Crustal structure and thermal anomalies of the Dunedin Region, South Island, New Zealand

Abstract

Seven geophysical data sets are used to investigate a transect along the southeast coast of South Island, New Zealand. The specific focus of this study is the Dunedin volcanic center, which last produced volcanics at the surface 13–10 Myr ago. Wide‐angle reflection/refraction seismic data along a two‐dimensional profile reveal a low‐velocity lower crust and mantle beneath the Dunedin volcanic center. The low‐velocity lower crust coincides with a highly reflective region on a nearby multichannel seismic line and may represent a hot, fluid‐rich region of the crust. In addition, high mantle helium ratios measured in the Dunedin region suggest a current or recent mantle‐melting event. High heat flow recorded in the Dunedin region is consistent with a hot body emplaced in the midcrust ∼10 Myr ago (Miocene) whose heat is just reaching the surface today. Uplift of an Oligocene limestone horizon in the Canterbury basin can be explained by a buoyant load beneath the Dunedin volcanic center and low flexural rigidity of the lithosphere beneath the volcanic center during the Miocene. We interpret the data as revealing two separate thermal events beneath the Dunedin volcanic center, one during the Miocene, when volcanism was last occurring at the surface, and the other occurring currently. Active volcanism associated with the current mantle‐melting event has yet to reach the surface.