Flow, deformation, and transport - Physical Processes in Earth and Environmental Sciences - page 145

fractures are less common and become infilled by mineral
precipitates from fluid solution, forming veins. Fractures
can be observed in rock outcrops in different shapes and
varieties, the commonest features are roughly planar sur-
faces or joints. These are cracks that extend from centimeters
to hundreds of meters. They vary in shape from irregular
discontinuous fractures to almost perfectly planar features.
Regularly spaced, planar fracture surfaces showing roughly
the same orientation form a set and are called systematic
joints. On the contrary, irregular, discontinuous, arbitrarily-
orientated fractures are called nonsystematic. Different
sets of joints can be seen cutting each other in many out-
crops. As they open perpendicularly to the fracture surface,
the trajectories of the individual joints are not much
affected by others. Nonetheless, adjustments of space once
the fractures are produced may cause some minor shear
displacements along the fracture surfaces. Joint surfaces
can be smooth or show some interesting features such as
plumose structures , fringes , or conchoidal structures which
are very useful in discerning and describing fracture prop-
agation. Plumose structures are linear irregularities, some-
times curved or wavy, arranged as in a feather or fan
fashion, parting from a single point and ending in a narrow
band or fringe at both sides. The fringe is formed by an array
of discrete en échelon fractures (Fig. 4.72). Plumose struc-
tures form in the propagation direction of the cracks,
whereas conchoidal structures are formed perpendicularly to
the general direction of the plumes and can be envisaged as
discontinuities in the fracture propagation. These structures
can be in the form of steps (Fig. 4.72b), ribs, or ripples.
Sheet joints are subhorizontal fractures, having a ten-
dency to parallel the topography. This arrangement gives
two basic interpretations for their origin. First the paral-
lelism can be seen as the cause of the topography, because
the preexisting fractures are sites of weakness in the rock
and thus control denudation patterns. A more likely expla-
nation is that topography, being a shear free surface, affects
the orientation of stresses in a principal stress plane con-
taining two of the principal stresses. Either way, sheet
joints must form when the main compressive stress is hor-
izontal and the minimum compression is vertical which
happens during crustal uplift or in general compressive
tectonic settings.
Columnar joints form in lava flows and shallow intru-
sions as the rocks experience a volume loss by cooling in
discrete domains. Commonly three conjugate sets of frac-
tures develop, arranged in geometric patterns, mostly
hexagonal as in desiccation cracks in drying wet clays. The
cracks initiate at some point in the lava flow due to tensile
stresses which arise because of differences in temperature
and volume from the surface where the lava flow is cooler

(a)

Mode I

(c)

(b)

Mode II Mode III

Fig. 4.71 (a) Extension fractures (Mode I) open normal to the crack,
whereas, shear fractures (b) and (c) show displacements of blocks
parallel to the fracture. Mode II shear fractures (b) move normal to
the fracture edge and Mode III (c) move parallel to the crack edge.

types: extension fractures and shear fractures . Extension
fractures or Mode I fractures (Fig. 4.71a), open perpen-
dicularly to the fracture surface and do not experience any
displacement parallel to the fracture plane. By way of con-
trast, shear fractures are characterized by a noticeable dis-
placement of the blocks along or parallel to the fracture
plane (Fig. 4.71b, c). This displacement can be produced
perpendicular to the fracture edge (Mode II) or parallel to
the fracture edge (Mode III). Faults are large shear frac-
tures (surfaces extending from several meters to the scale
of plate boundaries); the term shear fracture is mostly used
for centimeter-scale fractures.

4.14.2 Extension fractures (Mode I)

Mode I fractures or extension fractures form in the princi-
pal plane of stress containing the principal axes 1 and 2 ,

perpendicular to the direction of minimum stress 3 . It is

important to remember that in these surfaces there are no
shear stress components and so there is no shear move-
ment or displacement of blocks along the fracture surface.
Extension fractures do not show offsets either side of the
fracture, even at a microscopic scale. Thus extension frac-
tures form by opening or pulling apart of blocks of rock at
each side of the fracture when the tensile stress exceeds the
strength of the rock and brittle deformation occurs. Such
brittle deformation is characteristic of the upper part of the
crust. At depth, moving into a more ductile field, extension