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.