Collaborative Research: Roles of lithology and water on deep continental crustal rheology from a natural setting and laboratory experiments

地震通常在地球上陆壳的浅层（15公里）或俯冲洋板块的更深处（50-660公里）观测到。最近发现的岩石，只有在地震滑动事件在深（20-50公里）的大陆地壳断层表面熔化形成的，表明地震可以发生在以前被认为是地震的地区。在地震剪切带中发生地震的机制还不清楚。然而，最近的观察表明，这些岩石中许多矿物的强度对矿物颗粒中夹带的水量很敏感，水分流失可能会使它们增强。来自科罗拉多大学博尔德分校和阿克伦大学的研究人员将进行一系列综合的建模、现场和实验室调查，以量化水流失如何影响深部地壳断裂带的强度。他们将对加拿大萨斯喀彻温省科拉湖断层带的过程进行实地调查，测量断层带所有岩石的含水量，并进行实验以表征含水量对石英强度的影响，石英是断层带中的主要阶段之一。这些研究的结果将被用来建立一个预测模型的过程，造成深地壳地震。预期的社会成果将包括我们对地震过程的理解的进步，以及早期职业博士后研究员，研究生和本科生的支持。该项目将使用三种互补的方法来推进我们对“干燥”深部大陆地壳流变学的理解。首先，研究人员将对从加拿大地盾35-25公里古深度挖掘出的一个更大规模的剪切带进行实地和微观结构表征。该结构承载同运动假玄武玻璃，名义上无水麻粒岩-上角闪岩相条件下开发。这项工作将包括对名义上无水矿物（特别是石英和长石）的含水量测量，以将其流变学与干、湿或湿变形实验和流变学建模相关联。其次，通过压力步进变形实验，确定含水量和逸度对石英蠕变强度的影响。还将对低含水量石英岩进行剪切实验，以将微观结构和变形/恢复机制与剪切带的观察结果进行比较。第三，粘弹性数值模拟，使用干，湿矿物流动规律，从上述实验的参数，将测试的假设，应力增强，由于岩性和含水量的变化，导致在剪切带的幕式脆性破坏和pseudotachylyte代。该模型还将通过突出剪切带中需要更准确描述现场关系的区域来指导现场工作。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Earthquakes are generally observed at shallow depths (15 km) in Earth’s upper continental crust or at greater depths (50-660 km) in subducting oceanic plates. Recent discoveries of rocks that are only formed by melting on fault surfaces during seismic slip events in the deep (20-50 km) continental crust indicate that earthquakes can occur in zones previously thought to be aseismic. The mechanisms that can cause earthquakes to occur in aseismic shear zones are not understood. However, recent observations indicate that the strength of many minerals in these rocks is sensitive to the amount of water entrained inside the mineral grains and that water loss may strengthen them. Researchers from the University of Colorado Boulder and the University of Akron will perform an integrated series of modeling, field, and laboratory-based investigations to quantify how water loss affects the strength of a deep crustal fault zone. They will perform field-based investigations of the processes operating in the Cora Lake Fault Zone, Saskatchewan, Canada, measure water contents in all of the rocks in the fault zone, and perform experiments to characterize the effect of water content on the strength of quartz, one of the dominant phases in the fault zone. The results of these studies will be used to create a predictive model of the processes that caused the deep crustal earthquakes. Desired societal outcomes will include advancement of our understanding of the processes that cause earthquakes, as well as the support of an early-career post-doctoral researcher, graduate students, and undergraduate students.This project will advance our understanding of the rheology of “dry” deep continental crust using three complementary approaches. First, the researchers will conduct field-based and microstructural characterization of a kilometer-scale shear zone exhumed from 35–25 km paleodepths in the Canadian shield. The structure hosts synkinematic pseudotachylyte that developed under nominally anhydrous granulite- to upper amphibolite-facies conditions. The work will include water content measurements on nominally anhydrous minerals, particularly quartz and feldspar, to correlate their rheology to dry, damp, or wet deformation experiments and rheologic modeling. Second, pressure-stepping deformation experiments will determine the effects of water content and fugacity on quartz creep strength. Shear experiments on low water content quartzites will also be conducted to compare microstructure and deformation/recovery mechanisms with observations from the shear zone. Third, viscoelastic numerical modeling using dry and wet mineral flow laws, with refined parameters from the above experiments, will test the hypothesis that stress enhancement due to lithologic and water content variations led to episodic brittle failure and pseudotachylyte generation in the shear zone. The models will also guide field work by highlighting areas in the shear zone where more accurate characterization of field relationships is needed.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.