Geology Reference
In-Depth Information
horsetail splay
pull-apart
basin
releasing bend
principal displacement
zone (PDZ)
restraining bend
step-over
with pull-apart
basin
Explanation
normal separation fault
reverse separation fault
fault with sense of strike-slip
fold
overturned fold
areas of subsidence and
sediment accumulation
rotated
folds
thrusting
and uplift
Extension and Contraction Along
an Irregular Strike-Slip Fault
Fig. 4.19
Regional-scale structures along a strike-slip fault.
These structures can include restraining bends associated with thrusts and mountain building, releasing bends
associated with basin development and rapid subsidence, and horsetail splays of either normal or reverse faults where
deformation is spread over a broader zone. Right-stepping step-overs in a dextral shear zone (as shown here) create
pull-apart basins as the fault tips curve toward the continuing fault trace and generate normal slip. Modified after
Christie-Blick and Biddle (1985).
either side such that extensional structures, like
normal faults and basins, form between them
(Fig. 4.19). Alternatively, strike-slip faults may
display major lateral step-overs, in which one
fault trace ends and a second with the same
sense of displacement commences. A combina-
tion of the sense of motion on the strike-slip
fault and the geometry of the step-over
determines whether this intervening zone will
be one of compression or extension. With a
compressional step-over, a zone of uplift forms
between the fault tips, and thrust faults may
form to accommodate shortening across the
uplift. With an extensional step-over, normal
faults form to accommodate subsidence within a
pull-apart
basin (Fig. 4.19).
The geomorphological expression of strike-
slip faults can be viewed at the scale of individ-
ual seismic events or as a result of long-term
strain accumulation. Because individual faulting
events vary so widely in terms of the magnitude
of offset, length of rupture, and material that
they deform, a single set of geomorphic features
that characterize them cannot be specified. At
the scale of a few meters or less,
en echelon
faults commonly rupture the surface, small
collapsed basins form at releasing step-overs,
and small, thrusted uplifts appear at restraining
step-overs. Apparent vertical displacements
along the fault trace are also common. Some of
these displacements result from simple horizon-
tal translation of higher topography into an area
of lower topography on the opposite side of the
fault or vice versa. Many scarps, however, result
from some component of vertical displacement
along the fault that is a consequence of the fact
that the fault is not a simple vertical plane or
that the movement is not wholly strike-slip.
Along a steeply dipping, but undulating, fault
plane, one can visualize small-scale releasing
and restraining bends that cause subsidence or
uplift of one side of the fault with respect to the
other. If such a configuration were to persist
over long intervals, large basins and mountains
would develop that would be restricted to one
side of the fault. Along fresh ruptures of strike-
slip faults, however, uplifted scarps commonly
do not face in the same direction consistently,
