throughgoing crustal faults along the northern margin of the south china sea and their role in crustal extension a study of the rifled northern margin of the south china sea based on new single ship and two ship multichannel seismic data has revealed the presence of highly reflective steeply dipping normal faults which cut through the entire crust these faults with dips of 25 40 are associated with localized crustal thinning of 5 15 km and represent horizontal extension of the order of tens of kilometers a range of 5 20 km of horizontal extension has been measured along individual faults although the relative amount of horizontal extension taken up by these faults is modest compared to the total extension across the margin only about 15 or less the amount of localized crustal thinning which takes place along these fault surfaces is quite significant at one fault location the crustal thickness is reduced by 40 although the faults described have been recognized at three separate locations along the margin their trends appear to differ at each in two examples the faults are oblique to the inferred general north south extension direction of the margin this characteristic taken with the fact that some of the crustal faults also appear to serve as abrupt boundaries between broad regions of differing crustal thickness and reflectivity of the lower crust suggests that at least some of these faults were present prior to rifting such faults are postulated to have been former thrust faults perhaps associated with a triassic suture zone and or cretaceous subduction margin which have been reactivated as normal faults during subsequent rifting alternatively some of the throughgoing faults may represent crustal detachment surfaces that first formed as the result of a simple shear response to crustal extension

frictional viscous flow of phyllosilicate bearing fault rock microphysical model and implications for crustal strength profiles it is widely believed that around the brittle ductile transition crustal faults can be significantly weaker than predicted by conventional two mechanism brittle ductile strength envelopes factors contributing to this weakness include the polyphase nature of natural rocks foliation development and the action of fluid assisted processes such as pressure solution recently ring shear experiments using halite kaolinite mixtures as an analogue for phyllosilicaterich rocks for the first time showed frictional viscous behavior i e both normal stress and strain rate sensitive behavior involving the combined effects of pressure solution and phyllosilicates this behavior was accompanied by the development of a mylonitic microstructure a quantitative assessment of the implications of this for the strength of natural faults has hitherto been hampered by the absence of a microphysical model in this paper a microphysical model for shear deformation of foliated phyllosilicate bearing fault rock by pressure solution accommodated sliding along phyllosilicate foliae is developed the model predicts purely frictional behavior at low and high shear strain rates and frictional viscous behavior at intermediate shear strain rates the mechanical data on wet halite kaolinite gouge compare favorably with the model when applied to crustal materials the model predicts major weakening with respect to conventional brittle ductile strength envelopes in particular around the brittle ductile transition the predicted strength profiles suggest that in numerical models of crustal deformation the strength of high strain regions could be approximated by an apparent friction coefficient of 0 25 0 35 down to depths of 15 20 km

motion of the pacific plate relative to eurasia and its potential relation to cenozoic extension along the eastern margin of eurasia extension along the eastern margin of eurasia has been regarded commonly as a far field effect of the india eurasia collision however some aspects of the timing and location of this extension make a link between the collision and extension difficult we suggest that some extensional features commonly interpreted as effects of the collision may be related more simply to changes in plate convergence rates along the eastern plate boundary of eurasia an analysis of the motion of the pacific plate relative to eurasia suggests that the rate of pacific eurasia convergence varied significantly during the tertiary period from a late cretaceous convergence rate of 120 140 mm yr the rate declined substantially during early tertiary time and reached a minimum in eocene time of 30 40 mm yr in oligocene to earliest miocene time the average convergence rate increased moderately to 70 95 mm yr then decreased again to 65 70 mm yr during early to middle miocene time from late miocene to the present the rate of convergence increased to an average of 100 110 mm yr the paleocene through middle miocene episode of relatively slow convergence correlates with a period of widespread extension along the eastern margin of eurasia decreased convergence may have been related to a net reduction in horizontal compressional stress transmitted between the pacific and eurasian plates which resulted in widespread extension adjacent to the margin of eurasia

the structure of southwest taiwan the development of a fold and thrust belt on a margins outer shelf and slope d b j a m and c b acknowledge funding provided by the spanish ministerio de economia y competividad grant cgl2013 43877 p h k c acknowledges funding by most 104 2628 m 008 005 my3

the structure evolution and symmetry of the magma poor rifted margins of the north and central atlantic a synthesis abstract magma poor rifted margins consistently show extreme crustal thinning accompanied by normal faulting the serpentinization of the mantle beneath crust thinned to less than 8 2 km and the unroofing of a broad zone of mantle within the continent ocean transition accompanied by the development of detachment and other large offset faults these observations are the logical result of the progressive extension of cool lithosphere away from thermal anomalies such as plumes although the paucity of magmatism may be explained by depth dependent extension of the lithosphere and by pre depleted sub crustal lithosphere rifting above initially cool sub lithospheric mantle also may explain the subsidence deficit observed at some margins synrift subsidence is buffered by the simultaneous influx of warmer oceanic asthenosphere as it extends thins and cools ductile creeping layers in the mid and deep crust become progressively more brittle resulting in increased coupling between the upper and lower crust and eventually the embrittlement of the entire crust faults cutting from the surface across the moho bringing water into the mantle and causing its serpentinization increased coupling and the development of serpentine detachments predict the development of late stage asymmetry once the entire crust is brittle such detachments are imaged on some margins and inferred on others analysis of the crustal structure across conjugate margins shows that these are approximately symmetric until this late stage when they become markedly asymmetric similar analyses show that depth dependent stretching of the crust is insufficient to explain the discrepancy between the amount of the visible extension along faults and the amount of crustal thinning instead this extension discrepancy may be related to the complex evolution of brittle deformation through multiple phases and styles of faulting related to the changes in the rheological character and strength of the lithosphere as it is thinned complex polyphase faulting continues after complete crustal separation resulting in the exhumation of broad expanses of peridotitic basement the top of which is everywhere marked by an exhumed slip surface similar to the corrugated surface observed at mid ocean ridges the similarity in processes between mantle unroofing and seafloor spreading makes the distinction between the cot and true oceanic crust difficult and possibly moot

the snake range decollement an exhumed mid tertiary ductile brittle transition the snake range decollement srd in east central nevada separates supracrustal rocks extended by normal faulting from ductilely deformed igneous and metamorphic rocks a well known stratigraphy unaffected by earlier faulting permits analysis of both upper and lower plate strain leading to a better understanding of how vastly different rock types and deformational styles are juxtaposed along low angle faults in metamorphic core complexes middle cambrian to permian upper plate rocks are cut by two generations of ne trending east directed normal faults both generations were initiated as high angle 60 planar faults that flattened abruptly into the srd and rotated domino style to low angles yielding a total rotation of bedding of about 80 90 faulting is tertiary in age as 35 m y old volcanic rocks are involved and resulted in about 450 500 extension in a n55w s55e direction the srd developed as a subhorizontal surface 6 7 km deep at the top of the cambrian pioche shale lower plate granitic rocks and their late precambrian cambrian metamorphic country rocks were involved in progressive ductile to brittle extension at low greenschist grade forming a penetrative subhorizontal foliation and n55 70w lineation that increases in intensity eastward and upward toward the srd stretching and thinning in the lower plate is coaxial and comparable in magnitude to upper plate extension and is interpreted as synchronous k ar ages ranging from 20 to 40 m y in the lower plate suggests the n snake range represents a tertiary thermal anomaly we conclude that the srd developed as a ductile brittle transition zone at 6 7 km depth gravity data suggests that the gently domed srd is cut by younger basin and range faults but the geology of adjacent ranges suggests that the srd does not continue more than 60 km in any given direction the lack of stratigraphic omission across the srd rules out large amounts of movement on a surface that originally cut downsection and we suggest that extensional detachment faults such as the srd can be developed locally as boundaries between brittlely extended rocks and underlying ductile extension and intrusion

crustal structure of mainland china from deep seismic sounding data abstract since 1958 about ninety seismic refraction wide angle reflection profiles with a cumulative length of more than sixty thousand kilometers have been completed in mainland china we summarize the results in the form of 1 a new contour map of crustal thickness 2 fourteen representative crustal seismic velocity depth columns for various tectonic units and 3 a pn velocity map we found a north south trending belt with a strong lateral gradient in crustal thickness in central china this belt divides china into an eastern region with a crustal thickness of 30 45 km and a western region with a thickness of 45 75 km the crust in these two regions has experienced different evolutionary processes and currently lies within distinct tectonic stress fields our compilation finds that there is a high velocity 7 1 7 4 km s layer in the lower crust of the stable tarim basin and ordos plateau however in young orogenic belts including parts of eastern china the tianshan and the tibetan plateau this layer is often absent one exception is southern tibet where the presence of a high velocity layer is related to the northward injection of the cold indian plate this high velocity layer is absent in northern tibet in orogenic belts there usually is a low velocity layer lvl in the crust but in stable regions this layer seldom exists the pn velocities in eastern china generally range from 7 9 to 8 1 km s and tend to be isotropic pn velocities in western china are more variable ranging from 7 7 to 8 2 km s and may display azimuthal anisotropy

anisotropic rayleigh wave phase velocity maps of the sunda plate abstract we explore the rayleigh wave phase velocity and shear velocity structures beneath sunda plate over a broad period range of 30 150 s and 5 100 km the two stations technique was used to measure dispersion curves from the waveforms of 19 233 teleseismic events 1998 2018 recorded worldwide by 75 broadband seismic stations spread around the sunda plate the interstation path specific dispersion curves are then inverted for high resolution isotropic and azimuthally anisotropic phase velocity maps in the frequency range 30 150 s as well as for shear velocity model between 5 and 100 km main findings include a strong dichotomy with fast velocities located beneath the south china sea and the sunda shelf compared with slow velocities found beneath the surrounding regions at short periods our model also exhibits very slow velocities beneath the hainan plume at lithospheric and asthenospheric depths the complex anisotropy in the region at shorter periods contrasts with the pattern observed at longer periods sampling the asthenosphere and is consistent with the absolute plate motions in the region

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

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

how do deep water volcanoes grow abstract deep water volcanoes are emplaced in water depths 1 0 km and are widespread along continental margins and in ocean basins whilst the external morphology of deep water volcanoes can be mapped using bathymetric surveys their internal structure and true volume remain enigmatic it is thus difficult to determine how deep water volcanoes grow we investigate 13 late miocene to quaternary deep water volcanoes that are imaged in 3d by seismic reflection data from the northern south china sea which allow us to quantify their external morphology and examine their internal structure these deep water volcanoes were emplaced in water depths 1 5 km are relatively small 15 mpa which are typical of water depths 1 5 km inhibited degassing and fragmentation of ascending magma and thus erupted lava this lack of degassing and fragmentation permitted effusive eruptions during the latter stages of volcanism our models for volcano growth in the deep submarine realm demonstrate the power of using 3d seismic data when investigating the internal structure and total volume of deep water volcanoes

the continental extension discrepancy and anomalous subsidence pattern in the western qiongdongnan basin south china sea abstract passive margins are first order tectonic features on earth formed by polyphase extension and breakup of continental lithosphere it is accepted that mildly extended basins in the proximal margin form largely by uniform continental thinning but the continental extension in deepwater is much debated here we choose a line from the western qiongdongnan basin in the magma poor south china sea margin to further explore the style of continental extension and the corresponding subsidence patterns where high resolution seismic and heat flow data are available structural analysis reveals that the basin architecture is extremely asymmetric and mainly controlled by the se directed normal faults soling into a low angle detachment fault however fault timings nicely show that the faults form through polyphase rifting and from inside outwards and do not follow a simple sequential faulting like the west iberia margin where the upper and lower crustal deformation are strongly coupled perhaps the fault order here reflects a much weaker and decoupled lower crust then we divide the basin into five zones and calculate the average fault derived extension factors moreover the whole crustal extension factors are also estimated after assessing the initial crustal thickness on basis of the backstripped subsidence and the lithospheric extension factors are numerically modeled using a coupled lithosphere and basin scale model constrained by the heat flow and sedimentation data results show that along the profile the overall lithospheric extension is much less than that of the whole crust and only at the south end of the profile does lithospheric extension exceeds the whole crust showing lithospheric depth dependent thinning the whole crustal extension is roughly equal to the faults derived extension in the northern part displaying no extension discrepancy in the middle part the whole crustal extension is less than the faults derived extension denoting inverse extension discrepancy while the whole crustal extension is in far excess of the faults derived extension in the southern part showing obvious positive extension discrepancy we infer that the inverse and positive discrepancy of crustal extension was likely to be caused by landward directed lower crustal flow driven by the horizontal pressure arising from the combination of the differential extension in the faulted upper crust in the mantle lithosphere and the differential sedimentation during the syn rift period

lower crustal intrusion on the north atlantic continental margin near the faroe islands in the north atlantic the majority of basalt is intruded into the continent ocean transition rather than extruded onto the surface this melt is intruded into the lower crust as sills which cross cut the continental fabric rather than as an underplate of 100 melt as has often been assumed when continents break apart the rifting is sometimes accompanied by the production of large volumes of molten rock1 2 3 the total melt volume however is uncertain because only part of it has erupted at the surface furthermore the cause of the magmatism is still disputed specifically whether or not it is due to increased mantle temperatures we recorded deep penetration normal incidence and wide angle seismic profiles across the faroe and hatton bank volcanic margins in the northeast atlantic here we show that near the faroe islands for every 1 km along strike 360 400 km3 of basalt is extruded while 540 600 km3 is intruded into the continent ocean transition we find that lower crustal intrusions are focused mainly into a narrow zone 50 km wide on the transition although extruded basalts flow more than 100 km from the rift seismic profiles show that the melt is intruded into the lower crust as sills which cross cut the continental fabric rather than as an underplate of 100 per cent melt as has often been assumed evidence from the measured seismic velocities and from igneous thicknesses are consistent with the dominant control on melt production being increased mantle temperatures with no requirement for either significant active small scale mantle convection under the rift or the presence of fertile mantle at the time of continental break up as has previously been suggested for the north atlantic ocean4 5 6

structure of the se greenland margin from seismic reflection and refraction data implications for nascent spreading center subsidence and asymmetric crustal accretion during north atlantic opening 1 seismic reflection and refraction data from the se greenland margin provide a detailed view of a volcanic rifted margin from archean continental crust to near to average oceanic crust over a spatial scale of 400 km the sigma iii transect located 600 km south of the greenland iceland ridge and the presumed track of the iceland hot spot shows that the continent ocean transition is abrupt and only a small amount of crustal thinning occurred prior to final breakup initially 18 3 km thick crust accreted to the margin and the productivity decreased through time until a steady state ridge system was established that produced 8 10 km thick crust changes in the morphology of the basaltic extrusives provide evidence for vertical motions of the ridge system which was close to sea level for at least 1 m y of subaerial spreading despite a reduction in productivity from 17 to 13 5 km thick crust over this time interval this could be explained if a small component of active upwelling associated with thermal buoyancy from a modest thermal anomaly provided dynamic support to the rift system the thermal anomaly must be exhaustible consistent with recent suggestions that plume material was emplaced into a preexisting lithospheric thin spot as a thin sheet exhaustion of the thin sheet led to rapid subsidence of the spreading system and a change from subaerial to shallow marine and finally to deep marine extrusion in 2 m y is shown by the morphological changes in addition comparison to the conjugate hatton bank shows a clear asymmetry in the early accretion history of north atlantic oceanic crust nearly double the volume of material was emplaced on the greenland margin compared to hatton bank and may indicate east directed ridge migration during initial opening

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

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

origin of the west taiwan basin by orogenic loading and flexure of a rifted continental margin 1 seismic and well data suggest that the west taiwan basin developed by orogenic loading and flexure of a rift type continental margin the most likely source of the loading is taiwan where oblique convergence between the eurasian and philippine sea plates has produced an orogenic belt up to 300 km in length 100 km in width and 4 km in height flexure modeling shows that surface loading is unable to explain the depth of the west taiwan basin other subsurface or buried loads are required combined surface and buried loading explains the depth and width of the basin it also accounts for a bouguer gravity anomaly high and flanking low over the orogenic belt a lateral offset of 20 30 km between the peak topography and the maximum depth to the seismic moho and evidence for tectonic uplift in the penghu islands the depth of the base of the foreland sequence in the northern part of the west taiwan basin can be explained well by an elastic plate model with an effective elastic thickness te of 13 km while this value is low when compared to most other foreland basins it is within the range of values derived from rifted continental margins the northern part of the west taiwan basin unconformably overlies a passive margin sequence and therefore appears to have inherited the long term 1 myr flexural properties of the margin in the southern part of the basin however the depth to the base of the foreland sequence dips too steeply to be explained by elastic plate models this part of the basin therefore appears to be yielding rather than flexing differences in the flexural behavior along strike of the west taiwan foreland basin lithosphere are reflected in seismicity patterns west of the thrust front the northern part of the basin is associated with a low level of seismic activity while the south correlates with an abundance of earthquakes especially at shallow 25 km depths there is a cluster of earthquakes along two extensional faults that were active during rifting of the underlying margin therefore lithospheric flexure and fault reactivation may be important contributors to the seismicity of the taiwan region

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

seismic characteristics and evolution of post rift igneous complexes and hydrothermal vents in the lingshui sag qiongdongnan basin northwestern south china sea abstract the study of morphology distribution and characteristics of igneous complexes has great significance to the understanding of magma plumbing processes geodynamics and tectonic evolution of continental margins previous studies concentrated partly on the magma rich rifted basins where the lateral magma transport mainly affects the igneous complexes connection and distribution however due to seismic wave shielding effects of the large shallow magmatic bodies the underlying igneous complexes and their corresponding magma plumbing systems in the magma poor rifted margins are still in debate in this study 2d 3d seismic data and well data are utilized to describe the morphology architecture and spatial temporal distribution of igneous complexes in the lingshui sag of the qiongdongnan basin northwestern south china sea margin the identified igneous complexes include 98 intrusive sills and feeder dykes beneath some of the isolated sills twenty six cone shaped mounds that overlie intruded sills through internal disturbed conduits were also described drilled well samples and seismic expressions suggest that these mounds are hydrothermal vents a uniform bottom mounds horizon of these vents suggests that they probably formed at the same time constrained by biostratigraphic data and sedimentation rate of underlying and overlying sedimentary layers the magma emplacement was dated to the middle miocene ca 14 6 ma most of the hydrothermal vents are distributed along the f2 fault zone and have direct linkage with the underlying sills while the large sill complexes that are connected with limited vents are mainly present above the hyperextended continental crust where the crust thins to 6 10 km the sills intruded into different layers from the lower oligocene to the lower miocene and the emplaced depth of sills is 1 2 6 3 km whether or not they feed any vents above unlike most of the large volume and laterally linked sills found in the magma rich rifted margins the scattered distribution of sills at different levels indicates that dykes probably play an important role in magma transport which might coexist with numerous polygonal or small faults and interference reflections this work highlights the critical role of basin structures in controlling the distribution of post rift igneous complexes in magma poor margins including thinned continental crust sedimentary thickness and faults

rapid transition from continental breakup to igneous oceanic crust in the south china sea continental breakup represents the successful process of rifting and thinning of the continental lithosphere leading to plate rupture and initiation of oceanic crust formation magmatism during breakup seems to follow a path of either excessive transient magmatism magma rich margins or of igneous starvation magma poor margins the latter type is characterized by extreme continental lithospheric extension and mantle exhumation prior to igneous oceanic crust formation discovery of magma poor margins has raised fundamental questions about the onset of ocean floor type magmatism and has guided interpretation of seismic data across many rifted margins including the highly extended northern south china sea margin here we report international ocean discovery program drilling data from the northern south china sea margin testing the magma poor margin model outside the north atlantic contrary to expectations results show initiation of mid ocean ridge basalt type magmatism during breakup with a narrow and rapid transition into igneous oceanic crust coring and seismic data suggest that fast lithospheric extension without mantle exhumation generated a margin structure between the two endmembers asthenospheric upwelling yielding mid ocean ridge basalt type magmatism from normal temperature mantle during final breakup is interpreted to reflect rapid rifting within thin pre rift lithosphere

continental rifting and sediment infill in the distal part of the northern south china sea in the western pacific region challenge on the present day models for the passive margins abstract well known difficulties in imaging structures and sediment infill in the distal margins covered by deepwater and thick sediment in the western pacific have limited our ability to interpret rifting to drifting processes of these margins we used a wealth of multi channel seismic profiles and drilling data to construct a regional stratigraphic framework for the cenozoic strata in the xingning jinghai subbasin which is now located to the north of the apparent oceanic crust in the south china sea eight sequence bounding unconformities segmented seven sequence stratigraphy can be confidently recognized in our survey using mostly standard criteria for identification the particular arrangement of strata elements in the syn rift succession presents two contrast geometry sedimentary packages whose pattern is different with that bounded by high angle faults on the continental shelf observed by the previous studies in the xingning jinghai subbasin strata pattern on the hangingwall of the low angle faults shows that the faults bounding the depocenters were initially formed by high angle faults which is similar to the present day high angle faults on the continental shelf subsequently the high angle faults in the deepwater region were rotated to be extensional detachment low angle faults which extremely thinned the crust from 33 km to 8 10 km the low angle detachment fault systems can be used to explain the magmatic addition during the post rift stage on the hyper extended crust in the northern margin the hyper extended crust of the south china sea with the characteristics of landward dipping low angle faults high velocity lower crust and moderately magmatism challenges the magma poor or magma rich models constructed from the atlantic margins

paleostress analysis as a key to margin extension the penghu islands south china sea abstract the penghu islands belong to the eurasian passive margin west of the collision zone of taiwan from south to north the south china sea basins become narrower and disappear in the taiwan strait area the presence of the penghu islands allow description of the tectonic evolution of the margin through the reconstruction of tectonic paleostress orientation based on analysis of fault slip data collected in the field brittle structures have been studied on four islands in neogene basalt flows with thin interbedded sediments sets of minor striated faults tension gashes dikes and joints enabled us to reconstruct the orientation of late cenozoic stresses two main extensional tectonic events were thus identified a n s extension dominated during the middle late miocene especially in the northern penghu islands and a more recent nw se extension prevailed during the late miocene especially in the southernmost island the latter extension induced complicated patterns of perpendicular normal fault systems and strike slip conjugate and transfer fault systems through permutations between principal stress axes 1 2 and 2 3 modes the ages of these tectonic events are ascertained by the existence of syndepositional normal faults dike injections and superposed brittle structures a third less important compressional tectonic event is correlated with the plio quaternary collision in adjacent taiwan ene wsw compression the comparison between the results of marine studies in the south china sea basins and our paleostress reconstruction in the penghu islands suggests that repeated changes between n s and nw se directions of extension occurred in the past the counterclockwise change from n s to nw se recorded in the penghu islands took place between about 12 and 8 ma ago indicating that significant extensional phenomena along the south china margin may postdate the seafloor spreading activity in the central ridge

toward a robust workflow for deep crustal imaging by fwi of obs data the eastern nankai trough revisited crustal scale imaging by the full waveform inversion fwi of long offset seismic data is inherently difficult because the large number of wavelengths propagating through the crust makes the inversion prone to cycle skipping therefore efficient crustal scale fwi requires an accurate starting model and a stable workflow minimizing the nonlinearity of the inversion here we attempt to reprocess a challenging 2d ocean bottom seismometer obs dataset from the eastern nankai trough the starting model is built by first arrival traveltime tomography fat which is fwi assisted for tracking cycle skipping we iteratively refine the picked traveltimes and then reiterate the fat until the traveltime residuals remain below the cycle skipping limit subsequently we apply laplace fourier fwi in which progressive relaxation of time damping is nested within frequency continuation to hierarchically inject more data into the inversion these two multiscale levels are complemented by a layer stripping approach implemented through offset continuation the reliability of the fwi velocity model is assessed by means of source wavelet estimation synthetic seismogram modeling ray tracing modeling dynamic warping and checkerboard tests although the visco acoustic approximation is used for wave modeling the synthetic seismograms reproduce most of the complexity of the data with a high traveltime accuracy the revised fwi scheme produces a high resolution velocity model of the entire crust that can be jointly interpreted with migrated images derived from multichannel seismic data this study opens a new perspective on the design of obs crustal scale experiments amenable to fwi however a further assessment of the optimal obs spacing is required for reliable fwi

subducted seamount imaged in the rupture zone of the 1946 nankaido earthquake the nankai trough is a vigorous subduction zone where large earthquakes have been recorded since the seventh century with a recurrence time of 100 to 200 years the 1946 nankaido earthquake was unusual with a rupture zone estimated from long period geodetic data that was more than twice as large as that derived from shorter period seismic data in the center of this earthquake rupture zone we used densely deployed ocean bottom seismographs to detect a subducted seamount 13 kilometers thick by 50 kilometers wide at a depth of 10 kilometers we propose that this seamount might work as a barrier inhibiting brittle seismogenic rupture

the deep structure of south central taiwan illuminated by seismic tomography and earthquake hypocenter data abstract the taiwan mountain belt is generally thought to develop above a through going basal thrust confined to within the sedimentary cover of the eurasian continental margin surface geology magnetotelluric earthquake hypocenter and seismic tomography data suggest however that crustal levels below this basal thrust are also currently being involved in the deformation here we combine seismic tomography and earthquake hypocenter data to investigate the deformation that is taking place at depth beneath south central taiwan in this paper we define the basement as any pre eocene rifting rocks and use a p wave velocity of 5 2 km s as a reference for the interface between these rocks and their sedimentary cover we found that beneath the coastal plain and the western foothills clustering of hypocenters near the basement cover interface suggests that this interface is acting as a detachment this detachment is located below the basal thrust proposed from surface geology for this part of the mountain belt inherited basement faults appear to determine the geometry of this detachment and their inversion in the alishan area result in the development of a basement uplift and a lateral structure in the thrust system above them however across the shuilikeng and the chaochou faults earthquake hypocenters define steeply dipping clusters that extend to greater than 20 km depth above which higher velocity basement rocks are uplifted beneath the hsuehshan and central ranges we interpret these clusters to form a deeply penetrating east dipping ramp that joins westward with the detachment at the basement cover interface it is not possible to define a basal thrust to the east beneath the central range

geodynamics of the south china sea abstract the beginning of seafloor spreading in the south china sea scs is now established from iodp drilling leg 349 at 33 ma chron 12 32 ma is the oldest chron identified in the scs the nature of the crust of the northeastern part of the scs located north of chron c12 where chrons 15 to 17 were previously identified is not oceanic but thinned continental crust intruded by volcanic elongated features emplaced 17 22 ma ago based on magnetic anomaly identifications the end of the scs spreading could be either 15 5 20 5 ma briais et al 1993 barckhausen et al 2014 or something else however as post spreading magmatic activity 13 3 5 ma largely masks the spreading fabric in particular near the axis of the east sub basin published locations of the axial magnetic anomaly extinct spreading axis and spreading rates are not reliable a contoured map of the extremely dense set of magnetic data shows that a few magnetic lineations belonging to the magnetic seafloor spreading fabric are still preserved and parallel to the n055 bathymetric seafloor spreading trends identified on swath bathymetric maps in the central part of the scs suggesting that the extinct ridge axis is n055 trending with potential n145 transform faults based on published swath bathymetric data oceanic domains with different seafloor spreading lineaments have been delimited n055 n075 and n085 and provide important constraints used to propose a kinematic sketch of the scs opening as a consequence the zhongnan faults zone located between the east and southeast sub basins acted as a major fracture zone system during the scs opening with horizontal offsets varying from 50 km to 140 km the flow line pattern defined from the seafloor spreading lineaments and the few identified fzs have been used to highlight conjugate segments of continental margins during the first phases of opening of the scs from the fit of continents to chron c10 30 ma the n175 extension observed in the xisha trough and eventually south of the macclesfield bank extends to the qui nhon ridge located along the eastern margin of vietnam in the southward prolongation of the red river fault system normal faults curve toward the south with a horsetail geometry interpreted as evidence for a few tens of kilometers of dextral motion along the qui nhon ridge since chron 10 and until the end of scs opening the plate boundary located between the southern south china sea sscs and eu plates jumped westward several times from the location of the ulugan fault near palawan to the western limit of the southwest basin explaining the progressive formation of the scs from east to west and giving the characteristic v shape of the scs the opening of the whole scs is linked and occurred simultaneously with the northward subduction of the proto scs whose suture is located south of palawan and extends westwards in north borneo

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

inversion of a hyper extended rifted margin in the southern central range of taiwan seismic reflection and wide angle data acquired across south and west of taiwan show that extended to hyper extended continental crust of the chinese continental margin is present more than 200 km south of the shelf and is subducting at the manila trench furthermore crustal scale tomographic velocity models show that this crust is underthrusted to 15 km depth below the accretionary prism where it then is structurally underplated to the base of the prism we document an increasing volume of accreted crust from south to north and in our northern transect high velocity material of the accretionary prism can be directly linked to outcrops of central range basement rocks in map view the central range of taiwan is clearly contiguous with the hengchun peninsula and hengchun submarine ridge to the south accordingly we propose a new model in which the central range forms directly from the accretionary prism including the basement core which originates from subducted and then accreted extended to hyper extended continental crust

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

crustal structure and contact relationship revealed from deep seismic sounding data in south china abstract a 400 km long wide angle seismic profile in the south china crosses three tectonic domains southeast continental margin of yangtze block northwest continental margin of cathaysia block and south china suture zone separating these two blocks the crustal velocity model constructed from traveltime fitting shows that the crust thickness thins from the northwest to the southeast along the profile it also reveals the crustal contact relationship between the yangtze and cathaysia blocks where the wuchuan sihui fault is the boundary between them combining this profile with two more profiles in the continental south china and three profiles in the northern margin of south china sea constructs a continent ocean transition section this cross section reveals that the moho depth shallows gradually along the cross section from the yangtze block to the cathaysia block and the northern margin of south china sea but shallows abruptly in the continent ocean transition to the south china sea

deep crustal structure of the conjugate margins of the sw south china sea from wide angle refraction seismic data abstract the south china sea is the largest marginal basin of se asia yet its mechanism of formation is still debated a 1000 km long wide angle refraction seismic profile was recently acquired along the conjugate margins of the sw sub basin of the south china sea over the longest extended continental crust a joint reflection and refraction seismic travel time inversion is performed to derive a 2 d velocity model of the crustal structure and upper mantle based on this new tomographic model northern and southern margins are genetically linked since they share common structural characteristics most of the continental crust deforms in a brittle manner two scales of deformation are imaged and correlate well with seismic reflection observations small scale normal faults grabens horsts and rotated faults blocks are often associated with a tilt of the velocity isocontours affecting the upper crust the mid crust shows high lateral velocity variation defining low velocity bodies bounded by large scale normal faults recognized in seismic reflection profiles major sedimentary basins are located above low velocity bodies interpreted as hanging wall blocks along the northern margin spacing between these velocity bodies decreases from 90 to 45 km as the total crust thins toward the continent ocean transition the continent ocean transitions are narrow and slightly asymmetric 60 km on the northern side and no more than 30 km on the southern side indicating little space for significant hyper stretched crust although we have no direct indication for mantle exhumation shallow high velocities are observed at the continent ocean transition the moho interface remains rather flat over the extended domain and remains undisturbed by the large scale normal faults the main decollement is thus within the ductile lower crust

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

lateral crustal variation and post rift magmatism in the northeastern south china sea determined by wide angle seismic data abstract the northeastern margin of the south china sea experienced continental rifting followed by magmatism in the cenozoic this evolution has strongly influenced the heterogeneity of its crustal structure to gain insights into the spatial variation in crustal structure data from 30 ocean bottom seismometers were used to forward model ten p wave velocity models along both dip and strike directions the cenozoic strata 1 7 3 5 km s the mesozoic strata 3 5 5 5 km s the crystalline crust 5 5 7 5 km s and high velocity lower crust 7 0 7 5 km s were identified based on p wave velocities overall the crust thins from 25 km on the continental shelf to 15 km on the slope with thicker crust underneath the dongsha basin a mesozoic basin but thinner crust underneath the pre cenozoic nw trending fault zones this difference in thickness is possibly controlled by different strength of inherited structures i e the dongsha basin and the fault zones furthermore a widespread high velocity lower crust layer which resulted from the intrusion of a mafic sill complex was observed over the shelf to slope mafic intrusions within the lower crust have uplifted the caprock on the shelf and magma eruptions have formed many post rift seamounts on the slope the evidence suggests that post rift magmatism overprinted the vertical crustal structure we therefore propose that the heterogeneous crustal structures in this passive margin are an integrated result of segmental extension followed by magmatic overprinting

continental lithospheric thinning and breakup in response to upwelling divergent mantle flow application to the woodlark newfoundland and iberia margins abstract depth uniform stretching is not the dominant deformation process for thinning continental lithosphere leading to breakup it cannot explain the observed depth dependent lithosphere stretching and mantle exhumation at rifted continental margins depth dependent lithosphere thinning in which stretching of the lower crust and lithosphere mantle greatly exceeds that of the upper crust has been observed at many non volcanic and volcanic rifted continental margins including conjugate margin pairs passive continental margins show a paucity of brittle deformation in the upper crust during continental lithosphere thinning leading to breakup and sea floor spreading initiation a new model of rifted continental margin formation has been developed that assumes that deformation and thinning of continental lithosphere leading to breakup occurs in response to an upwelling divergent flow field within continental lithosphere and asthenosphere and that this deformation evolves into sea floor spreading the new model successfully predicts depth dependent stretching of continental margin lithosphere for both non volcanic and volcanic margins and mantle exhumation at non volcanic margins both of which are observed but are not explained by existing depth uniform continental lithosphere stretching models the new model provides a balance of extensional strain supplies an explanation for the paucity of synrift brittle deformation and offers a simple transition from prebreakup lithosphere thinning to sea floor spreading the observed diversity of rifted continental margin structure and width of the ocean continent transition can be explained by variability in the form of the upwelling divergent flow field the new upwelling divergent flow model of continental lithosphere thinning leading to continental breakup successfully predicts the observed bathymetry and margin geometry for the most recent segment of sea floor spreading initiation in the woodlark basin in the western pacific and the observed bathymetry and free air gravity anomaly for the newfoundland and iberian margins

seismic evidence of hyper stretched crust and mantle exhumation offshore vietnam abstract we study the evolution of the eocene recent phu khanh basin opened during the rifting of the south china sea scs this sub basin formed when continental crust ruptured along the east vietnam boundary fault evbf at the western edge of the scs using high quality long streamer seismic lines we interpret structures that highlight the different phases of the scs rifting and processes related to crustal boudinage extreme crustal thinning and mantle uplift that sometimes places sediments in contact with the moho discontinuity mark the central part of the basin the mantle is shallowest there and marks the final rupture of the continental crust during an intense phase of mantle upwelling there a low angle detachment fault separates several crustal blocks from the moho the cylindrical axis of the moho rise is roughly parallel to the trend of the south china sea propagator above the mantle the upper and lower crusts form large crustal boudins the network of normal faults is dense in the upper crust and occasionally propagates into the lower crust however the lower crust is missing at some places the seismic facies above the moho rise is poorly stratified and might have been affected by a certain degree of metamorphism at the apex of mantle uplift there are frequent indications of fluid circulations including volcanic edifices and gas escapes features three stages of extension are clearly identifiable with ages of the two youngest constrained by well calibration the first and oldest rift sequence is situated between the tilted pre rift basement and the oligocene horizons 32 ma the second is delimited by the oligocene to the mid miocene 15 5 ma horizons and the third is bound by the mid miocene and the upper miocene before 10 5 ma horizons these three rift episodes formed in at least two extension directions the first n s and the second nw se the distinct mid miocene 15 5 ma horizon is tilted and the above layers show a diverging reflection these are in turn sealed by an erosional unconformity before 10 5 ma although tectonic activity appears diachronous from north to south we suggest that cessation of rifting did not occur before 12 10 5 ma this differs from models derived from magnetic anomalies observed in the south china sea 15 5 to 20 ma

geometry internal architecture and evolution of buried volcanic mounds in the northern south china sea abstract high resolution seismic and borehole data were used to characterize a volcanic mound in the northern south china sea including its internal architecture lithological characteristics and evolutionary history on seismic profiles the mound shows a convex geometry similar to an isolated carbonate buildup it was therefore previously seen as a carbonate buildup and a significant hydrocarbon target until an industrial well by7 1 proved its volcanic nature ten seismic facies are identified to analyze the internal architecture of the mound under the constraint of borehole data petrologic analysis based on cuttings and thin sections reveals that the mound is composed of volcanic tuffs siliciclastics carbonates and basalt layers depositions and architecture of the mound are interpreted as a result of an interplay between tephra accumulation during volcanic eruptions wave induced re deposition of volcaniclastics when the volcanic mound emerged above sea level and subsequent siliciclastic carbonate deposition when the volcanic mound was flooded during the phases of volcanic quiescence furthermore the correlation of seismic and well data allows us to reconstruct the entire evolutionary history of the mound from its formation to its demise and final burial this work presents a detailed description of the mound architecture and its genesis and thus provides an analogue for detailed interpretations of similar structures encountered during petroleum explorations in the south china sea or elsewhere

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

evidence for a thick oceanic crust adjacent to the norwegian margin the oceanic crust created during this first few million years of accretion in the norwegian greenland sea lies at an unusually shallow depth for its age has a smooth upper surface and in many places the results of multichannel seismic reflection profiling reveal that its upper layers comprise a remarkable sequence of arcuate seaward dipping reflectors these have been attributed to lava flows generated during a brief period of subaerial seafloor spreading we describe the results of inversions of digitally recorded sonobuoy measurements and two ship expanded spread profiles collected over the oceanic crust adjacent to the norwegian passive margin we find that the crust of the deep lofoten basin is indistinguishable from normal oceanic crust in thickness and structure closer to the margin we observe up to a four times expansion in thickness of layers with velocities equal to those of oceanic layer 2 while the layer 3 region retains approximately the same thickness the area over which the seaward dipping reflectors can be observed on reflection profiles corresponds to the region of greatest expansion in layer 2 thickness in the very oldest crust immediately adjacent to an escarpment that probably marks the continent ocean boundary we see evidence for a low velocity zone overlying an indistinct reflector that may mark the dyke lava interface in the thick crust comparing the structure of the thick crust to that of eastern iceland we find a strong resemblance especially in the expansion in thickness of material with layer 2 velocities these results support the suggestion that during the earliest stages of spreading extrusive volcanism at the ridge crest was unusually voluminous building a thick pile of lavas erupted from a subaerial spreading center

cenozoic plate reconstruction of the south china sea region abstract reconstructions of the south china sea region at 60 ma 40 ma 30 ma 20 ma 10 ma and 5 ma are presented we have attempted to place the south china sea basin in a regional tectonic framework the tectonic evolution of the major blocks surrounding the south china sea were analyzed as well as the relative motions of the indian and australian plates we have tried to correct the tectonic models available in this region a 3 d graphics terminal was used to derive rotation poles for the different tectonic blocks and our model was then tested to determine its self consistency when the model conflicted with previous interpretations the input data were evaluated for alternative explanations at least two and possibly three stages of extension can be recognized in this region the earliest one active in the late cretaceous to eocene involved nw se extension the second one active from the late eocene to early miocene involved north south extension the third stage of extension which probably trended nw se can be dated as post oligocene the first extensional event produced the ne sw trending proto south china sea and a series of sedimentary basins along the south china margin following the southeastward extrusion of indochina the proto south china sea was mostly consumed at the palawan trough renewed north south extension in the south china continental margin started the present day south china sea spreading in the oligocene the southeastward extrusion of indochina blocked by sundaland resulted in the nw se trending opening of the south china sea basin in the early miocene collision of the north palawan microcontinental block with the west philippines block stopped the opening of the south china sea at the end of early miocene spreading activity switched to the sulu sea basin in the middle miocene but collision between the sulu ridge and the west philippines at mindanao halted the opening of the sulu sea at the end of the middle miocene in the late miocene greater india continued its northward path and seems to have ripped open the andaman sea in the pliocene subduction along the northern manila trench placed the north luzon arc on a collision path with the east asia continental margin at taiwan our reconstructions along with detailed geological and geophysical information may be used as a predictive tool for basin evolution models and block interactions in this region the development of the south china sea basin the gulf of thailand the malay basin and the central thailand basins are the result of collision induced extensional forces the sulu celebes and sumatra basins were formed as a consequence of prolonged subduction the opening of the pearl river mouth west natuna south china sea sulu and possibly celebes basins were terminated by various plate collisions during the course of plate reorganizations major boundary faults have changed their slip senses during different stages of evolution

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

seismic imaging of a mid crustal low velocity layer beneath the northern coast of the south china sea and its tectonic implications abstract low velocity layer lvl within the continental crust is usually closely associated with tectonic events thus it may reflect ongoing processes or offer a chance to backtrack the evolutional history mid crustal lvl has been reported in the northern coast of the south china sea scs by several geophysical studies but the 3d distribution of lvl in the whole coastal area is still poorly known here we perform a receiver function study based on 67 broadband seismological stations we use the common conversion point method to construct structures of the moho as well as intra crustal interfaces we image sharp intra crustal negative velocity gradient broadly distributed in the eastern cathaysia block with depth increasing from 10 km in the nw to 12 16 km in the se the corresponding amplitude of the negative interface shows variable values from 0 03 to 0 1 with an evidential trend of strengthening eastwards we interpret the negative gradient as top of the lvl produced by accumulation of fluids from dehydration of the paleo pacific subduction plate the mid crustal lvl tends to act as weak domains during continental rift and the lateral variation of the lvl may be one possible reason explaining the along strike variation in crustal stretching in the northern scs rifted margin

reply to chang et al 2014 evolution of the south china sea revised ages for breakup and seafloor spreading abstract chang et al introduce some new lines of evidence for an end of seafloor spreading in the south china sea not before 15 ma however we find strong indications that their interpretation is not conclusive the origin of the east taiwan ophiolite from the scs as assumed by chang et al is not in agreement with recent studies which find continental crust in the deep water domain of the northeastern south china sea the age dating of the east taiwan ophiolite is based on one single k ar age of questionable accuracy the calculation of te derived ages leaves a thermal rejuvenation of the oceanic crust in the south china sea by post spreading volcanism out of consideration biostratigraphically determined ages of sediments recovered directly above igneous crust of the south china sea during iodp 349 drilling expedition are inconclusive so far the final key to unlocking the spreading history of the scs will be the ar ar age dating of the basaltic rocks recovered during iodp expedition 349

a mechanism to thin the continental lithosphere at magma poor margins crustal thinning where continental plates break apart can be accomplished by a system of conjugate concave downward faults instead of multiple normal faults through exhumation of mid crustal and mantle material where continental plates break apart slip along multiple normal faults provides the required space for the earth s crust to thin and subside1 after initial rifting however the displacement on normal faults observed at the sea floor seems not to match the inferred extension2 here we show that crustal thinning can be accomplished in such extensional environments by a system of conjugate concave downward faults instead of multiple normal faults our model predicts that these concave faults accumulate large amounts of extension and form a very thin crust 10 km by exhumation of mid crustal and mantle material this transitional crust is capped by sub horizontal detachment surfaces over distances exceeding 100 km with little visible deformation our rift model is based on numerical experiments constrained by geological and geophysical observations from the alpine tethys and iberia newfoundland margins3 4 5 6 7 8 9 furthermore we suggest that the observed transition from broadly distributed and symmetric extension to localized and asymmetric rifting is directly controlled by the existence of a strong gabbroic lower crust the presence of such lower crustal gabbros is well constrained for the alpine tethys system4 9 initial decoupling of upper crustal deformation from lower crustal and mantle deformation by progressive weakening of the middle crust is an essential requirement to reproduce the observed rift evolution this is achieved in our models by the formation of weak ductile shear zones

the red river fault zone in the yinggehai basin south china sea abstract the red river fault a major strike slip fault of onshore southeast asia is suggested to be closely related to the formation of the south china sea and uplift of the tibetan plateau yet published documentation of the offshore segment of the red river fault is sparse high resolution seismic reflection and borehole data obtained by the petroleum industry along the northwestern margin of the south china sea permit mapping of the offshore segment of the red river fault in the yinggehai basin as well as demonstration of the timing of its shear movement and slip reversal two boundary faults and two basin center faults are mapped three successive deformation phases for the offshore red river fault zone include sinistral movement from 30 to16 ma slip reversal between 16 and 5 5 ma and dextral movement with low rates after 5 5 ma although no robust piecing points were observed for the sinistral movement period the horizontal displacement of dextral movement is estimated to be tens of kilometers this study supports a two stage model for the evolution of the india asia collisional orogen before the middle miocene sinistral movement of the red river fault likely was linked to spreading of the south china sea consistent with the concept of continental extrusion after the middle miocene distributed shortening better explains a low rate of dextral accommodation of the red river fault in response to the continuing india asia collision

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

rheology and strength of the eurasian continental lithosphere in the foreland of the taiwan collision belt constraints from seismicity flexure and structural styles 1 deformation in western taiwan is characterized by variable depth frequency distribution of crustal earthquakes which are closely connected with along strike variations of tectonic styles thin or thick skinned around the peikang high a major inherited feature of the chinese margin to fit the calculated high crustal geotherm and the observed distribution of the crustal seismic activity a qz diorite and granulite composition for the upper and the lower crust is proposed we then model the plate flexure through te estimates using brittle elastic ductile plate rheology flexure modeling shows that the best fit combination of te boundary condition is for thrust loads acting at the belt front the calculated te vary in the range of 15 20 km these values are primarily a reflection of the thermal state of the rifted chinese margin inherited from the oligocene spreading in the south china sea however other mechanical properties such as the degree of crust mantle coupling and the thickness of the mechanically competent crust and mantle are considered south of the peikang high flexure modeling reveals lower te associated with thinner mechanically strong layers variable stress strain distribution associated with a higher degree of crust mantle decoupling is examined to explain plate weakening we first show that plate curvature cannot easily explain strength reduction and observed seismic activity additional plate boundary forces arising from the strong coupling induced by more frontal subduction of a buoyant crustal asperity i e the peikang high with the overriding plate are required favorably oriented inherited features in the adjacent tainan basin produce acceleration of strain rates in the upper crust and hence facilitate the crust mantle decoupling as attested by high seismic activity and thick skinned deformation the relative weakening of the lower crust and mantle then leads to weaken the lithosphere by contrast to the north more oblique collision and the lack of inherited features keep the lithosphere stronger this study suggests that when the eurasian plate enters the taiwan collision tectonic inheritance of the continental margin exerts a strong control on the plate deformation by modifying its strength

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

inferring crustal structure in the aleutian island arc from a sparse wide angle seismic data set compressional seismic travel times from a relatively sparse wide angle data set hold key information on the structure of a 800 km long section of the central aleutian arc since the source and receiver locations form a swath along the arc crest that is similar to50 km wide we trace rays in 3 d for a collection of 8336 seismic refraction and reflection arrivals we investigate variations in seismic velocity structure parallel to the aleutian arc assuming that our result represents average crustal structure across the arc we explore seismic velocity models that consist of three crustal layers that exhibit smooth variations in structure in the 2 d vertical plane we consider the influence of additional constraints and model parameterization in our search for a plausible model for aleutian arc crust a tomographic inversion with static corrections for island stations reduces the data variance of a 1 d starting model by 91 our best model has seismic velocities of 6 0 6 5 km s in the upper crust 6 5 7 3 km s in the middle crust and 7 3 7 7 km s in the lower crust and a total crustal thickness of 35 37 1 km a resolution analysis shows that features having a horizontal scale less than 20 km cannot be imaged but at horizontal length scales of similar to50 km most model features are well resolved the study indicates that the aleutian island arc crust is thick compared to other island arcs and strongly stratified and that only the upper 60 of the arc crust has seismic velocities that are comparable to average seismic velocities in continental crust

the topographic and gravitational expression of density anomalies due to melt extraction in the uppermost oceanic mantle abstract evidence suggests that basaltic extrusion can decrease the density of the depleted source region by up to 2 for 20 partial melting this hypothesis is tested by calculating the effect of the density anomaly between the depleted and undepleted mantle on the topography gravity and geoid fields and comparing the anomalies to the observed topography gravity and geoid anomalies over volcanic features in the oceans if the source region lies at the base of the lithosphere we find that the gravity and geoid anomalies are proportionally more sensitive to the density contrast than the topography anomaly this effect is most pronounced for wavelengths less than 1000 km since mid ocean ridge volcanism the largest source of volcanism in the oceanic plates is fairly uniform along the ridge the depleted material in the oceanic plates and the corresponding topography and gravity anomalies should also be somewhat uniform in extent any long wavelength anomalies will be obscured in the topography and geoid fields by other anomalies at similar wavelengths such as slab and deep mantle anomalies however at mid plate swells and seamounts if the density contrast is greater than 0 4 the topography gravity and geoid anomalies are measurable to be consistent with the wavelengths represented in the geoid and topography data at hawaii the depleted material must be spread into a thin layer by convective flow at mid plate swells even in the thin layer model the topography and geoid anomalies due to the depleted material would still be as large as 200 m and 2 m respectively after the convective flow producing the bulk of the swell is removed the anomaly due to the depleted material would remain and would be apparent in the geoid and topography fields at seamounts if the source region is near the base of the plate then the presence of depleted material should be distinguishable in the geoid field but be somewhat obscured by flexural effects therefore although a topographic or geoid anomaly due to depletion has not been observed the density anomaly cannot be discounted because only careful scrutiny and modelling of mid plate swells and seamounts would distinguish its effect

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

tectonic characteristics and evolution history of the pearl river mouth basin abstract on the basis of the new understandings from tens of thousand of kilometres of geophysical material and over 60 exploration and evaluation wells providing data on the pearl river mouth basin prmb this paper analyses the special tectonic position of the prmb where three major plates i e the eurasian the pacific and the indo australian plates meet and interact the characteristics of the basin geological structure and its tectonic evolution are also discussed situated on continental and transitional crust the prmb is a mesozoic cenozoic oil and gas bearing basin developed on caledonian folding basement which was reformed during the mesozoic in the process of its formation and evolution the prmb underwent extension slight compression and re extension with the eruption of intermediate acid and basic magma meanwhile it formed double structural layers with faulting below and depression above and sediments were laid down with marine facies following continental facies hydrocarbons accumulated from continental source rocks in the marine reservoir rocks by using a fracture structure model of the uplift and depression the mechanism of the prmb and its evolution history have been interpreted

the control of precursor brittle fracture and fluid rock interaction on the development of single and paired ductile shear zones abstract ductile shear zones can occur as relatively isolated single structures as arrays or as characteristic paired zones in continuous glaciated exposures of metagranodiorites from the tauern window eastern alps the control of initial dilatant brittle fracture and associated fluid rock interaction on the geometry of subsequent ductile shear zones can be unequivocally established shearing occurred under amphibolite facies conditions fractures in weakly deformed metagranodiorites are often less than 1 mm thick but extend for tens of metres many are healed joints without shear offset others show minor mm cm discrete dextral offset such brittle faults commonly display a low angle en echelon arrangement with displacement transferred between discrete fracture segments by ductile compressive bridges the geometry of more strongly reactivated zones depends on the degree and heterogeneity of fluid rock interaction which is related to fluid infiltration and veining along the primary fractures with little fluid rock interaction reactivation produces single heterogeneous ductile shear zones centred on and immediately flanking the pre existing fracture with increased fluid rock interaction a bleached halo is developed symmetrically to either side of a central epidote quartz garnet calcite vein ductile shear zones commonly flank this bleached zone to develop a characteristic paired pattern strain is partitioned localizing in the central fracture vein and the flanking shear zones paired zones may anastomose in accordance with changes in the width of the central bleached zone but are always symmetrically spaced with regard to the central fracture vein with increasing deformation the ductile shear zones broaden into the adjacent metagranodiorite but not into the bleached zone which remains preserved as a low strain region paired shear zones can also develop to either side of aplite dykes examples of characteristic paired shear zones usually with a clear central vein are found in many areas ranging from greenschist to eclogite facies suggesting that the mechanism of their formation is quite general

the s reflector west of galicia spain evidence from prestack depth migration for detachment faulting during continental breakup the mechanism of continental extension and breakup and the role of detachment faults in these processes are currently the subject of intense debate one possible detachment fault is the s reflector imaged as an undulating in time locally discontinuous reflection on existing seismic profiles west of iberia here we present new images in depth of the four margin normal profiles across the west galicia rifted margin where the s reflector was originally defined and is best imaged it is shown that faults bounding wedge shaped units of late synrift sediments which hence were active during rifting immediately prior to breakup appear to detach at shallow levels onto the s reflector s itself appears as a continuous locally domal feature and does not generally appear offset the waveform of s is compatible with a reflection from a single interface such as a sharp tectonic boundary the depth sections show that s was active at 1 3 km below the seafloor during final rifting s is interpreted as a brittle detachment fault which controlled the final breakup of the continent west of galicia furthermore the data provide constraints on the sense of shear of s analogy with detachment terranes the present synrift and structural dips of s and the identification of a breakaway to s imply that s accommodated top to the west shear toward the east of the profiles s becomes more complex possibly because of different phases of detachment faulting and the development of both incisement and excisement structures from the geometry of wedge shaped sedimentary units deposited during faulting above s it also appears that s was active at an angle of 20 or less and hence may be considered a genuine low angle normal fault

magmatism at rifted continental margins the intense volcanism and uplift observed on many rifted continental margins forming basaltic seaward dipping reflector sequences is accompanied by the emplacement of a thick igneous section at depth partial melting by decompression of passively upwelling asthenosphere that is hotter than normal because it is near a hotspot explains both the thickness of the new igneous crust and the initial elevation of the rifted margins

shallow anatomy of a continent ocean transition zone in the northern south china sea from multichannel seismic data the cenozoic tectonic evolution of the south china sea is reflected in diverse tectonic processes including continental rifting seafloor spreading subduction terrane collision and strike slip fault movement a continent ocean transition zone in the northern south china sea caused by the lithospheric extension when the continent underwent break up rifting and later seafloor spreading is clearly imaged in the multi channel seismic data presented in this study the morphological units of the continent ocean transition zone are the rift depression the volcanic zone and tilted fault blocks the volcanic zone represents a highest extension zone within the continent ocean transition zone and is mainly distributed in the southern slope uplift zone along the northern passive margin of the south china sea the large listric normal faults bounding the dongsha rise and baiyun sag are evidenced in the seismic image the passive margin in the northern south china sea underwent the wide rift to narrow rift process in the transition zone as inferred from the relation between the surface heat flow and initial rifted crustal thickness the continent ocean transition zone in the continental margin of the south china sea is consistent with high heat flow zone average 90 mw m 2 observed in the previous heat flow measurements and at odp site 1148 and is manifested in a sharp change of the p wave velocity the rifted margin of the south china sea is a non typical magma poor passive margin or an intermediary form between the iberian type non volcanic and the greenland type volcanic margin compared to the world s typical passive margins c 2012 elsevier b v all rights reserved

the continent ocean transition at the southeastern margin of the south china sea abstract the south china sea formed by magma poor or intermediate volcanic rifting in the paleogene we investigate the structure of the continent ocean transition cot at its southern margin off nw palawan between the continental blocks of reed bank and the islands of palawan and calamian several surveys recorded by the bgr from 1979 to 2008 established a comprehensive database of regional seismic lines accompanied with magnetic and gravity profiles we interpret two major rifted basins extending in the ne direction across the shelf and slope separated by a structural high of non volcanic origin the continent ocean transition is interpreted at the seaward limit of the continental crust when magnetic spreading anomalies terminate some 80 100 km farther north the area in between displays extensive volcanism as manifest by extrusions that occasionally reach and cut the seafloor by dykes and by presumed basaltic lava flows occurring after break up the cot is highly variable along the nw palawan slope one type shows a distinct outer ridge at the cot with a steep modern seafloor relief the other type is characterised by rotated fault blocks bounded by listric normal faults ramping down to a common detachment surface half grabens developed above a strongly eroded pre rift basement the seafloor relief is smooth across this other type of cot we suggest the pre rift lithospheric configuration had major influence on the formation of the cot besides transfer zones volcanic domains confined to the north of competent crustal blocks correlate with the style of the cot gravity modelling revealed an extremely thinned crust across the shelf we propose a depth dependent extension model with crust being decoupled from mantle lithosphere explaining the discrepancy of subsidence observed across the south china sea region

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

small scale convection and divergent plate boundaries this paper explores the suggestion that observations of large igneous crustal thickness at rifted volcanic margins may in part be explained by small scale convection in the upper mantle this may increase the delivery of magma to the overlying lithosphere without the need for anomalously high mantle temperatures this concept is quantitatively assessed by numerical modeling of the flow caused by divergent plate motions in a viscous temperature and pressure dependent nonlinear fluid significant time dependent small scale convection is generated at the lower model viscosities and or higher temperatures and or sharper rift geometries these models can provide the required thick igneous crust observed at volcanic margins however they also give excessive variations in the thickness of igneous crust within the ocean basin and are therefore unacceptable an additional factor we have explored is the recent suggestion that the oceanic mantle may become dehydrated when melt is generated and removed this increases its viscosity by 2 to 3 orders of magnitude in the melting zone above 80 km the high viscosity lid stabilizes the flow and provides a more uniform oceanic crustal thickness while at the same time allowing for vigorous small scale convection during the early history of the rift and the delivery of thick igneous crust against the margin this factor coupled to the models for flow described above allow the main features of volcanic margins to be simulated besides the thick igneous crust predicted at the margins these models suggest that time dependent small scale convection below the margin may persist for tens of million years following the onset of seafloor spreading and that there may be coupling between this flow at the margin and that at the ridge axis the periodicity of this time dependent convection is primarily dictated by the model viscosities the models suggest that episodicities of tens of million years are reasonable these episodicities may be reflected in the record of vertical motions at rifted margins

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

spatial variation of seismogenic depths of crustal earthquakes in the taiwan region implications for seismic hazard assessment abstract this paper presents the first whole taiwan scale spatial variation of the seismogenic zone using a high quality crustal seismicity catalog the seismicity onset and cutoff depths i e seismogenic depths are determined by the earthquake depth moment distribution and used to define the upper and lower boundaries of the seismogenic zone respectively together with the published fault geometries and fault area moment magnitude relations the depth difference in the onset and cutoff depths i e seismogenic thickness is used as the fault width to determine the moment magnitudes of potential earthquakes for the major seismogenic faults results show that the largest mw7 9 8 0 potential earthquake may occur along the changhua fault in western taiwan where the seismic risk is relatively high and seismic hazard mitigation should be a matter of urgent concern in addition the first motion focal mechanism catalog is used to examine the relation between the seismogenic depths and earthquake source parameters for crustal earthquakes 50 km the shallowest onset and cutoff depths are observed for normal and strike slip events respectively this observation is different from the prediction of the conventional continental rheology model which states that thrust events have the shallowest cutoff depth thus a more sophisticated rheology model is necessary to explain our observed dependence of the seismogenic depths on faulting types meanwhile for intermediate to large crustal mw 4 depth 50 km earthquakes thrust events tend to occur at the bottom region of the seismogenic zone but normal and strike slip events distribute at a large depth range

tectono stratigraphic terranes and detachment faulting of the south china sea and sulu sea abstract geoscientific data acquired from the south china sea and sulu sea areas by bgr since 1977 are compiled and interpreted as comprehensive data sets together with available information of commercial wells and drillsites of the ocean drilling program odp generalized stratigraphy and lithology are discussed and correlated with the interpreted seismic profiles on the basis of the stratigraphic and structural analysis 5 main tectono stratigraphic terranes are defined for the south china sea and 4 main terranes for the sulu sea respectively the dangerous grounds reed bank palawan nw borneo trough and palawan island represent terranes of continental origin which developed on the proto china margin by uniform sense low angle simple shearing within the crust in late cretaceous to early paleocene this rift was abandoned and a new westward propagating rift system developed north of the dangerous grounds reed bank area from the middle eocene to the oligocene early miocene together with seafloor spreading in the south china sea basin the leading edge of the southward drifting continental terranes collided with the late cretaceous to early eocene subduction complex of the northernmost terrane of the proto sulu sea continuous convergence of these terranes back arc spreading creating the se sulu sea terrane and the assumed anti clockwise rotation of borneo are responsible for the complex collision compression structures of the sulu sea terranes including the formation of splinters of oceanic crust major right lateral wrench systems are nnw sse running up to 500 km long and cut across most of the identified terranes wrenching presumably ceased 12 to 16 myr ago

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

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

the influence of inherited continental margin structures on the stress and strain fields of the south central taiwan fold and thrust belt in this paper we test whether or not structural and morphological features inherited from the eurasian continental margin are affecting the contemporary stress and strain fields in south central ta

controls on the thermomechanical evolution of hyperextended lithosphere at magma poor rifted margins the example of espirito santo and the kwanza basins high quality long offset seismic data from many distal rifted margins show evidence for hyper extended 10 km thick crust direct observation of such domains is challenging as they lie at great water depth buried beneath thick sedimentary sequences and formed by rock assemblages that are hydrated and geophysically indistinguishable only a few drill holes have penetrated basement at ultradistal rifted margins these observations together with outcrops of preserved analogs exposed in collisional orogens suggest that the complex interaction of detachment faults rooted in a subhorizontal shear zone in the hyperextended crust or in the serpentinized mantle controls the formation of the ocean continent transition while depth dependent thinning controls the early phases of rifting conforming to classical rift models we still have a superficial understanding of how normal faults and subhorizontal shear zones form and evolve during rifting and lithospheric breakup here we develop a rheological parameterization to simulate the formation of and slip on large offset normal faults rooted in growing brittle to ductile shear zones the evolution of these structures leads to the creation of a hyperextended crust and eventually exhumed serpentinized mantle we also propose a simplified formulation to simulate magmatic underplating and seafloor spreading the resulting numerical models provide a self consistent picture for the evolution of magma poor rifted margins from initiation of rifting to seafloor spreading the model results are compared with first order observations of the kwanza and espirito santo conjugate margins in the south atlantic as well as of magma poor margins globally

crust upper mantle seismic velocity structure across southeastern china abstract one in line wide angle seismic profile was conducted in 1990 in the course of the southeastern china continental dynamics project aimed at the study of the contact between the cathaysia block and the yangtze block this 380 km long profile extended in nw se direction from tunxi anhui province to wenzhou zhejiang province five in line shots were fired and recorded at seismic stations with spacing of about 3 km along the recording line we have used two dimensional ray tracing to model p and s wave arrivals and provide constraints on the velocity structure of the upper crust middle crust lower crust moho discontinuity and the top part of the lithospheric mantle p wave velocity s wave velocity and v p v s ratio are mapped the crust is 36 km thick on average albeit it gradually thins from the northwest end to the southeast end offshore of the profile the average crustal velocity is 6 26 km s for p waves but 3 6 km s for s waves a relatively narrow low velocity layer of about 4 km of thickness with p and s wave velocities of 6 2 km s and 3 5 km s respectively marks the bottom of the middle crust at a depth of 23 km northwest and 17 km southeast at the crust mantle transition the p and s wave velocity change quickly from 7 4 to 7 8 km s northwest and 8 0 to 8 2 km s southeast and from 3 9 to 4 2 km s northwest and 3 9 to 4 5 km s southeast respectively this result implies a lateral contrast in the upper mantle velocity along the 140 km sampled by the profile approximately the average v p v s ratio ranges from 1 68 1 8 for the upper crust to 1 75 for the middle and 1 75 1 85 for lower crust with the interpretation of the wide angle seismic data jiangshan shaoxin fault is considered as the boundary between the yangtze and the cathaysia block

cenozoic rifting and volcanism in eastern china a mantle dynamic link to the indo asian collision abstract the indo asian continental collision is known to have had a great impact on crustal deformation in south central asia but its effects on the sublithospheric mantle remain uncertain studies of seismic anisotropy and volcanism have suggested that the collision may have driven significant lateral mantle flow under the asian continent similar to the observed lateral extrusion of asian crustal blocks here we present supporting evidence from p wave travel time seismic tomography and numerical modeling the tomography shows continuous low velocity asthenospheric mantle structures extending from the tibetan plateau to eastern china consistent with the notion of a collision driven lateral mantle extrusion numerical simulations suggest that at the presence of a low viscosity asthenosphere continued mass injection under the indo asian collision zone over the past 50 my could have driven significant lateral extrusion of the asthenospheric mantle leading to diffuse asthenospheric upwelling rifting and widespread cenozoic volcanism in eastern china