西秦岭早三叠世沉积特征及其构造控制作用

Early Triassic Sediments and Their Tectonic Controlling Factors,Western Qinling

西秦岭早三叠世沉积由深水浊积岩、角砾岩、砾岩、滑塌堆积、深水灰岩和钙质泥岩六个岩相组成，在垂向上表现为受构造控制而形成的四个沉积旋回。浊积岩及滑塌堆积的指向表明，大陆边缘的斜坡倾向西南，古水流方向２００—２６０°、说明沿碌曲－成县以北的北方板块边缘在早三叠世存在一引张构造背景，这与扬子板块向北俯冲引起的拖拉滚动有关。

The early Triassic deposits consist of chaotic megabreccias,conglomerates, slump deposits,turbidites, deep-water limestones and calcareous mudstones.From bottom to top, sediments show four sedimentary cycles controlled by tectonic process. A single sedimentary cycle is composed of chaotic megabreccias, conglomerates,turbidite, slump deposits,deep-water limestone and calcareous mudstones upward. Based on the directional structures of turbidite and slump deposits, continental margin slope dipping to the west-south, the palaeocurrent direction ranges from 220°to 260°.This indicates that on the north plate margin existed an extensional tectonic setting along Luqu-Chenxian in Early Triassic, which is related to the rollback caused by the subduction of the Yangtze plate towards the north.The subduction causing the roll-back process made the north plate margin form a series of normal faults. This also made the early formed shallow water platform factes break,collapse and slide along the steep slope,at last they deposited in the deep water basin and formed the chaotic megabreccias,conglomerates deposits which slid and slumed at the later,and the turbidite,at the relatively stable period, formed the deep water limestone and calcareous mudstone.In another tectonic active period,the similar sediments deposited in the deep water basin.So,the early Triassic deposits were obviously controlled by the tectonic process.