may very rapidly steepen during early stages of the
basin formation and that significant leakage of heat
may start before the syn-rift stage is passed. This may
be accompanied by fast burial and maturation (and
perhaps over-maturation) of the source rock, and also
make basin modelling difficult. Secondly, the geometry and tectonic position of strike-slip systems are such
that the likelihood for accumulation of marine source
rocks is less than for extensional basin systems now
situated in passive margin settings.
On the other hand, the structural complexity and
variability may develop structural and stratigraphic
trap types that are not found in extensional basin
systems. Examples are flower-structures and arrays
of anticlines that may be found along the strike-slip
fault at regular intervals.
12.5 Contractional Regimes
When reading the following, it is important to remember that compression characterises the stress system,
whereas contraction describes the physical process
of shortening. Contractional regimes are associated
with large-scale orogenic processes (formation of
mountain chains), but areas of shortening also may
occur on local scales within regional extensional
or strike-slip realms. Compression is characterised
by the principal stresses being oriented so that
σ 1 > σ 2 > σ 3 ¼ σ h max > σ h min > ρgz.
In
other
words, the smallest stress acts in the vertical plane so
that the most energy-efficient way of shortening is by
transferring excess mass towards the surface. Also,
due to the orientation of the principal stresses, the
dip of the plane of maximum shear is 30
, promoting
the development of thrust faults (Fig. 12.2).
Orogens are extremely mobile tectonic zones and
such systems may accommodate displacements that
are several orders of magnitude greater than that
which is typical for extensional basins. This implies
that the pattern of deformation may be very complex
and involve a number of stages or “phases”, each
phase representing a unique set of stress conditions
and p,T-relations. Still, even the most intricate pattern
can be analysed by the utilisation of relatively simple
geometrical methods and modelling techniques.
The major strain in a large-scale contractional system like an orogen is generally associated with plate
margins. In the deep and central parts of the oregen,
deformation takes place under high to extremely high
p,T-conditions. Such settings are not optimal for the
generation and accumulation of petroleum resources
and will therefore not be considered further here.
However, in the upper (shallow) parts of an orogen
as well as along its frontal parts, sedimentation and
structuring take place during p,T-conditions that are
compatible with the generation and accumulation of
hydrocarbons. Here basins related to the interior
development and collapse of the central part of the
orogen will occur, particularly in the later stages of the
mountain building, whereas foreland basins may be
active throughout the entire life of the orogen.
12.5.1 The Architecture of Thrust Systems
The most important building blocks of orogens are
folds and contractional faults. Both these types of
structures affect rocks volumes, so that they may be
constrained from their surroundings by a certain
geometry and intrinsic style of deformation. Thrust
faults tend to climb up-section, because this is the
direction of σ 3 . Still, contractional faults are
characterised by shallow dip and a tendency to flatten
over greater distances, particularly where they follow
beds of low mechanical strength. They are also frequently seen to merge with other faults along horizontal fault strands and surfaces between mechanically
weak beds, and they regularly intimately affiliate
with folds. This is so because the folds and faults
frequently are seen to develop in concert. One example can be initial buckling of a bed followed by a fault
breakthrough along the fold hinge. In other cases folds
can develop in front of an advancing fault (faultpropagation folding). In both these examples, the
folds would be asymmetrical with the longer fold
limb dipping at a shallow angle away from the transport direction, and the shorter fold limb at a steeper
angle. The fold axes would be oriented transverse to
the direction of transport and may constitute structural
traps of considerable magnitude. By continued
shortening the faults may link up, trapping isolated,
lens-shaped rock bodies, which commonly incorporate
folded beds. The lenses are referred to as horses and
where they are stacked between a horizontal floor fault
and a roof fault, they constitute a duplex (Fig. 12.14a).
In cases where the faults propagate systematically
in the direction of the front of the contractional
340
R.H. Gabrielsen
basin formation and that significant leakage of heat
may start before the syn-rift stage is passed. This may
be accompanied by fast burial and maturation (and
perhaps over-maturation) of the source rock, and also
make basin modelling difficult. Secondly, the geometry and tectonic position of strike-slip systems are such
that the likelihood for accumulation of marine source
rocks is less than for extensional basin systems now
situated in passive margin settings.
On the other hand, the structural complexity and
variability may develop structural and stratigraphic
trap types that are not found in extensional basin
systems. Examples are flower-structures and arrays
of anticlines that may be found along the strike-slip
fault at regular intervals.
12.5 Contractional Regimes
When reading the following, it is important to remember that compression characterises the stress system,
whereas contraction describes the physical process
of shortening. Contractional regimes are associated
with large-scale orogenic processes (formation of
mountain chains), but areas of shortening also may
occur on local scales within regional extensional
or strike-slip realms. Compression is characterised
by the principal stresses being oriented so that
σ 1 > σ 2 > σ 3 ¼ σ h max > σ h min > ρgz.
In
other
words, the smallest stress acts in the vertical plane so
that the most energy-efficient way of shortening is by
transferring excess mass towards the surface. Also,
due to the orientation of the principal stresses, the
dip of the plane of maximum shear is 30
, promoting
the development of thrust faults (Fig. 12.2).
Orogens are extremely mobile tectonic zones and
such systems may accommodate displacements that
are several orders of magnitude greater than that
which is typical for extensional basins. This implies
that the pattern of deformation may be very complex
and involve a number of stages or “phases”, each
phase representing a unique set of stress conditions
and p,T-relations. Still, even the most intricate pattern
can be analysed by the utilisation of relatively simple
geometrical methods and modelling techniques.
The major strain in a large-scale contractional system like an orogen is generally associated with plate
margins. In the deep and central parts of the oregen,
deformation takes place under high to extremely high
p,T-conditions. Such settings are not optimal for the
generation and accumulation of petroleum resources
and will therefore not be considered further here.
However, in the upper (shallow) parts of an orogen
as well as along its frontal parts, sedimentation and
structuring take place during p,T-conditions that are
compatible with the generation and accumulation of
hydrocarbons. Here basins related to the interior
development and collapse of the central part of the
orogen will occur, particularly in the later stages of the
mountain building, whereas foreland basins may be
active throughout the entire life of the orogen.
12.5.1 The Architecture of Thrust Systems
The most important building blocks of orogens are
folds and contractional faults. Both these types of
structures affect rocks volumes, so that they may be
constrained from their surroundings by a certain
geometry and intrinsic style of deformation. Thrust
faults tend to climb up-section, because this is the
direction of σ 3 . Still, contractional faults are
characterised by shallow dip and a tendency to flatten
over greater distances, particularly where they follow
beds of low mechanical strength. They are also frequently seen to merge with other faults along horizontal fault strands and surfaces between mechanically
weak beds, and they regularly intimately affiliate
with folds. This is so because the folds and faults
frequently are seen to develop in concert. One example can be initial buckling of a bed followed by a fault
breakthrough along the fold hinge. In other cases folds
can develop in front of an advancing fault (faultpropagation folding). In both these examples, the
folds would be asymmetrical with the longer fold
limb dipping at a shallow angle away from the transport direction, and the shorter fold limb at a steeper
angle. The fold axes would be oriented transverse to
the direction of transport and may constitute structural
traps of considerable magnitude. By continued
shortening the faults may link up, trapping isolated,
lens-shaped rock bodies, which commonly incorporate
folded beds. The lenses are referred to as horses and
where they are stacked between a horizontal floor fault
and a roof fault, they constitute a duplex (Fig. 12.14a).
In cases where the faults propagate systematically
in the direction of the front of the contractional
340
R.H. Gabrielsen