Two-phase, reciprocal, double trapdoor collapse at Hannegan caldera, North Cascades, Washington, USA

Abstract

The intracaldera Hannegan volcanics were erupted during two collapse episodes of the Hannegan caldera in the North Cascade mountains of Washington State. The first eruption yielded a down-to-the-north trapdoor style collapse at 3.722 ± 0.020 Ma (40Ar/39Ar) that is bounded by a horseshoe-shaped ring fault. The second collapse, most probably also trapdoor style, followed a short period of sedimentation, and completed the elliptical ring fault around the southern margin of the caldera. Post caldera plutons, with U-Pb ages of 3.42 ± 0.10 and 3.36 ± 0.20 Ma, intruded the intracaldera ignimbrite.