generation of continental rifts basins and swells by lithosphere instabilities 1 continents may be affected simultaneously by rifting uplift volcanic activity and basin formation in several different locations suggesting a common driving mechanism that is intrinsic to continents we describe a new type of convective instability at the base of the lithosphere that leads to a remarkable spatial pattern at the scale of an entire continent we carried out fluid mechanics laboratory experiments on buoyant blocks of finite size that became unstable due to cooling from above dynamical behavior depends on three dimensionless numbers a rayleigh number for the unstable block a buoyancy number that scales the intrinsic density contrast to the thermal one and the aspect ratio of the block within the block instability develops in two different ways in an outer annulus and in an interior region in the outer annulus upwellings and downwellings take the form of periodically spaced radial spokes the interior region hosts the more familiar convective pattern of polygonal cells in geological conditions such instabilities should manifest themselves as linear rifts striking at a right angle to the continent ocean boundary and an array of domal uplifts volcanic swells and basins in the continental interior simple scaling laws for the dimensions and spacings of the convective structures are derived for the subcontinental lithospheric mantle these dimensions take values in the 500 1000 km range close to geological examples the large intrinsic buoyancy of archean lithospheric roots prevents this type of instability which explains why the widespread volcanic activity that currently affects western africa is confined to post archean domains

upper mantle structure of the northern cascadia subduction zone previous knowledge of the structure of the cascadia subduction zone north of the canada united states border has been derived from a variety of geophysical studies that accurately delineated the downgoing juan de fuca plate from the offshore deformation front to depths of 50 60 km beneath south central vancouver island and the georgia strait little is known however of the structure of the cascadia subduction zone farther westward and to greater depths in the upper mantle we have assembled a set of some 1100 teleseismic traveltimes from events recorded on the western canadian telemetered network to augment a previously existing data set recorded on the washington regional seismograph network the composite data set is inverted for upper mantle structure below washington oregon and southwestern british columbia we analyze the new northern portion of the model between 48 5 50 n and 118 127 w which provides the first images of the deep slab structure in this region the model is parameterized using s

numerical comparison of different convergent plate contacts subduction channel and subduction fault summary at convergent plate boundaries the properties of the actual plate contact are important for the overall dynamics convergent plate boundaries both mechanically decouple and link tectonic plates and accommodate large amounts of strain we investigate two fundamental physical states of the subduction contact one based on a fault and the other based on a subduction channel using a finite element method we determine the specific signatures of both states of the subduction contact we pay particular attention to the overriding plate in a tectonic setting of converging plates where the subducting plate is freely moving the subduction channel reduces compression relative to the fault model in a land locked basin setting where the relative motion between the far field of the plates is zero the subduction channel model produces tensile stress regime in the overriding plate even though the amount of slab roll back is small the fault model shows a stronger development of slab roll back and a compressive stress regime in the upper plate based on a consistent comparison of fault and channel numerical models we find that the nature of the plate contact is one of the controlling factors in developing or not of backarc extension we conclude that the type of plate contact plays a decisive role in controlling the backarc state of stress to obtain backarc extension roll back is required as an underling geodynamic process but it is not always a sufficient condition

new insights on 3 d plates interaction near taiwan from tomography and tectonic implications abstract recent tomographic results are used to trace the south china sea and philippine sea subducting slabs south and northeast of taiwan along the manila and ryukyu trenches respectively in particular the 3 d plates interaction beneath taiwan is discussed based on a close up view of the tomographic sections together with earthquake hypocenters distribution our study indicates that 1 the east dipping south china sea slab can be followed to the north up to the latitude of hualien 2 the eurasian plate subducts beneath most part of the taiwan island down to the 670 km depth discontinuity 3 the north dipping philippine sea slab can be followed slightly west of the longitude of hualien both plates thus interact beneath northern taiwan where the arc continent collision is paroxysmal 4 slab detachment is envisaged at the northern edge of the subducted eurasian plate beneath the coastal range of taiwan which may facilitate the northwestward motion of the philippine sea plate with respect to eurasia slabs geometries obtained from tomographic sections allow us to reconstruct the late neogene plate kinematics and dynamics in this region our main conclusions are 1 the size of the original south china sea was about twice its present size 2 the subducted part of the west philippine basin i e the largest oceanic basin of the philippine sea plate extends only 400 km north of the ryukyu trench 3 slab detachment might have occurred 3 5 my ago beneath the central and northern ryukyu arc along a weak zone that is aligned with the gagua ridge an ancient plate boundary 4 the ryukyu trench has propagated westward from 126 e of longitude southeast of miyako island to the locus of the present arc continent collision along a major lithospheric tear that cut through the continent ocean boundary first and then through the continental lithosphere as a consequence the southern ryukyu margin developed progressively from east to west as a subduction zone during the last 8 my it has been elaborated onto a passive margin which corresponded to the edge of the eurasian continental shelf bordering the south china sea prior to pliocene 5 the southern okinawa trough rifted during the last 2 1 ma following the propagation of the subduction zone 6 subduction of the eurasian margin probably stopped recently beneath northern taiwan and a flip of subduction from west to east is being initiated near hualien

birth of a major strike slip fault in sw japan a 500 km long strike slip fault the north chugoku shear zone ncsz is identified in sw japan responsible for four m 7 earthquakes in the past 130 years a new geodynamic mechanism of increased interplate coupling above an obliquely subducting flat slab is presented to explain the transfer of trench parallel motion 400 km inland from the nankai trough to the level of the quaternary adakitic volcanic arc while quaternary dextral strike slip motion is widely believed to have occurred along the median tectonic line mtl near the miocene volcanic arc the central segment in shikoku has shown little seismicity for the last 1000 years pure dextral strike slip focal mechanisms for 15 modern and historical events indicate that part of the transcurrent motion is being transferred from the mtl to the ncsz

relations among subduction parameters clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones based on a critical appraisal of the geophysical and geological literature 26 parameters are estimated for each of 39 modern subduction zones to isolate causal relationships among these parameters multivariate analysis is applied to this data set this analysis yields empirical quantitative relations that predict strain regime and strike slip faulting in the overriding plate maximum earthquake magnitude benioff zone length slab dip arc trench gap and maximum trench depth excellent correlation is found between length of the benioff zone and the product of convergence rate and age of the downgoing slab this relationship is consistent with the conductive heating model of molnar et al 1979 if the model is modified in one respect the rate of heating of the slab is not constant it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate the structural style in the overriding plate is determined by its stress state though the stress state of overriding plates cannot be quantified one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional class 1 back arc spreading to strongly compressional class 7 active folding and thrusting this analysis indicates that strain class is probably determined by a linear combination of convergence rate slab age and shallow slab dip interplate coupling controlled by convergence rate and slab age is an important control on strain regime and the primary control on earthquake magnitude arc parallel strike slip faulting is a common feature of convergent margins forming a forearc sliver between the strike slip fault and trench optimum conditions for the development of forearc slivers are oblique convergence a compressional environment and a continental overriding plate the primary factor controlling presence of strike slip faulting is coupling strongly oblique convergence is not required the rate of strike slip faulting is affected by both convergence obliquity and convergence rate maximum trench depth is a response to flexure of the underthrusting plate the amount of flexural deflection at the trench depends on the vertical component of slab pull force which is very sensitive to slab age and shallow slab dip shallow slab dip conforms to the cross sectional shape of the overriding plate which is controlled by width of the accretionary prism and duration of subduction deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike slip faulting and strain regime empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones

the dynamical control of subduction parameters on surface topography the long wavelength surface deflection of earth s outermost rocky shell is mainly controlled by large scale dynamic processes like isostasy or mantle flow the largest topographic amplitudes are therefore observed at plate boundaries due to the presence of large thermal heterogeneities and strong tectonic forces distinct vertical surface deflections are particularly apparent at convergent plate boundaries mostly due to the convergence and asymmetric sinking of the plates having a mantle convection model with a free surface that is able to reproduce both realistic single sided subduction and long wavelength surface topography self consistently we are now able to better investigate this interaction we separate the topographic signal into distinct features and quantify the individual topographic contribution of several controlling subduction parameters results are diagnosed by splitting the topographic signal into isostatic and residual components and by considering various physical aspects like viscous dissipation during plate bending performing several systematic suites of experiments we are then able to quantify the topographic impact of the buoyancy rheology and geometry of the subduction zone system to each and every topographic feature at a subduction zone and to provide corresponding scaling laws we identify slab dip and slightly less importantly slab buoyancy as the major agents controlling surface topography at subduction zones on earth only the island arc high and the back arc depression extent are mainly controlled by plate strength overall his modeling study sets the basis to better constrain deep seated mantle structures and their physical properties via the observed surface topography on present day earth and back through time

constraints on the kurile slab from shear wave residual sphere analysis we analyze residual spheres for teleseismic s wave arrival times from 10 intermediate and deep focus earthquakes in the kurile subduction zone long period wwssn and csn recordings of transverse component s and scs arrivals provide good azimuthal coverage for each event with arrival time estimates that compare well with short period observations in order to isolate the near source structure substantial lower mantle and receiver path effects are suppressed both by using empirical station path statics derived from s ss scs and sscs arrivals for a total of 28 northwestern pacific events and by constructing differential residual spheres ray tracing through three dimensional aspherical shear velocity models supports the inferred large magnitude of the deep mantle and near receiver effects although existing models do not appear to provide accurate corrections the events are relocated in the prem structure and a modest spatial smoothing of the residuals is applied the resulting residual spheres are modeled using a finite difference travel time method that accounts for three dimensional ray path perturbations our starting models are derived from slab structures determined by previous p wave residual sphere analyses we explore the compatibility of the p and s wave observations to constrain the vs vp velocity ratio of the slab and to test whether the s wave arrival times place additional constraints on the slab geometry the modeling supports the previous results favoring penetration of the kurile slab to at least 800 km depth for most of the length of the arc with some evidence that the northern portion of the slab broadens by as much as a factor of 3 upon entering the lower mantle the preferred slab shear velocity models have relatively low 2 3 shear velocity contrasts below 670 km depth which can be accounted for by slab temperature reductions of 300 400

numerical modeling of stress and topography coupling during subduction inferences on global vs regional observables interpretation abstract the mechanical strength of the oceanic lithospheres exerts a fundamental control on the dynamics and tectonics of convergent margins here we use numerical models of coupled subducting upper plates and mantle to investigate the impact of the subducting lithosphere properties on the parameters commonly measured at subduction zones the models predict a correlation among parameters such as subducting and convergence velocities slab dip and curvature and stress coupling with the upper plates such signature emerges in a dynamic system through the slab buoyancy similar trends are found when assessing the impact of laboratory constrained non linear rheologies uncertainties on the mechanical strength and mantle flow although some features are more affected when the role of upper plate thickness is tested we find a positive correlation of upper plate subsidence and fore arc topography with the buoyancy of the down going plate however plasticity in the crust limits the development of topography becoming increasingly relevant with thinner upper plates so that no correlation can be established similar control is found on the width of the long wavelength dynamic subsidence due to flexure which increases non linearly with the upper plate thickness the models constrain relations among parameters within the dynamic system driven that allow inferences on the observable parameters the comparisons supports the idea that the subduction dynamics emerges as a global phenomenon in plate motions bending and dynamic subsidence yet andean type margin tectonics is best addressed in a regional context

overriding plate shortening and extension above subduction zones a parametric study to explain formation of the andes mountains mountain building above subduction zones such as observed in the andes is enigmatic and the key parameter controlling the underlying dynamics remains a matter of considerable debate a global survey of subduction zones is presented here illustrating the correlation between overriding plate deformation rate and twelve physical parameters overriding plate velocity subducting plate velocity trench velocity convergence velocity subduction velocity subduction zone accretion rate subducting plate age subduction polarity shallow slab dip deep slab dip lateral slab edge proximity and subducting ridge proximity all correlation coefficients are low r 0 39 irrespective of the global reference frame relative plate motion model or overriding plate deformation model except for the trench velocity 0 33 0 68 exact value depends on adopted global reference frame and subduction velocity which shows an anticorrelation 0 55 0 57 this implies that no individual parameter can explain overriding plate deformation except that trench retreat generally corresponds to extension while an approximately stable trench or trench advance generally corresponds to shortening understanding of the variety of strain patterns is obtained when slab edge proximity and overriding plate velocity are combined orogenesis occurs in overriding plates bordering central regions of wide subduction zones 4000 km when the overriding plate is moving trenchward at 0 2 cm yr e g andes japan because the center of a wide slab offers large resistance to lateral migration the overriding plate effectively collides with the subduction hinge forcing the slab to attain a shallow dip angle e g nazca and japan slabs overriding plate extension is only found close to lateral slab edges or during overriding plate motion away from the center of a wide subduction zone but in the latter scenario maximum extension velocities are much lower than in the former scenario for subduction settings close to lateral slab edges overriding plate motion plays no significant role in overriding plate deformation thus for rapid overriding plate extension the key ingredient is rapid trench retreat which only occurs close to lateral slab edges while for overriding plate shortening the key ingredients are 1 the resistance to rapid trench and hinge retreat which occurs far from lateral slab edges and 2 trenchward overriding plate motion

velocity and q structure of the great valley california based on synthetic seismogram modeling of seismic refraction data the modified reflectivity method is used here to interpret seismic refraction data from the west side of the great valley california in terms of a onedimensional compressional wave velocity model of the crust in addition the apparent attenuation qp has been estimated for the 6 km thick sedimentary section the reflectivity method was used to calculate the prominent free surface multiples that propagate in this sedimentary section modeling of the normalized amplitudes of these multiples yielded average qp values of 100 from 0 to 2 83 km depth and 200 from 2 93 to 6 20 km depth the velocity structure in the crust is complicated it is characterized by positive velocity gradients within the sediment to a depth of 4 8 km and by a 1 4 kin thick low velocity zone below this depth the sediment overlies a basement with a velocity of 5 7 km sec at its top which increases to 6 3 km sec at 12 km depth ambiguity of the data in critical regions leads to two equally valid interpretations of the lower crust at depths of 16 to 27 km simple step increases or a more complex structure consisting of alternating high and low velocity zones

slab mantle flow interaction influence on subduction dynamics and duration we investigate the influence of mantle flow relative to the lithosphere on subduction dynamics we use 2d thermo mechanical models assuming incompressible non newtonian fluid rheology different mantle flow velocities consistent with absolute plate motion models are tested as well as both directions of flow either sustaining or opposing slab dip the effects of different inflow outflow velocity profiles slab strengths and upper lower mantle viscosity contrasts are also evaluated slab dip deviations between models with opposite mantle flow directions range from 37 for relatively strong slabs max 1025 pa s to 50 for weaker slabs max 1024 pa s accounting for a significant amount of natural slab dip variability for imposed mantle flow supporting the slab the initial stage of slab steepening is followed by a stage of continuous slab dip decrease this slab shallowing eventually leads to mantle wedge closure subduction cessation and slab break off possibly driving subduction flips

trench curvature and deformation of the subducting lithosphere summary the subduction of oceanic lithosphere is generally accompanied by downdip and lateral deformation the downdip component of strain is associated with external forces that are applied to the slab during its sinking namely the gravitational force and the mantle resistance to penetration here we present theoretical arguments showing that a tectonic plate is also subject to a predictable amount of lateral deformation as a consequence of its bending along an arcuate trench zone independently from the long term physical processes that have determined the actual curvature of the subduction zone in particular we show that the state of lateral strain and the lateral strain rate of a subducting slab depend from geometric and kinematic parameters such as trench curvature dip function and subduction velocity we also demonstrate that the relationship between the state of lateral strain in a subducting slab and the geometry of bending at the corresponding active margin implies a small component of lateral shortening at shallow depths and may include large extensional lateral deformation at intermediate depths whereas a state of lateral mechanical equilibrium can only represent a localized exception our formulation overcomes the flaws of the classic ping pong ball model for the bending of the lithosphere at subduction zones which lead to severe discrepancies with the observed geometry and style of deformation of the modern subducting slabs a study of the geometry and seismicity of eight modern subduction zones is performed to assess the validity of the theoretical relationship between trench curvature slab dip function and lateral strain rate the strain pattern within the eight present day slabs which is reconstructed through an analysis of harvard cmt solutions shows that tectonic plates cannot be considered as flexible inextensible spherical caps whereas the lateral intraslab deformation which is accommodated through seismic slip can be explained in terms of deviations from the mechanical equilibrium

numerical modelling of lithosphere asthenosphere interaction in a subduction zone we developed a new 2 d numerical approach to study solid fluid coupling applied to subduction zones the lithosphere is characterised by an elastic or elastoplastic behaviour and the asthenosphere by a homogeneous isoviscous fluid the temperature effects are ignored and viscosity and density are constant in time the solid and the fluid problem are discretised by the finite elements method fem the same solid code used in hassani et al hassani r jongmans d chery j study of plate deformation and stress in subduction processes using two dimensional numerical models j geophys res 1997 102 17951 17965 has been used to compute the solution of the solid problem the stokes problem is solved by a direct solver with a stabilisation procedure we used a very simple staggered coupling method where the fluid domain is regularly re meshing we observed numerical instabilities when the time step is not sufficiently small especially when strong coupling between the solid and the fluid occurs we have tested different configurations where the lithosphere is elastic or elastoplastic and show how the slab geometry the topography and the stress regime in the plates are affected by the viscous resistance of the mantle we observed that the asthenosphere viscosity is a fundamental parameter in the subduction process for subduction with an extensional regime in the upper plate we observe a linear decrease of the extensional stress as a function of the asthenospheric viscosity

collisional model for rapid fore arc block rotations arc curvature and episodic back arc rifting in subduction settings 1 we review geophysical and geological data from 23 active and ancient plate boundaries to document a compelling spatial and temporal relationship between collision plate boundary curvature rapid tectonic rotations and the occurrence of back arc rifting our observations support a conceptual model where the change from subduction to collision causes rapid fore arc rotation when viewed in a reference frame relative to the bounding tectonic plates leading to marked plate boundary curvature our global compilation reveals that most active back arc rifts are associated with rapidly rotating fore arcs and nearby collisions thus we propose that collision and rapid fore arc rotations play a major role in the evolution and kinematics of back arc basins we conduct numerical modeling to better understand the physical processes behind these observed rapid tectonic rotations at convergent margins our results suggest that the presence of an indentor or choke point in the subduction system e g collision can generate rapid rotation of the fore arc about a nearby pole and lead to back arc rifting the rate of fore arc rotation and back arc rifting depends on the incoming indentor velocity and can be greatly enhanced by slab rollback and the presence of a low viscosity back arc where viscosity of the back arc is low fore arc rotation dominates where back arc viscosity is high the formation of strike slip faults and tectonic escape dominates our observational and model derived results illustrate that shallow crustal forces produced by the entry of buoyant features into subduction zones are a fundamental mechanism for the generation of fore arc block rotations plate boundary curvature back arc rift evolution and tectonic escape in subduction systems rollback of the subducting slab within the mantle will certainly enhance the processes we describe but it is not the only driving force

a new insight on the geometry of subducting slabs in northern luzon philippines abstract we used hypocentral and focal mechanism data in order to characterize the tectonic configuration of northern luzon and propose a model for describing the geometry of the subducted slab of the eurasian plate beneath the northern segment of the manila trench we took into consideration some of the observed bathymetric features i e the bend in the trench line of the manila trench at 20 n lat and the collision and subduction of an extinct mid oceanic ridge at 16 17 n lat and the intraplate deformation pattern in the north luzon ridge region in depicting a model that is consistent with most of the observed features and phenomena in the study area an earthquake catalog covering the period from 1619 to 1997 was used in studying the distribution of strain energy released while earthquakes covering the instrumental years 1963 1997 were used to analyze vertical distributions of earthquakes in sections we refined the location of some of the historical earthquakes using macroseismic data the inclusion of historical records despite their inaccurate locations could prevent misidentification of some regions as aseismic zones or gaps in seismicity especially along structures with long return periods a focal mechanism database that covers the period from 1963 to 1997 was used to decipher intraplate deformation pattern and subduction process of the slab the focal mechanism database consists of first motion solutions from previous authors and cmt solutions from harvard university seismicity strain energy release and focal mechanism maps and cross sections were drawn in order to have a three dimensional visualization of the geometry and stress regimes of seismogenic zones a new model of the subducted slab of the eurasian plate beneath the manila trench is proposed the model suggests the collision and subsequent partial subduction of a buoyant plateau at around 20 n lat to explain the sharp bend in the trench line the complicated deformation pattern on the overriding plate fronting the bend and the shallow dip of the subducted slab beneath this zone a tear in the slab is also inferred to be present as evidenced by the observed gap in strain energy release and the abrupt change in dip from shallow to steep south of 18 n lat the gap in seismicity and strain energy release 65 300 km depth at around 17 n lat may be used to infer the trajectory and location of the subducted extinct mid oceanic ridge mor the aseismic behavior is probably caused by the subducted ridge which is still hot and is deforming plastically this is supported by the heatflow data which shows high values along the extinct mor the subducted part of the mor may serve as the weakest zone where this tear could be localized the tear may also explain the cause of the abrupt termination of the eastern chain of volcanoes south of 18 n lat the above model which is a refinement of the model introduced by yang et al tectonophysics 258 1996 85 is consistent with the observed seismicity and deformation pattern observed bathymetric features spatial distribution and geochemical character of volcanism in northern luzon

effects of plate bending and fault strength at subduction zones on plate dynamics for subduction to occur plates must bend and slide past overriding plates along fault zones because the lithosphere is strong significant energy is required for this deformation to occur energy that could otherwise be spent deforming the mantle we have developed a finite element representation of a subduction zone in which we parameterize the bending plate and the fault zone using a viscous rheology by increasing the effective viscosity of either the plate or the fault zone we can increase the rates of energy dissipation within these regions and thus decrease the velocity of a plate driven by a given slab buoyancy we have developed a simple physical theory that predicts this slowing by estimating a convecting cell s total energy balance while taking into account the energy required by inelastic deformation of the bending slab and shearing of the fault zone the energy required to bend the slab is proportional to the slab s viscosity and to the cube of the ratio of its thickness to its radius of curvature the distribution of dissipation among the mantle lithosphere and fault zone causes the speed of a plate to depend on its horizontal length scale using the observation that earth s plate velocities are not correlated to plate size we can constrain the lithosphere viscosity to be between 50 and 200 times the mantle viscosity with higher values required if the fault zone can support shear tractions 50 mpa over 300 km these subduction zone strengths imply that the mantle fault zone and lithosphere dissipate about 30 10 and 60 of a descending slab s potential energy release if the slab is 100 km thick the lithospheric component is highly dependent on slab thickness it is smaller for thin plates but may be large enough to prevent bending in slabs that can grow thicker than 100 km ubduction zone strength should be more stable than mantle viscosity to changes in mantle temperature so the controlling influence of subduction zones could serve to stabilize plate velocities over time as the earth cools because the details of convergent plate boundaries are so important to the dynamics of plate motion numerical models of mantle flow should treat subduction zones in a realistic way

effect of lateral viscosity variations in the top 300 km on the geoid and dynamic topography summary we have investigated the impact of lateral viscosity variations lvv in the top 300 km of the mantle on the long wavelength gravitational response of the earth in contrast to the previous studies we demonstrate that the lvv may play a crucial role if a model with imposed plate velocities and partial layering is considered assuming lvv in the top mantle associated with the continental roots and the oceanic asthenosphere we are able to explain a significant portion of the geoid 85 per cent and of the free air gravity data 55 per cent without invoking a complex radial viscosity profile or assuming a lithosphere with an unrealistically low uniform viscosity the best fit to the data is found for models in which mass anomalies located at the boundary between upper and lower mantles reduce the mass flux across the 660 km discontinuity by a factor of 3 these models are characterized by a low average value of viscosity in the asthenosphere below the oceans 10 19 pa s and a large viscosity contrast 100 between the young suboceanic mantle and the deep continental roots the viscosity in the deep upper mantle is found to be around 3 10 20 pa s while a value higher by at least two orders of magnitude is typical for the lower mantle the predicted dynamic surface topography is influenced by the surface plate velocities and its amplitude amounts to a few hundred metres besides investigating the viscosity structures that reflect the different thicknesses of the lithosphere in continental and oceanic regions we have searched for a large scale pattern of lateral viscosity variation in the top 300 km that yields the best fit to the geoid the pattern obtained by this inversion shows a good correlation with the ocean continent distribution and 90 per cent of the geoid and 67 per cent of the free air gravity can be explained

structural expression of forearc crust uplift due to subducting asperity abstract new structural observations and mapping of reefal terraces carried out both on the field and on satellite remote sensing data indicate that sumba island is presently undergoing a large amount of extension associated with a significant regional uplift this crustal uplift may have been created by a major thrust emerging in the south of the island the uplift which partly accommodated the australian plate south west banda arc convergence is associated with the general northeastward tilt of the island the consequent anomalous positive topography along the southern coast of the island is being compensated by significant tectonic erosion along large scale curvilinear normal faults in the southeastern half of the island the most important expression of this gravitational collapse is located at the receding side of an advancing circular dome showing striking similarities with accretionary wedges being affected by seamount subduction the part of the forearc basin known as the savu basin is moderately deformed mostly in its central part and appears to act as a rigid buttress in the convergence between the banda arc and the australian plate as a result the convergence appears to be transferred northward within the actively shortening back arc domain which goes from the north of the flores island to the southwest of the timor block the convergent plate boundary shows a transition between a stable domain west of sumba and a tectonized domain east of sumba the latter coinciding with the subduction of the outer australian passive margin the subduction of the ocean continent boundary of the austral indian plate below the banda arc since the lower pliocene may have incorporated some crustal fragments in the plunging benioff zone most likely the integration of the stretched continental lithosphere in the subduction zone caused the uplift the entire forearc domain exhibiting inherited structures of the upper plate

digital isochrons of the world s ocean floor we have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self consistent set of global isochrons and associated plate reconstruction poles the age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models we have constructed an age grid with error estimates for each grid cell as a function of 1 the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid 2 the distance of a given grid cell to the nearest magnetic anomaly identification and 3 the gradient of the age grid i e larger errors are associated with high age gradients at fracture zones or other age discontinuities future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere the heat loss of the earth ridge push forces through time asymmetry of spreading and providing constraints for seismic tomography and mantle convection models

slab breakoff insights from 3d thermo mechanical analogue modelling experiments abstract the detachment or breakoff of subducted lithosphere is investigated using scaled three dimensional thermo mechanical analogue experiments in which forces are measured and deformation is monitored using high speed particle imaging velocimetry piv the experiments demonstrate that the convergence rate in a subduction zone determine if and when slab detachment occurs slow subduction experiments with scaled convergence rates 1 cm yr 1 have lower peclet numbers and are characterized by lower tensile strength subducted lithosphere causing detachment to occur when the downward pull force exerted by a relatively short subducted slab is relatively low therefore when continental collision is preceded by slow oceanic subduction the hot and weak subducted lithosphere need not be very long or extremely negatively buoyant to cause detachment under such conditions detachment may occur sooner after the onset of continental subduction than previously predicted in contrast if collision is preceded by rapid subduction 10 cm yr 1 breakoff will be delayed and occur only when the convergence rate has slowed sufficiently to thermally weaken the slab and cause its eventual failure the analogue experiments further confirm that slab detachment occurs diachronously as it propagates along the plate boundary stereoscopic piv reveals a characteristic strain pattern that accompanies the detachment horizontal contraction and subsidence with scaled values up to 1200 m in the trench and forearc area precedes the passage of the detachment and is followed by horizontal extension and uplift up to 900 m high frequency monitoring captures rapid propagation of the detachment along the plate boundary at scaled rates of up to 100 cm yr 1 however this rate is not constant and interaction between the slab and lower mantle or opening of a backarc basin in the upper plate can reduce or stop slab breakoff propagation altogether

geomorphic evidence for plio quaternary compression in the andean foothills of the southern neuquen basin argentina the southern central andes foothills are made of imbricate thick and thin skinned fold and thrust belts they were formed during cretaceous and miocene compressive periods by successive eastward propagation episodes of the orogenic front we analyze remnant terraces and alluvial deposits of the neuqun river and its tributaries to study the plio quaternary tectonic regime in the southern neuqun basin topographic profiles of remnant terraces show a crustal scale tens of kilometers anomaly above the chihuido south anticline this anomaly is accompanied by sedimentary aggradations downstream and upstream of the anticline another anomaly in the terrace profiles correlated with a knickpoint in the current neuqun river is observed farther east near aelo we interpret these anomalies to be the consequence of a plio quaternary uplift of the chihuidos and aelo anticlines folding and surface faulting of plio quaternary alluvial deposits show that this uplift is concomitant with the reactivation of the miocene orogenic front and of the predominantly thin skinned agrio fold and thrust belt from these data we infer that the southern neuqun basin is still shortening under a compressive stress regime and that the andean orogenic front is migrating farther to the east extensional features in the study area correspond mainly to thin skinned gravitational gliding resulting from crustal uplift thus we argue that the tectonic compressive regime is preserved in the southern neuqun basin throughout the plio quaternary though the slab may have steepened during this period

4 d evolution of se asia s mantle from geological reconstructions and seismic tomography abstract how the collision between india and asia is related to processes deeper in the mantle is unclear here we compare geological reconstructions of block motions within asia since 50 ma with the tomographically imaged three dimensional 3 d morphology of subducted lithosphere to obtain insight into the spatiotemporal evolution of mantle structure past positions of the convergent margin show remarkable similarities with slab geometry at specific depths the striking change in slab geometry from a linear structure beneath 1100 km to an increasingly distorted shape at depths of less than 700 km results from collision the slab contours match the progressive deformation of asia s margin including india s indentation and sundaland s extrusion ever since the onset of collision the indian plate appears to have overridden its own sinking mantle and it does not seem at present to underthrust tibet significantly north of the zangbo suture if correct this observation would provide further evidence against models of plateau build up involving indian lithosphere the tomographic images beneath india confirm that asian deformation has absorbed at least 1500 km of convergence since collision began from the match between the southeastward motion of sundaland between 40 and 20 ma and the principal change in slab structure between 700 and 1100 km depths we infer that lateral advection in the mantle is small and that the sinking rate beneath sunda was 2 cm yr in the lower mantle and 5 cm yr above the transition zone

influence of the subducting plate velocity on the geometry of the slab and migration of the subduction hinge abstract geological observations indicate that along two active continental margins east asia and mediterranean major phases of overriding plate extension resulting from subduction hinge retreat occurred synchronously with a reduction in subducting plate velocity in this paper results of fluid dynamical experiments are presented to test the influence of the velocity of the subducting plate on the hinge migration velocity and on the geometry of the slab results show that hinge retreat decreases with increasing subducting plate velocity in addition phases of hinge retreat alternate with phases of hinge advance for relatively high subducting plate velocities due to interaction of the slab with the bottom of the box simulating the upper lower mantle discontinuity such slab kinematics could explain the episodic behaviour of back arc opening observed in convergent settings the geometry of the slab and the kinematics of subduction are significantly affected by the velocity of the subducting plate three subduction modes with accompanying slab geometry can be recognized a relatively low subducting plate velocity is accompanied by relatively fast hinge retreat with backward sinking of the slab and a backward draping slab geometry with increasing subducting plate velocity hinge migration is relatively small resulting in subvertical sinking of the slab and a folded slab piling geometry for a very high velocity the hinge migrates forward resulting in forward oriented subduction vectors and a forward draping slab geometry

multiple active spreading centres in the hot north fiji basin southwest pacific a possible model for archaean seafloor dynamics abstract the north fiji basin nfb in the southwest pacific is one of the most evolved back arc basins within the basin active spreading is taking place along several ridges some of which display the characteristics of spreading ridges observed in mature large oceans characterized by a range of spreading rates segmented accretion centres organized magnetic lineations and propagators for example however the great cumulate length of the spreading centres of the nfb relative to the basin s surface makes it an unique example among the world s oceanic basins we have reviewed a comprehensive body of geological geophysical and geochemical evidence multiple spreading centres of various trends high heat flow high geoid and slow upper mantle seismic velocities which collectively demonstrate that a hot upper mantle underlies the north fiji basin active upper mantle convection is the key process controlling the large crustal and lithospheric production in the nfb which would be thus largely independent from the evolution of even the largest structural features such as the vitiaz and new hebrides subduction zones petrological studies of the archaean greenstone belts have led to a similar picture for the entire earth during the archaean numerous short active ridges connected by unstable triple junctions would have at this time provided an efficient convective mechanism allowing for a large heat dissipation we propose that the presently active processes in the north fiji basin are modern analogues to archaean seafloor dynamics

the role of the overriding plate thermal state on slab dip variability and on the occurrence of flat subduction 1 slab dip varies significantly both between different and along single subduction zones provided that old subducting plates are colder and denser than young plates variations in the slab dip should correlate with slab age however recent statistical analyses do not show this expected correlation we present the results of non newtonian numerical dynamic models where subduction is driven by means of a kinematic boundary condition we systematically vary the age of both the overriding and subducting plates in order to test these effects on the slab dip at different depth ranges we find that colder overriding plates result in shallower slab dips and episodes of flat slab subduction as a result of the increased suction force in the mantle wedge the influence of the thermal state of the overriding plate on slab dip is shown here to be more important than that of the age of subducting lithosphere modeling results are qualitatively compared to the large dip variability of the cocos slab including a flat slab segment we suggest that this variability is likely related to the change of the thermal state of the overriding plates with flat subduction occurring under cold lithosphere in southwestern mexico and steep subduction under the warmer lithosphere of the northwestern caribbean plate

episodic slab rollback fosters exhumation of hp uhp rocks summary the burial exhumation cycle of crustal material in subduction zones can either be driven by the buoyancy of the material by the surrounding flow or by both high pressure and ultrahigh pressure rocks are chiefly exhumed where subduction zones display transient behaviours which lead to contrasted flow regimes in the subduction mantle wedge subduction zones with stationary trenches mode i favour the burial of rock units whereas slab rollback mode ii moderately induces an upward flow that contributes to the exhumation a regime that is reinforced when slab dip decreases mode iii episodic regimes of subduction that involve different lithospheric units successively activate all three modes and thus greatly favour the exhumation of rock units from mantle depth to the surface without need for fast and sustained erosion

differential rotation between lithosphere and mantle a consequence of lateral mantle viscosity variations the description of plate motions in the so called hotspot reference frame introduces a global rotation of the lithosphere with respect to the mantle this rotation called toroidal field of degree 1 is roughly westward it reaches an amplitude of about 2 cm yr and has been consistently found in the different generation of plate tectonic models various authors have tried to relate this observation to the deceleration of the earth s rotation to polar wander or to tidal drag however these different physical mechanisms cannot explain the requested amplitude in this paper we compare the values of this rotation vector using different relative plate motion models expressed in the hotspot reference frame in a model earth with lateral viscosity variations a differential rotation is predicted the observed net lithospheric rotation is consistent with the dynamics of a model earth where the asthenospheric viscosity below the oceans is at least one order of magnitude lower than underneath the continents this relative westward drift of the lithosphere may account for the significant structural differences between east or west dipping subduction zones

the driving mechanism of plate tectonics relation to age of the lithosphere at trenches sea floor age depth and age trench depth relations suggest that the oceanic lithosphere continues to thicken and subside with age beyond 80 ma if so slab pull fs and ridge push fr forces can be calculated from the age of the sea floor at trenches t and if the shear stress acting on the base of the plate increases with absolute velocity a the approximate force balance equation is dva2 svat32 rt where d s and r are constants related to drag slab pull and ridge push respectively the relation between velocity and age is va2 s vat12 r we have tested this model by linear regression using rate and age data for 15 trenches va mm a 2 9 20 32 vat12 26 250 the strength of this correlation r 0 98 is strong evidence in favor of the validity of this simple model these results suggest that ridge push is just sufficient to overcome drag at the base of the plate and does not contribute significantly to the motions of oceanic plates though the value of r is consistent with the motions of plates not attached to subducted slabs

thermal modeling of the geometry of subduction with implications for the tectonics of the overriding plate the path which subducting lithosphere follows can be broken into three zones zone i a subhorizontal region of nearly constant dip near the trench zone ii a region of rapidly increasing dip and zone iii a region generally deeper than 125 km again of nearly constant dip we have modeled the thermal evolution of slabs as they pass through these three zones using the method of lines a numerical technique well suited to the solution of time dependent partial differential equations by use of appropriate initial conditions and time dependent boundary conditions we can simulate subduction with variations in age of subducting lithosphere slab velocities and paths followed by the slab thermal modeling results indicate that the geometry of the slab in region ii can be empirically related directly to the thermal structure of the slab in that region and thus indirectly to age and velocity it follows that low angle subduction can extend several hundred kilometers inland only in cases of high convergent velocities 15 cm yr 1 and old 120 m y lithosphere we further suggest that rapid variations in the convergence velocities can provide an enabling mechanism for back arc spreading while slow changes in convergence velocities and or age variations in the subducting lithosphere lead to migration of volcanic arcs

the bending mechanics in a dynamic subduction system constraints from numerical modelling and global compilation analysis abstract lithospheric plates bend at subduction zones where the vertical motions of the slabs are converted to surface plate motions to understand the mechanics of plate bending we derive scaling laws for the deflection at the margin i e radius and dip from numerical models of a subducting viscoelastic plate in such dynamic system we find that the buoyancy and the stiffness of the plates control the radius and the dip as well as the plate motions toward the trench this mechanical model successfully predicts the curvature of published three dimensional laboratory and numerical models for a thorough comparison with the observable we have also implemented forces additional to the slab pull such as the suction force and far field stresses by increasing or resisting the torque applied at the trench by the slab these forces can largely rearrange the dip and the radius of slabs and the inherent plate motions although they do not alter the observed anticorrelation between radius and dip similar inverse correlation relationship and dip radius ranges are shown by most of the subduction zones analysed from a global compilation radii in the range of 100 350 km and dips of 30 70 for slabs that extends to the bottom of the upper mantle are compatible with the models and allow estimating an average lithospheric viscosity contrast of 200 in the bending with respect to the ambient mantle radii and dips outside of this range are in good agreement with the trends and the magnitudes of models that include suction and far field forces in all these subduction zones the correlation between dip radius and plate velocity is found to be compatible with that of the models showing how relevant bending is for the dynamics of earth

controls of subduction geometry location of magmatic arcs and tectonics of arc and back arc regions most variation in geometry and angle of inclination of subducted oceanic lithosphere is caused by four interdependent factors combinations of 1 rapid absolute upper plate motion toward the trench and active overriding of the subducted plate 2 rapid relative plate convergence and 3 subduction of intraplate island seamount chains aseismic ridges and oceanic plateaus anomalously low density oceanic lithosphere cause low angle subduction under conditions of low angle subduction the upper surface of the subducted plate is in contact with the base of the overlying plate the wedge of low density asthenosphere is replaced by subducted lithosphere and the width of the arc trench gap either is significantly increased or a magmatic arc is not developed within the overlying plate the fourth factor is age of the subducting lithosphere subduction of young lithosphere produces two opposing tendencies 1 low angle subduction and increased arc trench distance owing to its low density and 2 decreased arc trench distance owing to its higher temperature two factors of secondary importance contribute to variation in subduction zone geometry and arc trench distance accretion of sediment in trenches depresses the upper portion of the subducting oceanic plate and causes the trench axis to migrate seaward prolonged subduction thickens the upper plate depresses the isotherms in the subducted plate and may create a broader arc both factors increase the arc trench gap the four primary factors also control development of other tectonic elements such as regional subsidence for example the amazon basin and a portion of the cretaceous interior seaway of western united states intra arc extension for example the basin and range province foreland fold and thrust belts and laramide style tectonics

a complex pattern of mantle flow in the lau backarc shear wave splitting analysis of local events recorded on land and on the ocean floor in the tonga arc and lau backarc indicate a complex pattern of azimuthal anisotropy that cannot be explained by mantle flow coupled to the downgoing plate these observations suggest that the direction of mantle flow rotates from convergence parallel in the fiji plateau to north south beneath the lau basin and arc parallel beneath the tonga arc these results correlate with helium isotopes that map mantle flow of the samoan plume into the lau basin through an opening tear in the pacific plate

control of forearc density structure on megathrust shear strength along the chilean subduction zone abstract unveiling how the interaction of friction pore pressure p and normal stress n controls the shear strength distribution along megathrust faults at convergent plate boundaries is crucial to our understanding of processes leading to the occurrence of devastating earthquake and the construction of mountains an existing density model of the chilean margin 18 45 s allows the computation of the so called vertical stress anomaly vsa that accounts for the component of normal stress due to the weight of the overlying crustal column a spatial correlation between vsa surface geology and several earthquake parameters suggests a significant control exerted by the geologically dominated density structure of the crust on megathrust shear strength the very seismogenic forearc in front of the high central andes 18 34 s has positive vsa values up to 60 mpa and a mafic dominated crust which suggest high values of shear strength at the megathrust that are partially controlled by the weight of a dense forearc crustal column in addition to a likely dry subduction channel characterized by high and low p in contrast the southern andean forearc 34 45 s shows neutral to negative vsa down to 30 mpa correlated with a felsic dominated crust which added to a presumably wet subduction channel low and high p configures a rather weak megathrust below a light forearc earthquakes here are concentrated around the arauco peninsula a first order morpho structural anomaly characterized by comparatively higher values of vsa than the rest of this segment which is the nest of the giant mw 9 5 valdivia 1960 earthquake and the southern rupture termination of the recent mw 8 8 maule earthquake the ultimately giant size of the 1960 earthquake resulted from rupture propagation along a 1000 km long region characterized by negative vsa values the correlation between vsa and the along strike segmentation of the andean orogen indicates that the geologically inherited density structure of the forearc also rules long term mechanical coupling between converging plates and prescribes the possible structure of the orogen because central and southern andes represent two extremes in terms of orogenic structure and seismogenic behavior results presented here have significant implications for the general comprehension of processes occurring at short and long time scale at convergent margins

contrasted east asia and south america tectonics driven by deep mantle flow east asia and south america have both experienced long term subduction since at least the jurassic but they have had contrasting tectonic evolutions since the late cretaceous east asia was dominated by extensional tectonics with many marginal sea basins forming during the cenozoic while south america was dominated by compressional tectonics building the andean mountains the mechanism controlling this contrast in tectonic style on opposite sides of the pacific has been unclear we suggest that the deep mantle flow revealed by seismic tomography plate reconstructions and geodynamic models contributed to the pan pacific tectonic disparity our geodynamic models suggest that the atlantic ocean opening plays an important role in promoting compression dominated tectonics and andean building along the south american margin by allowing fast trench ward motion of south america on the other hand the long standing downwelling flow beneath the interior of asia since asian assembly in the paleozoic and the early mesozoic helps to inhibit atlantic type ocean opening in asia and restrain trench ward motion of east asia promoting extension dominated tectonics along the asian margin the restrained trench ward motion suggests a low probability of flat slab subduction occurrence in east asia during the late mesozoic

mantle rheology and the scaling of bending dissipation in plate tectonics 1 plate tectonics on earth involves the bending deformation of plates at subduction zones and because plates are generally considered to be stiff owning to the rheology of mantle minerals the role of energy dissipation by plate bending in the global energy balance has been frequently debated in the recent literature here we consider how bending dissipation should scale with slab parameters such as dip angle plate age the radius of curvature and plate velocity by systematically exploring the parameter space with instantaneous stokes flow calculations we derive the scaling of bending dissipation for a range of mantle viscosity functions including pseudoplastic rheology with olivine flow laws our results indicate that as we move away from the isoviscous case the scaling gradually deviates from what has commonly been assumed in previous studies most notably for the radius exponent which exhibits more than threefold reduction and even a sign reversal in some cases these modifications in scaling exponents originate in the complication of the deformation field caused by viscosity variations within the bending plate approximating the lithospheric rheology by a single effective viscosity in the dynamical models of subduction has been a common practice but we suggest that such approximation may limit the geological relevance of modeling studies in particular when estimating the significance of bending dissipation

signature of remnant slabs in the north pacific from p wave tomography summary a 3 d ray tracing technique was used in a global tomographic inversion in order to obtaintomographicimagesofthenorthpacific thedatareportedbythegeophysical surveyofrussia 1955 1997 wereusedtogetherwiththecataloguesoftheinternational seismologicalcenter 1964 1991 andtheusgeologicalsurveynationalearthquake information center 1991 1998 and the recompiled catalogue was reprocessed the final data set used for following the inversion contained 523430 summary ray paths the whole of the earth s mantle was parametrized by cells of 2 2 and 19 layers the large and sparse system of observation equations was solved using an iterative lsqr algorithm asubhorizontalhigh velocityanomalyisrevealedjustabovethe660kmdiscontinuity beneaththealeutiansubductionzone thishigh velocityfeatureisobservedatlatitudes of up to 70 n and is interpreted as a remnant of the subducted kula plate which disappeared through ridge subduction at about 48ma a further positive velocity perturbation feature can be identified beneath the chukotka peninsula and okhotsk sea extending from 300 to 660km depth and then either extending further down to 800km chukotka or deflecting along the 660km discontinuity okhotsk sea this high velocity anomaly is interpreted as a remnant slab of the okhotsk plate accreted to siberia at 55ma

plate kinematics slab shape and back arc stress a comparison between laboratory models and current subduction zones a combination of statistical studies on present day subduction zones and three dimensional 3d laboratory models is performed with the aim to clarify the way that plate kinematics control the geometry of the slab and the overriding plate deformation in subduction zones in 3d laboratory models the analogue of a two layer linearly viscous lithosphere upper mantle system is achieved by means of silicon putty glucose syrup tank experiment the subducting and overriding plate velocities are systematically changed by exploring the variability field of natural plate kinematics both statistical and modelling approaches recognize the importance of overriding plate motion on subduction process behavior 1 trenches migrate at a rate close to the overriding plate motion but always move slower than the overriding plates the mechanism at work is a direct consequence of slab anchoring opposed by both lithosphere and mantle viscous resistance and is responsible for overriding plate deformation and slab geometry variability 2 an overriding plate shortens when the overriding plate moves toward the trench and conditions that are favourable for overriding plate extension are created when the overriding plate moves away from the trench 3 shallow and steep dips are found if the overriding plate moves toward and away from the trench respectively

plate motions slab dynamics and back arc deformation back arc extension or compression is often related to a particular behavior of the slab dip change and or forward backward migration with respect to the upper plate slabs act either passively when anchored in the deep mantle whereas back arc deformation accommodates the backward or forward motion of the upper plate with respect to the trench or actively when slab pull increasing with slab age forces the trench to rollback in addition to these two mechanisms i e slab anchor and slab rollback numerous observations support the existence of dynamic mantle flow that can exert an overpressure on one side of the slab causing its forward or rearward migration with respect to the arc based on a compilation of upper plate absolute motion trench absolute motion back arc deformation rate upper plate strain regime and slab age for all oceanic subduction zones excluding any kind of collision with continents arcs or plateaus we have examined how the combined effects of these parameters can account for the observed back arc deformations our main results are 1 a global correlation exists between upper plate absolute motion and back arc deformation i e back arc extension when upper plate retreats and vice versa 2 there are as many advancing trenches as retreating ones with trench motion globally limited to 50 mm y 1 furthermore there is no positive correlation between trench retreat and slab age and 3 upper plate absolute motion often fails to explain the back arc deformation rates and the trench motions observed at several subduction zones as tonga new hebrides sandwich or ryukyu we propose that the trench migration of these subduction zones are on the influence of mantle flows we conclude that spontaneous trench rollback related to slab pull is negligible with respect to upper plate motion back arc deformation regime is mostly controlled by the upper plate absolute motion relative to a partly anchored slab trench motion is limited to 50 mm y 1 even if locally mantle flows force slab to move

the lithospheric architecture of two subterranes in the eastern yidun terrane east tethys insights from hf nd isotopic mapping abstract a long standing controversy exists regarding the tectonic division lithospheric architecture and evolution of the eastern yidun terrane in the late triassic a compilation of geochronological geochemical and isotopic data 91 whole rock nd and 413 laser points of zircon hf for volcanic and intrusive rocks across the entire eastern yidun terrane has allowed for a detailed investigation into its lithospheric architecture two subterranes were identified using hf nd isotopic mapping they include a high hf t 3 0 and nd t 3 5 domain constrained in the southern yidun terrane syt and a low hf t

sunda java trench kinematics slab window formation and overriding plate deformation since the cretaceous the kinematics and time dependence of back arc extension or compression is one of the most poorly understood aspects of plate tectonics and has nearly exclusively been studied from snapshots of present day observations here we combine absolute and relative plate motions with reconstructions of now subducted ocean floor to analyse subduction kinematics and upper plate strain from geological observations since 80 ma along the 3200 km long sunda java trench one of the largest subduction systems on earth combining plate motions and slab geometries enables us to reconstruct a time dependent slab window beneath sundaland formed through wharton spreading ridge subduction we find that upper plate advance and retreat is the main influence on upper plate strain but subduction of large bathymetric ridges and slab window effects also play a significant and at times dominant role compression in the sundaland back arc region can be linked to advance of the upper plate extension of the sundaland backarc region correlates with two patterns of upper plate motion a retreat of the upper plate and b advance of the upper plate combined with more rapid advance of the sundaland margin due to hinge rollback subduction of large bathymetric ridges causes compression in the upper plate especially wharton ridge subduction underneath sumatra over the period 15 0 ma our reconstructions unravel the evolving geometry of a slab window underlying the java south sumatra region and we propose that decreased mantle wedge viscosities associated with this slab window exacerbated palaeogene extension in the java sea region via active rifting and enabled sumatran continental extension to continue at 50 35 ma when upper plate advance would otherwise have led to compression

orogens and slabs vs their direction of subduction abstract subduction zones appear primarily controlled by the polarity of their direction i e w directed or e to nne directed probably due to the westward drift of the lithosphere relative to the asthenosphere the decollement planes behave differently in the two end members in the w directed subduction zone the decollement of the plate to the east is warped and subducted whereas in the e to nne directed it is ramping upward at the surface there are w directed subduction zones that work also in absence of active convergence like the carpathians or the apennines w directed subduction zones have shorter life 30 40 ma than e or ne directed subduction zones even longer than 100 ma the different decollements in the two end members of subduction should control different ptt paths and therefore generate variable metamorphic assemblages in the associated accretionary wedges and orogens these asymmetries also determine different topographic and structural evolutions that are marked by low topography and a fast eastward migrating structural wave along w directed subduction zones whereas the topography and the structure are rapidly growing upward and expanding laterally along the opposite subduction zones the magmatic pair calc alkaline and alkaline tholeiitic volcanic products of the island arc and the back arc basin characterise the w directed subduction zones magmatic rocks associated with e or ne directed subduction zones have higher abundances of incompatible elements and mainly consist of calc alkaline shoshonitic suites with large volumes of batholithic intrusions and porphyry copper ore deposits the subduction zones surrounding the adriatic plate in the central mediterranean confirm the differences among subduction zones as primarily controlled by the geographic polarity of the main direction of the slab the western margin of the adriatic plate contemporaneously overridden and underthrust europe toward the west to generate respectively the alps and the apennines while the eastern margin subducted under the dinarides hellenides these belts confirm the characters of the end members of subduction zones as a function of their geographic polarity similarly to the pacific subduction zones

rapidness and robustness of finite source inversion of the 2011 mw 9 0 tohoku earthquake by an elliptical patches method using continuous gps and acceleration data the kinematic rupture process of the 2011 mw 9 0 tohoku earthquake is inverted with an elliptical patches method using a genetic algorithm for the purpose of rapid and robust estimation of the source parameters of a mega earthquake we use the ground displacement field provided by a continuous gps network and the ground velocity field recorded by acceleration networks in addition to the typical inversion procedure in which a data duration long enough to cover the whole rupture process is used inversions based on shorter signals giving an incomplete view over the ground shaking sequence are also carried out how fast can a robust estimation of the source parameters be obtained using the elliptical approximation we find that robust solutions of mw 9 0 earthquakes are rather quickly obtained regardless of the frequency band and the elliptical patch description it is also confirmed that because of the absence of off shore recording stations on the east side of the fault some uncertainties in the rupture process cannot be completely removed in fact at the very low frequencies considered the western part of the fault deeper portion contributes more significantly to the recorded signals than does the other end close to the trench this problem also prevents refinement of the description of the rupture process in particular when using more than one ellipse

abrupt upper plate tilting during slab transition zone collision abstract the sinking remnant of a surface plate crosses and interacts with multiple boundaries in earth s interior here we specifically investigate the prominent dynamic interaction of the sinking plate portion with the upper mantle transition zone and its corresponding surface elevation signal we unravel for the first time that the collision of the sinking slab with the transition zone induces a sudden dramatic downward tilt of the upper plate towards the subduction trench unraveling this crucial interaction was only possible thanks to state of the art numerical modelling and post processing the new model that is introduced here to study the dynamically self consistent temporal evolution of subduction features accurate subduction zone topography robust single sided plate sinking stronger plates close to laboratory values an upper mantle phase transition and simple continents at a free surface to distinguish the impact of the new physical model features three different setups are used the simplest model setup includes a basic high viscosity lower mantle the second adds a 660 km phase transition and the third includes additionally a continental upper plate common to all models is the clear topographic signal upon slab transition zone interaction the upper plate tilts abruptly towards the subduction trench by about 0 05 and over around 10 ma this dramatic increase in upper plate tilt can be related to the slab induced excitation of the high viscosity lower mantle which introduces a wider flow pattern a large change in horizontal extent of inundation of up to 900 km is observed as a direct consequence of the upper plate tilting such an abrupt variation in surface topography and inundation extent should be clearly visible in temporal records of large scale surface elevation and might explain continental tilting as observed in australia since the eocene and north america during the phanerozoic

retracted signatures of downgoing plate buoyancy driven subduction in motions and seismic coupling at major subduction zones this article has been retracted at the request of the editor in chief and authors please see elsevier policy on article withdrawal http www elsevier com locate withdrawalpolicy reason after publication errors were discovered in the plate motion database that it was based on this dataset was an updated version of the dataset presented in sdrolias and muller 2006 provided to us by the first author the errors in this version were in the away from trench towards trench assignment for subduction zones with back arcs and also due to the fact that the next generation plate model had only partially been completed these errors affect the conclusions about seismic coupling they also change some of the points in most of the other plots and although this does not invalidate the other conclusions the discussion to reach them would be altered

eolian transported freshwater diatoms and phytoliths across the equatorial atlantic record temporal changes in saharan dust transport patterns the deposition of freshwater diatoms fd and phytotliths p was determined from sediment traps for a wide region 20 n 7 s in the tropical and equatorial atlantic along a n s transect in the eastern equatorial atlantic around 10 w and in the western equatorial atlantic around 25 w these siliceous organisms are derived from the sahara and sahel regions of africa and eolian transport with direct settling over the open ocean is assumed to be the transport agent depositional rates of fd and p revealed strong coupling with seasonal changes in saharan dust transport that are associated with seasonal precipitation patterns major wind systems and the geographical extension of the dust plume across the atlantic mean daily fluxes were highest south of cape verde fd 9 104 valves m 2 d 1 p 2 104 bodies m 2 d 1 moderately high off cape blanc and in the guinea basin north of the equator of the order of 3 104 valves m 2 d 1 for fd and 0 7 104 bodies m 2 d 1 for p and consistently low south of the equator and in the western equatorial atlantic in traps north of the equator seasonal changes were marked aulacoseira granulata and a islandica were the most abundant fd in the traps regardless of trap location and season however the number of fd species was higher in the cape blanc and cape verde areas the morphological diversity shape and size of the p assemblage decreased with increasing distance from the african continent patterns of fd and p accumulation rates in surface sediments coincided with those in the traps robust freshwater diatom and phytolith records associated with seasonal eolian transport from saharan and sahelian regions into the atlantic furnish clues that can help in our present understanding of the processes linking transport between land atmosphere and ocean

parameters controlling dynamically self consistent plate tectonics and single sided subduction in global models of mantle convection recent advances in numerical modeling allow global models of mantle convection to more realistically reproduce the behavior at convergent plate boundaries in particular the inclusion of a free surface at the outer boundary has been shown to facilitate self consistent development of single sided subduction this allows for a more extensive study of subduction in the context of global mantle convection as opposed to commonly used regional models our first study already indicated important differences between mantle convection with single sided subduction and mantle convection with double sided subduction here we further investigate the effect of various physical parameters and complexities on inducing earth like plate tectonics and its evolution in time results reinforce the previous finding that using a free surface instead of a free slip outer boundary dramatically changes subduction style with free surface cases displaying many episodes of single sided subduction which leads to more realistic slab dip stress state trench retreat rate and slab induced mantle flow longevity of single sided subduction is promoted by a layer of hydrated crust with a low yield strength to lubricate the subduction channel a low viscosity asthenosphere and a high strength of the slab determined by a combination of high diffusion creep viscosity and intermediate friction coefficient although its effective viscosity is in the observationally constrained range in the bending region the time evolution displays interesting events including subduction polarity reversals subduction shut off and slab break off

a plate cooling model relating rates of plate motion to the age of the lithosphere at trenches previous studies have shown that the observed correlation between the age of the lithosphere and rate of absolute motion of subducting oceanic plates at trenches can be explained by a net slab pull force derived from a halfspace cooling model for the thermal lithosphere rates of motion are proportional to the square root of age however depth age trends suggest that the plate thermal structure applies to much of the seafloor rate and age data for trenches where the oldest seafloor is subducted in the western pacific should reflect the plate structure fitting rate and age data from 16 trenches to net slab pull models based on both halfspace and plate cooling models shows that rates of plate motion at the five western pacific trenches where the lithosphere entering the trench is oldest are systematically lower than the rates predicted by the best fitting halfspace model in contrast these data do fit a slab pull model based on plate cooling with r 0 97 and an rms error of 4 7 mm a thus the plate model represents a statistically significant improvement over the halfspace model and the plate like flattening of depth age trends is reflected in the relationship between the rate of subduction and the age of the seafloor at pacific trenches

great earthquakes and slab pull interaction between seismic coupling and plate slab coupling great earthquakes the few largest earthquakes that account for most of the earth s seismic energy release have occurred at only a few subduction zones around the world strong locking or seismic coupling of the interface between plates at certain subduction zones is often invoked to explain these great earthquakes although past studies have correlated strong seismic coupling with a compressional stress environment that is characterized by back arc compression and caused by trenchward motion of the overriding plate the consequences of this compressional environment for the tectonic forces that drive global plate motions are not yet clear to examine these consequences we compared subduction zone earthquake magnitudes to tectonically constrained estimates of the degree to which each slab transmits its excess weight as a direct pull force on a subducting plate at seismically uncoupled subduction zones that generate only moderate sized earthquakes we find that slabs must transmit nearly their entire upper mantle weight as a pull force on the subducting plate at seismically coupled subduction zones that produce great earthquakes however we find that slabs must be nearly completely detached from their subducting plates this suggests that slabs subducting in a compressional environment experience stress induced weakening that prevents the effective transmission of the slab pull force convergent mantle flow above a descending slab that becomes decoupled from its surface plate may induce additional surface compression that further locks the subduction zone and leads to additional slab decoupling and detachment the resulting redistribution of plate driving forces may be responsible for rapid changes in plate motion c 2003 elsevier b v all rights reserved

shape of the subducted rivera and cocos plates in southern mexico seismic and tectonic implications the geometry of the subducted rivera and cocos plates beneath the north american plate in southern mexico was determined based on the accurately located hypocenters of local and teleseismic earthquakes the hypocenters of the teleseisms were relocated and the focal depths of 21 events were constrained using a body wave inversion scheme the suduction in southern mexico may be approximated as a subhorizontal slab bounded at the edges by the steep subduction geometry of the cocos plate beneath the caribbean plate to the east and of the rivera plate beneath north america to the west the dip of the interplate contact geometry is constant to a depth of 30 km and lateral changes in the dip of the subducted plate are only observed once it is decoupled from the overriding plate on the basis of the seismicity the focal mechanisms and the geometry of the downgoing slab southern mexico may be segmented into four regions 1 the jalisco region to the west where the rivera plate subducts at a steep angle that resembles the geometry of the cocos plate beneath the caribbean plate in central america 2 the michoacan region where the dip angle of the cocos plate decreases gradually toward the southeast 3 the guerrero oaxaca region bounded approximately by the onshore projection of the orozco and o gorman fracture zones where the subducted slab is almost subhorizontal and underplates the upper continental plate for about 250 km and 4 the southern oaxaca and chiapas region in southeastern mexico where the dip of the subduction gradually increases to a steeper subduction in central america these drastic changes in dip do not appear to take place on tear faults suggesting that smooth contortions accommodate these changes in geometry the inferred 80 and 100 km depth contours of the subducted slab lie beneath the southern front of the trans mexican volcanic belt suggesting it is directly related to the subduction thus the observed nonparallelism with the middle american trench is apparently due to the changing geometry of the rivera and cocos plates beneath the north american plate in southern mexico and not to zones of weakness in the crust of the north american plate as some authors have suggested

provenance and tectonic evolution of ganderia constraints on the evolution of the iapetus and rheic oceans we provide estimates for the width timing and rates of opening and closing of the iapetus and rheic oceans the evolution of which profoundly infl uenced paleozoic global paleogeography these estimates are primarily derived from the transfer of ganderia and avalonia from gondwana to laurentia which led to closure of the iapetus ocean and opening of the rheic ocean ganderia a long lived arc terrane separated from the paleo caribbean margin of amazonia at 505 ma with a latitudinal speed of 9 cm a northward initiating the rheic ocean as a backarc basin ganderia s trailing edge was reduced to 5 cm a following opening of a 600 800 km wide backarc basin within ganderia at 475 ma opening and closing of the iapetus ocean was largely driven by far fi eld stresses slab pull in some places and slab rollback in others

how mantle slabs drive plate tectonics the gravitational pull of subducted slabs is thought to drive the motions of earth s tectonic plates but the coupling between slabs and plates is not well established if a slab is mechanically attached to a subducting plate it can exert a direct pull on the plate alternatively a detached slab may drive a plate by exciting flow in the mantle that exerts a shear traction on the base of the plate from the geologic history of subduction we estimated the relative importance of pull versus suction for the present day plates observed plate motions are best predicted if slabs in the upper mantle are attached to plates and generate slab pull forces that account for about half of the total driving force on plates slabs in the lower mantle are supported by viscous mantle forces and drive plates through slab suction

a new view into the cascadia subduction zone and volcanic arc implications for earthquake hazards along the washington margin in light of suggestions that the cascadia subduction margin may pose a significant seismic hazard for the highly populated pacific northwest region of the united states the u s geological survey usgs the research center for marine geosciences geomar and university collaborators collected and interpreted a 530 km long wide angle onshore offshore seismic transect across the subduction zone and volcanic arc to study the major structures that contribute to seismogenic deformation we observed 1 an increase in the dip of the juan de fuca slab from 2 7 to 12 where it encounters a 20 km thick block of the siletz terrane or other accreted oceanic crust 2 a distinct transition from siletz crust into cascade arc crust that coincides with the mount st helens seismic zone supporting the idea that the mafic siletz block focuses seismic deformation at its edges and 3 a crustal root 35 45 km deep beneath the cascade range with thinner crust 30 35 km east of the volcanic arc beneath the columbia plateau flood basalt province from the measured crustal structure and subduction geometry we identify two zones that may concentrate future seismic activity 1 a broad because of the shallow dip possibly locked part of the interplate contact that extends from 25 km depth beneath the coastline to perhaps as far west as the deformation front 120 km offshore and 2 a crustal zone at the eastern boundary between the siletz terrane and the cascade range

trench motion slab geometry and viscous stresses in subduction systems summary a semi analytic 3 d model for subduction within a newtonian viscous upper mantle provides a dynamically consistent means of computing viscous stress trench motion and slab geometry in subduction systems although negative slab buoyancy provides the basic driving force for subduction slabs that extend from the surface to the base of the upper mantle are oversupported by viscous stresses in the shallow 100 150 km mantle and undersupported by viscous stresses at greater depth in the upper mantle these deeper parts of the subduction system act as an engine for subduction while shallower parts act as a brake on trench motion trench migration rates and slab geometry reflect a competition between these two effects during steady state subduction trench migration rates vary approximately linearly with slab buoyancy and model rates of trench motion are in good agreement with the range of observed rates for a two layer upper mantle viscosity of 2 10 20 pa s above 300 km and 5 10 20 pa s below steady state slab dip increases as slab density decreases especially for very low density slabs which dip significantly more steeply than high density slabs the horizontal velocity at the top of the lower mantle measured relative to the foreland has a very large effect on trench migration rates rivalling or even exceeding that of slab buoyancy slab width parallel to the trench also has a significant effect on trench migration rates due to the viscous pressure of toroidal flow around the slab the stiffness of the subducting lithosphere does not exert a significant effect on trench migration rates or slab geometries for rigidities compatible with oceanic lithosphere very stiff slabs with elastic plate thicknesses more that 40 km or viscosities in the range of 10 25 10 26 pa s subduct significantly more slowly than weak slabs with trench migration rates in the range of half to a third that of weak slabs large unexpected effects on trench migration rates and slab geometry are exerted by the structure and density of the frontal prism and overriding plate indicating that local geology can exert important constraints on slab dynamics during non steady state subduction rates of trench migration respond rapidly as variably buoyant lithosphere penetrates into the asthenosphere in the absence of other driving forces for convergence trench migration rates can change by a factor of two or more in as little as 2 3 myr for example from 35 to 70 mm yr 1 when an oceanic piece of slab follows a continental one into the subduction system subduction of variable buoyancy lithosphere is accompanied by changes in slab dip with depth and through time

a double island arc between taiwan and luzon consequence of ridge subduction analysis of geomorphological geochronological geochemical and geophysical features in the segment of the taiwan luzon arc between taiwan and luzon the bashi segment allows the recognition of a double arc structure the two volcanic chains are separated by 50 km just north of luzon 18 n and converge near 20 n islets in the western chain are older and largely composed of volcanic rocks of miocene to pliocene age they all show low relief lateritic platforms and wave cut terraces and are covered by massive recrystallized limestone in contrast all active volcanoes in this segment of the taiwan luzon arc belong to the eastern chain where most islets are quaternary in age the volcanoes have well developed cone shapes and well preserved deposits of near vent facies magmas of the eastern chain have higher ksi la n la yb n and lowerend than their counterparts at the same latitude in the western chain therefore the magmas erupted in the eastern chain were derived from more enriched mantle sources than the magmas erupted in the western chain moreover the available seismological data seem to suggest an abrupt increase of the dip angle from 30 at 18 n to 80 at 20 n thus the double arc structure is located in the region where the benioff zone suddenly changes in analogy with the lesser antilles arc we propose a geodynamic model in which the double arc in the bashi strait is the tectonic manifestation of the subduction of the aseismic scarborough seamount chain the extinct mid ocean ridge of the south china sea before that ridge reached the manila trench the western chain was the volcanic front when the ridge reached the subduction zone at 5 4 ma its buoyancy temporarily interrupted the subduction thus causing a time gap in magmatic activity furthermore this ridge arc collision was probably also responsible for regional uplift causing extensive sub aerial weathering and erosion as well as massive reef formation in the western chain when subduction started again the dip angle became shallower in response to the extra buoyancy of the downgoing ridge if the depth of magma generation remained constant the shallower dip angle would have naturally led to an eastward shift of the volcanic front thus producing the younger eastern chain moreover we speculate that the abrupt change of the dip angle may have torn the downgoing slab thus allowing more enriched continental lithospheric material to invade the mantle wedge from the northwest thus imprinting a geochemically more enriched signature on the magmas of the eastern chain

comment on the potential influence of subduction zone polarity on overriding plate deformation trench migration and slab dip angle by w p schellart abstract the schellart s schellart w p 2007 the potential influence of subduction zone polarity on overriding plate deformation trench migration and slab dip angle tectonophysics 445 363 372 paper uses slab dip and upper plate extension for testing the westward drift his analysis and discussion are misleading for the study of the net rotation of the lithosphere since the first 125 km of subduction zones are sensitive also to other parameters such upper plate thickness geometry and obliquity of the subduction zone with respect to the convergence direction the deeper 125 km part cannot easily be compared as well because e or ne directed subduction zones have seismic gaps between 270 630 km moreover the velocity of subduction hinge cannot be precisely estimated and it does not equal to backarc spreading due to accretionary prism growth and asthenospheric intrusion at the subduction hinge it is shown here that hinge migration in the upper plate or lower plate reference frames supports a general global polarization of the lithosphere in agreement with the westward drift of the lithosphere the w directed subduction zones appear controlled by the slab mantle interaction with slab retreat imposed by the eastward mantle flow the opposite e ne directed subduction zones seem rather mainly controlled by the convergence rate plus density thickness and viscosity of the upper and lower plates finally the geological and geophysical asymmetries recorded along subduction and rift zones as a function of their polarity with respect to the tectonic mainstream are not questioned in the schellart s paper but they rather represent the basic evidence for the westward drift of the lithosphere

mantle wedge asymmetries and geochemical signatures along w and e ne directed subduction zones article i nfo subduction zone kinematics predict that assuming a fixed lower plate the velocity of the subduction equals the velocity of the subduction hinge vs vh in all subduction zones the subduction hinge migrates toward the lower plate however two main types of subduction zones can be distinguished 1 those where the upper plate converges toward the lower plate slower than the subduction hinge mostly w directed and 2 those in which the upper plate converges faster than the subduction hinge generally e or ne directed along the first type there generally is an upward flow of the asthenosphere in the hanging wall of the slab whereas along the opposite second type the mantle is pushed down due to the thickening of the lithosphere the kinematics of w directed subduction zones predict a much thicker asthenospheric mantle wedge larger volumes and faster rates of subduction with respect to the opposite slabs moreover the larger volumes of lithospheric recycling the thicker column of fluids rich hotter mantle wedge all should favour greater volumes of magmatism per unit time the opposite e ne directed subduction zones show a thinner if any asthenospheric mantle wedge due to a thicker upper plate and shallower slab along these settings the mantle wedge where the percolation of slab delivered fluids generates melting mostly involves the cooler lithospheric mantle the subduction rate is smaller andesites are generally dominant and the lithosphere thickens there appears to be a greater contribution to the growth of the continental lithosphere another relevant asymmetry that can be inferred is the slab induced corner flow in the mantle along w directed subduction zones and an upward suction of the mantle along the opposite e or nne directed slabs the upward suction of the mantle inferred at depth along e ne directed subduction zones provides a mechanism for syn subduction alkaline magmatism in the upper plate with or without contemporaneous rifting in the backarc positive 11 ba nd high 143 nd 144 nd characterize w directed subduction zones where a thicker and hotter mantle wedge is present in the hanging wall of the slab however this observation disappears where large amounts of crustal rocks are subducted as along the w directed apennines sub duction zone

oligocene to present shallow subduction beneath the southern puna plateau abstract the southern puna plateau is a conspicuous example of a high elevation orogenic plateau in a non collisional setting this orogenic sector is currently located above an anomalously shallow subduction segment in which timing and relation to upper plate tectonics have been widely overlooked this subduction segment here referred to as the southern puna shallow subduction spss is characterized by a 200 km wide shallow area located at 300 km from the trench at a depth of 100 120 km and dipping 10 12 to the east to determine the onset of the spss and its link to the tectonic and magmatic activity in this region we analyzed the tectonomagmatic record of the southern puna plateau from preexisting datasets also we present a new approach based on global subduction data that provides a straightforward methodology to extract potential paleo slab angles from the bedrock arc record this analysis reveals that a pronounced eastward arc front migration and magmatic broadening took place at 26 ma and was preceded by 4 ma of reduced magmatic activity which we link to the inception of the spss as expected in shallow subduction settings a change to basement cored distributed deformation south of 25 s in the southern puna plateau coincides with the beginning of shallow subduction also the spss is coincident with the enigmatic post eocene intraplate deformation of the otumpa hills located at 950 km from the trench we suggest that this succession of events is not fortuitous and that the development of the spss impacted directly the overriding plate since the oligocene contributing to the building of one of the largest topographies 3 km and thickest orogenic crusts 70 60 km on earth the shallow subduction would have acted jointly with cenozoic changes in plate kinematics and climate enhancing andean orogenesis at studied latitudes

subducted slabs beneath the eastern indonesia tonga region insights from tomography tomographic images of mantle structure beneath the region north and northeast of australia show a number of anomalously fast regions these are interpreted using a recent plate tectonic reconstruction in terms of current and former subduction systems several strong anomalies are related to current subduction the inferred slab lengths and positions are consistent with neogene subduction beneath the new britain and halmahera arcs and at the tonga and the new hebrides trenches where there has been rapid rollback of subduction hinges since about 10 ma there are several deeper flat lying anomalies which are not related to present subduction and we interpret them as former subduction zones overridden by australia since 25ma beneath the bird s head and arafura sea is an anomaly interpreted to be due to north dipping subduction beneath the philippines halmahera arc between 45and 25ma a very large anomaly extending from the papuan peninsula to the new hebrides and from the solomon islands to the east australian margin is interpreted to be the remnant of south dipping subduction beneath the melanesian arc between 45and 25ma this interpretation implies that a flat lying slab can survive for many tens of millions of years at the bottom of the upper mantle in the lower mantle there is a huge anomaly beneath the gulf of carpentaria and east papua new guinea this is located above the position where the tectonic model interprets a change in polarity of subduction from north dipping to south dipping between 45and 25ma we suggest this deep anomaly may be a slab subducted beneath eastern australian during the cretaceous or subducted north of australia during the cenozoic before 45ma the tomography also supports the tectonic interpretation which suggests little neogene subduction beneath western new guinea since no slab is imaged south of the new guinea trench however one subduction zone in the tectonic model and many others that associated with the trobriand trough east of papua new guinea and the miocene maramuni arc is not seen in the tomographic images and may require reconsideration of currently accepted tectonic interpretations 4 2002 elsevier science b v all rights reserved

geochemical evidence for the melting of subducting oceanic lithosphere at plate edges most island arc magmatism appears to result from the lowering of the melting point of peridotite within the wedge of mantle above subducting slabs owing to the introduction of fluids from the dehydration of subducting oceanic crust1 volcanic rocks interpreted to contain a component of melt not just a fluid from the subducting slab itself are uncommon but possible examples have been recognized in the aleutian islands baja california patagonia and elsewhere2 3 4 the geochemically distinctive rocks from these areas termed adakites are often associated with subducting plates that are young and warm and therefore thought to be more prone to melting5 but the subducting lithosphere in some adakite locations such as the aleutian islands appears to be too old and hence too cold to melt6 7 this implies either that our interpretation of adakite geochemistry is incorrect or that our understanding of the tectonic context of adakites is incomplete here we present geochemical data from the kamchatka peninsula and the aleutian islands that reaffirms the slab melt interpretation of adakites2 but in the tectonic context of the exposure to mantle flow around the edge of a torn subducting plate we conclude that adakites are likely to form whenever the edge of a subducting plate is warmed or ablated by mantle flow the use of adakites as tracers for such plate geometry may improve our understanding of magma genesis and thermal structure in a variety of subduction zone environments

azimuthal seismic anisotropy constrains net rotation of the lithosphere 1 net rotation nr of the lithosphere is found in hot spot reference frames important for tectonics and plume models but difficult to constrain using mantle flow and crystallographic texture modeling i show that nrs lead to mantle shearing which is recorded in azimuthal seismic anisotropy the nr amplitude in some hot spot models is so large that it degrades the model fit to anisotropy significantly smaller nrs such as predicted by flow models with stiff continental keels are consistent with seismology however

andean tectonics related to geometry of subducted nazca plate seismological and geological data show that tectonic segmentation of the andes coincides with segmentation of the subducted nazca plate which has nearly horizontal segments and 30 east dipping segments andean tectonics above a flat subducting segment between 28 s to 33 s are characterized by from west to east 1 a steady topographic rise from the coast to the crest of the andes 2 no significant quaternary and possibly neogene magmatism 3 a narrow belt of eastward migrating apparently thin skinned neogene to quaternary shortening of the andes and 4 plio pleistocene uplift of the crystalline basement on reverse faults in the pampeanas ranges from about 15 to 24 s over a 30 dipping subducted plate a west to east andes cross section includes 1 a longitudinal valley east of coastal mountains 2 an active neogene and holocene andesitic volcanic axis 3 the altiplano puna high plateau 4 a high neogene but inactive thrust belt eastern cordillera and 5 an active eastward migrating subandean thin skinned thrust belt tectonics above a steeply subducting segment south of 33 s are similar west of the volcanic axis but quite different to the east early cenozoic tectonics of western north america were quite similar to the neogene andes however duration of segmentation was longer and the width of deformation was greater in the western united states patterns of crustal seismicity are systematically related to plio quaternary structural provinces implying that current deformational processes have persisted since at least the pliocene horizontal compression parallel to the plate convergence direction is indicated to a distance of 800 km from the trench above flat subducting segments crustal seismicity occurs over a broad region whereas over steep segments it is confined to the narrow thrust belt strain patterns in the forearc region are complex and perhaps extensional and a broad region of the altiplano puna and eastern cordillera appears to be aseismic

variations of slab dip and overriding plate tectonics during subduction insights from analogue modelling we present small scale laboratory models of oceanic subduction in which plates motion is imposed by lateral boundary conditions the oceanic plate moves trenchward at constant speed and subducts below a fixed overriding plate in this configuration the long term process of subduction is not steady state slab interaction with the upper mantle lower mantle boundary results in periods of slab flattening during which the dip of the slab diminishes followed by periods of slab steepening the overriding plate tectonic regime is influenced by the dynamics of subduction slab anchoring favouring trench perpendicular shortening when the slab is anchored slab flattening further favours shortening while slab steepening favours extension or smaller shortening rates non steady state long term subduction may explain part of the variability of slab geometries evidenced by statistical analyses of present day subduction zones experiments suggest that despite boundary conditions applied on the converging plates do not change tectonics pulses within the overriding plate may be caused by this non steady state behaviour

on the relationships between slab dip back arc stress upper plate absolute motion and crustal nature in subduction zones 1 statistical analysis of modern oceanic subduction zone parameters such as the age of a downgoing plate or the absolute plate motions is performed in order to investigate which parameter controls the dip of a slab and conversely what the influence of slab geometry is on upper plate behavior for that purpose parameters have been determined from global databases along 159 transects from all subduction zones that are not perturbed by nearby collision or ridge plateau seamount subduction on the basis of tomographic images slabs that penetrate through or lie on the 670 km discontinuity are also identified the results of the statistical analysis are as follows 1 back arc stress correlates with slab dip i e back arc spreading is observed for deep dips deeper than 125 km larger than 50 whereas back arc shortening occurs only for deep dips less than 30 2 slab dip correlates with absolute motion of the overriding plate the correlation is even better when the slab lies on or even more penetrates through the 670 km discontinuity 3 slabs dip more steeply by about 20 on average beneath oceanic overriding plates than beneath continental ones 4 slabs dip more steeply on average by about 10 near edges 5 slab dip does not correlate with the magnitude of slab pull the age of subducting lithosphere at the trench the thermal regime of the subducting lithosphere the convergence rate or the subduction polarity east versus west the present study provides evidence that the upper plate absolute motion plays an important role on slab dip as well as on upper plate strain retreating overriding plates are often oceanic ones and thus may partially explain the steeper slab dips beneath oceanic upper plates one can infer that low slab dips correlate well with compression in continental advancing upper plates whereas steep dips are often associated with extension in oceanic retreating upper plates excess weight of old slabs is often counterbalanced by other forces probably asthenospheric in origin such as lateral mantle flow near slab edges or anchor forces to determine slab dip

lithospheric flexure and thermal anomalies plots of effective elastic thickness te of the oceanic lithosphere as a function of age when elastically deformed show a general increase in te with the square root of age although there is much scatter some of the scatter can be eliminated by correcting for the effect of finite yield strength in rocks which makes te for more sharply bent plates underestimate the true depth tm to the rheological boundary at the base of the high strength mechanical lithosphere after converting effective values te to true thicknesses tm it appears that the mechanical lithosphere is always thinner beneath islands and seamounts compared to lithosphere of similar age flexed at subduction zones from several other lines of evidence such as depth anomalies seamount subsidence and heat flow data it has been suggested that the thermal structure in plates subjected to hot spot type volcanism is reset to values appropriate for much younger lithosphere the reduction in mechanical plate thickness beneath hot spot volcanoes is also consistent with this concept of thermal rejuvenation since the strength of rocks decreases exponentially with rising temperature this dependence implies that variations in mechanical plate thickness can be used to map temperature anomalies in the lithosphere since the mechanical plate thickness depends principally upon the temperature at one depth specifically the depth at which the temperature reaches 550 600 c depending on strain rate flexural data are particularly sensitive indicators of the vertical structure of lithospheric reheating compared to quantities dependent on depth integrals of temperature such as swell height and subsidence the thermal rejuvenation hypothesis predicts that the flexural profile should mimic viscoelastic relaxation in the first 10 ma after loading if initially reheating is confined to depths close to and below the 550 c isotherm the flexural data combined with depth anomaly information for hawaii point to extremely rapid reheating localized in the lower lithosphere rejuvenation in the society islands on the other hand is compatible with slower methods of heat injection involving the entire lithospheric column

slow subduction of a thick ultrahigh pressure terrane 1 high precision lu hf and sm nd ages reported here cover a large portion of the western gneiss region wgr ultrahigh pressure uhp terrane norway and range from 413 9 3 7 to 397 1 4 8 ma collectively new and existing geochronologic data for eclogites demonstrate that eclogite facies metamorphism occurred over a large area 60 000 km2 for an unexpectedly long period of 20 ma a thermal model using slow subduction 2 4 mm a 1 of a relatively warm and thick slab can reproduce the observed p t t history of the wgr uhp terrane indicating that large uhp terranes with protracted peak or near peak histories probably reflect slow subduction of relatively thick crustal sections

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

early paleozoic tectonic evolution of the north qinling orogenic belt in central china insights on continental deep subduction and multiphase exhumation abstract the north qinling orogenic belt is an important component of qinling composite orogen in central china high pressure and ultra high pressure hp uhp rocks occur as lenses or layers surrounded by gneissic rocks in the northern central and southern qinling complex of the north qinling belt considered to have formed through deep continental subduction the magmatic protoliths of the hp uhp metamorphic rocks show formation ages ca 800 ma geochemical characteristics and pb nd isotopic compositions similar to those of the neoproterozoic igneous rocks of the south qinling belt continental materials of the south qinling belt were dragged down to mantle depths by the shangdan oceanic crust subducted and subjected to hp uhp metamorphism at ca 500 ma the erlangping and kuanping backarc basin formed in response to subduction of the shangdan ocean and might have developed into a limited small ocean basin at ca 500 ma the hp uhp rocks yielded retrograde metamorphic ages of ca 470 450 ma and ca 420 400 ma these ages are identical to the age of magmatic events in the north qinling hp uhp belt at 500 ma 450 ma and 420 ma related to deep subduction collision slab breakoff and crustal thinning during post collisional extension the dominant ca 500 400 ma ages of detrital zircons from the liuling group of the south qinling belt match well with those from the three stages of magmatic rocks and hp uhp rocks in the qinling complex this correlation suggests that the magmatic rocks and hp uhp metamorphic rocks in the north qinling belt initially exhumed to the surface eroded and were then deposited in the liuling basin in a post orogenic extensional setting during middle to late devonian new evidence suggests that the qinling complex is a tectonic complex rather than a uniform stratigraphic unit or a microcontinent as previously believed and is mainly composed of the exhumed hp uhp metamorphic rocks deep subduction exhumation related magmatic rocks and the early neoproterozoic granites together with the host rocks from the over riding plate at an active continental margin the early paleozoic tectonic history of the nqb includes oceanic slab subduction and formation of arc backarc spreading continental deep subduction arc continent collision break off and multi stage exhumation of the deep subducted slab as well as extension and thinning and associated erosion and deposition

the effect of slab gaps on subduction dynamics and mantle upwelling abstract gaps within a subducting plate can alter the surrounding mantle flow field and the overall subduction zone dynamics by allowing hot sub slab mantle to flow through the gaps and into the mantle wedge this through slab flow can produce melting of the slab gap edges as well as significant upwelling that can lead to anomalous alkaline volcanism and or dynamic uplift in the overriding plate while the altered mantle flow patterns affect the trench evolution numerous geodynamic models have investigated the processes that form slab gaps but few studies have examined the dynamics of slab gap altered mantle flow its effects on trench morphology and kinematics or the controlling parameters on these processes here laboratory subduction models with a pre cut gap in a subducting silicone plate are used to explore how slab gap size and slab gap depth influence the surrounding mantle flow field and trench dynamics results suggest that both the vertical extent and the depth of the top trailing edge of the slab gap are crucial parameters for modulating overall subduction dynamics they show that a slab gap which occurs near the surface and initially comprises 30 of the subducting plate width can extend enough vertically in the slab to produce significant vertical flow through the gap changes to the trench geometry and kinematics are also evident in the models such that double and triple arc geometries are formed during subduction of a shallow slab gap all of these results are consistent with observations of slab gaps and their induced surface expressions or the lack thereof in eastern anatolia east java italy and argentina

dynamics of subduction and plate motion in laboratory experiments insights into the plate tectonics behavior of the earth 1 three dimensional laboratory experiments have been designed to investigate the way slab bearing plates move during subduction inside the mantle in our experiments a viscous plate of silicone lithosphere subducts under its negative buoyancy in a viscous layer of pure honey mantle varying thickness width viscosity and density of the plate and mantle three characteristic modes of subduction are observed a retreating trench mode mode i a retreating trench mode following a transient period of advancing trench mode ii and an advancing trench mode mode iii these modes are characterized by different partitioning of the amount of subduction into plate and trench motion our experiments show that the velocity of subduction can be modeled by the dynamic interaction between acting and resisting forces where lithospheric bending represents 75 95 of the total resisting forces however our experimental results also show the impossibility to predict a priori the plate velocity only from the velocity of subduction without considering trench migrations we find experimentally that the lithospheric radius of curvature which depends upon plate characteristics stiffness and thickness and the mantle thickness exerts a primary control on the trench behavior our results suggest that the complexity of the style of subduction could be controlled by geometrical rules of a plate bending inside a stratified mantle the earth system is in the crucial range for the interplay between the rigidity of the plate and the mantle stratification this setting may be the responsible for the complexity of the past and present tectonic styles

metastable mantle phase transformations and deep earthquakes in subducting oceanic lithosphere earth s deepest earthquakes occur as a population in subducting or previously subducted lithosphere at depths ranging from about 325 to 690 km this depth interval closely brackets the mantle transition zone characterized by rapid seismic velocity increases resulting from the transformation of upper mantle minerals to higher pressure phases deep earthquakes thus provide the primary direct evidence for subduction of the lithosphere to these depths and allow us to investigate the deep thermal thermodynamic and mechanical ferment inside slabs numerical simulations of reaction rates show that the olivine spinel transformation should be kinetically hindered in old cold slabs descending into the transition zone thus wedge shaped zones of metastable peridotite probably persist to depths of more than 600 km laboratory deformation experiments on some metastable minerals display a shear instability called transformational faulting this instability involves sudden failure by localized superplasticity in thin shear zones where the metastable host mineral transforms to a denser finer grained phase hence in cold slabs such faulting is expected for the polymorphic reactions in which olivine transforms to the spinel structure and clinoenstatite transforms to ilmenite it is thus natural to hypothesize that deep earthquakes result from transformational faulting in metastable peridotite wedges within cold slabs this consideration of the mineralogical states of slabs augments the traditional largely thermal view of slab processes and explains some previously enigmatic slab features it explains why deep seismicity occurs only in the approximate depth range of the mantle transition zone where minerals in downgoing slabs should transform to spinel and ilmenite structures the onset of deep shocks at about 325 km is consistent with the onset of metastability near the equilibrium phase boundary in the slab even if a slab penetrates into the lower mantle earthquakes should cease at depths near 700 km because the seismogenic phase transformations in the slab are completed or can no longer occur substantial metastability is expected only in old cold slabs consistent with the observed restriction of deep earthquakes to those settings earthquakes should be restricted to the cold cores of slabs as in any model in which the seismicity is temperature controlled via the distribution of metastability however the geometries of recent large deep earthquakes pose a challenge for any such models transformational faulting may give insight into why deep shocks lack appreciable aftershocks and why their source characteristics including focal mechanisms indicating localized shear failure rather than implosive deformation are so similar to those of shallow earthquakes finally metastable phase changes in slabs would produce an internal source of stress in addition to those due to the weight of the sinking slab such internal stresses may explain the occurrence of earthquakes in portions of lithosphere which have foundered to the bottom of the transition zone and or are detached from subducting slabs metastability in downgoing slabs could have considerable geodynamic significance metastable wedges would reduce the negative buoyancy of slabs decrease the driving force for subduction and influence the state of stress in slabs heat released by metastable phase changes would raise temperatures within slabs and facilitate the transformation of spinel to the lower mantle mineral assemblage causing slabs to equilibrate more rapidly with the ambient mantle and thus contribute to the cessation of deep seismicity because wedge formation should occur only for fast subducting slabs it may act as a parachute and contribute to regulating plate speeds wedge formation would also have consequences for mantle evolution because the density of a slab stagnated near the bottom of the transition zone would increase as it heats up and the wedge transforms to denser spinel favoring the subsequent sinking of the slab into the lower mantle