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

合作研究：冰岛热点-山脊相互作用地震观测的地球动力学解决方案

• 批准号: 0855767
• 负责人: Aibing Li
• 金额: $ 15.35万
• 依托单位: University of Houston
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2009
• 资助国家: 美国
• 起止时间: 2009-05-01 至 2012-04-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0855767&HistoricalAwards=false
• 关键词: Collaborative; Research; Geodynamic; Solutions; Seismic

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.

本研究有三个主要目标：(1)测试冰岛地下的脱水流变岩石圈；(2)明确冰岛热点-脊相互作用的性质；(3)修正了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的水增加了残留物的粘度，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的存在尚未得到观测结果的检验。冰岛热点是进行这项测试的一个杰出地点，因为这里的DRL厚度可能与（薄）热岩石圈的厚度最不同，这里有对流地幔流的来源，该地区有广泛的高质量地震数据集。冰岛地下的对流源被地震成像为一个低速体，人们普遍认为它是上地幔中热的、羽状的上升流。羽流假说预测这种上涌是为了供给沿大西洋中脊（MAR）横向的快速地幔流，但目前关于这种现象的地震证据尚不明确。DRL的论点来自地幔柱脊相互作用的地球动力学研究，这些研究要求DRL阻碍地幔上升/融化的速度，以便成功预测冰岛的厚度。地壳。然而，地震证据是矛盾的。为了支持DRL，最近的断层扫描研究揭示了一个延伸到冰岛南部的低速层，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。与之相竞争的证据来自剪切波分裂（SWS）和地震斜度的表面波测量，这些测量结果被解释为反映了由于浅层地幔流动造成的晶体结构，完全在假设的非对流DRL范围内。SWS的各向异性也被解释为是由大规模的地幔流动引起的，而不需要首先有地幔柱。上述目标激发了新一代地震反演方案，通过整合地幔对流和地震结构的地球动力学模型，直接测试物理过程。热点-脊相互作用模型将模拟一系列完整的动力学行为：从强劲的（羽流）到弱的（非羽流）上升流；从低黏度地幔（无DRL）的浅层横向流动到高黏度地幔（有DRL）的深部横向流动。预测的晶体结构模式、温度和保留的熔体将用于计算弹性张量的三维变化，从中我们将生成合成地震图。合成数据和真实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。拒绝或确认地幔上涌和沿轴流动的能力，由羽流理论预测将具有广泛的重要性，以了解热点。对DRL的阳性测试将表明，脱水可以在广泛的条件下主导地幔流变，包括冰岛的剧烈岩浆活动。如果结果是否定的，人们就会质疑DRL对地幔对流的总体重要性，并可能需要重新思考熔体是如何产生的，以及如何沿着大西洋中脊输送的。更广泛的影响：该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。该研究还将推动新一代基于地球动力学的地震解决方法。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）的合作。对更广泛的社区的好处包括在跨学科地球物理学研究中培养研究生和博士后学者，并支持pi正在进行的外展活动。我们两所大学都位于多民族社区，都是各自地区社会、文化、科学和技术资源的重要贡献者。该研究将丰富我校的教学和科研项目，为不同文化背景的理科生提供先进的技术培训和指导。