Scientists have discovered a rare geological process in progress: the Earth’s crust beneath the Sierra Nevada is peeling off. The study was published in Geophysical Research Letters.
The research, led by Deborah Kilb from the Scripps Institution and Vera Schulte-Pelkum from the University of Colorado, analyzed anomalous earthquakes and deep deformations suggesting active lithospheric subsidence in this iconic Californian mountain range.
“In northern California, earthquakes are usually recorded up to about 10 kilometers deep, while in the south they reach 18 kilometers,” explained Kilb, a researcher at the Scripps Institution of Oceanography at the University of California, San Diego. However, the seismic events observed in the central area of the range were up to twice as deep.
“The fact that seismic activity is detected between 20 and 40 kilometers is very unusual,” noted Kilb. “It’s not common in the Earth’s crust.”
To analyze the phenomenon, Kilb collaborated with Vera Schulte-Pelkum, a researcher at the Cooperative Institute for Research in Environmental Sciences and adjunct professor at the University of Colorado Boulder. Using a method called receiver function analysis, the scientists generated images of the Sierra Nevada and detected lithospheric subsidence, a process in which the Earth’s crust peels off and sinks into the mantle.
Evidence of lithospheric subsidence
This discovery supports previous hypotheses that suggested the region had experienced lithospheric subsidence. This geological process occurs when the Earth’s outermost layer sinks into the lower mantle. According to the study’s data, this phenomenon is underway and advancing towards the northern part of the range.
This finding provides a unique perspective on how continents have formed and could help identify other regions where this process occurs. It also allows for a better understanding of seismic activity and the geological evolution of the planet.
How these processes influence continent formation
The lithosphere, the Earth’s outermost layer, is composed of the rigid crust and the upper part of the mantle. This layer is in a denser but fluid state and contains both continental and oceanic crust. However, the mechanisms that allow the existence of these sub-layers are still not fully explained, according to Schulte-Pelkum.
The images generated beneath the Sierra Nevada revealed a distinctive layer within the mantle, located between 40 and 70 kilometers deep. This layer shows signs of deformation gradually advancing northward.
In the southern Sierra Nevada, dense rocks appear to have completed the crust separation process, while in the central zone, it is still in progress. In the northern part, no signs of deformation have been detected.
These changes explain the deep seismic activity detected by Kilb, as the Earth’s crust in the region is exceptionally thick and colder than the surrounding mantle. The rock takes time to thermally adjust, allowing these earthquakes to occur in an unexpected area.
A geologically challenging process to observe
Lithospheric subsidence is a slow and complex phenomenon, difficult to detect from the surface. According to the study, the Southern Sierra Nevada would have completed this process between 3 and 4 million years ago.
These events have occasionally occurred in different parts of the world. “From a geological perspective, lithospheric subsidence is a rapid process, followed by long periods of stability,” explained Schulte-Pelkum. “It probably began when continents started to form and has contributed to their growth over time.”
Scientific debate on the origin of the phenomenon
The geological behavior of the Sierra Nevada has been a subject of debate in the scientific community. An anomaly detected in the mantle beneath the Great Valley has led to different interpretations.
Some experts argue that lithospheric subsidence is the cause, while others suggest that the anomaly may be due to a subduction process, where an oceanic plate sinks beneath a less dense continental plate and alters the topography.
Future implications and landscape evolution
If lithospheric subsidence continues in the Sierra Nevada, the terrain could continue to expand vertically, altering the current landscape. However, this change would take hundreds of thousands or millions of years to complete.
Similar events have occurred in large mountain ranges or regions with batholiths, such as the Andes, where some scientists have proposed that lithospheric subsidence took place in the past and could still be ongoing.
Studying these processes is key to understanding the Earth’s evolution. It could also have applications in other geological research, such as seismic activity linked to these phenomena.
Even on other planets, like Venus, evidence of lithospheric subsidence events has been found. Understanding how it occurs on Earth could help interpret similar processes on other celestial bodies.
