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

vp vs ratio along the voring margin ne atlantic derived from obs data implications on lithology and stress field a total of 13 regional ocean bottom seismograph obs profiles with an accumulated length of 2207 km acquired on the voring margin ne atlantic have been travel time modelled with regards to s waves the vp vs ratios are found to decrease with depth through the tertiary layers which is attributed to increased compaction and consolidation of the rocks the vp vs ratio in the intra campanian to mid campanian layer 1 75 1 8 in the central voring basin is significantly lower than for the layers above and beneath suggesting higher sand shale ratio this layer was confirmed by drilling to represent a layer of sandstone this mid cretaceous anomaly is also present in the northern voring basin as well as on the southern lofoten margin further north the vp vs ratio in the extrusive rocks on the voring plateau is estimated to be 1 85 conformable with mafic basaltic rocks landward of the continent ocean transition cot the vp vs ratio in the layer beneath the volcanics is estimated to be 1 67 1 75 these low values suggest that this layer represents sedimentary rocks and that the sand shale ratio might be relatively high here the vp vs ratio in the crystalline basement is estimated to be 1 67 1 75 in the basin and on the landward part of the voring plateau indicating the presence of granitic granodioritic continental crust in the lower crust the vp vs ratio in the basin decreases uniformly from southwest to northeast from 1 85 1 9 to 1 68 1 73 suggesting a gradual change from mafic gabbroic to felsic granodioritic lower crust significant 3 5 azimuthal s wave anisotropy is observed for several sedimentary layers as well as in the lower crust all these observations can be explained by invoking the presence of liquid filled microcracks aligned vertically along the direction of the present day maximum compressive stress nw se

comparison of the tectonics and geophysics of the major structural belts between the northern and southern continental margins of the south china sea abstract a comparison of the tectonics and geophysics of the major structural belts of the northern and the southern continental margins of south china sea has been made on the basis of measured geophysical data obtained by ourselves over a period of 8 years 1984 1991 this confirmed that the northern margin is a divergent one and the southern margin is characterized by clearly convergent features the main extensional structures of the northern margin are from north to south 1 1 the littoral fault belt a tectonic boundary between the continental crust and a transitional zone along the coast of the provinces of guangdong and fujian in south china it is characterised by earthquake activities high magnetic anomalies and a rapid change in crustal thickness 2 2 the northern and southern depression zones i e the pearl river mouth basin this strikes ne ene and is a very large cenozoic depression which extends from offshore shantou westwards to hainan island 3 3 the central uplift zone this includes the dongsha uplift shenhu uplift and may be linked with the penghu uplift and taiwan shoals to the east forming a large ne striking uplift zone along the northern continental slope it is characterized by high magnetic anomalies 4 4 southern boundary fault belt of the transitional crust this has positive gravity anomalies on the land side and negative ones on the sea side 5 5 the magnetic quiet zone this is located south of the southern boundary fault belt and between the continental margin and the central basin of the south china sea magnetic anomalies in this belt are of small amplitude and low gradient we consider the magnetic quiet zone to be a very important tectonic zone the major structures of southern continental margin southwards are 1 1 the northern fault belt of the nansha block this extends along the continental slope north of the liyue shoal reed bank and zhongye reef and is a tectonic boundary between oceanic crust and the nansha block continental crust 2 2 the nansha block uplift zone due to the development of reefs and shoals there are many channels and valleys our long distance multichannel seismic profiles indicated that there are thick paleogene sediments and thin neogene sediments all over the central part of the block 3 3 the nansha trough a nappe structure formed by the southeastward drifting of nansha block and northwestward overthrusting of palawan northwest borneo 4 4 zengmu shoal basin southwest of the nansha block the maximum thickness of cenozoic strata is over 9 km in this important petroliferous basin

s wave velocities and anisotropy of typical rocks from yunkai metamorphic complex and constraints on the composition of the crust beneath southern china abstract in order to constrain the interpretation of seismic data from receiver functions and deep profiles of the crust beneath southern china cathaysia and yangtze blocks we have measured s wave velocities v s and splitting as a function of hydrostatic confining pressure up to 650 mpa for 22 representative samples i e granite diorite felsic gneiss and mylonite amphibolite schist and marble from the yunkai metamorphic complex china that represent the crystalline basement beneath the region the experimental data were combined with electron backscattering diffraction ebsd analysis of rock forming minerals to constrain variations of v p v s ratios and understand the origin of seismic anisotropy the crusts beneath the yangtze and cathaysia blocks have different average thicknesses h 35 4 6 3 km and 29 8 1 8 km respectively but display almost the same v p v s values 1 73 0 08 and 1 74 0 04 respectively these ratios correspond to an average of bimodally distributed granitic and gabbroic lithologies which are dominant respectively in the upper and lower crusts instead a homogeneous andesitic composition of the overall crust positive and negative correlations between h and v p v s occur in west and east parts of southern china respectively the negative correlation indicates basaltic underplating from a partially molten mantle wedge above the subducting pacific plate into the southern china crust whereas the positive correlation implies that much larger thinning strain has taken place in the high temperature mafic lower crust high temperature than in the low temperature felsic upper crust during mesozoic cenozoic tectonic extension

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

late mesozoic tectonic structure and evolution along the present day northeastern south china sea continental margin abstract the northeastern south china sea continental margin holds the key to understanding late mesozoic tectonics and evaluating hydrocarbon potentials in mesozoic tectonic and stratigraphic structures offshore southeast china with newly obtained and processed seismic data and new drilling and logging data we correlate regional mesozoic stratigraphy and analyze major mesozoic tectonic events and structures in particular we focus our study on the three major tectonic units in the area the chaoshan depression the tainan basin and the dongsha penghu uplift which are separated by basement high thrust fold and or faults stratigraphic correlations suggest a major phase of southeastward regression spanning in time from the late early jurassic 180 ma to the early cretaceous 120 ma seismic data reveal two major tectonic events with the first one in the late jurassic to the early cretaceous contemporary with the regression and the second one in the late cretaceous regional magnetic anomaly map after the reduction to the pole clearly reveals the boundary between the dongsha penghu uplift and the chaoshan tainan depositional system the seismic and magnetic data also suggest that while the dongsha penghu uplift has highly magnetized sources buried mostly in the upper crust at depths from about 2 km to about 20 km the chaoshan tainan depositional system has thick mesozoic sediments of low magnetization

crustal structure of bohai sea and adjacent area north china from two onshore offshore wide angle seismic survey lines abstract in 2010 and 2011 onshore offshore surveys along two lines in the bohai area were carried out this paper presents the deep structure from wide angle seismic data along the two lines and try to improve the reliability of deep interfaces by using of the cmp prospecting method the result is as follows 1 the environmental noise of records from obss in shallow water area is stronger than that from deep waters 2 the cmp stacking method is potential for high density obs surveys 3 the thickness of sedimentary layer in bohai sea and surrounding area is 3 5 km the velocity increases with depth from about 2 km s at the seafloor to about 4 5 km s on the sedimentary basement 4 the upper crust thins toward the sea area from the land area the middle and lower crust also vary insignificantly the middle crust is about 4 km and the lower crust is about 15 km the velocities of the middle and lower crust shows gentle variation in vertical but obvious in lateral due to the impact of fault zones 5 the velocity contours nearby the tancheng lujiang tlfz and zhangjiakou penglai zpfz fault zones are u shaped where the velocity changes laterally and the interface undulates significantly velocity disturbances exist in the crust mantle transition zone at the fault zones the tlfz and zpfz could be the upwelling channels of deep material 6 in the eastern part of north china craton ncc the moho gently ranges at the depth of about 30 km with local rises lithosphere thinning occurred predominantly at the top of upper mantle the east boundary of lithosphere thinning is inferred to be on the east of bohai sea

deep structure of the ocean continent transition in the southern iberia abyssal plain from seismic refraction profiles ocean drilling program legs 149 and 173 transect we present a wide angle seismic refraction study of an 80 40 km region of the southern iberia abyssal plain south of galicia bank an intersecting grid of two e w and four n s wide angle reflection refraction profiles is used to define variations of the basement velocity structure within this unusually wide ocean continent transition oct these structures can be systematically linked to variations in acoustic basement morphology and to results from ocean drilling program odp boreholes lateral changes in the velocity structure of the basement occur abruptly over distances of 20 km where complex variations may be found thinned upper continental crust 2 5 km thick with velocities of 5 0 6 6 km s is limited to a series of n s fault blocks immediately south of galicia bank this crust is underlain by a high velocity layer 7 3 7 9 km s of weakly serpentinized i e 0 25 peridotite which exists throughout the eastern part of the survey area basement within the oct appears to consist dominantly of a broad region of exposed upper mantle that has been serpentinized heterogeneously both vertically and horizontally in the southeast sector of our survey where basement topography deepens and becomes subdued continental fault blocks are absent instead basement contains an upper layer of more pervasively serpentinized i e 25 45 peridotite that is 2 km thick this layer is characterized by low velocity at the top of basement 4 2 km s that increases rapidly with depth and it probably corresponds to a seismically unreflective layer previously identified in reflection profiles to the south of our survey in the western section of our survey beneath a series of elevated basement ridges velocities are reduced within both the upper basement layer 3 5 6 0 km s and lower layer 6 4 7 5 km s these changes suggest that both upper and lower layers have become more highly serpentinized with values of 60 100 in the upper layer and 25 45 in the lower layer probably during the last stages of rifting and immediately before formation of oceanic crust a normal or slow spreading oceanic crustal structure is not found within the survey region thus it appears that the onset of seafloor spreading occurs in the region west of the peridotite ridge sampled at odp site 897 and east of the j magnetic anomaly

seismic wide angle constraints on the crust of the southern urals a wide angle seismic reflection refraction data set was acquired during spring 1995 across the southern urals to characterize the lithosphere beneath this paleozoic orogen the wide angle reflectivity features a strong frequency dependence while the lower crustal reflectivity is in the range of 6 15 hz the pmp is characterized by frequencies below 6 hz after detailed frequency filtering the seismic phases constrain a new average p wave velocity crustal model that consists of an upper layer of 5 0 6 0 km s which correlates with the surface geology 5 7 km depths at which the velocities increase to 6 2 6 3 km s 10 30 km depths at which on average the crust is characterized by velocities of 6 6 km s and finally the lower crust from 30 35 km down to the moho which has velocities ranging from 6 8 to 7 4 km s two different s wave velocity models one for the n s and one for the e w were derived from the analysis of the horizontal component recordings crustal sections of poisson s ratio and anisotropy were calculated from the velocity models the poisson s ratio increases in the lower crust at both sides of the root zone a localized 2 3 anisotropy zone is imaged within the lower crust beneath the terranes east of the root this feature is supported by time differences in the sms phase and by the particle motion diagrams which reveal two polarized directions of motion velocities are higher in the central part of the orogen than for the siberian and eastern plates these seismic recordings support a 50 56 km crustal thickness beneath the central part of the orogen in contrast to moho depths of 45 km documented at the edges of the transect the lateral variation of the pmp phase in frequency content and in waveform can be taken as evidence of different genetic origins of the moho in the southern urals

compressional velocity structure and poisson s ratio in marine sediments with gas hydrate and free gas by inversion of reflected and refracted seismic data south shetland islands antarctica abstract a bottom simulating reflector bsr on the south shetland margin antarctic peninsula was surveyed within the progetto nazionale di ricerche in antartide pnra high resolution multichannel mcs reflection profiles were acquired and a three component ocean bottom seismograph obs was deployed where the bsr was particularly strong the compressional velocity field associated to the bsr was determined by the inversion of travel times of reflected mcs data and refracted obs data events information on shear wave velocities was extracted from the horizontal components of the obs records the analysis indicates the presence of velocity anomalies both above and below the bsr that are not related to the lithologic characteristics of the sedimentary column but can be associated to gas hydrate and free gas presence in pore space amplitude versus offset avo analysis was performed on common depth point gathers which correspond to the obs position the study determined reflection coefficient trends that provided qualitative information about gas hydrate and free gas abundance a theoretical model for elastic porous media was applied to quantify amounts of gas hydrate and free gas the resulting theoretical velocity curves were compared to interval velocity obtained by seismic inversion a quantitative estimation of free gas and gas hydrates trapped in the sediment pore spaces was made by interpreting velocity anomalies with respect to reference velocity curves normally consolidated marine terrigenous sediments the comparison of the results obtained by these independent methods seismic inversion avo analysis and theoretical model gives consistent values of poisson s ratio and gas hydrate and free gas estimation

oxygen deficient ti oxides natural magneli phases from mudstone xenoliths with native iron from disko central west greenland mudstone xenoliths in a strongly reduced andesitic subaquatic breccia with native iron from the asuk member on disko contain ti oxides which are oxygen deficient relative to rutile ore microscopy reveals that the mudstone xenoliths contain former clastic oxide grains which have equilibrated to blue ti oxides and grey aluminous ti oxides they also contain still immature coal fragments in a glassy matrix with native iron the blue oxides are compositionally similar to magneli phases tino2n 1 within the range n 4 to 7 and several grains contain more than one natural magneli phase two other phases found are 1 pale orange blebs in magneli phases with a composition approaching al1ti 1 3 ti 2 4 o7 alti phase b and 2 grey oxide rims on magneli phases or independent grains of the compositional series al2 x ti 0 5 4 mg fe 0 5 xti n 2 4 o2n 1 with n mostly between 7 and 10 alti phase a the natural magneli phases equilibrated at oxygen fugacities 4 to 5 log units below the fe w oxygen buffer at igneous temperatures and represent the most reduced high temperature environment yet recorded among native iron bearing rocks from disko the extremely reducing conditions were met in rocks where coal fragments were still in a state of degassing hydrocarbon components at the time of quenching field geology and carbon barometry indicate equilibration at pressures of less than 10 bars

crustal plumbing system of post rift magmatism in the northern margin of south china sea new insights from integrated seismology abstract the origin and evolution of magmatism in rifting margins are a fundamental geological subject and remain the focus of intense studies unlike the classical magma poor and magma rich rifting margins the northern margin of the south china sea scs exhibits a strong post rift magmatism but the evolution and mechanism of this behavior are still poorly understood in this work we investigate the detailed structural features of the post rift magmatism in the crust of the scs northern margin our analysis is based on wide angle and multichannel reflection seismic data combined with previous seismological results our results show a wide distribution of shallow igneous sills and intrusions these features are spatially and tectonically linked to the lower crustal high velocity bodies hvbs and submarine volcanism indicating a causative connection between the three features considering the existence of an obvious low velocity branch of the hainan plume in the northern scs and uplift of the lithosphere asthenosphere boundary we propose that the hvbs reflect consolidated mafic intrusions which formed the lower crustal reservoirs feeding the overlying igneous sills and intrusions in the scs northern margin where abundant extensional and detachment faults can act as magmatic channels such a dynamic process might cause post rift volcanism our results suggest that the crustal magmatic system of post rift volcanism has a multilevel upward migration mode and the hvbs in the lower crust could be the product of post rift magmatism further indicating that the scs northern margin had a magma poor property at the rifting phase but has undergone a strong magmatic rejuvenation by the subsequent mantle plume

rock lithology and porosity determination from shear and compressional wave velocity data examined in this study are previously published laboratory shear s and compressional p wave velocity measurements on water saturated sandstone calcareous sandstone dolomite and limestone cores as well as laboratory porosity measurements on the sandstone and limestone cores sandstone and limestone porosities range from 092 to 299 and from 006 to 229 respectively differential pressure was varied from 500 to 6000 psi corresponding to approximate burial depths from 290 to 3460 m respectively sandstone limestone and dolomite are effectively separated by poisson s ratio or equivalently by the ratio of p to s wave velocity separation of sandstone and limestone appears to result from the difference in of the matrix material namely quartz 056 and calcite 316 respectively an empirical function 1 v a b was fit by regression analysis to sandstone and limestone velocity vs and vp versus porosity values at each differential pressure in this equation a and b are const

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

lower crustal composition and crustal lineaments on the voring margin ne atlantic a review abstract a prominent lower crustal high velocity layer observed in the voring basin ne atlantic has been investigated with regard to composition and spatial link to crustal scale lineaments in the southernmost part of the basin the lower crustal layer is interpreted as eclogites based on anomalously high p wave velocities 8 4 km s in the remainder of the basin the layer is preferably interpreted as mafic intrusions emplaced during the last phase of rifting but it cannot be excluded that the body consists of older caledonian mafic rocks direct link with high grade caledonian rocks to the east and westwards to east greenland cannot be made since these rocks are mainly of felsic composition the crustal scale lineaments inferred from wide angle seismic data appear to represent the oceanward extension of post caledonian detachments the spatial variation in the thickness of the layer is within the late rifting magmatic hypothesis partly interpreted in terms of pre magmatic variations in thickness of the lithosphere in addition lateral variations in extension mantle upwelling and decompressional melting during the last phase of rifting may also have contributed

seismic imaging of the transitional crust across the northeastern margin of the south china sea abstract crustal structure across the passive continental margin of the northeastern south china sea scs is presented based on a deep seismic survey cooperated between taiwan and china in august 2001 reflection data collected from a 48 hydrophone streamer and the vertical component of refraction reflection data recorded at 11 ocean bottom seismometers along a nw se profile are integrated to image the upper 1 6 2 4 km s lower 2 5 2 9 km s and compacted 3 4 5 km s sediment the upper 4 5 5 5 km s middle 5 5 6 5 km s and lower 6 5 7 5 km s crystalline crust successively the velocity model shows that the thickness 0 5 3 km and the basement of the compacted sediment are strongly varied due to intrusion of the magma and igneous rocks after seafloor spreading of the scs furthermore several volcanoes and igneous rocks in the upper middle crust 7 10 km thick and a high velocity layer 0 5 km thick in the lower crust of the model are identified as the ocean continent transition oct below the lower slope in the northeastern margin of the scs a thin continent nw of the oct and a thick oceanic crust se of the oct in the continental margin of the northeastern scs are also imaged but these transitional crusts cannot be classified as the oct due to their crustal thickness and the limited amount of the volcano the magma and the high velocity layer the extended continent next to the gravity low and a sag zone extended from the sw taiwan basin may have resulted from subduction of the eurasian plate beneath the manila trench whereas the thick oceanic crust may have been due to the excess volcanism and the late magmatic underplating in the oceanic crust after seafloor spreading of the scs

an integrated geophysical study on the mesozoic strata distribution and hydrocarbon potential in the south china sea abstract a series of drilling dredge and seismic investigations indicate that mesozoic sediments exist in the south china sea scs which shows a bright prospect for oil and gas exploration in order to study the distribution of mesozoic strata and their residual thicknesses in the scs we carried out an integrated geophysical study based mainly on gravity data gravity basement depth and distribution of residual mesozoic thickness in the scs were obtained using gravity inversion constrained with high precision drilling and seismic data in addition the fine deep crustal structures and distribution characteristics of mesozoic thicknesses of three typical profiles were obtained by gravity fitting inversion mesozoic strata in the scs are mainly distributed in the south and north continental margins and have been reformed by the later tectonic activities they extend in ne trending stripes are macro controlled by the deep and large ne trending faults and cut by the nw trending faults which were active in later times the offset in nw direction of mesozoic strata in nansha area of the southern margin are more obvious as compared to the north margin in the pearl river mouth basin and southwest taiwan basin of the north continental margin the mesozoic sediments are continuously distributed with a relatively large thickness in the nansha area of the south margin the mesozoic strata are discontinuous and their thicknesses vary considerably according to the characteristics of mesozoic thickness distribution and hydrocarbon potential analyses from drilling and other data dongsha uplift chaoshan depression southwest taiwan basin peikang uplift and liyue bank have large thickness of the mesozoic residual strata have good hydrocarbon genesis capability and complete source reservoir cap combinations show a bright prospect of mesozoic oil gas resources

vp vs ratio structure of the lofoten continental margin northern norway and its geological implications summary in 1988 an extensive seismic refraction and wide angle reflection study was performed at the lofoten passive continental margin off northern norway by using three component ocean bottom seismographs the lofoten margin was formed by continental rifting and sea floor spreading with extensive volcanism which is responsible for landward flood basalts and seaward dipping reflectors sdr the p wave structure beneath the seaward side of the voting plateau escarpment at the lofoten margin is characterized by a thick oceanic lower crust and the existence of an additional high velocity layer at the bottom of the crust in this study the distribution of the p wave velocity vp to s wave velocity vs ratio down to the uppermost mantle is obtained by studying p to s converted shear wave data vp vs ratios of 3 0 5 5 are obtained in the sedimentary layers these high ratios are attributed to unconsolidated sediments in the upper crust a significant difference in vp vs ratios is found between oceanic side profiles and land side profiles i e 1 86 oceanward of the sdrs and 1 76 landward of the sdrs this variation may have been caused by a difference in porosity due to the difference in extrusive processes the upper crust of the oceanic side of the sdrs was created by submarine eruptions while the land side of the sdrs was formed by submarian eruptions the vp vs ratios in the middle crust and lower crust are estimated as 1 76 1 78 and 1 80 respectively comparing vp and vsv data from the lower crust to those in previous studies clearly indicates the presence of mafic gneisses this result represents new seismic evidence for the hotspot hypothesis concerning the formation of passive volcanic continental margins clear s wave upper mantle refractions can be observed at offsets of up to 100 km even where p wave arrivals cannot be identified in order to model these s wave arrivals a vs 4 8 km s 1 layer is necessary 2 km below the moho

deep structures and breakup along volcanic rifted margins insights from integrated studies along the outer voring basin norway three dimensional multichannel seismic survey and well data allows a deeper insight into rifting mechanisms and crustal structure of the norwegian volcanic rifted margin a surprising high velocity lower crustal dome marked by a strong amplitude reflection t reflection is described below the north gjallar ridge ngr near the volcanic plateau formed during the late paleocene early eocene breakup faulting along the ngr occurs during early campanian paleocene and is controlled by the crustal dome the t reflection was previously related to either the top of mafic underplated crust or soft lower crust updomed during the continental breakup the influence and timing of the crustal dome on the basin structure shows clearly that the dome predates the continental breakup based on the structural relationship and the results of the modelling it is therefore concluded that the t reflection does not necessarily originate from an anomalous tertiary magmatic event related to the breakup underplated material but could be also partly attributed to pre breakup mafic material or and inherited high pressure granulite eclogite rocks in the continental domain q 2004 elsevier ltd all rights reserved

continental interior and edge breakup at convergent margins induced by subduction direction reversal a numerical modeling study applied to the south china sea margin the dynamics of continental breakup at convergent margins have been described as the results of back arc opening caused by slab rollback or drag force induced by subduction direction reversal although the rollback hypothesis has been intensively studied our understanding of the consequence of subduction direction reversal remains limited using thermo mechanical modeling based on constraints from the south china sea scs region we investigate how subduction direction reversal controls the breakup of convergent margins the numerical results show that two distinct breakup modes namely continental interior and edge breakup edge refer to continent above the plate boundary interface may develop depending on the maturity of the convergent margin and the age of the oceanic lithosphere for a slab age of 15 45 ma increasing the duration of subduction promotes the continental interior breakup mode where a large block of the continental material is separated from the overriding plate in contrast the continental edge breakup mode develops when the subduction is a short duration event and in this mode a wide zone of less continuous continental fragments and tearing of the subducted slab occur these two modes are consistent with the interior relic late mesozoic arc and edge relic fore arc rifting characteristics in the western and eastern scs margin suggesting that variation in the northwest directed subduction duration of the proto scs might be a reason for the differential breakup locus along the strike of the scs margin besides a two segment trench associated with the northwest directed subduction is implied in the present day scs region

joint p and s wave velocity determination from reflected pp ss and converted ps sp phases from large aperture seismic reflection measurements abstract as less aliased better sampled seismic studies are acquired more ss reflections and especially ps sp converted phases are recognized we use a synthetic p data set to demonstrate the viability of using pp ss reflections and ps sp conversions in a joint inversion scheme to obtain average one dimensional p and s wave velocity models we achieve a coupling between the p and s wave velocity depth functions by introducing all these wave types in a single joint inversion the new information introduced by the converted phases provides additional constraints and better resolved velocity depth models can be estimated in our synthetic data case the resolution kernels become sharper decreasing the width of the main lobe by 10 50 and increasing their height by 20 40 this increase in resolution can result in a significant decrease in the uncertainty estimates of poisson s ratios in our synthetic test the uncertainties decrease approximately 50 we use this inversion scheme to invert p sections from horizontal components recorded in a seismic experiment along the southwest coast of greenland the joint inversion results resolve different levels of anisotropy within the crust these anisotropic fabrics can be responsible for the time difference delay between the radial and transverse p sections that causes a decrease in the amplitude of the s wave field when the two p sections radial and transverse are stacked

rifting and magmatism in the northeastern south china sea from wide angle tomography and seismic reflection imaging we present a new travel time tomography velocity model and seismic reflection images that delineate the rift architecture and magmatic features of the rifted margin in the northeastern south china sea these data reveal moderately stretched crust 25 km thick along the continental shelf and thin but laterally variable crustal thickness in the distal margin along the continental slope crust rapidly thins to 4 km in a basin characterized by tilted fault blocks that sole into a low angle detachment strain was localized to a degree within the highly stretched basin but failed to progress to breakup and seafloor spreading crust in the distal margin is 12 15 km thick few extensional structures are apparent in the distal margin but seismic velocities are suggestive of highly thinned and magmatically intruded continental crust the magmatic features we interpret include volcanic zones at the top of the basement that deform or disrupt overlying postrift strata sills intruded into the postrift sedimentary section and a high velocity 6 9 7 5 km s lower crustal layer that we take to be magmatic underplating or pervasive lower crustal intrusions these features primarily occur in the distal margin and may have been emplaced during postrift seafloor spreading the postrift magmatism may have been induced by convective removal of continental lithosphere following breakup and the onset of seafloor spreading in the south china sea

shear wave splitting in the lower crust beneath the archean crust of southwest greenland abstract slant stacks of seismic data from rifted ancient archean crust along the southwest coast of greenland indicate that the lower crust is strongly seismically anisotropic the slant stack data show that a lower crustal wide angle s wave reflection has a different intercept time and ray parameter on the radial and transverse components from a receiver gather recorded on passcal reftek three component seismometers from the s wave analysis the continental crust is clearly seismically anisotropic above a high velocity wedge in the lower crust possibly magmatic underplating during late cretaceous labrador sea rifting heated the pre existing lower crust promoting plastic flow and enabling alignment of anisotropic minerals to produce the seismic anisotropy

composition of the crust in the grenville and appalachian provinces of north america inferred from v p v s ratios we use the ratios between p and s wave velocities vp vs derived from seismic refraction data to infer the composition of the crust in the grenville and the appalachian provinces of north america the crust exhibits vp vs increasing with depth from 1 64 to 1 84 there is a clear distinction between the grenville province average vp vs 1 81 and the appalachian province average vp vs 1 73 which persists at all depths the boundary between these provinces is east dipping extending for 100 km east of the champlain thrust in the appalachian province the increase in vp vs ratios with depth from 1 67 to 1 74 0 02 may reflect a normal decrease of silica content in the continental crust in the grenville province beneath the central granulite terrane an anomalous vp vs ratio of 1 82 0 02 is observed extending to a depth of 10 km this correlates with the abundance of ca plagioclase in the marcy anorthosite at greater depth 15 20 km where seismic lamination and high electrical conductivity is observed vp vs is 1 84 0 02 and correlates with the tahawus complex a layered mafic intrusion within the 25 km thick lower crust of the grenville province the vp vs is 1 84 0 02 and p velocity is 7 0 0 1 km s which are typical for plagioclase bearing rocks gabbro norite the high vp vs ratio in the grenville province has not been reported in crust of any other age since the grenville province contains 75 of the world s known anorthosites high vp vs ratio is related to high plagioclase we suggest that the composition of the grenville lower crust was significantly modified by the emplacement of the anorthosites in the mid proterozoic

crustal structure of the central costa rica subduction zone implications for basal erosion from seismic wide angle data on the pacific margin off central costa rica an anomalous lens shaped zone is located between the overriding plate and the subducting oceanic lithosphere approximately 25 km landward of the deformation front this feature was previously recognized in reflection seismic data when it was termed megalens its origin and seismic velocity structure however could not unambiguously be derived from earlier studies therefore during rv sonne cruise so163 seismic wide angle data were acquired in 2002 using closely spaced ocean bottom hydrophones and seismometers along two parallel strike and two parallel dip lines above the megalens intersecting on the middle slope the p wave velocities and structure of the subducting oceanic cocos plate and overriding caribbean plate were determined by modelling the wide angle seismic data in combination with the analysis of coincident reflection seismic data and the use of synthetic seismograms the margin wedge is defined by high seismic velocities 4 3 6 1 km s 1 identified within a wedge shaped body covered by a slope sediment drape it is divided into two layers with different velocity gradients the lower margin wedge is clearly constrained by decreasing velocities trenchward and terminates beneath the middle slope at the location of the megalens seismic velocities of the megalens are lower 3 8 4 3 km s 1 relative to the margin wedge we propose that the megalens represents hybrid material composed of subducted sediment and eroded fragments from the base of the upper plate upward migrating overpressured fluids weaken the base of the margin wedge through hydrofracturing thus causing material transfer from the upper plate to the lower plate results from amplitude modelling support that the megalens observed off central costa rica is bound by a low velocity zone documenting fluid drainage from the plate boundary to the upper plate

composition of the uralide crust from seismic velocity v p v s heat flow gravity and magnetic data abstract p wave velocity vp s wave velocity vs poisson s ratio heat flow potential field and surface geological data are integrated to constrain a model for the composition of the uralide crust along the urseis transect the model is constructed using published laboratory measurements of vp vs and density for a variety of crustal rock types these laboratory data have been corrected for depth pressure and the uralides temperature depth function the model shows clear differences between the composition of the old continental crustal nucleus of the east european craton and the newly added crust of the accreted arc terranes to the east the crust of the east european craton is more felsic than that of the magnitogorsk and east uralian zones and the latter two have a lowermost crust whose characteristics indicate a high garnet content mafic garnet granulite and or the presence of hornblendite the overall composition of the arc terranes is basaltic physical properties suggest that eclogite is not present in the arc terranes or if present it exists in small amounts that are below the resolution of the data set the lack of eclogite in the lower crust favours an intracrustal differentiation model for the evolution of the bulk composition of the continental crust nevertheless the absence of surface uplift the lack of metamorphism and late orogenic mantle melts and the current crustal thickness indicate that crustal thinning did not affect the bulk composition of the uralide crust

a crustal structure profile across the northern continental margin of the south china sea an ocean bottom seismometer obs geophysical experiment was carried out across the northern continental margin of the south china sea two dimension kinematic ray tracing was used to construct synthetic records and to define the geological model along the obs profile according to the calculation and interpretation of refracted p wave phases the crustal thickness decreases from 22 km obs 1 in littoral area to 8 km obs 15 in deep sea the lower slope structured by volcanism changes sharply in moho depth from 22 to 9 km the volcanoes over the lower slope were markers of ocean continental transition zone a high velocity layer hvl with an average thickness of 4 km was found in the base of the lower crust over the shelf and slope the south china sea was originated by the passive rifting of south china continent and seafloor spreading however geological and geophysical data collected in the northern margin of the south china sea show a small amount of magma intruded into or erupted onto the crust during pull apart and breakup further analysis shows that the hvz in the base of the thinned continental crust was related to magmatism that occurred after the cessation of seafloor spreading of the south china sea magmatism during the crustal extension and breakup is very weak therefore the northern continental margin of the scs is classified as non volcanic passive margin the southern margin of the south china sea has a crust with a steady thickness of 18 24 km which is different from the nor them margin of the south china sea with a gradually thinned crust they constitute an asymmetrically conjugated pair of passive margins c 2001 elsevier science b v all rights reserved

deep penetration seismic soundings across the northern margin of the south china sea twenty reversed two ship expanding spread profiles esps with maximum source receiver offsets of 100 km were collected in three transects across the rifted northern margin of the south china sea source receiver offset versus two way travel time x t data were mapped into the intercept time versus ray parameter p domain and velocity depth solutions were obtained by a combination of sum inversion in the p domain and ray tracing in both the p and x t domains arrivals from the moho were detected on 17 of the esps the depths to moho determined for individual esp interpretations have reproducibilities of 0 1 km to 3 km in most cases the moho depth has been determined to within 1 5 km moho depths determined in this investigation represent a significant improvement over previous estimates of moho along the margin from gravity data variations in present day crustal thickness measured from top of prerift basement to moho are one measure of the amount and nature of the crustal thinning associated with the rifting of continental crust preceding the formation of the adjacent south china sea basin the esp interpretations reveal that across the eastern portion of the south china margin the crust appears to thin more or less continuously toward the continent ocean boundary in the west esp interpretations also show a general trend of seaward crustal thinning but in addition indicate at least two instances of focused localized crustal thinning crustal velocities and the relative proportion of upper crust vp 6 4 km s are used to identify areas of the south china margin with similar and contrasting crustal structures variations in these properties are believed to result primarily from contrasting prerift crustal structure across the margin however magmatic underplating during rifting depth dependent extension and pleistocene igneous intrusions may also have contributed to the variations in present crustal structure reliable information about variations in crustal thickness and velocity structure across and along the south china margin is an important prerequisite to understanding better the nature of the spatially variable rifting processes which dominated the formation of this margin

deep structure of the ocean continent transition in the southern iberia abyssal plain from seismic refraction profiles the iam 9 transect at 40 20 n we present a crust and mantle velocity structure for the west iberia passive continental margin derived from a 320 km long wide angle seismic profile acquired in the southern iberia abyssal plain we observe a 170 km wide ocean continent transition zone which includes a pair of overlapping peridotite ridges and is bounded by oceanic crust and landward by fault bounded blocks of continental crust the profile lies 40 km south of the transect sampled by ocean drilling program odp legs 149 and 173 the transition zone structure can be divided into an upper layer 2 4 km thick with velocities of between 4 5 and 7 0 km s 1 and generally a high velocity gradient 1 s 1 and a lower layer up to 4 km thick with a velocity of 7 6 km s 1 and a low velocity gradient a weak moho reflection in this zone was seen only on wide angle profiles at an offset of 30 km the upper layer has a distinctly lower velocity than thinned continental crust adjacent to the continental slope conversely the lower layer has too high a velocity to be magmatically intruded or underplated lower continental crust on the coincident seismic reflection profile fault bounded crustal blocks identified in unequivocal extended continental crust are not observed in the transition zone the upper layer has velocity bounds and gradient similar to oceanic layer 2 observed west of the peridotite ridges but no oceanic layer 3 velocity structure is present while magnetic anomalies have been identified within the transition zone they have not been modeled successfully as seafloor spreading magnetic anomalies nor do they generally form long linear margin parallel features finally odp boreholes 40 km north of our profile and within the interpreted transition zone have recovered up to 140 m thick sections of serpentinite and serpentinized peridotites with little evidence of mafic igneous material we conclude that the transition zone cannot be dominantly composed of either extended continental crust or oceanic crust although current melting models predict a considerably thicker crust of decompression melt products we interpret this region as exposed upper mantle peridotite with little or no synrift extrusive material and limited amounts of synrift material intruded within the serpentinized peridotite

3d analogue modeling of the south china sea a discussion on breakup pattern the opening of the south china sea is one of the most important cenozoic events in se asia in order to investigate its tectonic evolution three analogue modeling experiments were compared the modeling results suggest that rifting pattern and orientation change of the rift zone were related to initial theological stratification affected by the ductile flow of silicone lower crust and honey asthenosphere the faults became flattened especially close to the breakup area we conjecture that the slope area may bear relatively hotter and thinner lithosphere than the shelf area from the beginning of rifting due to stretching and mantle upwelling associated with this regional extension which in turn lead to the change in initial rheological stratification and therefore the rifting pattern from shelf to slope in the experiments breakup developed first at isolated points which grew and coalesced to become a single spreading area the conjugate boundaries were either all concave or all convex where a rigid massif was located at the divergent boundary stretching history was different in this case the northern and southern areas thinned rapidly and developed into two deep troughs which may finally evolve into spreading centers the shape of the massif controls the orientation and boundary shape of the spreading area it is interpreted that crustal breakup was more viscous in style and the nw sub sea basin developed along the northern trough of the zhongsha xisha massif macclesfield bank c 2008 elsevier ltd all rights reserved

continent ocean transition on the voring plateau ne atlantic derived from densely sampled ocean bottom seismometer data 1 a 180 km long seismic wide angle profile was acquired across the voring plateau ne atlantic using a tuned air gun array and three component ocean bottom seismometers deployed with 5 km spacing the seismic p wave data have been modeled by ray tracing inversion and the model has been constrained by s wave and gravity modeling the data and modeling have allowed us to depict the crustal structure and nature of the continent ocean transition cot in a classical volcanic margin case the p wave velocity near the top of the main crustal layer is estimated to 6 0 km s landward of the 25 km wide cot the seaward increase to 6 5 km s in the cot is conformable with heavily intruded continental crust within this zone farther seaward the velocity increase to 6 9 km s in the same layer suggests the presence of oceanic crust the abundant magmatism landward of and within the cot is primarily observed as extrusives forming wedges of seaward dipping reflectors and mafic lower crustal intrusions underplating the maximum thickness of the oceanic crust is measured to 23 5 km at the seaward termination of the cot

deep seismic structure of the northeastern south china sea origin of a high velocity layer in the lower crust we present a 2 d seismic tomographic image of the crustal structure along the obs2012 profile which delineates the moho morphology and magmatic features of the northeastern south china sea margin the image was created by forward modeling rayinvr and traveltime tomographic inversion tomo2d overall the continental crust thins seaward from 27 km to 21 km within the continental shelf across the zhu i depression and dongsha rise with slight local thickening beneath the dongsha rise accompanying the increase in the moho depth the dongsha rise is also characterized by 4 7 km thick high velocity layer hvl 7 0 7 6 km s in the lower crust and exhibits a relatively high velocity 5 5 6 4 km s in the upper crust with a velocity gradient lower than those of the zhu i depression and tainan basin across the continental slope and continent ocean transition cot which contain the tainan basin the crust sharply thins from 20 km to 10 km seaward and a 2 3 km thick hvl is imaged in the lower crust we observed that volcanoes are located only within the cot but none exist in the continental shelf the dongsha rise exhibits a high magnetic anomaly zone and different geochemical characteristics from the cot based on those observations we conclude that the hvl underlying the cot is probably extension related resulting from the decompression melting in the cenozoic whereas the hvl beneath the dongsha rise is probably arc related and associated with the subduction of the paleo pacific plate these findings are inconsistent with those of some previous studies

three dimensional magnetic structure and genesis of the oceanic crust of the northern south china sea abstract the north continental margin of the south china sea is an area conducive to examining the deep structure tectonic evolution and dynamic mechanisms of this region as there is an influence of oblique magnetization in mid latitude areas on the magnetic data pertaining to this area this study develops and validates an inversion method for magnetic anomalies that accounts for inclination and declination the inversion of the total magnetic intensity t in the northern south china sea was then used to reveal the distribution of magnetic bodies in this area the inversion results show that there is significant tectonic zoning in the area under study with north to south and east to west belts showing a pattern consistent with the main tectonic direction in this area there are also distinct differences between the high magnetic anomaly belt in the continental margin and the magnetic inversion features of the magnetic quiet zone in the south combined with ocean bottom seismology profiles and geothermal data the observed phenomena may be closely related to magmatic underplating activity in the deep crust caused by plate subduction

shear wave velocity structure of the shallow sediments in the bohai sea from an ocean bottom seismometer survey abstractto investigate the seismic velocity structure of the shallow sediments in the bohai sea of china we conducted a shear wave velocity inversion of the surface wave dispersion data from a survey of 12 ocean bottom seismometers obss and 377 shots of a 9000 in3 air gun with obs station spacing of approximately 5 km and air gun shot spacing of approximately 190 m high quality scholte wave data were recorded by the obss within 0 4 5 km offset we retrieved the scholte wave phase velocity dispersion for the fundamental mode and first overtone in the frequency band of 0 9 3 0 hz with the phase shift method and inverted for the shear wave velocity structure of the shallow sediments with a damped iterative least squares algorithm pseudo 2d shear wave velocity profiles with a depth of approximately 400 m revealed coherent features of relatively weak lateral velocity variation we also estimated the uncertainty in shear wave velocity structure based on the pseudo 2d profiles from six trial inversions wi

crustal structure across the xisha trough northwestern south china sea located at the northwestern part of the south china sea scs between the hainan and xisha paracel islands the xisha trough represents a failed rift in conjunction with the opening of the scs between 32 and 17 ma from west towards east within a scale of several hundred kilometers it presents all major stages of the rifting process and thus provides an ideal place to study the rifting process in great details in the autumn of 1996 a joint team of sino german scientists carried out a wide angle seismic experiment across the xisha trough with 10 ocean bottom hydrophones obh along a 237 km nnw sse oriented profile which was surveyed in 1987 with multi channel seismic mcs method by bgr of germany and soa of china favorable weather conditions and the powerful 4 x 12 1 air gun array rendered very good quality data with seismic signals observed at the offset of up to 110 km a detailed velocity depth model was obtained by using an interactive trial and error 2d ray tracing method interpretation of the nics data published by bgr provides very good geometrical constraints of the complex upper crustal structure which is characterized by fault blocks half horsts and half grabens filled with syn and post rift cenozoic sediments the velocity model in turn confirms the major structure outlined by the interpretation of the mcs data showing a varying sedimentary layer between i and 4 km of thickness and velocities between 1 7 and 4 5 km s the p wave velocity of 5 5 km s on the top of the crystalline basement is relatively low suggesting strong weathering within the crystalline crust the velocity increases downward continuously to 6 8 km s at the bottom of the crust without a clear differentiation in the middle crust showing clearly its continental nature even beneath the xisha trough the moho is marked by a sharp first order interface with a velocity of 8 0 8 1 km s at the uppermost mantle the moho depth is 15 km beneath the center of the trough and increases gradually to more than 25 kin towards north and south corresponding to an extreme thinning of the pre rift continental crust from more than 25 kin under the coast line to only 8 kin beneath the xisha trough the similar velocity structure of the continental nature on both sides of the xisha trough suggests a homogeneous pre rift continental setting the sharp moho and the lack of high velocity body hvb in the lower crust imply no magmatic underplating which is very different to interpretations across the eastern part of the continental margin in northern south china sea the intense faulting in the upper crust the strong but rather symmetrical crustal thinning centered at the xisha trough and the close neighborhood to the open scs nw subbasin suggest a pure shear rifting which failed most likely at the phase shortly before the continental breakup c 2001 elsevier science b v all rights reserved

the continent ocean crustal transition across the southwest greenland margin this paper examines the complete crustal transition across the nonvolcanic southwest greenland continental margin of the labrador sea using wide angle and coincident vertical incidence seismic profiles six ocean bottom seismometers and a sonobuoy record p and s wave first and multiple arrivals from the crust and upper mantle which are analyzed by two dimensional dynamic ray tracing and one dimensional reflectivity modeling the resulting seismic velocity model requires that the preexisting 30 km thick continental crust is thinned abruptly to 3 km across the continental slope primarily by removal of the lower crust farther seaward the crust thickens to 6 km primarily through the addition of a high velocity 7 0 7 6 km s layer in the lower crust this lower crustal layer is 4 5 km thick extends for a horizontal distance of 80 km and is interpreted as partially serpentinized upper mantle it is overlain by a low velocity 4 0 5 0 km s upper layer which is interpreted as highly fractured upper continental crust our model suggests that seafloor spreading did not start until chrons 27 28 13 ma younger than previously suggested this interpretation is supported by two dimensional modeling of gravity and magnetic data along the refraction line our results are consistent with a simple shear mechanism for the initial rifting with the sw greenland margin as the upper plate however a full characterization of the rifting mechanism must await comparison with a seismic model for the conjugate margin east of labrador

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

a regional shear wave velocity model in the central voring basin n norway using three component ocean bottom seismographs abstract a regional ocean bottom seismometer obs dataset was acquired in the central and northern part of the voring basin n norway in 1992 thirty three component obss were deployed along seven regional profiles and the shear waves recorded on the horizontal components have been modelled by use of 2 d ray tracing utilizing an established p wave model in the shallow sediments 1 4 km depth mean vp vs ratios of 2 1 2 2 and 2 3 2 5 are inferred along the nw se and ne sw trending profiles respectively the relatively high mean vp vs ratios in this interval are probably related to unconsolidated sediments in the uppermost layers the azimuthal variation in the vp vs ratios with the fast direction nw se indicates a shear wave anisotropy of 5 10 in this interval consistent with observations further north in the lofoten area in the 5 10 km depth interval high velocity sill intrusions in the sediments are mapped by the p wave modelling these high velocity intrusions act as efficient conversion interfaces confirmed by the shear wave modelling a regional decrease in the vp vs ratios to 1 7 1 8 are observed at 4 6 km depth possibly related to sand rich sediments in the middle cretaceous the upper crust is clearly mapped by the shear waves and the inferred vp vs ratio of 1 75 confirms that the crust in the voring basin is of continental origin an increase of the vp vs ratio 1 8 1 85 in the lower crust indicates a change in its composition possibly explained by a mixing of continental crust and magmatic underplating

seismotectonic characteristics of the taiwan collision manila subduction transition the effect of pre existing structures abstract based on the distribution of an earthquake swarm determined from an ocean bottom seismometer obs network deployed from 20 august to 5 september 2015 in the northernmost part of the south china sea scs and data from inland seismic stations in taiwan we resolved a ml 4 1 earthquake occurring on 1 september 2015 as a ne sw trending left lateral strike slip event that ruptured along the pre existing normal faults generated during the scs opening phases the direction of the t axes derived from the ml 4 1 earthquake and the background seismicity off sw taiwan present high consistency indicating a stable dominant nw se tensional stress for the subducted eurasian plate eup the distinct stress variations on the two sides of these reactivated ne sw trending features suggest that the presence of pre existing normal faults and other related processes may lessen the lateral resistance between the taiwan collision and manila subduction system and facilitate the slab pull process for the subducting portion which may explain the nw se tensional stress environment near the transition boundary in the northernmost part of the scs