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equal to the angle of internal friction for the
faulted material. These shears will have the
same sense of strike-slip motion as the principal
displacement zone, and they typically form a
suite of
en echelon
fractures (Fig. 4.18). As shear-
ing continues, the propagating tips of the Reidel
shears tend to curve to become parallel with the
normal faults and to display normal-fault
displacements with little strike-slip motion in
the tip zone. Conjugate Reidel shears (R
′
) can
form at 90
°
−
q
/2 (60-75
°
) to the principal
displacement zone. These conjugates have the
opposite sense of strike-slip motion as the main
fault. Finally, folds and thrust faults should form
with their axes or traces, respectively, approxi-
mately perpendicular to the main compressive
stress. The fold axes will initially be oriented at
about 45
°
to the main displacement zone, but, as
fault motion continues, they can be rotated into
more complete parallelism with the main fault
due to shearing adjacent to the principal
displacement zone. In Nature, some of these
structures may represent responses to different
generations of motion, and all of them can be
superimposed, making for a very complicated
array of structures within a broad shear zone
(Sylvester, 1988). Although the development of
structures along a newly formed strike-slip fault
does not follow an invariant pattern, a predicta-
ble succession of structures is common, whereby
discrete folds and tension gashes form initially
and eventually amalgamate into a through-
going, anastomosing shear zone (principal
displacement zone) (Sylvester, 1988; Anderson
and Rymer, 1983).
Whereas all of these structures can be associ-
ated with a relatively straight segment of a
strike-slip fault zone, many strike-slip faults
have traces that bend such that slip between the
adjacent blocks creates large compressive or
tensile stresses in the curved fault segments. For
example, in a restraining bend (Fig. 4.19), the
fault trace curves into the path of the blocks on
either side of the fault. The ensuing compres-
sion generates contractional structures that lead
to development of thrust faults, folds, and
ultimately mountains (Biddle and Christie-Blick,
1985). In the vicinity of a releasing bend, the
fault curves away from the path of the blocks on
Strike-Slip Fault Zones
extensional
fractures &
thrust faults
PDZ
R'
R
Reidel shears
& rotated folds
rotation parallel
to fault
superposition
of all
structures
Fig. 4.18
Orientation of structural features formed
in response to strike-slip shear couple.
Normal and thrust faults, folds (some rotated near the
shear zone), Reidel shears and conjugate shears (R
′
) form
at predictable angles with the principal displacement
zone (PDZ). Modified after Sylvester (1988).
Anatolian Faults, are strike-slip faults that have
caused widespread destruction in 20th-century
earthquakes. Strike-slip faults develop where
the maximum compressive stress (
s
1
) is
horizontal and there is a horizontally oriented
deviatoric tensile stress (Fig. 4.1C). Commonly,
this stress orientation prevails where two
crustal blocks are moving essentially horizon-
tally and approximately parallel to the boundary
between them, but in opposite directions. Thus,
a shear couple is created across this boundary
zone.
As observed in laboratory experiments and
field studies, a predictable geometry of structures
may form in shallow crustal rocks and alluvium
in response to this stress field (Fig. 4.18). A
principal displacement zone commonly forms
parallel to the shear couple. This orientation is
the one you would expect a strike-slip fault to
have, if the crustal blocks on either side had a
high rigidity with respect to a weak fault zone
between them. Normal faults should form
perpendicular to the direction of maximum
elongation and maximum tensile stress.
Reidel
shears (R) form at an angle of
q
/2 (
∼
15-20
°
)
with the principal displacement zone, where
q
is
