FOREARC VOLCANISM IN CASCADIA: THE BORING VOLCANIC FIELD

The Boring Volcanic Field consists of more than 80 small, mostly mafic volcanoes centered on the Portland Basin in northwestern Oregon and southwestern Washington. The area of the field is approximately 4000 km², of which about 500 km² is underlain by locally erupted volcanic rocks with a total volume near 10 km³. 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 main 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 paleomagnetic studies. Volcanism began with eruption of low-K tholeiite (LKT) from a half-dozen vents in the Oregon City area in the southern part of the Portland Basin at 2.6–2.4 Ma. At 1.6 Ma, following a hiatus of about 0.8 m.y., similar LKT lavas erupted a few kilometers to the north at Mount Scott. Subsequently, volcanic activity became widely dispersed, compositionally variable, and more or less continuous, with an average recurrence interval of 15,000-20,000 years (approximately 100 eruptions/1.6 m.y.). The youngest dated centers, ~50–200 ka (Beacon Rock, Elk Point, Mount Tabor, Bobs Mountain, Battle Ground), are all in the northern part of the field. Low-K to high-K calcalkaline compositions similar to those in the nearby Cascades volcanic arc dominate the field, but many erupted magmas exhibit little or no influence of fluids derived from the subducting slab. Something unusual about the subjacent mantle or crust must be responsible for the Boring field, and the timing and compositional characteristics of Boring volcanism suggest a genetic relationship with late Neogene intra-arc rifting to the east. Crustal structures related to clockwise rotation and northward translation of the Oregon Coast Range block may have provided pathways for the eruption of low-volume, predominantly mantle-derived melts in the Portland segment of the forearc.