Lithospheric Foundering in Progress Imaged Under an Extinct Continental Arc

A long‐standing question is how felsic continental crust is differentiated from its mafic parent mantle magmas. One currently proposed fundamental mechanism is lithospheric foundering and loss of dense material into the mantle. A type locality is the young extinct arc forming the Sierra Nevada, California. Here, we image a distinct anisotropic shear layer below the crust‐mantle boundary using receiver functions. The sense of shear is consistent with west‐ to southwestward removal of lithosphere. The shear signal is strongest in the southern Sierra, where lithospheric foundering was proposed to have concluded several million years ago, and is deeper and less pronounced in the central Sierra, where ongoing lithospheric foundering is corroborated by a band of unusually deep (40+ km) seismicity along the western foothills. Our observations provide progressive snapshots of a lithospheric foundering process spanning several million years and hundreds of kilometers, illuminating a fundamental differentiation process by which continents are built. Plain Language Summary The Earth's continents sit above sea level and thus make up the land surfaces we live on thanks to their large share of low‐density felsic minerals. The basaltic melts that continents are made from have a higher concentration of denser mafic minerals in comparison. To make continental crust, mafic portions have to be removed, a process called differentiation. A proposed mechanism for this is that dense portions of the lithosphere detach from more felsic portions above and are absorbed back into the Earth's mantle. We image the imprint on rocks at the current crust‐mantle boundary that recorded a dense portion below it peeling off under the Sierra Nevada in California, USA. The removal happened several million years ago in the southern part of the mountains and is still in progress under the central part, causing very deep small earthquakes, while the northern part still retains its dense layer. We have therefore captured snapshots of a fundamental continent‐building process. Key Points Anisotropy from receiver functions shows a pronounced sub‐Moho mantle shear layer under the central and southern Sierra Nevada, California Westward foundering of lithosphere has concluded in the south and is ongoing with concurrent deep seismicity in the central portion We capture continental crust differentiation spanning several million years in time and hundreds of kilometers in extent