Joints and Shear Fractures (D & R; p ) 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.

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 gashes right lateral or top-to-the-right from center to tips during rotation

Determining the sense of shear

Vein filling during crack opening

Significance: Economic Geology Alteration/Mineralization along fractures; Veins preserve dilational separation

Joints/Fractures: “no appreciable displacement”

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