The India-Asia Convergence Cannot Be Explained By Plate Tectonics Theory

Yep...! I am quoting geoscientist Fabio Antonio Capitanio from an article that appeared in the Deccan Herald on Jan 10.

The article describes a paper from the Jan 10  issue of Nature Geosciences. India–Asia convergence driven by the subduction of the Greater Indian continent

From the press release:

Almost 50 million years ago, the Greater Indian plate collided with the Eurasian plate leading to the formation of the mighty range. Usually, when two continental plates collide, the movement of the two plates stop. That did not happen with the Indian-Eurasian convergence.

The two plates continue to rub against each other generating stress, which is later released as earthquakes. An international team has now offered an explanation. In Sunday’s edition of “Nature Geoscience,” they suggest an internal mechanism that creates  the drag which pulls the Indian plate towards its boundary with Eurasia.

“The India-Asia convergence cannot be explained by plate tectonics theory, which is deeply challenged,” said team leader Fabio Antonio Capitanio, a geoscientist at the University of Monash, Australia.

 
I don't have access to the full paper but from the abstract I gathered that the research uses estimates of the density of the Indian lithosphere and then models the forces prevalent in the India-Asia convergent zone. The Indian lithosphere has delaminated at the convergence zone. The upper crust has been scraped off and has been stacked into the Himalayan frontal ranges. The lower continental slab is denser than the underlying mantle and sinks into the mantle providing the necessary drag that explains the continued convergence at the collision zone.

The schematic below show the major tectonic faults across the India- Asia collisional zone. The lower part of the figure is at best speculative and shows the Indian plate subducting underneath the Asian plate but until recently not much was really known about the nature, structure and extent of this subduction.

What this study suggests is that the traditional plate tectonics forces of ridge push and slab pull that are drivers of plate motion may have diminishing influence on convergence at late stages of plate convergence. Continental crust is lighter than the mantle and in continent continent collision zones crustal  buoyancy should be putting the brakes on subduction and convergence.  Instead the model results suggest that internally generated forces driven by density differences at the site of the collision may be providing the necessary force to drive convergence.

Its an interesting result but I have to say I find Dr. Capitanio comment that plate tectonic theory is "deeply challenged" a bit hyperbolic.

You can think of an analogy from biology where a structure, trait, feature and so on can be explained at two levels. A proximate explanation may include the immediate physiological or mechanical causes but there is a higher level ultimate explanation which is based on evolution.

I don't think geoscientists seek a proximate explanation using plate tectonic theory for every aspect of Himalayan tectonics and structure. For example the great relief of the Himalayan ranges and some internal folded structures are best explained using concepts of vertical movements of crustal blocks driven by denudational isostasy. This means that continents bob up in response to unloading or removal of overlying mass by erosion.

For the Himalayan ranges there may be many proximate explanations. Isostasy and in this case if the models are correct internal density differences which is dragging the Indian continental slab down. But it is plate tectonics that has created these conditions in the first place. It is in a sense the ultimate explanation.

The key here is the phrase continued convergence. The India-Asia plate convergence zone has a long history. Convergence initially  involved interaction of oceanic crust with oceanic crust, then as one oceanic crust was consumed by subduction, it was ocean crust with continental crust and finally the big Wham! Continental crust on both the Indian and Asian plates colliding. A recent study published in Science which seismically imaged the deep crustal and mantle roots of the Himalayas across the India-Asia collision zone found that the Indian plate has slid about 450 km underneath the Tibetian (Asian) plate and the upper mantle appears to shear off and sink. For most of this history conventional plate tectonic forces of ridge push and slab pull can explain the convergence.

It is in the latter mature stages of continent continent collision that this new study finds a role for autochthonous mechanisms.

Tip: Ontario-geofish