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We interpret seven two-dimensional deep-penetration and long-offset multi-channel seismic profiles in the northernmost South China Sea area, which were collected by R/V Marcus G. Langseth during the TAIwan GEodynamics Research (TAIGER) project in 2009. To constrain the crustal characteristics, magnetic inversion and forward magnetic modeling were also performed. The seismic results clearly show tilted faulting blocks in the upper crust and most of the fault plane connects downward to a quasi-horizontal detachment as its bottom in the south of the Luzon-Ryukyu transform plate boundary. North of the plate boundary, a small-scale failed rifted basin (minimum 5 km in crustal thickness) with negative magnetization probably indicates an extended continental origin. Significant lower crustal material (LCM) was imaged under a crustal fracture area which indicated a continent and ocean transition origin. The thickest LCM (up to 6.5 km) is located at magnetic isochron C15 that is probably caused by the magma supply composite of a Miocene syn-rift volcanic event and Pliocene Dongsha volcanic activity for submarine volcanoes and sills in the surrounding area. The LCM also caused Miocene crustal blocks to be uplifted reversely as 17 km crustal thickness especially in the area of magnetic isochron C15 and C16. In addition, the wide fault blocks and LCM co-existed on the magnetic striped area (i.e. C15-C17) in the south of the Luzon-Ryukyu transform plate boundary. Magnetic forward modeling suggests that the whole thick crustal thickness (>12 km thick) needs to be magnetized in striped way as oceanic crust. However, the result also shows that the misfit between observed and synthetic magnetic anomaly is about 40 nT, north of isochron C16. The interval velocity derived from pre-stack time migration suggests that the crust is composed of basaltic intrusive upper crust and lower crustal material. The crustal nature should refer to a transition between continent and ocean. Thus, the magnetic reversals may be produced in two possible ways: basaltic magma injected along the crustal weak zone across magnetic reversal epoch and because some undiscovered ancient piece of oceanic crust existed. The crustal structure discrimination still needs to be confirmed by future studies. [ABSTRACT FROM AUTHOR]

The outer rise on the distal periphery of a subduction system is caused by emplacement of an accreted load onto the flexed oceanic lithosphere. By examining the bathymetry and free-air gravity anomaly data collected by satellite observations and marine reflection seismic data collected during the TAIGER project, we demonstrate the characteristics of the flexural outer rise seaward of the Manila Trench. The region of the outer rise on the westernmost periphery of the Manila subduction system is characterized by the positive free-air gravity anomaly seaward parallel to the Manila Trench and the morphological rise at the south of the Manila subduction system. A flexure simulation is performed based on the flexural profiles along the southern Manila Trench-outer system and the resulting effective elastic thickness values may provide an alternative aspect for the spreading rates of the South China Sea basin. Since both the western periphery of the Taiwan collision belt and Manila subduction belt are dominated by the strain regime of extension of flexural origin, it appears that the strain regime of flexural extension associated with the flexural forebulge of the Western Taiwan Foreland Basin to the north, and the strain regime of flexural extension associated with the outer rise seaward of the Manila Trench to the south are meridionally interconnected. This revised understanding of the strain regime of flexural extension origin west of the Taiwan-Luzon convergent belt provides an alternative point of view on the strain regime offshore SW Taiwan. [ABSTRACT FROM AUTHOR]

Abstract: Off southwest Taiwan, a west-advancing orogenic wedge has obliquely impinged on the northern continental slope of the South China Sea (SCS) margin. We analyzed a dense grid of multi-channel seismic profiles to reveal the tectonic features in this oblique collision setting. In the upper SCS slope and adjacent to the accretionary wedge, the rifted continental margin is characterized by a deep-seated, oceanward-dipping, listric and active normal fault with rotated hangingwall strata beneath the slope. To the west of this slope segment, the mid-Oligocene to Recent post-breakup sediments show prograding and aggrading shelf-margin clinoforms that cover a series of small, possibly Paleogene, rift basins. In the submarine accretionary wedge, a series of west-vergent, NNW-striking fold-and-thrust structures characterizes the lower wedge, which can be further divided into frontal and rear segments with distinct structural features. The frontal segment is characterized by four west-vergent blind thrusts with gently folded limbs. In contrast, the rear, arcward segment is characterized by west-vergent, emergent and imbricate thrusts with tilted beds truncated at the hangingwalls. Strata within slope basins are tilted arcward, with dips that increase with depth, indicating continued relative uplift along thrust planes during sedimentation. Pulsed thrust activity is further evidenced by an array of arcward-dipping unconformable surfaces with westward onlapping strata in the basins. Longitudinal sedimentary tapering of pre-orogenic sediments correlates strongly with curvature of the submarine frontal accretionary belt, suggesting that pre-orogenic sediment thickness is the major control on the geometry of frontal structures. The preexisting SCS slope that lies obliquely in front of the advancing accretionary wedge has impeded the advancing of frontal folds resulting in a successive termination of folds against and along strike of the SCS slope. The existence of the SCS slope also leads the strike of impinging folds with NNW-trend to turn more sharply to a NE-strike, parallel to strike of the SCS slope. Our analysis shows that the pre-orogenic mechanical/crustal heterogeneities and seafloor morphology exert strong controls on the thrust-belt development in the incipient Taiwan arc-continent collision zone. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

Abstract: Surficial grain-size and down-core clay mineralogical data show that sediment along the inner-most part of the continental shelf in East China Sea is mainly derived from the Yangtze River (Changjiang), spanning from the Yangtze mouth (33°N) ~1000km southward to the southwestern corner of the Taiwan Strait (24°N). High-resolution CHIRP seismic profiles reveal an elongated mud wedge extending along the inner shelf, with a northern depocenter on the modern Yangtze delta and a southern depocenter at 27.5°N. Four distinct acoustic units are delineated within the mud wedge (from bottom up): unit I (late-Pleistocene, mainly valley fills), unit II (formed by transgressions, thin strata), unit III (11–2kyr BP, downlapping strata) and unit IV (2–0kyr BP, flat and opaque strata). Incised valleys, up to 15-m deep, are filled by flat-lying or inclined strata in unit I. The thin (<3m) and acoustically transparent unit II is only seen between 30 and 26°N in water depths between 40 and 90m. Separated by acoustically opaque strata or unconformities, units III and IV are widely distributed. During the past 11kyr Yangtze sediment accumulation has been unsteady, showing two high and one low accumulation-rate periods. The high-accumulation period at 5–8kyr BP may be related to maximum East Asian summer monsoon precipitation in the Yangtze basin; the other high-accumulation period, 0–2kyr BP, probably reflects intensive human activities in the river''s watershed. The low-accumulation-rate period at 2–5kyr BP, which is seen in both northern and southern Yangtze depocenters, is probably related to low river discharge and/or intensified Taiwan Warm Current and China Coastal Current. [Copyright &y& Elsevier]

Abstract: Southern Taiwan and its offshore area lie in the region where the Luzon volcanic arc initially collides with the rifted China continental margin. Because of the incipient arc-continent collision, the structures vary markedly along-strike the collision zone so as the patterns of seismicity. We use new seismic reflection profiles and integrate existing data to reveal major tectonic features and potential seismogenic faults of the study area. The accretionary wedge in the incipient arc-continent zone can be divided into the lower slope, upper slope, and backthrust domains, respectively. These structural domains reflect different aspects of wedge deformation, and exhibit significant structural variations along-strike. Reflection seismic data show that the prominent seismogenic structures in the Taiwan incipient collisional wedge include: (1) frontal decollement beneath the lower-slope domain, (2) out-of-sequence thrusts bordering the lower-slope and upper-slope domains, (3) megathrust that cuts into the oceanic (?) basement beneath the upper-slope domain, and (4) the Chaochou-Hengchun faults in the onshore upper-slope domain. Thermal regime for those structures indicates that the megathrust and part of frontal decollement are seismogenic. The geometry of the frontal decollement, out-of-sequence thrusts and megathrust is analogous to those observed along the Nankai prism of Japan, so that they are possibly capable of generating great earthquakes as shown in the Nankai Trough. Beneath the lower and upper-slope domains off SW Taiwan, the seismicity is characterized by mantle earthquakes with the accretionary wedge being largely aseismic. We interpret the lack of prominent seismicity within the accreted wedge to result from excess fluid pressure that has significantly weakened the wedge materials and fault zones and therefore results in less seismicity. The predominant mantle earthquakes beneath the accretionary wedge, however, may result from water-enriched mantle materials infiltrated during previous Mesozoic subduction event and later rift events. The volatile contents may have significantly reduced the rigidity of the mantle, leading to the mantle being more susceptible for brittle deformation and hence anomalously high seismicity. [Copyright &y& Elsevier]