timing kinematics and cause of aegean extension a scenario based on a comparison with simple analogue experiments abstract since the early 1970s most geodynamic models proposed for the eastern mediterranean consider that the initiation of extension in the aegean during the late middle miocene or later was a direct consequence of the westward extrusion of anatolia away from the arabia eurasia collision front in the late 1990s a few models have taken into account the increasing evidence for an earlier initiation of aegean extension arising mostly from the analysis of deformation within the various alpine metamorphic complexes of the area based largely on our own investigations we first present a review of the data that reliably constrain the minimum age for the initiation of extension in these complexes it shows that regional scale extension with a pattern of stretching orientations similar to that of the pliocene pleistocene was already strongly active in the aegean before the onset of arabian indentation into eurasia lower miocene vs middle or upper miocene this implies that the initiation of aegean extension did not result from the lateral extrusion of anatolia instead extension may have started owing to a process of gravity spreading of the continental lithosphere that had previously been thickened during alpine collision the main arguments favouring this interpretation are presented and a preliminary scenario is proposed for the neogene evolution of the eastern mediterranean in which lateral extrusion of anatolia occurred lately during southward spreading of the aegean lithosphere in order to test this hypothesis further physical experiments simulating the horizontal spreading of a continental lithosphere toward a free boundary have been carried out the analysis of various elements of the deformation field displacements strain rotations fault pattern and kinematics both in the experiments and in the aegean domain reveals an excellent agreement between the two not only do early stages of aegean extension appear to fit the gravity spreading hypothesis but so does the recent to present internal deformation of the aegean transverse e w active shortening observed in the northern aegean is commonly taken as one of the most striking pieces of evidence that aegean extension results from the lateral extrusion of anatolia the experiments indicate that such transverse shortening may equally develop as a result of pure gravity spreading associated with a pattern of faults and rotations that compares well with that of the northern aegean pure dextral shearing along some strands of the north aegean trough fault system and the related development of an asymmetric pattern of faults in the northern aegean are eventually the only features that evidently arise from the extrusion of anatolia since less than 3 ma finally we argue that during the early stages of aegean extension since at least 21 ma and until ca 11 ma the distribution of extensional strain on a regional scale has been controlled by the distribution of earlier thickening post thickening thermal weakening through partial melting of deep levels of the crust probably accounts for the localization of extensional strain within the areas of thick crust restoring the aegean in its most likely configuration at the onset of bulk crustal extension suggests that 1 the extensional strain rate remained roughly constant or has eventually decreased since the onset of aegean extension and 2 the amounts of extension before and after 16 ma have been roughly equal

deformation of subducted oceanic lithosphere summary the deformation of subducted oceanic lithosphere is revealed by detailed studies of the distribution of seismicity in benioff zones and by the increasingly well resolved images of the upper mantle obtained by seismic p wave tomography by using numerical experiments based on a simplified dynamical model of a subducted lithospheric slab we test the idea that the observed deformation of the subducted slab results from a balance between internal buoyancy forces and the viscous stresses associated with deformation the simplified model assumes that the lithosphere has uniform physical properties it is denser and much more viscous than the upper mantle and its penetration below 670 km depth is resisted by a step like density increase at that level the primary determinant of the style of deformation is the buoyancy number f a ratio of buoyancy stress generated by the mass anomaly in the slab to the stress associated with viscous deformation at a strain rate u l defined in terms of subduction rate u and slab thickness l for a small buoyancy number deformation of the slab is dominated by viscous flexure for large f down dip extension of the slab appears to dominate and a buckling instability of the slab is observed if there is resistance to penetration of the 670 km level the transition value of f at which this change in behaviour is observed is about 0 05 for constant viscosity and about 0 2 for stressdependent viscosity with a stress versus strain rate exponent of n 3 with n 3 a boudinage type instability of the slab is observed for large f penetration of the slab below the 670 km level depends on the density contrast between slab and lower mantle resisted only by the density difference the slab may initially penetrate several hundred kilometres below the 670 km level for plausible density parameters before a buckling instability causes this part of the slab to rotate and ascend comparison of the deforming slab geometry and stress field with seismicity distribution and tomographic images from the tonga subduction zone suggests that the effective buoyancy number f for this slab is approximately equal to the transition value that we determined experimentally we use this constraint to estimate average rheological parameters for the tonga slab which when compared with published creep deformation laws for olivine are consistent with the average temperature of the slab being about 0 4 to 0 45 times the melting temperature based on constitutive laws for olivine at shallow depths 100 km these temperatures correspond to around 590 c for wet olivine or around 700 c for dry olivine

motion of the pacific plate relative to eurasia and its potential relation to cenozoic extension along the eastern margin of eurasia extension along the eastern margin of eurasia has been regarded commonly as a far field effect of the india eurasia collision however some aspects of the timing and location of this extension make a link between the collision and extension difficult we suggest that some extensional features commonly interpreted as effects of the collision may be related more simply to changes in plate convergence rates along the eastern plate boundary of eurasia an analysis of the motion of the pacific plate relative to eurasia suggests that the rate of pacific eurasia convergence varied significantly during the tertiary period from a late cretaceous convergence rate of 120 140 mm yr the rate declined substantially during early tertiary time and reached a minimum in eocene time of 30 40 mm yr in oligocene to earliest miocene time the average convergence rate increased moderately to 70 95 mm yr then decreased again to 65 70 mm yr during early to middle miocene time from late miocene to the present the rate of convergence increased to an average of 100 110 mm yr the paleocene through middle miocene episode of relatively slow convergence correlates with a period of widespread extension along the eastern margin of eurasia decreased convergence may have been related to a net reduction in horizontal compressional stress transmitted between the pacific and eurasian plates which resulted in widespread extension adjacent to the margin of eurasia

dynamics of retreating slabs 2 insights from three dimensional laboratory experiments 1 a laboratory analogue of a three layer linear viscous slab upper mantle lower mantle system is established in a silicone putty honey and crystallized honey tank experiment the same setup as in the numerical investigation part 1 is used we focus on the interaction of the slab with the induced passive mantle flow by widely varying the mantle volume flux boundary conditions in our numerical experiments the lateral volume flux was set to zero in interpreting the results relative to the real earth the base of the box is taken as the bottom of the mantle convection system while the lateral boundaries may be associated with the presence of other nearby slabs dynamic force equilibrium assessed on the basis of an analytical review of forces is described for four different phases 1 the subduction initiation instability 2 the accelerating dynamic free fall phase of the slab 3 the dynamic interaction with the 660 km discontinuity and 4 a final phase of steady state trench retreat phase 3 is an important feature not observed in the numerical experiments this highly dynamic phase of interrupted trench retreat can therefore be attributed to boundary conditions on mantle volume flux on the basis of integration constants of force equilibrium in phases 2 and 4 we identify two different classes of volume flux one in which the lateral boundary can be considered open and the other class where it is closed closed boundary condition cases are obtained if any of the lateral box boundaries are 600 km away from the slab assuming a one to one relation between trench retreat and back arc spreading enigmatic observations of episodic opening of back arc basins can be explained by our experimental observations

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

oligo miocene rift of sardinia and the early history of the western mediterranean basin the geodynamic evolution of the western mediterranean basin in spite of many studies is still uncertain there is some consensus for interpreting this basin as a kind of small oceanic marginal basin its opening has generally been related to a subduction process which was active during the oligocene miocene somewhere east of sardinia corsica1 7 as the margins of the basin are deeply buried below miocene to present sediments direct lithological and stratigraphical data which could explain the events responsible for its formation are rare8 10 or missing altogether to obtain such data detailed field studies have been undertaken in sardinia fig 1 and the first results are presented here this approach is justified by the fact that in that island oligocene and miocene sediments were deposited in a rift fossa tettonica sarda of verdabasso11 which is the easternmost arm of the complex rift system that affected the european plate during oligocene and miocene times one of these arms evolved towards a small oceanic basin the western mediterranean or algero provencal basin while others such as the gulf of valencia and the sardinia rift aborted and remained at the rift stage its exceptional exposures make it possible to examine the sardinia rift to clarify the sequence of events which created it and to establish a sedimentological model which we believe is directly applicable to the western mediterranean basin

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

dynamics of slab rollback and induced back arc basin formation abstract we performed a numerical study to understand the dynamical mechanisms of back arc basin formation induced by slab rollback to this end we used two dimensional numerical models of an integrated plate mantle convection system with 410 and 660 km phase transitions retrograde slab migration occurs when the slab stagnates in the transition zone or when the deep section of the slab is vertical in both cases slab rollback occurs because the deep slab section obstructs the descending motion of the shallow slab section with an inclination buoyancy of the 660 km phase boundary acts as the obstructing force in the case of stagnant slab formation and an anchoring force against the horizontal motion works similarly in the case of vertical slabs to balance the horizontal component of the obstructing force a suction force at the plate boundary pulls the overriding plate toward the ocean back arc spreading is produced by means of slab rollback when the overriding plate with a weak area is fixed to the model boundary the back arc deformation becomes compressional when the overriding plate is freely movable despite trench retreat because the wedge mantle flow viscously drags the overriding plate toward the trench this implies that forces tending to actuate the overriding plate away from the trench are necessary to generate back arc extension even when trench retreat is generated by slab rollback

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

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

on subduction dip angles abstract in order to examine the main forces exerted on the slabs the dip angles of slabs are compared with other parameters related to the descending slabs as a result it is found that the main factors which control the dip angle are the relative velocity of two converging plates and the negative buoyancy force of the descending slab especially the additional negative buoyancy of the olivine spinel phase change a good correlation between the dip angle and the relative velocity suggests a significant role of dynamic support by flow stress

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

controls of the structure of subducted slabs numerical simulations of subducting slabs are formulated in which the shape and dip of the slab are determined by the dynamics of the flow rather than imposed a priori the dip of slabs is a function of the time since the initiation of subduction slabs fold develop a kink in dip and thicken on entry into a high viscosity lower mantle comparison of the simulations with seismic observations suggest that the lower mantle is at least 10 30 times more viscous than the upper mantle

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

geophysical characteristics of the southern mariana trough 11 50 n 13 40 n despite slow opening rates generally inferred for the mariana trough the southernmost part of the basin has fast spreading geophysical and morphologic characteristics that are unlike the features of the basin to the north a side scan sonar and geophysical survey maps the eastern part of the basin and the seafloor spreading center between 11 50 n and 13 40 n and identifies the following characteristics the ridge across axis profile forms a triangular to rounded high with relief of 100 to 500 m and cross sectional area variations of 1 to 7 km2 the along axis mantle bouguer gravity gradient is 0 2 mgal km axial segmentation occurs as overlapping axes and small deviation in trend no transform fault offsets exist despite significant changes in the trend of the spreading center characteristics of the surrounding basin include shallower overall depth than in the north no well developed frontal arc high in the southernmost trough the close proximity of submarine arc type volcanoes to the spreading center and tectonic fabric that is at a high angle to the trend of the spreading center on the eastern flank but is concordant on the western flank these characteristics imply different tectonic and magmatic conditions in the southern trough from the rest of the basin we propose that these effects are related to 1 the geometry of trench rollback in the southern trough leading to trench parallel extension generating inward radiating extensional faults 2 decoupling of the trench parallel extensional strain by the spreading center so that it primarily affects the eastern flank of the basin and 3 augmentation of the spreading center s magmatic budget by arc magmatic sources contributing to its fast spreading character although these effects may be accentuated in the southern mariana trough by the geometry of trench rollback and position of the slab which here underlies the spreading center they reflect distinct volcanic and tectonic processes which are varyingly expressed in back arc systems but are not normally found at mid ocean ridges

analogue modeling of arc and backarc deformation in the new hebrides arc and north fiji basin in most backarc basins extension is perpendicular to the arc thus individual spreading ridges extend approximately parallel to the arc in the north fiji basin however several ancient and active spreading ridges strike 70 90 to the new hebrides arc these high angle spreading ridges relocated southward during the asymmetric opening of the north fiji basin we have simulated the structural development of the north fiji basin and the new hebrides arc with scaled analogue models and the results have inspired us to come to several tentative conclusions we interpret the orientation of the high angle spreading ridges to be related to the asymmetric opening of the backarc basin around a hinge where they form close to the hinge relocation of these spreading ridges is most likely related to subduction of the west torres plateau along the new hebrides trench this resulted in localized collision retarded rollback of the subducting slab along the northwest corner of the trench and reduced extension and shearing in the northwest corner of the north fiji basin backarc extension continued in the rest of the north fiji basin owing to continued rollback of the southern part of the subducting slab here active extension was separated from the slightly or nonextending northwest corner by a zone striking at high angle to the new hebrides arc i e the hazel holme extensional zone moreover impingement of the d entrecasteaux ridge into the overriding plate led to local deformation and fragmentation of the arc

india eurasia collision chronology has implications for crustal shortening and driving mechanism of plates the motion of the indian plate is determined in an absolute frame of reference and compared with the position of the southern margin of eurasia deduced from palaeomagnetic data in tibet the 2 600 900 km of continental crust shortening observed is shown to have occurred in three different episodes subduction of continental crust intracontinental thrusting and internal deformation and lateral extrusion the detailed chronology of the collision and plate reorganizations in the indian and pacific oceans supports the hypothesis that slab pull is a dominant driving mechanism of plate tectonics

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

tomographic evidence of slab detachment beneath eastern turkey and the caucasus summary teleseismic phase readings from the eastern turkey seismic experiment etse have been inverted using teleseismic tomography in order to create a 3 d image of the underlying mantle beneath eastern turkey the aim was to investigate the existence of an upper mantle negative velocity anomaly that is used to explain the uplift of eastern anatolian plateau and the potential pieces of detached oceanic slabs related to neo tethyan subduction suggested by previous studies using teleseismic waveforms from the 29 stations of the eastern turkey seismic experiment 2926 direct p phases from 146 events were picked by using adaptive stacking techniques in order to increase the station coverage and resolve the surrounding area the phase readings from the international seismological center isc bulletin have also been added the data set consists of 9571 p and pkp phase readings of 79 stations from 503 teleseismic events this study develops the first highresolution 3 d upper mantle p wave tomographic model for this region the tomographic results show the existence of an upper mantle negative velocity anomaly to a depth of 200 km beneath the eastern anatolian accreationary complex eaac as commonly observed in the previous studies that suggest a shallow partially molten asthenosphere the slab like high velocity anomalies beneath the eaac pontides and caucasus are interpreted as the detached southern and northern neo tethys slabs

effects of relative plate motion on the deep structure and penetration depth of slabs below the izu bonin and mariana island arcs an increasing number of seismological studies indicate that slabs of subducted lithosphere penetrate the earth s lower mantle below some island arcs but are deflected or rather laid down in the transition zone below others recent numerical simulations of mantle flow also advocate a hybrid form of mantle convection with intermittent layering we present a multi disciplinary analysis of slab morphology and mantle dynamics in which we account explicitly for the history of subduction below specific island arcs in an attempt to understand what controls lateral variations in slab morphology and penetration depth central in our discussion are the izu bonin and mariana subduction zones we argue that the differences in the tectonic evolution of these subduction zones in particular the amount and rate of trench migration can explain why the slab of subducted oceanic lithosphere seems to be at least temporarily stagnant in the earth s transition zone below the izu bonin arc but penetrates into the lower mantle below the mariana arc we briefly speculate on the applicability of our model of the temporal and spatial evolution of slab morphology to other subduction zones although further investigation is necessary our tentative model shows the potential for interpreting seismic images of slab structure by accounting for the plate tectonic history of the subduction zones involved we therefore hope that the ideas outlined here will stimulate and direct new research initiatives

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

cordillera formation mantle dynamics and the wilson cycle we propose that the andes cordillera formed by a collision between the trenchward moving south american plate and the nazca slab and subslab mantle resistance to trenchward motion in the form of horizontal compressive normal stress deforms south america9s leading edge the advancing continent also perturbs the subslab mantle flow field as mantle is displaced this flow field detected via shear wave splitting includes trench parallel flow away from a central stagnation point and corner flow around the northern and southern ends of the continent the corresponding spatial variation in applied stress accounts for the large scale characteristics of andean deformation buckling at the stagnation point maximum normal stress forming the central andes and bolivian orocline and corner flow associated with the eastward motion of the caribbean and scotia plates the high stresses required to form and maintain the andes imply that south america9s trenchward motion is primarily driven by deep mantle flow coupled to its base no other driving force of sufficient magnitude is available similarities between north and south american plate motion history and large scale cordillera structure indicate that the western north american laramide rockies formed in a similar manner and that the north american plate is also driven westward by deep mantle flow we infer that atlantic spreading itself is similarly driven by contrast slab rollback in a closing ocean basin opens marginal basins at convergent margins as in the western pacific we suggest that there is a causal link between the opening and closing of ocean basins and the formation of cordilleras and marginal basins cordillera formation occurs at the leading continental margin during the spreading phase of the wilson cycle whereas marginal basins form during the closing phase

asymmetric deformation in the backarc region of the kuril arc northwest pacific new insights from analogue modeling 1 the eocene to middle late miocene tectonic evolution of the kuril arc and backarc region has been simulated with analogue experiments the experiments simulate asymmetric deformation in the overriding plate due to anticlockwise rollback of the subducting pacific plate the results show the formation of a n s to ne sw oriented dextral shear zone near the far edge of the retreating boundary analogous to the sakhalin hokkaido dextral shear zone contemporaneously normal faults and grabens form striking parallel to the retreating boundary near the far edge but striking more oblique near the hinge point and away from the retreating boundary this is similar to extensional structures observed in the kuril basin and the sea of okhotsk furthermore the model shows that the amount of extension progressively decreases away from the retreating boundary this appears to have also happened in the kuril okhotsk region as evidenced by crustal thickness variation in the region finally the model results show that extension is increasingly accommodated by the region close to the retreating boundary with progressive deformation this can account for the eocene early miocene extension in the sea of okhotsk followed by miocene spreading in the kuril basin

a laboratory investigation of effects of trench migration on the descent of subducted slabs a laboratory investigation of viscous slabs subducted from a migrating trench reveals a range of possible behaviours the slab dip in a uniform mantle is found to be steady or oscillatory depending on the rates of descent and trench migration in addition density and viscosity interfaces are used to model the increased resistance to sinking of stiff slabs through the seismic discontinuity at a depth of about 660 km in earth s mantle if the slab is denser than the lower layer and its dip in the upper layer is steady it can continue to descend through the lower layer in a tabular form and in either a steady or oscillatory manner or it can be laid horizontally on the interface for a distance before descending in a chain of diapirs resulting from gravitational instability at the interface if the slab dip in the upper layer is unstable the slab sinks into the lower layer in a chain of large diapirs at a spacing determined by the frequency of oscillations set by instability of the slab within the upper layer the style of penetration depends on the trench migration speed and the ratio of sinking velocities in the two layers estimates for mantle slabs indicate that they may range across the major regime transitions the experimental system provides a gross simplification of mantle conditions but the results are a testimony to the possibility of a range of complex behaviour of subducted lithosphere they also indicate that the relationship between the structural features in the lower mantle revealed by seismic imaging and present day tectonic processes at the surface may not be obvious tomographic images of western pacific subduction zones and data for the migration rates of associated trenches suggest a dependence of slab behaviour on migration rate similar to that seen in the experiments

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

control of seafloor aging on the migration of the izu bonin mariana trench article i nfo recent global kinematic studies reveal that most of the trenches roll back but a significant number of them advance toward the upper plate those advancing trenches are mostly located in the western pacific and correspond to the subduction of very old mesozoic oceanic lithosphere while retreating trenches are commonly explained by the slab pull action of the descending lithosphere the origin of advancing trenches is still debated since this relationship is dependent upon the adopted reference frame we select region where geological studies show the variability of trench migration style with time the izu bonin mariana ibm region represents a key example the detailed reconstruction of the trench migration of the ibm subducting system reveals that after a long episode of asymmetric rollback the ibm trench recently started advancing we propose that this change from retreating to advancing trench mode results from the subduction of progressively older and stiffer lithospheric material we test this hypothesis by means of two dimensional 2 d numerical models reproducing the effects of the lithospheric aging during subduction the result of our numerical tests shows that the entrance of old and stiff lithosphere forces the trench to advance because the increasing stiffness of the slab prevents the slab to unbend once it has subducted we adapt this physical result to the ibm evolution showing that the switch from trench retreat to trench advance can reconcile the shape of ibm slab as previously suggested finally we discuss the possibility that the subduction of the cretaceous lithosphere might have triggered the change of the motion of the pacific plate around 5 ma

pre spread rifting and heterogeneous volcanism in the southern havre trough back arc basin abstract the havre trough is an active back arc basin associated with pacific australian plate convergence two segments within the southern havre trough sht surveyed with gloria sea beam and sys09 swath data now provide the first detailed data to reliably document the structure and morphology of this complex back arc region at the southern segment 35 40 37 s along a strike length of some 120 km the sht structure comprises a series of segmented en echelon generally flat floored sediment filled axial rift grabens flanked by ridge and basin rift block topography prominent fault escarpments with relief of 500 1000 m and a marked zig zag arrangement mark the outer margins of the rift system within the rift grabens extensional tectonism is pervasive extensive shallow seismicity within the sht can only partly be associated with the rift graben system at present youthful constructional volcanic terrains within the rift grabens are not common and invariably have a restricted areal extent and a relief an identical rift morphology has been identified at the northern 33 34 s site data from both sites show individual rifts are 15 35 km in length 6 10 km wide and have axial depths of 3100 4000 m apart from at least one 15 20 km wide cross trough ridge available data indicate the rift graben system is contiguous between 33 s and 37 s this rift graben morphology by analogy with other known rifting back arc basins is considered here not to be consistent with true oceanic back arc spreading rather it is argued that the sht possibly indeed all of the havre trough is undergoing back arc rifting prior to true spreading further the rifting tectonic fabric within the sht and in the onshore taupo to the south extension is consistently dextrally oblique relative to the bounding colville and kermadec ridges the degree of obliquity increases from south to north if accepted that the sht is rifting rather than spreading magnetic anomaly data within the trough do not record the accretion of new oceanic crust and consequently can no longer be used to establish the spreading history of the sht the sht magnetic anomalies are here interpreted as pseudo linear magnetic anomalies resulting from the generally irregular spatial and temporal emplacement of magnetic sheeted lava and dike intrusives between older low magnetisation arc basement rift blocks flanking the axial rift proxy data from onshore north island new zealand and offshore sht suggest that the rate of extension and age of initiation of back arc rifting are between 35 and 37 s respectively 15 20 mm yr 1 and 5 ma

initial results from wing the continuous gps network in the western pacific area to investigate tectonic deformation in the western pacific a continuous gps tracking network has been established and named the western pacific integrated network of gps wing between 1995 and march 1997 we establised ten new sites data for the period july 1995 to october 1996 were analyzed together with data from international gps service for geodynamics igs global sites to estimate daily coordinates a fiducial free approach was used to obtain the most accurate baseline estimates to fix the estimated coordinates to the terrestrial reference frame the tsukuba igs site is assumed to be moving westward relative to the stable eurasian continent at 2 cm yr according to heki s 1996 estimate we find that 1 velocities of sites well within oceanic plates are in good agreement with rigid plate motion models 2 sites close to plate boundaries are all affected by the deformation at those boundaries among which back arc rifting spreading is clearly visible at the mariana and okinawa troughs 3 sites in eastern asia are moving east to east southeast relative to the stable eurasian continent suggesting long distance effects of the northward collision of india with asia

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

impact of regional mantle flow on subducting plate geometry and interplate stress insights from physical modelling summary physical models of subduction investigate the impact of regional mantle flow on the structure of the subducted slab and deformation of the downgoing and overriding plates the initial mantle flow direction beneath the overriding plate can be horizontal or vertical depending on its location with respect to the asthenospheric flow field imposed mantle flow produces either over or underpressure on the lower surface of the slab depending on the initial mantle flow pattern horizontal or vertical respectively overpressure promotes shallow dip subduction while underpressure tends to steepen the slab horizontal mantle flow with rates of 1 10 cm yr 1 provides sufficient overpressure on a dense subducting lithosphere to obtain a subduction angle of 60 while the same lithospheric slab sinks vertically when no flow is imposed vertical drag force due to downward mantle flow exerted on a slab can result in steep subduction if the slab is neutrally buoyant but fails to produce steep subduction of buoyant oceanic lithosphere the strain regime in the overriding plate due to the asthenospheric drag force depends largely on slab geometry when the slab dip is steeper than the interplate zone the drag force produces negative additional normal stress on the interplate zone and tensile horizontal stress in the overriding plate when the slab dip is shallower than the interplate zone an additional positive normal stress is produced on the interplate zone and the overriding plate experiences additional horizontal compressive stress however the impact of the mantle drag force on interplate pressure is small compared to the influence of the slab pull force since these stress variations can only be observed when the slab is dense and interplate pressure is low

laboratory experiments on the structure of subducted lithosphere abstract mantle seismic tomography images indicate a variety of shapes for slabs of oceanic lithosphere subducted into the transition zone in the vicinity of the 660 km discontinuity both horizontally deflected slabs and nearly vertical slabs have been inferred at subduction zones on the pacific and indian plates here we determine experimentally the behavior of a cold negatively buoyant and highly viscous slab incident on a fluid interface with a viscosity and a density increase the experiments are scaled to mantle conditions with realistic subduction peclet number ratio of trench migration to subduction velocities and physical properties of the interface a wide range of slab deformation styles is observed when slight changes in the parameters are imposed sinking slab stagnant slab spreading slab sinking pile and stagnant pile represent the five different modes identified we show that these deformation modes are governed by two velocity ratios which characterize the horizontal and the vertical components of the slab velocity near the interface slab penetration into the lower layer takes the form of folded slab piles for large values of the velocity ratios and stagnant behavior is promoted by rapid retrograde trench motion the sensitivity of slab behavior to transition zone density and viscosity changes and to trench migration can account for the coexistence of different modes of deformation in subducted lithosphere

episodic back arc extension during restricted mantle convection in the central mediterranean abstract in the central mediterranean during the last 80 myr africa has been slowly converging toward eurasia with an average relative velocity of less than 1 cm yr 1 for 50 myr this slow convergence led to the build up of the alpine orogenic belt and to the initiation of consumption of the former tethys ocean in the last 30 myr however the whole central western mediterranean has been swept by rapid episodes of trench migration up to 6 cm yr 1 and back arc opening which consumed the whole former tethys ocean we combine plate tectonic history geological timing and laboratory modelling to reconstruct the subduction history and the evolution of the central mediterranean over the last 80 myr we find that the dynamic evolution of the subducting oceanic lithosphere can reconcile the rapid episodic back arc migration with the slow african convergence but only if the subduction process is restricted to the upper mantle

spherical shell tectonics effects of sphericity and inextensibility on the geometry of the descending lithosphere the shape of a deep seismic zone is thought to represent that of the descending slab of lithosphere the lithosphere before subduction is a spherical shell and the shape of the descending slab is the result of the deformation of the spherical lithosphere at the subduction zone upon bending a spherical shell often deforms in a very different way from a simple plate we examine whether the actual shape of the descending slab can be explained by a simple bending of an inextensible spherical shell which shows little surface deformation under moderate stress this examination is made region by region for most of the subduction zones in the world by means of an analogue method the lithosphere is simulated by an inextensible spherical shell made of polyvinyl chloride resin the wadati benioff zone is shaped by plaster by referring to the reliable hypocentral data selected from the international seismological centre isc bulletins the spherical shell is forced to fit the miniature of the wadati benioff zone fitting is first attempted only by bending if a good fit is not attainable and if a discontinuity or gap in seismic activity is observed in the relevant region the spherical shell is torn along this discontinuity or gap and the goodness of fit is reexamined the results of the analysis are summarized as follows 1 the shape of the wadati benioff zone can be simulated largely by a simple bending of a spherical shell without surface deformation 2 in almost all of the regions of poor fit with bending a good fit can be achieved by tearing the spherical shell along the trace of low seismicity the sphericity of the lithosphere and the inextensibility upon deformation are the two essential factors in controlling the slab shape this means that the lateral constraint is most important for understanding the geometry of the downgoing slab of lithosphere and the stress state within it further several problems related to the deformational characteristics of the spherical lithosphere are also reviewed and discussed in connection with subduction tectonics

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

interseismic locking on the hikurangi subduction zone uncertainties from slow slip events interseismic locking on the hikurangi subduction zone in new zealand is examined in light of alternative assumed locking distributions and the impact of transients slow slip and volcanic sources on temporal and spatial resolution the modern pattern of locking in the north is poorly resolved and based on simulations of possible transient behavior may be an ephemeral feature of the subduction cycle while there appears to be some contemporary locking in the northern half of the hikurangi subduction zone hsz its location is model dependent and hence its relationship to structure slow slip or any transition zone there is unclear simulations of site velocities using the 14 year history of transient events reveal that in the timescale of the interseismic period the northern half of the hsz could be either locked or unlocked and this may not be resolvable for decades in the southern half there is strong contemporary locking in the 15 to 40 km depth range but again the slow slip history leads to uncertainty in the long term pattern slow slip events not only reduce the long term locking by aseismic slip but also greatly hinder our ability to see it it is within the range of possible models that the slip deficit rate at the hsz is more uniform along strike and the modern appearance is controlled by the particular pattern of transients over the past 10 to 20 years when the gps data were collected similarly uncertainties in surface velocities will be large at any subduction zone with large transients

closing the gap between regional and global travel time tomography recent global travel time tomography studies by zhou 1996 and van der hilst et al 1997 have been performed with cell parameterizations of the order of those frequently used in regional tomography studies i e with cell sizes of 1 2 these new global models constitute a considerable improvement over previous results that were obtained with rather coarse parameterizations 5 cells the inferred structures are however of larger scale than is usually obtained in regional models and it is not clear where and if individual cells are actually resolved this study aims at resolving lateral heterogeneity on scales as small as 0 6 in the upper mantle and 1 2 3 in the lower mantle this allows for the adequate mapping of expected small scale structures induced by for example lithosphere subduction deep mantle upwellings and mid ocean ridges there are three major contributions that allow for this advancement first we employ an irregular grid of nonoverlapping cells adapted to the heterogeneous sampling of the earth s mantle by seismic waves spakman and bijwaard 1998 second we exploit the global data set of engdahl et al 1998 which is a reprocessed version of the global data set of the international seismological centre their reprocessing included hypocenter redetermination and phase reidentification finally we combine all data used p pp and pwp phases into nearly 5 million ray bundles with a limited spatial extent such that averaging over large mantle volumes is prevented while the signal to noise ratio is improved in the approximate solution of the huge inverse problem we obtain a variance reduction of 57 1 synthetic sensitivity tests indicate horizontal resolution on the scale of the smallest cells 0 6 or 1 2 in the shallow parts of subduction zones decreasing to approximately 2 3 resolution in well sampled regions in the lower mantle vertical resolution can be worse up to several hundreds of kilometers in subduction zones with rays predominantly pointing along dip important features of the solution are as follows 100 200 km thick high velocity slabs beneath all major subduction zones sometimes flattening in the transition zone and sometimes directly penetrating into the lower mantle large high velocity anomalies in the lower mantle that have been attributed to subduction of the tethys ocean and the farallon plate and low velocity anomalies continuing across the 660 km discontinuity to hotspots at the surface under iceland east africa the canary islands yellowstone and the society islands our findings corroborate that the 660 km boundary may resist but not prevent present day large scale mass transfer from upper to lower mantle or vice versa this observation confirms the results of previous global mantle studies that employed coarser parameterizations

evolving force balance during incipient subduction nearly half of all active subduction zones initiated during the cenozoic all subduction zones associated with active back arc extension have initiated since the eocene hinting that back arc extension may be intimately associated with an interval several tens of myr following subduction initiation that such a large proportion of subduction zones are young indicates that subduction initiation is a continuous process in which the net resisting force associated with forming a new subduction zone can be overcome during the normal evolution of plates subduction initiation is known to have occurred in a variety of tectonic settings old fracture zones transform faults and extinct spreading centers and through polarity reversal behind active subduction zones although occurring within different tectonic settings four known subduction initiation events izu bonin mariana ibm along a fracture zone tonga kermadec along an extinct subduction boundary new hebrides within a back arc and puysegur fiordland along a spreading center were typified by rapid uplift within the forearc followed by sudden subsidence other constraints corroborate the compressive nature of ibm and tonga kermadec during initiation using an explicit finite element method within a two dimensional domain we explore the evolving force balance during initiation in which elastic flexure viscous flow plastic failure and heat transport are all considered in order to tie theory with observation known tectonic settings of subduction initiation are used as initial and boundary conditions we systematically explore incipient compression of a homogeneous plate a former spreading center and a fracture zone the force balance is typified by a rapid growth in resisting force as the plate begins bending reaching a maximum value dependent on plate thickness but typically ranging from 2 to 3 1012 n m for cases that become self sustaining this is followed by a drop in stress once a shear zone extends through the plate the formation of a throughgoing fault is associated with rapid uplift on the hanging wall and subsidence on the footwall cumulative convergence not the rate of convergence is the dominant control on the force balance viscous tractions influence the force balance only if the viscosity of the asthenosphere is 1020 pa s and then only after plate failure following plate failure buoyancy of the oceanic crust leads to a linear increase with crustal thickness in the work required to initiate subduction the total work done is also influenced by the rate of lithospheric failure a self sustaining subduction zone does not form from a homogeneous plate a ridge placed under compression localizes subduction initiation but the resisting ridge push force is not nearly as large as the force required to bend the subducting plate the large initial bending resistance can be entirely eliminated in ridge models explaining the propensity for new subduction zones to form through polarity reversals a fracture zone fz placed in compression leads to subduction initiation with rapid extension of the overriding plate a fz must be underthrust by the older plate for 100 150 km before a transition from forced to self sustaining states is reached in fz models the change in force during transition is reflected by a shift from forearc uplift to subsidence subduction initiation is followed by trench retreat and back arc extension moderate resisting forces associated with modeled subduction initiation are consistent with the observed youth of pacific subduction zones the models provide an explanation for the compressive state of western pacific margins before and during subduction initiation including ibm and tonga kermadec in the eocene and the association of active back arcs with young subduction zones on the basis of our dynamic models and the relative poles of rotation between pacific and australia during the eocene we predict that the northern segment of the tonga kermadec convergent margin would have initiated earlier with a progressive southern migration of the transition between forced and self sustaining states

seismic tomography with p and s data reveals lateral variations in the rigidity of deep slabs abstract regional seismic tomography of the northwest pacific island arcs using p and s wave arrival time data with similar path coverage reveals an oceanic lithospheric slab deflected in the mantle transition zone beneath the izu bonin region in good agreement with the results of earlier tomographic and other seismic studies in the region the s wave images however do not show clearly the deflection of the slab this implies that the stagnant lithospheric slab in the transition zone is more likely a bulk sound structure which is strongly supported by our results from joint inversions for bulk sound and shear wavespeeds for the izu bonin region where trench migration has been reported the properties of the deflected slab lying on top of the 660 km discontinuity show strong bulk sound and weak shear signatures in contrast to the descending slab itself there is an indication in the shear images that a component of the former oceanic lithosphere is penetrating into the top of the lower mantle suggesting that a slab split has occurred

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

seismic stratigraphy and structural analysis of the northern east china sea shelf basin interpreted from multi channel seismic reflection data and cross section restoration analysis of multi channel seismic data from the northern east china sea shelf basin ecssb reveals three sub basins socotra domi and jeju basins separated by structural highs hupijiao rise and faulted basement blocks these sub basins show a typical rift basin development faulted basement and syn rift and post rift sedimentation separated by unconformities four regional unconformities including the top of acoustic basement have been identified and mapped from multi channel seismic data faults in the acoustic basement are generally trending ne parallel to the regional structural trend of the area the depths of the acoustic basement range from less than 1000 m in the northwestern part of the domi basin to more than 4500 m in the socotra basin and 5500 m in the jeju basin the total sediment thicknesses range from less than 500 m to about 1500 m in the northwest where the acoustic basement is shallow and reach about more than 5500 m in the south interpretation of seismic reflection data and reconstruction of three depth converted seismic profiles reveal that the northern ecssb experienced two phases of rifting followed by regional subsidence the initial rifting in the late cretaceous was driven by the nw se crustal stretching of the eurasian plate caused by the subduction of the pacific plate beneath the eurasian plate extension was the greatest during the early phase of basin formation estimated rates of extension during the initial rifting are 2 6 5 and 3 5 in the domi jeju and socotra basins respectively a regional uplift terminated the rifting in the late eocene early oligocene rifting and extension although mild resumed in the early oligocene while fluvio lacustrine deposition continued to prevail the estimated rates of extension during the second phase of rifting are 0 7 0 8 and 0 5 in the domi jeju and socotra basins respectively a second phase of uplift in the early miocene terminated the rifting marking the transition to the post rift phase of regional subsidence regional subsidence dominated the study area between the early miocene and the late miocene an inversion in the late miocene interrupted the post rift subsidence resulting in an extensive thrust fold belt in the eastern part of the area uplift and subsequent erosion were followed by regional subsidence highlights this paper presents 2d seismic mapping of the northern east china sea shelf basin structural and stratigraphic evolution of the basin have been investigated we have also reconstructed three depth converted seismic reflection profiles cross section restoration provided quantitative information about extension rates

tethyan subducted slabs under india abstract tomographic imaging of the mantle under tibet india and the adjacent indian ocean reveals several zones of relatively high p wave velocities at various depths under the hindu kush region in northeastern afghanistan and southern tajikistan a regional northward dipping slab is seen in the entire upper 600 km of the mantle and is apparently still attached to the lithosphere of the indian plate under northern pakistan this same slab shows a roll over structure with the deeper portion overturned and dipping southward as can also be seen in the distribution of earthquake hypocenters farther east southeast e g in the vicinity of nepal a well resolved anomaly below 450 km depth is connected to the slab under the hindu kush but seems to be separated from the lithosphere above 350 km these upper mantle anomalies are interpreted as the remnants of delaminated sub continental lithosphere that went down when greater india continued to converge northward with asia after 45 ma the deeper high velocity anomalies under the indian sub continent appear clearly separated from the shallower ones as well as from each other and are inferred to represent remnants of oceanic lithospheric slabs that have sunk into the lower mantle and were subsequently overridden by the indian plate they occur at depths between 1000 and 2300 km and occasionally descend down to the core mantle boundary the anomalies form three parallel wnw ese striking zones we interpret the two southern zones as remnants of oceanic lithosphere that was subducted when the neo tethys ocean closed between india and tibet in the cretaceous and earliest tertiary the northern deep mantle zone under northern afghanistan the himalayas and the lhasa block in southern tibet may represent the last subducted remnant of the paleo tethys ocean which is thought to have closed before the hauterivian stage of the early cretaceous the middle zone continues southeastward as a rather straight high velocity zone towards sumatra where it becomes convex southward and parallel to the subduction zone under the sunda arc comparison of this straight middle zone near india with the shallower upper 600 1000 km northern zone which displays a cusp like shape near the yunnan sw china syntaxis of the eastern himalayas supports the notion that the shallow northern zone represents later subduction than the deeper middle zone the suggestion of a counterclockwise rotation 20 of the indian plate during tertiary indentation of asia is supported by these features the present day latitudes of 5 35 n of the deep slabs under india and adjacent areas correspond to the approximate paleolatitudes of the cretaceous subduction zones the slab remnants in the middle mantle occur therefore near the ancient locations where they started their downward journey which implies that lateral movements in the deeper mantle were not large

quantifying the net slab pull force as a driving mechanism for plate tectonics 1 it has remained unclear how much of the negative buoyancy force of the slab fb is used to pull the trailing plate at the surface into the mantle here i present three dimensional laboratory experiments to quantify the net slab pull force fnsp with respect to fb during subduction results show that fnsp increases with increasing slab length and dip up to 8 12 of fb making fnsp up to twice as large as the ridge push force the remainder of fb is primarily used to drive rollback induced mantle flow 70 to bend the subducting plate at the trench 15 30 and to overcome shear resistance between slab and mantle 0 8

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

an experimental study of subduction and slab migration results of fluid dynamical experiments on the behavior of subducted slabs are presented that address two important characteristics of subduction slab penetration through the transition zone and horizontal slab migration cold negatively buoyant molded slabs of concentrated sucrose solution with viscosities of 3 5 106 p were introduced into a more dilute two layered sucrose solution representing the upper and lower mantle the transition zone was modeled by a step increase in both density and viscosity ratios of slab thickness to upper layer depth the relative viscosities in each layer and the effective rayleigh number were close to mantle values the initial configuration consisted of two plates separated by a trench gap with one plate attached to a dipping slab simulating a developing subduction zone with an overriding plate two different boundary conditions were used at the trailing ridge end of the subducting plate 1 fixed end corresponding to zero ridge motion and 2 free end corresponding to zero ridge resistance based on results from 15 experiments with various combinations of upper layer lower layer and slab densities u l s and viscosities we find that slab penetration depth is sensitive to both the normalized slab density anomaly r s l s u and the dip angle three penetration regimes are recognized r 0 5 in which the slab sinks into the lower layer without distortion 0 2 r 0 5 which permits limited penetration the amount dependent on dip angle in this regime the slab develops a thick root just beneath the transition zone and r 0 2 in which the slab is deflected by the transition and little or no penetration occurs retrograde slab motion and trench migration occurred in nearly every case with the overriding plate moving parallel to but faster than the trench causing the gap between subducting and overriding plates to close with time prograde trench motion was never observed and the trench remained stationary only in the extreme case free ridge condition with equal upper and lower layer densities these results suggest that the transient extension found in many back arc basins may be a direct consequence of the tendency for retrograde subduction and the amount of extension may be governed in part by the degree of slab penetration through the transition zone

cenozoic global plate motions cenozoic global plate motions relative to the hot spots are investigated and compared to plate motions in a mean lithosphere reference frame plate motions were analyzed over six time intervals divided by ages 10 25 43 48 and 56 ma chosen as much as possible to coincide with key plate reorganizations alternative motion circuits and rotational parameters were considered and evaluated with paleomagnetic data from the pacific and north american plates the circuit found to be in best agreement with the paleomagnetic data is one in which the hot spots in the atlantic region are assumed to be fixed relative to the hot spots in the pacific region throughout the cenozoic the hot spot and mean lithosphere reference frames have been in continual slow relative motion the rate of motion is nonuniform however most of the motion having occurred during the middle cenozoic the net cenozoic rotation of the lithosphere relative to the hot spots is described by a right handed rotation of 7 about a euler pole at 46 s 87 e which yields a 5 displacement of the north poles of the two reference frames this motion is small enough that inferences drawn about plate speeds in one reference frame should be valid in the other analysis of the global motions resulting from our preferred model showed that many characteristics of current plate motions have persisted throughout the cenozoic plate speeds correlate with latitude plates moving faster near the equator than near the poles throughout the cenozoic as at present continental plates except for the indian plate moved slower than oceanic plates throughout the cenozoic even the structure of the velocity fields as revealed in a contour of root mean square velocities in equatorial bands persists throughout the cenozoic the migration of the paleomagnetic axis over time is also compared to the hot spot and mean lithosphere reference frames the paleomagnetic axis has shifted 5 10 relative to the hot spot frame and a lesser amount relative to the mean lithosphere frame

global paleo lithospheric models for geodynamical analysis of plate reconstructions we present a general framework to generate time dependent global subduction history models from kinematic plate reconstructions and explore their associated coupled plate mantle dynamic behaviour slabs are constructed by advecting material into the mantle by prescribing its radial velocity and following the absolute tangential motion of the subducting plate a simple geodynamic scenario where plates and slabs define isopycnic and isoviscous regions in an homogeneous or layered mantle was explored using the boundary element method based software bemearth the resulting dynamic behaviour was used to predict the absolute plate motion directions for the present day and a particular mid cretaceous 125 ma kinematic model we show how the methodology can be used to compare and revise kinematic reconstructions based on their effect on the balance of plate driving forces and the resulting euler poles of subducting plates as an example we compare the farallon plate dynamics at 125 ma in a global model with two reconstructions in the context of the evolution of the western north american cordillera our results suggest a method to identify episodes of absolute plate motions that are inconsistent with the expected plate dynamics

multiscale dynamics of the tonga kermadec subduction zone our understanding of mantle convection and the motion of plates depends intimately on our understanding of the viscosity structure of the mantle while geoid and gravity observations have provided fundamental constraints on the radial viscosity structure of the mantle the influence of short wavelength variations in viscosity is still poorly understood we present 2 d and 3 d finite element models of mantle flow including strong lateral viscosity variations and local sources of buoyancy owing to both thermal and compositional effects we first use generic 2 d models of a subduction zone to investigate how different observations depend on various aspects of the viscosity structure in particular the slab and lower mantle viscosity and the presence of a low viscosity region in the mantle wedge above the slab we find that 1 the strain rate provides a strong constraint on the absolute viscosity of the slab 10 23 pa s 2 stress orientation within the slab is sensitive to the relative viscosity of the slab lower mantle and the wedge and 3 the stress state and topography of the overriding plate depend on the wedge viscosity and local sources of buoyancy in particular the state of stress in the overriding plate changes from compression to extension with the addition of a low viscosity wedge we then use observations of strain rate stress orientation dynamic topography and the geoid for the tonga kermadec subduction zone as simultaneous constraints on the viscosity and buoyancy in a 3 d regional dynamic model together these observations are used to develop a self consistent model of the viscosity and buoyancy by taking advantage of the sensitivity of each observation to different aspects of the dynamics over a broad range of length scales the presence of a low viscosity wedge makes it possible to match observations of shallow dynamic topography and horizontal extension within the backarc and down dip compression in the shallow portion of the slab these results suggest that a low viscosity wedge plays an important role in controlling the presence of backarc spreading however for a model with a low viscosity and low density region that provides a good fit to the observed topography we find that a reduction of the slab density by a factor of 1 3 relative to the reference density model is required to match the observed geoid these results suggest that compensation of the slab by dynamic topography may be a much smaller effect at short to intermediate wavelengths than predicted by long wavelength modelling of the geoid

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

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

neogene and quaternary rollback evolution of the tyrrhenian sea the apennines and the sicilian maghrebides reconstruction of the evolution of the tyrrhenian sea shows that the major stage of rifting associated with the opening of this basin began at similar to10 ma it involved two episodes of back arc extension which were induced by the rollback of a west dipping subducting slab the first period of extension 10 6 ma was prominent in the northern tyrrhenian sea and in the western part of the southern tyrrhenian sea the second period of extension mainly affected the southern tyrrhenian sea began in the latest messinian 6 5 ma and has been accompanied by subduction rollback at rates of 60 100 km myr 1 slab reconstruction combined with paleomagnetic and paleogeographic constraints indicates that in the central apennines the latest messinian 6 5 ma arrival of a carbonate platform at the subduction zone impeded subduction and initiated a slab tear and major strike slip faults these processes resulted in the formation of a narrow subducting slab beneath the ionian sea that has undergone faster subduction rollback and induced extreme rates of back arc extension

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

tomographic constraints on the geodynamic evolution of the italian region in this paper we present p wave tomographic images of the mantle beneath italy obtained by inverting 6000 teleseismic p and pkp wave arrival times accurately repicked recorded in the time period 1988 1994 by the stations of the national seismic network of the istituto nazionale di geofisica we pay great attention in the data selection and picking procedure of seismic phases to obtain a very high quality data set the data were inverted with the well established aki christofferson husebye tomographic technique different reference models and residuals computation have been tried to verify the stability of the results the high quality of the repicked arrival times allows us to enhance the definition of the deep structures beneath both the alps and the apennines looking for their lateral and vertical continuity down to 800 km depth the main finding of this study is a continuous high velocity body located between 250 and 670 km depth beneath the entire apenninic system dipping toward the tyrrhenian area which continues upward segmented in two main anomalies in the northern apenninic and the calabrian arcs we interpret this high velocity feature as the subducted oceanic lithosphere between the eurasian and african plates dipping down to the upper lower mantle boundary beneath the tyrrhenian sea the retrieved images of the lithosphere subducting beneath apennines are reliable in terms of thickness about 80 90 km and p wave velocity contrast 2 4 higher than the normal mantle furthermore our tomographic images which focus on the deep geometry and continuity of the velocity structures provide new keys to understanding the geodynamic evolution of the italian region the segmentation of the high velocity slab upward suggests a complex evolution of the arc trench system and the initially continuous subduction of the ionian adriatic plate progressively developed in subordinate arcs probably due to lateral heterogeneity of the subducting lithosphere

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

the role of the east asian active margin in widespread extensional and strike slip deformation in east asia east asia is a region of widespread deformation dominated by normal and strike slip faults deformation has been interpreted to result from extrusion tectonics related to the india eurasia collision which started in the early eocene in east and se china however deformation started earlier than the collision latest cretaceous to palaeocene suggesting that extrusion tectonics is not the only driving mechanism for east asia deformation it is suggested that the east asian active margin has influenced deformation in east asia significantly along the margin cenozoic back arc extension took place behind several adjoining arcs implying eastward rollback of the subducting slab and collapse of the overriding plate towards the retreating hinge line we show that extension took place along a c 7400 km long stretch of the east asian margin during most of the cenozoic physical models are presented simulating overriding plate collapse and back arc extension the models reproduce important aspects of the strain field in east asia for geometrical and rheological conditions scaled to represent east asia modelling shows that the active margin can be held responsible for deformation in east asia as far west as the baikal rift zone located c 3300 km from the margin

the influence of rheological weakening and yield stress on the interaction of slabs with the 670 km discontinuity results of high resolution seismic tomography showing subducting slabs deflected in the transition zone and thickened in the lower mantle seem to call for slab material weaker than inferred from mineral physics deformation mechanisms a possible mechanism suggested by several authors could be the weakening due to grain size reduction which should occur in the cold portion of fast slabs after an exothermic phase transition at a depth of 400 km since the amount of weakening as well as the rate of subsequent strengthening due to the grain growth are not precisely known we present here a parametric study of slab behavior in the transition zone and upper part of the lower mantle we simulate a subducting slab in a two dimensional 2 d cartesian box in the numerical model with composite rheology including diffusion creep dislocation creep and a general stress limiting rheology approximating peierl s creep we concentrate on two rheologic effects the dynamic effect of slab weakening due to grain size reduction at the phase boundary and the effect of yield stress of stress limiting rheology the effect of trench migration on slab deformation is also included in our study results show that the slab ability to penetrate into the lower mantle is not significantly affected by a trench retreat in the absence of grain size weakening however in case of a 4 cm yr trench retreat grain size weakening provides a viable mechanism to deflect the slab in the transition zone provided that stress limiting deformation mechanism would limit the effective viscosity outside the areas of grain size weakening to about 10 24 pa s 2002 elsevier science b v all rights reserved

travel time tomography of the european mediterranean mantle down to 1400 km abstract the 3 d p wave velocity structure of the mantle below europe the mediterranean region and a part of asia minor is investigated this study is a considerable extension of an earlier tomographic experiment that was limited to imaging upper mantle structure only here the earth s volume under study encompasses the mantle to a depth of 1400 km and we increase the number of international seismological centre isc data for inversion by a factor of four by taking more years of observation and by including data from teleseismic events the most important departure from the earlier study is that we do not use the jeffreys bullen model as a reference model but an improved radially symmetric velocity model the pm2 model which is appropriate for the european mediterranean mantle our inversion procedure consists of two steps first the radial model pm2 is determined from the isc delay times by a nonlinear trial and error inversion of the data as opposed to the jeffreys bullen model the new reference model has a high velocity lithosphere a low velocity zone and seismic discontinuities at depths of 400 and 670 km next the isc data are corrected for effects related to the change in reference model and inverted for 3 d heterogeneity relative to the pm2 model we follow this two step approach to attain a better linearizable tomographic problem in which ray paths computed in the pm2 model provide a better approximation of the actual ray paths than those computed from the jeffreys bullen model hence the two step scheme leads to a more credible application of fermat s principle in linearizing the tomographic equations inversion results for the 3 d heterogeneity are computed for both the uncorrected isc data and for the pm2 data the data fit obtained in the two step approach is slightly better than in the inversion of isc data using the jeffreys bullen reference model a comparison of the tomographic results demonstrates that the pm2 data inversion is to be preferred to assess the spatial resolution an analysis is given of hit count patterns sampling of the mantle by ray paths and results of sensitivity tests with 3 d synthetic velocity models the spatial resolution obtained varies with position in the mantle and is studied both in map view and in cross section in the well sampled regions of the mantle the spatial resolution for larger scale structure can qualitatively be denoted as reasonable to good and at least sufficient to allow interpretation of larger scale anomalies a comparison is made of the results of this study with independent models of s velocity heterogeneity obtained in a number of investigations and with a prediction of the seismic velocity structure of the mantle computed from tectonic reconstructions of the mediterranean region in the context of this comparison interpretations of large scale positive anomalies found in the mediterranean upper mantle in terms of subducted lithosphere are given specifically addressed are subduction below southern spain below the western mediterranean and italy and below the aegean in the last region a slab anomaly is mapped down to depths of 800 km