十万大山盆地早期走滑拉分盆地原型及其地质意义

The Early Stage Pull-apart Basin Prototype of the Shiwandashan Basin and its Geological Significance

十万大山盆地位于中国西南部广西东南缘,大地构造属于古特提斯北支洋盆的北缘构造域,盆地形成演化可能记录了古特提斯俯冲碰撞闭合及印支期这一地区地球动力学过程。前人认为十万大山盆地是前陆盆地,并提出广西地区中晚二叠世界线之间存在东吴造山运动。本文对十万大山盆地下构造层(晚二叠世‒早三叠世,或称早期)进行了系统研究,提出十万大山盆地早期为走滑拉分盆地。主要证据如下:十万大山盆地早期长约200 km,宽80 km左右,平面上呈似菱形展布,剖面上为槽状,最大沉积厚度>5 km;野外观察到上二叠统彭久组与中二叠统为深海相的连续沉积,前人提出的广西东吴造山运动并不存在;上二叠统砾岩分布与盆地两条边界断裂(钦防断裂和小董-峒中断裂)相关,且两条断裂附近的砾岩中砾石成分各异,来源于盆地周缘,并不是前人定义的“磨拉石”沉积建造;盆地内部岩相古地理具相变快,相带窄的特点,与前陆盆地不同;另外盆地形成伴随着火山活动,后期又被大量花岗岩侵入,总体展现出强烈的岩浆活动特征,不同于前陆盆地缺乏岩浆活动。通过与典型前陆盆地和走滑拉分盆地的系统对比,认为十万大山盆地早期为走滑拉分盆地,其形成于印支板块向华南板块穿时碰撞的地球动力背景下,是两个板块初始碰撞的沉积记录。这一研究成果不仅恢复了十万大山盆地早期盆地原型,而且对于理清广西地区甚至华南古‒中生代之交的印支运动具有重要意义。

The Shiwandashan Basin is located at the tectonic junction of Paleo-Tethys,South China and Indochina blocks.The basin formation and evolution recorded the geodynamic processes of South China Block during the Indosinian.It is accepted that the Shiwandashan Basin is a foreland basin caused by the Dongwu orogenic movement during the Middle to Late Permian in Guangxi.In this paper we present a systematic and comprehensive study on the early tectonic layer and propose a prototype of pull-apart basin for the Shiwandashan Basin in the Late Permian.The main points are as follows:the prototype basin is about 200 km long and 80 km wide,rhombus shape on the plane and rift-like on the section with maximum deposition thickness of more than 5 km.Field observation showed that the sedimentation of the Upper Permian and the Middle Permian is characterized by the continuous abyssal sea facies,implying the Guangxi Dongwu orogenic movement suggested previously is virtually none existent.The distribution of the conglomerates in the Upper Permian is controlled by two boundary faults.Also,diverse gravel components of the conglomerates nearby the two faults came from the periphery of the basin.Hence,the“molasses”defined by previous studies might be wrong.Unlike the foreland basin,the lithofacies paleogeography in the Shiwandashan Basin is characterized by shape variation and narrow bands,and extensive magmatism(251-245 Ma).By comparing the Shiwandashan Basin with the foreland basin and the pull-apart basin,we concluded that the prototype of the Shiwandashan Basin in the early stage is a pull-apart basin.It is formed by the differential collision between the Indochina and South China blocks.These understandings are of considerable significance to the tectonic evolution of the Indosinian in Guangxi and even in South China.