Roundness of clasts in pseudotachylytes and cataclastic rocks as an indicator of frictional melting

Abstract

We examine the roundness (Rd) of clasts derived from fault rocks and discuss its possible application as an indicator for fracturing and chipping or frictional melting. The roundness of quartz and feldspathic clasts derived from four frictional melting-originated pseudotachylytes and one crushing without melting-originated pseudotachylyte as well as three cataclastic rocks was measured. Between 230 and 270 clasts >10 μm in size were measured for each sample. The analyses show that Rd is lower than 0.4 in all the clasts included in the cataclastic rocks and crushing-originated pseudotachylyte, whereas it varies from 0.1 to 1.0 and 35–90% of clasts have a roundness >0.4 in the melting-originated pseudotachylyte. It is suggested that the clasts having a roundness >0.4 were rounded by frictional melting rather than by fracturing or chipping in pseudotachylytes, and that the roundness of clasts may be used as a special index of disintegration attributed to attrition and melting.