PERVASIVE MID-CRUSTAL EXTENSIONAL STRAIN FOLLOWED BY RAPID LOCALIZATION: AN EXAMPLE FROM THE PIONEER CORE COMPLEX, IDAHO

The Pioneer core complex (PCC) in south-central Idaho consists of mid-crustal igneous/metamorphic rocks juxtaposed with upper crustal rocks along a WNW-directed brittle-ductile detachment. Current mapping provides new insight into the nature and distribution of extensional strain and the history of strain localization in the detachment. The west half of the core complex footwall consists of a lower plate of Precambrian gneisses and a middle plate of largely lower Paleozoic metasedimentary rocks. The middle and lower plates are separated by a sheet of Eocene granodiorite of the Pioneer intrusive suite (PIS). In the northwest part of the footwall the middle plate comprises a highly-strained section with a pervasive WNW lineation parallel to motion on the detachment. Eocene dikes show a range of relationships with the strain fabric ranging from extensively folded, transposed and boudinaged, to sharply cross-cutting with only minor top-WNW deflections. The PIS sheet is homogeneously foliated with a magmatic/high-T strain fabric and displays top-WNW shear-sense indicators. The lower plate mapped thus far displays melt-filled boudins and isoclinally folded melt injections that are locally strung out into isolated pods. These features are often associated with WNW stretching and where present, shear-sense indicators indicate a top-WNW shear sense. Many outcrops, however, also display late-stage injections that cross-cut the strain fabric. Although much mapping remains, taken together, these observations suggest:(1) WNW extensional strain was pervasive during emplacement of the ca. 48 Ma PIS, affecting perhaps all exposed levels. (2) Much of the extensional strain accumulated during intrusion of the PIS at mid-crustal levels (3) Pervasive extensional strain ended abruptly during the waning stages of intrusion and strain was rapidly localized in the detachment. These conclusions conflict with previous models for the PCC in terms of timing, directions and distribution of extensional strain. We suggest that pervasive WNW-ESE extensional strain in the middle crust was facilitated by intrusion-related heat and that waning of the heat source led to abrupt strain localization recorded by mylonites and brittle fault rocks at the top of the section. We are currently dating the intrusives to provide precise constraints on the duration and rates of strain accumulation.