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 overriding 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 AppalachianCaledonian 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.
By the Silurian the turbidites had reached as far south as Moffat and
approximately 8km of flysch type sediments were deposited, some of these
may have been derived from the ‘Solwayland’ island arc. A third island arc is
also suggested to the south of the Southern Uplands Fault known as
‘Cockburnland’.
The lack of volcanic activity during the Silurian has led to suggestions that the
oceanic plate had now been over-ridden or covered by sediment during the
Silurian (c.f. the over-riding of the East pacific Rise by the North American
plate) which was then folded by the collision of the plate margins.
This concept of plate tectonics could be used to explain the different faunal
provinces which existed during the Lower Palaeozoic.
ALPS
K.J. Hsu (1971) has suggested that the Alpine chain was not the result of
simple compression at plate margins of sediments deposited in the Tethys
Ocean. The African plate first moved eastwards because it broke away from
S. America earlier that the Laurasian plate which was still attached to N.
America. Later the Laurasian plate was separated from the American plate
with the result that the African plate moved westwards relative to the
Laurasian plate. The final northward drift of the African plate compressed the
Tethyan zone and in places thrust up part of the oceanic floor ie. the Troudos
Mountains in Cyprus. These movements would explain the numerous bends
and convergences of the Alpine belt.
DATING OF CONTINENTAL DRIFT
The JOIDES drilling programme confirmed that spreading had been
continuous throughout the Caenozoic at a relatively constant rate of 2cm per
year ridge flank. This had been originally deduced by the study of magnetic
reversals parallel to the ridges. In the Pacific these spreading rates are higher
– 3cm per year.
The initiation of drifting in the S. Atlantic is thought to have occurred during
the Middle Cretaceous (100 m.y.).
The dating of the igneous activity of the Stromberg lavas and the Deccan
traps etc., give dates of approximately 200 m.y. and it is suggested that this
world wide igneous phase preceded, by about 20 m.y., the onset of the
separation of the continents.
CAUSES OF PLATE MOTION
Any suggested cause must explain:
1) The high heat flow at oceanic ridges and the low heat flow in oceanic
trenches; it must also explain the high heat flow in the adjacent island
arcs.
2) Rates of motion ranging from less than 1cm to 6cm per year.
3) The formation of oceanic trenches.
4) The isostatic equilibrium of the ridges and the strong negative
anomalies over the trenches.
5) The apparent movement of plates over great distances without
significant deformation. Neither compressional nor tensional stresses
could be transmitted from one end of the plate to the other.
D.L. Turcotte and E.R. Oxburgh suggest that in a convection cell the lower
flow is gradually heated up and, due to the poor conductivity of the material,
there is only a gradual build up of hot material which increases in thickness as
it moves sideways towards the ascending current. As the convection cell
rises and meets the cold upper surface of the earth it starts to cool and moves
away from the ascending position. The thickness of the cooling current
increases resulting in a relatively rigid slab which increases in thickness away
from the ascending current position – this rigid slab is the oceanic plate. The
lower boundary of this slab is the boundary between the semi-rigid oceanic
crust and the very viscous mantle. The plate may be imagined as having a
rigid and elastic upper layer which grades down into the viscous mantle.
At the ‘sink’, the plate has reached its maximum thickness and as it is denser
it must now sink. As the rigid plate descends, it forms an elongated cold
tongue and undergoes ‘thermal erosion’ and physical break up.
Most authorities accept that the cause of movement must be some form of
thermal convection. E.R. Oxburgh (1971) discusses the role of convection
movements in the earth and suggests a model to explain some of the points
outlined above.
The source of the heat of the Earth is not well known and although it is known
that there is internal heating, it is not known how much or where.
D.L. Tozer on the basis of experimental work, has suggested that the internal
heating of a shallow layer would produce wide shallow cells and there are
some authorities who believe that there is no flow below the ‘Low Velocity
Layer’, alternatively the ‘Low Velocity Layer’ may be part of the upper
horizontal flow of a convection cell and move in the same direction as the
plate, the return flow being slow and diffuse extending down into the mantle
below the ‘Low Velocity Layer’. This is supported by seismic evidence which
indicates that plates or parts of plates are recognisable at depths of 700 km.
This theory is also supported by fluid mechanics models. There is no reason
to believe that the descending material returns to the same cell.
PRESENT DAY GEOSYNCLINES
R.S. Dietz and others believe that present day geosynclines are located on
the eastern margin of America where thick Cretaceous and Tertiary strata of
the continental shelf will give rise to a miogeosyncline and the sediments of
the same age forming the continental rise will form a eugeosyncline. The
sediments of the rise are up to 10 km thick and are composed of turbidites
interbedded with shales. On the shelf the sediments thicken from 3 km to 5
km when traced seawards and are in the main well sorted. It is argued that if
the present direction of plate motion is reversed, then these sediments would
be folded into an orogenic belt.