Post-Shock Oscillatory Slip: Displacement and Vibration Melting without Offset

English Abstract:

A series of experiments have been performed on Westerly granite using linear friction welding apparatus. For this application, two 24.8 x 47 x 63.5 mm machined blocks of the granite were brought into contact along their 24.8 x 63.5 mm faces. Westerly granite is a medium grained quartz-orthoclase-plagioclase (An<20)-biotite igneous assemblage, with accessory/secondary chlorite, clinozoisite, epidote, titanite, zircon, apatite and opaques. The friction apparatus was operated at a frequency of 20 Hz, an amplitude of 1 mm (wavelength of 4 mm), for 8.33 s, under a load of 5 kN. Total displacement was 666.40 mm and the mean velocity 80 mm/s. Following the experiment, the samples were mounted in epoxy and diamond-saw cut, and polished thins made of the slip face sections. The thin sections were examined using binocular and petrographic optical microscopes, and an Hitachi SU-70 field emission scanning electron microscope equipped with an energy-dispersive X-ray spectrometer. Oscillatory slip caused comminution and localized melting. Also, the transfer and "welding" of material occurred between the sliding faces. Two zones of damage are noted: a central partial melting zone (<300 microns wide) and a subsurface comminution/fractured zone developed either side of the slip surface (up to 50m microns wide). The partial melting zone comprises more rounded fragments, which are dominated by quartz, bonded by a submicroscopic matrix (seemingly amorphous). Biotite underwent breakdown via “bubbling” and dehydration, and has been preferentially incorporated into the melt. Orthoclase then plagioclase follow, leaving predominantly quartz with some plagioclase as clasts. Oscillatory slip may explain the production of friction melts in faults displaying negligible offset: the effective displacement can be substantial, yet the apparent slip is minimal. A scenario for facilitating this type of behaviour in nature can be sonic and subsonic vibrations associated with seismic events, including the post-shock phase of hypervelocity impact.