High Quality Content by WIKIPEDIA articles! The Rio Grande Rift is a rift valley extending north from Mexico, near El Paso, Texas through New Mexico into central Colorado. The upper Rio Grande flows south following the rift valley, but did not incise it. The Rio Grande Rift represents the easternmost mainfestation of widespread extension in the western U.S. during the past 35 million years. After the Farallon Plate subduction-associated compressive forces of the Laramide orogeny ended during the Eocene Epoch, erosion of these uplands filled the area of the Raton Basin with abundant sediments. In the late Oligocene Epoch regional tensional forces became dominant and rifting was initiated as the crust and uppermost mantle began extending.