New Mexico Geological Society Annual Spring Meeting — Abstracts

Proterozoic shear zones are key features of southwestern North America’s orogenic belts that extend over 1,000 km along strike from Mexico to eastern Canada. The dominant fabric of these shear zones is a subvertical, northeast-trending foliation expressed in a wide range of temperature regimes, from migmatites to mylonites to pseudotachylytes, that have previously been envisioned to have formed during the Paleoproterozoic (1.7-1.6 Ga Yavapai and Mazatzal orogenies). An emerging addendum is that the lower temperature shearing possibly represents reactivation of earlier shear zones during the later deformation such as the ~1.45 Ga Picuris and ~1.1 Ga Grenville orogenies. The research plan for this project is to use ⁴⁰Ar/³⁹Ar thermochronology to date fine-grained mica found in the grain-reduced shear zones as well as coarse-grained mica from the coarser grained shear zones to attempt to parse 1.65 versus ~1.45 Ga, or younger, tectonites. Previous studies have found that Paleoproterozoic shear zones in the Manzano Mountains of New Mexico may have been reactivates. Other ultramylonites in areas, away from the center of Picuris-age deformation in the Picuris Mountains, such as in the Cimmaron Range, remain undated. This study will date in-hand samples from shear zones in the Cimmaron Range and Manzano Mountains at the New Mexico Geochronology Lab to test the sensitivity of this methodology. This methodology will also be applied more regionally to an important tectonic boundary between the Mojave and Yavapai provinces that is present in the Lower Granite Gorge of the Western Grand Canyon where discrete mylonite – ultramylonite zones, some with pseudotachylyte, are embedded within the ~13-km-wide higher temperature Gneiss Canyon shear zone. For the Lower Granite Gorge shear zones, we plan to integrate U-Th-Pb monazite and xenotime in-situ geochronology to help parse out high temperature versus low temperature deformation. Using these combined field laboratories, the nature and timing of regional shear zone reactivation and this ⁴⁰Ar/³⁹Ar methodology will be tested.