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I. Introduction to Ductile Deformation A. Ductile Deformation = "solid state flow" 1.2. Permanent, coherent solid-state deformationno loss of cohesion at scale of mineral crystals
- no evidence of brittle fracturing a. e.g. glacial / ice deformation b. metal rolling in steel plants B. Ductile Characteristics
- smoothly varying deformation 2.3. no evidence of discontinuitiesno indication of discrete shear planes or fracture planes
- excludes soft sediment deformation, since sediments are not yetconsolidated in coherent state
C. General Process Conditions 1.2. Thermally activated process (Temps = 50% melting point)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
- folds: wave-like undulations of rock layers a. Commonly result from ductile deformation of sedimentary strata
- Fractal nature of folds a. microscopic to mesoscopic to megascopic
- Folds common to large-scale orogenic belts a. Fold and thrust complexes (1)(2) Appalachian valley and ridgeCanadian Rockies (3)(4) HimalayaAlps
b. Outer zone of orogenic belts = fold and thrust c. Inner zone of orogenic belts
(1)(2) > depths, > temperaturesmetamorphic mountain cores
- Other occurrences of folds a.b. glacial icesalt dome complexes c. folded beds, veins, dikes, igneous complexes
- Importance of folds a. traps for oil and gas b. influence deep hydrologic regime c.d. economic mineral accumulationsrecord of tectonic process
B. Fold Geometries and Morphology
- 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)(3) Antiforms: convex up foldSynforms: 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)(2) circle = constant radius of curvatureflat plane = 0 radius of curvature
f. Hingeline or hinge: imaginary line connecting points of maximumcurvature along a fold (1)(2) hinge may be curved or straighthinge 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
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”)
- Attitude of Folds: Descriptive Terminology a. Classification based on Axial Surface (axial plane if planar) (1)(2) upright: axial plane verticalsteeply, mod., gently inclined: axial plane between (3) horizontal and verticalrecumbent: axial plane horizontal
b. Classif. based on plunge of hinge line (1)(2) gently, mod., steeply plunging = inclined plungehorizontal: 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 dipover a regional area (2) Monocline: fold pair with long limbs of similar dip, and a (3) short step of increased dipOverturned folds: one of the limbs of the fold is inverted upside down (stratigraphy inverted) D. Elements of Fold Style (form description)
- 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
- Cylindricitya. 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?
- 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)(2) Z-folds: looking down plunge, fold profile has shape of ZS-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
- 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 of acuteness of interlimb angle(1) gentle, open, close, tight c. Bluntness = radius of curvature of fold at point of closure (1) sharp, rounded, blunt
- Ramsay’s Fold Classification a. Basic Concepts (1) dip isogon: line drawn across the folded layer connecting two points of equal dip (2) orthogonal thickness: thickness of layer measured perpendicular to bedding planes (3) axial trace thickness: thickness of folded layer measuredparallel to axial trace of fold
_______________________________________________________________________b.^ Ramsay's Classification Class Dip Isogon Geometry(from convex to concave) Orthgonal Thick(from hinge to limb) (^) (from hinge to limb)Axial trace thick.
a.b. ramsay's class 1 B foldcommonly plunging structures, e..g Valley and Ridge province of Appalachians
- Similar Folds a. ramsy's class 2 fold b. fold nappes: large-scale, recumbent isoclinal folds
- Other a.b. Chevron: sharp angular foldsKink Folds: step folds c. Ptygmatic folds: common disharmonic folds found in met. rocks; d. "convolute folding", axial surface is curvedBox folds: box shape fold geometry Summary of Fold Geometries_______________________________________________________________________
HingeLine SurfaceAxial Tightness Symmetry Shape Size Class
plunging upright open Symmetric kink λ+ amp. concentric Nonplunging inclinedrecumbant tight Asymmetric circularchevron similarparallel box disharm.flow
III. Kinematic Models of Folding A. Introduction
- 2-dimensional deformation models a. homogeneous deformation: straight and parallel lines remain b. straight and parallel after deformationInhomogeneous deformation: straight and parallel lines become c. curved and nonparallel after deformationSimple Shear- two-dimensional, constant volume deformation (1) Simple shear = rotational shear (2) analogy: sliding cards in a deck (a) homogeneous: square sides become parallelogram (b) after deformationInhomogeneous: sides become curved
d. Pure Shear = "flattening" (1) Pure Shear = Non-rotational shear (2) analogy: flattening a square (a)(b) max compression = direct shorteningmin stress = lengthening, NO ROTATION
- Competence - measure of rheology of rock material = "resistance toductile deformation" a. Competent material: deforms ductilely at relatively low rate (resistant) (e.g. sandstone) b. Incompetent material: deforms ductilely at relatively high rate(non-resistant) (e.g. shale)
B. Flexural Folding of a Layer
- Flexural Folding: orthogonal thickness of a given rock layer remainsconstant throughout the folding process a. Produces "Parallel folds" b. Stress Processes (1) Bending = Non-compressional stress (a) Vertical torque stress i) opposing stress components that produceequal and opposite pressure that bend the layer into a fold ii) No horizontal compression or tension is felt (b) Sources of vertical bendingby layer i) magmatic uplift ii) draping over faulted basement rocks (2) Buckling = active compressive stress parallel to layer (a) e.g. fold-thrust complexes i) complex bending and buckling as beds areramped up onto thrust planes c. Strain Response in Folding (1) Orthogonal Flexure (a) lines drawn perpendicular to bedding planes,
(3) Fold Characteristics (a) axial surface of fold parallel to shear planes ofdeformation (b)(c) Orthogonal thickness of layer changes across foldthickness of layer parallel to shear planes remains constant (d) shape of fold is same on both convex and concaveside of fold
- Volume-Loss Folding a. Process: solution and removal of rock material, results inshortening and folding of layers (1) Discrete zones of volume loss due to solution
C. Flexure Shear and Flow Folding of Multilayers
- Problem: complexity of multilayered rock systems a. varying competence between interbeds (e.g. chert, shale,sandstone, limetone) (1) Each lithology possesses unique rheology and competence (2) Net result: complex fold systems with mixed modes
- Scenario 1: stack of rocks with high competence of similar magnitude (e.g. stack of sandstone beds) a. mechanical model: rheologic isotropy b. Flexure-slip folding (1) each competent layer slips over the other via shear alongbedding planes (a) shear greatest on limbs (b) shear = 0 at hinges (c) sense of shear reverses at hinge i) convex side of fold: shear converges fromboth limbs toward the hinge
ii) concave side of fold: shear diverges alongboth limbs away from hinge
- Scenario 2: stack of mixed competence rocks (high competence contrast between layers... e.g. sandstone and shale interbeds) a. mechanical model: rheologic anisotropy b. Processes (1)(2) Competent layers slip as coherent massIncompetent layers become zones of shear accommodation (a)(b) deformed greatest on limbsflattening at hinge zone c. If thickness of competent layer > incompetent layer (1) disharmonic, ptygmatic folds may develop in incompetentlayer
- Scenario 3: multilayer system comprised entirely of incompetent rocks (e.g. shales) a. mechanical model: rheologic isotropy b. Net process: flow folding (passive shear folding) (1) similar folds D. Formation of Kink and Chevron Folds
- what are they? a. Kink folds: symmetric folds with sharp angular hinges and straight b. limbsChevron Folds: asymmetric folds with sharp angular hinges and straight limbs
- Kink Folds a. Kink band: one short limb between two axial surfaces b. process models (1) kink band propagation with axial surfaces migrating into undeformed material (2) Stationary kinks develop in response to simple shear inhinge zones
- Chevron Folds a. Intersection of two kink bands
(1) f1, f2, f3... fn c. Examples (1) F1So = first generation of axial plane of original beddingplanes
- Complex fold patterns a. Type 1 (1)(2) fold 1 upright, fold 2 upright, axial surfaces at right anglesnet pattern = "egg carton"
b. Type 2 (1) fold 1 recumbent, fold 2 upright, axial surfaces at rightangles (2) net pattern= crescents, mushrooms or m-shape patterns
c. Type 3 (1) Fold 1 inclined, fold 2 upright, axial surfaces not at right (2) anglesnet pattern = wavy s-patterns H. Diapiric Flow
- Diapirs defined a.b. circular or elliptical structuresprocess: bouyant uplift of elliptical mass via density constrasts (Density of diapir < density of country rock) (1) net effect: rising diapiric mass, sinking country mass c. Common diapiric materials(a)^ e.g. lava lamp (1)(2) salt / salt domesshale / shale diapirs (3) magma diapirs (granites) / gneiss domes
- Folding and Diapirs a. internal folding, deformation and shear flowage within diapir b. uplift and buckling of country rocks above diapir(1) faulting of country rock as well.
