New Zealand consists of a cluster of islands, the three largest being North, South, and Stewart, in the southwest Pacific Ocean. They have a total land area of about 270 000 km2 (similar to that of the British Isles or Japan). The islands are the small emergent parts of a much larger submarine continental mass (Fig. 0.1) that was rafted away from Australia and Antarctica by sea-floor spreading in the proto-Tasman Sea between 85 and 60 Ma. Much of this New Zealand subcontinent is a remnant of the former eastern margin of Gondwanaland, the ancient southern supercontinent. The mainland islands form a long, narrow, NE-SW trending archipelago bisected by an active, obliquely converging, boundary between the Australian and Pacific lithospheric plates (Fig. 0.2), which has evolved over the last 25 million years (Kamp 1992). The plate boundary is marked by active seismicity and volcanic arcs, illustrating New Zealand's position as part of the Circum-Pacific Mobile Belt -the so-called "Pacific Ring of Fire". The NE-SW trend of the modem plate boundary cuts across mainly NW-SE oriented structural features inherited from earlier (mid-Cretaceous) rifting events.