Posts Tagged ‘structure’

This blog addresses the petrology and PT history of metapelitic gneisses of the Kanskiy granulite complex in the Southern Yenisey Range (Eastern Siberia).

Regional position (a) and geological structure (b) of the Southern Yenisey Range, Eastern Siberia. (a) Main geological units of the Yenisey Range. 1, cover rocks; 2–4, Pre-Riphean metamorphic complexes: 2, Kanskiy and Yeniseyskiy granulitic–gneissic complexes, amphibolite- to granulite-facies metamorphism (>1900 Ma)

Check out the rest of this article at this link:



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(Wednesday was not a good day internet-wise for our campus–we had none for most of the afternoon.   Thursday, my laptop decided to try and die one month after its extended warranty ended.   Result is that this week’s structure post is late–sorry!)

This week’s structure pictures come compliments of a trip the Petrology class took last fall with Callan Bentley‘s crew at NOVA to Shenandoah National Park.   For the sequence of pictures:

  • determine the relative order of how the rocks formed & deformed
  • name the specific types of primary & deformation structures
  • give the general orientation of the principal stresses for each step
  • (Hint: I came up with four stages)

Lens cap is 72 mm.

Lens cap is 72 mm.   Picture taken at a 45 degree angle to the last one.

Rock not in place, but a good example of this structure.   Hard to see in the sunlight, so I’ve modified the image into B&W with a strong blue filter:

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Well, the EQ&V students did a fairly good job of identifying what kind of fault & the sense of shear on it for our last image (#4), but no one successfully ID’d the location.   A few got the state correct, but were still lacking a more precise location.   Due to that, I’m going to open up the “location” question to students currently enrolled in either EQ&V or Structural Geology here at UPJ.   If you think you know the exact location, write a comment to this post (make sure that your identity is not set at anonymous!) and I’ll award 5 pts worth of extra credit to one quiz grade.   Here’s the image again as a reminder:

As for structure #3:

The image is of a quartz (the blockier crystals both large & small) – muscovite (very thin elongate minerals that frequently cut across the blocky crystals) mylonite from the Picuri Range in New Mexico.   The large quartz grains (e.g. the big purple ones in the middle) have undulose extinction (in this view, a single grain has multiple shades of the same color) and occasional subgrains (e.g. the pale-yellow grain within the large purple grain in the left side of the image).   The smaller quartz grains that make up the matrix are elongate, but do not have aligned c-axes.   Quartz has four crystal axes: three are at 120 degrees to each other (a1, a2, a3) and the fourth is at 90 degrees to the other three (c-axis).   The orientation of the c-axis determines quite a few things about what that grain will look like both in XPL and with the accessory plate inserted.   If all of c-axes were aligned for quartz (this doesn’t work for all other minerals), with the accessory plate, all of quartz grains would be about the same color.   Since the grains have a variety of colors, the c-axes are randomly oriented.   The muscovite is present as ribbons (very thin, very elongate) and is aligned parallel to the elongation of the quartz grains.

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Because we’re currently discussing microstructural deformation in structure, I’m going to post another photomicrograph today 🙂   This one is compliments of the 2002 Microstructures class I took with Jane Gilotti at UIowa and is of a mylonite in New Mexico.   The photomicrograph was taken with the accessory plate inserted, which is a bit unusual (I’ll explain why we used it in the answer).   The structure students should:

  1. Identify the minerals present to the best of your ability
  2. Identify what the evidence of deformation is & name each type of microstructure present
  3. Explain why I took this image with the accessory plate inserted

NM-99.   (My notes don’t include how wide the field of view is… hmmm)

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Structure Wednesday #2

(Most of the UPJ students should remember this site from Field Methods last May.)

For this image, indicate two things:

  1. This fold is comprised basically of two lithologies–which one is the competent layer?
  2. If we were to return to the fold today to determine the orientation of the fold axis, how many strikes & dips would you want to take?  give an approximate location for each measurement.

Fold on the north side of 199 just before Kingston-Rhinecliff Bridge on the east side of the Hudson River.   The road sign (on the right side of the image) is about 4 ft tall.

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Answer Tuesday #2

So, what was Structure Wednesday #1 depicting?   A sinistral shear fracture from Piseco Lake Shear Zone (PLSZ).*   I have a number of pictures from this field trip, so a few more may pop up over the next few weeks.

(oh, since I forgot earlier–the lens cap in the picture is for a 72 mm lens. )

EQ&V Friday #2 is a picture of the ocean-ocean convergent boundary between the Atlantic and South Sandwich plates.   The South Sandwich Islands are located on the microplate and are a chain of volcanic islands.   The last eruption was of Montagu Island in 2007.

(image from GeoMapApp)


  • Piaschyk, D., Valentino, D., Solar, G., and Chiareenzelli, J.R., 2005, Variations in L- and S-tectonite on the northern boundary of the Piseco Lake Shear Zone, Adirondack Mountains, New York: Guidebook – New York State Geological Association, 77, pp. 75-91.
  • Valentino, D., Solar, G., Chiarenzelli, J., Gates, A., and Freyer, P., 2004, L- versus S-tectonite fabric variations within the southern Adirondack shear zone system: progressive deformation associated with a sinistral conjugate to a Grenville syntaxis: Guidebook – New York State Geological Association, 76, pp. 191-220.

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Structure Wednesday #1

For the following picture, determine 1) what is the name of the structure and 2) what is the sense of the offset.

(Picture was taken in 2007 on a Structural Geology field trip while I was teaching at Vassar.)

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