Presentation on theme: "Joints and Shear Fractures"— Presentation transcript:
1Joints and Shear Fractures Chapter 8Joints and Shear Fractures
2Joints“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
3JointsJoints – Fractures along which there is no appreciable displacement parallel to the fracture and only slight movement normal to the fracture plane.
4Importance of JointsEconomically important minerals can be found in jointsJoints act as the plumbing system for ground-waterHelp to demonstrate the tectonic history of an area which is important for the construction of dams, bridges, power plants, and buildings.
5Three modes of fractures Three types of fractures have been identified with each one formed by a separate kind of motion.Mode I – Opening of fracturesMode II – Sliding of fracturesMode III – Tearing of fractures
6Joint Systems Systematic Joints – Parallel joints with regular spacing Joint Set – Joints that share a similar orientationJoint System – Two or more joint sets in the same areaNonsystematic Joints – Do not share a common orientation, can be curved, and can form irregular fracture surfacesThey occur in many areas but do not appear to be related to a recognizable stress field
11Joint SetsConjugate 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 timeIf you can prove they are conjugate then σ1 bisects the acute angle
12Fracture AnalysisStudy 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.
14Regional TectonicsRegional joint-orientation patterns may be determined by measuring strike and dip of mesoscopic-scale joints over a wide area.Bearing of linear stream systemsSatellite imageryTopographic mapsAerial photos
19The 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 rapidlyArrest 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.
23Glacially produced joint surface in Killarney Granite, Ontario
24Controlling 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.
25Four 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.
26Joints in PlutonsJoints form in plutons in response to cooling and later tectonic stressOrientations of joints may be influenced by the boundary of the pluton
27SheetingSheeting (Same as unloading joints) – Form parallel to surface topography in massive rocks.Spacing between sheets increases with depth.Can be used in quarrying stone.
28Columnar JointsColumnar Joints – Form in response to cooling and shrinkage in magma.Form in Flows, Dikes, Sills, and Volcanic NecksHexagonal prisms are the most efficient geometric shape.
29Contraction to Form Columnar Joints or Mud Cracks