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In-Depth Information
The differences between the predictions of these models are small, but their
values for mantle temperature and plate thickness are rather different (Table 7.4).
The variability in the depth and heat-flow data resulting from hotspot proximity,
mantle thermal structure and hydrothermal circulation means that it is not possible
to establish an unequivocal global thermal model that can simultaneously account
for all the depth and heat-flow data at every age. The variations of depth and heat
flow with age for the half-space, PSM and GDH1 models are summarized in
Table 7.5.
Thermal structure of the oceanic lithosphere
Both plate and boundary-layer models of the lithosphere provide heat-flow values
that are in reasonable agreement with the measured values, but the ocean depths
predicted by the plate model and boundary-layer models differ, with the plate-
model predictions being overall in much better agreement with observed ocean
depths. Other geophysical evidence on the structure of the oceanic lithosphere
also shows that the oceanic lithosphere thickens with age, but they cannot dis-
tinguish amongst the thermal models (Fig. 5.17). The effective elastic thickness
(determined from studies of loading and a measure of the long-term strength of
the lithosphere) increases with age approximately as the 400-
◦
C isotherm. The
maximum focal depth of intraplate earthquakes (a measure of the short-term
strength of the lithosphere) increases with age approximately as the 600-700-
◦
C
isotherm. Results of surface-wave-dispersion studies show that the depth to the
low-velocity zone (the top of the asthenosphere) also deepens with age with
plate-model isotherms. However, while all these parameters clearly increase with
lithospheric age and broadly follow isotherms for the plate models, they are not
well enough determined to allow one to distinguish amongst the various thermal
models.
The observations could be reconciled with the boundary-layer model if some
mechanism to slow the cooling of the boundary layer model for ages greater than
∼
70 Ma were found, so that it would resemble the plate model. Two mechanisms
for maintaining the heat flux at the base of the lithosphere have been proposed:
shear-stress heating caused by a differential motion between lithosphere and
asthenosphere; and an increasing rate of heat production in the upper mantle.
These mechanisms are both somewhat unlikely; perhaps a better proposal is
that small-scale convection occurs in the asthenosphere at the base of the older
lithosphere. This would increase the heat flux into the base of the rigid lithosphere
and maintain a more constant lithospheric thickness.
The lithospheric plate is thought to consist of two parts: an upper rigid layer and
alower viscous thermal boundary layer (Fig. 7.10). At about 60 Ma this thermal
boundary layer becomes unstable; hence small-scale convection develops within
it (see Section 8.2), resulting in an increase in heat flow to the base of the rigid
layer and a thermal structure similar to that predicted by the plate model. Very
detailed, accurate measurements of heat flow, bathymetry and the geoid on old
