joints and shear fractures (d & r; p. 205-226) figures not from d & r are from: earth...
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Joints and Shear Fractures (D & R; p. 205-226)
Figures not from D & R are from: Earth Structure: An Introduction to Structural Geology and Tectonics, by Ben van der Pluijm and Stephen Marshak. Copy available in TA office, Rm. 314.
Joint: A natural fracture that forms by tensile loading- walls of fracture move apart slightly as joint develops
Joints/Fractures: Geometry
Planar and often smooth; no appreciable displacement. Most abundant structural element in crust. What do the surfaces look like?
Moscow Kremlin - Bell Tower of Ivan the Great. Fractured in 1737 due to uneven cooling
Plumose structure: A subtle roughness on surface of some joints; resembles imprint of a feather. Due to inhomogeneity of rock.
Joints: commonly elliptical
Close-up views of hackles in plumose structure. Plumose structure is more prominent away from origin due to stress concentrations at crack tips
Joints/Fractures: Kinematics
ribs are arrest lines- opening is not instantaneous, but rhythmic,
like splitting wood
Griffith cracks: preexisting microcracks and flaws in a rock
The largest properly oriented Griffith crack propagates to form a through-going crack
Joint arrays
Three competing mechanisms that contribute to joint formation during uplift and erosion:
(1) Contraction during cooling
(2) Poisson effect- e.g., rock expands in vertical direction and contracts in horizontal direction during unloading
(3) Membrane effect- expansion due to increase in curvature of layer
Cooling joints: form by thermal contraction
Exfoliation joints: Form by unloading of bedrock through erosion.They form parallel to topography
Exfoliation joints: Form by unloading of bedrock through erosion.They form parallel to topography
Tectonic joints: Form by tectonic stresses as opposed to stresses induced by topography.
Joint analysis
Significance: determine orientation of tectonic stresses
Significance for Engineering Planes of weakness!
Significance: Geologic Hazards
Joints and Geomorphology
Shear fracture: A fracture that grows in association with a component of shear
Shear fractures
en echelon tension gashes-form ~45 degrees from plane of max. shear stress-preexisting vein material rotates while new vein material grows
What is it?
What are these structures?What is the sense-of-shear?Describe how the veins grew.
en echelon tension gashesright lateral or top-to-the-rightfrom center to tips during rotation
What is it?
Determining the sense of shear
Vein filling during crack opening
Significance: Economic GeologyAlteration/Mineralization along fractures; Veins preserve dilational separation
Joints/Fractures: “no appreciable displacement”
Next: Geometry and Kinematics: Faults (Read D&R, p. 269-279; 286-296)
Important terminology/concepts
Joints- what are they?
Joint ornamentation- plumose structure
Joint kinematics: opening, sliding, scissoring
Griffith cracks and tensile crack formation
Tectonic joints
Exfoliation/unloading joints
Cooling joints
Joint arrays and joint analysis
Shear fracture formation
- en echelon tension gashes
- sense-of-shear indicators
Significance
- tectonics
- engineering
- economic geology
- hazards