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:

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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.

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.

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