Galatea Basin: the geophysical exploration of a transtensional basin in the southern Bay of Plenty, New Zealand

Abstract

The Galatea Basin, located in southern Bay of Plenty, straddles the eastern margin of the extensional Central Volcanic Region and is juxtaposed against strands of Waiohau and Te Whaiti faults. These faults are normal dextral oblique-slip faults defining the western limit of the North Island Dextral Fault Belt. A -27 mGal residual gravity anomaly centred over the Galatea Basin is resolved by 171 new gravity measurements. Forward modelling of the residual gravity anomaly in 2 3/4 and 3 dimensions, and interpretation of 2.4 km seismic reflection and 5 km seismic refraction profiles reveal a basin reaching depths greater than 3000 m. Basin infill is interpreted to be 900 m of Pleistocene volcanic and lacustrine sediments mantling 2000 ± 200 m of Tertiary sediments. A sequence of Cretaceous sediments up to ~2000 m thick may lie beneath Tertiary sediments. Displacement on the Te Whaiti Fault Zone is partitioned onto at least two strands. Te Whaiti (A), coincident with the present fault scarp and prominent Ikawhenua range front, dips 45 ° W and is dominantly normal. The subsurface Te Whaiti (B) fault dips 80 ± 10 ° W and is interpreted to have behaved as both a dip-slip and strike-slip fault throughout its history. Deformation and structural features within the Galatea Basin are consistent with a transtensional mode of deformation in the Pleistocene. Extension in the Central Volcanic Region, interacting with strike-slip in the North Island Dextral Fault Belt, controls the evolution of the Galatea Basin as a hybrid pull-apart basin.