THERMO-RHEOLOGICAL FEEDBACKS IN SILICIC LAVAS AND IGNIMBRITES
The lava-like and rheomorphic Grey’s Landing ignimbrite, Idaho, provides abundant field evidence supporting the upward migration of a transient, 1-2 m thick, sub-horizontal ductile shear zone at the interface between the pyroclastic density current and deposit, through which all of the deposit passed. Using rheological experiments and thermo-mechanical modeling, we demonstrate that syn-depositional welding and ductile flow is achievable within a very restricted field of likely time-temperature-strain space where rapid high-strain deformation (1000%) is favored by higher emplacement temperatures (850 °C). The field of ductile deformation is broadened significantly by accounting for strain-heating, which permits a sustained temperature increase up to 250 °C within the shear zone, and helps to explain the enormous extents of lava-like lithofacies and intense rheomorphism recorded in extremely high-grade ignimbrites. We conclude that strain heating, an inevitable result of magma transport that feeds back to rheology and transport, should be taken into account in thermal modeling of volcanic processes at both high and low strain rates, and both pre- and post-eruption.