1. Energy Balance From a Mantle Pseudotachylyte, Balmuccia, Italy
- Author
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Ferrand, Thomas P., Labrousse, Loïc, Eloy, Grégoire, Fabbri, Olivier, Hilairet, Nadège, and Schubnel, Alexandre
- Abstract
In the Balmuccia massif (NW Italy), a pseudotachylyte vein network (N068 trending) in a spinel lherzolite is interpreted as the product of frictional melting during a single Mw> 6 earthquake. The subvertical fault underwent a metric dextral coseismic displacement, raking 60°SW. The average width of the main slip surface is ~5 mm. A dense network of thin (20–200 μm) injection and ultramylonite‐like veins decorates the fault walls. In the injection veins, Raman microspectrometry mapping reveals pockets of still preserved amorphous silicate, containing ≈ 1% of structurally bound H2O. In the ultramylonite‐like veins, electron backscattered diffraction mapping reveals that ultrafine (0.2–2 μm) olivine grains exhibit a strong fabric with (010) planes parallel to shearing, consistent with temperatures above 1250°C during deformation and suggesting fast recrystallization from the frictional melt. The veins also exhibit pyroxene and recrystallized spinel, which proves that the earthquake occurred at a minimum depth of 40 km. The energy balance demonstrates that complete fault lubrication must have occurred during coseismic sliding (i.e., dynamic friction coefficient ≪ 0.1). Because of the low viscosity of slightly hydrated ultramafic liquids (≈ 1 Pa·s), we argue that lubrication was only transient, as the melt could rapidly flow into tensile fractures, which led to rapid cooling and promoted strength recovery and sliding arrest. Combined together, our observations suggest that this pseudotachylyte is the frozen record of a deep (>40 km) earthquake of 6 < Mw< 7. Its focal mechanism is deduced from the crystal preferred orientation due to late coseismic creep in ultramylonite‐like veins and deciphered by electron backscattered diffraction. In active mountain ranges, rocky massifs gradually rise to the surface under the combined effect of convergence and erosion. It is then possible to observe in the field the fossilized traces of earthquakes that have occurred several kilometers deep, tens of millions of years ago. Thus, it can be seen that contrary to what we experience on the surface, the rock melts when it tears. A magma forms and facilitates relative ground displacement. The traces discovered by the geologists are the solidified remains of this magma. Here by field observation and detailed analysis of samples in the laboratory, we describe the traces of an earthquake that occurred more than 40 km deep in the Earth's mantle. Its magnitude was greater than 6, the sliding on the fault greater than 1 m, and yet the trace is thinner than 5 mm. And, against all odds, the most talkative witnesses are extremely small: micrometric crystals, which formed from the magma upon the rupture, record crucial information unveiling its focal mechanism. We still do not entirely know what an earthquake exactly is, and nobody will ever be able to see what happens on the fault at depth during an earthquake. Hence, these fossils are key. The studied pseudotachylyte is the fossilized trace of an earthquake which occurred in the mantle, > 40 km deep; its magnitude was >6Coseismic total melting leads to complete fault lubrication; the ultramafic melt flows in “injection” veins, inducing fault restrengtheningMicrometric crystals, forming during fault restrengthening, record the sliding history, P‐Tconditions, and focal mechanism of the earthquake
- Published
- 2018
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