BIRTH AND EVOLUTION OF THE RIO GRANDE FLUVIAL SYSTEM IN THE LAST 8 MA: PROGRESSIVE DOWNWARD INTEGRATION AND INTERPLAY BETWEEN TECTONICS, VOLCANISM, AND RIVER EVOLUTION

New basalt ages coupled with new detrital zircon and sanidine age data for terrace deposits reveal that the ancestral Rio Chama (RC) developed by 8 Ma and the birth of the ancestral Rio Grande (RG) took place ~5 Ma with headwaters in the San Juan Mountains of Colorado. We report 17 new ⁴⁰Ar/³⁹Ar basalt ages and detrital mineral (zircon and sanidine) age distributions for 20 samples of RG-RC alluvium. The “run-out” geometry of 4.8 Ma basalts and underlying mainstem gravels at Black Mesa suggests that the course of the northern RG was established by then. Ancestral RG detrital zircon age spectra have peaks at 21 Ma (Servilleta Plaza volcanics), 28 Ma (southern San Juan volcanic field), 30-35 Ma (northern San Juan volcanic field), and 70-100 Ma (recycled San Juan Basin sediment). Detrital sanidine ages reveal age peaks at 19.8, 25.4 (Latir Volcanic Field), 28.72, 29.3, and 34.8 Ma (San Juan volcanic field). The larger percentage (10-12%) of 37-27 Ma grains in the alluvium is evidence for an early RG with headwaters in the San Juan Mountains. The 5 to 3 Ma building of the Taos Plateau Volcanic Field reduced through-flowing discharge from the San Juan Mountains relative to the Sangre de Cristo Mountains until spillover of Lake Alamosa re-integrated upper RG drainage to the system between 0.69 and 0.44 Ma. However, inset relationships at Black Mesa show that the RG was incising a gorge from 5 to 3 Ma in downstream reaches as basalts aggraded on the Taos Plateau. Temporal relationships and pre-volcanic field knickpoints suggest that surface uplift associated with the construction of the Taos Plateau volcanic field drove downward integration of the RG to southern NM by 4.8 Ma. River damming events resulted from local volcanism; lake spillover events were driven by a combination of pluvial climates and continued headwater uplift in the southern Rocky Mountains. Changes in ancestral RG sediment provenance from 2.6 Ma to 1.6 Ma document a northward shift of the RG-RC confluence due to surface uplift of the Valles Caldera magmatic system (1.6 to 0.5 Ma), which also facilitated integration of the river to the Gulf of Mexico by ~1 Ma. Increased amplitudes of glacial-interglacial cycles increased discharge and incision rates during the Pleistocene, however magmatic and tectonic drivers dominated the drainage evolution of the RG-RC system over the last ~8 Ma.