How Earthquakes transform the Earth’s lower crust

In this study, researchers propose a new mechanism of "top-down" crustal geodynamics, from the upper crust to the lower crust.

Researchers from the IGE, Institute of Earth Sciences of Grenoble / OSUG (Université Grenoble Alpes / CNRS / IRD / IFSTTAR / Université Savoie Mont Blanc), the University of Oslo and the University of Southern California propose a new mechanism for the geodynamics of the Earth’s crust. It links for the first time the changes of the lower crust with the deformations of the upper crust of Earth. The evolution of the composition and structure of the rocks forming the lower crust largely controls the behavior of the lithosphere during tectonic processes such as mountain formation, subsidence of sedimentary basins, opening of rifts or extension of high topography regions. Understanding these mechanisms is essential; however, their analysis, published on April 26, 2018 in Nature, suggests that it is the earthquakes occurring in the upper crust that significantly damage the lower crust, making possible the circulation of fluids and the metamorphism of the rocks.

Recent studies have shown that before the formation of a mountain (orogeny), the lower crust is composed of granulite-type rocks that are rich in iron and magnesium, dry, impervious and mechanically resistant. During an orogenic event, it is the infiltration of fluids along the shear zones or fractures that progressively hydrates these rocks, turning them into eclogites, which are denser and mechanically weaker rocks. The observation of this metamorphism testifies to an early damage of seismic origin, hitherto inexplicable under the high-pressure conditions of the lower crust where the rocks should not theoretically produce earthquakes.

For François Renard, professor at the UGA and researcher at the Grenoble Institute of Earth Sciences and his international colleagues, it is the activity regular seismic in the upper seismogenic crust that would maintain a natural mechanism of "pulses of elastic energy" inducing aftershocks in the lower crust, causing its damage. They confirmed their hypothesis by analyzing granulite outcrops of the Earth’s continental crust, exhumed in the Bergen Arch in western Norway.

The researchers identified traces of fossil earthquakes, called pseudotachylytes, which occurred between 30 and 60 km underground during a tectonic collision 400 million years ago. Locally, by increasing the permeability of the rock, seismic fracturing allowed the circulation of fluids and resulted in a transformation of granulites into eclogites. The presence of pseudotachylites in these shear zones initiated by faults generated by earthquakes provided them with evidence that lower cost deformations and shear zones were controlled by earthquakes from the upper crust.

The researchers were able to model that the hypocenters of the aftershocks could reach regions under the upper seismogenic crust. They notice that the volume of the lower crust affected by these replicates was very significant, greater than 1% of the total volume of the lower crust per million of years of orogenic activity. The overall process could therefore affect the mechanical behavior of the entire lower crust.

Challenging the traditional "bottom-up" mechanism, where deep shear in the mantle and lower crust controls the spatial distribution of crustal faults, the researchers propose in this study a new mechanism of “top-down” crustal geodynamics, from the upper crust to the lower crust.

Granulites from the lower continental crust exhumed in the Bergen Arcs, Norway.
These rocks recorded fossil earthquakes that occurred between 30 and 60 km underground during the Caledonian collision 400 million years ago. Locally, seismic fracturing has brought these rocks into contact with fluids and led to their transformation into more dense eclogites. This granulite-eclogite transformation, caused by earthquakes, controls the mechanical strength of the Earth’s lower crust.

Source

Earthquake-induced transformation of the lower crust. B. Jamtveit, Y. Ben-Zion, F. Renard, H. Austrheim. Nature, 26 Avril 2018, doi:10.1038/s41586-018-0045-y

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Updated on 20 August 2018