40Ar/39Ar geochronology, paleomagnetism, and evolution of the Boring volcanic field, Oregon and Washington, USA

Abstract

The 40 Ar/ 39 Ar investigations of a large suite of fi ne-grained basaltic rocks of the Boring volcanic fi eld (BVF), Oregon and Washington (USA), yielded two primary results. (1) Using age control from paleomagnetic polarity, stratigraphy, and available plateau ages, 40 Ar/ 39 Ar recoil model ages are defi ned that provide reliable age results in the absence of an age plateau, even in cases of signifi cant Ar redistribution. (2) Grouping of eruptive ages either by period of activity or by composition defi nes a broadly northward progression of BVF volcanism during latest Pliocene and Pleistocene time that refl ects rates consistent with regional plate movements. Based on the frequency distribution of measured ages, periods of greatest volcanic activity within the BVF occurred 2.7-2.2 Ma, 1.7-0.5 Ma, and 350-50 ka. Grouped by eruptive episode, geographic distributions of samples defi ne a series of northeast-southwest-trending strips whose centers migrate from south-southeast to north-northwest at an average rate of 9.3 1.6 mm/yr. Volcanic activity in the western part of the BVF migrated more rapidly than that to the east, causing trends of eruptive episodes to progress in an irregular, clockwise sense. The K 2 O and CaO values of dated samples exhibit well-defi ned temporal trends, decreasing and increasing, respectively, with age of eruption. Divided into two groups by K 2 O, the centers of these two distributions defi ne a northward migration rate similar to that determined from eruptive age groups. This age and compositional migration rate of Boring volcanism is similar to the clockwise rotation rate of the Oregon Coast Range with respect to North America, and might refl ect localized extension on the trailing edge of that rotating crustal block.