Lithospheric Structure and Deformation in an Oblique Continental Collision Zone, South Island, New Zealand

Abstract

Distributed deformation in both crust and upper mantle due to oblique continental collision in New Zealand's central South Island is investigated by analysing seismic wide-angle and gravity data. A 600-km long and 65-km deep P-wave velocity structure perpendicularly across the Alpine fault, the major transpressional fault between the Australian and Pacific tectonic plates, is constructed by fitting 34,000 measured arrival times from 400,000 seismograms. Evidence of distributed lithospheric deformation is manifest in: zones of anomalous P-wave velocities reduced by several percent over tens of kilometres wide and deep in both tectonic plates; 17 km of crustal thickening beneath the Southern Alps; seismic P-wave anisotropy of at least 10% in the upper mantle; and, from gravity, lithospheric mantle thickening below the crustal root. These are interpreted as evidence for lowered lithospheric strength in the plate boundary zone. Reduced crustal velocities relative to average values nearby of at least 4% west of the Alpine fault correlate with flexurally induced extension stresses in a moderately strong Australian crust. At least 8% lower than average crustal velocities immediately east of the Alpine fault are associated with a SW dipping fault zone of low strength in the Pacific plate caused by high pore fluid pressures. Off the east coast the Pacific plate resists flexure from imposed sedimentary loading, indicating an effective elastic thickness of at least 25 km. A velocity reversal is derived for mid-crustal material below 15 km depth in the crustal root, possibly related to heating. The Pacific lower crust exhibits strong thickening within the crustal root, interpreted as a sign of its highly ductile behaviour. The root is about twice as wide as the 100-km wide mountain range, yet they both share the same western margin, the surface trace of Alpine fault. The deepest part of the root is offset 10-20 km to the east of the highest elevations, and is about 10 km deeper than required to isostatically compensate the mountain load, presumably caused by a gravitational instability due to lithospheric thickening. Analysis of gravity data suggests that, despite increased plate convergence in the north, lithospheric thickening is more pronounced southwards. In contrast to the asymmetric deformation of the crust, the shape of the mantle anomaly due to lithospheric thickening appears symmetric, centred at the deepest part of the crustal root. This would imply that no intra-continental subduction has occurred. Further evidence for symmetric mantle deformation comes from seismic anisotropy in the upper mantle. P-wave anisotropy of at least 10 (± 3)%, up to 13 (± 5)%, is estimated to occur up to 100 km NW of the Alpine fault, possibly extending a similar distance SE of the fault. The fast orientation of the anisotropy is roughly parallel to the Alpine fault and therefore consistent with SKS results from most of New Zealand. Its depth range is inferred to be up to 200 km. Anisotropy has likely been caused by the strike-slip motion between the plates, where olivine crystals have become alligned or even dynamically recrystallised as suggested by the large magnitude of anisotropy and possible fast orientations. In summary, evidence for a weakened plate boundary zone and lithospheric deformation distributed up to 300 km laterally and 200 km vertically is found, and asymmetric crustal and symmetric mantle deformation due to transpressional plate motion is inferred.