Potassium-argon dates for basaltic rocks from the Rim Rock dike swarm of Trans-Pecos Texas range from 18 to 23 m.y. Because these dikes are closely associated with major normal faults, they indicate the age of Basin-Range faulting in the region. Two sets of dikes are distinguishable not only by strike and petrology, but also by age. The older, basaltic and trachy-basaltic set and the younger, more alkalic set have average ages of 22.3 and 19.2 m.y., respectively. Extensive deuteric alteration has not modified significantly the K-Ar dates of the rocks.

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