Page 264 - Physical Principles of Sedimentary Basin Analysis

246 Subsidence

Solution: (a) The tectonic subsidence becomes

m − (φ)
w t = w 0 + S
m − w

m − s
= w 0 + eζ 0 − ζ 0 . (7.179)
m − w
(b) The tectonic subsidence deceases with
w t = (e 2 − e 1 )ζ 0 (7.180)
when the water depth is constant and the void ratio decreases from e 1 to e 2 .Thewater
depth must therefore increase with (e 1 − e 2 )ζ 0 in order for the tectonic subsidence to be
constant. (See Exercise 7.1.)

7.15 Subsidence of the Vøring margin, NE Atlantic
In this section we will apply some of the models we have been through so far and compute
the subsidence history of a profile across the Vøring margin (west of mid-Norway). The
Vøring profile is shown in Figure 7.32a, and it spans the distance from the continental–
oceanic boundary to mainland Norway. The direction of extension has been along the
profile. There are four sub-basins, Hel Graben, Naagrind syncline, Træna basin and Hel-
geland basin, from the west to the east. The Vøring margin has developed by regional
subsidence from several episodes of lithospheric extension and crustal thinning, since the
end of the Caledonian orogeny. It began with a post-Caledonian extension, which con-
trolled the formation and sedimentation of Devonian sedimentary basins. Then followed
a Permian extension, a Late Jurassic extension and finally a Late Cretaceous to Paleocene
rift phase, which led to continental break-up and the formation of the North Atlantic ocean.
We use the following four rift phases in the modeling of the Vøring margin:
Devonian -400 Ma to -360 Ma
Permian -310 Ma to -260 Ma
Late Jurassic -160 Ma to -140 Ma
Late Cretaceous and Paleocene -80 Ma to -56 Ma

in line with earlier work (see for instance Lundin and Doré, 1997, Mjelde et al., 1997, Doré
et al., 1999, Reemst and Cloetingh, 2000, Ren et al., 2003, Gernigon et al., 2006, Wangen
et al., 2007). We notice from Figure 7.32a that the Paleozoic, Triassic and Jurassic are
present across the entire profile, and that thick Cretaceous formations are deposited mainly
to the west of Nordland ridge. The upper part of the basin has Pliocene and Pleistocene
sediments that covers most of the profile.
Figure 7.32a shows the crust at the present time, which also includes a lower crustal body
(LCB). The origin of the LCB is debated – it has been taken to be magmatic underplating
during the time of continental break-up, alternatively it could be Caledonian crust, or a
mixture of magmatic underplating and Caledonian crust. See Gernigon et al. (2003)for