Chapter 1 - Plate Tectonics

by rapid, complex motions. Examples are 
the Turkish-Aegean, Adriatic, Arabian, and Iran plates 
located along the Eurasian-African continent-continent 
collision boundary, and several small plates along the 
continent-arc collision border of the Australian-Pacific 
plates. The motions of small plates are controlled largely 
by the compressive forces of larger plates. 
Continental margins are of two types: active and pas-
sive. An active continental margin is found where 
Plate tectonics 
FAST 
(>45cm/a) 
(EPR 3«S) 
0 10 
DISTANCE (km) 
Figure 1.5 Axial 
topographic profiles across 
three ocean ridges. EPR, 
East Pacific rise; MAR, 
Mid-Atlantic ridge. V and F 
indicate widths of zones of 
active volcanism and 
faulting, respectively. After 
Macdonald (1982). 
either a subduction zone or a transform fault coincides 
with continent-ocean interface. Examples are the An-
dean and Japan continental-margin arc systems and the 
San Andreas transform fault in California. Passive con-
tinental margins occur along the edges of opening ocean 
basins like the Atlantic basin. These margins are char-
acterized by minimal tectonic and igneous activity. 
Divergent boundaries (ocean ridges) 
The interconnected ocean-ridge system is the longest 
topographic feature on the Earth's surface, exceeding 
70 000 km in length. Typical ocean ridges are 3000-
4000 km wide, with up to several kilometres of relief 
in the axial rift zone. Ocean ridges are characterized 
by shallow earthquakes limited to axial rift zones. These 
earthquakes are generally small in magnitude, commonly 
occur in swarms and appear to be associated with intru-
sion and extrusion of basaltic magmas. First-motion stud-
ies indicate that rift earthquakes are produced dominantly 
by vertical faulting as is expected if new lithosphere is 
being injected upwards. Most faulting occurs in the depth 
range of 2-8 km and some ruptures extend to the sea 
floor. 
The median valley of ocean ridges varies in geologi-
cal character due to the changing importance of tectonic 
extension and volcanism. In the northern part of the 
Mid-Atlantic ridge, stretching and thinning of the crust 
dominate in one section, while volcanism dominates in 
another. Where tectonic thinning is important, faulting 
has exposed gabbros and serpentinites from deeper crustal 
levels. Volcanic features range from large ridges (> 50 
km long) in sections of the median valley where 
volcanism has dominated, to small volcanic cones in 
sections dominated by extension. The axial topography 
of fast- and slow-spreading ridges varies considerably. 
A deep axial valley with flanking mountains character-
izes slow-spreading ridges, while relatively low relief, 
and in some instances a topographic high, characterize 
fast-spreading ridges (Figure 1.5). Model studies sug-
gest that differences in horizontal stresses in the oceanic 
lithosphere may account for the relationship between 
ridge topography and spreading rate (Morgan et al., 
1987). As oceanic lithosphere thickens with distance from 
a ridge axis, horizontal extensional stresses can produce 
the axial topography found on slow-spreading ridges. In 
fast-spreading ridges, however, the calculated stresses 
are too small to result in appreciable relief. The axis of 
ocean ridges is not continuous, but may be offset by 
several tens to hundreds of kilometres by transform faults 
(Figure 1.1/Plate 1). Evidence suggests that ocean ridges 
grow and die out by lateral propagation. Offset mag-
netic anomalies and bathymetry consistent with propa-
gating rifts, with and without transform faults, have been 
described along the Galapagos ridge and in the Juan de 
Fuca plate (Hey et al., 1980). 
Transform faults and fracture zones 
Transform faults are plate boundaries along which plates 
slide past each other and plate surface is conserved. They 
uniquely define the direction of motion between two 
bounding plates. Ocean-floor transform faults differ from 
transcurrent faults in that the sense of motion relative 
to offset along an ocean ridge axis is opposite to that 
predicted by transcurrent motion (Wilson, 1965) (Figure 
1.6). These offsets may have developed at the time 
spreading began and reflect inhomogeneous fracturing 
of the lithosphere. Transform faults, like ocean ridges, 
are characterized by shallow earthquakes (< 50 km deep). 
Both geophysical and petrological data from ophiolites 
cut by transforms suggest that most oceanic transforms 
are ieaky', in that magma is injected along fault sur-
faces producing strips of new lithosphere (Garfunkel, 
1986). Transforms cross continental or oceanic crust and 
may show apparent lateral displacements of many hun-
dreds of kilometres. First-motion studies of oceanic trans-
form faults indicate lateral motion in a direction away 
from ocean ridges (Figure 1.6). Also, as predicted by 
seafloor spreading, earthquakes are restricted to areas 
8 Plate Tectonics and Crustal Evolution 
pH - • 
* ^ - • 
TRANSFORM 
H 1 Aseismic ^ Extension 
* ^ | | - ^ 
^ 
- -* 
TRANSCURRENT 
1 ^ 
— 
Figure 1.6 Motion on transform and transcurrent faults 
relative to an ocean ridge axis (double vertical lines). Note 
that the amount of offset increases with transcurrent motion, 
while it remains constant with transform motion. Bold 
arrows refer to spreading directions, small arrows to plate 
motions. 
between offset ridge axes. Transform faults may pro-
duce large structural discontinuities on the sea floor, and 
in some cases structural and topographic breaks known 
as fracture zones mark the locations of former ridge-
ridge transforms on the sea floor. There are three types 
of transform faults: ridge-ridge, ridge-trench, and trench-
trench faults. Ridge-ridge transform faults are most 
common, and these may retain a constant length as a 
function of time for symmetrical spreading, whereas 
ridge-trcnch and trench-trench transforms decrease or 
increase in length as they evolve. 
Studies of oceanic transform-fault topography and 
structure indicate that zones of maximum displacement 
are very localized (< 1 km wide) and are characterized 
by an anatomizing network of faults. Steep transform 
valley walls are composed of inward-facing scarps asso-
ciated with normal faulting. Large continental transform 
faults form where pieces of continental lithosphere are 
squeezed within intracontinental convergence zones such 
as the Anatolian fault in Turkey. Large earthquakes 
(M > 8) separated by long periods of quiescence occur 
along 'locked' segments of continental transforms, 
whereas intermediate-magnitude earthquakes character-
ize fault segments in which episodic slippage releases 
stresses. Large earthquakes along continental transforms 
appear to have a period of about 150 years, as indicated 
by records from the San Andreas fault in California. 
Ridge segments between oceanic transforms behave 
independently of each other. This may be caused by 
instability in the convective upcurrents that feed ocean 
ridges, causing these upcurrents to segment into regularly-
spaced rising diapirs, with each diapir feeding a differ-
ent ridge segment. Transforms may arise at the junctions 
of ridge segments because magma supply between diapirs 
is inadequate for normal oceanic crustal accretion. The 
persistence" of transforms over millions of years indi-
cates that asthenospheric diapirs retain their integrity for 
long periods of time. It appears from the use of fixed 
hotspot models of absolute plate motion that both ridge 
axes and transforms migrate together at a rate of a few 
centimetres per year. This, in turn, requires that mantle 
diapirs migrate, and suggests that ocean-ridge segments 
and diapirs are decoupled from underlying mantle flow. 
Triple junctions 
Triple junctions are points where three plates