Cenozoic rift-related sedimentation and faulting, northern Culebra Range, southern Colorado

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.