Iraqi Kurdistan RegionTishk International University (TIU)

Faculty of EngineeringDepartment of Petroleum and Mining (PETM)

Erbil

Structural Geology Lecture NotesByDr. Fadhil Ali GhaebThird Semester2021/2022

Text Books: 1-M.P,Billings, 1977, Structural geology.2-Twiss and Moores, 1992, Structural geology

Lecture 5

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Lines

To describe a linear feature, student should know:1- Trend: a general term to show the direction of a line, in general projected on a horizontal plane. It is also called azimuth. It is a horizontal angle (between 0 and 360°) measured clockwise from true north, which has an azimuth of 000. As a couple of examples, E (East) has an azimuth of 090o while S45W (South 45 West) has an azimuth of 235.

2- Plunge (inclination): The vertical angle, measured downward, between the horizontal and a line. If the plunge angle is 30o then it is written as: 30 SE

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3- The pitch is the angle between a line on a plane and the strike of that plane. The pitch has a measurement from 0 to 90° (0 = horizontal; 90 = vertical). It also has a direction, which corresponds to the direction of the strike.

090270strike

Dip60oS

50o

EastNorth

Hence, lines are commonly represented by their trend and plunge. We will use the convention that the trend of lines is given with three digits from 000 to 360. So, 000 is N, 090 is E, 180 is S, 270 is W, 360 is again N. The plunge is given with two digits from 00 to 90°. A trend and a plunge give the orientation of any line in space.

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These are generally written as plunge -> trend. You can remember how to write a plunge & trend measurement down by recalling that a line plunges in the direction of the trend –hence we write the measurement with an arrow pointing from the plunge value toward the trend value.

Examples of line orientations: plunge -> trend, 00 -> 000, (Horizontal, N-oriented,), 00 -> 084 (horizontal, in direction 084, 42 -> 084 (the trend is the same as line above, but the line plunges only 42°)

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2- Slickensides Are rock surfaces naturally scratch by motion of rocks along each other.

1- Intersection lineationSuch as the intersection of an inclined layer with ground surface. (i.e. trace of the bedding plane on the Earth). This line is or is not parallel to the strike line.

Some Types of Lines in geological structures:

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4- Axes of foldsHinges of cylindrical folds are linear structures.

3- Stretching lineationsFormed by the parallel alignment of individual detrital grains.

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5- Mineral lineationsMetamorphic minerals often grow with a preferred crystallographic and dimensional orientation, i.e. with their long axes in parallel alignment.

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Ex. Palygorskite Mineral in MaqlubA case Study

The plant is known as Verbascum LitumIt is distributed on land as lineaments along the traces

(shear zones) of the mineral

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Thickness of strata and Depth:

(1) True thickness: the perpendicular distance between the parallel planes bounding a tabular body, as displayed on any section perpendicular to these planes; also called the stratigraphic thickness.

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(2) Apparent thickness: it is vertical distance between an upper and lower contact in a non-horizontal unit. The apparent thickness is equal to the true thickness only when theattitude of the unit is horizontal. It is always greater than true thickness. When vertically measured in a hole it is drilling thickness.

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(3) Outcrop width: the strike-normal distance between the traces of the parallel bounding planes measured at the earth’s surface. It may be measured horizontally or on an incline (apparent thickness). It is also called apparent thickness.

(4) Depth: the vertical distance from a specified level (commonly the earth’s surface) downward to a point, line or plane.

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• Note that where an individual rock unit intersects the land at a high angle, it crops out over a smaller area. Where beds are steep and the topography is gentle, such as at the left side of the diagram, the outcrop width is approximately the true thickness of the unit. By contrast, the outcrop width increases when the topography and the dip of the rock approach parallelism. In the case of the blue unit near the center of the diagram, it forms a "dip slope" and outcrops over a fairly large area even though it is dipping moderately. Where both the rock and the topography are nearly horizontal, units can crop out over very extensive areas, even though they are relatively thin.

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