Distribution, form, and structural control of the Alexandra Volcanic Group, North Island, New Zealand

Abstract

Karioi, Pirongia, Kakepuku, and Te Kawa which show a northwesterly (300°) alignment. Volcanism was predominantly basaltic and produced approximately 55 km³ of lava, volcanic breccia, and scoria from at least 40 centres. A new formation is proposed within the Alexandra Volcanic Group, the Okete Volcanic Formation, which shows differences in form, petrography, petrochemistry, and eruptive mechanisms from the rest of the Alexandra Volcanics. The Okete Volcanics form numerous small partially eroded scoria cones and thin lava flows with occasional tuff rings, and are alkalic in nature, consisting of finegrained basanites, basalts, and rare hawaiites. The Pirongia, Karioi, Kakepuku, and Te Kawa Volcanics form large low-angle composite cones constructed of thick lava flows and volcanic breccias, with minor dikes, scoria, tuff and lahars. They are composed of tholeiitic or subalkaline coarse-grained basalts, basaltic andesites, with rare andesites. Volcanism in the Alexandra field is Plio-Pleistocene in age, spanning the interval between 3.79 m.y. and 1.80 m.y., although some volcanic centres may be considerably younger. Geomorphological evidence suggests the volcanoes of Karioi, Pirongia, Kakepuku, and Te Kawa become progressively younger to the southeast. The Okete Volcanics intercalate stratigraphically with lavas from Pirongia and Karioi, and show no distinct temporal relationships. The regional northwesterly alignment may be caused by rising magma which encountered a major northwesterly fracture at deeper crustal levels and was channelled in part along the system of north and northeast-striking faults at upper crustal levels.