Pluton Intrusion Styles, Roof Subsidence and Stoping, and Timing of Extensional Shear Zones in the City of Rocks National Reserve, Albion Mountains, Southern Idaho

The 29 Ma Almo Pluton in the southern Albion Mountains is one of three Oligocene plutons that intrude the western part of a metamorphic core complex that includes the Albion, Raft River, and Grouse Creek Mountains. These granite plutons intrude, and are involved in, mylonitic shear zones; as a result, they provide key timing information for extensional shear that in part created the complex. In the Albion Mountains, the Almo pluton intruded Archean granitoids and the base of unconformably overlying layered Neoproterozoic rocks, which presently form a north-trending arch about 8 km wide. Along the west side of the Albion Mountains and westward to Middle Mountain, structurally and stratigraphically higher rocks are mylonitic in the down-to-the-northwest Middle Mountain composite shear zone. A 7 km-long zone along the west margin of the Almo pluton contains common stoped blocks and sags of these mylonitic higher stratigraphic units. We studied these stoped blocks and sags to determine their origin and elucidate timing of shear zones. Several kinds of roof subsidence are preserved within and above the Almo pluton, including inclusions of widely varying sizes that we infer to represent stoped blocks, monoclinal flaps, and downfaulted sags similar to grabens. Stoped blocks typically consist of steeply dipping panels of strata, but in a few cases gently dipping strata steepen with distance from the roof. These relations indicate that the blocks peeled from the roof of the pluton. Supporting this inference is the observation that several flap-like monoclinal folds rim the pluton margin, representing sags of strata into the pluton, with the sagging strata apparently frozen in place before detaching and subsiding into the pluton. Graben subsidence along pairs of faults, which are partly intruded by granite dikes, also is observed. The gravity-driven roof subsidence may have been aided by dikes inflating lenses above roof rocks. Stoped blocks mostly consist of highly sheared rocks of units stratigraphically higher than the Elba Quartzite, despite the nearby roof of the pluton typically consisting of nearly horizontal, undeformed Elba Quartzite. In stoped blocks, the Elba is only slightly deformed but the higher units are strongly sheared and diked by undeformed granite, indicating that an older shear zone, only partly exposed in nearby roof rocks, was preserved in the foundered blocks. The sheared rocks probably represent a latest Eocene shear zone identified in the same stratigraphic units in the nearby Vipont Mountain area. The top-to-the-northwest extensional shear zone exhumed crust in the future location of the Almo pluton, perhaps producing the arch now occupied by the pluton. Where the pluton intruded this shear zone it preferentially stoped blocks of layered, sheared rocks in the zone. During the early Miocene (~20-25 Ma), rocks farther west were sheared again nearly parallel to shear directions in the Eocene, an event only locally expressed in the Albion Mountains and rarely observed as mylonitic fabrics in the Almo pluton. Plutonism in the southern Albion Mountains thus occurred between extensional shear events, contrary to relations in many metamorphic core complexes.