Is the Antelope Flats Fault an Antithetic Rupture of the Teton Fault?

Date: 4/24/2019

Time: 06:00 PM

Room: Fifth Avenue

The Antelope Flats fault is a west-dipping normal fault zone, antithetic to the central section of the Teton fault in eastern Jackson Hole Valley, Wyoming. Fault scarps along the Antelope Flats fault are ~1–2 m high––far smaller than those marking the Teton fault––and form a discontinuous, ~6-km-long zone in an undated glacial outwash fan associated with the earliest phase of Pinedale glaciation (Pd-1; ~21–18 ka). Although the prehistoric rupture history and displacement of the Antelope Flats fault remain unconstrained, we hypothesize that because of the limited total length and height of scarps along the fault, it only ruptures to the surface in association with large, surface-faulting earthquakes on the Teton fault, located 9-km to the west. To determine the paleoseismic history of the fault as well as its kinematic relation to the Teton fault, we hand-excavated two trenches about 100 m apart across a <2-m high, west-facing fault scarp. The trenches exposed coarse gravel and cobbles that we interpret as sourced from the latest Pleistocene glacial outwash. These latest Pleistocene sediments have been vertically displaced by a single steeply west-dipping zone of shearing and are overlain by a <=0.6-m-thick deposit of scarp-derived colluvium in the hanging wall. These relations suggest that only a single postglacial surface rupture has occurred along the Antelope Flats fault. Pending radiocarbon (n=9) and luminescence (n=12) ages for the outwash and scarp colluvium will establish the first direct ages for the Pd-1 glacial outwash fan, constrain the timing of Antelope Flats fault rupture, and permit comparisons of paleoseismic records for the Teton and Antelope Flats faults.

Presenting Author: Glenn D. Thackray

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