Comments on: Thrust and fold

By: Colin

Colin — Fri, 12 Dec 2008 21:45:23 +0000

My father (a geophysicist) took an incredible photo in Switzerland of a perfect s-fold above a farm house. He came to several different conclusions about what could have caused it, and all of his friends had their own ideas.

Every time I bike past this particular thrust-fold (or what I’ve assumed was a thrust-fold) I’m reminded of that trip and pulling over to take that photo. So incredible to see terra firma flex like that.

By: Andrew

Andrew — Fri, 12 Dec 2008 06:03:52 +0000

No, not at all. This is Tertiary mudstone, siliceous endmember, part of the classic Monterey Formation. Search my site for Franciscan melange stuff.

By: Dave Hunter

Dave Hunter — Thu, 11 Dec 2008 21:05:43 +0000

Callan Bentley has the right idea on this one. The drag relationships suggest another thrust fault, imbricate (as stacked faulting or other features with the same sense of motion are described by us geologists) to the one visible in the middle of the road cut and beneath the elevation of the road grade.

The shear energy simply found several planes of weakness rather than transfering all the energy along one plane. Somebody with more structural experience than me may have a different opinion, but my guess is that these thrust slices splay off the end of a zone of faulting as the energy approaches the ground surface from depth. As the confining pressure lessens the energy can more easily shatter the material into the several imbricate faults.

Is this what you west coasters would consider classic Franciscan Melange?

By: Andrew

Andrew — Wed, 03 Dec 2008 22:21:12 +0000

There you go. Thrust slices!

By: Callan Bentley

Callan Bentley — Wed, 03 Dec 2008 12:35:18 +0000

Could be a second drag fold being dragged along a second fault underground. The sense of shear would be consistent with the exposed fault/fold combo: top to the right, with the rocks being broken into three chunks: the uppermost (left) block, with a bottom drag fold, the middle block with an upper and a lower drag fold, and an unseen lower block beneath the road… You don’t even need a second fault; but I wouldn’t be surprised to find one beneath the road. Anyhow: the point is, take a stack of strata and shove them to the right; they buckle, they break. The outcrop shows a consistent story.

By: Ken Clark

Ken Clark — Sun, 23 Nov 2008 07:32:54 +0000

If I had to hazard a guess, I would say that the more distal folding is simply a feature of a more plastic deformation than normal, while the upper fold is clearly a drag feature, the lower could come from more of the formation getting pushed along, rumpling up. If it continued it would most probably detached, creating a stacked series of horses. Wish I had some pictures handy; there are some nice examples of stacked horses in the Heart Mountain Detachment. Thats my $0.02, worth every penny you paid for it:)

By: Silver Fox

Silver Fox — Sun, 23 Nov 2008 04:42:15 +0000

I can see in the first photo how the upper fold below the thrust could be a drag fold – but that second fold makes things look a little more complicated. Also, is there a bit of a Z fold above the thrust? And I’m wondering what happens under the road? More questions that answers from me!

Great roadcut!

By: Farmlady

Farmlady — Sat, 22 Nov 2008 23:29:20 +0000

I don’t know much about Geology but that 2nd. photo is amazing. Think of the forces of the earth that caused that rock to move and create that beautiful terrain. Great photo!