collisional orogeny

collisional orogeny Orogeny forms mountains by thickening the continental crust. Collisional orogenies are caused by the convergence of two plates of continental lithosphere when the intervening oceanic lithosphere is destroyed by subduction. Continental lithosphere cannot be subducted wholesale, for it is buoyant with respect to the underlying asthenosphere; further convergence is therefore accommodated by lithospheric thickening, which produces a mountain belt and converts the lower continental crust to denser metamorphic phases that allows partial subduction of continental lithosphere. The formation of a mountain belt is the more important process here. The subducting slab is attached to one plate and descends below the other, giving an inherent asymmetry to collisional orogens (Fig. 1). Collision is driven by the consequent slab pull or by ridge push from oceans elsewhere on either plate. Crustal thickening and orogeny above a long-lived subduction zone, as in the Andes, are also caused by the addition of andesitic or granitic material, or both (Fig. 1c).

Two fundamental types of continental lithosphere are involved in collisional orogeny in an island arc bordered on both sides by oceanic lithosphere and normal continental lithosphere (Fig. 1a). There are therefore three fundamental types of collisional orogeny: arc-arc, as in the Moluccan Sea; arc-continent, as in Taiwan; and continent-continent, as in the Alps or the Himalayas. This might give the impression that collisional orogenies have a simple structure. However, they show great variability and complexity owing to the nature of the continental margins, the width of the ocean destroyed, the number and polarity of subduction systems, and the presence of microcontinents and oceanic plateaus.

Lithospheric thickening associated with collision is caused by foreland-verging thrusts in the upper crust, by ductile deformation and folding in the lower crust, and by flow beneath the Moho that develops a mantle root (Fig. 2c). Continental lithosphere is about 2.5 per cent less dense than the asthenosphere. An increase of 40 km in its thickness will therefore produce an uplift of 1 km. Structural and geophysical data indicate that in many mountain belts the lithosphere has thickened by a factor of 2, giving mountains 3 km high for lithosphere initially 120 km thick. This factor was as high as 3 in the Siluro-Devonian Caledonides of western Norway and as low as 1.4 in the Caledonides of the British Isles. Thickening stops when there is a change in the configuration of the plates driving the collision or when the potential energy of the mountains is sufficient to stop further convergence. Metamorphic transformation of lower crustal rocks to denser garnet-granulite or eclogite-facies assemblages reduces the buoyancy of the continental lithosphere. In some orogenies this has allowed thickening by a factor of more than 2 without producing excessively high mountains. Lithospheric thickening generally takes a few tens of millions of years in continent- continent collision zones. In the Alps, convergence began in the mid-Cretaceous and continues today; the slow northward motion of Africa has taken 90 Ma to thicken the rifted Mesozoic southern European margin. Convergence was assisted by subduction of eclogitized European lower crust. Pressure increases with thickening in continent-continent collisions (Fig. 2e). The increased thickness of rocks relatively rich in radiogenic heat-producing elements (potassium, uranium, and thorium) causes a slow rise in temperaturethat peaks after some 20–30 Ma to produce moderate-temperature, moderate-pressure Barrovian metamorphic assemblages (Fig. 2e).

In arc collision events, the oceanic lithosphere attached to the oceanward side of the arc is compressed and thickened by the collision. It is denser than the asthenosphere and it will therefore subduct. This produces a change in subduction polarity, allowing the back-arc oceanic lithosphere to be destroyed by subduction beneath the continent. The subducting slab attached to the continental margin will detach, and shortening will then most probably cease (Fig. 1b). These ‘subduction flip’ events are, therefore, rapid, taking less than 10 Ma, as in the Bismark Ranges of Papua New Guinea or the Grampian orogeny of the Caledonides. Thickening and uplift are due to the overthrusting of the arc and fore-arc upon the passive continental margin. The rapid nature of this event means that there is not enough time for radiogenic heating of the orogenic pile. However, the detachment of the slab of oceanic lithosphere during subduction flip is accompanied by voluminous plutonism that advects heat into the orogen causing Barrovian metamorphism (Fig. 1b).

The thick, dense mantle root beneath a continent- continent collision zone is neither thermally nor mechanically stable (Fig. 2b, c). Its removal greatly increases the buoyancy of the orogen, leading to a rapid uplift, or ‘morphogenic’ phase (Fig. 2c). This can produce mountains up to 5 km high which spread laterally as in the 700-km-wide Tibetan Plateau. In an arc-collision event, buoyancy is gained by the detachment of the heavy subducting slab from the continental margin causing rapid uplift (Fig. 1b). The increased gravitational potential of the mountains will cause large fold nappes to be thrust over the foreland of the orogen and produce foreland basins filled with molasse. Such nappes are thrust both towards the foreland and the hinterland (Fig. 1d, 2c), widening the collision zone, as in the Alps. and causing rapid uplift of metamorphic rocks (Fig. 2c, d, e). These rocks do not have time to cool, but pressure is dropping as they are uplifted, and the earlier Barrovian metamorphism is followed by a later moderate-temperature, low-pressure, Buchan event (point c in Fig. 2e). The thinning of the orogen reduces buoyancy and leads to subsidence or ‘collapse’ of the mountain belt (Fig. 2d), sometimes below sea level, as in the Alboran Sea in the core of the Betic–Atlas orogen. This event is also associated with production of late orogenic granite magma (Fig. 2e, point c). Orogenic collapse is relatively rapid; for example it took some 20–30 Ma in the Variscan orogeny of Europe, reducing the height of mountains much faster than erosion.

Many mountain belts record several collisional events. The Himalayas resulted from the closure of the 5000-km-wide Tethyan Ocean during the late Mesozoic-early Tertiary. The Caledonides have a record of subduction throughout Ordovician and Silurian times, suggesting that a wide ocean was destroyed. These events inevitably involved the production and collision of island arcs before final closure of the ocean. Volcaniclastic flysch sediments are deposited during these events. In long orogens these events will be diachronous; for example, in Taiwan, the Luzon Arc is colliding obliquely with Asia. They may also be missing along strike; for example, the Ordovician arc-collision event recorded in the British Isles is not present in the Caledonides of Greenland. The Alps were formed by convergence in extended continental crust and closure of ocean basins so small that they could not produce significant island arcs; there was therefore no early arc-collision event there.

The lines marking the closure of oceans are called sutures (Fig. 1d) and are usually marked by ophiolitic remnants (e.g. the Indus-Tsangpo suture of the Himalayas), by subduction-related metamorphism (e.g. the Variscan suture), by a marked change in fossil fauna and flora, and by a fundamental change in the geophysical properties of the crust (e.g. the lapetus suture in Britain). Recognition of these sutures is critical to the understanding of orogens that are older than the modern ocean floor and cannot be reconstructed using the principles of plate tectonics; compare, for example, the models presented for the Variscan and Alpine orogenies.

It is extremely unlikely that convergence will be orthogonal to the strike of the orogen. Thickening is therefore likely to be associated with strike-slip displacements and the orogeny will undergo transpression. Different plates colliding with one margin at different times can produce changes in shear sense. For example, Ordovician arc collision on the Laurentian margin of the Caledonides was associated with dextral motion, but later Silurian continent-continent collision was associated with sinistral motion. During the collapse phase this motion became transtensional and opened strike-slip basins within the orogen or allowed emplacement of large granite batholiths such as the Donegal batholith of Ireland.

Many collisional orogenies follow the Wilson Cycle, in which oceans open along the lines of earlier mountain belts whose lithosphere is weaker than that of the foreland. The time interval between closure and re-opening can be up to 400 Ma. The relative weakness of the orogen cannot therefore be attributed to thermal effects of collision, for the lithosphere would have cooled after such a long interval. It may be due to the presence of partially subducted continental eclogites within the mantle (Fig. 2d), which not only produce radiogenic heat but are weaker than the mantle they displace.

Collisional orogens form at continental margins and are therefore long and commonly linear. A linear form could reflect the original geometry of the margin, or be superimposed by shear, parallel to the orogen during collision. Arcuate sectors in orogen can occur because one plate, usually a microcontinent, acted as an indentor, as in the Iberian Variscides, or because of original embayments in the continental margin, as in the Appalachians. The role of indentors in the Himalayan orogeny is controversial and two models are proposed. One model suggests that India indented Asia extruding material eastwards along strike-slip faults. The other argues that Asian lithosphere behaved as a viscous slab, allowing the Indian collision to produce a wide zone of deformation; the strike-slip faults are related to dextral transpression along the eastern margin. Orogens with complex geometries can form around remnant oceans, such as those marginal to the Mediterranean, as a result of the interference of several plate processes such as microcontinent or arc collision, subduction roll-back, and back-arc spreading. Final closure may remove much of this complexity, producing a deceptively simple linear orogen.

Paul D. Ryan

Bibliography

Dewey, J. F. and and Bird, J. M. (1970) Mountain belts and the new global tectonics. Journal of Geophysical Research, 75, 2625–47.
Windley, B. F. (1995) The evolving continents. John Wiley and Sons, Chichester.
Burg, J.- P. and and Ford, M. (1997) Orogeny through time. Geological Society Special Publication No. 121.