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.
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
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
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:
(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.
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
Labels: orogenic belt, orogenic event, orogeny