Abstract:

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.

Key words: columnar joint, secondary joint, Sai Kung, Hong Kong, High Island Formation, rhyolitic porphyroclastic lava