INTERACTIONS BETWEEN VOLCANISM, RIVER INCISION, AND NORMAL FAULTING IN WESTERN GRAND CANYON AND IMPLICATIONS FOR NEOTECTONIC MODELS

Western Grand Canyon is an excellent location to examine the interactions of normal faulting, far-traveled intra-canyon basalt flows, and river incision. Datable basalt remnants that overlie river gravels and are offset by faults allow for the simultaneous calculation of fault slip rates and bedrock incision rates at numerous locations along the river profile. Use of the river profile as a datum combined with improved data on variation in incision rate across fault blocks lead to new models for geometry and history of Quaternary normal faulting. Incision rates are lowest adjacent to faults in the hanging wall (HW) and highest adjacent to faults in the footwall (FW). Data in the proximity of the Toroweap fault confirm earlier models that the sum of the HW incision rate plus the fault slip rate is subequal to the FW incision rate. This relationship breaks down away from the fault as apparent incision rates also vary within the Uinkaret fault block due to the formation of a hanging wall anticline above the listric Toroweap fault. Downstream, in the Shivwits fault block, incision rates are fairly constant (~ 65 m/my) for at least 30 km, indicating regional lowering of this block relative to the Colorado Plateau at about 75 m/my.

3D mapping of basalt remnants, using aerial photography and LIDAR data, along with 40Ar/39Ar ages, has allowed for a better understanding of the timing and geometry of different intra-canyon flows and eruptive centers. Initial findings indicate a need to reassess the correlation and timing of intra-canyon basalt flows. Available data indicate four major periods when lava flowed into the canyon. From around 700-500 ka basalt flows appeared to have entered the canyon near the Toroweap fault, in part from cinder cones built within the canyon, and flowed down stream over 100 km to become the Black Ledge flows. By about 300 ka volcanic activity was concentrated downstream, in the Whitmore Wash area. 40Ar/39Ar ages on the Grey Ledge remnants indicate two distinct flows dated at about 200 and 100 ka. LIDAR analysis and field observations point to the incorporation of volcanic debris into lava dams, which would likely add to edifice instability.