EVIDENCE FOR CATASTROPHIC EMPLACEMENT OF THE MARYSVALE GRAVITY SLIDE COMPLEX AIDED BY THERMAL PRESSURIZATION OF SHEAR ZONE FLUIDS

Numerous mechanisms have been proposed to reduce the frictional resistance and aid in the high-velocity emplacement of large-volume landslides. Here, we present field evidence from below and within the slide deposits and numerical modeling results which support a thermal pressurization mechanism in the Oligocene to Miocene mega-scale (> 8000 km²) Marysvale gravity slide complex (MGSC) of southwestern Utah. With frictional sliding, temperature increase at the slide base can result in the thermal expansion of pore fluids under saturated conditions. When the expansion of fluids is greater than can be accommodated by compression of the surrounding rock matrix, fluids become pressurized and reduce the effective normal stress acting on the basal slide plane. Throughout the MGSC, features such as clastic dikes, pseudotachylyte (solidified frictionally-induced melts), extensive damage, hydrothermal alteration local to the basal shear zone, and basal breccia development are consistent with high-velocity emplacement. Furthermore, the occurrence or absence of these specific features appears to be directly related to the lithologies adjacent to the slide plane. Features consistent with high fluid pressures (clastic dikes, thin deformation zones, and hydrothermal alteration) are limited to regions where shear localization occurred at the base of impermeable ash-flow tuffs. Conversely, features indicative of abrasive wear (diffuse deformation and pseudotachylyte) occur where the shear zones lack impermeable barriers and intersect highly permeable and damaged volcaniclastic sandstone. To evaluate the thermal pressurization mechanism in the MGSC, we measured lithologic unit permeabilities using a steady state flow test in a Triaxial Rock Deformation apparatus under predicted in situ effective confining pressure conditions. Using the constrained permeabilities, 1D numerical simulations of excess pore pressure generation and diffusion into the surrounding substrate indicate the shear zone lithology controls the efficacy of the thermal pressurization mechanism. Impermeable basal vitrophyres of ash-flow tuffs create confined, undrained conditions at the slide base where high fluid pressures are maintained, while diffusion of excess pore pressures outpaces generation in volcaniclastic sandstones.