Natural polish in granitic rocks

Shalev Siman-Tov, Emily E. Brodsky, & Greg M. Stock

Published August 11, 2016, SCEC Contribution #6594, 2016 SCEC Annual Meeting Poster #054

Fault mirrors are highly smooth and reflective rock surfaces that are found in many shear zones around the world. Recent studies suggest that fault mirrors are formed during high velocity slip on faults and therefore may serve as an indicator for seismic slip. In contrast, other studies suggest that fault mirrors may form under high normal stress at sub-seismic velocities and at room temperature. Fault mirrors are observed within the fault core of many rock type environments including limestone, dolomite, chert and rhyolite. However, to the best of our knowledge, they are missing in faults hosted in granite. Moreover, mirror-like surfaces form during high velocity rotary shear experiments in many types of rock but not in sheared granite blocks.

The absence of fault mirrors in granite is surprising, particularly since there exists extensive glacial polish on granitic bedrock. Glacial polish describes the smooth and reflective rock surfaces formed at the base of glaciers that carved the underlying bedrock. In addition to their import for studies of glacial dynamics and geomorphology, glacially polished surfaces may hold some significance for fault mechanics. Glacial polish and fault mirrors share many similarities. At field exposures they both present highly smooth surfaces and striations that clearly point in the slip direction. Studies on carbonate fault mirrors showed that individual highly reflective surfaces are composed of a thin nanograin layer. Preliminary SEM observations on samples collected from granitic rocks at Yosemite National Park suggest that these polished surfaces are also coated by an ultrathin cohesive layer composed of nanograins.

Although there are clear differences between glacial and fault-zone environments, the similarity between these textures, and the fact that both are formed during shear, suggest that a similar mechanism is responsible for their formation. The comparison raises questions about the importance of high fluid contents and extremely mobilized gouge in forming polish.

Key Words
glacial, polish, granite, fault, mirror

Siman-Tov, S., Brodsky, E. E., & Stock, G. M. (2016, 08). Natural polish in granitic rocks. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)