subduction zone trench migration slab driven or overriding plate driven abstract subduction zones on earth and their associated trenches and hinges migrate with respect to the overriding plate as indicated by overriding plate deformation e g backarc extension or backarc shortening and migrate with respect to hotspot and no net rotation reference frames global or absolute reference frames three geodynamic models exist that attempt to explain how absolute trench migration velocity correlates with overriding plate deformation in one model trench and hinge migration result from lateral migration of the slab with trench retreat causing overriding plate extension and trench advance causing overriding plate shortening in the second model trench migration is forced by the overriding plate with trenchward motion causing trench retreat and overriding plate shortening and motion away from the trench causing trench advance and overriding plate extension in the third model the trench and subduction hinge are thought to be static while overriding plate extension shortening is accommodated by landward trenchward overriding plate motion in this paper the conflicting geodynamic predictions made by these three models are tested by using global kinematic calculations of trench migration velocity v t overriding plate velocity v op and overriding plate deformation in different global reference frames the dependence between v t and overriding plate deformation as well as v op and overriding plate deformation was investigated correlation coefficients r and confidence limits were calculated using a quantitative approach with overriding plate deformation velocity v opd using a semi quantitative approach with an overriding plate strain classification approach and using a spearman rank correlation approach for the quantitative approach the correlation between v t and v opd is consistently positive r 0 33 0 68 where trench retreat corresponds to extension and statistically significant at 95 confidence level for all but one reference frame for v op and v opd the correlation is consistently negative but much less significant r 0 29 to 0 17 and statistically not significant at 95 confidence level for all reference frames these findings indicate that trench migration results predominantly from lateral migration of the slab rather than from overriding plate motion such lateral slab migration is most likely driven by the negative buoyancy force of the slab itself as demonstrated by geodynamic models

anelastic response of the earth to a dip slip earthquake the deformation of the earth s surface immediately following an earthquake is well modeled by a dislocation in an elastic half space some time after the event however the elastic stresses begin to relax due to flow in the more fluid regions of the upper mantle the asthenosphere this relaxation causes further slow displacements observable at the earth s surface this paper examines the pattern and timing of these postseismic motions for a vertical dip slip fault and a 30 dip thrust fault a variety of rheological models are studied these models encompass variations in asthenosphere thickness mesosphere viscosity fault depth and most important power law flow both newtonian n 1 and non newtonian n 3 maxwell rheologies are considered for the asthenosphere and mesosphere the results for a vertical dip slip fault show that a peripheral upwarp develops within a distance of a few lithosphere thicknesses from the rupture zone on the downthrown side of the fault a comparable downwarp develops on the upthrown side these peripheral warps grow to a maximum height of about 5 of the total fault slip some time after the earthquake event and then collapse at much greater times the vertical motions for a 30 dip thrust fault show that after the earthquake a general uplift occurs in the region near the rupture zone horizontal displacements propagate asymmetrically away from the rupture zone the largest displacements occurring in the overthrust plate the principal result of the comparison of n 1 and n 3 rheological laws is that the basic pattern of uplifts and horizontal motions is very similar the major difference is the time dependence of the development of the pattern in spite of the complex geometrical structure of the subduction zone the displacements ui 1 t for newtonian rheology are nearly the same as ui 3 t for the non newtonian rheology if t t3 thus although the qualitative pattern of the displacements is not diagnostic of power law flow the great difference in time dependence allows a clear discrimination of the two flow laws

a recent tectonic reorganization in the south central mediterranean abstract new geodetic data combined with seismicity geologic and geochemical information document a major tectonic reorganization in the central mediterranean around 0 8 0 5 ma when rapid trench migration and consequent tyrrhenian back arc extension which dominated the region s evolution since the tortonian 10 8 ma essentially stopped in response the african convergence in sicily was transferred to a back thrust in the southern tyrrhenian characterized by frequent m5 6 earthquakes in the last 20 years a diffuse transform boundary formed across northeastern sicily to connect the sicilian and calabrian plate boundaries causing opening of the messina straits home to large historical earthquakes and unusual volcanism in the aeolian island arc and the intraplate mount etna a divergence of the ionian region from the main part of africa with northeastward motions that reflect the joint influence of african push and hellenic slab pull and are transmitted to calabria and adria could explain motions observed in the sicily channel calabria and apulia as well as the high level of southeastern sicilian seismic activity

lithospheric stress states predicted from long term tectonic models influence of rheology and possible application to taiwan abstract the long term stress state of the lithosphere results from a combination of boundary forces surface loads and tractions from mantle flow here we have employed a numerical technique which allows for the self consistent treatment of the interaction between the lithosphere and mantle and takes into account the deformation of the free surface we have used this to study lithospheric dynamics in order to gain insights on the present day conditions of stress and deformation of the lithosphere our numerical approach takes the observed topography and far field plate velocities as input and computes the resultant deformation as a function of the rheological stratification our model application is inspired by taiwan tectonics stress data and earthquake focal mechanisms indicate that the upper crust in taiwan may be predominantly under compression the recent pingtung earthquake in 12 26 2006 however suggests that the sub moho mantle lithosphere under southern taiwan may be under extension if this earthquake indeed represents the local state of stress this raises the question as to why extension and compression can occur simultaneously in this region for addressing this issue we have performed a number of numerical simulations in which we studied the effects of rheological stratification boundary conditions and geometry on the stress state and lithospheric dynamics we have employed both layered viscosity models and models in which we use laboratory based non newtonian and temperature dependent creep laws results show that the rheological stratification plays an important role in lithospheric deformation the relatively hot lithospheric thermal state in taiwan makes it likely that part of the lower crust is relatively weak which results in decoupling from surface near deformation with deformation at depth bending of a subducting slab underneath taiwan causes an extensional stress state in the sub moho mantle lithosphere whereas the collision of the philippine sea plate with the eurasian margin is responsible for a compressional state of stress in the crust it is demonstrated that models with a weak lower crust are also in reasonable agreement with available gps data as well as with constraints on uplift rates from long term geodynamic modelling studies

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

foredeeps versus subduction zones the geologic characteristics of foredeeps and accretionary wedges suggest that these features are distinguishable on the basis of the direction of the associated subduction east north east dipping subduction related accretionary wedges show high relief and broad outcrops of metamorphic rocks they are associated with shallow foredeeps with low subsidence rates in contrast west dipping subduction related accretionary wedges show low relief and involve mainly sedimentary cover the related foredeeps are deep and have high subsidence rates this differentiation is useful both for oceanic and continental subductions e g eastern vs western pacific subductions or east dipping alpine vs west dipping apenninec subductions in a cross section of the alps the ratio of the area of the orogen to the area of the foredeeps is at least 2 1 whereas this ratio is 0 22 1 for the apennines these ratios explain why foredeeps related to east or northeast dipping subduction are quickly filled and are bypassed by clastic rocks whereas foredeeps related to west dipping subduction maintain a deep water environment longer these differences support the presence of an eastward asthenospheric counterflow relative to the westward drift of the lithosphere detected in the hot spot reference frame even in the mediterranean where no hot spots are present in this interpretation the apennines foredeep was caused by the eastward push of the mantle acting on the subducted slab whereas the foredeeps in the alps were caused by the load of the thrust sheets and the downward component of movement of the upper adriatic plate these forces contrast with the upward component of the eastward mantle flow

numerical modeling of frontal and basal accretion at collisional margins 1 we investigate the deformation of orogenic wedges that form in the early stages of continent continent collisions using a two dimensional numerical model limited to the upper lithosphere our models show that deformation at the plate margins is influenced by rheology surface processes and the balance between inward mass flux and outward subduction flux as controlled by the subduction load which represents the effects of slab pull and resistive forces and flexural downbending we find three characteristic deformation modes 1 near pure subduction with little or no accretion 2 frontal accretion with development of an accretionary wedge built up by offscraping of the sediment layer at shallow depth and 3 independent frontal and basal accretion where a retrothrust allows stacking of basement nappes at crustal to mantle depths near pure subduction is enabled for ordinary rheology materials characterized by brittle and viscous material behavior approximating a christmas tree type depth profile and almost zero friction along the subduction shear zone frontal accretion occurs when slightly increased friction along the subduction shear zone allows offscraping of the sediment layer from the subducting plate independent frontal and basal accretion develops in strong rheology models with an almost fully brittle material behavior major surface erosion or a reduction of the subduction load promote the development of large basement nappes frontal accretion is favored by major sedimentation during convergence a large backstop and in the case of a lateral transition from a strong rheology to an ordinary rheology subducting plate our numerical models develop first order characteristics as observed in natural orogenic wedges for example upper crustal nappe stacks frontal and basal accretion or extension in the core of an orogen frontal and basal accretion are interdependent and tend to stabilize the subduction system

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

feasibility of reconstructing paleoatmospheric records of selected alkanes methyl halides and sulfur gases from greenland ice cores seven short lived atmospheric trace gases were measured in 25 ice core samples from summit greenland samples were selected from contemporaneous sections of fluid and dry drilled ice cores to examine what effects using n butyl acetate as the drill fluid would have on the measurements the gases include three light alkanes c2h6 c3h6 and n c4h10 two methyl halides ch3cl and ch3br and two sulfur compounds ocs and cs2 with gas ages from 125 to 325 years before present alkane levels are comparable to measurements in modern arctic air although c2h6 exhibits greater variability than expected compared with c3h6 and n c4h10 these results are not consistent with the idea that the alkanes are primarily of anthropogenic origin suggesting that the ice cores may not truly record a paleoatmospheric signal with respect to these gases the ch3br results are consistent with previous observations of excess ch3br in greenland firn air in situ production processes appear to overwhelm the paleoatmospheric signal of this gas ch3cl exhibits the same effect to a lesser extent ocs levels are similar to those in antarctic ice cores and appear to reflect paleoatmospheric levels cs2 results are similar to the limited database of modern atmospheric measurements only c3h8 and n c4h10 exhibit clear evidence of contamination because of the presence of the drill fluid the results indicate that it is possible to analyze many trace gases in fluid and dry drilled ice samples however it appears that in situ production may significantly alter the levels of some trace gases in greenland ice cores

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

the mechanical state of the lithosphere in the altiplano puna segment of the andes abstract information about topography the shape of the geoid seismicity neogene deformation and volcanism in the region of altiplano puna of western south america is used to analyse the state of stress across the convergent plate margin in terms of the effects of topography and simple models of its compensation an average elevation near 4 km is consistent with compensation by a yet unresolved combination of crustal root and hot uppermost mantle producing a geoid high of 22 27 meters average horizontal compressive stress in excess of a reference sea level lithostatic value of 390 bars in a 150 km thick lithosphere and an average shear stress of 170 bars along a 30 dipping interplate boundary the basis for these estimates is evidence for a neutral to extensional stress regime within the high plateau contrasted with a compressional regime on the eastern slopes and along the interplate boundary itself comparison with other plateaus in a convergent plate tectonic setting suggests an evolutionary sequence from compressional to extensional tectonics as elevation of the plateau increases

trench migration and subduction zone geometry seismic tomography results show a large variety in upper mantle structure along convergent plate boundaries we numerically investigate the effect of trench migration on the evolution of a slab with temperature dependent viscosity encountering a viscosity interface we find that subduction zone geometry is sensitive to even small rates 1 cm yr of retrograde motion increase in trench migration rate decreases the dip angle of the slab and its ability to penetrate the lower mantle upon including a background mantle flow it turns out that trench migration relative to the upper mantle flow is more decisive than the absolute plate velocities subduction zone geometry appears to be strongly time dependent we conclude that individual tectonic setting and time dependent slab behaviour can account for many different types of observable subduction zone anomalies our model temperature can adequately account for magnitudes and patterns of seismic anomalies as obtained from seismic tomography

overriding plate velocity control on surface topography in 2 d models of subduction zones we study the dynamically induced flexural topography in subduction numerical mechanical models we focus on the topographic changes at the overriding plate op surface induced by variations in op kinematics particularly when the subducting plate sp has a stationary motion after having reached the rigid base of the upper mantle our models consist of two viscoelastic plates with free surfaces and an isoviscous mantle friction is imposed along the planar subduction interface we first characterize the main topographic features at a constant op velocity using spatial definitions based on geometrical estimations of the volcanic arc position the models exhibit the formation of a bulge in the forearc area followed landwards by a depression and a smaller second bulge both bracketing the arc region the steady state distance to the trench of these features increases with op velocity their amplitude is affected by the far field op tectonic regime that depends on kinematics and plates and subduction interface strength we next test the effect of sudden changes in op velocity an op acceleration yields a transient topographic tilt during which the outer forearc quickly subsides whereas the arc region uplifts and that is followed by reverse slower motions an op slowdown induces opposite motions the rates of elevation change during the tilt are approximately proportional to velocity variations and mainly sensitive to the sp strength the rates are higher than 0 1 mm yr for velocity changes higher than 1 cm yr we suggest that topographic accommodations of op velocity changes should be considered when quantifying nonisostatic topography

seismicity and shape of the subducted nazca plate an updated compilation of earthquake locations and focal mechanism solutions from the international seismological centre and preliminary determination of earthquakes is the basis of a comprehensive study of the geometry of the wadati benioff zone beneath western south america the new data support previous mapping of a sharp flexure rather than a tear in the subducted nazca plate beneath southern peru and provide evidence for a similar flexure in the southward transition from nearly horizontal subduction to a slab with 30 dip at latitude 33 s in contrast the transition from 30 slab dip beneath bolivia to a nearly horizontal dip in the region between 28 s 32 s is more gradual occurring over several hundred kilometers of along strike distance between 20 s and 32 s this southward flattening corresponds to a broadening of a horizontal benchlike part of the subducted plate formed between 100 and 125 km depth the transition in continental tectonic style near 27 s 28 s from a wide volcanically active plateau to a narrow nonvolcanic cordillera appears not to be associated with the main slab flattening which begins to the north of these latitudes but with a more abrupt change in curvature of the subducted slab from convex upward to concave upward immediately below the plate boundary interface the concept of gaussian curvature is applied to slab bending to explain how subduction geometry is affected by the shape of the south american plate we hypothesize that the polarity of vertical curvature in the subducting slab is governed by the orientation of lateral curvature of the plate margin focal mechanism solutions for intermediate and deep earthquakes are grouped by geographic region and inverted for the orientation and relative magnitudes of the principal stresses results of the inversion indicate that downdip extension dominates in the slab above 350 km while downdip compression dominates at greater depths

dependency of slab geometry on absolute velocities and conditions for cyclicity insights from numerical modelling summary the aim of this study is to quantify the relationship between the kinematics of subduction deformation in the overriding plate and the evolution of slab geometry a 2 d finite element numerical code is used and a first objective consists in benchmarking previously published analogue models far field plate velocities are applied and once the subducting plate reaches the 660 km discontinuity modelled as a rigid base we obtain two different forms or styles of subductionthatdependontheoverridingplatevelocity vop ifvop 0 theslabliesforwardson the 660km discontinuity style 1 and if vop 0 the slab lies backwards on the discontinuity style 2 we also obtain a cyclic pattern with the slab folding on itself repeatedly when vsp 0 and 2vop vsp 0 where vsp is the subducting plate velocity these conditions result from the analysis of several simulations in which the subduction velocities and plate viscosities are varied when the slab periodically folds on the 660km discontinuity periods of shallow slab dip and compression in the overriding plate are followed by periods of slab steepening and relative extension in the overriding plate folding periodicity is controlled by the slab viscosity and subduction velocity when a low viscosity zone is included in the overriding plate the trench motion is effectively decoupled from the overriding plate velocity therefore allowing it to be directly controlled by the deep dynamics of the slab for the cyclic style 2 corresponding to forward folding of the slab the low viscosity region in the overriding plate increases the stress amplitudes oscillations the trench motion and the folding periodicity with time therefore the strength of the entire overriding plate is shown to directly control the dynamics of subduction using the nazca and south american plate velocities we produce models of cyclic folding with a period of ca 22ma and a minimal dip angle of ca 10 episodic folding of the slab on the 660km discontinuity would produce the necessary changes in slab dip and overriding plate deformation that explain episodes of volcanic quiescence alternating with greater rates of shortening along the andes

overriding plate deformation and variability of fore arc deformation during subduction insight from geodynamic models and application to the calabria subduction zone in nature subducting slabs and overriding plate segments bordering subduction zones are generally embedded within larger plates such large plates can impose far field boundary conditions that influence the style of subduction and overriding plate deformation here we present dynamic laboratory models of progressive subduction in three dimensional space in which the far field boundary conditions at the trailing edges of the subducting plate sp and overriding plate op are varied four configurations are presented free both plates free sp fixed op fixed and sp op fixed we investigate their impact on the kinematics and dynamics of subduction particularly focusing on overriding plate deformation the results indicate that the variation in far field boundary conditions has an influence on the slab geometry subduction partitioning and trench migration partitioning our models also indicate that in natural narrow subduction zones assuming a homogeneous overriding plate the formation of back arc basins e g tyrrhenian sea aegean sea and scotia sea is generally expected to occur at a comparable location 250 700 km from the trench irrespective of the boundary condition in addition our models indicate that the style of fore arc deformation shortening or extension is influenced by the mobility of the overriding plate through controlling the force normal to the subduction zone interface trench suction our geodynamic model that uses the sp op fixed setup is comparable to the calabria subduction zone with respect to subduction kinematics slab geometry trench curvature and accretionary configuration furthermore the model can explain back arc and fore arc extension at the calabria subduction zone since the latest middle miocene as a consequence of subduction of the narrow calabrian slab and the immobility of the subducting african plate and overriding eurasian plate this setting induced strong trench suction driving fore arc extension and forced subduction to be accommodated almost entirely by slab rollback not trenchward subducting plate motion while trench retreat was accommodated almost entirely by back arc and fore arc extension not trenchward overriding plate motion comparable to our sp op fixed model

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

current plate velocities relative to the hotspots incorporating the nuvel 1 global plate motion model nuvel 1 is a new global model of current relative plate velocities demets et al 1990 which differ significantly from those of prior models here we incorporate nuvel 1 into hs2 nuvel1 a new global model of plate velocities relative to the hotspots hs2 nuvel1 was determined from the hotspot data and errors used by minster and jordan 1978 to determine am1 2 which is their model of plate velocities relative to the hotspots am1 2 is consistent with minster and jordan s relative plate velocity model rm2 here we compare hs2 nuvel1 with am1 2 and examine how their differences relate to differences between nuvel 1 and rm2 hs2 nuvel1 plate velocities relative to the hotspots are mainly similar to those of am1 2 minor differences between the two models include the following 1 in hs2 nuvel1 the speed of the partly continental apparently non subducting indian plate is greater than that of the purely oceanic subducting nazca plate 2 in places the direction of motion of the african antarctic arabian australian caribbean cocos eurasian north american and south american plates differs between models by more than 10 3 in places the speed of the australian caribbean cocos indian and nazca plates differs between models by more than 8 mm yr although 27 of the 30 rm2 euler vectors differ with 95 confidence from those of nuvel 1 only the am1 2 arabia hotspot and india hotspot euler vectors differ with 95 confidence from those of hs2 nuvel1 thus substituting nuvel 1 for rm2 in the inversion for plate velocities relative to the hotspots changes few euler vectors significantly presumably because the uncertainty in the velocity of a plate relative to the hotspots is much greater than the uncertainty in its velocity relative to other plates

nature of the plate contact and subduction zones diversity abstract in recent studies we showed that the nature of the plate contact in subduction zones is an important physical feature in both oceanic lithospheric subduction and continental collision we investigated two fundamental states of the plate contact one based on a fault and the other based on a subduction channel using geodynamic modeling we determined the speci c signatures of both states of the subduction contact we established that the nature of the plate contact influences the dynamic response of the overriding and subducting plate and is one of the controlling factors whether back arc extension develops or not in the present study we combine results of our previous numerical experiments with a re analysis of published observations overall our synthesis connects seismic moment release with back arc deformation and tectonic processes at the margin it leads us to identify four classes of subduction zones the rst two classes result directly from our numerical experiments in class 1 subduction zones are characterized by a plate contact that is largely fault like with an accretionary margin in class 2 the plate contacts are largely channel type and have an erosive margin class 3 where the plate contact is entirely channel like consists of accretionary margins with a high sediment supply subduction zones of class 4 mostly characterized by an erosive convergent margin northern chile peru honshu and kuril are more complicated they can be explained by incorporating regional observations

dynamic models of subduction geophysical and geological evidence in the tyrrhenian sea summary predictions based on a 2 d finite element model for subduction underneath the calabrian arc in southern italy are compared with a variety of geophysical and geological data such as the present day stress pattern within the slab uplift from the elevation of marine terraces in calabria and subsidence in the tyrrhenian marsili basin from odp leg 107 we model the behaviour of the slab driven by slab pull in agreement with the present tectonic style in this part of the mediterranean as suggested by several investigators the model accounts for the crustal lithospheric and mantle structures in a vertical cross section perpendicular to the calabrian subduction zone the shape of the slab is constrained on the basis of new tomographic images in the southern tyrrhenian sea which were obtained from the regional seismic stations of the istituto nazionale di geofisica while the rheological properties of the mantle are taken from global dynamic models density contrasts between the subducted slab and the surrounding mantle based on petrological models drive the flow in our viscoelastic model stress values displacements and vertical velocities at the surface are sampled at different times after loading until dynamic equilibrium is reached our estimates are appropriate for a time window of 100 kyr the validity of our comparison with the geological record is based on the assumption that the tectonic configuration in the past was not substantially different from that of the present day two families of models with unlocked and locked subduction faults are considered the unlocked models allow for roll back of the trench of about 20 mm yr in agreement with some geological estimates the same family of models predicts uplift of the calabrian arc of about 1 mm yr l and subsidence in the marsili basin of 1 2 mm yr in agreement with geological surveys the deviatoric stress obtained from the unlocked model is consistent with the continuous distribution of deep seismicity in the southern tyrrhenian sea with minor concentration within the lithospheric wedge locked models fail to reproduce these geophysical and geological observations predictions derived from a detached slab model are not consistent with the continuous hypocentral distribution of deep seismicity in the southern tyrrhenian sea deformation at the surface and the stress patterns at depth for a detached slab differ substantially from those of a continuous plate dynamic topography and horizontal motions are reduced when compared with the continuous plate with deviatoric stresses concentrated within the relict slab our results indicate that subduction is a major tectonic process in the southern tyrrhenian sea

geodetic displacements and aftershocks following the 2001 mw 8 4 peru earthquake implications for the mechanics of the earthquake cycle along subduction zones we analyzed aftershocks and postseismic deformation recorded by the continuous gps station areq following the m w 8 4 23 june 2001 peru earthquake this station moved by 50 cm trenchward in a n235 e direction during the coseismic phase and continued to move in the same direction for an additional 15 cm over the next 2 years we compare observations with the prediction of a simple one dimensional 1 d system of springs sliders and dashpot loaded by a constant force meant to simulate stress transfer during the seismic cycle the model incorporates a seismogenic fault zone obeying rate weakening friction a zone of deep afterslip the brittle creep fault zone bcfz obeying rate strengthening friction and a zone of viscous flow at depth the ductile fault zone dfz this simple model captures the main features of the temporal evolution of seismicity and deformation our results imply that crustal strain associated with stress accumulation during the interseismic period is probably not stationary over most of the interseismic period the bcfz appears to control the early postseismic response afterslip and aftershocks although an immediate increase by a factor of about 1 77 of ductile shear rate is required placing constraints on the effective viscosity of the dfz following a large subduction earthquake displacement of inland sites is trenchward in the early phase of the seismic cycle and reverse to landward after a time t i for which an analytical expression is given this study adds support to the view that the decay rate of aftershocks may be controlled by reloading due to deep afterslip given the ratio of preseismic to postseismic viscous creep we deduce that frictional stresses along the subduction interface account for probably 70 of the force transmitted along the plate interface

two dimensional simulations of surface deformation caused by slab detachment detachment of the deeper part of subducted lithosphere causes changes in a subduction zone system which may be observed on the earth s surface constraints on the expected magnitudes of these surface effects can aid in the interpretation of geological observations near convergent plate margins where detachment is expected in this study we quantify surface deformation caused by detachment of subducted lithosphere we determine the range of displacement magnitudes which can be associated with slab detachment using numerical models the lithospheric plates in our models have an effective elastic thickness which provides an upper bound for rapid processes like slab detachment to the surface deformation of lithosphere with a more realistic rheology the surface topography which develops during subduction is compared with the topography shortly after detachment is imposed subduction with a non migrating trench system followed by detachment leads to a maximum surface uplift of 2 6 km while this may be higher for the case of roll back preceding detachment in the latter situation the back arc basin may experience a phase of compression after detachment within the context of our elastic model the surface uplift resulting from slab detachment is sensitive to the depth of detachment a change in friction on the subduction fault during detachment and viscous stresses generated by sinking of the detached part of the slab overall surface uplift of these magnitudes is not diagnostic of slab detachment since variations during ongoing subduction may result in similar vertical surface displacements d 2002 elsevier science b v all rights reserved

flat subduction dynamics and deformation of the south american plate insights from analog modeling received 14 june 2007 revised 13 january 2008 accepted 12 march 2008 published 21 june 2008 1 we present lithospheric scale analog models investigating how the absolute plates motion and subduction of buoyant oceanic plateaus can affect both the kinematics and the geometry of subduction possibly resulting in the appearance of flat slab segments and how it changes the overriding plate tectonic regime experiments suggest that flat subductions only occur if a large amount of a buoyant slab segment is forced into subduction by kinematic boundary conditions part of the buoyant plateau being incorporated in the steep part of the slab to balance the negative buoyancy of the dense oceanic slab slab flattening is a long term process 10 ma which requires the subduction of hundreds of kilometers of buoyant plateau the overriding plate shortening rate increases if the oceanic plateau is large enough to decrease the slab pull effect slab flattening increases the interplate friction force and results in migration of the shortening zone within the interior of the overriding plate the increase of the overriding plate topography close to the trench results from 1 the buoyancy of the plate subducting at trench and 2 the overriding plate shortening experiments are compared to the south american active margin where two major horizontal slab segments had formed since the pliocene along the south american subduction zone flat slab segments below peru and central chile nw argentina appeared at 7 ma following the beginning of buoyant slab segments subduction in northern ecuador and northern chile the process of slab flattening resulting from the carnegie and iquique ridges subductions respectively seems to be active but not completed the formation of flat slab segments below south america from the pliocene may explain the deceleration of the nazca plate trenchward velocity citation espurt n f funiciello j martinod b guillaume v regard c faccenna and s brusset 2008 flat subduction dynamics and deformation of the south american plate insights

carbon disulfide measurements in the atmosphere of the western north atlantic and the northwestern south atlantic oceans carbon disulfide cs2 measurements were made over the western and equatorial north atlantic ocean and the northwestern and equatorial south atlantic ocean carbon disulfide was in the range 0 4 50 pptrv in the atmosphere of the western north atlantic ocean emissions from anthropogenic sources and wet lands were found to be important although anthropogenic sources were 4 6 times larger than biogenic sources the flux of cs2 from eastern north america between 30 and 39 latitude was estimated to be 2 108g yr 1 of sulfur the anthropogenic contribution was 1 8 108g yr 1 of sulfur whereas the contribution of marshes was 0 2 108g yr 1 of sulfur sources of cs2 at high latitudes in the northern hemisphere were comparatively weak carbon disulfide levels in the western south atlantic ocean between 5 and 1 latitude were in the range 0 2 6 pptrv most of the cs2 appeared to come from biomass burning in africa carbon disulfide was much higher close to shore suggesting that the south american continent was a significant source although too few data were available to quantify it on ferry flights from wallops virginia to natal brazil cs2 levels at the ferry altitude of about 6 km averaged 1 2 pptrv this background cs2 was adequate to account for all the ocs in the atmosphere

high rates of arc consumption by subduction processes some consequences new estimates of long term consumption of japanese and peruvian continental fore arc margins together with recent drilling data ocean drilling program on some other erosional island arcs such as izu bonin mariana or tonga allow the calculation of a mean rate of 7 3 km m y landward migration of the arc trench system on both east and west sides of the pacific basin during cenozoic time such relative arcward retreat counterbalances seaward migration of the slab hinge line to accommodate opening of the atlantic ocean retreat can thus be interpreted as a delay in seaward or retrograde migration due to upper mantle resistance to contraction of the pacific basin mass transfers in subduction zones must be reconsidered in the light of these new estimates especially the balance between net growth and loss of continental crust in recent geologic time a 50 m y period is long enough to erode the initial width of the volcanic arc and fore arc massif of any subduction zone assuming a mean trench volcanic arc landward migration of 5 km m y and a mean trench volcanic arc distance of 250 km this circumstance thus raises the question of whether the izu bonin mariana and tonga arcs are older than their reported eocene age

narrow subducting slabs and the origin of backarc basins abstract both tectonic and seismic data suggest that horizontally wide in the direction parallel to the axis of the trench dipping subducting slabs typically move stably at fixed angles to the horizontal and that their associated trenches migrate very slowly or not at all in contrast where backarc basins babs exist the associated trenches typically migrate at rates of several cm year this behavior may be related to the finite width of the slabs under babs analysis shows that hydrodynamic suction created by the downdip motion of a wide slab is sufficient to balance the pull of gravity on the heavy slab however for a narrow slab sideways asthenospheric flow into the slab overriding plate corner effectively eliminates the suction which results in the rapid sinking of the slab hydrodynamic modeling is used to quantify this hypothesis modeling is used also to show that wide slabs are stable in their inclined position not because of the gravitational wing like sliding but due to the lifting force produced by the negative pressure suction in the slab overriding plate corner finally the model results confirm that for relatively narrow slabs this lifting force is small and cannot counterbalance gravity therefore when bending moments in a narrow slab exceed the plastic yield limit the slab becomes unstable this instability can cause the rapid migration of the slab s hinge and the associated arc at rates of 10 20 cm year and thus form the bab

effects of slab mineralogy on subduction rates although velocities of subducting slabs should be controlled primarily by their negative thermal buoyan cies their mineralogy can also have signicant eects we explore this by using thermo kinetic modeling to predict mineralogy and compare the resultant buoyant driving force to the opposing viscous drag phase transitions of mg fe 2sio4 in subducting slabs depend on thermal struc ture in two ways first equilibrium phase boundaries should be deflected causing local buoyancy anomalies whose sign depends upon that of the clapeyron slope as slabs rst enter the transition zone negative anomalies should acceler ate them but positive anomalies that arise when they fully penetrate the transition zone should slow them such ef fects may induce geologically abrupt changes in plate mo tions second olivine that persists metastably in slabs will form regions of positive buoyancy which should reduce slab velocities the coldest and fastest slabs should be slowed more greatly thus narrowing the range of feasible subduc tion rates decreased descent rates however allow slabs to warm and metastable wedges to thermally erode such neg ative feedback mechanisms may serve to regulate subduction rates

how weak is the subduction zone interface several lines of evidence suggest that subduction zones are weak and that the unique availability of water on earth is a critical factor in the weakening process we have evaluated the strength of subduction zone interfaces using two approaches i from empirical relationships between shear stress at the interface and subduction velocity deduced from laboratory experiments and ii from a parametric study of natural subduction zones that provides new insights on subduction zone interface strength our results suggest that subduction is only mechanically feasible when shear stresses along the plate interface are relatively low less than 35 mpa to account for this requirement we propose that there is a feedback mechanism between subduction velocity water released from the subducting plate and weakening of the fore arc mantle that may explain how relatively low shear stresses are maintained at subduction interfaces globally

lithospheric deformation under pre and post seismic stress fields along the nicobar sumatra subduction margin during 2004 sumatra mega event and its tectonic implications abstract a high resolution study was carried out under pre seismic i e static and post seismic stress fields i e dynamic in a space time frame along the nicobar sumatra margin the study reveals that the descending lithosphere records minimum stress obliquity predominant thrust movement and down dip least compressive stress axis under static stress field in northwest sumatra sector iii the imbalance between down dip component of slab pull force and viscous resistive force possibly caused cyclic stress loading in compressive field around the flexing zone 25 km and that undergone brittle failure through generation of mega thrust event on 26th december 2004 a sharp decrease in stress obliquity towards north sector ii redressing of least compressive stress axes from horizontal to down dip direction and increasing thrust movements under dynamic stress field account for continued upward shortening of the lithosphere the weak thinner zone i e between 159 and 217 km depth an age discontinuity portion possibly was collapsed through rapid enhancement of stress induced weakening and strain localization following the 2004 sumatra mega shock it is also well appreciated in the literature that such shallow disruption of the lithosphere is inevitable in the upper mantle if the slab is weakened or broken there and this phenomenon is not uncommon below the sumatra

the tectonic expression slab pull at continental convergent boundaries examination of five thrust belt systems developed at continental subduction boundaries suggests that they comprise two distinct groups that display pronounced and systematic differences in structural style topographic elevation denudation metamorphism postcollisional convergence and foredeep basin geometry and facies the distinctive geological features developed within each thrust belt group appear to be causally linked to the relative rates of subduction and convergence via the magnitude of horizontal compressional stress transmitted across the subduction boundary at subduction boundaries where the rate of overall plate convergence is less than the rate of subduction termed here retreating subduction boundaries the transmission of horizontal compressive stress across the plate boundary is small and regional deformation of the overriding plate is by horizontal extension the tectonic expression of these retreating subduction boundaries includes topographically low mountains little erosion or denudation low grade to no metamorphism little to no involvement of crystalline basement in shortening little to no postcollisional convergence anomalously deep foredeep basins and a protracted history of flysch deposition within the adjacent foredeep basin analysis of deflection and gravity data across three retreating subduction boundaries apennine carpathian and hellenic systems shows that subduction is driven by gravitational forces acting on dense subducted slabs at depths between about 40 and 80 km carpathians 50 and 150 km apennines and 50 and 250 km hellenides the total mass anomalies represented by the slabs are approximately 3 1012 6 1012 and 12 1012 n m respectively the slabs are partially supported by flexural stresses transmitted through the subducted lithosphere to the foreland and partially supported by dynamic viscous stresses in the asthenosphere at subduction boundaries where the rate of overall plate convergence is greater than the rate of subduction termed here advancing subduction boundaries the transmission of horizontal compressive stress across the plate boundary is large and regional deformation of the overriding plate is by horizontal shortening the tectonic expression of these advancing subduction boundaries includes topographically high mountains antithetic thrust belts large amounts of erosion and denudation exposure of high grade metamorphic rocks at the surface extensive deformation of crystalline basement to midcrustal depths protracted postcollisional convergence tens of millions of years and a protracted history of molasse deposition within the adjacent foredeep basins analysis of gravity and deflection data across two advancing subduction boundaries developed within the continental lithosphere western to eastern and southern alps and himalayas shows that the thrust sheets have been translated for great distances over the foreland lithosphere relative to the point at which the subduction forces are applied thus obscuring any flexural and gravity signals from the subducted slab however it appears that far field stresses presumably related to global plate motions drive most of the convergent motion across these subduction boundaries the concept that orogenic belts formed above retreating subduction boundaries have recognizable tectonic signatures that differ from those of orogenic belts formed above advancing subduction boundaries suggests that it may be possible to interpret the plate boundary settings in which ancient orogenic belts evolved appendix b is available with entire article on microfiche order from the american geophysical union 2000 floridaavenue n w washington d c 20009 document t92 004 2 50 payment must accompany order

influence of lithosphere asthenosphere interaction on the overriding lithosphere in a subduction zone numerical modeling 1 several factors influence the topography of the overriding plate in a subduction zone mantle wedge flow caused by motion of the descending slab lithosphere elasticity and interaction between the solid lithosphere and the viscous asthenosphere this paper presents the results of new subduction modeling that incorporates both elastic elastoplastic deformation in the overriding plate and viscous deformation in the asthenosphere the effects of fluid solid coupling and interaction were considered using an arbitrary lagrangian eulerian technique that solves for the continuous deformation of fluid flow in contact with the rigid plate modeling results show that thicker lithosphere which is harder to deform contributes to the development of broadly shallower back arc basins as lithospheric strength decreases its ability to withstand outside force fluid load resulting from flow in the mantle wedge also declines leading to the development of deep and narrow basins large scale deformation of the lithosphere modifies both the mantle wedge geometry and the wedge flow pattern which in turn increases the fluid load and thereby enhances topographic variation generally the effect of interaction is enhanced with lithospheric deformation the elastoplastic model also leads to a weaker lithosphere and a correspondingly larger deflection in the lithosphere similarly to previous studies back arc basins are deeper when mantle viscosity is higher for a 50 km thick elastic lithosphere the lithosphere asthenosphere interaction is negligible when the asthenospheric viscosity is less than 4 1019 pa s increasing the rate of subduction or decreasing the dip angle will also lead to the development of deeper basins

stresses and plate boundary forces associated with subduction plate margins primary tectonic stress in the lithosphere is predominantly caused by lateral density variations within the earth and associated topographical loading when such a stress system caused by major sublithospheric density anomalies is intersected by a weak zone which cuts across the lithosphere plate boundary forces develop and modify the plate interior stresses in this paper unite element analysis is used to model the stresses and plate boundary forces associated with subduction plate boundaries with dipping and vertical slabs extending to about 270 and 400 km depths having the subduction fault both locked and unlocked in such regions several types of horizontal deviatoric stress may occur including 1 local compression in the trench arc region caused by the dense sinking slab and the associated surface downflexure 2 plate interior tension which occurs when this compression is intersected by a weak subduction fault 3 local tension associated with thickening of the crust at the arc and elsewhere 4 local tension in the back arc region produced by the underlying low density upper mantle and 5 downbending stresses in the subducting slab thermal stresses and transmitted ridge push which are not included in the modelling here a gradient from compression in the forearc to tension in the back arc can be modelled in terms of these stresses when the fault is partially locked it is however the intersection of the local compression 1 by an unlocked or partially locked subduction fault that modifies the plate interior stresses and gives rise to the slab pull and trench suction plate boundary forces the state of stress in the interior of the overriding plate is also crucially influenced by back arc spreading where this occurs plate boundary forces have been evaluated for each of the models it is shown that slab pull and trench suction may be of comparable magnitude

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

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

the development of slabs in the upper mantle insights from numerical and laboratory experiments we have performed numerical and laboratory experiments to model subduction of oceanic lithosphere in the upper mantle from its beginnings as a gravitational instability to the fully developed slab a two dimensional finite element code is applied to model newtonian creep in the numerical experiments scaled analog media are used in the laboratory a sand mixture models the brittle crust silicone putty simulates creep in the lower crust and mantle lithosphere and glucose syrup is the asthenosphere analog both model approaches show similar results and reproduce first order observations of the subduction process in nature based on density and viscosity heterogeneities in a stokes flow model subduction nucleates slowly and a pronounced slab forms only when the viscosity contrast between oceanic plate and mantle is below a threshold we find that the subduction velocity and angle are time dependent and increase roughly exponentially over tens of millions of years before the slab reaches the 670 km discontinuity the style of subduction is controlled by the prescribed velocity of convergence the density contrast between the plates and the viscosity contrast between the oceanic plate and the mantle these factors can be combined in the buoyancy number f which expresses the ratio between driving slab pull and resisting viscous dissipation in the oceanic plate variations in f control the stress in the plates the speed and the dip of subduction and the rate of trench retreat reproducing the contrasting styles of subduction observed in nature the subduction rate is strongly influenced by the work of bending the lithosphere as it subducts

holocene emerged notches and tectonic uplift along the jalisco coast southwest mexico this paper presents the preliminary results from a study of holocene emerged shorelines marine notches and their tectonic implications along the jalisco coast the pacific coast of jalisco sw mexico is an active tectonic margin this coast has been the site of two of the largest earthquakes to occur in mexico this century the 1932 mw 8 2 jalisco earthquake and the 1995 mw 8 0 colima earthquake measurement and preliminary radiocarbon dating of emergent paleoshorelines along the jalisco coast provide the first constraints upon the timing for tectonic uplift along this coastline uplifted holocene marine notches and wave cut platforms occur at elevations ranging from ca 1 to 4 5 m amsl in situ intertidal organisms dated with radiocarbon the first ever reported for the jalisco area provide preliminary results that record tectonic uplift during at least the past 1300 years bp at an average rate of about 3 mm year we propose a model in which coseismic subsidence produced by offshore earthquakes is rapidly recovered during the postseismic and interseismic periods the long term period is characterized by slow tectonic uplift of the jalisco coast we found no evidence of coastal interseismic and long term subsidence along the jalisco coast

buckling of a pervasively faulted lithosphere we investigated the buckling response of a faulted elastic plate under horizontal compression using the finite element technique to better understand the effect of faults on the elastic behavior of a plate we studied the effect of changes in fault spacing depth and dip on the effective young s modulus buckling stress and wavelength our model consists of a thick elastic plate whose entire upper surface is cut by evenly spaced faults we impose either an initial sinusoidal deformation with a fixed wavelength or a random deformation to the grid a fault is represented as a free surface with no resolved shear stress and is allowed to slip in a specified direction using the method of slippery nodes with the assumption of free slip on the faults our model results represent an end member case in which the buckling wavelength and buckling stress are minimized by the presence of the faults

mechanisms for the origin of mid ocean ridge axial topography implications for the thermal and mechanical structure of accreting plate boundaries to better understand the implications of ridge axis morphology for the thermal and mechanical structure of accreting plate boundaries we examine mechanisms for the creation of stress supported axial topography and its dependence on parameters such as spreading rate asthenosphere viscosity and plate thickness near the ridge axis three basic mechanisms are considered 1 asthenospheric upwelling in a narrow steady state conduit beneath a ridge axis 2 mantle flow stresses due to plate spreading extending ridge axis lithosphere or melt segregation and associated mantle compaction and 3 topography supported by moments due to horizontal extensional stresses in a thickening lithosphere if mid ocean ridge topography is due to asthenosphere rising in a narrow conduit at the ridge axis then the conduit structure cannot be thermally controlled as the associated thermal structure has relatively broad flat isotherms instead of a conduitlike shape mantle flow associated with plate spreading alone induces no stress supported topography thus the intuitive notion that diverging flow at the ridge axis due solely to plate spreading produces the axial valley is incorrect vertical mantle flow at the base of a stretching accreting lithosphere and mantle flow associated with melt segregation can however lead to stress supported axial topography however for these mantle flow related processes to produce significant amounts of axial relief the mantle viscosity beneath the ridge axis must be on the order of 1020 pa s 1021 p poise significantly higher than generally accepted values of 1018 pa s 1019 p horizontal extensional stresses in a strong brittle lithosphere that thickens with distance from the ridge axis can produce axial topography of the form and amplitude observed on slow spreading ridges a continuum idealization of deformation due to faulting is used to examine this mechanism this model predicts the axial valley width is controlled by the plate thickness near the ridge axis an 8 km thick plate near the ridge axis would produce the 30 km wide axial valley observed at slow spreading ridges an increased lithosphere thickness of only several kilometers over the half width of the axial valley can produce kilometer scale relief to examine whether a plate 8 10 km thick at the ridge axis that thickens by several kilometers within 15 km of the axis is plausible at a slow spreading ridge thermal models that include both the heat of magmatic crustal accretion on the ridge axis and upwelling mantle flow beneath the axis are formulated hydrothermal cooling that increases the effective conductivity by about a factor of 10 is required for an 8 km thick plate to exist at a slow spreading ridge axis however even with this enhanced hydrothermal cooling the plate thickness at a fast spreading ridge axis and the resulting horizontal force in the plate are too small to result in appreciable topography while mid ocean ridge axial topography is strongly influenced by spreading rate other factors also shape axial structure for example the anomalously shallow slow spreading reykjanes ridge has an axial structure more like that of a fast spreading ridge and the anomalously deep intermediate spreading australian antarctic discordance has a median valley axial structure typical of a slower spreading ridge in addition within a given spreading segment there is often a systematic deepening of the ridge axis toward transform offsets these non spreading rate dependent axial topographic variations may be produced by variations in magmatic heat input associated with crustal emplacement which shapes the thermal structure and lithosphere thickness near the ridge axis

vertical motions of the puerto rico trench and puerto rico and their cause 1 the puerto rico trench exhibits great water depth an extremely low gravity anomaly and a tilted carbonate platform between reconstructed elevations of 1300 m and 4000 m i argue that these features are manifestations of large vertical movements of a segment of the puerto rico trench its forearc and the island of puerto rico that took place 3 3 m y ago over a time period as short as 14 40 kyr i explain these vertical movements by a sudden increase in the slab s descent angle that caused the trench to subside and the island to rise the increased dip could have been caused by shearing or even by a complete tear of the descending north american slab although the exact nature of this deformation is unknown the rapid 14 40 kyr and uniform tilt along a 250 km long section of the trench is compatible with scales of mantle flow and plate bending the proposed shear zone or tear is inferred from seismic morphological and gravity observations to start at the trench at 64 5 w and trend southwestwardly toward eastern puerto rico the tensile stresses necessary to deform or tear the slab could have been generated by increased curvature of the trench following a counterclockwise rotation of the upper plate and by the subduction of a large seamount

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

dynamics of retreating slabs 1 insights from two dimensional numerical experiments from the effects of mantle flux part 2 therefore in this paper we apply forces to the slab using simple analytical functions related to buoyancy and viscous forces in order to isolate the role of rheology on slab dynamics we analyze parameters for simplified elastic viscous and nonlinear viscoelastoplastic single layer models of slabs and compare them with a stratified thermomechanical viscoelastoplastic slab embedded in a thermal solution the near surface behavior of slabs is summarized by assessing the amplitude and wavelength of forebulge uplift for each rheology in the complete thermomechanical solutions vastly contrasting styles of slab dynamics and force balance are observed at top and bottom bends however we find that slab subduction can be modeled using simplified rheologies characterized by a narrow range of selected benchmark parameters the best fit linear viscosity ranges between 5 10 22 pa s and 5 10 23 pa s the closeness of the numerical solution to nature can be characterized by a deborah number 0 5 indicating that elasticity is an important ingredient in subduction index terms 8120 tectonophysics dynamics of lithosphere and mantle general 8159 tectonophysics rheology crust and lithosphere 8160 tectonophysics rheology general keywords subduction numerical models lithospheric rheology

impact of rock salt creep law choice on subsidence calculations for hydrocarbon reservoirs overlain by evaporite caprocks accurate forward modeling of surface subsidence above producing hydrocarbons reservoirs requires an understanding of the mechanisms determining how ground deformation and subsidence evolve here we focus entirely on rock salt which overlies a large number of reservoirs worldwide and specifically on the role of creep of rock salt caprocks in response to production induced differential stresses we start by discussing available rock salt creep flow laws we then present the subsidence evolution above an axisymmetric finite element representation of a generic reservoir that extends over a few kilometers and explore the effects of rock salt flow law choice on the subsidence response we find that if rock salt creep is linear as appropriate for steady state flow by pressure solution the subsidence response to any pressure reduction history contains two distinct components one that leads to the subsidence bowl becoming narrower and deeper and one that leads to subsidence rebound and becomes dominant at later stages this subsidence rebound becomes inhibited if rock salt deforms purely through steady state power law creep at low stresses we also show that an approximate representation of transient creep leads to relatively small differences in subsidence predictions most importantly the results confirm that rock salt flow must be modeled accurately if good subsidence predictions are required however in practice large uncertainties exist in the creep behavior of rock salt especially at low stresses these are a consequence of the spatial variability of rock salt physical properties which is practically impossible to constrain a conclusion therefore is that modelers can only resort to calculating bounds for the subsidence evolution above producing rock salt capped reservoirs

carbon disulfide in the north atlantic and pacific oceans carbon disulfide cs2 was determined in surface waters of the north atlantic and pacific oceans the mean concentrations and ranges of cs2 in open ocean waters were 13 4 7 8 26 1 pm s picomol sulfur per liter for the north atlantic and 14 6 7 2 27 5 pm s for the pacific the concentrations in the coastal waters of the north atlantic averaged 26 4 pm s and ranged from 17 9 to 40 4 pm s warm waters generally contained higher levels of cs2 than did cold waters all the study areas were found to be supersaturated in cs2 relative to the atmosphere based on calculations from published cs2 mixing ratios in the marine boundary layer and their henry s law constants sea to air fluxes of cs2 were estimated using exchange velocities for spot and climatological wind speeds the global oceanic flux extrapolated from this study is 0 18 tg cs2 yr 1 and in the range 0 13 0 24 tg cs2 yr 1 it is suggested that microbial processes photochemical reactions and phytoplankton activity are potential sources for oceanic cs2

episodicity in back arc tectonic regimes abstract the evolution of back arc basins is tied to the development of the dynamics of the subduction system they are a part of we present a study of back arc basins and model their development by implementing 3d time dependant computer models of subduction including an overriding plate we define three types of episodicity pseudo quasi and hyper episodicity and find evidence of these in nature observations of back arc basin ages histories of spreading quiescence and compression in the overriding plate give us an understanding of the time development of these subduction zones and back arc basins across the globe today a number of trenches are advancing the izu bonin trench the mariana trench the japan trench the java sunda trench and the central portion of the peru chile trench the andes subduction zone the izu bonin mariana and japan all have established back arc basins while the others have documented episodes of spreading quiescence compression or a combination of these the combination of advancing and retreating trench motion places these subduction zones in the category of hyper episodicity quasi episodicity in which the back arc shifts between phases of rifting spreading and quiescence is the dominant form of episodic back arc development in the present we find this type of episodicity in models for which the system is dynamically consistent that we have allowed the subducting plate s velocity to be determined by the sinking slabs buoyancy quasi and hyper episodicity are only found in subduction zones with relatively high subducting plate velocities between 6 and 9 cm year finally those subduction zones for which the subducting plate is moving slowly such as in the mediterranean or the scotia sea experience only pseudo episodicity where the spreading moves linearly towards the trench but often does so in discrete ridge jump events

dynamics of subduction initiation at preexisting fault zones we show that a new subduction zone can initiate at a preexisting dipping fault zone with reasonable plate forces consistent with what is observed in the western pacific the dynamics of subduction initiation within a viscoelastic medium has been systematically explored with the finite element method we investigate the compression of oceanic lithosphere with both force and velocity boundary conditions and track the thermal structure in which heat is transported by both advection and diffusion the viscosity of the medium is non newtonian and temperature dependent we also examine the influence of a perfectly plastic yield stress a new method to model the preexisting weak zone as a graded fault with remeshing is introduced the first 400 km of plate convergence is examined during which time there are profound changes in plate boundary dynamics initially topography across the plate boundary is characterized by over 1000 m of uplift close to the trench subsequently a bathymetric depression develops on the overriding plate with an initial subsidence rate of 125 m myr for a slab subducting at 2 cm yr tectonic subsidence of the overriding plate is primarily dependent upon the depth of slab penetration for a plate force of 4 10 12 n m subduction initiates even with a fault shear stress of 3 mpa the resistance most consistent with seismological observations however subduction will not initiate when this fault shear stress is 5 mpa our results are consistent with plate reconstructions which predict the initiation of subduction across preexisting weak zones within oceanic basins

crustal structure of the kermadec arc from mango seismic refraction profiles three active source seismic refraction profiles are integrated with morphological and potential field data to place the first regional constraints on the structure of the kermadec subduction zone these observations are used to test contrasting tectonic models for an along strike transition in margin structure previously known as the 32 s boundary we use residual bathymetry to constrain the geometry of this boundary and propose the name central kermadec discontinuity ckd north of the ckd the buried tonga ridge occupies the forearc with vp 6 5 7 3 km s 1 and residual free air gravity anomalies constrain its latitudinal extent north of 30 5 s width 110 20 km and strike 005 south of 25 s south of the ckd the forearc is structurally homogeneous down dip with vp 5 7 7 3 km s 1 in the havre trough backarc crustal thickness south of the ckd is 8 9 km which is up to 4 km thinner than the northern havre trough and at least 1 km thinner than the southern havre trough we suggest that the eocene arc did not extend along the current length of the tonga kermadec trench the eocene arc was originally connected to the three kings ridge and the ckd was likely formed during separation and easterly translation of an eocene arc substrate during the early oligocene we suggest that the first order crustal thickness variations along the kermadec arc were inherited from before the neogene and reflect mesozoic crustal structure the cenozoic evolution of the tonga kermadec hikurangi margin and along strike variations in the duration of arc volcanism

free surface formulation of mantle convection ii implication for subduction zone observables summary viscous and viscoelastic models for a subduction zone with a faulted lithosphere and internal buoyancy can self consistently and simultaneously predict long wavelength geoid highs over slabs short wavelength gravity lows over trenches trench forebulge morphology and explain the high apparent strength of oceanic lithosphere in trench environments the models use two different free surface formulations of buoyancydriven flows see for example part i lagrangian viscoelastic and pseudo free surface viscous formulations the lower mantle must be stronger than the upper in order to obtain geoid highs at long wavelengths trenches are a simple consequence of the negative buoyancy of slabs and a large thrust fault decoupling the overriding from underthrusting plates the lower oceanic lithosphere must have a viscosity of less than pa s in order to be consistent with the flexural wavelength of forebulges forebulges are dynamically maintained by viscous flow in the lower lithosphere and mantle and give rise to apparently stiffer oceanic lithosphere at trenches with purely viscous models using a pseudo free surface formulation we find that viscous relaxation of oceanic lithosphere in the presence of rapid trench rollback leads to wider and shallower back arc basins when compared to cases without viscous relaxation moreover in agreement with earlier studies the stresses necessary to generate forebulges are small 100 bars compared to the unrealistically high stresses needed in classic thin elastic plate models

quantifying the energy dissipation of overriding plate deformation in three dimensional subduction models in a subduction system the force and the energy required to deform the overriding plate are generally thought to come from the negative buoyancy of the subducted slab and its potential energy respectively such deformation might involve extension and back arc basin formation or shortening and mountain building how much of the slab s potential energy is consumed during overriding plate deformation remains unknown in this work we present dynamic three dimensional laboratory experiments of progressive subduction with an overriding plate to quantify the force fopd that drives overriding plate deformation and the associated energy dissipation rate opd and we compare them with the negative buoyancy fbu of the subducted slab and its total potential energy release rate bu respectively we varied the viscosity ratio between the plates and the sublithospheric upper mantle with sp um 157 560 and the thickness of the overriding plate with top 0 5 2 5 cm scaling to 25 125 km in nature the results show that fopd fbu has average values of 0 5 2 0 with a maximum of 5 3 and opd bu has average values of 0 05 0 30 with a maximum of 0 41 the results indicate that only a small portion of the negative buoyancy of the slab and its potential energy are used to deform the overriding plate our models also suggest that the force required to deform the overriding plate is of comparable magnitude as the ridge push force furthermore we show that in subduction models with an overriding plate bending dissipation at the subduction zone hinge remains low 3 15 during steady state subduction

rates of neogene and quaternary tectonic movements in the southern banda arc based on micropalaeontology it is generally considered that the outer banda arc islands are composed in part of strongly deformed sediments that accumulated at the north australian continental margin the onset and completion of folding nappe emplacement imbrication and uplift that produced the islands of the outer banda arc have been dated by means of planktonic foraminifera the dating of the onset of uplift of timor from a deep submarine position at the end of the neogene nappe emplacement to mountains now reaching nearly 3 km high indicates the rate of post collision uplift to have been initially 3 mm yr then it slowed to about 1 5mm yr where the australian continental margin meets the eastern end of the present java trench there are indications that the australian margin has overridden the trench in the timor region by 240 km micropalaeontological dating of the neogene collision between the australian continental slope proximal rise and the trench provides constraints for checking the overriding hypothesis by calculating rates of convergence derived from plate motions 7 5cm yr against the rates based on the tectonic model 7 4cm yr after nappe emplacement possibly faster than 6 25 cm yr shortening of the continental crust appears to have migrated towards the australian continent so that the shelf itself has become involved in the imbrication with shortening of the cover rocks between the nappes and the present shelf edge amounting to about 40 km during the last 2 ma

large trench parallel gravity variations predict seismogenic behavior in subduction zones we demonstrate that great earthquakes occur predominantly in regions with a strongly negative trench parallel gravity anomaly tpga whereas regions with strongly positive tpga are relatively aseismic these observations suggest that over time scales up to at least 1 million years spatial variations of seismogenic behavior within a given subduction zone are stationary and linked to the geological structure of the fore arc the correlations we observe are consistent with a model in which spatial variations in frictional properties on the plate interface control trench parellel variations in fore arc topography gravity and seismogenic behavior

mechanics of graben formation in crustal rocks a finite element analysis small 1 10 km wide grabens are ubiquitous tectonic structures that have been observed on the surfaces of all the terrestrial planets and satellites in spite of their frequent occurrence there is no fundamental understanding of how they form other than that they are characteristic of extensional terranes this paper examines the mechanics of the initial stages of graben formation and demonstrates that the configuration of a pair of antithetically dipping normal faults a graben is the energetically most favorable fault configuration in elastic brittle rocks subjected to pure extension a corollary to this hypothesis is that the occurrence of synthetic normal faults is indicative of a stress field other than purely extensional or of nonhomogeneous characteristics of the rock the analysis proceeds by computing the stress field in the vicinity of a single initial normal fault using a two dimensional finite element model it is shown that slip on this fault substantially lowers the proximity to failure of the surrounding rocks except for two small regions one near the surface of the downthrown block and the other near the fault tip the region on the downthrown block is located a horizontal distance from the initial fault slightly less than the fault s depth the proximity to failure actually increases here and a secondary fault develops an antithetic secondary fault would link this region with the other failure zone near the fault tip to demonstrate the preference for antithetic secondary faults more rigorously we show that this fault configuration results in a smaller total strain energy than the synthetic configuration in addition a two layer model is used to show that the graben s width is only weakly dependent upon the presence of mechanically distinct layers in the crust both weak over strong and strong over weak configurations are investigated the depth of the initial fault and the thickness of the top layer are varied and the width of the resulting graben is evaluated by computing the distance between the initial fault and the region of highest proximity to failure near the surface this analysis indicates that the depth of the initial fault is the major factor controlling graben width

dynamics of continental collision influence of the plate contact summary observations shows that continental collision may evolve in different ways resulting in a wide range of tectonic responses in search of the controlling conditions and parameters we start from the results of our previous work which demonstrated that the properties of the plate contact are important for the overall dynamics of convergent plate margins two fundamental types of subduction plate contact can be distinguished one based on a fault and the other based on a weak subduction channel in this study we investigate how the plate contact affects the initial stage of continental collision we use a finite element method to solve the heat and the time dependent momentum equations for elastic power law viscous and plastic rheologies for the same rheological properties and driving forces varying the nature of the plate contact leads to three types of responses the presence of a subduction channel promotes coherent and when the boundary conditions allow it plate like subduction of the continental margin in models with a subduction fault coherent subduction of the incoming continental lithosphere occurs when the colliding passive margin has a gentle slope the approaching continental sliver starts to subduct and the subduction is characterized by a non plate like behaviour slower subduction velocity than in channel models and strong slab deformation if the continental margin is steep and the strength of the incoming continental crust is high fault models result in locking of the trench eventually leading to slab break off if the crustal strength is relatively low shear delamination of part of the crust is expected in the channel model this type of delamination never occurs the tectonic settings used in our experiments prescribed plate velocity of the subducting plate versus fixed subducting plate corresponding to a landlocked basin setting do not significantly influence the nature of the model response we conclude that initial stages of continental collision are strongly affected by whether the subduction contact is a fault or a channel neither the slab pull magnitude nor the tectonic setting is very important to the overall geodynamics at this stage the plate contact type along with the slope of the incoming passive margin and the rheology of the continent controls whether the incoming crust 1 subducts entirely 2 separates partially or entirely from the lithospheric mantle or 3 blocks the trench likely leading to slab break off

global trench migration velocities and slab migration induced upper mantle volume fluxes constraints to find an earth reference frame based on minimizing viscous dissipation sincetheadventofplatetectonicsdifferentglobalreferenceframeshavebeenusedtodescribethemotionofplatesandtrenches thedifferencein plate motion and trench migration between different reference frames can be substantial up to 4 cm yr this study presents an overview of trench migration velocities for all the mature and incipient subduction zones on earth as calculated in eight different global reference frames calculations show that irrespective of the reference frame 1 trench retreat always dominates over trench advance with 62 78 of the 244 trench segments retreating 2 the mean and median trench velocity are always positive retreating and within the range 1 3 1 5 cm yr and 0 9 1 3 cm yr respectively 3 rapid trench retreat is only observed close to lateral slab edges b1500 km and 4 trench retreat is always slow far from slab edges n2000km thesecalculationsarepredictedbygeodynamicmodelswithavaryingslabwidth inwhichplatemotion trenchmotionandmantleflow result from subduction ofdense slabs suggesting that trenchmotion is indeedprimarily drivenby slab buoyancyforces andthat proximity to alateral slab edge exerts a dominant control on the trench migration velocity despite these four general conclusions significant differences in velocities between such reference frames remain it is therefore important to determine which reference frame most likely describes the true absolute velocities togetanunderstandingoftheforcesdrivingplatetectonicsandmantleconvection itishereproposedthat basedonfluiddynamicconsiderationsand predictions from geodynamic modelling the best candidate is the one which optimises the number of trench segments that retreat minimizes the trench perpendicular trench migration velocity vt in the centre of wide n4000 km subduction zones maximizes the number of retreating trench segments located within 2000 km of the closest lateral slab edge minimizes the average of the absolute of the trench perpendicular trench migration velocity vt forallsubductionzonesonearth andminimizestheglobaluppermantletoroidalvolumeflux to thatresultsfromtrenchmigration and associated lateral slab migration i e slab rollback or slab roll forward calculations show that these conditions are best met in one particular indo atlantic hotspot reference frame where 75 of the subduction zones retreat vt in the centre of wide subduction zones ranges between 3 5 and 1 8 cm yr 83 of the trench segments located within 2000 km of the closest lateral slab edge retreat the average of vt is 2 1 cm yr and to 456 km 3 yr lower limit and 539 km 3 yr upper limit inclusion of all the incipient subduction zones on earth results in slightly greater fluxes of 465 km 3 yr lower limit and 569 km 3 yr upper limit it is also found that this reference frame is close to minimizing the total sub lithospheric uppermantlevolumeflux k associatedwithmotionofcontinentalkeelslocatedbelowthemajorcratons itisstressed however that kisanorder of magnitude smaller than to and thus of subordinate importance in conclusion the indo atlantic hotspot reference frame appears preferable for calculating plate velocities and plate boundary velocities

subduction of the lithosphere and back arc dynamics insights from physical modeling physical modeling of the subduction of an elastico plastic lithosphere underlained by a low viscosity asthenosphere and driven by both horizontal compressional force and gravitational sinking of the subducting plate has shown the subduction regime to be strongly affected by 1 a where a and 1 are the asthenosphere and lithosphere densities at 0 a steady state process develops which requires a horizontal lithosphere compression of h 0 3 0 4 s 108 pa where s is the lithosphere yield limit the central part of the leading wedge of the overriding plate is nonisostatically elevated in this case especially in the region of the frontal arc which corresponds to a free air gravity anomaly maximum of 200 400 mgal a decrease in 0 compression of the overriding plate is considerably less from the onset of subduction and then decreases during subduction at some point compression changes into tension tensional nonhydrostatic stresses grow during the process to a value near h 0 1 0 2 s 0 4 108 pa both subsidence of the overriding wedge below the isostatic equilibrium level and an increase in the benioif dip angle accompany this process the gravity anomaly maximum is displaced into the volcanic arc in this subduction regime where it is normally less than 150 mgal if some weaknesses are present in the overriding plate tensional stresses result in a failure of the lithosphere at these locations and the broken lithosphere segment the arc plate starts to move counter to the subduction plate the arc plate slides along the underthrusting plate s surface without loss of contact and causes back arc opening hydrostatic suction prevents the plates from being separated this effect is caused by the peculiarity of a mechanical system whereby a low viscosity asthenosphere and a plastic lithosphere which effective yield limit is considerably lower than the hydrostatic stresses in the plate if the overriding lithosphere is not weakened enough to fail under the existing tension then the hanging slab breaks down under its own weight when it reaches a specific length both interacting plates undergo isostatic recovery in this case and the whole cycle is repeated again the hydrostatic suction effect also provides back arc tension under other conditions such as when asthenospheric flow forces the subducted plate to move away from the island arc

carbonyl sulfide and carbon disulfide large scale distributions over the western pacific and emissions from asia during trace p 1 an extensive set of carbonyl sulfide ocs and carbon disulfide cs2 observations were made as part of the nasa transport and chemical evolution over the pacific trace p project which took place in the early spring 2001 trace p sampling focused on the western pacific region but in total included the geographic region 110 e to 290 e longitude 5 n to 50 n latitude and 0 12 km altitude substantial ocs and cs2 enhancements were observed for a great many air masses of chinese and japanese origin during trace p over the western pacific mean mixing ratios of long lived ocs and shorter lived cs2 showed a gradual decrease by about 10 and a factor of 5 10 respectively from the surface to 8 10 km altitude presumably because land based sources dominated their distribution during february through april 2001 the highest mean ocs and cs2 levels 580 and 20 pptv respectively based on 2 5 2 5 latitude bins were observed below 2 km near the coast of asia at latitudes between 25 n and 35 n where urban asian outflow was strongest ratios of ocs versus co for continental se asia were much lower compared to chinese and japanese signatures and were strongly associated with biomass burning biofuel emissions we present a new inventory of anthropogenic asian emissions including biomass burning for ocs and cs2 and compare it to emission estimates based on regional relationships of ocs and cs2 to co and co2 the ocs and cs2 results for the two methods compare well for continental se asia and japan plus korea and also for chinese cs2 emissions however it appears that the inventory underestimates chinese emissions of ocs by about 30 100 this difference may be related to the fact that we did not include natural sources such as wetland emissions in our inventory although the contributions from such sources are believed to be at a seasonal low during the study period uncertainties in ocs emissions from chinese coal burning which are poorly characterized likely contribute to the discrepancy

controls on trench topography from dynamic models of subducted slabs a finite element method with constrained elements and lagrange multipliers is used to study tectonic faults in a viscous medium a fault representing the interface between overriding and subducting plates has been incorporated into a viscous flow model of a subduction zone in which both thermal buoyancy and the buoyancy associated with the phase change from olivine to spinel are included the fault causes stress to concentrate in its vicinity giving rise to a weak plate margin and a mobile plate if a power law rheology is used surface dynamic topography with either a newtonian or a power law rheology and with typical subduction zone parameters is characterized by a narrow and deep trench and a broadly depressed back arc basin this suggests that oceanic trenches and back arc basins over subduction zones are dynamically compensated by viscous flow our models show that trench depth increases with fault dip angle slab dip angle slab length and age of oceanic lithosphere just prior to subduction the influence of fault dip angle and age of lithosphere on trench depth is greater than the influence of slab dip angle and slab length these relationships of trench depth versus subduction zone parameters explain well the statistics of observed trench depths for those subduction zones with oceanic lithosphere on both sides of the trench observed trench depths have been corrected for fault and slab dip angles based on the relationships from the dynamic models after correction to a common set of parameters trench depth correlates linearly with age of lithosphere prior to subduction with a slope which is close to what models having high viscosities within the transition zone and lower mantle predict comparison between the trench depths corrected for fault and slab dip angles and model trench depths suggests that the resisting tangential stress on faults in subduction zones may range from 15 mpa to 30 mpa depending on model details

the influence of trench migration on slab penetration into the lower mantle abstract a two dimensional numerical convection model in cartesian geometry is used to study the influence of trench migration on the ability of subducted slabs to penetrate an endothermic phase boundary at 660 km depth the transient subduction history of an oceanic plate is modelled by imposing plate and trench motion at the surface the viscosity depends on temperature and depth a variety of styles of slab behaviour is found depending predominantly on the trench velocity when trench retreat is faster than 2 4 cm a the descending slab flattens above the phase boundary at slower rates it penetrates straight into the lower mantle although flattening in the transition zone may occur later leading to a complex slab morphology the slab can buckle independent of whether it penetrates or not especially when there is a localised increase in viscosity at the phase boundary flattened slabs are only temporarily arrested in the transition zone and sink ultimately into the lower mantle the results offer a framework for understanding the variety in slab geometry revealed by seismic tomography

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

stress change and effective friction coefficient along the sumatra andaman sagaing fault system after the 26 december 2004 m w 9 2 and the 28 march 2005 m w 8 7 earthquakes the 2004 aceh and 2005 nias events are the two greatest earthquakes of the past 40 years with a total rupture of 1700 km long and a coseismic slip reaching up to 25 m these two earthquakes have caused large stress perturbations which significantly altered seismic activity in the sumatra andaman region using both detailed mapping of failure planes and various slip distributions we calculate this stress change along the sumatra andaman sagaing fault system from central sumatra to southern myanmar the static coulomb stress change delta cff and the observed seismic activity are in very good agreement with a coulomb index similar to 20 greater than the one obtained for random events compared to previous studies this high coulomb index confirms two important issues on the use of static stress change criterion unsuited to study near field aftershocks and only relevant for aftershocks analysis on large and mature faults at a time scale of several months the calculated delta cff distribution suggests that the 2004 and 2005 earthquakes inhibit failure on the north andaman rift and on the sagaing fault while failure is encouraged along the transform andaman zone the central andaman rift the west andaman fault the sumatra fault system and the offshore thrust faults west of sumatra island the maximum value of similar to 15 20 bar 1 5 2 mpa for delta cff is reached in the northern part of the sumatra fault system this high value together with the lack of major earthquake in the last 170 years result in a high seismic hazard for this region our results are also consistent with temporal evolution of both earthquakes location and focal mechanism prior to and after the events in particular we explain the occurrence and the mechanism of seismic swarms observed in the central andaman rift and along the west andaman fault finally our calculations reveal that the seismicity in the andaman rift zone can only be explained if mu 0 5 this result leads to two end member models one with a constant and high fault friction and one with spatial variations for which friction may depend on either the nature of the lithosphere oceanic versus continental or the fault type

kinematics of subduction and subduction induced flow in the upper mantle 1 results of fluid dynamical experiments are presented to model the kinematics of lithospheric subduction in the upper mantle the experiments model a dense high viscosity plate subducting lithosphere overlying a less dense low viscosity layer upper mantle the overriding lithosphere is not incorporated several important features of slab behavior were investigated including the temporal variability of hinge line migration the kinematic behavior of the slab and the subduction induced upper mantle flow both fixed and free trailing edge boundary conditions of the subducting plate were investigated results show that hinge line retreat is a natural consequence of subduction of a negatively buoyant slab the migration rate increases until the slab approaches the upper lower mantle discontinuity resulting in a decrease in migration rate followed by a renewed increase and finally approaching a steady state slab retreat results in mantle flow with material initially located underneath the slab flowing around the lateral slab edges toward the mantle wedge experimental results indicate that all rollback induced flow occurs around the lateral slab edges forcing the hinge line to attain a convex shape toward the direction of retreat no signs for poloidal flow underneath the slab tip have been detected only a small component of toroidal type flow was observed underneath slanting slab tips for a fixed trailing edge the slab does not sink vertically downward but sinks at an angle in a regressive manner for a free trailing edge slab sinking is oriented more vertically while the surface part of the subducting plate is pulled into the subduction zone

unraveling topography around subduction zones from laboratory models the relief around subduction zones results from the interplay of dynamic processes that may locally exceed the iso static contributions the viscous dissipation of the energy in and around subduction zones is capable of generating kilometer scale vertical ground movements in order to evaluate dynamic topography in a selfconsistent subduction system we carried out a set of laboratory experiments wherein the lithosphere and mantle are simulated by means of newtonian viscous materials namely silicone putty and glucose syrup models are kept in their most simple form and are made of negative buoyancy plates of variable width and thickness freely plunging into the syrup the surface of the model and the top of the slab are scanned in three dimensions a forebulge systematically emerges from the bending of the viscous plate adjacent to the trench with a large wavelength dynamic pressure offsets the foreside and backside of the slab by 500 m on average the suction that accompanies the vertical descent of the slab depresses the surface on both sides at a distance equal to the half width of the slab the topographic depression amounts to 500 m on average and becomes negligible at a distance that equals the width of the slab in order to explore the impact of slab rollback on the topography the trailing edge of the plates is alternatively fixed to fixed mode and freed from free mode the end wall of the tank both the pressure and suction components of the topography are 30 lower in the free mode indicating that slab rollback fosters the dynamic subsidence of upper plates our models are compatible with first order observations of the topography around the east scotia tonga kermadec and banda subduction zones which exhibit anomalous depths of nearly 1 km as compared to adjacent sea floor of comparable age

detachment of part of the downgoing slab and uplift of the new hebrides vanuatu islands several seisniological observations suggest that there is a gap within the downgoing slab of australian lithosphere plunging beneath the new hebrides and ages of elevated coral tenaces on the new hebrides islands suggest that the islands arc rising rapidly we suggest that the creation of the gap in the slab within the last 1 m y occurred by part of the slab detaching from the rest and sinking rapidly into the underlying asthenosphere this lowered the downward force on the slab at shallower depths and therefore on the overriding plate and the continued decrease of this force as the deeper slab sinks freely has allowed the islands above it to rise although this suggestion cannot account for all aspects of the uplift of the islands it provides a simple mechanism for explaining the fairly young uplift and its distribution along the new hebrides arc

interplay between subduction and continental convergence a three dimensional dynamic model for the central mediterranean summary the mediterranean region is attracting considerable attention due to the complexities of its tectonic setting which is considered worldwide a unique natural laboratory for studying the occurrence of extensional tectonics in a general context of continental convergence the tyrrhenian apennine system is controlled by the west dipping subduction of the adria ionian lithosphere and by the near north south convergence between the african and eurasian plates we provide the first 3 d dynamic model of the central mediterranean that quantifies the effects of subduction and convergence on surface deformation in simplified geometry the axis of the model extends from sicily to the alps along the subduction hinge line a convergence rate of 1 cm yr 1parallel to the subduction hinge has been applied to the tyrrhenian block in agreement with global plate motion models density contrasts within the slab cause the gravitational sinking and roll back of the slab in the southern tyrrhenian domain modelling results show a gradual decrease of hinge retreat from south to north with values ranging between 8 and 2 mm yr 1 indicating that the arculate geometry of the hinge line along the italian peninsula is ultimately controlled by the interplay between subduction and convergence the pattern of vertical velocity along directions perpendicular to the hinge with subsidence in the foredeeps and uplift at the eastern border of the tyrrhenian domain is maintained along the whole italian peninsula with higher values in the southern areas

some remarks on the origin of foredeeps abstract geological observations show that there are two main types of foredeep basins on the base of their subsidence rate and their relationship with the associated orogen or accretionary wedge high rates of subsidence up to 1600 m myr and a ratio lower than 1 between the area of the elevated belt and the area of the basin characterize the foredeeps associated with w dipping subduction low rates of subsidence up to 300 m myr and a ratio higher than 1 between the area of the orogen and the area of the basin characterize the foredeeps associated with e or ne dipping subductions this observation enables us to interpret the slow filling of foredeeps of the first type and the much faster filling of foredeeps of the second type moreover w dipping subductions have a steeper and faster monocline bounding the base of the deep foredeep in contrast to e or ne dipping subductions which have a shallower monocline this differentiation supports the notion of the eastward mantle flow with respect to the lithosphere detected in the hot spot reference frame in this view the foredeep depth in w dipping subductions is mainly controlled by the roll back of the subduction hinge pushed by the relative eastward mantle flow while foredeep depth in e or ne dipping subductions is instead mainly generated by the load of the thrust sheets and by the roll back of the subduction hinge due to the advancing upper plate contrasting the upward push of the mantle the shape of the foredeeps is regularly arcuate in case of w dipping subduction while is linear or following the shape of the inherited continental margin in case of e or ne dipping subduction fold development in the two opposite foredeeps is significantly different in w dipping subduction the folds are transported down in subduction while they are forming and consequently they are poorly eroded syntectonic sediments drape a preserved fold in the e ne dipping subduction folds and thrust sheets are instead uplifted and deeply eroded this is also predicted by the envelope of the crests in the two distinct accretionary wedges in the w dipping case the envelope may dip toward the subduction while it is rising toward the hinterland in the other cases this approach may explain the differences in terms of geometries amount and rates of subsidence between the foredeeps around the adriatic plate for instance the foredeep of the southern alps back thrust belt foredeep is deformed in the western part by the later apenninic foredeep development associated with the sinistral oblique ramp of the w dipping adriatic plate subduction the relationship in the frontal parts of a thrust belt between accretion and eustacy has to be analyzed in terms of when where and how fast thrusts propagation took place with respect to the randomly distributed sea level third order fluctuations and by the type of subduction which is connected to the foredeep and thrust belt or accretionary wedge therefore the stratigraphic patterns in foredeep basins are peculiar and casual successions that record the combination of regional tectonic evolution with independent wider scale sea level changes

topography of the overriding plate during progressive subduction a dynamic model to explain forearc subsidence overriding plate topography provides constraints on subduction zone geodynamics we investigate its evolution using fully dynamic laboratory models of subduction with techniques of stereoscopic photogrammetry and particle image velocimetry model results show that the topography is characterized by an area of forearc dynamic subsidence with a magnitude scaling to 1 44 3 97 km in nature and a local topographic high between the forearc subsided region and the trench these topographic features rapidly develop during the slab free sinking phase and gradually decrease during the steady state slab rollback phase we propose that they result from the variation of the vertical component of the trench suction force along the subduction zone interface which gradually increases with depth and results from the gradual slab steepening during the initial transient slab sinking phase the downward mantle flow in the nose of the mantle wedge plays a minor role in driving forearc subsidence

pliocene pleistocene sedimentary tectonic development of the mesaoria mesarya basin in an incipient diachronous collisional setting facies evidence from the north of cyprus

a modelling study of vertical surface displacements at convergent plate margins summary during the evolution of a subduction zone system variations are likely to occur in for example surface plate velocities and buoyancy of the subducting lithosphere we quantify vertical surface displacements at convergent plate margins resulting from such imposed variations for this purpose we use a 2 d numerical model in which the lithospheric plates have an effective elastic thickness we first define a model in which the subducting plate is driven by its negative buoyancy and a velocity at its surface side boundary its equilibrium topography after around 2 myr is the reference level for examination of surface displacements resulting from variations in buoyancy velocity of the surface plates friction along the interplate contact and subduction zone roll back we find that a decrease increase in buoyancy of the subducting material leads to a deepening uplift of the plate margins an increase in friction along the subduction fault deepens the overriding plate margin subduction zone roll back due to sinking of the negatively buoyant subducting plate induces subsidence of the overriding plate margin this subsidence is reduced when roll back takes place in a land locked basin setting trench retreat forced by the motion of the overriding plate is characterized by higher topography of the overriding plate margin than the case of retreat due to the sinking of the negatively buoyant slab in the first case in plane stress in the back arc region is compressive while it is tensional for roll back due to the sinking of the slab we conclude that vertical surface displacements during ongoing subduction may reach a magnitude of a few kilometres on the overriding and subducting plate margins