Collaborative Research: Quantifying melt in the mantle and controls on lithosphere-asthenosphere dynamics and intraplate magmatism: a joint seismic and EM survey of the Cocos plate

合作研究：量化地幔熔化并控制岩石圈-软流圈动力学和板内岩浆作用：科科斯板块的联合地震和电磁调查

• 批准号: 2146896
• 负责人: Samer Naif
• 金额: $ 100.39万
• 依托单位: Georgia Tech Research Corporation
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2022
• 资助国家: 美国
• 起止时间: 2022-09-01 至 2026-08-31
• 项目状态: 未结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2146896&HistoricalAwards=false
• 关键词: Collaborative; Research; Quantifying; melt; mantle

The existence of a weak, ductile asthenosphere, the upper layer of Earth’s mantle below the lithosphere, is fundamental for the operation of plate tectonics on Earth since it enables the motion of the overlying mobile lithospheric plates. Since the discovery of plate tectonics more than 50 years ago, the dominant mechanism responsible for this weakening remains unresolved. Debate centers on two competing mechanisms: partial melting and hydration of the mantle. In theory, geophysical imaging of seismic velocity and electrical resistivity could distinguish between these competing mechanisms, but inherent trade-offs and uncertainties associated with inferring physical properties from the field observations result in significant ambiguity. This project will image the seismic and electric structure of the mantle beneath a portion of the Cocos Plate in the eastern equatorial Pacific Ocean, where the presence of asthenospheric melt is confidently known. The new observations will provide an independent calibration for the effect of asthenospheric melt on seismic and electric properties and constrain the regional extent and geometry of the melt. This will in turn enable improved understanding of the source of abundant and anomalous volcanism in the region. The project will support the research activities of four graduate students and several undergraduate students, as well as data sharing and collaboration with partners in the neighboring countries of Nicaragua, El Salvador, and Costa Rica. More than 20 cruise participants will be trained in geophysical data acquisition during two research expeditions, including students, international partners, and community participants.This project will utilize passive-source seismic and magnetotelluric imaging to quantify the seismic velocity and electrical resistivity structure of the Cocos Plate lithosphere and asthenosphere. A co-located array of ocean-bottom seismic and electromagnetic receivers will be deployed over a 500 km by 500 km section of the Cocos Plate on both sides of the Nicaragua Fracture Zone (NFZ). The seafloor in the survey area shows abundant evidence of past intraplate magmatism, including elevated seafloor bathymetry, numerous seamounts significantly younger than the plate age, and prominent volcanic sills within the sediment column. The presence of a partial melt channel at the lithosphere-asthenosphere boundary is clearly imaged by a previous small-scale magnetotelluric profile. Observations from this study will provide new insights on the dominant controls on asthenosphere rheology and the mechanisms that produce intraplate magmatism by addressing the following key questions: (1) What is the spatial and depth extent of the high-melt region? Does the bathymetry contrast across the NFZ reflect a change in melt content and/or volcanic productivity? (2) What is the seismic signature of the previously inferred melt-rich channel? Are seismic observations consistent with magnetotelluric results and how can they be calibrated to each other? Given this calibration between melt content and velocity, what does it imply for the weakening mechanisms in the asthenosphere globally? (3) Does the mantle fabric change across the NFZ, both within and below the lithosphere? Does the implied flow field suggest a Galápagos plume influence on past seafloor spreading, and/or recent melt productivity beneath the plate?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.

岩石圈下地球上地幔的上层弱的，延性的软圈的存在对于地球上板块构造的运行至关重要，因为它可以使上覆的移动岩石圈板的运动。自从50年前发现板块构造学的发现以来，导致这种弱化的主要机制仍未解决。辩论以两种竞争机制为中心：地幔的部分融化和水合。从理论上讲，地震速度和电阻的地球物理成像可以区分这些竞争机制，但是继承了与从现场观察中推断物理性质相关的权衡和不确定性，从而导致了明显的歧义。该项目将在东部赤道太平洋中的一部分可可板下面的地幔的地震和电结构图像，在那里，微弱的熔体的存在是自信的。新的观察结果将提供独立的校准，以使小圈熔体对地震和电性能的影响，并限制熔体的区域范围和几何形状。反过来，这将使对该地区丰富和异常火山的来源的理解得到改善。该项目将支持四名研究生和几名本科生的研究活动，以及与尼加拉瓜，埃尔·萨尔瓦多和哥斯达黎加的邻国合作伙伴的数据共享和合作。在两次研究探险期间，包括学生，国际合作伙伴和社区参与者在内的地球物理数据获取的20多名巡航参与者将接受培训。该项目将利用被动源地震和磁性成像来量化地震速度以及Cocos Plate Plate lithosplosit lithosphere and lithophophere andsthophophere osthophophere的地震速度和电阻结构。在尼加拉瓜裂缝区（NFZ）两侧，将在500 km x 500 km的截面（NFZ）的500公里乘500 km的截面上部署一组海底地震和电磁接收器。调查区域中的海底显示了过去板岩岩浆岩的广泛证据，包括海底的测深升高，比板时代年轻得多的海底，以及沉积物柱中著名的火山窗台。先前的小型磁电曲线清楚地拍摄了岩石圈 - 心圈边界处的部分熔体通道的存在。这项研究的观察结果将提供有关软圈流变学的主要控制以及通过解决以下关键问题产生板岩岩浆作用的机制的新见解：（1）高融化区域的空间和深度范围是多少？ NFZ的浴室对比是否反映了熔体含量和/或火山生产力的变化？ （2）先前推断的融化频道的地震标志是什么？地震观测是否与磁电纤维效果一致？如何彼此校准？鉴于熔体含量和速度之间的这种校准，全球软圈的机制削弱了什么暗示？ （3）地幔织物是否在岩石圈内部和下方都改变了NFZ？隐含的流场是否暗示了加拉帕戈斯梅子对过去海底蔓延的影响，并且/或盘下面的近期融化生产力？该奖项反映了NSF的法定任务，并通过使用该基金会的知识分子优点和更广泛的影响来评估NSF的法定任务，并被认为是宝贵的支持。