Investigation of Anomalous Travel Times in the Central Andes: Possible Evidence for a Lithospheric Root Trapped Above a Flat Slab

板块会聚边界的大陆变形研究对于增进我们对大陆如何演化、自然资源如何集中以及地震和火山等自然灾害如何发生的理解具有根本重要性。 由于广泛的过程和丰富的数据，沿着南美洲西海岸的安第斯山脉是研究这种大陆变形的最佳场所之一。该项目试图利用过去几十年在南美洲记录的大量地震数据档案，调查智利和阿根廷边界附近的高安第斯山脉特定部分下如何发生变形，在那里，纳斯卡板块俯冲似乎在覆盖的南美洲岩石圈下变平。 这一地区特别令人感兴趣，因为以前的研究确定了一个地震信号，表明异常高的弹性波速，这表明南美洲岩石圈正在增厚的平板和变形的过程中。 虽然这种增厚是预期在这种类型的收敛边缘，它的存在以上的平板挑战我们目前的理解的物理和化学过程中所涉及的。该项目将开发最先进的地震成像方法，并将其应用于一个组合数据集，以推断这一特征是什么，它如何影响板块和大陆的变形，以及它如何影响南美洲的演变以及该地区地震和火山的产生。该项目涉及美国，智利和阿根廷的学生和高级科学家之间的广泛国际合作。该项目将支持研究生的工作，这些研究生既是女性，也是STEM研究中代表性不足的人口成员。它还将为2-3名本科生提供为期一个学期的研究机会。该项目的主要重点是加强美国，智利和阿根廷之间的国际研究联系，因为涉及RPI，智利和圣地亚哥大学以及阿根廷圣胡安国立大学的科学家和学生之间的高度合作。板块和岩石圈根是安第斯大陆边缘岩石圈生长和演化的基本特征，但它们很少被认为是相互交织的现象。平板上的地幔被认为是冷的和静态的，而根部的形成通常被认为是表面上更动态的软流圈地幔楔环境的产物。根据调查人员之前在2015年伊亚佩尔地震余震中在该地区所做的工作，有证据表明，位于高安第斯山脉下方的帕姆托平板部分拥有这样的根。此外，该团队假设，该根的较高密度导致俯冲的纳斯卡板块的底层“平坦”岩石圈局部向下偏转15-20公里。在Pamerican平板之上的岩石圈根的识别将迫使我们重新思考根如何在压缩构造环境中演化。几个因素，通常被认为是至关重要的根的形成，包括一个水合地幔楔和有力的角流楔内将大大减少，如果不是完全没有，在平板。此外，现存的岩石圈根是一种相对罕见的现象;它们通常只有在被移除时才能被发现。因此，现有的根的积极识别提出了一个难得的机会，在原位调查。虽然以前对这一领域的研究暗示存在这样一个实体，但在大多数情况下，由于分辨率或采样不足以及在通用背景模型中可能将信号误分类为离群值，因此它没有被检测到。初步分析表明，目前的数据集没有得到充分利用。国际团队计划生成增强的事件目录，然后应用标准（例如，到达时间层析成像、衰减层析成像、来自剪切波双折射的各向异性）和更先进的（例如，该奖项反映了NSF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。

Studies of continental deformation at convergent plate boundaries are of fundamental importance for advancing our understanding of how continents evolve, how natural resourses are concentrated, and how natural hazards such as earthquakes and volcanoes occur. Because of the broad spectrum of processes represented and the abundance of data available, the Andes mountains along the western coast of South America provide one of the best venues to investigate such continental deformation. This project seeks to take advantage of the extensive archive of seismic data recorded over the past several decades in South America to investigate how deformation is occurring beneath a particular part of the high Andes close to the border between Chile and Argentina where the subducting Nazca plate appears to flatten under the overriding South American lithosphere. This region is of particular interest because previous studies identified a seismic signal indicating unusually high elastic wavespeeds that suggest that the South American lithosphere is thickening above the flat slab and deforming it in the process. While such thickening is expected in this type of convergent margin, its existence above a flat slab challenges our current understanding of the physics and chemistry involved in such processes. This project will develop and apply state-of-the-art seismic imaging methodologies to an assembled dataset to allow inferences about what this feature is and how it is influencing the deformation of the both the slab and continent, and by extension how it affects the evolution of South America and the generation of earthquakes and volcanos in the region. This project involves an extensive international collaboration between students and senior scientists in the US, Chile, and Argentina. This project will support the work of graduate students who are both female and a member of an underrepresented demographic in STEM research. It will also provide semester-long research opportunities for 2-3 undergraduate students. A primary emphasis of this project is strengthening international research ties between the US, Chile and Argentina, as in involves highly collaborative work between scientists and students at RPI, the Universidad de Chile and Santiago, and the National University of San Juan in Argentina. Flat slabs and lithospheric roots are both fundamental features in the growth and evolution of continental lithosphere at Andean margins, but rarely, if ever, are they considered interreleated phenomena. The mantle above flat slabs is presumed to be cold and static, while root formation is generally believed to be the province of the ostensibly more dynamic environment of an asthenospheric mantle wedge. Based on previous work the investigators have done in this area with aftershocks of the 2015 Illapel earthquake, there is evidence that the part of the Pampean flat slab beneath the high Andes hosts such a root. Moreover, the team hypothesizes that the higher density of this root is causing the underlying “flat” lithosphere of the subducted Nazca plate to deflect downwards locally by 15-20 km. The identification of a lithospheric root above the Pampean flat slab would force us to rethink how roots evolve in a compressive tectonic environment. Several factors that are commonly cited to be critical to root formation, including a hydrated mantle wedge and vigorous corner flow within that wedge would be greatly reduced, if not completely absent, above a flat slab. Additionally, an extant lithospheric root is a relatively rare phenomenon; they often make their presence known only by their removal. Hence, the positive identification of an existing root presents a rare opportunity to investigate one in situ. While previous studies of this area hint at the existence of such an entity, for the most part it has escaped detection due to a combination of insufficient resolution or sampling and the potential misclassification of signals as outliers in generic background models. Preliminary analysis suggests that current datasets have been underutilized. The international team plans to generate enhanced catalogs of events, and then apply a combination of standard (e.g., arrival time tomography, attenuation tomography, anisotropy from shear wave birefringence) and more advanced (e.g., coda migration, anisotropic receiver functions, joint inversions) analysis techniques to this data to identify and characterize the hypothesized root.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.