thickness of gravel (Figure 7.13). They form only under conditions where the rate of piedmont erosion is greater than the rate of uplift across a range-bounding fault. Where rock control is not a factor, pediments represent a period of tectonic inactivity and thus are binary in nature. Pediment widths are dependent on the time since active faulting along the range front and also on drainage basin slope and length. Estimates of pedimentation rates are generally in the range of 300–1000 m per million years (m.y.) (Young and Brennan, 1974; Wallace 1978). A pediment 2 km wide may therefore represent a period of tectonic quiescence that lasted greater than 2 m.y. Pediments that form in conjunction with cliff retreat from fault-generated escarpments can be used in an analogous fashion.

Basalt flows and volcanic ash deposits provide an unique opportunity to determine the ages of tectonic events and the rates of geomorphic processes. They can preserve a datum that records previous river levels or a prefaulting topography. For example, if a basalt flowed down a river channel and subsequent tectonically induced entrenchment results in topographic inversion with the old channel preserved by the basalt, then the amount and geometry of downcutting can be determined (Figure 7.14).

The Grand Wash in northwestern Arizona is an example of topographic inversion (Figure 7.15). Basalts flowed down the Grand Wash valley about 7 m.y. ago (Hamblin et al., 1981). Base-level fall, perhaps related to the downcutting by the Colorado River, resulted in entrenchment. When downcutting by large streams is caused by tectonic uplift, the total amount of downcutting approaches the total amount of uplift. Because a former river level is both preserved and dated by basalt flows, average downcutting rates can be calculated, which turn out to be 26 m/m.y. for the Grand Wash (Hamblin et al., 1981). Similar calculations indicate that the Hurricane fault, near the town of Hurricane, Utah, has a minimum vertical slip rate of 300 m/m.y. for the last 0.3 m.y.

The amount of downcutting, however, decreases upstream from the base-level fall. Rice (1980), using four dated basalt flows, estimated a 95 m/m.y. average downcutting rate of the Little Colorado River over the past 2.5 m.y.

Alternatively, if an area containing basalt flows has

FIGURE 7.13 Pediment surface in the Mohave Desert of California exposing bedrock. The pediment-mountain front junction is abrupt and characterized by a sharp break in slope.

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