QUANTIFYING THE INTERACTION OF RIVER INCISION AND NORMAL FAULTING ACROSS THE WESTERN EDGE OF THE COLORADO PLATEAU VIA 40AR/39AR AND FIELD STUDIES OF QUATERNARY BASALTS IN GRAND CANYON

40Ar/39Ar dates on basalts of Grand Canyon provide a record of the interplay among volcanism, differential canyon incision, and neotectonic faulting. New 40Ar/39Ar ages are all < 723 ka, with age probability peaks at 602, 534, 348, 192, and 102 ka. Strategic sampling of basalts provides a semi-continuous record for quantifying late Quaternary incision and fault slip rates during progressive deepening of Grand Canyon.

The Eastern Grand Canyon block (east of Toroweap fault) has bedrock incision rates of 150- 175 m/Ma (last ~500 ka); western Grand Canyon block (west of Hurricane fault) has bedrock incision rates of 50-75 m/Ma (last ~720 ka). Fault displacement rates are 97-107 m/Ma on the Toroweap fault (last 500-600 ka) and 70-100 m/Ma on the Hurricane fault (last 200-300 ka). As the river crosses each fault, apparent incision rate is lowest in the immediate hanging wall and this rate, plus displacement rate, is subequal to incision rate in the footwall. At the reach scale, variation in apparent incision rates delineates ~100 m/Ma of cumulative vertical lowering of the western Grand Canyon block relative to the eastern block and 70-100 m of slip accommodated by formation of a hanging wall anticline.

Data from the Lake Mead region extend the fault-dampened incision model back to ~6 Ma. Extrapolation of Quaternary fault displacement and incision rates linearly back 6 Ma only accounts for ~ 2/3 of eastern and ~1/3 of western Grand Canyon incision. This "incision discrepancy" for carving Grand Canyon is best explained by higher rates during early (5-6 Ma) incision in eastern Grand Canyon and the existence of Miocene paleo-canyons in western Grand Canyon. Differential incision evidence for relative vertical displacement across Neogene faults of the Colorado Plateau- Basin and Range transition provides key data for evaluating uplift and incision models. These data indicate that the Lower Colorado River block has lowered at least 25-50 m/Ma (150-300 m) relative to the western Grand Canyon block and 125-150 m/Ma (750-900 m) relative to the eastern Grand Canyon block in 6 Ma. Using the paleo-river profile as a datum, the best model explaining the constrained reconstruction of the 5-6 Ma Colorado River paleo- profile, and geologic data, involves 750-900 m of vertical block motion due to Neogene surface uplift of the Colorado Plateau.