LANDSCAPE DENUDATION ALONG THE WESTERN EDGE OF THE RIO GRANDE RIFT USING BASALT PALEOSURFACES

The western edge of the Rio Grande rift, and the adjacent eastern Colorado Plateau, expose several Pliocene basalts that flowed down paleodrainages and preserve those drainages’ positions as inverted topography. In particular, in the Carrizo Arroyo area, the Carrizo Arroyo incises through the 3.7 ± 0.4 Ma Mesa Carrizo basalt flow (Bachman and Mehnert, 1978, K-Ar age) that flowed SE, subparallel to the modern Rio Puerco. The basalt caps a gravel deposit that consists predominantly of pebble to cobble sized, subangular to subrounded clasts of limestone, sandstone, and siltstone that were derived from local Paleozoic outcrops, as well as minor amounts of basalt and quartzite with unknown sources. The deposits contain on average 45 percent limestone and 38 percent sandstone. Clast imbrications in the gravel deposits indicate SE paleoflow direction, similar to the basalt paleosurface and the modern Rio Puerco. From map view it can be seen that this basalt may have filled a drainage that flowed east from a vent on top of the Colorado Plateau, crossed the rift flank near Carrizo Arroyo, and then turned south as it entered the Rio Grande rift. The total run-out of the flow was about 20 km. The base of the basalt is now 183 m above Carrizo Arroyo where the drainage cuts across the flow, giving a long term average incision rate of 50m/Ma from the base of the basalt to the local base level over the last 3.7 Ma. On a larger scale, using the Rio Puerco as base level, the landscape has been denuded about 370 m in the last 3.7 Ma, giving a regional incision rate of about 100 m/Ma. These rates give an estimate of the long-term, local landscape denudation and are similar to those reported by Love and Connell 2005 for denudation of the eastern Colorado Plateau. Other basalt run outs include the Cerro Verde flow and Rio San Jose flow, which also give similar denudation magnitudes of about 100m/Ma. Gradients of the basalt paleosurfaces and improved data on differential incision rates and magnitudes can help decipher any landscape affects associated with the Jemez lineament, Rio Grande rift, and Socorro Magma body.