CONTINENTAL SUBDUCTION, AS A MECHANISM FOR PALEOZOIC EMPLACEMENT OF THE ROBERTS MOUNTAINS THRUST, AND A DRIVER OF SUBSEQUENT TECTONISM IN WESTERN NORTH AMERICA

The Antler orogeny was the first tectonic event to affect western ancestral North America. From the Neoproterozoic to Late Devonian, post-rift subsidence led to growth of a kilometers-thick shallow-water carbonate platform on the western shelf of Laurentia. In the Late Devonian, carbonate deposition was interrupted by a rapid increase in water depth and the eventual drowning of the carbonate platform. The Mississippian Antler orogeny emplaced deepwater Lower to Middle Paleozoic shales atop the shallow-water carbonate platform. The contact between the dissimilar rock masses was named the Roberts Mountains Thrust.

In 1981, Jess Johnson of Oregon State U. proposed that the Antler orogeny involved the descent of continental crust into an ocean trench at a west-dipping subduction zone. The deepwater assemblage was an accretionary wedge, while the carbonate assemblage was a continental-shelf platform. As the former passive margin descended into the trench, the shallow-water platform slid under the accretionary prism. Speed and Sleep (1982) refined the model, and Ingersoll (2008) summarized the consensus on western subduction.

I base this talk on my work with modern seismic data from Indonesia that shows descent of continental crust into subduction trenches. Slab-rollback impels lateral movement of trenches across ocean plates. As the gravity-well of a trench nears a passive margin, the shelf descends a gentle ramp and enters the trench. Sediments scraped off the basinward side of the descending plate pile-up into an accretionary prism. Ultimately, shallow-shelf carbonate slides under deepwater sediments at the bottom of the trench. Gravity is the principal stress (σ1) driving the process. Nothing is laterally pushed-up or “thrusted” on top of the platform.

These analogs are consistent with the model that the Roberts Mountains thrust resulted from the collision of a back-rolling subduction trench with the passive margin of western Laurentia. The juxtaposition of these rock masses formed the first tectonic suture of Western North America. A consequence of the event was a break in the weld between Laurentian continental-lithosphere and oceanic-lithosphere. Slab breakoff led to a change in polarity from west-dipping to east-dipping subduction along this newly active margin; a state that persists to Recent time.