Paleogeology

Paleogeology, Paleoclimate, in relation to Evolution of Life on Earth

orogeny

cross-section diagram depicting Taconic Orogeny of eastern North America from 543-440 MaOrogeny is the tectonic process by which mountain chains (orogenic belts, orogens) are/were built.

(image at left - click to enlarge - Taconic Orogeny of eastern North America from 543-440 Ma).
Whereas a mountain chain is a geographic structure, an orogen may have been completely destroyed by erosion, exhuming ancient mountain-root rocks that display the folding, faulting, and metamorphic traces of the orogeny.
Orogenic belts are typically long, arcuate bands of crystalline rocks in terranes or blocks of deformed rocks separated by dipping thrust faults. Long parallel strips of rock exhibit similar characteristics along the length of the belt, but differ across the belt. The crystalline metamorphic rocks may lie below overthrust younger sediments that dip away from the orogenic core. Thin nappes (slices) of sediments from the sea bottom and near shore are thrust from the margins of compressing, alpinotype orogens toward the core, so are intimately associated with folds and metamorphism.

Orogeny can be associated with continental collision and volcanic activity. Where orogeny results from continental-continental collision, very high mountain chains such as the Himalayas and Alps can be thrust upward. Orogenic belts are associated with subduction zones at continental-oceanic collision margins, at which oceanic crust is consumed, producing volcanoes and island arcs. Over the tens of millions of years of orogenic activity, island arcs and their associated ocean trenches may be accreted to the collisional continental margin.

The topographic height of orogenic mountains is compensated (principle of isostasy) by a thickening of the underlying continental crust. Isostacy operates to balance the weight of upthrust mountains chains as they 'float' atop the denser mantle.

Characteristic sub-types of orogeny:

Alpinotype (ocean trench style);
Relatively narrow orogens with rapid, high uplift and predominant nappe structures including abyssal sedimentary rocks (black shale, chert, etc.) sitting atop deep, high pressure metamorphic zones with many facies. Abundant ophiolite complexes with ultramafic rocks, and few granites or migmatites.

Hercynotype (back-arc basin type):
Very wide orogens with minimal, slow uplift and shallow, low-pressure/high temperature metamorphism in thin metamorphic zones with abundant granite and migmatite. Rare nappe structures, few ophiolites, and almost no ultramafic rocks.

Cordilleran (arc) type;
Low-pressure/low geothermal gradient metamorphism with moderate uplift, dominated by calc-alkaline igneous rocks, andesites, and granite batholiths. Structures display a lack of nappes and migmatites, and lack of ophiolite and abyssal sedimentary rocks.

Variations:
Collisional tectonics modified by a transform plate boundary (New Zealand). Island arc orogenies occuring at a distance from the continental backstop (New Guinea). Proterozoic continent-continent collisional orogens (Musgrave Block in Australia).

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