Selective clast survival in an experimentally-produced pseudotachylyte. (June 2021)

• Record Type: Journal Article
• Authors: Papa, Simone; Spagnuolo, Elena; Di Toro, Giulio; Cavallo, Andrea; Favero, Marco; Camacho, Alfredo; Pennacchioni, Giorgio
• Is Part Of: Journal of structural geology. Volume 147(2021)
• Publication Date: 2021-06
• Subjects: Pseudotachylyte -- Laboratory earthquake -- Rotary shear apparatus -- Frictional melting -- Clast size distribution -- Thermal shock; Geology, Structural -- Periodicals; Géomorphologie structurale -- Périodiques; Geology, Structural; Periodicals; 551.805
• DOI: 10.1016/j.jsg.2021.104328
• Languages: English
• ISSNs: 0191-8141

Abstract

Abstract: On-fault processes during earthquakes contribute to seismic rupture propagation and slip. Here we investigate clast fragmentation in an experimental pseudotachylyte (solidified seismic melt) produced with a rotary shear machine. We slid for 0.44 m (corresponding to Mw ≥ 6 earthquakes), at slip rates > 1 m/s, pre-cut samples of quartz + phyllosilicates + plagioclase + sillimanite + garnet -bearing ultramylonite, that hosts pseudotachylytes in nature. The ultramylonite minerals extensively preserved as clasts in the experimental pseudotachylyte are quartz, plagioclase, and sillimanite. Garnet is scarcely preserved, despite having a melting temperature similar to plagioclase, probably due to having low thermal shock resistance. This selective clast survival is identical to the one found in the natural pseudotachylytes. Based on these experimental observations and assuming non-equilibrium melting, the preservation of a mineral, as a clast, in the melt appears to be controlled by its thermal shock properties as well as by its melting temperature. Since the mechanical effects of rupture propagation in these experiments were negligible, we conclude that, for Mw ≥ 6 earthquakes, (i) frictional slip and heating of the slipping zone plus (ii) thermomechanical properties of minerals, rather than fault rupture processes, control mineral comminution and clast survival in frictional melts. Highlights: Clast survival to melting of the natural pseudotachylytes reproduced experimentally. Clast survival depends on mineral melting temperature and thermal shock resistance. Preferential melting of garnet clasts is enhanced by thermal shock comminution. For large earthquakes, frictional sliding and heating control clast survival.