The measurements from eighteen GPS (Global Positioning System) surveys and four terrestrial surveys were used to calculate the velocities of 406 survey stations throughout the South Island, Stewart Island and the southernmost North Island. Repeated GPS measurements are available at 350 stations. The calculation of the velocities for the remaining stations is made possible through the use of terrestrial measurements.
The velocity was modelled under the assumption that the displacements of the stations are either linear with time or linear punctuated by discontinuities. The discontinuous model was used to estimate the coseismic displacements of stations in the vicinity of the 1994 Arthur�s Pass earthquake (M 6.7). The maximum station displacement was estimated to be ca. 40 cm, and significant displacements are seen to a range of ca. 70 km from the earthquake epicentre. Station displacements were also calculated for two later earthquakes in the vicinity of the Arthur�s Pass earthquake, but it was not possible to separate these from the postseismic displacements due to the earlier earthquake.
A continuous velocity field was estimated from the discrete station velocity measurements through a stochastic model based on the concept of minimum curvature. The selection of the basic stochastic model was effectively arbitrary; however, the model was refined to better suit the velocity field in the South Island. This was achieved through estimating the correlation between the velocity components (east and north) and the anisotropy of the velocity field. The stochastic model has the advantage over other models (e.g. polynomials or splines) in that only the probable shape of the velocity field is assumed. Therefore, the shape of the velocity field is not restricted by a priori model assumptions.
The measurement of the differential velocity across the South Island plate boundary between Christchurch and Cape Farewell is less than 85% of the interplate velocity calculated from NUVEL-1A. One possibility is that the NUVEL-1A model may not be an accurate representation of the motion at this plate boundary. Alternatively, deformation (occurring during the period of survey measurements) may extend a total distance of 150 km or more (assuming that the spatial velocity differential is less than 5x10⁻⁷/year) offshore from Christchurch and Cape Farewell. In the southern South Island there is evidence for as much as 22 mm/year of east directed motion being accommodated between Fiordland�s west coast and the stable interior of the Australian Plate. An accretionary wedge has been imaged west of Fiordland (Davey and Smith, 1983; Delteil et al., 1996); therefore, some of this deformation may be related to slip on the subduction interface.
The shear strain rates are clearly influenced by the dominant fault elements in the South Island, i.e. the southern and central Alpine Fault, and the eastern Hope Fault. The maximum measured shear strain rate in the South Island, 6(±1) x10⁻⁷/year, occurs adjacent to the Alpine Fault at (1 70.5°E, 43.3°S), ca. 40 km northeast of Mt Cook, and is coincident with a local dilatational strain rate minimum, -7 (±4.5) x 10⁻⁸/year. This is the only location where the measured strain rate is compatible with strike-slip and dip-slip motion on the Alpine Fault. Shear strain rates decrease eastwards along the Hope Fault: from 5(±0.7) x10⁻⁷/year at the Alpine Fault, to 3(±0.8) x10⁻⁷/year close to the Jordan Thrust. The zone of deformation broadens with a concomitant decrease in shear strain rate, such that within the northeast South Island there is no distinct maximum over any particular fault.
A band of contraction and shear has been imaged at a distance of 100 km southeast of, and parallel to, the Alpine Fault. The deformation at this location may be related to a frontal thrust zone similar to that described in the two-sided wedge models. The band of deformation continues north of Christchurch, intersecting the Porters Pass Fault Zone.
Significant contraction rates are seen in the measurements from four other zones. The first of these is situated towards the northeast (on land) ends of the Clarence, Awatere and Hope Faults. Some of this signal is presumably related to the uplift of the Seaward and Inland Kaikoura Ranges. The three remaining zones of significant negative dilatational strain rate are located north of the Wairau Fault, close to Jackson Bay and within central Otago.
A zone of significant shear strain rate is measured along the eastern side of, and within southern Fiordland. The deformation measurements probably partially reflect the existence of an important fault running through Lake Te Anau, which accommodates the motion of the Fiordland block relative to the Pacific Plate. The remainder may be due to internal deformation of the Fiordland block.
A new velocity differential measurement has been introduced, the rotational excess. This function of the shear strain rate, vorticity and dilatational strain rate should be sensitive to tectonic rotation (as measured by paleomagnetic data). Point estimates of the rotational excess are insignificant throughout the South Island. Also, there are no easily defined regions in which spatially averaged measurements are significant. If the rotational excess is assumed to be a direct measurement of tectonic rotation then the measurements place a bound on the size of the region and the rate at which it rotates. For example, the rate of tectonic rotation within a square region with side lengths of 50 km located adjacent to Cape Campbell is unlikely to be greater than 4°/Ma. However, greater tectonic rotation rates are possible within smaller regions.
| Identifer | oai:union.ndltd.org:ADTP/217478 |
| Date | January 2006 |
| Creators | Henderson, Christopher Mark, n/a |
| Publisher | University of Otago. Department of Geology |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://policy01.otago.ac.nz/policies/FMPro?-db=policies.fm&-format=viewpolicy.html&-lay=viewpolicy&-sortfield=Title&Type=Academic&-recid=33025&-find), Copyright Christopher Mark Henderson |
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