ERUPTIVE HISTORY OF THE EARLY TO MID MIOCENE WAITAKERE VOLCANIC ARC, AND PALEOGEOGRAPHY OF THE WAITEMATA BASIN, NORTHERN NEW-ZEALAND

Geophysical anomalies, volcaniclastic basinal sediments and exposures of intrusive and extrusive igneous rocks (Waitakere Group) indicate that a line of volcanoes (Waitakere Volcanic Arc) was active along west Northland during early and mid Miocene times (22–15 m.y. B.P.). The Arc is inferred to have been generated above a west-dipping Benioff zone that paralleled the boundary between the Indian and Pacific plates. The region between the Arc and a postulated eastern trench (arc-trench gap) consisted of a terrestrial basement high,an elongate basin (Waitemata Basin) and a northern land mass.Calc-alkaline volcanism was active at at least three major centres along the Arc and began in the Waitakian and early Otaian, supplying volcanic detritus to the Waitemata Basin. The two southern centres were the most active, continuing to erupt well into the Altonian. Eastward advance of the active margins of these centres and associated uplift, first at Kaipara and later in the Waitakere Ranges, saw bathyal sedimentation on the west side of the Waitemata Basin supplanted by shelf and terrestrial accumulation of coarse volcanic debris and lava flows.The northern centre, Waipoua, had an early sporadic phase (mid to late Otaian) followed by uplift, eastward advance and eruption of terrestrial, high-alumina basalt flows (Alton ian to Clifdenian). They mark the last phase of activity along the Arc. © 1979 Taylor & FrancisGroup, LLC.