Folds in Meta-Greywacke, New Zealand - PDF

Folds Ductile folds that formed under high temperature and pressure conditions in meta-greywacke, New Zealand  A.N.Hughes Folds Folds are bends or flexures of layered rock that...

Folds Ductile folds that formed under high temperature and pressure conditions in meta-greywacke, New Zealand  A.N.Hughes Folds Folds are bends or flexures of layered rock that form in response to motion along faults, diapirism, compaction, ductile flow, and/or regional subsidence or uplift. Folds are expressed as one or more regions of dipping features that correspond to inclined foliations (usually stratigraphic bedding). Folding under moderately-low pressure and temperature conditions, Raplee Ridge Monocline, near Mexican Hat, UT Fold terminology A.N.Hughes Folding under intermediate pressure and temperature conditions, West Castleton Syncline, Vermont Fold terminology Fold Limb Fold Limb Fo ld Ax is A.N.Hughes Folding under intermediate pressure and temperature conditions, West Castleton Syncline, Vermont Fold terminology Fold Limb Fold Limb Fo cu ld A r v xi at s/ Fo ur Hi e t ng ld ha e : t s Lo Ax ep ca ar tio is at n es of fo m ld ax lim im bs um Fold limb (dip domain): area where rocks all have a relatively constant dip A.N.Hughes Folding under intermediate pressure and temperature conditions, West Castleton Syncline, Vermont Elements of a fold Hinge Line: line of maximum curvature on a fold (parallel to fold axis in cylindrical folds) Axial surface: Plane connecting hinge lines for different layers in the fold, which separates regions of different dip Fold terminology Fold Limb Fold Limb Fo cu ld A r v xi at s/ Fo ur Hi e t ng ld ha e : t s Lo Ax ep ca ar tio is at n es of fo m ld ax lim im bs um Fold limb (dip domain): area where rocks all have a relatively constant dip A.N.Hughes Folding under intermediate pressure and temperature conditions, West Castleton Syncline, Vermont  Cylindrical vs. non-cylindrical folds Parallel synclinal and Non-Parallel anticlinal synclinal and hinges anticlinal hinges A cylindrical fold can be described geometrically In a non-cylindrical fold, rotation or by passing a line (fold hinge line) parallel to itself distortion of a line is necessary to through space (without rotation or distortion) trace out the form of the fold  Fold tightness Unfolded 180o Gentle 170-180o Open 90-170o interlimb angle Tight 10-90o Folded limestones, Kaikoura, New Zealand *these are words you use to describe the relationships between the TRUE DIP of fold limbs (think best-oriented cross section)—NOT map view Isoclinal 0-10o patterns or apparent dips  Curved-hinge folds Fold Angular-hinge folds curvatur e Chevron Circular Box Elliptical  A.N.Hughes Example: Angular Chevron Folds Folding under moderately-low pressure and temperature conditions in interbedded shales and limestones, Kaikoura, New Zealand (C. Connors for scale) Example: Barnhardt Canyon, Mazatzal Wilderness, AZ Tight Chevron Fold  A.N.Hughes Kink Bands Kink bands (parallel folding, parallel axial surfaces) form in rocks that are well bedded (thin, high contrast layers) Example: Southern Rincons Mountains, Arizona Boo on you, M. Dailey. You are no friend of structural geology  Intersection of axial surfaces As axial surfaces converge, they form a new axial surface orientation (that is the angle bisector, if layer thickness is maintained (which it usually is for map-scale structures that form under low to moderate pressure and temperature conditions)  Intersecting axial surfaces in practice Suppe, 1985 Characterizing folds by their geometry… Folds that share a common axial surface orientation Folds that share a common hinge line orientation …requires defining the orientation of the axial surface and hinge line in order to uniquely describe a given fold. Fold classificat ion by dip of hinge and axial surface Ramsay classification of folds Dip Isogons: lines connecting points of equal dip between top and bottom layer. Pattern of dip isogons Is used to classify fold type. Class 1: Inner curvature>Outer curvature 1A—rare 1B—common in low-temperature folding dominated by flexural slip—parallel layers—”parallel” fold 1C—common in competent layers deformed at high temperatures Class 2: Inner curvature=Outer Curvature Common in rocks with low ductility contrasts folded by passive amplification—parallel isogons—”similar” fold Class 3: Outer curvature>Inner Curvature Common in weak layers deformed at high temperature Harmonic vs. disharmonic folding Harmonic folding: no, or very gradual, changes in fold shape between successive layers “in phase” Disharmonic folding: pronounced changes in fold shape between successive layers Van Der Pluijm and Marshak, 1997 Harmonic Folding, Canadian Rockies Front Ranges, Lake Minnewanka, Alberta Harmonic Folding, Asturias, Spain C.D. Connors Example of disharmonic folds in layered carbonates and evaporites in the core of the Barbastro Anticline, Aragon, Spain A.N. Hughes  Parasitic/subsidiary folds: minor folds that develop on Parasitic folds the flank of a larger fold that reflect the overall shear sense in the fold limb Dextral ”Z” folds Sinistral ”S” folds The asymmetry of parasitic folds points toward the crest of the larger fold Parasitic folding example: Galicia, Spain Dextral ”Z” Sinistral ”S” folds folds The asymmetry of parasitic folds points toward the crest of the larger fold C.D. Connors  A.N.Hughes Folding of interbedded shales and limestones, Kaikoura, New Zealand Anticlines and synclines syncline: concave upward, with youngest rocks in center. N anticline: concave downward, with oldest rocks in center. J.H.Shaw Anticlines and synclines Anticline, Map View Syncline, Map View N N N J.H.Shaw Plunging anticlines and synclines Plunging Anticline, Map View N N N Plunging Syncline, Map View Barnhardt Canyon, Mazatzal Wilderness, Arizona Tight, disharmonic chevron folds A.N.Hughes  Barnhardt Canyon, Mazatzal Wilderness, Arizona Fold hinge line / Fold Axis FOLD FOLD LIMB LIMB Tight, asymmetric, A.N.Hughes inclined fold Folded interbedded limestones and marls Schluchhorn, Bernese Alps, Switzerland (photo by Bernhard Edmaier)