Slickenside

A slickenside is a smoothly polished surface found on rocks. This feature forms when two rock masses rub against each other along a fault. A fault is a crack in the Earth's crust where movement occurs. When these rocks slide, the friction polishes the surfaces. These surfaces are often marked by linear features called slickenlines. These lines act as kinematic indicators, which means they show the direction of movement.

The geometry of a slickenside can vary significantly. It may appear as a single smooth surface between two hard rocks. In other cases, the material between the fault surfaces, called gouge, may contain many tiny slip surfaces. These surfaces are incredibly thin, often around 100 micrometers thick. The grains making up these surfaces are ultra-fine, ranging from 0.01 to 1 micrometer in diameter. These grains have irregular boundaries and very few crystal lattice defects, which are known as dislocations.

There are several specific mechanisms that create these polished surfaces. One process is called asperity plowing. An asperity is a small bump or point that sticks out from the rock surface. As the rocks move, the asperity digs into the opposing rock. This creates permanent marks like troughs, grooves, or scratches. Another process is debris streaking. This occurs when the asperity wears down the rock into fine debris. This wear product accumulates in long shapes. If the asperity is hard, the debris piles up in front of it. If the asperity is soft, the debris trails behind.

Sometimes, a slickenside becomes so smooth that it looks like a mirror. This is called a fault mirror. This can happen when the grinding between rocks reduces particle sizes to the nanometer scale. It can also happen if fluid is present during the slip. Once the movement stops, this fluid can solidify into a shiny silica gel. Another mechanism is erosional sheltering. This happens when hard particles within a rock resist wear better than the surrounding softer material. The area behind these hard particles is protected from wear, creating an elongated tail parallel to the movement.

In some environments, minerals grow directly on the fault plane. These are called slickenfibres. These are secondary minerals, meaning they are not part of the original rock. They form when rocks undergo aseismic creep, which is a very slow movement rather than a sudden earthquake. Slickenfibres often have a stepped appearance due to irregularities in the fault plane. While slickenlines might only suggest two possible directions, slickenfibres can preserve the true direction of the slip.

Geologists use these features to understand complex earthquake processes. Slickenlines can help determine the timing of a fault slip. They can also show the complex geometry of an earthquake rupture. For example, researchers have used calcite slickenfibres in the Zagros Mountains. These fibers helped them find the depth of aseismic creep and the orientation of stress on the fault. If a fault shows many different orientations of slickenfibres, it might indicate that the direction of slip is not constant.

Slickensides are not only found in deep bedrock. In the field of pedology, which is the study of soils, slickensides appear in soils with high amounts of swelling clays. These are a type of feature called a cutan. In the Australian Soil Classification, they help identify a specific soil type called a vertisol. Interestingly, these features are not unique to Earth. During the Apollo 15 mission, astronauts photographed a boulder with slickensides on the Moon. This was found in a small crater near Rima Hadley.