Question

Which one of the following stereographic projections represents a reclined fold?

• A.

  

• B.

  

• C.

  

• D.

  

Hint:

A reclined fold's hinge line plots on the axial-plane great circle at its point of maximum dip.

Answer 1

The Correct Option is D

Step 1: Recall Fleuty's classification of folds.
Fleuty (1964) classified folds using two angles read straight off a stereographic projection: the plunge of the hinge line (fold axis) and the dip of the axial surface. On the stereonet, the axial surface plots as a great circle and the hinge line plots as a point. Where that point sits relative to the great circle tells us the fold style.

Step 2: Recall the main fold styles from this scheme.
A recumbent fold has an almost horizontal axial surface and an almost horizontal hinge line, so both angles are close to \(0^{\circ}\). An upright fold has an almost vertical axial surface with a gently plunging or horizontal hinge line. An inclined fold has a moderately dipping axial surface with a gently plunging hinge line, so the hinge line pitches at a low angle inside the axial surface. A reclined fold is special: the hinge line plunges in the true dip direction of the axial surface, and its plunge amount is close to the dip of the axial surface itself, so the hinge line pitches at close to \(90^{\circ}\) within the axial plane.

Step 3: Translate the reclined geometry onto the stereonet.
Because the hinge line of a reclined fold lies in the dip direction of the axial surface, the point marking the hinge line plots exactly on the great circle of the axial surface, at the position of maximum dip of that great circle, rather than off to one side near the strike line. This is the diagnostic signature that separates a reclined fold from an inclined or upright fold on a stereographic projection.

Step 4: Compare the four projections.
Each of the four stereographic projections given in options (A) to (D) plots an axial surface great circle together with a hinge line point in a different relative position, corresponding to a different one of the recumbent, upright, inclined and reclined styles. Only the projection in option (D) places the hinge line point on the axial surface great circle at its point of maximum dip, matching the reclined fold geometry described in Step 3.

Step 5: Final conclusion.
Therefore, the stereographic projection that represents a reclined fold is
\[ \boxed{\text{Option (D)}} \]