Plate tectonics

A mobile-lid mode of planetary evolution in which coherent pieces of the lithosphere move, form at spreading regions and can recycle through subduction. Slab pull, ridge push and mantle flow compete with deformation resistance. It affects heat transport, volcanism, earthquakes and the carbonate-silicate cycle. It is not guaranteed solely by an exoplanet's mass.

Repeated assembly, reorganization and dispersal of supercontinents. It is not necessarily periodic or synchronous across all continents. A Wilson cycle concerns the opening and closure of an individual ocean basin, whereas a supercontinent cycle describes global-scale changes in continental configuration.

A continental assembly containing most of the Earth's continental area or major cratons. Its reconstruction requires age-compatible geological connections and paleomagnetism; similarity of modern continental outlines alone is insufficient.

Separation and sinking of a relatively dense lower lithospheric layer from material above. It can recycle material without requiring a sustained global system of tectonic plates.

Opening and closure of ocean basins through continental rifting, oceanic spreading, convergence and continental collision.

A continental-oceanic transition that is not an active plate boundary during the interval being considered.

Convergence involving continental regions after consumption or closure of an intervening basin.

A boundary marking the joining of previously separated tectonic regions, often associated with remnants of an intervening oceanic system.

Localized extension that separates or thins a region of lithosphere.

Extension and thinning of continental lithosphere, potentially preceding ocean-basin formation.

Emplacement of a portion of oceanic lithosphere over another plate, commonly a continental margin.

Movement of a tectonic plate relative to another body or a specified reference frame.

Traction transmitted between the base of a tectonic plate and the mantle. Its component parallel to the plate can drive or resist plate motion, depending on the relative motion and surrounding mantle flow. Its direction should not be inferred from the word “drag” alone.

A region separating tectonic plates, accommodating divergent, convergent or relative lateral motion.

A boundary region where adjacent plates approach one another, potentially involving subduction or continental collision.

A geological fault is a fracture or fracture zone across which rock has undergone relative displacement. Its local geometry is described by a fault plane, fault strike, fault dip and fault rake. A locked fault can store elastic strain while surrounding rock deforms; some faults also accommodate displacement by slow creep.

A lateral separation between adjacent or overlapping fault strands. Displacement transfer can be accommodated by distributed deformation, a tilted relay ramp, or a connecting fault. Its releasing or restraining effect depends on both the stepping direction and the sense of fault slip.

A polished or striated surface on a fault, produced or modified by sliding. Striation direction constrains the orientation of fault slip; determining the sense of motion can require additional asymmetric indicators.

A finite-width region containing the main slip surface, subsidiary fractures and mechanically or chemically modified rock associated with a fault. It can include a comminuted core and a surrounding damaged region. Porous permeability can be enhanced along connected fractures while fine fault gouge reduces cross-fault flow.

One of a pair of fault orientations that can form under a common principal stress field with opposite senses of resolved shear stress. For newly formed, optimally oriented faults obeying the same Mohr-Coulomb failure criterion, their acute intersection angle is $90^{\circ }-\varphi$, where $\varphi$ is the friction angle. The maximum compressive principal stress bisects that acute angle. This permits an estimate $\mu =\tan \varphi$ only under the common-formation and optimal-orientation assumptions, rather than for arbitrary inherited faults.

A local principal stress model associated with Ernest Masson Anderson in which one principal stress is vertical, as suggested by negligible shear stress on a flat free surface. With compression positive and ${\sigma }_1\ge {\sigma }_2\ge {\sigma }_3$, vertical ${\sigma }_1$ gives normal faulting, vertical ${\sigma }_2$ gives strike-slip faulting, and vertical ${\sigma }_3$ gives reverse faulting. The other two principal stresses are horizontal. Topography, mechanical layering and local fault geometry can violate the idealization.

For a cohesionless, optimally oriented normal fault with vertical maximum compression and horizontal minimum compression,where $\delta$ is fault dip and $\phi$ the internal friction angle. This is a geometric inference under the model, not a measurement of friction from dip alone.

For a compression-positive Mohr circle, put $C=({\sigma }_1^{\prime }+{\sigma }_3^{\prime })/2$ and $R=({\sigma }_1^{\prime }-{\sigma }_3^{\prime })/2$. Tangency to the cohesionless Mohr-Coulomb failure criterion $\tau ={\mu }_f{\sigma }_n^{\prime }$ gives $R/C=\sin \phi$. The contact point is $({\sigma }_n^{\prime },\tau )=(C{\cos }^2\phi ,C\sin \phi \cos \phi )$. The physical plane-normal angle to ${\sigma }_1^{\prime }$ is half the corresponding central angle, giving the stated dip. Equivalently the plane itself makes $45^{\circ }-\phi /2$ with ${\sigma }_1^{\prime }$.

The associated stress ratio is ${\sigma }_1^{\prime }/{\sigma }_3^{\prime }=(1+\sin \phi )/(1-\sin \phi )$. A reactivated nonoptimal plane, nonzero cohesion or unknown principal stress directions defeats the simple dip-only inversion.

An opening crack formed by local extension. Near a normal fault it may accompany primary shear, but shaking, slope instability or compaction can also produce fissures.

Rock whose structure and composition record deformation within a fault zone.

A cohesive fault rock produced predominantly by mechanical grain fragmentation and granular rearrangement. It differs from a loose fault breccia or fault gouge in cohesion, while sharing a brittle crushing mechanism rather than requiring bulk melting.

Coarse angular rock fragments produced by crushing during fault slip, commonly in a finer matrix. Fine-grained, poorly cohesive comminuted material is fault gouge; cohesive material formed by similar grain-size reduction can be called cataclasite.

Deformation involving slip on geological faults.

A major continental strike-slip fault system in California; its strength has been studied with boreholes, heat flux and seismic observations.

Fine-grained fault rock produced by comminution and alteration; its mineralogy and fluid state can strongly affect friction.

A crustal fault system beneath the Seattle region capable of producing damaging earthquakes and local tsunamis.

The block below an inclined fault plane. It is paired geometrically with the hanging wall.

The block above an inclined fault plane. In reverse faulting it moves upward relative to the footwall.

A topographic step associated with displacement across a surface-reaching fault. Erosion and distributed deformation can modify its height and shape.

An exposed fault plane revealed as fault slip, differential motion and removal of cover bring it to the surface. On a normal fault, lowering the hanging wall can expose a planar footwall face. Continued exposure may preserve a bedrock scarp while weathering, erosion and collapse progressively modify its older portions. A hillslope following the plane therefore need not be an independent erosional surface that accidentally has the same orientation.

A simple model of hillslope smoothing in which sediment flux is proportional to negative slope. For constant transport coefficient $\kappa$, an ideal initial step broadens on scale $\sqrt{\kappa t}$. Real free-face collapse, nonlinear transport, changing climate and deposition can invalidate the linear diffusion equation approximation.

The relationship between characteristic accumulated fault slip and fault-system length. Approximately $D\propto L$ follows in self-similar elastic models at comparable driving stress, but natural faults can accumulate displacement at nearly constant length over part of their history.

A fault with displacement predominantly parallel to its strike.

Shortening accommodated by a hanging wall moving up relative to the footwall. The centre of its dip-slip focal mechanism is compressional under the usual lower-hemisphere convention. Low-angle reverse faulting is commonly called thrust faulting.

A fault on which the hanging wall moves up relative to the footwall, accommodating shortening. A low-angle reverse fault is a thrust fault. The fault dip is a geometric property and should be distinguished from the direction of fault slip.

A low-angle reverse fault. Near-surface splays may accompany either a surface-breaking or a blind main rupture.

A mechanically weak detachment along which an overlying package can move relative to its substrate. A décollement can follow weak bedding and connect to steeper thrust ramps, so its orientation need not equal the optimum dip for a new fracture in homogeneous rock.

A very large plate-interface thrust fault at a subduction zone.

A reverse fault dipping oppositely to the dominant thrust system and accommodating part of its shortening.

A fault with reverse slip whose upper tip remains beneath the ground surface. It can deform the overburden through folding, localized fold scarps, and secondary faults.

Extension accommodated by a hanging wall moving down relative to the footwall. Its focal mechanism has an extensional dip-slip pattern, with the centre unshaded under the convention that shaded quadrants denote compression.

A tilted region transferring displacement between overlapping normal fault segments. Opposing gradients of fault throw can produce an unbroken ramp. Continued extension can breach the ramp with a connecting fault, generating a linked segmented trace.

A relatively down-dropped block bounded by opposing normal faults. A small secondary graben can form within the distributed surface rupture zone of one earthquake.

A fault on which the hanging wall moves downward relative to the footwall. Local bending or collapse can create small normal faults even during a regional reverse-faulting event.

A long-lived low-angle normal fault that can exhume deep crust or mantle. Its mapped scarp need not be a plate-motion arrow.

Fault slip with both dip-slip and strike-slip components. The focal mechanism can have an off-centre nodal intersection, with the central polarity indicating its dip-slip sense under a specified shading convention. A reverse component with horizontal shear is compatible with transpression.

Predominantly horizontal relative motion along a fault. A pure strike-slip double couple has two vertical nodal planes; a focal mechanism gives both possible planes, and independent evidence selects the actual one.

Strike-slip faulting in which the opposite side moves to an observer's right when looking across the fault.

Strike-slip faulting in which the opposite side moves to an observer's left when looking across the fault. The sense is the same from either side.

A point where three plate boundaries meet. Kinematic stability means that one instantaneous junction velocity is compatible with all three moving boundaries, so a connected junction can persist. It is not a guarantee of dynamical stability for arbitrary future plate motion.

A junction whose persistence depends on the two overriding-plate trench constraints and the transform velocity line meeting at one point. Specifying only the boundary-type letters does not specify the compatibility geometry or trench polarities.

For pure normal symmetric spreading, the two ridge constraints are perpendicular bisectors of two edges of the plate velocity triangle. The transform constraint lies along the third edge. For a nondegenerate triangle their common intersection exists only when the angle opposite that third edge is a right angle; the junction velocity is then the midpoint of the transform edge.

For a boundary with in-plane normal $n$, an admissible junction velocity $U$ obeys $(U-v_{\mathrm{boundary}})\cdot n=0$. A ridge uses the mean of adjacent plate velocities in the symmetric model, a trench uses the overriding plate velocity, and a transform uses either adjoining plate velocity. Compatibility requires a common intersection of the three velocity-space lines.

The three plate velocities plotted in a common reference frame form a triangle whose directed edges are the pairwise relative velocities. Adding the same reference velocity translates the triangle without changing its edges.

An accreting divergent plate boundary. In the symmetric normal-spreading model, its normal velocity equals the mean normal velocity of the adjoining plates; motion parallel to the ridge does not change that normal compatibility condition.

Seafloor spreading creates oceanic crust at a spreading ridge and carries it away with the adjoining plates. With symmetric steady spreading, an age interval maps to distance by the half-spreading rate, not the full relative opening speed. A time sequence of field polarities can therefore become paired marine magnetic stripes.

The speed of one side of newly formed crust relative to the ridge axis in a symmetric spreading model. It is half the full relative opening speed; the width formed during a polarity interval is this speed times the interval duration.

A plate boundary at which relative motion is tangential to the boundary. It neither creates nor consumes lithosphere in the ideal kinematic model. The admissible junction velocities form a line parallel to the fault through either adjacent plate velocity in velocity space.

The seafloor continuation beyond the actively slipping ridge-to-ridge transform fault. Its two sides can differ in age without continuing relative plate-boundary slip.

The direction of the instantaneous angular-velocity vector describing relative rigid-plate motion on a sphere. At position $\mathbf{r}$, the relative velocity is $\mathbf{\omega }\times \mathbf{r}$. The antipodal pole represents the same physical rotation if the angular-velocity sign is reversed.

Relative angular velocities add as vectors: ${\mathbf{\omega }}_{B/C}={\mathbf{\omega }}_{B/A}+{\mathbf{\omega }}_{A/C}$. Geographic pole latitudes and longitudes must first be converted into Cartesian vector directions.

A region of lithosphere approximated as rigid on the spatial and temporal scale of a plate-kinematic model. Its instantaneous motion on a sphere is a rotation; deformation within boundary zones limits the rigid-plate approximation.

The tectonic plate containing most of North America and adjacent parts of the ocean floor.

A young oceanic tectonic plate entering the Cascadia subduction zone.

The oceanic tectonic plate subducting beneath much of western South America.

A prominent oceanic ridge on the Nazca Plate entering the Peruvian subduction zone. Its association with variations in slab dip motivates tests using seismic geometry, plate reconstructions and the thermal and compositional contributions to slab buoyancy.

A volcanic ridge system entering the South American subduction zone near central Chile, associated with the southern transition of the Pampean flat-slab region. Its crustal structure, hydration and past geometry constrain explanations of the slab dip; present-day bathymetry alone does not give the buoyancy of its already subducted continuation.

Gravitational sliding of lithospheric material away from elevated spreading regions. It supplies a driving contribution to plate tectonics together with slab pull and mantle stresses.

The ridge-push force in a plate model is the depth integral of the horizontal pressure difference between hot and cold compensated columns. To leading order, the old plate's linear excess density gives $F=g{\int }_0^{a}s\delta \rho (s)ds={\rho }_{m}g\alpha \Delta T{a}^2/6$. Here $s$ is depth below the old seafloor. Retaining the geometrical contribution from its lowered surface adds $g({\rho }_m-{\rho }_w)h^2/2$ within the linear-density model. Force per ridge length has units $\mathrm{N}{\mathrm{m}}^{-1}$, distinct from stress.

Downward recycling of a lithospheric plate into the mantle. A sufficiently cold, dense slab can provide slab pull, while bending and friction resist motion. The resulting material and volatile recycling connects plate tectonics with volcanism and long-term climate.

Release of water-rich fluid by reactions that destabilize hydrated minerals in a descending slab. It can promote flux melting of the mantle wedge and contribute to dehydration embrittlement. The released water is not evidence that the whole slab has melted.

The effect of slab-induced mantle flow, pressure and traction on the motion of neighbouring plates and their boundary. It is part of the coupled response to a sinking slab, rather than an unrelated energy source that can always be added independently to slab pull.

Migration of a subduction trench relative to the surrounding mantle as a slab retreats; the associated mantle flow can advect thermal anomalies.

Partial melting of material within a descending slab, distinct from flux melting of the mantle wedge by fluids released from the slab.

A proposed buoyant ascent of mixed sedimentary, crustal and hydrated mantle material from an interface region into the mantle wedge.

Removal of material from the overriding plate into a subduction zone.

Mechanical addition of solid slices beneath the overriding plate. Distinct from crystallization of new magma by magmatic underplating.

Establishment of sustained downward plate motion, distinct from its later continuation under existing plate forces.

The region containing a descending plate, its interface and the associated upper-plate and mantle wedge system.

A coupled phenomenon in which transient slow fault slip occurs with low-frequency seismic tremor, often near the downdip transition from locked to steadily creeping parts of a megathrust. Geodetic displacement and seismic signals provide complementary observations. Along-strike differences in recurrence, migration and cumulative slip help characterize subduction zone segmentation; tremor counts alone do not uniquely give fault slip or coupling.

Along-strike changes in the structure, deformation, coupling or thermal state of a subduction zone. Segments inferred from megathrust locking, episodic tremor and slip, past earthquake ruptures, slab dip and seismic tomography need not have identical boundaries. A common spatial pattern can motivate a physical hypothesis, but does not alone prove a persistent rupture barrier or identify a unique cause.

The subduction zone beneath the Pacific Northwest of North America, where the Juan de Fuca Plate and adjacent oceanic plates descend beneath the North American Plate.

An accreted basaltic terrane forming important parts of the forearc basement in the Pacific Northwest. Spatial variations in its thickness, stiffness and permeability can affect the interpretation of gravity anomalies and the fluid/stress environment above the megathrust. A proposed relation to central Cascadia creep uses gravity, geodetic and paleoseismic observations, rather than lithology alone.^[ref]

The wedge-shaped mantle above a descending slab, whose circulation and chemical modification influence arc magmas.

The upper-plate region between the subduction trench and volcanic arc.

A wedge of material transferred from the incoming plate to the overriding plate by offscraping or underplating.

The mantle of the overriding plate beneath the forearc, potentially hydrated by slab-derived fluids.

A sedimentary basin within the forearc.

A descending portion of lithosphere in a subduction zone.

The local angle of a descending slab relative to a horizontal surface; it can change with depth and along the margin.

A regime in which an initially descending slab becomes nearly horizontal beneath the overriding plate for a substantial inland distance before descending again. It can narrow or eliminate a hot mantle wedge, shift deformation inland and coincide with a volcanic gap. Incoming buoyant crust, hydration, upper-plate motion and mantle forces can all contribute; association with an oceanic ridge does not isolate one cause.

Net gravitational effect of thermal and compositional density contrasts between a slab and its surroundings, including phase changes.

The upper plate above a subducting plate. Its motion determines the normal velocity of a non-rolling-back trench in the idealized triple-junction compatibility construction.

A convergent margin where one plate descends beneath another. In a simple junction-kinematic model the trench normal motion is tied to the overriding plate; rollback or distributed deformation can modify that rule.

A gravitational driving force associated with a dense descending slab. The slab's buoyancy and ability to remain coherent matter, so increased planetary gravity alone does not determine whether subduction occurs.