Abundant rich phase transitions in step skew products

We study phase transitions for the topological pressure of geometric potentials of transitive sets. The sets considered are partially hyperbolic having a step skew product dynamics over a horseshoe with one-dimensional fibers corresponding to the central direction. The sets are genuinely non-hyperbolic containing intermingled horseshoes of different hyperbolic behavior (contracting and expanding center). We prove that for every $k\ge 1$ there is a diffeomorphism $F$ with a transitive set $\Lambda$ as above such that the pressure map $P(t)=P(t\, \varphi)$ of the potential $\varphi= -\log \,\lVert dF|_{E^c}\rVert$ ($E^c$ the central direction) defined on $\Lambda$ has $k$ rich phase transitions. This means that there are parameters $t_\ell$, $\ell=1,...,k$, where $P(t)$ is not differentiable and this lack of differentiability is due to the coexistence of two equilibrium states of $t_\ell\,\varphi$ with positive entropy and different Birkhoff averages. Each phase transition is associated to a gap in the central Lyapunov spectrum of $F$ on $\Lambda$.