EOCENE STRUCTURAL EVOLUTION OF THE BOEHLS BUTTE ANORTHOSITE AND CLEARWATER CORE COMPLEX, NORTH CENTRAL IDAHO: A BASEMENT-INVOLVED EXTENSIONAL STRIKE-SLIP RELAY

New geologic mapping in north central Idaho has identified the structure responsible for forming the Clearwater core complex and clarified its age. The Clearwater core complex is a metamorphic complex of anorthosite and upper amphibolite-facies gneisses that locally contain all three aluminosilicates. The core complex lies in an extensional pull-apart associated with the northwest trending dextral Lewis and Clark fault zone. The eastern boundary of the core complex is marked by the north-trending, east-dipping Jug Rock mylonite zone, which separates the anorthosite and associated gneisses from less metamorphosed Belt Supergroup strata to the east. Footwall gneisses record rapid decompression (kyanite pseudomorphed by andalusite) and metamorphic thermobarometry is consistent with a 6-kilobar drop in metamorphic pressure across the zone. The mylonite zone is marked by a 0.5-km wide zone of mylonitized gneiss that contains bands of ultramylonite and unannealed S-C fabrics. Kinematic indicators show a down-dip, top-to-the-east sense of shear. The mylonitic foliation is locally defined by chlorite and muscovite and augen of kyanite are pseudomorphed by andalusite. Together, these data demonstrate that the Jug Rock mylonite zone is an extensional shear zone. The base of the mylonite zone is defined by a mylonitized breccia composed of plagioclase and anorthosite clasts surrounded by mylonitic schist that record shearing of the intrusive contact between the anorthosite and surrounding country rocks. The top of the mylonite zone is a gradational boundary; no chloritic breccia zone has been observed. The location of the mylonite zone was controlled by the mechanical boundary between the stiff anorthosite and more incompetent gneisses that overly it. The northern boundary of the core complex is defined by a northwest trending zone of dextral strike-slip faults that deform the Middle Eocene Roundtop pluton. We infer that Middle Eocene dextral movement along the Lewis and Clark fault zone and extension along the Jug Rock mylonite zone are responsible for tectonically unroofing the core complex and exposing the Boehls Butte anorthosite and related metasediments.