DIACHRONOUS EXHUMATION IN THE COLORADO ROCKIES – A PRODUCT OF SERIAL LITHOSPHERIC DRIPS?

Low-temperature thermochronologic data record Miocene exhumation within a ~100-km-diameter dome – the Gothic Dome – in Colorado’s Elk and West Elk ranges. We attribute that exhumation to Miocene epeirogenic surface uplift. By contrast, new data corroborate our previously published conclusion that the Sawatch range, directly east of the Gothic Dome, experienced earlier, Eocene epeirogenic surface uplift. The discovery of diachronous exhumation across the Colorado Rockies is significant because – although consensus exists that the modern mountains owe their current height in part to epeirogenic uplift that postdates the Laramide Orogeny – when and why post-Laramide epeirogeny occurred remain open questions.

Application of apatite (U-Th)/He (AHe), apatite fission track (AFT), and zircon (U-Th)/He (ZHe) thermochronometers facilitates reconstruction of a rock’s trajectory from ~4-6 km burial depth to the surface. We compiled such data from 14 elevation transects and 19 other spot locations, the combined result of our work and previous studies. Most rocks analyzed within the Gothic Dome are Oligocene granitoids that intruded a Paleozoic-Paleogene sedimentary section. Today these granitoids form mountains with 1-2 kilometers of topographic relief thanks to their greater resistance to erosion than the surrounding sedimentary rocks. Oligocene granites are largely absent outside the dome’s perimeter, so there we analyzed Precambrian crystalline rocks instead. These data reveal that the center of the Gothic Dome has experienced ~4-6 kilometers of exhumation since 16±3 Ma. Exhumation magnitude diminishes toward the dome’s perimeter in all directions, as revealed by progressively older, but always emplacement or post-emplacement AHe, AFT, and ZHe dates in all directions away from the dome’s center. Samples from outside the dome’s perimeter possess pre-emplacement AHe dates. Such dates, combined with nearby ca. 11 Ma basalt to the north, west, and south, document that Miocene exhumation outside the dome’s perimeter was minimal.

We hypothesize that Eocene foundering of a lithospheric drip may have triggered Eocene surface uplift and exhumation in the Sawatch range; post-drip lithospheric heterogeneity spawned a second drip beneath the adjacent Elk and West Elk mountains to produce the Gothic exhumation dome.