Lower crustal earthquake facilitated by overpressurized frictional melts

Earthquakes are among the most catastrophic geological events that last only several to tens of seconds. During earthquakes, many processes may occur including rupturing, frictional sliding, pore fluid pressurization and occasionally frictional melting. However, little direct records of these fast processes remain preserved through geological time. During rapid shearing, frictional melt may form that lubricates the rocks and facilitates further sliding. The frictional melt layer may quench quickly within seconds to minutes depending on its thickness. After quenching, the product pseudotachylyte preserves valuable information about the conditions when the frictional melt was generated. Here, we study pseudotachylyte from Holsnøy Island in the Bergen Arcs of Western Norway, an exhumed portion of the lower continental crust. The investigated pseudotachylyte vein is ca. 1-2 cm thick and free of injection veins along the 2 m visible length of the fault. The pseudotachylyte matrix is made up of fine-grained omphacite (Jd₃₈), sodic plagioclase (Ab₈₃) and kyanite with minor rutile and sulphides. Many dendritic garnets are found within the pseudotachylyte showing gradual grain size reduction towards the wall rock. This suggests that the garnets crystallized during rapid quenching. The stability of epidote, kyanite and quartz in the wall rock plagioclase, and omphacite and albitic plagioclase together with quartz in the pseudotachylyte matrix constrains the ambient P ca. 1.5-1.7 GPa and T ca. 650-750°C. Using Raman elastic barometry, the constrained pressure condition from quartz inclusions in the dendritic garnets in the pseudotachylyte is > 2 GPa. Based on an elastic model (Eshelby’s solution), it is not possible to maintain 0.5 GPa overpressure within a thin melt layer by thermal pressurization or melting expansion. A potential explanation is that GPa level differential stress was present in the wall rocks and the melt pressure approached the normal stress when shear rigidity vanished during frictional melting. Our study illustrates how overpressure can be created within frictional melt veins under conditions of high differential stress, and offers a mechanism that facilitates co-seismic weakening during lower crustal earthquakes.