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The Identification Of The Pioneer Fragment, A Remnant Of Subducted Oceanic Plate About 30 Million Years Ago, At The Mendocino Triple Junction, Expands The Contact Area Between The Pacific, Cascadia, And North American Plates And Introduces New Elements In The Assessment Of Regional Seismic Risk
A fragment of tectonic plate considered lost has been identified beneath the North American continent, in the southern portion of the Cascadia subduction zone, according to research published on January 15 in the journal Science. The finding points to possible implications for regional seismic dynamics.
The research reveals that the so-called Pioneer Fragment, a remnant of an ancient oceanic plate subducted about 30 million years ago, is currently stuck to the base of the Pacific Plate and is moving northwest with it in a tectonically complex region.
The fragment was identified at the Mendocino triple junction, the point where the San Andreas Fault meets the Cascadia subduction zone. This is an area where different plate movement regimes coexist and interact directly.
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Along the San Andreas Fault, the North American and Pacific plates slide parallel to each other. In the Cascadia zone, which extends from Cape Mendocino to Vancouver Island, the oceanic plates of Juan de Fuca and Gorda are diving beneath the North America.
This subduction process is capable of generating earthquakes of magnitude 9 or higher, according to data from the Pacific Northwest Seismic Network. Previous evidence suggests that major events in Cascadia may trigger earthquakes along the San Andreas Fault.
Although the new study does not directly quantify the increased risk, the lead author of the research, David Shelly, from the United States Geological Survey, states that the discovery helps to better understand the interaction between these tectonic structures.
According to Shelly, the Pioneer Fragment expands the contact area between what is effectively the Pacific Plate and the Cascadia subduction zone, altering the known geometry of the interfaces between the plates in the region.
Seismic Analysis Revealed Subtle Movements Of The Plates
To investigate the Mendocino triple junction, researchers analyzed small earthquakes and low-frequency tremors, deep seismic events that are not perceptible without sensitive seismographs, but occur frequently along major faults.
From this data, it was possible to determine directions of subtle movements of the plates. In the region, the Pacific Plate slides northwest against the North American Plate while colliding with the Gorda Plate, which is moving under the continent.
Until now, there were competing explanations about the exact location of the plates and the tracing of the faults. The study indicates that the configuration is even more complex due to the presence of an unexpected fragment of the extinct Farallon Plate.
The Farallon Plate began to experience subduction beneath North America about 200 million years ago, during the fragmentation of the supercontinent Pangaea. The Juan de Fuca Plate is one of its known remnants.
Additional Fragments Alter The Tectonic Model Of The Region
Researchers identified that another remnant of the Farallon Plate remained attached to the Pacific Plate. This fragment, called the Pioneer Fragment, is not subducting but is moving laterally against the North American continent.
Additionally, parts of the Gorda Plate that were scraped and transferred to the North American Plate during collisions appear to have returned to the Gorda Plate, possibly diving again beneath the continent.
This dynamic, which Shelly described as a “tectonic hot potato,” may help explain why the 1992 Cape Mendocino earthquake occurred at a shallower depth than scientists expected.
According to the researcher, the fault associated with these fragments may not follow the original oceanic crust, lying at shallower levels, which alters the understanding of the depth of local seismic processes.
Between the Pioneer Fragment and the North American Plate, there is an almost horizontal fault, compared by Shelly to the layers of a cake. This structure is not currently included in seismic risk models.
“We don’t know if this fault can generate large earthquakes, but it’s something we need to consider in the future,” Shelly said, emphasizing that the discovery expands the uncertainties about the seismic behavior of the region and requires new analyses.
This article was based on research published on January 15 in the journal Science and statements from geophysicist David Shelly, from the United States Geological Survey, published by Live Science, based on seismic analyses of the Mendocino triple junction, between the San Andreas and Cascadia faults.
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