Deep controls on intraplate basin inversion - Intraplate Earthquakes - page 269

predicted and observed stress directions where observed stress data are available. There is
even the possibility of a vague correlation between intraplate seismicity (cf. central Europe)
and regions where this good fit is seen, although we have made no attempt to determine
any quantitative significance of this. In any case, it is fair to infer that intraplate stresses in
Europe (that is, away from zones of convergence) are those being generated by potential
energy effects rather than plate boundary effects. And, although there is some seismicity in
such regions, there is no evidence, nor do our results suggest, that geologically significant
intraplate deformation (i.e., basin inversion) is occurring at this time.
These correlations (good fit between modelled and observed stresses at the present day
in intraplate settings and bad fit in tectonic plate boundary settings) in the context of the
geological record of Late Cretaceous-Paleocene basin intraplate inversion in Europe allow
us to make some inferences about cause and effect in intraplate deformation.
First, the potential energy stress field ( Figure 10.2 ) is not in itself sufficient (for typical
continental lithosphere composition, heterogeneity, and thermal structure) to produce geo-
logically significant deformation, as recorded by what Nielsen and Hansen ( 2000 ) called
“primary inversion” of basins in Europe (Figures 10.3 - 10.5 ) . Rather, the main source of
intraplate stress in driving intraplate strain expressed as such is that derived at plate bound-
aries. These stresses are of course superimposed upon the stresses derived from potential
energy variations within the lithosphere. However, primary inversion occurs only when
very high plate boundary forces combine favourably with potential energy “background”
intraplate stresses (e.g., Figure 10.2 a ) in combination with the existence of pre-existing
lithosphere-scale weaknesses that are also favourably orientated. The crustal geometries
that are behind this are to some extent themselves a consequence of the processes that led to
basin formation in the region in the first place. Nevertheless, intraplate primary basin inver-
sion in Europe results not only from stresses building up - mainly from forces developed
at a plate boundary convergent setting - but also depends on finding favourable rift-like
structures amenable to reactivation as well as a favourable interaction (interference) of
those stresses with the intrinsic “background” potential energy stress field.
Second, there is clear evidence to link the dissipation (relaxation) of the extraordinary
stress field that caused primary inversion, derived mainly from plate boundary forces, to
the occurrence of what Nielsen and Hansen ( 2000 ) called “secondary” basin inversion.
This indirectly supports the inference that it was indeed the plate boundary derived stresses
causing the primary inversion in the first place. Nielsen et al .( 2007 ) , following discussion by
Nielsen et al .( 2005 ) , demonstrated that the relaxation of plate convergence derived stresses
was the key factor explaining very precisely dated shifts in deposition patterns within the
Danish Basin in the mid Paleocene (62 Ma). Possible causes for such a sudden plate-wide
stress change suggested by Nielsen et al .( 2005 ) were the Paleocene slowing down of the
African-European convergence (e.g., Rosenbaum et al ., 2002 ) or the creation of a new plate
margin in the North Atlantic (e.g., Harrison et al ., 1999 ) at the time of arrival of the proto
Icelandic plume at the base of the North Atlantic lithosphere during the late Danian, slightly
prior to the Danian-Selandian boundary (

62 Ma). However, the potential energy derived
lithosphere stresses thought to be developed at this time with the postulated arrival of the