Friday fold: granite dikes, Barberton greenstone belt

Folded & boudinaged granite dikes in tonalitic gneiss, Barberton granite-greenstone belt, South Africa. From Passchier, CW, Myers, JS, and Kroner, A., (1990). FIELD GEOLOGY OF HIGH GRADE GNEISS TERRANES.

Very crudely annotated:

This is a sweet example of how you can get different structures developing in different orientations relative to the principal stress directions. In this particular part of the Barberton Greenstone Belt, compression (orange arrows) operated from the top of the photo towards the bottom, and the rock stretched out from left to right (green arrows). Folds formed where granite dikes were compressed, but the same rock in a different orientation was boudinaged… Cool, eh?

So that’s your Friday fold! The boudinage is just a little bonus for you, because, hey, it’s Friday.

Suevite from Vrederfort

One of my students brought this sample in the other day:

She said her father collected it in South Africa. It was labeled “suevite.” I learned the term suevite about a year ago, while touring the USGS Chesapeake Bay Impact Crater coring project samples at the USGS Headquarters in Reston, Virginia. Wright Horton taught me that suevite is impact-generated melt that chills with other chunks of the pre-impact rock mixed into it. Sometimes, it is glassy. There was a bunch of it deep in the Chesapeake Bay Impact Crater core.

So I put this together in my head and asked “Where in South Africa did this come from?” The student couldn’t remember, but it was “some weird name.” “Was it the Vrederfort Structure?” I asked? Her eyes lit up: “Yes! That’s it! How did you know?” I didn’t know: but that’s the only impact site in South Africa I could name offhand. So I think that’s what this is. A pre-Vrederfort Impact granite smashed and melted during the impact, with individual mineral grains breaking off and mixing into the melt, which then solidified into suevite. Pretty neat little sample! I’d love to have one of my own, but settled for making a digital scan of hers. E-mail me if you want a bigger copy.