Abstract

40Ar/39Ar dates on basalts of Grand Canyon provide one of the best records in the world of the interplay among volcanism, differential canyon incision, and neotectonic faulting. Earlier 40K/40Ar dates indicated that Grand Canyon had been carved to essentially its present depth before 1.2 Ma. But new 40Ar/39Ar data cut this time frame approximately in half; new ages are all <723 ka, with age probability peaks at 606, 534, 348, 192, and 102 ka. Strategic sampling of basalts provides a semicontinuous record for deciphering late Quaternary incision and fault-slip rates and indicates that basalts flowed into and preserved a record of a progressively deepening bedrock canyon.

The Eastern Grand Canyon block (east of Toroweap fault) has bedrock incision rates of 150–175 m/Ma over approximately the last 500 ka; western Grand Canyon block (west of Hurricane fault) has bedrock incision rates of 50–75 m/Ma over approximately the last 720 ka. Fault displacement rates are 97–106 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, the apparent incision rate is lowest in the immediate hanging wall, and this rate, plus the displacement rate, is sub-equal to the incision rate in the footwall. At the reach scale, variation in apparent incision rates delineates ∼100 m/Ma of cumulative relative 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 indicate that our refined fault-dampened incision model has operated over the last 6 Ma. Bedrock incision rate has been 20–30 m/Ma in the lower Colorado River block in the last 5.5 Ma, and displacement on the Wheeler fault has resulted in both lowering of the Lower Colorado River block and formation of a hanging-wall anticline of the 6-Ma Hualapai Limestone. In modeling long-term incision history, extrapolation of Quaternary fault displacement and incision rates linearly back 6 Ma only accounts for approximately two-thirds of eastern and approximately one-third of western Grand Canyon incision. This “incision discrepancy” for carving Grand Canyon is best explained by higher rates during early (5- to 6-Ma) incision in eastern Grand Canyon and the existence of Miocene paleocanyons in western Grand Canyon.

Differential incision data provide evidence for relative vertical displacement across Neogene faults of the Colorado Plateau-Basin and Range transition, a key data set for evaluating uplift and incision models. Our data indicate that the Lower Colorado River block has lowered 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. The best model explaining the constrained reconstruction of the 5- to 6-Ma Colorado River paleoprofile, and other geologic data, is that most of the 750–900 m of relative vertical block motion that accompanied canyon incision was due to Neogene surface uplift of the Colorado Plateau.

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