Keplerian shear for Chacon Transformations

The concept of keplerian shear was introduced by Damien Thomine recently. It is useful for non ergodic systems, and can be seen as strong mixing conditionally on invariant fibers. The notion is particularly interesting when a.e. fiber is not strongly mixing. We develop here an approach appropriate for systems such that a.e. fiber is weakly mixing, and apply it to a family of rank one transformations. Each transformation is a kind of Chacon map, built with a random number of spacers at each step of the Rochklin tower. We prove that this new dynamical system exhibits keplerian shear. The method relies on a version of a local limit theorem for time dependent Birkhoff sums along the fullshift.