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

origin and emplacement of mariana forearc seamounts large seamounts occur on the outer half of the mariana forearc they represent a new class of seamounts consisting of horsts and diapirs of metamorphosed forearc material the degree of metamorphism in this material depends on the amount of water available and the pressure temperature regime of the forearc wedge the major source for the water involved in the metamorphism is most likely the descending slab theoretical models for thermal regimes in convergent margins suggest that the lower grade metamorphic facies will be restricted to the outermost part of a forearc zeolite and chlorite facies rocks predominate in dredge hauls from horsts on the outer 50 km of the mariana forearc thermal models indicate that higher grade greenschist facies should occur farther from the trench seamounts that were probably formed in response to diapiric emplacement of serpentinite predominate from 50 to 120 km from the trench uplift of the horsts and emplacement of the serpentinite diapirs were probably facilitated by vertical tectonic movement in response to subduction of plate seamounts and by fracturing of the mariana forearc

deformation and sedimentation along a developing terrane suture eastern sunda forearc indonesia the collision of the eastern sunda arc with northwest australia has resulted in the development of a suture between the sumba ridge and sawu timor terranes along a zone of intraforearc convergence the developing suture varies from the low angle sawu thrust with attendant mud diapirs in the sumba basin to high angle reverse faults near a basement high of the underthrust sumba ridge terrane bottom currents associated with the flow of pacific ocean deep water into the indian ocean have eroded the terranes and subsequently deposited the detritus in an assemblage of contourites along the suture this study reveals the high structural variability of a terrane suture and the oceanographic influence on the deposition of overlap assemblages

implications of loess and fluvial deposits on paleoclimatic conditions during an icehouse hothouse transition capitanian upper quanzijie low order cycle bogda mountains nw china abstract permian terrestrial sedimentary records provide important clues on paleoclimatic conditions and changes in a vegetated planet during the last icehouse hothouse transition seven stratigraphic sections of middle permian upper quanzijie low order cycle qzj lc in bogda mountains nw china are studied to interpret the depositional environments and paleoclimatic conditions within a multi order stratigraphic framework the upper qzj lc contains distinct laterally persistent thick and massive red mudrocks interbedded with laterally discontinuous conglomerates the mudrocks are interpreted as loess deposits on the basis of their massive structure uniform multi modal grain size distribution lateral extent mineral composition and uniform detrital zircon ages thin conglomerates with a low relief erosional base are interpreted as ephemeral braided stream deposits whereas thick conglomerates with a high relief base that display a clear fining upward trend as meandering stream deposits in tarlong taodonggou area in the south common protosols in loess deposits indicate dust accumulation under arid to semiarid conditions in dalongkou 70 km to the north argillic protosols indicate subhumid conditions alternating with arid semiarid episodes gleysols in the upper part in both areas indicate a change to subhumid humid conditions at the end of qzj time the results strongly suggest that persistent aridity eolian processes and dust trapping mechanisms had existed in ne pangea during capitanian time the gradual but dramatic climatic change from arid semiarid to subhumid humid conditions at the end of capitanian correlates to and is the result of the global demise of the late paleozoic ice age

seafloor spreading in the north fiji basin southwest pacific abstract the seapso leg 3 cruise december 1985 was carried out in the north fiji basin aboard the ifremer r v jean charcot the main purpose of the cruise was to carry out a geophysical and geological study of the central part of the basin between 16 s and 22 s to this end bathymetric profiles with the help of the seabeam multichannel echosounder single channel seismic reflection profiles and magnetic and gravimetric profiles were made moreover geological samples were taken by dredging and piston coring as well as water samples with the help of a multiprobe one of the aims of the cruise was to explore the present day spreading axis of the north fiji basin we were able to show that accretion the rate of which is about 7 cm yr in the north fiji basin has been constrained in a n s direction for 3 ma very recently between 0 6 and 0 7 ma ago the accretion process changed resulting in the creation of a ridge ridge trench triple junction centered at 173 30 e and 16 40 s water samples taken in the vicinity of this triple junction show significant anomalies of methane and manganese which are probably linked to hydrothermal events in its morphological characteristics transversal as well as longitudinal the ridge of the north fiji basin shows striking similarities to the east pacific rise it presents the same differences in level the same sizes of structures and the same particular features overlapping spreading centers offsets etc another aim of the seapso cruise was to study the complex deformation area located immediately to the west of the fiji islands which we interpret as a distensive type deformation in a strike slip system

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

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

structure and evolution of the himalaya tibet orogenic belt the 1981 french chinese expedition to tibet focused on the lhasa block extending earlier coverage 400 km north of the tsangpo suture the lhasa block stood between 10 and 15 n latitude over most of the upper cretaceous and eocene and if gondwanian in origin had detached from gondwana by early permian seismic profiles reveal a complex moho topography resulting both from multiple continental thrusting and large scale strike slip faulting subduction related granitoids representing mixtures of mantle and crustal components and anatectic granitoids have been analysed and dated this study emphasizes the role of smaller blocks in the accretion of the continental mosaic

a new type of active margin the convergent extensional margin as exemplified by the middle america trench off guatemala the results of dsdp legs 67 and 84 during which a transect of holes was drilled across the middle america trench off guatemala show no accretion at the front of that subduction zone since early eocene time the tectonic evolution of the trench includes extensional structure on the landward as well as the seaward trench slopes the guatemalan margin is proposed as the model of a new type of active margin the convergent extensional active margin ce margin

the okinawa trough genesis of a back arc basin developing along a continental margin abstract the okinawa trough is the only example of a marginal back arc basin type at a young stage of evolution that developed along a continental margin the first phase of crustal stretching and subsidence occurred during late miocene early pliocene times a second major phase occurred after the latest pliocene or early pleistocene with en echelon rift axes along the axes of the okinawa trough oblique to the general trend of the arc and trench crustal separation and active spreading has occurred in the central and southern okinawa trough but the north okinawa trough is only in a rifting stage the development of the basin has several correlations with the general tectonism of the arc system 1 1 there was a synchronism between opening and subsidence of the okinawa trough and tilting and subsidence of the fore arc terrace 2 2 back arc rifting was initiated in the volcanic arc and propagated along it during the neogene remnants of a volcanic arc are found on the continental side of the basin the beginning of the back arc spreading observed in the central and southern okinawa trough correspond to a relative minimum in the activity of the volcanic arc 3 3 the timing and kinematic evolution of the taiwan collision and the south okinawa trough opening suggest a connection between these two events such a collision lateral back arc opening model could explain the initiation of the opening of back arc basins but additional phenomena such as thermal convection retreating trench model or anchored slab model are indispensable to maintain extension in the back arc basin

normal faulting of the daiichi kashima seamount in the japan trench revealed by the kaiko i cruise leg 3 abstract a detailed topographic and geophysical survey of the daiichi kashima seamount area in the southern japan trench northwestern pacific margin clearly defines a high angle normal fault which splits the seamount into two halves a fan shaped zone was investigated along 2 4 km spaced 100 km long subparallel tracks using narrow multi beam seabeam echo sounder with simultaneous measurements of gravity magnetic total field and single channel seismic reflection records vertical displacement of the inboard half was clearly mapped and its normal fault origin was supported the northern and southern extensions of the normal fault beyond the flank of the seamount were delineated materials on the landward trench slope are displaced upward and to sideways away from the colliding seamount canyons observed in the upper landward slope terminate at the mid slope terrace which has been uplifted since start of subduction of the seamount most of the landward slope except for the landward walls aside the seamount comprises only a landslide topography in a manner similar to the northern japan trench wall this survey was conducted on r v jean charcot as a part of the kaiko i cruise leg 3 in july august 1984 under the auspices of the french japanese scientific cooperative program

late twentieth century simulation of arctic sea ice and ocean properties in the ccsm4 abstractto establish how well the new community climate system model version 4 ccsm4 simulates the properties of the arctic sea ice and ocean results from six ccsm4 twentieth century ensemble simulations are compared here with the available data it is found that the ccsm4 simulations capture most of the important climatological features of the arctic sea ice and ocean state well among them the sea ice thickness distribution fraction of multiyear sea ice and sea ice edge the strongest bias exists in the simulated spring to fall sea ice motion field the location of the beaufort gyre and the temperature of the deep arctic ocean below 250 m which are caused by deficiencies in the simulation of the arctic sea level pressure field and the lack of deep water formation on the arctic shelves the observed decrease in the sea ice extent and the multiyear ice cover is well captured by the ccsm4 it is important to note however that the temporal evolution of the simulated arctic sea ice cover over the

the japan trench and its juncture with the kuril trench cruise results of the kaiko project leg 3 this paper presents the results of a detailed survey combining seabeam mapping gravity and geomagnetic measurements as well as single channel seismic reflection observations in the japan trench and the juncture with the kuril trench during the french japanese kaiko project northern sector of the leg 3 on the r v jean charcot the main data acquired during the cruise such as the seabeam maps magnetic anomalies pattern and preliminary interpretations are discussed these new data cover an area of 18 000 km2 and provide for the first time a detailed three dimensional image of the japan trench combined with the previous results the data indicate new structural interpretations a comparative study of seabeam morphology single channel and reprocessed multichannel records lead to the conclusion that along the northern japan trench there is little evidence of accretion but instead a tectonic erosion of the overriding plate the tectonic pattern on the oceanic side of the trench is controlled by the creation of new normal faults parallel to the japan trench axis which is a direct consequence of the downward flexure of the pacific plate in addition to these new faults ancient normal faults trending parallel to the n65 oceanic magnetic anomalies and oblique to the japan trench axis are reactivated so that two directions of normal faulting are observed seaward of the japan trench only one direction of faulting is observed seaward of the kuril trench because of the parallelism between the trench axis and the magnetic anomalies the convergent front of the kuril trench is offset left laterally by 20 km relative to those of the japan trench this transform fault and the lower slope of the southernmost kuril trench are represented by very steep scarps more than 2 km high slightly south of the juncture the erimo seamount riding on the pacific plate is now entering the subduction zone it has been preceded by at least another seamount as revealed by magnetic anomalies across the landward slope of the trench deeper future studies will be necessary to discriminate between the two following hypothesis about the origin of the curvature between both trenches is it due to the collision of an already subducted chain of seamounts or does it correspond to one of the failure lines of the america eurasia plate boundary

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 eastern and western ends of nankai trough results of box 5 and box 7 kaiko survey abstract seabeam mapping and detailed geophysical surveying have been conducted over the eastern and western ends of the nankai trough the eastern survey covers the transition between the large izu bonin izu ogasawara ridge collision with honshu and the nankai trough subduction it includes a northeast trending basement ridge the zenisu ridge to the southeast of nankai trough as well as two large channel systems one following the trench the nankai channel and the other coming from the north the tenryu canyon the zenisu ridge is a zone of recent intra plate shortening consisting of three distinct segments with an increasing deformation from southwest to northeast we interpret this gradient of deformation as a way to absorb the kinematic discontinuity between the diffuse shortening prevailing over the izu bonin ridge and the concentrated shortening along the nankai trough the shortening axis appears to change from north south along the zenisu ridge to northwest southeast along the nankai trough and finally to east west within honshu the western survey covers the junction between the nankai and ryukyu trenches and the kyushu palau ridge which is subducted without major internal deformation the ridge appears to act as an indenter upon the margin the tip of the indenter is presently situated under the upper accretionary prism

a synthesis of the geologic evolution of taiwan abstract the island arc of taiwan is composed of cenozoic geosynclinal sediments more than 10 000 m thick lying on a pre tertiary metamorphic basement pleistocene to miocene andesitic islands surround the main island and are related mostly to arc magmatism the penghu island group in the taiwan strait is covered with pleistocene flood basalt neogene shallow marine clastic sediments are exposed mainly in the western foothills with pleistocene andesitic extrusives at the northern tip and the northeastern offshore islands a thick sequence of paleogene to miocene argillitic to slaty metaclastic rocks underlies the western central range and forms the immediate sedimentary cover on the pre tertiary metamorphic complex to the east which represents an older mesozoic arc trench system the coastal range in eastern taiwan is a neogene andesitic magmatic arc including also a large variety of volcaniclastic and turbiditic sediments cenozoic taiwan is the site of arc continent collision where the luzon arc on the philippine sea plate overrides the chinese continental margin on the eurasian plate east and northeast of taiwan the polarity of subduction changes whereby the oceanic philippine sea plate is subducting beneath the ryukyu arc system on the eurasian plate continent arc collision in taiwan island is anomalous and may occur in a broad belt of deformation rather than along a well defined plate boundary or subduction zone

oxygen isotopic constraints on the origin of magnesian chondrules and on the gaseous reservoirs in the early solar system abstract we report in situ ion microprobe analyses of the oxygen isotopic composition of the major silicate phases olivine low ca pyroxene silica and mesostasis of 37 magnesian porphyritic type i chondrules from cv vigarano usnm 477 2 vigarano uh5 mokoia and efremovka and cr eet 92042 eet 92147 eet 87770 el djouf 001 mac 87320 and gra 95229 carbonaceous chondrites in spite of significant variations of the modal proportions of major mineral phases in cr and cv chondrules the same isotopic characteristics are observed i olivines are isotopically homogeneous at the level within a chondrule although they may vary significantly from one chondrule to another ii low ca pyroxenes are also isotopically homogeneous but systematically 16o depleted relative to olivines of the same chondrule and iii all chondrule minerals analyzed show 16o enrichments relative to the terrestrial mass fractionation line enrichments that decrease from olivine spinel to low ca pyroxene and to silica and mesostasis the observation that in most of the type i chondrules studied the coexisting olivine and pyroxene crystals and glassy mesostasis have different oxygen isotopic compositions implies that the olivine and pyroxene grains are not co magmatic and that the glassy mesostasis is not the parent liquid of the olivine the 18o and 17o values of pyroxene and olivine appear to be strongly correlated for all the studied cr and cv chondrules according to 18 o pyroxene 0 72 0 16 0 13 18 o olivine 1 21 0 54 0 79 17 o pyroxene 0 81 0 15 0 13 17 o olivine 0 60 1 00 0 95 these correlations are consistent with mass balance of oxygen isotopes calculated in the case of chondrule formation according to the following gas melt mineral reactions mg 2 sio 4 olivine sio 2 melt mg 2 si 2 o 6 pyroxene sio 2 melt sio gas 1 2 o 2 gas these equations imply that 2 3 of the oxygen incorporated by low ca pyroxene during its crystallization in the melt is derived from the dissolution of precursor olivine and 1 3 from the addition of chemical components such as sio from the nebular gas the composition of the nebular gaseous reservoir in which type i cr and cv chondrules formed is determined from the present data set to be 18o gas 3 6 1 and 17o gas 1 8 1 this isotopic composition is in excellent agreement with that of the gaseous reservoir previously inferred from the oxygen isotope systematics of bulk chondrules from various chondrite groups i e cv cr unequilibrated ordinary and eh thus the oxygen isotopic systematic strongly supports the idea that most of the olivines in type i chondrules are relict grains and that gas melt interaction played a major role in the formation of type i chondrules

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

sedimentation and subsidence history of the east mariana basin western pacific drilling results and seismic reflection data from the east mariana basin are used to establish the presence of very thick sediments 1200 m in the probable jurassic basin the dominance of redeposited material derived from the surrounding seamounts and an anomalously great depth of crust 6900 m pelagic sediments dominate the interval 0 590 m and volcanogenic and shallow water carbonate turbidites occur in the middle interval 590 893 m the lower 300 m of sediments are unsampled the great depth to crust suggests that this region has had an unusual subsidence history or that present models are not adequate to explain the behavior of crust of this age

nature and distribution of deformation across the banda arc australian collision zone at timor recently acquired seismic reflection and seamarc ii side scan and swath bathymetry profiles near timor show that the banda arc australia collision zone has a tectonic framework similar to that of a typical oceanic subduction system deformation is occurring at present most intensely at the foot of the inner slope of the timor trough this deformation front is discontinuously advancing southward as new thrust slices develop within the subducted australian margin strata in contrast present deformation is apparently negligible in the savu basin the complex fore arc basin north of timor a possible significant exception is a postulated right lateral northeast trending fault zone offsetting the outer arc high between savu and roti although back arc thrusting has been documented north of the volcanic arc this component of convergence is minor compared with the scale of ongoing deformation in the timor trough the detailed nature of these surveys has also led to the recognition of along strike variations in deformation in the timor trough and in the savu basin these variations may be related to the variable degree of involvement of the australian continental margin along the arc

the west pacific geodynamic model abstract between the asian and australian continents on the western side and the general subduction of the central pacific on the eastern side the west pacific presents two types of subduction towards the continents and towards the ocean as a consequence at the present day there are two main types of marginal seas the japan sea and the south china sea and in the geological past two types of orogenesis the japanese and the taiwanese types these are the models of the two main orogenic belts respectively the circum pacific and tethyan for a long period of the past the alps and the mediterranean sea were the tectonic model of reference however with the increase in geological studies oceanographic research geophysical surveying the west pacific gradually became an alternative model which grew more useful with each passing day the present day trench island arc marginal sea system appears to be a good model for the first stages of orogenic belts whereas the mountains of the east eurasian margins provide an example of the completion of formation of such belts the coexistence of different stages of mountain building is the main reason for the increasing interest shown by earth scientists in the west pacific

middle oligocene oceanic crust of south china sea jammed into mindoro collision zone philippines mindoro island south of luzon philippines is a complex junction between the manila trench and the collision zone of the north palawan block with the western philippines mobile belt middle oligocene ophiolites recognized in this suture are coeval with the oldest magnetic anomalies of the south china sea basin these ophiolites are part of a complex pile of terranes thrust above the north palawan block at the lower middle miocene boundary these ophiolites are interpreted as fragments of south china sea oceanic crust jammed between two distinct continental blocks during the counterclockwise rotation of luzon

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

geology and tectonics of the ryukyu islands abstract the ryukyu islands are divided morphologically and geologically into three island groups north central and south ryukyus north central ryukyu represents the geological continuation of the outer belt of southwest japan composed of mesozoic eocene sedimentary sequences whereas south ryukyu is characterized by high pressure metamorphic rocks eocene volcanics and limestone and lower miocene sediments the geological and structural contrasts between north central and south ryukyus are conspicuous before the late miocene transgression covered the whole area the ryukyu islands have been established since then the successive developments of the goto tunghai senkaku basins since the paleogene of the okinawa trough since the miocene and of the grabens near the island group since the pliocene signify a southeastward shift of the basins in relation to the activity in the granite diapir zones accompanied by volcanism

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

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