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

中国哀牢山-红河剪切带组构演化的综合构造、岩石学和同位素研究

• 批准号: 1019682
• 负责人: Robert Wintsch
• 金额: $ 16.28万
• 依托单位: Indiana University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2010
• 资助国家: 美国
• 起止时间: 2010-08-15 至 2015-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1019682&HistoricalAwards=false
• 关键词: Integrated; Structural; Petrologic; Isotopic; Study

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.

这个断层容纳了数百公里的位移，在最近的地质过去，大约3000万年前。被检验的假设是剪切加热。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加热到这样的程度，那么它们就会变得脆弱，不那么容易发生地震。这个NSF项目将资助在中国进行的结构观测和样品采集的实地工作，以及样品的实验室工作，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，这将增加重大的国际影响。更具体地说，一些人认为，这个断裂带的岩石是在印度与亚洲渐新世-中新世碰撞期间，印度支那地块挤压过扬子地块时，由于剪切加热而变质的。一些热的证据来自渐新世到中新世的温度表，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，可以得出，岩石在30至20 Ma之间经历了700°C的温度，之后迅速冷却。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。我们将对白云母和钾长石进行表征，以确定这些矿物是岩浆的还是变质的。然后，我们将使用40Ar/39Ar年龄谱法确定这些矿物的年代。如果结果显示年龄大于~30 Ma，那么至少在这些岩石中，剪切加热的假设可以作为一种可行的弱化机制而被驳回。