CENOZOIC MAGMATISM AND PLATE TECTONICS IN WESTERN NORTH AMERICA: HAVE WE GOT IT WRONG? (Invited Presentation)

Many fundamental concepts of plate tectonics were developed from observations in western North America in the 1960s and 1970s. In particular, Warren Hamilton and Tanya Atwater placed Mesozoic and Cenozoic magmatism into a plate-tectonic framework in truly pioneering papers. Their basic understanding—that the magmatic and tectonic history of the area resulted from subduction of oceanic plates followed by ongoing conversion of the continental margin into a transform boundary—inspired a blast of creative new postulates, including the northward retreat of an ancestral Cascade Range, shallowing and then rapid steepening of the subducted Farallon plate to produce sweeps of magmatism, and opening of slab windows and consequent alkali-basaltic volcanism. These ideas led a callow undergraduate (me) to investigate the K₂O contents of lavas to determine the dip of the Miocene slab. There seemed to be no end to links between magmatism and plate tectonics, and the ideas of the 70s still underlie tectonic interpretations today.

The 1970s papers that followed Hamilton and Atwater were based on limited data that had to be hand-compiled on paper, but we can now test them with NAVDAT, which has extensive data for over 65,000 samples. Alas, space-time patterns of magmatism examined via time animations in many ways contradict the classic plate tectonic history. For example, there is no evidence for an ancestral Cascade arc in central and southern California; the eastward/westward sweeps of magmatism that should mark Late Cretaceous/Neogene changes in slab dip are not evident; and patterns of basaltic magmatism bear little if any relationship to postulated slab windows. In and around southern California the Tertiary record of magmatism is nearly opposite that predicted by the ancestral Cascade hypothesis: no magmatism after the Late Cretaceous shutdown of the Sierran arc until subduction ended at a given latitude, followed by a huge blast of subduction-like volcanism.

Given the misfit between the magmatic record and our current understanding of the plate tectonic history, it is time to reexamine that history. It is hard to see how the plate-circuit reconstructions can be in significant error, but it is difficult to reconcile the magmatic record with them. Something fundamental is wrong.