Page 230 - Physical Principles of Sedimentary Basin Analysis

212 Subsidence

2.0
β = 2
1.8

1.6
q [−]
^ 1.4

1.2

1.0
0.0 0.2 0.4 0.6 0.8 1.0
^ t [−]
Figure 7.12. The scaled surface heat flow after instantaneous stretching with a factor β = 2.

the surface heat flow is a factor β higher than the steady state heat flow. Only 20% is
left of the transient after ˆ t ≈ 0.2, and it has almost died out at ˆ t ∼ 0.5. The half-life of
the series (7.62) was estimated in the previous section to be ˆ t = 0.07, which is in good
agreement with Figure 7.12. The characteristic time for the lithosphere is roughly 470 Ma,
and the time ˆ t ∼ 0.2 is therefore ∼94 Ma. The increased surface heat flow from rapid
stretching of the lithosphere with a factor β = 2 will therefore last up to 100 Ma after a
stretching event.
The surface heat flow (6.50) in the case of radioactive heat generation in the crust is

q s = Sc + q m (7.63)
where S is the heat production per volume of rock, c is the thickness of the crust and q m is
the heat flow from the mantle into the base of the crust. It is now possible to make simple
estimates of what the surface heat flow is right after and a long time after a stretching event,
when relation (7.63) gives the surface heat flow before stretching. The surface heat flow
right after instantaneous stretching becomes
c
q s ≈ S + βq m (7.64)
β
because the heat producing crust is thinned by the β-factor and the mantle heat flow
increases by factor β. A long time after, when the thermal transient has died out, the surface
heat flow becomes
c
q s ≈ S + q m . (7.65)
β
The stationary surface heat flow is therefore reduced by a rift phase because a stretched
and thinned crust produces less heat than the unstretched crust. The loss in heat production
from the crust is often counteracted by heat production in the sediments that fill the basin.