Abstract

Dating of basalts of late Cenozoic age that have been displaced by recurrent movement along fault systems of the western Colorado Plateau indicates that perennial streams downcut quickly after uplifts. The rate of downcutting of the offset basalts upstream from a fault is largely a function of the amount of uplift. Minimum uplift rates can be estimated by comparing present stream profiles to the profiles that existed before extrusion of the lava flows. A preliminary study of the valleys of the Virgin River and adjacent drainage systems indicates that the eastern margin of the Basin and Range province has been rising in relative altitude at roughly 25 m/m.y. in the Grand Wash area, at least 90 m/m.y. in the block between the Grand Wash and Hurricane faults, and 390 m/m.y. in the block east of the Hurricane fault.

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