Structural Geology

• Structural geology


  The study of factors such as origin, occurrence, classification, type and effects of various secondary structures like folds, faults, joints, rock cleavage and are different from those primary structures such as bedding and vesicular structure, which develop in rocks at the time of their formation
• Outcrop


  Any Geological formation exposed on the surface
• Strike


  The direction in which a geological structure is present. The strike direction may be defined as the direction of the trace of the intersection between the bedding plane
• Dip


  The slope or inclination of a geological structure. Dip is expressed both as direction and amount. The dip direction is the direction along which the inclination of the bedding plane occurs
• Dip direction

  • S70°W
  • N75°E
  • S75°E
  • S15°W
• Slickenside

  • Foliation
  • Bedding
  • Lineation
  • Fault Plane
• Folds


  • They are one of the most common geological structures found in rocks
  • When a set of horizontal layers are subjected to compressive forces, they bend either upward or downward. The bend noticed in rocks are called folds
  • Folds may occur as single local bends or may occur repeatedly and intricately folded to the tectonic history of the region
• Classification and types of folds

  • Symmetrical and asymmetrical folds
  • Plunging and non-plunging folds
  • Open and closed folds
  • Similar and parallel folds
  • Overturned fold
  • Chevron folds
  • Isoclinal folds
  • Fan folds
  • Domes and basins
  • Geanticlines and geosynclines
  • Drag folds
• Anticline


  A fold where the beds are bent upwards, resulting in a convex upwards structure with the older beds occurring towards the concave side
• Syncline


  A fold where the beds are bent downwards, resulting in a convex downwards structure with the younger beds occurring towards the concave side
• Symmetrical fold


  A fold where the axial plane divides it into two equal halves, with one half being the mirror image of the other
• Asymmetrical fold


  A fold where the compressive forces responsible for folding are not of the same magnitude
• Plunging fold


  A fold where the fold axis is inclined to the horizontal plane
• Non-plunging fold


  A fold where the fold axis is horizontal
• Open fold


  A fold where the beds have uniform thickness throughout
• Closed fold


  A fold where the beds are thinner in the limb portions and thicker at the crest and trough
• Similar fold


  A fold where the shape or pattern remains the same at depth
• Parallel fold


  A fold where the crest and trough become pointed or angular with depth
• Overturned fold


  A fold where one of the limbs is overturned, resulting in the order of superposition of beds in that limb being in reverse order
• Chevron fold


  A fold with sharply bent, angular crest and troughs
• Isoclinal fold


  A fold where the limbs are mutually parallel to a great extent
• Fan fold


  A fold where in anticlines the limbs dip towards each other and in synclines the limbs dip away from each other with reference to the axial plane, resulting in a fan-shaped structure
• Dome


  A fold where the beds are locally pushed up, resembling an upper hemisphere with dips found in all sides from a common central top point, a type of anticline
• Basin


  A fold where the beds are locally pushed down, resembling a bowl with slopes in the opposite direction to a dome
• Geanticline


  A large-scale anticline
• Geosyncline


  A large-scale syncline
• Drag fold


  Minor asymmetrical folds within major folds, confined only to incompetent beds sandwiched between competent formations, developing due to shearing/dragging effect
• Mechanisms of folding


  1. Slips or shear occur in between the beds, similar to slipping of cards when a set is folded

  2. If beds are not allowed to slip over one another, folding cannot take place, as seen in hard and competent rocks like quartzites

  3. Another kind of fold mechanism involves accommodation of stress without slippage between beds
• Geanticlines


  Anticlines with a normal shape but a very large magnitude
• Geosynclines


  Synclines with a normal shape but a very large magnitude
• Drag Folds


  • Minor asymmetrical folds within major folds but confined only to incompetent beds which are sandwiched between competent formations
  • Develop because of the shearing/dragging effect
• Mechanisms of Folding


  1. Slips or shear occur in between the beds

  2. Similar to slipping of cards which occurs when the set is fold

  3. If they are not allowed to slip over one another, folding of the set cannot take place

  4. Folding generally occurs in the case of hard and competent rock like quartzites

  5. Folds are characterized by thinning of the limbs and thickening of crest and troughs, commonly in weak and incompetent rocks like shales
• Parallel Folds


  • Orthogonal thickness is same all around the fold
  • Thickness is same parallel to the axial plane of the fold
  • Outer and inner arc have same curvature
  • Dip isogons are parallel to each other
• Faults


  Fractures along which relative displacement of adjacent blocks has taken place
• Faults are the most unfavorable and undesirable geological structures for construction purposes
• Joints


  Fractures in rocks where relative displacement of adjacent blocks has not taken place
• Fault Plane


  Plane along which the adjacent blocks were relatively displaced
• Foot Wall


  The faulted block which lies below the fault plane
• Hanging Wall


  The faulted block which rests above the fault plane
• Faulting may recur at the same place as it offers the least resistance for the readjustment of the blocks and release of accumulated energy