Collaborative Research: CSEDI--The Dynamics of Plume-Trench Interaction: Samoa-Tonga

尽管过去几年我们对地球深部地幔的动力学和演化的了解取得了显著进展，但其中一个主要方面的争议越来越大。热点火山作用是不是由深部地幔热柱提供的？三十年来，这一直是标准模型的核心原则，但现在与“自上而下”的模型形成对比，后者通过地表板块的断裂构造，严格从上地幔获得热点火山活动。虽然夏威夷的热点链是羽流模型的“原型”，但萨摩亚的热点链经常被引用为自上而下模型的证据。萨摩亚最近的研究表明，尽管断裂构造在火山活动中发挥了重要作用，但它只是一个由羽流驱动的热点的“终结”。该项目使用萨摩亚热点和邻近的汤加俯冲带的独特并置来说明这种相互作用的动态，并对更广泛的关于羽流的辩论作出重要的理解。随着汤加板块迅速向东回滚，它迫使板块和太平洋板块之间的铰链断层向东传播。这个项目的中心论点是，这种倒退驱动了地幔从板块下方向北“逃逸”，迫使萨摩亚热柱逐渐向北移动，留下了一系列梯队状的火山构造。与此同时，汤加-太平洋枢纽的传播，以及相关的地幔返回到弧后地区，催生了与萨摩亚热点线形相毗邻的重新焕发活力的火山活动。这项研究由一个多学科、多机构的小组组成，成员包括动力学家(M.Billen，UC Davis和C.Kincaid，URI)、地震学家(M.Behn和J.Collins，WHOI)、地球化学家和矿物物理学家(S.Hart和G.Hirth，WHOI)。采用三维计算模型和实验“水槽”模型研究了板坯回弹与(假定)羽流管道的动力相互作用。沿萨摩亚链部署了四个地震台，并将与现有的常设台站一起用于进行剪切波分裂测量。这些区域地震各向异性观测(以及斐济和刘弧后公布的值)在数值和实验流动模拟的背景下进行了解释，并约束了汤加板块周围的地幔流动模式。这些台站的PS和Sp接收功能提供了岩石圈厚度的估计，以及410-660公里过渡带的厚度。正在描绘(假定的)烟柱-板块相互作用的性质，并结合地球化学和构造特征测绘，可以区分板块-断裂模型和烟柱驱动的热点模型。这些研究为我们的萨摩亚-汤加互动概念中明确提出的一系列具体问题提供了明确和直接的检验。这三个机构的研究生都参与并积极参与了该项目的所有方面。所有与会者参加的会议定于2007年6月8日在伍兹霍尔海洋研究所举行。来年，这些学生还将被招待对其他院校进行短期访问。频繁的电话会议和视频会议被用来进一步提高学生的参与度。学生们被鼓励参加2008年的苹果酒暑期项目，作为一种为他们的学习提供背景和广度的方式。晶格优先取向的运动学模型正被集成到CITCOM 3-D对流程序中，这些模型将通过地球动力学计算基础设施提供给整个社区。在更广泛的外联方面，计划于2007年7月初首次前往萨摩亚为地震台站服务；计划访问萨瓦伊和塔乌的高中，一天的科学课程将侧重于火山、羽流和板块构造。鉴于当地学校周围精致的火山地形，将为学生和教师进行非正式的半天实地考察。

Despite remarkable advances over the past few years in our understanding of the dynamics and evolution of Earth's deep mantle, one major aspect has become increasingly contentious. Is hotspot volcanism fed by deep mantle plumes or not? This has been a central tenet of the Standard Model for three decades, but is now contrasted with a "top down" model that derives hot spot volcanism strictly from the upper mantle, via fracture tectonics of the surface plates. While the Hawaiian hotspot chain is the "archetype" for the Plume model, the Samoan chain is cited frequently as evidence for the Top Down model. Recent studies in Samoa show that, while fracture tectonics plays an important role in the volcanism, it is only the "end-game" of a plume-driven hotspot. This project uses the unique juxtaposition of the Samoa hotspot and adjacent Tonga subduction zone to illuminate the dynamics of this interaction, and to contribute critical understanding to the debate about plumes more generally. As the Tonga slab rolls back rapidly to the east, it forces an eastward propagation of the hinge fault between the slab and the Pacific plate. The central thesis of this project is that this roll-back drives a northward flow of mantle "escaping" from beneath the slab, forcing the Samoa plume to step progressively northward, leaving behind a series of en echelon volcanic lineaments. Concomitantly, the propagation of the Tonga-Pacific hinge, and associated return flow of mantle into the back-arc region, spawns rejuvenated volcanism that is incidentally contiguous with the Samoan hotspot lineament. This study comprises a multi-disciplinary multi-institutional group of dynamicists (M. Billen, UC Davis, and C. Kincaid, URI), seismologists (M. Behn and J. Collins, WHOI), a geochemist and a mineral physicist (S. Hart and G. Hirth, WHOI). 3-D computational modeling and experimental "tank" modeling is being used to study the dynamic interaction of slab rollback and the (putative) plume conduit. Four seismic stations have been deployed along the Samoa chain and, in conjunction with existing permanent stations, will be used to make shear-wave splitting measurements. These regional observations of seismic anisotropy (along with published values from Fiji and the Lau back-arc) are interpreted in the context of the numerical and experimental flow modeling, and constrain the pattern of mantle flow around the Tonga slab. Ps and Sp receiver functions from these same stations provide estimates of lithospheric thickness, and thickness of the 410-660 km transition zone. The nature of (putative) plume-plate interaction is being delineated and, combined with the geochemistry and mapping of tectonic features, allows distinctions between plate-fracture models and plume-driven hotspot models. These studies provide clear and direct tests of the specific set of questions explicit in our concept for the Samoa-Tonga interaction.Graduate students from each of the three institutions are involved and are active participants in all aspects of the project. A meeting involving all participants has been scheduled for June 8, 2007 at the Woods Hole Oceanographic Institution. The students will also be hosted for short visits to the other institutions in the coming year. Frequent conference calls and video-conferences are used to further enhance the student participation. The students are encouraged to attend the 2008 CIDER Summer Program, as a way of providing context and breadth to their studies. Kinematic models for lattice-preferred orientation are being integrated into the CITCOM 3-Dconvection code, and these will be made available to the entire community through the Computational Infrastructure for Geodynamics. On a broader outreach front, the first trip to Samoa to service the seismic stations has been scheduled for early July 2007; visits to high schools on Savai`i and Ta`u are planned, with a day of science classes that will focus on volcanoes, plumes and plate tectonics. Given the exquisite volcanic terrain surrounding the local schools, informal half-day field trips will be run for both students and teachers.