Presentation on theme: "Joints and Shear Fractures (D & R; p. 205-226) FEBRUARY 4 / Joints, shear fractures and faults: geometry / 5&6 6 / Mechanics of faulting / 3&6 11 / The."— Presentation transcript:
Joints and Shear Fractures (D & R; p. 205-226) FEBRUARY 4 / Joints, shear fractures and faults: geometry / 5&6 6 / Mechanics of faulting / 3&6 11 / The Mohr stress diagram / 5 13 / Folds: geometry and strain / 7 18 EXAM #1 20 / Fold and thrust belts / 6 25 / Other compressional regimes / 6 27 / No class today (instructor out of town) Joints, shear fractures, And intro to mechanics Instead
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.
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