THE STRUCTURAL MODEL OF TRANSFER ZONES IN ONSHORE AND OFFSHORE AREAS IN NORTHWESTERN TAIWAN

Structural transfer zones in the frontal part of a fold-and-thrust belt mark the end of segments of major thrusts and represent the lateral variation in magnitude of displacement, slip direction and style of fault-related structure.

We use seismic profiles and well bore data to interpret subsurface structural geometry, build a three-dimensional structural mode, and integrate trishear modeling to analyze the structural evolution.

In the Tiehchanshan structure, the two anticlines gradually become symmetric toward transfer structure zone. Based on trishear modeling, the P/S ratio increases toward the transfer structure zone. As the dip of fault plane moderates, the influence of apical angle of trishear zone becomes weaker on fold geometry.

In the Yunghoshan-Chinshui structure, the Luchukeng fault is characterized by flat-ramp-flat structural geometry in the Yunghoshan structure, but transformed into a high-angle thrust in the Chinshui structure. The trishear modeling shows that the P/S ratio of main folding stage is similar to the result of the Tiehchanshan structure.

In the offshore Hsinchu, the E-W striking transcurrent faults of Hukou fault system in the eastern part change to NE-SW striking westward. And these transcurrent faults accompanied with symmetric fold geometries are high-angle reverse faults which resulted from normal faults, whose arrangement is en echelon and reactivated by late compression.

On the whole, three types of structural transfer zone can be identified based on their distinct development: 1. formation of the structural transfer zones in the inversion tectonic belt were controlled by the arrangement and linkage of early normal faults; 2. the continuity of fold geometry was first influenced by lateral variation in dip angle of low-angle thrust and in turn broken by high-angle transcurrent faulting, forming segmented fold structures during the late compression; and 3. slip along two parallel thrusts with opposite vergence and decreasing dip angle toward the transfer zone formed complex fault-related folds. The variation of P/S ratio along the strike of some thrusts also plays important role in shaping the features of the transfer zone.