In the vertical direction, panels a), c), and e) show the time evolution of the 3D out-of-plane current density (j_ez) at t = 337ω_pe,t = 362ω_pe, and t = 387ω_pe. Current density is represented as an isosurface of 2/3 the maximum current density at the respective time. Again, in the vertical direction, panels b), d), and f) show isosurfaces of the Q-value (see Eq. (5)) at the same times in the simulation. For clarity, isosurfaces left of the current sheet are shown in blue and isosurfaces right of the current sheet are in yellow. In all panels, in-plane magnetic field-lines are superimposed on flux-tubes, showing the magnetic field at several horizontal slices through the simulation box. The simulation times shown were chosen around the time of island/vortex formation, which occurred slightly earlier in the 3D case.