Joints and Shear Fractures Chapter 8 Joints and Shear Fractures

Joints “Joints are the most ubiquitous structure in the Earth’s crust, occurring in a wide variety of rock types and tectonic environments…They control the physiography of many spectacular landforms and play an important role in the transport of fluids…” Pollard and Aydin 1988

Joints Joints – Fractures along which there is no appreciable displacement parallel to the fracture and only slight movement normal to the fracture plane.

Importance of Joints Economically important minerals can be found in joints Joints act as the plumbing system for ground-water Help to demonstrate the tectonic history of an area which is important for the construction of dams, bridges, power plants, and buildings.

Three modes of fractures Three types of fractures have been identified with each one formed by a separate kind of motion. Mode I – Opening of fractures Mode II – Sliding of fractures Mode III – Tearing of fractures

Joint Systems Systematic Joints – Parallel joints with regular spacing Joint Set – Joints that share a similar orientation Joint System – Two or more joint sets in the same area Nonsystematic Joints – Do not share a common orientation, can be curved, and can form irregular fracture surfaces They occur in many areas but do not appear to be related to a recognizable stress field

Systematic and Nonsystematic Joints

Joint Appearance Joints Unfilled – Generally recent May have smooth surfaces May have irregular surfaces May have concentric ridges May have a feathered texture called Plumose Joints

Joint Appearance Joints Filled – Also called Veins Feldspar or Aplite – High temperature Quartz, Calcite, Chlorite, and Epidote – Low temperatures Ore Minerals – Low temperatures

Calcite filled joints in Vermont

Joint Sets Conjugate Joint Systems – Paired joint sets that form at acute angles and are thus shear fractures. Difficult to make certain that the acute joints formed at the same time If you can prove they are conjugate then σ1 bisects the acute angle

Fracture Analysis Study of joint systems in an area reveals the sequence and timing of tectonic events. The orientations of systematic fractures provides information about the orientation of the principal stress directions involved in brittle deformation.

Joints and Principle Stress Axes

Regional Tectonics Regional joint-orientation patterns may be determined by measuring strike and dip of mesoscopic-scale joints over a wide area. Bearing of linear stream systems Satellite imagery Topographic maps Aerial photos

Aerial photo of Precambrian granite in Wyoming

Graphical depictions of joint sets in Pigeon Forge, TN Lower-hemisphere equal-area plot Rose diagram Contour diagrams of a

Jointing related to the strain ellipsoid

Fracture and Lineament Orientation in Italy

The Anatomy of Joint Surfaces Various features provide information on the rate and direction of propagation of joints. Hackle Marks – Form in the zone where the joint traveled rapidly Arrest Lines – For parallel to the advancing edge of the fracture and perpendicular to direction of propagation. Origin – Can often determine the initial site of the joint. Joints always begin at a preexisting flaw in the rock such as a grain of atypical size or hardness, fossil, or concretion.

Plumose joint surface showing primary surface features. 1. Main joint face 2. Twist hackle fringe 3. Origin 4. Hackle plume 5. Inclusion hackle 6. Plume axis 7. Twist-hackle face 8. Twist-hackle step 9. Arrest lines 10. Constructed fracture-front lines

Features associates with the propagation of a joint

Plumose Joint Face, Ontario

Glacially produced joint surface in Killarney Granite, Ontario

Controlling Factors of Joint Propagation Bedding and foliation planes in coarse-grained rocks act as barriers to joint propagation. Bedding in fine-grained rocks are often not barriers. Variation in bed thickness also affects propagation direction.

Four Categories of Joints Tectonic – Form at depth and are driven by tectonic forces. Hydraulic – Form at depth during burial and compaction. Unloading – Form near the surface when ½ of the overlaying sediment is removed by erosion. Release Joints – Form after the release of horizontal stress and are controlled by existing rock fabric.

Joints in Plutons Joints form in plutons in response to cooling and later tectonic stress Orientations of joints may be influenced by the boundary of the pluton

Sheeting Sheeting (Same as unloading joints) – Form parallel to surface topography in massive rocks. Spacing between sheets increases with depth. Can be used in quarrying stone.

Columnar Joints Columnar Joints – Form in response to cooling and shrinkage in magma. Form in Flows, Dikes, Sills, and Volcanic Necks Hexagonal prisms are the most efficient geometric shape.

Contraction to Form Columnar Joints or Mud Cracks