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

山脉是由地球最外层较低的密度支撑的：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支撑高地。该项目探索了加州内华达山脉是如何由低密度材料支撑的。科学家们仍然不知道高密度物质是如何被去除的。对这种地震波特性的历史观察，最初使地震学家得出了内华达山脉有厚地壳的错误结论；虽然我们知道这不是真的，但这些观察结果仍然没有得到解释。研究人员认为，这些物质可能仍然存在，或者一直集中在山脉的西部。他们将利用地震信息来探测他们预测的地方是否存在致密物质。这种比较是通过扩展软件来完成的，该软件可以计算复杂地质结构下的地震波形。因此，这个项目提高了我们对山是如何形成的理解，也改善了我们的地震学工具，用于探索大陆地壳和地幔之间的边界发生了什么。在全球许多地区，人们用岩石圈沉降来解释造山运动，但在大多数情况下，岩石圈沉降的方式在很大程度上仍然是未知的。加州东部的内华达山脉经常被认为是这些地区之一，关于岩石圈如何在那里沉没的假设从岩石圈分层到局部对流不稳定不等。区分这些相互竞争的假说的一个关键证据是存在或不存在某些与弧相关的岩性（又称地层岩性）。“弧长岩”)，以及西喜马拉雅山麓下如此密集的岩性的几何形状。弧长岩的存在会导致下地壳和上地幔的厚度和波速发生巨大且可能突然的变化，这些变化可能会影响Pn能量在塞拉山脉上的传递。事实上，这样的影响可以为地震学中一个长期存在的难题提供解释，即：是什么原因导致了塞拉利昂产生的Pn阴影带？该项目的具体目标是应用最近开发的全波形反演（FWI）技术来评估塞拉利昂地壳和上地幔的试验模型，这些模型可能是造成Pn阴影带的原因。有一种假说认为，这一区域是由烃源岩岩性形成的，因此，对该区域的性质进行建模将有助于确定其几何形状和在塞拉山脉下的范围。这反过来又会限制该地区分层和对流不稳定的作用。要分析的主要数据来源是2005年5月至2007年中期内华达山脉地镜项目（SNEP）收集的，辅以2007年夏天约塞米蒂国家公园（~37.8°N）密集的宽带地震仪阵列的记录。该分析将结合远震接收函数和表面波与局部和区域（Pn）到达的完整波形建模，以确保产生的任何模型都与最可能对壳幔过渡敏感的观测结果一致。该项目得益于该地区先前研究的大量试验模型，该方法将使用这些模型作为正式联合反演的起始模型，以检查它们在解释Pn阴影带的同时仍与远震观测结果一致的程度。最终，这项研究的结果将有助于解决“弧长岩”在岩石圈沉降中的作用，从而确定岩石圈分层或对流不稳定是否能更好地解释塞拉山脉下的岩石圈沉降。

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.