PALEOSTRESS ANALYSIS AS A KEY TO MARGIN EXTENSION - THE PENGHU ISLANDS, SOUTH CHINA SEA

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 (σ1/σ2 and σ2/σ3 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-Quaternary 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. © 1990.