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

发生在海底环境中的火山区没有陆地上的火山区那么好研究，但海底喷发的规模和岩浆产量可能是巨大的。 该项目的重点是西太平洋的沙茨基海隆大火成岩省（LIP）。 沙茨基隆起的体积为320 - 690万立方公里，面积与加州大致相同。 海底磁性和测深数据表明，这个LIP在1.4亿至1.5亿年前开始形成，位于或接近板块构造系统中称为脊-脊-脊三联点的位置。 当三个分散的扩张中心在一个区域相遇时，就会出现脊-脊-脊的三联点;与沙茨基海隆有关的三联点标志着太平洋板块、伊佐那木板块和法拉隆板块的交叉。 推测沙茨基隆起，特别是塔穆地块在高原南部的高岩浆通量，导致以前的调查表明，热羽或化学不均匀性的地幔是岩浆活动的来源。 本项目旨在研究三重连接板块边界构型在沙茨基海隆形成中的重要性。 了解构造板块如何控制沙茨基隆起下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。 两名本科生将参与该项目。 一名学生将专注于该项目的建模部分，而另一名学生将开发关于生成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）什么是板驱动的，三维地幔流的性质在三重连接处的沙茨基隆起是侵位？(2)三联点的几何形状是否导致了沿沿着洋脊轴的流动，从而影响了地幔柱的空间分散或地球化学的不均匀性？(3)在三重连接处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？与只有两个板块分开的情况相比，与脊有关的上涌是否延伸到异常深度？（5）与正常洋中脊玄武岩相比，其熔融程度如何？ 揭示沙茨基海隆就位的地质过程具有挑战性，特别是因为自LIP开始形成以来已经过去了相当长的时间，并且没有直接的观测方法可以用来描述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海隆地区，其使得能够对板边界相关过程进行建模。 该项目使用太平洋-Izanagi-Farallon三联点的有限元模型来约束地幔如何响应三个板块的发散而上涌。 模型结合了板块重建的太平洋-Izanagi-Farallon三重连接点的几何形状，现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。 模型的一个子集还包括地幔脱水粘度和三重连接板块边界迁移的影响的治疗。 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，以改善对沙茨基隆起形成中所涉及的异质地幔物质的性质、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.