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

合作研究：校准石英织物强度作为应变大小的函数：在内华达州 Snake Range 核心复合体中进行的现场调查

• 批准号: 2022973
• 负责人: Sean Long
• 金额: $ 17.71万
• 依托单位: Washington State University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2020
• 资助国家: 美国
• 起止时间: 2020-08-15 至 2024-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2022973&HistoricalAwards=false
• 关键词: Collaborative; Research; Calibrating; quartz; fabric

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.

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彼此相对移动。了解板块构造运作的一个重要因素是记录在板块边界及其附近发生的岩石变形的大小。然而，在高温和地壳深处变形的岩石通常不会保留可以测量变形程度的地质特征。为了解决这个问题，从而更好地描述在板块边界运行的岩石变形过程，最近的研究提出，可以用测量岩石样品中矿物石英的排列程度(或“强度”)来圈定地壳内相对较高或较低幅度的变形区域。这代表了一种很有前途的新技术，可以用来理解变形的空间模式。然而，石英的排列强度与变形的大小并没有直接的关系。这个项目的目标是生成一个方程式，将石英的排列强度与变形的大小联系起来。为了实现这一目标，我们将记录石英强度模式，并测量内华达州斯内克山脉富石英岩石中与每种强度模式相关的形变幅度。这些岩石是这项研究的理想之选，因为它们在过去的研究中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，并且它们显示出从山脉西部的低强度变形到东部的高强度变形的变化。通过收集整个范围内的强度和变形幅度测量，将生成一个将这两个参数联系起来的方程，然后可以在全球范围内应用该方程来了解任何包含在高温下变形的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。该项目还将通过在每年将有1500名大学生参加的地质学入门课程中展示成果，在大盆地国家公园以及华盛顿东部和爱达荷州北部的社区大学发表演讲，以及通过两所参与大学的本科生的实地考察，来促进STEM教育。了解应变在形变过程中如何以及在哪里变得局部化，是阐明构造作用期间大陆地壳内的走滑剪切变厚、变薄或适应的过程的基础。研究人员提出，根据石英晶体组构计算的统计强度参数，通过将组构强度解释为有限应变大小的替代，有可能描绘出高应变区。喜马拉雅地区最近的几项研究已经成功地利用圆柱度等强度参数来阐明主要剪切带上相对应变震级的空间模式。然而，由于这些研究是在渗透重结晶岩石包中进行的，这些岩石缺乏可以测量有限应变的变形标记，因此它们不能定量地将组构强度与绝对应变大小联系起来。我们认为，生成一个将织物强度表示为有限应变大小的函数的校准方程，将是扩展这一新方法的实用价值的关键一步。在这个项目中，我们建议通过调查内华达州北蛇山变质核杂岩中新生代伸展剪切带中暴露的岩石来进行这样的校准。这种场域位置是理想的，因为剪切带中含有一个石英岩单元，它产生了发育良好的晶体组构，保存了微观、中尺度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30公里长的跨走向断面采集的石英岩样品获得圆柱度的值，并使用微观和细观应变分析来生成该断面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将织物强度表示为有限应变大小的函数的校准方程，这将为延性变形的运动学和结构分析提供一个不可或缺的新工具。这个项目的结果将具有深远的影响，因为我们提议测试一项技术的准确性，然后该技术可以在全球范围内应用于任何收缩、伸展或走滑造山系统，无论是活动的还是古老的。圆柱度和有限应变大小之间的校准可以作为任何包含富石英构造岩的造山带的基准，以阐明普遍存在重结晶的剪切带内应变局部化的空间模式。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。

项目成果

Extreme ductile thinning of Cambrian marbles in the Northern Snake Range metamorphic core complex, Nevada, USA: Implications for extension magnitude and structural evolution

• DOI: 10.1016/j.jsg.2023.104912
• 发表时间: 2023-07
• 期刊: Journal of Structural Geology
• 影响因子: 3.1
• 作者: S. Long;Jeffrey Lee;N. Blackford

The low-angle breakaway system for the Northern Snake Range décollement in the Schell Creek and Duck Creek Ranges, eastern Nevada, USA: Implications for displacement magnitude

美国内华达州东部 Schell Creek 和 Duck Creek 山脉北 Snake Range décollement 的低角度脱离系统：对位移幅度的影响

• DOI: 10.1130/ges02482.1
• 发表时间: 2022
• 期刊: Geosphere
• 影响因子: 2.5
• 作者: Long, Sean P.;Lee, Jeffrey;Blackford, Nolan R.
• 通讯作者: Blackford, Nolan R.