mechanics of low stress forearcs nankai and cascadia the nankai and cascadia subduction zones have low stress forearcs where the margin normal horizontal compressive stress is similar to or less than the vertical stress except in the accretionary prisms in the absence of active back arc spreading the forearc margin normal stress is controlled mainly by the total shear force along the subduction fault or the plate coupling force and the gravitational force the plate coupling force creates compression and the gravitational force in the presence of margin topography creates lateral tension we give a simple expression that relates the forearc stress to topography and average fault strength with the latter represented by an effective coefficient of friction we also use a finite element model of two converging plates in frictional contact to model the forearc stresses the analyses indicate that a low stress forearc requires not only the subduction fault to be very weak but also the mechanically coupled fault area to be narrow in the downdip direction and shallow in depth for nankai and cascadia where the coupled fault areas are narrow and shallow because of the young and hot subducting plates is about 0 05 or less and in any case less than 0 09 the low fault strength is probably not a unique property of low stress forearcs since the same low values combined with wider and deeper coupled areas can be shown to create large margin normal compression in addition there does not seem to be any relation between subduction earthquake magnitudes and plate coupling forces great earthquakes occur at subduction zones having low stress forearcs such as nankai and cascadia

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

mechanics of fold and thrust belts and accretionary wedges cohesive coulomb theory a critically tapered fold and thrust belt or submarine accretionary wedge is one that is on the verge of coulomb failure everywhere including its base where frictional sliding along a decollement is assumed to be occurring cohesion within a wedge can add significantly to the overall strength near the toe the effect of this is to decrease the near toe taper leading to a critical topographic profile that is concave upward if the decollement is planar we obtain an approximate self consistent solution for the state of stress within a thin skinned cohesive critical coulomb wedge and determine the relationship between the wedge taper and its strength and basal friction the theory is then applied to the presently deforming fold and thrust belt of western taiwan fitting of theoretical critical wedge shapes to topographic profiles and measurements of the step up angles of thrust faults from the basal decollement are used to constrain the taiwan wedge strength parameters an attractive assertion fully consistent with all the observations is that the mechanics of fold and thrust belts and accretionary wedges is governed by normal frictional and fracture strengths of rocks measured in the laboratory in particular if byerlee s law b 0 85 is adopted as the coefficient of sliding friction on the base we find a coefficient of internal friction 0 9 1 0 in the wedge and a wedge cohesion so 5 20 mpa other solutions having strengths and ambient stresses up to 4 times lower than this can also however satisfy the data

constraints on the stress state of the san andreas fault with analysis based on core and cuttings from san andreas fault observatory at depth safod drilling phases 1 and 2 stress state constraints on the saf

stratigraphic signatures of forearc basin formation mechanisms tectonic deformation of accretionary wedges is often interpreted using stratigraphic patterns observed in forearc basins however similar stratigraphic patterns could evolve from different deformational processes making it difficult to uniquely reconcile stratigraphic patterns and the geodynamic history of natural accretionary wedges therefore it is important to test the dynamic consistency of interpreted deformation histories one approach is to compare synthetic stratigraphy of forearc basins generated in numerical accretionary wedge models to the stratal patterns observed in natural forearcs we present a simple method to simulate synthetic stratigraphy in numerical accretionary wedge models comparable to the stratigraphic patterns observed in seismic reflection data as calibration we use reflection seismic and borehole data for the kumano forearc basin in nankai we observe that the stratigraphy in retro forearc basins remains predominantly undisturbed and unaffected by wedge deformation sediment stabilized wedge top basins in wedges with trenchward surface slope typically form landward of an active out of sequence thrust leading to a landward tilting of the basin these stratigraphic characteristics are diagnostic of the forearc basin forming mechanism we also infer that sedimentation in the trench can potentially influence the generation reactivation of megasplay fault activity in the nankai accretionary wedge activity on the megasplay fault helps create accommodation space landward to it that leads to the formation of kumano forearc basin

fault zone structure and weakening processes in basin scale reverse faults the moonlight fault zone south island new zealand abstract the 200 km long moonlight fault zone mfz in southern new zealand was an oligocene basin bounding normal fault zone that reactivated in the miocene as a high angle reverse fault present dip angle 65 75 regional exhumation in the last c 5 ma has resulted in deep exposures of the mfz that present an opportunity to study the structure and deformation processes that were active in a basin scale reverse fault at basement depths syn rift sediments are preserved only as thin fault bound slivers the hanging wall and footwall of the mfz are mainly greenschist facies quartzofeldspathic schists that have a steeply dipping 55 75 foliation subparallel to the main fault trace in more fissile lithologies e g greyschists hanging wall deformation occurred by the development of foliation parallel breccia layers up to a few centimetres thick greyschists in the footwall deformed mainly by folding and formation of tabular foliation parallel breccias up to 1 m wide where the hanging wall contains more competent lithologies e g greenschist facies metabasite it is laced with networks of pseudotachylyte that formed parallel to the host rock foliation in a damage zone extending up to 500 m from the main fault trace the fault core contains an up to 20 m thick sequence of breccias cataclasites and foliated cataclasites preserving evidence for the progressive development of interconnected networks of partly authigenic chlorite and muscovite deformation in the fault core occurred by cataclasis of quartz and albite frictional sliding of chlorite and muscovite grains and dissolution precipitation combined with published friction and permeability data our observations suggest that 1 host rock lithology and anisotropy were the primary controls on the structure of the mfz at basement depths and 2 high angle reverse slip was facilitated by the low frictional strength of fault core materials restriction of pseudotachylyte networks to the hanging wall of the mfz further suggests that the wide phyllosilicate rich fault core acted as an efficient hydrological barrier resulting in a relatively hydrous footwall and fault core but a relatively dry hanging wall

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

the coulomb critical taper theory applied to gravitational instabilities the critical coulomb wedge theory has been widely applied to the structural evolution of accretionary wedges and fold and thrust belts but it also predicts the spreading of a wedge under gravity this solution may be applied to the formation of gravitational spreading and gliding along passive margins where elevated pore fluid pressure is common following the initial hypotheses of the theory we provide an alternative expression of the exact solution for a noncohesive wedge better suited to slope instabilities our formulation allows a direct calculation of the fluid pressure required for the system to deform and predicts two kinds of gravitational deformation shallow slumping and or deep gravitational spreading rooting on the basal detachment to verify the predictions of the model we performed scaled experiments with pore pressure under conditions close to the critical taper hypotheses the good agreement between our experimental results and the theory confirms the applicability of the critical taper model to the formation of gravitational structures in sedimentary wedges subjected to pore fluid pressure

smectite diagenesis pore water freshening and fluid flow at the toe of the nankai wedge the presence of low chloride fluids in the lowermost sediments drilled at ocean drilling program site 808 at the nankai accretionary wedge has been considered as prime evidence for long distance lateral fluid flow from depth here we re evaluate the potential role of in situ reaction of smectite s to illite i in the genesis of this low chloride anomaly this reaction is known to be occurring at site 808 with both the s content and s to i ratio in the mixed layer clays decreasing substantially with depth we show that the bulk of the chloride anomaly can generate by in situ clay dehydration particularly if pre reaction smectite abundances ai approach 10 15 of the bulk sediment the ai values however are not well constrained at ai values 10 15 an additional source of low cl fluid centered close to the decollement could be required thus there remains the important possibility that the observed low cl anomaly is a compound effect of both lateral flow and in situ smectite dehydration

patterns of mineral transformations in clay gouge with examples from low angle normal fault rocks in the western usa abstract neoformed minerals in shallow fault rocks are increasingly recognized as key to the behavior of faults in the elasto frictional regime but neither the conditions nor the processes which wall rock is transformed into clay minerals are well understood yet understanding of these mineral transformations is required to predict the mechanical and seismogenic behavior of faults we therefore present a systematic study of clay gouge mineralogy from 30 outcrops of 17 low angle normal faults lanf s in the american cordillera to demonstrate the range and type of clay transformations in natural fault gouges the sampled faults juxtapose a wide and representative range of wall rock types including sedimentary metamorphic and igneous rocks under shallow crustal conditions clay mineral transformations were observed in all but one of 28 faults one fault contains only mechanically derived clay rich gouge which formed entirely by cataclasis clay mineral transformations observed in gouges show four general patterns 1 growth of authigenic 1m d illite either by transformation of fragmental 2m 1 illite or muscovite or growth after the dissolution of k feldspar illitization of fragmental illite smectite is observed in lanf gouges but is less common than reported from faults with sedimentary wall rocks 2 retrograde diagenesis of an early mechanically derived chlorite rich gouge to authigenic chlorite smectite and saponite mg rich tri octahedral smectite 3 reaction of mechanically derived chlorite rich gouges with mg rich fluids at low temperatures 50 150 c to produce localized lenses of one of two assemblages sepiolite saponite talc lizardite or palygorskite chlorite quartz and 4 growth of authigenic di octahedral smectite from alteration of acidic volcanic wall rocks these transformation groups are consistent with patterns observed in fault rocks elsewhere the main controls for the type of neoformed clay in gouge appear to be wall rock chemistry and fluid chemistry and temperatures in the range of 60 180 c

frictional properties of megathrust fault gouges at low sliding velocities new data on effects of normal stress and temperature abstract friction data used in modelling studies of subduction zone seismogenesis are often poorly representative of in situ conditions we investigated the influence of in situ effective stresses and temperatures on the frictional properties of simulated fault gouges prepared either from nankai odp material or illite shale at sliding velocities approaching those relevant for earthquake nucleation and sses biaxial double direct shear experiments were performed at room temperature normal stresses of 5 30 mpa and sliding velocities of 0 16 18 m s all materials exhibited velocity strengthening under these conditions along with an increase in the friction coefficient and slip hardening rate with increasing normal stress illite gouge showed increased velocity strengthening towards higher normal stresses the effect of temperature was investigated by means of ring shear experiments on illite gouge at 200 300 c an effective normal stress of 170 mpa a pore fluid pressure of 100 mpa and sliding velocities of 1 100 m s these experiments showed a transition from velocity strengthening to velocity weakening at 250 c our results provide a possible explanation for the updip seismogenic limit within subduction zone megathrusts and imply an enhanced tendency for earthquake nucleation and sses at low effective normal stresses

fluid accumulation and channeling along the northern barbados ridge decollement thrust a volume of three dimensional seismic reflection data acquired in 1992 imaged the decollement beneath the northern barbados ridge accretionary prism revealing reflection amplitude and waveform variations attributed to fluid accumulations along the plate boundary fault we model the seismic reflection by inversion for seismic impedance the product of velocity and density throughout the 5 x 25 km survey area and thus map physical property variations in 1997 ocean drilling program leg 171a penetrated the protodecollement and decollement at five sites with a logging while drilling lwd tool to log density and other physical properties of the decollement we construct a regional map of density and inferred porosity within the decollement from seismic models calibrated with lwd density data in the sediments out in front of the trench the protodecollement forms in a radiolarian rich miocene mudstone with an anomalously high porosity 70 75 that appears as a pervasive inherent characteristic of this interval seaward of the deformation front in the decollement beneath the wedge a consolidation trend of decreasing porosity runs perpendicular to the deformation front with porosity decreasing from 70 at the wedge toe to 50 4 km from the wedge toe a second distinct trend also forms along a 10 km long 1 to 2 km wide ne sw zone in which porosity is 70 as high as it is in the protodecollement this zone can be explained as an area of the decollement where fluid accumulations develop by maintaining high fluid content we postulate that high fluid content is maintained by continuous recharge flowing into and along this channel this porosity distribution within the decollement also strongly influences fluid migration into the overlying accretionary wedge and is directly associated with fluid charging of ramps and out of sequence thrusts above the decollement

estimating basal friction in accretionary wedges from the geometry and spacing of frontal faults elastic theory applied to the deformation in accretionary wedges is used to calculate the condition for slip along an active frontal fault and the basal decollement the equations for calculating the stresses can be solved for the coefficient of basal friction in the situation of the formation of a new frontal thrust fault this allows us to calculate the efficient coefficient of basal friction which includes the weakening effect of pore fluid pressure from geometric parameters and material properties only the geometric parameters like fault dip and layer thickness can be derived from high resolution seismic cross sections application of our analysis to the makran and the nankai accretionary wedge allows us to estimate the upper limit of the effective coefficient of basal friction wbw0 16 and wbw0 2 in these two areas respectively fl 2001 elsevier science b v all rights reserved

thermal constraints on the seismogenic portion of the southwestern japan subduction thrust for coastal cities an important factor in earthquake hazard from subduction zone earthquakes is the landward extent of the seismogenic portion of the subduction thrust fault in this study we test the hypothesis that the maximum downdip extent is defined by a critical temperature we have developed a transient thermal model for the nankai subduction zone of southwest japan to allow comparison of the thermally estimated downdip extent of the seismogenic zone with that from 1 seismicity and tsunami data for two great subduction earthquakes 2 the coseismic faulting extent of these events estimated from geodetic deformation data and 3 the interseismic locked zone determined from interseismic geodetic data the nankai margin has extensive heat flow and heat production data to control thermal models and thus crustal temperatures it has earthquake tsunami and geodetic data that constrain the coseismic rupture portion of the subduction thrust fault for past great earthquakes and the portion of the thrust fault that is locked and storing interseismic elastic strain on the nankaido margin off shikoku island the thermal model indicates that a temperature of 350 c taken to be the limit for seismic initiation from laboratory and field data is reached on the subduction thrust fault 150 km from the trench a transition zone into which rupture may extend with decreasing offset taken to be 450 c extends an additional 45 km downdip the thermal model results are in excellent agreement with the maximum downdip extent of coseismic displacement for the 1946 nankaido ms 8 2 earthquake off shikoku island and with the downdip extent of the present locked zone in the region of the 1944 tonankai ms 8 2 earthquake to the northeast the subduction angle is much steeper and the thermal models indicate a narrower downdip seismogenic extent the seismogenic locked zone from earthquake and geodetic data is also narrower thus our analysis of the southwest japan margin indicates that all three constraints on the downdip extent of the seismogenic zone thermal coseismic and interseismic geodetic data are in general agreement the study also supports the hypothesis that the seismogenic portion of subduction thrust faults is limited primarily by temperature the thermal control implies that subduction thrust faults with shallow dip have wider seismogenic zones compared to those with steep dip the subducting plate age and thus heat flow and the thickness of the insulating sediments on the incoming plate are also very important to the thermal regime and thus to the seismogenic width the relation of the maximum seismic rupture area to the interseismic locked zone is particularly important for earthquake hazard estimation on subduction margins such as cascadia where there have been no great historical events

numerical investigation of deformation mechanics in fold and thrust belts influence of rheology of single and multiple decollements 1 thin skinned fold and thrust belts related to convergence tectonics develop by scraping off a rock sequence along a weaker basal decollement often formed by water saturated shale layers or low viscosity salt horizons a two dimensional finite element model with a viscoelastoplastic rheology is used to investigate the structural evolution of fold and thrust belts overlying different types of decollements in addition the influence of multiple weak layers in the stratigraphic column is studied model shale decollements are frictional with lower friction angles as the cover sequence model salt layers behave linear viscous due to a lower viscosity as the cover sequence or with a power law rheology single viscous decollement simulations have been compared to an analytical solution concerning faulting versus folding results show that fold and thrust belts with a single frictional basal decollement generate thrust systems ramping from the decollement to the surface spacing between thrust ramps depends on the thickness of the cover sequence the structural evolution of simulations with an additional low frictional layer depends on the strength relationship between the basal and the intersequential decollement tectonic underplating and antiformal stacking occur if the within sequence decollement is weaker in the frontal part of models deformation is restricted to the upper part and imbrication occurs with a wavelength depending on the depth of the intermediate weak layer salt decollement with a viscosity of 10 18 pa s leads to isolated box folds detachment folds multiple salt layers 10 18 pa s result in long wavelength folding our results for both frictional and viscous decollements are in bulk agreement with the mohr coulomb type critical wedge theory citation ruh j b b j p kaus and j p burg 2012 numerical investigation of deformation mechanics in fold and thrust belts influence of rheology of single and multiple decollements tectonics 31 tc3005 doi 10 1029 2011tc003047

high pore pressure generation in sediments in front of the barbados ridge complex dsdp data for the barbados ridge complex show the existence of high pore pressures in sediments both oceanward and arcward of oceanic trenches we model the evolution of pore pressure by using a finite element procedure three consecutive stages are recognized 1 in the oceanic basin sedimentation rate is low there is little excess pore pressure 2 approaching the trench but before being incorporated into the accretionary prism sediments are subjected to increasing tectonic compression as well as greater sediment overburden thus high pore pressure can be created an important result is that the lowest effective stress occurs within the sedimentary layer instead of at its base and a decollement may develop along this weakened zone 3 arcward of the trench sediments above the decollement are offscraped while those beneath it are subducted fast sedimentation combined with tectonic compression can create pore pressures so high as to cause porosity to increase with depth this facilitates the formation of mud volcanoes even oceanward of the deformation front

the eastern and western ends of nankai trough results of box 5 and box 7 kaiko survey abstract seabeam mapping and detailed geophysical surveying have been conducted over the eastern and western ends of the nankai trough the eastern survey covers the transition between the large izu bonin izu ogasawara ridge collision with honshu and the nankai trough subduction it includes a northeast trending basement ridge the zenisu ridge to the southeast of nankai trough as well as two large channel systems one following the trench the nankai channel and the other coming from the north the tenryu canyon the zenisu ridge is a zone of recent intra plate shortening consisting of three distinct segments with an increasing deformation from southwest to northeast we interpret this gradient of deformation as a way to absorb the kinematic discontinuity between the diffuse shortening prevailing over the izu bonin ridge and the concentrated shortening along the nankai trough the shortening axis appears to change from north south along the zenisu ridge to northwest southeast along the nankai trough and finally to east west within honshu the western survey covers the junction between the nankai and ryukyu trenches and the kyushu palau ridge which is subducted without major internal deformation the ridge appears to act as an indenter upon the margin the tip of the indenter is presently situated under the upper accretionary prism

frictional properties of the south china sea oceanic basalt and implications for strength of the manila subduction seismogenic zone abstract we conduct friction experiments on a basalt sample no u1431e41r6w from international oceanic discovery program iodp expedition 349 in the temperature range of 100 600 c and effective normal stress of 50 mpa with 100 mpa pore fluid pressure loading rates are stepped up and down in the range of 0 04 1 0 m s to obtain the velocity dependence of friction based on the rate and state friction law we numerically fit the experimental curves to acquire constitutive parameters that cannot be obtained by direct measurement experimental results show that basalt friction coefficient varies from 0 70 to 0 74 similar to the range of gabbro documented in a previous study he et al 2007 stable velocity strengthening behavior is observed at temperatures of 100 c and 200 c and unstable velocity weakening at temperatures above 300 c with occurrences of periodic stick slips both a and b values inferred by numerical fittings show positive temperature dependence in the temperature range of 100 600 c the stick slip motions above 300 c seem to be due to a dramatic decrease in the characteristic slip distance dc from 11 to 15 m to 0 4 2 5 m based on our results and plastic flow laws of mafic rocks from a previous study fault strength profile and slip stability of manila subduction zone are discussed with calculations in the context of a unified friction flow model the inferred lower boundary of seismogenic zone of manila subduction fault is around 40 km which is in a reasonable range compared with seismicity data

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

residual strength of clays in landslides folded strata and the laboratory the post peak drop in drained shear strength of an overconsolidated clay may be considered as taking place in two stages first at relatively small displacements the strength decreases to the fully softened or critical state value owing to an increase in water content dilatancy second after much larger displacements the strength falls to the residual value owing to reorientation of platy clay minerals parallel to the direction of shearing if the clay fraction is less than about 25 the second stage scarcely comes into operation the clay behaves much like a sand or silt with angles of residual shearing resistance typically greater than 20 degrees conversely when the clay fraction is about 50 residual strength is controlled almost entirely by sliding friction of the clay minerals and further increase in clay fraction has little effect the angles of residual shearing resistance of the three most commonly occurring clay minerals are approximately 15 degrees for kaolinite 10 degrees for illite or clay mica and 5 degrees for montmorillonite when the clay fraction lies between 25 and 50 there is a transitional type of behaviour residual strength being dependent on the percentage of clay particles as well as on their nature the post peak drop in strength of a normally consolidated clay is due only to particle reorientation measurements of strength on natural shear surfaces agree within practical limits of variation with values derived from back analysis of reactivated landslides this field residual strength can be recovered by multiple reversal shear box tests on cut plane samples but in high clay fraction materials it is typically somewhat higher than the strength measured in ring shear tests residual strength is little affected by variation in the slow rates of displacement encountered in reactivated landslides and in the usual laboratory tests but at rates faster than about 100 mm min qualitative changes take place in the pattern of behaviour a substantial gain in strength is followed with increasing displacement by a fall to a minimum value in clays and low clay fraction silts this minimum is not less than the slow or static residual but in clayey silts with clay fractions around 15 25 according to tests currently in progress the minimum can be as low as one half of the static value author trrl

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

depth of seismic coupling along subduction zones underthrusting at subduction zones can cause large earthquakes at shallow depths but it is always accommodated by aseismic deformation below a certain depth the maximum depth of the seismically coupled zone or seismogenic zone is a transition from unstable to stable sliding along the plate interface we have determined the depth of this stability transition for the circurn pacific subduction zones of honshu kuriles kamchatka aleutians alaska mexico and chile these subduction zones have experienced great interplate earthquakes and the aftershock regions are well located depth estimates of interplate events that are located at the downdip edge of the aftershock regions are used to determine the maximum depth of seismic coupling for an average p wave velocity of 6 7 km s 1 above the plate interface we find that for most subduction zones the stability transition occurs at 40 5 km depth there are however several exceptions at the hokkaido trench junction where the japan trench and the kurile trench intersect seismic coupling is deep and extends down to 52 55 km deep coupling was also found in the coquimbo region in central chile the mexico subduction zone has shallow coupling the transition occurs at 20 30 km depth previous studies of micro earthquakes in honshu hokkaido the aleutians and alaska show that earthquakes within the upper plate extend no deeper than the downdip edge of the coupled zone that we find given our measurements of seismic coupling depth we then explore the mechanism that may determine coupling depth the concept of critical temperature has been used to explain the depth of seismic coupling in other tectonic environments thus we first test whether a critical temperature can explain our results temperatures at the plate interface are dependent on many variables but two that are poorly determined are shear stress and radiogenic heat generation shear stress has been constrained by inversion of heat flow data assuming a crustal radiogenic heat production rate of 3 1 exp z 8 5 wm 3 and a constant coefficient of friction we find two critical temperatures of about 400 c and 550 c the lower critical temperature may be characteristic of regions with a relatively thick continental crust and the higher temperature of regions with a relatively thin continental crust on the other hand one single critical temperature of about 250 c can explain the coupling depths if shear stresses are constant with depth

modeling consolidation and dewatering near the toe of the northern barbados accretionary complex at the toe of the northern barbados accretionary complex temperature and pore water chemistry data indicate that fluid flow is channeled along the decollement and other shallow thrust faults we examine mechanisms that may prevent consolidation and maintain high permeability over large sections of the decollement high resolution bulk density data from five boreholes show that the decollement is well consolidated at some sites while other sites remain underconsolidated underconsolidated decollement behavior is associated with kilometer scale negative polarity seismic reflections from the decollement plane that have been interpreted to be fluid conduits we use a coupled fluid flow consolidation model to simulate the loading response of a 10 km long by 680 m thick slice of sediment as it enters the accretionary complex the simulations capture 185 ka 5 km of subduction with a load function representing the estimated effective stress of the overriding accretionary prism 3 8 taper angle simulation results of bulk density in the decollement 3 2 km arcward of the deformation front are compared with observations the results show that persistent high pore pressures at the arcward edge of the simulation domain can explain underconsolidated behavior the scenario is consistent with previous modeling results showing that high pore pressures can propagate intermittently along the decollement from deeper in the complex simulated seaward fluxes in the decollement 1 14 cm yr 1 lie between previous estimates from modeling studies of steady state 1 m yr 1 flow maximum simulated instantaneous fluid sources 2 5 10 13 s 1 are comparable to previous estimates the simulations show minor swelling of incoming sediments fluid sources 3 10 15 s 1 up to 3 km before subduction that may help to explain small scale shearing and normal faulting proximal to the protodecollement

three dimensional model of hellenic arc deformation and origin of the cretan uplift 1 the hellenic arc of greece is the most seismically active part of europe but little is know about its mechanics we modeled deformation along the arc using a finite element model the model was intended to capture large scale 3 d structure of nubian plate subduction beneath the aegean block and its deformational consequences the shape of the interface was developed using mapped traces at the surface and earthquake hypocenters at depth model block motions were constrained by recent compilations of gps velocity vectors we simulated a 10 ka period of convergence between nubia and the aegean and calculated the strain field in the overriding plate as well as the spatial distribution and orientation of differential stress 1 3 from these calculations we derived testable quantities such as the expected seismic moment rate on the interplate contact uplift pattern and distribution of strain modes our relatively simple model broadly reproduced observed uplift patterns earthquake activity and loci of extension and contraction the model showed a localization of uplift near the island of crete where the fastest aegean uplift rates are well documented comparison of calculated expected seismic moment and observed earthquake catalogs implies a nearly fully coupled interplate contact on the basis of our modeling results we suggest that south aegean deformation is driven primarily by the fast moving 33 mm a 1 aegean upper plate overriding a nearly stalled 5 mm a 1 nubian lower plate this tectonic setting thus more closely resembles a continental thrust than it does a typical oceanic subduction zone

laboratory results indicating complex and potentially unstable frictional behavior of smectite clay a central problem in explaining the apparent weakness of the san andreas and other plate boundary faults has been identifying candidate fault zone materials that are both weak and capable of hosting earthquake like unstable rupture our results demonstrate that smectite clay can be both weak and velocity weakening at low normal stress 30 mpa our data are consistent with previous work which has focused on higher normal stress conditions 50 mpa and greater and found only velocity strengthening if natural fault zones contain significant smectite one key implication of our results is that localized zones of high pore pressure which reduce effective normal stress could be important in controlling potential sites of earthquake nucleation our experiments indicate that friction of smectite is complex and depends upon both sliding velocity and normal stress this complexity highlights the need for detailed experiments that reflect in situ conditions for fault gouges

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

updip limit of the seismogenic zone beneath the accretionary prism of southwest japan an effect of diagenetic to low grade metamorphic processes and increasing effective stress off southwest japan the seaward limit of coseismic displacement or updip limit of the seismogenic zone of the 1946 mw 8 3 thrust earthquake reaches to 4 km depth and 40 km landward of the trench this limit coincides with the estimated location of the 150 c isotherm and has been linked to changes in physical properties associated with the smectite to illite clay mineral transition here we show that this limit correlates with a suite of diagenetic to low grade metamorphic processes characterized by 1 declining fluid production and decreasing fluid pressure ratio and 2 active clay carbonate and zeolite cementation and the transition to pressure solution and quartz cementation these diagenetic to low grade metamorphic changes cause the onset of velocity weakening during thrust faulting an increase in effective stress and strengthening of the hanging wall which together combine to produce recordable earthquakes

effect of hydration state on the frictional properties of montmorillonite based fault gouge 1 we report on laboratory experiments examining the effect of hydration state on the frictional properties of simulated clay and quartz fault gouge we tested four mixtures of ca montmorillonite and quartz 100 70 50 and 30 montmorillonite at four hydration states dry 16 0 wt water we controlled the hydration state using either oven drying for 13 wt h2o under conditions of controlled relative humidity for each clay quartz mixture and hydration state we measured frictional properties over a range of normal stresses 5 100 mpa and sliding velocities 1 300 m s we observe a systematic decrease in the coefficient of friction with increasing water content normal stress and clay content values of for 50 50 mixtures range from 0 57 to 0 64 dry and decrease to 0 21 0 55 for the most hydrated cases wet for layers of 100 montmorillonite ranges from 0 41 0 62 dry to 0 03 0 29 wet as water content is increased from 0 to 20 0 wt the friction rate parameter a b becomes increasingly positive variation in a b values decreases dramatically as normal stress increases if our experimental results can be applied to natural fault gouge the combination of stress state hydration state and quartz content that facilitates unstable fault behavior implies that the onset of shallow seismicity in subduction zones is more complicated than a simple transition from smectite to illite

splay fault branching along the nankai subduction zone seismic reflection profiles reveal steeply landward dipping splay faults in the rupture area of the magnitude m 8 1 tonankai earthquake in the nankai subduction zone these splay faults branch upward from the plate boundary interface that is the subduction zone at a depth of approximately 10 kilometers approximately 50 to 55 kilometers landward of the trough axis breaking through the upper crustal plate slip on the active splay fault may be an important mechanism that accommodates the elastic strain caused by relative plate motion

the effect of compliant prisms on subduction zone earthquakes and tsunamis abstract earthquakes generate tsunamis by coseismically deforming the seafloor and that deformation is largely controlled by the shallow rupture process therefore in order to better understand how earthquakes generate tsunamis one must consider the material structure and frictional properties of the shallowest part of the subduction zone where ruptures often encounter compliant sedimentary prisms compliant prisms have been associated with enhanced shallow slip seafloor deformation and tsunami heights particularly in the context of tsunami earthquakes to rigorously quantify the role compliant prisms play in generating tsunamis we perform a series of numerical simulations that directly couple dynamic rupture on a dipping thrust fault to the elastodynamic response of the earth and the acoustic response of the ocean gravity is included in our simulations in the context of a linearized eulerian description of the ocean which allows us to model tsunami generation and propagation including dispersion and related nonhydrostatic effects our simulations span a three dimensional parameter space of prism size prism compliance and sub prism friction specifically the rate and state parameter b a that determines velocity weakening or velocity strengthening behavior we find that compliant prisms generally slow rupture velocity and for larger prisms generate tsunamis more efficiently than subduction zones without prisms in most but not all cases larger more compliant prisms cause greater amounts of shallow slip and larger tsunamis furthermore shallow friction is also quite important in determining overall slip increasing sub prism b a enhances slip everywhere along the fault counterintuitively we find that in simulations with large prisms and velocity strengthening friction at the base of the prism increasing prism compliance reduces rather than enhances shallow slip and tsunami wave height

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

loci and maximum size of thrust earthquakes and the mechanics of the shallow region of subduction zones earthquakes do not extend updip along the plate interface at most subduction zones all of the way to the plate boundary at the trench axis or deformation front rather the shallowest part of that interface moves mainly through stable aseismic slip that part of the plate boundary referred to here as the aseismic zone occurs along the base of an accreted wedge of young sediments the probable primary cause for the existence of this aseismic zone is the stable slip properties of the unconsolidated and semiconsolidated sediments in that zone subducted sediment is progressively dewatered and underplated to the base of the overriding plate through this process more consolidated rocks eventually come into contact at depth across both sides of the plate boundary from the point where sufficiently hard rock is found across the plate interface that interface will change its slip behavior to unstable stick slip sliding which is characteristic of consolidated material under most conditions this type of motion is accommodated seismically as episodic slip in large earthquakes the location of this transition to seismic behavior referred to as the seismic front marks the deep end of the aseismic zone and the top of the seismogenic zone i e that part of the plate interface that moves primarily in thrust earthquakes several convergent plate margins are discussed to illustrate the seismic front and the aseismic zone we find that the seismic front defined by smaller earthquakes that occur during the interval between large events is nearly the same as that for large events as inferred from the locations of their aftershocks the location of the seismic front is important for making estimates of the maximum possible size of thrust earthquakes along the plate boundary because it delimits the trenchward limit of the potential rupture area of these events the size of large thrust earthquakes is proportional to rupture area and to at least the cube of the downdip width w of that area thus by defining the location of the seismic front for convergent margins it is possible to make better estimates of w and hence to deduce the maximum size of future interplate earthquakes the average repeat time of such events is also related to w and so better estimates of average repeat time are possible from improved knowledge of w the hypothesis that the location of the seismic front is related to the maximum depth of subduction of unconsolidated sediment has implications for forearc mechanics we use a mechanical analysis laboratory modeling and multichannel seismic information to develop a simple model explaining the growth of forearcs the outer arc high or trench slope break that occurs arcward of most accretionary wedges represents an abrupt change in the critical taper of the accretionary wedge we argue that that change is caused by a large arcward increase in the strength of the material within the overriding plate that stronger material is called the backstop our laboratory modeling experiments indicate that a backstop with a trenchward dipping upper surface results in the development of an overlying structure that includes all of the primary morphologic features observed in modern forearcs in our modeling an accretionary wedge develops trenchward of the backstop an outer arc high develops above the trenchward toe of the backstop and farther arcward a passive forearc basin forms above the stronger material of the backstop this model is consistent with the observation that earthquakes do not extend updip along the plate interface all the way to the trench axis we hypothesize that plate motion is accommodated seismically along the base of the consolidated backstop and mostly or entirely aseismically trenchward of the toe of the backstop along the base of the accretionary wedge at several margins the seismic front is approximately coincident with the outer arc high supporting this interpretation

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

origin of a zone of anomalously high porosity in the subduction inputs to nankai trough abstract one poorly understood feature of the subduction inputs to the nankai trough subduction zone sw japan is a stratigraphic interval with an anomalously high porosity zone hpz which is up to 240 m thick and located within the clay and volcanic ash rich shikoku basin facies to investigate the origin of their peculiar physical properties we integrated logging while drilling lwd data shipboard density measurements and visual descriptions of core samples recovered from four drill sites of the ocean drilling program and integrated ocean drilling program we combined those observations with scanning electron microscopy sem and laboratory consolidation tests on both hpz samples and artificial mixtures of ash glass shards smectite and vesicular pumice smectite lwd data indicate that the hpz mudstones have a large proportion of dispersed volcanic ash 20 30 the consolidation tests show that the rate of porosity loss with increasing effective stress consolidation behavior is consistent among hpz specimens and matches artificial mixtures containing up to 60 volcanic material however absolute values of porosity remain higher for hpz samples compared to artificial mixtures so processes in addition to the mechanical effects of volcanic clasts must be contributing we suggest that hydration and partial dissolution turns clusters of volcanic glass into aggregates with distinct microfabric sem images confirm the presence of strengthened grain to grain contacts which probably inhibit the collapse of the intra aggregate pore space the aggregates behave like grains so that cohesive strength of the bulk sediment and rate of porosity loss remain nearly unchanged during burial the two step diagenetic process of dissolution and precipitation depends critically on a threshold abundance of fine grained dispersed volcanic ash pumice older units in the shikoku basin with only traces of dispersed ash show no such effects hpzs should be expected in other subduction zones with similar compositional and diagenetic prerequisites

numerical investigation of localization and micromechanics in a stratified soil specimen abstract for many geological deformation processes the existence of a mechanically weak layer is assumed for these weak layers clay sediments or clayey fault gouge seem to be likely candidates they are supposed to be a focus of displacement and thus crucial for slip plane and shear zone development there is an ongoing debate concerning the exact nature of shear localization in these layers to address this question we use numerical shear box tests utilizing the discrete element method dem our focus lies on the localization patterns and micromechanical properties of stratified silt clay silt and clay clay clay specimens we systematically vary the mechanical strength of each layer via the coefficient of particle friction to analyse localization and the micromechanics of the material our results indicate that the deformation of a material package is the result of different deformation modes the ability of single particles to slide roll or rotate is influenced to a large extent by layer strength and particle friction which also govern the layers ability to dilate or contract during deformation we show that the localization switches from a given layer to adjacent layers even if differences in the material properties are very small furthermore the localization is often concentrated in only one layer leading to a complete decoupling of the other layers and thus building a shear zone where high displacement can be accumulated

anomalously high porosities in the proto decollement zone of the barbados accretionary complex do they indicate overpressures elevated pore pressures have been hypothesized to play a key role in the development of the decollement zones of accretionary complexes drilling within the proto decollement zone of the barbados accretionary complex revealed anomalously high porosities up to 6 km seaward of the deformation front defining whether the high porosities are a reflection of overpressures is critical for understanding the seaward propagation of the decollement zone in this investigation we used numerical modeling to assess whether the high porosities reflect overpressures due to sedimentation from this work we concluded that reported permeabilities are not low enough to produce overpressures even if additional fluid were contributed from areas of greater sedimentation furthermore overpressures within the proto decollement zone are demonstrated to be inconsistent with normally compacted underlying sediments locally anomalous sediment strength provides a more likely explanation for the observed high porosities

influence of particle shape on the frictional strength of sediments a numerical case study abstract numerous direct and ring shear tests supply that composition and texture of sediments are crucial for shear and frictional strength we simulate particle interactions on a micro scale level during shearing with a numerical shear box using the discrete element method dem based on the granular model approach detailed information about slip localisation and rates forces at particle contacts as well as particle rotation can be quantified as function of particle shape stress conditions and shear rate our numerical experiments show that the deformation behaviour of clay is largely controlled by their particle shape two key factors sphericity and roughness could be identified as relevant for frictional strength shear zone development and particle rotation with sphericity dominating over roughness decreasing sphericity leads to complex initial microfabrics whose breakdown with increasing strain is caused by particle rotation preferred particle orientation favours low friction decreasing volume strain and the evolution of particle domains of similar orientation decreasing roughness results in particle interlocking of different degrees which obstructs slip and rotation to preferred orientations so that friction is high and volume strain positive spatial and temporal shear zone development depends on the roughness of particles increasing roughness precludes shear zone development and particle domain evolution shear zone localisation depends on particle sphericity increasing sphericity leads to more localised but vertically dilated deformation

the marine geochemistry of dissolved gallium a comparison with dissolved aluminum abstract dissolved ga concentrations in the pacific ocean range from 2 to 30 picomolar they are low in surface waters 2 12 pm with a subsurface maximum at 150 300 m 6 17 pm a mid depth minimum from 500 to 1000 m 4 10 pm and increasing values with depth to a maximum in the bottom waters 12 30 pm the highest concentrations are in the central gyre with lower values toward the north and east where productivity and particle scavenging increase dissolved ga concentrations in the surface waters of the northwest atlantic are nearly an order of magnitude higher than in the central north pacific with higher values in the gulf stream than in the continental slope boundary region dissolved al concentrations in the high latitude north pacific are also lower than in the central gyre and are the lowest yet reported for this element the features are similar however to other dissolved al profiles a surface maximum 0 8 nmol kg a mid depth minimum 0 06 nmol kg and increasing concentrations toward the sediment water interface 0 4 nmol kg the vertical distributions and horizontal transects indicate three sources of dissolved ga to the oceans the surface distribution reflects an eolian source with no net fluvial input to the open ocean the subsurface maximum a feature not seen for north pacific dissolved al is attributed to vertical exchange processes the source for the deep waters of the north pacific is from a sediment surface remineralization process or a pore water flux scavenging removal throughout the water column is evident in the vertical profiles for both dissolved ga and al with intensified removal in the boundary regions where productivity and particle scavenging are at a maximum residence times of dissolved ga in surface waters are nearly an order of magnitude longer than the corresponding values for al

experimental constraints on the relationship between clay abundance clay fabric and frictional behavior for the central deforming zone of the san andreas fault the presence of smectite saponite in fault gouge from the central deforming zone of the san andreas fault at parkfield ca has been linked to low mechanical strength and aseismic slip however the precise relationship between clay mineral structure fabric development fault strength and the stability of frictional sliding is not well understood we address these questions through the integration of laboratory friction tests and fib sem analysis of fault rock recovered from the san andreas fault observatory at depth safod borehole intact fault rock was compared with experimentally sheared fault gouge and different proportions of either quartz clasts or safod clasts extracted from the sample nano textural measurements show the development of localized clay particle alignment along shear folia developed within synthetic gouges such slip planes have multiples of random distribution mrd values of 3 0 4 9 the mrd values measured are higher than previous estimates mrd 1 5 that show lower degrees of shear localization and clay alignment averaged over larger volumes the intact fault rock exhibits less well developed nano clay fabrics than the experimentally sheared materials and mrd values decrease with smectite content we show that the abundance strength and shape of clasts all influence fabric evolution via strain localization quartz clasts yield more strongly developed clay fabrics than serpentine dominated safod clasts our results suggest that 1 both clay abundance and the development of nano scale fabrics play a role in fault zone weakening and 2 aseismic creep is promoted by slip along clay shears with 20 wt smectite content and mrd values 2 7

thick strong sediment subduction along south central chile and its role in great earthquakes abstract the south central chile margin experienced the largest and sixth largest earthquakes ever recorded the 1960 mw 9 5 valdivia and 2010 mw 8 8 maule megathrust earthquakes respectively in early 2017 we conducted a seismic survey along 1 000 km of south central chile to image these rupture zones using a 15 15 km long multi channel seismic streamer we processed these data using pre stack depth migration which provides the best look at the shallow part of the south central chile margin to date relative to other sediment dominated subduction zones where sediment is typically accreted at the toe an unusually large percentage of the thick trench sediments are consistently subducted beneath the slope with little thrust faulting or deformation analysis of the sediment p wave velocities and structure in the trench and outer wedge leads us to conclude that most of south central chile contains well drained strong sediments an exception in the vicinity of the subducting mocha fracture zone mfz has trench sediments that appear to experience localized delayed compaction thus lowering their strength and allowing the development of protothrusts similar to what is seen in other accretionary subduction zones the very shallow decollement along the south central chile allows more sediment to pass beneath the lower slope than almost all other subduction zone many of which have much thicker trench sections we conclude that subduction of the strong well drained thick sediment layer beneath the lower slope is typical for nearly all of the south central chile comparison to other thick sedimented subduction zones worldwide reveals that the subduction of such a large fraction of the trench sediment is particularly unusual this strong thick subducting sediment is likely a determining factor for developing a smooth plate interface located well above the subducted crust topography that ultimately becomes a broad megathrust with high homogeneous frictional properties and generates particularly large earthquakes along the south central chile margin

implications of basement rock alteration in the nankai trough japan for subduction megathrust slip behavior abstract the nankai trough is an exceptionally well studied convergent margin known to host damaging megathrust earthquakes as well as various forms of slow fault slip outcrop studies of exhumed analogues for the modern subduction thrust as well as seismic reflection images of the active subduction zone suggest that the basaltic basement participates in plate boundary shearing at depths of a few km and greater we obtained altered basaltic upper basement from the modern nankai trough seaward of the trench recovered by drilling on iodp expedition 333 we performed laboratory friction experiments on bare surfaces and gouge powders of this material in addition to and in combination with other materials for comparison the altered basalt exhibits predominantly velocity strengthening frictional behavior indicating a tendency for stable slip for bare surface experiments few instances of velocity weakening friction occur and are restricted to slip rates of

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

modeling megathrust earthquakes across scales one way coupling from geodynamics and seismic cycles to dynamic rupture abstract taking the full complexity of subduction zones into account is important for realistic modelling and hazard assessment of subduction zone seismicity and associated tsunamis studying seismicity requires numerical methods that span a large range of spatial and temporal scales we present the first coupled framework that resolves subduction dynamics over millions of years and earthquake dynamics down to fractions of a second using a two dimensional geodynamic seismic cycle sc method we model 4 million years of subduction followed by cycles of spontaneous megathrust events at the initiation of one such sc event we export the self consistent fault and surface geometry fault stress and strength and heterogeneous material properties to a dynamic rupture dr model coupling leads to spontaneous dynamic rupture nucleation propagation and arrest with the same spatial characteristics as in the sc model it also results in a similar material dependent stress drop although dynamic slip is significantly larger the dr event shows a high degree of complexity featuring various rupture styles and speeds precursory phases and fault reactivation compared to a coupled model with homogeneous material properties accounting for realistic lithological contrasts doubles the amount of maximum slip introduces local pulse like rupture episodes and relocates the peak slip from near the downdip limit of the seismogenic zone to the updip limit when an sc splay fault is included in the dr model the rupture prefers the splay over the shallow megathrust although wave reflections do activate the megathrust afterwards

the strength of large scale plate boundaries constraints from the dynamics of the philippine sea plate since 5ma abstract friction controls most of the strength of tectonic plate boundaries and thus the force mutually exchanged between plates estimates of the plate boundary friction coefficient are therefore of paramount importance to our understanding of the lithosphere torque balance however several lines of evidence indicate that the friction coefficient of plate margins is significantly lower than is measured in laboratory experiments on fractured rocks this poses a significant limit to extrapolation from the laboratory scale to the large scale of earth s lithosphere the record of past and present day plate motions represents an efficient probe into the torque balance of plate tectonics because plate motions readjust virtually instantaneously to temporal changes in driving and resisting forces here i derive inferences on the strength of large scale plate boundaries by focusing on the convergence of the fast moving philippine sea plate towards eurasia since subduction initiation 5 myr ago because at the present day the philippine slab reaches depths shallower than the 410 km transition zone in the upper mantle its weight is unlikely to provide sufficient driving force to shear the trailing plate over the viscous mantle at the observed rates using global models of the coupled mantle lithosphere system i argue that frictional coupling with the pacific and eurasia plates is the main driver for the dynamics of the philippine sea plate this allows me to infer that the friction coefficient of large scale plate boundaries is in range 0 01 0 07 and that the amount of sediments entering plate margins efficiently modulates the friction coefficient contributing significantly to its lateral variations importantly i corroborate my conclusions by looking at the present day vertical deformation of the philippine sea eurasia margin evident from observed trench parallel gravity anomalies

toward 4d modeling of orogenic belts example from the transpressive zagros fold belt abstract latest developments in computer power and computational solutions open new ways to envision complex natural systems advances in high resolution 3d geodynamic modeling in particular allow investigating tectonic processes such as transpression which involve normal and tangential kinematics with respect to the bulk orogenic trends we demonstrate that implementing the 4 dimensions space and time provides and constrains new answers to long lasting discussions as case study we have chosen the zagros simply folded belt in iran which exhibits spatial variations of structural patterns from the homogeneously shortened wide fold belt in the fars to the narrower higher elevated area in the izeh high resolution 4d numerical modeling shows how a low viscosity decollement becoming frictional toward the dezful embayment influences the exposed fold patterns results also emphasize the importance of an oblique backstop to produce en echelon arranged folds where convergence related and backstop controlled folds are mingled in the transpressional orogen

the southern ocean s role in carbon exchange during the last deglaciation changes in the upwelling and degassing of carbon from the southern ocean form one of the leading hypotheses for the cause of glacial interglacial changes in atmospheric carbon dioxide we present a 25 000 year long southern ocean radiocarbon record reconstructed from deep sea corals which shows radiocarbon depleted waters during the glacial period and through the early deglaciation this depletion and associated deep stratification disappeared by 14 6 ka thousand years ago consistent with the transfer of carbon from the deep ocean to the surface ocean and atmosphere via a southern ocean ventilation event given this evidence for carbon exchange in the southern ocean we show that existing deep ocean radiocarbon records from the glacial period are sufficiently depleted to explain the 190 per mil drop in atmospheric radiocarbon between 17 and 14 5 ka

the relationship of undrained shear strength and porosity with depth in shallow 50 m marine sediments abstract in this paper we review the relationship between undrained shear strength and porosity with depth of marine sediments in the upper 50 m below the seafloor based on a compilation of data from 46 ocean drilling program odp legs we find that only about half of the drill sites show the anticipated increase in strength 59 and associated decrease in porosity 49 down hole moreover a significant correlation coefficient for both strength and porosity with depth is found at only 31 of the sites four categories of sediments are distinguished based on the significance of correlation coefficients we observe an influence of sediment composition and texture on the relationship between strength and porosity with depth fine grained siliciclastic sediments clayey and silty sediments and nannofossil oozes with less than 50 carbonate frequently show significant correlation coefficients for strength and porosity with depth while fine grained carbonates with high carbonate concentrations calcareous muds and nannofossil oozes with an average carbonate content of 87 as well as reef facies carbonates lack significant trends down hole moreover correlation patterns vary considerably with geological or tectonic setting for instance delta deposits of the amazon fan and sediments from the atlantic passive continental margins frequently show significant correlation coefficients for both properties extensional arcs often show significant correlation only for one property shear strength or porosity correlation patterns are discussed in the light of sediment composition geological and tectonic setting the methods used for shear strength measurement and previous work published on compaction behaviour