The EarthScope Rio Grande Rift GPS Experiment: Measuring Active Tectonics in Colorado and New Mexico

The Rio Grande Rift is the easternmost deforming province within the tectonically active western margin of North America. Located east of the Colorado Plateau and cutting through the southern Rockies, surface evidence for rifting includes extensional basins, fault scarps with late Quaternary offsets, high surface heat flow, and volcanism. The processes driving Rio Grande Rift extension are not well understood, and accurate measurements of crustal motion are an important step towards better understanding how and why rifting occurs and the resulting earthquake and volcanic hazards. Previous estimates of Rio Grande Rift extension range from sub-mm to 5 mm/yr, but do not exceed uncertainties. Using precise GPS measurements of 25 new stable monuments (installed 2006-2007) and sites installed for EarthScope's Plate Boundary Observatory (PBO), we will accurately determine present day crustal motion of the Rio Grande Rift and southern Rocky Mountains. Slow rates make this a challenging goal, so geodetic monumentation, measurement sampling, and post-processing strategies are optimized to achieve sub-mm/yr accuracy. Network data are analyzed using GAMIT/GLOBK, with further analyses planned using the Gipsy and Bernese packages. EarthScope PBO and a selection of other continuous GPS stations in the region are included in the data analysis, as well as some sites outside the region, to improve network resolution and reference frame stability. Coordinate time series exhibit excellent monument stability (1 mm daily horizontal and 5 mm vertical precision using standard processing). We will improve our estimates of extension rates through further mitigation of atmospheric effects and multipath in GPS positions. Current results are still preliminary, but we anticipate having robust estimates of crustal deformation by the end of the five-year network occupation. Future work will include modeling of surface deformation, particularly spatially variable extension rates, and exploration of the relationship between surface motions and the lower crust and mantle lithosphere.