New Mexico Geological Society Annual Spring Meeting — Abstracts

A compilation of thermochronologic age-elevation transects shows temporally and spatially partitioned Cenozoic cooling indicative of multistage uplift of the Rocky Mountain region. These data lead to a testable hypothesis that explains the present data: 1) Laramide contraction and cooling of the basement, recorded by hanging wall blocks near the eastern and western Front Range uplifts and some Colorado Plateau monoclines, with typical cooling rates of 60m/Ma from 75-45 Ma; 2) Relative tectonic quiescence from 45-35 Ma during which the Rocky Mountain Erosion Surface developed; 3) Initial stages of long wavelength uplift driven by the Oligocene ignimbrite flare-up resulted in the deposition of the “Eocene” conglomerates in latest Eocene/earliest Oligocene and creating the Great Plains topographic ramp. Typical exhumation rates from 38-25 Ma were 100 m/Ma; 4) Relative tectonic quiescence from 25-10 Ma, inferred to reflect changing far field stress regimes culminating in, 5) Neogene acceleration of cooling and exhumation to rates >120, driven by small scale mantle convection.

This study focuses on the transition from post-Laramide quiescence into the Oligocene by examining five geologic units that span the southern Rocky Mountains. We hypothesize these units are related in that they record the initial pulse of uplift associated with Oligocene volcanism. Recent studies indicate that deposition of the Telluride Conglomerate initiated just prior to and ended just after the onset of Oligocene San Juan volcanism, supported by the presence of volcanic clasts in the upper portions of the unit. The Blanco Basin may be the eastward expression of similar early unroofing of the Needles Mountains. The coeval El Rito and Galisteo Formations may also record uplift in the southern Rockies. We have begun detrital zircon analysis and plan apatite fission track analysis on these units to test if they can be temporally and spatially linked to create a surface datum recording the Rocky Mountain landscape just prior to and in early stages of the ignimbrite flare-up. Beginning with the model of the Rocky Mountain Erosion Surface, and combining geochronologic and thermochronologic histories, our goal is to reconstruct the pre-Oligocene Rocky Mountain topography to constrain postOligocene and Neogene Rocky Mountain uplift histories.