Deep Structure of Lithospheric Fault Zones

该项目将研究在发震上地壳以下的岩石圈中，形成板块边界的类型和规模的断裂带是如何随着深度的变化而变化的。本项目旨在重建一个经过充分研究的主要断裂带在运动时的特征，目的是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、流变学、应力、应变率和温度。该项目将重点研究瑞士阿尔卑斯山脉的Simplon断层，该断层具有良好的位移约束速率，因此可以计算不同构造水平的应变率，并确定剪切带流变学。这项研究的结果将对岩石圈对板块运动响应的地球动力学建模、跨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解释以及了解发震断层是如何加载的具有价值。拟议的活动将通过女性充分参与STEM，通过虚拟实地考察公众参与STEM，以及通过本科生和研究生培训以及本科实地课程培养多样化，具有全球竞争力的STEM劳动力，从而促进预期的社会成果。容纳相对板块运动的断层必须横切大部分岩石圈，但人们对它们的宽度、内部结构和力学特性随时间和深度的变化知之甚少。这一提议旨在重建一个已得到充分研究的主要断裂带在运动时的特征，目的是确定剪切带宽度、流变学、应力、应变率和温度之间是否存在标度关系。瑞士阿尔卑斯山脉的Simplon断层将使用现场和实验室技术进行调查，以确定其原始宽度，流动应力，应变率和流变学作为温度和深度的函数。该断层是一个正常意义上的剪切带，在25 - 3 Ma之间以3-5毫米/年的速度活动，在运动过程中挖掘出下盘，暴露出深度达30公里的高度变形的岩石。分析方法将包括：(a)使用光学和扫描电镜方法进行微观结构分析；(b)测量石英的动态再结晶晶粒尺寸，以确定变形过程中的应力；(c)使用电子背散射衍射方法测量石英的晶体学优选取向，以确定主动滑移系统；(d)利用石英的Ti含量、碳质材料的激光拉曼光谱和共存矿物的多平衡热气压测定变形过程中的温度和压力；(e)对角闪石、白云母和黑云母进行39Ar/40Ar分析，对锆石和磷灰石进行U-Th/He分析；(f)开挖剪切带热演化的数值分析。变形时的应变率由宽度和位移率计算；与应力一起，这可以用来校准剪切区流变。

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.