FAULT ROCKS: A FOURTH CLASS OF ROCKS

Geology students learn three classes of rocks: igneous, sedimentary, and metamorphic, and that they all fit neatly into the rock cycle. A class of rocks that does not fit any of the three—or the rock cycle—is fault rocks derived from the other three classes. Some consider mylonites metamorphic rocks, but what about fault rocks formed by cataclasis, scaly fabrics that form in unconsolidated sediments in subduction zones, and hypervelocity impacts? Fault rocks should be recognized as a separate class on an equal footing with the other classes of rocks, because they have unique textures and are produced at sites ranging from the surface into the mantle under conditions dominated by simple (rarely pure) shear—fault and shear zones, and impact structures.

Mylonites form over a range of temperatures, pressures, and strain rates at both high temperature and retrograde conditions, involving crystal-plastic deformation, mostly as products of heterogeneous simple shear, and at high temperatures under prograde conditions. Cataclasites also form by heterogeneous simple shear near the surface at lower temperatures and pressures along faults and their associated damage zones. They produce fragments that are progressively reduced in size with no change in mineralogy, texture, or internal composition from the host materials. They also form under increased strain rates along ductile faults and in hypervelocity impacts. Pseudotachylite (glass) forms under high strain rates by frictional heating in a fault zone or impact structure. Fluids influence the movement rate on faults and their lithologic products. Hypervelocity (bolide) impacts produce abundant cataclasite, pseudotachylite, unique microstructures (shocked quartz), and ultra-high-pressure minerals (e.g., stishovite).

Minerals (quartz, carbonates, chlorite, feldspars, etc.) crystallize over a range of temperatures in slickensides on movement surfaces. Clear quartz may precipitate from fluids on moving fault surfaces following pressure dissolution at slightly elevated temperatures. These also are fault rocks/minerals.

Fault rocks have not been given the recognition they deserve as a separate class of rocks; it is time we revise our thinking and give them this recognition.