How Important Were Mid-Ocean Ridge Processes in the Formation of Shatsky Rise?

发生在海底环境中的火山省并没有像陆地上的火山省那样得到很好的研究，但海底喷发的规模和岩浆输出可能是巨大的。本项目重点研究西太平洋沙斯基隆起大火成岩省（LIP）。沙茨基隆起的体积为320万至690万立方公里，面积与加利福尼亚州大致相当。海底磁学和测深数据表明，这个LIP在1.4亿至1.5亿年前开始形成，在板块构造系统中被称为脊-脊-脊三联结的位置或附近。脊-脊-脊三联结发生在三个分散的扩散中心在单一区域相遇时；与沙茨基隆起相关的三重交界处标志着太平洋、伊扎那吉和法拉隆板块的交汇处。沙斯基隆起的高岩浆通量，特别是高原南部的塔木地块的高岩浆通量，使得之前的研究表明，地幔中的热柱或化学非均质性是岩浆活动的来源。本项目旨在探讨三结板块边界配置在沙氏隆起形成中的重要性。了解构造板块如何控制沙斯基隆起下的上地幔的三维流动模式，对于解开这个巨大的LIP是如何形成的至关重要。本项目将有两名本科生参与。一名学生将专注于项目的建模部分，而另一名学生将开发有关产生lip的地质过程的STEM教育材料，以Shatsky Rise为例。该项目解决了几个悬而未决的问题：(1)在沙斯基隆起所在的三重交界处，板块驱动的三维地幔流的性质是什么？(2)三结几何是否导致沿脊轴流动，从而影响地幔柱的空间弥散或地球化学非均质性？(3)三联点上升流速度的大小是多少？(4)经历熔融的三维地幔体积的形状和大小是什么？与只有两个板块分离的环境相比，山脊相关的上升流是否在异常深度上延伸？(5)与正常的洋中脊玄武岩相比，其融化程度如何？解开导致沙斯基隆起就位的地质过程是具有挑战性的，特别是自从LIP开始形成以来已经过去了相当长的时间，而且没有直接的观测方法可以用来表征地幔柱或可能与之相关的地球化学异质性。然而，有相对丰富的海洋地球物理和地质数据可用于沙茨基隆起地区，这使得建模与板块边界有关的过程。本项目使用太平洋-伊扎那吉-法拉隆三结的有限元模型来约束地幔如何响应三个板块的分离而上涌。模型结合了太平洋-伊扎那吉-法拉伦三结的几何形状，包括板块重建、真实的相对板块运动矢量和粒子跟踪，以跟踪地幔物质的平流。模型的子集还包括地幔脱水粘度的处理和三结板块边界迁移的影响。模型计算可以与现有的海底数据和地球化学分析进行比较，以改进对沙斯基隆起地层中非均质地幔物质的性质、位置和空间分散的推断。

Volcanic provinces that occur in the submarine environment are not as well studied as those on land, but size and magmatic output of submarine eruptions can be immense. This project focuses on the Shatsky Rise large igneous province (LIP) in the western Pacific Ocean. Shatsky Rise has a volume of 3.2-6.9 million cubic kilometers and an area roughly the same as California. Seafloor magnetic and bathymetric data suggest that this LIP began to form 140-150 million years ago, at or near a setting in the plate tectonic system called a ridge-ridge-ridge triple junction. Ridge-ridge-ridge triple junctions occur when three divergent spreading centers meet in a single area; the triple junction associated with Shatsky Rise marked the intersection of the Pacific, Izanagi, and Farallon plates. The high magmatic fluxes inferred for Shatsky Rise, particularly for Tamu Massif in the southern portion of the plateau, have led previous investigations to suggest that a thermal plume or chemical heterogeneity in the mantle was the source of the magmatism. This project aims to investigate the importance of triple junction plate boundary configuration in the formation of Shatsky Rise. Understanding how the tectonic plates controlled the three-dimensional flow patterns in the upper mantle beneath Shatsky Rise is crucial to unraveling how this voluminous LIP formed. Two undergraduate students will participate in this project. One student will concentrate on the modeling component of the project, while the other will develop STEM education materials on geological processes that generate LIPs, with Shatsky Rise as the example. The project addresses several outstanding questions: (1) What was the nature of plate-driven, three-dimensional mantle flow at the triple junction where Shatsky Rise was emplaced? (2) Did triple junction geometry cause flow along the ridge axes that could have influenced the spatial dispersion of a mantle plume or geochemical heterogeneity? (3) What was the magnitude of upwelling velocity at the triple junction? (4) What was the shape and size of the three-dimensional mantle volume that experienced melting, and did ridge-related upwelling extend over anomalous depths compared to a setting where only two plates diverge? and (5) What was the degree of melting, compared to normal mid-ocean ridge basalt? Unraveling the geological processes that contributed to the emplacement of Shatsky Rise is challenging, particularly since considerable time has elapsed since this LIP began to form and no direct observational method can be used to characterize the mantle plume or geochemical heterogeneity that may have been associated with it. There are, however, relatively abundant marine geophysical and geological data available for the Shatsky Rise region, which enable modeling of plate-boundary-related processes. This project uses a finite element model of the Pacific-Izanagi-Farallon triple junction to constrain how mantle upwelled in response to the divergence of three plates. Models incorporate the geometry of the Pacific-Izanagi-Farallon triple junction from plate reconstructions, realistic relative plate motion vectors, and particle tracking to follow the advection of mantle material. A subset of models also includes treatment of mantle dehydration viscosity and the effect of triple junction plate boundary migration. The model calculations can be compared to existing seafloor data and geochemical analyses to improve inferences about the nature, location, and spatial dispersion of heterogeneous mantle material involved in Shatsky Rise formation.