Evolution of the southeastern San Luis Basin margin and the Culebra embayment, Rio Grande rift, southern Colorado
— A. R. Wallace

The Culebra half-graben and horst in southern Colorado form the southern half of the asymmetric San Luis graben-horst system along the Rio Grande rift. The Blanca Peak horst forms the northern margin of the Culebra half-graben and is linked with the Sangre de Cristo horst to the north. These elements comprise the Culebra reentrant, an eastward recess in the rift margin south of Blanca Peak. Rifting in the Culebra horst-graben system began at about 25 Ma, and Santa Fe Group sediments were shed westward into alluvial fans and the developing depocenter. Clasts of progressively older source lithologies were carried to the depocenter, reflecting early uplift of the Culebra horst through at least 18.7 Ma, and sedimentation continued to at least 3.7 Ma. The drainage divide in the rising horst shifted westward, eventually limiting the alluvial sources to the western side of the modern range. Blanca Peak was not a topographic high during these stages of sedimentation. The structurally active part of the Culebra horst-graben system coincides with the thrust-faulted west side of a Laramide arch. Normal faulting along the western side of the Culebra horst began in the late Oligocene and was most active from the late Miocene to the present. The dominant faults strike northeast and are down-dropped into the basin, creating at least 4600 m of total offset. Rifting reactivated Laramide tear faults to form northwest-striking normal faults. The interaction between the rift-parallel faults and the oblique reactivated tear faults created a compartmentalized fault system and demonstrates a clear Laramide influence on rift-related faulting. Initial uplift of the Blanca Peak horst began in the early Miocene, but substantial uplift did not take place until the late Miocene and Pliocene. Since that time, uplift of the Culebra horst and Blanca Peak horsts overlapped in time and, in part, in space. The Culebra reentrant is a product of contrasting uplift styles along the margins of the two horsts and conjecturally some clockwise rotation of the Blanca Peak horst.