Riedel-shears are synthetic to the major shear (meaning that they are sub-parallel and have similar shearsense), whereas the Riedel’-shears are oriented at a
large angle to the regional orientation of the fault zone
and also display an opposite (antithetic) shear-sense.
In addition to the Riedel- and Riedel’-shears, contractional structures (reverse fault, thrusts and folds) with
their axes oriented 90
to the compressional stress
component may develop. Due to their favourable orientation and synthetic sense of shear, with continued
movement the Riedel-shears tend to become dominant
at the expense of the antithetic Riedel’-shears. In
accordance with the orientation of the tensional
vectors, a set of tensional fractures (T-fractures) may
be initiated with orientation parallel to the compressional component of the system.
By continued displacement there will be a tendency
for both Riedel- and Riedel’-shears to rotate and for
the tips of Riedel-shears to become joined to constitute
a system of linked fractures striking parallel to the
main shear trend. These Y-shears require that some
displacement has taken place and are accordingly not
present at the initial state of shear. Another set of shear
fractures, P-shears, also occurs at a more advanced
stage of development. These are oriented at an angle
of 60
to the contractional component (Fig. 12.13a)
and are accordingly symmetrical with the Riedelshears, with the Y-shear direction as the plane of
symmetry. Dynamically, the P-shears nucleate at the
profound, principal fault trace and develop up-section
to create an array of fractures with an en echelon
geometry.
In total, the interaction between the sets of secondary fractures (R, R’, T, P and Y) contributes to the
complex geometry of the strike-slip fault and
generates an uneven and step-like morphology. The
arrangements of the steps in left-stepping and rightstepping arrays generate contrasting stressconfigurations along the strike of the strike-slip fault,
depending on the relative shear-sense (Fig. 12.13b).
Thus a right-lateral (dextral) shear that affects a rightstepping system of strike-slip fault branches causes
extension in the overlap-zones (ramps or bridges)
between the individual fault branches, whereas a leftstepping system generates overlap zones of contraction for the dextral system. For a left-lateral (sinistral)
shear-sense, the relations are opposite.
This implies that a variety of structures, and hence a
variety of hydrocarbon trap types, are likely to develop
along a strike-slip fault. In the ideal case, where the
fault trace is planar and the movements of the opposing fault blocks are absolutely parallel, the trace would
be one vertical plane. But since this is the case only for
very restricted segments of strike-slip faults, there will
be segments where material is squeezed up and out of
the fault zone, and cases where slivers of the footwall
and hangingwall fall into the fault zone. In both cases
the faults are likely to have a steeply dipping root,
creating diagnostic geometries for strike-slip faults
called (positive and negative) flower-structures
(Fig. 12.13c). In cases where distinct fault segments
overlap, but are not in direct contact, zones with pullapart basins or turtle-back structures will occur,
whereas extensional and contractional duplexes will
develop where the fault-segments are in contact in
zones of releasing or restraining bends.
Movements in shear-zones are in many cases not
entirely parallel, so that a contractional or extensional
component adds to the shear. These situations are
called transpressional and transtensional, respectively,
and contribute to exaggerating the morphological
expressions of the structures described above. In
such cases there is a tendency for forces to decompose
along weak beds in the deforming units so that strain is
taken up in separate systems. Thus a transpressional
stress can be decomposed into a pure contractional
regime and a pure strike-slip regime. The process is
called strain partitioning and is well known for e.g.
the transform delineating the western Barents shelf,
where a dextral transpression is decomposed into collision in the West Spitsbergen Fold- and Thrust Belt
and shear along the Hornsund Fault Zone.
12.4.1 Hydrocarbon Prospectivity
in Strike-Slip Regimes
Large-scale strike-slip systems may be highly
dynamic depositional systems for sediments and also
offer a great variety of structural and sedimentary
traps. However, compared to extensional basins,
there are two significant differences. Firstly, the thermal development is different in that the steep dips and
deep roots of the master faults are likely to cause very
significant and fast thinning. This involves the substratum of the basin down to the level where the faults
detach, which may be top of the lowermost crust or
even the Moho. This implies that the thermal gradient
338
R.H. Gabrielsen
large angle to the regional orientation of the fault zone
and also display an opposite (antithetic) shear-sense.
In addition to the Riedel- and Riedel’-shears, contractional structures (reverse fault, thrusts and folds) with
their axes oriented 90
to the compressional stress
component may develop. Due to their favourable orientation and synthetic sense of shear, with continued
movement the Riedel-shears tend to become dominant
at the expense of the antithetic Riedel’-shears. In
accordance with the orientation of the tensional
vectors, a set of tensional fractures (T-fractures) may
be initiated with orientation parallel to the compressional component of the system.
By continued displacement there will be a tendency
for both Riedel- and Riedel’-shears to rotate and for
the tips of Riedel-shears to become joined to constitute
a system of linked fractures striking parallel to the
main shear trend. These Y-shears require that some
displacement has taken place and are accordingly not
present at the initial state of shear. Another set of shear
fractures, P-shears, also occurs at a more advanced
stage of development. These are oriented at an angle
of 60
to the contractional component (Fig. 12.13a)
and are accordingly symmetrical with the Riedelshears, with the Y-shear direction as the plane of
symmetry. Dynamically, the P-shears nucleate at the
profound, principal fault trace and develop up-section
to create an array of fractures with an en echelon
geometry.
In total, the interaction between the sets of secondary fractures (R, R’, T, P and Y) contributes to the
complex geometry of the strike-slip fault and
generates an uneven and step-like morphology. The
arrangements of the steps in left-stepping and rightstepping arrays generate contrasting stressconfigurations along the strike of the strike-slip fault,
depending on the relative shear-sense (Fig. 12.13b).
Thus a right-lateral (dextral) shear that affects a rightstepping system of strike-slip fault branches causes
extension in the overlap-zones (ramps or bridges)
between the individual fault branches, whereas a leftstepping system generates overlap zones of contraction for the dextral system. For a left-lateral (sinistral)
shear-sense, the relations are opposite.
This implies that a variety of structures, and hence a
variety of hydrocarbon trap types, are likely to develop
along a strike-slip fault. In the ideal case, where the
fault trace is planar and the movements of the opposing fault blocks are absolutely parallel, the trace would
be one vertical plane. But since this is the case only for
very restricted segments of strike-slip faults, there will
be segments where material is squeezed up and out of
the fault zone, and cases where slivers of the footwall
and hangingwall fall into the fault zone. In both cases
the faults are likely to have a steeply dipping root,
creating diagnostic geometries for strike-slip faults
called (positive and negative) flower-structures
(Fig. 12.13c). In cases where distinct fault segments
overlap, but are not in direct contact, zones with pullapart basins or turtle-back structures will occur,
whereas extensional and contractional duplexes will
develop where the fault-segments are in contact in
zones of releasing or restraining bends.
Movements in shear-zones are in many cases not
entirely parallel, so that a contractional or extensional
component adds to the shear. These situations are
called transpressional and transtensional, respectively,
and contribute to exaggerating the morphological
expressions of the structures described above. In
such cases there is a tendency for forces to decompose
along weak beds in the deforming units so that strain is
taken up in separate systems. Thus a transpressional
stress can be decomposed into a pure contractional
regime and a pure strike-slip regime. The process is
called strain partitioning and is well known for e.g.
the transform delineating the western Barents shelf,
where a dextral transpression is decomposed into collision in the West Spitsbergen Fold- and Thrust Belt
and shear along the Hornsund Fault Zone.
12.4.1 Hydrocarbon Prospectivity
in Strike-Slip Regimes
Large-scale strike-slip systems may be highly
dynamic depositional systems for sediments and also
offer a great variety of structural and sedimentary
traps. However, compared to extensional basins,
there are two significant differences. Firstly, the thermal development is different in that the steep dips and
deep roots of the master faults are likely to cause very
significant and fast thinning. This involves the substratum of the basin down to the level where the faults
detach, which may be top of the lowermost crust or
even the Moho. This implies that the thermal gradient
338
R.H. Gabrielsen