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.

Darwin- Patagonian Christmas

A friend recently returned from a trip to Patagonia, Argentina. I mentioned to her that Charles Darwin had spent some time exploring the Patagonian coastline and had made some interesting observations about the local fauna and the native people.

Being a bit vague on the details I dug into one of the most reliable source on Darwin's life and work, the Darwin Online archive.  I accessed his Beagle Diary, scrolling down to the time the Beagle docked at Port Desire on December 8, 1833. Below is his entry from December 24, 1833:

24th Took a long walk on the North side: after ascending some rocks there is a great level plain, which extends in every direction but is divided by vallies. — I thought I had seen some desart looking country near B. Blanca; but the land in this neighbourhead so far exceeds it in sterility, that this alone deserves the name of a desart. — The plain is composed of gravel with very little vegetation & not a drop of water. In the vallies there is some little, but it is very brackish. — It is remarkable that on the surface of this plain there are shells of the same sort which now exist. — & the muscles even with their usual blue colour. — It is therefore certain, that within no great number of centuries all this country has been beneath the sea. —1 Wretched looking as the country is, it supports very many Guanacoes. — By great good luck I shot one; it weighed without its entrails &c 170 pounds: so that we shall have fresh meat for all hands on Christmas day.

John van Wyhe, ed. 2002-. The Complete Work of Charles Darwin Online- Beagle Diary.

As always his keen geology eye had spotted shells which were similar to living shelly creatures, leading him to the conclusion that this region must have been under the sea in the past. All along the South American coastline he noticed oyster and shell beds a meter or two above the current sea level. He concluded that there must have been vertical movement of the crust. 

He was a good hunter too. Apart from the Guanacoe, one of this shipmates shot a rhea. The crew ate that too. Darwin realized only too late that he wanted the entire bird to compare its form to rhea elsewhere in S.America. Eating valuable zoological samples became a common occurrence all through the voyage of the Beagle. 

The Beagle then sailed south to Port Julian. On his walks there Darwin came across bits of spine and hind legs of a largish creature, later identified as a Megatherium or ground sloth. That led him to muse about what could have killed off this variety, as nothing like it existed today. The sedimentary layers entombing the fossil did not indicate any kind of flood. How could an entire species die off?  Taking inspiration from cuttings of apple trees which were clearly part of the parent, Darwin wondered if all individuals of animal species too shared some life force. If so, could a common disturbance or event kill all of them off? 

In February 1834 the Beagle sailed into the Straits of Magellan and Tierra del Fuego. He immediately formed a low opinion of the natives whom he though of as uncivilized savages, living naked and unkempt in an utterly desolate landscape: 

Their country is a broken mass of wild rocks, lofty hills & useless forests, & these are viewed through mists & endless storms. In search of food they move from spot to spot, & so steep is the coast, this must be done in wretched canoes. — They cannot know the feeling of having a home — & still less that of domestic affection; without, indeed, that of a master to an abject laborious slave can be called so. — How little can the higher powers of the mind come into play: what is there for imagination to paint, for reason to compare, for judgement to decide upon. — to knock a limpet from the rock does not even require cunning, that lowest power of the mind. Their skill, like the instinct of animals is not improved by experience; the canoe, their most ingenious work, poor as it may be, we know has remained the same for the last 300 years. Although essentially the same creature, how little must the mind of one of these beings resemble that of an educated man. What a scale of improvement is comprehended between the faculties of a Fuegian savage & a Sir Isaac Newton — Whence have these people come?

John van Wyhe, ed. 2002-. The Complete Work of Charles Darwin Online- Beagle Diary.

Darwin compared the Fuegian natives with natives he had met further north along the Patagonian coast. Those tribes appeared better dressed, with some European contact spoke a smattering of English and Spanish, and had learned to eat with a fork and knife. 

Could the savage Fuegian change their life habits too? Darwin was doubtful. He thought that hundreds of years of living in this habitat had formed an instinct that could not change, a view reinforced when Jemmy a Fuegian native who had lived with Europeans was seen to have gone back to his native state. Perhaps, Darwin thought,  differences between human populations were more deep than realized.

Much more of South America remained to be explored. At this time his interest lay primarily in geology, but questions about species extinction and fixity, and the nature of human differences in life habits and intellect, lingered on in his mind. 

If you would like to read more about Darwin I will recommend Adrian Desmond and James Moore's fine biography, Darwin: The Life Of A Tormented Evolutionist.

Landscapes: Sunderdunga Valley Kumaon Himalaya

In mid November, I explored the Sunderdunga valley in the Kumaon region of Uttarakhand. It was a good rigorous walk through some extraordinarily beautiful landscapes. This area is better known for the famous Pindari Glacier trek. Kafni Glacier is another option for trekkers. All three routes begin at village Khati. The picture taken of the high ranges from nearby Dhakori shows the three glacial valleys.

And here are some more photos of the route with a brief commentary.

The entrance to Sunderdunga valley with the vigorous Sunderdunga river flowing through.

The first day walk to Jatoli village was through golden and green forests.

Village Jatoli in the mid November sun. We stayed there overnight at the Kumaon Mandal tourist guesthouse. They provide excellent clean accommodation. 

About 4 km walk upstream from Jatoli the next day and the land cover changes abruptly. The forest is gone. There is no marked trail from here on and a walk over a rugged boulder strewn region begins. 

We navigate our way over steeply dipping metamorphic rocks and scree cones. 

Numerous rock falls make for tricky passages. You can spot my companions climbing their way up the steep slope. 

After slogging for about 8 km through this terrain we arrive at Kathaliya, situated at about 10,500 feet ASL. We have climbed about 2500 feet from Jatoli to Kathaliya. A small trekkers shed has been constructed here. We stayed there for the next couple of days. 

Next morning, ahead of Kathaliya camp, we encountered the full glory of Sunderdunga valley. Here, it is a occupied by a wide boulder strewn river bed with several small active channels. The earthy colors of rock and grass were in stunning contrast to the blue sky. A solitary shepherd's hut can been seen in the lower right. 

Another view of the valley.

Boulder bed! The surrounding Greater Himalaya are made up of high grade metamorphic rocks. You can spot quartzo- feldspathic gneiss, amphibolite gneiss, mica schist and gneiss, and mica, garnet, and kyanite bearing schist and gneiss in the river bed. Quite a treat to walk along this metamorphic treasure! 

 Crossing the wider channels on rickety wooden bridges was fun! 

Here we are near Maiktoli Top, a high vantage point. Jagdish Bisht, me, Ratan Singh Danu, Lucky, and Kapil. They made my trip safe, comfortable, and memorable.

Why I go to these places. A clear view of the bands of metamorphic rocks exposed along the spectacular cliff face of the Sunderdunga ridge!

 

Village Khati is such a pretty place.

The high bare peaks in the background speak of a worrying trend. Everyone I talked to told me that this trek would have been impossible a few years ago in mid November. The upper part of the valley and the rocky ridges would have been blanketed in a thick snow pack. This area still had not received a single snowfall when I left on 22nd November. The two or three big snowfalls of the year now occur mostly in January and February. The pastoral and agriculture economy depends on a healthy winter snow cover to rejuvenate the high meadows, and to replenish springs and streams.

My guide tells me that Sunderdunga valley is the tougher route amongst the three treks to the nearby glaciers. I am so glad I walked this valley!

Darwin's House Plants, Water Diviners, Geology Podcast

 A couple of good articles and a geology podcast.

1) “Spontaneous Revolutions” Darwin’s Diagrams of Plant Movement: Darwin's unbounded curiosity for nature led him down many unexpected research pathways. Towards the end of his long career, his restless mind noticed the growth patterns of his house plants. Determined to understand more about their motion and the stimuli, he spent hours tracking tendrils grow and came up with innovative ways to record their movements on paper. Natalie Lawrence has written a lovely essay on this lesser known chapter of Darwin's life and work. 

2) Trust, cost go greater depths to sustain unscientific water divining practice: Large swaths of Indian agriculture is desperately dependent on access to groundwater. Simrin Sirur explores the reliance on water diviners in south India. Diviners use sticks, coppers tongs, coconuts, magnetic compass, and chains with keys as their instruments for sensing groundwater. Despite all this unscientific baggage, many diviners are not all that ignorant. They have a knowledge of the local landscape and groundwater availability. Their prediction relies more on their past experience and a dollop of common sense. 

I must tell you about my experience with a diviner. My neighbor requested that I accompany her to a plot of land outside Pune. She had hired a diviner to help her locate groundwater. We picked him up en route. He was the late Pandit Bhimsen Joshi's son! On reaching my friend's property he got to work with copper tongs. After a few minutes of walking  up and down the site the copper tongs started shaking. He indicated the spot to drill and suggested going down to a depth of 150 feet. On the way back he cheerfully told us that he knew that the adjacent plot owner had struck water at 150 feet. Past experience and common sense go a long way! 

3) Geology Bites Podcast:  Conversations with Geologists: Oliver Strimpel has had quite an unusual career beginning with a doctoral degree in astrophysics. He later became the director of the Computer Museum in Boston and then a patent attorney. But geology beckoned him. He has worked alongside geology researchers trying to date rocks and unravel the timing of movement of the Karkoram fault in Ladakh. Geology Bites grew out of his passion for the subject. You will find a wide range of geology topics discussed on this site. 

I have so far listened to experts talk about radioactive waste disposal, continental crust composition, the inherent bias in the global sedimentary record, and on the evolution of minerals through geologic time. All have been excellent. The talks are about half hour, so they don't tax your patience too much. 

If you have free time coming up this Diwali, I recommend you dive into this collection of geology talks.

The Garden Of Ediacara

I came across this lovely evocative passage in Nick Lane's book Transformer: The Deep Chemistry of Life and Death.

"You are not completely spineless. You have a notochord: a flexible rod made of cartilage, which in your descendant, millions of generations hence, will develop into a proper backbone. For now, you flex your rod like an eel to undulate through the water, never quite fast enough. Better to stay submerged in the soft mud at the bottom, with only your head visible, while you filter out grains of food from the swell. You have a wormlike head, with a small bulging of nerves that will one day become your brain. Your eyes aren't much use, but at least you can make out the looming of a monster, and swiftly bury your head again. Oh, times have changed. Not long ago, the world was full of gently filter feeders, swaying their fronds softly in unison, never harming a soul. Not that you remember, except in some hazy instinctive yearning for the garden of Ediacara. But now there are vast armour-plated war machines, bristling with claws and spikes and rows upon rows of crystalline eye facets fixing you from every dimension. You are a tender morsel, barely a couple of inches long, protein-rich muscle strapped to a crispy rod; a tasty snack for Anomalocaris. Better pull in your head again, just in case- being a little bit spineless might help you survive in this fearsome new world, outnumbered a thousand to one by spiny monsters".

The passage describes the early Cambrian world (540-510 million years ago) which saw the rapid diversification of the animal biosphere. The ancestors of vertebrates had worm like bodies, and Anomalocaris, an early arthropod, was top predator. Much before, the garden of Ediacara was a very different place. Complex multicellular life appears in the fossil record from about 570 million years ago in the Ediacaran Period. These creatures were sessile (fixed to the sea floor) filter feeders with body shapes resembling large leaves and fronds. Evidence of mobile animals manifests by 550 million years ago. Their tracks, trails, and burrows are preserved in soft sediment.

The graphic shows this  'Edicaran biota', a term that includes a diverse and unrelated groups of organisms. Three distinct phases, termed Avalon, White Sea, and Nama,  showing different community assemblages and increasing ecologic specialization are recognizable through the Ediacaran Period. Notice that mobile bilateral creatures first appear in the White Sea (B) assemblage. 

 Source: Rebecca Eden, Andrea Manica, Emily G. Mitchell: Plos Biology 2022.

Later in the chapter Nick Lane elaborates on how many of these Ediacaran filter feeders, simpler creatures without specialized tissues for different functions,  could not cope with the anoxic sulpur rich environments and died out. Sponges notably did survive. Mobile animals though had evolved a rudimentary circulatory system and molecules like myoglobin and haemoglobin, capable of storing oxygen and removing carbon dioxide. When oxygen levels increased in the Cambrian their descendants had metabolic machinery to take advantage of this high-octane environment. The radiation of animals utterly transformed our world. 

I highly recommend this book. It has a fair bit of chemistry in it. Nick Lane explains much of it using easier to follow diagrams instead of the dreaded chemical equations of our college years. He is a firm advocate of the metabolism first (as against a RNA/genes first) view of the origin of life and provides elegant explanations of energy flow and the evolution of metabolic pathways that build organic molecules to form biomass and breaks them up to power respiration. Disease and ageing is the inevitable consequence of the eventual degradation of these metabolic reactions. 

At the heart of all this is the Kreb's cycle, a series of reactions which burn sugars in oxygen to generate energy for cellular functions. But the surprise is that much of life can get by with only a partial Kreb's cycle. In many microbes, it is not a cycle at all, but a short linear path. Not at all what our biochemistry book taught us and a lesson for creationists who insist that systems like the Kreb's Cycle are irreducible complex, a sign of intelligent design, which could not have evolved through incremental steps.

Nick Lane- Transformer: The Deep Chemistry of Life and Death.