223-3 Constraining metamorphic conditions and deformation mechanisms along a sheared Paleoproterozoic contact in the San Juan Mountains, southwestern Colorado

Session: Rock Deformation and the Dynamics of Mountain Building: A Session Honoring the Scientific Contributions of John P. Platt (Posters)

Poster Booth No.: 208

Presenting Author:

Carolyn Tewksbury-Christle

Authors:

Tewksbury-Christle, Carolyn M.¹, Hawthorne, Sophia², Peck, Harper³, Pollock, Michael⁴, Scott, Avery⁵

(1) Fort Lewis College, Durango, CO, USA, (2) Fort Lewis College, Durango, CO, USA, (3) Fort Lewis College, Durango, CO, USA, (4) Ensolum, LLC, Durango, CO, USA, (5) Fort Lewis College, Durango, CO, USA,

Abstract:

Precambrian basement rocks exposed in the San Juan Mountains in southwestern Colorado provide a window into mountain-building processes during the Yavapai-Mazatzal Orogeny. A shallowly-dipping Paleoproterozoic shear zone exposed on Snowdon Peak records top to the south thrusting of the 1.7 Ga Uncompaghre Formation over the older Twilight Gneiss and Irving Formation. Uncompaghre Formation geochemistry suggests erosion of exhumed gneisses, deposition uncomformably on the basement, and subsequent rapid burial to greenschist facies (450°C, 9-12 km) by 1.65 Ga. Ductile quartz deformation along the unconformity accommodates motion in this Paleoproterozoic shear zone, but previous P-T estimates are either not from the shear zone directly or are poorly constrained, and kinematic analyses focused on shear sense indicators.

We used Titanium-in-Quartz (TitaniQ), microstructural data, EBSD data, and quartz paleopiezometry and flow laws to constrain 1) deformation temperatures and 2) quartz rheology. Average TitaniQ temperatures of small quartz grains are 460 ± 30°C, but large quartz grain cores preserve higher temperatures (550-600°C). In micaceous zones, higher temperature quartz cores are beheaded by mica-rich domains. Quartz exhibits core and mantle structures, bimodal grain size distributions, well-developed subgrains, sutured and irregular grain boundaries, and undulose extinction, but most samples have weak crystallographic preferred orientations (CPOs).

Temperatures support previous conclusions that shearing occurred during initial metamorphism and not during later high-grade metamorphism associated with regional intrusions (ca. 1.43 Ga). Weak CPOs and beheaded grains suggest pressure-solution creep as the dominant deformation mechanism. Evidence of dislocation creep via grain boundary migration (GBM) despite unfavorable temperatures, along with high temperature cores, are likely relict from higher-grade gneisses (500-600°C) that the Uncompaghre Formation was sourced from. If weak CPOs are due to overprinting CPOs from the relict gneissic fabrics and younger shear zone fabrics, then dislocation creep may also be an important mechanism. To evaluate this, we calculated 1) stress with the quartz paleopiezometer assuming dynamic recrystallization of strain-free grains, 2) strain rate using a wet quartz flow law (10^-12-10^-10 s^-1), and 3) slip rates using shear zone thicknesses (0.1-2 mm/year). At these strain rates, dislocation and pressure solution creep require similar differential stresses, consistent with co-occurring microstructures and potentially supporting the interpretation of weak CPOs caused by overprinting textures. Our data are consistent with minor slip along this shear zone during the Yavapai-Matzatzal Orogeny.

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

doi: 10.1130/abs/2025AM-8759

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