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Introduction to Folds (p. 372-413). Nomenclature Geometric analysis More nomenclature Fold mechanisms - flexural-slip folding - buckling - kink folding. Why are folds important? fundamental to deformation of the Earth’s crust.

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## Introduction to Folds (p. 372-413)

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**Introduction to Folds**(p. 372-413) • Nomenclature • Geometric analysis • More nomenclature • Fold mechanisms - flexural-slip folding - buckling - kink folding**Why are folds important?**fundamental to deformation of the Earth’s crust Sensitive markers to tectonic stresses- everywhere!! tectonic transport direction**Structural traps for oil**Controls ore geometry in strata-bound deposits**Main types of folds**Anticline: fold that is convex in the direction of the youngest beds Syncline: Fold that is convex in the direction of the oldest beds *requires that you know facing direction (direction of youngest beds); know stratigraphy! Antiform: convex up Synform: convex down *simply describes geometry anticline syncline synformal anticline antiformal syncline**Geometric analysis**inflection point: point of opposing convexity median surface: imaginary surface connecting inflection points fold width, fold height symmetrical vs. asymmetrical concept of vergence**Geometric analysis cont.**hinge zone – hinge line: zone of max. curvature fold axis: imaginary line, which when moved parallel to itself can define the form of a fold**Geometric analysis cont.**axial surface: surface that passes through successive hinge lines axial trace: line of intersection of axial surface and ground surface symbology: anticline, syncline, antiform, synform**names of common fold shapes**Chevron: planar limbs meet at discrete hinge point**Circular fold**"lift-off" fold**symmetrical: median trace and axial trace are perpendicular**asymmetrical: median trace and axial trace intersect at oblique angle symmetric asymmetric symmetric asymmetric**open (70-120)**gentle (>90) isoclinal (~0) tight (~10)**Regional structures that have limbs**folded by smaller folds**overturned: at least one of the limbs exhibits overturning,**or a downward facing direction**parallel/concentric folds: layer thickness does not change**(lower T) similar folds: layer thickness changes; thickening in hinge and thinning along limbs (higher T)**Cylindrical: well-defined axial surface**non-cylindrical: too complex to have a single well-defined axial surface**Fold mechanisms for "free folds", where fold shapes depend**on layer properties (1) Flexural-slip folding- accommodates buckling by layer-parallel slip -direction of relative slip is perpendicular to hinge -individual displacement small, but sum is enough to accommodate bending of rock -marked by strong stiff layers with contacts of low cohesive strength -occurs in uppermost levels of crust**minor structures related to flexural-flow folding**occur at higher temperature**(2) Kink folding**-Occurs when there is strong cohesion between layers -Marked by sharp hinge, straight limbs (one short, two long)**Another mechanism: (3) Buckling**-Instability develops when layers of different mechanical properties are subjected to layer-parallel stresses -Wavelength depends on layer thickness and stiffness- thinner layers yield shorter wavelengths ptygmatic folds: high competence contrast cuspate-lobate folds: low competence contrast**Important terminology/concepts**ptygmatic fold circular fold fold tightness recumbent fold box fold parallel/concentric vs. similar folds cylindrical vs. non-cylindrical folds monocline flexural-slip folding parasitic folds kink folding buckling instability anticline syncline antiform synform anticlinorium synclinorium inflection point hinge line fold axis symmetrical vs. asymmetrical axial plane and trace chevron fold

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