the initiation of subduction by crustal extension at a continental margin summary we investigate how subduction may be triggered by continental crust extension at a continental margin the large topography contrast between continental and oceanic domains drives the spreading of continental crust over oceanic basement subduction requires the oceanic plate to get submerged in mantle so that negative buoyancy forces may take over and drive further descent this is promoted by two mechanisms loading by continental crust bends the oceanic plate downwards extension in the continental domain induces crustal thinning which acts to raise mantle above the oceanic plate in this model the width of the continental region undergoing extension is an important control parameter the main physical controls are illustrated by laboratory experiments and simple theory for elastic flexure coupled to viscous crustal spreading three governing dimensionless parameters are identified one involves the poorly constrained oceanic plate buoyancy we find that the oceanic plate can be thrust to depths larger than 40 km even if it is buoyant enabling metamorphic reactions and density increase in the oceanic crust another parameter is the ratio between the width of the continental extension region and the flexural parameter for the oceanic plate initiating subduction is easier if the continent thins over a short lateral distance or if the oceanic plate is strong the third important parameter is the ratio of oceanic plate thickness to initial continental crust thickness such that a weak plate and a thick crust do not favour subduction thus the change from a passive to an active margin depends on the local characteristics of the continental crust and is not determined solely by the age and properties of the oceanic lithosphere it is shown that the spreading of continental crust induces uplift of the margin as the adjacent seafloor subsides evidence for the emplacement of continental crust over oceanic basement at passive margins is reviewed

early jurassic carbon isotope excursion in the qiangtang basin tibet the eastern tethys implications for the toarcian oceanic anoxic event abstract the early jurassic was characterized by a global disturbance of the carbon cycle known as the toarcian oceanic anoxic event t oae this event is recorded by a large negative carbon isotope excursion cie in marine carbonates and marine organic matter as well as fossil wood here we present new high resolution carbonate 13 c carb and organic 13 c org carbon isotope records and total organic content from the bilong co section in the qiangtang basin the eastern tethys this section is high in organic carbon and black shale facies are also identified the bilong co section shows heavy 13 c org values compared to other reported marine organic carbon isotope values for the t oae this is explained by variable mixing of terrestrial and marine organic matter compared to european sections the magnitude of the negative cie in the bilong co section is slightly different which could be explained by different local paleoenvironmental conditions and strata missing although these the carbon isotope profile from the bilong co area displays a similar trend to those in contemporaneous strata in the western tethyan and boreal realms of nw europe which is characterized by a distinct negative excursion in the early toarcian our new data from the bilong co section in n tibet provide the first high resolution record of the early toarcian oceanic anoxic event in the eastern tethys

a slab in depth three dimensional geometry and evolution of the indo australian plate geoscientists use plate tectonics to explain many aspects of both continental evolution and evolution of the planet as a whole the subduction of material at convergent plate boundaries forms a fundamental component to the theory of plate tectonics plates continents subduction zones and spreading centers all exhibit motion and geometric evolution so to try and resolve the past geometries of the planet geologists have utilized plate tectonic reconstructions here we present a three dimensional image of the subducted indo australian plate below southeast asia and show that the geometry of the subducted slab at depth is intimately related to the geometric evolution of se asia over the past 50 ma including the collision of india with the asian continent we show how the once semicontinuous subducting indo australian plate has been segmented during collision between india australian and the subduction margin to the north thus we have found that the geometry of the subducted plate should form a key component to the interpretation of the evolution of earth s surface as complexities and evolution of the subducted plate are manifest in the evolution of the overriding plate

evidence for a submerged and displaced continental borderland north banda sea indonesia the banda sea is separated into two oceanic fragments the south and north banda basins by a large intervening region of ridges and basins called the banda ridges the north and south banda basins appear to be trapped pieces of pre tertiary oceanic crust whereas the banda ridges are a submerged and displaced continental borderland structurally very similar to the southern california offshore borderland analysis of dredged rocks from the banda ridges suggests an origin from western irian jaya

comment on dating the geologic history of oman s semail ophiolite insights from u pb geochronology by c j warren r r parrish d j waters and m p searle

neogene sediment thickness and miocene basin floor fan systems of the celebes sea abstract in the celebes sea basin the middle miocene turbidites were correlated from odp site 767 throughout the studied area differences in their regional thickness variations and distribution indicate two source areas the middle miocene turbidite fan complexes of the central and southern celebes sea basin are controlled by the paleo tarakan delta system the tectonic events and the basin floor morphology respectively the main source area for the time correlative turbidites along the southern sulu arc is assumed to be mindanao the correlation of the middle miocene to pleistocene sequences exhibit tentative ages for the development of the accretionary wedges along the cotabato trench and along the north sulawesi subduction a post middle miocene to pre pliocene age is inferred for the cotabato wedge and a plio pleistocene age is assumed for the north sulawesi wedge

magnetic lineations constraints for the back arc opening of the late neogene south banda basin eastern indonesia abstract the south banda basin is located within eastern indonesia near the triple junction between the eurasian pacific and indo australian plates it is underlain by oceanic crust but its origin and age were not well established it has been interpreted as a mesozoic trapped piece of indian ocean or as a cretaceous eocene basin related to the celebes and sulu basins but a neogene back arc origin was also considered recent geochemical and geochronological studies strongly support the latter hypothesis in this paper we present a new analysis of the magnetic data from the eastern part of the south banda basin the damar basin we used magnetic field measurements collected during nine oceanographic cruises from various institutions looking for magnetic correlation in the time span given from recent geochronological data the comparison between measured profiles and theoretical profiles deduced from the reversals of earth s magnetic field during neogene time allows us to infer an opening of south banda basin during late miocene early pliocene time from 6 5 to 3 5 ma magnetic lineations 2an 2ar 3n 3r 3an and possibly 3ar are recognized with an extinct spreading centre trending ene wsw at least five segments are identified each segment being separated by inactive transform faults perpendicular to the extinct spreading centre a half spreading rate of about 3 cm yr is calculated based on spacing of magnetic lineations the opening history of the basin is discussed the cause of cessation of spreading is likely the arc continent collision dated at about 3 ma however the onset of opening is less determined we suggest that damar basin began to open at about 6 5 ma during magnetic period 3an as an intra arc basin separating the banda volcanic arc to the south from the incipient lucipara volcanic arc to the north the latter was probably created at the beginning of rifting of damar basin as shown by both magnetic and geochronological data the young age of the south banda sea basin is contradictory with its great depth we discuss this problem in view of thermal and tectonic considerations we finally conclude that our magnetic model allows us to more precisely describe the opening of the basin

new paleomagnetic constraints on the cenozoic tectonic evolution of the north arm of sulawesi indonesia abstract extensive paleomagnetic sampling 34 sites was carried out in cenozoic formations of the north arm of sulawesi between 120 and 124 e consistent results obtained between 120 and 122 e suggest the existence of a unique post miocene clockwise rotation of about 20 25 of the western part of the north arm this rotation probably occurred during the northward drift of the north arm along the palu matano sinistral transcurrent fault between 122 5 and 124 e both clockwise and counterclockwise rotations with various amplitudes from 6 to 85 are encountered and these very likely correspond to individual microblock rotations due to pervasive deformation along shear zones this result is consistent with the complex fault system of the gorontalo and kotamobagu shear zones that have been recognized on radar imagery and by spot image analyses

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

collision rotation and the initiation of subduction in the evolution of sulawesi indonesia the island of sulawesi indonesia has been shaped and deformed as a result of collision with the sula platform a sliver of continental material from the northern margin of australia new guinea the collision has resulted in rotation of the north volcanic arm of sulawesi and the development of the accretionary wedge of the north sulawesi trench the north sulawesi trench changes laterally from a zone of no active deformation in the eastern part to a wide accretionary wedge in the west early stages of thrusting produce a steep frontal slope 8 16 indicative of relatively high basal shear stress whereas the more advanced western zone of thrusting produces a gentle 2 slope consistent with low basal shear stress reported paleomagnetic data suggest post late eocene counter clockwise rotation of the north arm and the offshore geophysics are explained by a pivot of the north arm with respect to the celebes basin about the eastern end of the arc convergence between the north banda basin and southeast sulawesi is documented by the presence of the tolo thrust its outcrop is strongly arcuate and its accretionary wedge varies in width from a minimum of a few kilometers at each end to a maximum of 30 40 km in the central part the northern end transforms to the leftlateral matano fault with a reported offset of 20 km the southern end of the thrust projects toward the deformed rocks of buton but the structural relations there are not clear the matano fault zone appears to connect westward with the palu fault which forms the western transform of the north sulawesi trench the palu matano fault system acts as a trench trench transform between the north sulawesi trench and the tolo thrust and this system is described by the same rotation pole as that for the sulawesi north arm

paleomagnetic data from the philippine sea plate and their tectonic significance new paleomagnetic data from guam and saipan have been obtained and are used in conjunction with previously published results from around the philippine sea plate to interpret the origin and history of the philippine sea plate a lower miocene direction i 3 5 d 54 5 a95 17 4 k 20 3 and a middle oligocene direction i 5 3 d 68 4 a95 12 8 k 52 4 have been obtained from the palau islands only one reliable result a middle oligocene direction i 15 1 d 66 1 a95 11 1 k 20 1 has been obtained from guam many characteristic remanent magnetic directions have been obtained from the lower eocene facpi formation however the results are too scattered k 4 5 to be considered reliable two reliable directions have been obtained from saipan a middle miocene result i 30 7 d 28 1 a95 8 4 k 64 6 and a lower eocene result i 12 0 d 43 0 a95 12 5 k 16 0 two lower eocene results from chichijima i 9 5 d 105 3 a95 12 6 k 6 1 and anijima i 0 8 d 91 7 a95 16 9 k 4 9 are the reliable results from the bonin islands there are two end member interpretations for these data 1 small scale local rotation of blocks along the plate margin or 2 rotation of the philippine sea plate as a whole to distinguish between these interpretations we reconstructed the east philippine sea province back to its prerifting configuration the rotation caused by deformation can be estimated by comparing the paleomagnetism of guam saipan and the bonin islands to palau prior to reconstruction the older declination values are scattered closing the marginal basins reduces the declination scatter and all of the islands show increasing clockwise deflection with time this suggests that the philippine sea plate has rotated up to 80 clockwise and moved northward around 20 since the eocene and deformational rotation is a second order feature the finite pole of rotation of the philippine sea plate relative to eurasia must be somewhere near the southeast comer of the plate because this is the only location that is compatible with both the change in orientation and paleolatitude for the plate these data cannot distinguish between a backarc origin or trapped crust origin for the west philippine sea province the existing models require major modification to be consistent with the paleomagnetic data

the palu metamorphic complex nw sulawesi indonesia origin and evolution of a young metamorphic terrane with links to gondwana and sundaland abstract the palu metamorphic complex pmc is exposed in a late cenozoic orogenic belt in nw sulawesi indonesia it is a composite terrane comprising a gneiss unit of gondwana origin a schist unit composed of meta sediments deposited along the se sundaland margin in the late cretaceous and early tertiary and one or more slivers of amphibolite with oceanic crust characteristics the gneiss unit forms part of the west sulawesi block underlying the northern and central sections of the western sulawesi province the presence of late triassic granitoids and recycled proterozoic zircons in this unit combined with its isotopic signature suggests that the west sulawesi block has its origin in the new guinea margin from which it rifted in the late mesozoic it docked with sundaland sometime during the late cretaceous u th pb dating results for monazite suggest that another continental fragment may have collided with the sundaland margin in the earliest miocene high pressure hp and ultrahigh pressure uhp rocks granulite peridotite eclogite are found as tectonic slices within the pmc mostly along the palu koro fault zone a major strike slip fault that cuts the complex mineralogical and textural features suggest that some of these rocks resided at depths of 60 120 km during a part of their histories thermochronological data u th pb zircon and 40ar 39ar from the metamorphic rocks indicate a latest miocene to mid pliocene metamorphic event which was accompanied by widespread granitoid magmatism and took place in an extensional tectonic setting it caused recrystallization of and new overgrowths on pre existing zircon crystals and produced andalusite cordierite sillimanite staurolite assemblages in pelitic protoliths indicating ht lp buchan type metamorphism the pmc was exhumed as a core complex at moderate rates c 0 7 1 0 mm yr accompanied by rapid cooling in the plio pleistocene some of the uhp rocks were transported to the surface at significantly higher rates 16 mm yr the results of our study do not support recent plate tectonic reconstructions that propose a nw australia margin origin for the west sulawesi block e g hall et al 2009

surface deformation and slab mantle interaction during banda arc subduction rollback the reason for the spectacular curvature of the banda subduction zone is debated tomographic images and plate reconstructions reveal subduction of a single slab the ancient geometry of the australian plate as well as the interaction between the slab and the mantle caused the deformation of the slab the spectacularly curved banda arc comprises young oceanic crust1 2 enclosed by a volcanic inner arc outer arc islands and a trough parallel to the australian continental margin3 4 5 strong seismic activity in the upper mantle defines a folded surface6 7 for which there are two contrasting explanations deformation of a single slab5 8 or two separate slabs subducting from the north and south6 9 here we combine seismic tomography with the plate tectonic evolution of the region to infer that the banda arc results from subduction of a single slab our palaeogeographic reconstruction shows that a jurassic embayment which consisted of dense oceanic lithosphere enclosed by continental crust once existed within the australian plate banda subduction began about 15 million years ago when active java subduction tore eastwards into the embayment the present morphology of the subducting slab is only partially controlled by the shape of the embayment as the australian plate moved northward at a high speed of about 7 cm yr 1 the banda oceanic slab rolled back towards the south southeast accompanied by active delamination separating the crust from the denser mantle increasing resistance of the mantle to plate motion progressively folded the slab and caused strong deformation of the crust the banda arc represents an outstanding example of large scale deformation of the earth s crust in response to coupling between the crust slab and surrounding mantle

subduction zone infancy examples from the eocene izu bonin mariana and jurassic california arcs a new model for the earliest stages in the evolution of subduction zones is developed from recent geologic studies of the izu bonin mariana ibm arc system and then applied to late jurassic ophiolites of cailfornia the model accounts for several key observations about the earliest stages in the evolution of the ibm system 1 subduction nucleated along an active transform boundary which separated younger less dense lithosphere in the west from older more dense lithosphere to the east 2 initial arc magmatic activity occupied a much broader zone than existed later 3 initial magmatism extended up to the modern trench over a region now characterized by subnormal heat flow 4 early are magmatism was characterized by depleted tholeiitic and ultra depleted boninitic magmas indicating that melting was more extensive and involved more depleted mantle than is found anywhere else on earth 5 early arc magmatism was strongly extensional with crust forming in a manner similar to slow spreading ridges and 6 crust production rates were 120 to 180 km 3 km ma several times greater than for mature arc systems these observations require that the earliest stages of subduction involve rapid retreat of the trench we infer that this resulted from continuous subsidence of denser lithosphere along the transform fault this resulted in strong extension and thinning of younger more buoyant lithosphere to the west this extension was accompanied by the flow of water from the sinking oceanic lithosphere to the base of the extending lithosphere and the underlying asthenosphere addition of water and asthenospheric upwelling led to catastrophic melting which continued until lithosphere subsidence was replaced by lithospheric subduction application of the subduction zone infancy model to the late jurassic ophiolites of california provides a framework in which to understand the rapid formation of oceanic crust with strong arc affinities between the younger sierran magmatic arc and the franciscan subduction complex provides a mechanism for the formation and subsidence of the great valley forearc basin and explains the limited duration of high t high p metamorphism experienced by franciscan melanges

accretion subduction erosion and tectonic extrusion during late paleozoic to mesozoic orogenesis in ne china abstract the high pressure heilongjiang complex records a long and complicated evolution from the late paleozoic to mesozoic in ne china which is still subject to controversy in this paper we present a new detailed study including structural geochemical and geochronological analyses ar ar dating on metamorphic minerals u pb sims dating on zircons two main periods of oceanward accretionary growth are identified between 305 ma and 195 ma and around 112 101 ma these periods of accretionary growth were associated with the emplacement of voluminous magmatism slab roll back with continental back arc extension and growth of a forearc accretionary prism between these periods of accretionary growth a major phase of regional subduction erosion took place between 195 ma and 142 ma at the eastern edge of the songliao block the main effects of this subduction erosion phase were that regional jurassic early cretaceous extrusion overlapped with mesozoic accretion and erosion in ne china which disturbed the initial orogenic architecture with a review of relevant zircon ages of this accretionary orogen we unravel the complicated geodynamic history of accretion subduction erosion and tectonic extrusion that shaped ne china from the late paleozoic to mesozoic

the role of deformation in the formation of metamorphic gradients ridge subduction beneath the oman ophiolite two tectonic scenarios have been proposed for genesis and emplacement of the oman ophiolite one suggests that the ophiolite was generated at a spreading center the other suggests generation within an intraoceanic arc an integrated thermal and kinematic model of the temperature stress rock type and displacement fields during early stages of the emplacement of the oman ophiolite was developed to test these two possibilities the thermal evolution was calculated by a finite difference algorithm for heat conduction considering heats of metamorphic reactions deformational heating heat advection by flowing rock mantle heat flow and radioactive heating the stress and displacement fields were calculated by an analytical model using a velocity boundary condition power law constitutive relations and a brittle frictional sliding relationship field observations in oman can be satisfied with the spreading center model but not with the arc model moreover simulations indicate that the ophiolite was probably 2 m y old at the time of intraoceanic thrusting and that shear stresses of 100 mpa were attained during thrusting in addition to satisfying currently available field constraints these simulations also indicate specific further work that will help resolve the controversy over thevolution of this unique orogen future work should be directed toward 1 using thermobarometry and thermochronology to constrain the spatial variation in ptt paths of the metamorphic sole 2 dating the igneous genesis of the ophiolite and 3 using quartzite and dunite recrystallized grain size paleopiezometers to infer stresses during thrusting these simulations point out the important effect synmetamorphic deformation can have on the formation of metamorphic field gradients in contractional fault zones metamorphic field gradients may be normal or inverted and contracted or extended relative to the initial thermobarometric gradient and adjacent fault zone rocks may display crossing pt paths unusually contracted or extended metamorphic field gradients may also develop in the footwalls of normal fault zones where portions of the heating hangingwall accrete to the uplifting cooling footwall

the west philippine basin an eocene to early oligocene back arc basin opened between two opposed subduction zones 1 based on geological and geophysical data collected from the west philippine basin and its boundaries we propose a comprehensive cenozoic history of the basin our model shows that it is a back arc basin that developed between two opposed subduction zones rifting started around 55 ma and spreading ended at 33 30 ma the initial spreading axis was parallel to the paleo philippine arc but became inactive when a new spreading ridge propagated from the eastern part of the basin reaching the former one at an r r r triple junction spreading occurred mainly from this second axis with a quasi continuous counter clockwise rotation of the spreading direction the gagua and palau kyushu ridges acted as transform margins accommodating the opening arc volcanism occurred along the palau kyushu ridge eastern margin during the whole opening of the basin whereas the paleo philippine arc decreased its activity between 43 and 36 ma the western margin underwent a compressive event in late eocene early oligocene time leading to the rising of the gagua ridge and to a short subduction episode along eastern luzon in the western part of the basin the spreading system was highly disorganized due to the presence of a mantle plume overlapping spreading centers and ridge jumps occurred toward the hot region and a microplate developed shortly after the end of the spreading a late stage of amagmatic extension occurred between 30 and 26 ma in the central part of the basin being responsible for the deep rift valley that cut across the older spreading fabric

cenozoic motion of the philippine sea plate palaeomagnetic evidence from eastern indonesia the history of motion of the philippine sea plate is poorly known because it is isolated from the oceanic ridge system interpretation of palaeomagnetic results from the plate has been controversial because declination data have been obtained only from the eastern margin where subduction related tectonic processes may have caused local rather than plate wide rotations new palaeomagnetic data relevant to the problem have been obtained from 34 sites north of the sorong fault and 29 sites within the sorong fault system these sites record south ward movement during the eocene and northward movement during the neogene sites within the sorong fault system record both counterclockwise and clockwise rotations interpreted as the result of neogene block movements at the southern boundary of the philippine sea plate north of the sorong fault all sites record clockwise declinations neogene rocks have small deflections consistent with rotation about the present day eurasia philippine sea plate pole oligocene middle eocene rocks show consistent clockwise declination deflections of 40 o declinations of lower eocene rocks indicate 90 o of clockwise rotation we propose that the entire area north of the sorong fault in east indonesia has always been part of the philippine sea plate and that the whole plate has rotated clockwise in a discontinuous manner by approximately 90 o since the early eocene the new data from north of the sorong fault provide a basis for determining rotation poles which satisfy all the palaeomagnetic data from the philippine sea plate and permit its reconstruction

tertiary and quaternary magmatism in mindanao andleyte philippines geochronology geochemistry and tectonic setting abstract a set of 230 igneous rock samples from leyte and mindanao philippines has beenstudied for their major and trace element chemistry mineralogy and geochronology 40k 40ar dating several volcanic sectors have been distinguished according to their geographic position tectonic framework and geologic history a in leyte the ophiolitic basement at 50 ma is morb type but characterized by negative nb 0 anomalies typical of back arc basalts miocene 20 and 11 ma and late pliocene to quaternary magmas 2 5 0 4 show typical island are talc alkaline characteristics in eastern mindanao eocene 47 46 ma magmatic rocks are arc tholeiitic while those of miocene 18 6 ma and early pliocene 4 5 ma age are generally talc alkaline dacites of 12 ma age have adakitic compositions the young 3 ma volcanics in surigao and north davao are mostly adakitic although talc alkaline basalts and basaltic andesites dated at 0 9 ma overlie adakites in surigao b in daguma oligocene 30 ma arc tholeiitic diorites intrude older arc sequences miocene0 17 0 7 7 ma dikes and flows in both daguma and sarangani peninsula are mostly talc alkaline andesites although minor adakites dated back to 18 ma have been found overlying undated breccias in daguma in southeast daguma the tale alkaline volcanic substratum of pliocene recent volcanoes is 8 6 ma old quaternary 1 83 ma to present andesitec and dacitic volcanics in mt parker and mt matutum southwest daguma are either talc alkaline or adakitic while mt blit northwest daguma is built up by 2 5 ma old hfse enriched basalts 1 ma old normal talc alkaline andesites and 0 8 ma old adakites c in central mindanao miocene andesites yield ages of 20 and 16 ma pliocene 2 5 ma 0 volcanism is dominated by talc alkaline and hfse enriched basalts and basaltic andesites quaternary lavas i ma to present range in character from tale alkaline to shoshonitic and exhibit wide variations in their trace element behavior d miocene lavas of 19 11 ma age in zamboanga are mostly talc alkaline though a lone 0 18 ma old adakite was also encountered pliocene quaternary basalts and basaltic andesites dated back to 2 2 1 ma are hfse enriched such that some display positive nb anomalies in spidergrams adakites of 3 8 0 7 ma age are located nearer the sulu trench the compositional variety of tertiary recent island arc magmatism in leyte and mindanao couldbe linked to the tectonic settings prevailing during each magmatic phase and is here utilized to refine paleokinematic reconstruction of the tectonic history of the philippine archipelago the identification of adakites at certain time periods is used to date timings of subduction initiations and arc polarity reversals major and abrupt changes in geochemical compositions are well exhibited by the pliocene pleistocene volcanism involving the collision related shift from talc alkaline to shoshonitic volcanism in central mindanao and adakite production in all the volcanic sectors defined the origin of the hfse enrichments among lavas younger than 3 ma could be attributed to the presence in their mantle source of an oib component or alternatively to the enrichment of this mantle source through percolation of slab derived melts

age depth correlation of the philippine sea back arc basins and other marginal basins in the world abstract statistical analysis of underway bathymetric profiles of the philippine sea back arc basins the mariana trough the shikoku basin the parece vela basin and the west philippine basin reveals an age depth correlation in the basins the basement depths of the philippine sea range from 3200 to 6000 m with their ages from 0 to 60 ma the age depth curve of the whole philippine sea can be represented by the following equation depth m 3222 366 age ma the equation shows that the basement depth of the philippine sea is about 800 m deeper than that of the major ocean floors of the same age the coefficient of 366 in this equation is close to that for major oceans 350 this suggests that the cooling processes of the lithospheres in the philippine sea and major oceans are similar to each other whereas the structure and composition of the deeper layers of the philippine sea seem to be different from those of major oceans our analysis of etop digital bathymetric data however indicates that this relationship does not hold for some other marginal basins marginal basins other than back arc basins generally follow the age depth curve of major oceans young back arc basins

genetic relations between the central and southern philippine trench and the sangihe trench we surveyed the junction between the central and southern philippine trench and the sangihe trench near 6on using swath bathymetry gravity and magnetics these data along with seismicity allow us to discuss the genetic relations between these trenches and the forces acting on converging plates our final model favors the northern extension of the halmahera arc up to 8on with three segments offset left laterally along nw se transform faults accretion of the northern segment to mindanao island 4 to 5 m y ago resulted in the failure within the philippine sea plate east of the arc initiation of the philippine trench between 7on and 10on agrees with the maximum recorded depth of the philippine trench floor 10 000 m below sea level and philippine sea slab 200 km south of 6on trench junction another segment of the arc is being subducted beneath the sangihe margin while south of 3on the southern segment of the halmahera arc is still active the rapid southward shallowing of the trench floor along the southern philippine trench the type of faulting affecting both sides of the trench the lack of significant interplate seismicity and the concentration of the seismicity beneath the miangas talaud ridge are interpreted as a slowing down of the subduction along this branch of the philippine trench compared with the rest of the subduction zone the sangihe deformation front has been recognized up to 7on but seems active only south of 6on

contraction and extension in northern borneo driven by subduction rollback abstract during the paleogene the proto south china sea was subducted beneath northern borneo subduction ended with early miocene collision of the dangerous grounds reed bank north palawan block and the sabah cagayan arc much of northern borneo then became emergent forming the top crocker unconformity later in the early miocene subsidence resumed it is proposed that northward subduction of the celebes sea initiated formation of the sulu sea backarc basin followed by subduction rollback to the se this formed a volcanic arc which emerged briefly above sea level and collapsed in the middle miocene as rollback continued the sulu arc was active during middle and late miocene between sabah and the philippines rollback drove extension in northern borneo and palawan accompanied by elevation of mountains crustal melting and deformation offshore there were two important extensional episodes the first at about 16 ma is marked by the deep regional unconformity and the second at about 10 ma produced the shallow regional unconformity both episodes caused exhumation of deep crust probably on low angle detachments and were followed by granite magmatism the nw borneo palawan trough and offshore sabah fold and thrust belt are often interpreted as features resulting from collision regional compression or subduction however there is no seismicity dipping slab or volcanicity indicating subduction nor obvious causes of compression the trough developed after the middle miocene and is not the position of the paleogene trench nor the site of neogene subduction inboard of the trough is a thick sediment wedge composed of an external fold and thrust belt and internal extensional zone with structures broadly parallel to the trough the trough is interpreted as a flexural response to gravity driven deformation of the sediment wedge caused by uplift on land that resulted from extension with a contribution of deep crustal flow

magmatic evolution of sulawesi indonesia constraints on the cenozoic geodynamic history of the sundaland active margin abstract tertiary and quaternary magmatic rocks from west sulawesi record the complex history of part of the sundaland margin where subduction and collision have been and are still active the present study based on petrographic data major and trace element chemistry and 40 k 40 ar dating aims to document the age and chemical characteristics of the magmatic formations from west sulawesi and to determine the corresponding constraints on the geodynamic evolution of the sundaland border the west sulawesi magmatic province includes the south arm of sulawesi ujung pandang area the western part of central sulawesi with the toraja and palu areas and finally the north arm extending from palu to manado which includes the tolitoli and manado areas paleocene magmatic activity seems to be restricted to an episode of calc alkaline magmatism in the ujung pandang area 61 59 ma the major eocene 50 40 ma magmatic event is tholeiitic and is documented in all areas except in ujung pandang it led to the emplacement of tholeiitic pillow lavas and basaltic dykes of back arc basin bab affinity these rocks are potential equivalents to the celebes sea basaltic basement from oligocene to miocene magmatic eruptions produced successively island arc tholeiitic iat and calc alkaline ca rock series the youngest iat activity occurred around 18 ma in the central part palu area and around 14 ma in the north arm tolitoli area while ca magmas were emplaced in the north arm at ca 18 ma tolitoli and manado areas typical calc alkaline activity resumed only in the north arm tolitoli and manado areas during the late miocene 9 ma and is still active in the manado region in other areas palu toraja and ujung pandang areas an important and widespread magmatic event occurred between 13 and 10 ma and emplaced k rich magmas either silica undersaturated alkali potassic basalts ak ultrapotassic basanites uk or shoshonites sh k rich activity continued in the south until the pleistocene 0 77 ma with alkali potassic ultrapotassic and shoshonitic magmas in central sulawesi toraja and palu areas the most recent magmatic event occurred between 6 5 and 0 6 ma the corresponding products are granitic rocks and widely distributed rhyolitic pyroclastic flow deposits all these rocks are acidic in character sio 2 60 with trace element and isotopic signatures sr nd pb typical of a strong continental imprint the most striking tectonic implication of this magmatic evolution is that west sulawesi can no longer be considered as a typical magmatic arc as previously assumed with the exception of the manado area beneath which subduction is still active calc alkaline and island arc tholeiitic lavas and plutonics are volumetrically minor with respect to k rich magmas their occurrence through time is also fairly restricted mostly to the period between 30 and 15 ma another important feature is the occurrence of island arc tholeiitic and calc alkaline magmas crosscutting an older terrane of bab affinity the tinombo formation manado tolitoli and palu areas as this formation is being regarded as an equivalent to the celebes sea floor the most likely explanation for this feature is the hypothesis of tectonic erosion linked to the nw dipping subduction beneath the north arm the late miocene high k magmatic activity in central and south sulawesi reflects the prevalence of a post collisional tectonic regime following the docking of microcontinents of australian origin to central sulawesi during neogene times the incompatible element enriched character of these high k rocks might reflect their derivation from a mantle source enriched through metasomatism related to a previous subduction event such a model cannot account for the plio pleistocene cak magmatism of central sulawesi the acidic composition of which does not support a derivation from an ultrabasic source the trace element patterns of the cak rocks are very similar to those of the high grade metamorphics of central sulawesi suggesting that the latter might represent their possible source such an anatectic model implies a collisional to post collisional tectonic regime limited to central sulawesi while a post subduction regime prevailed in the south

anomalously fast convergence of india and eurasia caused by double subduction prior to collision with eurasia the indian plate rapidly accelerated northwards numerical simulations show that the combined pull of two slabs in two parallel north dipping subduction systems could have caused this pulse of rapid movement

microblock rotations and fault coupling in se asia triple junction sulawesi indonesia from gps and earthquake slip vector data the island of sulawesi eastern indonesia is located within the triple junction of the australian philippine and sunda plates and accommodates the convergence of continental fragments with the sunda margin we quantify the kinematics of sulawesi by modeling gps velocities and earthquake slip vectors as a combination of rigid block rotations and elastic deformation around faults we find that the deformation can be reasonably described by a small number of rapidly rotating crustal blocks relative to the sunda plate the southwestern part of sulawesi makassar block rotates anticlockwise at 1 4 myr the northeastern part of sulawesi the bangai sula domain comprises three blocks the central north sula block moves toward the nnw and rotates clockwise at 2 5 myr the northeastern manado block rotates clockwise at 3 myr about a nearby axis and east sulawesi is pinched between the north sula and makassar blocks along the boundary between the makassar block and the sunda plate gps measurements suggest that the trench accommodates 15 mm yr of slip within the makassar strait with current elastic strain accumulation the tectonic boundary between north sula and manado blocks is the gorontalo fault moving right laterally at about 11 mm yr and accumulating elastic strain the 42 mm yr relative motion between north sula and makassar blocks is accommodated on the palu koro left lateral strike slip fault zone the data also indicate a pull apart structure in palu area where the fault shows a transtensive motion and may have a complex geometry involving several active strands sulawesi provides a primary example of how collision can be accommodated by crustal block rotation instead of mountain building