QUATERNARY REACTIVATION OF LARAMIDE FAULTS IN NORTH-CENTRAL NEW MEXICO

Northern New Mexico preserves a complex deformational history. Regional tectonic deformation spans the Paleozoic through the Cenozoic, with shortening during the Laramide Orogeny in the late Cretaceous/early Tertiary and extension from the Rio Grande rift in the Tertiary. Laramide-aged faults crosscut the region, resulting in pre-existing weaknesses in the crust. In north-central New Mexico, most of these faults are assumed to be inactive based on limited historical seismicity and a lack of identified offset Quaternary deposits. Analysis of recently available high-resolution (~1m) bare-earth lidar data and existing geologic and geomorphic maps reveals at least three faults that displace Quaternary deposits: the Willow Creek and East and West Brazos Peak faults. These faults appear to have a primarily normal sense-of-motion with subtle evidence for lateral offsets.

The Willow Creek fault, ~15 km west-southwest of Chama, NM, extends for nearly 50 km NW-SE with down-to-the-northeast displacements. Scarp heights range from <0.5 to 8.5 m on five different Quaternary surfaces, with displacement increasing with relative surface age and elevation above the Rio Chama. Based on a ~5-m-high scarp on the ~250 ka Brazos basalt, the Willow Creek fault has an estimated minimum vertical slip rate of ~0.02 mm/yr but may be as high as ~0.05 mm/yr using estimated ages for other faulted surfaces along strike. Evidence for oblique slip is recorded by right-lateral offset of a middle Quaternary (?) terrace riser, and by a linear and en-echelon stepping fault pattern.

The West Brazos Peak fault, located ~15 km southeast of Chama, also strikes NW-SE and extends for ~15 km. The fault displaces a late Quaternary (?) terrace by <4 m down-to-the-southwest. The East Brazos Peak fault, located ~20 km southeast of Chama, has clear geomorphic expression across the landscape for ~20 km, forming a generally down-to-the-east escarpment, but does not clearly cut any mapped Quaternary surfaces. The clear geomorphic expression and offset of Quaternary surfaces on the Willow Creek and West Brazos Peaks faults in relationship to folded Mesozoic stratigraphy indicates that Laramide-aged faults have been reactivated as normal or dextral-oblique faults in the Quaternary. Although the faults appear to have low slip rates, we suggest they should be included in regional and local seismic hazard assessments.