Oligocene alkaline igneous activity in NE Mexico

Abstract
Potassium-argon age determinations on rocks from the Sierra de San Carlos, State of Tamaulipas, México, which are petrographically very similar to other bodies in north-eastern México and the south-central U.S.A., indicate that a NW-trending zone of alkaline complexes, at least 1,400 km in length, stretches from the Gulf coast to southern New Mexico. This zone may be structurally related to the parallel, but somewhat younger, Rio Grande rift.