It's been a while since I last posted about my time in Bishop, California, back in September, when I attended a GSA field forum on the structural and neotectonic evolution of the volcanic tableland.

For reference, here's a list of the previous posts about that trip:
...Faults of the volcanic tableland
...The Bishop Tuff
...The flipping fault

So, picking up where I left off, I thought it would be worth a post to mention the gorgeous drainage channels one sees etched into the top "Ig2" welded layer of the Bishop Tuff. These channels are interpreted as being Pleistocene in age, when the area was wetter than it is now.

Here is a photograph of the most spectacular of these channels, as viewed from the rim:

We visited this vantage on our second day in the field. A hiking path at the bottom of the dry channel imparts a sense of scale.

Here's a Google Map of the area from the perspective of a hawk:

Where the road comes most closely tangential to the canyon is the point where we stopped to take a look at it, and where the above photograph was captured.

Further upstream along the channel, we find it broken by normal faulting. Check out the view across this graben (a graben is a normal-fault-bounded valley, downdropped relative to the highlands next to it). There, you see the distinctive crescent-shaped profile of the drainage channel, but offset along several fault scarps:

There are three scarps on the far side of the graben, and an additional one that Peter is standing on, on this side of the graben. Just behind Peter, you can see a broken relay ramp, too. View is to the northwest; those are the Sierras in the distance.

Here is a Google Map of the area, showing the drainage channel crossing the graben. This conclusively shows that the channel must be older than the faulting which produced the graben.

This Google Map shares its southeastern corner with the northwestern corner of the first one I showed. You can see this for yourself by dragging either one in the appropriate direction. They both share the white-knuckled place where the road goes straight down the fault scarp, rather than sensibly down a relay ramp. That wasn't my favorite thing to drive.

Here's another drainage channel, similarly bone dry, that we visited in our fourth day in the field. Perspective is to the east: those are the White Mountains in the distance:


The Google Map shows a more interesting relationship this time. Instead of the faulting cross-cutting the channel's orientation, this channel approaches the graben to the southeast, curves around (deflecting from its original downhill course) and drops down the relay ramp to the northeast, into the graben (breaking up into multiple channels en route). There, it resumes its original downhill trajectory to the southeast:

This suggests that at least some of these faults were rupturing the "Ig2" layer at the same time that water was flowing over the surface (i.e. before the Owens Valley's climate dried out, post-Pleistocene). The stream's course and the faulting were coeval.

So what was the source of these streams? Did they originate on the volcanic tableland, or were they derived from the Sierra Nevada, prior to incision by the Owens River (which makes a deep canyon a mile or two west of here)? Fred Phillips, of New Mexico Tech, holds up a piece of evidence:

That is not a rounded cobble of the Bishop Tuff. That's a rounded cobble of granite. While the majority of cobbles in these channels are locally-derived chunks of the Bishop Tuff, there are also clasts which originated elsewhere, beyond the volcanic tableland itself. This suggests a source area with a granitic outcrop. One candidate location is Casa Diablo Mountain, north of the (south-sloping) volcanic tableland. Another possibility is the Sierras, to the west.

Another possibility entirely is that the source of the cobbles could be anywhere, and they were brought to the volcanic tableland not by streams but by paleoindians, who used them as grain-grinding stones in their metates.

Labels: california, conferences, faults, granite, pleistocene, rivers, travel, volcano, weathering

I spent last week in the Owens Valley of California, attending a GSA field forum on the structure and neotectonics of the Owens Valley and the Volcanic Tableland north of Bishop. It was really cool, and I learned a lot. I'll be sharing images and ideas on the blog in days and weeks to come.

Twenty-four people attended, plus the three conveners: David Ferrill, Alan Morris, and Nancye Dawers. Here's the team getting an orientation on Monday morning, looking west towards the Sierra Nevada. Note the time-honored geology field tradition of using magnets to hold posters and maps to the side of the van:


David discusses the tectonics and geomorphology of the "Eastern California Shear Zone," a transtensional zone between the Sierra Nevada and the typical Basin & Range. This area ranges tremendously in elevation: from Mount Whitney in the Sierras (14,494' elevation) to Badwater in Death Valley (-282'). The lurid colors on this elevation map show that:


A Landsat photo comes out at the next stop, looking northeast towards the Volcanic Tableland:


And yet another image, this one a beautiful side-scanning radar image of the Volcanic Tableland, which David and Alan (here assisted by Wes Hildreth) pulled out at a stop overlooking the Owens River Gorge (a canyon which dissects the Volcanic Tableland):


This image shows east-dipping normal faults as white stripes, and west-dipping normal faults as dark stripes:


This is the main reason we're all here: the young welded ashflow deposits of the Bishop Tuff (760 ka) record brittle strain as a result of the past 760,000 years of extensional and strike-slip tectonics. Due to the low rainfall and this excellent marker unit, you can really get a sense of how such systems operate. The faults are expressed topographically: a lovely marriage of structure and geomorphology.

Our first overview of the Volcanic Tableland, looking northeast from the Sierra Nevada over the fractured Bishop Tuff, towards the White Mountains in the distance:


Here's a Google Map of the Volcanic Tableland, showing the orange upper ignimbrite layer of the Bishop Tuff, and the north-south trending faults which rupture it. Green stripes are the Round Valley (southwest), Owens River (southern border, trending east-west), and Fish Slough (far east, trending north-south):


Here's another Google Marp, zooming in on some of the faults. Conveniently, Google opted for morning sunlight in this image, so it's "color-coded" the same way as the side-scanning radar image I showed you earlier: east-dipping fault scarps are light-colored, while west-dipping fault scarps are in shadow:

(Another very cool thing about this image is the northwest-southeast trending Pleistocene drainage channel -- more on that later!)

Many of the faults in the Volcanic Tableland are arranged in en echelon arrays, reflecting a broader zone of deformation:


In en echelon arrays of these normal faults, we find the individual fault segments are linked up with intermediary flexures of the the ignimbrite layer, called "relay ramps." This was a new term to me, but once I learned it, I saw them everywhere. Here's one atop the Volcanic Tableland:

(It's the shallowly-sloped bit in the middle, dipping towards us, bounded by two west-dipping fault scarps: the intensely-shadowed areas.)

Here, in Fish Slough, we see a couple of 'relict' relay ramps that have gotten cut off as the small fault segments linked up into a larger through-going fault. Pretty cool!


The group descends a relay ramp on their way back from a field excursion to the vehicles:


Annotated version of the photo above:


Relay ramps occur on many scales. This 'scaling' of fault systems (and deformation in general) was a theme at the field forum. Here's a Google Terrain Map of the Owens Valley area. Notice how, just west of Bishop, the Sierra Nevada front jumps to the west? That's a much larger relay ramp, the Coyote Warp Relay Ramp:


Looking west from the first stop at the Sierras, with the Coyote Warp Relay Ramp descending from upper left towards lower right:


Annotated version of the photo above:

That's a little taste to get you started. More geology from the Owens Valley in future posts...

Labels: california, faults, structure, volcano

On our hike to Grinnell Glacier this past July in Glacier National Park, I found lots of cool cobbles of float, mainly of the Mesoproterozoic metasedimentary rocks that make up the bulk of the park: the Belt Supergroup. One of these formations, the Helena Formation, is intruded by a diorite sill known as the Purcell Sill. It's a prominent rock unit showing up as a black stripe within the lighter-colored Helena Formation, exposed high on the glaciated walls throughout the park. Occasionally, you'll find pieces of it as float, and I noticed that the higher we climbed up, the more of it we saw. Here's one of my favorites among these pieces of the Purcell Sill:


This cobble shows beautiful slickenlines, small gouges into the rock as neighboring rock ground across its surface, along a fault. These physical gouges are decorated with a chemical accoutrement: the metamorphic* mineral epidote, which is a gorgeous grassy green.

Labels: faults, igneous, metamorphism, minerals, montana, national parks, proterozoic, structure

(deep voice) "Previously, on NOVA Geoblog..."

...We were looking at the structure of the Bridger Range in Montana, near Bozeman. We discussed the concept of Pumpelly's Rule, which suggests that outcrop-scale structures (meso-scale) can help us understand the regional structure (mega-scale), and that the asymmetry of certain kinds of folds can tell us where we are on that structure (vergence). [Link to post]



So the Bridgers are an anticline, overturned in the southern part of the range... but that's not the whole story!

Starting during the Miocene, the west began to widen. The Bridger anticline cracked in half along its axis and the western half slid down relative to the eastern half. The downdropped western half became buried in younger sediments, and that's the Gallatin Valley, where Bozeman is located. When the block of rock above a fault plane slides down relative to the block of rock below the fault plane, we call it a "normal fault." (It would be normal for a kindergardener to slide down a playground slide, but the reverse of normal for them to slide up it!)



A Google Map "terrain" view to show how this is expressed physiographically - Bridger Range on the east, downdropped Gallatin Valley on the west:



And, zoomed out a bit to get some more regional context on how Basin & Range extension has left its mark on the physiography of western Montana, eastern Idaho, and western Wyoming:



I've visited some of these normal faults myself (solid lines); the rest I'm just inferring from the landscape (dashed lines). Basin & Range extension is one of the main reasons the west is so beautiful: those wide open spaces with mountains rising to define the horizon...

(sigh) ...I'm glad I got to spend so much of my summer out there. I'm looking forward to it again next summer. But in the meantime, this is the first week of classes at NOVA, and I'd best get back to work!

Labels: faults, montana, structure

There's an article on the Burgess Shale and another on inventorying all the plants on Plummers Island (home of the Plummers Island Thrust Fault, between DC and the Billy Goat Trail).

Labels: cambrian, dc, faults, fossils, plants

Labels: art, earthquakes, faults, teaching

Yesterday, I took a three Honors students and a colleague to Difficult Run, Virginia. This is a hiking trail that goes from Georgetown Pike, in the tony neighborhood of McLean, Virginia, down through a deep, steep river valley to the Potomac River.

As noted a couple days ago, the trail is right across the Potomac River from my beloved Billy Goat Trail. In a recap from that post, here's a map of the area... Feel free to switch it to "satellite" view.



Some discussion of the bedrock geology of Difficult Run can be found here, in an excellent field trip guide by Scott Southworth (USGS) and colleagues that's part of Excursions in Geology and History (Frank Pazzaglia, editor).

We began our trip by meeting up with Doug Dupin of the Palisades Museum of Prehistory, who joined us for our exploratory geohike. We walked a short distance down the trail and found a big (abandoned) quarry where it was rumored there was a good fault. This is one of these pieces of information that I heard somewhere, at some point. I couldn't find it in any literature, so maybe I heard it in discussion when I taught at George Mason University for a year between grad school and when I got my position at NOVA. Anyhow, I had never actually checked it out...

...So our first order of business was to review the criteria for identifying a fault: What would we look for? Fault breccia, fault gouge, slickensides, hydrous mineral veins, and of course, offset. However, here in the Virginia Piedmont, it's rare to have a good marker unit to compare on opposite sides of the fault: usually it's just schist on one side, schist on the other. In some places, you could add the presence of a fault scarp to that list, but being as how this was an old quarry, geomorphic features like that didn't seem likely. So our search focused on the search for fault breccia, fault gouge, veins of odd minerals, and slickensides.

A few minutes in, we found some slickensides on this boulder of float:

This is a boulder of migmatitic phyllonite, with a wavy texture due to mylonitic flow at depth. (The picture doesn't show this very well at all, though you can see faint undulations 'cascading' from the top of the photo towards the bottom. It's much clearer in cross-section.) Anyhow, the 'slicks' are a faint upper-left to lower-right lineation seen on this surface, one or two degrees off from the orientation of the ballpoint pen. The surface you're looking at here was a fault plane at some point in its history. Ballpoint pen for scale.

We did eventually locate the fault, uphill from this boulder. It was characterized by a zone of fault gouge (pulverized rock), three inches wide to a foot wide in places, and highly oxidized (presumably by oxygen-rich meteoric waters percolating along this fractured surface)... but there were no good marker units to judge the total offset.

Here's a different section through a similar rock (though I wouldn't apply the "phyllonite" textural description to this one). Instead of looking at the plane of foliation here, we're looking at a surface which is perpendicular to the foliation plane(s)....

Here in this image, you can see two cleavages... One which runs roughly upper-left to lower-right through the photo, defined by gneissic banding including bands of granite (light-colored; late Ordovician in age... Taconian Orogeny). A second cleavage runs roughly left-to-right through this photo. This second cleavage overprints the first. The overall interpretation is that the first cleavage developed due to lower-left-to-upper-right compression, forming the foliation defined by alternating bands of different compositions of minerals in an upper-left to lower-right direction. The second cleavage formed due to compressive stress sub-parallel to the pre-existing foliation, deforming it into a series of tight folds. The limbs of these folds line up parallel to one another, defining the second-generation, overprinting cleavage. Can anyone else add to this interpretation? Dime for scale.

Along Difficult Run itself, the outcrops were all relatively recently scoured (in 1972 by Hurricane Agnes), so there are some good exposures. As I noted earlier this week, the area shows some nice exposures of granite pegmatites (keys, and the edge of the Pazzaglia volume, for scale):


On our field trip yesterday, we took at closer look at these beautiful pegmatites, and the associated amphibolite bodies. Take a look at this close-up... Dime for scale.

What's going on here? You've got a beautiful (euhedral/subhedral) example of an orthoclase feldspar ("potassium feldspar") crystal amid a bunch of quartz. But look closer at the feldspar crystal... this sucker has been fractured in many places, and it's shot through with very small veins of quartz. Somehow, as this pegmatite dike was cooling, the earlier-crystallizing feldspar was broken and intruded by the presumably-still-fluid silica-rich magma. Anybody able to expand on this interpretation and shed some light on how this all played out? Or contradict it and give a different story to explain this relationship?

In the neighboring amphibolite, we checked out these cool ridges of resistant rock which are centered on thin fractures. Here, you see a couple of intersecting joint sets, each of which was the "plumbing system" for silica-rich hydrothermal fluids (my interpretation). These silica-rich hydrothermal fluids impregnated the surrounding amphibolite with quartz, which made the immediately-adjacent areas more silica-rich, and hence more resistant to weathering and erosion: Hence, now that they've made it to the surface, they're weathering out in high-relief. Dime for scale.


A bit further downstream, Doug showed us a 'cave' (central dark area, just to the right of the waterfall) between the bedrock and a big slab of sloughed-off migmatitic metagraywacke:

We each edged into the 'cave' to the end, where Doug has shown that a distinctly-rectangularly shaped hole admits a direct beam of sunlight during the fall and spring equinoxes. From the inside, it's a striking arrangement, enough to make you wonder whether it's anthropogenic. However, from the outside I was unconvinced that the hole's position was anything other than natural. Doug's initial intepretation of the site was strongly influenced by the fact that there are some unambiguous petroglyphs a short distance away from here, and based on this proximity, I think it's acceptable to infer that Native Americans may have visited this cave. However, I interpreted the opening to be completely natural, with no need to invoke anthropogenic modification in any way.

We hiked on along a ridge overlooking Mather Gorge, sighting a fox and an accipiter (Coopers? Sharp-shinned?) and a few vultures, and returned to the parking lot as the sun dipped low in the sky. On the way back to campus, Honors students Ana and Hope fed us Swiss cookies and cheese & crackers. Altogether, it was a pretty great way to spend a November afternoon...

Labels: birdies, faults, field trips, granite, igneous, metamorphism, ordovician, piedmont, rivers, virginia

This week, I'm taking some of my Honors students to Difficult Run, Virginia.

It's right across the Potomac River from my beloved Billy Goat Trail. Here's a map of the area:



Some discussion of the bedrock geology of Difficult Run can be found here, in an excellent field trip guide by Scott Southworth (USGS) and colleagues that's part of Excursions in Geology and History (Frank Pazzaglia, editor).

Here's a look at Difficult Run, looking upstream from below one of the several waterfalls there:



These outcrops were all relatively recently scoured (in 1972 by Hurricane Agnes), so there are some good exposures. We're going to look for a fault reported to be there, as well as the incision geomorphology of Difficult Run itself, and some nice exposures of granite pegmatites (keys for scale):





This field trip is less a guided tour, and more of an exploration, so I hope when we get back, I'll have some photos of new and interesting things to share.

Labels: faults, granite, igneous, metamorphism, ordovician, piedmont, rivers, virginia

Here's a couple of more photos from my travels out west this summer. This is in Jackson Hole, the large valley that abuts the Teton Range immediately to the east. If you've never been to the Tetons, you must go and check them out for yourself. They are an awesome, singular mountain range in the United States. Their shapes and sheer relief remind me of the Karakoram, or Torres del Paine, or some other awesome mountainous region of the world. It's really jaw-dropping.

Here's a shot of the Tetons from the northeast, visually pairing them with a line of coniferous trees in the foreground. Photographically, I like this parallelism in their shapes:



So what's up with the Tetons? What geologic processes give rise to their readily-apparent awesomeness? There's two main things going on here: faulting and glaciation. First, there's a major normal fault along the base of the range. The Tetons have moved up as a block while the Jackson Hole basin has dropped down as a block. As the rocks of the Tetons (some as old as 2.8 Ga) have been eroded, sediment was generated, and that dropped down to fill in the hole to the east. Jackson Hole is full of of sediment (over 20,000 feet deep), and then the peaks of the Tetons rise an additional 7000 feet beyond that. Based on offset of the Cambrian Flathead Sandstone on either side of the fault, total displacement is estimated to be 30,000 to 35,000 feet (Love, 1987). Even relatively young geologic units in Jackson Hole, like the Yellowstone-erupted Huckleberry Ridge Tuff (2.1 Ma), dip significantly towards the fault (Good and Pierce, 1996). Movement along this fault is ongoing, raising the mountains on average ~1 centimeter per year, with most movement having taken place over the past 9 million years. The Tetons are generally regarded as the youngest range in the Rockies.

Here's a shot coming north from the Gros Ventre landslide area (subject of a future post) towards the main road. A photogenic herd of bison was grazing on the grassy sagebrush flats, purposely maneuvering between me and the mountains so they would have a nice backdrop:



Once the Pleistocene ice ages began, the tall Tetons accumulated a lot of snow, which packed into glacial ice. Alpine glaciers started flowing downhill, and carving the rock of the mountains as they did so. That created the distinct U-shaped valleys seen in these photos, and left pointy little nubbins between them: the glacial horns like the Grand Teton and Mount Owen. The rocky debris scraped off the Teton block was deposited in Jackson Hole along with till from the Yellowstone ice cap to the north. These piles of glacial till are easily demarcated by the coniferous trees that grow on them, unlike the grasses and sage of the outwash plain.

References:

Good, John M., and Kenneth L. Pierce (1996). Recent and Ongoing Geology of Grand Teton and Yellowstone National Parks, Grand Teton Natural History Association, Moose, Wyoming, 58 pages.

Love, J. David (1987). "Teton mountain front, Wyoming." In: Geological Society of America Centennial Field Guide - Rocky Mountain Section, Stanley S. Beus, ed. Geological Society of America, pp. 173-178.

Labels: faults, geology, glacial landforms, travel, wyoming

Picking up from yesterday's post about my hike along Windy Run in Arlington, Virginia...

Just downstream from the waterfall (and crossing the trail) is a recent rockslide. Between D.C. and Great Falls (12 miles upstream), the Potomac River flows through a canyon called the Potomac Gorge. It's hundreds of feet deep overall, and consists of a series of nested straths (bedrock "terraces"), each shaped roughly like (half) a canoe. (At the tip of each canoe is a waterfall leading up to the next strath). Where the vertical distance between straths is great, as it is at Windy Run, mass wasting events serve to break down the cliffs and reduce the crisp profile of the straths.


This rockslide happened in 2005, and the area of "raw" rock up at the top of the cliff reveals the source area for the rock debris below. I wish I had taken a photo of this three years ago when it was really fresh -- it would be an excellent place to do repeat photography to show how the talus pile and cliff face change over time. Upstream are several examples of older talus aprons that have been overgrown by plants and buried in soil. Already, you can see that a few Ailanthus trees (single, upright pole-looking things) have taken root in this fresh landscape.


Once you get down from the Windy Run trail to the Potomac Heritage trail, here's the view of the river, looking upstream. Virginia's on the left; D.C. on the right. A slight "shelf" can be seen on the Virginia side where a notch has been cut to host the George Washington Parkway.


As I hiked along, I found this dead mole. It's a big fat sucker, and it must be quite fresh: probably a casualty from the previous 24 hours. Lens cap is 5 cm in diameter.


More critter evidence: here's a couple of small tree trunks that were decapitated by a beaver. Again, this is recent -- note the fresh curls of wood shavings at the base of the trunk.


But enough with these living entities: let's look at some rocks. This is the metagraywacke rock that makes up most of the Piedmont in our area. This rock is metamorphosed to various degrees up and down the Potomac River, in some places all the way to gneiss and migmatite. In some places, it's schisty, but in others primary sedimentary structures are still preserved. Upstream by Great Falls, for instance, we find graded bedding in isolated less-metamorphosed, less-deformed areas. Down along this stretch of the river, it preserves a diversity of sedimentary clasts, as shown in this image:

Here, you're seeing the graywacke matrix mixed in with a bunch of dark chunks. Today, these dark chunks are mostly biotite, but that's metamorphic. Originally, they were probably mud clasts. Little pebbles of granite and vein quartz are mixed in too. It's worth noting that not only are they metamorphosed, but they're also stretched out in the same direction: foliated and lineated. Many are squashed into X>Y>Z ellipsoidal shapes (where the letters refer to the lengths of the different axes of the ellipsoid), like a mango seed. Lens cap is 5 cm in diameter.

Let's pause for a moment and bring people up to speed if you haven't previously spent any time thinking about Appalachian geology. These rocks are part of the Appalachian mountain belt, which runs from Newfoundland to Georgia (by one definition) or from Texas to Scandanavia (by a more inclusive definition). The Appalachian mountain belt consists of three provinces: from west to east: the Valley and Ridge, the Blue Ridge, and the Piedmont. Two of these are topographically mountainous today: the Valley and Ridge and the Blue Ridge, as their ridgey names imply. But the Piedmont certainly counts as part of the ensemble, and if you compare it to the other two, you'll find that it experienced the most metamorphism, the most deformation, and is intruded in many places with syn-orogenic granites (which neither of "the Ridges" can claim, at least not for Paleozoic orogenies). The Blue Ridge and the Valley and Ridge are deformed, yes, and even lightly metamorphosed, but the Piedmont is really where the action is: this is the center of the ancient Appalachian mountain range. These rocks experienced some serious continental convergence.

So what was the Piedmont before it was the Piedmont? An ocean basin. Before the Atlantic, before Pangea, there was an ocean basin off the "east" coast (it was really the south coast at that point, but no matter...). We call this dead ocean the Iapetus Ocean. The Iapetus was closed via subduction throughout the Paleozoic, and it closed for good when Africa rammed into North America, metamorphosing these rocks and raising the Appalachians. As subduction narrowed the Iapetus, sediments atop the oceanic crust were scraped off in a big jumbled pile called an accretionary wedge. (It is for this mixed-up melange that the infamous geo-blog carnival is named.) You want to see an accretionary wedge being scraped up today? Dive down to the Peru-Chile Trench, off the west coast of South America. You want to see a fresh one at the surface? Visit California's coast ranges, which are a Mesozoic accretionary wedge, raised above sea level. You want to see what an accretionary wedge looks like after it's been tectonically squeezed between two continents? Come to the Piedmont!

Our metamorphosed accretionary wedge consists of a bunch of the sediments that were deposited in the Iapetus Ocean, including what was originally graywacke (a mix of sand & mud). Occasionally, you find a sedimentary clast that's a bit more intriguing, like this one (white arrow):

What intrigues me about this little sedimentary cobble is the fact that it's foliated, which indicates metamorphism and differential pressure, but its foliation does not line up with Appalachian foliation. This cobble was foliated before it was deposited in the accretionary wedge. Therefore, it was derived from some area that had previously experienced mountain building & regional metamorphism (presumably a continent). That ancestral orogenic episode produced a source rock from which this cobble was derived. Then that cobble was deposited by sedimentary processes somewhere and (possibly later) incorporated into the accretionary wedge, which then was metamorphosed (& foliated) itself. Lens cap is 5 cm in diameter.

Here's another one, which shows its foliation a bit better:

When I see something like this, I start to wonder, where did this cobble come from? What was its sedimentary provenance? Is this a North American cobble that attained its foliation in the Grenville Orogeny (~1 Ga)? Is this an African cobble that got squeezed in some pre-Pangea Gondwanan orogeny? Is it derived from a nameless microcontinent that was formerly marooned in Iapetus oceanic crust (a la Madagascar) and is now accreted to some continent as an exotic terrane? Do the answers to these questions change how we think about the (1) closure of the Iapetus, (b) Appalachian Orogeny, (c) assembly of Pangea?

Elsewhere in the Potomac Gorge, there are other clasts in the accretionary wedge complex that encourage similar thoughts (for instance, you can check out the photos at the top of this page). Another question raised by these clasts is this: Does their position amidst such relatively fine grained sediments (the mud and sand of the graywacke) represent original deposition? Or is that simply tectonically-induced "shuffling" in the blender-like environment of the accretionary wedge? The rocks in an accretionary wedge are not stratigraphically coherent, but sometimes they have little areas that are. If these clasts are in their original depositional position relative to the graywacke matrix, what does that tell us? Are these landslide deposits? Or are these "Snowball Earth"-related glacial dropstones? Without the original sedimentary bedding (destroyed via orogenic metamorphosis & deformation), it's impossible to answer these questions, but it sure would be nice to know.

Lastly, I'll note that everything I've talked about so far (metagraywacke, mysterious clasts, quartz veins, granite intrusions, and regional foliation) are all cut by a series of joints, brittle fractures in the rock. These joints are arranged in a series of joint sets which intersect one another, resulting in the "blocky" nature to bedrock exposures in the Potomac Gorge (example). Here, along one Gorge-bounding cliff, I saw that the joints had begun to accomodate some sliding of the blocks of rock on either side. Technically, they aren't joints any longer, but faults, instead. Total offset is only a few inches, but it shows up well in a photo like this. Note the similar sense of motion on the more distant fault "scarp." A housekey (with pink ribbon attached!) is jammed into the closer fault to give a sense of scale.


All in all, an hour strolling along Windy Run provides some terrific opportunities for reflection on the checkered geologic past of the Piedmont and the Appalachians, and the continuing geomorphic evolution of the Potomac Gorge landscape. I enjoyed my little stroll. It was with reluctance that I turned around and headed back to the house to grade exams...

Labels: appalachians, faults, geology, mass wasting, plate tectonics, virginia

This is a fault in a quarry near Ensenada, Baja California, Mexico. My friend Annie Kammerer of the NRC sent me the image, and then I annotated it.

First off, notice that the rock is light-colored, with grey and pink tones. I suspect it's a granitoid of the Coast Ranges Batholith (one of many batholiths of the Mesozoic west coast, like the Sierra Nevada and much of Idaho.) The Coast Ranges Batholith extends from "mainland" Mexico to the Baja Peninsula, and up into southern California.

Second, a prominent cross-hatching pattern is seen in the rock. These fractures are two intersecting joint sets. Joints are fractures in rock along which there has been no movement. If the rock on opposites sides of the fracture does move, then it's not a joint; it's a fault. Joints are caused by stresses the rock experiences. Because tectonic stresses are often distributed over a large volume of rock, the rock often develops many joints in the same orientation. A bunch of parallel joints is called a joint set (here's an example from Utah). Joint sets are much more interesting than mere joints because (let's face it) joints are extremely common in rock: they are the most common geologic structure. Joint sets, on the other hand, speak about larger forces and bigger patterns.

The third and final reason for enjoying this photo is that it shows a fault well. Running right down the middle is a prominent fault. Note that the fault is wider than the joints: it's filled with some sort of pulverized goo. This is a material called fault gouge. Faults may or may not have fault gouge in them. It's essentially ground-up rock: any bits that stick out get crushed as the fault grinds over them: like a mortar and pestle smashes up spices. Sometimes when the fault is more planar, the rock rubs directly against its neighbor, producing slickensides. Sometimes, asperities (knobs & bumps on the fracture surface) get snapped off, but not ground into pulp: this produces a fault breccia (like this celebrated example in Death Valley).

I'm struck by how vertical this fault is. Faults come in all sorts of orientations, but there are four really common ones: (1) high-angle normal faults which dip at an average of 60º into the Earth, (2) low-angle reverse faults which dip at an average of 30º into the Earth, (3) extremely-low-angle thrust faults, which can be close to horizontal, and (4) vertical strike-slip faults, which dip at 90º, or close to it. This appears to be the latter. Annie, the photographer, tells me that there was a substantial (~100 m) offset along this fault. If you looked down on it from a bird's-eye view, you might be able to tell that, but it's impossible to gauge the offset from this cross-sectional view.

Final observations: the fault is oriented the same way as one of the joint sets. It's likely that the rock was jointed first, and then when tectonic stresses required strike-slip faulting, one of those joints (a pre-existing plane of weakness) was utilized as the site of movement. Note too up in the upper-left another "joint" seems filled with fault gouge, meaning it's really another (parallel) fault. That would be entirely expected if a jointed rock was being tectonically smeared out. The vertical "slices" of rock migrate past one another like a sheared loaf of (sliced) bread. Some of the slices adhere along their cut surface, whereas others move. Some of the joints become faults, but once they start moving, the stress is accommodated, and there's no reason for every joint to become a fault. The weakest link takes all the strain.

Labels: faults, joints, mexico