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 belts70 (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 structure71 (1) Culiminations: crest lines at max. el. (2) Depressions: trough lines at min. el. j. Fold Profile (1) shape of fold examined in plane perpendicular to hinge line k. Fold Types (1) cylindrical folds: folds in which lines can be moved parallel to the fold surface without losing contact with the surface (a) Fold Axis: unique, imaginary reference line of constant orientation that can be moved along the fold surface without losing contact with the surface i) 0 degree angle between fold axis and any part of fold (2) Non-cylindrical folds: do not possess fold axis (a) essentially undulatory surfaces 2. Parts of Folded Layers and Multilayers a. Multilayers: nested stack of two or more folded surfaces (1) Inflection surfaces: planes connecting inflection lines on multiple fold levels (2) Axial Plane: plane connecting mult-level hinge lines (3) Axial Surface Trace: Axial surface intersects ground surface as a line. b. Anticlines and Synclines (1) Anticline: antiform structure with oldest rocks on concave side of fold (2) Syncline: synform structure with youngest rocks on concave side of fold (a) Overturned anticline = upside down “synformal” anticline (b) Overturned syncline = upside down “antiformal” syncline C. Fold Scale and Attitude 1. Components of scale a. wavelength: crest to crest distance (1) controls: commonly according to bed thickness (a) > thickness, > wavelength (b) < thickness, < wavelength72 b. amplitude (1) median surface: line connecting inflexion points on fold (2) A = distance between crest or trough, and median surface (i.e. “height” of fold”) 2. Attitude of Folds: Descriptive Terminology a. Classification based on Axial Surface (axial plane if planar) (1) upright: axial plane vertical (2) steeply, mod., gently inclined: axial plane between horizontal and vertical (3) recumbent: axial plane horizontal b. Classif. based on plunge of hinge line (1) gently, mod., steeply plunging = inclined plunge (2) horizontal: plunge = 0 degrees (3) vertical plunge = 90 degrees (4) Doubly plunging fold: hinge line plunges in two directions c. Domes and Basins (1) Dome: antiformal, dome structure (2) Basin: synformal, bowl structure d. Other Terms (1) Homocline: uniformly dipping beds with no change in dip over a regional area (2) Monocline: fold pair with long limbs of similar dip, and a short step of increased dip (3) Overturned folds: one of the limbs of the fold is inverted upside down (stratigraphy inverted) D. Elements of Fold Style (form description) 1. Measures of folding a. fold angle: (1) angle between two lines drawn perpendicular to inflection points of fold b. Interlimb angle (1) angle between two lines drawn as tangents to the inflections points of fold (2) angle < with increasing tightness of fold 2. Cylindricity a. measure of degree to which fold approximates ideal cylindrical fold (1) practical measure: on stereonet, how closely do poles to planes of fold fit a great circle distribution?73 3. Symmetry a. symmetric folds: in profile, shape on one side of hinge is mirror image to that on other side of hinge (1) limbs on both sides of fold have same dip angle b. asymmetric folds: no mirror plane symmetry in profile view (1) Z-folds: looking down plunge, fold profile has shape of Z (2) S-folds: looking down plunge, fold profile has shape of S (a) m-folds: small scale M-shaped folds found in the core of larger scale folds (3) Vergence: up dip direction of axial surface on an asymmetric fold 4. Style of Folded Surface (quantitative descriptors) a. Aspect Ratio = Amplitude/distance between inflection pts (1) wide, broad, equant, short, tall b. Tightness = measure of degree