A new Global Tectonic Paradigm--An extended thought experiment


(Summary: First principles and what we can see of the geology of Earth's simpler twin, Venus, inform us of a different way of understanding Earth processes.)

>>>>This is a draft--a work in progress.<<<<
My goal is to present this at the 2008 GSA conference

Since people started thinking about geology, they have applied overarching principles to attempt to understand smaller scale phenomenae. Early workers sought evidence of the hand of God and explained features as remnants of the Great Flood. Later, local geology was explained by the geosyncline theory--though the causes of these mega-features weren't ever convincingly explained. Still later, plate tectonics driven by mantle convection was used to explain features in the oceans such as trenches and ridges and features on the continents such as mountain ranges and rift valleys. Plate tectonics, like geosynclines and the Great Flood before it, is the paradigm in which contemporary scientists work to incrementally advance geology.

But all is not well. While the plate tectonic model seems to fit some things it doesn't fit others or fails to explain important phenomena. Some things, such as the nature of hot spots and subduction and other things as well seem to have serious problems. Perhaps the paradigm needs some tweaking, or perhaps it needs an overhaul. At the same time, knowledge of the deeper regions of the Earth and the physical principles that govern the Earth have increased substantially. Perhaps we need to take this new knowledge and start over.

This page is my attempt to do that. Being uneducated in geology on a global scale, I attempt here to figure out why the rocks are the way they are--that is what global tectonic processes drive creation and placement of the rocks we see. So here we go . . .

First principles:

  • The mantle is internally heated by radioactive decay
  • The crust is internally heated by radioactive decay and cooled by exposure to the atmosphere and space
  • Viscosities are such that heat generated in the mantle and crust will cause the mantle and crust to move--convection will happen
  • To the extent convection causes material to move up, material will also have to move down
  • Pressure and temperature increase from the surface to the core and mineralogy and viscosity respond to those changes
  • The mantle and deeper crust are more ductile and less brittle and the upper crust is less ductile and more brittle.
  • The character of the crust will affect heat flow from the mantle.
A note on the "ethic" of having read the "literature"

The scientific disciplines I address on this page are defined by tens of thousands of detailed peer reviewed scientific papers. The scientists that advance these fields have spent years and decades absorbing these papers. I admit that I have not read much at all. I have read a few papers--some are cited. I have skimmed some more. And if a paper is on the web, it gets more attention from me--probably anyone. (Paying $39 is ridiculous. Interlibrary loan takes time.) Does this disqualify me? Obviously, I don't think so, or I wouldn't be writing this. Sometimes too much knowledge is a hindrance to finding the answers. I suspect that since the first map was made, school kids have been saying the continents drifted. It took Alfred Wegener, a meteorologist, to really make the case for continental drift. But even then the geophysicists killed his idea because the viscosities weren't right--or some other thing. So nothing is sacred here. I will go where the ideas lead. That is the only ethic.


Working Assumptions:

  • Instabilities will develop within and below the crust. This is because of cooling or mineral phase changes or both. In particular, more dense units may overlie less dense units. To the right is a column of densities by Don Anderson.
  • Instabilities will create conditions favorable for Rayleigh-Taylor drip-like movements. The result of this will be a style of convection that favors top-down drips. The cartoon below illustrates how this convection would work. (Again, thanks to Don Anderson.)

from: D.L. Anderson, Top-Down Tectonics, Science, V. 293, P. 2016. http://www.mantleplumes.org/WebDocuments/TopDown.pdf

Three ways to flow. (Top) A fluid layer cooled from above or from the side, or heated from within, develops narrow cold downwellings that cool the interior. The downwellings are terminated by phase changes, density increases due to composition, or high viscosity. There are no active or hot upwellings. This resembles the upper mantle. (isolated chemical layer Middle) A high viscosity or cooled from above develops large cool downwellings. This mimics the mid-mantle (1000 to 2000 km). (Bottom) A deep, dense, high-viscosity layer with low thermal expansion overlying a hot region develops large, sluggish upwellings. This mimics the deep mantle.
(Figure description changed per personal communication with DLA)


from Don Anderson, Large Igneous Provinces, Delamination, and Fertile
Mantle, Elements, Dec 2

 

A simpler planet . . . http://www.solarviews.com/cap/venus/venusint.htm

 

The Earth suffers from continual continual resurfacing on a very short time scale. On the continents, erosion, continental collision and a myriad of other tectonic forces conspire to wipe out features that might simply illustrate the mechanics crust mantle interaction. The ocean basins are better with their subduction zones, but even there much remains hidden and much disappears into those subduction zones. And the floor of the ocean is all very young being created and destroyed in a very short time.

Venus doesn't have this problem. It is an Earth-sized planet that should have many of the characteristics of Earth, quite likely including a crust and mantle. There are differences, such as the absence of a magnetic field indicating a solid core and thus perhaps lower and the lack of a moon, eliminating a potentially significant source of energy. (More about these later.) There may be elemental differences, such as lower potassium, that reduce heat flow as well. See whamilton But for now the difference to focus on is that there are no subduction zones to consume the evidence of crust mantle interaction.

Of course Venus is a long way away and we don't really know all that much about it. We do have radar imagery of the surface which has allowed us to produce topographic and gravity maps. Some data has been acquired from __ Russian landers as well. While we don't know a lot, we can make some inferences.

Venus has several kinds of features, but the most prominent are circular features called coronae (corona is the singular) and rift-like features called chasmae. I will focus on these two. Other features such as arachnoids and novas may be different or may actually be phases of coronae.

Coronae

Most geologists have interpreted coronae to be hot spots--that is they are the results of convective plumes rising from the core mantle boundary. Of course this is just speculation. There is no information on the interior of Venus other than the radar images. In fact, what has happened is that geologists have just transferred a theory of geology from the Earth--the existence of mantle plumes--to Venus. But mantle plumes are increasingly questioned even here on earth. (See mantleplumes.org) Another well-respected geologist has rejected the notion of mantle plumes on Venus and contended that the Coronae are merely the remnants of craters. _________________.

Here is a slice of a larger photo of Venus (full photo here 4.713MB) showing coronae and some chasma and craters too. The colors represent elevations based on the Magellan radar altimetry.

So, to those two lines of speculation, I will add a third--the idea that Coronae are the result of Rayleigh-Taylor instabilities--or Rayleigh-Taylor drips. Actually, this isn't really my idea, it was previously investigated by Trudi Hoogenboom and Gregory Houseman. Hoogenboom, T & Houseman, G., Rayleigh-Taylor instability as a mechanism for corona formation on Venus, Icarus 180 (2006) 292-307. (See also Hamilton and Stofan (1996), Tackley and Stevenson (1991) and Tackley et al. (1992)) While Hooganboom and Houseman conclude that at least some coronae are more likely caused by plumes, they find that it is possible that most if not all could be explained by Rayleigh-Taylor drips.

Above is a table from Hoogenboom and Houseman showing size, topographic deflection, number, and gravity anomaly by topographic profile. Diameters range from 60 kilometers to 2600 kilometers and topographic deflections range from +3500 meters to -3800 meters. These are numbers to keep in mind as we follow this idea.

 

Hoogenboom and Houseman use numeric modeling to show that Rayleigh-Taylor instabilities can produce the observed corona profiles as they evolve over time. Thus a corona that was a depression at one time might be a rim, dome or plateau at another. They use dimensionless time units, but other numeric modeling has been using assumptions from Earth and real time units.

The figure at the right shows topography over time for one set of assumptions for a central downwelling based on Earth assumptions. As can be seen in the lower graph, the instability results in a rapid down-drop and a short period of negative topography followed by a longer period gradually declining net positive topography. The graph depicting topography as a function of distance from the center, shows potential dome and rim configurations.

Prior to Hoogenboom and Housman's work, the most successful model was that of Smrekar and Stofan (1997). This involved a rising plume followed by lithospheric delamination to cause the various topographic shapes. Give the complexity of this idea and the speculativeness of the sequence--not to mention the plumes themselves, this idea seems less likely.

In looking at any of these numeric models, it is important to recognize that the result is very much dependent on the initial assumptions. And those assumptions can be speculative to say the least--especially in the case of Venus where we know so little.

Another matter that bears some mention is volcanism. Coronae are often referred to as volcanic features, but this characterization is a bit misleading. Volcanism is associated with coronae--or at least most coronae, but it has not been traced to a given phase, location, or feature.


From: Time-dependent surface topography in a coupled
crust–mantle convection model, Russell N. Pysklywec and
M. Hosein Shahnas, July 12, 2003 15:21 Geophysical
Journal International
http://www.physics.utoronto.ca/~shahnas/Publication/gji1987[268-278].pdf

Here is a perspective view of Atete Corona (foreground). From
http://photojournal.jpl.nasa.gov/jpegMod/PIA00096_modest.jpg
These things don't look like craters to me.

The variety of topographic forms of coronae and the differences in timing and association of volcanism suggest a complex evolution for these features. Some have suggested that they begin as domes and evolve to depressions and then to rings. __________________. It seems likely to me that this sequence may be more related to a starting assumption that coronae are caused by plumes than observable facts. Historical interpretations of Venetian landforms are little more than speculation. I would posit that it is just as likely that evolution is from depressions to domes (or plateaus) to rings as suggested by the example shown above. Whatever the sequence, observation of Venus suggests a complex evolutionary sequence for coronae. If Rayleigh-Taylor instability is the cause--as I have assumed--then the first phase is down-drop. This will be significant to our considerations of Earth.

 

Chasmae

To the right, is another slice of a Venus photo (full photo here 4.518MB) showing the Dali Chasma, Atahensik Corona (left center) an Zemina Corona (upper right).

Chasma are rift-like features, but they are more complex than that. They are associated with chains of Corona, though the chains don't seem to have any particular time sequence. There are some compressional as well as extensional aspects to chasma. Hamilton and Stofan 1996. (See also Swafford & Kiefer 2004)

Chasmae and coronae are closely associated. There tend to be more coronae in the area of chasmae though some coronae are found in areas without chasmae. There is also a rough correlation of chasmae with geoid highs.

 

 

So with that we will return to Earth.

Back to Earth . . . http://www.solarviews.com/eng/earth.htm

Start with Africa, that's the easiest

If what appears to be happening on Venus really relates to the Earth, then even with Earth's resurfacing, it ought to be apparent somewhere. More than a few workers have noted the similarities to of the corona-chasma chains of Venus--an example of which was shown above--to the rift system and superswells of East Africa. A series of approximately circular features resembling coronae are intertwined with the East African Rift. In the diagram, the red lines are simplified traces of the rift.

(From Davis and Slack http://www.ess.ucla.edu/faculty/davis/2001GL013676.pdf I am looking for a better picture to illustrate the topographic and tectonic system of the East African Rift. The USGS has produced a particularly lame map that appears in everything from children's books to academic articles, but it shows almost nothing.)

Before we explore the East African rift more, I would like to return to the physics of Rayleigh-Taylor instabilities--or at least a rough approximation. Pysklywec and Shahnas (cited above) give some interpretation to the results of their numeric models. Two figures copied from their work show the results at an intermediate time. The figure to the right is a cartoon showing how uplift occurs. The one below gives more detail on deformation.

From: Russell N. Pysklywec and M. Hosein Shahnas, Time-dependent surface topography in a coupled crust–mantle convection model, July 12, 2003 15:21 Geophysical Journal International http://www.physics.utoronto.ca/~shahnas/Publication/gji1987[268-278].pdf

 

The Pysklywec and Shahnas interpretations shown here reflect only snapshot of a system that changes over time. The graphical results they present show how some situations evolve. It should be possible to plot similar cartoons accurately from their data. To reflect how the system might evolve I have created a series of cartoons. While they take their inspiration from Pysklywec and Shahnas, they represent only a personal intuition of the time sequence.

 

Beginning with no disturbance in the first frame an instability develops and the crust, lithosphere, and mantle are depressed. As the system evolves, flow in the crust and lithosphere cause uplift. As the drip continues it draws crust to its maximum depth. Isostatic forces resist the pulling and rebound affects both deformation and topography. Finally, the drip releases and the depression inverts as the isostatic forces take over.

So what evidence is there that a Rayleigh-Taylor drip might be responsible for the geologic and tectonic pattern seen in the East African Rift? The three figures below are from Weeraratne, Dayanthie S., Forsyth, Donald W. , and Fischer, Karen M.; Evidence for an upper mantle plume beneath the Tanzanian craton from Rayleigh wave tomography, JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. B9, 2427, 2003 http://www.dtm.ciw.edu/weeraratne/tanzanproof.pdf The one in the upper left shows seismic radial anisotropy that one might expect from a Rayleigh-Taylor drip that had the characteristics shown in the numeric simulations above. Anisotropy is caused by deformation of minerals. Weeratne et al, attribute the pattern to a mantle plume. There are some problems with this. First of all, the seismic velocities shown in the other two figures show that the area under the Tanzanian Craton is cold--cool colors show faster velocities which are interpreted by seismologists as hotter rocks. Second, their other data shows the depths of the anisotropy is relatively shallow and within the cool areas shown in Figure 11. This suggests to me that they are looking at fossil anisotropy that is more likely caused by a prior Rayleigh Taylor instability.

Tomography suggests that this area is not hot, and thus not a hotspot. Amal Sebai, El eonore Stutzmann, Jean-Paul Montagner, Deborah Sicilia, Eric Beucler; Anisotropic structure of the African upper mantle from Rayleigh and Love wave tomography, Physics of the Earth and Planetary Interiors 155 (2006) 48–62 http://www.ipgp.jussieu.fr/~stutz/sebai_stutzmann_al_pepi_2006.pdf

I hope to develop this topic more later.

Other Examples . . .

Here is a photo from the Council area on the Seward Peninsula of Alaska. Circular features showing recent uplift (note the drainage anomalies in the center of the circle) on the Seward were among the first things that lead me to the ideas expressed here. The circles are of interest because of their correlation with gold deposits. From a more academic point of view, they are of interest because of their relationship to gneiss domes. It could be that gneiss domes represent the inverted centers of rebounds illustrated above. Again, I hope to develop these ideas more thoroughly in the future.

Yet other examples that I hope to be able to develop are the Gulf of Mexico, the Athabasca Basin, the Williston Basin, Hudson Bay, the Rio Grande Rift, and others. It is possible that Rayleigh-Taylor instabilities can be induced by meteorite impacts. Examples of this that I am looking into now include the Witwatersrand, the Athabasca, and Beaverhead. Important economic mineral deposits are associated with many of these structures.

 

Complicating factors . . . The Earth is different

(>>Draft<< The following discussion is skeletal.)

While Venus is about the same size as the Earth, its surface looks very different. In fact the crust of the Earth is moving while the crust of Venus appears to be stationary. Earth's crustal movement has been documented in studies of plate tectonics.

Why would Earth's crust move while it's twin Venus' would not? There is one obvious possibility and some that may not be obvious. First, the obvious one. The Earth has a moon, while Venus does not. The Moon will exert an east to west drag on the crust that could, given proper viscosities, move the crust. This paper expresses the idea far better than I can: The westward drift of the lithosphere: A rotational drag?; B. Scoppola, D. Boccaletti, M. Bevis, E. Carminati, and C. Doglioni GSA Bulletin; January/February 2006; v. 118; no. 1/2; p. 199–209

(Less obvious differences between Earth and Venus involve speculation on lower abundance of radiogenic isotopes in planets closer to the sun and thus lower heat flow to drive convection on Venus. There is no hard evidence that this is the case and I do not address that hypothesis here.)

The following figures show one interpretation of movement of the Earth's crust relative to the mantle:

 

Figure 8. Present-day plate velocities relative to the shallow-hotspot reference frame, option 2 of Figure 3, incorporating the NUVEL1A relative plate motions model. Note that in this frame all plates have a westward component. From Carlo Doglioni & Marco Cuffaro, The hotspot reference frame and the westward drift of the lithosphere, http://www.mantleplumes.org/Hotspots.html.

 

 

Figure 9. Connecting the directions of absolute plate motions that we can infer from large-scale rift zones or convergent belts from the past 40 Ma, we observe a coherent sinusoidal global flow field along which plates appear to move at different relative velocities in the geographic coordinate system (after ). From Carlo Doglioni & Marco Cuffaro, The hotspot reference frame and the westward drift of the lithosphere, http://www.mantleplumes.org/Hotspots.html.

The effect of crustal movement is to distort the shape of Rayleigh-Taylor circular features in many places on Earth. The movement of the crust relative to the mantle also cuts the roots off of R-T collapses causing fossil anisotropy like that seen in the African example above.

Conclusions

Rifts and Coronae on Venus can be related to convection in that planet's interior. The same principles apply to Earth and result in--at least in the first instance--similar features on Earth. However, because unlike Venus, the surface of the Earth is moving, relative to the Earth's mantle, the features are distorted and hard to recognize. Once we understand the origin of features on the surface of the Earth as being related to mantle convection--and particularly Rayleigh-Taylor instability-- reinterpretation of geology should lead to a much better understanding of the Earth.

 

References

 

Carlo Doglioni & Marco Cuffaro, The hotspot reference frame and the westward drift of the lithosphere, http://www.mantleplumes.org/Hotspots.html

Hamilton, V.E. and E.R. Stofan (1996), The geomorphology and evolution of Hecate Chasma, Venus, Icarus, 121, 171-194. http://www.higp.hawaii.edu/~hamilton/papers/Icarus_Hecate.pdf

Hoogenboom, T. and G.A. Houseman (2006), Rayleigh-Taylor instability as a mechanism for corona formati

 

 

First posted: June 6, 2007

Last update: September 26, 2007 (some glitches were added then and haven't been corrected. Sorry.)

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