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