Abstract: We interpreted two multi-channel seismic profiles across the Southwest Sub-Basin (SWSB) of the South China Sea to clarify the oceanic basement types based on the regional morphological and structural features. We calculated half-spreading rates in different areas and analyzed their relationship with the basement types. Our geological interpretations recognized two typical types of the oceanic basement: type Ⅰ represents flat basement influenced by localized volcanic eruptions; and type Ⅱ features rotated blocks controlled by normal faulting. The calculated spreading rates showed that the half-spreading rate in the northeastern SWSB, represented by seismic profile N3, was between 13 and 36 mm/a; whereas in the middle SWSB, represented by the seismic profile NH973-1, the rate was largely stable at about 19 mm/a. The correlation between basement type and half-spreading rate is clear: typeⅠbasement corresponds to higher spreading rate and type II to lower spreading rate. The spreading rate in the northeastern SWSB varies periodically, following the inter-phase distribution pattern of the two basement types. The middle SWSB is dominated by type Ⅱ basement with a stable spreading rate. We suggest that the longer spreading history and higher magma budget in the northeastern SWSB may be the controlling factors for the complex basement morphology in this area.

Key words: Southwest Sub-Basin, oceanic basement type, spreading rate, magmatic-amagmatic process, South China Sea