A NUMERICAL MODELING INVESTIGATION OF THE DURATION OF POST-EMPLACEMENT RHEOMORPHISM IN A HIGH-GRADE IGNIMBRITE: GREY'S LANDING, IDAHO

A finite-difference thermal numerical model¹ was applied to the ~8 Ma extremely high-grade Grey's Landing ignimbrite (GL) from the Rogerson Graben of south-central Idaho to investigate the unit’s cooling behaviour, and cooling-induced constraints on the post-emplacement rheomorphism that affected the thickest sections of the unit.

The GL is a >10 km³ rhyolite (72% SiO₂) that shows a two-part vertical, structural zonation. Post-emplacement rheomorphism occurred in the upper 60% of sections greater than 30m thick. Post-emplacement structures consist of 1m- to 10m-scale, upright to (rarely) isoclinal, open to recumbent, flow folds with amplitudes from ~1m to 5m. Post-emplacement structures re-fold earlier isoclinal, recumbent flow folds (amplitudes <1m, wavelength £20m) that are interpreted to represent syn-emplacement structures; in the lower 40% of the unit the syn-emplacement structures are not modified by rheomorphism. Sections <30m thick show only syn-emplacement deformation. The GL has a massive lithoidal (devitrified) centre, with a 4m-thick lower vitrophyre, and an upper, deformed 6 m-thick vitrophyre. Rheomorphism presumably began as soon as the unit's particulate emplacement ceased. Assuming a 60m-thick section with emplacement temperatures of 1000 - 1100 C, consideration only of persistent temperatures > 650 C (roughly the glass transition temperature) implies that rheomorphism deep in the unit might have lasted as long as 13 - 18 years after emplacement. However, the vitrophyric uppermost 10m would have cooled so rapidly that deformation would have ceased after 4 years.

The homogeneous distribution of flow folds throughout the zone of rheomorphism in the upper portion of the unit – vitrophyre, spherulitic vitrophyre, and crystalline interior – suggests that deformation occurred contemporaneously throughout. For this to be the case, the onset of crystallization in the flow interior must nearly have matched the duration of cooling in the upper vitrophyre. Onset of crystallization thus must have been rapid (<4 yrs) in the core and likely limited how long the upper portion of the unit continued to deform.

¹ Manley, C.R., 1992, Extended cooling and viscous flow of large, hot rhyolite lavas: Implications of numerical modeling results. Journal of Volcanology and Geothermal Research 53:27–46.