Prac Two - Folds, cross sections and measuring rocks Flashcards
What are the basic parts of a fold?
Hinge - this separates two limbs that have different orientations.
Hinge point - is the point at which the rock is most curved (the peak or trough of the fold).
Hinge line or fold axis - is the line that is formed along the peak of the hing. If you were to draw a line from hinge point to hinge point (on the same fold) this would form a hinge line. Essentially the ridge of the fold.
Axial surface - the plane defined by the hinge line and the axial trace (the vertile line from the hinge to the ground).
What are the basic fold shapes?
Synforms - are folds where the limbs dip upwards, away from the hinge (U shaped).
Antiforms - are fold where the limbs dip downwards, away from the hinge (A shaped).
If we know the age of the beds we can determine whether the rocks are still in the order they were deposited (younging upwards) or if they have been inverted (younging downwards).
If rocks have younging upwards (right way up), we can used the term syncline instead of synform, and anticline instead of antiform.
We can also have synformal anticlines and antiformal synclines, where rocks show younging downwards (that is, they have been overturned and are now back the right way up again).
What are the basic fold orientations?
If the axial plane is verticle the fold is called an upright fold
If the axial surface is horizontal, the fold is called recumbent fold.
If it is any orientation between verticle and horisontal it is called an inclined fold.
If the hinge line of the fold is horisontal it is a non-plunging fold.
If the hinge line is inclined it will plung upwards or downwards, and this is called a plunging fold.
The plunge of the fold determines how the beds appear in map view.
Plunging folds make V-shapes in a map view.
The angle between the horizontal and the fold axis is called the angle of plunge, and this varies from less that 1 degree (non-plunging fold) to 90 degrees.
Describe fold cylindricity and fold symmetry.
Fold that have a straight hinge line (fold axis) are called cylindrical folds.
Folds that have curved hinge lines are called non-cylindrical folds.
Define symmetrical and asymmetrical folds.
Folds that are symmetrical can be cut along the axial plane and the two sides are mirror images of one another.
Folds that are asymmetrical when cut along the axial plane are not mirror images of one another.
How are the fold tightness determined?
Folds can be described according to their degree of tighness.
The tightness of a fold is measured by the size of the angle between the fold limbs, called the interlimb angle.
The size of the interlimb angle allows the fold to be classified in the following scheme.
How are fold shapes determined?
Some folds have particularly diagnostic hinge shapes, which are important to mention when writing descriptions.
Most folds are rounded to angular in the hinge zone, but the following are unusual hinge shapes.
(A) Chevron - planar limbs and very narrow angular hinge zone.
(B) Cuspate - curved limbs, that are curved in the opposite sense of most folds, and a very narrow hinge zone.
(C) Box folds - two distinct hinges and three planar limbs.
(D) Teardrop - continuously curved surfaces that curve bac on themselves.
Define the different kinds of faults.
Normal faults: Form when the rock on the hanging wall of the fault moves down relative to the rock on the foot wall. Normal faults form when the rock is pulled apart, so they are a result of extension in the crust.
Reverse faults: Form when the rock on the hanging wall fo the fault moves up relative to the rocks on the foot wall. When revese faults have a dip of 45 degrees or less they are called thrust faults. However, some geologists tend to used the terms thrust and reverse interchangably. Reverse faults for when rock is pushed together, so it is a result of shortening and compression in the crust.
Strike slip faults: Form when the rock on the hanging wall moves laterally relative to the footwall. In these faults, one block moves to the left (sinistral movemement) or right (dextral movement) relative to the footwall. These faults occur most commonly at MORs and places where one plate is sliding past another.
What are the different types of unconformities?
An unconformity is an erosional surface in a rock package that separates rocks of significantly different ages.
Unconformities occur when rocks are deposited followed by a period of non-deposition and erosion.
When more rocks are finally deposited on the erosional surface, there is a significant gap between the ages of the two layers.
The erosional surface is called an unconformity and there are four types.
