STRUCTURAL AND DEFORMATIONAL HISTORY OF THE MEGA-SCALE SEVIER GRAVITY SLIDE, GARFIELD COUNTY, UTAH: COW CREEK AND DEEP CREEK 7.5-MINUTE QUADRANGLES: EDMAP

Structural geology of a portion of the Sevier gravity slide (SGS) was studied through detailed field mapping, petrographic and geochemical analyses, and detrital zircon geochronology of two adjacent USGS 7.5' quadrangles (Cow Creek and Deep Creek) located in the NE portion of the Panguitch 1:100,000 sheet. Geologic mapping was carried out as part of a USGS EDMAP project to better understand the structural history of the SGS. Additional funding for LA-ICPMS at the University of Arizona Laserchron Center was provided by a GSA Graduate Student Research Grant. The SGS in southwest Utah is one of the largest terrestrial landslides on the planet. The slide is more than 1,300 km³ in volume (andesitic lava flows, lahars and regional ash- flow tuffs) and covers an area of ~2,500 km². This mega-scale gravity slide represents collapse of a large portion of the Marysvale volcanic field, coeval with Oligocene-Miocene igneous activity.

The study area contains portions of three different structural segments of the SGS: (1) a bedding plane segment, (~55 km long), typically located near the top of the volcaniclastic Brian Head Formation, (2) a ramp segment, ~1–2 km wide, that cuts obliquely upward across bedding, and (3) a former land surface segment (i.e., distal runout area) along which upper-plate rocks moved southward more than 32 km over the erosional land surface. The lower portion of the SGS contains a 50 to 100-meter-thick, highly deformed basal zone with clastic dikes and pseudotachylytes (injectites and frictional melts) beneath a relatively undeformed highly welded ash-flow tuff. The ash-flow tuff appears to have acted as a fluid barrier and concentrated deformation below it during catastrophic movement. Where the ash-flow tuff is absent, deformation continues upward for 100s of meters. The base of the slide consists of a basal layer of matrix supported conglomerate mix of volcanic and sedimentary rock fragments that originated from slide emplacement. This basal layer ranges from millimeters to meters in thickness and injected into the upper plate as clastic dikes. Two samples of the basal layer were collected for detrital zircon dating to determine a maximum depositional age (MDA) of the slide. The basal layer at Rock Creek has an MDA of 25.30 ± 0.41 Ma while at South Prospect Creek it has an MDA of 25.73 ± 0.33 Ma.