香港西贡粮船湾组火山岩石柱区次生节理研究

Secondary Joints in Columnar Area of High Island Formation in Sai Kung,Hong Kong

在香港世界地质公园西贡火山岩区,白垩纪粮船湾组发育有目前世界已知面积最大的流纹质碎斑熔岩石柱群。在详细的野外地质调查基础上,对粮船湾组火山岩石柱区的次生节理进行了研究,并探讨了柱状节理岩体的构造变形特征及过程。粮船湾组火山岩石柱区次生节理主要包括陡倾的纵节理及缓倾的横节理。前者多具有共轭剪节理的特征,形成于不同方向的挤压构造环境下;后者切割早期构造面理,形成于重力垮塌的构造环境,多发展为正滑断层。粮船湾组火山岩石柱总体上受次生节理改造明显,共轭节理反演的构造应力环境表明石柱区在140 Ma左右经历了快速的构造转换,主挤压应力由近NE—SW向转换为近NW—SE向,可能与莲花山断裂的构造活动相关。

The columnar jointed High Island Formation are widely distributed in the Sai Kung volcanic rock region of the Hong Kong Global Geopark. It is the largest columnar jointed rhyolitic porphyroclastic lava in the world. On the basis of field geological investigation,we study the secondary joints in the columnar area of High Island Formation,and investigate the structural deformation characteristics and process. The secondary joints in the columnar area can be divided into steep-dip longitudinal joints and gentle-dip horizontal joints. The former are with the characteristics of conjugate shear joints,which formed in the extrusion environment of multi directions. The latter cut the former structural foliation,developed into normal slip faults,which formed in the environment of gravitational collapse. The columnar was obviously reformed by the secondary joints. The stress inversion of conjugate shear joints reveals the rapid tectonic transition in about 140 Ma. The principal compression stress changed direction from NE-SW to NW-SE,associated with the structure activities of the Lianhuashan fault.