Early Cenozoic exhumation and paleotopography in the Arkansas River valley, southern Rocky Mountains, Colorado

Abstract

New thermochronometric, geochronologic, and clumped isotope data from the Mosquito Range, Arkansas Hills, and Arkansas River valley (Colorado, USA) constrain the magnitude and timing of Laramide deformation in this region, as well as the development of a low-relief Eocene erosion surface found throughout the southern Rocky Mountains. Apatite (U-Th-Sm)/He thermochronometry from seven vertical transects near the lower Arkansas River valley were collected to assess exhumation histories. New paleomagnetic data from the latest Cretaceous Whitehorn Granodiorite is presented to assess the effect of possible upper crustal tilting on these transects. These data, in combination with new zircon (U-Th)/He thermochronometry and 40Ar/39Ar and zircon U-Th-Pb geochronology from the Whitehorn Granodiorite support inverse thermal history models that imply ~3-5 km of differential (west side up) exhumation between the Mosquito Range-Arkansas Hills (5-7 km total exhumation from 80 and 60 Ma) and the Royal Gorge region to the east (< 1-2 km exhumation since ca. 120 Ma). Challenges in extracting reliable thermal histories from this data set include samples with significant grain-to-grain age variability, and the observation of upward younging age-elevation relationships in parts of several vertical transects. the former problem is common in Proterozoic crystalline rocks with protracted cooling histories, deriving from complications including helium implantation and radiation damage. We demonstrate, through the application of clumped isotopic data on colocated carbonate samples, that the latter complication likely arises from post-exhumation hydrothermal reheating driven by paleotopography and overlying late Eocene to early Miocene ignimbrite sequences. By comparing multiple closely spaced vertical transects in Proterozoic rocks including a transect collected in Cretaceous plutonic rocks and transects overlain by mid-Cenozoic ignimbrites with those that are not, we demonstrate that reliable thermal histories can be obtained from complex thermochronometric data sets through careful data evaluation and intertransect thermal history comparisons. Interpreting the thermochronometric data in the context of the spatial distribution of mid-Cenozoic ignimbrites in this region also provides new insights into the development of the low-relief Eocene erosion surface in the Rocky Mountains. We observe rapid and extensive Laramide exhumation while ignimbrite deposition is confined to narrow paleovalleys within a paleosurface of moderate relief in the Arkansas Hills and Mosquito Range. Where Laramide exhumation is minimal, the ignimbrites blanket a low-relief paleoerosion surface. the former paleolandscape was entirely formed in the Paleocene, based on our low-temperature thermochronometric data, while the latter paleosurface may well record a much longer evolutionary history, possibly partially inheriting an older paleolandscape. the compound nature of the Eocene erosion surface in this region may provide insight into the development of such surfaces throughout the Rocky Mountains.