Structural Geology: Foliation, Lineation, Faults, and Folds
Foliation in Metamorphic Rocks
Foliation is a broad term for any planar, parallel fabric or arrangement of elongated or flaky mineral grains developed within rocks.
- Slaty Cleavage: A very fine-grained, highly pervasive planar structural fabric common in low-grade metamorphic slates, causing them to split cleanly. It usually develops parallel to the axial plane of regional folds.
- Schistosity: A coarser-grained foliation developed in higher-grade metamorphic rocks (schists) where flaky minerals like mica expand in size and separate into clear layers.
- Gneissic Banding: High-grade segregation of minerals into alternating light (quartz/feldspar) and dark (biotite/amphibole) structural bands.
- Crenulation Cleavage: Formed when a pre-existing foliation fabric is micro-folded into small asymmetric alignments by a secondary deformation event.
Lineation and Structural Fabric
Lineation refers to any penetrative or non-penetrative linear, one-dimensional structural direction or fabric element aligned throughout a rock body.
- Slickenside / Stretch Marks: Grooves or scratch lineations tracking the directional movement vectors on a fault plane or between sliding fold beds.
- Mineral Lineation: Parallel alignment of linear, needle-like minerals (such as sillimanite or hornblende) generated under metamorphic pressure.
- Stretched Boulders / Pebbles: Conglomeratic fragments mechanically deformed and drawn out along the maximum elongation axis (X).
- Mullions: Large, regular fluted columns or undulating rods formed at the interface between structurally contrasting rock layers (like sandstone and shale).
- Boudinage (Sausage Structure): Formed when a competent rock layer embedded inside a weaker matrix undergoes tension and stretches, necking down or pulling apart into distinct sausage-shaped blocks (boudins).
Faults and Fracture Mechanics
Faults are fractures along which the rock masses on either side have undergone visible relative displacement.
Fault Anatomy and Terminology
- Fault Plane: The fracture surface along which displacement occurs.
- Hanging Wall: The block of rock situated directly above an inclined fault plane.
- Footwall: The block of rock situated directly beneath an inclined fault plane.
- Upthrown / Downthrown Block: The block shifted relatively upward is upthrown; the one shifted downward is downthrown.
- Hade: The angle of inclination of the fault plane measured from a vertical line (Hade = 90° – Dip).
- Net Slip (aa’): The total absolute displacement vector measured along the fault plane.
- Throw (ac): The absolute vertical component of the net slip.
- Heave (ca’): The absolute horizontal component of the net slip.
- Slickensides: Striated, polished, and shiny surfaces on a fault plane generated by the frictional rubbing action of the blocks.
Main Types of Faults
- Strike-Slip Fault: Displacement is strictly horizontal, parallel to the strike line of the fault plane.
- Dip-Slip Fault: Displacement is strictly parallel to the dip direction of the fault plane.
- Normal Fault (Gravity Fault): The hanging wall moves relatively downward with respect to the footwall (caused by σ₁ being vertical; tensional regimes).
- Reverse Fault: The hanging wall moves relatively upward with respect to the footwall (caused by σ₁ being horizontal; compressional regimes).
- Thrust Fault: A low-angle reverse fault where the fault plane dips at less than 45°.
- Overthrust Fault: An extreme thrust fault where the plane dips at less than 10°.
Folds and Compressive Stress
Folds are wavy undulations in rock layers resulting mostly from horizontal compressive stresses.
Anatomy of a Fold
- Limbs / Flanks: The sides of the fold stretching between zones of maximum curvature.
- Hinge Line: The line connecting points of absolute maximum curvature along a folded layer.
- Axial Plane: An imaginary surface that divides the fold symmetrically, connecting the hinge lines of all sequential layers.
- Crest & Trough: The highest topographical point of a convex fold is the crest; the lowest concave point is the trough.
- Inflection Point: The point on a limb where the direction of curvature switches.
Types of Folds
- Anticline: A topographically convex structure where rock layers dip away from each other; the oldest rocks are exposed in the core.
- Syncline: A topographically concave structure where rock layers dip toward each other; the youngest rocks are exposed in the core.
- Anticlinorium / Synclinorium: A massive regional anticline or syncline that contains numerous smaller, minor secondary folds on its limbs.
- Monocline: A simple step-like local bend in an otherwise uniformly dipping or horizontal rock sequence.
- Symmetrical Fold: The axial plane is vertical, and both limbs dip at approximately the same angle in opposite directions.
- Asymmetrical Fold: The axial plane is inclined, and the two limbs dip at completely different angles.
- Isoclinal Fold: A fold where both limbs dip at the exact same angle and in a parallel direction.
- Overturned Fold: An extreme asymmetrical fold where one limb is rotated past the vertical position, becoming inverted.
- Recumbent Fold: An extreme fold where the axial plane is virtually horizontal (0°). Large regional-scale recumbent folds are known as Nappe structures.
Ramsay’s Classification
Ramsay’s classification is based on dip isogons, which are lines connecting points of equal dip on the inner and outer boundaries of a folded layer.
- Class 1: Inner arc curvature is greater than outer arc curvature (isogons converge inward).
- Class 1A: Thickness is minimized at the hinge zone.
- Class 1B (Parallel / Concentric Folds): Thickness of the layer remains completely constant throughout the structure.
- Class 2 (Similar Folds): Inner and outer arc curvatures are completely identical. Dip isogons are parallel and equal in length.
- Class 3: Outer arc curvature is greater than inner arc curvature (isogons diverge heavily toward the core).
Classification based on Inter-limb Angle: