KEYNOTE: TIMING OF INTENSE MAGMATIC EPISODES IN THE NORTHERN AND CENTRAL SIERRA MADRE OCCIDENTAL, WESTERN MEXICO

The approximately 400,000-km² calc-alkalic Sierra Madre Occidental volcanic field (SMOVF) is the largest component of the Tertiary ignimbrite flare-up of western North America. The eruptive history of the SMOVF has been examined by laser-fusion ⁴⁰Ar-³⁹Ar dating of sanidine and anorthoclase for 112 samples selected from documented stratigraphic sections within three areas of concentrated mapping projects located in the Mexican states of Chihuahua and Durango. Extrapolation of detailed and precise age determinations from these three areas provides the first examination of fine scale (ca. 1 Ma) temporal fluctuations within the northern two-thirds of the SMOVF.

Overall the 112 ages range from 28 to 46 Ma with a separate group near 24 Ma. The oldest ages (>40 Ma, with a strong cluster at 44 to 46 Ma) are from the eastern margin of the field. The predominant age concentration (28 to 36 Ma) is found in two east-west traverses of the field that are separated by 500 km. Within this interval there are strong concentrations at 29-30 Ma, ca. 33 Ma, and 35-36 Ma in the northern traverse, and at 30-31 Ma, at 31.5 Ma in the southern traverse. Ages near 24 Ma are also present in the western part of both traverses, but precise dates have been obtained only in the south. Finally, a chemically distinct group of peralkaline tuffs near Chihuahua City are tightly clustered at 30 Ma.

These results demonstrate a clear tendency for eruptive activity to concentrate in discrete time intervals within the ignimbrite-dominated SMOVF. Although pulses of activity are not entirely coincident throughout the entire area studied, some are of regional significance. Activity at 29-30 Ma is common to all three areas studied. At 31 to 33.5 Ma activity appears to have been common across the medial part of the SMOVF. Ignimbrites as old as 46 Ma are exposed only in the eastern margins of the SMOVF. Ignimbrite sequences of similar age are oriented in belts that are generally parallel to the axis of the volcanic field and are younger to the west. This pattern fits with numerous studies that relate the timing and position of arc magmatism to the delamination/roll back of the subducted Farallon slab. However, a more difficult challenge is to reconcile discrete, brief (<5 Ma) and intense episodes of magmatism to behavior of the Farallon plate.