Sierra Crest Shear Zone: CONTEST
Callan is a busy boy these days working on his science education master's. But... (mainly through discussions in my Structural Geology class at George Mason University) I've been reminded of some of the cool stuff I saw when I did my geology master's thesis in the high Sierra of California. Here's a couple of neat images from my field work that ought to convey some of the magic of doing structural geology in the "Range of Light."
The challenge I now put to you: explain what's going on in these images. I've labelled them "A," "B," and "C" for easy reference. Winners get a "GEOLOGY ROCKS" bumper sticker. One winner per photo -- whoever comes closest to describing the geology most completely & accurately.
A
.jpg)
B
.jpg)
C
.jpg)
Just a taste of the magic that a summer of field work imparts... :)
Answers in a couple of days...
The challenge I now put to you: explain what's going on in these images. I've labelled them "A," "B," and "C" for easy reference. Winners get a "GEOLOGY ROCKS" bumper sticker. One winner per photo -- whoever comes closest to describing the geology most completely & accurately.
A
B
C
Just a taste of the magic that a summer of field work imparts... :)
Answers in a couple of days...
Labels: california, contest, structure
5 Comments:
For A, I wish I could crawl around the outcrop to make sure that the exposed surface isn't at some weird angle to the principle strain directions.
The outcrop is weird because what appears to be a fault gives two conflicting shear-sense indicators. The folding in the offset layer looks like the left side moved up, but the offset of the layer itself looks like the right side moved up.
One possibility is that we're not looking at a fault at all. The white plutonic-ish material is boundinaged, which suggests that the rock has shortened ~perpendicular to the "fault." If there was shortening exactly perpendicular to the "fault," it's possible to find an amount of movement that makes the offset layer line up. In addition, the fold in the layer could be the result of shortening.
But again, I would want to crawl around the entire outcrop and look for other indications of the local strain. I would want more evidence (and more clear-cut evidence) before publishing that photo as evidence of anything!
(However, it would be a great - though totally evil - exam question.)
For A, I hypothesize a reactivated fault. First a packet of siliciclastic rocks was brittlely left-offset, then under much hotter conditions it was ductilely right-offset along with compression across the fault and local intrusion of the granitic rock, which I will not call boudinage, at least not textbook boudinage.
Boy, those are pretty shots.
This is just a guess...
A) Igneous Intrusion
B) Glacial Deposits
C) Angular Unconformity
I'd call 'B' a mylonite. High ductile shear strain at great (20km?) depth - as in the basal shear plane of a listric fault.
'C' is harder to identify visually, but it looks like xenoliths in a granite. There also appears to be either a bit of imbrication or some ductile strain which has aligned the long axis. I'd lean toward the strain idea, but would want to verify with the surrounding granite. If I'm not mistaking, there are 'bands' of xenoliths, which are ~60 degrees off the long axis of the clasts. Perhaps related to the flow of the magma or an indication of vertical?
B looks like a metaconglomerate- or diamictite, as Misha guessed- that has been fractured by a strong Sigma 1 from roughly 285-105 degrees (taking top of photo as 0/360 degrees). I can't remember the name for the conjugate faults created this way; struture and stress and def was 25+ years ago. But the larger clasts, particularly the one to the lower left, show the characteristic fault pattern. The lower left clast, in addition, is fractured through the middle, more or less parallel to sigma 1; this indicates "tensile" stress/strain- or more accurately, much lower compressional stress.
For A, I would add that the region described as a boudinage or intrusion is likely (what we used to call, but obviously, I'm pretty out of date) a sigmoid gap, created when an offset along the original fault was opened up when the fault reversed its movement sense. I would suspect it may be more a "hydrothermal granite" rather than a true intrusion. Support for this could be found if there are open voids with euhedral qtz and k-spar terminations.
I can't really see enough detail on C to make a reasonable guess- this is an outcrop where two or more perspectives would really help. Growingtedium has already said everthing I might.
Post a Comment
<< Home