Dome collapse mechanisms and block-and-ash flow emplacement dynamics inferred from deposit and impact mark analysis, Mono Craters, CA

Abstract

Characteristics of the Panum block-and-ash flow (BAF) deposit, Mono Craters, CA, were analyzed to determine the mechanisms of collapse of the parent dome and dynamics of emplacement of the BAF. Granulometry, componentry, and obsidian water content data were used to define distinct facies of the Panum BAF deposit. These suggest a sequential, three-stage collapse model for the ancestral dome of the Panum vent, with destabilization first of its cold, brittle outer margins and then of its hot, ductile interior to an estimated depth of ~260m below the free-air surface.Impact marks on clast faces that resulted from clast-to-clast interactions recorded details of the impacting mechanism. We analyzed ~15,000 marks and report for the first time their size, shape, and sorting to infer the flow regime of the BAF during emplacement, to a distance of ~2.6km from the vent. Clast-to-clast interactions occurred over a broad spectrum from purely collisional (normal to clast faces) to purely frictional (clasts shearing in a unidirectional flow), and recorded a transition to a more frictional flow regime in distal reaches as the sliding head of the BAF decelerated and halted. Finally, we briefly differentiate a similar, but previously undescribed older deposit that predates the 1325–1350A.D. North Mono eruptive episode.