Extension in the Rio Grande rift

A positive gravity anomaly along the axis of the Rio Grande rift reflects a volume of anomalous mass added at the base of the crust and intruded into the crust. Part of this volume can be associated with vertical uplift of the crust. The remainder of this anomalous volume, plus the volume of surficial graben fill, can be associated with horizontal crustal extension. The volume of crustal uplift in the Rio Grande rift is unknown, but this term can be eliminated by means of an independent equation provided by assumption of generalized isostatic equilibrium. The volume and mass equations combined provide a solution for extension of the crust in terms of the following parameters: total anomalous mass deficiency in the mantle lithosphere, total anomalous mass excess in the crust and its density contrast, total anomalous mass deficiency of surficial graben fill and its density contrast, and the volume of material eroded from the uplift. Using standard density estimates and masses determined by equivalent-source modeling of gravity profiles, I obtained 1-km extension at 37°N (Colorado-New Mexico border), 13-km extension at 35°N (Albuquerque, New Mexico), and 24-km extension at 33°N in southern New Mexico. These estimates are of necessity very poorly constrained and may be as much as ±60% in error. Taking results at face value, the best fitting Euler pole occurs at about 41°N, in north central Colorado. The calculated opening angle is 1.32°. For an approximately 30-m.y. duration of the extensional system (late Oligocene to the present), calculated average angular velocity is 7.8×10⁻⁴ rad/m.y.; calculated average spreading half-rate at 33°N (for example) is a stately 0.04 cm/yr. The ratio of extension to uplift increases southward.