FOLDS I. FOLDS - type of deformation superimposed on some type of layering A. 1o layering - bedding, igneous layering = dikes and sills B. 2o layering - foliation, slaty cleavage, phyllitic cleavage, schistosity, gneissosity, mylonitic foliation, pressure solution, etc... C. Folds require heterogeneous strain II. TERMS: A. Hinge or hinge line - B. Maximum curvature - crest (highest point) C. l = wavelength from one hinge to another measured perpendicular to amplitude D. A = amplitude, distance from hinge to hinge measured parallel to a plane of symmetry E. Folds can be classified by interlimb Ð (ie. tight, open, gentle) Fleuty's classification of folds 180o > Ð > 120o gentle 120o > Ð > 70o open 70o > Ð > 30o close 30o > Ð > 0o tight 0o     =   isoclinal Ð < 0o     mushroom F. Fold axis - imaginary line swept through space to generate folds, not synonymous with hinge if swept parallel to itself, produces cylindrical folds (don't change shape from one to another) as opposed to conical folds where one end of axis is held stationary G. Hinge surface - connects hinges of each individual folded layer H. Axial surface - like the axis, not tied to a particular fold I. Anticline - oldest units in the core J. Antiform - concave down K. Syncline - youngest units in the core L. Synform - concave up M. Periodic - have well defined l N. Aperiodic - sporadic, no repeat distance e.g., kink folds O. Disharmonic - at depth, rock layers not in harmony ~ "room problem" III. DESCRIBING FOLDS IN THE FIELD: A. Need hinge line - bearing and plunge, read looking down plunge, can be horizontal, subhorizontal, vertical, subvertical or plunging B. Need hinge surface - take strike and dip, describe if axial surface is vertical (upright), inclined or horizontal/subhorizontal (recumbent) C. Measure limbs of the fold - strike and dip of short limb and long limb IV. DESCRIPTIONS OF FOLD STYLES A. Same hinge surface, must describe type of fold B. Ramsay developed dip isogons - lines of equal dip 1. Three classes: a. Convergent (with subclasses A,B,C) b. Parallel c. Divergent 2. Most common classes: 1B, 1C and 2 3. Classes do not explain fold style V. GEOMETRIC CLASSIFICATION OF FOLDS A. Based on: 1. Changes in bed thickness around the fold hinge 2. Difference in fold shapes in adjacent layers B. Concentric folds parallel folds - all bedding planes remain parallel to each other 1. Thickness is constant around fold hinge C. Similar folds - all folded surfaces have similar shapes 1. Line lengths between contacts measured parallel to hinge surfaces are constant VI. GENETIC CLASSIFICATION OF FOLDS A. Flexural slip fold - formed by flexural slippage mechanism (layers slip past one another during folding) 1. Micas behave or deform this way 2. Interlayered sandstones and shales (turbidites) B. Similar (or flow) folds - formed by shear folding mechanism two ways they can be produced: 1. Ductile deformation zone - form due to inhomogeneous simple shear 2. Modified concentric fold - start with a concentric fold and superimpose simple shear modified concentric folds are called: a. Shear folds b. Passive folds c. Flow folds d. Slip folds C. Other fold styles (exceptional) 1. Chevron folds a. Characterized by sharp hinges and straight limbs b. Require greater competency contrast between layers (ie. Interlayered sandstones and shales of a turbidite sequence) 2. Ptygmatic folds a. Flattening of veins or dikes b. Little if no kinematic significance 3. Kink folds a. Characterized by discrete bands of rotated layering b. High contact strength (micas) c. Locking angle - geometrically limits the amount of rotation d. Aperiodic e. Vergence of kinks is meaningless 4. Special types of kinks: a. Monoclines b. Box folds VII. FOLD ASYMMETRY A. Note orientations of minor folds mimic large scale structures B. Note orientation down plunge VIII. DISMEMBERMENT STRUCTURES (3 types) A. boudinage = "sausage" shaped in cross section 1. (ductility contrast) - A has the lowest ductility compared to B and C which are ductile e.g., A = conglomerate B and C = calcite limestone (metamorphosed to 5000 c) 2. Boudinage associated with folding a. Eventually the bed passes from the shortening to the elongation field when rotated 3. 2 types of boudinage related to folds a. Rods - fold axis parallel to rod length b. Chocolate tablet - 3D strain flowage B. Melange - generally pieces of sandstone floating in a shale matrix, but can be very complex with blocks of different rock types from different crustal levels and tectonic settings 1. Form in oceanic trenches e.g., west coast of U.S. Franciscan Belt e.g., sandstone, blueschist, and greenschist knockers 2. Broken Formation - transition is parallel to formation - sequence of bedding can still be seen C. Transposition - folding advances to a point where compositional layering is so folded that only individual hinges are recognizable, limbs are removed 1. Common in low grade metamorphic rocks a. Slaty and intersection lineation cleavage b. Pressure solution IX. BEDDING vs. CLEAVAGE RELATIONS: A. If folds are cylindrical - find axial planar cleavage B. Cleavage dip > bedding dip = upright C. Bedding dip > cleavage dip = overturned D. Tells plunge and bearing of the major fold involved E. Tells how to locate the fold hinge F. Tells stratigraphic up and down