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(a)
(b)
(c)
(d)
(e)
(f )
Fig. 4.103 Various kinematic models for deformations accompanying the development of normal listric faults (see text for explanations).
increase in block subsidence by sliding gives way to flat-
tening of the block as it reaches the subsided area,
whereas bedding or other initially horizontal layering
becomes progressively steeper. The progressive formation
of faults, younger toward the footwall is called back fault-
ing . Finally, a combination of synthetic and antithetic
listric faulting can be produced in the hangingwall, the
adjustment of the holes between the blocks being pro-
vided by ductile deformation or minor fracturing
(Fig. 4.103f).
Stepped faults showing flat and ramp geometries can
develop special deformation structures and involve distinc-
tive kinematics. The hangingwall block deforms over the
steps causing synclines or anticlines if the rocks are ductile
enough (Fig. 4.104). The flanks to ramp- or flat-related
folds formed by bending are areas where shear deforma-
tion increases and are preferred sites for secondary faulting
of the hangingwall block. Ramps change position as
extension progresses by cutting sigmoidal rock slices called
horses from the footwall block. Together all the horses
form a duplex structure bounded in the upper part by a
roof fault and at the bottom by a floor fault . The floor fault
is active (experiencing shear displacements along the sur-
face) as it is part of the main fault, whereas the roof fault
plays a secondary roll, being active only when the corre-
sponding horse forms.
4.15.5
Thrust and reverse faults
Thrust and reverse faults form in tectonic settings in which
a horizontal compression, defining the main principal
stress (
3 ) pro-
vides the vertical load. The main geotectonic settings in
which thrust and reverse faults form are convergent and
collision related plate boundaries. Thrusts and reverse
faults in continental settings form in fold and thrust belts
that can extend hundreds of kilometers. In oceanic envi-
ronments they appear in accretionary wedges or subduc-
tion prisms, between the trench located at the plate
boundary and a magmatic arc in both intra-oceanic and
continental active margins. Thrust faulting results in
crustal shortening and thickening (Fig. 4.100c, d). Thrust
and fold belts are limited in front (defined by the sense
of movement) by an area not affected by faulting, the
foreland , where a subsiding basin can form by tectonic
loading (Section 5.2). The area located at the back of the
thrust belt is the hinterland (Fig. 4.105). Structures in
1 ), is produced and a minor compression (
 
 
 
 
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