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

弱韧性软流圈的存在，即岩石圈下方地幔的上层，对地球上板块构造的运行是基本的，因为它使上覆的可移动的岩石圈板块能够运动。自从50多年前板块构造被发现以来，导致这种减弱的主要机制仍然没有解决。争论集中在两种相互竞争的机制上：地幔的部分熔融和水化。从理论上讲，地震速度和电阻率的地球物理成像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内在权衡和不确定性导致了显著的模糊性。该项目将对赤道太平洋东部科科斯板块下方地幔的地震和电性结构进行成像，那里确实存在软流层熔融。新的观测将为软流层熔融对地震和电学性质的影响提供一个独立的校准，并限制熔融的区域范围和几何形状。这反过来将使人们能够更好地了解该区域丰富和异常的火山活动的来源。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、萨尔瓦多和哥斯达黎加等邻国的合作伙伴共享数据和开展合作。20多名邮轮参与者将在两次研究探险期间接受地球物理数据获取方面的培训，其中包括学生、国际合作伙伴和社区参与者。该项目将利用被动震源地震和大地电磁成像来量化科科斯板块岩石圈和软流层的地震速度和电阻率结构。将在尼加拉瓜断裂带两侧500公里乘500公里的科科斯板块上部署一个共置的海底地震和电磁接收器阵列。调查区域的海底显示了大量过去板内岩浆作用的证据，包括海底水深上升，许多明显年轻于板块年龄的海山，以及沉积柱内突出的火山岩柱。以前的小尺度大地电磁剖面清楚地显示出岩石圈-软流圈边界存在部分熔融通道。这项研究的观察结果将通过解决以下关键问题，为软流圈流变学的主要控制因素和产生板内岩浆作用的机制提供新的见解：(1)高熔体区域的空间和深度范围是什么？NFZ上的水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的河道的地震特征是什么？地震观测与大地电磁结果是否一致？它们如何相互校准？考虑到熔体含量和速度之间的这种校准，这对全球软流圈的减弱机制意味着什么？(3)地幔组构是否在NFZ上发生变化，无论是在岩石圈内还是岩石圈之下？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/或板块下最近的熔体生产力产生了影响？这一裁决反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.