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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 Marysvale volcanic field in southwestern Utah hosts three large-volume gravity slides: the Sevier (SGS), the Markagunt (MGS), and the Black Mountains (BGS). The gravity slides are composed of lahar deposits, lava flows, and ash-flow tuffs erupted from former stratovolcanoes and other vents during the Oligocene and Miocene. The ash-flow tuffs are prime targets for dating to constrain the age of the gravity slides because some ash-flow tuffs are deformed within the slides, whereas others are undeformed and cap the slides. Furthermore, the gravity slides produced pseudotachylyte during slide motion, a direct indicator for the timing of each slide. This work provides new 40Ar/39Ar dates for several ash-flow tuffs and pseudotachylyte for the SGS, along with U/Pb zircon dates for one deformed tuff and alluvium near the slide plane. Results show that the slide was emplaced at 25.25 ± 0.05 Ma and was immediately followed by the eruption of the Antimony Tuff at 25.19 ± 0.02 Ma. The model presented here suggests that the intrusion of magma related to the Antimony Tuff acted as a triggering mechanism for the slide and that slide movement itself led to decompression melting and eruption of the Antimony Tuff. This sequence of events occurred on a geologically rapid timescale and may have been virtually instantaneous. 

Formation and evolution of the basal layer in large landslides has important implications for processes that reduce frictional resistance to sliding. In this report, we show that zircon geochronology and tectonic provenance can be used to investigate the basal layer of the gigantic-scale Markagunt gravity slide of Utah, USA. Basal layer and clastic injectite samples have unique tectonic chronofacies that identify the rock units that were broken down during emplacement. Our results show that basal material from sites on the former land surface is statistically indistinguishable and formed primarily by the breakdown of upper plate lithologies during sliding. Decapitated injectites have a different tectonic chronofacies than the local basal layer, with more abundant lower plate-derived zircons. This suggests clastic dikes formed earlier in the translation history from a structurally deeper portion of the slide surface and a compositionally different basal layer before being translated to their current position.