Friday, January 18, 2013

Geo 365: Jan. 19, Day 19: Not Quite Normal Normal Faults

Okay, it's not Jan. 19 yet, but I'm very pleased with the results of my puttering around today, and I can't wait to share. So here we go.
This is a panorama, stitched together in Hugin, of two shots of the same set of normal faults shown yesterday. Yesterday's photo, nice as it is, was mostly to give a sense of scale. Here there's no scale, just the gorgeous faults nicely highlighted by the great contrast in the colors of various sediments.

Now I presented this yesterday as "Basin and Range," and that's still my preferred explanation for this faulting. It's in a logical place for that to be true, and the mode of offset- normal, where the blocks on the upper side of the fault plane have moved down with respect to blocks on the lower side- is what I'd expect to see as we crossed out of Basin and Range onto the edge of the Cascade platform. But...

Here's an annotated version:
With the exception of two minor faults in the lower middle, marked in red, all are normal in offset. And aside from a couple of the larger ones, most of them don't seem to go all the way through. I was pondering this when I was there with Dana, but was A) kinda confused myself; B) not really confident in what I thought I was seeing, and C) really didn't want to confuse her. My suspicion is that I need to think about syndepositional deformation- faulting taking place as, or shortly after, the sediment was deposited. Now, soft-sediment "brittle deformation" is kind of a contradiction in terms, but I've seen it a number of times, both on the Oregon Coast and in glacial sediment in Ontario, Canada, so I know it can happen. (Followup: oh, yeah, also here.) So the answer? I just don't know. It might be one, the other, possibly even a combination of both- for example a Basin and Range-related earthquake may have led to more extensive slumping and extension of a delta front than just the simple tectonism caused by itself. I'm not even sure getting my nose up on the outcrop would help- though being able to look carefully at just how exactly those smaller faults peter out might.

I will say, spending an hour or better, squinting at my computer screen, and painstakingly highlighting the faults, while tedious, definitely convinced me that this is a real problem I can't just dismiss with a hand wave.

Photo unprocessed, other than stitching and cropping. August 18, 2011. FlashEarth location. (Cross-hairs on pull-out. The high tension power lines are the landmark to look for, here- this is the main conduit for taking Columbia River hydropower to California.) If you choose to visit this spot, please see Monday's safety comments; as I commented yesterday, we chose not to cross here. Just too dangerous.

Also, as a reminder, Blogger shrinks my photos somewhat, so some fine detail may be lost, even when you open them to "full size." I'm always happy to e-mail original copies for educational purposes. Followup note: I just checked, and at full size, the images above are 1600 x 640; the originals are 3297 x 1317- more than twice as big in linear dimensions.

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