Alpine schistosity

Question

I often find the term Alpine schistosity (schistosité alpine in French, scistosità alpina in Italian) in texts about Alpine geology. Chronologically ordered phases of Alpine schistosity producing different schistous foliations are named, in everything I have read, S$_1$, S$_2$ (and S$_3$).

I think that notations S$_1$, S$_2$, ..., S$_n$ may well refer to any ordered orogenetic phases producing schistosity in any rock, anywhere, at any time. Please correct me if I am wrong.

Nevertheless, I wonder whether, when talking about S$_1$ Alpine schistosity, or S$_2$ Alpine schistosity, one usually refers to a well defined temporal phase of Alpine orogeny producing a particular schistous foliation. Is that so?

Anyhow, I must remark that all schistosities that S$_1$ and S$_2$ in the geology of the Alps not necessarily refer to foliations generated during the Alpine orogeny, since for example in Briançonnais orthogneiss it is usual to distinguish a Silurian-Ordovician S$_1$ schistosity and a late Carboniferous S$_2$ schistosity, both generated before the Alpine orogeny. I thank you very much for any answer!

*Vv.Aa., Alpi liguri, edited by the Italian Geological Society.

Answer 1

I think that notations S$_1$, S$_2$, ..., S$_n$ may well refer to any ordered orogenetic phases producing schistosity in any rock, anywhere, at any time

Generally, yes. This is not only true for the Alps, but also for any other orogenic regions in the world (e.g. Himalayas, Arabian-Nubian Shield). Note that the numbering is ($n$) is used very loosely. It all depends on what's observable in the studied rocks and what is the emphasis.

...when talking about S1 Alpine schistosity, or S2 Alpine schistosity, one usually refers to a well defined temporal phase of Alpine orogeny producing a particular schistous foliation. Is that so?

For example, I've seen studies where original sedimentary foliation was termed S$_0$. In other places it was termed S$_1$. Talking about Alpine schistosities, you gave an example where S$_1$ and S$_2$ are pre-Alpine (Variscan). On the other hand, another Briançonnais study by Ganne et al (2003), refers to S$_1$ as the first true Alpine schistosity and uses S$_{-1}$ for the pre-Alpine Variscan schistosities.

Note that the S$_{n}$ terms are used to describe schistosities. These are visual features observed in petrographic examination of rocks under the microscope. These are commonly open to interpretation and debate. You have to distinguish them from the D$_{n}$ events (deformation). S$_{n}$ features are usually used as a proxy for D$_{n}$ events, but it's not a one-to-one relationships. Let's say you know that 3 deformation events occurred in a rock. However, you can only see two schistosities. How do you call them? S$_1$ and S$_2$? What if you know that the first schistosity was erased? Do you use S$_2$ and S$_3$ then?

Again, these terms are used loosely and they may not be correlated between different areas and with time, unless mentioned explicitly in the study you are reading.