Differential river incision due to Quaternary faulting on the Rio Salado-Jemez system at the million-year scale
— Cameron C. Reed, Karl E. Karlstrom, Ben Rodriguez, Nels A. Iverson, Matthew T. Heizler, Dylan Rose-Coss, Laura J. Crossey, Chris Cox, April Jean, Victor J. Polyak, and Yemane Asmerom, [eds.]

Abstract—Analysis of long-term average bedrock incision rates along the Rio Salado-Jemez system using fluvial terraces can be used to test and quantify the hypothesis that differential river incision reflects Quaternary fault slip during ongoing uplift of the Jemez and Nacimiento Mountains. Strath terrace flights, especially the 400–630 ka oldest straths, were correlated from the Arroyo Peñasco in the south-western Sierra Nacimiento to the Rio Salado near the southern nose of the Nacimiento and along the Rio Jemez from San Ysidro to the Rio Guadalupe confluence and Soda Dam. By focusing on highest/oldest (400–600 ka) river terraces, resulting bedrock incision values average out ~16 glacial/interglacial cycles and are interpreted here to reflect differential uplift. For previously mapped and correlated terraces, we applied high resolution topography (HRT) datasets (1-m lidar) to refine strath heights above river level. Terrace height alone cannot be used for correlation across active faults, so we applied new and published dating based on U-series dating of travertine-cemented fluvial deposits, tephrochronology on interbedded ash, and 40Ar/39Ar dating of detrital sanidines to constrain terrace ages across the system. Significant terrace correlation uncertainties remain because of variable terrace flight preservation, need for additional age control, complexities of applying maximum/minimum age constraints, and fault geometry. Our terrace correlation working hypothesis using new geochronology suggests that Quaternary fault slip rates are similar to river incision rates, as expected in neotectonically uplifting regions. We conclude from differential incision magnitudes that uplift of the Jemez Mountains and southern Sierra Nacimiento is taking place at ~150 m/Ma relative to the RioGrande rift and San Juan Basin over the past ~500 ka and that this surface uplift is driving some of the fastest fluvial bedrock incision ratesin New Mexico at the southern nose of the Sierra Nacimiento (~300 m/Ma) and at Soda Dam on the Rio Jemez (200-250 m/Ma). Quaternary faulting is interpreted to be enhancing upthrown-side incision rates by ~100 m/Ma relative to downthrown-side bedrock incision rates of ~150 m/Ma at the confluence of the Rio Jemez and Rio Guadalupe. The proposed mechanisms driving this interaction of differential river incision and Quaternary faulting is magmatic inflation of the Jemez Mountains and related reactivation of the network of Laramide and Miocene faults around the Nacimiento-Jemez uplifts.