Fractures are one of the most common geologic structures and are present in all rock outcrops. They influence the strength of rocks, and they are important passageways for the flow of fluids ranging from igneous magmas to water to oil and gas and surface pollutants. As water passageways they are also important influences on rock weathering. Joints are natural fractures in rock across which there has been little or no shear displacement
The study of the geologic history of joints can be quite difficult because evidence for the origin and the relative timing of joint formation is commonly ambiguous. Because joints are planes of weakness in the rock, joints can be reactivated during later tectonic events. As a result, some of the features that we observe in association with joints in the field may be related to later events and not the time of formation of the fracture.
The analysis of joints involves four different types of observations:
(1) the distribution and geometry of the joints system,
(2) the surface features of the joints,
(3) the relative timing of the formation of different joints, and
(4) the geometric relationship of joints to other structures. However, before considering these observations and their significance, we will first consider some of the general nomenclature used to describe joints in rock.
Geometric characteristics and classification of joints. 1. Geometric data to obtain In order to describe, classify, and analyze joints, we need to note several important geometric characteristics. These include:
a) Attitude (strike and dip)
b) Degree of preferred orientation
c) Scale and shape of joints
(1) Termination type
(a) Curving
(b) Branching
(c) Intersecting
(d) Segmentation (en echelon set)
(2) Shape
(a) Tabular most common in anisotropic rocks (e.g. seds.)
(b) Circular to elliptical most common in more isotropic rocks
(3) Extent (cm km) master joints
d) Regularity and density of spacing
(1) Average normal spacing between joints
(2) Average number of joints per distance normal to the joints
(3) May vary with rock type
e) Spatial pattern and distribution
f) Extent and cross cutting relationships
g) Nature of the fracture's surface, vein filling &/or rock alterations.
2. Some definitions a) Fracture a surface along which material has lost cohesion.
b) Joints fractures in rock along which movement has been negligible or absent.
c) Faults fractures in rock along which there has been appreciable movement.
d) Fissures fractures whose walls have moved apart
e) Set a group of related joints
f) Joint system two or more sets of joints that intersect at fairly constant angles.
g) Conjugate system a system of joints or fractures consisting of two or more sets that appear to have formed simultaneously
h) Systematic joints joints that form sets or systems or are otherwise related in a geometric pattern characterized by roughly planar geometry, regular parallel orientations, and regular spacing (V&M Fig. 7.5).
i) Nonsystematic joints joints with appreciably curved surfaces that have an irregular distribution.
j) Joint zone quasi continuous fracture that is composed of a series of closely associated parallel fractures.
k) Dihedral angle the angle between two joints or joint sets
l) Crossing joints systematic joints of one set that consistently terminate against the joints of another set.
m) Localized fractures restricted to fairly narrow zones.
3. Experimental classification of joints This classification is based on the orientation of joints relative to the three principal stresses responsible for their formation and the sense of motion across the joint.
a) Shear joints (S) form in a non principal plane, commonly at an angle of about 30 degrees to the greatest principal stress (sigma 1). Because they form in a non principal plane their formation involves a component of shear stress, although the shear joint may or may not display a shear offset.
b) Extension joints (E) form in the principal plane perpendicular to the least principal compressive stress (sigma 3>0) (i.e. in the sigma 1 sigma 2 plane). Their formation involves zero shear stress but a relative tensile stress.
c) Tension joints (T) form in the principal plane perpendicular to the greatest principal tensile stress (sigma 3<0). Their formation involves zero shear stress and absolute tensile stress. Tensile joints may not be distinguishable from extension joints in many cases, but the two are distinguished because rocks are an order of magnitude weaker in tension than in compression.
d) Mode of joint formation this is related to the experimental classification of joints in the sense that it depends on the relative displacement of material on opposite sides of the fracture: .
(1) Mode I extension mode
(2) Mode II shear mode with displacement normal to the edge of the fracture
(3) Mode III shear mode with displacement parallel to the edge of the fracture
4. Descriptive classification of jointsa) Classification by pattern
(1) radial
(2) ring
(3) en echelon (note relationship to stress state)
(4) sigmoidal
(5) columnal joints (associated with cooling igneous bodies)
b) Classification by geographic orientation (NSEW)
c) Classification by relationships to other structures
(1) longitudinal joints have their traces aligned parallel to regional structural trends
(2) cross or transverse joints transect regional structural trends at a high angle
(3) oblique or diagonal joints attitudes intermediate between longitudinal and cross joints
(4) bedding joints parallel to bedding
(5) strike joints parallel to the strike of the country rock
(6) dip joints strike parallel to the dip of the country rock
(7) sheet joints or exfoliation joints a set of joints that lies subparallel to the topographic surface
(8) Joints related to preexisting fabrics
(a) Joints parallel to preexisting planes of weakness
In many cases, joints tend to form parallel to planes of weakness, such as bedding, layering, foliation, or cleavage.
(b) normal joints joints that form perpendicular to preexisting planes of weakness
(c) cross joints joints that form normal to well developed linear fabric elements (e.g. mineral lineations or fold hinges)
(9) Relationships to folds, faults, igneous bodies
C. Surface features of joints and fractures 1. Plumose structure or hackle plume composed of grooves (called hackle).
2. Hackle fringe en echelon set of extension fractures
3. Rib marks cuspate or step planes that occur as smoothly curved ramps connecting adjacent parallel surfaces of the joint face, tend to be perpendicular to hackle lines.
4. Ripple marks (not in a sed. sense) like rib marks, but not perpendicular to hackle lines.
5. Slickenside lineations parallel sets of ridges and grooves or linear mineral fibers indicative of shear on the fracture surface.
6. Vein mineralization filling the fracture (qtz, calcite etc.)
D. The origin and tectonic interpretation of joints1. The Problem – under what conditions in the brittle upper part of the crust are stress sufficient to fracture rock?
2. Our interpretations are complicated because:
a) Different joints in the same outcrop may have formed at different times and for different reasons.
b) Local variations in the stress field may cause joints that form at the same time to have different orientations at different locations.
c) Since joints are planes of weakness in the rock, they can be reactivated during later tectonic events. As a result, some of the features that we observe in association with joints in the field may be related to later events and not the time of formation of the fracture.
3. Geometric relationship of joints to other structures
a) Joints related to faults
(1) Joints associated with faulting are related to the same stress system as that responsible for the fault.
(2) Pinnate fractures (also called feather joints), occur in the vicinity of the fault but at an angle to it (V&M Fig. 7.17). The acute angle between the feather joints and the fault points in the direction of relative movement of the block on which it lies. They may form before or during the faulting process and have a relationship to the regional stress field that is the same as that for en echelon extension fractures.
(3) Conjugate shear fractures (T&M Fig. 3.16)
b) Joints related to folds (T&M Fig. 3.17 or V&M Fig. 7.6b)
(1) The orientation of the joints is described with respect to the orthogonal reference axes a, b, c .
(a) b // hingeline
(b) a normal to hingeline (b) // to folded surface (e.g. bed)
(c) c normal to the ab plane
(2) Commonly formed joint patterns:
(a) ac joints
(b) bc joints
(c) strike parallel joints
(d) cross-strike joints
(e) fanning joints
conjugate shear joints (hk0 joints)
(3) The joints are the result of the folding process and not necessarily the regional stresses that initiated folding.
c) Joints associated with the forceful emplacement of igneous plutons
Each of the following sets does not necessarily occur in the same setting.
(1) One set at a high angle to the contact (radial)
(2) One set parallel to the surface to topography (sheeting)
(3) One set of ring fractures
4. Joints related to uplift and unroofinga) The joints are subvertical and form in response to subhorizontal stresses.
b) Based on the release of stresses that are stored in rocks buried at dept in the crust
c) Stresses at depth are a result of:
(1) Lithostatic pressure (gh)
(2) Thermal expansion due to the Earth’s geothermal gradient
d) Regional uplift leads to erosion and unroofing of the underlying rock.
e) As depth of burial decreases the stresses on the rock change due to: cooling, Poisson effect, and membrane effect
f) Cooling – as rocks cool, they contract. Although the contraction is easily accommodated in the vertical direction, rocks are constrained in the horizontal direction and cannot easily contract. As a result, horizontal tensile stress builds up in the rock.
g) Poisson effect – as part of their elastic behavior, rocks that are compressed in one direction will tend to extend in the directions normal to the direction of extension (and vice versa). As rocks are unroofed and compression is released vertically, the rocks will attempt to contract laterally, and horizontal tensile stress will build up.
h) Membrane effect – as rocks move from depth to the outer part of the crust, their radius of curvature decreases creating subhorizontal membrane stresses
5. Sheeting joints
a) Joints that are subhorizontal or parallel to topographic surfaces.
b) Typically form in massive igneous rocks (such as granite) that do not have bedding or other planar anisotropies.
c) Unlike unroofing joints, these joints form as a result of subviertical tensional stresses that are believed to be a result of residual stresses in the rock.
d) Residual stress can develop due to differing thermal properties of adjacent rocks resulting in differential expansion or contraction.
6. Hydraulic fracturing – occurs when pore pressures are high enough to exceed the tensile strength of the rocks
7. Release joints – related to tension produced during regional release and elastic relaxation of tectonic stress
8. Joints from regional warping – similar to those associated with folds.
