lithospheric structure buoyancy and coupling across the southernmost ryukyu subduction zone an example of decreasing plate coupling abstract the okinawa trough is a backarc basin located behind the ryukyu arc trench system the southernmost part of the okinawa trough spot displays different tectonic features from the rest of the okinawa trough the spot area includes abundant seamounts with active hydrothermal venting and high heat flow values to understand better the rifting and magmatism context of the spot area we examine the lithospheric structure buoyancy and coupling across the southernmost ryukyu subduction zone the results show that beneath the spot area the continental crust and mantle lithosphere thickness of 25 30 and 120 km respectively are thick with little continental thinning the analysis of mantle lithosphere buoyancy across the southernmost ryukyu subduction zone shows strong plate coupling between the overriding and subducting plates however the two plates are actually decoupled as indicated by present day interface earthquakes this situation indicates that the southernmost ryukyu subduction zone displays a transitory case of a changing plate coupling from a strong to a weak plate coupling such a coupling decoupling transition is probably associated with the collision of the luzon arc with the asian continental margin additionally the curve of the mantle lithosphere buoyancy across the southernmost ryukyu subduction zone indicates that the submarine magmatism in the spot area is located within the volcanic arc area suggesting early arc magmatism in the spot area

seismicity and structure in central mexico evidence for a possible slab tear in the south cocos plate the morphology of the transition from flat to normal subduction in eastern central mexico is explored using intraslab earthquakes recorded by temporary and permanent regional seismic arrays observations of a sharp transition in slab dip near the abrupt end of the trans mexican volcanic belt tmvb suggest a possible slab tear located within the subducted south cocos plate the eastern lateral extent of a thin ultra slow velocity layer usl imaged atop the cocos slab in recent studies along the meso america subduction experiment array is examined here using additional data we find an end to this usl which is coincident with the western boundary of a zone of decreased seismicity and the end of the tmvb near the sharp transition in slab dip waveform modeling of the 2 d structure in this region using a finite difference algorithm provides constraints on the velocity and geometry of the slab s seismic structure and confirms the location of the usl analysis of intraslab seismicity patterns reveals clustering sudden increase in depth variable focal mechanism orientations and faulting types and alignment of source mechanisms along the sharp transition in slab dip the seismicity and structural evidence suggests a possible tear in the south cocos slab this potential tear together with the tear along the orozco fracture zone to the northwest indicates a slab rollback mechanism in which separate slab segments move independently allowing for mantle flow between the segments

melting features along the ryukyu slab tear beneath the southwestern okinawa trough 1 the present day active volcanic front associated with the ryukyu subduction zone extends from japan to the ilan plain northern taiwan and is located within the okinawa trough 80 100 km above the ryukyu slab an abnormal amount of arc volcanism which consists of basalt andesite and rhyolite occurs within the southwestern okinawa trough above a slab tear of the ryukyu subduction zone cbvt the power spectrum analysis of magnetic data shows the occurrence of a thin crust above the slab tear and a thick crust beneath this volcanic area we suggest that an excess of h2o rich fluid might occur at the slab tear and might increase the melt flux both are conveyed obliquely to the uppermost mantle and lower crust cbvt magmas after interactions basaltic magmas would rise up accounting for the contrast of magnetization between this volcanic body and the adjacent ot crust

gaps tears and seismic anisotropy around the subducting slabs of the antilles seismic anisotropy in and beneath the subducting slabs of the antilles is investigated using observations of shear wave splitting we use a combination of teleseismic and local events recorded at three component broadband seismic stations on every major island in the area to map anisotropy in the crust the mantle wedge and the slab sub slab mantle to date this is the most comprehensive study of anisotropy in this region involving 52 stations from 8 seismic networks local event delay times 0 21 0 12 s do not increase with depth indicating a crustal origin in anisotropy and an isotropic mantle wedge teleseismic delay times are much larger 1 34 0 47 s with fast shear wave polarisations that are predominantly parallel to trend of the arc these observations can be interpreted three ways 1 the presence of pre existing anisotropy in the subducting slab 2 anisotropy due to sub slab mantle flow around the eastern margin of the nearly stationary caribbean plate 3 some combination of both mechanisms however there are two notable variations in the trench parallel pattern of anisotropy trench perpendicular alignment is observed in narrow regions east of puerto rico and south of martinique these observations support previously proposed ideas of eastward sublithospheric mantle flow through gaps in the slab furthermore the pattern of anisotropy south of martinique near saint lucia is consistent with a previously proposed location for the boundary between the north and south american plates

structure of the southernmost okinawa trough from reflection and wide angle seismic data abstract during a passive seismic experiment in the okinawa trough the shots of two reflection profiles were recorded by ocean bottom seismometers obs both profiles include 3 ocean bottom instruments are about 65 km in length and located in the axial portion of the southwestern okinawa trough processing of the reflection seismic data images recent deformation of the sedimentary units forward modelling of the wide angle data on both profiles reveals a 1 2 km thick sedimentary infill overlying an acoustic basement characterised by seismic velocities between 3 2 and 3 5 km s crustal thickness could only be modelled on one profile and was determined to be around 10 km thickening towards the ryukyu arc in the south gravity modelling was used to additionally constrain both profiles especially the deep structure of profile 1

how was taiwan created abstract since the beginning of formation of proto taiwan during late miocene 9 ma the subducting philippine ph sea plate moved continuously through time in the n307 direction at a 5 6 cm year velocity with respect to eurasia eu tearing the eurasian plate strain states within the eu crust are different on each side of the western ph sea plate boundary extensional in the okinawa trough and northeastern taiwan versus contractional for the rest of taiwan island the b feature corresponds to the boundary between the continental and oceanic parts of the subducting eurasian plate and lies in the prolongation of the ocean continent boundary of the northern south china sea strain rates in the philippines to northern taiwan accretionary prism are similar on each side of b contractional though with different strain directions perhaps in relation with the change of nature of the eu slab across b consequently in the process of taiwan mountain building the deformation style was probably not changing continuously from the manila to the ryukyu subduction zones the luzon intra oceanic arc only formed south of b above the subducting eurasian oceanic lithosphere north of b the luzon arc collided with eu simultaneously with the eastward subduction of a portion of eu continental lithosphere beneath the luzon arc in its northern portion the lower part of the luzon arc was subducting beneath eurasia while the upper part accreted against the ryukyu forearc among the consequences of such a simple geodynamic model i the notion of continuum from subduction to collision might be questioned ii traces of the miocene volcanic arc were never found in the southwestern ryukyu arc we suggest that the portion of eu continental lithosphere which has subducted beneath the coastal range might include the miocene ryukyu arc volcanoes formed west of 126 e longitude and which are missing today iii the 150 km wide oceanic domain located south of b between the luzon arc and the manila trench above the subducting oceanic eu plate south china sea was progressively incorporated into the eu plate north of b

crustal structure of the southern okinawa trough symmetrical rifting submarine volcano and potential mantle accretion in the continental back arc basin back arc basins are a primary target to understand lithospheric evolution in extension associated with plate subduction most of the currently active back arc basins formed in intraoceanic settings and host well developed spreading centers where seafloor spreading has occurred however rift structure at its initial stage a key to understand how the continental lithosphere starts to break in a magma rich back arc setting is poorly documented here we present seismological evidence for structure of the southern okinawa trough an active rift zone behind the ryukyu subduction zone we find that the southern okinawa trough exhibits an almost symmetric rift system across the rift axis yaeyama rift and that the sedimentary layers are highly cut by inward dipping normal faults the rift structure also accompanies a narrow 2 7 km wide on axis intrusion resulted from passive upwelling of magma on the other hand an active submarine volcano is located 10 km away from the rift axis the p wave velocity vp model derived from seismic refraction data suggests that the crust has been significantly thinned from the original 25 km thick arc crust and the thinnest part with 12 km thickness occurs directly beneath the rift axis the velocity model also reveals that there exists a thick layer with vp of 6 5 7 2 km s at lower crustal levels and may indicate that mantle materials accreted at the bottom of the crust during the crustal stretching the abrupt crustal thinning and the velocity depth profile suggest that the southern okinawa trough is at a transitional stage from continental rifting to seafloor spreading

initiation of arc magmatism in an embryonic continental rifting zone of the southernmost part of okinawa trough the okinawa trough is a young intracontinental backarc basin that has formed behind the ryukyu arc trench system since late miocene time in the southernmost part of the okinawa trough spot a cluster of active submarine volcanoes delineates a volcanic belt which is located only 100 km above the wadati benioff zone we report herein new major and trace element data for the spot volcanic rocks these rocks show a compositional range from medium k andesite to rhyolite their geochemical characteristics are similar to those of pre backarc rifting volcanic rocks from the central ryukyu arc and different from those of backarc basin lavas from the middle okinawa trough and the post backarc rifting ryukyu arc volcanics therefore despite being topographically contiguous with the rest of the trough the spot that developed in the quaternary is not a simple backarc basin but instead an embryonic rift zone in which early arc volcanism occurs as a result of the ryukyu subduction

curie point depths of the island of kyushu and surrounding areas japan as part of a comprehensive nationwide evaluation of geothermal resources for japan the first of the curie point depth maps covering the island of kyushu has been prepared the map was created by inverting gridded regional aeromagnetic data two satisfactory algorithms were developed to invert the gridded data based upon a distribution of point dipoles the first algorithm estimates x0 y0 and z0 the coordinates of the centroid of the distribution by computing a least squares fit to the radial frequency of the fourier transform the second algorithm estimates centroid depth only by computing a least squares fit to the squared amplitude of the frequency estimates the average depth to the top zt of the collection of point dipoles was estimated by a variation of the second algorithm the depth to the bottom of the dipoles inferred curie point depth is zb 2z0 zt the depth estimates are hand contoured to produce the final map the curie point depth map is then compared to regional geology and heat

velocity field of gps stations in the taiwan area abstract the 131 stations of the taiwan gps network were surveyed 4 6 times from 1990 to 1995 with dual frequency geodetic receivers the standard deviation of an observed baseline length with its linear trend removed is in the range of 6 10 mm for a 3 120 km long baseline the average rates of length change for all baselines of the network and those from nine continuously monitoring permanent stations are used in a least squares adjustment to estimate the velocities of the gps stations relative to paisha penghu situated at the chinese continental margin to the south of fengping in the northern coastal range the velocity vectors of stations in lanhsu lutao and the coastal range trend in the directions of 306 322 with rates of 56 82 mm yr in contrast there is a dramatic decrease in the rates to the north of fengping this may be caused by the motion along the ne sw trending thrusts which obliquely cut the northern coastal range a discontinuity of about 30 mm yr in the rates along with a remarkable change in the directions of station velocity is observed across the longitudinal valley then the moving directions gradually shift to the west for the stations in the western foothills in the kaohsiung pingtung coastal area the station velocities are even directed toward the southwest to the north of the peikang high the velocity vectors of the stations change direction from the west gradually to the north and finally to the east and southeast significant nw se extensional deformation is found in the ilan plain and northern taiwan in general the pattern of the velocity field for gps stations in the taiwan area is quite consistent with the directions of present day tectonic stress

history of sedimentation in the northwestern lake superior basin and its relation to lake agassiz overflow following the marquette glacial advance which blocked the eastern outlets of lake agassiz and reached northern michigan about 10 000 years bp the ice margin wasted back toward the northeast eventually allowing lake agassiz to overflow into the lake superior basin through a series of channels sediments deposited east of thunder bay near the mouth of the wolf wolfpup shillabeer and black sturgeon channels reflect three phases in the history of lake superior and provide the basis for reconstructing the early postglacial history of the region the lower part of the sedimentary sequence in the northwestern superior basin consists of a distinctive red stoney sandy till deposited during the marquette glacial advance and is overlain by pink rhythmites deposited in lake superior when it was a deep proglacial lake at the minong level the nearly 300 rhythmites deposited at this time typically consist of 4 cm thick silt clay couplets which are punctuated by silt laminae and sandy turbidites that probably

evidence for early cretaceous oceanic crust trapped in the philippine sea plate abstract the huatung basin is a small oceanic basin located east of taiwan previous age estimates from magnetic lineation studies indicated an eocene age for the basin and formation from the central basin spreading center of the west philippine basin during the last phase of spreading in middle eocene new ar ar ages obtained on gabbros dredged on oceanic basement highs of the huatung basin are early cretaceous these old ages are consistent with early cretaceous ages determined on radiolarian assemblages from lanyu island luzon arc we have performed magnetic anomalies modeling for an early cretaceous oceanic crust our results are in good agreement with new ar ar ages determinations the best fit is indeed obtained with an opening of the huatung basin during the early cretaceous from 131 to 119 ma with a half spreading rate varying between 25 and 30 mm yr the spreading center appears to be located south of the actual basin the abnormal depth 5500 m instead of 5900 m and thickness 12 km instead of 6 km of the crust beneath the basin indicate that there was probably an excess supply of magma during its formation we propose that the basin is a fragment of the former proto south china sea or possibly the new guinea basin that has been trapped by the philippine sea plate

origin of the southern okinawa trough volcanism from detailed seismic tomography 1 magmatism associated with subducting plate edges or slab tears has been suggested in the southern okinawa trough the cross back arc volcanic trail which consists of a cluster of about 70 seamounts is located above a ryukyu slab tear lying along the 123 3 e meridian in november 2003 more than 3300 earthquakes recorded in this area by 15 ocean bottom seismometers and surrounding land stations during a period of 12 days were used to determine the three dimensional vp and vs velocity structures and vp vs ratios a mantle inflow characterized by low vp and vs and high vp vs passing through the slab tear is imaged the fluid and or melt component is rising obliquely from the slab tear in the directions of the cross back arc volcanic trail the northern slope of the southern okinawa trough and to north of iriomote island the asthenospheric intake is also imaged by an inclined chip like high vp vs and low vp and vs body dipping northerly which might be linked to the slab retreat west of the slab tear most of the earthquakes are located around low vp and vs and high vp vs bodies which suggests that the seismicity is related to magmatic and or fluid activities east of it earthquakes are concentrated in an area characterized by high vp and vs velocities and low vp vs suggesting that the magma chamber is absent beneath the axial part of the trough and that normal faulting is the main factor controlling the seismicity

the crustal deformation of the ilan plain acted as a westernmost extension of the okinawa trough abstract we analyze the current deformation in ilan plain of northeastern taiwan considered as the western extension of okinawa trough based on 34 geodetic sites observed between 2002 and 2006 the station velocities are between 1 9 mm yr and 43 0 mm yr in a direction between 38 and 143 relative to stable continent margin of eurasian plate this largest velocity is moving southeastward 143 at a rate of 43 mm yr was enhanced by a seismic doublet that occurred on march 6 2005 the station velocities at the northern margin of ilan plain are insignificant with a magnitude of 1 9 mm yr to 7 5 mm yr on the contrary the station velocities increase approximately southward along the southern margin of ilan plain the stations are moving roughly e w with a magnitude of 5 8 mm yr to 21 8 mm yr further southward the station velocities increase to a value of 43 0 mm yr in a direction of 143 the most prominent feature of the strain distribution patterns in ilan plain are the remarkable extensional strain rates observed in central and southern flank of the ilan plain the largest extensional rate is found 2 66 strain yr in 147 with a shortening rate in the perpendicular direction 1 28 strain yr in 57 indicating transtensional deformation mode in the northeastern flank of ilan plain insignificant or minor deformations are observed overall the extensional rates in this area clearly increase from north to south the extension directions in study area mostly trend e w to nw se directions using the velocity field of 34 gps stations and its strain rate filed we suggest the lateral extrusion process towards mechanically weak domain adjacent ryukyu subduction zone the lateral extrusion is facilitated by the opening of the okinawa trough we conclude that the northeast taiwan is subjected to both the compressional force exerted by the indentation of philippine sea plate and the tensile force induced by trench retreat related to the suction force at the ryukyu trench due to the interaction of two forces the stress regime rapidly changes from pure compression in the taiwan collision belt to transtension near ilan plain and its offshore area

okinawa trough backarc basin early tectonic and magmatic evolution the okinawa trough lying between japan and taiwan is a backarc basin formed by extension within the continental lithosphere behind the ryukyu trench arc system stress directions associated with the two last extensional phases in the southwestern okinawa trough have been deduced from a comparison with analog modeling the direction of extension is n 150 o for the pleistocene phase of extension 2 0 1 ma and n 170 o for the late pleistocene holocene phase of extension 0 1 0 ma the present day ryukyu volcanic arc a narrow continuous feature extending from japan to taiwan is located on the eastern side of the okinawa trough 80 100 km above the wadati benioff zone the minimum depth for emplacement of arc magmatism scarce present day backarc volcanism appears in the middle and southern okinawa trough within linear en echelon bathymetric depressions a n045 o oriented seamount volcanic chain cuts across obliquely the southwestern okinawa trough and lies in the direct line of extension of the gagua ridge a n s linear volcanic feature of the philippine sea plate associated with this extension of the gagua ridge a large reentrant located at the base of the ryukyu prism the uplift of part of the nanao forearc basin and the deformation of the sedimentary arc suggest that the voluminous cross backarc volcanism could be tied to the subduction of the gagua ridge located there at a depth of 80 100 km beneath the backarc basin a second area of anomalous volcanism has been identified in the middle okinawa trough in the ene extension of the daito ridge a wnw ese 400 km long volcanic feature of the philippine sea plate we suggest that the gagua and daito ridges initially induced stress at the base of the arc which is still brittle and cracks propagated through the overlying brittle lithosphere allowing magmas with arc affinities to erupt at the seafloor this excessive magmatism reaches the seafloor through conduits which preferentially follow in their shallowest portion the crustal normal faults of the backarc rifts the okinawa trough is consequently still in an early stage of evolving from arc type to backarc activity

tectonic implications of early miocene oib magmatism in a near trench setting the outer zone of sw japan and the northernmost ryukyu islands abstract the outer zone of the sw japan and northernmost ryukyu arcs was affected by intense igneous activity during the miocene characterized by morb like basalts alkaline basalts and s type with subordinate i type felsic to intermediate volcano plutonic complexes these igneous rocks are inferred to be the products of near trench magmatism early miocene 18 ma alkaline basalt dikes from the shingu otoyo area in central northern shikoku and an alkaline lamprophyre dike from tanegashima one of the northernmost ryukyu islands pre date the middle miocene felsic to intermediate igneous rocks the basalts and lamprophyre have compositions of basanite basalt trachybasalt and phonotephrite they are characterized by elevated large ion lithophile elements liles e g sr ba and th and high concentrations of high field strength elements hfses e g tio 2 nb and zr the geochemical signatures of the basalts and lamprophyre suggest an ocean island basalt type oib type mantle source the occurrence of alkaline basalts and lamprophyre with oib type intraplate geochemical signatures in a near trench setting is unusual with regard to plate tectonic processes we propose that trench ward motion of the overriding plate during the period around the early miocene resulted in a shallowly dipping slab and interplate coupling between the subducting philippine sea plate psp and the overlying crust beneath most of the outer zone in the western part of sw japan and the northernmost ryukyu islands the oib type magmatism in the near trench environment is most plausibly explained by the upwelling of asthenospheric material from beneath the subducting slab which migrated through fractures and or tears in the slab we envisage two possible scenarios for the formation of these fractures or tears 1 the shallowing dip angle of the subducted psp resulted in concave upwards flexure of the slab generating fractures in the flexed region and 2 differential motion within the subducting slab induced slab tearing and segmentation

earthquakes along the ryukyu kyushu arc strain segmentation lateral compression and the thermomechanical state of the plate interface we systematically determined the focal depths and mechanisms of 49 large to moderate sized earthquakes mb 5 4 that occurred along the ryukyu kyushu arc since 1963 by inverting the waveforms and amplitudes of p and sh wave trains at teleseismic distances the results are sufficiently precise to delineate seismogenic structures near the plate interface in detail revealing features not predicted by plate kinematics in contrast to previous studies of this arc and those along other subduction zones shallow seismicity along the plate interface is systematic showing two distinct layers of activities the second layer of seismicity is delineated by a few earthquakes that occurred at depths between 50 and 65 km some 10 20 km directly beneath the seismogenic portion of the interplate thrust zone these earthquakes indicate lateral compressional strain within the subducted slab as their p axes are subparallel to the local strike of the arc not perpendicular to the arc as one would expect from the direction of plate convergence the occurrence of these events cannot be accounted for by membrane stress due to the geometry of the subducted slab to our knowledge similar earthquakes occurred only beneath the northern indoburman ranges and along the banda arc where subducted slabs as part of the indian plate are being dragged northward with their northern edges bumping into east west trending collision zones nearby by drawing an analogy between the tectonic settings of these three regions we interpret events beneath the plate interface along the ryukyu kyushu arc as a consequence of ongoing collision between the philippine sea plate and eurasia near taiwan the interplate thrust zone is largely aseismic down to a depth of approximately 30 km a large number of earthquakes showing low angle thrust faulting commence at this depth and are accompanied by two events that show antithetic thrust faulting at a slightly shallower depth of 20 25 km these observations suggest that the strength of the plate interface increases significantly below the depth of proximately 25 5 km based upon available heat flow measurements between the trench axis and the volcanic arc we estimated the temperature field and magnitude of shear traction along the interplate thrust zone the deepest interplate earthquakes 40 50 km correspond to temperatures of approximately 730 980 c comparable to the limiting temperature of intraplate mantle earthquakes 800 c and that of intermediate and deep focus earthquakes at wadati benioff zones potential temperature 900 k therefore the cessation of seismicity in general is probably controlled by similar temperature conditions regardless of the tectonic settings of source regions the magnitude of shear traction along the seismogenic portion 40 50 km depth of the interplate thrust zone seems to be of the order of 100 mpa which in turn implies a low average coefficient of friction of only 0 10 0 05 considerably less than those observed for laboratory specimens the inception of intermediate focus earthquakes within the subducted slab is at a depth of about 100 km whereas shallow earthquakes show no apparent variations along the entire arc intermediate depth earthquakes indicate downdip extension along the northern end of the arc near kyushu but abruptly change to downdip compression along the rest of the arc all available evidence indicates that this is a spatial pattern unrelated to earthquake cycles at the moment tectonic interpretation of such a distinct strain segmentation along the arc is enigmatic without the presence of clear discontinuities in the age of the subducted slab the sudden switch in the strain field of the slab is difficult to explain by the continuously varying rate of subduction along the arc in any case this pattern of strain segmentation requires the presence of a major accommodation structure such as a tear fault in the subducted slab near the tokara channel appendix is available with entire article on microfiche order from american geophysical union 2000 florida ave n w washington dc 20009 document b91 004 2 50 payment must accompany order

curie point depths of macedonia and thrace n greece the aeromagnetic data of macedonia and thrace were used to produce curie point estimates the data were high pass filtered to remove components arising from topography and magnetic core fields which were not adequately modeled by a dgrf the depth to the centroid z 0 of the deepest distribution of the magnetic dipoles was obtained by computing a least squares fit to the lowest frequency segment of the azimuthally averaged log power spectrum the average depth to the top of the deepest crustal block was computed as the depth to the top z t of the second lowest frequency segment of the spectrum the depth to the bottom of the deepest magnetic dipoles the inferred curie point depth was then calculated from z b 2z 0 z t the curie depth estimates for macedonia and thrace range between 11 2 and 17 3 km these results are consistent with the depths inferred by extrapolating known geothermal gradient and heat flow values

source and path of magma for volcanoes in the subduction zone of northeastern japan the assumption that the source of magma for volcanoes in subduction zones is located in the mantle immediately above the top of the subducting slab at approximately 100 km depth and straight beneath the volcanoes is incorrect in northeastern japan the combination of evidence from velocity tomography in the mantle wedge above the slab and mapping of earthquake size distribution within it strongly points to a source of fluids at the top of the slab at 140 to 150 km depth from where material rises along an inclined path to the volcanoes

dead sea sinkholes an ever developing hazard sinkhole development along the western shore of the dead sea became a major concern in 1990 with the appearance of a series of holes 2 15 m diameter and up to 7 m deep in the newe zohar area one of these sinkholes below the asphalt surface of the main road along the western shore of the dead sea was opened by a passing bus repeated infilling and collapse of these holes indicated the extent of this ongoing process and the significance of this developing hazard since then sinkholes have developed in other areas including qalia ein samar ein gedi and mineral beach

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

magnetic inversion in the east china sea and okinawa trough tectonic implications abstract in order to better understand continental rifting along the eastern margin of the eurasian plate we have performed a magnetic inversion to reveal the distribution and characteristics of the belts and basins in the east china sea and okinawa trough equivalent magnetizations are obtained by assuming a constant thickness of 5 km of the magnetized layer whose top surface is assigned to the sea floor and taking into account the present day geomagnetic field because of absence of magnetic reversals in the study area a high magnetization zone can reflect a shallow and or highly magnetized basement in contrast a low magnetization zone generally corresponds to a rifted basin with thick sediments the belts in the east china sea shelf basin are in general below sea level and correspond to shallow basement with thin sediments the inversion results show that the belts are parallel and trending about n075 this regular trend suggests a simple southeastward rifting pattern of the east china sea shelf basin the belt trend changes from n075 to n095 at the eastern side of the taiwan sinzi belt southwest of the taiwan sinzi belt the belts in the ryukyu subduction system are parallel to the ryukyu trench it implies that rifting of the middle and northern okinawa trough was initiated along the eastern side of the taiwan sinzi belt then the rifting has been mainly controlled by the oblique subduction of the philippine sea plate beneath the ryukyu arc the southern end of the taiwan sinzi belt terminates at the nw se trending miyako yandang high magnetization zone the southern okinawa trough rifted since early pleistocene is distinct from the middle and northern okinawa trough by the miyako yandang high magnetization zone the southern okinawa trough consists of two rifting patterns east of 123 5 e the backarc rifting is quasi symmetric as evidenced by symmetrically high magnetization zones on both sides of the yaeyama depression in contrast west of 123 5 e the volcanism or high magnetization zones developed along existing nw se trending strike slip faults have generally occurred at the northern margin of the southern okinawa trough the ryukyu arc west of 123 5 e exhibits low magnetization reflecting its non volcanic nature of the upper crust east of 123 5 e the present day ryukyu arc volcanism is expressed by relatively high magnetization