A BRIEF LIFESPAN FOR ASH CREEK BUTTE, CALIFORNIA

How long does it take to build a regional volcano in the Cascade Arc? Distributed regional volcanism within the volcanic arc has a broad footprint. Shield volcanoes of mafic and intermediate chemistry with eruptive volumes of multiple cubic kilometers are common edifices, but little is known about their rates of growth. Field geology, geochemistry, and absolute radiometric dating are useful tools to construct volcanic eruptive histories, but uncertainties on absolute dates are on the order of thousands of years. However, paleomagnetism – the record of the past geomagnetic field – can resolve variations on the order of decades. Paleomagnetic data were obtained from lava flows of the Ash Creek Butte shield volcano in northern California. This ~11 km³ shield volcano formed at ~230 ka, was heavily glaciated, and lacks significant unconformities (i.e. hiatuses) in its eruptive sequence. Data from twelve separate flows, positioned both early and late in the stratigraphic context of the volcano, yield a precise mean direction at D=7.9°, I=+60.1° (a95=1.9°, k=533; n=12). These tightly clustered paleomagnetic results suggest that this archetypal regional shield volcano could have formed in as little as 59 to 84 years, assuming modern to Holocene rates of secular variation are representative. Geochemical data from each flow are highly similar, and trace element and petrologic data supports the same eruptive order as field relationships. Our results from Ash Creek Butte corroborate paleomagnetic data from a number of small, regional volcanic edifices in western North America and implies sub-century build times may be common within the Cascade Arc and elsewhere. Some of these rapidly built volcanoes, such as Ash Creek Butte, are substantially larger than the typical definition for the volume of a monogenetic volcano (≤1 km³). Thus, paleomagnetic evidence poses an interesting conundrum as to what counts as ‘monogenetic’.