Lithospheric structure across oblique continental collision in New Zealand from wide‐angle P wave modeling

Abstract

[1] Oblique convergence between the Australian and Pacific plates in South Island, New Zealand, has resulted in crustal thickening and distributed deformation within both plates. We measure this thickening and image the deformation with seismic wide-angle data along a 600 km long transect that spans the plate boundary. P wave arrival times from 34,000 rays are used to construct a two-dimensional model for seismic velocity with depth. Crustal velocities average 6.1 and 6.2 (±0.23) km/s for the Australian and Pacific sides of the boundary, respectively. Upper mantle velocities average 8.1 (±0.25) km/s. Distributed deformation is indicated by the following observations: a reduction in upper crustal and midcrustal velocities by 4% west of the plate boundary, ascribed to flexural bending stresses; a velocity reduction of 8% immediately east of the Alpine fault, linked to high pore fluid pressures; 17 km of crustal thickening to form a 44(±1.4) km deep crustal root that is offset 10-20 km SE of the highest mountains; a velocity reversal below the midcrust in the eastern part of the crustal root, caused either by heating or an artifact from seismic anisotropy; strong thickening of a 7.0(±0.4) km/s fast lower crust within the crustal root; and a low-velocity zone in the Australian upper mantle due to sampling the slow orientation of upper mantle anisotropy.

Mapping the Moho beneath the Southern Alps continent‐continent collision, New Zealand, using wide‐angle reflections

“…East of the Alpine fault and beneath the South Island, the synthetic wavefields and wide-angle stacks are marked by a classic bow-tie; a function of the steep sided flanks of the crustal root between distance 250 to 300 km. Our image for Transect 1 suggests that both the eastern and western side of the root are steeper than the $25°dip modeled by [Van Avendonk et al, 2004] but are less steep than the 45°dip modeled by [Scherwath et al, 2003] on Transect 2. By using the synthetic image of Transect 2 as a guide, the pattern of lower crustal reflectivity on the intermediate profile (Figure 3b) is best matched using dips of at least 40°and 25°for the western and eastern flanks of the crustal root.…”

“…The accuracy of this image and interpretation can be verified by forward modelling of the velocity models for Transects 1 and 2. The compressional velocity models for Transect 1 [Van Avendonk et al, 2004] and Transect 2 [Scherwath et al, 2003] were used to compute normal-incidence wavefields using acoustic finite differences, and then superimposed on the CDP image (Figures 3a and 3c).…”

“…These transects were designed to cross the Southern Alps, where the orogen is relatively unaffected by neighboring tectonic regimes, and to provide optimal raypath coverage for 2-D crustal illumination . Seismic velocity models derived from refracted and reflected arrivals along Transects 1 [Van Avendonk et al, 2004] and 2 [Scherwath et al, 2003], and first arrival travel-time tomography of passive seismic data [Eberhart-Phillips and Bannister, 2002] provide an image of a crustal root about 100 km wide, that deepens from approximately 40 km under Transect 1 to a maximum depth of 45 km under Transect 2. The deepest part of the crust lies about 20 km east of the main topographic divide and about 40 km east of the Alpine fault.…”

Crustal seismic reflection profile across the alpine fault and coastal plain at Whataroa, South Island

“…The imaging of a horizontal Moho under the Alpine Fault supports arguments for a strong Australian plate (e.g. Scherwath et al 2003) Conclusions The Whataroa seismic profile has delineated a deep (2.5Á3 km) sedimentary basin under the coastal plain of Westland, near Whataroa. Three main sedimentary layers have been defined with seismic velocities of 1.6Á2.0 km s (1 , 2.6 (3.0 km s (1 and 3.4 km s (1 .…”

A comparison between the transpressional plate boundaries of South Island, New Zealand, and southern California, USA: The Alpine and San Andreas Fault Systems

“…Late Cretaceous to Cenozoic sedimentary basins offshore of South Island vary in thickness between AUS and PAC, with a common thickness range and a maximum thickness of 1-2 km and 4 km, respectively, for AUS, and 2-3 km and 6 km, respectively for PAC [Scherwath et al, 2003;van Avendonk et al, 2004;Plates 1a, 1b]. Cenozoic sedimentary basins offshore of southern California are typically shallow along the LARSE lines [0.5 km; Baher et al, 2005] but deepen toward the shore, reaching variable depths of 1.5 to 4 km [Wright, 1991] (Plates 1c, 1d).…”