reconstructing subducted oceanic lithosphere by reverse engineering slab geometries the northern philippine sea plate subducting slabs commonly acquire complex geometries from the migration of subduction trenches slab mantle interaction slab tearing and collision of slabs at depth although it is possible to construct three dimensional models of subducted slabs using earthquake hypocenter locations and tomographic models it is often not possible to rigorously test their accuracy here we present a methodology for performing such a test by reverse engineering the presubduction configuration of a slab of oceanic lithosphere from interpretations of its present day morphology we illustrate our approach for the ryukyu and shikoku slabs northwest philippine sea having simulated them as viscoelastic sheets that we unfolded and floated to the surface the net strain distribution of the floated mesh indicated which parts of the original slab model were geometrically viable minimal net strain and which parts of the mesh required additional tears and or zones of localized ductile extension to have enabled the slab to deform during subduction in the instance of the ryukyu and shikoku slabs the palau kyushu and gagua ridges are shown to have both acted as planes of weakness that broke into major vertical slab tears these subducted ridges are connected by a trench parallel tear that represented the former contact between the huatung and west philippine basins the fossil spreading center of the shikoku basin formed a separate slab window upon subduction along the nankai trough the methodology presented herein is a powerful tool to evaluate complex slab morphologies infer the locations of slab tears and therefore reconstruct intricate configurations of subducted oceanic lithosphere

controls on back arc basin formation the relationship between subduction and back arc spreading has been well known since the early days of plate tectonics however the reasons why back arc basins are associated with some subduction systems but not all has remained elusive we examine the kinematic controls on subduction and back arc basins for both the present day and cenozoic to differentiate between the major competing hypotheses for back arc basin formation and to explain their temporal and spatial distribution our new data set of subduction and back arc basin parameters uses a new set of paleo oceanic age grids muller et al 2005 associated with a moving atlantic indian ocean hot spot reference frame o neill et al 2005 the plate model includes detailed reconstructed spreading histories of back arc basins based on marine geophysical and satellite gravity data our combined rotation and oceanic paleo age model provides the age distribution of subducting lithosphere through space and time convergence rates and the absolute motion of the downgoing and overriding plates we find that back arc basins develop when the age of subducting normal oceanic lithosphere is greater than 55 million years additionally we establish an age dip relationship showing that the intermediate dip angle of the subducting slab is always greater than 30 with back arc spreading our results suggest that back arc basin formation is always preceded by an absolute motion of the overriding plate away from the subduction hinge thereby creating accommodation space between the overriding and subducting plates once back arc extension is established it continues regardless of overriding plate motion indicating back arc spreading is not a simple consequence of overriding plate behaviour the landward migration of the overriding plate as a precursor to back arc extension may indicate that extension on the overriding plate is influenced by the oceanward lateral flow of the mantle however once back arc extension is established rollback of the subduction hinge appears to be the primary force responsible for the continued creation of accommodation space our analysis indicates the driving mechanism for back arc extension is a combination of surface kinematics properties of the downgoing slab the effect of lateral mantle flow on the slab and mantle wedge dynamics

constraints on the lateral strength of slabs from three dimensional dynamic flow models if the viscosity of subducted lithosphere is described purely by temperature and pressure it should remain considerably more viscous than the surrounding material as it descends through the upper mantle many dynamic models of the long wavelength geoid associated with slabs require an increase in viscosity from the upper to the lower mantle but assume the perturbation to the lateral viscosity structure arising from the slabs themselves can be ignored previous studies indicate that strong and localized viscosity variations should have a dramatic influence on the geoid we present 3d finite element models of the regional geoid of the western pacific subduction zones slab buoyancies and viscosities are defined using the distribution of seismicity the geoid is very sensitive to the lateral strength of the slab very viscous slabs penetrating a low viscosity mantle generate significant 10 50 m long wavelength geoid lows opposite to the geoid high which is observed over slabs to obtain a geoid high comparable to that observed the lower mantle viscosity must be 60 200 times greater than the upper mantle viscosity and the slab must be in contact with the lower mantle these strict requirements suggest that slabs have been weakened and cannot act as stress guides from the deep mantle to the surface

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

central japan triple junction revisited we show that the two dimensional kinematic discussion of mckenzie and morgan s ttt type triple junction is not sufficient as it ignores the interaction of the sinking plates we derive a relation for the three dimensional existence of ttt triple junctions and show that it is satisfied at the unstable central japan triple junction we then use the geometry of the subducting pacific plate beneath the philippine sea and eurasian plates to show that the present situation did not hold before about 3 ma before that time the triple junction was probably located north of the present one and very close to the pole of relative motion finally we discuss the geological implications of this model

an expression of philippine sea plate rotation the parece vela and shikoku basins abstract the philippine sea plate located between the pacific eurasian and australian plates is the world s largest marginal basin plate the motion of the philippine sea plate through time is poorly understood as it is almost entirely surrounded by subduction zones and hence previous studies have relied on palaeomagnetic analysis to constrain its rotation we present a comprehensive analysis of geophysical data within the parece vela and shikoku basins two oligocene to miocene back arc basins which provide independent constraints on the rotational history of the philippine sea plate by means of their seafloor spreading record we have created a detailed plate model for the opening of the parece vela and shikoku basins based on an analysis of all available magnetic gravity and bathymetric data in the region subduction along the izu bonin mariana trench led to trench roll back arc rupture and back arc rifting in the parece vela and shikoku basins at 30 ma seafloor spreading in both basins developed by chron 9o 28 ma and possibly by chron 10o 29 ma as a northward and southward propagating rift respectively the spreading orientation in the parece vela basin was e w as opposed to ene wsw in the shikoku basin the spreading ridges joined by chron 6by 23 ma and formed a r r r triple junction to accommodate the difference in spreading orientations in both basins at chron 6no 20 ma the spreading direction in the parece vela basin changed from e w to ne sw at chron 5ey 19 ma the spreading direction in the shikoku basin changed from ene wsw to ne sw this change was accompanied by a marked decrease in spreading rate cessation of back arc opening occurred at 15 ma a time of regional plate reorganisation in se asia we interpret the dramatic change in spreading rate and direction from e w to ne sw at 20 1 3 ma as an expression of philippine sea plate rotation and is constrained by the spacing between our magnetic anomaly identifications and the curvature of the fracture zones this rotation was previously thought to have begun at 25 ma as a result of a global change in plate motions our results suggest that the philippine sea plate rotated clockwise by about 4 between 20 and 15 ma about a pole located 35 n 84 e this implies that the majority of the 34 clockwise rotation inferred to have occurred between 25 and 5 ma from paleomagnetic data may have in fact been confined to the period between 15 and 5 ma

potential field imaging of palaeozoic orogenic structure in northern and central europe abstract the trans european suture zone tesz is the most fundamental lithospheric boundary in europe separating the ancient crust of the fennoscandian shield east european craton from the younger crust of central europe and extending deep into the mantle geophysical potential field images provide an overview of the entire palaeozoic orogenic system of northern and central europe for the first time the tesz is largely concealed by sedimentary basins of permian cenozoic age geological observations are largely restricted to local basement highs and deep boreholes and the coverage of deep seismic surveys is widely spaced despite experiments recently acquired within the europrobe programme by contrast the potential field data offer a relatively detailed coverage of standardised observations throughout the tesz while some features of the images may be sourced in the near surface particularly in the gravity image much of their content reflects the structure of the underlying palaeozoic basement at the scale presented the images highlight the most fundamental features of the crustal structure of the tesz these include the strong contrast between the highly magnetic crust of the east european craton and the less magnetic palaeozoic accreted terranes of central europe the lateral continuity of terranes and their internal structure particularly where arc magmatic complexes are involved and the location and geometry of the terrane boundaries oceanic sutures and strike slip zones that separate them

a new model of slab tear of the subducting philippine sea plate associated with kyushu palau ridge subduction we suggest that the kyushu palau ridge kpr plays a key role in the subduction process of the philippine sea plate psp and the origin of the abu volcano in the southwestern japan the 3 d p wave velocity structure was imaged to approximately 300 km beneath the abu volcano using a large number of p wave arrivals from local earthquakes and teleseismic events our results indicate that a high velocity anomaly beneath the abu volcano is associated with the subducting psp however the anomaly is not continuous being interrupted apparently by a low velocity anomaly zone extending northwestwards from 80 km to great depth the psp appears to be tearing and then forms a slab window corresponding to kpr subduction at ca 2 ma the low velocity anomaly may indicate hot upper mantle material rising through the slab window and causing partial melting both of the lower crust of the overriding plate and the oceanic crust of the subducted kpr a new model is presented for slab tearing of the psp associated with the subduction of the buoyant wide and thick kpr and directional change in the motion of the plate contributing to better understanding of the abu volcanism c 2014 elsevier b v all rights reserved

postmagmatic tectonic evolution of the outer izu bonin forearc revealed by sediment basin structure and vein microstructure analysis implications for a 15 ma hiatus between pacific plate subduction initiation and forearc extension international ocean discovery program expedition 352 recovered sedimentary volcaniclastic successions and extensional structures faults and extensional veins that allow the reconstruction of the izu bonin forearc tectonic evolution using a combination of shipboard core data seismic reflection images and calcite vein microstructure analysis the oldest recorded biostratigraphic ages within fault bounded sedimentary basins late eocene to early oligocene imply a 15 ma hiatus between the formation of the igneous basement 52 to 50 ma and the onset of sedimentation at the upslope sites u1439 and u1442 extension led to the formation of asymmetric basins reflecting regional stretch of 16 19 at strain rates of 1 58 10 16 to 4 62 10 16 s 1 downslope site u1440 closer to the trench is characterized by a symmetric graben bounded by conjugate normal faults reflecting regional stretch of 55 at strain rates of 4 40 10 16 to 1 43 10 15 s 1 mean differential stresses are in the range of 70 90 mpa we infer that upper plate extension was triggered by incipient pacific plate rollback 15 ma after subduction initiation extension was accommodated by normal faulting with syntectonic sedimentation during late eocene to early oligocene times backarc extension was assisted by magmatism with related shikoku and parece vela basin spreading at 25 ma so that parts of the arc and rear arc and the west philippine backarc basin were dismembered from the forearc this was followed by slow rift to postrift sedimentation during the transition from forearc to arc rifting to spreading within the shikoku parece vela basin system

a regionalized upper mantle rum seismic model seismic velocity heterogeneity in the earth s mantle is strongly concentrated near its top the shallow heterogeneity of the mantle correlates strongly with surface tectonics we use these observations as constraints of a tomographic experiment aimed at building a regionalized upper mantle rum reference model we use a select set of teleseismic travel times to minimize the mapping of mislocation into structure the data selection emphasizes the robustness of individual picks the form of the rum model is a set of velocity profiles as functions of depth through the upper mantle for each of the different tectonic provinces of earth together the profiles constitute a three dimensional model which incorporates considerable structural detail but is described by only 90 parameters and has only about 22 degrees of freedom this is achieved by irregularly sampling a detailed regionalization of the globe by detailed mapping of subducted lithosphere in the mantle as defined by seismicity and by combining these structures in an irregular grid in which bookkeeping is efficiently handled the resulting rum model includes subducting slabs as sharp fast features in the upper mantle old continents are fast young oceans are slow models have been derived for both compressional and shear velocity the rum model is designed to represent as much of upper mantle heterogeneity as seen by body wave travel times as possible with a simple model it can be useful as a reference model for individual tectonic regions travel times are efficiently generated for the rum model mislocations of explosions of known location are significantly reduced when corrections for the rum model are applied to travel time residuals for a spherically symmetrical earth model

divergent behavior of the swash zone in response to different foreshore slopes and nearshore states abstract ultrasonic distance sensors were used to measure changes in bed elevation to a resolution of 0 1 mm following individual swash excursions in the high tide swash zone of an intermediate foreshore changes in bed elevation are related to depth averaged eularian swash velocities estimated by assuming mass balance of the swash discharge also measured using the ultrasonic distance sensors early in the study may 21 25 2008 the alongshore uniform foreshore had a cross shore slope of 0 18 and was fronted by a nearshore bar that forced wave breaking at low tide towards the end of the study june 5 9 2008 the foreshore had a slope of 0 12 and was fronted by a nearshore terrace that developed as the innermost bar migrated landward despite similar inshore wave heights and periods during these times the steeper foreshore eroded by 0 69 m 3 m 1 while the more gently sloped foreshore accreted by 0 21 m 3 m 1 the divergent behavior of the swash zone is attributed to the welding of the innermost bar to the foreshore creating a shallow nearshore terrace that saturated the inshore wave field this in turn led to a faster uprush velocity that promoted the landward advection of sediment picked up over the shallow terrace it is argued that accretion in the upper swash zone was the result of a larger supply of sediment at the base of the foreshore and a transformation of the wave field by the shallower depths of the welded feature in this respect swash hydrodynamics and morphological change can only be made if the swash zone is considered within the context of the broader nearshore state

subduction zone interaction controls on arcuate belts the interaction of neighboring subducting lithospheres is a characteristic feature of many tectonically complex areas here we use numerical modeling to study the interactions between two oppositely dipping adjacent subducting lithospheres and to understand their impact on the subduction evolution mantle flow and stress propagation through the mantle as slabs subduct rollback and approach they strongly affect each other if plate edges are at distances 600 km in this case the mantle flow around slabs combines into a single large convective cell trench migration is delayed and stress increases progressively with decreasing slabs distance the stress increases at depth as the slabs approach each other and it shallows up to the near surface as the slabs diverge a similar setting with two neighboring slabs migrating and passing each other is found in areas such as the alps apennines europe and the ryukyu manila subduction zones around taiwan where the arcuate trenches suggest the controls of deep slab interactions

seismic anisotropy in the izu bonin subduction system 1 we evaluated shear wave splitting in local s and teleseismic ss phases from earthquakes that sample the izu bonin subduction system backarc to examine the dynamics of slab deformation and mantle flow splitting times for local s phases range from 0 6 to 1 7 s and 0 9 to 3 2 s for teleseismic ss phases fast polarization directions range from nw se to ene wsw and suggest complex mantle deformation across the region mantle flow induced by both plate motion and trench parallel flow likely combine with fossil slab fabric to control the shear wave splitting observations in the southern portion of izu bonin strong variations in fast polarization directions beneath northern izu bonin suggest a diversion to trench parallel mantle flow behind the philippine sea plate this study demonstrates that many regions of subduction previously inaccessible by conventional shear wave splitting techniques may now be feasibly examined through the use of new complementary shear wave splitting data sets

imaging changes in morphology geometry and physical properties of the subducting pacific plate along the izu bonin mariana arc abstract recent studies on slab structure in the northwest pacific have imaged variable geometries of the subducted oceanic lithosphere along the plate margin tomographic images show portions of the subducted pacific plate penetrate vertically into the lower mantle below the mariana arc whereas below the izu bonin arc the slab appears to be lain down horizontally on top of the 670 km discontinuity using new technology to grid and visualize regional seismic tomography data even more detail about the structure of the subducting plates can be extracted from the three dimensional images we investigate the morphology and geometry of the subducting pacific plate as it changes between horizontal to nearly vertical using p wave tomographic images the most profound result from these new models is the presence of a distinct change of seismic property in the downgoing pacific slab beneath the izu bonin arc at a depth of 350 400 km the position of this anomaly corresponds to a region north and west of the ogasawara plateau we propose the change in morphology and physical property of the slab is related to the distortion of the pacific plate geometry as its shape converts from near horizontal to vertical together with the subduction of the ogasawara plateau

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

seismotectonics beneath the tokyo metropolitan area japan effect of slab slab contact and overlap on seismicity 1 we first determine the configuration of the upper surface of the pacific pac slab beneath kanto japan from the distribution of interplate earthquakes relocated by an appropriate 1 d velocity model then traveltime tomography is carried out to estimate three dimensional seismic velocity structures around kanto using 735 520 p wave and 444 049 s wave arrival times from 6508 local earthquakes the obtained results suggest that the philippine sea phs slab is subducting to depths of 130 140 km without a gap even to the northwest of the izu collision zone we subsequently define the lateral extent of the contact zone between the bottom of the phs slab and the upper surface of the pac slab phs pac interface and reveal that the slab contact zone underlies a wider area beneath kanto in harmony with the kanto plain the downdip limit of interplate thrust type earthquakes on the pac slab is deepened by 30 km locally under the slab contact zone this deepening is probably caused by a lower temperature environment in the pac slab resulting from the overlap with the phs slab subducting above and consequent thermal shielding by the phs slab from the hot mantle wedge we detect an extremely low velocity anomaly in the easternmost portion of the phs slab which is probably attributable to serpentinization of mantle peridotite interplate earthquakes are almost absent along the phs pac interface overlain by the serpentinized mantle in the phs slab suggesting that ductile deformation takes place along the interface because of low viscosity of the serpentine

mountain building in taiwan insights from 3 d geodynamic models taiwan is widely considered to be a typical example of an arc continent collision surrounded by two opposite dipping subduction zones the manner by which the interaction of the two neighboring slabs caused plate collision and mountain building is insufficiently understood various hypotheses have been proposed but the geodynamic feasibility of those remains to be tested here we present 3 d thermomechanical models to study the geodynamic evolution process of a taiwan like setting after an initial transform fault was consumed in our model setup the boundary between the eurasian plate and the south china sea is northeast trending the results show that all simulations result in toroidal mantle flow around the slab edges and that slab breakoff as well as a small scale mountain belt with high topography and crustal exhumation occurs in most cases the eurasian continental crust is exhumed in a dome like manner exposing higher grade metamorphic rocks facilitated by high erosion rates and a weak continental lower crust rheology but inhibited by the presence of a weak arc a high topography within the orogen as well as continental slab detachment can develop for the convergence direction of n307 degrees and large convergence rates our modeling results are thus generally consistent with the eurasian slab tearing model proposed for taiwan based on seismic tomographic studies and we suggest that the main characteristic features in taiwan can be explained by the combined effects of fast erosion a weak lower crust fast convergence and a small convergence azimuth

predicting trench and plate motion from the dynamics of a strong slab the motion of oceanic plates is commonly related to the subduction of cold and dense oceanic material into the mantle these models predict plate velocities from subduction velocities but the trench motion is not directly included in the computation here using a recent compilation of a global data set we found that the motion of trenches either advancing or retreating with respect to upper plates scales with their corresponding subducting plates motion based on simple experimental tests we found that subduction of strong slabs inside the upper mantle correctly predicts these kinematic relationships we deduce that the motion of the trenches represent the surface manifestation of the resistance encountered by the subducting lithosphere to bend and penetrate within the upper mantle

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

age of the subducting pacific slab beneath east asia and its geodynamic implications abstract we study the age of the subducting pacific slab beneath east asia using a high resolution model of p wave tomography and paleo age data of ancient seafloor our results show that the lithosphere age of the subducting slab becomes younger from the japan trench 130 ma to the slab s western edge 90 ma beneath east china and the flat stagnant slab in the mantle transition zone mtz is the subducted pacific plate rather than the proposed izanagi plate which should have already collapsed into the lower mantle the flat pacific slab has been in the mtz for no more than 10 20 million years considerably less than the age of the big mantle wedge beneath east asia 110 million years hence the present flat pacific slab in the mtz has contributed to the cenozoic destruction of the east asian continental lithosphere with extensive intraplate volcanism and back arc spreading whereas the destruction of the north china craton during the early cretaceous 140 110 ma was caused by the subduction of the izanagi or the paleo pacific plate

slab interactions in 3 d subduction settings the philippine sea plate region abstract the importance of slab slab interactions is manifested in the kinematics and geometry of the philippine sea plate and western pacific subduction zones and such interactions offer a dynamic basis for the first order observations in this complex subduction setting the westward subduction of the pacific sea plate changes along strike from single slab subduction beneath japan to a double subduction setting where pacific subduction beneath the philippine sea plate occurs in tandem with westward subduction of the philippine sea plate beneath eurasia our 3 d numerical models show that there are fundamental differences between single slab systems and double slab systems where both subduction systems have the same vergence we find that the observed kinematics and slab geometry of the pacific philippine subduction can be understood by considering an along strike transition from single to double subduction and is largely independent from the detailed geometry of the philippine sea plate important first order features include the relatively shallow slab dip retreating stationary trenches and rapid subduction for single slab systems pacific plate subducting under japan and front slabs within a double slab system philippine sea plate subducting at ryukyu in contrast steep to overturned slab dips advancing trench motion and slower subduction occurs for rear slabs in a double slab setting pacific subducting at the izu bonin mariana this happens because of a relative build up of pressure in the asthenosphere beneath the philippine sea plate where the asthenosphere is constrained between the converging ryukyu and izu bonin mariana slabs when weak back arc regions are included slab slab convergence rates slow and the middle philippine plate extends which leads to reduced pressure build up and reduced slab slab coupling models without back arcs or with back arc viscosities that are reduced by a factor of five produce kinematics compatible with present day observations

aseismic deep subduction of the philippine sea plate and slab window abstract we have made great efforts to collect and combine a large number of high quality data from local earthquakes and teleseismic events recorded by the dense seismic networks in both south korea and west japan this is the first time that a large number of korean and japanese seismic data sets are analyzed jointly as a result a high resolution 3 d p wave velocity model down to 700 km depth is determined which clearly shows that the philippine sea phs plate has subducted aseismically down to 460 km depth under the japan sea tsushima strait and east china sea the aseismic phs slab is visible in two areas one is under the japan sea off western honshu and the other is under east china sea off western kyushu however the aseismic phs slab is not visible between the two areas where a slab window has formed the slab window is located beneath the center of the present study region where many teleseismic rays crisscross detailed synthetic tests were conducted which indicate that both the aseismic phs slab and the slab window are robust features using the teleseismic data recorded by the japanese stations alone the aseismic phs slab and the slab window were also revealed zhao et al 2012 though the ray paths in the japanese data set crisscross less well offshore the slab window may be caused by the subducted kyushu palau ridge and kinan seamount chain where the phs slab may be segmented hot mantle upwelling is revealed in the big mantle wedge above the pacific slab under the present study region which may have facilitated the formation of the phs slab window these novel findings may shed new light on the subduction history of the phs plate and the dynamic evolution of the japan subduction zone

structural development of sumisu rift izu bonin arc geophysical swath mapping multichannel seismic profiling and ocean drilling data are used to document the structural evolution of sumisu rift and to analyze the pattern of strain resulting from extension of an intraoceanic island arc the 120 km long 30 50 km wide sumisu rift is bounded to the north and south by structural and volcanic highs west of the sumisu and torishima calderas and longitudinally by curvilinear border fault zones with both convex and concave dip slopes the zig zag pattern of normal faults average strikes 337 and 355 indicates extension oriented 076 10 orthogonal to the volcanic arc three oblique transfer zones divide the rift along strike into four segments with different fault trends and uplift subsidence patterns differential strain across the transfer zones is accommodated by interdigitating rift parallel faults and sometimes by cross rift volcanism rather than by strike or oblique slip faults from estimates of extension 2 5 km the age of the rift 2 ma and the accelerating subsidence we infer that sumisu rift is in the early synrift stage of back arc basin formation following an early sag phase half graben formed with synthetically faulted structural rollovers facing large offset 2 2 5 km throw border fault zones in the three northern rift segments the largest faults are on the arc side and dip 60 75 w whereas in the southern segment they are on the west side and dip 25 50 e the present full graben stage is dominated by hanging wall antithetic faulting basin widening by footwall collapse and a concentration of subsidence in an inner rift the hanging wall collapses but not necessarily as a result of border fault propagation from adjacent rift segments whereas the border faults may penetrate the theologically weak lithosphere te 3 km many of the hanging wall and footwall collapse structures are detached only a few kilometers below the seafloor back arc volcanism usually erupted along faults occurs in the rift and along the protoremnant arc during both stages where drilled the arc margin has been uplifted 1 1 0 5 km concurrently with 1 1 km of rift basin subsidence extremely high sedimentation rates up to 6 m kyr in the inner rift have kept pace with synrift faulting created a smooth basin floor and resulted in sediment thicknesses that mimic the differential basin subsidence a linear zone of weakness caused by the greater temperatures and crustal thickness along the arc volcanic line controls the initial locus of rifting rifts are better developed between the arc edifices intrusions may be accommodating extensional strain adjacent to the arc volcanoes no obvious correlations are observed between the rift structures and preexisting cross arc trends

philippine sea and east asian plate tectonics since 52 ma constrained by new subducted slab reconstruction methods we reconstructed philippine sea and east asian plate tectonics since 52 ma from 28 slabs mapped in 3 d from global tomography with a subducted area of 25 of present day global oceanic lithosphere slab constraints include subducted parts of existing pacific indian and philippine sea oceans plus wholly subducted proto south china sea and newly discovered east asian sea mapped slabs were unfolded and restored to the earth surface using three methodologies and input to globally consistent plate reconstructions important constraints include the following 1 the ryukyu slab is 1000 km n s too short to account for 20 philippine sea northward motion from paleolatitudes 2 the marianas pacific subduction zone was at its present location 200 km since 48 10 ma based on a 1000 km deep slab wall 3 the 8000 2500 km east asian sea existed between the pacific and indian oceans at 52 ma based on lower mantle flat slabs 4 the caroline back arc basin moved with the pacific based on the overlapping coeval caroline hot spot track these new constraints allow two classes of philippine sea plate models which we compared to paleomagnetic and geologic data our preferred model involves philippine sea nucleation above the manus plume 0 150 e near the pacific east asian sea plate boundary large philippine sea westward motion and post 40 ma maximum 80 clockwise rotation accompanied late eocene oligocene collision with the caroline pacific plate the philippine sea moved northward post 25 ma over the northern east asian sea forming a northern philippine sea arc that collided with the sw japan ryukyu margin in the miocene 20 14 ma

continental interior and edge breakup at convergent margins induced by subduction direction reversal a numerical modeling study applied to the south china sea margin the dynamics of continental breakup at convergent margins have been described as the results of back arc opening caused by slab rollback or drag force induced by subduction direction reversal although the rollback hypothesis has been intensively studied our understanding of the consequence of subduction direction reversal remains limited using thermo mechanical modeling based on constraints from the south china sea scs region we investigate how subduction direction reversal controls the breakup of convergent margins the numerical results show that two distinct breakup modes namely continental interior and edge breakup edge refer to continent above the plate boundary interface may develop depending on the maturity of the convergent margin and the age of the oceanic lithosphere for a slab age of 15 45 ma increasing the duration of subduction promotes the continental interior breakup mode where a large block of the continental material is separated from the overriding plate in contrast the continental edge breakup mode develops when the subduction is a short duration event and in this mode a wide zone of less continuous continental fragments and tearing of the subducted slab occur these two modes are consistent with the interior relic late mesozoic arc and edge relic fore arc rifting characteristics in the western and eastern scs margin suggesting that variation in the northwest directed subduction duration of the proto scs might be a reason for the differential breakup locus along the strike of the scs margin besides a two segment trench associated with the northwest directed subduction is implied in the present day scs region

subduction hinge migration abstract it has been argued that back arc tectonics is controlled principally by the absolute motion of the overriding plate and the migration of the hinge of the downgoing plate thus hinge migration rollback is fundamentally important to understanding the geodynamics of back arc regions but even rates of hinge migration are unknown the rate of hinge migration can be estimated if the rate of back arc extension and the motion of the overriding plate are known rates of rollback in a hotspot reference frame range from near zero in the case of the new hebrides arc to 50 70 mm a at the south sandwich arc and may be as high as 100 mm a at the tonga trench but the hinge is actually advancing along the izu bonin trench as evidenced by the fact that there is no significant active extension behind the izu bonin arc the rate of hinge advance increases from north to south along the izu bonin trench and correlates well with the dip of the subducted slab active spreading in the mariana trough may be attributed to a rate of hinge advance which is lower than the rate of retreat of the philippine plate all of the possible motions of overriding plates and subduction hinges which might give rise to back arc opening are represented among the examples considered thus there is no unique circumstance in which back arc extension occurs finally there appears to be no clear correlation between hinge motion and the age of the subducted lithosphere

the west philippine basin an eocene to early oligocene back arc basin opened between two opposed subduction zones 1 based on geological and geophysical data collected from the west philippine basin and its boundaries we propose a comprehensive cenozoic history of the basin our model shows that it is a back arc basin that developed between two opposed subduction zones rifting started around 55 ma and spreading ended at 33 30 ma the initial spreading axis was parallel to the paleo philippine arc but became inactive when a new spreading ridge propagated from the eastern part of the basin reaching the former one at an r r r triple junction spreading occurred mainly from this second axis with a quasi continuous counter clockwise rotation of the spreading direction the gagua and palau kyushu ridges acted as transform margins accommodating the opening arc volcanism occurred along the palau kyushu ridge eastern margin during the whole opening of the basin whereas the paleo philippine arc decreased its activity between 43 and 36 ma the western margin underwent a compressive event in late eocene early oligocene time leading to the rising of the gagua ridge and to a short subduction episode along eastern luzon in the western part of the basin the spreading system was highly disorganized due to the presence of a mantle plume overlapping spreading centers and ridge jumps occurred toward the hot region and a microplate developed shortly after the end of the spreading a late stage of amagmatic extension occurred between 30 and 26 ma in the central part of the basin being responsible for the deep rift valley that cut across the older spreading fabric

age spreading rates and spreading asymmetry of the world s ocean crust we present four companion digital models of the age age uncertainty spreading rates and spreading asymmetries of the world s ocean basins as geographic and mercator grids with 2 arc min resolution the grids include data from all the major ocean basins as well as detailed reconstructions of back arc basins the age spreading rate and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments the age uncertainties for grid cells coinciding with marine magnetic anomaly identifications observed or rotated to their conjugate ridge flanks are based on the difference between gridded age and observed age the uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges resulting in ridge jumps toward hot spots we also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age depth relationships the new set of grids helps to investigate prominent negative depth anomalies which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow a combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems

asthenospheric outflow from the shrinking philippine sea plate evidence from hf nd isotopes of southern mariana lavas abstract at subduction zones sinking of the downgoing lithosphere is thought to enable a return flow of asthenospheric mantle around the slab edges so that the asthenosphere from underneath the slab invades the ambient mantle flowing underneath the volcanic arc and the backarc basin for instance at the northern end of the lau basin trench retreat and slab rollback enable toroidal return flow of samoan mantle beneath a transform margin to provide a supply of fresh undepleted indian mantle that feeds the backarc spreading center questions however arise about the sense of mantle flow when plate kinematics predict that the trench is advancing as seen in the mariana convergent margin does the mantle flow in or does it escape outward through slab tears or gaps here we address the origin and sense of asthenospheric mantle flow supplying the southern mariana convergent margin a region of strong extension occurring above the subducting pacific plate does the asthenosphere flow northward from underneath the pacific plate and caroline hotspot through a slab tear or gap or does it flow outward from the mariana trough which possesses a characteristic indian ocean isotopic signature to address these questions we integrate geodetic data along with new hf nd isotopic data for fresh basaltic lavas from three tectonic provinces in the southernmost marianas the fina nagu volcanic complex the malaguana gadao backarc spreading ridge and the se mariana forearc rift our results indicate that indian mantle flows outward and likely escapes through slab tears or gaps to accommodate shrinking of the philippine sea plate we thus predict that asthenospheric flow around the pacific slab at the southern mariana trench is opposite to that predicted by most subduction driven mantle flow models

geodynamics double dip the indian plate moved north unusually quickly during the late cretaceous numerical simulations suggest that this rapid migration was caused by the pull of two coupled narrowing subduction zones

pacific trench motions controlled by the asymmetric plate configuration 1 we present a novel explanation for absolute trench normal motions of slabs surrounding the pacific rapid subduction zone retreat on the eastern side of the pacific and slow advance in the west can result from the large scale asymmetric plate configuration we use simple fluid dynamics to explain the mechanical background of this hypothesis and we use the results of a simple finite difference scheme to estimate the effect on trench motion velocities the hypothesis is based on two key assumptions first we follow the concept of plate scale horizontal counterflow in the asthenosphere driven by accretion of asthenosphere into lithosphere and by plate motion second we assume that horizontally wide slabs without large slab windows drift passively in the mantle flow field and do not retreat as a result of flow around the slab if the asthenosphere transfers flow related horizontal shear stress into deeper levels of the mantle an asymmetry in the plate configuration leads to different net pressure forces on the two slabs and thus affects the retreat behavior in an ocean with an asymmetric ridge position the slab of the smaller plate should retreat faster than the slab of the large plate which may even advance also the domain of a slower moving plate should collapse faster than the domain of the faster plate our model explains the counterintuitive negative correlation between slab age and retreat velocity observed in the pacific it also accords with the topographic asymmetry of the ridge flanks along the pacific rise

spatial and temporal evolution of the subducting pacific plate structure along the western pacific margin 1 tomographic images of the subducting pacific plate beneath the izu bonin mariana arc illustrate a progression of geometries from shallow dipping to vertical from north to south along the arc recent advances in technology and inversion techniques have improved resolution of slab structure beneath the western pacific island arcs but reasons for the variation in geometry and morphology are still poorly understood by comparing high resolution tomographic images of the western pacific and updated paleogeographic reconstructions we are able to link the spatiotemporal evolution of the subducting pacific plate back to the mid miocene we have reconstructed tectonic motions along the kurile japan izu bonin mariana arc system and the ryukyu arc to provide an independent additional interpretation of the subducting pacific plate using the most current plate motion data we then investigate the plausibility of our model and three other proposed models based on the interpreted slab structure from tomographic images the new reconstruction agrees with the basic characteristics of former trench retreat models but illustrates the importance of the collision of the ogasawara plateau with the trench in the mid miocene and its subsequent effect on the slab structure at depth and the impact of other aseismic ridge collisions along the plate boundary the combination of evidence in changed physical properties imaged with tomography and the current interpreted slab morphology can be analyzed with the past plate motions to understand subduction zone processes

submarine lava flow emplacement and faulting in the axial valley of two morphologically distinct spreading segments of the mariana back arc basin from wadatsumi side scan sonar images 1 high resolution deep tow side scan sonar data were collected over two distinct spreading segments in the central part of the mariana back arc basin these data allow mapping of small fissures and faults and the distinguishing of hummocky from smooth lava flows using these data we observe spatial variations in seafloor deformation and volcanic activity within each segment and also significant differences in the degree of tectonic deformation between the two segments one segment characterized by an hourglass shape suggestive of magmatic processes dominating over tectonic processes is in fact currently dominated by intense deformation rather than volcanism the other segment which exhibits morphology wide and deep linear axial valley typical of magma starved segments is subjected to very limited deformation and is covered by mostly unfaulted recent flows each segment also displays along axis variations in the degree of tectonic deformation and in lava flows freshness we observe a decrease of lava effusion rate from segment centers toward their ends we also investigated the apparent asymmetry of the mariana basin on the southern segment of the study area azimuths of tectonic structures are divided into two groups one segment parallel and one 15 oblique to strike of the segment these two trends of linear features developed synchronously with volcanic activity currently oblique structures seem to be the most active ones the tectonic structures that are parallel to the overall valley trend are distributed over the entire valley whereas the oblique structures are only located proximal to the eastern valley wall they are likely related to changes in the local stress field related to the obliquely trending eastern axial valley wall asymmetric character such as nonuniform spacing and throw of faults was not observed these observations suggest that the asymmetry of the basin is not due to asymmetric spreading but rather to eastward ridge jumps of several kilometers such small amplitude ridge jumps likely occurred frequently during basin development because the asymmetric character of the basin is strongly pronounced and no abandoned rift valley is recognizable within the entire basin

crustal velocity field of southwest japan subduction and arc arc collision we investigate crustal deformation in southwest japan over a 3 year period revealed by a permanent dense global positioning system gps array southwest japan is a part of the amurian plate a microplate moving about 10 mm yr toward the east with respect to the eurasian plate it overrides the philippine sea plate at the nankai trough and collides with the northeast japan arc in the central part of japan in this paper we first derive gps site velocities relative to the stable part of the amurian plate in order to isolate signals of crustal deformation caused by the subduction and or the collision the velocity field has a conspicuous feature indicating the interseismic elastic loading by the philippine sea plate slab at the nankai trough characterized by the northwestward movements of points throughout the studied area their amplitudes are the largest at the pacific coast and decay toward the japan sea coast with a subtle systematic shift of azimuths a model assuming an elastic half space the convergence rate at the nankai trough based on a refined euler vector and the strength of the coupling inferred from a thermal model could explain the velocity field in the western part of the studied area to a large extent those in the eastern part systematically deviate from them and the residual components there show east west shortening and north south extension this may represent crustal thickening and trenchward extrusion of crustal blocks caused by the collision between southwest and northeast japan this suggests that the collision between southwest and northeast japan gives rise to not only crustal thickening but also trenchward extrusion of crustal block a velocity contrast was found across the median tectonic line the largest inland active fault in japan but the current permanent gps network is not dense enough for us to discuss its coupling depth

origin and motion history of the philippine sea plate abstract the philippine sea plate is the one major plate whose tertiary motion is poorly constrained and whose origin is problematical its southern boundary is the sorong fault system which is part of a major left lateral fault system at the northern margin of the australian plate the southern part of the plate in eastern indonesia has been neglected in most syntheses but includes some of the oldest rocks within the plate which are separated from remnant arcs of the daito ridge province of the northern philippine sea by the west philippine central basin the east indonesian islands of the halmahera waigeo region contain a good mesozoic and tertiary stratigraphic record indicating a long arc history for the southern part of the plate new palaeomagnetic data from these islands define two sub areas an area forming part of the philippine sea plate north of the sorong fault and an area within the sorong fault system the area north of the fault records a long term clockwise rotation history whereas that within the fault zone records local rotations interpreted as due to deformation at the plate edge rocks of philippine sea plate origin within both areas record similar latitudinal shifts the rotation of the area north of the sorong fault is considered to represent the motion of the southern part of the philippine sea plate the new data indicate large tertiary clockwise rotations similar to earlier suggestions for other parts of the plate but record a discontinuous and more complex motion history than previously suggested for the southern part of the plate there was 40 rotation with northward translation between 0 and 25 ma no significant rotation between 25 and 40 ma and there was 50 rotation with southward translation between 40 and 50 ma we show that the new palaeomagnetic data form part of a single set with earlier palaeomagnetic data from elsewhere in the plate the translation history of the southern part of the plate in eastern indonesia can be reconciled with northward motions recorded elsewhere and can be used to determine rotation poles for the plate 15 n 160 e for the interval 5 25 ma and 10 n 150 e for the interval 40 50 ma reconstructions based on these poles predict that at 45 ma the palau kyushu ridge had a wnw ese orientation which is very different from that postulated by many models used to explain the widespread boninite volcanism in the izu bonin marians forearc at this time the long arc history of the southern part of the plate and the reconstructions based on the rotation poles calculated from the palaeomagnetic data favour an origin for the west philippine basin by spreading in a backarc basin

imaging the subducting slabs and mantle upwelling under the japan islands summary a high resolution p wave tomography of the crust and mantle down to 700 km depth beneath the japan islands is determined using a large number of high quality arrival time data from local earthquakes and teleseismic events simultaneously the tomography shows that the philippine sea slab is subducting aseismically down to 430 km depth under southwest japan though the seismicity within the slab ends at 180 km depth a low velocity low v zone in the mantle wedge under tohoku and kyushu is found to extend westward from the volcanic front to the backarc under the japan sea and east china sea significant low v anomalies are revealed in the deep portion of the mantle wedge 400 500 km depth above the pacific slab under southwest japan which may reflect hot mantle upwelling associated with fluids from the deep dehydration of the pacific slab low v anomalies appear at 420 700 km depths beneath the pacific slab under eastern japan which may reflect hot mantle upwelling associated with the deep subduction of the pacific slab and its collapsing down to the lower mantle

dynamics of the ryukyu izu bonin marianas double subduction system trench motions represent the surface expression of the interaction between subducting plates and the underlying mantle but the inherent dynamics are not fully understood one interesting case is the migration of the izu bonin marianas trench ibm that accommodates the subduction of the pacific beneath the philippine sea plate psp which is in turn subducting beneath the eurasian plate along the ryukyu trench the history of the ibm trench is dominated by fast episodic retreat from 40 to 15 ma however around 10 5 ma the ibm trench reversed its motion from retreat to advance the switch in trench motion occurred soon after the breakoff of the psp slab along the ryukyu trench and the onset of new subduction and represents a fundamental change in the dynamics of the western pacific subduction zones here following the modelling study of ci kova and bina 2015 which suggested a link between ibm trench advance and ryukyu subduction we run 2 d numerical experiments to test the influence of a newly formed ryukyu slab on the established ibm subduction zone we run two dimensional numerical experiments to test the influence of this newly formed ryukyu slab on the ibm subduction zone the results from our geodynamic model compare favourably with the reconstructed trend indicating that the switch in trench motion along the ibm trench may indeed be related to the onset of a new subduction zone along the ryukyu trench our analysis substantiates the idea that advancing trench motions in the western pacific are due to the establishment of a double subduction system further analysis of such dynamics provides insights for the mechanisms controlling subducting plate and trench motions and mantle force transmission

revel a model for recent plate velocities from space geodesy 1 we present a new global model for recent plate velocities revel describing the relative velocities of 19 plates and continental blocks the model is derived from publicly available space geodetic primarily gps data for the period 1993 2000 we include an independent and rigorous estimate for gps velocity uncertainties to assess plate rigidity and propagate these uncertainties to the velocity estimates the velocity fields for north america eurasia and antarctica clearly show the effects of glacial isostatic adjustment and australia appears to depart from rigid plate behavior in a manner consistent with the mapped intraplate stress field two thirds of tested plate pairs agree with the nuvel 1a geologic 3 myr average velocities within uncertainties three plate pairs caribbean north america caribbean south america and north america pacific exhibit significant differences between the geodetic and geologic model that may reflect systematic errors in nuvel 1a due to the use of seafloor magnetic rate data that do not reflect the full plate rate because of tectonic complexities most other differences probably reflect real velocity changes over the last few million years several plate pairs arabia eurasia arabia nubia eurasia india move more slowly than the 3 myr nuvel 1a average perhaps reflecting long term deceleration associated with continental collision several other plate pairs including nazca pacific nazca south america and nubia south america are experiencing slowing that began 25 ma the beginning of the current phase of andean crustal shortening

toward a generalized plate motion reference frame an absolute plate motion apm model is required to address issues such as the thermochemical evolution of earth s mantle all apm models have to rely on indirect inferences including those based on hot spots and seismic anisotropy each with their own set of uncertainties here we explore a seafloor spreading aligned reference frame we show that this reference frame fits azimuthal seismic anisotropy in the uppermost mantle very well the corresponding euler pole is close to those of hot spot reference frames ridge motion minimizing models and geodynamic estimates of net rotation and predicts clear trench motion patterns we conclude that a net rotation pole guided by the spreading aligned model at 64 e 61 s with moderate rotation of 0 2 0 3 myr could indeed represent a standard comprehensive reference frame for present day plate motions with respect to the deep mantle

geodynamics of trench advance insights from a philippine sea style geometry abstract for terrestrial parameter sets trench retreat is found to be nearly ubiquitous and trench advance quite rare largely due to rheological and ridge push effects recently updated analyses of global plate motions indicate that significant trench advance is also rare on earth being largely restricted to the marianas izu bonin arc thus we explore conditions necessary for terrestrial trench advance through dynamical models involving the unusual geometry of the philippine sea region in this subduction system a slab pull force from distal subduction is transmitted to the overriding plate at the pacific trench our 2d modeling demonstrates that trench advance can occur for terrestrial rheologies in such special geometries we observe persistent trench advance punctuated by two episodes of back arc extension characteristic features of the model such as time interval between extensional episodes high back arc heat flow and stress state of philippine plate correspond to processes recorded in the region

anomalously fast convergence of india and eurasia caused by double subduction prior to collision with eurasia the indian plate rapidly accelerated northwards numerical simulations show that the combined pull of two slabs in two parallel north dipping subduction systems could have caused this pulse of rapid movement

dynamics of double subduction numerical modeling abstract double subduction is a geodynamic process in which two plates following each other are synchronously subducted double subductions are known for both modern izu bonin marianas and ryukyu arcs and ancient west himalaya collision zone plate tectonics however our knowledge about this process is limited to conceptual schemes and some restricted analogue experiments in order to fill this gap we performed 2d numerical experiments using a coupled petrological thermomechanical approach based on finite differences and marker in cell techniques combined with thermodynamic database for the mantle we investigated the influence of convergence rate intermediate plate length activation volume of the mantle dislocation creep and age of the lithosphere based on these experiments we conclude that a subduction rates at two zones running in parallel differ and vary in time even when the total convergence rate remains constant supremacy of either subduction zone depends on physical parameters such as i relative rates of the plates ii slab ages and iii length of the middle plate b subduction dynamics of the double subduction system involves several processes unknown in simple subduction systems such as i eduction i e un subduction ii subduction re initiation iii subduction flip triggered by shallow slab breakoff and iv turn over of detached slabs to up side down attitudes c simulated tomographic structures related to slab propagation account for both penetration and non penetration of the 660 km discontinuity non penetration is favored by i low convergence rate ii faster relative movement of the overriding plate iii young age of the subducting slab and iv up side down turn over of detached slab

paleogeographic reconstruction and origin of the philippine sea abstract our reconstruction of the philippine sea suggests that it formed by two distinct episodes of back arc spreading each of which resulted from seaward retreat of the trench in the first episode the protoizu bonin trench retreated northward and the west philippine basin formed behind the northern half of the palau kyushu ridge in the second episode the izu mariana trench retreated eastward and the shikoku and parece vela basins formed behind it during the last 17 ma the philippine sea basin has been moving northwestward with respect to eurasia shifting the ttt triple junction off central japan westward by about 50 km the motion of the philippine sea with respect to eurasia at the triple junction changed from north northwestward to west northwestward 10 5 ma ago for the period before 17 ma ago we construct two models retreating trench model and anchored slab model the izu bonin trench migrated from south to northeast rotating in a clock wise sense since 48 ma ago in the retreating trench model in the anchored slab model the trench has been fixed with respect to eurasian margin since 43 ma ago we prefer the retreating trench model because the deformation of the plate boundary along the eastern margin of eurasia during 30 17 ma ago is much simpler for this model than for the anchored slab model furthermore rotations of the bonin mariana islands are consistent with those predicted from the retreating trench model the 48 ma ages of the northern part of the palau kyushu ridge and of chichi jima of the bonin islands indicate that there was subduction beneath the northern half of the ridge beginning at least 48 ma ago from this and the subparallelism in trend between the northern part of the palau kyushu ridge and the central basin ridge we propose that the major part of the west philippine basin formed by back arc spreading in a n s direction behind the northern part of the palau kyushu ridge the pacific plate was moving northward with respect to hot spots from 48 to 43 ma ago which implies that the pacific plate is not likely to have been subducting beneath the west philippine basin during this time we speculate that another plate existed south of the pacific plate and thai it was subducting beneath both tile west philippine basin and australia the annihilation of this plate might he a cause for the sudden change of the pacific plate motion at 43 ma ago

the role of slabs and oceanic plate geometry in the net rotation of the lithosphere trench motions and slab return flow 1 absolute plate motion models with respect to a deep mantle reference frame e g hot spots typically contain some net rotation nr of the lithosphere global mantle flow models for the present day plate setting reproduce similarly oriented nrs but with amplitudes significantly smaller than those found in some high nr pacific hot spot reference frames it is therefore important to understand the mechanisms of nr excitation which we attempt here with two dimensional cylindrical models of an idealized pacific domain we study the influence of slab properties oceanic ridge position continental keels and a weak asthenospheric layer on nr and trench migration fast slab return flow develops in models with stiff slabs and moderate slab dips rapid nrs comparable to the high nr pacific hot spot reference frames are primarily induced by asymmetric slab dips in particular a shallow slab beneath south america and a steep slab in the western pacific a scaling relationship links the amplitude of nr to plate size slab dip angle and slab viscosity asymmetric ridge positions also promote nr through asymmetric plate sizes continental keels have less impact in contrast to what has been found in earlier global studies several models yield unidirectional pacific trench motions such as slab advance in the western pacific and simultaneously slab retreat in the eastern pacific our model provides a physical explanation for nr generation in the present day pacific setting and hints at mechanisms for the temporal evolution of the basin