Lineation and foliation
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Lineation refers to any linear structure that occurs repetitively in rock, such as elongated pebbles or the intersection of two foliations. Lineations can indicate the direction of tectonic transport, though opinions differ on whether they are parallel or perpendicular to transport. Foliation refers to repetitive layering in metamorphic rocks caused by shearing forces or pressure differences. Types of foliation include fracture cleavage, crenulation cleavage, slaty cleavage, and schistosity. Lineations and foliations provide important clues about a rock's deformation history.
Lineation and foliation
- 2. Contents •Introduction •Lineation Slickenside Straie Intersection Mineral Crenulations Stretching •Foliation Fracture cleavage Crenulation cleavage Slaty cleavage Schistosity •Conclusion •References
- 3. Introduction
- 4. Tectonites: Rocks that are pervaded by foliation and or lineation- flowed in solid state Schistosity (foliation) only due to flattening- no lineation. L: Lineation only, due to unidirectional stretching or constriction. LS: Foliation and Lineation, related to non coaxial strain- shearing.
- 5. Lineation • The word lineation is used to describe any linear structure that occurs repetitively in a sample of rock; for example, it may refer to an array of elongate pebbles, oriented with their long dimensions mutually parallel, or it may refer to the lines of intersection of two foliation. Diorite gneiss
- 6. Contd.. • Lineation may be defined in a more precise way as any linear fabric element; it may be primary igneous or sedimentary structure or a secondary related to deformation • Lineation are ubiquitous in deformed rocks and is common for more than one lineation to be visible on a given foliation plane.
- 7. Origin of lineation • In the period 1935 to 1960 lineations were used, more or less indiscriminately, as indicators of “direction of tectonic transport.” • The precise meaning of this term was always obscure; to some it was synonymous with the direction of maximum finite extension in a deformed rock mass, to others, the “direction of tectonic transport” was the direction that one rock mass has been displaced relative to another. • some writer believed that lineations were always parallel to the direction of tectonic transport; others believed lineations were always normal to this direction and others again believed that they could be at any angle.
- 8. •Types of lineation Slickenside Striae Intersection Mineral Crenulations Stretching
- 9. Slickenside striae • Slickenside striae are common linear structure in many rocks but they are not generally a penetrative features and therefore not a fabric element. • a slickenside is a smoothly polished surface caused by frictional movement between rocks along the two sides of a fault. This surface is normally striated in the direction of movement. The plane may be coated by mineral fibres that grew during the fault movement, known as slickenfibres, which also show the direction of displacement. • How slickenside is form and show sense of movement on a fault
- 10. Contd.. Slickenside on a sample of dextral slickenside of pyrite sandstone
- 11. Intersection • Intersection of two planar features- an "apparent" lineation in that there is no fabric that is linear. • e.g., intersection between cleavage and planar surface
- 12. Mineral lineation • Mineral lineation can be defined by the preferred dimensional orientation of inequant grains or by elongate mineral aggregates Hornblende lineation in orthogeiness
- 13. Crenulations • Crenulation lineation: Intersection between fold hinges and foliation
- 14. Stretching lineation • elongation of minerals due to "stretching" deformation
- 16. Foliation • Foliation in geology refers to repetitive layering in metamorphic rocks. Each layer may be as thin as a sheet of paper, or over a meter in thickness. The word comes from the Latin folium, meaning "leaf", and refers to the sheet-like planar structure.It is caused by shearing forces (pressures pushing different sections of the rock in different directions), or differential pressure (higher pressure from one direction than in others) • foliation is any penetrative planar fabric present in metamorphic rocks. Rocks exhibiting foliation include the standard sequence formed by the prograde metamorphism of mud rocks; slate, phyllite, schist and gneiss. • Foliated rock is also known as S-tectonite in sheared rock masses.
- 17. Contd.. • Gneiss , foliated metamorphic
- 18. Contd.. Fabric elements that define foliations. In each case the foliation is parallel to the top of the block. (a) compositional banding; (b) parallel discontinuities; (c) elongate mineral grains; (d)inequant mineral grains
- 19. Types of foliation Fracture cleavage Crenulation cleavage Slaty cleavage Schistosity
- 20. Fracture cleavage • Fracture cleavage consist of closely spaced microfaults or fractures that divides the rock into a series of tabular bodies • Fracture cleavage is generally formed in low and medium metamorphic grade rocks and is common within sandstone beds in folded sequences of alternating sandstone and pelites where the cleavage in the pelites is a crenulation cleavage.
- 21. Crenulation cleavage • Crenulation cleavage varies considerably in morphology but the various forms share one diagnostic feature. In all cases an earlier foliation, generally defined by preferred orientation of layer silicates, is folded(crenulated) on a micro scale. • It may be defined as microfaults developed parallel to the fold limbs
- 22. Slaty cleavage • The word slate originated as a quarryman’s term for fine- grained rocks that were sufficiently fissile to be split into thin, planar slabs suitable for roofing. • slates are typically rich in layer silicates but the term slaty cleavage is also used to describe foliations in low grade metamorphic rocks of entirely different composition.
- 23. Schistosity • Schistosity is a term used to describe the foliation in rocks that have a grain size coarse enough to be called schists. • It is most common in high grade metamorphic rocks but also occurs in low-grade rocks, particularly in greenschist facies rocks of retrograde origin.
- 24. Phyllitic structure is a type of foliation
- 25. Conclusion • Lineation is a linear structure that occurs repetitively in a sample of rock and it may refer to an array of elongate pebbles, oriented with their long dimensions mutually parallel, or it may refer to the lines of intersection of two foliation. • Some writer believed that lineation were always parallel to the direction of tectonic transport; others believed lineation were always normal to this direction and others again believed that they could be at any angle.
- 26. Contd.. • Foliation in geology refers to repetitive layering in metamorphic rocks. Each layer may be as thin as a sheet of paper, or over a meter in thickness.It is caused by shearing forces (pressures pushing different sections of the rock in different directions), or differential pressure (higher pressure from one direction than in others) • foliation is any penetrative planar fabric present in metamorphic rocks. Rocks exhibiting foliation include the standard sequence formed by the prograde metamorphism of mudrocks; slate, phyllite, schist and gneiss.
- 27. Referances • Bruce E.hobbs, Winthrop D.means, Paul F.Williams (1976), An Outline of Structural Geology, John Wiley and Sons, INC. U.S.A. Pp 213-288 • Marland P.Billings, (2013), Structural Geology, PHI learning private limited, Delhi. Pp 408-419 • http://en.wikipedia.org/wiki/Lineation_(geology) • https://www.google.co.in/search?q=image+of+foliation