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1 Department of Geology, Utah State University, Logan, Utah 84322, USA
2 Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA
3 Berkeley Geochronology Center, Berkeley, California 94709, USA
4 New Mexico Institute of Mining and Geology, Socorro, New Mexico 87801, USA
Incision of the Colorado River in the Grand Canyon, widely thought to have happened between ca. 6 and 1.2 Ma, has continued at variable rates along the canyon over the past 
500 k.y., based on measurements of bedrock incision combined with U-series and 40Ar/39Ar ages. River incision rates downstream of the Toroweap fault in the western Grand Canyon are about half the 140 m/m.y. incision rate calculated for a distance of at least 200 km upstream of the fault. We hypothesize that this differential incision is due to west-down slip on the Toroweap fault of 94 ± 6 m/m.y. based on measured offset of the newly dated Upper Prospect basalt flow, which is the major middle-late Quaternary slip evident along the river. Regional incision has been driven mostly by base-level fall related to drainage reversal off the Colorado Plateau ca. 6 Ma. Because local normal faulting is lower in rate than this regional incision and is likely an expression of Basin and Range extension and subsidence rather than uplift, this is a case where active faulting diminishes, but does not drive, incision. Quaternary incision rates are insufficient to have carved the Grand Canyon in 6 m.y., suggesting either that rates have decreased through time as the original base-level signal has attenuated, or that some component of the canyon relief we see today existed prior to Colorado River integration.
Key Words: faulting • geochronology • geomorphology • Grand Canyon • incision
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