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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.
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Joint: A natural fracture that forms by tensile loading- walls of fracture move apart slightly as joint develops
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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
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Plumose structure: A subtle roughness on surface of some joints; resembles imprint of a feather. Due to inhomogeneity of rock.
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Joints: commonly elliptical
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Close-up views of hackles in plumose structure
Close-up views of hackles in plumose structure. Plumose structure is more prominent away from origin due to stress concentrations at crack tips
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Joints/Fractures: Kinematics
ribs are arrest lines- opening is not instantaneous, but rhythmic, like splitting wood
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Griffith cracks: preexisting microcracks and flaws in a rock
The largest properly oriented Griffith crack propagates to form a through-going crack
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Joint arrays
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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
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Cooling joints: form by thermal contraction
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Exfoliation joints: Form by unloading of bedrock through erosion.
They form parallel to topography
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Exfoliation joints: Form by unloading of bedrock through erosion.
They form parallel to topography
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Tectonic joints: Form by tectonic stresses as opposed to
stresses induced by topography.
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Joint analysis Significance: determine orientation of tectonic stresses
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Significance for Engineering
Planes of weakness!
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Significance: Geologic Hazards
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Joints and Geomorphology
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Shear fracture: A fracture that grows in association with a component of shear
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Shear fractures en echelon tension gashes -form ~45 degrees from plane of max. shear stress -preexisting vein material rotates while new vein material grows
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What is it? What are these structures? What is the sense-of-shear? Describe how the veins grew. en echelon tension gashes right lateral or top-to-the-right from center to tips during rotation
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What is it?
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Determining the sense of shear
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Vein filling during crack opening
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Significance: Economic Geology
Alteration/Mineralization along fractures; Veins preserve dilational separation
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Joints/Fractures: “no appreciable displacement” Next: Geometry and Kinematics: Faults (Read D&R, p ; )
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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
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