New Mexico Geological Society Annual Spring Meeting — Abstracts

The Rio Grande rift is the easternmost active tectonic feature of the western US. To date, rates of extension are estimated to range from sub-mm to 5 mm/yr, but uncertainties in these data are as large as the motion itself. The installation of a dense, semi-permanent global positioning system (GPS) network across the rift will provide more accurate measurement of active extension over the next 5 years. Integration of these data with other datasets (i.e. seismic velocities in the crust and mantle, gravity, surface heat flow, and geologic data) together with geodynamic models will shed light on processes that control continental rifting.

Over Fall 2006, we installed 21 of 25 planned GPS monuments in New Mexico and Colorado, with field engineering support and equipment from UNAVCO. Each site across the rift was chosen based on the presence of exposed bedrock, and a good sky view. The bedrock location was chosen carefully to minimize movements from freeze-thaw or poroelastic effects (i.e. no shale or pervasive fracturing). Currently, the 21 installed stations are recording continuous data that will be post-processed to be accurate to a mm. These data will allow us to determine how far north the rift propagates, its width from north to south, and heterogeneities in spreading rates. We can then begin to answer other questions regarding the seismic hazards of the rift zone, the character of the deformation, and how the rift zone affects surrounding tectonic provinces.

We also present preliminary results from simple numerical experiments on crustal extension using a finite-element model that combines Lagrangian and Eulerian approaches. The finite-element package, GALE v.1.1.1, is designed for long-term tectonic modeling and is distributed and supported by Computational Infrastructure for Geodynamics (CIG). Our models investigate the patterns of strain developed during extensional deformation of simple rheologic analogs for the crust, e.g., a fluid with temperature-dependent viscosity overlain by a brittleplastic layer. Our models to date are preliminary and we hope to develop more complex coupled crust-mantle models to help us understand the behavior of the whole lithospheric column during continental rifting.