THE BORING VOLCANIC FIELD OF THE PORTLAND, OREGON AREA: GEOCHRONOLOGY AND NEOTECTONIC SIGNIFICANCE

Dozens of young monogenetic volcanoes lie scattered west of the axis of the Cascade volcanic arc in NW Oregon and SW Washington. They are most abundant near Portland, where they have been informally called the Boring Lava. Here we refer to this collection of vents as the Boring volcanic field (BVF). Most flows are relatively primitive olivine-phyric basalts and basaltic andesites. Compositions range widely from low-K tholeiitic to high-K calcalkaline.

New ⁴⁰Ar/³⁹Ar incremental-heating whole-rock age determinations from the northern part of the BVF generally confirm the late Pliocene and Quaternary age of volcanism as indicated by previous K-Ar dating. Paleomagnetic polarities from dated flows corroborate the ⁴⁰Ar/³⁹Ar ages. However, ⁴⁰Ar/³⁹Ar results also show that many of the basalts contain excess ⁴⁰Ar, yielding eruptive ages younger than K-Ar ages from the same flows. Patterns of Ar release suggest at least three discrete "reservoirs" of ⁴⁰Ar in these samples, with the excess component resulting in U-shaped age spectra. The new results demonstrate that volcanic activity is more recent than previously thought. Three eruptive centers (Battle Ground Lake, Rocky Butte, and in the Portland Hills) yield ages of about 100-125 ka. Combined with available K-Ar ages, these ages suggest that the area of active volcanism is expanding westward and northward with time, with the three youngest ages being obtained from vents near the NW margin of the BVF.

The BVF occupies an anomalously near-trench, fore-arc setting within the Cascadia subduction system. Vents are not evenly distributed within the fore-arc but are concentrated within and along the periphery of the Portland basin, which has been interpreted as a Neogene pull-apart structure. We suggest that the distribution of vents in the BVF demarks a zone of localized crustal extension related to northward translation and clockwise rotation of the Oregon Coast Range microplate relative to interior North America. South of the BVF, extension is concentrated along the axis of the volcanic arc, whereas to the north tectonism is compressional. The BVF occurs in a transitional zone in which extension becomes less intense and more widely distributed. The age pattern in the BVF suggests that this transitional regime is slowly propagating to the NW with time.