the principles are similar to those of the pure-shear
model. The simple-shear model is based on
observations in the Basin-and-Range of North America and was formulated by Brian Wernicke in 1981.
The Basin-and-Range basin system displays a particular geometry in that the lithosphere is extended to the
degree that the lower crust, described as a metamorphic core complex, has become uplifted and exposed
in the central part of the basin. The asymmetrical
configuration of the basin particularly influences the
pattern of isostatic response to extension. An important factor is the relative thickness of the upper mantle/
lithosphere. This is because the lower crust commonly
is denser than the upper astenosphere, causing largescale contrasts in differential subsidence and uplift
across the basin. Superimposed on this are more
local isostatic effects, associated with contrasting
thicknesses of layers with different densities and the
topography of the basin.
Since the same tectono-thermal principles that
apply for the pure-shear basin also are valid for
simple-shear basins, the main basin stages and the
conditions for hydrocarbon generation and entrapment
are also the same. Even though the simple-shear model
was inspired by analysis of the Basin-and-Range basin
system it has proved relevant for many other basins
too, suggesting that simple shear is a common component in the formation of basins.
The delamination model can be seen as a combination of the simple- and pure-shear models. In this case
the upper and middle crust extends by simple shear. At
depth, the master fault flattens and merges with the
lower crust, which becomes thinned by pure shear.
The Viking Graben of the northern North Sea
seems to have a configuration that fits the delamination
model (Fig. 12.6). Also in this case, the thermomechanical pure-shear model applies and, with some
modifications, can be used to model the basin
development. However, the delamination model
makes it necessary to take into account an additional
variable parameter, namely that the two parts of the
lithosphere situated above and beneath the delamination surface have undergone different amounts of
extension.
12.3 The Structural Architecture
of Extensional Basins
Comparison of many extensional basins reveals that
they have many architectural elements in common.
These include the position and geometry of the dominant fault systems, the position of the most prominent
terraces or platforms, and the position of structural
highs. This does not imply that all basins are similar
or that all contain the same types of structures. Nevertheless, a systematisation suggested in the following,
is useful in the analysis of extensional basins and their
exploration for hydrocarbons. Before looking in detail
at the exploration leads that are typical for extensional
basins, it is therefore instructive to examine the principal structural building blocks of the extensional
basin (Fig. 12.10a, b).
The extra-marginal fault complex. The distal part
of the basin is separated from the foreland by an
enhanced fault frequency (as compared to the foreland
of the basin) and a set of planar to listric faults commonly arranged in an en echelon geometry and sometimes generating a horst-and-graben-system with a
moderate relief. In some cases dike systems affiliated
with the initial stage of extension are filling in some of
the faults of the extra-marginal fault complex. In many
cases, a horst is seen to separate the extra-marginal
fault complex from the platform, defining a topographic threshold between the foreland and the basin
itself.
a
b
Fig. 12.9 Pattern of rotation of sedimentary units in the (a) syn- and (b) post-rift stages. After Gabrielsen et al. (1995)
330
R.H. Gabrielsen
model. The simple-shear model is based on
observations in the Basin-and-Range of North America and was formulated by Brian Wernicke in 1981.
The Basin-and-Range basin system displays a particular geometry in that the lithosphere is extended to the
degree that the lower crust, described as a metamorphic core complex, has become uplifted and exposed
in the central part of the basin. The asymmetrical
configuration of the basin particularly influences the
pattern of isostatic response to extension. An important factor is the relative thickness of the upper mantle/
lithosphere. This is because the lower crust commonly
is denser than the upper astenosphere, causing largescale contrasts in differential subsidence and uplift
across the basin. Superimposed on this are more
local isostatic effects, associated with contrasting
thicknesses of layers with different densities and the
topography of the basin.
Since the same tectono-thermal principles that
apply for the pure-shear basin also are valid for
simple-shear basins, the main basin stages and the
conditions for hydrocarbon generation and entrapment
are also the same. Even though the simple-shear model
was inspired by analysis of the Basin-and-Range basin
system it has proved relevant for many other basins
too, suggesting that simple shear is a common component in the formation of basins.
The delamination model can be seen as a combination of the simple- and pure-shear models. In this case
the upper and middle crust extends by simple shear. At
depth, the master fault flattens and merges with the
lower crust, which becomes thinned by pure shear.
The Viking Graben of the northern North Sea
seems to have a configuration that fits the delamination
model (Fig. 12.6). Also in this case, the thermomechanical pure-shear model applies and, with some
modifications, can be used to model the basin
development. However, the delamination model
makes it necessary to take into account an additional
variable parameter, namely that the two parts of the
lithosphere situated above and beneath the delamination surface have undergone different amounts of
extension.
12.3 The Structural Architecture
of Extensional Basins
Comparison of many extensional basins reveals that
they have many architectural elements in common.
These include the position and geometry of the dominant fault systems, the position of the most prominent
terraces or platforms, and the position of structural
highs. This does not imply that all basins are similar
or that all contain the same types of structures. Nevertheless, a systematisation suggested in the following,
is useful in the analysis of extensional basins and their
exploration for hydrocarbons. Before looking in detail
at the exploration leads that are typical for extensional
basins, it is therefore instructive to examine the principal structural building blocks of the extensional
basin (Fig. 12.10a, b).
The extra-marginal fault complex. The distal part
of the basin is separated from the foreland by an
enhanced fault frequency (as compared to the foreland
of the basin) and a set of planar to listric faults commonly arranged in an en echelon geometry and sometimes generating a horst-and-graben-system with a
moderate relief. In some cases dike systems affiliated
with the initial stage of extension are filling in some of
the faults of the extra-marginal fault complex. In many
cases, a horst is seen to separate the extra-marginal
fault complex from the platform, defining a topographic threshold between the foreland and the basin
itself.
a
b
Fig. 12.9 Pattern of rotation of sedimentary units in the (a) syn- and (b) post-rift stages. After Gabrielsen et al. (1995)
330
R.H. Gabrielsen