Question

In the following illustration, PQRS is an exposed axial planar cleavage, striking north-south and dipping 40° east. B1 and B2 are the traces of two folded bedding planes on this axial planar cleavage. The fold can be classified as

• A. Horizontal fold
• B. Inclined fold
• C. Plunging fold
• D. Upright fold

Hint:

A plunging fold is characterized by an axial plane that is tilted and the fold axis that dips into the ground, as seen in the diagram where the bedding planes dip at 40°.

Answer 1

The Correct Option is C

Step 1: Understanding fold types.
- Horizontal fold: The axial plane of the fold is horizontal. In this case, the bedding planes would be horizontally oriented, which is not the case here as the fold has a 40° dip.
- Inclined fold: The axial plane is inclined but not horizontal, and the fold does not have a significant plunge.
- Plunging fold: A plunging fold occurs when the axis of the fold is tilted or "plunges" into the ground. In this case, the presence of traces B1 and B2, which dip 40° east, suggests that the fold axis is not horizontal, indicating that the fold is plunging.
- Upright fold: An upright fold is one where the axial plane is nearly vertical. The description in the image suggests an inclined axial plane, not a vertical one.

Step 2: Analyzing the diagram.
The diagram shows a folded structure with traces of bedding planes (B1 and B2) dipping at 40° east, which indicates the fold axis is inclined and plunging in nature. This is characteristic of a plunging fold, where the axis of the fold is not horizontal, but instead plunges into the ground at an angle.

Step 3: Conclusion.
Based on the explanation and the diagram, the fold described in the question is a plunging fold, making option (C) the correct answer.
\[ \boxed{\text{Plunging fold}} \]