INSIGHTS ON PROVENANCE AND PLIO-EARLY PLEISTOCENE DRAINAGE EVOLUTION OF THE ANCESTRAL RIO GRANDE RIVER IN CENTRAL AND SOUTHERN NEW MEXICO

Axial-fluvial strata exposed throughout the Rio Grande rift corridor preserve a ~4 m.y. record of provenance and drainage-network configurations from the Plio-early Pleistocene ancestral Rio Grande river just prior to incision and the arrival of the modern river system at ~1 Ma. Presented here are U-Pb detrital zircon ages from 8 samples (N=2382) collected from the Camp Rice and equivalent Palomas Formation in the Socorro, Hatch/Rincon-Jornada del Muerto, and Mesilla basins in central and southern New Mexico. Samples were collected from the three basins at equivalent stratigraphic horizons (5.0, 3.1, and 1.6 Ma) where known age constraints exist.

The Socorro basin is the furthest upstream locality in this study and records peak ages at 1684, 1442, 1076, 520, 421, 168, 87, 34, and 5 Ma. The Hatch/Rincon-Jornada del Muerto basin is downstream and south of the Socorro basin and exhibits peak ages at 1679, 1431, 1072, 618, 514, 421, 217, 166, 83, 35, 28, and 5 Ma. The Mesilla basin is the southernmost downstream locality in this study and preserves peak ages at 1687, 1432, 1034, 602, 522, 430, 224, 189, 165, 95, 64, 35, and 28 Ma. Peak ages from all samples overlap with Precambrian source areas of the Yavapi-Mazatzal, A-type granite, and Grenville provinces. The strongest Phanerozoic peaks overlap with the Permian–Cretaceous Cordilleran arc and late Eocene–Oligocene calderas of southern New Mexico.

Although peak ages are similar at each stratigraphic horizon across all three basins, there are distinct upsection and down-system spatial changes in percent occurrence of zircon populations. The highest percentage of latest Eocene–Oligocene ages occur in the oldest parts of each basin whereas grains of this age drop out at the expense of Jurassic–Cretaceous age zircons in younger parts of the basins. We interpret this trend to reflect an upsection transition from initial, caldera-dominated sources available during the Pliocene, to more Colorado plateau-dominated recycled sources during the Pleistocene stage of drainage development. Miocene–Pleistocene ages are common in the Socorro and Hatch/Rincon-Jornada del Muerto basins but absent in the Mesilla basin. The lack of young zircons in the Mesilla basin likely reflects a down-system decreased in availability of detritus from Plio-Pleistocene rocks of the Valles Calldera.