Deformed and undeformed dikes: tools to date extension in the Pioneer metamorphic core complex

Markstein 303

The deepest rocks of the Pioneer metamorphic core complex in Idaho record varying directions of crustal extension. In the lower section, the rocks indicate N-S to NE-SW extension. Above these rocks, the upper section records the regional NW extension consistent with the Wildhorse detachment fault. Understanding the timing of this deformation will help reveal the underlying processes responsible for the discrepancy in recorded extension direction. Perhaps the lower and upper sections represent varying deformation styles at different crustal levels in response to a single extension event. Alternatively, deformation could have occurred during two separate events and strain was localized at different crustal levels. Suites of deformed and undeformed dikes provide an opportunity to resolve the timing of deformation. Deformed dikes were emplaced either before or during deformation and undeformed dikes were emplaced after deformation. Five weeks of field work provided spatial and geologic context for these dikes while confirming crustal extension and thinning. Evidence for crustal thinning includes a ~1,300m stratigraphic section thinned to 250m near a large normal fault. Structural mapping and analysis of stereonets revealed a NW stretching lineation slightly oblique to a NE-striking and SW-dipping foliation, consistent with prior results. I am currently completing U-Pb analysis of eight dikes sampled from the upper section of the complex. In addition to geochronology, I will perform x-ray fluorescence and petrographic thin sections analyses to better characterize the composition and strain of the dikes. The deformation age constraints I create will be compared with similar work being done in the lower section of the core complex. Collectively, these results will reveal either synchronous extension or a sequence of multiple deformation events and help differentiate between formation models of the Pioneer metamorphic core complex. More broadly, these results will contribute to understanding processes underlying crustal deformation and extension.