Evidence of Faulting:
We can recognize faults by means of three different methods; (i) geological evidence of faulting, (ii) evidence of fault planes, (iii) physiographic evidence.
(1). Geological Evidence:
The very first and common most method to recognize a fault is the geological evidence of faultings. The geological evidence is seen very clearly on the geological maps. The most important geological evidence of faulting are (i0 offsets of rock units, (ii) repetition and omission of strata, (iii) abrupt termination of structures along their trend, (iv) strata out of stratigraphic sequence, and (v) abrupt change in the attitude of strata.
- Offsets of Rock Units: Displacement of rock beds, dykes, veins, etc occur on opposite sides of a fault.
- Repetition or Omission of Strata: In a traverse line, the outcrop of a bed may be repeated in cyclic order or it may disappear altogether. Such a repetition or omission of beds often establishes a fault.
- Abrupt Termination of Structures: The strata, dykes, veins, folds, or other structures may end abruptly against a fault line. Such termination of structures along their trend is usually caused by the dip and diagonal faulting. The lost portion of the structure may be concealed under the water, alluvium, lava, or younger series of beds.
- Strata out of Stratigraphic Sequence: The normal stratigraphic sequence of a region may be disturbed by faulting. When the strike and dip of strata change suddenly, faulting is indicated.
(2). Fault plane evidence:
The “fault plane evidence” includes the minor structure, which is found associated with faults. They are observed in the immediate vicinity of fault planes. The most important fault plane evidence are (i) slickensides, (ii) drag, (iii) fault breccia and gouge, (iv) silicification and mineralization, and (v) feather joints.
- Slicken Sides: The movement of one wall against another results in polishing and grooving of fault surfaces. These grooves and striations are called the “slickensides”. Slickensides are useful in identifying the direction of the latest movement on a fault surface. Slickensides are commonly associated with faults and are formed along the fault planes. In addition to striations, slickensides may also contain small steps-like features facing in the same direction and oriented more or less normal to striations. More pronounced parallel furrows along the fault planes are “mullions”. Mullions are larger than slickensides and are not easily erased by subsequent movements.
- Drag: In the immediate vicinity of a fault, the ends of strata may bend up and down. This local bending, which is caused by the fault displacement is called the “drag”. It indicates the direction of movement along a fault.
- Fault Breccia and Goage: Along some faults, the rocks are found highly fractured or even crushed to angular fragments. Angular fragments embedded in a matrix of finely grounded rock are called “fault breccia”. The fragment size of fault breccia may range from microscopic to several centimeters. Secondary minerals such as quartz, calcite, or sometimes pyrite may fill open spaces within the fault breccia. When the dislocating forces are very severe as is frequently the case in thrusting the rocks may be ground to a fine clay-like powder called the “gouge”. The gouge is frequently polished and striated by the fault movements. A micro-breccia that fills faults in wider zones of intense deformation is called the “mylonite”.
- Silicification and Mineralization: Circulating water while percolating through a fault zone may deposit fine-grained quartz causing silicification. Fault planes also act as passages for mineralizing solutions and many mineral deposits are formed in the fault plane itself.
- Feather Joints: Feather joints are the tension fractures formed due to fault displacements. They commonly develop adjacent to major faults. The feather joints indicated the direction of movement along the fault.
(3). Phsysiogrphic Evidence:
The physiographic pieces of evidence are seen clearly from a distance or on an aerial photograph. It many be noted that physiographic pieces of evidence are not conclusive. The chief physiographic pieces of evidence are: (i) fault scarp, (ii)fault line scarp, (iii) offset ridges, (iv) fault control of streams, and (v) lines of pond, springs or water seeking plants.
- Fault Scarp: A steep straight slope of a ridge is called the ”scarp”. Fault scarps are the scarps formed as a result of faulting. They are found only in those areas where faulting has been geologically very recent. Fault scarps face in the direction of the down-throw side.
- Fault Line Scarp: Fault frequently brings together resistant and non-resistant rock beds. A ridge may be formed along a fault due to the process of unequal erosion. Such ridges are called the “fault line scarps”. They may face either in the same direction as the original scarp or in the opposite direction. In the later case, the scarp is called the “obsequent fault line scarp”.
- Off-set Ridge: In regions where resistant rock beds are displaced along a fault, “offset ridges” are formed.
- Fault Control Streams: Streams may be guided in the direction and course of their flow by faulting. Such a stream may follow a nearly straight line or make approximately right-angle turns.
- Lines of Ponds, Springs, or Water Seeking Plants: Linear arrangement of ponds springs or water seeking plants may coincide with the alignment of faults.
The various criteria, discussed in the above paragraphs, are mostly indicative of faulting. Individually they do not give any conclusive evidence. Besides some of these criteria also suggest other structures, such as unconformities and folds. Usually, a combination of several criteria is required for the establishment of a fault.
- Dip and Strike (geological structure)
- Folds (elements of folds, types of folds,)
- Mechanics of Folds
- Recognition of a Fold
- Introduction to faults
- Classification/types of the Faults