Structural Geology

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Structural Geology

Crustal deformation comes in many forms and is there for all to see. Uplift and submergence of shore-lines, incised river meanders, and limestone seabeds at more than a mile high in the center of continents show broad patterns of uplift. More directly earthquakes and the displacement of rock that causes them show us a constant pattern of structural deformation of the rocks of the crust. Many rock formations contain obvious evidence of deformation on both large and small scale such as the folded rocks in major mountain belts.

Dip and Strike

Planar features such as fault plains, bedding surface, and joints can be oriented by measurement of the downward inclination of the plane surface (dip) and the direction or trend of the plane (strike). If we look at an outcrop of tilted beds in water, which gives us a level surface, it is easier to visualize dip and strike dip and strike . The angle between the bedding plane and the water surface is the dip and the trend of the waterline along the bedding plane is the strike. The true or maximum dip must be measured at right angles to the strike. These are measured in the field with a Brunton compass that was designed for this purpose. Symbols for dip and strike are added to a geologic map geologic map.

Folds

Folds are large warps in rock strata that range in size from microscopic to large domes and basins hundreds of kilometers wide. Three general types of folds can be distinguished:

Monoclines are folds in which horizontal or gently dipping beds are modified by simple step like bends
Anticlines are strata arched upward with the limbs dipping away from the crest. If the rock has been eroded the strata are progressively older toward the interior of the fold
Synclines are downfolds with the limbs dipping toward the center, and in eroded synclines the strata are younger toward the center of the fold.

For description of folds we describe an imaginary plane, the axial plain, that divides a fold into tow equal parts. The line of the intersection of the axial plane and a bedding plane is called an axis, and any downward inclination of the axis is referred to as the plunge.

Folds may vary from large to small, and simple to complex. Where deformation is more intense, such as in mountain belts, the rocks are deformed into a series of tight folds. Two more terms are used in description of these structures: an asymmetric anticline has one side dipping more steeply than the other; an overturned or recumbent fold has the axial plane almost or actually horizontal.

Faults

In the case of a fold, the rocks are responding to forces by bending and warping in a plastic manner. If the stress is too great, or applied too rapidly, the rocks may respond by breaking with slippage or displacement along fractures. The fault plane is described in terms of dip and strike. Faults grow by a series of small displacements (centimeters to as much as 90 meters) which occur as built-up stress is suddenly released.

Three basic types of faults are recognized.

Normal faults in which movement is mainly vertical and the rocks above the fault plane (hanging wall) move downward in relation to those beneath the fault plane (foot wall). The use of terms such as gravity and pull-apart faults helps visualize these. These are easily seen in some road cuts. Occurrence of these faults in groups may produce a graben, which is a downdropped block between two normal faults.
Thrust faults are lower angle faults in which the hanging wall is moved up and over the footwall. These faults are associated with intense folding caused by horizontal compression in the earth's crust. If the fault dip angle is high, these are often called reverse faults.
Strike-slip faults are high angle fractures in which the displacement is horizontal, parallel to the strike of the fault plane.

The vertical movement is low in relation to the horizontal movement. These are often seen as a straight, low ridge extending across the surface. Although movement along such faults rarely exceeds a few meters during a single earthquake episode, total movement on some of these faults exceeds 500 km. A simple terminology is used to describe movement along a strike-slip fault; standing on one side of a fault facing the other side, if it has moved to your left, it is left lateral, to the right, right lateral.