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

Abstract

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.