Tavşanlı Zone field trip, part 3

Picking up where we left off last time, we were in some partly-serpentenized peridotite, part of the Burham Ophiolite in Turkey’s Tavşanlı Zone, an ancient tectonic suture.

Our next stop on the field trip allowed us to visit some diabase dikes:

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Here’s a close-up of the right contact of the dike with the host peridotite:

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The field notebook’s long edge is ~18 cm. And here it is again, annotated:

tav_3_02anno

Near the village of Oranheli, we stopped to examine a jadeite meta-granitoid, a rock only a metamorphic petrologist could love. There were, however, a lot of metamorphic petrologists on the trip, and they were very keen on checking it out. This was the first of many occasions when random Turkish citizens would stroll up to our odd group to find out just what the hell we were doing:

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Further along, we saw a meta-basite (meta-basalt) within the meta-granitoid, and there I got a refreshing whiff of structure. Here’s a random isoclinal fold of a meta-granitoid dike cross-cutting the meta-basite, with a Turkish 1-lira coin (about the same size as a U.S. quarter) for scale:

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Next up were some very cool rocks: marbles with extremely elongated calcite crystals.

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These needle-like crystals are interpreted as being pseudomorphs of aragonite, the form of CaCO3 which is stable at high pressures and low temperatures.

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A bit further on, we return to metamorphosed shale and graywacke (now schist and “grayfels”), sheared out and pervasively deformed at blueschist conditions. I took a few photos of charismatic folds in the unit:

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Annotated, roughly showing the trace of foliation:

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Sandy layer folded over into a recumbent position, set in a sheared mass of meta-shale:

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Thicker sandy layer, in a recumbent isoclinal fold (white pen, 14 cm long, for scale):

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Zooming in on the above photo, to show the lovely, smaller wavelength parasitic folds which decorate the snout of the big fold:

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Extensional fractures along an isoclinally-folded, recumbent sandy layer:

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Small S-folds in the sheared shale (just above hammer):

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Coming down onto this roadside outcrop of sheared shale and graywacke were cobbles and boulders of float from somewhere up above. They were of a quartz-pebble conglomerate that showed a stretching lineation. Check out these two faces of typical samples:

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Now, here they are again, with the X, Y, and Z axes of the strain ellipsoid (longest, intermediate, and shortest, respectively) labeled for your benefit.

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This conglomerate has been sheared into a lovely L-S tectonite, with X>Y~Z. In other words, it’s mostly lineated, with only a weakly-defined foliation, indicating the stress field was mostly constrictional. (I collected a muddy sample of this stretched-pebble meta-conglomerate, and when I washed it off in the hotel shower the next morning, I was delighted what a cool sample I had selected. It has some awesome structural features; I’ll show it to you some other time…)

Our final stop of Day 1 of the trip was this spectacular overview of the Kocasu Gorge, a canyon which cuts across the structural trend of the area at approximately a right angle. (The canyon cuts north-south; the strike of the folded & thrusted rock units runs approximately east-west.)

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As the sun set, Aral showed us where we were, and the overall synclinal structure of the area.

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I recorded it in my field notebook like this:

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With this context established, we loaded back on the bus and drove for a couple of hours to get to a town with a decent hotel. We dined and slept, and the next morning got up ready for more suture-zone rocks.

Friday fold: twice-folded turbidites at Black Pond

Today’s Friday fold comes to us courtesy of Gary Fleming, botanist extraordinaire and brother of Tony Fleming, geological Jack of All Trades. Together, the Fleming brothers led a field trip for the Geological Society of Washington. While I was on that field trip, the topic of polyphase deformation came up, which led a couple of weeks later to Gary sending me this photograph. He took this photo in the Black Pond area, on the Virginia side of the Potomac River near the property of Madeira School:

mathergorgefm_blackpond

That’s a set of twice-folded folds. The earlier generation of folds are quite tight enough that their limbs are parallel; we call this “isoclinal.” They display axial planes that run left-to-right across the photo. They are overprinted by a second generation of folds which are more open and broad. The second generation folds have axial planes which run top-to-bottom across the photo. Here’s an annotated copy showing the undulating form of the folds:

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And here I’ve tacked on some color-coded axial plane traces: the first generation of folding (F1) is in yellow; the second generation (F2) is in blue:

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The rocks in question are turbidites of the Mather Gorge Formation, folded up during the late-Ordovician episode of mountain building called the Taconian Orogeny. Relative to the orientation of this photograph, the F1 folds would have resulted from top-to-bottom compression, while the F2 folds would have resulted from a later episode of side-to-side compression.

It’s also worth noting the collection of small parasitic F2 folds in the schisty section at the top of the photo (greenish-gray, and partially obscured by mud).

Happy Friday! If your week has left you as contorted as these rocks, I hope you have a relaxing weekend…

Thanks to Gary Fleming for sharing this image and letting me publish it here.

Tavşanlı Zone field trip, part 2

Yesterday, I shared a few thoughts about the first couple of stops on the field trip I took earlier this month from Istanbul to Ankara, prior to the Tectonic Crossroads conference. Today, we’ll pick up with some images and descriptions from the next few stops.

After lunch, our next stop brought us to a relatively low-metamorphic-grade outcrop of sheared graywacke (dirty sandstone) and shale. As you can imagine, it wasn’t particularly photogenic. Bedding was continuous only over a scale of a meter or two. It’s what suture-zone workers call “broken formation,” part way between undeformed rocks and a full-blown mélange. (It’s internally sheared up, but not yet mixed with adjacent formations.)

Looking back the way we had driven in, though (i.e., looking to the north), we could see the west-ward dipping limb of a large syncline exposed on the mountainside over yonder:

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Annotated version:

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The Orhanler Formation is the lowermost unit, layers of graywacke and shale that are probably Triassic in age. It is overlain by the thin sandstones of the Bayırköy Formation (Liassic), and then the limestone which is proving so irresistible to quarry excavators, the upper Jurassic Bilecik Limestone.

Our fourth stop was one of the ones that got me really excited. In fact, almost everyone on the trip seemed to get pumped up from visiting this outcrop. Check it out:

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The yellow field notebook’s long edge measures ~18 cm. Behind the notebook, my friends, is a layered gabbro. The stripes you see result from differing ratios of light and dark colored minerals — plagioclase and pyroxene, mainly. But why is it layered? Is this an example of a cumulate texture; a primary igneous structure resulting from the settling of crystals onto the floor of a magma chamber? Or is this a tectonic foliation, resulting from strain the rock has accumulated? It was introduced to the participants on the field trip as an example of the former, but several of us found this argument less than totally convincing, as the size of this rock body is ~200 km long and ~2 km thick. It’s awfully hard to envision a magma body that size. I found it easier to imagine this as a chunk of the mantle, as Alain Tremblay suggested to the group.

As I poked around the outcrop, I found something which was consistent with a deformational (rather than cumulate) origin to the layering…

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That’s an S-fold! Turn this cobble around, and on the other side, you can see a Z-fold:

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I suppose that tight little folds like this could have come in some stage of ductile deformation after an original cumulate layer formed, but that would require an episode of deformation not required by the foliation hypothesis. If these are planes formed by mantle flow, I’d expect a few small folds in those layers at the time that flow was forming them. Besides the blueschists and eclogites, the Tavşanlı Zone includes an ophiolitic suite, and having chunks of mantle there would in no way be a shocker.

Regardless of the origin of the mineralogical layering, I think we can all be pleased to learn that it is deformed. A series of “reverse” ductile shear zones cut across the layering, as you may be able to discern in this photo:

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Notice how the gabbro’s layers deflect towards the fault(s) in a “drag fold” fashion, tipping over to the left. Close up:

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Left of the notebook, you can see this gentle deflection quite nicely:

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This is sweet, right? I’m loving it.

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A close-up shot that particularly satisfies me:

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Note the thinning and rotation of the mineralogical layers as you get closer to the shear band at the center of the shear zone itself (far right of photo). Pen for scale.

We also stopped at a proper peridotite outcrop (no one’s arguing that this one isn’t mantle), which had serpentine veins cutting though it:

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More later

By the way, this blog’s move to the AGU servers has been postponed until probably Monday.

Friday fold: wavelength contrast

I scored this photo off the Internet more than five years ago, the first time I taught Structural Geology at George Mason University. I failed to note the website I got it from, and now that website has apparently disappeared, at least as far as the view from Google is concerned. If anyone knows the provenance of this image, please let me know so that I can properly attribute it.

I hesitate to post something like this without knowing who took it, but I did note to myself that it came from the Point Lake Greenstone Belt in the Northwestern Territories of Canada. This image and its implications follow so nicely on to our discussion last week about fold wavelength and the Ramberg-Biot equation that I can’t resist it. Ready? Brace yourself…

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I think that this is one of the coolest structural geology photos ever taken. Here it is graced with some annotations:

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Maximum compressive stress was in this case from the back to the front. The same vein, oriented ~parallel to σ1, is folded in two very different ways, depending on which rock type it is cutting across. As with a week ago, we can explain this behavior using the Ramberg-Biot equation:

L = 2 π t (η / 6ηo)

where L is the wavelength of the fold (in other words, the distance from one fold hinge to the next fold hinge); t is the thickness of the folded layer; η is the viscosity (resistance to flow) of the quartz vein (or, in general, the more competent of the two layers); and ηo is the viscosity of the rock unit (sandstone or shale) that the quartz vein cuts across.

If you keep t and η constant (for say, the rightmost of the two quartz veins), then the only thing left to vary would be ηo. So sandstone will have one ηo, while shale will have another ηo. The sandstone is more resistant to flowing than the shale is. The viscosity contrast between the quartz vein and the sandstone is less (they’re both made of quartz) than the viscosity contrast between the quartz vein and the shale (which have very different material properties).

The high viscosity contrast with the shale makes for a very big number, which raised to the ⅓ power (i.e., you take the cube root) makes for a very small number. This small number, multiplied by the constants of 2, π, and t, gives you L, which will also be a small number: hence the wavelength is small, and as a result, the folds are crunkled up next to one another like sardines in a can.

On the other hand, the low contrast between the viscosities of the quartz vein and the quartz sandstone means that you get a rather small number. Say η = 3. If ηo is also about 3, then you have: (3/(6*3)), or the fraction 1/6. Expressed as a decimal instead of a fraction, this is 0.167. Take the cube root of that, and you end up with a bigger number, in this case 0.55. Multiply that by 2, π, and t, and you get your new wavelength, L. Because you have a larger number in the (η / 6ηo) part of the equation, and everything else is the same, you end up with a larger wavelength. The result is only one fold antiform in the sandstone. In the neighboring shale, ~23 antiforms are packed into the same distance along strike of the vein.

Wild stuff, right? Happy Friday. Let’s hope your weekend is of sufficiently high contrast to the sludge of the week that you get all loose and wiggly, like the top part of the photo… : )

Rumeli Hisarı

Right after I got to Istanbul on this most recent trip, I took a taxi from my hotel down to the Bosphorus, to check out the Rumeli Hisarı, a fort complex built in 1452 by Sultan Mehmet the II in anticipation of the following year’s siege of Constantinople. It’s constructed at the narrowest point on the Bosphorus (660 m wide), with the aim of controlling boat traffic coming from the Black Sea. This narrow spot is today where they have the second of two bridges spanning the Bosphorus. It looks like this:

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It’s in Europe; that’s Asia on the far right of the photo. A few more shots of the fortress’s pattern of towers and interconnecting walls:

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Inside, I was pleased to note the variety of building stones. Here’s a nice porphyritic andesite which was a common constituent of the walls:
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And a folded limestone:

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Here are some yellowish blocks that are weathering away faster than the mortar which holds them in place. There is a Turkish 1-lira coin in front of the dark block near the center, to provide a sense of scale:

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Here’s a similar phenomenon playing out with some bricks used to make an archway, except here the mortar is the more rapidly weathering component:

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Check out this slab of brick… it’s got a curious adornment:

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Zoomed in to show this detail:

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Dog prints! Sometime a long time ago, maybe more than 500 years ago, a brick maker put out slabs of clay to dry, and some long-dead dog walked across it. The dog’s footprints are a kind of “historical trace fossil” that was then incorporated into this ancient structure.

Visiting the Rumeli Hisarı was a pleasant experience. I walked down along the Bosphorus next, peering into its surprisingly clear waters and counting jellyfish, then got a pide at a cafe. I caught another cab back to the hotel, and eventually fell asleep, a victim of jet lag…

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Photos from Virginia Geological Field Conference

For the second year in a row, more exotic travel plans meant that I wasn’t able to attend the superb Virginia Geological Field Conference. I see that they have now posted some photos on the group’s Facebook page, so go check them out to see what we both missed last weekend. Here’s a taste:

Sheared meta-conglomerate:

Metamorphosed mantle (?) xenoliths:

Güvem geoheritage site, Turkey

Looks like I’m late to the party…

While I was away, apparently the geoblogosphere went on a rampage of cooling columns. Everyone was posting images of their favorite columnar joints, and I was left out in the cold. Let me remedy that now. As it turns out, I was visiting some columns while everyone else was writing about them. Here are some images from the Güvem area of Turkey, north of Ankara, where there are a mix of late Miocene lake sediments and intercalated volcanic rocks, including these basalt flows. We stopped to visit them last Wednesday on our way to the North Anatolian Fault:

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The dark entablature looms above:guvem_columns01

A nice central panel with a good cross-section of the flow: guvem_columns03

Around the corner, some more:
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I ran across the street (and a stream) to check out a similar exposure there:guvem_columns05

Zooming in:guvem_columns06

Close-up of a few columns (with my hand for scale):guvem_columns08

Looking up along the columns:guvem_columns09

And a few more shots of the scene:guvem_columns10

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A full list of Turkish geoheritage sites may be found at the end of this document. Lockwood maintained a list of the other blog posts in this meme here, which I’ll quote below since it’s so nicely laid out already:

Geotripper, here, here and here,
Sam at Geology Blues
Phillip, also at Geology Blues
Silver Fox, and another columnar post here.
Glacial Till and another!
Life in Plane Light: Squashed columns!
Aaron at Got The Time
Geology Rocks
Dana at En Tequila Es Verdad
Cujo 359 (see comment on Dana’s post for description)
Wayne at Earthly Musings has a gorgeous photo of columns below the rapids at Lava Falls in Grand Canyon.
MB Griggs at The Rocks Know has photos of what may well be the most perfect columns in the world.
Jessica, AKA Tuff Cookie, showcases a variety in different rock types.
Hypocentre finds columns in a very unlikely place, as well as a spectacular photo of radiating columns.
Dave Tucker at Northwest Geology Field Trips displays precisely one slew of columnar displays in Washington State.
Dave Bressan at History of Geology shares the first printed image of columnar basalts, from 1565.
A couple more variations from Dana’s and my driving about W. Oregon.
Dr. Jerque has some spectacular examples from the bottom of the Grand Canyon.
Silver Fox Has another (better than mine) photo of horizontal columns in a set of dikes, and points out a couple more links to columny goodness (not to be confused with calumny, which is not good)
Dan McShane offers some more Washington State columns.
Garry Hayes, who deserves credit for starting this meme (see first links in the list, above), adds yet another set of photos from the opening of the Atlantic Ocean, and a lovely guest photo by Ivan Ivanyvienen, of columnar jointing in rhyolite at the San Juan Precordillera.
Update, October 4: Eric Klemetti- who did his doctoral work just down the street from where I’m sitting- has joined the fray. (Also, check out the links readers have left in the comments)
Helena Heliotrope at Liberty, Equality and Geology shows off some more Washington columns.
Chris and Anne at Highly Allochthonous each toss in a photo- Tokatee Falls looks awesome!
Some more Cape Perpetua jointed dike photos from Cujo359, and Devil’s Churn- again, numerous dikes with horizontal columns.

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