Deep reflection seismic data for improved imaging of crust structure: 2D case study of the southwest subbasin in the South China Sea

Seismic imaging of crustal structures becomes difficult in the presence of rough basements or complex seabed topography. Here, a 900 km deep seismic reflection profile collected across the southwest subbasin (SWSB) of the South China Sea is evaluated. By analyzing the types and distinctions of noise and effective signals, deep structure migration techniques are used to improve crustal structure imaging, wide-line processing is used to predict 3D space multiples, and f-k domain time-space variable adaptive deghosting and different offset stacking are used to enhance the deep events. The new imaged continental crustal structure in the Penxi Bank exhibits moderate thinning, down to 15 km, and is intersected by continental-ward low-angle detachment or normal faults. Within the limitations of the ocean bottom seismic P-wave velocity model, we find the subhorizontal lower crustal reflections (LCRs) 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 crusts. Based on the narrow continent-ocean transitions (COTs), the presence of more continental-ward detachment faults, and the calculation of high lithosphere heat flow, we deduce that the mantle lithosphere breakup occurred earlier than the crust in the SWSB. Moreover, the COTs 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 km to 6 km, indicating a thinner than normal crust. LCRs with a ridge-ward dipping pattern terminate at the Moho reflections and are partly connected to synspreading faults, hinting at their possible generation through synspreading faulting.