Vesicles, amygdales and similar structures in fault-generated pseudotachylytes

Abstract

Amygdales in fault-generated pseudotachylytes from the Outer Hebrides Thrust, Scotland, and the Ikertôq Shear Belt, West Greenland, contain mineral assemblages dominated by K-spar + sphene + epidote + quartz and carbonates respectively. These contrasting assemblages indicate that vesicle infilling took place under differing environmental conditions. Other ‘filled cavities’, resembling amygdales in form and mineralogy, are recognized as replacement pseudomorphs after porphyroclasts. Using published data relating vesicle abundance and solubility functions for H2O and CO2 to pressure, a palaeoseismic depth estimate (equivalent to the depth of vesiculation) of about 1.6 km is inferred for the Greenland pseudotachylytes described in this paper.