Lecture Four - Fold morphology Flashcards
What is structural geology?
The study of how rocks deform from their original states.
What scales can folds be found?
All scales from mountains to microscopic.
Folds can be found at mutliple scales - you can have a fold within a fold.
it is the same rule for faults.
What is the general rule for whether or not rocks will fold?
If the rocks are hot they are ductile and can therefore move and flow.
If the rocks are cold, they are brittle and will break.
There are acceptions to this rule.
E.g. folds can occur in cold, but unconsolidated sedimentary rocks.
Define a fold.
Folds are wavelike undulations in rock layers such as sedimentary strata.
They are the most obvious structures that demonstrate ductile deformation.
What are the names used for different fold orientations?
Draw a 2D section of a fold and lable it.
What do syncline, anticline, synform, antifor, aniformal syncline and synformal anticline mean/look like?
When an anticlinal fold is younging upwards (the right way up) it is considered antiformal.
When a syncline is younging upwards it is considered synformal.
When an antiform fold has been turned upside down, so it is now younging downwards, it is considered synclinal antiform.
When a synform fold has been turned upside down, so it is now younging downwards, it is considered anticlinal synform.
What is cylindricity?
When a fold has a linear hinge axis, it is cyclindrical.
When a fold has a curved hinge axis, it is considered non-cylindrical.
A strongly curved non-cylindrical fold is called a sheath fold.
What is symmetry, with regards to folds?
If the limbs of a fold are mirror images of one another, the fold is symmetrical.
If the limbs of a fold are not mirror images of one another, then the fold is asymmetrical or non-symmetrical.
What styles of the folded surface can a fold take?
It is necissary to know all the names of each style of fold, but not necissary to know the exact numerical barriers of each characteristic.
Tighness:
The tightness is dictated by the amount of strain and shortening placed on the rocks, if a fold is very tight, it has been placed under a large amount of strain.
Aspect ration A/M:
The ratio between the height (A) and the width (M) of a fold.
Bluntness:
Bluntness is essentially the pointyness of the fold hinge.
What styles of the folded layers can a fold take?
Measured by comparing curvature of inner surface (i) and outer surface (o), orthogonal thickness (to) and axial thickness (ta).
Orthogonal thickness - The thickness of the rock layer. Measured from the bottom of one layer to the bottom of the next.
Axial thickness - The thickness of the rok layer along the lenght of the fold axis.
What styles can multifple fold layers have?
Harmonic - folds with continuity between fold layers.
Disharmonic - fold layers not uniform, usuallly formed at higher temperatures.
Convoluted - non-planar axial surface.
What are foliation planes in folds?
Foliation planes are formed by the flattening of round grains, and the turning of flat grains to be at right angles to the direction of shortening.
What are some names given to different fold formations?
What are some fold mechanisms?
Buckling - inner part of layer undergoes shortening, outher part undergoes extension = produces parlallel folds.
Flexural slip - like folding a stack of cards = produces parallel folds.
Passive folding - like relative movement within a stack of cards = produce similar folds.
