Collaborative Research: Reconstructing the Plastic-to-Brittle Exhumation History of the Taiwan Metamorphic Core

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Taiwan represents one of the most active tectonic environments on Earth. The island marks the collision between the Luzon volcanic arc and the passive margin of Asia and for decades the collision has motivated geodynamic models to explain the evolution of many mountain belts around the world. This collision drives deformation and active uplift of the Taiwan mountain belt. Because of the very rapid uplift and high erosion rates the metamorphic core of Taiwan is exposed in the core of Taiwan's Central Range and its eastern margin. Here, a joint team of U.S. and Taiwanese scientists and students is exploring the idea that exhumation of the metamorphic rocks is facilitated by extension rather than the collision-driven shortening model that is usually invoked. The researchers will examine the geometry of the metamorphic rocks and the movement patterns of active faults to determine the mechanisms by which the rocks are exhumed. In addition to providing new insights into mountain building processes, the project advances desired societal outcomes through participation of women and underrepresented minorities in STEM; (b) development of a diverse, globally competitive STEM workforce through undergraduate and graduate student training plus a workshop for Taiwan undergraduate students; and (c) increased partnerships through international collaboration.More specifically, the objective of this project is to document the plastic-to-brittle exhumation history of the metamorphic core of Taiwan. The metamorphic core (the Tananao schist) in the eastern Central Range is marked by characteristics that are difficult to explain in the context of a steady-state, doubly-vergent orogenic wedge; the current prevailing model. In most parts of the mountain belt, metamorphic foliations are steeply dipping, forming a regional cleavage fan that is east dipping in the west and west dipping in the east. Close to the collisional suture (Longitudinal Valley) this simple pattern breaks down and the youngest metamorphic foliation is commonly gently dipping. Moreover, most mesoscale faults observed to date in this area record normal displacement and accommodate NE-SW extension, the same extension direction as recorded in crustal earthquakes. The researchers aim to test the hypothesis that normal displacement faults and shear zones accommodate syn-collision exhumation of the metamorphic core in eastern Taiwan. This will entail systematically documenting the geometry and kinematics of the youngest metamorphic fabrics and overprinting brittle structures in the Tananao Schist, and establishing the relation between the plastic-to-brittle deformation patterns and ongoing deformation as constrained by seismotectonics and seismic tomography. The research team composed U.S. and Taiwanese scientists and graduate and undergraduate students will collect metamorphic fabric data, and oriented rock samples for petrographic and XCT analyses. Kinematic data for ductile shear zones and overprinting brittle structures, e.g., faults and joints, will also be collected. The existing catalog of earthquake focal mechanisms will be supplemented with new mechanisms generated from Taiwan Central Weather Bureau data. The results will be integrated with seismic tomography and seismotectonic constraints on active deformation to better understand the mechanisms behind exhumation of Taiwan?s metamorphic core.