WHAT MECHANISM CREATED THE MIOCENE-AGED GOTHIC EXHUMATION DOME IN COLORADO’S ELK MOUNTAINS?

Low temperature thermochronology reveals a ~70 km-diameter dome of kilometer-scale exhumation, the Gothic Dome, in Colorado’s Elk Mountains. Exhumation magnitude in the dome’s center is 3.5-6 km and decreases in all directions to 1-2 km, the minimum detectable magnitude using thermochronology. Rocks exposed in the dome’s center passed through ~180°C at 16±3 Ma and through ~65°C at 10±2 Ma. Thermochronologic dates increase in all directions away from the center.

What mechanism could trigger such a spatially restricted exhumation dome? Three processes can produce kilometer-scale exhumation: 1) climate change; 2) drainage reorganization; 3) rock uplift. It is implausible that climate change would trigger kilometer-scale exhumation in the dome and leave surrounding areas untouched. Provenance data suggest the river network draining the dome’s interior is unchanged since the Miocene. Thus, rock uplift likely triggered Gothic Dome exhumation.

Two processes could create this Miocene dome-shaped exhumation pattern: 1) Miocene batholith intrusion; 2) a mantle drip. A regional gravity low probably reveals a batholith at depth, and small Miocene granite stocks and mafic dikes are exposed within the dome. If the granites are cupolas protruding above a Miocene batholith, then batholith emplacement likely triggered dome uplift and exhumation. But previous workers considered the batholith Oligocene, an interpretation we favor based on the area’s abundant exposed Oligocene granites. If correct, then both Miocene igneous activity and rock uplift were likely caused by a mantle drip.

If the dome’s thermochronologically defined (~70 km) diameter represents the entire flexural response to a drip, the region’s flexural rigidity is extremely low, ~10²⁰ Nm. But small magnitude uplift near the dome’s perimeter would produce minimal erosion and hence be thermochronologically undetectable. Today’s Colorado and Gunnison rivers trace a ~120 km diameter arc centered on the Gothic Dome. Ash flow tuffs derived from the south at 25.5 Ma increase in elevation north of the modern Gunnison River, suggesting post-25.5 Ma rock uplift northward, into the dome. These data suggest the complete Gothic Dome is ~120 km in diameter, corresponding to a flexural rigidity of ~3-4x10²¹ Nm, which is a plausible value for a drip-formed dome.