131-2 Oligocene Mylonites in the Northern Rio Grande Rift

Session: Going with the Shear - New Insights into Lithospheric Extensional and Strike-Slip Systems

Presenting Author:

John Singleton

Authors:

Singleton, John¹, Malavarca, Samantha L.², Primus, Miriam³, Rahl, Jeffrey M.⁴, Sitar, Michael Cole⁵, Frawley, Dylan Liam⁶, Broeder, Hunter⁷

(1) Department of Geosciences, Colorado State University, Fort Collins, CO, USA, (2) Department of Geosciences, Colorado State University, Fort Collins, CO, USA, (3) Department of Geosciences, Colorado State University, Fort Collins, CO, USA, (4) Washington and Lee University, Lexington, VA, USA, (5) Massachusetts Water Resources Authority, Boston, MA, USA, (6) Department of Geosciences, Colorado State University, Fort Collins, CO, USA, (7) Department of Geosciences, Colorado State University, Fort Collins, CO, USA,

Abstract:

The western flank of the Sangre de Cristo Range in southern Colorado was exhumed from a depth of ~8 km during Neogene extension associated with the range-bounding normal fault system, resulting in exposure of some of the deepest Cenozoic structural levels in the Rocky Mountain region. Multiple locations along the western flank record mylonitic and/or brittle-plastic deformation that we have investigated through detailed geologic mapping and microstructural analysis. In some locations, such as along the Deadman Creek thrust and Mosca Creek thrust, narrow (<10 m thick) crystal plastic deformation is localized in metasedimentary rocks and associated with top-NE directed contraction that is likely Laramide in age (Late Cretaceous-Early Paleogene). In other locations gently- to moderately-dipping mylonitic fabrics in crystalline rocks are up to ~100 m thick and record top-SW normal-sense shear. These normal-sense shear zones locally reactivate thrust faults (e.g., the Independence Mine shear zone and Deadman Creek thrust) and appear to be concentrated near Oligocene intrusions. Examples include mylonitic quartz veins and protomylonitic fabrics within the ~30 Ma Chokecherry granite, mylonites involving a ~28 Ma intrusion near Deadman Creek, and a chloritic brittle-plastic shear zone that cuts a ~26 Ma gabbro near Cottonwood Creek. Quartz crystallographic preferred orientations indicate a significant component of coaxial strain in some of the gently-dipping mylonitic fabrics with top-SW kinematic indicators. Microstructures, mineral assemblages, and quartz c-axis opening angles indicate deformation temperatures ranging from ~300–500°C in these Oligocene mylonites, and K-feldspar 40Ar/39Ar thermochronology data indicate that the range cooled to ≤300°C by the early Miocene. We interpret these shear zones to be largely driven by thermal softening associated with Oligocene magmatism and locally guided by older contractional structures. These shear zones are at least 10 Myr older than the middle Miocene initiation of the SW-dipping brittle normal fault system responsible for exhumation of the range, indicating that they represent a distinct phase of rifting.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9757

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.