Deformation of Rocks

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Traditional Aboriginal Knowledge

Deformation of Rocks

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Stress and Strain
Stages of Deformation
Brittle-Ductile Properties of the Lithosphere
Deformation in Progress
Evidence of Former Deformation
Fracture of Brittle Rocks
Normal Faults
Horsts and Gabens
Half-Grabens
Reverse Faults
Thrust Fault
Strike Slip Faults
Transform-Faults
Evidence of Movement on Faults
Folding of Ductile Rocks
Monoclines
Anticlines
Synclines
Geometry of Folds
Classification of Folds
The Relationship Between Folding and Faulting
Folds and Topography

see also deformation images in Geological Processes..


 
adapted to HTML from lecture notes of Prof. Stephen A. Nelson Tulane University

Within the Earth rocks are continually being subjected to forces that tend to bend them, twist them, or fracture them. When rocks bend, twist or fracture we say that they deform (change shape or size). The forces that cause deformation of rock are referred to as stresses (Force/unit area). So, to understand rock deformation we must first explore these forces or stresses.

Stress and Strain


Stress is a force applied over an area. One type of stress that we are all used to is a uniform stress, called pressure. A uniform stress is a stress wherein the forces act equally from all directions. In the Earth the pressure due to the weight of overlying rocks is a uniform stress, and is sometimes referred to as confining stress.  
 
stress1.gif

If stress is not equal from all directions then we say that the stress is a differential stress. Three kinds of differential stress occur. When rocks deform they are said to strain. A strain is a change in size, shape, or volume of a material.

Stages of Deformation

When a rock is subjected to increasing stress it passes through 3 successive stages of deformation.
 
  stressstrain.gif We can divide materials into two classes that depend on their relative behavior under stress. brittle.gif

How a material behaves will depend on several factors.
Among them are:

Brittle-Ductile Properties of the Lithosphere



ducttrans.gif


We all know that rocks near the surface of the Earth behave in a brittle manner. Crustal rocks are composed of minerals like quartz and feldspar which have high strength, particularly at low pressure and temperature. As we go deeper in the Earth the strength of these rocks initially increases. At a depth of about 15 km we reach a point called the brittle-ductile transition zone. Below this point rock strength decreases because fractures become closed and the temperature is higher, making the rocks behave in a ductile manner. At the base of the crust the rock type changes to peridotite which is rich in olivine. Olivine is stronger than the minerals that make up most crustal rocks, so the upper part of the mantle is again strong. But, just as in the crust, increasing temperature eventually predominates and at a depth of about 40 km the brittle-ductile transition zone in the mantle occurs. Below this point rocks behave in an increasingly ductile manner


Deformation in Progress


Only in a few cases does deformation of rocks occur at a rate that is observable on human time scales. Abrupt deformation along faults, usually associated with earthquakes caused by the fracture of rocks occurs on a time scale of minutes or seconds. Gradual deformation along faults or in areas of uplift or subsidence can be measured over periods of months to years with sensitive measuring instruments.


Evidence of Former Deformation


Evidence of deformation that has occurred in the past is very evident in crustal rocks. For example, sedimentary strata and lava flows generally follow the law of original horizontality. Thus, when we see such strata inclined instead of horizontal, evidence of an episode of deformation is present. In order to uniquely define the orientation of a planar feature we first need to define two terms - strike and dip

For an inclined plane the strike is the compass direction of any horizontal line on the plane. The dip is the angle between a horizontal plane and the inclined plane, measured perpendicular to the direction of strike

strikedip.gif

In recording strike and dip measurements on a geologic map, a symbol is used that has a long line oriented parallel to the compass direction of the strike. A short tick mark is placed in the center of the line on the side to which the inclined plane dips, and the angle of dip is recorded next to the strike and dip symbol as shown above.  For beds with a 900 dip (vertical) the short line crosses the strike line, and for beds with no dip (horizontal) a circle with a cross inside is used as shown below.

horizvert.gif

Fracture of Brittle Rocks



Normal Faults -


Are faults that result from horizontal tensional stresses in brittle rocks and where the hanging-wall block has moved down relative to the footwall block

normflt.gif

Horsts and Gabens -



 Due to the tensional stress responsible for normal faults, they often occur in a series, with adjacent faults dipping in opposite directions. In such a case the down-dropped blocks form grabens and the uplifted blocks form horsts. In areas where tensional stress has recently affected the crust, the grabens may form rift valleys and the uplifted horst blocks may form linear mountain ranges. The East African Rift Valley is an example of an area where continental extension has created such a rift. The basin and range province of the western U.S. (Nevada, Utah, and Idaho) is also an area that has recently undergone crustal extension. In the basin and range, the basins are elongated grabens that now form valleys, and the ranges are uplifted horst blocks

horstgraben.gif

Half-Grabens -

A normal fault that has a curved fault plane with the dip decreasing with depth can cause the down-dropped block to rotate. In such a case a half-graben is produced, called such because it is bounded by only one fault instead of the two that form a normal graben.

halfgrab.gif

Reverse Faults -

are faults that result from horizontal compressional stresses in brittle rocks, where the hanging-wall block has moved up relative the footwall block

reverse.gif

Thrust Fault -


is a special case of a reverse fault where the dip of the fault is less than 15o. Thrust faults can have considerable displacement, measuring hundreds of kilometers, and can result in older strata overlying younger strata.

thrust.gif

Strike Slip Faults -


are faults where the relative motion on the fault has taken place along a horizontal direction. Such faults result from shear stresses acting in the crust. Strike slip faults can be of two varieties, depending on the sense of displacement. To an observer standing on one side of the fault and looking across the fault, if the block on the other side has moved to the left, we say that the fault is a left-lateral strike-slip fault. If the block on the other side has moved to the right, we say that the fault is a right-lateral strike-slip fault. The famous San Andreas Fault in California is an example of a right-lateral strike-slip fault. Displacements on the San Andreas fault are estimated at over 600 km

strslip.gif

Transform-Faults



are a special class of strike-slip faults. These are plate boundaries along which two plates slide past one another in a horizontal manner. The most common type of transform faults occur where oceanic ridges are offset. Note that the transform fault only occurs between the two segments of the ridge. Outside of this area there is no relative movement because blocks are moving in the same direction. These areas are called fracture zones. The San Andreas fault in California is also a transform fault.

transform2.gif

Evidence of Movement on Faults


Folding of Ductile Rocks


When rocks deform in a ductile manner, instead of fracturing to form faults, they may bend or fold, and the resulting structures are called folds. Folds result from compressional stresses acting over considerable time. Because the strain rate is low, rocks that we normally consider brittle can behave in a ductile manner resulting in such folds. We recognize several different kinds of folds.


Monoclines


are the simplest types of folds. Monoclines occur when horizontal strata are bent upward so that the two limbs of the fold are still horizontal

monocline.gif

Anticlines

  Anticlines are folds where the originally horizontal strata has been folded upward, and the two limbs of the fold dip away from the hinge of the fold

anticline.gif


Synclines


Synclines are folds where the originally horizontal strata have been folded downward, and the two limbs of the fold dip inward toward the hinge of the fold. Synclines and anticlines usually occur together such that the limb of a syncline is also the limb of an anticline.

syncline.gif


Geometry of Folds


Geometry of Folds - Folds are described by their form and orientation. The sides of a fold are called limbs. The limbs intersect at the tightest part of the fold, called the hinge. A line connecting all points on the hinge is called the fold axis. In the diagrams above, the fold axes are horizontal, but if the fold axis is not horizontal the fold is called a plunging fold and the angle that the fold axis makes with a horizontal line is called the plunge of the fold. An imaginary plane that includes the fold axis and divides the fold as symmetrically as possible is called the axial plane of the fold

 foldgeom.gif

Note that if a plunging fold intersects a horizontal surface, we will see the pattern of the fold on the surface.


plunganti.gif

Classification of Folds Folds can be classified based on their appearance.



foldclass.gif

The Relationship Between Folding and Faulting



foldfault.gif

Because different rocks behave differently under stress, we expect that some rocks when subjected to the same stress will fracture or fault, while others will fold. When such contrasting rocks occur in the same area, such as ductile rocks overlying brittle rocks, the brittle rocks may fault and the ductile rocks may bend or fold over the fault
Also since even ductile rocks can eventually fracture under high stress, rocks may fold up to a certain point then fracture to form a fault.

 foldthrust.gif

Folds and Topography


Since different rocks have different resistance to erosion and weathering, erosion of folded areas can lead to a topography that reflects the folding. Resistant strata would form ridges that have the same form as the folds, while less resistant strata will form valleys

Mountain Ranges - The Result of Deformation of the Crust

One of the most spectacular results of deformation acting within the crust of the Earth is the formation of mountain ranges. Mountains originate by three processes, two of which are directly related to deformation. Thus, there are three types of mountains: