新生代华夏岩石圈减薄与东亚边缘海盆构造演化的年代学与地球化学制约研究

Geochronological and geochemical constraint on the Cenozoic extension of Cathaysian lithosphere and tectonic evolution of the border sea basins in East Asia

年代学与地球化学制约表明,华南的茂名与三水盆地从92～38Ma期间有近连续的火山喷发,并在(56±2)Ma火山作用源区发生了急剧的转折。这与东亚边缘海盆地最早伸展在时间上和构造方向上是一致的,形成了一系列NE向左旋剪切伸展盆地以及南海南西海盆、苏拉威西海、西菲律宾海等洋盆。35～17Ma期间印支块体沿红河断裂向东南挤出,使东亚沿NE向左旋剪切伸展受阻。因此,这一期间华夏块体上没有出现与拉张有关的玄武质岩浆作用,始新世洋盆NE向扩张也逐渐被终止。关于南海的晚第三纪扩张,年代学与构造证据是与根据磁异常确定的32～17Ma扩张期相矛盾的。17Ma以后,华夏、东海至日本海岩石圈均出现了强的NE向伸展,也使印支块体沿红河断裂从左旋挤出转向右旋走滑。南海在这一期间再次出现扩张,应是一个合理的构造格局。

Geochronological and geochemical evidence indicate that there occurred nearly continuous volcanism during 92～38 Ma in the Maoming and Sanshui basins in South China, however, their source regions sharply changed from the lowercrust to the upper mantle at (56±2) Ma. It is consistent with the first stageopening of the border sea basins in East Asia in age and tectonic direction, thus a series of pull apart basins in Cathaysian Block as well as Proto SouthChina Sea, Celebes Sea and West Philippine Sea with NE direction were formed during Eocene. Indochina Block was extruded through left lateral slip along the Red River fault during 35～17 Ma, which interrupted the shear extension of NE direction in East Asia. Thus, there was no basaltic magmatism related extension during this interval, the Eocene sea basins with NE direction terminated opening as well. The geochronological and tectonic evidence for the extension of South China Sea in Late Tertiary is in contradiction to the opening interval of 32～17Ma deduced from magnetic anomaly. There occurred strong extension of lithosphere with NE direction in Cathaysian Block, East Sea and Japan Sea since 17 Ma, which also turned the movement direction of Indochina block from left lateral to right lateral slip. That the South China Sea opened again during this stage isa reasonable inference.