A zone of high-angle normal faults trends eastward across southeastern Utah and southwestern Colorado. Locally these faults bound huge blocks of downthrown sedimentary strata to form grabens of considerable length. Two of these grabens cross the Abajo Mountains, one of a group of laccolithic centers that dot the Colorado Plateau. Evidence from prior work suggested that the age of one of these grabens–and, by inference, of the other grabens–was the age of the uplift of the Abajo Mountains. New evidence, including two dikes along one of the faults that bound a graben and a laccolith wholly confined to a graben, suggests that the grabens were in existence before the Abajo Mountains were formed.

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