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

该项目的目标是建立红河剪切带中地壳岩石的历史，该剪切带形成一条弧形断层，从西藏东部以时钟方向扫过中国西南部，然后沿着红河穿过河内到达北部湾。在3000万年前的地质历史中，这条断层容纳了数百公里的位移。正在检验的假设是剪切加热假设。断层中的岩石可能受到与这些大位移有关的摩擦的加热。如果这个断裂带的岩石被加热到这样的温度，那么它们就会变得脆弱，不太容易发生地震。美国国家科学基金会的这个项目正在资助中国的实地工作，以便进行结构观测和样本采集，以及对样本进行实验室工作，以检测上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，这增加了重大的国际影响。更具体地说，一些人认为，在印度与亚洲的渐新世-中新世碰撞期间，印度支那地块被挤出扬子地块时，该断裂带的岩石因剪切加热而变质。一些加热的证据来自渐新世至中新世的温度计时年龄，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，可以推断这些岩石在30~20 Ma之间经历了700℃的温度，之后迅速冷却。然而，这些年龄也有可能是结晶年龄，在这种情况下，剪切加热的论点变得相当薄弱。我们将对白云母和钾长石进行表征，以确定这些矿物是岩浆矿物还是变质矿物。然后我们将使用40Ar/39Ar年龄谱方法测定这些矿物的年龄。如果结果显示任何年龄大于~30 Ma，那么剪切加热假说可以被认为是一种可行的减弱机制，至少在这些岩石中是这样。

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.