Folds and foldingFolds and folding

In Ductile processes, deformation is retained.In Ductile processes, deformation is retained.

Plastic if permanent strain increases at same stressPlastic if permanent strain increases at same stress

Some types of folds

Folds and FoldingFolds and Folding

Folds and foldingFolds and folding

Convexity and Age of Beds

• Anticline - a fold that is convex in the direction of youngest bedsAnticline - a fold that is convex in the direction of youngest beds

• Syncline - a fold that is convex in the direction of oldest bedsSyncline - a fold that is convex in the direction of oldest beds

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!

Direction of Fold ClosingDirection of Fold Closing

inflection point - change in curvature (i.e., concave to

convex)

inflection point

Folds - Definitions

• Overturned folds are those who have a limb that is upside down, it has rotated beyond vertical, i.e. dipping past 90o from its original horizontal position°.

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.

• In normal profile view, folded surfaces can be divided up into limbs and hinges.

•Hinge: Region of Greatest Curvature

• 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.

Folds - Geometric Properties

Folds - Geometric Properties

Multiple 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.

• The hinge line of a fold is defined by successively connecting the hinge points of the fold.

• The orientation of the hinge line is recorded as a lineation (plunge (angle) & trend (compass direction)). Hinge lines are typically not straight and their orientations can vary considerably.

• To establish the orientation or attitude of a fold, it is necessary to know its hinge orientation and the orientation of the axial plane or axial surface.

trend and plunge of hingeline of a fold is not uniquely define the overall

oreintations of the foldThe trend and plunge of a hinge line of a fold does not uniquely define the orientation of the fold

Folds - Geometric Properties

Folds with the same hinge lineFolds with the same hinge line

Same hinge line direction Same axial surface

We need more information to distinguish these, maybe orientations of the limbsWe need more information to distinguish these, maybe orientations of the limbs

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.

Determining the Fold Axial Determining the Fold Axial

SurfaceSurface

Determining the Fold Axial Determining the Fold Axial

SurfaceSurface

Folds - Hinge lines & Axial Surfaces

Hinge lines are lines described by a lineation that lies on the axial surface, which is itself described by local 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

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 can we measure the axial surface?

Profile Plane of a Fold

Interlimb angle “fold tightness”, four categories

Angularity of Interlimb Angle

Attitude of Axial Surface

Cylindrical or Non-Cylindrical Cylindrical or Non-Cylindrical

FoldsFolds

Stratigraphic Facing

Graded BeddingGraded Bedding

Parasitic FoldsParasitic Folds

Parasitic folds

Give 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.

VergenceVergence