The boundary between the Earth's mantle and crust has long fascinated geologists. This region, both accessible and elusive to scientific instruments, is characterized by dynamic interactions among the planet's outer layers, making it geologically active.
A recent study conducted beneath the Sierra Nevada in California has unveiled new insights into this interaction, revealing evidence of how the mantle 'peels' the Earth's crust.
The process, known as delamination, occurs when fragments of the lithosphere detach and sink into the upper mantle layers. This phenomenon may account for significant differences in the thickness of oceanic versus continental crust, among other geological and geographical features.
Typically viewed as a 'drip,' this process involves heavier crustal rock losing integrity and detaching from the lithosphere to descend into the less dense mantle. However, researchers suggest that delamination may be more abrupt, akin to the mantle 'peeling' the crust.
As part of the study, the team analyzed seismic wave patterns through the Earth's inner layers to assess factors like composition and density. The seismically active Sierra Nevada region facilitated data compilation.
The researchers combined various seismic data sources, starting with a receiver function analysis. This was supplemented by data from the Comprehensive Catalog of the Advanced National Seismic System (ComCat), which identified a 'band of seismicity' located about 40 kilometers below the surface, featuring small earthquakes with magnitudes ranging from 1.9 to 3.2.
Through the differences detected in the receiver functions, the team identified a differentiated layer in the mantle, which becomes less differentiated towards the north. This finding aligns with the hypothesis that part of the lithosphere in the southern region detached from the crust millions of years ago.
According to the study's authors, the small earthquakes may indicate that this detachment occurred through rupture rather than dripping. The details of this analysis were published in the journal Geophysical Research Letters.
While the evidence is not yet conclusive, the findings support the hypothesis that the Mohorovičić discontinuity (the boundary between the Earth's crust and upper mantle) is not abrupt beneath the Sierra Nevada mountain range, but rather gradual.