TIMING AND KINEMATICS OF AN EOCENE-MIOCENE SHEAR ZONE: GROUSE CREEK MOUNTAINS, UTAH

New geologic and ⁴⁰Ar/³⁹Ar thermochronometric studies clarify the relationship between multiple west vergent shear zones, detachment faults, and a late Oligocene pluton along the west side of the metamorphic core complex in the Grouse Creek Mountains of northwestern Utah.

Two ductile fabrics of Tertiary age occur in wall and roof rocks of the 25.3 Ma Red Butte stocks within the footwall of the Middle detachment fault. The older exhibits stretching lineations and fold hingelines trending 305°, records top-to-305 shear, is cut by the Red Butte stocks, and becomes stronger upwards toward the detachment. The second fabric exhibits a foliation parallel to the first and a 275°-trending stretching lineation, records top-to-west shear, deforms the Red Butte stocks and associated dikes and overprints the older fabric within 50 meters beneath the detachment.

The first period of motion occurred in Eocene time, based on muscovite, biotite, and hornblende ⁴⁰Ar/³⁹Ar ages within and beneath the shear zone of 47 to 37 Ma. The second fabric is imparted on the 25.3 Ma stocks, and exhibits upper greenschist to lower amphibolite facies conditions of shearing as constrained by plasticity of feldspars and activity of prism slip in quartz as shown by lattice-preferred orientation study. Muscovite (dikes) and biotite (stocks) ⁴⁰Ar/³⁹Ar ages of 20.86 ± 0.21 Ma and 21.05 ± 0.15 Ma, respectively, provide a lower limit on the age of shearing. Multidomain modeling of K-feldspar, combined with mica ages and deformation mechanism study suggest a rapid cooling event (100’sºC/m.y.) from ~25 to ~23 Ma interpreted to represent post-intrusive cooling and partial extensional denudation of the core complex. We interpret the second shearing event as initiating during or shortly after intrusion of the stocks. K-feldspar models suggest a third period of rapid cooling initiating at 19 –17 Ma, interpreted to represent motion along the Middle detachment or a structurally higher detachment that places Permian to Miocene rocks on Pennsylvanian and older rocks. These data suggest that the Red Butte stocks intruded into the shear zone between discrete periods of extensional movement.