NEWLY MAPPED EOCENE STRIKE-SLIP FAULTS SOUTH OF THE BOEHLS BUTTE ANORTHOSITE, NORTHERN IDAHO

Recent mapping has identified two west-northwest striking faults 60 km in length south of the Boehls Butte anorthosite between the St. Joe and Clearwater rivers in northern Idaho. These faults separate rocks of the Belt Supergroup on the north from the Syringa metamorphic sequence on the south. To the west the faults connect to the north-south White Rock fault, a down-to-the-west normal fault. To the east they merge with the Kelly Forks fault. Mylonite is more common in the west and chlorite breccia in the east, indicating increasing paleodepth to the west. Sheared plutons along the faults contain subhorizontal stretching lineations and have s-c fabrics indicating right-lateral motion. Sheath folds are locally well developed in the metasedimentary rocks.

Mylonitic fabric is developed where the northern of the two faults intersects the northern boundary of the Beaver Creek pluton. That pluton has a concordant U-Pb zircon age of 46.4 ± 0.5 Ma, indicating that at least some of the motion along this fault was Eocene in age. The concept of a fault zone that transferred extensional motion from the northern end of the Bitterroot core complex west to the southern end of the Priest River core complex has been suggested by previous workers. It is likely these two faults are part of this extensional transfer system.