Shallow Seismic Survey of the Whataroa Glacial Valley in the vicinity of the Alpine Fault, Westland

Author:

Lepine, Patrick Rafe Wadworth (Patrick)

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Abstract:

The glacio-fluvial sediments of the Whataroa Valley contain a geological record of environmental change that occurred as the valley was filled by sediments following the retreat of Pleistocene glaciers. This record is greatly affected by the significant motion that has occurred on the Alpine Fault over the same period. The Alpine Fault is the boundary between the Pacific and Australian tectonic plates, with uplift along the fault giving rise to the formation of the Southern Alps. For much of its length, the trace of the Alpine Fault is covered in sediments, so its precise position is not known. A series of hammer and weight drop seismic reflection lines collected in 2011, 2013 and 2015 image the sediments of the Whataroa Valley overlying the (presumably) glacially carved basement. This thesis uses these high-resolution seismic data for three purposes: 1) characterisation of sedimentary strata deposited in the valley while contributing to the multidisciplinary dataset characterising site 2 of the Deep Fault Drilling Project, 2) constraining the position of the Alpine Fault near surface trace on the coastal plain and 3), comparing and contrasting two different seismic sources that been used in data acquisition. The WhataDUSIE 3D survey conducted in 2011, consists of seven separate profiles on the eastern side of the Whataroa River on the hanging wall of the Alpine Fault. Each of these profiles was approximately 750 m in length, with a receiver spacing of 4 m and a shot spacing of 8 m. This survey was aimed at characterising the sedimentary strata in the Whataroa Valley in 3D. The Whataroa 2013 survey consists of four profiles totalling approximately 3 km in length. Geophone spacing was 5 m with shot points located between every second geophone (at a spacing of 10 m). Two of these profiles (4 and 5) were collected on the coastal plain running approximately perpendicular to the Alpine Fault. These lines were collected in an attempt to image the Alpine Fault in the near surface sediments. The other two profiles (1 and 2) lie on a river terrace and are orientated parallel to the Alpine Fault, and perpendicular to the valley wall. These two profiles characterise the sediments in the lowe r section of the Whataroa Valley and were collected as part of a larger data set for characterisation of the DFDP-2 drill site. The Whataroa 2015 survey was a re-shoot and extension of line 5 of the Whataroa 2013 survey. It made use of a weight drop seismic source and had a shot spacing of 10 m, with a 5 m receiver spacing. A comparison between the hammer seismic source and weight drop seismic source showed that while the frequencies of both sources were similar, the weight drop seismic source produced five times the energy and was a more repeatable source, leading to better quality data. Results of this study identified five main seismic facies, and the depth to basement within Whataroa Valley at the DFDP-2 drilling location. Offset reflections were also identified in the vicinity of the Alpine Fault surface trace. The sedimentary strata in Whataroa Valley have been divided into four facies: fluvial gravels (facies 1), Pleistocene marine sediments (facies 2), Pliocene marine sediments (facies 3) and a transition to basement (facies 4). Facies 5 represents a sudden change in signal strength in the northern sections of lines 4, 5 and 7. Within the marine and most recent glacio-fluvial deposits are offset reflections that do not propagate to the surface. These reflections have been interpreted to represent old Alpine Fault surface traces with offsets on the order of 10 to 25 m. There is a strong signal strength change identified in lines 4 and 7 that coincides with the proposed location of the Alpine Fault surface trace. Basement was only conclusively observed i n lines 1 and 2, occurring at a depth of 240 m near the DFDP-2 drill site. This was confirmed by rock cuttings from the borehole itself.

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xiv, 153 pages A4

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OU geology Identifier:

2016Lepine

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Open Access

Location (WKT, WGS84):

MULTIPOLYGON (((170.38748098797 -43.265149244074,170.424934473365 -43.2659990625108,170.422899020371 -43.3141954428501,170.385416011598 -43.3133442005652,170.38748098797 -43.265149244074)))

Abstract PDF File

http://download.otagogeology.org.nz/temp/Abstracts/2016Lepine.pdf

Collection

Citation

Lepine, Patrick Rafe Wadworth (Patrick), “Shallow Seismic Survey of the Whataroa Glacial Valley in the vicinity of the Alpine Fault, Westland,” Otago Geology Theses, accessed July 21, 2018, http://theses.otagogeology.org.nz/items/show/633.