FIGURE 7.14 Profile of stream channel showing the relation between the present channel and portions of an older channel preserved by basalt flows. Downcutting induced by faulting has left the older channel remnants much higher than the present channel.

been tectonically warped or tilted, the basalts may record irregularities, anomalous gradients, or gradient reversals. Classic examples of basalt-filled channels being used as long-term tiltmeters can be found in studies of late Cenozoic tectonism in the Sierra Nevada Mountains of California (Bateman and Wahrhaftig, 1966). Use of volcanic ashes to date erosional or depositional surfaces are also abundant (Young and Brennan, 1974). Field relations using dated basalts have supported tectonic geomorphologic inference regarding classical geomorphic problems including the development of the western margin of the Colorado Plateau and the evolution of the Grand Canyon (McKee and McKee, 1972; Hamblin et al., 1981).

DISCUSSION

The tectonic geomorphology of fault-generated topographic fronts can be used to describe long-term tectonic

FIGURE 7.15 Aerial oblique view looking north at Grand Wash. The Virgin Mountains are located at the northernmost part of the photograph. The flat-topped mesas are capped by basalt flows that record a time when the base level in Grand Wash was much higher. The basalt flows provide a chronostratigraphic datum that can be used to calculate average rates of stream downcutting and make inferences about tectonic uplift.



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