Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site. 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. Highlights ► Northeastern South China Sea crust is thinned continental crust intruded by volcanics ► Postspreading magmatic activity (< 13 Ma) largely masks the spreading fabric ► N055°, N075° and N085° seafloor spreading oceanic domains are identified ► Extension in Xisha trough and south of Macclesfield bank extends to Qui Nhon Ridge ► South China Sea (SCS) opening is compensated by northward proto-SCS subduction

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[1] We use offshore multichannel seismic (MCS) reflection and wide-angle seismic data sets to model the velocity structure of the incipient arc-continent collision along two trench perpendicular transects in the Bashi Strait between Taiwan and Luzon. This area represents a transition from a tectonic regime dominated by subduction of oceanic crust of the South China Sea, west of the Philippines, to one dominated by subduction and eventual collision of rifted Chinese continental crust with the Luzon volcanic arc culminating in the Taiwan orogeny. The new seismic velocity models show evidence for extended to hyperextended continental crust,~10-15 km thick, subducting along the Manila trench at 20.5°N along transect T1, as well as evidence indicating that this thinned continental crust is being structurally underplated to the accretionary prism at 21.5°N along transect T2, but not along T1 to the south. Coincident MCS reflection imaging shows highly stretched and faulted crust west of the trench along both transects and what appears to be a midcrustal detachment along transect T2, a potential zone of weakness that may be exploited by accretionary processes during subduction. An additional seismic reflection transect south of T1 shows subduction of normal ocean crust at the Manila trench.

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Magnetic data suggest that the distribution of the oceanic crust in the northern South China Sea (SCS) may extend to about 21°N and 118.5°E. To examine the crustal features of the corresponding continent-ocean transition zone, we have studied the crustal structures of the northern continental margin of the SCS. We have also performed gravity modeling by using a simple four-layer crustal model to understand the geometry of the Moho surface and the crustal thicknesses beneath this transition zone. In general, we can distinguish the crustal structures of the study area into the continental crust, the thinned continental crust, and the oceanic crust. However, some volcanic intrusions or extrusions exist. Our results indicate the existence of oceanic crust in the northernmost SCS as observed by magnetic data. Accordingly, we have moved the continent-ocean boundary (COB) in the northeastern SCS from about 19°N and 119.5°E to 21°N and 118.5°E. Morphologically, the new COB is located along the base of the continental slope. The southeastward thinning of the continental crust in the study area is prominent. The average value of crustal thinning factor of the thinned continental crust zone is about 1.3-1.5. In the study region, the Moho depths generally vary from ca. 28 km to ca. 12 km and the crustal thicknesses vary from ca. 24 km to ca. 6 km; a regional maximum exists around the Dongsha Island. Our gravity modeling has shown that the oceanic crust in the northern SCS is slightly thicker than normal oceanic crust. This situation could be ascribed to the post-spreading volcanism or underplating in this region.

During arc-continent collision, buoyant sections of sediments and rifted continental crust from a subducting plate will accrete to the forearc of the upper plate as long as this backstop remains intact. Deformation of the oceanic arc and forearc block may ultimately lead to accretion of these mafic rock units to the new orogen. The Taiwan mountain belt, which formed at ∼6.5 Ma by oblique convergence between the Eurasian passive margin and the overriding Luzon arc in northern Taiwan, offers important insight in this process, since the collision is more advanced in the north than in the south. The incipient stage of arc-collision can be studied in southern Taiwan, while the northern portion of the orogen is presently undergoing collapse due to a flip in the subduction polarity between the Eurasian Plate and the Philippine Sea Plate. In this study, we seismically image the structure of the northern section of the mountain belt with a tomographic inversion. We present marine and land-based seismic refraction data, as well as local earthquake data, from transect T6 of the Taiwan Integrated Geodynamic Research (TAIGER) program across the Taiwan mountain belt and the adjacent Ryukyu arc. Our 2-D compressional seismic velocity model for this transect, which is based on a tomographic inversion of 10 213 P-wave arrival times, shows that the Eurasian crystalline continental crust thickens from ∼24 km in the Taiwan Strait to ∼40 km beneath the eastern Central Range of Taiwan. The detailed seismic velocity structure of the Taiwan mountain belt shows vertical continuity in the upper 15 km, which suggests that rocks are exhumed to the surface here from the middle crust in a near-vertical path. The continental crust of the westernmost Ryukyu arc is almost as thick (∼40 km) as in the adjacent northern Central Range of Taiwan, and it appears to override the leading edge of the Philippine Sea Plate offshore northeastern Taiwan. If we assume that the western Ryukyu arc crust also thickened in the collision, then the mountain belt is wider and less thick in northern Taiwan than in central Taiwan (∼50 km), which may be the result of post-collisional extension in the north.

During a passive seismic experiment in the Okinawa Trough the shots of two reflection profiles were recorded by ocean bottom seismometers (OBS). Both profiles include 3 ocean-bottom instruments, are about 65 km in length and located in the axial portion of the southwestern Okinawa Trough. Processing of the reflection seismic data images recent deformation of the sedimentary units. Forward modelling of the wide-angle data on both profiles reveals a 1-2 km thick sedimentary infill overlying an acoustic basement characterised by seismic velocities between 3.2 and 3.5 km/s. Crustal thickness could only be modelled on one profile and was determined to be around 10 km, thickening towards the Ryukyu Arc in the south. Gravity modelling was used to additionally constrain both profiles especially the deep structure of Profile 1.

Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site. northern Bay of Bengal area. Highlights ► Refraction velocity models show a sedimentary cover of 13 km minimum beneath the shelf and 7 km minimum beneath the deep ocean. ► The crust is thinned continental crust, 10-20 km thick. ► The thinned continental crust is intruded by volcanics, sills and SDRs. ► Seismic velocities of 7.4 km/s found at the base of the crust is interpreted as magmatic underplating. ► The thinned continental crust extends from 19°N (northern Bay of Bengal) to 25°N (Shillong Plateau).