Paleogene Tectonic Evolution of the Pacific Northwest

Three consecutive tectonic regimes are recorded in the Paleogene of the North American Cordillera between 40° and 60° latitude. The Paleocene (65-53 Ma) regime produced crustal shortening along the eastern edge of the Cordillera, as well as a weak magmatic arc in British Columbia. The lower and middle Eocene regime (53-42 Ma) produced a robust volcanic arc superimposed on a network of strike-slip and normal faults and large north-northeast trending extensional metamorphic reset terranes. The Late Eocene and Oligocene regime (42-30 Ma) produced a north-south Cascade arc south of 50° latitude, and tectonic quiescence to the north and east. This sequence of events is consistent with the following tectonic model. In Paleocene time, continuous subduction took place from 40° to 60° latitude; low-angle subduction south of 49° created the Laramide magmatic gap. At about 53 Ma, amalgamation of the Pacific, Kula and northern Farallon plates created a Pacific-North America transform boundary north of 47° latitude. Transform motion was in part taken up on the Fraser-Tintina strike-slip system, cross-cutting the volcanism produced from the remnant of the subducted slab. At about 42 Ma, inland transform motion ceased, and all transform motion took place on the Queen Charlotte transform. South of 47°, subduction continued with a steady steepening of the subduction angle from north to south with time.