Tectonophysics, 183 (1990) 161-176 Elsevier Science Publishers B.V., Amsterdam 161 Paleostress analysis as a key to margin extension: The Pen&u Islands, South China Sea Jacques Angelier a, Franqoise Bergerat a, Hao Tsu Chu b, Wen Shing Juang bandChiaYuLuc a Tectonique quantitative, URA 1315 CNRS, T25-26 El, Universit& P.&M. Curie, 75252 Paris Cedex 05 (France) b Central Geological Survey, M. 0. E.A., P. 0. Box 968, Taipei (Taiwan, China) ’ Department of Geology, National Taiwan University, 245 Choushan Road, Taipei (Taiwan, China) (Received April 28, 1989; revised version accepted October 6, 1989) ABSTRACT Angelier, J., Bergerat, F., Chu, H.T., Juang, W.S. and Lu, C.Y., 1990. Paleostress analysis as a key to margin extension: The Penghu Islands, South China Sea. In: J. Angelier (Editor), Geodynamic Evolution of the Eastern Eurasian Margin. Tectonophysics, 183: 161-176. The Penghu Islands belong to the Eurasian passive margin, west of the collision zone of Taiwan. From south to north, the South China Sea basins become narrower and disappear in the Taiwan Strait area. The presence of the Penghu Islands allow description of the tectonic evolution of the margin through the reconstruction of tectonic paleostress orientation based on analysis of fault slip data collected in the field. Brittle structures have been studied on four islands in Neogene basalt flows with thin interbedded sediments. Sets of minor striated faults, tension gashes, dikes and joints enabled us to reconstruct the orientation of late Cenozoic stresses. Two main extensional tectonic events were thus identified: a N-S extension dominated during the Middle-Late Miocene (especially in the northern Penghu Islands), and a more recent NW-SE extension prevailed during the Late Miocene (especially in the southernmost island). The latter extension induced complicated patterns of perpendicular normal fault systems and strike-slip conjugate and transfer fault systems, through permutations between principal stress axes (q/q and ~*/a, modes). The ages of these tectonic events are ascertained by the existence of syndepositional normal faults, dike injections and superposed brittle structures. A third, less important compressional tectonic event, is correlated with the Plio-Quatemary collision in adjacent Taiwan (ENE-WSW compression). The comparison between the results of marine studies in the South China Sea basins and our paleostress reconstruction in the Penghu Islands suggests that repeated changes between N-S and NW-SE directions of extension occurred in the past. The counterclockwise change from N-S to NW-SE recorded in the Penghu Islands took place between about 12 and 8 Ma ago, indicating that significant extensional phenomena along the South China margin may postdate the seafloor spreading activity in the central ridge. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Introduction zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA The Penghu Islands, previously called the Pescadores, are located in the southern Taiwan Strait, between Taiwan and the mainland (Fig. 1). The Penghu archipelago includes sixty-four is- lands and islets on both sides of the Tropic of Cancer, and it constitutes part of the continental shelf, in water depths shallower than 100 m. Chimei, the southernmost island, is close to the ENE-WSW trending slope of the South China Sea passive margin; the distance between Chimei and the top and the base of the steep continental slope averages 50 and 100 km respectively (Fig. 1). Due to this particular location close to the upper part of the passive margin, it is possible to study some characteristics of the Neogene exten- sional tectonics related to major rifting processes in the South China Sea by means of field observa- tion and data collection in the Penghu Islands. Note that the Penghu Islands are close to, but outside the Taiwan orogen (Ho, 1979); as a result, oo40-1951/90/$03.50 0 1990 - Elsevier Science Publishers B.V.