a subducting seamount beneath the nankai accretionary prism off shikoku southwestern japan a multi channel seismic mcs reflection survey was conducted to study the structure of the nankai convergent margin off shikoku island in july 1997 based on reflection characteristics we could identify three major seismic reflection units i e units a b and c the mcs data as well as swath bathymetric data reveal a buried circular seamount subducting beneath the nankai accretionary prism the subducting seamount is responsible for deformation of the accretionary wedge resulting in a compressed uplifted sediment knoll and many steep escarpments with ne sw strike a possible thrust fault was identified on the seaward flank of the seamount indicating compressive deformation a tectonic model for subduction of the seamount is presented based on the interpretation of the mcs data

subduction of the daiichi kashima seamount in the japan trench lallemand s culotta r and von huene r 1989 subduction of the daiichi kashimi seamount in the japan trench in j p cadet and s uyeda editors subduction zones the kaiko project tectonophysics 160 231 247 in 1984 1985 the kaiko consortium collected seabeam single channel seismic and submersible sampling data in the vicinity of the daiichi kashima seamount and the southern japan trench we performed a prestack migration of a shell multichannel seismic profile that crosses this area and examined it in the light of this unusually diverse kaiko dataset unlike the frontal structure of the northern japan trench where mass wasting appears to be the dominant tectonic process the margin in front of the daiiclr kashima shows indentation imbrication uplift and erosion emplacement of the front one third of the seamount beneath the margin front occurs without accretion we conclude that the daiichi kashima seamount exemplifies an intermediate stage between the initial collision and subduction of a seamount at a continental margin

formation of forearc basins by collision between seamounts and accretionary wedges an example from the new hebrides subduction zone seabeam data reveal two deep subcircular reentrants in the lower are slope of the new hebrides island arc that may illustrate two stages in the development of a novel type of forearc basin the malekula reentrant lies just south of the partly subducted bougainville seamount this proximity as well as the similarity in morphology between the reentrant and an indentation in the lower arc slope off japan suggests that the malekula reentrant formed by the collision of a seamount with the arc an arcuate fold thrust belt has formed across the mouth of the reentrant forming the toe of a new accretionary wedge the efate reentrant may show the next stage in basin development this reentrant lies landward of a lower slope ridge that may have begun to form as an arcuate fold thrust belt across the mouth of a reentrant this belt may have grown by continued accretion at the toe of the wedge by underplating beneath the reentrant and by trapping of sediment shed from the island arc these processes could result in a roughly circular forearc basin basins that may have formed by seamount collision lie within the accretionary wedge adjacent to the aleutian trenches

neotectonic deformation of the central chile margin deepwater forearc basin formation in response to hot spot ridge and seamount subduction 1 underlying the midslope valparaiso terrace the valparaiso basin constitutes a unique and prominent deepwater forearc basin on the central chile margin the basin contains a 3 3 5 km thick sediment fill of late cenozoic age the seismic stratigraphic architecture of the valparaiso basin fill shows that margin wide subsidence above the underthrusting juan fernandez ridge an aseismic volcanic swell crested with seamounts on the nazca plate characterized the basin evolution contemporaneously with margin subsidence and seaward rotation of the upper slope and shelf the outer part of the forearc was deformed by thrust faulting above the leading flank of underthrusting seamounts that rotated the middle slope to a more horizontal attitude and formed seaward vergent outer forearc highs subsidence of the margin above the underthrusting juan fernandez ridge is ascribed to processes of subduction erosion enhanced by factors related to subduction of the ridge that caused dismemberment of upper plate material along the shallow aseismic part of the plate interface and transfer of material to the downgoing nazca plate accelerated midslope subsidence 55 75 km landward of the trench coincides with the seaward limit of interplate seismicity suggesting that enhanced basal erosion is related to the transition from aseismic to stick slip subduction behavior since subduction of the juan fernandez ridge began near 33 s 10 ma ago the chile trench north of 33 s is inferred to have retreated 3 km myr and the midslope valparaiso basin subsided 0 3 0 5 km myr approximating analogous neogene rates for the peruvian margin

high resolution 3d numerical modeling of thrust wedges influence of decollement strength on transfer zones 1 the mechanics and dynamics of thin skinned compressible thrust wedges with prescribed offsets in the backstop i e transfer zones are investigated using a three dimensional finite difference numerical model with a visco brittle plastic rheology the main questions addressed are as follows i what is the influence of the initial length of the backstop offset and ii what is the effect of the frictional strength of the main decollement on the structural evolution of the brittle wedges along such transfer zones results show that the shorter the backstop offset the earlier these two thrust planes connect forming a curved frontal thrust along the entire width of the model younger in sequence thrusts are formed parallel to this curved shape long backstop offsets produce strongly curved thrust faults around the indenting corner simulations with a weak basal friction evolve toward almost linear frontal thrusts orthogonal to the bulk shortening direction increased basal drag in models with a strong decollement favors propagation of the backstop offset into a transfer zone up to the frontal thrust these simulations revealed that surface tapers of the wedge in front of the backstop promontory are larger than what the critical wedge theory predicts whereas the tapers on the other side of the transfer zone are smaller than analytical values this difference is amplified with increasing length of the backstop offset and or strength of the decollement modeled surface elevation schemes reproduce well the topographic patterns of natural orogenic systems such as the topographic low along the minab zendan transform transfer fault between the zagros and makran

when seamounts subduct data from an array of seismometers on the sea floor show the complex pattern of earthquakes around subducted seamounts

the giant ruatoria debris avalanche on the northern hikurangi margin new zealand result of oblique seamount subduction despite convergent margins being unstable systems most reports of huge submarine slope failure have come from oceanic volcanoes and passive margins swath bathymetry and seismic profiles of the northern hikurangi subduction system new zealand show a tapering 65 30 km wide by 65 km deep margin indentation with a giant 3150 630 km3 blocky debris avalanche deposit projecting 40 km out across horizontal trench fill and a debris flow deposit projecting over 100 km slide blocks are well bedded up to 18 km across and 1 2 km high the largest being at the avalanche deposit s leading edge samples dredged from them are mainly miocene shelf calc mudstones similar to those outcropping around the indentation cores from cover beds suggest that failure occurred 170 40 ka possibly synchronously with a major extension collapse in the upper indentation however the northern part of the indentation is much older the steep straight northern wall is close to the direction of plate convergence and probably formed around 2 0 0 16 ma as a large seamount subducted leaving in its wake a deep groove obliquely across the margin and an unstable triangle of fractured rock in the 60 angle between groove and oversteepened margin front the triangle collapsed as a blocky avalanche leaving a scalloped southern wall and probably causing a large tsunami tentative calculations of compacted volumes suggest that the indentation is over 600 km3 larger than the avalanche supporting a two stage origin that includes subduction erosion since failure convergence has carried the deposits 9 km back toward the margin causing internal compression the eventual subduction accretion of the ruatoria avalanche explains the scarcity of such features on active margins and perhaps the nature of olistostromes in fold belts

seamount statistics in the pacific ocean we apply the wide beam sampling technique of jordan et al 1983 to approximately 157 000 km of wide beam profiles to obtain seamount population statistics for eight regions in the eastern and southern pacific ocean population statistics derived from wide beam echograms are compared with seamount counts from sea beam swaths and with counts from bathymetric maps we find that the average number of seamounts with summit heights h h is well approximated by the exponential frequency size distribution h oe h the exponential model for seamount sizes characterized by the single scale parameter 1 is found to be superior to a power law self similar model which has no intrinsic scale in describing the average distribution of pacific seamounts and it appears to be valid over a size spectrum spanning 5 orders of magnitude in abundance large scale regional variations in seamount populations are documented we observe significant differences in seamount densities across the murray fracture zone in the north pacific and the eltanin fracture zone system in the south pacific the eltanin discontinuity is equally evident on both sides of the pacific antarctic ridge in the south pacific regions symmetrically disposed about the ridge axis have very similar seamount densities despite the large difference between pacific plate and antarctic plate absolute velocities evidently any differences in the shear flows at the base of the pacific and antarctic plates do not affect seamount emplacement systematic variations in o and are observed as a function of lithospheric age with the number of large seamounts increasing more rapidly than small seamounts these observations have been used to develop a simple model for seamount production under the assumptions that 1 an exponential size frequency distribution is maintained 2 production is steady state and 3 most small seamounts are formed on or near the ridge axis the limited data available from this study appear to be consistent with the model but they are insufficient to provide a rigorous test of the assumptions or determine accurately the model parameters however the data from the south pacific indicate that the off axis production of large seamounts probably accounts for the majority of seamounts with summit heights greater than 1000 m

robust characteristics method for modelling multiphase visco elasto plastic thermo mechanical problems abstract we have extended our previous 2d method gerya t v yuen d a 2003 characteristics based marker in cell method with conservative finite differences schemes for modeling geological flows with strongly variable transport properties phys earth planet interiors 140 295 320 which is a combination of conservative finite differences with marker in cell techniques to include the effects of visco elasto plastic rheology self gravitation and a self consistently derived evolving curvilinear planetary surface this code is called i2elvis and can solve a new class of computationally challenging problems in geodynamics such as shear localization with large strains crustal intrusion emplacement of magmas bending of realistic visco elasto plastic plates and core formation by vigorous shell tectonics activities related to a global rayleigh taylor instability of a metal layer formed around silicate rich lower density primordial core during planetary accretion we discuss in detail the computational strategy required the rheological constraints to be satisfied at each time step and spatial location we show analytical benchmarks and examples drawn from comparing between numerical and analogue experiments in structural geology subducting slab bending with a visco elasto plastic rheology and equilibrium spherical configurations from self gravitation we have also tested possibilities of future applications by addressing 3d geometries based on multigrid method and including inertial effects in the momentum equation with tracers in order to simulate meteoritic impact events and eventually earthquake instabilities

mechanical and electrical response due to fluid pressure equilibration following an earthquake 1 the mechanical and electrical response in a uniform porous crust are determined following a shear dislocation earthquake on an internal slip surface a uniform crust is studied because many exact analytical relations hold between the various response fields in this case the initial stress field that is created immediately following the earthquake subsequently relaxes through time as the fluid pressure equilibrates by fluid flow maps of the electric field generated due to the fluid flow electrokinetic coupling are presented using conservative estimates of the various parameters involved electric fields significantly larger than the diurnal magnetotelluric fields are generated quantitative results for how various components of the stress tensor change due to the fluid equilibration are also presented the coulomb stress can easily change by 100 and more in the months following an earthquake due to the fluid pressure equilibration

3d effects of strain vs velocity weakening on deformation patterns in accretionary wedges abstract shear zones are weaker than surrounding rocks whether weakening depends on accumulated strain or strain rate is debated we used a three dimensional numerical code with a visco plastic brittle rheology to investigate the influence of strain and strain rate weakening often referred to as velocity weakening on the evolution of thin skinned fold and thrust belts two model setups are used i a uniform setup to recognize the effects of each weakening mode on the structural evolution and dynamics of fold and thrust belts and ii a dual setup with two adjacent domains of decollement strength to investigate the structural response of laterally varying systems strain weakening and velocity weakening lead to remarkably different structural patterns in particular strain weakening favours out of sequence shear zones whilst some amount of finite plastic strain has to be achieved before the structural development initiates weakening in contrast velocity weakening starts with the onset of high strain rate bands and does not favour out of sequence deformation we also tested separately the influences of shortening velocity cover sequence thickness and weakening style and amount on the structural evolution of the dual systems the shortening velocity has a minor effect in strain weakened models velocity weakening and a thinner cover sequence enhance the occurrence of strike slip shear zones we conclude that the weakening mode strongly influences the fault patterns and the dynamics of thrust wedges inversely natural fault patterns are keys to discriminate weakening processes

coulomb wedge model applied to the subduction of seamounts in the japan trench results from the french japanese kaiko exploration program of the japan trench and complementary data are used to propose a model of subduction of seamounts examples in two seamount chains one at the junction between the japan and kuril trenches and the other in the southern part of the japan trench off inubo cape illustrate the stages of subduction the japan trench margin evolved as a homogeneous wedge of deformable noncohesive coulomb material during seamount subduction this is illustrated by the subduction of the daiichi kashima seamount in the southern japan trench our model predicts that the first consequence of seamount subduction is a compressive thickening of the toe of the wedge above the advancing flank of the seamount because of the oversteepening of the rigid base of the wedge this agrees with our observations in the daiichi kashima area this thickening shifts landward across the wedge with the advancing flank of the seamount erosion dominates the wedge above the trailing flank of the seamount because of the corresponding understeepening of the rigid base of the wedge a reentrant of the toe of the wedge is thus created it is largest when the oceanic flank of the seamount coincides with the base of the landward slope at this time the trench axis will probably have an abnormally large amount of sediments as illustrated at the junction between the japan and kuril trenches after the subduction of the seamount the edge of the wedge should progressively move back to its initial location frontal accretion is absent or very limited

deformation of accretionary wedges in response to seamount subduction insights from sandbox experiments sandbox experiments using a two dimensional and a three dimensional approach are used to study the deformation of margins in response to seamount subduction successive mechanisms of deformation are activated during the subduction of conical seamounts first reactivation of the frontal thrusts and compaction of the accretionary wedge is observed then back thrusting and conjugate strike slip faulting develops above the leading slope of the subducted seamount the basal d6collement is deflected upward in the wake of the subducting high and a large shadow zone develops behind the seamount trailing slope consequently frontal accretion is inhibited and part of the frontal margin is dragged into the subduction zone when the main d6collement returns to its basal level in the wake of the seamount the margin records a rapid subsidence and a new accretionary wedge develops closing the margin reentrant the sediments underthrusted in the wake of the seamount into the shadow zone are underplated beneath the rear part of the accretionary wedge substantial shortening and thickening of the deformable seaward termination of the upper plate basement associated with basal erosion is observed seamount subduction induces significant material transfer within the accretionary wedge favors large tectonic erosion of the frontal margin and thickening of the rear part of the margin the subduction and underplating of relatively undeformed water ladden sediments associated with fluid expulsion along the fractures affecting the margin could modify the fluid pressure along the basal ddcollement consequently significant variations of the effective basal friction and local mechanical coupling between the two plates could be expected around the subducting seamount

pacific plate subduction beneath the central mariana and izu bonin fore arcs new insights from an old margin 1 multichannel seismic mcs profiles and bathymetric data from the central mariana and izu bonin subduction systems image the subducting pacific plate from the outer trench slope to beneath serpentinite seamounts on the outer fore arc subducting oceanic crust varies along the mariana margin from 5 3 to 7 km thick and is covered by 0 5 2 km thick sediments and numerous seamounts oceanic crustal thickness east of the izu bonin trench is 6 km faulting resulting from flexure of the incoming pacific plate begins up to 100 km east of the trench axis near the 6 km depth contour the plate is cut by normal faults that reactivate inherited tectonic fabric where that fabric strikes 25 incoming crust breaks along new faults with a trench parallel strike the mariana trench axis is commonly a graben that accommodates an abrupt change within 8 beneath the outer fore arc we infer that the plate fails there rather than simply bends under the applied loads along portions of the mariana margin subducting seamounts displace the trench axis westward and uplift the toe of the slope surprisingly west of the toe there is no geophysical evidence of disturbance of the upper plate in response to seamount subduction nor of significant subduction erosion or sediment underplating mcs profiles across the base of the mariana inner trench slope provide evidence for both complete subduction and small scale accretion of pacific plate sediments however we found no evidence for long term sediment accretion the subducting plate dips 9 12 beneath serpentinite seamounts on the izu bonin and mariana fore arcs along the mariana margin the majority of these seamounts are located 50 70 km west of the trench where the mantle wedge is 3 7 km thick between 8 10 km thick fore arc crust and the top of the subducting plate the apparent lack of significant deformation of the mariana fore arc crust by subducting seamounts may be the result of a weak serpentinized mantle wedge and or progressive fracturing as the subducting plate increases in dip as it passes through the trench graben

geology and tsunamigenic potential of submarine landslides in santa barbara channel southern california abstract a large submarine landslide complex and four small landslides developed under the santa barbara channel suggesting a potential hazard from landslide generated tsunamis we integrate offshore stratigraphy and geologic structure multibeam bathymetric information and several kinds of seismic reflection data to understand how and when the submarine landslides formed seismic reflection data show that mass failure along the slope began at least 200 ka ago landslides appear as zones of poor reflectivity having an irregular upper surface and these zones alternate vertically with strong parallel reflections the emplacement ages of two of the three main landslide lobes are well established at 8 and 10 ka the source material for the youngest part of the landslide complex was sediment of probable late pleistocene and holocene age that accumulated in a shelf edge delta directly under this delta growth of faults and anticlines was particularly intense and tended to oversteepen the deltaic deposits these active structures also formed migration pathways and reservoirs for aqueous and hydrocarbon fluids from the deep basin tsunami deposits have been described from a low lying area near santa barbara and numerical modeling of tsunamis generated by hypothetical landslides in santa barbara channel indicates a moderate to severe threat borrero j c dolan j f and synolakis c e 2001 tsunamis within the eastern santa barbara channel geophys res lett 28 4 643 646 involving wave runups of 2 20 m for a range of assumed landslide volumes inundation from these waves however is expected to be highly focused so that only narrow 10 km sections of the shoreline would be affected

effects of oceanic ridge subduction on accretionary wedges experimental modeling and marine observations sandbox modeling is used to study the deformation of accretionary wedges caused by the subduction of oceanic ridges the first experiment incorporates a massive ridge within a sand wedge the wedge thickens and shortens when the forward propagation of the basal decollement ceases the wedge thickening results in taper change reactivation of preexisting thrusts and retreat of the frontal part of the sand wedge similar mechanisms may have affected some margins that have undergone ridge subduction such as the tonga margin after the subduction of the oblique louisville oceanic ridge the second experiment shows the effects of an active basement thrust slice as it enters a subduction zone this process may have happened in the eastern nankai accretionary wedge initially the wedge in this experiment behaved similarly to that of the first experiment rapidly the topographic slope changed the wedge thickened above the basement slice generating a slope break in the topography a deeper propagating accretionary wedge again characterized by a small taper developed these results when compared with observations made in the eastern nankai trench are in agreement with the past subduction of a basement thrust slice in this area

the effect of seamount subduction on seismic coupling it has long been speculated that the subduction of seamounts may have profound mechanical consequences particularly on the generation of large subduction zone earthquakes we consider this problem as a special case of the seismic coupling model of scholz and campos the model predicts that the subduction of a large seamount will increase the normal stress across the subduction interface and hence will enhance seismic coupling in the case of coupled arcs this increased coupling will result in a large increase in the recurrence intervals of earthquakes such events will therefore appear rare in the instrumental record as is observed for decoupled arcs the subduction of a large seamount should cause them to become locally coupled three of the latter localities have been found with satellite altimetry data two on the tonga kermadec trench and one on the izu bonin trench the diagnostic feature is a shallowing or obstruction of the trench axis adjacent to a chain or group of large seamounts on the subducting plate in each case the small to moderate plate interface seismicity typical of decoupled seismic zones is almost completely absent in a 100 200 km arc segment indicating that the subduction zone is locally locked as predicted by the model the occurrence of large interplate earthquakes at both tonga sites confirms that they are seismically coupled

global distribution of seamounts from ship track bathymetry data the distribution of submarine volcanoes or seamounts reflects melting within the earth and how the magma generated ascends through the overlying lithosphere globally 60 latitude we use bathymetry data acquired along 39 5 106 km of ship tracks to find 201 055 probable seamounts an order of magnitude more than previous counts across a wider height range 0 1 1 km implying that 24 000 60 remain to be discovered

did a submarine landslide contribute to the 2011 tohoku tsunami many studies have modeled the tohoku tsunami of march 11 2011 as being due entirely to slip on an earthquake fault but the following discrepancies suggest that further research is warranted 1 published models of tsunami propagation and coastal impact underpredict the observed runup heights of up to 40 m measured along the coast of the sanriku district in the northeast part of honshu island 2 published models cannot reproduce the timing and high frequency content of tsunami waves recorded at three nearshore buoys off sanriku nor the timing and dispersion properties of the waveforms at offshore dart buoy 21418 3 the rupture centroids obtained by tsunami inversions are biased about 60 km nne of that obtained by the global cmt project based on an analysis of seismic and geodetic data together with recorded tsunami waveforms we propose that while the primary source of the tsunami was the vertical displacement of the seafloor due to the earthquake an additional tsunami source is also required we infer the location of the proposed additional source based on an analysis of the travel times of higher frequency tsunami waves observed at nearshore buoys we further propose that the most likely additional tsunami source was a submarine mass failure smf i e a submarine landslide a comparison of pre and post tsunami bathymetric surveys reveals tens of meters of vertical seafloor movement at the proposed smf location and a slope stability analysis confirms that the horizontal acceleration from the earthquake was sufficient to trigger an smf forward modeling of the tsunami generated by a combination of the earthquake and the smf reproduces the recorded on near and offshore tsunami observations well particularly the high frequency component of the tsunami waves off sanriku which were not well simulated by previous models the conclusion that a significant part of the 2011 tohoku tsunami was generated by an smf source has important implications for estimates of tsunami hazard in the tohoku region as well as in other tectonically similar regions

a subducting seamount beneath the mediterranean ridge abstract two transverse morphological tracks across the front of the mediterranean ridge were swath mapped with a hydrosweep instrument system along the tracks ridges and troughs at the front of the accretionary prism are disrupted and bent in the direction of relative plate convergence the longer of the two tracks extends to bannock basin a brine filled depression with a closed outer moat around a central mound seismic reflection data and a gravity anomaly show a buried circular seamout beneath bannock basin whose summit is the mound this indicates an origin of the larger transverse track and bannock basin from a subducting seamount beneath bannock basin the seamount has displaced about 340 km 3 of the messinian section and yet the seafloor has not been uplifted therefore considerable evaporitic material is inferred to have been displaced by the seamount and bannock basin is a likely vent for large quantities of dissolved materials from the disrupted messinian evaporitic sequence

do subducting seamounts generate or stop large earthquakes seamount subduction is a common process in subduction zone tectonics contradicting a widely held expectation that subducting seamounts generate large earthquakes seamounts subduct largely aseismically producing numerous small earthquakes on rare occasions when they do produce relatively large events the ruptures tend to be complex suggesting multiple rupture patches or faults we explain that the seismogenic behavior of these seamounts is controlled by the development and evolution of an adjacent fracture network during subduction and cannot be described using the frictional behavior of a single fault the complex structure and heterogeneous stresses of this network provide a favorable condition for aseismic creep and small earthquakes but an unfavorable condition for the generation and propagation of large ruptures

from seamount to oceanic island porto santo central east atlantic the uplifted and deeply eroded volcanic succession of porto santo central east atlantic is the product of a wide spectrum of dynamic processes that are active in shoaling to emergent seamounts two superimposed lapilli cones marking the base of the exposed section are interpreted as having formed from numerous submarine to subaerial phreatomagmatic explosions pyroclastic fragmentation being subordinate the lower basaltic and the upper mugearitic to trachytic sections are dominated by redeposited tephra and are called lapilli cone aprons vertical growth due to accumulation of tephra voluminous intrusions and minor pillowed lava flows produced ephemeral islands which were subsequently leveled by wave erosion as shown by conglomerate beds periods of volcanic quiescence are represented by abundant biocalcarenite lenses at several stratigraphic levels the loose tephra piles became stabilized by widespread syn volcanic intrusions such as dikes and trachytic to rhyolitic domes welding the volcanic and volcaniclastic ensemble into a solid edifice shattering of a submarine extrusive trachytic dome by pyroclastic and phreatomagmatic explosions accentuated by quench fragmentation resulted in pumice and crystal rich deposits emplaced in a prominent submarine erosional channel the dome must have produced an island as indicated by a collapse breccia comprising surf rounded boulders of dome material subaerial explosive activity is represented by scoria cones and tuff cones basaltic lava flows built a resistant cap that protected the island from wave erosion some lava flows entered the sea and formed two distinct types of lava delta 1 closely packed pillow lava and massive tabular lava flows along the southwestern coast of porto santo and 2 a steeply inclined pillow hyaloclastite breccia prism composed of foreset bedded hydroclastic breccia variably shaped pillows and thin sheet flows capped by subhorizontal submarine to subaerial lava flows along the eastern coast of porto santo the facies architectures indicate emplacement 1 on a gently sloping platform in southwestern porto santo and 2 on steep offshore slopes along high energy shorelines in eastern porto santo growth of the pillow hyaloclastite breccia prism is dominated by the formation of foreset beds but various types of syn volcanic intrusions contributed significantly submarine flank eruptions occurred in very shallow water on the flanks of the hyaloclastite prism in eastern porto santo the island became consolidated by intrusion of numerous dikes and by emplacement of prominent intrusions that penetrate the entire volcanic succession volcanic sedimentation ended with the emplacement of a debris avalanche that postdates the last subaerial volcanic activity

seafloor structural geomorphic evolution of the accretionary frontal wedge in response to seamount subduction poverty indentation new zealand abstract high quality swath bathymetry and seismic reflection data reveal that the steep forearc slope along the northern sector of the obliquely convergent hikurangi subduction zone is characteristic of non accretionary and tectonically eroding continental margins with reduced sediment supply in the trench relative to further south and the presence of subducting seamount relief on the hikurangi plateau these seamounts influence the subduction process and the structurally driven geomorphic development of the over riding margin of the australian plate frontal wedge the origin of the poverty indentation on the inboard trench slope is attributed to multiple seamount impacts over the last c 1 myr period accompanied by canyon incision thrust fault propagation into the trench fill and numerous large scale gravitational collapse structures with multiple debris flow and avalanche deposits ranging in downslope length from a few hundred metres to more than 40 km the indentation is directly offshore of the waipaoa river which is currently estimated to have a high sediment yield into the marine system the poverty canyon stretches 70 km from the continental shelf edge directly offshore from the waipaoa to the trench floor incising into the axis of the indentation the sediment delivered to the margin from the waipaoa catchment and elsewhere during sea level high stands including the holocene has remained largely trapped in a large depocentre on the poverty shelf while during low stand cycles sediment bypassed the shelf to develop a prograding clinoform sequence out onto the upper slope the formation of the indentation and the development of the upper branches of the poverty canyon system has led to the progressive removal of a substantial part of this prograding wedge by mass movements and gully incision sediment has also accumulated in the head of the poverty canyon and episodic mass flows contribute significantly to continued modification of the indentation by driving canyon incision and triggering instability in the adjacent slopes prograding clinoforms lying seaward of active faults beneath the shelf and overlying a buried inactive thrust system beneath the upper slope reveal a history of deformation accompanied by the creation of accommodation space the middle to lower poverty canyon represents a structural transition zone within the indentation coincident with the indentation axis the lower to mid slope south of the canyon conforms more closely to a classic accretionary slope deformation style with a series of east facing thrust propagated asymmetric anticlines separated by early stage slope basins north of the canyon system seamount impact has resulted in frontal tectonic erosion associated with the development of an over steepened lower to mid slope margin fault reactivation and structural inversion and over printing

weak interplate coupling by seamounts and repeating m 7 earthquakes subducting seamounts are thought to increase the normal stress between subducting and overriding plates however recent seismic surveys and laboratory experiments suggest that interplate coupling is weak a seismic survey in the japan trench shows that a large seamount is being subducted near a region of repeating earthquakes of magnitude m 7 both observed seismicity and the pattern of rupture propagation during the 1982 m 7 0 event imply that interplate coupling was weak over the seamount a large rupture area with small slip occurred in front of the seamount its northern bound could be determined by a trace of multiple subducted seamounts whereas a subducted seamount itself may not define the rupture area its width may be influenced by that of the seamount

seamount subduction erosion in the nankai trough and its potential impact on the seismogenic zone seamount subduction along subduction zone plate boundary thrusts has long been implicated as a mechanism for abrasion and tectonic erosion of the base of the overriding plate however tectonic erosion processes have not been examined in detail with high quality three dimensional 3 d seismic reflection imaging in 1999 we acquired 3 d seismic reflection data from the nankai trough subduction zone to image the plate boundary fault and the overlying accretionary wedge structure fortuitously these data reveal a small to 1 km high basement ridge that has subducted to 7 km subseafloor updip from the basement ridge a 1 km thick sequence of sediment from the base of the accretionary wedge appears to be missing we interpret these data as evidence for tectonic erosion of the base of the accretionary wedge following the basement ridge subduction tectonic erosion has removed more than 25 km 3 from the updip edge of the seismogenic zone and carried it down into the seismogenic zone the tectonically eroded sediments are presumed to enhance fault zone fluid content potentially reducing fault zone effective stress and may temporarily inhibit earthquake rupture potential after the passage of the ridge the boundary fault returns to its former position in a period of enhanced underplating

thrust type subduction zone earthquakes and seamount asperities a physical model for seismic rupture a thrust type subduction zone earthquake of m sub w 7 6 ruptures an area of approximately 6 000 km sup 2 has a seismic slip of approximately 1 m and is nucleated by the rupture of an asperity approximately 25km across a model for thrust type subduction zone seismicity is proposed in which basaltic seamounts jammed against the base of the overriding plate act as strong asperities that rupture by stick slip faulting a m sub w 7 6 event would correspond to the near basal rupture of a approximately 2 km tall seamount the base of the seamount is surrounded by a low shear strength layer composed of subducting sediment that also deforms between seismic events by distributed strain viscous flow planar faults form in this layer as the seismic rupture propagates out of the seamount at speeds of kilometers per second the faults in the shear zone are disrupted after the event by aseismic slow viscous flow of the subducting sediment layer consequently the extent of fault rupture varies for different earthquakes nucleated at the same seamount asperity because new fault surfaces form in the surrounding subducting sediment layer during each fast seismic rupture

upper plate deformation associated with seamount subduction in many active margins severe deformation is observed at the front of the overriding plate where seamounts or aseismic ridges subduct such deformation appears to be a main tectonic feature of these areas which influences the morphology and the seismicity of the margin to better understand the different stages of seamount subduction we have performed sandbox experiments to study in detail the evolution of deformation both in space and time and thus complement seismic images and bathymetry interpretation we focus in this paper on the surface deformation directly comparable with seafloor morphology two types of subducting seamounts were modelled relatively small conical seamounts and larger flat topped seamounts the indentation of the margin by the seamount inhibits frontal accretion and produces a re entrant the margin uplift includes displacement along backthrusts which propagate from the base of the seamount and out of sequence forethrusts which define a shadow zone located on the landward flank of the seamount when the seamount is totally buried beneath the margin this landward shielded zone disappears and a larger one is created in the wake of the asperity due to

mineralogy and paragenesis of the co ni arsenide ores of bou azzer anti atlas morocco the co ni arsenide ore deposits at bou azzer morocco represent an uncommon type of co mineralization several co ni as au ag mines are active in bou azzer and adjacent areas the co ni arsenide ores are structurally controlled and concentrated mainly within quartz carbonate veins along the boundaries of neoproterozoic serpentinized mantle peridotites quartz diorite and precambrian volcanic rocks they have varying shapes and sizes however all deposits have the same general mineralogy in decreasing order of abundance arsenides sulfarsenides and sulfides arsenide minerals include skutterudite coas3 safflorite coas2 loellingite feas2 nickeline nias rammelsbergite nias2 and s rich ni co diarsenide co rich ores are predominant over ni rich ones in almost all deposits arsenopyrite and to a lesser extent gersdorffite are the main sulfarsenide minerals in almost all samples studied bornite an unidentified cu zn sulfide chalcocite chalcopyrite pyrite pyrrhotite sphalerite and tetrahedrite in decreasing order of abundance are the main sulfide minerals present in bou azzer ores gold silver alloys with about 15 wt percent ag on average are commonly associated with ni co arsenides and or disseminated in quartz veins in the ni rich assemblage pure nickeline with high ni content 48 wt and very low co content 0 1 wt grades outward to compositions with less ni 20 wt and very high co up to 18 wt this suggests a depositional sequence from ni monoarsenide nickeline to ni co diarsenide to cobalt triarsenides i e a progressive increase in co and as the ore forming elements co ni fe and as were originally leached from serpentinites by acidic magmatic fluids under moderately reducing conditions at high fluid rock ratios deposition of ore minerals occurred in response to increasing ph and decreasing oxygen fugacity of the mineralizing fluids caused by mixing between the magmatic brines and meteoric water the predominance of co over ni arsenide minerals in the bou azzer mineralization despite the high ni co ratio of the serpentinites may be attributable to the different solubilities of ni and co in the hydrothermal system

tsunamis within the eastern santa barbara channel several locally generated tsunamis have been reported in southern california during the past 200 years yet the hazard from locally generated tsunamis has received considerably little attention we consider here tsunamis generated by coseismic displacements on the channel islands thrust cit system as well as waves generated by slope failures along the walls of the santa barbara channel we find that purely tectonic sources could generate regional tsunamis with 2m runup whereas combinations of tectonic sources and submarine mass movements could generate local runup as large as 15m