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

地震通常发生在地球上大陆地壳的浅层(15公里)或俯冲的大洋板块的较深处(50-660公里)。最近发现的岩石只有在深部(20-50公里)大陆地壳发生地震滑动事件时，断层表面才会熔化形成岩石，这表明地震可能发生在以前被认为是无地震的地区。导致地震发生在无地震剪切带的机制尚不清楚。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，失水可能会加强它们。来自科罗拉多博尔德大学和阿克伦大学的研究人员将进行一系列综合的建模、野外和实验室调查，以量化水分损失如何影响地壳深层断裂带的强度。他们将对加拿大萨斯喀彻温省科拉湖断裂带的运行过程进行实地调查，测量断裂带所有岩石的水分含量，并进行实验以表征水含量对断裂带主要相之一石英强度的影响。这些研究的结果将被用来建立引起地壳深部地震的过程的预测模型。期望的社会成果将包括促进我们对地震发生过程的理解，以及对职业生涯早期的博士后研究人员、研究生和本科生的支持。这个项目将通过三种互补的方法来促进我们对“干燥”大陆深部地壳流变学的理解。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条千米尺度的剪切带进行现场和微观结构表征。该构造赋存在名义上无水麻粒岩-上角闪岩相条件下发育的同动假玄武岩。这项工作将包括测量名义上的无水矿物，特别是石英和长石的水分含量，以将它们的流变性与干、湿或湿变形实验和流变学模型相关联。其次，压力阶跃变形实验将确定含水率和逸度对石英蠕变强度的影响。还将进行低含水率石英岩的剪切实验，将微观结构和变形/恢复机制与剪切带的观测结果进行比较。第三，采用干、湿矿物流动规律的粘弹性数值模拟，结合上述实验得到的精炼参数，将验证岩性和含水率变化引起的应力增强导致剪切带幕式脆性破坏和假硅质岩生成的假设。这些模型还将通过突出剪切带中需要更准确地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智力优势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.