You’re told from a young age that you can’t judge a book by its cover. It’s what’s inside that counts, true beauty comes from within, etc. Given the nature of my PhD, which looks at processes occurring deep within the earth, I couldn’t agree more. Such phrases as “beauty comes from within” can also, in my humble opinion, be applied to faults. Which is why I’m particularly fascinated by the Alpine fault in New Zealand where it is possible to see the deep structure of an active fault. But I’m getting ahead of myself – first a few basic points.
The Earth’s surface is divided up into tectonic plates, the boundaries of which are made up of faults. Faults are approximately planar features in the Earth’s cold, and therefore brittle, upper crust that deform via rapid, localised movement. The types of faults can be divided into three end-members; strike-slip faults, normal (extensional) faults and reverse (thrust) faults. Figure 1 shows the way that these three fault types move.
Combinations of these different end-members are also possible and are referred to as oblique faults. The Alpine fault is an example of one such hybrid fault – specifically a combination of a reverse fault and a strike-slip fault1. This means that during an earthquake, material is moved both upwards and also sideways. In the case of the Alpine Fault, it’s the upward movement that interests me. Or what I should say is, it’s the material that this upward movement exhumes.
As you follow a fault deep into the earth, the temperature increases, and as a result the rocks become softer and ‘squishier’. Eventually, a point is reached at which, when a force is applied, the rocks are actually able to flow. This is known as ductile deformation and results in what are known as shear zones. This can be understood by imagining the earth as a Mars bar which you break in half. At the surface the chocolate (brittle) part breaks along clear lines (aka faults) whereas the inside filling (ductile part) oozes and squishes out (aka shear zones). The important point to note is that at the movement of the continental plates is thought to form broad shear zones (Figure 1) rather than occurring on discrete faults, though the horizontal extent of these zones is still greatly debated.
Exhumed shear zones are found all over the world but rarely does one know the actual tectonic environment (e.g the plate configuration, rate of deformation) in which they are formed. Therefore the theory that shear zone are broad rather than localised structures cannot easily be backed up by evidence from outcrops.
The Alpine Fault
Despite its name, the Alpine fault sits a long way from Europe. Running 600 km up the spine of the South Island of New Zealand, it is one of the world’s major geological features. Representing the on land segment of the boundary between the Pacific and Australian Plates2, it is this fault to which we owe thanks for the picturesque landscape of the Southern Alps or maybe more importantly the landscape of Tolkien’s Middle Earth.
The Southern Alps of New Zealand are part of a young convergence zone (< 10 Million years) that has experienced 20 km of uplift and subsequent rapid erosion1. This uplift and erosion has resulted in rocks that originally sat deep within the earth now sitting at its surface. And it is these rocks in which I’m interested…
Finally down to the good stuff…
Sitting immediately adjacent to the fault surface is a 1-2 km wide, 5 million year old, shear zone. In geological terms this is extremely young and, therefore, you can assume it is comparable to what currently sits below the fault. This material originated 25 km beneath the fault surface. Analyses focused on interpreting deformed structures within this shear zone have found that most of the surface displacement occurring on the fault in the last 5 million years has been accommodated, at depth, on this narrow 1-2 km shear zone. This shows extreme localisation at depth, something that’s not generally considered in standard models and which is therefore extremely ground breaking (excuse the pun)1.
There are of course limitations; no model is perfect and further questions need to be addressed. For example, the area exposed may not represent the whole shear zone. In addition, such localisation may be the result of local factors such as rock type or erosion rate. However, understanding the deep structure of faults is extremely important and the Alpine fault gives a brilliant insight into a fault’s signature at depth. Such knowledge may help scientists develop a greater understanding of fault systems as a whole, including their elusive earthquake cycles. To me just the fact that you are essentially able to view the profile of a fault is a pretty cool thing.
The sad reality is that we live in a world where first impressions count and we inherently, if not intentionally, judge. I just hope that next time you’re standing on a plate boundary (as I’m guessing you obviously do so very regularly!) you ask yourself “I wonder what’s going on below?”
- Norris, R. J. & Cooper, A. F. Very high strains recorded in mylonites along the Alpine Fault, New Zealand: implications for the deep structure of plate boundary faults. J. Struct. Geol. 25, 2141–2157 (2003).
- Little, T. A., Prior, D. J., Toy, V. G. & Reid, Z. The link between strength of lattice preferred orientation , second phase content and grain boundary migration: A case study from the Alpine Fault zone , New Zealand. J. Struct. Geol. 81, 59–77 (2015).
“I am a first year PhD student in the Earth Sciences Department researching the partition of stress in multi-mineral rocks. Other than my PhD topic, my interests lie in large scale structural geology, particularly in large magnitude earthquakes and their corresponding faults.”