Chapter 1 - Plate Tectonics

stress regime in the overriding plate from 
dominantly compressional to extensional, and in the 
opening of a back-arc basin (Uyeda and Miyashiro, 
1974). The subduction of active ridges may lead to for-
mation of new ridges in the descending plate at great 
distances from the convergent margin. For instance, the 
rifting of Antarctica from Australia, which began almost 
50 Ma, coincided with the subduction of a ridge system 
along the northern edge of the Australian-Antarctic plate. 
In the case of the Chile ridge, which is being subducted 
today, there are few effects until the ridge arrives at the 
trench axis. As the ridge approaches the Chile trench, 
the rift valley becomes filled with sediments and finally 
disappears beneath the toe of the trench (Cande et al., 
1987). Also, the landward slope of the trench steepens 
and narrows in the collision zone. At the trench-ridge 
collision site, the accretionary prism is reduced in size 
by more than seventy-five per cent and part of the base-
ment beneath the Andean arc appears to have been eroded 
away and subducted. Two factors seem important in 
decreasing the volume of the accretionary prism: 
1 the topographic relief on the ridge may increase the 
rate of subduction erosion carrying material away 
from the bottom of the accretionary prism 
2 subduction of oceanic ridges caused by transform 
faults may mechanically weaken the base of the 
accretionary prism, making it more susceptible to 
removal by subduction erosion. 
Heat flow increases dramatically in the collision zone 
and then decays after collision to typical arc values. 
Also, an ophiolite (fragment of oceanic cmst) was 
tectonically emplaced in the Andean fore-arc region 
during an earlier stage of the collision 3 Ma, supporting 
the idea, more fully discussed in Chapter 3, that ridge-
trench collisions may be an important way in which 
ophiolites are emplaced. The intriguing question of what 
happens to the convective upcurrent beneath the Chile 
ridge as it is subducted remains poorly understood. 
The Wilson Cycle 
The opening and closing of an oceanic basin is known 
as a Wilson Cycle (Burke et al., 1976), named after 
J. Tuzo Wilson who first described it in 1966. He pro-
posed that the opening and closing of a proto-Atlantic 
basin in the Paleozoic accounted for unexplained changes 
in rock types, fossils, orogenies, and paleoclimates in 
the Appalachian orogenic belt. A Wilson Cycle begins 
with the rupture of a continent along a rift system, such 
as the East African rift today, followed by the opening 
of an ocean basin with passive continental margins on 
both sides (Figure 1.12, a and b). The oldest rocks on 
passive continental margins are continental rift assem-
blages. As the rift basin opens into a small ocean basin, 
as the Red Sea is today, cratonic sediments are depos-
ited along both of the retreating passive margins (b), and 
abyssal sediments accumulate on the sea floor adjacent 
to these margins. Eventually, a large ocean basin such 
as the Atlantic may develop from continued opening. 
When the new oceanic lithosphere becomes negatively 
buoyant, subduction begins on one or both margins 
and the ocean basin begins to close (c and d). Complete 
closure of the basin results in a continent-continent 
collision, (e), such as occurred during the Permian when 
Baltica collided with Siberia forming the Ural Moun-
tains. During the collision, arc rocks and oceanic crust 
are thrust over passive-margin assemblages. The geo-
logic record indicates that the Wilson Cycle has occurred 
many times during the Phanerozoic. Because lithosphere 
is weakened along collisional zones, rifting may open 
new ocean basins near older sutures, as evidenced by 
Plate tectonics 13 
Collisional 
Orogen 
\ \ 
Arc \ \ Passive Continental 
C m. \ Sea Level \ Margin 
a 
^•''/^'^'-'-''-'fo/•-
^— 
V J 
m':i&;^ 
Lithosptiere ^ ^ 
/ 
^ ^ 5 ^ ^ o n t i n enta 1 Crust \ 
\ ^ _ ^ ^ 
Figure \.12 Idealized sequence of events in a Wilson 
Cycle (beginning at the bottom). 
the opening to the modem Atlantic basin approximately 
along the Ordovician lapetus suture. 
Stress distribution within plates 
Stress distributions in the lithosphere can be estimated 
from geological observations, first-motion studies of 
earthquakes, and direct measurement of in-situ stress 
(Zoback and Zoback, 1980). Stress provinces in the crust 
show similar stress orientations and earthquake magni-
tudes and have linear dimensions ranging from hundreds 
to thousands of kilometres. Maximum compressive 
stresses for much of North America trend E-W to NE-
SW (Figure 1.13). In cratonic regions in South America 
most stresses trend E-W to NW-SE. Western Europe 
is characterized by dominantly NW-SE compressive 
stresses, while much of Asia is more nearly N-S. Within 
the Australian plate, compressive stresses range from 
N-S in India to nearly E-W in Australia. Horizontal 
stresses are variable in Africa but suggest a NW-SE 
trend for maximum compressive stresses in West Africa 
and an E-W trend for the minimum compressive stresses 
in East Africa. Except at plate boundaries, oceanic litho-
sphere is characterized by variable compressive stresses. 
The overall pattern of stresses in both the continental 
and oceanic lithosphere is consistent with the present 
distribution of plate motions as deduced from magnetic 
anomaly distributions on the sea floor. 
Examples of stress provinces in the United States are 
shown in Figure 1.13. Most of the central and eastern 
parts of the United States are characterized by compres-
sive stresses, ranging from NW-SE along the Atlantic 
Coast to dominantly NE-SW in the mid-continent area. 
In contrast, much of the western United States is char-
acterized by extensional and transcurrent stress patterns, 
although the Colorado Plateau, Pacific Northwest and 
the area around the San Andreas fault are dominated by 
compressive deformation. The abrupt transitions between 
stress provinces in the western United States imply lower 
crustal or uppermost mantle sources for the stresses. The 
correlation of stress distribution and heat-flow distribu-
tion in this area indicates that widespread rifting in the 
Basin and Range province is linked to thermal processes 
in the mantle. On the other hand, the broad transitions 
between stress provinces in the central and eastern United 
States reflect deeper stresses at the base of the lithosphere, 
related perhaps to drag resistance of the North American 
plate and to compressive forces transmitted from the 
Mid-Atlantic ridge. 
Plate motions 
Cycloid plate motion 
The motion of a plate on a sphere can be described in 
terms of a pole of rotation passing through the centre of 
the sphere. Because all plates on the Earth are moving 
relative to each other, the angular difference between a 
given point on a plate to the pole of rotation of that plate 
generally changes with time. For this reason, the trajec-
tory of a point on one plate as observed from another 
plate cannot be described by a small circle around a 
fixed pole of rotation. Instead, the shape of a relative 
motion path is that of a spherical cycloid (Cronin, 1991). 
The trajectory of a point on a moving plate relative to 
a point on another plate can be described if three vari-
ables are known during the period of displacement be-
ing considered: 
1 the position of the pole of rotation 
2 the direction of relative motion 
3 the magnitude of the angular velocity. 
An example of cycloid motion for a hypothetical plate 
is given Figure 1.14. Plate 2 rotates around pole P2 with 
a given angular velocity. P2 appears to move with time 
relative to an observer on plate 1, tracing the path of a 
small circle. When the motion of plate 2 is combined 
with the relative motion of P2, a reference point M on