QUATERNARY FAULTING ON THE PUYE FAULT ZONE, ESPAÑOLA BASIN, NEW MEXICO

The Española Basin in the central Rio Grande Rift (RGR) of northern New Mexico is a slowly deforming, 60-km-wide region, with low geodetic extension rates (~1 mm/yr) and low-slip-rate (0.01 – 0.1 mm/yr) Quaternary-active faults. While previous work suggests that deformation is concentrated on the Pajarito fault on the western side of the basin, new neotectonic mapping on 1-m bare earth lidar data indicates that additional faults within the basin may slip at higher rates than previously thought, including the Puye, Picuris-Pecos, La Cañada del Amagre, and Cañones faults. We focus on the Puye fault, a distributed fault system in the central-western side of the Española Basin. The fault zone comprises at least four subparallel primary strands that offset the same geomorphic surface, with a primarily down-to-the-east sense of motion. The mapped fault strands trend north-south and are at least 12 km long. The highest scarps in our study site record 12 m of net vertical separation of the surface. Although geochronology results are still pending, we estimate the surface could be as old as Middle Pleistocene (~130 - 774 ka) based on soil development and correlation with regional surfaces, which translates to middle-late Quaternary slip rates that may exceed known rates on the Pajarito fault (0.1 mm/yr). In a wash that crosses the base of the largest scarp, ~1 m net vertical separation of younger inset deposits suggests repeated events on this strand of the fault system during the late Quaternary. We excavated three 1- to 2-m-deep pits to collect cosmogenic ¹⁰Be and optically stimulated luminescence (OSL) samples to date the age of the surface, describe the soils, and calculate a slip rate on the Puye fault system. In addition, we collected OSL samples to date the youngest offset deposits in the wash. The results of this work will be used to characterize the geologically observable distribution of strain at this latitude (~36 degrees north) in the RGR to reduce uncertainties in inputs for site-specific probabilistic seismic hazard analyses at nearby dams and regional seismic hazard models.