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Joints are fractures without displacement that play a crucial role in controlling landscape morphology and affecting rock strength and permeability. They reflect the history of stress and strain in a region.
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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.
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.
Nonsystematicjoints 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.
