Lecture index: Structural significance of joints. / Examples of fracture and joint patterns. / How to describe tensile structures. / Some common joint associations. / Some models for joint formation. / Veins ('filled joints').

Types of tensional geologic features:

• joints.
• veins.
• igneous dike and sills.
• sandstone dikes.
• earth fissures, crevices.
• ice crevasses.

The focus of this lecture will be on joints since they are the most common type of structure.

They are some of the simpler structures found in rocks, but also the most common.

• mass wasting surface failure plane; slope stability, dam stability, tunnel stability.
• strength anisotropy: later reactivation?
• fracture porosity/permeability - hydrologic modeling, mineralization.
• imp. geomorphic control, trellis drainage, lineaments.
• hydrocarbon migration.
• easy to interpret paleostress system (with caution), but difficult to date.
• igneous rocks provide an exception to difficulty in dating because of late juices that can fill the joints.
• pervasive phenomena, and one structure that occurs here in Nebraska

Above are some jointed basalts from Giant's Causeway, Ireland. Classic columnar jointing is developed here. Note that not all the columns are perfectly hexagonal. Also note how subhorizontal fractures also segment the columns along their length. These fractures have a distinctive disk shape, and are both convex upward and downward. The mechanics of the hexagonal columns are well studied, but those of the dish shaped fractures are which segment the columns are not. Think of how fluids might flow through such an array of fractures.

This is a USGS photo looking northwest showing the joint pattern evident in the sandstones of Arches National Monument. Salt anticline valley is in the upper left. The dominant fracture set is parallel to the fold trace, a common association. It is this joint set that controls the development of the the striking erosional forms that are a centerpiece of the valley.

This of course depends on why you are describing them.

• orientation - strike and dip.
• sets - preferred orientations, statistical description, as a population (using stereoplots).
• width, length, aspect ratio.
• density of (how to measure?).
  • intersections per unit length.
  • aggregate length per unit area.
  • aggregate area per unit volume.
  • the ease of which bias can creep in.
• associated brittle surface features (plumose marks and hackle structures).
• intersection geometries (T junctions and cross joints).
• tip geometries (they can bend).

In different geologic situations you get different charateristic joint patterns.

• joints in volcanic rocks.
• joints in plutonic rocks.
• joints associated with folds and tectonism: usually at least two sets in symmetry, but up 5.
• intracratonic regional joint sets: an amazing Great Plains example.
• joint sets associated with point phenomena.
• conjugate sets or not?

Joints are polygenetic.

• unloading. This is the most common model, and in general is simple and in detail complex.
• topographic forces.
• thermal contraction.
• tectonic stresses.
• fluid migration.
• impacts.
• tidal stresses.
• reactivation.
• implications of regional fractures sets in young loess.
• will return to when we have a better understanding of stress.

These differ from veins in that they are filled with hydrothermal precipitations, and can either form by wall rock replacement or by dilation (opening up and volume increase). In the latter case they respresent a greater magnitude of strain also.

• fibrous or directed mineral growth can track movement history.
• crack-seal history - cycles of growth.
• syntaxial vs. antitaxial growth.