Structural Geology
Lecture 19
Joint Patterns
(The orientation and spacing of joints, veins, and
dikes)
Two
parameters influence fracture patterns: the orientation of the the fractures
and their frequencies. Orientation of
fractures is based on the state of stress within the rock -- both stress
difference and orientation of the principal stresses. In contrast, the frequency or spacing of
fractures is based on the properties of the rocks in which the fractures have
formed (Fig. 19-1).
Fig. 19-1
The solution to the fracture spacing problem is
found by recalling that the critical stress intensity factor (KI) is
KI = (s3)(πc)1/2
where
c is the crack length. Here stress
intensity is a function of crack length and, thus, is independent of the
lithology. However, in a bedded
quartzite-limestone sequence, we predict that a quartzite will have more
closely spaced joints if a given extensional strain is responsible for
generating larger tensile stress. The
Young’s Modulus for quartzite is greater than for limestone. Hence, by applying Hooke’s law we predict
that tensile stress in a quartzite bed might be larger for a given unit of
strain. If tensile stress increases as a
consequence of the thermal cooling of rock the following equation for the
development of thermal stresses might apply:
where
E is the Young’s Modulus, a is the coefficient of thermal expansivity and DT is the change
in temperature. This equation also
suggests that when quartzite and an adjacent limestone bed are subject to the
same thermal change, tensile stress in the quartzite builds up faster because
Young’s Modulus and thermal expansivity for quartz are larger, and hence,
sqtz > sls
So,
the fracture intensity of the quartzite is larger and jointing will form on a
closer spacing.
Fig. 19-2
Fracture nomenclature is in part tied to the type of
rock in which the fractures have formed.
Orientation also plays a role in fracture nomenclature. For example, Balk documented several types of
fractures in granite bodies (Fig. 19-2).
The type of fractures are based on their relationship to the flow lines
of the granite as well as the orientation of the surface of the granite
body. It is fair to assume that most
joints in granites originate as extension fractures. Sheeting joints for parallel to the
topographic surface and are generally subhorizontal. Longitudianl joints are steeply dipping
which parallel the flow lines. Cross
joints in granites form perpendicular to flow lines.
Granite quarries are laid out based on the direction in
which the granite breaks in tension most easily. The quarrymen call this direction of easy
breaking the rift. Flow lines and
the longitudinal joints are ofter parallel to the rift of a granite. The grain of a granite is the
direction of next easiest breaking.
Sheeting joints may well parallel the grain of the granite. In New England granites the orientation of
the grain and rift may appear in the opposite orientation as described (i.e.
sometimes the rift parallels the topography.
The hardway of the granite is the direction in which the granite
is most difficult to break. The hardway
is vertical in all New England granites.
In sedimentary rocks the joint nomenclature depends on
the orientation of local structures, most commonly the fold axis. Stearns pointed out the folded sediments may
have four fracture sets (Fig. 19-3).
(Fig. 19-3)
Each set may consist of up to three fractures: an
extension fracture and conjugate shear fractures. Conjugate shears form so that the acute angle
between the shears faces s1. Each of the sets has no particular name,
however, the extension fractures may be referred to as either cross joint (i.e. that group of joints
perpendicular to the fold axis) or a strike joint (i.e. that group of joints
parallel to the fold axis). Here the term
cross joint in a sedimentary rock refers to a different type of joint than that
found in a granite.
A different set of nomenclature was developed for
joints in flat-lying sedimentary rocks.
Hodgson observed that in the absence of shear fractures in sedimentary
rocks there may be several joint patterns.
The most common three patterns are shown in Figure 18-4.
Extension joints are not one long discontinuity but
rather several joints that form end-to-end in a joint zone. Some authors preferred to restrict the term
joint to extension fractures. This is
the case with Hodgson. The line that an
individual joint makes on the outcrop surface is called the joint trace. Joint zones are often parallel to other
zones within the outcrop. These joint
zones make up one joint set and are called systematic joints. Often there are joints that form roughly
perpendicular to the systematic joints.
If these latter joints are themselves systematic joints they may be
called cross joints. Otherwise
they are non-systematic joints.
Here we see a third use of the term cross joint.
(Fig. 19-4)
(Fig. 19-5)
Conjugate shear fractures may be used to map s1is the vicinity of a
fault zone. For the Bonita Fault, a
normal fault in New Mexico, the s1 is vertical as witnessed by the orientation of
shear fractures in the fault zone (Fig.
19-5). Yet, as the fault is approached
the local rocks are seen to drag into the fault zone. Likewise, the conjugate shears rotate into
the main Bonita Fault suggesting that the stress field was not homogeneous near
the Bonita Fault. In the case of the
Bonita Fault each of conjugate shear fracture is inself a smaller normal fault.
Shear fractures in and around anticlines and synclines
may vary in orientation. For some
limestones in Morocco, de Sitter showed that the acute angle of the conjugate
shears faced parallel to the fold axes over anticlines yet faced perpendicular
to the fold axes over the synclines (Fig. 19-6).
(Fig. 19-6)
On the New York portion of the Appalachian Plateau
Engelder has used extension fractures to map the stress field during the
Alleghanian Orogeny (Fig. 19-7). Here
there appear to be a conjugate set of shear fractures. Yet, surface morphology studies and butting
relationships show that all fractures are extension in origin. These extension fractures may be called
cross-fold joints. The double pattern of
cross-fold joints indicates that there were at least two phases of compression
during the Alleghanian Orogeny.
Evidence that
joint sets are an accurate indicator of the maximum principal stress within the
crust come from two areas. Volcanic
dikes may be thought of as equivalent to extension fractures. In the Aleutian Peninsula of Alaska volcanic
dikes parallel the direction of subduction of the Pacific Plate under North
America (Fig. 19-8). It is well known that the orientation of
maximum principal stress in the lithosphere is parallel to subduction
directions.
Dikes around the Spanish Peaks area of Colorado can
also be used for stress markers at the time of injection. Ordinarily the stress around a hole in an
elastic plate would be radial. However,
if there is a tectonic stress in the vicinity of the hole then the stress
trajectories is deflected away from this radial pattern. The Sangre de Cristo Mountains served as a
ram forcing the ß⁄ into the east-west direction. This can be seen by the deflection of some of
the dikes from the Spanish Peaks into the east-west orientation (Fig. 19-9).
(Fig. 19-7)
(Fig. 19-8)
Fig. 19-9
