LOW-TEMPERATURE THERMOCHRONOMETRIC RESULTS FROM COLORADO’S SOUTH PARK AND ARKANSAS HILLS AREAS: CONSTRAINTS ON LARAMIDE PLUTON EMPLACEMENT DEPTH, THICKNESS OF THE WESTERN INTERIOR SEAWAY SEDIMENTARY SECTION, AND THE TIMING OF A POST-LARAMIDE UNROOFING EVENT

Researchers debate the relative contributions of the Laramide Orogeny and post-Laramide events in the creation of today’s Southern Rocky Mountain topography. We report apatite fission track (AFT) and (U-Th)/He thermochronology results for apatite (AHe), zircon (ZHe), and titanite (THe) for six granitic samples from a 60 km west to east transect from the Arkansas Hills near Salida through southern South Park to near Cripple Creek. All samples directly underlie paleochannels filled with Wall Mountain Tuff, which requires that they were at the surface by 37 Ma. Three Precambrian basement samples from South Park (in the east) yield pre-Laramide AHe and AFT ages. One basement sample from the Arkansas Hills, to the west, yields Laramide AHe, AFT, and ZHe ages. Also in the Arkansas Hills, two samples of Laramide-age Whitehorn Granodiorite, emplaced at 67 Ma, yield ca. 67 Ma ZHe, THe, and AFT ages and ca. 47 Ma AHe ages. These results lead to the following conclusions: 1) either the Whitehorn pluton was emplaced at 3-4 km depth, shallower than some paleobarometeric work indicates, or Laramide tectonism unroofed it soon after emplacement; 2) At least 3-4 km of latest Cretaceous to Cenozoic exhumation has occurred in the Arkansas Hills but less than 2 km has occurred in South Park. A recent, nearby study saw similar differential exhumation; 3) Precambrian basement in South Park is unconformably overlain by Mesozoic Western Interior Seaway (WIS) sediments. Our results place a 2 km upper bound on the thickness of the WIS sedimentary package in southern South Park, which requires thinning of the package from north to south across central Colorado; 4) The Whitehorn pluton was exhumed to the surface from a depth of 1.5-3 km between 47-37 Ma. This exhumation post-dates Laramide orogenic activity in the Sawatch Uplift, to which the Arkansas Hills belong. An alternative explanation for this unroofing event is required. This last conclusion differs from a recent interpretation that post-Laramide Whitehorn Granodiorite AHe ages result from middle Cenozoic hydrothermal circulation triggered by the flow of ignimbrites through the paleochannels. That interpretation was invoked to explain poor age reproducibility in the study’s samples. Our samples, by contrast, yield reproducible ages, obviating the need to invoke hydrothermal age resetting.