Tectonic analysis and the evolution of a curved collision belt: The Hsüehshan Range, Northern Taiwan

Detailed studies of fracture patterns and paleostress reconstructions based on analyses of fault-slip data sets allow identification of four main patterns of compressional paleostress orientations in the Cenozoic rocks of northern Taiwan. These patterns correspond to five successive tectonic events dominated by (1) N-S, (2) WSW-ENE, (3) WNW-ESE, (4) NW-SE and (5) WNW-ESE directions of compression. The first two events are characterized by relatively minor structures in the foreland fold-and-thrust belt; the first event probably reflects major left-lateral shear along NNE-SSW trends which occurred in the adjacent eastern metamorphic belt during the middle-late Miocene. These first two events do not affect the Plio-Pleistocene in the localities studied. In contrast, the last three events created the major compressional structures of the foreland belt during the Plio-Pleistocene collision in Taiwan. Preliminary studies suggest that the magnetic fabric of the rocks has been significantly influenced by compressional deformation along WNW-ESE and NW-SE trends, especially in the Slate Belt, thus confirming the importance of these last three events. On the basis of paleomagnetic work in progress the northernmost segment of the Taiwan Belt has been affected by a clockwise rotation of about 20°. Taking into account the distribution of successive compressional paleostress patterns, we conclude that of the 55° angle of the sharply curved belt of northern Taiwan, about 20° probably correspond to actual clockwise rotation during the Plio-Pleistocene whereas the remaining 35° reflect the initial shape of the curved Plio-Quaternary belt. The paleostress history of northern Taiwan is examined in terms of geodynamic evolution and kinematic reconstructions. The Plio-Quaternary development of the collision between the Luzon Arc and Eurasia followed a late Miocene predominantly transform motion from south to north of the Philippine Sea Plate relative to Eurasia. The succession of Plio-Pleistocene paleostress patterns in northern Taiwan is found to be consistent with the longitudinal shift (from northeast to southwest) and the transverse migration (from the inner to the outer zones) of compressional phenomena related to the evolution of oblique collision. Some major changes in compressional trends are also accounted for by variations in the kinematics of plate convergence: the most recent charge comprises a counterclockwise change in the direction of Philippine Sea Plate-Eurasia convergence.