are predominantly formed by the deformation of sedimentary sequences as a result of subduction of oceanic plates at active margins gravitational failure at passive margins or a combination of these two driving mechanisms at both types of margin a key question is is the ftb driven by gravitational failure basically the same as the ftb driven by plate convergence or are there fundamental differences to address this we reviewed ftb examples from different tectonic settings end members and hybrid systems in terms of geometry structure deformation shortening rate and tectonic process the energy source in gravity driven systems lies within the sedimentary material itself that is being deformed by updip extension and downdip contraction whereas in the plate convergence driven systems it lies outside the local sediment pile and within the broader undeformed crust and lithosphere the energy in gravity driven systems is resupplied by sediment input from large river deltas and therefore deformation tends to be episodic linked to episodes of major sediment input whereas in a system driven by plate motions the energy is resupplied by movement of a boundary upon which force is acting and tends to be stable and less episodic therefore the sedimentation plays an important role in the gravity driven deformation but a less important role in the plate convergence driven deformation in gravity driven systems it promotes upward propagation of normal faulting at upslope by increasing maximum principal stress but hinders upward propagation of thrusting i e mostly buried structures by increasing minimum principal stress at downslope while in plate convergence systems thrusts and folds tend to deform seafloor as a result of large compression with less effect from sedimentation despite the varying basal strength the observed thrust faults in both systems are mostly basinward verging in slight contrast to the theoretical model predication with homogeneous material implicating the control of likely variability of stratigraphic strength on thrust vergence the shortening rate across plate convergence driven systems is high several tens mm yr and continuous on a long timescale whereas across the gravity driven fold thrust belts it is generally slow several mm yr and more variable or even catastrophic through time despite overall seaward younging of thrusting fault activity differs slightly among different systems focused activity at the frontal structures of ftbs is more common at the plate convergence system while activity focused in the rear to middle of ftbs is more common in the gravity driven system plate convergence driven system is primarily limited by rate of plate motion i e the rate at which the plate is fed into the ftb whereas in gravity driven system the movement is limited resisted by the strength of sediments and detachment

does permanent extensional deformation in lower forearc slopes indicate shallow plate boundary rupture abstract seismic rupture of the shallow plate boundary can result in large tsunamis with tragic socio economic consequences as exemplified by the 2011 tohoku oki earthquake to better understand the processes involved in shallow earthquake rupture in seismic gaps where megathrust earthquakes are expected and investigate the tsunami hazard it is important to assess whether the region experienced shallow earthquake rupture in the past however there are currently no established methods to elucidate whether a margin segment has repeatedly experienced shallow earthquake rupture with the exception of mechanical studies on subducted fault rocks here we combine new swath bathymetric data unpublished seismic reflection images and inter seismic seismicity to evaluate if the pattern of permanent deformation in the marine forearc of the northern chile seismic gap allows inferences on past earthquake behavior while the tectonic configuration of the middle and upper slope remains similar over hundreds of kilometers along the north chilean margin we document permanent extensional deformation of the lower slope localized to the region 20 8 s 22 s critical taper analyses the comparison of permanent deformation to inter seismic seismicity and plate coupling models as well as recent observations from other subduction zones including the area that ruptured during the 2011 tohoku oki earthquake suggest that the normal faults at the lower slope may have resulted from shallow possibly near trench breaking earthquake ruptures in the past in the adjacent margin segments the 1995 antofagasta 2007 tocopilla and 2014 iquique earthquakes were limited to the middle and upper slope and the terrestrial forearc and so are upper plate normal faults our findings suggest a seismo tectonic segmentation of the north chilean margin that seems to be stable over multiple earthquake cycles if our interpretations are correct they indicate a high tsunami hazard posed by the yet un ruptured southern segment of the seismic gap

the 25 october 2010 mentawai tsunami earthquake mw 7 8 and the tsunami hazard presented by shallow megathrust ruptures the 25 october 2010 mentawai indonesia earthquake m w 7 8 ruptured the shallow portion of the subduction zone seaward of the mentawai islands off shore of sumatra generating 3 to 9 m tsunami run up along southwestern coasts of the pagai islands that took at least 431 lives analyses of teleseismic p sh and rayleigh waves for finite fault source rupture characteristics indicate 90 s rupture duration with a low rupture velocity of 1 5 km s on the 10 dipping megathrust with total slip of 2 4 m over an 100 km long source region the seismic moment scaled energy release is 1 4 10 6 lower than 2 4 10 6 found for the 17 july 2006 java tsunami earthquake m w 7 8 the mentawai event ruptured up dip of the slip region of the 12 september 2007 kepulauan earthquake m w 7 9 and together with the 4 january 1907 m 7 6 tsunami earthquake located seaward of simeulue island to the northwest along the arc demonstrates the significant tsunami generation potential for shallow megathrust ruptures in regions up dip of great underthrusting events in indonesia and elsewhere

new tectonic divisions of the grenville province southeast canadian shield on the basis of geological geophysical and geochronological data the grenville province has been divided into three first order longitudinal belts the parautochthonous belt pb allochthonous polycyclic belt apb and allochthonous monocyclic belt amb these are set apart by three first order tectonic boundaries the grenville front gf allochthon boundary thrust abt and monocyclic belt boundary zone mbbz the belts are subdivided into terranes based on internal lithological character the gf separates the archean to proterozoic foreland northwest of the orogen from reworked equivalents to the southeast continuous at the scale of the orogen its main characteristic is that of a crustal scale contraction fault the pb although less clearly identified along the length of the orogen in most places represents upgraded and tectonically reworked rocks of the adjacent foreland the boundary between the pb and the apb to the southeast the abt is most clearly delineated in the eastern half of the province it is the locus of major crustal delamination along which high grade mostly middle proterozoic polycyclic terranes were tectonically transported northwest toward and onto the pb the amb comprises two separate areas underlain by the wakeham supergroup and what is currently known as the grenville supergroup respectively its basal contact the mbbz is a decollement zone of variable kinematic significance between older polycyclic rocks and tectonically overlying monocyclic rocks this first order zonation implies a tectonic polarity to the grenville province superimposed on which are second order features evident from contrasting tectonic styles and radiometric ages these characteristics are consistent with a diachronous or oblique collisional model for the grenville orogen

stable creeping fault segments can become destructive as a result of dynamic weakening faults in earth s crust accommodate slow relative motion between tectonic plates through either similarly slow slip or fast seismic wave producing rupture events perceived as earthquakes these types of behaviour are often assumed to be separated in space and to occur on two different types of fault segment one with stable rate strengthening friction and the other with rate weakening friction that leads to stick slip the 2011 tohoku oki earthquake with moment magnitude m w 9 0 challenged such assumptions by accumulating its largest seismic slip in the area that had been assumed to be creeping here we propose a model in which stable rate strengthening behaviour at low slip rates is combined with coseismic weakening due to rapid shear heating of pore fluids allowing unstable slip to occur in segments that can creep between events the model parameters are based on laboratory measurements on samples from the fault of the m w 7 6 1999 chi chi earthquake the long term slip behaviour of the model which we examine using a unique numerical approach that includes all wave effects reproduces and explains a number of both long term and coseismic observations some of them seemingly contradictory about the faults at which the tohoku oki and chi chi earthquakes occurred including there being more high frequency radiation from areas of lower slip the largest seismic slip in the tohoku oki earthquake having occurred in a potentially creeping segment the overall pattern of previous events in the area and the complexity of the tohoku oki rupture the implication that earthquake rupture may break through large portions of creeping segments which are at present considered to be barriers requires a re evaluation of seismic hazard in many areas

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

partial rupture of a locked patch of the sumatra megathrust during the 2007 earthquake sequence the great sumatra andaman earthquake and tsunami of 2004 was a dramatic reminder of the importance of understanding the seismic and tsunami hazards of subduction zones 1 2 3 4 in march 2005 the sunda megathrust ruptured again producing an event 5 of moment magnitude mw 8 6 south of the 2004 rupture area which was the site of a similar event in 1861 ref 6 concern was then focused on the mentawai area where large earthquakes had occurred in 1797 mw 8 8 and 1833 mw 9 0 6 7 two earthquakes one of mw 8 4 and twelve hours later one of mw 7 9 indeed occurred there on 12 september 2007 here we show that these earthquakes ruptured only a fraction of the area ruptured in 1833 and consist of distinct asperities within a patch of the megathrust that had remained locked in the interseismic period this indicates that the same portion of a megathrust can rupture in different patterns depending on whether asperities break as isolated seismic events or cooperate to produce a larger rupture this variability probably arises from the influence of non permanent barriers zones with locally lower pre stress due to the past earthquakes the stress state of the portion of the sunda megathrust that had ruptured in 1833 and 1797 was probably not adequate for the development of a single large rupture in 2007 the moment released in 2007 amounts to only a fraction both of that released in 1833 and of the deficit of moment that had accumulated as a result of interseismic strain since 1833 the potential for a large megathrust event in the mentawai area thus remains large

evolution of deep crustal normal faults constraints from thermobarometry in the grenville orogen ontario canada abstract the robertson lake shear zone rlsz is a low angle 30 dip plastic to brittle extensional shear zone located in the southeastern grenville orogen ontario canada thermobarometric data have been collected to evaluate the effects of extension on regional metamorphic field gradients upper greenschist facies metamorphic conditions are preserved in the footwall and upper amphibolite facies metamorphic conditions in the hanging wall however steep gradients in metamorphic pressure are present adjacent to the robertson lake shear zone in the footwall pressures increase from 550 mpa to 800 mpa within 8 km of the rlsz and in the hanging wall pressures decrease from 900 mpa to 500 mpa within 12 km of the rlsz the pressure gradients are interpreted to be a product of isostatic flexural rotations in response to extension along the zone the degree of rotation indicated by the baric gradients in the hanging wall and footwall are compatible and require an initial shear zone orientation of 60 90 which suggests reactivation of an initially high angle terrane boundary previous studies provide constraints of a depth extent of 40 km for the rlsz and crustal thickness of 60 km during extension thus extensional rotations were accommodated by decoupling and regional scale flow in the lower third of the crust

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

deep crustal extensional faulting in the ivrea zone of northern italy abstract the ivrea zone in northern italy consists of amphibolite and granulite facies metasedimentary metabasic and ultrabasic rocks and is thought to represent a section through the deep continental crust detailed mapping of the high grade part of this zone has revealed a series of high temperature shear zones which after the effects of later low temperature faulting and folding have been removed appears to accommodate extension of the sequence the shear zones formed at temperatures similar to those at which thermal equilibration occurred in the host rocks and locally show evidence for development under prograde conditions these observations fit well with thermal effects calculated from a simple heat flow model for extensional faulting in this model the effects of shear heating and displacement of cool hanging wall rocks against hotter footwall rocks are calculated these observations indicate that faulting as a mode of failure may be important during extension of the lower continental crust especially when basic rocks are the dominant components of the rock pile a restored section through the extended lower crust as observed in the ivrea zone is presented and the extent to which the features observed are able to explain seismic reflectivity of the lower crust is discussed

controls on spatial and temporal evolution of prism faulting and relationships to plate boundary slip offshore north central sumatra across and along strike variations in the morphology and structure of the north central sumatran forearc 1 5 s to 1 n are broadly coincident with subducting plate topography and an earthquake segment boundary zone below the batu islands we present a detailed interpretation of multichannel streamer seismic reflection data collected offshore north central sumatra to better characterize the morphological and structural variations provide insight into fault development and relate structure to plate boundary rupture and seismicity patterns we interpret two relatively continuous major fault structures that divide the prism into three strike parallel belts that can be characterized by the relative fault slip rates along major and minor fault structures the midslope break fault s and upper slope bounding fault s are major potentially out of sequence thrusts accommodating a significant component of the compressional strain we propose that the upper slope bounding fault represents the more mature end member of an evolving fault system landward vergent structures are associated with a relatively thin sedimentary section near the deformation front in the center of our study area and suggest a potentially weak shallow plate boundary associated with the subducting wharton fossil ridge

relations between normal fault geometry tilting and vertical motions in extensional terrains an example from the southern gulf of suez abstract earthquakes and data from subsurface oil exploration suggest that large active normal faults in the southern gulf of suez are approximately planar with dips of 30 40 from the surface to around 10 km depth these faults and the blocks they bound appear to rotate about a horizontal axis as they move causing tilting this tilting is seen both in young vertical movements of the coastline such as raised beaches and marshlands and in the distribution of middle miocene marine rocks which are uplifted to elevations of 400 500 m in footwalls of faults and found at depths of around 3500 m in the adjacent grabens the absolute amplitude of the observed vertical motions can be approximately modelled by planar rotating normal faults that impose a saw tooth topography on a regional subsidence caused by crustal and lithospheric thinning the observations required for this simple model are the present day fault dip the amount of tilting and the width of the rotating blocks the virtues of the model are its simplicity and its compatibility with our knowledge of how large active normal faults move elsewhere on the continents

strength of stick slip and creeping subduction megathrusts from heat flow observations subduction faults called megathrusts can generate large and hazardous earthquakes the mode of slip and seismicity of a megathrust is controlled by the structural complexity of the fault zone however the relative strength of a megathrust based on the mode of slip is far from clear the fault strength affects surface heat flow by frictional heating during slip we model heat flow data for a number of subduction zones to determine the fault strength we find that smooth megathrusts that produce great earthquakes tend to be weaker and therefore dissipate less heat than geometrically rough megathrusts that slip mainly by creeping

relationship link between landward vergence in accretionary prisms and tsunami generation the very large slip up to the subduction front of the 2011 tohoku oki japan earthquake has challenged our classic view of the megathrust undergoing only aseismic slip at shallow depth furthermore the enhancement of tsunamis during frontal rupturing has increased concern about tsunami risks recent seismic reflection images from the sumatra subduction zone show frontal landward vergent thrusts in the accretionary prism in the area of supposed shallow ruptures and enhanced tsunamis using mechanical analysis we here show that sudden and successive decreases of the effective friction along the megathrust are required to form landward vergent frontal thrusts these decreases are most likely caused by dynamic weakening mechanisms such as thermal pressurization of the pore fluids related to the propagation of earthquakes to the seafloor therefore landward vergence in accretionary prisms is indicative of past seafloor frontal ruptures and consequent tsunamis the presence of landward verging structures in the cascadia and sumatra accretionary prisms might indicate future frontal rupture of the shallowest portion of the megathrust resulting in large tsunamis

seismic history and seismotectonics of the sunda arc historic records of the last 300 years reveal two great interplate earthquakes 1833 mw 8 1861 mw 8 8 which ruptured major segments of the sumatra fore arc in western indonesia a significant percentage of interplate slip along this portion of the plate boundary appears to occur seismically the ends of these rupture zones are coincident with clusters of large ms 7 and moderate 6 ms 7 shocks and with heterogeneities in the plate interface as inferred from geologic and geophysical data the northern extent of the 1833 rupture zone is coincident with a group of moderate and large earthquakes a structural arch in the fore arc and a fracture zone on the subducted plate the southern portion is coincident with a very prominent cluster and the along strike extension of a sediment filled trough on the subducted plate prior to this study sumatra was characterized as relatively aseismic as inferred from the lack of great earthquakes in the instrumental record of this century java and the lesser sunda islands had major ms 6 earthquakes in the historic record but none are of the same magnitude as the great events near sumatra there is no clear evidence that shocks in the java fore arc were interplate events yet geophysical data indicate recent uplift of the trench slope break and a change in strike of the trench axis where the northernmost flank of the roo rise a prominent bathymetrie high appears to be interacting with the fore arc this feature was formed at a ridge crest and is more buoyant and thus more difficult to subduct than the surrounding seafloor there is no increase in seismicity associated with this tectonism however that may indicate a lack of contact between the crystalline portions of the plates at this time we infer that a majority of slip on the plate interface near java occurs aseismically the variation in occurrence of great interplate earthquakes along the sunda arc may be interpreted in terms of a model that attributes variation in interplate coupling to the age of the subducted lithosphere great interplate earthquakes occur near sumatra where the age of the youngest crust is 46 ma the characteristics of the sunda arc and analogies with pacific arcs imply that the entire length of sumatra has the potential to produce great thrust earthquakes its seismic potential should be considered high as there has been no recurrence of the great events of 1833 and 1861 in contrast no such events have been reported off java and the lesser sunda islands where the oldest crust is 152 ma the plate interface near java and the lesser sunda islands should be considered to have low seismic potential

calculation and application of clinopyroxene garnet plagioclase quartz geobarometers recently published thermodynamic and experimental data in a variety of chemical systems have been evaluated to derive gibbs free energies for hedenbergite and pyrope these were used to calculate the geobarometric equilibria

time and size of a giant earthquake in cascadia inferred from japanese tsunami records of january 1700 geological evidence shows that great earthquakes have occurred in the recent prehistoric past in the cascadia subduction zone off the pacific coast of north america the most recent event or series of events is dated at about 300 years ago1 4 but the precise date and magnitude have not been determined geological investigations have not been able to distinguish a single giant earthquake from a series of great earthquakes occurring over a timespan of a decade or two4 although this information is important for the assessment of future hazard5 we have found several tsunami records in japan from ad 1700 with no indication of a local cause historical earthquake records and palaeoseismic evidence indicate the absence of a large earthquake in 1700 in south america alaska or kamchatka leaving cascadia as the most likely source of this tsunami the estimated time of the earthquake is the evening about 21 00 local time of 26 january 1700 the magnitude is estimated as 9 from the tsunami heights in which case the earthquake ruptured the entire length of the cascadia subduction zone2 these estimates are consistent with native american legends that an earthquake occurred on a winter night6

seismic evidence for broken oceanic crust in the 2004 sumatra earthquake epicentral region the 2004 sumatra earthquake was one of the largest events to occur in a subduction zone in the past 50 years seismic reflection data for this subduction zone reveal thrust faults cutting across the entire oceanic crust this observation coupled with the hypocentres of aftershocks suggests that the megathrust the interface between the indo australian plate and the sunda plate currently lies in the oceanic mantle

updip rupture of the 2004 sumatra earthquake extended by thick indurated sediments sediments accumulated in a subduction trench are usually unconsolidated and impede the updip propagation of fault rupture during an earthquake seismic images of the southern sumatra andaman trench reveal blocks of consolidated sediment that may have enabled fault rupture in 2004 to propagate up fault dip thus further seaward increasing the tsunami magnitude

non coulomb wedges wrong way thrusting and natural hazards in cascadia landward vergence in accretionary wedges is an uncommon phenomenon not readily explained by classical mohr coulomb critical wedge theory predominantly landward vergent thrust faults are observed along the cascadia convergent margin from 45 n to 48 n we present depth migrated multichannel seismic images of the internal structure of the accretionary wedge offshore washington collected during the orwell project in 1996 these reveal a high p wave velocity 4 km s basal layer that thickens landward and serves as a decollement for a series of overlying landward vergent thrust faults analog modeling using a ductile basal layer consisting of silicone putty produces an array of trenchward propagating landward vergent thrusts and offers a plausible mechanical model for the evolution of these structures the rheological properties of a basal calcareous mudstone layer offshore cascadia are discussed in relation to the mechanics of landward vergence and to rapid loading due to the prograding nitinat and astoria deep sea fans a viscoelastic layer beneath the accretionary wedge is considered to be capable of rupturing during great interplate earthquakes and thus represents an increased risk for both the maximum size of such an event and for the generation of tsunamis

links between sediment consolidation and cascadia megathrust slip behaviour at sediment rich subduction zones megathrust slip behaviour and forearc deformation are tightly linked to the physical properties and in situ stresses within underthrust and accreted sediments yet the role of sediment consolidation at the onset of subduction in controlling the downdip evolution and along strike variation in megathrust fault properties and accretionary wedge structure is poorly known here we use controlled source seismic data combined with ocean drilling data to constrain the sediment consolidation and in situ stress state near the deformation front of the cascadia subduction zone offshore washington where the megathrust is inferred to be strongly locked we find over consolidated sediments near the deformation front that are incorporated into a strong outer wedge with little sediment subducted these conditions are favourable for strain accumulation on the megathrust and potential earthquake rupture close to the trench in contrast offshore central oregon a thick under consolidated sediment sequence is subducting and is probably associated with elevated pore fluid pressures on the megathrust in a region where reduced locking is inferred our results suggest that the consolidation state of the sediments near the deformation front is a key factor contributing to megathrust slip behaviour and its along strike variation and it may also have a significant role in the deformation style of the accretionary wedge consolidated sediments in the cascadia subduction zone may create conditions favourable for megathrust earthquake ruptures over long distances and close to the trench according to analyses of seismic velocity of sediments from the region less consolidated sediments instead may promote aseismic slip of the plate boundary