The Boring Volcanic Field of the Portland-Vancouver Area, Oregon and Washington: Tectonically Anomalous Forearc Volcanism in an Urban Setting
Downloaded from fieldguides.gsapubs.org on April 29, 2010 The Geological Society of America Field Guide 15 2009 The Boring Volcanic Field of the Portland-Vancouver area, Oregon and Washington: Tectonically anomalous forearc volcanism in an urban setting Russell C. Evarts U.S. Geological Survey, 345 Middlefi eld Road, Menlo Park, California 94025, USA Richard M. Conrey GeoAnalytical Laboratory, School of Earth and Environmental Sciences, Washington State University, Pullman, Washington 99164, USA Robert J. Fleck Jonathan T. Hagstrum U.S. Geological Survey, 345 Middlefi eld Road, Menlo Park, California 94025, USA ABSTRACT More than 80 small volcanoes are scattered throughout the Portland-Vancouver metropolitan area of northwestern Oregon and southwestern Washington. These vol- canoes constitute the Boring Volcanic Field, which is centered in the Neogene Port- land Basin and merges to the east with coeval volcanic centers of the High Cascade volcanic arc. Although the character of volcanic activity is typical of many mono- genetic volcanic fi elds, its tectonic setting is not, being located in the forearc of the Cascadia subduction system well trenchward of the volcanic-arc axis. The history and petrology of this anomalous volcanic fi eld have been elucidated by a comprehensive program of geologic mapping, geochemistry, 40Ar/39Ar geochronology, and paleomag- netic studies. Volcanism began at 2.6 Ma with eruption of low-K tholeiite and related lavas in the southern part of the Portland Basin. At 1.6 Ma, following a hiatus of ~0.8 m.y., similar lavas erupted a few kilometers to the north, after which volcanism became widely dispersed, compositionally variable, and more or less continuous, with an average recurrence interval of 15,000 yr.
[Show full text]