Potassium-argon ages from the Mount Taylor Volcanic Field, New Mexico

Fourteen new K-Ar dates for volcanic rocks of the Mount Taylor field, New Mexico, indicate that most activity occurred between 4.3 and 1.5 m.y. (million years) ago. Peak activity was at about 3.0-2.5 m.y., both on the central andesite-rhyolite shield volcano and on the surrounding alkali basalt-trachyte volcanic plateau, and occurred concurrently with an episode of NNE-trending basin-range faulting. The K-Ar dates also indicate that the regional Ortiz pediment surface, graded to the ancestral Rio Grande, existed in the Mount Taylor area as recently as 3 m.y. ago and that 250-400 m of erosional downcutting has occurred in subsequent time. Growth of the Mount Taylor field was also concurrent with peak volcanic activity along the northeast-trending Springerville-Raton zone, a major late Cenozoic volcanic belt that is considered to reflect the presence of a regional structural discontinuity of Precambrian age in the North American craton.