Fractures are a consequence
of brittle deformation and
define the strength of rock
material and provide
information about its tectonic
history. They are important
pathways of fluids and provide
a space for minerals to grow.
Understanding how fracture
orientations form due to stress
is essential to historical
geology and resource
exploration.
Forms
either
pinnate/feather
features en echelon (French for
“rung of a ladder”). Seen
below:

Fracture: separation of a rock or
mineral that creates two surfaces that
did not exist before.
Fracture Failure Modes:
 Mode 1- Extension (joints)
 Mode 2- Shear (faults)
 Mode 3- Tear (faults)
Classifying Extensional Fractures
 Set: Many fractures in same area
with similar orientation.
 Joint: Small displacement normal to
their surfaces and virtually no
displacement parallel to their
surfaces.
 Systematic Joints: Regular parallel
orientations and spacing.
 Non-Systematic Joints: Curved,
irregular joints. Nearly always
terminate against older joints.
 Sheet Joints: Sub-parallel to
topography. Characterized
by
rounded
topography,
late
formation. Also form from plutonic
cooling.
 Columnar Joints: Separated into
hexagonal or pentagonal columns.
 Gash Fractures: Mineral-filled Sor Z-shaped extensional fractures
formed in ductile shear zones.
Veins: Extension fractures filled
with mineral deposits.
Geometry of Fracture Systems
Many fractures develop in sets
characterized by a consistent fracture
orientation. Determining the preferred
orientations of different fracture sets in a
rock is done by objectively collecting data
from all visible fractures. Plotting the
strikes of fractures for many exposures on
a Rose diagram is a useful practice for
finding regional patterns, though single
orientation data is not enough to
determine if fractures are genetically
related, as single fracturing events may
produce multiple orientations. Genetically
related fractures may also differ
depending on lithology, so separating
joint data by lithology can be important in
deciphering history of joint formation.
Joints terminate in a number of ways.
Some simply die out, while others curve,
kink, branch out, segment into en echelon
sets, and intersect other joints. Where
fractures end, they have definite tip lines.
The spacing of fractures is measured
as the average perpendicular distance
between fractures. Spacing depends on
lithology and increases with bed
thickness.
Mapping the location and orientation
of fractures is the essential for studying
spatial patterns of fractures. Regionally
consistent joint set orientations reflect
regional stress patterns. If the fractures are
Mode 1, the minimum principal stress is
normal to the joints, while the max
principal stress is in the direction of the
trajectories. Mode II fractures have a
shear stress perpendicular to the plane of
fracture, while Mode III fractures have a
1
shear stress parallel to the plane.
Features of Fracture Surfaces
Features on the surface of a fracture
provide information about the fracture’s
origin. Hackle fringe describes a set of
extension fractures that are aligned en
echelon and rotated away from the joint
axis. A plumose structure, or hackle
plume, has a characteristic feather
pattern. Rib marks are curved features
perpendicular to the lines of hackle of the
fracture face. All these features indicate
extensional fracturing, as opposed to
slickenside lineations, which indicate
shear fracturing.
Timing of Fracture Formation
When more than one set of joints are
developed, younger joints terminate
against older joints. Many sets of older
joints are orderly, while younger joints
increase connectivity of the fracture
system. Joints may cross-cut one another
if pressure across the earlier fracture is
high (closing the joint), or if the early
fracture has been cemented by mineral
deposits. A deep joint may also cut a
shallow, pre-existing joint, making it very
difficult to determine relative ages of
joints. Fractures that cross-cut a geologic
boundary or other geologic structure
postdate the formation of that boundary or
structure. Determination of relative ages
is only possible through careful
examination of hackle plume geometry on
the younger joint. It is, however, possible
for two joint sets to form during the same
fracturing event.
For example, if a fracture runs through
a dike, then the dike came first. If a joint
maintains constant orientation across
folded layers, it must have formed after
the folding, etc.
Relationships of Fractures to Other
Structures
Fractures are associated with faults, folds,
and igneous intrusions. Conjugate shear
fractures are two sets of small-scale
shear fractures that form in fault zones at
roughly 60 to each other. Ac and bc
fractures represent fractures parallel to
planes on folded beds. At the center of a
syncline, fractures are often parallel to the
fold axis, making a low angle with the
bedding, whereas folds at the peak of an
Billy Gange, 2011
Edited by Ryan Hardenburger, 2013
anticline make a high angle with the
bedding. Columnar, sheet, foliation and
lineations are all associated with igneous
intrusions.
References & Resources
Robert J. Twiss, Eldridge M. Moores,
Structural Geology 2nd edition, (W. H.
Freeman), p. 37-59, 2006
http://en.wikipedia.org/wiki/Fracture
http://eps.berkeley.edu/~burgmann/EPS11
6/reading/Lecture_2.pdf
Billy Gange, 2011
Edited by Ryan Hardenburger, 2013
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