Does the Phillips Valley Fault Rupture with the Teton Fault?

Session: Cryptic Faults: Assessing Seismic Hazard on Slow Slipping, Blind or Distributed Fault Systems [Poster]
Type: Poster
Date: 4/28/2020
Time: 08:00 AM
Room: Ballroom
Description: 

The Phillips Valley fault (PVF) is an east-dipping normal fault at the southern end of the Teton Range in northwestern Wyoming. The fault vertically offsets glacial and postglacial sediments up to ~5 m. Scarps along the PVF delineate an arcuate north-to-northeast surface trace that extends ~10 km between Teton Pass and the mouth of Phillips Canyon, where it intersects the southern section of the ~70-km long Teton fault. At Phillips Canyon, the PVF branches from the Teton fault and continues as a subparallel trace ~4 km to the west. The paleoseismic history of the PVF is unconstrained, so questions about the rupture timing, kinematic relation to the Teton fault and accommodation of slip during Teton fault ruptures remain unanswered. To determine the paleoseismic history of the PVF, we hand-excavated two trenches near Ski Lake where faulting is concentrated across a single ~3 m high scarp. The trenches were dug ~60 m apart on a deglacial surface within an undated set of moraines that we presume are of Pinedale age (12–20 ka). The northern trench exposed gravelly silt that has been vertically offset ~1.5–3 m across two narrow east-dipping shear zones. We interpret two to three events based on structural relations and scarp-derived colluvial wedges on the hanging wall. The southern trench was excavated adjacent to a shallow pond dammed by the innermost moraine. Although the main fault was not exposed in this trench, thin folded beds of lacustrine organic-rich silt and clay were exposed. The beds are warped-upward against an east-dipping shear zone and buried by a thick package of silty colluvium. We interpret folding of these sediments and subsequent burial by scarp-derived colluvium to have resulted from the most-recent rupture event. Bayesian analysis of pending radiocarbon (n=20) and luminescence (n=11) ages will provide the first constraints on timing of PVF ruptures and permit comparisons against the paleoseismic record of the Teton fault.

Presenting Author: Mark Zellman

Authors