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FOLD MOUNTAINS AND MOUNTAIN BUILDING (OROGENY)

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Mountain 🏔️🗻 Types

  1. Volcanic Mountain 🌋
  2. Fold Mountain 🏞️
  3. Block
  4. Escarpment 🏔️
  5. Residual Mountain 🗻

Fold Mountain 🏞️

  • True mountains.
  • Sedimentary rocks of Marine origin.
  • compressive forces fold the Sedimentary beds
  • these are never isolated - group of parallel ranges.
  • They are the most extensive system on the continent.
  • Unique to earth - No planet has fold mountains.

Mountain Building 🗻👷‍♀️🏗️ - Orogeny - Reference to fold Mountain building. but also can be used for other mountain formation.

Fold Mountains are of Three types.

  1. Himalayan Fold mountain
  2. Andean Fold mountains.
  3. Aleutian Fold mountains

The volcanic archipelago related to O-O collission may be considered as the initial stage in fold Mountain formation. They are the simplest type

EXPLAINING FOLD MOUNTAIN FORMATION.

The unique composition and their extensive presence have been a mystery for a long time. Most of their attempt to explain the earth's crust are those related to explaining the fold Mountain upliftment. Some of the earliest theory in fold Mountain origin are.

  1. Thermal contraction🌡️ theory of Harrold Jeffrey - mountains are wrinkles on earth's surface.
  2. Collisson💥 - N - Laurasia and S-Gondwana land. movement - Tidal pull of sun 🌞 and moon🌚. Taylor
  3. Wegener - Sial while drifting over the sima scrapes the ocean floor - accumulates the marine sediments as fold Mountain along the leading edge
  4. Geosyncline
  5. The modern tectonic Theory.

GEOSYNCLINES -

The concept of GEOSYNCLINES is considered a geological innovation in the attempt to explain the origin and features associated on the fold Mountain. fold Mountain have marine sedimentary rocks that have uplifted into long linear chains and was difficult to imagine oceans of today uplifting into fold Mountains.

Geo syncline may be defined as much elongated massive depressions relatively narrow like features like valleys. The concept was given by Hall and Dana who also suggested the floor of the geosyncline to be mobile capable of subsidence under the weight of accumulating sediments.

Types of Geosyncline

  • J. A. Steers
    • Geo Syclines are massive elongated depressions always between two continents
    • Eg Mediterranean sea or he may be referring to tethys sea the remnants of which is Mediterranean sea.
  • Evans -
    • GEOSYNCLINES can either be between 2 continent or they can also be along the margins of 2 continents.
    • eg: The hypothetical Geo syncline uplifted into the rockies and the andes Mountains.

Evaluating the concept of GEOSYNCLINES

In the modern concept of plate techtonies, we do not need the concept of GS to explain the upliftment and formation of fold Mountain

Eg upliftment of himalayas can be considered from a Geosyncline like tethys sea but the upliftment of Rockies and andes did not start from any such structures from the edges of continent.

2 The concept of volcanic intrusion (ophiolites) is not explained by geosynclinal Theory

In tectonic theory, the origin of opiolites is from the process obduction where broken fragments of ocean basalt rocks can be thrust into continental rocks and Melange which then can be uplifted with the upliftment of accentionary prism sediments.

Mountain building according to techtonic theory.

All fold mountains are essentially Convergent boundaries. There are 3 type of convergent boundary and so we may consider there ar 3 types fold mountains.

  1. The archipelago - Aleutian type - OO
  2. Andean type - OC
  3. The Himalayan type / The Aleutian type CC

The archipelago - Aleutian type - OO

  • (These are the simplest ones)
  • They don't look like fold Mountains
  • but they may be considered as the first stage in the upliftment of fold Mountain

Andean type - OC.

  • -o-c Collision , Melange -
  • Subduction related related to Ophiolitic Suits (They are the broken basaltic rocks of the ocean.
  • Andesitic eruption because of subducting ocean slab.
  • Massive volcanic intrusion ie granitic dioritic intrusion.
  • Base becomes the core of fold Mountain
  • Because of strong compression, there can be low angled thrust faults.

The Himalayan type / The Alpine type CC

  • c-c collision
  • Most imposing (extensive) features
  • Enormous sediminents found.
  • Strong compressive forces.
  • Very high doubling of crust.
  • Under thrusting of continent
  • No active volcanos.
  • Can also have reverse faults and complex structures.
  • over turned folds, Isoclinal folds, Recumbent folds, nappe structures
  • Many of these complex structures are found in the midde himalayas because of intense forces of the meocene times that uplifted the Himalayas.

Episodes of fold Mountain upliftment (Arch of Calcium is Her Territory)

  1. Archean 2- 2.5 bya Aravalli and Eastern Ghats
  2. Caledonian - Silurian and Devonian
  3. Hercynian - Carboniferous and Permian, Urals great dividing range and Pennines
  4. Tertiary Alpine - latest, Himalayan, Rockies, Andes. Started in late Cretaceous

Upliftment Of THE APPALACHIANS.

  1. Before Pangea - Before the pangea super continent, there was another super continent (1-1.1) Billion years B.P
  2. Pangea split and ancestral Atlantic Ocean Around (800 - 700) Million years ago, this pre Cambrian super continent split. First broke N. America from Africa which created an ancestral Atlantic Ocean.
  3. Marginal sea - When N. America and Africa drifted apart, an eastern chunk of N. America - broke up creating a marginal sea along with ancestral Atlantic Ocean.
  4. marginal sea and ancestral Atlantic Ocean close Around 6oo to 7oo Million years ago, the ancestral Atlantic Ocean start to close. At around the same time, the marginal sea also starts to close and the volcano eruption zone coincides with the continental fragments.
  5. tachonic orogeny - About 500 million years B.P, the marginal sea had entirely closed, forming the first Belt of the Appalachians, Called as tachonic orogeny of the appalachians.
  6. acadian orogeny - At around ( 4oo to 500) M years BP, the Western slab of ancestral Atlantic Ocean gets entirely subducted and the volcanic island arc collides with The N. American plate called acadian orogeny.
  7. hercynian upliftment - the remaining part of ancestral Atlantic Ocean gets entirely subducted and african plate Collides with N Americam plate. This happened around (250 - 350) M years ago, called Allegheny upliftment. This was the main hercynian upliftment.
  8. pangea again split atlantic ocean formed. Around 200 million years before present pangea again split to form the atlantic ocean of today.

The Appalachian mountain of today have 3 zones from West to East.

  • Valley Rich zone - unmetamorphozed palaeozoic sediments
  • Blue ridge zone - relatively more metamorphosed rocks along with precambrian crystalline continental rocks.
  • Pied mont zone/ province - metamorphosed sediments and intruded granitic plutons