A STRAIN GRADIENT WITHIN THE FOOTWALL OF THE PIONEER CORE COMPLEX, IDAHO: COMPLEX OVERPRINTING OF CRETACEOUS STRUCTURES BY EOCENE EXTENSIONAL STRAIN AND POSSIBLE SYN-EXTENSIONAL SHORTENING

New field mapping and U-Pb geochronology in the Pioneer core complex (PCC; central Idaho) allows us to delineate the distribution of extensional footwall strain and to identify the complex overprinting relationships between Cretaceous shortening-related structures and Eocene extensional strain. The PCC consists of a WNW-directed curviplanar detachment and a footwall comprising a Middle plate of metasedimentary rocks, a Lower plate of high-grade metamorphic rocks, and an intervening sheet of granodiorite. The Lower plate and ~48 Ma granodiorite sheet form an elongate asymmetric dome. Our focus is on the Middle plate on the west flank of the dome, which contains quartzite, marble, calc-silicate, and pelite that generally form a SW-dipping panel. These units are duplicated by thrust faults of probable Cretaceous age and folded into asymmetric, overturned, NE-vergent anticlines of uncertain age.

Unlike many core complexes, the PCC exposes a downward transition from relatively unstrained rocks near the detachment to pervasively strained rocks. The strain gradient is marked by a downward increase in the development of a WNW-trending lineation, tight/isoclinal folding, and transposition of Eocene dikes and melt. The strain intensity appears to be controlled by proximity to the granodiorite and cuts obliquely across unit contacts. This is demonstrated by a continuous band of Clayton Mine quartzite that displays isoclinal folds, transposed dikes, and a strong lineation, but preserves extensive cross-bedding as the unit cuts obliquely away from the granodiorite. In some areas, multiple units are missing along an apparent extensional fault that is locally concordant with the high-strain extensional fabric.

The age of the NE-vergent folds has been considered to be Cretaceous. However, the general coincidence between the tightness of the folds and the intensity of the development of the WNW lineation raises doubts about this interpretation. Furthermore, dikes are locally involved in asymmetric folding similar in style to that throughout the area. These dikes are likely Eocene in age but are currently being dated. If the folds are Eocene, it suggests a component of horizontal shortening during NW-directed extension and also provides a mechanism to create the larger scale asymmetric dome that is overturned to the NE.