In this article we will discuss about:- 1. Features of Geosynclines 2. Concept of Geosynclines 3. Stages.
Features of Geosynclines:
The geological history of the continents and ocean basins denotes the fact that in the beginning our globe was characterized by two important features viz.:
(i) Rigid masses and
Rigid masses representing the ancient nuclei of the present continents, have remained stable for considerably longer periods of time.
These rigid masses are supposed to have been surrounded by mobile zones of water characterized by extensive sedimentation. These mobile zones of water have been termed ‘geosynclines’ which have now been converted by compressive forces into folded mountain ranges.
On an average, a geosyncline means a water depression characterized by sedimentation. It has now been accepted by majority of the geologists and geographers that all the mountains have come out of the geosynclines and the rocks of the mountains originated as sediments were deposited and later on consolidated in sinking seas, now known as geosynclines.
If we consider the height and thickness of sediments of the young folded mountains of Tertiary period (e.g. Rockies, Andes, Alps, Himalayas etc.), then it appears that the geosynclines should have been very deep water bodies but the marine fossils found in the sedimentary rocks of these folded mountains belong to the category of marine organisms of shallow seas. It is, thus, obvious that the geosynclines are shallow water bodies characterized by gradual sedimentation and subsidence.
Based on above facts geosynclines can now be defined as follows:
‘Geosynclines are long but narrow and shallow water depressions characterized by sedimentation and subsidence’.
J.A. Steers (1932) has aptly remarked, ‘the geosynclines have been long and relatively narrow depressions which seem to have subsided during the accumulation of sediments in them.’
The following are the general characteristics of geosynclines:
(1) Geosynclines are long, narrow and shallow depressions of water.
(2) These are characterized by gradual sedimentation and subsidence.
(3) The nature and patterns of geosynclines have not remained the same throughout geological history rather these have widely changed. In fact, the location, shape, dimension and extent of geosynclines have considerably changed due to earth movements and geological process.
(4) Geosynclines are mobile zones of water.
(5) Geosynclines are generally bordered by two rigid masses which are called forelands.
Concept of Geosynclines:
The concept of geosynclines was given by James Hall and Dana but the concept was elaborated and further developed by Haug. J.A. Steers (1932) has remarked, “while the theory of geosynclines is due to Haug, the concept of idea belongs to Hall and Dana”.
It is desirable to discuss the concepts of geosynclines developed by different exponents:
(1) Concept of Hall and Dana:
Dana studied the folded mountains and postulated that the sediments of the rocks of folded mountains were of marine origin. These rocks were deposited in long, narrow and shallow seas. Dana named such water bodies as geosynclines. He defined, for the first time, geosynclines as long, narrow and shallow and sinking beds of seas. Hall elaborated the concept of geosynclines as advanced by Dana.
He presented ample evidences to show relationship between geosynclines and folded mountains. He opined that the rocks of folded mountains were deposited in shallow seas. According to Hall the beds of geosynclines are subjected to subsidence due to continuous sedimentation but the depth of water in the geosynclines remains the same (fig. 11.2). Geosynclines are much longer than their widths.
(2) Concept of E. Haug:
If the idea of geosynclines is due to Hall and Dana, the theory of their development is really due to Haug’. He defined geosynclines as long and deep water bodies. According to Haug ‘geosynclines are relatively deep water areas and they are much longer than they are wide’. He drew the palaeogeographical maps of the world and depicted long and narrow oceanic tracts to demonstrate the facts that these water tracts were subsequently folded into mountain ranges (fig. 11.3).
He further postulated that the positions of the present-day mountains were previously occupied by oceanic tracts i.e. geosynclines. Geosynclines existed as mobile zones of water between rigid masses.
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.
According to Haug there are systematic sedimentation in the geosynclines. The littoral margins of the geosynclines are affected by transgressional and regressional phases of the seas. The marginal areas of the geosynclines have shallow water wherein larger sediments are deposited whereas finer sediments are deposited in central parts of the geosynclines. The sediments are squeezed and folded into mountain ranges due to compressive forces coming from the margins of the geosynclines.
He has further remarked that it is not always necessary that all the geosynclines may pass through the complete cycle of the processes of sedimentation, subsidence, compression and folding of sediments. Sometimes, no mountains are formed from the geosynclines inspite of continuous sedimentation for long duration of geological time.
Though the contributions of Haug in this regard are praiseworthy as he developed the concept of geosynclines but his theory suffers from certain serious drawbacks and confusing ideas about them. His palaeogeographical map (fig. 11.3) of Mesozoic era depicted unbelievable larger extent of rigid masses (land areas) in comparison to geosynclines (oceanic areas).
Questions arise, as to what happened to such extensive land masses after Mesozoic era? Where did they disappear? Haug could not explain these and many more questions. His geosyclines as very deep oceanic tracts are also not acceptable because the marine fossils found in the folded mountains belong to the group of marine organisms of shallow seas.
(3) Concept of J.W. Evans:
According to Evans the geosynclines are so varied that it becomes difficult to present their definite form and location. The beds of geosynclines are subjected to gradual subsidence because of sedimentation. The form and shape of geosynclines change with changing environmental conditions. A geosyncline may be narrow or wide. It may be of different shapes.
There may be several alternative situations of geosynclines e.g.:
(i) It may be between two land masses (example, Tethy s geosyncline between Laurasia and Gondwanaland),
(ii) It may be in front of a mountain or a plateau (for example, resultant long trench after the origin of the Himalayas, this depression was later on filled with sediments to form Indo-Gangetic Plains),
(iii) It may be along the margins of the continents,
(iv) It may be in front of a river mouth etc. According to Evans all the geosynclines irrespective of their varying forms, shapes and locations are characterized by twin processes of sedimentation and subsidence. Geosynclines, after long period of sedimentation, are squeezed and folded into mountain ranges.
(4) Views of Schuchert:
He attempted to classify geosynclines on the basis of their characteristics related to their size, location, evolutionary history etc.
He has divided the geosynclines into 3 categories:
(i) Monogeosynclines are exceptionally long and narrow but shallow water tracts as conceived by Hall and Dana. The geosynclinal beds are subjected to continuous subsidence due to gradual sedimentation and resultant load. Such geosynclines are situated either within a continent or along its borders. These are called mono because they pass through only one cycle of sedimentation and mountain building.
Applachian geosyncline is considered to be the best example of monogeosynclines. In place of the Applachains (USA) there existed a long and narrow Applachain geosyncline during Pre-cambrian-period. The geosyncline was bordered by highland mass known as Applachia in the east. Applachian geosynclines were folded from Ordovician to Permian periods.
(ii) Polygeosynclines were long and wide water bodies. These were definitely broader than the monogeosynclines. These geosynclines existed for relatively longer period than the monogeosynclines and these have passed through complex evolutionary histories.
“These are considered to have experienced more than one phase of orogenesis, consequently they may have been diversified by the production of one or more parallel geanticlines arising from their floors in the squeezing process”. They originated in positions similar to those of monogeosynclines. Rocky and Ural geosynclines are quoted as the representative examples of polygeosynclines.
(iii) Mesogeosynclines are very long, narrow and mobile ocean basins which are bordered by continents from all sides. They are characterized by great abyssal depth and long and complex geological histories. These geosynclines pass through several geosynclinal phases e.g., phases of sedimentation, subsidence and folding. Mesogeosynclines are similar to the geosynclines conceived by Haug.
Tethys geosyncline is the typical example of such type. Mediterranean Sea is the remnant of Tethys geosyncline. This geosyncline was folded into Alpine mountains of Europe and the Himalayas of Asia. The unfolded remaining portion of Tethys geosyncline became Mediterrancean Sea, an example of median mass of Kober.
(5) Concept of Arthur Holmes:
Besides describing main characteristics of geosynclines, A. Holmes has also elaborated the causes of the origin of different types of geosynclines. He has also described the detailed processes and mechanisms of sedimentation and subsidence and consequent orogenesis.
According to him no doubt 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. He further pointed out that the process of subsidence of the geosynclinal beds was not a sudden process rather it was a gradual process.
The deposition of sediments upto the thickness of 12,160 m (40,000 feet) in the Applachian geosyncline could be possible during a long period of 300,000,000 years from Cambrian period to early Permian period at the rate of one foot of sedimentation every 7,500 years.
Holmes has identified 4 major types of geosynclines and has described the mode of their origin separately as given below:
(a) Formation of geosynclines due to migration of magma:
According to Holmes the crust of the earth is composed of 3 shells of rocks. Just below the outer thin sedimentary layer lies:
(i) outer layer of granodiorite (thickness, 10 to 12 km), followed by
(ii) an intermediate layer of amphibolite (thickness, 20-25 km), and
(iii) a lower layer of eclogite and some peridotite.
He has further pointed out that 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. It may be summarized that some geosynclines are formed due to displacement of light magmas and consequent subsidence of crustal surface. Present Coral Sea.
Tasman Sea, Arafura Sea, Weddell Sea and Ross Sea have been quoted as typical examples of such geosyncline. This concept of Holmes has been severely criticised because the transfer and displacement of magmas cannot cause subsidence to form geosynclines.
(b) Formation of geosynclines due to metamorphism:
According to Holmes the rocks of the lower layer of the crust, as referred to above, 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.
Caribbean Sea, the western Mediterranean Sea and Banda Sea have been quoted as examples of this category of geosynclines. This concept has been rejected on the ground that compression caused by convergent convective currents would not cause metamorphism rather it would cause melting of rocks due to resultant high temperature.
(c) Formation of geosynclines due to compression:
Some geosynclines are formed due to compression and resultant subsidence of outer layer of the crust caused by convergent convective currents. Persian Gulf and Indo-Gangetic trough are considered to be typical examples of this group of geosyclines.
(d) Formation of geosynclines due to thinning of sialic layer:
According to Holmes there may be two possibilities if a column of rising convective currents diverges after reaching the lower layer of the crust in opposite directions:
(i) 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. The former Tethys geosyncline is considered to have been formed in this manner,
(ii) Alternatively, the continental mass may be separated due to enormous tensile force generated by divergent convective currents. Former Ural geosyncline is supposed to have been formed due to this mechanism.
(6) View of Others:
Dustar has classified geosynclines into 3 types on the basis of structure of mountain ranges:
(i) Inter-continental geosynclines arc always situated between two continental or land masses. Schuchert’s mesogeosy cline is similar to this type. Ural geosyncline is quoted as the representative example.
(ii) Circum-continental geosynclines are generally situated along the margins of the continents. Schuchert’s monogeosyncline is the example.
(iii) Circum-oceanic geosynclines are generally 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 orthogeosynclines.
Stille has further classified the geosynclines on the basis of intermittent volcanic activity during their infilling into:
(i) eugeosynclines and
Eugeosynclines have relatively high amount of volcanic products (Greek prefix eu means high status of igneous activity) while miogeosynclines have low volcanic products (mio means low).
Stages of Geosynclines:
The geosynclinal history is divided into three stages viz.:
(i) Lithogenesis (the stage of creation of geosynclines, sedimentation and subsidence of the beds of geosynclines, fig. 11.4).
(ii) Orogeneis (the stage of squeezing and folding of geosynclinal sediments into mountain ranges, figs. 11.5 and 11.6), and
(iii) Gliptogenesis (the stage of gradual rise of mountains, and their denudation and consequent lowering of their heights). These stages would be elaborated during the discussion of geosynclinal theory of Kober.