PLATE TECTONICS
The Earth’s crust can be divided on the basis of seismological evidence into areas which only rarely have earthquakes, these aseismic plates are separated by narrow active seismic belts.
These plates may be composed of mainly continental material e.g. the Eurasian plate or largely oceanic material e.g. Pacific plate.
These plates are relatively rigid and have a thickness of between 100-150 km. Nearly all volcanic, seismic and tectonic activity is restricted to narrow zones around plate margins.
The six major plates are: a) Pacific
b) Indian
c) Eurasian
d) American
e) African
The minor plates which are considerably smaller are:
a) Arabian
b) Scotia
c) Nasca
d) Cocos
e) Phillipine
f) Caribbean
The type of plate margin may be divided into three -
a) constructive
b) destructive
c) conservative
Along constructive margins new crust is created along oceanic ridges, this new crust, along with a layer of the uppermost part of the mantle, moves away from the ridge and new material is added to the trailing edge.
The destructive margins (sinks) are marked by the presence of oceanic trenches and island arc systems. Along these margins the plates approach each other and one of the plates (the Oceanic plate) is forced down into the mantle at an angle of approximately 45o
Along conservative margins there is neither a loss nor a gain and the plates slip past each other, these margins are marked by transform faults.
In 1948, B. Gutenberg proposed the existence of a seismic ‘Low Velocity Zone’ in the upper part of the mantle; later work has confirmed the presence of this world wide zone at a depth of 100 km to 200 km. In this ‘layer’ seismic waves travel more slowly and this suggests that the zone is abnormally hot. It is suggested by some writers that not only are the rocks in this zone weak due to high temperatures, but the 5% of this layer may be molten.
The upper 100-150 km of the crust is relatively strong and is known as the Lithosphere, the underlying weak zone is known as the Asthenosphere, while the mantle underneath the Asthenosphere is referred to as the Mesosphere. If the outer part of the Earth moves with respect to the inner part, then this may be the zone along which this ‘ungluing’ takes place. It is deduced that the moving plates must involve material at least as deep as layer 2 of the oceanic crust.
The concept of plate tectonics indicates that the surface area of the Earth remains the same and therefore the addition of crustal material along the oceanic ridges must be balanced by the destruction of other plate margins which is achieved by melting and absorption of crustal material as it is forced down into the mantle.
Constructive Margins
The constructive margins are the mid-oceanic ridges. The occurrence of small earthquakes along the crests of the mid-oceanic ridges, the lack of sediment, the high heat flow and the presence of active volcanoes led Hess (1960) to postulate that the mid-oceanic ridges are situated over the rising limbs of convection currents in the mantle, and that the oceanic crust was chemically modified mantle. In his hypothesis, which is now known as ‘sea floor spreading’, Hess suggested that the new crust was being generated at the ridge crests and that the present deep ocean floor had been formed within the past 200 million years. He also suggested that the Earth was not expanding but that the trench systems of the Pacific were the sites of descending limbs of a convection system in which the oceanic crust was thrust down and absorbed by the mantle. An important aspect of Hess’s hypothesis was that the oceanic crust was composed of hydrated mantle (serpentinite) which reverted to mantle material when thrust down along the trench system. F.J. Vine and D.H. Mathews (1963) postulated that as the new sea floor forms and spreads laterally away from the ridge crests, the Earth’s magnetic field reverses its polarity at certain intervals and produces zones of alternate magnetised material parallel to the ridge crests. Refined potassium-argon dating made it possible to date the flows. This spreading has been a constant 2cm per year per flank (S. Atlantic) and the oldest part of the oceanic areas are no older than the Cretaceous. In the parts of the oceanic areas further away from the crests, the zones of magnetic reversals do not appear.
Conservative Margins
The ridge system is displayed by what appears to be tear faults. In 1965 J. Tuzo Wilson identified these as transform faults. These faults offset the ridges up to several hundred kilometres. These faults are only seismically active between the ridges and Wilson recognised that the offset was original and unchanging with time. The neutral or conservative margins are marked by transform faults such as the San Andreas, and Atlantic systems. Study of seismic wave patterns produced by the earthquakes along the transform faults, has confirmed the difference between the movement along these faults
and movement along tear faults. Two-thirds of the major earthquakes originate from transform faults and the investigation of P waves has shown that the major and mean stresses are horizontal. One of the problems associated with transform faults is the convection current patterns on either side of the faults; how do two convection cells exist so closely together with their rising limbs offset by the fault. F.J. Vine has suggested that the drag on the plates would produce a lifting effect along the ridge crests, this coupled with the thinning of the lithosphere plates along the ridges, could exert some kind of control on the position of the rising limb of the convection cells. The offset of the mid-oceanic ridges has been an argument used against the concept of convection currents, but most authorities believe that thermal convection in some form or other is the only known process at depth by which plate movement could be maintained.
Destructive Margins
Deeper foci earthquakes are restricted to oceanic trench-island are systems and these regions are believed to be zones along which the oceanic plates dip into the mantle. Zones of deep seismic activity characterise all active island are systems and are recorded at depths of 700 km. Records of these seismic belts show that they are curved both in plan and section. These Benioff zones dip on average at angles of 45o, but many are associated with zones dipping between 58o-64o. Earlier it was thought that the earthquakes activity was generated along the slip zone between the descending plate and the continental plate, but L.R. Sykes’s work has indicated that the descending plates, after passing through a tensional stage, are in fact under compression with the seismic activity taking place within the descending slab. At the oceanic trenches the crust is consumed at rates varying between 5cm to 15cm per year. There does not have to be the same number of ‘sinks’ as there are constructive margins, what is necessary is that there is a balance between the generation and the destruction of crustal material. This explains the lack of balance between plates, e.g. the African plate is growing while the Philippine plate has two destructive margins.
Along the destructive margins it is suggested that a process of continental accretion takes place due to:
a) accumulation of oceanic sediments in a chaotic mass at the leading edge of the continental plate, the estimated rate of growth is low – approximately 1mm per year.
b) partial melting of the outer layer of the lithosphere along the slip zone, this material may give rise to andesitic, dioritic and granodioritic material along the island arcs (calc-alkaline along the mid oceanic ridges).
The size and distribution of the plates vary with time as some plates increase in size while others decrease, and perhaps completely destroyed with the result that the mid-oceanic ridges and the trench systems must be free to migrate relative to each other. Following this line of argument, it is therefore suggested by several workers, that the Pacific oceanic plate is being destroyed while the Atlantic is growing. Physical arguments indicate that continental crust cannot be destroyed; this suggests that when a continental plate arrives at a destructive margin, the oceanic plate is pushed down, but if two continental plates meet at a ‘sink’, a single plate is formed with one over-riding the other as with the Indian sub-continent.
There is evidence that the trench systems are associated with tangential movements as well as compressional phenomena, e.g. the Alpine fault in New Zealand, Atacama fault etc. with a general movement of the Pacific in an anticlockwise direction. When a continental plate over-rides a mid-oceanic system thee also appears to be lateral movement e.g. San Andreas Fault, to the north of the Californian area the N. American continental plate appears to have covered the northern continuation of the East Pacific Rise.
The subduction zones of the American and Eurasian plates are marked by island arc systems; these may be single or double arcs. They are associated with vulcanicity of Tertiary and Quaternary age and separate oceanic basalt associations from the andesitic continential associations (the Andesite line). The island arcs are zones of high heat flow while the associated trenches are zones of low heat flow. Recent work has shown that these systems are characterised by paired metamorphic belts – an outer high temperature/low pressure. The low temp/high pressure zone develops blueschist facies and is associated ophiolites and cherts (Steinmann Trinity). The inner high temp/low pressure zone is characterised by high grade schists, gneisses and granitisation, these being followed by post tectonic granites, e.g. Cordillera type, Sierra Nevada and Japanese areas. Japanese geologists have recognised two periods during which paired belts developed – Triassic and Tertiary.
PAST PLATE MARGINS
Recognition of past destructive margins is based on sedimentary, igneous and metamorphic evidence.
It is claimed that subduction zones are recognised by the pressure of thin pelagic shales, cherts and ophiolites which mark the position of the oceanic plate which is being forced down under the continental plate. These sediments mark the position of the trench and the adjacent oceanic floor, and are affected by low temp/high pressure metamorphism producing a blueschist facies where the oceanic plate is forced under the continental plate. This may explain the lack of thick sedimentary accumulation in the present day trench systems – these rocks being carried under the continental plate edge. Recent seismic work has shown evidence of chaotic folding on the seaward side of several island arc systems.
The descending oceanic plate produces friction along the Benioff zones and the resulting heat produces rising andesitic magmas and geothermal gradients, at the leading edge of the continental plates this produces volcanoes and granitisation, the oceanic sediments being converted to ecolgites and basalts. The highlands produced by this activity at the leading edge of the continental plate are eroded to provide flysch wedges which extend on to the oceanic margin and also fill syn-tectonics and post-tectonic basins on the continental plate.
CALEDONOIDES
J.F. Dewey and others have suggested a plate model for the Appalachian-Caledonian orogenic event and relate the stages of the Caledonian orogeny in Britain to plate tectonics.
It is envisaged that a proto-Atlantic ocean existed with the Scottish Highlands situated on one margin (the Northern Zone) and the Welsh area situated on the other side of the Proto-Atlantic (the Southern Zone).
During the Late Precambrian, Torridonian and Moine sediments were deposited in the Northern Zone which at this time was a passive margin. In the Soutern Zone, the Longmyndian were being deposited but nearer the plate margin the Irish Sea land mass developed (island arc) with a subduction zone in the Anglesey area, this is marked in Anglesey by the development of glaucophane schists and wedges of hydrated mantle (serpentinite) were intruded along fault zones, later high temp/high pressure conditions developed which produced granitisation and intrusive granites. The volcanic activity associated with this subduction zone is late Precambrian in age and may explain the later Precambrian igneous activity of the Anglesey, Malvern and S. Wales areas.
During the Cambrian the Dalradian sedimentation continued with the Durness Limestone succession being deposited in a shallow shelf on the continental plate.
By the Ordovician the northern zone had also become a subduction zone with the margin of the plate now being marked by the Ballantrae complex which is associated with high pressure / low temperature metamorphic facies. The simple Japanese case of paired metamorphic belts is not seen in this Northern Zone although there is a gradual increase in metamorphism to high temperature conditions to the north.
In the Southern Zone on the other side of the Proto-Atlantic or Caledonian Ocean, the widespread volcanic activity of Welsh and Lake District areas indicate that the subduction zone was still active with turbidite deposition producing the Lower Ordovician slates of the isle of Man and the Lake District. The presence of tholeiitic basalt in the Lake District suggests that a third ‘sink’ or subduction zone developed in that area and there was a general advance of the Ordovician sea over the leading edge of the southern continental plate.
In the Ordovician, the subduction zone appears to have been well developed along the line of the present Southern Uplands Fault and up to 6000 metres of turbidites, conglomerates and mudstones were deposited in the Girvan area, and these probably marked the trench at the leading edge of the northern continent. South in the Moffat area only thin sediments accumulated and this area must have been positioned on the oceanic plate and marked by the deposition of fine grained pelagic sediments continuing graptolites.