the philippine sea new survey results reveal the structure and the history of the marginal basins the hydrographic department of japan initiated a long term geological and geophysical survey of the philippine sea in 1983 we here summarize the bathymetry and magnetic anomalies of this area focusing on the structure and the evolutionary process of the marginal basins the shikoku and parece vela basins developed as one backarc system in the later phase of their formation the significant difference between the two basins however is clearest near the extinct spreading centers the steep oki daito escarpment located in the northern west philippine basin was mapped for the first time during this survey the n s trending spreading fabric north of the escarpment is in striking contrast to the nw se to e w lineament in the south our data reveal a more complicated history of the western philippine sea than has been proposed in previous studies

heat flow along the nantroseize transect results from iodp expeditions 315 and 316 offshore the kii peninsula japan mal conductivity varies between about 1 0 w m 1 k 1 near the seafloor to 1 6 w m 1 k 1 at a depth of 1 km thermal conductivity generally increases with depth and correlates with variations in porosity and lithology thermal gradients along the transect are characterized by 48 sediment temperature measurements from 6 sites thermal corrections for the effects of bathymetric relief and sedimentation improve the confidence with which the heat flow values can be interpreted heat flow generally decreases with landward distance from the deformation front and varies from 70 mw m 2 just landward of the deformation front to 54 mw m 2 at sites characterizing the outer fore arc high and to 57 mw m 2 at the kumano basin site iodp heat flow measurements are significantly lower than nearby seafloor heat flow measurements this difference is most likely due to variations in bottom water temperature that have a large effect on values of seafloor heat flow thus the heat flow of the nankai accretionary prism is lower than previously thought we present thermal models of subduction along this transect and explore the impact of the initial geotherm conductive plate cooling based on the age of subducting seafloor 20 myr under predicts the observed heat flow we find a good fit to the data using a geotherm appropriate for 10 myr seafloor the extra heat is interpreted in terms of back arc thermal environments

ultrasonic velocity and attenuation during co2 injection into water saturated porous sandstone measurements using difference seismic tomography we undertook laboratory based seismic measurements with dense sensor array at ultrasonic frequencies during the injection of co2 into a water saturated sandstone specimen the resulting high quality seismic data enabled detailed determination of the relative velocity and attenuation coefficient of the compressional wave using difference seismic tomography which directly inverses time lapse changes in rock properties from time lapse changes in observed data co2 migration and water displacement were clearly mapped using tomographic images of relative velocity and the attenuation coefficient the final and largely stabilised volume fraction of co2 in the pore space of the sample is about 30 40 on average the p velocity fell by 7 5 12 and 14 5 and the attenuation coefficient q 1 increased by factors of 3 3 2 7 and 3 7 as a result of the replacement of water with co2 during the injection of gaseous liquid and supercritical co2 respectively as a function of gas saturation both the velocity and attenuation data are in good agreement with results obtained using the white and dutta ode model for partial saturation indicating that viscous losses due to fluid diffusion are of significant importance for compressional waves travelling at ultrasonic frequencies in porous rocks

release of mantle helium from forearc region of the southwest japan arc abstract shikoku japan is an island located on the forearc region of the western sector of the southwest sw japan arc that was formed by subduction of the philippine sea plate beneath the eurasian plate noble gas isotope analyses were carried out for 30 bubbling gas samples and spring water samples collected in shikoku in addition chemical and carbon isotope analyses were conducted for six gas samples observed 3 he 4 he ratios were 0 17 r r a to 3 56 r r a showing that several samples contain mantle helium components although some samples contain helium of crustal origin geographical distributions of 3 he 4 he ratios show that high 3 he 4 he ratios originate mainly from two areas one along the median tectonic line mtl a major active fault in japan the other coinciding with a region in which non volcanic long period tremors occur about 30 km deep obara k 2002 nonvolcanic deep tremor associated with subduction in southwest japan science 296 1679 1681 this fact indicates that fluids liberated from the slab in the forearc region cause deep tremors and fracturing within the crust thereby easing the transfer of fluids to the surface that are mixed with mantle helium to the surface an active fault system can also provide an efficient path for mantle helium transfer

long distance fluid and heat transport in the oceanic crust entering the nankai subduction zone nantroseize transect abstract i examine the potential causes of anomalous seafloor heat flux on the oceanic plate in the nankai trough seismogenic zone experiment nantroseize transect offshore southern japan the most prominent anomaly is a 50 mw m 2 change in heat flux between integrated ocean drilling program sites c0011 and c0012 over a distance of 30 km wide transition at the seafloor the observed surface heat flux pattern is indicative of hydrothermal circulation in the basement aquifer and advection of heat from the subducted crust into the aquifer on the incoming plate for a 600 m thick aquifer the permeability is likely 7 10 11 m 2 and hydrothermal circulation transports at least 300 times more heat than conduction alone the heat flux from the subduction zone seaward to the incoming plate is consistent with hydrothermal circulation in the subducting crust persisting to 100 km landward of the deformation front vigorous fluid circulation in the basaltic basement is consistent with both the seafloor thermal anomalies and geochemical anomalies near the sediment basement interface

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

deep shallow coupling response of the cenozoic bohai bay basin to plate interactions around the eurasian plate abstract the bohai bay basin bbb is a meso cenozoic intracontinental petroliferous basin where four pre existing continental scale fault zones intersect in east china the bbb has experienced complex deformational and evolutionary history that records significant information about how the lithosphere there has responded to the coeval plate interactions around the eurasian plate and or their resulting deep crust mantle processes in the cenozoic now both deep and shallow deformational and evolutionary processes of the bbb have been well studied but many geodynamic mechanisms previously proposed for those processes are too general to explain them well based on various data and principles we first summarized both deep and shallow responses and processes of the bbb in the cenozoic and then matched reasonable geodynamic mechanisms for each of them the results show as follows the cenozoic differential reactivation of pre existing continental scale weak zones controlled the coeval deep shallow differential coupling evolution of the bbb the paleogene extension in the bbb was mainly caused by crust mantle processes triggered by the stagnant izanagi slab the cenozoic strike slip faulting was influenced by various plate interactions in which the oblique subduction and anticlockwise rotation of young pacific plate resulted in the complex pre oligocene strike slip history of the tan lu fault zone to the south of the zhang peng fault zone while the india eurasia collision has caused the intense and synchronous strike slip faulting of various faults there since 35 ma which was then weakened by the oblique subduction and clockwise rotation of young philippine sea plate in the neogene the compression resulting from both the india eurasia collision and the philippine sea plate s subduction contributed to the latest paleogene tectonic inversion in the bbb the arrival of mantle flow triggered by both the india eurasia collision and the stagnant slab of the pacific plate resulted in the tectonic reactivation there in the late neogene

review of monitoring issues and technologies associated with the long term underground storage of carbon dioxide large scale underground storage of co2 has the potential to play a key role in reducing global greenhouse gas emissions typical underground storage reservoirs would lie at depths of 1000 mor more and contain tens or even hundreds of millions of tonnes of co2 a likely regulatory requirement is that storage sites would have to be monitored both to prove their efficacy in emissions reduction and to ensure site safety a diverse portfolio of potential monitoring tools is available some tried and tested in the oil industry others as yet unproven shallow focused techniques are likely to be deployed to demonstrate short term site performance and in the longer term to ensure early warning of potential surface leakage deeper focused methods notably time lapse seismic will be used to track co2 migration in the subsurface to assess reservoir performance and to calibrate validate site performance simulation models the duration of a monitoring programme is likely to be highly site specific but conformance between predicted and observed site performance may form an acceptable basis for site closure the geological society of london 2009

time lapse walkaway vertical seismic profile monitoring for co2 injection at the sacroc enhanced oil recovery field a case study abstractgeologic carbon storage involves large scale injections of carbon dioxide into underground geologic formations changes in reservoir properties resulting from co2 injection and migration can be characterized using monitoring methods with time lapse seismic data to achieve economical monitoring vertical seismic profile vsp data are often acquired to survey the local injection area we investigated the capability of walkaway vsp monitoring for co2 injection into an enhanced oil recovery field at sacroc west texas vsp data sets were acquired in 2008 and 2009 and co2 injection took place after the first data acquisition because the receivers were located above the injection zone only reflection data contain the information from the reservoir qualitative comparison between reverse time migration images at different times revealed vertical shifts of the reflectors center indicating the presence of velocity changes we examined two methods to quantify the changes in velocity standard full wa

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

volcano tectonic interactions during rapid plate boundary evolution in the kyushu region sw japan evolution of the local plate tectonic and volcanic system relationship at kyushu island is defined by major changes in tectonics and volcanic style at ca 15 10 6 and 2 ma plate reconstructions presented here suggest that prior to 15 ma the pacific plate subduction dominated kyushu tectonics from 15 to 6 ma the evolving relative plate motions shifted the triple junction between the pacific plate philippine sea plate and southwest japan northwards so that the philippine sea plate was subducted beneath kyushu we suggest that a lack of subduction related volcanism from 10 to 6 ma is due to shallow subduction of the young shikoku basin lithosphere by 6 5 ma changes in the philippine sea plate motion led to more rapid nearly trench normal subduction of the eocene west philippine basin crust beneath kyushu this model is supported by an increase in arc like geochemistry of lavas since ca 6 5 ma subduction of fluid rich features such as the kyushu palau ridge introduced large volumes of fluids into the kyushu arc system leading to voluminous volcanism across kyushu focused particularly in areas where the ridge subduction occurs in tandem with local extensional tectonics key issues such as the timing of izu arc collision with central japan and the history of motion of the philippine sea plate are reassessed here resulting in a model that favors izu arc central japan collision at ca 8 6 ma rather than the more widely accepted date of ca 15 ma

obliquity along plate boundaries most of the plate boundaries are activated obliquely with respect to the direction of far field stresses as roughly only 8 of the plate boundaries total length shows a very low obliquity ranging from 0 to 10 sub orthogonal to the plate displacement the obliquity along plate boundaries is controlled by i lateral rheological variations within the lithosphere and ii consistency with the global plate circuit indeed plate tectonics and magmatism drive rheological changes within the lithosphere and consequently influence strain localization geodynamical evolution controls large scale mantle convection and plate formation consumption and re organization thus triggering plate kinematics variations and the adjustment and re orientation of far field stresses these geological processes may thus result in plate boundaries that are not perpendicular but oblique to the direction of far field stresses this paper reviews the global patterns of obliquity along plate boundaries using gplate we provide a statistical analysis of present day obliquity along plate boundaries within this framework by comparing natural examples and geological models we discuss deformation patterns and kinematics recorded along oblique plate boundaries

magnetic anomalies in the shikoku basin a new interpretation kaiko surveys over the nankai trough made available new magnetic and structural data for the northern shikoku basin a survey of the oceanic lithosphere subducting below southwest japan along the central nankai trough revealed the existence of several north south basement troughs they are probably transform faults related to a north south spreading system we exanune the possibility of a late phase of north south spreading limited to the axial northernmost shikoku basin active between 14 and 12 ma if this system was already active before that time i e during the n55 opening of the southeastern basin then a triple junction should be found between both areas based on these data and previous studies we present a new interpretation of magnetic anomalies over the whole basin from early east west rifting to late north south spreading opening of the shikoku basin proceeded through multiple episodes of spreading the analysis of magnetic anomalies constrains the kinematic evolution of the basin through time and space two successive counter clockwise rotations of the spreading direction are postulated at anomaly 6 19 ma and at anomaly 5b 14 ma involving segmentation and rotation of the spreading ridge

topographic and sedimentary features in the yap subduction zone and their implications for the caroline ridge subduction abstract the yap subduction zone in the western pacific presents some unique features compared to normal intra oceanic subduction zones such as the subduction of an oceanic plateau however due to the relative paucity of geophysical data the detailed structure remains unknown in this area in this study we present the latest high quality swath bathymetry and multi channel seismic data acquired synchronously in 2015 across the yap subduction zone the topographic and sedimentary features are intensively investigated and a modified evolutionary model of the yap subduction zone is proposed the two stage evolution of the parece vela basin pvb produced fabrics that are n s trending and nw se trending our seismic data clearly reveal landslide deposits at the upper slope break of the forearc to the north of the yap island which was identified as the fault notch denoting a lithological boundary in previous work the swath bathymetry and seismic profile reveal detailed horst and graben structures including a crescent shaped fault zone near the contact between the yap trench and the caroline ridge a simple geometric model is proposed to explain the structure formation indicating that the higher topography of the caroline ridge resulted in enhanced bending related extension a seismic angular unconformity named r1 is identified in the sorol trough marking the onset of rifting in the trough based on the sequence thickness and deposition rate by deep sea drilling project dsdp it is deduced that the sorol trough formed at 10 ma or even earlier a modified model for the yap subduction zone evolution is proposed incorporating three major tectonic events the proto yap arc rupture in the oligocene the collision of the caroline ridge and the yap trench in the late oligocene or middle miocene and the onset of the sorol trough rifting in the late miocene

melting of a subducting slab and production of high mg andesite magmas unusual magmatism in sw japan at 13 15 ma characteristic high mg andesite magmas were produced in the sw japan arc at 13 15 ma that was synchronous with the commencement of subduction of a very young 11 m y lithosphere of the shikoku basin numerical simulation suggests that temperature at the surface of such a young subducting plate is high enough for partial melting both of the subducting sediments and oceanic crust at the beginning of the subduction high mg andesite magmas were likely to be produced by interaction between silicic slab melts and the overlying mantle wedge hma magmas may be commonly produced in the archean subduction zones under relatively high mantle temperature conditions contributing to making continental crusts

subduction controls the distribution and fragmentation of earth s tectonic plates the theory of plate tectonics describes how the surface of the earth is split into an organized jigsaw of seven large plates 1 of similar sizes and a population of smaller plates whose areas follow a fractal distribution 2 3 the reconstruction of global tectonics during the past 200 my 4 suggests that this layout is probably a long term feature of our planet but the forces governing it are unknown previous studies 3 5 6 primarily based on statistical properties of plate distributions were unable to resolve how the size of plates is determined by lithosphere properties and or underlying mantle convection here we demonstrate that the plate layout of the earth is produced by a dynamic feedback between mantle convection and the strength of the lithosphere using 3d spherical models of mantle convection with plate like behaviour that match the plate size frequency distribution observed for earth we show that subduction geometry drives the tectonic fragmentation that generates plates the spacing between slabs controls the layout of large plates and the stresses caused by the bending of trenches break plates into smaller fragments our results explain why the fast evolution in small back arc plates 7 8 reflects the dramatic changes in plate motions during times of major reorganizations our study opens the way to use convection simulations with plate like behaviour to unravel how global tectonics and mantle convection are dynamically connected

double saloon door tectonics in the japan sea fossa magna and the japanese island arc abstract a number of criteria considered diagnostic of double saloon door rifting and seafloor spreading are matched by data from the japanese arc these include i a pair of terranes sw and ne honshu which rotated in opposite directions from 22 21 ma to 14 11 ma ii rotated terranes which comprise a retro arc fold thrust belt attached to an accretionary wedge intruded by a magmatic arc iii contemporaneous backarc extension from 24 to 21 ma which is brought to a halt by progressive collision of the izu bonin and japan arcs from 15 to 5 ma iv isolation of blocks of thicker continental crust by areas of thin continental or oceanic crust during backarc rifting v such isolation may be due to simultaneous rifting or to progressively seaward rifts associated with ridge jumps towards the subduction zone vi opposite rotations are accommodated by subduction rollback demonstrated by seaward migration of the volcanic front from 30 26 ma to 16 15 ma vii concurrent development of a major arc orthogonal rift the fossa magna from 23 18 ma to 14 ma which was thereafter inverted from 15 ma to the recent viii a northeast propagating rift in the northern japan basin demonstrated by the relationship of linear magnetic anomalies to the mapped continent ocean boundary driving mechanisms for double saloon door tectonics are discussed in relation to various reconstructions of the northwest pacific opposite rotational torques leading to opposite terrane rotations may be caused by rollback of a curved trench hingeline or by the divergent slab sinking forces of the pacific and philippine sea plates

japan sea opening history and mechanism a synthesis the respective tectonic effects of back arc spreading and continental collision in asia are considered either as two independent processes or as closely interrelated extrusion tectonics assumes that the opening of the south china sea and the left lateral motion along the red river fault are geometrically linked in a pull apart manner this model is not accepted by several workers because the structural link between the two processes is not clearly demonstrated in the case of the japan sea we can show without ambiguity that back arc opening was controlled by large intracontinental strike slip faults which can be easily understood as effects of the india asia collision far from the indenter the japan sea opened in the early miocene in a broad pull apart zone between two major dextral strike slip shear zones the first one extends from north sakhalin to central japan along 2000 km it has accommodated about 400 km of finite displacement deformation along it varies from dextral transpression in the north to dextral transtension in the south the second is between korea and sw japan and has accommodated a smaller displacement of about 200 km the extensional domain in between lies in the back arc region of japan distributed stretching of the arc crust resulted in the formation of most of the japan sea while localized oceanic spreading at the southern termination of the eastern transpressional shear zone shaped the japan basin the first oceanic crust was formed in a small triangle based on the eastern shear zone and spreading propagated westward inside the pull apart region timing of oceanic crust formation of formation of the dextral shear zones and of block rotation in between as well as the internal structure of the basins and the geometry of deformation along the master shear zones are used to reconstruct the opening history this evolution is discussed by comparison to other manifestations of the arc and back arc activity such as the history of sedimentation and volcanism the paper then suggests that the collision of india can have tectonic consequences as far north as japan and sakhalin and describes the geometrical relation of back arc opening there and diffuse extrusion

estimation of changes in saturation and pressure from 4d seismic avo and time shift analysis a reliable estimate of reservoir pressure and fluid saturation changes from time lapse seismic data is difficult to obtain existing methods generally suffer from leakage between the estimated parameters we propose a new method using different combinations of time lapse seismic attributes based on four equations two expressing changes in prestack avo attributes zero offset and gradient reflectivities and two expressing poststack time shifts of compressional and shear waves as functions of production induced changes in fluid properties the effect of using different approximations of these equations was tested on a realistic synthetic reservoir where seismic data have been simulated during the 30 year lifetime of a water flooded oil reservoir results found the importance of the porosity in the inversion with a clear attenuation of the porosity imprint on the final estimates in case the porosity field or the vertically averaged porosity field is known a priori the use of a first order approximation of the gradient reflectivity equation leads to severely biased estimates of changes in saturation and leakage between the two different parameters both the bias and the leakage can be reduced if not eliminated by including higher order terms in the description of the gradient or by replacing the gradient equation with p and or s wave time shift data the final estimates are relatively robust to random noise as they present fairly high accuracy in the presence of white noise with a standard deviation of 15 the introduction of systematic noise decreases the inversion accuracy more severely

mid ocean ridge jumps associated with hotspot magmatism abstract hotspot ridge interaction produces a wide range of phenomena including excess crustal thickness geochemical anomalies off axis volcanic ridges and ridge relocations or jumps ridges are recorded to have jumped toward many hotspots including iceland discovery galapagos kerguelen and tristan de cuhna the causes of ridge jumps likely involve a number of interacting processes related to hotspots one such process is reheating of the lithosphere as magma penetrates it to feed near axis volcanism we study this effect by using the hybrid finite element code flac to simulate two dimensional 2 d cross section viscous mantle flow elasto plastic deformation of the lithosphere and heat transport in a ridge setting near an off axis hotspot heating due to magma transport through the lithosphere is implemented within a hotspot region of fixed width to determine the conditions necessary to initiate a ridge jump we vary four parameters hotspot magmatic heating rate spreading rate seafloor age at the location of the hotspot and ridge migration rate our results indicate that the hotspot magmatic heating rate required to initiate a ridge jump increases non linearly with increasing spreading rate and seafloor age models predict that magmatic heating itself is most likely to cause jumps at slow spreading rates such as at the mid atlantic ridge on iceland in contrast despite the higher magma flux at the galapagos hotspot magmatic heating alone is probably insufficient to induce a ridge jump at the present day due to the intermediate ridge spreading rate of the galapagos spreading center the time required to achieve a ridge jump for fixed or migrating ridges is found to be on the order of 105 106 years simulations that incorporate ridge migration predict that after a ridge jump occurs the hotspot and ridge migrate together for time periods that increase with magma flux model results also suggest a mechanism for ridge reorganizations not related to hotspots such as ridge jumps in back arc settings and ridge segment propagation along the mid atlantic ridge

the physics of gas chimney and pockmark formation with implications for assessment of seafloor hazards and gas sequestration pockmarks form where fluids discharge through seafloor sediments rapidly enough to make them quick and are common where gas is present in near seafloor sediments this paper investigates how gas might lead to pockmark formation the process is envisioned as follows a capillary seal traps gas beneath a fine grained sediment layer or layers perhaps layers whose pores have been reduced in size by hydrate crystallization gas accumulates until its pressure is sufficient for gas to invade the seal the seal then fails completely a unique aspect of capillary seals releasing a large fraction of the accumulated gas into an upward propagating gas chimney which displaces water like a piston as it rises near the seafloor the water flow causes the sediments to become quick i e liquefied in the sense that grain to grain contact is lost and the grains are suspended dynamically by the upward flow the quickened sediment is removed by ocean bottom currents and a pockmark is formed equations that approximately describe this gas piston water drive show that deformation of the sediments above the chimney and water flow fast enough to quicken the sediments begins when the gas chimney reaches half way from the base of its source gas pocket to the seafloor for uniform near surface sediment permeability this is a buoyancy control not a permeability control the rate the gas chimney grows depends on sediment permeability and the ratio of the depth below seafloor of the top of the gas pocket to the thickness of the gas pocket at the time of seal failure plausible estimates of these parameters suggest gas chimneys at blake ridge could reach the seafloor in less than a decade or more than a century depending mainly on the permeability of the deforming near surface sediments since these become quick before gas is expelled gas venting will not provide a useful warning of the seafloor instabilities that are related to pockmark formation however detecting gas chimney growth might be a useful risk predictor any area underlain by a gas chimney that extends half way or more to the surface should be avoided c 2009 elsevier ltd all rights reserved

paleomagnetic data from the philippine sea plate and their tectonic significance new paleomagnetic data from guam and saipan have been obtained and are used in conjunction with previously published results from around the philippine sea plate to interpret the origin and history of the philippine sea plate a lower miocene direction i 3 5 d 54 5 a95 17 4 k 20 3 and a middle oligocene direction i 5 3 d 68 4 a95 12 8 k 52 4 have been obtained from the palau islands only one reliable result a middle oligocene direction i 15 1 d 66 1 a95 11 1 k 20 1 has been obtained from guam many characteristic remanent magnetic directions have been obtained from the lower eocene facpi formation however the results are too scattered k 4 5 to be considered reliable two reliable directions have been obtained from saipan a middle miocene result i 30 7 d 28 1 a95 8 4 k 64 6 and a lower eocene result i 12 0 d 43 0 a95 12 5 k 16 0 two lower eocene results from chichijima i 9 5 d 105 3 a95 12 6 k 6 1 and anijima i 0 8 d 91 7 a95 16 9 k 4 9 are the reliable results from the bonin islands there are two end member interpretations for these data 1 small scale local rotation of blocks along the plate margin or 2 rotation of the philippine sea plate as a whole to distinguish between these interpretations we reconstructed the east philippine sea province back to its prerifting configuration the rotation caused by deformation can be estimated by comparing the paleomagnetism of guam saipan and the bonin islands to palau prior to reconstruction the older declination values are scattered closing the marginal basins reduces the declination scatter and all of the islands show increasing clockwise deflection with time this suggests that the philippine sea plate has rotated up to 80 clockwise and moved northward around 20 since the eocene and deformational rotation is a second order feature the finite pole of rotation of the philippine sea plate relative to eurasia must be somewhere near the southeast comer of the plate because this is the only location that is compatible with both the change in orientation and paleolatitude for the plate these data cannot distinguish between a backarc origin or trapped crust origin for the west philippine sea province the existing models require major modification to be consistent with the paleomagnetic data

cenozoic plate reconstruction of southeast asia abstract the india eurasia collision alone set up a series of chain reactions and caused the formation and destruction of sedimentary basins within the domain of the collision belt changes in the rate and angle of convergence between the india and eurasia plates reflect different stages of tectonic development in southeast asia for example extrusion of the indochina block induced the consumption of the pre existing proto south china sea along northeastern kalimantan and led to the eventual opening of the south china sea along the south china margin subsequent motions of the sino burma thailand malay peninsula sumatra and kalimantan blocks have produced a succession of basins stretching from north sumatra to central thailand and on to the natuna area we present reconstructions of the southeast asia region at 60 ma 50 ma 40 ma 30 ma 20 ma 15 ma 10 ma and 5 ma it is clear from the reconstructions that the impact between greater india and southeast asia took place in the northwestern part of southeast asia the duration for the impact was probably from the middle eocene to early miocene this timing is in good agreement with the dating of the main red river fault motion wu et al 1989 scharer et al 1990 since the impact between greater india and southeast asia was basically west of the burma block there is no reason to assume that the sumatra malay peninsula and kalimantan should extrude to the southeast first along the left laterally displaced mae ping and three pagodas fault zones as suggested by peltzer and tapponnier 1988 if the opening of the central thailand basins the gulf of thailand and the malay basin are taken into consideration a dextral megashear zone is required to compensate the relative motion between indochina and the malay peninsula this dextral megashear zone might even extend into western kalimantan and serve as a boundary between the indochina block and kalimantan

north luzon and the philippine sea plate motion model insights following paleomagnetic structural and age dating investigations results of one of the most comprehensive paleomagnetic and supporting geological programs ever carried out in offshore se asia on north luzon northern philippines are reported six new results based on 66 sites are reported from a total collection of 243 individual sites declinations in the data subset are sometimes scattered likely reflecting combinations of major plate and local rotations in both clockwise and counterclockwise directions and thus have a somewhat limited value for tectonic modeling the inclination data are however much more valuable and can be best explained if north luzon traveled as part of the philippine sea plate for most of its history a scenario which is compatible with the known geology of the eastern philippines and broader region in the proposed model for all of its eocene pliocene history north luzon is placed on the western edge of the philippine sea plate effectively always just to the west of the site where the benham plateau formed 40 ma the paleomagnetic data indicate a substantial northward migration of the area since the start of the neogene with an earlier interval stretching back to at least the mid early cretaceous when this part of the plate occupied equatorial latitudes post 15 ma motion of the plate has involved the indentation of the palawan microcontinental block into the western side of the philippine archipelago deformations induced by this process offer the most likely explanation for the scattered declinations observed in north luzon and areas a short distance to the south

surface wave tomography and azimuthal anisotropy of the philippine sea plate abstract long period and broadband waveform data from 1984 to 2007 extracted mainly from the iris data center with additional data from 1997 to 2007 extracted from the ocean hemisphere project is used to perform a detailed mapping of rayleigh wave group velocity and anisotropic structures of the philippine sea plate psp our results show prominent high rayleigh wave group velocity anomalies and heterogeneity in three marginal basins of the psp at period up to 30 s the convergent boundaries around the psp are associated with low velocity anomalies heterogeneity of velocity distribution at deep depths along the ryukyu trench is also observed our results indicate that the azimuthal anisotropy in the psp is depth dependent the fast direction in the west philippine basin and the shikoku basin is in ne sw at shorter periods which is correlated with fossilized fabric produced by final phase of deformation whereas the fast direction in the parece vela basin is complex at shorter periods and becomes uniformly ne sw oriented at longer periods at the depths near the lithosphere asthenosphere boundary the fast directions become parallel to the absolute plate motion for various parts of the psp the fast directions along subduction zones around the psp are predominately trench normal the relatively simple patterns of azimuthal anisotropy may imply that the observed anisotropy originates from the a type lpo of olivine fabric at shallow depths in the back arc region and with slab entrained mantle flow at deep depths

using image warping for time lapse image domain wavefield tomography abstracttime lapse seismic data are widely used for monitoring subsurface changes a quantitative assessment of how reservoir properties have changed allows for better interpretation of fluid substitution and fluid migration during processes such as oil and gas production and carbon sequestration full waveform inversion fwi has been proposed as a way to retrieve quantitative estimates of subsurface properties through seismic waveform fitting however for some monitoring systems the offset range versus depth of interest is not large enough to provide information about the low wavenumber component of the velocity model we evaluated an image domain wavefield tomography idwt method using the local warping between baseline and monitor images as the cost function this cost function is sensitive to volumetric velocity anomalies and it is capable of handling large velocity changes with very limited acquisition apertures where traditional fwi fails we described the theory and workflow of our method la

studying co2 storage with ambient noise seismic interferometry a combined numerical feasibility study and field data example for ketzin germany seismic interferometry applied to ambient noise measurements allows the retrieval of the seismic response between pairs of receivers we studied ambient noise seismic interferometry ansi to retrieve time lapse reflection responses from a reservoir during co2 geologic sequestration using the case of the experimental site of ketzin germany we applied ansi to numerically modeled data to retrieve base and repeat reflection responses characterizing the impedances occurring at the reservoir both with and without the injection of co2 the modeled data represented global transmission responses from band limited noise sources randomly triggered in space and time we found that strong constraints on the spatial distribution of the passive sources were not required to retrieve the time lapse signal as long as sufficient source location repeatability was observed between the base and the repeat passive survey to illustrate the potential of the technique ansi was applied to three days of passive field data recorded in 2012 at ketzin comparison with the modeled results illustrated the potential to retrieve key reflection events using ansi on field data from ketzin this study supports the idea that the geologic setting and characteristics of ambient noise at ketzin may be opportune to monitor co2 sequestration

olivine fabric transitions and shear wave anisotropy in the ryukyu subduction system recent shear wave splitting measurements from the fore arc region of the ryukyu subduction system show large magnitude 0 3 1 6 s trenchparallel splitting in both local and teleseismic phases the similarity of splitting parameters associated with shallow local s and teleseismic phases suggests that the source of anisotropy is located in the fore arc mantle one explanation for this pattern of shear wave splitting involves a transition from commonly observed high temperature olivine fabrics with flow parallel seismically fast directions to a flow normal b type olivine fabric in the cold fore arc mantle of the ryukyu wedge we test the b type fabric hypothesis by comparing observed splitting parameters to those predicted from geodynamic models that incorporate olivine fabric development the distribution of olivine fabric is calculated with high resolution thermomechanical models of the ryukyu subduction zone that include realistic slab geometry and an experimentally based wet olivine rheology we conclude that b type fabric can explain the magnitude and trench parallel orientation of deep local s phases that sample the core of the forearc mantle however our calculations show that b type fabric alone cannot account for large magnitude trench parallel splitting associated with teleseismic phases that sample the shallow tip of the fore arc mantle alternative models for trench parallel teleseismic splitting in the shallow tip of the fore arc mantle involve the addition of crustal or slab anisotropy and highly anisotropic foliated antigorite serpentinite 2008 elsevier b v all rights reserved

geochemistry of a continental saline aquifer for co2 sequestration the guantao formation in the bohai bay basin north china the neogene guantao formation in the beitang sag in the bohai bay basin bbb of north china a mesozoic cenozoic sedimentary basin of continental origin has been chosen as a candidate for a pilot field test of co2 sequestration hydrogeological and geochemical investigations have been carried out to assess its suitability taking advantage of many existing geothermal wells drilled to 2000 m or greater depths water samples from 25 wells and drill cores of three sections of the guantao formation were collected for measurements of mineralogy water chemistry and isotopes d

seismic morphologic and scale variabilities of subsurface pipes and vent complexes in a magma rich margin subsurface pipes related to fluid flow are observed in seismic reflection data as vertical to sub vertical features of low amplitude reflections linking chaotic reflections at the base root zones of volcanic constructs to their summits which can comprise craters mounds or eye shaped vents to date uncertainties remain regarding the linear correlation and scaling relationship between pipes and their overlying vent structures using 3d seismic reflection data from a magma rich basin the vigrid syncline in the voring basin offshore norway this study provides a seismic morphometric and statistical description of forty 40 pipes and their associated structures these pipes include conical bifurcated fault controlled and columnar types which are a consequence of the intrusion of two mappable magmatic sills of early eocene age the heights av 1098 m widths av 1740 m and slenderness ratios i e the heights widths of the pipes av 0 6 show low to moderate correlation coefficients with vent parameters such as height av 258 m area av 2 3 km2 and ellipticity av 1 5 the morpho statistical analyses provided here show that pipe formation mechanisms are mutually exclusive and that vent geometries are not particularly systematic in nature

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

tomographic evidence for the mantle upwelling beneath southwestern japan and its implications for arc magmatism abstract the seismotectonics and volcanotectonics of southwestern sw japan have been affected by the subduction of the old pacific and the young philippine sea slabs we have attempted to determine the three dimensional p and s wave velocity structures of the upper mantle in sw japan by using a large amount of high quality arrival time data recorded by the dense seismograph network on the japanese islands to understand the tectonic framework of sw japan our results show the existence of a large low velocity anomaly below the philippine sea slab in the chugoku and shikoku districts at least down to depths of 300 km we infer that the low velocity zone is an upwelling flow from depth two shallower low velocity zones derived from this low velocity zone are detected one is toward the quaternary volcanoes in the chugoku district and the other is toward the uppermost mantle beneath the kii peninsula the former suggests that the magma for the quaternary volcanoes in the chugoku district is attributable to the upwelling flow in the upper mantle this idea supports the geochemical and petrological observations that the erupted alkalic basalt in the chugoku district should have originated from a more primitive or enriched mantle the latter is responsible for the absence of a high velocity anomaly corresponding to the subducted philippine sea slab beneath the kii peninsula the philippine sea slab could have been more hydrated locally beneath the kii peninsula than that in the surrounding regions by the interaction with the mantle upwelling consequently dehydration reactions could be involved during the progressive metamorphism accompanied by intensive intraslab seismicity lowering the seismic velocity in the slab aqueous fluids thus released to the overriding plate might be a possible source of mantle type helium and trigger a long term swarm activity in the crust of the overriding plate our results indicate the importance of the heterogeneous structure below the philippine sea slab with regard to the tectonics of sw japan

evolution of tectono sedimentary systems in the kumano basin nankai trough forearc abstract sedimentary deposits in the distal kumano forearc basin of the nankai accretionary margin off kii peninsula japan have been imaged using three dimensional 3d seismic data the seismic data along with logging and core data from the integrated ocean drilling program iodp show that the unconformity between the accretionary prism and overlying forearc sediments is time transgressive the unconformity at site c0002 separates 5 ma prism rocks from 3 65 ma basin deposits at site c0009 it separates 5 6 ma prism from 3 8 ma basin sediments acoustic reflections in the basal deposits are sub parallel to the underlying accretionary prism the acoustic facies varies in thickness from 50 to 750 m the mudstone deposits and laterally equivalent turbidites are interpreted as lower trench slope deposits the condensed slope sediment ss section decreases in age from 3 5 to 1 5 ma at site c0002 to 1 5 0 9 ma at c0009 acoustic sequences within the lower forearc basin lfb contain higher proportions of silt and sand turbidites and progressively onlap the ss unit along a low angle discontinuity kl in a landward direction because of the landward onlap of the lfb unit the oldest lfb strata at c0002 are older than 1 67 ma whereas those at c0009 are younger than 0 9 ma thus the amount of time missing or characterized by condensed sedimentation across the kl unconformity decreases in duration in the landward direction the landward onlapping sequences tilt progressively landward in response to regional uplift along an out of sequence thrust oost mega splay fault regional tilting shifted the basin s depocenter progressively landward expanding that part of the basin from 10 km in width to 30 km the onset of sand silt turbidite deposition in the distal basin began after more accommodation space was created by the uplift of the outer ridge along the splay fault at 1 9 ma conversely turbidites of the upper forearc basin ufb progressively onlap lfb in a seaward direction furthermore the respective thicknesses of the lfb and ufb units switch from the seaward side of the basin c0002 farther landward c0009 the lfb unit is 800 m thick in the seaward region whereas it is only 200 300 m thick in the landward region the ufb unit is

stratigraphy and facies distribution of the utsira formation and the pliocene sequences in the northern north sea abstract the upper cenozoic deposits of the northern north sea have been analysed in order to establish a regional and detailed stratigraphy the utsira formation is subdivided into four log units and mapped and two main depocentres are outlined the lower part of the utsira formation consists of thick marine mounded sand bodies interpreted as overall stacked lowstand fan deposits while the upper part of the formation consists of more clayey silty intervals indicating increased relative sea level the succeeding progradational pliocene deposits are subdivided into 13 high frequency depositional sequences and are mapped the sequences are grouped into four composite sequences each of the four pliocene composite sequences is composed of one or two rather locally distributed prograding sequences lowstand sequence set succeeded by one or two more widely distributed aggrading prograding sequences transgressive highstand sequence set boundaries between the composite sequences are recorded as marked changes in distribution of depocentres and sequence architecture the regional uplift of scandinavia is believed to be the main control on sediment input feeding the succeeding general prograding pliocene sequences oscillations of the eustatic sea level punctuated the tectonically controlled progradation and affected variations in the accommodation space and thus created the high frequency sequences

estimation of co2 saturation during both co2 drainage and imbibition processes based on both seismic velocity and electrical resistivity measurements summary in order to monitor injected carbon dioxide co2 simultaneous measurements of seismic velocity and electrical resistivity are employed during the drainage co2 injection and imbibition water injection processes of a berea sandstone supercritical co2 10 mpa at 40 o c was injected into a water saturated berea sandstone in the drainage stage and monitored via simultaneous measurements after the injection of supercritical co2 fresh distilled water was injected into the co2 injected sandstone during the imbibition stage electrical resistivity and p wave velocity measurements acquired during the drainage and imbibition stages were employed to evaluate co2 saturations sco2 based on the resistivity index and the gassmann fluid substitution equations respectively comparing estimated values for sco2 saturation against those from volume derived sco2 based on analysis on injected and drained fluid volumes in the drainage process we conclude that gassmann brie and resistivity index are suitable for the evaluation based on p wave velocity and electrical resistivity respectively rt based estimation properly tracks the variation in sco2 even when sco2 is large 0 15 while vp based estimation is sensitive to the variation in sco2 when sco2 is small 0 1 while properly estimating sco2 when sco2 is small 0 1 monitoring the co2 imbibition process confirms residual co2 saturation within the sample

spreading direction in the central south china sea recent work1 2 indicates that the south china sea is an atlantictype marginal basin of late tertiary age magnetic anomalies in the eastern part of the sea are consistent with seafloor spreading directed approximately north south1 2 we present here a new morphostructural study based on coupled seabeam mapping and single channel seismic reflection profiling which reveals dominant normal fault scarps striking n50 e between 113 and 119 e longitude near the axis of this basin such a structural fabric implies a nw se spreading direction at least in the 150 200 km wide axial region of the south china sea and places new constraints on geodynamic models for the formation of this basin in the tectonic and palaeogeographic framework of south east asia and the south west pacific

ava inversion of the top utsira sand reflection at the sleipner field amplitude variation with angle ava inversion is performed on the top utsira sand reflector at the sleipner field north sea norway this interface is of particular interest because of the accumulation of co 2 injected from a point deeper in the utsira sand the focus is on the postmigration processing of angle gathers together with the actual inversion procedure the processing treats amplitude extraction offset to angle mapping and global scaling in detail two algorithms are used for the inversion of ava data one that assesses uncertainties and one fast least squares variant both are very suitable for this type of problem because of their covariance matrices and built in regularization in addition two three parameter approximations of the zoeppritz equations are used one is linear approximation and the other is quadratic the results show significant signals for all three elastic parameters but the substitution of brine by co 2 using gassmann s equation indicates that the contrasts in s wave

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

community infrastructure and repository for marine magnetic identifications magnetic anomaly identifications underpin plate tectonic reconstructions and form the primary data set from which the age of the oceanic lithosphere and seafloor spreading regimes in the ocean basins can be determined although these identifications are an invaluable resource their usefulness to the wider scientific community has been limited due to the lack of a central community infrastructure to organize host and update these interpretations we have developed an open source community driven online infrastructure as a repository for quality checked magnetic anomaly identifications from all ocean basins we provide a global sample data set that comprises 96 733 individually picked magnetic anomaly identifications organized by ocean basin and publication reference and provide accompanying hellingerformat files where available our infrastructure is designed to facilitate research in plate tectonic reconstructions or research that relies on an assessment of plate reconstructions for both experts and nonexperts alike to further enhance the existing repository and strengthen its value we encourage others in the community to contribute to this effort

neotethyan intraoceanic microplate rotation and variations in spreading axis orientation palaeomagnetic evidence from the hatay ophiolite southern turkey abstract insights into tectonic processes operating in ancient ocean basins are provided by analyses of fragments of oceanic lithosphere preserved as ophiolites during collisional orogenesis here we present a palaeomagnetic analysis of the upper cretaceous hatay kizil dag ophiolite of turkey that provides evidence for intraoceanic microplate rotation and variations in ridge axis orientation in a neotethyan ocean basin magnetizations at 46 sites are shown to be pre deformational in origin and rotated from the relevant reference direction a net tectonic rotation approach to the analysis of the data provides information on permissible net rotation poles and angles and allows uncertainties in input vectors to be considered results demonstrate that all levels of the ophiolite have been rotated anticlockwise by angles in excess of 90 around steeply inclined axes the hatay ophiolite formed in the same supra subduction zone spreading system as the troodos ophiolite cyprus which is known to have rotated 90 anticlockwise in an intraoceanic setting in the late cretaceous to early eocene by considering our results in the context of the known timing of the troodos rotation we infer that 50 60 of rotation of the hatay ophiolite took place as part of an areally extensive troodos microplate this phase of rotation was triggered by initial impingement of the arabian continental margin with the neotethyan subduction trench consistent with models for modern day oceanic microplate rotation in complex convergent plate boundaries the hatay ophiolite then became detached from the actively rotating microplate and was emplaced onto the arabian margin in the maastrichtian undergoing a further 30 40 of anticlockwise rotation during thrusting back stripping of rotations allows correction of the hatay sheeted dykes to their initial orientations the restored dyke trend of 020 differs from that inferred previously for the troodos sheeted dyke complex demonstrating a primary variation in orientation of neotethyan spreading axes such variability is commonly observed in modern spreading systems in marginal basins these may act as analogues for the supra subduction zone spreading inferred for many ophiolites

reactive transport simulation study of geochemical co2 trapping on the tokyo bay model with focus on the behavior of dawsonite abstract a long term up to 10 ka geochemical change in saline aquifer co 2 storage was studied using the toughreact simulator on a 2 dimensional 2 layered model representing the underground geologic and hydrogeologic conditions of the tokyo bay area that is one of the areas of the largest co 2 emissions in the world in the storage system characterized by low permeability of reservoir and cap rock the dominant storage mechanism is found to be solubility trapping that includes the dissolution and dissociation of injected co 2 in the aqueous phase followed by geochemical reactions to dissolve minerals in the rocks the co 2 water rock interaction in the storage system mainly in the reservoir changes the properties of water in a mushroom like co 2 plume which eventually leads to convective mixing driven by gravitational instability the geochemically evolved aqueous phase precipitates carbonates in the plume front due to a local rise in ph with mixing of unaffected reservoir water the carbonate precipitation occurs extensively within the plume after the end of its enlargement fixing injected co 2 in a long geologic period dawsonite a na al carbonate is initially formed throughout the plume from consumption of plagioclase in the reservoir rock but is found to be a transient phase finally disappearing from most of the co 2 affected part of the system the mineral is unstable relative to more common types of carbonates in the geochemical evolution of the co 2 storage system initially having formation water of relatively low salinity the exception is the reservoir cap rock boundary where co 2 saturation remains very high throughout the simulation period

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

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

late permian paleomagnetic pole from dikes of the tarim craton china we have obtained a late permian paleomagnetic pole from dikes in the tarim craton of china directions of magnetization from 21 mafic and ultramafic dikes at three localities pass a fold test at the 95 confidence level the new paleomagnetic pole lies at lat 66 n long 181 e n 21 k 61 and 95 3 9 on the basis of a comparison with paleomagnetic data from adjacent tectonic units we conclude that 1 the late permian paleolatitude of the tarim craton differs from that of the sino korean craton suggesting that the two cratons were distinct tectonic units in late permian time 2 there was relative motion between the tarim craton and the siberian craton after late permian time and 3 negligible relative motions have occured between different thrust sheets in the keping bachu area of the north western tarim craton

south malvinas falkland basin hydrocarbon migration and petroleum system abstract we report a detailed seismic interpretational study of direct hydrocarbon indicators dhis using 2d multichannel seismic reflection data the study includes analyses of four distinctive types of dhi 1 a bottom simulating reflection bsr 2 enhanced reflections ers 3 pipes and vertical anomaly clusters which indicate focused migration of hydrocarbon fluids and 4 flat spots which indicate a deeper source for the shallower hydrocarbon occurrences the interpretation of these dhis included analysis of spatial distribution amplitude variation with offset avo and instantaneous spectral decomposition these analyses are set in a tectono stratigraphic context in order to evaluate the relationship between intra basinal structures hydrocarbon occurrences and migration pathways the recent discovery of a gas condensate accumulation in the study area darwin well ca 190mmbbl confirms the presence of a mature source rock in the basin and supports a model whereby the observed dhis are connected via an active hydrocarbon plumbing system

tectonics of the southern tip of the parece vela basin philippine sea plate we report new geophysical and petrological data collected at the southern tip of the parece vela basin in the philippine sea the parece vela basin which was formed as a backarc basin behind proto mariana arc trench system from late oligocene to middle miocene provides us a good opportunity to study the nature of successive backarc basin formations in the philippine sea and the relationship between are and backarc magmatisms regional bathymetric map derived from satellite altimetry shows that the southern tip of the basin now located just west of the yap arc trench system has unique morphological and tectonic features which include 1 the absence of spreading center or its trace 2 shallow average depth and 3 enigmatic curved structures our newly collected high resolution bathymetric data reveal that the spreading fabric similar to the central parece vela basin exists to the north of 9 degrees 20 n thus it appears that the present day yap arc and backarc region represent the western half of the seafloor that was produced by the early e w and the following ne sw spreading in the northern and central parece vela basin and that the eastern counterpart now lies west of the west mariana ridge unlike the northern parece vela basin there appears to be no evidence for a systematic propagation of spreading center in the southern part instead two rift segments one which extends from the central parece vela basin and the other which lies within the western remnant arc kyushu palau ridge overlap at the southern tip of the basin producing a complex seafloor that includes curvilinear deeps and deformed topographic highs c 2007 elsevier b v all rights reserved

a slab fragment wedged under tokyo and its tectonic and seismic implications a three dimensional evaluation of earthquake hypocentres beneath the kanto basin in japan reveals the presence of a distinct 25 km thick and 100 km wide body its fast seismic velocity and the presence of a double seismic zone suggest that it is a fragment of the pacific slab rather than an extension of the philippine sea slab this implies that the penetration of the philippine sea slab is much shallower beneath the kanto basin than was previously thought the philippine sea and pacific plate slabs both subduct beneath tokyo and so their configuration and seismic potential have been subject to intensive study1 2 3 4 5 6 7 8 previous work suggests that the philippine sea slab extends up to 100 km northwest of tokyo and subducts to a depth of 90 km beneath the kanto basin where it is folded against the underlying pacific slab here we evaluate seismic data in three dimensions and delineate a distinct 25 km thick and 100 km wide body beneath the kanto basin that has hitherto been considered to be part of the philippine sea slab we find that several of its characteristics such as its high seismic velocity and the presence of a double seismic zone point instead to an affinity with the pacific slab implying that the philippine sea slab penetrates to depths of only 35 40 km beneath the kanto basin we propose that the body is a fragment of pacific lithosphere that formed by the collision of two intersecting seamount chains with the japan trench 2 3 million years ago we suggest that such slab fragments may not be uncommon especially where seamount subduction deforms trenches and near triple junctions

near ridge initiation of intraoceanic subduction effects of inheritance in 3d numerical models of the wilson cycle abstract despite its global significance much of the wilson cycle which infers kinematic inversion from plate divergence to convergence remains conjectural in particular for what concerns subduction initiation we present a high resolution 3d thermomechanical numerical model of the wilson cycle evolving from continental rifting through breakup and oceanic spreading to convergence and self consistent subduction initiation in the models intra oceanic subduction initiates off ridge and is an intrinsically three dimensional process its location is controlled by convergence induced ridge swelling and the subduction geometry is controlled by three main factors 1 the inherited compositional and thermal heterogeneity of the ridge and oceanic lithosphere from rifting and seafloor spreading 2 the curvature obliquity of the ridge with respect to the convergence direction and 3 the duration of transition from plate divergence to convergence the modelled mechanisms are consistent with geological records for example in the oman subduction obduction system and are undervalued elements of the wilson cycle

a synthesis of heat flow determinations and thermal modeling along the nankai trough japan 1 we review marine heat flow data along the nankai trough and show that observations 30 km seaward of the deformation front are 20 below conductive predictions 129 94 mw m 2 but consistent with the global heat flow average for oceanic crust of the same age 16 28 ma heat flow values 30 km seaward of the deformation front are generally 20 higher than conductive predictions this heat flow pattern is consistent with the advection of heat by fluid flow in the subducting oceanic crust and explains both the high heat flux in the vicinity of the trench 200 and 140 mw m 2 and steep landward declines to values of approximately 60 mw m 2 over distances of 65 and 50 km along the muroto and kumano transects respectively along the ashizuri transect the lack of heat flow data precludes a definitive interpretation we conclude that fluid flow in the subducting oceanic crust leads to temperatures that are generally 25 c higher near the toe of the margin wedge and 50 100 c lower near the downdip limit of the seismogenic zone than estimated by purely conductive models

defining the 3d geometry of thin shale units in the sleipner reservoir using seismic attributes acknowledgments the seismic interpretation and image processing was carried out in the seislab facility at the university of aberdeen sponsored by bg bp and chevron seismic imaging analysis was performed using geoteric ffa and analysis of seismic amplitudes was performed in petrel 2015 schlumberger we would like to thank the nddc rg11766 10 for funding this research and statoil for the release of the sleipner field seismic dataset utilized in this research paper and also anne kari furre and her colleagues for their assistance we also thank the editor alejandro escalona and the two anonymous reviewers for their constructive and in depth comments that improved the paper

evolution of the boundary between the philippine sea plate and australia palaeomagnetic evidence from eastern indonesia abstract the boundary between the philippine sea and australian plates is the left lateral sorong fault system of eastern indonesia until recently modelling this boundary for the period before about 5 ma was difficult the tertiary motion of the philippine sea plate was uncertain and palaeomagnetic data from areas adjacent to the fault were lacking recent geological and palaeomagnetic studies of the area north of the sorong fault have elucidated the tertiary motion history of the philippine sea plate providing a reference for examining movements within the fault system we report new palaeomagnetic data from within the sorong fault zone from the islands of taliabu and obi taliabu is part of the sula platform and is considered to be derived from australia pelagic limestones from the upper cretaceous tanamu formation of taliabu yielded a direction of d 329 1 i 34 9 implying counter clockwise rotation and a formation latitude of 19 5 s sula and misool are postulated to be part of a single microcontinent which had a different late cretaceous mid tertiary movement history from australia the sula platform was transported to its present position by movement along the sorong fault system in the late miocene obi includes rocks of philippine sea and australian origin all the new sites are in rocks of philippine sea plate origin since the early neogene the philippine sea plate which includes all islands north of the sorong fault has rotated 40 clockwise and moved 10 15 northwards philippine sea plate rocks within the sorong fault zone record similar latitude shifts but different rotations in north obi the upper oligocene anggai river formation and the middle miocene woi formation record 60 and 30 counter clockwise rotations respectively the sense of rotation is consistent with motion within a left lateral fault system with the philippine sea and australian plates providing the shear couple in contrast the woi formation in southeast obi records 15 20 clockwise rotation this area is separated from the zone of counter clockwise movement in north obi by a strand of the sorong fault arc volcaniclastic rocks from the upper cretaceous leleobasso formation of northwest obi have a primary magnetisation with a mean direction of d 357 1 i 21 9 these rocks formed at 11 n or 11 s depending on the interpreted rotation history and indicate a pacific rather than indian ocean origin a volcanic arc at the southern edge of the philippine sea plate collided with eastern new guinea at 25 ma the philippine sea australia plate boundary then changed from subduction to strike slip as the philippine sea plate began its neogene rotation initiating the sorong fault system we suggest that many of the arc fragments in the new guinea orogenic belt originated in the southern philippine sea plate arc which has subsequently been dismembered by strike slip faulting

tertiary evolution of the shimanto belt japan a large scale collision in early miocene to decipher the miocene evolution of the shimanto belt of southwestern japan structural and paleo thermal studies we carried out in the western area of shikoku island all units constituting the belt both in its cretaceous and tertiary domains are in average strongly dipping to the nw or se while shortening directions deduced from fault kinematics are consistently orientated nnw sse peak paleo temperatures estimated with raman spectra of organic matter increase strongly across the southern tertiary portion of the belt in tandem with the development of a steeply dipping metamorphic cleavage near the southern tip of ashizuri peninsula the unconformity between accreted strata and fore arc basin present along the whole belt corresponds to a large paleo temperature gap supporting the occurrence of a major collision in early miocene this tectonic event occurred before the magmatic event that affected the whole belt at 15 ma the associated shortening was accommodated in two opposite modes either localized on regional scale faults such as the nobeoka tectonic line in kyushu or distributed through the whole belt as in shikoku the reappraisal of this collision leads to reinterpret large scale seismic refraction profiles of the margins where the unit underlying the modern accretionary prism is now attributed to an older package of deformed and accreted sedimentary units belonging to the shimanto belt when integrated into reconstructions of philippine sea plate motion the collision corresponds to the oblique collision of a paleo izu bonin mariana arc with japan in early miocene

two station measurement of rayleigh wave phase velocities for the huatung basin the westernmost philippine sea with obs implications for regional tectonics summary a broad band ocean bottom seismometer obs deployed 180 km east of taiwan provides a first glimpse into the upper mantle beneath the westernmost section of the philippine sea or the huatung basin hb we measured interstation phase velocities of rayleigh waves between the obs and stations on the eastern coast of taiwan the phase velocities show smooth variations from 3 8 to 3 9 km s 1 for periods of 25 40 s in this short period range phase velocities are comparable to those characterizing the 15 30 ma parece vela basin of the philippine sea modelling of the finite frequency effect proves the validity of the measurement for the average hb the shear wave velocity models inverted from the 25 to 40 s dispersion show a velocity at lithospheric depths about 0 1 km s 1 lower than that of the west philippine sea which agrees with the age effect derived from the pacific pure path model inversions incorporating the less reliable data above 40 s yield a shear velocity 4 0 km s 1 below 150 km an unrealistic value even for a hotspot plume environment the seismological evidence together with the correlation in seafloor depth suggests that the hb and the parece vela basin may have a similar age this is at odds with the previous geochronological study suggesting an early cretaceous age for the hb thermal rejuvenation of the lithosphere was examined as a potential solution to reconciling the two age models

methane occurrence is associated with sodium rich valley waters in domestic wells overlying the marcellus shale in new york state prior work suggests spatial parameters e g landscape position distance to nearest gas well can be used to estimate the amount of dissolved methane in domestic drinking water wells overlying the deep marcellus shale new york ny provides an opportunity to investigate methane occurrence prior to expansion of high volume hydraulic fracturing because unconventional gas production is currently banned in the state we sampled domestic groundwater wells for methane in 2013 n 137 across five counties of ny bordering pennsylvania and then resampled a subset of those wells in 2014 for methane concentrations and 13c ch4 and d ch4 the majority of waters from wells sampled 77 had low concentrations of methane 10 mg l dissolved methane concentrations did not change as a function of proximity to existing vertical gas wells nor other parameters indicating subsurface planes of weakness i e faults or lineaments methane levels were significantly higher in wells closer to hydrography flow lines and most strongly correlated to na hco3 water type the distribution of methane between ca hco3 n 76 and na hco3 n 23 water types significantly differed p 0 01 with median methane concentrations of 0 002 and 0 78 mg l respectively combined classification of sampled waters based on the dominant water cation well topographic position and geologic unit of well completion effectively identified wells with a greater than 50 probability of having methane concentrations exceeding 1 mg l such classification schemes may be useful as a screening tool to assess natural versus gas production related sources of methane in domestic wells

constraints of magnetostratigraphic and mineralogical data on the provenance of sediments in the parece vela basin of the western pacific abstract the sediment provenance and sedimentary processes of the parece vela basin in the western pacific during the last ca 1 myr were studied based on the analysis of magnetic properties mineral features and paleomagnetic ages of core c p19 the results show that the sedimentary sequence has a lower limit age younger than 1 072 ma and the sedimentation process can be divided into two stages with sedimentation rates of 4 0 mm kyr during ca 1 0 78 ma and 3 6 mm kyr since 0 78 ma these sedimentation rates are higher than those in the surrounding area where the sedimentation rate is generally less than 2 mm kyr taken together with the mineral features and magnetic parameters these findings show that volcanic sediments eroded from the nearby ridge were transported by currents to the sampling site these volcanic sediments represent the main source of sediment in the area with eolian inputs contributing little and nonbiogenic authigenic magnetite accounting for part of the magnetic fraction the sedimentary environment and sediment provenance have varied during the last approximately 1 million years during 997 880 ka volcanic activity increased and the input of volcanic clasts was high whereas during 880 443 ka the sedimentary environment tended to be stable and the sediment supply decreased with the increase in volcanic activity in the western pacific the sediment input increased again during ca 443 ka present

reservoir leakage along concentric faults in the southern north sea implications for the deployment of ccs and eor techniques high quality 3d seismic and borehole data in the broad fourteens basin southern north sea is used to investigate newly recognised concentric faults formed in salt withdrawal basins flanking reactivated salt structures throw depth and throw distance plots were used to understand the growth histories of individual faults as a result three families of concentric faults are identified a intra seal faults within a salt withdrawal basin b faults connecting the seal and the reservoir on the crest of an inverted anticline c raft bounding faults propagating into reservoir units they have moved obliquely and show normal throws even though they formed during a period of regional compression faults in the salt withdrawal basin and on the inverted anticline are highly segmented increasing the chances of compartmentalisation or localised fluid flow through fault linkages slip tendency analysis was carried out on the distinct fault families to compare the likelihood of slip along a fault at different pore fluid pressures and within different lithologies our results show that sections of the faults are optimally oriented with regards to maximum horizontal stresses hmax increasing the slip tendency the identified faults cut through a variety of lithologies allowing different values of pore fluid pressures to build up before faults reactivate within the vlieland sandstones pore fluid pressures of 30 mpa are not sufficient to reactivate pre existing faults whereas in the deeper posidonia shales faults might reactivate at pore fluid pressures of 25 mpa fluid flow features preferentially occur near fault segments close to failure heterogeneity in slip tendency along concentric faults and high degrees of fault segmentation present serious hazards when injecting co2 into the subsurface this study stresses the importance of high quality 3d seismic data and the need to evaluate individual fault systems when investigating potential reservoirs for carbon capture and storage and enhanced oil recovery

numerical analysis of wave induced fluid flow effects on seismic data application to monitoring of co2 storage at the sleipner field 1 in this work we analyze how patchy distributions of co2 and brine within sand reservoirs may lead to significant attenuation and velocity dispersion effects which in turn may have a profound impact on surface seismic data the ultimate goal of this paper is to contribute to the understanding of these processes within the framework of the seismic monitoring of co2 sequestration a key strategy to mitigate global warming we first carry out a monte carlo analysis to study the statistical behavior of attenuation and velocity dispersion of compressional waves traveling through rocks with properties similar to those at the utsira sand sleipner field containing quasi fractal patchy distributions of co2 and brine these results show that the mean patch size and co2 saturation play key roles in the observed wave induced fluid flow effects the latter can be remarkably important when co2 concentrations are low and mean patch sizes are relatively large to analyze these effects on the corresponding surface seismic data we perform numerical simulations of wave propagation considering reservoir models and co2 accumulation patterns similar to the co2 injection site in the sleipner field these numerical experiments suggest that wave induced fluid flow effects may produce changes in the reservoir s seismic response modifying significantly the main seismic attributes usually employed in the characterization of these environments consequently the determination of the nature of the fluid distributions as well as the proper modeling of the seismic data constitute important aspects that should not be ignored in the seismic monitoring of co2 sequestration problems

analysis of gravity anomalies in the ulleung basin east sea sea of japan and its implications for the crustal structure of rift dominated back arc basin abstract the ulleung basin ub one of three major basins in the east sea sea of japan is considered to represent a continental rifting end member of back arc basin system but is much less understood compared to the nearby yamato basin yb and japan basin jb this study examines the gravity anomalies of the ub since the variation in crustal thickness can provide important insights on the mode of extension during basin opening our analysis shows that the moho depth from the sea surface varies from 16 km at the basin center to 22 km at the edges however within the central part of the basin the crustal thickness not including sediment is more or less the same 10 12 km by varying only about 10 20 of the total thickness contrary to the previous suggestions our finding of anomalous but uniformly thick crust is consistent with the recent seismic results from the yb 14 km on average a mantle residual gravity anomaly high 20 mgal exists in the northeastern part of the ub this feature is interpreted as the location of maximum extension slightly thinner crust by 1 km together with another moderate gravity high to the southwest the two anomalies form a nne ssw line which corresponds to the direction of the major tectonic structures of the korean peninsula we argue that the a massive magmatic emplacement took place extensively in the lower crust of the ub during the opening significantly increasing its overall thickness to almost twice as that of the jb where a mid ocean ridge style seafloor spreading occurred two important post opening processes took place after the formation of uniformly thick crust post rift volcanic intrusions in the north especially in its northeast sections but had little effect on the residual gravity anomaly itself and the deflection of crust in response to differential sediment loading towards the south producing the median high in the basement in response to the flexural bending we also conducted a simple test to examine what effect the variations in the mantle potential temperature and degree of extension may have on the gravity anomaly according to our model the latter case is much more likely to cause the variations in gravity anomaly than the former

estimation of permeability anisotropy using seismic inversion for the co2 geological storage site of sleipner north sea since 1996 more than 11 mt of co2 have been injected into a deep saline aquifer the utsira sand formation at the norwegian sleipner field an unexpected application of the extensive seismic monitoring program over this field leads to the estimation of the depth dependence of the permeability anisotropy strength and direction time lapse seismic monitoring is used to follow the displacement of the injected co2 considered as a permeability tracer the upper half of the utsira sand formation exhibits large anisotropy with a ratio between the maximum and minimum horizontal permeabilities larger than six in contrast can be as small as two in the lower half the direction of the maximum horizontal permeability does not exhibit substantial depth dependence and lies between n18 e and n34 e this is in agreement with previous authors who pointed out a clear co2 plume structure markedly elongated in the north northeast south southwest direction the small topographical trap at the top of the utsira san

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

east asia plate tectonics since 15 ma constraints from the taiwan region abstract 15 ma ago a major plate reorganization occurred in east asia seafloor spreading ceased in the south china sea japan sea taiwan sea sulu sea and shikoku and parece vela basins simultaneously shear motions also ceased along the taiwan sinzi zone the gagua ridge and the luzon ryukyu transform plate boundary the complex system of thirteen plates suddenly evolved in a simple three plate system eu ph and pa beneath the manila accretionary prism and in the huatung basin we have determined magnetic lineation patterns as well as spreading rates deduced from the identification of magnetic lineations these two patterns are rotated by 15 they were formed by seafloor spreading before 15 ma and belonged to the same ocean named the taiwan sea half spreading rate in the taiwan sea was 2 cm year from chron 23 to 20 51 to 43 ma and 1 cm year from chron 20 43 ma to 5b 15 ma five plate kinematic reconstructions spanning from 15 ma to present show implications concerning the geodynamic evolution of east asia amongst them the 1000 km long linear gagua ridge was a major plate boundary which accommodated the northwestward shear motion of the ph sea plate the formation of taiwan was driven by two simple lithospheric motions i the subduction of the ph sea plate beneath eurasia with a relative westward motion of the western end a of the ryukyu subduction zone ii the subduction of eurasia beneath the philippine sea plate with a relative southwestward motion of the northern end b of the manila subduction zone the luzon arc only formed south of b the collision of the luzon arc with eurasia occurred between a and b east of a the luzon arc probably accreted against the ryukyu forearc

polyphase tectonics at the southern tip of the manila trench mindoro tablas islands philippines abstract the southern termination of the manila trench within the south china sea continental margin in mindoro is marked by a complex polyphase tectonic fabric in the arc trench gap area onshore southern mindoro the active deformation front of the manila trench is marked by parallel folds and thrusts grading southward to n50 w trending left lateral strike slip faults this transpressive tectonic regime active at least since the late pliocene has overprinted the collision of an early miocene volcanic arc with the south china sea continental margin san jose platform the collision is postdated by deposition of the late miocene early pliocene elastics of the east mindoro basin the tectonic and geological framework of this arc which overlies a metamorphic basement and eocene elastics suggests that it was built on a drifted block of the south china sea continental margin

reconstruction of subduction and back arc spreading in the nw pacific and aleutian basin clues to causes of cretaceous and eocene plate reorganizations the eocene 50 45 ma major absolute plate motion change of the pacific plate forming the hawaii emperor bend is thought to result from inception of pacific plate subduction along one of its modern western trenches subduction is suggested to have started either spontaneously or result from subduction of the izanagi pacific mid ocean ridge or from subduction polarity reversal after collision of the olyutorsky arc that was built on the pacific plate with ne asia here we provide a detailed plate kinematic reconstruction of back arc basins and accreted terranes in the northwest pacific region from japan to the bering sea since the late cretaceous we present a new tectonic reconstruction of the intraoceanic olyutorsky and kronotsky arcs which formed above two adjacent oppositely dipping subduction zones at 85 ma within the north pacific region during another pacific wide plate reorganization we use our reconstruction to explain the formation of the submarine shirshov and bowers ridges and show that if marine magnetic anomalies reported from the aleutian basin represent magnetic polarity reversals its crust most likely formed in an 85 to 60 ma back arc basin behind the olyutorsky arc the olyutorsky arc was then separated from the pacific plate by a spreading ridge so that the 55 to 50 ma subduction polarity reversal that followed upon olyutorsky ne asia collision initiated subduction of a plate that was not the pacific hence this polarity reversal may not be a straightforward driver of the eocene pacific plate motion change whose causes remain enigmatic

origin and evolution of the west philippine basin a new interpretation abstract combined analyses of the magnetic lineations and seafloor structures of the west philippine basin show that it was formed by spreading from the central basin spreading center in two distinctly different spreading phases before and after subduction was initiated along the palau kyushu trend at 45 ma b p these episodes are recognized by differences in the strike of the magnetic lineations spreading rates density and strike of fracture zones and basement relief from 60 45 ma b p spreading was ne sw relative to present orientations at a half rate of 44 mm a at about 45 ma b p the spreading direction changed to a more n s direction with a reconfiguration of the central basin spreading center into numerous short e w segments offset by closely spaced n s transform faults spreading slowed to a half rate of 18 mm a and ceased at 35 ma b p thus the west philippine basin originated at 45 ma b p by the trapping of normal ocean crust west of the initial subduction along the palau kyushu trend the 45 35 ma b p period represents the dying phase of spreading on the central basin spreading center following isolation of the west philippine basin from the plate driving forces of the pacific