藏东昌都地区侏罗纪岩石磁组构研究及其构造意义

Anisotropy of magnetic susceptibility study of the Jurassic rocks in the Qamdo Area,eastern Tibet,and its tectonic implications

本文对藏东昌都地区侏罗纪汪布组、东大桥组和小索卡组红层共71个采点开展了磁组构(AMS)研究。磁组构测试结果表明,早侏罗世汪布组岩石磁线理较磁面理发育,磁化率各向异性度较高,磁化率椭球最小轴K3散布于层面缩短方向,代表了与构造成因相关的磁组构;中侏罗世东大桥组和晚侏罗世小索卡组岩石则磁面理较磁线理发育,磁化率各向异性度较低,磁化率椭球最小轴K3与层面近垂直,指示了原生沉积磁组构。早侏罗世汪布组地层的磁组构揭示了其构造应力场方向为NE-SW向。中侏罗世东大桥组的磁组构指示了其沉积时的古水流方向为SE向(138.3°),而晚侏罗世小索卡组磁组构指示了其沉积时的古水流方向为NNW向(328.3°)。古水流方向的明显变化揭示了昌都地区从中侏罗世到晚侏罗世沉积物物源发生了相应的转变,表明昌都地区南早北晚的隆升过程。

In this paper,anisotropy of magnetic susceptibility(AMS)was measured on samples from 71sites of the Jurassic Wangbu Formation,Dongdaqiao Formation and Xiaosuoka Formation in the Qamdo area of eastern Tibet.The AMS of the Lower Jurassic Wangbu Formation indicates that the magnetic lineation is more developed than the magnetic foliation,the anisotropy degree is very high,and the minimum axis K3 of magnetic axis ellipsoid is scattered in the layer shortening direction,which is related to the structural cause.The AMS of the Middle Jurassic Dongdaqiao Formation and the Upper Jurassic Xiaosuoka Formation implies that the magnetic foliation is more developed than the magnetic lineation,the anisotropy degree is very low,and the minimum axis K3of magnetic susceptibility ellipsoid is near vertical to the beds.These findings indicate the primary sedimentary fabric for the Middle Jurassic Dongdaqiao Formation and the Upper Jurassic Xiaosuoka Formation.The AMS of the Lower Jurassic Wangbu Formation means that the direction of the tectonic stress field was close to NE-SW direction during the Early Jurassic.The obvious change of the paleocurrent direction between the Dongdaqiao Formation and the Xiaosoka Formation suggests that the uplifting process in the southern part of the Qamdo area was earlier than the north,and the provenance of the sediment had also undergone a corresponding transformation.