Collaborative Research: Geophysical Study of Ongoing Subduction Initiation Along the Matthew-Hunter Trench

俯冲是板块构造的一个基本组成部分，其中一个板块俯冲到另一个板块之下，并在覆盖的板块上产生火山弧。然而，人们对俯冲是如何开始的知之甚少，主要是因为目前还没有观察到这一过程。最近的研究表明，俯冲作用沿着南太平洋瓦努阿图、新喀里多尼亚和斐济之间的马修和亨特海沟才刚刚开始。该项目将对马修-亨特俯冲带进行两次科学巡航，以部署和恢复海底地震仪，并收集海洋地球物理数据集。地震数据将用于对下行板块和火山下的岩浆源区进行成像。这些航行还将绘制海底地图，并进行重力和磁力测量，以提供有关构造结构、火山活动、地壳变形和俯冲起始过程年龄的证据。该项目将与法国和新喀里多尼亚的科学家合作进行。俯冲起始可能是地球动力学中最持久的问题。大多数关于俯冲起始的知识来自于对过去的起始事件的研究，其中一些火山产物被保存了下来。现今缺乏包括弧前岩浆作用经典序列的俯冲启动的例子，限制了对这些过程如何发生的理解。然而，最近的研究表明，伴随着岩浆活动的俯冲开始发生在沿着新赫布里底海沟和斐济地台之间的马修和亨特海沟。因此，马修-亨特俯冲带提供了一个独特的机会，研究一个年轻的俯冲带的形成过程中，但足够先进，创造岩浆产物。该项目将部署一个由25个被动海底地震仪组成的阵列（20个美国和5个法国），并进行海洋地球物理调查，以确定与这一正在发展的俯冲带有关的板块几何形状、地震活动性、结构和地幔流动模式。地震仪将沿沿着部署，由法国合作巡航拍摄，以利用浅层结构的限制，该阵列的设计是为了成像前弧，火山猎人岭和延伸的蒙齐尔裂谷的结构。地震数据将使用环境噪声和地震的瑞利波层析成像、体波层析成像和剪切波分裂进行分析。此外，还将进行广泛的多波束测深、声学反向散射成像、重力和磁力调查，以查明构造结构、近期海底火山活动区域和磁性等时线。地球物理跟踪间隔将足够密集，以便对弧前区域进行全面测绘，并在部署主要地震仪的同一区域进行三维地球物理反演。这种近地表和重合的更深地壳和地幔成像的组合将为理解俯冲起始提供强有力的约束。这一项目将与法国海洋所和新喀里多尼亚的调查人员合作进行，法国海洋所和新喀里多尼亚的调查人员提议进行一次法国巡航，目前正在等待安排，以收集活动源地震数据以及热流和流体样本，这一奖项反映了国家科学基金会的法定使命，并被认为值得通过利用基金会的知识价值和更广泛的影响审查标准进行评价予以支持。

Subduction, where one plate plunges downward beneath another plate and produces a volcanic arc on the overriding plate, is a fundamental component of plate tectonics. However, there is little understanding of how subduction begins, largely because there have been no observations of this process happening at the present time. Recent studies show that subduction is just beginning along the Matthew and Hunter Trench, between Vanuatu, New Caledonia, and Fiji in the South Pacific. This project will conduct a pair of scientific cruises to the Matthew-Hunter Subduction Zone to deploy and recover ocean bottom seismographs, as well as gather marine geophysical datasets. The seismic data will be used to image the downgoing plate and magma source regions beneath volcanoes. The cruises will also map the seafloor and make gravity and magnetic measurements that will provide evidence concerning tectonic structures, volcanism, crustal deformation, and ages of subduction initiation processes. This project will be carried out in collaboration with scientists from France and New Caledonia. Subduction initiation may be the most enduring question in terrestrial geodynamics. Most knowledge of subduction initiation comes from the study of past initiation events where some of the volcanic products have been preserved. The lack of present-day examples of subduction initiation that include the classic sequence of forearc magmatism limits the understanding of how these processes occur. However, recent studies show that subduction initiation with accompanying magmatic activity is occurring along the Matthew and Hunter Trench between the New Hebrides Trench and the Fiji Platform. The Matthew-Hunter Subduction Zone thus offers a unique opportunity to study a young subduction zone in the process of formation but sufficiently advanced to create magmatic products. This project will deploy an array of 25 passive ocean bottom seismographs (20 US and 5 French) and carry out a marine geophysical survey to define the slab geometry, seismicity, structure, and mantle flow patterns associated with this developing subduction zone. The seismographs will be deployed along lines to be shot by a collaborative French cruise to take advantage of shallow structure constraints, and the array is designed to image the structure of the forearc, the volcanic Hunter Ridge, and the extensional Monzier Rift. Seismic data will be analyzed using Rayleigh wave tomography from ambient noise and earthquakes, body wave tomography, and shear wave splitting. In addition, an extensive multibeam bathymetric, acoustic backscatter imagery, gravity and magnetic survey will be carried out to identify tectonic structures, regions of recent seafloor volcanism, and magnetic isochrons. Geophysical track spacing will be sufficiently dense to fully map the forearc area and carry out three-dimensional geophysical inversions over the same regions as the main seismograph deployment. This combination of near surface and coincident deeper crustal and mantle imaging will provide powerful constraints for understanding subduction initiation. This project will be carried out in collaboration with investigators from IFREMER and New Caledonia, who have proposed a French cruise currently awaiting scheduling to collect active source seismic data as well as heat flow and fluid samples.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.