  The dynamical evolution of the Gould Belt has been modelled in 3D and
confronted to the spatial and velocity distributions of all HI and H2 clouds
found within a few hundred parsecs from the Sun and to the Hipparcos distances
of the nearby OB associations. The model describes the expansion of a shock
wave that sweeps momentum from the ambient medium and it includes the effects
of the Galactic differential rotation and its gravitational torque, as well as
interstellar density gradients within and away from the Galactic plane,
possible fragmentation and drag forces in the late stages, and an initial
rotation of the system. The best fit to the data yields values for the current
Belt semi-axes of 373 +- 5 pc and 233 +- 5 pc, and an inclination of 17.2 +-
0.5 degrees, in agreement with earlier results. A different Belt orientation,
however, results from new H2 data and distance information. The present Belt
rim is found to coincide with most of the nearby OB associations and H2 clouds,
but its expansion bears little relation to the average association velocities.
An initial kinetic energy of (1.0 +- 0.1) 10^(45) J and an expansion age of
26.4 +- 0.4 Myr are required, in agreement with earlier 2D estimates. The
factor of 2 discrepancy between the Belt dynamical age and its stellar nuclear
age could not be solved by adding a vertical dimension in the expansion, drag
forces, fragmentation, or an initial rotation. Allowing the Belt to cross the
Galactic disc in the early stages requires a longer age of 52 Myr, but yields a
very poor fit to the data.
