  We show that the full dynamical freedom of the well known Szekeres models
allows for the description of elaborated 3--dimensional networks of cold dark
matter structures (over--densities and/or density voids) undergoing "pancake"
collapse. By reducing Einstein's field equations to a set of evolution
equations, which themselves reduce in the linear limit to evolution equations
for linear perturbations, we determine the dynamics of such structures, with
the spatial comoving location of each structure uniquely specified by standard
early Universe initial conditions. By means of a representative example we
examine in detail the density contrast, the Hubble flow and peculiar velocities
of structures that evolved, from linear initial data at the last scattering
surface, to fully non--linear 10--20 Mpc. scale configurations today. To
motivate further research, we provide a qualitative discussion on the
connection of Szekeres models with linear perturbations and the pancake
collapse of the Zeldovich approximation. This type of structure modelling
provides a coarse grained -- but fully relativistic non--linear and
non--perturbative -- description of evolving large scale cosmic structures
before their virialisation, and as such it has an enormous potential for
applications in cosmological research.
