Microtectonics Masterclass Part II

In the 1970’s as a PhD student at Imperial College, London, Professor Richard Sibson struggled to describe the fault rocks he collected from the Outer Hebrides fault zone in a systematic way. He was inspired to define a non-genetic classification of the products of deformation within this fault zone. This classification of fault rocks (Fig. 1) was explained in the paper “Fault Rocks and Fault Mechanisms” (Sibson, 1977) and although it has been subsequently refined (e.g. Woodcock & Mort, 2008) it has never been fundamentally overturned. We still define cataclasite and mylonite series rocks, and amorphous materials based on refined versions of this classification.

Figure 1 (right): Sibson’s textural classification of fault rocks

Figure 2 (left): segmented porosity in a synchrotron CT scan of Alpine Fault Zone cataclasites

Prof. Sibson then carried out fieldwork in New Zealand’s Alpine Fault Zone, where this classification was refined and proven through examination of mylonites, cataclasites, and also pseudotachylytes (Sibson et al., 1981). The samples collected during both studies were made into a stunning set of thin-sections at Imperial College, London, and subsequently taken by Sibson to UC-Santa Barbara and then the University of Otago, New Zealand. These thin sections are finally on their way back toward Imperial College, but en route they are making a stop in Mainz, where they will be open for examination by participants in this year’s Micro-tectonics block course, guided by Prof. Virginia Toy who has carried out microstructural research in the Alpine Fault Zone for the last 15 years.

In the decades since “Fault Rocks and Fault Mechanisms” was published, we have developed and refined many new analytical methods and techniques. Coevally our interpretations of fault zone processes from their textural record have substantially advanced (e.g. Rowe et al., 2011). In both mylonite- and cataclasite-series rocks, we have refined methods to determine the arrangement and shapes of grains and particles, their textures, their crystallographic orientations, and the nature of porosity. Many of these methods have been applied to samples from the Alpine Fault Zone, yielding datasets (e.g. Fig. 2) that will be presented during this course as case examples – linked back to thin sections of the same samples – to demonstrate the newest analytical techniques and stimulate participants to ‘think outside the square’ about how these methods could be used in their future research to revolutionise our understanding of such classic sequences.

Figure 3: EBSD map of a quartzofeldspathic Alpine Fault Zone mylonite

Figure 4: Left: Thin section of Outer Hebrides Thrust (OHT) pseudotachylyte demonstrating cyclic brittle and ductile behaviour (from Sibson, 1980). This section is included in the material we will examine in Mainz. Right: A pseudotachylyte fault and injection vein in one of the Imperial College collection thin sections.

Course outline:

Day 1. Outer Hebrides hand specimen and thin section analysis – relation to the fault rock classification scheme.

Day 2. Alpine Fault Zone hand specimen and thin section analysis

Day 3.

AM: Crystallographic textures of both mylonites and cataclasites from EBSD and neutron diffraction

PM: High resolution, targeted microstructural imaging and compositional analysis by TEM

Day 4.

AM: 2D grain size and shape analysis in cataclasites and pseudotachylytes based on BSE images

PM: 3D porosity and permeability analysis from synchrotron-CT scans

Day 5.

AM: How have experimental studies helped us to refine our understanding of textural records?

PM: Future directions – planning how to implement classic and new methods in participant’s own research and what could be done in future based on the classic OHT and AFZ sequences to advance our understanding of fault zone rocks.

References:

Rowe, C.D., Meneghini, F., Moore, J.C. 2011. Textural record fo the seismic cycle: strain-rate variation in an ancient subduction thrust. Geological Society of London Special Publication 359, 77-96.

Sibson, R., White, S., Atkinson, B., 1981. Structure and distribution of fault rocks in the Alpine Fault Zone, New Zealand. Geological Society, London, Special Publication 9(1), 197-210.

Sibson, R.H., 1977. Fault rocks and fault mechanisms. Journal of the Geological Society, London, 133, 191-213.

Sibson, R.H., 1980. Transient discontinuities in ductile shear zones. Journal of Structural Geology 2(1/2), 165-171.

Woodcock, N.H., Mort, K.., 2008. Classification of fault breccias and related fault rocks. Geological Magazine, 145(3), 435-440.