Types of lineation in structural geology

 In geology, lineation refers to the orientation of a planar feature. A planar feature is an element that extends in some linear fashion for a given reference plane. These elements may be faults, fractures, dikes, or other features which are tabular in nature. All lineations are a type of cross-cutting relationship, but not all cross-cutting relationships are considered lineation. Within the study of structural geology, the term lineation most frequently refers to foliation or palisade texture. As scientists who study the earth's crust, we rely heavily on visual features to identify lineation. We refer to these features as bedding planes, fault planes, and foliation planes. When different sets of these planes line up at the same time we called it strike-slip or shear lineation. Strike-slip or shear lineation will also result in mylonite zones. Lineations are found throughout the world depending on the type of fold they belong to.

 These structures contain minerals and rocks which are aligned in a particular fashion. The alignment of these minerals could be parallel or perpendicular to the foliation planes or axial planes, indicating different types of folds in which they occur. Lineations are generally not considered litho-structural elements but they help in understanding the process of deformation occurring within an area at different levels. This article will discuss various types of lineations in geology that can be identified in the earth's crust.

Formation Of Lineation.

Lineations develop when one part of a rock mass moves with respect to another part, and there are significant shear stresses during this movement. Most often, the lineation is a zone of shear fracture (a fault), but sometimes it may be a zone of plastic deformation.

Significance Of Lineation

Lineations are important to geologists because they represent the orientation of the long axes of planar and linear structures, such as foliation in metamorphic rocks, mineral stretching lineations, and stretching lineations in sedimentary dikes. The study of lineations is termed LITHONECLINOMETRY. Lithoneclinometry is an important tool for understanding the structure and tectonic evolution of metamorphic terranes. It is also used to understand the tectonics associated with igneous intrusions, sedimentary dikes, and fault zones.The relationship between orientation and lineation helps us to understand the mechanisms controlling the formation of fractures. These fractures are formed by different stresses acting on rocks such as primary stresses, secondary stresses and residual stresses.The orientation of a lineation can be measured by an outcrop photograph. The orientation of the lineation can then be used to calculate the direction of maximum horizontal principal stress (SHmax) for the deformation event that produced the lineation. These measurements are useful for paleostress analysis which attempts to calculate the stress state of ancient tectonic processes.

The following are the types of lineation in geology:

Fold Lineation:

 It is a lineation associated with folds. These are a set of parallel planar or linear structures that are in the same direction as the axial plane and/or the fold hinge. These are not present in all folds. They may be caused by stretching, flattening, and shearing during deformation. Folds exhibiting this type of lineation are called L-type folds. This type of lineation is commonly observed along the hinge line of an anticline or a syncline. It is parallel to the foliation and therefore it is also called axial planar cleavage. It is the result of tangential stress during folding.

S-type lineation: 

It is a lineation associated with shear zones, which is an extensive zone of deformation in the rock where it experiences a state of shear resulting in distinct foliation or lineation. S-type lineations can be either planar or linear and have the same orientation as foliation within shear zones. This type of lineation is observed in rocks that have undergone shearing. This kind of shearing may take place along a thrust fault or a reverse fault or any other shear zone. The axis of the S-type lineation is parallel to the shear plane and perpendicular to the dilation plane. 

P-type lineation:

 It is a lineation associated with penetrative fabrics or structures, which are structures formed by pressure solution that result in right angle fractures or veins that form at right angles to the principal (axial) compressive stress. The term P-type comes from pressure solution, which causes these structures to form. P-type lineations are linear and perpendicular to axial planes of folds, shear zones, and other penetrative. This type of lineation is observed in rocks that have undergone the pressure solution process, because of high pressure and temperature conditions prevailing at depth in the Earth’s crust.  Along with p-type lineation, there also exists p-foliation which indicates the direction in which pressure was applied during the pressure solution process. P-type structures are commonly observed in migmatites, pegmatites, and rocks belonging to regional metamorphic terrains like gneisses and schist belts.

Tectonic Lineations:

Tectonic lineations are linear features that reflect the tectonic environment in which they formed. Their orientation and shape can be used to infer stress fields, rock rheology, and flow kinematics. The most common type of tectonically-produced lineation is cleavage or foliation that may result from simple shear, pure shear, or a combination of simple shear and pure shear. Tectonic lineations are structural features and can be planar, linear, or irregular. They are developed due to the movements of the earth's crust. Tectonic lineation is of three types:

1. Planar: These occur in sedimentary rocks and result from compaction and recrystallization of grains that initially have random orientation.

2. Linear: These are generally found in metamorphic rocks and form as a result of slaty cleavage or mineral growth during metamorphism. They also occur in igneous rocks when feldspars and micas align in the direction of applied stress.

3. Irregular: These develop in igneous, metamorphic, and sedimentary rocks when they have undergone deformation under stress conditions. These stress conditions may be tectonic or diagenetic in origin. They may range from large to small sizes depending on the intensity of the deformation.

Conclusion

I hope that this article was helpful for people trying to figure out how to identify lineation in their own map projects. Keep in mind that this article is a general overview of lineation and not an exhaustive treatment of the subject. I would like to thank you for taking the time to read what I have written so far, and I would also like to encourage geologists who are familiar with this topic to suggest any edits to me if they feel that some things could be explained better.