Tephrochronology of North America's most recent arc-sourced ignimbrite flare-up: The Deschutes Formation of the Central Oregon Cascades

Abstract

The dangerous juxtaposition of human populations and active volcanic arcs worldwide makes elucidating the history of explosive eruptions of these arcs a crucial endeavor. The well-preserved Deschutes Formation, which forms part of the Deschutes Basin of central Oregon, USA, contains a distal record of a period of unusually frequent explosive eruptions between 5.45 and 6.25 Ma, which is atypical of the Cascades arc as a whole. However, no previous study has established the chronology, frequency, or total volume of all of the ignimbrites and tephra-fall units exposed within the Formation in order to compare to baseline activity of the arc. We use multivariate statistics on major and trace element compositions of pumice glass to correlate pyroclastic units across the Deschutes Basin, and combine these with recently published 40Ar/39Ar dates to establish, for the first time, a detailed and comprehensive tephrochronology of the Deschutes Formation. Our results suggest that at least 78 statistically distinct explosive eruptions occurred between 6.25 and 5.45 Ma. Eruption frequency is not constant throughout the 800 kyr of pyroclastic volcanism; a maximum rate of 1.7 to 3.0 explosive eruptions per 10 kyr occurred between 5.76 and 5.68 Ma and decreased for 340 kyr prior to graben formation. We estimate that the volume for 51 ignimbrite eruptions is 290 to 565 km3 (170 to 280 km3 DRE), assuming an equal westward flow component and a fall:flow ratio of 0.5:1 to 1.9:1. Using a single isopach method, we also estimate that a minimum volume of 110 km3 (45 km3 DRE) was deposited as 27 pumice-fall units which have no correlated ignimbrites. Thus, we estimate that a cumulative pyroclastic volume of 400 to 675 km3 (210 to 330 km3 DRE) was erupted in the central Oregon Cascades during this time. The volumetric rate (2.7 to 4.1 km3/10 kyr, DRE) and average frequency of ignimbrite eruptions (1.0 to 1.5 eruptions/ 10kyr) are more than a factor of ten greater than the Quaternary Cascades arc. Thus, the Deschutes Formation records North America's most recent arc-sourced ignimbrite flare-up. Although smaller in volume than most well-recognized flare-up events, this represents an important category of flare-ups that may be more common in arcs than previously recognized. We hypothesize that a heightened flux of basalt, possibly induced by slab-rollback, was focused beneath the arc and into the shallow crust by minor amounts of crustal extension. This extension allowed for the high flux of basalt to be stored at shallow levels beneath a new arc locus within fertile crust, resulting in an unusually silicic and explosive period of Cascade arc history.