Orogeny

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The terms orogeny and orogenesis are synonymous for tectonic processes that result in the formation of mountain chains. Orogeny and orogenesis are derived from the Greek words oros, meaning mountain, and geneia, meaning born. Orogeny can also have a time connotation when used in naming periods of intense tectonic activity and mountain building, whilst orogenesis is only used to describe the process. For example, "Grenvillian Orogeny" is used to refer to the period of orogenesis in many parts of the world approximately one billion years ago, synchronous with the collision between Laurentia and Baltica in the Grenville Province of North America. Areas in which mountain building have occurred in the past, although no mountains may remain today, are called orogens or orogenic belts.

Orogeny or orogenesis most commonly involves the collision between two continental lithospheric plates or the collision between a continental plate and an island arc. When a continent collides with an island arc, arc rocks are thrust over continental crust. In some orogens (including the Grenville Province), collision between a continental plate and one or more island arcs has occurred prior to final continent-continent collision. Collision follows subduction of oceanic lithosphere beneath one continent (or possibly even both continents), or a continent and an island arc. Subduction progressively closes the ocean that previously separated them. Continental crust may be partially subducted following ocean closure. Evidence for this comes from deep reflection seismic profiles and seismic tomographic images. High-pressure metamorphic minerals such as coesite and glaucophane in exhumed continental crust formed during subduction. In continent-continent collisions, oceanic upper mantle and crust, along with sediments deposited on the seafloor and in the trench above the subduction zone, are thrust over continental crust as a series of imbricate slices in a process called obduction. The former oceanic material (collectively called an ophiolite) marks the suture between the two plates. Ophiolitic rocks may be overthrust by continental crust of the colliding plate. Continued shortening (resulting from protracted convergence following plate collision) further deforms the orogen. Early-formed thrust slices are folded and thrusts developed with both the same and opposite sense of tectonic transport to earlier thrusts. In transpressional orogens (e.g., the Mesoproterozoic Albany Mobile Belt of Western Australia or the Tertiary Spitsbergen fold and thrust belt, Svalbard, Norway), lateral displacement between plates occurs in addition to folding and thrusting. Transpressional orogens are created by the oblique convergence between two continental plates and do not necessitate closure of an ocean.

Both upper and exhumed lower crustal rocks in collisional and transpressional orogens frequently display evidence for late orogenic extension, or cyclic changes between regional shortening and extension. Uplifted, upper crust may collapse along normal faults and extensional detachments as a result of gravitational instability. Gravitational collapse of upper crustal rocks may take place even though regional shortening continues at deeper crustal levels. Faults formed during gravitational collapse may trend parallel, orthogonal or oblique to earlier-formed thrust faults. Extension in the middle to lower crust in a thrust-thickened orogen may result from convective removal or peeling away (delamination) of part of a lithospheric root. Extension may also be induced by gravitational instabilities within the lower crust. Where the thickened crustal root comprises buoyant material, lateral and upward gravity spreading of the lower crust (root rebound) results in horizontal flattening and formation of shallow extensional ductile shear zones. Late-to post-orogenic granitoids that intrude as large batholiths or as sheet-like bodies along older shear zones result from partial melting of the extended crust. In Precambrian orogens, AMCG-suite rocks (anorthositemangerite-charnockite-grano diorite) are also commonly the product of late-orogenic extension.

Mountain Chains; Plate Tectonics