folds - I. Introduction to Ductile Deformation A. Ductile...

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69 I. Introduction to Ductile Deformation A. Ductile Deformation = "solid state flow" 1. Permanent, coherent solid-state deformation 2. no loss of cohesion at scale of mineral crystals 3. no evidence of brittle fracturing a. e.g. glacial / ice deformation b. metal rolling in steel plants B. Ductile Characteristics 1. smoothly varying deformation 2. no evidence of discontinuities 3. no indication of discrete shear planes or fracture planes 4. excludes soft sediment deformation, since sediments are not yet consolidated in coherent state C. General Process Conditions 1. Thermally activated process (Temps = 50% melting point) 2. deformation processes a. dislocations through crystal lattices b. solid-state diffusion (molecular transport) D. Fold Deformation dramatic evidence of ductile deformation processes II. Description of Folds A. Introduction 1. folds: wave-like undulations of rock layers a. Commonly result from ductile deformation of sedimentary strata 2. Fractal nature of folds a. microscopic to mesoscopic to megascopic 3. Folds common to large-scale orogenic belts a. Fold and thrust complexes (1) Appalachian valley and ridge (2) Canadian Rockies (3) Himalaya (4) Alps b. Outer zone of orogenic belts = fold and thrust c. Inner zone of orogenic belts
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70 (1) > depths, > temperatures (2) metamorphic mountain cores 4. Other occurrences of folds a. glacial ice b. salt dome complexes c. folded beds, veins, dikes, igneous complexes 5. Importance of folds a. traps for oil and gas b. influence deep hydrologic regime c. economic mineral accumulations d. record of tectonic process B. Fold Geometries and Morphology 1. Single folded surface (e.g. bedding contact) a. crests and troughs of wave form (1) crest = convex up (2) trough = concave up b. Inflection line (1) Pt. of change in curvature from convex to concave c. Fold Train (1) series of folds of alternating curvature (a) alternating crests and troughs (2) Antiforms: convex up fold (3) Synforms: concave up fold d. Fold System (1) series of folds of similar geometry and common origin e. Curvature: measure of change in orientation per unit distance along fold structure (1) circle = constant radius of curvature (2) flat plane = 0 radius of curvature f. Hingeline or hinge: imaginary line connecting points of maximum curvature along a fold (1) hinge may be curved or straight (2) hinge may vary in magnitude of curvature (3) Hinge zone = vicinity of max. curvature g. Limbs of fold = regions of lowest curvature on folds h. Crest line: line connecting points of highest elevation on structure i. Trough line: line connecting points of lowest elevation on structure
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71 (1) Culiminations: crest lines at max. el. (2) Depressions: trough lines at min. el. j.
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This note was uploaded on 10/02/2011 for the course GEOLOGY 406 taught by Professor Taylor during the Spring '11 term at Western Oregon University.

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folds - I. Introduction to Ductile Deformation A. Ductile...

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