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Chapter 1 - Plate Tectonics

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discussed in Chapter 4, 
are caused by two important solid-state transforma-
tions: from olivine to wadsleyite at 410 km and from 
spinel to perovskite + magnesiowustite at 660 km. 
The lower mantle extends from the 660-km discon-
tinuity to the 2900-km discontinuity at the core-
mantle boundary. For the most part, it is characterized 
by rather constant increases in velocity and density 
in response to increasing hydrostatic compression. 
Between 220-250 km above the core-mantle inter-
face a flattening of velocity and density gradients 
occurs, in a region known as the D'' layer, named 
after the seismic wave used to define the layer. The 
lower mantle is also referred to as the mesosphere, 
a region that is strong, but relatively passive in terms 
of deformational processes. 
The outer core will not transmit S-waves and is 
interpreted to be liquid. It extends from the 2900-km 
to the 5200-km discontinuity. 
The inner core, which extends from 5200-km dis-
continuity to the centre of the Earth, transmits S-
waves, although at very low velocities, suggesting 
that it is near the melting point. 
Plate tectonics 5 
There are only two layers in the Earth with anomalously 
low seismic velocity gradients: the LVZ at the base of 
the lithosphere and the D" layer just above the core 
(Figure 1.2). These layers coincide with very steep tem-
perature gradients, and hence are thermal boundary lay-
ers within the Earth. The LVZ is important in that plates 
are decoupled from the mantle at this layer: plate tec-
tonics could not exist without an LVZ. The D" layer is 
important in that it may be the site at which mantle 
plumes are generated. 
Considerable uncertainty exists regarding the tempera-
ture distribution in the Earth. It is dependent upon such 
features of the Earth's history as: 
1 the initial temperature distribution 
2 the amount of heat generated as a function of both 
depth and time 
3 the nature of mantle convection 
4 the process of core formation. 
Most estimates of the temperature distribution in the Earth 
are based on one of two approaches, or a combination 
of both: models of the Earth's thermal history involving 
various mechanisms for core formation, and models in-
volving redistribution of radioactive heat sources in the 
Earth by melting and convection processes. 
Estimates using various models seem to converge on 
a temperature at the core-mantle interface of about 4500 
± 500 °C and the centre of the core 6700 to 7000 °C. 
Two examples of calculated temperature distributions in 
the Earth are shown in Figure 1.2. Both show significant 
gradients in temperature in the LVZ and the D" layer. 
The layered convection model also shows a large tem-
perature change near the 660-km discontinuity, since 
this is the boundary between shallow and deep convec-
tion systems in this model. The temperature distribution 
for whole-mantle convection, which is preferred by most 
scientists, shows a rather smooth decrease from the top 
of the D" layer to the LVZ. 
Seafloor spreading 
Seafloor spreading was proposed to explain linear mag-
netic anomalies on the sea floor by Vine and Matthews 
in 1963. These magnetic anomalies (Figure 1.3), which 
had been recognized since the 1950s but for which no 
satisfactory origin had been proposed, have steep flanking 
gradients and are remarkably linear and continuous, 
except where broken by fracture systems (Harrison, 
1987). Vine and Matthews (1963) proposed that these 
anomalies result from a combination of seafloor spread-
ing and reversals in the Earth's magnetic field, the record 
of reversals being preserved in the magnetization in the 
upper oceanic crust. The model predicts that lines of 
alternate normally and reversely magnetized crust should 
parallel ocean ridge crests, with the pronounced mag-
netic contrasts between them causing the observed steep 
linear gradients. With the Geomagnetic Time Scale deter-
mined from paleontologically-dated deep sea sediments, 
Vine (1966) showed that the linear oceanic magnetic 
Figure 1.3 Linear magnetic anomalies and fracture zones 
in the NE Pacific basin. Positive anomalies in black. After 
Raff and Mason (1961). 
anomalies could be explained by seafloor spreading. No 
other single observation in the last 50 years has had 
such a profound effect on geology. With this observa-
tion, we entered a new scientific era centred around a 
dynamic Earth. 
Ocean ridges are accretionary plate boundaries where 
new lithosphere is formed from upwelling mantle as the 
plates on both sides of ridges grow in area and move 
away from the axis of the ridge (Figure 1.1/Plate I, 
cross sections). In some instances, such as the South 
Atlantic, new ocean ridges formed beneath superconti-
nents, and thus as new oceanic lithosphere is produced 
at a ridge the supercontinent splits and moves apart on 
each of the ridge flanks. The average rate of oceanic 
lithosphere production over the past few million years is 
about 3.5 km^/y and, if this rate is extrapolated into the 
geologic past, the area covered by the present ocean 
basins (sixty-five per cent of tne Earth's surface) would 
be generated in less than 100 My. In fact, the oldest 
ocean floor dates only to about 160 Ma, because older 
oceanic plates have been subducted into the mantle. 
6 Plate Tectonics and Crustal Evolution 
I I -I \ 1 1 1 1 r—I r—n 1 1 1 1 1 1 r n I n 
Figure 1.4 Distribution of world earthquakes 1961-1969. From National Earthquake Information Center Map NEIC-3005. 
Plate boundaries 
Earthquakes occur along rather narrow belts (Figure 1.4), 
and these belts mark boundaries between lithospheric 
plates. There are four types of seismic boundaries, dis-
tinguished by their epicentre distributions and geologic 
characteristics: ocean ridges, subduction zones, trans-
form faults, and collisional zones. Provided a sufficient 
number of seismic recording stations with proper azi-
muthal locations are available, it is possible to deter-
mine the directions of first motion at sites of earthquake 
generation, which in turn provides major constraints on 
plate motions. 
Modem plates range in size from < 10"^ km^ to over 
10^ km^ and plate margins do not usually coincide with 
continental margins (Figure 1.1/Plate 1). Seven major 
plates are recognized: the Eurasian, Antarctic, North 
American, South American, Pacific, African and Aus-
tralian plates. Intermediate-size plates (10^-10^ km^) in-
clude the Philippine, Arabian, Nasca, Cocos, Caribbean 
and Scotia plates. In addition, there are more than twenty 
plates with areas of 10^-10^ km .^ Both plate theory and 
first-motion studies at plate boundaries indicate that plates 
are produced at ocean ridges, consumed at subduction 
zones, and slide past each other along transform faults 
(Figure 1.1/Plate 1, cross-sections). At collisional zones, 
plates carrying continents may become sutured together. 
Plates diminish or grow in area depending on the dis-
tribution of convergent and divergent boundaries. The 
African and Antarctic plates, for instance, are almost 
entirely surrounded by active spreading centres and hence 
are growing in area. If the surface area of the Earth is 
to be conserved, other plates must be diminishing in area 
as these plates grow, and this is the case for plates in the 
Pacific area. Plate boundaries are dynamic features, not 
only migrating about the Earth's surface, but changing 
from one type of boundary to another. In addition, new 
plate boundaries can be created in response to changes 
in stress regimes in the lithosphere. Also, plate boundaries 
disappear as two plates become part of the same plate, 
for instance after a continent-continent collision. Small 
plates (< 10^ km^) occur most frequently near continent-
continent or arc-continent collisional boundaries and are 

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