INVESTIGATING A BULL’S-EYE OF LATE MIOCENE EXHUMATION IN COLORADO’S ELK AND WEST ELK MOUNTAINS USING (U-TH)/HE THERMOCHRONOLOGY

We are conducting an apatite (U-Th)/He (i.e., AHe) study to investigate the timing, aerial extent, and magnitude of middle Cenozoic to recent exhumation in Colorado’s Elk and West Elk mountain ranges. Lithologic considerations indicate that a roughly circular bull’s-eye of exhumation with a radius of about 50 km stretches between the towns of Carbondale in the north and Crested Butte in the south and from Aspen in the east to Crawford in the west. The accumulation of spatially widespread constraints on the magnitude and timing of exhumation are prerequisites for testing proposed explanations for its cause.

Approximately two dozen granitic plutons intruded a Paleozoic through early Cenozoic sedimentary pile between ca. 35-29 Ma in the Elk/West Elk area. A second suite of felsic plutons intruded the same area between ca. 18-12 Ma. Both suites of plutons were subsequently exhumed from beneath their sedimentary cover and today stand as isolated peaks that rise 0.5-2 km above the surrounding landscape. Remnants of ca. 11 Ma basalt flows encircle the plutons to the north, west, and south. The presence of these basalts suggests that minimal post-11 Ma regional unroofing has occurred outside the bull’s-eye. However, the fact that these basalts cap mesas that rise 0.5-1.5 km above the modern river valleys requires significant, spatially focused, post-11 Ma river incision. Our work builds upon a recent AHe study that showed three plutons in the bull’s-eye passed through the ~70°C isotherm between about 5.6-15.0 Ma. Our data, which expand the geographic reach of earlier studies, corroborate the occurrence of a mid-Miocene exhumation event within the bull’s-eye; the first four Cenozoic plutons that we’ve analyzed all passed through the ~70°C isotherm between 8-16 Ma. A Precambrian granite outcrop from outside the bull’s-eye passed through ~70°C prior to 61 Ma, illustrating the low magnitude of middle Cenozoic to recent exhumation outside the bull’s-eye’s perimeter.