  Implosive formation of current sheets is a fundamental plasma process.
Previous studies focused on the early time evolution, while here our primary
aim is to explore the longer-term evolution, which may be critical for
determining the efficiency of energy release. To address this problem we
investigate two closely-related problems, namely: (i) 1D, pinched anti-parallel
magnetic fields and (ii) 2D, null point containing fields which are locally
imbalanced ('null-collapse' or 'X-point collapse'). Within the framework of
resistive MHD, we simulate the full nonlinear evolution through three distinct
phases: the initial implosion, its eventual halting mechanism, and subsequent
evolution post-halting. In a parameter study, we find the scaling with
resistivity of current sheet properties at the halting time is in good
agreement - in both geometries - with that inferred from a known 1D similarity
solution. We find that the halting of the implosions occurs rapidly after
reaching the diffusion scale by sudden Ohmic heating of the dense plasma within
the current sheet, which provides a pressure gradient sufficient to oppose
further collapse and decelerate the converging flow. This back-pressure grows
to exceed that required for force balance and so the post-implosion evolution
is characterised by the consequences of the current sheet `bouncing' outwards.
These are: (i) the launching of propagating fast MHD waves (shocks) outwards
and (ii) the width-wise expansion of the current sheet itself. The expansion is
only observed to stall in the 2D case, where the pressurisation is relieved by
outflow in the reconnection jets. In the 2D case, we quantify the maximum
amount of current sheet expansion as it scales with resistivity, and analyse
the structure of the reconnection region which forms post-expansion, replete
with Petschek-type slow shocks and fast termination shocks.
