Transect debrief 6: folding and faulting

Okay; we are nearing the end of our Transect saga. During the late Paleozoic, mountain building began anew, and deformed all the rocks we’ve mentioned so far. This final phase of Appalachian mountain-building is the Alleghanian Orogeny. It was caused by the collision of ancestral North America with the leading edge of Gondwana. At the latitude of Virginia, that means northwestern Africa (Morocco and/or Mauritania).

Whereas the first two pulses of Appalachian mountain building were relatively provincial affairs, this Alleghanian phase was a full-on continent-on-continent smackdown. The Himalaya (India colliding with Eurasia) would be a good modern analogue for the Pennsylvanian and Mississippian Appalachians.

When I was live-blogging the trip, I posted this photo of Judy Gap:

It was a bit hard to get it all into one measly iPhone frame (hence the tilted angle: those trees are in fact vertical!), but what you’re looking at here is the erosion-resistant Tuscarora Sandstone (Silurian in age; quartz-rich beach deposits) that outcrop as a ridge. However, here at Judy Gap, there are two ridges. What gives? This is where I was introduced to a new term that is apparently becoming a common phrase in the structural geology literature: contraction fault.

The story most Physical Geology students get about fault types is that tectonic extension causes normal faults, while tectonic compression causes reverse faults. Contraction faults are faults that display an apparent “normal” sense of motion, but were caused by a compressional tectonic regime. How the heck does that work, you may ask? Consider the following diagram:

So the deal with contraction folds is that they might start out “reverse” but are then rotated and tipped over as deformation proceeds. The former footwall becomes the new “hanging wall,” and the sense of motion is obscured by this new orientation. This means that they do represent contractional strain, but a freshman geology student is unlikely to spot it at first glance.

The Germany Valley to the east of Judy Gap is a big breached plunging anticline, as I attempted to show with this iPhone photo from the Germany Valley Overlook along Route 33:

It’s a bit easier to see if you jump up in the air 10 kilometers or so. Fortunately, that’s precisely why God created Google Earth:

The valley is hemmed in by a big V-shaped fence of mountains, all held up by the Tuscarora. It’s tough stuff. During Alleghanian folding, the crest of the anticline was breached, and water was able to get inside and gut the weaker rocks. The quarry annotated in the photo is mining the same Cambrian and Ordovician carbonates seen in the Shenandoah Valley back in Virginia (Lincolnshire and Edinburg Formation equivalents). A pattern geologists have noted with eroded anticlines is that older rocks are exposed in the middle of the structure, with younger rocks flanking them along the sides.

So that’s a glimpse of the big picture of deformation in the Valley & Ridge, but we can also see cool deformation at smaller scales… Stay tuned…

Transect debrief 4: transgression, passive margin

…So where were we? Ahh, yes: an orogeny, and then some rifting. What happened next to Virginia and West Virginia? Let’s consult the column…

After the rifting event opened up the Iapetus Ocean, seafloor spreading took place and tacked fresh oceanic crust onto the margin of the ancestral North American continent. As North America (“Laurentia”) moved away from other continental fragments (Congo craton, Amazonia craton), it got a little bit calmer ’round these parts. From the continent’s perspective, the spreading center moving further and further offshore.

This shift of the tectonic locus out to the middle of an ocean basin means that the edge of the ancestral North American continent could finally relax a bit. The magmatic intrusions became a distant memory, and the crust cooled, contracted a bit, and sank. This subsidence allowed seawater to lap up onto the edge of the continent, and with the seawater came sediments. Rivers draining the exposed North American continent brought sediments to the sea, and dumped them. We geologists call this “passive margin sedimentation,” and it results in relatively “mature” sediments: those that have been well-worked over, typically rich in quartz and well sorted and with more rounded component grains.

As time went by, the edge of the continent subsided more and more, and any given spot in the modern-day Blue Ridge transitioned from streams to beach to continental shelf. The sedimentary stack reflects this increasing distance from the shoreline: a transgressive sequence.

It starts at the bottom with Weverton Formation: conglomerates and sandstones (and as I discovered on the Transect Trip, siltstones too). Here’s a piece of the Weverton from a GSW trip several springs ago:

The Weverton is overlain by muddy deposits of the Harpers Formation, which can also include coarse sandy units, as I learned on the Transect Trip. Here’s a shot of the Harpers Formation at Harpers Ferry, West Virginia, the type locality. This was taken five years ago, back when I had just gotten out of grad school, and spent a year teaching at George Mason University (pre-NOVA). [The student pictured is Steve Elmore, who just earned his master’s from GMU, working with Bob Hazen. Congratulations, Steve!]

The Harpers is really important, because it contains some Olenellus trilobite fossils, which constrain its age to be Cambrian.

The Harpers is overlain by another sandstone: a clean, pure quartz package named the Antietam Formation. For me, the Antietam is a favorite local rock, because it is studded with Cambrian-aged Skolithos trace fossils. On the trip, I used the iPhone to upload a few photos of these, but here’s a higher-resolution image to savor:

You’re looking at the bedding plane of the Antietam in the above image, with your sight-line parallel to the paleo-vertical orientation of the tubes. Wow. Beyond all reason or deeper interest, I just love Skolithos tubes. I look at this outcrop, and I wonder: is this a palimpsest? or a small wormy Manhattan? In other words: was this multitude of burrows generated by a small population that dug in the same area over a long period of time, or by a huge population living cheek-to-jowl over a relatively brief moment?

Regardless, the sand-then-mud-then-sand-again picture painted by the succession of Weverton-Harpers-Antietam isn’t a “textbook” transgressive sequence, but it might make more sense if you consider the Antietam sands as barrier island deposits, with the Harpers being deposited in a Pamlico-Sound-type setting.

Finally, the transgression is complete when we get to the top of the Blue Ridge sequence and see the Tomstown Formation, a carbonate unit:

IMAGE CREDIT: USGS

The Tomstown tells of a time when sea level had gotten so high locally that the shoreline was way, way, way far away. There were no clastic sediments making it out to this location, and all that was available to precipitate were the ions dissolved in the seawater. No sand, no pebbles, no mud: only carbonate.

The sequence of sedimentary strata continues, but to follow its succession upwards, you’ll have to travel across the Blue Ridge Thrust Fault to the west, into the Valley & Ridge province. More on that in the next post. For the moment, let me share a cartoon sequence of images by Tom Gathright (1976), showing the overall stratigraphic evolution of the Blue Ridge province*:

*Note that Gathright used the outmoded names “Hampton” instead of Harpers, and “Erwin” instead of Antietam. Please forgive him and move on.

That last panel, showing Alleghanian deformation, is something we will attack in a future post. For now, I’m satisfied to have finally climbed to the top of the Blue Ridge stratigraphic stack.

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Gathright, Thomas M., 1976. Geology of the Shenandoah National Park, Virginia. Virginia Division of Mineral Resources Bulletin 86, Charlottesville, VA. [buy it from SNPA Bookstore]

Transect debrief 3: Rodinian rifting

The Grenville Orogeny, responsible for Virginia’s basement complex, was one mountain-building event among many that helped put together a Mesoproterozoic supercontinent called Rodinia. But Rodinia didn’t last: it broke apart during the Neoproterozoic to form the Iapetus Ocean basin. This rifting event is recorded in Virginia’s Blue Ridge province in the Swift Run Formation and the Catoctin lava flows.

It’s probably about time to start putting some of these rock units in stratigraphic context. Here’s my redrawing (and updating) of a cartoon Shenandoah National Park stratigraphic column based on an original by Tom Gathright (1976):

You’ll notice here that the Swift Run Formation is interbedded with the Catoctin Formation, a Neoproterozoic series of lava flows fed by fissure eruptions (kind of like what’s happening this week in Iceland).

Trickling downhill away from these fissure eruptions would have been flows of basaltic lava (tholeitic, indicating a mantle source chemistry). If you want a warmer modern analogue than Iceland, look to the Afar Triangle region of Ethiopia:

As with Neoproterozoic Virginia, the continental crust of modern Ethiopia is stretching, opening up topographic grabens which are being filled with clastic influx from the surrounding highlands and mafic lava which is formed from decompression melting in the underlying mantle, and funneled to the surface via feeder dikes. In places you will see streambed conglomerates interlayered with the mafic lava flows, and in places there are tuffs and rhyolites that are a (volumetrically-small) part of the package. Elsewhere there are lake sediments. The two bear a common geologic signature, despite being separated by thousands of miles and half a billion years. There’s that refrain again: Same as it ever was, same as it ever was.

Once on the surface, the lava cooled, and in some places, columnar jointing formed:

The cooling age on some of the rhyolitic upper units in the Catoctin Formation is 570-565 Ma (Rb/Sr on pyroxene by Badger and Sinha, 1988). Some mafic and felsic dikes (could be feeders) associated with the unit yield the same age via U/Pb (Aleinikoff and others, 1995).

At some point, ancestral North America (“Laurentia”) drifted away from the spreading center, and volcanism ceased. The crust cooled, subsided, and then a sequence of sedimentary rocks began to accumulate atop the cooled lava flows. This transgressive sequence of sediments (the Chilhowee Group) is the next thing up in the stack. More on that later.

Transect debrief 2: weathering the Grenvillian landscape

From the basement complex, the next unit up in the Blue Ridge province’s stratigraphic sequence is the Swift Run Formation. It rests atop an erosional unconformity. After the Grenville Orogeny (~1.1 Ga) added a swath of new crust along the margin of the North American continent, the landscape began to weather and erode. Eventually, an episode of rifting broke open rift valleys and a new ocean basin, the Iapetus. The Neoproterozoic rift valleys filled with sloughed-off detritus from the exposed Grenvillian rocks (granitoids, mainly), resulting in arkosic sediment. This arkose is mixed in with muddy layers: it looks very much like the much-younger rift valley sediments in the Culpeper Basin (Triassic rifting for those, not Neoproterozoic). This is the principle of uniformity at work. The same tectonics yield the same signature, even though they happen at different times. Same as it ever was, same as it ever was.

Here’s a reposted iPhone photo of some of the Swift Run, showing rip-up clasts of mudstone in the arkose:

Some of it is conglomeratic, with rounded quartz pebbles surrounded by immature-composition sand (reposted iPhone photo):

Later, during Paleozoic mountain-building (Alleghanian Orogeny), the Swift Run developed a penetrative cleavage. Here’s a photo showing bedding and cleavage intersecting in the Swift Run:

Annotated:

This is a cool outcrop: In spite of being polka-dotted with lichens, it shows primary bedding truncations (a primary geopetal sedimentary structure that tells us that up is “up” in this photo) as well as a small S-fold (top to the left) that probably resulted from Paleozoic Alleghanian deformation:

Annotated:

In spite of small folds and well-developed cleavage, I was shocked when someone on the field trip noticed this:

That’s two recumbent isoclinal folds! Annotated:

These folds may be just a local phenomenon formed as one layer of the Swift Run slipped over its neighbor… but they also may hint that deformation is more pervasive in this unit than a cursory glance would indicate. Quite interesting, if you ask me.

Take home lessons: (1) The Swift Run Formation is a post-Grenville rift-related sedimentary deposit. It is compositionally and texturally immature. (2) The Swift Run, like everything else in the Blue Ridge province, got deformed millions of years later during the Alleghanian phase of Appalachian mountain-building.

Transect debrief 1: starting in the basement

It is time to debrief the post-NE/SE-GSA field trip that I went on, affectionately dubbed the “Transect Trip” for the past 27 iPhone-uploaded “live”-geoblogged posts.

First off, I’d have to say that I enjoyed the live-field-blogging experiment overall, though I’ve got some critiques of the process and products. I think it’s amazing that I can upload photos and short blog posts from my iPhone to this site with a minimum of hassle. However, I can’t do much more than that. It’s not as easy to tag the posts or geotag the photos. I can’t compose annotations. In fact, I can’t even be sure the photos will be in focus, since the iPhone camera is a static lens. And there’s no macro feature on the iPhone camera, a source of some frustration for a guy like me that likes to photograph small things. Further, typing with my thumbs is laborious, keeping the live-geoblogged posts on the terse side.

So, when I asked what readers thought of the whole enterprise, I wasn’t surprised to get feedback that it would be nice to put things in a bit more context. I aim to start that process today, with the first rock we encountered, a charnockite (orthopyroxene-bearing granitoid). The rock type is named for Job Charnock, founder of Calcutta, India, whose tombstone is made of charnockite:

Charnockites are common rocks in the core of Virginia’s Blue Ridge “anticlinorium.” Here’s a nice photo of a fresh sample, showing the rusty/clayey weathering “rind” on the sample:

Compare that image with this version, the original that I uploaded from the field trip via my iPhone:

Pretty profound difference in quality, eh?

So, here’s the deal with these charnockites. Volumetrically, they are a big part of the “basement complex” that cores the Blue Ridge. There are also a bunch of other flavors of granitoid down there; about fifteen discernible rock units in all. Our understanding of the basement complex has gotten a thorough re-working in recent years thanks to the coordinated efforts of many geologists who have focused on reexamining the Blue Ridge. Chief among these scientists in Scott Southworth of the USGS in Reston, who led an effort to remap the area in and around Shenandoah National Park. Dick Tollo (GWU), Bill Burton (USGS), Joe Smoot (USGS), Chuck Bailey (W&M), and John Aleinikoff (USGS) were part of the effort, too. The rocks were found to be more diverse than previously thought, and thus “complex.” Aleinikoff was responsible for a suite of new dates on the granitoids and their metamorphic successors in the basement complex. They have crystallization ages ranging from 1,183 Ma (±11 Ma) to 1,028 Ma (± 9 Ma): all Mesoproterozoic in age, and thought to be related to the Grenville Orogeny.

Some of these granitoids were deformed during Grenvillian mountain-building and attained a foliation which strikes northwest, in contrast to the later (Paleozoic) Appalachian foliation, which strikes northeast.

The plutonic rocks of the Blue Ridge province’s basement complex are the oldest rocks in Virginia, and they were the first ones we encountered on this field trip. All through that first day, we climbed upward through the stratigraphic column, meeting younger and younger rocks.

Live geoblogging: assessment

What do you think, folks? Do you like seeing field photos as I take them? Should I continue the Transect Trip instant photoblog series tomorrow as we traverse the Valley & Ridge?

Transect Trip 11: bedding/cleavage

Bedding and cleavage intersect in the Weverton Fm.

Bedding = Cambrian

Cleavage = late Paleozoic (Alleghanian)

Update

Just got back from three days of geology conferencing at the Northeastern & Southeastern Joint Section Meeting of the Geological Society of America in Baltimore. No time to blog whilst there, though I shot a dozen or so tweets up to my Twitter feed: small beer compared to a nice meaty blog post. Apologies if it was insufficient for your needs. Great to meet up with everyone there… I’m off tonight for a two-day field trip in the Appalachians, and I’ll get back to blogging this weekend when I return.

Is this dike a feeder?

A new paper in the journal Geology examines an interesting question: how can you tell feeder dikes from non-feeder dikes?

The answer is, normally you can’t. Normally, there’s no way to tell for sure whether a given dike actually funneled magma to the paleo-surface, or whether it never reached the paleo-surface. The reason for this is that usually, the paleo-surface is gone by the time the dike is exposed at the modern surface to your scrutiny. In the new paper, a team of Japanese researchers examined the plumbing of Miyakejima Volcano, which collapsed during an eruption in the year 2000. The collapse opened up a view into the volcano’s “guts,” which showed the anatomical details of many dikes.

Here’s Figure 2B from the paper (reproduced, as with Figure 3 below, with permission of the publishers of Geology), showing the extraordinary exposures on this volcano. The authors report that they were able to trace an individual dike for more than 350 meters. In this example, you can follow a feeder dike up 150 m to find where it erupted at the paleosurface in a cinder cone!

So, given such an extraordinary exposure, how do you go about assessing the geometries of the dikes? The research team used photography as their tool. Hopefully it will be obvious that examining the dikes in person would be difficult and dangerous on a subvertical cliff many hundreds of meters tall — and on an unstable and crumbly volcano, to boot! So they took photos, and then did their measurements based on the photos. They claim a resolution of about 3 cm per pixel at a distance of about 1 km.

Filtering your data through a medium like photography is a good way to introduce error and bias to your study, and the authors took some steps to avoid that. They used good zoom lenses, aimed at the outcrop face from the safety of the opposite side of the caldera, and aimed them straight on to the dike outcrops (i.e., within 10° of the strike of the dikes, not necessarily orthogonal to the cliff face, since there is no guarantee the dike would intersect the cliff face at a right angle): so the apparent thickness was as close as reasonably possible to the true thickness. For each photo, they cut off a 20% margins on each side of the image (total cropped area: -40%), as a guard against the effects of lens distortion. Finally, they double-checked their accuracy by comparing in-person measurements of objects of known size on the caldera rim to their photo-measurements of those same objects.

They defined feeder dikes as those (as in the image above) which were observed to connect directly to the bottoms of spatter cones and diatremes.  They defined non-feeder dikes as those which terminated “either by tapering away inside layers or ending bluntly at layer contacts,” where the ‘layers’ being referred to are pyroclastics and lava flows within this stratovolcano. In total, they tallied up 165 dikes, 93% of which were “non-feeders.” Of these, they selected the 27 best-exposed (21 non-feeders and 6 feeders) for their analysis.

What did they find? To quote from their abstract:

A typical feeder thickness reaches a maximum of 2–4 m at the surface, decreases rapidly to ~1 m at a depth of 20–40 m, and then remains constant to the bottom of the exposure. By contrast, a typical non-feeder thickness reaches a maximum of 1.5–2 m at 15–45 m below the tip, and then decreases slowly with depth to 0.5–1 m at the bottom of the exposure.

Width vs. depth data from five representative non-feeder dikes are plotted in their Figure 3, top row, and three representative feeder dikes in the second row of Figure 3. Check it out:

Feeder dikes open up (get wider) at the surface, but the non-feeder dikes first get wider (gradually positive trend to these plots), and then abruptly pinch out up towards the tip (sudden leftward cant at the top of the plot). The authors ponder these dramatically different profiles, and offer an explanation.

They offer two equations which describe these dike profiles pretty accurately. If you’re not mathematically inclined, take a deep breath. We’ll translate in a few column-inches! The first equation is:

b = (2P_o(1-v²)L)/E

where b is the thickness of the dike, P_o is the magmatic overpressure (the pressure in excess of the normal stress on the dike at the point of measurement), v is Poisson’s ratio, a measure of how much volume is conserved during strain for the host rock. In other words, when a material is compressed in one direction, how much do the other directions pooch outward? Call it ‘poochiness.’  E is Young’s modulus, a measure of the elasticity of the host rock. L is the “dike-controlling dimension,” that is whichever of the dimensions of the dike (either the dip-dimension or the strike-dimension) is smaller. So, to translate this equation into “English” enough that even Rick Sanchez could understand it, equation #1 says, ” The thickness of a dike of a given height depends on how much pressure the magma opening and filling the dike is under, along with how ‘poochy’ and elastic the host rock is.”

The second equation is:

P_o = (ρ_r–ρ_m)gh + ρ_c + σ_d

where ρ_r is the density of the host rock, ρ_m is the density of the magma, and g is the acceleration due to gravity. The variable h is the dip dimension (height) of the dike (measured upward from the source magma chamber), ρ_c is the excess magmatic pressure in the source chamber before rupture (dike injection), and  σ_d is the difference between the maximum and minimum principal stresses. Let’s translate this one, too: “The pressure exerted by the magma filling a growing dike depends on the difference between the density of the magma and the host rock it’s intruding into, as well as the force exerted on the magma by gravity.  Another important factor is whether there are significant tectonic stresses impinging on the dike as it forms.”

So where does that leave us in interpreting Figure 3, showing those different profiles for feeder dikes versus non-feeder dikes? Equation #2 says that the magmatic overpressure in a dike (P_o) will increase as the dike propagates upwards (gets taller, in other words: h goes up). And equation #1 says, if the magmatic overpressure increases, then the dike will get thicker. That’s why the non-feeder dikes get thicker and thicker in a nice gradual way as you trace them upwards.

An additional factor is related to the density. You can lower the density of a magma if you allow the gases in it to expand under lower pressure regimes (i.e., at shallower depths). The basaltic lava from this volcano has been previously measured to have about 2% water by weight. As this water exsolves from the magma at shallow depths (lower pressures), it will make bubbles that expand, and lower the density of the magma. However, at shallower depths, the rock surrounding the dike is under less pressure too, so they both decrease their densities in tandem.

Deviations from the expected dike geometries can be observed in some of the field measurements. For instance, in the lower-right-hand corner of Figure 3, dike “110-01” flares out to a wider thickness right as it crosses a stratum of “poorly consolidated scoriaceous tuff” within the volcano. The authors suggest that this rock type has a lower Young’s modulus. Because it’s poorly consolidated, it’s less elastic. A lower E value in equation #1 results in a larger b value, the thickness of the dike. Cool!

Now, the feeder dikes have a constant thickness all the way up. To the authors of the paper, this suggests that in the course of the eruption, these dikes reached a stress equilibrium with the surrounding host rock. Magma, being fluid, flowed away from highly-pressurized zones, and the dike thickness “evened out.” And why do the feeder dikes abruptly get wider at the top? The authors postulate a couple of possible reasons: First to consider is the elastic free-surface effect, which is essentially saying that as a dike approaches the surface of the Earth, half of the surrounding rock elasticity is lost (replaced by air), and so that control “hemming in” the dike is lost, and the dike expands. Second, erosion is probably an important factor, as the flowing magma churns away at the wall rock, breaking it down thermally as well as dynamically. In other words, some of the rock that used to be there at the edge of the fissure has been abraded or melted away as a consequence of all that lava flowing out of the dike and away over the surface.

Take home message? To quote the authors, “Feeders propagate and grow as non-feeders before they reach the surface. Therefore, the geometric difference between these types of dike… is primarily a reflection of the feeders reaching the surface.”

I’m interested in feeder dikes because Neoproterozoic feeder dikes of the Catoctin Formation are a significant piece of the geologic story of Virginia’s Shenandoah National Park:

…But these are interpreted as feeder dikes. To my knowledge, no one has claimed any particular outcrop in the Blue Ridge province as a spot where you can actually see the dike flare out and transition into a Neoproterozoic spatter cone. I picked up the Geshi, et al. paper in the first place because I wanted to know whether there was some measurable aspect of the Shenandoah dikes’ geometries that could tell me if indeed they were feeder dikes. The problem is that the exposure in Virginia (especially vertically) isn’t quite as good as the exposure on the inside of Miyakejima’s caldera. We’re lucky if we get 10 meters of vertical exposure, and there’s no suggestion from the Miyakejima data that that 10 m is sufficient to “profile” the dike sufficiently precisely to say whether it’s got a feeder geometry or not, especially if you don’t know where in the dike’s profile that 10 m vertical segment lies. So maybe all we Virginians can do is just interpret: we’ve got a bunch of Neoproterozoic dikes cutting basement rock, and atop the basement rock a bunch of Neoproterozoic lava flows, therefore some of those dikes are likely to be feeders.

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Geshi, N., Kusumoto, S., & Gudmundsson, A. (2010). Geometric difference between non-feeder and feeder dikes Geology, 38 (3), 195-198 DOI: 10.1130/G30350.1