Fault propagation folds occur at the tip of thrust

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Fault propagation folds occur at the tip of thrust faults, where fault displacement dies out and must be accommodated by folding. Detachment folds die out downward at a bedding plane detachment and are not related to a ramp, although they could form above a detachment tip. They can also be formed by a buckling process as long as there is a thick ductile unit (salt or shale) at the detachment that can flow into anticlinal cores. Typically, they have an arcuate (bow shape) map pattern that reflects the amount of displacement from tips to the center. Maximum displacement is represented by the arrow (b) from the string connecting the tips (a) to the fault. When a thrust fault dies out, it’s displacement is usually transferred to an adjacent fault along a relay zone. Rule 1 - Admissibility: Structures that are not characteristic of the style of deformation are inadmissible. This is something that comes with experience, but you might hesitate to insert normal faults in a cross section that has been shortened. Rule 2 & 3 – Restoration of cross section to its undeformed configuration must be done keeping the areas of each layer constant. Alternatively, the lengths of layer boundaries can be kept constant. The problem of thrust faults: If you push on the end of a thrust block, the resisting forces along the fault are so great, that the end of the block will crush before the block can move along the thrust fault.
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