Accommodation zone: Zone between two overlapping fault segments where offset is transferred from one fault segment to the other. Used specifically for the zone connecting two oppositely dipping half-grabens.
Active diapirism: Diapirs forcing their way upward through the overburden, driven by differential, thermal or displacement loading. Weakening of the overburden by means of fracturing is generally involved.
Active folding: Folding of layers by layer-parallel shortening controlled by contrasts in viscosity between layers (buckling).
Active markers: Markers or structures that react mechanically to the stress field by having a different viscosity from their matrix. Such markers may buckle or boudinage, and do not give a representative image of the general state of strain in the rock.
Active rifting: Rifting as a response to upwelling of the asthenosphere, which generates tensile stresses that lead to normal faulting and stretching in the lithosphere.
Allochthonous: Tectonic unit that has been transported too far for direct correlation with the substrate. Derived from Greek: allo mean “different” and chthon means “ground”. Typically used for nappes that have moved tens of kilometers or more.
Allochthonous salt: Salt that has been detached from its source layer, usually by contractional deformation.
Angular shear: Change in angle for a pair of lines that were orthogonal before deformation. More specifically, the angular shear along a reference line is the change in angle of a line that was perpendicular to the reference line before deformation.
Anisotropic volume change: Volume change that is created by shortening or extension in one or two directions only.
Anticlinal: Fold where rock layers get younger away from the axial surface of the fold.
Anticrack: Engineering term for closing fracture, i.e. fractures that show compactional displacement.
Antiform: Fold where the limbs dip down and away from the hinge zone.
Antiformal syncline: A syncline (strata get older away from its axial surface) that has the shape of an antiform, i.e. a syncline turned upside down.
Antithetic fault: From the Greek word antithetos, meaning “placed in opposition to”. An antithetic fault is a fault dipping in the opposite direction to an adjacent master fault or dominating fault set.
Antithetic shear: Shear acting antithetic to the sense of displacement of a reference fault. Used as a section restoration technique.
Aperture: The distance between the two walls of a fracture.
Area change: Change in area due to deformation. Implies volume change unless compensated for in the third dimension.
Aseismic slip: Stable sliding, as opposed to stick-slip.
Aspect ratio: Long dimension divided by short dimension of an ellipse or rectangle.
Asperity: Irregularity along a fracture surface.
Autochthonous: Lithologic unit in or along an orogen- ic belt that has not been tectonically transported. The Greek word auto means “the same” in this connection.
Axial plane: A planar axial surface, not necessarily parallel to the bisecting surface.
Axial plane cleavage: Cleavage that is subparallel to the axial surface of a fold. The cleavage must have formed during the process of folding. Also called axial planar cleavage.
Axial surface: The theoretical surface connecting the hinge lines of consecutive surfaces in a fold structure.
Axial trace: The theoretical line that connects hinge points across a fold.
Axially symmetric extension: Extension in one principal direction (X-axis of the strain ellipsoid) and equal shortening in the other two (Y and Z). Implies perfect constrictional strain. Equal to uniform extension.
Axially symmetric flattening: Shortening in one principal direction (Z) and equal extension in the other two (Y and X). Implies perfect flattening strain. Equal to uniform shortening.
Back-thrust: Thrust displacing the hanging wall toward the hinterland, i.e. opposite to the general thrusting direction.
Backstripping: Isostatic basin restoration where focus is on the subsidence history of a basin by successively removing sedimentary sequences and balancing isostasy.
Backward modeling: Starting with the present state and modeling (restoring) back to the pre-deformational stage.
Balancing: The construction or interpretation of a geologic profile or 3-D model that can be reconstructed by means of geologically realistic processes to a geologically sound undeformed state.
Basin: In fold terminology, this is a dome turned upside-down.
Bending: Folding mechanism that occurs where forces are applied at a high angle to the layering.
Bisecting surface: Surface that divides a fold into two parts. When the bisecting surface is vertical, the limbs should have the same dip.
Blastomylonite: A mylonite that has recrystallized posttec- tonically. Grains show no preferred orientation (equant grains) and little or no internal strain.
Blind fault: Fault that terminates without reaching another fault or the surface. Traditionally used in thrust fault terminology (blind thrust).
Bluntness: The angularity or curvature of folds as observed in cross-sections perpendicular to the hinge line.
Body forces: Forces that affect the entire volume of a rock, the inside as well as the outside.
Bookshelf tectonics: Popular name for the development of rotated fault blocks forming according to the domino model.
Borehole breakouts: Stress method that uses the geometry of a borehole to estimate the maximum horizontal stress.
Boudinage: The process leading to the formation of boudins.
Boudins: Structures forming during systematic segmentation of preexisting layers. Classic boudins form by extension of layers that are more competent than the matrix. See also foliation boudinage.
Boundary drag: The restriction of flow in a layer by the viscous shear forces acting along the boundaries. Particularly relevant for salt and viscous magma.
Box fold: Fold with two axial planes and two hinge zones that formed simultaneously. Reminiscent of a (bottomless) box in cross-section.
Branch line: A line of intersection between two intersecting faults. Used for any type of fault (normal, reverse or strike-slip).
Branch point: Point in a section or map where two fault traces join.
Breached relay ramp: Relay ramp that has been cut by a fault, transforming it from a soft-linked into a hard-linked overlap structure.
Breccia: Cohesive or non-cohesive fault rocks consisting of randomly oriented fragments resulting from brittle fracturing. Breccia fragments must constitute more than 30% of the rock.
Brittle deformation mode: Deformation by means of brittle deformation mechanisms (fracturing, frictional sliding, cataclastic flow).
Brittle strain, brittle deformation: Deformation by fracturing (discontinuous deformation).
Brittle shear zone: Shear zones dominated by brittle deformation mechanisms. Also called frictional shear zones. Also used for shear zones that disrupt originally continuous markers.
Buckle folds: Folds that form by buckling. They show a certain regularity with regard to wavelength and amplitude as a function of layer thickness and the viscosity contrast between layer(s) and the matrix.
Buckling: A folding mechanism that occurs when layers that are more competent (higher viscosity) than the matrix are compressed parallel to the layering. As stress increases the layer becomes unstable and buckles through the amplification of minute irregularities along the layer interfaces.
Bulb: Upper thick part of a teardrop diapir.
Byerlee’s law: Relation between critical shear stress on a fracture and the related normal stress across it. The normal stress reflects the depth in the crust, hence this law models critical shear strength through the frictional upper crust.
Cataclasis: Brittle crushing of grains (grain size reduction), accompanied by frictional sliding and rotation. Derived from a Greek word for crushing.
Cataclasite: Cohesive and fine-grained fault rock. Cataclasites are subdivided into those that have 10–50% matrix (protocataclasite), 50–90% matrix (cataclasite) and > 90% matrix (ultracataclasite).
Cataclastic (deformation) band: Deformation band where cataclasis is an important deformation mechanism.
Cataclastic flow: Flow of rock during deformation by means of cataclasis, but at a scale that makes the deformation continuous and distributed over a zone.
Centrifuge: Spinning device used for physical modeling where gravity can be scaled.
Channel flow: Large-scale flow of relatively low-viscosity heated rocks in a “channel” from the continent–continent collision zone of an orogen. The kinematics is that of extrusion, with a thrust below and normal movement above the channel.
Characteristic earthquake model: Each slip event is equal to the others in terms of slip distribution and rupture length.
Chemical compaction: Compaction by means of wet diffusion, i.e. dissolution at grain contacts and/or stylolite formation.
Chevron fold: Fold with angular hinge and where the axial surface forms more or less perpendicular to σ1.
Chevron method: The vertical shear method used during restoration or balancing of sections.
Chocolate tablet boudinage: Boudinage in two directions (in the XY-plane), forming more or less square or rectangular boudins in three dimensions.
Christmas-tree folds: Secondary folds superimposed on a larger and preexisting upright fold, usually by means of gravity collapse. The Christmas-tree pattern emerges when the primary fold is an upright antiform.
Clay injection: Injection of clay along a fault, normally because a tensile fracture opens due to local overpressure.
Clay smear: Smearing or, less commonly, injection of clay along the fault core.
Clay smear potential (CSP): relationship between the thickness of a faulted clay layer and the distance from the clay layer along the fault in a sequence of sandstone with one or more clay layers. CSP is used in fault seal analysis.
Cleavage: A tectonic foliation formed at low-grade metamorphic conditions and related to folding. A cleaved rock breaks more easily along the cleavage.
Cleavage refraction: A change in cleavage orientation across an interface between layers of contrasting competence.
Climb: The process by which edge dislocations jump to another slip plane to get around obstacles in the lattice.
Coaxial deformation: Lines along ISA do not rotate during the deformation; Wk = 0. The principal strain axes (X, Y and Z) remain stationary throughout the deformation history.
Coble creep: See grain boundary diffusion.
Coefficient of sliding friction: The shear stress required to activate slip on a fracture divided by the normal stress across the fracture.
Cohesion: The solidness of a medium. A cohesive rock does not fall apart very easily, while a non-cohesive medium easily disintegrates. Cementation increases cohesion in sedimentary rocks and fault gouges.
Cohesive strength: The intersection of the fracture criterion or envelope with the vertical axis of the Mohr diagram. It is the theoretical value of the shear stress that it takes for a rock to fail in shear along a plane across which there is no normal stress. In many cases the cohesive strength of a rock is approximately twice its tensile strength (C = 2T).
Compaction: Shortening in one direction while the other two directions are unstrained.
Compaction band: Deformation band involving compaction without shear. Compaction bands are found in highly porous sandstones.
Compaction cleavage: Cleavage formed by lithostatic compaction of sediments into sedimentary rocks. Best developed in mudrocks.
Compaction curve: Graph expressing changes in porosity with burial depth for a given sediment(ary rock).
Compactional shear zones: Shear zones with a component of compaction across the walls.
Competency: A relative expression that compares the mechanical strength or resistance to flow of a layer or object to that of its adjacent layers or matrix. Competent objects are more resistant to flow than their matrix.
Compression: Expression used extensively for compressional stresses. Contraction or shortening should be used for strain.
Compressive strength: The amount of compression that a rock can withstand before fracturing, usually many times its tensile strength (eight times according to Griffith).
Concentric faults: Circular faults related to collapse of underlying rocks, for example due to salt collapse and collapse of karst structures.
Concentric folds: Folds whose well-rounded arcs approximate half-circles so that limbs and hinges are inseparable (or they can be said to consist of hinges only).
Conjugate faults: Two intersecting faults that formed under the same stress field. Such faults show opposite sense of shear and make 30° to σ1.
Conjugate folds: See box folds.
Constant area restoration: Restoration of map or cross-section where area is preserved.
Constant displacement restoration: Displacement is considered to be constant along the fault(s).
Constant-horizontal-stress reference state: Reference state of stress assuming that the lithosphere has no shear strength at a certain depth and that it behaves like a fluid below this compensation depth.
Constant length restoration: Restoration of one or several markers in a cross-section where the length of the marker is the same before and after restoration.
Constitutive laws: Laws or equations describing the relationship between stress and strain.
Continuous cleavage: Cleavage where the distance between individual cleavage domains is indistinguishable in hand sample, i.e. less than 1 mm.
Contraction: Reduction in length. Synonymous with shortening.
Contraction fracture: Closing fracture, usually stylolitic. Also known as anticrack.
Contractional fault: Fault that shortens a reference horizon, which may be lithologic layering or the surface of the Earth.
Corrugations: Cylindrical undulations on shear zones, faults or slip surfaces. Occur from micro- to map scale. Large-scale examples may be related to fault growth by segment linkage or shear zone-parallel folding; outcrop-scale also to frictional carving of fault walls.
Couette flow: Simple shear-type flow caused by translation of the overburden relative to the substrate.
Coulomb failure envelope: The linear failure envelope predicted by the Coulomb fracture criterion.
Coulomb material: Material that conforms to the Coulomb fracture criterion.
Coulomb wedge: Model of orogenic wedge where the Coulomb fracture criterion is applied to the wedge material.
Crack: An engineer-oriented term for fracture, commonly restricted to small (hand-sample or outcrop-size) fractures.
Creep: Generally used for slow geologic processes. More specifically used for the (slow) way that permanent plastic deformation accumulates at long-term constant stress by various microscale or atomic-scale deformation mechanisms (diffusion creep, dislocation creep, etc.).
Creep mechanisms: Deformation mechanisms at work in a crystal or crystal aggregate that respond to sustaining stress by the gradual accumulation of plastic strain. These are separated into diffusion creep (grain boundary diffusion, volume diffusion and pressure solution) and dislocation creep (dislocation climb, dislocation glide and recrystallization).
Crenulation cleavage: Cleavage formed by microfolding at low metamorphic conditions of phyllosilicate-rich and well-foliated rocks.
Crenulation lineation: Lineation formed by crenulation of phyllosilicate-rich layers. Closely related to intersection lineations.
Critical taper model: See critical wedge model.
Critical tensile strength: The tensile stress when the material is at the verge of failure.
Critical wedge model: Model of allochthonous units in a collision zone (subduction zone, foreland fold-and-thrust belt) where the units take on a wedge-shaped geometry controlled by basal friction, internal strength and erosion/deposition. Also called critical taper model.
Critically stressed: Stressed to the limit of its strength, so that the material is at the verge of failure.
Cross-slip: Process allowing dislocations to change slip planes in order to bypass obstacles.
Cylindrical fold: Fold with straight hinge line, so that an imaginary cylinder can be fitted to the hinge zone.
Damage zone: Zone of brittle deformation structures (fractures, deformation bands and/or stylolites) around a fault. The zone has a density of such structures that is higher than the surrounding rocks.
Décollement: Large-scale detachment, i.e. fault or shear zone that is located along a weak layer in the crust or in a stratigraphic sequence (e.g. salt or shale). The term is used in both extensional and contractional settings.
Décollement folds: Folds formed above a décollement or detachment, where sub-décollement layers are undisturbed by the folding. Identical to detachment folds.
Decoupling: Expression used in cases where the upper part of a section deforms in a different style than the lower part, or if the structures are not directly connected between the two levels. The two parts are separated by a décollement or very weak layer (typically salt).
Deformation: The change of the shape, position and/or orientation as a result of external forces. The deformation is found by comparing the undeformed and the deformed states and positions.
Deformation bands (I): Millimeter-thick zones of strain localization formed by grain reorganization and/or grain crushing. Shear bands with some compaction across the band form the most common type.
Deformation bands (II): Microscopic zones in a mineral grain with similar optical orientation (extinction), forming between dislocation walls.
Deformation matrix: Transformation matrix that relates the undeformed and the deformed states of a deformation. A deformation matrix describes a linear transformation and therefore homogeneous deformation. It represents a complete description of the deformation (but not the deformation history).
Deformation mechanisms: Mechanisms at the microscale that are active during deformation, including cataclasis, frictional grain boundary sliding, dry and wet diffusion, dislocation glide, dislocation climb, plastic grain boundary sliding, twinning and kinking. Recrystallization is a process that generally occurs by means of two or more mechanisms. The term deformation mechanism can also be used in a more general way.
Deformation phase: A time period during which structures form continuously within an area or region. The structures would show a common expression that can be linked to a particular stress or strain field or kinematic pattern, although the style may vary (for example; both open and tight folds may be related to the same phase because strain can be heterogeneous, but they share the same shortening direction).
Deformation twins: The result of mechanical twinning. Deformation twins are common in calcite crystals.
Delamination model: Model where the dense root of an orogen detaches and sinks into the underlying mantle.
Detachment: Low-angle or horizontal fault or shear zone separating an upper plate (hanging wall) from a lower plate (footwall). Detachments are typically reactivated weak layers or structures.
Detachment folds: Folds formed in competent layers above a detachment during shear or slip on the detachment.
Deviatoric stress: The difference between the total stress and the mean stress. Closely related to tectonic stress.
Dextral: Right-lateral, moving right relative to a point of reference.
Diapir: A body, usually of salt, magma or water-saturated mud or sand that gravitationally moves upward and intrudes the overburden.
Diapirism: The process associated with the formation of diapirs.
Differential compaction: Two or more areas experiencing different amounts of compaction, for example across a steep fault where new sediments preferentially accumulate on the downthrown fault block.
Differential loading: Unevenly distributed load, for instance delta lobes generating locally thick layers of sediments.
Differential stress: The difference between the largest and smallest principal stresses, i.e. the diameter of the Mohr circle.
Diffusion: The movement of vacancies (holes) in an atomic lattice. Volume diffusion occurs within the lattice, while grain boundary diffusion (Coble creep) occurs along the grain boundaries.
Dilation (US), dilatation (UK): Volume change, usually implying volume loss (negative dilation being volume gain). Isotropic dilation involves the same amount of extension in all directions. A common example of anisotropic dilation is uniaxial strain.
Dilation band: Deformation band where displacement is dilational (volume increase) without shear. Dilation bands show an increase in porosity and are relatively uncommon as compared to other types of deformation bands.
Dilational shear zones: Shear zones with a component of dilation (negative compaction) across the walls.
Dip isogons: Theoretical lines connecting points of equal dip on the upper and lower boundaries of a folded layer oriented in an upright position (vertical bisecting surface).
Dip separation: Apparent fault displacement as observed in a vertical section in the dip direction of the fault. Dip separation equals true displacement for dip-slip faults.
Dip-slip fault: Fault with the slip vector oriented along the dip direction of the fault surface, i.e. a perfectly reverse or normal fault.
Dipmeter diagrams: Well log showing dip and azimuth of planar features based on interpretations of resistivity measurements along the wellbore. Measurements are done by running a dipmeter tool through the wellbore, and the planar features represent bedding, deformation bands or fractures.
Disaggregation band: A deformation band formed by non-destructive granular flow (rotation and frictional sliding of grains). Commonly forms in sand and poorly consolidated sandstones.
Discrete crenulation cleavage: Crenulation cleavage with a sharp discontinuity between QF- and M-domains, in contrast to zonal crenulation cleavage.
Disharmonic folds: Folds in multilayered rocks that change shape and wavelength along the axial trace.
Disjunctive cleavage: Domainal cleavage that is independent of previous foliation(s). Typical for very low grade metasediments. May be divided into stylolitic, anastomosing, rough and smooth, according to the morphology of the domains. Disjunctive cleavage contrasts with crenulation cleavage, where a preexisting foliation is reworked by microfolding and solution.
Dislocation: An atomic-scale line defect within a crystal lattice. Dislocations can move by means of glide and climb mechanisms and dislocation formation and motion cause plastic deformation. Edge and screw dislocations are the principal types of dislocations in naturally deformed rocks.
Dislocation creep: Strain accumulation through the movement of dislocations through the crystal, where mechanisms called climb and cross-slip are used to bypass obstacles in the lattice.
Dislocation glide: The self-healing (and caterpillar-style) process by which edge dislocations move (see Figure 10.12).
Dislocation walls: Concentration of dislocations forming walls within a crystal. Dislocation walls mark the boundary between deformation bands and subgrains.
Displacement: The difference between the location of a point before and after deformation. For faults, displacement is the relative motion of two originally adjacent points on each side of the fault.
Displacement field: The field of vectors describing the distortion of points in a deformed medium, i.e. vectors connecting the pre- and post-deformational positions of particles.
Displacement–length ratio: The ratio between the maximum displacement of a fault and its length. Usually measured on a cross-section or a map, which introduces a sectional uncertainty.
Displacement loading: Loading by changing the lateral boundary conditions, for instance by compressing or extending a volume of rocks or sediments.
Displacement vector: Vector connecting the positions of a material point (e.g. a sand grain) before and after deformation.
Domainal cleavage: Cleavage composed of domains of different minerals, usually micaceous M-domains and quartzofeldspathic QF-domains. When individual domains are visible in hand sample, the domainal cleavage is a spaced cleavage.
Dome: Bowl-shaped geometry with layers dipping in every direction from a summit.
Domino (fault) model: Model where parallel normal faults define fault blocks that rotate like domino bricks during deformation. Also called bookshelf mechanism. In the classic domino model blocks are by definition rigid, but the soft domino model allows for internal block deformation.
Domino faults: Set of parallel normal faults separated by rotated fault blocks (domino blocks) where bedding is dipping antithetic to the faults.
Doubly plunging fold: Fold plunging in two directions because of curved hinge line.
Downbuilding: Term used for the process where sediments accumulate along a passive salt diapir. The sediments build downward with regard to the surface of the Earth.
Drag (folds): Zone of folding on one or both sides of a fault or salt structure. The folding must be related to the fault formation and/or growth. Originally a genetic term implying that the folding is controlled by frictional resistance along a fault. Now used as a purely descriptive term.
Ductile deformation: Continuous deformation at the scale of observation, resulting from any deformation mechanism (brittle or plastic). Some geologists restrict the term to crystal-plastic deformation.
Ductile shear zone: Shear zone with no internal shear-related discontinuities.
Duplex: Tectonic unit consisting of a series of horses that are arranged in a piggy-back fashion between a sole and a roof thrust. Also used for similar structures in extensional and strike-slip settings (extensional and strike-slip duplexes).
Dynamic analysis: The analysis that explores the relationship between stress and strain.
Dynamic recrystallization: Synkinematic recrystallization, i.e. continual crystallization during deformation. Revealed in shear zones by slightly non-equant grains that define a new fabric at an angle to the foliation.
Edge dislocation: Type of linear defect in a crystal lattice. See Figure 10.11.
Effective stress: The total stress minus the pore pressure in a porous rock or sediment.
Elastic deformation (strain): Deformation (strain) that disappears when the applied stress is removed.
Elastic material: Material that deforms elastically.
Elongation: e = (l − l0)/l0, where l0 and l are the lengths of the line before and after the deformation, respectively.
Enveloping surface: Surface enveloping or tangenting a series of geometric features, such as a series of fold hinges or Mohr circles.
Exfoliation joints: Also called sheeting joints. Joints formed near the surface in homogeneous rocks because of pressure release and nearness to a free surface. Typically subparallel to the surface.
Extension: A measure of how much longer a line or object has become due to deformation.
Extension fracture: Fracture formed by extension perpendicular to the fracture walls. The amount of extension can be minute, as for joints, or can be larger, as for veins.
Extensional duplex: Duplex forming along an extensional fault, where individual riders are separated by extensional faults and bound by a roof fault and a floor fault.
Extensional fault: Fault that extends (increases the length of) a reference horizon, which may be lithologic layering or the surface of the Earth.
Extrusion: Strain model where rock moves in a uniform direction with no strain along Y (plane strain). Applied to outcrop-scale shear zones, and also to orogens (particularly the Himalayan orogen) as a model where a unit (thrust nappe) in the orogenic wedge translates toward the foreland faster than the overlying unit. The result is an allochthonous unit bound by a thrust below and a normal fault above. See also channel flow.
Extrusive advance: The gravity-controlled mechanism by which an exposed salt sheet (salt glacier) advances.
Fabric: The configuration of planar and/or linear objects in a penetratively deformed rock. An L-fabric is composed of linear features, while an S-fabric consists of planar elements.
Failure envelope: The curve enveloping a series of Mohr circles representing different differential and mean stress values (different positions along the x-axis of the Mohr diagram). Each circle touches the envelope. The failure envelope describes the stress conditions at failure for different stress conditions in a given medium (rock). Also see Coulomb failure envelope and Mohr failure envelope.
Far-field stress: Equivalent to remote stress.
Fault: Surface or narrow tabular zone with displacement parallel to the surface (zone). Generally used for brittle structures (structures dominated by brittle deformation mechanisms).
Fault bend: Bend in the fault trace or fault surface. Although the term does not imply a particular evolution, many fault bends probably represent hard-linked structures formed by fault linkage.
Fault-bend fold: Fold forming in the hanging wall in response to a bend or kink in the fault surface. Traditionally a fold forming above a thrust fault ramp.
Fault core: Central high-strain zone of a fault where most of the displacement is taken up. Enveloped by the fault damage zone. The fault core can consist of non-cohesive rock flour or strongly sheared phyllosilicate-rich rock called fault gouge, or cohesive cataclasite, and can contain lenses of wall rock. The core thickness can vary from less than 1 mm for small (meter-scale) faults to around 10 m for large (kilometer-displacement) faults.
Fault cut: Stratigraphic section missing in a well due to omission by faulting. The fault cut is estimated based on stratigraphic information from nearby wells or outcrops.
Fault cutoff line: The line of intersection between a fault surface and another (usually stratigraphic) surface cut by the fault. There are two such lines, known as hanging-wall and footwall cutoff lines.
Fault damage zone: See damage zone.
Fault gouge: Fine-grained and clay-rich non-cohesive rock located in the core to faults, formed by crushing and chemical alteration of the host rock.
Fault grooves: Linear tracks or grooves carved out on a slip surface by asperities on one of the walls.
Fault linkage: The process where two adjacent fault seg- ments interfere and connect, forming a hard-linked or soft-linked structure.
Fault-plane solution: Stereographic projection containing information about the first motion caused by an earthquake based on seismic observations from a number of seismographs. It consists of two orthogonal planes separating compressional from tensional motion. One of these planes represents the fault orientation. The beachball style plots also give us the sense of slip (normal, reverse, etc.) and approximate locations of the principal stress axes.
Fault-propagation fold: Fold forming ahead of a propagating fault tip. Traditionally used for thrust faults, but can be used for any type of fault (normal, strike-slip or reverse).
Fault stepover: Link between two more or less parallel faults that are not aligned. The faults must be close to each other so that their stress fields can interfere. The two faults can be soft-linked or hard-linked.
Fault strain: Strain calculated for an area or volume affected by numerous faults. As a concept, two- and three- dimensional strain applies only to ductile (continuous) deformation, but since ductility is scale dependent, it can, as an approximation, be applied to fractured rocks with distributed fractures.
Fault trace: Intersection between a fault and any given surface, such as the surface of the Earth, a stratigraphic interface or a cross-section.
Fault zone: A series of closely spaced subparallel faults or slip surfaces, forming a zone. The thickness of the zone must be small relative to its length. In recent use it describes the central and most strained part of a fault, coinciding with or somewhat wider than the fault core.
Fenster: Erosional exposure of the rock unit underlying a nappe (window).
Fiber lineation: See mineral fiber lineation.
Fissure: Fluid-filled extensional fracture.
Flaps: Folded layers along the upper walls of salt diapirs, dipping away from the diapir (unless they have been inverted). Flaps form as a moving salt diapir breaks through its overburden, lifts, rotates and shoulders aside roof layers.
Flat-ramp-flat fault: Fault with a subhorizontal segment connected with steeper segments on each side. Used about extensional as well as reverse/thrust faults.
Flexural flow: See flexural shear.
Flexural folding: Folding by means of flexural slip, flexural shear or orthogonal flexure.
Flexural shear: Fold mechanism where layers are deformed by layer-parallel simple shear. The shear strain is zero at the hinge point, increasing toward the inflection points. Sense of shear is away from the hinge zone, i.e. opposite on the two limbs. Also called flexural flow.
Flexural slip: Slip along bedding interfaces during folding. As for flexural shear, slip increases away from the hinge line, being opposite on the two limbs. Typical in folded layers of high contrasts in strength.
Floor thrust: Thrust fault defining the base of a duplex structure or the basal thrust of a nappe or nappe complex, i.e. a sole thrust.
Flow: A term used for rocks in the perspective of geologic time. Given enough time and appropriate physical conditions (temperature, pressure, fluid availability), rocks flow by means of plastic or brittle deformation mechanisms. A distinction can be made between cataclastic flow and plastic flow.
Flow apophyses: Apophyses separating domains of different particle motion during flow (deformation).
Flow laws: Mathematical models describing the relation between deformation rate, stress and deformation mechanism for a given rock.
Flow parameters: Parameters describing the deformation at any instant or interval of the deformation history. For steady-state deformation, they are representative of the entire deformation history. Important flow parameters are the velocity field, flow apophyses, ISA and vorticity.
Flow pattern: The pattern outlined by the particle paths during flow (distributed deformation).
Flower structure: The upward-splitting and widening pattern of strike-slip faults as seen in cross-section.
Fold axis: The straight hinge line of a cylindrical fold.
Fold hinge: See hinge.
Fold limb: The two parts of the fold that are separated by the hinge zone, i.e. by the area of maximum curvature.
Fold nappe: Nappe that is internally folded throughout and appears to have originated by shearing of an inverted fold limb.
Folds: Structures formed by ductile deformation (folding) of more or less planar surfaces, such as bedding or tectonic foliations, into curved surfaces. Folding of already folded surfaces is called refolding.
Foliation: Usually a tectonic planar structure formed in the plastic regime. Foliations are characterized by flattening across the structure. Also used for primary structures such as bedding or magmatic layering, in which case the term primary (in contrast to secondary or tectonic) foliation should be used.
Foliation boudinage: The formation of boudins in strongly foliated metamorphic rocks, where the boudins are separated by en echelon arranged shear fractures, small shear zones or extension fractures.
Foliation fish: Volume in a strongly foliated rock that is back-rotated relative to the rest of the rock, displaying a fish-like geometry.
Footwall: The surface underneath a non-vertical fault.
Footwall collapse: The formation of one or more secondary faults in the footwall to a fault. Expression most commonly used for normal faults.
Footwall uplift: Uplift of the footwall of a normal fault, which typically is in the order of 10% of the fault throw.
Forced folds: Folds formed as basement blocks move along preexisting faults, forcing covering sedimentary layers to fold into monoclinal structures.
Foreland: The peripheral or frontal part of a thrust region or orogenic belt, dominated by thin-skinned tectonics and very low to non-metamorphic conditions.
Forward modeling: Modeling a process or the develop- ment of a cross-section etc. from the beginning to the end or present state, i.e. opposite to backward modeling.
Four-way dip closure: Dome structure.
Fracture: A sharp planar discontinuity. An ideal fracture is narrow (thinner than 1 mm), involves a displacement discontinuity as well as being a mechanical discontinuity, and is weak so that the rock preferentially breaks along the fracture. It also conducts fluids. A distinction is drawn between extension fractures and shear fractures, and sometimes also contraction fractures.
Fracture cleavage: A dense array of fractures that mimics a cleavage. While ordinary cleavage involves shortening across the cleavage, fracture cleavage involves shear along or extension across the structure. The term is a bit confusing and is best omitted.
Fracture toughness: A material’s resistance to continued growth of an existing fracture. High fracture toughness implies high resistance against fracture propagation and therefore low propagation rates.
Frictional regime: The regime where the physical conditions favor the brittle deformation mechanism in the crust, i.e. the upper part of the crust. Identical to the brittle regime, but emphasizes the dependence on deformation mechanism (and not deformation style).
Frictional shear zone: Shear zones dominated by brittle deformation mechanisms. Also called brittle shear zones.
Frictional sliding: Sliding on a fracture that has a certain friction without activation of plastic deformation mechanisms.
Frontal ramp: Ramp oriented perpendicular to the transport direction. Used traditionally about thrust ramps, but now also about ramps along extensional faults.
General shear: Deformation that is more complex than simple shear, usually involving three-dimensional strain. Used synonymously with subsimple shear by some geologists.
General shear zone: Shear zone that deviates from the ideal shear zone model.
Geometric striae: Linear irregularities on a slip surface, e.g. surface corrugations.
Gliding model: Orogenic model popular in the 1950s and 1960s in which nappes were thought to gravitationally glide from elevated parts of the orogen.
Gneissic banding: Banding or layering where individual bands consist of different minerals, often representing transposed and flattened dikes or other primary structures.
Graben: German for grave. A depression bounded by two more or less strike-parallel but oppositely dipping normal faults or vertical faults.
Grain boundary diffusion: Diffusion of crystal vacancies along grain boundaries – a plastic deformation mechanism. Also called Coble creep.
Grain boundary migration: Recrystallization of crystals by means of migration of grain boundaries.
Grain boundary sliding: Plastic deformation mechanism where grains slide as a result of diffusion (not to be confused with frictional grain boundary sliding, which is a brittle mechanism).
Granular flow: Particles flowing by frictional sliding and rolling (translation and rigid rotation). Typical for deformation of loose sand or soil. Also called particulate flow.
Gravitational orogenic collapse: Orogen collapsing under its own weight, occurring when gravitational forces created by the weight of the elevated top of the orogen exceed the strength of the orogenic edifice.
Griffith fracture criterion: σs = 2T + σn μ, which is non- linear (curved) in the Mohr diagram.
Groove lineation: Lineation defined by fault grooves.
Growth fault: A shallow normal fault that has moved during deposition of sediments on the hanging-wall side. The hanging-wall strata thicken toward the fault and may also be more coarse-grained close to the fault. The fault displacement increases downwards as fault dip decreases.
Hackles, hackle marks: Plumose curvilinear patterns defined by gentle relief on extension fractures, radiating from the nucleation point of the fracture or fanning away from a curvilinear axis. Identical to plumose structures.
Half-graben: Structural depression controlled by one master normal fault and typically also antithetic (oppositely dipping) minor normal faults in the rotated hanging-wall layers. An asymmetric graben is a graben structure somewhere between a perfect half-graben and a symmetric graben.
Halokinesis: The study of salt tectonics, i.e. the formation of salt diapirs and related structures due to flow of salt in the subsurface.
Halokinetic cycle: Cyclic sedimentary sequences along salt diapirs where cycles are separated by unconformities and caused by non-steady rise of the diapir. Halokinetic cycles are vertically stacked along the walls of diapirs.
Hanging wall: The rock volume above a dipping fault.
Hanging-wall collapse: The formation of one or more secondary faults in the hanging wall to a fault. Expression most commonly used about normal faults.
Hard link: Expression used in fault overlap zones where the overlapping faults are connected by at least one fault that is mappable at the scale of observation.
Harmonic folds: Folds that are repeated with similar shape along the axial trace.
Heave: The horizontal component of the dip separation of a fault. Equal to the horizontal component of the true displacement vector for dip-slip faults.
Heterogeneous deformation: Deformation varies within the area or volume in question. Also called inhomogeneous deformation.
Heterogeneous strain: The state of strain (strain ellipse or ellipsoid) varies within the area or volume in question. Also called inhomogeneous strain.
High-angle fault: Fault dipping more than 30°.
Hinge: The area of maximum curvature of a folded surface, i.e. the zone that connects the fold limbs.
Hinge line: The line of maximum curvature, i.e. the line defined by consecutive hinge points on a folded surface. Linear and known as the fold axis for cylindrical folds.
Hinge point: Point of maximum curvature on a folded surface.
Hinge zone: Zone of maximum curvature on a fold.
Hinterland: The central or internal zone of an orogen, as opposed to the foreland. The hinterland is characterized by basement involvement and locally high metamorphic grade.
Homogeneous deformation: Deformation is everywhere the same within the area or volume in question.
Homogeneous strain: The state of strain is everywhere the same within the area or volume in question, meaning that the strain in the entire area or volume can be represented by a single strain ellipse or ellipsoid.
Homologous temperature: The ratio of a material’s temperature T to its melting temperature Tm using the Kelvin scale.
Horizontal separation: The fault-related separation of layers observed on a horizontal surface across the fault.
Horse: The smallest tectonic unit in thrust terminology: a tectonic sheet bounded by thrust faults on each side and occurring in trains in duplex structures. S-shaped geometry is common. Now also used in normal fault terminology (horses in extensional duplexes).
Horsetail fractures: Fractures splaying off the tip of a larger fracture.
Horst: Elongated area that is stratigraphically elevated relative to rocks on each side. A horst is bounded by normal faults that are vertical or dipping away from the horst.
Hybrid fracture: Combination of shear (Mode II) and opening (Mode I) fractures.
Hydraulic crack, or fracture: Increasing the fluid pressure in an interval in a well until the formation pressure is exceeded and the rock fractures. Used to stimulate hydrocarbon producers or water injectors.
Hydrostatic stress: State of stress where the stress is the same in all directions (spherical stress ellipsoid). Occurs in fluids only, including magma.
Ideal shear zone: Ductile simple-shear zone with or without additional compaction or dilation across its planar and parallel walls.
Imbrication zone: A series of reverse faults dipping in the same direction and soling out on a floor thrust, but not necessarily bounded by a roof thrust. Also used for similar arrangement of normal faults.
Incremental strain ellipse: Strain ellipse for a small part of the total deformation history. The incremental strain ellipses sum up to the finite strain ellipse.
Inhomogeneous deformation/strain: See heterogeneous deformation/strain.
In-sequence thrusting: Thrusts getting younger toward the foreland.
Instantaneous stretching axes (ISA): Directions of maximum ongoing stretching (ISA1), minimum stretching (maximum shortening; ISA3) and an intermediate axis perpendicular to the other two (ISA2). These axes are defined at any instant during the deformation.
Interlimb angle: The internal angle between the two limbs of a fold.
Internal rotation component of strain: The difference (in degrees) between the orientation of a material line situated along the maximum principal strain axis and the orientation of the same line prior to deformation.
Interplate faults: Faults defining a plate boundary.
Intersection lineation: Lineation formed by the intersection between two planar structures, such as bedding and a cleavage.
Intertectonic: Between two phases of deformation.
Intraplate faults: Faults occurring within tectonic plates, i.e. not defining part of a plate boundary.
Inverse deformation: The deformation (transformation) that takes a deformed object back to its undeformed state. Also called reciprocal deformation or strain.
Inversion: (1) Turning stratigraphy upside-down by means of recumbent folding (inverted limb). (2) Reactivating a normal fault as a reverse fault. (3) Turning a basin into a high and vice versa (related to the previous definition).
ISA: See instantaneous stretching axes.
Isochoric: Having constant volume or area.
Isotropic medium: A medium that has identical mechanical properties in all directions, so that it reacts identically to stress regardless of its orientation.
Isotropic volume change: Volume change caused by shrinking or expanding a volume by the same amount in all directions. Also called volumetric strain.
Joint: Extensional fracture, often laterally extensive (up to hundreds of meters) with very small (microscopic) displacement.
Juxtaposition: Description of the lithologic contact relations on a fault. Commonly expressed in a juxtaposition or triangle diagram.
Juxtaposition seal: Seal where sand is in complete contact with shale across the fault.
Kinematic axes: A kinematic framework defined by three orthogonal axes, a, b and c, with the a-axis representing the transport direction and c being perpendicular to the shear plane.
Kinematic indicator: Any structure indicating the sense of shear or transport during a deformation event. Examples include shear bands in mylonites, rotated porphyroclasts, drag folds and Riedel shears associated with faults.
Kinematic vorticity number, Wk: dimensionless number representing the ratio between the rate of rotation of the strain ellipsoid and the rate of strain accumulation. Wk = 0 for pure shear and 1 for simple shear.
Kinematics: From Greek kinema, meaning motion. The description of how rock masses or objects in rocks move as a result of deformation.
Kink bands: More or less synonymous with kink folds, emphasizing the fact that one limb of such asymmetric folds commonly defines bands of anomalous dips.
Kink folds: Small (typical centimeter-scale) angular folds with straight hinges, thought to form with axial planes at an acute angle to σ1.
Klippe: An erosional remnant of a thrust nappe.
L-fabric: See linear fabric.
L-tectonite: Strongly deformed rock dominated by a linear fabric.
LS-tectonite: Strongly deformed rock dominated by both linear and planar fabric elements.
Lateral ramps: Ramps that form parallel to the transport direction of an allochthonous unit.
Leading branch line: The frontal branch line.
Line defect: Linear defect in the crystal lattice, known as a dislocation.
Lineament: Straight or gently curved line feature on the surface of the Earth (or another planet), identified and mapped by means of remote sensing imagery. A lineament is likely to represent a geologic structure or lithologic contact.
Linear fabric: A lineation that penetrates the rock.
Lineation: Linear structure formed by means of tectonic strain, e.g. rotated amphibole needles in an amphibolite, stretched aggregates of quartz and feldspar in a granitic gneiss, or striations on a fault surface. The linear objects are pervasive (metamorphic rocks) or limited to a fracture surface (brittle regime).
Listric (fault): From Greek listros, meaning “spoon-shaped”. Geometric term that describes the downward flattening geometry of some faults. Faults that steepen by depth are sometimes called antilistric.
Lithostatic pressure: The product of the density and height of the overlying rock layers, multiplied by g, the acceleration due to gravity.
Lithostatic reference state: Reference state of stress in the crust where the crust is considered a medium without shear strength (i.e. a fluid) and where stress in any direction is the product of density, depth and g.
Lithostatic stress: The state of stress caused by the overburden, equal in all directions and thus hydrostatic. A commonly used reference state of stress.
Low-angle fault: Fault dipping less than 30°.
Lower plate: The footwall to a large-scale detachment extensional fault or shear zone.
M-domains: Microscopic cleavage domains dominated by mica (M) and sometimes also other phyllosilicates and opaque phases. M-domains are separated by QF-domains.
M-folds: Symmetric folds, typically folds occurring in the hinge zone of a larger fold.
Master fault: The largest fault in an area.
McKenzie model: The pure shear model for rifting, where the lithosphere is stretched symmetrically in an overall pure-shear style.
Mean stress: The arithmetic mean (average) of the principal stresses.
Mechanical stratigraphy: Stratigraphy based on the mechanical properties of layers rather than lithology or sedimentary characteristics.
Mechanical twinning: The formation of twins in response to a directed stress.
Metamorphic core complex: Exposed portion (window) of the metamorphic rocks of the lower plate (footwall) of a low-angle normal fault (detachment). According to the original use of this term, the upper plate should be exposed to brittle deformation while the lower one contains mylonitic rocks formed in the plastic regime.
Mica fish: Mica grains with pointed and oppositely bent tails, typically delimited by shear bands. Characteristic of micaceous mylonites (or phyllonites).
Microstructures: Structures that range in size from the atomic scale to the scale of grain aggregates, observable under the optical or electron microscope.
Microtectonics: The study of small-scale deformation structures that yield information about strain, kinematics and the deformation history.
Mineral fiber lineation: Lineation formed by growth of fibrous or elongated minerals, such as quartz, serpentine and actinolite.
Mineral lineation: Lineation deformed by parallel-arranged minerals on a surface or throughout a given volume of rocks.
Minibasin: Small basin forming atop, between or adjacent to salt diapirs.
Missing section: See fault cut.
Mode I fracture: Opening-mode or extension fracture.
Mode II fracture: Shear fracture, where the movement is into and out of the plane of observation. Also known as the sliding mode.
Mode III fracture: Shear fracture with movement parallel to the edge, i.e. the plane of observation is parallel to the slip vector. Also called the tearing mode.
Mohr circle: Circle in the Mohr diagram that describes the normal and shear stress acting on planes of all possible orientations through a point in the rock.
Mohr diagram: Diagram where the horizontal and vertical axes represent the normal (σn) and shear (σs) stresses that act on planes through a point.
Mohr failure envelope: The failure envelope of a material found experimentally, regardless of whether it obeys the Coulomb fracture criterion or not.
Monoclinal fold (monocline): Sub-cylindrical fold with only one inclined limb (the other limb is the regionally horizontal layering).
Mullion: Linear deformation structures at the interface between a competent and an incompetent layer, where cusp shapes point into the more competent rock.
Mylonite: Well-foliated tectonic rock formed by intense plastic deformation, usually at middle crustal levels and deeper. Normally characterized by grain size reduction, although grain size increase can occur when fine-grained rocks become mylonitized. Subordinate brittle deform- ation can occur. Subdivided into protomylonite, blastomylonite, mylonite and ultramylonite depending on the degree of recrystallization.
Mylonite nappe: Nappe dominated by mylonitic rocks.
Mylonitic foliation: The foliation formed during mylonitization: usually a strong and compositional foliation defined by parallel minerals and mineral aggregates, lenses and parallel layers reflecting primary structures such as dikes and bedding.
Mylonitization: The process that transforms a rock to a mylonite. This occurs predominantly by means of plastic deformation mechanisms, commonly with subordinate brittle microfracturing.
Nabarro–Herring creep: See volume diffusion.
Nappe: Allochthonous unit, usually a thrust nappe, but the expression has also been used for extensional allochthonous units that are resting on extensional detachments.
Nappe complex: A collection of thrust nappes that share common lithological and/or structural features and form a single unit.
Neck: Term used to describe the narrow connection between elements in pinch-and-swell structures.
Necking: The formation of pinch-and-swell structures.
Neutral point: Point in a fold hinge of no stretching or shortening.
Neutral surface: Theoretical surface in a fold hinge zone separating layer-parallel extension in the outer part from layer-parallel shortening in the inner part of the hinge.
Newtonian fluid: A material that deforms so that shear stress and shear strain are linearly related. Also called linear or perfectly viscous material.
Non-coaxial deformation: Lines along the principal strain axes do not have the same orientation before and after the deformation.
Non-coaxial deformation history: Lines along ISA as well as the principal strain axes rotate during the deformation history: Wk ≠ 0.
Non-steady-state deformation: The particle paths and flow parameters such as ISA and Wk vary during the deformation history.
Normal drag: Rotation of layers in the walls of a fault so that the curvature is consistent with sense of offset on the fault.
Normal fault: Fault where the hanging wall has moved down relative to the footwall. Normal faults are extensional with respect to a horizontal layer or the surface of the Earth.
Normal stress: Stress or stress component acting perpendicular to the surface of reference.
Oblate: Disk-shaped geometry, used to describe the strain ellipsoid. For perfect oblate objects the maximum and intermediate axes (X and Y) are of the same length: X = Y >> Z.
Oblique ramp: Ramps oriented obliquely to the transport direction (in thrust systems or extensional fault systems).
Oblique-slip fault: Fault where the displacement vector is dipping at a lower angle than the dip of the fault, i.e. a mixture of strike-slip and either normal or reverse movement.
Oceanic metamorphic core complex: Metamorphic core complex formed in an oceanic rift setting, where the metamorphic core consists of serpentinites.
Open-toed advance: Salt sheet advance by means of extrusion at the toe while the rest of the sheet is being covered by sediments.
Orogenic root collapse: Model where the orogenic root is less dense than its surroundings so that it collapses upward and spreads out laterally.
Orogenic wedge: The wedge-shaped (as seen in cross-section) area of allochthons in an orogenic zone or mountain range: thickest in the hinterland and thinning toward the foreland.
Orthogonal flexure: Fold mechanism where lines originally orthogonal to the layering remain orthogonal, producing a neutral surface separating outer-arc stretching from inner-arc shortening. Also called tangential longitudinal strain.
Out-of-sequence thrusting: Thrusts not getting systematically younger (or older) toward the foreland.
Outlier: An erosional remnant of a thrust nappe. Identical to klippe.
Overcoring: Strain relaxation method where the expansion of a drill core is measured and related to stress.
Overlap zone: Zone between overlapping fault segments. Can be hard linked, where the overlapping faults are physically connected, or soft linked, where the strain in the overlap zone is ductile (continuous deformation). A relay ramp is a common expression of the latter.
Overlapping faults: Two faults with approximately the same strike orientation but laterally offset with respect to each other so that the fault tips are misaligned. The tips have grown past each other by a length that is much smaller than the fault length, and the two faults must be close enough that their elastic strain fields overlap.
Overpressure: The pore pressure in a stratigraphic unit exceeds the hydrostatic pressure. This can occur in a highly porous and permeable unit (e.g. sandstone) that is captured between two impermeable layers (e.g. shale) so that the pore fluid cannot escape. Further lithostatic loading (burial) will then generate an overpressure, which reduces the effective stress and counteracts physical compaction.
Overprinting relations: Relative age relations between structures that can be observed in the field, from aerial photos, satellite images or seismic sections, for example one fault cutting another, or a fold being refolded.
P-shear fractures, P-shears: Sets of subsidiary slip surfaces arranged oblique to the zone. When the section of observation (which for a strike-slip fault should be horizontal) is considered as a profile, P-shears define a thrust-type sense of movement.
P–T path: A path in a P–T diagram (diagram with pressure and temperature along the axes), with an arrow on the path showing the development. The path is based on identification of metamorphic parageneses indicating pressure and temperature at different times.
P–T–t path: A path in P–T space that contains points that have been dated, usually radiometrically.
Paleopiezometer: Use of recrystallized grain size to estimate the level of differential stress during recrystallization (or subgrain formation), based on experimental or theoretical data.
Parallel folds: Folds showing constant layer thickness (Class 1B).
Parasitic folds: Folds occurring on the limbs or hinge of a larger fold that formed during a single process of folding.
Parautochthonous: Almost autochthonous rocks in an orogen, with only short transportation. The rocks can easily be correlated with the autochthonous.
Particle path: The path traced out by particles in a deforming medium during progressive deformation.
Particulate flow: Identical to granular flow.
Passive diapirism: Diapirism where salt is at or near the surface and rises due to loading by sediments continually accumulating and downbuilding around the diapir, displacing salt into the diapir.
Passive folding: Folding where the layering has no mechanical influence (no competency contrast): the layers merely act as markers.
Passive marker: Structure or object that has no mechanical or rheological contrast to its surroundings so that it deforms identically to its surrounding rock. This implies that it will not buckle or boudinage.
Passive rifting: Rifting forming in response to, and driven by, far-field forces related to plate interactions. Passive rifts form along weak zones in the lithosphere.
Pencil cleavage: Two differently oriented cleavages in shales causing the shale to break into pencil-like fragments. Typically a combination of compaction and tectonic strain.
Perfect plastic deformation: Time-independent plastic deformation where strain rate has no influence on the stress–strain curve. Perfect plastic deformation does not show any work (strain) hardening or softening.
Perfect plastic material: Incompressible material that accumulates permanent strain at a constant stress level and where this level of stress (the yield stress) cannot be increased even if we try: the yield stress is independent of strain rate.
Periodic folds: Folds along a single layer showing constant wavelength–thickness ratio (L/h).
Permanent strain: Strain that remains after the removal of the stress field that caused the strain.
Phyllitic cleavage: Continuous cleavage formed under lower-middle greenschist facies conditions in phyllitic rocks.
Phyllonite: Micaeous mylonite, typically strongly sheared low-grade metapelite.
Phyllosilicate (deformation) bands: Deformation bands where phyllosilicates have reoriented to form a local fabric along the band. Common in phyllosilicate-bearing sands and sandstones. Also called phyllosilicate frame-work structures.
Pinch-and-swell structures: Necking of (usually) competent layers without the formation of clearly separated boudins.
Pinning: Expression used about the role of non-recrystallizing minerals as obstacles in a rock that undergoes recrystallization.
Pinpoint: Fixed point of reference during restoration or balancing.
Pitch: The angle between the strike of a slip surface and the slip lineation, measured with a protractor on the slip surface. Also called rake.
Planar fabric: Foliation, cleavage.
Plane strain: Strain where the intermediate strain axis (Y) remains unaffected by the deformation (Y = 1), shorten- ing in Z being compensated by extension in X. There is therefore no particle motion perpendicular to the XZ- plane. Plane strain produces strain ellipsoids that plot along the diagonal (k = 1) of the Flinn diagram.
Plastic behavior: Deformation that only occurs at a given yield stress. The deformation is permanent, and could be strain hardening or softening.
Plastic deformation: Ductile deformation resulting from plastic deformation mechanisms. Plastic deformation produces a non-recoverable change in shape (permanent strain) without failure by rupture. To some scientists (engineers in particular) plastic deformation is simply the accumulation of permanent deformation without fracturing in response to stress. Soil mechanics people therefore regard clay as a (quasi-)plastic material.
Plastic deformation mechanisms: Dislocation creep (glide + climb), twinning and diffusion. In a strict meaning of the term, diffusion is separated from both plastic and brittle deformation mechanisms.
Plastic shear zone: Shear zone dominated by plastic deformation mechanisms.
Plasticity: Deformation mechanisms where atomic bonds are broken while material coherency or cohesion is maintained. Note however that soil mechanicists relate plasticity merely to rheology (the accumulation of permanent strain).
Plumose structures: Subtle relief pattern on joints that resembles the structure of a feather, indicating the growth direction of joints. The pattern points toward the nucleation point of the joint. Commonly found on joints in fine-grained rocks. Also called hackle marks.
Poiseuille flow: Flow in a layer where the flow is fastest in the middle and decreases toward the margins due to friction.
Poisson’s ratio: The ratio between strain imposed on a sample in one direction and the resulting strain perpendicular to this direction. If they balance there is no volume change, the material is incompressible, and Poisson’s ratio is 0.5.
Polyclinal folds: Folds where axial surfaces show variable or contrasting dips.
Polyphasal: Having several phases; used about deformation that has occurred as discrete events, where the individual events or phases are named D1, D2, …
Pore pressure: The pressure in the fluid (water, oil or gas) filling the pore space in a porous rock.
Porphyroblast: Metamorphic mineral grain significantly larger than those constituting the matrix.
Porphyroclast: Larger relict mineral grains, typically feldspar or other “resistant” minerals, in a strongly sheared finer-grained mylonitic matrix. Commonly with asymmetric tails that reflect sense of shear.
Position gradient tensor: See deformation matrix.
Postkinematic: After deformation.
Postrift: After rifting; typically used about the sequence of sediments deposited during the phase of thermal subsidence after the cessation of rifting and related extensional faulting and stretching.
Posttectonic: Something that happened after the deformation phase in question.
Power-law distribution: Data defining a straight line in a log-log diagram.
Pressure difference: Used here for difference in pressure from one location to another, typically resulting from differential loading. Creates an unstable situation in viscous rocks (fluids) that drives flow, for instance in a salt layer or in the asthenospheric mantle.
Pressure solution: Wet diffusion along grain boundaries. The only type of diffusion that can occur at submetamorphic temperatures. Commonly called dissolution or solution when found in sedimentary rocks, because tem- perature and chemical factors seem to be more important than pressure during compaction of sediments.
Pressure solution cleavage: Tectonic cleavage defined by subparallel solution seams. Common in limestones.
Pretectonic: Prior to the deformation phase in question.
Primary fabric: Sedimentary and magmatic fabric formed as the rock was deposited or crystallized.
Primary foliation: Sedimentary and magmatic foliation formed as the rock was deposited or crystallized (e.g. bedding).
Principal planes of stress: The planes in a stress field that do not contain shear stresses. These planes are perpendicular to the principal stress axes.
Principal strain axes: The two (three) orthogonal axes of the strain ellipse (ellipsoid) that represent the directions and amounts of maximum extension (X) and shortening (Z). The strain ellipsoid has a third, intermediate axis (Y) that is orthogonal to the other two. Eigenvectors of the deformation matrix.
Principal stress axes, principal stresses: The three mutually orthogonal axes (two in two dimensions) defining the directions of the principal stresses and therefore the stress ellipsoid. Eigenvectors of the stress tensor. Values of the principal stresses are the lengths of these axes and the eigenvalues of the stress tensor.
Principal stretches: The lengths X, Y and Z of the principal strain axes.
Process zone: The zone ahead of a fracture where the formation of numerous microcracks softens the rock. The process zone propagates ahead of the fracture. Also called the frictional breakdown zone.
Prograde metamorphism: A metamorphic development toward higher metamorphic grade.
Progressive deformation: Ongoing and evolving deformation, producing a deformation history.
Prolate: Elongate three-dimensional shape reminiscent of a cigar. A perfectly prolate object has a major principal strain that is much longer than the other two axes, which are of equal length: X >> Y = Z.
Protolith: The original rock with structures or textures that have not been significantly altered by the deformation in question, e.g. a protolithic lens in a shear zone.
Protolithic lenses: Lens-shaped bodies of the protolith in deformed rocks. Also see protolith.
Pseudotachylyte (US), pseudotachylite (UK): Glass or devitrified glass formed as a result of frictional melting during faulting.
Pull-apart basin: Basin formed in an extensional overlap or releasing bend of a strike-slip fault or fault zone.
Pulverization: Process occurring during faulting where rock is crushed to submicrometer particle sizes so that the fractured particles remain in place such that the original (primary) textures of the rock are clearly evident in hand sample. Pulverization is thought to be associated with supershear rupture velocity quakes (very fast rupture rates), which have been observed along strike-slip faults.
Pure shear: Plane strain coaxial deformation where particles move symmetrically around the principal axes of the strain ellipse in the XY-plane. Flow apophyses are orthogonal and Wk = 0.
Pure shear model of rifting: See McKenzie model.
Pure shear zone: Shear zone deformed by pure shear.
QF-domains: Microscopic cleavage domains dominated by quartz (Q) and feldspar (F), separated by mica-rich M-domains.
R'-fractures: Fractures in a shear setting that are antithetic to Riedel shears.
R-shears: See Riedel shears.
Radial faults: Faults forming a radial pattern, ideally radiating from (or projecting towards) a common center point. Typically found in the overburden of domes, notably salt domes.
Raft: Fault block in an extensional fault system atop adécollement, where the fault block is separated from pre-faulting strata in its neighboring fault blocks. The process is called rafting.
Rake: See pitch.
Ramp: The steep and relatively short segment of a thrust fault as it climbs to a higher level in the stratigraphy. Now also used for steep segments connecting two low-angle normal fault segments. See also relay ramp.
Ramp-flat-ramp geometry: A fault with a subhorizontal central segment bounded by steeper segments at each end.
Random fabric: Fabric where fabric elements show no preferred orientation.
Reactive diapirism: The process of salt rising as a reaction to significant tectonic brittle strain, usually extension.
Reciprocal deformation: The deformation (transformation) that takes a deformed object back to its undeformed state. Also called inverse deformation.
Recovery: Removal or rearrangement of dislocations in a crystal to produce domains of fewer dislocations and hence lower energy. Leads to subgrain formation.
Recrystallization: Process occurring during deformation (and diagenesis) by means of grain boundary migration and/or the formation of new grain boundaries by the localization of dislocations and point defects. Recrystallization that occurs during deformation is known as dynamic, whereas temperature-controlled recrystallization is static. Unstable minerals can also recrystallize into new minerals with different compositions during changing metamorphic conditions.
Recumbent fold: Fold with subhorizontal axial surface and horizontal hinge line.
Reduced stress tensor: The tensor containing information about the orientation of the principal stress axes and the relative (but not absolute) length of the axes.
Relay: The zone between overlapping fault segments.
Relay ramp: Folded area in a relay formed by flexing of layers between the fault tips. Usually used for subhorizontal layers that are given a ramp-like geometry in the overlap zone. The folding is due to strain transfer between the two faults.
Releasing bend: A bend along a strike-slip fault that has generated local extension.
Releasing overlap zones: Overlap zones where the fault arrangement and sense of displacement cause stretching within the overlap zone. Also called extensional overlap zones.
Remote stress: The stress field that is regionally present away from anomalies set up by local structures such as weak faults, i.e. remote with respect to some structure(s) in question. Also called far-field stress.
Repeated section: Stratigraphic section that is repeated because of faulting, i.e. the opposite of missing section. Whether one observes repeated or missing sections in a well depends on the orientations of the fault and the well. A vertical well gives repeated sections across reverse or thrust faults.
Residual stress: Stress that has been locked into a rock so that it is preserved after the external stress field has been changed or removed.
Restoration: Expression used for the reconstruction of a geologic section, map or 3-D model to its predeformational situation.
Restraining bends: Bends on strike-slip faults that generate local contraction or transpression.
Restraining overlap zones: Overlap zones with shortening in the displacement direction. Also called contractional overlap zones.
Retrodeform: Undeform, transform back to the predeformational stage.
Retrograde metamorphism: A metamorphic development toward lower metamorphic grade.
Reverse drag: Rotation of layers in the hanging wall of a fault so that the curvature is inconsistent with sense of offset on the fault. Rollover structures, which are related to fault geometry, are the most common example of reverse drag.
Reverse fault: Fault where the hanging wall has moved up relative to the footwall, implying shortening of horizontal layers.
Rheologic stratigraphy: The rheological stratification of the lithosphere or part of the lithosphere, where a subdivision can be made into layers of contrasting rheological properties.
Rheology: The study of flow (rheo in Greek) of any rock and other material that deforms as a continuum under the influence of stress. Elasticity, viscosity and combinations of these are different rheological behaviors.
Ribs and rib marks: Elliptical (conchoidal) structures occurring on extension fractures, centered on the nucleation point of the fracture. They are ridges or furrows where the fracture has a slightly anomalous orientation and are perpendicular to hackle marks.
Riedel shear fractures, Riedel shears: Sets of subsidiary slip surfaces arranged en echelon, each Riedel or R-shear being oblique to the zone or main slip surface. When the section of observation (which for a strike-slip fault should be horizontal) is considered as a profile, R-shears appear extensional. An antithetic set (R′ or R prime) also occurs, although less commonly than R-shears.
Rim syncline: Syncline along the rim of a map-scale structure. Typically used about folds encircling and outlining salt diapirs.
Rodding: Elongated mineral aggregates (lineation) forming during deformation.
Rolling hinge model: Dynamic model where the upper part of an initially steep normal fault flattens as offset accumulates. The hinge of the curved fault moves or “rolls” toward the hanging wall due to isostatic adjustments during faulting. The model results in a metamorphic core complex.
Rollover: The fold structure defined by the steepening of otherwise horizontal hanging-wall layers toward a normal fault. Normally related to a downward-flattening (listric) fault.
Roof thrust: Low-angle fault that defines the upper limitation of a duplex structure.
Rotational deformation: Expression used for non-coaxial deformations.
S-C mylonites: Mylonites showing two sets of more or less simultaneous surfaces (S and C) with an oblique angle that indicates the sense of shear.
S-fabric: Planar fabric.
S-fold: Asymmetric fold where the short limbs appear to have been rotated anticlockwise with respect to their long limbs.
S-tectonite: Strongly deformed rock dominated by a planar fabric.
Salt anticline: Salt-cored anticline, usually formed by salt movement (flow into the anticlinal structure) and bending of overlying strata (no intrusion of salt).
Salt (stock) canopy: Salt from several upward-widening diapirs that have merged at a higher stratigraphic level than their source salt layer.
Salt expulsion: Another name for salt withdrawal.
Salt glacier: Sheet or tongue of salt flowing from a sur- facing salt diapir where salt is extruded, usually due to contractional strains.
Salt (wing) intrusion: Salt intruding a higher stratigraphic level below the surface. Sometimes called salt wing intrusion.
Salt nappe: Equivalent to salt sheet.
Salt pillow: A concentration of salt caused by flow of salt into a sink where the salt has not intruded the overburden.
Salt roller: Asymmetric triangular-shaped volume of salt forming in the footwall of a normal fault.
Salt sheet: Sheet of allochthonous salt formed by the lateral flow of salt from one or more diapirs, generally at or near the surface.
Salt stock: Salt diapir with more or less cylindrical geometry.
Salt tectonics: Term used when salt is involved in deformation to the extent that it influences the type, geometry, localization and/or extent of deformation structures that form. Does not imply a direct relationship with plate tectonic stress.
Salt wall: Elongated salt diapirs, where the length is many times the width of the structure.
Salt weld: Point or area where a salt layer becomes completely exhausted (removed by flow) so that the boundary layers become attached to each other.
Salt withdrawal: Salt flowing from a local area into an adjacent salt structure. Salt withdrawal can result in local sedimentary basins (so-called minibasins) above the area of withdrawal, and generate salt welds when all the salt has withdrawn.
Scale model: A model that has been properly scaled down (or up) from some natural example.
Scaling: Changing natural physical quantities to those appropriate for a given laboratory setup. Parallel to the scaling of lengths, such as the thickness of the crust, it is necessary to scale strain rate and quantities such as temperature, viscosity, stress, gravity or confining stress, cohesion and grain size.
Schistosity: Tectonic foliation defined by coarse-grained platy minerals in rocks deformed under upper greenschist and amphibolite facies conditions.
Screw dislocation: Type of linear defect in a crystal lattice. See Figure 11.12.
Sealing fault: Impermeable or very low permeability fault in porous and permeable rocks that prevents fluids from crossing the fault. Shale or clay smearing as well as a membrane of non-permeable fault gouge may seal faults, even if there is sand–sand contact along the fault surface.
Secondary foliation: Foliation formed after the rock was deposited or crystallized.
Seismogenic zone: The zone of frequent earthquakes in the crust, which is the middle and lower part of the brittle crust, where the crust is strongest.
Self-juxtaposed seal: Reservoir against reservoir across the fault. Sealing due to fault rock development, including cataclasis and cementation in or along the fault.
Semi-ductile: Both ductile and brittle deformation style, i.e. originally continuous layers are partly deflected and partly affected by discontinuities.
Shale gouge ratio (SGR): Ratio used in fault sealing analysis, relating the amount of shale that has passed a point at a fault surface to the local fault throw.
Shale smear factor (SSF): Ratio between fault throw and the thickness of the shale source layer. SSF is used in the evaluation of the sealing properties of faults.
Sharp discontinuity: Term used about fractures (including stylolites), i.e. structures that are very thin relative to their length and height. The term discontinuity relates to abrupt changes in displacement (rate) and also implies that the structures represent mechanical anomalies characterized by reduced shear strength, tensile strength and stiffness, and also high fluid conductivity relative to the host rock. See also tabular discontinuities.
Shear bands: Small-scale (millimeter- or centimeter-scale) shear zones, usually in well-foliated mylonitic rocks. Single sets of shear bands make an oblique angle to the foliation that indicates the sense of shear in sheared rocks.
Shear fold: Term used for similar (passive or Class 2) folds.
Shear fracture: Fracture with detectable wall-parallel displacement. Different from fault in that it only consists of a single fracture, while faults are composed of a number of linked fractures.
Shear sense indicators: See kinematic indicators.
Shear strain (γ): The tangent value of the change in angle ψ experienced by two originally perpendicular lines.
Shear strain rate: The rate at which shear strain accumulates.
Shear stress: Stress acting parallel to a plane of reference.
Shear zone: Tabular strain zone dominated by ductile (continuous) deformation, typically dominated by simple shear.
Shear zone walls: The two bounding sides of a shear zone. Also called shear zone margins.
Sheath folds: Highly non-cylindrical folds formed in zones of high shear strains.
Shortcut fault: Fault or fault segment that establishes a new and shorter or less painful trace, for instance during fault reactivation or due to geometric complications such as a kink in the fault plane. In the latter case a short-cut fault would cut across the kink and make the fault straighter.
Shortening: See contraction.
Similar fold: Fold where the inner and outer arcs have exactly the same shape, implying that the distance between the arcs parallel to the axial trace is everywhere the same. Similar folds are Class 2 folds according to Ramsay’s dip isogon-based classification.
Simple shear: Non-coaxial plane strain deformation where particles move along straight lines and Wk = 1.
Simple shear model of rifting: See Wernicke model.
Simple shear zone: Shear zone deformed by simple shear.
Sinistral: Left-lateral, moving left relative to the point of reference. Used about relative sense of movement on faults, indicating that the opposite fault block is offset or moves to the left.
Slaty cleavage: Tectonic cleavage in slate that forms by reorientation and pressure solution (diffusion) of minerals.
Slickenlines: Lineations found on slickensides, providing information about the direction of slip on faults.
Slickensides: The finely polished surface displayed by faults, formed by extensive grain crushing (milling) and/or synkinematic mineral growth.
Slickolites: The peaks or “teeth” on tectonic stylolites.
Slip: (1) Shear motion localized to a surface (slip surface). (2) Movement of a dislocation front within a crystallographic plane.
Slip plane: (1) Planar slip surface. (2) Crystal lattice plane along which dislocations move.
Slip surface: Well-defined surface or narrow (< 1 mm) zone along which slip has occurred. In principal, the amount of slip can be from a few centimeters to several kilometers, but large displacements tend to produce a zone (not surface) of brittle shearing known as the fault core. A slip plane and a fault are equivalent terms to some geologists.
Smear: Zone or membrane of fine-grained and commonly impermeable rock or sediment smeared along a fault. The smear comes from clay, shale or other fine-grained rock in the rock strata affected by faulting.
Soft domino model: The domino model modified to allow for ductile strain within fault blocks.
Soft link: Expression used for fault overlap zones where there is no fault connecting the two overlapping fault segments. Instead, a relay ramp may be present.
Soil mechanics: Engineering mechanics applied to soils.
Sole thrust: Low-angle thrust fault that marks the base of a tectonic unit.
Spaced cleavage: Cleavage where the width of the cleavage domains is at least 1 mm, i.e. distinguishable in a rock sample.
Splay faults: Smaller faults horse-tailing from a fault near its tip point.
Spreading: Gravitationally driven translation of rocks with a radial displacement or velocity pattern, similar to what happens when putting soft dough on a table.
Static recrystallization: Recrystallization of a rock or mineral grain (shortly) after deformation. Characterized by equigranular texture unstrained grains (no pronounced undulatory extinction under the microscope etc.).
Steady-state deformation: Deformation where particle paths and flow parameters such as ISA and Wk are constant throughout the deformation history.
Steady-state flow (or creep): Deformation of a material at constant strain rate. See also steady-state deformation.
Stem: Thin part of a teardrop diapir.
Stickolites: Tubular structures perpendicular to pressure solution seams in limestones.
Stick-slip: Sudden (seismic) slip events separated by periods of no motion.
Stiff: High Young’s modulus.
Strain: Change in length (1-D) or shape (2-D or 3-D) due to deformation. Primarily defined for continuous (ductile) deformation, but is also used in areas of faulting: see fault strain.
Strain axes: See principal strain axes.
Strain compatibility: Fulfilled when there is no slip along an interface separating differently strained layers.
Strain ellipsoid: The ellipsoid resulting from the deformation of a unit sphere passively present in the deforming medium. The strain ellipse has three principal axes that define the directions of maximum, minimum and intermediate strains (principal strain axes).
Strain energy: The energy stored within a crystal as dislocations accumulate during straining. Proportional to dislocation density.
Strain geometry: Shape of the strain ellipsoid or Flinn k-value, commonly displayed in the Flinn diagram.
Strain hardening: The effect in which the stress level must be increased in order to maintain a fixed strain rate. Best constrained in the laboratory during rock mechanics experiments. In a shear zone or deformation band setting, strain hardening means that it is easier to deform the wall rock than to continue the deformation in the zone or band. Strain hardening is also referred to as deformation hardening or work hardening.
Strain invariants: The principal strain axes and their lengths (eigenvectors and eigenvalues of the deformation matrix), since they are independent of our choice of coordinate system.
Strain markers: Objects reflecting the state of strain in a deformed medium, i.e. lines or objects of known undeformed length, shape and/or orientation.
Strain partitioning: The physical decomposition of strain into different components from micro- to mesoscale. For example: Transpression can be partitioned into zones of pure and simple shear, e.g. along obliquely convergent plate boundaries. At the centimeter-scale, simple shear can be partitioned into oblique shear bands and domains experiencing back-rotation.
Strain rate: The rate or speed at which strain accumulates. Two different types of strain rate are in common use: elongation rate and shear strain rate.
Strain shadows: Sectors on each side of a porphyroclast or other rigid object in a shear zone or mylonite zone where minerals may crystallize into tails. May be symmetric or asymmetric, in the latter case serving as sense-of-shear indicators. Related to sectors of instantaneous stretching. Also called pressure shadows and pressure fringes.
Strain softening: The effect in which the stress level must be decreased in order to maintain a fixed strain rate, i.e. opposite to strain hardening.
Stratigraphic separation: The missing or repeated section seen in a well that penetrates a fault.
Strength: The amount of stress that a rock can sustain before it fails or yields. Differential stress (σ1 − σ3) is commonly implied in geology. Unconfined rock samples can be tested in the laboratory under axial compression, axial tension or shear, which will give different values (compressive strength, tensile strength and shear strength). However, addition of a confining pressure increases the strength of a rock, or the differential stress that is required for fracture or permanent strain to accumulate.
Stress: Stress on a surface is the force over the area on which it is applied. Typically decomposed into a normal and a shear component. The complete state of stress at a point is given by the stress tensor and illustrated by the stress ellipsoid.
Stress ellipsoid: Ellipsoid that describes the state of stress at a point. A point represents the intersection of an infinite number of planes, and the normal stress to any given plane at that point is the distance from the point to the ellipse, measured perpendicular to the plane in question. The axes of the ellipse are the principal stresses (maximum, minimum and intermediate normal stresses, designated σ1, σ2 and σ3), oriented perpendicular to the principal planes of stress. A stress ellipsoid only exists when all three principal stresses are compressive or tensile. Stress ellipse in two dimensions.
Stress guide: Competent layer along which stress is preferentially transmitted. Such layers could be limestones in sedimentary rocks, or the lower part of the brittle upper crust at lithospheric scale.
Stress tensor (matrix): Tensor describing the state of stress at a point.
Stretching (s): s = 1 + e, where e is elongation (extension). Equal to the beta factor reported in regional estimates of the stretching of a basin.
Stretching lineation: Lineation formed by tectonic stretching of objects, such as mineral aggregates and conglomerate clasts.
Striations, striae: Linear scratches and grooves on a slip surface, formed by frictional movement of the hanging wall against the footwall of a fault.
Strike-slip duplex: Duplex formed along a strike-slip fault, similar to classic thrust- or extension-related duplexes turned on their side. Hence, there are two types: extensional and contractional strike-slip duplexes.
Strike-slip fault: Fault where the displacement is horizontal. Strike-slip faults are dextral (right-lateral) or sinistral (left-lateral).
Structural geology: The study of deformation structures in the lithosphere in order to understand their geometry, distribution and formation.
Structural style: The distinctive appearance of a set of structures. For example, folds having similar opening angle, vergence and/or metamorphic mineral association share a structural style that makes them distinct from other folds in the same rocks, or similar to folds in another area or unit. Tectonic setting, depth–temperature conditions and lithologic characteristics all influence the resulting structural style.
Stylolite: Compactional serrated surfaces formed perpendicular to σ1 by dissolution of minerals (usually carbonate or quartz) by wet diffusion (pressure solution) due to burial or tectonic stress.
Stylolitic cleavage: Cleavage consisting of stylolites (pressure solution seams). Also called pressure-solution cleavage.
Subgrains: Part of a mineral grain that has a uniform optical, and therefore also crystallographic, orientation that differs from that of the rest of the grain. At the crystal lattice scale, subgrains are accumulations of dislocations in the form of dislocation walls.
Subseismic faults: Faults that are too small to be imaged on a given seismic line or survey. Their size range is defined upward by the resolution of the seismic data in each case, and other data, such as cores, outcrop data or dipmeter data, must be used to identify them.
Subsimple shear: Planar deformation intermediate between simple and pure shear, i.e. 0 < Wk < 1 and the flow apophyses make an acute angle.
Subsimple shear zone: Shear zone where the deformation is subsimple shear.
Superplasticity: Microscale deformation process dominated by grain boundary sliding and favored by fine grain size. No longer widely used.
Surface forces: Forces that act on a surface (traction).
Surface lineations: Lineations limited to a surface (fault or fracture), i.e. not penetrative.
Stylolites: Wavy or serrated surfaces enriched in residual minerals, usually clays or micas. Most common in limestone, but also in sandstones and cherts. They are compactional structures that form perpendicular to the shortening direction.
Syncline: Fold where rock layers get older away from the axial surface of the fold.
Synform: Fold where the limbs dip down and toward the hinge zone.
Synformal anticline: An anticline (strata get younger away from its axial surface) that has the shape of a synform, i.e. an anticline turned upside down.
Synkinematic: Formed or occurring during deformation.
Synrift: Formed during rifting, typically used about sedimentary sequences that show thickening toward the hanging-wall side of extensional faults.
Syntectonic: During the deformation phase in question.
Synthetic fault: A small fault dipping in the same direction as an adjacent main fault.
Synthetic shear: Shear acting synthetically to the sense of displacement of a reference fault. Applied during restoration of cross-sections.
T-fractures: Extension fractures occurring in a brittle shear zone or fault. They initially indicate the instantaneous stretching direction, but easily rotate during continued deformation. Typically associated with R- and P-fractures in strike-slip fault zones.
Tabular discontinuity: Term used for strain localization structures with a measurable thickness in outcrop or hand specimen (i.e. thicker than sharp discontinuities) and a continuous change in strength or displacement across them. Deformation bands are examples of tabular discontinuities. Shear zones can be considered tabular discontinuities.
Tangent-lineation diagrams: Plots of fault data and their kinematics in stereonet that reveal paleostress orientations and possibly stress ellipsoid shape.
Tangential longitudinal strain: Fold mechanism where lines that were orthogonal to the folded layer before folding remain so after the folding is over. The mechanism implies outer-arc extension and inner-arc contraction.
Tear fault: See transfer fault.
Teardrop diapir: Allochthonous salt shaped like an inverted teardrop, formed by welding of the stem of an upward-widening diapir.
Tectonic fabric: Fabric resulting from a tectonic process, as opposed to primary fabric.
Tectonic overpressure: The non-lithostatic component of pressure deep in the crust or lithosphere.
Tectonic stress: Lithosphere-scale deviations from any chosen stress reference state that are caused by tectonics, typically plate tectonics.
Tectonics: From the Greek word tektos – to build. The knowledge of how the lithosphere is being built as rocks or sediments yield after a period of stress build-up. Examples are plate interactions (plate tectonics), salt movements (salt tectonics), the effect of glaciers (glaciotectonics) and collapsing orogens or fault scarps (gravity tectonics).
Tectonite: Strongly strained rock, usually a mylonite. A distinction is made between L-tectonites (strong lineation), S-tectonites (strong foliation) and LS-tectonites (pronounced foliation and lineation).
Tensile fracture, tensile crack: Extension fractures with small openings perpendicular to the walls, i.e. Mode I or opening-mode fractures. Most geologists find it useful to reserve the term tension for stress, in which case the term tensile fractures should be restricted to extension fractures formed under tension.
Tensile strength: The intercept of a fracture criterion or envelope with the negative horizontal axis. In general: the amount of tensile stress that a medium can withstand before failing.
Tension: Pull that can (but does not have to) result in extension.
Tensor: An nth order tensor is an object that has mn components, m being the dimension. Zero-order tensors are scalars, first-order tensors are vectors, while second- and higher-order tensors are matrices. Tensors have properties that are independent of coordinate systems. For instance, the deformation matrix (second- or third-order tensor in section or space) will describe the strain ellipsoid and dilation regardless of the related coordinate system.
Thermal loading: The (increased) buoyancy of salt caused by heating (salt expands and thus becomes lighter when heated).
Thermobarometry: The quantitative determination of the temperature and pressure at which a metamorphic (or igneous) rock reached chemical equilibrium. Metamorphic index minerals or mineral parageneses are used to estimate P and T, based on known reactions and stability fields.
Thick-skinned tectonics: Orogenic deformation involving the basement and not only the overlying sedimentary cover.
Thin-skinned tectonics: Orogenic deformation not involving the basement, only the overlying sedimentary cover. Basement nappes may still occur in the orogenic wedge, but these basement rocks must stem from an area far away from the area in question.
Throw: The vertical component of the dip separation of faults.
Thrust: Contractional low-angle plastic shear zone or brittle fault (thrust fault) separating an allochthonous unit (thrust nappe) from underlying or overlying rocks. Thrusts typically place older rocks on top of younger.
Thrust advance: The motion of salt sheets by thrusting (shear strain localization along the base of the salt sheet).
Thrust fault: Low-angle reverse fault with measureable displacement (often > 10 km). More informally used for any low-angle reverse fault, regardless of the amount of displacement.
Thrust nappe: Map-scale rock unit that has been transported on a thrust fault for tens of kilometers or more.
Tightness (of folds): Fold geometry depending on the fold’s interlimb angle. From gentle via open and tight to isoclinal.
Traction: Stress on a surface.
Trailing branch line: The rearmost branch line of a nappe or nappe complex.
Transcurrent faults: Strike-slip faults with unconstrained tip.
Transected fold: Fold whose axial plane cleavage is oblique with respect to the axial surface of the fold.
Transecting cleavage: The cleavage transecting the fold hinge of a transected fold.
Transfer fault: Fault that transfers offset from one fault to another.
Transform fault: Strike-slip fault that defines a plate boundary. Transform faults transfer movement between mid-ocean ridge segments or island-arc bounding faults.
Translation: Rigid movement without any rotation or strain. The displacement field consists of parallel and equally long displacement vectors.
Transmissibility: The ability of a fault to transmit fluids in a hydrocarbon or water reservoir.
Transposed layering: Layering formed by tectonic flattening of originally cross-cutting elements (dikes, beds, cross-beds, magmatic banding, tectonic foliation, etc.) into a composite foliation of subparallel elements. The process involves high strain and the foliation reflects the flattening (XY) plane of the strain ellipsoid. Commonly seen in gneisses formed in the lower crust.
Transpression: Strike-slip zone with an additional and simultaneous shortening across the zone. Usually a three-dimensional deformation where the strain ellipses plot off the k = 1 diagonal of the Flinn diagram.
Transtension: Strike-slip zone with additional and simultaneous extension across the zone, i.e. opposite to transpression.
Triangle diagram: Diagram showing the juxtaposition of beds for a continuously increasing offset. The input for the construction of a triangle diagram is the local stratigraphy. Factors such as SGR-values can be calculated and incorporated into triangle diagrams.
Trishear: Model for the deformation ahead of a propagating fault tip, where shear fans out into an upward-widening zone of heterogeneous ductile isochoric deformation expressed by (fault propagation) folding of horizontal layers. See Box 9.4.
Undulatory (or undulose) extinction: Uneven extinction of mineral grains seen under the optical microscope with crossed polarizers on rotation of the stage.
Uniaxial contraction/shortening: Contraction in one direction and no strain in the plane perpendicular to this direction. Compaction of sediments is an example.
Uniaxial extension: Extension in one direction and no strain in the plane perpendicular to this direction. Uniaxial extension implies volume increase.
Uniaxial-strain reference state: Model of the state of stress in the crust, where stress arises as a consequence of vertical compaction only (i.e. uniaxial compaction). Stresses arise because rocks cannot expand or contract in the horizontal plane and the stress level increases downward with the weight of the overburden.
Uniform extension: A state of strain where stretching in X is compensated for by equal shortening in the plane orthogonal to X.
Uniform flattening: A state of strain where shortening in Z is compensated for by identical stretching in all directions perpendicular to Z.
Uniform slip model: The slip at a given point is the same in each slip event, while the slipping area may vary.
Upper plate: The hanging wall to a large-scale extensional detachment fault or shear zone.
Upright fold: Fold with vertical axial surface and horizontal hinge line.
Variable slip model: Both the amount of slip and the rupture length vary from event to event.
Vein: Extension fracture filled with mineral(s).
Velocity field: A vector field describing the velocities of particles at any given moment during the deformation history.
Vergence: Term related to the geometry of asymmetric folds. The vergence direction is the direction of apparent shear if the fold is considered to be the result of simple shear.
Vertical shear: Simple shear with a vertical shear plane, i.e. particles moving along vertical lines. Commonly used to restore vertical sections.
Viscosity: A measure of the resistance of a fluid to deforming under shear stress or, less formally, of a medium’s resistance to flow. “Thick” fluids therefore have a higher viscosity than “thin” or runny fluids. Over geologic time, rocks in the middle and lower crust can be considered as fluids with very high viscosity.
Viscous material: Material showing a linear relationship between shear stress and shear strain. Viscous materials also show a linear relationship between shear stress and shear strain rate.
Volume change: See dilation.
Volume diffusion: Diffusion creep where vacancies migrate through the crystal lattice. Also known as Nabarro–Herring creep.
Volumetric strain: Isotropic volume change.
Vorticity: The local angular rate of rotation in a fluid, or a measure of how fast a fluid rotates or circulates. Applied to rocks deforming (flowing) in the plastic regime. Mathematically, vorticity is the curl of the velocity, and therefore a vector whose axis is along the axis of rotation. It can be visualized by considering a tiny portion of the fluid or flowing rock freezing into an undeformable sphere. The vorticity vector is the axis of rotation through that sphere and the angular velocity (an expression of rotation rate) is half the vorticity. If the sphere does not rotate there is no vorticity.
Wall rock: The rock on each side of a shear zone or a fault.
Wallace–Bott hypothesis: Slip on a given fracture occurs along the greatest resolved shear stress.
Wernicke model: The simple-shear model for rifting, where the lithosphere is stretched asymmetrically with a controlling low-angle simple-shear zone in the central part of the rift.
Wet diffusion: The type of diffusion occurring by transport of ions in water films between grain contacts, generally at shallow depths.
Window: Erosional exposure of the rock unit underlying a nappe. Identical to fenster.
Wing crack: Tensile fracture forming at the tip of a shear fracture during fracture growth or reactivation, oriented oblique to the host fracture.
Work hardening/softening: More or less the same as strain hardening/softening, but strictly relates to the work (energy) involved rather than the level of stress.
Yield point: The point on a stress–strain curve that marks the transition from elastic to permanent deformation.
Yield stress: The critical stress that it takes for a rock to flow (yield).
Young’s modulus: The ratio between stress and strain for an elastic material, which describes how much stress it takes to achieve a certain strain. Also called the elastic modulus. A measure of stiffness.
Z-folds: Folds whose short limbs appear to have been rotated clockwise with respect to their long limbs.
Zonal crenulation cleavage: Crenulation cleavage through which the earlier foliation can be traced continuously.