Fault is a planar discontinuity between blocks of rock that have been displaced past one another, in a direction parallel to the discontinuity. Or, faults are rupture along which the opposite walls have moved past each other. Some faults are only a few inches long, and the total displacement is measured in fractions of an inch. At the other extreme, there are faults that are 100 of miles long with a displacement measured in mile and even tens of miles.

Fault Zone: Fault zone is a tabular region containing many parallel or interweaving small faults. Shear Zone Shear Zone: Shear zone is a zone across which blocks of rock have been displaced in a fault like manner, but without prominent development of visible faults. Shear zones are thus regions of localized ductile deformation, in contrast to fault zones that are regions of localized brittle deformation. In another common usage the term shear zone refers to a tabular region of pervasively faulted rock – that is, a fault containing a very large number of closely spaced and anastomosing fault surface.

The intersection of the fault with the ground surface is known as the fault line, fault trace or fault outcrop. The strike and dip of a fault are measured in the same way as they are for bedding or jointing. The strike of the fault is the intersection of the fault plane with a horizontal line in the plane of the fault. The dip of the fault is the angle between the horizontal plane and fault plane. The block above the fault plane is known as hanging wall. The block below the fault plane is the foot wall. Dip & Strike UP THROW BLOCK FOOT WALL HANGING DOWN THROW BLOCK FAULT PLANE fault line

A person standing upright in a tunnel along a fault would have his feet on the foot wall, and hanging wall would hang over him. It is obvious that vertical faults have neither a foot wall nor a hanging wall. Hanging wall Foot wall

Three basic types of faults: Diagram Name Mode of Formation... Normal Fault Extension (tension) Reverse Fault (or Thrust) Compression Strike Slip Shear

Normal Dip-Slip Movement Normal2.gif Blocks move to illustrate movement

Reverse Dip-Slip Movement Reverse2.gif Blocks move to illustrate movement

Right-Lateral Movement Rightlat2.gif Blocks move to illustrate movement

Left-Lateral Movement Leftlat2.gif Blocks move to illustrate movement

Oblique-Slip Movement Oblique2.gif Blocks move to illustrate movement

Animation of Blind Fault Blindani2.gif’ The resulting fold is called a monoclinal fold

Dip-slip – normal (horst & graben)

Dip-slip – normal (horst & graben)

. . Nature of Movement Along Faults Translation Movements: are those in which all straight lines on opposite sides of the fault and outside the dislocated zone that were parallel before the displacement are parallel afterwards. in translational movement there has been no rotation of blocks relative to each other. b c c’ d . . a a’

Nature of Movement Along Faults Rotational Movements: are those in which some straight lines on opposite sides of the fault and outside the dislocated zone, parallel before the displacement, are no longer parallel afterwards. e d c’ c a d c a axis of rotation centre of rotation b b b’ b’ The centre of rotation is a point in the fault plane and the axis of rotation is a line that is perpendicular to the fault plane at this point. In a sense all faults have a certain amount of rotational movement. The displacement increases or decreases along the strike of all faults, and the blocks must rotate somewhat relative to one another. But if the rotation is not too great, the movements at any one locality may be treated as if the fault were a translational one.

Types of Fault Movement Three types: 1. Relative movement 2. Apparent movement 3. Absolute movement

1. Relative movement Faults in themselves never offer any direct evidence as to which block actually moved. Thus in the figures: Hanging wall block may have gone down and the foot wall block may have remained stationary. Foot wall block may have gone up and the hanging wall block have moved down. Both blocks may have gone down, but the hanging wall block may have gone down more than the foot wall block. Or, both blocks may have gone up, but the foot wall block may have gone up more than the hanging wall block.

2. Apparent movement The preceding discussion has been confined to the relative movements along faults, and it has not considered the effects on disrupted strata or veins. The apparent movement of the disrupted stratum may be very different from the net slip. This point is very important and cannot be overemphasized. The apparent movement is a function of many variables, and depends not only on the net slip, but also on the strike and dip of the fault, the strike and dip of the disrupted stratum, and the attitude of the surface on which the observations are made. It is possible for the apparent movement to be zero, although the net slip may be great. The following figures show the relationship between the net slip and the apparent movement under different conditions.

Apparent movement

Apparent movement

Apparent movement

Apparent movement

3. Absolute movement

Fault Terminology Because in most cases no direct evidence is available concerning the absolute movements, the following terminology is based chiefly on relative movements. SLIP: The term slip is used to indicate the relative displacement of formerly adjacent points on opposite sides of the fault, and is measured in the fault surface. NET SLIP: Is the total displacement; it is the distance measured on the fault surface between two formerly adjacent points situated on opposite walls of the fault. ab = net slip

ab = net slip = strike slip Fault Terminology STRIKE SLIP: is the component of net slip parallel to the strike of the fault. DIP SLIP: is the component of the net slip measured parallel to the dip of the fault plane. ab = net slip = strike slip ab = net slip = dip slip

Fault Terminology RAKE: is the angle that a line in a plane makes with a horizontal line in that plane. ab = net slip cab = rake

ad = perpendicular slip Fault Terminology TRACE SLIP: is that component of the net slip measured parallel to the trace of the bed on the fault plane. ab = net slip ac = strike slip bc = dip slip db = trace slip ad = perpendicular slip PERPENDICULAR SLIP: is that component of the net slip measured perpendicularly to the trace of the bed on the fault.

ab = net slip ae = vertical slip throw heave Fault Terminology VERTICAL SLIP: is the vertical component of the net slip and dip slip, called throw by many geologist. ab = net slip ae = vertical slip throw ed = horizontal throw dip slip heave heave HORIZONTAL DIP SLIP: is the horizontal component of the dip slip, called heave by many geologist.

Fault Terminology HORIZONTAL SLIP: is the horizontal component of the net slip. f ab = net slip af = horizontal slip

Fault Terminology SHIFT: The term shift is used to refer to the displacement on opposite sides of the fault and outside the dislocated zone. Dip slip ab = net slip; net shift is ef = dip shift, strike shift is zero.

Fault Terminology SEPERATION: The distance between the displaced parts of any recognizable geological surface which has been offset along a fault, measured in any specified direction.

Fault Terminology SEPERATION:

Fault Terminology SEPERATION: Strike Separation: is measured along the fault. Dip Separation: is measured down the fault dip. Vertical Separation: is the vertical distance between the disrupted stratum. Horizontal Separation: is measured at right angles to the strike of the faulted surface within a horizontal plane. Perpendicular Separation: is the shortest or perpendicular distance between the two parts of the faulted surface. This becomes the stratigraphic separation or stratigraphic throw if the faulted surface is bedding.