The trans-Challis fault system (TCFS) is a new structural province in central Idaho mapped during the Challis CUSMAP program conducted by the U.S. Geological Survey. The TCFS contains numerous northeast-trending faults, several eruptive centers for the Challis Volcanics, and many precious metal deposits. Major movement occurred in the fault system during Eocene extension that affected the area from the Snake River Plain north into British Columbia.

The eastern Snake River Plain is the southern boundary of Eocene extension, and in Idaho the Lewis and Clark line is the northern boundary. The eastern Snake River Plain was “welded” together by passage of the plain over the Yellowstone hotspot. Younger Basin and Range faults were then able to extend across the plain and now terminate at the next major Eocene crustal break north of the plain, the TCFS. The TCFS is an extensional feature equal in importance to the Lewis and Clark zone and the eastern Snake River Plain and should be considered in Tertiary reconstructions for the Pacific Northwest.

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