Tuesday, October 13, 2020

Himalaya: Critical Wedge

 A while ago a friend commented that she had learned in school geography class that Himalayan faults are the youngest. Now, earthquakes in Bhuj, Gujarat, and Latur,Maharashtra, in recent times tell us that there is active faulting going on elsewhere in India too. But I could understand what she was trying to say, that the Himalaya is a growing mountain chain with active faulting. 

I think a lot of non-geology folks appreciate this point. What is not that well known is that within the Himalaya the locus of active faulting has shifted southwards over geologic time. The Himalaya is the northern margin of the Indian continental crust which has been broken up into blocks or litho-tectonic units and stacked by major faults. From north to south these faults are, South Tibetan Detachment, Main Central Thrust, Main Boundary Thrust, and Main Frontal Thrust. There are plenty of minor faults between these major breaks. 

As the Indian continent collided and underthrust Asia, slices of its crust were pushed up in the following order; The Tethyan Himalaya along the South Tibetan Detachment (45-35 million years ago), the Greater Himalaya along the Main Central Thrust (24-15 million years ago), the Lesser Himalaya by the Main Boundary Thrust (and many subsidiary faults, 11-5 million years ago), and the Siwaliks by the Main Frontal Thrust (and some subsidiary faults, 1 million years ago to recent). All these faults merge at depth with a north dipping (sloping) master fault known as the Main Himalaya Thrust along which the India plate is underthrusting or sliding underneath Tibet. The Himalaya is deformed Indian crust riding atop the MHT.

Why did faulting activity migrate southwards in this growing orogenic (fold and thrust) mountain belt? Geologists give a mechanical explanation of this style of mountain growth using the Critical Wedge Model. 

It will be worth pausing this post to watch a video made by Middlebury Plate Tectonics on Critical Wedge Theory. Email subscribers who cannot see the embedded video may watch it at this link- Critical Wedge Theory- Himalaya.


To summarize, the Himalaya may be abstracted as a wedge of crust which is thicker in the north and thinner towards south. The ratio of normal stress to shear stress controls whether a fault can slip. As crust thickens beyond a threshold value of the ratio, increased normal stress can pin down and lock a fault. Subsequently, the locus of active fault slip migrates towards the region with a more favorable stress ratio, which in the case of the evolving Himalaya orogen has been progressively southwards.

This is a very informative video but I know of many geologists who would protest. Their objection will not be that the Critical Wedge model is wrong but that it doesn't explain all of the Himalaya. They argue that the upper structural levels of the Greater Himalaya were extruded by a different mechanism. The growth of compressional mountain belts involves crustal thickening due to folding and thrusting. The Critical Wedge model explains this as taking place by the brittle breakage of slices of underthrusting crust along faults and their continuous accretion to a growing wedge. Rocks of the Greater Himalaya though show signs of ductile deformation. High grade gneisses were partially melted to form migmatites. Pods, lenses and sheets of granite magma was injected along fractures and planar rock fabric (schistosity).

The picture above shows leucogranite sills (white layers) intruding high grade gneisses near the village of Naagling in the Kumaon Himalaya, Uttarakhand. This partial melting and magma injection was contemporaneous with the extrusion of rock from deeper to shallower levels of the crust. 

All this took place beginning about 24 million years ago. Geologists have termed the movement of this hot mushy ductile rock mass as 'channel flow', literally to mean a channel of semi-solid rock that is being squeezed upwards like toothpaste from its container. In this case, the container were two bounding fault systems, the South Tibetan Detachment as the roof, and the Main Central Thrust as the floor. Supporters of channel flow say that the pervasive ductile deformation observed in the Greater Himalaya doesn't support the Critical Wedge mechanism of orogen growth. Instead, they propose that 'channel flow' was a unique phase in Himalaya development, restricted to the Miocene when deeply buried hot crust was being extruded. Over time  shallower levels of the crust were incorporated into the growing orogen where colder temperatures permitted brittle breakage of the crust and critical wedge growth. The Lesser Himalaya and the Siwalik ranges can be more satisfactorily explained by this mechanism of southwards fold and thrust propogation. 

'Critical Wedge' and 'Channel Flow' are statements on how crust with contrasting mechanical properties responds to compressional forces of tectonic origin and/or surface directed pressure gradients generated due to removal of overburden by erosion.

One final point. The video mentions 'out of sequence' thrusting referring to rejuvenation of extinct fault zones in the rear of the wedge. In case of the Himalaya this means renewed faulting in locales much to the north of the Main Frontal Thrust. This out of sequence thrusting manifest by low level earthquakes is taking place near and just southwards of the Main Central Thrust zone and seems to be driven by enhanced erosion stripping away rock, thereby reducing crustal thickness and normal stress.

Interestingly, I came across a paper by Paramjit Singh and colleagues which has used Apatite Fission Track (AFT) to reveal a pattern to this exhumation. Fisson Tracks is a kind of radiation damage in uranium bearing crystals. It is an ongoing process, but the tracks get preserved only below a critical temperature. The density of tracks is correlated to the time since the rock cooled below the healing temperature. In mineral apatite, fission tracks records the time when the rock cooled below 120 deg C. A young AFT date means that the rock was at around 4 km depth at a more recent time and has been exhumed to the surface much rapidly than a rock recording an older AFT date.

AFT dates taken along a north south profile in the Kumaon-Gharwal Himalaya from the Vaikrita Thrust to the Berinag Thrust  show a southward younging of dates, indicating sequential uplift and exhumation from north to south since Pliocene times (<5 million years ago). The 'out of sequence' faulting regime seems to be a second cycle of an 'in-sequence' pattern developing in the footwall (structurally underneath) of the Main Central Thrust zone. Similiar studies done by this team of scientists across nearby transects in the same climatic zone in the High Himalaya show that rocks are following different exhumation patterns. Contrary to what the video depicted, that does not sound like a climate controlled phenomenon. Rather variations in local tectonics may be dictating this style of exhumation. 

Himalaya never cease to be a mystery and a wonder.

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