Session n°2 (Tuesday, June 11th) : Basics in structural Geology

Field measurements : planes, lines. Analysis of deformation in the field. The use of stereonets and fabric diagrams, how to read them ? Simple shear, pure shear and vorticity number. Training in structural geology.

Field measurements

In the field, you are in front of rocks that you need to analyse in terms of mineralogy, petrography and structure.

What structure ?

The rock may present a (stratigraphic) bedding, may be layered (mineralogical layering), banded : it has a structure. Whatever the complexity of this structure, it will be characterized geometrically, at least locally,

- by a plane : bedding, cleavage plane, foliation plane, fracture, fault, shear plane etc …

- or by a line, mineral lineation, striation on fault plane, intersection between two planes etc.

The change, or rate of change, of the structure will be described and taken into account in your analysis, BUT, fundamentally, a structure can be reduced to a plane and a line.

How do we measure a plane and a line ?

Using a compass, and using the local (magnetic) North (always called « the North » -corrections for declination are performed afterwise-) and the (local) horizontal plane as references.

Measurement of a plane

The strike, or azimuth of a plane is « the angle (measured in the horizontal plane) between the North and any horizonral line of the plane ». All the angles are measured in the horizontal plane (i.e. in the plane tengent to the Earth). This angle is (usually) measured by starting from the North and rotating clockwise (toward the East) up to the horizontal of the plane. Since this horizontal line is not oriented (positive/negative sense), the maximum value of this angle is 180°. Hence the strike of a plane is an angle between 0° and 180°.

The dip of a plane is the (dihedral) angle between the geological plane and the horizontal plane. Hence, the dip is an angle between 0° (horizontal) and 90° (vertical).

In conclusion, the (structral) measurement of a plane will be like :

N120°E, N30° = 120° N30° : direction 120°, dip 30° to the North.

Note that a horizontal plane has no direction (since there is an infinity of horizontal lines in the plane) ; it has just a 0° dip. It will be noted « horizontal », or « subhorizontal » since planes having low dips are very difficult to measure.

---- A few training exercices ----

Measurement of a line

The trend, or azimuth, of a line, is the strike (angular value, see above) of the vertical plane containing the line. Like for the strike, the trend is an angle between 0° and 180°.

The plunge of a line is the angle between the line and the horizontal plane, measured in the vertical plane containing the line. The plunge of a line is therefore an angle between 0° (horizontal line) and 90° (vertical line).

---- A few training exercices ----

Date: 2015-01-29; view: 1091