Heat flow and gravity interpretation across the Rio Grande Rift in southern New Mexico and west Texas

Abstract

Recent heat flow measurements at 10 separate localities in southern New Mexico and West Texas range from 0.9 to 3.6 HFU (1 HFU = 1 × 10−6 cal/cm2 s). Radiogenic heat production determinations at eight of these localities range from 2.2 to 4.9 HGU (1 HGU = 1 × 10−13 cal/cm3 s). When the heat flow measurements are reduced by the heat from bedrock radioactivity, the Rio Grande rift and immediately adjacent areas are characterized by very high reduced flux (≥1.8 HFU), which contrasts strongly with 1.2 to 1.5-HFU values in bordering portions of the Basin and Range province and a 0.8-HFU value assumed for the southern Great Plains. Analyses of 2000 gravity observations in the Basin and Range province and the Great Plains reveal that a positive gravity ridge of about +30 mGal occurs along the Rio Grande rift in the Franklin and San Andres mountains and the Tularosa Valley. This relative gravity high can be caused by crustal thinning and/or shallow sills and dikes of mafic rocks in the crust under the rift. Gravity anomalies and a thin crust are compatible with a low-density upper mantle beneath most of the Basin and Range province. The new heat flow and radioactivity data indicate that the Basin and Range-Great Plains heat flow transition is marked by unusually high observed and reduced flux in the neighborhood of the Rio Grande rift. The combined thermal and gravity observations may be explained by shallow crustal intrusions, a hot abnormal upper mantle, and relative crustal thinning beneath the rift.