Thermal pressurization and fluidization of pulverized cataclastic rocks formed in seismogenic fault zones.

Vein networks of pseudotachylytes and ultracataclastic rocks are composed mainly of ultrafine-to fine-grained materials, including fault gouge and microbreccia, and are widely considered indicators of past seismic faulting events. I show that such vein networks of pseudotachylyte with both melt and crush origins, as well as ultracataclastic rocks, form by the rapid injection of ultrafine-to fine-grained material sourced from pulverized ultracataclastic rocks in seismogenic fault zones under thermal pressurization and fluidization during seismic events. The thermal expansion of seismic slip zones caused by frictional heating results in the rapid fluidization of ultrafine-to fine-grained materials along with expanded fluids and gases (e.g., water vapor and melt) that are injected under pressure into fracture void spaces within fault zones in a gas–solid–liquid system during large earthquakes. I propose that the thermal expansion of water vapor and ultrafine-to fine-grained materials caused by frictional heating in the fault slip zone is the main mechanism responsible for the dramatic increase in pore pressure that results in the dynamic coseismic weakening of faults. • Vein networks of pseudotachylyte (Pt) are indicators of seismic faulting. • Pt veins are sourced from by pulverized ultracataclastic rocks in seismic fault zones. • Thermal pressurization and fluidization occurred in fault zone during seismic faulting. • Thermal expansion of seismic slip zones caused by frictional heating in fault zones. • Thermal expansion is a main mechanism of dynamic coseismic weakening of faults. [ABSTRACT FROM AUTHOR]