Sinistral and dextral

This article is about the geologic usage, for human physiology usage see the articles laterality.

The terms sinistral and dextral (or left-handed and right-handed) are also used to describe the direction of coiling in gastropod shells.

Schematic illustration of the two strike-slip fault types. The view is of the Earth's surface as from space.

The terms sinistral and dextral refer to the horizontal movement of blocks on either side of a fault or the sense of movement within a shear.

Sinistral or left lateral movement

Movement is sinistral (left-handed) if the block on the other side of the fault moves to the left, or if straddling the fault the left side moves toward the observer.

Dextral or right lateral movement

Movement is dextral (right-handed) if the block on the other side of the fault moves to the right, or if straddling the fault the right side moves toward the observer.

Naming origin

Sinistral gains its origins from the Latin "sinister" for left-handed. The opposite sense of movement dextral gains its name from the Latin "dexter" for right-handed.

Identifying sense of throw

Identifying the sense of throw of a fault or the sense of shear can be achieved via several methods:

• Mappable offset of rock units
• Offset of visible bedding, cross cutting dikes
• Folding of strata proximal to a fault caused by frictional drag
• Shear sense indicators in sheared rocks such as
  • Mica fish
  • Extensional veins
  • Shear deflections and stair-stepping of shear folia

Examples

Banded gneiss with dike of granite orthogneiss with sinistral shear

Boudinaged jasperoid in sheared basalt, Fortnum Gold Mine, Australia. Example of dextral shear

Asymmetric boudinaged quartz vein in zone of sinistral shear, Starlight Pit, Fortnum Gold Mine, Western Australia

See also

• Fault (geology)
  • Thrust fault
• Shear (geology)
• Transform boundary

References

• Schlumberger Oilfield Glossary