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Chapter 1 - Plate Tectonics

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Coupling also varies 
between descending slabs. In some continental-margin 
subduction zones (e.g., Peru-Chile, Alaska), coupling is 
very strong, resulting in large earthquakes and a rela-
tively low dip of the descending slab (Figure 1.10). In 
other areas, such as the Kurile and Mariana arc systems 
(Figure 1.8), slabs are largely decoupled from overrid-
ing plates and extend to great depths, and earthquakes 
are smaller and less frequent. In some instances, sub-
ducting slabs are forced beneath the lithosphere in the 
overriding plate, a situation known as buoyant sub-
duction (Figure 1.10). Buoyant subduction occurs when 
the lithosphere is forced to sink before it becomes nega-
tively buoyant (i.e., in < 50 My today), and thus it tends 
to resist subduction into the asthenosphere. Underplated 
buoyant slabs eventually sink into the mantle when they 
cool sufficiently and their density increases. 
Seismic tomographic studies of subduction zones re-
veal a detailed three-dimensional structure of descend-
ing slabs. For instance, P-wave tomographic images of 
the Japan subduction zone correlate well with major 
surface geological features in Japan (Figure 1.11). Seis-
mic velocities are several percentage points higher in 
the descending slab than in the surrounding mantle, and 
results indicate that the slab boundary is a sharp seismic 
discontinuity (Zhao et al., 1992). Moreover, low-
velocity anomalies occur in the crust and mantle wedge 
over the descending slab, a feature also common in other 
subduction zones. The two low-velocity zones in the 
crust correlate with active volcanism in the Japan arc, 
and probably reflect magma plumbing systems. Deeper 
low-velocity zones (> 30 km) may represent partly-melted 
ultramafic rocks, formed in response to the upward trans-
Plate tectonics 11 
a 1501-
•• > .^ / 
Figure 1.10 Buoyant subduction beneath 
the Peru-Chile arc in central Peru. 
Earthquake first-motions shown by arrows 
are tensional. Black dots are earthquake 
hypocentres. Modified after Sacks (1983). 
200 400 600 
Figure 1.11 Cross-section of the Japan subduction zone 
showing perturbations of P-wave velocities from normal 
mantle (in percentages). Crosses and circles show fast and 
slow velocities respectively. Solid triangles are active 
volcanoes. Base of crust shown at 25-30 km. Units on 
horizontal axis are degrees of E longitude. After Zhao et al. 
fer of volatiles from the descending plate, which lowers 
the melting points of mantle-wedge silicates. 
Another interesting question about subduction is, what 
happens when a submarine plateau or aseismic ridge 
encounters a subduction zone? Because they resist sub-
duction they may produce a cusp in the arc system, as 
illustrated for instance by the intersection of the Caroline 
ridge with the Mariana arc south of Japan (Figure 1.1/ 
Plate 1). Paleomagnetic and structural geologic data from 
the Mariana arc support this interpretation, indicating 
that the arc was rotated at its ends by collision of these 
ridges between 30 and 10 Ma (McCabe, 1984). Also, 
volcanic and seismic gaps in arc systems commonly occur 
at points of collision between submarine plateaux 
and ridges with arcs (McGeary et al., 1985). Examples 
are the Tehuantepec, Cocos, Carnegie, Nazca and Juan 
Fernandez ridges along the Middle American and Peru-
Chile subduction systems. When a plateau or ridge en-
counters an arc, subduction stops and a volcanic/seismic 
gap forms in the arc. In most instances, plateaux/ridges 
accrete to arcs, and only small ridges and some volcanic 
islands are negatively buoyant and can actually be sub-
ducted. As we shall see in Chapter 5, this may be an im-
portant mechanism by which continents grow laterally. 
A commonly asked question is, just where and how 
are new convergent boundaries initiated? Because of the 
very high stress levels necessary for the oceanic litho-
sphere to rupture, it is likely that pre-existing zones of 
weakness in the lithosphere provide sites for new sub-
duction zones. Of the three proposed sites for initiation 
of new subduction zones, i.e., passive continental mar-
gins, transform faults/fracture zones, and extinct ocean 
ridges, none can simply convert to subduction zones by 
the affect of gravitational forces alone (Mueller and 
Phillips, 1991). Hence, additional forces are needed to 
convert these sites into subduction zones. One possible 
source is the attempted subduction of buoyant material 
(such as a submarine plateau) at a trench, which can 
result in large compressional forces in both subducting 
and overriding plates. This is the only recognized tectonic 
force sufficient to trigger nucleation of a new subduction 
zone. Transform faults and fracture zones are likely sites 
for subduction initiation in that they are common in the 
vicinity of modem subduction zones and are weaker than 
normal oceanic lithosphere. 
Collisional boundaries 
Deformation fronts associated with collisional bounda-
ries are widespread, as exemplified by the India-Asia 
boundary which extends for at least 3000 km northeast 
of the Himalayas. Earthquakes are chiefly < 100 km deep 
and first-motion studies indicate a variety of fault types. 
Thrust fault mechanisms generally dominate near sutures, 
such as the Indus suture in the Himalayas. Transcurrent 
faulting is common in the overriding plate as illustrated 
by the large strike-slip faults produced in China and 
12 Plate Tectonics and Crustal Evolution 
Tibet during the India collision. In addition, extensional 
faulting may extend great distances beyond the suture in 
the overriding plate. For instance, the Baikal rift in south-
em Siberia appears to have formed in response to the 
India collision 55 Ma. 
A plate boundary in the early stages of an arc-
continent collision is illustrated by the Sunda arc system 
in eastern Indonesia (Figure 1.1/Plate 1). Australia is 
beginning to collide with this arc as the Australian plate 
is subducted beneath the arc. In fact, a large bend in the 
descending slab beneath the island of Timor may be 
produced by subduction of continental crust. Numerous 
hypocentres at 50-100 km depth are interpreted to re-
flect the beginning of detachment of the descending slab 
as continental crust resists further subduction. Farther to 
the east, Australia collided with the arc system and is 
accreted and sutured to the arc on the northern side of 
New Guinea. Earthquakes and active volcanism in north-
em New Guinea are interpreted to reflect initiation of a 
new subduction zone dipping to the south, in the oppo-
site direction of subduction prior to collision with the 
Australian plate. 
Trench-ridge interactions 
It is interesting to consider what happens when an ocean 
ridge approaches and finally collides with a subduction 
zone, as the Chile and Juan de Fuca ridges are today 
(Figure 1.1/Plate 1). If a ridge is subducted, the arc 
should move 'uphill' and become emergent as the ridge 
crest approaches, and it should move 'downhill' and 
become submerged as the ridge passes down the 
subduction zone (DeLong and Fox, 1977). Correspond-
ing changes in sedimentation should accompany this 
emergence-submergence sequence of the arc. Ridge 
subduction may also lead to cessation of subduction-
related magmatism as the hot ridge is subducted. This 
could be caused by reduced frictional heating in the 
subduction zone or by progressive loss of volatiles from 
a descending slab as a ridge approaches. Also, the outer 
arc should undergo regional metamorphism as the hot 
ridge crest is subducted. All three of these phenomena 
are recorded in the Aleutian arc and support the subducted 
ridge model. Ridge subduction may also result in a 
change in

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