Modular subvarieties and birational geometry of $S{U}_C(r)$

Let $C$ be an algebraic smooth complex genus $g>1$ curve. The object of this paper is the study of the birational structure of the coarse moduli space $U_C(r,0)$ of semi-stable rank r vector bundles on $C$ with degree 0 determinant and of its moduli subspace $S{U}_C(r)$ given by the vector bundles with trivial determinant. Notably we prove that $U_C(r,0)$ (resp. $S{U}_C(r)$) is birational to a fibration over the symmetric product $C^{(}rg)$ (resp. over $P^{(r-1)g}$) whose fibres are GIT quotients $(P^{r-1}{)}^{rg}\mathrm{/}\mathrm{/}PGL(r)$. In the cases of low rank and genus our construction produces families of classical modular varieties contained in the Coble hypersurfaces.