Curvature of Fold and Thrust Belt in NE Taiwan: Paleomagnetic Results and Their Implications on Plate Indentation and Back-Arc Opening Processes

The Taiwan orogen is the result of the collision of the Luzon volcanic arc pushed northwestward by the Philippine Sea plate against the Eurasian plate since Late Miocene. The Taiwan orogeny is also accompanied by the opening of the Okinawa Trough. The fold-and-thrust belt in NE Taiwan shows a convex shape. What are the roles of plate indentation and back-arc opening that played in the curvature of the NE Taiwan salient?

Our paleomagnetic study indicates both a complex pattern of local block rotations characterized by pre- and post-folding magnetizations, and a consistent, homogeneous pattern of magnetic lineations (supported by the anisotropy of magnetic susceptibility) which remain parallel to the regional fold axis all along the salient. Such homogeneity of the trends of the structures favors the interpretation of the rotational pattern as a succession of large-scale regional clockwise then local anticlockwise rotations.

The pre-collision structure trends corrected by the block rotations in different areas reveals a syn-collision curvature that mimics the current shape of the Central Range and an inherited convex shape only bent significantly at its northern tip. We propose to associate the main clockwise bending of the NE Taiwan salient to the indentation of the Luzon volcanic arc against the Eurasian plate and the anticlockwise "un-bending" to the opening of the Okinawa Trough.