  I present a model of discrete gravity, which is formulated in terms of a
topological gauge theory with defects. The theory has no local degrees of
freedom and the gravitational field is trivial everywhere except at a number of
colliding null surfaces, which represent a system of curvature defects
propagating at the speed of light. The underlying action is local and it is
studied in both its Lagrangian and Hamiltonian formulation. The canonically
conjugate variables on the null surfaces are a spinor and a spinor-valued
two-surface density, which are coupled to a topological field theory for the
Lorentz connection in the bulk. I discuss the relevance of the model for
non-perturbative approaches to quantum gravity, such as loop quantum gravity,
where similar variables have recently appeared as well.
