depositional history of quaternary sediments on the continental shelf off the southeastern coast of korea korea strait abstract the depositional history of quaternary sediments on the continental shelf off the southeastern coast of korea korea strait was investigated by means of analysing high resolution seismic profiles and sediment samples quaternary sediments up to 50 m thick overlie the irregular surface of the acoustic basement in the inner shelf the sedimentary column is comprised of two main sequences separated by a mid reflector which is interpreted as the pre holocene surface the holocene sequence reaching up to 30 m in thickness with a depocenter near the present river mouth is characterized by deposition in a prodelta environment while the upper pleistocene sequence represents deposition in deltaic or shallow water environments during the late glacial period most of the shelf was subaerially eroded and deposition took place only in the trough region the present location of marine built terraces indicates that the lowest position of sea level was about 130 m below its present level the subsequent postglacial transgression reworked pleistocene sediments to form a thin veneer of transgressive facies sea level rose rapidly in the early stages of the postglacial transgression and approached the present level at approximately 7000 5000 yrs b p recent sedimentation the holocene sequence probably began at that time

peculiar geometry of northern luzon philippines implications for regional tectonics of new gravity and paleomagnetic data 1 the northern termination of the philippine archipelago is remarkably abrupt the distinctive almost rectangular shape of northern luzon is a consequence of the almost simultaneous disappearance near 18 n of the central cordillera the much narrower sierra madre and the broad floodplain of the cagayan river the floodplain is underlain by a deep sedimentary basin that is closed off just inland of the coast by an ene wsw structural high known as the sicalao ridge gravity surveys show that the ridge has steep flanks that are almost certainly defined by faulting in the same region there is also a dramatic change in the luzon arc which is the volcanic expression of subduction of the south china sea the volcanic centers in the central cordillera the north luzon segment of the arc are all now inactive but further north eruptions are frequent on the small eastern islands of the bashi segment the differences between the north luzon and bashi segments suggest that they had very different geological histories until the late neogene but the evolution of northern luzon is so poorly understood that it is not even known on which plate it originated new paleomagnetic data and the existence of the sicalao ridge provide important constraints on reconstructions and when combined with geological data favor the possibility that in the paleogene the central cordillera and sierra madre were combined as parts of an arc at the southern margin of the philippine sea the sicalao ridge can be interpreted as a rift margin feature created during the detachment of luzon from continental sundaland

the marinduque intra arc basin philippines basin genesis and in situ ophiolite development in a strike slip setting the marinduque basin is a marine intra arc basin in the north central philippine volcanic arc system a suite of marine geophysical observations show the basin as a whole to be rhombic in shape with its long axis trending north northwest a conspicuous volcanic ridge trends east northeast across the basin dividing it into two smaller active depocenters symmetric linear magnetic anomalies striking parallel to and centered over the central volcanic ridge indicate that it formed by extension in a north south direction by a process analogous to sea floor spreading geometric and temporal relations between the central volcanic ridge and several en echelon sets of north trending left stepping faults indicate that sea floor spreading occurred in the extensional stepover zone of a left lateral strike slip fault system and that the marinduque basin as a whole is a composite pull apart basin whose floor is in part composed of oceanic type crust the evolution of the central volcanic ridge presents an actualistic model for in situ development and emplacement of ophiolites in an island arc setting the presently active left lateral philippine fault zone truncated the eastern flank of the basin as well as the central volcanic ridge following their initial formation shortening associated with a probable restraining bend in the philippine fault has resulted in uplift of basin flank sediments which have been thrust back toward the basin interior the philippine fault has also displaced fragments of the ridge and basin floor toward the north the overall history of the marinduque basin further suggests that strike slip processes may play an important role in the origin and growth of intra arc basins such basins may ultimately become isolated in a forearc or backarc setting

seismic reflection evidence for a dangerous grounds miniplate no extrusion origin for the south china sea the collision of india and asia has caused large strike slip faults to form in east asia resulting in the extrusion of crustal blocks toward the southeast since the eocene as a result of the indentation of rigid india into asia it has been suggested that the south china sea opened as a result of relative motion between a rigid indochina sundaland block and china alternative models propose that rifting and seafloor spreading were driven by trench forces to the south we test these competing models by analysis of seismic reflection profiles across the boundary between sundaland and the southern rifted margin known as the dangerous grounds we show that the southern boundary of the dangerous grounds is a subduction zone that jammed in the middle miocene to the west the dangerous grounds are bounded by a strike slip zone also active until similar to 16 ma that becomes diffuse south of the now inactive south china sea seafloor spreading center we place the western edge of the dangerous grounds just to the east of the natuna arch lupar line the west baram line is confirmed as originating as a major strike slip fault within the dangerous grounds and is continuous with the red river fault zone because the dangerous grounds were independent of sundaland until similar to 6 ma its motion cannot have been constrained by motion of this block making extrusion impossible as a mechanism to rift the south china sea se motion by both the dangerous grounds and sundaland suggests subduction forces were the primary trigger for plate motions our reconstruction places a similar to 280 km upper limit on the motion on the red river fault and a similar to 1400 km width to the paleo south china sea

collisional orogenesis in the northern canadian cordillera implications for cordilleran crustal structure ophiolite emplacement continental growth and the terrane hypothesis during upper triassic to lower jurassic time arc magmatic rocks of the stikine terrane arc marginal sediments of the whitehorse trough igneous and mantle rocks of the cache creek ophiolite and kutcho assemblage and oceanic sedimentary and volcanic rocks of the cache creek terrane represented a magmatic arc forearc basin forearc basement and subduction complex respectively the cache creek subduction complex was thrust to the southwest over the whitehorse trough forearc basin and the stikine terrane in the middle jurassic during collision with an inboard continental domain interpretation of the various lithotectonic assemblages of the northern intermontane belt in terms of a new plate tectonic model has a number of important implications for the canadian cordillera a the model allows comparisons to be drawn with available seismic reflection interpretations of cordilleran crustal structure b ophiolite emplacement was achieved by ramping of forearc oceanic lithosphere onto thick crustal parts of a subducting plate during collisional orogenesis c island arc collision and accretion were the principal mechanisms for continental growth with relatively minor contributions from sliced off oceanic seamounts and or plateaux and d some terrane bounding faults such as the nahlin fault do not represent major lithospheric scale boundaries and their importance in tectonic reconstructions has been overemphasised

tertiary stratigraphy and basin evolution southern sabah malaysian borneo abstract new mapping and dating of strata in the southern part of the central sabah basin in northern borneo has made it possible to revise the lithostratigraphy and chronostratigraphy of the area the recognition in the field of an early miocene regional unconformity which may be equivalent to the deep regional unconformity recognised offshore has allowed the development of a stratigraphic framework of groups and formations which correspond to stages in the sedimentary basin development of the area below the early miocene unconformity lies ophiolitic basement which is overlain by an accretionary complex of eocene age and a late paleogene deep water succession which formed in a fore arc basin the late paleogene deposits underwent syn depositional deformation including the development of extensive melanges all of which can be demonstrated to lie below the unconformity in this area some localised limestone deposition occurred during a period of uplift and erosion in the early miocene following which there was an influx of clastic sediments deposited in delta and pro deltaic environments in the middle miocene these deltaic to shallow marine deposits are now recognised as forming two coarsening upward successions mapped as the tanjong and kapilit formations the total thickness of these two formations in the central sabah basin amounts to 6000 m only half of the previous estimates although the total stratigraphic thickness of cenozoic clastic strata in sabah may be more than 20 000 m

cenozoic plate interaction of the australia and philippine sea plates hit and run tectonics abstract recent studies in northwest new guinea have shown the presence of at least two marginal basins of different age both of which formed in back arc settings the older basin opened between the middle jurassic and early cretaceous a remnant of which is now preserved as the new guinea ophiolite its obduction started at 40 ma and it was finally emplaced on the australian margin at 30 ma the younger basin was active during the oligocene to middle miocene and was obducted in the early pliocene studies of the western edge of the philippine sea also reveal an important deformation of the philippine arc in the oligocene which hitherto has remained unexplained using information from these systems paleomagnetic results kinematic reconstructions and geochemistry of the supra subduction ophiolite we present a plate model to explain the region s eo oligocene development we suggest that an extensive portion of oceanic crust extended the australian plate a considerable distance north of the australian craton as australia began its steady 7 8 cm year northward drift in the early eocene this lithosphere was subducted thus the portion of the philippine sea plate carrying the taiwan philippine arc to its present site may have actually been in contact with the ophiolite now in new guinea and obduction led to deformation of the philippine sea plate itself neogene plate kinematics transported the deformed belt in contact with the sunda block in the late miocene and pliocene this interpretation has implications for the origin for the philippine sea plate and the potential incorporation of continental fragments against its boundaries

plate kinematics origin and tectonic emplacement of supra subduction ophiolites in se asia abstract a unique feature of the circum pacific orogenic belts is the occurrence of ophiolitic bodies of various sizes most of which display petrological and geochemical characteristics typical of supra subduction zone oceanic crust in se asia a majority of the ophiolites appear to have originated at convergent margins and specifically in backarc or island arc settings which evolved either along the edge of the sunda eurasia and australian cratons or within the philippine sea plate these ophiolites were later accreted to continental margins during the tertiary because of fast relative plate velocities tectonic regimes at the active margins of these three plates also changed rapidly strain partitioning associated with oblique convergence caused arc trench systems to move further away from the locus of their accretion we distinguish relatively autochthonous ophiolites resulting from the shortening of marginal basins such as the present day south china sea or the coral sea and highly displaced ophiolites developed in oblique convergent margins where they were dismantled transported and locally severely sheared during final docking in peri cratonic mobile belts i e the philippine mobile belt we find a series of oceanic basins which have been slightly deformed and uplifted varying lithologies and geochemical compositions of tectonic units in these basins as well as their age discrepancies suggest important displacements along major wrench faults we have used plate tectonic reconstructions to restore the former backarc basins and island arcs characterized by known petro geochemical data to their original location and their former tectonic settings some of the ophiolites occurring in front of the sunda plate represent supra subduction zone basins formed along the australian craton margin during the mesozoic the philippine sea basin the huatung basin south of taiwan and composite ophiolitic basements of the philippines and halmahera may represent remnants of such marginal basins the portion of the philippine sea plate carrying the taiwan philippine arc and its composite ophiolitic continental crustal basement might have actually originated in a different setting closer to that of the papua new guinea ophiolite and then have been displaced rapidly as a result of shearing associated with fast oblique convergence

paleomagnetic results from luzon and the central philippines samples were collected from 86 paleomagnetic sites from the islands of luzon marinduque mindoro panay negros cebu and mindanao in the philippine arc the sampling sites range in age from pleistocene to jurassic characteristic directions of magnetization of the samples were determined by the use of vector plots curie temperature determinations thin section studies and hysteresis studies showed that remanence of these samples is carried by fine grained pseudo single domain magnetite positive fold tests from miocene data from panay jurassic data from mindoro and cretaceous data from cebu suggest that the magnetization of these regions was acquired prior to folding rotations reported below are measured with respect to the axial goecentric dipole field the plio pleistocene data set shows no resolvable rotation for the 22 sites this data set suggests that the various terranes that make up the philippine arc have behaved as a single unit during the past 5 m y or that deformation has been below the limits of resolution the inclination data from the plio pleistocene sites have anomalously shallow inclination and are consistent with other plio pleistocene data from vietnam taiwan and the marianas these data support earlier suggestions for a late neogene offset dipole effect the late miocene sites fall into two separate groups ten sites from western luzon show evidence for around 20 of clockwise rotation in contrast to this late miocene samples from the bicol region negros marinduque and mindanao are not rotated the cause of the postlate miocene clockwise rotation of luzon is unknown but a pliocene collision of the north luzon arc with taiwan is suggested early neogene results also separate into two different populations the population from marinduque shows evidence for a large counterclockwise rotation the second early neogene population comes from panay cebu and mindanao and clearly shows evidence for a clockwise rotation the validity of this rotation is further supported by a fold test and a reversal test these early neogene data sets are consistent with a middle to late miocene collision of the palawan continental terrane and the central philippine arc data from six dikes of possible oligocene age from the zambales ophiolite are highly discordant from the present field being rotated approximately 60 clockwise the directions from these dikes are similar to a direction reported earlier from late oligocene sediments also from the zambales region these two data sets support the interpretation that the eocene direction from zambales is recording a large clockwise rotation of the region data from the mesozoic sites are from two regions data from the cretaceous pandan formation of cebu are discordant with data from the upper jurassic from mindoro the presence of a fold test from each region and a reversal test from mindoro supports the interpretation that each of these data sets is reliable the vgp of mindoro is displaced southward from the late jurassic vgp of south china suggesting a post jurassic southward migration of mindoro

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

ocean plate stratigraphy in east and southeast asia abstract ancient accretionary wedges have been recognised by the presence of glaucophane schist radiolarian chert and melange recent techniques for the reconstruction of disrupted fragments of such wedges by means of radiolarian biostratigraphy provide a more comprehensive history of ocean plate subduction and successive accretion of ocean floor materials from the oceanic plate through offscraping and underplating reconstructed ocean floor sequences found in ancient accretionary complexes in japan comprise from oldest to youngest pillow basalt limestone radiolarian chert siliceous shale and shale and sandstone similar lithologies also occur in the melange complexes of the philippines indonesia thailand and other regions this succession is called ocean plate stratigraphy ops and it represents the following sequence of processes birth of the oceanic plate at the oceanic ridge formation of volcanic islands near the ridge covered by calcareous reefs sedimentation of calcilutite on the flanks of the volcanic islands where radiolarian chert is also deposited deposition of radiolarian skeletons on the oceanic plate in a pelagic setting and sedimentary mixing of radiolarian remains and detrital grains to form siliceous shale in a hemipelagic setting and sedimentation of coarse grained sandstone and shale at or near the trench of the convergent margin radiolarian biostratigraphy of detrital sedimentary rocks provides information on the time and duration of ocean plate subduction the ages of detrital sediments becomes younger oceanward as younger packages of ops are scraped off the downgoing plate ops reconstructed from ancient accretionary complexes give us the age of subduction and accretion direction of subduction and ancient tectonic environments and is an important key to understanding the paleoenvironment and history of the paleo oceans now represented only in suture zones and orogenic belts

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

mass balance modeling of mineral weathering rates and co2 consumption in the forested metabasaltic hauver branch watershed catoctin mountain maryland usa mineral weathering rates and a forest macronutrient uptake stoichiometry were determined for the forested metabasaltic hauver branch watershed in north central maryland usa previous studies of hauver branch have had an insufficient number of analytes to permit determination of rates of all the minerals involved in chemical weathering including biomass more equations in the mass balance matrix were added using existing mineralogic information the stoichiometry of a deciduous biomass term was determined using multi year weekly to biweekly stream water chemistry for a nearby watershed which drains relatively unreactive quartzite bedrock at hauver branch calcite hosts 38 mol of the calcium ion ca2 contained in weathering minerals but its weathering provides 90 of the stream water ca2 this occurs in a landscape with a regolith residence time of more than several ka kiloannum previous studies indicate that such old regolith does not typically contain dissolving calcite that affects stream ca2 na ratios the relatively high calcite dissolution rate likely reflects dissolution of calcite in fractures of the deep critical zone of the carbon dioxide co2 consumed by mineral weathering calcite is responsible for approximately 27 with the silicate weathering consumption rate far exceeding that of the global average the chemical weathering of mafic terrains in decaying orogens thus may be capable of influencing global geochemical cycles and therefore climate on geological timescales based on carbon balance calculations atmospheric derived sulfuric acid is responsible for approximately 22 of the mineral weathering occurring in the watershed our results suggest that rising air temperatures driven by global warming and resulting in higher precipitation will cause the rate of chemical weathering in the hauver branch watershed to increase until a threshold temperature is reached beyond the threshold temperature increased recharge would produce a shallower groundwater table and reduced chemical weathering rates copyright 2013 john wiley sons ltd

the cretaceous southeast bohol ophiolite complex central philippines a highly disaggregated supra subduction zone ophiolite abstract the southeast bohol ophiolite complex seboc represents a complete ophiolite sequence with a pelagic chert cap yielding an early cretaceous age the residual peridotite suite of this ophiolite complex is dominated by harzburgite with rare occurrences of lherzolite and dunite pods layered ultramafic cumulate rocks made up of clinopyroxenite wehrlite dunite and massive to layered mafic cumulate rocks consisting of gabbro and norite are present although disaggregated the ophiolite complex includes a sheeted dike complex and a preponderance of sheet flows over pillow lavas in the volcanic section geologic observations such as gabbro pods included within residual harzburgite and deep water sedimentary and volcanic rocks directly overlying the harzburgite have often been reported from the mid atlantic ridge and other slow spreading centers however such situations are never encountered in the seboc geochemical data on seboc show that this ophiolite complex exhibits transitional mid ocean ridge basalt island arc tholeiite geochemical affinities this suggests formation in a subduction related marginal basin the field geological characteristics of seboc show that aside from being a harzburgite ophiolite type this crust mantle sequence was formed in a fast spreading center later tectonic events resulted in its emplacement as a forearc ophiolite

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

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

paleomagnetism of palawan philippines abstract paleomagnetic studies have been carried out on palawan and on the island of busuanga to the north results from the cretaceous espina basalts of the calatuigas ophiolite in the south palawan block spb pass a fold test yield normal and reversed directions with a magnetic intensity and af demagnetization characteristics consistent with a primary trm the mean direction is 293 9 and an inclination of 5 8 with a k of 37 7 and an 95 of 12 6 this suggests that these ophiolites have moved northward and rotated counterclockwise by 66 13 with respect to the geocentric axial dipole gad field it also suggests that they were obducted from the south paleomagnetic directions from the jurassic busuanga cherts and the cretaceous guinlo formation from the island of busuanga in the north palawan block npb and from the guinlo on the main island of palawan are similar fail regional fold tests and have af demagnetization characteristics consistent with secondary magnetization their inclinations are statistically indistinguishable at a 95 significance level but variation in declination suggests differential local rotation about a vertical axis the paleolatitude is comparable to that of regions of pervasive cretaceous remagnetization in the south china borderland and may reflect similar remagnetization consistent with the npb s proposed south china origin

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

philippine fault a key for philippine kinematics on the basis of new geologic data and a kinematic analysis we establish a simple kinematic model in which the motion between the philippine sea plate and eurasia is distributed on two boundaries the philippine trench and the philippine fault this model predicts a velocity of 2 to 2 5 cm yr along the fault geologic data from the visayas provide an age of 2 to 4 ma for the fault an age in good agreement with the date of the beginning of subduction in the philippine trench the origin of the philippine fault would thus be the flip of subduction from west to east after the locking of convergence to the west by the collision of the philippine mobile belt with the eurasian margin

the palaeozoic tectono metallogenic evolution of the northern tasman fold belt system australia interplay of subduction rollback and accretion abstract the tasman fold belt system in eastern australia provides a record of the palaeozoic geological history and growth of the australian continent along the proto pacific margin of gondwana inboard of an extensive and long lived subduction system the hodgkinson and broken river provinces represent prominent geological elements of this system and together form the northern tasman fold belt system geochronological age dating of the timing of gold formation in the amanda bel goldfield in the broken river province and the hodgkinson goldfield in the hodgkinson province provides constraints on the occurrence of a deformation and mineralisation episode in the late devonian early carboniferous integration of these newly obtained data with petrogenetic constraints and a time space evaluation of the geological evolution of the hodgkinson and broken river provinces as well as other terranes in the northern tasman fold belt system allows for the development of a geodynamic model for the palaeozoic evolution of the northern tasman fold belt system our model indicates that three cycles of extension contraction occurred during the palaeozoic evolution of the northern tasman fold belt system episodes of extension were controlled by rollback of the subduction system along the proto pacific margin of gondwana whereas episodes of contraction resulted from accretion following the arrival of positively buoyant segments i e micro continental blocks oceanic plateaus at the subducting trench our composite interpretative model on the geodynamic evolution of the northern tasman fold belt system integrates the timing of the development of mineral deposits throughout this part of the system and provides a significant advancement in the understanding of palaeozoic geodynamics along the margin of gondwana in northeast australia and allows comparison with the southern part of the tasman fold belt system

pre eocene synmetamorphic structure in the mindoro romblon palawan area west philippines and implications for the history of southeast asia the structure of the pre eocene rocks considered as the basement of the philippines has been investigated in the mindoro lubang romblon tablas sibuyan and north palawan areas in the former two areas the same pre eocene succession of units is recognized from top to bottom 1 a pre eocene olistostrome 2 an ophiolitic nappe 3 a schistose sequence pelites sandstones basic schists and marbles and 4 a gneissic unit the nature of the contact between the olistostrome and the underlying units is unclear but the ophiolite and the schistose sequence form two thrust sheets of oceanic origin thrust upon the gneissic unit that is considered a part of a continental basement called the west philippines block small scale structures show that the early deformation stage is characterized by a submeridian 0 n40 e lineation formed in greenschist to amphibolite facies conditions during the thrusting kinematic analysis show that the thrusting was from north to south in north palawan metamorphic rocks with similar microtectonic and kinematic characteristics are found they are overlain by a late jurassic olistostrome which is correlated with the olistostromes found in calamian north mindoro carabao and buruanga peninsula north panay the microstructural features and the presence of the olistostrome suggest that the north palawan mindoro tablas romblon sibuyan and carabao islands belong to the same north palawan block of hamilton 1979 which is a continental fragment rifted from asia in cenozoic times it is assumed that the western panay and zamboanga areas which are characterized by mesozoic ophiolites and metamorphic rocks also belong to the north palawan block all these islands experienced to some extent the same mesozoic geohistory the south verging thrusting is interpreted as the result of an oblique collision of the west philippines block with asia the contemporaneous left lateral strike slip faulting and the calc alkaline magmatism widespread along the chinese margin are also included in the geodynamic model

cretaceous tectonics and geological environments in east russia abstract by the late jurassic the northern part of the east asian continental margin was diversified principal structural constituents were the siberian or north asian craton bounded on the east by a miogeosynclinal fold belt a system of smaller ancient blocks and a collage of terranes that had been attached to the craton from the east and southeast at different times such a diverse structural environment caused variability of cretaceous landscapes and related sedimentary systems global processes of lithospheric plate interaction and related regional tectonic processes played a leading role in cretaceous environmental changes in the continental margin of east russia during the early cretaceous the oblique plate convergence produced a transform continental margin over a long period of time during the middle albian a collage of terranes were attached to the continental margin of east russia then during the late albian period the angle of convergence increased subduction resumed and a giant east asian volcanic belt formed along the continental margin this belt was morphologically represented by a chain of mountain ridges up to 3000 m thus creating a sublongitudinal tectonic and climatic zonation the active continental margin with a typical environmental arrangement of marginal seas island arcs the open sea persisted until the end of the cretaceous the late maastrichtian tectonics was mostly characterized by overall uplift of the region

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

rapid emplacement of the oman ophiolite thermal and geochronologic constraints in understanding of ophiolite emplacement requires knowledge of the elapsed time between igneous crystallization and intraoceanic thrusting and the rate and duration of that thrusting hornblende 4oar 39ar ages demonstrate that the igneous oceanic crust in oman crystallized and cooled to 825 k in 1 2 m y hornblende ages from metamorphic rocks and from cross cutting dikes require that the basal metamorphic thrust fault beneath the ophiolite also cooled below 825 k in 1 2 m y motion along the sole thrust accounted for 200 km of displacement at a rate of 100 200 mm yr on the basis of age relationships and thermal considerations we favor a two stage model for the initial stages of samall ophiolite emplacement intraoceanic thrusting over 2 m y old lithosphere at 150 km m y parallel to a spreading ridge for 1 2 m y followed by equally rapid and brief thrusting over cold and old lithosphere preservation of the samail ophiolite is a direct result of its young age and positive buoyancy at the time of ocean closure and we propose that all ophiolites that originated near spreading centers and were emplaced onto continents were young at the time of intraoceanic thrusting

a new insight on the geometry of subducting slabs in northern luzon philippines abstract we used hypocentral and focal mechanism data in order to characterize the tectonic configuration of northern luzon and propose a model for describing the geometry of the subducted slab of the eurasian plate beneath the northern segment of the manila trench we took into consideration some of the observed bathymetric features i e the bend in the trench line of the manila trench at 20 n lat and the collision and subduction of an extinct mid oceanic ridge at 16 17 n lat and the intraplate deformation pattern in the north luzon ridge region in depicting a model that is consistent with most of the observed features and phenomena in the study area an earthquake catalog covering the period from 1619 to 1997 was used in studying the distribution of strain energy released while earthquakes covering the instrumental years 1963 1997 were used to analyze vertical distributions of earthquakes in sections we refined the location of some of the historical earthquakes using macroseismic data the inclusion of historical records despite their inaccurate locations could prevent misidentification of some regions as aseismic zones or gaps in seismicity especially along structures with long return periods a focal mechanism database that covers the period from 1963 to 1997 was used to decipher intraplate deformation pattern and subduction process of the slab the focal mechanism database consists of first motion solutions from previous authors and cmt solutions from harvard university seismicity strain energy release and focal mechanism maps and cross sections were drawn in order to have a three dimensional visualization of the geometry and stress regimes of seismogenic zones a new model of the subducted slab of the eurasian plate beneath the manila trench is proposed the model suggests the collision and subsequent partial subduction of a buoyant plateau at around 20 n lat to explain the sharp bend in the trench line the complicated deformation pattern on the overriding plate fronting the bend and the shallow dip of the subducted slab beneath this zone a tear in the slab is also inferred to be present as evidenced by the observed gap in strain energy release and the abrupt change in dip from shallow to steep south of 18 n lat the gap in seismicity and strain energy release 65 300 km depth at around 17 n lat may be used to infer the trajectory and location of the subducted extinct mid oceanic ridge mor the aseismic behavior is probably caused by the subducted ridge which is still hot and is deforming plastically this is supported by the heatflow data which shows high values along the extinct mor the subducted part of the mor may serve as the weakest zone where this tear could be localized the tear may also explain the cause of the abrupt termination of the eastern chain of volcanoes south of 18 n lat the above model which is a refinement of the model introduced by yang et al tectonophysics 258 1996 85 is consistent with the observed seismicity and deformation pattern observed bathymetric features spatial distribution and geochemical character of volcanism in northern luzon