Accretionary prism: Accretionary prisms are wedge-shaped stacks of oceanic sediment and some volcanic rock scraped from the top of the subducting lithosphere and plastered over the trench slope. Older autochthonous deposits on the upper plate may also be incorporated. Each slice of sediment is separated by landward-dipping thrusts (i.e. verging towards the trench). Accretion begins at the frontal taper. Landward stacking of thrust panels occurs above a décollement, where the oldest panels are farthest from the trench.
Active rifting: Extension and stretching of the lithosphere, and development of rift basins promoted by a rising mantle plume. Cf. Passive rifting.
Actualistic models: Models based on the principle that natural processes and laws we witness today have acted in the past. This does not mean that the products of such processes, for example some environmental condition, will be the same today and in the distant past, but that the laws governing such processes will be the same. cf. Uniformitarianism
Advective fluid flow: The flow of fluids through a porous medium; in this case only the fluids move. Advective flow via aquifers is the most efficient mechanism for mass transfer of dissolved solids in the shallow crust. cf. convective flow, groundwater flow.
Afar depression: Also called the Afar Triangle. The northernmost sector of the East African Rift System, characterised by alkaline volcanism and hypersaline-hyperacidic lakes. It is the most advance sector of continental rifting that appears to be transitional to incipient production of oceanic crust and sea floor spreading. It borders Ethiopia, Eritrea, and Djibouti. Its shores are washed by Red Sea and Gulf of Aden. In a plate tectonic context It is considered to be a triple junction – the confluence of EARS, and the Red Sea – Gulf of Aden sea floor spreading.
Allochthonous: In geology this means a sediment, rock, lithospheric block, or fluid body that has moved from the place where it formed to the place where it is now found. Cf. Autochthonous.
Allochthonous terrane: See Terrane.
Aspect ratio: In basin analysis this applies to the ratio of areal extent represented as along-strike length, to down-dip width. It is more commonly used in description of strike-slip basins that tend to be long and narrow.
Back-arc basins: Basins behind magmatic arcs, floored by oceanic or continental crust. Oceanic back-arc basins are commonly formed by sea floor spreading; their continental cousins by rifted continental crust. In continental varieties, high heat flow is manifested as volcanic centres, and fault-controlled geothermal activity.
Back-bulge: A shallow basin between a flexural forebulge and the adjacent continent. It is regarded as a separate depozone in foreland basin systems.
Backstripping: A numerical geohistory analysis that calculates the contribution to total subsidence by tectonics – the method was developed by Watts and Ryan, 1976. The tectonic subsidence component is determined by subtracting the isostatic effect of sediment load. It is usually done in conjunction with decompaction and bathymetric corrections.
Basin axis: Applies to elongate basins usually corresponding to the deepest part of a basin. The axis usually parallels orogenic belts (e.g. foreland basins) or volcanic arcs (forearc basins), rifted plate boundaries as in passive margins, or parallels fault displacement as in strike-slip basins. A basin axis may migrate laterally with changes in tectonic and/or sediment loads.
Basin-wide evaporites: Accumulate on subsiding basin floor and as a consequence can reach several 100 m thick. Associated facies range from shallow marine shoreface to deeper slope and base-of-slope environments. Precipitation takes place either within the water column with crystals sinking to the basin floor (meromict), or at the sea floor (holomict). The particular mode of precipitation depends on the degree of brine stratification and evaporitic drawdown.
Benioff zones: Identified in 1930s (before plate tectonics) as a zone of earthquakes associated with oceanic trenches, some as deep as 650 km.. Benioff (1949) suggested that they originated on large, continentward-dipping reverse faults where the direction of slip placed oceanic crust beneath continental crust. The Zone is now recognised as a subducting slab of oceanic crust.
Bottom simulating reflector (BSR): In some oceanic regions, methane hydrate layers (methane clathrate) just below the sea floor have much lower density than the overlying and underlying sediment. They present as a prominent reflector on seismic profiles.
Breakup unconformity: A significant, commonly angular stratigraphic discordance between syn-rift deposits that are confined to fault-bound grabens and half grabens, and the base of more widespread post-rift strata that signal the change to a passive margin. In a plate tectonic context it signifies the transition from rifting to sea floor spreading (rift-drift).
Brittle behaviour (rheology): Most Earth materials behave elastically up to their elastic limit, beyond which deformation is irreversible. If the strain rate is high, this deformation will take place as sudden fracturing (e.g. broken glass). Brittle deformation is also enhanced by low confining pressures and low temperatures – these are the conditions that lead to faulting and fracturing during an earthquake. Cf. Ductile flow.
Buoyancy: Buoyancy is the result of fluid forces acting on a body immersed in a fluid. If the resultant force is greater than the gravitational force acting on the body (that itself is a function of its density), then the body will rise (positive buoyancy – negative buoyancy is the opposite). Buoyancy plays an important role in many processes – the rise of mantle plumes and magmas, diapirism, density and temperature stratification in the oceans, the support of clasts in sediment gravity flows and pyroclastic flows.
Caledonian Orogeny: The period of mountain building beginning in the Ordovician and continuing to the Early Devonian, resulting from closure of Iapetus Ocean and consequent collision tectonics. The earliest deformation began phase about 490-475 Ma with collision between Laurentia and Grampian Arc. The later phase saw closure of Iapetus and collision with Baltica and Avalonia continental blocks.
Carbonate platform: Also called carbonate shelf. Thick successions of carbonate rock, that occupy shelf-like structures attached to continental landmasses, or as stand alone, isolated platforms surrounded by relatively deep ocean basins; also called carbonate banks. Heterotrophs and autotrophs contribute to carbonate production. Evaporites may form part of the stratigraphic succession in arid climates. The proximity to landmasses will determine the degree of mixing with siliciclastic sediment. Islands, banks and bars, and reefs generate significant relief across a platform. Platform-margin reefs mark the transition to slope and deep ocean basins.
Compaction: The process where sediment particles, once deposited, are pushed closer together to form a more tightly knit framework. Compaction begins almost immediately following deposition and continues during sediment burial. The normal compressive stress in this case is applied by the overlying sediment. Because porosity is also reduced, an additional requirement for compaction to take place is the release of interstitial water through aquifers. If fluid cannot escape (for example because of permeability barriers) then the rock body will not compact, and internal fluid pressures will rise – this is called overpressure. Mudrocks can compact to less than a tenth their depositional thickness. More rigid frameworks like sandstones compact far less. See also pressure solution, lithic fragments.
Compensation depth: Applies to models of isostasy where, at equilibrium there is a common depth at which lithostatic pressures are equal across all components of the lithosphere.
Composite terrane: See Superterrane.
Conduction: This is a diffusive process where heat is transferred via molecular vibrations. Conduction does not involve the transfer of mass, cf. convection, advection. It is a less efficient mechanism of heat transfer than convection.Consolidation: Is broadly synonymous with compaction of sediment that results in a loss of porosity and bulk volume. It is the main physical process involved in sediment diagenesis.
Continental rifts: Basins formed by lithospheric stretching and thinning, where the primary subsidence mechanism is faulting. Rifts commonly have high heat flow, manifested as volcanism, resulting from shallow mantle plumes (in part an isostatic response to crustal thinning). Sediment fill early in rift history tends to be terrestrial and coarse grained.
Continental rise: The bathymetric transition from continental slope to abyssal plain. Gradients are less then those of continental slope, merging with the deep basin beyond. Water depths are commonly >3000 m. Much of the rise are is made up of submarine fans that are fed by submarine canyons and gullies on the adjacent slope. Mass transport deposits derived from the slope generally move across the rise.
Continental shelf: The submarine extension of a continent. Shelf inclinations are generally <1o averaging about 0.1o . Water depths range from about 60 m to 200 m. Shelves and their environments are sensitive to sea level fluctuations. During low sea levels (e.g. during glaciations) the shorelines migrate seawards and the shelf thus exposed is subjected to weathering and fluvial erosion . A significant change in slope at their seaward margin is called the slope break – it marks the bathymetric transition to continental slope. It also corresponds to the transition from continental to oceanic crust.
Continental slope: The bathymetric region beyond the shelf and shelf break, extending from about 100m to 3000 m, with gradients between 2o – 5o . Slopes are commonly transected by gullies and submarine canyons that focus sediment transport, some of which remains on the slope (finer-grained sediment), and some bypassing the slope on its way to the basin beyond; in this case sediment transport is commonly via turbidity currents and other types of sediment gravity flow. Gravitational failure also shapes the slope. Hemipelagic sediment is important to slope accumulations.
Convection: The flow of fluids en masse resulting from temperature and buoyancy gradients. Convection is the primary mechanism for transferring heat from Earth’s mantle to the lithosphere. Cf. conduction, advection.
Craton: The stable interior of continents, usually made up of very old geological provinces including Precambrian shields and distinguished from more marginal mobile belts or collisional orogens. Note however that Cratonic rocks may be structurally involved during orogenic deformation.
Critical taper theory: A mechanical theory used to explain the formation of wedge-shaped fold-thrust belts and accretionary prisms. During compression the slope, and therefore the angle of the wedge taper reaches a critical point depending on the strength of the materials, the frictional forces along the décollement, and the slope of that surface. As the critical slope or taper angle is approached, the materials within the wedge will deform – once the critical taper is reached, the entire mass slides along the décollement and there is little subsequent internal deformation of materials.
Crust: The outer layer of Earth. The boundary between crust and mantle is defined by the Moho. Oceanic crust is produced at oceanic spreading ridges. It averages 6 km in thickness, and consists primarily of basalt, gabbro, peridotite and ultramafic rock types. Continental crust averages 35 km thick but ranges to 70 km beneath some Cratonic blocks. Continental crust tends to be more felsic than oceanic crust. The crust is underlain by the mantle lithosphere.
Décollement: Also detachment, sole thrust. Names given to the thrust at the base of a thrust stack, that is the common surface of detachment for all thrusts. It overlies undeformed rocks (e.g. Cratonic platform, crystalline basement). An example is shown in the fold-thrust cross section below.
Decompaction: A numerical method for restoring a stratigraphic unit to its original thickness and porosity at deposition. It is an important part of geohistory determinations that tease apart the different components of basin subsidence and basin accommodation. Also backstripping.
Delamination: The decoupling, or separation of upper crust from lower crust and/or mantle lithosphere. The main driving mechanism is negative buoyancy in the mantle lithosphere such that it detaches from the crust and sinks into the asthenosphere. Delamination commonly occurs above crustal-scale thrust faults.
Depocenter: The term applied to the region of thickest sediment accumulation in a basin, that also corresponds to the region of maximum subsidence. The depocenter may be axial, as is the case for passive margins, foreland basins and accretionary prisms, or more central as in intracratonic basins. It may also migrate as basin dynamics evolve, for example the craton-advancing load of a foreland fold-thrust belt. Depocenters can be determined from sediment isopach maps.
Depositional sink: That part of a sediment routing system at the endpoint for deposition. It can occur anywhere in a sediment routing system, including the transfer zone, where there is sufficient accommodation to enable its preservation. See source to sink.
Detachment: see décollement
Diapir: A buoyant, mobile body acting as a fluid that intrudes to shallower levels of the crust. Salt diapirs are common, but the process also occurs with mudstones and magmas. Positive buoyancy occurs when fluid forces acting on the body exceed the gravitational forces. Diapirism in salt produces many kinds of intrusive geometries, from dome-shaped, to laterally extensive walls, sheets, and salt-cored anticlines. During intrusion the stress on the surrounding strata is accommodated by faulting and folding. Salt diapirism results in salt withdrawal from stratiform evaporites at depth imposing a kind of supply and demand limit to the size and number of diapirs that might be generated from a particular evaporite unit. The increasing overburden load plays a critical role in initiating salt instability (buoyancy disparities) and diapir rise.
Ductile deformation (flow): Deformation (strain) beyond a material’s elastic limit that is permanent (not reversible), but does not result in fracturing (brittle failure) – i.e. the material is behaving as a plastic. Materials that deform ductily appear to flow or bend. Ductile deformation of rock is enhanced under conditions of high confining pressures, high temperatures, and low strain rates.
Duplex: An imbricate stack of horses bound above and below by through-going thrusts; these are the roof and floor thrusts. Duplexes represent progressive, incremental formation of ramps and bending folds (anticline-syncline pairs). Duplexes can take several geometric forms.
Dynamic topography: Long wavelength topography that is produced by mantle convection beneath the lithosphere. Wavelengths are measured in 100s of kilometres. The topography can be positive (uplift) or manifested as subsidence. Theoretically this effect can produce shallow basins in the absence of tectonism. Conversely, the subsidence of sedimentary basins that originate primarily by tectonics, such as foreland basins, may be enhanced by down-welling mantle convection cells.
Earthquake focus: The actual point beneath the surface where an earthquake is focused. Cf epicentre.
Earthquake magnitude: Magnitude (M) reflects the severity of ground roll and shaking, and on seismograms the amplitude of the signal (usually of surface waves). M is expressed as a number (M1.8, M4.6, M7.8) up to a maximum of 10. The scale is logarithmic, such that a magnitude of 4 (104) is 100 times smaller and less energetic than M6 (106).
Elastic behaviour: This rheological behaviour describes materials that respond to stress by deforming but can return to their original state when the stress is removed. The principle was developed by Robert Hooke– Hookes Law (1660); the classic physics experiment involves a spring. The principle can also be applied to most sediments and rocks. The level of stress at which deformation becomes irreversible is called the elastic limit. Beyond the elastic limit deformation will occur as brittle failure or ductile flow.
Endorheic lake: A water body that has no surface outflow drainage, and it surrounded by drainage divides. In most cases inflow from surface runoff and groundwater discharge is balanced or exceeded by evaporation.
Epicenter: The projection of an earthquake focus (at depth) to the surface.
Episodic tremor: Swarms of very low magnitude earthquakes at a subduction interface and its associated faults, barely felt, if at all. None of the displacements results in major earthquakes. Associated with slow slip displacements.
Eugeosyncline: See Geosyncline, and Miogeocline.
Eustasy: Allen & Allen (2005) define it as “…global sea level measured from a fixed datum, such as the centre of the Earth”. A rise or fall in sea level requires either a change in ocean water volumes, or a change in the size of ocean basins (determined by plate tectonics). Fluctuations in ocean volumes are caused by changes in glacial ice volumes, by steric effects (heating and cooling of water), and over longer geological periods tectonic plate configurations. Note that the relative change in sea levels from eustatic causes is not the same everywhere because of gravitational-isostatic effects.
Exotic terrane: A term for terranes in general, emphasizing its distinctiveness compared with other terranes or an allochthon. See Terrane, Suspect terrane.
Flexural wave: Flexure of oceanic and continental lithosphere produces a foredeep, an adjacent forebulge and a shallow back-bulge basin. Together, the three components are wave-like, having amplitude and wavelength that depend on the properties of the lithosphere. Wavelengths are commonly measured in 100s of kilometres; amplitudes up to 8 km.
Flexure: The mechanical response to loading an elastic plate. In a plate tectonic context it can occur in oceanic and continental lithosphere. The amount of flexure depends on the elastic thickness and strength of the lithosphere. Flexure is responsible for creating sedimentary basins at plate margins, such as oceanic trenches at subduction zones, moats around large volcanic edifices, and foreland basins at collisional margins.
Fluid pressure: The pressure within a fluid (liquid and gas phases), usually expressed as a compressive stress – in its simplest form: P = ρgz
where P is the pressure of interstitial fluids at some depth measured vertically, ρ is the density of the fluid, g = the gravitation constant, and z the depth from the surface to the point of interest. Fluid pressures generally increase with depth in the crust. Cf. hydrostatic pressure, lithostatic pressure.
Flux melting: A term derived from welding and glass making. A flux is a substance that lowers the melting point of solids. It applies to magma generation in the mantle where water, derived by dehydration of mica, glaucophane, and serpentinite minerals, lowers melting points by 200°C and more. Flux melting is a critical stage in the formation of partial melts.
Fold-thrust belt: A major thrust system developed during lithosphere-scale plate convergence, with cumulative shortening of 100s of kilometres, that usually results in mountain building. The resulting topographic results in flexure and formation of a foreland basin. Thrust faults are generated in pre-existing strata, but usually evolve to include the proximal parts of the foreland basin and its sediments. An example is shown below.
Forearc basin: Basins in the upper plate between the ocean trench and magmatic arc at convergent margins. Subsidence is due to crustal flexure resulting from tectonic loading by an accretionary wedge. The wedge consists of structural slivers of oceanic crust and sediment scraped from the subducting slab. Thrusts usually dip towards the arc. Thrust slivers are added to the bottom of the thrust stack.
Forebulge: A positive, low amplitude uplift outboard of the foredeep, that is a flexural response to loading of an elastic plate. Also called a peripheral bulge. It forms the outer limit to the foredeep. Deposits over the bulge are thin or condensed; unconformities are common. The bulge migrates in tandem with the topographic (orogenic) load and the foredeep axis.
Foredeep: The component of a foreland basin immediately outboard of the orogenic load (fol-thrust belt); also referred to as a foredeep depozone. It lies between the wedge top and forebulge depozones. The foredeep axis is approximately parallel to the orogenic belt. Sediment is sourced from topography in the orogen.
Foreland: The continent side of an orogen that contains a miogeoclinal prism (that thins towards the craton) overlying crystalline basement. In a collisional orogen the older strata are involved in thin-skinned thrusting and tectonic shortening, manifested as a fold-thrust belt and foreland basin. The foreland is separated from the hinterland by a collisional suture zone that commonly contains high pressure metamorphic rocks (e.g. blue schist) and lots of evidence for ductile deformation.
Foreland basins: Basins formed on continental crust as a result of continent-continent or continent-arc collision across a convergent plate boundary. In both cases, basin subsidence is the result of tectonic loading and flexure of continental lithosphere by massive foreland thrust stacks. The foredeep axis tends to migrate away from the thrust-fold belt in concert with developing thrusts, or back towards the thrust-fold belt with erosion of the mountain topography. Foreland basins resulting from continental collision are also called peripheral foreland basins; those associated with magmatic arcs are called retroarc foreland basins.
Geohistory: A numerical analysis that attempts to tease apart the different components of basin subsidence and basin accommodation space. The method developed by Van Hinte (1978) calculates the effects of compaction (decompaction), bathymetry, and sea level change. The contribution to total subsidence by tectonics is determined by backstripping the sedimentary column – this method was developed by Watts and Ryan, 1976.
Geoid: A hypothetical surface of equal gravitational potential, that coincides with sea level in the absence of tides, waves, currents, and changes in air pressure. Sea level in this context is an ideal surface. Because it depends on gravitational potential, the geoid, and therefore sea level will not be a smooth surface, but will have long wavelength hill and valley like relief. Satellite altimetric measurements if sea level are referenced to the geoid.
Geostatic pressure: An alternative term for lithostatic pressure.
Geosyncline: The term coined in the 19th century for basins containing great thicknesses of sedimentary rock. Early hypotheses, popularized by folk like J. Dana, James Hall, and Marshall Kay, linked geosynclines to orogenic belts as cause and effect, rather than just being exposed in mountain uplifts. Eugeosynclines were deep-water oceanic basins, commonly bordered by an uplifted ridge or geanticline; Miogeosynclines, or miogeoclines were relatively shallow water sedimentary depocenters, likened to passive margin sedimentary prisms. Interpretations of thick sedimentary successions were frequently forced to comply with geosynclinal models. One of the major problems with these models was a general lack of modern analogues. The advent of plate tectonics and the recognition of basins associated with plate interactions, rendered the terms redundant.
Geothermal gradient: Temperature generally increases with depth in the crust; the gradient for a particular location is stated as the temperature increase per unit depth. The global average is 3o C/ 100 m although there can be large departures from these values in regions of geothermal and volcanic activity, or regions that have cooled significantly over geological time, such as old oceanic crust.
Glacio-isostatic rebound: The isostatic response where a landmass elevation rises following the melting and removal of an ice sheet. Evidence for rebound is commonly observed as raised beaches. Well-known examples of post-glacial rebound where uplift continues today are Scandinavia and Hudson Bay.
Gondwana: The southernmost of two very large continental lithospheric blocks comprising Pangea – Laurasia was the northern block. It consisted of what would become South America, Africa, India, Madagascar, Australia, Antarctica, and Zelandia (the New Zealand centered block). It was named by Eduard Suess based on fossils from its namesake in India, that he recognised being similar to those in other southern continents (particularly Australia, South America). gondwana was central to Alfred Wegener’s theory of continental drift. Gondwana eventually split from Laurasia (i.e., Pangea) in the Late Triassic and into the Jurassic. The intervening seaway was the Tethys. Gondwana itself also began to break up into its constituent continental blocks from about the Mid-Jurassic, continuing well into the Cenozoic.
Graben: A fault depression or basin formed between paired, normal dip-slip faults. The downward displaced hanging wall block is common to both faults. The adjacent upthrown blocks are horsts. They form in regions of extensional tectonics, such as rift zones. Cf. Half graben.
Guyot: The name given by Harry Hess (1946) to flat-topped seamounts, commonly found at dpeths of 100s of metres. Hess’ original definition was based on seafloor acoustic data collected during World War 2. The truncated morphology of these seamounts is attributed to erosion and wave attack during subaerial exposure followed by isostatic subsidence.
Half graben: A depression or basin formed by extension, and (usually) rotation of a hanging wall block above a single, normal dip-slip fault (cf. paired faults in grabens). They commonly form above listric faults in continental rift zones, but analogous structures also form in rotational slumps.
Halokinesis: Halokinesis, or salt tectonics, studies the movement of (stratiform) salt during burial, the kinds of diapiric structures that form, the response of the surrounding bedrock (such as faulting), their impact on depositional processes, and their influence on stratigraphic architecture.
Heat flow: The transfer of heat from Earth’s core and deep mantle to the surface, primarily by conduction and convection. It is expressed as milli-Watts per square metre (mWm-2).
Hinterland: In collisional orogens, the mountainous region in the upper plate underlain primarily by metamorphic core complexes and crystalline rocks. The hinterland is separated from the cratonward foreland by a collisional suture zone. Hinterland lithosphere may be arc or continent related.
Hotspot: The surface or shallow subcrustal expression of a magma plume, usually associated with submarine eruption of seamounts and volcanic islands in ocean basins (oceanic lithosphere).
Hotspot track: Linear or arcuate trails or chains of submarine seamounts and volcanic islands across the ocean floor. Many of the better-known examples show a relatively consistent age progression, from an active or recently active volcano to volcanoes as old as Cretaceous; e.g. Hawaii-Emperor chain (Pacific), New England chain (Atlantic).
Hydrostatic pressure: At any depth, the pressure exerted by a (theoretical) overlying column of water having unit-area cross-section, is calculated from the expression P = ρgz where ρ = density of water, g = gravity constant, and z = depth from some datum, commonly sea level. Note that, assuming a cross-section of unit-area reduces volume to units of depth. It is analogous to lithostatic pressure.
Iapetus ocean: The ancient ocean (and oceanic lithosphere) separating Laurentia, Baltica, and Avalonia during the latest Precambrian and Early Paleozoic. Ocean closure and collision tectonics during the Ordovician through Early Devonian gave rise to Caledonian mountain building.
Inboard basin: Basins on the leading edge of a terrane. Includes oceanic basins in the intervening ocean between terrane and autochthon, forearc basins and trenches associated with subduction, or intermontane basins that overlie the incoming terrane. All these basins are deformed during terrane accretion. Cf. outboard basins.
Intra-arc basins: Basins located within magmatic arcs, between and over the flanks of volcanic edifices. They occur in oceanic and continental crust. Crustal extension may be involved in subsidence, caused by transtension at the convergent margin or the more localised effects of caldera collapse. Loading by growth of volcanic edifices will also contribute to subsidence. There will likely be multiple depocenters along the line of the volcanic arc. Heat flow is high.
Intracratonic basins: Also called intracratonic sags. They occur in continent interiors, generally remote from the direct effects of plate boundary tectonics and heat flow. Subsidence may in part be induced by far-field stresses that can generate long-wavelength buckling, the down-welling of mantle and cooling. Subsidence is aided by sediment loads.
Iridium anomaly: Anomalously high concentrations of Ir derived from meteorite impacts incorporated into sediments, particularly mudrocks. It was first recognized at the Cretaceous-Tertiary (K-T) boundary, corresponding to the widespread distribution in aerosols generated by the Chicxulub impact.
Island arcs: Volcanic chains and the associated magmatic rocks beneath, that build on the upper plate above subduction zones. Thus, they face oceanic crust, and behind have backarc basins, that may also be floored by oceanic crust. The greatest modern concentration of island arcs is the circum-Pacific (Ring of Fire). Cf. Continental arcs.
Isopach: A contour that delineates a sedimentary, volcanic or volcaniclastic unit thickness, either as a single bed or succession of beds. Unit thickness is measured directly in the field, from core or borehole logs (gamma and SP logs are commonly used to do this), or from seismic reflection traces. Isopachs are used to map thickness trends.
Isostasy: Isostasy describes the tendency to equilibrium of a lighter lithosphere floating on a more dense and perhaps more ductile mantle asthenosphere; it describes the state of balance between the lithosphere and asthenosphere. The two foundational models are Pratt Isostasy that allows density to vary from one lithospheric block to another, and Airy Isostasy where density is the same across all blocks. A later modification of the theory recognises the elastic nature of the lithosphere, allowing it to bend under loads – this is flexural isostasy, also known as regional isostasy. At equilibrium the lithosphere and asthenosphere are isostatically compensated. See also compensation depth, glacio-isostatic rebound.
Kinematics: The branch of classical mechanics that studies movement. In Earth sciences this centres on deformed rock, the kind that produces fault zones and landslides, thrust sheets and folds, or entire mountain belts and the evolving boundaries of tectonic plates. A kinematic analysis can probe single crystals or entire mountains.
Laurentia: Laurentia and Gondwana were the two major continental blocks that split from Pangea supercontinent. Laurentia originally was an amalgam of North American, Greenland, and northwest Scotland. Beginning about 175 Ma, the North American-Greenland portion of Laurentia split from Pangea, leaving the Scottish sliver behind.
Liquidus: The temperature at which there is complete melting of a solid; it is the maximum temperature at which crystals can coexist with their liquid melt. Note that the liquidus occurs at higher temperatures than the solidus.
Lithosphere – the compositional version: Consists of the crust and mantle lithosphere. The lithosphere-asthenosphere boundary is defined by a low seismic velocity discontinuity.
Lithosphere – the rheological version: The lithosphere – asthenosphere boundary is defined by the solidus, a isotherm at 1100o-1330oC, that marks the transition to partial melting of mantle rock. This is manifested as the transition to mechanically weak ductile-viscous behaviour in the asthenosphere. Defined in this way, oceanic lithosphere is about 5 km thick at spreading ridges increasing to 90 km thick in older, colder oceanic lithosphere. Continental lithosphere commonly ranges from 100-250 km thick.
Lithostatic pressure: At any depth, the pressure exerted by the overlying column of rock and sediment having unit-area cross-section, is calculated from the expression P = ρgh where ρ = density of the rock column, g = gravity constant, and h = depth from some datum, commonly sea level. Note that, assuming a cross-section of unit-area reduces volume to units of depth. Also called overburden pressure. It is analogous to hydrostatic pressure.
Magmatic arc: Also called volcanic arcs. A chain of volcanoes and associated intrusions that form in the plate above a subduction zone. Arcs generally parallel the deep oceanic trenches. At mantle depths, dewatering of oceanic crust in the subducting slab lowers the melting point of mantle rock. The partial melts rise because of buoyancy.
Magnetic field: Earth’s magnetic field is generated by a hot (4000-5000oC), fluid-like, iron-nickel rich outer core that moves slowly around a solid iron inner core. The field is forced into a tear-drop shape by solar winds, with the head of the ‘drop’ towards the sun (extending about 65,000 km), tapering over 600,000 km away from Earth. The magnetic field protects us from harmful components of the solar spectrum, like cosmic rays. The field has North and South poles that occasionally reverse over geological periods of 104 -105 years. The field intensity also waxes and wanes.
Magnetic poles: The points in a magnetic field where lines of equal magnetic intensity converge. On Earth, these poles are close to, but not coincident with the geographic poles; the magnetic poles also wander. During magnetic reversals the N and S magnetic poles switch places.
Magnetic reversals: Reversal of Earth’s magnetic field has occurred many times, and over the last few million years this has happened about every 200,000 to 300,000 years. The last reversal took place 780,000 years ago; this is called the Brunhes-Matuyama Reversal. Reversals are recorded by iron-bearing minerals in volcanic and sedimentary rocks where the minerals act as tiny magnets – the direction of polarity (i.e. magnetic N and S) is locked in mineral at the time of lava solidification or sedimentation, and this remnant magnetism can be measured.
Mantle plumes: A process of mantle convection involving the buoyant rise of asthenosphere; From lab experiments it has been learned that the shape of the plume depends on its viscosity – high viscosity leads to relatively narrow bodies, high viscosity to mushroom-shape bodies with tails. They are important forms of heat transfer and from partial melting, sources of magma.
Mélange: An extensive, mappable body of brecciated and sheared rock and sediment, chaotically mixed. They tend to form in compressive, accretionary tectonic settings and are more common in subduction-related accretionary prisms. However, they are also know from strike-slip and extensional regimes where more local compressive stresses are possible.
Mid-Ocean Ridge: The elevated ridge-like bathymetry with a central rift in oceanic crust, created by upwelling mantle plumes, intrusion of basic igneous rocks such as gabbros, and eruption of basaltic flows and pillow lavas during sea floor spreading. The total length of these ridges in all Earth’s oceans is more than 64,000 km. The ridge system was discovered by Marie Tharpe in the early 1950s.
Miogeocline (miogeosyncline): The part of a geosyncline that represents the continental margin. It consists of seaward thickening wedge or prism of sediment, commonly more than 10 km thick, that contains fluvial, coastal, and shelf facies. It is analogous to a passive margin succession. According to geosynclinal theory, a eugeosyncline occupies the oceanic realm seaward of the prism. Although the term geosyncline is no longer used, miogeocline does occasionally creep into modern literature.
Mohorovičić (Moho) discontinuity: The Moho defines the base of the crust that is recorded as an abrupt seismic P-wave velocity discontinuity from an average 6.7–7.2 km/s above, to 7.6–8.6 km/s in the mantle below. The increased seismic velocities are due to an increase in density, from basalt-gabbro lithologies in the lower crust, to peridotites in the uppermost mantle. See also Lithosphere – asthenosphere.
Oceanic trench: A narrow, relatively steep-sided depression in the ocean floor, the axis of which is at or close to the juncture where oceanic lithosphere bends into a subduction zone. One side of the trench is floored by oceanic crust; the other side by rocks of the overlying plate. Mariana Trench (near Guam) contains the deepest point on Earth’s surface at 10,916 m (35,814 feet) and is associated with subduction beneath Mariana volcanic arc. Trenches are seismically active.
Orogen: A very general term for regional deformation, usually along a convergent plate boundary, and associated with episodes of mountain building. Deformation styles include ductile and brittle behaviour, metamorphism, magmatic intrusion and arc volcanism, formation of accretionary prisms or fold-thrust belts, and associated sedimentary basins. Collisional orogens contain broad hinterland and foreland components, separated by a suture zone.
Orographic precipitation: Warm, humid air masses that are forced up mountain fronts will cool at higher elevations, promoting rain or snow fall. Also called orographic uplift.
Outboard basin: Basins on the trailing edge of a terrane. Cf. inboard basins. The effects of terrane accretion will vary depending on the size of the basin and its distance from the deformation front. However, these basins will be subject to deformation associated with any new incoming terrane.
Overlap assemblage (terranes): An assemblage of sediment and/or volcanic rocks that postdates the docking of two allochthonous terranes and provides an upper age limit for the terrane accretion event.
P & S waves (seismology): Seismic body waves generated by an impulse (earthquake, TNT, meteor impact) that travel through Earth from the energy source. P waves push and pull materials in the same direction as the propagated waves (also called compressional waves). S waves, or shear waves produce sideways motion – motion at right angles to the propagation direction. Shear waves do not travel through liquid. P waves travel fastest (up to 7.97 km/sec in upper mantle rocks) and are the first to appear on a seismogram. See also Surface waves.
Paleothermometer: Geological, paleontological and chemical tools used to determine the temperature conditions and thermal history of ancient environments, and more deep-seated processes associated with sedimentary basins, igneous and metamorphic events. They are components of rocks such as minerals, isotopes, fossils, and fluids that provide us with either a direct measure or proxies of paleotemperatures. Common examples include vitrinite reflectance of coals, fossil colour, radiogenic blocking temperatures, stable isotopes of oxygen and carbon, fission tracks, and fluid inclusions.
Pangea: A supercontinent borne of the amalgamation of Laurentia (North America + Greenland + northwest Scotland and Ireland) and the Grampian Arc 475-465 Ma (Ordovician), primarily via collision tectonics, initiating the first stage of the Caledonian Orogeny. Continued collisions with other continental block like Baltica, Avalonia, Africa, Australia, Antarctica, India, and New Zealand (Zelandia) during the 2nd stage of the Caledonian, created the final amalgam of Pangea. The Moine Thrust in the Scottish Hebrides is one of the defining megastructures of this event. Pangea eventually gave rise to Gondwana and Laurasia continental blocks.
Partial melting: Most rocks consist of several minerals, each of which has a different melting point. When rocks begin to melt, those minerals with the lowest melting points will be the first to contribute to magmas – the rock will be partially melted, producing a kind of crystal mush. Partial melting is a critical stage of magma formation in the mantle. See Flux melting.
Passive margins: Thick sedimentary wedges or prisms that border the trailing edge of continental margins during sea floor spreading. They present as shelves, platforms, and plateaus, with a continental slope at the seaward transition to ocean basins and oceanic crust. Passive margins follow directly on the heels of continental rifting, once stretching has ceased; there is commonly a major unconformity between the rift and passive margin succession, called a breakup unconformity. Subsidence is primarily a flexural response to lithospheric cooling (once mantle heating has been turned off), aided by a sediment load that can be more than 10 km thick. Depositional systems cover the gamut of siliciclastic and carbonate environments.
Passive rifting: Rift basins formed by stretching and extension of the lithosphere in response to stress fields generated by far-field plate motions. The rise of mantle plumes is a passive response to extension. Cf. Active rifting.
Pericratonic terrane: A terrane on attenuated continental lithosphere and sandwiched between the craton and surrounding allochthonous terranes, in which sediment is partly derived from the craton and neighbouring terranes.
Plutonic rocks: Igneous rocks cooled from magma that has intruded and remains in the crust. Some of this magma may find its way to the surface via dike feeders and erupted as volcanic products.
Pop-up ridge: Uplifts formed by contraction along strike-slip restraining bends. They are commonly bordered by reverse or thrust faults. The ridges may become important sources for sediment in adjacent strike-slip basins.
Post-glacial rebound: Bedrock beneath thick ice sheets is depressed by the weight of the ice load; ice loading effects the entire lithosphere. Ice melt reduces the load such that the lithosphere rises (rebounds) – this is an example of the lithosphere acting elastically. Rebound is an isostatic response, that begins as soon as melting begins. Classic regions currently undergoing rebound (since the last glaciation) are Scandinavia, Scotland, central Canada and northern USA.
Post-rift stratigraphy: Stratigraphy that accumulates after the main phase of lithospheric extension and rifting. It is usually associated with sea floor spreading and the accumulation of a passive margin across the continental-oceanic crust transition. It may be separated from syn-rift stratigraphy by a breakup unconformity that records the transition from ‘rift to drift’.
Postrift subsidence: The exponential decrease in tectonic subsidence resulting from the isostatic response to lithosphere cooling and mantle densification. It follows the synrift subsidence stage that results from mechanical processes. In numerical models, this component of subsidence is calculated by backstripping. The subsidence signature is typical of passive margins.
Precession: One of the Milankovitch astronomical orbitals. As Earth spins about its axis, an equatorial bulge causes the axis to wobble, a bit like a spinning top. A single wobble takes from 19,000 to 23,000 years, depending on variations in the gravitational interaction of the Sun, Moon, and planets (especially Jupiter). Precession is directly responsible for changes in solar intensity (insolation) on time scales of 100s to 1000s of years. See also Obliquity, Eccentricity.
Primordial heat: Heat that is left over from the time when planetary accretion took place (for Earth this is about 4.6 Ga). It is one of three major sources of heat in the Earth; the other sources are radioactive decay (radiogenic heat) and gravitational friction (mainly between the Sun and Moon).
Prodelta: Develops basinward of the steeper gradient delta front, as gently dipping stratal units that eventually merge with the basin floor. The prodelta is below wave base. It derives its mainly muddy-silty sediment from the distal limits of turbidity currents, from suspension, and from hypopycnal flows of mud.
Progradation: The basinward accretion of sediment when sediment supply keeps pace with or exceeds the generation of accommodation, either at the beginning or end of sea level rise. In a sequence stratigraphic context, it occurs during normal regression. The shoreline trajectory is approximately horizontal.
Provenance: Provenance studies the origins of detrital clasts in sedimentary rocks. It determines where they came from and when they were derived; i.e. provenance attempts to determine the composition of the original source rock, the tectonic environment from which it was delivered, and how far the source rock or sediment has traveled. Potentially it permits the reconstruction of ancient plate locations and plate motions.
Pull-apart basins: Also called strike-slip and wrench basins. These basins form between pairs of strike-slip faults, or at abrupt bends along single strike-slip fault strands. Basin subsidence is entirely tectonic. At releasing bends, strain is extensional and the crust is ‘pulled’ apart producing transtensional basins. Transtension and transpression is also generated along transform faults where relative plate motions are oblique rather than strike-slip.
Radiogenic heat: One of three heat sources in Earth’s interior, it is the heat generated by radioactive decay of natural isotopes, principally Uranium-238 and -235, Thorium-232, and Potassium-40. See also gravitational friction, primordial heat.
Releasing bend basin: Strike-slip basins formed by extensional subsidence at releasing bends. The type is represented by the iconic Ridge Basin in southern California.
Rheology: Describes the mechanical response of materials to stress. It applies to solids and fluids in Earth systems and is usually expressed as a relationship between stress and strain, or in the case of viscosity the strain rate. The three end-member behaviours are elastic, plastic (including ductile flow), and viscous behaviour. The principles can be applied to materials at the scale of the lithosphere and asthenosphere, to the behaviour of fluids in a single turbidity current.
Rift basin: In a plate tectonic context, an elongate basin bound by normal, commonly listric faults and related grabens that forms during the stretching of continental lithosphere. Volcanism is common in active rifts where mantle plumes manifest at as effusive and explosive eruptions. Sediments include coarse grained alluvial and fluvial deposits, associated with the elevated fault topography and rift shoulders. Evaporites accumulate in arid settings, from saline lakes or marine incursions. See rift drift, rift shoulder.
Rift-drift: Refers to the transition from continental rifting during lithospheric extension where basin subsidence is a function of brittle failure of the crust, to the production of oceanic crust and initiation of sea floor spreading. The drift stage is accompanied by accumulation of a passive margin on the continent side (trailing edge) in concert with an ocean basin. The transition may be recorded stratigraphically by a breakup unconformity.
Rift shoulder: An area of elevated topography in the footwall of rift basin boundary faults. Elevations can be 100s of metres above the plateau or plain adjacent to the rift basin. The elevated topography is a primary source of clastic sediment shed into the rift basin.
Sea level: Eustatic sea levels are measured with reference to a fixed datum like the centre of the Earth. For any datum that is not fixed, like a shoreline, sea level must be considered relative. Relative sea levels are affected by changes in glacio-eustasy, steric, and tectonic processes. Sea level is not the same everywhere because of differences in the gravitational potential. This is the main reason for the system of locks in Panama and Suez canals. Sea level equates with baselevel for depositional systems.
Seamount: Submarine volcanoes usually associated with oceanic crust, and formed from deep mantle-derived magma plumes and possibly from shallow rejuvenated plumes. Seamount heights range from a few 100 m, to over 8000 m – they are called volcanic islands where the edifice pokes its vent above the waves. They are similar to terrestrial shield volcanoes in size and gross morphology. They commonly occur in straight or arcuate chains. Iconic examples like Hawaii-Emperor chain and Louisville chain also show consistent age progression. Active seamounts are found in the middle of oceanic plates and close to plate margins; flanking mid-ocean ridges, subduction zones, and along fracture zones. Seamounts that broach the surface may provide habitats for coral reefs. Once volcanic activity ceases, the edifice will gradually sink under its own weight (an isostatic response).
Sediment routing system: The path that sedimentary particles follow from their source to final destination in a depositional sink. That path, or route, may be relatively direct, or circuitous with stopovers at sites of temporary storage (for example, a fluvial point bar). The character of the system depends feedbacks among landscapes, tectonics (denudation, erosion), climate, travel time, and depositional processes.
Sediment transfer zone: The zone of sediment dispersal between its source area (dominated by erosional processes), and depositional sink. In this zone there is generally a balance between sediment transport and deposition. Sediment dispersal in the transfer zone takes place via many different terrestrial and marine processes.
Sedimentary basin: From a geodynamic perspective, a region of prolonged subsidence, dependent on the rheology and thermal structure of the crust and lithosphere mantle. The four most common mechanisms promoting subsidence are: lithospheric stretching, cooling and densification, flexure from loading the crust with sediment and volcanic edifices, and flexure from tectonic loads.
Shoreface: The shallow marine environment extending from the low tide zone to fairweather wave base. The sea floor in this region is constantly impinged by wave orbitals. Bedforms of various sizes will form, depending on wave energy and tidal currents.
Shoreline trajectory: A 2-dimensional plot of shoreline excursions, either landward or seaward, based on interpretation of sedimentary facies and stratigraphy, using the stacking patterns of successions or parasequences that contain evidence for proximity to paleoshorelines. For example, a progradational trajectory is horizontal; a forced regression trajectory downsteps seaward.
Sink: See depositional sink.
Slow slip events: Small displacements along a subduction zone and its associated faults as a result of continued build-up of strain. The events occur in conjunction with episodic tremor – swarms of very low magnitude earthquakes, barely felt, if at all. None of the displacements results in major earthquakes.
Solidus: At temperatures below the solidus, a material remains solid. In geodynamics it represents the temperature above which partial melting will occur; the solidus isotherm at 1100-1300oC is used to define the lithosphere-asthenosphere boundary. The variability within this temperature range reflects the degree of hydration in the mantle lithosphere; hydration generally lowers the melting point. Note that the solidus occurs at lower temperatures than the liquidus; i.e. the two do not coincide.
Source to sink: An expression that describes, in very general terms, the journey sediment takes from its source area to its final destination. It encompasses:
- Source area topography, tectonic setting, paleoclimate;
- The changes in sediment composition by physical (abrasion, winnowing) and chemical processes;
- Changes in depositional environments en route,
- The stratigraphic position of sediment at its destination; and
- Post-depositional changes (diagenesis).
Spreading ridge: Also called a mid-ocean ridge. New oceanic crust and mantle lithosphere are created at spreading ridges. The oceanic lithosphere is stretched to a few km thin along the ridge axis; in its place, convecting asthenosphere plumes provide the magmas that erupt along the ridge. Spreading ridges are major plate tectonic boundaries.
Stratigraphic shingling: A term introduced by John Crowell (1974) applicable to strike-slip basins. It involves strike-slip fault controlled migration of a basin depocenter that produces a succession of dipping stratigraphic packages that onlap the basin floor. The packages are stacked like roofing shingles. At any one location the thickness of overlapping shingles might be measured in 100s of m; the cumulative thickness over the life of a strike-slip basin may be >10 km.
Stretching factor: β is used in models of lithospheric stretching and rifting. It is the ratio of the stretched width of the crust-mantle lithosphere and original pre-rift width.
Strike-slip basins: Also called pull-apart and wrench basins. Elongate, rhomboid- or sinusoidal-shaped basins formed by extensional subsidence, most commonly at releasing bends or extensional fault stepovers. Strike-slip basins have high aspect ratios, and are relatively deep compared with their areal dimensions. Cumulative stratigraphic thicknesses can be >10km where shingling of dipping sedimentary panels keeps pace with fault-induced depocentre migration. Sediment composition may change over the life of a strike-slip basin because of lateral shifts in source rock.
Subcretion: Slices of sediment and shallow crustal rocks that are accreted to the base of crustal slabs or accretionary prisms during compression, usually associated with subduction.
Subduction zone: Subduction zones are the ‘recycle bins’ of oceanic lithosphere. Cold, dense oceanic lithosphere will sink beneath less dense continental lithosphere at convergent plate boundaries, whereupon it is recycled into the mantle. The process may also be initiated by the negative buoyancy of the dense, oceanic rock, sinking under its own weight. The subducting plate was first identified by dipping zones of earthquake epicentres beneath island arcs and convergent continental margins, such as the Andes; these are Benioff Zones. Epicentres as deep as 640 km have been recorded.
Submarine canyon: Like their terrestrial counterparts, they are narrow, deep, steep sided valleys that extend from a continental shelf or platform to the slope, terminating near the base of slope or rise, where they merge with submarine channels. Their location may be structurally controlled, initiated by paleodrainage, or focusing of sediment gravity flow during low sea levels. They are important conduits for sediment delivery to submarine fans. Canyon wall collapse may produce significant tsunamis. Canyon heads may approach within a few 100 m of shorelines (e.g. Monterey Canyon, California, Hikurangi Canyon, New Zealand).
Submarine fan: Fan-shaped depositional systems that accumulate at the base of slope, continental rise and adjacent basin floor. Sediment is usually fed via a large submarine channel or canyon that may bifurcate into multiple channels down gradient. The channels feed sediment to lobes that prograde basinward; lobes may be inactive for a period. Deposition is dominated by sediment gravity flows – turbidity currents, debris flows. Mass transport deposits (slumps, slides) are common in some fan systems.
Successor basins: Post orogenic, relatively undeformed basins that overlie fold belts and terrane suture zones. The timing of successor basin fill helps bracket the end of (exotic) terrane accretion, or docking (e.g. by plate collision). They usually consist of terrestrial sediments, the provenance of which will reflect the compositions of the lithospheric blocks involved in terrane accretion.
Superterrane: The composite of two or more terranes that were amalgamated prior to its accretion to an orogen.
Suspect terrane: The evocative name given to any terrane, particularly those of unknown or indefinite origin. Generally synonymous with exotic terrane and allochthonous terrane.
Syn-rift: At the time of rifting. It is usually applied to structures such as listric faults that develop from brittle failure during crustal stretching, and to the sedimentary deposits that accumulate during the various rift processes particularly deformation and volcanism, commonly in fault-bound basins. Cf. Post-rift.
Synrift subsidence: The initial, rapid stage of subsidence resulting from lithospheric stretching, thinning, and faulting. The amount of subsidence attributed purely to these tectonic processes is determined by backstripping. cf. postrift subsidence, geohistory.
Tectonic province: Regions of tectonism or lack of it, magmatism, volcanism, and metamorphism in a plate tectonic context. Such provinces include collision orogens, magmatic arcs, forearc and foreland basins, strike-slip basins, oceanic basins, and stable cratons. The concept was popularized by W.R. Dickinson to relate provenance to plate tectonics.
Tectonic transport: The movement of crustal or lithospheric blocks along major faults or fault zones. It can occur at the scale of a single thrust duplex, and entire fold-thrust belt, and obducted or delaminated segments of the lithosphere.
Tectonic subsidence: The component of total subsidence resulting from crustal-lithosphere scale processes such as rifting, the isostatic response to lithosphere cooling and densification (such as passive margins, oceanic crust), and fault-controlled strike slip pull-apart subsidence. It is calculated using the backstripping geohistory method. Tectonic subsidence curves commonly have signatures that are common to a specific subsidence mechanism, for example the synrift and postrift stages of passive margin subsidence.
Terrain: A general term for land and landforms. Common qualifications include rough terrain, flat terrain, mountainous terrain, subdued terrain. Cf terrane.
Terrane: Stratigraphic and structurally distinct blocks, ranging in size from lithospheric scale to thin crustal slivers, kilometres to 1000s of km in extent, and in fault contact with neighbouring terranes or autochthonous cratons. All terranes are allochthonous, emplaced by plate tectonic processes, mostly contractional or strike-slip. Many orogens consist of a collage of disparate terranes. See Terrane stitching, terrane docking, overlap assemblage.
Terrane docking: The accretion of an allochthonous terrane to another terrane, stable craton, or orogenic belt. The timing of docking events are a critical part of unraveling the history of an orogen.
Terrane stitching: Intrusive magmatic rocks that crosscut the faulted boundary between two terranes. Dating the intrusives provides an upper age limit for terrane docking.
Tethys sea: The body of tropical seas between Gondwana and Laurasia dating from Late Paleozoic (Permian), while the two continental blocks were attached as Pangea. The seaway expanded during the separation of Gondwana and Laurasia, but by about 50 Ma (Early Eocene) had disappeared as India, Arabia, and Apulia (parts of Italy, Greece, Turkey and the Balkan states) collided with the Eurasia forming the Himalayan – Alpine mountain belts.
Thermal subsidence: Subsidence of the lithosphere resulting from cooling and densification, following an earlier rifting and heating event. Cooling is exponential over periods commonly exceeding 100 Myrs. Subsidence takes place by lithospheric flexure. Thermal subsidence is usually initiated at the start of sea floor spreading. Passive margin successions provide a stratigraphic record of thermal subsidence. Also called Thermo-isostatic subsidence
Thin-skinned deformation: A reference to crustal-scale deformation, such as fold-thrust belts, that structurally overlie basement rocks that have not been involved in the deformation. The cross-section shown below (Alberta Front Ranges) is a classic example. In that example, the fold-thrust belt is part of the allochthon, and the Paleozoic cratonic strata and crystalline basement are part of the autochthon.
Tidal bulge: The bulge in ocean water mass caused by the gravitational pull of the moon, that develops on the side facing the Moon and the immediate opposite side of Earth. The bulge corresponds to high tide. Earth’s rotates through this tidal bulge resulting in the high tide to move along ocean margins; (in other words, the bulge stays in place and Earth moves). See tidal wave, tidal range.
Tidal gauge: A device for measuring the relative changes in sea level. Measurements are specific to the location of the gauge and in themselves do not account for uplift or subsidence of the land, unless coupled with a GPS instrument or satellite altimetry. Corrections need to be made for the kind of coast (open sea or sheltered harbour), air pressure, water temperature, storm surges, and tectonic-seismic events. The measuring device can be as simple as a graduated post, or sophisticated satellite radar altimetry.
Tidal range: This is the range between mean high water and mean low water. It varies from place to place because of coastal geomorphology and bathymetry. In some places it can be amplified (Bay of Fundy has a range to 14 m) or weakened – ranges in the Mediterranean are very low. A commonly used scale for tidal ranges is:
- Micro-tidal < 2 metres.
- Meso-tidal 2 – 4 metres.
- Macro-tidal > 4 metres.
Tidal wave: The cycle of tidal highs and lows that move along a coastline. If the waves have a period of 12 hours (i.e. two tides per day) then they are semidiurnal. Movement of tidal waves around ocean margins is caused by Earth’s rotation relative to the tidal bulge produced by gravitational forces from the Moon and Sun. Movement is counterclockwise in the northern hemisphere, and clockwise in the southern hemisphere. Tidal waves are NOT synonymous with Tsunami.
Topography driven flow: Groundwater flow that is driven by topographic gravitational potential. It is the dominant mechanism of groundwater flow at shallow levels of Earth’s crust, to depths of 2-3 km. It is usually expressed as hydraulic potential, or hydraulic head (H), where:
HTotal = h (the elevation head) + P (pressure head)/ρg, relative to a datum (commonly taken as sea level).
Total basin subsidence: The total amount of subsidence resulting from tectonic processes (the tectonic subsidence component), sediment compaction, and the isostatic effects of sediment load. Geohistory analysis attempts to tease apart these different processes.
Transcurrent fault: Synonymous with strike-slip fault. See also Transform fault.
Transform fault: One of the major types of plate boundary where two plates slide past each other in a strike-slip motion. If relative plate motion is oblique, then components of transtension and transpression will occur. They are lithosphere-scale structures. Classic examples include San Andreas Fault in California that separates the North American plate from the Pacific Plate, and the Alpine Fault in New Zealand, separating the south Pacific and Australian plates. Dextral (right-lateral) strike-slip displacement along Alpine Fault is about 450 km.
Triangle zone: In a foreland fold-thrust belt context, it defines a wedge-shaped, subsurface deformation front having a basal thrust (the main décollement) and a hinterland-dipping roof thrust. The roof thrust is commonly passive.
Triple junction (plate tectonics): A point where three plate boundaries converge. Potentially, it can be any combination of the three basic boundary types: spreading ridge (R), trench (T), and transform fault (F). Certain boundary combinations are more stable than others, for example a RRR triple junction is always stable; whereas a FFF is mostly unstable (because of opposing relative motions).
Unroofing (tectonics): Describes the uplift and erosion of rocks that in stratigraphic successions records the order that successive rock types or layers were exposed. For example, the stratigraphic expression of unroofing of a magmatic arc will be an initial pulse of volcanic and volcaniclastic sediment, overlain successively by exposure and erosion of progressively deeper intrusive igneous rocks.
Viscosity: Viscosity is used to describe a material in which its strength depends on the rate of deformation, or strain rate. From a practical point of view, it is a measure of its resistance to deformation, or flow. It is normally applied to fluids, including rocks that may behave as fluids under high confining pressures and low strain rates. In the Earth sciences, viscosity is applied to phenomena like mud flows and ice sheets, and to rocks in the mantle.
Volcanic island: Refers to seamounts that have breached the sea surface such that active volcanism involves subaerial processes. Good examples include Hawaii islands, Canary Islands, and Cook Islands.
