Low‐Rate Faulting on the Margin of the Colorado Plateau and Rio Grande Rift in North‐Central New Mexico

Abstract

AbstractIn northern New Mexico, seismic hazard outside of the Rio Grande Rift (RGR) is generally considered low due to limited historical seismicity and few mapped Quaternary faults. Although the region has a complex tectonic history, nearly all known faults are considered inactive because of a lack of faulted Quaternary deposits. Analysis of lidar and existing geologic and geomorphic mapping permits identification and characterization of four faults on the Colorado Plateau/RGR margin. The Gallina, Willow Creek, and East and West Brazos Peak faults are normal or dextral‐normal faults that strike NW‐SE with lengths from 15 to 50 km. The Willow Creek fault has scarp heights <0.5–6.4 m on five Quaternary surfaces, with displacement increasing with relative surface age. An ∼5‐m‐high scarp on the ∼250 ka Brazos basalt yields an ∼0.02 mm/yr minimum dip‐slip rate. Oblique slip is recorded by dextral offset (∼40 m) of a middle Quaternary (?) terrace riser, a linear and en‐echelon stepping fault pattern, and a consistent down‐to‐the‐northeast fault expression. The West Brazos Peak fault displaces late Quaternary (?) surfaces by <4 m down‐to‐the‐southwest. We interpret these faults are Laramide‐aged faults reactivated as normal or dextral‐oblique faults in the Quaternary, consistent with regional stress and geodetic data. Although the faults have low slip rates (<0.1 mm/yr), the fault lengths suggest they may rupture in M6‐7 earthquakes and may pose a previously unrecognized seismic hazard to the region. Dextral transtension within the eastern Colorado Plateau is consistent with geodetic strain rates and earthquake focal mechanisms.