LAUSR

Abstract The Rio Grande fluvial system has evolved dramatically over the past 8 Ma, undergoing channel migrations, drainage capture and integration events, volcanic damming, and carving and refilling of paleocanyons. This paper is motivated by the need for a synthesis aimed at understanding processes driving regional drainage development, especially the roles of rifting, volcanism, and climate change. Major conclusions supported by our synthesis of published data are as follows: 1) 20–8 Ma southward drainage from the San Juan volcanic field into the deepening Rio Grande rift resulted in deposition of thick basin fill of the Santa Fe Group by rivers flowing on a south-sloping alluvial plain in the north and by alluvial fans within fault-segmented internally drained rift basins. 2) The ancestral Rio Chama river system developed by ~ 8 Ma to link northern New Mexico basins (Espanola and northern Albuquerque basins). 3) The ancestral Rio Grande joined the Rio Chama by 5 Ma and linked drainage from the San Juan Mountain headwaters to central New Mexico as an integrated axial river system. 4) The Rio Grande extended its reach progressively southward to central New Mexico (Palomas basin) by 4.5 Ma, to southern New Mexico (Mesilla basin) by 3.1 Ma, to Texas (Hueco basin) by 2.06 Ma, and joined the Pecos River to reach the Gulf of Mexico by ~ 800 ka. 5) The change from aggradation to bedrock incision to carve the Rio Grande valley took place after 1 Ma with differential incision rates of 10 to 150 m/Ma. Volcanism concurrent with the development of the river system provides a unique opportunity to apply multiple geochronometers to the study of its incision and drainage evolution. This paper reports seventeen new and 79 previously unpublished 40Ar/39Ar basalt ages in the context of a compilation of published geochronology. We also report detrital zircon ages for nineteen samples of ancestral Rio Grande-Rio Chama sediment, and compare them to eleven detrital K-feldspar samples to evaluate this potentially powerful new detrital grain analysis. The elongate geometry of 4.8 Ma basalt mesas in the Espanola basin suggests that the course of the Rio Grande connecting northern and central New Mexico drainage was established by ~ 5 Ma. Detrital zircon age spectra for ancestral Rio Grande alluvium underlying these basalt flows contain 10–12% of 37–27 Ma grains suggesting that the ~ 5 Ma Rio Grande was carrying detritus from, and likely had its headwaters in, the San Juan Mountains of southwestern Colorado. The 5 Ma to 3 Ma accumulation of an ~ 240-m-thick section of basalt flows on the Taos Plateau was accompanied by inset relationships downstream (Espanola basin), documenting the existence of a developing 5 to 2.5 Ma Rio Grande valley with an axial river. Coincident timing and inferred pre-volcanic knickpoints suggest that the construction of the Taos Plateau volcanic field in northern New Mexico helped drive downward integration to southern New Mexico by 4.5 Ma. Changes in ancestral Rio Grande sediment provenance from 2.6 Ma to 1.6 Ma document a northward shift of the Rio Grande-Rio Chama confluence and indicate that surface uplift of the Jemez Mountains diverted and reconfigured the river system, and likely drove further downstream integration. The Taos Plateau volcanic field reduced through-flowing surface drainage from the San Juan Mountains relative to the Sangre de Cristo Mountains until the ~ 440 ka spillover of Lake Alamosa in south-central Colorado, and we view this event as a re-integration, not initial integration, of upper Rio Grande drainage. Progressive downward integration of Rio Grande rift basins from 8 to 1 Ma was facilitated by a combination of processes: increased river gradients in the upper basin due to construction of volcanic fields and potential epeirogenic uplift; increased discharge due to climate change events; waning rift extension that allowed aggradation to exceed subsidence; dampening of topographic divides between basins by aggradation; probable groundwater connectivity; and lake spillover events. Downward integration events may crudely correlate to climate change “events” at 6 Ma (onset of the southwestern monsoon) and 2.6 Ma (global change toward glacial-interglacial climate). Magmatic influences included the building of the 6–2.5 Ma Taos Plateau volcanic field, construction of the 10 to 0 Ma Jemez Mountains, and 6 Ma to 0 Ma Jemez lineament volcanism that was likely associated with mantle-driven surface uplift in a northeast-trending zone across northern New Mexico. River damming events were driven by volcanism in the northern Rio Grande rift, while basin spillover/groundwater sapping events were punctuated by a combination of pluvial climates and continued headwater uplift in the southern Rocky Mountains. Integration of the Rio Grande system to the Gulf of Mexico by ~ 800 ka was facilitated by headwater uplift as well as the onset of ~ 100 ka high-amplitude glacial-interglacial cycles at ~ 900 ka that provided higher discharge and bedrock incision rates during the Pleistocene. We conclude that magmatic and tectonic forcing dominated over the last ~ 8 Ma, but were amplified by climate change events to determine the fluvial evolution of the Rio Grande system.