Post-Laramide, Eocene epeirogeny in central Colorado—The result of a mantle drip?

Abstract

The Southern Rocky Mountains first rose during the Laramide Orogeny (ca. 75–45 Ma), but today’s mountains and adjacent Great Plains owe their current height to later epeirogenic surface uplift. When and why epeirogeny affected the region are controversial. Sedimentation histories in two central Colorado basins, the South Park–High Park and Denver basins, shifted at 56–54 Ma from an orogenic to an epeirogenic pattern, suggesting central Colorado experienced epeirogeny at that time. To interrogate that hypothesis, we analyzed thermal histories for seven samples from central Colorado’s Arkansas Hills and High Park using thermochronometers with closure temperatures below ~180 °C, enabling us to track sample exhumation from ~5–7 km depth. Three samples are from the Cretaceous Whitehorn pluton, and four are Precambrian granitoids. All zircon and titanite (U-Th)/He dates (ZHe and THe) and one apatite fission-track (AFT) date are similar to the 67 Ma pluton emplacement age. Whitehorn dates using the lower-temperature apatite (U-Th)/He (AHe) thermochronometer are 55–41 Ma. These data require two exhumation episodes, one ca. 67–60 Ma, the second beginning at 54–46 Ma. The pluton reached the surface by 37 Ma, based on the age of volcanic tuff filling a pluton-cutting paleovalley. The Precambrian samples do not further refine this thermal history owing to the comparatively higher He closure temperature of their more radiation-damaged apatite. Laramide crustal shortening caused 67–60 Ma exhumation. Arkansas Hills shortening ended before 67 Ma, so shortening could not have caused the exhumation event that began 54–46 Ma; thermochronology supports the Eocene epeirogeny hypothesis. Epeirogeny affected >2.0 × 104 km2, from the Sawatch Range to the Denver Basin. We attribute epeirogeny to an Eocene mantle drip that likely triggered subsequent drips, causing younger exhumation events in adjacent areas.

Hydrothermal Ore Deposits Record the Oxygen Isotope Composition of Meteoric Paleo‐Waters in the San Juan Volcanic Field, Colorado, USA

“…Uplift rates between 0.21 and 0.35 mm yr −1 are consistent with 2–3 km of surface uplift from 25 to 15 or 12 Ma (Figure 3). These rates are smaller than those expected from crustal thickening alone (Abbott et al., 1997), suggesting that other mechanisms might be responsible for driving surface uplift (Abbott et al., 2022).…”

Using Stochastic Point Pattern Analysis to Track Regional Orientations of Magmatism During the Transition to Cenozoic Extension and Rio Grande Rifting, Southern Rocky Mountains

“…This is except for time periods where the number of samples falls below approximately 10, and then uncertainty of our analysis increases. This impact is most notable for the 55 to 50‐Ma age range (Figure 4), perhaps because the numerous zircon fission track ages in this age range likely represent cooling related to epiorogenic uplift, which occurred between approximately 54–46 Ma (Abbott et al., 2022), or by partial thermal resetting of Late Cretaceous to Paleocene intrusions by younger magmatism (e.g., Rosera et al., 2021). Regardless, we conclude that removing the fission track data has little effect on the directional distribution results for time periods with well‐documented magmatic activity in our study area (i.e., approximately 70 to 55 Ma and 45 to 15 Ma; Figures 5a and 6a).…”

“…Recently, Abbott et al. (2022) proposed that shortening related to Laramide compression in the southern Rocky Mountains ceased by ca. 67 Ma, and that an epiorogenic pulse of uplift between 54 and 46 Ma occurred in response to a mantle drip event.…”

Evaluating the Compatibility of Hematite (U‐Th)/He Data and Hematite‐Carried Secondary Magnetizations: An Example From the Colorado Front Range

“…The Southern Rocky Mountains in the western United States is a physiographic province with a mean elevation >2,500 m above sea level and hosts the highest peaks of the entire Rocky Mountains. The area experienced extensive thick‐skinned, high‐angle reverse faulting during the Late Cretaceous‐Eocene Laramide orogeny (∼70–45 Ma), although some of the current high elevation is possibly attributed to post‐Laramide epeirogenic uplift (Abbott et al., 2022; Cather et al., 2012; Eaton, 2008). The high peaks of the Southern Rockies have since been denuded by fluvial processes and alpine glaciation during Pleistocene glacial periods (e.g., Benson et al., 2005).…”

“…The timing of cessation of Laramide contraction and attendant unroofing of the Precambrian crust varied across the Southern Rockies (Copeland et al., 2017; Tweto, 1975). Subsequent epeirogenic uplift and erosion (Abbott et al., 2022; Cather et al., 2012; Eaton, 2008) resulted in regional scale features such as the Rocky Mountain Erosion Surface (e.g., Chapin & Kelley, 1997; Epis & Chapin, 1974; Madole et al., 1987) and the modern topographic expression of the Southern Rockies.…”