INCLINED TRANSPRESSION IN THE TACONIC AND TAIWAN SLATE BELTS

Theoretical models of inclined transpression provide a framework for understanding strain distribution in accretionary wedges that form in response to oblique convergence. Low-temperature metamorphic rocks exposed in the Late Ordovician Taconic and modern Taiwan accretionary wedges can be compared in the context of such models, because broad analogies can be made between depositional and tectonic setting. Strata in the Giddings Brook thrust sheet of the Taconic Allochthon and in the Backbone Range of Taiwan consist of relatively thin successions of deep marine sediments dominated by fine-grained clastic material, and deformation of the strata is well established as the product of arc-passive margin collision.

In Taiwan, the strike of the deformation zone boundaries and the orientation of the relative plate convergence vector are known, because the collision is ongoing. Along the Southern Cross Island Highway, low-temperature metamorphic rocks in the Backbone Range are characterized by non-coaxial plane strain, a mean cleavage strike clockwise from strike of the deformation zone boundaries, and a mean mineral lineation trend approximately parallel to the relative plate convergence vector. Overall, the geometry and kinematics are intermediate between the predictions of models of inclined transpression and complete kinematic partitioning into dip- and strike-slip components. In the northern part of the Giddings Brook thrust sheet, the strata are similarly characterized by non-coaxial plane strain and a mean mineral lineation which, on the cleavage plane, is steeply raking but not down-dip. To the south in an area of anomalously NW-trending folds, the mean mineral lineation has a rake of ~50° on the cleavage plane, and the strata show evidence of flattening strain and triclinic strain symmetry consistent with oblique top-to-NW sinistral sense of shear. These observations can be explained by models of inclined transpression, if the angle in map view between the relative plate convergence vector and the deformation zone boundaries is relatively low. This suggests that the orientation of the deformation zone boundaries changes along strike of the Taconic Allochthon and may be controlled locally by fracture zones related to stepping of the Iapetus rift zone between the New York promontory and Quebec reentrant.