Slickensides
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In
geology Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Ear ...
, a slickenside is a smoothly polished surface caused by frictional movement between
rocks In geology, rock (or stone) is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its chemical composition, and the way in which it is formed. Rocks form the Earth's ...
along a fault. This surface is typically striated with linear features, called slickenlines, in the direction of movement.


Geometry of slickensides

A slickenside can occur as a single surface at a fault between two hard surfaces. Alternatively, the gouge between the fault surfaces may contain many anastamosing slip surfaces that host slickensides. These slip surfaces are on the order of 100 micrometers thick, and the size of the grains that constitute the surface are ultra-fine (0.01-1 micrometers in diameter).Power, William L., and Terry E. Tullis. "The relationship between slickenside surfaces in fine-grained quartz and the seismic cycle." Journal of Structural Geology 11.7 (1989): 879-893. These grains are unlike typical grains of fault rock in that they have irregular grain boundaries and few crystal lattice defects (termed dislocations). Slickensides have conspicuous shapes that can be used to determine the direction of movement along the fault. Straight slickenlines indicate linear-translational fault motion. They are parallel to the direction of fault motion and serve as a kinematic indicator. Curved slickenlines have recently been studied for their potential to preserve the direction of earthquake rupture propagation. Kearse, J., Kaneko, Y., Little, T., and Van Dissen, R., 2019, Curved slickenlines preserve direction of rupture propagation: Geology, https://doi.org/10.1130/G46563.1


Surface Roughness

Slickenside formation results in unique roughness on a slip surface. Fault surface roughness (or topography) is characterized by the aspect ratio of asperity height to scale of observation, and this roughness is a key parameter in the study of fault slip. In general, a fault surface appears rougher at smaller scales (i.e. rough and bumpy at approximately millimetre scales and smaller, and increasingly smooth with larger fields of view). This smoothing with larger observation scales is more pronounced in the slip-parallel direction than the slip-perpendicular direction and is commonly a result of slickenside formation.


Mechanisms to create slickensides

The unique geometry of a slickenside can be created in a variety of ways, but the precise mechanisms that create them is not well understood. The grinding between two rocks produces granular material, and there is a change in the behaviour of wear material when the particle size is reduced to nanometers.Toy, V. G., A. Niemeijer, F. Renard, L. Morales, and R. Wirth (2017), Striation and slickenline development on quartz fault surfaces at crustal conditions: Origin and effect on friction, J. Geophys. Res. Solid Earth, 122, 3497–3512, doi:10.1002/2016JB013498 When the particle size is reduced so dramatically that the surface becomes shiny, it can be characterized as a fault mirror. A fault mirror may also be the result of fluid being present at the fault surface during slip.Kirkpatrick, J. D., et al. "Silica gel formation during fault slip: Evidence from the rock record." Geology 41.9 (2013): 1015-1018. Once slip has stopped, this fluid solidifies as a silica gel, which appears shiny and hosts slickenlines.


Asperity plowing

An asperity on a fault surface is a bump or point with higher relief than the area around it. The asperity, when pressed into the opposing rock surface and then moved, digs into the opposing rock, forming troughs, grooves, and scratches.Means, W. D. "A newly recognized type of slickenside striation." Journal of Structural geology 9.5-6 (1987): 585-590. Asperity plowing is thus a result of permanent deformation in the brittle regime at a small scale.Kirkpatrick, J. D., & Brodsky, E. E. (2014, October 22). Slickenline orientations as a record of Fault Rock Rheology. Earth and Planetary Science Letters. Retrieved November 3, 2022, from https://www.sciencedirect.com/science/article/pii/S0012821X14006037


Debris streaking

When an asperity plows into the opposing rock, it wears itself and the opposing rock down and produces fine debris. This debris, or wear product, accumulates both in front of and behind the asperity in a long, elongated shape. If the asperity is relatively hard, the debris will accumulate in front of the asperity. If the asperity is relatively soft, the debris will trail behind. This debris hardens over time and is preserved as a form of slickenline.


Erosional sheltering

Some rocks may contain particles that are harder than the rest of the rock. When these rocks are worn, the harder particles will resist wear more than the softer rock, the rock on the
lee side Windward () and leeward () are terms used to describe the direction of the wind. Windward is ''upwind'' from the point of reference, i.e. towards the direction from which the wind is coming; leeward is ''downwind'' from the point of reference ...
of the hard particle will be protected from wear. This creates a ''tail'' that starts abruptly as a ''crag'' where the hard particle was located and is elongated parallel to the direction of movement down-slip from the particle.


Fiber growth

The fault plane may be coated by
mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. ( ...
fibres that grew in during the fault movement, known as ''slickenfibres''. Due to irregularities in the fault plane, exposed slickenfibres typically have a stepped appearance that can be used to determine the sense of movement across the fault. Slickenfibres are secondary minerals that make up the slickensides rather than the rock itself. Slickenfibres form in areas where the rock slowly creep past one another rather than sliding suddenly as a result of an
earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, from ...
.Ishii, Eiichi. (2017). Estimation of the highest potential transmissivity of discrete shear fractures using the ductility index. International Journal of Rock Mechanics and Mining Sciences. 100. 10-22. 10.1016/j.ijrmms.2017.10.017. Unlike slickenlines, which give two possibilities for slip direction, slickenfibres preserve the true slip direction.


Implications

Slickensides provide useful insight into earthquake processes.
Calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...
slickenfibres have recently been used to constrain the depth of aseismic creep in the
Zagros Mountains The Zagros Mountains ( ar, جبال زاغروس, translit=Jibal Zaghrus; fa, کوه‌های زاگرس, Kuh hā-ye Zāgros; ku, چیاکانی زاگرۆس, translit=Çiyakani Zagros; Turkish: ''Zagros Dağları''; Luri: ''Kuh hā-ye Zāgro ...
as well as the orientation of stress acting on the fault. It has also been suggested that when multiple slickenfibre or slickensteps orientations are present, it can indicate that the ongoing shear is not strain softening so slip does not have a constant direction. In addition to the direction of slip, slickenlines have also been used to constrain the timing of fault slip. They also preserve any complexity in the geometry of the earthquake rupture.


Other types of slickensides


Slickensides in soils

In
pedology Pedology (from Greek: πέδον, ''pedon'', "soil"; and λόγος, ''logos'', "study") is a discipline within soil science which focuses on understanding and characterizing soil formation, evolution, and the theoretical frameworks for modeling ...
, the study of
soils Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former term ...
in their natural environments, a slickenside is a surface of the cracks produced in soils containing a high proportion of swelling
clays Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...
. Slickensides are a type of cutan. In the
Australian Soil Classification The Australian Soil Classification is the classification system currently used to describe and classify soils in Australia. It is a general-purpose, hierarchical classification system, and consists of five categorical levels from the most general t ...
, slickensides, along with lenticular structural aggregates, are an indicator of a vertosol.


Slickensides on the Moon

On the
Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
, a boulder with slickensides, discovered in a debris-strewn small crater at Station 9 near
Rima Hadley Rima, also known as Rima the Jungle Girl, is the fictional heroine of W. H. Hudson's 1904 novel '' Green Mansions: A Romance of the Tropical Forest''. In it, Rima, a primitive girl of the shrinking rain forest of South America, meets Abel, a pol ...
, was photographed during a
moonwalk Moonwalk may refer to: Space travel * Moonwalk, an excursion on the Moon, see Moon landing ** For a specific event, see List of spacewalks and moonwalks 1965–1999 ** For a specific person, see List of people who have walked on the Moon *Extrave ...
by the crew of
Apollo 15 Apollo 15 (July 26August 7, 1971) was the ninth crewed mission in the United States' Apollo program and the fourth to Moon landing, land on the Moon. It was the first List of Apollo missions#Alphabetical mission types, J mission, with a ...
.https://history.nasa.gov/alsj/a15/A15SampleCat_1.pdf Catalog of Apollo 15 Rocks – Part 1. 15015-15299


Gallery


Notes


References

* Allaby, A. and Allaby, M. (Eds). 1990. ''The Concise Oxford Dictionary of Earth Sciences''. New York, USA: Oxford University Press. * McDonald, R. C. et al. 1990. ''The Australian Soil and Land Survey Field Handbook'', 2nd Ed. Melbourne, Australia: Inkata Press. * ''Microtectonics'', by C.W.Passchier and R.A.J.Trouw, 2nd rev. and enlarged ed., 2005, XVI, 366 p., 322 illus., with CD


External links

* {{Structural geology Pedology Structural geology