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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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slide1

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.

slide2

Joint: A natural fracture that forms by tensile loading- walls of fracture move apart slightly as joint develops

joints fractures geometry
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

slide4

Plumose structure: A subtle roughness on surface of some joints; resembles imprint of a feather. Due to inhomogeneity of rock.

slide6

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
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
Griffith cracks: preexisting microcracks and flaws in a rock

The largest properly oriented Griffith crack propagates to form a through-going crack

slide10

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

slide12

Exfoliation joints: Form by unloading of bedrock through erosion.

They form parallel to topography

slide13

Exfoliation joints: Form by unloading of bedrock through erosion.

They form parallel to topography

joint analysis
Joint analysis

Significance: determine orientation of tectonic stresses

slide16

Significance for Engineering

Planes of weakness!

slide20

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 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

slide26

Significance: Economic Geology

Alteration/Mineralization along fractures; Veins preserve dilational separation

slide27
Joints/Fractures: “no appreciable displacement”Next: Geometry and Kinematics: Faults (Read D&R, p. 269-279; 286-296)
slide28
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