KINEMATIC HISTORY OF THE WILDHORSE DETACHMENT FAULT, PIONEER MOUNTAINS, SOUTH-CENTRAL IDAHO

New mapping and structural analysis of the western Wildhorse Detachment Fault, south-central Idaho, was completed to document its kinematic history and relation to underlying ductile strain. The Eocene, down-to-the-northwest fault places previously folded upper Paleozoic strata in the upper plate against metamorphosed lower Paleozoic rocks and Eocene intrusives in the lower plate of the Pioneer Mountains metamorphic core complex.

The WDF zone is 5-10 m thick, most strongly developed in lower plate rocks, and characterized by breccia, gouge, polished fault surfaces, and striations. The WDF has an arcuate map trace over a distance of 20 km that reflects a progressive change in strike, from 344° in the north to 325° in the west to 290° in the southwest. Both the change in strike and an accompanying change in dip, from 24° in the north to 74° in the southwest, reflect a fault geometry shaped like the southwest limb of a gently northwest plunging antiform. Striations consistently plunge northwest and trend 295° ± 15°, except to the southwest (East Fork Wood River) where they trend 270°±10°. Lower plate rocks immediately below the WDF contain a stretching lineation oriented parallel to these striations. Upper plate strata are folded into an east-vergent overturned syncline whose hinge plunges gently southeast, except south of East Fork where the hinge plunges more steeply (45°) southeast. The southward change in striation trend, lineation trend, and fold plunge provide evidence that the southwest WDF and overlying rocks are tilted southward. This southward tilting occurs immediately west of the White Mountain fault, a normal fault that cuts lower plate rocks and the WDF but, according to our new mapping, does not project southwest into upper plate strata. Southward tilting appears to be an on-strike continuation of down-to-the-south slip accommodated by the White Mountain Fault, and is interpreted to reflect late-stage doming of the core complex.

Ductile foliations and lineations are strongly developed immediately below the northwestern WDF but noticeably weaker, and more dispersed, immediately beneath the southwestern WDF. One interpretation is that the WDF cuts down section to the northwest through the older ductile shear zone.