The Gothic Dome: Kilometer-scale Miocene Exhumation in Colorado’s Elk
and West Elk Mountains
Abstract
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.