Friday, February 6, 2015

Geo 1095: February 6, Day 767: Enigmatic Lines

Just a few minutes ago, I tweeted, "Struggling to figure out what, if anything, I can say about today's geophoto... or if I should skip it and go to next." I guess I've decided to go for it. There's a lot going on here, but a lot of it is confusing enough to me that I'm reluctant to hazard too much in the ways of guesses. Let's start with the things I am confident about: The more or less straight lines, which are narrower, are joints or faults- fractures of one kind or another. Given that this terrane has been folded quite thoroughly in some areas (though I'm not certain here- that's my confusion), it would make sense to presume these fractures followed- that is, came after- the episode of folding and metamorphism. There are at least two terranes outboard of this, including the Josephine Ophiolite (which I discussed in January of 2014), and The Otter Point melange (discussed in December of 2013), so there has definitely been deformation since this block was accreted.

Moving on to the trickier bits, we have those broader, lighter bands weaving around. Do those undulations represent folds? Maybe, maybe not. One part of the problem is that the camera flash tends to flatten things, so the third dimension is suppressed. What is the actual shape of the surface we're looking at here? A bigger problem is, what exactly are those bands? Bedding features? Cataclastically pulverized, then recrystallized, marble? Tensional veins filled with recrystallized calcite? Some combination of those, or something that hasn't occurred to me? Since I really don't have any idea, it's irresponsible for me to speculate on how they've been potentially deformed and distorted. I'll end by saying that folding is a possibility here, but I've seen situations where planes that appear to be wildly folded turn out to be basically flat when you look at the surface from a different perspective.

A big part of learning geology is learning to think about how things are oriented in three-dimensional space, and it takes lots of practice. A flip side of learning that is realizing when something might be happening in rock space that you can't really see on the surface from a single limited perspective. And that's what we have going on here, as far as I'm concerned.

Photo unmodified. May 9, 2013. FlashEarth Location. (Since we're underground, I have only a vague idea where this is with respect to the surface.)

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