Paper No. 12
Presentation Time: 11:20 AM


VOGL, James J., Department of Geological Sciences, University of Florida, Gainesville, FL 32611, RODGERS, David W., Department of Geosciences, Idaho State University, 921 South 8th Ave., Box 8072, Pocatello, ID 83209 and LINK, Paul K., Department of Geosciences, Idaho State University, Pocatello, ID 83209,

The Pioneer core complex (PCC) in central Idaho is bound by the NW-directed Wildhorse detachment. The footwall consists of a Middle plate (MP), a Lower plate (LP), and a 49-47 Ma intrusive suite. The MP comprises Ordovician quartzite, marble, calc-silicate, and pelitic schist cut by thrust faults and top-NW oblique-normal faults. The MP also contains syn-extensional NW-trending folds and spatially variable ductile extensional strain. The LP consists of ~2.6 Ga and ~695 Ma orthogneiss, as well as Meso-/Neoproterozoic paragneiss, all of which were extensively strained and melted in the Early Eocene and folded into a NNW-trending dome. Age relations suggest that the Archean gneiss was thrust over younger paragneiss prior to Eocene extension.

This pre-extensional geometry has influenced the extensional kinematic development of the PCC in several ways. For example, the MP/LP boundary provided a horizon into which an early detachment soled, as evidenced by large sections of the MP that have been structurally omitted along this boundary, compared to the LP stratigraphy, which remained intact. Significant motion on this early detachment pre-dated voluminous magmatism, suggesting lithologic, rather than magmatic/thermal, control on development. Additionally, lineation and fold orientations in paragneisses beneath the Archean orthogneiss have distinctly different orientations from the NW-trending lineations in overlying units. This relationship may indicate weak inflow of the lowermost units that was decoupled from the stronger overlying orthogneiss and/or superposition of vertical thinning on structures with different orientation beneath the thrust at the onset of extension.

On a regional scale, the PCC coincides spatially with the two largest thrust faults in the region, as well as large-scale folding. Combined with the recognition of several thrust faults in the PCC footwall, this suggests that the core complex developed in a region of localized crustal thickening.

In contrast to several core complexes in the region, the PCC was not reactivated during Miocene Basin-and Range extension despite its favorable location given the regular spacing of major normal faults to the east. Instead, our (U-Th)/He data show that Miocene faults die out toward the PCC, indicating that it resisted later extension.