Vesicles, amygdales and similar structures in fault-generated pseudotachylites - comment

Abstract

Fault-generated pseudotachylites form by friction-nduced melting during earthquakes. Their study may provide important information on earthquake processes (e.g., McKenzie and Brune, 1972; Sibson. 1975; Grocott, 1981). Maddock et al. (1987) proposed that the relative abundance of amygdales and vesicles in pseudotachylites could be used to constrain their formation depth, and estimated that pseudotachylites in the Ikertoq Shear Belt in Greenland formed at a depth of 1.6 km. Similarly, the absence of vesicles in the Harry Creek pseudotachylite in central Australia has been used to infer a formation depth in excess of 2 km (Allen, 1980). These depth estimates implicitly assume that bubble growth occurs under conditions of vapourmelt equilibrium, so that the volume of exsolved vapour (determined from vesicle abundance) represents the concentration of volatile elements in the original rock in excess of their solubilities in the melt at the pressure of melting. However, we will show that attainment of such equilibrium is extremely unlikely during the rapid formation of pseudotachylites. If the rate of quenching melt to glass is more rapid than that for bubble growth, the presence of large amygdales and vesicles in pseudotachylites will not be controlled by exsolution of CO_2 and H_2O from an oversaturated melt, and the occurrence or lack of amygdales or vesicles cannot be used to infer formation depth.

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