Pseudotachylites from Vogogna

This outcrop displays a pseudotachylite breccia, consisting of fragments of mafic granulites cemented by thin, dark veins resembling a basaltic glass known as tachylite, from which the name pseudotachylite is derived. The original glass is often devitrified and replaced by a very fine-grained matrix.

Pseudotachylites form along seismogenic faults during high-energy earthquakes. When two blocks of rock slide rapidly past one another at depth, the intense heat generated by friction causes the rocks to melt instantly along the fault surface. The presence of the molten material drastically reduces friction, facilitating the abrupt movement responsible for the seismic shock. The molten material is then injected into fractures in the surrounding rock, where it solidifies almost instantly, forming glassy veins. The transition between the brecciated rock and the pseudotachylite veins is generally very distinct.

As they provide direct evidence of friction-induced melting during a seismic event, pseudotachylites are regarded as genuine ‘fossil earthquakes’.

This pseudotachylitic breccia outcrops along the Canavese Line, the major fault that separates the stronalites to the west from a mafic-ultramafic complex to the east. It is thought to have formed during an earthquake associated with movements along this tectonic structure, some 20 million years ago.