FAULT REACTIVATION AND POLYPHASE DEFORMATION: PIONEER FAULT, SOUTH-CENTRAL IDAHO

Mineralization within the Pioneer fault damage zone provides a record of deformation history, and an opportunity to better understand deformation mechanisms. In south-central Idaho, a segment of the Pioneer fault exposed at Little Fall Creek has accommodated large-magnitude (Mesozoic) shortening overprinted by Paleogene extension. The resulting 30 m thick fault damage zone displays quartz vein mineralization, graphite concentration on slip surfaces, polyphase contractional and extensional microstructures and micro- to outcrop-scale corrugated, polished slip surfaces. The gently west dipping (207/14) fault separates Ordovician argillite in the hanging wall from Mississippian argillite and quartzite in the footwall, and is mapped as a significant regional thrust fault that likely accommodated east-northeast directed Sevier shortening. However, slip surfaces within the fault zone document top-to-the-west translation with a mean slip vector 15/272, consistent with extensional unroofing of the Pioneer Mountains metamorphic core complex.

The fault damage zone varies from proto- to ultra-cataclasite and provides evidence for overprinting of contractional strain fabrics by extensional fabrics. The fault damage zone is characterized by multiple anastomosing slip-surfaces which are corrugated from outcrop (10’s of meters) to millimeter-scale indicating a history of slip surface interactions, fault growth, and reactivation. Early deformation features include mm-scale euhedral quartz veins, graphite foliation, graphitic stylolites, SC foliations, and ptygmatic folds which are consistent with shortening. We observe that the ptygmatic folds and SC foliation are cut by open mode syntaxial quartz veins. These are overprinted by later stage features including open mode quartz veins, graphite shear fractures, mica fish, elongate quartz crystals in open mode veins, and slip surfaces coated with graphite and amorphous quartz. Graphite is concentrated throughout the fault damage zone as stylolites and is associated with amorphous-quartz coated slip-surfaces. The later stage features document dilation and west-directed extension.