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.

自板块构造被发现 50 多年来，弱的、延性的软流圈（岩石圈下方地幔的上层）的存在对于地球上的板块构造的运行至关重要，因为它使得上覆的移动岩石圈板块能够运动。几年前，造成这种减弱的主要机制仍未解决，争论集中在两个相互竞争的机制上：理论上，地幔的部分熔化和水化。地震速度和电阻率可以区分这些相互竞争的机制，但与从现场观测推断物理特性相关的固有权衡和不确定性导致了该项目将对部分地幔下方的地震和电结构进行成像。赤道东太平洋的科科斯板块，那里存在软流圈融化，新的观测结果将为软流圈融化对地震和电特性的影响提供独立的校准，并限制区域性。该项目将支持四名研究生和几名本科生的研究活动，以及数据共享和合作。尼加拉瓜、萨尔瓦多和哥斯达黎加邻国的合作伙伴将在两次研究考察期间接受地球物理数据采集方面的培训，其中包括学生、国际合作伙伴和社区参与者。该项目将利用被动源地震。和大地电磁成像，以量化科科斯板块岩石圈和软流圈的地震速度和电阻率结构。将在科科斯板块 500 公里 x 500 公里的部分部署同位海底地震和电磁接收器阵列。尼加拉瓜断裂带（NFZ）两侧的海底显示出过去板内岩浆活动的丰富证据，包括海底抬升​​。测深、许多比板块年龄年轻得多的海山，以及沉积物柱内突出的火山岩台，先前的小规模大地电磁剖面观测清楚地显示出岩石圈-软流圈边界处存在部分熔融通道。通过解决以下关键问题，对软流圈流变学的主导控制和产生板内岩浆作用的机制提供了新的见解：（1）高熔点的空间和深度范围是多少整个 NFZ 的测深对比是否反映了熔体含量和/或火山生产力的变化？ (2) 先前推断的富含熔体通道的地震特征是什么？地震观测结果与大地电磁结果一致吗？考虑到熔体含量和速度之间的这种校准，这对全球软流圈的弱化机制意味着什么？（3）整个 NFZ 内部和下方的地幔结构是否发生变化？隐含的流场是否表明加拉帕戈斯羽流对过去的海底扩张和/或板块下方的近期熔体生产力产生了影响？该奖项反映了 NSF 的法定使命，并通过使用基金会的知识和更广泛的评估进行评估，认为值得支持。影响审查标准。