The Rio Grande rift is the easternmost deforming province within the tectonically active western margin of North America. Located just east of the Colorado Plateau, the rift is undergoing stretching and extension manifested by slightly higher probabilities of earthquake occurrence than in surrounding regions. Rates of motion are estimated at sub-mm to 5 mm/yr, but uncertainties in measurements of total extension are typically of the same order as the estimates themselves. This project brings together a team of researchers from the University of Colorado Boulder and the University of New Mexico to establish current rates of motion along the Rio Grande rift with high precision. The team is measuring rift motions using state-of-the-art global positioning system (GPS) instruments at 24 sites in Colorado and New Mexico. Each measurement epoch lasts six months and data are being collected for four epochs, providing rates encompassing a three-year span. The slow rates of motion in the region make this project especially challenging. The monumentation, observing time, and innovative post-processing strategies are designed to optimally resolve deformation at the sub-millimeter per year scale. A primary goal of the project is to understand what stretching rates measured at the surface imply about the stretching of the plate at deeper levels. An intriguing possibility is that stretching in the Rio Grande rift is heterogeneous, with different processes operating in the crust and the mantle. Available geologic observations also suggest that the character of the rifting changes dramatically from north to south, with a narrow rift zone marked by linear topographic depressions in Colorado and northern New Mexico grading to a broad region of multiple valleys ("basin and range") in south-central New Mexico. The factors that drive this variable character of the Rio Grande rift are not well-understood, but could have significant bearing on our understanding of how and why tectonic plates undergo stretching and of earthquake and volcanic hazards within rift zones. To examine these issues, the team is comparing their measured rates of motion with all other available datasets, including seismic velocities in the crust and mantle, gravity, surface heat flow, and geologic data. These diverse datasets are being used to build computer models of processes that control how tectonic plates undergo rifting. The project involves training graduate and undergraduate students in GPS measurement and analysis and spreading the group's knowledge in a variety of ways, including: training K-12 school teachers in the research process (the 'Earthworks' project of the Cooperative Research in Environmental Sciences at CU Boulder); involving middle schools in the siting and "adoption" of GPS sites; and interactions with agencies interested in earthquake and volcanic hazards along the Rio Grande rift including state Geological surveys, local and federal emergency management agencies, local governments, city planners, and engineers.
