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السبت، 1 يونيو، 2013

Origin of Mountains

السبت, يونيو 01, 2013
Origin of Mountains
By: Dr. / Zaghloul El-Naggar
 Two main hypotheses were put forward to explain the formation of mountains: the vertical —tectonics hypothesis which claims the predominance of vertical movements in the Earth’s crust, and the horizontal-tectonics hypothesis, which states that the major land movements responsible for the building of mountains are primarily horizontal in nature and are directly connected with both plate tectonics and the drifting of continents.

  Both hypotheses, however, recognize the close association of orogenesis with geosynclines. As previously mentioned, geosynclines are very large, elongated troughs, several thousands of kilometers long and several hundred kilometers wide, that have been infilled with very thick accumulations of both sediments and layered volcanics (more than 15,000 m thick). Such infill becomes later squeezed and uplifted to form mountains, with or without a crystalline core of igneous and metamorphic rock.

  The vertical-tectonics hypothesis postulates that thermal expansion can cause gravity faulting (or sagging) to produce geosynclines in the form of half grabens or full grabens, while plate-tectonics assume that such troughs are formed by the subduction of one lithospheric plate below another as a result of a driving force in the underlying mantle such as convection currents or thermal plumes.

  The central idea of plate tectonics is that the solid, outermost shell of the Earth (the lithosphere) is riding over a weak, partially molten, low velocity zone (the asthenosphere). Continents are looked upon as raft-like inclusions embedded in the lithosphere, while only a thin crust (5 km thick) tops the lithosphere in ocean basins. The thickest continental crust, about 70 km, is reported to lie beneath theAlps.

 The lithosphere (about 100 km thick) is broken up into about 12, large, rigid plates by rift systems. Each of these plates has been moving as a distinct unit, diverging away or converging towards each other and slipping past one another.

  Along divergent junctions, plates spread apart, being accompanied by intensive volcanicity and earthquake activity. The resulting space between the receding plates is filled by molten, mobile, basaltic material that rises from below the lithosphere. This basaltic magma solidifies in the cracks formed by the rift, producing new sea-floor material that adds to the edges of the separating plates and hence, the name “sea­floor spreading” for the whole process which is continuously repeated over and over again.

  Most basaltic magmas are believed to originate from the partial melting of the rock peridotite, the major constituent of the upper mantle. Since mantle rocks exist under high temperature and high pressure, melting most often results from a reduction in the confining pressure, although the influence of increasing temperature cannot be excluded. This can result from the heat liberated during the decay of radioactive elements that are thought to be concentrated in both the upper mantle and the crust.

  Along convergent junctions, plates collide against each other, producing volcanic island-arcs, deep-sea trenches, both shallow and deep earthquakes and volcanic eruptions. In the framework of plate tectonics, orogeny occurs primarily at the boundaries of colliding plates, where marginal sedimentary deposits are crumpled and both intrusive and extrusive magmatism (volcanism) are initiated. However, mountain belts formed at such junctions differ with the different rates of spreading as well as with the nature of the leading edges of the colliding plates (continental or oceanic).

  When the abutting edges are ocean floor and continent, the heavy, oceanic lithosphere descends beneath the lighter, continental one to subduct into the underlying mantle. This downbuckling is marked by an offshore trench, while the edge of the over-riding plate is crumpled and uplifted to form a mountain chain parallel to the trench. Great earthquakes occur adjacent to the inclined contact between the two plates, and increasing in depth with the increase in the downward movement of the descending plate, while oceanic sediments may be scraped off the descending slab and incorporated into the adjacent mountains. Such zones of convergence, where the lithosphere is consumed are called subduction zones. Here, the lithospheric material is consumed in equal amount to the production of new lithosphere along the zones of divergence. Rocks caught up in a subduction zone are metamorphosed, but as the oceanic plate descends into the hot mantle, parts of it may begin to melt, and the generated magma may float upwardly, in the form of igneous intrusions and/or volcanic eruptions. The production of magma in the subduction zone may be a key element in the formation of granitic rocks, of which continents are mainly composed.

  Granitic magmas are thought to be generated by the partial melting of water-rich rocks, subjected to increased pressure and temperature. Therefore, burial of wet, quartz-rich material to relatively shallow depths is thought to be sufficient to trigger melting and generate a granitic magma in a compressional environment characterized by rising pressures. Most granitic magmas, however, loose their mobility before reaching the surface and hence, produce large intrusive features such as batholiths.

  Andesitic magmas are intermediate in both composition and properties between the basaltic and the granitic magmas. Consequently, both andesitic intrusions and extrusions are not uncommon, but the latter are usually more viscous and hence, less extensive than those produced by the more fluid, basaltic magma. A single volcano can, therefore, extrude lavas with a wide range of chemical compositions and hence of physical properties.

  Again, when an oceanic plate with a continent at its leading edge collides with another plate carrying a continent, convergence (accompanied by the gradual consumption of the oceanic lithosphere by subduction) gradually closes the oceanic basin in between, producing magmatic belts, folded mountains and mElange deposits on the over-riding continental boundary. This can continue until the two continents collide, when the plate motions are halted, because the continental crust is too light for much of its composition to be carried down to the mantle. Here, the descending oceanic plate may break off, with the complete cessation of subduction at the continent/continent suture, but this can start up again, els1ewhere on the colliding plate. Such continent/continent suture is marked by lofty mountainous chains, made up of highly folded and thrust-faulted rocks, coincident with or adjacent to the magmatic belt. Both giant thrusting and infrastructural nappes lead to considerable crustal shortening and are accompanied by much thickening of the continental crust. An excellent example of continent/continent collision is the Himalayan chain, which began forming some 45 million years ago. This magnificent mountainous chain, with the highest peaks on the surface of the Earth, was created when a lithospheric plate carrying India ran into the Eurasian plate in the Late Eocene time. This can explain how the very thick root underlying the Himalayas was formed.

  The plate tectonic cycle of the closing of an ocean basin by continued subduction of an oceanic plate under a continental one until a continent/continent collision takes place and an intra-continental (collisional) mountain belt is formed, has been called the “Wilson cycle,” after J.T. Wilson, who first suggested the idea that an ancient ocean had closed to form the Appalachian Mountain Belt, and then re-opened to form the present-day Atlantic Ocean. As partly mentioned by Dewey and Bird (1970), any attempt to explain the development of mountain belts must account for a large number of common features which are shared by most of the fully developed younger mountain chains such as:

1) Their overall long, linear or slightly arcuate aspect.

2) Their location near the edges of present continents or near former edges of old continents that are presently intra-continental.

3) The marine nature of the bulk of their sediments, and the intense deformation of such sediments.

4) Their frequent association with volcanic activity.

5) Some of their thick sedimentary sequences were deposited during very long intervals, in the complete absence of volcanicity.

6) Short-lived, intense deformation and metamorphism, compared with the lengthy time during which much of the sedimentary succession of mountain belts was deposited.

7) Their composition of distinctive zones of sedimentary, deformational, and thermal patterns that are in general, parallel to the belt.

8) Their complex internal geometry, with extensive thrusting and mass transport that juxtaposes very dissimilar rock sequences, so that original relationships have been obscured or destroyed.

9) Their extreme stratal shortening features and, often, extensive crustal shortening features.

10) Their asymmetric deformational and metamorphic patterns.

11) Their marked sedimentary composition and thickness changes that are normal to the trend of the belt.

12) The dominantly continental nature of the basement rocks beneath mountain belts, despite the fact that certain zones in these belts have basic and ultrabasic (ophiolite suite) rocks as basement and as upthrust slivers.

13) Presence of a thrust belt along the side of the mountainous chain closest to the continent, usually with thrust sheets and exotic blocks (or allochthons).

14) Presence of melange belts (composed of mappable rock units of crumpled, chaotic, contorted and otherwise deformed, heterogeneous mixtures of rock materials, with abundant slumping structures and ophiolitic complexes).

15) Presence of a complexly deformed metamorphic core, with severe metamorphism, magmatization and plutonic intrusions.

16) Presence of magmatic belts of both plutonic, hypabyssal and volcanic igneous activity.

17) Presence of folds of several stages and with unified or divergent trends.

18) Presence of block faulting, especially at the peripheries of the mountainous chain.

19) Presence of deep roots that are proportionately related to both the mass and elevation of the mountainous range, and can be as deep as 5 times the mountain’s height, or even more.

 These features are clearly suggestive of geosynclinal deposition, or deposition in mobile belts that are generally referred to as orthogeosynclines and are typically produced by the subduction of an oceanic plate below a continental one. Orthogeosynclines are usually separated into eugeosynclines (characterized by intensive volcanicity) and miogeosynclines (distinguished by being non-volcanic).
Eugeosynclinal belts (with their basic lavas, radiolarian cherts and graywackes, intermediate lava and fragmental volcanic rocks, as well as other sedimentary, volcanic and plutonic rocks that are metamorphosed to varying degrees) usually characterize the central cores of mountain systems. However, these can be notably narrow and may even be absent in some of the major mountains, probably due to severe tectonism in recurring phases of orogenesis. Extrusive lavas and agglomerates that fringe eugeosynclinal belts are identical to those currently being deposited in modern island arcs. Thick sequences of shallow-water sedimentary rocks without volcanic material (characteristic of miogeosynclines) sometimes occur in a belt parallel and adjacent to the eugeosynclinal belt. These usually occur on that side of the mountain chain nearer to the old cores of continents (known as the continental cratons), which are themselves believed to be old mountain roots.

 Such features of youthful mountains have strongly supported the contention that the present-day, paired island arc/trench systems, with their intensive seismicity and volcanicity, are quite probably mountain belts in the process of formation.

 Miyashiro (1967) observed that the mountainous islands of Japan belonged to an old island arc/trench system that had been compressed and subjected to metamorphism and uplift during the later pan of the Mesozoic era. These mountains exhibit a pair of different metamorphic belts parallel to the length of the islands and adjacent to one another. On the Pacific side, the main outcrops are schists containing minerals indicative of formation at relatively low temperature but high pressure (e.g. glaucophane, aragonite, lawsonite), and without any evidence of granitic basement. On the western side of the islands, the other belt does have granites and metasediments with minerals indicative of relatively high temperature and low pressure (e.g. sillimanite).

 Such paired metamorphic belts, also formed during a late Mesozoic orogeny, were found elsewhere around the Pacific (e.g. in both New Zealand and California), with the “glaucophane-schist’ (or “blue-schist”) belt always occurring on the ocean side, and the high-temperature, metamorphic belt (the “sillimanite-schist” belt) on the continental side.

 The “blue-schist” belt is interpreted to have formed under ocean trench conditions, where the required low temperature and high pressure are likely to be obtained. Similarly, the high temperature metamorphic belt is debated to represent uplifted island arcs, where high heat flows must have been obtained. This is especially true where a collisional suture zone marked by blue schist ophiolite melanges is recorded.

 Stemming from this, Dewey and Bird (1970) suggested that mountain belts are a consequence of plate evolution and that they develop by the deformation and metamorphism of the sedimentary and volcanic assemblages of Atlantic-type continental margins. These authors proposed two main types of mountain building. The first “island arc/cordilleran type,": is for the most part thermally driven and develops on leading plate edges above a descending plate (i.e. above a subduction zone) and is marked, by paired metamorphic belts, paired miogeosyncline (continental shelf) eugeosyncline (region between continental shelf edge and trench) relationship, and divergent thrusting. The second “collision type” results from continent/island arc or continent/continent collision. It is for the most part mechanically driven, lacks the paired metamorphic zonation, its metamorphism is dominantly of the low-temperature type (“blue schist” facies) and its thrusting is dominantly towards and onto the consumed plate. This often involves the complete remobilization of basement near the site of collision, and gravity slides further onto the site of the old continental shelf.

  Another essential difference between the two types of mountain belts is that the cordilleran type has a dense, basic root, probably related to the emplacement of basic intrusions beneath the high-temperature, volcanic, metamorphic axis, while the root of collision mountain belts is sialic and probably results from continental underthrusting and thickening.

  Ophiolite belts usually mark the presence of former zones of subduction between two colliding plates, and are a significant feature of most mountain belts. These are commonly associated with radiolarian cherts which are believed to be of deep marine origin. Ophiolites are said to be well-developed in cordilleran mountain belts and form extensive upthrust regions behind the “blue schist” trench terrains, in the form of huge thrust slices or slivers of peridotite, gabbro and basaltic pillow lava. The composition and structure of the rocks strongly suggest oceanic crust and mantle which have been forced upwardly into the overlying rocks by the subducting plate. These also occur as smaller, detached rafts in the melanges of trenches, representing blocks that might have slid down the inner trench wall, slices of oceanic crust, of upper mantle, or of both, and of seamounts torn off the descending plate. Thick, intensely deformed oceanic sediments might also have been scraped off the descending plate and plastered to the inner trench wall or incorporated into the adjacent mountains. Subsequent uplifts expose the so-called melange terrain of highly complicated nature, in which shear surfaces replace bedding as the dominant structural feature.

  In collisional mountain belts, ophiolite blocks are extruded from the trench during collision and lie in flysch-mElange suture zones that mark the collision “join lines." The composition of ophiolite pillow basalts may be a criterion for distinguishing between the crust of the main oceans (tholeiite and spilite) and the alkalic crust of small ocean basins, if the latter are produced by the separation of arcs from continents. These authors concluded that: “Although the cordilleran/island arc and collision mechanisms are probably the fundamental ways by which mountain building occurs, mountain belts are generally the result of complex combinations of these mechanisms.” They referred to the evolution of the Appalachian orogen which involved Ordovician cordilleran/island arc mechanisms, followed by Devonian continental collision.

  Dewey and Bird also mentioned that the Alpine—Himalayan system has been developing since the early Mesozoic times by multiple collision resulting from the sweeping of microcontinents and island arcs across the Tethyan—Indian Ocean. Similar inland mountain belts such as the Urals, were also looked upon as complex combinations of cordilleran belts, microcontinents, and volcanic arcs, of widely different ages, that became juxtaposed by the closing up of a major ocean basin.

  The possibility of expanding and contracting transform offsets of consuming plate margins was mentioned by these authors to raise the likelihood of distinctive belts of volcanism, deformation and metamorphism coming to an abrupt termination along the strike of a mountain belt.

   From the above discussion it becomes obvious that the two main types of mountain building suggested by Dewey and Bird (1970) which are: the “island arc/cordilleran type” and the “collision type" are no more than successive stages in the mountain-building cycle as each continent/continent collision must be preceded by closing the ocean basin in-between. In other words, collisional mountains represent the final stage in the development of these magnificent landforms, and must be preceded by both the island arc and the cordilleran stages. This is clearly demonstrated by the Himalayan orogeny, which is considered to be the product of a combination of both the cordilleran and the collisional types of mountain building. This author concluded that “The present boundary between the Indian Plate and the Eurasian Plate is delineated by the belt of ophiolites and colored melange rocks separating the ‘Tethys’ Himalayas from the Karakoram and Tibetan Plateau region of Central Asia…" and added: "…the Himalayan orogenic belt has resulted through a combination of the two principal mountain-building processes. The first phase of the Himalayan orogeny, taking place at the junction between the continental margin of the Indian Plate and the Tethyan oceanic crust, during the Upper Cretaceous to Eocene period, could be considered as of the “cordilleran” type. Available geological data appear to indicate that the subsequent phases in the Himalayan orogeny, commencing probably from Late Eocene, were the result of the collision between the Indian and the Eurasian Plate."

   Athavale also reiterated that both Hamilton (1970) and Bird and Dewey (1970) had already evolved similar models for each of the Ural Mountains and the Appalachian chain, respectively.

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