An Integrated Structural, Petrologic and Isotopic Study of Fabric Evolution in the Ailao Shan-Red River Shear Zone, China

该项目的目标是确定红河剪切带中地壳岩石的历史，该剪切带形成一个弧形断层，顺时针从西藏东部横扫中国西南部，然后沿着红河穿过河内到达东京湾。这个断层在地质学上最近的过去，大约3000万年前，容纳了数百公里的位移。被测试的假设是剪切加热。断层中的岩石可能会被与这些大位移相关的摩擦加热。如果这个断层带的岩石被如此加热，那么它们就会变得脆弱，不太容易发生地震。NSF的这个项目正在资助实地工作，以便在中国进行结构观测和样本收集，以及对样本进行实验室工作，以检测最后一次加热事件发生的时间。该项目还涉及与台湾研究人员的合作，这增加了重要的国际影响。更具体地说，一些人认为该断裂带的岩石是在印度与亚洲的渐新世-中新世碰撞期间，随着印度支那地块被挤出扬子地块，剪切加热而变质的。一些加热的证据来自渐新世到中新世的热年代计，特别是独居石，白云母和钾长石。根据这些年龄都是冷却年龄的解释，岩石在30到20 Ma之间经历了700°C的温度，然后迅速冷却。然而，也有可能这些年龄可能是结晶年龄，在这种情况下，剪切加热的论点变得相当弱。我们将描述白云母和钾长石的特征，以确定这些矿物是岩浆还是变质的。然后，我们将使用40 Ar/39 Ar年龄谱方法测定这些矿物的年龄。如果结果显示任何年龄超过30 Ma，那么剪切加热的假设可以被视为一种可行的弱化机制，至少在这些岩石中。

The goal of this project is to establish the history of the crustal rocks in the Red River shear zone that forms an arc-shaped fault that sweeps clock-wise from eastern Tibet across southwestern China and then follows the Red River through Hanoi to the Gulf of Tonkin. This fault accommodated hundreds of km of displacement in the geologically recent past, about 30 million years ago. The hypothesis being tested is that of shear heating. It is possible that the rocks in the fault might be heated by the friction associated with these large displacements. If the rocks in this fault zone had been so heated, then they would become weak and less prone to earthquakes. This NSF project is funding field work to allow structural observations and sample collection in China, as well as laboratory work on samples to detect when the last heating event occurred. This project also involves collaboration with Taiwanese researchers, which adds significant international impact.More specifically, the rocks in this fault zone are thought by some to have been metamorphosed by shear heating as the Indochina block was extruded past the Yangtze block during the Oligocene-Miocene collision of India with Asia. Some of the evidence for heating comes from the Oligocene to Miocene ages of thermochronometers, especially monazite, muscovite, and K-feldspar. With the interpretation that these ages are all cooling ages, it follows that the rocks experienced temperatures of 700°C between 30 and 20 Ma, and cooled quickly after that. However, it is also possible that these ages may be crystallization ages, in which case the argument for shear heating becomes considerably weaker. We will characterize muscovite and K-feldspar to determine whether those minerals are magmatic or metamorphic. We will then date these minerals using the 40Ar/39Ar age spectrum method. If the results show any ages older than ~30 Ma, then the hypothesis of shear heating can be dismissed as a viable weakening mechanism, at least in these rocks.