FLAT SUBDUCTION AND CENOZOIC MAGMATIC PATTERNS IN WESTERN NORTH AMERICA

Magmatic patterns evident in the NAVDAT database of western North American magmatism (navdat.geongrid.org) provide support for widespread flat subduction under the western United States in the early Tertiary, but the classic interpretations of this event require significant modification. Plate reconstructions require rapid subduction of a huge area of Farallon plate under western North America in the Late Cretaceous and early Tertiary, yet magmatism shut down along most of the arc in the Late Cretaceous, not returning in most areas until the late Tertiary. This event has classically been viewed as (1) an inland sweep of the arc as the subducted plate flattened, (2) a rapid westward sweep as the plate rapidly steepened, and (3) northward extinction of the ancestral Cascades arc as the Mendocino triple junction swept north.

These patterns are not well supported by the NAVDAT compilation, although there is weak evidence for an inland sweep in the early Tertiary. Magmatism returned to the western U.S. via three main events: a southward sweep from Montana to Nevada, a clockwise sweep from New Mexico to Nevada around the Colorado Plateau, and a mid-Miocene outburst in northern Nevada (the Yellowstone event). The sweep around the Colorado Plateau is not consistent with slab-steepening because magmatism occurred simultaneously from New Mexico to California, along and south of the subducted Mendocino fracture zone. There is no evidence of an ancestral Cascades arc south of latitude 38°N (north of Yosemite National Park); indeed, California, southern Nevada, and Arizona were largely devoid of magmatism in the Oligocene. The southernmost expression of an ancestral Cascades arc formed north of Yosemite ~10 Ma.

Although magmatic patterns support widespread flat subduction in the early Tertiary, isotopic ratios in Cenozoic volcanic rocks indicate extensive preservation of old lithosphere across the western U.S. These features can be reconciled if the top of the subducted flat slab was at ~100 km depth, as in the modern Andes, except in southern California where it was much shallower and bit into the crust. Key geodynamic questions include (1) what triggered the various renewals of volcanism; (2) how arc volcanism reestablished itself in central California, and (3) why volcanic patterns in Mexico do not match those in the U.S.