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