EOCENE EXTENSION IN NORTHWESTERN UTAH-SOUTHERN IDAHO: EARLY MOTION ON THE POLYPHASE MIDDLE MOUNTAIN SHEAR ZONE

The west side of the Raft River-Grouse Creek-Albion (RGA) metamorphic core complex of northwestern Utah and southern Idaho displays mylonitic rocks and stacked detachment faults with top-to-the-west shear that clearly record Late Oligocene-Early Miocene and Middle Miocene extension. Older Eocene extension, perhaps of larger magnitude than combined Oligo-Miocene extension, is more cryptic. We focus on two areas of detailed study surrounding Late Oligocene plutons - the central Grouse Creek Mountains (Red Butte area) and Vipont Mountain - and summarize a wide geographic distribution of 40Ar/39Ar thermochronologic data to demonstrate the Eocene ancestry for the shear zone.

Two ductile fabrics of Tertiary age occur in wall and roof rocks of the Late Oligocene Red Butte and Vipont Mountain plutons. The older and deeper fabric exhibits stretching lineation and fold hingelines trending 305° (Red Butte) and 297° (Vipont), records top-to-W-NW shear, and is cut by the Red Butte stock and its associated dikes. The second and higher fabric exhibits a foliation parallel to the first, a stretching lineation trending 275° (Red Butte) and 273° (Vipont), top-to-W shear, and deforms the Red Butte and Vipont plutons.

A Middle to Late Eocene age for early extensional shear is based on: (1) ⁴⁰Ar/³⁹Ar muscovite cooling ages of 40-50 Ma occur in a N-S, 35-km long swath of footwall rocks in the western Raft River and eastern Grouse Creek Mountains, and 50 km to the north in the northern Albion Mountains (Mt. Harrison). (2) Rocks within the shear zone and away from Oligocene plutons yield discordant hornblende age spectra of ~45 Ma and muscovite ages varying from 37 to 42 Ma. (3) Annealed mylonite in a roof "keel" between Oligocene plutons (Muddy Creek) yields a plateau age of 26.9 ± 0.1 Ma on muscovite, demonstrating pre-intrusion shear.

Eocene extension in the RGA closely followed the termination of shortening, and provides a link in the documented southward sweep in the onset of extension across the Snake River Plain and in constraining regional models of post-orogenic, synconvergent extension in the western US.