Karlstrom 0107065 Pederson Excavation of Grand Canyon by the Colorado River, previously thought to have happened between 6.0 and 1.2 Ma, is actually relatively young, ongoing, and happening at variable rates along the length of the Grand Canyon. Information from middle-late Quaternary stream terraces in eastern Grand Canyon indicates that strong side-stream and mainstream responses to glacial/interglacial climate changes have been superimposed on an overall incision rate of 0.2 to 0.6 mm/yr. This suggests that up to half of the present-day relief in eastern Grand Canyon has been generated in the Quaternary Period. In contrast, new data from western Grand Canyon indicate up to an order of magnitude lower incision rate over the same time period. This discrepancy is interpreted to be due to west down Quaternary slip on the Toroweap-Hurricane fault system modulating the incision of crustal blocks in the region. Recently recognized slip rates of ~0.25 mm/yr on this fault system are approximately equal to the difference between greater upstream rates (eastern Grand Canyon) and smaller downstream rates obtained within and immediately below the fault zone. A new picture is emerging of a younger, segmented Grand Canyon, which contains a sensitive record of the denudational history of the Colorado Plateau.
This project is a focused test of this hypothesis using detailed geomorphic and structural field studies combined with U-Series dating of travertine within alluvial terraces and Ar-Ar dating of basalts. These data provide an improved understanding of the age of the Grand Canyon and the forces that drive and modulate the incision of this and other high plateau regions. This work also has a broad educational and outreach impact; we are working closely with the Interpretive Division of Grand Canyon National Park to design the "Trail of Time" to communicate geologic understanding to the 5 million visitors that come to the Park annually. Additionally, we continue to train a host of graduate and undergraduate students in the context of a multi-university collaboration that deals with numerous aspects of the tectonic and landscape evolution of the southwestern U.S.A.
