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Long-runout landslides are enigmatic in the geological sciences, with large volumes of displaced material that are transported for distances far exceeding expectations grounded in the physics of frictional sliding. In these systems, long runout requires a mechanism that effectively reduces the frictional resistance of the basal slide plane well below values observed in rock during laboratory testing. Over the past several decades, numerous mechanisms have been proposed to account for this phenomenon, however few studies develop causal relationships between field observations and numerical models simulating these processes in controlling long runout. In this work, we describe the evolution of basal zone deformation within the exceptionally exposed Sevier gravity slide of southwestern Utah, identify signatures of frictional processes through zircon geochronology, and develop a numerical model that considers the effects of the evolving basal zone on dynamic weakening mechanisms to investigate the frictional processes controlling long runout. The late Oligocene-aged Sevier gravity slide is among the largest terrestrial landslides on Earth, with volume of displaced material exceeding 1000 km3 and transport distances of 35 km across the former land surface. This structure is the first of three gigantic-scale sector collapse events of the southern Marysvale volcanic field during late-stage volcanism in the region, and subsequent Basin and Range normal faulting provides exceptional exposure into the basal zone of this structure. As such, the Sevier gravity slide is an excellent field site to characterize and document the evolving structural complexity with translation distance and build conceptual models for better understanding emplacement mechanics. Combined with continuity across the entire mapped structure, outcrop-scale structures within the basal deformation zone, including frictional wear products, pseudotachylyte, deformed jigsaw clasts, and a variety of kinematic indicators indicate the gravity slide represents a single high-velocity emplacement event from north to south. Injection of basal material into the slide also suggests the high fluid pressures were present during translation. Based on the deformation styles and the source of wear products, both of which evolve systematically with distance along the length of the slide, we constrain spatial heterogeneity in the relative frictional strength of the basal slide plane and identify features of the slide that are related to the apparent strength of the slip surface. For example, highly effective weakening mechanisms resulting in slip localization and low overall damage to the slide block primarily occur when impermeable volcanic tuffs are present within the northern portions of the slide proximal to the ramp. With increasing translation distance, we observe greater slip delocalization, thicker accumulations of wear products, and more widespread damage to the slide block and underlying substrate as more permeable lithologies are exposed at the basal slide plane due to frictional wear and thinning of the allochthonous upper plate. Based on these observations, our conceptual model for emplacement envisions pressurization of basal zone fluids by thermal expansion during frictional heating (thermal pressurization). Early in the translation history, impermeable volcanic units trap fluids in the basal zone, resulting in shear localization, low frictional resistance to sliding, and simple translation deformation. The overall damage state of the slide increases with translation distance and the loss of impermeable volcanic units in the slide allow pressurized fluids to escape the basal zone. Under these conditions normal frictional contact is restored, leading to slip delocalization, greater damage and generation of basal wear products, and ultimately deceleration and cessation of the slide. |
