Flow, deformation, and transport - Physical Processes in Earth and Environmental Sciences - page 147

s 1

s 2

s 3

Fig. 4.74 Mineral-filled (calcite) gash fractures developed in Jurassic-
age limestone, Delphi, Greece. Coin is 2.5 cm diameter.

(a)

S 3

S 1

Fig. 4.76 3D geometry of gash fractures in a shear zone. Sigmoidal
gashes are older than the second generation of lenticular ones. Note
the orientation respect the principal stresses.

(b)

S 1

S 3

older sets will be more rotated and the sigmoid shape
more developed, due to several increments of deformation
in the shear zone (Fig. 4.75c). Thus, by observing the
morphology and orientation of gash fissures, the principal
strain axes and therefore the direction of the principal
extension, can be deduced. Also sigmoid and rotated gash
veins are important in the discrimination of simple shear
versus pure shear deformation.
Feather or pinnate fractures are another kind of exten-
sion fracture that form in the brittle domain associated
with shear fractures and faults. Sets are arranged in an en
echelon fashion forming an angle with the faults. As in all
extension fractures, they are orientated parallel to the prin-
cipal stress surface containing

1

(c)

S 1

S 3

1

2

Fig. 4.75 Gashes formed by (a) homogeneous flattening and
(b) simple shear, where the original fissures (1) rotate due to further
shear strain acquiring (c) a sigmoidal shape whereas new formed
gashes (2) are lenticular. Gashes, as all extension fractures, develop
in the plane e 1 - e 2 , perpendicular to the direction of maximum
lengthening ( e 1 ).

2 (Fig. 4.76) and so are
good indicators of shear or sense of block motion in the
fault (Fig. 4.77). The acute angle ( c. 30

1 -

) points in the
direction of block movement at either side of the fault
(Fig. 4.78a). Notice that nonrotated gash veins show the
same orientation (Fig. 4.78b).

-45

increment in strain, giving way to further stretching, will
provoke a rotation in the cracks due to shearing and an
extension of the cracks at the tips. These will open in the
perpendicular direction to maximum elongation (the
direction of the principal compressive stress axis 4.14.3 Shear fractures (Modes II and III)

3 )
according to the orientation of the strain ellipse, so the
cracks will show a sigmoid shape. Newly formed fissures
will be lenticular, nonrotated and oriented normal to the
new S 1

Shear fractures, like faults, are formed at an angle to the
principal stress surfaces; this fact being the chief difference
compared to extension fractures (Fig. 4.77). The term shear direction, cutting the older sigmoid gashes; so