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

这项研究有三个主要目标：（1）测试冰岛下面的脱水流变岩石圈;（2）确定冰岛热点-洋脊相互作用的性质;（3）调和该地区以前地震研究的模糊性。有人提出，在部分熔融开始时，地幔中的水被提取出来，这增加了残余物的粘度，以至于它可以抵抗对流。虽然这种脱水流变岩石圈（DRL）可以影响地幔对流的广泛过程，但DRL的存在还有待于通过观测进行检验。冰岛热点是一个出色的网站进行这项测试，因为这里的DRL的厚度可能是最不同的（薄）热岩石圈，有对流地幔流的来源，该地区有一个广泛的，高质量的地震数据集。对流源在冰岛下方被地震成像为低速体，这被广泛认为是上地幔中的热的、羽状的上涌。地幔柱假说预测这种上涌会沿中大西洋海岭（MAR）横向沿着供给快速地幔流，但目前这种现象的地震证据尚不明确。DRL的论据来自地幔柱-山脊相互作用的地球动力学研究，需要DRL来阻止地幔上升/熔融的速率，以便成功地预测冰岛的厚度？的外壳。然而，地震证据是矛盾的。最近的层析成像研究表明，冰岛南部有一层低速层，其宽度（600公里）和厚度（150公里）与具有日间行车线的模型的预测一致，并且明显大于没有日间行车线的模型。相互竞争的证据来自地震各向异性的剪切波分裂（SWS）和表面波测量，这些测量被解释为反映了由于浅地幔流而产生的结晶组构，以及假设的非对流DRL。来自SWS的各向异性也被解释为是由大规模的地幔流引起的，而不需要首先形成地幔柱。上述目标激发了新一代地震反演方案，该方案通过集成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-洋脊相互作用的模型将模拟一系列完整的动力学行为：从活跃（羽流）到弱（非羽流）上升;从低粘度地幔（无DRL）的浅侧流到预测的更粘地幔（有DRL）的深侧流。预测的晶体组构、温度和残留熔体模式将用于计算弹性张量的3D变化，我们将从中生成合成地震图。合成数据和真实的数据之间的不匹配将用于确定最可能和最不可能的基于地球动力学的解决方案。拒绝或确认地幔柱理论预测的地幔上涌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，脱水可以在很大范围的条件下控制地幔流变学，包括冰岛的强烈岩浆活动。一个负面的结果将导致的问题的一般重要性的DRL地幔对流，并可能需要一个戏剧性的重新思考如何融化产生和运输沿着大西洋中脊。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。该研究还将推进新一代基于地球动力学的地震求解方法。开发的数字代码将提供给社区，例如通过与地球动力学计算基础设施（CIG）合作。对更广泛的社区的好处包括培训一名研究生和一名博士后学者进行跨学科的物理学研究，并支持PI正在进行的外展活动。我们的两所大学都位于种族多元化的社区，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，并提供先进的技术培训和指导具有不同文化背景的理科学生。

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.