I work on some of the most beautiful rocks in the world. These rocks have remained solid throughout their history, and yet have deformed in such a way that they appear to have flowed like honey. In fact, they do flow, but very slowly, through the movement of tiny minerals within them. These rocks form in shear zones. Here is an example of a rock from the Zanskar shear zone in the NW Himalaya:

Orthogneiss from the Zanskar shear zone

And here is an example of what that rock looks like down a microscope:

Cross polarised photomicrograph of an orthogneiss from the Zanskar shear zone. Width of view is about 2 mm.

But first, what are shear zones?

The Earth moves under our feet constantly. Rocks slide past each other and, very slowly, are pushed into huge mountain ranges, such as the Himalayan mountain chain. While mountain chains can be thousands of kilometres long, there are only a handful of rock layers within them that are responsible for pushing the mountains upwards and these layers are exceedingly thin, just centimetres to metres in width. These rock layers are extremely deformed because they have acted like conveyor belts that shifted kilometres-thick packages of rocks upwards. These conveyor belts are called shear zones.

Shear zones can move quickly and abruptly, causing earthquakes and tsunamis. But more commonly, they creep slowly, the minerals within them changing shape, allowing the solid rock to flow. These slow movements gradually build up mountains and subduct entire oceans, at the rate of just a few centimetres a year, which is about the same rate as your fingernails grow.

I study these shear zone ‘rock conveyor belts’ to understand how solid rock flows and deforms.

My research focus

My research experience focuses on shear zones in a variety of geological settings. I have worked on a number of crustal shear zones within mountain ranges, including the Zanskar shear zone in NW Himalaya and the Pichao Shear Zone in Sierra de Quilmes, Argentina. These studies focussed on understanding the evolution of the shear zones and their place within the tecono-metamorphic regime.

My work on ductile shear zones led me to ask questions about exactly how the beautiful structures we see down the microscope are produced. While some structures have been well understood for a long time (e.g., S-C fabric and folds) others have proven more enigmatic. Specifically, I wondered why ductile C’ shear bands form in shear zones, and why some porphyroclasts recrystallise completely, while others remain intact to high strain. So began my foray into numerical modelling, using the microstructural modelling platform, Elle. During a postdoc at the University of Tuebingen (Germany) I explored the factors that led to the formation of C’ shear bands by modelling different rock microstructures.

After all that time in front of a computer I was keen to get back into the field so I returned to Australia on a teaching-research position at Monash University to look at links between deformation and metasomatism in shear zones. My current research has two main directions:

(1) Strain localisation, fluid flow and metasomatism in high pressure subduction channels

(2) Ductile shear zones as critical mineral superhighways

Short biography

Dr Melanie Finch is a structural and metamorphic geologist at James Cook University, Australia. She completed her PhD at Monash University in 2016 on the tectono-metamorphic evolution of a hot back arc and its thick shear zones. She was then awarded an Alexander von Humboldt Foundation Postdoctoral Fellowship at the University of Tuebingen, Germany. She returned to Australia in 2018 in a teaching-research position at Monash University and then moved to JCU in 2022. Her research focus is on the links between deformation and metasomatism in shear zones, focussing on subduction channels as well as in the crust on shear zones related to critical mineral deposits. Melanie is a 2021-2022 ‘Superstar of STEM’ and the President of Women in Earth and Environmental Sciences Australasia (WOMEESA).