The40Ar/39Ar age constraints on the duration of resurgence at the Valles caldera, New Mexico

Abstract

The40Ar/39Ar ages for rocks from the Valles caldera, New Mexico, imply that resurgent uplift of the caldera floor occurred within 27 ± 27 ka of caldera collapse. The structural resurgent dome of the Valles caldera was uplifted approximately 1000 m above the surrounding caldera floor during resurgence. The upper Bandelier Tuff, which yields an age of 1.256 ± 0.010 Ma (2σ; ages relative to Fish Canyon tuff sanidine at 28.02 Ma), is the lower constraint on the timing of resurgence. The oldest postcollapse ring fracture dome, Cerro del Medio, is the upper time constraint and yields an age of 1.229 ± 0.017 Ma for one of its oldest flow lobes. Therefore resurgent uplift probably occurred within 54 ka, at a minimum rate of 1.9 cm/yr. Sanidine phenocrysts from rhyolites of the postcollapse intracaldera Deer Canyon and Redondo Creek members yield40Ar/39Ar single‐crystal laser fusion ages ranging from 1.229 ± 0.013 Ma to 1.283 ± 0.017 Ma (Deer Canyon Member; n = 8) and 1.208 ± 0.017 Ma to 1.239 ± 0.017 Ma (Redondo Creek Member; n = 4). With one exception from each unit, these ages are statistically indistinguishable from the upper Bandelier Tuff, indicating that eruption of these postcollapse rhyolites probably commenced shortly after caldera formation. Melt inclusion hosted excess argon was not found to have a measurable effect on the age of sanidine from the upper Bandelier Tuff or the Deer Canyon Member.