Sub-surface geological mapping of the eastern Alexandra Volcanic Group, Waikato

The Alexandra Volcanic Group (AVG; 2.74–1.60 Ma) is a 65 km-long, cross-arc chain of basaltic volcanoes located ~100 km west of the Taupō Volcanic Zone. The field hosts >120 vents of both arc and alkaline composition and is the subject of ongoing geological mapping to decipher the age, structure and tectono-magmatic origin of its bimodal volcanism. This study focuses on the AVG east of Mt. Pirongia which lies closest to the arc front and is largely buried beneath ≤200 m of Hamilton Basin sediments. Outcropping arc-type volcanoes include Pukehoua (371 m), Puketōtara, Kakepuku (449 m), Te Kawa (214 m), and Tokanui. Alkaline basalt vents are identified at Turitea (2.21 Ma) and Ngāhape (1.90 Ma). Stratigraphic interpretations from over 170 water bore logs, coal exploration records, and petroleum core logs were integrated to reconstruct the subsurface volcanic architecture. The resulting map identifies 25 volcanic vents (19 new), most of which are small-volume and correlated with the Okete Volcanic Fm. These vents follow NE-SW alignments consistent with the regional fault pattern. Buried lava fields associated with Kakepuku (2.35 Ma), Te Kawa (2.21 Ma), and the Waikeria dacite (1.99 Ma) infill basin-and-range style depressions within the Waipapa terrane. These are overlain by ring plain deposits of Pirongia Volcano (~2.5 Ma) and ignimbrites from Mangakino Caldera. The new mapping and stratigraphic constraints from this work will permit further testing of proposed, but contrasting, hypotheses (e.g. mantle upwelling via slab rollback ± tearing, hotspots or deep crustal faults) that attempt to explain the compositional, geochronological and spatial patterns of the AVG volcanoes. More broadly, the identified vent distribution at the arc–cross-arc interface may offer valuable insights into the architecture of submerged cross-arc systems, such as those within the Kermadec Arc.