Specific questions to be addressed in this project include, 1) When was the main phase of extension in the Basin and Range of southeastern Arizona and southwestern New Mexico? 2) When was the main phase of extension in the southern Rio Grande rift of southern New Mexico and western Texas? 3) What is the nature of the boundary between the southern Rio Grande rift and adjacent Basin and Range? 4) What was the main driver of extension in the southern Rio Grande rift? To answer these questions, samples from thirteen separate locations will be "triple dated" using apatite (U-Th)/He (AHe), apatite fission-track (AFT), and zircon (U-Th)/He (ZHe) thermochronologic methods.