Subhorizontal stretching, oblique collision, and landscape evolution in the Central Range of Taiwan

Abstract:

The deformed schists and slates of the eastern Central Range of Taiwan (ECR) are characterized by a strike-parallel, or gently northeast-plunging stretching lineation. This first-order, regionally consistent characteristic could be explained by the early tectonic history, by strain partitioning, or by lateral extrusion. From west to east across the range, the stretching lineation varies from down dip to along-strike. Strain histories recorded by syntectonic fibers indicate that the western central Range experiences non-coaxial strain consistent with west-directed thrusting. In the ECR, fibers record down dip stretching followed by rotation of the stretching direction into parallelism with the subhorizontal lineation. Therefore, the rocks of the ECR record a history of deformation west of the divide that precedes advection of subsurface material from west to east across a regime of horiztontal stretching fixed relative to the orogenic topography. Thus, the lineation is the last fabric imparted on the rocks and not related to pre-collision tectonics. Two explanations remain: does material turn north relative to the orogenic topography as it advects beneath the divide, or does material turn to the south as the thermally weakened interior of the central range is extruded toward the region of lower topography? The first hypothesis is problematic given the lack of significant obliquity in the relative plate motion vector and the lack of consistency in the sense of strike slip from kinematic indicators such as quartz c-axis fabrics in granites of the Eastern Central Range. Extrusion of the core of the ECR could play a role in shaping of the landscape and crustal structure of the Hengchun peninsula in southern Taiwan. Topographic observations in the south are consistent with rock-uplift and horizontal strain pattern driven by on-going extrusion from the north. Such observations include progressively rotated drainage basins and a ‘V’ shaped pattern of high channel steepness that points to the south. We quantify these topographic patterns from a DEM, and model the topographic development using a landscape evolution model. The high obliquity between the arc and the Asian passive margin provides a mechanism for progressively squeezing the interior of the orogeny toward the south as the collision propagates laterally.