Lithospheric Foundering Under the Sierra Nevada: New Rare Evidence?

      Lithospheric foundering, or delamination, refers to the loss and sinking (foundering) of a portion of the lowermost lithosphere from the tectonic plate to which it was attached. The lower part of the lithosphere, called the mantle lithosphere, is denser than the asthenosphere below it. According to Wikipedia:

“Delamination occurs when the lower continental crust and mantle lithosphere break away from the upper continental crust. There are two conditions that need to be met in order for delamination to proceed:

·       The lower lithosphere must be denser than the asthenosphere.

·       The intrusion of more buoyant asthenosphere making contact with the crust and replacing dense lower lithosphere must occur.

Density inversions are more likely to occur where there are high mantle temperatures. This limits this phenomenon to arc environments, volcanic rifted margins and continental areas undergoing extension.”

     There are two main geological effects of delamination: uplift of the crustal lithosphere into mountain ranges and volcanism as hot mantle material breaks through the thinned lithosphere.

     When Seismologist Deborah Kilb was studying California Earthquake records, she noticed a series of quakes that should have been too deep into the mantle for seismic activity due to the high pressures and temperatures. The source of these quakes was nearly twice as deep as the deepest California earthquake epicenters at about 11km (6 miles). The deeper quakes were centered 18km (11 miles) below the surface. Some quakes were 20-40km (12.4-25 miles) below the surface.

     Kilb shared the earthquake data with Vera Schulte-Pelkum, a research scientist at the Cooperative Institute for Research in Environmental Sciences and an associate research professor of geological sciences at the University of Colorado Boulder, who was studying deep rock deformations under the Sierra Nevada. They then utilized a seismic technique known as receiver function analysis to image the rocks below the Sierra Nevada. The scientists found that in the central region of the mountain range, Earth’s crust is currently peeling away from the bottom in the process of delamination, or lithospheric foundering, with the lowest layers melting deeper into the mantle.

     Lithospheric foundering has been studied in several places, including under the Andean plateau and under Tibet. The process has been associated with batholiths – igneous rock intrusions, usually granitic, and found near mountain fold belts. Knowledge of lithospheric foundering can offer insights into the formation of continents. There are indications of lithospheric foundering on Venus, where there is no plate tectonics. Below is a map of faults and lithospheric structures under Tibet, and below that is a similar map under the Central Andes.

“Lithospheric foundering is the process of the denser materials being pulled to the bottom, while the less dense material emerges at the top, resulting in land creation. “It’s dumping some of this denser stuff into this gooey, solid mantle layer underneath and sort of basically detaching it so it stops pulling on the less dense stuff above,” she {Kilb} explained.”

     Under Sierra Nevada, they found a distinct layer about 40 to 70 kilometers (25 to 43 miles) deep with characteristics different from those of the rock around it. This is the layer being peeled off.

     There is an ongoing debate about whether the mantle anomaly identified under the Sierra Nevada is due to lithospheric foundering or subduction. There is scant evidence of lithospheric foundering due to the difficulty of imaging at great depths. Thus, the new research may offer some rare evidence. Graphics from the paper are below.   

 

  

References:

 

Scientists stumble across rare evidence that Earth is peeling underneath the Sierra Nevada. Taylor Nicioli, CNN. April 18, 2025. Earth is peeling underneath the Sierra Nevada, rare evidence shows | CNN

Earth's crust is peeling away under California. Stephanie Pappas. Live Science. February 1, 2025. Earth's crust is peeling away under California | Live Science

Lithospheric Foundering in Progress Imaged Under an Extinct Continental Arc. Vera Schulte-Pelkum and Deborah Kilb. Geophysical Research Letters. Volume51, Issue 24. December 13, 2024. Lithospheric Foundering in Progress Imaged Under an Extinct Continental Arc - Schulte‐Pelkum - 2024 - Geophysical Research Letters - Wiley Online Library

Crustal bobbing in response to lithospheric foundering recorded by detrital proxy records from the central Andean Plateau. B. Carrapa; G. Jepson; P.G. DeCelles; S.W.M. George; M. Ducea; C. Campbell; R.R. Dawson (née Canavan). Geology (2025) 53 (1): 29–33. Crustal bobbing in response to lithospheric foundering recorded by detrital proxy records from the central Andean Plateau | Geology | GeoScienceWorld

Lithospheric foundering and underthrusting imaged beneath Tibet. Min Chen, Fenglin Niu, Jeroen Tromp, Adrian Lenardic, Cin-Ty A. Lee, Wenrong Cao & Julia Ribeiro. Nature Communications volume 8, Article number: 15659 (2017). Lithospheric foundering and underthrusting imaged beneath Tibet | Nature Communications

Delamination (geology). Wikipedia. Delamination (geology) - Wikipedia

Plateau Formation Controlled by Lithospheric Foundering Under a Weak Crust. M. McMillan, L. M. Schoenbohm, A. R. Tye. Geophysical Research Letters. Volume 50, Issue16. August 22, 2023. Plateau Formation Controlled by Lithospheric Foundering Under a Weak Crust - McMillan - 2023 - Geophysical Research Letters - Wiley Online Library