Fractures&Joints.lec1

Fractures, Joints, and Veins

Reading: Chapter 7 in Van der Pluijm and Marshak; Recommended Chap. 2 Maley

Basic Types:

Fracture: Surface of discontinuity, generally with little displacement

Extensional Fractures: Displacement perpendicular to fracture, tensional
Shear Fractures: Displacement parallel to fractures

Joint: Natural Extensional Fracture

Vein: Fracture filled with mineral precipitate or rarely mud.

Fractures formed in Santa Cruz Mudstone, south of Panther Beach. Small mud-filled veins, formed as sediment was consolidating. Mudstone was elastic enough to fracture, but still draining muddy fluid that formed darker material in cracks.

Calcite and quartz veins in a zone of distributed faulting

Dike: Fracture filled with igneous rock or remobilized clastic sedimentary rock.

Dike along an extensional joint, Unimak Volcano, Aleutian Islands

Columnar Joints. Devils Postpile, Eastern Sierra

Exfoliation Joints. Tenaya Lake area, Sierra.

Patterns of Natural Fractures

Systematic and Nonsystematic

Systematic joints going from lower left to upper right with another apparently non systematic joints.Note alteration along joints indicating fluid movement. Purisima Fm, Point Reyes, CA

Orthogonal

Who Cares?

Applied Geology: Cuts and cliffs

Hydrogeology-Petroleum Geology: Directional permeability. Permeability ~ cube of fracture width. Cracks and make a huge difference. Directionality of cracks also important in connectivity and in designing a well to cross as many as possible

Ore Deposits
Quarrying: Largest block size depends on joint frequency. Michelangelo needed a huge block of marble to sculpt the David, whereas abundant fractures desirable in quarrying for crushed rock.
Academic geologists: Stress orientations and understanding tectonics
People who enjoy scenery

Surface Morphology or Ornamentation of Joints (Extensional Fractures)

Extensive surface morphology that illustrated joint growth: Origin, Arrest lines (also called ribs), and Hackles that in aggregate comprise Plumose structures (look like feathers).
Morphology of joint surfaces resembles that of a broken glass or plexiglas and can be experimentally produced as extensional fractures. Fluid pressure > confining stress and tensional strength
Key point is that identification of arrest lines and hackles specify that the fracture is extensional and not a shear fracture
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Arrest marks on sandstone in Utah. courtesy of Ty Kennedy-Bowdoin

Surface Morphology or Ornamentation of Shear Fractures

Slickenlines

This is the faulted surface of limestone block showing well developed slickenline lines. Slickenline lines on a shear fracture with mm to cm of displacement may be much more subtle

Shear veins: Mineral precipitate fibers formed during movement.
Conjugate geometry or intersecting two planar joints at about 60 degree angle

Image from Structural Geology of Rocks and Regions by Davis and Reynolds, 1996

Relationship to Stress

The joint is an extensional fracture with the deformation involving opening of the fracture. Intuitively you know that this must occur perpendicular to the minimum principal stress.

Sometimes have extension in more than one dimension. In this case radially to to expansion of soil layer by explosions. Fractures formed during shot explosions in seismic exploration (Sebkha region, Tunisia, 1984, from SEG calendar, 2000)

Shear failure at about +30 degrees to Sigma 1.
Hydrofracture due to fluid pressure exceeding magnitude of total stress plus tensional strength of rock--causes tensional stress and extensional failure or jointing. (Effective Normal Stress = Total Normal Stress - Fluid Pressure).

Relationships of Joint Frequency to Bedding Thickness in Sedimentary Rocks

Joints more closely spaced in thinner sedimentary layers
Why? Formation of joint relieves tensile stress in layer over a lateral distance proportional to the joint length. Joints end at layer boundaries, typically as they are discontinuities. Therefore the longer joints in thicker layers need to be spaced less frequently

Relationship to Rock Strength:

Stronger, "stiffer", or more brittle rocks have more closely spaced joints.
Strain equivalent along layers of different types.
A more brittle or stronger bed will fail at a lower strain than one that is of lesser strength or more elastic. Therefore it most fail more often to achieve the same amount of strain
If a bed is stronger (stiffer, higher absolute magnitude Young's modulus, E, the proportionality constant between stress and strain) higher stresses are required to achieve the same amount of strain in the stronger layers, therefore they fracture more frequently. (Stress = E *Strain)

Highly fractured (veined) cherts, a more brittle lithology interbedded with shales with few fractures. Kodiak Island,

Tectonic Interpretation of Joints and Fractures:

Joints perpendicular to minimum principal stress
Shear Fractures form related sets, intersecting at less than 90 degrees, and symmetrical to Sigma 1.

Joints due to Regional Deformation:

(a) Stretching due to normal faulting.

(b) Movement of fault wall over non-planar fault surface causes extensional fracture.

Note Error in (b). Offset bed shows reverse fault motion, but arrows show normal fault

Sheeting Joints or Exfoliation: Earth surfaces is eroded and relieves vertical stress but lateral stress is not reduced proportionally. Therefore vertical stress becomes minimum principal stress and joints form perpendicular to land surface. Shrinkage of cooling plutons (large homogenous igneous masses masses may also produce sheeting joints or exfoliation.
Columnar Joints: shrinkage of tabular igneous bodies parallel to maximum extent.
Role of fluid Pressure or Hydraulic Fracturing: At depth in the earth all stresses should be compressive. However fluids in a crack and oppose lateral forces on solids in the rock and place the rock in a state of stress that approximates much shallower levels. If fluid pressure exceeds stress on rock solids a fracture opens
Natural Hydraulic Fracturing:

If wall of crack is impermeable, fluid pressure can act against it and form a hydrofracture once external applied stress is overcome (see hydrofractured plexiglass above).

If crack wall is permeable then fluid can flow into crack walls and counteract fluid pressure in crack, providing sediment grains can move independently

If wall rock is cemented or cohesive then fluid part of pressure of fluid in rock is counteracted or offset by internal rock strength. Higher fluid pressure in crack wins and rock cracks. The resistance of the matrix in transmitting internal fluid pressure to the crack wall is called the poroelastic effect.

Veins

A fracture filled with a precipitate, commonly quartz or calcite.
Extensional and shear veins
Fibers tell history of vein growth. Fibers may grow towards walls (Antitaxial) or towards center of vein (Syntaxial).
Veins good strain indicators. Preserve history of fluids moving through fractures, as microscopic fluid inclusions and as chemical signals in precipitates
Fluid inclusions and minerals in veins can record conditions that veins and associated fractures formed at: Pressure and temperature and fluid composition
Veins seal fractures, reducing permeability. They also strengthen rock and restore continuity across the fracture.

Working with Joints and Veins in the Field

Measure in an unbiased manner, using a scan line or representative area
Type, Orientation, Frequency
Timing: Younger joints commonly terminate into older, existing joints. Because fracture stops at free surface of the preexisting joint and cannot propagate across it. Also, joints tend to become parallel or more commonly perpendicular to the free surface because the free surface cannot support shear stress and therefore is a principal plane of the stress ellipsoid. Remember extensional joints are perpendicular to the minimum principal stress and therefore in a principal plane of the stress ellipsoid. If the joint is cemented or otherwise strong, then it is not a free surface and the above arguments would not apply.