Browsing by Author "Kleffmann, Stefan"
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Item Restricted Crustal Structure Studies of a Transpressional Plate Boundary - the Central South Island of New Zealand(Te Herenga Waka—Victoria University of Wellington, 1999) Kleffmann, StefanSeismic 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.Item Restricted Effects of surficial till on seismic reflection data in Westland, New Zealand(Te Herenga Waka—Victoria University of Wellington, 1994) Kleffmann, StefanRefraction static corrections, based on a surface-consistent decomposition of shot and receiver delay times, are used to improve seismic reflection sections recorded in Westland, New Zealand. The technique utilizes the redundancy of first breaks obtained in multifold reflection profiling to derive a best-fit solution of static corrections. Large statics, originating from glacial and fluvioglacial deposits in the near surface, were believed to deteriorate reflection signals because of insufficient compensation by previous correction techniques. A comparison is made between applying field statics only, and applying field plus refraction statics. A clear improvement in the reflection quality is obtained with the latter. However, a comparison was also made between a final stacked section with field plus residual statics, to a section with field, refraction and residual statics applied. No significant difference is apparent. This suggests that statics due to near-surface velocity inhomogeneities (i.e. refraction statics) were sufficiently small to be accommodated by the entirely empirical approach of residual statics. The application of refraction statics does, however, have some advantages. Refraction statics are applied earlier in the processing sequence than residual statics, therefore early recognition of reflection events is facilitated, which in turn assists an optimum set of processing parameters such as stacking velocities. It is found that deterioration of reflection signals is attributed to wave-propagation effects in the glacial surface layer that cannot be compensated for by the application of simple static time shifts. These effects are simulated by two wave-equation modelling techniques, operating in the time and frequency domains respectively. Modelling results identify shot-generated noise in the form of groundroll as a major factor in the deterioration of reflection amplitudes. Its response to the variable surface velocity across the profile lengths results in complex arrival patterns on the seismograms. These surface waves cannot be effectively attenuated by conventional filtering techniques, hence, useful reflection information remains hidden. Also, interference of waves generated at the irregular glacial depth interface causes attenuation of reflections from deeper layers. Wave-equation modelling indicates a relationship of increasing frequency content of the recorded seismograms with increasing velocity and decreasing layer thickness below the receiver station. Depth and velocity parameters of the wave-equation model are determined from refraction statics calculations which are based on ray-path theory. The match of the modelling results to the effects observed on the field data demonstrates the complementary character of raypath and wave-equation methods.