LOW-T THERMOCHRONOLOGY OF SOUTH-CENTRAL IDAHO: INTERACTIONS BETWEEN UPLIFT, EXTENSION, AND HOTSPOT PROCESSES

We have applied (U-Th)/He dating of apatite (AHe) to the Pioneer-Boulder Mtns (PBM) in central Idaho to investigate the topographic effects of the passage of the Yellowstone hotspot (YSHS). Our study will document spatial variations in the timing, amounts and rates of exhumation to better understand the interactions between thermal uplift, flexural bulging, and extensional faulting associated with the hotspot. The PBM on the north flank of the Eastern Snake River Plain (ESRP) represent a broad region of high elevation not bound by a major normal fault. A topographic and structural high is defined by numerous ridgelines above 3 km and elevations that slope down to the ESRP at ~1600 m and erosive removal of the 50-45 Ma Challis volcanics. (Fig. 1) Challis volcanics are preserved on the flanks of the culmination to the SE and NW.

A 1200 km transect through the Pioneer Mountains has produced AHe ages between 11 and 9 Ma, suggesting a significant increase in uplift-related exhumation following >30 Ma of little exhumation. Samples dated thus far to the northwest have ages of ~7.7 Ma (2365m) and ~8.7 Ma (2697m). Two possible models can explain the observed data: (1) both areas are part of the same exhumation event yielding a rate of 0.4 mm/yr between ~11 and 7.7 Ma, or (2) uplift/exhumation propagated to the northwest producing slightly younger ages.

The Garfield Stock, 20 km SE, is in the footwall of a Copper Creek fault (N-S) and yields a scatter of ages between ~9 Ma and ~16 Ma. The Mackay Mine pluton sits in a horst ~30 km east of the Pioneers and produced AHe ages of ~44 Ma, indicating an older exhumation event in this area. The WC Peaks, 55 km to the NW, have an AHe age of ~24 Ma and are in the footwall of a SSW-NNE trending normal fault.

Overall, ages related to the Pioneers strongly suggest that this exhumation event is related to the passage of the YSHS, either as a broad thermal swell (as observed around Yellowstone today) and/or as part of a flexural bulge from downwarping of the ESRP caused by emplacement of mid-crustal mafic sills. Ongoing AHe dating across the Sawtooth Range and Boulder Mountains will provide a more complete view of the age of faulting and exhumation rates to the W and NW. Furthermore this will add constraints to our working models for topographic evolution as a result of hotspot process.