The South Fork slide (SFS) is exposed over an area of 800 km2 in northwest Wyoming, and its evolution is connectedin space and time to the adjacent and overlying 48.87 Ma Heart Mountain slide (HMS). The youngest rocks deformedas part of the SFS are the Eocene Willwood Formation sands and mudstones, which have a U/Pb detrital zirconyoungest depositional age (TuffZirc calculation) of 51.80 11.04/21.08 Ma as well as a spectrum of older zircons thatwere eroded from the Sevier Highlands to the west and the Archean Beartooth uplift to the north (np379 U-Pb zirconages). The SFS is overlain by allochthonous Paleozoic carbonate and Eocene volcanic rocks of the HMS. The SFSdeformation involves shallowly plunging thin-skinned folds that trend northeast-southwest, where the slip surfacecould have been the top of the Jurassic Gypsum Springs Formation. Detailed mapping reveals the absence of anycleavage or joints and a plethora of minor structures unreported in any thin-skinned belt as well as the fact that nearlya third of the exposed allochthonous sediments are overturned. Calcite in Sundance Formation limestones is mechanicallytwinned, and the resultant strain analysis reveals that the predetachment Sevier layer-parallel shorteningstrain axes (31/31 samples; n p 1231 twins) are now in a random orientation. Synemplacement calcite veins (3207,907; n p 2) record a horizontal shortening strain parallel to the veins. Neither calcite strain data set has any record ofa strain overprint (low negative expected values). Rare slip indicators in the basal Sundance suggest northwestsoutheastmotion. Poorly constrained cross sections and palinspastic restorations indicate 125 km of shortening (anduncertain transport distances), and the above evidence suggests that the SFS formed as an Eocene landslide slightlyearlier than the HMS. The SFS, then, represents the largest single allochthon (terrestrial or marine) that maintainedits internal stratigraphy and did not disaggregate while in motion.
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