43 Unraveling the History of the Rio Grande Rift: From Contraction to Extension and Evolution of the Rift

Thursday, 17 March 2022
1:30 PM - 5:30 PM
CBC-A 110

GEOLOGIC MAPPING AND KINEMATIC ANALYSIS OF THE BRITTLE-PLASTIC INDEPENDENCE MINE SHEAR ZONE IN THE SANGRE DE CRISTO RANGE, SOUTHERN COLORADO: OLIGOCENE-MIOCENE EXTENSIONAL REACTIVATION OF A LARAMIDE REVERSE FAULT

Paper number: 43-4
Program: Theme Sessions
Time: 2:30 PM – 2:50 PM

Authors

Abstract

The Sangre de Cristo Range in southern Colorado records some of the deepest Cenozoic structural levels in the Rocky Mountain region. Exposures of presumed Laramide-age contractional mylonites show evidence for brittle-plastic extensional overprinting associated with the Rio Grande rift. We investigated the relation between Laramide contraction and Rio Grande rift extension by detailed geologic mapping and kinematic analysis in a 50 km2 area centered on the Independence Mine shear zone (IMSZ), formerly called the Independence Mine thrust. The IMSZ is a 15- to 100-meter-thick, shallow-to-moderately (23°–62°), WSW-dipping brittle-plastic shear zone near the topographic base of the western flank of the range. It displays microstructural evidence for initiation as a top-NE contractional mylonite zone consistent with Laramide kinematics but is pervasively overprinted by deformation fabrics indicating top-SW extensional reactivation. Top-SW microstructures are characterized by mica-lined C- and C’-shear bands and mixed brittle-plastic deformation of quartz. Our mapping shows that the IMSZ is the thickest member of a system of mylonitic shear zones that dip shallowly to moderately (24°–55°) to the WSW and are hosted primarily within Proterozoic gneiss. Shear zones in amphibole-rich gneiss are commonly dominated by chlorite whereas those in quartzo-feldspathic gneiss have abundant white mica. Many of the thinner shear zones also record top-SW overprinting of top-NE fabrics. Extensional overprinting appears to be mostly restricted to mylonites where secondary micas form an interconnected weak phase. We interpret these relations as fluid-mediated, reaction-weakening gradients where lithologically controlled rheological contrasts were variably sensitive to extensional reactivation. One top-SW shear zone adjacent to the IMSZ cuts a diorite stock that we dated at 26 ± 1 Ma using LA-ICP-MS zircon U-Pb geochronometry. This date is consistent with extensional reactivation occurring during late Oligocene to early Miocene time. The IMSZ and associated reactivated shear zones may represent a mid-crustal extension mechanism that was widespread in the earliest stages of Rio Grande rifting before extension shifted to high-angle brittle-regime normal faults along the range front.

Geological Society of America Abstracts with Programs. Vol. 54, No. 2

doi: 10.1130/abs/2022CD-373821

© Copyright 2022 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.