Mechanical twinning in quartz: Shock experiments, impact, pseudotachylites and fault breccias
Mechanical twinning in quartz: Shock experiments, impact, pseudotachylites and fault breccias
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DOI：
10.1016/j.tecto.2011.06.016
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发表时间：
2011-09
期刊：
影响因子：
2.9
通讯作者：
H. Wenk;C. Janssen;T. Kenkmann;G. Dresen
中科院分区：
文献类型：
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作者：
H. Wenk;C. Janssen;T. Kenkmann;G. Dresen
越来越多地使用衍射方法来研究矿物的首选取向，已经确定变形岩石中的石英不仅表现出典型的c轴取向模式，而且在正、负菱形取向上也普遍存在明显的差异。在三角石英晶体结构中，正菱形和负菱形在结构上是不同的，特别是负菱形（如{011¯1}）比正菱形（如{101¯1}）要硬得多。在这里，我们将重点放在应力作用下机械dauphin<s:1>孪晶作为这种差异的原因，并通过EBSD测量说明在高应力作用下含石英岩石中普遍存在的孪晶。在南非弗里德堡陨石撞击地点的实验中，观察到激波砂岩和含硅辉石石英岩的特征孪生。南加州圣罗莎糜棱岩带中与伪铜绿岩相关的石英也有类似的孪晶记录，而下伏韧性糜棱岩中的石英或多或少没有孪晶。这表明孪晶是由局部地震应力引起断层表面断裂和摩擦熔化而产生的。来自SAFOD（圣安德烈亚斯断层深度观测站）钻探项目的含石英角砾岩也显示出孪生的证据，表明在Parkfield的圣安德烈亚斯断层目前的爬行段存在高地震应力。从这些观察结果来看，dauphin<s:1>孪晶显微结构可以诊断高局部应力和瞬态应力。
Increasing use of diffraction methods to study preferred orientation of minerals has established that quartz in deformed rocks not only displays characteristic c-axis orientation patterns, but that there is also generally a distinct difference in the orientation of positive and negative rhombs. In the trigonal quartz crystal structure positive and negative rhombs are structurally different, and particularly negative rhombs (e.g. {011¯1}) are much stiffer than positive rhombs (e.g. {101¯1}). Here, we focus on the role of mechanical Dauphiné twinning under stress as a cause of this difference and illustrate with EBSD measurements ubiquitous twinning in quartz-bearing rocks subjected to high stresses. Characteristic twinning is observed in experimentally shocked sandstones and stishovite-bearing quartzites from the Vredefort meteorite impact site in South Africa. Similar twinning is documented for quartz associated with pseudotachylites from the Santa Rosa mylonite zone in Southern California, whereas quartz in underlying ductile mylonites are more or less twin-free. It suggests that twinning was produced by local seismic stresses that caused fracture and frictional melting on fault surfaces. Quartz-bearing breccias from the SAFOD (San Andreas Fault Observatory at Depth) drilling project also show evidence of twinning and suggest high seismic stresses in the currently creeping segment of the San Andreas Fault at Parkfield. From these observations it appears that Dauphiné twin microstructures can be diagnostic of high local and transient stresses.