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Foliation and Lineation Questions and Answers

1.

A pervasive parallel foliation of fine-grained, platy minerals in a direction perpendicular to the direction of maximum finite shortening by deformation and low grade metamorphism is called :

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: B

It is called slaty cleavage.

Slaty cleavage is a penetrative, closely spaced foliation formed in fine-grained rocks such as shale during low-grade metamorphism. Platy minerals (mica and chlorite) grow or rotate perpendicular to the direction of maximum shortening, so the rock splits into thin, smooth slates along the cleavage, usually parallel to the axial planes of folds.

Foliation is the general term for any planar fabric. Schistosity is the coarser foliation of higher-grade schists with visible mica flakes, and S-tectonite is a rock whose fabric is dominated by a foliation of any kind.

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2.

In an S-tectonite the strain ellipsoid will be :

Exam Year(s)

MH SET - 2023, SET Exam - 2023,

Answer: C

In an S-tectonite the strain ellipsoid is Oblate finite strain.

Tectonites are classed by their dominant fabric. An S-tectonite has a strong planar fabric (foliation) and little or no lineation, which records flattening strain: the finite strain ellipsoid is oblate, shaped like a pancake, with X roughly equal to Y and both much greater than Z.

A prolate ellipsoid (cigar-shaped) gives an L-tectonite with a dominant lineation, and plane strain gives an LS-tectonite with both fabrics. 'Infinite plane strain' is not a real category.

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3.

Plotting pitch is useful when rocks contain :

Exam Year(s)

MH SET - 2024, SET Exam - 2024,

Answer: A

Plotting pitch is useful when rocks contain a Lineation.

Pitch (or rake) is the angle between a linear feature and the strike line, measured within the plane that contains it. It is used for lineations such as slickenlines, mineral elongation or fold hinges lying on a foliation or fault plane, especially when the plane dips steeply and the plunge is hard to measure directly.

Foliation, gneissosity and parting are planar structures, recorded by strike and dip, so pitch, a measure for lines within a plane, does not apply to them.

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