New Mexico Geological Society Annual Spring Meeting — Abstracts

Detailed geologic mapping and ⁴⁰Ar/³⁹Ar dating reveal that megabreccia blocks as large as 600x400m were emplaced within intracaldera, Upper Bandelier Tuff (UBT) during eruption and collapse of the Valles caldera, and suggest that subsequent resurgence probably was complete within at most, 90 ky of caldera formation at approximately 1.23 Ma. The distribution and sources of intracaldera megabreccia blocks are highly variable. ⁴⁰Ar/³⁹Ar ages of selected megabreccia blocks indicate that they are composed of older pre-caldera units that include the 1.6 Ma Lower Bandelier Tuff (LBT; clasts yield ages of 1.64±0.04 Ma (2σ), and a dacitic tuff dated at 8.08±0.08 Ma.

The timing of resurgence is constrained by the eruption of the UBT and emplacement of the oldest ring fracture dome, Cerro Del Medio; ⁴⁰Ar/³⁹Ar dating places these time constraints at 1.232 ± 0.018 Ma (UBT) and 1.19 ± 0.03 Ma (Cerro Del Media). The Deer Canyon and Redondo Creek members of the Valles Rhyolite formation, erupted prior to and during resurgence, respectively, stratigraphically overlie the UBT. However, these units are not shown by ⁴⁰Ar/³⁹Ar dating to be measurably younger than the UBT, suggesting that resurgence commenced soon after collapse of the Valles caldera and eruption of the UBT.

Abundant clasts of LBT, derived from the reworking of LBT-bearing megabreccia blocks, are found within caldera fill volcaniclastic deposits. Fieldwork shows that these 1.6 Ma clasts are not syneruptive with Deer Canyon lavas associated with resurgence, as previously suggested by Phillips et al. (2002, Proc. NM Geol. Soc. Spring Meeting).