11.2A Folds, Faults, and Mountains
Folds and Faults
Folds
Over millions of years, stress forces can bend rock like a ribbon or soft dough.
Steady pressures of stress over long periods of time affect sedimentary layers and can fold them into dramatic forms.
Folds
Folds :
During mountain building, compressional stresses often bend flat-lying sedimentary rocks into wavelike ripples called folds.
Folds of sedimentary strata come in three main types Anticlines Synclines Monoclines
Anticlines and Synclines
Anticlines and Synclines :
An anticline is usually formed by the upfolding, or arching of rock layers.
Often found in association with anticlines are downfolds, or troughs, called synclines.
The anticlines are the folds that go upwards and the synclines are the folds that go downward.
Dips
The angle that a fold or fault makes with the horizontal is called the dip of the fold or fault.
The more the bend in the fold or fault, the stronger the dip.
In the figure at right, folds, faults and dips are visible in B.
In C, the folds are starting to overturn and D and E the folds have overturned all the way and folded over completely.
Monoclines
Monoclines
Folds are generally closely related to faults in the Earth’s crust. Examples of this close association can be found in monoclines.
Monoclines are large step-like folds in otherwise horizontal sedimentary layers.
Monoclines occur as sedimentary layers get folded over a large faulting-block of underlying rock.
Monoclines are a prominent feature of the Colorado Plateau region.
Hanging walls and footwalls
Faults
Recall that faults are fractures in the Earth’s crust along which movement has taken place.
The rock surface immediately above the fault is called the hanging wall.
The rock surface below the fault is called the footwall.
Types of Faults
Faults
The major types of faults are
Normal faults Reverse faults Thrust faults Strike-slip faults
Types of Faults
Faults Normal faults occur due to
tensional stress and reverse and thrust faults occur due to compressional stress.
Compressional forces generally produce folds as well as faults, resulting in a thickening and shortening of rocks.
Shearing stresses produce strike-slip faults.
Faults are classified according to the type of movement that occurs along the fault.
Normal Faults
Normal Faults
A normal fault occurs when the hanging wall block moves down relative to the footwall block.
Most normal faults have steep dips of about 60 degrees. These dips often flatten out with depth.
The movement in normal faults is mainly in a vertical direction, up-down, with some horizontal movement as well.
Because of the slide down of the hanging wall block, normal faults result in the lengthening, or stretching, of the crust.
Tensional stress pulls the blocks apart and lets the hanging wall drop downward
Reverse Faults
Reverse Faults:
A reverse fault is a fault in which the hanging block moves up (instead of down) relative to the footwall block.
Reverse faults are high angle compressional faults with dips greater than 45 degrees.
Thrust Faults
Thrust Faults:
Thrust faults are reverse faults with dips of less than 45 degrees.
Because the hanging wall block moves up and over the footwall block, reverse and thrust faults result in a compression, squeezing and shortening, of the crust.
Thrust Faults
Thrust Faults:
Most high-angle reverse faults are small in scale. They cause only local displacements in regions that are already filled with other types of faulting.
Thrust faults, however, exist at all scales. Many can be quite large.
In the Swiss Alps, the northern Rockies, Himalayas, and Appalachians, thrust faults have displaced layers as far as 50 kilometers.
The result of this type of movement is that older rocks end up on top of younger rocks.
Strike-Slip Faults
Strike-Slip Faults:
Faults in which the movement is horizontal and parallel to the line of the fault is called a strike-slip fault.
Because of their large scale, and linear nature ( in a line) many strike-slip faults produce a trace that can be seen over a great distance.
Rather than a single fracture, large strike-slip faults usually consist of a zone of roughly parallel fractures.
Strike-Slip Faults
Strike-Slip Faults:
The zone of parallel fractures created by a strike-slip fault may be up to several kilometers wide.
The most recent movement is often along a section only a few meters wide and may offset features such as stream channels.
Crushed and broken rocks produced during faulting are more easily eroded, often producing linear valleys or troughs that mark strike-slip faults.
Fence break created by strike-slip fault
