Zhao Liang et al.-NC: Seismic detection discovered a huge amount of serpentinite layers at the plate subduction interface. Plate subduction is the engine that drives plate tectonics and is a deep control factor in the evolution of the earth and even the habitability of the blue earth.
The dynamic process and mechanism of the initial initiation and continuous subduction of plate subduction are controlled by the rheological characteristics of the subducting interface.
The viscosity of serpentine is several orders of magnitude lower than that of ordinary mantle rocks. Therefore, serpentinization of mantle rocks at the subduction interface can significantly change the rheological properties of the lithosphere and promote the "positive feedback" of continuous subduction of the slab.
In addition, serpentine is also the main carrying mineral for fluids such as water and CO 2 , and is therefore an important medium for mass and energy transport between deep and shallow parts of the earth.
Petrological evidence shows that serpentinite belts are widely exposed at the ancient/new subduction boundary. For example, the Alpine Viso unit exposes ~10km thick serpentinite belts.
However, since the subduction interface is only a few kilometers thick, it is difficult to detect the existence of deep subduction boundary serpentinite layers using traditional seismological methods.
With funding from the National Natural Science Foundation of China’s “Continental Evolution and Monsoon System Evolution” basic science center project and the “Tethys Geodynamic System” major research program, researcher Zhao Liang from the State Key Laboratory of Lithosphere Evolution organized the China-Italy-France Alps
The Seismic Observation Program (CIFALPS) team carried out high-resolution seismic imaging studies in the Western Alps, a classic continental deep subduction area (Figure 1).
The application of a new generation of cross-dimensional inversion algorithm significantly improves the observation precision and accuracy of the subduction interface.
The new three-dimensional shear wave velocity image found that there is a 15-25km thick low-velocity layer with shear wave speed ≤3.7km/s in the 50-70km depth range of the European continent-Adrian plate subduction interface (Figure 2).
Comprehensive analysis of rock physics experimental data, it is inferred that the best reasonable explanation for stable low-velocity properties of ≤3.7km/s under P-T conditions of 50-70km depth is serpentinite, which may be caused by thin layers of subducted ocean floor serpentinite and fluids.
Formed by the accumulation of petrified mantle.
It is this accumulation and the improvement of imaging accuracy that enable seismological imaging to detect its existence: the volume of the low-velocity body beneath the Western Alps reaches ~300,000km 3 (Fig. 1c, Fig. 1d).
On the one hand, serpentinite layers promote the deep subduction and exhumation of continental plates; on the other hand, they provide a huge container for deep water and carbon cycles in the subduction zone.
The fine images of the subducting slab interface obtained in this study help to construct a composition model of the subducting interface, and provide quantitative evidence for understanding the deep continental subduction and deep water and carbon cycle processes and dynamic mechanisms.
Figure 1 Geological structure map of the Western Alps (a and b) and shear wave velocity distribution maps at 40km (c) and 60km (d) depth.
Light green and bright yellow highlight the distribution of massive low-velocity bodies Figure 2 Shear wave velocity distribution along the China-Italy-France Alps seismic observation line.
(a) Velocity cross section; (b) Velocity profile below a typical control point.
The green five-pointed star shows the location of the low-velocity layer. The research results were published in the top international academic journal Nature Communications.
(Zhao L, MalusàM G, Yuan H, Paul A, Guillot S, Lu Y, Stehly L, Solarino S, Eva E, Lu G, Bodin T, CIFALPS groupGroup, AlpArray Working Group. Evidence for a serpentinizedplate interface favoring continental subduction[J
]. Nature Communications, 2020, 11: 2171. DOI: 10.1038/s41467-020-15904-7) (See the end of the article for the original link).
This is the 15th international journal paper result obtained by the China-Italy-France Alpine Seismic Observation Program led by the State Key Laboratory of Lithosphere Evolution.