Melt–clast interaction and power-law size distribution of clasts in pseudotachylytes

Before the onset of melting along frictional interfaces, the wall rocks of seismic faults are crushed to generate a power-law grain-size distribution pattern. Theoretical analysis with the help of numerical models shows that, under conditions of uniform rim melting of the grains, the pre-melting power-law pattern survives in a modified form in the relicts, which occur as clasts in the quenched product, i.e. pseudotachylyte. The size distribution of clasts in experimentally produced as well as natural pseudotachylytes, including those from the Sarwar–Junia fault zone in western India, shows a pattern similar to that predicted by theoretical analysis. A micron-scale mantle of glass±microlites around a large number of clasts, and minute beads of glass set within glassy matrix, are the observed evidence of rim melting. Post-melting clast size distribution in the numerical models follows the relationship N∝(1+z/z′)−D, where N is number of clasts of size ≧z. Size z may be represented by mean diameter r, or sectional area a, or volume v, and D(=Dr, Da or Dv) is the modified power-law exponent. z′ (=r′, a′ or v′) is a constant that depends on thickness (ω) of uniform rim melting, and r′=ω, a′=ω2, and v′=ω3. The analysis suggests that the modified power-law clast-size distribution pattern is a characteristic feature of all fault-related pseudotachylytes.