JOINTS
Review
Definitions
Joints are fracture surfaces along which there has been imperceptible movement.
Readings
Ch. 5
Lecture : SLIDES OF THE FOLLOWING FEATURES
Joints have play important roles in determining the patterns of surficial weathering.
In many instances joints can be very eye-catching because the best developed joints are systematic, that is they are planar, parallel and evenly spaced. Each of these groups of joints that displays these common features is known as a set.
If the joints are sufficiently closely spaced and regular, their predictable nature can be used for quarrying.
What do joints look like close up?
Have you ever snapped a piece of cold dark chocolate? The fracture face you form often shows distinctive feather-like patterns. Similarly, some joint faces are marked by plumose markings.
O'head
Fractures with plumose structures are formed explosively, with the V opening up in the direction of propagation of the fracture.
Spatial relationship of joints to regional principal stresses.
There are two types of joints based on their spatial relation to regional stresses.
If there is absolutely no shear movement on the fracture and we have conditions of pure tension (negative normal stress- Fracture diagram) the tension gashes form at right angles to the direction of minimum principal stress.
If there is some shear movement across the joints they form at a similar orientation to the principal stress directions as faults.
Include Figure
Such a model predicts two or three sets of joints which can be formed in folds
Include Figure
EXAMPLES FROM THE ALABAMA FILED TRIP
Veins
On the field trip we observed quartz veins related to regional tectonic events. Veins are joints where minerals have precipitated. Mineralizing solutions invade the rock bodies along joints. We clearly veins that invaded tension fractures in the flanks of folds. However, there some of these veins showed a pronounced curvature in the form of an s( sigmoidal). Sigmoidal tension gashes are double curved and point opposite to the direction of relative shear. They form because an original vein keeps growing and rotating progressively. They form because the deformation is progressive. They're a great indicator of the type of internal deformation that rocks have experienced. In the examples we saw, the small sigmoidal tension gashes indicated the sense of internal deformation of the fold that was in turn consistent with the regional patterns of plate collision.
Exercise: Look at the Oman Field Trip Web Site and determine the sense of shear from the sigmoidal tension gashes shown on the vertical outcrop.
On the field trip we also noticed spheroidal weathering (onion-skin weathering) As large bodies of rock are unroofed through erosion and continued uplift, the release of the overburden weight can created hairline decompression fractures.
In Louisiana clayey soils can expand and contract throughout the year creating large(tens of feet in the long dimension) vertical contraction joints. tens of feet.