Deformation Geology: Folds and Faults
by Owen Borville
January 30, 2021
Learning, Geology, Science
Deformation is a general term for the folding, faulting, and other processes applied to rocks and strata resulting from shear stress, compression, and extension (tension).
Types of Deformation
Stress is the force per unit area acting on a plane surface within a body. Types of stress include tensional (pulling), compressional (pushing), and shear stress.
Strain is the small changes in length and volume associated with deformation of the Earth’s crust by tectonic stresses or by the passage of seismic waves.
Compression is the stress component perpendicular to a given surface, such as a fault plane, that results from forces applied perpendicular to the surface (squeezing) or from remote forces transmitted through the surrounding rock.
Tension (tensile stress) is a type of stress that pulls rock apart in opposite directions. Tension increases the length of the rock and causes faulting, such as at spreading ridges.
Shear stress is the stress component parallel to a given surface, such as a fault plane, that results from forces applied parallel to the surface or from remote forces transmitted through the surrounding rock.
Elasticity is a property that allows a material to be deformed, or have its shape changed, but is returned to its original shape when the force applied is released. Elastic strain or elastic deformation allows a material to be returned to its original shape after the deformation force is removed. The elastic limit is the maximum amount of elastic deformation possible before permanent deformation occurs.
Plastic Strain or plastic deformation is a permanent change in the size or shape of solid rock by tectonic folding or glacial flowing without fracturing.
Fracture is any break in rock along which no significant movement has occurred. Ductile rocks exhibit a large amount of plastic deformation before fracture while brittle rocks show little or no plastic deformation before fracture.
Strike and dip are used to measure the orientation or angle of a tilted sedimentary layer. The strike is measured from a horizontal line lying within the plane of the layer. The dip is the angle between a line perpendicular to the strike line and the horizontal plane.
Folding or folds occur where originally horizontal sedimentary strata are deformed at an angle. A monocline is a regional fold with one very steep side in an area of otherwise nearly horizontal layers. An anticline is a geologic structure that forms when horizontal sedimentary strata or layers are folded during geologic processes and produce an arching shape. A syncline is an upward-curving (concave) fold in rock that resembles a trough.
The axial plane of a fold is a plane surface intersecting the fold at the hinge line of each strata or layer. The fold axis (hinge line) is the imaginary line located where the axial plane intersects the fold layer. The limbs of the fold are the two legs of folded strata joined by the hinge line.
Several types of geologic folds occur in the subsurface strata in addition to normal folds. A plunging fold is a fold in which the fold axis or hinge line is tilted at an angle or not horizontal. An inclined fold is a fold where the axial plane is inclined with respect to the surface of the Earth. An overturned fold occurs when both limbs dip in the same direction and the axial plane of the fold is tilted so that the strata on one limb of the fold becomes nearly upside down. A recumbent fold is an overturned fold with an axial plane that is nearly horizontal. Other types of folds include chevron folds, drag folds, and fault propagation folds. Chevron folds have strait limbs and sharp hinges. A fault propagation fold is a fold that occurs at the top or tip of a fault and is enlarged as the fault surface grows.
Domes are geologic structures formed by tectonic activity with the shape of an upside down bowl and are the opposite of basins. A dome can also refer to a large mass of granite where the top layers of soil and vegetation have been removed by erosion and weathering, leaving the granite exposed. A basin is a relatively large bowl shaped depression in the Earth's surface that collects sediment.
A joint is a narrow crack in rock formed by tension (pull-apart forces) along which there has been no significant parallel movement on either side of the crack. Joints are a type of permanent deformation and commonly form in parallel sets.
A fault is a break in the rock in the Earth’s crust which is caused by tectonic forces and along which rocks on either side of the break have moved past each other.
Dip-slip faults are inclined fractures where the block on the top side of the fault has shifted parallel to the fault plane. The hanging wall is the side of a dip-slip fault that slides on top of the footwall. The footwall is the bottom side of a dip-slip fault where the hanging wall rests and slides.
If the block or rock mass above a dip-slip fault moves down, the fault is termed a normal fault, whereas if the block above the fault moves up, the fault is termed a reverse fault. A thrust fault is a reverse fault with a dip of 45° or less.
Block faulting occurs when the Earth’s crust is stretched by tectonic forces, forms cracks in the crust, and causes blocks of crust to either slide down the fault or to be pushed up the fault. Block faulting causes geologic features called horst and graben. A horst is an elongate block of rock uplifted between two roughly parallel faults when tension forces are applied (pulling apart). A graben is an elongate block of rock down-dropped between roughly parallel faults when tension forces are applied (pulling apart). These grabens form valleys.
A fault scarp is a steep slope or cliff formed when movement along a fault exposes the fault surface. Slickenlines are the shiny linear lines produced by a rock mass moving across another on a fault surface or on a glacial surface. Slickensides are polished striated rock surfaces caused by one rock mass moving across another on a fault surface or on a glacial surface.
Cataclastic rock is a rock having more than 10 percent of its volume consisting of fragments bounded by fractures related to brittle deformation. Cataclastic rocks are commonly associated with faults. Fault gouge is a similar term referring to rock fragments produced by faults.
A mélange is a mixture of rocks formed by tectonic disruption, such as during multiple faulting, which brings disparate rock types together. A mélange usually consists of a matrix of weak material, like shale, with hard pieces of exotic rocks, such as gneiss or igneous rocks.
Salt domes occur when deeply buried salt rock rises toward the surface because of its lower density, deforming the rock layers above. Salt domes commonly produce petroleum reservoir traps. Cap rock is a low permeability rock layer which overlies a petroleum reservoir and blocks the flow of liquids from above and below.
An orogeny is an episode of mountain building or intense rock deformation that produces orogenic belts. Orogenies are caused by igneous activity such as plutons and batholiths and include intense deformation and metamorphism. Orogenies are also influenced by plate tectonics.
A shield is the exposed basement rock of a continent consisting of igneous and metamorphic rock which can be 2.6 to 4 billion years old. Cratons are the stable continental cores or basement rock containing older Precambrian metamorphic and igneous rock that is mostly covered by relatively thin layers of younger sedimentary rock.
by Owen Borville
January 30, 2021
Learning, Geology, Science
Deformation is a general term for the folding, faulting, and other processes applied to rocks and strata resulting from shear stress, compression, and extension (tension).
Types of Deformation
Stress is the force per unit area acting on a plane surface within a body. Types of stress include tensional (pulling), compressional (pushing), and shear stress.
Strain is the small changes in length and volume associated with deformation of the Earth’s crust by tectonic stresses or by the passage of seismic waves.
Compression is the stress component perpendicular to a given surface, such as a fault plane, that results from forces applied perpendicular to the surface (squeezing) or from remote forces transmitted through the surrounding rock.
Tension (tensile stress) is a type of stress that pulls rock apart in opposite directions. Tension increases the length of the rock and causes faulting, such as at spreading ridges.
Shear stress is the stress component parallel to a given surface, such as a fault plane, that results from forces applied parallel to the surface or from remote forces transmitted through the surrounding rock.
Elasticity is a property that allows a material to be deformed, or have its shape changed, but is returned to its original shape when the force applied is released. Elastic strain or elastic deformation allows a material to be returned to its original shape after the deformation force is removed. The elastic limit is the maximum amount of elastic deformation possible before permanent deformation occurs.
Plastic Strain or plastic deformation is a permanent change in the size or shape of solid rock by tectonic folding or glacial flowing without fracturing.
Fracture is any break in rock along which no significant movement has occurred. Ductile rocks exhibit a large amount of plastic deformation before fracture while brittle rocks show little or no plastic deformation before fracture.
Strike and dip are used to measure the orientation or angle of a tilted sedimentary layer. The strike is measured from a horizontal line lying within the plane of the layer. The dip is the angle between a line perpendicular to the strike line and the horizontal plane.
Folding or folds occur where originally horizontal sedimentary strata are deformed at an angle. A monocline is a regional fold with one very steep side in an area of otherwise nearly horizontal layers. An anticline is a geologic structure that forms when horizontal sedimentary strata or layers are folded during geologic processes and produce an arching shape. A syncline is an upward-curving (concave) fold in rock that resembles a trough.
The axial plane of a fold is a plane surface intersecting the fold at the hinge line of each strata or layer. The fold axis (hinge line) is the imaginary line located where the axial plane intersects the fold layer. The limbs of the fold are the two legs of folded strata joined by the hinge line.
Several types of geologic folds occur in the subsurface strata in addition to normal folds. A plunging fold is a fold in which the fold axis or hinge line is tilted at an angle or not horizontal. An inclined fold is a fold where the axial plane is inclined with respect to the surface of the Earth. An overturned fold occurs when both limbs dip in the same direction and the axial plane of the fold is tilted so that the strata on one limb of the fold becomes nearly upside down. A recumbent fold is an overturned fold with an axial plane that is nearly horizontal. Other types of folds include chevron folds, drag folds, and fault propagation folds. Chevron folds have strait limbs and sharp hinges. A fault propagation fold is a fold that occurs at the top or tip of a fault and is enlarged as the fault surface grows.
Domes are geologic structures formed by tectonic activity with the shape of an upside down bowl and are the opposite of basins. A dome can also refer to a large mass of granite where the top layers of soil and vegetation have been removed by erosion and weathering, leaving the granite exposed. A basin is a relatively large bowl shaped depression in the Earth's surface that collects sediment.
A joint is a narrow crack in rock formed by tension (pull-apart forces) along which there has been no significant parallel movement on either side of the crack. Joints are a type of permanent deformation and commonly form in parallel sets.
A fault is a break in the rock in the Earth’s crust which is caused by tectonic forces and along which rocks on either side of the break have moved past each other.
Dip-slip faults are inclined fractures where the block on the top side of the fault has shifted parallel to the fault plane. The hanging wall is the side of a dip-slip fault that slides on top of the footwall. The footwall is the bottom side of a dip-slip fault where the hanging wall rests and slides.
If the block or rock mass above a dip-slip fault moves down, the fault is termed a normal fault, whereas if the block above the fault moves up, the fault is termed a reverse fault. A thrust fault is a reverse fault with a dip of 45° or less.
Block faulting occurs when the Earth’s crust is stretched by tectonic forces, forms cracks in the crust, and causes blocks of crust to either slide down the fault or to be pushed up the fault. Block faulting causes geologic features called horst and graben. A horst is an elongate block of rock uplifted between two roughly parallel faults when tension forces are applied (pulling apart). A graben is an elongate block of rock down-dropped between roughly parallel faults when tension forces are applied (pulling apart). These grabens form valleys.
A fault scarp is a steep slope or cliff formed when movement along a fault exposes the fault surface. Slickenlines are the shiny linear lines produced by a rock mass moving across another on a fault surface or on a glacial surface. Slickensides are polished striated rock surfaces caused by one rock mass moving across another on a fault surface or on a glacial surface.
Cataclastic rock is a rock having more than 10 percent of its volume consisting of fragments bounded by fractures related to brittle deformation. Cataclastic rocks are commonly associated with faults. Fault gouge is a similar term referring to rock fragments produced by faults.
A mélange is a mixture of rocks formed by tectonic disruption, such as during multiple faulting, which brings disparate rock types together. A mélange usually consists of a matrix of weak material, like shale, with hard pieces of exotic rocks, such as gneiss or igneous rocks.
Salt domes occur when deeply buried salt rock rises toward the surface because of its lower density, deforming the rock layers above. Salt domes commonly produce petroleum reservoir traps. Cap rock is a low permeability rock layer which overlies a petroleum reservoir and blocks the flow of liquids from above and below.
An orogeny is an episode of mountain building or intense rock deformation that produces orogenic belts. Orogenies are caused by igneous activity such as plutons and batholiths and include intense deformation and metamorphism. Orogenies are also influenced by plate tectonics.
A shield is the exposed basement rock of a continent consisting of igneous and metamorphic rock which can be 2.6 to 4 billion years old. Cratons are the stable continental cores or basement rock containing older Precambrian metamorphic and igneous rock that is mostly covered by relatively thin layers of younger sedimentary rock.
