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

最近在弗吉尼亚州发生的5.8级地震影响了美国东海岸的主要大都市地区，引发了对更好地了解北美东部边缘（ENAM）地质特征的需求。在弗吉尼亚州和西弗吉尼亚州，有100多个火山体，大约有4700万到4900万年的历史，是美国东海岸已知最年轻的火山喷发。这些岩浆和它们在上升和喷发期间从地壳和地幔中收集的岩石碎片是最近地质时代地壳和地幔的唯一直接样本。这项研究的结果将与目前部署在东海岸的EarthScope可移动地震仪阵列的数据以及其他地震研究相结合，以创建美国东部地壳和地幔状态的综合图像，为未来潜在的地震危险提供背景。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的研究。两所大学都靠近现场，这使得现场表征和采样非常方便。除了普通教育的课程材料外，还将为初高中学生开展实地考察，并通过终身学习研究所开展拓展课程。将为弗吉尼亚理工大学的地球科学博物馆举办一个展览。这项研究的数据将与公众、地球科学协会和地球观测协会共享。目前关于ENAM下软流圈和岩石圈的组成和结构的限制很少。地球化学和岩石学数据对于解释该地区的地震数据以及了解ENAM以及全球裂谷边缘的裂谷向漂移转变的长期持续演变至关重要。暴露在弗吉尼亚州和西弗吉尼亚州的始新世火山群是美国东部已知最年轻的岩浆活动，也是研究ENAM地幔和下地壳新生代过程的唯一岩石学窗口。我们假设：1)始新世岩浆是由浅层软流圈绝热熔融（如岩石圈剥离、边缘驱动对流或深俯冲作用）产生的，2)熔融与岩石圈对泛大陆分裂和/或Farallon俯冲的响应有关，这种反应至少在始新世持续在这个被动边缘下。我们将通过一系列地球化学、光谱和岩石学观测和建模来检验这些假设。熔体和基底捕虏体的年代学将评估熔融过程，约束岩石圈的结构和演化，并相对于捕虏体的年龄评估火山活动的年龄。EarthScope可移动阵列目前正沿着东海岸部署到2013年，我们的项目位于？里士满样吗?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组成、岩石圈和软流圈结构以及挥发性成分的重要约束。

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.