Collaborative Research: Lithospheric foundering beneath the Sierra Nevada constrained by analysis of an anomalous Pn shadow zone

合作研究：异常 Pn 阴影区分析限制了内华达山脉下方的岩石圈沉没

• 批准号: 1547149
• 负责人: Steven Roecker
• 金额: $ 7.98万
• 依托单位: Rensselaer Polytechnic Institute
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2016
• 资助国家: 美国
• 起止时间: 2016-08-15 至 2019-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1547149&HistoricalAwards=false
• 关键词: Collaborative; Research; Lithospheric; foundering; beneath

Mountain ranges are supported by lower than usual densities in the outermost parts of the Earth: that is, a lower density crust, thicker crust, or low densities in the shallowest mantle will support elevated terrain. This project explores how the Sierra Nevada mountains in California came to be supported by the low density materials. Understanding of how higher density materials were removed is still unknown to scientists. Historical observations of peculiarities of such seismic waves originally led seismologists to the erroneous conclusion that the Sierra Nevada had a thick crust; although we know that to not be true, those observations still have not been explained. The researchers suggest that this material likely remains or has been focused in the western part of the mountain range. They will use seismic information to detect whether dense material exists where they predict. This comparison is accomplished by expanding software that can compute seismic waveforms in the presence of complex geologic structures. Thus this project improves our understanding of how mountains can be made as well as improving our seismological toolkit for exploring what happens at the boundary between the crust and mantle in continents.Lithospheric foundering has been invoked to explain orogenesis in a number of regions around the globe, but in most cases the means by which lithosphere founders remains largely unknown. The Sierra Nevada in eastern California is often cited as one of these regions, and hypotheses for how lithosphere foundered there range from lithospheric delamination to localized convective instability. A key piece of evidence for discriminating between these competing hypotheses lies in the presence or absence of certain arc-related lithologies (a.k.a. "arclogite") and the geometry of such dense lithologies beneath the western Sierran foothills. The presence of arclogite should result in large and potentially abrupt changes in the thickness and wavespeed of the lower crust and upper mantle, and these are likely to influence the passage of Pn energy across the Sierra. Indeed, such effects can provide an explanation for a long-lived conundrum in seismology, namely: what is the cause of a Pn shadow zone produced by the Sierra? The specific objective of the project is to apply recently developed full waveform inversion (FWI) techniques to evaluate trial models of the crust and upper mantle beneath the Sierra that may be responsible for the Pn shadow zone. One hypothesis is that arclogitic lithologies are responsible for this zone, and hence that modeling the nature of the zone will allow determination of their geometry and extent beneath the Sierra. This in turn will place constraints on the roles of delamination and convective instability in this region. The primary source of data to be analyzed was collected during the Sierra Nevada Earthscope Project (SNEP) from May 2005 to mid-2007, supplemented by recordings from a dense array of broadband seismometers across Yosemite National Park (~37.8°N) during the summer of 2007. The analysis will combine full waveform modeling of teleseismic receiver functions and surface waves with local and regional (for Pn) arrivals to ensure that any model produced is consistent with observations that are mostly likely to be sensitive to the crust-mantle transition. This project benefits from an abundance of trial models from prior studies in the region, and this approach will be to use these as starting models in a formal joint inversion to examine how well they can explain the Pn shadow zone while still being consistent with teleseismic observations. Ultimately, the study's results will help resolve the role of "arclogite" in lithospheric foundering and hence determine whether delamination of the lithosphere or a convective instability can better explain lithospheric foundering beneath the Sierra.

地球最外层的山脉由低于平常的密度支撑：也就是说，较低密度的地壳、较厚的地壳或最浅地幔的低密度将如何支撑加利福尼亚州的内华达山脉。科学家们仍然不知道如何去除高密度材料。对这种地震波特性的历史观察最初导致地震学家得出了内华达山脉厚度的错误结论。地壳；尽管我们知道这不是真的，但研究人员认为这种物质可能仍然存在或集中在山脉的西部，他们将利用地震信息来探测是否存在致密物质。这种比较是通过扩展软件来完成的，该软件可以在存在复杂地质结构的情况下计算地震波形，因此该项目提高了我们对山脉如何形成的理解，并改进了我们的地震学工具包以探索发生的情况。地壳和地幔之间的边界岩石圈沉没已被用来解释全球许多地区的造山作用，但在大多数情况下，岩石圈沉没的方式仍然很大程度上未知，加利福尼亚州东部的内华达山脉经常被认为是这些地区之一，并且关于岩石圈如何在那里形成的假设范围从岩石圈分层到局部对流不稳定性，区分这些相互竞争的假设的关键证据在于是否存在某些与弧相关的岩性（又称岩石圈）。 “arclogite”）以及塞拉山脉西部山麓下方如此致密岩性的几何形状，弧辉岩的存在应该会导致下地壳和上地幔的厚度和波速发生巨大且潜在的突然变化，而这些可能会影响通道。事实上，这种效应可以解释地震学中一个长期存在的难题，即：内华达山脉产生 Pn 阴影区的原因是什么？最近开发了全波形反演（FWI）技术来评估山脉下方的地壳和上地幔的试验模型，这些模型可能是造成 Pn 阴影区的原因，一个假设是弧逻辑岩性是造成该区域的原因，因此可以对自然进行建模。该区域的分布将允许确定其在山脉下方的几何形状和范围，这反过来又会限制该地区的分层和对流不稳定的作用。要分析的数据的主要来源是在内华达山脉地球范围项目期间收集的。 SNEP) 2005 年 5 月至 2007 年中期，并辅以 2007 年夏季优胜美地国家公园（~37.8°N）上密集的宽带地震仪阵列的记录。该分析将把远震接收器函数和表面波的完整波形建模与本地地震相结合。和区域（对于 Pn）到达，以确保生成的任何模型与最有可能对壳幔转变敏感的观测结果一致。该项目受益于大量的试验模型。该方法将使用这些作为正式联合反演的起始模型，以检验它们如何解释 Pn 阴影区，同时仍与远震观测保持一致，最终，该研究的结果将有助于解决这一问题。 “arclogite”在岩石圈形成中的作用，从而确定岩石圈分层或对流不稳定是否可以更好地解释山脉下方的岩石圈形成。