Subduction zones, convergent plate tectonic boundaries where oceanic lithosphere is overridden by continental lithosphere, are the only places on Earth capable of hosting great (magnitude greater than 8.5) earthquakes. Stretching from northern California to Vancouver Island, the Cascadia Subduction Zone, where the Juan de Fuca plate is subducting under the North American Plate, poses a serious seismic and tsunami hazard to populations along the coast of North America. The rocks that make up this extensive subduction zone are exposed at the surface only in the central-western Olympic Mountains within Olympic National Park. Here, a record of subduction is preserved in outcrops of mélange, highly chaotic and jumbled masses of sedimentary rock. These rocks formed in a three-stage process of initial subduction to a depth of 15-20 kilometers beneath the Cascadia plate boundary; transfer to the base of the North American plate accretionary wedge through underplating as new faults formed; and uplift and exhumation to the surface over millions of years. The geology of the region surrounding Mt Olympus and Mt Tom preserves evidence of these key processes, and we hypothesize that it specifically preserves examples of prominent subduction faults from depths relevant to great earthquakes. We conducted geologic mapping and extensive sampling of outcrops in the Lake of the Gods region in summer of 2023. We present initial evidence that these rocks document subduction zone processes through analysis of microscopic structures, mineral composition, and chemistry of rock samples.
Anna Ledeczi
University of Washington
Presentation given at North Coast & Cascades Science Days, 2024
go.nps.gov/sciencedays
[OLYM]
28 сен 2024
