Geosynclines

The Concept of Geosynclines

Introduction

  • The concept of geosyncline was given by James Hall and Dana and elaborated by E. Haug.
  • Geosynclines are long, narrow and shallow depressions; generally bordered by rigid Forelands or Kratogen; sinking due to gravity.
  • Geosynclines are the water depressions characterized by sedimentation and subsidence. – A. Steers (1932).

  • The mobile zone (Geosyncline or Orogen): It is the place of mountain building. g. Tethys sea, Tarin basin, Indo Gangetic plains.
  • Rigid zone (Foreland or Kratogen): It surrounds mobile zone. (Deccan plateau, Angaraland).
  • Marginal range (Rand Ketten): Parallel ranges formed on both sides of geosynclines. g. Kunlun mountain, Himalayas.
  • Median mass (Zwischen gebirge): It can be plateau, plain or sea. g. Tibetan Plateau.

Development of Concept

  • Haug (Mesozoic era map).
  • W. Evans;
  • Schuchert (based on evolutionary history – number of cycles of sedimentation);
  • Holmes (Convection current).

Characteristics of geosynclines

  • These are characterized by gradual sedimentation and subsidence.
  • Geosynclines are generally bordered by two rigid masses which are called forelands.
  • The nature and patterns of geosynclines have changed throughout geological history, due to earth movements and geological process.
  • These are now converted into folded mountain ranges.

Evaluation and Conclusion

  • All great fold mountains stand on the sites of former Thus, geosynclines have been cradles of mountains. – P.G. Worchester.
  • The geosynclines give the glimpse of root formation as postulated in the Airy’s isostasy
  • Geosynclines gives a glimpse of continental drift and plate tectonics.

Geosynclines and Mountain Building – Kober’s Theory

Kober, a German geologist explained his theory of of geosynclines and mountain build using stage concept in his book ‘Der Bau der Erde’. His theory is based on the Force of Contraction produced by the cooling of the earth.

As per Kober, there are two zones:

  • Orogen or mobile zone: the place of mountain building.
  • Kartogen or rigid zone: Orogen is surrounded by Kartogen.

Basis of theory: Orogenetic Force

Kober's geosynclinal theory is based on the forces of contraction produced by the cooling of the earth.

According to J.A. Steers (1932), ‘Kober is definitely a contractionist; and for him contraction provides the motive force for the compressive stress’.

The theory: Stages of Geosyncline

As per Kober, there are three stages involved in mountain building:

1. Lithogenesis:

  • Creation of geosynclines: Geosynclines are created due to the cooling and contraction of the earth.
  • Sedimentation and subsidence: Deposition is started.
  • It is the preparatory stage of mountain building.

2. Orogenesis:

  • In this stage, the process of mountain-building starts.
  • There is squeezing and folding into mountain range due to compressive force.
  • Huge sediment deposits in the orogen zone from Kartagen led to an increase in pressure and weight.
  • Orogen zones are subjected to gradual subsidence due to more pressure.

3. Gliptogenesis:

  • Due to the contraction force, in the last stage, there is gradual rise of the mountain ranges.
  • Later, agents of denudation start acting. Eg. weathering, erosion, and deposition make third order relief features.

Criticism of the theory

  • The force of contraction by cooling is not sufficient to cause mountain building.
  • Kober's theory explains the west-east extending mountains, but north-south extending mountains (Rockies and Andes) cannot be explained.

Evaluation

  • The concept of Kober invalidated the earlier views, like that of Suess.
  • According to Suess only one side of the geosyncline moves, whereas the other side remains stable. The moving side is the backland, whereas stable side is the Eg. the Himalayas were formed due to southward movement of Angaraland, and the Gondwanaland remained stationary.
  • After the postulation of plate tectonic theory, this view of Suess has become meaningless.
  • The concept of Kober, that both the forelands move together, is validated. Eg. Ample evidence of palaeomagnetism and sea-floor spreading have shown that both Asiatic and Indian plates are moving towards each other.

Conclusion

  • According to J.A. Steers (1932) ‘Kober’s views are a based on the geosynclinal hypothesis of Hall and Dana, which includes his own views on orogenesis.’
  • Inspite of a few inherent limitations, Kober is given credit for advancing the idea of the mountain formation from geosynclinal sediments.

Other theories and Evolution of the Concept

James Hall and Dana

  • The sediments of the rocks of folded mountains were of marine origin. These rocks are deposited in long, narrow, and shallow seas. Dana named such water bodies as

E. Haug

  • Haug defined geosynclines as long and deep-water bodies.
  • Haug drew the palaeogeographical maps of the world and depicted long and narrow oceanic tracts to demonstrate that these water tracts were subsequently folded into mountain ranges.
  • He identified 5 major rigid masses during Mesozoic era e.g. (i) North Atlantic Mass, (ii) Sino-Siberian Mass, (iii) Africa-Brazil Mass, (iv) Australia-India-Madagascar Mass and (v) Pacific Mass.
  • He located 4 geosynclines between these ancient rigid masses e.g. (i) Rockies geosyncline, (ii) Ural geosyncline, (iii) Tethys geosyncline and (iv) Circum-Pacific geosyncline.

Formation of geosynclines

  • Sedimentation: The marginal areas of the geosynclines have shallow water where larger sediments are deposited. Finer sediments are deposited in the central parts.
  • The sediments are squeezed and folded into mountain ranges due to compressive forces.
  • It is not always necessary that the geosynclines may pass through the complete cycle of sedimentation;e., subsidence, compression and folding of sediments.

Criticisms

  • His palaeogeographical map of Mesozoic era depicted unbelievable larger extent of rigid masses in comparison to geosynclines or oceanic areas.
  • Questions arise, as to what happened to such extensive land masses after Mesozoic era? Where did they disappear?

J.W. Evans

  • The form and shape of geosynclines change with changing environmental conditions.
  • There may be several alternative situations of geosynclines e.g.
    • it maybe between two land masses (example, Tethys geosyncline between Laurasia and Gondwanaland),
    • it may be in front of a mountain or a plateau
    • it may be along the margins of the continents
    • it may be in front of a river mouth etc.

Schuchert

He has divided geosynclines into 3 categories.

Mono-geosynclines

  • These are exceptionally long and narrow, but shallow water tracts as compared to Hall and Dana.
  • They pass through only one cycle of sedimentation and mountain building.
  • Example: Applachian There existed a long and narrow Appalachian geosyncline during pre-Cambrian period. It was bordered by highland mass known as Applachia. Applachian geosynclines were folded from Ordovician to Permian periods to form its orogeny.

Poly-geosynclines

  • These were broader than the mono-geosynclines.
  • They have experienced more than one phase of orogenesis. Hence, they may have been diversified by the production of one or more parallel geanticlines arising from their floors in the squeezing process.
  • Rocky and Ural geosynclines are the representative examples of polygeosynclines.

Meso-geosynclines

  • These are very long, narrow and mobile ocean basins which are bordered by continents from all sides.
  • These geosynclines pass through several geosynclinal phases. Phases of sedimentation, subsidence and folding.
  • These are similar to the geosynclines conceived by Haug.
  • Example: Tethys geosyncline. This geosyncline was folded into Alpine mountains and the Himalayas. The unfolded remaining portion of Tethys geosyncline became Mediterranean Sea, an example of median mass of Kober.

Arthur Holmes

According to A. Holmes, sedimentation leads to subsidence, but this process cannot account for the greater thickness of sediments in geosynclines. Rather, earth movements can cause subsidence of high magnitude in the geosynclinal beds.

Holmes has identified 4 major types of geosynclines and has described their mode of their origin.

Magmatic Geosynclines

  • Migration of magmas from the intermediate layer to neighbouring areas causes collapse and subsidence of upper or outer layer and thus is formed a geosyncline.
  • Examples: Coral Sea, Tasman Sea, Arafura Sea, Weddell Sea and Ross Sea.
  • Criticism: The transfer and displacement of magmas cannot cause subsidence to form geosynclines.

Metamorphic Geosynclines

  • Rocks of the lower layer of the crust are metamorphosed due to compression caused by converging convective currents.
  • This metamorphism increases the density of rocks, with the result the lower layer of the crust is subjected to subsidence and thus a geosyncline is formed.
  • Examples: Caribbean Sea, western Mediterranean Sea and Banda Sea.
  • Criticism: Compression caused by convergent convective currents would not cause metamorphism rather it would cause melting of rocks and finally the igneous rocks.

Compressional Geosynclines

  • These are formed due to compression and resultant subsidence of outer layer of the crust caused by convergent convective currents.
  • Example: Persian Gulf and Indo-Gangetic trough.

Geosynclines formed due to thinning of sialic layer

According to Holmes there may be two possibilities.

  • The sialic layer is stretched apart due to tensile forces exerted by diverging convective currents. This process causes thinning of sialic layer which results in the creation of a geosyncline. Example: former Tethys geosyncline.
  • The continental mass may be separated due to enormous tensile force generated by divergent convective currents. Example: Former Ural geosyncline.

Dustar’s views

He has classified geosynclines into 3 types based on structure of mountain ranges.

Inter-continental geosynclines

  • These are always situated between two land masses. Example: Ural Geosyncline.

Circum-continental geosynclines

  • These are generally situated along the margins of the continents. Example: Schuchert’s mono-geosyncline.

Circum-oceanic geosynclines

  • These are found along the marginal areas of the oceans where continental margins meet with the oceanic margins. Stille has named such geosyncline as marginal geosyncline while others have called it special type of geosyncline or unique geosyncline. More extensive geosynclines have been named by Stille as

Stille’s views

  • He has further classified the geosynclines based on intermittent volcanic activity during their infilling into (i) eugeosynclines and (ii) miogeosynclines.
  • Eugeosynclines have relatively high number of volcanic products while miogeosynclines have low volcanic products.

Types of geosynclines

There are seven types of geosynclines:

  1. Ortho-geosyncline: These are elongated basins which are filled with great thickness of sediments, which is subsequently deformed to form a fold-mountain chain.
  2. Eu-geosyncline: The piles of sediments are found with an abundance of volcanic rocks. These are formed at some distance from the shield areas or the Kratons.
  3. Mio-geosyncline: These are formed adjacent to the Kraton, where there is a thinner development of sediments which lack volcanic rocks.
  4. Taphro-geosyncline: This is an elongated depression, formed due to faulting. These are also known as graben or rift valley.
  5. Para-geosyncline: This is the geosyncline which lies within the Kraton.
  6. Zeugo-geosyncline: These are parageosyncline with marginal uplifts.
  7. Auto-geosyncliie: It is a para-geosyncline without marginal uplifts.

Most orogenic-belts arise on the sites of geosynclines and the resulting mountains, therefore, consist of sediments and volcanic rocks deformed and metamorphosed to a greater or lesser extent according to their position and depth in the orogenic belt.