摘要
本文解释了横穿南海西南次海盆的两条多道地震测线,对南海西南次海盆的基底形态特征进行了描述和分类,并讨论了基底形态与扩张速率之间的对应关系。研究结果表明,西南次海盆基底可以分为两种类型:类型1为岩浆作用主导的平坦基底,局部有岩浆侵入;类型2为构造作用主导的掀斜断块。扩张速率的计算表明:N3测线所代表的西南次海盆北东段的半扩张速率在13～36mm/a之间周期性变化,而在NH973-1所代表的中段半扩张速率基本稳定在19mm/a左右,未有明显变化。海盆基底的类型变化与扩张速率的对应关系明显,在扩张速率较快的区域以类型1为主,而在扩张速率慢的区域以类型2为主。西南次海盆北东段扩张速率呈现周期性变化,相应海盆基底也表现为相间排列;而西南次海盆中段的基底比较单一,以类型2为主。南海西南次海盆北东段扩张时间更久,并可能存在周期性活动的岩浆房;而海盆中段海底扩张发生较晚,岩浆作用较弱,从而造成西南次海盆从北东到南西不同的基底形态特征。
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:typeⅠrepresents 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 typeⅡ 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.