Geosyncline Theory – UPSC

• A geosyncline is a long, narrow and relatively shallow marine depression in which sediments piled up for millions of years while its floor kept subsiding under their weight and under earth movements.
  • It lies between rigid masses (forelands) that supply the sediment and later squeeze it.
• The classical view held that “out of geosynclines have come the mountains”: the thick sediment prism is later compressed, folded and uplifted into fold mountains.
  • The Tethys (Himalaya, Alps), the Appalachian trough (North America) and the Caledonian trough (Britain, Scandinavia) are the standard examples.
• The idea ruled mountain-building thought for about a century, until plate tectonics recast geosynclines as sediment wedges of continental margins and ocean basins.

Concept of Geosyncline

Meaning and Characteristics

• Rigid masses and mobile zones: the early globe is pictured as stable continental nuclei surrounded by mobile belts of water receiving heavy sedimentation.
  • These mobile belts are the geosynclines; compressive forces later converted them into folded ranges.
• Working definition: geosynclines are long but narrow and shallow water depressions marked by sedimentation and subsidence.
• Core characteristics:
  • Length far exceeds width, often by ten times or more.
  • Twin processes: sedimentation causes subsidence, and subsidence makes room for more sediment, so the two stay roughly in balance.
  • Mobile, not permanent: location, shape and extent changed through geological time with earth movements.
  • Bordered by two rigid forelands, which are eroded to fill them.

Evidence from Fold Mountains

• Marine sedimentary rocks: fold mountains are built mainly of sandstone, shale and limestone that carry marine fossils.
• Great thickness but shallow-water fossils: sediment piles exceed 12 km, yet the fossils belong to organisms that live only in shallow seas.
  • The paradox is solved only if the sea floor sank steadily as it filled, keeping the water shallow throughout.
• Shape: fold belts are long and narrow; the Himalaya runs about 2,400 km but is only 150–400 km wide, the outline of a narrow trough.
• Position: they lie along continental margins, trending north–south (Rockies, Andes) or east–west (Alps, Himalaya), and are usually arcuate, convex on one side and concave on the other.
• Indian example: the summit of Mount Everest is Middle Ordovician marine limestone, about 450 million years old, with brachiopod and crinoid fossils laid down in a warm, shallow Tethyan sea.
  • The Jurassic Spiti shales of the Tethys Himalaya, rich in ammonites, tell the same story of a sea where the high Himalaya now stands.

Evolution of the Concept

• The idea came from James Hall and James Dwight Dana; the theory of how geosynclines develop came from Émile Haug.

James Hall (1859) and James Dwight Dana (1873)

• James Hall, working on the northern Appalachians, found that the folded Palaeozoic strata are shallow-water marine sediments about 12 km thick.
  • They are ten to twenty times thicker than unfolded strata of the same age in the interior lowlands to the west.
  • He argued that the floor subsided as sediment accumulated, so water depth stayed nearly constant; mountain building followed this long down-warping.
• James Dwight Dana found such thickening common in most ranges and in 1873 named the sinking trough a “geosynclinal”, later shortened to geosyncline.
  • He defined it as a long, narrow, shallow and sinking sea bed.

Émile Haug (1900)

• Émile Haug defined geosynclines as relatively deep-water belts, much longer than wide, and drew palaeogeographic maps to show present fold ranges on the sites of former geosynclines.
• Mesozoic rigid masses and the geosynclines between them:

• Systematic sedimentation: coarse sediment settles at the shallow margins and finer sediment in the centre; the littoral margins shift with marine transgression and regression.
• Folding is by compression from the margins, but not every geosyncline completes the cycle; some receive sediment for ages and never yield mountains.
• Criticism:
  • His map shows land far larger than sea, with no account of where those landmasses went after the Mesozoic.
  • Deep-water geosynclines clash with the shallow-water fossils of fold mountains.

J. W. Evans

• J. W. Evans held that geosynclines vary too much in form, width and location to fit one model; all share sedimentation and subsidence, and all end in folding.
• Possible settings:
  • Between two landmasses: Tethys between Laurasia and Gondwanaland.
  • In front of a mountain or plateau: the trough south of the rising Himalaya, later filled to form the Indo-Gangetic plains.
  • Along continental margins.
  • Off a river mouth.

Charles Schuchert (1923)

• Charles Schuchert classified geosynclines by size, location and evolutionary history:

Arthur Holmes

• Arthur Holmes argued that loading alone is not enough: sediment weight cannot produce such thickness, so earth movements must drive the subsidence, and it is gradual, not sudden.
  • About 12 km of Appalachian sediment at roughly 30 cm per 7,500 years implies some 300 million years, from the Cambrian to the early Permian.
• He pictured the crust as granodiorite over amphibolite over eclogite, and gave four modes of origin, most of them driven by his mantle convection currents:

• The last two modes are the most durable part of his scheme, since they foreshadow subsidence at convergent and divergent plate margins.

Hans Stille and Marshall Kay

• Hans Stille divided the crust into stable kratons (his 1936 shortening of Kober’s kratogen) and mobile orthogeosynclines.
  • Kratons are hochkraton (stable continental crust) and tiefkraton (stable oceanic crust).
• Within an orthogeosyncline he separated two belts, often divided by a geanticline:
  • Eugeosyncline (eu, well developed): abundant volcanic rocks, deeper water, farther from the craton.
  • Miogeosyncline (mio, less): little volcanism, thinner sediment, next to the craton.
  • Miogeosynclines are now read as former continental margins like those fringing the Atlantic; eugeosynclines as deformed small ocean basins like the marginal seas of the western Pacific (Sea of Japan, Sea of Okhotsk).
• Marshall Kay (1951), in North American Geosynclines, extended the scheme to troughs within or beside cratons:

• A structural grouping is also used: inter-continental (between two landmasses; Ural), circum-continental (along continental margins; Appalachian) and circum-oceanic (where continental and oceanic margins meet; Stille’s marginal geosyncline).

Kober’s Geosynclinal Orogen Theory

Base and Driving Force

• Leopold Kober, Austrian geologist, set out the theory in Der Bau der Erde (1921), aiming to link ancient rigid masses with the mobile zones between them and to trace the geological history of mountains.
• Orogen and kratogen: the mobile water belts on the sites of today’s mountains are orogens (places of mountain building); the rigid masses around them are kratogens.
  • Kratogens include the Canadian, Baltic, Siberian and African shields, the Chinese massif, Peninsular India, and the Brazilian, Australian and Antarctic masses.
  • Continents grew outwards from these rigid nuclei; a mid-Pacific geosyncline once separated north and south Pacific forelands that later foundered.
• Driving force: contraction of a cooling Earth pushes the forelands towards each other, squeezing and folding the geosynclinal fill.
  • James Alfred Steers (1932) called Kober “definitely a contractionist” and saw the theory as Hall and Dana’s geosyncline, Haug’s development and Kober’s own orogenic ideas combined.

Stages of Geosynclinal Evolution

Lithogenesis

• Creation, sedimentation and subsidence: contraction forms the geosyncline; the forelands are eroded and their debris spreads over its floor.
• The load deepens the floor, making room for more sediment, and the pile thickens over millions of years.

Orogenesis

• Squeezing and folding: the forelands converge, and the compressed sediment is folded into ranges.
• Two outcomes:
  • Very strong compression folds the whole fill into one continuous range.
  • Moderate compression folds only the edges into two marginal ranges, leaving an unfolded median mass (Zwischengebirge) between them.
  • Kober’s median masses: the Tibetan Plateau (between Kunlun and Himalaya), the Hungarian basin (between Carpathians and Dinaric Alps) and the Mediterranean (unfolded Tethys).

Gliptogenesis

• Gradual rise and denudation: the new ranges keep rising while weathering and erosion lower their height.
  • As erosion unloads the range, the crust rebounds isostatically.
• The three stages are thus also the three phases in the life of a geosyncline.

Periods of Mountain Building

• Indian example: the Aravallis, among the world’s oldest fold belts, are a relict Precambrian range worn down by long denudation, while the Himalaya is the young Alpine chain.

Evaluation

• Merits:
  • Ties the thick marine sediments, their folding and the long, narrow shape of fold belts into one sequence.
  • The median mass usefully explains plateaus and basins enclosed by ranges.
  • Recognises mountain building as episodic, grouped into distinct orogenic periods.
• Criticism:
  • Thermal contraction is far too weak to shorten the crust by hundreds of kilometres, and radioactive heat undercuts the cooling-Earth premise.
  • It cannot explain the one-sided movement shown by nappes, or why rigid forelands should move at all.
  • The Tibetan Plateau is not an unfolded median mass but thickened, uplifted crust.

Plate Tectonic Reinterpretation

Why the Classical Concept Failed

• Geosynclinal theory had no mechanism for moving forelands; seafloor spreading and plate motion supplied one in the 1960s.
• Robert S. Dietz (1963) read ancient geosynclines as collapsed continental rises, like the sediment prisms of today’s Atlantic margin.
• John F. Dewey and John M. Bird (1970) mapped geosynclinal belts onto plate margins, explaining mountains by subduction and collision.
• Active vs passive margins: a margin on a plate boundary (subduction, collision or transform) is active; one carried away from a spreading ridge, such as the east coast of North America, is passive.

Geoclines of Passive Margins

• As a passive margin drifts from the ridge, the lithosphere cools, becomes denser and subsides, and sediment keeps accumulating on the shelf, slope and rise.
• The body is a wedge parallel to the continent and open towards the ocean, not a two-sided trough fed from bordering mountains.
  • Dietz and Holden (1966) therefore coined miogeocline, and geocline replaced geosyncline.

• Northern Gulf of Mexico: a miogeocline more than 10 km thick at the outer shelf, built as the crust sank under sediment load since the Jurassic.
• Economic value: miogeoclines host major petroleum reserves.
  • Indian example: Mumbai High, discovered in 1974, lies in the Mumbai Offshore Basin, a pericratonic rift basin on India’s western passive margin.
• Bengal Fan: fed by the Ganga and Brahmaputra, it is the largest submarine fan on Earth, about 3,000 km long with sediment up to 16.5 km thick, a present-day “geosyncline” in the making.

Classical Terms in Plate Settings

• Tethys retold: India drifted north as the Tethys ocean floor subducted beneath Asia; the shelf sediments of its northern margin were scraped up and thrust south, and slivers of ocean floor survive as ophiolites along the suture.
• Current view: the geosyncline is obsolete as a theory but survives as a descriptive term for thick linear sediment belts.
  • A. M. Celâl Şengör (1982) urged dropping it altogether because of its link to discredited ideas.
  • Mountains are now explained by the Wilson cycle of ocean opening and closing.