Structural and Metamorphic Geology

Lineation

Lineation: a fabric element in which one dimension is considerably logner than the other two. 

Lineations can be surficial lineations (seen only on the surface) or they can be penetrative lineations (seen throughout the structure). 

Mineral lineations

Lineations made up of aligned prismatic minerals either by prismatic or elongate minerals, or mineral aggregates. This is MINERAL GRAIN LINEATION. 

Minerals and mineral aggregates can form a linear fabric by means of recrystallisation, dissolution/precipitation or rigid rotation.

• Physical rotation of rigid prismatic crystals can occur during deformation
• Synkinematic recrystallisation occurs via plastic deformation or dissolution/reprecipitation process that will reshape the minerals
• Cataclasis, pressure solution and recrystallisation all contribute to change the shape of minerals and mineral aggregates during deformation
• FOR EXAMPLE: in a homogeneously strained rock, a spherical aggregate of minerals after deformation wouuld represent the shape of the strain ellipse

Rodding describes elongated mineral aggregates that are easily distinguished from the rest of the rock, can be considered stretching lineations but are influenced by other structure forming processes. This is POLYCRYSTALLINE LINEATION. 

Structural lineations 

• Intersection lineation: in situations where planar structures intersect, the line of intersection is called the intersection lineation. For example when the first tectonic cleavage S1 cuts the primary bedding S0, forming intersection lineation L1
• Fold axes and crenulations: fold axes are considered to be linear structures, crenulation lineations follow the hinge line of a fold 
• Boudinage: competant rock layers that have been stretched into segments, as they are longer in one dimension they can define a lineation. Boudins tend to occur in the limbs of a fold.
• Mullions: deformation structures that form in the interface of the competant and incompetant rock. Lineations here will form along the hinge of the mullion structures. 
• Pencil structures: occur as a result of interference between compaction cleavage and tectonic cleavage. 
• Other structural lineations include spherical features like ooids, reduction spots etc which we know the original shape of, and therefore can see the deformation that has taken place

Lineations in the brittle regime 

Some lineations occur only on fracture surfaces, characteristics of the upper crust brittle regime. These lineations form by mineral growth in extension fractures, striations carved on the walls of shear fractures or faults, by intersections between…