ANISOTROPY OF MAGNETIC SUSCEPTIBILITY (AMS) AND PALEOMAGNETIC ANALYSIS OF A MID-CRUSTAL PLUTONIC SUITE: A RECORD OF EXTENSION AND DOMING IN THE FOOTWALL OF THE PIONEER CORE COMPLEX

The Pioneer Core Complex (PCC) of south-central Idaho consists of WNW-directed brittle-ductile detachment that separates an un-metamorphosed hanging wall from a footwall of igneous and metamorphic rocks. The footwall comprises a lower plate of high-grade Precambrian rocks that define an elongate NNW-trending dome and a middle plate of metasedimentary rocks on the west flank. Eocene (~47-50 Ma) granitoids of the Pioneer intrusive suite (PIS) surround the dome forming a large stock-like body on the east and a sheet separating the middle and lower plates on the west flank. Foliations in the PIS wrap around and parallel the dome. It has been suggested that the western PIS sheet was intruded along an active SW-side-down shear zone juxtaposing the middle and lower plates. However, no lineation data from the PIS have been reported and SW-NE extension is orthogonal to upper crustal extension directions reported for the upper plate. We are testing this shear-zone hypothesis as well as models for mid-crustal dome formation through an investigation of the strains recorded by the PIS. Strain accumulation was documented by mapping of strain fabrics, contact relationships, and dike orientations, as well as through anisotropy of magnetic susceptibility (AMS) analysis.

Mapping shows that most of the PIS, including the contact regions has a homogeneous magmatic foliation with little evidence of significant solid-state strain. Preliminary AMS work show that on the NW flank of the dome, the maximum susceptibility directions (K₁) plunge shallowly to the WNW or NW, approximately parallel to the direction of motion on the detachment. On the NE flank of the dome, K₁ directions plunge gently NNW or SSE and are parallel to macroscopic lineations developed in the adjacent gneisses. These data suggest the western PIS was emplaced into a WNW-directed shear zone and that, overall, the PIS records the last increments of strain recorded in the adjacent metamorphic rocks. AMS cores have been collected from the other parts of the dome and are pending analysis; these additional data will allow us to more completely characterize the strains associated with mid-crustal extensional and doming processes. In addition to AMS analysis, paleomagnetic directions are also being analyzed around the dome to determine the timing and thermal state during doming.