## Clast-size Distribution in Natural and Experimental Pseudotachylytes

Presenter: Chien-Cheng Hung

Date: 2017/02/16

**Abstract**

The high-velocity frictional experiments has been successfully reproduced artificial pseudotachylyte which contains clasts of various sizes in an ultrafine matrix, similar to texture observed for natural pseudotachylytes. In these two papers, they presented a new line of argument based on the size analysis of clasts contained in the natural and experimental pseudotachylytes to examine whether the origin of pseudotachylyte is crush or melt origin. The size distribution of clasts in both pseudotachylytes obeys a power law of the form: N = N' r –D or 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 constant, N' is a constant that depends on the total number of measurements. The total area of ultrafine-grained clasts in a thin section of both pseudotachylytes, estimated from the size distribution is less than 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, and that the ratio of actual area of the ultrafine-grained matrix to the hold product represent almost the ratio of the melt (glass). In my research, micro-analytical results of slip-stepping experiment show that chemical compositions of the melt matrix were varied among experimental products. Flash heating and comminution are proposed to result in melting of asperities and reduce particle size of mineral grains during initial sliding. Therefore, in our future work, we will utilize the method of analysis of clast-size distribution from these papers to observe whether the occurrence of clast-size distribution among slip-stepping experimental products are different.

**Reference**

Shimamoto, T., Nagahama, H., (1992). An argument against a crush origin for pseudotachylytes based on the analysis of clast-size distribution. Journal of Structural Geology 14, 999–1006.

Tsutsumi, A., (1999). Size distribution of clasts in experimentally produced pseudotachylytes. Journal of Structural Geology 21, 305–312.