NEW 40AR-39AR DATES ON BASALTS IN GRAND CANYON: CONSTRAINTS ON RATES OF QUATERNARY RIVER INCISION AND SLIP ON THE TOROWEAP FAULT, AND IMPLICATIONS FOR LAVA DAMS

Preliminary 40Ar-39Ar dates from Quaternary basalts that erupted from the Uinkaret volcanic field and flowed into the Grand Canyon near Lava Falls reveal a history of basaltic volcanism from 630 ka to present. These new dates necessitate substantial reinterpretation of Grand Canyon incision history, Quaternary slip rates, correlation of basalt flow remnants, and interpretation of lava dams. The oldest reliably dated flows include new dates of 630 ± 40 ka (all errors 2 sigma) on the lower Prospect flow at mile 179 (left); 630 ± 70 and 600 ± 60 ka on a high basalt remnant (labeled "Toroweap flow" by Hamblin, 1994) at mile 177 (left), and the previously dated 603 ± 16 ka "Black ledge" flow of Granite Park (Lucchitta et al., 2000). These dates are all indistinguishable in age, in contrast to the disparate stratigraphic positions and ages assigned by Hamblin (1994). New dates from the upper Prospect flow are: 483 ± 22 ka, 526 ±30 ka, 528 ± 83 ka, 529±29 ka, and 534 ±56 ka, giving a mean age of 509 ± 30 ka. These dates agree well with a new date of 520 ± 50 ka on a flow at mile 208 (labeled "Black Ledge" by Hamblin, 1994) and with the "upper Black Ledge" flow at Granite Park (524 ± 14 ka; Lucchitta et al., 2000). The Upper Prospect flow is offset 48 m by the Toroweap fault giving slip rates of 95 m/my. The mile 208 flow downstream of the Toroweap fault has its base 36 m above river level on main-stem channel gravel and, with a mean pool depth of 14.3 m, gives an incision rate of 96 m/my. A basalt at mile 177 (immediately upstream of the Toroweap fault) yields a date of 346 ± 33 ka, has its base 31.2 m above river level, where mean pool depth is 14.7 m, yielding a bedrock incision rate of 133 m/my. These figures indicate that incision rates are lower downstream of the Toroweap fault, supporting the hypothesis that slip on the fault accounts for differential Canyon incision. Dates of 110 ± 30 and 97 ± 32 ka on the "Gray Ledge" flow are in close agreement. A locally underlying flow also mapped by Hamblin as "Gray Ledge" is 193 ± 46 ka, suggesting multiple eruptions within this mapped unit. These new geochronologic data indicate a need for additional careful dating and petrologic correlation of flow remnants in the Grand Canyon before the extent, height, and chronology of postulated lava dams can be confidently assessed.