Researchers have documented an unusual tectonic process occurring beneath the Pacific Northwest, where a subduction zone is fragmenting into smaller pieces. The study, published in Science Advances, analyzed data collected during a 2021 research effort that used seismic imaging to examine structures hidden beneath the seafloor. By sending sound waves into the ocean floor and measuring returning signals, scientists created detailed underground maps that revealed large faults and fractures cutting through the sinking tectonic plate.
The affected area involves the Juan de Fuca and Explorer plates, which are being pushed beneath the North American plate in the Cascadia subduction zone along the Pacific Northwest coast. Researchers identified several significant tears running through the oceanic plate, including one massive feature where part of the slab has dropped approximately five kilometers. A tear stretching roughly 75 kilometers shows varied earthquake activity, with some portions continuing to produce seismic events while others have become unusually quiet. Scientists interpret the absence of earthquakes in certain areas as evidence that sections of the plate have already separated.
Rather than failing in a single catastrophic event, the subduction zone appears to be terminating through a series of smaller breakups occurring over extended periods. As individual sections tear away at different times, the larger tectonic system gradually dismantles. Transform boundaries, which are faults where sections of Earth’s crust move sideways past one another, appear to play a crucial role in this process by acting like geological scissors that isolate individual fragments. Once separated, these pieces can form microplates that move somewhat independently from surrounding major plates.
This discovery may help explain geological features observed in other regions worldwide. Scientists have previously identified abandoned plate fragments and unusual volcanic rock sequences that appear to record final stages of ancient subduction systems, but lacked clear information about how such breakups actually occurred. The Cascadia findings offer a potential explanation for how remnants such as fossil microplates off Baja California formed through gradual unraveling rather than sudden collapse.
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