ANISOTROPY AND THE RHEOLOGY OF AN ARC-CONTINENT COLLISION, TAIWAN

In the Taiwan arc-continent collision, continental crust within the collision zone is accreted within a wedge that faces Asia (i.e., the pro-wedge), but parcels of rock advect west to east relative to the Luzon arc and, in the case of the basement and deepest passive margin sediments, are ultimately exposed at the steep surface of a wedge that faces the arc (i.e., the retrowedge). The rheology of any point in the system is dependent on the rock fabric that has developed along this particle trajectory. This consideration leads to predictions about the rheology, kinematics, and fabrics that can be tested against observations of the Taiwan mountain belt. For example, the strength of an anisotropic material depends on the orientation of fabric relative to Sigma1 (Phi), with high strength for Phi values of 0 or 90 degrees, and lower strength when there is larger resolved shear stress on cleavage planes. For frictionless deformation, when Phi is between 45 and 90 degrees, additional increments of strain are stable, and fabric rotates without development of a second cleavage. When Phi is between 0 and 45 degrees, deformation is unstable, and buckling leads to development of crenulation cleavage. Doubly vergent wedge models predict a reversal in the direction of plunge for Sigma1 from prowedge to retrowedge. Material accreted within the prowedge develops a single arc-ward-dipping cleavage within a stable regime, but advection into the retrowedge leads to a higher strength followed by buckling instabilities and crenulation cleavage development. In an obliquely convergent setting such as Taiwan, movement through the prowedge can lead to strengthening with respect to orthogonal shortening while weakening of the wedge with respect to strike-parallel simple shear—a process that inherently leads to strain partitioning.

In Taiwan, a single east-dipping cleavage dominates the prowedge slate belt, and this fabric is overprinted by a west-dipping crenulation cleavage in the retrowedge that increases in intensity eastward. In samples from the retrowedge, strain histories associated with the early slaty fabric record downdip extension followed by sinistral shear. These results are consistent with the model predictions and illustrate potential positive and negative feedbacks between anisotropy and strength that could be significant for the mechanics of mountain-building.