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Chapter 1 - Plate Tectonics

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Such junctions are a necessary consequence of rigid plates 
on a sphere, since this is the common way a plate bound-
ary can end. There are sixteen possible combinations 
of ridge, trench, and transform-fault triple junctions 
(McKenzie and Morgan, 1969), of which only six are 
common. Triple junctions are classified as stable or 
unstable, depending on whether they preserve their 
geometry as they evolve. The geometric conditions for 
stability are described with vector velocity triangles in 
Figure 1.7 and only RRR triple junctions are stable for 
all orientations of plate boundaries. It is important to 
understand evolutionary changes in triple junctions, be-
cause changes in their configuration can produce changes 
that superficially resemble changes in plate motions. 
Triple junction evolution is controlled by the lengths of 
transform faults, spreading velocities, and the availabil-
ity of magma. 
Convergent boundaries (subduction zones) 
Convergent plate boundaries are defined by earthquake 
hypocentres that lie in an approximate plane and dip 
beneath arc systems. This plane, known as the seismic 
zone or Benioff zone, dips at moderate to steep angles 
and extends in some instances to the 66()-km seismic 
discontinuity. The seismic zone is interpreted as a brittle 
region in the upper 10-20 km of descending lithospheric 
slabs. Modem seismic zones vary significantly in hypo-
centre distribution and in dip (Figure 1.8). Some, such 
as the seismic zone beneath the Aleutian arc, extend to 
depths < 300 km while others extend to the 660-km 
discontinuity (Figure 1.8, a and b respectively). In gen-
eral, seismic zones are curved surfaces with radii of 
curvature of several hundred kilometres and with irregu-
larities on scales of < 100 km. Approximately planar 
seismic zones are exceptional. Seismic gaps in some 
zones (e.g., c and e) suggest, although do not prove, 
fragmentation of the descending slab. Dips range from 
30 ° to 90 °, averaging about 45 \ Considerable varia-
tion may occur along strike in a given subduction zone, 
as exemplified by the Izu-Bonin arc system in the West-
ern Pacific. Hypocentres are linear and rather continu-
ous on the northern end of this arc system, (d), becoming 
progressively more discontinuous toward the south. Near 
the southern end of the arc, the seismic zone exhibits a 
pronounced gap between 150 and 400 km depth, (c). A 
large gap in hypocentres in descending slabs, such as 
that observed in the New Hebrides arc, (e), may indicate 
that the tip of the slab broke off and settled into the 
mantle. Because some slabs appear to penetrate the 660-
km discontinuity (Chapter 4), the lack of earthquakes 
below 700 km probably reflects the depth of the brittle-
ductile transition in descending slabs. An excellent cor-
Plate tectonics 
Velocity triangle 
All orientations 
Stable if ab,ac 
form a straight 
l ine, or if be is 
parallel to the 
slip vector CA 
Stable if ac,bc 
form a straight 
l ine, or if C lies 
on ab 
Stable if C lies 
on ab,or if ac, 
be form a 
straight line 
Stable if ab, be 
form a straight 
line, or if ae,be 
do so 
Stable if ab goes 
through C, or if 
ac, be form a 
straight line 
East Pacific Rise 
and Galapagos 
Rift zone. 
Intersection of 
the Peru-Chile 
Trench and the 
West Chile Rise. 
Owen fracture 
zone and the 
Carlsberg Ridge 
West Chile Rise 
and the East 
Pacific Rise. 
San Andreas 
Fault and Men-
docino Fracture 
Mouth of the 
Gulf of California. 
Figure 1.7 Geometry and 
stability requirements of six 
common triple junctions. 
Dashed lines ab, be, and ac 
in the velocity triangles join 
points, the vector sum of 
which leave the geometry 
of AB, BC, and AC, 
respectively, unchanged. 
The junctions are stable 
only if ab, be, and ac meet 
at a point. Key: track 
symbol, trench; double line, 
ocean ridge; single line, 
transform fault. 
relation exists between the length of seismic zones and 
the product of plate convergence rate and age of the 
downgoing slab. 
First-motion studies of earthquakes in subduction zones 
indicate variation in movement both with lateral dis-
tance along descending slabs and with slab depth. Sea-
ward from the trench in the upper part of the lithosphere 
where the plate begins to bend, shallow extensional 
mechanisms predominate. Because of their low strength, 
sediments in oceanic trenches cannot transmit stresses, 
and hence are usually flat-lying and undeformed. Seis-
mic reflection profiles indicate, however, that rocks on 
the landward side of trenches are intensely folded and 
faulted. Thrusting mechanisms dominate at shallow 
depths in subduction zones (20-100 km). At depths < 25 
km, descending slabs are characterized by low seismicity. 
Large-magnitude earthquakes are generally thrust-types 
and occur at depths > 30 km (Shimamoto, 1985). Dur-
ing large earthquakes, ruptures branch off the slab and 
extend upwards forming thrusts that dip away from the 
trench axis. Calculations of stress distributions at < 300 
km depth show that compressional stresses generally 
dominate in the upper parts of descending slabs, whereas 
tensional stresses are more important in the central and 
lower parts (Figure 1.9). At 300-350 km depth in many 
slabs compressional stresses are very small, whereas at 
depths > 400 km a region of compressional stress may 
be bounded both below and above by tensional stress 
The seismicity in descending slabs is strongly cor-
related with the degree of coupling between the slab 
and the overriding plate (Shimamoto, 1985). The low 
seismicity in descending slabs at depths < 25 km may 
reflect relatively high water contents and the low strength 
of subducted hydrous minerals, both of which lead to 
decoupling of the plates and largely ductile deformation. 
At greater depths, diminishing water and hydrous min-
eral contents (due to slab devolatilization) result in greater 
10 Plate Tectonics and Crustal Evolution 
^ 200 
i 300 
H 400 
§ 500 
T V 
t * 
Aleutian Islands 
- (I74'W ITT^W) -
1 1 1 1 1 
T V 
• * 
- Northern ** • 
.:. . 
_ L _l 1 i» 
T V 
! » " • 1 1 1 I 1 1 
'^W^, 1 
Vl*. 1 
Southern :''..*•*•-•• . - • I 
. Izu-Bonin Arc J 
c 1 
. J 1 1 1 1 1 1 
J 300 
X 4 0 0 
UJ 500 
6 0 0 
7 0 0 
100 200 
^ i . M I 
0 100 
S T A 
200 300 
N C E 
Northern Izu-Bonin Arc 
0 100 200 
( knn ) 
300 400 500 600 
t->:- New Hebrides 
100 200 300 400 500 600 0 
D I S T A N C 
100 200 300 400 500 600 700 
E (km) 
Figure 1.8 Vertical cross-sections of 
hypocentre distributions beneath 
modem arc-trench systems. Each 
diagram shows earthquakes for 7-10 
year periods between 1954 and 1969. 
T = trench axis; V = recently active 
volcanic chain. Distance is measured 
horizontally from each trench axis in 
kilometres. Hypocentre data from many 
sources, principally from National 
Earthquake Information Center, US 
Coast and Geodetic Survey. 
^ 400 
Q 600 
V T 
A • 
1 1 \ ^ ^ 1 
/&jtm^^ j^/S^^ 1 
/ y/W^ 
//^f0^ /^t/wW^ 1 
/ y^^r 
/ /^^^ 
^ ^ 1 
n i l Tension 
O Connpression J 
\ \ \ 1 
300 200 100 0 
100 200 
Figure 1.9 Calculated distribution of stresses in a 
descending slab. Subduction rate, 8 cm/y and dip = 45 °. 
After Goto et al. (1985). V, volcanic front; T, trench. 
coupling of overriding and descending plates, and thus 
to the onset of major earthquakes.

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