Download Chapter 6 and 7 Joints

CHAPTER 7
JOINTS,
VEINS AND LINEAMENTS
Joint: is a fracture without displacement.
The important of the joints not only their important
in controlling landscape morphology, but also
because they affect rock strength and
permeability. As well as they reflect the history
of stress and strain in a region.
Longitudinal crakes
Joint: is a fracture without displacement.
They are tensile fracture that form ┴ to
σ3 and // to the principle plane that
contain σ1 and σ2.
Joints: they form during
mode I loading.
Surface morphology of joint
• Smooth joints
• Plumose structure along the joint plane forms in
fine-grained rocks (i.e., shale , siltstone, sandstone,
chalk and basalt ), but might not be seen obviously
in very coarse-grained rocks like conglomerate and
granite.
Characteristic of joint arrays
• Systematic joints are a group of joints that are
parallel or subparallel to
one another, and have the
same spacing.
* Systematic joints reflect
regional tectonic stress.
• Nonsystematic joints have an irregular spatial
distribution, they do not parallel neighboring
joints, and they tend to be non-planar. Both
systematic and non-systematic joints can
occur in the same outcrop. Nonsystematic joints
reflect very local heterogeneity of stress field.
Joint Sets and Joint Systems
• Two or more joint sets (group of systematic
joint) intersect each other forms a joint system.
• The angle between two joint sets in a joint
system is the dihedral angle.
• Orthogonal system, where the two joint sets
are perpendicular (where the dihedral angle is
90°).
• Conjugate system: The dihedral angle 60°.
• The through-going joints are master joints and
the short joints are the cross joints.
• Within the folded sedimentary rocks three main sets
of joints are common.
1- The strike parallel joint
2- Cross-strike joint with an angle between
60°-90° to the bedding strike.
3- Joints fanning around maximum ductility.
• Sheeting joints or exfoliation within the
intrusive and metamorphic rocks without
schistosity.
• Columnar joints formed in the hypabyssal
intrusive igneous rock bodies and lava flows.
• Is it important to determine the joint
spacing in sedimentary rocks.
Joint spacing depends on four parameters:
1. Bed thickness
2. Stiffness
3. Strain
4. Tensile strength
• Joints are more closely spaced in thinner beds, and
are more widely spaced in thicker beds.
• Relation between joints spacing and lithology
(stiffness). Stiffer beds have smaller joint spacing,
because stress is larger in stiffer bed under the same
strain.
• Relation between joint spacing and tensile
strength. Rocks with the smaller tensile
strength develops more closely spaced joints.
• Relation between joint spacing and the
magnitude of extensional strain. A bed that
has been stretched more contains more joints
than a bed that has been stretches less. More
stretches bed develops more closely spaced
joints.
Joint studies in the field
• Why do we study joints?
• For engineering, hydrologic, mining, paleostress
fields, and geomorphology studies.
The main characteristics of the joint
sets in the field are:
• Surface morphology of the joint plane:
Any structural features superimposed on joints
(stylolitic pits or slip lineations).
• Dimensions of the joints:
Length of the joint trace. Large joints reflect
regional tectonic stress conditions.
• Joint spacing and density in outcrop:
Indicate mechanical property of the rock.
Joint density depends on both the length and the
spacing of the joints.
• The relation between joint distribution and
lithology:
Joint cut relationship with single or many beds or
the entire outcrop reflect the physical
characteristics of the bed rock.
• The relation between joints and other
structural features:
Orientation of joint with tectonic foliations, folds,
stress fault.
Dealing with field data about joints:
• Field study for joint can be carried out in two
ways:
1. Inventory method. (Define a representative
region and do all the measurement on the joints).
2. Selection method. (Use to define systematic
sets of joints). But the disadvantage of this
method, it needs very careful observation on the
different set of joint. Restricted with few sets,
without taking consideration of all joint sets.
May miss important joint sets.
• How do we plot the field joint data?
There are 4 different ways:
• Plot the strike and the dip of the joint sets on
the geological map.
• Drawing representative joint trajectories on a
map.
• Drawing statistical diagrams (histogram)
• or using a polar diagram called rose diagram.
Fig. (1). Rose diagram of fracture strikes in a quarry near Hashemite University.
Different shadings represent different domains.
Origin and tectonic interpretation of
joint
Joint developed when stress exceed the
tensile fracture strength of a rock and
Griffith cracks begin to propagate. This
related to:
1. Uplifting and unloading
2. Sheeting
3. Hydraulic fracturing
4. Tectonic deformation
Veins and veins arrays
• Vein is a fracture that has filled with minerals
precipitated from water solutions that passed
through the fracture. Quartz, calcite and
gypsum form the most common vein fill, but
other minerals do occur in veins, including
numerous ore minerals such as gold, silver,
copper, zeolites and chlorite.
Lineaments
• Lineament is a regional-scale linear feature
recognized on aerial photos, satellite imagery
or topographic maps.
• Structural lineaments are defined by
structurally controlled alignments of
topographic features like ridges, depressions,
and/or escarpments.
• Most lineaments are the geomorphological
manifestation of joint arrays, faults, folds,
dikes, or contacts between different lithology.