Abstract

The aerial distribution, dissection, and stratigraphic relations of Pliocene and younger mafic volcanic flows indicate that the Rio Grande in central New Mexico became a throughflowing drainage system during late Pliocene time (between about 3.0 and 4.5 m.y. ago). K-Ar dates indicate that the course of the Rio Grande was established more than 4.0 m.y. ago in central New Mexico and about 3.0+ m.y. ago in north-central New Mexico. The Ortiz geomorphic surface was probably graded to the ancestral Rio Grande about 3 m.y. ago and is considerably older than the Llano de Albuquerque surface. The central Rio Grande region has been warped and faulted since late Pliocene time. A complex fault block in the southeastern part of the Albuquerque-Belen Basin includes volcanic rocks more than 20 m.y. old that originated from southwestern New Mexico before rifting of the Rio Grande depression.

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