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.
 

Geology Outcrop Quiz: Milam Glacier Area

In the outcrop below, which direction are the strata dipping (tilting)? North is to the right. 

If your answer was towards the right (north), you would be wrong.

The apparent northward dip of the rock layers is an illusion caused by fractures intersecting the plane of the exposure. The outcrop below from a nearby spot shows the real geometry of the strata.

This is an across the bedding exposure. You can see that the strata are almost vertical and are warped into small scale folds. Look at the black arrows on the left centre of the image. They point to sub horizontal cracks (fractures) penetrating the rocks. These are axial plane fractures as they follow the plane that divides the folds symmetrically. To the top right, the red arrow points to the distinctive planar fabric that has arisen due closely spaced fractures. 

In contrast to this perspective, when the outcrop is a bedding plane exposure i.e. a slice parallel to the bedding, as it is in the first picture, you won't see the folding. Instead you can only observe a bedding surface cut by horizontal fractures that divides the rock wall into layers and gives the impression from afar of horizontal or gently tilted beds.

The photo below is a close up of an outcrop with prominent nearly vertical thin orange-brown and dark grey folded beds. Notice the white horizontal bands? These are the axial plane fractures which later were filled with quartz, as mineral saturated groundwater circulated through the rocks.  

The outdoors is such a good classroom. This final outcrop illustrates how erosion along a hillslope has sliced the rock body at different angles with respect to the bedding, exposing both, the true orientation of the strata as well as the pseudo bedding. 

To the right of the waterfall is an across the bedding cut, exposing the nearly vertical gently folded strata. To the left of the waterfall is an along the bedding exposure. Only a sheer rock wall is observed with the axial plane fractures imparting a crude layering to the rock face, giving the illusion of bedding.

All these exposures are along the road side trail near Milam village in Kumaon, Uttarakhand. You can test your observational skills when you next visit that area. 

Trekking in the Himalaya is always a treat.  And when you come across these small geology puzzles, it is with a double scoop topping.

Readings: Gagging Indian Scientists, Human Evolution, Sand Mining

From the past couple of weeks:

1) Slow Subsidence of Scientific Institutions: As land movement and destruction of homes in Joshimath Uttarakhand became a prominent talking point, the Indian government reacted like it usually does when faced with an awkward situation concerning its own accountability. It imposed a gag order on its scientists, forbidding them from talking to the media until publication of a final report. Dinesh C. Sharma offers a thoughtful perspective on the corrosion of autonomy of India's scientific institutions and the damage this withholding of information does to open and informed debate. 

2) Teeth Reveal How Brains Developed In Utero: How fast did our ancestors brain grow before birth? When did patterns of brain growth become more human like? Teeth start developing very early in a fetus at about 20 weeks old and they fossilize well too. Researchers found a relationship between molar length and prenatal brain growth by studying teeth from skeletons of various primate species and comparing them with gestation length and mass at birth of each species. The final conclusion was that rates of pre natal brain development increased during hominid evolution and became more human like about one million years ago.  By anthropologist Tesla Monson.

3) Grains of Sand: Too Much and Never Enough. This is a topic of great concern in India too. Unregulated sand mining is stripping river valleys barren of sand, in turn changing river morphology and devastating habitats. Alka Tripathy-Lang writes about the global demand for sand and its impact on environment and livelihoods. What is the future for this resource? Will we learn to use it sustainably?  I'll also recommend this Planet Money podcast episode on Peak Sand featuring a stolen beach! - Episode 853: Peak Sand. 

A Geological Map Of The Himachal Himalaya

Digging through my collection of Himalaya geology papers, I came across this geological map of the Himachal Himalaya. It is from a study on the tectonic history of the Himachal mountains by A. Alexander G. Webb and colleagues, published in Geosphere in the year 2011.

The paper itself is quite a detailed work using both field mapping and various geochemical and geochronologic methods. It will be hard reading and likely incomprehensible for non-geologists. I won't discuss the specifics here. I will however simplify the most interesting findings. 

1) The Greater Himalaya are the tallest of the ranges, and they are made up of rocks which were buried the deepest during the mountain building process. Based on the geometric relationship of the fault zones that contain this rock unit, and the timing of fault activity,  the authors propose a mechanism for the emplacement of these rocks from deeper to shallower levels. 

2) By establishing a chronology and comparing their geochemistry,  rock units displaced by faults and dismembered by tectonics and erosion are shown to have been contemporaneously deposited.

3) There are rare instances of preservation of the depositional contacts between major rock groups. Using this as a guide to their original location and supplementing it with geochronologic information, the pre mountain-building geographic locations of these rock units in relation to the northern shoreline of the Indian subcontinent is proposed. I found this paleo-geographic reconstruction most useful. It really helped clarify my thoughts about the origin and relationships of the different Himalayan sub-divisions.

4) Himalayan mountain building beginning around 35-45 million years ago led to metamorphism of sedimentary layers that were deposited on the northern margin of India. The mineral monazite (phosphate mineral) forms during such metamorphic reactions.  It also contains radioactive elements like thorium which geologists can use to estimate the timing of its formation. Dates of monazite formation from some low grade metamorphic rocks has unearthed an even older phase of metamorphism that affected these rocks.  It shows that the northern margin of India was involved in an earlier phase of mountain building around 500-600 million years ago.

Travelers, do download a high resolution version of this map here - Geological Map of the Himachal Himalaya.

Kangra, Chamba, Manali, Spiti, and Shimla. These are all popular places to visit in the Himachal region. Here are a few tips for a broad understanding of the terrain that you will be driving or trekking through. The descriptions below are by reference to the map legend where all the rock groups are tabulated. 

1) Sub-Himalayan Sequence. These are sedimentary rocks deposited in the Himalayan foreland basin. As the Himalayan mountain building progressed from around 45 million years ago, the crust in front of the rising mountains bent to form a depression.  Debris from this eroding mountain chain was deposited in these foreland basins and then folded and uplifted to form the frontal ranges, including the familiar Siwaliks. Jwalamuki Temple, where a flame is powered by natural gas emanating from deeply buried strata, is located within the Siwaliks. These sub-Himalayan ranges were uplifted between 5 to 0.5 million years ago. Lookout for a lot of sandstone, shale, and pebbly and gravel rich layers.

2) Tethyan Himalayan Sequence. These are low grade metamorphic and sedimentary rocks ranging in age from 800 million to around 70 million years old. The fossil bearing strata that you find in the Spiti valley belong to this group of rocks. The Tethyan Himalaya were the first mountain ranges to form following the India-Asia collision, beginning about 45 million years ago. You will see slates, limestone, sandstone. The older part of this sequence is made up of metamorphic rocks like shiny phyllites and mica schists with garnets.

3) Igneous Rocks. These are of various ages , ranging from 1.8 billion years ago to about 470 million year ago and point to magmatic activity that affected the northern margin of the subcontinent from time to time. The spectacular Dhauladhar range near the town of Dharamsala is made up mostly of granites which intruded the crust around 500-470 million years ago.

4) Greater Himalaya Crystalline Complex. As the name suggests these rocks are high grade crystalline metamorphic varieties like gneiss and schist. They range in age from about 1000 million to 500 million years ago. These were rocks that formed at depths of about 25 kilometers and then were uplifted about 25-16 million years ago. These rocks have a typical banded appearance and contain pink and red garnets and shiny mica rich layers.

5) Tertiary Leucogranite. This granite formed by partial melting of metamorphic rocks during mountain building. It ranges in age from about 40 million to 8 million years ago. You will see them in the higher reaches of the Greater Himalaya. They are easy to spot. Look for white bands cutting across (dikes) dark banded rock. At places the white bands will be parallel (sills) to the rock layers.

6) Outer Lesser Himalaya. These are low grade metamorphic and sedimentary rocks ranging in age from about 1000 million to 500 million years ago. The common rock types will be the familiar slate and limestone.

7) The rest of the Lesser Himalayan units are among the oldest rocks in the Himalaya. The oldest among them, the Munsiari Group, have been dated to about 1.9 billion years ago. They comprise high to low grade metamorphic rocks. Keep a watch for banded metamorphic rocks as well as quartzites and limestones.

The Lesser Himalayan rocks were uplifted between 16 and 5 million years ago.  

8) Indus Suture Zone. If you wander into the Indus valley. These are rocks that formed in the zone of collision between India and Asia. Ophiolites are fragments of the ocean crust thrust up when the Indian plate dove underneath Asia. The Indus Molasse are beds of sand and gravel derived from the erosion of nearby mountain ranges and deposited in lakes and streams.

To briefly summarize the geology. The Greater Himalaya, the Lesser Himalaya, and the Tethyan Himalaya are rock groups made up of sediments that were deposited on the northern continental shelf of India and intermittently intruded by granitic magmas. This sequence developed across a vast time span ranging from 1.8 billion years ago to 70 million years ago. Sediments of the older units of the Tethyan Sequence (Haimanta), the rocks of the Greater Himalaya, and the units making up the Outer Lesser Himalaya were deposited roughly at the same time but at different geographic locales. All these rock units were metamorphosed to varying degrees during Himalayan mountain building. 

Two cross sections from the paper depicts these units restored to their original locations across the Indian margin and then subsequently disrupted by tectonics. 

Compressive forces have deformed this stratigraphy into a complicated structure made up of folded rock sheets stacked by thrust faults. Erosion, by selectively removing portions of these thrust sheets, and by exhuming deeper levels of the crust, has played a big role in producing the present day rock outcropping pattern.

The paper is open access if  you want to dive in. A. Alexander G. Webb et. al. 2011 : Cenozoic tectonic history of the Himachal Himalaya (northwestern India) and its constraints on the formation mechanism of the Himalayan orogen.

Joshimath Sinking

By now, the unfolding tragedy in the mountain town of Joshimath in the Uttarakhand Himalaya is national news. Land subsidence is destroying homes. Many families have been left with no choice but to leave and resettle elsewhere. 

Journalist Kavita Upadhyay has written a good explainer on Joshimath's predicament. 

How heavy, unplanned construction and complex geology is sinking Joshimath.

And yesterday, News9 Plus with Kartikeya Sharma invited Kavita Upadhyay, along with another journalist Nivedita Khandekar, and geologist Hars Vats, to discuss not just Joshimath but also the vulnerabilities of Himalayan towns across the mountain arc. 

Their conversation is on Spotify- Joshimath Sinking. 

Repeated warnings by geologists were ignored for decades. As Sunita Narain points out in this Down To Earth article, engineers and technocrats with little understanding of the environmental and ecological issues are in charge of passing plans for big power projects. For Joshimath, all the relief measures are probably too late to make the town safe again, but if the destruction of this important historic town does not usher in a national soul searching about the way in which the Himalaya are being developed for tourism and hydropower, then I fear there will be much more pain and suffering in the future.