high resolution numerical modeling of stress distribution in visco elasto plastic subducting slabs abstract we employ high resolution 2d thermomechanical modelling with complex elasto visco plastic rheology to analyse stresses in subducting and overriding plates the model contains a dynamic subduction channel defined as a few km thick zone of specific weak visco plastic rheology between the slab and the overriding plate our high resolution model corresponds to the zoom in of the large scale thermomechanical model of the central andes with the 4 fold resolution increase initial model configuration material temperature and stress distributions was interpolated from the parent model boundary conditions including slab pull and push velocities as well as velocity of the overriding plate were also re mapped from the large scale model we demonstrate that for shallow dipping overridden slabs the dominant stress is unbending stress at 50 100 km depth this stress reaches 1 1 5 gpa and strongly prevails over the slab bending stress near the trench which is limited by the frictional plastic yield unbending creates double 5 10 km distance between maximums compression zone in the upper part of the slab and a zone of extension some 20 30 km below pressure in these zones differs from the lithostatic pressure to as much as plus minus 0 8 gpa details of the distribution and magnitude of bending and unbending stresses in the slab are mainly controlled by the overriding velocity and direction of the slab pull force overpressure in the subduction channel in all models does not exceed 100 mpa large over and under pressures in the slab do not however lead to significant more than 10 km offsets of main dehydration reactions in crust and mantle of the slab

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

past forearc deformation in nicaragua and coupling at the megathrust interface evidence for subduction retreat 1 deformation at the boundary between the cocos and caribbean plates in nicaragua is currently focused in the volcanic arc in the form of arc parallel shearing in the middle miocene however there was widespread subsidence with local uplift in the outer forearc and inner forearc broad uplift to understand this complex deformation we use numerical modeling to investigate the effect of inhomogeneous friction at the megathrust fault on strain localization in the upper plate a good fit is obtained when 1 the interface between the slab and upper plate is not situated at the current wadati benioff zone but rather is moved 50 km landward to where subduction may have been active in the early tertiary and 2 there is high stress accumulation between 15 and 25 km depths and low stress accumulation updip and downdip of the thrust interface the results are consistent with a high velocity block being the remnant of an oceanic plateau that subducted in the eocene and was later incorporated into the upper plate we suggest that this geometry persisted into the middle miocene before the slab broke off and the thrust interface jumped seaward

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

rock uplift and exhumation of continental margins by the collision accretion and subduction of buoyant and topographically prominent oceanic crust understanding the causes of rock and surface uplift is important because they control the location of mountain building depocenters and drainage characteristics and can influence climate here we combine previous thermochronological data with field observations to determine the amount of exhumation rock and surface uplift that occurs in the upper plate of central and south american subduction zones during the collision accretion and subduction of oceanic plateaus and aseismic ridges the collision of buoyant and topographically prominent oceanic plateaus and ridges can drive at least 5 km of rock uplift within 2 ma uplift appears to be an immediate response to collision and is generally independent of the slab dip the amount of rock uplift is controlled mainly by excess topography associated with the ridge ultimately linked to buoyancy and erosion while it is also influenced by the strength of the subduction interface related to the presence of volcanic asperities and overpressured sediments in the subduction channel the quantity of exhumation is strongly dependant on climate induced erosion and the lifespan over which the topography is uplifted and supported sediment draining into the trench may leave the elevated ridge axis sediment starved increasing the shear stresses at the ridge subduction interface leading to positive feedback between ridge subduction rock uplift and exhumation trench parallel variations in exhumation have a direct impact on exploration paradigms for porphyry related metalliferous deposits and it is likely that porphyry systems are completely eroded by the impingement of plateaus and aseismic ridges within temperate and tropical climates

quantification of interplate coupling in subduction zones and forearc topography the effect of long term coupling in subduction zones on forearc topography is studied using a 2 dimensional finite element model a curved fault representing the interface between subducting slab and overriding plate has been incorporated into a model with an elastic crustal layer a viscoelastic upper mantle an initial topography and an effective coefficient of friction on the fault we assume that traction arising from friction is one of the stresses acting on topography for various friction coefficients the change of topography after some equilibration time is computed with specific geometry and kinematic boundary conditions for three subduction zones northern chile northern japan and tonga while the intrinsic coefficient of friction for small rock samples is high 0 6 0 8 the observed topography of forearc regions are inconsistent with an effective friction coefficient larger than 0 2

morphologic and geologic effects of the subduction of bathymetric highs morphologic and geologic observations suggest that subduction of bathymetric highs such as aseismic ridges chains of seamounts and fracture zones are important in the development of many forearc features and that those features form during relatively brief episodes of intense tectonism a bathymetric high obliquely entering a subduction zone tends to compress sediments along its leading edge resulting in arcward compression of the accretionary wedge a landward deflection of the trench axis and a steepened inner wall result from this deformation if a significant component of oblique slip occurs along the subduction zone then along strike movement of the accretionary wedge may also occur stresses resulting from subduction of bathymetric features with sufficient buoyancy or high relief extend farther landward than in the case of smaller less buoyant features inducing uplift of the leading edge of the overriding plate tectonic erosion of the base of the overriding plate and along strike transport of are material may also occur the accelerated tectonism observed along several convergent margins can be attributed to the consumption of bathymetric irregularities on the seafloor rather than temporally abrupt changes in rates and directions of plate motions or other episodic events in the accretionary prism

deformation of erosive and accretive forearcs during subduction of migrating and non migrating aseismic ridges results from 3 d finite element models and application to the central american peruvian and ryukyu margins subduction of aseismic oceanic ridges causes considerable uplift and deformation of the upper plate and may lead for example to the indentation of the forearc the formation of marine terraces or distinct fault patterns in the upper plate depending on the orientation of the ridge relative to the plate convergence direction the ridge may either be stationary or migrate along the margin here we use three dimensional numerical models to investigate the tectonic evolution of forearcs affected by ridge subduction in different experiments we distinguish between migrating non migrating ridges and accretive erosive margins respectively our results reveal that displacement and strain fields above migrating and non migrating ridges are asymmetric with respect to the ridge axis unless both ridge and plate convergence direction are perpendicular to the trench as the asymmetric deformation pattern shifts along the margin through time uplift caused by the underthrusting ridge is followed by subsidence when the ridge crest passed by and regions initially experiencing shortening may subsequently undergo extension and vice versa if the forearc comprises an accretionary prism the ridge induced reentrant is larger than those in models with erosive forearcs and strain localizes in the frontal part of the wedge additional models with a setup adjusted to the cocos and gagua ridges provide constraints on the onset of their subduction at the central american and ryukyu margins at 2 ma and 1 ma respectively displacement and strain fields from a model for the nazca ridge collision zone show good agreement with geological data from marine terraces and quaternary faulting

focused subaerial erosion during ridge subduction impact on the geomorphology in south central peru data obtained from low temperature thermochronometers such as apatite fission track and u th he are combined with morphometric information extracted from digital elevation models this combination shows several geomorphological effects that are caused by the migration of the nazca ridge along the peruvian coastal margin offshore the depth of the deep sea trench decreases by 1500 m where the nazca ridge collides with the continental south american plate onshore the ridge causes an uplift of at least 800 m in the coastal cordillera this uplift results in a westward shift of the coastline thereby focusing and increasing erosion in the uplifted areas at the trailing edge the shelf subsides and the coastline retreats eastwards producing at least part of the indentation observed between paita and pisco the ridge acts therefore like a wave uplifting the andean margin as it traverses inland and southwards leaving a clear fingerprint on the topographic evolution of the peruvian coastal margin

quaternary normal and reverse faulting and the state of stress in the central andes of south peru field studies in the andes of southern peru show that in the high andes and pacific lowlands quaternary and recent faults are normal this extensional tectonics postdates compressional deformations of pliocene early quaternary age in the sub andes the observed deformations are compressional they affect early quaternary deposits some of the faults separate quaternary deposits from the bedrock and thus are clearly of tectonic origin and not landslide effects striations on the fault planes indicate n s trending extension in the high andes and pacific lowlands the total amount of crustal stretching is small probably of the order of 1 during the last 1 2 m y in the sub andes folds and faults affecting neogene and early quaternary deposits result from n s shortening nevertheless it is supposed that this n s shortening is of early quaternary age the present day compression probably strikes e w judging from focal mechanisms in the sub andes of central peru southern bolivia and northwest argentina data from structural analysis of faults and from earthquake focal mechanisms allow us to surmise the state of stress in the andes of southern peru the high andes and pacific lowlands subjected to n s trending extension are bounded by two zones of e w trending compression the sub andes to the east and the contact between the convergent nazca and south america plates to the west in our model the maximum horizontal compressive stress trajectory hmax is roughly parallel with the e w convergence between the two plates hmax corresponds to 1 in the sub andes and to 2 in the high andes the latter situation is caused by the elevated mass of the high andes where zz the vertical stress is inferred to be 1 thus the third principal stress axis being orthogonal to the other two axes it is oriented n s allowing extension to occur in that direction on the other hand in the sub andes zz is 3 and horizontal e w shortening occurs the state of stress in the andean continental crust above the 30 dipping slab appears to be different from that in the andes of central peru situated above the flat subducting segment in this region compressional deformantion affect a wider part of the cordillera

observations on microstructure of pumice particles using computed tomography pumice occurs widely in new zealand and is important in infrastructure construction but is not readily characterized in situ it is a unique material the particles of which crush easily under finger pressure making it arguably the most delicate of the suite of crushable soils found in various locations around the world in this study we scanned pumice particles of different sizes using an x ray computed tomography ct scanning machine to investigate particle microstructure using image analyses we were able to determine the solid density of pumice more accurately than the standard method which showed increasing density as particle size decreased results from the ct scanning showed a solid density of about 2 2 g cm3 which is similar to the highest value obtained using the standard method on the other hand the apparent density from ct scanning was 0 89 g cm3 which is lower than the density of water therefore confirming that almost all pumice particles float on water in addition the volume of

the migration history of the nazca ridge along the peruvian active margin a re evaluation abstract the collision zone of the 200 km wide and 1 5 km high nazca ridge and the peruvian segment of the convergent south american margin between 14 s and 17 s is characterized by deformation of the upper plate and several hundred meters of uplift of the forearc this is evident by a narrowing of the shelf a westward shift of the coastline and the presence of marine terraces as the nazca ridge is oblique with respect to both trench and convergence direction of the nazca plate it migrates southward along the active plate boundary for reconstructing the migration history of the nazca ridge this study uses updated plate motion data resulting from a revision of the geomagnetic time scale the new model suggests that the ridge crest moved laterally parallel to the margin at a decreasing velocity of 75 mm a before 10 8 ma 61 mm a 10 8 4 9 ma and 43 mm a 4 9 ma to present intra plate deformation associated with mountain building in the peruvian andes since the miocene reduces the relative convergence rate between nazca plate and peruvian forearc taking an intra plate deformation at a rate of 10 mm a estimated from space geodetic and geological data into account does not significantly reduce these lateral migration velocities constraining the length of the original nazca ridge by its conjugate feature on the pacific plate yields a length of 900 km for the subducted portion of the ridge using this constraint ridge subduction began 11 2 ma ago at 11 s therefore the nazca ridge did not affect the northern sites of ocean drilling program odp leg 112 located at 9 s this is supported by benthic foraminiferal assemblages in odp leg 112 cores indicating more than 1000 m of subsidence since at least middle miocene time and by continuous shale deposition on the shelf from 18 to 7 ma recorded in the ballena industrial well at 11 5 s the model predicts the passage of the ridge crest 9 5 ma ago this agrees with the sedimentary facies and benthic foraminiferal stratigraphy of odp leg 112 cores which argue for deposition on the shelf in the middle and late miocene with subsequent subsidence of a minimum of several hundred meters onshore at 12 s the sedimentary record shows at least 500 m uplift prior to the end of the miocene also in agreement with the model

effect of aseismic ridge subduction on slab geometry and overriding plate deformation insights from analogue modeling abstract we present analogue models simulating the subduction of a buoyant ridge oriented perpendicularly or obliquely with respect to the trench beneath an advancing overriding plate the convergence velocity is imposed by lateral boundary conditions in this experimental set we analyze the three dimensional geometry of the slab the deformation and topography of the overriding plate experiments suggest that ridge subduction diminishes the dip of the slab eventually leading to the appearance of a horizontal slab segment in case boundary conditions impose a rapid convergence this result contrasts with that obtained in free subduction experiments in which ridge subduction diminishes the convergence velocity which in turn increases the dip of the slab beneath the ridge the slab dip decrease is accompanied by the indentation of the overriding plate by the ridge resulting in arc curvature experiments suggest that indentation is larger for small convergence velocity and large slab dip ridge subduction also uplifts the overriding plate uplift first occurs close to the trench fore arc area and is accompanied by the flexural subsidence of the overriding plate behind the uplifted area back arc subsidence the uplifted area migrates within the overriding plate interiors following the appearance of a horizontal slab segment these results are compared with natural examples of ridge subduction in the circum pacific area they explain why ridge subduction may have contrasted effects on the overriding plate dynamics depending on the global conditions that constrain the converging system

from oceanic plateaus to allochthonous terranes numerical modelling abstract large segments of the continental crust are known to have formed through the amalgamation of oceanic plateaus and continental fragments however mechanisms responsible for terrane accretion remain poorly understood we have therefore analysed the interactions of oceanic plateaus with the leading edge of the continental margin using a thermomechanical petrological model of an oceanic continental subduction zone with spontaneously moving plates this model includes partial melting of crustal and mantle lithologies and accounts for complex rheological behaviour including viscous creep and plastic yielding our results indicate that oceanic plateaus may either be lost by subduction or accreted onto continental margins complete subduction of oceanic plateaus is common in models with old 40 ma oceanic lithosphere whereas models with younger lithosphere often result in terrane accretion three distinct modes of terrane accretion were identified depending on the rheological structure of the lower crust and oceanic cooling age frontal plateau accretion basal plateau accretion and underplating plateaus complete plateau subduction is associated with a sharp uplift of the forearc region and the formation of a basin further landward followed by topographic relaxation all crustal material is lost by subduction and crustal growth is solely attributed to partial melting of the mantle frontal plateau accretion leads to crustal thickening and the formation of thrust and fold belts since oceanic plateaus are docked onto the continental margin strong deformation leads to slab break off which eventually terminates subduction shortly after the collisional stage has been reached crustal parts that have been sheared off during detachment melt at depth and modify the composition of the overlying continental crust basal plateau accretion scrapes oceanic plateaus off the downgoing slab enabling the outward migration of the subduction zone new incoming oceanic crust underthrusts the fractured terrane and forms a new subduction zone behind the accreted terrane subsequently hot asthenosphere rises into the newly formed subduction zone and allows for extensive partial melting of crustal rocks located at the slab interface and only minor parts of the former oceanic plateau remain unmodified oceanic plateaus may also underplate the continental crust after being subducted to mantle depth u hp terranes are formed with peak metamorphic temperatures of 400 700 c prior to slab break off and subsequent exhumation rapid and coherent exhumation through the mantle along the former subduction zone at rates comparable to plate tectonic velocities is followed by somewhat slower rates at crustal levels accompanied by crustal flow structural reworking and syndeformational partial melting exhumation of these large crustal volumes leads to a sharp surface uplift

geophysical characteristics of the ninetyeast ridge andaman island arc trench convergent zone abstract the convergence tectonics of the ninetyeast ridge ner upon the andaman island arc trench system is examined through an analysis of etopo2 bathymetry satellite derived free air gravity and seismic data oblique subduction and the buoyancy forces arising from subduction of the ner render the subduction processes near the andaman arc highly complex the bathymetric expression of the ner is visible up to lat 10 n but seismic reflection data indicate that it extends up to about lat 17 n the gravity anomalies are strongly positive over the exposed segment of the ridge but are subdued over the buried portion there is a prominent break in the continuity of the trench gravity low where the ner seems to impinge upon the island arc further a strong curvilinear belt of negative anomalies just behind and running parallel to the island arc associated with the forearc basin is a dominant feature of the gravity map an offset in the continuity of this strong negative anomaly occurs at about the same latitude where the ner seems to be converging upon the island arc seismic reflection data indicate that the ner is very close to the trench flexural modeling of the gravity anomalies for the subducting indian ocean lithosphere loaded by sediments and the ner indicates that the ner is at the starting phase of its collision with the island arc and may not have started affecting the subduction process itself we infer that the en echelon block structure of the ner in the proximity of the convergent zone is a consequence of complex strike slip and subduction related tectonic forces

late pleistocene crustal uplift and gravity anomaly in the eastern part of kyushu japan and its geophysical implications the miyazaki plain eastern part of kyushu japan is characterized by both significant negative gravity anomalies and aseismic crustal uplifting 1 mm year in the late pleistocene and holocene we examine the relationship between these two phenomena which may provide important constraints on the interaction between the collision and or subduction of the kyushu palau ridge and the forearc we estimate the mass deficiency below 11 km depth by using the gravity anomalies and p wave velocity structure of the upper crust the onset of the load accumulation 0 5 0 4 ma is inferred from the movement of the fluvial terraces considering the tephrochronology the loading history is assumed to be a linear function of time we evaluate the crustal rebound by assuming a viscoelastic plate deformation with an underplating load existing at 20 or 30 km depth the predicted crustal movement for models with a lithospheric crustal viscosity of 1023 1024 pa s can explain the observed altitudes of the shoreline of the marine terraces formed at the last interglacial of about 125 kyr bp and the middle holocene of 5 6 kyr bp although we cannot restrict the origin of the buoyant body the subduction of the kyushu palau ridge remnant arc associated with back arc opening of the shikoku basin may be related to the buoyancy for the uplifting region examined here on the other hand the buoyant body off the miyazaki plain probably plays an important role in the interaction between the subducting oceanic slab and the overriding forearc crust thus the observed lateral variation of the interplate coupling on the convergent boundary along the nankai trough may be attributed to the existence of the buoyant body

forearc response to subduction of the cocos ridge panama costa rica stratigraphic paleontologic and structural data from two panama costa rica forearc peninsulas located landward of the aseismic cocos ridge along the southernmost middle america trench document rapid pliocene subsidence and basin infilling followed by quaternary deformation and uplift on the burica peninsula an approximately 3 000 m thick pliocene pleistocene clastic sedimentary sequence characterized by fine grained turbidite deposits and volumetrically minor coarser grained channel fill slump and debris flow deposits bears depth diagnostic foraminifera that document rapid pliocene subsidence and deposition on a south dipping paleoslope new paleobathymetric and age estimates from foraminifera in these rocks indicate shallowing of depositional paleodepths from 2 000 to 1 200 m during the late pliocene present day exposure of these rocks indicates an average quaternary uplift rate of 1 mm yr the turbidite section on the burica peninsula is interpreted to represent a trench slope deposit and is correlated to a similar marine sedimentary section on the adjacent osa peninsula of costa rica structures developed within pliocene pleistocene strata of the outer forearc burica and osa peninsulas record minor 5 subhorizontal margin perpendicular shortening and margin perpendicular extension bedding orientations on the osa and burica peninsulas generally record arcward tilting by 10 30 of these outer forearc blocks on the burica peninsula the medial fault zone a north striking high angle right lateral strike slip fault has truncated and reoriented the pliocene pleistocene sedimentary sequence by fault bend forced folding a linear margin parallel fault herein named the ballena celmira fault zone bounds the arcward side of the osa and burica peninsulas and has accommodated vertical displacements between the outer forearc burica osa area and inner forearc terraba area throughout the tertiary structural and seismicity observations from the forearc region suggest that neogene deformation of the leading edge of the arc in this area reflects the effects of isostatic uplift and possibly increased interplate shear stresses as a result of subduction of the cocos ridge beneath this part of the arc since 1 m y ago

quaternary uplift of the peruvian coast related to the subduction of the nazca ridge 13 5 to 15 6 degrees south latitude abstract marine terraces between 13 5 and 15 6 s latitude on the peruvian coast preserve a history of quaternary sea level fluctuations and forearc movement above the subducting nazca ridge field mapping and amino acid racemization epimerization electron spin resonance radiocarbon and uranium series disequilibrium ages from mollusk shells from terrace deposits are used to define a marine terrace chronology terraces on the peruvian coast formed during global high stands of quaternary sea level in a similar way to the marine terraces on the west coast of the united states and baja california mexico a last interglacial marine terrace 125 ka is preserved on nearly every terrace flight along the coast from northeast to southwest each terrace varies in elevation because of variations in the rate of coastal uplift variations in uplift rate from northeast to southwest in the study area are explained by a simple geometric model in which the variables are the rate of plate convergence morphology of the nazca ridge and orientation of the ridge relative to the peruvian coast the oblique northeastward orientation of the ridge relative to the east west direction of plate convergence results in a ca 70 mm yr southeastward migration of the zone of nazca ridge subduction beneath the peruvian coast the coastal area above the northern flank and crest of the nazca ridge is stable or has undergone net subsidence while the coast above the ridge crest is rising at about 0 3 m kyr above the southern flank of the nazca ridge the coast is rising at a rate of ca 0 5 m kyr the fastest rate determined from studies of marine terraces on the coasts of peru and chile south of the influence of the nazca ridge uplift rates are 0 2 m kyr the pattern of faster uplift above the southern flank and slower uplift above the northern flank of the ridge is a predictable consequence of oblique ridge subduction

helium isotopes in paleofluids and present day fluids of the larderello geothermal field constraints on the heat source 1 the he isotope composition of paleofluids entrapped in fluid inclusions of hydrothermal minerals is compared with the present day fluid composition of the larderello geothermal field almost constant values of 3he 4he m 3he 4he air r ra over time indicate that no important changes have occurred in the deep source of gases at least during the last 3 8 million years on a regional scale a correlation has been found between the r ra spatial distribution heat flow and bouguer gravity anomaly high values of r ra and heat flow and low bouguer anomaly values indicate that the larderello field is an area of preferential escape for mantle derived fluids a positive correlation has also been found between the r ra spatial distribution and a major seismic reflector named the k horizon a deep magma source refilled by periodic gas input from the mantle is the most likely source of 3he enriched fluids and the anomalously high heat flow the nearly constant value of r ra clearly indicates that input of fresh mantle material has occurred up to recent times clear evidence of mixing between mantle and crustal fluids indicates that the high r ra is the lower limit of the actual mantle value which is suggested to be similar to the subcontinental european mantle the decrease of r ra over time in the peripheral part of the larderello field indicates that important changes in the feeding fracture system and or cooling rate have occurred in these areas

three dimensional finite element models on the deformation of forearcs caused by aseismic ridge subduction the role of ridge shape friction coefficient of the plate interface and mechanical properties of the forearc abstract geological and geophysical data show that the forearc of subduction zones experiences strong deformation during the subduction of aseismic oceanic ridges in order to better understand ridge related forearc deformation patterns we performed a series of three dimensional finite element models in which we varied the ridge shape the friction coefficient of the plate interface and the mechanical strength of the forearc experiments were carried out for migrating non migrating ridges and accretive erosive margins respectively our results show that the subducting ridge uplifts the forearc and induces horizontal displacements that alter the strain regime of both erosive and accretive forearcs generally shortening prevails in front of the ridge while domains of shortening and extension exist above the ridge models with stationary ridges show high uplift rates only above the ridge tip whereas the forearc above migrating ridges experiences uplift above the leading ridge flank and subsequent subsidence above the trailing flank the height and width of the ridge as well as the friction coefficient of the plate interface have the largest effect on the forearc deformation patterns whereas the mechanical strength of the forearc plays a lesser role forearc indentation at the trench is largest for high and broad ridges high friction coefficients and or weak forearc material shortening and extension of the forearc above the ridge are more intense for high and narrow ridges our model results provide information about the distribution of ridge induced displacements and strain fields and hence help to identify deformation patterns caused by subducting aseismic ridges in nature

vergence boundaries an extension of the vergence concept abstract the difficulty in distinguishing between vergence changes on adjacent limbs of tight primary folds and the similar effects produced by non coaxial refolding of primary folds by larger scale folds is outlined it is suggested that a distinction should be made between typical and atypical vergence boundaries atypical vergence boundaries falsely suggest the presence of congruent major folds whereas major folds attributed to typical vergence boundaries are real practical rules are formulated that should be applied to vergence boundaries in non coaxially refolded regions in order to recognize atypical vergence boundaries

the impact of the south american plate motion and the nazca ridge subduction on the flat subduction below south peru 1 flat subduction near peru occurs only where the thickened crust of the nazca ridge subducts furthermore the south america continent shows a westward absolute plate motion both the overriding motion of south america and the subduction of the nazca ridge have been proposed to explain the flat slab segment below south peru we have conducted a series of numerical model experiments to investigate the relative importance of both mechanisms results suggest that the average upper mantle viscosity should be about 3 5 1020 pa s or less and basaltic crust should be able to survive 600 to 800 c ambient temperature before transforming into eclogite to explain the slab geometry below peru the effect of the overriding plate is estimated to be as large or twice as large as that of the plateau subduction

andean coastal uplift and active tectonics in southern peru 10be surface exposure dating of differentially uplifted marine terrace sequences san juan de marcona 15 4 s article i nfo along the san juan de marcona bay of southern peru two spectacular sequences of preserved marine terraces record net quaternary uplift previous geomorphic analysis of these paleo shorelines has revealed evidence of upper plate deformation and regional uplift however in the absence of a robust absolute dating method these studies contain substantial uncertainties concerning the numerical dating of these marine markers and thus the corresponding calculated surface uplift rates however field mapping surveying the neotectonic features and 10 be dating of abraded surfaces contained within two sequences of marine terraces along this margin should allow for the robust calculation of pleistocene uplift rates the san juan de marcona bay lies on the southern flank of the subducting and south migrating aseismic nazca ridge where the maximum rates of coastal uplift are expected in this locality we measure high uplift rates ranging from 0 4 m ka to 0 9 m ka during the pleistocene margin parallel normal faults displace several marine terraces and influence the development of bays thereby contributing to the configuration of paleo and present day coastlines the faults have relatively low slip rates b0 1 m ka over 400 ka and have been inactive for the last 80 ka the presence and activity of these normal faults can be directly linked to subduction zone processes with the release and accommodation of short term coseismic compression during megathrust subduction zone events in contrast the regional permanent uplift is probably controlled by post seismic and or interseismic strain accumulation over longer time scales due to inelastic behavior of the upper plate since at least the latest pliocene the san juan de marcona area has experienced long term regional tectonic uplift that has increased since about 800 ka due to the southward migration of the subducting nazca ridge based on migration velocity and geometry of the ridge the influence of the nazca ridge on the uplift of the forearc should account for 0 4 m ka within 145 km south of the ridge axis hence most of the post 400 ka mean uplift rate of the san juan de marcona area could be explained by the nazca ridge subduction

study of plate deformation and stress in subduction processes using two dimensional numerical models two dimensional finite element modeling is used to model subduction of an oceanic lithospheric plate beneath continental lithosphere the subduction process is initiated along a preexisting inclined fault and continues until reaching 400 km of total convergence the lithosphere is assumed to be underlain by an in viscid asthenosphere different rheological laws have been considered for the lithosphere including elasticity and elastoplasticity the modeling shows that both the stress system in the plates and the surface topography are strongly dependent on two main parameters the density contrast between lithosphere and asthenosphere l a and the coefficient of friction along the subduction plane varying these two parameters allows explanation of the main characteristics of real subduction zones and results in two major regimes manifested by extension or compression in the arc back arc system extension and back arc rifting corresponds to a positive density contrast and a low coefficient of friction while negative values and or high friction leads to a compressional regime the coexistence of trench arc compression and back arc tension is only possible for a coefficient of friction lower than 0 1 the results of the numerical experiments agree with those of experimental modeling conducted under similar physical assumptions

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

enigmatic highly active left lateral shear zone in southwest japan explained by aseismic ridge collision global positioning system gps site velocities and earthquake focal mechanisms reveal an active left lateral shear zone cutting across kyushu in southwest japan surprisingly no active faults have been identified in association with this zone of rapid contemporary deformation to explain the existence of this shear zone we propose a model comprising subduction of an aseismic ridge kyushu palau ridge at the southwest end of the nankai trough because of rapid 40 mm yr along strike migration of the ridge we suggest that the ridge subduction point and resulting left lateral shear zone is never in one place long enough to enable the development of a through going fault zone that can be identified at the ground surface reconciling the mismatch between the gps seismological and geological data in this region our conceptual model is supported by numerical modeling results we also suggest that the along strike change in subducting plate buoyancy explains the recent counterclockwise rotation of the kyushu forearc documented in paleomagnetic studies as is found in many other western pacific subduction margins

backstop geometry and accretionary mechanics of the sunda margin the convergent sunda margin off indonesia displays all geological features characteristic of an accretion dominated subduction zone a combined interpretation of prestack depth migrated seismic reflection data and velocity information gained from refraction studies is supplemented by high resolution bathymetric data and for the first time allows the exact mapping of backstop regimes initially the outer high evolved as material was pushed against a static rigid arc framework backstop underlying a forearc basin increasing material strength of the outer high due to lithification formed a dynamic backstop which controls accretion today an out of sequence thrust marks the transition from the recent active frontal accretionary prism to the outer high and may be traced in the seismic and bathymetric data over the whole extent of the study area the existence of a static as well as a dynamic backstop controls the forearc geometry and is associated with the segmentation of the forearc which is observed in regimes of frontal as well as of oblique subduction mass balance calculations which account for porosity changes and metamorphism indicate a subduction history dominated by accretionary processes since the late eocene accretion is associated with the low values of basal friction inferred for the sunda margin structural investigations of conjugate fault planes indicate a very weak basal detachment effective stress analyses reveal that intrinsically weak material causes the high strength ratio of the detachment to the overlying sediments whereas overpressuring within the frontal accretionary prism is negligible

what drives orogeny in the andes the andes the world9s second highest orogenic belt were generated by the cenozoic tectonic shortening of the south american plate margin overriding the subducting nazca plate we use a coupled thermomechanical numerical modeling technique to identify factors controlling the intensity of the tectonic shortening from the modeling we infer that the most important factor was accelerated westward drift of the south american plate changes in the subduction rate were less important other important factors are crustal structure of the overriding plate and shear coupling at the plates9 interface the model with a thick 40 45 km at 30 ma south american crust and relatively high friction coefficient 0 05 at the nazca south american interface generates 300 km of tectonic shortening during 30 35 m y and replicates the crustal structure and evolution of the high central andes the model with an initially thinner

constraints on inner forearc deformation from balanced cross sections fila costena thrust belt costa rica 1 the fila costena thrust belt in the forearc of costa rica is accommodating a significant portion of the convergence of the cocos plate and panama microplate geologic mapping of the thrust belt depicts a duplex with three horses that incorporate eocene limestones and oligocene to early miocene clastics inboard of the subducting cocos ridge axis by constructing a cross section at this location along a ne sw trending transect perpendicular to the thrust belt we constrain a shortening rate of approximately 40 mm a and propose that as much as 50 of the total plate convergence rate is taken up in the inner forearc the eocene limestones at the base of the thrust sheets pinch out in both directions away from the onland projection of the cocos ridge axis owing to decrease in slip on faults and a lateral ramp in the basal decollement the thrust belt terminates near the panama border at the onland projection of the subducting panama fracture zone these observations suggest that shortening is propagating to the east with the migration of the panama triple junction and the onset of shallow subduction of the thickened edge of the cocos plate the absence of similar features in the nicaraguan forearc where the subducting crust is older subducts more steeply and lacks incoming ridges and seamounts indicates that deformation of the forearc basin in costa rica reflects greater coupling between the converging plates inboard of the cocos ridge

response of the tectonically erosive south peruvian forearc to subduction of the nazca ridge analysis of three dimensional analogue experiments 1 the collision zone of the obliquely subducting nazca ridge and the erosive peruvian forearc is characterized by enhanced tectonic erosion normal faulting forearc uplift and a minor indentation we apply two three dimensional sandbox experiments monitored by an optical particle imaging velocimetry system to decipher the spatial and temporal evolution of forearc deformation during oblique convergence the rigid model ridge causes a wave of uplift followed by subsidence to shift along the forearc by varying the strength of the analogue material we demonstrate that the forearc response to ridge subduction in particular the amount and rate of uplift and wedge indentation is controlled by the mechanical strength of the forearc the model using the high strength material yields results more compatible with the geological record of the peruvian forearc first the modeled amount of uplift scales to 1 2 km in nature which agrees with 1 0 km of uplift recorded by marine terraces second the calibrated model uplift rate of 0 9 km myr is similar to the natural rate of 0 7 km myr third the development of normal faults above the model ridge is in accordance with the style of faulting above the nazca ridge finally oversteepening of the uplifted wedge and local slope failure as observed in the experiment has also been identified off southern peru a comparison between the erosive peruvian and accretive margins emphasizes that the style of ridge induced deformation depends on the forearc strength and crustal structure which in turn are controlled by the long term evolution of the margin s mass transfer regime

rapid pulses of uplift subsidence and subduction erosion offshore central america implications for building the rock record of convergent margins integrated ocean drilling program iodp expedition 334 to southern costa rica central america documented unprecedented subduction erosion in an area of active seismic slip widespread subduction erosion of the upper plate initiated when the cocos ridge an overthickened aseismic ridge arrived at the middle america trench subduction erosion was coeval with the rapid formation of deposition centers on the forearc of the upper plate the completely recovered shelf sequence constrains a short 2 m y interval of extreme subsidence 1200 m with a rapid pulse occurring during the first 0 3 m y this event removed an estimated 1 2 106 km3 of forearc material at a rate of 1690 km3 m y km of trench during a time of rapid 1035 m m y shelf sediment accumulation at this erosive margin a sediment starved trench persisted in spite of abundant sediment supply because subduction erosion led to the creation of forearc basins similar rapid pulses of subduction erosion may punctuate the evolution of many margins contributing disproportionately to crustal recycling at subduction zones with implications for the evolution of continental crust and mountain belts and recycling of continental material into the mantle

ridge subduction at an erosive margin the collision zone of the nazca ridge in southern peru the 1 5 km high obliquely subducting nazca ridge and its collision zone with the peruvian margin have been imaged by wide angle and reflection seismic profiles swath bathymetry and gravity surveying these data reveal that the crust of the ridge at its northeastern tip is 17 km thick and exhibits seismic velocities and densities similar to layers 2 and 3 of typical oceanic crust the lowermost layer contributes 10 12 km to the total crustal thickness of the ridge the sedimentary cover is 300 400 m thick on most parts of the ridge but less than 100 m thick on seamounts and small volcanic ridges at the collision zone of ridge and margin the following observations indicate intense tectonic erosion related to the passage of the ridge the thin sediment layer on the ridge is completely subducted the lower continental slope is steep dipping at 9 and the continental wedge has a high taper of 18 tentative correlation of model layers with stratigraphy derived from ocean drilling program leg 112 cores suggests the presence of eocene shelf deposits near the trench continental basement is located 15 km landward of the trench normal faults on the upper slope and shelf indicate extension a comparison with the peruvian and northern chilean forearc systems currently not affected by ridge subduction suggests that the passage of the nazca ridge along the continental margin induces a temporarily limited phase of enhanced tectonic erosion superposed on a long term erosive regime

the collision zone between the north d entrecasteaux ridge and the new hebrides island arc 1 sea beam morphology and shallow structure sea beam bathymetric data closely spaced single channel seismic reflection sections and geopotential field data were collected aboard the r v j charcot over the collision zone between the north d entrecasteaux ridge ndr and the new hebrides island arc the ndr trends east at a small angle 14 to the plate convergence direction so that the ridge creeps northward along the trench at about 2 5 cm yr requiring a continual structural adjustment of the accretionary complex in this report we study the shallow structure tectonic erosion and consequent mass wasting of the accretionary complex all of which were produced by the collision of the ndr in a complex deformational environment the accretionary complex in this collision zone can be divided into northern central and southern parts the northern part of the collision zone which lies north of the leading flank of the ridge shows a lobate lower accretionary complex that is structured by east to southeast dipping thrust and reverse faults and n60 w trending strike slip faults a middle accretionary complex that bulges seaward and appears to be rotated 45 e from the regional arc trend and an upper accretionary complex that is shaped by slumps and a canyon network the central part of the collision zone which directly overlies the crest of the ridge forms a broad shallow protrusion that is bounded on its northern and southern sides by steep scarps rocks forming this protrusion have been uplifted possibly by as much as 1500 2500 m and tilted to the north causing northward block sliding along an extensional detachment surface the southern part of the collision zone which lies in the wake of the ridge is deformed by large slumps and normal faults that trend parallel to the ridge axis rocks forming this southern part collapse causing widespread mass wasting these geophysical data indicate that normal trench convergence has apparently produced only a few trench parallel structures confined to the toe of the accretionary complex whereas the along trench motion of the ndr has resulted in well developed structures that extend obliquely across the arc slope and form an asymmetric tectonic pattern the area of the accretionary complex that is disturbed by collision appears to be about twice as wide as the width of the ndr and the time required to heal the disturbance may be about 0 8 m y

continental margin deformation along the andean subduction zone thermo mechanical models the chilean andes extend north south for about 3000 km over the subducting nazca plate and show evidence of local rheological controls on first order tectonic features here rheological parameters are tested with numerical models of a subduction zone incorporating slab pull and upper plate convergence which calculate the development of stress and strain over a typical period of 4 myr the models test the effects of subduction interface strength arc and fore arc crust rheology and arc temperature on the development of superficial near surface faulting as well as viscous shear zones in the mantle deformation geometries are controlled by the intersection of the subduction interface with continental rheological heterogeneities upper plate shortening and trench advance are both correlated and favored to a first order by upper plate weakness and to a second order by interface strength in cases of a strong interface a weak forearc crust is dragged downward by tectonic erosion a scenario for which indications are found along the northern chilean margin in contrast for a resistant forearc the slab pull force transmits to the surface and produces topographic subsidence this process may explain present day subsidence of the salar de atacama basin and or the persistence of a central depression specific conditions for northern chile produce a shear zone that propagates from the subduction zone in the mantle through the altiplano lower crust into the subandean crust as proposed by previous studies models with a weak interface in turn allow buoyant subducted material to rise into the continental arc in case of cessation of the slab pull this buoyant material may rise enough to change the stress state in the continental crust and lead to back arc opening in a case of young and hydrated oceanic plate forced by the slab pull to subduct under a resistant continent this plate is deviated and indented by the continental mantle and stretches horizontally at 100 km depth this situation might explain the flat wadati benioff zone of central chile

the updip and downdip limits to great subduction earthquakes thermal and structural models of cascadia south alaska sw japan and chile this study examines thermal and structural controls of the updip and downdip rupture limits of great subduction thrust earthquakes data on past great earthquake seismic limits have been compiled for four continental subduction zones cascadia sw japan south alaska and chile these limits have then been compared to the predictions of several models for what constrains great earthquake rupture temperatures on the subduction thrusts have been estimated by finite element numerical models the landward limits of the observed updip aseismic zones correspond to the position where the thrust temperature reaches about 100 c that is depths of about 2 to 10 km for the subduction zones studied this temperature agrees with the dehydration of stable sliding smectite clays to illite chlorite the temperatures in this region are controlled mainly by the thickness of sediment on the incoming crust and by the crustal age and thus heat flow the downdip limits correspond to the depth on the thrust where either 1 the temperature reaches about 350 c which corresponds to thermally activated stable sliding behavior for crustal rocks with a transition to 450 c or 2 about 40 km depth if 350 c is reached at greater depth depths of about 40 km approximately correspond to the intersection of the thrust with the continental forearc moho and this downdip limit may be a consequence of stable sliding serpentinite or talc and other hydrated forearc mantle rocks the primary temperature controls on the downdip region are the age of the subducting oceanic plate and the thrust dip profile secondary control comes from the thickness of incoming sediment the convergence rate and the radioactive heat generation in the overlying forearc the 100 c updip limit occurs near the trench for young subducting plates with a thick sediment section such as cascadia 6 8 ma and up to 80 km landward for older oceanic crust such as south alaska 50 ma the 350 c downdip thermal limit is applicable for young oceanic plates e g cascadia and nankai whereas the forearc mantle limit applies for older plates e g south alaska and chile except near the chile rise for the margins studied that have experienced great earthquakes there is generally good agreement between the postulated thermal and moho limits and the rupture or seismogenic zone as defined by the distribution of aftershocks and by waveform tsunami and dislocation modeling the downdip limit of the interseismic locked zone from dislocation modeling is also in agreement with these limits

quaternary uplift astride the aseismic cocos ridge pacific coast costa rica the pacific coast of costa rica lies within the central american forearc and magmatic arc region that was created by northeastward subduction of the cocos plate beneath the caribbean plate at the middle america trench from the peninsula de nicoya south eastward toward the peninsula de osa and the peninsula de burica on the panamanian border the middle america trench loses its physiographic expression where it intersects the aseismic cocos ridge interaction between subduction of the buoyant aseismic cocos ridge and the overriding caribbean plate is invoked to explain the variation in rates of vertical crustal uplift along a coastal transect from nicoya to burica the pliocene and pleistocene stratigraphic record and holocene marine terraces and beach ridge complexes indicate that maximum rates of crustal uplift have occurred on the peninsula de osa immediately landward of the aseismic cocos ridge crustal uplift rates decrease northwest toward the peninsula de nicoya and to a lesser extent southwest toward the peninsula de burica the late quaternary stratigraphy on the peninsula de osa is subdivided into two major chronostratigraphic sequences from groupings of radiocarbon dates crustal uplift rates calculated from these sequences systematically decrease from 6 5 to 2 1 m ka north east across the peninsula deformation of the peninsula is modeled as uplifted and down to the northeast tilted fault blocks with an angular rotation rate of 0 03 to 0 06 per thousand years although less well constrained crustal uplift rates on the peninsula de nicoya 200 km to the northwest of the peninsula de osa vary from 1 m ka for pliocene and pleistocene sediments to 2 5 m ka for holocene marine terraces in the quepos region 100 km to the northwest of the peninsula de osa calculated uplift rates derived from incision of late quaternary fluvial terraces range from 0 5 to 3 0 m ka on the peninsula de burica only 60 km to the southwest of the peninsula de osa calculated uplift rates range from 4 7 m ka for a late holocene marine terrace to 1 2 m ka for post late pliocene deep sea sediments the variations in calculated uplift rates on the peninsula de osa constrain a dynamic model for subduction of the cocos ridge and the resulting uplift of the overriding caribbean plate deflection of the caribbean plate is modeled using various effective elastic thicknesses as the response of an elastic plate to the buoyant force of the subducted cocos ridge because the shape of the subducted end of the cocos ridge is unknown two scenarios are evaluated 1 a radially symmetric ridge with a slope similar to the slope of the flanks of the ridge and 2 a ridge where the subducted end was truncated by the panama fracture zone the best fit model utilizes a truncated ridge that has been subducted during the past 0 5 m y 50 km beneath the overriding caribbean plate which has an effective elastic thickness of 5 km the model predicts that the highest uplift rate should be 3 7 m ka and occur on the southwest coast of the peninsula de osa the rate of uplift slows considerably to the northeast and indicates that the peninsula de osa is tilting to the northeast which agrees with observations in that region the predicted uplift rate attributed to aseismic ridge subduction also decreases along the coast both north and south of the peninsula de osa resulting in little uplift that can be attributed to cocos ridge subduction in the northwestern portions of the peninsula de nicoya

tectonic control of the subducting juan fernandez ridge on the andean margin near valparaiso chile near the latitude of valparaiso chile a fundamental change in configuration of the benioff zone volcanic arc activity and the structure of the continental margin occurs opposite the subducting juan fernandez ridge upper plate tectonics related to subduction of the ridge were studied by an international group of geoscientists in the two degree segment offshore valparaiso extending from the shelf edge seaward across the eastern end of juan fernandez ridge near the o higgins group of seamounts the juan fernandez ridge strikes northeast rather than continuing its east west trend across the pacific basin the ridge uplifts the upper plate and sediments of the valparaiso basin are deformed against its southern flank this deformation is consistent with the southward migration required by an oblique trending ridge and the nearly trench normal vector of plate convergence in the trench axis the ridge forms a basement barrier behind which sediments 2 5 km deep have ponded the lower slope over the ridge appears eroded whereas the margin not yet affected by ridge subduction is fronted by an accretionary prism about 25 km wide nazca plate relief clearly influences tectonism of the margin where it subducts beneath thin continental crust its relation to deeper processes segmenting the andean orogen appears to involve prior tectonic events

crustal density structure in northwestern south america derived from analysis and 3 d modeling of gravity and seismicity data abstract this paper presents a three dimensional 3 d interpretation of new gravity and seismicity datasets for northern south america a 3 d forward density model was constructed on the basis of deep wide angle seismic refraction sections moho depth from receiver functions and surface geology density values were estimated from published borehole data for sediments by using empirical velocity density functions and considering mineralogical chemical composition variations under typical pressure temperature conditions for upper and lower crustal rocks the modeled 3 d density structure was kept as simple as possible the continental and oceanic plates were formed by two sedimentary bodies one crustal body and one mantle lithosphere body overlying a sub lithospheric mantle the caribbean plate was modeled with an atypical crustal thickness of 18 km including sediments the geometry of the caribbean plate was modeled using a combination of gravity modeling and analyses of the seismicity and focal mechanism solutions intermediate seismicity and the orientation of the t axes appeared aligned along the predicted position of the slab as a result the estimated slab dip angle under maracaibo and the merida andes was 15 and increases up to 20 after 100 km depth the model shows two orientations in the slab strike n150 e 5 in western colombia and southward underneath the maracaibo block the modeling results suggest that the northern south american upper and lower crusts are relatively light and the density of the caribbean crust is typical for an oceanic crust

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

physical characteristics of subduction interface type seismogenic zones revisited 1 based on global earthquake catalogs the hypocenters nodal planes and seismic moments of worldwide subduction plate interface earthquakes were extracted for the period between 1900 and 2007 assuming that the seismogenic zone coincides with the distribution of 5 5 m 7 earthquakes the subduction interface seismogenic zones were mapped for 80 of the trench systems and characterized with geometrical and mechanical parameters using this database correlations were isolated between significant parameters to identify cause effect relationships empirical laws obtained in previous studies were revisited in light of this more complete accurate and uniform description of the subduction interface seismogenic zone the seismogenic zone was usually found to end in a fore arc mantle rather than at a moho depth the subduction velocity was the first order controlling parameter for variations in the physical characteristics of plate interfaces determining both the geometry and mechanical behavior as such the fast subduction zones and cold slabs were associated with large and steep plate interfaces which in turn had large seismic rates the subduction velocity could not account for the potential earthquake magnitude diversity that was observed along the trenches events with mw 8 5 preferentially occurred in the vicinity of slab edges where the upper plate was continental and the back arc strain was neutral this observation was interpreted in terms of compressive normal stresses along the plate interface large lateral ruptures should be promoted in neutral subduction zones due to moderate compressive stresses along the plate interface that allow the rupture to propagate laterally

three dimensional numerical models of the influence of a buoyant oceanic plateau on subduction zones abstract we have investigated potential effects on a subduction zone during oceanic plateau trench collision using three dimensional numerical models an oceanic plateau of varied density was embedded in the slab which was pinned at one end the plateaus strongly influenced the shape of the trench for a plateau with a higher density the trench retreat rate was reduced in the region surrounding the plateau and the plateau subducted along with the slab for lower density plateaus the trench in the region of the plateau advanced and the plateau compressed and resisted subduction spreading laterally along the trench with a weaker slab rheology the arcuate shape of the trench towards the free end of the trench was enhanced beneath the most buoyant plateaus a tear formed in the subducted portion of the slab soon after the slab tip reached the top of the lower mantle we compare the model results with a region in the northwest pacific where the ogasawara plateau meets the trench of the izu bonin mariana subduction zone

subduction of a late cretaceous seamount of the louisville ridge at the tonga trench a model of normal and accelerated tectonic erosion the hotspot generated louisville ridge is a 4000 km chain of seamounts typically 2 2 5 km high and 10 40 km in diameter and an underlying crustal swell 1 5 km high and 100 km wide trending nnw across the southwestern pacific the northwest end of the ridge collides with the north trending tonga trench 26 s which just north of that point is exceptionally deep 10 8 km and lacks both a turbidite wedge and a bordering accretionary complex the collision zone is moving rapidly southward multichannel seismic reflection data in the collision zone show a west dipping subsurface platform 2 3 km beneath the lower western trench slope which is interpreted as the flat summit of a subducted guyot motuku of the louisville chain projected eastward the summit plain passes 1 2 km above the trench floor dredging of the nearby inner trench wall recovered uppermost cretaceous maestrichtian oceanic pelagic sediments interpreted to be fragments of the sedimentary mantle of a subducted louisville seamount the principal effects of hotspot ridge collision with a sediment starved trench are 1 the impacting seamounts are subducted rather than accreted and 2 although some seamount rocks are temporarily accreted the inner trench wall is tectonically eroded arcward at rates possibly as high as 50 km m y accelerated tectonic erosion is related to 1 fracturing shearing and general weakening of arc substrate rocks as they are lifted by the swell penetrated by impacting seamounts and left to collapse as the ridge moves away 2 a more effective removal of weakened rock in underthrusting grabens which are larger in the crustal swell 3 a possible elevation of the subduction decollement to account for the removal of as much as 30 000 km of material from a 400 km sector of the trench in 1 million years and 4 a reduction in supply of arc derived debris resulting from the gap in arc volcanism accompanying subduction of the ridge normal tectonic erosion in the tonga trench is apparently minor and we conclude that the bulk of the 37 000 km of material which fills subducting grabens each million years is arc derived volcanic and pelagic sediment

the state of stress in an overriding plate situated above a flat slab the andes of central peru in the andes of central peru that are situated above a flat dipping slab analysis of both structural data collected in the field and available focal mechanisms show that the quaternary and recent state of stress is characterized by n s trending extension in the western cordillera and along the pacific coast whereas e w trending compression prevails in the sub andean lowlands and at the contact between the nazca and south american plates the eastern cordillera deformations suggest an intermediate state of stress i e strike slip faulting characterized by e w trending compression and n s trending extension thus major fault zones striking nw se i e parallel to the central peruvian andes are normal with a sinistral component in the western cordillera e g cordillera bianca and reverse sinistral in the eastern cordillera e g cordillera de huaytapallana this state of stress displays similarities with that of southern peru and may be also interpreted as an effect of compensated high topography in this model the vertical stress zz increases with the topography and hmax is considered as fairly constant trending e w roughly parallel to the convergence on both edges of the andes tectonics being compressional zz is 3 and hmax is 1 in the high andes zz becomes 1 then hmax is 2 allowing hmin striking n s to be 3 along the pacific coast tectonics is not compressional as expected and a nearly neutral state of stress may be due to a topographical effect related to the nearby deep peru chile trench however a conspicuous difference occurs in the andes of central peru in respect to the southern peru andes compressional strike slip tectonics is observed in the eastern cordillera indeed the mean elevation of the eastern cordillera is 3 7 km whereas that of the western cordillera is 4 2 km allowing an intermediate state of stress to occur therefore in central peru a lower mean andean topography and a stronger coupling consequence of the flat subduction seems to explain that compressional tectonics is distributed in a more widespread area than in southern peru superficial deformations related to the subduction of the aseismic nazca ridge are restricted to the coastal domain opposite to this ridge they support the fact that submarine topography of large extension may subduct without producing compressional deformation in the overriding plate which can be reminiscent of a collisional setting

thermal structure coupling and metamorphism in the mexican subduction zone beneath guerrero summary temperature is one of the most important factors that controls the extent and location of the seismogenic coupled and transition partially coupled segments of the subduction interplate fault the width of the coupled fault inferred from the continuous gps observations for the steady interseismic period and the transient width of the last slow aseismic slip event m w 7 5 that occurred in the guerrero subduction zone in 2001 2002 extends up to 180 220 km from the trench previous thermal models do not consider this extremely wide coupled interface in guerrero subduction zone that is characterized by shallow subhorizontal plate contact in this study a finite element model is applied to examine the temperature constraints on the width of the coupled area the numerical scheme solves a system of 2 d stokes equation and 2 d steady state heat transfer equations the updip limit of the coupling zone is taken between 100 and 150 c while the downdip limit is accepted at 450 c as the transition from partial coupling to stable sliding from the entire coupled zone the seismogenic zone extends only up to 82 km from the trench inferred from the rupture width of large subduction thrust earthquakes corresponding to the 250 c isotherm only a small amount of frictional heating is needed to fit the intersection of the 450 c isotherm and the subducting plate surface at 180 205 km from the trench the calculated geotherms in the subducting slab and the phase diagram for morb are used to estimate the metamorphic sequences within the oceanic subducting crust a certain correlation exists between the metamorphic sequences and the variation of the coupling along the interplate fault