James J. Vogl

Detrital mineral chronology of the Uinta Mountain Group: Implications for the Grenville flood in southwestern Laurentia

Numerous studies have shown that large quantities of Grenville-age detritus dominate Neoproterozoic to Cambrian arenites in southwest Laurentia (southwestern United States). U-Pb ages and Hf isotopic compositions of zircons and 40 Ar/ 39 Ar ages of white mica from clastic sedimentary rocks of the Neoproterozoic Uinta Mountain Group also indicate significant Mesoproterozoic detritus mixed with a variably abundant Archean component. Zircons with ages representative of the Paleoproterozoic basement in the eastern Uinta Mountains or the younger Paleoproterozoic rocks of the adjacent Yavapai-Mazatzal terranes were not observed. A limited range of initial ϵ Hf (∼90% between –3 and +3) for Mesoproterozoic zircons suggests derivation from a source region (or regions) characterized by mixing between juvenile and reworked older crust during Grenville orogenesis. The enriched Grenville-age basement proposed to underlie much of southeastern North America may be this source based on similarities of Hf isotopic data from Mesoproterozoic zircons in Mississippi River sand and available paleocurrent data. If so, then disruption of this supply in the Cambrian may be related to Iapetan rifting and, perhaps, the separation of the Precordillera terrane from Laurentia.

Miocene regional hotspot-related uplift, exhumation, and extension north of the Snake River Plain: Evidence from apatite (U-Th)/He thermochronology

Passage of North America over the Yellowstone hotspot has had a profound infl uence on the topography of the northern Rocky Mountains region. One of the most prominent hotspot-related topographic features is the Yellowstone crescent of high terrain, which consists of two elevated shoulders bounding the eastern Snake River Plain and converging at a topographic swell centered on the Yellowstone region. We have applied single-grain (U-Th)/He dating to apatites (AHe) collected from the Pioneer-Boulder Mountains on the northern arm of the Yel- lowstone crescent of high terrain to constrain the timing, rates, and spatial distribution of exhumation. These data provide constraints on the timing and processes responsible for uplift related to passage of the hotspot. The Pioneer-Boulder Mountains represent a topographic and structural culmination defi ned by elevation and by the geometry of preserved strata of the Eocene Challis volcanic province. AHe ages indicate that ≥2–3 km of exhumation has occurred in the core of the Pioneer-Boulder Mountains culmination, where no Challis volcanics are preserved, since ca. 11 Ma. Challis volcanics are extensively preserved and Eocene topographic highs are locally preserved to the north and south of the Pioneer-Boulder Mountains, indicating minimal erosion in those areas. Age-elevation relationships suggest an exhumation rate of ~0.3 mm/yr between ca. 11 and 8 Ma for the culmination core; this relatively rapid interval of exhumation followed a period of >30 m.y. during which little to no regional-scale exhumation occurred. Spatial patterns of both exhumation and topography indicate that faulting was not the primary control on uplift and exhumation of the culmination. Instead, NNW-trending normal faults are superimposed on the culmination, with the AHe ages from the footwall of the Copper Creek fault indicating that faulting began at or after ca. 10–9 Ma. Regional exhumation at 11–8 Ma was synchronous with silicic eruptions from the ca. 10.3 Ma Picabo volcanic fi eld located immediately to the south and with S tilting of the southern fl ank of the Pioneer-Boulder Mountains culmination, which was likely the result of loading of the eastern Snake River Plain by midcrustal mafi c intrusions. This synchroneity suggests a causal relationship between hotspot processes and exhu- mation through potential contributions of fl exure and mantle dynamics to uplift, as well as changes in drainage networks and base level