OBLIQUE CONVERGENCE AND SYN-OROGENIC EXTENSION IN ACTIVE ARC-CONTINENT COLLISION: CONSTRAINED BY EARTHQUAKE ANALYSIS IN SOUTH CENTRAL TAIWAN

We analyze seismicity and earthquake focal mechanisms in south central Taiwan to understand the mechanics of oblique convergence and syn-orogenic extension in an active arc-continent collision. Seismicity in arc-continent collision zone occurred throughout the upper crustal levels above a gently east-dipping horizon from the Western Foothills (WF) in the west to the Backbone Range (BR) and the eastern Central Range (ECR) in the east. Across the border between ECR and BR in the hinterland, earthquakes appear with mostly normal faulting mechanisms with T-axes directing NE-SW at a depth range from near-surface to 15 km. In the BR of the collision zone, earthquakes with a mixture of normal and strike-slip mechanisms striking mostly NW-SE with a series of E-W trending secondary sub-sets also with NE-SW trending T-axes appear above a 10 km depth. Focal mechanisms stress inversion results indicate normal motions with the minimum compressive stress sub-parallel to the strike of the Taiwan orogen under the BR and ECR. In the BR series of strike-slip faults resembled conjugate Riedel shears in their geometry and their senses of shear. Orogen-parallel extension in BR and ECR indicates lateral extrusion of the collisional orogen. The tectonic features of Riedel shears and orogen-parallel extension in the arc-continent collision zone indicate strain partitioning in an actively deforming shear zone produced by oblique convergence. The earthquake analysis in south central Taiwan provides critical constraints on the tectonic mode switches and stress permutations within an active arc-continent collisional wedge.