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▪ accretion (veins) ▪ accretion (tectonic) ▪ accretionary prism ▪ allochthonous nappe ▪ angular unconformity ▪ anatexis ▪ anticline ▪ aphanitic texture ▪ aplite, aplite dike (◙ aplite) ▪ argillaceous, argillic ▪ assimilation ▪ assymetric fold ▪ assymetric vein ▪ attitude of structures ▪ autochthonous ▪ axial plane ▪ axis (fold) ▪ azimuth ▪

Ancient continents ▫ Baltica

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accretionary prism

An accretionary prism, accretionary wedge, or accreted mélange is a wedge-shaped body of faulted and folded material accreted (added) to a continental margin in a subaqueous thrust zone.

A mélange is a body of mappable-sized blocks of different rocks jumbled together with little continuity of contacts. The origins of mélanges are either tectonic, submarine sliding (olistrosomes), or diapirism.

Olistostromes are mélanges formed by accumulation of submarine, gravitational flow as semi-fluid bodies, so are stratigraphic units that lack true bedding, yet are intercalated between normal sedimentary bedding sequences.

Accretionary prisms can arise by:
▪ Having been scraped off subducting oceanic crust and accreted to an island arc or continental margin at a subduction zone.
▪ Accumulation, at greater depth, of a mass of deformed trench sediments and ocean floor sediments on the underside of the tectonic plate that lies above the subducting plate (tectonic underplating). Trenches infilled with thick turbidite sediments typically have listric thrust faults that flatten into a decollement horizon that is often close to the top of the pelagic sediments.
▪ Some accretionary prisms with few reflectors on seismic profiles may contain chaotically deformed sediments (broken formation or mélange) similar to those known from ancient orogens.

Accretionary prisms and accreted terranes are not equivalent to tectonic plates, but rather are associated with tectonic plates and accrete as a result of tectonic collision. Materials incorporated in accretionary prisms include:
▪ Ocean-floor basalts – typically seamounts scraped off the subducting plate
▪ Pelagic sediments – typically immediately overlying oceanic crust of the subducting plate
▪ Trench sediments – typically turbidites that may be derived from:
--- ▪ Oceanic, volcanic island arc
--- ▪ Continental volcanic arc and cordilleran orogen
--- ▪ Adjacent continental masses located along strike (such as Barbados).
--- ▪ Material transported into the trench by gravity sliding and debris flow from the forearc --------ridge (olistostrome)
--- ▪ Piggy-back basins, which are small basins located in surface depression on the accretionary -----prism.
--- ▪ Material exposed in the forearc ridge may include fragments of oceanic crust or high------------pressure metamorphic rocks thrust from deeper in the subduction zone.

◙ subduction zone magmas ◙

[links: images: formations: accretion prisms: Jurassic and Cretaceous Franciscan Formation - an accreted melange or accretionary prism that formed in a trench along a subduction zone; low-grade metamorphosed accretionary complex with greenschist facies metamorphism, shear deformation, crenulation cleavage, kink band, Otaki Group;
diagrams: accretionary prism and olistostrome associated with Santiago Peak Volcanics and associated intrusives; structural zones and lithologic zones and metamorphic zones associated with subduction zones and orogeny; webpages: Geology of the Point Reyes Area, California - accretionary complex of a subduction zone, transform plate boundary, granitic basement of a continental magmatic arc; webpages: trenches and mélanges]

anatexis

Anatexis (loss of texture) is partial melting of rock under extreme conditions of temperature and pressure.

Anatexis more often results from tectonism (regional metamorphism) than from magmatism (contact metamorphism) because magmas typically carry too little excess heat to melt significant quantities of surrounding rock while themselves continuing to remain molten.

Anatexis provides a common form of magmatic mixing in areas of active magmatism, wherby adjacent magma bodies can develop transient subsurface communications before their eruption or final subsurface emplacement. Anatexis-related magmatic mixing involves the secondary melting of mid- to lower crustal rocks upon contact with much hotter, rising mafic melts of mantle origin, and produces felsic (feldspar- and quartz-rich) magmas in arc and continental rift settings. Such melts may reach high crustal levels carrying both mantle heat and mantle material.

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anticline

Anticlines are upwardly convex folded structures that form arches with the youngest (last deposited) strata at the hinge.

Anticlines are A-shaped structures that develop during crustal deformation as the result of compression that accompanies orogenic mountain building.

(images at left - click to enlarge - top, schematic of an anticline; botttom, road cut exposure of anticlinal fold with syncline to right)

Anticlines are of particular interest to oil-exploration geologists because the Earth's largest oilfields occur in large, gentle anticlines in thick sedimentary rock sequences.

Click here for an image that shows an anticline with a hinge that has a flat top and two steep limbs, creating a box-like shape. Folds with this shape are called box folds. Both synclines and anticlines can be box folds.

[links: images: formations: anticline; anticline and syncline near Calico Ghost Town, Yermo, California; anticline; Teton anticline; anticline; satellite: anticlines near Paradox, UT, and anticline crossed by transverse stream, and Uncompahgre uplift; plunging anticlines and synclines north of Moab, UT; plunging anticlines and synclines, Dinosaur National Monument, UT]

argillic

Argillic or argillaceous refers to clay.

Argillic alteration of rocks involves conversion of certain minerals to minerals of the clay group, such as kaolinite (below right) and montmorillonite (bottom right).

Clays are hydrous aluminium phyllosilicates, typically less than 2 μm (micrometres) in diameter, and are distinguished from other small soil particles, such as silt. Clays may be residual or transported, and generally result from:
▪ the chemical weathering of aluminosilicate-bearing rocks (such as granite, containing feldspars),
▪ solution of rocks containing clayey impurities, such as limestone,
▪ disintegration and solution of shales,
▪ hydrothermal alteration.

Clays exhibit the smallest size of soil particles, flake or layered shape, affinity for water, and a tendency toward high plasticity.

In soils, argillic horizons are diagnostic clay accumulations, often designated as Bt (B horizon dominated by deposited clay, "t").

For example, "alfisols have an argillic, a kandic, or a natric horizon and a base saturation of 35% or greater. They typically have an ochric epipedon, but may have an umbric epipedon. They may also have a petrocalcic horizon, a fragipan or a duripan." Glossary, Map, Soil Orders.

links: Micromorphology of argillic horizons / Soil Formation and Classification, What is Soil?, Soil Science Glossary, USDA gallery of soil profiles, soil facts, soil education

assimilation

Assimilation is that process of magmatic differentiation whereby ascending magmas evolve chemically by recruiting easily melted or dissolved components (fusibles) from the walls of their conduits.

Under the agency of heat and magmatic fluids, ascending magmas pick up volatiles, silica, trace elements, and occassionally fragments of wall rock. The heat that the melt gains by leaving behind quick-freezing refractories (an exothermic process) is typically sufficient to compensate for heat lost in the endothermic reactions required for the assimilation (melting) of country rock components. This trade-off ensures that assimilation can proceed without causing the melt to freeze (solidify).

Any wall rock fragments that survive more or less intact, without completely melting or dissolving into the magma, are called xenoliths. Surviving wall rock crystals are called xenocrysts. Together, xenoliths and xenocrysts provide invaluable information about rarely exposed lower crust and mantle levels by carrying these materials up within the ascending magma.

▪ Bowen's Reaction Series

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