Extensions of Veech groups I: A hyperbolic action

Abstract

Given a lattice Veech group in the mapping class group of a closed surface $S$, this paper investigates the geometry of $\Gamma$, the associated ${\pi }_{1}S$-extension group. We prove that $\Gamma$ is the fundamental group of a bundle with a singular Euclidean-by-hyperbolic geometry. Our main result is that collapsing “obvious” product regions of the universal cover produces an action of $\Gamma$ on a hyperbolic space, retaining most of the geometry of $\Gamma$. This action is a key ingredient in the sequel where we show that $\Gamma$ is hierarchically hyperbolic and quasi-isometrically rigid.