Collaborative Research: Calibrating quartz fabric intensity as a function of strain magnitude: a field-based investigation in the Snake Range core complex, Nevada

16个巨大的构造板块覆盖着地球表面，它们以每年几毫米到几十毫米的速度相互移动。了解板块构造运动的一个重要因素是记录发生在板块边界及其附近的岩石的变形幅度。然而，在高温下和地壳深处变形的岩石往往不能保留可以测量变形幅度的地质特征。为了解决这一问题，从而更好地描述在板块边界处运行的岩石变形过程，最近的研究提出，测量岩石样品中矿物石英的排列程度（或“强度”）可以用来描绘地壳内相对高或低震级变形的区域。这代表了一种很有前途的理解变形空间模式的新技术。然而，石英的排列强度并没有与变形的大小直接相关。该项目的目标是生成一个方程，该方程将石英的排列强度与变形幅度联系起来。为了实现这一目标，我们将在内华达州斯内克山脉的富含石英的岩石中记录石英强度模式并测量与每种强度模式相关的变形幅度。这些岩石是这项研究的理想选择，因为它们在过去的研究中已经产生了明确的强度模式，它们保留了允许测量变形幅度的特征，并且它们表现出从西部的低幅度变形到东部的高幅度变形的变化。通过收集整个范围内的强度和变形幅度测量值，将生成一个将这两个参数关联起来的方程，然后可以在全球范围内应用该方程，以了解任何包含高温变形岩石的区域的变形幅度。该项目将为研究生和本科生提供研究项目，从而培养下一代地球科学家。该项目还将通过每年向1500名大学生展示地质学入门课程的成果，在大盆地国家公园和华盛顿州东部和爱达荷州北部的社区大学进行讲座，以及为两所参与大学的本科生进行实地考察，为STEM教育做出贡献。了解形变过程中应变是如何以及在哪里局部化的，对于阐明在构造运动期间大陆地壳增厚、变薄或容纳走滑剪切的过程是至关重要的。研究人员提出，通过将织物强度解释为有限应变量级的代理，从石英晶体织物中计算出的统计强度参数有可能划定高应变区域。最近在喜马拉雅地区的一些研究已经成功地利用强度参数，如圆柱度来阐明跨主要剪切带的相对应变大小的空间模式。然而，由于这些研究是在普遍再结晶的岩石包裹中进行的，缺乏可以测量有限应变的变形标记，因此无法定量地将织物强度与绝对应变量级联系起来。我们建议生成一个将织物强度表示为有限应变量级函数的校准方程，这将是扩大这种新方法实用性的关键一步。在这个项目中，我们建议通过研究内华达州北蛇岭变质核杂岩的新生代伸展剪切带内暴露的岩石来进行这样的校准。这个区域是理想的，因为剪切带包含一个石英岩单元，产生发育良好的晶体结构，保留微观、中观和区域尺度的应变标志，并在有限应变量级上表现出显著的跨走向梯度。我们建议从沿着30公里的横走向样带收集的石英岩样品中获得圆柱度值，并使用微尺度和中观尺度应变分析来生成该样带上该石英岩单元的三维有限应变的详细模型。这两个数据集的集成将允许计算一个校准方程，该方程将织物强度表示为有限应变量级的函数，这将为接近韧性变形的运动学和结构分析提供不可或缺的新工具。该项目的结果将产生深远的影响，因为我们建议测试一种技术的准确性，然后可以在全球范围内应用于任何收缩、伸展或走滑造山系统，无论是活跃的还是古老的。圆柱度和有限应变量级之间的校准可以作为任何含石英构造岩的造山带的基准，以阐明剪切带内普遍表现出再结晶的应变局部化的空间模式。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。

Sixteen large tectonic plates cover the Earth’s surface and move relative to one another at rates of several millimeters to tens of millimeters per year. An important element for understanding the operation of plate tectonics is documenting the magnitude of deformation of rocks that has occurred at and near the boundaries of plates. However, rocks that have been deformed at high temperatures and deep within the crust often do not preserve geologic features that allow measuring the magnitude of deformation. To address this issue, and thereby better characterize the rock deformation processes that operate at plate boundaries, recent research has proposed that measuring the degree (or ‘intensity’) of alignment of the mineral quartz within rock samples can be used to delineate areas of relatively high- or low-magnitude deformation within the crust. This represents a promising new technique for understanding the spatial patterns of deformation. However, the intensity of alignment of quartz has not been directly correlated to the magnitude of deformation. The goal of this project is to generate an equation that relates the intensity of alignment of quartz to the magnitude of deformation. To accomplish this goal, we will document quartz intensity patterns and measure the magnitude of deformation associated with each intensity pattern in quartz-rich rocks in the Snake Range in Nevada. These rocks are ideal for this study because they have yielded well-defined intensity patterns in past studies, they preserve features that allow measuring the magnitude of deformation, and they exhibit a change from low-magnitude deformation in the western part of the range to high-magnitude deformation in the east. By collecting intensity and deformation magnitude measurements across the range, an equation relating these two parameters will be generated, which can then be applied globally to understand deformation magnitudes in any region that contains rocks deformed at high temperatures. This project will provide research projects for graduate and undergraduate students, thereby training the next generation of geoscientists. This project will also contribute to STEM education by presenting results in introductory geology courses that will reach 1,500 college students each year, giving talks at Great Basin National Park and at community colleges in eastern Washington and northern Idaho, and through a field trip for undergraduate students from both participating universities.Understanding how and where strain becomes localized during deformation is fundamental for illuminating the processes that thicken, thin, or accommodate strike-slip shearing within continental crust during tectonism. Researchers have proposed that statistical intensity parameters calculated from quartz crystallographic fabrics have the potential to delineate zones of high strain, by interpreting fabric intensity as a proxy for finite strain magnitude. Several recent studies in the Himalaya have successfully utilized intensity parameters such as cylindricity to elucidate the spatial patterns of relative strain magnitude across major shear zones. However, as these studies were performed within packages of pervasively recrystallized rocks that lack deformed markers from which finite strain can be measured, they cannot quantitatively relate fabric intensity to absolute strain magnitude. We propose that generating a calibration equation that expresses fabric intensity as a function of finite strain magnitude would be a critical step forward for expanding the utility of this new approach. In this project, we propose to perform such a calibration by investigating rocks exposed within a Cenozoic extensional shear zone in the Northern Snake Range metamorphic core complex in Nevada. This field locality is ideal because the shear zone contains a quartzite unit that yields well-developed crystallographic fabrics, preserves micro-, meso- and regional-scale strain markers, and exhibits a dramatic across-strike gradient in finite strain magnitude. We propose to obtain cylindricity values from quartzite samples collected along a 30 km-long across-strike transect, and to use micro- and meso-scale strain analyses to generate a detailed model of 3-dimensional finite strain of this quartzite unit across this transect. Integration of these two datasets will allow calculation of a calibration equation that expresses fabric intensity as a function of finite strain magnitude, which will provide an indispensable new tool with which to approach the kinematic and structural analysis of ductile deformation. The results of this project will have far-reaching implications, as we propose to test the veracity of a technique that can then be applied globally within any contractional, extensional, or strike-slip orogenic system, active or ancient. The calibration between cylindricity and finite strain magnitude can be applied as a benchmark within any orogen that contains quartz-rich tectonites, in order to illuminate the spatial patterns of strain localization within shear zones that exhibit ubiquitous recrystallization.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.