Waitemata Tectonic Event(25 Ma ago) created a subduction system from the northeast
of Northland, causing: (1) the emplacement of the Northland Allochthon (24-23
Ma) which provided argillaceous limestone and serpentine to Northland, and minor
copper to both there and East Cape (located off Northland then); and (2)
Northland volcanism (from 22 Ma), which lacked economic mineralisation. The
Kiwitahi Tectonic Event (1714 Ma) caused: (1) a realignment of volcanism - from
coastal western Northland, to west of the Hauraki Plains (Kiwitahi); (2) a start
to the migration southwards from Northland of (a) the Alpine Fault System
towards Marlborough today, (b) subduction towards Cook Strait, and (c) volcanism
towards Taranaki-to-Bay of Plenty (initially through Coromandel, Kiwitahi, and
offshore west coast); and (3) a start to rhyolitic volcanism & associated
hydrothermal activity. The latter caused epithermal mineralisation, providing in
Coromandel the best known North Island metalliferous deposits. Andesitic
volcanism has been active in both of these two contrasting, subequal belts since
22 Ma. Rhyolitic volcanism and hydrothermal activity was wholly restricted to
the eastern Marshall Belt, with an abrupt limit to economic mineralisation at
the Volcanic Divide (within Hauraki Plains) and a major change there to basaltic
volcanism of the western Searle Belt. The Kaimai Tectonic Event (5-1.5 Ma)
caused: (1) a temporary reversal in volcanic migration in the Searle Belt, and a
duplication of rhyolitic & hydrothermal activity at Waihi (Marshall Belt);
(2) the rotational rifting of both the Central Volcanic Region (by 30) and the
Hauraki Rift (10), causing major curvature of the North Island and New Zealand,
and the transfer of the offshore subduction system from Coromandel to East
Coast; and (3) an effective end to Coromandel mineralisation. Post-Kaimai
Volcanism (1.5-0 Ma) included: (1) andesitic volcanism in the Searle Belt,
yielding the western coastal titanomagnetite deposits; and (2) andesitic, and
rhyolitic & hydrothermal activity in the Marshall Belt - the latter
providing geothermal energy, sulphur, and pumice/perlite deposits.
Post-Subduction (Intraplate) Volcanism (10-0
Ma) developed in a steadily expanding northwest region of relative tectonic
quiescence, as subduction and its volcanism migrated southwards. Intraplate
volcanism was predominantly basaltic, providing excellent aggregate; but less
common rhyolitic & hydrothermal activity provided geothermal potential and
mercury at Ngawha and at Puhipuhi, and halloysitic clays of extreme brightness
elsewhere in Northland. This volcanism has reached as far SE as Mayor Island,
providing an early New Zealand mineral resource -
obsidian.