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Simple Structural Geology Lab

Introduction

Structural geology is the branch of geology that deals with the three-dimensional shapes of rocks and how these shapes form. These shapes, or structures, develop when rocks are deformed as a result of some force applied to the rock. The force that acts upon a rock is the stress. As a result of the applied stress, the rock will deform; that is, it will undergo some type of change in shape or volume. This deformation is called the strain. The three-dimensional shape that results is called a geologic structure.

Strike and Dip
Folds and faults are three dimensional structures that may have various orientations in space. We must have some method of describing the orientation of these structures on a geologic map. The method that geologists use to describe the location in space of a geologic structure is the strike and dip. The strike of a bed or structure is given by the compass direction of an imaginary line at the intersection of a horizontal plane and the inclined bed or structure. The dip of a bed or structure is the angle of intersection between a horizontal plane and the inclined bed or structure. Dip is always measured perpendicular to strike. The dip determination will also have a direction, i.e., that direction toward which the layer is inclined down from the horizontal plane.

Folds
If a rock undergoes compressional stress and plastic strain, the resultant geologic feature will be a fold. Most folds develop at deeper levels within the crust, where rocks are under a high enough temperature and pressure to deform plastically. Folds are defined by the relationship of the dipping layers to an imaginary plane that divides the fold in half. This plane is called the axial plane. The line of intersection between the axial plane of a fold and the fold is called the axis.

Anticlines are folds in which the limbs of the fold dip away from the axis. Synclines are folds in which the limbs of the fold dip toward the axis. It is possible to distinguish between anticlines and synclines on the basis of the age of the bed exposed in the middle of the eroded fold. In an eroded anticline, older beds will be exposed in the middle; in an eroded syncline, younger beds will be exposed in the middle.

Faults
Faults occur when a rock is subjected to enough stress that it reaches the limit of its strength and breaks. Different types of faults will form depending on the type of stress to which the rock is subjected. Strike-slip faults develop when rocks are subjected to shear stress. In strike-slip faults, the movement is purely horizontal, with no up-and-down displacement. We classify strike-slip faults as either right lateral or left lateral. Imagine yourself standing on one side of a strike-slip fault, so that you are facing the fault. If the block on the other side is displaced to your right, the fault is a right lateral strike-slip fault . If the block on the other side of the fault is displaced to your left, the fault is a left lateral strike-slip fault.

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Dip slip faults form when rocks break from compressional or tensional stress, so that there is vertical displacement. When distinguishing between the types of dip slip faults, we define the block of rock above the fault plane as the hanging wall block and the block of rock below the fault plane as the footwall block (Fig.5). The terms "hanging wall" and "footwall" are old mining terms. The hanging wall block was the block of rock where miners would hang their lanterns; the footwall block was where miners would walk. It is easy to determine which side of the fault is the hanging wall if you imagine a miner standing on the fault plane. The hanging wall will be the block above the miner1s head; the footwall will be the block below the miner1s feet.

If rocks break under tensional stress, the hanging wall will move down relative to the footwall and a normal fault forms (see figure above). In this situation, the crust actually extends and lengthens. When rocks break under compressional stress, the hanging wall moves up relative to the footwall, and a reverse fault or thrust fault forms. In a reverse or thrust fault, the crust is shortened. Thrust faults are simply reverse faults in which the angle formed by the fault plane and the surface is quite shallow.

Commonly, the fault plane itself can become grooved and polished as one block of rock scrapes against the other. The scratches on the fault plane surface are called slickensides. Slickensides may record the slip orientation of the fault plane, and may even feel smoother in the direction of slip.

EXERCISES

1. Use the written or symbolic form of the strike and dip measurement to express the following orientations. Use a protractor to determine strike. For the problems in this exercise, assume that north is at the top of the page unless otherwise indicated.

a. Strike: north thirty-seven degrees west; Dip: twenty-five degrees southwest

b.

c. Strike: north eighty-nine degrees east, Dip: five degrees northwest

d.

2. Determine if the fault below is a right or left lateral strike-slip fault.

3. Name the structures shown in the figures below.

4. Use the structure models to answer the following questions. You will have to print the models out to complete the lab.

  • Name the structure.
  • Label the oldest to youngest beds X-Y-Z.
  • Complete the sides.
  • Draw a line indicating the fold axis on the model when appropriate.


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