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

The Toroweap and Hurricane faults, considered to be the most active in Ariz ona, cross the Uinkaret volcanic field in the western Grand Canyon. These n ormal faults are down-thrown to the west, and the Colorado River crosses th ese faults as it flows west in the Grand Canyon. Cosmogenic He-3 (He-3(c)) dates on basalt flows and related landforms are used to calculate vertical displacement rates for these faults. The two faults cross unruptured alluvi al fans dated as 3 ka (Toroweap) and 8 ka (Hurricane), and 10 other landfor ms that range in age from 30 to 400 ka are displaced. Middle and late Quate rnary displacement rates of the Toroweap and Hurricane faults are 70-180 an d 70-170 m/m.y., respectively. On the basis of these rates, the combined di splacement of 580 m on these faults could have occurred in the past 3 to 5 m.y. All He-3(c) dates are younger than existing K-Ar dates and are consist ent with new Ar-40/Ar-39 dates and existing thermoluminescence (TL) dates o n basalt flows. These different dating techniques may be combined in an ana lysis of displacement rates. Downcutting rates for the Colorado River in th e eastern Grand Canyon (400 m/m.y.) are at least double the downcutting rat es west of the faults (70-160 m/m.y.). Faulting probably increased downcutt ing in the eastern Grand Canyon relative to downcutting in the western Gran d Canyon during the late Quaternary.