Early to middle Pleistocene tephrochronology of North Island, New Zealand: Implications for volcanism, tectonism, and paleoenvironments

Abstract

Thick volcaniclastic sequences of early to middle Pleistocene age in southern North Island, New Zealand, contain rhyolitic tephra beds that record the early history of the Taupo volcanic zone (TVZ). At least 54 different tephra beds are recorded, and their chronology is defined by glass fission-track ages and paleomagnetism. The tephra beds span the interval ca. 2.0−0.6 Ma and provide an event frequency of 1/19 k.y., significantly higher than the frequency of sheet-forming ignimbrites preserved in the TVZ at this time (≈1/100 k.y.). The distal tephra beds thus provide a record of volcanism not revealed in the proximal volcanic region and suggest a major period of explosive activity at ca. 1.79−1.60 Ma. Several important marker horizons are identified: Pakihikura tephra (1.63 Ma), Mangatewaiiti tephra (1.24 Ma), Potaka tephra (1.00 Ma), Kidnappers B tephra (1.00 Ma), and Kaukatea tephra (0.88 Ma). These tephra beds allow direct correlation between (1) marine and nonmarine facies and (2) the fore-arc and back-arc regions of New Zealand. The tephra beds provide a framework for a paleoenvironmental reconstruction of the southern North Island. Volcaniclastic transport routes from the TVZ to basins in the south and southeast, and through the site of present mountain ranges, supplied material to a terrestrial lowland fore-arc area in the interval 1.64−0.7 Ma. Uplift and deformation since 0.7 Ma have disrupted paleodrainage routes, diverting them to the north and southwest.