Название: Structural Analysis and Synthesis
Автор: Stephen M. Rowland
Издательство: John Wiley & Sons Limited
Жанр: География
isbn: 9781119535485
isbn:
There is a problem measuring declinations because they are meant to be from the geographic North Pole, but a compass or transit measures from the magnetic North Pole. These devices must be adjusted to correct for the difference in location between the poles. The closer the measurement is made to the poles, the more pronounced the correction may be, and the current rapid wandering of the magnetic North Pole is further complicating data collection. There are apps available for smartphones to measure orientations that do not require corrections, but they tend to be less accurate and subject to cell phone coverage. Because inclinations are relative to the earth surface at a specific location, they are only comparable locally. Relative to a fixed point in space, a 30° dip at one location would not be parallel to a 30° dip measured 1000 km away.
Definitions
The following terms are used to describe the orientations of lines and planes. All of these are measured in degrees, so values are typically followed by the degree symbol (°).
AttitudeThe orientation in space of a line or plane. By convention, the attitude or orientation of a plane is expressed as its strike and dip; the attitude of a line is expressed as trend and plunge. (syn: orientation).BearingThe declination or horizontal angle between a line and a specified coordinate direction, relative to compass directions or in azimuth.StrikeThe bearing of the line of intersection of an inclined plane with the horizontal plane or surface (Figure 1.1). The strike is a line of equal elevation on a dipping plane.DipThe vertical angle between an inclined plane and a horizontal line perpendicular to its strike. The maximum angle of inclination on an inclined plane. The direction of dip can be thought of as the direction water would flow down the plane (Figure 1.1).TrendThe bearing (compass direction or declination) of a line (Figure 1.2). Non‐horizontal lines trend in the down‐plunge direction.PlungeThe vertical angle of inclination between a non‐horizontal line and the horizontal (Figure 1.2).PitchThe angle measured within an inclined plane between a horizontal line and the line in question (Figure 1.4). Also called rake.Apparent dipThe vertical angle between an inclined plane and a horizontal line that is not perpendicular to the strike of the plane or in true dip direction (Figure 1.2). For any inclined plane, the true dip is always greater than any apparent dip. Note that an apparent dip may be defined by its trend and plunge or by its pitch within a plane.
Figure 1.4 Pitch (or rake) of a line in an inclined plane.
Problem 1.1
Translate the azimuth convention into the quadrant convention, or vice versa.
1 N12°E
2 298°
3 N86°W
4 N55°E
5 126°
6 N37°W
7 233°
8 270°
9 083°
10 N3°W
Problem 1.2
Circle those attitudes that are impossible (i.e. a bed with the indicated strike cannot possibly dip in the direction indicated).
1 314°, 49°NW
2 086°, 43°W
3 N15°W, 87°NW
4 345°, 62°NE
5 062°, 32°S
6 333°, 15°SE
7 089°, 43°N
8 065°, 36°SW
9 N65°W, 54°SE
Problem 1.3
Fault surfaces can contain slip lineations (fault striae). Such slip lineations can be used to determine the orientation of a slip on a fault and, therefore, whether the motion on the fault was strike‐slip, dip‐slip, or oblique‐slip. A geology student who was just learning to use a pocket transit recorded the orientations of five slip lineations on one fault surface. The strike and dip of the fault surface is 320°, 47°NE. The student’s five recorded lineation orientations are recorded in the table below.
Determine which lineation orientations are feasible and which ones must represent a mistake on the part of the student because the given orientation does not lie within the fault plane. Give a brief explanation for each of your five answers. For the valid lineation orientations, indicate which type of fault motion is indicated.
Lineation (plunge and trend) | Feasible? (yes or no) | Explanation | Type of motion indicated (strike‐slip, dip‐slip, or oblique‐slip) |