LATE CRETACEOUS OROGENESIS ACROSS THE IDAHO BATHOLITH OF NORTH-CENTRAL IDAHO

The Late Cretaceous Atlanta lobe of the Idaho batholith has generally been characterized to include an early deformed phase restricted to a zone along the Salmon River suture zone on the western margin and scattered roof pendants, and a later voluminous undeformed main-phase which intruded most of central Idaho, and is thus considered post-tectonic. However, recent fieldwork in the Elk City area of north-central Idaho has identified a series of structural zones that deform main-phase Idaho batholith plutons, and thus questions this basic tenet. This region consists of Mesoproterozoic metasedimentary rocks and augen gneiss that are folded about shallowly north- and south-plunging fold axes and intruded by granodiorite of the Idaho batholith yielding a U-Pb zircon age of ~86 Ma. Our recent mapping has delineated the Orogrande shear zone and a series of other km-wide north-striking deformation zones with the following features: (1) tight to isoclinal folding that includes dikes and sills of the Idaho batholith, (2) ductile contractional shear fabrics with kinematic indicators that suggest a component of dextral strike-slip shear that are developed in both main-phase batholith and older rocks; (3) later phases of undeformed Idaho batholith and Eocene intrusive rocks that cross-cut earlier ductile structures; (4) reactivation by Miocene north-striking brittle faults; and (5) vein-gold mineralization. Age-control brackets deformation to between 86 Ma and the 80-67 Ma age range for the Atlanta lobe peraluminous suite.

The timing and geometry of these deformation zones are consistent with other dextral transpressional structural zones documented in central Idaho to the south, including the post 78 Ma, pre-Eocene Deadwood/Johnson Creek zone, and 95-77 Ma Sawtooth metamorphic complex. We propose that following 100-90 Ma dextral transpressional shear on the western Idaho shear zone, which overprints the suture zone in western Idaho, dextral transpression became more distributed across much of central Idaho. Although difficult to recognize because it is largely overprinted by younger intrusions, this late phase of deformation may have played an important role in orogenic crustal thickening beneath central Idaho in the Late Cretaceous.