Processes and Rates of Arc Crust Growth and Differentiation in the Southern Sierra Nevada Crustal Section

内华达山脉南部地壳剖面弧形地壳生长和分异的过程和速率

• 批准号: 1552202
• 负责人: Oliver Jagoutz
• 金额: $ 32万
• 依托单位: Massachusetts Institute of Technology
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2016
• 资助国家: 美国
• 起止时间: 2016-01-01 至 2019-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1552202&HistoricalAwards=false
• 关键词: Processes; Rates; Arc; Crust; Growth

The construction of magmatic arcs at subduction zones plays a critical role in the creation of new continental crust on Earth. In particular, a number of different mechanisms in the lower crust (30 km depth) of arcs are hypothesized to be critical for the formation of felsic continental crust. However the restricted number of locations worldwide where rocks formed at these depths are exposed on the surface is a fundamental limitation to improving our understanding of the chemical and physical controls on these processes and the time scales over which they occur. This proposal aims to evaluate these processes through a detailed study of a relatively little studied exposure of lower crustal rocks in the southernmost Sierra Nevada batholith, combining field observations, major, trace element and isotope geochemistry, and high precision geochronology. The research activities in this project will be integrated with field and analytical experiences for undergraduates while findings will be incorporated into undergraduate courses. Through collaboration with the Tejon Ranch Conservancy, it is planned to help develop outreach programs and education materials for the Tejon Ranch property. These programs include geologic tours of the ranch, and will cover both the basics of plate tectonics and arc magmatism, as well as the geologic history of California. The programs will be available to the general public, and be specifically targeted to public schools serving the primarily underrepresented minority populations in the Tehachapi/Bakersfield region.The Sierra Nevada Batholith is one of the best-studied examples of continental arc plutonism, but nearly the entire exposed range samples rocks formed at shallow to middle crustal pressures, and as a result our understanding of lower crustal processes in the Sierra Nevada are limited. However, the Sierra Nevada also contains a unique crustal section, exposing a structurally continuous section extending from ~7-37 km depth, and containing a suite of igneous rocks emplaced contemporaneously at ~100 Ma, coinciding with the Late Cretaceous Sierra Nevada magmatic flare-up event. This section provides an ideal location to evaluate different models for lower crustal processes including fractional crystallization and partial melting dominated deep crustal hot zones, cyclic underthrusting and relamination of buoyant material. The combination of structurally continuous and contemporaneously emplaced outcrop will allow us to better characterize these processes and their relevant timescales, and to assess their importance for the generation of felsic continental crust. These findings will have additional important implications for our understanding of the Mesozoic history of the western US, and for validating seismic observations of continental lower crust.

俯冲带岩浆弧的构造在地球新大陆地壳的形成中起着至关重要的作用。特别是，一些不同的机制在地壳下（30公里深）的弧被假设是对长英质大陆地壳的形成至关重要。然而，在世界范围内，在这些深度形成的岩石暴露在地表的地点有限，这是一个根本性的限制，无法提高我们对这些过程的化学和物理控制以及它们发生的时间尺度的理解。本提案旨在通过详细研究内华达山脉最南端的下地壳岩石的相对较少的暴露，结合野外观测，主要，微量元素和同位素地球化学以及高精度地质年代学来评估这些过程。该项目的研究活动将与本科生的实地和分析经验相结合，而研究结果将纳入本科课程。通过与Tejon Ranch Conservancy的合作，它计划帮助开发Tejon Ranch财产的外展项目和教育材料。这些项目包括农场的地质之旅，并将涵盖板块构造和弧岩浆作用的基础知识，以及加州的地质历史。这些项目将向公众开放，并特别针对为特哈查皮/贝克斯菲尔德地区主要代表性不足的少数民族提供服务的公立学校。内华达山脉岩基是大陆弧深部作用研究得最好的例子之一，但几乎所有暴露的岩石样本都是在浅层到中地壳压力下形成的，因此我们对内华达山脉下地壳作用的了解有限。然而，内华达山脉也包含一个独特的地壳剖面，暴露出一个从~7-37公里深度延伸的结构连续剖面，其中包含一套火成岩，同时位于~100 Ma，与晚白垩世内华达山脉岩浆爆发事件相吻合。这一节提供了评估下地壳过程的不同模型的理想位置，包括以分离结晶和部分熔融为主的深部地壳热区、旋回逆冲和浮力物质再分层。构造连续和同期侵位露头的结合将使我们能够更好地描述这些过程及其相关的时间尺度，并评估它们对长英质大陆地壳形成的重要性。这些发现将对我们了解美国西部中生代历史和验证大陆下地壳的地震观测具有重要意义。