New Mexico Geological Society Annual Spring Meeting — Abstracts

Geodetic and stress-field studies of the Rio Grande rift (RGR; Aldrich et al., 1986; Kreemer et al., 2010; Berglund et al., 2012) have aimed at precisely measuring active crustal deformation and understanding the associated dynamic processes and stresses. How is deformation distributed spatially across a large area spanning the RGR from the Great Plains to the Basin and Range province and how does this relate to lithospheric structure and the stress field? We review results and interpretations from previous studies and provide a summary of our current knowledge of the active tectonics related to the RGR.

With sufficiently long time series and noise characterization, modern techniques for processing Global Positioning System (GPS) phase data allow us to estimate a GPS receiver’s relative horizontal position and velocity with sub-millimeter accuracy. Spatially differentiating these velocities along a given profile gives the strain rate in that dimension. With respect to stable North America, a low, westward velocity gradient exists from near zero velocity in the Great Plains up to ~4 mm/yr just west of the Colorado Plateau. Average station velocities within the basin-bounding normal faults of the RGR are just above 1 mm/yr. This results in an average strain-rate of ~1.2 nanostrain/yr along five ~1000 km long east-west profiles. We used a statistical F-test to determine whether or not strain in these profiles is better described using a single linear model, or if introducing more parameters in a segmented model is a better statistical fit to the data. We found that a single two-parameter regression was a better statistical representation than any of the segmented models, which supports previous studies’ conclusion that strain is broadly distributed across a large area extending well beyond the surface expression of the RGR.

However, we cannot disregard the clear anomalies present in both GPS and stress measurements. We find localized velocity and strain anomalies in or near the RGR. For example, a zone of possible east-west contraction east of the RGR in southern Colorado and northern New Mexico corresponds spatially to ~NNE horizontal minimum stress orientations (S_h) defined by aligned Plio-Quaternary volcanic vents in the eastern Jemez lineament. These differ markedly from generally east-west extension and S_h in and near the rift, and the paleostress field suggests this situation has existed for a few million years. These may reflect E-W contraction and compressional horizontal stress caused by lithospheric flexure due to buoyant mantle or magma emplacement in the eastern Jemez Lineament. The E-W oriented S_h closer to the center of the rift provides evidence of rift-related extension beyond its surface expression which is congruent with both geodetic results and tomographic images of the lower crust and mantle. In addition, the best-fit E-W velocity gradient in parts of the rift is higher than in adjacent Colorado Plateau or Great Plains, but error is larger due to low station density (3 or 4 in each transect). Longer observation times and higher station density is required to resolve if the rift does correspond to measurably higher E-W velocity gradients.