  Global magnetic field extrapolations are now revealing the huge complexity of
the Sun's corona, and in particular the structure of the boundary between open
and closed magnetic flux. Moreover, recent developments indicate that magnetic
reconnection in the corona likely occurs in highly fragmented current layers,
and that this typically leads to a dramatic increase in the topological
complexity beyond that of the equilibrium field. In this paper we investigate
the consequences of reconnection at the open-closed flux boundary ("interchange
reconnection") in a fragmented current layer. We demonstrate that it leads to a
situation in which magnetic flux (and therefore plasma) from open and closed
field regions is efficiently mixed together. This corresponds to an increase in
the length and complexity of the open-closed boundary. Thus, whenever
reconnection occurs at a null point or separator of the open-closed boundary,
the associated separatrix arc of the so-called "S-web" in the high corona
becomes not a single line but a band of finite thickness within which the
open-closed flux boundary is highly structured. This has significant
implications for the acceleration of the slow solar wind, for which the
interaction of open and closed field is thought to be important, and may also
explain the coronal origins of certain solar energetic particles. The
topological structures examined contain magnetic null points, separatrices and
separators, and include a model for a pseudo-streamer. The potential for
understanding both the large scale morphology and fine structure observed in
flare ribbons associated with coronal nulls is also discussed.
