DIFFERENTIAL RIVER INCISION DUE TO QUATERNARY FAULTING AT THE NEXUS OF THE RÍO GRANDE RIFT AND JEMEZ LINEAMENT

We use long-term average bedrock incision rates along the Río Salado-Jemez system of northern New Mexico to test and quantify the hypothesis that differential river incision reflects Quaternary fault slip on the margins of the Río Grande rift and Jemez lineament. By focusing on the oldest (600–400 ka) river terraces, resulting bedrock incision values average out glacial-interglacial cycles and are interpreted 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. New and published dating based on U-series geochronology of travertine-cemented fluvial deposits, tephrochronology on interbedded ash, and ⁴⁰Ar/³⁹Ar dating of detrital sanidines are used to constrain terrace ages across the system. Our terrace correlation working hypothesis using new geochronology suggests that differential incision magnitudes, and the Quaternary fault slip rates inferred from them, are 100-150 m/Ma, similar to river incision rates in neotectonically active regions. Incision rates are 200-250 m/Ma in the upper Jemez River and ~300 m/Ma along the Rio Salado across the southern nose of the Sierra Nacimiento. These are some of the fastest fluvial bedrock incision rates in New Mexico. High rates are on the upthrown-side of the San Ysidro-Jemez fault system and are 100-150 m/Ma higher than the incision rate of ~150 m/Ma constrained by the 630 ka Lava Creek B terraces at the confluence of the Río Jemez and Río Guadalupe on the downthrown side of the fault. 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 demonstrating the expansion of the Río Grande rift westwards and encroaching on the margins of the Colorado Plateau.