GEODETIC MEASUREMENTS OF DEFORMATION IN THE RIO GRANDE RIFT REGION

We use 3 years of measurements from 25 continuous GPS stations across the Rio Grande Rift in New Mexico and Colorado to estimate surface velocities, time series and strain rates. The stations are part of the EarthScope Rio Grande Rift GPS experiment, a collaboration between researchers at the University of Colorado at Boulder, the University of New Mexico, and Utah State University. The network is arranged in 5 east-west profiles transecting the rift, with the southernmost line in southern New Mexico and the northernmost line in northern Colorado. Most of the stations have shallow-drilled braced monuments installed in 2006-2007 and will remain occupied until 2010-2011 or longer. Position estimates from regional Plate Boundary Observatory (PBO) stations and data from Continuously Operating Reference Stations (CORS) are included in the processing to increase station density and extend profiles across the Colorado Plateau to the west and further east into the Great Plains. Time series from the first three years of the experiment show excellent monument stability. Velocity gradient estimates from five east-west profiles suggest that the strain rate field across the physiographic provinces of the Colorado Plateau, the Rio Grande rift and the Western Great Plains can be characterized by east-west extension with an average gradient of ~1.2 nstr/yr. We find that east-west extension rates for the four northern profiles that transect northern Colorado, southern Colorado, northern New Mexico and central New Mexico increase from north to south, which is consistent with a clockwise rotation of the Colorado Plateau around a pole of rotation north of the plateau. Our southernmost profile, which lies to the south of the Colorado Plateau, shows that the rate of extension within the boundaries of the Rio Grande rift in southern New Mexico is 0.5 ± 0.4 mm/yr. Velocity gradients for the four northern profiles suggest that deformation may not be concentrated in a narrow zone but distributed broadly across a region spanning from the western edge of the Colorado Plateau to several hundred km east of the Rio Grande rift. This unexpected broadly distributed deformation at the surface has important implications for lithospheric strength, fault mechanics, and earthquake hazards.