2. Orogenesis is the most complex of tectonic
processes and interpreting ancient mountain
belts is one of the greatest challenges geologists
face today.
As originally defined by Gilbert (1890), an
orogeny (from the classical Greek Oros meaning
'mountain' and gens meaning 'stemming from')
is simply a period of mountain building.
3. To field geologists the term orogeny represents a
penetrative deformation of the Earth's crust
associated with phases of metamorphism
and igneous activity along restricted,
commonly linear zones and within a limited time
interval (Dennis 1967).
However our increasing understanding of the
rheology of lithospheres allows geologists today
to view orogenesis on a larger scale as the
interaction of a series of geodynamic processes.
4.
5. orogeny literally, the process of formation of
mountains. The term came into use in the middle of
the 19th Century, when the process was thought
to include both the deformation of rocks within the
mountains, and the creation of the mountainous
topography.
Only much later was it realised that the two
processes were mostly not closely related, either in
origin or in time.
Today, most geologists regard the formation of
mountainous topography as postorogenic.
6. By present geological usage, orogeny is the process by
which structures within fold-belt mountainous areas
were formed, including thrusting, folding, and faulting
in the outer and higher layers, and plastic folding,
metamorphism, and plutonism in the inner and deeper
layers.
orogeny or orogenesis—the formation of mountains.
It is brought about by the movements of the rigid
plates making up the Earth’s crust (described by plate
tectonics).
Where two plates collide at a destructive margin rocks
become folded and lifted to form chains of fold
mountains(such as the young fold mountains of the
Himalayas).
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14. • Formation of an orogen is accomplished in part
by the tectonic processes of subduction (where
a continent rides forcefully over an oceanic
plate (noncollisional orogens)) or convergence of
two or more continents (collisional orogens).
• Orogeny usually produces long arcuate (from the
Latin arcuare, "to bend like a bow") structures,
known as orogenic belts.
• Generally, orogenic belts consist of long parallel
strips of rock exhibiting similar characteristics
along the length of the belt. Orogenic belts are
associated with subduction zones, which
consume crust, produce volcanoes, and
build island arcs.
15. • The processes of orogeny can take tens of millions of years
and build mountains from plains or from the seabed.
• The topographic height of orogenic mountains is related to
the principle of isostasy, that is, a balance of the
downward gravitational force upon an upthrust mountain
range (composed of light, continental crust material) and
the buoyant upward forces exerted by the dense
underlying mantle.
• Frequently, rock formations that undergo orogeny are
severely deformed and undergo metamorphism. Orogenic
processes may push deeply buried rocks to the surface.
• Sea-bottom and near-shore material may cover some or all
of the orogenic area.
• If the orogeny is due to two continents colliding, very high
mountains can result (see Himalayas).
16. An orogenic event may be studied:
(a) as a tectonic structural event,
(b)as a geographical event, and
(c) as a chronological event.
Orogenic events:
cause distinctive structural phenomena related
to tectonic activity affect rocks and crust in
particular regions, and happen within a specific
period
17. Orogen or "orogenic system"
• An orogeny produces an orogen, or (mountain) range-foreland
basin system.
• The foreland basin forms ahead of the orogen due mainly to loading
and resulting flexure of the lithosphere by the developing mountain
belt.
• A typical foreland basin is subdivided into a wedge-top basin above the
active orogenic wedge, the foredeep immediately beyond the active
front, a forebulge high of flexural origin and a back-bulge area beyond,
although not all of these are present in all foreland-basin systems.
• The basin migrates with the orogenic front and early deposited
foreland basin sediments become progressively involved in folding and
thrusting. Sediments deposited in the foreland basin are mainly
derived from the erosion of the actively uplifting rocks of the mountain
range, although some sediments derive from the foreland.
• The fill of many such basins shows a change in time from deepwater
marine (flysch-style) through shallow water to continental (molasse-
style) sediments.
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19. The Persian Gulf – the foreland basin produced by the
Zagros orogenic belt
