Tethyan Himalaya And Trans Himalaya

What is the difference between Tethyan Himalaya and Trans Himalaya?

I've seen the two terms being used interchangeably, but geologists recognize them as geologically distinct terrains. Their geographic locations and geologic context has been annotated in the satellite imagery below. 

The Indus Suture is the zone of collision between the Indian and Asian tectonic plates. It contains broken pieces of  oceanic crust and deep sea sediments which were uplifted and jammed between the colliding continents, forming a sort of a geologic no-man's land. The Tethyan Himalaya are the ranges immediately south of the Indus Suture. They are the deformed rocks of the Indian plate. The Trans Himalaya are the ranges north of the Indus Suture made up of a variety of rocks of the Asian plate.

The Tethyan Himalaya is a pile of Paleozoic and Mesozoic sedimentary rocks which was deformed into a fold and thrust belt during the early stages (45-35 million years ago) of the India Asia collision. At places, the sedimentary cover has been stripped away by erosion and high grade metamorphic rocks formed deep in the crust have been exposed. These 'windows' are known as gneiss domes since the sedimentary cover rocks have been arched up during uplift and exhumation of the high grade rocks. The area around the famous Tso Moriri lake is one of the best examples of a gneiss dome.

If your are traveling in Zanskar, Spiti, Lahaul, upper reaches of Kinnaur, and near about Milam and Panchachuli Glaciers,  you are in the Tethyan Himalaya. 

In the late Cretaceous (100 million  years ago), the leading edge of the Indian plate began subducting underneath Asia. As the plate dove deeper it heated up and released water, which triggered the formation of magma in the upper mantle of the Asian plate. This magma rose and assimilated rocks from the Asian lower crust. It then intruded older sedimentary and metamorphic rocks of the Asian crust and solidified as giant bodies of granites and granodiorites (containing calcium rich feldspars). These large granitic intrusions or 'batholiths' range in age from 100 million years to about 50 million years ago. One example is the Ladakh Batholith on which the town of Leh sits. Some of this magma also erupted on the surface through volcanoes. The rocks of Khardungla Pass are remnants of this ancient volcanic terrain. 

A similar situation today is along the western South American margin. There, subduction of the Nazca Plate underneath South America has triggered large scale magmatism and formation of giant batholiths of the Andes Mountains. 

Another impressive geologic feature of the Trans Himalaya is the Karakoram Fault Zone. It is a NW-SE aligned right lateral strike slip fault where crustal blocks have been sliding past each other since about 18 million years ago, resulting in a 150 km of offset of rocks. There has been some vertical movement also along this fault and this uplift has resulted in the formation of the Pangong Ranges where high grade metamorphic rocks have been exhumed from a deeper crustal level. The Pangong Lake is a drowned river valley formed by the damming of the river on its western end due to fault uplift. 

Strike slip faults have been in the news recently. The devastating earthquake in Turkey and Syria was caused by movement along the left lateral strike slip East Anatolian Fault. 

The India Asia collision resulted in the partial melting of deeply buried rocks of the Asian crust in the Miocene (21-16 million years ago) and the resulting granitic magmas have intruded the upper levels of the crust as dikes and sills. These melt channels also coalesce to form plutons and batholiths. Granitic intrusions of Miocene age which formed as crustal melts differ in their composition from the older Ladakh batholith which has a mixed mantle and crustal origin. There is a lot of interesting and complicated geology in the Trans Himalaya too! 

There are lots of technical papers on this topic. For good popular style book I recommend Mike Searle's Colliding Continents: A Geological Exploration of the Himalaya, Karakoram and Tibet. 

If you want a short answer to the question I posed, it is this: The Himalaya (including the Tethyan Himalaya) is the deformed northern edge of the India Plate. The Trans Himalaya is the deformed southern edge of the Asian Plate.
 

Quiz: Spot The Granite Intrusion

I came across this glacially transported boulder in the Dugtu village valley near the Panchachuli Glacier in the Kumaon Himalaya.

It is a block of high grade gneiss intruded by a granite. Without scrolling beyond the first photograph, try to work out the contact between the gneiss and the granite.

Answer:

The boulder is encrusted by moss. There is some mineral staining too. And sunlight falling on the rock gives it a speckled appearance.. All this reduces the contrast in color between the gneiss and the granite.

But there is a vital clue in the orientation of structures. Both the gneiss and the granite have a planar fabric imprinted on them.

The fabric of the gneiss is due to the orientation of platy minerals like micas stacked in layers, alternating with layers richer in quartz and feldspars. Assume this is the original disposition of the rock as well. The gneiss layering you see is due to the trace of horizontal planes of separation of different mineral layers. I have outlined some of this planar fabric in brown lines.

The granite has a planar fabric too, but this is due to near vertical fractures. The rock has been broken in to thin slabs  by fractures (red lines) which may have formed during the cooling of the magma. These fractures don't pass into the surrounding host gneiss. Two arms of the granite have penetrated between the gneiss layers forming mini sills.

You can see the contact (black line) between the gneiss and the granite roughly where my wallet is. Here, the horizontal planar fabric of the gneiss abruptly juxtaposes against the vertical planar fabric of the granite.

The Serpents Of Nagling- Granite Intrusions Into Greater Himalayan Sequence Metamorphics

Over chai, elders told us about large serpents invading their village. A curse, they said. Only the correct prayers and purification rituals saved them, forcing the serpents to retreat deep into the forest. Some serpents remain trapped in the rock faces near the village, which was renamed Nagling (Nag means cobra..or more generically serpent).

The picture below are the entombed serpents of Nagling (trekkers for scale).

Geologists recognize them to be granite dykes (intrusions cutting across host rock layering) and sills (intrusions parallel to host rock layering) intruding the high grade metamorphic rocks of the Greater Himalayan Sequence (GHS).

The GHS is a block of the Indian crust bounded between the Main Central Thrust (MCT) at the base and the South Tibetan Detachment System (STDS) at the top. It represents mid crustal material which was metamorphosed and then was extruded and exhumed during Himalayan orogeny between 25 million years ago to about 16 million years ago. These dates vary somewhat along the strike of the Himalaya. Thrusting along the MCT took place earlier in the western Himalaya. Eastern regions like the Sikkim Himalaya record younger dates for the movement of the MCT.

The grade of metamorphic varies within the GHS. The figure below is a schematic section of the Greater Himalayan Sequence. It is from a study on the nature of the MCT by Michael Searle and colleagues from the Nepal Himalaya and is a very useful guide to think about the internal structure of the GHS.

 Source: Searle et. al. 2008

From the base of the MCT the grade of metamorphism increases towards higher structural levels. This is recognized as an "inverted metamorphic gradient", since minerals that are formed at higher and higher temperatures and pressures are occurring at structurally higher and by implication apparently shallower levels of the crust. The inverted gradient is recognized by the successive appearance of  biotite, garnet, sillimanite and finally kyanite. The sillimanite-kyanite zone transitions into the zone of partial melting and granite intrusives. This is the zone where the crust experienced conditions that lead to the formation of in situ melts and their mobilization and intrusion into surrounding rock. Above this zone the grade of metamorphism reduces towards the STDS. In the figure, the granite intrusion zone is directly overlain by the STDS and the Tethyan sequence. However, there is variation in this theme across the Himalaya. In the Kumaon region where I was, the "melt zone" is overlain by a sequence of lower metamorphic grade phyllite rocks.

What caused this melting and production of granitic magma? Many geologist point to the STDS. They suggest that this zone of extentional faulting stretched and thinned the crust, resulting in " decompression-related anatexis". This means that when extentional faulting along the STDS and exhumation reduced the overburden on deeply buried hot rocks, the release in pressure resulted in the lowering of rock melting point. This led to a partial melting of the crust (anatexis). Other geologists disagree with this explanation. They point out that since decompression has a minor effect on melting the likely source rock compositions you would require unreasonably large amounts of denudation along the STDS.  Rather, they suggest that crustal thickening by the continued convergence of India with Asia elevated temperatures in the middle levels of the crust to a range where partial melting began. These melts then moved along weak planes and intruded the surrounding GHS above the sillimanite and kyanite grade gneisses. The main pulses of this magma generation took place between 24 million years and 19 million years ago.

Geologists estimate the temperatures of this melt zone to be around 650 deg C to 750 deg C, corresponding to a  burial depth of about 20-25 km. Yes, the GHS represents crust that has traveled from that depth to the Himalayan heights it now commands by a combination of thrust faulting and erosional unroofing i.e. the stripping away of shallower levels of the crust!

During one of my previous treks in the Kumaon region I had walked across the GHS from the base of the MCT to the sillimanite zone in the Goriganga valley from the town of Munsiari to village Paton. This time, one valley to the east,  we began our trek at village Nagling in the zone of  partially melting. All around us were rock faces intruded by sill complexes and dykes. The picture below shows multiple sills of granite cross cut by dykes.

High up from Nagling village towards Nagling Glacier I saw this granite dyke complex (outlined by red dotted lines ) cutting across metamorphic banding (black lines).

And in the stream near Nagling Glacier I came across this rounded stream boulder showing granite cross-cutting banded migmatitic gneiss.

We traveled north and  reached Duktu. Earlier, somewhere near the village of Baaling, we had crossed the zone of partial melting and were in the uppermost levels of the GHS made up of phyllite grade metamorphic rocks. The phyllites are not intruded by granite.

However, granite was present at Dugtu too, but only in the Dhauliganga river bed. This river emerges from the Panchachuli Glacier. The Panchachuli ranges which fall lower in the GHS are made up of high grade gneiss intruded by granite.

As a result, the Dhauliganga river bed near Duktu village is choked with boulders of granite and migmatite rocks.

This is a very distinctive  biotite-tourmaline granite. The picture below shows blocks of granite with tabular black tourmaline.

Here is a picture of me looking intently at a block of GHS made up of a granite intruding in to a gneiss.

And another close up of light colored granite intruding dark grey banded gneiss and encircling and enclosing rafts of the metamorphic host rock (red arrows).

And finally, from the sheer rock faces near Nagling Glacier, one of my favorite examples of the granite intrusions. A near vertical dyke (red broken outline) cut and displaced by a fault (yellow broken lines). Metamorphic banding shown in black lines.

... Pleistocene-Holocene glacial deposits of the Panchachuli Glacier area.. coming up next!