Cite this article as Long, M. D., M. H. Benoit, R. L. Evans, J. C. Aragon, and J. Elsenbeck (2020). The MAGIC Experiment: A Combined Seismic and Magnetotelluric Deployment to Investigate the Structure, Dynamics, and Evolution of the Central Appalachians, Seismol. Res. Lett. XX, 1–16, doi: 10.1785/ 0220200150. The eastern margin of North America has undergone multiple episodes of orogenesis and rifting, yielding the surface geology and topography visible today. It is poorly known how the crust and mantle lithosphere have responded to these tectonic forces, and how geologic units preserved at the surface related to deeper structures. The eastern North American margin has undergone significant postrift evolution since the breakup of Pangea, as evidenced by the presence of young (Eocene) volcanic rocks in western Virginia and eastern West Virginia and by the apparently recent rejuvenation of Appalachian topography. The drivers of this postrift evolution, and the precise mechanisms through which relatively recent processes have modified the structure of the margin, remain poorly understood. The Mid-Atlantic Geophysical Integrative Collaboration (MAGIC) experiment, part of the EarthScope USArray Flexible Array, consisted of collocated, dense, linear arrays of broadband seismic and magnetotelluric (MT) stations (25–28 instruments of each type) across the central Appalachian Mountains, through the U.S. states of Virginia, West Virginia, and Ohio. The goals of the MAGIC deployment were to characterize the seismic and electrical conductivity structure of the crust and upper mantle beneath the central Appalachians using natural-source seismic and MT imaging methods. The MAGIC stations operated between 2013 and 2016, and the data are publicly available via the Incorporated Research Institutions for Seismology Data Management Center. Introduction The eastern North American margin (ENAM), today a passive continental margin, has been modified by multiple episodes of orogenesis and rifting through two complete cycles of supercontinent assembly and breakup over the past billion years of Earth history. Major tectonic events that have affected the margin include the Grenville orogeny, associated with the formation of the Rodinia supercontinent (e.g., Rivers, 1997; McLelland et al., 2010); the subsequent breakup of Rodinia (e.g., Li et al., 2008); the various phases of the Appalachian orogeny (e.g., Hatcher, 2010; Hibbard et al., 2010), which were associated with the accretion of multiple terranes onto the edge of Laurentia and which culminated in the formation of Pangea; and the breakup of Pangea during the Mesozoic (e.g., Frizon de Lamotte et al., 2015). In addition to this established tectonic history, there are hints that the passive margin has undergone substantial postrift evolution since supercontinental breakup. For example, there is a temporally extensive history of postrift 1. Department of Geology and Geophysics, Yale University, New Haven, Connecticut, U.S.A.; 2. National Science Foundation, Alexandria, Virginia, U.S.A.; 3. Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, U.S.A.; 4. Now at Earthquake Science Center, U.S. Geological Survey, Menlo Park, California, U.S.A.; 5. Now at Lincoln Laboratories, Massachusetts Institute of Technology, Lexington, Massachusetts, U.S.A. *Corresponding author: maureen.long@yale.edu © Seismological Society of America Volume XX • Number XX • – 2020 • www.srl-online.org Seismological Research Letters 1 Data Mine Downloaded from https://pubs.geoscienceworld.org/ssa/srl/article-pdf/doi/10.1785/0220200150/5084304/srl-2020150.1.pdf by Yale University user on 01 July 2020 volcanism along the margin (e.g., Mazza et al., 2017), including a localized occurrence of magmatic activity during the Eocene in what is now western Virginia and eastern West Virginia (e.g., Mazza et al., 2014). Furthermore, there is evidence from geomorphologic investigations that Appalachian topography has undergone relatively recent rejuvenation (e.g., Miller et al., 2013). This may be due to flow in the deep mantle (e.g., Moucha et al., 2008; Spasojevic et al., 2008) or to temporal changes in the density structure of the crust (e.g., Fischer, 2002); alternatively, landscape transience may be due solely to surface processes. Finally, like many passive continental margins, ENAM plays host to significant seismicity, as evidenced by the 2011 magnitude 5.8 Mineral, Virginia, earthquake (e.g., Wolin et al., 2012; McNamara et al., 2014), which occurred in the central Virginia seismic zone (CVSZ). With the advent of new geophysical data sets in eastern North America, such as the EarthScope USArray and the GeoPRISMS ENAM Community Seismic Experiment (Lynner et al., 2020), we are in a position to gain new insights into the structure and dynamics of this passive margin. In this article, we describe the Mid-Atlantic Geophysical Integrative Collaboration (MAGIC) experiment, a combined seismic and magnetotelluric (MT) deployment across the central Appalachian Mountains that was part of the USArray Flexible Array. This project is aimed at studying the detailed structure of the crust, lithospheric mantle, asthenosphere, and mantle transition zone across this portion of ENAM. Our choice of target region in the central Appalachians (Fig. 1) affords us the opportunity to probe structure across Virginia, West Virginia, and Ohio, from the Coastal Plain in the east, across the present-day Appalachian Mountains, and extending west across the Grenville Front. The Grenville Front has typically been thought to represent the westward extent of deformation during Grenville orogenesis (e.g., Whitmeyer and Karlstrom, 2007), although recent work has proposed that this feature is instead associated with the Midcontinent Rift (e.g., Stein et al., 2018; Elling et al., 2020). We are particularly interested in understanding how the various episodes of orogenesis and rifting, associated with two complete Wilson cycles of supercontinent assembly and breakup, have affected the structure of the crust and lithospheric mantle, and in characterizing to what extent deep structures correspond to geologic units at the surface. An example of a specific target is the nature of the putative Grenville Front at depth, as constraints on its geometry and extent in the midcrust may shed light on its origin. Figure 1. Map of Mid-Atlantic Geophysical Integrative Collaboration (MAGIC) seismic and magnetotelluric (MT) station locations. Inset map shows geographic location. Locations for seismic stations are shown with red triangles, andMT stations are shown with yellow squares. Background colors indicate topography (m), as shown by color bar at right. Thin black lines indicate state boundaries. Dashed lines show major tectonic features, including the putative Grenville Front (GF; black), as in Stein et al. (2018), boundaries of the Rome trough (RT; magenta), and the Appalachian Front (AF; orange) from Whitmeyer and Karlstrom (2007). Black star indicates the epicenter of the 2011 Mineral, Virginia, earthquake. White star indicates the location of Mole Hill, a topographically prominent volcanic neck of the Eocene (Mazza et al., 2014). The color version of this figure is available only in the electronic edition. 2 Seismological Research Letters www.srl-online.org • Volume XX • Number XX • – 2020 Downloaded from https://pubs.geoscienceworld.org/ssa/srl/article-pdf/doi/10.1785/0220200150/5084304/srl-2020150.1.pdf by Yale University user on 01 July 2020 Furthermore, we wish to understand the processes associated with postrift evolution of the margin, including those that caused the Eocene magmatic event and those that may be responsible for the recent rejuvenation of Appalachian topography. Finally, we are interested in understanding the potential links between the structure of the crust and mantle lithosphere and the seismically active CVSZ. Motivated by these scientific questions, the MAGIC field experiment was carried out across the central Appalachians between 2013 and 2016. This deployment, funded by the EarthScope and GeoPRISMS programs of the National Science Foundation (NSF), was a collaborative effort among seismology principal investigators (PIs) Maureen Long (Yale University) and Margaret Benoit (then at the College of New Jersey) and MTs PI Rob Evans (Woods Hole Oceanographic Institution [WHOI]). We deployed a linear array of (mostly) collocated broadband seismometers and long-period MT instruments across the U.S. states of Virginia, West Virginia, and Ohio (Fig. 1). Seismic data were collected starting in October 2013 and ending in October 2016. We deployed 28 broadband seismic sensors, with continuous data recording, and relied on natural (passive) earthquake sources. MT data were collected between October 2015 and May 2016, with each of the 25 MT sites being occupied for ∼3 weeks; as with the seismic data, the MT experiment relied on natural sources. The MAGIC array crossed a number of geologic terranes and physiographic provinces, extending past the Grenville Front at its western end, and also intersected the CVSZ near its eastern end, passing close to the epicenter of the 2011 Mineral, Virginia, earthquake (Fig. 1). Our array also sampled the region affected by anomalous volcanic activity during the Eocene, with several stations deployed within a few kilometers of Eocene volcanic outcrops. Instrument Deployment and Details A map of all seismic and MT stations operated as part of the MAGIC experiment is shown in Figure 1. The MAGIC seismic array extended from Charles City, Virginia, to Paulding, Ohio, for an array aperture of ∼770 km. A total of 29 sites were eventually occupied, with a maximum of 28 stations deployed at any given time (one station [TRTF] was relocated to a nearby site [MOLE] in summer 2015). The nominal station spacing was therefore just under 30 km, although we designed the array such that th

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MAGIC 实验：结合地震和大地电磁部署来研究中央阿巴拉契亚山脉的结构、动力学和演化

将本文引用为 Long、MD、MH Benoit、RL Evans、JC Aragon 和 J. Elsenbeck（2020 年）。MAGIC 实验：结合地震和大地电磁部署，研究中央阿巴拉契亚山脉地震的结构、动力学和演化。水库 莱特。XX, 1–16, doi: 10.1785/ 0220200150. 北美东部边缘经历了多次造山作用和裂谷作用，形成了今天可见的地表地质和地形。地壳和地幔岩石圈如何对这些构造力作出反应，以及在地表保存的地质单元如何与更深的结构相关，我们知之甚少。自泛大陆分裂以来，北美东部边缘经历了显着的后裂谷演化，维吉尼亚西部和西维吉尼亚东部的年轻（始新世）火山岩的存在以及阿巴拉契亚地形最近明显恢复活力都证明了这一点。这种后裂谷演化的驱动因素，以及相对较新的过程改变边缘结构的精确机制，仍然知之甚少。中大西洋地球物理综合协作 (MAGIC) 实验是 EarthScope USArray 灵活阵列的一部分，由横跨阿巴拉契亚中部的宽带地震和大地电磁 (MT) 站（每种类型 25-28 台仪器）的并列密集线性阵列组成山脉，穿过美国弗吉尼亚州、西弗吉尼亚州和俄亥俄州。MAGIC 部署的目标是使用天然震源地震和 MT 成像方法表征阿巴拉契亚山脉中部下方地壳和上地幔的地震和电导率结构。MAGIC 台站于 2013 年至 2016 年间运行，数据可通过联合研究机构地震数据管理中心公开获取。介绍 北美东部边缘 (ENAM)，今天是一个被动的大陆边缘，在过去十亿年的地球历史中，已经通过多次造山和裂谷经历了超大陆组装和分裂的两个完整周期而改变。影响边缘的主要构造事件包括与罗迪尼亚超大陆的形成相关的格伦维尔造山运动（例如，Rivers，1997；McLelland 等，2010）；Rodinia 随后的解体（例如，Li et al., 2008）；阿巴拉契亚造山运动的各个阶段（例如，Hatcher，2010；Hibbard 等，2010），这与多个地体在 Laurentia 边缘的增生有关，并最终形成 Pangea；以及中生代期间盘古大陆的分裂（例如，Frizon de Lamotte 等，2015）。除了这一既定的构造历史之外，还有迹象表明，自超大陆破裂以来，被动边缘经历了大量的后裂谷演化。例如，后裂谷的历史时间很长。 1. 美国康涅狄格州纽黑文耶鲁大学地质与地球物理系；2. 美国弗吉尼亚州亚历山大市国家科学基金会；3. 伍兹霍尔海洋研究所地质与地球物理系，伍兹霍尔，美国马萨诸塞州；4. 现在在美国加利福尼亚州门洛帕克美国地质调查局地震科学中心；5. 现在在美国马萨诸塞州列克星敦麻省理工学院林肯实验室 *通讯作者：maureen.long@yale.edu © 美国地震学会第 XX 卷 • 编号 XX • – 2020 • www.srl-online.org Seismological Research Letters 1 Data Mine 耶鲁大学用户于 2020 年 7 月 1 日沿边缘火山运动从 https://pubs.geoscienceworld.org/ssa/srl/article-pdf/doi/10.1785/0220200150/5084304/srl-2020150.1.pdf 下载（例如，Mazza 等人，2017），包括始新世期间在现在的西弗吉尼亚州和西弗吉尼亚州东部发生的局部岩浆活动（例如，Mazza 等人，2014 年）。此外，来自地貌调查的证据表明，阿巴拉契亚的地形经历了相对较新的复兴（例如，Miller 等人，2013 年）。这可能是由于深部地幔中的流动（例如，Moucha 等，2008；Spasojevic 等，2008）或地壳密度结构的时间变化（例如，Fischer，2002）；或者，景观瞬变可能完全是由于表面过程。最后，像许多被动大陆边缘一样，ENAM 承载着重大的地震活动，正如 2011 年发生在中部的 5.8 级矿物，弗吉尼亚州地震（例如，Wolin 等人，2012 年；McNamara 等人，2014 年）所证明的那样弗吉尼亚地震带 (CVSZ)。随着北美东部新地球物理数据集的出现，例如 EarthScope USArray 和 GeoPRISMS ENAM Community Seismic Experiment (Lynner et al., 2020)，我们能够对这种被动边缘的结构和动态获得新的见解。在本文中，我们描述了中大西洋地球物理综合协作 (MAGIC) 实验，这是一项横跨阿巴拉契亚山脉中部的地震和大地电磁 (MT) 组合部署，是 USArray 柔性阵列的一部分。该项目旨在研究跨越这部分 ENAM 的地壳、岩石圈地幔、软流圈和地幔过渡带的详细结构。我们在阿巴拉契亚山脉中部选择的目标区域（图 1）使我们有机会从东部的沿海平原、今天的阿巴拉契亚山脉向西延伸穿过弗吉尼亚州、西弗吉尼亚州和俄亥俄州格伦维尔阵线。Grenville Front 通常被认为代表了 Grenville 造山过程中向西变形的范围（例如，Whitmeyer 和 Karlstrom，2007），尽管最近的工作提出该特征与中大陆裂谷相关（例如，Stein 等人， 2018 年；Elling 等人，2020 年）。我们特别有兴趣了解与超大陆组装和分裂的两个完整威尔逊旋回相关的造山和裂谷的各种事件如何影响地壳和岩石圈地幔的结构，以及表征深部结构在多大程度上对应于地质单元在表面。一个特定目标的例子是假定的深部格伦维尔锋的性质，因为对其几何形状和中地壳范围的限制可能会揭示其起源。图1。中大西洋地球物理综合协作 (MAGIC) 地震和大地电磁 (MT) 站位置地图。插图地图显示了地理位置。地震台站位置用红色三角形表示，MT台站用黄色方块表示。背景颜色表示地形 (m)，如右侧的颜色条所示。细黑线表示状态边界。虚线显示了主要的构造特征，包括推定的 Grenville Front（GF；黑色），如 Stein 等人所述。（2018 年）、Whitmeyer 和 Karlstrom（2007 年）的罗马海槽边界（RT；洋红色）和阿巴拉契亚阵线（AF；橙色）。黑星表示 2011 年弗吉尼亚矿产地震的震中。白星表示鼹鼠山的位置，鼹鼠山是始新世地形上突出的火山颈（Mazza 等，2014）。此图的彩色版本仅在电子版中提供。2 地震研究快报 www.srl-online.org • 第 XX 卷 • 第 XX 号 • – 2020 下载自 https://pubs.geoscienceworld.org/ssa/srl/article-pdf/doi/10.1785/0220200150/5084304/srl- 2020150.1.pdf 耶鲁大学用户 2020 年 7 月 1 日 此外，我们希望了解与边缘后裂谷演化相关的过程，包括那些导致始新世岩浆事件的过程以及可能导致阿巴拉契亚地形最近恢复活力的那些过程。最后，我们有兴趣了解地壳和地幔岩石圈结构与地震活动 CVSZ 之间的潜在联系。受这些科学问题的启发，MAGIC 野外实验于 2013 年至 2016 年在阿巴拉契亚中部进行。这项部署由美国国家科学基金会 (NSF) 的 EarthScope 和 GeoPRISMS 计划资助，是地震学首席研究员 (PI) Maureen Long（耶鲁大学）和 Margaret Benoit（当时在新泽西学院）和 MT 之间的合作努力PI Rob Evans（伍兹霍尔海洋研究所 [WHOI]）。我们在美国弗吉尼亚州、西弗吉尼亚州和俄亥俄州部署了（主要是）并置的宽带地震仪和长周期 MT 仪器的线性阵列（图 1）。地震数据采集于2013年10月至2016年10月。我们部署了28个宽带地震传感器，数据连续记录，并依赖于自然（被动）震源。MT 数据是在 2015 年 10 月至 2016 年 5 月期间收集的，25 个 MT 站点中的每一个都被占用了约 3 周；与地震数据一样，MT 实验依赖于天然资源。MAGIC 阵列穿过了多个地质地体和自然地理省，在其西端延伸经过格伦维尔阵线，并在其东端附近与 CVSZ 相交，靠近 2011 年弗吉尼亚矿产地震的震中（图 1） ）。我们的阵列还对始新世期间受异常火山活动影响的地区进行了采样，在始新世火山露头几公里内部署了几个站点。仪器部署和细节 图 1 显示了作为 MAGIC 实验一部分运行的所有地震和 MT 台站的地图。 MAGIC 地震阵列从弗吉尼亚州的查尔斯市延伸到俄亥俄州的鲍尔丁，阵列孔径约为 770 公里。最终共29个站点被占用，在任何给定时间最多部署 28 个站点（一个站点 [TRTF] 在 2015 年夏季搬迁到附近的站点 [MOLE]）。因此，标称站间距不到 30 公里，尽管我们设计的阵列使得