It's shape is because of the way it has moved over the margin, and subduction along the western edge of Zealandia isn't it?
Yes and no. NZ is the shape it is because of the subduction zones, however, i's only in the South Island that the subduction occurs in the west, off the North Island, it occurs to the east. My understanding is that it all used to occur to the east, because the pacific plate is older, and less bouyant than the Indo-australian plate, however, the Fiordland Terrane 'rafted' into the subduction zone, and gummed the whole thing up, so the subduction shifted focus, and moved to the west coast - where the Alpine fault now is, and of course, needless to say, the top most part of the south Island, and the bottom most part of the North Island are emeshed in a suitably complex set of faults where the plates make their transition.
The Alpine fault is (I believe) largely transform, however, there is obviously substantial compression involved (cif the Southern Alps), my understanding is that south of Fiordland Subduction resumes, but it's Indoaustralia that subducts.
The North Is. in the chain is largely volcanic because of where the fault is, and how it now divides the chain?
Not quite, my recollection is that the North Island gets a lot of back ark vulcanism (which appears to have shifted focus over time) and that elements of the North Island Vulcanism are as a result of the fact that there's a back arc spreading zone.
This is from an article by J Tuzo Wilson (1963 SciAm), "Convection currents in the earth's mantle may move blocks of material with different effects. Continental mountain chains and island arcs could form where [convection] currents sink and blocks meet; mid-ocean ridges [could form] where currents rise and blocks are [rifted] apart."
And : "Two convection currents perpendicular to each other suggest a mechanism for producing large horizontal faults such as the one that has offset western New Zealand 300 miles northward."
Yeah, the Geology of the west coast is smeared in a northern direction, and the area that is now Nelson used to be part of Fiordland
NZ is the only island chain directly over the margin of two plates, [that are moving transversely].
Yeah, I think this is right.