The routine

It’s that time of year for me… summer’s here, and I’m winding up my duties at NOVA in preparation for some travels. We leave Sunday night for two weeks in Turkey, followed by my regional field geology course in Montana (also two weeks), followed by some family time and mountain climbing in New Hampshire (three weeks), including hiking the Presidential Range.

This summer, as I have done for the past several summers, I’ll be subletting my home while I’m gone. Among the many disadvantages of living in DC (high taxes, high crime, lots of noise, all those politicians), this is a big advantage: you can sublet your apartment soooooo easily. It’s a cinch! The city’s government and nonprofit sectors draw in swarms of interns every summer, and I’ve been very fortunate to find great subletters via Craigslist to come pay my rent/mortgage and take care of my cat while I’m away.

The first summer I did this was the most extreme: I was gone for three months in 2006 on a road trip up to Alaska and back; but since then I’ve subletted for at least two months each summer, mainly while I was out west. The 2010 summer is the shortest sublet I’ve so far had: a mere seven weeks. Still, the routine each summer is roughly the same: stock up on cat food and litter, pack up my clothes and store them somewhere (mom’s attic; my office at NOVA), clean the place up, and then clear the heck out. The packing has been more complicated this year since I’m essentially packing for three trips with overlapping gear needs all at once. But it’s a nice annual tradition: right about the time that DC gets to be sweltering hot and humid, I can decamp for exotic locales and cooler climes. I feel very lucky not only to travel like this, but also to have my home and cat cared for in my absence, and bring in cash to pay the mortgage, too. It’s a sweet deal.

More immediately, I’ll be in an all-day workshop starting tomorrow night, and through Sunday. It’s a SERC workshop on the role two-year colleges like NOVA play in geoscience education. Between that and packing, this might be the last you hear from me for a while.

Blogging will likely be light around here for the next two months as I’m flitting about. I’ll do my best to log on and post some travelogues when I can, but I can’t promise too much. When I have phone service (will my iPhone work in Turkey?), I can offer a series of short posts to my Twitter account. Beyond that, you’re on your own!

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DC fossil website is now live

Chris Barr’s informative website of the “Accidental Museum of Paleontology” that can be found in D.C. building stones is now live. You should go and check it out, and if you ever visit the city, you can use it as a guide for your tourism.

Uniformitarian

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Heat-stressed map of the Chesapeake Bay / Washington, DC region, as seen at Kenilworth Aquatic Gardens. Looks like mudcracks, eh?

Similar stresses; similar strains.

Vector maps by Eric Fischer


“The Geotaggers’ World Atlas #8: Washington, DC” by Eric Fischer

If you like maps, you should go check out these images by photographer Eric Fischer. (via here) The different colors represent different modes of transportation: Black is walking (less than 7mph), red is bicycling or equivalent speed (less than 19mph), blue is motor vehicles on normal roads (less than 43mph); green is freeways or rapid transit.

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):

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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:

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

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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:

cren_1

…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.

Triassic rifting in the Capitol

My girlfriend’s mom was in town in January, and we took her down to visit the Capitol Building. The tour had a good bit of history, but definitely missed the opportunity to talk geology. I was particularly struck by the columns in the Hall of Statuary:
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Close up of one column, with my hand for scale:
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That’s the Leesburg Conglomerate, a Triassic-aged deposit found in the western part of the Culpeper Basin of Virginia. (Technically, it’s “the Leesburg Member of the Balls Bluff Siltstone.”) The photos I showed readers in May 2008 were from the east side of Route 15, just north of Leesburg itself. The Culpeper Basin is a failed rift valley from the time of Pangea’s breakup. I say “failed” in the sense that it failed to become an ocean basin like the Red Sea or the Labrador Sea. While it may have failed to rend the metamorphic rocks underlying Reston, Annandale, and D.C. from the North American continent, it succeeded in accumulating continental sediments for two periods of geologic time, preserving a detailed record written in siltstones, conglomerates, basalt flows, diabase intrusions, dinosaur footprints and fish fossils.

Among the strata that the basin accumulated, the Leesburg Conglomerate stands out as the real rock star. It’s a gorgeous looking rock, a poorly-sorted and well-oxidized mishmash of (mainly) limestone chunks derived from the weathering of the young Appalachian Mountains. Visually striking as it is, it’s not surprising that someone tried to use it as a building stone. However, it’s not well-suited to being sculpted. Rumor has it that after countless episodes of pebbles popping out of otherwise pristine, finished columns, the column-carver swore he would never touch this particular stone again. To my knowledge, the Capitol’s Hall of Statuary is the only place in the world where the Leesburg Conglomerate has been used as a building stone.

Transtensional quartz vein

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

gsw_sp_FT_09_04

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.”

gsw_sp_FT_09_04_anno

“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:
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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:
MOR

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