Lithospheric Shortening and Ductile Deformation in a Back-Arc Setting: South Wanganui Basin, New Zealand

<p>South Wanganui Basin (SWB), New Zealand, is located behind the southern end of the Hikurangi subduction system. One of the most marked geophysical characteristics of the basin is the -150 mGal Bouguer/isostatic gravity anomaly. Sediment fill can only partly explain this anomaly. 3-D gravity models show that the gravity anomaly associated with the basin is generally consistent with a downwarp model of the entire crust. However, the downwarp of the Moho has to be 3-4 times larger than the downwarp of the sediment-basement interface to fit the observed gravity anomaly. Hence a model of lithospheric shortening where ductile thickening of the crust increases with depth is proposed. Finite element modelling demonstrates that the crust, in order to produce the ductile downwarp, is best modelled with at least two distinct different layers. The model requires the top 15-20 km of the crust to behave purely elastic and the lower part (10 km thick) to be viscoelastic with a viscosity of 10[to the power of 21 pascal-seconds]. The existence of this ductile lower continental crust can be explained due to fluids released from the subducting slab accumulating in the lower crust. This is supported by receiver function analysis results. These results propose a 10+/-2 km thick low S-wave velocity layer in the lower crust. The vertical loading necessary to create the basin is high (up to 200MPa) and is difficult to explain by slab pull forces transmitted via a strongly coupled subduction interface alone. An additional driving mechanism proposed is a thickened mantle lithosphere inducing normal forces on the base of the crust. However, the exact origin of the basin remains a puzzling aspect. Receiver function analysis shows that the crust of the subducting Pacific plate underneath the mainland in the lower North Island is abnormally thick ([approximates]10 km) for oceanic crust. This matches with results from the 3-D gravity modelling. Further features discovered with the receiver function analysis are an up to 6 km thick low-velocity layer on top of the slab, which is interpreted as a zone of crushed crustal material with subducted sediments. Furthermore, a deep Moho (39.5+/-1.5 km) is proposed underneath the northern tip of theMarlborough sounds. Shallow seismic and gravity investigations of the southeastern corner of the SWB reveal a complex faulting regime with high-angle normal and reverse faults as well as a component of strike slip. The overall style of faulting in the SWB changes from the west to the east. There are the low-angle thrust faults of the Taranaki Fault zone in the west, the high-angle mostly reverse faults in the eastern part of the basin and the strike slip faults, with a component of vertical movement, at the eastern boundary within the Tararua Ranges.</p>