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truly dry conditions, the lower continental crust
will have the strength that is not too different
from that of the (dry) upper mantle (Mackwell
et al
., 1998; Rybacki & Dresen, 2004; Bystricky
& Mackwell, 2001). However, the influence of
water is stronger for SiO
2
-rich minerals (or rocks)
(e.g., Chen
et al
., 2006b; Katayama & Karato,
2008a; Post
et al
., 1996) and therefore when the
continental lower crust has some water, its creep
strength will be lowered substantially. I conclude
that the strength contrast at the crust-upper
mantle boundary will vary from one region to
another particularly due to the heterogeneity in
the water content.
the strength in the shallow regions, but if one
uses a simple model of strength profile using the
friction law for the shallow region and the ductile
flow law (corresponding to power-law creep of dry
olivine), then one will have a strong oceanic litho-
sphere with a peak strength exceeding 500MPa
(Kohlstedt
et al
., 1995) (Figure 4.17).
In order for plate tectonics to occur, the strength
of the lithospheremust be smaller than
100MPa
on average (e.g., Richards
et al
., 2001; Tackley,
2000). For a strong oceanic lithosphere such as
shown in Figure 4.17, the style of convection
should be ''stagnant lid convection'' where a near
surface strong layer remains stagnant and con-
vection occurs only in the deep interior of planets
(e.g., Solomatov & Moresi, 1997) (such a style
of convection occurs in most of other terrestrial
planets such as the Moon, Venus, Mars and Mer-
cury (Schubert
et al
., 2001)).
Because plate tectonics occurs on the Earth,
there must be some mechanisms by which the
∼
4.6.2 Upper mantle
(a) How strong is the lithosphere? Or why do
plate tectonics operate on the Earth?
Viscos-
ity of minerals and rocks depends strongly on
temperature and therefore viscosity is high in re-
gions near the surface. Brittle fractures reduce
Differential Stress (MPa)
400
Differential Stress (MPa)
600
0
100
200
300
500
600
0
200
400
800
0
0
0
0
Diabase
Quartz
10
10
200
200
OCEANIC
LITHOSPHERE
20
20
400
400
CONTINENTAL LITHOSPHERE
30
30
Olivine
600
600
K & T
L & P
G & T
Olivine
40
40
•
=
10
−
15
s
−
1
Wet Rheologies
ε
•
=
10
−
15
s
−
1
Dry Rheology
50
50
ε
800
800
60
60
(a)
(b)
Fig. 4.17
The strength profile of the lithosphere (after Kohlstedt
et al
., 1995). (a) oceanic lithosphere; (b) continental
lithosphere. In the shallow region, brittle failure controls the strength, whereas in the deep region, plastic flow
controls the strength. Kohlstedt
et al
. assumed dry (water-free) oceanic lithosphere and wet (water-saturated)
continental lithosphere.
