7 September 2012
Friday folds: Selkirk Fan
Posted by Callan Bentley
The Friday fold is a guest submission from Marek Cichanski of De Anza College, who writes:
I’m a structural geologist who teaches at a community college in the San Francisco Bay area, and I thought you might enjoy these outcrop photos from the Selkirk Mts of BC, Canada:
I don’t study that particular area, but I was there this summer as a participant on the Alpine Club of Canada’s 2012 General Mountaineering Camp. As it happened, this year’s camp site was located on some beautiful outcrops of metamorphic and igneous rocks. As soon as I got off the helicopter and saw the outcrops, my head spun around and exploded, and I spent part of a day taking pictures and video. Here’s a video that I shot while I was walking around on the outcrops:
The rocks were mostly schist, gneiss, calc-silicate rock, amphibolite, and pegmatite of the Omineca Crystalline Belt. Structurally, the camp site was along the crest of the `Selkirk Fan’.
The complex ductile deformation seen in these outcrops is probably the result of the collision of a composite island-arc terrane (the Intermontane Superterrane) with cratonic North America.
There’s a lot more to be seen at this mountainous site, including bizzaro brecciation and shearing. Check out Marek’s full Flickr photo set here.
I love this one, for instance: multiple wavelengths of folds in granitic veins of different thicknesses, with some boudinage thrown in for good measure:
And speaking of boudinage, check out these multiple sizes of boudins, again apparently controlled by the thickness of the layer undergoing boudinage:
Lastly, check this one out.
Here’s what Marek has to say about it:
Layers of schist and marble (grey) on the flank of Mt. Sir Sandford. (This ridge leads up and to the right to the summit of M.S.S..) It’s a bit hard to tell, but there appears to be an isoclinal infold of the brown schist in the grey marble, suggesting large-scale transposition.
Great folds amid great scenery. …And Marek’s a great Mountain Beltway reader, since he wrote to me to share these lovely structures with the world. Well done, Marek!
To the rest of you: Consider throwing me a fold photo or two in the future. Share the wonders of ductile deformation with the world!
And have a good weekend.
Thanks very much for posting these images, Callan!
I should have written a clearer caption for that last image, though. The high point shown in the image isn’t actually the summit of Mt. Sir Sandford… the ridge leads up and to the right, to the true summit, which is an intimidating, glacier-clad massif of marble. (The mountain was out of condition for most of the summer, but a few days after my group flew out, another group managed to get to the summit… I’m so jealous!)
Anyone looking for a relevant reference might want to start with Gibson et al., Journal of Structural Geology, 2005, v.27, p. 1899-1924. Dan Gibson at Simon Fraser University, along with colleagues like Richard Brown and Sharon Carr, have been doing mapping and geochronology in this part of the Selkirks.
Oh gosh, I go to De Anza! I took Intro to Geology with Dr. DiLeonardo though. 🙂