EXHUMATION HISTORY OF COLORADO'S SOUTH PARK BASIN AND ITS POTENTIAL CAUSES FROM COUPLING APATITE (U-TH)/HE THERMOCHRONOLOGY WITH CONSTRAINTS FROM SURFACE GEOLOGY

We use apatite (U-Th)/He (AHe) thermochronologic data along a west-to-east transect across South Park, an intramontane basin in Colorado’s Rocky Mountains, to constrain the last ~2 km (cooling below about 65 °C) of the basin’s exhumation history. That history contributes to the larger effort to unravel the mechanisms behind the region’s major Cenozoic topographic changes. The basin’s flanking ranges were uplifted during the Laramide Orogeny (ca. 70-50 Ma) – at that time South Park was a local low receiving synorogenic sediment shed from both ranges. By 37 Ma South Park’s topography sloped gently eastwards, as revealed by abundant erosional remnants of the eastward-flowing Wall Mountain Tuff. The area’s topography was subsequently modified again by normal faulting associated with the Rio Grande Rift (ca. 28Ma) and possibly by an epeirogenic surface uplift event during the last 5-10 m.y. Today South Park forms a local topographic low at 3000m elevation flanked to the west by the Mosquito Range and to the east by the Front Range, with peaks in both ranges exceeding 4200m. We specifically sampled Precambrian granites that directly underlie the Wall Mountain Tuff to constrain the pre-37 Ma exhumation history. This sampling strategy is a key component of our data interpretation, as it provides an unambiguous constraint on the time-temperature history of the samples. Our westernmost sample yields a late Laramide AHe age (mean of 58±4 Ma), indicating approximately 2 km of unroofing between then and 37 Ma. The sample just 20 km to the east yields pre-Laramide ages; rocks that lay on the surface there at 37 Ma remained below 65^oC throughout the Mesozoic. Possible explanations for these significantly different exhumation histories include differential uplift between the two sites during the Laramide Orogeny or dramatically different erosion rates engendered by the accumulation of a thick Paleozoic sedimentary sequence on the western side of South Park (the Central Colorado Trough) and its absence to the east.