olelog

Dinochick featured the paper Constraints on the early uplift history of the Tibetan Plateau by Wang & al. in this post. The good news is that it is freely available online through the PNAS open access option, so that everybody can read it. I appreciate that.

Let me just quote the Concluding Remarks from the paper:

We propose a temporally and spatially differential surface-uplift history of the Tibetan Plateau . Our integrated study suggests that the central plateau (the Lhasa and southern Qiangtang terranes) was uplifted by the Late Paleogene. A high proto-Tibetan Plateau may have contributed to climatic changes farther north in central Asia. Intriguingly, this timing also corresponds to a period of pronounced global cooling and changes in ocean chemistry. The plateau subsequently expanded as a result of the continued northward collision of India with Asia. To the south, the Himalayan rose during the Neogene. To the north, the Qilian Shan rapidly uplifted in the Late Cainozoic. These ranges constitute the modern southern and northern margins, respectively, of the Tibetan Plateau.

Late Paleogene - The Paleogene period ended around 23 million years ago
Neogene - The Neogene period started around 23 million years ago
Late Cainozoic - The Cainozoic Era covers the 65.5 million years since the Cretaceous–Tertiary extinction event at the end of the Cretaceous that marked the demise of the last non-avian dinosaurs and the end of the Mesozoic Era. The Cainozoic era is ongoing. The Cainozoic is divided into two periods, the Paleogene and Neogene.

The surface uplift history of the Tibetan Plateau and Himalaya is among the most interesting topics in geosciences because of its effect on regional and global climate during Cainozoic time, its influence on monsoon intensity, and its reflection of the dynamics of continental plateaus.

One of the many analytical techniques used in the study was Fission Track Analyses. This technique seems to have become quite popular lately - at least in many of the papers I have been reading. Apatite fission track analysis has become an important and successful tool in low-temperature thermochronology and during the last decade it became applied in numerical tectonic modelling, assessing tectonic hazards, landscape development, tectonic geomorphology, dating processes of mountain building, hydrocarbon exploration, sedimentary burial history, and much more.

Here is a paragraph from Wikipedia’s article on Fission Track Dating:

Fission track dating is a radiometric dating technique based on analyses of the damage trails, or tracks, left by fission fragments in certain uranium bearing minerals and glasses. Uranium-238 undergoes spontaneous fission decay at a known rate. The fragments emitted by this fission process leave trails of damage in the crystal structure of the minerals enclosing the uranium. Etching of polished surfaces of these minerals reveals the spontaneous fission tracks for counting by optical microscopic means. The number of tracks correlates directly with the age of the sample and the uranium content. To determine the uranium content the sample is annealed by heating and exposed to a barrage of thermal neutrons. The neutron bombardment produces an induced fission of the uranium-235 in the sample and the resulting new induced tracks are used to determine the uranium content of the sample as the U-235:U-238 ratio is well known. Alternatively, a uranium-free piece of mica, the external detector, is attached to the sample and both sample and mica are exposed to a barrage of thermal neutrons. The resulting induced fission of the uranium-235 in the sample creates new induced track in the external detector, which are revealed by etching. The ratio of spontaneous tracks to induced tracks is proportional to the age.

• http://paleochick.blogspot.com/2008/03/new-findings-from-tibetan-plateau.html
• http://www.terradaily.com/reports/New_Findings_From_Tibetan_Plateau_Suggest_Uplift_Occurred_In_Stages_999.html
• http://www.physorg.com/news125598631.html
• http://geology.cr.usgs.gov/capabilities/gronemtrac/geochron/fission/tech.html
• http://www.mnsu.edu/emuseum/archaeology/dating/dat_fission.html

On Monday, 3 March 2008, a Magnitude 5.4 earthquake struck the Owen Fracture Zone Region (according to USGS). This reminded me of a paper in the first issue of Nature Geoscience.

In the Arabian Sea the 1,100 km long Owen fracture zone marks the boundary between the Indian and Arabian tectonic plates (see map). Both plates are colliding with the southern edge of Eurasia but the Arabian plate is generally considered to be moving north-eastward slightly faster than the Indian plate, and it is this difference in motion that is accommodated by the Owen fracture zone. This motion seems to have started between 3 and 8 million years ago.

The 3 March earthquake is marked with an orange coloured star on the USGS map of historic earthquakes shown below.

On this map the Sheba Ridge and the Owen transform fault down to the Carlsberg ridge are clearly delineated by historic earthquakes (1990 - present). Lesser earthquakes are seen along the Owen fracture zone (green line) - this is after all one of the slowest plate boundaries on Earth with a moving rate estimated as only about 2 mm/year. There is a conspicuous seismic gap in the southern end of “the green line” (marked with a question point on my map above) with some diffuse earthquakes West of the line.

Fournier et al. treated this area in a paper titled “In situ evidence for dextral active motion at the Arabia–India plate boundary” published in Nature Geoscience 1, 54 - 58 (2008) - Published online: 2 December 2007 | doi:10.1038/ngeo.2007.24 .

Their study suggest that a wedge of the Arabian plate, approximately corresponding to my orange coloured triangle, has been transferred to the Indian plate at some time in the last 10 million years. This is now a 50 km wide pull-apart basin, where an ultraslow divergent boundary (“spreading ridge”) has been developing. As the diffuse earthquakes seem to show the deformation is not yet clearly localised, but correspond to a transient state preceding the birth of a new plate boundary, and a new triple junction approximately where the earthquake on Monday occurred (orange star). A more stable ridge-ridge-ridge triple junction than the recent situation.

• http://www.nature.com/ngeo/journal/v1/n1/full/ngeo.2007.24.html
• http://www.nature.com/ngeo/journal/v1/n1/full/ngeo.2007.56.html

How deep subducting plates (slabs) penetrate into the mantle is subject of debate. An article by Saskia Goes et al. titled "Evidence of lower-mantle slab penetration phases in plate motions” published in Nature of 20 February 2008 may improve scientists understanding of tectonic plate movements and thereby potentially improve their ability to assess earthquake risks.
When oceanic lithosphere cools it gets denser. This means that dense, strong lithosphere overlies light, weak asthenosphere. At subduction zones denser oceanic crust sinks into lighter mantle, and many geologists today believe that this density difference is what drives plate tectonics. The slab is not being pushed into the mantle, but pulled into it. It is a top-down drive and not a bottom-up drive (by up-welling magma at mid ocean ridges).

The authors found surprisingly that, contrary to common scientific predictions, dense plates tend to be held in the upper mantle, while younger and lighter plates sink more readily into the lower mantle.

Subduction of younger lithosphere (of ages less than about 60 million years) often subducts up to two times faster than older, fundamentally denser, lithosphere. Old, dense and relatively stiff plates tend to flatten upon reaching the boundary between the upper and the lower mantle (at a seismic discontinuity at around 660 km down), 'draping' on top of it. By contrast, younger more malleable plates tend to bend and fold above the boundary of the lower mantle for tens of millions of years until they form a critical mass that can sink rapidly into the lower mantle. When this mass moves into the lower mantle, the part of the plate still at the surface is pulled along at high speed.

Although it may be a common view that slabs penetrate the 660 km discontinuity and sink deep into the lower mantle, it is still controversial. The new findings may be seen as evidence of plates transiting from the upper into the lower mantle. The authors also believe their findings to be consistent with seismic images of the distribution of subducted material in upper and lower mantle.


Reference:
Saskia Goes, Fabio A. Capitanio & Gabriele Morra, "Evidence of lower mantle slab penetration phases in plate motions", Nature, 21 February 2008. Volume 451 Number 7181 pp865-1028. doi:10.1038/nature06691

• http://www.nature.com/nature/journal/v451/n7181/abs/nature06691.html
• http://www3.imperial.ac.uk/newsandeventspggrp/imperialcollege/newssummary/news_21-2-2008-13-50-20?newsid=28714
• http://www.scientificblogging.com/news_releases/odd_tectonic_plate_movement_lighter_ones_sink_into_the_lower_mantle
• http://www.alphagalileo.org/index.cfm?_rss=1&fuseaction=readrelease&releaseid=527313
• http://www.terradaily.com/reports/Imperial_Scientists_Explain_Tectonic_Plate_Motions_999.html

I had just started writing a new post concerning a paper that could change our view on mantle plumes, when I became aware of a weird piece in Der Spiegel. The post on which I began will now have to wait till tomorrow, but in some ways the two articles are related.

On January 25 2008 Spiegel Online International brought an article titled Where Continents Go To Die - A New Look into the Center of the Earth.

Let me start with saying that I don’t believe in this theory (yet), but find it worth discussing. So here we go.

Old, cold plates are pushed down into the Earth's mantle on the continental edges, where they collect large amounts of iron. Weighted down by the iron, the plates sink farther and farther into the Earth's mantle. There, at a depth of 2,900 km, they settle into "plate graveyards”. Heat and pressure in the depths trigger chemical processes, causing the plates to deposit their load of heavy elements. Once liberated of this burden (a few hundreds of millions years later), they become lighter than their surroundings, causing them to rise and as mantle plumes they make their way toward the surface (at hot spots etc.). Well, that is the biggest convection cell that I have ever heard of, and I must stress that this is only a (new) theory, that in no way has been proven by facts, as far as I know.

More about the mantle plume bit in my next post here.


• http://www.spiegel.de/international/world/0%2C1518%2C531023%2C00.html
• http://www.spiegel.de/international/world/0,1518,grossbild-1078154-531023,00.html




PS:

1. In another context (science journalists/journalism) Chris (goodSchist) had an indirect comment to this post over at Clastic Detritus (Response#2).
   
2. There is a dedicated mantle plume website at http://www.mantleplumes.org/ with links to lots of (free) papers on mantle plumes.
   

Apart from the question how early in Earth’s history plate tectonics and thereby subduction processes began, it seems clear that the way subduction works has changed over time.

1) 4 billion years ago the interior was hotter than now, leading to melting of the subducted crust at a more shallow depth.
2) The dominant composition of the magmas forming above subduction zones has changed over time.

The evolution of the subduction zones may have followed the pattern described by Martin & Moyen in 2002, and I shall briefly summarise some of it with reference to 4 diagrams.

A. In early Archaean (around 4 billion years ago) the temperature gradient along the Benioff zone was very high, and therefore the subducted oceanic crust melts at shallow depth. The subduction itself was also shallow compared with the situation today.
B. In middle to late Archaean Earth was cooler, the temperature gradient was lower, and the melting occurred at lower depth.
C. At the Archaean-Proterozoic transition the temperature gradients are too low to allow high degree of melting in the subducted crust. Instead melting in the mantle wedge plays a greater role.
D. Is the situation we have today.
The evolution was gradual, and there is an overlapping of the different phases. But each step had its own peak.

And now to the sanukitoids. Sanukitoids are a variety of granitoids (granite-like crystalline rocks) with a high magnesium (Mg) content. They are called "sanukitoid" because of their similarity in bulk chemical composition to high-magnesium andesite from the Setouchi Peninsula of Japan, known as "sanukites" or "setouchites". Sanukitoids are found in 3.0–2.7 billion years old Neoarchean terrains in many parts of the world, including the Karelian craton in northern Finland.

We are here at the transition from step B to step C in the subduction evolution diagrams. The forming of sanukitoids has been explained with a two stage process. Firstly, fluids and/or melts from subducting oceanic crust enriched the mantle wedge peridotite during subduction. Secondly, the enriched (metasomatised) mantle wedge partly melted after or at the end of subduction. The high magnesium content in sanukitoids is explained by magnesium from peridotite in the mantle wedge. The composition of sanukitoids and their occurrence in a narrow time span means a unique change in plate movements near the Archean-Proterozoic boundary.

Over 20 sanukitoid intrusions have been found in the Karelian craton. Understanding of the compositional variety of the sanukitoid series gives further knowledge of the evolution of the Karelian craton and possible Archaean subductions. Comparing the geochronological and geochemical data of sanukitoids from different cratons may be one way of tracking Archaean supercontinents. Further geophysical studies and geochemical and isotope data will be needed to enhance our understanding of the beginning of modern style plate tectonics.

References:
Heilimo & Halla : Sanukitoid series granitoids in the western Karelian craton – implications to Neoarchaean plate tectonics (Abstract Volume, 28th Nordic Geological Winter Meeting. January 7 - 10, 2008, Aalborg, Denmark)

Martin H., Moyen J.-F. 2002: Secular changes in TTG composition as markers of the progressive cooling of the Earth. Geology, 30(4), 319-322. ( http://www.gsajournals.org/perlserv/?request=get-abstract&doi=10.1130%2F0091-7613(2002)030%3C0319%3ASCITTG%3E2.0.CO%3B2 )

Zeszyt 26, 2005; Volume 26, 2005 Hervé MARTIN, Jean-François MOYEN The Archaean-proterozoic Transition: Sanukitoid and Closepet Type Magmatism (free available as pdf file - download: http://www.geo.uw.edu.pl/PTMINSP/2005/057.pdf )

Wikipedia - http://en.wikipedia.org/wiki/Sanukitoid