SPATIAL VARIATION IN GRAND CANYON INCISION: NEW GEOCHRONOLOGY ON RIVER TERRACES AND THE SURPRISE VALLEY LANDSLIDE

The relative importance of different tectonic and geomorphic controls on Grand Canyon incision can be tested by analyzing the spatial and temporal distribution of Quaternary incision rates throughout Grand Canyon. To this end we report new incision rates from 4 localities in eastern Grand Canyon where incision data was previously lacking. All incision rates are calculated from the paleo-strath to the modern strath. U-series dating of perched river gravels near Palisades (river mile 65) gives an incision rate of ca. 175 m/Ma averaged over ca. 530 ka. Four miles downstream at Tanner, gravels over 70 m above river level are outside of U-series range suggesting a maximum incision rate of ca. 190 m/Ma. At Elves Chasm (river mile 116.5) U-series and ²³⁴U model ages on gravel terraces between ca. 50 and 150 m above river level suggest steady incision of about 120-135 m/Ma over the last 1.5 Ma. Although post-depositional travertine cementation occurs, statistically indistinguishable ages on travertine coatings around gravel clasts and detrital travertine blocks deposited along with the gravel allows for determination of depositional ages for most Elves Chasm terraces. Near Deer Creek, cosmogenic burial ages have been obtained for well-shielded main-stem gravels that underlie landslide deposits. An age of 940 ± 240 ka (1σ) was obtained for gravels, 70 m above river level, in a buried channel about 1 mile upstream from Deer Creek (RM 135). Half a mile downstream from Deer Creek an age of 978 ± 287 ka (1σ) was obtained for gravels, 61 m above river level, under a landslide block (RM 136.6) that initiated on the north side of the canyon and was thrust 100s of m up the south side of the canyon. Although separate landslides likely cover these gravel deposits, the similar height and consistent ages suggest that the Colorado River was 60-70 above present river level at ca. 950 ka resulting in an incision rate of ca. 90 ± 20 m/Ma. Overall, new values suggest that Quaternary incision rate increase from ca. 90 m/Ma to ca. 175 m/Ma across eastern Grand Canyon, with high incision rates in the footwall of the Toroweap fault due to local footwall uplift. High incision rates in eastern Grand Canyon suggest a component of tilting during epeirogenic uplift, as supported by tomographic data, rather than uniform uplift of the eastern Grand Canyon block.