Timing and driving mechanisms for multi-stage uplift of the Southern Rocky Mountains: Evidence from thermochronology and detrital zircon analysis

Abstract:

A compilation of apatite fission track (AFT) and (U-Th)/He (AHe) thermochronologic data from the southern Rocky Mountains reveals spatial and temporal patterns in Cenozoic cooling corresponding to discrete uplift pulses: 1) 70-45 Ma cooling ages are best preserved in hangingwall blocks of Laramide uplifts, especially in the northern Rockies with apparent exhumation rates (AER) of ~ 60 m/Ma based on age-elevation transects and modeled cooling paths; 2) Relative tectonic quiescence from 45-35 Ma coincided with the establishment of the Rocky Mountain erosion surface; 3) 35-25 Ma cooling ages variably overprint Laramide cooling histories south of central Colorado and much of the Great Plains topographic ramp; this episode had apparent exhumation rates of ~100 m/Ma and was driven by heating and mantle buoyancy during the ignimbrite flare-up concentrated beneath the San Juan and Mogollon Datil volcanic fields, with associated long wavelength uplift in central Colorado; 4) 20-10 Ma shorter wavelength uplifts in Rio Grande rift flanks at rates of ~ 150 m/Ma from Wyoming to Mexico in response to simultaneous opening of the Rio Grande rift; 5) Ongoing post-10 Ma uplift at rates > 120 m/Ma especially in areas of faulting and areas overlying marked mantle velocity transitions due to small scale upper mantle convection and response to land surface uplift, river integration, and isostatic rebound due to differential erosion.

To investigate the sedimentary record of mid-Tertiary events, detrital zircon analysis of the Telluride Conglomerate and Blanco Basin Formation of the western and southeastern flanks of the San Juan volcanic field reveals youngest grain clusters of 28 Ma and 33 Ma, respectively. These units record unroofing, first of Paleozoic sedimentary units, then Proterozoic basement and Oligocene volcanics. AHe of these units constrains burial to less than 1-1.5 km. We interpret these units to record the uplift and erosion associated with the inflating San Juan volcanic field caldera complexes ~ 35-30 Ma. The combination of low-temperature thermochronology and detrital zircon data allow the constraint of timing and location of uplifting mountain ranges and the drainages that were both a response and driver for continued relief generation.