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.

Hol(e)y basalt, Batman!

Today, our theme is vesicles. Here are some images of vesicles in basaltic lava flows in the Owens Valley of California, the same spot where we saw the baked fanglomerate that I showcased a few days back.


In this photo (and the zoomed-in detail shot below), you can see a couple of things. One is the size difference of the vesicles as you go up in the flow. Bigger bubbles represent larger loci of low density, and hence will be more likely to rise in a fluid batch of lava. This is the inverse of the phenomenon that causes graded bedding (heaviest grains sinking first). The result is a “graded vesicular lava flow.”

Also visible are several cooling joints that intersect to form columns. At the lower part of these columns, you can see arrest lines perpendicular to the column. Each of these subhorizontal lines represents a single instance of fracture propagation as the column separated from the rest of the flow. In composite, they form a “crack panel” like others showcased here in the past.

Let’s take a closer look at these distinctive features:


…And here’s some big vesicles, big enough to host a Swiss Army knife for scale:


They aren’t as big as some I’ve shown here in the past, but they were the largest vesicles I saw on the Owens Valley Field Forum last September. One thing I find interesting about this batch of vesicles is how they deform one another. The big one in the upper right has several smaller ones above it that “wrap around” its left edge. I envision this as the small bubbles hanging out with ~neutral buoyancy (ascendancy power), when up from below comes this massive bubble. As it pushes up (with its greater buoyancy), they smear out to the side, out of the way.

Likewise with the pair of large vesicles at lower right: it looks like the big flat one was there first, with the smaller “egg-shaped” one rising up from below and impinging on its larger upstairs neighbor. If the lava has been less viscous, the two may have merged into one, as blobs in lava lamps may be seen to do: a minimizing of surface tension, a lowering of the surface-area-to-volume ratio. Why would the smaller impinge on the larger? As I’m envisioning it, there would be a viscosity gradient in the cooling flow, with cooler temperatures towards the top (and hence higher resistance to flow). Deeper in the lava, temperatures would remain warmer, and hence the lava would be less viscous. I’m thinking that the big flat bubble had essentially risen as far as it could, but its top side was cooler than its more ductile bottom side, and so the bottom side was less resistant to the nosy intrusions of upstart bubbles from below.

Do you see anything else worth discussing in these photos?

Easter egg

Searching through my photo archives this morning for something suitably “Eastery”… something in pastel colors, perhaps? … a petrified lagomorph? … how about an egg, or something egg-shaped?

This is as close as I got:


This is in the Owens Valley of eastern California, showing a boulder of the Mesozoic Sierra Nevada Batholith bearing a faulted xenolith. I love outcrops like this, with a combination of primary structures (like the xenolith) and secondary structures (like the fault). And the fault surface appeared to have hosted some fluid flow, encouraging epidotization (hydrous metamorphism) along its surface. How appropriate, considering both the “cracked egg” implication of the round xenolith and the pastel tones of the green epidote.

I’ll annotate it up for you, because I know you love it when I do that:


Happy Easter, folks. Focus on the bunnies and candy, and not the zombies.

Baked fanglomerate

A quick post to share a few images of an outcrop I visited last September out in California’s Owens Valley. This is a spot where alluvial fans coming off the eastern Sierra Nevada were overrun by a basaltic lava flow (Jeff, Kim, Fred, and Kurt for scale):


The unofficial term for these conglomerates deposited by alluvial fans is “fanglomerate,” and it’s pretty cool to see the contact metamorphism at the top of the fanglomerate. There’s also some weakly-developed columnar jointing in the basalt. Here’s an annotated version, in case the contact wasn’t quite obvious enough:


Here’s a close up (Doug for scale), showing the orange zone of thermal metamorphism at the top of the fanglomerate as the lava flow above baked the hell out of it:


Groovy, eh? Where’s your favorite example of contact metamorphism?

Crucifix site 2: Horsts & grabens

So, those sediments we saw yesterday? They’re faulted in the area around the Crucifix Site.

As this image shows, the style of faulting is normal faulting:chalk_bluff_stuff_05

Annotated with some color to jazz things up a bit:chalk_bluff_stuff_05anno

In normal faults, the upper block moves downward with respect to the lower block. They are typical of extensional tectonic settings.


Numerous small faults here — a little bit of displacement on each:chalk_bluff_stuff_01

Ditto for this exposure (same place, just zoomed out a bit):chalk_bluff_stuff_18

Nice! Two horsts and a central graben:chalk_bluff_stuff_02

The normal faults exposed at the Crucifix Site are a lot like those we observe a kilometer to the north, up on the Volcanic Tableland. The difference is that the Volcanic Tableland faults disrupt the stiff upper layer of the Bishop Tuff (“Ig2″), while these Crucifix faults are smaller and more subtle — cutting through weaker stuff.

Crucifix Site 1: Sediments

On the September 2009 GSA field forum in the Owens Valley, the final stop of our first day was to check out the so-called “Crucifix Site,” along Chalk Bluff Road (north of Bishop, California, at the southern margin of the Volcanic Tableland). It’s called the “Crucifix Site” because there is a metal cross erected there:chalk_bluff_stuff_13

This is the site of some pre-Bishop-Tuff volcaniclastic sediments. The place is interesting on several levels, including the sediments themselves, and the subsequent deformation they have experienced. Here’s a look at the outcrop:chalk_bluff_stuff_03

Some annotations help to call one’s attention to primary sedimentary structures and interpretations:chalk_bluff_stuff_03anno

Lots of the sediment itself was made of little beads of obsidian, usually surrounded with a “chalky” weathering rind:chalk_bluff_stuff_07

A cool little channel cross-section was visible, too:chalk_bluff_stuff_04

For some reason, this is what pops into my mind when I run into a well-exposed Cheshire channel cross-section:

Birds and wasps had tunneled into the softer layers, resulting in horizontal rows of holes. I tried to ignore these modern bioturbations so I could focus on the ancient tale in the rocks themselves. Some cool soft sediment deformation was visible, like these flame structures (upper part of the central gray layer):

Zoomed in on a pair of flame structures, and the down-sagged material between them:chalk_bluff_stuff_11


…And another set:chalk_bluff_stuff_12


This was odd: The lowermost layer (upon which my field notebook rests) is unperturbed, but the layers above it are all churned up in one small area (center), flanked by a couple of bird holes:

Zoomed-in on the area in question:chalk_bluff_stuff_09

Annotated, for your viewing pleasure; green is top of undisturbed layer; red shows boundaries of zone of disrupted sediment:chalk_bluff_stuff_09anno

I would be pleased to hear from anyone who knows more about sedimentology than me about the wavy bedding in the second (& third) photo, and this weird sediment disturbance in the lowermost photo. Also: with the flame structures, it looks like coarser material in the lower layer (gray) is the less viscous participant, while finer-grained (white) material is sinking downwards. Isn’t this the opposite of the way it usually works?

Plutonic contacts in eastern Sierras

Last September, at the location of the faulted moraine (eastern Sierra Nevada, California), I took some photos of some of the sexier plutonic contacts exposed in big boulders (erratics) of the glacial till composing the moraine. Check them out. What do you see here?




Transtensional quartz vein

On last May’s GSW spring field trip to Chain Bridge Flats, I saw a quartz vein:


Surely, upon looking at this photograph, you will be struck by the way the vein is not the same thickness along its length, and parts of it appear to be a white line transitioning into a parallelogram, and back into a white line again. What, you make ask, gives?

I think what you’re looking at here is a transtensional quartz vein. Like all veins, this one formed when the host rock (in this case, metagraywacke of the “Sykesville Formation”) cracked open and hot fluids squirted into that fracture. Elements dissolved in the fluid organized themselves into mineral crystals, and precipitated in the void space of the crack, sealing it shut with quartz “glue.”


“Transtension” is the word used to describe a kinematic regime which contains elements of transform “shear” (in this case, right-lateral) and tensional stress. Because of the jagged shape of the fracture here, some parts of the fracture are grinding past their neighbors, while other parts are dilating. The dilating parts are only dilating because of the shape of the fracture. The actual motion of the blocks of rock is uniform and non-rotational. We call these little pulling-apart areas “releasing bends.”

On a much larger scale (lithosphere-scale), releasing bends near the surface create pull-apart basins like the Dead Sea. Deeper in the crust, pull-aparts may serve to accommodate pluton emplacement, as has been suggested by Tikoff & Teyssier (1992) for the Tuolumne Intrusive Suite of the high Sierra in California.

This “part-sliding, part-extension” pattern is actually quite common. Here’s another example, this one in a brick sidewalk on Capitol Hill:

The same pattern also shows up at the Mid-Atlantic Ridge, where the extensional segments (north-south-oriented) are sites of new oceanic crust being formed, and the fracture networks (east-west-oriented) are sides of transform faults, where the South American Plate slides laterally past the African Plate:

Where else have you seen this pattern? Use the comments section to share an example or two.

Dalmatian pluton

Continuing with some photos from eastern California…

After checking out the faulted moraine, but before heading up the hill to check out the indurated shear zone (which you can just see in the background of this photo), we stopped to check out this visually-striking outcrop:


Look at the glee on the faces of Kurt (green shirt) and Marcos (running; he’s so excited!). We were all pretty jazzed by this polka-dotted outcrop. But in spite of being titillated, we weren’t quite able to figure this thing out.

Here’s Jeff expressing his understanding of just what we’re looking at:


Wes Hildreth, our Bishop Tuff man from the USGS, takes a closer look:


So here’s an even closer look, with my pen for scale:


We’re looking at two main rock types: a dark colored one which is present in grapefruit- to basketball-sized chunks, most with high sphericity and round cross-sections, and a light-colored granite which was present between the dark orbs. There were some more angular dark chunks, too.


Our best bet was that these were xenoliths, stoped off the edges of the granitic magma chamber as it intruded, then piled up on the floor of the magma chamber. If you went “in” to this rock face by a few feet, the ‘xenoliths’ disappeared and you would be swimming in pure granite. The problem with the chamber-floor-debris-layer interpretation is that this “layer” of putative xenoliths was oriented subvertically. So if it represents the floor of an ancient magma chamber, then the whole pluton would have to be rotated by ~90° in order to produce this particular outcrop.

An alternative explanation is that the subvertical orientation of this layer of dark blobs was the wall of a magma chamber, and we’re looking at an outcrop face that was ~parallel to that wall. Probing fingers of granitic magma were working their way into the surrounding rock, perhaps thermally eroding it as they went, producing rounded shapes. Later, once everything had solidified, a fault developed through the transitional zone which divided pure pluton from pure wall rock, and gave us this dual-composition outcrop.

I think I prefer the xenolith interpretation, but to be honest, none of us were really sure what was going on here. We were all struck by the beautiful polka-dot pattern, but among this crew of 30 professional geologists, not one could say for sure just what the hell was going on with this outcrop.

…And I think that’s pretty cool.

Mini debris flow


Small-scale debris flows as a muddy slurry flowed over weathered-out chunks of the Bishop Tuff. Probably about two days old. Southern edge of the Volcanic Tableland, last September.


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