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Displacement rates on the Toroweap and Hurricane faults: Implications for Quaternary downcutting in the Grand Canyon, Arizona

¹ Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112, USA
² U.S. Geological Survey, 1675 W. Anklam Road, Tucson, Arizona 85745, USA
³ Arizona Geological Survey, 416 W. Congress Street, Tucson, Arizona 85701, USA
⁴ Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112, USA
⁵ Department of Earth and Environmental Sciences, University of Rochester, Rochester, New York 14627, USA

The Toroweap and Hurricane faults, considered to be the most active in Arizona, cross the Uinkaret volcanic field in the western Grand Canyon. These normal faults are downthrown to the west, and the Colorado River crosses these faults as it flows west in the Grand Canyon. Cosmogenic ³He (³He_c) dates on basalt flows and related landforms are used to calculate vertical displacement rates for these faults. The two faults cross unruptured alluvial fans dated as 3 ka (Toroweap) and 8 ka (Hurricane), and 10 other landforms that range in age from 30 to 400 ka are displaced. Middle and late Quaternary displacement rates of the Toroweap and Hurricane faults are 70–180 and 70–170 m/m.y., respectively. On the basis of these rates, the combined displacement of 580 m on these faults could have occurred in the past 3 to 5 m.y. All ³He_c dates are younger than existing K- Ar dates and are consistent with new ⁴⁰Ar/³⁹Ar dates and existing thermoluminescence (TL) dates on basalt flows. These different dating techniques may be combined in an analysis of displacement rates. Downcutting rates for the Colorado River in the eastern Grand Canyon (400 m/m.y.) are at least double the downcutting rates west of the faults (70–160 m/m.y.). Faulting probably increased downcutting in the eastern Grand Canyon relative to downcutting in the western Grand Canyon during the late Quaternary.

Key Words: cosmogenic elements • Colorado River • normal faults

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