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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:
, , , and
Edited by:
Jim O'Connor, Rebecca Dorsey, and Ian P. Madin
https://doi.org/10.1130/2009.fld015(13)

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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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Additional publication details

Publication type:
Book chapter
Publication Subtype:
Book Chapter
Title:
The Boring Volcanic Field of the Portland-Vancouver area, Oregon and Washington: tectonically anomalous forearc volcanism in an urban setting
DOI:
10.1130/2009.fld015(13)
Year Published:
2009
Language:
English
Publisher:
Geological Society of America
Publisher location:
Boulder, Colo.
Contributing office(s):
Geology and Geophysics Science Center
Description:
18 p.
Larger Work Type:
Book
Larger Work Title:
Volcanoes to vineyards: geologic field trips through the dynamic landscape of the Pacific Northwest
First page:
253
Last page:
270
Country:
United States
Other Geospatial:
Portland/Vancouver
Online Only (Y/N):
N
Additional Online Files (Y/N):
N