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

composite transform convergent plate boundaries description and discussion abstract the leading edge of the overriding plate at an obliquely convergent boundary is commonly sliced by a system of strike slip faults this fault system is often structurally complex and may show correspondingly uneven strain effects with great vertical and translational shifts of the component blocks of the fault system the stress pattern and strain effects vary along the length of the system and change through time these margins are considered to be composite transform convergent ctc plate boundaries examples are given of structures formed along three ctc boundaries the aleutian ridge the solomon islands and the philippines the dynamism of the fault system along a ctc boundary can enhance vertical tectonism and basin formation this concept provides a framework for the evaluation of petroleum resources related to basin formation and mineral exploration related to igneous activity associated with transtensional processes

a geophysical study of the manila trench luzon philippines 2 fore arc basin structural and stratigraphic evolution the pattern of sediment distribution within the manila trench subduction zone west of luzon exerts a strong influence on the tectonic evolution of the accretionary prism and the fore arc basins of this convergent plate margin multichannel common depth point seismic reflection data reveal up to 2600 m of turbidite sediments overlying a thin pelagic hemipelagic sediment layer and oceanic basement in the northern part of the manila trench these turbidites were transported from the uplifted collision zone of taiwan southward along the manila trench by gravity controlled processes similarly thinner turbidite deposits in the southern end of the trench were transported northward from the collision zone involving the north palawan block and the central philippines sedimentation within the manila trench is dominated by sources at the ends of the trench with longitudinal transport along the trench axis in contrast the fore arc basin system of the manila trench receives sediment through submarine canyons incised into the west luzon continental slope the rate of sediment supply to the fore arc basin system is not directly related to the rate of sediment supply to the trench the size of the accretionary prism varies significantly along strike of the trench and it strongly correlated with the local thickness of turbidite sediments within the trench a thickness of more than 200 m of trench turbidites is probably required before accretion and uplift become the dominant process affecting the fore arc region this generalization may apply to other convergent plate margins as well the uplift rate of the accretionary prism probably varies dramatically along the trench and is largely independent of variations in the local sediment accumulation rates in the fore arc basins this variability results in the formation of several different types of fore arc morphology along the manila trench the overall seismic stratigraphy of the west luzon trough fore arc basin is remarkably uniform with seismic sequences continuous for up to 110 km along strike the unconformities that bound these seismic sequences are probably the result of a complex interplay of relatively long term approximately 10 ma uplift of the accretionary prism and relatively short term approximately 1 2 ma variations in sediment supply to the fore arc basin modulated by eustatic sea level changes

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

crustal thickness variation from a continental to an island arc terrane clues from the gravity signatures of the central philippines abstract offshore and ground gravity data were utilized to estimate crustal thickness across the central philippines where a transition from continental to island arc terrane occurs significant differences in gravity anomalies were observed between the palawan microcontinental block pcb and the philippine mobile belt pmb two major terranes that came together through arc continent collision islands of the pcb mindoro tablas romblon sibuyan and western panay made up of an assortment of continent derived sedimentary and igneous rocks and slivers of ophiolitic bodies register lower bouguer anomalies compared to that displayed by masbate island in the pmb the calculated crustal thickness of this region exhibits a complex moho topography of non uniform depth across the collision zone with the thickest parts 32 km corresponding with ophiolitic units emplaced consequent to arc continent collision on the other hand relatively thinner crust 21 km within the collision zone coincides with areas surmised to have undergone attenuation following intra arc rifting the same characteristics are observed offshore of western mindoro and within the marinduque basin areas known to have experienced crustal thinning following regional tectonic rearrangements that triggered riftings and intra basin openings

complexities in the tectonic evolution of the west philippine basin abstract the oceanic west philippine basin can be subdivided into a main basin a northwest sub basin and a southern sub basin these basins have undergone different tectonic histories magnetic anomalies 21 17 are present in the main basin and indicate that it formed by two limbed seafloor spreading the now extinct spreading center is located within a zone of rough topography associated with the northwest trending central basin fault cbf the morphology of the cbf is primarily the result of post spreading tectonism the magnetic anomaly pattern of the main basin does not extend into the southern sub basin or the northwest sub basin geometrical constraints indicate that the southern sub basin did not form as part of the southern limb of the main basin the southern sub basin may have formed immediately before a ridge jump to the main basin spreading axis alternatively possible shallower crustal depth in the southern sub basin may indicate that the sub basin is younger than the main basin and formed separately from it in the northwest sub basin which originated as part of the main basin the dominant topographic trends are perpendicular to those in the main basin this difference as well as higher heat flow and shallower depth in the northwest sub basin indicate that it has undergone a deformational event which did not affect the main basin this deformation may be related to subduction along the east luzon margin in mid tertiary time the gagua ridge an uplifted sliver of oceanic crust along the western edge of the northwest sub basin may have originated during this event

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

platinum group minerals in the santo tomas ii philex porphyry copper gold deposit luzon island philippines mineralized quartz diorites of the santo tomas ii porphyry copper gold deposit carry high au contents average 1 8 ppm as well as 160 ppb pd and 38 ppb pt values of other platinum group elements pge and rhenium are below the analytical detection limits there is a significant positive correlation between au and cu the highest pd values were detected in the most au and cu rich rocks platinum group minerals pgm occur exclusively as inclusions in chalcopyrite and bornite potential pd and pt contents in sulphide concentrates are estimated at 1 5 g t and 0 4 g t respectively the precious metal assemblages consist of merenskyite main pgm kotulskite moncheite native gold electrum hessite and petzite polyphase fluid inclusions in quartz veinlets associated with a pgm bearing bornite chalcopyrite magnetite assemblage are characterized by high salinity 35 to 60 eq wt nacl and high trapping temperatures between 380 and 520 c they may represent primary magmatic hydrothermal fluids which have been responsible for the transport of pd pt and au as chloride complexes

onland signatures of the palawan microcontinental block and philippine mobile belt collision and crustal growth process a review abstract the collision of the palawan microcontinental block with the philippine mobile belt had significantly influenced the geological evolution of the philippines multiple collisions involving several fragments through space and time resulted into the collage of terranes of varying origin exposed in this part of central philippines cusping of the overriding plate volcanic arc gap ophiolite emplacement incipient back arc rifting island rotation and tilting raised coastal terraces metamorphism intrusion of igneous rocks and steepened subducted slab as seen in focal mechanism solutions are some of the manifestations of this collision a late early miocene to early middle miocene age 20 16 ma is proposed for the major collision between the palawan indenter and the philippine mobile belt the collision boundary is located from the northern part of mindoro through the central mountain range swinging east of sibuyan island in the romblon island group and finally threading along the buruanga peninsula and eastern side of the antique ophiolite complex before exiting and connecting with the negros trench the collision through accretion and crustal thickening has contributed to the crustal growth of the philippine archipelago

the three dimensional attenuation structures beneath the philippine archipelago based on seismic intensity data inversion abstract attenuation structures were determined for the philippine archipelago by inversion of earthquake intensity data set from 1957 to 1992 the intensity data based on the adapted rossi forel intensity scale were translated into acceleration values using a modified acceleration intensity equation a damped least squares method was used in our inversion we correlated the inversion results with other data such as observed anomalous intensity distribution present seismicity and the location of trenches and volcanic centers our results strongly indicate the association of low attenuation values with subducted slabs along the known location of trenches in the philippines for example the configuration of the philippine trench is suggested by the concentration of low attenuation values along the east side of central and southern philippines while on the western side of luzon island a low attenuation structure is consistent with the location of the manila trench the extent and depth of low attenuation values are also apparent along the eastern side of luzon in southern luzon through negros island in the central philippines the site where anomalous intensity distribution was observed coincided with the location of a noticeable low attenuation structure determined in this study moreover possible locations of an upwelling magmatic body along the numerous areas in the philippines are also indicated by the concentration of high attenuation values furthermore a possible aseismic slab which probably represents a structure that connects the manila trench with the negros trench was also inferred in eastern mindoro however it is also possible that these low attenuating values are due to the presence of continental material in this area on the basis of these initial results we suggest some modifications of the configuration of tectonic structures in several portions of the archipelago

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

new features from the romblon island group key to understanding the arc continent collision in central philippines abstract a complete ophiolite sequence and melange are reported for the first time in the romblon island group located at the central part of the philippine archipelago the study area comprises tablas romblon and sibuyan islands built on amalgamated crust and mantle rocks that are typically fault bounded and stratigraphically disrupted the sibuyan ophiolite complex soc consists of harzburgite and dunite layered clinopyroxenite layered and isotropic gabbro diabase dike swarm and basaltic andesitic pillow lava and flow deposits a jurassic cretaceous age of formation and late early miocene age of emplacement are suggested for the soc by available data emplacement of these crust mantle units is believed to have been facilitated through displacements along east verging thrust faults developed consequent to the early miocene arc continent collision in central philippines along with recent k ar isotopic ages 12 ma obtained for some of the metamorphic rocks in the area these lithologic units and structural features provide critical information regarding the arc continent collision which is one of the significant events in the evolution of the philippine island arc system the usefulness of potential field geophysical methods to studying lithologic and structural relationships in an extremely disrupted terrane is evaluated in this study observed variations in gravity and magnetic values correlated with lithologic distributions and geologic structures higher relative bouguer and magnetic anomalies are observed over members of the complete ophiolite sequence and younger igneous materials exposed in tablas and sibuyan islands their geophysical characteristics contrast markedly with the less dense and magnetically inferior sedimentary rocks exposed only on tablas island on the other hand metamorphic units in tablas romblon and sibuyan islands display variable magnetic and gravity signatures that reflect differences in composition and alteration or weathering thrust faults related to the emplacement of the ophiolite as a result of the collision are inferred from sudden changes in gravity and magnetic values

neotectonic regime on the passive continental margin of the northern south china sea abstract between 1989 and 1994 more than 6600 km of reflection seismic profiles were obtained in the south china sea off hong kong with the german research vessel sonne during cruises so 50b so 72a and so 95 a seismo stratigraphic interpretation of this data set leads to a new age assignment of the unconformity t0 which we place within the pleistocene both neogene unconformities t1 and t0 are generated by uplift of the dongsha rise and truncation of their overlying strata this uplift is caused by intrusion of magma into the upper crust our seismic profiles show plutons which have penetrated the sedimentary cover whereby their original stratification in the contact zone is eliminated these magmato tectonic events may be correlated to the two main collision phases between taiwan and the continental margin of east china 5 3 and 3 0 ma ago the collisional events subsequent to the nnw to wnw drift of taiwan transformed the compression into strike slip movements along the continental margin of southeastern china the accompanying stress regime is transtensional with subsidence of the cooling oceanic crust since the cessation of rifting and its consumption beneath the manila trench providing the extensional stress the strike slip movements remobilized many of the rift and drift faults providing pathways for magma ascent the tectonic framework of the northern south china sea is characterized by miocene faults trending ne sw these faults are scarce but are distributed throughout the study area pliocene faults striking ene wsw to ne sw are concentrated west of the dongsha islands and are mostly strike slip in character recent faults are generally oriented ne sw subparallel to the synrift faults they result in part from local uplifts where they are normal in character but strike slip motion also occurs most of the faults involve the basement and represent reactivated zones of weakness of the rift and drift phases

new results from physical modelling of arc continent collision in taiwan evolutionary model abstract both the analysis of the geological data and the results from 2d and 3d experimental modelling of arc continent collision in taiwan reveal the following major stages in this process 1 failure and thickening of the subducting crust of the chinese continental margin under the frontal part accretionary prism of the philippine sea plate psp since ca 6 7 ma 2 uplift squeezing and exhumation of the subducted crustal slice of the chinese margin and initiation of the central range about 5 ma ago 3 flexural buckling subsidence of the luzon arc fore arc under strong horizontal compression and sedimentary infilling of this area by the material shed from the growing and exhuming central range 4 failure of the psp along the west vergent longitudinal valley fault dipping beneath the arc and closure of the fore arc 5 subduction reversal and stoppage of continental subduction which has already occurred in northern taiwan and propagates to the south the southward propagation of the collision is not stationary currently occurring initial collision in southern taiwan and offshore to the south does not follow the same described above evolution as in central and northern taiwan the evolution of the collision in time and space should be considered in the framework of a 3d model where the psp rupture is preparing in the central and northern taiwan during a long time passing through a number of evolutionary stages once it has occurred the rupture longitudinal valley fault rapidly propagates to the south into the domain which did not undergo the pre rupture evolution the rupture front is currently located near 21 30 n

geologic and paleomagnetic evidence for a possible miocene collision in western panay central philippines the palawan metamorphic terrane represented by paleozoic basement on the northwestern tip of panay central philippines collided with the remainder of the island between late oligocene and middle miocene time subduction continued north and south of the collision zone along the manila and negros trenches respectively evidence for the collision is 1 the juxtaposition along a major fault of old melange terrane against highly metamorphosed paleozoic continental sediments similar to those exposed on palawan island itself and 2 miocene thrusting in the melange terrane oriented parallel to the suture paleomagnetic studies show that since the early miocene the island of panay has rotated 20 clockwise declination 20 9 inclination 22 1 for six sites whereas the northern philippines has rotated counterclockwise these contrasting rotations are consistent with a collision of the philippine arc with a northeastward converging palawan block

formation of arcs and backarc basins inferred from the tectonic evolution of southeast asia since the late cretaceous abstract results of the geological and geophysical surveys in the daito ridges and basin in the northern west philippine basin suggest that the daito ridge was an arc facing toward the south from the late cretaceous to the early tertiary the late cretaceous and tertiary history of southeast asia is evaluated based on these data in the daito ridges and basins and reconstructed based on overall plate kinematics that have operated in this area during the late cretaceous the daito ridge and the east philippine islands were positioned along the boundary between the indian and pacific plates the western half of the philippines setting on the indian plate approached from the south and collided with the east philippine daito arc either during the latest paleocene or the earliest eocene it is inferred that the bulk of the philippine archipelago rotated clockwise and borneo spun counterclockwise during the tertiary from the reconstruction the formation of backarc basins and their spreading direction are assessed as a result some primary causes and significant characteristics are suggested for the opening of backarc basins in southeast asia first opening of some backarc basins commenced with or was triggered by collisions second backarc basins opened approximately parallel to oceanic plate motion third the formation of some backarc basins was triggered by the approach of a hot spreading center fourth the spreading mode or direction of backarc basins was greatly affected by the configuration of the surrounding continent and was also rearranged to spread approximately parallel to oceanic plate motion the formation of backarc basins and their spreading direction can be reasonably explained by plate kinematics however the generative force responsible for their formation is possibly within the subduction system particularly to form horizontal tensional force in backarc side

tomographic imaging of lithospheric structures under taiwan abstract tomographic images of the crustal and mantle velocity structures under taiwan are obtained by simultaneous inversion of local earthquake p wave arrival times for hypocenters and p wave velocity structures in northern taiwan a high velocity zone coinciding with the wadati benioff zone can readily be identified as the subducted philippine sea plate the imaged zone dips toward the north at an angle of 40 from a depth range of 20 55 to 100 130 km an upper mantle low velocity wedge ranging from 40 to 80 km in depth exists above the subducted slab above this wedge is the ilan plain of northern taiwan which lies at the west end of the okinawa trough a well recognized back arc basin the crustal velocities under the plain are also relatively low the well defined high velocity zone and the low velocity wedge provide some constraints on the thermal structures of the subduction system under northern taiwan in tomographic images across the central section of taiwan thickening of the crust and up arching of the lower crustal materials under the central range are commonly observed the crust under the western foothills region is clearly thinner and the near surface low velocity layers are well developed the structures under the central range show that although the taiwan orogeny is quite young a root deeper in the north and shallowing to the south has formed the results of our tomography show that a significant portion of the lithosphere is involved in the taiwan orogeny

zircon u pb geochronology of the zambales and angat ophiolites luzon philippines evidence for an eocene arc back arc pair two basement terranes the zambales ophiolite in the west and the angat ophiolite in the east are exposed on the island of luzon separated by a circa 10 km thick and circa 100 km wide sedimentary basin the structural and age relationships between the two ophiolitic blocks are central to understanding the geologic and tectonic development of the northern philippines and evaluating models of terrane evolution proposed for this area of the western pacific we analyzed zircons from the zambales and angat terranes to better constrain their origin two zircon fractions from tonalite in the acoje block of the zambales ophiolite give concordant u pb ages at 44 2 0 9 ma two zircon fractions from plagiogranite and one fraction from diorite in the coto block of the zambales ophiolite give concordant u pb ages of 45 1 0 6 ma these results provide a middle eocene age for the zambales ophiolite in agreement with the minimum late eocene age of the overlying aksitero formation no age difference is discerned between the arc like acoje block and morb like coto block of the zambales ophiolite four zircon fractions from two sample sites in the angat ophiolite give concordant ages of 48 1 0 5 ma this age is considerably younger than the late cretaceous age based on radiolarian fauna derived from a sheeted dike pillow lava sediment sequence south southeast of the main ophiolite the small age difference between the zambales and angat ophiolites suggests a common origin and obviates the need for a major structural discontinuity west of the southern sierra madre beneath the central valley of luzon the cretaceous biostratigraphic ages of the ophiolitic rocks southeast of the eocene angat ophiolite implies that there are two ophiolitic basements exposed in the southern sierra madre the relationship between the two ophiolites is constrained by the overlying stratigraphic relations which indicate that an eocene volcanic arc and associated volcaniclastic apron was built on both the eocene and cretaceous ophiolitic basement this suggests that the zambales angat ophiolite represents a preserved eocene back arc basin that opened behind an eocene arc that developed within cretaceous oceanic basement in this model the zambales angat ophiolites are therefore not allochthonous terranes but part of a single plate generated in situ forming part of the autochthonous basement of luzon

miocene to recent calc alkalic volcanism in eastern taiwan k ar ages and petrography abstract k ar radiometric datings are presented for volcanic rocks from the coastal range of taiwan and from lanhsu and lutao islands the samples involved are basalts andesites and dacites which show the main petrological and geochemical characteristics of island arc magmas the k ar data show that volcanic activity occurred from early miocene to early pliocene times in the coastal range and in lanhsu island and during pliocene times ca 1 3 4 3 ma in lutao island the geological significance of early and middle miocene ages is discussed with respect to hydrothermal metasomatic alterations which have affected most of the samples a significant increase in incompatible elements e g k and sr is shown to have occurred during pliocene to recent times and is exemplified by the compositions of the lanhsu and lutao volcanic rocks the origin of these chemical variations is related to the magmatic effects of crustal thickening linked to the transition from subduction to collision regimes

north palawan block philippines its relation to asian mainland and role in evolution of south china sea the island of mindoro the northern part of palawan island and the reed bank area southwestern philippines together constitute a continental fragment the north palawan block lying within an island arc oceanic setting the permian to paleogene rocks of these areas indicate a geologic origin and history for the block contrasting with that of the rest of the philippine archipelago these rocks also suggest that the north palawan block once occupied a pre drift position contiguous with the south china mainland four prominent pre neogene regional unconformities are recognized both onshore and offshore the china mainland in taiwan and in the north palawan block the synchrony of these unconformities and the facies relation of the unconformity bounded sedimentary units strongly suggest a common pre neogene history for all these areas by contrast an important regional unconformity occurring at the top of the middle miocene throughout the palawan area is absent from the asian mainland extension in the south china sea basin since the mesozoic which has separated the north palawan block from the asian mainland has been approximately uniform from west to east however there is strong evidence to suggest that this extension has been achieved by temporally separated phases of continental crustal attenuation and more recent sea floor spreading from the foregoing observations and using the most recent magnetic spreading anomaly data for the south china sea a suite of palinspastic reconstructions has been prepared which shows the evolution of the south china sea area from the late triassic to the pliocene the reconstructions illustrate 1 the convergent continental margin setting of the north palawan block during much of the jurassic and cretaceous 2 the late cretaceous inception of the philippine island arc system 3 the subsequent counter clockwise rotation of the arc system from the late eocene onward 4 the paleocene to middle miocene opening of the south china sea and 5 the early to middle miocene collision between the north palawan block and the palawan subduction system

constraints of sea beam data on crustal fabrics and seafloor spreading in the south china sea abstract the south china sea is a mid late tertiary marginal basin the magnetic anomaly lineations in the eastern part of the basin trend approximately east west 1 2 suggesting a north south direction of spreading in the spring of 1985 two cruises on the french research vessel jean charcot provided sea beam coverage seismic reflection magnetic and gravity profiles the sea beam data exhibit two major structural trends scarps striking n50 e 15 interpreted as normal faults and scarps striking n140 e 15 interpreted as fracture zones this fabric implies a northwest southeast direction of spreading up to about 100 km north and south of the inferred spreading axis 3 dense sea beam coverage of a roughly 1 square area northwest of the scarborough seamounts chain shows that the emplacement of these seamounts was at least in its initial stage controlled by faulting in two orthogonal directions n50 e and n140 e magnetic and gravimetric maps of the same area also reveal anomalies trending roughly n50 e which are disrupted by transform zones striking n140 e this detailed study indicates that the fracture zones may be closely spaced less than 20 30 km east of macclesfield bank furthermore magnetic anomalies identified as 6 and 6a 20 myr along two north south profiles located at both edges to the north of this detailed study area may be correlated with the n80 e trend characteristic of such anomalies in the eastern part of the south china sea the east west trend of magnetic anomalies 6 and 6a south of scarborough seamount chain recognised by taylor and hayes 1983 1 is incompatible with the trend of the fault scarps observed on sea beam data we infer that progressive right lateral offsets of the ridge across closely spaced discontinuities may account for the nearly east west average trend of some of the magnetic anomalies and of the scarborough seamount chain which represents the location of the relict spreading axis in spite of a n50 e spreading direction the whole central part of the basin east of 115 e may have formed in this way since n130 140 e striking fracture zones are observed on the sea beam swaths this model may correspond to the second of two successive spreading phases with n s and nw se directions of extension respectively as was presented by the authors in a previous paper pautot et al 1986 3 alternatively we may assume that the direction of extension did not change radically during the opening history 3 implying that oblique spreading occurred along n80 e trending ridge segments creating the n80 e trending scarps and magnetic anomalies observed to the north and to the south of the study area such a direction of extension is compatible with mid late tertiary left lateral movements along large n130 150 e strike slip faults such as the red river and wang chao faults in south china and sundaland

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

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

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

comment on investigating the macrodispersion experiment made site in columbus mississippi using a three dimensional inverse flow and transport model by heidi christiansen barlebo mary c hill and dan rosbjerg

the geochemistry and tectonic setting of the northern section of the luzon arc the philippines and taiwan abstract the luzon arc consists of a 1200 km chain of stratovolcanoes and volcanic necks stretching from mindoro 13 n to the coastal range of taiwan 24 n this study is concerned with three of the five major segments along the arc the northern luzon babuyan and taiwan segments the late tertiary to quaternary volcanics of these segments are primarily andesitic but range in composition from basalt to rhyolite and are typical arc volcanics porphyritic plagioclase textures primarily calc alkaline with a few tholeiitic volcanic centers low tio 2 concentrations and low high field strength element hfse to large ion lithophile element lile ratios there is a large range in k 2 o and other liles from low k tholeiites to high k calc alkaline suites calayan island and mt tabungon volcanic substratum of mt cagua volcanics in the babuyan segment and most rocks from the taiwan coastal range are low k tholeiites e g nearly flat ree patterns low lree and lile concentrations and low th u ratios the dominant calc alkaline series ranges from medium k to high k rocks the high k calc alkaline rocks are mainly young and are found on batan and lutao islands when the low k tholeiitic and medium and high k calc alkaline rocks are associated in the same region the low k rocks are usually older there is a general relationship between k 2 o and other lile concentrations and k ar radiometric dates to some extent 87 sr 86 sr ratios also increase with time throughout the arc there is a correlation between latitude and 87 sr 86 sr and a positive correlation between 87 sr 86 sr and la sm cn an increase in both these geochemical parameters has been associated with an increase in the input of continental crustal material in other arc regions the lowest 87 sr 86 sr ratios found along the entire luzon arc the samples from baguio are equivalent to those found in samples from mid tertiary plutons from northern luzon and the bicol arc both groups of rocks are associated with westward subduction along the philippine trench where presumably deep oceanic sediments with little or no continental crustal component have been subducted the absence of continental crust below the arc in the northern luzon and babuyan segments has been suggested by several groups of researchers this together with the fact that metasomatized ultramafic nodules probably of mantle origin in disequilibrium with their batan host lavas but with similar 87 sr 86 sr and la sm cn ratios to the host rocks suggests that upper level assimilation afc processes does not appear to be the major influence on the geochemical signatures a collision zone between the upper crustal block of eastern china and taiwan and the manila trench has been recorded in the northern section of the arc taiwan sediments have also been shown to decrease in thickness along the south china sea basin nearly parallel to the manila trench from north to south the sediment source region is most probably taiwan and perhaps eastern china this latitudinal in variation sediment thickness may explain the crustal signature related to both geochemical and age parameters we suggest that this crustal input has taken place via subduction of sediment rich in a crustal component

palaeomagnetism and k ar ages of neogene rocks of northern taiwan tectonics of the arc junction of ryukyu and luzon arcs summary more than 100 block samples were collected for palaeomagnetic and geochronological studies from 14 sites in miocene and plio pleistocene volcanic rocks in the western foothills province of northern taiwan in an attempt to understand the tectonic evolution of the junction of the ryukyu and the luzon arcs potassiumargon dating yields ages of 7 9 f 0 5 ma and 10 1 f 0 6 ma for two samples from the miocene chiopanshan volcanic series and 1 2 f 0 1 ma for a sample from the plio pleistocene chilung volcanic group the palaeomagnetic direction from seven sites of the miocene volcanics shows no deflection of declination value d 5 y i 43 8 cxqs 14 0 the direction from the volcanic rock of 1 2 ma is d 2 5 and i 53 3 the reliability of the direction is ascertained through thermal demagnetization positive fold test and the presence of normal and reversed polarities comparison of neogene palaeopoles between northern taiwan and china indicates that little relative tectonic movement has occurred in northern taiwan since 20ma these results establish that the arcuate shape of geological belts in northern taiwan is not due to tectonic bending but to an inherited geometry reconstruction of the 10ma palaeoposition of taiwan and the south ryukyu arc implies that the cusp shape at the arc junction has already existed prior to 10 ma

sequential thrusting model for mountain building constraints from geology and heat flow of taiwan in this study we analyze the thermal evolution of the taiwan fold and thrust belt based on the available geological information a finite element program is used to simulate the complex kinematics of the observed sequential thrusting and the interplay of uplift erosion and sedimentation processes the discrete sequential thrusting model successfully reproduces the observed decrease in heat flow at the deformation front in northern taiwan this reveals that the heat flow drop may be due to the recent stacking of cold thrust sheets at the deformation front this inference is consistent with the high seismicity and dense quaternary fault distribution in western taiwan the model also successfully reproduces the observed high heat flow in the central ranges suggesting that the observed high heat flow may be a result of tectonic unroofing due to active underplating uplift and erosion these conclusions are insensitive to uncertainties in the frictional coefficient on the thrust plane

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

reconstructing the mesozoic early cenozoic evolution of northern philippines clues from palaeomagnetic studies on the ophiolitic basement of the central cordillera summary the first reliable palaeomagnetic data from the cretaceous to eocene ophiolitic basement rocks in the philippines are presented a total of 12 drill core sites from five localities in the central cordillera in northern luzon philippines were sampled eight drill core sites were from pillow basalts and four were from diabase feeder dykes combining the characteristic remanent magnetization direction from these sites gives a mean in situ direction of dec 162 2 i nc 21 4 95 17 0 k 21 1 and a tilt corrected direction of dec 159 3 inc 12 5 95 6 0 k 162 5 along with other lines of palaeomagnetic evidence this clustering suggests that the magnetization is primary and that the ophiolitic basement rocks of the central cordillera formed at subequatorial latitudes 6 3 n 3 1 this information

genesis of the kinabalu sabah granitoid at a subduction collision junction the kinabalu batholith is a late neogene granitoid in northwestern sabah east malaysia apparently marking the locus at which subducted south china basin lithosphere interacted with roots of the northern sabah collision suture the exposed batholith comprises a relatively small core of biotite quartz monzodiorite bqm grading out to dominant hornblende quartz monzonite hqm both lithologies contain mafic igneous and metasedimentary inclusions and are cut by late stage aplite dikes major element data indicate the bqm k2o na2o ratios ranging 0 72 1 03 represents a low k type while hqm k2o na2o ratios ranging 1 35 5 58 is a distinct high k type magmaphile element distributions support this distinction hqm showing higher k rb lree and lower ta and nb contents than bqm indicating the more extensive interaction of hqm with sial least squares mass balance models suggest that hqm evolved through the combined effects of fractional crystallization and crustal assimilation while bqm was dominated by fractional crystallization however similar plagioclase zoning patterns in both lithologies suggest they are comagmatic rather than generated by melting of separate sources it is concluded that low k type melts formed by subduction induced remelting of underplated lower crust underwent high pressure sialic contamination with development of high k character these provided access to later formed low k melts which were less contaminated and consolidated to form the pluton core the unusual zonation from inner low k to outer high k type compositions may indicate that the cessation of subduction prevented upward migration of the melting anomaly and thermal maturation of the pluton

decelerating precursory seismicity in vrancea abstract preshock seismic excitation followed by seismic quiescence has been observed in the seismogenic region of strong shallow mainshocks the strain released by such preshocks is decelerating with the time to the mainshock and is fitted by a power law with a power value larger than unit this model is tested in the present work for the intermediate depth earthquakes of the vrancea region generated in an isolated seismogenic zone proper for such testing a backward application of this decelerating preshock strain model for the case of 4 3 1977 m 7 5 earthquake for which reliable data are available shows a good fit of the power law pattern to the seismic activity preceding the main shock the occurrence rate of recent intermediate depth shocks in vrancea indicates that this region is currently in a state of decelerating seismic deformation which may lead to the generation of a strong intermediate depth mainshock there at about the beginning of the third decade of the present century the respective uncertainties are unknown due to lack of previous relative studies

new geomorphic data on the active taiwan orogen a multisource approach a multisource and multiscale approach of taiwan morphotectonics combines different complementary geomorphic analyses based on a new digital elevation model dem side looking airborne radar slar and satellite spot imagery aerial photographs and control from independent field data this analysis enables us not only to present an integrated geomorphic description of the taiwan orogen but also to highlight some new geodynamic aspects well known major geological structures such as the longitudinal valley lishan pingtung and the foothills fault zones are of course clearly recognized but numerous previously unrecognized structures appear distributed within different regions of taiwan for instance transfer fault zones within the western foothills and the central range are identified based on analyses of lineaments and general morphology in many cases the existence of geomorphic features identified in general images is supported by the results of geological field analyses carried out independently in turn the field analyses of structures and mechanisms at some sites provide a key for interpreting similar geomorphic features in other areas examples are the conjugate pattern of strike slip faults within the central range and the oblique fold and thrust pattern of the coastal range furthermore neotectonic and morphologic analyses drainage and erosional surfaces have been combined in order to obtain a more comprehensive description and interpretation of neotectonic features in taiwan such as for the longitudinal valley fault next at a more general scale numerical processing of digital elevation models resulting in average topography summit level or base level maps allows identification of major features related to the dynamics of uplift and erosion and estimates of erosion balance finally a preliminary morphotectonic sketch map of taiwan combining information from all the sources listed above is presented

structural variety and tectonic evolution of strike slip basins related to the philippine fault system northern luzon philippines the northern part of the philippine fault zone in luzon corresponds to a complex braided system of left lateral strike slip faults the nw oriented main active branch which emerges from the philippine sea splits into an array of north striking splays responsible for the tectonic evolution of the central cordillera this complex fault pattern has favored local stress field variations strike slip basins have evolved in this framework along or in the vicinity of the main splays of the philippine fault apart from the classical pull apart tectonics on a releasing fault termination overlap or bend we describe other mechanisms such as the strike slip tilting or the warping of a strip limited by two strike slip faults the strike slip basins are good recorders of the evolution of the philippine fault system those located along the north striking cordilleran faults individualized in late early pliocene to pleistocene time when the present day philippine fault initiated but their main control is the fault shape acquired in upper middle miocene time the recent tectonic evolution of the fault system is best recorded in central luzon where the active basins trace an asymmetrical v shape with the longest branch trending ne parallel to the east luzon trough and the shortest one trending nw related to the philippine fault both the fault pattern and the basin distribution demonstrate the influence of the benham rise in the tectonic evolution of luzon the structural setting is interpreted as the result of an early miocene collision event between the benham rise and the eastern margin of luzon and subsequent inception of the nw striking strand of the philippine fault the present locations of the basins result from the interaction between the structural heritage and the present day regional plate motion