26 April 2013
Near the end of the stellar pre-GSA-Minneapolis-meeting “Structural Geology of the Sub-Province Boundaries” field trip in the southern Superior Province, we visited a small but intriguing pavement exposure of biotite schist intruded by small dikelets of the Burntside trondhemite. These dikes were then deformed through boudinage and folding. Here, for example, was a lovely “S” fold:
So my gut reaction on seeing such a thing, is to interpret it thus:
In other words, my instinct was “S folds indicate sinistral (top to the left) shear.”
…But this outcrop was going to provoke a moment of insight. It turns out not to be so simple. Peter Hudleston suggested that asymmetric folds aren’t reliable indicators of simple shear. Audaciously, he suggested to me that in fact, this “S” fold could instead mean this:
What sacrilege was this?!?
Think about progressive simple shear, Peter suggested…
I sat down and thought about it for a minute, and sketched a series of folds in my notebook, and realized that if shear strain was high, Peter was right: the long limbs and short limbs would swap places!
This was an important realization for me. An insight like that is a moment of illumination that makes a person look at the rocks differently than they did before the realization. I see it as an important development in the evolution of my own understanding of rock structure: one insight like that is worth the cost in time and money for a trip like the Superior Province field excursion before GSA. Here, a few feet away from the “S” fold (with its “sinistral” implications) are plenty of indications of dextral kinematics:
We can zoom in there (at upper right) on a “big fish,” for instance, which is basically bounded top to bottom by C surfaces and left to right by S surfaces:
Kinematically, this is the standard interpretation:
This makes us re-examine the fold we started off with: instead of being a rock-solid (pun very much intended) indicator of sinistral simple shear, it’s in fact entirely consistent with what the rest of the outcrop was suggesting: dextral simple shear!
Son of a gun. I’ll never trust those S folds again…
Anyhow, I am prompted to blog this now, this week, because last week a new article came out in Geology that looks at a similar phenomenon: the unfolding of folds during simple shear.
The idea is this: if you have an original layer oriented at a certain angle relative to a simple-shear-dominated shear zone, you can first fold that layer, then rotate it backwards and unfold it!
As with my example from Minnesota, this phenomenon is a case of how a moderate amount of strain yields one structure, while a higher amount of strain produces another, totally non-intuitive structure. In Figure 1 of the paper (produced above, with color added), compare the unstrained left box with the very strained right box. They look like mirror images!
In particular, if the viscosity contrast between the folded layer (black) and the surrounding medium (green) is high (above ~50), the folded layer is capable of being entirely unfolded given a sufficiently high state of shear strain. Modeling results from Maria-Gema Llorens and her co-authors indicate that the presence of intrafolial folds and cusp-like folds in the matrix adjacent to otherwise straight layers are indicators that the “straight” layer may once have been folded.
Here’s a detail from their Figure 3, showing the relation of these apparently sinistral folds to the larger dextral shear zone that contains them:
This is an exciting new development in the study of folded layered rocks – and layered rocks that seem to be mostly non-folded.
Llorens, M., Bons, P., Griera, A., & Gomez-Rivas, E. (2013). When do folds unfold during progressive shear? Geology, 41 (5), 563-566 DOI: 10.1130/G33973.1