Thomas Brian Phillips, Craig Magee 

Intraplate volcanism is widely distributed across continental interiors. Yet controls on the location, 3D geometry, and longevity of many individual intraplate volcanic systems are often poorly understood. Geophysical and geodetic data provide insights into active intraplate magma plumbing systems, particularly when coupled with petrological and chemical analyses of volcanic products, but these techniques are relatively low resolution and reveal little about the host rock structure. It is therefore difficult to evaluate whether magma transiting the lithosphere is directed by the complex and highly heterogeneous structure of continental interiors. Here, we use borehole-constrained 2D seismic reflection data to characterise the 3D geometry of the newly-discovered, ancient, intraplate Tuatara Volcanic Field in the Great South Basin, offshore the South Island of New Zealand, and investigate its relationship with pre-existing crustal and lithospheric structures. The ~270 km2 Tuatara Volcanic Field is dominated by a dome-shaped edifice of tabular lavas, surrounded and overlain by ~69 distinct volcanoes connected and likely fed by a network of >70 sills. Seismic-stratigraphic analyses reveal the volcanic system developed over 40 Myr, between the Late Cretaceous and Early Eocene (~85 Ma–45 Ma). We find that the Tuatara Volcanic Field is located directly above the Livingstone Fault, which separates the Caples and Dun Mountain-Maitai basement terranes; the recently active (600 yr before present) Auckland Volcanic Field is similarly located above the Livingstone Fault along-strike on the North Island. We suggest the Livingstone Fault controlled the location of the Tuatara Volcanic Field by: (i) producing relief at the base lithosphere, which we propose focused small-scale convection and/or lithosphere detachment, promoting periodic melting over ~40 Myr; and (ii) providing a pathway that facilitated magma ascent. Our work highlights that (recently) active intraplate volcanic systems, including the Auckland Volcanic Field, may comprise distributed volcanoes interconnected by sill-complexes, and be spatially and temporally controlled by pre-existing crustal and/or lithospheric structures.