OLIGOCENE-MIOCENE BASIN FORMATION AND MODIFICATION IN THE NORTHERN RIO GRANDE RIFT; CONSTRAINTS FROM 40AR/39AR, FISSION TRACK, AND TEPHROCHRONOLOGY

The morphology of the south-central Colorado Rocky Mountains is the product of deformational events/processes from the Precambrian to the present. The most prominent morphological and structural feature in this part of Colorado is the northern Rio Grande rift including the San Luis and Upper Arkansas basins. Using a variety of geochronological methods we have been working to constrain the timing of basin formation (including exhumation of rift flanks), basin deposition, and modification of basin architecture by the youngest phases of deformation. 40Ar/39Ar ages of hornblende from rocks of the southern Sawatch range west of the Poncha Pass region of the Rio Grande rift suggest that these rocks have remained at temperatures less than ~500°C since ~1600 Ma. Biotite and muscovite separates from the same samples yield 40Ar/39Ar ages of ~1300 to ~1400 Ma indicating that Phanerozoic tectonic activity was not accompanied by heating of these samples above temperatures of greenschist-facies conditions. Apatite fission track ages for three samples from the southern Sawatch range west of Poncha Pass range from 19.8±2.3 to 29.0±4.0 Ma suggesting rift-related cooling at that time. Two samples from slightly higher elevations in the same part of the range had apatite fission track ages of ~46-299 Ma indicating possible preservation of the base of the Mesozoic partial annealing zone that is also seen in the Front Range. The San Luis basin, the Poncha Pass transfer zone, and the Upper Arkansas basin all have exposures of poorly-consolidated, Tertiary age sandstones, conglomerates, and volcanic ash deposits. These basin fill deposits have been folded and faulted during continued deformation following the initial rift formation. Using tephrochronology we have determined ages on volcanic ash deposits from two locations. Just south of the Upper Arkansas basin in the northern edge of the Poncha Pass transfer zone two ash layers yield ages of 13.3±0.2 and 14.2±1.4 Ma. To the east, just south of the town of Salida, ash deposits correlate with Yellowstone hotspot ashes with an age range of 8.5-10.5 Ma. The two ash localities are separated by a fault that bounds the southern edge of the Upper Arkansas valley suggesting that deformation responsible for the current basin morphology is younger than 8.5 Ma.