DETRITAL ZIRCON PROVENANCE TRENDS FROM PLIO-PLEISTOCENE, AXIAL-FLUVIAL STRATA OF THE RIO GRANDE RIFT: A ~4 M.Y. HISTORY OF THE ANCESTRAL RIO GRANDE FLUVIAL SYSTEM, NEW MEXICO

Axial-fluvial strata exposed in the Rio Grande rift (RGR) corridor throughout New Mexico preserve a ~4 m.y. record of drainage evolution during the Plio-early Pleistocene integration of the 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 Palomas formations (Upper Santa Fe Group equivalents) in the Socorro, Hatch/Rincon, Jornada del Muerto, and Mesilla basins in central and southern New Mexico.

The northernmost Socorro basin records peak ages at 1684, 1442, 1076, 520, 421, 168, 87, 34, and 5 Ma. To the south, the Hatch/Rincon and Jornada del Muerto basins exhibit 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 Yavapai-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.

Comparison of detrital zircon trends with previous studies in the northernmost portions of the RGR in northern New Mexico and southern Colorado provide a spatial and temporally extensive record of drainage configuration during the Plio-Pleistocene phase of drainage development, and record distinct up-section and spatial variations in percent occurrence of zircon populations. Provenance trends record an upsection transition from initial, Eocene-Oligocene caldera-dominated sources widespread during the Pliocene, to more Colorado Plateau-dominated recycled sources during the Pleistocene stage of drainage development. Trends are interpreted to reflect denudation of the Colorado Plateau possibly as a result of headward erosion of the Rio Puerco and Rio San Jose during the late Pliocene. The absence of zircons that overlap in age with late Cenozoic volcanic fields at the youngest stratigraphic horizons may reflect hydrologic closure of the Upper San Luis basin following the emplacement of the Taos Plateau volcanic field in northern New Mexico and southern Colorado.