FROM BRITTLE TO DUCTILE: THE RELATIONSHIP BETWEEN FOOTWALL FAULTS, MAGMA EMPLACEMENT AND ITS ASSOCIATED HEAT

The Pioneer metamorphic core complex in Idaho formed during Eocene NW-SE extension. The Wildhorse detachment juxtaposes metamorphic footwall, composed of the lower and middle plates, and the unmetamorphosed hangingwall. Extension prior to initiation of the Wildhorse detachment was accommodated by a footwall fault system comprised of three faults, as well as a high temperature ductile strain zone; these features record an evolving rheology as a result of intrusion of a ~48.6 Ma granodiorite sheet.

The middle plate-lower plate (MP/LP) detachment omits section and has mylonitic blocks of quartzite in undeformed granodiorite suggesting that intrusion followed fault formation. The MP/LP detachment is intruded and mostly-concealed by the granodiorite body; heat from intrusion weakened the overlying metasedimentary section leading to high-temperature top-NW strain immediately above the solidified stronger granodiorite.

The footwall fault system also includes the Hyndman and Paymaster faults of the middle plate which are Eocene in age and share similar geometries to the MP/LP detachment. The Hyndman fault has a discrete character and is in closest proximity to the granodiorite. The discrete appearance of the fault implies brittle deformation, yet this fault records top-WNW motion with subhorizontal WNW-trending sillimanite and other high-temperature deformation features facilitated by contact metamorphism. Post-kinematic muscovite replacing andalusite, along with the lack of brittle deformation indicated deformation ceased before cooling. More distant from the granodiorite, the Paymaster fault also has a discrete character. Brittle faults zone features are annealed and overgrown by hornblende and other high-temperature minerals associated with contact metamorphism. Thus, deformation in this zone ceased by the time of granodiorite emplacement. The Hyndman and Paymaster faults likely sole into the MP/LP detachment.

Our observations suggest that a brittle detachment system developed prior to intrusion of a granodiorite sheet at ~48.6 Ma into the main detachment and that heat from the intrusion led to the evolution of ductile strain zones. Thus, the area records an evolution from brittle to ductile deformation, prior to initiation of the Wildhorse detachment after ~47 Ma.