STRUCTURAL RELATIONSHIPS ACROSS THE SEVIER GRAVITY SLIDE AND IMPLICATIONS FOR EMPLACEMENT MECHANICS

Field mapping in the southern Marysvale volcanic field (Utah) provides evidence of three mega-scale (>2,000 km³) gravitational collapse events. Here, we summarize key field observations and structural relationships across the exceptionally preserved Sevier gravity slide, the oldest structure in the Marysvale gravity slide complex, collected along a transport-parallel transect of the former land surface. These observations provide insight to the mechanics that permitted >30 km translation during emplacement and identifying features of large-scale gravitational collapse structures that may be misinterpreted as tectonic features elsewhere. Across the slide, intense deformation, pseudotachylyte, and consistency of kinematic indicators support the interpretation that emplacement occurred during a single, high-velocity event. Injection of basal material into the slide mass also suggests high fluid pressures beneath the slide. With translation distance, we observe greater slip delocalization at the base of the slide, thicker accumulations of basal wear products, and more widespread damage to both allochthonous rock and the underlying substrate. Large-scale deformation styles are largely translation- or extension-dominated over much of the structure. However, some portions of the slide display significant compressional deformation representing progressive aggradation of slide material during deceleration near the slide terminus or interactions between slide blocks and paleo-topography on the land surface.

From these observations, our conceptual model for emplacement envisions the 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.