Andy Cross
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Andy CrossUK PhD title: A study of the interplay between brittle and ductile processes on the Alpine Fault Zone, New Zealand, and implications for the seismic cycle. Supervisors: Dave Prior, Virginia Toy Email: ajcross89@gmail.com |
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Geological Background
The Alpine Fault Zone (AFZ) forms the onshore section of the transpressional Pacific-Australian plate boundary, extending for over 400 km along the west coast of New Zealand’s South Island between Milford Sound and Arthur’s Pass. The AFZ accommodates as much as 70% of the motion between the Pacific and Australian plates (Norris & Cooper, 2001) and forms a significant topographic boundary along the South Island between the Southern Alps to the east of the fault, and flat-lying coastal plains to the west, resulting from the ramping of the Pacific plate over the Australian plate. Oblique plate motion can be resolved into 27±5 mm yr-1 dextral strike-slip and 10 mm yr-1 reverse dip-slip components (Norris & Cooper 2001), with greatest uplift along the central Alpine Fault, in the region of Mt. Cook. Motion vectors along the NNE-striking, 45° SE-dipping fault zone have remained relatively unchanged over the past 5 Ma, meaning that the fault rocks currently exposed at the surface are representative of those currently deforming at depth (Beavan et al., 1999; Norris and Cooper 2003). The hanging-wall fault-rock sequence comprises a thin layer (~5 cm) of fault gouge, overlain by up to 50 m of cataclasite and a 1 km sequence of ultra-mylonites to proto-mylonites.
Since European settlement of New Zealand in ca. 1800 AD, no major earthquakes (Mw >7) on the central Alpine Fault have been experienced (Scholz et al., 1973; Sutherland et al., 2007). Geological evidence supports dates of AD 1717, AD 1620 and AD 1430 for the three most recent major earthquakes on the Alpine Fault (Sutherland et al., 2007), with coseismic strike-slip estimates of 8 m consistent with Quaternary strike-slip rates and an interseismic period of 300 years.
Project Description and Aims
The aim of my PhD research is to investigate the transition between pressure-sensitive brittle fracturing and temperature-sensitive ductile flow along fault zones, using the Alpine Fault as a case study.
There are three main focuses of this research at present, looking at the interplay of brittle and ductile processes throughout the crust:
- A field study of secondary gouge zones observed in the hanging-wall mylonite sequence above the principal slip zone of the Alpine Fault at Stony Creek.
- An investigation into the influence of stress changes during the seismic cycle on the balance between grain-size sensitive (GSS) and grain-size insensitive (GSI) deformation mechanisms. This will involve a collaboration with Susan Ellis and Phaedra Upton (GNS Science), to build upon existing code which models post-seismic stress changes in the mid- to lower-crust (Ellis and Stockhert, 2004; Ellis et al., 2006), and adapt it to incorporate variable grain size as a function of stress.
- Numerical modelling of creep controlled by frictional heating. The aim of this is to construct forward models of frictional heat dissipation and deformation, to reproduce microstructural observations of creep fabrics adjacent to fault slip zones (Smith et al. (2011)). Ultimately, it is hoped that these models will give insight into the kinematics of fault slip in natural examples.
Figure 1: A gridded section of the mylonite sequence at Stony Ck., approximately 20 m (across foliation) above the principal slip zone. Secondary zones of gouge are shown in mint-green.The overall aim of this research is to contribute towards understanding the accommodation of strain through creep mechanisms, and the implications for this on the seismic cycle and earthquake energy budget, primarily for the Alpine Fault, but also for fault zones in general.
Methodology
A combination of fieldwork, lab work and numerical modelling will be used to achieve these aims. Two weeks of fieldwork on the West Coast in January 2012 provided preliminary data which has raised questions to be pursued (notably on the secondary gouge zones of the mylonite sequence), and samples which will provide data relevant to more than one of the three topics outlined above. Lab work will consist of time spent on the SEM at the University of Otago, for electron-backscatter diffraction (EBSD) and XRD, to characterise the microstructure and geochemistry of samples from the Alpine Fault Zone.
MATLAB scripts to model the dissipation of friction-generated heat and the effects on active deformation mechanisms are a work in progress.
Structural observations at Lake Hawea
The Alpine Fault at Gaunt Creek