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Abstract The exceptional transport distance of long-runout landslides requires a mechanism for reduced frictional resistance to sliding. Here, we use zircons in the frictional wear products generated during emplacement of the Sevier gravity slide (southwest Utah, USA) to identify how the source of material evolves with transport distance and discuss how changes in frictional strength are reflected in this data set. Across the ~38 km runout distance of the slide, basal wear products have unique zircon age distributions, or tectonic chronofacies, which capture changes in material sources and indicate poor mixing across the structure. Over much of this distance, basal material forms by breakdown of slide blocks, with little input from the underlying substrate. This suggests the basal slide plane has low frictional strength, buffering the substrate from deformation. We also observe a decrease in the mean age of zircons within the basal layer with increasing transport distance as abrasive wear is localized at the base of the overlying block during slip. Toward the distal portion of the slide, the amount of substrate zircons in the basal layer increases, consistent with greater frictional coupling during deceleration. Tying the unique tectonic provenance recorded by zircons within the basal layer of the Sevier gravity slide to larger deformation styles, we argue that the observed spatial evolution in frictional strength is consistent with widespread fluid pressurization. 

Abstract The physical processes that facilitate long‐distance translation of large‐volume gravity slides remain poorly understood. To better understand these processes and the controls on runout distance, we conducted an outcrop and microstructural characterization of the Sevier gravity slide across the former land surface and summarize findings of four key sites. The Sevier gravity slide is the oldest of three mega‐scale (>1,000 km²) collapse events of the Marysvale volcanic field (Utah, USA). Field observations of intense deformation, clastic dikes, pseudotachylyte, and consistency of kinematic indicators support the interpretation of rapid emplacement during a single event. Furthermore, clastic dikes and characteristics of the slip zone suggest emplacement involved mobilization and pressurized injection of basal material. Across the runout distance, we observe evidence for progressive slip delocalization along the slide base. This manifests as centimeter‐ to decimeter‐thick cataclastic basal zones and abundant clastic dikes in the north and tens of meters thick basal zones characterized by widespread deformation of both slide blocks and underlying rock near the southern distal end of the gravity slide. Superimposed on this transition are variations in basal zone characteristics and slide geometry arising from interactions between slide blocks during dynamic wear and deposition processes and pre‐existing topography of the former land surface. These observations are synthesized into a conceptual model in which the presence of highly pressurized fluids reduced the frictional resistance to sliding during the emplacement of the Sevier gravity slide, and basal zone evolution controlled the effectiveness of dynamic weakening mechanisms across the former land surface.