Tectonic exhumation of a metamorphic core in an arc-continent collision during oblique convergence, Taiwan

Abstract

Observations over the last few decades from a number of orogenic systems have highlighted the possible importance of tectonic exhumation, i.e., ductile thinning and normal faulting, in exhuming rocks once buried in high-pressure conditions. Taiwan is one of the few active orogens in the world where rocks that once experienced high-pressure metamorphism (> 50 km) are exposed at the Earth’s surface, providing a natural laboratory for advancing our understanding of exhumation processes. We integrate previously published studies of the Taiwan orogen with new structural, geochronological, and fluid inclusion microthermometry data to argue that tectonic extrusion and structural thinning played a critical role in exhuming the metamorphic core of the orogen until very recently, ca. 0.7 Ma. We propose a two-stage process for exhuming the high-pressure metamorphic rocks of the Yuli Belt: an initial stage where exhumation is driven primarily by pressure gradients in a subduction channel and a second stage that is initiated as an orogen-parallel regional-scale strike-slip zone, the Tailuko shear zone, is offset by an orogen-normal strike-slip zone. The offset generates an extensional bend that is filled with extruding high-pressure rocks as the upper crust is structurally thinned. Evidence for tectonic thinning comes primarily from a low-angle penetrative foliation that records significant vertical shortening and a suite of sub-vertical late-stage, mineral-filled veins. Isotopic dating indicates that the second stage started ca. 2.4–3.1 Ma and ended at ca. 0.7 Ma when the northern Backbone Range orocline started to form. We propose the low-angle foliation formed in the footwall of a regional-scale extensional shear zone that rooted to the east, beneath the forearc. Combined tectonic and erosional processes may have limited the topographic growth of the orogen from ~3.0 to < 1.0 Ma.