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3
a
Compression
Eroded during primary inversion
Primary marginal trough
In-plane stress
0
-4
3
b
Secondary marginal trough
0
Relaxation
Eroded during secondary inversion
No in-plane stress
-4
200
150
100
Horizontal distance (km)
50
0
Figure 10.5 The primary and secondary inversion mechanisms: (a) basin fill after primary inversion,
which occurs as a response to in-plane compression of a pre-existing rift zone; (b) basin fill after
secondary inversion, which occurs as a response to relaxation of the earlier in-plane compression.
Details of the modelling can be found in Nielsen et al .( 2005 ) .
On the other hand, the thinning of the crust during rifting, and the filling of the rift
with sediments reduce the overall load-carrying capacity of the lithosphere because (1)
less competent sediments have replaced more competent crustal rocks; (2) sediments are
less dense than crustal rocks, reducing the confining pressure and thereby the strength of
the crust below the sediments as compared to crust at the same depth outside the basin; (3)
the crust beneath the sediments is likely to be warmer and therefore weaker than crust at the
same depth outside the basin because of sediment blanketing; and (4) formation of crustal
scale extensional faults that can be reactivated potentially have strong implications for
the possibility of later inversion. Furthermore, the thermal refraction aspect of thermally
equilibrated rift basins can in itself promote a strain energy favourable mode of basin
inversion (Stephenson et al ., 2009 ) .
Numerical modelling allows for investigating the relative importance of such oppositely
directed mechanisms. Thus, assuming that a rift remains weak after its formation because of
the wealth of faults that are produced during rifting, Nielsen and Hansen ( 2000 ) constructed
a numerical thermo-mechanical model of compressional basin inversion that reproduced
the fundamental observational features of inversion structures on the European continent
( Figure 10.5 ) . The model response to compression was localised shortening, thickening, and
uplift of the upper crust and sediments within the rift, while the lower crust and upper mantle
became slightly depressed. Simultaneously, syn-compressional and asymmetric marginal
 
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