Deep controls on intraplate basin inversion - Intraplate Earthquakes - page 263

The principal horizontal stresses are the vertically averaged deviatoric stresses relative
to the lithostatic stress state, in which stresses (or pressure) at any depth are equal in any
direction and equal to the weight of the overburden. To obtain the associated tectonic forces
(N/m), the stresses should be multiplied by the thickness of the elastic shell, which is
100 km. The principal horizontal stress directions are compared to observed values (where
available, from the Global Stress Map; Heidbach et al . [2007a, b], spatially averaged and
extrapolated) and seismicity since 1973 in Figure 10.2 b .
In the present context we wish to highlight the relationship between the stress field
of oceans and that of the adjacent continental areas. Mature oceanic areas such as the
central Atlantic Ocean clearly demonstrate the existence of ridge push in the form of
relative compression of the older and deep parts of the oceanic lithosphere. Ocean ridges
exhibit relative extension perpendicular to the spreading axis, but in the North Atlantic the
Icelandic melt anomaly and the associated anomalous elevation of the ocean floor bear
witness to a much more active spreading system. This anomaly produces a SE-directed
maximum horizontal compressional stress field, which radiates from the Icelandic area
through the British Isles and into central Europe, a model prediction that is in excellent
agreement with observed directions of maximum compressional stress in this region ( Figure
10.2 b ). However, it appears that the effect of the high potential energy and basal pressure
in the North Atlantic around Iceland is not sufficient to place the highlands of southern
Norway under significant compression. Rather, there is slight extension, which was also
the conclusion of Pascal and Cloetingh ( 2009 ) using a one-dimensional approach.
We note that the conjugate Norwegian and Greenland margins exhibit very different
stress states in the present model. While the Norwegian coastal areas generally are in a
neutral to slightly compressive state of stress, the east Greenland coastal areas are in a state
of relative extension. Apparently, this pattern correlates with the occurrence of extensive
North Atlantic breakup magmatism ( < 62 Ma), which profoundly affected the central East
Greenland oceanic and continental areas, but was far offshore on the continental shelf
of western Norway. The dominant NW-SE direction of the axes of the intrusions of the
British Tertiary Igneous Province (England, 1988 ) delivers further evidence of a correlation
between the present-day potential energy related stress field and Paleocene North Atlantic
magmatism. As dyke emplacement preferentially occurs within planes perpendicular to the
direction of the minimum principal stress, σ 3 (Anderson, 1951 ) , it appears that the predicted
stress field of the British Isles ( Figure 10.2 a ) with NE-SW relative extension has changed
little from the stress field that furnished the overriding control on dyke emplacement in the
Paleocene.
10.3 Past intraplate basin inversion in Europe
10.3.1 Style of Late Cretaceous-Paleocene basin inversion in Europe
Common to the west-central European structures are that the zones of inversion are Late
Paleozoic-Mesozoic sedimentary basins and rifts that formed during the breakup of Pangaea