Subduction Denialism, Part 3: Sedimentation in the Cascadia subduction zone
Note: This post is the third in a three-part series — please see Part 1 and Part 2 from earlier today before continuing — you’ll need the context. The numbering of the figures in this post continues from Part 2.
Geophysical Imaging of the Cascadia Continental Margin
Let’s get back to the Cascadia region — if you need a refresher of the area, go back to Part 2 and look at Fig. 1a for the regional bathymetric/topographic map of this continental margin. The seismic-reflection data shown below (from Calvert, 2004) is from the northern part of this region, near Vancouver Island.
Click on it for a bigger and less fuzzy version. Calvert interprets the package of higher amplitude reflectors (darker color) as the underthrusting Juan de Fuca plate. Earthquakes, shown by the black dots, along with the P-wave velocity structure shown in color are depicted with the reflection profile in the lower figure.
A USGS study from 1999 by Stanley et al. investigates crustal-scale structure of this region to evaluate models of deformation as they relate to earthquakes (this one is an Open File report that can be found here).
The profile above (one of many transects shown in report) is again showing the seismic wave velocity structure across this continental margin.
The two illustrations above are just a couple of many crustal-scale studies of the Cascadia region (see references within each for more). Now, let’s zoom in a bit on the accretionary complex.
I didn’t put annotation on the map above on purpose, no interpretation, no arrows, nothing – this is the physiography of this margin, plain and simple. The region in the reddish-orange to yellow-green colors marks the boundary between the oceanic Juan de Fuca plate (to the west) and the continental North American plate (to the east).
Sedimentation History of Cascadia Convergent Margin
A great paper on the evolution of the Cascadia margin is a 1978 paper from Barnard in Marine Geology. The paper presents numerous seismic-reflection profiles that were acquired across the continental slope — the image below is just one of the many in the paper.
You definitely want to click on this one to see the details. This profile is essentially at the latitude of the Columbia River mouth, which is the large east-west trending estuary near the upper-right corner of the map in Fig. 17. Note the ridge and basin topography. The reflectors in the ridges are concave-down suggesting incremental uplift and folding of older strata. The areas between the ridges are relatively flat with horizontal to slightly inclined reflectors that ‘onlap’ the basin margins indicating that sediment as filled in these basins.
The next image below, which I’ve shown before in the Sea-Floor Sunday series, shows this ridge and basin topography very clearly.
Also note the submarine fan that has developed (near the bottom of the image) outboard of the deformation front. It has a beautiful fan valley/channel that connects back to the shelf area, which is consistent with previous observations mentioned above regarding terrigenous sediment being transported across the accretionary wedge and out onto the Juan de Fuca plate. If you look back up at the bathymetric map in Fig. 17, you can see this submarine fan and the submarine canyon that fed it.
Underwood & Moore (1995) synthesize the history and magnitude of sedimentation in the Cascadia region:
…the continental margin of Oregon and Washington has several impressive submarine canyons. Barnard (1978) calculated the late Pleistocene sediment volume for Cascadia Basin (seaward of the deformation front) and compared that value with estimated volume for the Washington slope; he concluded that roughly two-thirds of the sediment delivered to the shelf edge bypasses the slope completely through the dominant submarine canyons.
In other words, two-thirds of the continentally-derived sediment makes it beyond the continental slope (where those trench-slope basins shown in Barnard’s seismic-reflection profiles shown in Fig. 18 are).
Underwood & Moore (1995) continue:
Rates of turbidite accumulation within Cascadia Basin have been exceptionally high (Kulm et al., 1973), particularly during lowstands. The most important features are the Nitinat and Astoria fans, and the Vancouver and Cascadia channels … The basin floor is unusually shallow (2,400-3,000 m) because the subducting oceanic crust (generated by sea-floor spreading along the nearby Gorda/Juan de Fuca Ridge system) is young, warm, and buoyant; the rate of plate convergence is also relatively slow. As a consequence, the trench is completely obscured as a bathymetric feature.
Emphasis mine. They go on to discuss more details of component depositional systems, their respective thicknesses, histories, and relationships to the coastal staging areas in that review paper.
So, why is there so much sediment being transported to and deposited along the Cascadia margin? The size of the Columbia River drainage basin is a major factor. While many continental arc margins have numerous and high sediment-flux rivers along there length, few have rivers the size of the Columbia. It is the largest river flowing into the Pacific Ocean from North America (as measured by volume of flow). Big rivers generally deliver a lot of sediment.
By contrast, if you look at a map of Peru and Chile (another oceanic-continental convergent margin) you’ll notice that the continental divide runs down the spine of the Andes Mountains, which is pretty close to the plate boundary. The Columbia River watershed reaches ~900 km back into the North American continent from the plate boundary. The headwaters for rivers that flow from the Andes into the Pacific Ocean, however, are generally 200-300 km from the plate boundary. But it’s not just distance — the Columbia River drainage basin area is huge and has several large tributaries. While the river systems coming off the Andes may have high sediment flux locally, they are numerous and are spread across the orogen.
If you go back up and look at the map in Fig. 17 you’ll notice that the mouth of the Columbia River is not very close to the continental slope. Remember that the last million years or so has fluctuated between ice ages and ‘interglacials’ — sea levels were >100 m lower than they are today just 18,000 years ago in the latest Pleistocene. The lower sea level pushes the shoreline out to the modern continental shelf edge. In other words, the mouth of the Columbia River emptied directly into (and helped create) the submarine canyon head you see near the continental shelf edge.
A great paper by Normark and Reid (2003) discusses Pleistocene-aged turbidite deposits in the greater Cascadia region. You have most likely heard about those famous catastrophic late Pleistocene mega-floods that created the Channeled Scablands. What happened when these mega-floods reached the sea?
The map above summarizes the extent of turbidite sediments showing them nearly 1000 km (!) to the west, beyond the Blanco Fracture Zone, and on the Pacific Plate. The inset diagram in the lower left (click on it to see bigger version) shows the volumes for the initial outburst flood. Normark & Reid also show an ODP core in Escanaba Trough has a >300 m thick section of turbidites that have been deposited since ~30,000 years ago. The details and implications of this paper are endlessly fascinating (to me), so I’ll save a more thorough discussion of it for another time. But what’s important to reiterate for this post is that the Cascadia region has received ridiculous amounts of sediment in recent geologic history.
This series of posts is in no way a comprehensive review of subduction — it is not a comprehensive review of the Cascadia example. It is not even a comprehensive review of sedimentation in the Cascadia region for the Pleistocene. Although these posts are lengthy as blog posts go, I’m barely scratching the surface.
One of the last comments from Anaconda in that long exchange was:
Why is it so hard for you to admit a harmless mistake (jumping to the conclusion that Cascadia has a “trench overfilled w/ sediment”), or that subduction theory has weaknesses for that matter?
First of all, I hope that this series of posts demonstrates that I did not “jump to a conclusion” regarding Cascadia’s history of sedimentation. If my summary of the numerous studies doesn’t convince you, then I encourage you to look at the specific studies and evaluate them yourself.
Secondly, I have no problem discussing the uncertainties with respect to subduction — obviously there is a lot we don’t know. But, Anaconda is talking about “weaknesses” in the entire concept … as in weaknesses compared to a model of the Earth that has no subduction. This is confusing logic (at least to me) … the subduction denialists must, at some point, explain the physiography of plate boundaries within the context of their conceptual model of the Earth. They can try and debunk plate tectonics theory all they want … be my guest … but that won’t create a paradigm shift or revolution in thinking. That will only happen when a comprehensive and integrated model (with data and observations from all over the Earth) is presented, tested, and evaluated by others. If they want to resurrect geosynclinal theory for the formation of orogenic belts — go for it. But do it with data and do it systematically. Broad assertions don’t cut it.
Of course they will shout back “No, the burden of proof is on you!”. And then when I point them to either a single paper, several papers, or nearly 70 papers and several textbooks full of thousands of references from several decades of research, they’ll rebut “Don’t give me a laundry list! If you can’t convince me in your own words, you fail!” or something like that. And then, of course, there will be a rebuttal that discusses dogma, history/philosophy of science, other meta-science arguments, persecution from the ivory tower, or even conspiracy theories that, in my opinion, simply evade actually doing any real research.
I’ve been called arrogant and disgrace to my profession by Anaconda before (really) … and before that, when I attempted to have a civil discussion, he accused me or “trying to appear reasonable”. So, I think it’s safe to assume it’s a no-win situation with them.
So, let me state this clearly – I don’t believe OIM or Anaconda (or any other hard-core subduction denialist for that matter) will be swayed in any way by these posts. It’s not in their nature. This post isn’t really for them — I put all this together for: (1) genuinely interested people that may have come across contrasting ideas while surfing the web and (2) for myself because I’m a nerd and think it’s fun.
But, if Anaconda, OIM, or any other subduction denialist wishes to respond to this post I would ask that you do as I have done — provide full citations and links (if available) for any data, figures/illustrations, and papers/studies you reference. Being thorough and diligent with references may seem trivial but is necessary to avoid confusion.
References Cited (below are cited in this post, for longer list of subduction references, see here):
Barnard, 1978, The Washington continental slope: Quaternary tectonics and sedimentation: Marine Geology, v. 27.
Calvert, 2004, Seismic reflection imaging of two megathrust shear zones in the northern Cascadia subduction zone:Nature, v. 428, p. 163-166.
Normark and Reid, 2003, Extensive Deposits on the Pacific Plate from Late Pleistocene North American Glacial Lake Outbursts: The Journal of Geology, v. 111, p. 617-637.
Stanley et al., 1999, Subduction zone and crustal dynamics of western Washington: A tectonic model for earthquake hazards evaluation: USGS Open File Report 99-311; http://pubs.usgs.gov/of/1999/ofr-99-0311/
Underwood, M.B. and Moore, G.F., 1995, Trenches and trench-slope basins: in Busby & Ingersoll, eds.; Tectonics of Sedimentary Basins, Blackwell, p. 179-219.
Postscript: While most of the papers I cite (or are cited within the papers I cite) are not freely and easily available to the public, there is a wealth of data that is available for anyone to do science. In the past, sifting through some of these data portals was cumbersome and time-consuming. These days, many have GoogleEarth portals so you can search in the area of interest.
Two of these are the Integrated Ocean Drilling Project (IODP), and its legacy progams DSDP and ODP, and the USGS National Archive of Marine Seismic Surveys. The screenshot below shows both of these data portals for the greater Cascadia region.
When you are within GoogleEarth and click on the information, a window pops up that has links to where you can download the data and read the reports with details of acquisition. These datasets are available for you to do some science!! If you have a hypothesis that needs testing, get at it.
UPDATE #1 (11/17/2008): Instead of addressing any specific place or dataset presented in these posts, OilIsMastery has responded with generalities. He is suggesting that the western Pacific Plate boundary is a spreading center. He pointed to this map of oceanic crustal ages as support for this. As you can see in the comments, I’ve asked him repeatedly how the older oceanic crust (~160-120 million years old) came to be juxtaposed against the boundary — if this was a spreading center, where is the young (i.e., new) crust?
To help focus and avoid confusion … the map below zooms in on the area of interest.
UPDATE #2 (11/18/2008): Okay … OilIsMastery is now saying the above plate boundary is not divergent, but is called ‘orogenic’ or ‘volcanic’ … or something like that. The Nazca-South American boundary, which he originally claimed was divergent (because those are the only kind that exist, remember?), also has volcanoes and is an orogenic belt … so, I’m not quite sure how to distinguish. Hopefully he will clarify his classification scheme for everybody – bonus points if data is involved!