Geophysical study of the Hauraki depression, North Island, New Zealand

Abstract

Geophysical data show that the southern part of the Hauraki Depression is a rift structure. In W—E cross section it is made up of a fault angle depression, a median horst, and a graben. A maximum thickness of about 3 km of Quaternary and Tertiary sediments fill the two depressions. The fault angle depression and the graben are bounded at the eastern side by major normal faults which dip about 70 ± 10° to the west, namely, the recently discovered Kerepehi Fault in the centre and the Hauraki Fault which forms the eastern boundary of the depression. A minor hinge fault (Firth of Thames Fault) probably runs along the western boundary. Transverse faults cross the depression causing horizontal offsets of the major normal faults of up to 3 km. Recent rifting of the southernmost part is indicated by active faulting and by shallow earthquake activity beneath the depression (focal depth ≤ 12 km). The chemistry of numerous hot springs implies that high temperatures (250–300°c) occur at depths of less than 5 km; these springs occur over the boundary faults and over the median basement ridge. The Hauraki Depression continues north up the Hauraki Gulf, as indicated by the parallel course of the Firth of Thames shore lines. Sea gravity data across the Hauraki Gulf, and sea seismic data between Leigh and the Moko Hinau Islands suggest a basement structure similar to that of the southern part of the depression. The Hauraki Depression is, therefore, a large tectonic feature about 20–30 km wide and at least 220 km long. A comparison between major geological and geophysical features of the Hauraki Depression and those of other well-studied continental rifts shows striking similarities. It is inferred that the Hauraki Depression is an active rift which was formed after the collision of the Indian and the Pacific plates. © 1979 Taylor and Francis Group, LLC.