Rifted continental margins form by lithospheric extension and break-up. The continent to ocean transition (COT) architecture depends on the interplay between tectonic and magmatic processes, and thus, to study the COT variability of different systems is key to understand rifting. We use MCS data and magnetic lineations across the Pearl River margin (PRM) of South China Sea to investigate a previously poorly defined COT. The structure of the PRM presents different amounts of extension allowing the study of conjugate pairs of continental margins and their COT in a relative small region. We reprocessed about 2250 km of MCS data along 4 regional, crustal-scale lines and found that 3 of them possibly display the COT. The time-migrated seismic sections show differences in internal reflectivity, faulting style, fault-block geometry, the seismic character of the top of the basement, in the geometry of sediment deposits, and Moho reflections, that we interpret to represent clear continental and oceanic domains. The continental domain is characterized by arrays of normal faults and associated tilted blocks overlaid by syn-rift sedimentary units. The Moho is imaged as sub-horizontal reflections that define a fairly continuous boundary typically at 8-10 s TWT. Estimation of the thickness of the continental crust using 6 km/s average velocity indicates a ~22 km-thick continental crust under the uppermost slope passing abruptly to ~9-6 km under the lower slope. Comparatively the oceanic crust has a highly reflective top of basement, little-faulting, not discernible syn-tectonic strata, and fairly constant thickness (4-8 km) defined by usually clear Moho reflections. The COT occurs across a ~5-10 km narrow zone. Rifting resulted in asymmetric conjugate margins. The PRM shows arrays of tilted fault blocks covered by abundant syn-rift sediment, whereas the conjugate Macclesfield Bank margin displays abrupt thinning and little faulting. Seismic profiles also show a change in the tectonic structure from NE to SW. On the two NE-most lines, crustal thinning and break-up occur over 20-40 km wide segments. To the SW, continental extension occurred over a comparatively broader ~100-110 km segment of tilted fault-block structure. We interpret, that this 3D structural variability and the narrow COT was caused by the lateral NE to the SW propagation of a spreading center. In the NE, early spreading center propagation during ongoing rifting stopped continental stretching, causing an abrupt break-up and a narrow COT to seafloor spreading. Later arrival of spreading center propagated to the SW, resulted in a comparatively broader segment with fault-block structures of extended continental crust. However, the COT to clear oceanic crust is narrow. Spreading center propagation in the basin is however not a simple phenomena and ridge jumps and abrupt cessation of spreading contributed to form narrow COT that laterally change to highly thinned continental crust segments. We suggest that the tectonic architecture of continental extension and the abrupt COT along the PRM have been controlled by 3D oceanic spreading center propagation to a degree larger than by the local lithospheric structure during rifting

American Geophysical Union (AGU) Fall Meeting, 9-13 December 2013, San Francisco

Peer reviewed