Geoscience Reference
In-Depth Information
Figure 8.20. Possible
forces acting on the
lithospheric plates: F DF ,
mantle-drag; F CD , extra
mantle-drag beneath
continents; F RP ,
ridge-push; F TF ,
transform-fault resistance;
F SP , slab-pull; F SR , slab
resistance on the
descending slab as it
penetrates the
asthenosphere; F CR ,
colliding resistance acting
on the two plates with
equal magnitude and
opposite directions; and
F SU ,asuctional force that
may pull the overriding
plate towards the trench.
(From Forsyth and Uyeda
(1975).)
Continental plate
F
Oceanic plate
TF
F DF + F CD
F
SU
F
F
RP
DF
F
CR
F
SP
F
SR
Driving forces
The ridge-push force acts at the mid-ocean ridges on the edges of the plates. It
is made up of two parts: the pushing by the upwelling mantle material and the
tendency of newly formed plate to slide down the sides of the ridge. Of these two,
the sliding contribution is approximately an order of magnitude smaller than the
upwelling contribution.
An estimate of the total ridge-push per unit length of the ridge axis, F RP ,is
F RP = ge ( ρ m ρ w ) L
e
2
3 +
(8.38)
where e is the elevation of ridge axis above the cooled plate,
ρ m the density of the
mantle at the base of the plate,
ρ w the density of sea water and L the plate thickness
(Richter and McKenzie 1978). Equation (8.38)gives F RP as 2
10 12 Nm 1 (N,
×
10 4 m; e ,3
10 3 m;
ρ w ,10 3 kg m 3 ;
newton) for the following values: L , 8.5
×
×
10 3 kg m 3 ; and g , 9.8ms 2 .
The other main driving force is the negative buoyancy of the plate being
subducted at a convergent plate boundary. This arises because the subducting plate
is cooler and therefore more dense than the mantle into which it is descending.
This force is frequently known as slab-pull .Anestimate of the slab-pull force
per unit length of subduction zone, F SP ( z ), acting at depth z and caused by the
density contrast between the cool plate and the mantle is given by
ρ m , 3.3
×
exp
exp
8 g αρ m T 1 L 2 Re t
π
2 z
2 Re t
2 d
2 Re t
π
π
F SP ( z ) =
(8.39)
4
where z is the depth beneath the base of the plate,
α
the coefficient of thermal
expansion, T 1 the temperature of the mantle, d
L the thickness of the upper
mantle and Re t the thermal Reynolds number ,given by
+
ρ m c P vL
2k
Re t =
(8.40)
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