Abstract
The nature of the lower crust and the crust-mantle transition is fundamental to Earth sciences. Transformation of lower crustal rocks into eclogite facies is usually expected to result in lower crustal delamination. Here we provide compelling evidence for long-lasting presence of lower crustal eclogite below the seismic Moho. Our new wide-angle seismic data from the Paleoproterozoic Fennoscandian Shield identify a 6–8 km thick body with extremely high velocity (Vp ~ 8.5–8.6 km/s) and high density (>3.4 g/cm3) immediately beneath equally thinned high-velocity (Vp ~ 7.3–7.4 km/s) lowermost crust, which extends over >350 km distance. We relate this observed structure to partial (50–70%) transformation of part of the mafic lowermost crustal layer into eclogite facies during Paleoproterozoic orogeny without later delamination. Our findings challenge conventional models for the role of lower crustal eclogitization and delamination in lithosphere evolution and for the long-term stability of cratonic crust.
Highlights
The nature of the lower crust and the crust-mantle transition is fundamental to Earth sciences
Eclogitization of the lower crust may lead to the formation of a seismic Moho, which does not coincide with the petrological crust–mantle transition[10,25,26,27,28]: the seismic Moho is marked by a sharp increase in velocity at the top of an eclogitic lower crustal body, and the petrological crust–mantle boundary is marked by a change in composition from mafic to ultramafic rocks at the bottom of an eclogitic body, possibly without notable change in seismic velocity
Possible traces of a fossil subduction zone are imaged offshore in the BABEL seismic profile B as strong NE-dipping mantle reflectors at depths 50 to 80 km associated with a strong Pn-velocity contrast[50] (Fig. 1a)
Summary
The nature of the lower crust and the crust-mantle transition is fundamental to Earth sciences. Our new wide-angle seismic data from the Paleoproterozoic Fennoscandian Shield identify a 6–8 km thick body with extremely high velocity (Vp ~ 8.5–8.6 km/s) and high density (>3.4 g/cm3) immediately beneath thinned high-velocity (Vp ~ 7.3–7.4 km/s) lowermost crust, which extends over >350 km distance We relate this observed structure to partial (50–70%) transformation of part of the mafic lowermost crustal layer into eclogite facies during Paleoproterozoic orogeny without later delamination. Delamination requires weak upper mantle rheology (e.g., temperatures >1500 °C3) and lower crustal temperature >900–1000 °C31,35,36, and the presence of decoupling zone may be important to initiate delamination[33,37] If these conditions are not satisfied, thick lower crustal roots may be preserved, dependent on exact composition
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