A
couple of days weeks ago, I posted three photographs (reproduced below) and asked you to explain them. My sincere apologies that I haven't gotten the answers up sooner... it's been a crazy time. I've been swamped. And the explanations are not brief. Anyhow...
Here are my explanations -- and the winners for the contest!
A.jpg)
This is a kink band that got reactivated fault. The tectonic stresses acting on these rocks changed over time, and with them deformation took different paths.
This kink band occured in an area of highly foliated metavolcanic rocks, which developed their transposed foliation (running left to right across the photo) due to transpression in the late Mesozoic. The orientation of the kink bands suggests that the second generation of deformation (the kinking) was caused by a maximum stress oriented at an angle of ~30 degrees to foliation. (see Figure 56, page 100 of
my geology master's thesis). Some of the resulting deformation was taken up by (See Figure 52, page 95 of
my geology master's thesis) kinking. If the second generation of deformation (kinking) were directed parallel to the foliation, we would expect to see conjugate pairs of kink bands, both at the same angle to foliation. But that ain't what we see... we see kinks in only one angular relationship to foliation. This tells us that the maximum stress (sigma-1 in part C of the diagram below) must have been coming in at an angle of about ~30 degrees to the foliation:
Later, those kink bands/faults were reactivated under a third generation of deformation, which then allowed those fault surfaces to open "void spaces" which instantly filled with whatever fluids were available. In this case, that appears to have been a quartz-saturated water, which filled in the void space with a deposit of milky quartz.
Winner? Kim came closest -- and also pointed out that this story is reinforced by looking around the area at similar exposures which show the same story. Kim, you win a bumper sticker!
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This is a strained metaconglomerate, and it provides a nice case-study in strain localization.
This photo speaks volumes to me, because my geology master's thesis was a "real life" check on the predictions of a forward numerical model. My advisor wanted to try and understand the development of lineation in shear zones (ductile faults) via computer modeling. So he came up with a cool model that made predictions about the orientation of lineation relative to foliation and relative to the shear zone's boundaries, and he sent me out into the real world to see if real shear zones played by those rules. And the two didn't match up perfectly.
One issue that may contribute to the lack of agreement between the Sierra Crest shear zone system and modeling predictions is that models distribute strain systematically across a shear zone, whereas it is instead localized in natural systems. The shear zone is itself a localization of strain, of course. The question is, 'how local?' In other words, at which scale(s) is strain being accommodated? Possible triggers for strain localization are many: rheological contrasts between lithologies (Nadin and Saleeby, 2004), variations in temperature or fluid flux (due perhaps to proximity to an intruding magma body) (McCaig, 1984; O'Hara, 1988; Tobisch et al., 1991), variations in stress (due perhaps to salients of wall rock which project into the shear zone or the presence of resistant blocks inside the shear zone), presence of fluids, and / or pre-existing structural heterogeneities. For whatever reason, certain areas within a shear zone may accommodate more strain than neighboring areas. Shear localization may occur on many scales.
Photo B above shows cm-scale localization of strain as small pebbles in a metaconglomerate wrap around a larger, central, less deformed clast. Pebbles immediately across strike from the large clast are more deformed than pebbles along strike from the large clast (i.e. those in the rigid clast's 'pressure shadow'). As a result, the orientations of the long axes of the surrounding pebbles (
i.e. lineation) occur in a variety of orientations, a condition also seen in traces of the foliation. On a shear-zone-segment (km) scale, strain localization may be noted in the appearance of pods of relatively undeformed rock surrounded by well-foliated and lineated rock more typical of the shear zone. In the Gem Lake and Mono Pass segments of the Sierra Crest Shear Zone system, for instance, lozenge-shaped pods of clast-rich volcanic breccia (
See thesis Figures 14, 15, and 21) were far less deformed than neighboring rock. The implication is that the deforming portions of the shear zone 'flowed' around these pods of more resistant material.
Winner? Growing Tedium came closest, though nobody wrote about the strain localization.
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I took this last photograph because it demonstrates well the relationship between bedding and foliation in these rocks. Bedding runs from the lower-left of the outcrop towards the upper-right. But within those beds, you'll notice that all the clasts are elongated vertically into elliptical shapes (ellipsoidal in three dimensions). That's because these rocks got squeezed from the sides when they were hot enough and under enough pressure to flow into new shapes. At this location, deformation played a light enough touch that we can still see relict bedding, but in most of the Sierra Crest Shear Zone, the rocks are much more pervasively deformed: they exhibit a transposition foliation, where no traces of their primary structures can be still be seen. So in some ways, Photo C is the opposite of Photo B: it's a zone of lesser deformation surrounded by a zone of greater deformation: a less-disturbed pocket of rock in an area defined by its disturbed rocks.
Here's how I interpreted this outcrop in my thesis:
Winner? Again,
Growing Tedium came closest, by referencing the long axes of these clasts and the "bands" (bedding planes) which run through the outcrop at a 60-degree angle to the long axes. GT, please send me an e-mail with your mailing address, and I'll put your bumper sticker(s) in the mail to you ASAP.
Thanks to everyone for playing, and my sincere apologies for taking this long to get the answer up. (Is it apparent why it took me a while, now that you've read through this whole thing?) I've got a new, simpler contest planned for later in the week.
__________________________________________________________________References cited
McCaig, A.M., 1984. "Fluid rock interaction in some shear zones from the Pyrenees."
Journal of Metamorphic Geology 2, 129-141.
Nadin, E.S., and Saleeby, J.B., 2004. "Localization of shear along a compositional discontinuity: the Proto-Kern Canyon Fault, Sierra Nevada, California."
GSA Annual Meeting Abstracts: Denver 2004.
O'Hara, K., 1988. "Fluid flow and volume loss during mylonitization: An origin for phyllonite in an overthrust setting, North Carolina, U.S.A."
Tectonophysics 156, 21-36.
Tobisch, O.T., Barton, M.D., Vernon, R.H., and Paterson, S.R., 1991. "Fluid-enhanced deformation: Transformation of granitoids to banded mylonites, western Sierra Nevada, California, and southeastern Australia."
Journal of Structural Geology 13, 1137-1156.
Labels: california, contest, structure
