Collaborative Research: Structure and dynamics of the Alaska mantle wedge

合作研究：阿拉斯加地幔楔的结构和动力学

基本信息

批准号：
1829447
负责人：
Carl Tape
金额：
$ 19.18万
依托单位：
University of Alaska Fairbanks Campus
依托单位国家：
美国
项目类别：
Standard Grant
财政年份：
2018
资助国家：
美国
起止时间：
2018-09-01 至 2022-08-31
项目状态：
已结题

来源：
https://www.nsf.gov/awardsearch/showAward?AWD_ID=1829447&HistoricalAwards=false
关键词：
Collaborative Research Structure dynamics Alaska

项目摘要

Subduction zones are the most active geologic features on the planet - oceanic plates descend into the earth's interior at geological speeds and carry with them fluids and materials from the sea floor. As this material heats, fluids such as water are released, lubricating faults at shallow depths and fluxing the warm Earth's deeper mantle to make magmas. As a result, subduction zones host the planet's largest earthquakes and most of the violent volcanic eruptions. In North America, the Alaska-Aleutian subduction system is by far the largest such system - Alaska has hosted the largest earthquake in North America, and the planet's largest 20th century eruption. The eastern end of this subduction zone lies directly beneath most of the population of the state so creates a major natural hazard. At the same time this eastern end is geologically complex, making the pathways of fluid difficult to understand and complicating the underlying theories of volcanism. This project aims to significantly advance our understanding of the nature of the mantle in this complex transition, using a revolutionary new data set. Over the last several years a vast amount of high-quality earthquake signals has been collected from new seismometers in Alaska - the entire state has been covered since 2015-16 by the EarthScope Transportable Array that places state-of-the-art instrumentation every 85 km, accompanied by a number of smaller, dense deployments over areas of interest. All these projects ensonify with earthquake signals the interior of the planet in this highly active and complex region. This specific project aims to capitalize on these data to address and test several hypotheses that will help better understand the ways in which large volcanoes form and more generally the variations in temperature of the Earth's mantle. The results will provide a framework for interpreting rocks that come from similar environments in the geologic record. This project focuses on three generic hypotheses regarding geodynamic process in subduction zones: 1) Variability in fluid release from subducting plates correlates with variability in the degree of melting in the overlying mantle wedge - tested through a variety of seismic proxies. 2) Rock fabric as revealed by seismic anisotropy is controlled by distance from the edge of the slab as expected if three-dimensional flow controls it. 3) The depth at which the mantle wedge transitions from cold forearc to hot subarc is globally constant. Measurements of seismic attenuation at mantle depths provides a proxy for temperature in all of these regions, and local-earthquake shear-wave splitting will complement new teleseismic (SKS) splitting measurements to infer anisotropy. Parallel observations of seismicity and high-frequency phases that interact with the slab surface then allow inferences about the mantle wedge to be compared with slab dehydration. High-frequency wavefield simulations of split shear waves will assess the maximum depth of a supra-slab anisotropic slow layer, a probable signature of slab-mantle coupling depth. At the same time, petrologically-driven models provide a framework for making predictions that test each hypothesis. These hypotheses will be tested via comparison of three distinct corridors within Alaska for which EarthScope and related projects provide unusually good sampling: (a) the Cook Inlet corridor where normal Pacific lithosphere subducts and the arc is robust; (b) the nearly amagmatic Denali corridor where the Yakutat oceanic plateau subducts and generates intermediate-depth earthquakes; and (c) the Wrangell Volcanic Field corridor where slab seismicity is nearly absent but there is very high volume volcanism. These comparisons take advantage of Alaska Transportable Array combined with several dense portable broadband experiments (BEAAR, SALMON, MOOS, WVLF), previous projects conducted by the PIs and which sample each of these corridors. This project addresses EarthScope science objectives and emphasizes interdisciplinary work at the interface between petrology, seismology, and geodynamics. It leverages education and outreach opportunities through the EarthScope National Office, notably those available through the EarthScope website and social media. All project participants - including graduate students supported at two institutions- will work with the EarthScope National Office to maximize scientific outreach of the project. The project will generate improved predictions of amplitudes of seismic waves in south-central Alaska, including within the Anchorage metropolitan region; therefore the project can contribute toward seismic hazard assessments and ground motion prediction.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.

俯冲带是地球上最活跃的地质特征——海洋板块以地质速度下降到地球内部，并携带来自海底的流体和物质。当这种物质受热时，水等液体就会释放出来，润滑浅层断层，并推动温暖的地球深层地幔形成岩浆。因此，俯冲带发生了地球上最大的地震和大多数猛烈的火山喷发。在北美，阿拉斯加-阿留申俯冲系统是迄今为止最大的此类系统 - 阿拉斯加曾发生过北美最大的地震，以及地球上 20 世纪最严重的火山喷发。该俯冲带的东端位于该州大部分人口的正下方，因此造成了重大的自然灾害。与此同时，这个东端地质复杂，使得流体的路径难以理解，并使火山作用的基本理论变得复杂。该项目旨在利用革命性的新数据集，显着增进我们对这一复杂转变中地幔性质的理解。在过去的几年里，阿拉斯加的新型地震仪收集了大量高质量的地震信号 - 自 2015-16 年以来，EarthScope 可移动阵列已覆盖整个州，该阵列每 85 次放置一次最先进的仪器公里，同时在感兴趣的地区进行一些规模较小、密集的部署。所有这些项目都与地球内部这个高度活跃和复杂的地区的地震信号相呼应。这个具体项目旨在利用这些数据来解决和测试几个假设，这些假设将有助于更好地了解大型火山的形成方式以及更普遍的地幔温度变化。研究结果将为解释地质记录中类似环境的岩石提供一个框架。该项目重点关注有关俯冲带地球动力学过程的三个一般假设：1）俯冲板块流体释放的变化与上覆地幔楔的熔化程度的变化相关——通过各种地震代理进行了测试。 2）地震各向异性揭示的岩石结构由距板块边缘的距离控制，正如预期的那样，如果三维流动控制它的话。 3）地幔楔从冷弧前过渡到热弧下的深度全局恒定。地幔深度地震衰减的测量为所有这些区域的温度提供了一个替代指标，而局地地震剪切波分裂将补充新的远震（SKS）分裂测量以推断各向异性。通过对地震活动和与板块表面相互作用的高频相位进行并行观测，可以将地幔楔与板块脱水进行比较。分裂剪切波的高频波场模拟将评估板片上各向异性慢层的最大深度，这是板片-地幔耦合深度的可能特征。与此同时，岩石学驱动的模型提供了一个框架来进行预测，以检验每个假设。这些假设将通过比较阿拉斯加境内的三个不同走廊进行检验，EarthScope 和相关项目为这三个走廊提供了异常良好的采样：(a) 库克湾走廊，这里正常的太平洋岩石圈俯冲且弧形强劲； (b) 几乎无岩浆的德纳利走廊，雅库塔特海洋高原在此俯冲并产生中深度地震； (c) 兰格尔火山田走廊，这里几乎不存在板片地震活动，但火山活动量非常大。这些比较利用了阿拉斯加可移动阵列以及多个密集便携式宽带实验（BEAAR、SALMON、MOOS、WVLF）以及 PI 之前进行的项目并对每个走廊进行了采样。该项目旨在解决 EarthScope 的科学目标，并强调岩石学、地震学和地球动力学之间的跨学科工作。它通过 EarthScope 国家办公室利用教育和外展机会，特别是通过 EarthScope 网站和社交媒体提供的机会。所有项目参与者——包括两个机构支持的研究生——将与 EarthScope 国家办公室合作，最大限度地扩大该项目的科学范围。该项目将对阿拉斯加中南部（包括安克雷奇大都市区）的地震波振幅进行改进的预测；因此，该项目可以为地震灾害评估和地面运动预测做出贡献。该奖项反映了 NSF 的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。