3HE AND 21NE SURFACE EXPOSURE DATING ALONG THE RIO GRANDE GORGE AND ITS TRIBUTARIES: IMPLICATIONS FOR INCISION RATES AND RIO GRANDE GORGE FORMATION IN SOUTHERN COLORADO AND NORTHERN NEW MEXICO

Prior to ~420 ka the Rio Grande (RG) was sourced south of the San Luis Hills (SLH). After the draining of paleo Lake Alamosa at ~420 ka, the RG captured 25,000 km² of watershed associated with the northern San Luis Valley drainage basin. We present ³He and ²¹Ne cosmogenic surface exposure ages for several sites along the RG gorge and its tributaries to determine the timing of this event and to unravel the geomorphic history of the northern RG rift. An upper beach deposit related to Lake Alamosa yielded a ³He age of 419 ka and ²¹Ne age of 441 ka indicating that incision of the upper RG initiated at or soon after 420 ka. ³He results downstream at the junction of the RG/Red River (RR) suggest that minor incision by the RG began at ~600 ka before deep canyon incision started northward towards the SLH. Exposure ages along the gorge progressively decrease from 300 ka at Arroyo Hondo (AH) to 180 ka at the SLH. Exposure ages presented here enable us to determine regional incision rates. Calculated incision rates tend to decrease from 0.5 mm/yr at AH to 0.03 mm/yr at the SLH. Incision rates at AH appear to have remained constant at 0.5 mm/yr since 300 ka. The formation of the RG gorge appears to have taken 218 ka at AH and 180 ka at the SLH. However, ³He exposure ages of ~600 ka at the RG/RR confluence indicates that minor incision occurred earlier.