ZIRCON GEOCHRONOLOGY RECORDS FRICTIONAL WEAR PROCESSES ACTIVE DURING EMPLACEMENT OF THE GIGANTIC-SCALE SEVIER GRAVITY SLIDE

Observations of long runout in large-volume landslides require a mechanism to reduce the frictional resistance at the basal slide plane during transport. Basal layer, or the comminuted material generated by wear and deposited beneath slide blocks during emplacement of these large-scale landslides, shows evidence for widespread fluidization and can provide insights to the frictional processes active during transport. However, the history of basal layer formation, such as timing, source, transport distance, and longevity, that is needed to understand the mechanics of these natural hazards are difficult to discern based on field observations alone. Here, we describe a novel application of zircon geochronology and maximum depositional ages to interpret the translation history of the gigantic-scale (> 1000 km³) volcanic Sevier gravity slide of southwestern Utah and the frictional wear processes operating during its emplacement. Basal layer samples collected in a transport-parallel profile across the 35 km runout distance of the Sevier gravity slide reveal unique tectonic chronofacies in the U/Pb age distributions of zircon grains incorporated during sliding. This indicates the basal layer is not well-mixed across the structure and wear products are quickly deposited after formation without significant reworking. Over much of the runout segment of the slide, the basal layer contains zircons only from the allochthonous upper plate, suggesting the substrate is buffered from deformation, consistent with low to no frictional strength of the sliding interface required to achieve high slip velocities and long runout. This continues until the most distal samples, where a rapid increase in the proportion of zircon grains in the basal layer sourced from the underlying substrate demonstrates a return to high frictional strength during the final deceleration phase of the slide and resulting in scour and deformation to the substrate. We argue the observed spatial distribution of zircon sources is consistent with widespread thermal pressurization during transport followed by rapid fluid pressure loss during the deceleration phase, consistent with field observations of clastic injectites, hydrothermal alteration, and highly localized shear.