DEEP REFLECTION SEISMIC DATA FOR IMPROVED IMAGING OF CRUST STRUCTURE: 2D CASE STUDY OF THE SOUTHWEST SUB-BASIN IN THE SOUTH CHINA SEA

Abstract

Seismic imaging of crustal structures becomes difficult in the presence of rough basements or complex bathymetry. Here, we present a 900 km deep seismic reflection profile collected across the Southwest Sub-basin (SWSB) of the South China Sea. By analyzing the types and distinctions of noise and effective signals, we employed deep structure migration techniques to improve crustal structure imaging, wide-line processing to predict 3D space multiples, and F-K domain time-space variable adaptive de-ghosting and different offset stacking to enhance the weak signals reflected from deep strucutures. The imaged continental crustal structure in the Penxi Bank exhibits moderate thinning, down to 15 km, and is intersected by continental-ward low-angle normal faults. Within the limitations of the OBS P-wave velocity model, we detected sub-horizontal lower crustal reflections that may be indicative of a weak lower crust. Two small-scale rollover structures along detachment faults rooted and rafted to the top of these weak lower crust. Based on the presence of narrow continent-ocean transitions(COTs), continental-ward detachment faults, and high lithosphere heat flow, we deduced that the mantle lithosphere breakup occurred earlier than the crust in the SWSB. Moreover, the continent-ocean transitions and oceanic crust domains demonstrate rough basements with numerous faults and approximately 20% diffuse or weak Moho reflections. From the southern COT to the initial oceanic domain, the thickness of the crust gradually reduces to only 3-5 km. This suggests a relatively low magmatic budget and protracted tectonic extension from the continental breakup to the onset of seafloor spreading. Within the oceanic crust domain, the crust thickness ranges from approximately 4-6 km, indicating a thinner oceanic crust than normal crust. Lower crustal reflections with a ridge-ward dipping pattern terminate at the Moho reflections and are partly connected to syn-spreading faults, hinting at their possible generation through syn-spreading faulting.