| The Middle Mountain shear zone (MMSZ) and the brittle Upper detachment, two west-dipping normal fault systems, and the 25.3 Ma Red Butte stocks are exposed in a metamorphic core complex in northwest Utah, in the central Grouse Creek Mountains. 40Ar/39Ar thermochronology and kinematic studies indicate two periods of motion along the MMSZ in Eocene and Late Oligocene to Early Miocene time and motion along the sub-parallel Upper detachment in Middle Miocene time. Eocene fabrics record top-to-the-305 noncoaxial shear and are cut by the Red Butte stock and associated leucocratic dikes and sills. The Oligo-Miocene fabric exhibits a foliation parallel to the first, records top-to-the-275 noncoaxial shear, deforms the Red Butte stocks and overprints the Eocene fabric within 50 meters beneath the Middle detachment fault. Improvements are proposed to an existing model depicting roughly simultaneous bivergent exhumation and support is given to ‘thermally triggered’ models of extension for metamorphic core complexes. |
