Collaborative Research: Geodynamic Solutions for Seismic Observations of Iceland Hotspot-Ridge Interaction

这项研究有三个主要目标：(1)测试冰岛之下的脱水流变学岩石圈；(2)确定冰岛热点-海脊相互作用的性质；(3)调和该地区以往地震研究的模糊性。有人提出，在部分熔融开始时从地幔中提取水会大大增加残留物的粘度，使其能够抵抗对流。虽然这种脱水的流变岩石圈(DRL)可以影响地幔对流的各种过程，但DRL的存在还有待于观测结果的验证。冰岛热点是进行这项测试的杰出地点，因为在这里，DRL的厚度可能与(薄的)热岩石圈的厚度最不同，那里有对流地幔流动的来源，并且该地区拥有广泛的、高质量的地震数据集。对流的来源在地震上被成像为冰岛下方的低速体，人们普遍认为它是上地幔的热羽状上升流。羽流假说预测，这种上升流将沿着中大西洋海脊(MAR)横向输送快速的地幔流动，但目前对这种现象的地震证据并不明确。支持DRL的论据来自对地幔柱-脊相互作用的地球动力学研究，这些地球动力学研究需要DRL来阻止地幔的隆升/熔融速率，从而成功地预测冰岛-S地壳的厚度。然而，地震证据是相互矛盾的。为了支持DRL，最近的断层扫描研究显示，冰岛南部有一个低速层，其宽度(600公里)和厚度(150公里)与有DRL的模型的预测一致，并且明显大于没有DRL的模型。相互竞争的证据来自剪切波分裂(SWS)和地震各向异性的面波测量，这些测量被解释为反映了由于浅地幔流动而产生的晶体组构，很好地在假设的非对流DRL内。来自SWS的各向异性也被解释为是由大规模的地幔流动引起的，而不需要首先出现地幔热柱。上述目标推动了新一代地震反演方案的提出，该方案通过整合地幔对流和地震结构的地球动力学模型来直接测试物理过程。热点-脊线相互作用模式将模拟一系列完整的动力学行为：从剧烈(羽状物)到弱(非羽状物)上升流；从低粘性地幔浅侧向流动(无DRL)到较粘性地幔深侧向流动(有DRL)。然后将使用晶体组构、温度和保留的熔体的预测图案来计算弹性张量的3D变化，由此我们将生成合成地震记录。合成数据和真实数据之间的不匹配将被用来确定最有可能和最不可能的基于地球动力学的解决方案。拒绝或证实地幔柱理论所预测的地幔上升流和沿轴流动的能力，将对理解热点具有广泛的重要性。DRL的阳性测试将表明，在包括冰岛强烈岩浆活动在内的广泛条件下，脱水可以主导地幔流变学。否定的结果将导致人们对DRL对地幔对流的普遍重要性产生疑问，并可能需要对熔融是如何产生和沿中大西洋海脊输送的进行戏剧性的重新思考。更广泛的影响：这个项目具有很强的推广潜力，因为热点起源的主题对理解绝对板块运动、地幔对流和地表火山活动具有深远的重要性。这项研究还将提出新一代基于地球动力学的地震求解方法。开发的数值代码将提供给社区，例如通过与地球动力学计算基础设施(CIG)的合作。对广大社区的好处包括对一名研究生和一名博士后学者进行跨学科地球物理研究方面的培训，并支持正在进行的宣传活动。我们两所大学都位于不同种族的社区，是各自地区社会、文化、科学和技术资源的重要贡献者。拟议的研究将丰富我们的教学和研究计划，并为不同文化背景的理科学生提供先进的技术培训和指导。

This study has three major goals: (1) Test for a dehydrated rheological litho- sphere beneath Iceland; (2) define the nature of Icelandic hotspot-ridge interaction; and (3) reconcile the ambiguities of previous seismic studies in the area. It has been proposed that the extraction of water from the mantle at the onset of partial melting increases the viscosity of the residue so much that it can resist convection. While such a dehydrated rheological lithosphere (DRL) can impact a wide range of processes in mantle convection, the existence of a DRL has yet to be tested against ob- servations. The Iceland hotspot is an outstanding site to perform this test because here the thickness of the DRL is likely to be most distinct from that of the (thin) thermal lithosphere, there is a source of convective mantle flow, and the area has an extensive, high-quality seismic data set. The source of convective flow is imaged seismically beneath Iceland as a low-velocity body, which is widely be- lieved to be a hot, plume-like upwelling in the upper mantle. The plume hypothesis predicts this up- welling to feed rapid mantle flow laterally along the Mid-Atlantic Ridge (MAR) but current seismic evidence for such a phenomenon is ambiguous. Arguments for a DRL come from geodynamic stud- ies of mantle plume-ridge interaction that require a DRL to hinder the rate of mantle upwell- ing/melting so as to successfully predict the thickness of Iceland?s crust. Seismic evidence, however, is contradictory. In support of a DRL, recent tomography studies reveal a layer of low velocities that extends south of Iceland with a width (600 km) and thickness (150 km) that are consistent with predictions of models with a DRL and distinguishably larger than those without one. The competing evidence comes from shear-wave splitting (SWS) and surface-wave measurements of seismic anisot- ropy that were interpreted to reflect crystallographic fabric due to shallow mantle flow, well within the hypothesized, non-convecting DRL. Anisotropy from SWS was also interpreted to be caused by large-scale mantle flow without the need of a mantle plume in the first place. The above goals motivate a new generation of seismic inversion scheme that directly tests the physical processes by integrating geodynamic models of mantle convection and seismic structure. Models of hotspot-ridge interaction will simulate a complete range of dynamic behaviors: from vig- orous (plume) to weak (non-plume) upwellings; and from cases with shallow lateral flow of low- viscosity mantle (no DRL) to deep lateral flow predicted for more viscous mantle (with a DRL). Pre- dicted patterns of crystallographic fabric, temperatures, and retained melt will then be used to com- pute 3D variations in elasticity tensors, from which we will generate synthetic seismograms. Misfits between the synthetic and real data will be used to identify the most and least probable geodynamics- based solutions. The ability to reject or confirm the mantle upwelling and along-axis flow that are predicted by plume theory will have broad importance to understanding hotspots. A positive test for a DRL will show that dehydration can dominate mantle rheology over a wide range of conditions, in- cluding the vigorous magmatism at Iceland. A negative result will lead to questions of the general importance of a DRL to mantle convection and could require a dramatic re-thinking of how melt is generated and transported along the Mid-Atlantic Ridge. Broader Impacts: This project has strong outreach potential because the topic of the origin of hotspots has far reaching importance to understanding absolute plate motions, mantle convection, and surface volcanism. The study will also advance a new generation of geodynamic-based seismic solution methods. The numerical codes developed will be made available to the community, such as through collaboration with the Computational Infrastructure for Geodynamics (CIG). Benefits to the broader community include the training of a graduate student and a post-doctoral scholar in interdis- ciplinary geophysics research, and supporting ongoing outreach activities of the PIs. Both of our uni- versities are located in ethnically diverse communities and are important contributors to the social, cultural, scientific, and technical resources of their respective regions. The proposed research will enrich our teaching and research programs, and provide advanced technological training and mentor- ing of science students with diverse cultural backgrounds.