Page 268 - Physical Principles of Sedimentary Basin Analysis

250 Subsidence

the Vøring profile. These β-factors shows that the depocenters have undergone the most
stretching. They also show that the Helgeland basin (in the interval 200 km to 300 km)
formed primarily from the Devonian and Permian rift phases. The next two rift phases
(Late Jurassic and the Late Cretaceous) are mainly responsible for the sub-basins to the
west of the Nordland ridge.
It is not straightforward to use relation (7.190) to compute factor β i when the thermal
uplift is important. The average mantle density at the beginning of rift phase i + 1 then
depends on the transient temperature at time t i+1 , which again depends on β i . The only
way to deal with this situation is to model each time interval between the rift phases itera-
tively until we match the tectonic subsidence at time t i+1 . A simple strategy is to try out all
β-factors in the interval from 1 to 5 with a step size 0.2. One can then continue with a finer
search in the interval ±0.2 around the β-factor that gives the best match, and then yet a finer
search, and so on. This scheme is not computationally demanding in 1D, although better
options exist. Figure 7.33a shows the results of the optimization of tectonic subsidence for

C P T J C P E O M P Q
−2
3
0
subsidence [km] 2 4 4

modeled tectonic subsidence (1)
isothermal subsidence (2) 1 2
6
thermal uplift (3)
observed tectonic subsidence (4) (lateral pos 151 km)
8
−400 −300 −200 −100 0
time [Ma]
(a)
C P T J C P E O M PQ
7
6
5
beta [−] 4

3
2
(lateral pos 151 km)
1
−400 −300 −200 −100 0
time [Ma]
(b)

Figure 7.33. (a) The tectonic subsidence of the position 151 km. (b) Lithospheric stretching at position
151 km.