STRESS AND STRAIN RATE ALONG A SHEARED PRECAMBRIAN CONTACT BETWEEN THE UNCOMPAGHRE GROUP AND IRVING FORMATION, SOUTHWESTERN COLORADO

Precambrian basement exposed in the Needle Mountains in southwestern Colorado records a complex metamorphic and deformation history across heterogeneous lithologies, primarily during the Yavapai and Mazatzal orogenies. Contacts amongst these lithologies are possible strain localization zones, so characterization of strain accommodating minerals and deformation mechanisms will provide insights into the paleo stresses and strain rates during assembly.

The Irving Formation (1.80-1.78 Ga) and Twilight Gneiss (1.77-1.76 Ga) represent arc-related igneous and sedimentary rocks accreted to Laurentia and metamorphosed during the Yavapai orogeny (ca. 1.71 - 1.68 Ga). Subsequent slab roll back and extension exhumed these rocks, allowing for deposition of 2.5 km of sedimentary protoliths (ca. 1.705 Ga) that were later metamorphosed at greenschist to amphibolite facies to form quartzites, metapelites, and metaconglomerates (the Uncompaghre Group). Following reburial, deformation resulted in macro- to micro-scale folding, foliation development, and variable shearing along the contact. In many locations, Mesoproterozoic plutons intruded the suite, resulting in a final stage of deformation and metamorphism. Where exposed on the shoulder of Snowdon Peak, CO, however, the Uncompaghre-Irving contact is not modified by subsequent intrusions and provides a window into strain accommodation during basement assembly. At this location, previous researchers documented polyphase deformation, increased strain towards the contact, and evidence of quartz crystal plasticity, but detailed microstructural analysis of these samples has not been conducted since the late 1980s.

We are investigating 55 legacy thin sections of the Uncompaghre Quartzite and Irving Formation (collected across the contact by B. Tewksbury and T. Olson, 1978-1983) with a focus on applying modern microstructural techniques to characterize strain-accommodating minerals, as well as the deformation mechanisms responsible. We will present results from our microstructural, quartz paleopiezometry, and quartz flow law analyses addressing the conditions, strain rate, and effective strength and viscosity during deformation. This research will have implications for heterogeneous lower crustal deformation during terrane assembly.