14 December 2018
It’s time for another guest Friday fold! This one comes to us from Nathan Niemi of the University of Michigan.
Nathan writes that this is
an aerial view of the Titus Canyon Anticline in Titus Canyon, Death Valley National Park, looking to the northwest. The fold is cored in the lower Paleozoic Zabriskie Quartzite and Wood Canyon Formation (brick-red bed at right), and the overlying middle Paleozoic Carrara Formation (shales and limestones) is detaching from the Zabriskie and forming the tight, prominent fold in the center of the photo. The dark gray rocks to the left are overturned upper Paleozoic carbonates of the Bonanza King Formation.
Thanks for sharing, Nathan – what a great view of a great structure! I found it in Google Maps if anyone is interested in exploring the local context with that tool.
Happy Friday, all.
7 December 2018
The American Geophysical Union’s Fall Meeting begins this weekend in my hometown of Washington, D.C.
To prepare for the influx of geoscience-oriented visitors to our region, my colleagues and I have prepared a number of field trips and introductory articles about our local four-dimensional landscape. The Mid-Atlantic region has a tremendous variety of different geological stories to tell, revealed in the rocks of several closely-packed geological provinces, as shown in this lovely cross-section by Kat Cantner:
In collaboration with the Geological Society of America and the Geological Society of Washington, I helped organize a three-day field trip transecting the Appalachian mountain belt, but unfortunately we got the word out too late to garner many registrants, and it was cancelled. Fortunately, there are several other trips with less of a time commitment, that AGU attendees should consider taking advantage of.
Only have time to read an article, perhaps on your flight or train ride into town? My NOVA colleague Ken Rasmussen and I wrote the cover story in this month’s EARTH Magazine, “Touring the Capital Geology of Washington, D.C.,” which covers the basis of the rocks of the Piedmont / Coastal Plain interface as well as stories locked in the building stones of the National Mall area. There’s a lot to discover there, and this final version of the piece is half the length of the article we submitted! (Thank goodness for editors!)
Enjoy these resources, and enjoy visiting Washington, D.C. I hope to see/meet some of you at the Fall Meeting.
23 November 2018
Octavia Sawyer is back with a fresh Friday fold from her haunts out in Utah! This is:
Maxfield limestone, which would make it Cambrian. Where the fold is located, the outcrop comes to a point, sort of like the corner of a building, so it’s hard to show the whole thing in one photo. The “center” photo was taken from directly in front of the corner; the “left” photo was taken from the left side, looking along the face/wall out toward the corner; etc. Location is in the foothills east of Provo, Utah, in a disused gravel pit and limestone quarry.
Pretty nice outcrop! Thanks for sharing, Octavia!
21 November 2018
A few days ago, I showed you some flow banding in pseudotachylyte near Dobbins Lookout in the South Mountains, south of Phoenix, Arizona. I’d like to return to the South Mountains today for a more comprehensive look at the rocks exposed there. The South Mountains offer a geometrically-relatively-simple example of a metamorphic core complex.
Photo-wise, let’s start with this outcrop, which offers a good starting point for thinking about metamorphic core complexes.
There you see a granite, and a deformed granite. Granite is an igneous rock: one of those rocks that forms from the slow crystallization of magma, deep underground. The undeformed granite is equigranular, meaning its crystals are about the same size and the same shape in every direction. But as you work your way down through the photo, the rock takes on more foliation, and the size of the big chunky feldspar crystals is reduced dramatically. Quartz crystals are “ribboned” out into long wraithlike things. This is what a granite looks like when you smear it out. The exact boundary between the granite and the smeared-out version of the granite is impossible to put your finger on here – the deformation is progressively more and more pronounced from top to bottom in this photo. By the time we’re at the bottom, though, we’ve got a mylonite.
Mylonite is a structural rock term; it’s independent of the composition of the rock that got deformed. Mylonites can form from igneous rocks (as here) or from sedimentary rocks, or from metamorphic rocks. If they can smear out, they can hope to someday be lucky enough to be transformed into mylonite!
Mylonites were first described at Loch Eriboll in Scotland’s Northwest Highlands, where Charles Lapworth gave them their name and interpreted their fine grain size and foliation to be the result of thrust faulting. Rather than faults being crisp breaks in the crust, sometimes they could be broader zones of smeared-out rocks that accomodated the relative movement between two big blocks of rock in a diffuse zone of deformation. Lapworth’s insights helped solve the mystery of the Northwest Highlands (how Moine schist ended up on top of a Cambrian sedimentary sequence), and it gave the world a new way of looking at faulting – not only in terms of orientation (bedding-parallel) but also in terms of diffusivity.
In the South Mountains, the massive phaneritic granite you find at lower levels transitions at Dobbins Lookout into something very fine indeed:
There are also darker varieties, that may have started off as some other lithology, but are just as smeared-out:
This little outcrop (of granitic-protolith mylonite) has a couple of tiny kinks running through it. These kink bands are like very crisp folds, and they only occur in rocks that have a strong mechanical layering, as here with the mylonitic foliation. Kinks are structures on the border zone between ductile and brittle, a taste of what’s to come…
(There is also at least one proper microfault in that image. See if you can find it!)
It’s worth mentioning that mylonites are often lineated as well as foliated. While I’ve been showing you photos of the trace of the foliation surface so far, I can also aim the camera lens right down onto that foliation surface, and look at the lineations lying within it, like pencils aligned on the surface of a desk. These are mineral stretching lineations, and they point in the direction in which the rock was elongated. In the South Mountains, that’s an east-northeast/west-southwest orientation, and it’s the direction the hanging wall block slid off the top of the metamorphic core complex. In other words, it’s the direction of crustal extension.
Here’s a comparison between lineations, both coarse and fine:
There’s some serious grain size reduction between the first lineation photo and the second. That’s mylonitization for you! Grain sizes are dramatically reduced as the rock’s constituent minerals are “milled out” and smeared into wispy blebs.
Okay, now let’s zoom out and look at the overall geology of the mountain range, before diving back in to outcrop photos. Here is a geologic map of the South Mountains by Steve Reynolds and Julia Johnson of Arizona State University, shared with geologists attending the January 2018 Structural Geology and Tectonics Forum at ASU:
Reynolds & Johnson, 2018
The key things I’d like to call your attention to are:
- How simple the geology is in this range: there are essentially two rock types.
- How the mylonitic foliation in the cross-section is almost parallel with the surface in the middle of the section, but dips ever-so-gently more steeply to the right (east-northeast).
- There are scraps of breccia atop the mylonitized granite in several places. Brittle deformation, in other words.
Steve Reynolds interprets the structural geology of the range as representing an episode of crustal extension, with deep, warm rocks experiencing mylonitization at first, then rising toward the surface and progressively colder conditions.
Reynolds & Johnson, 2018
When rocks are cold and shallow (under low confining pressures), they break rather than flow, and so in the South Mountains, we’d expect to see brittle structures (which came later) overprinting or cross-cutting ductile mylonitic foliation and lineation (which developed earlier). The overall consequence on the large scale is tectonic extension: note how the cross-sections in this trio of block diagrams get wider from earlier to younger.
So now, with that set-up, it’s finally time for some photographs of the brittle deformation that overprints the ductile… The next few photos show brittle faulting offsetting well-developed mylonitic foliation:
Such cute little faults!
I’ve switched the orientation of the trace of foliation in the next three photos. If you look closely, you’ll see that both the light-colored rock (deformed granite) and the dark colored rock (deformed some-darker-colored-rock) are foliated:
…Zooming in closer…
Here’s another outcrop, quite striking, especially with that foliation-perpendicular contact between foliated-dark and foliated-light so crisp and sharp:
I can only imagine that contact is a fault. There are lots of little microfaults in there too, disrupting the contacting and giving it a stair step sort of morphology.
I think this was the most profound example I saw: Everything you’re looking at in this next photo is mylonite, just in two different colors, and the orientation of the trace of foliation is ~horizontal in this photo in both rock types:
I was really struck in the field by how profoundly crisp the vertical contacts were, a clear indication there was no more subsequent mylonitization of these rocks after faulting shuffled them into the arrangement we see today. Furthermore, faulting didn’t seem to have significantly altered the orientation of the foliation (assuming it was parallel in both rock units to start with as well as after faulting had concluded), which is a little surprising.
So, annotated, I think we have to call all those light/dark foliation-perpendicular contacts faults:
There might be some foliation-parallel slip, too. We saw that in the orientation of the pseudotachylytes I mentioned the other day. These faults, if I’m right in calling them that, are fully annealed; the modern outcrop breaks cleanly across them. They aren’t crumbly. They don’t show any cataclasis. There’s plenty of that up (structurally) above in the (map-scale) chloritic breccia.
The little train of three boudin-like fault blocks at upper right was nice, too:
All told, these are an intriguing set of rocks and structures, and I’m glad I got the chance to explore them in more detail than when I first visited this site in January.
17 November 2018
Pseudotachylyte is fault glass. It’s rock that got melted due to frictional heat generated “in the heat of the moment” as an earthquake occurs, then froze before having the chance to crystallize. My friend and colleague Christie Rowe likes to call pseudotachylyte a “fossil earthquake,” and I think that’s a lovely way of thinking of it. Here’s an example I saw Thursday afternoon, in the South Mountains of Phoenix, Arizona:
The South Mountains are a metamorphic core complex: a large sort of structure that is moderately common in the mountains of southwestern Arizona, but also present in other areas of the world. These metamorphic core complexes are characterized by a dome-like blob of exhumed rock rising up, and overlying rocks slipping off the top. The rocks that pooch upward in the middle of the complex go from deep, hot conditions to shallow, colder conditions. The deformation they experience thus starts off ductile, and then transitions to brittle as they make their way up to the surface.
At Dobbins Lookout in the South Mountains, you can see this for yourself. There, a Tertiary-aged granite pluton shows mylonitic fabric with a dominantly horizontal foliation and an east-west-trending lineation, cross-cut by veins of pseudotachylyte. Here is an example:
The principle of cross-cutting relationships reminds us that when one geologic unit or structure cuts across another one, the cutter must be younger. At this site, the ductile fabric of the mylonite is transected by the veins of brittle pseudotachylyte. This shows in miniature the structural imprint of the granite’s journey upward through different deformational conditions through time.
Here is another outcrop showing the same sort of thing:
Now check this out:
Do you see what I see?
Let’s zoom in…
Note the “sideways diapir-shaped” set of concentric bands within a small pocket of pseudotachylyte –
It almost looks like a thumbprint, or a surficial deposit of mud. But it’s not! It’s part of the rock.
It struck me that this could be flow banding within the melt, recording the expansion of the earthquake-frictionally-melted rock into a ~square dilational pocket, as the block of granite to the left slid to the left, relative to the block of granite to the right, something like this:
In the annotation, the pseudotachylyte vein’s contact with the mylonitized granite is outlined in black, the concentric flow banding is traced out in white, and my interpretation of flow direction is shown with the yellow arrows.
What do you think? I thought this was very cool. I’ve seen flow banding before in igneous rocks, both felsic and mafic, volcanic and plutonic, but this was my first time seeing it in pseudotachylyte. I ran my interpretation by Steve Reynolds of Arizona State University, the geologist who mapped out these rocks and first successfully interpreted them as a metamorphic core complex, and he concurs, so therefore I know it has to be legitimate! 🙂
I first saw these rocks in January on a field trip that Steve led for the Structural Geology and Tectonics Forum, but I didn’t have nearly as much time to explore the site then as I did this past Thursday, when I was lucky enough to find myself in Phoenix with a free afternoon, and good company in the person of my former student Stephanie Sparks, now working on a PhD at ASU under the supervision of Kip Hodges. Stephanie and I saw some other cool outcrops on our field trip, and I’ll feature them in future posts later this week.
16 November 2018
He is back in the States now, and has been kind enough to share his geology-themed photos with me, so I can share them with you. Take a gander:
Several people more familiar with these rocks than me used the comments on the previous post to identify these rocks. Darrel Cowan specifically nailed them as belonging to ‘the Macigno,’
the earliest of the three foredeeps that record the eastward advance of the Ligurian nappe. Age: about Oligocene to early Miocene. Erik is correct: I’ve seen excellent olistostromal melange on the Via dell’amore near Riomaggiore.
Thanks to all for sharing – both photos and knowledge. Happy Friday.
9 November 2018
Reader and former student Paxton DeBusk shared this lovely folded gneiss with me at the conclusion of the Virginia Geological Field Conference a few weeks ago:
That’s a lovely hand sample, with a high folding:volume ratio!
Happy Friday, all
6 November 2018
I’m at the GSA Annual Meeting in Indianapolis this week, and I got an award today: the James Shea Award from the National Association of Geoscience Teachers.
I was introduced at the awards luncheon today by my co-nominator Kaatje Kraft, who had many nice things to say. I followed her comments with this little speech:
Thank you, Kaatje, and also to Joshua Villalobos for nominating me.
I’m getting this award for blogging. I write a blog, a geo-blog. This award is simultaneously an important validation of the blog as a medium for geoscience communication and outreach, in addition to honoring my particular individual contributions — but also something that also feels a little like a requiem. I say that because geology blogs in general, and mine in particular, have diminished in their productivity/presence/importance over the past several years, and particularly The Past Two.
Sharing science has been important to me from an early age. In college, I wrote a natural history column for the William & Mary student newspaper called “Wild Williamsburg.” That, I think, was my initial foray into communicating science to a nonscience audience, and I daresay it emboldened me.
In 2007, I had been hearing about blogs quite a bit, and decided that I could use the blog format to funnel news items and other cool digital ephemera to my students. I started NOVA Geoblog, and I found that almost instantly, it was being read much more by people who were not my students. These people were spread out all over, and they quickly became an extended community to me, a farflung network of colleagues and compatriots.
This was during the geoblogosphere’s “Cambrian Explosion,” and I was not alone in writing a blog about geology. Other geobloggers adopted social media platforms too to “flesh out their outreach portfolio,” and I followed as far as Twitter. This was when my blogging efforts changed their name to Mountain Beltway, and AGU elected to host it. Twitter has sapped some of the vitality from blogs, it would seem, and that platform has grown more dynamic and useful even as many blogs gather the digital equivalent of dust.
I’ve kept at mine, though, and celebrated a decade of writing it last fall. During those ten years, I’ve become a contributing editor at EARTH magazine, helped write textbooks, and even pushed out a few peer-reviewed articles into the scholarly literature. Each of these, plus that whole “email lifestyle,” saps a little bit from the drive to write on the blog. I also became a parent, and it turns out that cuts into your writing time, too! While my productivity on the blog has dropped off over the years, I can at least claim honestly that I’m more blogorific than most of my geoblogger peers have been able to maintain. I am proud of the body of work I’ve developed over the years, all of it instantly accessible via a Google search. Even if my writing has slackened, readership has stayed constant, as web searches bring new readers to material I published years ago. I can also point to an expanded reach in Twitter, which gets out the easy-to-write stuff without much effort.
There’s been another reason for my productivity decline.
A source of dismay for me, and a reason I find it hard to stay perky enough to write constantly about how cool geology is, is the erosion of norms in American governance. Our democracy has been shown to be unexpectedly fragile, and today’s midterm elections are the first great test of whether the citizenry will accept these changes, or will reject them.
Even if today’s election results turn out to be on the whole sane and science-affirming, I fear that we’re never going back to the way things used to be. Therefore, to tease a positive affirmation from this heartbreaking, nasty situation, let me say this: we have important work ahead of us. Democracy, like science, is a participant sport, and it only moves forward if it’s constantly maintained. The currents of counterfactualism and anti-intellectualism are stronger than ever, and we are the ones who must swim upstream. We teachers must communicate our understanding of the world and the way we have come to that understanding through nonpartisan means that anyone can practice, no matter their politics, their skin color, their level of physical ability, or their gender.
It is imperative that we be successful at this task. Nothing less than the fate of the planet’s biosphere (which includes, but is not limited to, human civilization) hinges on whether we succeed. The impartiality of science is a way forward for our civilization, and because of humanity’s immense intellectual power, for the rest of the Earth’s organisms as well. We need to protect science, this important tool, and the insights we derive from it, from being weakened through underfunding or the newly elevated primacy of ideology over reality.
Our work is essential. Let us be lucky enough to do it well.
2 November 2018
Happy Friday, Mountain Beltway readers!
Here is a guest Friday fold to ease you into the weekend:
It comes to us from Corsica via Darrel Cowan of the University of Washington.
I was in a workshop and field trips on Corsica–CourseAlp 2011–and made this photo on the coastal road near Erbalunga on Cap Corse. I present it as a refolded fold in Mesozoic marble of the Schistes Lustrés.
Do you see the twice-folded central layering? Here, I’ll highlight it in lurid shades of green and pink, and trace out the axial surface of the first-generation fold (F1) and the second generation fold that overprints it (F2):
Super cool: what lucky rocks, to have enjoyed folding twice!
Have a good weekend, all.
26 October 2018
It’s been a very Billy-Goaty week for me. Three times since Sunday afternoon, I’ve taken people to the Billy Goat Trail’s “A” loop in C&O Canal National Historical Park. On Tuesday and Thursday, it was my NOVA Physical Geology students. On Sunday, though, it was just my son and me.
Good news! He helped me discover a new fold by exploring some new rock outcrops and climbing on them. He led, I followed, and on the ground there was this:
It’s a nice boy-sized antiform in the Mather Gorge Formation, a sequence of ancient turbidites that were metamorphosed during Taconian mountain-building in the late Ordovician (~460 Ma). Here, based on the variation in thickness of the layers being folded, I think we’re looking at relict bedding. In other places along the trail, metamorphic foliation is the planar fabric that got folded:
Bonus fold: Here, on the shore of the C&O Canal, is another fold/fault complex with a granite dike and some boudinaged vein quartz, too:
Happy Friday! Enjoy the weekend.