Abstract
The Alpine Fault, the main onshore structure in the Pacfic-Australian plate boundary, runs along the West Coast of the South Island of New Zealand, where it accommodates ~70% of the relative plate motion. The fault accommodates dextral reverse oblique slip with 23 - 25 mm/yr strike slip and up to 10 mm/yr of dip slip. The structure of the Alpine Fault zone at the surface can be divided into two parts: (1) the range front structure which comprises faults and other features that have developed within the top few kilometres and (2) the mylonite zone structure which preserve features developed in the shear zone at depth and have been uplifted along the fault.
The study focuses on the range front structure of the Alpine Fault zone along the Waitangi-taona River, an area which covers a section approximately 6 km wide along strike of the Alpine Fault. Three parallel creek sections that feed into the Waitangi-taona River and the Waitangi-taona River headwaters, expose some of the most complete sections perpendicular to and through the fault rock sequence of the Alpine Fault.
Mapping and characterisation of the mylonite zone structure found that protolith variation has a strong influence on the characteristics of structural features developed including S-C’ shear bands. A field-based classification scheme for Alpine Fault mylonites is presented that incorporates the field observations made in this study.
A map of the entire field area at 1:20,000 and larger scale (1:2,000) maps and cross sections of the creek sections were produced. Additionally 3D maps of the fault zone structure and fault zone transitions were made using the field data. An increased thickness of the mylonite zone at the Waitangi-taona headwaters was identified. This is thought to be primarily related to imbrication of the mylonite sequence as it is thrust out shallowly onto gravels.
The range front structure was examined by combining field studies with remote sensing techniques such as aerial and satellite imagery. Two subvertical faults were identified that are sub-parallel or at a small angle (≤25°) to the 060° striking basal thrust of the Alpine Fault. These faults are found within 100 m south-east of the basal thrust (i.e. in the hanging-wall) and are thought to form in response to shallow thrusting of the hanging-wall out onto the range-front. The slip direction on these faults should lie parallel to the basal thrust, such that juxtaposition of footwall against hangingwall rocks does not occur.
Anisotropy of Magnetic Susceptibility studies were conducted on samples of fault gouge collected from the two sub-vertical faults and the Alpine Fault basal thrust. In this study AMS was found to be a useful proxy for penetrative gouge fabric elements forming at an angle to the gouge zone boundary, and a useful indicator for fault shear sense and direction. The most robust indicator for shear sense and direction was found to be the planar AMS fabric (the AMSF fabric). The source of the AMS fabric in the matrix is thought to be a combination of clay fabric (preferred basal orientation of phyllosilicates) and iron oxides. In some of the thin sections of fault gouge, and in one fault outcrop, ultramylonite clasts and the foliation within them were found to be aligned within the gouge zone (Foliation in Clast Preferred Orientation: F-CPO). This is also thought to contribute to the AMS.
In most cases the AMSF shear direction was consistent with the shear direction from slickenside striations. On one of the sub-vertical faults (the `MacDonalds-Galaxiid sub-vertical segment') the AMSF fabric had a shear direction close to being within the basal thrust, whereas the slickenside striations on this fault plunged 40/244°. Slickenside striations likely reflect the last increment of shear, whereas the AMSF data provide a time averaged bulk shear direction. In this case the change in kinematics for this fault is thought to come about as it is thrust out shallowly in the near surface.