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:

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

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

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:

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:

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

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

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:

Annotated, roughly showing the trace of foliation:

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

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

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

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

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

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:

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.

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.)

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

I recorded it in my field notebook like this:

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:

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:

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:

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:

Annotated version:

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:

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…

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

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:

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

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

This is sweet, right? I’m loving it.

A close-up shot that particularly satisfies me:

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:

More later…

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

Tavşanlı Zone field trip, part 1

Before the Tectonic Crossroads conference two weeks ago, I had the good fortune to participate in a Istanbul-to-Ankara geology field examining the Tavşanlı Zone, a tectonic suture zone where a portion of the Tethys Ocean basin closed. This paleo-convergent boundary is marked by a suite of interesting rocks, including blueschists, ophiolites, and eclogites. I’d like to share with you some of the things I saw along the trip.

This is one of the trip leaders, Aral Okay (pronounced “Oh-kai,” okay?), discussing the general geology of the area at our first stop. (The other trip leader was Donna Whitney.)

I think in general, you can make out the east-west trend of the rock units on Aral’s map (where they aren’t obscured by alluvium). This reflects the approximate north-south convergence of the Tethys closure in Turkey. To visualize this, I’d like to call your attention to a paleogeographic interpretation of the Tethys Ocean from Ron Blakey, the talented mapmaker from Northern Arizona University:

See all those colliding east-west-oriented crustal fragments in the northwestern Tethys? Those are the pieces that will comprise future Turkey. As you can imagine, rocks caught up in these tectonic collisions got both deformed and metamorphosed. Some of them were even subducted to ~80 km depth, and then brought back up to the surface! At our first stop, we saw some blueschist-grade rocks that had a phyllitic texture. Here’s two of them:

As usual, my eye was drawn towards the structures visible in these rocks. Here are a couple of nice little folds:

(The Turkish 1-lira coin is the same size as a U.S. quarter.)

I found this to be an interesting portion of the outcrop:

That’s green phyllite on the left, and blue phyllite on the right. Allow me to annotate it for you:

“Blueschist” and “greenschist” refer to two assemblages of minerals which supposedly represent different combinations of temperature and pressure. They are examples of metamorphic “facies,” as illustrated in this image:

Image redrawn and modified by me from Figure 3 of Bousquet, et al. (2008), which is itself modified from Oberhänsli, et al. (2004), and also from University of British Columbia (1997), which is modified from Yardley (1988).

Theoretically, blueschists and greenschists should be forming at different combinations of pressure and temperature. Blueschist forms at high pressures, but relatively low temperatures. But here we have an outcrop of blueschist that is right adjacent to a greenschist (medium temperature and pressure), with no faulting in between. It was suggested to me by a blueschist expert that this was likely a reflection in differences in the initial composition of the protoliths. I found this explanation less than completely satisfying, but there was no time to discuss, for we were being called back to the bus, already gunning its engine and ready to roll down the road.

At our second stop, we found some metamorphic rocks that showed clear textural evidence of having had pyroclastic protoliths:

There were lots of chunky bits in there.

So it wasn’t just pelitic (muddy) rocks that were getting metamorphosed in this Tethyan suture zone, but volcanic rocks too!

More later… when we move on to stop #3…

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…

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

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 / 6η_o)^⅓

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 η_o 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 η_o. So sandstone will have one η_o, while shale will have another η_o. 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 η_o 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 / 6η_o)^⅓ 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:

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:

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:

And a folded limestone:

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:

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:

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

Zoomed in to show this detail:

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…

Friday fold: multilayer buckle folding demo

Check out this video I found online whilst uploading last week’s Friday fold:

This video was produced and published on YouTube by Markus Beckers, Michael Ketterman, Dennis Laux and Janos Urai.

It’s a nice demonstration of how multiple layers of material of different properties and different thicknesses can yield up different flavors of folds. In the movie, there are two materials present: white silicone and gray foam. The silicone layers are stronger (“more competent”) than the foam. But the two silicone layers are different thicknesses. It turns out that this ends up being a decisive factor in determining the way they fold.

We can explain this behavior using the Ramberg-Biot equation:

L = 2 π t (η / 6η_o)^⅓

where L is the wavelength of the fold (in other words, the distance from one antiform fold hinge to the next antiform fold hinge); t is the thickness of the folded layer; η is the viscosity (resistance to flow) of the silicone layer (or, in general, the more competent of the two layers); and η_o is the viscosity of the foam layers.

In other words, the (η / 6η_o)^⅓ part of the equation reflects the viscosity contrast between the affected layers. In the video, this viscosity contrast is a constant, since we’re looking at two layers of the same stuff surrounded by the same matrix of other stuff. The only difference is the thickness of the two silicone layers.

So as far as our video up top is concerned, pay attention to the t value and the L value: the thicker the layer is, the larger the wavelength of the resulting fold. The thin layer has a lower t value, and so it ends up with a shorter wavelength: i.e., there are more folds packed into the same amount of vertical space as its stouter neighbor. The thick layer’s higher t value means it wıll have a proportıonately higher L value. It will have a longer wavelength, and fewer undulations will fit into the available vertical space.

Happy Friday, everyone! I’m heading back to DC tomorrow (from Turkey), so more regular posting wıll resume next week.

Friday fold: Archean gneiss from Montana

Deducing my first anticline

When I was done with my sophomore year at William & Mary, I embarked on a time-honored tradition among W&M geology majors: the Geology 310 Colorado Plateau field course. Jess alluded to this same course in her Magma Cum Laude contribution to this month’s Accretionary Wedge geology blog “carnival,” too.

My version of Geology 310 was led by the legendary Gerald Johnson (a.k.a. “Dr J”), a dynamic and enthusiastic educator who seemed particularly at home in the field. One day, he had us out in Utah (I think) somewhere, and pulled over to the side of the road so we could examine some tilted sandstone layers. We took a strike and dip reading, and plotted it on a map.

Then we descended into a narrow valley, where Dr. J did some “geology at 60 miles per hour,” pointing out shale outcrops in a few places in the valley. Then we drove up the opposite side. We pulled over again. Same sandstone strata: we again took a strike and a dip on the beds. The data was then recorded on our maps with a strike and dip symbol, a broad, squat “T” shape, where the upper bar of the “T” is parallel to the strike of the bedding, and the vertical prong of the “T” is pointing in the dip direction.

“Well,” Dr. J asked us, “What’s going on here?”

We were all silent, trying to puzzle it out. What’s the deal? What is he fishing for? Seconds ticked by, and no one had the right answer. We started to sweat… “Um, the sandstone beds are dipping to the west on the ridge west of the valley,” someone ventured, “and they are dipping to the east on the ridge east of the valley?”

“Yes, but what does that mean?” he replied. Silence…

Eventually, he relented, and spelled it out for us. Imagine this situation from the sides, he suggested, gesticulating the layers dipping off in opposite directions. “These are the same layers, so they were once laterally continuous…” He mimed a cross-sectional perspective:

How could we connect these disparately oriented strata together?

Bam! It hit me: I got the idea of an anticline at that point — the idea that a structure like an anticline could be so large that I couldn’t actually see it from my earthbound human-sized perspective, and I could only infer it from detailed measurements of the rock structures. It was a revelation to me: this valley and its surrounding ridges were part of a massive fold. The anticline must have breached in the middle, with the shale eroding away faster than the sandstone, producing a valley flanked by two ridges.

I’m grateful to Dr. J for putting us through all stages of this exercise: collecting the incremental pieces of data, being forced to think about it in an attempt to come up with an interpretation, and then finally giving us the proper interpretation, once it had become obvious we weren’t going to get it on our own. This last bit is particularly important to me as an educator: sometimes it’s okay to spell it out for students, particularly if it’s their first time walking down a particular path. By revealing the “answer,” Dr. J guided my thinking from data to big picture structure to geomorphological interpretation in a way that I can only describe as “opening up a new pathway” in my mind. Once he showed the way to think about this sort of thing, it was suddenly very easy for me to visualize this sort of complicated four-dimensional story. Once the pathway was there, it was almost effortless to let my thoughts flow along that pathway. Weird how one’s perspective can change in a moment, and how that influences everything that comes after.

For me, this exercise and ensuing discussion constituted an important moment in developing my ability to think like a geologist. I don’t think my brain will ever be the same.

Friday fold: Siccar Point, Scotland

As with last week, I’m going to show you someone else’s fold today. This one should have strong resonance with most geologists, because it’s a fold in the tilted (and contorted) older strata exposed below the famous unconformity at Siccar Point, Scotland:

I found this image on the British Geological Survey’s online repository of images, which are available for public use with attribution. I found out about the BGS photo repository via a post on StructuralGeology.org.

The photo was taken by T.S. Bain in 1979. Rock hammer (lower left) for scale.

The specific rock type here is shale, and their age is Silurian. Note the thinning of the limbs of the fold, and the relatively thick hinge area.

Happy Friday – may your workday rapidly thin (like the limbs of this “similar” fold), and your weekend be as thick as this fold hinge!