藏南晚白垩世宗卓组巨型混杂堆积的特征及其地裂——地震成因论证

Characters of Slumping Accumulation of Upper Cretaceous Zongzuo Formation and Demonstrate Its Caused by Large Break-up and Earthquakes, Southern Xizang (Tibet)

藏南晚白垩世宗卓组为超常规模的地裂—地震成因的混杂堆积,主体为一套延伸距离远、厚度巨大的巨震浊积岩。分为基质和岩块两个组成部分,基质为泥、砂质浊积复理石,岩块的岩性有不同时代的砂岩、泥晶灰岩、结晶灰岩、火山岩和硅质岩等,物源主要来自印度板块北缘和洋壳本身。宗卓组中广泛发育有同沉积角砾岩、同沉积塑性变形、微断层、液化脉和砂岩脉等震积标志。依据岩块大小和碎屑沉积物粒径,这套巨震浊积岩可划分为8个“巨浊积旋回”,表现为8个地震周期。宗卓组震积岩形成于会聚板块边缘,是被动伸展构造环境的产物。

A large group of E-W stretching megaturbidites （seismoturbidites） were accumulated in northern Tethys-Himalayan sedimentary belt and distributing along southern to the Indus Tsangpo suture zone, which referred to the Upper Cretaceous Zongzuo Formation. It can be recognized as matrix and block mass. The matrix is composed of muddy and sabulous clastic turbidites, and its sequence is in order. The fossils in matrix are Cretaceous. The block masses is composed of sandstone, micritic limestone, crystalline limestone, volcanic and siliceous rocks. The blocks are of different ages that are form Permian to later Cretaceous. They mostly came from the northern part of the Indian passive continental margin or ocean crust itself. The megaturbidites were produced by large break-up and earthquakes in southern Xizang （Tibet）. The upper Cretaceous Zongzuo Formation is characterized by synsedimentary breccia, hydroplastic deformations, fault grading, ＂molar tooth＂ and sedimentary dykes. According to the size of blocks and clastics, the megaturbidites can be divided into 8 sedimentary cycles, relating to 8 earthquake cycles. The Zongzuo Formation deposited on convergent plate boundary, and produced under tectogenesis extending environment. Seismites that caused by large break-up and earthquakes are suggested as one of the composition of old plate boundary and one of the characters of suture zone.