Glossary of terms

Acicular

Needle shaped (of mineral grains)

Acive continental margin

A transition or boundary between continental and oceanic lithosphere that coincides with a plate boundary (ridge, trench or transform fault).

Allochthon

A body of rock that has been moved from its original position, usually in the hanging wall of a thrust fault

Allochthonous

Moved from its original position, usually in the hanging wall of a thrust fault

Angular unconformity

An unconformity characterised by an angular discordance (a difference of strike or dip or both) between older strata below and younger strata above.

Anticline

A fold in which the younging direction is away from the inside of the fold.

Antiform

A fold where the limbs dip away from the hinge so that the fold closes upward.

Antitaxial vein

A vein filled with mineral fibres, in which the fibres have grown outwards, toward the walls.

Antithetic Riedel shears (R' shears)

Subsidiary fractures that form at about 75° to a fault, with sense of displacement opposite to that of the fault.

Apparent dip

The apparent dip of a surface is its dip measured on a cross-section that is not perpendicular to the strike of the surface. Apparent dip is always less than true dip.

Arc construction

Busk construction

Attitude

Orientation

Autochthon (autochthonous)

A body of rock that has not been moved from its original position, usually in the footwall of a thrust fault

Axial planar foliation

Planar fabric that is approximately parallel to the axial surface of a fold.

Axial surface (hinge surface)

Imaginary surface that passes through all the hinge lines of a fold

Azimuth

A bearing measured clockwise from north from 0 - 360°.

Bar

A force concentration of one million dynes per square centimetre, or 100,000 Pa

Bathymetric contour

Contours that show the depth of the sea floor below sea level .

Bearing

A direction relative to north.

Beds

Sedimentary layers thicker than 1 cm.

Bioturbation structures (trace fossils)

Structures produced by organisms that have disturbed sediment.

Boudin

A structure formed by a layer that has separated into pieces, typically as a result of extension.

Boudinage (pinch-and-swell structure)

The process of formation of boudins.

Branch point

A point where one fault meets and joins another.

Breccia

A rock composed of typically angular fragments, typically from a fault. (The term breccia is also sometimes used in sedimentary geology for an angular conglomerate.)

Brittle fracture (brittle failure)

Non-recoverable deformation characterized by loss of strength across a surface.

Busk construction

A construction for folds in profile view in which the traces of surfaces are assumed to be concentric arcs of circles. Also known as the arc construction.

C-plane

A surface within a shear zone that is characteristically parallel to the zone boundaries and which shows the most intense deformation; from the French word "cisaillement" meaning shearing.

C-prime (C') foliation

Shear bands that form later in a history of a shear zone after a strong foliation has been developed throughout. They are typically oriented oblique to the shear zone boundaries, with the same sense of rotation (clockwise or counterclockwise) as the overall shear zone.

C-S foliation

Foliation in a shear zone characterized by intensely strained zones roughly parallel to the shear zone (C planes) and less strained zones where the foliation is oblique (S-planes).

Cataclasite

Fragmented material in a fault zone that is sand-sized or smaller.

Chill zone

Typically finer-grained material found near the edge of an igneous intrusion, typically formed by rapid coolling

Chocolate tablet structure

Boudins that are roughly equant in plan view, produced by extension of a layer in all directions

Cleavage refraction

A change in the orientation of cleavage caused by differences in the mechanical properties of layers. Cleavage typically bends so that it is more perpendicular to competent layers, and more parallel to incompetent layers.

clinometer
Clinometer (inclinometer)

A device that measures inclinations.

Coaxial deformation

Deformation in which the strain axes remain fixed to the rock material throughout deformation

Coesite

A monoclinic form of silica in which the silica tetrahedra are linked in a manner similar to feldspars. It requires pressures above 2 GPa to form.

Columnar joints

Joints that separate elongated bodies of rock that are typically polygonal (roughly hexagonal) in cross-section. These typically form perpendicular to the base and top of an igneous flow or sill.

Compaction

Volume loss due to the expulsion of water as rocks are buried.

Compass

A tool which uses the Earth's magnetic field to measure directions parallel to the Earth's surface.

Competence

A general term for resistance to stress.

Complex crater

A crater that show a central uplift and a terraced outer wall.

Compromise boundaries

Approximately planar boundaries produced when mineral grains grow against one another. Compromise boundaries are not controlled by the crystal structure of either mineral grain.

Concentric joints

Joints parallel to the surfaces of an intrusion.

Conjugate shear fractures

Fractures formed when when rocks fail simultaneously along two families of planes approximately 60° apart in orientation.

Contour

A curving line on a map that separates higher values of some quantity from lower values.

Contour interval .

The numerical difference in value between contours on a map

Contour spacing

The horizontal distance between contours recorded on a map.

Convolute lamination

A soft-sediment deformation structure in which laminae are deformed into complex chaotic folds.

Core zone

The central part of a fault, in which brittle deformation has destroyed the continuity of older structures.

Crater

A roughly circular depression formed by a meteorite impact or volcanic activity.

Crenulation cleavage

Planar fabric represented by closely spaced fold axial surfaces. Almost always a second or later generation of fabric.

Crenulation lineation

Linear fabric represented by closely spaced fold hinges.

Crest point

The highest point on the cross-section of an antiform.

Crest surface

A surface containing multiple fold crest lines.

Cross bedding

Cross stratification caused by dunes.

Cross lamination

Cross stratification produced by ripples.

Cross stratification

A configuration of laminae produced by migration of bedforms during sedimentation, in which laminae are oriented at an angle to the depositional horizontal.

Culmination point

The highest point on the crest line of a fold.

Cutoff line

The line where a fault intersects an older surface.

Cutoff point

The point on a cross-section where the trace of a fault cuts the trace of an older surface.

Cylindrical fold

A fold with a well defined axis that is everywhere parallel to the folded surface. Parallel cross-sections cut in multiple positions through the fold appear identical to each other.

Damage zone

The outer part of a fault, in which remnants of pre-existing structures can still be traced.

Décollement

A very extensive flat in on a thrust fault, where there has been a large amount of movement.

Deformation

The movements of parts of the Earth's crust relative to each other.

Delta pyroclast

A prophyroclast around which foliation curves in a tightly curved shape, formed as the porphyroclast and adjacent foliated rock are rotated in a shear zone.

Depression point

The lowest point on the trough line of a synform.

Descriptive (geometric) analysis

Analysis comprising records of the positions, orientations, sizes, and shapes of structures that exist in the Earth's crust at the present day.

Detachment fold

A fold produced when wall rocks accommodate variations in slip between different parts of a fault.

Deviatoric stress

The state of stress obtained when the mean stress is subtracted from each of the principal stresses. The deviatoric stress is the part of the stress system that acts to change shape.

Dextral

Right-lateral movement: relative to an observer looking toward a fault or shear zone, the far side appears to have moved right.

diaplectic glass

Glass generated by the action of a shockwave on common minerals.

Differential stress

The difference between the largest and smallest principal stress.

Differentiated crenulation cleavage

Crenulation cleavage combined with pressure solution cleavage, producing a foliation that is characterized both by folds and by parallel domains of different composition.

dike

Dyke (U.S. spelling)

Dilation

Change in size (area or volume)

Dip

The value of the inclination of a plane. Represents the inclination of steepest line that can be drawn on the plane.

Dip direction

The azimuth of the steepest inclined line that can be drawn on the plane.

Dip separation

The distance along a fault, measured in the direction of fault dip, between two cutoff lines of the same severed surface.

Dip-slip fault

A fault in which the component of slip parallel to the fault dip exceeds that parallel to the fault strike.

Disconformity

An unconformity between strata of different ages but identical orientation.

Discordance

A difference of orientation between planes: difference of strike or dip or both.

Disharmonic fold

A fold where the hinges and limbs do not match with those in adjacent layers..

Dislocations

A linear defect in the structure of a crystal where the atoms of the crystal structure are out of alignment.

Distortion

Change in shape

Domain

A region within a rock that has a distinctive composition or texture.

Drag fold

A fold adjacent to a fault in which layers are bent in the direction of movement of the opposite wall. Most 'drag' folds probably form during fault propagation and not by 'drag' after the fault has formed.

Ductile

Flow of material in the solid state, typically without fracture.

Duplex

A structure in which imbricate inclined thrust faults merge into flat faults both upward and downward.

Dyke

A minor intrusion where magma has filled a crack that is discordant to layers in the surrounding rocks. (Spelled 'Dike' in the U.S.A.)

Dynamic analysis

Structural methods that involve the determination of force, stress, strength, or energy involved in deformation.

Dynamic recrystallisation

Breakdown of original mineral grains and growth of new mineral grains as a result of extreme ductile deformation.

Earthquake

Rapid relative motion of parts of the solid Earth due to brittle failure on a fault.

Effective stress

Overall stress minus the fluid pressure.

Elastic

A stress-strain relationship in which stress and strain are proportional, and the strain is recoverable.

En echelon

An arrangement of linear or planar structures where each structure offset from adjacent structures in a consistent sense that is oblique to the structures themselves. (Literally: as a squadron of cavalry.)

Endocontact

The region within an intrusion where it is affected by contact with the host rock.

Enveloping surface

A surface tangent to multiple folds in a layer.

Equal angle projection

A spherical projection in which angles are preserved. Also known as a sterographic projection. Great and small circles project as circular arcs. Constructed using a Wulff net.

Equal area projection

A spherical projection in which area is preserved.Great and small circles project as complex, non-circular arcs. Constructed using a Schmidt net.

Exocontact

The region in the host rock (country rock) affected by contact with an intrusion.

Extension

A measure of longitudinal strain equal to the fractional change in length.

Extension fracture

A fracture where rock masses on either side of a fracture have moved apart slightly, without significant movement parallel to the fracture.

Extrapolation

The process of estimating values outside the range of a data set.

Fabric

Any penetrative structure that gives a rock different properties in one direction relative to another.

Fabric element

Features within a rock that are aligned to give the rock a fabric.

Facing direction

Direction of younging of strata in a fold axial surface.

Fault

A fracture showing significant displacement of one wall relative to the other, parallel to the fracture plane.

Fault bend fold

A fold produced by movement of a curved fault, in which movement of the fault caused bending of the hanging wall, footwall, or both.

Fault cutoff

Intersection line between a fault plane and an older planar rock unit.

Fault propagation fold

A fold that forms at the propagating tip of a fault, marking a region where there are rapid changes in both the dip and the slip of the fault.

Fault regime

A tectonic environment in the Earth's upper crust characterized by the orientation of the principal stresses.

Fault scarp

A steep topographic slope marking a fault, where one wall of the fault forms higher ground than the other.

Fault tip

The linear boundary at the edge of a fault surface, separating faulted from unfaulted rock.

fault zone

A set of sub-parallel faults that may branch and joint along strike or down dip.

Fenster (tectonic window)

An area of footwall on a geologic map that is entirely surrounded by hanging wall.

Fibrous vein fill

Mineral grains within a vein that are strongly elongated, typically tracking the direction of movement as the fracture opened.

Finite deformation

The total deformation that a part of the Earth has undergone, between its original configuration and the present-day state.

Finite strain

The total strain that a part of the Earth has undergone, between its original configuration and the present-day state.

Fissility

The property of splitting easily parallel to a plane, usually the result of sedimentary compaction.

Flame structure

A narrow, pointed soft-sediment deformation structure consisting of mud forced up into overlying sand.

Flat

In fault terminology, a region where a fault is parallel to layers in the wall rocks.

Flattening foliation

A foliation defined by tabular domains, produced by shortening.

Floor thrust

The lower flat of a thrust duplex structure.

Fluidise (Fluidize)

A process whereby previously stable sediment is behaves as a fluid when upward-flowing water passes through it from below.

Flute

A scoop-like depression in mud that becomes less distinct in a down-current direction.

Flute cast (Flute mold)

A sedimentary structure formed when a flute is filled by sand and preserved in relief on the base of a sandstone bed.

Fold axis

The orientation of a line that is everywhere parallel to a folded surface. The fold axis is the direction of the hinge, crest, trough, and inflection lines.

Fold hinge

A line of maximum (tightest) curvature on a folded surface.

Fold interference pattern

A pattern formed by layers that have been folded twice during their deformation history.

Folding line construction

A construction in which differently dipping surfaces are rotated about their line of intersection in order to represent them on a plane sheet of paper.

Foliation

Planar fabric

Footwall

The body of rock directly below a fracture.

Foreland basin

A sedimentary basin that forms adjacent to an orogen, and which deepens toward the orogen, typically formed by bending of the lithosphere in response to the weight of the growing orogen.

Formation

The primary unit of mapping in stratified rock; must be mappable, defined by lithological characteristics, have a type section, and be named for a place or geographical feature.

Fracture

A surface produced by brittle failure at some point in the history of a rock.

Fracture tip

The linear boundary at the edge of a fracture surface, separating fractured from unfractured rock.

Geologic map

A map showing the extent of, and boundaries between, different units of rock.

Geological boundary

A surface in 3D space, or a line on a 2D map, where one type of rock contacts another.

Glacial striae

Lineation on the Earth's surface formed by abrasion during flow of ice, which reveals the direction of ice movement.

Gneissic banding

Foliation characterised by layered domains of different composition, and the parallel preferred orientation of minerals, typicall of high-grade metamorphic rocks.

Gouge

Clay-rich fault material, typically produced by faulting of fine-grained sedimentary rocks.

Graben

A block between two normal faults that dip towards each other. The rocks in a graben are offset downward relative to the rocks on either side.

Graded bed

A bed, usually with a sharp base, in which grain-size becomes finer toward the top.

Gravity regime

A state of stress in which the maximum principal stress is vertical. Normal faults are common.

Great circle

A circular line that divides a sphere into two precisely equal parts.

Groove

A sedimentary structure formed when an object is dragged by a current across a sediment surface.

Groove cast (Groove mold)

A sedimentary structure formed when a groove is filled by sand and preserved as a mold on the base a bed.

Group

A lithostratigraphic unit consisting of several formations.

Hackles

Feather-like striae radiating from a central point or line on a fracture. Part of plume structure.

Half-graben

A tilted block bounded by a normal fault on one side, along which the block is offset downward.

Hanging wall

The body of rock immediately above a fracture.

Harmonic folds

Folds in which each layer is folded in step with the adjacent layers, so that hinge points can be matched between layers

Heave

The horizontal component of dip separation.

Heterogeneous strain

Strain that is not the same everywhere within a rock. Straight lines may become bent and parallel lines may becomeg non-parallel.

Hinge

A line or point of maximum (tightest) curvature on a folded surface.

Hinterland

The region adjacent to a thrust belt away from which the thrust sheets appear to have moved.

Homogeneous strain

Strain that is the same everywhere within a rock. Straight lines remains straight, parallel lines remain parallel.

Horst

A block between two normal faults that dip away from each other. The rocks in a graben are offset upward relative to the rocks on either side.

Ideal plastic deformation

A ductile behaviour in which a material shows no deformation a certain stress (yield stress) is reached, and then deforms rapidly so as to prevent the stress from rising futher.

Imbricate fan

A configuration of multiple listric faults that branch upward from a single flat.

Impact breccia

Breccia resulting from a meteorite striking the Earth.

Inclination

An angle of slope measured downward relative to horizontal.

Incremental strain

The strain that occurred during a small interval in the history of deformation.

Infinitesimal strain

The strain that was occurring a single instant during the history of deformation. (The limit of incremental deformation as the time interval tends to zero.)

Inflection line

A line on a folded surface where the curvature is zero, typically marking a change from convex-up to convex-down, or from antiform to synform.

Inflection surface

A surface containing multiple inflection lines.

Inlier

A region on a geologuc map where older strata are surrounded by younger strata.

Interpolation

The process of estimating intermediate values between the points in a sparse set of data.

Intersection lineation

A lineation defined by the intersection of two differently-oriented foliations.

Invariant feature

A feature that is independent of fold orientation.

Inverse grading

Grain-size variation within a bed in which coarser grains are concentrated at the top.

Isobar

A contour line separating higher pressures from lower; a line joining points of equal pressure.

Isopach

A contour line based on the thickness of a stratified rock unit, separating a thicker from a thiner part of a layer; a line joining points of equal thickness.

Joint

A fracture where the movement parallel to the failure surface is minimal.

Joint set

Many joints in approximately the same orientation.

Joint system

A combination of joint sets, cross-cutting each other in a regular way.

Kinematics

The study of how parts of the Earth moved over geologic time. Include changes in position, orientation, size, and shape that occurred between the formation of the rocks and their present day configuration.

Kink construction

A construction for folds in profile view, in which fold limbs are assumed to be perfectly planar, parallel surfaces, and hinges are perfectly angular

Klippe

A region on a geologic map where the hanging wall of a fault is completely surrounded by footwall.

Lamina (laminae)

A layer thinner than 1 cm; plural "laminae".

Layers

Domains that are very extensive and parallel-sided.

Lineation

Linear fabric; fabric defined by elements that are parallel to a line in space.

Liquidize, liquidise

A process whereby previously deposited sediment behaves as a liquid, when the sand grains become separated by fluid

Listric

Describes a fault with dip that decreases with depth.

Lithosphere

The rigid outer part of the Earth that is divided into plates. Comprises the crust and the uppermost part of the mantle.

Lithostatic stress

A state of stress in which all principal stresses are equal, due to the weight of overlying rock.

Lithostratigraphic unit

A named rock layer recognized on the base of its lithological characteristics.

Load structure (load cast)

A soft-sediment deformation structure caracterized by rounded bulges, typically on the base of a sandstone bed.

LS tectonites

Rocks with a strong tectonic lineation that lies in the plane of a strong tectonic foliation.

Macroscopic (map-scale) structures

Structures that are too big to see in one view. Must be mapped, or imaged from an aircraft or satellite.

Magnetic contour

A contour line based on characteristics of the Earth's magnetic field; a line that separates stronger magnetic fields from weaker.

Magnetic declination

The azimuth of the Earth's magnetic field.

Map-scale (macroscopic) structures

Structures that are too big to see in one view. Must be mapped, or imaged from an aircraft or satellite.

Mapping

The process of recording and interpreting data on a two-dimensional plan view such as a topographic base map.

Mean stress

The average of the three principal stresses.

Member

A smaller mappable unit  recognized within a formation.

Mesoscopic (outcrop-scale) structure

Structures visible in one view at the Earth's surface without optical assistance.

Metamorphic aureole

Baked zone around an intrusion that is often recognizable from changes in texture or mineralogy.

Microbreccia

Fine-grained breccia; breccia with particles 2-4 mm in diameter.

Microscopic structure

Structure that requires optical assistance to be visible.

Mineral lineation

Linear fabric defined by parallel alignment of acicular mineral grains; linear equivalent of slaty cleavage and schistosity.

Mortar structure

Microscopic structure characterized by rims of fine-grained recrystallised material surrounding remnants of original crystals.

Mudcracks

Thin fractures on the surface of beds that thin downward, form by shrinkage of mud as it dries.

Mylonite

Fine-grained rock formed through ductile shearing and dynamic recrystallisation in a shear zone.

Mylonitic lineation

Stretching lineation formed during extreme ductile shearing.

Natural scale

The geometry of a cross-section when the vertical and horizontal scales are equal.

Necking

Process whereby layers start to thin at points of weakness during extension.

Negative flower structure

A zone of faults that steeped downwards and merge into a single fault or shear zone at depth, in which central blocks are offset downwards; typical of transtensional and strike-slip faults.

Neotectonics

The study of recent fault movements.

Newtonian (viscous) deformation

Deformation in which the strain rate is proportional to the stress.

Non-coaxial deformation

Deformation in which the strain axes vary in orientation relative to the rock material over time.

Non-cylindrical fold

A fold in which there is no line that lies parallel to all parts of the surface, so there is no fold axis. Cross-sections intersecting the fold at different points show contrasting geometries.

Non-penetrative fabric

A fabric that is not present everywhere in the rock, when observed at a given scale. Individual fabric planes or lines can be counted and the spaces between them can be discerned.

Non-rigid deformation

Dilation and distortion

Nonconformity

An unconformity formed by the contact between younger sedimentary strata deposited upon an eroded surface of older crystalline rock.

Normal stress

The part of the stress (or traction) on a plane that acts perpendicular to the plane.

Oblate

The shape of an ellipsoid with one axis much shorter than the other two. Informally, a "pancake".

Oblique-slip fault

A fault that shows both significant dip slip and significant strike-slip.

Offset

Difference in the cutoff location of a surface on either side of a fault; an informal term for separation.

Onlap

The relationship of successively younger beds that extend farther geographically onto an unconformity surface, generally produced by progressive burial of topography during transgression.

Orientation

The angles between a structure and a frame of reference (typically defined by the north and vertical directions)

Orogenic belt

Regions where the Earth's lithosphere has undergone shortening as a result of plate movements; usually mountainous, or mountainous in the past.

Orthographic projection

A projection method in which structures are projected perpendicularly onto a sheet of paper, often accompanied by folding line constructions.

Outcrop

A region of exposed rock at the present-day erosion surface.

Outcrop map

A map on which observations of exposed rock types and structures are recorded.

Outcrop-scale (Mesoscopic) structure

Structure visible in one view at the Earth's surface without optical assistance.

Outlier

A region on a geologic map where younger strata are completely surrounded by older strata.

Overstep

A relationship involving strata below an unconformity, that describes the way the younger succession rests on a variety of units in the lower succession.

Paleogeologic map

A map of subcrop units below and unconformity; the geologic map that would be produced if all rocks above an unconformity were stripped away.

Paraconformity

A disconformity where the only evidence for a time-gap is from detailed paleontological investigation.

Parallel fold

A fold in which the thickness of a layer (measured perpendicular to the layer boundaries) is constant.

Pascal

A force concentration of 1 Newton per square metre

Passive continental margin

A transition or boundary between continental and oceanic lithosphere that does not coincide with a plate boundary.

Penetrative fabric

A fabric that is present everywhere in the rock, as far as can be observed at a given scale. Individual fabric planes or lines cannot be counted.

Phyllitic foliation

Penetrative foliation intermediate between slaty cleavage and schistosity. There is no universal agreement on the definition, but typically defined by mineral grains between 0.1 and 1 mm diameter.

Piercing point

The point where an older linear feature intersects a fault.

Pillows

Balloon-like structures formed by rapid chilling of lava erupted under water.

Pitch (rake)

Orientation of a line that lies in a plane, measured from the strike direction, within the plane.

Planar surface

A surface with constant orientation (strike and dip); contours are equally spaced, parallel, straight lines.

Plate tectonics

A theory describing the large-scale movements of the the lithosphere using simple mathematical and geometrical methods to describe the movement of rigid plates.

Plume (Plumose) structure

Feather-like and concentric markings on joint surfaces produced during fracture propagation.

Plunge

Value of inclination of a line, measured downward relative to horizontal.

Pole to plane

A line that is perpendicular to a given plane.

Porphyroblast

A large mineral grain in a metamorphic rock that has grown during metamorphism to be larger than the surrounding material.

Porphyroclast

A large mineral grain in a metamorphic rock that is surrounded by finer-grained material produced by grain-size reduction during deformation.

Positive flower structure

An array of downward-steepening faults with similar strike, in which central blocks are offset upwards relative to the surrounding area, typical of transpressional and strike-slip faults.

Pressure

The state of stress in a stationary fluid, or the mean stress.

Pressure solution

A phenomenon whereby minerals in a rock are selectively dissolved in response to stress.

Pressure solution cleavage

A fabric defined by planar domains that originated as a result of pressure solution.

Primary fabric

A fabric that originated as the rock is formed.

Primary structure

A geologic structure formed at the same time as the rock in whichit is found.

Primitive

The outer circle on a stereographic projection. Represents a horizontal plane.

Principal planes of stress

Three mutually perpendicular planes that experience no shear stress.

Principal stresses

Three normal stresses acting along the poles to the principal planes of stress

Principle of Original Horizontality

The principle that most stratified rocks were deposited in layers approximately parallel to the Earth's surface.

Principle of Superposition

The principle that stratified rocks form with the oldest layers at the bottom, and youngest at the top.

Profile plane

The plane perpendicular to a fold axis.

Progressive simple shear

A type of noncoaxial flow in which a single plane (the shear plane) undergoes no rotation, dilation, or distortion; typical of shear zones.

prolate

The shape of an ellipsoid with one axis much longer than the other two. Informally, a "cigar".

Protolith

The original rock from which a metamorphic rock was formed by the action of heat, pressure, and deformation.

Pseudotachylite

Material melted by heating during fault movement. Is typically dark and very fine grained or glassy.

Pull apart basin

A localised subsiding area formed at a releasing bend on a strike-slip fault.

Pure shear

A special type of coaxial deformation in which there is no dilation and in which the intermediate strain axis stays the same length.

pure strain

Coaxial deformation: deformation in which the rock material is not rotated relative to the strain axes.

Quadrants

A method of measuring bearings, popular in the United States, where angles are specified clockwise or counterclockwise from N or S, towards either E or W. E.g. S37E

R'-shears

Antithetic Riedel shears.

R-shears

Synthetic Riedel shears.

Radial joint

Joint formed in outside an igneous intrusion. Orientation is approximately perpendicular to the boundary of the intrusion.

Rake (pitch)

Orientation of a line that lies in a plane, measured from the strike direction, within the plane.

Ramp

In fault terminology, a region where a fault cuts across layers in the wall rocks.

Refolded folds

Folds that have been subject to a second folding event, producing a fold interference pattern.

Regression

The retreat of the sea from the land surface.

Relay ramp

A region of distortion between the tips of two faults, where slip is transferred from one fault tip to another.

Releasing bend

A bend in a strike-slip fault that produces a component of extension.

Representative fraction

The scale of a map stated as a ratio of the length of a feature on the map to the length of the same feature in the real world.

Restraining bend

A bend in a strike-slip fault that produces a component of shortening.

Reverse drag fold

An obsolete term for a rollover fold or fault bend fold formed in the hanging wall of a listric normal fault.

Ribs

Concentric ridges, approximately perpendicular to hackles on a joint surface. Part of plumose structure.

Riedel fracture (Riedel shear)

Small fractures that develop in response to stresses in fault walls during fault propagation and movement. Rieldel shears are typically oriented at ~15 and ~75° to the main fault.

Right-hand rule

An orientation-measuring convention for planes. When facing the strike direction, the plane dips your right. (Or, dip direction is 90° clockwise from the right-hand-rule strike direction.)

Rods

Linear fabric elements that are elongated, continuous domains.

Rollover anticline

A fault bend fold formed in the hanging wall of a listric normal fault.

Roof thrust

The upper flat of a thrust duplex structure.

Rotation

Change in orientation, typically measured in degrees, about a particular axis of rotation.

Rotational deformation

Deformation during which the strain axes have rotated.

S-plane

Foliation in a shear zone, oriented oblique to the shear zone boundary, representing less deformed regions; from the French word "schistosité".

S-tectonites

A strong secondary planar fabric or foliation. (The S is for 'schistosity'.)

Scalar

A physical quantity that can be represented by a single number.

Schistosity

Penetrative foliation defined by mineral grains coarser than ~1 mm. Coarser-grained version of slaty cleavage.

Secondary structure

A structure formed well after the rock in which it is occurs.

Section balancing

Preparation of two consistent cross-sections showing the geometry before and after deformation. An important part of checking cross-sections through thrust belts.

Separation

The distance between two fault cutoff lines, measured in a specified direction on a fault surface.

Sequence

A package of strata bounded above and below by unconformities.

Shatter cone

A conical joint surface diagnostic of impact structures, often decorated with plumose markings.

Shear fracture

A fracture where the two walls have slid past each other. (More or less synonomous with a fault, but typically used for small-scale features of the Earth and of experimentally deformed rocks.)

Shear stress

The part of the stress (or traction) on a plane that acts parallel to the plane.

Shear zone

The ductile equivalent of a fault zone. A belt of ductile deformation across which movement has caused significant offset between the two sides. Shear zones are typically formed at depths greater than brittle faults.

Sheath fold

A fold having a strongly curved hinge, so that the geometry resembles a finger of a glove. (Sometimes called condom folds.)

Sigma porphyroclast

A structure formed by foliation that sweeps around a porphyroclast forming curved, rhombic shape, indicating the sense of shear.

Sill

A tabular igneous intrusion where magma has intruded parallel to strata in the host rock.

Simple crater

A crater with with simple bowl shape and raised rims. Usually less than 5 km in diameter.

Simple shear

A type of non-coaxial deformation in which particles move along lines parallel to a single plane

Sinistral

Left-lateral movement: relative to an observer looking toward a fault or shear zone, the far side appears to have moved left.

Slickenlines

Scratches (striae) or fibres on the fault surface. Indicate direction of net slip.

Slip

The displacement vector of a fault.

Slump structure

A soft sediment structure formed in sediments deposited on a slope that undergo catastrophic slope failure, and move under the influence of gravity. Beds may be tightly folded, boudinaged, or both.

Small circle

lines on a stereonet that reach from east to west, like lines of latitude.

Sole markings

Sedimentary structures, such as flute and groove casts, preserved when coarse sediment is deposited rapidly on a muddy substrate.

Stereogram

A stereographic projection of a geologic structure or structures. A powerful tool for solving geometric problems in structural geology.

Stereonet

A grid of curves, the 3-D equivalent of a protractor. Used to measure angles on a stereographic projection.

stishovite

A high pressure tetragonal polymorph of quartz, in which silicon is in octahedral coordination, surrounded by six oxygen atoms.

Strain

Change in size and shape (dilation and distortion).

Strain axes

Three mutually perpendicular lines in a strain ellipsoid representing the maximum, minimum, and intermediate stretches; also, the poles to three planes of zero shear strain.

Strain ellipse

The shape of a deformed circle that originally had unit radius.

Strain ellipsoid

The shape of a deformed sphere that originally had unit radius.

Strain partitioning

Deformation in which strain is heterogeneous and different parts of the rock show different parts of the strain history.

Strain rate

Strain per unit time.

Strain ratio

The ratio between the long and short axis of the strain ellipse, a convenient measure of the amount of distortion in two dimensions.

Stratified

Organised in layer (strata) that were originally horizontal.

Stratigraphy

The study of the organization and history of stratified rocks.

Stress

Force concentration or force per unit area; also, the concentration of all the forces acting at a point within the Earth.

Stress axes

The directions of the principal stresses; also, the directions of the poles to the principal planes of stress.

Stress ellipse

The envelope in a plane of all the stresses (tractions) acting at a point. Also, the ellipse having the principal stresses as its axes.

Stress ellipse (2-D) or Stress ellipsoid (3-D)

The envelope in 3D of all stresses (tractions) acting at a point. Also, the ellipsoid having the principal stresses as its axes.

Stretch

A measure of longitudinal strain equal to the deformed length divided by the original length.

Stretching lineation

Linear fabric defined by elongated domains, produced by extension.

Strike

Azimuth of a horizontal line that lies in a plane.

Strike separation

The distance along a fault, measured in the direction of fault strike, between two cutoff lines of the same severed surface.

Strike-slip fault

A fault in which the component of slip parallel to the fault strike exceeds that parallel to the fault dip.

Structural geology

The study of structures within the Earth and their origin; in practice, structural geology mainly focusses on secondary structures and the deformation processes that formed them.

Structure contour

A contour based on the elevation of a geological surface, separating higher and lower parts of the surface; a line joining points of equal elevation on a geological surface.

Subcrop

A feature on an unconformity surface formed where the unconformity cuts off (intersects) a older rock unit or surface.

Subcrop limit

A line on an angular unconformity surface making the boundary of an older unit that was partially removed by erosion at the unconformity surface.

Subsurface

The region below the Earth's topographic surface.

Suevite

A fine-grained rock with a breccia texture, consisting of a mixture of rock fragments, glass, and melt, produced during impacts of extra-terrestrial objects.

Surface trace

The line along which a geological surface intersects the topographic surface.

Surface trace (outcrop trace)

The intersection of a geological surface with the topographic surface.

Syncline

A fold where the younging direction is towards the centre of the fold.

Synform

A fold where the limbs dip towards the hinge and the fold closes downward.

Syntaxial vein

A vein where repeated cracking has occurred in the centre of the vein and mineral fibres are typically in crystallographic continuity with the grains in the wall rock.

Synthetic Riedel shears (R-shears)

Fractures that form at 15° to a fault, with the same sense of displacement as the fault.

Tabular mineral grains

Platy or flake-shaped mineral grains (e.g. mica) that are often aligned to produce a fabric.

Tectonic wedge

A pair of oppositely vergent thrusts that meet in the subsurface.

Tectonic window (Fenster)

On a geologic map, an area of footwall that is entirely surrounded by hanging wall.

Tectonics

The mathematical study of structures; commonly applied to large-scale movements of the lithosphere and the structures that these have produced (plate tectonics).

Tensor

A physical quantity that varies in magnitude with orientation, and can be represented by an ellipse or ellipsoid, or by a square matrix of numbers. (Strictly speaking this definition describes a 2nd-order tensor; 1st-order tensors are here referred to as vectors.)

Threading contours

The process of drawing contours separating higher and lower values.

Throw

The vertical component of dip separation.

Thrust regime

A stress regime in which the minimum principal stress is vertical. Reverse faults are common.

Tip line

The linear boundary at the edge of a fracture surface, separating fractured from unfractured rock.

Topographic contours

Contours that show the elevation and shape of the land surface. Contours that separate points of higher and lower elevation.

Topographic surface

The land surface of the Earth.

Trace

The line formed by the intersection of a geologic surface with the topographic surface or a cross-section.

Traction

Force per unit area; also known as stress.

Transform fault

A strike-slip fault that is also a plate boundary.

Transgression

The advance of the sea over the land surface.

Translation

Change in position

Transpression

A combination of strike-slip and shortening.

Transtension

A combination of strike-slip and extension.

Trend

The azimuth of a line, measured in the direction of downward plunge.

Trough point

The lowest point on the trace of a synform.

Trough surface

A surface containing multiple fold trough lines.

Unconformity

An ancient surface of erosion and/or non-deposition that indicates a gap in the stratigraphic record.

Variant feature

A feature in a fold that changes with orientation.

vector

A physical quantity that has magnitude and direction, and can be represented by an arrow.

Vein

An joint that is filled with minerals, typically deposited from groundwater.

Vergence

The direction in which rocks near the surface have moved relative to rocks deeper down.

Vertical exaggeration

Describes the distortion of a cross-section in which the vertical and horizontal scales are not equal. The vertical exaggeration is the ratio between the length of the representation of a vertical unit line and the representation of a horizontal unit line.

Viscous (Newtonian)

A deformation mode in which the strain rate is proportional to the stress.

Wrench regime

A state of stress in which the intermediate principal stress is vertical. Strike-slip faults are common.

Xenolith

A piece of host rock that is surrounded by an intrusion.

Zenith

The highest point on a spherical or curved surface.

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Geological Structures: a Practical Introduction by John Waldron and Morgan Snyder is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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