PALEOSTRESS INVERSION FOR THE ONSHORE NORTH MARGIN OF SOUTH CHINA SEA SINCE LATE MESOZOIC:TAKING ZHANGMUTOU,GUANGDONG PROVINCE AS A CASE

Widely spreaded in the upper-crust, brittle fractures are one of the most important forms of rock deformation. They are very useful for paleostress inversion in certain areas. We made a paleostress field reconstruction in the area of the Zhangmutou Town and adjacent areas, applying the Multiple Inverse Method developed by Yamaji. We obtained 125 valid fault-slip data at 5 valid measurement sites, and got inversion results-the orientation of principal axis of stress and φ, a parameter representing the shape of stress ellipsoid-with Multiple Inverse Method software package. Based on our field observation and the inversion results, combined with tectonic analysis, four stages of paleostress were subdivided since Late Mesozoic in the research area:①the stress regime which led to NNW-SSE extension,within which the principal axis of σ₁was nearly vertical, and the azimuth of σ₃ diverted from 254.9° to 355.7°; ②the stress regime which led to ENE-WSW extension, within which the principal axis of σ₁was nearly vertical and the azimuth of σ₃was around 64.9°; ③strike-slip type stress regime within which the azimuth of σ₁is NE-SW(46.8°~59.9°); ④strike-slip type stress regime within which the azimuth of σ₁ is NW-SE(329.5°~340°). The tectonic implications of the paleostress regime are discussed at the same time. 1) the first stage of extension(NNW-SSE) had same mechanism as the rifting of South China Sea. It belongs to the continental margin extension in East Asia which resulted from the rollback of paleo-Pacific plate; 2) the second stage of extension(NNW-SSE) was associated with the clockwise rotation of the Indochina Block during the late stage of its extrusion; 3)the third stage of strike-slip (NE-SW) stress regime might subject to the convergence of India-Australia Plate and Eurasian Plate; 4) the last stage of strike-slip (NW-SE) stress regime may controlled by the subduction of the Philippine Sea Plate beneath the Eurasian Plate.