GEO M5: Structural Geology and GeoPhysical Methods

• Structural geology is a subdiscipline of geology that studies how rocks deform in response to the stresses that act within the Earth
• in the mountainous and sub-mountainous tracts and also in shallow plains, exposures of rocks may be easily seen forming sides of valleys or caps of hills or even uplands and slopes in level fields
• An outcrop is simply defined as an exposure of a solid rock on the surface of the earth
• Outcrop Dimensions

  The width or breadth of the outcrop of a particular bed is given by the distance between the top and bottom edges of the bed as measured on the surface of the ground in a direction perpendicular to the strike of that particular bed.
• Outcrop Dimension
• Outcrop Dimension Many variations are induced in the breadth of an outcrop of a rock by the topography of the area.
• The thickness of a particular layer or bed is the perpendicular distance between the top and bottom surface of the same layer as seen in a vertical section at right angles to the strike of the layer.
• The depth to a particular layer or bed at any place from the surface, if believed to be present on the

  basis of general geology and dip of the formation, is given by the perpendicular distance between

  the ground surface and the top surface of that particular layer.
• FOLDS may be defined as undulations or bends or curvatures developed in the rocks of the crust

  as a result of stresses to which these rocks have been subjected from time to time in the past

  history of the Earth.
• The folds may develop in any type of rock and may be of any shape and geometry ranging from

  simple up arched bends or downward curvatures to completely overturned flexures
• The ultimate shape and extent of a fold depends upon a number of factors like the nature,

  magnitude and the direction of and duration for which these forces act upon the rocks and also the

  nature of the rocks being affected
• The process of development of folds in the rocks is called Folding
• It isa very slow geological process and indicates an effort of the rocks in a particular environment to

  adjust themselves to the changing force fields operating on, within or around them
• Folding is a ductile type of deformation expenenced by the rocks compared to the brittle

  deformation where the rocks actually get broken and displaced when stressed
• Limbs - these are the sides or flanks of a fold. An individual fold will have a minimum of two limbs

  but when the folds occur in groups, as they very often do, a middle limb will be common to two

  adjacent folds
• Hinge - In a folded layer, a point can be found where curvature is maximum and one limb ends and

  the other limb starts from that point.
• This is the hinge point When rocks occur in a sequence and their all-hinge points are joined together, they make a

  line, called the hinge line
• When the hinge line is traced throughout the depth of a folded sequence a surface is obtained

  which may be planar or non-planar. It is referred to as axial surface.
• Axial plane is the imaginary plane that passes through all the points of maximum

  curvature inclined or horizontal in nature.
• fold surface is planar in nature, otherwise, it in a folded sequence.
• Axial Plane It may be vertical, sometimes called a planar fold if the axial is a non-planar fold
• Axis of a Fold - Itis simply defined as a line drawn parallel to the hinge line of a fold.
• Amore precise definition of an axis of a fold would be the line representing the intersection of the

  axial plane of a fold with any bed of the fold.
• the angle of inclination of the fold axis with the horizontal as measured in a vertical plane is

  term ed the plunge of the fold
• Most folds are variations of two general forms; up arched and down arched bends. The line

  running through the highest points in an up arched fold defines its crest.
• A corresponding line running through the lowest point in a down arched fold makes its

  trough. The crest and trough may or may not coincide with the axis of the fold
• Anticlines are defined as those folds in which

  the strata are uparched, that is, these become CONVEX UPWARDS ,
• anticlines - the geologically older rocks occupy a position in the interior of the fold, oldest being

  positioned at the core of the fold and the youngest forming the outermost flank,
• anticlines - he limbs dip away from each other at the crest in the simplest cases
• synclines - the strata are downarched, that is, these become CONVEX DOWNWARDS, the geologically

  younger rocks occupy a position in the core of the fold
• synclines - the older rocks form the outer flanks, provided the normal order of superposition

  is not disturbed,
• synclines - the limbs dip towards a common center
• Depending upon the nature and direction of the stresses the axial plane in a resulting fold may acquire any

  position in space, that 1s, it may be vertical, inclined or eve n horizontal Following main types are re cognized on

  the basis of position of the axial plane in the resulting fold:
• Symmetrical Folds - These are also called normal or upright folds In such a fold, the axial plane is essentially

  vertical
• Symmetrical Folds - The limbs are equal i n length and dip equally in opposite directions.
• t may be an anticline or syncline and when classified, may be described as symmetrical

  anticline / syncline as the case may be
• All those folds, anticlines or synclines, in which the limbs are unequal in length and this dip unequally on either side from the hinge line are termed as asymmetrical folds
• Overturned Folds - These are folds with inclined axial planes in which both the limbs are dipping essentially in

  the same general direction
• Overturned Folds - The amount of dip of the two limbs may or may not be the same.
• Overfolding indicates very severe degree of folding.