Oldest Himalaya Rocks

In Peninsular India, the most significant change in rock type occur across what is known as the Archean Proterozoic boundary.

Archean rocks, older than 2.5 billion years, are typically varieties of granite and granite gneiss. They formed when the earth was much hotter and silica rich continental crust was growing by injections of magma from the uppermost mantle and by partial melting of older mafic (silica poor) crust. At places the crust subsided by vertical movements, and lava and sediment filled the narrow depressions. These volcano-sedimentary rocks were deformed and metamorphosed to form linear schist enclaves within the granitic crust. The term granite greenstone terrain describes this rock association.

This phase of continental crust building petered out by around 2. 5 billion years ago. The thick crustal blocks or cratons became the nuclei for future continent growth. By 2 billion years ago or so in the Paleoproterozoic (the Proterozoic Eon spans from 2.5 billion years ago to 538 million years ago) , the Archean crust became the floor for several sedimentary basins. Erosion of the Archean rocks provided sediments that accumulated in these basins over the next 1 billion years, with long hiatuses punctuating pulses of sediment deposition. The names of these sites of deposition will be familiar to many readers and travelers.  Aravallis, Vindhyans, and Cuddapah, to name a few, represent this younger Proterozoic phase of crustal recycling. 

There was limited development in Peninsular India of younger sedimentary basins and as a result Archean and Proterozoic crustal sections are widely exposed all across the country.

The satellite imagery posted below shows one classic locality of the Archean Proterozoic boundary. This is from the Cuddapah Basin of South India.  

The Archean granitic terrain has a rough texture due to the bouldery nature of the landscape formed by weathering of fractured granite. Towards the east north east, the layering of sedimentary strata of the Cuddapah Basin is prominent and unmistakable.  The following graphic is a geologic log prepared to describe the succession of rock types from the Cuddapah Basin.

Source: Vivek S Kale and coworkers; Proc. Indian. Nat. Sci. Academy 2020.

Archean 'basement' and Proterozoic 'cover sequence' is a common stratigraphic motif of the Precambrian geology of India.

A few weeks ago I found an old paper from the 1970's on the sedimentology and stratigraphy of Tethyan sequences from the Kali valley area, near the Kumaon Nepal border. These are, as the name suggests, sediments deposited in the Tethyan Ocean along the  northern margin of the Indian continent. They range in age from the Proterozoic to the Mesozoic.

Here is the stratigraphic column from the paper. I have shown only the Precambrian (Archean and Proterozoic collectively make up the Precambrian) section of the column. The Central Crystallines are assigned an Archean age, while the Tethyan Sequence Martoli Formation is Early Precambrian. The name Proterozoic was not in use in the 1970's when this paper was written.

 Source: S. Kumar and coworkers; Journal of Paleontological Society of India 1977.

'Crystalline' in this context refers to the rock texture made up of large interlocking minerals formed during slow cooling of a magma or during high temperature metamorphism of a sedimentary rock.

The geologic sequence I described earlier took place along the northern margin of India too where the future Himalaya would form. At first glance the Himalaya sequence seems a replica of the geology of Precambrian Peninsular India. It records an Archean  'crystalline'  basement, succeeded by variably deformed and metamorphosed Precambrian (Proterozoic) sediments.

Except that it is wrong. There are no Archean age rocks exposed anywhere in the Himalaya. And the rock units immediately in contact with the Archean are not Early Precambrian.

When this paper was published there was precious little geochronology work done in the Himalaya. Geologists knew from sporadic absolute dating of Peninsular rocks that the granite and granite gneiss terrains are older than 2.5 billion years  (Archean).  The thick sedimentary sequences overlying the granite basement also were Precambrian as ascertained by dating intrusive granites and interbedded lava layers. The lack of any shelly fossils in them was another indicator of the Precambrian age of the cover sedimentary sequences. 

Given this familiarity with Peninsular geology, it would have been natural to assign the same chronology to a Himalaya rock sequence of high grade gneiss in contact with unfossiliferous sedimentary rocks. 

The Central Crystallines are now known as the Greater Himalaya Sequence and they are not Archean but Neoproterozoic (Late Precambrian) in age. Detailed work has shown that they represent sediments deposited roughly between 1 billion and 600 million years ago along the northern continental shelf of India. A paleogeographic reconstruction of the Himachal Himalaya by Alexander Webb and coworkers shows the original disposition of the different Himalaya rock divisions. Observe (A) that the Greater Himalaya (GHC) and the lower part of the Tethyan Himalaya (Haimanta/Martoli Formation) were deposited synchronously in adjacent areas of the continental shelf.  

 Source: Alexander Webb and coworkers; Geosphere 2011.

They look very different from each other today because they experienced different conditions during Himalaya mountain building. The Central Crystallines which began their life as marine sediments became crystalline gneisses and schists during high grade Cenozoic metamorphism 35 to 20 million years ago, while the Tethyan Sequence escaped being buried deep in the crust and retained much of their original sedimentary character. 

What about the oldest Himalaya rocks? These are Paleoproterozoic in age, dated to be about 1.9 to 1. 8 billion years old. The units Damtha, Berinag, Wangtu, Jeori and Baragaon in the Himachal Himalaya cross section are the Paleoproterozoic age rocks. They are remnants of a magmatic arc and associated basins which formed along the northern margin of India when continental blocks were colliding and suturing into an early supercontinent named Colombia.

Underneath these Paleoproterozoic rocks would have been the Archean 'basement'. But where is it now? 

We can take a step back and understand how the Himalaya are constructed. As the Indian continental crust collided and pressed into Asia, a crack or a fault initiated in the collision zone started propagating southwards, slowly splitting the Indian crust.  As India kept getting pushed under Asia along this master fault, slices of Indian crust get scraped off  and thrust upwards along subsidiary faults to form a growing mountain range. This tectonic evolution is depicted as stages B, C, D, E, F. You can also read my post Himalaya: A Critical Wedge for more details on the mechanisms of mountain building.

If as shown in the cross section, the faults that break and transport crustal sheets are located entirely within the Proterozoic and younger layers then the Archean rocks won't get incorporated into the Himalayan orogen. They lie below the basal detachment/master fault. Alternatively, this model may not be applicable everywhere in the Himalaya and there may be slivers of Archean rocks buried deep under the thrust pile, but erosion hasn't exposed them yet. 

In the geologic future, the plate tectonic engine that made the Himalaya will change track. The mountain ranges will stop growing. Erosion will wear down the Himalaya and eventually lay bare its roots. The Archean 'crystalline basement' so familiar all over Peninsular India will also be visible at the base of the gentle rolling hills that were once the mighty Himalaya.

Geology Infographics

I read a lot of technical literature on various geology topics. The papers are usually long and written in jargon filled language. It can be tough to hold your concentration and read through the paper in one sitting. What can help is a well complied figure which summarizes the ideas and the results of the study. By figure, I don't mean a graph or tabular display of data, but a graphic that presents the data with a combination of symbology, line art, text, and even images. Such infographics help in grasping the gist of the study and make reading the elaborate explanations easier (you still have to read them).

In this post I will showcase three infographics that I liked from my readings. I won't write long explanations about them, since the idea is to see if you can understand the broad findings by looking at a picture. Read the abstract of the paper to assess how effective the figure is.

1) Ediacaran Extinction and Cambrian Explosion.

The distribution through time and the changes in diversity of early complex multicellular life is depicted in this infographic. The evolutionary history of two distinct 'biotas' are tracked. The Ediacaran 'biota' is a catchall phrase that includes a diverse range of extinct large fossil organisms which may include some early animals as well. Metazoans ancestral to living animal groups are the second category. The carbon isotope curve shows two prominent deflections towards negative values, termed the 'Shuram' and "BACE" (Basal Cambrian Carbon Isotope Excursion) excursions. They are thought to indicate global environmental crises. Bookending this graphic are two diversity measures. On the left is the diversity of body fossils. On the right is the diversity of trace fossils, such as imprints, tracks, and burrows. 

Take home point. The Cambrian 'Explosion' is not about the origin of animals but their geologically rapid diversification whose roots lie a good 20 to 30 million years preceding the Cambrian events. Pulses of diversity expansion and collapse took place during that time period.

2) Dating Cave Art.

Humans have left some breathtaking artwork on the walls of caves all over the world. But how do we know when they were created? The pigments used in the drawings cannot be directly dated. One can use associated cultural artifacts to narrow down the time period. Or if lucky, mineral layers that entomb the artwork can be dated directly. This method still brackets the maximum and minimum age of the artwork. This infographic explains how artwork in a Spanish cave was dated using uranium and thorium isotopes. 

3) Angiosperm and Insect Coevolution.

I had written about this topic is detail in a previous post, but thought I'll share this infographic again. The Cretaceous was a time of great environmental shifts and changes in terrestrial biodiversity. Gymnosperms gave way to a dominance of angiosperms. The diversification of flowering plants had a large collateral impact on earth. The history of angiosperms and insect groups through the Cretaceous and Cenozoic is explained in this beautifully compiled infographic. 

If you have come across a science infographic that you particularly like, do share the link in the comments section.

Early Animals, Hominin Diets, Groundwater Governance

 A few links to interesting listening and reading.

1) Tracking the first animals on earth:  Unequivocal evidence of animals is preserved in soft sediments from about 570 million years ago. The fossil record of the Ediacaran to early Cambrian times (570 to 500 million years ago) has yielded rich information about the patterns of animal evolution. Apart from fossils, comparative genetic studies have given insights into how different animal groups are related to each other and the order of branching of these groups. Amazingly, organic molecules recovered from enigmatic fossilized taxa have been used to differentiate between animal and non-animal remains. Zoologist Matthew Cobb explains all this and much more about early animal evolution in about 30 minutes. Give it a listen! 

2) Plant-eating and meat-eating in Australopithecus: What did our ancient relatives eat? By ancient, I mean going back a million years or more. We can use isotopes of nitrogen to tease out information about diets. Carnivores have more nitrogen-15 enriched tissue than plant eaters. Carbon isotopes (C13 and C12) also yield information about the diet of herbivores. Grazers munching on grass take in more of the heavier isotope of carbon than browsers eating leaves and stems. Paleoanthropologist John Hawks discusses some recent work on nitrogen and carbon isotopes of Australopithecines and how the patterns of isotopic variation extracted from tooth enamel can be interpreted in terms of diets and life history. Fascinating stuff. 

3) Addressing Depletion in Alluvial Aquifers: Why Context Matters in Participatory Groundwater Management: India relies a lot on groundwater for agriculture. There are signs from many parts of the country of acute groundwater distress. Participatory Groundwater Management initiatives have had some success in addressing this distress. Pratik Kumar and Veena Srinivasan point out that these cooperative movements have been more successful in hard rock aquifers from different parts of the country than alluvial aquifers of northwest India. Geology matters. Aquifer properties matter. Hard rock aquifers are more sensitive to abstraction and are rapidly de-watered and recharged seasonally. Alluvial aquifers are spread over vast areas and water levels are less sensitive to abstraction. The amount you can extract doesn't vary with lowering of water level. 

People depending on hard rock aquifers experience the limitation of the resource yearly and are more willing to join cooperative initiatives to manage the resource.

I have just given a gist of the more elaborate arguments in the paper. The graphic below very neatly compares hard rock and alluvial aquifers. 

 Source: Pratik Kumar and Veena Srinivasan 2025

The paper is open access.

Lithospheric Dehumidifier

A few years ago a friend decided to demolish his old house due to extensive and expensive water damage to the ground floor. 

Could this be the explanation for the damage?

xkcd comics

A spanking new apartment building now stands at the spot of the old bungalow. There are no signs of any water damage so far. A giant lithospheric dehumidifier may have been used during construction.

There is no end to the add on and perks developers promise these days.

Oil Hunters Of India

At Independence in 1947, India had just a few operational oil wells situated in the northeastern region of the country. Most of the subcontinent's sedimentary basins remained unexplored for their hydrocarbon potential. The Oil Hunter: Journey of a Geologist for India's Oil Exploration by Dr. Shreekrishna Deshpande is a personal recollection of the immense effort undertaken by Indian geologists to re imagine these basins as hydrocarbon source and reservoir rocks. It is the story of the development of India's oil industry told with unconcealed pride.

Russia, France, and the U.S. offered personnel and technical help along the way, but the lion's share of the credit goes to geologists of the Oil and Natural Gas Corporation for their perseverance and resilience in the face of immense challenges.

Dr. Deshpande joined the ONGC in its early days in 1961 and describes vividly his field experience in remote locales all across the country, from scorching Kutch, to steep Himalaya terrain, to facing personal danger during an insurgency in Assam. There were inevitable career challenges along the way due to changing institutional structure and unrealistic political expectations. Their impact on company work culture and productivity is described in honest and direct language.

One of my favorite passages comes towards the end of the book where he explains the divergence between geologists and management in their basic understanding of exploration and discovery.

"Subsurface discoveries of oil reserves cannot be projected with certainty. The inputs to the discoveries are always deterministic, but the result is never so, and there is a strong factor of probability. Exploration efforts are to reduce the risk factor and increase the probability of discovery. Methods for direct detection of  hydrocarbons from the surface, are  yet to be evolved. This contrasts with any other industry, where the output is more predictable and proportionate to the input. 

.... As a simplistic approach the management decides the cost of discovery of one tonne of oil by dividing the amount of discovered oil by the expenditure met. It is expected by them that similar expenditure should proportionately result in additional discoveries.

When a sedimentary basin turns old and mature, the addition to the existing stock of subsurface oil becomes increasingly difficult. Non-geoscientists then blame geologists for such uncertainty. Only the high profitability in the oil industry is visible; its probabilistic nature and the risks involved are not so obvious. The stochastic nature of oil discoveries is not appreciated by non-explorationists". 

My one complaint about this book is that there is very little geology in it! Dr. Deshpande describes the geology work he and others undertook in very broad strokes. I felt that a few examples of how specific types of geologic data is useful for petroleum exploration would be illuminating for the non specialist reader. 

Let me give an example. Early in his career he is sent to Osmania University, Hyderabad, to analyze some sedimentary rock samples using Differential Thermal Analysis. He simply mentions that the results were used by ONGC in their exploration efforts, but how so? DTA is a way to understand whether the sedimentary rock was baked during burial to temperatures that are conducive for hydrocarbon formation. 

Another example, and one that involves his specialization, could have been a brief passage describing his work on limestones. What is a carbonate sedimentologist looking for in these rocks? The main reservoir of Mumbai High, India's biggest oilfield, are Miocene age limestone beds which were deposited repeatedly during phases of oscillating sea level. Among other things, exploration geologists want to know how open spaces or porosity in these rocks has evolved over time and whether its occurrence can be predicted throughout the sedimentary section. There was a geological detective story waiting to be told there. 

But these are mere quibbles. Overall, this is a very readable account of the productive and remarkable career of a pioneer exploration geologist of India.  Popular accounts of Indian geology and industry are rare. Recently, Himalaya geology expert Dr. Om N. Bhargava released his memoir, Travails and Ecstasy of a Geologist Addicted to the Himalaya, on his experiences of working in the Himalaya. Indian earth scientists are beginning to share the good work they have done with a more general audience, bringing a much needed familiarity with a lesser appreciated but critical field of study. 

20,000 Days In The Life Of A Clam

My Whatsapp profile description says, "what's a million years here and there".

It is a tongue in cheek acknowledgment of the vast spans of time geologists often have to contend with. If I am studying a rock that formed more than a billion years ago, a 5 to 10 million year uncertainty in nailing down its exact time of formation is acceptable. Uncertainty in estimating the time of formation may occur due to our as yet not so perfect understanding of the decay rate of various radioactive isotopes being used for dating, or due to limits of sensitivity of the instruments measuring the radioactive isotopes in the mineral. We are making great strides in measurement techniques with a 0.1 % accuracy now achievable. 

Sedimentary rocks are harder to date than igneous rocks since minerals with radioactive elements rarely form in them at the time of their deposition (some limestones and black shales are exceptions). Often, using some indirect methods we can bracket their maximum and minimum age. Take the example of the Alwar Group sedimentary rocks which occur in the northern Aravalli mountains in Rajasthan. They are estimated to have been deposited sometime between 2.1 billion years ago and 1.8 billion years ago, an uncertainty of 300 million years! We know from our understanding of sediment accumulation processes that deposition was not uniformly spread out over the 300 million years. Rather, the sequence of sediments would have been deposited in discrete pulses lasting 10 to 20 million years, separated by long phases of non-deposition. 

Amazingly, even though we don't know exactly when during the 300 million year interval these sediments came to be deposited, we can track fairly accurately what was happening then on a daily basis. Some strata of the Alwar Group are of shallow marine origin. As the daily tide flooded in and ebbed, a thin layer of sand was deposited during each of these high energy phases. During the slack phase in between, a thin layer of mud was deposited. Stacks of these tidal bundles made of a sand and mud couplet record the passage of daily tides. Observing the stacking pattern closely reveals even more details. Sets of bundles of thicker sand-mud couplets alternate with sets of thinner bundles. Each set formed during alternating spring (thick layers) and neep tide (thin layers) cycles. 

We can interpret the ancient record by comparing the patterns with those forming today in different settings. The principle "present is the key to the past" is used with some caution, but it works well in this case.

Such tidal rhythmites are fairly common in the geologic record. The pictures to the left is a section of a core from  Cretaceous age sediments laid down in an estuary. The sedimentary section is made up of sets of thicker silt layers capped by a darker mud layer, overlain by a set of thinner silt and mud couplets. Each silt layer represents deposition during the flood or ebb tide. During the slack period, stirred up organic rich mud settled down. Again, we don't know the exact age of the rock to a certainty of few hundred thousand years, but we can track daily events. Image source: G. Shanmugan; AAPG Bulletin v. 84, 2000.

Speaking of tides, the earth's rotation is slowing down due to tidal friction. Marine organisms like corals, brachiopods, and clams build a calcium carbonate skeleton to house their soft tissues. Their shell grows by a daily addition of a thin mineral layer. Geologists have studied the pattern of skeleton growth of Paleozoic corals. Besides daily growth bands, they can identify seasons too, as corals lay down thicker bands during the dry season and thinner bands during the wet phase. The number of days in the year are estimated by counting the number of daily bands in each season. It turns out that there were about 420 days in a year during the early and middle Silurian (between 443 million to 419 million years ago). By middle Devonian (roughly 370 million years ago), some 50 million years later, the number of days had reduced to about 410. Using bivalve shells, scientists estimate that there were 370 days in a year by Late Cretaceous times (about 80 to 70 million years ago). 

There are other examples closer to our own existence on earth of natural rhythms being preserved in rock. Geologists and climate experts routinely use mineral bands in cave stalagmites to understand variations in rainfall. A recent study by Gayatri Kathayat and colleagues from Uttarakhand, North India, reveals details of the course of Indian monsoons over thousands of years, mapping dry and wet phases lasting few centuries each. Despite an uncertainty of a few decades in the absolute age of each layer, it gives us a broad picture of climate change through the Holocene. 

We accept the fuzziness of our estimates of the age of an event while being able to sharply resolve the changes taking place in that cloud of uncertainty.

I could have named this post "Ode to Laminae", an appreciation of thin layers that form in tune with earth cycles and which preserve in their layering valuable information on ancient tides, earth moon dynamics, changing of seasons, and longer term climate change.  I just thought the title of the post and the paper it refers to is better click bait. 

Sometime in the Late Miocene (about 10 million years ago, what's a few tens of thousands of years here and there) a giant clam living on the western margin of the Makassar Strait (Indonesia) built a shell with daily growth increments. I will post the entire abstract of the paper below so you can get an idea of the details of ocean conditions scientists can tease out today with sophisticated instrumentation.  

 Iris Arndt et.al., 20,000 days in the life of a giant clam reveal late Miocene tropical climate variability.

Giant clams (Tridacna) are well-suited archives for studying past climates at (sub-)seasonal timescales, even in ‘deep-time’ due to their high preservation potential. They are fast growing (mm-cm/year), live several decades and build large aragonitic shells with seasonal to daily growth increments. Here we present a multi-proxy record of a late Miocene Tridacna that grew on the western margin of the Makassar Strait (Indonesia). By analysing daily elemental cycle lengths using our recently developed Python script Daydacna, we build an internal age model, which indicates that our record spans 20,916 ± 1220 days (2 SD), i.e. ∼57 ± 3 years. Our temporally resolved dataset of elemental ratios (El/Ca at sub-daily resolution) and stable oxygen and carbon isotopes (δ18O and δ13C at seasonal to weekly resolution) was complemented by dual clumped isotope measurements, which reveal that the shell grew in isotopic equilibrium with seawater. The corresponding Δ47 value yields a temperature of 27.9 ± 2.4 °C (2 SE) from which we calculate a mean oxygen isotopic composition of late Miocene tropical seawater of −0.43 ± 0.50 ‰. In our multi-decadal high temporal resolution records, we found multi-annual, seasonal and daily cycles as well as multi-day extreme weather events. We hypothesise that the multi-annual cycles (slightly above three years) might reflect global climate phenomena like ENSO, with the more clearly preserved yearly cycles indicating regional changes of water inflow into the reef, which together impact the local isotopic composition of water, temperature and nutrient availability. In addition, our chronology indicates that twice a year a rainy and cloudy season, presumably related to the passing of the ITCZ, affected light availability and primary productivity in the reef, reflected in decreased shell growth rates. Finally, we find irregularly occurring extreme weather events likely connected to heavy precipitation events that led to increased runoff, high turbidity, and possibly reduced temperatures in the reef.

Tell me geology isn't the coolest field of study.

Plastic In Sediment, Antarctica Ice Core, Alfred Wallace

A few interesting readings:

1) Sedimentation Shifted - How rivers move sediment along their course to the sea is an important aspect of sedimentology research. Grain size, shape, and density, all affect how currents move sediment, and where and in what proportions sand, silt, and mud particles come to be deposited. Now there is a new kid on the block: plastic. Catherine Russell has written a fascinating article diving deep into experimental work on how plastic impacts sediment transport. The work she describes has important implications for our understanding of plastic pollution in rivers, and the role plastic particles plays in enhancing erosion rates and sediment redistribution in riverbeds. 

2) Antarctica: 1.2-Million-Year-Old Ice- Scientists use gases trapped in old ice to measure ancient atmospheric composition and estimate past climatic conditions. A long running drilling program in Antarctica had so far recovered 800,000 year old ice. That record has been recently broken. Scientists have reached the very bedrock of the Antarctica continent. The oldest ice at the very bottom is 1.2 million years old. This is the longest continuous record of our climate that we have so far.  It hold much valuable information on climate fluctuations through the Pleistocene and Holocene. This article is a press release of the University of Bern. 

3) Beyond Evolution: Alfred Russel Wallace’s critique of the 19th century world- Alfred Russel Wallace is the co-discover of evolution through natural selection along with Charles Darwin. He was a brilliant naturalist and made foundational contributions to natural history. But he also was very sympathetic to the plight of local people suffering under colonialism and the environmental degradation the race to strip the land of resources was causing. Marshall A. summarizes nicely Wallace's observations on the impact of environmental damage, both in his native Wales and also during his travels in the far away Malay archipelago. 

Let me take this opportunity to share again this lovingly crafted documentary on the life and work of Alfred Wallace. It is made as a paper-puppet animation, produced by Flora Litchman and Sharon Shattuck and narrated by George Beccaloni of the  Natural History Museum London and Andrew Berry of Harvard University.

 

What a fine example of science outreach. 

Shear Luck Near Sunderdunga River

As I settled down for lunch by the Sunderdunga riverside during my recent Kumaon trek, I noticed a polished boulder nearby. It had a striking appearance dominated by a large crystal of feldspar set in a much finer grained material. This finer matrix had a pronounced streaky fabric, as if made up of very fine layers. Upon closer examination, these layers or foliation was due to the planar arrangement of minerals like amphiboles, mica, quartz, and feldspar. The larger eye catching feldspar grain in the center of the boulder seems a little flattened along one axis and elongated along the orthogonal, giving it a crude sigmoid shape.

I had chanced upon a rock caught up in a shear zone. These are fault zones where movement of the crust causes intense rock deformation. The type of deformation I observed typically occurs at a deeper level where high temperatures make rocks soft and ductile. Rocks caught up in fault zones at shallower levels undergo brittle deformation. They have a broken appearance, made up of sharp edged fragments set in a crushed finer matrix. The rock is fractured, and these cracks get filled with minerals like calcite and quartz. 

This typical brittle like deformation was absent in this rock. There was no sign of any fracturing and breakage of the rock. Instead, the finer grained minerals seemed to flow around the larger feldspar crystal. Grain size reduction occurs by plastic rearrangement of atomic layers and recrystallization of softer minerals during deformation. The stronger resistant minerals which remain as large crystals are called porphyroclasts. Since rocks are sliding past, there is a rotational component to deformation also. Larger grains often show signs of being rotated, while finer groundmass wraps around.

The end stage of such ductile deformation are rocks known as mylonites. These have a flinty or glassy appearance due to the extreme grain size reduction.

I suspect this particular rock has not quite reached the mylonite stage. Let us call it a protomylonite. It does show a clear contrast between the finer matrix made up of stretched and elongated minerals and a large porphyroclast.  

The asymmetry of the porphyroclast gives geologists an idea of the sense of motion along faults. The annotated photo below shows the relative sense of shear or motion.  

Hundreds of such measurements have been made in the Greater Himalaya. When measured in-situ,  the direction of relative motion is 'top to the south', indicating the general direction of movement of Himalaya thrust faults.  Deformation is not uniformly distributed throughout the Greater Himalaya but appears restricted to narrow zones. These zones of intense shearing containing deformed rocks including mylonites have allowed the recognition of  major thrust fault zones such as the Main Central Thrust which emplaces the Greater Himalaya slab on top of the Lesser Himalaya.

There are minor shear zones too. I think this rock was eroded from one such shear zone in the Sunderdunga valley. 

Coming back to the brittle versus ductile deformation regimes. Almost all the deformation you observe in the Greater Himalaya took place in the ductile regime. Here are a few examples from the Greater Himalaya of ductile deformation seen in schists and gneisess. These are my observations from various treks in the Kumaon. 

A cross section of the Himalaya is presented here to showcase the metamorphic gradients along the Greater Himalaya slab (green). For this reading, you can ignore the rest of the Himalaya orogen shown in the figure. Temperature gradient increases towards the core of the slab with kyanite (k) and sillimanite (sill) as the prime high grade metamorphism indicators. This example is from the Nepal Himalaya, but the arrangement of the different Himalaya divisions is identical in adjacent Kumaon. 

 Source: Mike Searle et.al. Tectonophysics 2017

Notice the localization of mylonites along the Main Central Thrust zone. Metamorphism of rocks above around 600 degree centigrade during the Eocene (~35 million years ago) and in the Miocene (~25-16 million years ago) has resulted in the ubiquity of ductile deformation observed in the Greater Himalaya. In hotter pockets in the core, metamorphic rocks partially melted and the resulting granitic magma was injected along penetrative weak planes, forming dikes, sills, and small plutons.

Channeled between two great fault zones, the Main Central Thrust at the base and the South Tibetan Detachment as roof, this hot mushy crustal material was then tectonically extruded to shallower levels, its ductile fabrics frozen and preserved as the rocks cooled. Subsequent tectonism has superimposed brittle deformation on the Greater Himalaya ductile structures. 

Finally, another beautiful example of a gneiss showing ductile shearing.  Fish shaped white feldspar are set in a biotite mica and quartz matrix which flows around the porphyroclasts. Can you guess the sense of relative motion?

Observing features that you have seen only in a textbook - that is the great joy of going out in the field.