the deep water fold and thrust belt offshore nw borneo gravity driven versus basement driven shortening the deep water region offshore nw borneo is an active fold and thrust belt that hosts a signifi cant number of proven hydrocarbon accumulations in the past two mechanisms have been discussed as primary control for neogene to holocene folding and thrusting in this deep water province 1 basement driven crustal shortening and 2 gravity related delta tectonics in this study new balanced interpretations of regional crustal scale depth migrated twodimensional 2 d multichannel seismicrefl ection profi les are presented that provide for the fi rst time quantitative data on tectonic shortening throughout the entire deep water fold and thrust belt of nw borneo we use our tectonic restorations to compare the amount of deep water shortening on the nw borneo slope to the amount of extension across the nw borneo shelf a key result of this balancing study is the observation that pliocene to holocene gravity driven shortening decreases from south to north while the total amount of shortening increases slightly to the north consequently the amount of purely basement driven compression along nw borneo is strongly inferred to increase toward the north because most of the shortening is late pliocene and younger we interpret the tectonic shortening to be ongoing

seismic and geologic characteristics of drowning events on carbonate platforms carbonate platform drowning events are recorded in upper jurassic lower cretaceous carbonates of the baltimore canyon area offshore united states east coast and lower miocene carbonates of the pearl river mouth basin offshore people s republic of china cursory examination of seismic data from both areas would indicate that the platforms fit the classic definition of a drowning unconformity however detailed lithologic and paleontologic data indicate that the depositional erosional hiatuses vary widely in each area from 0 25 m y in the baltimore canyon area to a condensed section with no hiatus in the pearl river mouth basin the sedimentary sequence produced during drowning the drowning sequence in both areas is gradational with underlying shallow platform carbo ates and in some places overlying deeper marine limestones and shales the generalized facies models proposed for drowning events suggest that they may appear as instantaneous or gradual changes in geologic and seismic data however high resolution seismic data across the baltimore canyon and pearl river mouth basins show that some drowning sequences may be masked on conventionally processed seismic sections because of this resolution problem a distinction should be made between seismically and geologically defined unconformity surfaces the geologic characteristics of drowning events on carbonate platforms include a gradational lower and sometimes upper contact chemical sedimentation glauconite phosphate open marine shelf sediments and a variable hiatus at the upper boundary late growth shelf margin reefs also can be a diagnostic feature of drowned platforms seismic characteristics include horizontal to sub horizontal basinal marine onlap basin parallel reflector continuity above the carbonate sequence boundary and downlap of later highstand units evidence from platforms adjacent to continental margins baltimore canyon indicates that drowning occurs in a step wise fashion advancing more or less symmetrically across the platform from shelf to nearshore environments data from isolated open ocean atolls pearl river mouth basin indicate that they respond differently to drowning showing an asymmetric decrease in shallow platform environments as drowning occurs proper recognition by a geoscientist of the seismic and geologic characteristics of drowning events can lead to better reservoir and seal estimates and the correct reconstruction of the depositional tectonic and eustatic histories of an area

a p wave velocity model of earth s core present earth core models derived from the retrieval of global earth structure are based on absolute travel times mostly from the international seismological centre isc and or free oscillation eigenfrequencies many core phase data are left out of these constructions e g pkp differential travel times amplitude ratios and waveforms this study is an attempt to utilize this additional information to construct a model of core p wave velocity which is consistent with the different types of core phase data available in conjunction with our waveform modeling we used 150 differential time measurements and 87 amplitude ratio measurements which were the highest quality observations chosen from a large population of global digital seismograph network gdsn records as a result of fitting these various data sets a one dimensional p wave velocity model of the core prem2 is proposed this model modified from the preliminary reference earth model prem dziewonski and anderson 1981 shows a better fit to the combined data set than any of the existing core models major features of the model include a sharp velocity discontinuity at the inner core boundary icb with a large jump 0 78 km s and a low velocity gradient at the base of the fluid core the velocity is nearly constant over the lower 100 km of the outer core the model features a depth dependent q structure in the inner core such that a constant t for the inner core fits the amplitude ratios and waveforms of short period waves moderately well this means the top of the inner core is more attenuating than the deeper part of the inner core in addition the p velocity in the lowermost mantle is reduced from that of prem as a baseline adjustment for the observed separations of the df and ab branches of pkp at large distances

global marine gravity from retracked geosat and ers 1 altimetry ridge segmentation versus spreading rate 1 three approaches are used to reduce the error in the satellite derived marine gravity anomalies first we have retracked the raw waveforms from the ers 1 and geosat gm missions resulting in improvements in range precision of 40 and 27 respectively second we have used the recently published egm2008 global gravity model as a reference field to provide a seamless gravity transition from land to ocean third we have used a biharmonic spline interpolation method to construct residual vertical deflection grids comparisons between shipboard gravity and the global gravity grid show errors ranging from 2 0 mgal in the gulf of mexico to 4 0 mgal in areas with rugged seafloor topography the largest errors of up to 20 mgal occur on the crests of narrow large seamounts the global spreading ridges are well resolved and show variations in ridge axis morphology and segmentation with spreading rate for rates less than about 60 mm a the typical ridge segment is 50 80 km long while it increases dramatically at higher rates 100 1000 km this transition spreading rate of 60 mm a also marks the transition from axial valley to axial high we speculate that a single mechanism controls both transitions candidates include both lithospheric and asthenospheric processes

basin superposition on the northern margin of the south china sea abstract basin superposition on the northern margin of the south china sea is a product of pulsed basin development with geometries which changed through space and time of changing strain fields and of tectonically inherited pre existing zones of crustal weakness this region of the south china sea was welded together or assembled prior to the cretaceous three distinct episodes of continental disassembly and accompanying basin evolution ensued each episode commencing with rifting and the formation of grabens or half grabens and each terminating with a gentle subsidence phase the major trends of the faults and basins varied from predominantly ne for the first episode through ne ew for the second to ew nw for the third the first episode occurred during the late cretaceous to paleocene as a back arc extensional domain induced by the dip steepening of a subducted oceanic slab the second basin episode of the late eocene to early oligocene was initiated by the collision of the indian plate with the eurasian plate the continued penetration of the former into the latter led to the tectonic escape and subsequent clockwise rotation of the indochina block creating regional transtension and left lateral motion along the red river fault the third episode of basin development occurred during the middle miocene as a reactivated product of this collision owing to the clockwise rotation of indochina the approximately e w trend of the simple shear principal displacement zone changed to northwest by the pliocene the eastward extrusion of south china had exceeded that of indochina reversing the sense of red river fault displacement to right lateral a clockwise rotation of the red river geosuture together with considerations of the influence of zones of inherited crustal weakness provides a model for the observed episodic changes in regional strain such a model best explains the post cretaceous evolution of superimposed basins on the northern margin of the south china sea

kinematics of cenozoic extension on the south china sea continental margin and its implications for the tectonic evolution of the region abstract in the northern south china sea several large sedimentary basins developed during the cenozoic characterized by episodic rifting clockwise rotation of the rifts an eastward aging breakup unconformity and intensifying crustal extension to the east these features may be summarized by a scissors model the maximum rifting occurred north of the maximum thermal subsidence which was in turn north of the seafloor opening in contrast the nansha microcontinent south of the south china sea basin was dominated by compressional deformation during the cenozoic extension was of subordinate significance and manifested mainly by block faulting and tilting in the late early oligocene maximum tilting occurred in the southern part of the nansha microcontinent at distance from the deep sea basin of the south china sea the asymmetric development of extensional structures around the south china sea basin may be explained by the simple shear model proposed by wernicke 1981 the tectonic development of the south china sea was the consequence of various interactions of three major plates and may be summarized into three stages with dominating influences of respectively the retreat of the west pacific subduction zone in the late cretaceous the hard collision and impinging of india to tibet since the late eocene and the fast northward subduction of the indian ocean australian plate since the late early miocene

scattered pkkp further evidence for scattering at a rough core mantle boundary abstract pkkp signals from novaya zemlya recorded at lasa at distances around 60 show consistent anomalies in both slowness and azimuth the observed anomaly suggests that the signal is a bc branch arrival although the arrival time corresponds to the df branch the bc branch however does not extend back to this distance the azimuth of approach is in the range 229 245 instead of the expected 186 these anomalies are associated only with pkkp analysis of the core phases pkikp and p p bc from the same events show that they arrived at lasa with the appropriate slownesses and azimuths the pkkp signals can be interpreted as scattered pkkp the scattering occurs on underside reflection at the core mantle boundary and is probably caused by topographic irregularities on the boundary itself the calculated scattering region has a surface projection at about 60 s 134 e which is outside the diametral plane through source and receiver and about 21 from the expected pkkp reflection point at 76 s 95 e both the direct and scattered arms of the pkkp signal have a pk path close to that of the c end of the bc branch the unexpectedly large amplitude of the arrival suggests that there may be a focusing of energy at c which would indicate a change in velocity gradient just above the inner core boundary the observations nevertheless require on the scattering interpretation lateral variations in the topography of the core mantle boundary and a region of relatively large topography responsible for the anomalous pkkp observations

post rift uplift and focused fluid flow in the passive margin of northern south china sea an important post rifting tectonic event with extensive uplift and fluid flow took place at the northern margin of the south china sea scs during the late miocene based on an analysis of two and three dimensional seismic data plus drilling well data the tectonic event mainly affected the dongsha rise and the adjacent depressions tectonic deformation likely occurred in the dongsha rise the eastern part of the panyu swell and several depressions of the zhu i sag the post rifting deformation in the study area was characterized by faulting erosion igneous activity and hydrothermal fluid flow the tectonic movement caused abundant secondary nw and wnw tensional and transtensional faults with lengths of 1 to 10 km it also created a clear angular unconformity which marks the time of the tectonic movement late neogene igneous rocks are found in the strata in the dongsha rise only close to the continent ocean transition zone geothermal activity occurred in the liuhua carbonate platform and adjacent depressions deformation is more intense in the eastern area than in the western area according to sequence stratigraphy we infer that the tectonic event mainly occurred in the late period of late miocene and ended at the mio pliocene boundary 5 5 ma the dynamic mechanism for tectonic movement could be associated with subduction of the south china sea slab beneath the philippine sea plate at the manila trench when subduction of lower density crust within the continental ocean transition zone was initiated large resistance stresses may have led to lithosphere bending and crustal uplift in the dongsha area c 2014 elsevier b v all rights reserved

origins of high frequency scattered waves near pkkp from large aperture seismic array data this article identifies the likely origin of 1 hz scattered waves in the vicinity of pkkp by comparing measurements of slowness and onset time to ray theoretical predictions the measurements are obtained from slant stacks of large aperture seismic array lasa data from 36 earthquakes and six explosions in the range 30 116 three types of scattered waves explain the main features seen in the stacks including p scattered to pkp near the earth9s surface p pkp pkkp scattered near its core mantle boundary cmb reflection point pk kp and skkp scattered near its cmb reflection point sk kp the lasa stacks image the amplitude and slowness variations of the scattered waves with time they also show where these waves can be detected and where they are free from contaminating arrivals sk kp waves rise above the noise approximately 100 sec before the onset time of the main skkp arrival near 113 observations of pk kp span 30 100 however at distances greater than 50 they suffer from p pkp contamination at distances less than 40 the pk kp last for about 280 sec this is approximately 130 sec longer than the maximum ray theoretical prediction for waves scattered at the cmb indicating a possible combination of near surface scattering and contributions from the overlying mantle

deep shallow coupling response of the cenozoic bohai bay basin to plate interactions around the eurasian plate abstract the bohai bay basin bbb is a meso cenozoic intracontinental petroliferous basin where four pre existing continental scale fault zones intersect in east china the bbb has experienced complex deformational and evolutionary history that records significant information about how the lithosphere there has responded to the coeval plate interactions around the eurasian plate and or their resulting deep crust mantle processes in the cenozoic now both deep and shallow deformational and evolutionary processes of the bbb have been well studied but many geodynamic mechanisms previously proposed for those processes are too general to explain them well based on various data and principles we first summarized both deep and shallow responses and processes of the bbb in the cenozoic and then matched reasonable geodynamic mechanisms for each of them the results show as follows the cenozoic differential reactivation of pre existing continental scale weak zones controlled the coeval deep shallow differential coupling evolution of the bbb the paleogene extension in the bbb was mainly caused by crust mantle processes triggered by the stagnant izanagi slab the cenozoic strike slip faulting was influenced by various plate interactions in which the oblique subduction and anticlockwise rotation of young pacific plate resulted in the complex pre oligocene strike slip history of the tan lu fault zone to the south of the zhang peng fault zone while the india eurasia collision has caused the intense and synchronous strike slip faulting of various faults there since 35 ma which was then weakened by the oblique subduction and clockwise rotation of young philippine sea plate in the neogene the compression resulting from both the india eurasia collision and the philippine sea plate s subduction contributed to the latest paleogene tectonic inversion in the bbb the arrival of mantle flow triggered by both the india eurasia collision and the stagnant slab of the pacific plate resulted in the tectonic reactivation there in the late neogene

comment on barckhausen et al 2014 evolution of the south china sea revised ages for breakup and seafloor spreading abstract we re examined the cessation of the south china sea seafloor spreading by using radioactive dating and fossil assemblage of the eto and the te derived age the youngest available age for the fragment of the south china sea is 15 ma and is more consistent with those suggested by previous studies moreover recent iodp expedition preliminary results suggested that the hemipelagic claystone layer stratigraphically bounded by igneous sequences of massive lava flow rock is early miocene in age therefore the previous interpretations are more inclusive than those provided by barckhasen et al 2014 and indicate that a slow spreading ridge was more plausible to have occurred during the seafloor spreading of the south china sea

the global short period wavefield modelled with a monte carlo seismic phonon method summary at high frequencies 1 hz much of the seismic energy arriving at teleseismic distances is not found in the main phases e g p pp s etc but is contained in the extended coda that follows these arrivals this coda results from scattering off small scale velocity and density perturbations within the crust and mantle and contains valuable information regarding the depth dependence and strength of this heterogeneity as well as the relative importance of intrinsic versus scattering attenuation most analyses of seismic coda to date have concentrated on s wave coda generated from lithospheric scattering for events recorded at local and regional distances here we examine the globally averaged vertical component 1 hz wavefield 10 range for earthquakes recorded in the iris farm archive from 1990 to 1999 we apply an envelopefunction stacking technique to image the average time distance behavior of the wavefield for both shallow 50 km and deep 500 km earthquakes unlike regional records our images are dominated by p and p coda owing to the large effect of attenuation on pp and s at high frequencies modelling our results is complicated by the need to include a variety of ray paths the likely contributions of multiple scattering and the possible importance of p to s and s to p scattering we adopt a stochastic particle based approach in which millions of seismic phonons are randomly sprayed from the source and tracked through the earth each phonon represents an energy packet that travels along the appropriate ray path until it is affected by a discontinuity or a scatterer discontinuities are modelled by treating the energy normalized reflection and transmission coefficients as probabilities scattering probabilities and scattering angles are computed in a similar fashion assuming random velocity and density perturbations characterized by an exponential autocorrelation function intrinsic attenuation is included by reducing the energy contained in each particle as an appropriate function of traveltime we find that most scattering occurs in the lithosphere and upper mantle as previous results have indicated but that some lower mantle scattering is likely also required a model with 3 to 4 per cent rms velocity heterogeneity at 4 km scale length in the upper mantle and 0 5 per cent rms velocity heterogeneity at 8 km scale length in the lower mantle with intrinsic attenuation of q 450 above 200 km depth and q 2500 below 200 km provides a reasonable fit to both the shallow and deep earthquake observations although many trade offs exist between the scale length depth extent and strength of the heterogeneity

numerical modeling of failed rifts in the northern south china sea margin implications for continental rifting and breakup abstract failed rifts record important information of continental extension and breakup process in the northern south china sea scs margin the tainan southern depression and the baiyun sag to the east are characterized with lower crust high velocity anomalies lchva and intracrust detachment faults whereas the xisha trough to the west develops on a larger scale with crust cutting normal faults and absence of lchva these contrasts indicate different rifting processes between the northeastern and northwestern scs 2d numerical modeling is performed to understand the formation mechanism of these failed rifts two types of mechanisms are proposed i syn rift competitive type and ii rift migration type with a half extension rate of 2 cm yr and 1 5 cm yr respectively in type i two rifts develop initially on the shoulders of the weak zone but they compete with each other during extension one rift becomes dominant to furnish the final breakup whereas the other one is abandoned the crust structure of this type fits the observations in the baiyun sag and the tainan southern depression however in type ii only one rift develops at the beginning the initial rifting center will migrate and the final continental breakup will occur at a place far from the initial rifting location in this type normal faults cut through whole crust and wide extensional margins will form such as observed in the xisha trough our results suggest that the depth dependent extension of the scs is strongly heterogeneous resulting primarily from varying extensional rates

revised pacific antarctic plate motions and geophysics of the menard fracture zone a reconnaissance survey of multibeam bathymetry and magnetic anomaly data of the menard fracture zone allows for significant refinement of plate motion history of the south pacific over the last 44 million years the right stepping menard fracture zone developed at the northern end of the pacific antarctic ridge within a propagating rift system that generated the hudson microplate and formed the conjugate henry and hudson troughs as a response to a major plate reorganization 45 million years ago two splays originally about 30 to 35 km apart narrowed gradually to a corridor of 5 to 10 km width while lineation azimuths experienced an 8 counterclockwise reorientation owing to changes in spreading direction between chrons c13o and c6c 33 to 24 million years ago we use the improved pacific antarctic plate motions to analyze the development of the southwest end of the pacific antarctic ridge owing to a 45 counterclockwise reorientation between chrons c27 and c20 61 to 44 million years ago this section of the ridge became a long transform fault connected to the macquarie triple junction following a clockwise change starting around chron c13o 33 million years ago the transform fault opened a counterclockwise change starting around chron c10y 28 millions years ago again led to a long transform fault between chrons c6c and c5y 24 to 10 million years ago a second period of clockwise reorientation starting around chron c5y 10 million years ago put the transform fault into extension forming an array of 15 en echelon transform faults and short linking spreading centers

what are the spratly islands abstract seismic records combined with dredged samples and a core indicate that the spratly islands of the dangerous ground province are constructed of presently active carbonate build ups known to extend back continuously at least to the pleistocene and presumed to have initiated in the miocene most likely upon the crests of sea floor cuestas that trend north east south west parallel to the sea floor spreading magnetic anomalies of the contiguous abyssal plain of the southern part of the south china sea the cuestas range from spectacular to subdued constructed of triassic and cretaceous strata and no older rocks have been identified from dredges the cuesta axes plunge towards the south west away from the islands suggesting that the reefs began colonising their more elevated parts but the timing is uncertain the highest seismically recorded cuesta crest is in 440 m of water and the islands and reefs are generally closely surrounded by water deeper than 1500 m since the so called mid miocene unconformity mmu the region has been undergoing post rift thermal subsidence however the nearby seismic lines show no evidence of drowned carbonate reefs it is suggested that the coral algal reefs colonised the crests of the most elevated cuestas that have maintained stability as shown by the 165 m core of one reef indicating periodic exposure with caliche horizons deepening water has protected the build ups from extinction by post rift draping strata in contrast to the central luconia province and the build ups have been able to keep up with regional thermal subsidence the dredged mesozoic strata indicate that the dangerous ground is not exotic and should be interpreted as an integral part of the pre rift sundaland continent that included south china vietnam peninsular malaysia western sarawak and possibly part of sabah igneous and metamorphic samples have been dredged although individual spot k ar dates cannot be accepted at face value such rocks can also be interpreted as an integral part of sundaland post mmu dredged samples are predominantly deep water calcareous mudstones typified by the draping strata of the ocean drilling program odp site 1143 cored from recent to late miocene

collisional model for rapid fore arc block rotations arc curvature and episodic back arc rifting in subduction settings 1 we review geophysical and geological data from 23 active and ancient plate boundaries to document a compelling spatial and temporal relationship between collision plate boundary curvature rapid tectonic rotations and the occurrence of back arc rifting our observations support a conceptual model where the change from subduction to collision causes rapid fore arc rotation when viewed in a reference frame relative to the bounding tectonic plates leading to marked plate boundary curvature our global compilation reveals that most active back arc rifts are associated with rapidly rotating fore arcs and nearby collisions thus we propose that collision and rapid fore arc rotations play a major role in the evolution and kinematics of back arc basins we conduct numerical modeling to better understand the physical processes behind these observed rapid tectonic rotations at convergent margins our results suggest that the presence of an indentor or choke point in the subduction system e g collision can generate rapid rotation of the fore arc about a nearby pole and lead to back arc rifting the rate of fore arc rotation and back arc rifting depends on the incoming indentor velocity and can be greatly enhanced by slab rollback and the presence of a low viscosity back arc where viscosity of the back arc is low fore arc rotation dominates where back arc viscosity is high the formation of strike slip faults and tectonic escape dominates our observational and model derived results illustrate that shallow crustal forces produced by the entry of buoyant features into subduction zones are a fundamental mechanism for the generation of fore arc block rotations plate boundary curvature back arc rift evolution and tectonic escape in subduction systems rollback of the subducting slab within the mantle will certainly enhance the processes we describe but it is not the only driving force

basement structures of east and south china seas and adjacent regions from gravity inversion abstract the satellite derived gravity gives us an opportunity to investigate the basement structures of the east and south china seas since the satellite gravimetry could provide large scale gravity data with high resolution of 1 arc min by 1 arc min we isolate the residual gravity anomaly corresponding to the basement by subtracting the gravity anomalies of sediments and moho undulations from satellite derived free air gravity anomalies two methods namely gravity inversion method and convolution method based on flexure isostasy model are used to calculate the moho undulations in order to guarantee the accuracy of the moho undulations since it occupies large percentages of the gravity anomalies we invert the isolated gravity anomaly for the basement depths of east and south china seas and adjacent areas with resolution of 1 arc min by 1 arc min the basement depths of east and south china seas range from 0 5 km to 12 km and the moho depths vary between 6 km and 32 km the basement topography reveals many tectonic depressions and two spreading axes concealed by the sediments which are unseen in the bathymetry the two spreading axes correspond to the spreading ridges derived from magnetic anomaly and the sw ne oriented spreading axis extends sw much farther than that identified from magnetic anomaly almost reaching to the nam con son basin we also find that the faults constrain the distributions of basement depressions since the faults usually lie along the places where large changes of basement depth take place reversely the basement map could be used to identify the unknown faults besides according to the four profiles in the east and south china seas the mirror image relation was found between the basement topography and the underlying moho undulations that when the basement depth increases or decreases the corresponding moho depth decreases or increases

carbonate platform growth and demise offshore central vietnam effects of early miocene transgression and subsequent onshore uplift abstract miocene carbonate platforms cover a large part of the central vietnamese south china sea margin early carbonate deposition took place on two regional platforms separated by a narrow depression developed along the trace of the east vietnam boundary fault zone west of the east vietnam boundary fault zone the tuy hoa carbonate platform fringes the continental margin between da nang and nha trang here platform growth initiated during the early miocene and continued until middle miocene time when regional uplift led to subaerial exposure termination of platform growth and karstification east of the fault zone the triton carbonate platform was also initiated during the early miocene carbonate growth thrived during early and part of middle miocene time and a thick clean lower and middle miocene carbonate succession cover the triton horst and the qui nhon ridge during the middle miocene partial drowning resulted in the split up of the triton carbonate platform repeated partial drowning events throughout the middle and late miocene resulted in westwards retreat of platform growth and eventual platform drowning and termination of carbonate deposition modern carbonate growth continues on isolated platforms hosting the paracel islands farther seawards the onset of widespread carbonate deposition largely reflects the early miocene transgression of the area linked with early post rift subsidence and the opening of the south china sea the mid neogene shift in carbonate deposition is interpreted as a consequence of regional uplift and denudation of central and south indochina starting during middle miocene time when the tuy hoa carbonate platform became subaerially exposed stressed carbonate growth conditions on the triton carbonate platform probably resulted from increased inorganic nutrient input derived from the uplifted mainland possibly enhanced by deteriorated climatic conditions and rapid sea level fluctuations promoting platform drowning

the temporal and spatial distribution of volcanism in the south china sea region cenozoic magmatism in the south china sea the associated continental margins coastal south china and indochina is reviewed volcanism contemporaneous with rifting and sea floor spreading in the south china sea was very weak in the margins and adjacent areas there was only very limited magmatic activity most probably after the cessation of sea floor spreading the south china sea has non volcanic margins large basement relief caused by extension formed major basins and a continental oceanic transitional zone in the northern margin reflects high crustal rigidity during rifting and drifting the paucity of a magma supply and presence of rigid fault blocks are unfavorable to the mantle flow model previously proposed to account for the opening of the south china sea which was supposedly linked to the escape of indochina induced by indo eurasia collision c 2005 published by elsevier ltd

prolonged post rift magmatism on highly extended crust of divergent continental margins baiyun sag south china sea three dimensional 3d seismic borehole and geochemical data reveal a prolonged phase of post rift magmatism on highly extended crust of the baiyun sag south china sea two volcanic complexes are identified and described in the context of continental rifting and diachronous continental breakup of the south china sea biostratigraphic data from exploration wells by7 1 and by2 complemented by k ar datings from core samples confirm that magmatic activity in the baiyun sag occurred in two main stages 1 a first episode at the base of the miocene 23 8 ma and 2 a second episode occurring at the end of the early miocene 17 6 ma the relative location of volcanic complexes in the baiyun sag and their stratigraphic position reveals prolonged magmatism inboard of the ocean continent transition zone during continental breakup we suggest that magmatism in the baiyun sag reflects progressive continental breakup in the south china sea with the last volcanic episode marking the end of a breakup sequence representing the early post rift tectonic events associated with the continental breakup process seismic and borehole data from this breakup sequence records diachronous magma emplacement and complex changes in depositional environments during continental breakup c 2016 the authors published by elsevier b v

inner core fine scale structure from scattered waves recorded by lasa 1 recent observations of inner core scattering ics waves provide evidence that the outermost 300 km of the inner core has strong heterogeneities with a length scale of a few kilometers these waves follow a path similar to that of the inner core reflected waves pkikp and were originally observed in data from 16 events in the distance range 58 to 73 recorded by the large aperture seismic array lasa here we present additional observations of the ics waves from a total of 78 events recorded by lasa at distances from 18 to 98 we use a modified version of the generic array processing software package to identify ics waves on the basis of travel time back azimuth ray parameter amplitude and coherence there are 44 events that produce clear ics waves we then perform forward modeling of the observed ics waves using a monte carlo seismic phonon method that allows for multiple scattering along the raypath most of the ics waves appear without a visible pkikp phase initially grow in time and have a spindleshaped envelope the duration risetime and decay rates of the observed ics waves can be best explained by small scale volumetric heterogeneities in the outermost few hundred kilometers of the inner core the average qc value for the 44 events is 600 most clear ics waves are found for raypaths sampling the pacific ocean and asia and relatively few observations are from the atlantic ocean roughly consistent with the recently observed hemispheric difference in the inner core structure

inhomogeneities near the core mantle boundary inferred from short period scattered pkp waves recorded at the global digital seismograph network we analyze short period pkp precursor wave trains recorded at global digital seismograph network stations in the distance range 120 140 to infer the nature of inhomogeneities near the core mantle boundary travel times and particle motions are consistent with predictions of single scattered waves near the cmb the dominant frequencies are around 1 hz the regions best sampled are beneath indonesia north america central africa and south america based on first order scattering theory the characteristics of the wave number spectrum of the structural inhomogeneities are obtained for two hypotheses volume inhomogeneities and topographic irregularities for the range of wavelengths sampled by these data between 10 and 70 km the spectrum of inhomogeneities is best represented by a power law of index 5 3 for volume inhomogeneities and 6 8 for topographic irregularities if the scattering is due only to volumetric inhomogeneities we estimate the strength of the structural variations to be about 0 5 in p wave velocity for a 200 km thick layer if the scattering is due only to topographic irregularities the topography has an rms height of 280 m at present we can not distinguish between these two types of inhomogeneities

thrust tectonics along the north western continental margin of sabah borneo widely accepted plate tectonic models suggest that an inactive subduction zone lies along the north west continental margin of sabah in contrast interpretation of reflection seismic data acquired by bgr shows an autochthonous continental terrane comprising an oligocene to early miocene carbonate platform being progressively overthrust by an allochthonous rock complex progressive compression resulted in the development of four structural zones imbricated thrust sheets zone iii two thrust sheet systems one on top of the other zone iv a complex zone with multiphase deformation zone v and piercement ridges zone vi

presumption of large scale heterogeneity at the top of the outer core basal layer abstract a layer of reduced p velocity gradient with thickness of about 100 200 km has been identified at the base of the liquid core from seismological methods it has been interpreted as a dense layer resulting from partial re melting of the inner core which is depleted in light elements with respect to the liquid core during freezing in an attempt to specify where freezing and re melting occur the structure of this basal layer is investigated with the seismological core phase pkpbc which has its turning point in the lower third of the outer core the large pkpbc data set of the ehb catalog distributed by the international seismological centre is analyzed in order to compensate for the uneven distribution of the data and to minimize the influence of mantle heterogeneities the travel time anomalies are binned inside equal area and equal azimuth sectors sampling the base of the liquid core at different depths most of the observed variations in the binned travel time residuals are not significant according to their confidence level the only features which could be significant are a large patch with a velocity increase of about 0 5 located at the top of the basal layer beneath the eastern hemisphere and the complementary velocity decrease beneath the western hemisphere and the south pole this observation suggests that some freezing or re melting processes occur at the top of the basal layer with a hemispherical dissymmetry if confirmed it may give strong constraints on the fate of the light elements during the freezing and re melting process and on their interaction with the basal layer and the overlying liquid core

seismic sequence analysis and reservoir potential of drowned miocene carbonate platforms in the madura strait east java indonesia seismic analysis of four miocene carbonate buildups the porong ke 11c kd 11e and bd buildups in the madura strait region indonesia shows that they are constructed of four seismic units 1 4 all the buildups are located atop an east west trending oligocene fault block which through its uplift favored the deposition and aggradation of shoal water carbonates aggradation platform narrowing and backstepping of leeward reefs into the upper parts of individual buildups typify the growth profiles adjacent off buildup sedimentary rocks deposited at the same time as the buildups show clear horizontal onlap relationships in combination these observations imply that all the platforms were drowned the uppermost parts unit 4 of each of four major drowned buildups in the trend have been drilled to test for the presence of hydrocarbons an analysis of the drilling results in combination with our seismic analysis shows that those buildups that have relatively high levels of hydrocarbons have suitable trap configurations but lack seal integrity for example compactionally induced strain and gravitational gliding created leaky collapse graben faults extensional structures atop some buildups porong ke 11c which allowed hydrocarbons to drain likewise the updip inclination of some potential seal beds facilitated leakage ke 11e in terms of potential targets in similar drowned buildups in the region higher priority should be given to targets that 1 show evidence of ongoing platform backstepping and so called give up reef growth profiles associated with incipient drowning 2 contain all four seismic units units 1 4 in the buildup 3 lack an extensional collapse graben over the crest of the drowned structure and 4 demonstrate bed inclinations in the sealing beds that indicate likely closure over the crest of the buildup begin page 214

mesozoic and cainozoic rocks dredged from the south china sea reed bank area and sulu sea and their significance for plate tectonic reconstructions abstract outcrops of pre quaternary rocks were sampled by dredging and coring during two cruises of rv sonne in the south china sea and sulu sea 1982 1983 late triassic deltaic sandstones wih plant remains are the oldest sediments recovered late jurassic early cretaceous regional metamorphism is indicated by gneiss and garnet mica schists exposed in the northwestern flanks of reed bank these rocks and occurrences of gabbro diorite dacite rhyolite and siliceous shale prove that the dangerous grounds and reed bank are underlain by a continental fragment during the period of seafloor spreading in the south china see a late oligocene to early miocene carbonate platform developed on this southwards drifting continental fragment which subsided in the middle miocene middle miocene andesite and early to middle miocene reefal carbonates were recovered from the cagayan ridge in the sulu sea the new data are in accord with the plate tectonic history developed by taylor and hayes 1980 1983 and holloway 1981

revision of paleogene plate motions in the pacific and implications for the hawaiian emperor bend comment according to literature summarized by wright et al 2015 it is generally accepted that hotspots have moved relative to each other since 80 ma and that the hawaiian hotspot moved south relative to the spin axis during formation of the emperor seamounts prior to 48 ma koivisto et al 2014 advanced the alternative view that the minimum relative motion between hotspots is much smaller than generally accepted implying errors in the global plate circuit are larger than generally accepted in particular they suggest that the global circuits are incorrect for motion prior to 48 ma between east and west antarctica in this view southward motion of the hawaiian hotspot is evidence for motion of the entire lower mantle relative to the spin axis wright et al defend conventional wisdom using freshly calculated relative motion solutions for plates in the pacific basin they conclude that the hawaiian emperor bend heb formed at ca 50 42 ma and sharpest at 47 5 ma is a result of the cessation of southward motion of the hawaiian hotspot and is not a result of change of absolute motion of the pacific plate the purpose of this comment is to point out that their results are at least as consistent with a large change in motion of the pacific plate that was responsible for a significant part of the heb wright et al calculate pacific farallon finite half rotations from the pacific plate record at more frequent intervals than previous workers they recognize an acceleration from 75 mm yr full rate prior to 60 ma to 180 mm yr after 40 ma according to their preferred cande and kent 1995 time scale this acceleration is spread out over 57 40 ma but according to the ogg 2012 time scale the acceleration is contained within the interval of 50 43 ma a time interval entirely within the age span of the heb their statement that their results are not strongly dependent on the time scale used is hard to understand in the discussion associated with wright et al s figure 3 they suggest that this acceleration is dominated by eastward acceleration of the farallon plate in an absolute reference frame any conclusion that pacific absolute motion didn t change much is circular because they assume that one of three published models for pacific plate absolute motion is correct and for all of these published models changes in pacific plate absolute motion are small this discussion does not address the possibility that all three of these models for absolute reference frames share common errors in the global plate circuit wright et al briefly mention their analysis of pacific antarctic motion presented in their figure 2 in terms of rate and direction of motion at the pitman fracture zone in the far south pacific in discussion of northern hemisphere events possibly related to pacific farallon acceleration they suggest that westward acceleration of the pacific plate would correspond to increased pacific antarctic spreading rates because they observe a rate decrease at the pitman fracture zone they suggest that change in pacific absolute motion was relatively minor here they make a serious error in failing to consider spherical geometry the pitman transform fault active at 48 ma was 90 100 of arc away from the molokai and murray fracture zones and the heb their discussion frames the most relevant issue as how the motion of the pacific plate changes at the location of the hawaiian hotpsot the velocity of the pacific plate relative to antarctica when calculated at relevant northern hemisphere locations does increase sharply near 48 ma calculated at the location of hawaii the velocity of the pacific plate changes from slow 35 mm yr northwest motion to fast 83 mm yr west northwest motion

core mantle boundary topography and its relation to the viscosity structure of the lowermost mantle abstract two large areas of anomalously low seismic velocities are visible in all tomographic models of the lowermost mantle depending on the density structure of these large low shear velocity provinces llsvps the core mantle boundary cmb will deform upwards or downwards due to isostatic and dynamic topography the latter being sensitive to the viscosity structure of the lowermost mantle heterogeneities in the viscosity structure although difficult to constrain might be especially important if the llsvps are thermochemical piles with elevated intrinsic viscosity as suggested by mineral physics based on numerical models we identify a short wavelength about 80 120 km wide up to a few km deep topographic depression that forms around the pile edges if the pile is more viscous than the surrounding mantle the depression forms when a wedge of thermal boundary layer material becomes compressed against the viscous pile and is enhanced by relative uplift of the cmb beneath the pile by plumes rising above it the depth and asymmetry of the depression constrain the magnitude of the viscosity contrast between pile and the surrounding mantle furthermore periodic plume initiation and pile collapse at the pile margin systematically modify the characteristic depression with a maximum in asymmetry and depth at the time of plume initiation core reflected waves or scattered energy may be used to detect this topographic signature of stiff thermochemical piles at the base of the mantle

tectonic stratigraphic division and blind fold structures in nansha waters south china sea extensive multiple channel reflection seismic data have been collected in the nansha spratley islands waters the southern margin of the south china sea stratigraphic correlation is shown with focus on a comprehensive geophysixcal survey line run front offshore nw sabah to offshore se vietnam according to the varying tectono stratigraphy from southeast to northwest five tectonic belts can be determined i e the palawan borneo nappe nansha trough nanwei liyue compressive belt zheng he extensional belt and the circum southwest subbasin belt in the palawan borneo nappe neogene quaternary deposits were highly upthrust northwestwards resulting in a series of moderately to tightly folded anticlines separated by open synclines the nansha trough was a narrow deep water belt filled with thick undisturbed neogene quaternary deposits the nanwei liyue compressive bell was dominated by strongly folded paleo anticlines overlain by an undeformed sedimentary cap with a pronounced hiatus of paleogene sediments the zheng he extensional belt consisted of a rugged topography and paleogene half grabens bounded by listic faults the major extensional faults were reactivated to cut through the overlying neogene quaternary deposits over the circum southwest subbasin belt neogene quaternary deposits draped on the largely subsided fault blocks related to the late oligocene mid miocene seafloor spreading based on the regional stratigraphic correlation the prominent paleo anticlines found within the nanwei liyue compressive belt are deduced to consist of mainly mesozoic marine sediments that were compressed in the late mesozoic era therefore the nansha microcontinent block is shown to be a collision complex assembled during late mesozoic era c 2004 elsevier ltd all rights reserved

continentward dipping detachment fault system and asymmetric rift structure of the baiyun sag northern south china sea abstract the rift architecture and deep crustal structure of the distal margin at the mid northern margin of the south china sea have been previously investigated by using deep seismic reflection profiles however one fundamental recurring problem in the debate is the extensional fault system and rift structure of the hyperextended rift basins baiyun sag and liwan sag within the distal margin because of the limited amount of seismic data based on new 3d seismic survey data and 2d seismic reflection profiles we observe an array of fault blocks in the baiyun sag which were tilted towards the ocean by extensional faulting the extensional faults consistently dip towards the continent beneath the tilted fault blocks and extensional faults a low angle high amplitude and continuous reflection has been interpreted as the master detachment surface that controls the extension process during rifting the continentward dipping normal faults evolved in a sequence from south to north generating the asymmetric rift structure of the baiyun sag the baiyun sag is separated from the oceanic domain by a series of structural highs that were uplifted by magmatic activity in response to the continental breakup at 33 ma and a ridge jump to the south at 26 24 ma therefore we propose that magmatism played a significant role in the continental extension and final breakup in the south china sea

basement control on flexural profiles and the distribution of foreland facies the dwyka group of the karoo basin south africa flexural partitioning of a foreland system into foredeep forebulge and backbulge is the result of the interplay of lithospheric deflection under orogenic loading flexural tectonics and the structure of the underlying basement flexural tectonics is commonly invoked as the sole control on flexural profiles which are modeled as dampened sine curves with wavelengths and magnitudes controlled by a number of physical parameters of the lithosphere and the applied loads basement tectonics is shown here to be an equally important control on the scale and shape of flexural provinces by controlling the location of flexural hinge lines and the tilt directions within the foreland system the case study for this paper shows that basement heterogeneity coupled with the selective reactivation of major crustal faults may lead to distortion of the ideal flexural profile by modifying the wavelength and shape of individual flexural provinces

marginal basin evolution the southern south china sea abstract the southern south china sea is divided into contrasting morphology by the west baram line to the west is the sundaland extinct passive margin in which rifting began in the eocene 46 ma and ceased at 19 21 ma anomaly 6 where sea floor spreading began much later than along the shelf of china the break up hiatus lasted 3 5 ma marked by the mid miocene unconformity also preserved on land sarawak the post rift strata date from 16 ma and drape over the rifted topography to the east is a convergent margin that became a collision zone in the middle miocene the sunda shelf is of uniform 30 km thickness except in localised deep basins and extends to a water depth of 200 m the continental slope is narrow the continental rise dangerous grounds is covered by water ranging from 500 m to 3 5 km depth at the continent ocean transition its width ranges from 170 to 330 km the rajang delta extends over the shelf and continental slope its post rift sediments drape over the rifted proximal topography of the dangerous grounds to the east the drape is thinner and has not completely buried the rifted topography the cuestas appear to have supported the carbonate build up infrastructures of the spratly islands whose slopes rise abruptly from a sea floor of 2 3 km depth the sabah and brunei margin is a collision zone that was formerly convergent the on land geology indicates a mesozoic ophiolitic basement the main collision feature is the western cordillera constructed mainly of sandy eocene to lower miocene turbidites predominantly oligocene to lower miocene in the west crocker 32 18 ma uplifted episodically throughout the upper miocene and pliocene 14 8 ma the 2 km deep northwest borneo trough may be a relict of the convergence phase but could also be a collisional foredeep the oil prolific baram delta resulting from uplift and erosion of the western cordillera has built out as far as the northwest borneo trough it is suggested that the passive margin continental rise dangerous grounds has been underthrust beneath sabah to cause uplift of the western cordillera the west baram line accordingly abruptly separates the collision zone from the western extinct passive margin and is a now extinct major right lateral transform fault

tectonic zonation of the central himalaya and the crustal evolution of collision and compressional belts abstract the central himalayan segment is divided from south to north into sub parallel structural facies zones the main boundary fault mbf plays the role of an important tectonic element dividing the siwalik molasse along the foothills of the himalaya against the para autochthonous and allochthonous tectonic units of the southern himalaya on the basis of recent researches i suggest that the himalayan region should be divided geologically into two major geologic zones i propose to call the dividing structural line between them the main axial zone maz of the himalaya which is situated in the crystalline complex of the higher himalaya this deep seated structure in the himalaya has played a crucial role in the history of geological development as well as incorporating the root zone of southward pushed thrust sheets the main central thrust mct tectonically separates the carbonate para autochthonous zone of southern himalaya from the metamorphosed crystalline vaikrita complex the vaikrita central crystalline complex in its turn is separated from the huge pile of the sedimentary tethyan complex by the tethyan thrust tt farther north the great himalaya suture ghs called the indus tsangpo suture divides the northern extremity of himalaya from the karakoram erogenic belt the ghs has provided excellent conditions for the study of mantle remains on the oceanic crust with the occurrence of ophiolitic melange which was the result of subduction of a continental type plate and obduction of oceanic material in recent years a large number of data has been recorded especially as a result of sino french and other scientific expeditions in the tibetan region reversed critical wide angle reflection profiles of the crust mantle boundary south of the great himalaya suture ghs or the indus tsango yarlung zangbo suture in tibet reveal a deep 70 km moho extending north of the higher himalaya whilst to the south the moho is 15 km higher thus in the southern region of tibet the crust gets thinner gradually towards the south and reduces to about 38 km in the gangetic plain of india the 300 km of terrain between the ghs and the gangetic plain represent the transitional collision and compressional belt the magnetolluric and explosive seismographic data indicate that the layered structures are developed within the crust transisting to the upper mantle the moho discontinuity exhibits step terraces the low velocity and resistance layer within the crust indicates that the crust is overlapping and superimposed thus the intense compression from south to north caused by the indian plate caused the crust to overlap shorten thicken and become isostatically adjusted to give rise to the tibetan plateau the successive occurrence of nappe shear zones led to progressive superimposing and thickening of the crust it also indicates intra continental subduction and destruction of the lithosphere after plate collision

morphological indicators of structural control relative sea level fluctuations and platform drowning on present day and miocene carbonate platforms abstract the morphology of carbonate platforms may be influenced by tectonic activity and eustatic variations 3d seismic data and satellite imagery are used in order to investigate the morphological similarities between present day carbonates platforms east of borneo island and miocene carbonate platforms of the south china sea the morphological similarities exhibit platform fragmentation that could be caused by subtle faulting sufficient to drown reef rims platform contraction which is a result of back stepping of the reef margin during a relative sea level rise and polygonal patterns in internal lagoons described as mesh reefs in modern platforms and possibly interpreted as karst in miocene platforms vertical movements may trigger the formation of new geomorphological conditions that modify the distribution of coral growth with respect to the new hydrodynamic conditions in space and time these movements uplift and tilting reduce and localize the space necessary for the coral ecosystem explaining the contraction leading to drowning of parts of and ultimately the whole platform

velocity structure near the inner core boundary from waveform modeling a velocity model of the inner core boundary icb region is developed from broadband wave form modeling the data consist of long period and short period record sections gathered from the u s arrays wwssn and lrsm deep events beneath indonesia were used these sampled the icb structure beneath the north pacific the cagniard de hoop technique was used to generate the synthetics which allowed considerable flexibility in forward modeling these individual pkp phases our preferred model was developed from modifications of the preliminary reference earth model prem it contains a relatively low velocity gradient above the icb a sharp jump at the icb of about 0 78 km s and a relatively large gradient underneath these features are constrained largely by the differential travel times and amplitude ratios of pkikp and pkikp of the short period section further modification of prem in the fluid core was required to explain the long period observations associated with pkp waveforms our modeling results suggest a low velocity gradient at the bottom roughly 400 km of the fluid core a low q zone at the top of the inner core is required to fit the relative amplitudes of pkikp phases similar to earlier studies this model indicates possible inhomogeneity above the icb as suggested by other authors

the structural evolution of folds in a deepwater fold and thrust belt a case study from the sabah continental margin offshore nw borneo se asia abstract the deepwater fold and thrust belt offshore sabah nw borneo is located on a continental margin that hosts a number of proven hydrocarbon accumulations reprocessed and depth migrated regional 2d seismic data reveal the occurrence of a wide range of compressional syn depositional deformation features including large scale thrust faults and prominent thrust hangingwall anticlines this paper focuses on the documentation and detailed description of the style of deepwater folds and the associated stratal patterns within a study area of ca 3000 km 2 offshore sabah nw borneo this region exhibits a variety of fold systems including 1 anticlines with large interlimb angles that exhibit no seafloor expression 2 anticlines with medium interlimb angles seafloor relief and a faulted anticline crest 3 anticlines with small to medium interlimb angles a seafloor expression and flanks associated with slides and slumps and finally 4 anticlines with small interlimb angles that are commonly buried by a thick sedimentary overburden and in places modified by superimposed syn sedimentary deltaic normal faults if analysed section per section perpendicular to the strike of the fold and thrust belt the widest anticlines occur in the most seaward parts of the fold belt near the present day thrust front whereas the narrower anticlines are located further landward however if viewed along the strike of the deepwater fold belt there are significant variations in the geometry of laterally corresponding contemporaneously developing fold systems to explain these differences factors such as rock inhomogeneities in the deformation wedge variable surface slope irregular basement topography differential sediment input or the presence or absence of subsurface gas hydrates need to be taken into account these factors vary significantly within the studied system and are therefore difficult to generalise and transfer to other submarine fold and thrust belts

s to p conversions from mid mantle slow scatterers in slab regions observations of deep stagnated oceanic crust the fate of a subducted slab is a key ingredient in the context of plate tectonics yet it remains enigmatic especially in terms of its crustal component in this study our efforts are devoted to resolve slab related structures in the mid mantle below eastern indonesia the izu bonin region and the peru area by employing seismic array analysing techniques on high frequency waveform data from f net in japan and the alaska regional network and the usarray in north america a pronounced arrival after the direct p wave is observed in the recordings of four deep earthquakes depths greater than 400 km from three subduction systems including the philippines the izu bonin and the peru this later arrival displays a slightly lower slowness compared to the direct p wave and its back azimuth deviates somewhat from the great circle direction we explain it as an s to p conversion at a deep scatterer below the sources in the source region in total five scatterers are seen at depths ranging from 930 to 1500 km those scatterers appear to be characterised by an 7 km thick low velocity layer compared to the ambient mantle combined evidence from published mineral physical analysis suggests that past subducted oceanic crust possibly fragmented is most likely responsible for these thin layer compositional heterogeneities trapped in the mid mantle beneath the study regions our observations give a clue to the potential fate of subducted oceanic crust

development and controlling factors of miocene carbonate platform in the nam con son basin southwestern south china sea miocene carbonate platforms are located on intrabasinal structural highs and along the southern margin of the nam con son basin also referred to as the wanan basin by chinese southwestern south china sea these isolated platforms together cover a gross area of over 25 000 km 2 we map the platforms reconstruct their evolution and evaluate the controls on platform growth and demise within the regional geological setting using available seismic and well data the platforms initiated in the late early miocene and reached their widest extent during the middle miocene the platform margins were controlled by synsedimentary faults associated with carbonate slope failure deposits and or reef talus the platforms formed as isolated platform platform growth history involves six phases start up second start up keep up exposure renewal and drowning phases quantitative analysis of the basin evolution indicates that tectonism eustasy the supply of both terrigenous sediment and nutrients were the dominant controls on platform development and demise c 2013 elsevier ltd all rights reserved

seismic stratigraphy and tectonic structure from a composite multi channel seismic profile across the entire dangerous grounds south china sea abstract we interpret a more than 500 km long composite multi channel seismic reflection profile across the entire dangerous grounds the south china sea five tectonostratigraphic units are determined together with seven sequence boundaries detailed analysis of extensional features based on measurements of fault heaves revealed two major phases of extension separated by a distinct unconformity which likely correspond with the beginning of sea floor spreading in the south china sea early extension occurred during continental rifting late cretaceous early oligocene and resulted in formation of half grabens and rotated blocks controlled by a deeply rooted detachment system extension continued in our study area during the drifting phase of the east subbasin of the south china sea late oligocene early miocene until the subsequent opening of the southwest subbasin at about 25 ma but its intensity decreased markedly thereafter deeply rooted detachment systems evolved possibly during this second phase of extension at the continent ocean transition area from a coincidence of the lowermost reflections and moho depth as derived from gravity inversion we tentatively interpret the flattened part of this detachment system as the crust mantle boundary it is suggested that the dangerous grounds reflect a magma poor rift system at the initial stage of mantle unroofing we suggest that a widespread carbonate platform developed across the dangerous grounds concurrent with the period of seafloor spreading in the southwest subbasin of the south china sea fault related stretching factors f along the line show discrepancies with the whole crustal stretching factors c the continental crust of the dangerous grounds may have experienced nonuniform thinning

upper mississippian lower carboniferous carbonate stratigraphy and syndepositional faulting reveal likely ouachita flexural forebulge effects eastern kentucky u s a abstract stratigraphic analysis of late middle to early late mississippian early carboniferous late meramecian early chesterian carbonates in east central and northeastern kentucky u s a shows six unconformity bound sequences the earliest late meramecian st louis sequence shows a relatively uniform distribution and a typical eastward appalachian gradient along the ene trending outcrop belt on the western margin of the appalachian basin however abrupt uplift and exposure of deeper water st louis deposits indicated by erosional thinning and well developed exposure features at the meramecian chesterian transition signal inception of regional early chesterian uplift that intensively altered the nature and distribution of the five overlying early chesterian carbonate sequences uplift was related to the syndepositional reactivation of wnw trending basement faults that broke up the st louis platform and generated a mosaic like distribution of the overlying five sequences due to structurally related erosional incision and paleoslope change to the southwest these early chesterian events coincide with a major phase of northeast directed ouachita orogeny on the southern margin of laurussia and concomitant forebulge migration to the ene modeling of regional stress distribution as well as timing and location during bulge migration strongly indicate that a migrating ouachita bulge reactivated basement structures and generated the noted stratigraphic anomalies in eastern kentucky and nearby areas the likelihood of low amplitude flexural waves like the ouachita bulge migrating across areas where they were not previously considered has major implications for understanding structural stratigraphic and eustatic anomalies in supposedly stable cratonic areas far removed from active tectonism

episodic rifting and subsidence in the south china sea the south china sea experienced at least three stages of rifting and two intervening stages of sea floor spreading since the early cretaceous its evolution can be described by an episodic model of tectonism one of thermal cooling and subsidence pulsed by temporally and spatially confined heating events analysis of regional geologic and geophysical data suggests episodes of rifting and associated thermal activities initiated during the late cretaceous the late eocene and the late early miocene the rift system corresponding to the first episode trends northeast southwest whereas those of the second and third trend east west these two trends coincide with the orientations of the major tectonic lineations within the basin age estimates from heat flow and bathymetric data suggest the oceanic crust in the southwest subbasin is considerably older 55 ma than that in the northwest 35 36 ma or east 32 ma subbasins the episodic tectonic model is supported by basin subsidence analysis subsidence especially subsidence rate curves derived from the regional well data demonstrates that unlike a classic atlantic passive margin thermal subsidence in the south china sea was punctuated by rapid subsidence events which are chronologically consistent with the rifting episodes in terms of hydrocarbon potential the episodes of rifting and drifting would be conducive to the development of overprinted structures and the deposition of several discrete transgressive packages of source rocks and reservoirs separated by widespread unconformities the thermal maturity of sedimentary organic matter affected by episodic rifting and subsidence may be greater than expected on a purely passive margin of equivalent age that had not experienced repeated heating

paleoclimatic controls on sedimentation diagenesis and reservoir quality lessons from miocene carbonates miocene carbonates worldwide were deposited in a wide range of paleogeographic and tectonic settings and form important reservoirs in several regions the reservoir quality of these carbonates appears to have been primarily controlled by the prevailing paleoclimate which exerted a major control on both the depositional patterns and diagenetic pathways two end members are discussed here 1 humid oceanic tropical subtropical settings and 2 arid land locked temperate subtropical settings in humid oceanic tropical subtropical settings e g miocene of southeast asia a warming trend paralleled by generally rising sea level allowed thick coral reefs and skeletal banks to develop most of which are seismically resolvable these carbonates typically represent several stages or cycles of development largely third order separated by discontinuities in platform growth with episodic subaerial exposure development of economic reservoirs is mainly related to relative sea level falls and associated meteoric diagenesis trends and patterns of reservoir distribution can be predicted within a sequence stratigraphic framework basal transgressive carbonates are mostly tight because of their relatively fine grained textures intense compaction and isolation from meteoric water influence best reservoir quality is commonly developed beneath subaerial unconformities in highstand buildups where effects of meteoric water leaching and karstification are most intense in contrast in arid land locked subtropical temperate settings unfavorable environmental conditions caused by periodically increased salinity and probably relatively low temperature restricted the full growth of carbonate buildups as a result relatively thin and narrow fringing coral reefs with small poorly developed lagoons in rhodalgal ramps are the most common facies type in the upper miocene of the mediterranean and the middle miocene of the gulf of suez and red sea whereas the lower miocene of the middle east is characterized by low energy shallow shelfal and lagoonal carbonates under arid climatic conditions recharge of fresh groundwaters is limited thus minimizing meteoric dissolution and cementation during subaerial exposure instead sea level fall and lowstand ommonly resulted in extensive evaporitic conditions hypersaline brines originated within evaporitic basins or lagoons were the main cause of the pervasive dolomitization leaching of metastable skeletal grains and generation of moldic vuggy and intercrystalline porosity often associated with these arid land locked carbonates is the widespread anhydrite cement that precipitated over a wide range of burial depths porosity distribution in these carbonates thus depends not only on the original depositional fabrics but more importantly on the degree to which the porosity occluding anhydrite cement was dissolved or the low permeability carbonates were fractured these miocene examples illustrate the contrasting diagenetic patterns and reservoir styles controlled by paleoclimatic and paleoceanographic trends and demonstrate the importance of the often overlooked hypersaline diagenesis in arid land locked subtropical temperate settings the development of exploration and production strategies for carbonates that were deposited in different climatic settings should acknowledge and use these fundamentally different styles of diagenesis and reservoir characteristics

pacific plate apparent polar wander hot spot fixity and true polar wander during the formation of the hawaiian island and seamount chain from an analysis of the skewness of magnetic anomaly 20r 44 ma

observations of high frequency scattered energy associated with the core phasepkkp stacks of global seismic network data reveal emergent high frequency 1 hz energy extending back from the pkkpc caustic at 72 to a distance of about 60 the arrival s emergent onset long duration 150 s apparent slowness and proximity to the core phase pkkp indicate a deep scattering origin however the amplitude and timing of the observations are difficult to explain with simple models of pkkp scattering

revision of paleogene plate motions in the pacific and implications for the hawaiian emperor bend reply understanding the relative motion between the pacific plate and its neighboring plates in the paleogene has important consequences for deciphering the relationship between absolute and relative plate motions in the pacific ocean basin the history of circum pacific subduction and the cause of the hawaiian emperor bend heb we quantitatively model the farallon vancouver pacific antarctic seafloor spreading history from 67 to 33 ma based on a comprehensive synthesis of magnetic anomaly and fracture identifications we find a well constrained increase from 75 5 mm yr to 101 5 mm yr in pacific farallon full spreading rates between 57 6 ma and 55 9 ma followed by a stepwise increase to 182 2 mm yr from 49 7 to 40 1 ma the increases in pacific farallon spreading rates are not accompanied by any statistically significant change in spreading direction the 57 6 55 9 ma surge of pacific farallon spreading reflects an eastward acceleration in farallon plate motion as it precedes west pacific subduction initiation and is not associated with any significant change in pacific antarctic spreading we interpret the increase in pacific farallon spreading rates after ca 50 ma as a consequence of further acceleration in farallon plate motion we find no indication of a major change in pacific plate absolute motion at this time our model suggests that changes in relative motion direction between the pacific and farallon and pacific and antarctic plates were insignificant around the formation time of the heb ca 47 5 ma and the bend is largely a consequence of hawaiian hotspot motion which ceased rapid motion after 47 ma

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

architecture development and geological control of the xisha carbonate platforms northwestern south china sea newly acquired seismic data allow improved understanding of the architecture and evolution of isolated carbonate platforms on the continental slope of the northern south china sea the xisha carbonate platforms were initiated on a basement high the xisha uplift in the early miocene and have remained active up to the present their distribution is limited to pre existing localized fault bounded blocks within the xisha uplift so individual platforms were small in size at the beginning of the miocene however during the middle miocene the platform carbonate factories flourished across an extensive area with 55 900 km 2 the platforms began to backstep in response to a relative sea level rise in the late miocene platform edge reefs patch reefs pinnacle reefs atoll reefs and horseshoe reefs all developed on various platforms the distribution of platform carbonates shrank significantly during pliocene quaternary time to isolated carbonate platforms represented today by xuande atoll and yongle atoll tectonics and eustasy were the two main controls on platform development tectonics controlled both the initial topography for reef growth and the distribution of platforms including those that survived the drowning event associated with the late miocene rapid relative sea level rise eustasy controlled high frequency carbonate sequence development crown copyright c 2014 published by elsevier b v all rights reserved

methods for identification of isolated carbonate buildups from seismic reflection data isolated carbonate buildups icbs are commonly attractive exploration targets however identifying icbs based only on seismic data can be difficult for a variety of reasons these include poor quality two dimensional data and a basic similarity between icbs and other features such as volcanoes erosional remnants and tilted fault blocks to address these difficulties and develop reliable methods to identify icbs 234 seismic images were analyzed the images included proven icbs and other features such as folds volcanoes and basement highs which may appear similar to icbs when imaged in seismic data from this analysis 18 identification criteria were derived to distinguish icbs from non icb features these criteria can be grouped into four categories regional constraints analysis of basic seismic geometries analysis of geophysical details and finer scale seismic geometries systematically assessing the criteria is useful because it requires critical evaluation of the evidence present in the available data working from the large scale regional geology to the fine details of seismic response it is also useful to summarize the criteria as a numerical score to facilitate comparison between different examples and different classes of icbs and non icbs our analysis of scores of different classes of features suggests that the criteria do have some discriminatory power but significant challenges remain

seismic velocity structure in the earth s outer core 1 we constrain seismic velocity structure in the earth s outer core by analyzing differential travel times waveforms and amplitude ratios of several pairs of core phases we use differential travel times and waveforms of pkpbc pkikp at 144 148 pkikp pkikp at 120 141 pkikp pkpbdiff at the pkp caustics distance range 141 145 and differential travel times and amplitude ratios of pkpab pkpbc at 146 159 to avoid the influence of the inner core anisotropy in velocity we only use the pkikp pkikp and the pkpbc pkikp observations whose pkikp phases sample the inner core along the equatorial paths paths with their ray angles being larger than 35 from the earth s rotation axis these observations show the following characteristics 1 both the observed pkpbc pkikp and pkikp pkikp differential travel times show a distinct east west hemispheric pattern pkikp phases arrive about 0 7 s earlier for those sampling the eastern hemisphere 40 e 180 e than those sampling the western hemisphere 180 w 40 e 2 the observed differential pkikp pkpbdiff travel time residuals also exhibit a hemispheric difference pkikp pkpbdiff differential travel times are about 0 9 s larger for those sampling the western hemisphere than those sampling the eastern hemisphere and 3 both the observed pkpab pkpbc differential travel times and pkpbc pkpab amplitude ratios show scatter overall these observations can be best explained by two one dimensional p velocity models one for each hemisphere at the bottom of the outer core the seismic data sampling the eastern hemisphere can be explained by prem at the bottom of the outer core while those sampling the western hemisphere can be explained by a lower velocity gradient at the bottom of the outer core which has reduced velocities relative to prem linearly increasing from 0 at 200 km above the inner core boundary icb to 0 35 at the icb different velocity gradients at the bottom of the outer core indicate that there may exist a compositional difference and or a large scale temperature difference there and that inner core formation processes may be different between the two hemispheres different inner core formation processes may produce different geometric inclusions of melt in the top of the inner core and thus may provide an explanation to the hemispheric variation of seismic velocity and attenuation in the top of the inner core the seismic data also suggest that the variation of seismic velocity in the tangential cylinder of the outer core if it exists is less than 0 1 we use a compressional wave tomographic model to study the travel time delays caused by the seismic heterogeneities in the mantle the tomographic model is unable to explain the scatter of the travel time data suggesting the existence of unknown strong small scale seismic heterogeneities in the mantle

a review of indian phanerozoic palaeomagnetism implications for the india asia collision abstract an overview is presented of the indian apparent polar wander path apwp for the phanerozoic and in particular for post late palaeozoic times this apwp is compiled on basis of data available at october 1981 from peninsular and extrapeninsular indo pakistan and from dsdp cores from the indian plate one of the more important and newly recognized features of this apwp is a large scale triassic jurassic loop this loop indicates a changeover from a late palaeozoic early mesozoic northwards and counter clockwise rotational movement with greater india reaching moderately low southern latitudes into a southwards and clockwise rotational movement during the early to middle jurassic recognizable likewise in apwp s from other gondwana continents this loop reflects the opening of the neotethys studies of extrapeninsular regions up to and north of the indus tsangpo suture zone have shown wide spread presence of magnetic overprints which delineate two regionally confined age groups younger overprints 20 40 m y predominate in the more external thrust zones older overprints 50 60 m y in contrast are found in the more internal zones both north and south of the indus tsangpo suture zone the latter are interpreted to reflect a late phase of relaxation in the early tertiary collision of greater india with south central asia or off shore island arcs which occurred at equatorial to low northern palaeolatitudes 0 10 n subsequent northwards movement over 2500 3000 km or more and impingement of greater india into southern asia resulted into large scale underthrusting of greater india along the main central thrust beneath southern tibet and to clockwise rotation of thrust units in the western himalaya a discrepancy between indian palaeomagnetic data and results available todate from southern tibet is discussed

the structure of the base of the outer core inferred from seismic waves diffracted around the inner core 1 we systematically searched for seismograms of waves diffracted around the inner core pkpcdiff from all the temporary seismic arrays with data currently available at the iris dmc as well as some permanent regional seismic arrays including f net in japan and grf in germany to assemble the largest high quality pkpcdiff database ever created pkpcdiff waves preferentially sample the base of the outer core and so contain important clues about earth structure in this region we measured pkpdf pkpcdiff differential traveltimes and pkpcdiff pkpdf amplitude ratios in the distance range of 154 160 and modeled the observations using grid searches and full wave theory synthetic seismograms the optimum model found by fitting the differential traveltimes has relatively low velocity at the base of the outer core as in ak135 which is consistent with many previous traveltime studies however the optimum model found by fitting the amplitude ratios pkpcdiff pkpdf does not exhibit this feature and instead is closer to prem the discrepancy may be explained by two likely causes one is that small scale topography or roughness on the icb tends to scatter energy away from pkpcdiff waves by generating trailing coda waves the other is that there exists a thin layer with relatively low q at the base of the outer core this might be expected if there are suspended solid particles at the base of the outer core as proposed decades ago both mechanisms could generate smaller pkpcdiff amplitudes without significantly affecting pkpcdiff traveltimes

geometric and depositional responses of carbonate build ups to miocene sea level and regional tectonics offshore northwest australia abstract the geometric and depositional responses of isolated carbonate build ups to miocene sea level change and regional tectonics are investigated using 3d seismic and borehole data from the browse basin north west australia combined with outcrop information from the cariatiz reef southeast spain the interpreted seismic volume documents five 5 miocene sequence boundaries and five 5 main seismic facies seismic attribute analyses proved to be a highly effective tool for interpreting carbonate facies but when compared with outcrop information from southeast spain attribute data are limited to features larger than 16 4 m vertically and 18 75 m horizontally estimations of reservoir potential are thus significantly underestimated when only based on the interpretation of seismic data as a corollary of the structural analysis in this work growth patterns suggest messinian structural partitioning across the browse basin with deformation associated with plate collision focused on preferentially oriented faults i e only influencing carbonate build up evolution at a local scale

crustal growth and two stage india eurasia collision in ladakh abstract the post palaeozoic history of crustal growth between india and the karakoram lhasa accreted southern margin of asia begins with the creation of the oceanic crust during the northward drift of the karakoram lhasa block away from the northern margin of india after permo triassic rifting data suggest initiation of northward subduction and the development of the island arc close to the southern margin of asia sometime in upper jurassic 160 m y the northward retreat of the overriding eurasian plate due to movements along transform faults which subsequently changed into transcurrent faults resulted in the opening of the shyok back arc basin in the kohistan ladakh region during pre upper cretaceous time the mature and thickened tholeiitic island arc lying between the subducting indian plate and the spreading shyok back arc basin suffered compression deformation emplacement of ophiolitic melanges and metamorphism during the upper cretaceous 80 m y the intrusion of the calc alkaline magma into the deformed rocks of the island arc and the back arc basin during the paleocene eocene 60 40 m y was followed by explosive calc alkaline volcanism 40 25 m y resulting in further crustal growth in this part of the trans himalaya the collision of the dras island arc with the approaching margins of the indian and eurasian plates took place in two stages in the first stage it collided with the northern margin of india during the paleocene along the indus suture the final collision of the island arc accreted northern margin of india with the eurasian plate took place along the shyok suture during the early oligocene 30 m y

time constraints on the evolution of southern palawan island philippines from onshore and offshore correlation of miocene limestones abstract the link between the deformation of southern and central palawan island philippines and the deformation of the adjacent offshore wedge is investigated the wedge is a continuation of the palawan fold and thrust belt and bounds the borneo palawan trough to the dangerous grounds and to palawan island key parameters for the understanding of the formation and development of this wedge are two limestone formations the oligocene to middle miocene nido limestone and the upper miocene to lower pliocene tabon limestone the initiation of the thrust belt formation is constrained by the nido limestone which was deposited from shortly before the breakup of the eastern south china sea 35 ma until the early miocene age data available from wells offshore central palawan gives an age of early miocene close to the base of the nido limestone while cropping out onshore north palawan these limestones were overthrusted by the wedge in southern and central palawan seismic images show gently east dipping carbonates below the wedge the seismic data show that these limestones are only mildly affected by the wedge formation the end of the wedge development can be constrained by the tabon limestone with an age of 9 to 4 ma this limestone sequence overlies unconformably the offshore wedge a detailed biostratigraphic correlation of the tabon limestone along the southwest palawan shelf using well data combined with multichannel seismic data and investigations onshore southern and central palawan shows a time and space transgressive development of these limestones they are progressively younging towards the west we propose that the formation of the tabon limestone is directly linked with the development of the wedge that tectonically controls the formation of this carbonate platform this constrains the time for the final phase of the formation of the palawan thrust belt after the final compressional phase and wedge formation in the lower early pliocene the wedge underwent a phase of subsidence based upon the detailed correlation of these limestones we propose that the wedge did not form in the southern palawan area prior to 18 ma using the sealing tabon limestone as time constraint we suggest that the development of the wedge in the south palawan area started in the lower middle miocene 15 ma and continued developing towards the west until the upper late miocene to early pliocene 5 ma after the wedge propagation stopped the wedge front collapsed in several places due to gravitational sliding southern and central palawan were uplifted above sea level during a second phase of compression in the late pliocene onshore outcrops give indications to a working spleothem since 1 2 ma

the structure and stratigraphy of deepwater sarawak malaysia implications for tectonic evolution abstract the structural stratigraphic history of the north luconia province sarawak deepwater area is related to the tectonic history of the south china sea the sarawak basin initiated as a foreland basin as a result of the collision of the luconia continental block with sarawak sarawak orogeny the foreland basin was later overridden by and buried under the prograding oligocene recent shelf slope system the basin had evolved through a deep foreland basin flysch phase during late eocene oligocene times followed by post oligocene molasse phase of shallow marine shelf progradation to present day seismic interpretation reveals a regional early miocene unconformity emu separating pre oligocene to miocene rifted basement from overlying undeformed upper miocene pliocene bathyal sediments seismic well data and subsidence analysis indicate that the emu was caused by relative uplift and predominantly submarine erosion between 19 and 17 ma ago the subsidence history suggests a rift like subsidence pattern probably with a foreland basin overprint during the last 10 ma modelling results indicate that the emu represents a major hiatus in the sedimentation history with an estimated 500 2600 m of missing section equivalent to a time gap of 8 10 ma the emu is known to extend over the entire nw borneo margin and is probably related to the sabah orogeny which marks the cessation of sea floor spreading in the south china sea and collision of dangerous grounds block with sabah gravity modelling indicates a thinned continental crust underneath the sarawak shelf and slope and supports the seismic and well data interpretation there is a probable presence of an overthrust wedge beneath the sarawak shelf which could be interpreted as a sliver of the rajang group accretionary prism alternatively magmatic underplating beneath the sarawak shelf could equally explain the free air gravity anomaly the sarawak basin was part of a remnant ocean basin that was closed by oblique collision along the nw borneo margin the closure started in the late eocene in sarawak and moved progressively northeastwards into sabah until the middle miocene the present day nw sabah margin may be a useful analogue for the oligocene miocene sarawak foreland basin

a tectonic model for the tertiary evolution of strike slip faults and rift basins in se asia abstract models for the tertiary evolution of se asia fall into two main types a pure escape tectonics model with no proto south china sea and subduction of proto south china sea oceanic crust beneath borneo a related problem is which if any of the main strike slip faults mae ping three pagodas and aliao shan red river asrr cross sundaland to the nw borneo margin to facilitate continental extrusion recent results investigating strike slip faults rift basins and metamorphic core complexes are reviewed and a revised tectonic model for se asia proposed key points of the new model include 1 the asrr shear zone was mainly active in the eocene oligocene in order to link with extension in the south china sea the asrr was less active during the miocene tens of kilometres of sinistral displacement with minor amounts of south china sea spreading centre extension transferred to the asrr shear zone 2 at least three important regions of metamorphic core complex development affected indochina from the oligocene miocene mogok gneiss belt doi inthanon and doi suthep around the asrr shear zone hence paleogene crustal thickening buoyancy driven crustal collapse and lower crustal flow are important elements of the tertiary evolution of indochina 3 subduction of a proto south china sea oceanic crust during the eocene early miocene is necessary to explain the geological evolution of nw borneo and must be built into any model for the region 4 the eocene oligocene collision of ne india with burma activated extrusion tectonics along the three pagodas mae ping ranong and klong marui faults and right lateral motion along the sumatran subduction zone 5 the only strike slip fault link to the nw borneo margin occurred along the trend of the asrr fault system which passes along strike into a right lateral transform system including the baram line

observations of pkkp precursors used to estimate small scale topography on the core mantle boundary stacks of global seismic data reveal the time and distance dependence of high frequency 1 hertz precursors to the seismic phasepkkp synthetic seismogram modeling shows that scattering from random small scale topography at the pkkpcore mantle boundary reflection point generates precursory arrivals similar to those seen in the data models with a root mean square core mantle boundary topography of 250 to 350 meters and correlation length of 7 to 10 kilometers explain the main features of the data however a systematic range dependent misfit between observed and predicted precursor power suggests that inner core scattering may contribute to the precursors

oligocene miocene carbonates and their role for constraining the rifting and collision history of the dangerous grounds south china sea abstract the attenuated continental crust of the dangerous grounds is located in the southeastern part of the south china sea it was affected by unconformities as identified by several authors cullen et al 2010 hinz and schluter 1985 hutchison 2010 hutchison and vijayan 2010 in the northeastern dangerous grounds a prominent reflector in seismic data is associated with the top of a widespread oligocene to early miocene 18 20 ma carbonate platform this reflector and the underlying carbonates can be used to constrain the timing of the unconformities and the rifting history of the dangerous grounds by carefully interpreting seismic reflection lines we trace the platform carbonates based upon their appearance in the seismic image this platform is continuous in the palawan borneo trough and gets patchy toward the dangerous grounds in the dangerous grounds the image of this key reflector changes and here it merely forms the top of a clastic layer carbonates remain abundant but mainly as isolated reefs that grew on top of tilted fault blocks in the southwestern dangerous grounds the prominent unconformity sealing the tectonic activity is known as the middle miocene unconformity this in fact is an early miocene unconformity which represents a sequence boundary in the borneo palawan trough and in various parts of the dangerous grounds while in other parts of the dangerous grounds it represents a major angular unconformity the unconformity characteristics supplemented with tentative ages indicate that luconia and the southern dangerous grounds were sub aerial during the early miocene while the reed bank the northern dangerous grounds and parts of the central dangerous grounds were mostly submerged except for some islands concentrated on the western edge of the borneo palawan trough this trough is interpreted as a foreland basin where the flexural forebulge provided shallow marine conditions that promoted reef growth as the carbonate deposition migrated from the borneo palawan trough toward the dangerous grounds we suggest that the flexural forebulge provided shallow water conditions for further reef growth on the eastern dangerous grounds

global scale observations of scattered energy near the inner core boundary seismic constraints on the base of the outer core abstract we have collected a global dataset of several thousands of high quality records of pkpdf pkpbc pkpbc diff and pkpab phase arrivals in the distance range 149 178 within this collection we have identified an energy packet that arrives 5 20 s after the pkpbc or pkpbc diff and represents a phase that is not predicted by 1d reference seismic models we use array analysis techniques to enhance the signal of these scattered phases and show that they originate along the great circle path in a consistent range of arrival times and narrow range of ray parameters we therefore refer to this scattered energy the m phase using the cross correlation technique to detect and measure the scattered energy arrival times we compiled a dataset of 1116 records of this m phase there are no obvious variations with source or station location nor with the depth of the source after exploration of possible location for this m phase we show that its origin is most likely in the vicinity of the inner core boundary a tentative model is found that predicts an m like phase and produces good fits to its travel times as well as those of the main core phases in this model the p velocity profile with depth exhibits an increased gradient from about 400 km to 50 km above the icb i e slightly faster velocities than in ak135 or prem and a 50 km thick lower velocity layer right above the icb

seismic sequence stratigraphic analysis of the carbonate platform north offshore taiping island dangerous grounds south china sea abstract taiping island also known as itu aba is the largest natural terrestrial landmass in the south china sea and is centrally located using bathymetry and marine multi channel seismic data we explored the seismic stratigraphic features of the offshore and isolated carbonate platform north of taiping island the western flank of the carbonate platform is characterized by an intercalation between high amplitude and low amplitude reflections showing the landward and seaward migration of the platform foreslope deposits in addition there are two offshore carbonate build ups that are underlain by normal faults six sequence boundaries and five depositional sequences caused by eustatic sea level cycles are identified and correlated with the eustatic sea level change chart although the evolution of the seismic sequences is partly controlled by local tectonics the overall stacking pattern of the sedimentary strata in our study area reveals five third order cycles and one second order cycle which is in accordance with the eustatic sea level chart additionally the formations of the western taiping seamount group and the zhenghe daoming trough are preliminarily analyzed based on seismic data

a 9 million year long astrochronological record of the early middle eocene corroborated by seafloor spreading rates the early middle eocene ca 56 41 ma is recorded in the pelagic scaglia rossa and variegata formations of the umbria marche basin central italy geochemical and magnetostratigraphic alignment between the bottaccione section gubbio central italy and the smirra core cagli central italy allows us to generate a continuous and wellpreserved new record that combined with previously published data from the same area creates a continuous high resolution record from the paleocene eocene thermal maximum ca 56 ma to the lower part of chron c21n comparison with carbon isotope records from ocean drilling program sites 1258 and 1263 reveals a satisfactory match providing further evidence of the global significance of the long term trend and superposed perturbations captured by the 13c records the identification of astronomically forced geochemical cycles allows us to develop a 405 k y tuned age model thereby extending the astrochronology from ca 56 0 to ca 47 5 ma marine magnetic anomaly profiles from major oceanic basins characterized by high seafloor spreading rates were used to independently test the astronomical polarity time scale associated with our tuning as well as other polarity time scales our age model suggests the existence of periods of relatively constant seafloor spreading rates separated by rapid changes while the other time scales generate more gradual variations and also include large and short term deviations in spreading rates that occur simultaneously in different oceanic basins implying errors in polarity reversal ages the umbria marche age model further contributes to the closure of the middle eocene gap in the astronomical time scale

crustal structure north of the taiping island itu aba island southern margin of the south china sea abstract based on the multi channel seismic mcs and gravity data offshore north of taiping island itu aba island in the spratly islands nansha islands we revisited the crustal structures in the northern part of the southern margin of the southwest sw sub basin of the south china sea scs the mcs data suggest that the basement structural highs in the southwest margin of the scs are dominated by both fault blocks and volcanic basement structures that probably formed along with the basement faults the gravity modeling results reveal that these volcanic basement structures were probably associated with the high velocity or density lower crust hvdlc suggesting that the southern margin of the sw sub basin serves as an intermediate margin based on the tectonic features in the scs margins including 1 the transition between the magma poor margins of the northwest nw sub basin and intermediate margins of the east and sw sub basins 2 the cessation of the seafloor spreading in the nw sub basin being succeeded by a major ridge jump event which was very likely to reflect an active mantle upwelling event 3 the extensive distribution of the hvdlc beneath in the well investigated northern margin of the east sub basin and as explored in the sw sub basin in this study and 4 basement faulting and volcanism shown in this study that may be related to the hvdlc and hyper extension we proposed a conceptual model to interpret the origin of the hvdlc in the southern margin of the sw sub basin the margin of the scs basin was magma poor at the time it occurred an active mantle upwelling event caused by small scale mantle convection possibly related to the influence of the nascent hainan mantle plume was formed subsequently resulting in a southward ridge jump and the hvdlc beneath the current margins of the east and sw sub basins the east and sw sub basins continued to spread latitudinally dividing their hvdlcs as the margins separated accordingly afterwards the hyper extension in the distal margins may also be responsible for a crust weakening zone wherein the subsequent volcanic events could occur

dry climate near the western pacific warm pool pleistocene caliches of the nansha islands south china sea abstract the nansha spratly islands are located in the middle of the south china sea at about 10 n near the northwestern margin of the western pacific warm pool wpwp a borehole was drilled at an atoll at nansha and cores were taken the upper 165 4 m of the borehole consists entirely of limestone of reef facies and lagoon facies which had been influenced by extensive meteoric diagenesis petrographic and geochemical studies have identified at least four subaerial exposure surfaces ses in the pleistocene carbonate sequence these ses are thought to have resulted from global sea level changes and characterized by caliche formation when developed in limestone caliches typically form in areas where annual precipitation ranges from 500 mm up to about 1200 mm compared with 1800 2200 mm of annual rainfall of the present day nansha islands the nansha caliche therefore indicates the existence of several dry climate episodes during pleistocene sea level lowstands lower rainfall and higher evaporation during such dry conditions may explain the higher sea surface salinities reported elsewhere in the south china sea the nansha caliche may also indicate reduced extent of the wpwp and southerly shift of the intertropical convergence zone during major sea level lowstands

evolution of the south china sea revised ages for breakup and seafloor spreading abstract the continental breakup which gave way to the formation of the oceanic south china sea scs basin began in the latest cretaceous in the northeastern scs and propagated in southern and western direction over a long period of time possibly more than 40 m y the seafloor spreading history of the south china sea has been interpreted in different ways in the past and the debate over the correct timing of the major tectonic events continues we review the different models that have been published and present a revised interpretation of seafloor spreading anomalies based on three datasets with documented high quality which cover all of the scs but the northernmost and southernmost parts we can precisely date the onset of seafloor spreading in the central part of the scs at 32 ma after a ridge jump at 25 ma spreading also began in the southwestern sub basin and spreading ended at 20 5 ma in the entire basin followed by a phase of magmatic seamount formation mainly along the abandoned spreading ridge spreading rates vary from 56 mm yr in the early stages to 72 mm yr after the ridge jump to 80 mm yr in the southwestern sub basin we find indications for a stepwise propagation of the seafloor spreading from northeast to southwest in segments bounded by major fracture zones seafloor spreading ended abruptly probably because the subduction zone along the eastern and southern boundary of the scs of which today the manila trench remains was blocked by collision with a continental fragment possibly the northern part of palawan or a part of the dangerous grounds

post rift submarine volcanic complexes and fault activities in the baiyun sag pearl river mouth basin new insights into the breakup sequence of the northern south china sea abstract post rift volcanisms and fault activities are significant for understanding the tectonic sedimentary evolution of rifted continental margins and associated hydrocarbon entrapments in this study a series of post rift fault associated volcanic activities have been found in the northern continental margin of the south china sea scs however their episodes and relationships with the progressive continental breakup are still doubtful here new 2d 3d seismic borehole dating and biostratigraphic data are used to investigate the characteristics of the post rift volcanism and fault activities in the baiyun sag bys pearl river mouth basin the newly found fault associated volcanic complexes include thirteen submarine volcanic mounds and associated lava flows together with igneous sills that are mainly composed of basalts and tuffs seismic stratigraphic analysis combined with k ar dating from cores confirms that post rift volcanism in the bys climaxed along with the breakup of the scs mainly in three episodes 1 the first episode near oligocene miocene boundary 23 3 ma 2 the second at the middle early miocene 17 6 ma and 3 the new found third episode at the late early miocene 17 1 ma also fifty three syn sedimentary faults are interpreted which are characterized by a wnw trending normal fault group separated by an ene trending transcurrent fault group and there is evidence that these fifty three faults were long lived from ca 32 17 1 ma and served as important conduits for the upward migration of the magma the three episodes of volcanism and tectonic fault activities are related to the progressive continental breakup in the northern scs margin and they represent the tectonic signature of the early stage of the post rift subsidence after the lithospheric breakup which extends to the top of a hypothetical breakup sequence 32 17 1 ma in the bys furthermore this study proves that the diachronous continental breakup in the scs margin can result in corresponding changes in depositional architectures along with a breakup sequence characterized by the deposition of a sandstone carbonate shale regressive transgressive cycle the sedimentological changes and associated tectonic features recorded in the baiyun sag suggest an exceptionally active tectonic setting in the time of the formation of the breakup sequence early oligocene early miocene along the northern scs margin

seismic structure in the northeastern south china sea s wave velocity and vp vs ratios derived from three component obs data a nearly 500 km long seismic profile with reflective and refractive wide angle ocean bottom seismometer obs data and multi channel seismic mcs data was acquired across the northeastern continental margin of the south china sea scs the s wave crustal structure and vp vs ratios have been obtained based on a previously published p wave model using the software raylnvr modeling of vertical and horizontal component obs data yields information on the seismic crustal velocities lithology and geophysical properties along the obs 2001 seismic profile s wave velocities in the model increase generally with depth but exhibit high spatial variability particularly from the shelf to the upper slope of the northeastern scs margin vp vs ratios also reveal significant lithological heterogeneity dongsha penghu uplift dpu is a tectonic zone with a thicker crust than adjacent areas and a high magnetic anomaly with a vp vs of 1 74 and a p wave velocity of 5 0 5 5 km s the dpu primarily consists of felsic volcanic rocks in the upper crust and is similar to the petrology of zhejiang fujian volcanic provinces which perhaps is associated with a mesozoic volcanic arc the ocean continent transition oct in the northeastern scs is characterized by a thinning continental crust volcanoes in the upper crust and a high velocity layer hvl in the lower crust the s wave velocity and vp vs ratio suggest that the hvl has a mafic composition that may originate from underplating of the igneous rocks beneath the passive rifted crust after the cessation of seafloor spreading c 2009 elsevier b v all rights reserved

fine scale heterogeneity in the earth s inner core the seismological properties of the earth s inner core have become of particular interest as we understand more about its composition and thermal state1 2 observations of anisotropy and velocity heterogeneity in the inner core are beginning to reveal how it has grown and whether it convects3 4 the attenuation of seismic waves in the inner core is strong and studies of seismic body waves5 6 have found that this high attenuation is consistent with either scattering or intrinsic attenuation5 the outermost portion of the inner core has been inferred to possess layering and to be less anisotropic than at greater depths7 8 9 10 here we present observations of seismic waves scattered in the inner core which follow the expected arrival time of the body wave reflection from the inner core boundary the amplitude of these scattered waves can be explained by stiffness variations of 1 2 with a scale length of 2 kilometres across the outermost 300 km of the inner core these variations might be caused by variations in composition by pods of partial melt in a mostly solid matrix or by variations in the orientation or strength of seismic anisotropy

tests of fixity of the indo atlantic hot spots relative to pacific hot spots rates of inter hot spot motion have been debated for decades herein we present updated predictions for the tracks of the tristan da cunha reunion and iceland hot spots assuming them to be fixed relative to pacific hot spots uncertainties in pacific hot spot rotations which include uncertainties in the current locations of hot spots of 100 200 km are combined with uncertainties in relative plate motions accumulated through the plate circuit to obtain the final uncertainty in the predicted positions including uncertainties of 150 200 km in the current locations of the indo atlantic hot spots improvements to reconstruction methods to relative plate reconstructions to age dates along the tracks and to the geomagnetic reversal timescale lead to significant changes from prior results when compared with the observed tracks the predicted tracks indicate nominal rates of motion of only 2 6 mm a 1 of these indo atlantic hot spots relative to pacific hot spots over the past 48 ma within the uncertainties the rates range from no motion to rates as high as 8 13 mm a 1 for reconstructions prior to 48 ma b p however the apparent rates of inter hot spot motion are much larger 46 55 20 mm a 1 if the motion occurred entirely between 68 ma b p and 48 ma b p either hot spots moved rapidly before 48 ma b p and slowed drastically at 48 ma b p or global plate circuits through antarctica become less reliable as one goes increasingly further into the past most paleomagnetic data favor the latter explanation

using direct and coda wave envelopes to resolve the scattering and intrinsic attenuation structure of southern california characterizing scattering and absorbing properties and the power spectrum of crustal heterogeneity is a fundamental problem for informing strong ground motion estimates at high frequencies where scattering and attenuation effects are critical we perform a comprehensive study of local earthquake coda waves in southern california to constrain scattering and intrinsic attenuation structure we analyze data from 1195 spatially distributed earthquakes from 1981 2013 at source depths of 10 to 15 km and epicentral distances from 0 250 km with magnitudes larger than 1 8 we stack envelope functions from 28 127 vertical component and 27 521 transverse component seismograms filtered from 2 to 4 hz we model these observations using a particle based monte carlo algorithm that includes intrinsic attenuation as well as both p and s wave scattering and both single and multiple scattering events we find that spatially averaged coda wave behavior for southern california can be explained only with models containing an increase in scattering strength and intrinsic attenuation within the uppermost crust i e they are poorly fit with half space models of constant scattering strength a reasonable fit to our data is obtained with a two layer model composed of a shallow crustal layer with strong wide angle scattering and high p and s intrinsic attenuation and a deeper layer with weaker scattering and lower intrinsic attenuation top 5 5 km qi 250 qi 125 heterogeneity correlation length a 50 m fractional velocity heterogeneity e 0 4 lower crust qi 900 qi 400 a 2 km e 0 05

inhomogeneities near the core mantle boundary evidenced from scattered waves a review statistical properties of small scale inhomogeneities wavelengths between 20 and 70 km near the core mantle boundary are inferred from scattered core waves observations of scattered core waves at large seismic arrays and worldwide networks indicate that the inhomogeneities have a global nature with similar characteristics however there may exist a few regions having markedly stronger or weaker strengths scattering by volumetric inhomogeneities of about 1 inp wave velocity in the lower mantle or by about 300 m of topographic relief of the core mantle boundary can explain the observations at present it is not possible to rule out either of these two alternatives or a combination of both

drowning history of a miocene carbonate platform zhujiang formation south china sea abstract the identification and interpretation of drowning events in the geologic record can aid significantly to the reconstruction of the depositional tectonic and eustatic history of a study area and often improve reservoir and seal prediction in carbonate rocks the differentiation between drowned platforms showing a record of continuous deepening and those with a record of exposure followed by rapid deepening remains however problematic the zhujiang carbonate platform liuhua 11 1 field south china sea study shown here provides an example of an integrated approach combining high resolution geochemistry microfacies analyses and foraminiferal biostratigraphy in order to improve the reconstruction of environmental conditions prior during and after platform demise and drowning the zhujiang carbonate platform displays the following vertical succession of four facies types i skeletal grain facies with a miogypsinid lepidocyclinid dominated fauna deposited in a moderately deep heterostegina sp and spiroclypeids possibly capped by a subaerial exposure surface well rounded rhodoids representing a mesotrophic lagoon dominate the upper portions of the rhodoid facies iv pelagic marine shales of the hanjiang formation burying the carbonate platform after drowning this facies succession in combination with geochemical evidence suggests a deepening upward trend this trend might have been interrupted by transient subaerial exposure but no evidence for meteoric diagenesis was found at the drowning unconformity topping the carbonate platform instead microfacies analyses suggest that platform demise may be related to progressive changes in environmental conditions including increasing nutrient levels and or decreasing temperature up core towards the drowning unconformity these findings are of significance for those concerned with miocene carbonate factories and more specifically the demise of carbonate platforms in general