uncertainties in the relative positions of the australia antarctica lord howe and pacific plates since the late cretaceous we determined parameters that describe finite rotations and their uncertainty regions for relative plate motion at the spreading centers between the pacific and antarctica plates between australia and antarctica and between the lord howe rise and australia we combined these to yield a range of possible finite rotations describing the relative positions of the pacific australia antarctica and lord howe plates since the late cretaceous if the pacific australia plate boundary has had its present trend since anomaly 18 time reconstructions show 330 110 km of motion of the pacific plate relative to the lord howe rise since anomaly 5 time 9 8 m y 420 110 km since anomaly 6 time 19 5 m y 770 330 km since anomaly 13 time 35 6 m y and 820 260 km since anomaly 18 time 43 0 m y we examined two cases for times prior to anomaly 18 assuming a late cretaceous age of australia antarctica separation if a plate boundary existed between the lord howe rise and pacific plates since the late cretaceous with no plate boundary in antarctica reconstructions with the lord howe rise fixed predict 610 200 km of westward motion of the pacific plate between the times of anomalies 31 and 22 followed by 260 100 km of northward motion between the times of anomalies 22 and 18 if the lord howe rise was fixed to the pacific plate until the eocene but a plate boundary existed between east and west antarctica reconstructions show very little motion across this boundary between the times of anomalies 31 and 22 followed by convergence between the times of anomalies 22 and 18 this second case also brings 70 80 m y paleomagnetic poles from the pacific and east antarctica plates into better agreement than the first case but large uncertainties in the reconstructions do not allow the first case to be conclusively eliminated

texture of the uppermost inner core from forward and back scattered seismic waves body waves interacting with the boundary of the solid inner core at narrow and wide angles of incidence provide independent constraints on a heterogeneous texture that may originate from the process of solidification the equatorial quasi eastern hemisphere of the uppermost 50 100 km of the inner core is characterized by a higher isotropic p wave velocity lower q s inferred from pkikp and simpler pkikp pulses compared to adjacent regions in the western hemisphere and polar latitudes compared to this region the adjacent western primarily pacific equatorial region is characterized by higher q s and a higher level of coda excitation following pkikp lateral variations in both inner core q inferred from transmitted pkikp and inner core heterogeneity inferred from the coda of reflected pkikp can be modeled by lateral variations in a solidification fabric in an actively crystallizing eastern equatorial region characterized by upwelling flow in the outer core fabrics that explain strong attenuation and the absence of attenuation and velocity anisotropy in short range 120 140 pkikp and weak pkikp codas have an anisotropy of scale lengths with longer scale lengths in the vertical direction perpendicular to the inner core boundary in less actively solidifying regions in the equatorial western hemisphere longer scale lengths tend to be more parallel to the inner core boundary consistent with outer core flow tangent to the inner core boundary or viscous shearing and recrystallization in the horizontal direction away from more actively crystallizing regions in the eastern hemisphere this texture is less effective in attenuating pkikp by forward scattering lateral variation in the equatorial western hemisphere between vertical versus horizontal oriented plate like textures may explain lateral variations from weak to strongly back scattered pkikp coda and from strong to weak velocity and attenuation anisotropy in short range pkikp 2007 elsevier b v all rights reserved

middle miocene deduction and late miocene beginning of collision registered in the hengchun peninsula geodynamic implications for the evolution of taiwan abstract taiwan is located in the axis of the manila trench it results from an oblique collision between the northernmost part of the luzon arc and the chinese passive margin this active collision follows the subduction of the oligocene miocene oceanic crust of the south china sea along the manila trench the tectonized chinese margin emerged in the hengchun peninsula south taiwan gentle folds which are delineated by the quaternary reefal limestones demonstrate recent deformations these folds deformed a thick detrital sequence of miocene age ssuchung chi series which was previously strongly folded and thrust westward axis ns n20 upon the renting melange of latest miocene age these main deformations sealed by the middle pliocene are the evidence for the onset of collision in this part of taiwan at the end of the miocene because of its obliquity the collision started already in the northern part of taiwan during the late miocene 6 7 8 ma the ssuchung chi series a sequence of proximal turbidites has contained since the middle miocene nn 6 13 ma fragments of an oligocene to lower miocene oceanic crust this ophiolitic material is very similar to the east taiwan ophiolite of the coastal range it originated most probably from a slice of south china sea crust obducted in middle miocene times 13 14 ma upon the chinese margin north of the hengchun peninsula this obduction occurred 7 to 8 ma before the beginning of collision these results make it possible to propose an evolutionary model for taiwan from the oligocene to the recent with the different phases of a collision between a volcanic arc and a passive margin

japan sea a pull apart basin abstract recent field work in the hokkaido central belt and marine geology studies along the eastern margin of japan sea in addition to previously published data lead us to propose a new model of opening of the japan sea the synthesis of both on land and offshore structural data gives new constraints about the structural evolution of the system the rhombohedral shape of the japan basin and the particular tectonic behaviour of the margins on both east and west sides can be explained by an early eo oligocene rifting of a pull apart basin accommodated along two large right lateral shear zones east of korea and west of northeast japan and sakhalin it is followed during upper oligocene lower miocene by the main opening of the japan basin as a mega pull apart then a back arc spreading probably related to the subduction process induced the creation of the yamato and tsushima basins at the end of lower miocene and in middle miocene clockwise rotation of southwest japan larger than 20 or major bending of honshu mainland deduced from paleomagnetic studies is unlikely at this time since 1 or 2 my b p to present compression prevails along the eastern margin of the japan sea the generation of marginal basins as pull apart basins along intracontinental strike slip faults is a mechanism which has been proposed by other authors concerning the south china sea the question then is whether the fragmentation of the asiatic continent is an intracontinental deformation related process as proposed here or a subduction related one

a p wave velocity model of earth s core present earth core models derived from the retrieval of global earth structure are based on absolute travel times mostly from the international seismological centre isc and or free oscillation eigenfrequencies many core phase data are left out of these constructions e g pkp differential travel times amplitude ratios and waveforms this study is an attempt to utilize this additional information to construct a model of core p wave velocity which is consistent with the different types of core phase data available in conjunction with our waveform modeling we used 150 differential time measurements and 87 amplitude ratio measurements which were the highest quality observations chosen from a large population of global digital seismograph network gdsn records as a result of fitting these various data sets a one dimensional p wave velocity model of the core prem2 is proposed this model modified from the preliminary reference earth model prem dziewonski and anderson 1981 shows a better fit to the combined data set than any of the existing core models major features of the model include a sharp velocity discontinuity at the inner core boundary icb with a large jump 0 78 km s and a low velocity gradient at the base of the fluid core the velocity is nearly constant over the lower 100 km of the outer core the model features a depth dependent q structure in the inner core such that a constant t for the inner core fits the amplitude ratios and waveforms of short period waves moderately well this means the top of the inner core is more attenuating than the deeper part of the inner core in addition the p velocity in the lowermost mantle is reduced from that of prem as a baseline adjustment for the observed separations of the df and ab branches of pkp at large distances

distinct layering in the hemispherical seismic velocity structure of earth s upper inner core the existence of hemispherical variation in the earth s inner core is well documented but consensus has not yet been reached on its detailed structure the uppermost layers are a region of particular importance as they are directly linked to the growth processes and post solidification mechanisms of the inner core here we use a large pkikp pkikp differential travel time residual data set to derive a model for the upper inner core providing new constraints on its isotropic and anisotropic velocity and the amount of scattering we find that the eastern and western hemisphere are separated by sharp boundaries this is incompatible with the recently proposed inner core translation model but might be explained by differences in outer core convection and inner core solidification rates the eastern hemisphere displays weak anisotropy of 0 5 1 0 the western hemisphere on the other hand is characterized by the presence of an isotropic upper layer with a thickness of 57 5 km with anisotropy of 2 8 appearing at deeper depths the boundary between the isotropic layer and the deeper anisotropy appears sharp we also detect for the first time a high velocity layer at the top of the eastern hemisphere with a thickness of 30 km which we interpret as being due to an increased amount of light elements there appears to be no relationship between the layered structure in the two hemispheres with abrupt changes in velocity with depth in one hemisphere without any significant change at the same depth in the other hemisphere our results indicate that there is a difference in composition and mineral structure between the hemispheres resulting in differing responses to external processes

a glassy lowermost outer core summary new theories for the viscosity of metallic melts at core pressures and temperatures together with observations of translational modes of oscillation of earth s solid inner core suggest a rapid increase in the dynamic viscosity near the bottom of the liquid outer core if the viscosity of the lowermost outer core f region is sufficiently high it may be in a glassy state characterized by a frequency dependent shear modulus and increased viscoselastic attenuation in testing this hypothesis the amplitudes of high frequency pkikp waves are found to be consistent with an upper bound to shear velocity in the lowermost outer core of 0 5 km s 1 at 1 hz the fit of a maxwell rheology to the frequency dependent shear modulus constrained by seismic observations at both low and high frequency favours a model of the f region as a 400 km thick chemical boundary layer this layer has both a higher density and higher viscosity than the bulk of the outer core with a peak viscosity on the order of 10 9 pa s or higher near the inner core boundary if lateral variations in the f region are confirmed to correlate with lateral variations observed in the structure of the uppermost inner core they may be used to map differences in the solidification process of the inner core and flow in the lowermost outer core

kinematics of the philippine sea plate in this study new instantaneous rotation vectors describing the motion of the philippine sea plate relative to surrounding plates are determined that most nearly satisfy all available geological geophysical and seismological observations along the plate margins pacific philippine and caroline pacific poles are generated from a set of trial eurasia philippine poles of rotation using published determinations of the eurasia pacific and caroline philippine poles the pacific philippine caroline pacific and eurasia philippine poles are then checked for agreement with 1 slip vectors determined from focal mechanisms of earthquakes along the ryukyu trench and the nankai trough 2 convergence at the sagami trough 3 extension in the mariana trough 4 extension behind the bonin trench 5 extension in the western sorol trough and 6 convergence at the mussau trench trial eurasia philippine poles are eliminated if predicted plate motions do not match observed plate motions the locations and rotation rates of the three best fitting poles are eurasia philippine 37 0 n 141 0 e 1 60 ma pacific philippine 3 8 n 130 4 e 1 68 ma caroline pacific 16 9 n 141 4 e 0 54 ma the plate motions predicted by these new poles are compared with the spatial distribution of earthquakes and new focal mechanism solutions in the complex region where the philippine pacific and caroline plates intersect intermediate and deep focus earthquake hypocenters along the southern mariana arc show a southward decrease in the length of the seismic zone which qualitatively agrees with the predicted convergence rate as the pole of rotation is approached and as the effect of back arc spreading in the mariana trough disappears the absence of volcanism and intermediate depth seismicity along the southernmost mariana yap and palau sections of the plate boundary is attributed to very low convergence rates a new focal mechanism solution along the trench south of guam is the first clear underthrusting solution found along the mariana trench and agrees with the convergence direction implied by the pacific philippine pole of rotation no clear underthrusting focal mechanism solutions were found along the yap and palau trenches although three high angle thrust solutions demonstrate that compression is occurring in the upper plate perpendicular to the plate boundary

paleomagnetic evidence for the miocene counter clockwise rotation of northeast japan rifting process of the japan arc paleomagnetic results have been obtained from more than 700 oriented samples in igneous rocks of cretaceous to miocene age from northeast japan the remanent magnetizations of welded tuffs with age between 32 ma and 21 ma old from 17 widely distributed sampling sites in northeast japan are fairly well grouped with a mean ofd 41 2 i 56 5 95 7 2 the remanent magnetization of welded tuffs of 14 ma and 11 ma from 4 sites shows a northerly direction d 11 7 i 60 5 95 15 3 these data imply that northeast japan was apparently subjected to a counter clockwise rotation through 47 14 about a vertical axis on the representative point of northeast japan 141 e 39 n during the past 21 ma comparing with the period of the clockwise rotation of southwest japan the differential rotation between northeast japan and southwest japan is concluded to have occurred concurrently during the period between 21 ma and 12 ma a rifting model of the japan arc is proposed northeast japan rotated counter clockwise through 50 about a northern pivot at 146 e 44 n while southwest japan rotated clockwise through 54 about a southern pivot at 129 e 34 n the differential rotation about different rotation pivots gives rise to the opening of a diamond shaped back arc basin the japan sea

geodynamics and tectonic evolution of the panxi rift abstract this paper gives a report on the geophysical geological and geochemical studies of the panxi rift the results of wide scale gravity measurements deep seismic soundings aeromagnetic geomagnetic geothermal and seismicity investigations which have been carried out by the institute of geophysics of the academia sinica are presented and analysed thereby the results from geotraverse i are discussed in detail the following rift associated features have been found an intracrustal low velocity and resistivity layer lower than normal velocities of 7 6 7 9 km s and low resistivity of 250 m in the upper mantle elevated velocities and resistivities in the central region doming as analysed from the gravity and seismicity data n s rift oriented positive magnetic anomalies in the central region n s striking faults and n s oriented distribution of basic and ultrabasic proterozoic rocks and grabens it is shown that the region containing the panxi rift was subjected to extensional and compressional tectonics which started in the late proterozoic and found their climax in the late palaeozoic the recent compressional tectonics are considered to relate to the collision of the indian and eurasian plates rifting and igneous processes during the tectonic evolution of the panxi rift are compared with the major continental rifts and conceptual models for the various rifting episodes are presented

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

formation and evolution of the yilan yitong graben abstract the chinese continental crust became a part of the large eurasian plate since the triassic and then entered a new developing stage in the way of tectonic evolution unique to the continental crust the eastern part of china was affected by the progradation force of the pacific and indian plates extensional faults trending in a nne direction occurred moreover as materials from the mantle flowed upward and the horizontal tension increased a huge mesozoic cenozoic rift system was formed as a part of this rift system the yilan yitong graben has four features as follows 1 1 basement faults developed well in the graben controlled by the boundary faults 2 2 during its evolution the graben underwent both yenshan and himalaya movements which were apparently characterized by downfaulting and depression 3 3 two groups of fault systems which mainly consisted of normal faults resulted in numerous regularly distributing uplifts and sags 4 4 material from the mantle erupted frequently along the fault zones the geotherm turned upwards

paleomagnetic constraints on the late cretaceous and cenozoic tectonics of southeastern asia abstract many features of the cenozoic tectonic history of central and southeastern asia can be understood as direct consequences of the thrust and penetration of india into asia recent indentation experiments with plasticine tapponnier et al 7 have extended this idea and have led to the prediction of a pattern of large rotations and displacements of continental blocks that can be tested by paleomagnetism the available cretaceous and cenozoic paleomagnetic data from this part of the world have been reviewed and a new apwp for eurasia has been constructed for reference the negligible rotation of south china and large clockwise rotation of indochina are consistent with the model i e with an history of large scale left lateral strike slip motion along the altyn tagh and red river faults data from malaya and borneo can be reconciled with the model although in a less straightforward fashion the large counter clockwise rotation of south tibet implies that it rotated in sympathy with india during the collision and suggests that future indentation experiments should include this feature finally a middle cretaceous reconstruction of the south margin of asia is proposed one interesting result is the restored continuity of geological features in tibet and indochina with active subduction of oceanic indian plate crust taking place to the south at subtropical latitudes

bent structural trends of japan flexural slip folding related to the neogene opening of the sea of japan the trend of the pre miocene structures of japan is bent in kyushu and central japan on land geologic features such as the trend of synmetamorphic jurassic lineations neocomian basins and late cretaceous strike slip faults have distorted trends as a result of the miocene opening of the sea of japan accentuation of the bending in central japan could be due to the pliocene quaternary collision of japan with the izu ridge a preopening reconstruction of japan is proposed the bent structural trends of kyushu and central japan along with neogene fault motions are explained by a two dimensional model of broad scale flexural slip folding

inner core freezing and melting constraints from seismic body waves abstract hypotheses for the mechanisms of growth of earth s inner core can be tested from observations of lateral variations of elastic and anelastic properties near the inner core boundary differential travel times of pkikp pkikp and the apparent travel time of p waves diffracted around the inner core are consistent with the existence of a stably stratified globally uniform region of reduced velocity gradient at the bottom of the outer core this layer may be a zone of iron enrichment relative to the bulk of the outer core representing a zone either of crystallization or melting of the inner core weak backscattered pkikp in the equatorial eastern hemisphere can be made consistent with either a hypothesis of freezing or melting in a melting hypothesis the eastern hemisphere is texturally older and smoother with larger crystals or smaller elastic contrasts across grain boundaries providing weaker backscatter to the coda of pkikp additional observations consistent with higher homologous temperature and melting in this region are the existence of a thin low velocity layer in the uppermost inner core inferred from multipathing of pkikp pkikp waves and a bright spot in the amplitude of pkiikp waves observed at antipodal distances alternatively a freezing hypothesis in the eastern hemisphere is an inner core that is growing radially outward with crystals stretched in the vertical direction this hypothesis can simultaneously explain weak backscattered pkikp coda and high attenuation of transmitted pkikp from the loss of backscattered energy at angles perpendicular to vertically oriented crystals strong backscattering observed in the coda of pkikp sampling the equatorial western hemisphere of the inner core agrees best with a heterogeneity having an isotropic distribution of scale lengths in the uppermost inner core

unzipping of the volcano arc japan abstract a working hypothesis for the recent evolution of the southern volcano arc japan is presented which calls upon a northward progressing sundering of the arc in response to a northward propagating back arc basin extensional regime this model appears to explain several localized and recent changes in the tectonic and magrnatic evolution of the volcano arc most important among these changes is the unusual composition of iwo jima volcanic rocks this contrasts with normal arc tholeiites typical of the rest of the izu volcano mariana and other primitive arcs in having alkaline tendencies high concentrations of light ree and other incompatible elements and relatively high silica contents in spite of such fractionated characteristics these lavas appear to be very early manifestations of a new volcanic and tectonic cycle in the southern volcano arc these alkaline characteristics and indications of strong regional uplift are consistent with the recent development of an early stage of inter arc basin rifting in the southern volcano arc new bathymetric data are presented in support of this model which indicate 1 1 structural elements of the mariana trough extend north to the southern volcano arc 2 2 both the mariana trough and frontal arc shoal rapidly northwards as the volcano arc is approached 3 3 rugged bathymetry associated with the rifted mariana trough is replaced just south of iwo jima by the development of a huge dome 50 75 km diameter centered around iwo jima such uplifted domes are the immediate precursors of rifts in other environments and it appears that a similar situation may now exist in the southern volcano arc the present distribution of unrifted volcano arc to the north and rifted mariana arc to the south is interpreted not as a stable tectonic configuration but as representing a tectonic snapshot of an arc in the process of being rifted to form a back arc basin

block rotation by strike slip faulting structural and paleomagnetic evidence geometric analysis shows that motion of sets of strike slip faults should cause block rotation otherwise the distorted fault domain would not fit with its surrounding the sense of the block rotation depends on the sense of fault slip and the spacing and orientation of the faults all of which can be obtained from structural data the predictions of the geometric model can then be independently tested by paleomagnetic measurements the model was tested in northern israel structural data reveal several domains of contemporaneous right and left lateral strike slip faults paleomagnetic measurements show that in a domain of nnw trending left lateral faults blocks rotated 23 3 8 2 clockwise and in a domain of right lateral faults blocks rotated 22 4 9 0 anticlockwise these results are in agreement with the prediction of the geometric analysis of the structure the combined results of the heterogenous deformation of the area resembles a pure shear which allows n s extension by about 1 3 in other domains rotations of 34 6 9 1 and 53 1 11 0 were found in their original orientation the left and right lateral faults intersected at angles of 60 70 enclosing the principal axis of shortening in accordance with theories of brittle failure as a result of block rotations this angle is now larger and may reach 110 the results demonstrate large block rotations in different senses in domains of strike slip faults the independent structural and paleomagnetic data can be interpreted within the framework of a simple geometric model northern israel is considered to be a good model for this efficient mechanism of intraplate deformation which is expected to occur in similar tectonic settings elsewhere

regional variations in the uppermost 100 km of the earth s inner core 1 the structure of the uppermost 100 km of the inner core was examined from pkikp and pkikp waveforms in the distance range of 118 140 we found evidence of a low velocity layer in the uppermost inner core in the equatorial region predominantly located between longitude 20 w to 140 e in the latitudinal direction the anomaly is detectable from 35 s beneath the indian ocean to 60 n underneath asia the maximum thickness of the low velocity layer inferred from waveform modeling is 40 km with velocity jump of about 3 we speculate that this layer may represent newly solidified core in the area where vigorous compositional convection in the outer core coincides with new crystal growth in the inner core

a geophysical study of the manila trench luzon philippines 1 crustal structure gravity and regional tectonic evolution the manila trench subduction zone is an active convergent plate margin between the south china sea and the northern philippines the trench trends northerly and is associated with a volcanic arc an east dipping benioff zone beneath luzon and a well developed fore arc basin system the luzon trough fore arc basins lie landward of the manila trench and contain up to 4 5 km of cenozoic sediments offscraping and accretion of turbidite sediments deposited in the manila trench have resulted in the upbuilding and outbuilding of an accretionary prism which forms the seaward margin of the fore arc basin system the uplifted zambales ophiolite and its offshore extension form the landward side of the fore arc basin multichannel seismic reflection profiles reveal that folding and thrust faulting of trench strata occur at the base of the trench slope the major structural decollement at the manila trench usually forms near the unconformity separating the hemipelagic sediments from the turbidite sediments subduction and perhaps underplating of the basal hemipelagic trench sediments accompany the deformation and accretion of the overlying clastic deposits faulting and uplift occur within a narrow zone of the fore arc where seamounts associated with the relict spreading center of the south china sea basin have presumably been subducted north of lingayan gulf the fore arc is disrupted by active fault systems that trend north and northwest offshore from the luzon central valley and from the southern cordillera central based on regional geological and tectonic observations we infer that subduction probably began at the manila trench in late oligocene to middle miocene time the long term average convergence rate at the manila trench is estimated to be between 10 and 20 mm yr and may be slowing in the north owing to the collision of taiwan with eurasia

sunda strait and central sumatra fault sunda strait marks the limit between java trench frontal subduction and sumatra trench oblique subduction the right lateral central sumatra fault accommodates the oblique subduction it does not pass sunda strait but ends between sumatra and java in a complex pattern of dominantly normal faults associated with subsidence seismicity and volcanism we examine the implications of this phenomenon in sunda strait and in the adjacent fore arc basin and accretionary prism which underwent north south compression due to subduction as well as east west extension we propose that the strait is a consequence of the northwestward motion of the southwestern sumatra block along the central sumatra fault we also propose that the extensional zone in the sunda strait widens considerably to the southwest toward the trench and changes into a composite zone of deformation affected by strike slip as well as normal faulting

anomalously strong observations of pkikp pcp amplitude ratios on a global scale the inner core boundary marks the phase transition between the solid inner core and the fluid outer core as the site of inner core solidification the boundary provides insight into the processes generating the seismic structures of the inner core in particular it may hold the key to understanding the previously observed hemispherical asymmetry in inner core seismic velocity anisotropy and attenuation here we use a large pkikp pcp amplitude ratio and travel time residual data set to investigate velocity and density contrast properties near the inner core boundary although hemispherical structure at the boundary has been proposed by previous inner core studies we find no evidence for hemispheres in the amplitude ratios or travel time residuals in addition we find that the amplitude ratios are much larger than can be explained by variations in density contrast at the inner core boundary or core mantle boundary this indicates that pkikp is primarily observed when it is anomalously large due to focusing along its raypath using data in which pkikp is not detected above the noise level we calculate an upper estimate for the inner core boundary icb density contrast of 1 2 kg m 3 the travel time residuals display large regional variations which differ on long and short length scales these regions may be explained by large scale velocity variations in the f layer just above the inner core boundary and or small scale topography of varying magnitude on the icb which also causes the large amplitudes such differences could arise from localized freezing and melting of the inner core

back arc extension in the okinawa trough the okinawa trough lying to the east of china is a back arc basin formed by extension within continental lithosphere behind the ryukyu trench arc system middle to late miocene uplift associated with normal faulting of the initially adjacent ryukyu nonvolcanic arc and the taiwan sinzi folded belt corresponds to the first rifting phase the timing of rifting is supported by the presence of marine sediments of corresponding age drilled in the northern okinawa trough the rifting occurred after a major early miocene change in the motion of the philippine plate with respect to eurasia and ceased during the pliocene a second rifting phase started about 2 m y ago at the plio pleistocene boundary and has continued until the present time it has proceeded to a more advanced stage in the middle and southern okinawa trough than it has farther north detailed bathymetric sea beam seismic reflection and magnetics data collected during the pop 1 cruise of the r v jean charcot reveal the principal features of the extensional processes the back arc spreading phase started very recently in the southern and middle okinawa trough as exemplified by several en echelon and in some cases overlapping active central graben oriented n70 e n80 e some of these depressions are intruded by volcanic ridges of fresh back arc basalt with associated large magnetic anomalies transform faults between these en echelon active rifts are not obvious we suggest that the major part of the southern okinawa trough is underlain by a thinned continental crust and that except for the system of en echelon rifts of the southern okinawa trough the back arc basin oceanic domain is limited to a width of a few tens of kilometers or less in the axial portion of the trough the system of axial back arc volcanic ridges that occur in the rifts ends at the latitude of okinawa island whereas active volcanoes in the ryukyu arc occur only north of okinawa island we refer to this transition between active arc and back arc volcanism as the volcanic arc rift migration phenomenon vamp globally back arc volcanism propagated from the southern okinawa trough to the okinawa vamp area rifting continues to occur in the northern okinawa trough but is not yet accompanied by associated volcanism the okinawa vamp area is characterized by a series of parallel basaltic ridges oriented n75 e with associated linear magnetic anomalies characteristic of dyke intrusions we suggest that the formation of the back arc oceanic domain took place along the axial back arc extensional zone trending n75 e and that this zone presently ends at the southern extremity of the active volcanic chain the initial phase of formation of back arc basin oceanic crust is non steady state and is characterized by the lack of a developed fracture zone pattern the termination of the vamp area in the direction of the volcanic zone of the arc is consistent with the suggestion of molnar and atwater that the volcanic arc is a fundamental line of weakness which determines where initial back arc spreading occurs apparently back arc extension initially occurred within the continental lithosphere located westward of the ryukyu arc along its whole length but the subsequent back arc volcanism was initiated in the southernmost portion of the region and then moved northward this migration was accompanied by the shutting down of volcanic activity along the abandoned portions of the arc it is this transfer of volcanism that we call the vamp process thus arc and back arc basin volcanism seem to be associated in such a manner that spreading tends to migrate simultaneously with a cessation in volcanic activity along the arc this interplay of arc and back arc activity is probably linked to changes in the parameters of plate convergence since the plate motion in the phillippine sea is oblique to the trench at least in the southern part of the okinawa trough we suggest that the oblique resisting force applied to the edge of the overriding plate engenders the development of en echelon extensional features within and behind the arc the motion of the ryukyu platelet with respect to eurasia is consequently an extensional strike slip motion trending roughly parallel to the okinawa trough

on the inner outer core density contrast from pkikp pcp amplitude ratios and uncertainties caused by seismic noise summary the inner core boundary icb of the earth is characterized by a discontinuous change in elastic properties between the liquid outer and solid inner core in the ray theory approximation a measure of the density contrast at the icb is given by the amplitude ratio of p waves reflected from the core mantle boundary cmb pcp waves and the icb pkikp waves since that ratio conveniently appears in an explicit form in the transmission reflection coefficient equations the results for inner outer core density contrast derived from direct amplitude picks of these waves in the time domain have varied significantly among different authors the transmission reflection coefficients on the liquid solid and solid liquid boundaries derived from ground displacements enable a direct comparison between the amplitude measurements on displacement seismograms in the time domain and theoretical values a new approach is proposed and applied to integrate effects of microseismic and signal generated noise with the amplitude measurements thus providing a direct maximal uncertainty measure to suppress the effects of varying radiation pattern and distinctively different ray paths at longer epicentral distances this new method was applied to high quality arrivals of pcp and pkikp waves from a nuclear explosion observed at epicentral distances 10 20 from recording stations the resulting uncertainties are high precluding precise estimates of the icb density contrast but provide a robust estimate of an upper bound from body waves of about 1100 kg m 3 median values of two amplitude ratios observed around 17 epicentral distance indicate a small density contrast of 200 300 kg m 3 and suggest the existence of zones of suppressed density contrast between the inner and the outer core a density contrast stronger than 5000 kg m 3 at the cmb or a combination of both

incipient subduction and deduction along the eastern margin of the japan sea the eastern margin of the japan sea has been under convergent tectonics since sometime after the pliocene the suture line is on the ocean continent boundary between the japan yamato basins and the tohoku northeast japan arc incipient subduction and obduction of the oceanic and sub oceanic crust are observed along the suture line with the occurrence of numerous n s trending fault bounded ridges and troughs focal mechanism solutions of several compressional type earthquakes with magnitudes of m 6 9 to 7 7 which have occurred along the zone are consistent with the observation of geological thrust faults the convergent stresses in the japan sea may be due to india eurasia collision and its associated intra plate or inter microplate movements in east asia the eastward movement of the amurian plate causes baikal extension along its western margin and the japan sea convergence along its eastern margin

thermochemical flows couple the earth s inner core growth to mantle heterogeneity the uppermost 100 km of the earth s inner core is subject to an east west divide seismic waves travel more rapidly and are attenuated more severely in the eastern than in the western hemisphere and the west is more anisotropic that is seismic waves travel at different speeds in different directions than the eastern hemisphere the origin of this hemispherical dichotomy has remained enigmatic aubert et al now show that a model incorporating thermochemical convection and dynamo action can account for these effects by producing a large scale long term outer core flow that couples the heterogeneity of the inner core with that of the lower mantle seismic waves sampling the top 100 km of the earth s inner core have revealed that the eastern hemisphere is seismically faster more isotropic and more attenuating than the western hemisphere it is now shown that a single model of thermo chemical convection and dynamo action can account for all these effects by producing a large scale long term outer core flow that couples the heterogeneity of the inner core with that of the lower mantle seismic waves sampling the top 100 km of the earth s inner core reveal that the eastern hemisphere 40 e 180 e is seismically faster1 2 more isotropic2 3 and more attenuating4 than the western hemisphere the origin of this hemispherical dichotomy is a challenging problem for our understanding of the earth as a system of dynamically coupled layers previously laboratory experiments have established that thermal control from the lower mantle can drastically affect fluid flow in the outer core5 which in turn can induce textural heterogeneity on the inner core solidification front6 the resulting texture should be consistent with other expected manifestations of thermal mantle control on the geodynamo specifically magnetic flux concentrations7 8 in the time average palaeomagnetic field9 10 over the past 5 myr and preferred eddy locations11 in flows imaged below the core mantle boundary by the analysis of historical geomagnetic secular variation12 here we show that a single model of thermochemical convection and dynamo action can account for all these effects by producing a large scale long term outer core flow that couples the heterogeneity of the inner core with that of the lower mantle the main feature of this thermochemical wind is a cyclonic circulation below asia which concentrates magnetic field on the core mantle boundary at the observed location and locally agrees with core flow images this wind also causes anomalously high rates of light element release in the eastern hemisphere of the inner core boundary suggesting that lateral seismic anomalies at the top of the inner core result from mantle induced variations in its freezing rate

the fenwei rift and its recent periodic activity the fenwei rift on the southern sector of the jin shaan rift system of china is marked by a crescent shaped valley 600 km in length and 30 90 km in width depressed up to 10 km and filled with about 7000 m of cenozoic deposits bounded on both northern and southern sides by majestic mountain ranges the geometry of the rift valley is characterized by six branch depressions and five intervening swells extending east northeastward in a dextral en echelon pattern and bounded on both sides by abrupt topographic slopes reflecting the underlying faults these are typically a system of growth faults having downthrows ranging from 800 m to 10 km and dipping toward the centre of the valley forming an asymmetric graben structure the geometry kinematics and evolution of these faults have had controlling influences on the neotectonic movement of the rift and its recent periodic activity as the present overall form of the rift valley estimates of the amount of extension across the rift for various recent geological periods were obtained from calculations made on the fault separation of corresponding stratigraphie horizons the total amount of extension in response to tensile stresses acting in a direction varying from 25 nw on the west to 70 nw on the northeast is estimated to be 9065 m since the beginning of the rift formation in the eocene whereas the rate of extension in the recent is 4 5 mm yr and in modern times it is 8 24 mm yr the amount of left lateral displacement across the rift during various stages of its development was also calculated from the observed effects of strike slip movement on the drainage system the left lateral offset since the mid pleistocene is approximately 7170 m and the offset rate in modern times is 6 mm yr these estimates suggest that the fenwei rift has been a place of intense neotectonic activity details of more recent activity of the rift were investigated in terms of the various rift related phenomena such as seismic events ground fissuring epeirogenic movement shifting of streams or lakes and climatic changes which have occurred in the historic period seven highly periodic cycles of seismicity are recognized from available historic records of the earthquakes that have occurred in the rift region during the last 4000 yrs each cycle appears to begin with a period of intense seismicity consisting of a series of violent shocks chiefly of m6 8 and lasting approximately 200 yrs and then passes on to a prolonged period of quiescence of about 600 yrs during which only minor seismic events occur the 22 events of ground fissuring recorded in the historic annals are also concentrated in several distinctive periods of intense activity which coincide with seven corresponding periods of active seismicity suggesting that seismic events and ground fissures resulted from the same cause both being indications of periodic tectonic activity of the rift moreover periodic and coincident with periods of tectonic activity are the occurrences of the natural phenomena related to rifting such as the appearance and disappearance of lakes the shifting of streams and changes of river water from clear to turbid

presumption of large scale heterogeneity at the top of the outer core basal layer abstract a layer of reduced p velocity gradient with thickness of about 100 200 km has been identified at the base of the liquid core from seismological methods it has been interpreted as a dense layer resulting from partial re melting of the inner core which is depleted in light elements with respect to the liquid core during freezing in an attempt to specify where freezing and re melting occur the structure of this basal layer is investigated with the seismological core phase pkpbc which has its turning point in the lower third of the outer core the large pkpbc data set of the ehb catalog distributed by the international seismological centre is analyzed in order to compensate for the uneven distribution of the data and to minimize the influence of mantle heterogeneities the travel time anomalies are binned inside equal area and equal azimuth sectors sampling the base of the liquid core at different depths most of the observed variations in the binned travel time residuals are not significant according to their confidence level the only features which could be significant are a large patch with a velocity increase of about 0 5 located at the top of the basal layer beneath the eastern hemisphere and the complementary velocity decrease beneath the western hemisphere and the south pole this observation suggests that some freezing or re melting processes occur at the top of the basal layer with a hemispherical dissymmetry if confirmed it may give strong constraints on the fate of the light elements during the freezing and re melting process and on their interaction with the basal layer and the overlying liquid core

the banda celebes sulu basin a trapped piece of cretaceous eocene oceanic crust the banda celebes and sulu basins are poorly understood marginal seas located at the junction of the eurasian indian australian and pacific philippine sea plates the incompleteness of the data sets the complex geology of the surrounding islands and the late cenozoic evolution involving subduction rifting transform faulting and island arc collision have complicated tectonic interpretation of the region the banda basin is underlain by oceanic crust1 3 bowin et al 2 and lapouille et al 3 have suggested that it is a trapped oceanic basin which was once continuous with the late jurassic argo abyssal plain off north west australia similarly the celebes and sulu basins are also underlain by oceanic crust4 6 on the basis of newly identified magnetic reversal ages fig 1 heat flow data and on land geological studies we suggest that the celebes and sulu seas may have been continuous with the banda basin this cretaceous eocene basin has been dissected into its present configuration by tertiary tectonic processes

wide scale detection of earthquake waveform doublets and further evidence for inner core super rotation summary we report on more than 100 earthquake waveform doublets in five subduction zones including an earthquake nest in bucaramanga colombia each doublet is presumed to be a pair of earthquakes that repeat at essentially the same location these doublets are important for studying earthquake physics as well as temporal changes of the inner core particularly our observation from one south sandwich islands ssi doublet recorded at station ink in canada shows an inner core traveltime change of 0 1 s over 6 yr confirming the innercore differential motion occurring beneath central america observations from one aleutian islands doublet recorded at station bosa in south africa and from one kuril islands doublet recorded at station bdfb in brazil show an apparent inner core traveltime change of 0 1 s over 7 yr and 6 yr respectively providing evidence for the temporal change of inner core properties beneath central asia and canada respectively on the other hand observations from one tonga fiji solomon islands doublet recorded at station ptga in brazil and from one bucaramanga doublet recorded at station wrab in australia and station chto in thailand show no little temporal change no more than 0 005 syr 1 if any of inner core traveltimes for the three corresponding ray paths for which the path in the inner core is nearly parallel to the equatorial plane such a pattern of observations showing both presence and possible absence of inner core traveltime change can be explained by the geometry and relative directions of ray path lateral velocity gradient and inner core particle motion due to an eastward super rotation of a few tenths of a degree per year

inner core boundary topography explored with reflected and diffracted p waves abstract the existence of topography of the inner core boundary icb can affect the amplitude phase and coda of body waves incident on the inner core by applying pseudospectral and boundary element methods to synthesize compressional waves interacting with the icb these effects are predicted and compared with waveform observations in pre critical critical post critical and diffraction ranges of the pkikp wave reflected from the icb these data sample overlapping regions of the inner core beneath the circum pacific belt and the eurasian north american and australian continents but exclude large areas beneath the pacific and indian oceans and the poles in the pre critical range pkikp waveforms require an upper bound of 2 km at 1 20 km wavelength for any icb topography higher topography sharply reduces pkikp amplitude and produces time extended coda not observed in pkikp waveforms the existence of topography of this scale smooths over minima and zeros in the pre critical icb reflection coefficient predicted from standard earth models in the range surrounding critical incidence 108 130 this upper bound of topography does not strongly affect the amplitude and waveform behavior of pkikp pkikp at 1 5 hz which is relatively insensitive to 10 20 km wavelength topography height approaching 5 km these data however have a strong overlap in the regions of the icb sampled by pre critical pkikp that require a 2 km upper bound to topography height in the diffracted range 152 topography as high as 5 km attenuates the peak amplitudes of pkikp and pkpcdiff by similar amounts leaving the pkpcdiff pkikp amplitude ratio unchanged from that predicted by a smooth icb the observed decay of pkpcdiff into the inner core shadow and the pkikp pkpcdiff differential travel time are consistent with a flattening of the outer core p velocity gradient near the icb and iron enrichment at the bottom of the outer core

inner core rotation and its variability from nonparametric modeling 1 we present a new approach for studying the inner core rotation and its variability by separating the underlying structure from its time evolution this is achieved by fitting a large set of existing seismic bc df traveltime data with a smoothing spline analysis which is implemented in the statistics package r this method allows us to separate the time independent mantle contribution from the time dependent core contribution without any a priori constraints and to also estimate the error of the fit we add our newly acquired seismic data from the passcal experiment arctic in northern alaska to pkp differential traveltime measurements previously obtained from south sandwich islands earthquakes and recorded at college col and the alaskan seismic network this nearly doubles the number of measurements to a total of 1165 and increases the data coverage of the inner core structure and our time coverage to about 55 years the large number of measurements allows us to use the standard statistical method of bootstrapping to derive the rotation rate and thus to further separate the core structure from its time evolution this approach has been successfully tested with synthetic data sets that feature both a nonlinear structure and variable rotation rates while sharing the same earthquakes and stations geometry as our real data applying this method to the real data yields an average rotation rate of 0 39 0 090 22 yr 1 at 68 confidence level our results also suggest a nonzero acceleration of the inner core with an apparent change in the rotation rate from 0 24 to 0 56 yr 1 within the last 55 years the minimum torque acting on the inner core is estimated 1 19 1016 n m which could result from the imbalance of the electromagnetic and gravitational torques

cenozoic extensional tectonics in china abstract this paper gives a brief review of the cenozoic extensional structures of china during the cenozoic eastern china was in a rifted setting where divergent plate motion and extensional structures were dominant subsidence has occurred initially in response to attenuation of the crust and was enhanced later by subsidence due to thermal contraction in western china particularly in the qinghai tibetan plateau where rifting occurs in an orogenic setting in which convergent plate movements are dominant the rifting is associated with continental collision and lateral escape and large intra continental transform boundaries extensional structures in china may be grouped into eleven graben systems in eastern china and four in western china they occur in different styles linear rift valley basin and range structures and pull apart basins associated with strike slip faults the geometric characteristics kinematics of extension and the mechanism of continental extension in china are discussed the cenozoic extrusion structural model proposed by tapponnier et al 1982 elegantly accounts for the gross tectonics and the formation of the graben systems of western china however in considering the overall strain pattern during the extension of the early tertiary basin and range region in eastern china a large scale simple shear kinematic model may be preferable to the pure shear rift model

two modes of back arc spreading back arc spreading is initiated with rifting along the volcanic zone of an island arc under extensional tectonics the width of the volcanic zone of the arc which is closely related to the dip angle of the subducting slab should cause a variation in the style of initial rifting and subsequent back arc spreading two modes of back arc spreading single rift type and multirift type are introduced from the comparative study of the bonin arc and the japan sea divergent tectonics in an island arc above a steeply dipping slab will cause a single rift system in a narrow volcanic zone but will cause a multirift system in a broad volcanic zone which is related to a shallow dipping angle of subduction

role of strike slip faulting in the evolution of allochthonous terranes in the philippines concepts of allochthonous terrane transport and emplacement are dominated by the assumption that most terranes originate on the subducting plate collide with the upper plate and are emplaced there movement of terranes along the convergent margin is recognized but is generally attributed to postcollision slip in the northern philippines allochthonous terranes originate primarily within the arc system have been translated along it by strike slip faults and were emplaced by cessation of that slip the authors suggest that in the philippines some originally vertical strike slip boundaries may have evolved into shallow dipping sutures marked by fold and thrust systems this mode of terrane evolution may be more common than generally appreciated particularly in orogenic belts developed in response to oblique convergence

the cenozoic rift system of the north china plain and the deep internal process abstract the cenozoic north china plain rift system generated on the site of a mesozoic rift system is the largest rifted basin in east china it trends nne with a total length of about 1000 km maximum width up to 400 km in its middle part and a width of 50 100 km at its south and north the largest thickness of the cenozoic deposits reaches 12 000 m and the accumulated thickness of the cenozoic basalts about 2000 m this rift system is a region of strong seismicity it has quite considerable differential crust movement at the present day the geophysical data indicate that the thickness of the crust and lithosphere beneath the rift system is approximately 4 8 km and 40 km thinner than that of the surrounding mountains this rift system has rather high heat flow values and residual gravity anomalies so far no low velocity anomaly of pn has been found in this region we infer that the formation of this rift system is caused by upwelling of the asthenosphere which is probably related to the convective roll induced by subduction of the pacific plate underneath the asiatic continent and the disturbance of the upper mantle in this region during the mesozoic

velocity structure near the inner core boundary from waveform modeling a velocity model of the inner core boundary icb region is developed from broadband wave form modeling the data consist of long period and short period record sections gathered from the u s arrays wwssn and lrsm deep events beneath indonesia were used these sampled the icb structure beneath the north pacific the cagniard de hoop technique was used to generate the synthetics which allowed considerable flexibility in forward modeling these individual pkp phases our preferred model was developed from modifications of the preliminary reference earth model prem it contains a relatively low velocity gradient above the icb a sharp jump at the icb of about 0 78 km s and a relatively large gradient underneath these features are constrained largely by the differential travel times and amplitude ratios of pkikp and pkikp of the short period section further modification of prem in the fluid core was required to explain the long period observations associated with pkp waveforms our modeling results suggest a low velocity gradient at the bottom roughly 400 km of the fluid core a low q zone at the top of the inner core is required to fit the relative amplitudes of pkikp phases similar to earlier studies this model indicates possible inhomogeneity above the icb as suggested by other authors

using pkikp coda to determine inner core structure 2 determination of qc 1 despite the similar and rather uniform composition of two components of the core the solid inner part presents a much larger attenuation compared to the liquid outer part more over several authors have reported evidence of scattered seismic energy coming from the inner core ic this implies that the ic has lateral variations in structure or composition with a scale length from one to tens of kilometers these seismic scatterers could be caused by changes in crystal grain size variations in the orientation of anisotropy or the presence of partial melt and or impurities this work pursues the characterization of inner core scattering ics using a global data set of high frequency pkikp waveforms recorded at short period small aperture seismic arrays we apply a methodology used in the study of heterogeneities in the crust and upper mantle that consists of curve fitting of the observed scattered energy with a standard model along with a stripping technique to isolate the pkikp coda from the rest e g p pcp and pp codas from the observed pkikp codas we found an average qc 500 for the inner core reflecting a scattering attenuation at least comparable to the intrinsic attenuation we found a geographical dependence in the observations of pkikp coda with most of them coming from the pacific ocean and asia and relatively few observations coming from the atlantic ocean the simplest explanation is that the ics has a quasi hemispherical variation similar to other properties such as p wave velocity jump at the inner core boundary attenuation in the upper ic strength of anisotropy and or thickness of an overlaying isotropic layer this phenomenon is probably related to the solidification texturing of iron crystals showing hemispherical differences in the growth of the inner core

kinematic model for the opening of the sea of japan and the bending of the japanese islands we have analyzed paleomagnetic data from japan reported over the past 25 years in combination with marine geophysical and on land geologic data to reconstruct the opening of the sea of japan and the bending of honshu from these data we propose that the sea of japan opened by a two stage back arc spreading event 1 in the early to middle tertiary japan began to rift off the asian coast as a single rigid block rotated clockwise approximately 25 and formed the japan basin 2 in the miocene the yamato and tsushima basins opened and shortly thereafter 19 ma the japan arc collided with another island arc in central hokkaido at this stage honshu began to behave as two rigid blocks southwest honshu and northeast honshu southwest honshu continued to rotate clockwise approximately 45 as the yamato and tsushima basins continued to open northeastern honshu rotated counterclockwise approximately 40 as hokkaido was held fixed by the arc collision while the yamato basin continued to open these two events the opening of the tsushima and yamato basins and the arc collision in hokkaido resulted in the early to middle miocene bending of the japan arc observed in the paleomagnetic data after 12 ma back arc spreading ceased

processes of allochthonous terrane evolution mindoro island philippines on mindoro two tectonostratigraphic terranes the north palawan block and the mindoro block are separated by a steeply dipping shear zone the north palawan block was rifted off southern china in the oligocene and comprises most of the shallow southern end of the south china sea the eastern part of this terrane exposed on mindoro includes jurassic and paleogene strata probable rift basalts of oligocene age and oligo miocene graben fill deposits in the late oligocene and early miocene the eastern north palawan block underwent crustal stretching with grabens oriented at a high angle to synchronous spreading anomalies in the south china basin we hypothesize that grabens formed in an overall transtensional environment near a transform boundary at the eastern margin of the opening south china basin the mindoro block consists of a pre upper cretaceous low grade metamorphic basement overlain by upper cretaceous and upper eocene strata the mindoro block records several episodes of intense deformation of mesozoic and tertiary age and contrasts with the north palawan block both in terms of stratigraphy and structural evolution the mindoro suture which separates the mindoro and north palawan blocks is a complex structural boundary comprising slices of diverse lithologies separated by steeply dipping anastomosing faults deformed tectonic slices in the suture zone include serpentinized ultramafic rock amphibolite and rocks derived from both of the bounding terranes we interpret this suture as a zone of mostly transcurrent faulting that was active through much of mid tertiary time paleogeographic considerations for early and middle miocene time support the premise that the west side of the central philippine archipelago was a left lateral transform boundary the mindoro and north palawan blocks were initially juxtaposed along the transcurrent mindoro suture by late miocene time subsequent deformation on mindoro includes crustal shortening associated with the southern end of the manila trench which projects onshore on southwest mindoro the onset of shortening here probably reflects a late miocene change in philippine eurasian plate motion the mindoro suture zone is now locked and it acts as a rigid buttress against which strata of the north palawan block have been structurally stacked the mindoro suture zone is similar to other terrane boundaries in the northern philippines the history of terrane transport in this region was apparently dominated by strike parallel shuffling of terranes along major transcurrent boundaries within the philippine arc transcurrent motion has taken place in extensional and compressional regimes and it is ongoing today this may be a dominant process by which complex island arc systems are disrupted prior to accretion onto a continental margin

measuring the seismic velocity in the top 15 km of earth s inner core abstract we present seismic observations of the uppermost layer of the inner core this was formed most recently thus its seismic features are related to current solidification processes previous studies have only constrained the east west hemispherical seismic velocity structure in the earth s inner core at depths greater than 15 km below the inner core boundary the properties of shallower structure have not yet been determined because the seismic waves pkikp and pkikp used for differential travel time analysis arrive close together and start to interfere here we present a method to make differential travel time measurements for waves that turn in the top 15 km of the inner core and measure the corresponding seismic velocity anomalies we achieve this by generating synthetic seismograms to model the overlapping signals of the inner core phase pkikp and the inner core boundary phase pkikp we then use a waveform comparison to attribute different parts of the signal to each phase by measuring the same parts of the signal in both observed and synthetic data we are able to calculate differential travel time residuals we apply our method to data with ray paths which traverse the pacific hemisphere boundary we generate a velocity model for this region finding lower velocity for deeper more easterly ray paths forward modelling suggests that this region contains either a high velocity upper layer or variation in the location of the hemisphere boundary with depth and or latitude our study presents the first direct seismic observation of the uppermost 15 km of the inner core opening new possibilities for further investigating the inner core boundary region

america eurasia plate boundary in eastern asia and the opening of marginal basins abstract the opening of the arctic ocean during the past 55 ma resulted in relative rotation of america with respect to eurasia about a pole located in eastern siberia near the plate boundary the extensional plate boundary enters deep inside the eurasian continent up to the rotation pole on the opposite side of the pole on the pacific side of the plate boundary compressive tectonics are recorded along sakhalin and hokkaido from the oligocene to middle miocene the relative movement was accommodated by strike slip motion along sakhalin and hokkaido although the rotation pole was not located at a significatively different position from now in this paper we explain this by independent motion of the southernmost tip of the american plate towards the pacific margin which behaves as a free boundary this oceanward motion resulted in an extension of the american plate giving rise to the wedge structure of the okhotsk sea the japan sea opened as a pull apart basin along the strike slip boundary finally the increasing extension in the okhotsk sea led to the opening of the oceanic kuril basin

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

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

the cenozoic tectonic evolution of the great north china two types of rifting and crustal necking in the great north china and their tectonic implications abstract the great north china underwent widespread continental rifting during cenozoic time that caused extension and necking of the crust in this area and formed four major rift systems in this paper we provide geological and geophysical evidence that rifting and extension in the western part of the great north china differs from that in its eastern part in age and nature the space time relationships show that the origins of the western rift group of the great north china are probably related to the tibet plateau tectonics whereas those of the eastern rift group are related to the subduction events in the western margin of the pacific plate furthermore the quaternary tectonic regime in the eastern part of the great north china differs from the neogene tectonic pattern and shows relations with western rifts we speculate that during the quaternary the processes that caused rifting in the west have spread further east and now affect the eastern part of the great north china

seismic velocity structure in the earth s outer core 1 we constrain seismic velocity structure in the earth s outer core by analyzing differential travel times waveforms and amplitude ratios of several pairs of core phases we use differential travel times and waveforms of pkpbc pkikp at 144 148 pkikp pkikp at 120 141 pkikp pkpbdiff at the pkp caustics distance range 141 145 and differential travel times and amplitude ratios of pkpab pkpbc at 146 159 to avoid the influence of the inner core anisotropy in velocity we only use the pkikp pkikp and the pkpbc pkikp observations whose pkikp phases sample the inner core along the equatorial paths paths with their ray angles being larger than 35 from the earth s rotation axis these observations show the following characteristics 1 both the observed pkpbc pkikp and pkikp pkikp differential travel times show a distinct east west hemispheric pattern pkikp phases arrive about 0 7 s earlier for those sampling the eastern hemisphere 40 e 180 e than those sampling the western hemisphere 180 w 40 e 2 the observed differential pkikp pkpbdiff travel time residuals also exhibit a hemispheric difference pkikp pkpbdiff differential travel times are about 0 9 s larger for those sampling the western hemisphere than those sampling the eastern hemisphere and 3 both the observed pkpab pkpbc differential travel times and pkpbc pkpab amplitude ratios show scatter overall these observations can be best explained by two one dimensional p velocity models one for each hemisphere at the bottom of the outer core the seismic data sampling the eastern hemisphere can be explained by prem at the bottom of the outer core while those sampling the western hemisphere can be explained by a lower velocity gradient at the bottom of the outer core which has reduced velocities relative to prem linearly increasing from 0 at 200 km above the inner core boundary icb to 0 35 at the icb different velocity gradients at the bottom of the outer core indicate that there may exist a compositional difference and or a large scale temperature difference there and that inner core formation processes may be different between the two hemispheres different inner core formation processes may produce different geometric inclusions of melt in the top of the inner core and thus may provide an explanation to the hemispheric variation of seismic velocity and attenuation in the top of the inner core the seismic data also suggest that the variation of seismic velocity in the tangential cylinder of the outer core if it exists is less than 0 1 we use a compressional wave tomographic model to study the travel time delays caused by the seismic heterogeneities in the mantle the tomographic model is unable to explain the scatter of the travel time data suggesting the existence of unknown strong small scale seismic heterogeneities in the mantle

independent estimate of velocity structure of earth s lowermost outer core beneath the northeast pacific from pkikp pkpbc differential traveltime and dispersion in pkpbc the presence of a low velocity layer at the base of earth s outer core has been proposed however the seismic profile of the basal layer indeed has been poorly constrained in previous seismic studies the model parameters of the layer are substantially nonunique and there are tradeoffs between the seismic velocity of the layer and the properties of the mantle and inner core a more tightly constrained profile of the layer helps further examine the composition and dynamics of the layer in this study we obtained the p wave velocity profile of the basal layer beneath the northeast pacific based on two new seismic observations by analyzing seismograms of the hi net in japan the new observations are particularly sensitive to the layer structure and are relatively insensitive to the structure of the other parts of the earth 1 the frequency dispersion in p waves that graze or are diffracted at the inner core boundary pkpbc and pkpc diff and 2 differential traveltimes between the p waves reflected from the inner core boundary pkikp and those that turn above the boundary pkpbc the resulting velocity model of the lowermost outer core called f layer velocity model for the western hemisphere fvw has p wave velocities that lie between those of ak135 and the preliminary reference earth model prem and a velocity gradient that is slightly gentler than that of prem models with a uniform p wave velocity value within the layer are not supported by the observations for the region investigated which appears not to support the presence of a thick basal layer that is fe rich and dense there

structure and dynamics of earth s inner core abstract the seismological exploration of the earth s inner core has revealed unexpected and puzzling structural complexities its elastic anisotropy is now well established and has been shown to vary spatially there is a well defined hemispherical dichotomy in anisotropy and attenuation and significant variations in the radial direction with weaker anisotropy in the uppermost regions and possibly different properties in an innermost inner core of radius 300 600 km perhaps even more puzzling is the observation of a stably stratified layer at the base of the outer core which has recently been proposed to result from melting of inner core material if confirmed the presence of this layer has major implications for our understanding of convection in the outer core inner core differential rotation with respect to the mantle remains controversial we will review here recent progress in understanding the dynamics of the inner core a number of different models have been proposed for the development of the inner core s structure and formation of the stable layer at the base of the outer core and particular care will be taken to clarify under which conditions the proposed mechanisms can be active

seismotectonics of new guinea a model for arc reversal following arc continent collision the structure and evolution of the northern new guinea collision zone is deduced from international seismological center isc seismicity 1964 1985 new and previously published focal mechanisms and a reexamination of pertinent geological data a tectonic model for the new guinea margin is derived which illustrates the sequential stages in the collision and suturing of the bewani toricelli adelbert finisterre huon new britain arc to central new guinea followed by subduction polarity reversal in the west east of 149 e the solomon plate is being subducted both to the north and south bringing the new britain and trobriand forearcs toward collision west of 149 e the forearcs have collided and together they override a fold in the doubly subducted solomon plate lithosphere which has an axis that is parallel to the strike of the ramu markham suture and that plunges westward at an angle of 5 beneath the coast ranges of northern new guinea active volcanism off the north coast of new guinea is related to subduction of the solomon plate beneath the bismarck plate active volcanism of the papuan peninsula and quaternary volcanism of the new guinea highlands are related to slow subduction of the solomon plate beneath the indo australian plate along the trobriand trough and the trough s former extension to the west respectively from 144 148 e seismicity and focal mechanisms reveal that convergence between the sutured bismarck and indo australian plates is accommodated by thrusting within the finisterre and adelbert ranges and compression of the new guinea orogenic belt together with basement involved foreland folding and thrusting to the south the finisterre block overthrusts the new guinea orogenic belt whereas the adelbert block is sutured to new guinea and overthrusts the oceanic lithosphere of the bismarck sea along the new guinea trench west of 144 e seismicity defines a southward dipping wadati benioif zone and focal mechanisms indicate oblique subduction only this oldest westernmost portion of the collision has progressed past suturing to a full reversal in subduction polarity

the structure of the base of the outer core inferred from seismic waves diffracted around the inner core 1 we systematically searched for seismograms of waves diffracted around the inner core pkpcdiff from all the temporary seismic arrays with data currently available at the iris dmc as well as some permanent regional seismic arrays including f net in japan and grf in germany to assemble the largest high quality pkpcdiff database ever created pkpcdiff waves preferentially sample the base of the outer core and so contain important clues about earth structure in this region we measured pkpdf pkpcdiff differential traveltimes and pkpcdiff pkpdf amplitude ratios in the distance range of 154 160 and modeled the observations using grid searches and full wave theory synthetic seismograms the optimum model found by fitting the differential traveltimes has relatively low velocity at the base of the outer core as in ak135 which is consistent with many previous traveltime studies however the optimum model found by fitting the amplitude ratios pkpcdiff pkpdf does not exhibit this feature and instead is closer to prem the discrepancy may be explained by two likely causes one is that small scale topography or roughness on the icb tends to scatter energy away from pkpcdiff waves by generating trailing coda waves the other is that there exists a thin layer with relatively low q at the base of the outer core this might be expected if there are suspended solid particles at the base of the outer core as proposed decades ago both mechanisms could generate smaller pkpcdiff amplitudes without significantly affecting pkpcdiff traveltimes

joint inversion for inner core rotation inner core anisotropy and mantle heterogeneity observational evidence for differential rotation of the inner core has recently been reported from time dependent travel times of the seismic inner core phase pkp df these analyses are hindered by potential biases from the mantle heterogeneity and systematic earthquake location errors which have raised doubts about the reality of the inferred differential rotation here we report time dependent pkp df pkp bc differential travel times of a total of 611 measurements obtained from 92 earthquakes in the south sandwich islands ssi over the past nearly half a century recorded at the college alaska station col and alaska seismic network asn the events are relocated using the joint hypercenter determination jhd technique dewey 1971 travel time residuals at col from ssi earthquakes over a period of 45 years and at three additional stations from ssi earthquakes over periods of about 22 years show steady increases with time the temporal trends are demonstrated to be statistically very significant with dense samples of the pathway at different time periods we use a joint inversion technique that allows us to separate the time dependent inner core structure from the time independent mantle biases surprisingly the inner core rotation seems resolvable even from digital data observed at asn in the 1990s spanning over 8 years alone in such a joint inversion the results provide strong support for a differential inner core rotation the inferred rotation rate ranges from 0 3 to 1 1 per year faster than the mantle when we take into account the uncertainty in the tilt of the anisotropy axis of the patch of the inner core that was sampled a westward inner core rotation can be ruled out

seismic structure near the inner core boundary in the south polar region 1 it is crucial to obtain good spatial coverage of seismic data points for better understanding the earth s core but the core beneath the polar regions remains largely unexplored we analyzed differential traveltimes and amplitude ratios of core phases whose raypaths run beneath antarctica for determining the vp and qp structure near the inner core boundary in the south polar region the model we obtained south polar region model spr is described relative to the preliminary reference earth model prem as follows a 0 05 km s lower vp value at the top of the inner core a 1 5 times steeper vp gradient in the upper 300 km of the inner core a smaller qp 300 in the upper 300 km of the inner core and a 0 04 km s lower vp at the bottom of the outer core the vp values of spr in the lowermost outer core lie between those of prem and ak135 being closer to those of ak135 the lowermost outer core vp inside the tangent cylinder is thus close to the global average in the upper inner core spr has lower vp than ak135 and prem the spr vp profile is close to that of previous models for the western hemisphere although most of our data sample the eastern hemisphere of the inner core our results indicate that the inner core does not have a simple hemispherical variation as usually supposed our data support an eyeball shaped high vp anomaly with compressional velocity higher than in 1 d reference earth models concentrated to a smaller region beneath eastern asia

okinawa trough genesis structure and evolution of a backarc basin developed in a continent abstract the okinawa marginal basin was opened by crustal extension into the asian continent north of the taiwan collision zone it is located behind the ryukyu trench subduction zone and the ryukyu active volcanic arc if we except the andaman sea the okinawa trough is the only example of marginal backarc basin type opened into a continent at an early stage of evolution active rifting and spreading can be observed synthesis of siesmic reflection seismic refraction drilling dredging and geological field data has resulted in interpretative geological cross sections and a structural map of the ryukyu okinawa area the main conclusions of the reconstruction of this backarc basin volcanic arc evolution are 1 backarc rifting was initiated in the volcanic arc and propagated along it during the neogene it is still active at both ends of the basin remnants of volcanic arc are found on the continental side of the basin 2 there was synchronism between opening and subsidence of the okinawa trough and tilting and subsidence of the forearc terrace the late miocene erosional surface is now 4000 m below sea level in the forearc terrace above the trench slope retreat and subsidence of the ryukyu trench line relative to the asian continental plate could be one of the causes of tilting of the forearc and extension in the backarc area 3 a major phase of crustal spreading occurred in pliocene times 1 9 my ago in the south and central okinawa trough 4 en echelon rifting and spreading structures of the central axes of the okinawa trough are oblique to the general trend of the arc and trench the ryukyu arc sub plate cannot be considered as a rigid plate rotation of 45 to 50 of the southern ryukyu arc since the late miocene is inferred the timing and kinematic evolution of the taiwan collision and the south okinawa trough opening suggest a connection between these two events the indentation process due to the collision of the north luzon arc with the china margin could have provoked lateral extrusion clockwise rotation 45 to 50 according to palaeomagnetic data and buckling of the south ryukyu non volcanic arc tension in the weak crustal zone constituted by the south ryukyu volcanic arc and opening of the south okinawa trough similar lateral extrusions rotations buckling and tensional gaps have been observed in indentation experiments additional phenomena such as thermal convection retreating trench model or anchored slab model could maintain extension in the backarc basin such a hypothetical collision lateral backarc opening model could explain the initiation of opening of backarc basins such as the mariana trough bonin trough parece vela shikoku basin and sea of japan a new late cenozoic palaeogeographic evolution model of the philippine sea plate and surrounding areas is proposed

collision rotation and back arc spreading in the region of the okhotsk and japan seas the india eurasia collision caused extensive deformation in the eurasia continent since late eocene time we propose that back arc spreading in the okhotsk and japan seas was related to the movement of microplates relevant to the india eurasia collision the kuril basin of the okhotsk sea and the basins of the japan sea were formed as back arc basins simultaneously their late oligocene to middle miocene age is constrained by sediment stratigraphy basement depth and heat flow data the movements of two microplates the okhotsk and amuria plates are significant for the tectonics of the okhotsk and japan seas simultaneous retreat of both microplates from trench hinge lines caused back arc spreading in the kuril basin and the japan sea the amuria plate moved north northeastward due to the india eurasia collision this caused pull apart opening of the baikal rift along a transform boundary between the siberia and amuria plates and a collision along the stanovoy range at its northern margin this movement triggered a clockwise rotation of the okhotsk plate with a component of dextral collision between the okhotsk and amuria plates which is well observed along the central zone of the sakhalin hokkaido islands between the two back arc plates the clockwise rotation of the okhotsk plate is suggested by the southwestward fan shaped opening of the kuril basin the kuril basin narrows to the northeast and terminates south of the kamchatka peninsula where a relative rotation pole between the okhotsk and kuril forearc plates is located the kamchatka peninsula was a zone of collision on the other side of the clockwise rotating okhotsk plate during the opening of the kuril basin the sredinny range in the kamchatka peninsula is one of the results of the collision between the okhotsk plate and the forearc plate of the kuril arc the bending of the japanese islands occurred in association with the back arc spreading of the yamato basin of the japan sea

geodynamic evolution of the taiwan luzon mindoro belt since the late eocene abstract the structural framework of the taiwan luzon mindoro belt or festoon is described following three major transects the luzon transect with active subduction and active island arc the taiwan transect with active collision the mindoro transect with active subduction and inactive collision based on this geological study and on available geophysical data a model for the geodynamic evolution of this portion of the philippine sea and eurasia plates boundary is proposed in a succession of reconstructions between the late eocene and the present the major geodynamic events are 1 1 beginning of the opening of the south china sea s c s in lower oligocene times contemporaneous with obduction of the zambales and angat ophiolites on luzon 2 2 subduction of a mesozoic oceanic basin along the proto manila trench from the upper oligocene to the lower miocene 3 3 obduction of the south china sea oceanic crust onto the chinese and reed bank calamian passive margins in middle miocene time 14 15 ma related to a major kinematic reorganization end of opening of the s c s 4 4 beginning of collision between the luzon microblock and the two margins of the s c s in the upper miocene 7 ma collision is still active in taiwan whereas it stopped in mindoro during the pliocene

plate boundary evolution in the halmahera region indonesia abstract halmahera is situated in eastern indonesia at the southwest corner of the philippine sea plate active arc arc collision is in process in the molucca sea to the west of halmahera new stratigraphic observations from halmahera link this island and the east philippines and record the history of subduction of the molucca sea lithosphere the halmahera basement complex and the basement of east mindanao were part of an arc and forearc of late cretaceous early tertiary age and have formed part of a single plate since the late eocene early oligocene there is no evidence that halmahera formed part of an oligo miocene arc but arc volcanism associated with eastwards subduction of the molucca sea beneath halmahera began in the pliocene and the pliocene arc is built on a basement of the early tertiary arc arc volcanism ceased briefly during the pleistocene and the arc shifted westwards after an episode of deformation the present active arc is built upon deformed rocks of the pliocene arc the combination of new stratigraphic information from the halmahera islands and models of the present day tectonic structure of the region deduced from seismic and other geophysical studies is used to constrain the tectonic evolution of the region since the miocene diachronous collision at the western edge of the philippine sea plate which began in mindanao in the late miocene impeded the movement of the philippine sea plate and further motion has been achieved by a combination of strike slip motion along the philippine fault subduction at the philippine trench and subduction of the molucca sea lithosphere beneath halmahera

magnetic anomalies and tectonic fabric of marginal basins north of new zealand detailed airborne magnetic studies conducted over the region of the s w pacific marginal basins extending from the solomon islands to new zealand suggest that three major phases of basin formation and island arc development have occurred in this region development of the tasman sea took place during the late cretaceous paleocene development of the basins to the east of the tasman sea occurred predominantly during the oligocene as well as during the upper miocene to recent the south fiji basin consisting of the kupe and minerva abyssal plains is marked by the presence of possibly two rrr triple junction spreading centers that were active between the times of anomalies 13 to 7 36 25 5 m y the kupe abyssal plain shows the presence of residual magnetic anomalies 7 to 13 of the eastern limb of the proposed spreading center the western limb appears to have been subducted beneath the present site of the three kings rise this seafloor spreading phase calculated half spreading rate of 35 mm yr was coincident with the overthrusting phase of the new caledonia ultramafic rocks during that period active volcanism along the then continuous solomons new hebrides fiji lau island arc was taking place magnetic anomalies from 1 to 4 0 8 m y b p are seen to extend along a clearly defined lineation pattern over the north fiji basin analysis of the magnetic anomalies suggests that during the upper miocene at about the time of magnetic anomaly 4 8 m y the solomons lau island arc commenced breaking up through the development of an active spreading center calculated half spreading rate of 35 mm yr located between the new hebrides and fiji portions of the island arc portions of the northeastern sector of the minerva abyssal plain were subducted beneath the north fiji basin during this period between 0 8 m y b p at the time of about anomaly 3 to 2 4 5 3 5 m y the development of the north fiji basin extended into the lau basin and havre trough thus separating the tonga and lau ridges as a result a distinct series of active spreading centers developed southward along the axis of the havre trough lau basin the havre trough is marked by present seismic and submarine volcanic activity residual magnetic anomalies 1 2 0 1 8 m y b p form a symmetrical pattern around the axis of the trough extending south from the lau basin to the active taupo white island volcanic zone of new zealand phases of development of the marginal basins described in this paper may be correlated with phases of changes in the direction and rate of pacific plate motion

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

is northern honshu a microplate abstract seismic slip vectors along the japan trench the eastern margin of the japan sea and the sagami trough are compared with global relative plate motions rm2 minster and jordan 1978 to test a new hypothesis that northern honshu japan is part of the north american plate this hypothesis also claims that the eastern margin of the japan sea is a nascent convergent plate boundary kobayashi 1983 nakamura 1983 seismic slip vectors along the japan trench are more parallel to the direction of the pacific north american relative motion than that of the pacific eurasian relative motion however the difference in calculated relative motions is too small avoid to the possibility that a systematic bias in seismic slip vectors due to anomalous velocity structure beneath island arcs causes this apparent coincidence seismic slip vectors and rates of shortening along the eastern margin of the japan sea for the past 400 years are also consistent with the relative motion between the north american and eurasian plates calculated there seismic slip vectors and horizontal crustal strain patterns revealed by geodetic surveys in south kanto beneath which the philippine sea plate is subducting indicate two major directions one is the relative motion between the north american and philippine sea plates and the other that between the eurasian and philippine sea plates one possible interpretation of this is that the eastern margin of the japan sea may be in an embryonic stage of plate convergence and the jump of the north american eurasian plate boundary from sakhalin central hokkaido to the eastern margin of the japan sea has not yet been accomplished in this case northern honshu is a microplate which does not have a driving force itself and its motion is affected by the surrounding major plates behaving as part of either the eurasian or north american plate another possibility is that the seismic slip vectors and crustal deformations in south kanto do not correctly represent the relative motion between plates but represent the stresses due to non rigid behaviors of part of northern honshu

tectonic record of convergence changes in a collision area the boso and miura peninsulas central japan abstract late cenozoic formations in boso and miura have been affected by several tectonic events tectonic analysis enables us to reconstruct six different paleostress types 1 early extension affecting the oligocene early miocene mineoka group 2 and 3 relatively minor compressional and extensional events probably early pliocene in age 4 major nne ssw compression dominating prior to 2 or 3 ma ago and 5 and 6 more recent major nnw sse compression to the west and wnw ese extension to the east both types affecting the pleistocene and prevailing since 1 2 ma ago the counterclockwise change from nne ssw to nnw sse compression is not accurately dated but very likely occurred between 2 and 3 ma ago it is compared to similar evolutions in other areas of the izu collision zone we conclude that it corresponds to a major counterclockwise change in the direction of plate convergence from sse nnw to se nw the relationships between the directions of convergence and the distributions of plio quaternary compressional paleostresses in and around the collision zone are described through a simple analogy for the two stages of plio quaternary collision this counterclockwise change in stress fields and relative motions also described in the taiwan collision zone along the same philippine sea plate eurasia boundary is interpreted as a major event at the scale of the plate the possible significances of the other paleostress types identified in boso are discussed we conclude that tectonic analysis in and along collision boundaries provides a key for understanding kinematic evolution

neogene displacements in the solomon islands arc abstract the geology and present configuration of the solomon island arc can be explained in terms of the neogene displacement of a single linear chain of islands the central part of an original arc consisting of bougainville choiseul santa ysabel guadalcanal and san cristobal was displaced to the northeast as a consequence of the attempted subduction of the woodlark spreading system malaita arose on the northeastern side of the arc as a result of interaction between the arc and the pacific ocean floor and the volcanic islands of the new georgia group formed to the southwest in response to the subduction of a spreading ridge thus giving rise to the present double chain structure of the arc

intricate heterogeneous structures of the top 300 km of the earth s inner core inferred from global array data i regional 1d attenuation and velocity profiles abstract we apply a waveform inversion method based on simulated annealing to complex core phase data observed by globally deployed seismic arrays and present regional variation of depth profiles of attenuation and velocity for the top half of the inner core whereas measured attenuation parameters exhibit consistent trends for data sampling the eastern hemisphere of the inner core for the western hemisphere there is a remarkable difference between data sampling the inner core beneath africa w1 and beneath north america w2 obtained attenuation profiles suggest that intricate heterogeneities appear to be confined in the top 300 km the profile for the eastern hemisphere has a high attenuation zone in the top 150 km that gradually diminishes with depth conversely for the western hemisphere the profile for w1 shows constant low attenuation and that for w2 represents a gradual increase from the inner core boundary to a peak at around 200 km depth velocity profiles obtained from differential traveltimes between pkp df and pkp cd bc phases for the eastern and western hemispheres are respectively about 0 8 faster and 0 6 slower than the reference model at the top of the inner core and the difference nearly disappears at about 200 km depth our result suggests the presence of intricate quasi hemispherical structures in the top 200 300 km of the inner core

paleogeographic maps of the continents bordering the indian ocean since the early jurassic paleomagnetic data from the african plate from the early jurassic onward are critically re evaluated and updated resulting in a set of 30 paleopoles from which an apparent polar wander path is derived using 20 ma intervals this path is then used in conjunction with kinematic models of the indian and atlantic oceans to place other continents bordering the indian ocean eurasia india islands and plateaus madagascar seychelles mascarene and blocks involved in the collision of india with eurasia south tibet indochina in an absolute paleogeographic frame fixed with respect to africa paleogeographic maps are presented for the early jurassic pre breakup the jurassic cretaceous boundary anomaly ml5 the early cretaceous m2 the late cretaceous a 34 the cretaceous tertiary boundary a 29 the early eocene a 24 the middle eocene a 21 and the oligocene miocene boundary a 6 observed and predicted paleomagnetic directions for landmasses other than africa i e for western europe china korea india and australia which are not used in deriving the maps are found to be in very satisfactory agreement the quality of reconstructions as measured by rms discrepancies between observed and predicted paleolatitudes is of the order of 5 the main features of the paleogeographic maps are discussed with the aim of relating oceanic and on land geodynamic events the constraints provided by tibetan data on the india asia collision and the 20 ma reorganization of geology and plate boundaries in a band extending from the south china sea through tibet to the red sea system are discussed in more detail we emphasize the episodic nature of the breakup of gondwanaland and formation of the indian ocean and subsequent interaction with eurasia in which six major discrete events are identified separated by 30 ma phases of rather uniform plate motion

tectonic setting of western pacific marginal basins abstract global kinematics as well as magnetic anomalies of marginal basins with continental geology and paleomagnetic data as additional constraints are used to present a set of reconstructions of the western pacific marginal basins between 56 ma and the present at key periods 56 43 32 20 12 and 3 ma our model accounts for the rapid motion of exotic terranes along the whole of the western pacific convergent zone marginal basins appear to open in a great variety of tectonic settings the two extreme examples being the mariana trough where trench suction may be the predominant driving force and the south china sea where intracontinental deformation appears to be the major mechanism the study of marginal basins is a 3 d problem which must take into account the whole tectonic context subduction related tectonics in cross section and upper plate deformation in map view and not only a 2 d problem the classical trench arc back arc problem

short period pkp phases and the anelastic mechanism of the inner core abstract short period seismograms are synthesized for pkp phases in anelastic earth models the synthetics were constructed using a synthetic technique valid at grazing incidence a source time function appropriate for deep focus earthquakes and an instrument response for either a short period wwssn or sro seismograph the agreement between predicted and observed amplitudes and spectral ratios requires neither a low q zone at 0 2 2 hz nor a low or negative p velocity gradient at the bottom of the outer core thin low q zones beneath the inner core boundary fit spectral ratio data that sample the upper 200 km of the inner core but fail to fit data that sample the lower inner core only a model having q 1 0 003 0 004 at 0 2 2 hz nearly constant with depth in the inner core satisfies all of the spectral ratio and amplitude data the assumption of a bulk viscosity of 10 10 3 pa s for the liquid phase of a partially molten inner core combined with the observation of low shear attenuation in the inner core at frequencies less than 0 005 hz limit the physical parameters associated with two possible attenuation mechanisms 1 fluid flow and viscous relaxation due to ellipsoidally shaped inclusions of melt and 2 the solid liquid phase transformation induced by the stress change during the passage of a seismic wave both mechanisms require an order of 0 1 partial melt to reproduce the observed q 1 in the outer core the time constant of the mechanism of phase transformation is predicted to be 10 4 10 6 s confirmation of small shear attenuation in the inner core in the frequency band of seismic body waves would favor the mechanism of phase transformation

origin of the sula platform eastern indonesia the sula platform is a fragment of continental crust that is generally regarded as a piece of new guinea that was detached from western irian jaya in late cenozoic time the assumption is based on the apparent correlation of the stratigraphy of the sula platform with that of western irian jaya new data from the sula platform and western irian jaya show that this correlation is probably not valid the best correlation of the sula platform stratigraphy with that of the former northern margin of the australian craton is found in central papua new guinea between long 141 and 145 e more than 1200 km farther east than any previously proposed site of origin implying a displacement of more than 2500 km

paleomagnetic evidence for the clockwise rotation of southwest japan abstract over 500 oriented samples of felsic rocks of cretaceous to middle miocene age were collected along the go river in the central part of southwest japan in an attempt to detect the process of tectonic rotation of southwest japan from the paleomagnetic view point thermal demagnetization was successful in isolating characteristic directions from the remanent magnetization of samples reliability of the paleomagnetic direction is ascertained through the agreement of directions from different kinds of rocks as well as the presence of both normal and reversed polarities the paleomagnetic results establish that southwest japan began to rotate clockwise through58 14 later than 28 ma and ceased its motion by about 12 ma southwest japan has undergone no detectable north south translation since 28 ma these results imply that southwest japan was rotated about the pivot around 34 n 129 e between 28 ma and 12 ma in association with the opening of the japan sea

western irian jaya the end product of oblique plate convergence in the late tertiary abstract the island of new guinea is the surface expression of the deformed northern margin of the australian continental block to the south undeformed continental crust is overlain by flat lying platform sediments that form a conformable sequence from late palaeozoic to late tertiary along the new guinea cordillera these platform sediments have been tightly folded and overthrust to the south as a result of crustal shortening caused by convergence between the australian plate and the pacific plate the earth movements resulting from this convergence started at the end of the miocene and continue to the present day it is postulated that all the major structures of western irian jaya were caused by this orogeny and can be explained by the convergence directed southwest northeast between the australian and pacific plates it is further postulated that the convergence has remained essentially constant since the start of the earth movements this convergence is consistent with that postulated in the circum pacific plate tectonic map aapg 1981 of 12 5 cm yr directed about 15 south of west only if the effects of the sorong yapen fault system are taken into account this complex east west trending fault system forms the southern boundary of the pacific plate and has responded to the convergence by left lateral strike slip displacement of at least 370 km since the late miocene as a consequence south of the sorong yapen fault system the convergence has been directed more to the south i e at least 45 south of west in this region south of the fault zones the north south component of the convergence has been accommodated in the east by folding and overthrusting over a wide zone of interaction called the new guinea mobile belt dow 1977 in the west on the other hand the australian continental crust has been a stable buttress kepala burung fig 2 which following convergence has protruded into the pacific plate its northeastern margin has therefore been exposed to the full brunt of collision with the pacific plate and large segments of continental crust have been broken off and rafted westwards in order to accommodate the difference in crustal shortening between the new guinea mobile belt and kepala burung two fundamental crustal fractures formed in the intervening region the waipoga and wandamen faults these fractures make angles of about 65 with the regional structural trend fig 2 and they were the focus of most of the resulting differential crustal displacement two of the more spectacular examples of this differential movement have been the thrusting of large slabs of crustal rocks tens of kilometres southwards over the australian continental crust weyland overthrust and lengguru fold belt

accreted terranes in the northern part of the philippine archipelago the philippine archipelago consists of a complex array of ophiolites continental fragments and island arc elements that can be identified as accreted terranes similar to those increasingly recognized as components of orogenic zones the northern philippines including luzon mindoro and nearby smaller islands can be divided into at least six of these tectonic elements from west to east across central luzon there are the neogene accretionary prism of the west luzon arc the eocene zambales ophiolite the cretaceous angat ophiolite and a late cretaceous early paleogene volcanic arc built on an older metamorphic basement this arc terrane is repeated by offset along the philippine fault to the south the western luzon terranes are juxtaposed against the metamorphic basement block of mindoro which itself is colliding with the north palawan microcontinental fragment the ophiolites appear to have originated in back arc basins rather than in oceanic plateaus and the metamorphic belts have protoliths indicating deformation along active margins perhaps deep within accretionary prisms the philippine terranes are interpreted as fragments that originated within the complex pacific eurasian plate boundary rather than as intra pacific basin units these terranes have been assembling at least since the oligocene by strike slip and convergent displacements strike slip displacement either along transcurrent faults or as a component of convergence played a very important if not dominant role in this assembly most terranes identified in the northern philippine appear to be plate fragments with attached crust and upper mantle their characteristics contrast with those usually interpreted for terranes in the north american cordillera where near orthogonal convergence and shallow dipping terrane sutures are favored

hemispherical variations in seismic velocity at the top of the earth s inner core knowledge of the seismic velocity structure at the top of the earth s inner core is important for deciphering the physical processes responsible for inner core growth1 2 3 previous global seismic studies4 5 6 7 8 9 have focused on structures found 100 km or deeper within the inner core with results for the uppermost 100 km available for only isolated regions10 11 12 here we present constraints on seismic velocity variations just beneath the inner core boundary determined from the difference in travel time between waves reflected at the inner core boundary and those transmitted through the inner core we found that these travel time residuals observed on both global seismograph stations and several regional seismic networks are systematically larger by about 0 8 s for waves that sample the eastern hemisphere of the inner core 40 e to 180 e compared to those that sample the western hemisphere 180 w to 40 e these residuals show no correlation with the angle at which the waves traverse the inner core this indicates that seismic anisotropy is not strong in this region and that the isotropic seismic velocity of the eastern hemisphere is about 0 8 higher than that of the western hemisphere

a laboratory model for solidification of earth s core abstract to better understand the influence of rotating convection in the outer core on the solidification of the inner core we have constructed a laboratory model for solidification of earth s core the model consists of a 15 cm radius hemispherical acrylic tank concentric with a 5 cm radius hemispherical aluminum heat exchanger that serves as the incipient inner core onto which we freeze ice from salt water long exposure photographs of neutrally buoyant particles in illuminated planes suggest reduction of flow parallel to the rotation axis thermistors in the tank near the heat exchanger show that in experiments with rotation the temperature near the pole is lower than near the equator unlike for control experiments without rotation or with a polymer that increases the fluid viscosity the photographs and thermistors suggest that our observation that ice grows faster near the pole than near the equator for experiments with rotation is a result of colder water not readily convecting away from the pole because of the reversal of the thermal gradient we expect faster equatorial solidification in the earth s core such anisotropy in solidification has been suggested as a cause of inner core elastic and attenuation anisotropy though the plausibility of this suggestion will depend on the core nusselt number and the slope of the liquidus and the effects of post solidification deformation previous experiments on hexagonal close packed alloys such as sea ice and zinc tin have shown that fluid flow in the melt can result in a solidification texture transverse to the solidification direction with the texture depending on the nature of the flow a comparison of the visualized flow and the texture of columnar ice crystals in thin sections from these experiments confirms flow induced transverse textures this suggests that the convective pattern at the base of the outer core is recorded in the texture of the inner core and that outer core convection might contribute to the complexity in the seismically inferred pattern of anisotropy in the earth s inner core

change in motion of pacific plate at 5 myr bp alignments of volcanoes such as the hawaiian seamount chain record the passage of oceanic plates over mantle plumes or hot spots and changes in the orientation of a seamount chain may indicate changes in the direction of motion of the plate within the hotspot frame of reference1 this hypothesis can be tested for the pacific plate by comparing data from the hawaiian volcanoes with tectonic movements inferred from the geology of adjacent plates we have now found that the motion of the pacific plate changed to a more northerly direction 5 myr ago probably as a result of the detachment of a piece of pacific sea floor which had been subducted beneath the fiji plateau during the tertiary

on the evolution of marginal basins this paper presents a critique of the models proposed to explain the formation of back arc basins those marginal basins located behind active or inactive trench systems and whose origin is inferred to be subduction related several marginal basins are not generically back arc but rather are related to the initiation of subduction continental rifting or plate boundary readjustments models of back arc basin formation must be consistent with the observed characteristics of back arc basins which include the following 1 the crustal structure and magnetic lineation patterns of back arc and major ocean basins are similar implying that the processes of crustal accretion in both tectonic environments are also similar 2 arc trench systems without currently spreading back arc basins are in the majority 3 although petrographically similar to and within the compositional range of midocean ridge basalts morb back arc basin basalts babb show consistent geochemical differences from n type morb and are in many respects transitional toward island arc tholeiites iat 4 distinct babb and iat mantle sources may be in extremely close proximity during the initial rifting of an island arc 5 rifting of the volcanic arc may occur on either side of the line of active volcanoes 50 km and may vary from one side to the other along strike 6 a remnant arc is not always developed 7 back arc spreading may be initiated above subducted lithosphere but with time the center of spreading migrates away from the arc and no longer overlies a benioff zone and 8 there is no simple correlation between the timing of global plate reorganizations and the formation of back arc basins the three main classes of models proposed to explain back arc basin formation are mantle diapirism induced aesthenospheric convection and global plate kinematics because the driving mechanism for the first two classes of models is provided by the local subducting slab these models fail to explain the temporal and spatial distribution of back arc basins the third class of models proposes that back arc basins should form whenever global plate interactions require divergence between the overriding plate and the trench line the latter being observed to move only in a seaward direction with respect to a hot spot reference frame however the tectonic setting of several back arc basins suggests that they represent more than just a passive response to kinematic boundary conditions for example some dynamic back arc force seems required to move the new hebrides tonga and south sandwich forearc plates seaward neither the local nor the global models adequately specify the necessary and or sufficient conditions for back arc basin formation a better understanding of the processes associated with the initiation of rifting and of subduction is required arc rifting and back arc spreading are both secondary phenomena to subduction but they are also primary extensional phenomena fundamentally analogous to continental rifting and the initial spreading of major ocean basins

the inner inner core of the earth texturing of iron crystals from three dimensional seismic anisotropy abstract we present a three dimensional model of the seismic anisotropy and texturing of iron crystals in the inner core the form of the anisotropy changes suddenly at slightly less than half of the inner core radius the outer part is composed of iron crystals of a single phase with different degrees of preferred alignment along the spin axis of the earth the inner part may be composed of a different phase of crystalline iron or have a different pattern of alignment

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

oblique subduction and collision forearc tectonics of the kuril arc the tectonics of the southwestern kuril arc are a result of the oblique subduction of the pacific plate at the kuril trench in association with forearc sliver migration caused by the oblique subduction collision tectonics occur at the leading margin of the sliver and tensional tectonics take place at the tapering margin as a result of the collision a deep crustal section of island arc is observed at the leading margin of the forearc sliver tectonics of the kuril arc related to oblique subduction are different from those of the western sunda arc where backarc spreading occurs at the leading margin this difference is due to margin morphology of the oblique subduction zone

erosional modification of bahama escarpment the precipitous 28 to 40 bahama escarpment is characterized by abundant low energy restricted peritidal and lagoonal rocks and scarce high energy bank margin rocks of early and middle cretaceous age at depths of 2 000 m this indicates significant erosional modification of the slope all models of carbonate platforms have high energy facies at platform edges with low energy and restricted facies confined to platform interiors erosion has removed marginal deposits that constituted the original slope of the bahama platform and exposed peritidal and lagoonal facies rocks on the basis of widths of facies belts of middle cretaceous carbonate platforms in eastern mexico and of erosional benches apparent in seismic profiles across the bahama and blake escarpments an estimated 5 km of material has been eroded from the base of the escarpment greatest erosion occurred between the cenomanian and maastrichtian with additional erosion in the paleocene and oligocene as documented by tertiary and late cretaceous pelagic and forereef deposits that unconformably drape and fill fissures in the older shallow water rocks it is unclear whether erosion of the escarpment is occurring at present erosion of the slope deeper than 2 500 m is effected in part by spalling of joint blocks from shattered exposures jointing probably results from expansion of previously buried rocks exposed at the sea floor by erosion erosion of the bahama escarpment may have occurred by other processes acting in concert or individually at different times as well dissolution by corrosive water below the calcite compensation depth ccd and removal of sediments at the base of the slope by abyssal currents probably contributed most to erosional modification of the bahama escarpment bioerosion and jointing may aid dissolution by increasing surface area exposed to corrosive waters tectonic activity related to movements along fracture zone trends might have contributed locally to erosion of the bahama escarpment erosion of the bahama escarpment shallower than 2 500 m has been less extensive and apparently proceeds by slumping of large allochthonous blocks as indicated by the presence of an apparently intact eocene forereef deposit in the interval from 3 590 to 3 430 m off eleuthera island