PRELIMINARY TRANSIENT HEAT-FLOW MODEL OF RIO-GRANDE RIFT IN SOUTHERN NEW-MEXICO

A positive gravity anomaly, anomalous seismic structure, and abnormally high reduced heat flow occur in a possible extension of the Rio Grande rift in southern New Mexico. The reduced heat flux is explained by the transient effects of basaltic intrusions with dimensions based on interpretations from gravity and seismic data. Ages of volcanic activity in the rift area are used to determine times of thermal events. The effect of latent heat and sharp boundaries of the reduced heat flow anomaly also restrict modeling. The thermal model consists of three intrusions at the base of the crust at 35 m.y., 10 m.y. and 5 m.y. The initial temperatures of these intrusions would differ from those in the surrounding crust and upper mantle by about +800°C, +800°C, and +600°C, respectively. The 35-m.y. intrusion is consistent with Oligocene volcanism and tectonic activity in southwestern New Mexico. This mass also contributes to the Bouguer gravity high in the rift area, as do the intrusions at 10 and 5 m.y. The intrusions at 10 and 5 m.y. explain the excess reduced flux, with the 5-m.y. pulse accounting for most of the curvature in the sharp boundaries of the anomaly. The idealized model is consistent with late Tertiary andesites, the periods of little extrusive activity, and Pliocene-Quaternary basalts in the rift area. © 1978.