Discuss the types of Penetrative and Non-penetrative lineations with the help of suitable diagrams and throw light on their genesis.

Types of Lineations

Lineations are broadly classified into two categories based on their pervasive nature within the rock body: Penetrative and Non-penetrative.

1. Penetrative Lineations

Penetrative lineations are those linear structures that occur pervasively throughout the rock mass, visible at all scales from microscopic to outcrop. They represent a homogeneous distribution of linear fabric elements.

• Mineral Lineation: This involves the preferred alignment of elongate mineral grains (e.g., hornblende, tourmaline, sillimanite needles) or mineral aggregates within metamorphic rocks. The long axes of these crystals are aligned sub-parallel, defining a prominent linear fabric.

Diagram: Imagine a rock cross-section with numerous elongated amphibole crystals all pointing in the same direction.

• Stretching Lineation: Formed by the elongation and alignment of clasts, pebbles, ooids, fossils, or reduction spots within a deformed rock. These indicate the direction of maximum extension or stretching during ductile deformation.

Diagram: Picture initially spherical pebbles deformed into cigar-shaped ellipsoids, all aligned parallel to each other.

• Crenulation Lineation: These are micro-fold hinges developed on an earlier foliation surface (e.g., schistosity or cleavage). The hinges of these small, tightly spaced wrinkles or crenulations create a linear fabric. They often indicate a superposed deformation event.

Diagram: A wavy line representing a folded foliation plane, with arrows indicating the parallel orientation of the hinge lines of the micro-folds.

• Intersection Lineation: Formed by the geometric intersection of two planar fabric elements, most commonly the intersection of bedding (S0) and a later-formed cleavage or foliation (S1), or the intersection of two different generations of cleavage. This lineation typically parallels local fold axes.

Diagram: Two intersecting planes (e.g., bedding and cleavage) creating a visible line on their common surface.

2. Non-Penetrative Lineations (Surface Lineations)

Non-penetrative lineations are localized linear features confined to specific surfaces, such as fault planes or bedding planes, and do not pervade the entire rock mass uniformly. They often form in the brittle deformation regime.

• Slickenlines (Slip Lineations): These are striations, grooves, or ridges found on fault surfaces (slickensides) formed by frictional sliding of one rock mass against another during fault movement. They indicate the direction of fault slip. There are two main types:
  • Groove Lineations: Linear abrasions or grooves on fault surfaces caused by the physical grinding of asperities during fault movement.
  • Fibrous Lineations: Linear growths of minerals (e.g., quartz, calcite fibers) on fault surfaces, oriented parallel to the direction of fault displacement. These form when there is an extensional component during deformation, allowing mineral growth in the opening fracture.

Diagram: A flat surface (fault plane) with parallel scratches or grooves extending across it in one direction.

• Boudinage: This refers to the segmentation and elongation of a more competent rock layer (e.g., sandstone) within a less competent matrix (e.g., shale) due to extension. The resulting sausage-like or rectangular segments (boudins) are typically longer in one dimension, defining a linear fabric at the layer scale.

Diagram: A continuous rock layer broken into several elongated, separated "sausage-like" segments due to stretching.

• Mullions: Coarse, columnar, or ridged corrugations that typically form at the interface between competent and incompetent layers, often in folded sequences. They are typically parallel to fold axes.

Genesis of Lineations

The genesis of lineations is intrinsically linked to the stress and strain conditions experienced by rocks, primarily during tectonic deformation and metamorphism.

Genesis of Penetrative Lineations:

• Ductile Deformation: Most penetrative lineations, such as mineral, stretching, and crenulation lineations, are formed under ductile deformation conditions, typically at higher temperatures and pressures in metamorphic environments. They reflect intense bulk strain and material flow.
• Oriented Growth and Recrystallization: Mineral lineations arise from the preferential growth or recrystallization of elongated minerals or the rotation of existing platy/prismatic minerals into alignment parallel to the direction of maximum finite extension (X-axis of the strain ellipsoid) or perpendicular to the principal plane of shortening.
• Passive and Active Rotation: During deformation, existing grains or aggregates can passively rotate into alignment, or actively grow in preferred orientations due to local deviatoric stress.
• Progressive Strain: Stretching lineations develop as rocks undergo significant stretching, causing initially equidimensional features to deform and elongate parallel to the direction of maximum extension. This can occur in various faulting regimes (extensional, compressional, transpressional).
• Superposed Deformation: Crenulation lineations are a result of later deformation folding an existing planar fabric (foliation or bedding), with the hinge lines of these micro-folds defining the lineation.
• Geometric Intersection: Intersection lineations are a direct geometric consequence of the intersection of two non-parallel planar fabrics. Their orientation is controlled by the attitude of the intersecting planes, often parallel to fold axes in folded terrains.

Genesis of Non-Penetrative Lineations:

• Brittle Deformation and Frictional Sliding: Slickenlines are characteristic of brittle deformation, forming along discrete fault surfaces. Frictional movement between rock blocks generates the physical abrasions (grooves) and can induce mineral growth (fibrous lineations) in the direction of slip.
• Localized Extension: Boudinage forms due to localized extension within a layered sequence where a competent layer stretches and breaks into segments. The elongation direction of these boudins is parallel to the maximum extension direction within that layer. Mullions are also formed by a combination of buckling and stretching at layer interfaces.

Diagram: Simplified representation of Lineation types

Penetrative Lineation (Mineral Lineation):
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| \ | \ | \ |
| \| \| \| |
| \ \ \ |
| \ \ \ |
| \ \ \ |
--------------------->
(Elongated minerals aligned parallel to the stretching direction)

Non-Penetrative Lineation (Slickenlines on a fault plane):
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|||||||||||||||||||||| (Fault plane surface)
\\ \\ \\ \\ \\ \\
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// // // // // //
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(Parallel grooves/striations on a fault surface indicating slip direction)