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The Boring Volcanic Field of the Portland-Vancouver area, Oregon and Washington: Tectonically anomalous forearc volcanism in an urban setting

By
Russell C. Evarts
Russell C. Evarts
U.S. Geological Survey, 345 MiddlefieldRoad, Menlo Park, California 94025, USA
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;
Richard M. Conrey
Richard M. Conrey
Geo Analytical Laboratory, School of Earth and Environmental Sciences, Washington State University, Pullman, Washington 99164, USA
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Robert J. Fleck
Robert J. Fleck
U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA
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;
Jonathan T. Hagstrum
Jonathan T. Hagstrum
U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA
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Published:
January 01, 2009

Abstract

More than 80 small volcanoes are scattered throughout the Portland-Vancouver metropolitan area of northwestern Oregon and southwestern Washington. These volcanoes constitute the Boring Volcanic Field, which is centered in the Neogene Portland 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 monogenetic volcanic fields, 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 field 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. The youngest centers, 50-130 ka, are found in the northern part of the field. Boring centers are generally monogenetic and mafic but a few larger edifices, ranging from basalt to low-SiO2 andesite, were also constructed. Low-K to high-K calc-alkaline compositions similar to those of the nearby volcanic arc dominate the field, but many centers erupted magmas that exhibit little influence of fluids derived from the subducting slab. The timing and compositional characteristics of Boring volcanism suggest a genetic relationship with late Neogene intra-arc rifting.

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GSA Field Guide

Volcanoes to Vineyards

Jim E. O’Connor
Jim E. O’Connor
U.S. Geological Survey Oregon Water Science Center 2130 SW 5th Avenue Portland, Oregon 97201 USA
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Rebecca J. Dorsey
Rebecca J. Dorsey
Department of Geological Sciences 1272 University of Oregon Eugene, Oregon 97403-1272 USA
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Ian P. Madin
Ian P. Madin
Oregon Department of Geology and Mineral Industries 800 NE Oregon Street #28, Suite 965 Portland, Oregon 97232-2162 USA
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Geological Society of America
Volume
15
ISBN electronic:
9780813756158
Publication date:
January 01, 2009

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