Thrust and fold

The same day I was up at Redwood looking for the bent trees, I ran across this fine example of a thrust fault right next to the Huckleberry Botanic Reserve.


Today I finally got around to putting it in my gallery at—not as a thrust fault (I already have a good one), but as an example of a drag fold. Looks pretty good there, almost textbook quality. But here’s a secret: look at this view of the fault.


I can’t figure out what the double curvature means! I can’t figure out the relationship of the fault to the folds. I feel like a freshman in his first field course. A real geologist would crawl all over this, including the hillside on top, until everything was clear. But I tell myself, the key to being a good scientist is to admit when you’re mystified because enlightenment comes that way. A certain set of people can visualize things ten times as complicated, and I hope one of them will pipe up.

By the way, my spread about Oakland’s geology is in the new Oakbook, the printed one. Go get one for free.

8 Responses to “Thrust and fold”

  1. Colin Says:

    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.

  2. Andrew Says:

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

  3. Dave Hunter Says:

    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?

  4. Andrew Says:

    There you go. Thrust slices!

  5. Callan Bentley Says:

    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.

  6. Ken Clark Says:

    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:)

  7. Silver Fox Says:

    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!

  8. Farmlady Says:

    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!

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