Post-Oligocene river incision, southern Sierra Madre Occidental, Mexico

The Sierra Madre Occidental of western Mexico consists of a granitic basement covered by Oligocene ignimbrites that define a reference surface from which to estimate late Cenozoic river incision. A 90-m-grid digital elevation model was used to characterize contemporary topography and interpolate the Late Oligocene surface of the ignimbrite plateau from a surface fit to the highest points in the relatively undissected uplands between major river valleys. Long-term river incision rates calculated from the difference between this reference surface and longitudinal profiles of 11 rivers that flow toward the Tepic-Zacoalco rift zone range from about 0.01 to 0.2 mm year−1. River profiles of this region also show evidence of river capture driven by flexural uplift along the flank of the rift zone. River profile concavity values (θ) in the Sierra Madre Occidental range from 0.22 to 0.63, a range similar to that reported previously for a wide range of environments. In contrast, the empirically constrained ratio of exponents in the stream power model of river incision (m/n) ranges from 0.44 to 0.52, close to the expected theoretical value of 0.5. The wider range of observed θ values may illustrate how θ can differ from the driving values of m/n in non-steady-state bedrock river systems.