A PALEOMAGNETIC FRAMEWORK FOR BAJA BRITISH COLUMBIA MODELS

Among Paul Umhoefer’s enduring contributions to understanding the tectonics of the North American Cordillera are his early work to synthesize paleomagnetic data into a unified geological and tectonic framework (Umhoefer, 1987) and his continued work to evaluate and reconcile many of the large-scale displacements that paleomagnetic data require through the lens of a skilled and insightful geologist (e.g., Umhoefer, 2003).

Cretaceous paleomagnetic data establish a paleogeography of North America which is then used as a comparison for the paleomagnetic and paleogeographic data from Cordilleran terranes. Paleomagnetic data from Cretaceous rocks of stable North America (Housen, 2021, and Tikoff et al., 2022) are robust and define two age-grouped paleopoles. The NA pole from 145 to 85 Ma is 71.8 N, 192.7 E, A₉₅=2.4, N=27, placing NA at high latitude. The 84-65 Ma pole for North America is 82.6 N, 184.1 E, A₉₅ = 3.5, N=3, which indicates a southward shift of NA after 80 Ma. The N-S orientation and well-defined set of paleopoles provide an ideal reference for definition of displacements of Cordilleran terranes.

Paleomagnetic data from the Intermontane and Insular Terranes have well-defined ages, magnetizations, and paleohorizontal control. For sedimentary units, the effects of inclination shallowing are considered and corrected for. These data indicate that the Intermontane and Insular Terranes have different amounts and histories of their displacement: 1) The Intermontane Terrane experiencing post-100 Ma displacements of ~700 to 1400 km; and 2) The Insular Terrane experiencing larger post-100 Ma displacements (2000-3000 km). This pattern suggests a separate Late Cretaceous tectonic history for these two terranes. The analysis here also finds significantly less displacement of the Insular Terrane from units younger than 80 Ma. This demonstrates that paleomagnetic studies of these rocks have captured its northward motion. The transition from the lower paleolatitudes recorded by ~90 Ma rocks to progressively higher paleolatitudes recorded by rocks that are younger than 80 Ma provides useful constraints for tectonic models. The Stikine-escape concept of Umhoefer (2003) will be discussed and compared with other collisional tectonic models for NA orogenic events (Tikoff et al., this session).