The physiography of southeastern Arizona is characterized by long, thin mountain ranges separated by broad, fault-bounded valleys. This physiography, which is unique on the planet, is the result of crustal extension that occurred between 8- to 12 million years ago.
The story of the evolution of SE Arizona is the subject of a featured article in the new Fall-Winter 2012 issue of Arizona Geology magazine, published by the Arizona Geological Survey. The paper is “Post-Tectonic Landscape Evolution in Southeastern Arizona: When Did a River Start to Run Through It?” by Matthew C. Jungers.
Initially, the basins had internal drainage and were not connected. Jungers’ story shows how geologic forces gradually connected the basins and how the Gila-Santa Cruz-San Pedro river system developed. The article also describes how he figured it out.
The graphic below shows the sequence of events according to Jungers:
His figure caption reads:
“Figure 2. Final stages of the Basin and Range disturbance. (A) Structural basins were filled with sediment, and most basins were still internally drained. (B) Following the cessation of extensional tectonics in the region, basins continued to fill with sediment and faults were buried. Basins began to integrate with the main stem Gila River via a combination of basin spillover and headward drainage capture. (C) Following integration with an adjacent basin, sedimentary fill was incised as its basin adjusted to a new, lower base level. (D) As a new, through-flowing drainage network was established, integrated basins graded to the Gila River. The shift to an oscillating climate in the Quaternary may be preserved in flights of terraces that record alternating periods of floodplain stability followed by rapid incision. Figure adapted from Menges and Pearthree, 1989.”
Read the full article here.