Plate Tectonics of Montana and Northern Rockies

Basin and Range Extension

The plate configuration responsible for the most of the mountain ranges in Montana, Wyoming, daho was subduction of the oceanic (Farallon) plates under the western margin of North America. The Farallon plate was generated at a oceanic ridge in the Pacific Ocean and consumed at the trench adjacent to the West Coast. However, with the rapid westward advance of North America (due to the opening of the North Atlantic with the separation of Europe and North America, the Farallon plate was being consumed faster than it was being genrated. The Farallon plate shrunk in size. (It had split into two parts--the Kula and the Farallon plate proper.) The Kula plate has completely disappeared (under Alaska and British Columbia).

Beginning in the Eocene the Cordillera have been undergoing extension oriented approximately east-west. The extension is consistent with the motion of the Pacific plate to the northwest and the role of the San Andreas fault as a transform fault.

Basin and Range extension is currently active in Western Montana and accounts for most of the seismicity. The 1935 Helena Earthquake is a notable example where movement on a normal fault allowed the Helena Valley to be downdropped. Normal faults have been active on both sides of the Smith River Valley during the Quaternary and in many other valleys in Western Montana.

Post-Eocene volcanism in Montana is associated with basin and range extension. Haystack Butte west of Augusta and basalt flows by Lingshire and at Lake Sutherlin in the Little Belt Mountains are some of the many occurrences.

Plate Tectonics of Montana (Page 13 of 14)