TRANSVERSE STRUCTURES IN FOLD-AND-THRUST BELT, NORTHWESTERN TAIWAN

Transverse structures are predominant in the outer fold-and-thrust belt in northwestern Taiwan. The transverse fault extends toward the hinterland and is terminated on the surface by structural settings of the inner fold-and-thrust belt. The main purpose of this study is to investigate the characteristics of subsurface structural styles of the transverse structures in northwestern Taiwan. This study used seismic data and built several balanced cross sections through the fold-and-thrust belt to delineate a 3-D structural geometry in the study area. In the outer fold-and-thrust belt, the surface thrust and fold settings are offset by the transverse fault apparently in a left-lateral sense. However, heave measured along low angle thrust fault on both sides of the transverse fault indicates right-lateral relative displacement along the transverse fault. Some isolated subsurface fold structures in the footwall of the low angle thrusts were mapped and are arranged in a right-lateral en echelon, in consistent with the right-lateral strike-slip component along the transverse fault. On the other hand, throw along the low angle thrusts decreases laterally toward the transverse fault. Such lateral variation in structural feature suggests that low angle thrusts and the associated structures have been developing and propagating laterally until they met with and were confined and affected by the transverse fault, which is inferred, based on regional tectonics and coexisting normal faults, as a reactivated normal fault. The major structural features in the inner fold-and-thrust belt are tight folds and are characterized by fault-propagation folds, some of which are with steep or even overturned and thinned forelimb. A series of parallel balanced cross sections show that the sole slip surface of the fault-propagation fold structure changes in depth laterally and may form a subsurface lateral ramp. In some places the sole slip surface may step up into higher level across a blind transverse fault. Such features manifest different role played by the transverse fault in the outer and inner fold-and-thrust belts, respectively.