WIDESPREAD 11-9 MA EXHUMATION ALONG THE NORTHERN MARGIN OF THE SNAKE RIVER PLAIN REVEALED THROUGH (U-TH)/HE THERMOCHRONOLOGY: A RECORD OF NORMAL FAULTING, FLEXURE, AND BASE-LEVEL CHANGES

Although best known for prolific volcanism, the Yellowstone hotspot has also had profound effects on the topography of the northern Rocky Mountains region. One of the most prominent topographic features is the Yellowstone crescent of high terrain (YCHT), which comprises elevated shoulders bounding the eastern Snake River Plain (ESRP). We have applied apatite (U-Th)/He dating (AHe) to samples from the Pioneer-Boulder Mtns. (PBM) on the northern arm of the YCHT and Boise Mtns. to the west to constrain the timing, rates, and spatial distribution of exhumation and to provide constraints on the processes responsible for uplift related to passage of the hotspot.

The PBM represent a topographic-structural culmination. AHe ages indicate that >2-3 km of exhumation has occurred since 11 Ma in the core of the culmination. Challis volcanics and Eocene topographic highs are preserved to the north and south, indicating minimal erosion in those areas. Age-elevation relationships suggest an exhumation rate of ~0.3 mm/yr between ~11 and 8 Ma for the culmination core. This relatively rapid exhumation follows a period of >30 m.y. of little regional-scale exhumation. Spatial patterns of both exhumation and topography indicate that faulting was not the primary control on uplift and exhumation. Exhumation was synchronous with S-tilting of volcanics to the south, perhaps suggesting a component of flexural uplift associated with loading of the ESRP.

The Boise Mtns. are underlain by rocks of the Idaho batholith and are cut by NW-trending faults. Initial AHe data has yielded numerous 11-9 Ma ages, as well as older ages ranging up to Oligocene. Several of the Late Miocene ages come from some of the highest elevations and from the footwalls of normal faults. However, hangingwalls also appear to have undergone extensive Miocene exhumation. The 11-9 Ma AHe ages are synchronous with the main period of downfaulting of the western Snake River plain (WSRP) to the SW. These relations may suggest that widespread exhumation in the Boise Mtns. may have been controlled by both normal faulting and base-level fall in the WSRP.

Published data suggest that hotspot-related magmatism may have also controlled faulting and downwarping along the entire length of the ESRP at ~10 Ma. Thus, the effects of the hotspot may have been particularly widespread at this time.