Faults - Faulting and Folding 1. Stress () is force...

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Faulting and Folding 1. Stress ( σ ) is force exchanged per unit of area; types include pressure (P) with forces perpendicular to the surface, and shear stress ( σ s ) with forces parallel to the surface. Strain ( ε ) is change-in- dimension/dimension; types include volumetric strain ( ε v ) and shear strain ( ε s ). 2. Mechanisms of deformation include elasticity under all conditions, and either “ductile” viscous flow (at high T ) or “brittle” fracture and frictional sliding (at low T and high σ s ). * elasticity: v ~ P in all matter; s ~ σ s in solids. * viscous flow: s ~ σ s × time at over 60% of melting temperature. * frictional sliding occurs if s = f × ( P - P fluid ) + 3. Faults are surfaces of frictional sliding which extend down to the base of the lithosphere, where they turn into viscous flow zones in the asthenosphere. 4. Faults form at 30 ° to the shortening axis and 60 ° to the extension axis, so they appear in 3 varieties: * normal faults (60 ° dip) where crust is stretched (ridges). * thrust faults (30 ° dip) where crust is shortened (subduction zones). * strike-slip faults dip vertically and slip horizontally. Equivalent to transforms. 5. A fourth deformation mechanism is solution transfer, in the special case of water-soluble minerals sitting in water for long times. Such sedimentary rocks often fold instead of faulting. Most large folds are now thought to lie over thrust faults in the brittle metamorphic “basement” layer of the crust, and they may be seismically dangerous!
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1 c:\work\classes\ess1\faults.doc FAULTING AND FOLDING To talk about mechanics, we need some definitions/vocabulary/concepts: 1. Stress ( σ ) is force exchanged per unit of area; types include pressure (P) with forces perpendicular to the surface, and shear stress ( σ s ) with forces parallel to the surface . -We talk of force exchanged across a surface because of Newton’s 3 rd Law: “For every “action” [force transmitted one way] there is an equal and opposite “reaction” [force transmitted back].” -Because stress is force/area, its metric units are N/m 2 = Pa. Superior to confusing “pounds per square inch” in English tradition.
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This note was uploaded on 10/14/2010 for the course ESS ESS1 taught by Professor Bird during the Spring '10 term at UCLA.

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Faults - Faulting and Folding 1. Stress () is force...

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