Collaborative Research: A Seismic Study of Oceanic-Arc Crustal Construction Processes at the Archetypal Andreanof Segment of the Aleutian Arc

这个项目旨在了解俯冲带上方的火山弧是如何形成地壳的。火山弧内的地壳形成是一个极其重要的过程，它不断地补充大陆的质量，大陆不断的风化和侵蚀提供了推动地球生物地球化学循环的元素营养的来源。值得注意的是，新大陆物质的产生和物质回收回到地幔的过程在地球上保持了数亿年的平衡。理解这种平衡是理解地球宜居性的关键。所有的地壳都是由更深的地幔的部分熔融形成的，地幔向上上升，然后冻结形成地壳。当俯冲构造板块释放的水降低了地幔的融化温度，产生了上升并聚集在火山弧中的岩浆时，大陆地壳的积木就形成了。弧壳形成过程一直是许多研究的焦点，但我们对这些过程的认识还远远不够完整。问题之所以存在，是因为弧形过程是复杂和时断时续的，它们发生在一系列空间尺度上，并取决于变量，如俯冲板块的年龄和给定弧形的演化阶段，这可能永远不会达到稳定状态。该项目的研究地点是阿留申弧段的安德烈诺夫段。巧合的是，这个地点恰逢1957年阿留申大地震的震中，这是上个世纪第三大地震，也是有记录以来最大的余震区。活动震源地震技术将被用来成像沿这一弧段和跨越这一弧段的整个地壳的地震速度结构，以解决弧壳形成的许多悬而未决的问题。这些方法可以用来解决与火山弧常见的地震和火山灾害有关的问题。该项目的教育部分将培训一批下一代科学家，使他们能够获取和深入分析这些类型的数据。通过这笔赠款，将在阿留申弧安德里安诺夫段进行有源地震实验，以研究弧壳形成过程。关于弧壳形成过程的许多基本问题仍然存在，包括原生弧岩浆的来源和变异性，这些岩浆在哪里以及如何分离以产生地壳岩石的特征弧形地球化学特征，水在所有这些过程中的作用，弧壳的整体组成及其基本结构和与大陆地壳的相似性，所有这些过程的强迫作用和这些强迫的时空变异性，以及整个系统的物质平衡。通过对弧段进行重点、多学科的研究，可以取得实质性的进展，因为弧段具有简单、众所周知的岩浆和构造史，对地球化学进行了很好的研究，并有足够的沿走向的可变性，以捕捉许多据信控制弧壳地球化学和构造演化的过程。安德里安诺夫地区是少数几个满足这些标准的地点之一。它由一个完整的弧形地块组成，形成于三个可识别和确定年代的岩浆时期；它只是轻微变形，在活火山活动中表现出从东部的主要玄武岩到西部的安山岩/英安岩的沿走向的变化。这笔赠款支持的地震实验将包括沿整个管段长度的一个横断面和两个弧形交叉横断面。将沿每个断面采集记录在8公里长的拖缆上的多通道地震数据和仪器间距为10至15公里的海底地震数据，并将使用先进的层析成像和海洋地震数据成像方法对这些数据进行分析，以产生地震速度和反射率图像，从而首次揭示完整弧形地块在分段尺度上的完整地壳结构。这些图像将提供成分估计，为熔体侵位和分馏模型提供信息，并使一些具体问题得以解决，包括：洋弧地壳的整体组成、厚度和基本结构，以块状弧形地壳组成和结构表示的原始岩浆组成和地壳处理等因素的变化，以及镁铁质/超镁铁质堆积物的命运如何？这一奖项反映了美国国家科学基金会的法定使命，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。

This project aims to understand how crust is formed in volcanic arcs above subduction zones. Crustal formation within volcanic arcs is a profoundly important process that continually replenishes the mass of the continents, whose constant weathering and erosion provide the source of elemental nutrients that drive Earth's biogeochemical cycles. Remarkably, the creation of new continental material and the recycling of material back into the mantle have remained balanced on Earth over hundreds of millions of years. Understanding this balance is key to understanding the habitability of the planet. All of Earth's crust is created from partial melts of the deeper mantle that rise upward and then freeze to form the crust. The building blocks of continental crust form when water released from a subducting tectonic plate lowers the melting temperature of the mantle, creating magma that rises and accumulates in volcanic arcs. The process of arc-crust formation has been the focus of many studies, but our understanding of these process is far from a complete. Questions persist because arc processes are complex and episodic, they occur over a range of spatial scales and depend on variables such as the age of the subducting plate and the evolutionary stage of a given arc, which may never reach a steady state. The study location of this project is the Andreanof segment of the Aleutian arc. Coincidentally, this location coincides with the epicenter of the 1957 Great Aleutian earthquake, the third largest earthquake of the last century and with the largest aftershock zone of any earthquake ever recorded. Active-source seismic techniques will be used to image the seismic velocity structure of the entire crust along and across this arc segment to address many of the unanswered questions of arc-crust formation. These same methodologies can be used to address questions related to earthquake and volcanic hazards common to volcanic arcs. The education component of this project will train a cohort of the next generation of scientists in the acquisition and advance analyses of these types of data.Through this grant an active-source seismic experiment will be conducted in the Andreanof segment of the Aleutian arc in order to study arc-crust formation processes. Many basic questions about arc-crust formation processes remain, including the source and variability of primary arc magmas, where and how these magmas fractionate to produce the characteristic arc geochemical signatures of crustal rocks, the role of water in all of these processes, the bulk composition of arc crust and its basic architecture and similarity to continental crust, the forcing functions for all of these processes and the temporal and spatial variability of those forcings, and the mass balance of the entire system. Substantial advance can be made by focused, multi-disciplinary study of an arc segment that has simple, well known magmatic and tectonic histories, well studied geochemistry, and sufficient along-strike variability to capture many of the processes that are believed to control arc-crust geochemical and structural evolution. The Andreanof segment is one of very few locations that satisfy these criteria. It consists of an intact, arc massif formed during three identifiable and dated magmatic episodes; it is only modestly deformed and exhibits along-strike variability in active volcanism from primarily basaltic in the east to andesitic/dacitic in the west. The seismic experiment supported by this grant will consist of one transect along the entire length of the segment and two arc-crossing transects. Multi-channel seismic (MCS) data, recorded on an 8-km-long streamer, and ocean-bottom seismic data, with instrument spacings of 10 to 15 km, will be acquired along each transect, and these data will be analyzed with advanced tomographic and MCS imaging methodologies to produce seismic velocity and reflectivity images that will reveal, for the first time, the full crustal architecture of an intact arc massif at a segment scale. These images will provide estimates of composition that will inform models of melt emplacement and fractionation, and enable a number of specific questions to be addressed, including: What is the bulk composition, thickness and basic architecture of oceanic-arc crust, How are variations in factors such as primary magma composition and crustal processing of magmas expressed in bulk arc crustal composition and structure, and What is the fate of mafic/ultra-mafic cumulates?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.