Crustal Structure and Lithospheric Doming: Aspects of Deformation Along an Obliquely Convergent Plate Margin, New Zealand

Abstract

Lithospheric deformation along and adjacent to the Pacific-Australian Plate boundary through New Zealand has resulted in different expressions in North and South Islands. This thesis investigates some aspects of crustal and upper mantle structure in New Zealand and is divided into two distinct parts. The first examines the structure of the obliquely compressional crustal plate boundary in South Island using seismic techniques; the second focuses on the domed topography of central North Island and its relationship to mantle processes.

High density active source, one and three-component, seismic data from a transect across the Southern Alps provides information on the deformation of the crust across the Australia-Pacific plate boundary of South Island. These data show 0-0.08 s (∼ 0.25 %) delay times between the radial and transverse directions for shear waves (Sg and SmS phases), with maximum possible delays of 140 ms and the fast direction aligned with the transverse direction (approximately parallel to the plate boundary). The transect is perpendicular to the Alpine Fault, which is slightly oblique to the fast mantle directions determined from SKS phases. The small values of crustal splitting may result from the oblique angle of the ray paths to the actual crustal structure at depth, or the complex nature of the deformation as observed at the surface, which though on a small scale can be strongly anisotropic, may not add constructively over a large region.

Poisson’s ratio, determined from forward modelling of both P and S phases, shows low values of 0.21 - 0.24 for the crust of South Island. A broad region of low values (σ=0.15) exists at 10-20 km depth under the Southern Alps, which corresponds to a previously identified body of low Vp and high resistivity. The low σ is interpreted as low pore fluid pressure and high silica composition rocks. This contrasts with previous interpretations of high pore fluid pressure at this depth.

The topography of central North Island, New Zealand, describes a 250 km wide and ∼ 500 m high dome. Exhumation estimates from mudstone porosity measurement indicate an increase in exhumation from ∼ 500 m at the coast to 2 km in the region of the present topographic high. Combining these values gives an estimate of rock uplift of over 2.5 km for central North Island, since 4 Ma, a rate of 0.6 mm/yr. Tectonic uplift of 1.25 km indicates that ∼ 50 % of the rock uplift occurs in response to exhumation. An independent local estimation of differential erosion in central North Island gave 300 m of exhumation since at least 500 ka, a rate of ≥ 0.6 mm/yr. Using a digital elevation model of New Zealand the fluvial incision of the landscape was calculated and ∼169 m of rebound can be attributed to incision. Contouring maximum incision elucidates a region of high incision ∼ 50 km south of the present centre of domed rock uplift. Using incision as a proxy for rock uplift, it is hypothesised that the incision signal is recent and demonstrates the southward migration of the centre of rock uplift. Rebound of sedimentary basins due to a reduction in plate coupling forces can also account for some of the observed rock uplift.

Buoyancy forces required to create the pattern and magnitude of rock uplift are investigated using a 3 D loading model of the lithosphere. Strong upward forces (65 MPa) are required under the Central Volcanic Region, combined with broad uplift (36 MPa) over western North Island, to fit the observed rock uplift. Low Pn velocities under the Central Volcanic Region indicate temperatures 500◦C hotter than that of normal mantle. This temperature anomaly corresponds to 60 kg/m3 less dense than normal mantle, which is consistent with the change in density of 66 kg/m3 estimated from the loading model and assuming the density change occurs over a 100 km depth range. The southern extent of buoyancy forces does not correspond well to regions of high seismic attenuation in the lithosphere but instead with the region of high incision.