雪峰造山带南段构造变形研究

Study on the Deformation in the Southern Xuefeng Orogenic Belt

前人对雪峰造山带构造变形的认识尚存在较多分歧。本文通过构造剖面测制并结合区域地质调查,对雪峰造山带南段的不整合特征、形变类型、变形时代和强度、剖面结构以及构造变形的运动学特征、成因机制、构造体制等进行了较全面的分析研究,取得主要进展如下:(1)雪峰造山带南段形变类型主要有板劈理、膝折及折劈理、褶皱及与褶皱同走向的逆断裂、正断裂、左行和右行走滑断裂等;(2)广西运动和早中生代构造运动(印支运动与早燕山运动)中均存在明显的挤压变形,在NW-NWW向挤压下形成NE-NNE向褶皱和逆断裂;广西运动还形成了板劈理;(3)雪峰造山带南段以溆浦–靖州断裂为界分为东带和西带,分别为加里东期雪峰逆冲推覆构造的根带和中带,广西运动中东带变形强度显著大于西带,表现在东带板劈理发育且褶皱相对紧闭,而西带板劈理不发育且褶皱相对开阔,岩层倾角、不整合特征、抬升幅度、先期构造线偏转等表明早中生代构造运动中东带变形较强,而西带变形微弱;(4)劈理优势倾向以及褶皱轴面和逆断裂的倾向,反映雪峰造山带东带广西运动和早中生代构造运动中均具有背冲构造样式,但后者背冲构造的中轴相对前者显著向西迁移约20 km;(5)溆浦–靖州断裂以东在广西运动中成为雪峰推覆构造的根带,主要与其块体强度低及南华纪早期为一断陷深槽有关。

There exist different views about the deformation of the Xuefeng Orogenic Belt. The authors studied the uncomformity, deformation timing and strength, geometrical and kinematic properties, dynamic mechanisms and tectonic regimes in the southern Xuefeng Orogenic Belt through profile measurements and regional geological survey, and reached the conclusions as follows：（1） There occurred main deformation types such as slaty cleavages, folds, thrusts, normal faults, sinistral and dextral strike-slip faults.（2） There existed notable compressional deformations with NW to NWW compression and NE to NNE-trending folds and thrusts in both the Caledonian event and Early Mesozoic tectonic events（Indosinian event and Early Yanshanian event）. Slaty cleavages were formed in the Caledonian event.（3） Southern segment of the Xuefeng Orogenic Belt can be divided into the east and west zones by the Xupu-Jingzhou Fault, and they were the root and middle zone of the Xuefeng thrust system respectively. The deformation in the east was stronger than that of the west zone in the Caledonian event, as shown by slaty cleavages and tight folds in the east zone, whereas the west zone lacks slaty cleavage and with more open folds. Differences of layer dip angles, uncomformity, uplift, and rotation of former lineaments indicated that there existed stronger deformation in the east than west zones in Early Mesozoic tectonic events.（4） Dip directions of slaty cleavages, fold axial surfaces and thrusts indicated that there existed back thrusts in both Caledonian event and Early Mesozoic tectonic events in the east zone of the Xuefeng Orogenic Belt, however, the axis of the back thrust in the Early Mesozoic shifted about 20 km westwards.（5） The eastern segment of the Xupu-Jingzhou Fault turned into the root of the Xuefeng thrust system, which was related to its lower strength and it was a deep fault during early Nanhuan.