HISTORY OF UPLIFT, EXTENSION AND SEDIMENTATION IN THE NORTHERN RIO GRANDE RIFT USING 40AR/39AR GEOCHRONOLOGY

The Rio Grande rift in the southwestern U.S. is a large Cenozoic mid-continental rift. Pre- and syn-rift volcanic rocks and basin-fill sedimentary rocks in the northern part of the rift are now exposed in uplifted blocks along rift margins, where related basins preserve the record of uplift, extension, and sedimentation. Field relationships and ⁴⁰Ar/³⁹Ar data for a sequence of middle to late Cenozoic volcanic rocks from the Sangre de Cristo Mountains and adjacent San Luis Valley constrain inception of northern Rio Grande rifting to circa 25 Ma.

Between 25 and 15 Ma, based upon geologic mapping and age constraints, the northern Rio Grande rift was still relatively broad and shallow. At 15 Ma large-scale uplift and block faulting occurred along rift margins. Deposition of sediments and volcanic rocks continued within the rapidly developing central depression of the rift to 4.7 Ma, when young intrabasin basalts were deposited with upper Santa Fe sediments.

We present two uplift rates for the northern Rio Grande rift based upon an offset 26-25 Ma surface and offset 4.6 Ma Servilleta basalts of the northern Taos Plateau. Based upon the displacement (~1450 m) and age of the 26-25 Ma surface, we have determined a first order uplift rate of 0.058 km/m.y. An uplift rate of 0.087 km/m.y. was determined for the 400 meters of offset Servilleta (4.7 Ma) basalts. The second uplift rate is similar to several studies using apatite fission-track dating north and south of the Colorado-New Mexico border that give rates of 0.28-0.14 km/m.y. These uplift rates suggest that there were variable rates of uplift during the 26 m.y. of rift history.