DETAILED STRAIN AND CPO ANALYSIS OF A QUARTZITE METACONGLOMERATE REVEALS A METAMORPHIC DISCONTINUITY ACROSS THE PLOMO DUCTILE SHEAR ZONE IN THE PICURIS MOUNTAINS OF NORTHERN NEW MEXICO

The Picuris Mountains of Northern New Mexico may provide some of the best evidence for large-scale tectonism in Laurentia during the Mesoproterozoic (e.g., Daniels et al., 2013). Recent studies have concluded that significant tectonism and metamorphism in the Picuris Mountains occurred around 1.4 Ga, rather than during the more typical 1.5-1.6 Ga Mazatzal time span. The Picuris Mountains contain two map-scale ductile thrusts that are believed to have developed during the 1.4 Ga event. The southern thrust zone, the Plomo Fault, juxtaposes highly strained quartzite metaconglomerate and aluminous schist. The metaconglomerate provides a natural laboratory for quantifying strain, deformation partitioning, and rheology within the deformed quartz pebbles. Macroscale calculations of the strain ellipse based on deformed quartzite clast shapes indicate oblate strain, with a Lode’s parameter of 0.5 – 0.7 and octahedral shear value of 1.0 – 1.3. Orientated thin sections were analyzed using electron backscatter diffraction (EBSD) to quantify crystallographic preferred orientation (CPO) in the deformed quartzite clasts. EBSD results also demonstrate flattening strain at the microscale. The opening angles between crossed girdle patterns of quartz c-axis pole figures were measured at 86-88 degrees. When compared to published quartz opening-angle calibrations, these values correlate to conditions of ~650 °C and 6 kbar, consistent with deformation and metamorphism of the metaconglomerate in a deep orogenic setting. The conditions based on opening angles are ~100 °C hotter and ~1 – 1.5 kbars greater than metamorphic conditions determined from the metasedimentary units juxtaposed immediately across the Plomo Fault. This suggests that the metaconglomerate unit was exhumed from a structurally deeper level by slip on the Plomo Fault. We conservatively estimate 3.5 to 5.5 km of exhumation, which supports the interpretation that the Plomo Fault was a major orogenic structure.