Lineation

(Fig. 4.2, Passchier and Trouw, 1996)
Any linear feature,
 primary or secondary, that occurs
 penetratively (at the outcrop and/or hand specimen scale) and
 homogeneously in a body of rock.

If lineation is not penetrative and is present only along a discrete surface,
 the lineation is surficial and is called a surface lineation). Examples:


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slickenside striae defined by the alignment of mineral fibres or grooves.
sedimentary groove casts in a bedding plane.
flute casts and other linear primary structures on bedding of a turbidite.
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Types of Lineation

Intersection lineation - Formed by intersecting foliations, for example:

(bxf), or (SoxS1) - Bedding intersecting a foliation; (reads: bedding cross foliation)
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
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If the foliation is axial planar, then the (bxf) defines the fold axis.
If the foliation is a slaty cleavage, then (bxc) is the fold axis.
(S1xS2) - A younger foliation S2 intersecting an earlier foliation S1).
 Pencil lineation - rock cleaves parallel to both foliations producing elongate
rhombic prisms or “pencils”.
Crenulation lineation

Defined by the alignment of hingelines of pervasive microfolds in a foliation plane.
.
 (S1xS2) - A younger crenulation foliation S2 intersecting an earlier foliation S1.
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Stretching lineation

Defined by deformed, constricted (i.e., elongated)
 Grains of minerals such as quartz (that commonly are equidimensional).
 Aggregates of equidimensional grains.
 Pebbles in a deformed conglomerate.

Forms by crystal plastic deformation.
Mineral lineation - Defined by the preferred orientation of:

Euhedral or subhedral long (not because they are deformed and elongated) mineral grains such as
amphibole, tourmaline, and sillmanite.

Planar minerals such as micas that share a common axis.

The alignment of the elongate minerals may be due to:
1. Passive rotation toward a principal strain direction during progressive deformation,
2. Growth, i.e., directional crystallization parallel to foliation and 1.

Comminution and rotation of larger clasts may also produce alignment of grains in
cataclasites during cataclastic flow.

Mineral lineations commonly occur in the:
 plane of a metamorphic foliation
 shear surfaces
 mylonitic C-foliation
Pressure shadow

Is a kind of mineral lineation where
 new minerals (e.g., quartz, calcite) precipitate (crystallize out of a solution)
 on the contact between (i.e., on either sides of ):
 an earlier stiff mineral inclusion
 mineral aggregate, or grain (e.g., pyrite)
 and its less competent matrix,
 producing an elongate structure on a foliation.

The fibres are parallel to the direction of stretching.

Pressure shadow is a growth-related mineral lineation and forms by preferential growth.
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Linear Structures:
Large, parallel alignment of elements at the outcrop scale, for example:
stretched pebbles, mullions, pencil structure, boudin.

Discrete linear structure  Formed by the deformation of discrete objects such as:
 stretched ooids
 pebbles,
 fossils
 reduction spots.

If the objects where originally spherical, then the ellipsoidal grains give the strain ratio
and orientation of the strain ellipsoid (or ellipse in 2D).
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Boudins

Sausage-shaped segments of extended competent layers surrounded by less competent
matrix.

Linear segments of a layer that has been pulled apart along periodically-spaced lines of separation
called boudin lines (necks).
 pinch-and-swell structure
 chocolate tablet structure

If ductility contrast is large, boudins will be subangular with rectangular forms.

As ductility contrast diminishes, boudins become more lens-shaped in profile.
Mullion

Rod
Linear fluted structures
 developed within a rock
 or at lihologic interfaces.(look like mullions in Gothic cathedrals)

Mullion cross section (few cm in dimension) has:
 convex surfaces with intervening cusps.
 or by alternating convex and concave surfaces.

Mullion length is indefinite.
 When intense deformation detaches the limbs of folds such as occurs during fold
transposition, only fold hinges may be left in the rock.

These hinges are called rod.

Rodding typically occurs:
 In a multilayer composed of phyllite or schist and quartzite.

The quartz layers are relatively rigid and define visible folds.

The limbs of these quartz layers may become severely pinched out, and quartz
may flow in the fold hinge zone to form a rod.
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Slip lineations

Form on surfaces on which there has been sliding, e.g.,
 a fault plane
 on the interface of beds in a flexural slip fold.

Are a type of surface lineation.
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Trend parallel to the sliding direction on the faulted or folded surface.
Two basic types of slip lineation:
1. Groove lineations -
form by plowing of surface irregularities due to friction.
2. Fiber lineations -
form when vein mineral fibres precipitate along a sliding surface.
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Types of Tectonites and Strains
Tectonite:
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
Rock with pervasive foliation and/or lineation
deformed ductiley by solid-state microscopic crystal plasticity or cataclastic flow.
Flattening:
 Produces S-tectonite.
 Spheres are deformed into oblate (pancake-shaped) strain ellipsoids: S1 = S2 > S3
Constriction
 Produces L-tectonite
 Spheres are deformed into prolate (cigar-shaped) strain ellipsoids (cigar): S1 > S2 = S3
Plane Strain
 May produce LS-tectonite
 Form in non-coaxial strain conditions; stretching in one direction is compensated by flattening in
another; S1 > S2 > S3 assume S2 = 1.

Lineations are commonly associated with foliation (in LS tectonites).
 In this case, the relative age of the lineation and foliation must be established.
 If of the same age, the lineation may furnish information on the direction of tectonic transport
(e.g., shear direction on a mylonitic C-foliation).
 This requires cutting oriented thin section.

If the rock only possesses lineation, it is called a L tectonite

If the rock only has foliation, it is called S tectonite

Like foliation, lineation may be:
 primary (linear tool marks, flute cast, ripple marks on turbidite bedding)

secondary (e.g., crenulation).
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