Abstract

The South Fork Fault System (SFFS) extends more than 35 km along the valley of the South Fork of the Shoshone River, southwest of Cody, Wyoming. Tear faults, a triangle zone, and flat-ramp geometries mark the leading edge of the system. Transport was southeast, down a shallow slope during the early to middle Eocene, approximately coeval with the Heart Mountain Fault System (HMFS). Detachment of the SFFS is in Lower Jurassic strata, rich in gypsum–anhydrite, overlain by about 1250 m of Jurassic through Tertiary sedimentary and volcanic rocks. Movement between 5 km and 10 km to the southeast spread the allochthonous mass over an area exceeding 1400 km2. A newly discovered break-away fault and an area of Eocene tectonic denudation mark the upper (northwestern) part of the system. The SFFS is interpreted to have developed in a style similar to the HMFS. The exposed, denuded surface was buried by additional Eocene-age volcanic rocks soon after slip. Seismic data and well control indicate that the Castle Tear Fault is a high-angle tear in the SFFS that merges at depth with the detachment in Jurassic strata. Structural relationships suggest that the Castle Tear Fault folded the emplaced HMFS during transport of the SFFS, indicating the SFFS is younger than the HMFS. Catastrophic rear-loading during emplacement of HMFS may have initiated subsequent movement on the SFFS, with dehydration processes trapping water in a nearly frictionless, anhydrite–water slurry within the Jurassic Gypsum Spring Formation. Rapid movement along this boundary caused “piggy-back”-style spreading of several large carbonate blocks of the HMFS as the SFFS slid to the southeast. Tear faults in the developing SFFS created variable structural geometries within the slide mass and caused transport distance to vary between 5 km and 10 km.

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