Deep controls on intraplate basin inversion - Intraplate Earthquakes - page 266

of the secondary inversion phase occurred during the late Danian (lasting approximately
from 62 to 61 Ma) when the depocentre shifted away from the inversion ridge to a more
distal position and became more symmetrical. Simultaneously, the inversion ridge and the
proximal areas of the asymmetrical marginal trough experienced a gentle doming, the ero-
sion of which is revealed by the occurrence of Late Cretaceous cocolites in early Selandian
sediments (Clemmensen and Thomsen, 2005 ; Nielsen et al ., 2005 ; Steuerbaut, 1998). It
is apparent from Figure 10.3 how the secondary phase of inversion has exerted control
on the Late Paleocene (and Eocene) deposits along the margins of the more prominent
European inversion structures, although some of the depocentres that formed (e.g., along
the eastern margin of the Mid Polish Swell and the Weald-Boulonnais area) initially were
in a non-marine setting.
10.3.2 Modelling intraplate basin inversion
The typical width of inversion zones is on the order of a couple of lithosphere thicknesses,
i.e., 200-250 km, when the extent of the marginal troughs is included. This order of
magnitude wavelength points to a whole lithosphere involvement. The challenge is that a
quantitative model of basin inversion must address both the relatively narrow localisation
of shortening in the deeper parts of former sedimentary basins, the formation of marginal
troughs, and the change of inversion style seen in the mid Paleocene in Europe.
The process of basin formation modifies the overall thermal and rheological structure
of the lithosphere and it is obvious that this modification somehow is relevant to the later
localisation of structural inversion. One fundamental question is simply why sedimentary
basins are readily reactivated in compression. Although the mere existence of a sedimentary
basin indicates the presence of a structural weakness in the continental lithosphere, it is
not trivial how sedimentary basins can be reactivated a long time after formation, as is the
case in Europe where the inversion structures generally are associated with Paleozoic and
Mesozoic rift systems (Ziegler et al ., 1995 ) .
The thermal and structural changes implied by extensional basin formation involve
processes that both reduce and increase the load-carrying capacity of the lithosphere. For
example, Braun ( 1992 ) , Ziegler et al .( 1995 ) , vanWees and Beekman ( 2000 ) , and Sandiford
et al .( 2003 ) pointed out that the formation of a rift basin elevates and strengthens (over time)
the mantle beneath the basin because of the long-term cooling effected by the shallowing
of the mantle and the attenuation of crustal heat production. This mainly thermal aspect
shouldwork against a later reactivation of the basin centre. Indeed, analysis of the subsurface
temperature field in thermally equilibrated rifts has revealed (Sandiford, 1999 ; Sandiford
et al ., 2003 ; Hansen and Nielsen, 2003 ) that a wide range of plausible values for the
controlling parameters (thermal conductivities, heat production rates and their distribution,
and the basin aspect ratio) result in a cooler upper mantle beneath the rift, while the mantle is
warmer and therefore weaker beneath the margins of the rift. The fundamental mechanism
here is refraction of heat around the relatively poorly conducting sediments and reduced
crustal heat generation where the crust has been thinned.