During the Middle to Late Miocene era the Albuquerque and Espanola basins formed one basin, an irregular half-graben tilting west, formed by high-angle faulting on reactivated structures from the Laramide orogeny. The Embudo-Pajarito-La Bajada-San Francisco-Rincon fault system became active at the end of the Miocene. When this happened, the Albuquerque basin reversed its half-graben tilt from west to east, and now slopes down to the base of the newly formed Sandia Mountains. The northern part of the Albuquerque Basin was left with the appearance of a symmetrical basin, and is sometimes regarded as a separate geologic basin (Santo Domingo basin).
