INFLUENCES OF TECTONISM AND PALEOCLIMATE ON 15-10 MA DRAINAGE EVOLUTION IN THE NORTHERN RIO GRANDE RIFT, NEW MEXICO

Well-dated, synrift deposits of the Española Basin (EB), extending north into the southern San Luis Basin (SSLB), provide a record of Miocene drainage evolution in the northern Rio Grande rift. Building on previous work, we map and correlate distinct depositional belts, describe how the position and character of drainages changed during 15-10 Ma, and assess the role external controls may have played in drainage evolution. Tilting rates of the west-dipping EB half-graben and activity of its master fault, which links with the east-tilted SSLB via a transfer structure, are interpreted to have increased between 15 and 11 Ma. Volcaniclastic sedimentation, with much input from the eroding Plaza volcanic field in the central SSLB, dominated the SSLB in the early Miocene. These streams flowed south onto a wide basin floor in the EB, where they merged with west-flowing piedmont drainages sourced in the Sangre de Cristo Mountains to the east. Although western basin-floor streams likely continued southward to the Albuquerque Basin, eastern basin-floor streams may have ended in playa lakes in a fault-bounded arm of the southeastern EB. At ~14-13 Ma, a paleoriver draining the Colorado Plateau supplied sand to the 13.5-10 Ma Ojo Caliente eolian dune field. This river shifted northward to a position slightly south of the modern Rio Chama and carried a gravelly bedload after 12 Ma. During 14-13 Ma, bedload coarsening occurred in the western EB and on the eastern SSLB and EB piedmonts. Between 16-10 Ma, the eastern EB piedmont prograded westward ~8 km, with progradation rates highest at 13.5-11 Ma. Tectonics influenced piedmont progradation by: 1) increasing subsidence rates in the western hanging wall of the EB master fault during a time of higher throw rates on this structure and increased tilt rates of the EB half-graben, concomitant with decreased subsidence rates in the southeastern arm of the EB; 2) elevating headwater topography along the eastern rift flank; 3) increasing subsidence in the SSLB, which facilitated burial of the inactive Plaza volcanic field and reduced sediment supply from there. Paleoclimatic changes at 15-13 Ma may have also promoted bedload coarsening, piedmont progradation, and integration of the Rio Chama by increasing stream competency and sediment discharge.