Geometry and Kinematics of Complex Structures in the Fold-and-Thrust Belt, Northwest Taiwan

Kenn-Ming Yang¹, Shiu-Tsann Huang¹, Ching-Weei Lin², Jong-Chang Wu¹, Wan-Chi Chiang², Hsin-Hsiu Ting¹, Chen-Jung Chou², and Wen-Wei Mei^1
1 Chinese Petroleum Corporation, Miaoli, Taiwan
² National Chengkung University, Tainan, Taiwan

The fold-and-thrust belt is the major gas-producing area in NW Taiwan. Hydrocarbons are trapped in fault-related structures that were formed during the latest phase of orogeny. The foreland areas are imprinted with pre-existing structures, which resulted in the syn-orogenic complex structures. The complex structures can be manifested by transverse structures that have originated from the normal fault reactivation predominantly in the outer fold-and-thrust belt and by lateral discontinuities between tight and breached fault-propagation fold structures in the inner fold-and-thrust belt. The transverse faults extend toward the hinterland and are cut on the surface by structural settings of the inner fold-and-thrust belt. Some discontinuities, surface and subsurface, between the fault-propagation fold structures in the inner fold-and-thrust belt might be connected with the transverse structures in the outer fold-and-thrust belt.

Structural analyses indicate that, in the beginning of each episode of major thrusting, low-angle thrusting in the inner fold-and-thrust belt and the normal fault reactivation in the outer fold-and-thrust belt might happen coevally but with transverse faults propagating toward the front of the developing low-angle thrust system in the inner fold-and-thrust belt. During the next stage of the episode, low-angle thrusts started to develop but were confined by the transverse faults. In the meaning time, transverse faults continued to develop underneath the low-angle thrust system and resulted in the surface and subsurface discontinuities between the fault-propagation fold structures in the inner fold-and-thrust belt. Such kinematic model provides the best explanation for the structural pattern of the fold-and-thrust belt in NW Taiwan.