Geoscience Reference
In-Depth Information
Figure 10.4.
Yield-strength envelopes for the oceanic and continental lithosphere in
compression at strain rate 10
−15
s
−1
. Since rocks are considerably stronger in
compression than they are in tension, the yield-strength envelopes for oceanic and
continental lithosphere in tension have a broadly similar shape to those shown here
but with the differential stress reduced by a factor of two to three. (From Kohlstedt
et al
., Strength of the lithosphere: constraints imposed by laboratory measurements,
J. Geophys. Res.
,
100
,17587-602, 1995. Copyright 1995 American Geophysical
Union. Reprinted by permission of American Geophysical Union.)
rate (higher strain rates increase the depth). In contrast to the continental litho-
sphere, the yield-strength envelope for the oceanic crust and uppermost mantle is
simple - strength increases linearly with depth down to
∼
35 km and below this
the lithosphere deforms by solid-state creep. (The yield-strength envelope shown
here is for 60-Ma-old oceanic lithosphere; young oceanic lithosphere is much
weaker - the depth of the maximum in strength is approximately proportional
to the square root of the plate age.) The maximum stress that can be transmitted
by the lithosphere can be estimated by calculating the area under the yield-
strength curves. For the oceanic lithosphere this is about (2-3)
10
13
Nm
−
1
×
10
12
Nm
−
1
when it is under compression, but about 8
×
when it is under
10
13
Nm
−
1
and
tension. For the continental lithosphere the values are
∼
(0.5-2)
×
10
12
Nm
−
1
, respectively, depending upon assumptions of composition
and age. These values are considerably in excess of the values for the plate-driving
and resistive forces (Section 8.2.4), confirming that the plates are indeed strong
enough to transmit such forces without fracturing.
Table 10.2 gives some idea of the worldwide extent of continental crust of
various ages. Only 30% of current basement rocks are younger than 450 Ma; the
remaining 70% are older. Continental growth rates are discussed in Section 10.2.4.
It is immediately apparent from a map of the ages of the continents that the
oldest material tends to concentrate towards the centre of a continent with younger
material around it. These old continent interiors are termed
cratons
(Greek
cratos
,
meaning strength, power or dominion). On the North American continent these
cratons are the stable, flat interior regions (Fig. 10.5). To the east of the Archaean
∼
(1-3)
×
