Subduction Denialism, Part 2: Subduction zones, trenches, and accretionary complexes
This blog has moved to Wired Science (as of Sept 14, 2010)
Alrighty … so the overall objective here is to discuss the Cascadia subduction zone and why it does not have a distinct and discernible trench like other subduction zones do. My claim to Anaconda was that (1) there has been a significant amount of sedimentation across this margin, and (2) the Cascadia margin has a robust accretionary wedge — you can see my original comment as I stated it here. These two aspects have a lot to do with the physiography of this plate boundary as we see it today.
But before I focus on Cascadia, I wanted to show some data from various modern subduction zones. I show a lot of data below without too much interpretation or synthesis. This is on purpose. For those that are skeptical of or claim subduction is a “myth”, you are welcome to interpret these data differently.
IMPORTANT: This post is not a comprehensive review of subduction, this post is not ‘one-stop shopping’ for everything you’ve ever wanted to know about subduction — what I show below is the tip of the iceberg. I list more references at the bottom of the post and at the bottom of Part 3. I strongly encourage you to familiarize yourself with the literature.
Maps and Profiles of Convergent Plate Boundaries
Let’s take a quick look at several of Earth’s subduction zones — to get a sense of which ones have distinct trenches and which ones do not. The following series of maps and topographic profiles were created in GeoMapApp, which is a FREE web tool for exploring Earth’s bathymetry/topography. Again … this is free … it’s not behind subscription, it’s not only for those with a university/corporate license, it’s available to the public to use for research and education. Did I mention that this tool is free?
All maps are more-or-less at the same scale.
First, the focus area for this series of posts — the Cascadia plate boundary region of the northwest United States and southwest Canada. In fact, there are three plates in this view — Pacific, Juan de Fuca, and North American.
Below is a topographic profile across the margin (its position marked by the white horizontal line in the map above).
Next is a plate boundary that does have a significant trench — the Peru-Chile trench of the Nazca/South American margin.
The trench is clearly visible in both the bathymetry (darkest blue color) and in the profile below.
The Sumatran subduction zone got a lot of mainstream press back in 2004 because of the devastating tsunami. As a result, there are some web resources available to the public to learn more about this area (e.g., USGS summary poster on the earthquake).
The Sunda Trench is not as distinct as the Peru-Chile trench but is still discernible. It becomes a much more recognizable trench in the southeast corner of the map and then off the map to the east.
The Aleutian system has a pretty decent trench.
Note that depending on where you put the profile generator (it’s fun, you should try it) the arc barely pops out above sea level.
Here’s the Mariana system. It’s important to note that this is a full-fledged island arc system (two oceanic plates) whereas the Cascadia and Nazca-South American are continental arcs (one oceanic plate, one continental plate). The Sumatran and Aleutian systems are a bit different as their backarc regions are flooded shelfal areas (e.g., see Aleutian profile just above).
As you probably know, Mariana trench is the deepest/lowest place on the surface of the Earth (at nearly 11 km below sea level).
Finally, the last one in this series of maps and profiles is the Nankai Trough region offshore southeastern Japan.
The Nankai Trough is perhaps not as distinct as either the Chile trench and certainly not the Mariana, but is still noticeable in the profile below.
These maps and profiles were presented to show the spectrum of physiographic styles of subduction zones.
Next, I will show a few examples of data from beneath the surface. I’ll start with relatively new data acquired for the Nankai Trough region.
Geophysical Imaging of Subduction Zones
Observing the structure and configuration of the Earth’s crust requires geophysical imaging techniques. Reviewing geophysical theory and the various types of data acquisition and processing methods is way beyond the scope of this post. There are numerous textbooks written on the subject and I found a few good websites as well with some simple googling (e.g., here, here, here, and here).
To my knowledge, the highest-resolution subduction zone imaged to date is the last map/profile I showed above, the Nankai Trough. This is largely a result of this area being the focus area for a research project called the Nankai Trough Seismogenic Zone Experiment (or NanTroSEIZE). This program is in progress — they’ve already done a lot of seismic acquisition and some drilling into the accretionary wedge. I’ve posted about this a year ago (here) but will show the images again right now.
The first image is an absolutely gorgeous bathymetric map of the trough and accretionary complex region. The “frontal thrust” area marks the boundary between the trough and the ‘crumpled up’ (i.e., folded and faulted) accretionary wedge.
This map pattern suggests a compressional structural regime. How can we test that? What does this area look like in cross section?
The perspective block image above is from the 3D seismic-reflection survey. Click on it for a full resolution version. The seismic reflectors clearly show the reverse and thrust faults within the accretionary complex as well as the top of the oceanic crust underneath it. A model of converging plates explains these compressional features. A model of an Earth where there is no convergence must account for these observations (both conceptually and specifically right here on this spot on the Earth). To put it bluntly, how did the Nankai accretionary complex form without convergence?
The Moore et al. (2007) paper goes into much more detail, but if you don’t have access you can find much of these data in the IODP Expedition 314 Preliminary Report, which summarizes one of the drilling legs. In the introduction of this report, there is the following:
The Nankai Trough is a subducting plate boundary, where the Philippine Sea plate underthrusts the southwestern Japan margin at a rate of ~4.1–6.5 cm/y along an azimuth of 300°–315°N (Seno et al., 1993; Miyazaki and Heki, 2001) down an interface dipping 3°–7° (Kodaira et al., 2000).
The Nankai subduction zone forms an “end-member” sediment-dominated accretionary prism. In the toe region off Muroto, a sedimentary section ~1 km thick is accreted to or underthrust below the margin (Moore et al., 2001).
I’ve put the full citations for these references at the bottom of the post for those interested in the details about plate movement rates and azimuth measurements. As you’ll see later in Part 2, the Cascadia subduction zone shares this sediment-dominated characteristic.
The figures below are in the same general area but at a much different scale — instead of several kilometers deep into the subsurface, these seismic wave velocity profiles reveal structure 10s to 100s of km into the subsurface.
Although the authors of these figures, which can be found on the NSF-MARGINS site here, denote their interpretation of the subducting Pacific Plate with the black lines, one can clearly see the distinct west-dipping ‘fast’ (blue color) velocity structure. In addition, the dots show the location of earthquakes. This dipping planar zone of earthquakes that have been documented in association with plate margins interpreted as subduction zones is called the Benioff zone (or Wadati-Benioff zone).
The general relationship of earthquakes to plate boundaries is well established. The figure below (from this seismology textbook) shows global seismicity for all depths in the top figure and just the deep earthquakes in the bottom figure.
These deeper earthquakes are those associated with that dipping Benioff zone as seen above in Fig. 9. These are the measurements, the observations — a model of the Earth where there is no convergence (only divergent boundaries) must account for these observations if the hypothesis is to be further tested.
The image below is the P-wave velocity structure of a transect of the Izu-Bonin-Mariana (or IBM) arc system. This image and much more can be found on the NSF-MARGINS ‘Subduction Factory’ initiative’s webpage for the IBM system.
I’m not going to try and explain all the details of seismic velocity studies here — firstly, the details of it are beyond my expertise and, secondly, that’s not really the point. The P-wave velocity structure data shown above is what it is. Interpret it.
The next image (below) is from the Indonesia-Tonga region (north of Australia) and shows anomalous P-wave velocity structure over a 30 degree transect going down 1500 km (east to the right).
The white dots in the distinct inclined blue band represent earthquake hypocenters. Again … this is just another example of data. You’ll have to read the paper for more details about their methods and how they interpret these data.
Okay … one more — the image below is beneath Costa Rica and again displays seismic wave velocity structure.
Based on these (and more) data, the authors of this paper conclude:
Both the 3-D P-wave velocity structure and petrological modeling indicate existence of low-velocity hydrous oceanic crust in the subducting Cocos Plate beneath central Costa Rica. Intermediate-depth seismicity correlates well with the predicted locations of hydrous metamorphic rocks, suggesting that dehydration plays a key role in generating intermediate-depth earthquakes beneath Costa Rica.
That statement brings up composition — an additional aspect that I’m not going to address in these posts. I’m also not going to talk about arc magmatism, high-pressure/low-temperature metamorphism, and so on. Perhaps other geobloggers can chime in about those topics. But I would point out that, any conceptual model of how the Earth works must integrate ALL the information available.
At the risk of sounding like a broken record … if you disagree with the interpretations in the studies above, please study the papers in detail and evaluate the methods and conclusions. Seismology and tomography are highly technical and quantitative fields of study that take years of training and experience. If necessary, engage the authors for any clarification of acquisition, processing, or other computation related to the data presented. The vast majority of researchers respond well to sincere (and polite) inquiries about their own research. If you think their methods and data are in order, then reinterpret and present a mechanism for the observed patterns.
In addition to seismologic-based investigation of the Earth’s interior, geophysicists also use gravity and magnetics. Addressing the wealth of geophysical data from around the planet from the thousands of researchers over several decades is beyond the scope of this measly little blog post. I encourage everyone to familiarize themselves with the concepts and the data that support these concepts.
Before moving on to Part 3, which deals with the Cascadia plate margin in a bit more detail, I’ll conclude this post with some general words about accretionary wedges.
Accretionary wedges (also known as accretionary complexes or accretionary prisms) are essentially compressional fold-thrust belts composed primarily of oceanic sediment and, in many cases, continentally-derived sediment from the nearby continental plate. The faults and folds, in general, verge towards the oceanic plate (i.e., look at the black and blue lines that show the faults on Fig. 8 above). Anticlinal structures that include some landward-verging reverse faults (referred to as antithetic) are produced, with create the tectonic ridges that you can see in the bathymetric map in Figs. 7 and 8 above. Deformation and sedimentation occur concurrently and incrementally throughout the evolution of the system. There are numerous studies that examine accretionary wedges from around the world and compare and contrast their structural styles (e.g., Scholl et al., 1980 and Moore, 1989 are just two).
If you are somewhat new to the study of the relationships of tectonics and sedimentation I enthusiastically recommend the 1995 textbook ‘Tectonics of Sedimentary Basins’ edited by Busby & Ingersoll (which is apparently now out of print, but used copies can be found). The chapter on trenches and trench-slope basins (by Underwood & Moore) is a fantastic synthesis and great starting point for learning more. Regarding accretionary-style subduction zones, they state:
In accretionary subduction zones, trench-floor and oceanic-plate deposits are added to the toe of the landward trench slope (or inner slope) by imbricate thrusting. A detachment surface, or décollement, separates the upper part of the accreted section (i.e., zone of offscraping) from material that is underthrust beyond the base of the slope. Above the décollement, offscraped sediment is transferred to the accretionary prism (or accretionary wedge), and this prism displays a rugged and irregular seafloor morphology governed by numerous tectonic ridges that form by folding and fault dislocation.
Take one more look at Figures 7 and 8 above of the Nankai accretionary wedge to get a sense of these general patterns.
The figure below is a sketch that illustrates accretionary wedges quite nicely. This is a line drawing of seismic-reflection data that you can find on this site.
It’s important to note that although accretionary wedges from around the planet share certain general characteristics, each one has unique and idiosyncratic features as well.
Part 3 investigates the Cascadia plate boundary region with special emphasis on the sedimentation history and accretionary wedge.
References Cited (note: this is the list for the specific papers referred to above, see this longer list for more about subduction in general):
Busby and Ingersoll, 1995, Tectonics of Sedimentary Basins, Blackwell.
Kinoshita, M., et al., 2008, IODP Expedition 314 Preliminary Report, NanTroSEIZE Stage 1A; doi:10.2204/iodp.pr.314.2008
Kodaira, S., Takahashi, N., Park, J., Mochizuki, K., Shinohara, M., and Kimura, S., 2000. West-ern Nankai Trough seismogenic zone: results from a wide-angle ocean bottom seismic survey. J. Geophys. Res., 105:5887–5905.
Miyazaki, S., and Heki, K., 2001. Crustal velocity field of southwest Japan: subduction and arc-arc collision. J. Geophys. Res., 106(B3):4305–4326. doi:10.1029/2000JB900312
Moore et al., 2007, Three-dimensional splay fault geometry and implications for tsunami generation: Science, v. 318, p. 1128-1131. DOI: 10.1126/science.1147195
Moore, G.F., et al., 2001. New insights into deformation and fluid flow processes in the Nankai Trough accretionary prism: results of Ocean Drilling Program Leg 190. Geochem., Geophys., Geosyst., 2(10). doi:10.1029/2001GC000166
Moore, G.F., Taira, A., Klaus, A., et al., 2001. Proc. ODP, Init. Repts., 190: College Station, TX (Ocean Drilling Program). doi:10.2973/odp.proc.ir.190.2001
Moore, J.C., 1989, Tectonics and hydrogeology of accretionary prisms: role of the decollement zone: Journal of Structural Geology, v. 11, p. 95-106.
NSF-MARGINS SubFac (subduction factory) research initiative website: http://www.nsf-margins.org/SF/SF.html
Scholl, D.W., vonHuene, R., Vallier, T.L., Howell, D.G., 1980, Sedimentary masses and concepts about tectonic processes at underthrust ocean margins: Geology, v. 8, p. 564-568.
Seno, T., Stein, S., and Gripp, A.E., 1993. A model for the motion of the Philippine Sea plate consistent with NUVEL-1 and geological data. J. Geophys. Res., 98:17941–17948.
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.
Other References on Note:
List of research projects from Harvard Seismology group.
Fowler, 2004, The Solid Earth: An Introduction to Global Geophysics: Cambridge University Press, 704 p.
Lowrie, 1997, Fundamentals of Geophysics: Cambridge University Press, 368 p.
Shearer, 1999, Introduction to Seismology: Cambridge University Press, 204 p.
Stein and Wysession, 2003, An Introduction to Seismology, Earthquakes, and Earth Structure: Blackwell Publishing.
Turcotte, 2001, Geodynamics: Cambridge University Press, 528 p.
A great popular/nontechnical book is Naomi Oreskes ‘Plate Tectonics: An Insider’s History of the Modern Theory of the Earth’.