Cooling and welding history of the Bishop Tuff in Adobe Valley and Chidago Canyon, California

Abstract

A numerical model for cooling and welding of ash-flow tuffs was developed to obtain a quantitative understanding of the welding compaction that occurs during the cooling of ash-flow sheets and to evaluate the relative effects of emplacement temperature, thickness, and emplacement histories on compaction zonation (density profiles) of large ash-flow sheets. This model includes a set of two-phase, compressible Navier–Stokes equations for the transport of mass, momentum, and energy for a variable density volatile outfluxing through a porous media (ash-flow tuff). With this model we calculated theoretical compaction profiles (density vs. elevation) to match actual data measured in sections of Bishop Tuff. Compaction profiles from the Adobe basin indicate that there were 3 periods of non-deposition (5, 10, and 40 days) separating 4 distinct rapidly emplaced packets (REP) of pyroclastic flow deposition. In contrast, the 3 periods of quiescence (400, 20, and 800 days) between the 4 REPs in Chidago Canyon were considerably longer. These data constrain models of eruption for the Bishop Tuff by implying an eruption cycle that lasted longer than 3 years. The average calculated deposition temperatures for all 4 REPs in the Adobe basin are fairly uniform at 667±33 °C. In Chidago Canyon two of the REPs had a higher deposition temperature (∼660 °C) than the other two (534 °C).