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

合作研究：自然环境和实验室实验中岩性和水对深部大陆地壳流变学的作用

• 批准号: 2234125
• 负责人: Kevin Mahan
• 金额: $ 46.45万
• 依托单位: University of Colorado at Boulder
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2023
• 资助国家: 美国
• 起止时间: 2023-08-01 至 2026-07-31
• 项目状态: 未结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2234125&HistoricalAwards=false
• 关键词: Collaborative; Research; Roles; lithology; water

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.

地震通常在地球上大陆地壳的浅层深度（15公里）或大洋板块的俯冲深度（50-660公里）观察到。最近发现的岩石仅是在地震滑动事件中（20-50公里）大陆地壳深部断层表面融化形成的，这表明地震可能发生在以前被认为是地震带的地区。导致地震发生在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，而水分的损失可能会增强它们的强度。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建模、实地和实验室调查，以量化水分损失如何影响地壳深部断裂带的强度。他们将对加拿大萨斯喀彻温省科拉湖断裂带的工作过程进行实地调查，测量断裂带中所有岩石的含水量，并进行实验，以表征含水量对石英强度的影响，石英是断裂带中的主要阶段之一。这些研究的结果将被用来建立一个导致地壳深处地震的过程的预测模型。期望的社会成果将包括我们对导致地震的过程的理解的进步，以及对早期职业博士后研究员、研究生和本科生的支持。该项目将利用三种互补的方法推进我们对“干”大陆地壳深部流变学的理解。首先，研究人员将对加拿大地盾中35-25公里古深度的一公里尺度剪切带进行实地和微观结构表征。该构造含有在名义上无水麻粒岩-上角闪岩相条件下发育的同动伪岩。这项工作将包括对名义上无水矿物的含水量测量，特别是石英和长石，将它们的流变学与干燥、潮湿或潮湿的变形实验和流变学模型联系起来。其次，通过压力步进变形实验确定含水率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，将其微观结构和变形/恢复机制与剪切带观测结果进行比较。第三，采用干湿矿物流动规律的粘弹性数值模拟，结合上述实验的精细参数，将验证由岩性和含水量变化引起的应力增强导致剪切带中偶发脆性破坏和伪木酸酯生成的假设。这些模型还将通过突出显示剪切带中需要更准确地描述场关系的区域来指导现场工作。该奖项反映了美国国家科学基金会的法定使命，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。