(U-TH)/HE EVIDENCE FOR MIOCENE YELLOWSTONE HOTSPOT-RELATED UPLIFT IN SOUTH-CENTRAL IDAHO
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 high is defined by numerous ridgelines above 3 km and elevations that slope down to the ESRP at ~1600 m and decreases more gently to the northwest. The topographic high generally coincides with a structural high defined by extensive erosional removal of the 50-45 Ma Challis volcanics that are much more extensively preserved on the flanks of the culmination.
AHe ages from the central Pioneer Mountains through >1 km of elevation difference all yield ages between 11 and 9 Ma, suggesting a significant increase in uplift-related exhumation following >30 my of little exhumation. A single sample dated thus far to the northwest at slightly lower elevation yielded ages of ~7.7 Ma. Two possible models can explain the data collected thus far: (1) both areas are part of the same exhumation event yielding a rate of ~0.3-0.4 mm/yr between ~11 and 7.7 Ma, or (2) uplift/exhumation propagated to the northwest producing slightly younger ages. These two model are being tested through a more detailed elevation transect in the northwest.
Overall, these ages 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. Previous studies of the tilt history of Miocene volcanic rocks to the south indicate subsidence of the SRP at the same time as uplift recorded by our AHe ages, perhaps suggesting a flexural component. Ongoing AHe dating across a complete SE-NW transect away from the ESRP will provide a more complete view of the uplift profile and better understanding of hotspot processes in the region.