The Colorado Rockies were initially raised during the Laramide Orogeny; ca. 70-45 Ma. But consensus exists that the range experienced a second,; post-Laramide episode of surface uplift; the timing and cause of that; post-Laramide surface uplift event remains enigmatic. Low-temperature; thermochronologic studies conducted by us and others using apatite; (U-Th)/He (AHe), apatite fission track (AFT), and zircon (U-Th)/He (ZHe); techniques reveal that a dome of kilometer-scale exhumation occurred in; Colorado’s Elk and West Elk mountains between ca. 18-6 Ma. We call this; feature the “Gothic Dome” because it is centered on Gothic Mountain,; near the town of Crested Butte. We suggest the; ~100-km-diameter Gothic Dome likely experienced Miocene; surface uplift, which triggered the dome-shaped exhumation pattern; documented by the low-temperature thermochronometry. The exhumation; magnitude exceeds 4 km in the center of the dome (as revealed by a 16 Ma; ZHe date on an Oligocene pluton) and diminishes toward its perimeter.; This diminution of exhumation magnitude toward the perimeter is revealed; by progressively older AHe, AFT, and ZHe dates in all directions away; from Gothic Mountain. AHe dates for samples that lie outside the; perimeter are Laramide-age or older, further documenting the dome-shaped; nature of this Miocene exhumation event and illustrating the low; magnitude of Miocene to recent exhumation outside the dome’s perimeter.; Outcrops of ca. 11 Ma basalt surround the Gothic Dome to the north,; west, and south, requiring that Miocene exhumation outside the dome’s; perimeter was minimal. A suite of alkaline, low-volume, felsic plutons; and ultramafic lamprophyres intruded the Gothic Dome between ca. 18-12; Ma. This alkalic magmatism began either immediately prior to or; contemporaneous with the onset of Gothic Dome exhumation, hinting that; the same root cause might be responsible for both. Workers elsewhere,; including Tibet, the Altiplano, and California’s Sierra Nevada; mountains, have attributed small-volume alkalic magmatism, surface; uplift, and exhumation to the activity of lithospheric drips. We offer; that Miocene activity of such a drip beneath Colorado’s Elk and West Elk; mountains is an appealing mechanism to explain the near simultaneity of; those same phenomena here.
