A symmetrical fold, where both sides (limbs) dip away from the fold axis at roughly the same angle, can become overturned when asymmetrical forces continue to act upon it, pushing one limb significantly further than the other.
The Transformation Process
Initially, geological layers may be bent into symmetrical folds under compression. However, if the stress is unevenly distributed or continues intensely from one direction, it can cause one limb of the fold to be tilted more steeply than the other. As this asymmetrical tilting progresses, that limb is pushed closer and closer to a vertical position.
The Defining Condition of an Overturned Fold
An overturned fold is specifically defined by the extent of this tilting on one side. As referenced, if the fold is sufficiently tilted that the beds on one side have been tilted past vertical, and are sloping in the same direction as the beds on the other limb, the fold is considered overturned. This means that the younger beds on the overturned limb are now stratigraphically below older beds.
Key Factors Leading to Overturning
- Asymmetrical Pressure: Uneven geological forces pushing harder on one side of the fold.
- Continued Compression: Sustained pressure that progresses beyond simple bending, leading to extreme tilting.
- Differential Stress: Variations in stress intensity across the fold structure.
In essence, a symmetrical fold transitions into an overturned fold as asymmetrical stress forces one of its limbs to rotate beyond a vertical orientation, resulting in beds on that limb dipping in the same direction as the other limb, albeit with a significantly different orientation relative to their original position.
