Northward translation of “BAJA British Columbia” along the Late Cretaceous to Paleocene margin of western North America

Abstract

Paleomagnetic data suggest that a large part of British Columbia and southern Alaska was at the latitude of Baja California, Mexico, in early Late Cretaceous time. This allochthonous block (Baja BC) includes the Insular (Talkeetna in Alaska) and Intermontane composite terranes, the miniterranes between them, and possibly part of the Omineca metamorphic belt. Assuming that Baja BC was adjacent to North America by middle Cretaceous, an analysis using plate kinematics of the Pacific basin produces a model of northward movement of Baja BC which is compatible with the known geology of the reconstructed margin of western North America. Baja BC moved north 2400 km as part of the KuIa plate along a transform margin with North America from 85 to 66 Ma. This time interval is marked by the cessation of igneous activity in the Sierra Nevada, development of wrench faults and basins along coastal California, and initiation of Laramide‐style tectonics to the east. Within Baja BC, diminished igneous activity to the west was coeval with possible wrench‐tectonic basins along its eastern margin. By the end of the Cretaceous, Baja BC was positioned from Oregon to northern British Columbia. Between 66 and 56 Ma, Baja BC became attached to North America in a dextral‐transpressive stage, during which the Kula‐‐North America plate boundary expanded to encompass the whole block with a series of northwest trending strike‐slip faults, a fold and thrust belt in the east, and large‐scale uplifts and the formation of an incipient volcanic arc on the west. Southern Alaska was the site of convergence during the whole of the period of northward translation of Baja BC. A Late Cretaceous subduction zone in Alaska evolved into a major compressional belt as the basin and miniterranes on the northern margin of Baja BC collided with interior Alaska. The collision culminated in the Paleocene with formation of the McKinley granites and coeval dextral slip on the Denali fault system.