EOCENE TO MIOCENE HISTORY OF THE MIDDLE MOUNTAIN SHEAR ZONE SYSTEM

New mapping and geochronology, combined with prior studies, clarifies the N-S continuity of the Cenozoic Middle Mountain shear zone, the relationship between stacked detachment faults that overlie the shear zone, and the polyphase extensional history. Mapping has established that the Middle Mountain shear zone is continuous from Ingham Pass in the southern Grouse Creek Mountains, Utah, to the north end of Middle Mountain, Idaho. The ductile shear zone experienced two periods of down-to-WNW ductile extension manifest in two orientations of stretching lineation: 295-305° (Eocene) and 265-275° (Oligo-Miocene). Motion on the shear zones brackets intrusion of 25-29 Ma granites, which exhibit the Oligo-Miocene fabrics along their western flanks. Muscovite and biotite 40Ar/39Ar cooling ages support the two-stage history of ductile extension as a N-S belt of 40-47 Ma cooling ages occurs in the immediate footwall of the shear zone, east of a N-S belt of 20-22 Ma cooling ages within the shear zone.

Two detachment faults cap the ductile shear zone: the Middle detachment, which juxtaposes the Chainman-Diamond Peak and Oquirrh Formation over Ordovician and older rocks; and a fault, which we term the Twin Peaks detachment, that places a hanging wall of dominantly Tertiary sedimentary and volcanic rocks, but locally Permian and Triassic rocks, over the Pennsylvanian-Permian Oquirrh Formation and older rocks. South of Ingham Pass, the Twin Peaks detachment lies above rocks of the hanging wall to the Middle detachment. North of Ingham Pass, with few exceptions, the Twin Peaks detachment either reactivates the Middle detachment or it cuts into variable structural levels of the footwall of the Middle detachment, including the Archean basement. The Twin Peak detachment carries rhyolite and rhyolite porphyry associated with the Twin Peaks rhyolite that we have dated at 12.97 ± 0.10 Ma by 40Ar/39Ar on sanidine. Thus, the Middle Mountain shear zone system represents a polyphase top-to-west extensional system that was active episodically for ~30 m.y. during the Cenozoic.