Class Notes

on
Framework of Plate Tectonics
Part of subject GPM 203 Geodynamics
by
Dr. Sandeep, Assistant Professor, Department of Geophysics,
Institute of Science, BHU, Varanasi

General
The continents drift slowly (the timescale for substantial change is 10-100 million years), but the
process is at all is remarkable. The Fig. 1 below illustrates the general structure of the first 100-
200 kilometers of the Earth's interior and form basis for the origin of plate tectonics.
The crust is thin, varying from a few tens of kilometers thick beneath the continents to less than
10 km thick beneath the many of the oceans. The crust and upper mantle together constitute the
lithosphere, which is typically 50-100 km thick and is broken horizontally into large plates. These
plates rest on the asthenosphere.
The asthenosphere is mostly in plastic state (deformable) due to the heat generated by radioactive
decay. The material that is decaying is primarily radioactive isotopes of light elements like
aluminum and magnesium. Though this heat source is small on an absolute scale, this is sufficient
to maintain the consistent plastic nature of asthenosphere because of the insulating properties of
the Earth's rocks (the corresponding heat flow at the surface out of the Earth is only about 1/6000
of the Solar energy falling on the surface).

Fig. 1: A general layered structure of the earth up to 200 km depth (top) and a
hypothetical model showing role of convection currents for plate movement (bottom).

Dr. Sandeep, Assistant Professor, Department of Geophysics, Institute of Science, BHU, Varanasi
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Convection Currents
Very slow convection currents flow in plastic layered asthenosphere, and these currents provide
horizontal forces on the lithospheric plates (similar to as convection in a pan of boiling water
causes a piece of cork on the surface of the water to be pushed sideways as shown in Fig. 1. The
timescale for convection in the pan is seconds and for plate tectonics is 10-100 million years, but
the principles are similar. Thus, it is evident that differentiation is crucial to plate tectonics on the
Earth, because it is responsible for producing an interior that can support tectonic motion.

Plate Tectonic Model

Within the basic theory of plate tectonics, plates are considered to be internally rigid, and to act as
extremely efficient stress guides. A stress applied to one margin of a plate is transmitted to its
opposite margin with no deformation in the interior of the plate. Deformation, then, only takes
place at plate margins. This feature is rather surprising that the plates are only 80-150 km thick
but may be many kilometers in width. On detailed examination of plates, it is recognized that
there are some locations where intraplate deformation does occur. Mountain belts are
characterized by extensive thrust faulting and movements along large transcurrent fault zones.
Even extensional deformation may be found in such belts. Within oceanic areas there are regions
of crustal extension and accretion in the back arc basins that are located on the landward sides of
many destructive plate margins.
Plates are mechanically decoupled from each other, although plate margins are in intimate
contact.

In order to explain the distribution of earthquakes, the processes of seismic zones, seismic
activity, tectonics, major geological formations etc., plate tectonic model was proposed (Fig. 2):

Fig. 2: Model of plate tectonics showing the role of lithosphere and asthenosphere. Arrows
indicate relative movements. Arrows in asthenosphere indicate compensating flow in response to
the downward movement of lithosphere. A denotes rift zone; C-oceanic crust; T-transform fault;
R- Ridge; B- Benioff zone of the earthquakes dipping along contacts of P1 & P3.

Plate tectonic model was put forward to explain seismic activity, tectonics, young mountain belts,
major geological formations of the earth during the last 180 my. Based on this, new global
tectonic theory was proposed. The combination of concept of transform faults with the hypothesis
of sea floor spreading led to the construction of plate tectonics. According to this model the top
layer of the earth (i.e. lithosphere) surface is divided into a number of a seismic plates or rigid
blocks bounded by the seismicity associated with active ridge crest, faults, trenches and mountain
systems. These plates are composed of continental crust or oceanic crust or combination of both.

Dr. Sandeep, Assistant Professor, Department of Geophysics, Institute of Science, BHU, Varanasi
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In seafloor spreading hypothesis, the plates are assumed to be in constant relative motion and the
boundary zones reflect:
 Creation or destruction of oceanic crust
 Extension and compression of oceanic crust
 Lateral movement of crustal plate along faults without change in their surface area.

The boundaries between the layers are gradational in the earth. The asthenosphere corresponds to
LVZ (low velocity zone) of seismology. It attenuates seismic waves particularly high frequency
shear waves. The lithosphere is discontinuous at the principal zones of tectonic activity within the
earth (ocean ridges, island arcs and major strike slip faults) and it is continuous elsewhere. The
major tectonic features are the results of relative movement and interaction of these blocks which
spread apart at ocean ridges (where new crust is created), slide past one another at large strike-
slip faults (Transform Fault/ Transcurrent fault- TF/TC) (where accretion or consumption does
not take place), and are under thrust at island arcs (where lithosphere is destroyed) and similar
other structures. Plate tectonic hypothesis and related features is shown in Fig. 2.

NEW GLOBAL TECTONICS

The concept of global geology was developed in the late 1960’s with the help of plate tectonic
hypothesis. According to this, the upper mantle is in dynamic state and all the geological
formations of the last 180 my can be explained by this hypothesis. The basic processes of plate
tectonic are still not known and it has certain limitations.
In new global tectonics, it is supposed that the top layer i.e. lithosphere is a mobile layer which
plays a key role in geological processes.
Lithospheric plate includes “the crust and the uppermost mantle”. It has significant strength and
about 100 km thick. The asthenosphere, over which lithosphere rests, has comparatively less
strength. It extends from the base of the lithosphere to a depth of 250 km. The mesosphere, which
has comparatively higher strength, extends from 250 km to 2900 km. It is passive and inert, and
hence it does not take part in tectonic processes. The nature of the mobile lithosphere is shown in
Fig. 3.

Fig. 3: Lithospheric plates and their boundaries. Most plates have portions capped by
Dr. Sandeep, Assistant Professor, Department of Geophysics, Institute of Science, BHU, Varanasi
thick continental crust and portions with thinner oceanic crust. Cau: Caucasus
Mountains; carp.: Carpathian Mountains; H.K.: Hindu Kush; Pam.: Pamirs; P.B.:
Pannonian Basin; Py: Pyrenees.
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In this hypothesis, the earth’s surface can be divided into seven major plates and 14 minor plates.
The seven major plates are:
1) Pacific plate
2) North American plate
3) South American plate
4) African plate
5) Eurasian plate
6) Indo-Australian plate
7) Antarctic plate
These plates are drifting in different directions with variable drift rates and generate different
types of boundaries (Fig. 4).

Fig. 4: Earth’s major tectonic plates with their direction and rates (in mm a-1) of
relative motion shown at selected points on their boundaries. Circled numbers
represent absolute motions (in mm a-1) of plates relative to the hotspot reference frame.

Earthquakes and their distribution

Earthquakes occur because earth materials are stressed to their breaking point. Two factors are
important: (1) the presence of brittle materials; and (2) motion that builds stress in the brittle
materials. Practically the only parts of the Earth that meets both these conditions is where the rigid
lithospheric plates are in motion, building stresses where they are in contact with each other. Most
earthquakes therefore occur along or near plate boundaries, within the brittle regime near the top
of the rigid plates (Fig. 5).

At divergent and transform boundaries, the rigid lithospheric plates normally do not extend deeper
than about 100 km (Fig. 5a, b). The cold brittle part of the plates is generally in the upper 20 km,
so that only shallow earthquakes occur.

Dr. Sandeep, Assistant Professor, Department of Geophysics, Institute of Science, BHU, Varanasi
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Virtually all deep, and most very large, earthquakes occur at convergent plate boundaries, where a
rigid plate can extend downward to a depth of about 700 km (Fig. 5c). Shallow earthquakes
(upper 70 km) are associated with compression and other contortion (twist or bend) on the top of
both the plates. Very large earthquakes occur due to sudden release of stress where the two plates
are locked together, at their boundary. Moderate to large earthquakes can occur deeper, if the
lower plate descends so quickly that it is still cold enough to undergo brittle failure.

Fig. 5: Occurrence of earthquakes

at plate boundaries. a,b) shallow
earthquakes of small to moderate
size occur along divergent and
transform fault boundaries. c)
Small to moderate size
earthquakes at convergent plate
boundaries occur at shallow
depths in the descending and
overriding plates. Very large
earthquakes occur where the
plates lock together, and at
greater depths in the upper
portion of the descending plate.

Earthquake activity throughout the world is largely confined to young mountains, trench system
of the Alpine-Himalayan, Circum Pacific belts and the crest of the mid-oceanic ridges (Fig. 6).
Volcanoes are also distributed along the trenches only. Seismological observations shows that a
large number of earthquakes occur in the Circum-Pacific belt in association with the trench
system. Frequency of earthquakes reduces in the Alpine-Himalayan and it further reduces to the
mid-oceanic ridges. The current seismicity of the earth is confined to narrow linear zones. The
seismic observations further reveals that the seismic activity of the ridge crest and strike-slip
faults is 10-20 km. The young mountains and trench systems show shallow focus earthquakes.
These features are also associated with intermediate (70<h<300 km) and deep focus earthquakes
up to 700 km. The intermediate and deep focus earthquake zones reflect different processes that
can not be explained by ocean floor spreading hypothesis.

Dr. Sandeep, Assistant Professor, Department of Geophysics, Institute of Science, BHU, Varanasi
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Fig. 6: World wide distribution of epicenters of large magnitude earthquakes (mb >4)
for the period 1961-67.

Types of Plate Boundary

Lithospheric plates have three different types of boundaries as given below (Fig. 7):
1) Ocean ridges: Diverging boundary or accreting boundary or constructive boundary
2) Trenches: Converging boundary or destructive boundary- lithosphere is consumed and
destroyed.
3) Transform faults: Conservative boundary- these boundaries are also known as ridges,
trenches and transform faults where the plate is neither created nor destroyed.
Diverging Boundary
This boundary is also called accreting boundary where plates are diverging i.e. they move apart.
Diverging boundaries are mid-ocean ridges where two plates move apart from each other. The
molten magma from the Upper Mantle (UM) and depleted mantle upwell between the separating
plates and a new oceanic crust (lithosphere) is formed. In such region, the thickness of lithosphere
is very small and exactly at the ridges it is nearly zero. This is also called constructive boundary.
The nature of ridges and lithosphere is depicted in Fig. 7a.

Dr. Sandeep, Assistant Professor, Department of Geophysics, Institute of Science, BHU, Varanasi
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Fig. 7: Three general

types of plate boundaries,
highlighted by bold lines
between two plates. (a)
plate rip apart and grow
at divergent boundaries;
(b) compress and are
destroyed at convergent
boundaries; and (c) slide
past one another at
transform boundaries,
neither creating nor
destroying plate
materials.

The divergent motion of the plates is frequently perpendicular to the strike of the boundary,
although this is not the case and not a geometrical necessity (Fig. 8). In Pacific Ocean it appears
to be intrinsic characteristics of spreading whenever a steady direction has been established for
some time.

Fig. 8: Cross section across the

ridges showing direction of
movement of lithosphere and
accretion of magma from the
upper mantle.

Convergent Boundary
The convergent boundaries are formed where two lithosphere plates move each other and come
closer by convergence (Figs. 7b & 9). This is also known as destructive boundary by the
mechanism of convergence where one of the oceanic lithospheric plates being thrust under the
other, eventually to become reabsorbed into sub-lithospheric mantle. Since the Earth is not
expanding significantly, the rate of lithospheric destruction at trenches must be virtually the same
as the rate of creation at ocean ridges.
Also Himalayan types orogens, caused by the collision of two continental plates (Fig. 9c), are
included in this category where continued compressional deformation may be occurring. The
direction of motion of the underthrusting plate need not be at right angles to the trench, that is,
oblique subduction can occur.
The collision of two plates would lead the formation of young mountains and/ or subduction of
lithosphere into the upper mantle. The sinking of lithosphere in to the upper mantle may be
shallow to 700 km depth forming island arc resulting earthquake activity up to 700 km. The
lithospheric collision can be of three types as given below:
1) Continent-continent collision
2) Ocean-continent collision, and
3) Ocean-ocean collision
The pictorial representation of possible causes of collision, subduction of lithosphere and
formation of mountains (young) is shown in Fig. 9. If a continent resting on a lithospheric plate is

Dr. Sandeep, Assistant Professor, Department of Geophysics, Institute of Science, BHU, Varanasi
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collided to an oceanic lithospheric plate, it will consume it (Fig. 9b). Since the continental crust is
made up of lighter material, it can not sink in to the mantle. If the trench and island arcs are
oceanic crust on both sides, then the island arc will “flip” (to turn over with rapid movement) so
as to consume the oceanic crust behind the arc (Fig. 9a). A continental arc is formed (e.g. western
ridge of America) consuming oceanic crust along the edge of the continent from the ocean side. In
such cases, the subduction of lithospheric plate (LP) is normally 300 to 400 km (continent-oceanic
collision).

Fig. 9: Volcanism at
convergent plate boundaries.
(a) Subduction zone with
two oceanic crust; (b)
boundary where oceanic
crustal plate subducts
beneath continental crustal
plate; (c) continental -
continental crustal plate
collision zone- volcanisms
ceases during continental
collision.

Collision between the oceanic and continental regions takes place if the oceanic slab goes down
below the continental lithosphere (Fig. 9b), the lithosphere may sink up to several hundreds of
kilometers. If the collision of two continental plates takes place over a wide zone of over thrusting
and regeneration of sinking slab of lithosphere plate ceases (Fig. 9c).
Conservative Boundary
The plate boundary at which neither subduction nor creation of lithosphere takes place is called
conservative boundary (Fig. 7c). The boundaries are transform faults or transcurrent faults and
motion along them is shown in Fig. 10. Along these boundaries, two plates slide past each other in
tangential motion and hence the plate materials are neither created nor consumed. The relative
motion is usually parallel to the fault.

Dr. Sandeep, Assistant Professor, Department of Geophysics, Institute of Science, BHU, Varanasi
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Fig. 10: Possible motion of

lithospheric plate on Transform
fault and transcurrent fault.

The tectonic activity of the earth has been explained in the light of rifting of lithospheric plates
Figs. 8 & 10). The earth’s lithosphere is broken into a number of big and small plates which are in
relative motion to each other. The boundaries of the plates have been identified as:
1) Ridges
2) Trenches
3) Transform and Transcurrent faults (TF/TC)
At the ridges, the upper mantle materials up wells to the surface, solidifies and create a new crust
(Figs. 8 & 10). At the trenches, the lithosphere sink into the upper mantle and the magnitude of
sinking is found to be different at different collision sites. At the TF/TC faults (trasnform fault/
transcurrent fault), lateral movement of the lithosphere takes place where the lithosphere is neither
created nor destroyed. The global pattern of source and sink is shown in Figs. 3 & 4. It shows
division of earth’s surface into a limited number of rigid block which are in relative motion with
each other. The drift rates and the direction of different plates are also different (Fig. 4). Dotted
lines show lines of divergence, and lines of convergence are marked by arrowheads showing the
direction of motion of the down going slab. These are the zones of strongest seismic activities.
The plate boundaries coincide with the seismic activity of the globe. Intermediate (70 <h < 300
km) and deep focus (h= 300-700 km) earthquakes are observed at trenches and shallow focus (h=
10-20 km) earthquakes at ridges and transform faults. The rate of divergence at oceanic ridges has
been derived from magnetic strip anomaly pattern. A convincing configuration of the spreading
rate has been obtained from deep drilling programme.

A collision zone of two continental blocks is marked by formation of young mountains (Fig. 9c).
The formation of Himalayas is the result of collision of Indian plate with the Asian plate. Since
there is no subduction of lithosphere in the upper mantle, collision of India with Eurasia (both
continent-continent type of collision) produced massive range of folded mountains. Active
volcanoes are distributed on the convex side of the subducting lithosphere. Resistance offered by
the colliding plate with the subducting lithosphere would melt the surface of lithospheric materials
resulting volcanic eruption.
References:
Lowrie: Fundamentals of Geophysics
Gubbins: Seismology and plate tectonics

Dr. Sandeep, Assistant Professor, Department of Geophysics, Institute of Science, BHU, Varanasi