Rift extension andfault slip rates in the southern San Luis Basin, New Mexico
— P. W. Bauer and K. I. Kelson

New estimates of a minimum, average slip rate on the Embudo fault zone (EFZ) and a minimum, average, eastwest, extension rate across the southern San Luis Basin of the Rio Grande rift for the last 3 m.y. yield values of 102 m/m.y. and 83 m/m.y., respectively. The estimates are based on the vertical offset of ca. 3 Ma Servilleta Basalt flows across the EFZ southwest of the Village of Pilar, and an average slip vector for that segment of the fault zone. A previous estimate of 130 m/m.y. average, east-west Quaternary (last 1.8 Ma) extension rate across the southern San Luis Basin was based on the slip rates on the Sangre de Cristo and Los Cordovas faults. If these independent estimates of average east-west extension are approximately correct, then the rate of extension across the southern basin may have increased slightly since the late Pliocene. Several other published estimates of slip on the Sangre de Cristo fault can be used to approximate average extension rates as far back as about 28 Ma. Although some of these estimates are poorly constrained, they can be used to speculate that: 1) average San Luis Basin extension rates may have increased since rift inception; and 2) average extension rates may have been greater across the southern San Luis Basin than across the central part of the basin near the Colorado border. The first speculation is difficult to evaluate, as extension rates have certainly varied through time. The second speculation is consistent with published conclusions that the cumulative amount of extension increases northward among rift basins, and that rifting has progressed northward through Neogene time.