Deep Structure of Lithospheric Fault Zones

该项目将研究形成板块边界的断裂带的类型和规模如何随孕震上地壳下岩石圈的深度而变化。该项目旨在重建一个主要断裂带在运动时的特征，以确定控制断裂带行为的重要因素之间是否存在标度关系：剪切带宽度、流变性、应力、应变率和温度。该项目将把重点放在瑞士阿尔卑斯山的Simplon断层上，该断层的位移速率受到很好的限制，因此可以计算不同构造层次的应变率，并确定剪切带的流变性。这项研究的结果将对岩石圈对板块运动的响应进行地球动力学模拟、解释跨越板块边界剪切带的地球物理数据、解释地震后变形的大地测量结果以及理解孕震断层是如何加载的具有价值。拟议的活动将通过以下方式促进预期的社会成果：妇女充分参与STEM，通过虚拟实地考察公众参与STEM，并通过本科生和研究生培训以及本科生实地课程发展多样化的、具有全球竞争力的STEM劳动力队伍。适应相对板块运动的断层必须横跨岩石圈的大部分地区，但其宽度、内部结构和力学性质随时间和深度的变化尚不清楚。这一建议旨在重建一个主要断裂带在运动时的特征，目的是确定剪切带宽度、流变性、应力、应变率和温度之间是否存在标度关系。将使用野外和实验室技术对瑞士阿尔卑斯山的Simplon断层进行调查，以确定其原始宽度、流动应力、应变率和作为温度和深度函数的流变性。该断裂是一条正常意义上的剪切带，在25-3 Ma之间以3-5 mm/年的速度活动，在运动过程中剥离下盘，暴露出深达30公里的高度变形的岩石。分析方法将包括：(A)使用光学和扫描电子显微镜方法进行微观结构分析；(B)测量石英的动态重结晶粒度以确定变形过程中的应力；(C)使用电子背散射衍射法测量石英的晶体择优取向，以确定活动滑移系；(D)利用石英的钛含量、碳质材料的激光拉曼光谱和共存矿物的多平衡温压分析来确定变形过程中的温度和压力；(E)使用39Ar/40Ar分析角闪石、白云母和黑云母以及锆石和磷灰石的U-Th/He分析的热年代学；(F)折返剪切带热演化的数值分析。变形过程中的应变率将根据宽度和位移速率计算出来；与应力一起，这可以用来校准剪切带的流变性。

This project will investigate how fault zones, of the type and scale that form plate boundaries, change character with depth in the lithosphere below the seismogenic upper crust. This project aims to reconstruct the characteristics of a well-studied example of a major fault zone at the time of motion, with the aim of determining if there is a scaling relationship between important factors that control fault zone behavior: shear zone width, rheology, stress, strain-rate and temperature. The project will focus on the Simplon fault in the Swiss Alps, a fault with a well-constrained rate of displacement, so that strain-rate at different structural levels can be calculated and the shear zone rheology determined. The results of this research will be of value for geodynamic modeling of the lithospheric response to plate motions, the interpretation of geophysical data across plate boundary shear zones, the interpretation of geodetic measurements of post-seismic deformation, and for understanding how seismogenic faults are loaded. The proposed activity will advance desired societal outcomes by full participation of women in STEM, public engagement with STEM through virtual field trips, and development of a diverse, globally competitive STEM workforce through undergraduate and graduate student training plus an undergraduate field course.Faults that accommodate relative plate motion must transect much of the lithosphere, yet their width, internal structure, and mechanical properties as a function of time and depth are poorly understood. This proposal aims to reconstruct the characteristics of a well-studied example of a major fault zone at the time of motion, with the aim of determining if there is a scaling relationship between shear zone width, rheology, stress, strain-rate and temperature. The Simplon fault in the Swiss Alps, will be investigated using field and laboratory techniques to determine its original width, flow stress, strain-rate, and rheology as a function of temperature and depth. The fault is a normal sense shear zone, active between 25 and 3 Ma at a rate of 3-5 mm/year, which exhumed its footwall during motion, exposing highly deformed rocks from depths of up to 30 km. Analytical methods will include: (a) microstructural analysis using optical and SEM methods; (b) measurement of the dynamically recrystallized grain size of quartz for determination of stress during deformation; (c) measurement of the crystallographic preferred orientation of quartz, using electron backscatter diffraction methods, to determine the active slip systems; (d) determination of temperature and pressure during deformation using the Ti content of quartz, laser Raman spectrometry of carbonaceous material, and multi-equilibrium thermobarometry of coexisting minerals; (e) thermochronology using 39Ar/40Ar analysis on hornblende, muscovite and biotite plus U-Th/He analysis on zircon and apatite; and (f) numerical analysis of the thermal evolution of the exhumed shear zone. Strain-rate during deformation will be calculated from the width and displacement rate; together with the stress, this can be used to calibrate the shear zone rheology.