Rates of river incision and scarp retreat in eastern and central Grand Canyon over the past half million years: Evidence for passage of a transient knickzone

Researchers have long debated whether a transient knickzone, accompanied by a wave of high incision, has migrated through the Grand Canyon in the geologically recent past or if, instead, canyon incision rate has been constant during the past several million years. Resolution of this debate has been hindered until recently by the absence of incision rate data for central Grand Canyon and the short duration (since 385 ka) of the eastern Grand Canyon rate history derived directly from river terraces. Here we constrain incision rate histories since ca. 500 ka at eastern Grand Canyon's Hermit Creek (river mile 96) and at river mile 159 in central Grand Canyon. At Hermit Creek, U/Th ages of travertine- cemented river terrace fill and other surficial deposits reveal an average incision rate of 519 _+55-58 m/m.y. since 506 ± 33 ka and a maximum rate of 210 _+42-49 m/m.y. since 394 ± 32 ka. These data require an incision rate of 1-4 km/m.y. between ca. 500 and ka followed by at least a fivefold decrease after 400 ka. We attribute this decrease to the migration of a transient knickzone past Hermit Creek between 500 and 400 ka. These same deposits also record a 600-800 m/m.y. retreat rate for the Redwall-Muav escarpment since 500 ka. At river mile 159, we analyze the relative age relationship between intrusion of the basaltic "159-Mile Dikes" and cutting of the Muav Gorge. We conclude that the dikes intruded prior to cutting of the gorge, which requires an average river incision rate of 763 m/m.y. since ca. 520 ka. We interpret this rapid rate (compared to ~100-160 m/m.y. over shorter time scales) as independent evidence of knickzone passage through central and eastern Grand Canyon at ca. 500-400 ka.