40AR/39AR DETRITAL SANIDINE AND BASALT GEOCHRONOLOGY CONSTRAINTS FOR THE BIRTH AND EVOLUTION OF THE RIO GRANDE RIVER

Contrasting published models for the birth and evolution of the Rio Grande are evaluated using ⁴⁰Ar/³⁹Ar basalt and detrital sanidine (DS) geochronology. Repasch et al. (2017) concluded that both Rio Grande and Rio Chama had headwaters in the San Juan volcanic field (SJVF), based in part on 37-27 Ma detrital zircon (DZ) grains (apparent SJVF ages) in post-5 Ma sediment. Taos Plateau volcanism was accompanied by downstream (Black Mesa) inset relationships suggesting a developing 5-3 Ma river valley connecting the San Luis and Espanola rift basins. In this model, Taos Plateau volcanism blocked or re-routed drainages and the ca. 400 ka Lake Alamosa spillover, for example, was the latest river-damming/spillover episode that reintegrated the upper Rio Grande to its SJVF headwaters. In contrast, Wells et al. (1987) suggest that the Ancestral Rio Grande had its headwaters in the modern Red River drainage until 600-300 ka, and more recently, Machette et al. (2013) proposed that the upper Rio Grande, above the Red River confluence, was only integrated to SJVF headwaters following spillover of the Ancestral Lake Alamosa at ca. 400 ka. Our DS studies show that modern to 5 Ma river deposits near and downstream of the confluence of the Red and Rio Grande rivers have detrital sanidine with significant Latir volcanic field detritus consistent with establishment of the Ancestral Red River as early as 5 Ma. These deposits also have significant 28 to 37 Ma DZ that suggested a SJVF connection. However, detrital sanidine data are substantially more precise than DZ data and have the added benefit of K/Ca data that allows for more accurate connections to volcanic sources. DS data from and below the Red River confluence region for deposits between 2.5 and >~5 Ma do not favor a SJVF connection, but rather indicate ultimate sources in the Mogollon-Datil volcanic field (MDVF). This poses the possibility that 28-37 Ma DS detritus was deposited into Rio Grande rift sediments from MDVF sources, then were reworked into a developing Rio Grande river system, but without a direct SJVF connection at 3-5 Ma. Lastly, a DS maximum deposition age of 1.2 Ma from river gravel overlying a 3.4 Ma basalt on the rim of the Taos Box near the Red River confluence indicates that the modern upper Taos Box was carved after 1.2 Ma at a minimum average incision rate of ~230 m/Ma.