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

软流圈是位于岩石圈下方的地幔上层，它的存在是地球板块构造运动的基础，因为它使上覆的可移动岩石圈板块运动成为可能。自从50多年前发现板块构造以来，造成这种弱化的主要机制仍未得到解决。争论集中在两个相互竞争的机制上：地幔的部分熔融和水化作用。理论上，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，但从现场观测推断物理性质的固有权衡和不确定性导致了严重的模糊性。该项目将对赤道东太平洋部分科科斯板块下地幔的地震和电结构进行成像，在那里软流圈融化的存在是有信心的。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一个独立的校准，并限制融化的区域范围和几何形状。这反过来又能使人们更好地了解该地区丰富的异常火山活动的来源。该项目将支持四名研究生和几名本科生的研究活动，并与尼加拉瓜、萨尔瓦多和哥斯达黎加等邻国的合作伙伴进行数据共享和合作。超过20名邮轮参与者将在两次研究考察中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。本项目将利用无源地震和大地电磁成像技术，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。在尼加拉瓜断裂带（NFZ）两侧的Cocos板块上，一个位于同一位置的海底地震和电磁接收器阵列将被部署在500公里乘500公里的区域。调查区域的海底显示了大量过去板块内岩浆活动的证据，包括海底测深升高，许多明显比板块年龄年轻的海山，以及沉积物柱内突出的火山断层。先前的小尺度大地电磁剖面清楚地显示了岩石圈-软流圈边界处存在部分熔融通道。本研究的观测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活动的机制提供新的见解：(1)高熔体区的空间和深度范围是什么？NFZ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)先前推断的富融水通道的地震特征是什么？地震观测是否与大地电磁测量结果一致？它们如何相互校准？鉴于熔体含量和速度之间的这种校准，它对全球软流层的减弱机制意味着什么？(3)岩石圈内和岩石圈下的地幔结构是否在NFZ内发生变化？隐含的流场是否表明Galápagos羽流对过去海底扩张的影响，和/或最近板块下的融化能力？该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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.