Abstract

The ∼400,000 km2 calc-alkalic Sierra Madre Occidental volcanic field is the largest component of the Tertiary ignimbrite flare-up of western North America. Laser-fusion 40Ar/39Ar geochronology of sanidine and anorthoclase from ignimbrites sampled within three areas of concentrated geologic mapping in the Mexican states of Chihuahua and Durango provides the first examination of fine-scale temporal fluctuations within this significant area of the ignimbrite flare-up. Overall, the 112 40Ar/39Ar ages range from 28 to 46 Ma, with a separate group near 24 Ma. The oldest ages (40–46 Ma, with a strong cluster at 44–46 Ma) are found only along the eastern margin of the field. The predominant age concentration (28–36 Ma) is found across two east-west traverses of the volcanic field that are separated by 500 km. Within this interval, there are strong concentrations at 28–30 Ma, ca. 33 Ma, and 35–36 Ma in the northern traverse, and at 30–31 Ma and at 31.5 Ma in the southern traverse. A cluster of ages near 24 Ma is present in the western part of both traverses. Ignimbrite sequences of similar age are oriented in belts generally parallel to the axis of the Sierra Madre Occidental volcanic field. The belts partly overlap but are progressively younger toward the west. This geographic pattern fits with numerous studies that relate the position of arc magmatism to the delamination/rollback of the subducted Farallon slab. However, a more difficult challenge is to reconcile a pattern of discrete, brief (∼2 m.y.) and intense episodes of magmatism to behavior of the Farallon plate.

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