Gallery of Slickensides

Fault slickensides — James St. John/Flickr/CC BY 2.0

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Updated on January 31, 2019

Slickensides are naturally polished rock surfaces that occur when the rocks along a fault rub against each other, making their surfaces smoothed, lineated, and grooved. Their formation may involve simple friction, or if the fault surface was once deeply buried, actual growth of oriented mineral grains may respond to the forces on the fault. Slickensides appear to lie between the grinding of shallow rocks that makes fault gouge (and cataclasite) and the deep-seated friction that melts rock into pseudotachylites.

Slickensides may be scattered surfaces as small as your hand or, in rare cases, thousands of square meters in extent. The corrugations show the direction of motion along the fault. Unusual minerals may occur given the combinations of fluids and pressures along slickensides. But even familiar rocks, as we will see, take on unusual features too.

Slickensides can range in size from tiny, as in a chert specimen, to gigantic. In all cases, you spot them by their telltale glint, and in all cases, they signify shear, the sideways motion of faulting.

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On an Outcrop

Slickenside on mountain crop — Andrew Alden

Slickensides may appear on an outcrop if you face the sun. This is part of the faulted and sheared western face of Point Bonita in Golden Gate National Recreation Area, near San Francisco.

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In Limestone

Most rock types can have slickensides. This limestone is also fractured and brecciated by the fault movements that created this slickenside.

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Sandstone, Wright's Beach, California

An uncommon host rock for slickensides — Andrew Alden

This site is very close to the San Andreas fault, and pervasive fracturing affects this already-jumbled tectonic megabreccia of Franciscan sandstone.

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Peridotite, Klamath Mountains, California

Serpentine slips in peridotite — Andrew Alden

Serpentine minerals form easily by alteration of peridotite, especially where faulting admits fluids. These readily form slickensides.

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In Serpentinite

Slickensides are very common in serpentinite. These ones are small, but whole outcrops glisten because slickensiding is so pervasive.

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In a Serpentinite Outcrop

Outcrop-scale slickenside — Andrew Alden

This larger slickenside is in a serpentinite body at Anderson Reservoir, California, near the Calaveras fault.

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In Basalt

A rare black flash of Slickensides — Andrew Alden

Where igneous rocks are tectonically deformed, as at this outcrop in northern San Quentin, California, even basalt can acquire slickensides.

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Closeup of Basalt Slickenside

A dark mirror of Slickenside — Andrew Alden

This sample from the previous outcrop displays the aligned mineral grains and polished surface that define a slickenside.

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Metabasalt, Isle Royale, Michigan

Slickensides in michigan — Ben+Sam/Flickr/CC BY-SA 2.0

This exposure from Raspberry Island can be mistaken for glacial striations, but the orientation is wrong. The green color is suggestive of serpentine minerals.

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In Chert

In San Francisco's Corona Heights near Peixotto Playground at 15th and Beaver streets is this world-class slickenside in Franciscan chert, exposed by quarrying.

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Corona Heights Slickenside, Beaver Street

slickenside an urban neighborhood — Andrew Alden

On the Beaver Street end of this slickenside, the higher surfaces reflect the sky. Slickensides are also called fault mirrors.

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Slickenlines

The individual streaks and grooves of a slickenside are called slickenlines. Slickenlines point in the direction of faulting and some features can indicate which side moved which way.

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Rock Near a Slickenside

A remnant block from the near side of the fault plane shows the undisturbed form of the chert.

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Chert Reflections

The slickenside surface appears hand-polished. Chert is tough enough to preserve this kind of polish against weathering.

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Slickenside in the French Alps

huge exposure on slickenside in the alps — Centre National de la Recherche Scientifique

This large slickenside is on the Vuache fault, at Mandalaz peak in Haute-Savoie.