High resolution crustal and upper mantle structure adjacent to a continental transform, South Island, New Zealand

Abstract

The onshore-offshore shooting technique provides arguably the highest resolution method to image the crust and upper mantle. This component of the South Island GeopHysical Transect (SIGHT) experiment is used to investigate two principal properties of the eastern South Island: crustal structure and Pn anisotropy. We present a 3-D P-wave seismic velocity model and find seismic velocity perturbations linked to surface volcanism. A mid crustal cylindrical low velocity zone, with a minimum 4% reduction in P-wave speed (vp = 5.85 ± 0.10 km/s) runs adjacent to and just south of the Rangitata River (measures 15 km by 100 km). The surface correlation of this exotic feature with sporadic Cretaceous rhyolites is suggestive of an emplaced rhyolitic pluton. An isolated 10 km by 10 km low velocity zone (vp < 7.5 km/s) in the upper mantle beneath the Quaternary Timaru basalts can be explained by ~2% partial melt. Evidence for a wide zone of lithospheric weakness at the plate boundary arises from: significant Pn anisotropy in the upper mantle 65 km offshore (6.5 ± 2.7%), and a westward rising Oligocene seismic horizon that climbs by 1.5 km over a lateral distance of 250 km. Assuming channel flow in the mid-lower crust, in the uplift profile can be replicated by a viscosity of 1018 - 1019 Pa.s. Deformation bounds on continental collision are presented. Increased apparent Pn speeds beneath the continental shelf may indicate an end to an anisotropic domain, and suggest a deforming zone in the mantle lithosphere that is at least 200 km wide. This is wider than the zone of Pliocene crustal deformation. The correlation with abundant SKS measurements onshore implies a 80 - 210 km thick anisotropic layer, which may confine the anisotropy to the lithosphere. Bulk rock properties of the Pacific plate can be inferred within this plate boundary zone. Low modal P-wave velocities (~6.2 km/s) and an anomalously low Poisson ratio (0.20± 0.01) are most realistically explained by a felsic crust of high quartz content. A case study of an exposed basement chunk at the centre of the Miocene Lyttelton volcano provides circumstantial evidence for its basement connectivity. In summary, observations of four seismic datasets reveal a heterogeneous crust. The pattern of lithospheric deformation contradicts the science concerning plate tectonics at continental plate boundaries, but supports the steadily evolving ideas embroiled in continental tectonics that overall, deformation below the plate boundary is pervasive and widespread.