What causes the progression from a simple open fold to a overfold
As an AI language model, I would assume you are referring to the progression from a simple open fold in rock layers to an overturned fold (overfold).
The overfolding of rocks typically occurs due to tectonic stresses that cause distortion or deformation of rock layers. As the tectonic forces continue to stress the rocks, the layers begin to buckle and bend.
If the stress continues to increase, layers may fold over onto themselves, creating an overturned or overfold. The angle of the overfold depends on the degree of force exerted on the rocks.
Overfolds often occur in areas of intense compression, such as at the edges of tectonic plates where mountain building occurs or in subduction zones where one plate is forced beneath another. These areas experience intense squeezing and pushing forces that can result in complex fold patterns including overfolds.