Linear patterns known as slickenlines streak the surfaces of many faults around the world. These smooth, polished features consist of tiny ridges and grooves that run parallel to the direction of fault slip. In a new paper, Toy et al. present data from laboratory experiments that shed new light on slickenline formation.
Unlike striations, which are scratched out of brittle material, and slickenfibers, which form by precipitation of minerals into parallel linear structures on the fault surface, slickenlines most likely form by deformation of rock at the fault surface. However, the precise mechanisms involved in slickenline formation and their effects on the fault’s slip behavior are unclear.
To investigate slickenline formation, the team recreated hydrothermal fault conditions in the lab. They embedded fragments of highly polished silica (found in nature as quartz) in quartz powder meant to simulate the finely ground rock produced by fault movement. Using a motor-driven shearing apparatus, they saturated these samples with water, squeezed them to high pressure, and sheared them at temperatures of 100°C or 450°C.
During the experiment, the polished silica fragments broke into much smaller chips. The researchers retrieved chips from each sample and examined them using high-resolution microscopy and white light interferometry, which revealed their surface texture.
All of the chips showed signs of striation marks caused by brittle mechanisms. However, only those that had been subjected to higher temperatures showed characteristics of slickenlines. Microscopic observations suggested that two specific deformation mechanisms, known as brittle wear and pressure solution transfer, enabled slickenline formation.
The researchers found nestled in the slickenline grooves round silica beads about 200 nanometers across in an amorphous matrix film. These probably precipitated from the water. The scientists propose that formation of beads and films in a fault setting could promote slip weakening, which can facilitate fault movement. They may also promote smoothing of the fault surface, development of the ridge-and-groove pattern, and maintenance of the direction of slip.
These findings could help improve understanding of fault behavior worldwide, especially for faults that are no longer active and whose past movements must be gleaned from indirect evidence. (Journal of Geophysical Research: Solid Earth, https://doi.org/10.1002/2016JB013498, 2017)
—Sarah Stanley, Freelance Writer