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

该项目旨在了解地壳是如何在俯冲带上方的火山弧中形成的。火山弧内的地壳形成是一个非常重要的过程，它不断地补充大陆的物质，大陆的不断风化和侵蚀提供了驱动地球生物地球化学循环的元素营养来源。值得注意的是，在数亿年的时间里，地球上新的大陆物质的产生和地幔物质的循环一直保持平衡。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融化形成的，这些地幔向上上升，然后冻结形成地壳。当从俯冲构造板块释放的水降低了地幔的融化温度，产生岩浆，岩浆上升并积聚在火山弧中时，大陆地壳的组成部分就形成了。弧壳的形成过程一直是许多研究的焦点，但我们对这一过程的认识还很不完整。问题仍然存在，因为弧的过程是复杂和偶发的，它们发生在一系列的空间尺度上，并取决于诸如俯冲板块的年龄和给定弧的演化阶段等变量，这些变量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线的安德里亚诺夫段。巧合的是，这个位置与1957年阿留申大地震的震中重合，这是上世纪第三大地震，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整个地壳沿弧段和横跨弧段的地震速度结构，以解决弧地壳形成的许多未解问题。这些相同的方法可用于解决与火山弧常见的地震和火山危害有关的问题。这个项目的教育部分将训练一批下一代科学家获取和提前分析这类数据。通过这笔拨款，将在阿留申弧的安德里亚诺夫段进行一项有源地震实验，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可变性，这些岩浆在何处以及如何分解产生地壳岩石的特征弧地球化学特征，水在所有这些过程中的作用，弧壳的总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以及这些强迫的时空变异性。以及整个系统的质量平衡。通过集中、多学科的弧段研究，可以取得实质性进展，弧段具有简单、已知的岩浆和构造历史，经过充分研究的地球化学，以及足够的沿走向变异性，以捕获许多被认为控制弧壳地球化学和构造演化的过程。Andreanof段是极少数满足这些标准的地点之一。它由一个完整的弧状地块组成，形成于三个可识别的岩浆期；它只有轻微的变形，并在活火山活动中表现出沿走向的变化，从东部的主要玄武岩到西部的安山岩/英安质。这项拨款支持的地震实验将包括沿整段长度的一个样条和两个弧形交叉样条。在8公里长的拖缆上记录的多通道地震（MCS）数据，以及仪器间距为10至15公里的海底地震数据，将沿着每个样带获取这些数据，并使用先进的层析成像和MCS成像方法进行分析，以产生地震速度和反射率图像，从而首次在段尺度上揭示完整弧形地块的完整地壳结构。这些图像将提供对成分的估计，为熔体就位和分块模型提供信息，并使许多具体问题得以解决，包括：大洋弧地壳的总体组成、厚度和基本结构是什么？在大洋弧地壳组成和结构中，原始岩浆组成和岩浆的地壳加工等因素的变化是如何表达的？基性/超基性堆积的命运是什么？该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。

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.