Orogeny: As it relates to Mountain Building
Orogenesis: The processes that collectively produce a mountain belt
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Mountains can be constructed by lavas and volcanic debris erupting at the
surface and by massive amounts of intrusive igneous which has been
solidified at depths. For example, the Andes
BUT! Most major mountain belts are a result of tectonic forces that have
shortened and thickened the crust called “compressional mountains”
which contain large amounts of preexisting sedimentary rock and
crystalline crustal fragments
 Folding and thrust faulting are most obvious signs of
orogenesis, yet metamorphism and igneous activity are
present as well.
Mountain building occurs at convergent plate boundaries
 tectonic forces collide and fold
 subduction of oceanic lithosphere triggers partial melting and intrusion
Subduction and Orogeny
 oceanic lithosphere subducts beneath an oceanic plate= island arc and
related tectonic features
 oceanic lithosphere subducts beneath a continental plate= volcanic arc
and related tectonic features (“Andean-type” plate margins)
 compressional forces are dominant
 major features of subduction zones are 1)deep ocean trench, 2) a volcanic
arc, 3) forearc region located between the trench and the volcanic arc,
and 4) the backarc basin located on the side of the volcanic arc opposite
the trench
1. Island Arcs= volcanic activity, pluton bodies, and accumulation of
sediment that has been scraped from subducting plate increases the
material
2. Andean-type Margins= volcanic arcs, accretionary wedges and forearc
basins
a. Volcanic arcs- need partial melting of mantle rock, usually
peridotite, which generates primary magmas with basaltic
compositions.
i. Basalt less dense and rises to the crust
ii. Magmatic differentiation allows the rocks to continue
ascending by crystalizing out the Fe rich minerals and
leaving the light silicas
iii. Now the andesitic or rhyolitic composition which produce
explosive eruptions (water and other volatiles = gas laden
magma) which produce volcanic arcs!
b. Emplacement of Plutonsi. Plutons metamorphose the host rock beneath the
continental crust= contact metamorphism
ii. With uplifting and erosion the plutons are exposed as
batholiths (Sierra Nevadas) of diorite to granodiorite
c. Development of Accretionary Wedge-