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

台湾代表着地球上最活跃的构造环境之一。该岛标志着吕宋火山弧和亚洲被动边缘之间的碰撞，几十年来，这种碰撞激发了地球动力学模型来解释世界各地许多山脉的演化。这种碰撞驱动了台湾山带的变形和活动隆升。由于台湾的快速隆升和高剥蚀率，台湾变质核部暴露在台湾中央山脉的核心及其东缘。在这里，一个由美国和台湾的科学家和学生组成的联合团队正在探索这样一种观点，即变质岩的折返是由伸展作用促进的，而不是通常被引用的碰撞驱动的缩短模式。研究人员将研究变质岩的几何形状和活动断层的运动模式，以确定岩石被挖掘出来的机制。除了对造山过程提供新的见解外，该项目还通过妇女和未被充分代表的少数民族参与STEM促进预期的社会成果；(B)通过本科生和研究生培训以及为台湾本科生举办研讨会，发展多元化的、具有全球竞争力的STEM劳动力队伍；以及(C)通过国际合作增加伙伴关系。更具体地说，该项目的目标是记录台湾变质核心区从塑料到脆性的挖掘历史。中东部山脉的变质核(塔纳脑片岩)具有一些特征，这些特征很难在稳定的、双叉的造山楔形背景下解释；这是目前流行的模式。在山带的大部分地区，变质面理呈陡峭的倾角，形成东倾西倾的区域性解理扇。靠近碰撞缝合线(纵谷)，这种简单的模式被破坏，最年轻的变质面理通常是缓缓倾斜的。此外，到目前为止，该地区观测到的大多数中尺度断裂都记录了正常位移，并适应了NE-SW向的伸展，与地壳地震中记录的伸展方向相同。研究人员的目的是验证这样的假设，即台湾东部的正向位移断层和剪切带适应变质核的同碰撞折返。这将需要系统地记录塔纳脑片岩中最年轻的变质组构和叠加的脆性结构的几何学和运动学，并建立受地震构造和地震层析成像约束的塑性-脆性变形模式和正在进行的变形之间的关系。由美国和台湾科学家以及研究生和本科生组成的研究小组将收集变质组构数据，以及用于岩相学和XCT分析的定向岩石样本。还将收集韧性剪切带和叠置脆性结构(如断层和节理)的运动学数据。现有的地震震源机制目录将由台湾中央气象局数据产生的新机制补充。研究结果将结合地震层析成像和地震构造对活动变形的约束，以更好地了解台湾-S变质核折返的机制。

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.