南海西北次海扩张时代和洋壳性质:沉积地层及重磁依据

摘要

Using sediment strata and the abnormity of gravity and magnetism, the spreading age and character of northwest sub-sea basement of South China Sea was discussed. The sea floor spreading age was deduced at early Oligocene at the beginning and at the forepart of late Oligocene in the end. Twice periods of sea-floor spreading were implicated by stratum deformation, passive overlap, oceanic crust configuration and its symmetry. The first one occurred at early Oligocene during which the upper Eocene was snapped, deformed and overturned to spreading center on account of sea floor expanding, subsiding, and its pushing and shoving. The second one took place at the forepart of late Oligocene during which lower Oligocene was pulled apart at a limited distance because of short time and lesser range of sea-floor expanding. As a result of the different degree of lithosphere thinning and extending, the northwest sub-sea basement show a triangle with a wide edge in northeast and becoming narrow and vanishing to the southwest. The ocean crust boundary was deduced at the turning of the northwest sub-sea basement in southwest region and at turning of the northwest sub-sea floor in northwest region based upon the upper Eocene snapped points and the abnormity of gravity and magnetism of northwest sub-sea basement. In addition, the abnormity of gravity and magnetism indicate that rudimental continental crust may exist in the neonatal ocean crust. These phenomena, mentioned above, illuminate that the northwest sub-sea of South China Sea came to an untimely end when the sea floor spreading was at its bud period.%利用沉积地层被动超覆和基底重磁异常特征对南海西北次海形成时代和洋壳性质进行了探讨.推断南海西北次海初始扩张时间为早渐新世,结束扩张时间为晚渐新世早期.地层变形、被动超覆特征、洋壳基底形态及对称性特点反映出两期洋壳扩张事件.第一期发生在早渐新世.由于洋壳扩张,上始新统被拉断,在洋壳边界处上始新统突然终止现象明显.受洋壳横向扩张推挤和纵向沉降作用影响,上始新统明显变形,并向扩张中心倾覆.第二期洋壳扩张发生在晚渐新世早期.该期洋壳扩张持续时间短,扩张幅度小,下渐新统被拉开的距离有限.由于南海西北次海形成期间不同部位地壳伸展减薄程度不同,南海西北次海洋壳基底呈北东部较宽,向南西方向变窄,并逐渐尖灭的不规则三角形.根据盆地边缘上始新统向海盆中心方向的断点/线和重磁异常资料,推测西北次海南西侧洋壳边界位于海盆基底坡角处附近,洋壳较窄；而北东侧洋壳边界位于海底坡角处附近,洋壳相对较宽.另外,重磁异常表明,在洋壳基底中有陆壳残留块体存在.上述这些现象说明南海西北次海在洋壳萌芽阶段就先天天折,停止发育.