LOW-RATE QUATERNARY FAULTING IN NORTHERN NEW MEXICO

Seismic hazard in northern New Mexico is generally low, based on limited historical seismicity, no large historical earthquakes, and slow-moving faults. Previous work suggests late Quaternary deformation is concentrated on faults bounding margins of the Rio Grande Rift, with faults on the eastern margin of the San Luis Basin to the north and the western margin of the Española Basin to the south, accommodating most of the strain. Faults within and on the eastern margin of the Española Basin, western margin of the San Luis Basin, and within the Colorado Plateau have been considered inactive during the late Quaternary. To better understand the evolution of Quaternary deformation and strain partitioning in the region, we assess the recency, style, and rate of faulting on eight faults (the Cañones, La Cañada del Amagre, Picuris-Pecos, Nambé-Santa Fe, Pojoaque, Puyé, Willow Creek, and Brazos Peak faults) within the Española Basin and eastern margin of the Colorado Plateau near the Chama Basin and Tusas Mountains. We analyzed lidar and optical imagery and integrated our remote observations with field observations and previous geologic and geomorphic mapping. Our observations indicate evidence for low-rate Quaternary activity on six of the eight faults in the region previously thought to be inactive in the late Quaternary. Faults within the Rio Grande Rift have distributed, short (<15 km), N-S to NNE-SSW-striking strands that vertically offset Pleistocene and possibly Holocene terraces and glacial deposits by <4 m, with limited evidence for oblique motion. Faults on the eastern margin of the Colorado Plateau consist of distributed, en echelon NW-SE-striking strands that vertically offset middle Pleistocene to Holocene terraces and glacial deposits, including the ~250 ka Brazos basalt, by <6.4 m, yielding potential late Quaternary vertical slip rates of <0.1 mm/yr, with evidence for up to 40 m of dextral offset of middle Pleistocene terraces. The presence of low-rate, cryptic, active faults away from the primary Rio Grande Rift fault system suggests that strain is distributed on several faults across the region, both within the Rio Grande Rift and on the eastern margin of the Colorado Plateau, and these faults may represent a previously uncharacterized seismic hazard in the region.