THE PIONEER CORE COMPLEX OF SOUTH-CENTRAL IDAHO: THREE-DIMENSIONAL STRAIN PATTERNS AND DOMING IN AN AREA OF LOCALIZED CONSTRICTION

The Eocene Pioneer core complex (PCC) in central Idaho is the most three-dimensional core complex in the northern U.S. and southern Canada region, being characterized by a curviplanar detachment and an elongate asymmetric footwall dome of high-grade rocks. Results of recent mapping and geochronology has led to a new model to explain the structural development of the PCC.

The deepest rocks of the footwall display an Eocene planar high-strain foliation that is folded into a 10km long, NNW-trending, doubly plunging, NE-vergent fold (Wildhorse dome). The dome is wrapped by an ~49-48 Ma granodiorite sheet that contours the dome, along with its magmatic fabric.

On the west flank of the dome, the granodiorite sheet is overlain by SW-dipping metasedimentary rocks of the Middle plate (MP). Eocene extensional strain is recorded in the MP by isoclinal folding and boudinage of Eocene dikes, foliation development, and WNW stretching lineations with a top-WNW shear sense. Eocene strain is limited to the basal part of the MP, except at the NW apex of the footwall, where the entire MP is highly attenuated.

WNW-trending NE-vergent folds developed at a variety of scales in the MP. WNW stretching parallel to fold hinges is shown by well-developed lineations that are spatially continuous with lineations that are demonstrably Eocene. The recognition of Eocene NE-vergent folds on the SW flank of the geometrically similar Wildhorse dome strongly suggests that the folds and dome formed from syn-extensional NE shortening. Thus, it appears that the MP/LP boundary and intervening granodiorite represent a top-NW mid-crustal shear zone that was folded into the the Wildhorse dome. As a result of tilting, normal-sense shear zones evolved into dextral strike-slip shear zones on the SW flank of the dome. This model requires clockwise vertical-axis rotation of at least part of the Wildhorse dome.

U-Pb geochronology indicates that ductile strain, plutonism, and folding/doming occurred between ~49 Ma and ~46 Ma when late dikes cut all features. After stiffening of the footwall, strain was localized into the curviplanar detachment fault.

The PCC formed in an area of localized constriction at a boundary between low-angle dextral strike-slip faults and NE-trending normal faults, both of which are kinematically similar to ductile fabrics in the footwall of the PCC.