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14 January 2019

A tale of three iguanas

As noted last week, I spent the week spanning New Year’s Eve in the enchanted isles of the Galapagos. The previous week (over Christmas) my family and I were in coastal Ecuador. I saw a total of three species of iguanas in the two locations, and they offer a neat little story of evolution. Let’s take a look.

First, let’s introduce the key players:

The green iguana, Iguana iguana
(coastal Ecuador, and many other places around the tropical Americas):

The land iguana, Conolophus subcristatus
(Galapagos only, in low-elevation inland locations where there are plants growing):

The marine iguana, Amblyrhynchus cristatus
(Galapagos only, along the shorelines):

What do these three species have in common? What differentiates them? What lessons do they have to teach us?

The green iguana likes trees, as you’ll see here:

It is a vegetarian, eating leaves and according to my observations, seems to especially enjoy hibiscus flowers!

Now let’s journey a thousand kilometers to the west, and examine the two species of iguana that are only found in the Galapagos Islands. The islands are of volcanic origin, and have never been connected to any continent. We think they have been formed (and continue to be) due to a volcanic “hotspot.” Like Hawai’i, they show an age progression – youngest in the west (closest to the East Pacific rise, where the Nazca plate is being formed through seafloor spreading) and older to the east (headed toward the subduction zone that has helped build the Andes on the western margin of the South American plate and continent).

We begin with the land iguana, which is most similar in lifestyle to the green iguana in that it lives 100% of its time on the land, and eats arboreal vegetation.

The differences in color and proportions are apparent at a glance.

What’s not necessarily apparent is that the land iguanas of the Galapagos don’t climb. Their ancestors gave up the arboreal acrobatics we see in the green iguana, and instead confine themselves to the ground in the dry lowlands of several of the main islands (though not others, as I discuss in a separate post).

That’s a bit of an issue, since this is what they eat:

That’s a prickly pear cactus (Opuntia). It makes up about 80% of their diet, though they’ll eat other vegetation, and have been documented opportunistically eating bugs and carrion, too. In the mature prickly pear, there can often be a significant vertical distance between the ground and the delectable fleshy paddles and fruit, as my wife Lily here demonstrates:

So the land iguanas have to either resign themselves to shorter, younger cactus, or to the chunks that fall due to being undermined by the vandalism of the cactus finch, which can fly to the upper part of the cactus and then jab its  massive beak repeatedly into the flesh:

This “bird-on-plant” violence likely generates some airborne leavings which gravity delivers to the lower levels of the ecosystem….

As a result, this land iguana seems to be saying, “Yum; thanks!”

They are handsome beasts. I regret the fact that these photos lack a sense of scale, but this individual for example is over a meter long:

The marine iguana is also a vegetarian, but with a twist: unique among modern lizards, it forages for its food in the sea! They have long, sharp claws that aid in clinging to rocks on shore, battered by waves, or underwater in powerful currents. In the water (for the big males) or in the intertidal zone (for the smaller individuals), they eat algae.

They are most often dark gray, which not only provides camouflage on the basalt of the islands’ coasts, but also due to its low albedo, helps them regain functional temperatures after being immersed in the water surrounding the islands, which is quite cold due to the Humboldt Current. Their heads often have a grayish tonsure, as salt crystallizes there, expelled through the nostrils in audible blasts by special glands that clean their blood of excess salt ingested during feeding. Their tails are relatively bladelike (laterally compressed, relatively vertically extended) so that, swept from side to side, they are useful for submarine locomotion, like the tail of a fish.

Most of the marine iguanas I observed spent a lot of time just lazing about, soaking up the rays:

Darwin found their appearance repellent, but I think they have a certain pug-like dignity to them.

My wife thought so, too. Here she is conversing with a rapt trio of them:

…Though they don’t appear impressed with what she has to say:

Several had alternative colorations that reminded me of forest camouflage, as deer hunters wear in the Appalachians each fall:

On Española Island, the marine iguanas have a distinctive “Christmas tree” color scheme of “red” and “green.”

Clearly, if the interpretation that the prevailing dark gray coloration is an adaption for heating up or camouflage on lava flows, then one of those variables doesn’t apply as acutely on Española as elsewhere. Or else an additional variable applies on Española Island that isn’t manifested as acutely elsewhere, overwhelming the influence of the previous variables, or perhaps sexual selection is more important among Española marine iguanas? Anyhow, they are gorgeous looking little demons:

There’s also a pink iguana in the Galapagos, but it’s only on a single remote island (Wolf) and has less than 100 individuals in the population, so I didn’t get to see that. It’s most closely related to the land iguana.

How are these species (actually genera!) related? They are all descended from a common ancestor iguana. The two Galapagos species represent a clade, a group of more related species, while the green iguana is more distantly related.

Using molecular genetics (comparing parts of their DNA sequences), Rasmann (1997) estimated the age of divergence between the Galapagos marine and land iguanas at ~10 Ma, which is significant because that’s more than double the age of the oldest islands in the archipelago. Assuming there was only 1 ancestral iguana population* that washed ashore on the archipelago, it implies that landing and the subsequent divergence took place on an island that no longer exists, now sunk (+ eroded) out of sight to the east, and that the lizards have used the Galapagos Islands as stepping stones, hopping to the west and colonizing as new islands emerge from the seafloor in the west, abandoning the older ones to the east (or alternatively, going down with the ship).

* “population” here would mean at the very least 1 pregnant female, and perhaps upwards to 1 female and 1 male, or multiple individuals.

So the implication would be something like this:

More recently, MacLeod, et al. (2015) also made an estimate of the divergence between not only these two groups but also included related lizards, including the genus of the green iguana. I present a modified form of their Figure 1 below:

Note the red lines I’ve added, which show the land and marine iguanas diverging much more recently, ~4.5 Ma (shorter red line to the right; this is about the age of the oldest igneous rocks on the oldest islands in the archipelago), and the last common ancestor of green iguanas, land iguanas, and marine iguanas as existing ~14 Ma (larger red line to the left). Note also that the green iguana (Iguana) is not the most closely related group of living lizards to the Galapagos clade (i.e., Conolophus + Amblyrhynchus). Instead, that honor falls to the spinytail iguanas, Ctenosaura.

Okay, that’s probably enough for one post… I think it’s time to…

References cited:

Amy MacLeod, Ariel Rodríguez, Miguel Vence , Pablo Orozco-terWengel , Carolina García, Fritz Trillmich , Gabriele Gentile, Adalgisa Caccone, Galo Quezada, and Sebastian Steinfartz. “Hybridization masks speciation in the evolutionary history of the Galápagos marine iguana,” Proceedings of the Royal Society B; 22 June 2015. https://doi.org/10.1098/rspb.2015.0425

Kornelia Rassmann, “Evolutionary Age of the Galápagos Iguanas Predates the Age of the Present Galápagos Islands,”
Molecular Phylogenetics and Evolution; Volume 7, Issue 2; 1997, Pages 158-172. https://doi.org/10.1006/mpev.1996.0386.

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11 January 2019

Friday fold: more from Utah

Octavia Sawyer offers us up another folded scene from Utah for our first Friday fold of the year:

What a beauty that is! I love those colors.

Octavia says:

I’m still sorting through all my hiking photos from last summer, and I just came across this set. This location is northeast of the huge anticline in Slate Canyon that you posted in September. The first photo (A) shows an anticline in the left foreground that looks a lot like the one from September. It’s not the same exposure, although it’s the same kind of rock (Big Cottonwood fm). The situation behind and to the right of the anticline is what I was actually trying to photograph. It looks like maybe another anticline(s) that got faulted after it was folded? The way it appears to undulate into and out of the ground reminded me of those sea serpents they used to draw on Renaissance maps. Photos A and B were both taken from halfway up the opposite (north facing) wall of the canyon; photo C was taken from the bottom of the canyon, looking downhill toward the “serpent.” The colors are a little wonky in the third photo; at that altitude (over 6,000 ft) the light is so bright that my camera sometimes has trouble dealing with it.

Zooming in on the upper right [her third photo], where these delightful parasitic folds (“the sea serpent”) can be seen:

Thanks for sharing, Octavia!

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8 January 2019

Geology of the Galapagos Islands

I spent a wonderful week over the new year in the Galapagos Islands, and saw many wondrous things there. Many of those fall into the realm of biology, but I wanted to take the time today to organize some of my observations about the geology of the place. It’s mostly volcanic, with a tangent into the sedimentology of beach sand. Enjoy these many photos, and I hope you are inspired to visit this place yourself someday…

Let’s begin with a prominent volcano called Daphne, east of Baltra (where the airport is) that I saw both from the air…

…and from the deck of the ship, once we had sailed:

We didn’t stop there, but I’m guessing that’s a tuff cone, a big pile of pyroclastics that built up from submarine eruptions, a la Surtsey in Iceland.

Continuing with the pyroclastic theme, here’s a look at Bartoleme, where tan tuff hosts a lens of something white. I note the curved bottom and flat top to this unit, and the red shadow of contact metamorphism beneath it, and so I think it’s a pyroclastic flow deposit. I suppose it could also be a rhyolite flow. Unfortunately, I haven’t been able to successfully fact-check this “armchair” geologizing from the resources available on the Internet, so I welcome your insights.

(Note also the basalt in the foreground. That’s the Sullivan Bay flow, of which more in a moment.)

Let’s stick with the tuffaceous islands for the moment. Kicker Rock is the eroded remains of a tuff cone from a violent Surtseyan-style eruption.

Zoom in closer to see the prominent vertical fracture set (as well as some boats for scale) as well as the subtle traces of pyroclastic bedding layers sloping to the left on the left, and to the right on the right:

Here’s a set of 6 photos (no scale, alas) of seaside cliffs across the water at Cerro Brujo (“Witch Hill”) showing similar tan tuff with black basalt inclusions:

Here is a cobble I found on the beach south of Cerro Brujo, where one of these dark, angular chunks is shown as an inclusion in the tan-colored tuff:

The cobbles of tuff weathered from these cliffs rapidly round into little ping pong balls which are most numerous close to the cliffs, mixed in with basalt cobbles, shells and sea urchin spines (long purple elements):

The sand on this beach has a significant olivine fraction:

The majority of the beach sand we saw was shell and coral bits: soft, white sand. The contrast with seaside outcrops of basalt can be quite striking, as here at Cerro Dragon:

Or here, at a beach on the shore of Santa Cruz:

A few examples of the sand in close-up:

This beach sand can get lithified and make coquina. In this case, here’s a storm- (or tsunami?-) tossed slab, several meters above sea level, with a boulder-sized inclusion of basalt of the left side.

Three close-up shots of textural details in the coquina:

But mostly, what I saw was basalt. Some of the basalt was decidedly porphyritic, with big chunky feldspar phenocrysts:

And some of those were zoned in a most delightful way:

The basalt was frequently vesicular in outcrop, as seen here, with a Sally Lightfoot crab for scale:

In some cases, like at Sullivan Bay, it is glorious pahoehoe basalt from a flow about 200 years ago:

There, basalt has oozed out over an orangey scoria, to dramatic effect:

It has built out a broad plain that is full of interesting details. Hiking over it with my family was the geological highlight of the trip for me:

The textures here were so photogenic; it was difficult to stop capturing them with my camera.

I find these overlapping “lava toes” (little basalt lobes) so charming:

Or how about this extraordinarily consistent braided texture?

Note the “brittle deflation” evidence seen here, these concave collapse features accommodated by fractures in the pahoehoe “skin” as the molten rock beneath flowed out of the way and stopped propping up the “ceiling.”

In this exposure, there’s a neat texture that reminds me of thin sections of schist, with its mica-rich and quartzofeldspathic domains. Note that many of the less-wrinkled areas host some sort of inclusion within them: these are potentially chunks of scoria or vesicular basalt that were there prior to the most recent eruption, and entrained in the flow, providing stiffness and strength to the surface layer as flow forced it to fold:

Speaking of inclusions, here are several other elegant inclusions in the Sullivan Bay basalt flow…

A vesicular basalt:

…a block of orange scoria:

And the tip of a bigger block in the lava’s subsurface, making a petrified eddy in the stream of the pahoehoe’s flow:

There were several spots where cracks in the lava apparently served as fumaroles, acting as conduits for steam vented from the cooling flow or perhaps water from whatever lay beneath it:

Here are some nice examples of big vesicles, distorted by the flow into ellipsoidal dimensions, some of them intact and some ruptured in the passage of time:

Another spectacular feature was this hornito (“little oven”) which my son (and his hammerhead shark toy) examine in this photo:

Very little in the way of life has colonized the Sullivan Bay flow in its two centuries of existence. We saw a lone Galapagos penguin at its edge, and here’s a cactus trying to establish a foothold:

Other than that, it looked pretty pristine. What a great thing to be able to see and traverse in person!

Next up (now that we’ve covered the rocks), I’ll share some imagery of the plants and animals I saw on my trip…

 

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1 January 2019

2018 Yard List

New year’s day is the time I tally up and report the bird species seen in my yard on the forested slope of Massanutten Mountain in Shenandoah County, Virginia. This is my seventh such annual list.

Here are the previous iterations:

It’s been a good year. Two new “seen for the first time ever” species for the yard are noted in bold type.

  1. Carolina wren
  2. American goldfinch
  3. Chickadee
  4. Dark-eyed junco
  5. Red-bellied woodpecker
  6. Mourning dove
  7. American robin
  8. White-breasted nuthatch
  9. Barred owl
  10. Pileated woodpecker
  11. Brown creeper
  12. Tufted titmouse
  13. American crow
  14. Downy woodpecker
  15. Red-tailed hawk
  16. Eastern phoebe
  17. Eastern bluebird
  18. Turkey vulture
  19. Pine warbler
  20. American raven
  21. Golden-crowned kinglet
  22. Sharp-shinned hawk
  23. Bald eagle
  24. Hairy woodpecker
  25. Yellow-bellied sapsucker
  26. Hermit thrush
  27. Black vulture
  28. Blue jay
  29. Red-eyed vireo
  30. Northern flicker
  31. Whippoorwill
  32. Brown-headed cowbird
  33. Wild turkey
  34. Canada geese
  35. Chipping sparrow
  36. Cooper’s hawk
  37. Red-shouldered hawk
  38. Pine siskin
  39. Blue-headed vireo
  40. Yellow-rumped warbler
  41. Ruby-throated hummingbird
  42. Ruby-crowned kinglet
  43. Blue-gray gnatcatcher
  44. Broad-winged hawk
  45. Eastern towhee
  46. Indigo bunting
  47. Tree swallow
  48. Northern cardinal
  49. Ovenbird
  50. Rose-breasted grosbeak
  51. Magnolia warbler
  52. Eastern wood-pewee
  53. Yellow-billed cuckoo
  54. Scarlet tanager
  55. Blackpoll warbler
  56. Great crested flycatcher
  57. Chimney swift
  58. Black-throated green warbler
  59. Purple finch
  60. White-throated sparrow

It was a respectable year on the yard bird count front. I hope for more quality time in my yard ogling my fine feathered friends in 2019…

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21 December 2018

Friday fold: Samaria Gorge, Crete II

Jay Brodsky, the new Chief Digital Officer for AGU, shared this photo of a folded outcrop he saw in Samaria Gorge in western Crete:

That is a spectacular outcrop! The folds’ axial planes are more or less horizontal, so we’d be justified in calling these “recumbent” folds. I love, the 3D exposure; how we see them sectioned both in profile and also parallel to the bedding plane. Very, very cool.

You know, the very first Friday fold ever (>8 years ago) was also from Samaria Gorge in Crete. My wife Lily was the source, before she was my wife! Jay suggested I get her to take me back there, and I’m starting to think he may be right.

Happy Friday to you, and happy holidays, too.

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19 December 2018

The 50 State Fossils: A Guidebook for Aspiring Paleontologists, by Yinan Wang

Here’s a kid’s book to consider for the holiday season: The 50 State Fossils: A Guidebook for Aspiring Paleontologists, written by Yinan Wang and illustrated by Jane Levy. It has a simple structure: each state gets a page, and that page is divided into four parts: a map of the state w/ areas highlighted showing where the fossil can be found, an illustration of the organism as it looked when it was alive, in a cartoony sort of style, a representative photograph of the fossil, and a few paragraphs of text describing the organism, its geological setting, and superlative aspects of its discovery or lifestyle. Not every state has a state fossil, but Wang rolls with the punches, presenting “state dinosaurs” where they occur, and the (fossil-derived) “state stone” in one case. For states where there are “none of the above,” Wang makes suggestions as to what would be an appropriate choice, considering the geology of the state, as well as proposed bills to designate state fossils that never made it out of their respective legislatures. This is a useful compilation of information about the range of organisms which are embraced / celebrated as state fossils. Duplicates become very apparent when reading through the whole book (as my son and I did on Monday): a lot of mammoths, a lot of trilobites, a lot of crinoids, and a lot of dinosaurs. The unique choices really stand out: belemnites (Delaware), the Tully monster (Illinois), petrified palm wood (Louisiana), and Jefferson’s ground sloth (West Virginia). My own state, Virginia, has the elegant Pliocene scallop Chesapecten jeffersonius as its state fossil for many good reasons. I have a couple of personal connections to the story of that bill, and am pleased that its story is included in the brief description. In 1687, it was the very first fossil from the Americas to be illustrated in the scientific literature. So illustrating it is key to its selection, and it pains me to say that the photo used to illustrate the fossil appears to be another species in the Chesapecten genus, perhaps C. middlesexensis or C. madisonius. You can count the ribs – C. jeffersonius should only have 8 or 9 ribs, but the individual shown in the photo has 12 ribs. I found one small typo too. These are relatively minor details, and for the intended audience of paleophile children, I don’t think they matter in any essential way. I think the book would make an admirable addition to the bookshelves of young paleontologists. My son (6 years old) really enjoyed it, and has gone back to it several times since to leaf through and re-examine the critters. He’s particularly taken with Shonisaurus, the Nevadan icthyosaur!

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14 December 2018

Friday fold: Titus Canyon Anticline

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.

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7 December 2018

Welcome to D.C. for #AGU2018

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!)

Many of the same sites were shared with AGU’s Lily Strelich, who put out this awesome “Self-Guided Tour of the Geology in D.C. Buildings” (with accompanying Google Map) in this week’s EOS.

Enjoy these resources, and enjoy visiting Washington, D.C. I hope to see/meet some of you at the Fall Meeting.

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23 November 2018

Friday fold: loopy limestone in Utah

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!

 

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21 November 2018

Brittle overprinting ductile in the South Mountains metamorphic core complex, Phoenix, Arizona

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.

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