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Van der Voo Lecture - Dr. Bernard Housen, Western Washington University

Whence British Columbia, Redux: The Baja-BC model and Collisional Orogeny
Friday, April 14, 2023
3:30-4:30 PM
1528 1100 North University Building Map
Recognizing the role and extent of large scale deformation, including rotations of continental margins (such as rotation of Iberia- Van der Voo, 1969) and the displacement and accretion of tectonostratigraphic terranes (van der Pluijm et al., 1993) have been among the most important applications of paleomagnetism to tectonics problems. An example of these processes are models for the growth and accretion of the North America Cordillera, and causes of regional orogenic events. The western margin of North America has been an accretionary margin for the past ~250 million years, with a complex history of terrane accretion, subsequent translation, and orogenic and magmatic processes during that time. An important, and to some controversial, element of the tectonic history of the Cordilleran margin has been the paleolatitude and timing of terrane accretion and interaction between these terranes and the margin- including the role and nature of subducting oceanic plates. Beginning with work published over the past 50 years, paleomagnetic results pointed to the accretion of large terranes at latitudes far to the south of their present locations along the NA margin. These models are commonly referred to as the Baja-BC hypothesis (Irving, 1985; Umhoefer, 1987), and remains an issue of active study (Housen, 2018; Mahoney et al., 2021; Tikoff et al., 2023). Evaluation of models for far-travelled terranes requires understanding the paleogeography of North America to be used as a comparison for data from Cordilleran terranes. Data for Late Jurassic to Eocene time will be presented, and questions- for example conflicting view of Late Jurassic paleopoles- will be highlighted. Paleomagnetic results from Cretaceous rocks of stable North America (Housen, 2021; Tikoff et al., 2023) indicate that the data is 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 terranes along the Cordilleran margin.

The paleomagnetic data from the Intermontane and Insular Superterranes indicate that the Intermontane Superterrane had post-100 Ma displacements of ~700 to 1400 km; and the Insular Superterrane had post-100 Ma displacements that are larger (2000-3000 km). This forms the basis for an updated collisional model for Late Cretaceous-Eocene orogenic events. The collisional phase (100-85 Ma) resulted in a nearly simultaneous collision from central Mexico to central Idaho, and the coastal magmatic arcs in this segment experience a rapid and intense episode of dextral tranpression/contractional deformation. This event also resulted in major contraction in the Sevier fold-thrust belt, foreland block uplifts in the northern Rocky Mountains, and significant foreland sedimentation in adjacent North America. A 85-55 Ma “run” phase resulted in continued contraction throughout North America in addition to dextral strike-slip faulting of coastal blocks, and is supported by paleomagnetic data from Paleocene rocks of the Chugach terrane. The contractional deformation in Wyoming and Montana is hypothesized to occur because of the clockwise rotation of a large lithospheric block (Blue Mountains terranes and adjacent Laurentia). This block rotation was accommodated by sinistral motion along the Lewis and Clark deformation zone, and resulted in significant shortening in SW Montana.
Building: 1100 North University Building
Event Type: Lecture / Discussion
Tags: Natural Sciences
Source: Happening @ Michigan from Earth and Environmental Sciences