Lineation and foliation

Structural geology - Lineation and foliation

1. 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

15. Contd.. Stretched calcite

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. Referance s 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