Volcanic alignments North and West of New Zealand's Central Volcanic Region

Abstract

Three major types of volcanic alignment are recognised. Volcanic vent alignments are inferred to be tensional features usually parallel to the local principal horizontal stress (PHS) direction, and include fissures, elongation of centres, and lines of closely spaced contemporaneous vents. Volcanic centre alignments are most likely related to underlying pre-existing faults that dilated at a point or as a fissure during volcanism. Regional volcanic alignments, which incl ude widely spaced centres and volcanic formations within a narrow belt, imply a fundamental relation between volcanism and structure, and may indicate the location of the major fractures that act as volcanic conduits from depth to near-surface minor fault systems. Regional alignments, and those centre alignments that strike west in Northland and north-west in Taranaki, reflect post-Hokonui structures. They show a 30° clockwise swing of strike in progressively more southern areas from the Bay of Islands to Bombay — North Hauraki, and perhaps a further 30° to Mt Egmont, comparable to that of Macpherson's regional structural arc. Of all alignments, they alone include pre-Pliocene and non-basaltic volcanic formations. On the other hand, vent alignments, and two other sets of centre alignments, striking south and south-west, show no comparable southwards swing in strike, on a regional basis, but have considerable local variations. They appear to reflect Late Cenozoic structures. Inferred PHS directions are: for the post-Hokonui structures, NE in the Bay of Islands swinging steadily to the S of E in Taranaki; and for Late Cenozoic structures between NNE and ENE, averaging NE, in all districts from the Bay of Islands to the South Waikato. Thus the PHS bearing appears to have swung anticlockwise (or the land rotated clockwise) since the folding of the post-Hokonui Orogeny, by a small amount in stable Northland, by 30° in Bombay and North Hauraki, and by possibly as much as 60° in Taranaki.