Ancient tectonic slabs are still hiding beneath the surface of California and Mexico

Nature · Published 3 October 2026 Swipe more like this →

In short: For millions of years, the massive Farallon plate was consumed by the North American continent. While most of it sank deep into the mantle, recent seismic research reveals that large, fossilized chunks of this ancient plate remain trapped just 100 kilometers beneath our feet, fundamentally changing our understanding of western geology.

About 100 million years ago, the Farallon oceanic plate was positioned between the converging Pacific and North American plates. As these plates collided, the Farallon plate was subducted underneath North America. While much of the plate eventually sank into the deep mantle, researchers have long observed a mysterious 'high velocity anomaly' near California's Sierra Nevada mountains. This feature, known as the Isabella anomaly, consists of a large mass of cool, dehydrated material sitting 100 to 200 kilometers below the surface.

Previously, geologists often explained this anomaly through delamination, a process where a portion of the lithospheric plate beneath the mountains breaks off and sinks. However, new research using seismic tomography—which measures the speed of seismic waves to determine subsurface temperature and composition—suggests a different origin. By comparing the Isabella anomaly to similar features found under Mexico’s Baja Peninsula and the Pacific Northwest, scientists discovered that these slabs are likely remnants of the Farallon plate that broke off at significant depths rather than near the surface.

The geometry of these slabs, which align nearly due east of known Farallon fragments, provides compelling evidence for this theory. Furthermore, the presence of high-magnesium andesite volcanic rocks on the surface near the eastern edge of these anomalies suggests that these spots mark where the Farallon plate finally gave way, melting as it detached. This discovery forces a major rethinking of the tectonic history of western North America, suggesting that the geological evolution of the continent is far more complex than previously assumed.

This research, led by geophysicists at Brown University, indicates that the way we understand the Sierra Nevada's formation and other continental margins must be thoroughly re-examined. By identifying these 'fossil' slabs, scientists are gaining a clearer picture of how ancient tectonic movements continue to shape the modern landscape, proving that the remnants of the distant past are still very much present beneath the crust.

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