Source: Geophysical Research Letters

The processes that form continental crust from the denser basaltic rocks of the upper mantle may make the lower lithosphere denser than the underlying mantle. One theory holds that the lower lithosphere splits away and sinks into the mantle in a process called foundering. Conclusive evidence of foundering, however, has been hard to come by.

Peering deep under California’s Sierra Nevada, Schulte-Pelkum and Kilb discovered new evidence of lithospheric foundering in progress. The team imaged the lower crust and uppermost mantle beneath the Sierra Nevada with receiver function analysis, which uses seismic waves that change as they cross structures beneath the surface.

They also studied earthquake data from the Advanced National Seismic System Comprehensive Earthquake Catalog, or ComCat. They found a band of seismicity in the central Sierra, in which small earthquakes (ranging from magnitude 1.9 to 3.2) occur at the unusual depths of 40 kilometers and greater.

Differences in receiver functions along the mountain range revealed a distinct layer in the mantle, which grows gradually less distinct farther north. This aligns with the existing hypothesis that a section of the lithosphere beneath the southern Sierra sank (foundered) millions of years ago.

A slab of colder continental lithosphere also has the capacity to crack, rather than to stretch and flow like hot material typically found at such depths. This likely also explains the presence of such deep earthquakes in the central Sierra, according to the authors.

The researchers found no evidence of this layer in the northern Sierra, indicating foundering has yet to progress to that region.

This work aligns with previous studies that found a gradient Moho, rather than a sharply defined crust-mantle boundary, under the Sierra. It also matches previous suggestions that a cold mantle anomaly under the Great Valley region to the west may be dense lithosphere lost to the foundering process. Foundering has been ongoing in the Sierra for at least 3 million years according to this hypothesis, and the researchers say it may be progressing northward. This region provides evidence of a process of differentiation that occurs throughout Earth’s crust, they argue. (Geophysical Research Letters, https://doi.org/10.1029/2024GL111290, 2024)

—Nathaniel Scharping (@nathanielscharp), Science Writer

Citation: Scharping, N. (2025), Deep beneath California’s Sierra Nevada, Earth’s lithosphere may be peeling away, Eos, 106, https://doi.org/10.1029/2025EO250020. Published on 17 January 2025.

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