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Chapter 1 - Plate Tectonics

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velocities 
Space geodesy is measuring the precise position of sites 
on the Earth's surface from sources in space, such as 
radio-wave sources and satellite tracking. Three meth-
ods are currently used: very long baseline radio inter-
ferometry (VLBI), satellite laser ranging (SRL), and the 
global positioning system (GPS). VLBI depends on the 
precise timing of radio-wave energy from extragalactic 
sources (chiefly quasars) observed by radio telescopes. 
The radio waves recorded at different sites on the Earth 
are correlated and used to determine site locations, ori-
entation of the Earth, and azimuths of the radio sources. 
Arrival times of radio waves are measured with extremely 
precise hydrogen laser atomic clocks (Robertson, 1991). 
SRL is based on the round-trip time of laser pulses re-
flected from satellites that orbit the Earth. Successive 
observations permit the position of the tracking station 
to be determined as a function of time. GPS geodesy 
uses several high-altitude satellites with orbital periods 
of twelve hours, and each satellite broadcasts its posi-
tion and time. When multiple satellites are tracked, the 
location of the receiver can be estimated to within a few 
metres on the Earth's surface. Accuracy of the results 
depends on many factors (Gordon and Stein, 1992), 
including the length of time over which measurements 
have been accumulated. To reach accuracies of 1-2 mm 
for sites that are thousands of kilometres apart requires 
many years of data accumulation. 
Space geodetic measurement are especially important 
in a better understanding of modem plate tectonics. 
Results have been used to verify that plate motions are 
steady on time scales of a few years, to estimate rates 
and directions of plate motions, to estimate motions of 
small regions within plate boundary zones, to better 
understand deformation around plate boundaries, and to 
estimate rotations about a vertical axis of small crustal 
blocks. Results are encouraging and indicate that plate 
velocities averaged over a few years are similar to vel-
ocities averaged over millions of years by the methods 
previously mentioned (Gordon and Stein, 1992; Smith 
et al., 1994). For instance, SRL velocities for the North 
Plate tectonics 17 
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Figure 1.17 Velocities (root mean square) of continental 
plates calculated from apparent polar wander paths for the 
last 300 My. Modified after Piper (1987). 
American plate of 1.5-2 cm/y compare favourably with 
magnetic anomaly results averaged over 3 My. Both 
VLBI and GPS data suggest that the motion of the 
Pacific plate relative to the Eurasian and North American 
plates is about ten per cent faster than that estimated 
from magnetic anomaly data, suggesting that the Pacific 
plate has speeded up over the past few millions of years 
(Argus and Heflin, 1995). 
Plate driving forces 
Although the question of what drives the Earth's plates 
has stirred a lot of controversy in the past, we now seem 
to be converging on an answer. Most investigators agree 
that plate motions must be related to thermal convection 
in the mantle, although a generally accepted model re-
lating the two processes remains elusive. The shapes 
and sizes of plates and their velocities exhibit large 
variations and do not show simple geometric relation-
ships to convective flow patterns. Most computer mod-
els, however, indicate that plates move in response chiefly 
to slab-pull forces as plates descend into the mantle 
at subduction zones, and that ocean-ridge push forces 
or stresses transmitted from the asthenosphere to the 
lithosphere are very small (Vigny et al., 1991; Lithgow-
Bertelloni and Richards, 1995). In effect, stress distribu-
tions are consistent with the idea that at least oceanic 
plates are decoupled from underlying asthenosphere 
(Wiens and Stein, 1985). Ridge-push forces are caused 
by two factors (Spence, 1987): 
1 horizontal density contrasts resulting from cooling 
and thickening of the oceanic lithosphere as it moves 
away from ridges 
2 the elevation of the ocean ridge above the surround-
ing sea floor. 
The slab-pull forces in subduction zones reflect the cool-
ing and negative buoyancy of the oceanic lithosphere as 
it ages. The gabbro-eclogite and other high-pressure 
phase transitions that occur in descending slabs also 
contribute to slab-pull by increasing the density of the 
slab. 
Using an analytical torque balance method, which 
accounts for interactions between plates by viscous cou-
pling to a convecting mantle, Lithgow-Bertelloni and 
Richards (1995) show that the slab-pull forces amount 
to about ninety-five per cent of the net driving forces of 
plates. Ridge push and drag forces at the base of the 
plates are no more than five per cent of the total. Com-
puter models using other approaches and assumptions 
also seem to agree that slab-pull forces dominate (Vigny 
et al., 1991; Carlson, 1995). Although slab-pull cannot 
initiate subduction, once a slab begins to sink the slab-
pull force rapidly becomes the dominant force for con-
tinued subduction. 
Geomagnetism 
Rock magnetization 
To understand the magnetic evidence for seafloor spread-
ing, it is necessary to understand how rocks become 
magnetized in the Earth's magnetic field. When a rock 
forms, it may acquire a magnetization parallel to the 
ambient magnetic field referred to as primary magneti-
zation. Information about both the direction and inten-
sity of the magnetic field in which a rock formed can be 
obtained by studying its primary magnetization. The most 
important minerals controlling rock magnetization are 
magnetite and hematite. However, it is not always easy 
to identify primary magnetization in that rocks often 
acquire later magnetization known as secondary mag-
netization, which must be removed by demagnetization 
techniques prior to measuring primary magnetization. 
Magnetization measured in the laboratory is called 
natural remanent magnetization or NRM. Rocks may 
acquire NRM in several ways, of which only three are 
important in paleomagnetic studies (Bogue and Merrill, 
1992; Dunlop, 1995): 
1 Thermal remanent magnetization (TRM). TRM 
is acquired by igneous rocks as they cool through a 
blocking temperature for magnetization of the con-
stituent magnetic mineral(s). This temperature, known 
as the Curie temperature, ranges between 500 °C 
and 600 °C for iron oxides, and is the temperature at 
which magnetization is locked into the rock. The 
direction of TRM is almost always parallel and pro-
portional in intensity to the applied magnetic field. 
18 Plate Tectonics and Crustal Evolution 
2 Detrital remanent magnetization (DRM). Clastic 
sediments generally contain small magnetic grains, 
which became aligned in the ambient magnetic field 
during deposition or during compaction and dia-
genesis of clastic sediments. Such magnetization is 
known as DRM. 
3 Chemical remanent magnetization (CRM). CRM 
is acquired by rocks during secondary processes if 
new magnetic minerals grow. It may be produced 
during weathering, alteration, or metamorphism. 
NRM is described by directional and intensity param-
eters. Directional parameters include declination, or the 
angle with respect to true north, and the inclination, or 
dip from the horizontal. A paleomagnetic pole can be 
calculated from the declination and inclination deter-
mined from a given rock. 
Reversals in the Earth's magnetic field 
Some rocks have acquired NRM in a direction opposite 
to that of the Earth's present magnetic field. Such mag-
netization is known as reverse magnetization in con-
trast to normal magnetization which parallels the Earth's 
present field. Experimental studies

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