Although perhaps more rare than in extensional
regimes, source rocks may be involved in orogens,
such as in the Canadian Cordillera. Detailed studies
here performed by Hardebol and his co-workers reveal
a complex maturation history, where both tectonic and
sedimentary burial have to be taken into account.
Although some mountain chains may reveal surprisingly homogeneous bulk geothermal gradient patterns,
the individual tectonic units may have undergone
contrasting histories of burial and uplift, making a
full tectonic restoration necessary in the evaluation
of the hydrocarbon maturation of the system
(Fig. 12.14).
12.6 Structural Inversion
By the term “structural inversion”, or simply “inversion”, we generally mean a system of extensional
structures that has subsequently undergone contraction. This implies that the principal axes of stress
have been changed from being orientated such that
σ 1 > σ 2 > σ 3
¼
σ v > σ h max > σ h min
¼
ρgz > σ h max > σ h min
switches to
σ h max > ρgz > σ h min
This implies that the dip of the plane of maximum
shear will switch from 60
to 30
(Fig. 12.15). Thus,
although an established zone of weakness will represent a potential zone of reactivation when structural
inversion occurs, it is unlikely that the already
established faults will be able to accommodate much
strain, meaning that new faults with lower angles of
inclination will be initiated. The most common
characteristics of an inverted system are:
• Reverse reactivation of (extensional) faults
• Generation of new, low-angle fault traces
• Development of secondary contractional structures
(folds, reverse faults, thrusts)
• Uplift of basin margins
• Uplift of central parts of basins.
The most common configuration at an early stage
of inversion of a fault is shown in Fig. 12.15a. In this
case, the accommodation space generated in the
hangingwall during extension is completely filled by
sediments. During inversion of the master fault, these
sediments will be squeezed out of their position in the
hangingwall and onto the footwall, accompanied by
uplift and folding. For inversion without any oblique
component, the fold axes will be oriented parallel to
the strike of the extensional fault and, accordingly,
orthogonal to the new σ h max . By continued deformation, the pre-existing fault may be squeezed against the
hangingwall and become steepened as a consequence,
whereas new, low-angle faults generated in the footwall may create local thrusts (Fig. 12.15b).
If one looks at the entire basin, the response on
inversion will depend on the geometry of the basin
and the mechanical properties of the crust and lithosphere. In the case of a basin that has already been
affected by thinning and thermal weakening, the central part may become overdeepened and the basin
shoulders uplifted. In contrast, in the case of a
mechanically strong basin fill, the central basin may
be uplifted, forming an inverted eye-shaped basin
geometry. Alternatively, the basin fill may be folded
and squeezed out of the basin, as described above for
faults.
12.6.1 Hydrocarbon Prospectivity in Basins
with Structural Inversion
From a petroleum exploration point of view, structural
inversion is an effect that comes on top of and
subsequent to the development of a regular extensional basin, and particularly affects the basin
margins. Inversion structures may provide additional
structural traps as very well exemplified in the midNorwegian margin by the Ormen Lange and Helland
Hansen structures. On the other hand, inversion
invokes an additional risk for breaking of the seal
and leakage though reactivated faults. Finally,
12 The Structure and Hydrocarbon Traps of Sedimentary Basins
343
regimes, source rocks may be involved in orogens,
such as in the Canadian Cordillera. Detailed studies
here performed by Hardebol and his co-workers reveal
a complex maturation history, where both tectonic and
sedimentary burial have to be taken into account.
Although some mountain chains may reveal surprisingly homogeneous bulk geothermal gradient patterns,
the individual tectonic units may have undergone
contrasting histories of burial and uplift, making a
full tectonic restoration necessary in the evaluation
of the hydrocarbon maturation of the system
(Fig. 12.14).
12.6 Structural Inversion
By the term “structural inversion”, or simply “inversion”, we generally mean a system of extensional
structures that has subsequently undergone contraction. This implies that the principal axes of stress
have been changed from being orientated such that
σ 1 > σ 2 > σ 3
¼
σ v > σ h max > σ h min
¼
ρgz > σ h max > σ h min
switches to
σ h max > ρgz > σ h min
This implies that the dip of the plane of maximum
shear will switch from 60
to 30
(Fig. 12.15). Thus,
although an established zone of weakness will represent a potential zone of reactivation when structural
inversion occurs, it is unlikely that the already
established faults will be able to accommodate much
strain, meaning that new faults with lower angles of
inclination will be initiated. The most common
characteristics of an inverted system are:
• Reverse reactivation of (extensional) faults
• Generation of new, low-angle fault traces
• Development of secondary contractional structures
(folds, reverse faults, thrusts)
• Uplift of basin margins
• Uplift of central parts of basins.
The most common configuration at an early stage
of inversion of a fault is shown in Fig. 12.15a. In this
case, the accommodation space generated in the
hangingwall during extension is completely filled by
sediments. During inversion of the master fault, these
sediments will be squeezed out of their position in the
hangingwall and onto the footwall, accompanied by
uplift and folding. For inversion without any oblique
component, the fold axes will be oriented parallel to
the strike of the extensional fault and, accordingly,
orthogonal to the new σ h max . By continued deformation, the pre-existing fault may be squeezed against the
hangingwall and become steepened as a consequence,
whereas new, low-angle faults generated in the footwall may create local thrusts (Fig. 12.15b).
If one looks at the entire basin, the response on
inversion will depend on the geometry of the basin
and the mechanical properties of the crust and lithosphere. In the case of a basin that has already been
affected by thinning and thermal weakening, the central part may become overdeepened and the basin
shoulders uplifted. In contrast, in the case of a
mechanically strong basin fill, the central basin may
be uplifted, forming an inverted eye-shaped basin
geometry. Alternatively, the basin fill may be folded
and squeezed out of the basin, as described above for
faults.
12.6.1 Hydrocarbon Prospectivity in Basins
with Structural Inversion
From a petroleum exploration point of view, structural
inversion is an effect that comes on top of and
subsequent to the development of a regular extensional basin, and particularly affects the basin
margins. Inversion structures may provide additional
structural traps as very well exemplified in the midNorwegian margin by the Ormen Lange and Helland
Hansen structures. On the other hand, inversion
invokes an additional risk for breaking of the seal
and leakage though reactivated faults. Finally,
12 The Structure and Hydrocarbon Traps of Sedimentary Basins
343