Numerical solutions of the time-dependent Ginzburg-Landau equations show a new mechanism for plastic motion of a driven vortex lattice in a clean superconductor. The mechanism, which involves the creation of a defect superstructure, is intrinsic to the moving vortex lattice and is independent of bulk pinning. Other structural features found in the solutions include a reorientation of the vortex lattice and a gradual healing of lattice defects under the influence of a transport current. {copyright} {ital 1996 The American Physical Society.}