Geol411 Joints and Veins

GEOL411: Joints and Veins

I) Introduction:

* Joint = planar fracture or crack in a rock, without shear displacement

(Shear fractures do not produce the features found in joints)

* Why Study Joints:

- Affect strength of rock bodies on escarpments.

- Can lead to rock falls.

- Affect regional permeability of rock bodies; direct groundwater or oil flow.

- Affects permeability of reservoirs.

- Controls natural erosion.  

- Controls caves.

- Paleostress indicators: form normal to σ3; the joint plane will contain both σ1 

and σ2.

II) Joint Sets and Systems:

* Systematic vs. Non-Systematic Joints:

- systematic joints:  there are several with a similar orientation and

approximately equal spacing; tend to be planar

        - non-systematic:  random-like arrangement and spacing; may be curviplanar.

*Joint set = a group of systematic joints

  * Joint system = two or more joint sets (may have formed at very different times!)

        - Orthogonal joint systems: dihedral angle ~ 90˚

        - Conjugate joint systems: dihedral angle < 90˚ (often 30 to 60˚)

                [Do not confuse with conjugate fractures!]

[pic 1]

* Timing relations – the first-formed set of joints will truncate later sets, because joints         cannot cross a free surface (free surfaces cannot transmit shear stresses, and must         be principle planes of stress!).

[pic 2]

        - e.g. for orthogonal joint sets, the “rungs” of the ladder-like pattern formed later

        - e.g. grid pattern suggests alternating stress field and concurrent cracking (σ2 and                         σ3 may have been similar in magnitude and flipped back and forth)

* Joint spacing:

        - Experiments suggest that joints form sequentially.

        - The often regular spacing of joints in sed rocks is explained by the formation of                         one joint relieving tensile stresses for a critical distance on either side of the                 joint, causing a “stress shadow”.

        - Width of stress shadow depends on bed thickness.

        - This explains why joints are usually closer spaced in thinly bedded sediments.

[pic 3]

        - Spacing depends on:

                [1] bed thickness

                [2] strain (more strain requires more joints)

                [3] stiffness (Young’s modulus)

                [4] tensile strength.

III) Surface morphology of joints

* Plumose structure:  Joint surfaces show a feather-like pattern.

        (This is best seen in fine-grained rocks.)

[pic 4]

        - Origin = small dimple where joint initiates (commonly an inclusion)

        - Mirror zone = very smooth zone around origin

        - Mist zone = area where surface roughens, around the mirror zone

        - Hackle zone = contains “barbs” which curve away from plume axis. Note barbs                         point towards the origin.

        - Plume axis is often wavy and diffuse.

         -Arrest lines may be seen – concentric rings around the origin where a joint                         formed in stages.

* Plumose structure forms due to:

        (i) rock inhomogeneity

        (ii) changing stress field at crack tip as joint propagates.

* Twist Hackle:  If a joint is not // principal plane of stress (inclusions, bedding planes,         etc.) then crack tip pivots into a new orientation. Joint may split into an en echelon         array (called “twist hackle”).