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

Indiana University, Bloomington IN

Investigators

Abstract

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.