Kinematic analysis of the Taiwan Slate Belt

Abstract

The Taiwan Slate Belt can be subdivided into two structural domains based on fold and cleavage geometries and finite and incremental strain histories. The two belts correspond generally to two topographic ranges in Taiwan: the Hsüehshan Range and the Backbone Range. Rocks in the Hsuehshan Range are characterized by upright, symmetric folds and a steeply SE dipping, axial planar pressure solution cleavage. Synkinematic fibers in pressure shadows are typically straight, parallel to cleavage, and plunge downdip, indicating that the rocks experienced a coaxial strain history. In contrast, rocks in the Backbone Range are characterized by inclined, asymmetric folds that verge toward the foreland and display a moderately SE dipping, axial planar pressure solution cleavage. Synkinematic fibers are straight and parallel to the downdip stretching lineation in XY sections. In XZ sections, however, the fibers typically display a clockwise curvature (viewed to the NE), indicating a noncoaxial strain history. We propose that the different strain histories are due, at least in part, to differences in the original tectonostratigraphic settings of the strata in the two ranges. Strata in the Hsüehshan Range are interpreted to have been deposited in a half‐graben associated with an east dipping normal fault. The strata of the Backbone Range, in contrast, are considered to have been deposited on the unfaulted Chinese continental margin. We suggest that the basement horst on the east side of the half‐graben functioned as a steep‐faced, rigid backstop that concentrated pure shear deformation throughout most of the Hsüehshan Range; shear strains were apparently limited to zones near the base of the sedimentary sequence and adjacent to the backstop. The noncoaxial strains characteristic of the Backbone Range are interpreted to be the result of a regional‐scale shear zone that formed as Chinese basement and its sedimentary cover were underthrust southeastward beneath the Luzon Arc.