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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.
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The concept of tectonic provenance: Case study of the gigantic Markagunt gravity slide basal layer
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.
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- Award ID(s):
- 2113155
- PAR ID:
- 10335611
- Date Published:
- Journal Name:
- Terra Nova
- ISSN:
- 0954-4879
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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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.more » « less
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Abstract We deployed a network of 68 three-component geophones on the slow-moving Two Towers earthflow in northern California. We compute horizontal-to-vertical spectral ratios (HVSRs) from the ambient seismic field. The HVSRs have two prominent peaks, one near 1.23 Hz and another between 4 and 8 Hz at most stations. The 1.23 Hz resonance is a property of the background noise field and may be due to a velocity contrast at a few hundred meters depth. We interpret the higher frequency peaks as being related to slide deposits and invert the spectral ratios for shallow velocity structure using in situ thickness measurements as a priori constraints on the inversion. The thickness of the shallowest, low-velocity layer is systematically larger than landslide thicknesses inferred from inclinometer data acquired since 2013. Given constraints from field observations and boreholes, the inversion may reflect the thickness of deposits of an older slide that is larger in spatial extent and depth than the currently active slide. Because the HVSR peaks measured at Two Towers are caused by shallow slide deposits and represent frequencies that will experience amplification during earthquakes, the depth of the actively sliding mass may be less relevant for assessing potential slide volume and associated hazard than the thicknesses determined by our inversions. More generally, our results underscore the utility of combining both geotechnical measurements and subsurface imaging for landslide characterization and hazard assessment.more » « less
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1111/ter.12608
