A simple thermal conduction model constrained by piezothermometric data obtained from coexisting minerals in both crustal and mantle xenoliths from Kilbourne Hole maar, New Mexico, is used to understand the development of rifting and the thermal evolution of the lithosphere in the southern Rio Grande rift. The one-dimensional finite difference code models crustal thinning and underplating by magmatic injection into the lower crust and uppermost mantle. The model is in good agreement with crustal and mantle thermometric arrays assuming a transient, pre-rift gradient superimposed on a stable geotherm and yielding a net gradient of 20-degrees-C km-1 in the 40-km-thick crust and 3-degrees-C km-1 in the upper mantle 30 million years ago. This is consistent with pre-rift volcanic activity in the area possibly related to Laramide orogenic events. A massive injection of basaltic dikes and sills (almost-equal-to 70 vol.%) in the uppermost mantle and at the base of the crust probably occurred in the early development of the southern Rio Grande rift, between 20 and 30 Ma. The heat contribution due to the injection of magma is treated as a temperature-dependent heat source distributed uniformly throughout the intruded region which extends from 25 to 45 km depth. Textural observations along with major and trace element compositions of the mantle and crustal xenoliths indicate that metasomatism and recrystallization may have occurred during liquid infiltration and partial melting 20 to 30 Ma. This is consistent with early rift bimodal volcanism associated with crustal contamination and linked to approximately 25% total NE-SW extension. Dike injection and crustal thinning ceased 20 Ma associated with the mid-Miocene lull in volcanic activity. The total volume of material injected, 100 km3 per km length of rift per million years, is approximately an order of magnitude less than for a typical mid-ocean ridge. Conductive cooling of an uppermost mantle (40-45 km depth) from temperatures of over 1200-degrees-C (22 Ma) leads to the formation of a granulite metamorphic crustal complex and generation of a surface heat flow of 82 mW m-2, in accordance with observed present-day values in the southern Rio Grande rift. Overall, the results from the dike injection model are consistent with both the crustal and mantle piezothermometric arrays and present-day geochemical and geophysical data from the southern Rio Grande rift.
