Structural Geology Flashcards
Active Folding
Folds form in layers of different competence due to layer parallel compression. The mechanical properties of the layers control the geometries of the folds. Also known as buckle folding.
Angular Shear Strain
The change in angles of a feature or strain in a plane.
Anticline
A fold with older rocks in its core
Antiform
A fold where the limbs close upwards
Asymmetric Folds
Folds where the limbs have different lengths.
Axial Plane
A plane that connects all the hinge lines through a fold known as the axial surface (3D term).
Axial trace
Where only an edge of the axial plane is seen, eg. when a fold is seen in cross section or on a map. The hinge line runs along the top of the axial plane and so is also an axial trace (2D term).
Anisotropic fabric
A rock having different properties in different directions eg layering.
Aseismic Slip
Continuous, steady displacement along a fault plane that does not result in significant earthquake activity.
Balanced cross section
A cross section that can be restored to a geologically reasonable state.
Basement
Deformed crystalline igneous and metamorphic rocks that have been involved in at least one orogenic event. Sedimentary rocks are generally not present in the basement.
Bedding cleavage lineation
Lineation formed on a bedding surface where the cleavage passes through it.
Bending
Occurs where layers are folded due to the movement of fault blocks. Unlike active folding the layering has no mechanical influence on the folds formed. Instead the fold shapes are controlled by the movement and geometries of fault blocks.
Blind thrust
A thrust fault that does not reach the surface or another fault. May cause folding that can be observed at the surface.
Boudin
‘Sausage-shaped’ segments of rock produced by layer parallel extension of a competent layer surrounded by less competent layers.
Boudinage
Process of formation of boudins.
Branch Line
Intersection between two faults. Usually the intersection between the floor thrust and the thrusts which branch from it.
Brittle Deformation
Occurs where a rock fractures due to applied stresses. It is localised deformation with the rocks either side of the fracture unaffected. Occurs in the upper crust where temperatures and pressures are relatively low.
Brittle-Ductile Zone
The zone in the crust where deformation style changes from brittle to ductile. Also known as brittle-plastic zone.
Buckle Fold
A fold formed by buckling or active folding. Shortening is taken up along the layers by tangential longitudinal strain.
Cataclasite
Fine-grained, cohesive fault rock, formed at depth >5km in the crust.
Chevron fold
A fold with an angular hinge and straight limbs
Cleavage
Cleavage is a secondary layering that develops in the rock as a result of deformation. It forms perpendicular to sigma one, is often associated with folding and can be localized or regional in extent.
Cleavage refraction
Change in the angle of dip of cleavage between layers of different competence.
Cover
Sedimentary rocks that overlie the basement.
Curvilinear fold
A fold with a curved hinge line. Also known as a non-cylindrical fold.
Cylindrical fold
A fold with a straight hinge line.
Damage zone
Zone of brittle deformation structures around a fault.
Decollement
A large scale fault or shear zone along a weak layer or sedimentary sequence also known as a detachment.
Deformation
The change in shape, position and/or volume of a rock in response to applied forces. It is determined by comparing the rock’s deformed and undeformed states. For most rocks the assumption is that their undeformed state was horizontal layers.
Differential Stress
Stress state where the principal stresses have different values leading to strain and deformation.
Dip
The maximum dip of a plane. Lies perpendicular to the strike.
Dip-Slip fault
A fault with displacement parallel to its dip.
Displacement
The relative offset of points once adjacent on either side of a fault.
Domino faults
A series of parallel normal faults and fault blocks which rotate during extension.
Drag zone
Zone of ductile deformation structures (usually folds) around a fault.
Ductile deformation
Rocks change shape smoothly, without breaking, in response to compression. This shape change is pervasive - it affects all of the rock.
Duplex
A series of thrusts and horses linked by a floor and roof thrust.
Elongation
The measurement of change in length of a line
Emergent thrust
A thrust fault that reaches the surface rather than detaching on to a roof thrust
Enhanced (engineered) geothermal systems
EGS creates/enhances natural permeability in hot dry rocks. Water is injected at high pressures enhancing the rocks natural permeability by triggering movement on small scale shear fractures.
Enveloping surface
An enveloping surface is constructed for a single layer. The antiform surface connects the tops of the folds and the synform surface connects the trough. An enveloping surface is like the bread for a fold sandwich.
Facing
The direction of younging along the fold axial plane.
Failed rift
A rift where extension ceased before formation of oceanic crust.
Fault
A planar surface across which displacement has occurred.
Fault bend fold
A fold formed in the hanging - wall of a thrust above a ramp.
Fault Breccia
Incohesive fault rock with <30% clay matrix.
Fault gouge
Incohesive fault rick with >30% fine grained material.
Fault plane solution.
Stereonet plot of the first p-Wave arrival, which represents the sense of displacement along a fault that generates an earthquake. Also known as a focal mechanism or beach ball plot.
Fault propagation fold
A fold formed in front of a propagating thrust fault.
Fault valve behaviour
The cyclic process where the build-up of fluid pressure due to migration of fluids into a sealed fault zone causes the fault to rupture. The accompanying drop in pressure causes the minerals in the fluids to precipitate out re-sealing the fault.
Fissure
Extension fracture filled with a gas or fluid.
Flat
The section of a thrust fault that runs parallel to bedding.
Flexural flow
Folding is accommodated by shearing within the beds in the limbs of a fold. Shearing dies out towards the hinge.
Flexural slip
Folding is accommodated by slip along bedding in the limbs of a fold. Slip dies out towards hinge.
Floor thrust
A low angle thrust that forms the base of a duplex or imbricate fault system.
Flower structure
Upward branching of a strike - slip fault as seen in cross section.
Fold and thrust belt
A region where crustal shortening is accommodated by thrust faults and associated folding.
Fault breccia/gouge
Forms where original rock is ground down by movement along the fault.
Cataclasites
Form by brittle deformation at the grain-scale and involve no loss cohesion at the macroscopic scale.
Pseudotachylytes
Form by localised melting along the fault plane.
Fold axis
An imaginary line that lies normal to the profile plane and so plots as its pole on a stereonet. It is an approximation of the hinge line and is used to estimate the position of the axial plane on a stereonet.
Footwall
Fault block beneath the fault surface.
Footwall cut-off
Where a layer of feature in the footwall is cut by the fault.
Foreland
The frontal part of the orogenic zone dominated by thin skinned deformation and with little/no metamorphism
Graben
Low ground bound by two parallel normal faults which dip towards the graben. The graben forms hanging wall of the normal faults.
Growth fault
A fault that develops and moves during sedimentation. A wedge shaped pattern of sediments forms against the fault. The amount of offset of the sediments decreases with time
Hanging wall
Fault block above the fault surface
Hanging wall cut off
Where a layer of feature in the hanging wall is cut by the fault.
Heave
Horizontal component of offset along a fault.
Heterogeneous Strain
Strain varies across a rock body - different parts of an object deform by different amounts.
Hinge
Point of maximum curvature on a fold (2D term)
Hinge collapse
The squeezing together of competent layers within the hinge zone of a chevron fold due to space problems with continued shortening.
Hinge line
Line of maximum curvature along a fold (3D term)
Hinterland
The more internal parts of the orogenic zone dominated by thick-skinned deformation.
Homogeneous Strain
Strain is constant across a rock body - different parts of an object deform by same amounts
Horse
A fault bound block within a duplex
Horst
High ground bound by two parallel normal faults which dip away from the horst. The horst forms the footwall of the normal faults.
Hydraulic Fracturing
Fluids are pumped at high pressure into gas-rich impermeable rocks to induce fracturing. Sand-sized particles in the fluids hold the fractures open thus creating permeability.
Hydrostatic Stress
Stress state where all the stresses (including the principal stresses) have the same value.
Imbricate fan
A series of thrust faults extending from a floor thrust to the surface
Induced seismicity
Seismicity caused by human activity altering the stress states in the crust.
Inclined fold
A fold with an inclined axial plane.
Interlimb angle
The angle between the two limbs of a fold. The smaller the interlimb angle the more tightly folded the rocks and the greater the amount of deformation.
Isotropic fabric
A rock having the same properties in all directions e.g. random distributions of grains in an igneous intrusion
Joint
Extension fracture with little/no displacement across it.
Kink bands
Rotated segment of well layered rocks which form asymmetric folds with sharp boundaries and angular hinges.
Limb
Bed segments between hinges.
Listric fault
A fault whose dip decreases with depth
Nodal Planes
Orthogonal great circles on a fault plane solution that separate the areas of compressional and tensional first motions. One of which represents the fault plane, although it is not possible to tell which from the fault plane solution alone.
Normal fault
A fault where the hanging-wall has moved down relative to the footwall.
Normal stress
Sigma n, Stress oriented normal to a surface such as bedding
Oblique slip
Displacement has a strike slip and a dip slip component
P-Waves
Compressional seismic waves usually generated by an earthquake. P-waves are the fastest seismic waves and so the first type of wave to arrive at a seismic station. P-waves are non-destructive seismic waves and their detection can be used to give up to 90 seconds warning of an earthquake
Passive Folding
Layers act as passive markers and have no mechanical influence on the geometry of the folds formed
Passive margin
The zone of transition between continental and oceanic crust on the edge of a former rift zone. The Atlantic coastlines of South America and Africa are passive margins from the opening of the Atlantic.
Pericline
Double plunging fold
Pi plot
A plot of poles to bedding readings for folds formed in the same stress regime fall on a great circle – best fit great circle.
Pin line
A line on the section where no deformation has occurred, usually at the edge of the foreland, and is used as a fixed point during restoration.
Plunging fold
A fold with a non horizontal hinge line
Pole
An imaginary line at perpendicular to a plane
Pressure solution
Pressure solution occurs where soluble minerals go into solution in areas of high pressure and are re-precipitated in areas of lower pressure.
Pressure solution cleavage
Cleavage formed by pressure solution alone
Principal stresses
The three normal stresses acting on the different sides of a cube and, therefore, orthogonal to each other.
Profile Plane
The profile plane is the true cross section through a fold. It is perpendicular to the fold axial plane.
Pseudotachylyte
Glassy fault rock produced by frictional melting along a fault
Pull apart basin
A localised basin formed along a releasing bend in a strike-slip zone.
Ramp
A thrust fault that cuts across bedding usually connecting two flats
Recumbent fold
A fold with horizontal axial plane.
Releasing bend
A bend in a strike-slip zone where displacement along the fault results in an area of extension
Restored cross section
A cross section that shows the original, pre-deformation geology.
Restraining bend
A bend in a strike slip zone where displacement along the fault results in an area of compression
reverse fault
A fault where the hanging wall has moved up relative to the footwall
Rift valley
A linear valley produced by crustal extension. The ideal geometry is a central graben (valley) flanked by domino faults which dip towards the valley
Rigid body strain
Rigid body strain occurs where a rock mass moves or rotates with no change of shape. Only detectable if there is an external reference frame
Roll over anticline
Anticline in the hanging wall of a listric normal fault formed by the movement along the fault.
Roof thrust
A low angle thrust along the top of a duplex
Rotation
Rigid rotation occurs when every point of a rock body undergoes the same displacement and rotation with no change in shape. Only detectable if there is an external reference frame
Sealing fault
A fault along which fluid cannot flow – an impermeable fault.
Sedimentary basin
A crustal depression in which sediments accumulate
Seismogenic zone
The zone in the crust between about 2 and 15km depth where the crust is strongest and most earthquakes occur.
Shear fracture
A fracture with displacement parallel to the fracture surface and
offset of layers across the fracture (a fault is a series of linked fractures).
Shear stress
σs (sigma s): Stress oriented parallel to a surface such as bedding.
Shear zone
Equivalent of a fault at depth. A narrow band of ductile deformation produced by shearing.
Sheath fold
Curvilinear folds formed in zones of high shear strain. Hinge lines rotate to parallel with direction of movement.
Similar folds
Folds formed by flexural flow, where the outer and inner arcs of the hinge zone have the same shape and where the limbs have thinned to accommodate this. Also known as shear folds.
Slickenfibres
Fibrous mineral growth along a fault plane
Slickenslides
Lineations on a fault plane indicating the last direction of movement.
Slip surface
Fault surface along which slip movement is concentrated. Often surrounded by a damage or drag zone. Also known as a slip plane.
Stereonet
A lower hemisphere graph on which three-dimensional data are plotted in two dimensions as great circles and two-dimensional data are plotted as one dimensional points.
Stick slip
Sudden movement along a fault plane generating an earthquake, followed and preceded by a period of quiescence
Strain
The permanent change in shape (in 1D, 2D or 3D) of a rock body as a result of deformation.
Strain Ellipse
The strain ellipse is a method of representing the amount of strain a rock has undergone using an initially circular marker. The axes of the ellipse are approximately the same as the directions of compression and extension.
Stress
Pair of equal and opposite forces acting on a unit area of a rock body.
Strike
The line of the horizontal on a plane
Strike slip fault
A fault with horizontal (along strike) displacement
Structural Geology
The study of how rocks deform and the processes of deformation.
Stylolites
Tooth-or-saw like surface (often a bedding plane) along which clay minerals and other insolubles are concentrated due to calcite dissolution
Symmetric Folds
Folds where the limbs have the same length.
Syncline
A fold with younger rocks at in the core
Synform
A fold where the limbs close downwards.
Tangential Longitudinal Strain
Shortening is accomodated by deformation in the hinge zone of the fold with no deformation along the limbs.
Tension fracture
Extensional fracture formed under tension
Tensile strength
The amount of tensile stress (extension) a rock can endure without failing. Depends on composition, temperature and pre-existing weaknesses
Thick-skinned deformation
Deformation involving both sedimentary cover and basement rocks
Thin skinned deformation
Deformation involving only sedimentary cover rocks.
Throw
Vertical component of offset along a fault
Thrust fault
Low angle reverse fault
Thrust nappe
Large-scale imbricate or duplex thrust system associated with thick-skinned deformation
Transfer fault
Transfer faults transfer displacement from one fault to another and can form at any scale.
Transform fault
Large-scale strike-slip fault that forms a plate boundary or transfer displacement from one plate boundary to another.
Translation
Rigid translation occurs when every point of a rock body undergoes the same displacement and there is no change of shape. Only detectable if there is an external reference frame
Transpression
Combination of strike-slip faulting and compression. Can occur at any scale but common along destructive plate boundaries.
Transtension
Combination of strike-slip faulting and extension
Upright fold
A fold with a vertical axial plane
Vein
Mineral-filled extension fracture.
Vergence
The orientation of a minor fold axial plane relative to the enveloping surface. Minor folds on the limbs of a major fold will verge towards the hinge zone of the major antiform. Minor folds in the hinge zone of a major fold have neutral vergence as their axial planes are perpendicular to the enveloping surface.
Slickensides
Are created by the two fault blocks polishing, abraiding and gouging the fault plane as they move. They give the direction of the last movement along a fault.
Slickenfibres
Are small voids that open along fault plane and info which new minerals are precipitated. These new minerals are fibrous and their direction of growth give the last relative slip direction.
Normal faults
Are dip slip faults where the hanging wall moves down relative to the foot wall. The usually form at a dip of 50-60 degrees.
Thrust faults
Dip slip faults where the hanging wall moves up the relative to the foot wall placing older rocks on top of younger. They can dip between 0-30 degrees often running parallel to bedding, cutting up through a layer then running parallel to bedding again.
Strike slip faults
Form where the fault blocks move parallel to each other along strike. They usually form close to vertical. Where the fault blocks move to the left relative to each other’s the fault is known as sinistral. Where the fault blocks move to the right relative to each other it is known as dexteal.
