Crustal Structure Studies of a Transpressional Plate Boundary - the Central South Island of New Zealand

Abstract

Seismic refraction/wide-angle reflection and vertical-incident reflection studies are used to investigate the crustal structure of the Australian-Pacific transpressional plate boundary in the central South Island of New Zealand. A crustal-scale two-dimensional velocity model, derived from refraction/wide-angle reflection data, reveals asymmetric crustal thickening of the Pacific crust in response to the oblique convergence. The velocity model is consistent with the observed negative Bouguer anomaly in the central South Island and shows that the thickest part of the Crust is offset to the southeast of the highest mountains of the Southern Alps by approximately 20 km. This crustal root is delineated by wide-angle reflections from a high velocity layer, with a velocity of 7.5 ± 0.4 km/s, which is interpreted to represent oceanic crust. This layer is overlain by a layer, with a velocity of 6.45 ± 0.15 km/s, which is interpreted to be composed of metamorphosed schists. Above this lies a sequence of Permian and Mesozoic greywackes that appear to be remarkably uniform. Within this sequence no lateral variations in P-wave velocity are apparent below 6 km depth, and only a slight increase, from 6.18 km/s at 6 km to 6.22 km/s at 12 km depth, is observed. Below this depth a constant, velocity of 6.22 km/s is ascribed. In the top of the greywacke crust, lateral variations in velocity from 4.4 km/s to 5.62 km/s are seen, and are attributed to different degrees of weathering and fracturing. The shallow greywacke basement shows a non-linear increase in velocity with depth, as determined from it high resolution refraction study at Lake Pukaki, a trend that is similar to laboratory measurements of seismic velocity in this type of rock. A mean Poisson's ratio for the greywacke crust of 0.22±0.01 is determined from shear-wave arrivals. The thickness of the entire crust increases from 25 km near the East Coast to 44 km beneath Lake Pukaki. The shape of the lower crustal deformation and the amount of crustal shortening since the onset of the convergence lead to an estimate of the total amount, of uplift of the Southern Alps that is larger by a factor of 2 than previous estimates. A seismic expression typical of a compressional orogen is observed on near-vertical incident reflection data recorded at Lake Pukaki. These reflections delineate two deep crustal interfaces that dip to the southeast at 35 ± 5° and define a zone approximately 8 km wide. The preferred explanation for this reflectivity is anisotropy, caused by either metamorphic layering or mylonites. This interpretation is suggestive of a ramp-like shear zone along which the schists that are exposed adjacent to the plate boundary arc being transported to the surface. Thus it is possible that the reflections delineate a downward extension of a broad Alpine fault zone. Time delays observed on arrivals which traverse the plate boundary reveal that the schists at the boundary are accompanied by a marked reduction in P-wave velocity below 5 km depth. This low-velocity zone is given a width of approximately 30 km, as defined by the smoothest variation in velocity that is in agreement with the data, and has a P-wave velocity that is 5 - 10 % less than a velocity in greywacke at similar depth. The preferred explanation for this velocity reduction is the presence of fluids al enhanced pore pressure. Combined seismic and gravity modelling of the McKenzie Basin, which lies just to the east of the plate boundary, reveals a sequence of Miocene or older Tertiary sediments underlying the surficial glacial deposits. The presence of a sequence of Tertiary sediments implies that the basin formed as a result of uplift of the mountain ranges east of the basin, indicating wide-spread deformation associated with the continental collision.