We present multichannel seismic reflection images of about 2250 km of reprocessed seismic records collected during Sonne cruise 49 across the northern margin of the South China Sea. The data were collected in four seismic lines striking perpendicular to the strike of the margin. The lines cross the outer continental shelf and slope, and two of them continue across the central region of the basin. The four lines have been carefully reprocessed to improve signal to noise ratio including pre-stack statistical predictive deconvolution, pre-stack multiple attenuation by radon filtering and by FK filtering. Semblance-based velocity picking was before and after processing with analyses every 5 km or closer in areas of rough basement topography. Constant velocity stacks were also used in areas of interest. The data were stacked and subsequently post-stack time migrated using an FD algorithm and time and space variant smooth velocity models. The resulting sections display in detail the structure of post-rift and syn-rift sediment, the basement structure of fault-bounded blocks, often fault reflections, and very often clear and fairly continuous reflections from crust – mantle boundary. The four seismic images show the tectonic structure formed during -now inactive- rifting. From west to east the tectonic structure changes considerably as the amount of extension attained increases towards the east. In the western region the lines show the structure of the Xisha Trough, from the region where continental rifting stopped before continental-crust break up and separation to the east where a extension may have led to break up and subsequent to seafloor spreading. The images show clearly how the continental crust-mantle boundary progressively shallows to reach close to the top of the basement. The conjugate continental flanks of the rift display rather different styles of faulting producing a very asymmetric structure. The conjugate flanks display rather different amounts of fault dimensions and associated extension. The area of intense necking is < 50 km wide and in one of the flanks is not associated to apparent faulting. There is evidence of important volcanism in the images, with the most spectacular evidence displayed as large conical volcanoes formed over continental crust. The continent ocean transition is characterized by complex faulting in some lines, and seems to occur relatively abruptly in < 30 km distance in all lines. The crust next to the transition appears oceanic in nature with crust-mantle boundary reflections at roughly 2 seconds below top basement (i.e. about 5-6 km thick crust). The data support a model of rapid transition from continental crust rifting to seafloor spreading. © 2014 American Geophysical Union. All Rights Reserved

AGU Fall Meeting 3–7 December 2012, San Francisco, California

Peer Reviewed