Seismic evidence of a detachment fault near the fossil spreading center in the Southwest Sub-basin, South China Sea

In 2010, we conducted an ocean bottom seismometer (OBS) experiment surveying the fossil spreading center in the Southwest Sub-basin of the South China Sea. The detailed velocity model, simulated by two-dimensional ray tracing, shows a strong lateral variation in velocity structure across the spreading ridge. Thus we divided the oceanic crust across the spreading center into three types: (1) normal oceanic crust (5.7–6.8 km thick with a velocity range of 4.8–7.1 km/s), (2) thinned oceanic crust on the NW side of the spreading center (4.3–5.7 km thick with a velocity range of 5.0–7.1 km/s), and (3) thickened oceanic crust at the spreading center affected by post-spreading magmatism(5.7–7.0 km thick with a velocity range of 3.9–7.1 km/s). In addition, a low-angle (24°) SE-dipping detachment fault has been identified on the NW flank of the fossil spreading center, corresponding to the portion of the upper mantle uplift beneath a thinned oceanic crust with a sharp change in P-wave velocity. The igneous crustal thickness of the detachment fault footwall is constrained by PmP reflections to vary between 4.3 and 5.7 km, that is approximately 10–30% thinner compared to the normal oceanic crust (average crustal thickness of ~6.2 km). According to the magnetic anomaly lines, the fault began at ~17 Ma (C5Cr) with a duration of ~1 Myr. Comparing to other detachment fault footwalls, we suggest that the nature of the exhumed footwall rocks is supposed to be basalt which may imply an initial formation stage of the detachment fault, and the existence of the detachment fault may suggest that the last extension of the SWSB was dominated by the tectonic process. Moreover, we suggest that a low-velocity (7.6–7.8 km/s) body located within the upper mantle beneath the detachment fault is caused by both mantle serpentinization and partial mantling.