The direction of absolute movement along the boundary faults of the Basin and Range–Colorado Plateau margin has been determined by comparing present stream profiles with profiles of ancestral streams preserved beneath basalt flows of late Cenozoic age that have been displaced by recurrent movement. Studies of the Grand Wash, Hurricane, and Toroweap faults in southern Utah and northern Arizona indicate that the eastern block or footwall has been the active block and has moved upward, whereas the western block or “downthrown” block has remained essentially stationary. A similar direction of absolute movement on the Wasatch fault in central and northern Utah is apparent from the manner in which Lake Bonneville shoreline terraces have been displaced. These data indicate that recurrent movement along the eastern boundary faults of the Basin and Range rift system is not produced by collapse or subsidence after inflation or uparching of the crust in the rift zone but results from upward-directed stresses. This could be an expression of the direction of mantle movement along the Colorado Plateau-Basin and Range margin.

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