A KINEMATIC MODEL FOR THE ORIGIN OF FOOTWALL SYNCLINES

The origin of footwall synclines at the base of thrust ramps can be related to the kinematic evolution of fault-propagation folds. The conventional fault-propagation fold model does not predict the development of footwall synclines because it assumes a high propagation to slip ratio for development of the lower flat which does not allow for folding before the thrust cuts up section. An alternative model, where the propagation to slip ratio remains finite throughout the development of the fault-propagation fold, produces a detachment fold at the thrust tip during propagation of the thrust along the lower flat. If the thrust cuts up through the ductile core of the detachment fold during ramp formation, it will leave behind the forelimb of the detachment fold as part of a footwall syncline.