Geoscience Reference
In-Depth Information
troughs formed on the former rift flanks. The model suggested that the shortening of crust
and sediments is accommodated by shortening also in the lithospheric mantle in the vicinity
of the rift, where the upper mantle is slightly warmer and therefore weaker. The occurrence
of the asymmetrical marginal troughs was explained in this model by flexural loading of
the lithosphere by the internal lithospheric load (as compared to the pre-inversion situation)
that formed along the inversion axis because of thickening of the crust in the inversion zone
and the replacement of less consolidated near-surface sediments with deeper and denser
sediments. The most compelling proof for the existence of this lithospheric load along
the inversion axis is the longevity of the asymmetric sedimentary depocentres flanking the
European inversion structures (e.g.,
Figure 10.3
)
. In some cases the axial load is also visible
in a slightly elevated Bouguer gravity anomaly along the inversion trend (Wybraniec
et al
.,
Nielsen and Hansen's (2000) model also predicts the occurrence of “secondary inver-
sion” (
Figure 10.5
)
. The mechanism here is that the compression during the primary
inversion phase over-deepens the lithospheric flexure that is produced by the axial load of
the inversion structure. During the convergence phase the lithosphere is relatively stiffer
because the ongoing straining works against the continuous viscous relaxation of the
stresses that are generated. When compressional stresses cease or decrease, the lithosphere
performs a vertical, elastic flexural adjustment to the new boundary condition (not dis-
similar to the mechanism described in Braun and Beaumont [1989]) in the form of an
upward doming (order of magnitude 10
2
m) of the central inversion zone and the proximal
areas of the primary marginal troughs, and a flexural down warp of smaller amplitude at a
greater distance. The undulation continues beyond the elastic flexural adjustment, driven
by viscous relaxation of stresses in the softer parts of the lithosphere and the associated
regional isostatic adjustment.
10.4 Discussion
The computed stresses described in Section 10.2 and illustrated in
Figure 10.2
are those
derived from models of lateral variations in the present-day density structure of the litho-
sphere and lateral pressure variations in the mantle below the lithosphere due to density
contrasts and related convection (i.e., variations of potential energy of the lithosphere and
basal pressure). We will refer to these stresses as the “potential energy” stresses in the
ensuing discussion.
Figure 10.2
b
shows that these, expressed in terms of principal stress
orientation, are largely incompatible with observed principal stresses, where available (e.g.,
is along or close to active plate boundaries, as exemplified by the Tethyan convergence
zone from the Mediterranean through central Asia.
This incompatibility demonstrates quite succinctly that interplate deformation (as
expressed here as seismicity near a plate boundary) is driven by stresses heavily domi-
nated by forces developed by plate interactions at plate boundaries. However, away from
active plate boundary zones,
Figure 10.2
b
shows that there is generally a good fit between
