Chapter 2: Structural Geology

accuracy

how close a measurement conforms with a standard or accepted value

apparent dip

(alpha) the inclination of a plane measured in a direction not perpendicular to strike

apparent dip direction

The bearing of the vertical plane containing the apparent dip angle

attitude

the orientation in space

azimuth

the bearing of a plane measured clockwise from north when dip is to the right

beta

the angle between strike and the apparent dip direction

bearing

The horizontal angle between a line and a specified coordinate direction, usually north or south

depth

(d) The vertical distance from the ground surface downward to a feature of interest

dip

(delta) The inclination of the line of greatest slope of an inclined pkane, measured perpendicular to strike

dip-slip fault

A fault in which the movement is parallel to the dip of the fault plane, such as a normal, reverse, or listric fault

directed line

a line described by its bearing and plunge

fenster

a "window" into rocks underlying a thrust fault

fold line

A hinge line along which a vertical cross section may be constructed and then flattened out to a horizontal or map view

footwall block

The block that underlies a non-vertical fault

growth fault

A fault that forms contemporaneously with deposition in sedimentary rock. The throw increases with depth and the sedimentary units on the downthrown side are thicker than the corresponding units on the upthrown side

hanging-wall block

The block that overlies a non-vertical fault

heave

The amount of horizontal displacement on a fault

hinge line

The axis along which curvature of a fold is greatest

inclination

A general term for the vertical angle between the horizontal and a plane or line, measured downward

klippe

An outlier of a thrust sheet completely surrounded by the exposed footwall

listric fault

A fault with a curved fault plane. Near the surface the fault plane is steeply dipping, but it becomes progressively flatter with depth. Listric faults may be normal or reverse.

nappe

A sheetlike rock unit that has moved over other rocks (allochthonous) on a predominantly horizontal surface

net slip

The distance between two formerly adjacent points on either side of the fault, measured on the fault surface

normal fault

A dip-slip fault in which the hanging wall has moved down relative to the footwall. The dip of the fault is between 45` and 90`

Oblique-slip fault

A fault in which movement is not parallel to the strike or dip of the fault plane

offset

the horizontal component of displacement measured perpendicular to the strike of the disrupted unit

outcrop width

(w) The width of an outcrop, as measured on a map view, perpendicular to strike

pole

a line perpendicular to a plane, and represented as a point on a stereographic projection

plunge

the vertical angle between a line and horizontal

precision

the degree of agreement or consistency of repeated measurements

projection

the process by which points on one surface are transferred to another surface, such that in each view (map or cross section) the points correspond

rake (pitch)

The angle measured in a specified plane, between a line and the horizontal

reverse fault

A dip-slip fault in which the hanging wall has moved up relative to the footwall. The dip of the fault is between 45` and 90`

riedel shear

subsidiary faults inclined between 10` and 20` from the trace of the dominant strike slip fault

slope angle

(sigma) the angle from the horizontal down to the slope

stereonet

A representation of angular relationships between planes and lines. In structural geology, a stereonet is synonymous with Wulff net, an equal angle stereographic projection

strike

the bearing of a horizontal line in an inclined plane

strike separation

The horizontal distance between a stratigraphic unit offset by a fault, measured along the strike of the fault. It is an apparent displacement.

strike-slip fault

A fault in which movement is parallel to the strike of the fault

thickness

(t) The perpendicular distance between the top and bottom of a geological unit

throw

the amount of vertical displacement on a fault, also, the vertical component of net slip

thrust fault

A reverse fault in which the fault plane dips less than 45`

trend

The bearing of the vertical plane containing a line