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The Boring volcanic field of the Portland-Vancouver area, Oregon and Washington; tectonically anomalous forearc volcanism in an urban setting

Russell C. Evarts, Richard M. Conrey, Robert J. Fleck and Jonathan T. Hagstrum
The Boring volcanic field of the Portland-Vancouver area, Oregon and Washington; tectonically anomalous forearc volcanism in an urban setting (in Volcanoes to vineyards; geologic field trips through the dynamic landscape of the Pacific Northwest, Jim E. O'Connor (editor), Rebecca J. Dorsey (editor) and Ian P. Madin (editor))
Field Guide (Geological Society of America) (December 2009) 15: 253-270

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, (super 40) Ar/ (super 39) Ar geochronology, and paleomagnetic 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 approximately 0.8 m.y., similar lavas erupted a few kilometers to the north, after which volcanism became widley 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-SiO (sub 2) andesite, were also constructed. Low-K to high-K calc-alkaline compositions similar to those of the near-by volcanic arc dominated 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.


ISSN: 2333-0937
EISSN: 2333-0945
Serial Title: Field Guide (Geological Society of America)
Serial Volume: 15
Title: The Boring volcanic field of the Portland-Vancouver area, Oregon and Washington; tectonically anomalous forearc volcanism in an urban setting
Title: Volcanoes to vineyards; geologic field trips through the dynamic landscape of the Pacific Northwest
Author(s): Evarts, Russell C.Conrey, Richard M.Fleck, Robert J.Hagstrum, Jonathan T.
Author(s): O'Connor, Jim E.editor
Author(s): Dorsey, Rebecca J.editor
Author(s): Madin, Ian P.editor
Affiliation: U. S. Geological Survey, Menlo Park, CA, United States
Affiliation: U. S. Geological Survey, Portland, OR, United States
Pages: 253-270
Published: 200912
Text Language: English
Publisher: Geological Society of America, Boulder, CO, United States
References: 63
Accession Number: 2010-060163
Categories: Quaternary geology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 3 tables, geol. sketch map
N45°00'00" - N45°45'00", W122°45'00" - W122°15'00"
Secondary Affiliation: University of Oregon, USA, United StatesOregon Department of Geology and Mineral Industries, USA, United StatesWashington State University, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 201033
Program Name: USGSOPNon-USGS publications with USGS authors
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