Geoscience Reference
In-Depth Information
Age (Ma)
0
50
100
150
200
RIGID
plate motion
Mechanical boundary layer
Thermal boundary layer
Onset of
instability
Thermal structure of plate
and small-scale convection
approach equilibrium
VISCOUS
Figure 7.10.
A schematic diagram of the oceanic lithosphere, showing the proposed
division of the lithospheric plate. The base of the mechanical boundary layer is the
isotherm chosen to represent the transition between rigid and viscous behaviour.
The base of the thermal boundary layer is another isotherm, chosen to represent
correctly the temperature gradient immediately beneath the base of the rigid plate.
In the upper mantle beneath these boundary layers, the temperature gradient is
approximately adiabatic. At about 60-70 Ma the thermal boundary layer becomes
unstable, and small-scale convection starts to occur. With a mantle heat flow of
about 38 × 10
−3
Wm
−2
the equilibrium thickness of the mechanical boundary layer
is approximately 90 km. (From Parsons and McKenzie (1978).)
oceanic crust and across fracture zones may improve our knowledge of the thermal
structure of the lithosphere.
7.6 Continental heat flow
7.6.1 The mantle contribution to continental heat flow
Continental heat flow is harder to understand than oceanic heat flow and harder
to fit into a general theory of thermal evolution of the continents or of the Earth.
Continental heat-flow values are affected by many factors, including erosion,
deposition, glaciation, the length of time since any tectonic events, local con-
centrations of heat-generating elements in the crust, the presence or absence of
aquifers and the drilling of the hole in which the measurements were made. Nev-
ertheless, it is clear that the measured heat-flow values decrease with increasing
age (Fig. 7.11). This suggests that, like the oceanic lithosphere, the continental
lithosphere is cooling and slowly thickening with time. The mean surface heat
flow for the continents is
65 mW m
−
2
. The mean surface heat flow in non-
reactivated Archaean cratons is 41
∼
11 mW m
−
2
,which is significantly lower
±
17 mW m
−
2
than the mean value of 55
±
for stable Proterozoic crust well away
from Archaean craton boundaries.
That all erosional, depositional, tectonic and magmatic processes occurring
in the continental crust affect the measured surface heat-flow values is shown in
