Relationship of the west coast, North Island, igneous bodies to the mid-Cenozoic Challenger Rift System and subduction of the Pacific plate

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Abstract
Possible origins of the igneous bodies off the west coast of the North Island, as revealed previously by geophysical surveys, are considered in the context of the wider Cenozoic geotectonic development of New Zealand. First it is suggested that the northwest-southeast-trending anomaly sources north of Manukau Harbour cannot be genetically related to the Miocene orogenic andesites and basalts onshore in western Northland, as earlier proposed, chiefy because the onshore subduction-related frontal arcs were oriented north-east-southwest. An alternative origin suggests that the offshore igneous bodies are of the correct age and position to be either rift volcanics or the products of early sea-floor spreading within the axial trough of the mid-Cenozoic Challenger Rift System. The geophysical character, and limited geologic data, of the anomaly sources in the southern field indicates that they may have a dual origin. At least some of these anomaly sources probably originated as frontal arc andesites as the subducted Pacific plate progressively extended to the southwest beneath northern New Zealand and concomitantly increased its dip; substantial intrabasement magnetic sources indicate that others have a rift or early sea-floor spreading origin.