THE PRE-RIFT EXHUMATION HISTORY ALONG THE ARKANSAS RIVER SEGMENT OF COLORADO’S RIO GRANDE RIFT

Several recent low-temperature thermochronologic studies have characterized kilometer-scale exhumation associated with normal faulting along segments of the Rio Grande Rift in Colorado that initiated around 28 Ma. Unfortunately however, in most samples the large magnitude of that rift-induced exhumation often obscures the record of pre-rift exhumation; the record of which is necessary in order to decipher the topographic consequences of the area’s complex Cenozoic tectonic history. The Whitehorn Granodiorite was intruded ca. 70 Ma in conjunction with the rise of the Sawatch anticline during the Laramide Orogeny. It is bisected by a paleochannel filled with mid-Tertiary volcanic rocks, the oldest of which is 36.7 Ma. Rio Grande Rift extension ensued after cessation of this volcanism, splitting the Sawatch anticline and placing the Whitehorn Granodiorite in the modern Mosquito Range, which constitutes the eastern portion of the rifted Sawatch anticline. We harnessed these field relationships to characterize the area’s pre-Rio Grande Rift exhumation history using a battery of low-temperature thermochronometers. These include apatite (U-Th)/He (AHe, which characterizes sample passage through about 65 °C), apatite fission track (AFT, approximately 110-120 °C), zircon (U-Th)/He (ZHe, about 180 °C), and titanite (U-Th)/He (THe, also about 180 °C). Previous paleobarometery indicates that the Whitehorn Granodiorite was emplaced at 10-11 km depth at about 70 Ma and the presence of the paleochannel reveals that our samples were at the surface by 37 Ma, revealing an average exhumation rate of 330 m/m.y. Thermal modeling of our age and track length data suggests that this exhumation was accomplished during two rapid exhumation episodes separated by a time of slower exhumation during which the samples lingered in the apatite partial annealing zone. The first episode of rapid exhumation occurred 70- 65 Ma, coincident with active Laramide thrusting in the Sawatch Range. The second episode, at 47-37 Ma, immediately preceded the voluminous ‘Ignimbrite Flare-up’ volcanic event, suggesting that the two events might be causally related.