Taiwan Slate Belt: Insights into the ductile interior of an arc-continent collision

The Slate Belt geologic province in Taiwan consists of three structural provinces with markedly different structural styles, histories, and finite strains: the Hsüehshan Range, the western Backbone Range, and the eastern Backbone Range. The Hsüehshan Range is a regional pop-up structure consisting of a biotite-grade core bounded on the west by the east-dipping Chüchih thrust fault and on the east by the westdipping Lishan fault, a passive-margin growth fault on which the slip direction has been inverted. The western Backbone Range is marked by west-vergent folds, a moderately southeast-dipping cleavage, and a downdip stretching lineation. Pressure shadows and chlorite-mica aggregates record 40%-460% extension during noncoaxial strain histories consistent with top-to-the-west shear where pre-Tertiary basement has been thrust over the Slate Belt. In contrast, the stretching lineation in the eastern Backbone Range plunges gently to the northeast, with a finite elongation of 180%-500%, subparallel to fold axes and the strike of structural fabrics. Thus, from west to east across the Backbone Range, the extension direction changes from downdip to along strike. In the Slate Belt, a comparison of ductile strain indicators with brittle structures, seismicity, and GPS (Global Positioning System) data indicates that there is vertical strain partitioning in the eastern part of Taiwan, whereby the ductile regime has significantly different kinematics than the near-surface brittle regime.