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.

S-C fabric in meta-ignimbrite

Here’s a sample from my 2004 geology M.S. thesis work in the Sierra Crest Shear Zone of eastern California. The rock is a sheared ignimbrite (ashflow tuff) tuff bearing a porphyritic texture and a nicely-developed “S-C” fabric.

With annotations, showing the S- and C-surfaces, and my kinematic interpretation:

S-C fabrics develop in transpressional shear zones: ~tabular zones of rock that are subjected both to compression and lateral shear (“transform” motion). The S-surfaces (foliation) initially form at about 45° to the shear zone boundary, and then progressively tilt over in the direction of shearing as deformation proceeds. This gives this sample (when viewed from this angle) a dextral (top to the right) sense of shear. (previous examples on Mountain Beltway) The C-surfaces are shear bands, where a large amount of shear strain (parallel to the shear zone boundary) is accommodated.

You should be able to click through (twice) for big versions of these images.

I polished up this little slab and made a refrigerator magnet out of it. I think it’s a lovely rock.

“Geology of Skyline Drive” w/JMU

I mentioned going out in the field last Thursday with Liz Johnson‘s “Geology of Skyline Drive” lab course at James Madison University.

We started the trip south of Elkton, Virginia, at an exposure where Liz had the students collect hand samples and sketch their key features. Here’s one that I picked up:

Regular readers will recognize those little circular thingies as Skolithos trace fossils, which are soda-straw-like in the third dimension. Rotate the sample by 90°, and you can see the tubes descending through the quartz sandstone:

This is the Antietam Formation, a distinctive quartz sandstone / quartzite in the Blue Ridge geologic province. But at this location, on the floor of the Page Valley and butted up against the Blue Ridge itself, we see something else in the Antietam:

Parts of this outcrop are pervasively shattered: a variety of sized clasts of Antietam quartzite are loosely held together in porcupine-like arrays of fault breccia. Turns out that this is the structural signature of a major discontinuity in the Earth’s crust: the Blue Ridge Thrust Fault. This is the fault that divides the Valley & Ridge province on the west from the Blue Ridge province on the east. And here, thanks to a roadcut on Route 340, we can put our hand on the trace of that major fault. Here’s another piece of the fault breccia:

After grokking on the tectonic significance of this fault surface, we drove up into Shenandoah National Park, to check out some outcrops along Skyline Drive itself, but it was really foggy. Here’s a typical look at the team in the intra-cloud conditions atop the Blue Ridge:

We checked out primary sedimentary structures in the Weverton Formation at Doyles River Overlook (milepost 81.9), like these graded beds (paleo-up towards the bottom of the photo)…

…and these cross-beds. You can see that it was raining on us at this point: hence the partly-wet outcrop and glossy reflection at right:

Cutting through this outcrop was a neat little shear zone where a muddy layer had been sheared out into a wavy/lenticular phyllonite, with a distinctive S-C fabric visible in three dimensions:

Finally, we went to the Blackrock Trail, which leads up to a big boulder field of quartzite described as Hampton (Harpers) Formation. In some places, exquisite cross-bedding was visible, as here (pen for scale):

Here’s a neat outcrop, where you can see the tangential cross beds’ relationship to the main bed boundary below them:

…And then if you spin around to the right, you can see this bedform (with internal cross-bedding) in the third dimension. I’ve laid the pen down parallel to the advancing front of this big ripple:

That last photo also shows the continuing influence of the fog.

Thanks much to Liz for letting me tag along on this outing! It was a great opportunity for me to observe another instructor leading a field trip, and also to discover some new outcrops in the southernmost third of the park.

Crenulation lineation

Hiking last Sunday in Rock Creek Park, DC, I saw this boulder and my eye was immediately drawn to the linear pattern running from upper left towards lower right (Swiss Army knife at upper right for scale):

Because that photo is not especially large, let’s zoom in a bit to two sections… Here is Photo 1, annotated to show the areas we will look at next:

Here’s a cropped and higher-resolution look at the diagonal lineations that caught my eye:

There are lots of different linear elements that can show up in rock fabric (as distinguished from the many kinds of planar elements that could be found). Some lineations are primary, but the ones that interest me are secondary (i.e., tectonic in origin). Let’s rotate our perspective, moving to the left of the first photo, and turning our head ~70° to look towards the right. This closer look at the left face of the boulder reveals the origin of these particular linear elements:

…They are crenulation lineations, essentially very small folds that deform the cleavage of these highly-foliated rocks. The crenulations’ fold axes were popping out in very slight 3D relief on the face of the boulder that initially caught my eye, like tectonic “ripple marks.” On the right side of Photo 3, you can see the lineations (fold axes) stretching away into the blurry distance.

In addition, some of the convex-outward crenulations had been breached, which means that the trace of the foliation was outcropping along the same trend as the fold axis. This is a variety of intersection lineation: two planar elements intersecting in a line. In this case the planar elements are [a] the foliation and [b] the outcrop surface.

(The other, more “classic” variety of lineation is a mineral stretching lineation, like the lineated gneiss I showcased last November.)

So, how should we interpret these rocks? I’d say that an initial foliation was imparted to them due to shearing along the Rock Creek Shear Zone, a prominent north-south-trending zone of smeared rocks in northwest DC; about 1 km wide. The foliation formed perpendicular to an original σ₁ maximum principal stress direction. Later, the stress field changed, and deformed this pre-existing foliation. The new σ₁ was oriented (using Photo 1 as our reference) from the lower left towards the upper right. The new σ₂ was oriented parallel to the crenulation fold axes (upper left towards lower right). And the new σ₃ was oriented in the direction perpendicular to the main outcrop face — that’s why the folds pooched out in that direction. (It offered the least resistance to being pushed.)

Recall that we saw something similar in the snow back in February.

Anyhow, I had just gotten through discussing lineations with my GMU structure students, so I figured I should photograph this particular outcrop for their benefit…

…and, I suppose, for your benefit as well, dear blog reader.

Shear bands in amphibolite

Check out these cool structures in one of the amphibolite bodies exposed along the Billy Goat Trail (C&O Canal NHP, near Potomac, Maryland):

Those are shear bands — basically small shear zones that are discretely localized within a larger body of less-deformed rock. Note the grain-size reduction visible in the shear bands, their dextral sense of offset, and their induration (making them more resistant to the forces of weathering and erosion: they stand up at least a centimeter higher than the rest of the amphibolite outcrop). We have seen a larger indurated shear zone before.

Note that the upper photo is truncated by the format of this blog template — click on it to go to the original image on Flickr, which allows you to see the sense of scale, and a wider view.

Here’s a cool YouTube video showing the process by which these things form (in a nice conjugate set given a homogenous material and plane strain):

A small shear zone

Back at NOVA Geoblog, I spent a portion of September and October 2009 reviewing the geological wonders I witnessed as part of a GSA field forum in the Owens Valley of California. However, I got distracted by other things, and never finished the series.

I’d like to pick up on that today, looking at a feature which is a typical part of mountain belts like the Mesozoic-aged Sierra Nevada magmatic arc. Here, we will look at a small shear zone exposed in the edge of the Sierra Nevada batholith, in the eastern Sierras, where they meet the Owens Valley, west of Bishop and north of the Tungsten Hills.

Let’s start off with a photo of the area: this is looking to the southeast, with the Sierras to the right, and the Tungsten Hills in the middle distance, and the White Mountain range beyond that. Simon Kattenhorn (blue t-shirt) provides a sense of scale, as do the vehicles parked at the more distant hairpin turn.

You can see those two hairpin turns in the road in this Google Map:

So you can see both in the Google Map and in the first photo that there is a prominent ridge poking up from the hillside there. (It’s the dark green stripe trending ~095° on the map.) From where the cars were parked, this looked like a dike, perhaps of granite, that was weathering out in positive relief. Several of us decided to climb up there and check it out. I’m glad we did, for it turns out to be a positively-weathered shear zone.

Here’s a decent shot that shows well the undeformed granite (bottom third) and the highly-foliated shear zone which cuts across it and deforms it to various degrees (upper two-thirds):

Shear zones are the deep, hot, ductile equivalent of faults. They were first described in the Scottish Hebrides in 1970 by John Ramsay and R.H. Graham¹. The idea is that two big blocks of rock move relative to one another, and if conditions are sufficiently high-temperature and high-pressure, in between will develop a zone of smooshed and squished rocks. The textural patterns that result are called a deformational “fabric,” and it is that fabric that calls our attention to the shear zone. You can see some of the more-deformed areas and the less-deformed areas in this photo:

The photo above also shows a top-to-the-left sense of shear, with the bands of dark minerals “tipping over” to the left.

Why this particular shear zone was weathered out in positive relief (standing up above the surrounding hillside) is unknown to me, but I guess that it may have to do with induration: the phenomenon that sometimes faulting or shearing makes rocks harder than they were pre-deformation.

Anyhow, let’s take a look at the various varieties of fabrics that this rock demonstrates. The shear zone cuts through granite, and here’s a relatively undeformed (~equigranular) exposure of the granite:

Small shear band running through the granite, again exhibiting the asymmetric fabric that suggests top-to-the-left kinematics.

A more pervasively-deformed sample, showcasing several decent augen (hard chunks, in this case of feldspar, that the foliation wraps around):

Nicely-developed S-C fabric, again with top-to-the-left shear sense:

Annotated version of the same photo, highlighting the orientation of the S- and C-foliation surfaces. S-C fabrics are typical of ductile deformation in transpressional shear zones…

Another sample, more pervasively deformed, showing smaller grain size (mylonitization):

Ditto, and with a more-fully-developed transposition foliation:

Lastly, here’s the same face that I annotated above, rotated 90°, which to me brings a different sense of perspective to the outcrop:

What really jumps out at my eye about this outcrop is the more-deformed (highly-foliated) and less-deformed (more-equigranular) domains. This is typical of my experience with shear zones: strain tends to be localized in certain bands, with other areas in the same shear zone being markedly less deformed (an old example from the old blog). This makes it tricky to accurately measure the amount of strain the rock has experienced, because no single square inch of this outcrop surface is “typical” of the overall strain in the shear zone.

To learn more about rock fabrics in shear zones, check out this site.

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1: Ramsay, J.G., and Graham, R.H., 1970. Strain variation in shear belts. Canadian Journal of Earth Sciences 7, 786-813.