CONSTRAINING THE TIMING OF INCISION OF THE VIRGIN RIVER GORGE

The Virgin River Gorge of northwestern Arizona is a dramatic feature, boasting high relief that begins abruptly at the mouth of the canyon, and spanning elevations ranging from about 750 meters at the floor of the gorge to 2443 meters at nearby Mt. Bangs. Although sitting on the eastern edge of Basin and Range extension and the northwestern edge of the Colorado Plateau, few of the numerous studies conducted on the landscape of the southwest United States have focused on the Virgin River Gorge, and consequently little is known regarding its origin. Two possible, although certainly not exclusive, mechanisms for initiation of incision of the gorge include the Piedmont Fault, a Miocene(?) normal fault clearly active into the Quaternary and bounding the western edge of the gorge, and the Escalante anomaly, which could indicate epeirogenic uplift of the headwaters of the Virgin River. Our approach invokes a detailed geomorphic analysis of the Virgin River fluvial network with a focus on channel steepness patterns and knickpoint identification coupled with thermochronologic and surface exposure dating to constrain the timing and magnitude of incision. The rocks of the Virgin River Gorge are capped by basalt and cut by tributaries of the Virgin River; our new ⁴⁰Ar/³⁹Ar whole rock dates on these basalt samples establish a maximum age of canyon cutting. Added to these data, cosmogenic radionuclide ³⁶Cl dating of limestone platforms that define a series of knickpoints within tributaries of the Virgin River provide constraints on incision rates. Together, these data begin to allow us to constrain uplift, erosion, and incision of a key location in the southwestern United States.