Active and Extinct Hydrothermal Systems of the North Island, New Zealand

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Abstract

A simplified geologic map of the North Island is shown in Figure 1. The oldest rocks which form the basement are of late Paleozoic to Mesozoic age. The New Zealand lithosphere only began to develop as a separate crustal entity in the late Cretaceous-early Tertiary when it broke away from the Gondwana supercontinent as the Tasman Sea opened (Sporli, 1987). Much of the geology of this continental fragment, which extends from New Caledonia to the Campbell Plateau, is obscured by the fact that about 70% of it is submerged (Fig. 2). New Zealand's Cenozoic history relates to its proximity to a major active boundary between the Indian and Pacific plates (Figs. 3 and 4). Accurate reconstruction of the plate boundaries for most of this era is difficult, although several versions are published (e.g. Cole and Lewis, 1981; Ballance et al., 1982; Brothers, 1984; Walcott, 1987). In the following summary, we focus on the products of arc magmatism and closely related hydrothermal activity (Fig. 5). We start with the present situation. The TVZ is a complex volcano-tectonic depression, filled with pyroclastic deposits and lavas, that is related to the westward dipping subduction zone of the Hik:urangi Trough (Fig. 6). It extends offshore into the Tonga-Kermadec arc and marks the start of the Pacific 'horseshoe of fire'. Convergence along the Hikurangi Trough is increasingly oblique southward to form a transform plate boundary as delineated by the Alpine Fault in the South Island (Figs. 2 and 3). The Benioff zone dips at a very shallow angle west of the Hikurangi Trough but becomes steeper westward to where it lies at about 80 km depth beneath the TVZ (Fig. 6). An accretionary prism, comprising Tertiary and younger sediments, lies above the shallower Benioff zone. Bounding the accretionary prism to the west are the Axial Ranges which are made up of Mesozoic greywackes and argillites of the Torlesse terrane; the North Island Shear Belt comprises a set of dextral north-trending faults that cuts across them (Fig. 7). The TVZ lies adjacent to the Axial Ranges, about 250 km west of the Hikurangi Trough, and extends from White Island to Tongariro. Its margins are defined by steep gravity gradients and distribution of volcanic vents, except to the northwest where it merges with the Coromandel Volcanic Zone (Rogan, 1982; Wilson et al., 1984). Northeast trending normal faults dominate the structural fabric, forming a series

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