Collaborative Research: Clockwise block rotation in the Pacific Northwest and sinistral movement on the Lewis & Clark zone

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

General The western portion of the North American continent was assembled by collision of large blocks of crust over the past 200 million years. These collisions are likely responsible for the formation of mountain belts, significant igneous activity, and production of mineral deposits throughout the western United States. This project will gather data from rocks in Idaho and Montana to test new ideas to explain the timing and origin of the mountain building events in the northern Rocky Mountains and Pacific Northwest. The ideas to be tested include the type of crustal movement, the timing of those crustal movements, and the deformation of rocks associated with it. Predictions resulting from these ideas include the existence of clockwise rotation of large portions of Idaho, Oregon, and Montana. The Lewis & Clark zone of Idaho and Montana - a large-scale strike-slip fault - may have resulted from these clockwise rotations. The work conducted for this project will include collection of samples to measure the direction of the Earth’s magnetic field recorded when the rocks formed, and to analyze minerals in these rocks to determine their age. This data will test predictions for the timing of deformation and of associated igneous activity. The work will be done by a team of faculty at two universities, three graduate students, and several undergraduate students. The proposal is a straightforward paleomagnetic test of whether the Blue Mountains and Adjacent Laurentia – the region of eastern Oregon, southeastern Washington, central Idaho, and possibly southwestern Montana – rotated ~30° clockwise as a coherent block in the Late Cretaceous-Paleogene. A series of paleomagnetic analyses will be conducted on different locations to test whether the clockwise rotation happened and, if so, what is the spatial extent of the crustal block. Paleomagnetic analyses of different ages of rocks will test when the clockwise rotation occurred, to correlate it to motions along the western margin of Laurentia and contractional deformation in the continental interior. The implications are three-fold. First, clockwise rotation could provide independent evidence for dextral terrane translation along the western margin of the Pacific Northwest. Second, the clockwise rotation may explain the activation of the Lewis and Clark line, a major strike-slip system that is oblique to the plate margin. The timing of clockwise rotation should act as a guide to the history of left-lateral vs. right-lateral movement on that structure. Third, the rotation of the Blue Mountains- Adjacent Laurentia block may be spatially and temporally correlated with Laramide-style deformation in its type locality, it could inform questions about foreland deformation. The work will support three graduate students from Western Washington University and the University of Wisconsin-Madison. It will also support undergraduate student research and a capstone experience for undergraduate students at each university. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.