Reclined Fold Geometry

Folds are geological structures representing deformed rock layers. Key components include:

• Hinge Line: The line marking the area of maximum curvature within a fold. It acts as the fold's axis.
• Axial Plane: An imaginary plane that divides the fold symmetrically, containing the hinge lines of successive layers.
• Strike Line: A line representing the horizontal direction within a geological plane, such as the axial plane. Its direction defines the plane's strike.

A reclined fold is a specific type where the axial plane is tilted significantly from the vertical, and consequently, the hinge line often plunges at an angle.

Angle Determination

The question asks for the angle between the hinge line and the strike line of the axial plane.

• The strike line is, by definition, a horizontal line within the axial plane. Its direction indicates the strike of the axial plane.
• The hinge line represents the axis of the fold.

A standard convention in structural geology defines the trend of the hinge line (fold axis) as being perpendicular to the strike of the axial plane. This geometric relationship holds true for various fold types, including reclined folds, considering the general orientation.

Therefore, the angle between the hinge line and the strike line of the axial plane in a reclined fold is $90^\circ$.