The platform is a relatively flat tectonic unit that is
less intensely faulted. The extension may be
concentrated on a limited number of faults, but even
those display only moderate throws as compared to the
major fault complexes that delineate the interior basin.
Hence, the platform is characterised by a few broad,
and only slightly tilted, structural traps. Where age
determinations are possible, the fault systems of the
platform area are seen to have been activated at an
early stage of the basin development, but the activity
slowed down or became arrested when the subsidence
accelerated in the inner part of the basin system. The
platform may also be delineated on its basinward side
by a horst.
A platform marginal horst is sometimes developed
at the basinward side of the platform, defining the
transition from the tectonically quiet platform to the
much more heavily faulted sub-platform or the inner
marginal fault complex. The platform marginal horst
is asymmetrical in outline in that its platform-facing
margin is defined by a few faults with moderate
throws, whereas its basinward border consists of a
complex system of faults with great throws, sometimes in the order of several 100 m. The fact that the
marginal platform horst is such a common feature in
many mature extensional basins suggests that it has an
important mechanical significance, the position of its
distal (antithetic) fault perhaps being determined by
the mechanical strength of the platform rocks.
The sub-platform is delineated by the marginal
platform high on its distal side and the interior graben
on the other. The inner marginal fault system that
b
Sub-platform
Internal graben
a
Deep
planar faults?
Deep
planar faults?
Limit of intense
faulting during
extension
Marginal
platform
high
Internal platform
fault complex
Extra-marginal
fault complex
Outer master
fault system
Axis of rotation
and area of faulting
during cooling stage
Interior
graben
Graben
axis
Inner master
fault system
Platform
Fig. 12.10 (a) Principal sketch of major structural elements in
graben systems. (b) Graben margin with its main structural
elements as seen in three dimensions. Note the change of
configuration as seen along the strike of the margin. Redrawn
from Gabrielsen (1986)
12 The Structure and Hydrocarbon Traps of Sedimentary Basins
331
less intensely faulted. The extension may be
concentrated on a limited number of faults, but even
those display only moderate throws as compared to the
major fault complexes that delineate the interior basin.
Hence, the platform is characterised by a few broad,
and only slightly tilted, structural traps. Where age
determinations are possible, the fault systems of the
platform area are seen to have been activated at an
early stage of the basin development, but the activity
slowed down or became arrested when the subsidence
accelerated in the inner part of the basin system. The
platform may also be delineated on its basinward side
by a horst.
A platform marginal horst is sometimes developed
at the basinward side of the platform, defining the
transition from the tectonically quiet platform to the
much more heavily faulted sub-platform or the inner
marginal fault complex. The platform marginal horst
is asymmetrical in outline in that its platform-facing
margin is defined by a few faults with moderate
throws, whereas its basinward border consists of a
complex system of faults with great throws, sometimes in the order of several 100 m. The fact that the
marginal platform horst is such a common feature in
many mature extensional basins suggests that it has an
important mechanical significance, the position of its
distal (antithetic) fault perhaps being determined by
the mechanical strength of the platform rocks.
The sub-platform is delineated by the marginal
platform high on its distal side and the interior graben
on the other. The inner marginal fault system that
b
Sub-platform
Internal graben
a
Deep
planar faults?
Deep
planar faults?
Limit of intense
faulting during
extension
Marginal
platform
high
Internal platform
fault complex
Extra-marginal
fault complex
Outer master
fault system
Axis of rotation
and area of faulting
during cooling stage
Interior
graben
Graben
axis
Inner master
fault system
Platform
Fig. 12.10 (a) Principal sketch of major structural elements in
graben systems. (b) Graben margin with its main structural
elements as seen in three dimensions. Note the change of
configuration as seen along the strike of the margin. Redrawn
from Gabrielsen (1986)
12 The Structure and Hydrocarbon Traps of Sedimentary Basins
331