TECTONIC GEOMORPHOLOGY OF THE JEMEZ SYSTEM: RESOLVING QUATERNARY RIVER RESPONSE TO DYNAMIC LANDSCAPES USING 40AR/39AR GEOCHRONOLOGY

Compilation of new and published incision rate data in the tributaries of the northern Río Grande system, the fourth longest river in the US, show differential incision (at a scale of 100 m/Ma) as rivers cross the Jemez lineament and Southern Rockies at long wavelengths that may reflect mantle buoyancy driving forces. Two major tributaries of the Jemez River, the Río Salado and Río Guadalupe, drain the western margins of the Valles Caldera and Sierra Nacimientos. Unpublished U-series dated travertines along the Río Salado suggest it is one of the fastest incising river systems in the Southern Rockies. Neotectonic forcings that may focus higher river incision at shorter, subregional wavelength include Pleistocene thermal inflation of the Valles Caldera and/or young faulting. We evaluate the extent to which epeirogenic uplift or local tectonics may be responsible for the differential incision rates across this dynamic system. Geomorphic signals from climate are minimized by measuring bedrock incision over million-year timescales that average out glacial-interglacial cycles, and by concentrating on this relatively small area within the broader Jemez Mountains.

We sampled five major previously mapped Quaternary Río Salado terraces (Qt₇, Qt₆, Qt₅, Qt₃, Qt₁) and found few young DS grains that usefully corroborate ages and rates obtained through U-series dating of travertines. Additional detrital sanidine (DS) geochronology sampling of ‘cryptic’ ashes from river terraces along the main stem stream provides an age-correlation for Qt₁ terraces of the Jemez River system. Newly sampled terraces at the confluence of the Río Guadalupe and Jemez River provide Lava Creek B (640 ka) maximum depositional ages (MDA) that may serve as a benchmark terrace to trace downstream. If DS ⁴⁰Ar/³⁹Ar geochronology supports high incision rates for all rivers, higher than regional average rates, this may implicate the young (1.61 and 1.23 Ma) Valles Caldera eruptions as the driver of enhanced young surface uplift within a broad region of potential mantle-driven uplift in the Southern Rockies. However, if rates in the Río Salado do not agree with other tributaries and terraces of the Jemez River, this may suggest young reactivation of the Nacimiento fault and neotectonic footwall uplift in its southernmost reaches near the Tierra Amarilla anticline.