Crustal Deformation and Magmatism around a Rigid Block under Continental Extension: A Case Study from the Zhongsha Islands, South China Sea

At rifted continental margin, crustal structure provides crucial evidence to study continental extension and magmatism characterizing rifting, breakup and onset of seafloor spreading. However, due to the lack of seismic surveys and rock samples from the northwestern South China Sea margin, the extension mode and style of the continent-ocean transition are still questionable, especially around the Zhongsha Islands. This study presents a newly collected wide-angle seismic profile OBS2017-2, across the Northwest Sub-basin, Zhongsha Islands and Southwest Sub-basin. The profile was acquired in June 2017 by R/V "Shiyan 2" of the South China Sea Institute of Oceanology, CAS. P-wave velocity structures around the Zhonsha Islands were modelled for the first time. Results suggest that: Zhongsha Islands and adjacent domains are characterized by heterogeneous crustal structure ranging in thickness from ~25 km under the islands to hyper-extended continental crust of just ~6 km to the north and south, with velocity ranges from 4.5 to 6.4 km/s. In contrast, the Northwest Sub-basin shows features typical for oceanic crust, with a crustal thickness of 5.5-5.8 km and velocity structure of 5.5-7.2 km/s. The Southwest Sub-basin indicates a more complex structure that vary from normal oceanic crust to thinned oceanic crust which is perhaps underlain by serpentinized mantle; crustal thickness ranges from ~5-6 km to 3.7-5 km. Beneath the continental slopes and the southern domain of the Northwest Sub-basin, a high velocity body and low upper-mantle velocities were found and may result from post-rift magmatic underplating. Cenozoic magmatism was found to mainly occur on the continental slope toe, rift and oceanic basin, where large normal faults were formed during continental rifting. In contrast, there is no magmatism within the Zhongsha Islands, indicating a rigid continental block less affected by rifting. Continent-ocean transition zones on both sides of the Zhongsha Islands are narrow (~30 km), suggesting a rapid transition from continental to oceanic crust. Our study provides crustal structure and hence constrains on crustal deformation and magmatism around the rigid block of Zhongsha Islands, and thus provide critical information for understanding rifting, breakup and seafloor spreading in the whole South China Sea.