REVISITING QUATERNARY FAULTING IN THE CENTRAL RIO GRANDE RIFT (Invited Presentation)

The central Rio Grande Rift (RGR) in south central Colorado and northern New Mexico has limited historical seismicity, no large historical earthquakes, low geodetic extension rates (<1 mm/yr) and low-slip-rate (0.01 – 0.1 mm/yr) Quaternary-active faults. Previous work suggests late Quaternary deformation within the RGR is concentrated on margin-bounding faults, specifically 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 elsewhere within the RGR and Colorado Plateau are considered currently inactive or to have low Quaternary slip rates (<0.01). To better understand the evolution of Quaternary deformation and strain partitioning in the region, we assess the recency, style, and rate of faulting on the Puyé, Picuris-Pecos, Cañones, and La Cañada del Amagre faults within the Española Basin of the RGR and the Willow Creek and Brazos Peak faults in the eastern margin of the Colorado Plateau. We analyzed 1-m lidar-derived topography and optical imagery and integrated our remote mapping with field observations, ¹⁰Be cosmogenic and luminescence chronologies, and previous geologic and geomorphic mapping. Our observations indicate low-rate Quaternary activity on these faults, with faults within the RGR characterized by distributed, short (<15 km), N-S to NNE-SSW-striking strands that vertically offset Pleistocene and possibly Holocene terraces and glacial deposits. The Puyé fault vertically offsets middle Pleistocene (~264 ka) fluvial terraces by up to 30 m and has a slip rate of ~0.1 mm/yr, an order of magnitude higher than previous estimates. Faults on the eastern margin of the Colorado Plateau consist of distributed, en echelon NW-SE-striking fault strands that vertically offset middle Pleistocene to Holocene terraces and glacial deposits. For example, the Willow Creek fault displaces the ~250 ka Brazos basalt by <6.4 m, yielding Pleistocene vertical slip rates of <0.1 mm/yr, with evidence for oblique (normal-dextral) offset of middle Pleistocene terraces. The presence of low-rate active faults away from the margins of the RGR suggests that strain is currently distributed on faults within the RGR and on the eastern margin of the Colorado Plateau, and these faults may represent a previously uncharacterized seismic hazard in the region.