A lower crustal “bright spot” under the Bay of Plenty, North Island, New Zealand

Marine seismic reflection measurements across the central Bay of Plenty, northern North Island, New Zealand, detected a strong seismic reflection in the lower crust at a depth of about 10 km (5 s twt). This reflection is observed on the stacked seismic section for about 8.5 km across and below a young actively rifting basin that lies along the offshore extension of the active Taupo Volcanic Zone. The reflection is unusual in that its maximum extent on any shot record is about 1250 m, about half the Fresnel zone diameter, suggesting a series of point reflectors or reflectors of limited extent. Wave equation modelling indicates that the reflection event probably represents a positive increase in acoustic impedance and thus does not represent a magma body or partial melt, as has been suggested previously. The preferred model suggests a thin layer, such as a mafic sill, with the large reflectivity and apparent wavelength caused by wavelet tuning, similar to that deduced for the Surrency bright spot in the USA. A mafic sill would be consistent with the geological setting, although other explanations, such as a highly altered shear zone, cannot be excluded. From the restricted extent of the reflections on the shot gathers and their spatial relationship, we infer that the sill is fragmented.