Cenozoic rift-related sedimentation and faulting, northern Culebra Range, southern Colorado
— Alan R. Wallace

The development of the Rio Grande rift along the margin of the Culebra Range in southern Colorado involved the complex interaction between orogenic sedimentation and high-angle normal faulting. Prior to rifting, the study area was covered by a partially eroded, intermediate-composition, 28 Ma stratovolcano. Early rifting began at about 26 Ma with the creation of a broad basin into which orogenic sediments were shed from the east. The basal units were in part volcanic rich, and continued downcutting into the rising source highland produced increasingly Paleozoic- and Proterozoic-rich sediments. Basalt flows locally were erupted across the sediments during sedimentation. High-angle normal faulting commenced in the middle Miocene and produced mutually offsetting NNE-trending, down-to-the-west, and WNW-trending, generally down-to-the-south fault systems. The rift-bounding uplift changes northward from an east-tilted homocline to a completely fault-bounded horst, and the mosaic of high-angle faults are present in the general transition zone. This change in structural style may reflect the presence of two similar-facing half-grabens rather than a single graben.