EVOLUTION OF CENOZOIC EXTENSIONAL BLOCK FAULTING AND SEDIMENTATION IN THE SOUTHERN RIO GRANDE RIFT, NEW MEXICO

Extensional block faulting and sedimentation in the southern Rio Grande rift, New Mexico, began in latest Eocene-early Oligocene time (35-28.5 Ma) with creation of the Goodsight-Cedar Hills half graben, where up to 250 m of sediment, fallout ash, ash-flow outflow sheets, and basalt of the Bell Top Formation was deposited. Eruption of basaltic andesite (Uvas Fm; 26-28 Ma) and southward progradation of a volcaniclastic apron (Thurman Fm; 27.4-27 Ma) dominated the late Oligocene. By latest Oligocene through most of the Miocene, volcanism had ceased, but faulting uplifted the Caballo Mts, Dona Ana Mts, Organ-San Andres Mts, and Sierra de las Uvas, with concomitant deposition of about 2 km of sediment of the Hayner Ranch and Rincon Valley Formations in closed basins floored by playa lakes. Another major pulse of deformation, beginning after 9.8 Ma, produced the modern distribution of uplifts and basins and which continues with diminishing intensity to the present day. Faulting stepped into the former Miocene basins, resulting in uplifts such as the Robledo Mts, Cedar Hills, Red Hills, Rincon Hills, San Diego Mt, and Animas Hills. Deposition of about 150 m of sediment of the Pliocene-early Pleistocene Palomas and Camp Rice Formations took place in the new, generally narrower basins. The latest pulse of deformation coincided with eruption of asthenosphere-derived basalt. The ancestral Rio Grande arrived in the southern Rio Grande rift ~5 Ma and periodically occupied six contiguous basins (Palomas, Hatch-Rincon, Corralitos, Jornada del Muerto, Mesilla, Tularosa-upper Hueco). The ancestral Rio Grande emptied into Lake Cabeza de Vaca in the Hueco basin of far west Texas, breaching the lake ~2.2 Ma. Palomas/Camp Rice deposition ended ~0.78 Ma, when the Rio Grande and its tributaries began to alternately incise and partially backfill the basins. Several faults have evidence of post-early Pleistocene movement, and the Organ and Caballo faults have demonstrable Holocene movement.