TIMING AND KINEMATIC SIGNIFICANCE OF FAULTS WITHIN THE FOOTWALL OF THE PIONEER METAMORPHIC CORE COMPLEX

The Pioneer metamorphic core complex (Idaho) formed during regional NW-SE Eocene extension. The curviplanar top-WNW Wildhorse detachment juxtaposes unmetamorphed rocks with igneous and metamorphic rocks of the footwall. Faults also occur in the southwest part of the footwall where NW-striking, SW-dipping faults were mapped by Dover (1983). These faults were all interpreted as thrust faults by Dover, however, on the basis of younger-on-older relationships, Silverberg (1990) suggested some of the faults are likely normal faults. The displacement, kinematics, and timing of motion remain unclear, but are critical to understanding the kinematic and mechanical evolution of both the PMCC and the hangingwall.

The footwall faults are parallel to older-on-younger thrusts indicating they have not been significantly rotated. Given that these faults strike parallel to the regional extension direction and to the approximate slip direction on the Wildhorse detachment, we envision two models for their development. (1) The faults are dip-slip faults that predate the dominant top-WNW motion indicating temporal orthogonal changes in the regional extension direction. (2) The faults are oblique-dextral faults with top-WNW motion, similar to shallowly dipping dextral faults mapped in the hangingwall immediately to the west and northwest; the footwall faults may be in part steeper, deeper equivalents of the mechanically enigmatic shallow faults in the hangingwall. Model (2) would suggest that the Pioneer metamorphic core complex formed at a 3D boundary where highly oblique dextral-normal faults roll over into NE-trending dip-slip faults.

We present results of new mapping and kinematic analysis that addresses the kinematics of these faults. Timing of motion is addressed by assessing the relative timing between development of fault fabrics and contact metamorphism produced by a ~48.5 Ma granodiorite at slightly deeper levels. We will further constrain the age of the faults by U-Pb dating of dikes showing specific timing relationships with fault motion.