FOLD CLASSIFICATIONS

Transcription

1 GG303 Lecture 28 9/4/01 1 FOLD CLASSIFICATIONS I Main Topics A Fold nomenclature B Ramsay's classification schemes C Interference of folds D Superposition of folds II Fold nomenclature and classification schemes A Anticlines, synclines, antiforms, synforms, and monoclines B Kinks: folds with sharp, angular hinge regions C "Tightness" of folds D Classification by orientation of axial plane and plunge of fold axis E Symmetrical folds vs. asymmetrical folds III Ramsay's classification scheme; single-layer folds in profile A Relates the curvature of the inner and outer s of a fold. B Dip isogons: lines that connect points of equal dip Fold class Curvature (C) Comment I Cinner > Couter Dip isogons converge 1A Orthogonal thickness on limbs exceeds thickness at hinge; uncommon 1B Parallel folds 1C Orthogonal thickness on limbs is less than thickness at hinge 2 Cinner = Couter Dip isogons are parallel Class 2 = similar folds) 3 Cinner < Couter Dip isogons are diverge Class 1C (or 1B) folds commonly are stacked with class 3 folds. Stephen Martel 28-1 University of Hawaii

2 GG303 Lecture 28 9/4/01 2 IV Mechanical interaction of folds (See Fig of Suppe) A Layers far apart will not interact as they fold B Layers of similar properties that are close together will tend to fold as a single fold C Layers "near" each other will interact V Superposition of folds A Can produce highly complicated geometries B Common in metamorphic rocks C "Demonstration" of z- and s- folds (parasitic) References Compton, R.R., 1962, Manual of Field Geology: Wiley, New York, 378 p. Ragan, D.M., 1973, Structural geology: Wiley, New York, 208 p. Ramsay, J.G., 1967, Folding and fracturing of rocks: McGraw-Hill, New York, 568 p. Ramsay, J.G., and Huber, M.I., 1987, The techniques of modern structural geology, volume 2: Folds and fractures: Academic Press, New York, 700 p. Stephen Martel 28-2 University of Hawaii

3 GG303 Lecture 28 9/4/01 3 NOMENCLATURE FOR FOLDS w Fig s Limb Hinge Positive curvature = concave up Negative curvature = concave down Limb w Inflection point; curvature = 0. s Radius of curvature is small(est) at the hinge, larg(est) on the limbs Symmetrical Folds Wavelength Enveloping Amplitude Enveloping Asymmetrical Folds Enveloping Crest Hinge Enveloping Hinge Trough Stephen Martel 28-3 University of Hawaii

4 GG303 Lecture 28 9/4/01 4 NOMENCLATURE FOR FOLDS Fig Anticline Antiformal syncline Monocline Syncline Synformal Anticline Syncline: fold where rocks become younger towards axial Anticline: fold where rocks become older towards axial Synform: fold where limbs dip towards axial Antiform: fold where limbs dip way from axial Monocline: gentle anticline-syncline pair with horizontal outer limbs Overturned folds Overturned syncline: one limb of syncline is overturned Overturned anticline: one limb of anticline: is overturned Stephen Martel 28-4 University of Hawaii

5 GG303 Lecture 28 9/4/01 5 Ramsay's Fold Classification Fig Dip Isogon: a line that connects points of equal dip on the top and bottom of a folded layer Class 1: Dip isogons converge towards axial ; Cinner > Couter 1A Limbs thicker than hinges Dip isogon 1B Layer thickness is constant (parallel folds) 1C Limbs thinner than hinges Class 2: Dip isogons parallel axial (similar folds); Cinner = Couter Inner and outer fold s have exactly the same shape Translate outer fold parallel to axial to match inner fold Class 3: Dip isogons diverge from axial ; Cinner < Couter Class 3 conditions can't extend "forever" otherwise the inner and outer fold s would cross Stephen Martel 28-5 University of Hawaii

6 GG303 Lecture 28 9/4/01 6 Terms for Describing the Tightness of Folds Fig Interlimb angle 180 o o 120 o - 70 o 70 o - 30 o 30 o - 0 o "0 o " Negative Description of fold Gentle Open Close Tight Isoclinal Mushroom Gentle 120 o 70 o Open 30 o 30 o Close Tight Isoclinal (limbs are parallel) Stephen Martel 28-6 University of Hawaii

7 GG303 Lecture 28 9/4/01 7 Fold Classifications Fig (modified from Ragan, 1973, Figure 7.10) Based on direction of fold concavity, axial suface orientation, and fold axis orientation Vertical Fold axis Fold opens horizontally Vertical axial Vertical fold axis Upright plunging Reclined Fold opens up or down Vertical axial Horizontal fold axis Inclined plunging Fold opens sideways Horizontal axial Horizontal fold axis Upright horizontal Inclined horizontal Recumbent First modifier (e.g., "upright") describes orientation of axial Second modifier (e.g., "horizontal") describes orientation of fold axis Stephen Martel 28-7 University of Hawaii