Building the US West Coast

About 365 million years ago, during the Devonian Period, a westward dipping subduction zone began west of the continent, creating a volcanic island arc chain. As the ocean crust between the arc and the continent subducted, the two landmasses began to collide. The effects were felt across present day Montana. This marked the beginning of tectonic activity along this boundary, and it resulted in the Antler orogeny. The arc accreted, and it can be seen today in the Klamath Mountains and northern Sierra Nevada Mountains of California and southern Oregon (Chernicoff, Stanley, et al. 2002). The Cordillera would continue to accumulate additions from the west.

The Sonomian arc started docking in Central California about 260 mya. By about 235 mya, the thrust sheet produced by the collision reached central Nevada.

Using maps and descriptions created by Dr. James Sears from the University of Montana, and used with his permission, we can get a visual idea of the sequence of tectonic events of the past 220 million years.

Click on each map to enlarge.

Late Triassic Hypothetical west-facing magmatic arc of same scale and curvature as modern Aleutian arc encloses a marginal sea floored by late Paleozoic-Triassic oceanic crust and marine sediments (Cache Creek, etc.). The arc collapses against North America beginning in late Triassic.