RETHINKING THE ANTLER AND SONOMA OROGENIES IN THE WESTERN UNITED STATES

The 1970's recasting of the Antler and Sonoma orogenies into a plate tectonic framework by W.R. Dickinson, B.C. Burchfiel, G.A. Davis, and others was a seminal event, but this general framework has changed little over the intervening years. This paper focuses on issues that may require us to revise our thinking about these orogenies.

Antler orogeny: The classic models for this latest Devonian-Early Mississippian event invokes arc-continent interaction, development of a single, thin, but wide obducted allochthon, and consequent flexural loading of a tapered continental margin. However, the western U.S. lower Paleozoic outer continental margin was comprised of a series of periodically reactivated, long-lived basins and highs of what can be described as a continental borderland. The Antler orogeny as seen in Nevada-Idaho portion of the continental margin must be modelled as the collapse of this borderland. An arc-continent collision may have been involved, but there never has been any direct evidence for this in the western U.S. Arguably, this also makes the Antler events in the western U.S. more compatible with Devonian-Mississippian events in the Canadian Cordillera.

Sonoma orogeny: The upper Paleozoic continental margin records periodic compressional and extensional tectonism. The classic latest Permian-earliest Triassic Sonoma orogeny can be viewed as a culmination of these upper Paleozoic events. The mechanism(s) for these tectonic episodes is unresolved and possibilities include: 1) Ancestral Rocky Mountain and related deformation; 2) Laramide-style far-field effects of an Andean-type southwestern continental margin; 3) periodic shifts in plate interactions between western North America and components of the paleoPacific, and, for the last event, 4) an arc-continent collision. However, whereas the“Sonoma orogeny” did occur in that Triassic units depositionally overlap deformed rocks of the Golconda allochthon, the final emplacement of this allochthon, and hence is “basal” thrust, the Golconda thrust, may be later Triassic or even Jurassic features. This begs the question of the significance of the classic Sonoma orogeny versus later Triassic and Jurassic events, and again, opens the door for more compatibility with events documented in the Canadian Cordillera.