The Rate of SW Pacific Cenozoic Tectonic Transitions Compared to Models of North American Cordilleran Tectonics: Are the Latter Too Simplistic?

The North American Cordillera includes multiple subduction sutures, and its proposed history is among the most complex of any orogenic belt in the world. In spite of the complexity of the various competing models, the pace of proposed tectonic transitions is vastly slower than that of the Cenozoic history of the SW Pacific. The most complex models for a Cordilleran transect at 40°N latitude have proposed the initiation of 4 subduction zones within about 500 m.y. In contrast, a typical transect across the SW Pacific exhibits that number of events within the last 50 m.y. Have we vastly underestimated the pace of orogenesis in the Cordillera? For the last subduction phase, probably not, because Franciscan Complex subduction went on uninterrupted for at least 140 m.y. Parts of the Sierra Nevada suggest greater complexity. The Feather River ultramafic belt is associated with high-temperature/high-pressure metamorphic rocks (metamorphic soles) that may reflect at two different, hot subduction initiation events separated by 100 m.y. (240, 340 Ma) in addition to an equally extended history of oceanic crust formation. If two hot subduction initiation events are recorded, this may imply two additional subduction initation events not long before them that resulted in the creation of the young oceanic crust on the hanging wall. This is because ophiolites with metamorphic soles universally show a supra subduction zone origin. Accordingly, this single zone of ultramafic and mafic rocks, previously thought to represent (at most) a single subduction suture, may in fact be a composite tectonic zone with the rock record of two to four or more discrete subduction zones. The Bear Mountains ophiolite zone is a another Sierran mafic-ultramafic belt that has a range of high-grade metamorphic and igneous ages. It may also reflect multiple subduction events, similar to the Feather River belt.