Page 143 - Global Tectonics

OCEAN RIDGES 129

ridge, it becomes removed from underlying heat These two models, for the cooling and contraction
sources and cools. This cooling has two effects. First, of oceanic lithosphere with age, are referred to as the
the lithosphere contracts and increases in density. half space and plate models respectively. In the former

Second, because the lithosphere–asthenosphere bound- the lithosphere cools indefinitely, whereas in the latter
ary is controlled by temperature (Section 2.12), the it ultimately attains an equilibrium situation deter-
cooling causes the lithosphere to increase in thickness mined by the temperature at the lithosphere–
away from the mid-ocean ridge. This latter phenom- asthenosphere boundary and the depth at which this

enon has been confirmed by lithosphere thickness esti- occurs as a result of convection in the asthenosphere.
mates derived from surface wave dispersion studies in Clearly the main constraints on these models are the

the Pacific Ocean, which indicate that the thickness observed depth (corrected for sediment loading) and
increases from only a few kilometers at the ridge crest heat flux at the ocean floor as a function of age. Stein

to 30 km at 5 Ma age and 100 km at 50 Ma (Forsyth, & Stein (1992), using a large global data set of depth

1977). and heat flow measurements, derived a model (GDH1

The cooling and contraction of the lithosphere – global depth and heat flow model 1) that gave the
cause a progressive increase in the depth to the top best fit to the observations. Any such model must

of the lithosphere away from the ridge (Sclater & make assumptions about the depth to the ridge crest

Francheteau, 1970), accompanied by a decrease in heat and the thermal expansion coefficient, the thermal

flow. It follows that the width of a ridge depends upon conductivity, the specific heat, and the density of the

the spreading rate, and so provides an explanation for lithosphere. However Stein & Stein (1992) showed
the relative widths of the rapidly spreading East Pacifi c that the crucial parameters in determining the best fi t
Rise and more slowly spreading Mid-Atlantic Ridge. to the data are the limiting plate thickness and the
Parsons & Sclater (1977) determined the nature of the temperature at the base of the lithospheric plate. In
age–depth relationships of oceanic lithosphere, and the GDH1 model these have the values 95 km and
suggested that the depth d (meters) is related to age t 1450°C respectively.
(Ma) by: A comparison of the age–depth relationship pre-
dicted by the half space model, the Parsons, Sclater &
d = 2500 + 350t 1/2 McKenzie model and GDH1, is shown in Fig. 6.9a and
the depth–age equations for GDH1 are:
It was found, however, that this relationship only
1/2
holds for oceanic lithosphere younger than 70 Ma. For d = 2600 + 365t for t < 20 Ma
older lithosphere the relationship indicates a more and d = 5650 − 2473exp(−t/36) for t > 20 Ma.
gradual increase of depth with age. In order to
explain this, Parsons & McKenzie (1978) suggested a
model in which the cooling layer comprises two units
rather than the single unit implied by Parsons & 6.5 HEAT FLOW AND
Sclater (1977). In this model the upper unit, through
which heat moves by conduction, is mechanically
rigid, and the lower unit is a viscous thermal bound- HYDROTHERMAL
ary layer. As the lithosphere travels away from a
spreading center, both units thicken and provide the CIRCULATION
relationship – depth proportional to the square root
of age – described above. However, the lower unit
eventually thickens to the point at which it becomes The half space model of lithospheric cooling with age

unstable and starts to convect. This brings extra heat predicts that the heat flux through the ocean fl oor on
to the base of the upper layer and prevents it thicken- ridge flanks will vary in proportion to the inverse square

ing at the same rate. They suggested that the age– root of its age, but across older ocean fl oor measured

depth relationship for oceanic lithosphere older than heat flow values vary more slowly than this, again favor-
70 Ma is then given by: ing a plate model. The GDH1 model of Stein & Stein
(1992) predicts the following values for heat fl ow,
−2
d = 6400 − 3200exp(−t/62.8) q (mWm ) as a function of age, t (Ma):