Phase transitions and cyclic pseudotachylyte formation in simulated faults

Abstract

Field evidence from faults containing pseudotachylytes has revealed cyclical episodes of frictional melting, ductile deformation, and overprinting at a later stage by a new generation of pseudotachylytes. Here we connect these cycles to earthquake dynamics using a development of a discrete element model with solid grains that can melt during frictional heating and viscous melts that can bond through solidification during cooling. A new earthquake episode initiates with the crushing of bonded clusters once the bond strength is exceeded, with frictional shear heating being activated again. We explore the competition between melting and solidification in terms of phase transitions using scaling laws dependent on the characteristic times for melting, thermal diffusion and loading rates. A phase diagram is constructed that is capable of explaining the tendencies towards pseudotachylytes associated to cataclasites or mylonites, depending on the fault conditions (its depth and thickness, crust motion and ambient temperature) and the mechanical and thermal parameters defining the grains within the fault and the host rock.

Keywords:

• fault dynamics
• pseudotachylytes
• discrete element method
• phase transitions
• melting
• shear localization

Acknowledgments

Financial support for this research from the Australian Research Council, through grant DP1096958, is gratefully appreciated. The authors would like to thank Dr. E. Veveakis for fruitful discussions and the reviewers for their generous and helpful comments.