Paleomagnetism and the Terranes Puzzle in Western North America: How it Started vs. How it is Going

Cristina García-Lasanta, Western Washington University
Bernie Housen, Western Washington University

Abstract

In 1983, a group of geoscientists, including Professor Myrl Beck from WWU and professionals from the USGS, published a map depicting suspect terranes of the Western margin of the North American plate that had been described until then. With the terrane map, they also represented the paleomagnetic data available until then: 77 localities (in 88 different rock formations) across 25 accreted terranes, compiled in 50 different published contributions. The map included arrows representing mean paleomagnetic directions and the expected paleomagnetic vector for each terrane (these based on the paleomagnetic references for stable North America available at the time and considering the geochronology results available then for each geologic formation). The different orientations and sizes of the arrows in each locality were used to illustrate the magnitudes and sense of vertical axes rotation and latitudinal displacement that the paleomagnetic results interpret for each terrane.

Following Prof Beck's steps, the Pacific Northwest Paleomagnetism Laboratory is currently working on gathering an up-to-date compilation of published paleomagnetic data collected in rock formations from Western North America. The final product we aim to create with this long-term project would be an interactive platform in which the interpretations that paleomagnetism results have provided concerning the accretion history of Western North America terranes can be easily accessed and visualized by users from all geoscientific disciplines.

One of the first stages in this ongoing project was to produce a georeferenced basemap of all the terranes defined across the North American Cordillera. After a preliminary version was presented at AGU24 in December 2023 (based on compilations by May et al., 1983 and Silberling et al., 1992), a second version of this map, after additional refining and adjustments, is presented here. The paleomagnetic localities identified so far for the updated paleomagnetic data compilation are also shown. We roughly estimate that around 80% of the published paleomagnetic results have been located (a total of 532 localities so far, if we attend to the authors' descriptions in each study, the geographic distribution of sites and the ages of the sampled rocks), however we are still corroborating and reevaluating the total number of discrete localities that are included to better depict the paleomagnetic components in each study.

Session

Cordilleran tectonics