Insights into the structure and tectonic history of the southern South Island, New Zealand, from the 3-D distribution of P- and S-wave attenuation.

The Pacific–Australian plate boundary in the South Island of New Zealand is a transpressive boundary through continental lithosphere consisting of multiple terranes that were amalgamated during previous periods of subduction and plate reorganization. The style and locus of deformation within the present-day plate boundary is controlled by the mechanical behaviour and distribution of these different lithospheric blocks. Geological studies are limited when it comes to illuminating lithospheric structure and rheology at depth. Imaging the 3-D seismic velocity and attenuation (1/ Q), with distributed local earthquakes, helps unravel regional structure and variations in strength, fractures and fluids. We determine the 3-D distribution of Qp and Qs, which show much more variation than seismic velocity (Vp), underlining the utility of Q (1/attenuation). The Haast schist belt, previously shown as ca. 25-km thick dry unit with moderate Vp and low Vp / Vs, is imaged with high Qs, and the highest Qs areas correlate with zones of higher grade schist. Below 25-km depth, the distribution of high Qp and Qs is markedly different from that of the overlying geological terranes. Both the strike and depth of the high Q regions indicate that they represent the subducted Hikurangi Plateau and its adjacent Cretaceous oceanic crust. The thickest part of the plateau, previously identified by Vp  > 8.5 km s−1 from seismic tomography and P -wave precursors and associated with an eclogite layer at the base of the plateau, also has the highest Q. This confirms that the strong plateau extends southwestwards as a narrow salient to the northern Fiordland subduction zone, where moderate- Q Eocene oceanic crust on the Australian plate is being subducted and bent to vertical. In the ductile crust, Q results suggest fluid saturation and elevated temperature conditions in the crustal root of the Southern Alps, and confirm that the shape of this crustal root is influenced by both the orientation and depth of the underlying plateau. Q also provides insight into the failed rifting that occurred in oceanic crust at the edges of the Hikurangi Plateau, with a region of relatively low Q at the on-land extension of the Bounty Trough and Canterbury Basin, at the narrowest part of the South Island. In the brittle crust above 10-km depth, low Q is related to regions of active recent seismicity that have high fracture density, with low Qs where fluids are present. In contrast, the locked Alpine Fault does not exhibit low Q in the brittle crust. [ABSTRACT FROM AUTHOR]