Geophysical characterisation of the Alpine Fault at Haast, Turnbull and Whataroa, New Zealand
The geometry of the upper-crustal portion of the Alpine Fault, the Australian-Pacific plate boundary on the South Island of New Zealand, is poorly constrained by geophysical studies where it cuts through the young sedimentary units of the coastal plain. Excellent accessibility on the vast mainly flat coastal floodplain at Haast, Turnbull and Whataroa region provides rare opportunity to undertake geophysical surveys along the Alpine Fault of New Zealand. In 2009, two seismic reflection lines were collected to better image the upper km or so of the Alpine Fault in the vicinity of the surface scarps of the most recent, but still prehistoric, ruptures south of the Haast River. Both profiles were running perpendicular to the fault extend to 2.5 km in length along the Turnbull River and 3.3 km at Haast. Analysis of these lines suggests that multiple strands of the fault have been active during the Holocene. The fault strands cut across sub-parallel strata, interpreted to represent at least seven regionally identifiable sequences that correspond to the accumulation of Holocene sediments following the last glacial retreat. Total combined vertical throw on the fault strands is consistent with expected uplift rates on the fault. Dip on the fault strands is interpreted to be between 53 and 74° SE. The WhataDUSIE-2D seismic profile was collected in early 2011. The project led by researchers from the University of Otago, TU Bergakademie Freiberg (Germany) and the University of Alberta (Canada), provided relatively high-resolution coverage (4-8 m geophone spacing, 25-100 m shot spacing) along a ~5.1 km long profile across the Alpine Fault in the Whataroa Valley. Several technical difficulties arose during data collection and data processing. However, combination of CMP stacking and ray-theoretical travel time inversion methods provided detailed insight of a dynamic and complex geological setting. High-resolution seismic data imaged the uppermost part (~2-3 km) of the hanging wall of the Alpine Fault and show the bedrock topography eroded by glacial processes from the Last Glacial Maximum, which went through a rapid deposition process and filled with 200-400 m thick post-glacial sediments and outwash gravels. Several dipping reflectors, which likely associate to the Alpine Fault damage zone have been modelled dipping to ~40 – 42° SE in 600 m wide zone at depths of 1500 – ~2500 m. Two additional phases were modelled against dipping reflectors distributed in a ~1000-m-wide area dipping ~16° SE at the SE end of the transect. These features are likely positioned oblique to the 2D profile, therefore projecting the exact location of the reflectors is not possible by the data. These reflectors may indicate a potentially wide fractured damage zone.
Advisor: Gorman, Andrew Robert
Degree Name: Doctor of Philosophy
Degree Discipline: Geology
Publisher: University of Otago
Keywords: Seismic; Gravity; AlpineFault; Tectonics; Paleoenviroment
Research Type: Thesis