A Reversible Rheology for Water-Weakened Quartz

本项目的总体目标是利用实验技术研究：1)化学环境（水逸度）、应变速率和温度对轴向压缩变形石英单晶强度的影响；2)相同参数对与单晶相同原料合成石英岩（石英集合体）强度的影响；3)单晶与多晶强度的基本关系。这些目标将通过对退火合成石英单晶和合成石英岩进行实验来实现，实验范围包括温度、水的流变性和应变率，在格里格斯型活塞-圆柱岩石变形装置中使用熔盐池精确测定力学数据。具体而言，研究人员将：1)对经过热处理的合成石英晶体的单晶芯进行温度、应变速率和压力步进实验，以建立恒定密度和均匀分布的可冻流体包裹体，研究基a、棱镜a和棱镜c滑移系统；2)将实验温度、应变速率和水逸度条件的顺序进行反转，检验这些速率规律是否可逆；3)用同样经过热处理的合成石英材料制备粉末制成热压多晶石英聚集体，并对这些多晶石英样品进行温度、应变速率和压力步进实验，测试其可逆性，并将结果与T、应变速率和基片和棱镜滑移的水易度灵敏度进行比较。从这些实验中得到的力学数据将用于开发以构造速率计算的天然石英岩变形的预测模型。中下地壳逆冲断层的位移和与碰撞构造和河道流动有关的低角度正脱离通常局限于富含石英的岩性中。因此，在板块规模的大陆断裂带下，石英长石的震后粘弹性松弛被认为是由石英强度控制的。由于石英变形对大陆岩石圈构造和流变学的重要性，人们进行了许多实验研究，以研究石英的变形和恢复机制，并量化可以应用于地壳构造载荷的力学关系。然而，水对石英的削弱机制仍然没有得到解决，并且很少有研究来确定水的挥发度、应变速率和温度对石英中常见滑移系统强度的影响。这个项目使用新的实验方法来解决这些问题。可以建立简单的模型，将单个滑动系统的强度与实验条件范围内的石英岩强度联系起来，并将结果外推到自然变形的石英岩上。这些实验的数据可用于大陆地壳在板块边界处粘度的实地研究和数值模拟，并有助于理解大陆地壳中引起与地震有关的变形的过程。

The overall goals of this project are to use experimental techniques to investigate : 1) the effect of chemical environment (water fugacity), strain rate, and temperature on the strength of quartz single crystals deformed in an axial compression; 2) the effect of the same parameters on the strength of synthetic quartzites (quartz aggregates) made from the same starting material as the single crystals; and 3) the fundamental relationship between the strengths of single crystals and polycrystals. These goals will be accomplished by performing experiments on annealed synthetic quartz single crystals and synthetic quartzites over a wide range of temperatures, water fugacities, and strain rates in a Griggs-type piston-cylinder rock deformation apparatus using a molten salt cell for precise determination of mechanical data. Specifically, the researchers will: 1) perform temperature-,strain rate-, and pressure-stepping experiments on single crystal cores in of synthetic quartz crystals, which have been heat-treated to establish a constant density and uniform distribution of freezable fluid inclusions, to investigate basal a, prism a and prism c slip systems; 2) reverse the sequence of experimental temperature, strain rate, and water fugacity conditions to test whether these rate laws are reversible; 3) hot-press polycrystalline quartz aggregates made of powders fabricated from the same heat-treated synthetic quartz materials, and subject these polycrystalline quartz samples to temperature-, strain rate-, and pressure-stepping experiments, testing for reversibility, and comparing the results with T, strain rate, and water fugacity sensitivities of basal and prism slip. The mechanical data from these experiments will be used to develop predictive models of deformation of natural quartzites at tectonic rates.Displacements of middle to lower crustal thrust faults and low-angle normal detachments associated with collisional tectonics and channel flow are commonly localized within quartz-rich lithologies. Postseismic visco-elastic relaxations in quartzo-feldspathic rocks below plate-scale continental fault zones are therefore thought to be governed by quartz strength. Motivated by the importance of quartz deformation to tectonics and rheology of the continental lithosphere, many experimental studies have been performed to investigate the deformation and recovery mechanisms of quartz, and quantify mechanical relations that can be applied to tectonic loading of the crust. Yet, the mechanism(s) of water weakening of quartz continues to be unresolved and little work has been performed to determine the effects of water fugacity, strain rate and temperature on the strength of the common slip systems in quartz. This project uses new experimental methods to address these problems. Simple models can be constructed that relate the strengths of the individual slip systems to the strengths of the quartzites over the range of experimental conditions and extrapolate the results to naturally deformed quartzites. Data from these experiments can be used in field-based research and numerical simulations of the viscosity of the continental crust at plate boundaries and may aid in the understanding of the processes in the continental crust that cause the deformations related to earthquakes.