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## Folds and folding

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**Outline**• Terms For Describing Folds • Fold systems • Fold geometries • Mechanics of folding • Kimematic models of folding**Convexity and Age of Beds**• Anticline - a fold that is convex in the direction of youngest beds • Syncline - a fold that is convex in the direction of oldest beds**Direction of Fold Closing**inflection point inflection point - change in curvature (i.e., concave to convex)**Folds - Definitions**• Antiforms are anticline-shaped folds (convex-down) whose stratigraphic order has not been determined. • Synforms are syncline-shaped folds (convex-up) whose stratigraphic order has not been determined. • We apply these terms to any fold in which facing direction and/or stratigraphic order is unknown or uncertain. • Determining stratigraphic succession - which way is up!**Folds - Definitions**• Overturned folds are those who have a limb that us technically upside down, it has rotated beyond vertical - dipping past 90°.**Folds - Geometric Properties**• The most basic element of a fold is the folded surface • We usually describe folds in normal profile view as seen by looking down the fold axis or down plunge.**Folds - Geometric Properties**• In normal profile view, folded surfaces can be divided up into limbs and hinges. • If the hinge is sharp, that point is called the hinge point otherwise it is called a hinge zone. • Fold limbs commonly curve, and the location where segments of opposite convexity join is called the inflection point. It is the place where the fold is setting up for the next hinge.**Folds - Geometric Properties**• The hinge line of a fold is defined by successively connecting the hinge points along the strike length of the fold. • The orientation of the hinge line is recorded as a lineation (plunge & trend). Hinge lines are typically not straight and their orientations can vary considerably. Take hinge points along a single folded surface, taken together define a hinge line The orientation of a folded surface can be defined by the orientation of a hinge line, using plunge & trend.**Hinge line**inflection point inflection point - change in curvature (i.e., concave to convex)**Folds - Geometric Properties**• To establish the orientation or attitude of a fold, it is necessary to know its hinge orientationand the orientation of the axial plane or axial surface. The trend and plunge of a hinge line of a fold does not uniquely define the orientation of the fold**Folds - Geometric Properties**• The axial surface of a fold connects all the hinge points in all successive layers. • It may be planar - an axial plane, or a curvi-planar surface - an axial surface.**Folds - Hinge lines & Axial Surfaces**Hinge lines are lines described by a lineation that lies on the axial surface, which is itself described by strike and dip.. Axial surface - Surface created by the hinge lines of consecutive layers within the fold area - it may be planar or curved. Described by strike and dip**How can we measure the axial surface?**• We can measure its dip direction and the angle of dip • Strike can always be determined by remembering that strike is perpendicular to dip • How is the AP shown on a stereonet?**Interlimb Angle**Four Categories: Gentle Open Tight Isoclinal**Cylindrical or Non-Cylindrical Folds**non-cylindrical fold**Fold Types: Cylindrical Folds**Cylindrical folds: Folds where the hinge line is straight. If traced far enough, few hinge lines are ever straight, but segments of the hinge lines are straight, so this is a useful concept.**Stereographic Determination of Fold Orientations**Cylindrical and non-cylindrical folds Poles to bedding planes are co-planar if the fold has a cylindrical geometry.**Stereographic Determination of Fold Orientations**• It is usually impossible to directly measure the axis and axial surface of large folds. • The trend and plunge of the hinge line (fold axis) and strike and dip of the axial surface can be calculated using a stereonet. An axial surface, by definition, passes through the hinge line of successive folded surfaces within a fold. The point representing the trend and plunge of the hinge line lies on a great circle that describes the orientation of the axial surface (great circle).**By definition, the fold axis (hinge line) lies upon the**axial plane, which bisects the fold-limbs.**Stereographic Determination of Fold Orientations**How to determine a fold axis and axial surface of a large fold in the field Two methods are: 1) Beta diagrams: -diagrams 2) Pi diagrams: -diagrams**Stereographic Determination of Fold Orientations**• Beta diagrams: b-diagrams • Intersection shows the trend and plunge of fold axis. The intersection of two bedding planes (e.g., great circles) represents a close approximation to trend and plunge of the hinge line. The intersection of the great circles is labeled beta (b). This is called a beta (b) diagram.**Stereographic Determination of Fold Orientations**• Pi diagrams: p-diagrams Another way to calculate the orientation of a fold. p plots uses at least 2 poles to bedding, results in the orientation of the fold axis. p uses multiple poles to bedding, fits a best-fit great circle to those poles, and also results in the orientation of the fold axis.**Stereographic Determination of Fold Orientations**• Pi diagrams: p-diagrams • Pole to pi great circle shows the orientation of the fold axis p plots uses at least 2 poles to bedding, results in the orientation of the fold axis. p uses multiple poles to bedding, fits a best-fit great circle to those poles, and also results in the orientation of the fold axis.**The angle between limb 1 and limb 2 and the axial plane are**the same - a bisector!**Bisecting surface:**Simple view in stereographic method that the bisecting surface approximates the axial surface. • The bisecting surface and the axial surface do not always coincide. • The axial surface connects individual hinge lines**Determining the orientation of the bisecting surface of a**fold. • Construct beta diagram • Plot poles to the fold limbs • Measure angle between the poles. • Fit a great circle to the bisector and b**Determining the orientation of the bisecting surface of a**fold Stereographic view of bisecting surface in proper orientation.**Parasitic Folds**Parasitic folds always verge towards anticlines and away from synclines**Parasitic Folds**Parasitic folds verge towards anticlines and away from synclines**Parasitic folds verge towards anticlines and away from**synclines**Vergence**The direction in which the next antiform can be found. Vergence occurs in the direction in which thrusting took place.**Vergence**Parasitic folds Gives us information about sense of shear on the fold limbs as well as the location of larger-scale fold hinges.. Think of S and Z folds, their asymmetry will give a sense of rotation, when viewed down plunge.**Vergence**Small scale folds define fold shape