Size distribution of clasts in experimentally produced pseudotachylytes

High-velocity frictional experiments on monzodiorite, at sliding rates of 1.3 m/s and at normal stresses of 2 MPa, produced dark 100 μm thick pseudotachylyte layers on the sliding surfaces. The experimentally produced pseudotachylyte contains clasts of various sizes in an ultrafine matrix, similar to the texture observed for natural pseudotachylytes. The size distribution of clasts in the experimental pseudotachylyte obeys a power law of the form: N=N′(1+r/r′)−D, where N is the cumulative number of clasts with a size greater than r, r′ and D are material constants, and N′ is a constant that depends on the total number of measurements. The total area of ultrafine-grained clasts in a thin section, estimated from the size distribution is less than 0.5%, whereas the actual area of ultrafine matrix is measured as 60–70% of the total product. This suggests that the matrix cannot be regarded as a product of only comminution processes, consistent with natural pseudotachylytes, and that the ratio of actual area of the ultrafine-grained matrix to the hole product represents almost the ratio of the melt (glass). This estimate is consistent with the ratio of glass determined from an X-ray diffraction analysis.