Definitions
anticline - fold that is convex in the direction of the youngest beds
antiform- "a fold that is a fold that points up": convex upward
syncline- fold that is concave in the direction of the youngest beds
synform - "a fold that points down": concave upward
overturned folds: when one limb is overturned, i.e. younger rocks are underneath
fold limb: flank of a fold on either side of a fold
fold hinge: zone of maximum curvature
crest: highest point of a folded surface
crest line: collection of crests along on a common folded surface
inflection points: Location on a fold where curved limb segments of opposing convexity meet.
hingeline: collection of hinge points along a single folded surface
axial surface: surface that passes through the hingelines of various folded layers in a sequence (axial trace is the intersecting of the axial surface with any other surface, e.g. outcrop)
fold axis is a line that when moved parallel to itself orchestrates a fold
cylindrical folds are folds that possess fold axes; every part of the folded surface is oriented such that it contains a line whose orientation is identical to that of the hingeline
Readings
Ch. 7 Folds
Lecture
FOLDS
slides:
(1) Folding can be recognized from the satellites.
(2) Folding can be related to faulting
(3) Folds can maintain the thickness of beds
(4) Folds can change the thickness of beds (greater internal strain)
(5) Sometimes sedimentary structures may be confused as tectonic folds
Folds are important in the oil industry because the zone of closure can serve as a structural trap.
* during thrust faulting: fault-bend folds
* igneous intrusions
* gravity-induced slumping
Folds are classified according to various schemes which later permit interpretation.
TIGHTNESS: gentle, open, tight, isoclinal//based on interlimb angles
SIZE
SYMMETRY: asymmetrical folds have limbs of different lengths.
LAYER THICKNESS: concentric ore parallel folds: individual layers maintain a constant thickness similar folds display significant thickening in the hinge and significant thinning on the limbs
interlimb angle : For a given folded surface, this angle is the angle between the flanks
bisecting surface: Surface which splits the interlimb angle in half
p-diagram: stereographic plot of the poles of folded surfaces
b-diagram: stereographic plot of the great circles folded surfaces.
The intersection of the great circles approximates the fold axis orientation