POST-OLIGOCENE RIVER INCISION AND FLEXURE-DRIVEN DRAINAGE CAPTURE ALONG RIFTED MARGINS IN THE SOUTHERN SIERRA MADRE OCCIDENTAL, MEXICO

A renewed appreciation of the importance of bedrock river incision in geomorphic landscape evolution has spurred research on the morphology and distribution of bedrock channels. In addition, a strong interest in feedback between erosion and rock uplift has focused attention on processes and rates of bedrock river incision. Here we report results of a study of long-term river incision into Oligocene ignimbrites in the southern Sierra Madre Occidental (SMO), western Mexico.

The SMO 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 yr^-1. In addition, river profiles of this region also show evidence of river capture because of flexural uplift along the flank of the rift zone.

The form of river profiles in the southern SMO ranged from smoothly concave to relatively linear and even convex in the lower reaches of some profiles. The convex reaches presumably reflect either active subsidence of the Tepic-Zacoalco rift zone, or uplift of the Sierra Madre Occidental.

River profile concavity values (Q) 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.

Ignimbrite sheets, such as the southern Sierra Madre Occidental, provide ideal locations for studying long-term river incision because of their relatively simple initial condition as a depositional surface and their amenability to conventional radiometric dating.