Uncovering ancient subduction mega-faults in the Olympic Mountains

On October 5, 2024 the Quimper Geological Society hosted a lecture by Harold Tobin, professor of Seismology and Geohazards at the University of Washington and Director of the Pacific Northwest Seismic Network. Harold’s research employed field mapping and geophysical techniques to investigate processes that operate in fault zones, particularly those associated with subduction zones. One of his study areas is the Olympic Mountains, where recently (2023 and 2024) he and his graduate students have been studying a remote area west of Mount Olympus, last visited by geologists when Rowland Tabor and Bill Cady where they mapped more than 50 years ago. Tabor and Cady identified the Olympic Structural Complex, the highly deformed sedimentary rocks of the accretionary wedge associated with the Cascadia subduction zone. In a region ranging from Snow Dome to Mt. Tom and the White Glacier, Tobin’s team mapped a zone of concentrated brittle and ductile deformation they interpret as a fossil megathrust fault, the plate boundary fault responsible for great Cascadia earthquakes. Harold discussed the evidence for this conclusion and how analysis of an ancient fault helps us understand how and why these megathrust faults slip and generate earthquakes, as well as how the Olympics were built.

About the Speaker

Professor Harold Tobin holds the Paros Endowed Chair in Seismology and Geohazards at the Department of Earth and Space Sciences at the University of Washington. He also serves as the Director of the Pacific Northwest Seismic Network and Washington’s State Seismologist. Despite that title, his scientific roots are in subduction zone geology and the structure of plate boundary fault zones. With a B.S. in geology and geophysics from Yale University and a PhD from University of California, Santa Cruz, Harold has held faculty positions at New Mexico Tech and the University of Wisconsin-Madison prior to moving to Seattle in 2018. His first taste of subduction geology was as an undergraduate field assistant in the Olympic Mountains in 1987, and after 30 years of offshore and onshore research in Japan, Alaska, New Zealand, Costa Rica, and California, he has come full circle to explore the core rocks of the Olympic mountains.

Baranof Island, Alaska, and Vancouver Island were neighbors 50 million years ago

Darrel Cowan discussed the evolution of hypotheses about the large-scale coastwise displacements of tectonic elements or terranes along the western margin of North America. He presented the geologic evidence he published in 1982, which links Vancouver Island with southeast Alaska. The still-contentious Baja British Columbia hypothesis, developed in the early 1980s, was based entirely on paleomagnetic data.

This IN-PERSON ONLY lecture was free and open to the public. The lecture was recorded and posted shortly after the presentation, as are all our events since 2020. 

About the Speaker

Darrel Cowan is Professor Emeritus in the Department of Earth & Space Sciences at the University of Washington. He refers to himself as a field structural geologist. Since 1974, Darrel has spent his career educating geologists in tectonics and structural geology. Darrel is a west coaster, originally from southern California. He graduated from Stanford in 1966 and finished his PhD in the Franciscan subduction complex there in early 1972. Darrel’s research projects and those of his graduate students have investigated structural and tectonic problems in the Pacific Northwest, including southwestern British Columbia and Vancouver Island, southern Alaska, Japan, and the northern Apennines and Sicily in Italy.