Collaborative Research: Virginia's Volcanoes: a Window into Eastern North America Mantle Processes

最近在弗吉尼亚州米纳尔市发生的5.8级地震影响了美国东海岸的主要大都市地区，促使人们需要更好地了解北美东部边缘(ENAM)的地质特征。在弗吉尼亚州和西弗吉尼亚州暴露的100多个火山体大约有47-4900万年的历史，是美国东海岸已知的最年轻的喷发。这些岩浆和他们在上升和喷发期间从地壳和地幔收集的岩石碎片是近代地质时代唯一直接的地壳和地幔样本。这项研究的结果将与目前部署在东海岸的EarthScope便携式地震仪阵列的数据以及其他地震研究一起使用，以创建美国东部地壳和地幔状态的全面图景，为未来可能发生的地震灾害提供背景。该项目将支持弗吉尼亚理工大学和詹姆斯·麦迪逊大学的研究生和本科生研究。实地地点离这两所大学很近，因此很容易进行实地鉴定和抽样。除了通识教育的课程材料外，还将为初中生和高中生进行实地考察，并通过终身学习研究所开展外展课程。将为弗吉尼亚理工大学的地球科学博物馆制作一个展览。这项研究的数据将与公众、GeoPRISMS和EarthScope社区共享。目前，对埃纳姆之下的软流圈和岩石圈的组成和结构几乎没有什么限制。地球化学和岩石学数据对于解释该区域的地震数据和了解埃纳姆地区裂谷向漂移过渡的长期、持续演化以及世界各地的裂谷边缘至关重要。在弗吉尼亚州和西弗吉尼亚州发现的始新世火山体群是美国东部已知的最年轻的岩浆作用，也是了解埃纳姆地幔和下地壳新生代过程的唯一岩石学窗口。我们推测：1)始新世岩浆是通过浅层软流圈的绝热熔融(如岩石圈拆沉、边缘驱动对流或深俯冲的影响)产生的；2)熔融与岩石圈对盘古大陆解体和/或法拉隆俯冲的响应有关，这种反应至少持续到始新世。我们将通过一系列的地球化学、光谱和岩石学观察和建模来检验这些假说。熔体和基底包体的地质年代学将评估熔融过程，约束岩石圈的结构和演化，并评估火山活动相对于包体年龄的年龄。EarthScope可移动阵列目前正沿着东海岸部署到2013年，我们项目的位置位于？里士满横断面？建议用于集中地震研究。我们的数据将产生深入ENAM的垂直横截面，这将为大规模地球动力学和地震研究提供关于基底和地幔成分、岩石圈和软流层结构以及挥发分含量的重要约束。

The recent magnitude 5.8 earthquakes in Mineral, VA, impacted major metropolitan areas on the East Coast of the U.S. and sparked a need to better understand the geologic characteristics the of Eastern North America Margin (ENAM). A group of more than 100 volcanic bodies approximately 47-49 million years old exposed in Virginia and West Virginia are the youngest known eruptions on the East Coast of the U.S. These magmas and the fragments of rock they collected from the crust and mantle during their ascent and eruption are the only direct samples of the crust and the mantle in recent geologic times. The results from this study will be used in conjunction with data from the EarthScope Transportable Array of seismometers currently being deployed along the East Coast as well as other seismic studies to create a comprehensive picture of the state of the crust and mantle underneath the Eastern U.S., providing context for the potential of future seismic hazards. This project will support graduate and undergraduate research at Virginia Tech and James Madison University. The proximity of the field site to both universities makes field characterization and sampling highly accessible. A field trip for middle and high school students and an outreach course through the Lifelong Learning Institute will be developed in addition to course materials for general education. An exhibit will be created for the Museum of Geociences at Virginia Tech. Data from this research will be shared with the public, the GeoPRISMS and EarthScope communities. Few constraints currently exist on the composition and structure of the asthenosphere and lithosphere under the ENAM. Geochemical and petrologic data are critical for interpretation of seismic data in the region and understanding the long-term, continued evolution of the rift-to-drift transition for ENAM, as well as for rift margins worldwide. A swarm of Eocene volcanic bodies exposed in Virginia and West Virginia are the youngest known magmatism in the Eastern U.S. and are the only petrologic window into Cenozoic processes in the mantle and lower crust in ENAM. We hypothesize that: 1) The Eocene magmas were generated through adiabatic melting of shallow asthenosphere (e.g., lithospheric delamination, edge-driven convection, or effects from deep subduction), 2) Melting is related to the lithospheric response to the breakup of Pangaea and/or Farallon subduction that continued under this passive margin at least through the Eocene. We will test these hypotheses with an array of geochemical, spectroscopic, and petrologic observations and modeling. Geochronology of the melts and basement xenoliths will evaluate melting processes, constrain the structure and evolution of the lithosphere, and evaluate the age of the volcanic activity relative to the age of the xenoliths. The EarthScope Transportable Array is currently being deployed along the East Coast through 2013 and the location of our project lies within the ?Richmond Transect? proposed for concentrated seismic studies. Our data will produce a vertical cross-section deep into the ENAM that will provide important constraints on basement and mantle composition, lithospheric and asthenospheric structures, and volatile contents for large-scale geodynamic and seismic studies.