The Transition From Frictional Sliding to Shear Melting in Laboratory Stick‐Slip Experiments

Pseudotachylites are thought to be caused by fault surface melting due to frictional heating during earthquakes. We report on pseudotachylite formation in the laboratory during spontaneous stick-slip on dry, bare-surface granite faults in room temperature triaxial experiments. A continuous melt layer averaging 7 microns in thickness was formed on sawcut surfaces during stick-slip events at 400 MPa confining pressure. At this pressure, dynamic weakening during stick-slip caused total stress drops that ranged from 172 to 414 MPa shear stress (peak normal stress was 249 to 639 MPa) with 1.2 to 4.2 mm slip. In contrast, repeated stick-slip cycles at 50 MPa confining pressure produced fine-grained fault gouge but showed no evidence of melting. Event duration ranged from 0.07 ms for low stress events to 0.32 ms at high stress, and average slip velocity ranged from 0.3 to 20 m/s. Based on thermocouple measurements within 3 mm of the fault, maximum temperatures in some 400 MPa events exceeded 1500°C. By operating at normal stresses 10 to 50 times greater than those used in unconfined rotary machines, triaxial stick-slip experiments are able to develop high transient temperatures and create pseudotachylites, even with limited total slip.