Lithospheric structure of the Fiordland plutonic block controls the transition from transpression to subduction along the southwestern New Zealand plate boundary

In southwestern Zealandia, the plate boundary transitions from the Puysegur oblique subduction zone to the 600-km long transpressive Alpine Fault and Southern Alps uplift zone.  Utilizing abundant earthquake observations, we construct a 3D seismic velocity model to 130-km depth that demonstrates that the strong lithosphere of the Fiordland block defines the character of deformation along the plate boundary zone.  Highly oblique convergence combined with the relatively-weak young Puysegur slab enables sharp slab bending as it is translated northward around the Fiordland block. 

The Fiordland block contains plutonic rock from the 500-100 Ma Gondwana Cordillera, and its Grebe shear zone is a long-lived boundary, with a geochemically indicated Precambrian lithospheric keel underlying the Eastern Domain.  The Grebe shear zone is imaged as a boundary to 80-km depth, with Eastern Domain lithosphere abutting the deeper Australian slab, where it bends to vertical below 75-km depth.  Western Fiordland Orthogneiss lower crust, uplifted in the Miocene along reactivated shear zones, is imaged as a rigid/strong high-velocity feature pushed up above the 30-70-km depth Australian slab. In the crust, seismicity is distributed from the offshore Alpine Fault to eastern Fiordland, with partitioning along various structures including reactivated shear zones.

In southernmost Fiordland, south of Dusky Sound, the Puysegur slab maintains its moderately dipping subduction continuous with its offshore extent, and the overlying Pacific plate shows moderate seismic velocity material with the deep keel located further east than the slab.  In northern Fiordland, the impacting Pacific lithospheric base has an additional strong component, with Cretaceous underplated Hikurangi igneous plateau. This collision further steepens the young Australian slab which exhibits abundant deep seismicity 70-150-km depth. Overlying the deep vertical slab, our model suggests crustal thickening between the George Sound and Indecision Creek shear zones with exhumed high-velocity orthogneiss (Vp~6.5 km/s) overlying mid-crustal Vp of ~6.0 km/s.